Overview of ligand versus metal centered redox reactions in tetraaza macrocyclic complexes of nickel with a focus on electron paramagnetic resonance studies

Abstract

Complexos de cobre(II) (3d 9 , S = 1/2) são estáveis e amplamente investigados por espectroscopia de ressonância paramagnética eletrônica (EPR). Já o isoeletrônico níquel(I) é muito menos comum e muito menos estudado. No entanto, níquel(I) tem interesse biológico, uma vez que o sítio ativo da metil coenzima M redutase (MCR) , 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, F 430 , which coordinates Ni I in its active form, MCR red1 . 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 Ni I from their stable, Ni II , precursors. Reduction of Ni II macrocyclic complexes can afford Ni I 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 Ni I are discussed in terms of the competition between metal-centered and ligandcentered reduction. EPR results are particularly important in making the distinction between these two reduction processes, as formation of Ni I gives characteristic EPR spectra similar to those for Cu II , 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 Ni I 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 (dx2-y2 1 ground state, which includes tetragonally distorted octahedral, square pyramidal and square planar geometries) and those for trigonal bipyramidal complexes (dz2 1 ground state). Previous work on Cu II was related to the situation for Ni I . The different types of EPR spectra for such systems are specifically discussed using previously unpublished examples of several tetraaza macrocyclic complexes of nickel, including F 430 and MCR itself.

Synthesis and Characterization of Heteromultinuclear Ni/M Clusters (M = Fe, Ru, W) Including a Paramagnetic (NHC)Ni–WCp*(CO)3 Heterobinuclear Complex

“…The X-band EPR spectrum of 7 in 2-methyltetrahydrofuran frozen solution is shown in Figure . The spectrum is rhombic; however, the g value pattern is characteristic of a d 9 metal ion with a singly occupied molecular orbital (SOMO) of d x 2 – y 2 character, as typically seen for Cu II , and of particular relevance here for Ni I . Particularly notable are two weak satellite features associated only with g mid .…”

Dianionic States of Trithiadodecaazahexaphyrin Complexes with Homotrinuclear MII3O Clusters (M = Ni and Cu): Crystal Structures, Metal- Or Macrocycle-Centered Reduction, and Doublet–Quartet Transitions in the Dianions

“…Previously, many reduced mononuclear Ni I species with tetrapyrrolic ligands were studied . These species show EPR signals with g ∥ and g ⊥ or even three-component signals with g x , g y , and g z , but the g -factors of these signals are higher than 2.000 ( g ∥ = 2.2 and g ⊥ = 2.05) . The Ni I state in 1 is delocalized over three nickel atoms within the Ni 3 O triangles, averaging out any asymmetry of this signal.…”

Influence of Ring-Expanded N-Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study

“…Note that the g -values of d 9 Ni­(I) metal centers are highly dependent on the ligand coordination environment and many EPR studies of complexes bearing Ni­(I) centers that are 4-, 5- or 6-coordinated have been reported over the years, , with some recent studies focusing on the catalytic role of Ni­(I). , By comparison, far fewer studies have been reported for low coordinate Ni­(I) complexes. ,− One characteristic trend observed with the 3-coordinate systems is that reversed g -values are frequently observed, compared to the higher coordinate systems, and the g -tensor components are very dependent on whether the complex adopts a T- or Y-shaped arrangement around the Ni­(I) center. , This case of reversed EPR g -values has been previously reported for CpNi­(I) complexes (Table ). − The observed g values are consistent with the spin density plots for 1 – 3 , which revealed considerable spin density localization on the nickel centers (see Figure S17 in the Supporting Information).…”

“…Note that the g-values of d 9 Ni(I) metal centers are highly dependent on the ligand coordination environment and many EPR studies of complexes bearing Ni(I) centers that are 4-, 5or 6-coordinated have been reported over the years, 104,105 with some recent studies focusing on the catalytic role of Ni(I). 106,107 By comparison, far fewer studies have been reported for low coordinate Ni(I) complexes.…”