The electronic structures of an isostructural series of octahedral nitrosyliron complexes {Fe-NO}6,7,8 elucidated by Mossbauer 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](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 {FeNO)(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)(N-3)(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 Mossbauer 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 NO2- (S = 1/2) In agreement with Solomon et al. (J. Am. Chem. Sec. 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 Mossbauer spectra.