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.