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-eta(1) -pyramidal (2), cis-eta(1)-pyramidal (3) and eta(1) 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-eta(1)-pyramidal SO2 coordination mode is the most stable in the three conformations of [Pt(Me4NCN)(SO2)]CI 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 I and 2, respectively. The coordination Of SO2 into Pt atom results in a significant blue shift in emission energy. (C) 2007 Elsevier B.V. All rights reserved.