Reactivity of Pt(0) bromosilylene complexes towards ethylene †

The base-free carbazolyl bromosilylene RSiBr (R = 1,8-bis(3,5-di-tert -butyl-phenyl)-3,6-di-tert -butyl-carbazolyl) reacts with ( η 2 - C 2 H 4 )Pt(PPh 3 ) 2 and Pt(PCy 3 ) 2 to form platinasilacyclobutane R(Br) Si(C 2 H 4 )Pt(PPh 3 ) 2 (1) and silylene platinum complex R(Br)SiPt (PCy 3 ) 2 (2), respectively. When silylene complex 2 is treated with C 2 H 4 , the six-membered metallasilacycle R(Br)Si(C 2 H 4 ) 2 Pt(PCy 3 ) 2 (3) is obtained. All compounds are characterised by XRD and multi-nuclear NMR spectroscopy.

As the first Pt(0) source (η 2 -C 2 H 4 )Pt(PPh 3 ) 2 was chosen since the silylene should be capable of replacing C 2 H 4 as a ligand at the Pt(0) centre.(η 2 -C 2 H 4 )Pt(PPh 3 ) 2 was reacted with one molar equivalent of carbazolyl bromosilylene at room temperature for 12 hours.The formation of a new product (1)  could be observed judging from the 1 H, 29 Si and 31 P NMR spectra of the reaction mixture (Scheme 2).However, the significant high-field shift of the 29 Si NMR resonance from 129.2 ppm in RSiBr to −35.9 ppm in the reaction mixture indicated the presence of a tetra-coordinated silicon atom rather than three-fold coordination.Orange crystals were obtained from a concentrated toluene solution via vapour diffusion with n-pentane.
We next probed the reactivity of bromosilylene RSiBr towards Pt(PCy 3 ) 2 .Here, no potentially reactive moiety is precoordinated to the platinum fragment and the reaction should lead to the targeted silylene complex R(Br)SiPt(PCy 3 ) 2 .Both compounds were mixed as solids and toluene was added.Upon sonication for 15 minutes the solution changed its colour from yellow to bright orange.In this case, the character-

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This istic down-field shift of the 29 Si NMR resonance from 129.2 to 235.5 ppm suggested the formation of silylene platinum complex 2. Orange crystals of 2 were obtained from immediate concentration of the reaction solution and storage at −40 °C for 24 hours, alongside yellow crystals which were identified as Pt(PCy 3 ) 2 .
The molecular structure of complex 2 was determined by X-ray diffraction (Fig. 3).The Si-Pt distance of 2.1858(5) Å is shorter than in previously reported platinum silylene complexes (I, III-V, 2.21-2.27Å), indicating Si-Pt double bond character in the solid state. 17,19,28,35Judging from the bond angles around silicon, the silylene ligand adopts a Y-shaped geometry.Such an arrangement was also observed for Tilley's [Mes 2 SivPt(PCy 3 ) 2 ] (III), while bromosilylene complex V shows a T-shaped arrangement. 19,35Summation of angles around Si and Pt confirms planarity for both atoms.The plane of the silylene ligand, defined by the Br, N and Si atoms and the coordination plane of platinum, defined by the Pt, P1 and P2 atoms, intersect at an angle of 83°.An ideal dihedral angle of 90°for π-donation from platinum to silicon is thus closely approached.In contrast, reported values for this angle were 69°in [Mes 2 SivPt(PCy 3 ) 2 ] (III) and 72°in [Bbt(Br)SivPt (PCy 3 ) 2 ] (V), most probably due to the greater steric congestion in these complexes. 19,35The Si-N bond in 2 (1.8218(16) Å) is elongated compared to the starting material RSiBr (1.8018( 14) Å). 36 This might be attributed to π-donation from Pt d-orbitals to the silylene p-orbital upon which the p-p-interaction between the carbazole-N and the Si atom decreases.Moreover, in platinum complex 2 the silicon atom is bent out of the plane, spanned by the carbazole unit, by approximately 73°, while in RSiBr the Si atom is situated in the carbazole plane between the two arene units.
The multinuclear NMR spectra of R(Br)SiPt(PCy 3 ) 2 (2) are displayed in Fig. 4. The 29 Si NMR resonance can be found in the characteristic low-field region at 235.5 ppm as a triplet signal with coupling to the phosphorus nuclei ( 2 J SiP = 147 Hz) and a set of 195 Pt satellites ( 1 J SiPt = 3570 Hz).The corres-ponding 31 P NMR singlet resonance is obtained at 59.8 ppm ( 1 J PPt = 3833 Hz), while in the 195 Pt NMR, the triplet signal arises at −4644.9 ppm.NMR spectroscopic parameters of compound 2 compare well to bromosilylene platinum complex V (δ Si = 298.11ppm, 2 J SiP = 137 Hz, 1 J SiPt = 3660 Hz, 1 J PPt = 3300 Hz). 35Relative to the silylene complexes I-IV, the 29 Si NMR resonance of 2 is high-field shifted (cf.δ Si = 309-394 ppm), while the 1 J SiPt value is larger (cf.19]28 Judging from these parameters the silylene RSiBr should exhibit enhanced σ-donor as well as π-acceptor character as compared to carbon-substituted silylene ligands. The UV-vis spectrum of silylene complex 2 in hexane solution exhibits an absorption band at 393 nm as well as several bands below 360 nm, explaining the yellow-orange colour of the compound.This is in accord with the spectra observed by Tokitoh, but here, no weak absorption in the range of 560 nm could be found. 35romosilylene platinum complex 2 decomposes in solution within one day at room temperature forming free Pt(PCy 3 ) 2 and a silicon-containing decomposition product (2-I) which was characterised by NMR spectroscopy (Scheme 3).While this compound could not be isolated from the mixture, independent structural proof could be obtained and allowed identifi-  cation (see ESI †).The insertion of the silicon atom into a C-H bond of flanking arene groups might be attributed to an increased acidity of the silicon atom upon coordination to the metal fragment.A possible mechanism for the decomposition proceeds via the π-coordination of one of the flanking arene moieties to the low-coordinated silicon centre.Subsequently, the C-H bond may be activated at Si and the Pt complex can dissociate readily, as a Si(IV) compound is generated.However, if complex 2 is freshly prepared and exposed to one atmosphere of C 2 H 4 , a colour change from orange to light yellow is observed after stirring at ambient temperature for 12 hours.Colourless crystals were grown from a concentrated n-heptane solution and the X-ray experiment confirmed the presence of the six-membered platinasilacycle R(Br)Si(C 2 H 4 ) 2 Pt (PCy 3 ) 2 (3).The insertion of π-systems into Pt-silylene bonds has already been described in 2015, though it has been demonstrated with bridging silylene ligands. 24The molecular structure of compound 3 is depicted in Fig. 5.During the reaction, two molecules of C 2 H 4 are added to the Si-Pt double bond, resulting in a cyclohexane type core with chair conformation and two C-C single bonds (1.557(11) Å and 1.537(11) Å).An early example of a six-membered Si-Pt-metallacycle was already isolated in 1992 within attempts to trap transient silylene species by reacting silyl platinum precursors with acetylene. 16Later, one platinasilacyclohexane type structure was crystallographically characterised among the work on cyclic Si-Pt compounds by Osakada. 37The Si-C (1.846(9) Å and 1.850(9) Å) and Pt-C (2.093(9) Å and 2.132(8) Å) bond distances in 3 show similar values to those reported for the literature known compound (Si-C(sp 3 ) 1.863(5) Å, Pt-C(sp 3 ) 2.148(5) Å). 37 However, because Osakada's metallacyle also comprises sp 2hybridised carbon atoms, further structural parameters are not compared at this stage.The Si-N bond in complex 3 (1.783(7)Å) is shorter than in the starting material 2 which might be due to electron donation from the carbazole-N atom into the p-type orbital at silicon upon dissociation of the Si-Pt bond.As judged from the bond angles, the expected tetrahedral coordination geometry around the silicon atom is only slightly deformed.At platinum, the P1-Pt-P2 angle is widened to 108.96(9)°, resulting in a distorted square-planar coordination environment.
The 29 Si NMR resonance of R(Br)Si(C 2 H 4 ) 2 Pt(PCy 3 ) 2 ( 3) is found at 15.5 ppm.In the 31 P NMR experiment, the magnetically equivalent 31 P nuclei give rise to one signal at 21.0 ppm with 195 Pt satellites that display P-Pt coupling of 1 J PPt = 1641.8Hz.The corresponding 195 Pt NMR resonance is obtained as a triplet signal at −4655.5 ppm.
After isolating compound 3, we wondered if the four-membered platinasilacyle 1 could also be expanded to a cyclohexane-like structure by insertion of another molecule of ethylene into the Pt-Si bond.Thus, 1 was reacted under one atmosphere of C 2 H 4 for 72 hours at 80 °C.Judging from the NMR spectra, an analogous platinasilacyclohexane (4) formed, as the spectra closely resemble those of compound 3.The 31 P NMR spectrum of 4 exhibits a singlet resonance at 26.8 ppm with a phosphorus platinum coupling constant of 1 J PPt = 1761.7 Hz.
In summary, we have shown the distinct reactivity of stable bromosilylene RSiBr towards two Pt(0) compounds.With (η 2 -C 2 H 4 )Pt(PPh 3 ) 2 , the platinasilacyclobutane-like product 1 is obtained.In contrast, the reaction of RSiBr with adduct-free Pt(PCy 3 ) 2 yields platinum silylene complex 2 which features a short Si-Pt bond and a down-field shifted 29 Si NMR resonance of 235 ppm.Though complex 2 is unstable in solution, it can be trapped via reaction with
C 2 H 4 to give six-membered platinasilacycle 3. The four-membered platinasilacyle 1 can further be expanded by insertion of another molecule of C 2 H 4 into the Si-Pt bond, yielding platinasilacyclohexane 4.