σ‐Silane Platinum(II) Complexes as Intermediates in C−Si Bond‐Coupling Processes

Abstract

Platinumc omplexes [Pt(NHC')(NHC)][BAr F ]( in which NHC' denotes ac yclometalated N-heterocyclic carbene ligand, NHC) react with primary silanes RSiH 3 to afford the cyclometalated platinum(II) silyl complexes [Pt(NHC-SiHR')(NHC)][BAr F ]t hrough ap rocess that involves the formation of CÀSi and PtÀSi bonds with concomitante xtrusion of H 2 .L ow-temperature NMR studies indicatet hat the process proceeds through initial formation of the s-SiH complexes [Pt(NHC')(NHC)(HSiH 2 R)][BAr F ], which are stable at temperatures below À10 8C. At highert emperatures, activation of one SiÀHb ond followed by aC ÀSi coupling reaction generates an agostic SiH platinum hydride derivative [Pt(H)(NHC'-SiH 2 R)(NHC)][BAr F ], whichu ndergoes as econd SiÀHb ond activation to afford the final products. Computationalm odeling of the reaction mechanism indicates that the stereochemistry of the silyl/hydride ligands after the first SiÀHb ond cleavage dictates the nature of the products, favoringthe formation of aC ÀSi bond over aC ÀHb ond, in contrastt op revious results obtained for tertiary silanes. Furthermore, the processi nvolves a trans-to-cis isomerization of the NHC ligand beforet he second SiÀHbond cleavage.

Reversible carbon–boron bond formation at platinum centers through σ-BH complexes

“…Moreover, one of the oxygen atoms of the pinacol moiety is interacting with the platinum center (Pt-O bond distance, 2.247(3) Å), trans to the hydride ligand, which explains the reduction of the 1 J H,Pt coupling constant of the hydride compared to that in [Pt(H)(I t Bu i Pr) 2 ][BAr F ]. [19,21] With respect to complexes 3b and 3c, a similar rearrangement is observed, although in a different range of temperatures. However, as the temperature increases, mixtures of these compounds together with intermediates 4b,c and the final boryl complexes 2b,c coexist, hampering full characterization of intermediates 4b,c (see Figures S25-S28 in the ESI).…”

“…[9] The Pt1-H1 and H1-B1 bond distances are 1.69(4) Å and 1. 19(3) Å, respectively. The latter is slightly elongated with respect to the B-H bond of boranes [13] and comparable to related σ-BH primary aminoborane complexes.…”

“…This value is smaller than that observed for complex [Pt(H)(I t Bu i Pr) 2 ][BAr F ] ( 1 J H,Pt = 2496 Hz). [19] In addition, the diastereotopic Pt-CH 2 protons in 3a merge into a singlet Please do not adjust margins Please do not adjust margins resonance at 2.44 ppm in 4a, [20] that correlates in the 1 H, 13 C-HSQC spectrum with a very broad resonance at 26.8 ppm. The 11 B{ 1 H} NMR spectrum shows a very broad signal at 32.9 ppm.…”

“…[23] In previous works we have shown that trans-cis isomerizations of Pt(NHC) 2 fragments can occur in their reactions with H 2 [24] and silanes. [19] Thus, we investigated mechanisms containing species with a cis geometry of the NHC ligands. Starting from complex 1, trans-cis isomerization requires overcoming a barrier of 12.8 kcal mol -1 and yields species 1-cis, 11.1 kcal mol -1 above the origin (see Figure S50 in the SI) presumably due to the increased steric bulk between the isopropyl chain of the cyclometalated NHC ligand and the other NHC fragment.…”

“…Recently, we applied this approach to analyze C−Si bond coupling reactions. [19] Figure 7 shows the relevant LMOs together with their centroids in three representative structures of the key chemical step of each process. The corresponding arrowpushing schemes are also depicted in Figure 7.…”