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 (Pr-i), 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 [Fe1(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)degrees in [Fe1(i)Pr(NO)](-) to 160.3(2)degrees in [Fe1(dmp)(NO)](-). The observed difference in angle is related to the cavity size: [Fe1(i)Pr(NO)](-) has the most restricted cavity with a small diameter (similar to 3 Angstrom) while [Fe1(dmp)(NO)](-) is more flexible and larger (similar to 8 Angstrom diameter). This angular difference is supported by EPR measurements which shows that [Fe1(dmp)(NO)](-) has a significantly more rhombic spectrum than that found for [Fe1(i)Pr(NO)](-). Magnetic moment, Mossbauer and EPR data on these {Fe-NO}(7) complexes agree with a electronic configuration of [Fe3+ d(5) HS (S = 5/2)-NO- (S = 1)] that lead to an S = 3/2 ground state via antiferromagnetic coupling.