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The Electronic Structures of an Isostructural Series of Octahedral Nitrosyliron Complexes {Fe−NO}6,7,8 Elucidated by Mössbauer Spectroscopy

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posted on 22.04.2000 by Christina Hauser, Thorsten Glaser, Eckhard Bill, Thomas Weyhermüller, Karl Wieghardt
From the reaction of cis-[(cyclam)FeIII(Cl)2]Cl (cyclam = 1,4,8,11-tetraazacyclotetradecane) with hydroxylamine in water the octahedral nitrosyliron complexes trans-[(cyclam)Fe(NO)Cl](ClO4) (1) and cis-[(cyclam)Fe(NO)I]I (2) have been isolated as crystalline solids. EPR spectroscopy and variable-temperature susceptibility measurements established that 1 possesses an S = 1/2 and 2 an S = 3/2 ground state; both species are of the {Fe−NO}7 type. Electrochemically, 1 can be reversibly one-electron oxidized yielding trans-[(cyclam)Fe(NO)Cl]2+, an {Fe−NO}6 species, and one-electron reduced yielding trans-[(cyclam)Fe(NO)Cl]0, an {Fe−NO}8 species. These complexes have been characterized in CH3CN solutions by UV−vis and EPR spectroscopy; both possess a singlet ground state. All of these nitrosyliron complexes, including [LFe(NO)(N3)2] (S = 3/2; L = 1,4,7-trimethyl-1,4,7-triazacyclononane) and [L‘Fe(NO)(ONO)(NO2)](ClO4) (S = 0; L‘ = 1,4,7-triazacyclononane), have been studied by variable-temperature Mössbauer spectroscopy both in zero and applied fields. The oxidation of 1 is best described as metal-centered yielding a complex with an FeIV (S = 1) coupled antiferromagnetically to an NO- (S = 1), whereas its reduction is ligand-centered and yields a species with a low-spin ferric ion (S = 1/2) antiferromagnetically coupled to an NO2- (S = 1/2). In agreement with Solomon et al. (J. Am. Chem. Soc. 1995, 117, 715) both {Fe−NO}7 (S = 3/2) species in this work are described as high-spin ferric (S = 5/2) antiferromagnetically coupled to an NO- (S = 1). Complex 1 is proposed to contain an intermediate spin ferric ion (S = 3/2) antiferromagnetically coupled to NO- (S = 1). The alternative descriptions as low-spin ferric antiferromagnetically coupled to NO- (S = 1) or low-spin ferric with an NO- (S = 0) ligand are ruled out by the applied field Mössbauer spectra.