Synthesis and characterization of ruthenium(II)-pyridylamine complexes with catechol pendants as metal binding sites

A novel tris(2-pyridylmethyl)amine (TPA) derivate having two catechol moieties linked by amide linkages at the 6-positions of two pyridyl groups was synthesized. The ligand, N,N-bis[6-{3,4-(dihydroxy)benzamide}-2-pyridyl-methyl]-N-(2-pyridylmethyl)amine (Cat(2)-TPA; L2), and its precursor, N,N-bis[6-{3,4-bis(benzyloxy)-benzamide}-2-pyridyl-methyl]-N-(2-pyridylmethyl)-amine ((Bn(2)Cat)(2)-TPA; L1), formed stable ruthenium(II) complexes, [RuCl(L2)]PF(6) (2) and [RuCl(L1)]PF(6) (1), respectively. The crystal structure of [RuCl(L2)]Cl (2') was determined by X-ray crystallography to show two isomers in terms of the orientation of one catechol moiety. In complex 2, the ligand bearing catechols acts as a pentadentate ligand involving coordination of one of the amide oxygen atoms in addition to that of the tetradentate TPA moiety and two metal-free catechol moieties as metal-binding sites. The coordination of L2 results in the preorganization of the two catechols to converge them to undergo intramolecular pi-pi interactions. The (1)H NMR spectrum of 2 in DMSO-d(6) revealed that only one isomer was present in the solution. This selective formation could be ascribed to the formation of an intramolecular hydrogen-bonding network among the hydroxyl groups of the catechol moieties, as suggested by X-ray analysis. This intramolecular hydrogen bonding could differentiate the pK(a) values of the hydroxy groups of the catechol moieties into three kinds, as indicated by spectroscopic titration with tetramethylammonium hydroxide (TMAOH) in DMF. The complexation of 2 with other metal ions was also examined. The reaction of 2 with [Cu(NO(3))(2)(TMEDA)] (TMEDA = N,N,N',N'-tetramethylethylenediamine) in methanol allowed us to observe the selective formation of a binuclear complex, [RuCl(L2(2-)){Cu(TMEDA)}]PF(6) (3), which was characterized by ESI-MS, UV-vis, and ESR spectroscopies. Its ESR spectrum in methanol suggested that the coordination of the Cu(II)-TMEDA unit to the converged catechol moieties would be different from conventional kappa(2)-O,O':eta(2)-coordination: it exhibits a novel bridging coordination mode, bis-kappa(1)-O:eta(1)-coordination, to form the binuclear Ru(II)-Cu(II) complex.