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 (Cat₂-TPA; L2), and its precursor, N,N-bis[6-{3,4-bis(benzyloxy)-benzamide}-2-pyridyl-methyl]-N-(2-pyridylmethyl) -amine ((Bn₂Cat)₂-TPA; L1), formed stable ruthenium(II) complexes, [RuCl(L2)]PF₆ (2) and [RuCl(L1)]PF₆ (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 π-π interactions. The ¹H 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(NO₃) 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)}]PF₆ (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 κ²-O,O′:η²-coordination: it exhibits a novel bridging coordination mode, bis-κ¹-O:η¹- coordination, to form the binuclear Ru(II)-Cu(II) complex.