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

From the reaction of cis -[(cyclam)Fe III (Cl) 2 ]Cl (cyclam = 1,4,8,11-tetraazacyclotetradecane) with hydroxylamine in water the octahedral nitrosyliron complexes trans -[(cyclam)Fe(NO)Cl](ClO 4 ) ( 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 CH 3 CN solutions by UV−vis and EPR spectroscopy; both possess a singlet ground state. All of these nitrosyliron complexes, including [LFe(NO)(N 3 ) 2 ] ( S = 3 / 2; L = 1,4,7-trimethyl-1,4,7-triazacyclononane) and [L‘Fe(NO)(ONO)(NO 2 )](ClO 4 ) ( 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 Fe IV ( 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 NO 2 - ( 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.