Modulation of an Anagostic Interaction in SiPSi-Type Pincer Platinum Complexes

The reactivities of tris(benzyldimethylsilyl)-phosphine [P(o-C6H4-CH2SiMe2H)(3)] (1) and tris(benzyldiphenylsilyl)phosphine [P (o-C6H4-CH2SiPh2H)(3)] (6) toward the same platinum precursor [Pt(PPh3)(3)] are strikingly different. The reaction with 1 renders the trans disilyl platinum(II) complex [Pt{P(o-C6H4-CH2SiMe2)(2) (o-C6H4CHSiMe2)}PPh3] (2) in which the ligand coordinates in a tridentate fashion while a new Si-C bond is formed from the third Si moiety. The most prominent feature is an anagostic interaction that is established at the apical position. In contrast, the reaction of [Pt(PPh3)(3)] with 6 yields the hexacoordinated hydrido trisilyl platinum(IV) complex [PtH{P(o-C6H4- CH2SiPh2)(3)}PPh3] (7). We have studied the effect of the variation of the monodentate ligand in 2 by simple substitution reactions. We found a systematic variation of the chemical shift of the anagostic hydrogen in the H-1 nuclear magnetic resonance spectrum of the corresponding PMe3, P(OPh)(3), and CO complexes that can in principle be ascribed to a varying degree of the pi acceptor character of the ancillary ligand. However, theoretical calculations at the density functional theory level show only slight changes in the frontier orbitals in line with predominantly closed-shell electrostatic interactions.