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\DOI{10.5802/crchim.328}
\datereceived{2023-08-31}
\daterevised{2024-04-29}
\dateaccepted{2024-07-23}
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\begin{DefTralics}
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\dateposted{2024-08-27}
\begin{document}

\begin{noXML}

%\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

\CDRsetmeta{articletype}{research-article}

\title{Synthesis and characterization of
\textit{cis}-bisdiphenylphosphinoethene gold(I) complexes}

\alttitle{Synth\`{e}se et caract\'{e}risation des complexes
\textit{cis}-1,2-bis(diph\'{e}nylphosphino)\'{e}th\`{e}ne or(I)}

\author{\firstname{Oc\'{e}ane Yvonne Odette} \lastname{Fayet}}
\address{Department of Chemistry \AA{}ngstr\"{o}m Laboratories -- Synthetic Molecular Chemistry, BOX 523 Uppsala (75120), Sweden}

\author{\firstname{Stefano} \lastname{Crespi}\CDRorcid{0000-0002-0279-4903}}
\addressSameAs{1}{Department of Chemistry \AA{}ngstr\"{o}m Laboratories -- Synthetic Molecular Chemistry, BOX 523 Uppsala (75120), Sweden}

\author{\firstname{Andreas} \lastname{Orthaber}\CDRorcid{0000-0001-5403-9902}\IsCorresp}
\addressSameAs{1}{Department of Chemistry \AA{}ngstr\"{o}m Laboratories -- Synthetic Molecular Chemistry, BOX 523 Uppsala (75120), Sweden}
\email[A. Orthaber]{andreas.orthaber@kemi.uu.se}

\keywords{\kwd{Aurophilic interactions}
\kwd{Gold(I) complex}
\kwd{Single crystal X-ray diffraction}}

\altkeywords{\kwd{Interaction aurophile}
\kwd{Complexe d'or}
\kwd{Diffraction des rayons X sur monocristal}}

\dedicatory{Oc\'{e}ane Yvonne Odette Fayette dedicates this manuscript
to her grandmother who never had the opportunity to finish her
chemistry degree} 

%\dedicatory{Oc\'{e}ane Yvonne Odette Fayet d\'{e}die ce manuscrit \`{a} sa grand-m\`{e}re qui n'a jamais eu l'occasion de terminer ses \'{e}tudes de chimie}

\shortrunauthors

\thanks{Swedish Research Council (grant nos.
2017-03727, 2021-05414, and 2022-06725), ERASMUS program}

\begin{abstract}
A series of solid-state structures of gold(I) complexes using the
semi-rigid \textit{cis}-1,2-{\ubreak}bis(diphenylphosphino)ethene 
(= \textit{cis}-\textbf{bdppe}) ligand are reported. The proximity of the
phosphine\unskip\break donor atoms of the bidentate ligand framework greatly favors
formation of mononuclear over di{\hbreak}nuclear complexes. Structural analysis
and theoretical studies shed further light on counter intermolecular
packing motifs in the solid state including interactions of solvent,
ligand and counterion fragments.
\vspace*{-1pt}
\end{abstract}

\begin{altabstract}
Une s\'{e}rie de structure de complexes d'or(I) \`{a} l'\'{e}tat
solide utilisant le ligand semi-rigide
\textit{cis}-1,2-bis(diph\'{e}nylphosphino)\'{e}th\`{e}ne est
rapport\'{e}e. La proximit\'{e} des atomes phosphines donneurs de la
structure du ligand bident\'{e} favorise grandement la formation de
complexes mononucl\'{e}aires par rapport aux complexes
dinucl\'{e}aires. L'analyse structurale et les \'{e}tudes
th\'{e}oriques ont apport\'{e} un \'{e}clairage suppl\'{e}mentaire sur
les motifs de contre-empilement intermol\'{e}culaire \`{a} l'\'{e}tat
solide, y compris sur les interactions du solvant, du ligand et des
fragments du contre-ion.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}

Coinage metals are well known for their versatile coordination
chemistry, yielding a large variety of structural motifs, which are
strongly influenced by ligating donor atoms, denticity and geometry of 
ligands~\cite{1,2,3,4}. Importantly, secondary interactions
like van der Waals, dipole, dispersive, metallophilic interactions
often play decisive roles in supramolecular arrangements and solid
state-packing. In particular, gold(I) is prone to form simple linear   
coordination compounds with coordination number two ($CN = 2$),
where the gold(I) ion can further engage in moderate to strong
metallophilic, i.e.\ aurophilic, interactions~\cite{5}. These
exhibit strengths comparable to strong hydrogen bonding and allow 
stabilization of inter- and intramolecular arrangements alike. The former
often leads to preferred conformations, while the latter can lead to
extended supramolecular structures observable in the solid-state
and solution~\cite{6,7}. Observation of these effects in
multinuclear clusters can also be interpreted using the premise of
cooperativity to further increase their strength, and  importance. 
\looseness=-1

Chelating effects are well understood for a variety of ligands possessing 
energetically favorable binding to a single metal (cation).
Nonetheless, multidentate phosphine ligands are also well known to
stabilize high nuclearity gold cluster compounds balancing the benefits of
aurophilic/metallophilic and chelating effects~\cite{8,9}.
However, subtle steric effects of the chelating ligand may have large
repercussions on the nuclearity and coordination
environment~\cite{10,11}, the latter also playing important
roles in catalytic applications~\cite{12}. Other
non-classical interactions such as halogen or hydrogen bonding
involving donors and acceptors other than oxygen and nitrogen are of
great interest. Halide interactions with acidic hydrogen atoms of
``non-coordinating'' solvents such as chlorinated alkanes offer a
variety of possibilities. In particular, chloroform and DCM have been
\mbox{subject} to extensive studies revealing their importance in
intermolecular stabilization and packing effects in the
solid-state~\cite{13}. For example, chloride (and other
halide) ions are often found in a hexacoordinated environment
stabilized by chloroform X${\cdots}$H--C hydrogen
bonding interactions~\cite{14,15}. Similar chloroform halide
interactions are found in a variety of organic and inorganic molecular
systems~\cite{16,17,18,19,20}. Spectroscopic and theoretical analyses
of such chloroform and bromoform--chloride interactions
clearly showed their affinity towards secondary solid-state
interactions~\cite{18,21,22}.

We detail the different aspects of primary and secondary factors that
govern the formation of mono- and dinuclear gold clusters based on the 
rigid  bidentate \textit{cis}-1,2-bis(diphenylphosphino)ethene (${=}$
\textit{cis}-\textbf{bdppe}) ligand. In this combined theoretical and
experimental/structural investigation of selected complexes, we elucidate 
the structural diversity of these deceivingly
simple coordination compounds~\cite{23}.


\section{Results}

Synthesis of linear phosphine gold halide complexes is typically
achieved by the straightforward ligand substitution reaction starting
from chloridotetrahydrothiophene gold(I), [AuCl(tht)], or similar
thioether precursors with phosphine ligands in a suitable solvent such
as DCM, chloroform or THF~\cite{24,25}. Our initial attempts
to prepare a dinuclear gold complex based on
\textit{cis}-1,2-bis(diphenylphosphino)ethene (${=}$~\textit{cis}-\textbf{bdppe}) in
a 2:1 gold:ligand ratio were unsuccessful. Reactions in concentrated
chloroform solutions resulted in the formation of a mononuclear complex
along with unreacted gold precursor (Scheme~\ref{sch1}).

\begin{scheme*}
\includegraphics{sc01}
\vspace*{5pt}
\caption{Synthesis of mono- and dinuclear gold complexes of diphosphine
ligand \textit{cis}-\textbf{bdppe}.\label{sch1}}
\vspace*{-.8pc}
\end{scheme*}



Single crystals of this complex have been obtained by slow evaporation
of the solvent, revealing the molecular structure of the complex as a
gold(I) center tetrahedrally coordinated by two
\textit{cis}-\textbf{bdppe} ligands. (Figure~\ref{fig1}) Complex
\textbf{1-Cl} crystallizes in the monoclinic space group \textit{C2/2}
with half of the molecule present in the asymmetric unit. The two
ligands form a slightly distorted tetrahedral arrangement around the
gold center (twist of the least squares planes 104.25(5)\textdegree).
The gold--phosphorus distances are in the expected range of gold(I)
phosphine complexes (P1--Au1 2.375(1)~\AA{} and P2--Au1 2.372(1)~\AA{}).
Notably, the chloride counterion is stabilized by four chloroform
molecules illustrating the ability of the acidic solvent proton to
engage in hydrogen bonded interactions
(C--H${\cdots}$Cl donor acceptor distance and angles of
3.294(3)/3.354(2)~\AA{}) and 149.1(2)\textdegree/144.3(1)\textdegree,
respectively. Moreover, slightly longer hydrogen bonding interactions
with the ethene bridge of the ligands are also observed
(C21${\cdots}$Cl 3.574(2)~\AA{} and
C1${\cdots}$Cl 3.639(2)~\AA{}). A Hirshfeld surface
analysis of the chloride fragment (Figure~\ref{fig1}b) illustrates the
hydrogen bonding interactions of the solvent molecules and the ligand
backbone. Notably, the yellow single crystals of \textbf{1-Cl} show
discoloration and lose their crystallinity over time, presumably due to
solvent loss. 

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig01}
{\vspace*{-2pt}}
\caption{(a) Solid-state structure of mononuclear complex \textbf{1}.
ORTEP style representation with ellipsoids drawn at a 50\% probability
level. Further crystallographic details are elaborated in the
experimental section. (b) Hirshfeld surface analysis highlighting the
hydrogen bonded interactions of the chloride fragment.\label{fig1}}
{\vspace*{-3pt}}
\end{figure*}

We utilized the geometry obtained from X-ray diffraction as the
starting point of computational analysis. We preoptimized the
geometry using the semiempirical tight-binding GFN2-xTB method, which
is known to provide optimal molecular geometries for gold
complexes~\cite{26}, followed by a second geometry optimization at the
r$^{2}$SCAN-3c level. As for the tight-binding approach, this composite
method is reported to provide fast and reliable geometries for
organo-gold species~\cite{27}. The structure was characterized as a
true minimum, with no relevant imaginary frequencies. 

We studied the orbital contributions governing the intermolecular
interactions in the metal complex by applying Weinhold's Natural Bond
Orbital (NBO) analysis. In particular, we studied the second-order
energy terms ($E(2)$) that arise from the interaction of two
unperturbed NBOs, which provide a quantitative value for the
stereoelectronic stabilization associated with that specific orbital
interaction. As~expected, one source of intermolecular stabilization
comes from the negative hyperconjugation of the lone pairs of the
chloride anion with the C--H $\rmsigma^*$ orbitals of chloroform.
We found an analogous negative hyperconjugative effect affording
$n_{\mathrm{Cl}}\rightarrow\rmsigma^*_{\mathrm{CH}}$ values ranging
between 2--4~kcal${\cdot}$mol$^{-1}$ (see Table~\ref{tab1} and
Figure~\ref{fig2}). Most interestingly, we found a hyperconjugative
effect between the halide and the C--H bonds~of the
\textit{cis}-\textbf{bdppe} ligand (see Figure~\ref{fig2}). The
stabilization associated with these perturbations is minimal 
(1--1.3~kcal${\cdot}$mol$^{-1}$), but still relevant to explain the peculiar
apical position that the halide assumes in the geometry of the complex.


\begin{table*}
\caption{Relevant $E(2)$ values associated with selected hyperconjugative
interactions in one of the orbitals depicted in Figure~\ref{fig3}\label{tab1}
\vspace*{-1pt}}
\begin{tabular}{ccccc}
\thead
\parbox[t]{2.7cm}{\centering  NBO number (Donor orbital)} & Donor orbital &
\parbox[t]{3cm}{\centering  NBO number (Acceptor orbital)} & Acceptor orbital &
\parbox[t]{2cm}{\centering  $E(2)$  (kcal${\cdot}$mol$^{-1}$)}\vspace*{2pt} \\
\endthead 
156 & LP Cl$_{102}$ & 350 & BD$^*$ C$_{4}$--H$_{5}$ & 1.25 \\
156 & LP Cl$_{102}$ & 447 & BD$^*$ C$_{78}$--H$_{79}$ & 1.03 \\
157 & LP Cl$_{102}$ & 350 & BD$^*$ C$_{4}$--H$_{5}$ & 0.25 \\
157 & LP Cl$_{102}$ & 447 & BD$^*$ C$_{78}$--H$_{79}$ & 0.46 \\
158 & LP Cl$_{102}$ & 350 & BD$^*$ C$_{4}$--H$_{5}$ & 0.28 \\
158 & LP Cl$_{102}$ & 447 & BD$^*$ C$_{78}$--H$_{79}$\vspace*{5pt} & 0.13 \\
155 & LP Cl$_{102}$ & 479 & BD$^*$ C$_{106}$--H$_{107}$ & 1.64 \\
157 & LP Cl$_{102}$ & 479 & BD$^*$ C$_{106}$--H$_{107}$ & 2.48 \\
158 & LP Cl$_{102}$ & 479 & BD$^*$ C$_{106}$--H$_{107}$\vspace*{5pt} & 4.32 \\
155 & LP Cl$_{102}$ & 483 & BD$^*$ C$_{111}$--H$_{112}$ & 0.71 \\
156 & LP Cl$_{102}$ & 483 & BD$^*$ C$_{111}$--H$_{112}$ & 1.86 \\
157 & LP Cl$_{102}$ & 483 & BD$^*$ C$_{111}$--H$_{112}$ & 3.11 \\
158 & LP Cl$_{102}$ & 483 & BD$^*$ C$_{111}$--H$_{112}$\vspace*{5pt} & 2.98 \\
155 & LP Cl$_{102}$ & 487 & BD$^*$ C$_{116}$--H$_{117}$ & 0.79 \\
156 & LP Cl$_{102}$ & 487 & BD$^*$ C$_{116}$--H$_{117}$ & 1.21 \\
157 & LP Cl$_{102}$ & 487 & BD$^*$ C$_{116}$--H$_{117}$ & 3.51 \\
158 & LP Cl$_{102}$ & 487 & BD$^*$ C$_{116}$--H$_{117}$\vspace*{5pt} & 3.00 \\
155 & LP Cl$_{102}$ & 491 & BD$^*$ C$_{121}$--H$_{122}$ & 1.59 \\
157 & LP Cl$_{102}$ & 491 & BD$^*$ C$_{121}$--H$_{122}$ & 1.92 \\
158 & LP Cl$_{102}$ & 491 & BD$^*$ C$_{121}$--H$_{122}$ & 4.46 
\botline
\end{tabular}
\tabnote{LP: lone pair; BD$^*$: antibonding orbital; the index as a
subscript for the atoms of each bond corresponds to the atom index in
the cartesian coordinate file.\vspace*{-2pt}}
\end{table*}

\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig02}
\vspace*{-3pt}
\caption{Geometry of the gold complex obtained at r$^{2}$SCAN-3c
level of theory. The main
$n_{\mathrm{Cl}}\rightarrow\rmsigma^*_{\mathrm{CH}}$ NBO interactions
(in kcal${\cdot}$mol$^{-1}$) are depicted: the chloride-chloroform (left) and
chloride-ligand (right) ones.\label{fig2}}
\end{figure*}

\begin{figure*}
\includegraphics{fig03}
\caption{Depiction of the relevant NBOs (isovalue 0.05) listed in 
Table~\ref{tab1}. Each number corresponds to the index value of the NBO.\label{fig3}}
\vspace*{-2pt}
\end{figure*}

Exclusive formation of this complex can be improved by maintaining a
strict 1:1 ratio of metal precursor to ligand and performing the reactions 
in chloroform (Scheme~\ref{sch1}). The crude product was purified by
washing with cold ethanol yielding 96\% of analytically pure compound
\textbf{1-Cl}. We hypothesized that the poor solubility of the gold
precursor and the chloride-stabilizing nature of chloroform are 
the main reasons for the formation of this product. Hence further reactions
were conducted in different coordinating solvents. Reactions in THF at
higher concentrations (${>}$0.1~M) gave a product mixture, while
reactions in THF at lower concentrations (1--2~mM) exclusively resulted
in formation of dinuclear complex \textbf{2} in good yields. Upon
removal of all volatiles and washing with ethanol, the THF insoluble
portion contained mainly \textbf{1-Cl}, and the soluble fraction left
for crystallization provided analytically pure material of complex
\textbf{2} (yield 78\%). The complex crystallized in the monoclinic
space group \textit{Cc} with one molecule in the asymmetric unit
(Figure~\ref{fig4}). The dinuclear complex \textbf{2} showed a slight
decrease of the gold phosphine distance to 2.231(1)~\AA{} and 
2.237(1)~\AA{} for P1--Au1 and P2--Au2, respectively. The spatial 
arrangement of the diphosphine and moderate aurophilic interactions led to an
intramolecular Au1--Au2 distance of 3.0313(3)~\AA{}---below
the sum of their van der Walls radii (3.32~\AA{})---which
indicates moderate aurophilic interactions. While the ligand backbone
is essentially co-planar, the diphenylphosphine-C single bond rotation
creates a twisted arrangement of the nearly linear phosphine gold(I)
chloride fragments (Cl--Au${\cdots}$Cu--Cl torsion angle
of 51.96(6)\textdegree).\looseness=-1

\begin{figure*}
\includegraphics{fig04}
\caption{Solid-state structure of mononuclear complex \textbf{2} and
Hirshfeld surface analysis. (a) ORTEP style representation with
ellipsoids drawn at a 50\% probability level. Further crystallographic
details are elaborated in the experimental section. Red areas indicate dominating
intermolecular CH${\cdots}\rmpi$ interactions. (b)
Hirshfeld surface plot. (c) Short intermolecular contacts; highlighted
in blue H${\cdots}$C interactions.\label{fig4}}
\end{figure*}


Removing the coordinating chloride ions from the dinuclear complex
\textbf{2} by non- or weakly coordinating anions using suitable 
silver(I) salts proved to be difficult and led to complex mixtures of
products, including a tetrahedrally coordinated gold ion, determined by 
$\mbox{}^{31}$P-NMR spectroscopy. Unambiguous identification of this species
was obtained by single crystal X-ray diffraction of crystal directly
obtained by slow evaporation of the crude reaction mixture. Targeted
synthesis of these tetrahedral coordination compounds \textbf{1-X} by 
treating \textbf{1-Cl} with suitable silver(I) salts
gave essentially quantitative conversions. Compound
\textbf{1-BF}$_{\boldsymbol{4}}$ and \textbf{1-PF}$_{\boldsymbol{6}}$
crystallized in the triclinic space group \textit{P-1} and monoclinic
space group \textit{P2}$_{1}$ with one and two molecules in the
asymmetric unit, respectively. The solid-state structure of
\textbf{1-PF}$_{\boldsymbol{6}}$ also contained disordered solvent molecules
which were treated as diffuse scattering using solvent masking.
Substituting the chloride of complex \textbf{1-Cl} with a
tetrafluoroborate (\textbf{1-BF}$_{\boldsymbol{4}}$) or hexafluorophosphate
(\textbf{1-PF}$_{\boldsymbol{6}}$) barely affected the direct gold(I)
coordination (P--Au distances ranging from 2.3859(4)~\AA{} to
2.3996(3)~\AA{}) and 2.356(6)~\AA{} to 2.394(6)~\AA{}, respectively.
However the folding angle of the P2Au planes varied significantly:
98.1(1)\textdegree~(\textbf{1-BF}$_{\boldsymbol{4}}$) and
110.2(2)\textdegree~(\textbf{1-PF}$_{\boldsymbol{6}}$) compared to
104.7(4)\textdegree~(\textbf{1-Cl}). The tetrafluoroborate anion is
positioned in between the diphenylphoshine groups from the two ligands.
Weak hydrogen bonding interactions of the tetrafluoroborate and
intermolecular $\rmpi$-stacking (centroid distances 3.373(1)~\AA{})
contribute to the solid-state packing of \textbf{1-BF}$_{\boldsymbol{4}}$.
Similarly, $\rmpi$-interactions \textbf{1-PF}$_{\boldsymbol{6}}$ (for
details, see Supporting Information Figures~S1--S2) dominate the
solid-state packing. In contrast to \textbf{1}-BF$_{4}$,
\textbf{1}-PF$_{6}$ and \textbf{1}-Cl have the counterions and solvent 
molecules (THF and CHCl$_{3}$, respectively) in close proximity
to the ethylene bridge. The differences of the counterion position and
additional solvation effects provide a rationale for the large
differences observed in the discussed folding angles. The gold (coinage
metal) coordination environment is in good agreement with previously
reported [M(I)(\textit{cis}-\textbf{bdppe})$_{2}$]PF$_{6}$ complexes (M
${=}$ Au and Cu)~\cite{28}. In a slight variation of this ligand, the
diphosphine \mbox{2,3-bis(diphenylphosphino)maleic} acid (\textit{dpmaa})
could also be targeted to form either a mononuclear tetrahedral or a
dinuclear gold complex. In view of the high solubility in water,
cytotoxicity tests revealed potential applications as anti-tumor
agents~\cite{29}. The related
2,3-bis(diphenylphosphino)maleic-N-phenylimide ligand showed similar
coordination behavior forming both mono- and dinuclear
complexes~\cite{30,31}.\looseness=-1

\begin{scheme*}
\includegraphics{sc02}
\vspace*{6pt}
\caption{Attempted synthesis of \textbf{3}, under the formation of
various side products including \textbf{1} and unidentified
species.\label{sch2}}
\vspace*{-8pt}
\end{scheme*}

Despite the easy of formation of the mononuclear complex, treatment of
\textbf{2} with silver triflate and excess of ligand partially
suppressed its formation and a dinuclear complex \textbf{3}
(Scheme~\ref{sch2} containing
two ligands was formed in an inseparable mixture with \textbf{1-OTf}.
Similarly, treatment of \textbf{2} with silver tetrafluoroborate 
led to the identification of \textbf{1-BF}$_{\boldsymbol{4}}$.
The mononuclear cations $\boldsymbol{1}^{\boldsymbol{+}}$ have been
identified by their characteristic $\mbox{}^{31}$P~NMR resonance, as
well as their crystal structure (see Supporting Information). Unambiguous
identification of the molecular structure of compound \textbf{3} was
provided by single crystal X-ray diffraction of colorless blocks that
crystallized directly from the reaction mixture (Figure~\ref{fig5}).
Dinuclear complex \textbf{3} crystallized in the centrosymmetric space
group \textit{P-1} with half of a molecule in the asymmetric unit. The
gold atoms are modeled with a positional disorder having site occupancy
factors of 89.5\% and 10.5\%. Two gold atoms bridge the ligands with an
Au${\cdots}$Au distance of 2.9908(4)~\AA{} and 3.051(4) for the major and
minor component respectively, which is comparable to that observed in
\textbf{2}, despite the shorter intra-ligand P1${\cdots}$P2 distance
(3.525(2)~\AA{}). \mbox{In addition,}\break $\rmpi$--$\rmpi$-interactions (centroid
distances 3.435(8)~\AA{}) \mbox{result} in a partially rigidified diphenylphosphine
backbone. The inter-ligand P${\cdots}$Au${\cdots}$P arrangement is slightly
contorted (162.3(1)\textdegree), forming a near ideal plane
\textit{plane1} (P1Au1P1$^{\prime}$P2$^{\prime}$Au1$^{\prime}$P2) with
an average deviation of 0.1~\AA{}, while the planes spanned by the
ligand backbone \textit{plane2} (P1C1C21P2) form an overall chair like
arrangement and a fold angle 48.8(3)\textdegree between \textit{plane1} and
\textit{plane2}. In a similar approach, \textbf{2}
was treated with silver trifluoroacetic acid (2.5 equiv) and
subsequently added to a solution of 4,4$^{\prime}$-bipyridine; however
predominant formation of tetrahedrally coordinated mono-nuclear gold
complex was observed.

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig05}
\vspace*{2pt}
\caption{Solid-state structure of dinuclear complex \textbf{3}. ORTEP
style representation with ellipsoids drawn at a 50\% probability level.
Further crystallographic details in the experimental
section.\label{fig5}}
\vspace*{2pt}
\end{figure*}

Neither the mononuclear nor the dinuclear complexes show any
appreciable emission with UV\unskip\break irradiation in solution and solid
state. This is, in particular, surprising given the short intramolecular
Au${\cdots}$Au distances of the dinuclear complexes,
which is indicative of aurophilic interactions and contrasts the
observed emission of higher nuclearity clusters using this and similar
ligand systems~\cite{32}.

\section{Conclusions}

The chemistry of these rigid diphosphine ligands having an unsaturated
spacer is dominated by the balance of chelating and aurophilic effects
that lead to mono and  dinuclear complexes. Primarily, reaction
conditions such as ligand:gold ratio, solvent and
concentration of reagents also lead to different products.
Further, non-covalent interactions play decisive roles in the
solid-state packing and may impact their solution behavior.
Notably, the prepared dinuclear gold(I) complexes are non-emissive,
which hints at dominating non-radiative decay pathways. With this study
we have elucidated few of the factors that direct the coordination
behavior, the simple bidentate phosphine ligand towards Au(I) centers,
and provide more insights into the rational synthesis of related
derivatives. \looseness=-1


\section{Experimental Section}

\subsection{General}

All chemicals were obtained from commercial suppliers and used as
received. All reactions and products were protected from prolonged
exposure to sunlight using aluminum foil or amberized glass. 

\subsection{X-ray crystallography}

Crystallographic data sets were collected from single crystal samples
mounted on a loop fiber and coated with high viscosity Fomblin PFPE
(Hampton Research). Collection was performed using a Bruker SMART APEX
diffractometer equipped with an APEXII CCD detector, a 3-circles
goniometer, and a MoK$_{\rmalpha}$-microfus sealed tube. The
crystal-to-detector distance was 5.0~cm, and the data collection was
carried out in 512 ${\times}$ 512-pixel mode. Cell refinement and data
reduction were performed with SAINT (Bruker AXS), absorption correction
was done by multi-scan methods using SADABS~\cite{33,34}. The structure
was solved by direct methods and refined using SHELX suite of
programs~\cite{35,36} using OLEX2~\cite{37}. Full details concerning
the data sets and crystal resolutions can be found in the respective
CIF files stored at the Cambridge Crystallographic Data Centre under
the allocated deposition numbers CCDC: 2291875-2291879.

\subsection{Computational methods}

Preliminary optimizations of the geometries were obtained using xTB
6.6.0~\cite{38} at GFN2-xTB level002E. Optimization and frequency
analysis at the DFT level were performed using ORCA 5.0.3~\cite{39}
with r$^{2}$SCAN-3c method~\cite{40}. Finally, NBO analysis was
performed using NBO 7.0.9 software package~\cite{41}. The cartesian
coordinates of the optimized geometry, the Molden file with the NBOs
and the ORCA output of the NBO analysis can be found in a figshare
repository~\cite{42}.


\subsection{Synthesis}

\subsubsection{Synthesis of [AuCl(tht)]}
Tetrahydrothiophene gold(I) chloride
was prepared in a slightly modified way than reported in literature~\cite{43}. Tetrahydrothiophene (C$_{4}$H$_{8}$S,
$45~\rmmu\mathrm{L}$, 0.51~mmol, 1.4~eq) was added to a suspension of
tetrachloroauric acid (H[AuCl$_{4}$], 123~mg, 0.36~mmol, 1~eq) in water
(1~mL)/EtOH (5~mL). The initial yellow precipitate changed to a white
precipitate within 15~min. The precipitate was filtered and washed
twice with cold EtOH (10~mL) to yield 94\% of analytically pure
product. The solution was filtered and subjected to slow evaporation of the solvent
protected from light, giving the corresponding complexes in good yields
(95\% and 92\%, respectively) as microcrystalline powders. Single
crystals suitable for X-ray diffraction were obtained by slowing down
the evaporation of THF.

\subsubsection{Synthesis of \textbf{1-Cl}}
Tetrahydrothiophene 
gold(I) chloride (44~mg, 0.14~mmol, 1~eq) and Ph$_{2}$P--C${=}$C--PPh$_{2}$
(\textit{cis}-\textbf{bdppe}, 108~mg, 0.27~mmol, 2~eq) were dissolved
in chloroform (5~mL) and stirred with protection from light for 24~h.
Filtration, followed by slow evaporation of all solvents gave
\textbf{1-Cl} in 96\% yield as large yellow crystalline material
suitable for single crystal diffraction. Upon further drying of the
material under reduced pressure, removal of residual tht also results
in complete discoloration of the material which is ascribed to the loss
of chloroform solvate molecules. NMR spectroscopic data is in agreement
with previously reported data of the cationic complex \textbf{1-OTf}
and \textbf{1-PF}$_{\boldsymbol{6}}$:$\mbox{}^{32}$~$\mbox{}^{1}$H NMR (400~MHz,
CD$_{2}$Cl$_{2}$) $\rmdelta$ 7.53--7.40 (m, 4H), 7.33 (dd, $J = 10.1$,
7.9~Hz, 8H), 7.15 (td, $J = 8.0$, 6.6~Hz, 25H) ppm. $\mbox{}^{31}$P NMR
(161.8~MHz, CD$_{2}$Cl$_{2}$) $\rmdelta$ 23.1~ppm.

\subsubsection{Synthesis of \textbf{1-BF}$_{\boldsymbol{4}}$
and \textbf{1-PF}$_{\boldsymbol{6}}$}
A solution
of \textbf{1-Cl} (40~mg) in THF (10~ml) is treated with 1 equivalent of
[Ag(MeCN)$_{4}$)]BF$_{4}$ or [Ag(MeCN)$_{4}$)]PF$_{6}$ for 24~h. The
solution is filtered and slow evaporation of the solvent protected from
light gives the corresponding complexes in good yield (95\% and 92\%)
containing single crystals suitable for X-ray diffraction. 

\subsubsection{Synthesis of \textbf{2}}
Importantly, [AuCl(tht)]
(70~mg, 0.22~mmol, 2~eq) must be fully dissolved in THF 
(ca.~80--100~ml) before \textit{cis}-\textbf{bdppe} (43~mg, 0.11~mmol, 1~eq
in 5~mL THF) is added. The reaction is stirred overnight, followed by
slow evaporation of the solvent (at all time protect from light), upon
which small colourless crystals suitable for x-ray diffraction are
obtained. Residual tht is removed under reduced pressure, while
impurities arising from formation of \textbf{1-Cl} can be easily 
removed by extracting \textbf{2} using a minimal amount of THF, which
upon solvent removal gives 78\% of \textbf{2}, while the THF insoluble
fraction consists mainly of \textbf{1-Cl}. 1H NMR (400~MHz,
CD$_{2}$Cl$_{2}$) $\rmdelta$ 7.59 (m, 16H) 7.52 (d, $J = 7.3$~Hz, 8H),
7.47--7.38 (m, 20H) ppm. $\mbox{}^{31}$P NMR (161.8~MHz, CD$_{2}$Cl$_{2}$)
$\rmdelta$ 13.6~ppm.\looseness=-1

\subsubsection{Attempted synthesis of \textbf{3}}
In an attempt to
remove the coordinating chloride complex \textbf{2} (50~mg, 0.06 mmol)
was stirred for 24~h with a silver salt, [Ag(OTf)], in THF (36~mg,
2.5 equiv.) in the presence of excess ligand (24~mg, 1.1 equiv.). The
reaction mixture was filtered, and after removal of all volatiles the
crude was crystallized from THF and chloroform, giving a product mixture
containing \textbf{3} (crystalline) and \textbf{1-X} (where X is either
OTf or Cl). Satisfactory spectroscopic data could not be obtained.
$\mbox{}^{31}$P NMR (THF): ${-}$22.6~ppm.

\section*{Declaration of interests}

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.

\section*{Funding}

The authors would like to thank the Swedish Research Council
(Vetenskapsr\aa{}det project grant 2017-03727 to AO, and 2021-05414
to SC) for financial support. OYOF would like to thank the
ERASMUS program for funding their exchange. The computations were
made possible by resources provided by the National Academic Infrastructure
for Supercomputing in Sweden (NAISS) at the National Supercomputer
Centre (Link\"{o}ping University, project NAISS 2023/22-567) partially
funded by the Swedish Research Council through grant agreement no.
2022-06725.

\CDRGrant[SRC]{2017-03727}
\CDRGrant[SRC]{2021-05414}
%\CDRGrant[NAISS]{2023/22-567}
\CDRGrant[SRC]{2022-06725}

\back{}

\section*{Supplementary data}

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

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-328-suppl1.pdf}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-328-suppl2.pdf}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-328-suppl3.pdf}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-328-suppl4.pdf}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-328-suppl5.pdf}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-328-suppl6.pdf}}

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