Structural comparison of ethylenediamine platinum(II) complexes containing thiourea and its di- and tetramethyl substituted derivatives

Three new platinum(II) complexes, [Pt(en)(tu)2](NO3)2 (1), [Pt(en)(dmtu)2] (NO3)2 (2) and [Pt(en)(tmtu)2](NO3)2 (3) (where en = ethylenediamine, tu = thiourea, dmtu = 1,3-dimethyl-2-thiourea and tmtu = 1,1,3,3-tetramethy-2-thiourea), have been synthesized and characterized using elemental analysis, IR spectroscopy and NMR spectroscopy. All complexes are soluble in high polarity solvents. Single crystals have been obtained by slow evaporation of methanol solution at room temperature. NMR studies show that the amide proton signals shift downfield while the thiocarbonyl carbon signals shift upfield as compared to free thioureas. These opposite chemical shifts suggest that there is a decrease in thiocarbonyl bond order and an increase in thiocarbonyl carbon-amide nitrogen bond order for the thioureas upon coordination to platinum(II). The molecular structures of the crystallized complexes have been identified by single crystal X-ray diffraction. Both 1 and 2 crystallize in the monoclinic space groups P2(1)/c and P2(1)/n, respectively, while 3 crystallizes in the orthorhombic space group C222(1). X-ray crystallographic analysis indicates that the geometry about the platinum atom is square-planar and the crystal packings are dominated by intermolecular en N − H⋅⋅⋅O (NO3-) and tu N − H⋅⋅⋅O (NO3-) hydrogen bonds.