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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/2504 |
| Date | 01 November 2005 |
| Creators | Mejia Rodriguez, Ma. del Rosario |
| Contributors | Darensbourg, Marcetta Y. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 1324652 bytes, electronic, application/pdf, born digital |
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