The nano-scale molecule with the longest delocalized metal-metal bonds: linear heptacobalt(II) metal string complexes Co-7(mu(7)-L)(4)X-2

Abstract

A new type of pyrazine-modulated oligo-a-pyridylamino ligands, N-2-(pyrazin-2-yl)-N-6-(6-(pyrazin-2-ylamino)pyridin-2-yl)pyridine-2,6-diamine (H(3)pzpz) (1) and N-2-(pyrazin-2-yl)-N-6-(6-(pyridin-2-ylamino)pyridin-2-yl)pyridine-2,6-diamine (H(3)tpz) (2), were synthesized and characterized by IR, H-1 NMR and MS(FAB). Using 1 and 2, the linear heptacobalt(II) metal string complexes [Co-7(mu(7)-L)(4)X-2] (L = pzpz(3-), X = Cl- (3), NCS- (4); L = tpz(3-), X = Cl- (5), X = NCS- (6)) were synthesized and structurally characterized. The structures showed the shortest Co-Co distance (2.194 angstrom) and the longest Co chain (13.5 A) obtained to date with direct Co-Co bonds. The Co-Co distances are in the range 2.194-2.309 angstrom. Electrochemical studies showed two reversible oxidations and one reversible reduction, while all the redox reactions of H(3)pzpz complexes, 3 and 4, occurred at higher potentials than H(3)tpz complexes, 5 and 6. The complexes 3-6 are fairly stable to oxidation. Temperature-dependent magnetic research on 3-6 revealed anomalous magnetic behavior with intermediate magnetic moment values between quartet and doublet states, and deviation from the Curie-Weiss law.