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\DOI{10.5802/crchim.456}
\datereceived{2026-02-10}
\daterevised{2026-04-29}
\dateaccepted{2026-04-30}
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\COI{The authors do not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
have declared no affiliations other than their research organizations.}

%\dateposted{2026-04-23}

\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\title{A three-decade journey in organometallic dendritic macromolecules}

\alttitle{Un voyage de trois d\'{e}cennies dans les macromol\'{e}cules
dendritiques organom\'{e}talliques}

\author{\firstname{Carmen Mar\'\i{}a} \lastname{Casado}\CDRorcid{0000-0003-2348-9366}\IsCorresp}
\address{Dpto. de Qu\'{i}mica Inorg\'{a}nica, Facultad de Ciencias,
U.A.M., Cantoblanco, 28049 Madrid, Spain}
\address{Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad
Aut\'{o}noma de Madrid, Madrid 28049, Spain}
\email[C. M. Casado]{carmenm.casado@uam.es}

\author{\firstname{Beatriz} \lastname{Alonso}\CDRorcid{0000-0001-9082-8466}\IsCorresp} 
\addressSameAs{1}{Dpto. de Qu\'{i}mica Inorg\'{a}nica, Facultad de Ciencias,
U.A.M., Cantoblanco, 28049 Madrid, Spain}
\addressSameAs{2}{Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad
Aut\'{o}noma de Madrid, Madrid 28049, Spain}
\email[B. Alonso]{beatriz.alonso@uam.es}

\author{\firstname{Mar\'\i{}a Pilar} \lastname{Garc\'{i}a-Armada}\CDRorcid{0000-0003-2410-3365}} 
\address{Dpto. Ingenier\'{i}a Qu\'{i}mica y Medio Ambiente, E.T.S.I.I.,
U.P.M., Jos\'{e} Guti\'{e}rrez, Abascal, 2, 28006 Madrid, Spain}
\email[M. P. Garc\'{i}a-Armada]{pilar.garcia.armada@upm.es}

%\shortrunauthors

\keywords{\kwd{Organometallic dendrimers}
\kwd{Heterometallic dendrimers}
\kwd{Ferrocene}
\kwd{Metallocenes}
\kwd{Electrochemistry}
\kwd{Biosensors}}

\altkeywords{\kwd{Dendrim\`{e}res organom\'{e}talliques}
\kwd{Dendrim\`{e}res h\'{e}t\'{e}rom\'{e}talliques}
\kwd{Ferroc\`{e}ne}
\kwd{M\'{e}talloc\`{e}nes}
\kwd{\'{E}lectrochimie}
\kwd{Biocapteurs}}

\begin{abstract}
This review summarizes contributions made by the authors between 1990
and 2025 in the design, synthesis, and application of redox-active
organometallic dendrimers and polymers. Emphasis is placed on the
progressive evolution from fundamental synthetic studies to functional
devices, highlighting how dendritic architectures based on ferrocene
and related units have provided a rich and adaptable platform at the
interface of inorganic, polymer, and materials chemistry.
\end{abstract}

\begin{altabstract}
Cette revue r\'{e}sume les contributions des auteurs r\'{e}alis\'{e}es
entre 1990 et 2025 dans la conception, la synth\`{e}se et
l'application de dendrim\`{e}res et de polym\`{e}res
organom\'{e}talliques \`{a} activit\'{e} r\'{e}dox. L'accent est mis
sur l'\'{e}volution progressive, depuis des \'{e}tudes
synth\'{e}tiques fondamentales jusqu'\`{a} des dispositifs
fonctionnels, en mettant en \'{e}vidence comment les architectures
dendritiques fond\'{e}es sur le ferroc\`{e}ne et des unit\'{e}s
apparent\'{e}es ont fourni une plateforme riche et adaptable \`{a}
l'interface de la chimie inorganique, de la chimie des polym\`{e}res
et de la chimie des mat\'{e}riaux.
\end{altabstract}

\editornote{Article submitted by invitation}
\alteditornote{Article soumis sur invitation}

\thanks{Direcci\'on General de Investigaci\'on Cient\'ifica y T\'ecnica
(87/0123, N 90/0227, 93/0287), Direcci\'on General de Ense\~{n}anza
Superior e Investigaci\'on Cient\'ifica (PB97-0001), Spanish
Direcci\'on General de Investigaci\'on (CTQ2004-07381-C02,
CTQ2009-12332-C02), Consejer\'ia de Educaci\'on, Comunidad de Madrid
(S-0505/PPQ-0328).}

%\input{CR-pagedemetas}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

Organometallic dendrimers and polymers have emerged over the past three
decades as highly versatile redox-active
macromolecules~\cite{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}. The
well-defined architectures of dendrimers, combined with the unique
electronic properties of metallocene units such as ferrocene and
cobaltocenium, offer unprecedented opportunities for tuning
electrochemical behavior, constructing molecular reservoirs for charge
storage, and developing functional interfaces for sensing and
catalysis.

{\advance\baselineskip1pt
From the early 1990s, the effort of our group was directed toward the
incorporation of \mbox{organometallic} moieties into silicon-based scaffolds,
such as carbosilanes, cyclosiloxanes and silsesquioxanes, yielding some
of the first examples of redox-active organometallic polymers. These
studies established the fundamental design principles of electronically
active macromolecular systems, in which the proximity and connectivity
of metal centers dictate the degree of intramolecular communication and
cooperative redox behavior~\cite{16}.

This chemistry expanded into the field of carbosilane and carbosiloxane
dendrimers, whose flexible frameworks enabled the controlled
positioning of organometallic units at the periphery or within the
interior of the macromolecule~\cite{17,18}. 
}

A parallel line of development emerged through the design of
amine-based organometallic dendrimers, which provided exceptionally
versatile platforms for peripheral functionalization across multiple
generations. Their branching amine frameworks enabled the systematic
incorporation of diverse terminal groups, including thiol and
organometallic amide and urea functionalities. The ability to tailor
the outer shell greatly expanded the scope of organometallic dendritic
materials, establishing amine-based dendrimers as key scaffolds for
coupling structural complexity with functional responsiveness. Such
architectures proved particularly attractive for constructing hybrid
systems in which the dendrimer periphery acts as a chemically
addressable interface, facilitating supramolecular recognition,
selective coordination to metal surfaces, and the formation of
electroactive thin films. 

A major turning point came with the application of these macromolecules
to sensing and bioelectrochemistry. Dendrimers and polymers were
exploited as mediators in amperometric biosensors, as scaffolds for the
immobilization of enzymes, and as nanoscale hosts for fluorescent or
electroactive probes. This transition marked the evolution from
structural organometallic chemistry to device-oriented applications,
with a strong impact on the detection of biologically and
environmentally relevant analytes~\cite{19}. 

\section{Early developments. Redox-active polymers}\label{sec2}

The early 1990s marked the emergence of redox-active organometallic
polymers as a coherent research field in Mor\'{a}n's group. Within this
framework, the fundamental synthetic principles that would later guide
dendrimer and macromolecular design were established. Initial efforts
focused on the strategic incorporation of organometallic units---such
as arene chromium tricarbonyl, ferrocene, and cobaltocenium
fragments---into silicon-based polymeric scaffolds. These studies
provided some of the earliest examples of macromolecules exhibiting
well-defined, reversible redox behavior (i.e., reversible
electrochemistry). 

One of the earliest and most versatile synthetic strategies was
Pt-catalyzed hydrosilylation, which enabled efficient formation of
Si--C bonds between Si--H-containing siloxanes,
silsesquioxanes or cyclosiloxanes and vinyl-substituted ferrocene
derivatives (Figure~\ref{fig1}). In 1993, Casado and coworkers used this
methodology to prepare the first ferrocenyl-functionalized
octasilsesquioxanes~\cite{20}, by reacting
octakis(hydrodimethylsiloxy)silsesquioxane with vinylferrocene or
divinylferrocene. These reactions, catalyzed by Karstedt's catalyst,
afforded mono- and poly-ferrocenyl silsesquioxanes and ultimately
poly(ferrocenyl-octasilsesquioxanes). This hydrosilylation route was
later extended to cyclosiloxane frameworks~\cite{21}, demonstrating
that Si--H-functionalized cyclotetrasiloxanes could also
serve as efficient platforms for organometallic incorporation.
Hydrosilylation of 1,3,5,7-tetramethylcyclotetrasiloxane with
vinylferrocene afforded a fully substituted tetranuclear ferrocenyl
model compound, confirming that clean and complete functionalization of
the Si--H groups could be achieved under mild Pt-catalyzed
conditions without siloxane ring degradation. Using analogous reactions
with divinylferrocene or divinyloctamethylferrocene, the corresponding
ferrocenyl and octamethylferrocenyl cyclosiloxane-based polymers were
prepared, representing a new class of silicon--ferrocene
hybrid materials in which the organometallic units are embedded
directly within a siloxane ring framework.

\begin{figure*}
\includegraphics{fig01}
\vspace*{-1pt}
\caption{\label{fig1}Representative examples of ferrocenyl polymers.}
\end{figure*}

These two families of cyclosiloxane- and silsesquioxane-based polymers
were shown to form stable free-standing films and exhibited distinct
electrochemical signatures that depended on the steric and electronic
properties of the cyclopentadienyl substituents. Films of the
ferrocenyl polymers displayed sharp, surface-confined redox waves,
whereas the permethylated analog exhibited broader,
diffusion-controlled responses and significantly more negative redox
potentials due to the strong electron-donating effect of the additional
methyl groups. This work not only expanded the scope of
hydrosilylation-based access to ferrocene--siloxane hybrid
architectures but also provided early insights into how siloxane
frameworks and Cp-ring substitution patterns govern redox behavior in
silicon-containing organometallic polymers. 

Parallel work explored condensation reactions to produce polysiloxanes
and polysilanes in which ferrocenyl groups were connected via amide
linkages (Figure~\ref{fig2}). These studies demonstrated that polymer
architecture could be modulated through linker choice and chain
topology~\cite{22}. Two general synthetic routes were explored. The
first method consisted in exploiting the ability of (chlorocarbonyl)
\mbox{ferrocene} and 1,1$'$-bis(chlorocarbonyl)ferrocene to undergo classical
condensation reactions, with amine-functionalized siloxanes in the
presence of a base to neutralize the acidic byproduct liberated in the
reaction. In the second approach, an organometallic moiety that
chemically behaves as a Lewis base was allowed to react with an
organosilane and a poly(methylsiloxane) functionalized with acid
chloride groups. In this method, the key starting ferrocene monomers
were (${\upbeta}$-aminoethyl)ferrocene and
1,1$'$-bis(${\upbeta}$-aminoethyl)ferrocene. These synthetic routes
produced polymers in which the ferrocene units could be positioned
either as main-chain elements or as pendant groups, allowing systematic
investigation of mobility, electronic communication, chain topology,
and redox behavior. Electrochemical measurements demonstrated that the
ferrocenyl centers behaved as independent \mbox{redox} sites, generating
single reversible waves corresponding to multiple simultaneous
one-electron oxidations---an early demonstration of ``molecular
multielectron'' behavior in macromolecules.

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Example of a polysiloxane with amide-linked
ferrocenyl moieties.}
\end{figure}

To extend the scope beyond ferrocene, synthetic efforts were also
directed toward chromium tricarbonyl--functionalized
derivatives~\cite{23}. Notably, early polymers were obtained by
reacting Cr(CO)\tsub{6} with preformed aromatic polymers such as
phenyl-substituted polysiloxanes. Polymers incorporating
$\upeta^{6}$-arene--Cr(CO)\tsub{3} fragments provided a distinct
electrochemical signature and established the viability of integrating
low-valent transition-metal carbonyl units into macromolecular
frameworks while retaining reversible redox behavior. These
chromium-containing systems broadened the conceptual landscape of
redox-active polymers beyond metallocenes, revealing alternative
strategies for tuning multielectron processes through metal--ligand
interactions.

A major milestone was achieved with the introduction of
cobaltocenium-based polymers (Figure~\ref{fig3})~\cite{24}, which
expanded the electronic versatility of the field by adding a
permanently charged, chemically robust redox unit. The key to
incorporating cobaltocenium units into polymeric scaffolds lay in the
high reactivity of (chlorocarbonyl)cobaltocenium and its
bis(chlorocarbonyl) analog toward amine- or alcohol-functionalized
organic and silicon-based precursors. Cuadrado and coworkers adapted
classical condensation chemistry---previously applied to amide-linked
ferrocene polysiloxanes---to access both pyrrole- and
allyl-functionalized cobaltocenium monomers as well as siloxane-based
macromolecules containing cobaltocenium fragments either in the polymer
backbone or as \mbox{pendant} redox-active side groups. In a complementary
advance, the authors introduced electropolymerizable cobaltocenium
monomers---notably the pyrrole-functionalized species---which, upon
oxidative electropolymerization, produced surface-confined polypyrrole
films embedding cobaltocenium units with well-defined, reversible
electrochemistry. These electrogenerated polymer films, displaying
robust adhesion and persistent redox activity in both organic and
aqueous media, provided one of the first demonstrations that cationic
metallocene motifs could be integrated into conducting polymer matrices
without loss of electrochemical stability. Taken together, these
studies established condensation-driven routes as a powerful complement
to hydrosilylation for preparing redox-active organosilicon polymers,
and they highlighted the distinctive opportunities offered by
cobaltocenium, whose intrinsic positive charge and accessible
Co(III)/Co(II) couple opened new design principles for multielectron
processes and electrode--polymer interfaces. These cationic
organometallic polymers offered new opportunities for studying charge
transport in polyelectrolyte systems, designing redox-responsive
materials with inherent ionic conduction, and creating positively
charged interfaces for electroanalytical applications. Their emergence
marked a transition from neutral metallocene systems toward
redox-active polymers with built-in ionic functionalities.

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}Example of a polysiloxane with amide-linked
cobaltocenium moieties.}
\end{figure}

A further expansion of hydrosilylation-based strategies emerged with
the incorporation of silyliron dicarbonyl fragments into linear and
cyclic siloxanes as well as dendritic carbosilane scaffolds,
demonstrating the versatility of Si--H-functionalized backbones for the
construction of multimetallic macromolecular architectures. In close
analogy to the ferrocene--siloxane systems prepared in the early 1990s,
Ram\'{i}rez-Oliva and coworkers employed Karstedt-catalyzed
hydrosilylation to attach the vinyl-substituted silyliron complex
($\upeta^{5}$-C\tsub{5}H\tsub{5})Fe(CO)\tsub{2}Si(CH\tsub{3})\tsub{2}CH${=}$CH\tsub{2} 
to Si--H-containing frameworks spanning cyclotetrasiloxanes,
poly(methylhydrosiloxane-\textit{co}-dimethylsiloxane) copolymers,
and carbosilane dendrimers~\cite{25}. This methodology parallels the
classical Pt-catalyzed hydrosilylation used for ferrocenyl
silsesquioxanes and cyclosiloxanes but adapts it to a distinct
organoiron fragment whose vinyl handle enables clean, quantitative
consumption of the Si--H functionality. The resulting tetrametallic
siloxane, tetrametallic carbosilane dendrimer, and polymeric siloxane
derivative (Figure~\ref{fig4}), represent a cohesive family of
multimetallic polymers in which the organometallic units are tethered
to the siloxane or carbosilane framework through a flexible
two-methylene spacer. Electrochemical studies revealed redox processes
characteristic of electron-deficient iron carbonyl fragments,
proceeding through EC-type mechanisms involving oxidative CO
loss---behavior distinct from the reversible redox waves of
metallocene-based polymers, yet equally instructive in demonstrating
how metal--ligand electronic structure governs redox accessibility
within hybrid silicon--organometallic materials. 

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Example of a polysiloxane with silicon-linked
cyclopentadienyl dicarbonyl iron moieties.}
\end{figure}

By the end of the 1990s, organometallic polymer chemistry had reached a
mature and well-defined stage. Subsequent decades would build upon
this foundation with more complex architectures, but the essential
principles of redox-active macromolecular design were solidly rooted in
this early period.

\section{Growth of organometallic carbosilane dendritic
architectures}\label{sec3}

In 1994, with Alonso's doctoral thesis, we moved toward more
structurally defined architectures, exploring dendritic systems and
focusing on organosilicon dendrimers as ideal platforms for
incorporating ferrocenyl units~\cite{26}. This shift allowed us to
exploit the pronounced reactivity of Si--Cl, Si--H, and Si--allyl or
Si--vinyl functionalities in dendritic frameworks, enabling efficient
attachment of diverse organometallic monomers at their peripheries and
ultimately providing access to different families of electroactive
dendrimers. The development of carbosilane dendrimers functionalized
with organometallic units marked a turning point in the construction of
redox-active macromolecules, providing unprecedented control over
molecular topology, surface functionality, and the spatial disposition
of organometallic groups. Unlike earlier siloxane- and
silsesquioxane-based frameworks, carbosilane dendrimers rely on robust
Si--C linkages and modular branching units, making them particularly
suited for stepwise elaboration under hydrosilylation, allylation, and
condensation conditions.

\subsection{Ferrocenyl-functionalized carbosilane dendrimers and
dendritic wedges}

Carbosilane dendrimers with peripheral ferrocenyl units were among the
earliest and most thoroughly studied families~\cite{26}. Initial growth
of carbosilane dendritic frameworks relied on a \textbf{divergent
strategy} centered on iterative hydrosilylation and allylation
cycles~\cite{27,28,29}. Using polyfunctional silicon nodes such as
tetraallylsilane, cyclotetrasiloxane or octasilsesquioxane derivatives
as initiation points, dendritic generations were expanded through
Pt-catalyzed hydrosilylation of terminal allyl or vinyl groups with
chlorosilanes (Me\tsub{2}SiHCl, MeHSiCl\tsub{2}), producing dendritic
intermediates bearing Si--Cl functionalities and further alkenylation
reactions via Grignard reagents, typically allyl or vinylmagnesium
halide, regenerating outward-pointing allyl or vinyl termini for
further growth. This methodology furnished up to three-generation
dendritic chlorosilanes with four, eight, and sixteen terminal Si--Cl,
Si--H and Si--allyl or Si--vinyl periphery sites
(Figure~\ref{fig5})~\cite{17,26,30,31}.

\begin{figure*}
\vspace*{3pt}
\includegraphics{fig05}
\vspace*{1pt}
\caption{\label{fig5}Representative examples of carbosilane dendritic
frameworks.}
\vspace*{4pt}
\end{figure*}

\begin{figure*}
\includegraphics{fig06}
\vspace*{1pt}
\caption{\label{fig6}Representative cyclic voltammograms of
polyferrocenyl carbosilane dendrimers (a) in dichloromethane solution
(b)~immobilized on a Pt disk electrode. (Adapted with permission from
\cite{17}. Copyright 1999 Elsevier.)}
\end{figure*}

For the functionalization with ferrocenyl units, three principal
synthetic routes were employed (Scheme~\ref{sch1}): (i) Reaction of
dendritic Si--Cl groups with ferrocenyllithium afforded polyferrocenyl
dendrimers, among the first examples of organometallic dendritic
molecules with a controlled number of identical redox
units~\cite{26,30}; (ii) Hydrosilylation of vinylferrocene with
Si--H-functionalized carbosilanes provided alternative access to
tetra-, octa-, and hexadeca-ferrocenyl dendrimers~\cite{18}; (iii) The
condensation reaction of (${\upbeta}$-aminoethyl)ferrocene with the
highly reactive Si--Cl peripheral groups in the dendrimers also offered
access to dendrimers with four, eight and sixteen ethylferrocenyl units
and Si--NH groups~\cite{26}. 

\begin{scheme*}
\includegraphics{sc01}
\caption{\label{sch1}Three synthetic routes to polyferrocenyl
carbosilane dendrimers.}
\vspace*{-8pt}
\end{scheme*}

Electrochemical studies revealed a single, reversible oxidation wave in
each dendrimer, corresponding to a simultaneous multielectron oxidation
of all ferrocenyl centers, four, eight, and sixteen electrons for
first-, second-, and third-generation examples, respectively, which
demonstrates their electronic independence (Figure~\ref{fig6}a). These
findings established a key principle in carbosilane dendrimer
electrochemistry: the dendritic scaffold electrically isolates
peripheral ferrocene units, enabling predictable multielectron
responses and facilitating the design of dendrimer-based electron
reservoirs and sensing interfaces. 


Interestingly, upon oxidation, these ferrocenyl-functionalized
carbosilane dendrimers were shown to spontaneously deposit onto
electrode surfaces, forming coherent electroactive films that preserved
the characteristic single multielectron redox wave of the molecular
precursors. Cyclic voltammetry of the surface-bound tetra- and
octaferrocenyl species revealed well-defined, symmetric redox responses
with the expected linear dependence of peak current on scan rate,
confirming true surface-confined electron transfer behavior
(Figure~\ref{fig6}b). Importantly, the formal potentials of the
immobilized dendrimers remained essentially identical to those measured
in solution, indicating that surface confinement does not perturb the
intrinsic redox properties of the peripheral ferrocenyl units. A
notable feature of these dendrimer-derived films is their exceptional
stability and robustness. Overall, these results confirmed the
practical feasibility of using organometallic carbosilane dendrimers as
well-defined multielectron platforms for electrode
modification~\cite{32}. 

Beyond their well-defined multielectron redox behavior,
ferrocenyl-functionalized carbosilane dendrimers bearing peripheral
Si--NH groups were shown to act as redox-responsive receptors for
anionic species. In their neutral state, anion recognition occurs
through cooperative hydrogen-bonding interactions, while upon
electrochemical oxidation of the ferrocenyl units, electrostatic
attractions further enhance guest binding. Notably, these dendrimers
can be readily immobilized onto electrode surfaces by electrooxidation,
yielding modified electrodes that display a clear and sensitive
electrochemical response to anions. This behavior demonstrated the
feasibility of employing such organometallic dendrimers as
electrochemical sensors, combining controlled multielectron redox
activity with selective molecular recognition~\cite{33}.

\begin{figure*}
\includegraphics{fig07}
\vspace*{-3pt}
\caption{\label{fig7}Silicon-based dendritic wedges.}
\end{figure*}


To improve synthetic efficiency and facilitate modular variation of
organometallic groups, carbosilane dendrimers were also constructed
using a \textbf{convergent approach}. This strategy involves the
preparation of dendritic ``wedges''---organometallic silanes containing
a single terminal vinyl, allyl or Si--H group---followed by attachment
to Si--H or alkenyl-functionalized carbosilane cores, respectively, via
hydrosilylation~\cite{31,34,35,36,37,38}. 

The smallest dendritic wedges were the silicon-bridged biferrocenes
\textbf{1} and \textbf{2}, and ferrocenylmethylphenylvinylsilane
(\textbf{3}) (Figure~\ref{fig7}), which were prepared by reaction of
ferrocenyllithium with vinylmethyldichlorosilane,
dichloromethylsilane~\cite{37}, and methylphenylvinylchlorosilane,
respectively. Further growth of the first-generation dendrons
\textbf{1} and \textbf{3} was achieved by Pt-catalyzed hydrosilylation
with phenylchlorosilane, resulting in dendrons which contain a reactive
chlorosilane functionality available for an ensuing alkenylation step
with allylmagnesium bromide, to afford the desired growth dendrons
\textbf{4} and \textbf{5} (Scheme~\ref{sch2}). 

\begin{scheme}
\includegraphics{sc02}
\vspace*{5pt}
\caption{\label{sch2}Growth of dendritic wedges.}
\vspace*{-11pt}
\end{scheme}


The availability of alkenyl or Si--H substituents at the focal point of
the dendritic wedges enabled their incorporation into different Si--H-
or alkenyl-polyfunctionalized carbosilane, cyclotetrasiloxane and
silsesquioxane dendritic cores as well as linear polysiloxanes via
hydrosilylation chemistry (Scheme~\ref{sch3}). The electrochemical
behavior observed for these dendritic molecules is consistent with the
presence of significant interactions between the two ferrocenyl units
bridged through silicon atoms. The cyclic voltammograms recorded in
dichloromethane solution are characterized by two well-separated,
reversible oxidation waves of equal intensity (Figure~\ref{fig8}). The
first oxidation occurs at nonadjacent ferrocene sites within the
dendritic wedges or dendrimers, making the subsequent removal of
electrons from the remaining ferrocenyl centers, which are adjacent to
those already oxidized, more difficult. As reported by Alonso et~al.,
these compounds~\cite{31,34} \mbox{constituted} the first examples of
organometallic dendritic molecules exhibiting electronic communication
between transition-metal centers, not only in solution but also when
confined to electrode \mbox{surfaces}. 

\begin{scheme*}
\includegraphics{sc03}
\caption{\label{sch3}Growth of an octasilsesquioxane dendritic core and
functionalization with silicon-bridged biferrocene \textbf{1}.}
\vspace*{-8pt}
\end{scheme*}

\begin{figure*}
\includegraphics{fig08}
\vspace*{-3pt}
\caption{\label{fig8}Representative cyclic voltammograms of carbosilane
dendrimers with electronically communicated ferrocenyl units (a) in
dichloromethane solution (b) immobilized on a Pt disk electrode.
(Adapted with permission from~\cite{34}. Copyright 1997
American Chemical Society.)}
\vspace*{-3pt}
\end{figure*}

By combining divergent and convergent methodologies, ferrocenyl or
ferrocenyl--aryl groups can be systematically incorporated at each
generation, \mbox{leading} to high surface densities of organometallic units.
As we will see in the following section, these transformations permit
the construction not only of homometallic dendrimers but also of
heterometallic arrays, a significant advancement in the design of
multielectron macromolecular systems. 


\subsection{Arene--Cr(CO)\tsub{3}-functionalized dendrimers and
heterometallic arrays}

Lobete and coworkers~\cite{39} reported one of the earliest examples of
silicon-based \mbox{organometallic} \mbox{dendrimers} bearing $\upeta^{6}$-arene metal
fragments, demonstrating how dendritic architectures can serve as
multidentate platforms for surface organometallic coordination. Using a
divergent synthetic strategy from tetrapropenylsilane, we prepared
first- and second-generation organosilicon dendrimers functionalized at
their periphery with phenyl groups. These terminal arene sites were
then exploited as $\upeta^6$-binding motifs for Cr(CO)\tsub{3} units,
enabling construction of a family of dendritic organometallic species.
Thermal treatment of the first generation phenyl-terminated dendrimer
with Cr(CO)\tsub{6} at 140~\textdegree C yielded either a tetrametallic
dendrimer---with full coordination of four arene groups---or a
monometallic analog, depending on stoichiometry. Reaction with the
second-generation dendrimer afforded a tetrachromium complex, although
full octa-functionalization could not be achieved under these
conditions due to decomposition at the elevated temperatures required. 


As part of our efforts to develop dendrimeric macromolecules bearing
redox-active organometallic units at predetermined positions,
polyfunctional ferrocenyl derivatives were explored as core building
blocks~\cite{40}. In a further step toward heterometallic dendrimers,
we combined in a single dendritic scaffold an electron-donating
ferrocenyl fragment with the electron-withdrawing
($\upeta^{6}$-aryl)Cr(CO)\tsub{3} moiety. In this context,
1,1$^{\prime}$-bis(dimethylvinylsilyl)ferrocene, synthesized by
reaction of 1,1$'$-dilithioferrocene with dimethylvinylchlorosilane, was
successfully employed as a two-directional core for the synthesis of
novel redox-active homo- and heterometallic pentanuclear systems, which
can be regarded as first-generation dendrimer models. Peripheral
functionalization with ferrocenyl and
($\upeta^{6}$-C\tsub{6}H\tsub{5})Cr(CO)\tsub{3} moieties was achieved
(Scheme~\ref{sch4}), and electrochemical studies revealed that the
extent of electronic communication between the metal centers bridged by
silicon atoms depends strongly on their chemical nature.\looseness=-1 

\begin{scheme}
\includegraphics{sc04}
\vspace*{5pt}
\caption{\label{sch4}Synthesis of homo- and heterometallic dendrimers
from ferrocene as core.}
\vspace*{-6pt}
\end{scheme}

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig09}
\vspace*{-2pt}
\caption{\label{fig9}Organometallic silicon-based dendrimers with
peripheral Si--cyclopentadienyl, Si--Co and Si--Fe $\upsigma$-bonds.}
\vspace*{-2pt}
\end{figure*}

Terminal aryl groups present in certain ferrocenyl aryl carbosilane
dendrimers or dendrons offer an orthogonal functionalization route via
$\upeta^{6}$-coordination to Cr(CO)\tsub{3}~\cite{36}. The resulting
heterometallic systems featuring Si-bridged Fc--Cr(CO)\tsub{3} pairs in
close proximity were accessed through a convergent growth strategy,
either by hydrosilylation of Cr(CO)\tsub{3}-functionalized dendrons or,
more reliably, by post-metallation of preformed \mbox{ferrocenyl} dendrimers
with Cr(CO)\tsub{6}, circumventing the reduced reactivity of
Cr(CO)\tsub{3}-bound vinyl groups. Electrochemical studies showed that
ferrocenyl and chromium centers oxidize at distinct potentials,
evidencing their electronic independence within the insulating
carbosilane matrix, while generation and metal loading modulate the
appearance of sequential or merged multielectron redox waves.

\subsection{Carbosilane dendrimers decorated with other organometallic
fragments}
\subsubsection{Si--cyclopentadienyl, Si--Co and Si--Fe $\upsigma$-bonds}

Given the central role of cyclopentadienyl ligands in organometallic
chemistry, our group explored their incorporation as peripheral
functionalities in well-defined dendritic architectures, with the aim
of generating versatile organometallic dendrimers~\cite{41}. In this
context, early generations of silicon-based dendrimers bearing
cyclopentadienyl, carbonylcobalt, and carbonyliron units at the
periphery were developed (Figure~\ref{fig9}).
Cyclopentadienyl-functionalized organosilicon dendrimers were obtained
via the reaction of alkali cyclopentadienides with a tetrafunctional
silicon dendrimer scaffold. The coordinating ability of the
surface-bound \mbox{cyclopentadienyl} ligands was demonstrated through
complexation with Co\tsub{2}(CO)\tsub{8}, yielding multinuclear cobalt
carbonyl derivatives. In parallel, direct reactions of dicobalt
octacarbonyl or iron carbonyl anions with Si--H- or
Si--Cl-functionalized dendrimers enabled the formation of
cobalt--silicon and iron--silicon $\upsigma$-bonded species,
respectively. \looseness=1

As a continuation of our work on multimetallic organometallic
architectures, our group extended hydrosilylation methodologies to the
construction of silane- and siloxane-based systems bearing multiple
iron--silicon bonds. Building on earlier carbosilane dendrimers in
which the metal centers are ${\upsigma}$-bonded to the dendritic
framework, a vinyl-functionalized silyliron fragment,
($\upeta^{5}$-C\tsub{5}H\tsub{5})Fe(CO)\tsub{2}Si(CH\tsub{3})\tsub{2}CH=CH\tsub{2},
was prepared and employed as a key building block~\cite{25}.
Platinum-catalyzed hydrosilylation of this fragment with
Si--H-functionalized linear and cyclic siloxanes, carbosilane
dendrimers, and polysiloxane backbones afforded a series of
multimetallic compounds with controlled incorporation of organometallic
units. These results demonstrate the versatility of hydrosilylation as
a general strategy for the assembly of complex iron-containing
macromolecular systems.

\subsubsection{Alkyne-complexed and cluster-functionalized dendrimers} 

Further diversification of carbosilane dendrimers has been achieved
through the use of alkynyl-functionalized architectures, which provide
an efficient entry to more complex organometallic and heterometallic
systems~\cite{42,43}. Ferrocenylalkynes, and in particular the
Fc--C${\equiv}$C motif, have long been recognized as valuable building
blocks in \mbox{molecular} electronics and organometallic synthesis, yet
dendritic systems bearing ferrocenylethynyl or butadiynyl units remain
comparatively rare. Exploiting this unit, first- and second-generation
carbosilane dendrimers were functionalized at their periphery with
ferrocenylalkynyl and ferrocenylbutadiynyl groups via lithiation of the
corresponding ferrocenylalkynes and subsequent reaction with
chlorosilane-terminated dendritic scaffolds, affording well-defined
homometallic dendrimers with controlled numbers of redox-active
termini.

These alkynyl-linked ferrocenyl dendrimers served as effective
precursors for the construction of heterometallic systems through
coordination of the C${\equiv}$C units to transition metal carbonyl
clusters. In particular, reaction with the activated triosmium cluster
Os\tsub{3}(CO)\tsub{10}(NCMe)\tsub{2} led to the clean formation of
novel heterometallic dendrimers in which each ferrocenylalkynyl arm is
selectively coordinated to a trinuclear osmium fragment~\cite{42}
(Scheme~\ref{sch5}). Electrochemical studies revealed that, in all
cases, the ferrocene units retain their reversible one-electron redox
behavior, while coordination to the Os\tsub{3} cluster induces a cathodic
shift of the formal potential, reflecting electronic communication
between the ferrocenyl donor and the carbonyl metal cluster without
compromising redox reversibility.

\begin{scheme}
\vspace*{4pt}
\includegraphics{sc05}
\vspace*{13pt}
\caption{\label{sch5}Synthesis of homometallic ferrocenylalkynyl and
heterometallic ferrocenyl-osmium cluster carbosilane dendrimers.}
\vspace*{-10pt}
\end{scheme}

Complementary studies explored the use of cobalt carbonyl chemistry.
Ferrocenylalkyne--dicobalthexacarbonyl complexes were examined as
precursors for hydrosilylation reactions, with the dual objective of
generating ferrocenyl-functionalized vinylsilanes and extending this
\mbox{approach} to carbosilane dendrimers~\cite{43}. While sterically
demanding silyl-substituted ferrocenylalkyne cobalt complexes proved
unreactive toward \mbox{hydrosilylation}, terminal
ferrocenylalkyne--Co\tsub{2}(CO)\tsub{6} complexes underwent smooth
transformation to ferrocenyl vinylsilanes upon reaction with
trialkylsilanes, demonstrating the feasibility of controlled
decomplexation--hydrosilylation pathways.

Attempts to translate this strategy to more elaborate diynyl dicobalt
carbonyl systems highlighted the intrinsic limitations of such
precursors, as hydrosilylation generally resulted in complete
decomplexation of both Co\tsub{2}(CO)\tsub{6} units rather than selective
mono-decomplexation. Nevertheless, these studies underscore the rich
coordination chemistry accessible at the dendrimer periphery and
illustrate how alkynyl linkers enable the modular introduction of
multiple metal fragments.

\section{Amine-based organometallic dendritic frameworks}\label{sec4}

Dendritic architectures incorporating nitrogen atoms within their
branching skeletons, have played a central role in expanding the
chemical and functional versatility of organometallic macromolecules.
Unlike purely carbosilane or siloxane frameworks, amine-based scaffolds
provide sites for coordination, protonation, hydrogen bonding, and
modular post-functionalization, making them exceptionally adaptable to
diverse applications. 

One of the key advantages of nitrogen-containing dendrimers lies in
their peripheral functionalization flexibility. Amide-, urea-, thiol-,
and other donor-functionalized dendrimers can be constructed from a
common amine-based core, enabling the generation of tailored outer
shells with specific recognition, binding, or anchoring capabilities.
This modularity has been exploited to create hybrid systems in which
the organometallic core governs redox behavior while the nitrogen-based
periphery mediates supramolecular interactions, immobilization on
electrode surfaces, or selective complexation of analytes. Such a
dual-level design---redox-active interior and functionally programmable
exterior---has been instrumental in bridging organometallic dendrimer
chemistry and supramolecular and materials science.

\subsection{Ferrocenyl-functionalized poly(propylenei\-mine) dendrimers}

One of the earliest organometallic families based on
poly(propyleneimine) (PPI) dendritic scaffolds involved the
incorporation of ferrocenyl amide termini. In 1996, we reported what
was the highest number of organometallic functionalities ever attached
to a dendritic surface at that time~\cite{44}. The synthetic strategy
mirrored the general condensation methodologies described in
Section~\ref{sec2}; however, the dendritic architecture enables precise
and highly controlled placement of the ferrocenyl groups
(Scheme~\ref{sch6}). 

\begin{scheme}
\vspace*{2pt}
\includegraphics{sc06}
\vspace*{10pt}
\caption{\label{sch6}Synthesis of ferrocenyl PPI dendrimers.}
\vspace*{-9pt}
\end{scheme}

These dendrimers exhibit single, reversible one-electron waves for all
generations, each corresponding to the collective oxidation of all
ferrocene units with no intramolecular electronic communication. Their
redox behavior in solvents such as dichloromethane or THF is
accompanied by \mbox{pronounced} changes in solubility upon oxidation, leading
to precipitation of the dendrimers onto the electrode surface
(Figure~\ref{fig10}). The adsorption thermodynamics and kinetics of
these ferrocenyl--amide dendrimers on Pt electrodes were subsequently
\mbox{investigated} by Abru\~{n}a~\cite{45}.  Adsorption of the reduced
dendrimers in CH\tsub{2}Cl\tsub{2} follows the Langmuir isotherm. 
Electrochemical quartz-crystal microbalance (EQCM)
measurements revealed that the oxidized form deposits onto the Pt
electrode due to its low solubility, whereas the reduced form readily
redissolves, except for the first monolayer, which remains strongly
adsorbed. Atomic force microscopy (AFM) provided molecularly resolved
images of high-generation dendrimers adsorbed on Pt(111), offering
direct insight into the organization of these surface-confined
organometallic assemblies.

\begin{figure}
\includegraphics{fig10}
\caption{\label{fig10}Cyclic voltammograms of ferrocenyl PPI dendrimers
in dichloromethane and THF solutions. (Adapted with permission
from~\cite{16}. Copyright 1999 Elsevier.)}
\end{figure}

Ferrocene is shown to be an excellent guest for $\upbeta$-cyclodextrin
inclusion complexation. In collaboration with Prof.\ Kaifer, we reported
the first example of dendritic terminal groups undergoing complexation
with cyclodextrins, demonstrating that moderately sized dendrimers act
as effective multivalent guests and form very large supramolecular
assemblies~\cite{46}. Despite the complexity of the equilibria,
electrochemical and spectroscopic data indicate that all ferrocene
units in these dendrimers are accessible to cyclodextrin hosts.
Conceptually, the dendrimer serves as a three-dimensional template that
organizes cyclodextrins at its periphery, giving rise to
high-molecular-weight supramolecular {complexes} (e.g., ${>}$11 kDa for an
octameric assembly). {Notably}, these assemblies can be reversibly
dissociated by electrochemical oxidation of the ferrocene units, which
significantly weakens cyclodextrin binding.\looseness=-1


Our group also reported the first examples of mesoporous silica
materials incorporating redox-active dendritic guests within their
ordered channels~\cite{47}. PPI amidoferrocenyl dendrimers were
successfully encapsulated in MCM-41, yielding stable dendrimer--silica
composites in which the structural integrity and electrochemical
activity of the dendrimers were preserved. Structural studies showed
efficient channel filling for the smaller dendrimers, with reduced
inclusion for larger macromolecules. Electrochemical measurements
demonstrated size-dependent redox behavior that differed markedly from
analogous nonporous silica systems, while preliminary results indicated
sensitivity toward dihydrogen phosphate anions, highlighting the
potential of these hybrid materials as redox-responsive platforms for
electrochemical applications.

Also in collaboration with Prof.\ Kaifer, our group reported a series of
PPI dendrimers bearing four, eight, sixteen, or thirty-two peripheral
ferrocenyl--urea units, designed to combine redox activity with
anion-binding capability~\cite{48}. These dendrimers were synthesized
from diaminobutane-based PPI scaffolds via reaction with
isocyanatoferrocene and were obtained in moderate yields, although they
display very low solubility (Scheme~\ref{sch7}). The incorporation of
urea linkages introduces dual hydrogen-bond donor sites, which
significantly strengthen supramolecular interactions with anionic
guests. Their electrochemical behavior in DMSO was found to be highly
sensitive to hydrogen phosphate anions at submillimolar concentrations,
as demonstrated by square-wave voltammetry (Figure~\ref{fig11}).
Notably, effective anion sensing was achieved in a polar solvent where
hydrogen-bonding interactions are typically weakened, underscoring the
stability of the dendrimer--anion complexes and the suitability of
these systems as redox-active platforms for anion recognition.

\begin{scheme}
\vspace*{3pt}
\includegraphics{sc07}
\vspace*{9pt}
\caption{\label{sch7}Synthesis of ferrocenyl--urea PPI dendrimers.}
\vspace*{-11pt}
\end{scheme}

\begin{figure}
\vspace*{2pt}
\includegraphics{fig11}
\vspace*{2pt}
\caption{\label{fig11}Square-wave voltammograms (SWV) of
ferrocenyl--urea PPI dendrimers. (Adapted with permission
from~\cite{48}. Copyright 2002 Royal Society of Chemistry.)}
\end{figure}

More recently, our group investigated in 2023 the effect of partially
replacing ferrocenyl units with thiol functionalities to improve the
sensing performance of electrode surfaces. To this end, first- and
third-generation ferrocenyl--urea-thiolated dendrimers were synthesized
via the reaction of ferrocenyl isocyanate and
\textit{N}-hydroxysuccinimide 3-mercaptopropanoyl derivatives with
diaminobutane-based PPI scaffolds. The introduction of thiol groups was
intended to promote stronger interactions with electrode surfaces,
thereby enhancing the electrochemical sensing properties of the
resulting dendrimer-modified electrodes~\cite{49}.\looseness=-1

We also developed a related new family of PPI dendrimers bearing
ferrocenyl--aza-crown-ether units, representing, to our knowledge, the
first examples of redox-active organometallic heteroditopic
dendrimers~\cite{50}. These systems were designed to position
ferrocenyl redox centers in close proximity to two distinct binding
sites, enabling the simultaneous complexation of cationic and anionic
guests. They were prepared via condensation of an
aza-crown-ether--functionalized ferrocenyl acid chloride with three
first-generation amine-terminated \mbox{dendrimers} (Scheme~\ref{sch8}). NMR
studies indicated cooperative binding of cations and anions through the
combined action of the aza-crown ether and amide functionalities,
highlighting the potential of these dendritic architectures as
electrochemical sensing platforms. \looseness=1

\begin{scheme}
\includegraphics{sc08}
\vspace*{12pt}
\caption{\label{sch8}Synthesis of PPI dendrimers bearing
ferrocenyl--aza-crown-ether units.}
\vspace*{-6pt}
\end{scheme}

As part of our ongoing efforts in the design of organometallic
dendrimers with electrochemical sensing capabilities, our group
developed in 2008 a synthetic approach to a ferrocenyl dendrimer
functionalized with pyrrole substituents~\cite{51} (Figure~\ref{fig12}).
This system was conceived to exploit the anodic electropolymerization
of pyrrole-substituted ferrocenyl derivatives, enabling the formation
of conducting polymer films immobilized on electrode surfaces. The
dendrimer was obtained via a stepwise condensation strategy involving a
pyrrole-containing \mbox{ferrocenyl} acid chloride intermediate and subsequent
coupling to a PPI dendrimer bearing terminal amino groups. Its
electropolymerization afforded redox-active films that were applied to
the electrochemical sensing of dihydrogen phosphate anions in aqueous
media, addressing the challenges associated with anion recognition in
water and highlighting the potential of these materials as functional
sensing interfaces.

\begin{figure}
\includegraphics{fig12}
\caption{\label{fig12}Ferrocenyl dendrimer functionalized with pyrrole
substituents.}
\end{figure}

Later on, a new family of PPI dendrimers bearing large and flexible
ferrocenylamidoalkyl chains at the periphery was designed to provide
readily accessible ferrocenyl units for anion binding
(Figure~\ref{fig13})~\cite{52}. Efficient synthesis of the target
dendrimers was achieved through amidation using acid fluoride
intermediates, thereby overcoming the limitations associated with
chlorocarbonylferrocene. Electrochemical studies demonstrated that
robust modified electrodes could be prepared using these materials and
that their voltammetric response is sensitive to both the presence and
concentration of anions in organic and aqueous media, with sensing
performance strongly dependent on film\break thickness.

\begin{figure}
\includegraphics{fig13}
\vspace*{-5pt}
\caption{\label{fig13}PPI dendrimer bearing thirty-two
ferrocenylamidoalkyl chains.}
\vspace*{-5pt}
\end{figure}

In 2012, we reported the anion-recognition properties of two PPI
dendrimers incorporating either isolated ferrocene units or biferrocene
motifs at the periphery~\cite{53}. The dendrimer with isolated
ferrocenes~\cite{44} and the biferrocene-containing dendrimer were
prepared via amidation of a first-generation PPI scaffold with
chlorocarbonylferrocene and a biferrocenyl diacyl chloride intermediate
(Scheme~\ref{sch9}), respectively, affording systems in which the
ferrocenyl centers are either independent or $\upsigma$-bonded
through C--C linkages. Electrochemical studies revealed stepwise and
reversible oxidations consistent with electronic interactions between
adjacent iron centers in the biferrocene-containing dendrimer. Both
dendrimers exhibited effective voltammetric sensing of hydrogen
phosphate anions in polar media such as DMSO, while the biferrocenyl
system additionally enabled hydrogen sulfate recognition. Notably,
anion sensing was achieved at submillimolar concentrations in solution,
and electrodes modified with dendrimer films displayed sensitivity to
HSO\tsub{4}\tsup{\tminus} over a broad concentration range, underscoring the
potential of these architectures for surface-confined electrochemical
sensing applications.

\begin{scheme}
\includegraphics{sc09}
\vspace*{10pt}
\caption{\label{sch9}Synthesis of first-generation PPI dendrimer
incorporating biferrocene units.}
\vspace*{-6pt}
\end{scheme}

While dendrimers bearing ferrocenyl units are well established, systems
decorated with permethylferrocenyl moieties remained comparatively
underexplored, largely due to synthetic challenges {associated} with
polymethylcyclopentadienyl \mbox{chemistry}. Motivated by the markedly
different electronic properties of polymethylferrocenes, our group
developed a series of dendrimers incorporating octamethylferrocenyl
units, with the aim of accessing redox-active macromolecules exhibiting
more negative redox potentials than their ferrocene analogs~\cite{54}.
These metallodendrimers were prepared by condensation of
octamethylferrocenyl aldehydes with PPI dendrimers of increasing
generation, followed by reduction to yield amine-linked derivatives
bearing up to thirty-two octamethylferrocenyl units per molecule
(Scheme~\ref{sch10}). Electrochemical studies revealed fully reversible
oxidation processes with significantly shifted formal potentials,
reflecting the strong electron-donating effect of the methyl
substituents. The dendrimers of increasing generation showed an
increasing tendency to adsorb onto electrode surfaces, enabling the
preparation of stable modified Pt or glassy carbon electrodes
displaying persistent electrochemical\break responses. 

\begin{scheme}
\vspace*{6pt}
\includegraphics{sc10}
\vspace*{16pt}
\caption{\label{sch10}Synthesis of octamethylferrocenyl dendrimers.}
\vspace*{-10pt}
\end{scheme}

\subsection{Cobaltocenium- and mixed
ferrocenyl--cobaltocenium-functionalized PPI dendrimers}\label{sec42}

Whereas ferrocene has been the most widely employed redox-active unit
in organometallic dendrimers, examples of polycationic redox-active
metallodendrimers remain comparatively scarce. In this context,
cobaltocenium constitutes a particularly attractive complementary
motif, as it is a highly stable, positively charged metallocene,
isoelectronic with ferrocene, and undergoes a reversible one-electron
reduction to neutral cobaltocene. Along these lines, we reported the
synthesis of multimetallic PPI dendrimers bearing four, eight, sixteen
and thirty-two peripheral cobaltocenium units, obtained via
condensation with excess \mbox{1-(chlorocarbonyl)cobaltocenium}
(PF\tsub{6}\tsup{\tminus} salt)~\cite{55,56}. Cyclic voltammetry and
EQCM measurements
established fully reversible redox chemistry associated with the
cobaltocenium--cobaltocene couple across all generations. A distinctive
feature of these dendrimers is their pronounced interfacial activity:
upon electrochemical reduction, the transformation of highly charged,
hydrophilic cobaltocenium peripheries into neutral, hydrophobic
\mbox{cobaltocene} units promotes adsorption and electrodeposition onto Pt and
glassy carbon, an effect that becomes increasingly pronounced with
dendrimer generation. EQCM experiments revealed monolayer/submonolayer
adsorption at open circuit, followed by reversible deposition of
multilayer equivalents upon scanning to sufficiently cathodic
potentials (${{\approx}{-}}0.75$~V), consistent with reduced
solubility of the neutral dendritic species; re-oxidation induces
partial desorption, leaving an approximately monolayer
coverage~\cite{56}. 

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig14}
\vspace*{-5pt}
\caption{\label{fig14}Cyclic voltammogram of an electrode modified with
mixed ferrocene--cobaltocenium dendrimers.}
\vspace*{-2pt}
\end{figure*}

Beyond their interfacial electrochemistry, these polycationic systems
also enabled an elegant example of redox-triggered supramolecular
self-assembly: although the oxidized dendrimers do not form inclusion
complexes with $\upbeta$-cyclodextrin ($\upbeta$-CD) in aqueous media,
electrochemical reduction drives the association of multiple peripheral
cobaltocene units with freely diffusing $\upbeta$-CD hosts, leading to
solubilization of the reduced form and suppression of the
characteristic anodic stripping features in cyclic voltammetry. Thus,
dendrimers in the presence of $\upbeta$-CD constitute a
high-molecular-weight multisite host--guest system in which complex
formation is effectively switched ``on'' by electrochemical activation
of the guest (Scheme~\ref{sch11})~\cite{55}.

\begin{scheme}
\vspace*{2pt}
\includegraphics{sc11}
\vspace*{10pt}
\caption{\label{sch11}Cyclodextrin binding by
cobaltocenium-functionalized dendrimers.}
\vspace*{-12pt}
\end{scheme}

Building on the complementary redox properties of neutral ferrocene and
cationic cobaltocenium, we reported the synthesis of mixed
ferrocene--cobaltocenium dendrimers in which both organometallic units
are simultaneously incorporated at the dendritic periphery~\cite{57}.
These heterometallic systems were prepared by treating the first four
generations of PPI scaffolds with an equimolar mixture of freshly
prepared 1-chlorocarbonylferrocene and the PF\tsub{6}\tsup{\tminus} salt of
chlorocarbonylcobaltocenium, enabling the controlled installation of
both metallocene fragments through amide formation at the terminal
amine groups. As expected from the competitive coupling, the reaction
afforded fractions containing different Fc/Co\tsup{\tplus} loadings;
however, the overall peripheral composition could be reliably assessed
by NMR (diagnostic {resonances} for Fc and cobaltocenium fragments) and
corroborated by TXRF (total reflexion X-ray fluorescence) and ESI mass
spectrometry, which reflected successive ionization states of the
polyelectrolytic dendrimers. Electrochemical studies showed reversible
redox activity for both organometallic units, with ferrocene oxidation
occurring as a single multielectron wave and cobaltocenium reduction
likewise giving rise to a well-defined collective process, consistent
with largely non-interacting peripheral sites. In addition, these
mixed-metal dendrimers readily modified electrode surfaces to form
durable, surface-confined electroactive films displaying two distinct
reversible redox systems, thereby establishing a robust platform for
multifunctional electrochemical interfaces and subsequent sensing
applications \mbox{(Figure~\ref{fig14}).}\looseness=-1


\section{Functional applications: biosensors and electrochemical
devices}\label{sec5.}

Our contributions to functional applications constitute a natural
extension of the fundamental electrochemical behavior of redox-active
organometallic dendrimers and related macromolecules, particularly
their ability to deliver predictable multielectron responses and to
generate robust surface-confined redox films~\cite{19}.  This
translation from molecular redox chemistry into practical
electrochemical platforms emerged progressively, starting with the use
of ferrocenyl dendrimers as mediators in amperometric biosensors, and
evolving toward multi-operational enzyme electrodes and, more recently,
hybrid dendrimer/polymer--nanoparticle interfaces for advanced sensing
and environmental \mbox{monitoring.} 

\subsection{Dendritic redox mediators in enzyme electrodes: from
proof-of-concept to multi-operational biosensors}

The first demonstration of the functional potential of dendritic redox
architectures was achieved through the development of mediated glucose
biosensors based on ferrocenyl dendrimers. In this pioneering work,
carbon-paste electrodes doped with glucose oxidase (GOx) and ferrocenyl
silicon-based dendrimers were shown to behave as efficient \mbox{amperometric}
glucose sensors, displaying rapid \mbox{current} \mbox{responses} and stable
steady-state signals under anaerobic conditions. The study demonstrated
that dendritic ``relay systems'' provide a tunable alternative to
classical monomeric ferrocenes, and that sensor performance depends not
only on the number of ferrocenyl redox centers but also on dendrimer
flexibility and framework topology~\cite{58}. A particularly relevant
analytical feature was that dendrimer-based sensors exhibit improved
operational stability relative to freely diffusing mediators,
consistent with the reduced solubility of oxidized dendritic species
and the reduced tendency of leaching from the electrode matrix.

After establishing this initial proof-of-concept, subsequent work
increasingly shifted toward the design of chemically modified
electrodes in which redox macromolecules provide stable electroactive
coatings. In this direction, heterometallic systems introduced
additional functionality beyond mediation, particularly when combining
neutral ferrocene with cationic cobaltocenium fragments. Both ideas
culminated in the construction of heterometallic
ferrocene--cobaltocenium dendrimer films used as multifunctional
electrode modifiers for enzyme-based glucose monitoring. In this study,
GOx was immobilized electrostatically on carbon and platinum electrodes
modified with the heterometallic dendrimers, enabling dual-mode
operation: under anaerobic conditions, the ferrocene units act as
efficient mediators for electron transfer between the reduced enzymatic
cofactor and the electrode, whereas under aerobic conditions the
cobaltocenium moieties electrocatalyze oxygen reduction, thus enabling
sensitive monitoring of oxygen consumption during enzymatic
turnover~\cite{59}. Importantly, this contribution represented a
transition from a mediator-containing electrode mixture to a genuine
functional biointerface, as it systematically explored the influence of
dendrimer generation, dendrimer film thickness, substrate
concentration, interferences, and storage stability on the analytical
response.

In parallel, we also explored the use of redox organosilicon
macromolecules as alternative mediator families, where film robustness
and redox-site density can be optimized through polymer design. In
particular, early investigations in 2003 already established that
organosiloxane redox polymers having electronically interacting
ferrocenyl units can serve as efficient electrode modifiers and
electrocatalytic matrices for hydrogen peroxide sensing, a process of
central relevance in oxidase-based biosensors~\mbox{\cite{60,61,62}.} 

A further advance toward analytically optimized devices was enabled by
introducing polymethylferrocenes, which provide lower redox potentials
and faster kinetics, thus supporting operation at milder potentials
with improved selectivity and reducing interference in complex
matrices~\cite{63,64,65}. 

Ultimately, this evolution culminated in the design of
multi-operational biosensors based on \mbox{carbosilane} dendrimers with
interacting ferrocenyl sites~\cite{66}. In these systems, immobilized
oxidases (GOx and lactate oxidase LOx) could be monitored through
different transduction regimes (mediated enzyme regeneration under
anaerobic conditions and peroxide electrocatalysis under aerobic
conditions), providing versatile platforms operating within
interference-free potential windows and demonstrating the maturity of
dendrimer-based bioelectrocatalytic interfaces. 

\subsection{Hybrid dendrimer/polymer--nanoparticle interfaces:
electrocatalytic amplification and new sensing modes}

Following the consolidation of dendrimer-derived redox films as enzyme
electrode platforms, our research progressively incorporated
nanomaterials design principles in order to enhance surface area,
catalytic activity and charge transport at the electrode
interface~\cite{19}. This stage was motivated by the well-established
limitation of purely molecular redox films, namely finite interfacial
electron transfer rates and limited catalytic amplification,
particularly in peroxide- or oxygen-coupled electrochemical assays.
Metallic nanoparticles (NPs) offer a direct route to overcoming these
constraints, but their use requires strategies to suppress aggregation
and ensure reproducibility. Here, dendritic and organosilicon redox
macromolecules provide a unique advantage: they can act simultaneously
as film-forming redox matrices, NP stabilizers, and nanoscale
templates.

The first systematic implementations of this strategy appeared in 2016,
in which ferrocenyl organosilicon macromolecules were combined with
metallic NPs to construct hybrid sensing interfaces. Polyferrocenyl
polycyclosiloxane--AuNP architectures were developed as robust
electrode coatings enabling favorable electron-transfer pathways and
supporting peroxide sensing based on horseradish peroxidase
(HRP)~\cite{67}. These hybrid materials established the basic rationale
that redox macromolecules provide structured and tunable electroactive
matrices, whereas AuNPs and PtNPs deliver catalytic amplification and
enhanced conductivity.

In 2017, dendrimer-templated routes to obtain monodispersed and
size-controlled AuNPs directly from electrodeposited dendrimer films
were \mbox{introduced}. This strategy enabled reproducible control over NP
dimensions, which is critical because electrochemical performance
strongly depends on AuNP size and dispersion. The resulting hybrid
interfaces were exploited for sensing applications including dopamine
and nitrite determination, demonstrating the versatility of
dendrimer-controlled AuNP platforms as general electrocatalytic
electrodes~\cite{68,69}. The approach was refined in 2018 through
systematic optimization of AuNP size using aminoferrocenyl dendrimer
templates, leading to highly efficient electrocatalysts for hydrogen
peroxide and enabling sensitive, non-enzymatic peroxide sensing with
improved analytical characteristics~\cite{70}.

Garc\'{i}a Armada et~al.\ employed thiolated DAB
(diaminobutane) dendrimers as bonding layers between electrodeposited
and colloidal AuNPs to form electrocatalytic self-assembled layers
optimized for the covalent immobilization and direct electrochemistry
of HRP~\cite{71}. This work provided a comprehensive kinetic and
analytical characterization of the hybrid films and established
optimized conditions for obtaining fast electron transfer and high
sensitivity toward hydrogen peroxide. The resulting interfaces were
conceived as modular platforms for more complex devices, including
oxidase/peroxidase bienzymatic architectures or inhibition biosensors.

The most recent stage of this development expanded the NP concept
toward new electrode architectures and sensing mechanisms. In 2021,
ferrocenyl and perferrocenyl polycyclosiloxanes were used not only as
electrode modifiers but also as templates for PtNP formation, taking
advantage of the synergistic coupling between ferrocene-mediated
electrocatalysis and PtNP surface properties. These PtNP-containing
interfaces were further applied as platforms for immobilization of
oxidase enzymes such as xanthine oxidase, enabling efficient
bioelectrocatalysis and reinforcing the value of the hybrid approach
for biosensing~\cite{72}. 

Finally, our work culminated in 2023 with electrocatalytic multilayer
structures based on thiolated ferrocenyl PPI dendrimers acting as
bonding layers between electrodeposited and colloidal AuNPs
(Figure~\ref{fig15}). A
particularly significant advance was the direct comparison between
ferrocenyl-thiolated dendrimers and thiolated analogs lacking
ferrocene, clearly demonstrating the beneficial role of redox-active
dendritic fragments for enhancing charge transfer and \mbox{catalytic}
performance. Beyond peroxide sensing, these HRP-based systems were
successfully converted into inhibition-based electrochemical devices
for heavy metal analysis, enabling the determination of Pb\tsup{2\tplus}
and Cu\tsup{2\tplus} by both amperometric and impedimetric detection
modes~\cite{49}. 

\begin{figure*}
\includegraphics{fig15}
\vspace*{-3pt}
\caption{\label{fig15}Schematic structure and operation of the biosensor.}
\vspace*{-3pt}
\end{figure*}

\section{Dendrimer--nanomaterial hybrids and fluorescent sensing
platforms}\label{sec6}

In parallel with our developments in electrochemical dendrimer films
and biosensing, a complementary research direction was established
through collaborative work with M. Algarra, focused on the integration
of dendritic scaffolds with semiconductor nanomaterials to produce
hybrid sensing platforms. In these studies, dendrimers were not only
employed as redox-active architectures, but also as multifunctional
nanoscale ligands able to stabilize quantum dots (QDs), control their
surface chemistry, and confer water solubility and analyte
responsiveness.

A representative example is the development of Hg(II) fluorescent
sensors based on cadmium sulfide QDs coated with the fifth-generation
PPI dendrimer~\cite{73}. The resulting CdS--PPI nanocomposites were
synthesized in aqueous media and characterized by EDXA (energy
dispersive X-ray analysis) and SEM, which revealed macroscopic
spherical nanocomposite structures and confirmed the coexistence of CdS
and the nitrogen-rich dendritic coating. The hybrid nanocomposites
displayed intense fluorescence, with an emission maximum around 535 nm
(excitation at 351 nm), and their luminescence could be selectively
modulated by metal-ion binding. Although Cu(II) and Pb(II) were
identified as potential interfering quenchers, several other common
cations such as Cd(II), Zn(II), Co(II), and Ni(II) displayed negligible
influence. Additionally, the work revealed the crucial role of
dendrimer conformation: changes in pH and ionic strength affect the
emission wavelength and intensity by modifying the dendrimer/QD
environment, thus highlighting both opportunities and limitations
associated with using dendritic scaffolds as adaptive nanostructured
hosts. 

This collaborative line was further expanded through the preparation of
thiolated PPI dendrimers coupled with CdSe QDs to afford fluorescent
nanocomposites responsive to heavy-metal ions~\cite{74,75}. In these
systems, the presence of thiol groups enhances coordination ability and
promotes strong interactions at the QD surface, resulting in sensors
capable of responding to Cd(II) and Pb(II) through enhancement or
quenching mechanisms, respectively, with analytically useful
concentration ranges and detection limits in the micromolar regime. 

This collaborative line was further extended to ZnSe-based QD
systems~\cite{76}. A water-soluble nanocomposite obtained by a
thiolated PPI-dendrimer coated with fluorescent ZnSe showed a set of
favorable properties to be used as a sensor for the selective
recognition of C-reactive protein in human serum samples at
concentrations of risk~\cite{77}.

\section{Conclusions and perspectives}\label{sec7}

Over the past three decades, our research on organometallic dendrimers
and related macromolecules has evolved from fundamental synthetic
developments to the demonstration of functional devices and hybrid
sensing platforms. Early efforts established efficient methodologies
for incorporating redox-active organometallic units into robust
dendritic frameworks---particularly carbosilane and
amine-based scaffolds---enabling precise control over
nuclearity, site isolation, and redox organization. These studies
revealed key electrochemical principles, notably the simultaneous
multielectron behavior of peripheral redox sites and the insulating
role of dendritic matrices in maintaining electronic independence.

Subsequent generations of work expanded the chemical scope toward
heterometallic dendrimers and redox-asymmetric architectures, where
different organometallic fragments introduce complementary and
addressable redox responses. Such systems offered insight into
cooperativity and redox communication, while also enabling new modes of
interfacial reactivity, including robust electrodeposition and
formation of electroactive dendritic films. The transition from
molecular systems to surface-confined assemblies ultimately provided a
direct bridge to applications, particularly in electrochemical devices
and biosensing, where dendritic films function as multielectron
mediators, enzyme immobilization matrices, and multifunctional sensing
interfaces.

Looking forward, organometallic dendrimers remain highly attractive as
programmable macromolecular platforms at the interface of molecular
electronics, electroanalysis, and functional materials. The modularity
of dendritic synthesis, combined with the broad palette of redox-active
\mbox{organometallic} fragments available, opens opportunities for rational
design of multi-state charge reservoirs, electrocatalytic interfaces,
and hybrid nanomaterials. Future efforts will likely benefit from
deeper integration with nanotechnology and surface science, including
dendrimer-derived electrode architectures, dendronized polymers, and
mixed redox/optical platforms that combine electrochemical
addressability with photophysical functionality. In this broader
context, our work illustrates how dendritic organometallic chemistry
can provide not only structurally elegant macromolecules but also
practical and versatile building blocks for next-generation
electrochemical and sensing technologies.

\section*{Acknowledgements}

The authors are greatly indebted to their colleagues and collaborators,
all the students and PhD students who have contributed to these works. 

\printCOI

\section*{Funding}

The Direcci\'{o}n General de Investigaci\'{o}n Cient\'{i}fica y
T\'{e}cnica (87/0123, N 90/0227 and 93/0287), the Direcci\'{o}n General
de Ense\~{n}anza Superior e Investigaci\'{o}n Cient\'{i}fica
(PB97-0001), the Spanish Direcci\'{o}n General de Investigaci\'{o}n
(CTQ2004-07381-C02 and CTQ2009-12332-C02) and the Consejer\'{i}a de
Educaci\'{o}n, Comunidad de Madrid (S-0505/PPQ-0328) are thanked for
their financial support.

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