Mechanistic Studies of Alkyl Chloride Acetoxylation by Pt–Sb Complexes

The bis-acetate complexes (SbQ₃)Pt(OAc)₂ (<b>1</b>) and (SbQ₂Ph)Pt(OAc)₂ (<b>2</b>) (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 (<b>1b</b>) and (SbQ₂Ph)PtCl₂ (<b>2b</b>), respectively. The first acetoxylation step produced the intermediates (SbQ₃)Pt(Cl)(OAc) (<b>1a</b>) and (SbQ₂Ph)Pt(Cl)(OAc) (<b>2a</b>). The reaction was studied using pseudo first order kinetics (excess DCE) in order to compare the rates of reaction of <b>1</b> and <b>2</b>, which revealed that <i>k</i> _obs = 2.44(6) × 10^-4 s^-1 for <b>1</b> and 0.51(2) × 10^-4 s^-1 for <b>2</b>. The intermediate <b>1a</b> 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 (<b>3</b>) (^tbpy = 4,4'-di-<i>tert</i>-butyl-2,2'bipyridine), was studied for the acetoxylation of DCE to form (^tbpy)Pt(Cl)(OAc) with <i>k</i> _obs = 0.46(1) × 10^-4 s^-1. Density Functional Theory (DFT) calculations were used to examine possible Pt-mediated mechanisms for the reactions of <b>1</b>, <b>2</b>, or <b>3</b> 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 <b>1</b> in heated dichloromethane-<i>d</i> ₂ and chloroform-<i>d</i> was determined (0.43(1) × 10^-4 s^-1 for dichloromethane-<i>d</i> ₂ and 0.37(1) × 10^-4 s^-1 for chloroform-<i>d</i>) and compared to the rate of acetoxylation of DCE.