Oxidation and reduction behavior of macrocyclic complexes of nickel. Electrochemical and electron spin resonance studies

Abstract

Electrochemical and electron spin resonance studies have been carried out on an extensive series of macrocyclic complexes of nickel, which vary in the nature and degree of ligand unsaturation, charge type, and ring size. Oxidation of complexes containing neutral and dianion ligand systems produces stable six-coordinate and square planar nickel(III) species, respectively. The one-electron reduction products of the parent nickel(II) macrocycles exist as either d9 nickel® complexes or as metal stabilized anion radicals, depending upon the nature of the ligand unsaturation. The overall redox behavior of the family of macrocycles is discussed in terms of their chemical reactivity patterns, stereochemistry, and charge type. Comparisons are made to other similarly structured complex types including the tetraphenylporphyrin complexes of nickel. The significant chemical behavior of transition metal complexes very often depends on their facile redox properties. This is true to a large degree for the natural and synthetic complexes involving macrocyclic ligands. These substances undergo a diverse array of chemical reactions, such as ligand oxidative dehydrogenation,1-5 metal alkylation,6-6 ligand substitution,10-12 and hydrogenation.13-15 The success of some of these reactions is intimately linked with the ability of higher and lower oxidation states of these complexes to function as reactive intermediates.The ability of macrocyclic ligands to stabilize a wide range of oxidation states of a coordinated metal ion has been amply demonstrated by the studies of Olson and Vasilevskis.16 Simultaneous and subsequent work conducted in our laboratories1 11 17 18 and by Endicott and coworkers19 has proven the generality of their observations. It becomes highly significant, therefore, when the availability of an extensive series of macrocyclic complexes permits an opportunity, to provide a sound chemical basis for the evaluation of redox-structure relationships.We present here a (1) J.

Soluble and membrane‐bound paramagnetic centers in Methanobacterium Bryantii

“…EPR examination of these Ni(II1) complexes in fact shows spectra very similar to the signal reported here [20,22,23], indicative of low-spin Ni(II1) in an octahedral geometry with both tetragonal and rhombic distortions. Nitrogen hyperfme lines in this sharp signal are not observed, as is common for the g, feature with model complexes having nitrogenous axial ligands [20,22,23]. This, however, does not rule out this possibility, since such splittings are not always seen.…”

“…With regard to the radical signal which appears upon dithionite addition, reduction of Ni(I1) model complexes containing conjugated systems (e.g., diimines) results in the appearance of a radical-type g = 2.002 signal, rather than the anisotropic signal of a Ni(1) d9 metal complex [20]. This indicates that the electron is highly delocalized and has predominantly ligand character.…”

“…The Ni(II)-Ni(II1) redox couple is normally very positive in potential, so that Ni(II1) would be quite an unlikely species. However, it is known that formation of coordinate complexes of Ni(I1) by chelation to a variety of compounds inclu~ng macrocyclic ligands [20,21] as well as small peptides f22,23] and also bleomycin [23] makes quantitative oxidation possible. EPR examination of these Ni(II1) complexes in fact shows spectra very similar to the signal reported here [20,22,23], indicative of low-spin Ni(II1) in an octahedral geometry with both tetragonal and rhombic distortions.…”

“…However, it is known that formation of coordinate complexes of Ni(I1) by chelation to a variety of compounds inclu~ng macrocyclic ligands [20,21] as well as small peptides f22,23] and also bleomycin [23] makes quantitative oxidation possible. EPR examination of these Ni(II1) complexes in fact shows spectra very similar to the signal reported here [20,22,23], indicative of low-spin Ni(II1) in an octahedral geometry with both tetragonal and rhombic distortions. Nitrogen hyperfme lines in this sharp signal are not observed, as is common for the g, feature with model complexes having nitrogenous axial ligands [20,22,23].…”

Copper (II) complexes of a macrocyclic binucleating ligand which exhibit two-electron oxidation and two-electron reduction. Structure of [Cu2(C24H34N4O2)(CH3OH)2](ClO4)2, a macrocyclic dicopper(II) complex involving coordinated methanol

“…2H20, and [Cu2(L1)(C104)21 (VI) (El,, = -0.43 V, -0.58 V , -0.76 V, respectively) (16). This trend can be interpreted in terms of a stabilising effect of the copper([) state with increasing unsaturation and has been observed before for macrocyclic (N4), mononuclear copper(I1) (34), and nickel(I1) (35) complexes. The copper-nitrogen bonds can be considered to be more strongly polarized when the nitrogen centres are unsaturated, which would lead to a situation where the copper(I1) ions would become reduced more readily.…”