Mechanistic Studies of Alkyl Chloride Acetoxylation by Pt–Sb Complexes

The bis-acetate complexes (SbQ3)Pt(OAc)2 (1) and (SbQ2Ph)Pt(OAc)2 (2) (Q = 8-quinolinyl) were used to study C–Cl acetoxylation of 1,2-dichloroethane (DCE) to generate 2-chloroethyl acetate and the complexes (SbQ3)PtCl2 (1b) and (SbQ2Ph)PtCl2 (2b), respectively. The first acetoxylation step produced the intermediates (SbQ3)Pt(Cl)(OAc) (1a) and (SbQ2Ph)Pt(Cl)(OAc) (2a). The reaction was studied using pseudo first order kinetics (excess DCE) in order to compare the rates of reaction of 1 and 2, which revealed that kobs = 2.44(6) × 10–4 s–1 for 1 and 0.51(2) × 10–4 s–1 for 2. The intermediate 1a was synthesized independently, and the solid-state structure was determined using single crystal X-ray diffraction. A non-Sb containing control complex, (tbpy)Pt(OAc)2 (3) (tbpy = 4,4′-di-tert-butyl-2,2′bipyridine), was studied for the acetoxylation of DCE to form (tbpy)Pt(Cl)(OAc) with kobs = 0.46(1) × 10–4 s–1. Density Functional Theory (DFT) calculations were used to examine possible Pt-mediated mechanisms for the reactions of 1, 2, or 3 with DCE. The lowest energy calculated substitution mechanism occurs with nucleophilic attack by the Pt center on the C−Cl bond followed acetate reaction with the Pt−C bond. However, close in energy and potentially also a viable mechanism is a direct substitution mechanism where the coordinated acetate anion directly reacts with the C−Cl bond of DCE. In addition, the rate of acetoxylation for complex 1 in heated dichloromethane-d2 and chloroform-d was determined (0.43(1) × 10–4 s–1 for dichloromethane-d2 and 0.37(1) × 10–4 s–1 for chloroform-d) and compared to the rate of acetoxylation of DCE.