Articles

Is Thorium a d Transition Metal or an Actinide? An Answer from aDFT Study of the Reaction between Pyridine N-Oxide and

Cp2M(CH3)2 with M ) Zr, Th, and U

Ahmed Yahia and Laurent Maron*

UniVersite de Toulouse. INSA, UPS, LPCNO, 135 AVenue de Rangueil, F-31077 Toulouse, France, andCNRS, LPCNO, F-31077 Toulouse, France

ReceiVed October 10, 2008

The reaction of various actinide and transition metal bisalkyl complexes with pyridine N-oxide hasbeen investigated at the DFT level for M ) Zr, Th, and U. Rather than the expected oxygen transferreaction, an ortho C-H activation is observed experimentally for Th and U and is explained theoretically.The oxygen transfer reaction implies the formation of ethane by coupling two methyl groups; however,the transition state lies very high in energy due to the electrostatic repulsion between the two negativelycharged methyl groups. In contrast, C-H activation leads to a six-membered-ring transition state at lowenergy with a perfect alternation of charges. The latter is found to be kinetically accessible for Th andU but not for Zr, in agreement with the experimental observations. This is related to the ability of Th andU to hybridize their unoccupied 5f orbitals with the 6d in order to form a covalent M-Me bond, leadingto strong M-Mefσ*(C-H) back-donation at the transition state. Thorium is thus behaving more likean actinide that can use the 5f orbitals for bonding than as a transition metal.

Introduction

Actinide chemistry has been of increasing interest over thepast decade, mainly due to environmental problems as well asthe problem of nuclear waste reprocessing. This has led to thedevelopment of the chemistry of organoactinide complexes,1

which is so far limited mainly to uranium and thorium. Somevery interesting achievements have been recently reported inactinide chemistry, such as the reduction of the uranyl ion2,3 orthe reactivity of the uranium-oxo bond4,5 of the Cp2UOcomplex. Pool et al.6 studied the reaction of pyridine N-oxidewith Cp*2MR2 (M ) Th, U and R ) CH3, CH2Ph). It shouldbe noticed that pyridine N-oxide has already been reported tobe an effective oxygen transfer reagent in both actinide7 andgroup III chemistry.2,8 Rather than the oxygen transfer reaction,an unexpected ortho C-H activation was reported, and the firstcyclometalated pyridine N-oxide complex has been crystallized.6

However, the term “oxygen transfer reaction” is not unambigu-ous. A concomitant oxygen transfer and hydrogen abstraction

can lead to an oxo-carbene complex, or an oxygen transferand the coupling of the two methyl groups can lead to an oxocomplex. The first reaction was studied by Pool et al., andnothing was reported on the second possible reaction, mainlydue to the expected difficulty of coupling the two methyl groups.For the observed C-H activation reaction, the formation of theuranium(VI) intermediate was ruled out by Pool et al.6 bystudying a similar reaction with the thorium complex Cp2ThR2,where only the oxidation state IV is available. This singularreactivity has thus attracted our attention, and we have decidedto study the reaction mechanism by theoretical approaches. Inparticular, the fact that thorium and uranium complexes displaysimilar reactivity was not obvious at first sight, as the 7s2 6d2

electronic configuration of thorium makes it more likely to reactas a group IV metal rather than an early actinide, for which the5f orbitals are often involved in bonding.9,10 Moreover, Faganet al.11 reported a difference of chemical behavior of uraniumand thorium complexes on benzene exchange reaction ofCp*2An(C6H5)2 (An ) U, Th). The exchange is found to beefficient at room temperature for uranium but requires highertemperature for thorium. Pool et al.6 also reported a differenceof reactivity between uranium and thorium, since no C-Hactivation of the methyl group could be obtained for uraniumwith the 2,6-lutidine N-oxide, whereas the thorium complexreacts. Thus, the question of the nature (group IV metal oractinide) of thorium remains. In a recent experimental study,

* Corresponding author. E-mail: laurent.maron@irsamc.ups-tlse.fr.(1) Ephritkhine, M. Chem. ReV. 1997, 97, 2193.(2) Arnold, P. L.; Patel, D.; Wilson, C.; Love, J. B. Nature 2008, 451,

315.(3) Berthet, J. C.; Siffredi, G.; Thuery, P.; Ephritikhine, M. Eur. J. Chem.

Soc. 2007, 4017.(4) Barros, N.; Maynau, D.; Maron, L.; Eisenstein, O.; Zi, G.; Andersen,

R. A. Organometallics 2007, 26, 5059.(5) Zi, G.; Jia, L.; Werkema, E. L.; Walter, M. D.; Gottfriedsen, J. P.;

Andersen, R. A. Organometllics. 2005, 24, 4251.(6) Pool, J. A.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2005,

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Organometallics 2009, 28, 672–679672

10.1021/om800943a CCC: $40.75 2009 American Chemical SocietyPublication on Web 01/07/2009

the group of Kiplinger12 reported that thorium has to beconsidered as an actinide atom and not as a group IV metal.Indeed, on the basis of comparative studies of the chemicalreactivity of Cp*2Zr(Me)2, Cp*2Hf(Me)2, and Cp*2Th(Me)2 withpyridine N-oxide or other pyridine derivatives, the authorsconcluded that thorium is not a group IV metal. More precisely,it was reported in this study that the thorium complex reactswith pyridine N-oxide, whereas the zirconium and hafniumcomplexes do not. This is in good agreement with the photo-electron spectroscopy observation made by Green,13-16 whereit was demonstrated that in thorocene and tetrakisborohydride-thorium complexes, the 5f orbitals were involved in the bonding.To enforce this comparison, we decided to compute at the DFTlevel and thus to compare the reaction profile obtained for areal group IV element such as zirconium(Zr), thorium, and areal actinide, uranium. Recently, Yang et al.17 reported a DFTstudy of the reaction between 2-picoline and Cp*2MMe2 withM ) Th, U. The authors showed that it was possible toreproduce the free energy of reaction if Cp* was modeled by aCp ligand, but nothing was reported on the kinetics. Moreover,the authors proposed that the C-H activation reaction occursvia an agnostic mediated transition state, implying an M · · · Hinteraction at the transition state. A difference of reactivitybetween uranium and thorium was also found computationally(even though the reported energy differences are within the errorrange of the method). It has been shown that theoreticalapproaches are capable of studying reaction mechanisms involv-ing either transition metal complexes18 or more recentlylanthanide centers.19-23 Moreover, theoretical approaches havebeen extensively used to predict coordination and reactivity ofactinide-containing molecules.24-30 However, the reactivity oforganoactinide complexes is much less developed, and theoreti-cal studies are rather limited.31-34 This is mainly due to the

difficulty of dealing with actinide complexes. As shown in theliterature, relativistic effects are important,9,10 as are electroniccorrelation effects, due to the large number of unpaired electronsin the ground state.35,36 Thus, elaborate theoretical studies havebeen mainly limited to small molecules such as uranylderivatives10,37,38 or more recently to actinide dimers39 sincethe use of the popular DFT method was not appropriate in suchcases. Recently, Barros et al.4 have proposed the first compre-hensive study of the reactivity of organo-uranium complexes(Cp2UO and Cp2UNMe) using DFT methods; the resultscompared very well with experiment. In particular, it has beenshown that DFT methods can be safely used in that case bycomparing the geometry and the DFT wave function withCASSCF ones, which include static electronic correlation. Thus,in a similar spirit to the study of Barros et al., DFT methodshave been used here to probe the reactivity of pyridine N-oxidewith Cp2MR2 complexes, where M ) Zr, Th, U and R ) CH3.The preference for C-H activation rather than oxygen transferof pyridine N-oxide will be explained and the nature (transitionmetal or actinide) of thorium will be discussed.

Computational Details

Zirconium, thorium, and uranium were treated with a small-coreStuttgart-Dresden pseudopotential in combination with the ap-propriate basis set.40,41 In all cases, the basis set was augmentedby a set of polarization function (f for Zr, g for Th and U).42 Carbon,nitrogen, and hydrogen atoms were described with a 6-31G(d,p)polarized double-� basis set.43 Calculations were carried out at theDFT level of theory using the hybrid functional B3PW91.44,45

Geometry optimizations were carried out without any symmetryrestrictions, and the nature of the extrema (minima) was verifiedwith analytical frequency calculations. For all transition states, theintrinsic reaction coordinate was followed to verify the directconnection between the transition state and the adducts. All thesecomputations were performed with the Gaussian 0346 suite ofprograms. Gibbs free energies were obtained at 298.15 K withinthe harmonic approximation. In the following, only Gibbs freeenergy will be discussed even though it is known that the calculatedentropy within this approximation may not be well described. Theelectronic density was analyzed using the natural bonding analysis(NBO) technique.47 The NBO analysis describes donor-acceptorinteractions according to a second-order perturbation theory. This

(12) Jantunen, K. C.; Scott, B. L.; Kiplinger, J. L. J. Alloys Compd.2007, 444, 363.

(13) Clark, J. P.; Green, J. C. J. Chem. Soc., Dalton Trans. 1977, 505.(14) Brennan, J. G.; Green, J. C.; Redfern, C. M. J. Am. Chem. Soc.

1989, 111, 2373.(15) Green, J. C.; Shinomoto, R.; Edelstein, N. Inorg. Chem. 1986, 25,

2718.(16) Green, J. C.; De Simone, M.; Coreno, M.; Jones, A.; Pritchard,

H. M. I. Inorg. Chem. 2005, 44, 7781.(17) Yang, P.; Warnke, I.; Matin, R. L.; Hay, P. J. Organometallics

2008, 27, 1384.(18) See for instance the entire volume: Chem. Rev. 2000, 100.(19) Maron, L.; Eisenstein, O. J. Am. Chem. Soc. 2001, 123, 1036.(20) Eisenstein, O.; Maron, L. J. Organomet. Chem. 2002, 647, 190.(21) Werkema, E. L.; Maron, L.; Eisenstein, O.; Andersen, R. A. J. Am.

Chem. Soc. 2007, 129, 2529.(22) Sherer, E. C.; Cramer, C. J. Organometallics 2003, 22, 1682.(23) Hunt, P. A. Dalton Trans. 2007, 18, 1743.(24) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green,

J. C.; Hazari, N. J. Am. Chem. Soc. 2006, 128, 9602.(25) Lyon, J. T.; Andrews, L.; Malmqvist, P.; Roos, B. O.; Yang, T.;

Bursten, B. E. Inorg. Chem. 2007, 46, 4917.(26) Evans, W. J.; Kozimor, S. A.; Ziller, J. W.; Kaltsoyannis, N. J. Am.

Chem. Soc. 2004, 126, 14533.(27) Castro-Rodriguez, I.; Nakai, H.; Gantzel, P.; Zakharov, L. N.;

Rheingold, A. L.; Meyer, K. J. Am. Chem. Soc. 2003, 125, 15734.(28) Spencer, L. P.; Yang, P.; Scott, B. L.; Batista, E. R.; Boncella,

J. M. J. Am. Chem. Soc. 2008, 130, 2930.(29) Graves, C. R.; Yang, P.; Kozimor, S. A.; Vaughn, A. E.; Clark,

D. L.; Conradson, S. D.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.;Hay, P. J.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130,5272.

(30) Li, J.; Bursten, B. E.; Zhou, M.; Andrews, L. Inorg. Chem. 2001,40, 5448–5460; Inorg. Chem. 2001, 40, 5448.

(31) Pepper, M.; Bursten, B. E. Chem. ReV. 1991, 91, 719.(32) Cao, X.; Dolg, M. Coord. Chem. ReV. 2006, 250, 900.(33) Vallet, V.; Macak, P.; Wahlgren, U.; Grenthe, I. Theor. Chem. Acc.

2006, 115, 145.(34) Kaltsoyannis, N. Chem. Soc. ReV. 2003, 32, 9.

(35) Vallet, V.; Maron, L.; Schimmelpfennig, B.; Leininger, T.; Teichteil,Ch.; Gropen, O.; Grenthe, I.; Walhgren, U. J. Phys. Chem. A 1999, 103,9285.

(36) Dolg, M.; Fulde, P.; Stoll, H.; Preuss, H.; Pitzer, R. M.; Chang, A.Chem. Phys. 1995, 195, 2050.

(37) Ismail, N.; Heully, J.-L.; Saue, T.; Daudey, J.-P.; Marsden, C. J.Chem. Phys. Lett. 1999, 300, 296.

(38) Wang, W.; Andrews, L.; Li, J.; Bursten, B. E. Angew. Chem., Int.Ed. 2004, 43, 2554.

(39) Gagliardi, L.; Roos, B. O. Nature 2005, 433, 848.(40) Andrae, D.; Haeussermann, U.; Dolg, M.; Preuss, H. Theor. Chim.

Acta 1990, 77, 123.(41) Moritz, A.; Cao, X.; Dolg, M. Theor. Chem. Acc. 2007(accepted)

http://www.teochem.uni-stuttgart.de/pseudopotentials/clickpse.en.html.(42) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth,

A.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.Chem. Phys. Lett. 1993, 208, 111–114.

(43) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,2257.

(44) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.(45) Burke, K.; Perdew, J. P.; Yang, W. Electronic Density Functional

Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G.,Das, M. P., Eds.; 1998..

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(47) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,899–926.

Is Thorium a d Transition Metal or an Actinide? Organometallics, Vol. 28, No. 3, 2009 673

refers, in a MO method, to estimating the interaction between twomolecular orbitals by calculating the term |⟨φ1|F|φ2⟩ |2/(ε1 - ε2).

Results and Discussions

The reaction of pyridine N-oxide with Cp2MMe2 gives riseto an interesting problem of regioselectivity associated with asubsequent chemioselectivity (see Scheme 1). From the mo-lecular orbital (MO) point of view, both coordination modesare likely to occur, since a bent Cp2M fragment (with M a dtransition metal) exhibits three in-plane orbitals (equatorialplane), namely, two a1 and a b2,

48 allowing the coordination ofup to three ligands. In the case of an actinide center, four forbitals are also in-plane, but the study of Maynadie et al.49

has shown that for a bent complex only three ligands can becoordinated. Several reactions can thus occur starting for eachkind of coordination of the pyridine N-oxide substrate to themetal center (labeled 1 and 2 on Scheme 1).

Coordination 1 (Scheme 1), which is in between the twomethyl groups, can lead to the formation of Cp2M(CH3)2O (anoxygen transfer reaction), which can evolve by oxygen migrationto Cp2M(OCH3)(CH3) or to the formation of orthometalatedCp2M(CH3)(ONC5H4) with the release of methane (ortho C-Hactivation reaction). The first reaction involves an oxidation ofthe metal center (up to VI) and is thus possible only for uranium.However, despite extensive efforts, only the transition associatedwith the ortho C-H activation could be located on the PES.

Coordination 2 can lead either to the formation of Cp2MOwith the release of ethane (an oxygen transfer reaction) or tothe formation of orthometalated Cp2M(CH3)(ONC5H4) with therelease of methane (ortho C-H activation reaction). Moreover,by oxygen transfer and R-hydrogen abstraction of one CH3, theformation of Cp2MO(CH2) with the release of methane couldalso be envisioned. Despite our efforts, the transition states thatwould lead to the two latter pathways could not be located onthe potential energy surface (PES). Indeed, all attempts to obtainsuch transition states failed, always leading to the transition stateassociated with the oxygen transfer reaction, and ethaneformation could be characterized. For this reason, only theoxygen transfer will be discussed in the following for thiscoordination. Thus, the ortho C-H activation is associated witha coordination of the pyridine N-oxide between the two methylgroups (coordination 1 in Scheme 1), whereas oxygen transferwould involve sideways coordination of the pyridine N-oxide(coordination 2 in Scheme 1).

Thus, the experimental preference for C-H activation is notrelated to the stability of the molecular orbitals in the equatorial

plane, and therefore a full investigation of the reaction pathwayhas been performed.

Ortho C-H Activation Pathway: The ExperimentallyObserved One. The experimentally observed reaction has beenstudied using DFT methods for Zr, Th, and U. The experimentalcomplexes Cp*2M(CH3)2 (Cp* ) C5Me5) have been modeledby the Cp2M(CH3)2 (Cp ) C5H5) systems since it has alreadybeen shown in both actinide3,17 and lanthanide chemistry19-23

that C5H5 is the best compromise to model C5Me5. Yang et al.17

have only considered the explicit Cp* ligand for thermodynamicpurposes but computed the activation barriers with a Cp modelfor the reactivity of Cp2M(CH3)2 (M ) Th or U) with 2-picoline.This model will be discussed at the end of this section bypresenting pathways obtained with explicit Cp* for Th. Thecalculated pathways are presented in Figure 1 for all atoms. Ascan be seen, the reaction is calculated to be thermodynamicallyfavorable for all metals (exergonic by 22 to 32 kcal · mol-1).From the kinetic point of view, the calculated activation barrieris high for Zr (42.0 kcal · mol-1) but low for Th and U (around15-17 kcal · mol-1). Thus, the reaction is calculated to bekinetically accessible for Th and U but not for Zr, which is ingood agreement with the experimental observations by theKiplinger group.6,12 Since the pathway is similar for all metals,the geometries of the stationary points will be described for Zr,and some selected geometrical parameters as well as chargesare reported in Tables 14 for Th and U.

The geometries of the optimized structures are presented inFigure 2 in the case of Zr. In the Cp2MMe2 complex, the twoM-Me distances are equivalent (see Table 1) and theMe-M-Me angle varies between 96° and 104°. The Zr-Medistance is found to be smaller than the Th-Me and U-Me

(48) Albright, T. A. Tetrahedron 1982, 38, 1339.(49) Maynadie, J.; Barros, N.; Berthet, J.-C.; Thuery, P.; Maron, L.;

Ephritikhine, M. Angew. Chem., Intl. Ed. 2007, 46, 2010.

Scheme 1. Schematic Representation of the Two PossibleCoordination Modes of Pyridine N-Oxide to the MetalFragment (1 corresponds to C-H activation and 2 to oxygen

transfer)

Figure 1. Free energy profile of the ortho C-H activation ofpyridine N-oxide catalyzed by Cp2M(CH3)2 with M ) Zr, Th, andU. The free energy is given in kcal · mol-1.

Table 1. Selected Geometrical Parameters for Cp2M(Me)2 as Wellas NBO Charges

MM-Me

(Å)M-Cp

(Å)Me-M-Me

(deg)Cp-M-Cp (deg) q(M) q(Me)

Zr 2.28 2.24 99 131 1.53 -0.43Th 2.49 2.55 104 131 2.78 -0.71U 2.42 2.50 96 125 2.40 -0.42

674 Organometallics, Vol. 28, No. 3, 2009 Yahia and Maron

ones, in agreement with a smaller ionic radius for Zr. Moreover,the Th-Me distance (as well as the Th-Cp ones) is found tobe the longest, in agreement with the larger ionic radius ofTh(IV) than of U(IV). This is related to the actinide contraction.An NBO analysis of the three complexes indicates that the bondis mainly covalent polarized toward CH3 (significant orbitaloverlap) for Zr but mainly ionic (small orbital overlap) for Thand U. Indeed, for the latter, it was not possible to find anybond at the first-order NBO level but only a significantinteraction between empty 6d orbitals and the sp3 lone pair ofCH3 at the second-order donor-acceptor level (around 160kcal · mol-1). A similar situation was reported by Barros et al.for the uranium-oxo bond in Cp2UO.4 The difference betweenZr and Th/U is associated with the larger energy differencebetween the 6d and the sp3 of CH3 than between the 4d and thesp3 CH3 (Scheme 2). Moreover, the bond is even more ionicfor Th than U, and this can simply be explained by the factthat the 6d is mixing with the 5f (lower in energy) for U.

As expected, regarding the NBO bond analysis, the thoriumand uranium atom carry a larger positive charge than thezirconium (see Table 1).

The reaction starts by formation of a pyridine N-oxide adduct3 (Figure 2). This adduct formation is calculated to beendergonic by 19.7 kcal · mol-1 for Zr, whereas the adduct ismuch stronger for Th and U (coordination energy of +5.1 and+1.7 kcal · mol-1, respectively). The coordination is found tobe quite weak in the case of Zr, since the loss of entropy (mainlytranslational) associated with the coordination of the pyridineN-oxide is estimated to be at least 10 to 15 kcal ·mol-1 accordingto Watson and Eisenstein.50 This is quantitatively confirmedby looking at the coordination enthalpy (-7.0 kcal · mol-1 forTh). On the other hand, the coordination of pyridine N-oxide isrelatively strong to Th and U since the loss of translationalentropy is almost fully compensated by the coordination to themetal center. This difference of behavior between Zr and Th/Uis associated with the difference of the electrostatic interactionbetween the pyridine N-oxide and the metal center. Indeed, asmentioned before, the charge is larger for Th and U than for Zr(2.62, 2.10, and 1.37, respectively) so that the electrostaticinteraction between the negatively charged oxygen (-0.60,-0.59, and -0.47, respectively) and the metal center is smallerin the latter case. The M-O distances are found to be quiteshort, in agreement with a significant electrostatic interaction

(50) Watson, L.; Eisentein, O. J. Chem. Educ. 2002, 79, 1269.

Table 2. Selected Geometrical Parameters for the Transition Statefor C-H Activation (see Scheme 3 for atom labels)

MM-Me1

(Å)M-Me2

(Å)Me1-H (Å)

Coxy-H (Å)

M-O(Å)

Me1-M-Me2 (deg)

Zr 2.58 2.33 1.47 1.33 2.38 137Th 2.73 2.52 1.57 1.27 2.48 137U 2.69 2.49 1.54 1.30 2.43 154

Table 3. Charges of the Atoms Involved in the Six-Membered-RingTransition State for C-H Activation (see Scheme 2 for atom labels)

metal q[M] q[Me1] q[Me2] q[H] q[Coxy] q[O] q[N]

Zr 1.22 -0.30 -0.41 0.26 -0.07 -0.50 0.08Th 2.50 -0.40 -0.63 0.23 -0.12 -0.62 0.08U 1.91 -0.38 -0.43 0.25 -0.09 -0.59 0.08

Table 4. Selected Geometrical Parameters for the OrthometalatedComplex 6 (see Scheme 2 for atom labels)

M M-Me (Å) M-Coxy (Å) M-O (Å)

Zr 2.32 2.39 2.29Th 2.50 2.67 2.42U 2.45 2.60 2.34

Scheme 2. Schematic Bond Diagram between M and CH3 inthe Case of a 4d Metal (left) or a 5f/6d Metal (right)

Figure 2. Optimized geometries of the reactants (1 and 2), product(6), intermediate (3 and 5), and transition state (4) involved in theC-H activation of the pyridine N-oxide

Is Thorium a d Transition Metal or an Actinide? Organometallics, Vol. 28, No. 3, 2009 675

between the metal fragment and the pyridine N-oxide (2.46 Åfor Zr, 2.56 Å for Th, and 2.42 Å for U). The Me-M-Meangle has increased to 156° for U (133° for Zr and 125° forTh), to allow the coordination of the pyridine N-oxide. As canbe seen from Figure 2, the ortho C-H bond is oriented towardthe methyl group, preparing the possible C-H activationtransition state. However, even though the coordination is strongfor Th and U, the ortho C-H bond is not yet elongated (1.08Å, to be compared to 1.08 Å in the pyridine N-oxide). 3 evolvesto the C-H activation transition state 4 (Figure 2).

The calculated activation barrier is 42.0 kcal · mol-1 for Zr,which implies a kinetically improbable reaction, but is reducedto 15 to 17 kcal · mol-1 for Th and U, in line with a kineticallyfacile reaction. As can be seen from Figure 2, at the transitionstate, a favorable planar six-membered ring is formed (Scheme3). This is very similar to the C-H activation transition statereported by Werkema et al.51 in the case of C-H activation inC6F5H, which was described as a proton transfer with nucleo-philic assistance of fluorine. A five-membered ring transitionstate was also found by Yang et al.17 in the case of the reactionof Cp2An(Me)2 with 2-picoline. The main geometrical param-eters are reported in Table 2.

The M-Me1 bond distance has increased by 0.3 Å, implyingthe breaking of this bond. The methyl group has oriented its C3

axis toward the incoming hydrogen. The Coxy-H bond is broken(around 1.30 Å, to be compared with 1.08 Å in the free pyridineN-oxide), but the Me-H bond is still long and not fully formed(around 1.50 Å). Thus, this transition state is better describedas a proton transfer (Table 3) between the pyridine N-oxide anda methyl group with a nucleophilic assistance of oxygen. Indeed,the M-O distance has decreased by 0.08 Å, inducing an increaseof the electrostatic interaction, already present in the adduct.The height of the activation barrier can be explained by theperfect charge alternation (see Tables 1 and 3 and Scheme 4)at the transition state. This alternation has already been reportedto be a more important feature than the strength of the bond tobe broken.51 As can be seen from Table 3, the charges arecalculated to be larger for Th and U than for Zr, in line withmore ionic transition states for the former than the latter.

Interestingly, the charge of Me1, involved in the reaction, isreduced compared to the Cp2MMe2 complex, and an NBOanalysis shows that the M-Me1 bond is mainly covalent in allcases. The charge of the Me2 group is the same as in the reactant.At the same time (Table 3), the charge at the metal center is

decreasing with respect to the reactants, whereas the totalnegative charge carried by the three equatorial ligands (the twomethyls and the oxygen of the pyridine N-oxide) is larger thanthe total negative charge carried by the two methyls in thereactant. Since the M-Cp distances have not changed between1 and 3, the decrease of the charge can be associated with thechange of the nature of (at least) one bond at the transition state.Indeed, Th and U are making covalent bonds (strongly polarizedtoward the methyl group) with Me1 by using their empty 5forbitals (mainly the fσ) mixed with the 6d ones. This now meansthat an M-Me1 bond is found at the first-order NBO level butno longer at the second order as in the Cp2MMe2 complex. Thisreinforces the observation made for the reaction energies thatthe thorium atom is behaving more as an actinide than atransition metal. In the case of Zr, the Zr-Me bonds are foundto be slightly less polarized toward methyl at the transition statethan in the reactant. As can be seen from Table 3, the chargedistributions in the transition states are rather similar for Zr,Th, and U, and thus the question arises as to the origin of thedifference in activation barrier. An NBO analysis at the second-order perturbation theory level indicates a marked differencebetween Zr, for which the reaction is kinetically difficult, andTh/U, for which the reaction is facile. Indeed, some donation σ(Coxy-H)fM and some back-donation M-Me1fσ*(Coxy-H)are found. The ratio between donation and back-donation iscrucial to predict the activation of the Coxy-H bond. If the ratiois around 1, then the bond is not really activated and theactivation barrier, for a subsequent C-H activation reaction, ispredicted to be high, whereas if the ratio is high (or small),then the bond is weakened and the activation barrier may besmall. The NBO analysis indicates that the ratio for Zr is 0.8and down to 0.2 for Th and U, in line with the calculatedactivation barrier. This agrees with the fact that the d orbitalsare higher in energy for Th/U than Zr, leading to reduceddonation from σ (Coxy-H) to the empty d orbital of metal forTh and U than for Zr (Scheme 5).

On the other hand, the mixing of the 5f and 6d (for M ) Thand U) allows the formation of a covalent M-Me1 bond,allowing more substantial M-Me1fσ*(Coxy-H) back-donationthan in the case of Zr (Scheme 5). Thus, the ratio is smaller forTh/U than for Zr, in line with a lower barrier found for theformer than for the latter. At the NBO level, even though theM · · · H distances are short (2.36 and 2.33 Å for Th and U,respectively), no evidence can be found for an interactionbetween H and M and, moreover, an electrostatic repulsion(Table 3) between the two positively charged entities is found,as already reported in lanthanide chemistry.19-22,24 Thus, thetransition state is a clear σ bond metathesis one, in agreementwith the results of the labeling experiments by Jantunen et al.12

4 connects to the methane adduct 5 (Figure 2). The formationof this adduct is calculated to be very exergonic (15 to 20kcal · mol-1 relative to separated reactants). Thus, the loss ofentropy associated with methane coordination is fully overcomeby the formation of the orthometalated complex. It thereforecannot properly be described as an adduct, in agreement with

(51) Werkema, E. L.; Messines, E.; Perrin, L.; Maron, L.; Eisenstein,O.; Andersen, R. A. J. Am. Chem. Soc. 2005, 127, 7781.

Scheme 3. Atom Labeling for the Complexes Scheme 4. Schematic Charge Repartition at the C-H BondBreaking Transition State

676 Organometallics, Vol. 28, No. 3, 2009 Yahia and Maron

the long distance found between the methane molecule and themetal center (the shortest distance is found to be 4.75 Å for Thand can be as high as 9.89 Å for U). However, a strongelectrostatic interaction between the negatively charged oxygen(-0.57 for Zr, -0.69 for Th, and -0.66 for U) and the metalfragment (+1.39 for Zr, +2.66 for Th, and +2.22 for U) isobserved, which is in line with pulling down the reaction energy.The former pyridine N-oxide ring has rotated by roughly 90°around the M-O axis and now lies perpendicular to theequatorial plane. Would a Cp*(C5Me5) ligand allow this changeof orientation of the pyridine ring? A study of this pathwaywas then carried out with the explicit methyl group on the Cpligand. The free-energy profile is found to be similar to the onepresented in Figure 1 (see Figure 1 Supporting Information).The free energy of reaction and the activation barrier obtainedwith the real Cp* ligand are close to the values obtained withthe model Cp ligand. Moreover, the orientations of the substrateat the transition state with both models are similar, so the Cpmodel can be safely used to discuss the reactivity. Yang et al.17

have reported a similar observation, but based only on thereaction free energy. The M-O distance has decreased withrespect to the transition state, in agreement with an orthometa-lated complex (2.34 Å for Zr, 2.42 Å for Th, and 2.22 Å forU). The M-Coxy bond is also formed, and the optimizeddistances are 2.35 Å for Zr, 2.67 Å for Th, and 2.60 Å for U.Then the release of the methane molecule is calculated to beexergonic by 6.8 kcal · mol-1 for Zr, 9.0 kcal · mol-1 for Th,and 6.2 kcal · mol-1 for U. This is in agreement with theexpected entropic gain. The orthometalated complex 6 is notsubstantially modified compared to the methane adduct 5 alreadydescribed. Indeed, the M-O, M-Coxy, and the orientations ofthe pyridine ring perpendicular to the equatorial plane are similarin 6 and 5 (see Figure 2 and Table 4).

Thus, the C-H activation pathway is predicted to bekinetically and thermodynamically accessible for Th and U butnot really for Zr. These findings are in agreement with theexperimental observation for Zr, Th, and U reported by theKiplinger group.6,12 The difference of reactivity of Th/U andZr can be explained by the ionicity of the transition state, whichis slightly more pronounced for the former, and also by the ratioof donation to back-donation, associated with the relative

energetic position of the d orbitals, which is much smaller atthe transition state for Th/U than for Zr. At this stage, it ispossible to conclude that the Th is behaving more as an actinidethan a transition metal. In order to understand the reasons ofthe experimental preference for the C-H activation, the putativeoxygen transfer reaction has been investigated for all three metalcenters. The energy profile will be analyzed.

Oxygen Transfer Pathway. The calculated energy profilesfor all three atoms are presented in Figure 3. As can be seen,the reaction is calculated to very exergonic (-75.3 kcal · mol-1

for Zr, -95.5 kcal · mol-1 for Th, and -111.6 kcal · mol-1 forU). It should be noticed that this reaction is more exergonic

Scheme 5. Schematic Representation of the Main Interactions for Donation and Back-Donation at the Transition State for Zr(right) and Th/U (left)

Figure 3. Free energy profile for oxygen transfer of pyridineN-oxide catalyzed by Cp2M(CH3)2 with M ) Zr, Th, and U. Thefree energy is given in kcal · mol-1.

Is Thorium a d Transition Metal or an Actinide? Organometallics, Vol. 28, No. 3, 2009 677

than the ortho C-H activation one. This is related to the strengthof the M-O bond. Indeed, as already demonstrated by Barroset al.4 in the case of Cp2UO, the M-O bond has a substantialelectrostatic contribution, which makes the bond very strong.An NBO analysis of complex 9 (oxo complex) shows that theM-O is a double bond for Zr, whereas it is a single bond withan electrostatic interaction for Th and U (already reported byBarros et al.4). This also indicates that Th is closer to U thanZr, in line with the results obtained in the previous section. Thecalculated activation barriers are found to be very high for Zr(62.5 kcal · mol-1) and high for Th (44.5 kcal · mol-1) and U(37.4 kcal · mol-1). Thus, the reaction is calculated to bekinetically very difficult for Th and U and impossible for Zr.

The reaction starts by formation of a pyridine N-oxide adduct(7). The optimized structures of complexes 7 to 9 are presentedin Figure 4. As already mentioned in the Introduction, thisadduct corresponds to reaction path 2, as shown in Scheme 1.

As can be seen from Figure 3, the formation of the adduct 7is highly endergonic for Zr, leading to a weak adduct, but lessendergonic for Th and U (stronger adduct). Despite thedifference of the position of the pyridine N-oxide in adducts 3and 7, the free energy of formation is almost equivalent forboth. As already mentioned in the previous section, thedifference between Zr and Th/U is associated with the differenceof charge of the metal center and consequently with the dif-ference of electrostatic interaction between the pyridine N-oxide

and the metal (+1.43 for Zr, +2.62 for Th, and +2.18 for U).To allow the coordination of the pyridine N-oxide, theMe1-M-Me2 angle has been reduced to 77° for Zr (83° forTh and 82° for U). 7 evolves to the oxygen transfer transitionstate 8. It should be noticed that this transition state couplesthe N-O breaking (to form the M-O bond) and the Me1-Me2

coupling to form an ethane molecule. The calculated activationbarriers are found to be high, in line with kinetically difficultreactions. As can be seen from Table 5, the Me1-M-Me2 isvery acute, preparing the coupling of the two methyl groups.The C3 axis of Me2 is still oriented toward the metal, whereasthe C3 axis of Me1 is no longer pointing toward the metal.Thus, the ethane molecule is not yet formed.

On the other hand, the N-O bond is elongated by up to 0.53Å for Th and can be considered as almost broken. At the sametime, the M-O bond is almost formed. An NBO analysis ofthe transition state indicates that the M-Me2 bond is found tobe covalent for all atoms and the M-Me1 bond is still ionic forTh and U. The bonding situation and the relationship betweenbonding and the charge at the metal center are similar to thatdiscussed in the previous section, as the bonding situation atthe transition state is similar to that for the C-H activationtransition state. The height of the activation barrier can beexplained by analyzing the charges at the transition state (Table6).

As can be seen from Table 6, the two methyl groups arenegatively charged, and it has already been mentioned that sincethey should couple to form the ethane molecule, theMe1-M-Me2 angle is acute. Thus, a strong electrostaticrepulsion occurs between the methyl groups, penalizing thistransition state and explaining the height of the activation barrier.This repulsion is not compensated by the strong electrostaticinteraction between the oxo group (Table 6) and the metalfragment. This latter interaction is found to be very substantialfor Th and U but less for Zr, since in the latter case both theoxygen charge and the metal charge are smaller than for theformer. It can also be noticed that the two methyl charges arenot equivalent for Th and U, whereas they are perfectly identicalfor Zr. This can be explained by the fact that the M-Me2 bondbecomes covalent for Th and U by the use of the 5f orbitals,allowing a decrease of the negative charge on the Me2 groupand thus slightly reducing the electrostatic repulsion betweenthe two methyl groups. This of course cannot be observed forZr, where the two bonds were already covalent. The strongerO-M interaction as well as the lowering of the electrostaticrepulsion between the two methyl groups by using their empty5f orbitals makes the activation barrier lower for Th and U thanfor Zr. This reinforces the fact that despite its electronicconfiguration, the thorium atom should be considered as anactinide rather than as a transition metal. Despite our efforts,following the intrinsic reaction coordinate, no ethane adduct

Figure 4. Optimized geometries of the product (9), intermediate(7), and transition state (8) involved in the oxygen transfer reactionof the pyridine N-oxide.

Table 5. Selected Geometrical Parameters for the Oxygen TransferReaction Transition State (see Scheme 2 for the atom labels)

MM-

Me1 (Å)M-

Me2 (Å)M-O

(Å)N-O(Å)

Me1-M-Me2 (deg)

N-O-M (deg)

Zr 2.43 2.47 1.98 1.80 62 137Th 2.63 2.57 2.05 1.83 68 159U 2.49 2.47 2.06 1.66 77 157

Table 6. Charges of the Atoms Involved in the Transition State ofOxygen Transfer (see Scheme 2 for atom labels)

metal q[M] q[Me1] q[Me2] q[O] q[N]

Zr 1.39 -0.22 -0.22 -0.64 -0.23Th 2.71 -0.51 -0.44 -0.91 -0.21U 2.26 -0.38 -0.36 -0.79 -0.13

678 Organometallics, Vol. 28, No. 3, 2009 Yahia and Maron

could be located on the potential energy surface. Thus, thetransition state directly connects to the products.

Conclusions

In this study, it has been possible to explain the experimen-tally reported unusual reactivity6 of pyridine N-oxide withCp2MMe2 complexes for M ) Th and U. In particular, it hasbeen possible to show that the oxygen transfer reaction, whichis the thermodynamically most favorable reaction, is kineticallyforbidden. The electrostatic repulsion of the two methyl groups,which should couple to form an ethane molecule, makes thereaction almost impossible. On the other hand, the ortho C-Hactivation is kinetically and thermodynamically favorable. Asix-membered-ring transition state is formed, rather than aclassical four-membered ring, due to the nucleophilic assistanceof the oxygen. A perfect alternation of charge is observed inthis transition that, as already reported,51 makes the transitionstate kinetically accessible. The nonreactivity of the zirconiumcomplex reported experimentally12 is also found from thetheoretical point of view.

By comparing the results for Zr, Th, and U, we have shownin this study that despite its electronic configuration, the thoriumatom behaves more as an actinide than as a transition metal.The ionic nature of the M-Me bonds, the use of the 5f orbitalsat the transition state, and the energetic data obtained for boththe energy profiles lead us to the conclusion that thorium isclearly an actinide. It was also possible to explain why thereaction should occur for the actinide atoms rather than for Zr.This is related to the energetic position of the d orbitals involvedin the donation/back-donation process at the transition state,which makes the bond breaking easy or difficult. The orbitalsare closer to the occupied ones for Zr but much higher for theactinide, making the ratio donation/back-donation close to 1.0

for Zr but close to 0.2 for the actinide. This study thereforereinforces the experimental conclusion drawn by Jantunen etal.12

The fact that Thorium is acting as an actinide metal is ofinterest in the field of using actinide metals as catalysts forreactions. Indeed, actinide atoms have some interesting proper-ties but can only difficultly be handled due to their radioactivity.Thorium, like uranium, has some stable isotopes (nonradioac-tive) and thus can be used in standard organometallic laboratories.

The orthometalated complex formed still contains a methylgroup, and work is in progress to see if an R-hydrogenabstraction can be observed to lead to a putative oxo-carbenecomplex. This has not been observed experimentally, and theanalysis of the energy profile would be useful to proposemodifications of either the substrate or the ligands attached tothe metal center that would allow this reaction. Moreover, theexperimentally observed different reactivity of the uranium andthorium methyl complexes toward the 2,6-lutidine N-oxide6 orthe 2-picoline N-oxide12 needs to be understood. Theoreticalwork is underway in that context.

Acknowledgment. We are grateful to the CNRS, UPS,and CEA for financial support of this work. CalMip (CNRS,Toulouse, France) and CINES are acknowledged for calcula-tion facilities. L.M. thanks the Institut Universitaire deFrance.

Supporting Information Available: Complete ref 42 citation;free energy profile for the Cp* complexes; Cartesian coordinatesfor the optimized complexes 1-9 for Zr, Th, and U. This materialis available free of charge via the Internet at http://pubs.acs.org.

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Is Thorium a d Transition Metal or an Actinide? Organometallics, Vol. 28, No. 3, 2009 679