Ligand effects on bioinspired iron complexes

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Title: Ligand effects on bioinspired iron complexes
Author: Mejia Rodriguez, Ma. del Rosario
Abstract: 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 .
URI: http : / /hdl .handle .net /1969 .1 /2504
Date: 2005-11-01

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Ligand effects on bioinspired iron complexes. Available electronically from http : / /hdl .handle .net /1969 .1 /2504 .

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