Bioinspired CNP Iron(II) Pincers Relevant to [Fe]-Hydrogenase (Hmd): Effect of Dicarbonyl versus Monocarbonyl Motifs in H Activation and Transfer Hydrogenation.

A set of bioinspired carbamoyl CNP pincer complexes are reported that are relevant to [Fe]-hydrogenase (Hmd). The species [(CNP)Fe(CO)I] [R = Ph, ; R = Pr, ] undergoes ligand deprotonation, resulting in the dearomatized complexes of formulas [(CNP)Fe(CO)] ( and ). The crystal structure and H{P} NMR spectroscopy of the iodide-bound dearomatized species [Na(18-crown-6)][(CNP)Fe(CO)I] () showed that the deprotonated moiety was the phosphoramine N(H) linkage. Separately, the complexes (CNP)Fe(CO)(MeCN) ( and ) synthesized, as well as deprotonated and dearomatized in similar fashion. Reactivity studies revealed that the parent complexes require more forceful conditions for H activation, compared with the complexes. The ligand backbone was not found to participate in H activation and H → hydride transfer to an organic substrate was not observed in either case. Density functional theory calculations revealed that the higher reactivity of the complex in H splitting could be attributed to its higher affinity for H. This behavior is attributed to two key points related to the requisite (Fe) → σ*(H) back-bonding interaction in a conventional M-H Kubas interaction: (i) generally, the weaker π donor capacity of the dicarbonyls, and (ii) specifically, the detrimental effect of a strongly π acidic CO ligand (versus weakly π acidic MeCN ligand) trans to the H activation site. The higher reactivity of the complex is also evidenced by the catalytic transfer hydrogenation by , whereas was ineffective. Overall, the results suggest that Nature uses the dicarbonyl motif in [Fe]-hydrogenase to the interaction between the Fe center and dihydrogen, thereby preventing premature H activation prior to substrate (HMPT) binding and any resulting nonspecific hydride transfer reactivity.