New azasilatranes: bidentate and tridentate coordination modes of the novel ligand EtOSi(Ph2PNCH2CH2)2(HNCH2CH2)N

Abstract

The synthesis and characterization of EtOSi(Ph2PNCH2CH)2(HNCH2CH2)N (ethoxy-N,N'·bis(di-phenylphosphino)azasilatrane, the title compound 3b) is reported. Its reactions with sulfur and MeI proceed easily via quaternization at the phosphorus sites to give the bis(phosphonium) salt and the bis(phosphine) sulfide, respectively. Synthesis and characterization of the coordination compounds (3b) Ni(CO)2, (3b) PtCl2, (3b) M(CO)4 (M = Mo, W), and (3b) M(CO)3 (M = Mo, W) give evidence for the existence of two possible coordination modes; one wherein 3b behaves as a bidentate (P,P') ligand and the other as a tridentate (P,P',O) ligand. The latter coordination mode provides a rare example of coordination of a silyl ether to a transition metal. NMR spectroscopic studies in solution and the solid state establish the presence of transannular Si-N bonding and hypercoordination at silicon. Solid-state 31P NMR spectra give direct evidence for the presence of a twisted, helically chiral conformation of the silatranyl cage, which in solution is lost as a consequence of rapid conformational mobility on the NMR time scale. These findings are further corroborated by the results of crystal structure determinations of the bis(phosphine) sulfide and (3b) PtCl2.