ARTICLE pubs.acs.org/JPCA
Theoretical Treatment of Redox Processes Involving Lanthanide(II) Compounds: Reactivity of Organosamarium(II) and Organothulium(II) Complexes with CO2 and Pyridine Stephanie Labouille,† Franc) ois Nief,*,† and Laurent Maron*,‡ † ‡
Laboratoire Heteroelements et Coordination, CNRS, Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau, France University of Toulouse, INSA, UPS, LPCNO, 135 avenue de Rangueil, F-31077 Toulouse, France, and CNRS, LPCNO UMR 5215, F-31077 Toulouse, France
bS Supporting Information ABSTRACT: An effective methodology to deal with the theoretical treatment on the redox chemistry of divalent organolanthanide complexes is reported and has been tested on two representative substrates, pyridine and CO2, with two different metals (samarium and thulium). An influence of the ancillary ligands, namely, C5Me5 (Cp*) or (2,3,4,5-tetramethylphospholyl) (Tmp), on the one- or two-electron oxidation processes is observed. The theoretical results are in excellent agreement with the experimental observations indicating the efficiency of the method.
A. INTRODUCTION The organometallic chemistry of lanthanides (CeLu, with the conventional addition of La, hereafter abbreviated as Ln) mainly involves synthetic methods and transformations in which the lanthanide element stays in the +III oxidation state (trivalent), which is the most stable, according to the position of these elements in the periodic table.1 These trivalent complexes have found many applications, especially in polymerization catalysis.2 However, stable organolanthanide complexes in the lower oxidation state +II (divalent) have been known for a long time, and their chemistry has been relatively less developed, partly because the +II oxidation state was once thought to be restricted to Sm, Yb, and Eu.3 Very recently, a number of isolable, but extremely reactive, divalent complexes with La,4 Nd,5 Dy,6 and Tm79 have been discovered, and thus divalent organolanthanide chemistry is expected to find new applications in the near future.10 The divalent complexes have the seemingly antagonistic properties of being Lewis acidic compounds and powerful reducing agents, and indeed divalent systems have even been shown to reduce dinitrogen into the diazenide (N2 2 ) dianion at room temperature and atmospheric pressure.10,11 A major concern of organometallic chemistry in general is to tailor the desired properties of a complex by ligand design, that is, to study the influence of the ligands on reactivity. In transitionmetal organometallic chemistry, this influence is well described by the consideration of covalent bonding through ligandmetal donation and metalligand retrodonation. In organolanthanide chemistry, and particularly with divalent compounds, it is generally admitted that the ligandmetal is mostly (if not exclusively) ionic12 and that the orbital-based concepts of donation and retrodonation are not adequate for the understanding of the lanthanideligand bond. In divalent lanthanide chemistry, the influence of the ligand environment is real. For instance, r 2011 American Chemical Society
Flowers et al. have shown by electrochemical techniques that addition of HMPA to solutions of SmI2 in organic solvents could boost the reducing power of SmII, and indeed, HMPA or other additives have proven necessary in many SmI2-based organic transformations.13 We have recently started to investigate the reductive behavior of TmII organometallic compounds, and we have observed ligand effects in this chemistry. Specifically, we found that bis[1,2,4-trist-butyl(cyclopentadienyl)]thulium(II) (A) in the presence of pyridine reacted instantaneously with reductive dimerization to give a bimetallic TmIII complex, whereas bis(2,5-bis-t-butyl-3,4dimethylphospholyl)thulium(II) (B) reacted much more slowly, since a simple pyridine adduct of TmII was observed at first.8 (Scheme 1) From this point, it seems fairly obvious that the key step is that of an electron transfer from TmII to pyridine, which is more effective when cyclopentadienyl is bonded to Tm than phospholyl. Another interesting reaction is the reductive dimerization of CO2 into oxalate promoted by bis(pentamethylcyclopentadienyl)samarium(II)14 in which the first step is also most likely an innersphere monoelectronic transfer to coordinated CO2. Therefore, the LnII/pyridine and the LnII/CO2 systems looked to us as good models to theoretically investigate mechanisms involving electron transfers in divalent lanthanide chemistry. The systems under investigation will be Cp*2Tm (A0 ) and (Tmp)2Tm (B0 ) (as simpler models of A and B, respectively), Cp*2Sm (C) and (Tmp)2Sm (D), and their interactions with CO2 and pyridine (Figure 1). The steric properties of the Cp* and Tmp ligands can be considered as similar. Received: April 14, 2011 Revised: June 15, 2011 Published: June 15, 2011 8295
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Scheme 1. Reactivity of Organothulium(II) Complexes with Pyridine
Figure 2. Gibbs free energy profiles for the reaction of Cp*2Tm with CO2 and pyridine (kcal 3 mol1). Enthalpies are given in parentheses. The spin multiplicity, the mean value of , and the spin density at the Tm center are also given. Figure 1. Organosamarium(II) and organothulium(II) complexes under study.
Noteworthy, to our knowledge, no theoretical studies of electron transfer in lanthanide(II) chemistry have appeared in the literature.
B. COMPUTATIONAL DETAILS Calculations were performed with the Gaussian 03 suite of programs.15 Density functional theory (DFT) was applied by the mean of the B3PW91 hybrid functional.16,17 Following our previous work,18 two kinds of relativistic effective core potentials (RECPs) have been used to describe the lanthanide center: small-core Stuttgart-Dresden RECP19 (which includes 1s, 2s, 2p, 3s, 3p, and 3d electrons) and large-core Stuttgart-Dresden RECP20 (which includes, in addition, 4s, 4p, 4d, and 4f electrons), depending on the size of the system. The large-core RECP was chosen according to the formal oxidation state of the lanthanide. The RECPs were used in combination with their optimized valence basis sets supplemented by an f polarization function for the large-core RECP. The Stuttgart-Dresden relativistic effective core potential21 was employed for phosphorus centers in association with its valence basis set and a d polarization function, while the 6-31+G(d) basis set was used for all other atoms.22 Geometry optimizations were performed on the whole system and without any symmetry constraints. The resulting stationary points were characterized by full vibration frequencies calculations as minima. All the following energies are given in kcal 3 mol1. C. RESULTS AND DISCUSSION 1. One-Electron Reduction of Substrates by Ln(II) Complexes. a. Case of a Heavy Lanthanide: Reduction by Cp*2Tm (A0 ) and (Tmp)2Tm (B0 ). As mentioned in the Introduction, the main
problem is to theoretically deal with the oxidation at the metal center that leads to an X2TmIII(L•) complex, starting from an X2TmII(L) adduct (X = Cp*, Tmp; L = pyridine, CO2). Indeed, in this case, it is necessary not only to treat explicitly the 4f
electrons, which were demonstrated to be inactive in pure LnIII reactivity,12,23 but also to locate the two complexes, namely, the X2TmIII(L•) complex and the X2TmII(L) adduct on the potential energy surface (PES) in order to determine the energetic cost of the oxidation. In the case of TmII, whose electronic configuration is 4f13, the situation is relatively simple. Indeed, since Hund’s rules are followed for the lanthanide complexes,23 a single electron transfer from the Tm to an L ligand induces a change in the spin multiplicity of the system, and the electronic configuration of the initial TmII complex is 4f13πL*0, with only one unpaired electron on the metal (2S + 1 = 2), whereas the electronic configuration of the final TmIII complex is 4f12πL*1, with now two unpaired electrons on the metal and one unpaired electron transferred in the LUMO (πL*) of the L ligand (2S + 1 = 4). The validity of the DFT approach to deal with f-element complexes, exhibiting a number of unpaired electrons in either their ground or excited states, is now well established and documented in the literature.2344 However, in the present case, the validity was tested on this process by comparing with CASSCF calculations, although it was already reported to be valid in the literature on a related system Cp*2Yb(bipy) by Booth et al.45,46 CASSCF calculations were carried out for both spin states by distributing the 13 electrons onto eight orbitals (the 7 f’s + π*L). In the case of the quartet spin state, the CASSCF wave function is a 4H with the two unpaired electrons occupying fø and fδ orbitals. Although two f orbitals are of ø and δ symmetries, the wave function appears to be mainly described by only one determinant (CI = 0.92 with only one of the two fø and two fδ are singly occupied, whereas the two others are doubly occupied). It is noteworthy that a similar situation was reported by Booth et al. in the case of the triplet excited state of Cp*2Yb(bipy) (4f13π*) and was attributed to a small ligand field effect in the Cp*2Ln complexes; the same explanation is of course valid in the present study. Thus, DFT can be safely used in the following. This procedure has been applied both for the reduction of pyridine and for the reduction of CO2 (Figures 2 and 3, pathways in red). The oxidation pathway was also verified by following the molecular orbitals (see the Supporting Information), the SOMO 8296
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Scheme 2. Definition of a Thermodynamic Cycle for the Oxidation of the Pyridine Substrate
Figure 3. Gibbs free energy profiles for the reaction of (Tmp)2Tm with CO2 and pyridine (kcal 3 mol1). Enthalpies are given in parentheses. The spin multiplicity, the mean value of , and the spin density at the Tm center are also given.
of the oxidized system being the LUMO of the reduced system. As already shown for uranium complexes,18 CO2 coordination involves monoelectronic oxidation of the metal center. A similar situation is found for Tm, and the oxidation is calculated in both cases to be an easy process (ΔG = 3.9 and 8.9 kcal 3 mol1, respectively, for X = Cp* and Tmp). On the other hand, the oxidation of the metal center upon coordination is more complicated for the pyridine substrate. Indeed, the reduction of pyridine appears more difficult for pyridine than for CO2, no matter what X ligand is considered (ΔG = 19.9 and 23.0 kcal 3 mol1, respectively, for X = Cp* and Tmp). This is in line with the energetic position of the πL* orbital of the ligand, which is lower for CO2 than for pyridine. An analogous situation was already reported by Yahia et al.47 on the reductive CC coupling in yttrium and lanthanum complexes. Moreover, the reduction of L is easier for Cp* than for Tmp ligands, meaning that phospholyl ligands provide a better stabilization of the formal +II oxidation state of Tm than Cp ligands. These findings are in excellent agreement with the experimental observation, but some information on the barrier are needed (a complete study of the reaction mechanism is currently under investigation and will be reported in a forthcoming publication together with the experiments). b. Case of an Early Lanthanide: Reduction by Cp*2Sm (C) and (Tmp)2Sm (D). In contrast to what occurs with Tm divalent complexes and, in general, with the late divalent lanthanides (end of the series), single electron transfer from early lanthanides to an L ligand does not imply any change in the spin multiplicity of the system (2S + 1 = 7 in the initial and final SmII and SmIII complexes). Therefore, it is necessary to use another approach to determine the energetic cost of the oxidation step. By using f-in-core RECPs, which are adapted to a given oxidation state, it is possible to determine a geometry of the complex involved in the oxidation step, namely, the L adduct to the SmII complex and the SmIII complex (in the latter case, the ligand is bearing an extra electron as shown by the inspection of the atomic spin density at the NBO level). However, because of the difference in the number of
Figure 4. Gibbs free energy profiles for the reaction of Cp*2Sm with CO2 and pyridine (kcal 3 mol1). Enthalpies are given in parentheses. The spin multiplicity, the mean value of , and the spin density at the Sm center are also given.
explicitly treated valence electrons (10 for a RECP associated with a +II oxidation state and 11 for a RECP adapted to a +III oxidation state), it is not possible to directly compare the total energies (or Gibbs free energies) of both complexes. Thus, a computational strategy has been defined. In the case of CO2 reduction, there is a clear change in the geometry of the complex18 that allows determining the energetic of the single electron transfer process. Unfortunately, this change in geometry is not always present for an oxidation/reduction process (in particular, when substrates like pyridine are involved). It is, however, possible to find a workaround by defining a thermodynamic cycle, involving CO2 for the oxidation step and then an isodesmic ligand exchange from the oxidized species (Scheme 2). Because there is no coordination of the substrates on the left of the two equations, then ΔG4 = 0 and ΔG2 = ΔG1 + ΔG3. ΔG3 can be obtained using f-in-core calculations as there is no change in oxidation state and ΔG1 can be obtained with a calculation involving an explicit treatment of the f electrons. This procedure is, in fact, general and can be applied to all lanthanide atoms. Thus, to validate this approach, calculations were carried out on the Tm complexes (Figures 2 and 3, pathways in blue) and compared to the classical procedure developed in the previous paragraph. The comparison between the reaction energies obtained with this approach and the direct one is excellent for both complexes (maximum deviation of 1.3 kcal 3 mol1), therefore, establishing the validity of this two-step approach. This general procedure was thus applied to the pyridine substrate (Figures 4 and 5) for the two SmII complexes, analogs of the TmII 8297
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Scheme 4. Binuclear Two-Electron CO2 Reduction Scheme Investigated. The Spin Multiplicity, the Mean Value of , and the Spin Density at the Sm Centers Are Also Given
Figure 5. Gibbs free energy profiles for the reaction of (Tmp)2Sm with CO2 and pyridine (kcal 3 mol1). Enthalpies are given in parentheses. The spin multiplicity, the mean value of , and the spin density at the Sm center are also given.
Scheme 3. Schematic Representation of the Two-Electron Reduction by Lanthanide(II) Complexes According to the Literature
complexes. As expected, the oxidation of SmII complexes is computed to be more demanding than that of TmII analogs: ΔG = 29.8 and 34.3 kcal 3 mol1, respectively, for Cp*2Sm(pyridine) and (Tmp)2Sm(pyridine), to be compared with ΔG = 19.9 and 23.0 kcal 3 mol1, respectively, for Cp*2Tm(pyridine) and (Tmp)2Tm(pyridine). This is in excellent agreement with the difference of oxidation potential of TmII and SmII, TmII being far easier to oxidize than SmII.48 As expected, the ligand influence on the divalent state, and, in particular, the stabilizing effect of Tmp, is found both for SmII and for TmII. 2. Reduction of CO2 by Two Electrons. Previous experimental and theoretical studies on the reduction of CO2,18,49 mainly by uranium complexes, have demonstrated the importance of the formation of a dinuclear complex bearing a doubly reduced CO2 molecule. In particular, such a complex was even crystallized by Brennan et al.50 in the case of the reaction of (C5H4Me)3U with CS2. The formation of a dinuclear complex was reported to be highly exergonic18 in the case of [(tBuArOH3)MesU]. However, the mechanism of double electron reduction was not investigated in detail, simply because the main interest of the authors was to investigate further reactivity from the dinuclear compounds and the formation of either a carbonate or an oxalate. Thus, as a highly reasonable approximation, it was considered that the double electron reduction could be accomplished by two subsequent single electron transfers, the first one being achieved by coordination of a CO2 molecule to a monometallic complex (as developed in the previous section) and the second one through the coordination of a second metallic fragment to the previously formed singly reduced complex (Scheme 3). In
Figure 6. Potential energy curves along the OCO angle in the binuclear two-electron CO2 reduction process.
this approximation, note that the first electron transfer has been shown to be endergonic,18 whereas the second electron transfer could be simply induced by the approach of the second metallic fragment, thus leading to a “barrierless” process. However, another type of two-electron reduction scheme can be considered starting from complex E, X2SmII CO2SmIIX2, where CO2 is bridging between two SmII units (Scheme 4). We investigated this alternative CO2 reduction pathway using the small-core RECP (4f electrons explicitly in the valence shell). The discussion will be made on variations of electronic energy between complexes of comparable size (E, F, and G), complex E being considered as a reference. Only the Cp* ligand has been considered in this study, because the results are expected to be strictly analogous with the Tmp ligand. The scheme depicted in Scheme 4 involves the formation of the dinuclear Sm(II) adduct E that will thus undergo a two-electron reduction. On the basis of this scheme, it appears that the OCO angle is the key geometrical parameter that can allow the study of this reduction scheme. Thus, the PES has been investigated by scanning along 8298
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Figure 7. HOMO-1, HOMO, and LUMO energetic variation along the reaction coordinate (OCO angle) for the binuclear two-electron CO2 reduction process.
this angle (Figure 6) for different spin multiplicities (2S + 1 = 11 and 2S + 1 = 13). The mechanism that leads to the reduction of CO2 from E can be divided into two distinct monoelectronic processes. A first electron transfer takes place for an OCO angle of 160° as there is an adiabatic crossing between the red and yellow curves. This crossing could be located because, although this transfer does not imply any change in the spin multiplicity of the whole system (the blue curve corresponding to 2S + 1 = 11 is still higher than the yellow and red curves that correspond to 2S + 1 = 13), it is associated with a change in coordination (from E to F) that stabilizes the radical developed on the carbon by delocalization in a vacant orbital on Sm. This first electron transfer is almost immediately followed by the second electron transfer occurring at an OCO angle around 145° (crossing between the yellow and blue curves). This crossing is easier to locate than the previous one as the second electron transfer is inducing a change in the spin multiplicity due to the formation of a negative charge localized on the carbon atom in CO2 (from F to G). This process can also be followed by monitoring the energetic and nature variation of the HOMO-1, HOMO, and LUMO orbitals (Figure 7). Indeed, an exchange between the HOMO (f lone pair in E, π* of CO2 in G) and the LUMO (π* of CO2 in E, f lone pair in G) takes place during the process, indicating that the two-electron reduction is occurring.
This two-electron reduction mechanism is predicted to be an easy process because the maximum in electronic energy is found around 5 kcal 3 mol1, corresponding to the first electron transfer (which is in excellent agreement with the value reported in the previous section) and a barrierless second electron transfer to form a very stable dinuclear complex. The energetic stabilization of the orbitals is in qualitative agreement with the energetic of the reaction.
D. CONCLUSION We have been successful in modeling for the first time metalto-ligand electron transfers in lanthanide(II) chemistry. For TmII (a late lanthanide with a more than half-filled f subshell), the “direct” theoretical treatment of the LnII/pyridine interaction is relatively straightforward at the small-core RECP level (explicit treatment of f electrons) because, although the geometries of the X2TmII(L) and X2TmIII(L•) forms are similar, their spin state is different [2S + 1 = 2 for X2TmII(L) and 2S + 1 = 4 for X2TmIII(L•)]. In addition, these calculations successfully reproduce the stabilizing effect of phospholyl for the TmII oxidation state. For an early divalent lanthanide, such as SmII (with a less than half-filled f subshell), small-core RECP treatment of the LnII/ pyridine interaction was unsuccessful, mostly because electron 8299
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The Journal of Physical Chemistry A transfer to the ligand does not induce a change either in spin state [2S + 1 = 7 for both X2SmII(L) and X2SmIII(L•)] or in geometry. However, a satisfactory workaround to this problem was found by an “indirect” treatment. Essentially, a thermodynamic cycle was considered in which we first contemplated CO2 reduction by SmII that, involving a change in geometry, was successfully treated at the small-core RECP level despite the constant 2S + 1 = 7 spin state, followed by a CO2•/pyridine• exchange that was treated at the large-core RECP level because this exchange no longer involved a change in oxidation state at Sm. Validation of this treatment was achieved by its application to thulium because the energies calculated by the “direct” and “indirect” treatments were in excellent agreement. The divalent state stabilization by phospholyl was also found with SmII. The stronger overall reducing power of TmII versus that of SmII was also successfully reproduced. Finally, the two-electron reduction of CO2 by two Cp*2Sm fragments was investigated. The energy of the Cp*2SmOCO SmCp*2 was examined as a function of the OCO angle and spin state of the complex. The mechanism that leads to the reduction of CO2 can be divided into two successive monoelectronic processes, which occur as the OCO angle bends from 180° to 160° (first electron transfer, same 2S + 1 = 13 spin state) to 145° (second electron transfer, change from 2S + 1 = 13 to 2S + 1 = 11), with two well-located adiabatic crossovers. This overall reduction is predicted to be a very easy process. We now plan to investigate other systems involving LnII reduction, both experimentally and theoretically.
’ ASSOCIATED CONTENT
bS
Supporting Information. Coordinates of all stationary point structures in xyz format. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the Institut Universitaire de France, CALMIP, and CINES for grant of computing time and CNRS, Ecole Polytechnique, and UPS for financial support of this work. ’ REFERENCES (1) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. Rev. 1995, 95, 865. (2) Hou, Z. M.; Wakatsuki, Y. Coord. Chem. Rev. 2002, 231, 1. (3) Izod, K. Angew. Chem., Int. Ed. 2002, 41, 743. (4) Hitchcock, P.; Lappert, M. F.; Maron, L.; Protchenko, A. V. Angew. Chem., Int. Ed. 2008, 47, 1488. (5) Jaroschik, F.; Momin, A.; Nief, F.; Le Goff, X.-F.; Deacon, G. B.; Junk, P. C. Angew. Chem., Int. Ed. 2009, 48, 1117. (6) Jaroschik, F.; Nief, F.; Le Goff, X.-F.; Ricard, L. Organometallics 2007, 26, 1123. (7) Cheng, J.; Takats, J.; Ferguson, M. J.; McDonald, R. J. Am. Chem. Soc. 2008, 130, 1544. (8) Jaroschik, F.; Nief, F.; Le Goff, X.-F.; Ricard, L. Organometallics 2007, 26, 355. (9) Evans, W. J.; Allen, N. T.; Ziller, J. W. Angew. Chem., Int. Ed. 2002, 41, 359.
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dx.doi.org/10.1021/jp205144h |J. Phys. Chem. A 2011, 115, 8295–8301
