Is Thorium a d Transition Metal or an Actinide? An Answer from a DFT

Jan 7, 2009 - An Answer from a DFT Study of the Reaction between Pyridine ... 6d in order to form a covalent M−Me bond, leading to strong M−Me→Ï...
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Organometallics 2009, 28, 672–679

Articles Is Thorium a d Transition Metal or an Actinide? An Answer from a DFT 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, and CNRS, LPCNO, F-31077 Toulouse, France ReceiVed October 10, 2008

The reaction of various actinide and transition metal bisalkyl complexes with pyridine N-oxide has been investigated at the DFT level for M ) Zr, Th, and U. Rather than the expected oxygen transfer reaction, 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 negatively charged methyl groups. In contrast, C-H activation leads to a six-membered-ring transition state at low energy with a perfect alternation of charges. The latter is found to be kinetically accessible for Th and U but not for Zr, in agreement with the experimental observations. This is related to the ability of Th and U to hybridize their unoccupied 5f orbitals with the 6d in order to form a covalent M-Me bond, leading to strong M-Mefσ*(C-H) back-donation at the transition state. Thorium is thus behaving more like an actinide that can use the 5f orbitals for bonding than as a transition metal. Introduction Actinide chemistry has been of increasing interest over the past decade, mainly due to environmental problems as well as the problem of nuclear waste reprocessing. This has led to the development of the chemistry of organoactinide complexes,1 which is so far limited mainly to uranium and thorium. Some very interesting achievements have been recently reported in actinide chemistry, such as the reduction of the uranyl ion2,3 or the reactivity of the uranium-oxo bond4,5 of the Cp2UO complex. Pool et al.6 studied the reaction of pyridine N-oxide with Cp*2MR2 (M ) Th, U and R ) CH3, CH2Ph). It should be noticed that pyridine N-oxide has already been reported to be an effective oxygen transfer reagent in both actinide7 and group III chemistry.2,8 Rather than the oxygen transfer reaction, an unexpected ortho C-H activation was reported, and the first cyclometalated pyridine N-oxide complex has been crystallized.6 However, the term “oxygen transfer reaction” is not unambiguous. A concomitant oxygen transfer and hydrogen abstraction * Corresponding author. E-mail: [email protected]. (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, 127, 1338. (7) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117, 9448. (8) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Inorg. Chem. 1994, 33, 1448.

can lead to an oxo-carbene complex, or an oxygen transfer and the coupling of the two methyl groups can lead to an oxo complex. The first reaction was studied by Pool et al., and nothing was reported on the second possible reaction, mainly due to the expected difficulty of coupling the two methyl groups. For the observed C-H activation reaction, the formation of the uranium(VI) intermediate was ruled out by Pool et al.6 by studying a similar reaction with the thorium complex Cp2ThR2, where only the oxidation state IV is available. This singular reactivity has thus attracted our attention, and we have decided to study the reaction mechanism by theoretical approaches. In particular, the fact that thorium and uranium complexes display similar reactivity was not obvious at first sight, as the 7s2 6d2 electronic configuration of thorium makes it more likely to react as a group IV metal rather than an early actinide, for which the 5f orbitals are often involved in bonding.9,10 Moreover, Fagan et al.11 reported a difference of chemical behavior of uranium and thorium complexes on benzene exchange reaction of Cp*2An(C6H5)2 (An ) U, Th). The exchange is found to be efficient at room temperature for uranium but requires higher temperature for thorium. Pool et al.6 also reported a difference of reactivity between uranium and thorium, since no C-H activation of the methyl group could be obtained for uranium with the 2,6-lutidine N-oxide, whereas the thorium complex reacts. Thus, the question of the nature (group IV metal or actinide) of thorium remains. In a recent experimental study, (9) Pyykko¨, P. Inorg. Chim. Acta 1987, 139, 989. (10) Vallet, V.; Schimmelpfennig, B.; Maron, L.; Teichteil, Ch.; Leininger, T.; Gropen, O.; Grenthe, I.; Walhgren, U. Chem. Phys. 1999, 244, 185. (11) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyman, A. M.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650.

10.1021/om800943a CCC: $40.75  2009 American Chemical Society Publication on Web 01/07/2009

Is Thorium a d Transition Metal or an Actinide?

the group of Kiplinger12 reported that thorium has to be considered as an actinide atom and not as a group IV metal. Indeed, on the basis of comparative studies of the chemical reactivity of Cp*2Zr(Me)2, Cp*2Hf(Me)2, and Cp*2Th(Me)2 with pyridine N-oxide or other pyridine derivatives, the authors concluded that thorium is not a group IV metal. More precisely, it was reported in this study that the thorium complex reacts with pyridine N-oxide, whereas the zirconium and hafnium complexes do not. This is in good agreement with the photoelectron spectroscopy observation made by Green,13-16 where it was demonstrated that in thorocene and tetrakisborohydridethorium complexes, the 5f orbitals were involved in the bonding. To enforce this comparison, we decided to compute at the DFT level and thus to compare the reaction profile obtained for a real group IV element such as zirconium(Zr), thorium, and a real actinide, uranium. Recently, Yang et al.17 reported a DFT study of the reaction between 2-picoline and Cp*2MMe2 with M ) Th, U. The authors showed that it was possible to reproduce the free energy of reaction if Cp* was modeled by a Cp ligand, but nothing was reported on the kinetics. Moreover, the authors proposed that the C-H activation reaction occurs via an agnostic mediated transition state, implying an M · · · H interaction at the transition state. A difference of reactivity between uranium and thorium was also found computationally (even though the reported energy differences are within the error range of the method). It has been shown that theoretical approaches are capable of studying reaction mechanisms involving either transition metal complexes18 or more recently lanthanide centers.19-23 Moreover, theoretical approaches have been extensively used to predict coordination and reactivity of actinide-containing molecules.24-30 However, the reactivity of organoactinide complexes is much less developed, and theoretical studies are rather limited.31-34 This is mainly due to the (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.

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difficulty of dealing with actinide complexes. As shown in the literature, relativistic effects are important,9,10 as are electronic correlation effects, due to the large number of unpaired electrons in the ground state.35,36 Thus, elaborate theoretical studies have been mainly limited to small molecules such as uranyl derivatives10,37,38 or more recently to actinide dimers39 since the use of the popular DFT method was not appropriate in such cases. Recently, Barros et al.4 have proposed the first comprehensive study of the reactivity of organo-uranium complexes (Cp2UO and Cp2UNMe) using DFT methods; the results compared very well with experiment. In particular, it has been shown that DFT methods can be safely used in that case by comparing the geometry and the DFT wave function with CASSCF ones, which include static electronic correlation. Thus, in a similar spirit to the study of Barros et al., DFT methods have been used here to probe the reactivity of pyridine N-oxide with Cp2MR2 complexes, where M ) Zr, Th, U and R ) CH3. The preference for C-H activation rather than oxygen transfer of pyridine N-oxide will be explained and the nature (transition metal or actinide) of thorium will be discussed.

Computational Details Zirconium, thorium, and uranium were treated with a small-core Stuttgart-Dresden pseudopotential in combination with the appropriate basis set.40,41 In all cases, the basis set was augmented by 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-ζ basis set.43 Calculations were carried out at the DFT level of theory using the hybrid functional B3PW91.44,45 Geometry optimizations were carried out without any symmetry restrictions, and the nature of the extrema (minima) was verified with analytical frequency calculations. For all transition states, the intrinsic reaction coordinate was followed to verify the direct connection between the transition state and the adducts. All these computations were performed with the Gaussian 0346 suite of programs. Gibbs free energies were obtained at 298.15 K within the harmonic approximation. In the following, only Gibbs free energy will be discussed even though it is known that the calculated entropy within this approximation may not be well described. The electronic density was analyzed using the natural bonding analysis (NBO) technique.47 The NBO analysis describes donor-acceptor interactions according to a second-order perturbation theory. This (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.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth, A.; Jonas, V.; Ko¨hler, 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.. (46) Frisch, M. J.; et al. Gaussian 03, ReVision B.05; Gaussian, Inc.: Wallingford, CT, 2004. (47) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899–926.

674 Organometallics, Vol. 28, No. 3, 2009

Yahia and Maron

Scheme 1. Schematic Representation of the Two Possible Coordination Modes of Pyridine N-Oxide to the Metal Fragment (1 corresponds to C-H activation and 2 to oxygen transfer)

refers, in a MO method, to estimating the interaction between two molecular orbitals by calculating the term |〈φ1|F|φ2〉|2/(ε1 - ε2).

Results and Discussions The reaction of pyridine N-oxide with Cp2MMe2 gives rise to an interesting problem of regioselectivity associated with a subsequent chemioselectivity (see Scheme 1). From the molecular orbital (MO) point of view, both coordination modes are likely to occur, since a bent Cp2M fragment (with M a d transition metal) exhibits three in-plane orbitals (equatorial plane), namely, two a1 and a b2,48 allowing the coordination of up to three ligands. In the case of an actinide center, four f orbitals are also in-plane, but the study of Maynadie´ et al.49 has shown that for a bent complex only three ligands can be coordinated. Several reactions can thus occur starting for each kind of coordination of the pyridine N-oxide substrate to the metal center (labeled 1 and 2 on Scheme 1). Coordination 1 (Scheme 1), which is in between the two methyl groups, can lead to the formation of Cp2M(CH3)2O (an oxygen transfer reaction), which can evolve by oxygen migration to Cp2M(OCH3)(CH3) or to the formation of orthometalated Cp2M(CH3)(ONC5H4) with the release of methane (ortho C-H activation reaction). The first reaction involves an oxidation of the metal center (up to VI) and is thus possible only for uranium. However, despite extensive efforts, only the transition associated with the ortho C-H activation could be located on the PES. Coordination 2 can lead either to the formation of Cp2MO with the release of ethane (an oxygen transfer reaction) or to the formation of orthometalated Cp2M(CH3)(ONC5H4) with the release of methane (ortho C-H activation reaction). Moreover, by oxygen transfer and R-hydrogen abstraction of one CH3, the formation of Cp2MO(CH2) with the release of methane could also be envisioned. Despite our efforts, the transition states that would lead to the two latter pathways could not be located on the potential energy surface (PES). Indeed, all attempts to obtain such transition states failed, always leading to the transition state associated with the oxygen transfer reaction, and ethane formation could be characterized. For this reason, only the oxygen transfer will be discussed in the following for this coordination. Thus, the ortho C-H activation is associated with a coordination of the pyridine N-oxide between the two methyl groups (coordination 1 in Scheme 1), whereas oxygen transfer would involve sideways coordination of the pyridine N-oxide (coordination 2 in Scheme 1). Thus, the experimental preference for C-H activation is not related to the stability of the molecular orbitals in the equatorial (48) Albright, T. A. Tetrahedron 1982, 38, 1339. (49) Maynadie´, J.; Barros, N.; Berthet, J.-C.; Thue´ry, P.; Maron, L.; Ephritikhine, M. Angew. Chem., Intl. Ed. 2007, 46, 2010.

Figure 1. Free energy profile of the ortho C-H activation of pyridine N-oxide catalyzed by Cp2M(CH3)2 with M ) Zr, Th, and U. The free energy is given in kcal · mol-1. Table 1. Selected Geometrical Parameters for Cp2M(Me)2 as Well as NBO Charges M

M-Me (Å)

M-Cp (Å)

Me-M-Me (deg)

Cp-MCp (deg)

q(M)

q(Me)

Zr Th U

2.28 2.49 2.42

2.24 2.55 2.50

99 104 96

131 131 125

1.53 2.78 2.40

-0.43 -0.71 -0.42

plane, and therefore a full investigation of the reaction pathway has been performed. Ortho C-H Activation Pathway: The Experimentally Observed One. The experimentally observed reaction has been studied using DFT methods for Zr, Th, and U. The experimental complexes Cp*2M(CH3)2 (Cp* ) C5Me5) have been modeled by the Cp2M(CH3)2 (Cp ) C5H5) systems since it has already been 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 thermodynamic purposes but computed the activation barriers with a Cp model for the reactivity of Cp2M(CH3)2 (M ) Th or U) with 2-picoline. This model will be discussed at the end of this section by presenting pathways obtained with explicit Cp* for Th. The calculated pathways are presented in Figure 1 for all atoms. As can be seen, the reaction is calculated to be thermodynamically favorable for all metals (exergonic by 22 to 32 kcal · mol-1). From the kinetic point of view, the calculated activation barrier is high for Zr (42.0 kcal · mol-1) but low for Th and U (around 15-17 kcal · mol-1). Thus, the reaction is calculated to be kinetically accessible for Th and U but not for Zr, which is in good agreement with the experimental observations by the Kiplinger 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 charges are reported in Tables 14 for Th and U. The geometries of the optimized structures are presented in Figure 2 in the case of Zr. In the Cp2MMe2 complex, the two M-Me distances are equivalent (see Table 1) and the Me-M-Me angle varies between 96° and 104°. The Zr-Me distance is found to be smaller than the Th-Me and U-Me

Is Thorium a d Transition Metal or an Actinide?

Organometallics, Vol. 28, No. 3, 2009 675

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

M-Me1 (Å)

M-Me2 (Å)

Me1H (Å)

CoxyH (Å)

M-O (Å)

Me1-MMe2 (deg)

Zr Th U

2.58 2.73 2.69

2.33 2.52 2.49

1.47 1.57 1.54

1.33 1.27 1.30

2.38 2.48 2.43

137 137 154

Table 3. Charges of the Atoms Involved in the Six-Membered-Ring Transition 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 Th U

1.22 2.50 1.91

-0.30 -0.40 -0.38

-0.41 -0.63 -0.43

0.26 0.23 0.25

-0.07 -0.12 -0.09

-0.50 -0.62 -0.59

0.08 0.08 0.08

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

M-Me (Å)

M-Coxy (Å)

M-O (Å)

Zr Th U

2.32 2.50 2.45

2.39 2.67 2.60

2.29 2.42 2.34

Scheme 2. Schematic Bond Diagram between M and CH3 in the 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 the C-H activation of the pyridine N-oxide

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

The reaction starts by formation of a pyridine N-oxide adduct 3 (Figure 2). This adduct formation is calculated to be endergonic by 19.7 kcal · mol-1 for Zr, whereas the adduct is much stronger for Th and U (coordination energy of +5.1 and +1.7 kcal · mol-1, respectively). The coordination is found to be quite weak in the case of Zr, since the loss of entropy (mainly translational) associated with the coordination of the pyridine N-oxide is estimated to be at least 10 to 15 kcal · mol-1 according to Watson and Eisenstein.50 This is quantitatively confirmed by looking at the coordination enthalpy (-7.0 kcal · mol-1 for Th). On the other hand, the coordination of pyridine N-oxide is relatively strong to Th and U since the loss of translational entropy is almost fully compensated by the coordination to the metal center. This difference of behavior between Zr and Th/U is associated with the difference of the electrostatic interaction between the pyridine N-oxide and the metal center. Indeed, as mentioned before, the charge is larger for Th and U than for Zr (2.62, 2.10, and 1.37, respectively) so that the electrostatic interaction between the negatively charged oxygen (-0.60, -0.59, and -0.47, respectively) and the metal center is smaller in the latter case. The M-O distances are found to be quite short, in agreement with a significant electrostatic interaction (50) Watson, L.; Eisentein, O. J. Chem. Educ. 2002, 79, 1269.

676 Organometallics, Vol. 28, No. 3, 2009 Scheme 3. Atom Labeling for the Complexes

between the metal fragment and the pyridine N-oxide (2.46 Å for Zr, 2.56 Å for Th, and 2.42 Å for U). The Me-M-Me angle has increased to 156° for U (133° for Zr and 125° for Th), to allow the coordination of the pyridine N-oxide. As can be seen from Figure 2, the ortho C-H bond is oriented toward the methyl group, preparing the possible C-H activation transition state. However, even though the coordination is strong for Th and U, the ortho C-H bond is not yet elongated (1.08 Å, to be compared to 1.08 Å in the pyridine N-oxide). 3 evolves to 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 reduced to 15 to 17 kcal · mol-1 for Th and U, in line with a kinetically facile reaction. As can be seen from Figure 2, at the transition state, a favorable planar six-membered ring is formed (Scheme 3). This is very similar to the C-H activation transition state reported by Werkema et al.51 in the case of C-H activation in C6F5H, which was described as a proton transfer with nucleophilic assistance of fluorine. A five-membered ring transition state was also found by Yang et al.17 in the case of the reaction of Cp2An(Me)2 with 2-picoline. The main geometrical parameters are reported in Table 2. The M-Me1 bond distance has increased by 0.3 Å, implying the 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 pyridine N-oxide), but the Me-H bond is still long and not fully formed (around 1.50 Å). Thus, this transition state is better described as a proton transfer (Table 3) between the pyridine N-oxide and a methyl group with a nucleophilic assistance of oxygen. Indeed, the M-O distance has decreased by 0.08 Å, inducing an increase of the electrostatic interaction, already present in the adduct. The height of the activation barrier can be explained by the perfect charge alternation (see Tables 1 and 3 and Scheme 4) at the transition state. This alternation has already been reported to be a more important feature than the strength of the bond to be broken.51 As can be seen from Table 3, the charges are calculated to be larger for Th and U than for Zr, in line with more ionic transition states for the former than the latter. Interestingly, the charge of Me1, involved in the reaction, is reduced compared to the Cp2MMe2 complex, and an NBO analysis shows that the M-Me1 bond is mainly covalent in all cases. 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 (51) Werkema, E. L.; Messines, E.; Perrin, L.; Maron, L.; Eisenstein, O.; Andersen, R. A. J. Am. Chem. Soc. 2005, 127, 7781.

Yahia and Maron Scheme 4. Schematic Charge Repartition at the C-H Bond Breaking Transition State

decreasing with respect to the reactants, whereas the total negative charge carried by the three equatorial ligands (the two methyls and the oxygen of the pyridine N-oxide) is larger than the total negative charge carried by the two methyls in the reactant. Since the M-Cp distances have not changed between 1 and 3, the decrease of the charge can be associated with the change of the nature of (at least) one bond at the transition state. Indeed, Th and U are making covalent bonds (strongly polarized toward the methyl group) with Me1 by using their empty 5f orbitals (mainly the fσ) mixed with the 6d ones. This now means that an M-Me1 bond is found at the first-order NBO level but no longer at the second order as in the Cp2MMe2 complex. This reinforces the observation made for the reaction energies that the thorium atom is behaving more as an actinide than a transition metal. In the case of Zr, the Zr-Me bonds are found to be slightly less polarized toward methyl at the transition state than in the reactant. As can be seen from Table 3, the charge distributions in the transition states are rather similar for Zr, Th, and U, and thus the question arises as to the origin of the difference in activation barrier. An NBO analysis at the secondorder perturbation theory level indicates a marked difference between Zr, for which the reaction is kinetically difficult, and Th/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 is crucial to predict the activation of the Coxy-H bond. If the ratio is around 1, then the bond is not really activated and the activation barrier, for a subsequent C-H activation reaction, is predicted to be high, whereas if the ratio is high (or small), then the bond is weakened and the activation barrier may be small. The NBO analysis indicates that the ratio for Zr is 0.8 and down to 0.2 for Th and U, in line with the calculated activation barrier. This agrees with the fact that the d orbitals are higher in energy for Th/U than Zr, leading to reduced donation from σ (Coxy-H) to the empty d orbital of metal for Th and U than for Zr (Scheme 5). On the other hand, the mixing of the 5f and 6d (for M ) Th and U) allows the formation of a covalent M-Me1 bond, allowing more substantial M-Me1fσ*(Coxy-H) back-donation than in the case of Zr (Scheme 5). Thus, the ratio is smaller for Th/U than for Zr, in line with a lower barrier found for the former than for the latter. At the NBO level, even though the M · · · H distances are short (2.36 and 2.33 Å for Th and U, respectively), no evidence can be found for an interaction between 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, the transition state is a clear σ bond metathesis one, in agreement with the results of the labeling experiments by Jantunen et al.12 4 connects to the methane adduct 5 (Figure 2). The formation of this adduct is calculated to be very exergonic (15 to 20 kcal · mol-1 relative to separated reactants). Thus, the loss of entropy associated with methane coordination is fully overcome by the formation of the orthometalated complex. It therefore cannot properly be described as an adduct, in agreement with

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

the long distance found between the methane molecule and the metal center (the shortest distance is found to be 4.75 Å for Th and can be as high as 9.89 Å for U). However, a strong electrostatic interaction between the negatively charged oxygen (-0.57 for Zr, -0.69 for Th, and -0.66 for U) and the metal fragment (+1.39 for Zr, +2.66 for Th, and +2.22 for U) is observed, 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 the equatorial plane. Would a Cp*(C5Me5) ligand allow this change of orientation of the pyridine ring? A study of this pathway was then carried out with the explicit methyl group on the Cp ligand. The free-energy profile is found to be similar to the one presented in Figure 1 (see Figure 1 Supporting Information). The free energy of reaction and the activation barrier obtained with the real Cp* ligand are close to the values obtained with the model Cp ligand. Moreover, the orientations of the substrate at the transition state with both models are similar, so the Cp model can be safely used to discuss the reactivity. Yang et al.17 have reported a similar observation, but based only on the reaction free energy. The M-O distance has decreased with respect to the transition state, in agreement with an orthometalated complex (2.34 Å for Zr, 2.42 Å for Th, and 2.22 Å for U). The M-Coxy bond is also formed, and the optimized distances 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 be exergonic 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 the expected entropic gain. The orthometalated complex 6 is not substantially modified compared to the methane adduct 5 already described. Indeed, the M-O, M-Coxy, and the orientations of the pyridine ring perpendicular to the equatorial plane are similar in 6 and 5 (see Figure 2 and Table 4). Thus, the C-H activation pathway is predicted to be kinetically and thermodynamically accessible for Th and U but not really for Zr. These findings are in agreement with the experimental observation for Zr, Th, and U reported by the Kiplinger group.6,12 The difference of reactivity of Th/U and Zr can be explained by the ionicity of the transition state, which is slightly more pronounced for the former, and also by the ratio of donation to back-donation, associated with the relative

energetic position of the d orbitals, which is much smaller at the transition state for Th/U than for Zr. At this stage, it is possible to conclude that the Th is behaving more as an actinide than a transition metal. In order to understand the reasons of the experimental preference for the C-H activation, the putative oxygen transfer reaction has been investigated for all three metal centers. The energy profile will be analyzed. Oxygen Transfer Pathway. The calculated energy profiles for 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 for U). It should be noticed that this reaction is more exergonic

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

678 Organometallics, Vol. 28, No. 3, 2009

Yahia and Maron Table 5. Selected Geometrical Parameters for the Oxygen Transfer Reaction Transition State (see Scheme 2 for the atom labels) M

MMe1 (Å)

MMe2 (Å)

M-O (Å)

N-O (Å)

Me1-MMe2 (deg)

N-OM (deg)

Zr Th U

2.43 2.63 2.49

2.47 2.57 2.47

1.98 2.05 2.06

1.80 1.83 1.66

62 68 77

137 159 157

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

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

than the ortho C-H activation one. This is related to the strength of the M-O bond. Indeed, as already demonstrated by Barros et al.4 in the case of Cp2UO, the M-O bond has a substantial electrostatic contribution, which makes the bond very strong. An NBO analysis of complex 9 (oxo complex) shows that the M-O is a double bond for Zr, whereas it is a single bond with an electrostatic interaction for Th and U (already reported by Barros et al.4). This also indicates that Th is closer to U than Zr, in line with the results obtained in the previous section. The calculated 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 be kinetically 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 presented in Figure 4. As already mentioned in the Introduction, this adduct corresponds to reaction path 2, as shown in Scheme 1. As can be seen from Figure 3, the formation of the adduct 7 is highly endergonic for Zr, leading to a weak adduct, but less endergonic for Th and U (stronger adduct). Despite the difference of the position of the pyridine N-oxide in adducts 3 and 7, the free energy of formation is almost equivalent for both. As already mentioned in the previous section, the difference between Zr and Th/U is associated with the difference of charge of the metal center and consequently with the difference of electrostatic interaction between the pyridine N-oxide

metal

q[M]

q[Me1]

q[Me2]

q[O]

q[N]

Zr Th U

1.39 2.71 2.26

-0.22 -0.51 -0.38

-0.22 -0.44 -0.36

-0.64 -0.91 -0.79

-0.23 -0.21 -0.13

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, the Me1-M-Me2 angle has been reduced to 77° for Zr (83° for Th and 82° for U). 7 evolves to the oxygen transfer transition state 8. It should be noticed that this transition state couples the N-O breaking (to form the M-O bond) and the Me1-Me2 coupling to form an ethane molecule. The calculated activation barriers are found to be high, in line with kinetically difficult reactions. As can be seen from Table 5, the Me1-M-Me2 is very acute, preparing the coupling of the two methyl groups. The C3 axis of Me2 is still oriented toward the metal, whereas the 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 0.53 Å for Th and can be considered as almost broken. At the same time, the M-O bond is almost formed. An NBO analysis of the transition state indicates that the M-Me2 bond is found to be covalent for all atoms and the M-Me1 bond is still ionic for Th and U. The bonding situation and the relationship between bonding and the charge at the metal center are similar to that discussed in the previous section, as the bonding situation at the transition state is similar to that for the C-H activation transition state. The height of the activation barrier can be explained by analyzing the charges at the transition state (Table 6). As can be seen from Table 6, the two methyl groups are negatively charged, and it has already been mentioned that since they should couple to form the ethane molecule, the Me1-M-Me2 angle is acute. Thus, a strong electrostatic repulsion occurs between the methyl groups, penalizing this transition state and explaining the height of the activation barrier. This repulsion is not compensated by the strong electrostatic interaction between the oxo group (Table 6) and the metal fragment. This latter interaction is found to be very substantial for Th and U but less for Zr, since in the latter case both the oxygen charge and the metal charge are smaller than for the former. It can also be noticed that the two methyl charges are not equivalent for Th and U, whereas they are perfectly identical for Zr. This can be explained by the fact that the M-Me2 bond becomes covalent for Th and U by the use of the 5f orbitals, allowing a decrease of the negative charge on the Me2 group and thus slightly reducing the electrostatic repulsion between the two methyl groups. This of course cannot be observed for Zr, where the two bonds were already covalent. The stronger O-M interaction as well as the lowering of the electrostatic repulsion between the two methyl groups by using their empty 5f orbitals makes the activation barrier lower for Th and U than for Zr. This reinforces the fact that despite its electronic configuration, the thorium atom should be considered as an actinide rather than as a transition metal. Despite our efforts, following the intrinsic reaction coordinate, no ethane adduct

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Organometallics, Vol. 28, No. 3, 2009 679

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

for Zr but close to 0.2 for the actinide. This study therefore reinforces the experimental conclusion drawn by Jantunen et al.12

Conclusions

The fact that Thorium is acting as an actinide metal is of interest in the field of using actinide metals as catalysts for reactions. Indeed, actinide atoms have some interesting properties but can only difficultly be handled due to their radioactivity. Thorium, like uranium, has some stable isotopes (nonradioactive) and thus can be used in standard organometallic laboratories.

In this study, it has been possible to explain the experimentally reported unusual reactivity6 of pyridine N-oxide with Cp2MMe2 complexes for M ) Th and U. In particular, it has been possible to show that the oxygen transfer reaction, which is the thermodynamically most favorable reaction, is kinetically forbidden. The electrostatic repulsion of the two methyl groups, which should couple to form an ethane molecule, makes the reaction almost impossible. On the other hand, the ortho C-H activation is kinetically and thermodynamically favorable. A six-membered-ring transition state is formed, rather than a classical four-membered ring, due to the nucleophilic assistance of the oxygen. A perfect alternation of charge is observed in this transition that, as already reported,51 makes the transition state kinetically accessible. The nonreactivity of the zirconium complex reported experimentally12 is also found from the theoretical point of view. By comparing the results for Zr, Th, and U, we have shown in this study that despite its electronic configuration, the thorium atom behaves more as an actinide than as a transition metal. The ionic nature of the M-Me bonds, the use of the 5f orbitals at the transition state, and the energetic data obtained for both the energy profiles lead us to the conclusion that thorium is clearly an actinide. It was also possible to explain why the reaction should occur for the actinide atoms rather than for Zr. This is related to the energetic position of the d orbitals involved in the donation/back-donation process at the transition state, which makes the bond breaking easy or difficult. The orbitals are closer to the occupied ones for Zr but much higher for the actinide, making the ratio donation/back-donation close to 1.0

The orthometalated complex formed still contains a methyl group, and work is in progress to see if an R-hydrogen abstraction can be observed to lead to a putative oxo-carbene complex. This has not been observed experimentally, and the analysis of the energy profile would be useful to propose modifications of either the substrate or the ligands attached to the metal center that would allow this reaction. Moreover, the experimentally observed different reactivity of the uranium and thorium methyl complexes toward the 2,6-lutidine N-oxide6 or the 2-picoline N-oxide12 needs to be understood. Theoretical work 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 calculation facilities. L.M. thanks the Institut Universitaire de France. Supporting Information Available: Complete ref 42 citation; free energy profile for the Cp* complexes; Cartesian coordinates for the optimized complexes 1-9 for Zr, Th, and U. This material is available free of charge via the Internet at http://pubs.acs.org. OM800943A