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]H(2)ase, the (FeFeI)-Fe-I organometallic complexes, (mu-pdt)[Fe(CO)(2)PTA](2), 1-PTA(2), (pdt = SCH2CH2CH2S; PTA = 1,3,5-triaza-7-phosphaadamantane), and (mu-pdt)[Fe(CO)(3)][Fe(CO)(2)PTA], 1-PTA, were synthesized and fully characterized. For comparison to the hydrophobic (mu-pdt)[Fe(CO)(2)(PMe3)](2) and {(mu-H)(mu-pdt)[Fe(CO)(2)(PMe3)](2)}(+) analogues, electrochemical responses of 1-PTA(2) 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 H-2 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 H-2 formation from HOAc (pK(a) in CH3CN = 22.6) at -1.4 V in CH3CN/H2O mixtures at the (FeFeI)-Fe-0 redox level. Functionalization of the PTA ligand via N-protonation or N-methylation, generating (mu-pdt)[Fe(CO)(2)(PTA-H+)](2), 1-(PTA.H+)(2), and (mu-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 H-2 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 eta(2)-H-2-(FeFeI)-Fe-II species, derived from reduction of the (FeFeI)-Fe-I parent complex to (FeFeI)-Fe-0 followed by uptake of two protons in an ECCE mechanism.