STEREOELECTRONIC PREFERENCES IN ELECTRON-TRANSFER SERIES OF NICKEL WITH TRIDENTATE LIGANDS CONTAINING HARD-SOFT DONOR SETS

A pair of redox-active Ni(II) complexes containing mixed hard-soft donor sets in tridentate ligands have been prepared in order to elucidate ligand properties that promote the stabilization of multiple metal oxidation states within a single complex. The ligands Na(pscoo) (5-(diphenylphosphino)-3-thiapentanoate(1-)) and pspyr (4-(diphenylphosphino)-1-(2-pyridyl)-2-thiabutane) were synthesized in good yield by alkylations of psh (2-(diphenylphosphino)ethanethiol). The corresponding complexes [Ni(pscoo)(2)] (6)and [Ni(pspyr)(2)(BF4)(2)] ([8](BF4)(2)) were readily prepared. Also prepared was [Ni(pset)(2)](BF4)2 ([4](BF4)(2)) by alkylation of [Ni(ps)(2)]. Compound 6 crystallizes in monoclinic space group P21/c with a = 9.092(2) Angstrom, b = 24.585(7) Angstrom, 18.785(3) Angstrom, beta 94.11(2)degrees, and Z = 4. It has a distorted octahedral structure with idealized C-2 symmetry. The compound [8] (BF4)2 was obtained in triclinic space group P (1) over bar with a 10.064(3) Angstrom, b = 10.063(4) Angstrom, c 11.582(5) Angstrom, alpha 96.35(4)degrees, beta = 10.45(3)degrees, gamma = 114.54(3)degrees, and Z = 1. Complex [8](2+) has a centrosymmetric distorted octahedral structure. In methanol, 6 undergoes a reversible oxidation at E(1/2) = 0.66 V (vs SCE) to the Ni(III) species [6](+), which can be chemically generated by oxidation with (NH4)2[Ce(NO3)(6)] at -55 degrees C, retains the stereochemistry of 6, and has a (d(z)2)(1) ground state. Complex 6 cannot be reversibly reduced. In contrast, [8](2+) in propylene carbonate exhibits three chemically reversible redox steps, at E(1/2) = 0.93, -0.47, and -1.01 V, which are metal-centered and correspond to the couples Ni(III/II), Ni(II/I), and Ni(I/0), respectively. Complexes [8](3+) and [8](1+) were chemically generated at -55 degrees C by reaction with (NH4)2[Ce(NO3)(6)] and Cp(2)Co, respectively; EPR properties confirmed metal-centered redox steps. Characteristics of the electron transfer series based on [8](2+) include close approaches to electrochemical reversibility for Ni(III/II) and Ni(I/0) couples but quasi-reversible behavior for the Ni(II/I) couple. When the electrochemical behavior of the latter couple is compared to that of [4](2+), which is reduced in two essentially reversible steps, a structural change is indicated. The following series (coordination number indicated) is consistent with the experimental results: Ni(III)(6)reversible arrow Ni(II)(6) reversible arrow Ni(I)(4)reversible arrow Ni(0)(4). These results demonstrate that multiple oxidation states of nickel can be stabilized with coordinatively flexible hard-soft ligands that respond to the stereoelectronic preferences of the metal.