Ligand effects on bioinspired iron complexes



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Texas A&M University


The synthesis of diiron thiolate complexes was carried out using two ligands that were expected to furnish improved catalytic activity, solubility in water, and stability to the metal complexes. The water-soluble phosphine 1,3,5-triaza-7- phosphaadamantane, PTA, coordinates to the Fe centers forming the disubstituted complex (m-pdt)[Fe(CO)2PTA]2, which presents one PTA in each iron in a transoid arrangement. Substitution of one CO ligand in the (m-pdt)[Fe(CO)3]2 parent complex forms the asymmetric (m-pdt)[Fe(CO)3][Fe(CO)2PTA]. Enhanced water solubility was achieved through reactions with electrophiles, H+ and CH3 +, which reacted with the N on the PTA ligand forming the protonated and methylated derivatives, respectively. The 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IMes, was reacted with (m-pdt)[Fe(CO)3]2 yielding the asymmetric (m-pdt)[Fe(CO)3][Fe(CO)2IMes], an electron rich, air stable complex that does not show reactivity with H+. Electrocatalytic production of hydrogen was studied for the all-CO, bis-PMe3, mono- and di-PTA FeIFeI complexes, as well as the PTA-protonated and -methylated derivatives. The all-CO species produce H2, in the presence of the weak HOAc, at their second reduction event, FeIFe0 ?? Fe0Fe0, that occurs at ca. ??1.9 V, through an EECC mechanism. The mono- and di-substituted phosphine complexes present electrocatalytic production of H2 from the Fe0FeI redox state; this reduction takes place at ??1.54 V for (m-pdt)[Fe(CO)3][Fe(CO)2PTA], and at ca. ??1.8 for the disubstituted PMe3 and PTA derivatives. A positive charge on the starting complex does not have an effect on the production of H2. It was found that the protonated and methylated derivatives are not the catalytic species for H2 production. At their first reduction event the neutral precursor forms, and catalysis occurs from the FeIFeI complex in all cases. The possibility of enhanced catalytic activity in the presence of H2 O was explored by conducting electrochemical experiments in the mixed CH3CN:H2O solvent system for the PTA-substituted complexes. The reduction potential of the catalytic peak is shifted to more positive values by the presence of H2 O. The cyclic voltammogram of {(m-pdt)[Fe(CO)2(PTA?? H)]2}2+ in CH3CN:H2O 3:1 shows the reduction of a more easily reduced species in the return scan. This curve-crossing event provides evidence for the (h2-H2)FeII intermediate proposed in the ECCE mechanism.