Experimental and computational tuning of metalla-N-heterocyclic carbenes at palladium(II) and platinum(II) centers

Abstract

Palladium(II) and platinum(II) complexes featuring metalla-N-heterocyclic carbenes (7–12) were synthesised via metal-mediated coupling between equimolar cis-[MCl₂(CNR)₂] (R = 2,6-Me₂C₆H₃ (Xyl), 2,4,6-Me₃C₆H₃ (Mes)) and 2-aminopyridine or 2-aminopyrazine. Thiocyanate complexes 13–18 with two thiocyanate ligands were obtained through the ligand exchange in the parent compounds 7–12 with NH₄CNS in acetone/CH₂Cl₂. Complexes 7–18 were isolated and characterised by HRESI⁺-MS, IR, ¹H and ¹³C{¹H} NMR spectroscopy and single-crystal X-ray diffraction (in the case of 11, 16, and 18). The UV-vis properties of 7–18 and the electrochemical properties of 7–12 were also evaluated. To study the electronic structure and bonding nature in the new compounds, the quantum theory of atoms in molecules (QTAIM) and Mayer bond order analysis together with the extended transition state with the natural orbitals for chemical valence (ETS-NOCV) method, were used. X-ray diffraction studies and theoretical considerations indicate that the thiocyanate derivatives 16 and 18 form supramolecular dimers by two symmetrical pairs M¹⋯C⁵ and S¹⋯C² with short intermolecular contacts between an electron-rich M^II-center and thiocyanate ligand on the one side and the electron-poor π-system of an azaheterocyclic ring on the other side. Representative carbenes 8, 11 and 12 were evaluated as photocatalysts for the hydrosilylation of diphenylacetylene with triethylsilane giving 1,2-(diphenylvinyl)triethylsilane in 98% yield under visible light irradiation (blue light, 445 nm).

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Introduction

Transition metal complexes with heteroatom-stabilised carbenes, i.e. N-heterocyclic carbenes (NHCs)¹ and acyclic diaminocarbenes (ADCs),² underpin a broad range of applications in the catalysis of organic processes;^1a,2a,3 they are studied as emissive materials in optoelectronics⁴ and potential anticancer drugs.^2a,5 The ligand properties of diaminocarbenes are manifested through the formation of the M–C_carbene σ-bond by transferring a pair of nonbonding electrons from the nucleophilic σ-orbital of the carbene carbon atom to the metal atom M. At the same time, the formation of a π-bond is also possible as a result of the interaction of a vacant electron-deficient pπ-orbital of a carbene carbon atom with a metal, whose orbital symmetry is suitable. The diaminocarbene ligand is usually an effective σ-donor and a weak π-acceptor, which is reflected in the catalytic and photophysical properties of the respective metal complexes. Tuning of the electron-donor capacity of diaminocarbenes, which is crucial for the success of their target applications, has been mainly accomplished by varying the substituents on the nitrogen atoms or the modification of the heterocyclic skeleton (Fig. 1A). It was shown that NHC complexes containing a metal atom within the NHC skeleton, named metalla-N-heterocyclic carbenes (MNHCs),⁶ exhibit a stronger electron-donor character by comparison with classical imidazole-2-ylidenes.⁷ In pursuit of our ongoing project on the chemistry of diaminocarbenes,^3d,e,8 we have reported that platinum(II)–MNHC species (Fig. 1B) work as self-photosensitising transition-metal photocatalysts for the hydrosilylation of alkynes under visible light.⁹ In this catalytic reaction the platinum(II)–MNHC catalyst has a dual function: it serves as a light-absorbing species and a transition metal catalyst enabling the catalytic process.

Fig. 1 (A) Examples of cyclic, N-heterocyclic and metalla-N-heterocyclic diaminocarbenes. (B) The platinum(II)–MNHC catalyst exhibiting photocatalytic properties under visible light irradiation.

Additional strategies for fine-tuning the properties of complexes of this type are linked to (i) the modification of the structure of the azaheterocyclic ring (highlighted with a yellow rectangle), (ii) substitution in the aryl rings (highlighted with a cyan rectangle), and (iii) the replacement of the anionic ligands at both metal centers (highlighted with a magenta rectangle). An interplay of these will define the application of the complex thus designed towards a specific goal.

The scenario of this work was as follows. We started by the preparation of a series of palladium(II)– and platinum(II)–MNHC complexes via the metal-mediated coupling of isocyanides with various 2-aminoazaheterocycles. The displacement of chlorides with thiocyanates led to the generation of previously unknown diaminocarbene-thiocyanate derivatives. The electronic structure and bonding nature in all new compounds were assessed by using the quantum theory of atoms in molecules (QTAIM), the natural bond orbital (NBO) analysis, and Mayer bond order analysis together with the extended transition state with the natural orbitals for chemical valence (ETS-NOCV) method. Analysis of the frontier molecular orbital character, energy gaps, and UV/vis spectra by density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches provided insights into the excited states involved in the light absorption and electrochemical processes, in turn, related to the potential photocatalytic properties. Finally, we assessed the catalytic properties of the thus prepared carbene complexes in the photocatalytic hydrosilylation of alkynes under visible light irradiation.

Results and discussion

Preparation and characterisation of metalla-N-heterocyclic diaminocarbenes

For this study, a series of Pd^II and Pt^II MNHC species 7–12 were prepared via the nucleophilic addition of 2-aminoazaheterocycles 5 and 6 to metal-bound isocyanides in cis-[MCl₂(CNR)₂] (1–4, M = Pd, Pt). The reaction of 1–4 and azaheterocycles 5, 6 proceeds in CH₂Cl₂ at RT for ca. 1 d and the subsequent workup provided the binuclear diaminocarbene species 7–12 in good (84–95%) isolated yields (Scheme 1, Route A). This process occurs via the intermediate generation of the corresponding mononuclear M-ADC species and its nucleophilic addition to the unreacted isocyanide material facilitated by the deprotonation of the M-ADC intermediate by an excess of the aminoazaheterocycle used.^9,10

Scheme 1 Preparation of 7–18.

MNHC complexes 7–12 are yellow solids stable in the 20–80 °C temperature range. New complexes 9–12 were characterised by elemental analyses (C, H, and N), accurate-mass ESI⁺-MS, IR, 1D (¹H, ¹³C{¹H}, and ¹⁹⁵Pt{¹H}) NMR and 2D (¹H,¹³C-HSQC and ¹H,¹³C-HMBC) NMR spectroscopy (see the Experimental section for more details), while the authenticity of the known species 7 and 8 was confirmed by ¹H NMR spectroscopy.^9,10 The intense absorption bands at 1619–1667 cm⁻¹ in the FTIR spectra of 7–12 were assigned to ν(C_carbene–N) of the NCN moiety. For 7–12, the important ¹³C resonances of the C atoms from two carbene ligands are different and are detected at δ_C 168 and 182 ppm. This evidences that two NCN groups in 7–12 are substantially structurally different; one of them (having the downfield resonance at δ_C 168 ppm and assigned to C⁷ carbon) is of the amidine type, whereas another one (having the downfield resonance at δ_C 182 ppm and assigned to C¹ carbon) is of the diaminocarbene type. These δ_C values for diaminocarbene carbon C¹ are downfield relative to those (δ_C 157–159) reported for the series of palladium complexes featuring chelated NHCs, such as bis(imidazolylidene) palladium dichloride complexes,⁸⁰ but are some upfield of the δ_C 197–201 range observed for the series of palladium complexes featuring chelating acyclic aminocarbene ligands, i.e. [PdCl{C(N=C(C₆R²R³R⁴R⁵CON))=N(H)R¹}(C=NR¹)].¹¹

The thiocyanate anion NCS⁻ enables several pathways to alter the properties of the metal centres due to its ambidentate nature, allowing the coordination via either S- or N-ends. To the best of our knowledge, only two examples of thiocyanate diaminocarbene complexes, viz. nickel(II)–NHC with N-coordinated thiocyanate¹² and gold(I)–NHC with S-coordinated thiocyanate,¹³ have been reported. Thiocyanate species demonstrate better donor abilities and increased dipole moment of complex molecules relative to halide analogues which may lead to better catalytic activity of thiocyanate species, e.g. in the Kumada–Corriu coupling reaction.¹² In addition, the better donor character of thiocyanate is related to the stabilisation of the HOMOs of the complexes and their red-shifted absorption in the UV-vis spectrum.¹⁴

The common method for the preparation of thiocyanate complexes from chloride substrates is a metathetic reaction with NaNCS or KSCN in an aqueous or alcohol solution, occasionally aided by AgNO₃ to facilitate chloride subtraction.¹⁵ Metathetic reactions of 7–12 with a three-fold excess of NH₄CNS in acetone/CH₂Cl₂ at RT for 24 h afforded yellow bis(N-coordinated thiocyanate) complexes 13–18 in good yields (Scheme 1, Route B). Owing to the poor solubility of 13–18 in the most common solvents, all 13–18 were characterized in solution by means of HR-MS (ESI⁺) and in the solid state by FTIR and solid-state ¹³C CP/MAS NMR. The structures of 16 and 18 were also elucidated by single-crystal XRD.

Complexes 13–18 gave satisfactory CHN data, which are consistent with the proposed formulae confirming the replacement of both chloride ligands with thiocyanate. The HRESI⁺-MS spectra of 13–18 exhibit sets of peaks with the characteristic isotopic distribution due to [M + H]⁺, [M + Na]⁺ and/or [M − NCS]⁺ ions. IR spectroscopy was used to identify the coordination modes of thiocyanate in metal thiocyanate complexes.¹⁶ As a general rule, the value of ν(CN) stretching for the thiocyanate of N-bonded complexes lies near 2050 cm⁻¹ (strong band) and that of S-bonded complexes is close to 2100 cm⁻¹ (weaker band).^16c The IR spectra of 13–18 measured in KBr revealed two intense absorption bands in the range 2030–2090 cm⁻¹ attributed to the ν(CN) stretching of the N-bonded thiocyanate. Additionally, the FTIR spectra of 13–18 displayed strong absorption overlapping bands at ca. 2200 cm⁻¹ which are also present in the spectra of parent compounds 7–12 and are attributed to the ν(C≡N) of the isocyanide ligands.

UV-Vis absorption properties

All prepared complexes are yellow (10–18) or yellowish (7–9) solids, which makes them capable of absorbing light in the visible range. UV-vis absorption spectra of 7–18 were measured at ambient temperature in CH₂Cl₂ solution (Fig. 2) and in KBr pellets (Fig. S6 and S7 in the ESI;† UV-vis data for 7–18 are summarised in Table S4 (ESI)).†

Fig. 2 UV-vis absorption spectra for 7–18 in CH₂Cl₂ at RT (0.03 mM for 7–12 and saturated solutions for 13–18).

Similar to other palladium¹⁷ and platinum^17a,d,18 cyclometallated complexes, the UV-vis spectra of 7–12 contain strong absorption bands below 340 nm [ε = (1.15–3.59) × 10⁴ M⁻¹ cm⁻¹] assigned to π–π* ligand centred transitions in the xylyl and mesityl moieties (Fig. 2).¹⁹ The wide intense absorption bands in the 340–450 nm range with characteristic molar extinction coefficients [ε = (0.60–1.13) × 10⁴ M⁻¹ cm⁻¹] can be assigned to mixed ¹L′LCT (ligand-to-ligand charge transfer) and ¹ILCT (intraligand charge transfer) transitions based on TD-DFT calculations (see the section TD-DFT calculations). The energy of the ¹L′LCT/¹ILCT absorption band is affected by the change of the azaheterocyclic fragment.²⁰ Complexes with pyridine rings 7–9 and 11 show blue-shifted ¹L′LCT/¹ILCT bands relative to their pyrazine counterparts 10 and 12 (e.g. 364–398 nm [ε = (0.60–0.83) × 10³ M⁻¹ cm⁻¹] in 7–9 and 11vs. 402–414 nm [ε = (1.06–1.13) × 10³ M⁻¹ cm⁻¹] in 10 and 12).

This could be explained by a decrease of the LUMO energy in the case of 10 and 12 with pyrazine species due to the additional intercalation electron negative nitrogen atom in the azaheterocyclic fragment. Variation of the aryl fragment of the CNR or the corresponding diaminocarbene ligands induces no significant changes in the positions of the ¹L′LCT/¹ILCT bands and only exerts minimal influence on the extinction coefficients (e.g. 364–393 nm [ε = (0.64–0.83) × 10³ M⁻¹ cm⁻¹] in 7 and 8vs. 378–414 nm [ε = (0.60–1.13) × 10³ M⁻¹ cm⁻¹] in 9–12).

The UV-vis spectra of 13–18 demonstrate similar absorption profiles to the spectra of the 7–12 (Fig. 2). Replacing both chloride ligands with thiocyanate ligands leads to minor changes in the absorption spectra. Complexes 7–18 are not fluorescent and/or phosphorescent at RT, and no emission bands were detected.

Frontier molecular orbital analysis

To gain insight into the electronic structures and optical properties of the obtained complexes time-dependent DFT calculations on 7, 8, 11, and 12 were undertaken. The theoretically predicted structures are in good agreement with experimental X-ray structures (Fig. S8 and Table S5 in the ESI†). In general, the computed bond lengths are close to the experimental ones indicating sufficient accuracy of the selected computational protocol.²¹ The main difference lies in slight elongation of coordination bonds (by 0.01–0.03 Å) and CN triple bonds (0.03–0.04 Å) in the optimized structures compared to experimental results, due to relaxation of the cyclic metal including systems.

Fig. 3 shows the shape and energy of the frontier molecular orbitals for 7, 8, 11, and 12, while the FMO compositions are given in Table S8.† In all cases, the highest occupied molecular orbitals (HOMOs) are delocalized by the π molecular orbitals on the aryl ring connected with the exocyclic nitrogen atom of the diaminocarbene species (contribution of aryls >80%), while the contribution of the metal d orbitals is insignificant (<5%). The energies of the HOMOs of 7, 8, 11, and 12 are similar, while the lowest unoccupied molecular orbitals (LUMOs) are localised over the entire diaminocarbene (22–34%) and azaheterocyclic (38–54%) species. The large contribution (12–16%) from the metal d orbitals in the LUMO may be related to the absence of luminescence for 7–12 at RT due to the population of a metal-centred state.²²

Fig. 3 Calculated molecular orbital energy HOMO–LUMO gaps and surface plots of the HOMO−1, HOMO and LUMO for 7, 8, 11 and 12.

The HOMO–LUMO energy difference varies considerably between 3.94 eV (7) and 3.31 eV (12). The small HOMO–LUMO gap of 12 with pyrazine species comes from the low-lying LUMO which is affected by the additional intercalation of the electron negative nitrogen atom in the azaheterocyclic fragment. There is a correlation between the HOMO–LUMO energy difference and the red-shifted absorption bands determined experimentally for 10 and 12 with pyrazine rings relative to their pyridine counterparts 7–9 and 11.

TD-DFT calculations

TD-DFT calculations were carried out to elucidate the nature of the transitions in the absorbance profile. The TD-DFT calculated UV-vis spectra of all complexes are in good agreement with the experimental ones. The calculated absorption spectra for 7, 8, 11, and 12 are reasonably consistent with the experimental spectral profiles (Fig. S9 and S11†). The absorption band in the 320–450 nm range results from the combination of the most intense transfers appearing at ca. 340–360 nm (f ca. 0.123–0.156) and ca. 393–410 nm (f ca. 0.031–0.038) associated correspondingly with the HOMO → LUMO (91–92%) and HOMO−1 → LUMO (91–94%) transitions (Tables S6 and S7 in the ESI†).

Electrochemical studies

The electrochemical properties of 7–12 were examined using cyclic voltammetry (CV) in dichloromethane solution (Fig. 4). Palladium complexes 7, 9, and 10 exhibit two irreversible oxidation waves in the ranges of 0.48–0.58 and 0.83–0.85 V, which correspond to the multistep oxidation processes (subsequent oxidation processes are probably assigned to the next step of the oxidation of the mono-oxidized species²³). Platinum complexes 8, 11, and 12 are oxidized at more positive potentials than the corresponding palladium counterparts and the first irreversible oxidation peak was observed in the range of 0.72–0.77 V.

Fig. 4 Cyclic voltammograms of 7 and 8 in CH₂Cl₂ at RT (potentials given in mV vs. Fc⁺/Fc). Oxidation potentials (mV) of 9: 483, 833; 10: 718, 968; 11: 566, 850; and 12: 740, 1028.

It is worth noting that complexes with mesityl substituent 9–12 are oxidized easier than their corresponding analogues 7 and 8 with xylyl substituent. To interpret the electrochemical behaviour of the complexes, quantum-chemical calculations were performed for 7, 8, 11, and 12 and their mono-oxidized forms at the level of the DFT theory. The plot of the spin density calculated for the oxidized forms of 7, 8, 11, and 12 (Fig. 5 and S12†) demonstrates that the first oxidation process could be associated with the oxidation of the aryl fragment connecting to the exocyclic nitrogen atom.

Fig. 5 A plot of the spin density calculated for [8]⁺.

X-Ray diffraction studies of chloride species

Single crystals of 11 were obtained by the slow evaporation of CH₂Cl₂ solution. Crystals of 11 are yellow and possess a low symmetry P1 space group (Fig. 6). In 11, both metal centres acquire the distorted square-planar coordination environment. The CNR ligands are at the cis-position to the C atoms of {C̲(NR)N(R)C̲N̲(N̲C₄H₃X)} (X = CH, N); the latter forms a dinuclear bicyclic framework.

Fig. 6 View of 11 with the atomic numbering schemes. Solvent molecule and hydrogen labels are omitted for simplicity. The crystal data, and selected bond lengths and angles are given in the ESI, section S1.†

The bond lengths of the two coordinated CN groups are in the interval of 1.136(8)–1.145(8) Å, typical of the CN triple bonds in isocyanide Pd^II and Pt^II complexes.^3e,24 The Pt–C_carbene distances 1.983(6)–1.989(5) Å are slightly smaller than those of typical Pt–C bonds (1.995–2.023 Å)^5d,25 reported for the relevant Pt^II–ADC complexes and are close to the Pt–C distances in Pt–NHC complexes (1.970–1.991 Å).²⁶ On the other hand, the Pt–C bond lengths are sufficiently larger than those in Pt–NHCs, where the NHC is included in the extended rigid π-system (1.906–1.970 Å)²⁷ and in Pt–CAAC complexes (1.942–1.967 Å).²⁸ The bond lengths in the first carbene moiety C¹–N¹ (1.349(8) Å) and C¹–N³ (1.341(7) Å) are similar and are between a single CN bond (e.g., 1.469(10) Å) in amines²⁹ and a double CN bond (e.g., 1.279(8) Å) in imines,²⁹ and the distances indicate a substantial delocalization of the electron density. In the second carbene, the endocyclic C⁷–N³ bond of the metallacycle is typically single (1.449(8) Å), while the exocyclic C⁷–N⁴ bond is typically double (1.264(8) Å) indicating a low degree of delocalisation in this NCN carbene fragment. The structure of this carbene moiety is, therefore, closer to the amino(imino)carbene or diaminocarbene-like structures reported previously.^11,30 All other bond lengths in 11 are normal, and their values agree with those for the previously reported 7,^10b8⁹ and other related diaminocarbene and isocyanide species.^8c,10a,31

DFT analysis of ligand properties in chloride species 7–12

To shed light on the nature of the bonding in these compounds, the quantum theory of atoms in molecules (QTAIM)³² and Mayer bond order analysis,³³ as well as the extended transition state and the natural orbitals for chemical valence (ETS-NOCV) method,³⁴ were used.

Bader's theory of atoms in molecules provides a unique way to analyse the electron density, ρ_и.³² The most important property to assign a bond is the presence of a bond critical point (BCP) which is a point of electron and energy densities along the bond path. It is quite interesting to compare the M¹–C¹ bonds, which involve the metalla-N-heterocyclic carbene C¹ atoms, with the almost equivalent M²–C⁷ bonds, which engage the only formal acyclic diaminocarbene C⁷ atom. The BCPs for both pairs of M–C_carbene bonds show a moderate value of electron density (ρ_b 0.070–0.091e) and a positive value of the Laplacian (Table S9†). Since a covalent bond in transition metal complexes is characterized by a small value of ρ_b and small and positive values of ∇²ρ(r) due to the diffuse character of the electron distribution,³⁵ the best descriptor to ascertain the type of interaction is the local energy electron density H(r).³⁶ All of these M–C_carbene have negative values of H(r) at the BCPs. The listed parameters indicate a significant degree of covalency. Another similar criterion was proposed by Espinosa³⁷ on the basis of the ratio |V(r)|/G(r) for closed-shell interactions, |V(r)|/G(r) < 1 and for shared shell interactions, |V(r)|/G(r) > 2. The interaction is taken as an intermediate type if the ratio falls between these two limits.³⁸ The BCPs of M–C_carbene bonds have |V(r)|/G(r) values of 1.47–1.70 and it is taken as evidence for a partially covalent nature. At the same time, the values for the ρ_b bond and |V(r)|/G(r) for all M–C_carbene bonds are higher than those for the M–Cl, M–C_CNR and M–N bonds; all this indicates that M–C_carbene is more covalent. The values for the local topological properties at the BCP of these two types of M–C_carbene bonds are close enough (Table S9†), and are comparable to those obtained from the theoretical electronic energy density for the M–C_carbene bonds of the other diaminocarbene complexes.^36,39 The calculated Mayer bond orders (MBOs) of the M–C_carbene bonds are similar for both M–C_carbene bonds of each complex (0.5491 and 0.5512 for 7 and 0.9907 and 1.0678 for 8) and are larger than the MBOs of the M–C_CNR bonds (0.2675 and 0.2753 for 7 and 0.9029 and 0.9058 for 8). The values of the topological parameters at BCPs of the C_carbene–N bonds are all typical of covalent bonds between nonmetal atoms with some degree of delocalization.⁴⁰ The MBOs for C_carbene–N bonds in the N¹–C¹–N³ moiety are higher than 1 and very similar (1.2570–1.3101), which support the previous proposal that the unsaturation of the C atom of diaminocarbene is alleviated by π-donation of electron density from the filled pπ orbitals of the N atoms to the empty pπ orbital of the carbene carbon.⁴¹ The MBOs for C_carbene–N bonds in the second N³–C⁷–N⁴ moiety are different (0.9715–0.9989 for C⁷–N³ and 1.6346–1.6752 for C⁷–N⁴) as evidenced by XRD data.

The coordination bonding in binuclear diaminocarbene complexes was rationalised by the extended transition state (ETS)⁴² together with the natural orbitals for chemical valence (NOCV)^34,43 method. The ETS-NOCV scheme in this manner gives quantitative (ΔE_orb) data on the strength of orbital interactions in chemical bonds.⁴⁴ The ETS-NOCV calculations for 7 and 8 were carried out using PBE0/ZORA-def2-TZVP geometries with the ADF program package.⁴⁵ The fragmentation pattern is shown in Fig. S13,† where the C¹–N¹, C⁷–N³ and M–L bonds were fragmented homolytically in the frozen geometry of the complex molecule for this purpose. The major contribution to the total orbital interaction comes from the three orbital terms ΔE_orb(1) − ΔE_orb(3) (90%), indicating a strong covalent bonding (Fig. 7). The strongest pairwise orbital interaction (ΔE_orb(1), −470 kcal mol⁻¹) comes from the ligand to metal σ-dative interactions, which is provided by 41% of total ΔE_Orb. Then, the second term (ΔE_orb(2), −362 kcal mol⁻¹) is associated with the coupling of p orbitals in diaminocarbene moieties. The last ΔE_orb(3) (−225 kcal mol⁻¹) is due to the π-back-donation from metal centers to carbene ligands and comprises only 20% of the total ΔE_Orb energy.

Fig. 7 Shape of the deformation densities Δρ_(1)–(3) that correspond to ΔE_orb(1)–(3). The direction of the charge flow of the deformation densities is blue (loss of electron density) → green (gain of electron density).

X-Ray diffraction studies of thiocyanate derivatives

Single crystals of 16 and 18 were obtained directly from the reaction mixture upon its slow evaporation (Fig. 8, 9, and S1†).

Fig. 8 View of 18 with the atomic numbering schemes. Solvent molecule and hydrogen labels are omitted for simplicity. Crystal structure of 16 is given in section S1 in the ESI† alongside selected bond lengths and angles.

Fig. 9 Views of dimers of 18. Dotted lines indicate the Pt¹⋯C⁵ and S¹⋯C² contacts. Thermal ellipsoids are shown with 50% probability.

Compounds 16 and 18 form isomorphs via the Pd/Pt atom exchange in the molecular crystals.^5d,24d,e Both thiocyanates in 16 and 18 are bound to the metals by the N atom in agreement with the IR spectroscopy results. The M¹–N⁸ and M²–N⁹ distances were within the range of typical values for Pd–N^16b,46 and Pt–N^16a,46 coordination bonds reported for relevant N-ligated thiocyanates. The M¹–N⁸–C⁴⁵ and M²–N⁹–C⁴⁶ angles due to the thiocyanate binding are considerably different: one thiocyanate in 16 and 18 is coordinated to the metal in a somewhat linear fashion (M¹–N⁸–C⁴⁵ 175.7(3)–178.5(3)°), while the other adopts a bent configuration (M²–N⁹–C⁴⁶ angle in the range of 147.1(3)–159.1(4)°). Subsequent theoretical studies (section S4.1†) and analysis of the CCDC (section S5†) showed that the differences are associated with the effect of crystal packing.

The polycyclic organometallic skeleton of thiocyanate complexes 16 and 18 is almost identical to that of chloride species (Fig. S2 and S3†). Thiocyanate ligands are explored as potential crystal-driving supramolecular generators in biochemistry⁴⁷ and materials⁴⁸ studies. The versatile nature of the interacting centers in NCS is mainly associated with sulfur and the nitrogen atoms allow it to be involved in hydrogen bonding,⁴⁹ halogen bonding,⁵⁰ and π-interactions.⁵¹ The Hirshfeld surface analysis of the XRD structures of 16 and 18 indicates the domination of the contacts that involve hydrogen atoms, specifically, H–H, C–H, S–H, and N–H (section S6 in the ESI†). The H–H contacts provide the largest contributions to the molecular Hirshfeld surfaces, because the fraction of these atoms is maximal and the Hirshfeld surface analysis does not disclose the attractive or repulsive nature of these contacts. Apart from the contacts involving H atoms, 16 and 18 exhibit two pairs M¹⋯C⁵ and S¹⋯C² short intermolecular contacts forming supramolecular dimers (Fig. 9). The distances between the M¹⋯C⁵ and S¹⋯C² interacting atoms are slightly larger than the sum of Bondi's vdW radii (102–109%),⁵² but are comparable with the sum of Alvarez's (86–89% for M¹⋯C⁵ and 98–99% for S¹⋯C²)⁵³ vdW radii, confirming the presence of noncovalent interactions. To further confirm the existence of noncovalent interactions between the M^II-center and C⁵ atom and between the S-center of thiocyanato and C² atom, a further computational study was carried out.

QTAIM analysis³² of model supramolecular associates (16)₂ and (18)₂ shows appropriate bond critical points (3, −1) (BCPs) for the discussed contacts (Fig. 10, 11, S15, S16 and Table S13†). In the case of (18)₂, the formally bifurcated μ₂-S⋯[Pt^II,C] contact (Fig. 11) and in the case of (16)₂ a S¹⋯Pd² contact (Fig. S16†) are formed. The low magnitudes of electron density (0.005–0.008 a.u.), the positive values of the Laplacian of electron density (0.015–0.021 a.u.), and the positive energy density close to zero (0.001 a.u.) in the BCPs, as well as the balance between the Lagrangian kinetic energy G(r) and potential energy density V(r) at the BCPs (ratio |G(r)|/V(r) > 1), reveal that these contacts are purely noncovalent.³⁷

Fig. 10 Contour line diagram of the Laplacian of electron density distribution ∇²ρ(r), bond paths, and selected zero-flux surfaces (left panel), visualization of the electron localization function (ELF, center panel) and reduced density gradient (RDG, right panel) analyses of Pt¹⋯C⁵ intermolecular contacts in (18)₂. Bond critical points (3, −1) are shown in blue, nuclear critical points (3, −3) in pale brown, and bond paths as pale brown lines, length units are Å, and the color scale for the ELF and RDG maps is presented in a.u.

Fig. 11 Contour line diagram of the Laplacian of electron density distribution ∇²ρ(r), bond paths, and selected zero-flux surfaces (left panel), visualization of the electron localization function (ELF, center panel) and reduced density gradient (RDG, right panel) analyses of μ₂-S⋯[Pt^II,C] intermolecular contacts in (18)₂. Bond critical points (3, −1) are shown in blue, nuclear critical points (3, −3) in pale brown, ring critical points (3, +1) in orange, and bond paths as pale brown lines, length units are Å, and the color scale for the ELF and RDG maps is presented in a.u.

The Electron Localization Function⁵⁴ (ELF) projections for the M¹⋯C⁵ contacts were plotted along with CPs and bond paths (Fig. 10, 11 and S15, S16†). ELF projections show increased ELF areas around Pd^II and Pt^II atoms near the bond paths connecting the metal centers and azaheterocyclic species that can be interpreted as filled d_z² orbitals. The M⋯C bond paths that connect the metal centers and C⁵ atom from the azaheterocyclic species go through these d_z² and π*-orbitals. In the ELF projections plotted for the S¹⋯C² contacts, a sulfur lone pair is visible near the bond paths; also, the electron density depletion regions associated with the π-holes are easily identified in the ELF basins. Reduced density gradient (RDG) analyses confirm the fact that all discussed contacts are attractive.

The comparison of the crystal structures of the thiocyanate derivatives with those of the chloride species (Fig. S4 and S5†) revealed significant changes that occurred on anion exchange. While the crystal structure of the chloride species is predominantly held by C–H⋯X (X = Cl, N) hydrogen bonds with no involvement of the M^II-center, the change from Cl to NCS favours the formation of M⋯C interactions.

Application of Pt^II–MNHC complexes as photocatalysts for hydrosilylation of alkynes

Representative platinum(II) diaminocarbene derivatives 8, 11 and 12 were evaluated as potential photocatalysts in the hydrosilylation of alkynes. As a model system, we have chosen the reaction of 1,2-diphenylacetylene with triethylsilane giving 1,2-(diphenylvinyl)triethylsilane under the photocatalytic conditions similar to those reported previously.⁹

Without the addition of a catalyst, no reaction was observed in the 20–60 °C range with or without visible light irradiation (Table 1, entries 1–3) indicating that irradiation itself is not sufficient to initiate this process. Furthermore, at 40–60 °C only an insignificant catalytic effect of any studied complexes was observed after 6 h without light (catalyst loading: 0.1 mol%, entries 4–5, 7–8 and 9–12). Under blue LED irradiation all catalysts 8, 11, and 12 showed moderate to good activity (62–89% product yields after 6 h, entries 6, 9 and 13); complex 11 was the most active (entry 9; 89% product yield). Increasing the reaction time to 12 h led to nearly quantitative conversion of the starting 1,2-diphenylacetylene to the respective silylated product (entry 14). Application of 0.5 mol% of catalyst shortens the reaction time to 4 h (entry 15), suggesting that the system with higher catalyst loading can be used if shorter reaction time is preferred over a larger TON. Although the exact reasons for the increased catalytic activity of mesityl carbene species require further studies, some first attempts suggest the optimum combination of donor and steric properties of these derivatives.⁵⁵

Table 1 Photocatalytic activities of 8, 11 and 12 in the model hydrosilylation reaction^a

Entry	Catalyst	Product yield^b
		At 40 °C, no LED	At 60 °C, no LED	At 40 °C, with LED light
a Reaction conditions: 1,2-diphenylacetylene (5.0 × 10^−4 mol), Et3SiH (7.5 × 10^−4 mol), selected catalyst (5.0 × 10^−7 mol, 0.1 mol%), toluene (2 mL), and time: 6 h; for entries with light irradiation a 3 × 2 W LED source with a 445 nm maximum was used. b Yield of 1,2-(diphenylvinyl)triethylsilane was determined by ^1H NMR spectroscopy using 1,2-dimethoxyethane as the internal standard; the product was obtained as a mixture of E/Z isomers (E 55–75%). c Time: 12 h. d Catalyst loading: 0.5 mol%, 4 h.
1–3	—	<5	<5	<5
4–6	8	<5	5	62
7–9	11	<5	6	89
10–13	12	<5	<5	81
14	11			98^c
15	11			98^d

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Conclusions

In the course of this study, a series of new binuclear palladium(II) and platinum(II) metalla-N-heterocyclic carbene (MNHC) type complexes were prepared via metal-mediated coupling between cis-[MCl₂(CNR)₂] (R = Xyl, Mes) and 2-aminopyridine or 2-aminopyrazine nucleophiles. Structural characterization of 7–12 thus achieved by HRESI⁺-MS, IR, and 1D (¹H, ¹³C{¹H}, ¹⁹⁵Pt{¹H}) and 2D (¹H,¹³C-HSQC, ¹H,¹³C-HMBC) NMR spectroscopy as well as by single-crystal XRD revealed the difference in diaminocarbene ligands where both C_carbene–N bonds in the MNHC moiety are in agreement with the data for classical diaminocarbenes, while the second NCN fragment is an ADC-like carbene due to the rather double and single C–N bond character. QTAIM reveals that the M–C_carbene bonds are characterized by low to moderate values of electron density ρ_b, positive values of the Laplacian ∇²ρ, and negative values of local electron energy densities, H(r), signifying that these M–C_carbene bonds have a significant degree of covalency. In the UV-vis absorption spectrum of the MNHC species, the wide intense absorption in the 320–414 nm range is assigned to ¹L′LCT/¹ILCT transitions which are important for visible-light photocatalysis. The absorption characteristics are governed by the nature of the azaheterocyclic fragment and complexes with pyrazine and pyridine rings (10 and 12) show 20–40 nm red-shifted bands relative to their pyrazine counterparts (7–9 and 11). In contrast, the parent CNR fragments have more effect on the electrochemical properties of the MNHC compounds, and species with mesityl substituents (9–12) are oxidized at a lower potential than their corresponding xylyl analogues (7 and 8).

Moreover, the first palladium(II) and platinum(II) diaminocarbene thiocyanate derivatives 13–18 were synthesized from the corresponding starting chlorides 7–12 by metathetic reaction with NH₄NCS. In these complexes, both thiocyanate ligands are N-bound as confirmed by IR spectroscopy and single-crystal XRD. Replacing both chloride ligands with thiocyanate ligands leads to minor changes in absorption spectra, however, significantly reducing the solubility of compounds in organic solvents. Isomorphous crystal structures of the thiocyanate derivatives 16 and 18 exhibit two symmetrical pairs M¹⋯C⁵ and S¹⋯C² short intermolecular contacts between the electron-rich M^II-center and thiocyanate ligand on the one side, and the electron-poor π system of the azaheterocyclic ring on the other side; the observed noncovalent interactions provide supramolecular dimers.

The photocatalytic properties of the prepared platinum–diaminocarbenes 8, 11 and 12 as catalysts for hydrosilylation of alkynes with hydrosilanes were evaluated. The highest catalytic efficiency was achieved with complex 11 that is derived from the platinum-mediated coupling of mesityl isocyanide with 2-aminopyridine. The catalytic system uses blue LED irradiation for 6–12 h in toluene with a typical catalyst loading of 0.1 mol% and allows the transformation of 1,2-diphenylacetylene and triethylsilane into the respective 1,2-(diphenylvinyl)triethylsilane with up to 98% yields.

Together, the experimental and computational data provide an insight into the flexible character of the MNHC species. We explored the photocatalytic properties of the [M]–MNHCs derived from reactions of metal-activated RNCs and various 2-aminoazaheterocycles in order to develop useful catalytic systems and to establish catalyst structure–activity relationships for other visible light activated systems,⁵⁶ including Pd-catalyzed cross-coupling reactions and Pt-catalyzed hydroarylation reactions.

Experimental

Materials and instrumentation

Solvents, organic and inorganic reagents were obtained from commercial sources and used as received. Isocyanide complexes cis-[MCl₂(CNR)₂] (M = Pd, R = Xyl 1,¹¹ Mes 2;⁸¹ M = Pt, R = Xyl 3,⁵⁷ Mes 4⁵⁸) were prepared as reported earlier. C, H, and N elemental analyses were carried out on a Euro EA 3028 HT CHNSO analyzer. Mass-spectra were acquired on a Bruker micrOTOF spectrometer equipped with an ESI source; a CH₂Cl₂/MeOH mixture was used as the solvent. The instrument was operated in positive ion mode using an 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 drying gas flow was 4.0 L min⁻¹. The most intense peak in the isotopic pattern is reported. Infrared spectra were recorded on a Bruker Tensor 27 FTIR instrument (4000–2400 cm⁻¹, resolution 2 cm⁻¹) using KBr pellets. The UV/vis absorption spectra in CH₂Cl₂ solution were recorded on a Shimadzu UV-2500 spectrophotometer in a quartz cuvette with l = 1.0 mm and the complex's concentration was 0.03 mM. A Shimadzu UV-1800 spectrophotometer was used to measure the UV/vis absorbance of solid-state complexes. The ¹³C CP/MAS NMR spectra were acquired using a double-resonance 4 mm MAS Bruker probe at a resonance frequency of 101 MHz under 14 kHz MAS. The CP contact time in all experiments was 3.5 μs with a delay between acquisitions of 1 s and the number of scans collected was 20 000. Suitable single crystals were selected and mounted on a MiTeGen tip via a crystallographic oil. Data were collected using a Rigaku XtaLAB Synergy, single source at home/near, HyPix diffractometer (monochromated CuKα radiation, λ = 1.54184 Å) at 100(0) K. In each case, the structure was solved with a ShelXT⁵⁹ structure solution program using Intrinsic Phasing and refined with a ShelXL⁵⁹ refinement package incorporated in the OLEX² program package⁶⁰ using Least Squares minimization. Empirical absorption correction was applied in the CrysAlisPro⁶¹ program complex using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The CVs were measured using an Autolab PGSTAT302N voltammetric analyzer at a scan rate of 100 mV s⁻¹. Glassy carbon and a platinum wire were used as the working and counter electrodes, respectively. The sample solutions (ca. 5 mM) were in 0.1 M [nBu₄N](BF₄)–CH₂Cl₂. Ag/AgNO₃ (0.01 M AgNO₃ in MeCN) was used as the reference electrode and the potential values were corrected against an Fc⁺/Fc couple (E_1/2 = 0.52 V vs. Ag/AgNO₃ in CH₂Cl₂).

Synthetic work

General procedure for the synthesis of 7–12. A mixture of solid [MCl₂(CNR)₂] (1–4, 0.10 mmol) and aminoazaheterocycles 5 and 6 (0.20 mmol) was dissolved in CH₂Cl₂ (5 mL) and left to stand at RT for 24 h. During the reaction time, the mixture turned from pale yellow to lemon yellow. After 24 h, the reaction mixture was filtered to remove precipitated 5·HCl or 6·HCl and then slowly evaporated in air at RT to give yellow crystals of 7–12. The solids of 7–12 were separated by decantation and washed with three 2 mL portions of Et₂O and then dried in air at RT.

9. Yellowish solid (37 mg, yield 78%). Anal. Calcd for C₄₅H₄₈N₆Cl₂Pd₂: C, 56.5; H, 5.1; N, 8.8; found: C, 56.7; H, 5.4; N, 9.0. [M − Cl]⁺ calcd for C₄₅H₄₈N₆ClPd₂⁺ 921.1703, found 921.1706. IR (KBr, selected bands, cm⁻¹): ν(C–H) 2920 (s), 2854 (s); ν(C≡N) 2189 (s); ν(C_carbene–N) 1629 (s); δ(C–H from Mes) 855 (s). ¹H NMR (CDCl₃, 400.13 MHz, δ): 9.10 (d, ³J_H,H = 8.5 Hz, 1H, H³), 9.06–8.99 (m, 1H, H⁵), 7.84 (ddd, ³J_H,H = 8.8 Hz, ³J_H,H = 7.4, ⁴J_H,H = 1.7 Hz, 1H, H⁴), 7.03–6.94 (m, 1H, H⁶), 6.91 (s, 2H, m-H from Mes), 6.82 (s, 2H, m-H from Mes), 6.68 (s, 2H, m-H from Mes), 6.46 (s, 2H, m-H from Mes), 2.41 (s, 6H, 2Me), 2.31 (s, 3H, Me), 2.26 (s, 15H, 3Me), 2.23 (s, 6H, 2Me), 2.02 (s, 6H, 2Me), 1.47 (s, 3H, Me), 1.46 (s, 3H, Me). ¹³C{¹H} NMR (101 MHz, CDCl₃, δ): 195.0 (C_carbene–N), 167.0 (C_carbene–N), 162.8 (C=N), 147.7 (p-C from Mes), 146.7, 141.5 (C³, C⁴), 140.7 (ipso-C from Mes), 140.0, 139.8, 138.9 (p-C from Mes), 136.2, 133.9, 133.9 (o-C from Mes), 133.7 (p-C from Mes), 128.5, 128.3, 128.1, 127.2 (m-C from Mes), 124.9, 124.2 (ipso-C from Mes), 118.3, 117.2 (C⁵, C⁶), 21.2, 21.1, 20.1, 20.0 (Me's), 19.5, 19.3 (2Me's), 18.4 (4Me).

10. Yellow solid (42 mg, yield 88%). Anal. Calcd for C₄₄H₄₇N₇Cl₂Pd₂: C, 55.2; H, 5.0; N, 10.2; found: C, 54.8; H, 5.0; N, 10.6. [M + H]⁺ calcd for C₄₄H₄₈N₇Cl₂Pd₂⁺ 958.1417, found 958.1412. IR (KBr, selected bands, cm⁻¹): ν(C–H) 2916 (s), 2853 (s); ν(C≡N) 2199 (s), 2189 (s); ν(C_carbene–N) 1632 (s); δ(C–H from Mes) 854 (s). ¹H NMR (CDCl₃, 400.13 MHz, δ): 10.37 (d, ⁴J_H,H = 0.7 Hz, 1H, H³), 8.89 (dd, ³J_H,H = 3.1 Hz, ⁴J_H,H = 1.0 Hz, 1H, H⁵), 8.32 (d, ³J_H,H = 3.3 Hz, 1H, H⁶), 6.93 (s, 2H, m-H from Mes), 6.83 (s, 2H, m-H from Mes), 6.69 (s, 2H, m-H from Mes), 6.48 (s, 2H, m-H from Mes), 2.40 (s, 6H, 2Me), 2.32 (s, 3H, Me), 2.26 (s, 3H, Me), 2.02 (c, 6H, 2Me), 2.26 (c, 3H, Me), 2.02 (s, 6H, 2Me), 1.47 (s, 6H, 2Me). ¹³C{¹H} NMR (101 MHz, CDCl₃, δ): 196.8 (C_carbene–N), 163.2 (ipso-C from Mes), 161.8 (C_carbene–N), 147.6 (p-C from Mes), 142.2 (C³), 140.6 (p-C from Mes), 140.2 (p-C from Mes), 139.1 (ipso-C from Mes), 137.9 (C⁵), 137.6 (C⁶), 135.9 (o-C from Mes), 134.0 (o-C from Mes), 133.9 (ipso-C from Mes), 133.9 (o-C from Mes), 128.6, 128.3, 128.2 (m-C from Mes), 127.0 (o-C from Mes), 124.7 and 123.9 (C≡N), 21.2, 21.1, 20.1, 20.0, 19.5 (Me's), 19.2 (2Me's), 18.4 (4Me's).

11. Yellow solid (47 mg, yield 83%). Anal. Calcd for C₄₅H₄₈N₆Cl₂Pt₂: C, 47.7; H, 4.3; N, 7.4; found: C, 47.2; H, 4.3; N, 7.6. [M + H]⁺ calcd for C₄₅H₄₉Cl₂N₆Pt₂⁺ 1134.2688, found 1134.2678. IR (KBr, selected bands, cm⁻¹): ν(C–H) 2915 (s); ν(C≡N) 2194 (s); ν(C_carbene–N) 1667 (s), 1619 (s); δ(C–H from Mes) 771 (s). ¹H NMR (CDCl₃, 400.13 MHz, δ): 9.36–9.07 (m, 2H, H⁵ + H³), 8.02–7.73 (m, 1H, H⁴), 7.05 (t, ³J_H,H = 6.6 Hz, 1H, H⁶), 6.92 (s, 2H, m-H from Mes), 6.82 (s, 2H, m-H from Mes), 6.68 (s, 2H, m-H from Mes), 6.42 (s, 2H, m-H from Mes), 2.40 (s, 6H, 2Me), 2.32 (s, 3H, Me), 2.27 (s, 15H, 3Me), 2.25 (s, 6H, 2Me), 2.04 (s, 6H, 2Me), 1.45 (s, 3H, Me), 1.40 (s, 3H, Me). ¹³C{¹H} NMR (101 MHz, CDCl₃, δ): 183.6 (C_carbene–N), 168.3 (C_carbene–N), 153.0 (C=N), 148.3 (p-C from Mes), 145.1, 141.7 (C³, C⁴), 140.1, (ipso-C from Mes), 139.9, 139.6, 138.6 (p-C from Mes), 136.2, 134.0, 133.9 (o-C from Mes), 133.1 (p-C from Mes), 128.5, 128.2, 128.1, 126.7 (m-C from Mes), 125.2, 124.6 (ipso-C from Mes), 117.7, 116.9 (C⁵, C⁶), 21.2, 21.1, 20.1, 19.9 (Me's), 19.5, 19.0 (2Me's), 18.4 (4Me's). ¹⁹⁵Pt{¹H} NMR (CDCl₃, 86.02 MHz, δ): −3756, −3796.

12. Yellow solid (53 mg, yield 94%). Anal. Calcd for C₄₄H₄₇N₇Cl₂Pt₂: C, 46.6; H, 4.2; N, 8.6; found: C, 45.8; H, 4.4; N, 8.4. [M + H]⁺ calcd for C₄₄H₄₈N₇Cl₂Pt₂⁺ 1135.2640, found 1135.2644. IR (KBr, selected bands, cm⁻¹): ν(C–H) 2915 (s); ν(C≡N) 2208 (s), 2190 (s); ν(C_carbene–N) 1634 (s); δ(C–H from aryls) 835 (s). ¹H NMR (CDCl₃, 400.13 MHz, δ): 10.50 (s, 1H, H³), 9.10 (d, ³J_H,H = 3.0 Hz, 1H, H⁶), 8.38 (d, ³J_H,H = 3.4 Hz, 1H, H⁵), 6.94 (s, 2H, m-H from Mes), 6.83 (s, 2H, m-H from Mes), 6.70 (s, 2H, m-H from Mes), 6.44 (s, 2H, m-H from Mes), 2.39 (s, 6H, 2Me), 2.33 (s, 3H, Me), 2.28 (s, 3H, Me), 2.27 (s, 6H, 2Me), 2.25 (s, 6H, 2Me), 2.04 (s, 6H, 2Me), 1.47 (s, 3H, Me), 1.41 (s, 3H, Me). ¹³C{¹H} NMR (101 MHz, CDCl₃, δ): 185.05 (C_carbene–N), 162.89 (ipso-C from Mes), 153.62 (C²), 148.11 (p-C from Mes), 141.98 (C³), 141.80 (p-C from Mes), 140.20 (p-C from Mes), 139.93 (ipso-C from Mes), 139.88 (C³), 138.73 (p-C from Mes), 137.06 (C⁵), 135.97 (o-C from Mes), 135.84 (o-C from Mes), 134.34 (C⁶), 134.00 (o-C from Mes), 133.30 (ipso-C from Mes), 132.64 (ipso-C from Mes), 129.08 (o-C from Mes), 128.58, 128.57, 128.26, 128.09 (m-C from Mes), 126.55 (o-C from Mes), 125.05 and 124.30 (C≡N), 21.19 (Me's), 21.07 (Me's), 20.06 (Me's), 19.90 (Me's), 19.42 (2Me's), 18.97 (2Me's), 18.32 (4Me's). ¹⁹⁵Pt{¹H} NMR (CDCl₃, 86.02 MHz, δ): −3718, −3803.

General procedure for the synthesis of 13–18. A mixture of 7–12 (0.05 mmol) with a three-fold excess of NH₄CNS (0.15 mmol) was suspended in an acetone/CH₂Cl₂ mixture (v/v 3 : 1, 2 mL) at RT. The reaction mixture was stirred overnight to give a yellow solution over the yellow or orange precipitate. These precipitates of 13–18 were separated by centrifugation, washed with two 2 mL portions of H₂O and dried in air at RT.

13. Yellow solid (35 mg, yield 74%). Anal. Calcd for C₄₃H₄₀N₈S₂Pd₂: C, 54.6; H, 4.3; N, 11.9; S, 6.8, found C, 54.3; H, 4.8; N, 11.5; S, 6.6. [M + Na]⁺ calcd for C₄₃H₄₀N₈S₂Pd₂Na⁺ 969.0784, found 969.0792. IR (KBr, selected bands, cm⁻¹): ν(C≡N) 2204 (s), ν(NC_thiocyanate) 2064, 2087 (s); ν(C_carbene–N) 1605, 1634 (s); δ(C–H from aryls) 778 (s). ¹³C CP MAS NMR: 136.75, 136.42, 134.39, 133.46, 132.51, 131.36, 130.76, 129.55, 128.80, 127.96, 127.16, 125.82 (C and CH from aryls), 118.49 and 117.77 (C≡N), 21.42, 20.78, 19.03, 18.30 (Me's).

14. Yellow solid (42 mg, yield 75%). Anal. Calcd for C₄₃H₄₀N₈S₂Pt₂: C, 46.0; H, 3.6; N, 10.0; S, 5.7, found C, 45.7; H, 3.3; N, 9.8; S, 5.6. [M − NCS]⁺ calcd for C₄₂H₄₀N₇SPt₂⁺ 1063.2363, found 1064.2355. IR (KBr, selected bands, cm⁻¹): ν(C≡N) 2202 (s), ν(NC_thiocyanate) 2072, 2094 (s); ν(C_carbene–N) 1611, 1627 (s); δ(C–H from aryls) 777 (s). ¹³C CP MAS NMR: 166.7, 151.0, 145.1, 144.5, 136.7, 133.7, 130.7, 128.7, 127.9, 126.8, 126.2 (C and CH from aryls), 118.4 and 117.0 (C≡N), 21.5, 20.9, 20.5, 19.0, 18.6, 18.4, 17.8 (Me's).

15. Yellow solid (34 mg, yield 68%). Anal. Calcd for C₄₇H₄₈N₈S₂Pd₂: C, 56.3; H, 4.8; N, 11.2; S, 6.4, found C, 55.8; H, 4.3; N, 10.8; S, 6.6. [M − NCS]⁺ calcd for C₄₆H₄₈N₇SPd₂⁺ 944.1766, found 944.1754. IR (KBr, selected bands, cm⁻¹): ν(C≡N) 2187 (s), ν(NC_thiocyanate) 2071, 2100 (s); ν(C_carbene–N) 1619 (s); δ(C–H from aryls) 763 (s). ¹³C CP MAS NMR: 166.2, 140.3, 136.1, 134.8, 129.3, 127.0 (C and CH from aryls), 118.5 and 115.6 (C≡N), 20.0, 19.0 (Me's).

16. Yellow solid (36 mg, yield 71%). Anal. Calcd for C₄₆H₄₇N₉S₂Pd₂: C, 55.1; H, 4.7; N, 12.6; S, 6.4, found C, 54.8; H, 4.6; N, 12.8; S, 6.4. [M – NCS]⁺ calcd for C₄₆H₄₈N₇SPd₂⁺ 945.1718, found 945.1716. IR (KBr, selected bands, cm⁻¹): ν(C≡N) 2193 (s); ν(NC_thiocyanate) 2057; ν(C_carbene–N) 1625 (s); δ(C–H from aryls) 855 (s). ¹³C CP MAS NMR: 140.1, 137.9, 135.5, 135.1, 134.2, 133.7, 132.7, 131.5, 128.8 (C and CH from aryls), 19.8, 19.3, 19.1 (Me's).

17. Yellow solid (54 mg, yield 77%). Anal. Calcd for C₄₇H₄₈N₈S₂Pt₂: C, 48.0; H, 4.0; N, 9.5; S, 5.4, found C, 47.6; H, 3.7; N, 9.5; S, 5.6. [M + H]⁺ calcd for C₄₇H₄₉N₈S₂Pt₂⁺ 1180.2814, found 1180.2820. IR (KBr, selected bands, cm⁻¹): ν(C≡N) 2205 (s), ν(NC_thiocyanate) 2030, 2062 (s); ν(C_carbene–N) 1608 (s); δ(C–H from aryls) 854 (s). ¹³C CP MAS NMR: 196.81 and 190.60 (C_carbene–N), 160.66, 154.03, 150.48, 141.46, 138.79, 135.41, 133.29, 130.58, 129.43, 127.34, 126.07, 124.60 (C and CH from aryls), 116.12 and 114.59 (C≡N), 20.81, 20.04, 18.11, 16.06 (Me's).

18. Yellow solid (47 mg, yield 80%). Anal. Calcd for C₄₆H₄₈N₉S₂Pt₂: C, 46.8; H, 4.0; N, 10.7; S, 5.4, found C, 46.3; H, 3.6; N, 10.6; S, 5.5. [M + H]⁺ calcd for C₄₆H₄₈N₉S₂Pt₂⁺ 1180.2750, found 1180.2754. IR (KBr, selected bands, cm⁻¹): ν(C≡N) 2202 (s), ν(NC_thiocyanate) 2055, 2097 (s); ν(C_carbene–N) 1604, 1629 (s); δ(C–H from aryls) 845, 860 (s). ¹³C CP MAS NMR: 185.6 (C_carbene–N), 161.9, 153.5, 147.5, 145.3, 142.2, 140.8, 140.2, 139.7, 138.0, 136.2, 135.7, 135.1, 134.9, 133.8, 131.2, 130.1, 128.9, 128.4, 128.0, 125.5, 122.7 (C and CH from aryls), 119.0 and 118.4 (C≡N), 24.1, 21.9, 21.4, 19.7, 19.1, 18.4, 17.1 (Me's).

Computational details

DFT/TD-DFT studies on complexes 7, 8, 11 and 12. The full geometry optimization of all complexes was carried out at the DFT level of theory using the PBE0⁶² functional with the atom-pairwise dispersion correction with the Becke–Johnson damping scheme (D3BJ)⁶³ with the help of the ORCA package (version 5.0.2).⁶⁴ Zero-order regular approximation (ZORA)⁶⁵ was employed to account for relativistic effects. The ZORA-def2-TZVP(-f)⁶⁵ basis sets were used for the H, C, N, F, O and S atoms while the SARC-ZORA-TZVP basis sets were used for the Pd and Pt atoms.⁶⁶ The Hessian matrix was calculated analytically for the optimized structures to prove the location of correct minima (no imaginary frequencies). A combination of the “resolution of identity” and the “chain of spheres exchange” algorithms (RIJCOSX)⁶⁷ in conjunction with the auxiliary basis sets SARC/J was used.⁶⁸ The SCF calculations were tightly converged (TightSCF). Numerical integrations during all DFT calculations were performed on a dense grid (DEFGRID3). TD-DFT calculations were performed for optimized geometries in the gas phase and in solution (CH₂Cl₂) using the conductor polarizable continuum model (CPCM)⁶⁹ solvation model. Calculation of 60 vertical transition energies to the lowest singlet excited states with significant oscillator strengths and their characteristics allowed us to simulate a large portion of the absorption spectra and are listed in Tables S6 and S7.† Calculations of the lowest 7 singlet–singlet excitation energies at the optimized geometries are satisfied to gain a better insight into the nature of the long wavelength light absorption of the complexes. QTAIM, MOs and spin density were calculated using the Multiwfn 3.8 software⁷⁰ and visualized with the VMD program.⁷¹

DFT studies on complexes 16 and 18. The single point calculations based on the experimental X-ray geometries of 16 and 18 and the full geometry optimization procedure for isolated model structures have been carried out at the DFT level of theory using the dispersion-corrected hybrid functional ωB97XD⁷² with also the help of the Gaussian-09⁷³ program package. For palladium and platinum atoms the quasi-relativistic pseudopotentials MWB28 and MWB60 and the appropriate contracted basis sets⁷⁴ were used, and the 6-31G* basis sets were used for other atoms. The topological analysis of the electron density distribution with the help of the atoms in molecules (QTAIM) method developed by Bader³² has been performed by using the Multiwfn program (version 3.7).⁷⁵ The Cartesian atomic coordinates for all model structures are presented in Table S14, ESI.†

Hirshfeld analysis. The Hirshfeld molecular surfaces were generated by the CrystalExplorer17 program.⁷⁶ The normalized contact distances, d_norm,⁷⁷ based on Bondi's van der Waals radii,⁵² were mapped into the Hirshfeld surface (Table S15 and Fig. S18, S19†). In the color scale, negative values of d_norm are visualized with the red color indicating contacts shorter than the sum of van der Waals radii. The white color denotes intermolecular distances that are close to van der Waals contacts with d_norm equal to zero. In turn, contacts longer than the sum of van der Waals radii with positive d_norm values are colored with blue.

General procedure for catalytic hydrosilylation of alkynes with hydrosilanes (specific conditions are provided in Table 1). The solution of the selected catalyst in CH₂Cl₂ (0.1 mL, 5.0 × 10⁻⁶ M) was placed in 5 mm tubes and the solvent was evaporated to dryness under a stream of dinitrogen. 1,2-Diphenylacetylene (5.0 × 10⁻⁴ mol), Et₃SiH (7.5 × 10⁻⁴ mol), and toluene (2 mL) were added into the tube. The tube was closed with a septum, kept at 40–60 °C or under LED irradiation with 445 nm LEDs (3 × 2 W; the LEDs were placed at a 2 cm distance from the tube) for 6–12 h. The contents of the tube were poured over silica-gel and extracted with hexane (5 mL). The extracts were evaporated under reduced pressure and the crude product was subsequently dissolved in 0.6 mL of CDCl₃ with 1,2-dimethoxyethane (1.0 equiv., used as an NMR internal standard) added, and then analyzed by ¹H and ¹³C NMR spectroscopy. The isomeric content was determined on the basis of the analysis and matching of ¹H and ¹³C chemical shifts for products against authentic samples of (E)-⁷⁸ and (Z)-1,2-(diphenylvinyl)triethylsilanes.⁷⁹ Quantifications were performed upon integration of the selected peaks of the product against peaks of 1,2-dimethoxyethane.

Author contributions

M.V.K. – conceptualisation, investigation (synthesis, physicochemical study and catalysis), and writing; K.V.L. – conceptualisation and writing; E.A.K. – investigation (electrochemical study and computational study of 7, 8, 11 and 12) and writing; A.S.N. – investigation (computational study of 16 and 18) and writing; M.A.K.– conceptualisation, investigation (catalysis), writing, and project administration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Russian Science Foundation (project 21-73-10083). The DFT/TD-DFT calculations for studies on 7, 8, 11 and 12 were performed by E.A.K. in the framework of the Russian Science Foundation project 21-73-00056, while the electrochemical studies were performed in the framework of the Russian Foundation for Basic Research project 19-29-08026. Measurements were performed at the Center for Magnetic Resonance, Center for X-ray Diffraction Studies, Center for Chemical Analysis, Thermogravimetric and Calorimetric Research Centre, Centre for Optical and Laser Materials Research, Department of Cryogenic Engineering and Materials Research, and Computing Centre (all belong to St Petersburg University).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2133432, 2133433 and 2133435. For ESI and crystallographic data in CIF or other electronic format see DOI: http://doi.org/10.1039/d2dt00252c

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