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

Abstract

AbstractUsing pyrazine‐modulated oligo‐α‐pyridylamido ligands N2‐(pyrazin‐2‐yl)‐N6‐[6‐(pyrazin‐2‐ylamino)pyridin‐2‐yl]pyridine‐2,6‐diamine (H3pzpz) and N2‐(pyrazin‐2‐yl)‐N6‐[6‐(pyridin‐2‐ylamino)pyridin‐2‐yl]pyridine‐2,6‐diamine (H3tpz), linear heptametal(II) extended metal atom chains (EMACs) [M7(μ7‐L)4X2] [L = pzpz3–, M = NiII, X = Cl– (1), NCS– (2); M = CrII, X = Cl– (3), NCS– (4); L = tpz3–, M = CrII, 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 1 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 CrII atom. Magnetic study revealed a quintet ground state resulting from the isolated, high‐spin CrII atom.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)