Abstract
Copper(II) (3d9, S = 1/2) complexes are stable and widely investigated by electron paramagnetic resonance (EPR) spectroscopy. In contrast, isoelectronic nickel(I) is much less common and much less investigated. Nickel(I), however, is of biological interest as the active site of methyl coenzyme M reductase (MCR) contains a tetraaza macrocyclic ligand, F430, which coordinates NiI in its active form, MCRred1. As result, the redox behavior and spectroscopy of tetraaza macrocyclic complexes of nickel is of importance in biomimetic chemistry. Such efforts are complicated by the difficulty in generating NiI from their stable, NiII, precursors. Reduction of NiII macrocyclic complexes can afford NiI in certain cases, but in many other cases can lead instead to reduction of the macrocycle to generate an organic radical anion. Previous studies on the formation of tetraaza macrocyclic complexes of NiI are discussed in terms of the competition between metal-centered and ligand-centered reduction. EPR results are particularly important in making the distinction between these two reduction processes, as formation of NiI gives characteristic EPR spectra similar to those for CuII, while ligand-centered reduction gives narrow EPR spectra at g = 2.00, typical of organic radicals. Even if metal-centered reduction occurs, the geometry of the resulting NiI macrocyclic complex is highly variable and, as a result, the EPR spectral appearance is highly variable. In this case, the comparison is between the extremes of spectra typical for tetragonally distorted complexes (<img src="/img/revistas/jbchs/v21n7/a02img11.gif" align=absmiddle> ground state, which includes tetragonally distorted octahedral, square pyramidal and square planar geometries) and those for trigonal bipyramidal complexes (<img src="/img/revistas/jbchs/v21n7/a02img12.gif" align=absmiddle> ground state). Previous work on CuII was related to the situation for NiI. The different types of EPR spectra for such systems are specifically discussed using previously unpublished examples of several tetraaza macrocyclic complexes of nickel, including F430 and MCR itself.
Highlights
Renner et al.44 prepared hexahydro- and octahydroporphyrins (structural diagrams shown below; note that there are two regioisomers of the hexahydroporphyrin (CSD code: KODHAM), depending on which one of the two meso alkenes is reduced; both are reduced in the octahydroporphyrin shown on the right; the hydrogens added to the meso positions are not shown). These tetraaza macrocycles are less p-conjugated than the iBCs and reproduce the structure of F430 as closely as one could reasonably hope for, yet they yield even more purely ligand centered (p-anion) radicals upon reduction, as shown by electron paramagnetic resonance (EPR) spectra that consist of a narrow signal at
We describe here EPR studies on several macrocyclic complexes of nickel that span a variety of tetraaza macrocycle coordination
The anaerobic organisms that are the source of methyl CoM reductase (MCR) can be grown on medium enriched in, e.g., 61Ni (I = 3/2, 1.13% natural abundance), whereas chemical synthesis using such isotopes is very expensive
Summary
Electron paramagnetic resonance (EPR) spectroscopy has been widely applied over the past six decades to the study of coordination complexes of the d block (transition metal) ions. Among the many possible dn electronic configurations found, the d9 configuration has been well studied. This is the case for several reasons, chemical and physical. The d9 (S = 1/2) configuration is very amenable to study by EPR spectroscopy since there are no complications from intermolecular electronelectron interactions in mononuclear complexes. Mononuclear complexes with multiple electron/holes, such as those with the d8 electronic configuration (NiII in many coordination environments, such as tetrahedral and octahedral), often exhibit complicated intramolecular electron-electron interactions that arise from spin-orbit and spin-spin coupling.. Mononuclear complexes with multiple electron/holes, such as those with the d8 electronic configuration (NiII in many coordination environments, such as tetrahedral and octahedral), often exhibit complicated intramolecular electron-electron interactions that arise from spin-orbit and spin-spin coupling.1,15 These effects can lead to significant zerofield splitting (zfs) and difficulty in obtaining EPR spectra at conventional microwave frequencies The crystal structure of the pentamethylester of F430, F430M, has been reported (as the 12,13-diepimer, since this is the thermally stable form; CSD code: KOBCEJ).
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