The electronic structures and reaction mechanisms of transition-metal complexes
can be calculated accurately by density functional theory (DFT) in cooperation with the
continuum solvation model. The palladium catalyzed Heck reaction, iron-model
complexes for cytochrome P450 and superoxide reductase (SOR), and tetrairon
hexathiolate hydrogenase model were investigated.
The DFT calculations on the catalytic Heck reaction (between phenyl-bromide
and ethylene to form the styrene product), catalyzed by palladium diphosphine indicate a
four-step mechanism: oxidative addition of C6H5Br, migratory insertion of C6H5 to
C2H4, b-hydride transfer/olefin elimination of styrene product, and catalyst regeneration
by removal of HBr. For the oxidative addition, the rate-determining step, the reaction
through monophosphinopalladium complex is more favorable than that through either
the diphosphinopalladium or ethylene-bound monophosphinopalladium. In further
study, for a steric phosphine, PtBu3, the oxidative-addition barrier is lower on monopalladium monophosphine than dipalladium diphosphine whereas for a small
phosphine, PMe3, the oxidative addition proceeds more easily via dipalladium
diphosphine. Of the phosphine-free palladium complexes examined: free-Pd, PdBr-, and
Pd(h2-C2H4), the olefin-coordinated intermediate has the lowest barrier for the oxidativeaddition.
P450 and SOR have the same first-coordination-sphere, Fe[N4S], at their active
sites but proceed through different reaction paths. The different ground spin states of the
intermediate FeIII(OOH)(SCH3)(L) model {L = porphyrin for P450 and four imidazoles
for SOR} produce geometric and electronic structures that assist i) the protonation on
distal oxygen for P450, which leads to O-O bond cleavage and formation of
(FeIV=O)(SCH3)(L) H2O, and ii) the protonation on proximal oxygen for SOR, which
leads to (FeIII-HOOH)(SCH3)(L) formation before the Fe-O bond cleavage and H2O2
production. The hydrogen bonding from explicit waters also stabilizes FeIII-HOOH over
FeIV=O H2O products in SOR.
The electrochemical hydrogen production by Fe4[MeC(CH2S)3]2(CO)8 (1) with
2,6-dimethylpyridinium (LutH ) were studied by the DFT calculations of proton-transfer
free energies relative to LutH and reduction potentials (vs. Fc/Fc ) of possible
intermediates. In hydrogen production by 1, the second, more highly reductive, applied
potential (-1.58 V) has the advantage over the first applied potential (-1.22 V) in that the
more highly reduced intermediates can more easily add protons to produce H2.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2009-05-283 |
| Date | 2009 May 1900 |
| Creators | Surawatanawong, Panida |
| Contributors | Hall, Michael B. |
| Source Sets | Texas A and M University |
| Language | English |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | application/pdf |
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