Addition versus substitution in the reaction of diiodo(2,9-dimethyl-1,10-phenanthroline)platinum(II) with aromatic bases. X-ray crystal structures of the substitution products with 2-amino-pyridine and 1-methyl-cytosine

Square-planar [PtX2(Me2PHEN)] complexes (Me2PHEN = 2,9-dimethyl-1,10-phenanthroline) behave as a spring trap and react with an extra ligand (L) to give the addition product [PtX2(L)(Me2PHEN)] in which the Me2PHEN plane is perpendicular to the coordination plane of the other three ligands. For X = I and L = PY or APY (PY, pyridine, APY, 2-amino-pyridine) the addition product has monocoordinated Me,PHEN. The free and coordinated ends of Me2PHEN are in rapid exchange at room temperature but the exchange becomes slow, in the NMR time scale, at low temperature (213 K). In chloroform solution and at room temperature, the addition product is indefinitely stable in the case of PY while in the case of APY displacement of a iodo ligand by the free end of Me2PHEN and formation of the cationic species [PtI(L)(Me2PHEN)]I take place in a few hours. An anchimeric assistance of the exocyclic aminic-group of APY in the release of the iodide ion is suggested. The displacement of the iodide is also favored by solvents of higher solvating ability and/or dielectric constants such as methanol. The single crystal X-ray structures of [PtI(APY)(Me2PHEN)]I.2CHCl(3) (1.2CHCl(3)) and [PtI(MeCY)(Me2PHEN)]I.CH3OH (2.CH3OH) (MeCY, 1-methyl-cytosine) have revealed that the steric repulsion towards cis-ligands is smaller for the APY and MeCY ligands than for I and that chloroform can give intermolecular interactions with the phenanthroline moiety which can compete with stacking interactions between phenanthroline ligands.