Phosphorus-31 nuclear magnetic resonance spectroscopic characterisation of tertiary phosphine palladium(0) complexes: evidence for 14-electron complexes in solution

Abstract

³¹ P N.m.r. spectroscopy has been used to investigate the solution structure of PdL_n[L = PMe₃, PMe₂Ph, PMePh₂, PPh₃, PEt₃, PBuⁿ₃, P(CH₂Ph)₃, PPrⁱ₃, P(C₆H₁₁)₃, and PBu^t₂Ph]. It is concluded that at room temperature the species existing in solution are [Pd(PMe₃)₄], [Pd(PMe₂Ph)₄], [Pd(PEt₃)₃], [Pd(PBuⁿ₃)₃], [Pd (PPh₃)₃], [Pd{P(CH₂-Ph)₃}₃], [Pd(PPrⁱ₃)₂], [Pd{P(C₆H₁₁)₃}₂], and [Pd(PBu^t₂Ph)₂]. At low temperatures (–60 to –100 °C) additional species [Pd(PMePh₂)₄], [Pd(PPh₃)₄], [Pd(PEt₃)₄], [Pd(PBuⁿ₃)₄], [Pd(PPrⁱ₃)₃], and [Pd{P(C₆H₁₁)₃}₃] have been identified. Variable-temperature ³¹P n.m.r. spectroscopy, coupled with lineshape analysis, has been used to derive ΔH^‡ and ΔS^‡ values for tertiary phosphine exchange between the complex and an excess of tertiary phosphine. In order to explain the kinetics it is necessary to postulate that [Pd{P(CH₂Ph)₃}₃] exchanges P(CH₂Ph)₃via dissociation to [Pd{P(CH₂Ph)₃}₂]. Thus there is evidence for a number of formally 14-electron complexes of the type PdL₂.