Theoretical studies on square-planar pincer platinum(II) complex and its SO2 adducts: SO2 detector and potential luminescent probe

We explored the electronic structures and spectroscopic properties of [Pt(Me4NCN)]Cl (1) (NCN− = [2,6-(CH2NMe2)2-C6H3]−) and its SO2 adducts, [Pt(Me4NCN)(SO2)]Cl, which show three penta-coordinated modes (trans-η1-pyramidal (2), cis-η1-pyramidal (3) and η1-coplanar (4)), using the theoretical methods. For 2, a comparative study of ab initio and density functional theory (DFT) shows that the DFT method cannot describe molecular structures appropriately, especially the Pt-S bonding. Therefore, the structures of 1-4 in the ground state were optimized by the ab initio MP2 method and those of the first two complexes in the lowest-lying triplet excited state were studied by the unrestricted MP2 method. The present studies reveal that trans-η1-pyramidal SO2 coordination mode is the most stable in the three conformations of [Pt(Me4NCN)(SO2)]Cl because of its lower total energy as well as the most overlapping between the HOMO and LUMO of SO2 and the filled Pt d orbital, and least repulsion between the lone pair orbitals of SO2 and the Lewis base (PtNCN). The spectroscopic calculations at the TD-DFT associated with the Polarizable Continuum Model (PCM) level indicate that the adsorption of SO2 changes the nature of low-lying absorptions of 1, agreeing well with the color change observed in experiments. We predicted the phosphorescent emissions in the CH2Cl2 solution at 778 and 552 nm for 1 and 2, respectively. The coordination of SO2 into Pt atom results in a significant blue shift in emission energy.