The Hydrophilic Phosphatriazaadamantane Ligand in the Development of H₂ Production Electrocatalysts: Iron Hydrogenase Model Complexes

As functional biomimics of the hydrogen-producing capability of the dinuclear active site in [Fe]H2ase, the FeIFeI organometallic complexes, (μ-pdt)[Fe(CO)2PTA]2, 1-PTA2, (pdt = SCH2CH2CH2S; PTA = 1,3,5-triaza-7-phosphaadamantane), and (μ-pdt)[Fe(CO)3][Fe(CO)2PTA], 1-PTA, were synthesized and fully characterized. For comparison to the hydrophobic (μ-pdt)[Fe(CO)2(PMe3)]2 and {(μ-H)(μ-pdt)[Fe(CO)2(PMe3)]2}+ analogues, electrochemical responses of 1-PTA2 and 1-(PTA·H+)2 were recorded in acetonitrile and in acetonitrile/water mixtures in the absence and presence of acetic acid. The production of H2 and the dependence of current on acid concentration indicated that the complexes were solution electrocatalysts that decreased over-voltage for H+ reduction from HOAc in CH3CN by up to 600 mV. The most effective electrocatalyst is the asymmetric 1-PTA species, which promotes H2 formation from HOAc (pKa in CH3CN = 22.6) at −1.4 V in CH3CN/H2O mixtures at the Fe0FeI redox level. Functionalization of the PTA ligand via N-protonation or N-methylation, generating (μ-pdt)[Fe(CO)2(PTA-H+)]2, 1-(PTA·H+)2, and (μ-pdt)[Fe(CO)2(PTA-CH3+)]2, 1-(PTA-Me+)2, provided no obvious advantages for the electrocatalysis because in both cases the parent complex is reclaimed during one cycle under the electrochemical conditions and H2 production catalysis develops from the neutral species. The order of proton/electron addition to the catalyst, i.e., the electrochemical mechanism, is dependent on the extent of P-donor ligand substitution and on the acid strength. Cyclic voltammetric curve-crossing phenomena was observed and analyzed in terms of the possible presence of an η2-H2−FeIIFeI species, derived from reduction of the FeIFeI parent complex to Fe0FeI followed by uptake of two protons in an ECCE mechanism.