Preparation and nuclear magnetic resonance studies of the dynamic processes in α, 1,2-η-triphenylmethyl- and monosubstituted-α, 1,2-η-tri-phenylmethyl-palladium and -platinum complexes

Abstract

Reaction of the zerovalent palladium or platinum complexes [M₂(dba)₃(s)](dba = PhCHCHCOCHCHPh; s = dba or solvent) with Ph₃CCl gives [{M(CPh₃)Cl}_n](n probably 2) which are converted into the pentane-2,4-dionates [M(CPh₃)(acac)][M = Pd (3a) or Pt (3b); acac = MeCOCHCOMe] by reaction with Tl(acac). The CPh₃ ligands in complexes (3a) and (3b) are η³-benzylically bound to the metal. Three different dynamic processes of progressively higher ΔG^‡ are undergone by complex (3a) and have been studied by n.m.r. spectroscopy: (A), where the metal moves between two η³-positions on a single phenyl via a suprafacial [1,5]-sigmatropic shift; (B), where the metal moves from one phenyl ring to another (ΔG^‡= 13.8 kcal mol^–1 at 300 K); and (C), where the ends of the acac ligand also become equilibrated [after remaining distinct during processes (A) and (B)]. Process (A)[but not (B) or (C)] also occurs in (3b)(ΔG^‡=ca. 6.4 kcal mol^–1 at 187 K) within the accessible temperature range, but it could not be frozen out for either (3a) or (3b). A p-substituent in one phenyl ring has a small effect on the rate of process (B), but the metal shows definite site preferences and favours the phenyl bearing the more electron-releasing substituent.