EFFECTS OF SULFUR SITE MODIFICATION ON THE REDOX POTENTIALS OF DERIVATIVES OF [N,N'-BIS(2-MERCAPTOETHYL)-1,5-DIAZACYCLOOCTANATO]NICKEL(II)

Cyclic voltammetry has been used to examine Ni(II/I) and Ni(II/III) redox potential data for a structurally characterized homologous series of six NiN2S2 complexes, in which the sulfur sites are systematically varied in donor ability: thiolate (RS-), thioether (RSR), sulfinate (RSO2-), and mixtures of these. These include [N,N'-bis(mercaptoethyl)-1,5-diazacyclooctanato]nickel(II) ((bme-daco)Ni(II), 1), [N-(mercaptoethyl)-N'-(sulfinoethyl)-1,5-diazacyclooctanato]nickel(II) ((mese-daco)Ni(II), 2), [N,N'-bis(sulfinoethyl)-1,5-diazacyclooctanato]nickel(II) ((bse-daco)Ni(II), 3), [N-(mercaptoethyl)-N'-(3-thiabutyl)-1,5-diazacyclooctanato]nickel(II) iodide ([(metb-daco)Ni][I], 4), [N-(sulfinoethyl)-N'-(3-thiabutyl)-1,5-diazacyclooctanato]nickel(II) iodide ([(setb-daco)Ni][I], 5), and [N,N'-bis(3-thiabutyl)-1,5-diazacyclooctane]nickel(II) diiodide ([(btb-daco)Ni][I]2, 6). X-ray crystallography has established that all complexes are largely square planar with distortions toward tetrahedral ranging from 1-degrees (strictly square planar) to 18.3-degrees. Throughout the series, the differences in Ni-S bond distances vary <0.08 angstrom, with the shortest distance at 2.133(3) angstrom for Ni-SO2R in 3 and the longest distance at 2.211(3) angstrom for Ni-SR2 in 6. All complexes show reversible reduction waves in CH3CN, assigned to the Ni(II/I) couple in complexes 2-6 by EPR of chemically reduced solutions. The Ni(II/I) potentials vary over a ca. 1.5-V range. Within the series the Ni(I) state is most accessible (-482 mV vs NHE in CH3CN) for the dithioether complex (6) and least for the dithiolate (1). Changes in the donor character of the sulfur ligands have a consistent and additive effect on the redox potentials: in CH3CN each methylation of the nickel-bound bound thiolates results in stabilization of Ni(I) by ca. 700 mV whereas each oxygenation stabilizes the Ni(II/I) couple ca. 300 mV. Potential measurements in water demonstrate large hydrogen-bonding effects for the complexes with thiolate and sulfinate donor sites. Reversible Ni(II/III) couples are observed only for complexes 3 and 6. For all complexes the separation between reduction and oxidation events is ca. 2 V. The crystal structure is given of previously reported (bse-daco)Ni(II) (3), which crystallizes in the orthorhombic P2(1)2(1)2 (No. 18) space group with a = 8.696(4) angstrom, b = 9.993(5) angstrom, c = 8.015(4) angstrom, V = 696.5(5) angstrom3, and Z = 2. The synthesis and structure of two new compounds are given: [(metb-daco)Ni(II)][I] (4) crystallizes in the triclinic P1BAR (No. 2) space group with a = 7.558(5) angstrom, b = 8.922(7) angstrom, c = 12.559(9) angstrom, alpha = 102.79(6)degrees, beta = 95.05(6)degrees, gamma = 107.23(6)degrees, V = 778.0(10) angstrom3, and Z = 2; and [(setb)-daco)Ni(II)] [I] (5) crystallizes in the monoclinic P2(1)/n (No. 14) space group with a = 9.012(2) angstrom, b = 12.891(3) angstrom, c = 14.055(3) angstrom, beta = 99.42(2)degrees, V = 1610.8(6) angstrom3, and Z = 4.