Platinum(II) phosphine and orotate complexes with aminopyridine co-ligands, and their molecular recognition via hydrogen bonding

Abstract

The new complexes [Pt(dppp)(py)₂][OTf]₂, 1, [Pt(dppp)(2-ap)₂][OTf]₂, 2, [(dppp)Pt(μ-OH){μ-NH(C₅H₃N)NH₂}Pt(dppp)][OTf]₂, 3 (py = pyridine, 2-ap = 2-aminopyridine, NH(C₅H₃N)NH₂ = 2,6-diaminopyridine anion, dppp = 1,3-bis(diphenylphosphino)propane, OTf = O₃SCF₃) have been prepared via reactions between [Pt(dppp)(OTf)₂] and pyridine, 2-aminopyridine or 2,6-diaminopyridine (2,6-dap) respectively. The amines exhibit a range of co-ordination modes. Pyridine and 2-aminopyridine co-ordinate to platinum through endo-nitrogen atoms in complexes 1 and 2, the latter existing as a pair of rotomers due to the steric hindrance introduced by the 2-substituent. However, 2,6-diaminopyridine co-ordinates to platinum through the exo-nitrogen of one amino group, to give the unusual μ-amido complex 3. Reaction of the known orotate chelate complex [Pt(PEt₃)₂(N,O-HL)] [HL = orotate, the dianion of 2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid (orotic acid)] with 2,6-dap gave [Pt(PEt₃)₂(2,6-dap)(N-HL)] 4, which contains an unconventional monodentate orotate ligand. In this co-ordination mode the orotate retains an ADA hydrogen bonding site and was found to co-crystallise with 2,6-dap via complementary ADA∶DAD triple hydrogen bonds to give [Pt(PEt₃)₂(N-HL)(2,6-dap)]·2,6-dap, 5. Complex 5 exhibits a helical chain structure of associated [1+1] adducts in the solid state.