Effects of the number of cyclometalated rings and ancillary ligands on the rate of MeI oxidative addition to platinum(ii)–pincer complexes

Abstract

Organoplatinum(II)–pincer complexes [Pt(C^N^C)(L)] (2a; L = PPh₂Me, 2b; L = pyridine (py), 2c; L = 4-picoline (pic)) are synthesized by the reaction of [Pt(C^N^C)(DMSO)], 1, with 1 equiv. of L, where HC^N^CH = 2,6-diphenylpyridine. Complexes 2 have 5d(Pt) → π*(pincer) metal-to-ligand charge-transfer bands in the visible region, which were used to easily follow the kinetics of their reactions with MeI by using UV-vis spectroscopy. An S_N2 mechanism is suggested and the proposed intermediates were supported by DFT calculations. Consistent with the proposed mechanism, the rates of the reactions at different temperatures were slower for L = PPh₂Me than those for L = N-donor ligands and large negative ΔS^‡ values were found for all oxidative addition reactions. The rates are almost 2 times slower for L = PPh₂Me as compared to L = 4-picoline because of the steric effects and the electronic effects transmitted through the ligands. To investigate the effect of the number of cyclometalated rings on the rate of MeI oxidative addition, the kinetics of the reaction of [Pt(Ph)(ppy)(py)] (Hppy = 2-phenylpyridine) and [Pt(Ph)₂(py)₂] (having 1 and zero cyclometalated ring, respectively) with MeI were also studied and a correlation between the number of cyclometalated rings and the rate of the oxidative addition reaction was found.