Preparation and fluxional behaviour of η3-methylbenzyl platinum and palladium complexes

Abstract

Protonation of the complexes [M(η²-CH₂CHPh)(L–L)][M = Pt, L–L =(C₆H₁₁)₂P(CH₂)_nP(C₆H₁₁)₂, n= 2 or 3 (1a or 1c), Bu^t₂P(CH₂)_nPBu^t₂, n= 2 or 3 (1b or 1d), and Bu^t₂PCH₂C₆H₄CH₂PBu^t₂(1e); M = Pd, L–L = Bu^t₂P(CH₂)_nPBu^t₂, n= 2 or 3 (1f or 1g) and Bu^t₂PCH₂C₆H₄CH₂PBu^t₂(1h)] with HBF₄ in diethyl ether affords a series of complexes, [M(η³-MeCHPh)(L–L)][BF₄](2a–2h), which contain an η³-methylbenzyl ligand. The complexes 2a–2h were characterized by ¹H, ¹³C and ³¹P NMR spectroscopy and all except 2a and 2f were found to undergo intramolecular rearrangement in solution at or below room temperature. A mechanism is proposed, on the basis of variable-temperature NMR studies, that involves an η³⇌σ conversion coupled with single-bond rotation and β-elimination/hydride migration processes. For 2a–2e, the influence of the chelating diphosphine on the nature of the η³-benzyl interaction was investigated by ³¹P-{¹H} NMR spectroscopy and it was found that the largest diphosphines induce the most asymmetric η³ interaction. Similarly, it was found that the activation barriers to intramolecular rearrangement are lowest for the complexes with the largest diphosphine ligands.