Structure and Magnetic Properties of Trigonal Bipyramidal Iron Nitrosyl Complexes

Non-heme Fe(II) proteins form stable nitrosyl adducts. However, there are few synthetic examples of structurally characterized non-heme iron nitrosyl complexes, especially those that are coordinatively unsaturated. Presented herein is a series of non-heme {Fe-NO}7 complexes with trigonal bipyramidal coordination geometry. These complexes were synthesized with tripodal ligands derived from tris(N-R-carbamoylmethy)amine [1R]3where R ) isopropyl (iPr), cyclopentyl (cyp), or 3,5-dimethylphenyl (dmp) groups. The R groups of these [1R]3ligands form cavities around the metal ion that can influence structural and functional properties. The nitrosyl complexes were synthesized by treating the Fe(II) precursors [Fe1R]with NO at room temperature. These iron nitrosyl complexes have similar molecular structures as determined by X-ray diffraction methods. They differ in their Fe-N-O angles which range from 178.2(5)° in [Fe1iPr(NO)]to 160.3(2)° in [Fe1dmp(NO)]-. The observed difference in angle is related to the cavity size: [Fe1iPr(NO)]has the most restricted cavity with a small diameter (∼ 3 Å) while [Fe1dmp(NO)]is more flexible and larger (∼8 Å diameter). This angular difference is supported by EPR measurements which shows that [Fe1dmp(NO)]has a significantly more rhombic spectrum than that found for [Fe1iPr(NO)]-. Magnetic moment, Mössbauer and EPR data on these {Fe-NO}7 complexes agree with a electronic configuration of [Fe3+ d5 HS (S ) /2)-NO (S ) 1)] that lead to an S ) /2 ground state via antiferromagnetic coupling.