Theoretical Studies of Structures and Mechanisms in Organometallic and Bioinorganic Chemistry: Heck Reaction with Palladium Phosphines, Active Sites of Superoxide Reductase and Cytochrome P450 Monooxygenase, and Tetrairon Hexathiolate Hydrogenase Model
Abstract
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.
Subject
PalladiumHeck reaction
Superoxide Reductase
Cytochrome P450
Hydrogenase
Density Functional Theory
Citation
Surawatanawong, Panida (2009). Theoretical Studies of Structures and Mechanisms in Organometallic and Bioinorganic Chemistry: Heck Reaction with Palladium Phosphines, Active Sites of Superoxide Reductase and Cytochrome P450 Monooxygenase, and Tetrairon Hexathiolate Hydrogenase Model. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /ETD -TAMU -2009 -05 -283.