Synthesis and Characterization of Neutral and Cationic Platinum(II) Complexes Featuring Pincer-like Bis(phosphino)silyl Ligands: Si−H and Si−Cl Bond Activation Chemistry

Abstract

The synthesis and characterization of a new series of Pt(II) complexes bearing the tridentate bis(phosphino)silyl ligands [κ3-(2-R2PC6H4)2SiMe]− ([R-PSiP], R = Ph, Cy) are reported. The tertiary silane [Ph-PSiP]H reacted with PtCl2(SEt2)2 to afford [Ph-PSiP]PtCl (1), which was structurally characterized. Alkyl derivatives of 1 were prepared in high yields by the reaction of 1 with either PhCH2K to afford [Ph-PSiP]PtCH2Ph (2) or MeLi to afford [Ph-PSiP]PtMe (3). The X-ray crystal structure of 2·CH2Cl2 was obtained and confirmed a square-planar geometry about Pt, with the benzyl ligand positioned trans to Si. Treatment of either 2 or 3 with 1 equiv of B(C6F5)3 in benzene solution led to the formation of the corresponding cationic species {[Ph-PSiP]Pt}+[RB(C6F5)3]− (R = CH2Ph, 4; R = Me, 5). X-ray quality crystals of 4(OEt2)·OEt2 were grown from an Et2O solution of 4; the crystal structure of 4(OEt2)·OEt2 features an Et2O molecule coordinated to the cationic Pt center, resulting in square-planar coordination geometry at Pt. Both 4 and 5 underwent a subsequent reaction to generate [Ph-PSiP]Pt(C6F5) (6). Treatment of 1 with 1 equiv of Li[B(C6F5)4](OEt2)2.5 in fluorobenzene afforded {[Ph-PSiP]Pt(OEt2)}+[B(C6F5)4]− (7(OEt2)). Although the reaction of 1 with AlCl3 led to a mixture of products, crystallization attempts enabled the isolation of a minute quantity of fluorobenzene-solvated [Ph-PSiP]PtCl·AlCl3 (1(AlCl3)·(C6H5F)2), which was crystallographically characterized and features a chloride ligand bridging the Pt and Al centers. The dicyclohexylphosphino derivative [Cy-PSiP]H reacted with PtCl2(SEt2)2 in the presence of Et3N to afford [Cy-PSiP]PtCl (8), which reacted with AgOTf to form [Cy-PSiP]PtOTf (9). Heating of either 8 or 9 for extended periods in benzene solution in the presence of amine bases did not result in benzene activation. Treatment of 8 with RLi reagents (R = Me, Ph) afforded the corresponding methyl ([Cy-PSiP]PtMe, 10) and phenyl ([Cy-PSiP]PtPh, 11) complexes, the latter of which was crystallographically characterized. As was observed for 3, compound 10 reacted with 1 equiv of B(C6F5)3 to form {[Cy-PSiP]Pt}+[MeB(C6F5)3]− (12), which upon heating in benzene afforded [Cy-PSiP]Pt(C6F5) (13) with loss of MeB(C6F5)2. Treatment of 8 with LiEt3BH led to the formation of [Cy-PSi(μ-H)P]Pt (14), which was identified on the basis of NMR and IR spectroscopic data as a bis(phosphino) Pt derivative of [Cy-PSiP]H that features η2-Si-H coordination involving the tethered silicon fragment. Although complexes 2, 3, and 10 did not react with an atmosphere of H2, these compounds reacted readily with 1 equiv of PhSiH3 to form the corresponding silyl complexes ([Ph-PSiP]PtSiH2Ph, 15; [Cy-PSiP]PtSiH2Ph, 16) upon loss of toluene or methane, respectively. While 2 and 3 also reacted with 1 equiv of Ph2SiHCl to form the silyl complex [Ph-PSiP]PtSiPh2Cl (17), 10 reacted with either Ph2SiHCl or iPr2SiHCl to form [Cy-PSiP]PtCl (8). Complex 10 also reacted with 1 equiv of Me3SiCl to form 8 and Me4Si upon heating. Although complex 11 also reacted with hydridochlorosilanes to form 8, these reactions were significantly slower relative to 10. In contrast to 10 and 11, the η2-Si-H complex 14 reacted with 1 equiv of either PhSiH3 or Ph2SiHCl to form the corresponding Pt silyl complexes 16 and [Cy-PSiP]PtSiPh2Cl (18), with loss of H2.