Advances in Inorganic Chemistry

273 Pages·2004·9.065 MB·English

by  R. van Eldik (Eds.)

CONTENTS
PREFACE . . . . . . . . . . . . . . . ix
Synergy BetweenTheory andExperiment asApplied toH/D Exchange
ActivityAssays in[Fe]H ase ActiveSite Models
2
JESSEW. TYE,MICHAELB.HALL,IRENEP.GEORGAKAKIAND
MARCETTAY.DARENSBOURG
I. Introduction . . . . . . . . . . . . 1
II. ExperimentalSection . . . . . . . . . . 4
III. ResultsandDiscussion . . . . . . . . . . 6
IV. Conclusions . . . . . . . . . . . . 23
References . . . . . . . . . . . . 24
Electronic Structureand SpectroscopicProperties of Molybdenum and
TungstenN ,NNH,NNH , andNNH ComplexesWith Diphosphineco-ligands:
2 2 3
Insightsintothe End-OnTerminal Reduction PathwayofDinitrogen
FELIXTUCZEK
I. Introduction . . . . . . . . . . . . 27
II. V|brationalSpectroscopyandQCA-NCA . . . . . . 30
III. ElectronicStructureandElectronicAbsorptionSpectroscopy . . 43
IV. Conclusions . . . . . . . . . . . . 50
References . . . . . . . . . . . . 52
QuantumChemical Investigationsintothe Problemof BiologicalNitrogen
Fixation: Sellmann-Type Metal–SulfurModelComplexes
MARKUSREIHERANDBERNDA.HESS
I. Introduction . . . . . . . . . . . . 56
II. TheActiveSiteofNitrogenase:FeMo-Cofactor . . . . . 57
III. BiomimeticSellmann-TypeMetalComplexes . . . . . 61
IV. TheFirst2-Electron^2-ProtonTransfer . . . . . . 64
v
vi CONTENTS
V. ReactionEnergeticsoftheReductionatMononuclearFe(II)
Centers . . . . . . . . . . . . . 74
VI. PhotoisomerizationofCoordinatedDiazene . . . . . 82
VII. StudyoftheFirstDinuclearN ComplexwithBiologically
2
CompatibleLigandSphere . . . . . . . . . 88
VIII. Conclusion . . . . . . . . . . . . 92
IX. Appendix:QuantumChemicalMethodology . . . . . 93
References . . . . . . . . . . . . 97
Protonand Electron Transfers in[NiFe] Hydrogenase
PERE.M.SIEGBAHN
I. Introduction . . . . . . . . . . . . 101
II. ComputationalDetails . . . . . . . . . . 104
III. ResultsandDiscussion . . . . . . . . . . 105
IV. Conclusions . . . . . . . . . . . . 123
References . . . . . . . . . . . . 123
Heterolytic Splittingof H–H, Si–H,andOthersBonds on
Electrophilic MetalCenters
GREGORYJ.KUBAS
I. Introduction . . . . . . . . . . . . 127
II. HeterolyticCleavageandAcidityofCoordinatedH . . . . 132
2
III. HeterolyticCleavageofSi^HBonds . . . . . . . 150
IV. HeterolyticCleavageofB^HandC^HBonds . . . . . 168
V. ConcludingRemarks . . . . . . . . . . 171
References . . . . . . . . . . . . 172
TetrapodalPentadentateNitrogen Ligands:Aspectsof Complex
Structureand Reactivity
ANDREASGROHMANN
I. Introduction . . . . . . . . . . . . 179
II. Overview . . . . . . . . . . . . 180
III. TetrapodalPentadentateCoordinationModules . . . . . 183
IV. Conclusion . . . . . . . . . . . . 206
References . . . . . . . . . . . . 206
CONTENTS vii
Efficient, Ecologically Benign, AerobicOxidationof Alcohols
ISTVA¤NE.MARKO¤,PAULR.GILES,MASAOTSUKAZAKI,ISABELLECHELLE¤-REGNAUT,
ARNAUDGAUTIER,RAPHAELDUMEUNIER,FREDDIPHILIPPART,KANAEDODA,
JEAN-LUCMUTONKOLE,STEPHENM.BROWNANDCHRISTOPHERJ.URCH
I. Introduction . . . . . . . . . . . . 212
II. FirstGenerationCopper-CatalyzedAerobicOxidationProtocol . . 212
III. SecondGenerationCopper-CatalyzedAerobicOxidationProtocol . 216
IV. ThirdGenerationCopper-CatalyzedAerobicOxidationProtocol . 223
V. TowardsaTrulyE⁄cient,Aerobic,CatalyticOxidationProtocol . 228
References . . . . . . . . . . . . 236
Visible LightPhotocatalysis byaTitania TransitionMetal Complex
HORSTKISCH,GERALDBURGETHANDWOJCIECHMACYK
I. Introduction . . . . . . . . . . . . 241
II. PreparationandCharacterizationofPhotocatalysts . . . . 242
III. PhotocatalyticDegradationwithArti¢cialV|sibleLight
(l(cid:1)355nm) . . . . . . . . . . . . 249
IV. PhotodegradationwithNaturalIndoorDaylight . . . . . 252
V. PhotodegradationwithSunlight . . . . . . . . 253
VI. Mechanism . . . . . . . . . . . . 254
VII. Summary . . . . . . . . . . . . 258
References . . . . . . . . . . . . 258
INDEX . . . . . . . . . . . . . . . 261
CONTENTSOFPREVIOUSVOLUMES . . . . . . . . . . 269
PREFACE
This thematic volume focuses on ‘Redox-active Metal Complexes’
and is dedicated to the late Dieter Sellmann.The theme of this issue
is taken from a collaborative research program, the so-called
‘‘Sonderforschungsbereich 583’’ initiated by Dieter Sellmann and
funded by the Deutsche Forschungsgemeinschaft. In this program,
active since July 2001, 16 research groups from di¡erent areas in
Chemistry and Physics collaborate at the University of Erlangen-
Nu« rnberg with the common goal to control reactivity of redox-active
metal complexes through molecular architecture. As part of this
activity an international symposium was organized in April 2003 on
the theme of this volume, and speakers were invited to contribute to
this thematic volume, scheduled to be co-edited by Dieter Sellmann.
Unfortunately, Dieter Sellmann died unexpectedly on 6 May 2003. In
order to acknowledge his important contributions in this area and
his remarkable input to initiate the‘‘Sonderforschungsbereich 583’’, it
wasdecidedtodedicatethisvolumetothememoryofDieterSellmann.
Dieter Sellmann studied chemistry in Tu«bingen and Mu« nchen, and
did post-doctoral work at Princeton University, New Jersey. He com-
pleted his Habilitation in1972 at theTechnical University in Mu« nchen
on dinitrogen ¢xation, a topic that fascinated him and his many
collaborators over many years to come. He held his ¢rst teaching
position in Paderborn, and in 1980 accepted a call to the Universityof
Erlangen-Nu« rnberg as Professor of Inorganic and General Chemistry.
He was a remarkably active researcher and published close to 250
research papers in top international journals. He had many friends
all over the world and was well respected by many inorganic and
bioinorganicchemists.
The present volume includes eight contributions. The ¢rst chapter
by Michael Hall, Marcetta Darensbourg and co-workers presents a
detailed account of the synergybetween experiment and theoryonthe
ix
x PREFACE
H/D exchange activity of [Fe]H ase active site models. In the subse-
2
quent chapter, FelixTuczek focuses on the end-on terminal reduction
pathwayof dinitrogen at Mo andWcenters. Markus Reiherand Bernd
Hess use quantum mechanical calculations to investigate biological
nitrogen ¢xation in Chapter 3. In another theoretical chapter, Per
Siegbahndescribesprotonandelectrontransferin[NiFe]hydrogenase.
In the following chapter, Gregory Kubas presents a detailed account
of the heterolytic splitting of H^H, Si^H and other s bonds. Subseq-
uently, Andreas Grohmann reports on the development of tetra-
podal pentadentate nitrogen donor ligands and their coordination
compounds. In Chapter 7, Istva¤n Marko¤ and his co-workers report on
the catalytic aerobic oxidation of alcohols, which is followed by the
¢nal chapter with a contribution from Horst Kisch and co-workers on
visiblelight photolysis catalysedby titaniatransition metalcomplexes.
I sincere appreciate the e¡ective collaboration with all authors in the
preparationofthisspecialissueoftheseries.
I thoroughly believe that these contributions cover important
advances in inorganic and bioinorganic chemistry with respect to
redox-active metal complexes, and trust that the inorganic chemistry
communitywillbene¢tfromthem.
RudivanEldik
UniversityofErlangen-Nu« rnberg
Germany
SYNERGY BETWEEN THEORY AND EXPERIMENT
AS APPLIED TO H/D EXCHANGE ACTIVITY ASSAYS
IN [Fe]H ase ACTIVE SITE MODELS
2
JESSE W. TYE, MICHAEL B. HALL, IRENE P. GEORGAKAKI and
MARCETTA Y. DARENSBOURG
DepartmentofChemistry,TexasA&MUniversity,CollegeStation,TX77843,USA
I. Introduction
II. ExperimentalSection
A. H/DExchangeinD/HOMixturewithFeIFeIComplexasCatalyst
2 2
B. Reactionsof{(m-H)(m-pdt)[Fe(CO)(PMe)]}þ[PF](cid:1)withAcetone
2 3 2 6
C. ComputationalDetails
III. ResultsandDiscussion
A. ChoiceandValidationoftheComputationalModel
B. CreationoftheOpenSite
C. DihydrogenComplexes
D. H/DExchangeInhibition
E. CleavageoftheH^HBond
F. H/DExchangeintothem-HPosition
G. TheOverallMechanism
H. D/HOScramblingCatalyzedby(m-pdt)[Fe(CO) (PMe)]
2 2 2 3 2
I. Reactionof{(m-H)(m-pdt)[Fe(CO)(PMe)]}þwithAcetone
2 3 2
J. CalculationofNMRShieldingTensors
IV. Conclusions
References
I. Introduction
In view of its application to fuel cell development, research into
hydrogen activation remains a forefront area for chemists, physicists,
and biologists (1). A rekindling of opportunity and excitement in this
¢eld of chemistry has come from the delineation of simple catalytic
sitesofhydrogenaseenzymesasdisplayedbyproteincrystalstructures
published within the last decade (2^10). These active sites hold out
promise of using complexes comprised of base metals such as iron or
1
ADVANCESININORGANICCHEMISTRY (cid:1)2004ElsevierInc.
VOLUME56ISSN0898-8838/DOI10.1016/S0898-8838(04)56001-4 Allrightsreserved.
2 J.W.TYEetal.
a combination of Fe/Ni instead of platinum metal as catalysts for such
important technicalprocesses.
The starting point for the chemist is the preparation of synthetic
analogues of composition and structure as similar as possible to the
natural active site, with the expectation that the electronic properties
of the latter might be reproduced in the model complex, ultimately
engendering similar function (11). In the case of Fe-only and [NiFe]
hydrogenases, the fortunate presence of diatomic ligands, well known
to serve as reporters of electron density, has facilitated a comparison
betweenthenaturalandthesyntheticactivesitesbyprovidingcredible
reference points for the use of spectroscopy in assigning redox levels
fortheenzymeatvariousstagesofcatalyticactivityordeactivation(9).
These comparisons have encouraged a unique synergism between
computations, spectroscopy and synthetic model development (9,12).
The work described herein is an attempt to move such interactions
even closer to the goal of predicting properties needed for synthetic
catalysts designed forhydrogenactivation.
Hydrogenases are biological catalysts responsible for H uptake or
2
production, in which the required H cleavage has been established to
2
occurinareversibleandheterolyticmanner(Hþ/H(cid:1))(13).Thisactivity
is typically assayed by H/D exchange reactivity in H /D O or H /D /
2 2 2 2
H O mixtures (13^17). The active site of iron-only hydrogenase,
2
[Fe]H ase (7^10), consists of a 2Fe2S butter£ycore in which the sulfur
2
atoms are linked by three light atoms of undetermined identity, but
typically modeled by either propane dithiolate (pdt), or (cid:1)SCH N(R)
2
CH S(cid:1). The active site is connected to the ¢rst 4Fe4S cluster of the
2
electron-transport chain via a bridging cysteine. Although unusual in
nature, the diatomic ligands (CO, CN(cid:1)) that ¢ll the remaining coordi-
nation sites of each metal center harken to the genesis of the ancient
organisms and the harsh terrestrial conditions under which these
enzymesevolved (18).
The [Fe]H ase enzyme exists in at least three di¡erent redox levels.
2
The oxidized-active form, assigned as FeIIFeI, is the state that takesup
and activates H (9). Inthis stateboth metals areinoctahedralcoordi-
2
nation geometry by virtue of a m-CO group, and the distal Fe (the one
further removed fromthe4Fe4S cluster),istentativelyassignedasFeII.
This iron is coordinated bya labile H O molecule in the oxidized form
2
(7), and a CO inthe CO-inhibitedoxidized form (10) as shown in Fig.1.
Photolytic (CO-loss) conditions allow the CO inhibited form ofenzyme
toregain activityas assayedby H/Dexchangein H /D Omixtures(19).
2 2
The rapid development of [Fe]H ase active site model chemistry
2
bene¢ted greatly from early organometallic studies of (m-S )[Fe(CO) ] ,
2 3 2
SYNERGYBETWEENTHEORYANDEXPERIMENT 3
FIG. 1. Stick drawing structures of (a) CO-inhibited oxidized form of
[Fe]H aseactivesite;and(b)FeIIFeIIfunctionalmodels.Thespeci¢corientation
2
ofthePMe ligandsisEdependent:E¼H,transoid;E¼SMe,cisoid.
3
(m-SRS)[Fe(CO) ] , and (m-SRS) [Fe(CO) (L)] . Reihlen reported the
3 2 2 2
synthesis of (m-SEt) [Fe(CO) ] in 1929 (20). In the 1960s Poilblanc (21)
2 3 2
examined the ligand exchange process for a series of complexes of the
form(m-SR) [Fe(CO) ] .Poilblanc(21)andTreichel(22)investigatedthe
2 3 2
attackofelectrophiles onthe metal^metalbondof (m-SR) [Fe(CO) (L)]
2 2 2
complexes to generate {(m-E)(m-SR) [Fe(CO) (L)] }þ. In the 1980s,
2 2 2
Seyferth (23) developed the chemistry of the bridged dithiolate com-
plexes of the form (m-S(CH ) S)[Fe(CO) ] .
2 x 3 2
Diiron(II) complexes of the type {(m-E)(m-pdt)[Fe(CO) (PMe )] }þ
2 3 2
(E¼H or SMe) as seen in Fig. 1 were examined as potential struc-
tural/spectroscopic models of the [Fe]H ase active site, using PMe as
2 3
a substitute for thereactive cyanideligands (24^26).
WiththeencouragementofProf.DieterSellmannin2001,andusing
his experimental protocol (27) we explored the reactivity of FeIIFeII
complexes toward D and D /H O mixtures. In order to establish the
2 2 2
factors a¡ecting such reactions, solutions of these complexes under
various conditions were pressurized with D in a medium pressure
2
NMRsampletube.The2HNMRspectroscopicmonitorofthereactions
indicated the build-up of D-incorporated species (24^26). Control
experiments established that the activation of D in these reactions
2
was facilitated by light and was inhibited by coordinating solvents or
the addition of CO (24,25).This last feature is in agreement with the
CO-inhibition of [Fe]H ase activity and strongly suggests the need for
2
creationof anopen siteprior to D binding to FeII.
2
Therelativelysimpleactivesiteof[Fe]H aseandthelimitedinvolve-
2
ment of the protein as ligands in the ¢rst coordination sphere has
appealed to computational chemists as an appropriate system to
explore by Density FunctionalTheory (12,28^32).The calculations pub-
lished to date have focused on correlating (cid:1)(CO)/(cid:1)(CN) vibrational
frequencies of the di¡erent redox levels of the diiron active site with
4 J.W.TYEetal.
model complexes, on de¢ning plausible possibilities for the unique
threelight-atom S to S linker, and on delineating mechanistic possibi-
lities for H activation (12,28^32). Until now, none of the published
2
computational models have attempted to explore how the [Fe]H ase
2
activesiteperformstheactivityassay,i.e.,theH/Dexchangereactivity
inH /D OorD /H Omixtures.Herein,DFTcalculationsaredescribed
2 2 2 2
that suggest reasonable mechanistic explanations for the experimen-
tally observed H/D exchange reactivity, not of the enzyme active site,
but of FeIIFeII functional modelcomplexes. Newexperiments have also
beencarriedout inorder totest thehypotheses impliedbysome of the
individual steps of the proposed mechanism, whichwere calculated to
be energetically feasible.
II. Experimental Section
Reagents used in the preparation of starting materials, procedures,
and instrumentationhavebeen describedearlier(24,25).
A. H/DEXCHANGEIND2/H2OMIXTUREWITHFeIFeICOMPLEXASCATALYST
In a typical experiment 0.8 mL portions of solutions made from
0.029 g (m-pdt)[Fe(CO) (PMe )] in 1 mL CH Cl were placed in
2 3 2 2 2
medium-pressureNMR sampletubes (Wilmad,528-PV-7) together with
2 mL H O. The tubes were degassed, pressurized with 10 bar D and
2 2
exposed to sunlight as shown in Fig. 2. 2H NMR spectra were taken
at timeintervals to follow the formationofHOD.
B. REACTIONSOF{(m-H)(m-pdt)[Fe(CO)2(PMe3)]2}þ[PF6](cid:1)WITHACETONE
A solution made from 0.095 g {(m-H)(m-pdt)[Fe(CO) (PMe )] }þ[PF ](cid:1)
2 3 2 6
in 10 mL acetone was exposed to sunlight for 50 min.The acetone was
removed under vacuum and the resulting solid was redissolved in
7^10 mL CH Cl .The IR spectrum ((cid:1)(CO) region only) of this solution
2 2
showed a mixture of the startingcomplex (bands at 2031(s) and1978(s)
cm(cid:1)1) and the presumed acetone complex (bands at 2031(s), 1989(m),
1978(s), and 1945(s) cm(cid:1)1). After bubbling CO through the solution for
5 min, the IR spectrum showed the disappearance of the (cid:1)(CO) bands
at 2031, 1989, 1978, and 1945 cm(cid:1)1, while the (cid:1)(CO) bands at 2031 and
SYNERGYBETWEENTHEORYANDEXPERIMENT 5
FIG. 2. Medium pressure NMR sample tubes containing solutions of the
diironcomplexes, pressurizedwith10 bar D andwereexposed to sunlight on
2
thewindowsill.
1978 cm(cid:1)1regained intensity. A similar reactionwas carried out in an
NMR sample tube using 10 mg of {(m-H)(m-pdt)[Fe(CO) (PMe )] }þ
2 3 2
[PF ](cid:1) in 0.8 mL acetone-d . After exposure to sunlight for 1 h the 1H
6 6
NMR spectrum showed two sets of resonances in the up¢eld region.
A quartet centered at (cid:1)7.7 ppm with J coupling constants of 29.7
H^P
and 21.3 Hz was assumed to be the acetone complex, {(m-H)(m-pdt)
[Fe(CO) (PMe )][Fe(CO)(PMe )(acetone)]}þ; and a triplet centered at
2 3 3
(cid:1)15.0 ppm with J 22.8 Hz, derived from the parent compound,
H^P
{(m-H)(m-pdt)[Fe(CO) (PMe )] }þ. The 31P{1H} NMR spectrum dis-
2 3 2
played a doublet centered at 24.3 ppm and another doublet centered at
22.4ppm,bothwithJ couplingof7.4Hz.ThreemicrolitersofCH CN
P^P 3
wereaddedandthesamplewas maintainedinthe darkfor30min.The
1H NMR spectrum of this sample showed the disappearance of the
hydride resonance at (cid:1)7.7 ppm and the appearance of a new hydride
resonance as a doublet of doublets centered at (cid:1)10.9 ppm.This hydride
signal was identical to that of a bona ¢de sample of {(m-H)(m-pdt)
[Fe(CO) (PMe )][Fe(CO)(PMe )(CH CN)]}þ,whosepreparationandfull
2 3 3 3
characterizationwas reportedearlier(33).