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

Using pyrazine‐modulated oligo‐α‐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 (μ 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 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 Cr II atom. Magnetic study revealed a quintet ground state resulting from the isolated, high‐spin Cr II atom.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)