| 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 spectroscopyand theoretically by ab initio calculations. Direct bonding of SO2 and formation of the thermodynamically moststable product [PtI(NCN)(SO2)], 2, was predicted by DFT calculations. Theoretical considerations further indicatedthe possibility of an alternative and competitive process involving an intermediate [Pt(I−SO2)(NCN)], 3, containingan 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 ΔH°= −36.6 (±0.8) kJ mol-1, ΔS° = −104 (±3) J K-1 mol-1, and K298 = 9 (±4) M-1. The exchange of SO2 on 2is fast at room temperature, but when the solution was cooled to low temperatures (167−188 K), rate constantswere obtained by line-shape analyses of the 1H NMR spectra. The temperature dependence of the exchange rateconstants afforded the activation parameters (ΔH⧧ = +36.2 (±1.0) kJ mol-1, ΔS⧧ = +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. Thisis in line with a direct binding of SO2 on the platinum center and suggests that the nature of the metal-boundhalide has only limited influence on the SO2 binding. This conclusion is particularly important when fabricatingsensor materials for the detection of SO2 gas that are based on this type of organoplatinum(II) complexes asactive sites.
- According to low-temperature NMR spectroscopy and DFT calculations, the reversible binding of SO2 on arylplatinum(II) complexes 1 containing the pincer ligand [C6H3(CH2NMe2)2-2,6]- occurs directly on the platinum center and does not involve any intermediates. The exchange of SO2 on adduct 2 is extremely fast (extrapolated k298 = 1.5 × 108 s-1) and follows a dissociative mechanism, hence involving 1 as an intermediate.
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