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

Cyclic voltammetry has been used to examine Ni(II/I) and Ni(II/III) re dox 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 (R SO2-), and mixtures of these. These include s(mercaptoethyl)-1,5-diaza cyclooctanato]nickel(II) ((bme-daco)Ni(II), 1), '-(sulfinoethyl)-1,5-d iazacyclooctanato]nickel(II) ((mese-daco)Ni(II), 2), is(sulfinoethyl)- 1,5-diazacyclooctanato]nickel(II) ((bse-daco)Ni(II), 3), N'-(3-thiabut yl)-1,5-diazacyclooctanato]nickel(II) iodide ([(metb-daco)Ni][I], 4), N'-(3-thiabutyl)-1,5-diazacyclooctanato]nickel(II) iodide ([(setb-daco )Ni][I], 5), and '-bis(3-thiabutyl)-1,5-diazacyclooctane]nickel(II) di iodide ([(btb-daco)Ni][I]2, 6). X-ray crystallography has established that all complexes are largely square planar with distortions toward t etrahedral ranging from 1-degrees (strictly square planar) to 18.3-deg rees. Throughout the series, the differences in Ni-S bond distances va ry <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, assigne d 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. Withi n the series the Ni(I) state is most accessible (-482 mV vs NHE in CH3 CN) 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 methylatio n of the nickel-bound bound thiolates results in stabilization of Ni(I ) by ca. 700 mV whereas each oxygenation stabilizes the Ni(II/I) coupl e ca. 300 mV. Potential measurements in water demonstrate large hydrog en-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 oxidat ion events is ca. 2 V. The crystal structure is given of previously re ported (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. T he synthesis and structure of two new compounds are given: [(metb-daco )Ni(II)][I] (4) crystallizes in the triclinic P1BAR (No. 2) space grou p with a = 7.558(5) angstrom, b = 8.922(7) angstrom, c = 12.559(9) ang strom, alpha = 102.79(6)degrees, beta = 95.05(6)degrees, gamma = 107.2 3(6)degrees, V = 778.0(10) angstrom3, and Z = 2; and [(setb)-daco)Ni(I I)] [I] (5) crystallizes in the monoclinic P2(1)/n (No. 14) space grou p with a = 9.012(2) angstrom, b = 12.891(3) angstrom, c = 14.055(3) an gstrom, beta = 99.42(2)degrees, V = 1610.8(6) angstrom3, and Z = 4.