Iron Hydride Detection and Intramolecular Hydride Transfer in a Synthetic Model of Mono-Iron Hydrogenase with a CNS Chelate

Abstract

We report the identification and reactivity of an iron hydride species in a synthetic model complex of monoiron hydrogenase. The hydride complex is derived from a phosphine-free CNS chelate that includes a Fe-C(NH)(═O) bond (carbamoyl) as a mimic of the active site iron acyl. The reaction of [((O═)C(HN)N(py)S(Me))Fe(CO)2(Br)] (1) with NaHBEt3 generates the iron hydride intermediate [((O═)C(HN)N(py)S(Me))Fe(H)(CO)2] (2; δFe-H = -5.08 ppm). Above -40 °C, the hydride species extrudes CH3S(-) via intramolecular hydride transfer, which is stoichiometrically trapped in the structurally characterized dimer μ2-(CH3S)2-[((O═)C(HN)N(Ph))Fe(CO)2]2 (3). Alternately, when activated by base ((t)BuOK), 1 undergoes desulfurization to form a cyclometalated species, [((O═)C(NH)NC(Ph))Fe(CO)2] (5); derivatization of 5 with PPh3 affords the structurally characterized species [((O═)C(NH)NC)Fe(CO)(PPh3)2] (6), indicating complex 6 as the common intermediate along each pathway of desulfurization.

Scaffold-based [Fe]-hydrogenase model: H2 activation initiates Fe(0)-hydride extrusion and non-biomimetic hydride transfer

“…Consistent with our previous work, 18 detection of the highly reactive (especially anionic) Fe–H species of [Fe]-hydrogenase synthetic models is difficult. Intriguingly, this reductive pathway contrasts the well-characterized intramolecular hydride transfer reaction resulting in methylthiol extrusion observed in another model system from our group ( mer -CNS; no scaffold), 35 likely due to the unbound state of the thioether-S Me unit downstream of 1 in this case.…”

Functional Hydride Transfer by a Thiolate‐Containing Model of Mono‐Iron Hydrogenase featuring an Anthracene Scaffold

“…First, as D 2 activation is not observed in the absence of base, this suggests that the iron‐bound sulfur is insufficiently basic to drive the heterolysis of D 2 ; a moderately basic proton acceptor must be included to drive the reaction. Second, in two previous reports we spectroscopically observed iron‐hydride species derived from thioether‐based ligands, thus formulated as neutral complexes; such species were not competent hydride transfer agents . However, the putative iron–hydride in this system is an anion (Scheme ) as a result of the thiolato‐S donor, and evidently is a competent hydride transfer agent.…”

“…Second, in two previous reports we spectroscopically observed ironhydride species derived from thioether-based ligands,t hus formulated as neutral complexes;s uch species were not competent hydride transfer agents. [9,13] However,the putative iron-hydride in this system is an anion (Scheme 3) as aresult of the thiolato-S donor, and evidently is acompetent hydride transfer agent.…”

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

“…415 Similar in structure to [ 60 ], the proposed kinetic product could not be characterized, as it rapidly converted to the cyclometalated species [ 62 ]. The latter product can also be prepared from hydride [ 61 (H)H] ( δ ( 1 H) −5.1 ppm), which, in turn, is generated from [ 61 HBr] and HBEt 3 − .…”