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\DOI{10.5802/crchim.292}
\datereceived{2023-08-31}
\daterevised{2023-11-09}
\dateaccepted{2024-01-24}
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\dateposted{2024-08-23}
\begin{document}

\begin{noXML}

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%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{French/Nordic Special Issue on Materials and Coordination Chemistry}
%\def\cdr@specialtitle@french{Num\'ero sp\'ecial franco-nordique sur les mat\'eriaux et la chimie de coordination}
%\makeatother

\title{1,2,4-triamino-5-nitrobenzene as air-stable aromatic polyamines
for coordination chemistry}

\alttitle{1,2,4-triamino-5-nitrobenz\`{e}ne comme polyamines
aromatiques stables \`{a} l'air pour la chimie de coordination}

\author{\firstname{Simon} \lastname{Pascal}\CDRorcid{0000-0001-8387-494X}\IsCorresp}
\address{Aix-Marseille Univ, Centre Interdisciplinaire de
Nanoscience de Marseille (CINaM), CNRS UMR 7325, Campus de Luminy, case
913, Marseille cedex 09 13288, France}
\address{Nantes Universit\'{e}, CNRS, CEISAM, UMR 6230, Nantes,
F-44000, France}
\email[S. Pascal]{simon.pascal@cnrs.fr}

\author{\firstname{Elena} \lastname{Zaborova}\CDRorcid{0000-0001-5212-9728}}
\addressSameAs{1}{Aix-Marseille Univ, Centre Interdisciplinaire
de Nanoscience de Marseille (CINaM), CNRS UMR 7325, Campus de Luminy,
case 913, Marseille cedex 09 13288, France}
%\email{elena.zaborova@univ-amu.fr}

\author{\firstname{Olivier} \lastname{Siri}\CDRorcid{0000-0001-9747-3813}\IsCorresp}
\addressSameAs{1}{Aix-Marseille Univ, Centre Interdisciplinaire
de Nanoscience de Marseille (CINaM), CNRS UMR 7325, Campus de Luminy,
case 913, Marseille cedex 09 13288, France}
\email[O. Siri]{olivier.siri@univ-amu.fr}

\shortrunauthors

\keywords{\kwd{Aromatic amines}\kwd{Acac ligand}\kwd{Metal
coordination}\kwd{X-ray structure}\kwd{Nickel complexes}}

\altkeywords{\kwd{Amines aromatiques}\kwd{Ligand acac}\kwd{Chimie de
coordination}\kwd{Structure RX}\kwd{Complexes de nickel}}

\thanks{CNRS, French ``Minist\`{e}re de la Recherche et de
l'Enseignement Sup\'{e}rieur''}

\begin{abstract}
We report the synthesis and characterization
of two nickel complexes \textbf{7} and \textbf{8} based on
1,2,4-triamino-5-nitrobenzene \textbf{6}. The formation of these
compounds is observed from the same precursor, their stoichiometry
depending on the nature of the solvent used for the crystallization.
The key role of the nitro group in \textbf{6} is discussed,
demonstrating its major impact on the stability and reactivity of the
ligand. Each nickel complex could be characterized by single crystal
X-ray analysis, highlighting the importance of the intramolecular
H-bonding interactions on the stability of the complexes incorporating
a monodentate aromatic amine-type ligand.
\vspace*{2pt}
\end{abstract}

\begin{altabstract}
Nous rapportons la synth\`{e}se et la caract\'{e}risation de deux
complexes de nickel \textbf{7} et \textbf{8} \`{a} base du ligand
1,2,4-triamino-5-nitrobenz\`{e}ne \textbf{6}. La formation de ces
compos\'{e}s est observ\'{e}e \`{a} partir du m\^{e}me pr\'{e}curseur,
leur st{\oe}chiom\'{e}trie d\'{e}pendant de la nature du solvant
utilis\'{e} pour la cristallisation. Le r\^{o}le cl\'{e} du groupement
nitro dans \textbf{6} est discut\'{e}, d\'{e}montrant son impact majeur
sur la stabilit\'{e} et la r\'{e}activit\'{e} du ligand. Chaque
complexe de nickel a pu \^{e}tre caract\'{e}ris\'{e} par diffraction
des rayons X, mettant en \'{e}vidence l'importance des interactions
hydrog\`{e}nes intramol\'{e}culaires sur la stabilit\'{e} des complexes
incorporant un ligand monodentate de type amine aromatique.
\end{altabstract}

\maketitle

\vspace*{20pt}

\twocolumngrid

\end{noXML}

\section{Introduction}

Phenylenediamine derivatives \textbf{1--3} (Figure~\ref{newfig1}) are
an important class of organic compounds with rich chemical and physical
properties~\cite{1,2,3} and remarkable applications as precursors in
color chemistry~\cite{4,5}. Their coordinating ability has been less
investigated because phenylenediamines do not form very stable
complexes with bivalent metal ions~\cite{6}. This poor stability can be
explained by the low basicity of the aromatic amines due to the
delocalization of the lone pair of the nitrogen atom toward the p
orbitals of the aromatic ring, decreasing the electron density on the
nitrogen atom available for the bond \mbox{formation} with a metal
center. The coordinating ability of aromatic diamines is also
conditioned by the position of the nitrogen atoms on the aromatic ring
(\textit{ortho}, \textit{meta} or \textit{para}).
\textit{o}-Phenylenediamine ligand~\textbf{1} coordinates either in a
monodentate, chelating bidentate and bridging bidentate fashion in
different dinuclear complexes, with possible oxidation of \textbf{1}
depending on the metal center~\cite{7}.\ Studies of coordination
compounds of \textbf{2} and \textbf{3} in solution and solid state are
rare, possibly because these ligands can only coordinate one nitrogen
atom to a metal center by means of a weak linkage~\cite{7}, but also
because of oxidation processes forming \textit{p}-benzoquinonediimine
ligands~\cite{8}.\ Falthouse and Hendrickson have reported a series of
binuclear Cu(II) complexes in which \textbf{2} acts as a bridging
bidentate ligand but the instability inherent to the phenylenediamines
made it difficult to fully characterize the complexes~\cite{9}.

\begin{figure}
{\vspace*{6pt}}
\includegraphics{fig01}
{\vspace*{6pt}}
\caption{\label{newfig1}Molecules studies in previous works and our work.}
{\vspace*{4pt}}
\end{figure}

As an extension of these studies, it appeared very attractive to
investigate 1,2,4-triaminobenzene~\textbf{4}, which combines the
structural elements of \textbf{1}, \textbf{2} and~\textbf{3}, as ligand
in coordination chemistry. However, bearing three amino functions,
molecule \textbf{4} is highly unstable in air and can be easily
oxidized, sacrificing its aromatic character in favor of a quinoidal
form. To overcome this limitation, we envisaged the use of analogs of
type \textbf{5} because the incorporation of an electron-withdrawing
nitro group should strongly reduce the electron density and
consequently prevent air oxidation of the molecule.

Herein, we wish to describe the synthesis and X-ray diffraction
analysis of two new complexes (\textbf{7}~and~\textbf{8}), based on the
air-stable 1,2,4-triamino-5-nitrobenzene derivative \textbf{6}.
Importantly, the key role of the nitro group on the stability and the
reactivity of the ligand is clearly emphasized, illustrating a further
aspect to be considered in the coordination chemistry of aromatic
amines.

{\vspace*{2pt}}

\section{Results and discussion} \label{sec2}

{\vspace*{2pt}}

Precursor \textbf{6} was first prepared as recently reported in the
literature~\cite{10}. Its metalation with Ni(acac)$_{2}$ in toluene at
room temperature led to the formation of a beige precipitate
(\textbf{BP}) that could be isolated by filtration (Scheme~\ref{sch1}).
IR analysis of this powder compared to the precursor \textbf{6}
(Figure~\ref{fig1}) revealed the appearance of strong bands around
1380, 1020 and 925~cm$^{-1}$, which could be attributed to
carbon--oxygen and carbon--carbon bonds. In addition, the~N--H
vibrations above 3250~cm$^{-1}$ for \textbf{6} are shifted for
\textbf{BP} whereas the C--H vibrations (2900~cm$^{-1}$) are not
affected. These observations are consistent with the coordination of
one or several NH$_{2}$ group(s) to the nickel center and the presence
of acetylacetonate (acac) moieties as ancillary ligand.

\begin{scheme*}
{\vspace*{1pc}}
\includegraphics{sc01}
{\vspace*{1.2pc}}
\caption{\label{sch1}Synthesis of complexes \textbf{7} and \textbf{8}
(acac $=$ acetylacetonate).}
{\vspace*{.2pc}}
\end{scheme*}

\begin{figure}
{\vspace*{4pt}}
\includegraphics{fig02}
{\vspace*{4pt}}
\caption{\label{fig1}IR spectra of precursor \textbf{6} (black) and
\textbf{BP} (red).}
{\vspace*{4pt}}
\end{figure}

Next, we characterized \textbf{BP} in solution. Its ${}^{1}$H NMR
spectra in acetone-$d_{\mathit{6}}$ and methanol-$d_{\mathit{4}}$ are
reported in Figure~\ref{fig2}, showing signals of aliphatic chains of
ligand \textbf{6} and peaks of the acac moiety in the 1--5~ppm region,
together with the presence of square planar low-spin complexes (i.e.,
diamagnetic character). Unfortunately, further investigations by mass
spectrometry could not afford usable data due to the presence of
numerous peaks in both methanol and acetone solutions.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig2}${}^{1}$H NMR (400~MHz) spectrum of \textbf{6} in
acetone-$d_{\mathit{6}}$ (top) and \textbf{BP} in
acetone-$d_{\mathit{6}}$ (middle) or methanol-$d_{\mathit{4}}$
(bottom). The solvent's residual peak is indicated with an asterisk.}
\end{figure*}

\looseness=-1
Interestingly, the UV--Vis spectroscopic data recorded in methanol and
acetone revealed the same absorption profile for \textbf{BP} and
\textbf{6} (Figure~\ref{fig3}). This observation proved the instability
of the complex \textbf{BP} in solution by means of a weak linkage,
suggesting that \textbf{6} probably coordinates only one nitrogen atom
to a metal center (unlike~\textbf{1})~\cite{7}.\ Crystallization
attempts of \textbf{BP} in different solvents were then performed to
gain insights into the nature of the coordination mode of\ \textbf{6}.

\begin{figure}
{\vspace*{.3pc}}
\includegraphics{fig04}
{\vspace*{.3pc}}
\caption{\label{fig3}UV--visible absorption spectra of \textbf{6} and
\textbf{BP} dissolved in acetone (top) or methanol (bottom).}
\end{figure}

Single crystals suitable for structural determination could be obtained
by slow evaporation of a \mbox{solution} of \textbf{BP} in methanol. The X-ray
diffraction analysis revealed the formation of a 1:2 (metal (Ni)/ligand
(\textbf{6})) complex \textbf{7} in which the nickel ion is found in an
octahedral geometry and surrounded by two acac \mbox{moieties}
coordinated in a planar fashion with respect to the metal center
(Figure~\ref{fig4}). 

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig05}
{\vspace*{1pt}}
\caption{\label{fig4}Single-crystal X-ray structure of \textbf{7}
(recrystallization of \textbf{BP} in methanol). Selected bond lengths
are indicated in~{\AA}.}
{\vspace*{4pt}}
\end{figure*}

The Ni coordination sphere is completed by two additional ligands
\textbf{6} in which a single amino group N(1) (and N$'$(1)) is
coordinated. The monodentate coordination mode of \textbf{6} can be
explained by a closer look at its molecular structure. Since only the
amine function N(1) in \textit{meta} position is not conjugated with
the nitro group, it coordinates specifically with a metal center due to
the availability of its lone pair, in contrast to the two other amine
functions in \textit{para} and \textit{ortho} positions, which are
highly conjugated with the nitro group. This hypothesis is confirmed
experimentally by examination of the bond lengths between the nitrogen
atoms of the amines and their corresponding aromatic carbons. The
distance N(1)${\cdots}$C(Ar) of 1.437~{\AA} clearly indicates a lack of
conjugation and availability of the lone pair whereas the distances
N(4)${\cdots}$C(Ar) and N(3)${\cdots}$C(Ar) are much shorter due to the
conjugation with NO$_{2}$ (1.355 and 1.350~{\AA}, respectively). A
close examination of the hydrogen-bonding interactions in \textbf{7}
revealed H-bonds involving N(1), N(2) and N(3) (as H-donor or
H-acceptor) that clearly stabilize the molecular architecture in the
solid state\break (Figure~\ref{fig4}).

Remarkably, we observed that recrystallisation of \textbf{BP} in
acetone instead of methanol led to the formation of complex \textbf{8}
having the same octahedral geometry but a different stoichiometry (2:2
{versus} 1:2) \mbox{(Figure~\ref{fig5}).} By analogy with \textbf{7},
ligand \textbf{6} is \mbox{monodentate} since only the amine function N(1), in
\textit{meta} position with respect to the nitro group, is coordinated
to the Ni center owing to the availability of its lone pair. Numerous
N--H${\cdots}$O bonding interactions involving N(1), N(2) and N(3),
ranging from 1.976 to 2.251~{\AA}, contribute to the stabilization of
the\break complex.

\begin{figure*}
\includegraphics{fig06}
{\vspace*{1pt}}
\caption{\label{fig5}Single-crystal X-ray structure of \textbf{8}
(recrystallization of \textbf{BP} in acetone).   For the ortep view on
the right, the aromatic amino ligand \textbf{6} is omitted for clarity
except for the amino function coordinated to the Ni center. Selected
bond lengths are indicated in~{\AA}.}
{\vspace*{2pt}}
\end{figure*}

The most striking feature of \textbf{8} is the coordination mode
adopted by two of the six acac ligands, one of whose oxygen atoms
coordinates to two metal centers, acting as a bridging ligand
(Figure~\ref{fig5}). As a result, the two Ni centers are in proximity
(d(Ni${\cdots}$Ni) ${=}$ 3.215~{\AA}). To the best of our knowledge, this
arrangement was only observed for modified acac in which
electron-donating substituents in the 3-position of the
pentane-2,4-dionato group were\break added~\cite{11}.

\section{Experimental section} \label{sec3}

\subsection{General} \label{sec3.1}

Solvents (HPLC grade) and reagents were purchased from Sigma Aldrich
and were used as received. \mbox{Compound} \textbf{6} was prepared
according to published procedures~\cite{10}. NMR spectra were recorded
on a JEOL ECS400 NMR spectrometer at room temperature. IR spectra were
recorded on an Agilent Cary 630 FTIR equipped with an attenuated total
reflectance (ATR) sampling. Suitable crystals were mounted on a Rigaku
Oxford Diffraction SuperNova diffractometer and measured at 293~K at
the Cu radiation ($\lambda = 1.54184$~{\AA}). Data collection,
reduction and multiscan ABSPACK correction were performed with
CrysAlisPro (Rigaku Oxford Diffraction). Using Olex2, the structures
were solved with the ShelXT structure solution program using Intrinsic
Phasing and refined with ShelXL using least-square\break minimization.

\subsection{Synthesis of \textbf{BP}} \label{sec3.2}

Ni(acac)$_{2}$ ($m = 204$~g, 0.792~mmol, 2~equiv) was added to an
orange--red solution of \textbf{6} (100~mg, 0.396~mmol, 1~equiv) in
20~ml of toluene under vigorous stirring. The reaction mixture was
stirred at room temperature for 4~h. The precipitate obtained was then
isolated by filtration on a sintered glass funnel and dried in vacuo to
afford \textbf{BP} as a beige solid\break ($m = 184$~mg).

\section{Conclusion} \label{sec4}

We have prepared and crystallized novel nickel complexes \textbf{7} and
\textbf{8} incorporating a new aromatic triamino ligand \textbf{6}.
This ligand behaves in a monodentate fashion in coordination chemistry
because of the presence of the electron-withdrawing group NO$_{2}$ that
``deactivates'' the two amine functions in \textit{ortho} and
\textit{para} position. Although scarce in the literature due to a weak
linkage inherent to the monodentate aromatic amines, complexes
\textbf{7} and \textbf{8} could be fully characterized by X-ray
diffraction, probably because of the presence of numerous
intramolecular H-bonding interactions. The structure determination
clearly established two different metal (Ni)/ligand (\textbf{6})
stoichiometry depending on the solvent used for crystallization, and
the formation of a 1:2 complex (\textbf{7}) or a 2:2 adduct
(\textbf{8}) could be observed in methanol and acetone, respectively.
This work might open new perspectives in coordination chemistry of
aromatic polyamines by exploiting the NH$_{2}$/NO$_{2}$ balance to tune
the reactivity of the ligand and/or the stability of the metal
complexes.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could \mbox{benefit} from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}

This work was supported by the CNRS and the French ``Minist\`{e}re de
la Recherche et de l'Enseignement Sup\'{e}rieur.''

\section*{Acknowledgements}

The authors thank M.~Giorgi (Spectropole, Marseille) for the X-ray
diffraction studies.

\back{}

\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes[application/zip]{supplementary-material}{\cdrattach{crchim-292-suppl.zip}}

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