EFFECTS OF SULFUR SITE MODIFICATION ON THE REDOX POTENTIALS OF DERIVATIVES OF [N,N'-BIS(2-MERCAPTOETHYL)-1,5-DIAZACYCLOOCTANATO]NICKEL(II)
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Cyclic voltammetry has been used to examine NiII/I and NiII/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)NiII,1), [N-(mercaptoethyl)-A-(sulfinoethyl)-1,5-diazacyclooctanato]nickel(II) ((mese-daco)NiII, 2), [N,N-bis(sulfinoethyl)-1,5-diazacyclooctanato] nickel(II) ((bse-daco)NiII, 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 ([(setbdaco)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 (strictly square planar) to 18.3. Throughout the series, the differences in Ni-S bond distances vary <0.08 , with the shortest distance at 2.133(3) for Ni-SO2R in 3 and the longest distance at 2.211(3) for Ni-SR2 in 6. All complexes show reversible reduction waves in CH3CN, assigned to the NiII/I couple in complexes 26 by EPR of chemically reduced solutions. The NiII/I potentials vary over a ca. 1.5-V range. Within the series the Ni1 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 thiolates results in stabilization of Ni1 by ca. 700 mV whereas each oxygenation stabilizes the NiII/I couple ca. 300 mV. Potential measurements in water demonstrate large hydrogen-bonding effects for the complexes with thiolate and sulfinate donor sites. Reversible NiII/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 (bsedaco)NiII (3), which crystallizes in the orthorhombic P21212 (No. 18) space group with a = 8.696(4) , b = 9.993(5) , c = 8.015(4) , V = 696.5(5) 3, and Z = 2. The synthesis and structure of two new compounds are given: [(metbdaco)NiII] [I] (4) crystallizes in the triclinic [formula omitted] (No. 2) space group with a = 7.558(5) , b = 8.922(7) , c = 12.559(9) , = 102.79(6), = 95.05(6), = 107.23(6), V= 778.0(10) 3, and Z = 2; and [(setb-daco)NiII][I] (5) crystallizes in the monoclinic P21/n (No. 14) space group with a = 9.012(2) , b = 12.891(3) , c = 14.055(3) , = 99.42(2), V= 1610.8(6) 3, and Z = 4. 1993, American Chemical Society. All rights reserved.
