Synthetic Advances Inspired by the Bioactive Dinitrosyl Iron Unit

Resulting from biochemical iron-NO interactions, dinitrosyl iron complexes (DNICs) are small organometallic-like molecules, considered to serve as vehicles for NO transport and storage in vivo. Formed by the interaction of NO with cellular iron sulfur clusters or with the cellular labile iron pool, DNICs have been documented to be the largest NO-derived adduct in cells, even surpassing the well-known nitrosothiols (RSNOs). Continuing efforts in biological chemistry are aimed at understanding the movement of DNICs in and out of cells, and their important role in NO-induced iron efflux leading to apoptosis in cells. Intrigued by the integrity of the unique dinitrosyl iron unit (DNIU) and the possibility of roles for it in human physiology or medicinal applications, the understanding of fundamental properties such as ligand effects on its ability to switch between two redox levels has been pursued through biomimetic complexes. Using metallodithiolates and N-heterocyclic carbenes (NHCs) as ligands to Fe(NO)(2), the synthesis of a library of novel DNICs, in both the oxidized, {Fe(NO)(2)}(9), and reduced, {Fe(NO)(2)}(10), forms (Enemark-Feltham notation), offers opportunity to examine structural, reactivity, and spectroscopic features. The raison detre for the MN2S2.Fe(NO)(2) synthesis development is for the potential to exploit the ease of accessing two redox levels on two different metal sites, a property presumably required for achieving two electron redox processes in base metals. Hence such molecules may be viewed as synthetic analogues of [NiFe]- or [FeFe]-hydrogenase active sites in nature, both of which use bridging thiolates for connection of the two centers. A particular success was the development of an Fe(NO)N2S2.Fe(NO)(2)(+/0) redox pair for proton reduction electrocatalysis. Monomeric, reduced NHC-DNICs of the L2Fe(NO)(2) type are synthesized via the Fe(CO)(2)(NO)(2) precursor, and oxidized thiolate-containing forms are derived from the dimeric (mu-RS)(2)[Fe(NO)(2)](2). Monomeric NHC-DNICs are four coordinate, pseudotetrahedral compounds with planar Fe(NO)(2) units in which the slightly bent Fe-NO groups are directed symmetrically inward at both redox levels. They serve as stable analogues of biological histidine binding sites. In agreement with IR data, Mossbauer spectroscopic parameters, and DFT computations, the prototypic NHC-DNICs indicate extensive delocalization of the electron density of iron via pi-backbonding. Such pi-delocalization presents an unusual reaction path for the one electron process of RS-/RSSR interconversion. Comparisons with imidazole-DNICs find NHCs to be the better ligands to Fe(NO)(2) and prompted investigations in (a) possible relationships between such imidazole- and NHC-containing DNICs, (b) systems that might mimic the reactivity of DNICs with the endogenous gaseotransmitter CO, and (c) mechanistic details of such processes. In a broader context, these studies aim to further describe the behavior of the {Fe(NO)(2)} unit as a single molecular entity when subjected to various ligand environments and reaction conditions.