Magnetic Exchange Coupling in Actinide-Containing Molecules

14 Pages·2009·4.17 MB·English

Inorg. Chem. 2009, 48, 3382-3395
Forum
Magnetic Exchange Coupling in Actinide-Containing Molecules
Jeffrey D. Rinehart, T. David Harris, Stosh A. Kozimor, Bart M. Bartlett, and Jeffrey R. Long*
Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460
ReceivedJuly12,2008
Recent progress in the assembly of actinide-containing coordination clusters has generated systems in which the
firstglimpsesofmagneticexchangecouplingcanberecognized.Suchsystemsareofinterestowingtotheprospects
forinvolving5felectronsinstrongermagneticexchangethanhasbeenobservedforelectronsinthemorecontracted
4forbitalsofthelanthanideelements.Here,wesurveytheactinide-containingmoleculesthoughttoexhibitmagnetic
exchangeinteractions,includingmultiuranium,uranium-lanthanide,uranium-transitionmetal,anduranium-radical
species.Interpretationofthemagneticsusceptibilitydataforcompoundsofthistypeiscomplicatedbythecombination
ofspin-orbitcouplingandligand-fieldeffectsarisingforactinideions.Nevertheless,forsystemswhereanalogues
featuringdiamagneticreplacementcomponentsforthenon-actinidespincenterscanbesynthesized,adatasubtraction
approachcanbeutilizedtoprobethepresenceofexchangecoupling.Inaddition,methodshavebeendeveloped
for employing the resulting data to estimate lower and upper bounds for the exchange constant. Emphasis is
placedonevaluationofthelinearclusters(cyclam)M[(µ-Cl)U(MePz)] (M)Co,Ni,Cu,Zn;cyclam)1,4,8,11-
2 42
tetraazacyclotetradecane; MePz- ) 3,5-dimethylpyrazolate), for which strong ferromagnetic exchange with 15
2
cm-1eJe48cm-1isobservedfortheCoII-containingspecies.Owingtothemodularsyntheticapproachemployed,
thissysteminparticularoffersnumerousopportunitiesforadjustingthestrengthofthemagneticexchangecoupling
and the total number of unpaired electrons. To this end, the prospects of such modularity are discussed through
thelensofseveralnewrelatedclusters.Ultimately,itishopedthatthisresearchwillbeofutilityinthedevelopment
of electronic structure models that successfully describe the magnetic behavior of actinide compounds and will
perhaps even lead to new actinide-based single-molecule magnets.
Introduction compounds,suchasstrongmagneticsuperexchange.While
themagneticpropertiesofactinide-containingcomplexesare
Interest in the magnetic properties of actinide-containing
ofinterestonafundamentallevel,theycanalsopotentially
compounds stems from their unique characteristics relative
beexploitedinproducingdiscretemoleculesthatexhibitslow
to transition metal- and lanthanide-containing magnetic
magnetic relaxation.1 Such molecules are known as single-
materials. In many ways, these characteristics can be seen
moleculemagnets(SMMs),andtheirunusualbehaviorarises
as a blending of the typical magnetic behavior associated
withlanthanidecompounds,suchasspin-orbitcouplingand from the influence of a negative axial magnetic anisotropy,
D, on a high-spin ground state, S. The resulting relaxation
relativistic effects, with that observed in transition metal
barrierofU)S2|D|forintegerSvalues[orU)(S2-1/ )|D|
4
* To whom correspondence should be addressed. E-mail: jrlong@ for half-integer S values] is, at most, 60 cm-1 for known
berkeley.edu.
(1) (a)Sessoli,R.;Tsai,H.L.;Schake,A.R.;Wang,S.;Vincent,J.B.; transitionmetalclusters.1dThepossibilityofincreasingthis
Folting,K.;Gatteschi,D.;Christou,G.;Hendrickson,D.N.J.Am. barrier height, and perhaps opening the way for potential
Chem.Soc.1993,115,1804.(b)Sessoli,R.;Gatteschi,D.;Caneschi, applications,1c,2providesimpetusforthedevelopmentofnew
A.;Novak,M.A.Nature1993,365,141.(c)Gatteschi,D.;Sessoli,
R.;Villain,J.MolecularNanomagnets;OxfordUniversityPress:New approaches to generating SMMs.
York,2006andreferencescitedtherein.(d)Milios,C.J.;Vinslava,
Transitionmetalshaveservedextremelywellforgenerat-
A.; Wernsdorfer, W.; Moggach, S.; Parsons, S.; Perlepes, S. P.;
Christou,G.;Brechin,E.K.J.Am.Chem.Soc.2007,129,2754. ing discrete clusters, wherein strong magnetic coupling
3382 InorganicChemistry, Vol.48,No.8,2009 10.1021/ic801303wCCC:$40.75 2009AmericanChemicalSociety
PublishedonWeb04/13/2009
MagneticExchangeCouplinginActinide-ContainingMolecules
betweenmanymetalcentersgivesrisetoconcertedbehavior withbridgingligandorbitals,therebyenhancingtheconcerted
withalargetotalspinquantumnumber,S.3However,owing magnetic behavior between bridged metal centers within a
to their large single-ion anisotropies, some of the systems single cluster unit.6a,7
exhibiting the largest relaxation barriers instead contain In this Forum, we survey recent developments in the
lanthanide ions.4 Indeed, the anisotropy associated with synthesisandcharacterizationofmolecularsystemsinwhich
lanthanideionssuchasTb3+andDy3+hasledtomanifesta- actinide ions potentially engage in magnetic exchange
interactions. Thus far, efforts have focused exclusively on
tionsofslowmagneticrelaxation,eveninmoleculescontain-
speciesincorporatinguraniumbecausethisactinideelement
ingjustonemetalcenter.5Theseexamplesdemonstratethe
offers minimal radioactivity (in depleted form) with acces-
greatpromiseoffelementsinthefutureofSMMchemistry,
sible oxidation states allowing for zero, one, two, or three
especiallyiftheirsingle-ionpropertiescouldbeincorporated unpairedelectrons.Researchershaveconfrontedtheintrica-
into high-nuclearity clusters where the spin and axial cies of the magnetic exchange in a number of interesting
anisotropy of many metals could contribute to the energy ways, often with the goal of identifying and, to the extent
barriertospininversion.However,itisdifficulttoenvision possible, quantifying ferro- or antiferromagnetic exchange
high-nuclearity lanthanide clusters with concerted spin coupling.Understandingtheseexchangeinteractionsnotonly
behavior because the 4f valence orbitals typically lack the is essential to the development of models for the basic
radial extension and energetic proximity required for sig- electronicstructureofthe5felementsbutalsomayrepresent
nificant overlap with bridging ligand orbitals.6 This results the key to producing the first actinide-based SMMs.
in small covalent interactions and weak pathways for Challenges in Interpreting the Magnetic Behavior of
magneticsuperexchangethroughdiamagneticbridgingligands. Actinide Compounds
In contrast, the greater radial extension of the 5f valence
Despite a growing number of varied synthetic systems
orbitalsofactinidescanpotentiallyprovideincreasedoverlap
incorporating paramagnetic uranium centers, unraveling
actinide magnetic behavior remains a challenge because of
thelackofatheoreticalfoundationforaccuratelymodeling
(2) (a)Garanin,D.A.;Chudnovsky,E.M.Phys.ReV.B1997,56,11102.
the complex interactions that govern actinide magnetic
(b)Leuenberger,M.N.;Loss,D.Nature2001,410,789.(c)Heersche,
H.B.;deGroot,Z.;Folk,J.A.;vanderZant,H.S.J.;Romeike,C.; susceptibility.Inthemolecularchemistryoffirst-rowtransi-
Wegewijs, M. R.; Zobbi, L.; Barreca, D.; Tondello, E.; Cornia, A.
tion metal ions, it is usually possible to treat the magnetic
Phys.ReV.Lett.2006,96,206801.(d)Jo,M.-H.;Grose,J.E.;Liang,
W.;Baheti,K.;Deshmukh,M.M.;Sokol,J.J.;Rumberger,E.M.; susceptibility as being due to the unpaired spins, with
Hendrickson,D.N.;Long,J.R.;Park,H.;Ralph,D.C.NanoLett. minimal effects from the orbital components owing to an
2006,6,2014.
(3) (a)Powell,A.K.;Heath,S.L.;Gatteschi,D.;Pardi,L.;Sessoli,R.; orbital angular momentum that is largely quenched by the
Spina,G.;DelGiallo,F.;Pieralli,F.J.Am.Chem.Soc.1995,117, ligand field. The “spin-only” approximation, which works
2491.(b)Zhong,Z.J.;Seino,H.;Mizobe,Y.;Hidai,M.;Fujishima,
reasonably well for most first-row transition metal species,
A.;Ohkoshi,S.;Hashimoto,K.J.Am.Chem.Soc.2000,122,2952.
(c) Murugesu, M.; Habrych, M.; Wernsdorfer, W.; Abboud, K. A.; loses its validity for a number of molecular systems of
Christou, G. J. Am. Chem. Soc. 2004, 126, 4766. (d) Ako, A. M.; interest for magnetism, including those containing actinide
Hewitt,I.J.;Mereacre,V.;Cle´rac,R.;Wernsdorfer,W.;Anson,C.E.;
Powell,A.K.Angew.Chem.,Int.Ed.2006,45,4926. ions.8 For example, a UIV center possesses two unpaired 5f
(4) (a)Osa,S.;Kido,T.;Matsumoto,N.;Re,N.;Pochaba,A.;Mrozinski, electrons, leading to a 3H (S ) 1, L ) 5) ground state.
J.J.Am.Chem.Soc.2004,126,420.(b)Mishra,A.;Wernsdorfer, Spin-orbit coupling produces an energy splitting based on
W.;Abboud,K.A.;Christou,G.J.Am.Chem.Soc.2004,126,15648.
(c) Zaleski, C. M.; Depperman, E. C.; Kampf, J. W.; Kirk, M. L.; thetotalangularmomentum,J,where|S-L|eJeL+S.
Pecoraro,V.L.Angew.Chem.,Int.Ed.2004,43,3912.(d)Mishra, For the UIV ion, which has a less than half-filled 5f shell,
A.;Wernsdorfer,W.;Parsons,S.;Christou,G.;Brechin,E.K.Chem.
Commun.2005,2086.(e)Mori,F.;Nyui,T.;Ishida,T.;Nogami,T.; this leads to a 3H4 designation for the spin-orbit-coupled
Choi,K.-y.;Nojiri,H.J.Am.Chem.Soc.2006,128,1440.(f)Tang, ground state. Depending on the degree of spin-orbit
J.;Hewitt,I.;Madhu,N.T.;Chastanet,G.;Wernsdorfer,W.;Anson, coupling,mixingwithexcited-stateRussell-Saundersterms
C.E.;Benelli,C.;Sessoli,R.;Powell,A.K.Angew.Chem.,Int.Ed.
2006,45,1729.(g)Ferbinteanu,M.;Kajiwara,T.;Choi,K.-y.;Nojiri, of the same J value can occur. Recent calculations on the
H.;Nakamoto,A.;Kojima,N.;Cimpoesu,F.;Fujimura,Y.;Takaishi, UIVionsuggestthatthe3H groundstatecanhavesignificant
S.;Yamashita,M.J.Am.Chem.Soc.2006,128,9008.(h)Tangoulis, 4
V.;Figuerola,A.Chem.Phys.2007,340,293.(i)Chandrasekhar,V.;
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T. J.; Cle´rac, R.; Murugesu, M. Angew. Chem., Int. Ed. 2008, 47, 2008,47,29.
8848. (8) Thediscussionoftheelectronicstructureofactinidespresentedhere
(5) (a)Ishikawa,N.;Sugita,M.;Ishikawa,T.;Koshihara,S.;Kaizu,Y. ismeanttogiveanoverviewofthecomplexityofsuchsystemsand
J. Am. Chem. Soc. 2003, 125, 8694. (b) Ishikawa, N.; Sugita, M.; isbynomeanscomprehensive.Forathoroughtheoreticalanalysis,
Ishikawa,T.;Koshihara,S.;Kaizu,Y.J.Phys.Chem.B2004,108, the reader is referred to the following: (a) Wybourne, B. G.
11265.(c)Ishikawa,N.;Sugita,M.;Wernsdorfer,W.Angew.Chem., Spectroscopic Properties of Rare Earths; Wiley: New York, 1965.
Int.Ed.2005,44,2931.(d)Takamatsu,S.;Ishikawa,T.;Koshihara, (b)Siddall,T.H.TheoryandApplicationsofMolecularParamagnet-
S.-y.;Ishikawa,N.Inorg.Chem.2007,46,7250.(e)AlDamen,M.A.; ism;Wiley:NewYork,1976.(c)Kanellakopulos,B.InOrganome-
Clemente-Juan,J.M.;Coronado,E.;Mart´ı-Gastaldo,C.;Gaita-Arin˜o, tallics of the f-Elements; Marks, T. J., Fischer, R. D., Eds.; NATO
A.J.Am.Chem.Soc.2008,130,8874. Advanced Study Institutes Series; D. Reidel: Dordrecht, The Neth-
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A.; Laurent, J.-P. Chem.sEur. J. 1998, 4, 1616. (c) Kahn, M. L.; TheChemistryoftheActinideandTransactinideElements,3rded.;
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Inorganic Chemistry, Vol. 48, No. 8, 2009 3383
Rinehart et al.
1G character (9% in the case studied).9 This mixing of ion electronic structure. The electronic complexity leads to
4
excited-state Russell-Saunders terms into the ground state astrongvariationinthethermalpopulationofthemanystates
invalidatesSandLasindividualquantumnumbersandcan energetically comparable to the ground state. This effect
alsorestoretheorbitalangularmomentumbymixingother becomes evident as the higher-energy Stark sublevels
f orbitals into the ground-state wave function. depopulate as the temperature is lowered. Depopulation of
The above methodology for describing the electronic these sublevels leads to a concomitant decrease in the
structure of the UIV ion, wherein spin-spin repulsions are magnitude of the total angular momentum vector. Two
considered, followed by coupling of the spin and orbital important consequences arise from this phenomenon. First,
angular momenta, and then allowing for the mixing of J thedecreaseintheangularmomentummanifestsitselfasa
states is known as an intermediate coupling scheme. While decrease in the magnetic susceptibility, which can obscure
wechoosetostartfromthemorefamiliarRussell-Saunders other simultaneous effects, such as magnetic exchange
couplingscheme,theelectronicstructureofactinidescould couplingbetweenbridgedmetalcenters.Additionally,ifsuch
be described equally well by first coupling the individual an exchange coupling between the actinide ion and other
spin and orbital angular momenta of each electron (j-j metalsintheclusterdoesexist,itsmagnitudewilldecrease,
coupling scheme), then applying the interelectronic repul- and in the case of low-symmetry non-Kramers systems,
sions,andfinallyallowingformixingoftheJstates.Because eventuallydisappearatlowtemperaturebecausethereisless
neither the Russell-Saunders method nor the j-j coupling angular momentum to which the spin of the non-actinide
method alone accurately describes the electronic structure center can couple. Dealing with these single-ion effects,
of actinides, this sort of intermediate coupling scheme is whichbothobscureandinterferewiththeeffectsofmagnetic
necessary. couplingbetweenmetalcenters,isofpreeminentimportance
The discussion thus far has neglected the effects of the to the progression of the field of actinide molecular mag-
ligand field. While the electronic structure perturbations of netism.
the ligand field are not nearly as noticeable for actinides as Attempts to model the ligand field and extract exchange
for transition metals, the radial extension of the 5f orbitals couplingdatabyfittingtoanappropriateHamiltonianhave
does allow for metal-ligand interaction. Therefore, it is been employed for both lanthanide and actinide systems,
necessarytoconsidertheligand-fieldeffectsasaperturbation which have a single, well-isolated Stark sublevel ground
on the spin-orbit-coupled ground-state configuration. This state. However, complications arise when modeling the
perturbation removes the (2J + 1)-fold degeneracy of the temperature-dependentmagneticeffectsduetosubtleshifts
groundstate.Theresultingsublevelsplittingisoftenreferred intheStarksublevels.10Attemptingtomodelmanyvariables
toastheStarksplitting,andthestatesthemselvesarereferred can easily lead to overparameterization, so several groups
to as Stark sublevels. The degeneracy of these sublevels is havecircumventedtheseissuesbyattemptingtosynthesize
determined by symmetry; however, in accordance with acompoundisostructuraltothemoleculeofinterest,wherein
Kramersrule,thedegeneracyofoddelectronsystemscannot thenon-f-elementparamagneticcenterhasbeenreplacedby
be completely lifted regardless of the ligand field. The adiamagneticone.Then,subtractionofthetwodatasetsis
orderingofthesesublevelsissubtlyaffectedbytheorienta- performed to simulate removal of the orbital and ligand-
tionandstrengthoftheligandfieldandcannotbepredicted field effects of the f-element ion, thereby exposing any
in a straightforward manner. The irregularity of the Stark magneticexchangecouplinginteractions.Althoughextremely
splittingisfurthercomplicatedbymixingbetweentheStark dependentonsmallchangesintheligandfield,suchamodel
sublevels of the spin-orbit-coupled ground state and the has been applied successfully to numerous lanthanide-
excited states. containing systems.11 This subtraction method is also cur-
The complexity of the electronic structure for a single rently the most widely employed method for interpreting
actinide ion is both exciting and frustrating in terms of its uranium magnetism and will be discussed in detail below.
effects on the magnetic properties of a molecular cluster.
Survey of Molecules Potentially Exhibiting Magnetic
Ofparticularnoteistheanisotropytotheorientationofthe
Exchange
angularmomentumcreatedbytheligand-fieldStarksplitting.
Whenmagneticallycoupledtothespinsofothermetals,this ADiuraniumComplex.Thefirstobservationofmagnetic
single-ionanisotropycouldgiverisetoalargeenergybarrier exchange coupling in an actinide-containing molecule was
betweenorientationsoftheground-statespinofthemolecule, reportednearly20yearsagoforthebinuclear,1,4-diimido-
thereby possibly leading to SMMs with a high blocking benzene-bridgedcomplex[(MeC H ) U] (µ-1,4-N C H ).12
5 4 3 2 2 6 4
temperature.Therealizationofsuchphenomenamaygreatly ThepresenceofcouplingbetweenthetwoUVcenterswithin
dependontheabilitytocharacterizethemagneticexchange this molecule became evident upon a comparison of its
coupling between ligand-bridged metal centers. However, variable-temperaturemagneticsusceptibilitytothatofstruc-
inordertoverifysuchaninteractionbetweenmetalcenters,
it is necessary to investigate the variable-temperature mag- (10) Kahn,M.L.;Rafik,B.;Porcher,P.;Kahn,O.;Sutter,J.-P.Chem.sEur.
J.2002,8,525.
netic susceptibility for evidence of magnetic exchange
(11) Sutter,J.-P.;Kahn,M.L.InMagnetism:MoleculestoMaterialsV;
coupling,ataskcomplicatedbytheintricacyoftheactinide- Miller, J. S., Drillon, M., Eds.; Wiley-VHC: Weinheim, Germany,
2001;pp161-187andreferencescitedtherein.
(9) Danilo,C.;Vallet,V.;Flament,J.-P.;Wahlgren,U.J.Chem.Phys. (12) Rosen,R.K.;Andersen,R.A.;Edelstein,N.M.J.Am.Chem.Soc.
2008,128,154310. 1990,112,4588.
3384 InorganicChemistry,Vol.48,No.8,2009
MagneticExchangeCouplinginActinide-ContainingMolecules
sponding to µ ) (3/ , where µ is the crystal quantum
2
number.8a Considering EPR selection rules, a spectrum is
expected for a sublevel with crystal quantum number µ )
(1/ , while no spectrum is expected for a sublevel with
2
crystalquantumnumberµ)(3/ .Theuncoupleddinuclear
2
complex,[(MeC H ) U] (µ-1,3-N C H ),gavenoEPRspec-
5 4 3 2 2 6 4
trumat4K,suggestingpopulationofonlythelowest-energy
Stark sublevel, µ ) (3/ . Thus, the drop in the magnetic
2
susceptibility of [(MeC H ) U] (µ-1,4-N C H ), at least at
5 4 3 2 2 6 4
such very low temperatures, can be attributed to magnetic
exchangeratherthantheusualeffectsoftheStarksublevel
depopulation. Figure 1 shows the resulting calculated and
experimentalsusceptibilitydata.Thedifferencesbetweenthe
twoexperimentaldatasetswereattributedtosampleimpurity
and,assuch,thecalculateddataweremodeledwithvarying
Figure1.Experimental(symbols)versuscalculated(lines)molarmagnetic amounts of paramagnetic impurity. On the basis of these
susceptibilityfor[(MeCH)U](µ-1,4-NCH).Eachcalculatedcurveis
5 43 2 2 6 4 parameters, the best fit was obtained with an exchange
modeled with a different amount of the paramagnetic impurity,
(MeCH)U(THF).Takenfromref12. constant of J ) -19 cm-1 and an estimated paramagnetic
5 43
impurity of 1 mol %.
turally similar compounds. The geometric isomer OtherMultiuraniumSystems.Althoughourfocusison
[(MeC5H4)3U]2(µ-1,3-N2C6H4), for instance, displays es- actinide-containing molecules for which the occurrence of
sentiallyconstantmagneticsusceptibility((cid:1)M)withdecreas- magnetic coupling has been directly probed, there are a
ing temperature from 300 K down to ca. 150 K, at which numberofintriguingmultiuraniumsystemswhereinmagnetic
point it begins to rise monotonically as the temperature is exchangeislikelybuthasnotbeenrigorouslyinvestigated.
decreased to 5 K. This behavior, typical of an isolated 5f1 Forexample,attemptstoproduceclustersthatmightfeature
center, is essentially the sum of that observed for two uranium-uranium bonds led to a number of simple diura-
(MeC5H4)3U(NPh)complexesandindicatesthelackofany nium alkoxides, including [U2(O2CMe3)9]0/1-,14 and the
magnetic exchange between the two UV centers.12,13 In chloro-bridgedspecies[(C Me ) U Cl ]-.15Onthebasisof
6 6 2 2 7
contrast,themagneticsusceptibilitydataobtainedfor[(MeC5- the assumption that nitrogenous ligands promote bridging
H4)3U]2(µ-1,4-N2C6H4)displaysimilarbehaviordowntoca. inactinides,16anumberofdi-,tri-,andtetranuclearuranium
75 K but then exhibit a downturn at lower temperatures, amide species were synthesized, including the dinuclear
indicative of antiferromagnetic coupling (see Figure 1). complex [U(η-C H )] [µ-η4:η4-HN(CH ) N(CH ) N(CH ) -
8 8 2 2 3 2 2 2 3
Inanattempttoobtainaquantitativedeterminationofthe NH], which contains the shortest U···U separation yet
couplingin[(MeC5H4)3U]2(µ-1,4-N2C6H4),theexperimental observed in a molecule.17 Diuranium systems featuring an
(cid:1)M vs T data were compared to calculated susceptibilities. arene bridge, such as [(Mes(tBu)N)2U]2(µ-η6:η6-C7H8) and
The magnetic interaction between the UV centers was [(Cp*)U](µ-η6:η6-CH),18andthepyrazolate-bridgeddimer
2 2 6 6
modeledbyemployingthefollowingIsingHamiltonianfor [U(Me Pz) ] (Me Pz- ) 3,5-dimethylpyrazolate),19 also
2 4 2 2
an isolated dinuclear complex: presentthestrongpossibilityofmagneticexchangecoupling.
While the foregoing examples constitute only a fraction
H)-2J(Sˆ ·Sˆ )+gµ Hˆ ·(Sˆ +Sˆ ) (1)
z1 z2 | B z z1 z2 ofthemolecularuraniumclustersthatmightexhibitmagnetic
where Sˆ is the effective spin operator for each S ) 1/ UV exchange coupling, they do serve to give an idea of how
zn 2
ion(thezdirectionisdefinedasalongtheU···Uaxis),Jis much synthetic work has already been accomplished in the
the exchange constant, g| is the Lande´ g factor, µB is the area. In addition, symmetric dinuclear complexes such as
Bohr magneton, and Hˆ is the magnetic field vector. Note thesewouldservewellintestinggeneralelectronicstructure
z
thatthisHamiltoniandoesnotaccountfordeviationsinthe modelsattemptingtoaccountfortheinfluenceofexchange
magnetic susceptibility resulting from depopulation of the coupling on the magnetic behavior of actinide ions. Given
uranium Stark sublevels with decreasing temperature but
ratheritassumessuchdeviationsarisesolelyfromexchange (14) Cotton,F.A.;Marler,D.O.;Schwotzer,W.Inorg.Chem.1984,23,
betweentwoS)1/ ions.Thisassumptionwasmadebased 4211.
2 (15) Cotton,F.A.;Schwotzer,W.Organometallics1985,4,942.
onananalysisoftheelectronparamagneticresonance(EPR) (16) Reynolds, J. G.; Zalkin, A.; Templeton, D. H.; Edelstein, N. M.;
spectrum,whichsuggestedthatonlythelowestStarksublevel Tempelton,L.K.Inorg.Chem.1976,15,2498.
is populated at low temperature. The J ) 5/ ground state (17) (a)Berthet,J.C.;Ephritikhine,M.Coord.Chem.ReV.1998,178,83.
2 (b) Borgne, T. L.; Lance, M.; Nierlich, M.; Ephritikhine, M. J.
for a UV center is split by the ligand field into three Stark Organomet.Chem.2000,598,313.
sublevels, two corresponding to µ ) (1/ and one corre- (18) (a)Diaconescu,P.L.;Arnold,P.L.;Baker,T.A.;Mindiola,D.J.;
2 Cummins,C.C.J.Am.Chem.Soc.2000,122,6108.(b)Diaconescu,
P.L.;Cummins,C.C.J.Am.Chem.Soc.2002,124,7660.(c)Evans,
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Inorganic Chemistry, Vol. 48, No. 8, 2009 3385
Rinehart et al.
thecurrentlackofreliablemodels,asignificantsteptoward
probing the presence of exchange coupling in such species
wouldbethedevelopmentofsyntheticmethodsforpreparing
mixed-actinide analogues, wherein one of the two actinide
centers is rendered diamagnetic. Here, the replacement of
oneoftheUIVcenterswithaThIVcenter,oroneoftheUIII
centers with an AcIII center,20 would enable a subtraction
approach of the type elaborated below to be applied in
providingaqualitativeassessmentoftheexchangecoupling.
Another type of uranium-containing molecule that offers
promise in the area of molecular magnetism is the high-
nuclearity uranium oxo cluster. While most oxo-bridged
uranium complexes are di- or trinuclear species,21 it was
recentlyshownthathydrolysisofUI (THF) inthepresence
3 4
of water and other ligands can result in higher-nuclearity
clusters.22Thelargestoftheseisthediscretedodecanuclear
speciesU (µ-O) (µ-OH)I(µ-OSCF) (CHCN),which Figure 2. Structure of Cp*2U[(NC(CH2C6H5)tpy)YbCp*2]2.23 Orange,
12 3 12 3 8 2 2 3 3 16 3 8 purple,blue,andgrayspheresrepresentU,Yb,N,andCatoms,respectively.
contains a double-decker square-antiprism U O (OH)
12 12 8 Hatomsareomittedforclarity.TheCp*ligandsandbenzylgroupsare
core.22d This type of cluster, while well beyond the scope drawntransparentlyforbettervisualizationofthecorestructure.
of current techniques for analyzing magnetic exchange
coupling, may offer prospects for observation of the SMM
behavior in uranium systems. Indeed, such clusters could
potentiallycombinethedesirablepropertiesoflargespinand
single-ionanisotropywiththehighcouplingstrengthofthe
oxo bridge.
AUranium-LanthanideSystem.Recently,evidenceof
exchangecouplingwasreportedforthebenttrinuclear4f-5f
cluster Cp* U[(NC(CH C H )tpy)YbCp* ] (UYb ; tpy )
2 2 6 5 2 2 2
terpyridyl).23Thestructureofthisspeciesfeaturesacentral
[Cp*UIV]2+unitconnectedthroughNC(CHCH)tpybridges
2 2 6 5
totwo[Cp* Yb]x+(x)0or1)moieties,asshowninFigure
2
2.Thecyclicvoltammetryandelectronicabsorptionspectra
oftheUYb clustersuggestthepresenceofbothCp* YbIItpy
2 2
and Cp* YbIIItpy• species at room temperature.24 The vari-
2
able-temperature magnetic susceptibility data obtained for
the cluster are plotted in Figure 3. Here, (cid:1) T follows a
M
Figure3.Variable-temperaturemagneticsusceptibilitydataforCp*U[(NC-
gradual downward trend from 350 K to ca. 25 K, followed 2
(CHCH)tpy)YbCp*].Inset:Variable-temperaturemagneticsusceptibility
2 6 5 22
dataobtaineduponsubtractionofdataforCp*U(NC(CHCH)tpy) and
2 2 6 5 2
(20) Note,however,thataccomplishingthissubstitutionwouldinvolvevery Cp*2Th[(NC(CH2C6H5)tpy)YbCp*2]2fromtheUYb2data.Takenfromref
seriousdifficultiesstemmingfromtheextremeradioactivityofactinium 23.
isotopes.
(21) (a)Berthet,J.-C.;LeMaréchal,J.-F.;Nierlich,M.;Lance,M.;Vigner, by a precipitous drop at lower temperatures, which can be
J.;Ephritikhine,M.J.Organomet.Chem.1991,408,335.(b)Lukens, understoodlargelyintermsoftheorbitalangularmomentum
W.W.,Jr.;Allen,P.G.;Bucher,J.J.;Edelstein,N.M.;Hudson,E.A.;
Shuh,D.K.;Reich,T.;Andersen,R.A.Organometallics1999,18, quenching discussed above for 5f1 systems. However, the
1253.(c)Korobkov,I.;Gambarotta,S.;Yap,G.P.A.Organometallics. gradual decline in (cid:1) T from its room temperature value is
M
2001,20,2552.(d)Castro-Rodriguez,I.;Olsen,K.;Gantzel,P.;Meyer,
characteristic of multielectron f-element-containing com-
K. Chem. Commun. 2002, 2764. (e) Karmazin, L.; Mazzanti, M.;
Pécaut, J. Inorg. Chem. 2003, 42, 5900. (f) Enriquez, A. E.; Scott, plexes and is generally attributed to thermal depopulation
B.L.;Neu,M.P.Inorg.Chem.2005,44,7403.(g)Salmon,L.;Thue`ry, oftheStarksublevels.8However,thebehaviorobservedhere
P.; Asfari, Z.; Ephritikhine, M. Dalton Trans. 2006, 24, 3006. (h)
Christopher, P.; Larch, F.; Cloke, G. N.; Hitchcock, P. B. Chem. is further complicated by the presence of both diamagnetic
Commun.2008,82. YbIIandparamagneticYbIIIions,inadditiontoanunpaired
(22) (a)Mokry,L.M.;Dean,N.S.;Carrano,C.J.Angew.Chem.,Int.Ed.
electron residing on the terpyridine fragment.
1996,35,1497.(b)Duval,P.B.;Burns,C.J.;Clark,D.L.;Morris,
D. E.; Scott, B. L.; Thompson, J. D.; Werkema, E. L.; Jia, L.; Inanattempttodeconvolutethemagneticdataandextract
Andersen,R.A.Angew.Chem.,Int.Ed.2001,40,3357.(c)Berthet, information regarding potential exchange interactions be-
J.-C.;Thuery,P.;Ephritikhine,M.Chem.Commun.2005,3415.(d)
Nocton,G.;Burdet,F.;Pe´caut,J.;Mazzanti,M.Angew.Chem.,Int. tween the UIV and YbIII centers, a stepwise series of
Ed.2007,46,7574. subtractions was performed on the UYb data. First, (cid:1) T
(23) Schelter,E.J.;Veauthier,J.M.;Thompson,J.D.;Scott,B.L.;John, 2 M
data collected for the precursor complex Cp* U(NC-
K.D.;Morris,D.E.;Kiplinger,J.L.J.Am.Chem.Soc.2006,128, 2
2198. (CH C H )tpy) were subtracted from the UYb data to
2 6 5 2 2
(24) Veauthier, J. M.; Schelter, E. J.; Kuehl, C. J.; Clark, A. E.; Scott, remove any orbital contribution from the UIV ion to the
B.L.;Morris,D.E.;Martin,R.L.;Thompson,J.D.;Kiplinger,J.L.;
John,K.D.Inorg.Chem.2005,44,5911. overallmagnetism.Then,toeliminatethemagneticcontribu-
3386 InorganicChemistry,Vol.48,No.8,2009
MagneticExchangeCouplinginActinide-ContainingMolecules
tion from YbIII, (cid:1) T data collected for Cp* Th[(NC(CH -
M 2 2
C H )tpy)YbCp* ] (ThYb ) were subtracted. The result of
6 5 2 2 2
thesesubtractions,shownas∆(cid:1) TintheinsetofFigure3,
M
is a data set that follows a monotonic increase with
decreasing temperature from 350 K to ca. 15 K and then
dropsprecipitouslyatlowertemperatures.Therisein∆(cid:1) T
M
is interpreted as evidence of exchange coupling within the
cluster,althoughthespecificnatureofthecouplingisunclear
because the UIV and YbIII ions and the terpyridine radical
representthreedistinctparamagneticcenters.Thecurvature
ofthedataabove60Kisattributedtoelectronicdifferences
betweentheUYb andThYb clusters,asevidencedincyclic
2 2
voltammetry,wheretheredoxpeaksforthetwoclustersare
shiftedrelativetooneanother.Furthermore,theauthorsnote
thatthenegativevaluesfor∆(cid:1) Trepresentanovercorrection
M
during the subtraction process. Thus, while qualitative
interpretation of ∆(cid:1) T vs T suggests the presence of
M
magnetic coupling, the complexity of this system may
prohibit a quantitative analysis.
Themagneticpropertiesofananalogoustrinuclearspecies,
Cp* U (NC(CH C H )tpy) , in which UIII replaces both of
6 3 2 6 5 2
theYbcenters,exhibitasimilartrendwhere(cid:1) Tdecreases
M
withdecreasingtemperature.25However,thecomplications
encounteredinthedataanalysisoftheUYb species,along
2
withthelackofdiamagneticanaloguestotheUIVUIII cluster,
2
have thus far made it impossible to deconvolute the many
Figure 4. Schematic representation of the ligand precursors HLi. Note
factorscontributingtothemagneticsusceptibilityandisolate 4
thetwo-carbonbackbonefori)1-5,three-carbonbackbonefori)6-8,
evidence of magnetic exchange coupling. andfour-carbonbackbonefori)9.
Uranium-TransitionMetalSystems.Todate,themost
comprehensively studied class of actinide-containing mol-
eculesexhibitingmagneticexchangeinteractionsisaseries
of trinuclear uranium-transition metal assemblies synthe-
sized by Ephritikhine and coworkers. These clusters have
theformULi M (py) (M)Cu,Zn;py)pyridine),where
2 2 n
Li is one of a series of nine Schiff-base bridging ligands,
each with a modified diimino hydrocarbon backbone (see
Figure4).26Thestructureofeachclusterconsistsofacentral
UIV ion coordinated linearly to two MII ions through
orthogonal (Li)4- bridges, as represented in Figure 5. The
UIV center resides in a dodecahedral coordination environ-
ment,encapsulatedbyeightOdonoratoms.EachMIIcenter
iscoordinatedtotwoNatomsandtwoOatomsoftheSchiff Figure 5. Structure of UL6Cu(py). Orange, green, red, blue, and gray
2 2
baseinadistortedsquare-planargeometryandisboundby spheres represent U, Cu, O, N, and C atoms, respectively. H atoms are
omittedforclarity.
zero, one, or two pyridine molecules, depending on the
bridging ligand. Importantly, the coordination environment The isolation of isostructural copper and zinc analogues
aroundtheUIVcenterremainsinvariantwithchangesinthe oftheULi M (py) clustersenabledtheuseofasubtraction
2 2 n
bridgingligandandnumberoftransitionmetal-coordinated methodsimilartotheonedescribedabove,therebyproviding
pyridine molecules, suggesting that differences in the aroutethroughwhichtoinvestigatethemagneticinteraction
magneticbehavioracrosstheseriesarenotduetoalterations between the UIV and CuII ions without contamination by
in the ligand field of the uranium ion. single-ion effects of UIV. Variable-temperature magnetic
susceptibilitydataforUL7 M (py)(M)Cu,Zn)areshown
2 2
(25) Schelter,E.J.;Wu,R.;Scott,B.L.;Thompson,J.D.;Morris,D.E.; at the top of Figure 6. For the UZn cluster, (cid:1) T remains
Kiplinger,J.L.Angew.Chem.,Int.Ed.2008,47,2993. 2 M
(26) (a)LeBorgne,T.;Rivie`re,E.;Marrot,J.;Girerd,J.-J.;Ephritikhine, essentiallyconstantat0.8cm3·K/molasthetemperatureis
M.Angew.Chem.,Int.Ed.2000,39,1647.(b)LeBorgne,T.;Rivie`re, lowered from 300 to 100 K and then drops precipitously at
E.;Marrot,J.;Thue´ry,P.;Girerd,J.-J.;Ephritikhine,M.Chem.sEur.
lower temperatures, tending toward zero at 2 K. This drop,
J.2002,8,774.(c)Salmon,L.;Thue´ry,P.;Rivie`re,E.;Girerd,J.-J.;
Ephritikhine,M.Chem.Commun.2003,762.(d)Salmon,L.;Thue´ry, typical of UIV complexes with a 5f2 valence electron
P.; Rivie`re, E.; Girerd, J.-J.; Ephritikhine, M. Dalton Trans. 2003,
configuration, can be attributed to the depopulation of the
2872.(e)Salmon,L.;Thue´ry,P.;Rivie`re,E.;Ephritikhine,M.Inorg.
Chem.2006,45,83. Starksublevelsandsubsequentquenchingofthetotalangular
Inorganic Chemistry, Vol. 48, No. 8, 2009 3387
Rinehart et al.
WhileferromagneticcouplingisobservedforUL7Cu(py),
2 2
thenatureoftheexchangeappearshighlydependentonthe
identityofthebridgingSchiffbaseand/ornumberofpyridine
molecules coordinated to the copper center, as found upon
acomparisonoftheentireseriesofUM clusters.Fori)6,
2
8, and 9, behavior similar to that of UL7 Cu (py) was
2 2
observed,indicativeofferromagneticcoupling.Incontrast,
for i ) 1-5, ∆(cid:1) T turns down below 100 K, indicating an
M
antiferromagnetic exchange interaction. The shift from
antiferromagnetic to ferromagnetic coupling occurs as the
backbone of the Schiff base increases from two C atoms (i
) 1-5) to three (i ) 6-8) or four C atoms (i ) 9). This
phenomenon is attributed to an increase in the Cu···U
distance, which is associated with a lengthening in the
diiminochain.Itshouldbenoted,however,thattheobserva-
tionofanincreasedmetalseparationisbasedonthestructural
characterizationofonlyfourUCu clusters(i)2,6,7,and
2
9),forwhichtheaverageCu···Udistancesare3.538,3.661,
3.641, and 3.647 Å, respectively. In addition, the magnetic
behavior may be affected by other exchange pathways, as
evidenced by a downturn in (cid:1) T below 15 K observed in
M
theanalogousThLi Cu clusters(i)1and2)anddeviation
2 2
ofthemagnetizationdataforULi Cu (i)1-5)at2Kfrom
2 2
the Brillouin function. This weak effect is attributed to a
long-rangeintramolecularCu···Cuinteractionandmayplay
an important role in influencing the overall magnetism. In
explaining this behavior, the authors note that similar
magnetostructural correlations have been documented in
gadolinium-transition metal species, where exchange in-
teractionswerefoundtovarywithfactorssuchasCu···Gd
distances and dihedral angles between O-Cu-O and
O-Gd-O planes.27
AnalogoustrinuclearclustersoftheformUL7 M (py) (M
2 2 2
) Co, Ni, Zn) were prepared to probe the effect of the
transition metal on the overall magnetic properties of the
cluster.26a,b,dApplicationofthesubtractionmethodtothese
Figure 6. Upper: Variable-temperature magnetic susceptibility data for
UL72Cu2(py) (filled circles) and UL72Zn2(py) (open triangles) clusters. systemsgave∆(cid:1)MTvsTplotsthatshowbehaviorsuggestive
Lower:Variable-temperaturemagneticsusceptibilitydata(∆(cid:1)MT)obtained of antiferromagnetic coupling between the central UIV ion
uponsubtractionoftheUZn datafromtheUCu data.Adaptedfromref
2 2 and the paramagnetic transition metal ions, in contrast to
26b.
the ferromagnetic coupling exhibited by the UCu cluster.
2
However,astheauthorsnote,spin-orbiteffectsassociated
momentum,asdescribedabove.The(cid:1) TdatafortheUCu
clusterexhibitasimilartrend,holdingcMonstantat1.7cm3·K2/ withhigh-spinCoIIcentersmaycomplicatetheinterpretation
moldownto100Kbeforedroppingto0.8cm3·K/molat2 of the magnetic data for the UCo2 cluster. Similarly, the
downturn in the data for the UNi cluster could potentially
K, close to the value of 0.75 cm3·K/mol expected for two 2
be attributed to zero-field splitting associated with the S )
noninteractingS)1/ CuIIcenterswithg)2.00.Subtraction
2 1 NiII centers.
of the UZn data from the UCu data (see Figure 6, lower)
2 2 Inadditiontosuperexchangeinteractions,therehasbeen
removesanycontributionfromtheUIVion,leavingonlythe
arecentreportsuggestingthatmagneticcouplingmayoccur
spin contribution of the two CuII ions together with any through direct metal-metal orbital overlap in the mixed-
vestigesofmagneticexchangecoupling.Indeed,theproduct
valence linear trinuclear cluster [UFeIIFeIII(C H NSi-
5 4
of the subtracted data sets displays a monotonic rise with (tBu)Me ) ]+.28 This intriguing molecule, prepared through
decreasing temperature, reaching a maximum at ∆(cid:1) T ) 2 4
M
0.95 cm3·K/mol. This increase in (cid:1) T is attributed to a
M (27) (a)Benelli,C.;Blake,A.J.;Milne,P.E.Y.;Rawson,J.M.;Winpenny,
ferromagneticexchangeinteractionbetweentheUIVandCuII R. E. P. Chem.sEur. J. 1995, 1, 614. (b) Costes, J.-P.; Dahan, F.;
Dupuis,A.Inorg.Chem.2000,39,165.(c)Costes,J.-P.;Dahan,F.;
centers. Although the subtracted data led to the qualitative
Dupuis,A.Inorg.Chem.2000,39,5994.(d)Costes,J.-P.;Dahan,F.;
determination of the sign of the exchange constant (J > 0 Donnadieu,B.;Garcia-Tojal,J.;Laurent,J.P.Eur.J.Inorg.Chem.
for ferromagnetic coupling), no attempts to quantify the 2001,363.
(28) Monreal,M.J.;Carver,C.T.;Diaconescu,P.L.Inorg.Chem.2007,
magnitude of the interaction have been put forth. 46,7226.
3388 InorganicChemistry,Vol.48,No.8,2009
MagneticExchangeCouplinginActinide-ContainingMolecules
starting at room temperature could be indicative of an
extremely strong ferromagnetic exchange interaction medi-
ated by direct orbital overlap between the metals. Note,
however,thatsuchalinearupwardtrendinthemomentwith
decreasing temperature could also potentially arise from
complications in applying corrections for the diamagnetic
contributionsoftheunusualsampleand/orthesampleholder.
If indeed the upward trend is the result of a strong
ferromagneticinteraction,asimplesubtractionoftheUFeII
2
datafromtheUFeIIFeIIIdatawouldnotprovideanappropri-
ate means of extracting the pure exchange interaction from
the overall magnetism because the added electron imposes
adifferentligandfieldonthecentralUIVion.Instead,access
to a diamagnetic analogue, such as an isostructural species
containingCoIIIinplaceofFeIII,couldperhapslenditselfto
theimplementationofthesubtractionmethodandestimation
Figure7.Structureof[UFeIIFeIII(CHNSi(tBu)Me)]+.28Orange,red,blue,
5 4 24 of the coupling strength for this interesting system.
cyan,andgrayspheresrepresentU,Fe,N,Si,andCatoms,respectively.
H atoms are omitted for clarity. The (tBu)MeSi groups are drawn Uranium-RadicalSystems.Thusfar,wehavediscussed
2
transparentlyforbettervisualizationofthemetalcoordinationenvironments. exchangeinteractionsbetweenuraniumandotherparamag-
netic metal centers; however, recent years have seen ex-
amples of uranium-radical systems showing evidence of
magnetic exchange coupling. Compelling evidence of such
an interaction was reported in 2005 in the radical complex
Cp* UIII(tpy·).29Thismolecule,whichwaspreparedthrough
2
a one-electron reduction of [Cp* UIII(tpy)]I, features a UIII
2
center coordinated by a terpyridyl ligand that houses an
additional,delocalizedelectron.StructuralanalysisandNMR
spectroscopysupporttheassignmentsofuraniumandligand
oxidation states.
Inanattempttoprobethepotentialexchangebetweenthe
UIII center and the unpaired electron of the reduced ligand,
magnetic susceptibility data were collected for both the
radical complex and the cationic precursor complex,
[Cp* UIII(tpy)]I. Structural analysis revealed very similar
2
ligand fields for the two species, enabling the use of the
Figure 8. Variable-temperature magnetic data for UFeII(CHNSi-
2 5 4
(tBu)Me) (blacksquares)and[UFeIIFeIII(CHNSi(tBu)Me)](BPh)(red subtractionmethod,whereinthecationiccomplexdatawere
24 5 4 24 4
circles).Adaptedfromref28. subtractedfromtheradicalcomplexdata.Theresulting(cid:1) T
M
the one-electron oxidation of UFeII (C H NSi(tBu)Me ) , dataremainessentiallyconstantasthetemperatureislowered
2 5 4 2 4
exhibitsastructureconsistingofacentralUIVioncoordinated from 300 K, before decreasing precipitously below 20 K.
totwo1,1′-bis(amido)ferrocenylderivatives(seeFigure7). This interaction is attributed to antiferromagnetic coupling
Coordination of UIV to the rigid ferrocenylamido moieties between the S ) 3/2 UIII center and the unpaired electron
enforces U···Fe distances of 2.9556(5) and 2.9686(5) Å. residingonthereducedterpyridylligand.Whilenoattempt
Variable-temperature magnetic moment measurements was made to quantify the coupling strength, the low
show very different behavior for the UFeII2 and UFeIIFeIII temperature at which the drop in (cid:1)MT is observed indicates
clusters (see Figure 8). In the case of the former species, thattheinteractionisrelativelyweak.Onepossibleexplana-
withdecreasingtemperature,µ followsthemonotonicdrop tion for the exchange being weak is the large separation
eff
typical for a UIV center with a 5f2 valence electron config- betweentheunpairedelectronandtheUIIIcenter.Whilethe
uration.Incontrast,fortheUFeIIFeIIIcluster,asthetemper- electron is delocalized throughout the terpyridyl ligand, no
ature is decreased from 300 K, µ begins to rise immedi- goodresonanceformexistswhereintheelectronresideson
eff
ately,followingaseeminglylineartrendbeforeturningover a uranium-coordinated N atom. Alternatively, the drop in
below20K.Theauthorsnotethattheobservedbehavioris (cid:1)MTmaybetheresultofintermolecularexchange,possibly
indicative of a magnetic interaction between UIV and FeIII between radical ligands on neighboring molecules.
centers. Indeed, the result is without precedent because the Asecondexampleofauranium-radicalcomplexhasbeen
spin-orbitcouplingandligand-fieldeffectsassociatedwith foundtobindandactivatecarbondioxide.30Thismolecule
aparamagneticuraniumcenterusuallygiverisetoasteady
decrease in the net magnetic moment as the temperature is (29) Mehdoui, T.; Berthet, J.-C.; Thue´ry, P.; Salmon, L.; Rivie`re, E.;
Ephritikhine,M.Chem.sEur.J.2005,11,6994.
lowered, even in systems exhibiting (weak) ferromagnetic
(30) Castro-Rodriguez,I.;Nakai,H.;Zakharov,L.N.;Rheingold,A.L.;
exchange interactions. Thus, the steady increase in µeff Meyer,K.Science2004,305,1757.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3389
Rinehart et al.
exchange interactions between the two paramagnetic centers
canbewhollyextractedfromtheoverallmagneticbehaviorof
themolecule.
Recently, it was found that a related UIII species could
reducedi-tert-butylbenzophenonetogive[((t-BuArO)tacn)UIV-
3
(OC•t-BuPh )].32 The crystal data and the results of density
2
functional theory (DFT) calculations are consistent with an
overall structure comprised of four resonance forms, three
containing a UIV center with an unpaired electron residing
onthedi-tert-butylbenzophenonefragment,andonefeaturing
aUIIIcenterboundtoadiamagneticdi-tert-butylbenzophe-
none.Thetemperaturedependenceofthemagneticsuscep-
Figure9.Structureof((AdArO)tacn)U(CO).30Orange,red,gray,andblue
3 2
spheresrepresentU,O,CandNatoms,respectively.Hatomsareomitted tibility data for this radical complex shows a trend similar
forclarity. to that of the CO complex, with the exception of a higher
2
momentat300K,whichisattributedtoacontributionfrom
was prepared by first encapsulating a UIII ion within the
the UIII resonance form (see Figure S1 in the Supporting
pocket of a bulky hexadentate ligand, (AdArO) tacn
3 Information).Calculationssuggestthatcouplingbetweenthe
[(AdArOH) tacn)1,4,7-tris(3-adamantyl-5-tert-butyl-2-hy-
3 UIV center and the radical ligand is at least physically
droxybenzyl)-1,4,7-triazacyclonane],togivetheelectron-rich,
reasonablebecausethecomputedsinglyoccupiedmolecular
coordinativelyunsaturatedcomplex[(AdArO) tacn]U.Expo-
3 orbital of the molecule possesses both metal and ligand
sure of this complex to an atmosphere of CO initiates a
2 character.
one-electron transfer from the UIII center to the CO ligand
2
to afford [(AdArO) tacn]UIV(CO ), as depicted in Figure 9.
3 2
The structure of the product reveals a remarkable η1-OCO Modular Approach to the Synthesis of Halide-Bridged
coordination to the UIV ion, with U-C-O and O-C-O Actinide-Transition Metal Clusters
bond angles of 171.1(2)° and 178.0(3)°, respectively. The
Motivated by the successes achieved and challenges
presenceofanunpairedelectronresidingontheCO carbon
2 encountered in the work presented above, our research has
was inferred largely from the differences in C-O vs
terminal focused on the development of a modular strategy for
C-O bond lengths and shifts in the IR spectra compared
U synthesizing halide-bridged 5f-3d clusters.19,33 This effort
tofreeCO ,whichsuggestabondingschemecomprisedof
2
the resonance forms UIVdOdC•-O- T UIV-OtC-O-. was spurred by our preparation of the pyrazolate-bridged
dimercomplex[U(Me Pz) ] ,obtainedfromthereactionof
Thevariable-temperaturemagneticsusceptibilitydataforthe 2 4 2
UCl with K(Me Pz) (Me Pz- ) 3,5-dimethylpyrazolate).
radical complex were compared to those taken for a related 4 2 2
Initial reactivity studies of this molecule revealed its
UIVcomplex,[(AdArO)tacn]UIV(N).Athightemperature,the
3 3
susceptibility to cleavage by bases such as tetrahydrofuran
µ vs T plots for the two compounds are virtually superim-
eff
posable.31 Asthetemperatureisdecreased,however,thetwo (THF) via displacement of the bridging pyrazolate ligands.
Ofparticularsignificancewasthefindingthatthedimercould
curvesbegintodivergeatca.120K.Belowthistemperature,
the data for the azido complex drop sharply, reaching a becleavedanalogouslybytheterminalchlorideligandofa
minimumofca.0.7µ at5K.Thisbehaviorisconsistentwith transitionmetalcomplextoaffordachloride-bridgedcluster.
B
an isolated 5f2 UIV center. The low-temperature data for the Thus, insertion of the trans-dichloro complexes (cyclam)-
radical complex display a quite different trend. While the MCl2(cyclam)1,4,8,11-tetraazacyclotetradecane;M)Co,
momentdropsasthetemperatureisdecreased,itdoessomore Ni, Cu, Zn) was found to generate the linear trinuclear
graduallythanwasobservedfortheazidoanalogueandreaches species (cyclam)M[(µ-Cl)U(Me2Pz)4]2, as represented in
aminimumofca.1.5µ at5K.Thedifferenceinthemagnetic Figure 10. Similar to the ULi M (py) clusters discussed
B 2 2 n
behaviorinthetwocomplexesisattributedtotheextraelectron above, this MU system is well-suited for probing the
2
residing on the CO ligand, which accounts for the added possibility of magnetic exchange coupling, owing to the
2
magneticmomentatlowtemperature.However,theobservation invariance in the coordination geometry of the UIV centers
thattheshapesofthetwoµ vsTcurvesdonotdeviateabove asthecentralMatomischanged.Importantly,theexistence
eff
120 K may suggest the presence of an exchange interaction of the ZnU member of the series, featuring an S ) 0 ZnII
2
between the UIV center and the unpaired electron on CO. center,enablessubtractionofthespin-orbitandligand-field
2
Unfortunately,thesubtractionmethodcannotbeappliedtothis effects associated with the UIV centers. The subtraction
system because of the lack of an analogue of the radical procedure utilized is identical with that described in detail
complexthateliminatestheradicalbutpreservestheligandfield above.
experienced by uranium. Thus, it seems unlikely that any
(32) Lam,O.P.;Anthon,C.;Heinemann,F.W.;O’Connor,J.M.;Meyer,
(31) Notethat(cid:1) Tprovidesasensitivemeasureofthemagneticmoment K.J.Am.Chem.Soc.2008,130,6567.
M
ofasampleandisrelatedtotheperhapsmorefamiliarquantityµ (33) Rinehart,J.D.;Bartlett,B.M.;Kozimor,S.A.;Long,J.R.Inorg.
eff
asfollows:µ )(8(cid:1) T)1/2µ . Chim.Acta2008,361,3534.
eff M B
3390 InorganicChemistry,Vol.48,No.8,2009
MagneticExchangeCouplinginActinide-ContainingMolecules
Figure10.Structureofthelinearcluster(cyclam)Co[(µ-Cl)U(MePz)].33
2 42
Orange,purple,green,gray,andbluespheresrepresentU,Co,Cl,C,and
Natoms,respectively.Hatomsareomittedforclarity.
Figure 11. Variable-temperature magnetic susceptibility data for the
Analysis of the magnetic behavior of (cyclam)Co[(µ- trinuclearcluster(cyclam)Co[(µ-Cl)U(Me2Pz)4]2(CoU2,purplesquares)and
(cyclam)Zn[(µ-Cl)U(MePz)] (ZnU,redcircles).33Bluediamondscor-
Cl)U(Me Pz) ] (CoU ) leads to what is perhaps the most 2 42 2
2 4 2 2 respondtoasubtractionoftheZnU datafromtheCoU data.Magnetic
2 2
clear-cut case to date for magnetic exchange coupling data for the precursor complex (cyclam)CoCl are depicted as green
2
between an actinide ion and a transition metal ion. Figure triangles.
11showsthevariationin(cid:1)MTwithtemperaturefortheCoU2 added to the ∆(cid:1)MT data to account for a spin-only (S ) 1)
andZnU2clusters.Inspectionofthedatashowsthatthetwo contributionfromeachofthetwoUIVcenters.Theresulting
species exhibit behavior similar to that of other molecules data are shown at the top of Figure 12 and were fit using
containing UIV centers with a 5f2 valence electron configu- MAGFIT3.134andanexchangeHamiltonianofthefollow-
ration.Atroomtemperature,the(cid:1)MTvaluesof2.06cm3·K/ ing form.
molforZnU and2.47cm3·K/molforCoU arereasonable
2 2
fortheuncoupledconstituentmetalcentersforeachsystem,
withthedifferenceof0.41cm3·K/molbeingattributableto
the presence of a low-spin CoII center with S ) 1/ in the
2
CoU species. At low temperature, (cid:1) T tends toward zero
2 M
for the ZnU cluster, as expected for depopulation of the
2
upperUIVStarksublevels.Withdecreasingtemperature,the
datafortheCoU clusteralsodropsteadily,butwitharather
2
differentcurvature.Thisdifferenceincurvatureisreflected
best in the ∆(cid:1) T values obtained upon subtraction of the
M
ZnU data from the CoU data. In distinct contrast to the
2 2
dataobtainedforthemononuclearcomplex(cyclam)CoCl ,
2
for which (cid:1) T remains essentially constant at 0.41 cm3·K/
M
mol, ∆(cid:1) T increases monotonically as the temperature is
M
lowered, reaching a maximum of 0.68 cm3·K/mol at 40 K
beforeturningover.Itisthisrisein∆(cid:1) Tthatindicatesthe
M
presence of ferromagnetic exchange coupling between the
UIVandCoIIcenterswithintheCoU cluster.Thedownturn
2
in ∆(cid:1) T below 40 K can be attributed to a loss of the
M
exchange coupling due to reduction of the total angular
momentumoftheUIVcentersupondepopulationoftheStark
sublevels. Note that this effect appears to be universal in
the ferromagnetically coupled uranium systems studied so
far using the subtraction method (see Figures 6 and 11).
Inviewofthesignificantriseobservedinthe∆(cid:1) Tdata
M
for the CoU cluster, efforts were made to quantify the
2
strengthofthemagneticexchangeinteraction.33Thisisnot
at all straightforward because the exchange will be signifi-
cantly attenuated by the gradual loss of coupling strength Figure12.Adjusted(cid:1) Tdatausedtoestimatetheboundsonthestrength
M
due to angular momentum depletion on the UIV centers as ofthemagneticexchangecouplingintheCoU2(redcircles),NiU2(blue
diamonds), and CuU (green squares) clusters.19,33 Black lines represent
thetemperatureislowered.Nevertheless,alowerboundfor 2
calculatedfitstothedata.Upper:∆(cid:1) Tdatafor(cid:1) T(CoU)-(cid:1) T(ZnU),
M M 2 M 2
theexchangeconstant,J,canbeobtainedbyfittingthedata (cid:1) T(NiU)-(cid:1) T(ZnU),and(cid:1) T(CuU)-(cid:1) T(ZnU)usedtoobtaina
M 2 M 2 M 2 M 2
assuming that no loss of coupling strength occurs with lowerbound.Avalueof2.00cm3·K/molhasbeenaddedtorepresenta
spin-onlycontributionfromeachofthetwoUIVcenters.Lower:(cid:1) Tdata
decreasing temperature. Working under this assumption, a M
obtaineduponmultiplicationbythereductionfunctionr(T))(2.00cm3·K/
temperature-invariant contribution of 2.00 cm3·K/mol was mol)/(cid:1) T(ZnU)andusedtoobtainanupperbound.
M 2
Inorganic Chemistry, Vol. 48, No. 8, 2009 3391