Synthetic and nuclear magnetic resonance studies on dialkyl- and diaryl-platinum complexes containing chelating, monodentate, or bridging Ph2PCH2PPh2 ligands

Abstract

Complexes of the type [PtR2(dppm-PP′)](R = Me, CH2CMe3, Et, CH2Ph, Ph, C6H4Me-p, C6H4OMe-2, C6H2Me3-2,4,6,1-naphthyl, C6F5, or C6H4Me-o; dppm = Ph2PCH2PPh2) have been prepared from [PtCl2(dppm-PP′)] and the corresponding alkyl-lithium or Grignard reagents. Equilibrium constants, K, for the conversion of [PtR2(dppm-PP)′] into cis-[PtR2(dppm-P)2] with dppm were studied using 31P n.m.r. spectroscopy at different temperatures. Equilibrium is rapidly established for R = Me, even at –60 °C, but more slowly for R = Ph, completion taking less than 1 h at –30 °C; for the sterically hindered (o-substituted) aryls equilibrium is only established after several days at 20 °C. The values of K increase as the temperature is lowered. Complexes of the type cis-[PtR2(dppm-P)2] were isolated for R = Me, C6H4Me-o, or 1-naphthyl. The o-tolyl or 1-naphthyl complexes exist as syn–anti mixtures in solution, due to restricted rotation around the platinum–aryl bonds. Treatment of several complexes of the type [PtR2(dppm-PP′)] with Mel gives [PtR2Me(I)(dppm-PP′)] with trans addition of Mel. Treatment of [PtR2(dppm-PP′)] with HCl gives [PtCl(R)(dppm-PP′)] for R = C6H2Me3-2,4,6,C6H4OMe-2, or 1-naphthyl, whereas [Pt(C6H4OMe-2)2(dppm-PP′)] with Mel appears to give [PtI(C6H4OMe-2)(dppm-PP′)]. The 1H, 31P, and 195Pt n.m.r. parameters for these complexes are discussed. For [PtR2(dppm-PP′)]δ(P) is much more negative (–30 to –40 p.p.m.) than for cis-[PtR2(dppm-P)2](+5 to +20) and the J values are much smaller. In contrast, platinum-195 chemical shifts are 600 p.p.m. to high frequency of those for complexes of type cis-[PtR2(dppm-P)2], similarly for 13C n.m.r. shifts. The δ(PCH2P) values for the chelates are 3.9–4.5 p.p.m., whereas for [PtR2(dppm-P)2] they are 1.8–3.0 p.p.m.