Mechanistic Aspects of the Reversible Binding of SO2 on Arylplatinum Complexes: Experimental and ab Initio Studies

Abstract

The detailed mechanism of the reversible binding and fast exchange of SO2 on the organoplatinum(II) complex [PtI(NCN)], 1, has been studied experimentally in solution (C2F4Br2) using low-temperature NMR spectroscopy and theoretically by ab initio calculations. Direct bonding of SO2 and formation of the thermodynamically most stable product [PtI(NCN)(SO2)], 2, was predicted by DFT calculations. Theoretical considerations further indicated the possibility of an alternative and competitive process involving an intermediate [Pt(I-SO2)(NCN)], 3, containing an I-S bond, prior to a 1,2-sigmatropic migration of the SO2 molecule to form ultimately adduct 2. Temperature-dependent (223-287 K) analysis of the equilibrium constant between complex 1 and the adduct 2 indicated H = -36.6 (±0.8) kJ mol-1, S = -104 (±3) J K-1 mol-1, and K298 = 9 (±4) M-1. The exchange of SO2 on 2 is fast at room temperature, but when the solution was cooled to low temperatures (167-188 K), rate constants were obtained by line-shape analyses of the 1H NMR spectra. The temperature dependence of the exchange rate constants afforded the activation parameters (H = +36.2 (±1.0) kJ mol-1, S = +33 (±6) J K-1 mol-1, k298 = 1.5 (±0.5) × 108 s-1, and k174 = 2.34 (±0.08) × 103 s-1). From these and concentration-dependent measurements, a dissociative mechanism, D, for the SO2 exchange on 2 has been deduced involving 1 as an intermediate. This is in line with a direct binding of SO2 on the platinum center and suggests that the nature of the metal-bound halide has only limited influence on the SO2 binding. This conclusion is particularly important when fabricating sensor materials for the detection of SO2 gas that are based on this type of organoplatinum(II) complexes as active sites.