Synthesis, Structures, and Electrochemical Properties of Nickel Complexes of Macrocyclic N2S2Aminothioethers

Macrocyclic complexes derived from the template synthesis of the cis -dithiolate nickel(II) complex [1,5-bis(mercaptoethyl)-1,5-diazacyclooctanato]nickel(II), I, and [ N, N ‘-bis(2,2-dimethyl-2-mercaptoethyl)-1,5-diazacyclooctanato]nickel(II), I*, with S to S linkers of C 2, II* Br 2, C 3, III* Br 2, and C 4, IV* Br 2, were synthesized and fully characterized as square planar complex ions. The systematic enlargement of the N 2 S 2 cavity size correlates with electrochemical measurements which find the Ni II/I couple to be more accessible through the series, while the Ni III oxidation state is stabilized in the smaller cavities, maintaining an approximate 2 V difference between the Ni III/II and Ni II/I couples. The dithioether compound [ I* Me 2 ]I 2 shows the most positive Ni II/I potential (−412 mV vs NHE, in CH 3 CN solution), consistent with the greater flexibility of the open-chain N 2 S 2 ligand. While the reduced macrocycles show little tendency to react with carbon monoxide, EPR, electrochemical, and infrared spectroscopic studies of the Ni I state of the open chain ligand complex support the reversible binding of CO in CO-saturated solutions. Cyclic voltammetry shows EC‘ activity in the production of H 2 from HBF 4 solutions in the presence of the macrocycle complexes and is interpreted in terms of an inner-sphere Ni III −H intermediate. Crystal data for II* Br 2, III* Br 2, and IV* Br 2 are as follows: II* Br 2 · 3 / 2 H 2 O crystallizes in the monoclinic system, space group P 2 1 / c with cell constants a = 7.900(5)Å, b = 18.41(2) Å, c = 15.612(14) Å, β = 97.53(6)°, V = 2251(3) Å 3, and Z = 4, R = 0.0972, R w = 0.1886; for III* Br 2 · 5 / 2 H 2 O, monoclinic system, space group P 2 1 / c with cell constants a = 18.821(5) Å, b = 7.844(2) Å, c = 32.968(8) Å, β = 97.84(2)°, V = 4824(2) Å 3, and Z = 8 and R = 0.077, R w = 0.125; for IV* Br 2 ·H 2 O, orthorhombic system, space group Pnma with cell constants a = 18.562(3) Å, b = 15.140(3) Å, c = 9.703(3) Å, V = 2726.8(11) Å 3, and Z = 4 and R = 0.044, R w = 0.099.