Challenges in the Synthesis of Active Site Mimics for [NiFe]-Hydrogenases

One of the more active areas in bioorganometallic chemistry is the preparation and reactivity studies of active site mimics of the [NiFe]-hydrogenases. One area of particular recent progress involves reactions that interconvert Ni(μ-X)Fe centers for X = OH, H, CO, as described by Song et al. Such reactions illustrate new ways to access intermediates related to the Ni-R and Ni-SI states of the enzyme. Most models are derivatives of the type (diphosphine)Ni(SR)2Fe(CO)3-n(PR′3)n. In recent work, the methodology has been generalized to include FeII(diphosphine) derivatives of Ni(N2S2), where N2S22- is the tetradentate diamine-dithiolate (CH2N(CH3)CH2CH2S-)2. Indeed, models based on Ni(N2S2) have proven valuable, but these studies also highlight challenges in working with heterobimetallic complexes, specifically the tendency of some such Ni-Fe complexes to convert to homometalliic Ni-Ni derivatives. This kind of problem is not readily detected by X-ray crystallography. With this caution in mind, we argue that one series of complexes recently described in this journal are almost certainly misassigned.