0FDB -9H9AMGH .C HD 3DA

PRIZE WINNERS 19CHIMIA 1999 53 No 12

Chimia 53 (1999) 19-28copy Neue Schweizerische Chemische Gesellschaft

ISSN 0009-4293

From Catalyst Design to MolecularDevices Theory and Experiments

Thomas R Ward Werner Prize Winner 1995a)

Abstract Three different topics are presented herein With the help of molecular-orbital analysis the uniquegeometry and catalytic properties of dO bent-metallocenes was analyzed in search for cyclopentadienylsubstitutesTo overcome the inherent racemization of coordinatively unsaturated piano-stool complexes a ten-electrondonor ligand was designed This ligand incorporates an arene bearing two tethers a phosphine and an imine(abbreviated PArN) It was shown that upon 176171171-coordination of PArN to ruthenium a configurationallystable piano-stool complex results [Ru(176171171-PArN)L]n+A tripodal dodecadentate ligand incorporating three soft bipyridine donors as well as three harder salicylamidechelates was synthesized This ligand allowed the investigation of the iron release from ferric enterobactinsuggesting a protonationtranslocation into a salicylate-binding mode In the presence of a single iron ion anddepending on its oxidation mode it was shown that this system displays switch-like properties

rn

lt=roI

o0

oo-

Thomas R Ward was born in Fribourg onJanuary 8th 1964 as the last of 6 children ofJohn E Ward and Ada Lovinger Ward As anAmerican citizen he obtained Swiss nationalityin 1976 He received his diploma in chemistry in1987 from the University of Fribourg with or-ganic chemistry as major and inorganic chem-istry as minor subjectsFrom 1988 to 1991 he was a doctoral studentin the group of Prof L M Venanzi at the ETH-ZOrich His PhD thesis dealt with the synthesisand coordination properties of C3-symmetricphosphine ligands and their use as acetaliza-tion catalysts This work benefited from a fruit-ful collaboration with Prof D Seebach as wellas with Ciba Geigy which patented these sys-temsFascinated by group theory hejoined the groupof Prof R Hoffmann at Cornell University as aSwiss National Science Foundation postdoc-toral fellow (1991-1992) This theoretical ex-cursion lead him into the fascinating field ofheterogeneous catalysis Why is rhodium soefficient at removing NO from car exhaustSoon after returning to Switzerland for a sec-ond postdoc in the group of Prof C Floriani atthe University of Lausanne he was awardedthe A Werner Fellowship and moved to Berneto undertake his independent career in Fall1993

1 Introduction

Catalysis is perhaps the word whichbest describes the spirit of chemistry themiracle of consumption and regeneration[1] Ever since my beginnings in scienceI have been fascinated by all aspects ofcatalysis bioinorganic theoretical orga-nometallic or as a tool to generate librar-ies [2]

After having identified a relevant prob-lem from the current literature I like to runqualitative molecular-orbital calculationsto rationalize the published observationsCoincidentally these form the basis for asynthetic project Three distinct projectsat different stages of achievement are pre-sented herein i) outlining a problem withthe help of molecular orbital theory ii)designing a ligand system and iii) appli-cations

2 The Geometry of dOPentacoordinate Complexes [3]

21 BackgroundIn the field of homogeneous catalysis

the most versatile catalysts may well bethe so-called bent-metallocene systemsThese contain a dO-metal flanked by twocyclopentadieny Is (abbreviated Cp) Mostoften the catalyst precursors are[MCp2L2]-likecompounds If one consid-ers Cp as a six-electron donor occupying a

single coordination site such complexesare four-coordinate distorted tetrahedrons(T-4) with a large CPcentroirM-CPccnlroidangle [4] To understand their unrivalledcatalytic properties however we focus onfive coordinate [MCp2L3]-complexesPentacoordinate complexes playa pivotalrole in transition-metal catalysis as five-coordinate transition states or intermedi-ates have often been postulated either forassociative reactions involving tetrahe-dral (T-4) or square planar (SP-4) com-plexes or for dissociative reactions in-volving octahedral (OC-6) complexes [5]

The square pyramid (SPY-5) and thetrigonal bipyramid (TB-5) represent pro-totypical geometries of five-coordinatecomplexes The [MCp2L3] compoundshowever cannot be categorized as eitherTB-5 or SPY-5 This is illustrated withthree examples [TaCp2H3]1is one of thefirst structurally characterized [MCp2L3]complexes [6] [zrCP2Cl(172-CH3CO)]2is an early example of an 172-boundacyl[7] and 3 contains a planar four-coordi-nate carbon [8] Interestingly the pro-posed transition state 4 for the a-olefinpolymerization catalyzed by bent metal-

Correspondence Dr TR WardDepartment of Chemistry and BiochemistryUniversity of BerneCH-3000 Berne 9E-Mail wardiacunibech

a) See Chimia 1998 52 744


a 130deg 120deg 110deg 100deg 90deg 80deg 70deg 60deg

Fig 2 Walsh diagram for the SPY-5--+TB-5--+EBT-5 interconversion of [TiHsI Dotted line Etotlabels in parentheses correspond to D3h irreducible representations

locenes also displays this unusual geome-try Structural features common to thesecomplexes are a bent MCp2-fragmentand a coplanar arrangement of the threeligands L with two acute L-M-L anglesWe call this unusual [MCP2L3] geometryedge-bridged tetrahedral abbreviated asEBT-S (Fig 1)

We reasoned that perhaps the uniquecatalytic properties of bent-metallocenecomplexes may be related to their unusualEBT-S geometry oftheir pentacoordinatecomplexes We thus set out to analyze theelectronic origin of this geometry withthe aim of finding Cp-substitutes whichwould also favor EBT-S geometries

22 Theoretical Analysis221 [TiHst

We begin our analysis with an extend-ed Huckel (eH) description of the bondingof a trigonal bipyramidal five-coordinatedO complex containing pure (J-donorsThese qualitati ve arguments were assessedby Density Functional Theory (DFT) Asimplified molecular-orbital diagram fora [TiHs]- model is sketched in the middleof Fig 2 The HOMO is a2 and is mostlyligand-centered The dxy and dyz orbitalsremain unperturbed and correspond to theLUMO (e) The second-order Jahn-Tell-er distortion (2OJTD) [9] of e symmetryallows mixing of HOMO and LUMOs(fHOMO reg fLUMO = a2 reg e = e) Byvarying the Hpiv- Ti-Hax angle 0 from130deg to 55deg we find two minima at 1200

(Etat = 000 eV) and 700 (Etat = +022 eV)which correspond to the SPY-S and EBT-S geometries respectively The TB-S (a=90deg Etat= + 100 eV) is a transition state inthis diagram The SPY-S vs EBT-S pref-erence can be traced back to a difference inligand-ligand interactions associated withthe 2a orbital artificially setting the LucLeg and LaCLpiv overlaps to zero reducesthe energetic advantage of the SPY -5 overthe EBT-S to only 005 eV It thus appearsthat the EBT-S geometry is a result of a20JTD of e symmetry As the forwarddistortion (0 gt 90deg) presented in Fig 2corresponds to the Berry distortion wesuggest that the EBT-S geometry resultsfrom a distortion along a reversed-Berrypathway

Since the 20JTD is doubly degenerate(e symmetry) we must investigate a sec-ond distortion also of e symmetry The a-distortion presented in Fig 2 correspondsto a rotation of both M-Lax vectors aroundthe x-axis We investigate a f3-distortionwhich corresponds to a rotation of bothM-Lax vectors around the y axis (see Fig3) The whole e distortion pathway wasprobed by independently varying 00

f3

H T ~H

H-Ti~ bullbullbullbullHH

3

CI--AIMeCpbull 7- 2zCp r Ph

SiM3

4

H Ta ~ -HH-1Ti~bullbullbullbullH

H

2

CICp cp~zro

Me

1

HCp

C Ta-Hp -H

2~_

-9~

-11 -

reg2a1

~b ~ GrcD-~ M = d metalU 1a1 LUMO

Fig 1 The three lowest-lying unoccupied orbitals of the bent dOMCp fragment dictate thecoplanar arrangement of the ML3J fragment in [MCP2L3l complexes 1-3 and transition state 5


I bull

I precession aXIsI

Lax I rl~f

U ~Leq

Lpv-f-M~Uy Leq

Lax I rl~f

2a = 180deg Y = 120deg TB-52ult 180deg ~ = 0deg120deg240deg EBT-52ult 180deg ~ = 60deg180deg300deg SPY-5

lG8CiiilGs -01

sect -02

-0

-04

-0

-050 -030 -010 010 030 050

cos(alpha)cos(beta)

s-CI)aCiiibulllGs -01

I -02

-0

-04

-0

--050 -030 -010 010 030 050

cos(alpha)cos(beta)

Fig 3 Potentia-energy surface E= f(a3)for the SPY-5--7TB-5-poundBT-5 interconversion of (TiHsl- (left) and of (Ti(NH2hHJ- (right) The coordinatesabscissa = cos amiddotcos 3 ordinate = cos amiddotsin 3 correspond to the position of an M-Lax unit vector projected onto the xz-plane containing theMLeqLeqLpiv fragment equienergy contours in eV

360deg and 55deg ~ a ~ 90deg The resultingpotential-energy surface (PES) is present-ed in Fig 3 with ordinate cos amiddot sin 3andabscissa cos amiddotcos 3These coordinatesrepresent the projection of M-Lax unitvectors onto the equatorial planeMLeqLeqLpivThis PES shows the expect-ed Mexican-hat features with three mini-ma and three saddle points correspondingto the SPY-5 (a = 60deg 3= 60deg 180deg and300deg) and EBT-5 (a = 70deg 3= 0deg 120degand 240deg) geometries respectively TheEBT-5 which appears asaminimumin theWalsh diagram E = f(a) (Fig 3) is in facta saddle point on the two-dimensionalsurface (E = f( af3) and lies well below themaximum representing the TB-5

We conclude that a dO complex con-taining five pure a-donors should displaySPY-5 geometry A Cambridge Struc-tural Database (CSD) search revealed asingle homoleptic compound containingfive pure a-donors around a dO metal[Ta(CH2(4-MeC6H4raquos] [10] This com-pound indeed displays an Spy -5 structure(2a= 1381deg 3= 1772deg) Recently thestructure of [TaMes] was determined by

gas-phase electron diffraction In contrastto its main group counterpart [SbMes] thedOcomplex has an Spy -5 geometry [11][12]

As can be appreciated from Fig 2 thetotal energy of the e-distortion followsmostly the fate of the a orbital in D3hsymmetry (bl irreducible representationin C2v point group) Introducing ligandscapable of interacting with this orbitalwhich contains a large contribution of thedyz orbital (in both C2v geometries) mayinvert the SPY -5 preference eventuallyleading to an EBT-5 ground state geome-try

222 [Ti(NH2Y2H31-Adding two equatorial ligands capable

of Jr-interaction with their Jr-systems per-pendicular to the equatorial plane fulfilsthis requirement the in-phase combina-tion of the two Py orbitals is of bl symme-try Let us replace two equatorial hydridesby two amides with their p-orbital per-pendicular to the xz-equatorial plane[Ti(NH2)2H3]- The Walsh diagram forthe a-distortion of [Ti(NH2hH3]- is pre-

sented in Fig 4 As suspected we com-pute the EBT-5 geometry (a = 70deg Etat =000 eV) to be preferred over the Spy -5 (a= 110deg Eta = 091 eV) with a slightlydistorted TB-5 (a= 95deg Elot= 100 eV) astransition state

Computing the PES E=f(af3) wefindan EBT-5 minimum with a soft potentialalong the f3-precession coordinate Thisfeature will be important in discussingexperimental structural data (Fig 3b)

Introduction of double-faced Jr-donorligands (rather than the single-faced Jr-donors presented above) does not alter theoverall picture This can be traced down tothe fact that the second Jr-interaction (typ-ically a Px orbital) does not have bl sym-metry and thus does not contribute signif-icantly to the fate of the total energy in Fig2

It thus appears that the geometry adopt-ed by five-coordinate dO-[MD2L3] com-plexes with strong single- or double-facedJr-donors D as well as that adopted by[MCp2L3] complexes is the result of thesame 20JTD along a reversed-Berry co-ordinate From our model calculations it


Fig 4 Walsh diagram for the SPY-5~TB-5~EBT-5 interconversion of [T7rNHz)2H3- Only theMOs containing Npy contributions are sketched All other MOs are very similar to those of Fig2 (Dotted line EtoJ

120deg) and six small (ca 90deg) interligandangles are expected After computing allten interligand angles for each complexthe largest angle was assigned as the LacM-Lax angle The remaining three lig-ands were taken to define the equatorialplane

In those cases where two large angles(ca 150deg) were computed thus suggest-ing an SPY -5 coordination the equatorialplane was defined in terms of the twostrongest Jr-donor ligands 0 and 0 and theremaining equatorial ligand Lpiv

The observed structural data for[M02L3] -complexes (0 = strong Jr-donorL = pure a-donor) are displayed in Fig 5along with the eH-isoenergy contour at01 eV for the model [Ta(NH2hH3]2- Inmost compounds of this class the axialsubstituents bend away from the Jr-donorsand towards the pivot (2a lt 1800 andf3 lt 30deg) Five of the seven compoundsare within the minimum-energy regioncalculated for the model complex[Ta(NH2hH3P-

Inspection of [Ta(CHBu1hMes-(PMe3h] (Entry 1) reveals an EBT -5 ge-ometry The presence of a very bulkymesitylene in the pivot position preventsan efficient 20JTO resulting in a large a-angle (2a = 1663deg)

Systems incorporating one strong andone weaker Jr-donor can be expected toadopt geometries intermediate betweenEBT-5 encountered with 2 strong Jr-do-nors and SPY -5 a geometry prevalentwith pentacoordinate compounds bearinga single Jr-donor [13][14] Such an inter-mediate geometry is observed not onl y for[Re02(Nph] (Entry 4) but also for[WO(NEt2)Np3] (Entry 7) [15] In bothcases the axial ligands precess towardsthe weaker Jr-donor the longest mostdistant oxo- and the amide-groups re-spectively

For [MCp2L3J complexes the 2a an-gles fall in the range 1480-1021deg It isinteresting to note that the complexeswhich are least bent are those with themost electronegative Lax Electropositiveaxial donors favour the 20JTD since inthe TB-5 geometry the HOMO is essen-tially located on the axial donor ligands(see Fig 2) Good axial donors raise theenergy of this orbital and thus favor anefficient 20JTO as this latter is inverselyproportional to the HOMO-LUMO gapThis is nicely reflected with compoundscontaining axial silanes which all displayvery acute LacM-Lax angles despite sig-nificant steric interactions with Lpiv

It should be noted that the minimumcomputed for our model [TaCp2H3] isvery deep and small distortions both in the

2~

Etal

232 Ligand Labelling and Distor-tion Mapping

To unambiguously determine the rele-vant distortion angles a consistent ligand-labelling scheme is required Since thecomplexes can be viewed as distorted TB-5 one large laquo 180deg) three medium (ca

our surprise this search yielded no morethan seven pentacoordinated dO-complex-es containing only two strong Jr-donors

Eventually we relaxed our stringentdefinition to incorporate complexes whichcontain two strong Jr-donors and up tothree weaker Jr-donors A total of36 com-plexes matched these requirements Theseresults are not presented here

For comparison all [MCp2L3]-likecomplexes were retrieved from the CSOAgain here only mononuclear complexesand those containing no chelates wereconsidered yielding a total of thirteen[MCp2L3]-compounds

1a2

~

3a11b2

2a1

1b1

-13

-15

gt~gt-

crwzw

-11

appears that all dOsystems containing onlytwo strong single-faced Jr-donors in theequatorial plane with their filled p orbitalperpendicular to this plane (Py) should dis-play an EBT-5 geometry In the followingsection we test this model with structuraldata retrieved from the CSO

-9

23 Structure Correlation231 Fragment Definition

In order to test the above hypothesiswe extracted all pentacoordinate dOcom-plexes from the CSO After defining andretrieving the structures of interest wemapped the available structures in the two-dimensional configuration space spannedby a and f3

To ensure that the geometry is not bi-ased by ligand constraints all polynuclearcomplexes were excluded as well as thosecontaining chelating- or It-arene-ligands(ngt 1) Only those compounds containingtwo strong Jr-donors were considered To


Fig 6 Mapping of [MCP2L3fx- structures The dotted circular line represents the 01 eV eHisoenergy contour computed for [TaCP2H31(Definition of coordinates see Fig 3)

Fig 5 Mapping of [M02L3fx- structures (0 = strong rr-donor L = pure a-donor) The dotted circularline represents the 01 eV eH isoenergy contour computed for [Ta(NHl2H3]2- (Definition ofcoordinates see Fig 3)

060

040

040

020

+

+7

-3 1 1~5 6 -----t- ---1- 5=f - - - - - - t -

I -t= II ~

I II I

00

cos(a)cos(~)

I

I

I

II

I

-020 00 020

cos(a)cos(~)

I

I

I

I

I

II

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

II

I

I

I

I

I

I

-020

-040-060

-040

060

040

-060

-040

020

a-c(f)~ 000~en0u

-020

-020

040

-040

020

ec(f)

000~(f)

ou

31 BackgroundIn the field of enantioselective transi-

tion-metal catalysis ligand design plays avery prominent role In the spirit of EFischers lock and key concept the ster-ic bulk on the chiral ligand is relayed by ametal template to a prochiral substrateeventually giving rise to chiral inductionTo simplify matters Cz-symmetric biden-tate ligands were tailored reducing thenumber of diastereomers from four to twoupon complexation of a prochiral sub-strate

In the early nineties however elec-tronically asymmetric bidentate ligandswere introduced and rapidly found manyapplications in enantioselective catalysis[21-23] In such systems in addition tosteric arguments electronicfactors playacritical role in determining which diaster-eoisomer (or diastereoisomeric site) re-acts faster to yield the enantiomericallyenriched product Such C1-symmetric bi-

3 Synthesis of ConfigurationallyStable Three-Legged Piano-StoolComplexes [20]

24 OutlookA theoretical analysis has revealed that

the EBT-5 geometry of dObent metal-locenes [MCp2L3] is the result of a re-versed-Berry distortion Such EBT-5 ge-ometries are predicted for all five coordi-nate dO complexes which incorporate twostrong IT-donors Our theoretical modelwas tested with a structure-correlation anal-ysis of all dOfive-coordinate complexesincorporating two strong IT-donors

In the spirit of Muetterties and Gug-genbergers mapping of the Berry path-way [16] the dOcomplexes containing twoIT-donorscan be arranged in a sequencethat maps a reversed-Berry pathway (Fig7)

Based on this study we predicted thatdObis-amide- systems should display bent-metallocene-like catalytic properties Wewere pleased to see that such systemsreported by McConville Schrock and Gib-son indeed display very promising olefin-polymerization properties [17-19]

a and [3directions are costly in energyThe distortion along the [3coordinate isdisfavoured purely on steric grounds at a= 65deg and [3=30deg the shortest HacHcp andHacCcp contacts are 205 A and 202 Arespectively By artificially setting the Hax-Hcp and Hax-Ccp overlaps to zero theshallow minimum observed for[Ti(NH2)2H3]- in EBT-5 is restored andthe [3-precession about the y-axis is soft


L MO oc d 2~ - (n) + (lr )

orbital of the allyllr-system Close inspec-tion of the dx2 orbital of Pd(PH3)-(NH2) + however reveals that this Frag-ment Molecular Orbital (FMO) is hybrid-ized away from the nitrogen a mere re-flection of the electronic asymmetry causedby the PAN ligand This hybridization al-lows mixing of both the lr and 7rorbitalsof the allyl-fragment into the LUMO astheir overlap with the hybridized dx2

orbital is no longer zero Because of thegood energy match between the 7rand theLUMO only this orbital contributes (andnot the lr orbital) significantly to theLUM O Perturbation theory allows to pre-dict that

As can be appreciated from Fig 8 theCrral1sP bears the greatest coefficient andthus is predicted to be most electrophilicIt should be pointed out that this picture isindependent of the all yIconformation (exoor endo) As both these conformationsinterconvert rapidly under true catalysisconditions we conclude that the nucle-ophile attacks preferentially the Cr(IIISP ofthe exo-conformation yielding the ob-served enantiomer This prediction wassubsequently substantiated by DFT meth-ods [28][29]

Qualitative MO arguments thus helprationalize the role of electronic asymme-try in enantioselective catalysis Similararguments were used with Fallers piano-stool complex [CpMo(CO)(NO)( 1J3-allyl)]5 to rationalize the site of nucleophilicattack on the coordinated allyl [25]

322 Geometry of CoordinativelyUnsaturated Two-Legged Piano-StoolComplexes [30]

Following the synthesis of chiral-at-metal piano-stool complexes initiated byBrunner et ai the question of configura-tional stability of such systems was ad-dressed by various groups [31] Of par tic-ular interest in the context of catalysis arecoordinatively unsaturated piano-stoolcomplexes as these would be invariablyinvolved in a catalytic cycle involvingpiano-stool complexes as catalysts

It had been suggested by Hofmann that16-electron complexes of the type [M(17-CnHn)LL] (n = 4-7) may have pyramidaland thus chiral-at-metal ground-state ge-ometries [32] Twenty years later no firmexperimental proof unambiguously vali-dated this prediction We thus re-analyzedthese systems in search of more quantita-tive predictions ie inversion barriers

Our calculations suggest that indeedcoordinatively unsaturated two-legged pi-

+

~ EBT-5

6

copyHI

Jr7eO~ PPh3

R a

Orientation of prochiral substratedictated by electronic asymmetry

fect on the enantiomeric excess of thereaction

Before studying the role of chirality-at-the-metal in enantioselective catalysiswe analyzed theoretically the role of elec-tronic asymmetry in the palladium-cata-lyzed allylic alkylation as well as the ge-ometry of coordinatively unsaturated two-legged piano-stool complexes Based onthese results we designed a chiral-at-met-al piano stool complex which displaysremarkable configurational stability

32 Theoretical Considerations [27]321 On the Regioselectivity of aNucleophilic Attack on [Pd(allyl)(phosphine)(imine)] Complexes

To understand the role of electronicasymmetry we analyzed the frontier or-bitals of a model [Pd(1J3-C3Hs)(PH3)-(NH2)] We reasoned that since the coor-dinated allyl fragment undergoes a nucleo-philic attack the carbon atom which bearsthe greatest coefficient in the LUMO ismost prone to be functionalized by theincoming soft nucleophile The interac-tion diagram for [Pd( 1J3_C3HS)(PH3)-(NH2)] is built from the Pd(PH3)(NH2)+and (1J3_C3HSt (see Fig 8)

The LUMO of such systems consistsessentially of the out-of-phase combina-tion of the dx2 with the non-bonding

Reversed-Berry Pathway

+

5

copyI

MoRON CO

~R

Site of nucleophilic attackdictated by electronic asymmetry

Berry PathwaySPY-5 ~ TBP-5

dentate ligands offer the interesting pro-spective of studying the role of metal-based chirality To our knowledge thisarea has received only little attention de-spite interesting preliminary reports

As early as 1979 Falleret ai reporteda chiral molybdenum-based promoter 5for the functionalization of allylic sub-strates [24] It was shown that when coor-dinated to the [CpMo(CO)(NO)j+-moie-ty a symmetric allyl could be functional-ized diastereoselectively at the carbon cis-positioned to the nitrosyl in the exo-con-formation [25] As a nitrosyl ligand is asbulky as a carbonyl ligand there remainslittle doubt that the diastereoselectivitymust be caused by electronic rather thansteric arguments Unfortunately howev-er the system was stoichiometric and notcatalytic Similarily Giadysz has exten-sively studied the Lewis-acidic [ReCp-(NO)PPh3]+ fragment 6 as a promoter forfunctionalizing prochiral substrates [26]Again here only stoichiometric applica-tions have been reported

One possible explanation for this factcould well be the configurational labilityof the coordinatively unsaturated piano-stool complexes which are invariably in-volved in catalytic cycles If a chiral-at-metal complex were to racemize duringcatalysis this would have a dramatic ef-

Fig 7 Mapping of the reversed-Berry pathway with five-coordinate dOcomplexes incorporatingtwo n-donors and three a-donors


Fig 8 Simplified interaction diagram between Pd(NH2YPH3l+ and (C3HcJ-

X

d 2 2X -y

H

~H

H

HbullbullJHH ~IN-Pd J H

H H ~H

H

--------

d orbitals

1t

X~---------

n33 Synthesis and Characteriza-tion of a Configurationally StablePiano-Stool Complex [34]

Having analyzed in detail phosphine-imine systems (vide supra) we set out tosynthesize a ten-electron donor ligand in-corporating an electron deficient imineand a phosphine tethered to an arene (ab-breviated PArN) The ligand synthesis aswell as its coordination to ruthenium aresummarized in Fig 10 After 716711-coor-dination a racemic planar chiral complex[Ru(71671-PArN)CI2J (8) was obtainedAfter many unsuccessful derivatization-and crystallization experiments we werepleased to find that the racemate could beresolved by preparative HPLC on Chiral-pak AD using EtOH to afford both en-antiomers in nearly quantitative yieldChloride abstraction in a coordinatingsolvent yields the chiral-at-metal complex[( 71671711-(PArN) Ru(OH2)](OTfh 9which displays remarkable configuration-al stability Its X-ray structure is depicted

ano-stool complexes possess a pyramidalground-state geometry In all cases how-ever the inversion barriers via a planarachiral geometry are low The best candi-date [FeCp(NO)SiR3]+ incorporates anelectropositive (J-donor (SiR3) and an ex-cellent 1r-acceptor (NO+) Unfortunatelyits inversion barrier is computed at 15kcalmiddot mol-J and thus is expected to readilyracemize in solution at room temperatureSuch systems have much in common withamines which are pyramidal but readilyracemize in solution as their inversionbarriers are low in most cases

For both N-based and metal-based chi-rality the pyramidalization is caused by a20JTD away from the planar achiral ge-ometry For N-based chirality electronictuning of the substituents on nitrogen (i eelectronegati ve substituents or incorpora-tion into a small ring) suffices to preventrapid racemization Incorporation of thenitrogen in a bicycIic framework locks theconfiguration and allows the separation ofenantiomers This was elegantly achievedwith the resolution of Tragers base byPrelog [33]

We reasoned that tethering of two do-nors on an arene would yield after7167171-coordination to a metal center abicyc1ic-like framework and thus preventracemization (see Fig 9)

34 OutlookBased on two theoretical analyses ad-

dressing the role of electronic asymmetryin enantioselective catalysis and the ge-ometry of coordinatively unsaturated pi-ano-stool complexes we have synthesized

a configurationally stable three-leggedpiano-stool complex which displayspromising catalytic activities in variousC-C-bond forming reactions (ie Mukai-yama aldol Diels-Alder reaction and cy-cIopropanation) Although a considerable

effort may be required to optimize theligand design to obtain excellent levels ofinduction we have shown that such sys-tems are amenable to address the role ofchirality at the metal in enantioselectivecatalysis


well as equimolar amounts of dioxygen asa by-product

Long before the appearance of dioxy-gen organisms had developed an addic-tion to iron for various purposes Thechoice of iron may well be due to itsabundance (fourth most abundant elementin earths crust) as well as its versatility asa catalyst thanks to its broad range ofaccessible oxidation states Photosynthet-ic activity dramatically decreased the avail-ability of iron in water as dioxygen oxi-dizes iron to its ferric state with subse-quent production of rust as illustrated in(Eqn2)

Thereafter the dioygen concentrationin the atmosphere rose steadily and stabi-lized at about 20 ca 300 million yearsago [37] This elicited the appearance ofaerobic cells that could not only withstandthis pollution but could even turn it to theiradvantage by developing respiratory andoxidative processes capable of extractingenergy more completely from nutrient mol-ecules

Paradoxically the iron required as cat-alyst for photosynthesis became scarcebecause this reaction produces dioxygenand indirectly rust Hard-pressed organ-isms eventually came up with an elegantsolution to this threat Iron-scavengingagents referred to as siderophores werereleased by organisms to collect the vitalmetal Siderophores are chelating ligandswhich display very high affinity for ironTypically the binding constants of theseligands are higher than the solubility prod-uct of rust under physiological conditionsallowing siderophores to extract ferric ionsfrom rust

Almost all bacteria and fungi secretelow-molecular-weight siderophores toscavenge iron from their environmentMost natural and synthetic siderophorescontain either three hydroxamate- or threecatechol-binding sites Enterobactin a tris-catecholate ligand is the most powerfulnatural siderophore known to date with anoverall stability constant of ca 1049 Withsuch high binding constants the iron-re-lease mechanism has attracted considera-ble attention [38]

To simulate the iron-uptake and -re-lease mechanism we designed a do dec a-dentate ligand which mimics both a si-derophore with high affinity for a hardferric ion as well as an octahedral por-

~($)-8 (R)-8

PArN7

Fe Id)middot)

Fecentrn-5~)CI Ph2

g)

a)-c)

M = d6melal

X Y 2 electron donors

configurationally stable

m))

Photosynthetic cells using light as an ener-gy source may well have been the re-sponse to the dearth of energy [36] Theremarkable ability of these primitive or-ganisms to switch to the use of H20 as areductant with the concomitant produc-tion of dioxygen probably produced theworst case of pollution in terrestrial histo-ry Indeed the photosynthesis reaction(Eqn 1) produces carbohydrates essen-tial feedstocks for higher organisms as

c

Br

-Einv lt 15 kcal mor 1

F3

Ct( -II ju pH20 Ph2

CF3

9

NR2 -=====~

R R1 Einvlt 5 kcal morl

o = free coordination site

4 An Iron-Based Molecular Switch [35]

41 BackgroundOnce upon a time ca 25 billion years

ago the atmosphere surrounding theEarth contained very little dioxygenlaquo 1) As a consequence the first multi-molecular units were anaerobic and usedthe surrounding organic compounds as thesource of building materials and energyGradually the primordial soup depleted

Fig 9 Anchoring a configurationally labile chiral center in a bicyclic framework results in aconfigurationally stable complex

Fig 10 Preparation and structural characterization of the enantiomerically pure complex[(T6TTL(PArN)Ru(OH~J(OTf)29 a) 35-Bis(trifluoromethyl)pyrazole NaH DMF rt 2 h then60deg 48 h (86) b) [Pd(PPh3)4] BU3Sn(CHCH2) Toluene 100deg8 h (92) c) HPPh2 AIBN CH2CI2hv (quant) d) 05 equiv [(T6-C6HsC02Et)RuCI212 CH2CI2 rt 05 h (82) e) CHP2 110deg24h (quant) f) HPLC on Chiralpak AD EtOH g) excess CF3S03Ag THFH20 (quant)


WAVELENGTH (nm)

Fig 11 Visible-absorption spectra resulting from the treatmentofthe ferric complex Fe(III)(NNOO)3l(Aax= 460 nm) with vitamin C yielding the ferrous complex [Fe(II)(NNOO)i12+ (Aax= 575 nm)

400 440 480 520 560 600 640 680

01f0I I0 siderophore-like

o 0 0

middot I 1N N N I I porphyrin-like

N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05

w 03lt)zlttlDao()lDltt 02

42 Results and DiscussionIn contrast to hydroxamate-based si-

derophores and due to its stability thereduction potential of [Fe(lll)( entero-bactin) ]3-lies outside the range accessiblewith natural reducing agents (ie NADHand FADH2) [41] Therefore alternativerelease mechanisms have been investigat-ed for [Fe(IlI)(enterobactin)p- The mainpathway seems to occur via ahydrolysis ofits tris-lactone backbone An interestingalternative is a protonation of a catecholoxygen with a concomitant translocationin a salicylate-binding mode [42]

To probe this we synthesized a tripo-dal dodecadentate ligand consisting ofthree salicylamide-binding sites and threeelectron-deficient 22-bipyridines (abbre-viated (NNOOh With this ligand at handwe showed that these systems are codedfor the oxidation-state-selective iron che-lation and iron transport The low-spinferrous ion binds selectively to the softtris-bipyridine pocket [Fe(II)(NNOOhF+while the high-spin ferric ion binds to theharder tris-salicylamide pocket [Fe(IlI)-(NNOOh] Moreover it was observedthat oxidation or reduction induces in-tramolecular (depending on conditions)reversible iron translocation between thesetwo sites thus revealing switch-like prop-erties This is best illustrated with visiblespectra resulting from the titration of theferric complex [Fe(Ill)(NNOOh] withaliquots of aqueous vitamin C resulting inthe formation of the ferrous complex[Fe(II)(NNOOhF+ (see Fig 11) Alterna-tively the ferrous complex may be oxi-dized with H202to yield the ferric com-plex These two series of spectra are su-perimposable

phyrin-like environment to accommodatethe softer ferrous ion [39][40] We rea-soned that in the presence of a single ironion and depending on its oxidation statethe metal ion would bind selectively toonesite or the other Oxidation or reductioncould be used to drive the metal reversiblyand intramolecularly from one site to theother as schematized in below

43 OutlookThe iron localization oxidation state

and translocation are conveniently ad-dressed by visible spectroscopy Further-more the Mossbauer spectrum for theferric complex is fully consistent with thatobtained by Raymond upon lowering thepH of [Fe(III)(enterobactin)]3- solutionsthus supporting the iron-release mecha-nism from enterobactin via the salicylate-binding mode [42] A summary of thespectroscopic data is presented in Fig 12

5 Conclusions

Three different projects were outlinedin this paper- i) What makes dObent-metallocenes so

unusualA MO analysis coupled with a struc-ture correlation revealed that [Cp2ML3]complexes can be viewed as trigonalbipyramidal structures which undergodistortion along a reversed-Berry path-

way Given the right electronic envi-ronment this distortion is energetical-ly favoured over the Berry distortionand is in fact quite common Exten-sions of this work to metals with differ-ent electron counts as well as the syn-thesis of novel cycIopentadienyl sub-stitutes are planned [43]

- ii) What is the role of metal-basedchirality in enantioselective catalysisTo probe this question we developed a

PRIZE WINNERS 28CHI MIA 1999 53 No 12

Fig 12 Summary of the spectroscopic data of the ferric complex [Fe(III)(NNOO)3J and the ferrouscomplex [Fe(II)(NNOOhj2+

ttltUVVis max 574 nm (E 2300) 460 nm (E 3200)

max 543 nm (E 2250)

Mbssbauer o (mms) 036 049AEa (mms) 037 099

CV +440 mV (vs SeE) -368 mV

[16] EL Muetterties LJ Guggenberger J AmChem Soc 1974 96 1748

[]7] R Bauman WM Davis RR SchrockJAm Chen Soc 1997 119 3830

[18] 10 ScaliaId DH McConville J AmChem Soc 1996 ]]8 10008

[19J U Siemeling T TUrk WW Schoeller CRedshaw VC Gibson Inorg Chem1998374738

[20] B Therrien TR Ward Angew Chem1998 in press

[21] CG Frost J Horwarth 1MJ WilliamsTetrahedron Asym 19923 1089 J SprinzG Helmchen ibid Lett 199334 1769 Pvon Matt A Pfaltz Angew Chen Int EdEngl 199332 566 A Togni U Burck-hardt V Gramlich PS Pregosin R Salz-mann 1 Am Clzem Soc 1996 118 1031

[22] TV RajanBabu AL Casalnuovo J AmChem Soc 19961186325

[23] K Inoguchi S Sakuraba K Achiwa Syn-lett 1992 ]69

[24] RD Adams DF Chodosh JW FallerAM Rosan 1 Am Clzem Soc 1979 1012570

[25] BER Schilling R Hoffmann JW Fail-eI J Am Chem Soc 1979 101592

[26] JA Gladysz BJ Boone Angew ClleInInt Ed Engl 199736550

[27] TR Ward Organometalics 1996 152836

[28] PE B1ochl A Togni Organometallics1996 154125

[29] F Gilardoni 1 Weber H Chermette TRWard J Phys Chen A 19981023607

[30] TR Ward O Schafer C Dau] P Hof-mann Organometaics 1997 163207

[31] H Brunner Adv Organomet Chen 198018151

[32] P Hofmann Angell Chem lilt Ed Engl197716536

[33] EL Eliel SH Wilen LN Mander Ste-reochemistry of Organic Compounds JohnWiley New York 1994

[34] B Therrien TR Ward M Pilkington CHoffmann F Gilardoni J Weber Org(-nomelallics 1998 17 330

[35] TR Ward A Lutz SP Parel J EnslingP GUtlich P Bugly6 e Orvig InorgChem submitted

[36] M Olumucki The Chemistry of LifeMcGraw-Hill New York 1993

[37] EC Theil KN Raymond in Bioinor-ganic Chemistry Eds l Bertini H-BGray SJ Lippard JS Valentine Univer-sity Science Books Mill Valley Califor-nia 1994 p 1-37

[38] BF Matzanke G MUller-Matzanke KNRaymond in Iron Carriers and Iron Pro-teins Ed TM Loehr VCH Weinheim1989 p 1-121

[39] A Lutz TR Ward Hell Chilll Acta 199881207

[40] A Lutz TR Ward M Albrecht Tetrahe-dronl996 5212]97

[41] For an excellent related report see L Ze-]ikovich 1 Libman A Shanzer Narure(London) 1995 374 790

[42] VL Pecoraro GB Wong TA KentKN Raymond J Am Chem Soc 19831054617

[43] TR Ward S Duclos B Therrien KSchenk Organometaics 1998172490

[oxidation]+ base

[reduction]+ H+

Received November 4 1998

[1] R Hoffmann The Same and Not the SameColumbia University Press New York1995

[2] C Briindli T Ward Helv Chim Acta199881 1616

[3] TR Ward H-B Burgi F Gilardoni 1Weber Am Chem Soc 1997 11911974

[4] Je Green Chem Soc Rev 199827263[5] T Auf del Heyde Angew Chem Int Ed

Engl 199433823[6] RD Wilson TF Koetzle DW Hart A

Kvick DL Tipton R Bau J Am ChemSoc 197799 1775

[7] G Fachinetti C Floriani F Marchetti SMerlino J Chen Soc Chem Commun1976522

[8] D RUttger G Erker Angew Chem InlEd Engl 199736812

[9] TA AlbrightJK BurdettM-H Whang-bo Orbital Interactions in Chemistry JohnWiley New York 1985

[10] eJ Piersol RD Profilet PE FanwicklP Rothwell Polyhedron 1993 12 1779

[II] e Pulham A Haaland A Hammel KRypda] HP Verne HV Volden AngewChem Int Ed Engl 199231 1464

[12] TA Albright H Tang Angew ChemInt Ed Engl 1992311462

[13] DL DuBois R Hoffmann New 1Chem1977 1479

[14] WA Nugent J M Mayer Meta]-LigandMultiple Bonds John Wiley amp Sons NewYork 1988

[15] JPL Ny M-T Youinou JA OsbornOrganometallics 1992 112413

general approach for the synthesis ofconfigurationally stable chiral-at-met-al three-legged piano-stool complex-es We are currently testing these asLewis acids in various C-C-bond form-ing reactionsiii) How is iron released from ferricenterobactinThe synthesis of tripodal ligands in-corporating salicylamide-binding siteshas allowed us to give strong spectro-scopic support for the iron release fromenterobactin via a salicylate bindingmode Tn addition we synthesized afully functional redox-triggered mo-lecular switch Future directions in-clude inclusion of this device in anartificial membrane as well as a de-tailed mechanistic investigation of thetranslocation mechanism

Herewith I wish to express my gratitude toProf Dr A Ludi as well as the whole faculty ofthe chemistry and biochemistry department fortheir unconditional support This research wouldnot have been possible without the financialsupport from the Stifttlng fur Stipendien auf demGebiete der Chemie (Award of an A WernerFellowship) as well as the Swiss National ScienceFoundation I wish to thank my coworkers whosenames appear in the publications as well as ProfDr H -B Biirgi for sharing his passion of sciencewith me


a 130deg 120deg 110deg 100deg 90deg 80deg 70deg 60deg

Fig 2 Walsh diagram for the SPY-5--+TB-5--+EBT-5 interconversion of [TiHsI Dotted line Etotlabels in parentheses correspond to D3h irreducible representations

locenes also displays this unusual geome-try Structural features common to thesecomplexes are a bent MCp2-fragmentand a coplanar arrangement of the threeligands L with two acute L-M-L anglesWe call this unusual [MCP2L3] geometryedge-bridged tetrahedral abbreviated asEBT-S (Fig 1)

We reasoned that perhaps the uniquecatalytic properties of bent-metallocenecomplexes may be related to their unusualEBT-S geometry oftheir pentacoordinatecomplexes We thus set out to analyze theelectronic origin of this geometry withthe aim of finding Cp-substitutes whichwould also favor EBT-S geometries

22 Theoretical Analysis221 [TiHst

We begin our analysis with an extend-ed Huckel (eH) description of the bondingof a trigonal bipyramidal five-coordinatedO complex containing pure (J-donorsThese qualitati ve arguments were assessedby Density Functional Theory (DFT) Asimplified molecular-orbital diagram fora [TiHs]- model is sketched in the middleof Fig 2 The HOMO is a2 and is mostlyligand-centered The dxy and dyz orbitalsremain unperturbed and correspond to theLUMO (e) The second-order Jahn-Tell-er distortion (2OJTD) [9] of e symmetryallows mixing of HOMO and LUMOs(fHOMO reg fLUMO = a2 reg e = e) Byvarying the Hpiv- Ti-Hax angle 0 from130deg to 55deg we find two minima at 1200

(Etat = 000 eV) and 700 (Etat = +022 eV)which correspond to the SPY-S and EBT-S geometries respectively The TB-S (a=90deg Etat= + 100 eV) is a transition state inthis diagram The SPY-S vs EBT-S pref-erence can be traced back to a difference inligand-ligand interactions associated withthe 2a orbital artificially setting the LucLeg and LaCLpiv overlaps to zero reducesthe energetic advantage of the SPY -5 overthe EBT-S to only 005 eV It thus appearsthat the EBT-S geometry is a result of a20JTD of e symmetry As the forwarddistortion (0 gt 90deg) presented in Fig 2corresponds to the Berry distortion wesuggest that the EBT-S geometry resultsfrom a distortion along a reversed-Berrypathway

Since the 20JTD is doubly degenerate(e symmetry) we must investigate a sec-ond distortion also of e symmetry The a-distortion presented in Fig 2 correspondsto a rotation of both M-Lax vectors aroundthe x-axis We investigate a f3-distortionwhich corresponds to a rotation of bothM-Lax vectors around the y axis (see Fig3) The whole e distortion pathway wasprobed by independently varying 00

f3

H T ~H

H-Ti~ bullbullbullbullHH

3

CI--AIMeCpbull 7- 2zCp r Ph

SiM3

4

H Ta ~ -HH-1Ti~bullbullbullbullH

H

2

CICp cp~zro

Me

1

HCp

C Ta-Hp -H

2~_

-9~

-11 -

reg2a1

~b ~ GrcD-~ M = d metalU 1a1 LUMO

Fig 1 The three lowest-lying unoccupied orbitals of the bent dOMCp fragment dictate thecoplanar arrangement of the ML3J fragment in [MCP2L3l complexes 1-3 and transition state 5


I bull

I precession aXIsI

Lax I rl~f

U ~Leq

Lpv-f-M~Uy Leq

Lax I rl~f


lG8CiiilGs -01

sect -02

-0

-04

-0

-050 -030 -010 010 030 050

cos(alpha)cos(beta)


I -02

-0

-04

-0

--050 -030 -010 010 030 050

cos(alpha)cos(beta)





















2~

Etal






1a2

~

3a11b2

2a1

1b1

-13

-15

gt~gt-

crwzw

-11


-9







060

040

040

020

+

+7

-3 1 1~5 6 -----t- ---1- 5=f - - - - - - t -

I -t= II ~

I II I

00

cos(a)cos(~)

I

I

I

II

I

-020 00 020

cos(a)cos(~)

I

I

I

I

I

II

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

II

I

I

I

I

I

I

-020

-040-060

-040

060

040

-060

-040

020

a-c(f)~ 000~en0u

-020

-020

040

-040

020

ec(f)

000~(f)

ou











L MO oc d 2~ - (n) + (lr )









+

~ EBT-5

6

copyHI

Jr7eO~ PPh3

R a








+

5

copyI

MoRON CO

~R









X

d 2 2X -y

H

~H

H


H H ~H

H

--------

d orbitals

1t

X~---------

















~($)-8 (R)-8

PArN7

Fe Id)middot)

Fecentrn-5~)CI Ph2

g)

a)-c)

M = d6melal



m))


c

Br


F3

Ct( -II ju pH20 Ph2

CF3

9

NR2 -=====~









WAVELENGTH (nm)


400 440 480 520 560 600 640 680


o 0 0


N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05








5 Conclusions








max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+




















I bull

I precession aXIsI

Lax I rl~f

U ~Leq

Lpv-f-M~Uy Leq

Lax I rl~f


lG8CiiilGs -01

sect -02

-0

-04

-0

-050 -030 -010 010 030 050

cos(alpha)cos(beta)


I -02

-0

-04

-0

--050 -030 -010 010 030 050

cos(alpha)cos(beta)





















2~

Etal






1a2

~

3a11b2

2a1

1b1

-13

-15

gt~gt-

crwzw

-11


-9







060

040

040

020

+

+7

-3 1 1~5 6 -----t- ---1- 5=f - - - - - - t -

I -t= II ~

I II I

00

cos(a)cos(~)

I

I

I

II

I

-020 00 020

cos(a)cos(~)

I

I

I

I

I

II

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

II

I

I

I

I

I

I

-020

-040-060

-040

060

040

-060

-040

020

a-c(f)~ 000~en0u

-020

-020

040

-040

020

ec(f)

000~(f)

ou











L MO oc d 2~ - (n) + (lr )









+

~ EBT-5

6

copyHI

Jr7eO~ PPh3

R a








+

5

copyI

MoRON CO

~R









X

d 2 2X -y

H

~H

H


H H ~H

H

--------

d orbitals

1t

X~---------

















~($)-8 (R)-8

PArN7

Fe Id)middot)

Fecentrn-5~)CI Ph2

g)

a)-c)

M = d6melal



m))


c

Br


F3

Ct( -II ju pH20 Ph2

CF3

9

NR2 -=====~









WAVELENGTH (nm)


400 440 480 520 560 600 640 680


o 0 0


N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05








5 Conclusions








max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+




























2~

Etal






1a2

~

3a11b2

2a1

1b1

-13

-15

gt~gt-

crwzw

-11


-9







060

040

040

020

+

+7

-3 1 1~5 6 -----t- ---1- 5=f - - - - - - t -

I -t= II ~

I II I

00

cos(a)cos(~)

I

I

I

II

I

-020 00 020

cos(a)cos(~)

I

I

I

I

I

II

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

II

I

I

I

I

I

I

-020

-040-060

-040

060

040

-060

-040

020

a-c(f)~ 000~en0u

-020

-020

040

-040

020

ec(f)

000~(f)

ou











L MO oc d 2~ - (n) + (lr )









+

~ EBT-5

6

copyHI

Jr7eO~ PPh3

R a








+

5

copyI

MoRON CO

~R









X

d 2 2X -y

H

~H

H


H H ~H

H

--------

d orbitals

1t

X~---------

















~($)-8 (R)-8

PArN7

Fe Id)middot)

Fecentrn-5~)CI Ph2

g)

a)-c)

M = d6melal



m))


c

Br


F3

Ct( -II ju pH20 Ph2

CF3

9

NR2 -=====~









WAVELENGTH (nm)


400 440 480 520 560 600 640 680


o 0 0


N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05








5 Conclusions








max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+






















060

040

040

020

+

+7

-3 1 1~5 6 -----t- ---1- 5=f - - - - - - t -

I -t= II ~

I II I

00

cos(a)cos(~)

I

I

I

II

I

-020 00 020

cos(a)cos(~)

I

I

I

I

I

II

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

II

I

I

I

I

I

I

-020

-040-060

-040

060

040

-060

-040

020

a-c(f)~ 000~en0u

-020

-020

040

-040

020

ec(f)

000~(f)

ou











L MO oc d 2~ - (n) + (lr )









+

~ EBT-5

6

copyHI

Jr7eO~ PPh3

R a








+

5

copyI

MoRON CO

~R









X

d 2 2X -y

H

~H

H


H H ~H

H

--------

d orbitals

1t

X~---------

















~($)-8 (R)-8

PArN7

Fe Id)middot)

Fecentrn-5~)CI Ph2

g)

a)-c)

M = d6melal



m))


c

Br


F3

Ct( -II ju pH20 Ph2

CF3

9

NR2 -=====~









WAVELENGTH (nm)


400 440 480 520 560 600 640 680


o 0 0


N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05








5 Conclusions








max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+




















L MO oc d 2~ - (n) + (lr )









+

~ EBT-5

6

copyHI

Jr7eO~ PPh3

R a








+

5

copyI

MoRON CO

~R









X

d 2 2X -y

H

~H

H


H H ~H

H

--------

d orbitals

1t

X~---------

















~($)-8 (R)-8

PArN7

Fe Id)middot)

Fecentrn-5~)CI Ph2

g)

a)-c)

M = d6melal



m))


c

Br


F3

Ct( -II ju pH20 Ph2

CF3

9

NR2 -=====~









WAVELENGTH (nm)


400 440 480 520 560 600 640 680


o 0 0


N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05








5 Conclusions








max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+





















X

d 2 2X -y

H

~H

H


H H ~H

H

--------

d orbitals

1t

X~---------

















~($)-8 (R)-8

PArN7

Fe Id)middot)

Fecentrn-5~)CI Ph2

g)

a)-c)

M = d6melal



m))


c

Br


F3

Ct( -II ju pH20 Ph2

CF3

9

NR2 -=====~









WAVELENGTH (nm)


400 440 480 520 560 600 640 680


o 0 0


N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05








5 Conclusions








max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+


























~($)-8 (R)-8

PArN7

Fe Id)middot)

Fecentrn-5~)CI Ph2

g)

a)-c)

M = d6melal



m))


c

Br


F3

Ct( -II ju pH20 Ph2

CF3

9

NR2 -=====~









WAVELENGTH (nm)


400 440 480 520 560 600 640 680


o 0 0


N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05








5 Conclusions








max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+




















WAVELENGTH (nm)


400 440 480 520 560 600 640 680


o 0 0


N N N

10

[Oxidation]

Fe(ll)

o Fe(lIl)

[Reduction]-

2+

04

01

o

05








5 Conclusions








max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+






















max 543 nm (E 2250)































[oxidation]+ base

[reduction]+ H+



















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