A Structural and Mössbauer Study of Complexes with Fe2(μ-O(H))2 Cores: Stepwise Oxidation from FeII(μ-OH)2FeII through FeII(μ-OH)2FeIII to FeIII(μ-O)(μ-OH)FeIII

Publisher Website
Google Scholar
Abstract
Dinuclear non-heme iron clusters containing oxo, hydroxo, or carboxylato bridges are found in a number of enzymes involved in O2 metabolism such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases. Efforts to model structural and/or functional features of the protein-bound clusters have prompted the preparation and study of complexes that contain Fe(μ-O(H))2Fe cores. Here we report the structures and spectroscopic properties of a family of diiron complexes with the same tetradentate N4 ligand in one ligand topology, namely [(α-BPMCN)2FeII2(μ-OH)2](CF3SO3)2 (1), [(α-BPMCN)2FeIIFeIII(μ-OH)2](CF3SO3)3 (2), and [(α-BPMCN)2FeIII2(μ-O)(μ-OH)](CF3SO3)3 (3) (BPMCN = N,N‘-dimethyl-N,N‘-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane). Stepwise one-electron oxidations of 1 to 2 and then to 3 demonstrate the versatility of the Fe(μ-O(H))2Fe diamond core to support a number of oxidation states with little structural rearrangement. Insight into the electronic structure of 1, 2‘, and 3 has been obtained from a detailed Mössbauer investigation (2‘ differs from 2 in having a different complement of counterions). Mixed-valence complex 2‘ is ferromagnetically coupled, with J = −15 ± 5 cm-1 (H = JS1·S2). For the S = 9/2 ground multiplet we have determined the zero-field splitting parameter, D9/2 = −1.5 ± 0.1 cm-1, and the hyperfine parameters of the ferric and ferrous sites. For T < 12 K, the S = 9/2 multiplet has uncommon relaxation behavior. Thus, MS = −9/2 ↔ MS = +9/2 ground state transition is slow while ΔMS = ±1 transitions between equally signed MS levels are fast on the time scale of Mössbauer spectroscopy. Below 100 K, complex 2‘ is trapped in the Fe1IIIFe2II ground state; above this temperature, it exhibits thermally assisted electron hopping into the state Fe1IIFe2III. The temperature dependence of the isomer shifts was corrected for second-order Doppler shift, obtained from the study of diferrous 1. The resultant true shifts were analyzed in a two-state hopping model. The diferric complex 3 is antiferromagnetically coupled with J = 90 ± 15 cm-1, estimated from a variable-temperature Mössbauer analysis.

This publication has 24 references indexed in Scilit: