Mutual Placement of Isocyanide and Phosphine Ligands in Platinum(II) Complexes [PtHal2L1L2] (Hal = Cl, Br, I; L1,L2 = CNCy, PPh3) Leads to Highly-Efficient Photocatalysts for Hydrosilylation of Alkynes

A series of platinum complexes featuring phosphine and isocyanide ligands [PtX2(PPh3)(CNCy)] (X = Cl, Br, and I) as well as their parent phosphine [PtX2(PPh3)2] and isocyanide [PtX2(CNCy)2] analogues have been prepared and evaluated as catalysts for the photocatalytic hydrosilylation of alkynes. Under violet light irradiation (λmax = 400 nm), phosphine–isocyanides complexes [PtX2(PPh3)(CNCy)] gave high yields of silylated products (product yield up to 99%, TONs up to 1.98 × 103). The blue light irradiation (λmax = 450 nm) was more suitable for the parent phosphine complexes [PtX2(PPh3)2], which showed comparable efficiency (product yield up to 99%, TON up to 1.98 × 103), while isocyanide complexes [PtX2(CNCy)2] were not active.


Introduction
Homogeneous catalysis by transition metal complexes has become an indispensable tool for organic chemists.Nowadays, it is almost unthinkable to consider synthetic strategies that do not rely on metal catalysis for C-C bond formation reactions, stereoselective synthesis, or regulation of redox processes.The prevalence of the late transition metal catalysts, mostly of the platinum group [1], viz.platinum, rhodium, iridium, and palladium, as well as their non-noble counterparts, viz.cobalt, iron, nickel, and manganese, can be rationalised by their expected two-electron transformations and intrinsic stability of metal centres under varying conditions.The catalytic activity of the transition metal catalysts can be tuned by the installation of appropriate ligands; the electron donor strength of the ligand affects the properties of the metal centre, such as its nucleophilicity and redox capability.The most common neutral ligands involved in transition metal catalysts are tertiary phosphines [2].The phosphorous centre of this ligand is bound to transition metals through the σ-donation of its electron pair to a metal orbital and the π-backdonation occurring from filled metal orbitals to empty orbitals of the phosphine.The stability of phosphine complexes with platinum metals, relative ease in their preparation, and tunability justify their widespread application in the development of effective homogeneous catalysts [3].
While phosphines and isocyanides are primarily responsible for the tuning of the donor properties and stability of the metal centre, secondary halide ligands neutralise the charge of the metal centre and therefore stabilise the intermediates of the catalytic cycle [21,22].For example, palladium Chugaev carbene complexes ([PdHal 2 (R 1 NCN(H)N(H)CNR 2 )]) bearing chlorides have shown superior catalytic efficiency to the bromide species as precatalysts for Suzuki-Miyaura cross-coupling [23].To rationalise the selection and mutual influence of ancillary ligands on the catalytic properties of metal complexes, we have prepared a series of platinum's dihalide complexes with varying sets of phosphine and isocyanide ligands and examined their catalytic activity for the hydrosilylation of alkynes under thermal and photocatalytic conditions.

Synthesys and Charactrization
As aliphatic isocyanides are known to have greater donor strength than their aromatic counterparts [24], cyclohexyl isocyanide was chosen for investigation, and triphenylphosphine was selected as one of the most commonly available tertiary phosphines.The series of new (4-6, 8, 9) and known (1-3 [25,26], 7 [27]) platinum(II) bisphosphine, bisisocyanide, and phosphine/isocyanide complexes (Table 1) were prepared and characterised by CHN elemental analyses, high-resolution mass spectrometry (HR ESI + -MS), FT-IR, and 1 H, 13 C{ 1 H}, 31 P{ 1 H}, and 195 Pt{ 1 H} NMR spectroscopy.The geometry of all the complexes in solution was assigned based on FT-IR and NMR spectroscopy (for detailed discussion, see Section S1 in the Supplementary Materials (SM)).All 1-9 are isomerically pure in the solution and the solid state.The Pt-P one-bond spin-spin coupling constants ( 1 J Pt,P ) are dominated by the Fermi contact interaction of nuclei with s-orbital electrons and are used as an estimate of the bond strength (Table S2, see Supplementary Materials).Since the magnitude of the 1 J P,Pt is related to the s-character of the Pt-P bond, they ultimately give information about the effective nuclear charge on the platinum atom in the complexes [26,28].The Pt-P bond strengths and 1 J Pt,P depend, among other things, on the nature of the other ligands, but the influence of the ligand in trans position is higher than that of the ligand in cis position [25,26].In both series cis-[PtX 2 (PPh 3 ) 2 ] and cis-[PtX 2 (PPh 3 )(CNCy)], the 1 J Pt,P decreases in order of Cl > Br > I (Table S2) and matches the gradually declining transinfluence of these ligands [29].The comparison of each pair cis-[PtX 2 (PPh 3 ) 2 ] and cis-[PtX 2 (PPh 3 )(CNCy)] with the same anionic ligands led to the conclusion that the 1 J Pt,P of 1-3 is bigger than the 1 J Pt,P of 4-6, being consistent with the greater σ-donation and π-acceptance abilities of phosphines compared to isocyanides [30,31].

DFT Analysis of Ligand Properties
Density functional theory (DFT) calculations were performed to determine if the bonding and electronic structure in the complexes could further explain the differences in the catalytical activity of 1-9 (Section S4, see Supplementary Materials).Geometry opti-

DFT Analysis of Ligand Properties
Density functional theory (DFT) calculations were performed to determine if the bonding and electronic structure in the complexes could further explain the differences in the catalytical activity of 1-9 (Section S4, see Supplementary Materials).Geometry optimisations in the ground state resulted in structures in good agreement with experimental crystallographic data: changes in coordination bonds and angles were less than 3% (Table S6, see Supplementary Materials).Information on the binding of various metal complexes can be found through the topological analysis of the electron density distribution (AIM method) [45].This approach was utilised in studying the nature of Pt-L coordination bonds in optimised equilibrium structures (Tables 2 and S6, Figures 3 and S4).The most important property to assign a bond is the presence of a bond critical point (BCP) between pairs of bonded atoms.The BCPs occur where the line of maximum density between two atoms, named the bond path, intersects the interatomic surfaces.The electron density value and its Laplacian at BCPs provide insights into the nature of interactions.As a rule, ρ(r) > 0.20 a.u. and ∇ 2 ρ(r) < 0 indicate a covalent bond, while ρ(r) < 0.10 a.u. and ∇ 2 ρ(r) > 0 indicate an ionic bond.More broadly, increasing values of ρ(r) and reductions in ∇ 2 ρ(r) indicate increasing covalent character and stronger bonding interactions.The values of ρ(r) and ∇ 2 ρ(r) parameters between platinum and the donor atom in these BCPs are typical for closed-shell dative interactions [46] (Table 2).Among 1-9, the Pt-C bonds have larger values of ρ(r) than those of the Pt-P bonds, and for both series 1-3 and 4-6, the values of ∇ 2 ρ(r) of the Pt-P bonds remain unaltered (Table 2 and Figure 3).At the same time, in the mixed-ligand complexes 4-6, the influence of the halogen's type on the value ∇ 2 ρ(r) Pt-C bonds can be seen with the maximum observed for 4 (0.171).However, all the BCP parameters for Pt-Hal bonds suggest no significant changes in ligand type variation.
Table 2. Bond orders and QTAIM of complexes 1-9, where values of the density of all electronsρ(r), Laplacian of electron density-∇ 2 ρ(r), energy density-H b , potential energy density-V(r), Lagrangian kinetic energy-G(r) (a.u.), MBO-Mayer bond order, and WI-Wiberg bond indices.Based on Espinosa's criterion [47] for characterising interactions at the BCPs by analysing the ratio of the potential and kinetic energy densities, the |V(r)|/|G(r)| values of Pt-L BCPs are 1.41-1.82(Table 2), indicating that these Pt-L bonding regions have a somewhat covalent character, which could well correspond to a dative bond type.In addition, the values for |V(r)|/G(r) for all Pt-P bonds are higher (1.69-1.82)than for the Pt-C bonds (1.53-1.56),indicating the Pt-P is more covalent.
The calculated Mayer bond orders (MBOs) [48] of the Pt-C bonds (0.90-1.02) are larger than those of the Pt-P bonds (0.65-0.89), demonstrating the weakened character of the latter.Meanwhile, Wiberg bond indexes at 1.28-1.43[49] show that the Pt-C bonds have a partially double character through π-donation of electron density from the filled d orbitals of the platinum to the empty p π orbitals of the C atoms.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 16 activity for different platinum(II) halides has been described beforehand.In [55][56][57], Pt II -NHC complexes featuring ancillary iodide ligands showed lower catalytic activity in the hydrosilylation process when compared to the bromide species; no comparison to chloride analogues has been provided.Visible light-induced catalytic reactions have become a powerful trend in organic synthesis [58][59][60][61].Extending this strategy to platinum(II)-catalysed hydrosilylation under visible light irradiation [10] brings up the photocatalytic system in which a transition metal catalyst serves as both a photo-absorber and a catalysis-enabling species [62,63].
At 40 °C, the application of 0.05 mol% of 1-6 showed differences between phosphine and mixed ligand phosphine-isocyanide complexes, and under these conditions, the mixed ligand complex 4 showed superior catalytic activity (Figure 4C).This example illustrates the complementary role of the isocyanide and phosphine ligands.The bis-isocyanide complex itself is not capable of catalysing the hydrosilylation, probably because the isocyanide is not a sufficient donor and cannot facilitate the oxidative addition step [52,53].However, the bis-phosphine complex exhibits weaker catalytic activity as well, presumably due to excessive shielding of the metal centre by bulky tertiary phosphine ligands [54].The combination of the phosphine and isocyanide ligands on one platinum centre represents an optimal compromise between nucleophilicity and spatial availability of the metal centre, leading to superior catalytic efficiency.
According to the data along the series Cl-Br-I, the complexes with chloride and bromide ligands were more effective in hydrosilylation than the iodide complexes (Table S8, Figure 4).To the best of our knowledge, only one example of a comparison of catalytic Screening the entire library of 1-9 as catalysts with loading of 0.5 mol% at 80 • C revealed large variations in catalytic activity as a function of catalyst structure, with yields of the 1,2-(diphenylvinyl)triethylsilane product ranging from 13% to 99% after 24 h (Figure 4A).Catalysts 1-6, containing one (4-6) or two (1-3) PPh 3 ligands, gave significantly higher yields than containing two isocyanides (7-9), possibly as a result of the greater donation ability of PPh 3 compared to CNCy [51].With lower catalyst loadings (0.05 mol%), all 1-6 showed elevated activity (97-99%, TONs 1.9-2.0× 10 3 ), and, surprisingly, no significant difference in activity was observed between phosphine complexes 1-3 and mixed ligand complexes 4-6 (Figure 4B).
At 40 • C, the application of 0.05 mol% of 1-6 showed differences between phosphine and mixed ligand phosphine-isocyanide complexes, and under these conditions, the mixed ligand complex 4 showed superior catalytic activity (Figure 4C).This example illustrates the complementary role of the isocyanide and phosphine ligands.The bis-isocyanide complex itself is not capable of catalysing the hydrosilylation, probably because the isocyanide is not a sufficient donor and cannot facilitate the oxidative addition step [52,53].However, the bisphosphine complex exhibits weaker catalytic activity as well, presumably due to excessive shielding of the metal centre by bulky tertiary phosphine ligands [54].The combination of the phosphine and isocyanide ligands on one platinum centre represents an optimal compromise between nucleophilicity and spatial availability of the metal centre, leading to superior catalytic efficiency.
According to the data along the series Cl-Br-I, the complexes with chloride and bromide ligands were more effective in hydrosilylation than the iodide complexes (Table S8, Figure 4).To the best of our knowledge, only one example of a comparison of catalytic activity for different platinum(II) halides has been described beforehand.In [55][56][57], Pt II -NHC complexes featuring ancillary iodide ligands showed lower catalytic activity in the hydrosilylation process when compared to the bromide species; no comparison to chloride analogues has been provided.
Visible light-induced catalytic reactions have become a powerful trend in organic synthesis [58][59][60][61].Extending this strategy to platinum(II)-catalysed hydrosilylation under visible light irradiation [10] brings up the photocatalytic system in which a transition metal catalyst serves as both a photo-absorber and a catalysis-enabling species [62,63].Despite the fact that all 1-9 have weak absorption bands in the visible light region, the species were evaluated as hydrosilylation catalysts under visible light irradiation [50].
The observed photocatalytic activity is not aligned directly with the photophysical properties of the complexes under study.Within the series of bisphosphine com-plexes 1-6, there is a noticeable increase in light absorption in the visible region on going from chlorine to bromine to iodine, while the catalytic activity of respective complexes decreases.The efficiency of 1-6 is lower compared to the most efficient protocols for light-induced hydrosilylation described previously, which were based on mixed-ligand Pt II -diaminocarbene/isocyanide complexes [10,50].

Instruments and Reagents
Solvents and organic and inorganic reagents were obtained from commercial sources and used as received.The complexes 1-3 and 7 were obtained using known methods [25][26][27]65].C, H, and N elemental analyses were carried out on a Euro EA 3028 HT CHNS analyzer.High-resolution mass spectra were acquired on a Bruker micrOTOF spectrometer equipped with an electrospray ionisation (ESI) source.MeOH was used as the solvent.The instrument was operated in positive ion modes using a m/z range of 50-3000.The capillary voltage of the ion source was set at −4500 V (ESI + ) and the capillary exit at +(70-150) V.The nebulizer gas pressure was 0.4 bar, and the drying gas flow was 4.0 L/min.The most intensive peak in the isotopic pattern is reported.Infrared spectra were recorded on the Shimadzu IRAffinity-1 FTIR instrument (4000-400 cm −1 , resolution 2 cm −1 ) in KBr pellets.NMR spectra were recorded on a Bruker AVANCE III 400 spectrometer in CDCl 3 at ambient temperature (at 400, 100, 162, and 86 MHz for 1 H, 13 C, 31 P, and 195 Pt NMR, respectively).Chemical shifts are given in δ-values [ppm] referenced to the residual signals of undertreated solvent (CHCl 3 ): δ 7.26 ( 1 H) and 77.2 ( 13 C). 1 H and 13 C NMR data assignment for 4-7 was achieved by using 2D ( 1 H, 1 H-COSY, 1 H, 1 H-NOESY, 1 H, 13 C-HMQC/HSQC and 1 H, 13 C-HMBC) NMR correlation experiments.The UV/vis absorption spectra in CH 2 Cl 2 solution were recorded on a Shimadzu UV-2500 (Shimadzu Corporation, Tokyo, Japan) spectrophotometer in a quartz cuvette with l = 1.0 mm and the complex's concentration of 0.03 mM.
X-ray Diffraction Study.X-ray diffraction studies of single crystals of complexes 4, 6, and 7. Single crystals for X-ray diffraction experiments were grown by the slow evaporation of the respective compounds in the CH 2 Cl 2 /Et 2 O mixture under air at room temperature.A single-crystal X-ray diffraction experiment was carried out on a SuperNova, single source at offset/far, HyPix3000 diffractometer with monochromated CuKα radiation.The crystal was kept at 100(2) K during data collection.The structures were resolved by ShelXT [66] (a structure solution programme using intrinsic phasing) and refined by means of the ShelXL [67] programme incorporated into the OLEX2 program package [68].The crystallographic details are summarised in Table S1.Empirical absorption correction was applied in the CrysAlisPro (Agilent Technologies, Yarnton, UK, 2012) program complex using the spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm.The crystal data, data collection parameters, and structure refinement data for 4, 6, and 7 are given in Table S3, and the plots and selected bond lengths and angles are given in Table S4 CCCDC numbers 1811734 (4), 1839000 (6), and 1839001 (7) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (access date: 23 October 2023).
Computational Details.The geometry of 1-9 were optimised at density functional theory (DFT) calculations at the M06 [69]/Def2SVP [70,71] level with the help of Gaussian-09 [72].The single-point calculations for optimised structures have been carried out using the PBE0 [73]-D3/def2-TZVP [74] level of theory with also help of the Gaussian-09 programme package.The topological analysis of the electron density distribution with the help of the atoms in molecules (QTAIM) method developed by Bader [45], MBO [75], and WI [49] analysis been performed using the Multiwfn programme (version 3.7) [76].The Cartesian atomic coordinates for all model structures are presented in Table S7.

Catalytic Tests
General procedure for the catalytic hydrosilylation of alkynes with hydrosilanes (specific conditions are provided in Table S8).The solution of the selected catalyst in CH 2 Cl 2 (0.1 mL, 5.0 × 10 −6 M) was placed in the 5 mm tubes, and the solvent was evaporated to dryness under a stream of dinitrogen.1,2-diphenylacetylene (5.0 × 10 −4 mol), Et 3 SiH (7.5 × 10 −4 mol), and toluene (2 mL) were added to the tube.The tube was closed with a septum, kept at 40-80 • C or under LED irritation with 400/450 nm LEDs (3 × 2.5 W, the LEDs were placed at 2 cm distance from the tube) for 6-24 h.The contents of the tube were poured over silica-gel and extracted with hexane (5 mL).Extracts were evaporated under reduced pressure, and the crude product was subsequently dissolved in 0.6 mL of CDCl 3 with 1,2-dimethoxyethane (1.0 equiv, used as an NMR internal standard) added, and then analysed by 1 H and 13 C NMR spectroscopy.The isomeric content was determined on the basis of the analysis and matching of 1 H and 13 C chemical shifts for products against authentic samples of (E)-1,2-(diphenylvinyl)triethylsilane [77] and (S)-1,2-(diphenylvinyl)triethylsilane [78].Quantifications were undertaken upon integration of the selected peaks of the product against peaks of 1,2-dimethoxyethane.
All 1-9 absorb light up to 400 nm, making these complexes suitable for visiblelight-activated reactions.The absorption maxima of 1-9 have been red-shifted in order bisphosphine > phosphine/isocyanide > bisisocyanide complexes and Cl > Br > I.The blue light irradiation (λ max = 450 nm) is more suitable for phosphine complexes 1-3 (up to 99% product yield, TON up to 1.98 × 10 3 ), while phosphine-isocyanide species 4-6 better operated under violet light (λ max = 400 nm, up to 99% product yield, TON up to 1.98 × 10 3 ).The isocyanide complexes 7-9 demonstrate low catalytic activity under all conditions.Future studies aimed at understanding the mechanism of the photocatalytic action of the phosphine and isocyanide complexes and widening the scope of their catalytic applications are currently being expanded within our group.
Figure S1.The structures of complex 5.The thermal ellipsoids are shown at 50% probability.Figure S2.The structures of complex 6.The thermal ellipsoids are shown at 50% probability.Figure S3.The structures of complex 8.The thermal ellipsoids are shown at 50% probability.Figure S4.View of optimised structures of complexes 1-9. Figure S5.UV/vis absorption spectra of complexes 1-3 in CH 2 Cl 2 at RT (2 × 10 −5 M).

Table 1 .
The numbering scheme of the prepared platinum(II) complexes used as catalysts in the hydrosilylation of alkynes.