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

Audria Stubna, Du Hwan Jo, Miquel Costas, William W. Brenessel, Hanspeter Andres, Emile L. Bominaar, Eckard Münck, Lawrence Que

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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](CF 3SO3)2 (1), [(α-BPMCN)2Fe IIFeIII(μ-OH)2](CF3SO 3)3 (2), and [(α-BPMCN)2Fe III2(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 ΔM s = ±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.

Original languageEnglish (US)
Pages (from-to)3067-3079
Number of pages13
JournalInorganic Chemistry
Volume43
Issue number10
DOIs
StatePublished - May 17 2004

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