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 [ 1 R ] 3 - where R = isopropyl ( i Pr), cyclopentyl (cyp), or 3,5-dimethylphenyl (dmp) groups. The R groups of these [ 1 R ] 3 - ligands form cavities around the metal ion that can influence structural and functional properties. The nitrosyl complexes were synthesized by treating the Fe(II) precursors [Fe 1 R ] - 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 [Fe 1 i Pr(NO)] - to 160.3(2)° in [Fe 1 dmp (NO)] - . The observed difference in angle is related to the cavity size: [Fe 1 i Pr(NO)] - has the most restricted cavity with a small diameter (∼ 3 Å) while [Fe 1 dmp (NO)] - is more flexible and larger (∼8 Å diameter). This angular difference is supported by EPR measurements which shows that [Fe 1 dmp (NO)] - has a significantly more rhombic spectrum than that found for [Fe 1 i Pr(NO)] - . Magnetic moment, Mössbauer and EPR data on these {Fe−NO} 7 complexes agree with a electronic configuration of [Fe 3+ d 5 HS ( S = 5 / 2 )−NO - ( S = 1)] that lead to an S = 3 / 2 ground state via antiferromagnetic coupling.