The significant role that nitric oxide plays in human physiology is linked to the ability of NO to bind to iron forming mono-nitrosyl iron complexes. Protein-bound and low-molecular-weight dinitrosyl iron complexes (DNICs) are known to form in excess NO. Studies of such biological DNICs have relied on their paramagnetism and characteristic EPR signal of g value of 2.03. It has been suggested that DNICs act in vivo as NO storage (when protein-bound) and transfer agents (when released by, for example, free cysteine). Biological DNICs, mainly resulting from iron-sulfur cluster degradation, are difficult to extract and isolate, thereby preventing their full characterization. Thus, development of synthetic DNICs is a promising approach to model and better understand the formation and function of biological DNICs, the scope of donor ligands that might coexist with Fe(NO)2 units, the redox levels of bio-DNICs, and establish other spectroscopic techniques appropriate for characterization. A series of N-heterocyclic carbene (NHC) and imidazole (Imid) complexes has been characterized as mimics of histidine-containing DNICs. The pseudo-tetrahedral L2Fe(NO)2 complexes have NO stretching frequencies and redox potentials that suggest the NHCs are slightly better donors than Imids, however the two types of ligands have similar steric properties. Both the EPR-active, {Fe(NO)2}9 and the EPR-silent, {Fe(NO)2}10 states can be accessed and stabilized by the NHC. Nitric oxide transfer studies have shown that only the {Fe(NO)2}9 complexes are capable of transferring NO to a suitable NO trapping agent. Deprotonation of the distal nitrogen functionality in the imidazolate ligands of [(Imidazole)2Fe(NO)2]- leads to aggregation forming molecular squares of {Fe(NO)2}9 units bridged by the imidazolates. These interesting tetrameric complexes are examined by X-ray diffraction, EPR, and Mossbauer studies. The paramagnetic tetrameric complexes have multiple redox events observed by cyclic voltammetry. Mossbauer spectral data of the tetrameric complexes are compared with Mossbauer data obtained for a series of NHC-containing DNICs. Iron and cobalt-containing mononitrosyl N2S2 model complexes of the nitrile hydratase enzyme active site demonstrate sulfur-based reactivity resulting in the formation of polymetallic complexes. In all cases, shifts in the nitrosyl stretching frequencies demonstrate substantial transfer of electron density from the (NO)M(N2S2) moiety to the metal-acceptor site.
