Electrochemical Reduction and Oxidation in Noncoordinating and Coordinating Solvents of Two Closely Related Binuclear Nickel(II) Complexes Containing either Sulfur or Oxygen Endogenous Bridging Centers

Alan M. Bond, Ian S. Creece, Masa aki Haga, Richard Robson, Jenny C. Wilson

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Abstract

Replacement of oxygen by sulfur as the endogenous bridging center of the nickel binuclear complexes L(X)Ni2(pz) (X = O, S; pz = pyrazolate; L = binucleating ligand component with peripheral tert-butyl substituents to solubilize the complexes in nonpolar solvents) leads to remarkable differences in their solution and redox chemistry. Whereas the diamagnetic planar complex L(S)Ni2(pz) shows no detectable specific solvent interaction in coordinating solvents such as dimethylformamide (DMF), the oxygen analogue forms the paramagnetic species L(O)Ni2(pz)(DMF)2 in solution from which the solid monosolvate L(O)Ni2(pz)(DMF) can be isolated. The reversible half-wave potentials, E1/2r, for the reduction processes [formula ommited] as measured by voltammetric studies at platinum, gold, glassy-carbon, and mercury electrodes are separated by several hundred millivolts. In the noncoordinating nonpolar solvent dichloromethane, the reduced binuclear complexes are highly reactive. Nonspecific effects in polar solvents provide stability in the kinetic sense, whereas thermodynamic redox potentials are essentially solvent independent. By contrast, the two oxidation processes [formula ommited] have similar E1/2r values and exhibit substantial specific solvent effects as does a further two-electron process to generate a highly reactive formally nickel(IV) complex [L(S)Ni2(pz)]4+. The solvent dependence of the cyclic voltammetry of the L(O)Ni2(pz) complex is substantially more complex than that for the sulfur analogue. Temperature and scan rate dependence as well as the presence of additional processes are observed in coordinating solvents that reflect the strong specific solvation terms associated with the coexistence of a range of [L(O)Ni2(pz)(solvent)n]m complexes ( n= 0-4, m = 2+/+/0/-/2-) in coordinating solvents. Since the majority of voltammetric studies on nickel binuclear complexes have been conducted in solvents such as dimethylformamide or acetonitrile, the present information can be used to rationalize unexplained complexities previously reported on this important class of binuclear complex. Interestingly, the sulfur binuclear complex is more difficult to oxidize than the oxygen analogue, which is contrary to normal expectations. This illustrates the importance of the nature of the endogenous bridging centers in binuclear complexes.

Original languageEnglish
Pages (from-to)712-721
Number of pages10
JournalInorganic Chemistry
Volume27
Issue number4
DOIs
Publication statusPublished - 1 Feb 1988
Externally publishedYes

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