Mechanistic aspects of the reversible binding of SO2 on arylplatinum complexes: Experimental and ab initio studies

The detailed mechanism of the reversible binding and fast exchange of SO2 o n the: organoplatinum(II) complex [PtI(NCN)], 1, bas been studied experimen tally in solution (C2F4Br2) using low-temperature NMR spectroscopy and theo retically 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 poss ibility 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 mi gration of the SO2 molecule to form ultimately adduct 2. Temperature-depend ent (223-287 K) analysis of the equilibrium constant between complex 1 and the adduct 2 indicated DeltaH degrees = -36.6 (+/-0.8) kJ mol(-1), DeltaS d egrees = -104 (+/-3) J K-1 mol(-1), and K-298 = 9 (+/-4) M-1 The exchange o f 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 a nalyses of the H-1 NMR spectra. The temperature dependence of the exchange rate constants afforded the activation parameters (DeltaH(double dagger) = +36.2 (+/-1.0) kJ mol(-1), DeltaS(double dagger) = +33 (+/-6) J K-1 mol(-1) , k(298) = 1.5 (+/-0.5) x 10(8) s(-1), and k(174) = 2.31 (+/-0.08) x 10(3) 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 platinu m center and suggests that the nature of the metal-bound halide has only li mited influence on the SO2 binding. This conclusion is particularly importa nt when fabricating sensor materials for the detection of SO2 gas that are based on this type of organoplatinum(II) complexes as active sites.