Download Introduction to Computational Chemistry

Chemistry 380.37
Fall 2015
Dr. Jean M. Standard
August 17, 2015
Introduction to Computational Chemistry
What is computational chemistry?
Computational chemistry involves the use of computers to model the structure, energy, physical and chemical
properties, and reactivity of atomic and molecular systems. Computational chemistry may involve the study of
individual molecules at the microscopic level, or it may involve the simulation of the bulk properties of molecular
systems at the macroscopic level. It may involve modeling of gas, solution, or solid phase systems, or even
processes at the interface between phases (for example, adsorption of a gas on a solid substrate).
In which areas of chemistry is computational chemistry used?
Computational chemistry methods are employed in all areas of chemistry: analytical, inorganic, organic, and
physical chemistry, as well as biochemistry. In addition, computational chemistry is very prevalent in other
subfields of chemistry, including materials and polymer chemistry. Journal articles in all areas publish research that
involves computational chemistry. Attached on the last several pages are some samples of title pages of some
articles from the ISU Chemistry Department published in scientific journals during the last several years. These
samples are just a few out of the numerous articles containing computational chemistry research to appear in current
journals. Computational chemistry is truly widespread in its utility as a predictive and analytical tool in all areas of
chemistry.
Where are computational chemists employed?
Computational chemists are hired by academic institutions, government agencies, and all types of industries. The
pharmaceutical industry in particular has embraced computational chemistry as an effective tool in the design of
new drugs. Computational chemists are employed to study all types of chemical processes from catalytic reactions
occurring on metal surfaces to gas phase reactions involved in destroying the earth’s stratospheric ozone layer.
Listed on the last page of this handout are a few job advertisements for computational chemists found in previous
issues of Chemical and Engineering News. These listings are just a random sampling of the types of job
opportunities available for computational chemists.
What sort of educational preparation do computational chemists need?
Computational chemistry often requires a Ph.D. However, more and more companies today also are hiring
computational chemists at the Master’s level. Students who want to pursue a degree in computational chemistry
should generally start with some extra courses in math (differential equations is particularly helpful) as well as at
least one course in a computer programming language (C or Fortran are the most useful).
Are there advanced degrees awarded in computational chemistry?
Most universities do not award a specific Ph.D. degree in computational chemistry. A Ph.D. degree is usually
awarded in physical chemistry for a student carrying out research under the direction of a physical chemistry faculty
member who specializes in computational chemistry; however, faculty members with specializations in
computational chemistry also may be found in the other subdisciplines of chemistry.
Some universities have special programs in computational chemistry and computer visualization. Two examples are
Wayne State University and Northeastern University. These programs may lead either to certificates or Master’s
degrees in computational chemistry.
What is molecular modeling?
Molecular modeling is sometimes used as a synonym for computational chemistry. However, computational
chemistry is often meant as a broader term that includes computer simulation of molecular systems at both the
microscopic and macroscopic levels. Molecular modeling may often refer solely to computer simulation of systems
at the microscopic level.
2
Brief Description of Methods Covered in CHE 380.37
I. Definitions and Terms
A. What is molecular modeling?
• the generation, representation, analysis, and prediction of molecular structures, properties,
interactions, and reactions via computer simulation.
B. What is the purpose of molecular modeling?
• predictive tool
• analytic tool
C. Who uses molecular modeling?
• molecular modeling is employed in every area of chemistry
• molecular modeling is used in academia, government, and industry
II. Force Field Methods - Molecular Mechanics
A. Description of the Method
• Molecular mechanics (MM) is based on an empirical force field representation of the interactions
between atoms.
• A force field is made up of stretching, bending, torsional, electrostatic, and nonbonded interactions
which yield the energy of the system as a function of atomic coordinates.
• The force field is parametrized using experimental properties such as heats of formation,
vibrational frequencies, and dipole moments.
B. Utility and Applications
• MM is particularly useful for large molecular systems (biomolecules, polymers) where quantum
calculations are not possible.
• MM may be used for prediction of relative energies of conformers.
III. Conformation searching
A. Description of the Method
• Conformation searching involves calculations to find the lowest energy conformer of a molecule
(or the lowest conformers within a specified energy range).
• An empirical force field is usually employed to represent the interactions between atoms.
However, quantum mechanical methods may also be used.
• Several methods, including systematic, Monte Carlo, distance geometry, and molecular dynamics,
are employed to carry out conformer searches.
B. Utility and Applications
• Conformation searching is useful for large, flexible molecules with many low-energy conformers.
3
IV. Quantum Mechanical Methods
A. Description of the Methods
• All the methods which employ quantum mechanics (QM) are based on solving the Schrödinger
equation (to some level of approximation) for the molecular system of interest.
• Ab initio ("from the beginning") methods involve no empirical parameters and therefore are the
most accurate techniques (and the most expensive computationally).
• Semiempirical methods rely on parametrization of some of the integrals that occur in the solution
of the Schrödinger equation using experimental data.
• Density functional methods are based on the specification of a certain functional form for the
electron density in the molecule.
B. Utility and Applications
• QM is most useful for smaller molecular systems, although semi-empirical methods have been
used for larger molecules. Density functional theory has proven especially useful not only for
organic molecules but also for molecules containing transition metals.
• Molecular geometries, energies, electron density, orbitals, and other properties (such as vibrational
frequencies) may be determined.
• Transition states, reaction paths, and mechanisms may be studied.
V. Molecular Dynamics
A. Description of the Method
• In molecular dynamics (MD), Newton's classical equations of motion are used to solve for the
motion of the atoms as a function of time.
• An empirical force field can be employed to represent the interactions between atoms. Some
recent studies have employed quantum mechanical force fields, or a mixture of classical and
quantum mechanics.
B. Utility and Applications
• Like molecular mechanics, MD is useful for large systems such as biomolecules and polymers
where quantum mechanics is prohibitive.
• MD is useful in the study condensed phase systems, such as biomolecules in the presence of
explicit solvent molecules.
• MD is also good for looking at condensed phases of smaller systems but with a large number of
particles (e.g. liquid Ar or H2O).
• MD yields not only information about how the molecular configuration varies in time, but it may
be used to calculate thermodynamic information, such as free energies, heat capacity, etc.
• MD can be used as a searching tool to find lowest energy conformation in large molecular
systems.
ARTICLE
pubs.acs.org/JPCA
Multireference Configuration Interaction Study of Bromocarbenes
Jean M. Standard,* Rebecca J. Steidl, Matthew C. Beecher, and Robert W. Quandt
Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160, United States
ABSTRACT: Multireference configuration interaction (MRCI) calculations of the lowest
~ (1A0 ) and triplet ~a(3A00 ) states as well as the first excited singlet A
~ (1A00 ) state have
singlet X
been performed for a series of bromocarbenes: CHBr, CFBr, CClBr, CBr2, and CIBr. The
MRCI calculations were performed with correlation consistent basis sets of valence triple-ζ
plus polarization quality, employing a full-valence active space of 18 electrons in 12 orbitals
(12 and 9, respectively, for CHBr). Results obtained include equilibrium geometries and
harmonic vibrational frequencies for each of the electronic states, along with ~a(3A00 ) r
~ (1A00 ) r X
~ (1A0 ) transition energies. Comparisons have been made with previous computational and
~ (1A0 ) singlet-triplet gaps and A
X
experimental results where available. The MRCI calculations presented in this work provide a comprehensive series of results at a
consistent high level of theory for all of the bromocarbenes.
I. INTRODUCTION
It has long been known that chlorine-containing hydrocarbons
have large ozone depletion potentials (ODPs) due to considerable release rates and long atmospheric lifetimes.1 It would be
expected that bromine-containing halocarbons, with release rates
and atmospheric lifetimes that are generally much smaller than
their chlorine-containing counterparts, would have negligible
ODPs. However, unstable reservoir molecules and synergistic
effects with chlorine lead to much greater than expected ODPs
for these species.2 Indeed, it has been estimated that, on a per
atom basis, bromine is almost 60 times more destructive to ozone
than chlorine.3 These larger than expected ODPs have led to a
renewed interest in bromine-containing hydrocarbons in recent
years. It has been found that, in addition to better known sources
such as Halons and methyl bromide, bromoform is a significant
source of reactive bromine in the stratosphere.4-6 Depending
upon the excitation wavelength, photodissociation of bromocarbons such as bromoform can follow two different routes:
dissociation into atomic bromine and a substituted methyl radical
or dissociation into molecular bromine and a singlet carbene. For
example, McGivern et al. observed that at 193 nm the primary
photoproduct was atomic bromine.7 They also observed secondary dissociation of the excited photoproduct, CHBr2*, to form
CBr and HBr. Xu et al. observed the formation of atomic halogen
in both the 2P1/2 and 2P3/2 spin states upon excitation at 234 and
267 nm.8 They also found significant formation of molecular
bromine with branching ratios into that channel of 0.16 and 0.26
at 267 and 234 nm, respectively. Quandt and co-workers recently
studied the 2 ! 193 nm photodissociation of CBr4 and CHBr3
via photoproduct emission.9 Observed emission was attributed
to the Swan system (d3Πg f a3Πu) of C2, which was formed via
reaction of electronically excited radicals, CH(A2Δ) and CBr(A2Δ). In addition, formation of CBr2 (or CHBr) and Br2 was
implied by secondary evidence. A computational study of this
dark channel was undertaken, and the results showed the presence of three transition states and an ion-pair isomer intermediate for both CBr4 and CHBr3 dissociation.
r 2011 American Chemical Society
A number of previous computational studies have been performed on bromocarbenes. A comprehensive study of the lowest
~ (1A0 ) and triplet ~a(3A00 ) states of all halocarbenes was
singlet X
carried out by Schwartz and Marshall in 1999 in which equilibrium geometries, harmonic vibrational frequencies, and ~a(3A00 )
~ (1A0 ) singlet-triplet gaps obtained at the QCISD/6-311GrX
(d) level were reported.10 A few years later, Drake et al. employed
CISD, CASSCF, and CASPT2 methods along with basis sets of
~ (1A00 ) r
DZP quality to determine geometries and adiabatic A
~ (1A0 ) and first excited
~ (1A0 ) transition energies for the ground X
X
~ (1A00 ) singlet states of a series of bromo- and iodocarbenes.11 In
A
particular, the CASPT2(18,12)/DZP method was shown to provide
a good balance of computational cost and predictive accuracy for the
~ (1A0 ) transition energies.11 CASSCF and
~ (1A00 ) r X
adiabatic A
CASPT2 calculations also have been employed in other studies of
the bromocarbenes CFBr 12 and CBr2 13 in order to obtain equilibrium geometries, harmonic vibrational frequencies, and other spectroscopic parameters for the ground and first excited singlet states as
well as the lowest triplet state.
Higher level multireference configuration interaction (MRCI)
calculations have been previously completed only for CHBr,14,15
CFBr,12 and CClBr.16 In work by Yu et al.,14 MRCI calculations were
carried out on CHBr with a cc-pVTZ basis set using state-averaged
full-valence active space CASSCF reference functions. Equilibrium
geometries and a detailed analysis of the potential surfaces of the
~ (1A00 ) singlet states as well the lowest
~ (1A0 ) and excited A
ground X
3 00
~a( A ) triplet state of CHBr were presented. In recent work by
Burrill and Grein,15 a TZP basis set with polarization and diffuse
functions was employed in order to carry out MRCI calculations on
the lowest six singlet and triplet electronic states of CHBr. For CFBr,
the MRCI method with an active space of two electrons in two
orbitals, MRCI(2,2), and a TZP quality basis set was employed to
~ (1A00 ) singlet state.12 For CClBr, a
study only the first excited A
similar MRCI(2,2) study was carried out to determine the geometry
Received: August 13, 2010
Revised:
December 31, 2010
Published: January 31, 2011
1243
dx.doi.org/10.1021/jp107688v | J. Phys. Chem. A 2011, 115, 1243–1249
pubs.acs.org/joc
β-Amino Alcohol Derived β-Hydroxy- and β(o-Diphenylphosphino)benzoyloxy(o-diphenylphosphino)benzamides: An
Ester-Amide Ligand Structural Model for the Palladium-Catalyzed
Allylic Alkylation Reaction
Geetanjali S. Mahadik, Stanley A. Knott, Lisa F. Szczepura, Steven J. Peters,
Jean M. Standard, and Shawn R. Hitchcock*
Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160
[email protected]
Received July 29, 2009
A commercially available collection of β-amino alcohols have been converted to their corresponding
β-hydroxy- and β-(o-diphenylphosphino)benzoyloxy(o-diphenylphosphino)benzamides 11a-f and
12a-f and have been employed in the Tsuji-Trost asymmetric alkylation reaction with 1,3-diphenylpropenyl acetate. With the exception of ligands 11b and 11f, the β-hydroxybenzoyloxy
(o-diphenylphosphino)benzamide ligands 11a-f primarily afforded the (R)-enantiomer of the product.
In contrast, the bis(phosphine) ligands 12a-f consistently afforded the (S)-enantiomer. The best ligand
(12c) was derived from cis-(1R,2S)-2-amino-1,2-diphenyl-1-ethanol, and when applied in the asymmetric allylic alkylation reaction, it yielded the product in an enantiomeric ratio of 97.8.22 favoring the
(S)-enantiomer. A computational study was conducted on the conformation that this ligand might
adopt in the palladium-catalyzed alkylation reaction as compared to that of the Trost ligand 1a.
1. Introduction
The palladium-catalyzed asymmetric allylic alkylation reaction1 known as the Tsuji-Trost reaction has been the subject of
intense studies2 directed toward the design, synthesis, and
application of a myriad of chiral, nonracemic ligand scaffolds.
Of the ligands that have been prepared and applied in this
reaction, the Trost modular phosphine ligands (1a,b)3 have
proven to be the benchmark for evaluating the efficacy of newly
developed phosphine ligands in the asymmetric allylic alkylation reaction. In fact, on the basis of their successful use in a
variety of applications, many of these modular ligands are
commercially available.4 The success of these ligands has
encouraged the synthesis and application of structurally novel
phosphines based on the binaphthyl type ligands,5 tartrate
derived systems,6 carbohydrates,7 paracyclophanes,8 and
(1) Tsuji, J. In Palladium Reagents and Catalysts: New Perspectives for the
21st Century; John Wiley & Sons: Chichester, UK, 2004; pp 431-518.
(2) (a) Lu, Z.; Ma, S. Angew. Chem., Int. Ed. 2008, 47, 258. (b) Trost, B.
M. J. Org. Chem. 2004, 69, 5813. (c) Trost, B. M.; Crawley, M. L. Chem. Rev.
2003, 103, 2921. (d) Trost, B. M.; Machacek, M. R.; Aponick, A. Acc. Chem.
Res. 2006, 39, 747. (e) Trost, B. M.; Van Vranken, D. L. Chem. Rev. 1996, 96,
395. (f) Trost, B. M. Acc. Chem. Res. 1980, 13, 385. (g) Trost, B. M.; Breit, B.;
Organ, M. G. Tetrahedron Lett. 1994, 35, 5817.
(3) (a) Trost, B. M.; Van Vranken, D. L.; Bingel, C. J. Am. Chem. Soc.
1992, 114, 9327. (b) Trost, B. M.; Van Vranken, D. L. Angew. Chem., Int. Ed.
1992, 31, 228.
(4) (a) Trost, B. M.; Fandrick, D. R. Aldrichim. Acta 2007, 40, 59.
(5) Feng, J.; Bohle, D. S.; Li, C. Tetrahedron: Asymmetry 2007, 18, 1043–
1047.
(6) Marques, C. S.; Burke, A. J. Tetrahedron: Asymmetry 2007, 18, 1804–
1808.
!
(7) Khiar, N.; Navas, R.; Alvarez,
E.; Fern!
andez, I. ARCHIVOC 2008, 8,
211-224.
(8) Jiang, B.; Lei, Y.; Zhao, X. J. Org. Chem. 2008, 73, 7833–7836.
8164
Published on Web 10/07/2009
J. Org. Chem. 2009, 74, 8164–8173
DOI: 10.1021/jo9016474
r 2009 American Chemical Society
9692
J. Phys. Chem. A 2009, 113, 9692–9699
Metal-Olefin Bond Energies in M(CO)5(C2H4-nCln) M ) Cr, Mo, W; n ) 0-4:
Electron-Withdrawing Olefins Do Not Increase the Bond Strength
Darin N. Schlappi and David L. Cedeño*
Department of Chemistry, Illinois State UniVersity, Box 4160, Normal, Illinois 61790-4160
ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: July 17, 2009
Metal-olefin bond dissociation enthalpies have been calculated for the series of complexes M(CO)5(C2H4-nCln),
M ) Cr, Mo, W; n ) 0-4 using density functional theory. Experimental values of the bond enthalpies have
been measured for M(CO)5(C2H4-nCln) M ) Cr, Mo, W; n ) 2 (vinyl chloride), 3, and 4 using laser
photoacoustic calorimetry in n-hexane solution. Experimental and calculated values indicate that the trend in
metal-olefin bond energies is opposite to the electron-withdrawing ability of the olefin, which is counter to
expectations based on the Dewar-Chatt-Duncanson model for metal-olefin bonding. An in-depth analysis
of the metal-olefin interaction using a bond energy decomposition scheme implies that the observed and
calculated decreasing trend is influenced by the increase in steric interactions and olefin reorganizational
energy which is concomitant to the increase of the number of electron-withdrawing halogen atoms.
Introduction
Many significant chemical processes such as olefin hydrogenation, isomerization, hydrocarbonlyation, hydroformylation,
polymerization, and metathesis among others are driven by the
presence of a metal catalyst and involve the formation of an
intermediate that contains a metal-olefin bond.1-6 It is beneficial
to be able to synthesize catalysts for these reactions that are
fine-tuned to the needs of a particular reaction or process because
the use of olefins and olefin-related products in industry has
become prevalent. The ability to control the properties of these
catalysts relies heavily on a complete understanding of the
thermodynamic factors that influence the strength of the bond
between a given metal complex and an olefin.
Contributing to a level of understanding that would allow
for an accurate prediction of the bond strength between a metal
complex and an olefin is the primary goal of our research. The
current picture of metal-olefin bonding is based on frontier
molecular orbital theory introduced by Dewar in 19517 and
expanded by Chatt and Duncanson in 1953.8 The approach is
known as the Dewar-Chatt-Duncanson (DCD) model of
metal-olefin bonding. The DCD model details the metal-olefin
bond as being a two way synergistic electron exchange between
a metal complex and an olefin. The bond consists of a σ
interaction in which the highest occupied molecular orbital
(HOMO) of the olefin donates electron density to an empty dσ
orbital on the metal complex. Additionally, there is a π bonding
interaction in which the metal donates electron density back to
the olefin from an occupied dπ orbital to the unoccupied
antibonding π* orbital of the olefin.
The electron population changes in the π and π* orbitals of
the olefin have the physical consequence of decreasing the bond
order of the carbon-carbon double bond. This is equivalent to
a partial sp2 to sp3 rehybridization of the olefinic carbons that
causes the lengthening of the CdC bond and the back-bending
of the substituents around the CdC bond away from the metal
complex and outside of the plane of the CdC bond. The DCD
model has been commonly used to rationalize the bonding
strength between a metal complex and an olefin.2,5,9 A wide* Corresponding author. Email: [email protected].
spread expectation of this rationalization is that for some metals
the π (or back-) bonding interaction is the dominating contribution to the metal-olefin bond. Therefore, if the hydrogen atoms
in ethylene were to be replaced with a more electron-withdrawing substituent such as a halogen (X ) F, Cl), then the backbonding would increase because a halogenated ethylene is a
better π acceptor than ethylene. Based on this rationalization,
the bonding energies between a metal and the olefin series C2H4,
C2F4, and C2Cl4 would be a function of the electron-withdrawing
ability of the substituents around the CdC bond of the olefin
and decrease in the order C2F4 > C2Cl4 > C2H4. Experimental
data on Cr(CO)5(C2X4) (where X ) H, F, Cl) indicate, however,
that the metal-olefin bond strength follows the trend Cr-C2H4
> Cr-C2F4 > Cr-C2Cl4.10 Cedeño and Weitz10 carried out a
Density Functional Theory (DFT) study of the series
Fe(CO)4(C2X4) and Cr(CO)5(C2X4) where X ) H, F, Cl, that
provides an explanation for the discrepancy between the
experimental data and the expectations based on the DCD
model.
Along the same line of reasoning, the DCD model may be
used to predict the trend of the metal bond strength for the olefin
series C2H4-nXn (X ) F or Cl). Given that the back-bonding
ability of an olefin is enhanced as halogenation increases, then
the metal-olefin bond strength should increase with an increase
in the number of halogens. Back in 1974, Tolman11 determined
the equilibrium bonding constants between bis(tri-o-tolyl phosphite)nickel(0) and 38 different olefins including the C2H4-nFn
(n ) 0-4) series. In his paper, Tolman found that none of the
fluoro olefins examined (with the exception of CH2dCHCF3)
were as good as C2H4 in coordinating to nickel(0), a surprising
result that was out of line with his expectation. Tolman hinted
that the reason for the inadequacy of the DCD picture of
metal-olefin bonding was due to the reorganization that occurs
in the olefin as the carbons of the double bond are forced to
rehybridize from sp2 to sp3. A computational DFT study by
Schlappi and Cedeño12 examined the bonding of the olefins
C2XnH4-n (X ) F or Cl, and n ) 0-4) to Ni(PH3)2(CO). It was
found that the olefins bound to the nickel with dissociation
energies that follow a trend very similar to the one shown in
Tolman’s study and confirmed his presumption. We concluded
10.1021/jp9027468 CCC: $40.75 © 2009 American Chemical Society
Published on Web 08/07/2009
Inorg. Chem. 2008, 47, 7852-7862
Catalytic Dioxygen Activation by
(Nitro)(meso-tetrakis(2-N-methylpyridyl)porphyrinato)cobalt(III) Cation
Derivatives Electrostatically Immobilized in Nafion Films: An
Experimental and DFT Investigation
John A. Goodwin,*,† Jennifer L. Coor,† Donald F. Kavanagh,† Mathieu Sabbagh,† James W. Howard,†
John R. Adamec,† Deidre J. Parmley,† Emily M. Tarsis,† Tigran S. Kurtikyan,‡
Astghik A. Hovhannisyan,‡ Patrick J. Desrochers,§ and Jean M. Standard|
Department of Chemistry and Physics, Coastal Carolina UniVersity, P.O. Box 261954,
Conway, South Carolina 29526-6054, Molecular Structure Research Centre, National Academy of
Sciences, YereVan, Armenia, Department of Chemistry, UniVersity of Central Arkansas,
Conway, Arkansas 72035, and Department of Chemistry, Illinois State UniVersity,
Normal, Illinois 61790
Received January 15, 2008
Complexes of the (nitro)(meso-tetrakis(2-N-methylpyridyl)porphyinato)cobalt(III) cation, [LCoTMpyP(2)(NO2)]4+, in which L )
water or ethanol have been immobilized through ionic attraction within Nafion films (Naf). These immobilized six-coordinate
species, [LCoTMPyP(2)(NO2)/Naf], have been found to catalyze the oxidation of triphenylphosphine in ethanol solution by
dioxygen, therefore retaining the capacity to activate dioxygen catalytically without an additional reducing agent as was previously
observed in nonaqueous solution for the non-ionic (nitro)cobalt porphyrin analogs. Heating these immobilized six-coordinate
species under vacuum conditions results in the formation of the five-coordinate nitro derivatives, [CoTMPyP(2)(NO2)/Naf] at 85
°C and [CoTMPyP(2)/Naf] at 110 °C. The catalytic oxidation of gas-phase cyclohexene with O2 is supported only by the
resulting immobilized five-coordinate nitro complex as was previously seen with the corresponding solution-phase catalyst in
dichloromethane solution. The simultaneous catalytic oxidation of triphenylphosphine and cyclohexene with O2 in the presence
of the Nafion-bound six-coordinate ethanol nitro complex is also observed; however, this process is not seen for the CoTPP
derivative in dichloromethane solution. The oxidation reactions do not occur with unmodified Nafion film or with Nafion-supported
[BrCo(III)TmpyP]/Naf or [Co(II)TmpyP]/Naf, indicating the necessity for the nitro/nitrosyl ligand in the oxidation mechanism. The
existence of a second reactive intermediate is indicated because the two simultaneous oxidation reactions depend on two
distinct oxygen atom-transfer steps having different reactivity. The absence of homogeneous cyclohexene oxidation by the
six-coordinate (H2O)CoTPP(NO2) derivatives in the presence of Ph3P and O2 in dichloromethane solution indicates that the
second reactive intermediate is lost by an unidentified route only in solution, implying that the immobilization of it in Nafion
allows it to react with cyclohexene. Although direct observation of this species has not been achieved, a comparitive DFT study
of likely intermediates in several catalytic oxidation mechanisms at the BP 6-31G* level supports the possibility that this intermediate
is a peroxynitro species on the basis of relative thermodynamic accessibility. The alternate intermediates evaluated include the
reduced cobalt(II) porphyrin, the dioxygen adduct cobalt(III)-O2-, the oxidized cobalt(II) π-cation radical, and the nitrito complex,
cobalt(III)-ONO.
Introduction
Development of robust heterogeneous catalysts for activation of molecular oxygen is important for a wide range of
applications including environmentally benign synthesis,
water purification, and the oxygen reduction reaction in fuelcell technology.1 Many heterogeneous catalysts based on
metalloporphyrins that are effective in electrocatalytic and
photocatalytic oxidations with dioxygen have been studied,
* To whom correspondence should be addressed.
[email protected].
†
Coastal Carolina University.
‡
National Academy of Sciences.
§
University of Central Arkansas.
|
Illinois State University.
(1) Centi, G.; Misono, M. Catal. Today 1998, 41, 287–296.
7852 Inorganic Chemistry, Vol. 47, No. 17, 2008
E-mail:
and porphyrin-catalyzed oxidation reactions using peroxides,
N-oxides, and other oxidants or co-reductants are also well
known.2 A relatively small subset of these metalloporphyrin
systems carries out catalytic oxidation reactions with molecular oxygen as the oxidant in the absence of a coreductant; although some of these have been shown to occur
by auto-oxidation radical-chain mechanisms.3 The use of
metalloporphyrin catalysts in supported heterogeneous systems4,5 and in microporous porphyrin assemblies6 has also
been widely investigated.1
The reactivity of five-coordinate (nitro)cobaltporphyrins
in the catalytic oxidation of alkenes, apparently through
secondary oxo-transfer from the coordinated nitro ligand, was
10.1021/ic8000762 CCC: $40.75
© 2008 American Chemical Society
Published on Web 07/30/2008