The reactivity of protein bound iron−sulfur clusters with nitric oxide (NO) is well documented, but little is known about the actual mechanism of cluster nitrosylation. Here, we report studies of members of the Wbl family of [4Fe−4S] containing proteins, which play key roles in regulating developmental processes in actinomycetes, including Streptomyces and Mycobacteria, and have been shown to be NO responsive. Streptomyces coelicolor WhiD and Mycobacterium tuberculosis WhiB1 react extremely rapidly with NO in a multiphasic reaction involving, remarkably, 8 NO molecules per [4Fe−4S] cluster. The reaction is 104-fold faster than that observed with O2 and is by far the most rapid iron−sulfur cluster nitrosylation reaction reported to date. An overall stoichiometry of [Fe4S4(Cys)4]2− + 8NO → 2[FeI2(NO)4(Cys)2]0 + S2− + 3S0 has been established by determination of the sulfur products and their oxidation states. Kinetic analysis leads to a four-step mechanism that accounts for the observed NO dependence. DFT calculations suggest the possibility that the nitrosylation product is a novel cluster [FeI4(NO)8(Cys)4]0 derived by dimerization of a pair of Roussin’s red ester (RRE) complexes.

Paramagnetic hydrides are likely intermediates in hydrogen-evolving enzymic and molecular systems. Herein we report the first spectroscopic characterization of well-defined paramagnetic bridging hydrides. Time-resolved FTIR spectroelectrochemical experiments on a subsecond time scale revealed that single-electron transfer to the μ-hydride di-iron dithiolate complex 1 generates a 37-electron valence-delocalized species with no gross structural reorganization of the coordination sphere. DFT calculations support and (1)H and (2)H EPR measurements confirmed the formation an S = ½ paramagnetic complex (g = 2.0066) in which the unpaired spin density is essentially symmetrically distributed over the two iron atoms with strong hyperfine coupling to the bridging hydride (A(iso) = -75.8 MHz).

Natural model: The synthesis of a close structural analogue of the active site of [Fe]‐hydrogenase is described (see structure; C gray, H dark blue, Fe green, N light blue, O red, S yellow). Nature most probably constructs the five membered ferracyclic ring to poise the 2‐hydroxy pyridine substituent in a position to assist the heterolytic cleavage of dihydrogen, and the accessibility of the analogue should now provide opportunities for probing this.

Results of density functional theory (DFT) calculations on the protonation of the [FeFe]-hydrogenase model complex, Fe2(µ-pdt)(CO)4(PMe3)2 (pdt = propane-1,3-dithiolate), show that diiron bridging-hydride species are more stable than iron terminal-hydride, sulfur-hydride, or formyl isomers. Consistent with experimental observation, the transoid basal/basal forms are more stable than other µ-H isomers. With an ether as the proton carrier, [Et2OH]+, the favoured reaction pathways appear to involve weak coordination to CO followed by transfer of the proton from ether to an iron terminal site rather than directly to the bridging site. These kinetically favoured terminal-hydride species isomerize through a low-energy Ray-Dutt twist to produce the apical/basal bridging-hydride isomer. This isomer rearranges over somewhat higher barrier Bailar twists to the cisoid and transoid basal/basal isomers, the former finally rearranging to the latter isomer