Redox Modification of EMACs Through the Tuning of Ligands: Heptametal(II) Complexes of Pyrazine-Modulated Oligo-alpha-pyridylamido Ligands

Abstract

Using pyrazine-modulated oligo-alpha-pyridylamido ligands N(2)-(pyrazin-2-yl)-N(6)-[6-(pyrazin-2-ylamino)pyridin-2-yl]pyridine-2,6-diamine (H(3)pzpz) and N(2)-(pyrazin-2-yl)-N(6)-[6-(pyridin-2-ylamino)pyridin-2-yl]pyridine-2,6-diamine (H(3)tpz), linear heptametal(II) extended metal atom chains (EMACs) [M(7)(mu(7)-L)(4)X(2)] [L = pzpz(3-), M = Ni(II), X = Cl(-) (1), NCS(-) (2); M = Cr(II), X = Cl(-) (3), NCS(-) (4); L = tpz(3-), M = Cr(II), X = Cl(-) (5), NCS- (6)] were synthesized and structurally characterized. Electrochemical studies showed that heptanickel(II) complexes can undergo one reversible oxidation at +0.46 V for I and +0.52 V for 2. Chromium(II) species 3 exhibited two reversible, one-electron oxidation peaks at +0.61 and +0.88 V, and 5 exhibited three reversible, one-electron oxidation peaks at +0.40, +0.68 and +1.07 V The redox peaks shifted positively when axial ligands changed from chloride to thiocyanate anions, at +0.67 and +0.92 V for 4 and +0.44, +0.73 and +1.11 V for 6. The introduction of electron-withdrawing pyrazine rings to the spacer ligand retarded oxidation of the heptametal EMACs and stabilized the complexes. In nickel(II) species 1 and 2, both terminal nickel atoms exist in spin state S = 1 whereas all the inner nickel atoms exist in spin state S = 0. Temperature-dependent magnetic research revealed an antiferromagnetic interaction between the two terminal atoms through a superexchange pathway along metal cores with a parameter of about -4 cm(-1). Chromium(II) species 3-6 showed a localized structure consisting of three quadruple Cr-Cr bonds and a single terminal Cr(II) atom. Magnetic study revealed a quintet ground state resulting from the isolated, high-spin Cr(II) atom. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)