Using a combination of electrochemical and NMR techniques, the oxidative addition of PhX to three closely related bis-diphosphine PPd complexes, where the steric bulk of just one substituent was varied, has been analysed quantitatively. For the complex derived from MetBuP, a rapid reaction ensued with PhI following an associative mechanism, and data was also obtained by cyclic voltammetry for PhOTs, PhBr and PhCl, revealing distinct relative reactivities from the related (PCx)Pd complex (Cx=cyclohexyl) previously studied. The corresponding EttBuP complex reacted more slowly with PhI and was studied by NMR spectroscopy. The reaction course indicated a mixture of pathways, with contribution from a component that was [PhI] independent. For the CxtBuP complex, reaction was again monitored by NMR spectroscopy, and was even slower. At high PhI concentrations reaction was predominantly linear in [PhI], but at lower concentrations the [PhI] independent pathway was again observed, and an accelerating influence of the reaction product was observed over the concentration range. The NMR spectra of the EttBuP and CxtBuP complexes conducted in CD shows some line broadening that was augmented on addition of PhI. NMR experiments carried out in parallel show that there is rapid ligand exchange between free phosphine and the PdPd complex and also a slow ligand crossover between different PPd complexes. DFT calculations were carried out to further test the feasibility of CD involvement in the oxidative addition process, and located Van der Waals complexes for association of the PPd complexes with either PhI or benzene. PhI or solvent-assisted pathways for ligand loss are both lower in energy than direct ligand dissociation. Taken all together, these results provide a consistent explanation for the surprising complexity of an apparently simple reaction step. The clear dividing line between reactions that give a di- or monophosphine palladium complex after oxidative addition clarifies the participation of the ligand in coupling catalysis. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
