# Electrostatic vs Electronic interactions within oxidized multinuclear Pt(bipyridine)(dithiolene) complexes

Khalil Youssef, Antoine Vacher, Thanaphon Krueawatthanawet, Thierry

Roisnel, Frédéric Barrière, Dominique Lorcý

► To cite this version:

Khalil Youssef, Antoine Vacher, Thanaphon Krueawatthanawet, Thierry Roisnel, Frédéric Barrière, et al.. Electrostatic vs Electronic interactions within oxidized multinuclear Pt(bipyridine)(dithiolene) complexes. Dalton Transactions, 2024, 53 (48), pp.19388-19402. 10.1039/d4dt02514h. hal-04832766

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Electrostatic *vs* electronic interactions within oxidized multinuclear
Pt(bipyridine)(dithiolene) complexes

Khalil Youssef, Antoine Vacher, Thanaphon Khrueawatthanawet, Thierry Roisnel, Frédéric
Barrière, Dominique Lorcy*

Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, F-35000 Rennes, France

Email : dominique.lorcy@univ-rennes1.fr

D. Lorcy https://orcid.org/0000-0002-7698-8452

**Abstract**

Bi- and trimetallic platinum(bipyridine)(dithiolene) complexes involving different organic
linkers between the redox active platinum(dithiolene) moieties have been synthesized and
studied by electrochemistry and spectroelectrochemistry. Cyclic voltammetry experiments
carried out on these multinuclear complexes in dichloromethane using [NBu₄][PF₆] as the
supporting salt show only one oxidation process involving the metallacycles. Nevertheless,
spectro-electrochemical studies, carried out under the same conditions, show a different
evolution of the spectra during oxidation depending on the position of the metallacycles on the
phenyl ring. For instance, only the 1,3-disubstituted (Pt₂1,3-P) and 1,3,5-trisubstituted (Pt31,3,5-
P) complexes show the growth of an absorption band in the NIR region upon oxidation while
it was not observed for the other complexes. In contrast, electrochemical studies carried out
using the poorly coordinating supporting electrolyte, [Na][B(C₆H₄(CF₃)₂)₄], indicate sequential
oxidation of the redox centers with ΔE values varying according to the nature of the bridge and
the distance between metal centers. For all the investigated multinuclear complexes, spectro-
electrochemical experiments performed in the presence of [Na][B(C₆H₄(CF₃)₂)₄] show an
absorption band in the NIR region consistent with appreciable electrostatic effects and charge
delocalization in the mixed valent intermediates.

**Introduction**

Electronic interactions across a bridging ligand between paramagnetic metal centers in bi- and trimetallic complexes are highly dependent on the length and nature of the polytopic ligand.¹,²,³,⁴,⁵,⁶,⁷,⁸ Non-innocent di- or tritopic ligands are interesting building scaffolds for that purpose as they participate into the electron delocalization thanks to the mixing of the ligand and metal orbitals. ⁹,¹⁰ For instance, bis and tris(dithiolene) ligands, with a fused conjugated
bridge, were shown to allow electronic delocalization in either molecular or polymeric materials.<sup>11,12</sup> To probe the influence of the bridging ligand on the electronic communication between two or more coordinated metallic moieties, it is important to investigate firstly molecular complexes before polymetallic ones. Among the various electroactive mononuclear complexes, platinum(diimine)(dithiolene) complexes have focused a lot of attention due to their unique electrochemical, and photophysical properties as well as their wide range of applications.<sup>13,14,15,16,17</sup> These complexes, with their intense color, have been shown to possess substantial molecular hyperpolarizability,<sup>18</sup> solution and solid-state luminescence,<sup>19,20,21,22,23</sup> solvatochromism,<sup>24</sup> and unusual redox behaviour.<sup>20,25,26</sup> They can display two reversible reduction waves corresponding to the successive reduction of the bipyridine ligand into the radical anion and the dianion while, on the anodic scan, two oxidation waves can be observed corresponding to the oxidation of the dithiolene metallacycle into the radical cation and then to the dication.<sup>27</sup> Depending on the substituents on the bipyridine and the dithiolene ligands, the four redox processes cannot always be observed on the cyclic voltammogram within the available electrochemical window. Nevertheless, to probe a potential electronic communication along an organic bridge between two or more platinum(diimine)(dithiolene) moieties, this electroactivity is of high interest and these multinuclear complexes, depending on the linker connecting the two metalladithiolene moieties, could exhibit properties different than those of the mononuclear complexes. These multinuclear complexes are rare as only one trimetallic complex has been reported so far involving the tritopic ligand 1,2,3,4,5,6-benzenehexathiolate coordinating three Pt(dbbpy) centers (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine), **Pt₃B** (Chart 1a).<sup>28</sup> However, this complex was obtained in very low yield and apart from the reporting of its absorption spectrum, no in-depth investigation has been performed on the interaction between the metal centers. Therefore, we decided to investigate bis- and tris-platinum(diimine)(dithiolene) complexes as redox active polynuclear complexes with various organic bridges between the coordinating sites. For that purpose, we prepared different ditopic and tritopic dithiolene ligands together with the corresponding multinuclear platinum(diimine)(dithiolene) complexes. Regarding the ditopic ligands, we studied the influence of the benzene ring substitution by two dithiolene moieties and we investigated the 1,3 (**1,3-Pdt**) and the 1,4 (**1,4-Pdt**) ligands (Chart 1b). We also looked at the impact of the length of the organic bridge using the 2,7 substituted fluorene (**Fldt**, Chart 1b), as this organic bridge can be viewed as a planar and rigid biphenyl linker. Concerning the tritopic ligand, we focused on a phenyl ring substituted by three dithiolene moieties at the 1,3,5 positions (**1,3,5-Pdt**). For comparison purposes, we also prepared and analyzed the mononuclear complex **PtP**
with only one dithiolene moiety on the phenyl ring (ligand **Pdt**, Chart 1b). Electrochemical and spectroelectrochemical investigations were carried out on these five new complexes in order to assess the nature and strength of potential electronic interactions between redox centers.

<figure><img src="image_2.png" /><figcaption>Chart 1</figcaption></figure>

## Results and discussion

*Synthesis of the proligands.* Among the different dithiolene proligands, dithiol-2-one can be used to generate metal dithiolene complexes because it is readily deprotected in the presence of an alcoholate into the desired dithiolene and further reacts with various metal halide.²⁹ Another protecting group which presents an interest for the formation of metal complexes is the inorganic dibutyltin group.³⁰,³¹,³² Indeed, through a transmetalation with the appropriate dihalogenated metallic fragment, this proligand allows for the formation of the desired complex in a cleaner way and with higher yields. Thus, we envisioned the preparation of the two types of proligands. The synthesis of the bis(dithiol-2-one) proligands with a 1,4-phenyl and a fluorenyl organic spacer group was performed according to previously published procedures.³³,³⁴ The synthetic strategy used to reach the two proligands with a dithiol-2-one heterocycle at the 1,3 and the 1,3,5 positions of a phenyl ring is outlined in Scheme 1. They were prepared through a two steps synthesis starting from 1,3-bis(2-bromoethanone) **1**³⁵ and 1,3,5-tris(2-bromoethanone) **2**.³⁶ The first step is a nucleophilic substitution reaction with potassium isopropyl xanthate. Then cyclisation and dehydration in the presence of concentrated sulfuric acid led to the 1,3-bis(1,3-dithiol-2-one) benzene **3**³⁷ and to the 1,3,5-tris(1,3-dithiol-2-one) benzene **4**.
<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Synthesis of proligand **3** and **4**</figcaption></figure>

Then, we converted all the dithiol-2-one proligands **3**, **4** and **5** into the corresponding dibutyltin protected dithiolene ligands **6**, **7** and **8** (Scheme 2a). This transformation was performed according to the same approach as the one we used to form the 1,4-bis(2,2-dibutyl-1,3,2-dithiastannol-4-yl)benzene **9**, (Scheme 2b).³⁴ In the presence of sodium methanolate (NaOMe) we easily removed the carbonyl protecting group of the dithiol-2-one moieties and generated the dithiolene ligands which react readily with dichlorodibutyltin, Bu₂SnCl₂, to afford the protected dithiolene ligands **6**, **7** and **8**.

<figure><img src="image_4.png" /><figcaption>**Scheme 2.** Synthetic pathway to protected dithiolene ligands **6**, **7**, **8** and **9**.</figcaption></figure>

Crystals of the proligands **3** and **7** were obtained by slow evaporation of dichloromethane solutions. The molecular structure of these two proligands are presented in Fig. 1. The 1,3-bis(dithiol-2-one) **3** is not fully planar and adopts a “boat” conformation with a dihedral angle of 14.2(2)° between the average planes defined by the dithiole and phenyl rings, Fig. S1. Proligand **7** is not planar either as there is a rotation of the metalladithiolene moieties on the phenyl ring with dihedral angles between the average planes defined by the metalladithiolene and phenyl rings in the range of 35.4(5)° - 49.1(3)° (Fig. S2). In addition, one of the metalladithiolene moiety is more distorted along the S...S hinge than the two others. Indeed
along the S21...S22 the dihedral angle amounts to 15.8(2)° while it does not exceed 2.3(3)° for the S11...S12 hinge and 1.2(6)° for S1...S2.

<figure><img src="image_5.png" /><figcaption>**Fig. 1** Molecular structures of the proligands **3** (top) and **7** (bottom).</figcaption></figure>

*Synthesis of the complexes.* For the synthesis of the Pt(diimine)(dithiolene) complexes we choose first the dithiole-2-one proligands as starting materials. The mononuclear reference complex, **PtP**, was synthesized starting from the carbonyl-protected dithiolene ligand **10**.38 After deprotection of the ligand in the presence of sodium methanolate (MeONa) followed by the addition of *t*Bu2bpyPtCl2, the **PtP** complex was isolated as a blue powder in 74% yield (Scheme 3).

<figure><img src="image_6.png" /><figcaption>**Scheme 3.** Synthesis of mononuclear complex **PtP**.</figcaption></figure>

All the attempts to generate the multinuclear platinum complexes starting from the various bis and tris dithiol-2-one proligands were unsuccessful in the presence of NaOMe followed by the addition of *t*Bu2bpyPtCl2. Having in hands four dibutyltin proligands, i.e., the three ditopic ones **6**, **8** and **9** and the tritopic one **7**, we performed the transmetalation reaction in the presence of *t*Bu2bpyPtCl2 (Scheme 4). After adding *t*Bu2bpyPtCl2 to the pale-yellow solution of the
proligands, the color of the solution quickly changes to dark blue. These reactions led to the desired bimetallic complexes, **Pt21,3-P**, **Pt21,4-P** and **Pt2Fl** together with the trimetallic complex **Pt31,3,5-P** in good yields. All these complexes were obtained as deep blue solids.

<figure><img src="image_7.png" /><figcaption>**Scheme 4.** Synthesis of the target bi- and trinuclear platinum complexes</figcaption></figure>

The purity and the molecular structure of all these complexes were confirmed by elemental analyses, ¹H NMR and ¹³C NMR analyses, mass spectrometry and UV-vis spectroscopy. It is worth mentioning that these complexes are stable in the solid state while a quick evolution occurs in solution especially upon ambient light exposure and under air atmosphere indicating that, as the mononuclear Pt derivatives, these complexes are photochemically reactive.³⁹,⁴⁰ The light sensitivity is more pronounced for the bimetallic and trimetallic complexes compared to the monometallic ones. Therefore, the complexes used for spectroscopic studies were freshly prepared and kept under inert atmosphere in a Schlenk tube and protected from light. Despite its relative instability in solution, we managed to obtain crystals of the mononuclear complex **PtP** by slow evaporation of a concentrated solution of dichloromethane. On the contrary, all the attempts to crystallize the bimetallic and trimetallic complexes were unsuccessful. The **PtP**
complex crystallizes in the triclinic system; P-1 space group. Analysis of the structure reveals that the complex is not fully planar as the phenyl ring forms a dihedral angle of 22.5(1)° with the metallacycle (Fig. 2).

<figure><img src="image_8.png" /><figcaption>**Fig. 2** Molecular structure **PtP**, H atoms are omitted for clarity.</figcaption></figure>

*UV-vis absorption studies.* The UV-vis absorption spectra of all the complexes were analyzed in eight solvents of different polarity (CH₃CN, DMSO, DMF, acetone, CH₂Cl₂, CHCl₃, THF, toluene) and the lowest energy absorption band together with the absorption coefficient are gathered in Table 1. This lowest energy band lying in the visible region is ascribed to mixed-metal/ligand-to-ligand charge transfer process (MMLL'CT) between the dithiolene moiety with a non-negligible participation of the central Pt ion to the bipyridine unit.⁴¹ Among those five complexes only three (**PtP**, **Pt21,3-P** and **Pt2Fl**) are soluble in all the solvents. For these compounds, the lowest λmax value was found in acetonitrile and the highest in toluene with difference reaching 160 nm for **Pt21,3-P** and 146 nm for **Pt2Fl**. In toluene, this CT absorption band is split in two bands for **Pt2Fl** and **PtP**, and in three bands for **Pt21,3-P**. Actually, the broadening or splitting of this MMLL'CT absorption band in aprotic non polar solvent such as toluene and benzene has already been observed for other monometallic Pt complexes.²⁰,²¹, ⁴² The shift amplitude of the CT band maxima towards higher energy in solvents of increasing polarity is of the same order of magnitude as what was observed for all the monometallic Pt(diamine)(dithiolene) complexes.²⁴,⁴² This negative solvatochromism effect is attributed to the existence of a polar ground state and non-polar excited state.²⁰ A linear correlation was found between the experimentally measured λmax absorption maxima and the empirical solvent parameters reported by Eisenberg and co-workers as exemplified in Fig. 3b for **Pt21,3-P** (linear regression coefficient R² = 0.95).²² For the three different bimetallic complexes and the trimetallic one analysed in CH₂Cl₂, it can be observed that the nature of the bridging organic spacer group has no significant influence on the wavelength of the MMLL'CT absorption band (Fig. 4 and Table 1). Regardless of the number of Pt atoms within the complex mono-, bi-, or
trimetallic complexes, only a slight shift of 14 nm is observed for the wavelength of the MMLL'CT band between **PtP** and **Pt31,3,5-P**. On the contrary, the extinction coefficient $ε$ fluctuates according to the number of Pt atoms. Indeed, it amounts to 7927 M⁻¹.cm⁻¹ for **PtP**, in the range of 10090-13070 M⁻¹.cm⁻¹ for the bimetallic complexes (**Pt21,4-P**, **Pt2Fl** and **Pt21,3-P**) and reaches 21066 M⁻¹.cm⁻¹ for **Pt31,3,5-P**. Thus, the extinction coefficient $ε$ for the trimetallic complex is almost three times higher for **Pt31,3,5-P** than for the monometallic one **PtP**.

<table><caption>**Table 1.** Charge transfer absorption band wavelength maxima $λ_{max}$ (nm) and absorption coefficient $ε$ (M⁻¹ cm⁻¹) : $λ_{max}(ε)$</caption><thead><tr><th>Solvent (a)</th><th>PtP</th><th>Pt21,3-P</th><th>Pt21,4-P</th><th>Pt2Fl</th><th>Pt31,3,5-P</th></tr></thead><tbody><tr><td>CH<sub>3</sub>CN (1.000)</td><td>580(5828)</td><td>586(4850)</td><td>-</td><td>594(7470)</td><td>-</td></tr><tr><td>DMSO (0.973)</td><td>586(7442)</td><td>592(12861)</td><td>604(12510)</td><td>596(12550)</td><td>604(20240)</td></tr><tr><td>DMF (0.901)</td><td>598(5458)</td><td>604(11541)</td><td>604(15400)</td><td>606(15050)</td><td>608(19480)</td></tr><tr><td>Acetone (0.797)</td><td>608(7294)</td><td>618(5806)</td><td>-</td><td>618(12200)</td><td>-</td></tr><tr><td>CH<sub>2</sub>Cl<sub>2</sub> (0.765)</td><td>608(7927)</td><td>620(10090)</td><td>622(13070)</td><td>618(10550)</td><td>622(21066)</td></tr><tr><td>CHCl<sub>3</sub> (0.610)</td><td>626(7991)</td><td>642(8110)</td><td>646(9676)</td><td>634(8400)</td><td>632(14923)</td></tr><tr><td>THF (0.494)</td><td>650(12914)</td><td>680(6260)</td><td>-</td><td>674(17350)</td><td>-</td></tr><tr><td>Toluene (0.172)</td><td>674(7661)</td><td>626(2620)</td><td>-</td><td>688(9000)</td><td>-</td></tr><tr><td></td><td>720(8177)</td><td>681(3030)</td><td>-</td><td>740(8650)</td><td>-</td></tr><tr><td></td><td></td><td>746(2160)</td><td>-</td><td></td><td></td></tr><tr><td>R<sup>2</sup></td><td>0.98</td><td>0.95</td><td>0.95</td><td>0.92</td><td>0.98</td></tr></tbody></table>

(a) Empirical parameter from ref 20
<figure><img src="image_9.png" /><figcaption>**Fig. 3** CT absorption band for the Pt2,1,3-P complexes in eight solvents of different polarity, absorbance maxima have been normalized (a) and correlation between Eisenberg parameters and experimentally measured λmax visible absorption maxima in the eight solvents (b). 22</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 4** MLCT of the five investigated complexes in CH2Cl2. Absorbance maxima have been normalized.</figcaption></figure>

It is worth mentioning that no luminescence was detected for all these complexes in both the solid and solution state when excited in the MLL'CT absorption band, which may be attributed to the nonradiative thermal deactivation in the Pt(II)-containing complexes. 43

*Electrochemical studies.* The redox properties of these complexes were investigated by cyclic voltammetry in CH2Cl2 in the presence of 0.1 M [NBu4][PF6] as supporting electrolyte. The redox potentials are given in V vs SCE (Saturated Calomel Electrode) and are collected in Table 2. All these complexes present only two redox processes as exemplified in Fig. 5 (black line)
representing the CVs of **Pt21,3-P** and **Pt21,4-P** (CVs of **PtP**, **Pt2Fl** and **Pt31,3,5-P** are given in SI, Fig. S3-S5). On the cathodic scan, one chemically reversible reduction process occurs at a very close redox potential for all the complexes and is attributed to the reduction of the bipyridine ligands to the radical anion species. Indeed, localization of the LUMO is mainly on the *tBu*2bpy ligands in such complexes as demonstrated by the various studies performed on different monometallic Pt(diimine)(dithiolene) complexes. Further investigations at lower potentials do not allow the observation of the second expected redox process corresponding to the reduction of the monoanion into the dianionic state. Usually, this second reduction process is easily observed when electron-withdrawing groups are present on the bipyridine,<sup>17,27</sup> which is not the present situation with *tBu* groups. On the anodic scan and for all the investigated complexes, only one oxidation process is observed which is assigned to the oxidation of the Pt-dithiolene moieties in the radical cation species. Here again, investigations carried out at higher oxidation potentials do not show any further oxidation process corresponding to the oxidation of the radical cation species to the dicationic one. Another interesting feature is that, even with the presence of two redox active platinum-dithiolene metallacycles, only one oxidation process is observed. Therefore, this implies that there is no electronic interaction between the electroactive moieties along the organic bridge or that the electronic interaction is too weak to be detectable by cyclic voltammetry. The fact that the reduced bipyridine ligands do not interact is not surprising, while the fact that there is only one oxidation process for the Pt-dithiolene moieties is surprising. Indeed, electrochemical study carried out on two bimetallic complexes, **Mo21,4-P** and **Mo2Fl** (Chart 2), involving the same organic spacers but different electroactive (Cp₂Mo) metallic fragments, led to a CVs where a splitting of each redox process was observed indicating some sizeable electronic interactions through the same spacer.<sup>33, 34</sup>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Chart 2.** Molybdenocene bimetallic complexes exhibiting electronic interactions between the two Mo-dithiolene moieties.</figcaption></figure>
<table><caption>**Table 2.** Redox potentials<sup>a</sup> of investigated complexes in 0.1 M CH<sub>2</sub>Cl<sub>2</sub>-[NBu<sub>4</sub>][PF<sub>6</sub>] (E in V vs SCE).</caption><thead><tr><th>Complexes</th><th>E<sub>red</sub> (ΔE<sub>p</sub><sup>b</sup>)</th><th>E<sub>ox</sub>(ΔE<sub>p</sub><sup>b</sup>)</th></tr></thead><tbody><tr><td><b>PtP</b></td><td>-1.42(70)</td><td>0.29 (60)</td></tr><tr><td><b>Pt21,4-P</b></td><td>-1.45(60)</td><td>0.19 (120)</td></tr><tr><td><b>Pt2Fl</b></td><td>-1.42(70)</td><td>0.26 (120)</td></tr><tr><td><b>Pt2 1,3-P</b></td><td>-1.43(70)</td><td>0.22 (180)</td></tr><tr><td><b>Pt3 1,3,5-P</b></td><td>-1.46 (60)</td><td>0.21 (200)</td></tr></tbody></table>

In terms of oxidation potentials, **Pt2Fl**, **Pt21,3-P**, and **Pt31,3,5-P** oxidize at a potential close to that observed for the monometallic **PtP** complex, while **Pt21,4-P** is the easiest to oxidize (Table 2). It can be noticed that for the oxidation process, the difference between the anodic peak potential and the cathodic peak potential, (ΔEp=Epa - Epc) depends also on the organic bridge and the number of Pt units. The ΔEp amounts to 60 mV for **PtP** while it increases to 120 mV for **Pt21,4-P** and **Pt2Fl** and reaches 180 mV and 200 mV for **Pt21,3-P**, and **Pt31,3,5-P** respectively. The oxidation process is electrochemically reversible (ΔEp = 60 mV) at this scan rate (100 mV.s-1) only for the monometallic complex while it is electrochemically quasi-reversible (ΔEp > 60 mV) for the multinuclear complexes in the conditions used for this analysis, namely CH<sub>2</sub>Cl<sub>2</sub>-[Bu<sub>4</sub>N][PF<sub>6</sub>]. Due to a very low ion-pairing strength with positively charged species in low polarity solvents, the use of the BArF<sup>-</sup> anion (B(C<sub>6</sub>H<sub>4</sub>(CF<sub>3</sub>)<sub>2</sub>)<sub>4</sub><sup>-</sup>) instead of PF<sub>6</sub><sup>-</sup> enables an increase of the repulsive electrostatic interactions between positive like charges.<sup>44,45</sup> As a consequence, a further increase of the redox splitting of two closely overlapping redox processes could occur. Therefore, this electrolyte is of interest for electrochemical investigations of mixed-valent species.<sup>46</sup> Accordingly, we performed the electrochemical studies on three complexes, **Pt21,4-P**, **Pt21,3-P** and **Pt31,3,5-P** in CH<sub>2</sub>Cl<sub>2</sub> in the presence of 0.005 M [NBu<sub>4</sub>][BArF] as supporting electrolyte. Upon the anodic scan, the CVs of these complexes in this medium show two oxidation processes (Fig. 5 for **Pt21,4-P** and **Pt21,3-P**, red curve, and Fig S6 for **Pt31,3,5-P**). In the cathodic region, as expected, no

<footnote><sup>a</sup> Oxidation and reduction processes, taken as the average of the anodic and cathodic peak potentials.</footnote>

<footnote><sup>b</sup> ΔE<sub>p</sub> (in mV): peak-to-peak separation.</footnote>
separation between the bipyridine-based redox processes can be detected. The oxidation potentials are collected in Table 3.

<figure><img src="image_13.png" /><figcaption>**Fig. 5** Cyclic voltammogram of **Pt21,3-P** (left) and **Pt21,4-P** (right) in CH2Cl2 using 0.1 M of [NBu4][PF6] (black line) or 0.005 M of [NBu4][BArF] (red line) as supporting electrolytes. v=100 mV.s⁻¹. Note the different current scale.</figcaption></figure>

<table><caption>**Table 3.** Oxidation potentialsª of investigated complexes in 0.005 M CH2Cl2- [NBu4][BArF] (E in V vs SCE).</caption><thead><tr><th>Complexes</th><th>E<sup>1</sup><sub>ox</sub>(ΔEp<sup>b</sup>)</th><th>E<sup>2</sup><sub>ox</sub> (ΔEp<sup>b</sup>)</th></tr></thead><tbody><tr><td><b>Pt21,4-P</b></td><td>0.20 (100)</td><td>0.38 (110)</td></tr><tr><td><b>Pt2 1,3-P</b></td><td>0.16 (70)</td><td>0.40 (190)</td></tr><tr><td><b>Pt3 1,3,5-P</b></td><td>0.12 (210)</td><td>0.57 (330)</td></tr></tbody></table>

It is interesting to note that a stepwise oxidation is observed in CH2Cl2-[NBu4][BArF], while no separation between the redox processes was detected in CH2Cl2-[NBu4][PF6]. This is

<footnote>ª Taken as the average of the anodic and cathodic peak potentials.</footnote>

<footnote>ᵇ ΔEₚ (in mV): peak-to-peak separation.</footnote>
indicative of repulsive electrostatic interactions between the Pt(dithiolene) moieties in these complexes in the dicationic state.⁴⁷ As expected, the low ion-pairing strength of the BArF⁻ anion makes the oxidation of an already charged species more difficult due to the electrostatic repulsion, explaining the step-by-step oxidation process observed. Another information which can be extracted from these data is the potential difference between the two sequential oxidative redox processes which amounts to E² - E¹ = 180 mV and 240 mV for **Pt₂1,4-P** and **Pt₂1,3-P** respectively. This difference indicates that the electrostatic interactions are stronger for **Pt₂1,3-P** than for **Pt₂1,4-P** presumably due to the shorter distance of the platinum(dithiolene) moieties in the *meta* substituted complex than in the *para* substituted one. Geometry optimization with DFT theoretical calculations (*vide infra*) indicate that indeed the Pt···Pt distance is shorter for **Pt₂1,3-P** (9.76 Å) than for **Pt₂1,4-P** (12.16 Å).

*Spectroelectrochemical investigation.* UV-vis-NIR spectroelectrochemical investigations were carried out on freshly prepared solutions of the five platinum complexes in order to get some insights into the spectroscopic signature of the oxidized species. This investigation was first performed on a dichloromethane solution of each complex containing 0.2 M [NBu₄][PF₆] as the supporting electrolyte. Upon gradual oxidation of the mononuclear complex, **PtP**, besides several modifications such as the growth of several new absorption bands in the UV-vis region at 388 and 462 nm, the MMLL'CT band gradually decreases and new absorption bands in the visible region centered at 718 nm grow (Fig. S8). As the oxidation process occurs mainly on the Pt(dithiolene) moieties (*vide supra*), it is consistent to observe a modification of the MMLL'CT absorption band upon the gradual oxidation of the compound corresponding to the formation of the oxidized complex. These results are consistent with the spectro-electrochemical study performed by Kitagawa on Pt(diimine)(dithiolene).⁴⁸

For the bi- and trinuclear complexes, upon gradual oxidation, two types of evolution of the spectra can be distinguished, one for **Pt₂1,4-P** and **Pt₂Fl** complexes (Fig. 6) and the other type for **Pt₂1,3-P** and **Pt31,3,5-P** (Fig. 7). Concerning the two complexes **Pt₂1,4-P** and **Pt₂Fl**, upon gradual oxidation, no variations occur in the NIR region contrary to the UV-vis region. Indeed, the charge transfer band to bipyridine (638-640 nm) progressively decreases while two main absorption bands, at a higher and lower energy, grow at 454 and 757 nm for **Pt₂1,4-P** and at 488 and 764 nm for **Pt₂Fl**.
<figure><img src="image_14.png" /><figcaption>**Fig. 6** Differential UV-vis-NIR spectra of **Pt21,4-P** (a) and **Pt2Fl** (b) monitored upon gradual oxidation of the neutral to the oxidized complexes in CH2Cl2-[NBu4][PF6].</figcaption></figure>

The evolution of the spectra upon gradual oxidation of these two bimetallic complexes is quite similar to what was observed for the mononuclear **PtP** besides the red shift noticed for the lowest energy absorption band by about 39-46 nm. Therefore, taking into account the electrochemical investigation together with the spectroelectrochemical one, no electronic interaction is detectable between the two Pt(dithiolene) moieties in this medium.

Contrariwise, in the case of **Pt21,3-P** and **Pt31,3,5-P** even if the gradual oxidation leads to a decrease of the MMLL'CT band and the growth of several new absorption bands in the UV-vis region (397, 472, 730 nm and 458, 548, 738 nm), a new broad band centered at 1650 nm in the NIR region appears on both spectra (Fig. 7). Usually, upon gradual oxidation of a multinuclear complex, the growth of a new broad absorption band in the NIR region is assigned to an intervalence charge transfer (IVCT) band corresponding to the formation of mixed-valence species.⁵,⁴⁹ Often, the formation of mixed-valence species can also be noticed during the electrochemical investigation by a splitting of the redox systems. Nevertheless, our electrochemical studies did not evidence a stepwise oxidation of the Pt(dithiolene) moieties for any of the multimetallic complexes in CH2Cl2-[NBu4][PF6]. Based on the structural difference between **Pt21,3-P**, **Pt31,3,5-P** and **Pt21,4-P**, **Pt2Fl**, we may consider that stronger electrostatic through-space interactions can occur between the two or three electroactive moieties in **Pt21,3-P** and **Pt31,3,5-P** due to the 1,3 and 1,3,5 positions on the phenyl ring and the shorter intermetallic distances.⁵⁰,⁵¹ Along this line, it exists in the literature some examples where, in
the case of too weak electronic interaction to be detected by an electrochemical study, it is still
possible to observe some optical signature of an IVCT.⁵² For these two *meta*-substituted
complexes, **Pt21,3-P** and **Pt31,3,5-P**, we also performed the chemical oxidation through the
addition of one, two or three equivalents of [Fc][BF₄] (ferrocenium tetrafluoroborate) as
oxidizing agent. After the addition of one equivalent of [Fc][BF₄] to a solution of the **Pt21,3-P**
or **Pt31,3,5-P** complexes in CH₂Cl₂, under an inert atmosphere, we isolated the mono(oxidized)
complex [**Pt21,3-P**][BF₄] and [**Pt31,3,5-P**][BF₄] complexes as dark blue powders. The UV-vis-
NIR spectra of these oxidized complexes exhibit a broad band in the NIR region at about 1640
nm, similar to the one observed during the spectroelectrochemical experiment (See Fig. 7 and
Fig. S9). With two equivalents of [Fc][BF₄] for generating the bis(oxidized) species, [**Pt21,3-
P**]²⁺ and [**Pt31,3,5-P**]²⁺, the NIR absorption band at 1640 nm disappeared.

<figure><img src="image_15.png" /><figcaption>**Fig. 7** Differential UV-vis-NIR spectra of **Pt21,3-P** (a) and **Pt31,3,5-P** (b) monitored upon gradual oxidation of the neutral complexes.</figcaption></figure>

The UV-vis-NIR spectro-electrochemical investigations were also performed on a
dichloromethane solution of three complexes, **Pt21,4-P**, **Pt21,3-P** and **Pt31,3,5-P**, containing
now 0.005 M [NBu₄][BArF] as the supporting electrolyte. Fig. 8 shows the differential UV-vis-
NIR spectra that were obtained during the oxidation of **Pt21,3-P** and **Pt21,4-P** (Fig. S10 for
**Pt31,3,5-P**). For **Pt21,3-P** and **Pt31,3,5-P** a similar evolution of the absorption spectra can be
observed during the progressive oxidation of the solution, although with a more intense
absorption band in the NIR region than that obtained in a [NBu₄][PF₆] supporting electrolyte.
This NIR absorption band is localized at 1640 and 1650 nm for **Pt21,3-P** and **Pt31,3,5-P**
respectively close to the wavelength observed with [NBu₄][PF₆] (1650 nm). On the other hand,
discrepancies were observed on the spectra obtained by slowly increasing the oxidation potentials applied on a solution of the **Pt21,4-P** complex using [NBu4][BArF] instead of [NBu4][PF6] as the supporting electrolyte (Fig. 8). Indeed, this time a broad absorption band centered in the NIR region at 2030 nm is observed and corresponds to the signature of the formation of mixed-valence species. Interestingly, the wavelength of the IVCT band for this complex is shifted to lower energy by about 400 nm compared to the IVCT band for **Pt21,3-P** and **Pt31,3,5-P** complexes. For all these complexes, further increase of the oxidation potential induces the disappearance of the IVCT band. (Fig. S11)

<figure><img src="image_16.png" /><figcaption>**Fig. 8** Differential UV-vis-NIR spectra of **Pt21,4-P** (left) and **Pt21,3-P** (right) monitored upon gradual oxidation of the neutral complex in CH2Cl2 with 0.005 M [Bu4N][BArF] as the supporting electrolyte.</figcaption></figure>

These electrochemical results using the low ion pairing [NBu4][BArF] electrolyte, i.e. stepwise oxidation of each electrophore, and the spectroelectrochemical study, i.e. observation of an IVCT band, are indicative of through space electrostatic interactions between the Pt(dithiolene) moieties in the mono oxidized bi- and trimetallic complexes.

*Theoretical calculations.* Geometry optimizations were carried out on three complexes namely, **Pt21,3-P**, **Pt21,4-P** and **Pt2Fl** in the neutral state (Gaussian03, B3LYP/LanL2DZ). The HOMO of **Pt21,4-P** and **Pt2Fl** complexes is delocalized over the whole bis(dithiolene) bridging ligand with a small metal contribution (Fig. 9). The LUMO of these two complexes are centered on
the Pt(bipyridine) fragments. Concerning the frontiers orbitals of the **Pt21,3-P** complex, a pair of quasi degenerated HOMOs lying at -4.03 and -4.09 eV show that the 1,3 substitution of the phenyl ring induces a non-delocalization of the HOMOs on the whole bis(dithiolene) bridging ligand but rather a relative localization on each dithiolene moiety. Another difference concerns the degenerated pair of LUMOs (-2.40 eV) as it is not fully symmetrically localized on each Pt(dithiolene)(bipyridine) (Fig. S12). Comparison of the calculated energy levels of the HOMOs, **Pt21,3-P** (-4.03 eV), **Pt21,4-P** (-3.89 eV) and **Pt2Fl** (-3.99 eV) show that the HOMO in **Pt21,4-P** is higher in energy by 0.14 eV in comparison to that of its 1,3-isomer, in perfect accordance with the relative redox potentials determined by cyclic voltammetry. Discrepancies can also be noticed on the monocationic states (Fig. 10). We can notice that there is an effect of connectivity, *meta* vs *para*, **1,3-Pdt** vs **1,4-Pdt**, of the two Pt(dithiolene)(bipyridine) units on the formation of the mixed-valence species. Only the SOMO of **Pt21,4-P** is fully delocalized over the whole bridging bis(dithiolene) ligand.

<figure><img src="image_17.png" /><figcaption>**Fig. 9.** Frontier molecular orbitals (HOMO and LUMO) and calculated energy levels for complexes **Pt21,3-P** (left), **Pt21,4-P** (middle) and **Pt2Fl** (right)</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. 10.** Frontier molecular orbitals (SOMO and LUMO) of the monooxidized species and calculated energy levels for complexes **Pt21,3-P**⁺ (left), **Pt21,4-P**⁺ (middle) and **Pt2Fl**⁺ (right)</figcaption></figure>

**Conclusion**

A series of electroactive di- and tri-platinum(bipyridine)(dithiolene) complexes have been prepared with various organic bridges between the Pt(dithiolene) moieties such as *meta* and *para* substituted benzene, fluorene and 1,3,5-trisubstituted benzene. Comparison of the optical properties of these bi- and trinuclear complexes with a mononuclear complex as a reference indicates that they all absorb strongly in the visible range with no significant variation in the wavelength of the MMLL'CT band with the number of units Pt. All the multinuclear Pt complexes exhibit the same negative solvatochromism as that observed for the mononuclear complex, attributed to the existence of a polar ground state and non-polar excited state. The electrochemical response is strongly dependent on the nature of the supporting electrolyte. In [NBu₄][PF₆]-CH₂Cl₂, only a single reversible oxidation process is observed on the cyclic voltammetry of the dinuclear complexes indicating a weak repulsive electrostatic interaction between the metal centers. In [NBu₄][BArF]-CH₂Cl₂ however, the cyclic voltammetry of **Pt21,3-P** and **Pt21,4-P** reveals a splitting of the oxidation into two closely spaced reversible waves and is assigned to the much weaker affinity for positive charges of the BArF⁻ anion versus PF₆⁻ in this solvent. In addition, the potential difference (E² - E¹) between the two consecutive oxidations is commensurate with the estimated through-space distance between the metal centers in these complexes: namely, 180 and 240 mV, 12.16 and 9.76 Å for **Pt21,4-P** and **Pt21,3-P** respectively. Interestingly, upon gradual oxidation of the *meta* substituted complexes **Pt21,3-P** and **Pt31,3,5-P** in [NBu₄][PF₆]-CH₂Cl₂ an NIR absorption band was evidenced while
no obvious splitting of the oxidation process occurred in their cyclic voltammogram. This
suggests a weak stabilization in the mixed valence state by through bond electronic
delocalization only evidenced for the *meta* vs. *para* substituted complexes. In [NBu₄][BArF]-
CH₂Cl₂ however, all complexes exhibited a NIR absorption band in their mixed valence state
thanks to the electrostatic splitting of the sequential redox processes and the generation of a
significant concentration of the mixed valent species upon gradual oxidation.

**Experimental**

*General.* All synthetic reactions were performed under argon atmosphere. Chemicals and materials from commercial sources were used without further purification. NMR spectra were obtained in CDCl₃ unless indicated otherwise. Chemical shifts are reported in ppm, ¹H NMR and ¹³C NMR spectra were referenced to residual CHCl₃ (7.26 ppm and 77.2 ppm respectively). Melting points were measured on a Kofler hot-stage apparatus and are uncorrected. Mass spectra were recorded by the Centre Régional de Mesures Physiques de l'Ouest, Rennes. Methanol, acetonitrile and dichloromethane were dried using Inert pure solvent column device. CVs were carried out on a 5 10⁻⁴ M solution of the studied complexes in CH₂Cl₂-[NBu₄][PF₆] 0.1 M and in CH₂Cl₂-[NBu₄][BArF] 5 10⁻³ M. CVs were recorded on a Biologic SP-50 instruments at 0.1 Vs⁻¹ on a platinum disk electrode. Potentials were measured *versus* KCl Saturated Calomel Electrode (SCE). The spectroelectrochemical setup was performed in CH₂Cl₂-[NBu₄][PF₆] 0.2 M using a Pt grid as the working electrode, a Pt wire as the counter electrode and SCE reference electrode. A Shimadzu 3600 plus spectrophotometer was used to record the UV-vis-NIR spectra.

*Synthesis of the proligands*

**4,4'-(1,3-phenylene)bis(1,3-dithiol-2-one) 3:** To a solution of 1,1'-(1,3-phenylene)bis[2-bromoethanone] **1** (1.84 g, 5.75 mmol) in 110 mL of dichloromethane, potassium isopropyl xanthate (2.5 g, 14.3 mmol) was added and the reaction mixture was refluxed for 8h. Then filtration through Celite and evaporation of the solvent afforded a yellow solid in quantitative yield (2.48 g, 5.75 mmol). The compound was used for the next step without further purification. ¹H NMR (300MHz): δ 8.64 (t, J = 3Hz, 1H, CH), 8.26 (dd, J = 3, 6Hz, 2H, CH), 7.67 (t, J = 9Hz, 1H, CH), 5.72 (h, J = 6Hz, 2H, CH), 4.66 (s, 4H, CH₂), 1.55 (d, J = 6Hz, 12H, CH₃). Then, the compound was added slowly to 55 mL of concentrated H₂SO₄ at 0 °C and the reaction mixture was stirred for 6h at 0 °C. The solution was poured into 500 mL of iced water and the precipitate was filtered off and washed with water, ethanol, and diethyl ether. The
precipitate was dried overnight in an oven to afford **3** as a beige powder (1.45 g, 4.67 mmol). Crystals of sufficient quality for X-ray diffraction were obtained by layering petroleum ether onto a dichloromethane solution of **3**. Yield 81%; Mp 200 °C; ¹H NMR (300 MHz) δ 7.48 - 7.40 (m, 4H), 6.92 (s, 2H); ¹³C NMR (75 MHz) δ 191.7, 133.8, 133.8, 130.1, 127.0, 124.2, 113.14.; IR: νCO = 1634 cm⁻¹; HRMS (ESI) calcd for C₁₂H₆S₄O₂Na [M+Na]⁺ 332.9142, Found 332.9141. Anal. Calcd for [C₁₂H₆O₂S₄]: C, 46.34; H, 1.95; S, 41.31. Found: C, 46.35; H, 2.03; S, 41.35.

**4,4',4''-(benzene-1,3,5-triyl)tris(1,3-dithiol-2-one) 4:** 1,3,5 tri(2-bromoethanone) benzene 2 (1.24 g, 2.81 mmol) and potassium isopropyl xanthate (2.2 g, 12.6 mmol) were dissolved in 40 mL of dry dichloromethane. The yellow suspension was stirred at 45 °C overnight. Then the suspension was filtrated over Celite and evaporation of solvent afforded yellow solid in quantitative yield (1.7 g, 2.8 mmol). The compound was used for the next step without further purification. ¹H NMR (300MHz): δ = 8.84 (s, 3H), 5.71 (h, *J* = 6.2 Hz, 3H, CH), 4.70 (s, 6H, CH₂), 1.38 (d, *J* = 6.2 Hz, 18H, CH₃); ¹³C NMR (75MHz): δ = 211.92, 191.29, 137, 132.23, 79.47, 42.97, 21.25. The precipitate (1.7 g, 2.8 mmol) was slowly added to 30 mL of concentrated sulfuric acid at 0 °C and the reaction mixture was stirred for 7h at 0 °C. The solution was poured into 150 mL of iced water and the precipitate was filtered off and washed with water, ethanol, and diethyl ether. The precipitate was dried overnight in an oven to afford **4** as a beige solid (1.1 g, 2.57 mmol). Yield 92%; Mp >176 °C (dec.); ¹H NMR (DMSO, 300MHz): δ = 7.89 (s, 3H, CH), 7.67 (s, 3H, CH); ¹³C NMR (75 MHz, DMSO) δ 191.8, 133.9, 131.3, 124.3, 117.2; IR: νCO = 1650 cm⁻¹; HRMS (ESI) calcd for C₁₅H₇S₆O₃ [M+H]⁺ 426.8714, found 426.8713; Anal. Calcd for [C₁₅H₆O₃S₆ + ¼ CH₂Cl₂]: C, 40.90; H, 1.46; S, 42.96. found: C, 40.93; H, 1.40; S, 44.24.

**1,3-bis(2,2-dibutyl-1,3,2-dithiastannol-4-yl)benzene 6:** To a Schlenk tube containing 4,4'-(1,3-phenylene)bis(1,3-dithiol-2-one) **3** (0.37 g, 1.2 mmol) a solution of NaOMe (freshly prepared from sodium (0.27 g, 12 mmol) in 35 mL of dry methanol) was added. The yellow suspension was stirred at 43 °C for 1.5 h until the yellow suspension changed to a brown solution, then Bu₂SnCl₂ (0.79 g, 2.6 mmol) was added and stirred at r.t. for 1 h. The excess of MeONa was quenched with 20 mL of water. After addition of dichloromethane, the organic phase was washed with water and dried over MgSO₄. The solvent was removed under reduced pressure. Compound **6** was purified by flash chromatography on silica gel using dichloromethane/petroleum ether (1/1) as eluent and obtained as a yellow solid (0.48 g, 0.67 mmol). Yield 56%; mp < 50 °C; ¹H NMR (300MHz): δ = 7.70 (t, *J* = 3Hz, 1H, CH), 7.40 (dd, *J* = 3, 6Hz, 2H, CH), 7.23 (dd, *J* = 3, 6Hz, 1H, CH), 6.82 (s, 2H, CH), 1.73 (m, 16H, CH₂), 1.42
(m, 8H, CH₂), 0.96 (t, *J* = 6Hz, 12H, CH₃); ¹³C NMR (75MHz): δ = 140.6, 136.9, 128.1, 126.0, 117.8, 27.9, 26.7, 22.5, 13.1; HRMS (ESI) calcd for C₂₆H₄₂S₄Sn₂ [M⁺] 722.02079, found 722.02300. Anal. Calcd for [C₂₆H₄₂S₄Sn₂]: C, 43.36; H, 5.88; S, 17.80. found: C, 43.69; H, 6.22; S, 18.19.

**1,3,5-tris(2,2-dibutyl-1,3,2-dithiastannol-4-yl)benzene 7** : To a Schlenk tube containing 4,4',4"-(benzene-1,3,5-triyl)tris(1,3-dithiol-2-one) **4** (0.5 g, 1.17 mmol) a solution of MeONa (freshly prepared from sodium (0.40 g, 17.5 mmol) in 30 mL of dry methanol) was added. The brown suspension was stirred at 43 °C for 1.5 h until the brown suspension changed to a brown solution. Then Bu₂SnCl₂ (1.17 g, 3.86 mmol) was added and the solution was stirred at r.t. for 2 h. The excess of MeONa was quenched with 30 mL of water. After addition of dichloromethane, the organic phase was washed with water. The organic phase was dried with MgSO₄ and the solvent removed under reduced pressure. Flash chromatography on silica gel using dichloromethane/petroleum ether (1/1) as eluent, afforded **7** as a pale-yellow powder (0.74g, 0.71 mmol). Crystals of sufficient quality for X-ray diffraction were obtained by slow evaporation of a dichloromethane solution: Yield 61%; Mp 109 °C; ¹H NMR (300MHz): δ = 7.58 (s, 3H), 6.82 (s, 3H), 1.70 (m, 24H), 1.41 (m, 12H), 0.96 (t, *J* = 6.0 Hz, 18H); ¹³C NMR (75MHz): δ = 140.45, 136.76, 124.91, 118.00, 27.95, 26.74, 22.59, 13.73; HRMS (ESI) calcd for C₃₆H₆₀S₆Sn₃ [M⁺] 1044.008, found 1044.009. Anal. Calcd for [C₃₆H₆₀S₆Sn₃]: C, 41.52; H, 5.81; S, 18.47. found: C, 41.53; H, 6.02; S, 19.00.

**4,4'-(9,9-dipropyl-9H-fluorene-2,7-diyl)bis(2,2-dibutyl-1,3,2-dithiastannole) 8** : In a flask containing 4,4'-(9,9-dipropyl-9H-fluorene-2,7-diyl)bis(1,3-dithiol-2-one) **5** (120 mg, 0.25 mmol) was placed under an inert atmosphere a solution of NaOMe (freshly prepared from sodium (60 mg, 2.6 mmol) in 10 mL of dry methanol). The reaction mixture was stirred for 2 h at 50 °C, then cooled to room temperature and Bu₂SnCl₂(152 mg, 0.5 mmol) was added. The reaction mixture was stirred overnight at r.t. The excess of MeONa was quenched with 30 mL of water. After the addition of dichloromethane, the organic layer was washed with water. The organic phase was dried over MgSO₄ and the solvent was removed under reduced pressure. Filtration was realized on silica gel using dichloromethane. After evaporation of the solvent compound **8** was isolated as a yellow to brown oil (200 mg, 2.24 mmol). Yield 89%; ¹H NMR (300 MHz) δ 7.56 (dd, *J* = 18.4, 9.1 Hz, 6H), 6.86 (s, 2H), 2.01 - 1.89 (m, 4H), 1.78 (q, *J* = 8.0 Hz, 8H), 1.72 - 1.55 (m, 8H), 1.42 (m, 8H), 0.96 (t, *J* = 7.2 Hz, 12H), 0.79 - 0.62 (m, 10H).
Synthesis of the Pt(bipyridine)(dithiolene) complexes

**PtP** : To a Schlenk tube containing the 4-phenyl-dithiol-2-one (100 mg, 0.51 mmol) in dry methanol (10 mL), sodium (60 mg, 5 mmol) was added and the mixture was stirred for 1.5 h. Then an equimolar suspension of the *t*Bu₂bpyPtCl₂ (275 mg, 0.514 mmol) in dry THF (20 mL) was cannulated. The reaction was covered with aluminum foil and the mixture was stirred for 5 days at room temperature. Evaporation of the solvents, and purification by flash column chromatography on silica gel, using dichloromethane/petroleum ether (8/2) as eluent, afforded the complex **PtP** as a blue solid (0.24 g, 0.38 mmol). Crystals of sufficient quality for X-ray diffraction were obtained by slow evaporation of a concentrated solution of dichloromethane in dark. Yield 74%, Mp > 260°C, ¹H NMR (400 MHz, DMSO) δ 9.10 (d, *J* = 6.1 Hz, 1H), 8.99 (d, *J* = 6.1 Hz, 1H), 8.61 (t, *J* = 2.1 Hz, 2H), 7.78 (dt, *J* = 6.2, 2.2 Hz, 2H), 7.71 – 7.64 (m, 2H), 7.26 (t, *J* = 7.7 Hz, 2H), 7.16 – 7.08 (m, 1H), 6.87 (s, 1H), 1.43 (d, *J* = 1.0 Hz, 18H); ¹³C NMR (101 MHz, DMSO) δ 163.1, 154.9, 148.1, 147.2, 139.4, 137.3, 128.1, 125.2, 124.9, 121.5, 121.2, 35.8, 29.8. HRMS (MALDI) calcd for C₂₆H₃₀N₂S₂Pt [M⁺] 629.149, found 629.149; Anal. Calcd for [C₂₆H₃₀N₂S₂Pt] C, 49.59; H, 4.80; N, 4.45; S, 10.18. Found: C, 49.06; H, 4.73; N, 4.38; S, 9.96.

**Pt2,1,3-P** : To a Schlenk tube containing *t*Bu2bpyPtCl2 (296 mg, 0.56 mmol) dissolved in 15 mL of CH2Cl2, a solution of **6** (200 mg, 0.28 mmol) in 15 mL dry CH2Cl2 was added by cannula. Rapidly the color of the solution changed from yellow to intense blue, and the mixture was stirred for 18 h at 45 °C. After partial evaporation of the solvent under vacuum, 25 mL of diethyl ether was added. The precipitate was filtered, washed with diethyl ether, then dried under vacuum to afford the bimetallic complex as a dark blue powder. (310 mg, 0.26 mmol). Yield 95 %, Mp >260 °C; ¹H NMR (300 MHz) δ 9.36 (d, *J* = 6.1 Hz, 2H), 9.07 (d, *J* = 6.1 Hz, 2H), 8.23 (s, 1H), 7.89 (dd, *J* = 8.5, 2.0 Hz, 4H), 7.53 (d, *J* = 7.7 Hz, 2H), 7.39 (dd, *J* = 17.2, 6.2 Hz, 4H), 7.18 (t, *J* = 7.7 Hz, 1H), 6.83 (s, 2H), 1.44 (s, 18H), 1.37 (s, 18H); C NMR (126 MHz) δ 162.1, 155.7, 155.5, 149.3, 148.1, 139.5, 127.6, 124.7, 124.5, 123.4, 123.4, 121.0, 119.1, 35.7, 35.6, 30.2, 30.2; HRMS (MALDI) calcd for C₄₆H₅₄N₄S₄Pt₂ [M⁺] 1180.252. Found 1180.249. Anal. Calcd for [C₄₆H₅₄N₄S₄Pt₂ + ½ CH₂Cl₂] C, 45.64; H, 4.53; N, 4.58; S, 10.48. Found: C, 45.43; H, 4.72; N, 4.41; S, 10.55.

**Synthesis of Pt2,1,4-P** : To a Schlenk tube containing *t*Bu2bpyPtCl2 (223 mg, 0.41 mmol) dissolved in 15 mL of CH2Cl2, a solution of proligand **9** (150 mg, 0.20 mmol) in 15 mL dry CH2Cl2 was added by cannula. Rapidly the color of the solution changed from yellow to intense blue, and the mixture was stirred for 18 h at 45 °C. The reaction mixture was concentrated by
half and after the addition of 30 mL of diethyl ether a precipitate was formed. Filtration through a cannula and drying under vacuum afforded the bimetallic complex as a blue powder. (172 mg, 0.14 mmol). Yield 70 %, Mp >260 °C; ¹H NMR (300 MHz) δ 9.37 (d, *J* = 6.2 Hz, 1H), 9.24 (d, *J* = 6.2 Hz, 1H), 7.90 (d, *J* = 2.2 Hz, 4H), 7.70 (s, 4H), 7.44 (dd, *J* = 6.0, 2.0 Hz, 4H), 6.87 (s, 1H), 1.44 (s, 36H); ¹³C NMR (126 MHz) δ 162.1, 155.8, 155.5, 149.3, 148.1, 124.9, 124.6, 119.3, 35.4, 35.7, 30.3, 30.3; HRMS (MALDI) calcd for C₄₆H₅₄N₄S₄Pt₂ [M⁺] 1180.252, found 1180.257. Anal. calcd for [C₄₆H₅₄N₄S₄Pt₂ + 1 CH₂Cl₂] C, 44.58; H, 4.46; N, 4.42. Found: C, 44.72; H, 4.71; N, 3.88.

**Synthesis of Pt₂Fl** : To a Schlenk tube containing *t*Bu₂bpyPtCl₂ (360 mg, 0.672 mmol) dissolved in 20 mL of CH₂Cl₂, a solution of proligand **8** (300 mg, 0.336 mmol) in 20 mL dry CH₂Cl₂ was added by cannula. Rapidly the color of the solution changes from yellow to blue intense, and the mixture was stirred for 24h at 50 °C. After partial evaporation of the solvent to a solution of 5 mL, 12 mL of diethyl ether were added. The precipitate was filtered and washed with diethyl ether to afford the complex as a deep blue powder (0.44 mg, 0.325 mmol). Yield 96 %; mp >260 °C; ¹H NMR (400 MHz) δ 9.29 (d, *J* = 6.1 Hz, 2H), 9.15 (d, *J* = 6.1 Hz, 2H), 7.96 - 7.88 (m, 4H), 7.74 (d, *J* = 8.0 Hz, 4H), 7.55 (d, *J* = 7.9 Hz, 2H), 7.48 - 7.38 (m, 4H), 6.92 (s, 2H), 2.07 - 1.92 (m, 4H), 1.44 (d, *J* = 5.3 Hz, 36H), 0.69 (d, *J* = 6.0 Hz, 10H); ¹³C NMR (101 MHz) δ 162.4, 162.3, 155.7, 150.9, 149.1, 148.2, 142.8, 139.6, 139.1, 138.5, 124.8, 120.9, 120.5, 119.5, 118.9, 55.3, 43.2, 35.8, 35.8, 30.5, 30.3, 17.3, 14.7. UV-vis (CH₂Cl₂) λ (nm) (ε [L.mol⁻¹cm⁻¹]) 618 (10550), 384 (26600); HRMS (ESI) calcd for C₅₉H₇₀N₄S₄Pt₂ [M⁺] 1352.37791. Found 1352.378; Anal. calcd for C₅₉H₇₀N₄ S₄ Pt₂: C, 52.35; H, 5.21; S, 9.47. Found: C, 52.32; H, 5.23; S, 9.74.

**Synthesis of Pt3,1,3,5-P** : To a Schlenk tube containing *t*Bu₂bpyPtCl₂ (333 mg, 0.623 mmol) dissolved in 20 mL of CH₂Cl₂, a solution of **7** (216 mg, 0.207 mmol) in 20 mL dry CH₂Cl₂ was added by cannula, and the color changed from yellow to intense blue. The reaction mixture was refluxed for 20 h at 45 °C. After partial evaporation of the medium to 15 mL, 10 mL of diethyl ether was added. The precipitate was filtered, washed with diethyl ether (20 mL x 3), then dried under vacuum to afford the trimetallic complex as a dark blue powder (305 mg, 0.176 mmol). Yield 85 %, Mp >260 °C; ¹H NMR (300 MHz, CD₂Cl₂) δ 9.41 (d, *J* = 6.1 Hz, 3H), 9.23 (d, *J* = 6.1 Hz, 3H), 8.00 (d, *J* = 2.0 Hz, 6H), 7.88 (s, 3H), 7.53 (dt, *J* = 6.1, 2.3 Hz, 6H), 6.91 (s, 3H), 1.45 (d, *J* = 2.7 Hz, 54H); ¹³C NMR spectrum could not be obtained due to limited solubility of the compound; HRMS (MALDI) calcd for C₆₆H₇₈N₆S₆Pt₃ [M⁺] 1731.356. Found 1731.350; Anal. Calcd for [C₆₆H₇₈N₆S₆Pt₃ + 1 CH₂Cl₂] C, 44.27; H, 4.44; N, 4.62; S, 10.58. Found: C, 44.06; H, 4.49; N, 4.59; S, 11.04.
## Crystallography

Suitable crystal for X-ray diffraction single crystal experiment were selected and mounted on the goniometer head of a D8 Venture (Bruker-AXS) diffractometer equipped with a CMOS-PHOTON70 detector, using Mo-Kα radiation (λ = 0.71073 Å, multilayer monochromator). Crystal structures were solved by dual-space algorithm using SHELXT program,⁵³ and then refined with full-matrix least-squares methods based on F2 (SHELXL program).⁵⁴ All non-Hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions and treated as riding on their parent atom with constrained thermal parameters. Crystallographic data on X-ray data collection and structure refinements are given in Table 4. CCDC 2362193-2362195 for compounds **3**, **7** and **PtP** respectively.

<table><caption>**Table 4.** Crystallographic data of proligands **3** and **7** and the complex **PtP**</caption><thead><tr><th></th><th>3</th><th>7</th><th>PtP</th></tr></thead><tbody><tr><td>Formulae</td><td>C<sub>12</sub>H<sub>6</sub>O<sub>2</sub>S<sub>4</sub></td><td>C<sub>36</sub>H<sub>60</sub>S<sub>6</sub>Sn<sub>3</sub></td><td>C<sub>26</sub>H<sub>30</sub>N<sub>2</sub>PtS<sub>2</sub></td></tr><tr><td>FW (g.mol<sup>-1</sup>)</td><td>310.41</td><td>1041.27</td><td>629.73</td></tr><tr><td>System</td><td>monoclinic</td><td>monoclinic</td><td>triclinic</td></tr><tr><td>Space group</td><td>P2<sub>1</sub>/m</td><td>P2<sub>1</sub>/c</td><td>P-1</td></tr><tr><td>a (Å)</td><td>3.9743(6)</td><td>12.488(2)</td><td>11.8012(12)</td></tr><tr><td>b (Å)</td><td>24.267(4)</td><td>35.420(5)</td><td>12.4958(11)</td></tr><tr><td>c (Å)</td><td>5.9919(9)</td><td>10.2719(14)</td><td>18.4065(19)</td></tr><tr><td>α (deg)</td><td>90</td><td>90</td><td>92.581(3)</td></tr><tr><td>β (deg)</td><td>90.252(7)</td><td>109.935(5)</td><td>90.574(3)</td></tr><tr><td>γ (deg)</td><td>90</td><td>90</td><td>116.854(4)</td></tr><tr><td>V (Å<sup>3</sup>)</td><td>577.88(15)</td><td>4271.2(11)</td><td>2417.7(4)</td></tr><tr><td>T (K)</td><td>150(2)</td><td>150(2)</td><td>150(2)</td></tr><tr><td>Z</td><td>2</td><td>4</td><td>4</td></tr><tr><td>D<sub>calc</sub> (g.cm<sup>-3</sup>)</td><td>1.784</td><td>1.619</td><td>1.730</td></tr><tr><td>μ (mm<sup>-1</sup>)</td><td>0.808</td><td>2.058</td><td>5.993</td></tr><tr><td>Total refls</td><td>5374</td><td>42187</td><td>26580</td></tr><tr><td>Abs corr</td><td>multi-scan</td><td>multi-scan</td><td>multi-scan</td></tr><tr><td>Uniq refls (R<sub>int</sub>)</td><td>1338 (0.0363)</td><td>9698 (0.0280)</td><td>11002(0.0250)</td></tr><tr><td>Uniq refls (I > 2σ(I))</td><td>1282</td><td>9390</td><td>9759</td></tr><tr><td>R<sub>1</sub>, wR<sub>2</sub></td><td>0.0516, 0.1090</td><td>0.0336, 0.0740</td><td>0.0303, 0.0797</td></tr><tr><td>R<sub>1</sub>, wR<sub>2</sub> (all data)</td><td>0.0543, 0.1102</td><td>0.0346, 0.0745</td><td>0.0364, 0.0828</td></tr><tr><td>GOF</td><td>1.218</td><td>1.136</td><td>1.062</td></tr></tbody></table>
Acknowledgements

This work was granted access to the HPC resources of TGCC/CEA/CINES/IDRIS under the
allocation 2024 AD010814136R1 awarded by GENCI. Financial support was obtained from
ANR (Paris France) under contracts n°19-CE08-0029-02 and a PhD grant (to K. Youssef) from
Région Bretagne.

Authors contribution

Conceptualization and designed experiments: D. L. Experimental synthetic work and cyclic voltammetry experiments performed by K. Y and T. K. Spectroelectrochemical experiments carried out by A. V and K. Y. Theoretical calculations performed by F. B. X-Ray crystallography investigation: T. R. Writing, review and editing: D. L, F. B., and A. V. All authors have approved the final draft of the manuscript.

Competing interest

The authors declare no competing interests

Conflict of interest

The authors declare that they have no conflict of interest

Data Availability

Data will be made available on request.

Supplementary material

Electronic supplementary information (ESI) available. Figures S1–S25. CCDC 2362193-2362195 and Molecular structures of compounds **3, 7** and **PtP**. For ESI and crystallographic data in CIF or other electronic formats see DOI:

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