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Description:
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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 . |