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

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Abstract
31 P N.m.r. spectroscopy has been used to investigate the solution structure of PdLn[L = PMe3, PMe2Ph, PMePh2, PPh3, PEt3, PBun 3, P(CH2Ph)3, PPri 3, P(C6H11)3, and PBut 2Ph]. It is concluded that at room temperature the species existing in solution are [Pd(PMe3)4], [Pd(PMe2Ph)4], [Pd(PEt3)3], [Pd(PBun 3)3], [Pd (PPh3)3], [Pd{P(CH2-Ph)3}3], [Pd(PPri 3)2], [Pd{P(C6H11)3}2], and [Pd(PBut 2Ph)2]. At low temperatures (–60 to –100 °C) additional species [Pd(PMePh2)4], [Pd(PPh3)4], [Pd(PEt3)4], [Pd(PBun 3)4], [Pd(PPri 3)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.