Synthesis and vibrational spectroscopy of 57Fe-labeled models of [NiFe] hydrogenase: first direct observation of a nickeliron interaction

Despite our extremely low atmospheric concentration of dihydrogen (B1 ppm), this substrate is a key metabolite of many anaerobic bacteria. In such living systems can be found the most prevalent enzymes for hydrogen processing, the nickel–iron hydrogenases ([NiFe]–H2ases). 1,2 These electrocatalysts specifically mediate the redox reaction H2 " 2H + + 2e at several hundred turnovers per second. Their heterobimetallic active sites exist in several states, some of which are summarized below (Fig. 1, left and centre). The active sites feature Ni bound to four cysteinato residues, two of which bridge to an Fe(CO)(CN)2 fragment. In the Ni–C state, Ni(III)Fe(II) centres bind a bridging hydride (H ), reductive elimination of which affords Ni–L. Thus, H is abstracted by a terminal cys ligand (Fig. 1, centre top), leaving a Ni(I)Fe(II) core with a 2e bond between the metals. The use of vibrational spectroscopy to study [NiFe]–H2ase is convenient in that its active site features chromophores easily identifiable by such techniques. Spectral analyses are often aided by comparison to data from synthetic models whose structures are well understood. Specificity for Fe-coupled modes is afforded by nuclear resonance vibrational spectroscopy (NRVS, vide infra). This has recently enabled observation of characteristic Fe–CN/ Fe–CO bending and stretching modes in [NiFe]–H2ase (Ni–A and Ni–R) and the Fe subsite model [Fe(benzenedithiolato)(CN)2CO] 2 . However, no NRVS studies have reported on metal–metal bonding, which is expected for low-valent clusters like Ni–L and itsmodels. A near-complete [NiFe]–H2ase mimic is the Ni(II)Fe(I) species [(OC)3Fe(pdt)Ni(dppe)] + ([1], pdt = S(CH2)3S ; dppe = 1,2-bis(diphenylphosphino)ethane), a model for Ni–L, albeit with metal oxidation states reversed (Fig. 1, right). This S = 1/2 model is prepared from (OC)3Fe(pdt)Ni(dppe) (1), 9 itself the subject of density functional theory (DFT) and resonance Raman investigations. Disclosed here is methodology for Fe-labeled prototypes [(OC)3 Fe(pdt)Ni(dppe)] ([1]), enabling the study of metal– metal bonding with NRVS. The Ni(I)Fe(I) complex 1 is usually accessed by interaction of (pdt)Ni(dppe) with an Fe carbonyl such as Fe2(CO)9 or Fe(CO)4I2. 11–14