Experimental and Theoretical Studies of the Hydrogenation of α, β

Article pubs.acs.org/Organometallics

Experimental and Theoretical Studies of the Hydrogenation of α,βUnsaturated Acids by an 18e Hydride Carbonylniobocene Complex Antonio Antiñolo,* Khédidja Dorani, Santiago García-Yuste, Isabel López-Solera, Antonio Otero,* and María Teresa Tercero-Morales Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Química, Campus Universitario, Universidad de Castilla-La Mancha, 13071-Ciudad Real, Spain

Gábor Kovács, Gregori Ujaque, and Agustí Lledós* Departament de Química, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallés, Barcelona, Spain S Supporting Information *

ABSTRACT: The reaction of the niobocene complex [Nb(η5-C5H4SiMe3)2(H)(CO)] (1) with α,β-unsaturated acids, namely, fumaric and maleic acids, and methylfumarate ester yielded the corresponding carboxylato-containing niobocene derivatives [Nb(η5C5H4SiMe3)2(κ1-O-OOC-CH2-CH2-R)(CO)] (R = COOH (2); R = COOMe (3)), resulting from the hydrogenation of the CC bond and the consequent coordination of a carboxylato moiety on the Nb center. The process represents the first reported example of the hydrogenation of α,β-unsaturated acids mediated by a hydride niobocene complex. Spectroscopic and theoretical studies at the DFT level have allowed for proposing a plausible reaction mechanism. Initial coordination of the CC bond to the Nb center is followed by the insertion of this bond into the Nb−H bond, and the reaction is completed by a proton transfer from the carboxylic moiety to the α-carbon of the alkyl moiety, giving rise to the final κ1-carboxylato ligand.

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Nb−H bond. Additionally, the reactivity of [Nb(η 5 C5H4SiMe3)2(H)(L)] (where L = CO, isocyanides) toward activated alkynes bearing electron-withdrawing substituents was studied,6 and as a consequence of the selective insertion process into the Nb−H bond, a series of alkenyl-containing complexes were isolated. On the basis of these precedents, our recent investigations have turned to the study of the interactions of α,β-unsaturated carboxylic acids with niobocene hydride derivatives. This class of organic molecules has been involved in a series of important chemical processes,7 one of the main interesting reaction being their catalytic hydrogenation by late transition metal catalysts. This class of processes catalyzed by early transition metal catalysts has never been described. Herein we report our results concerning the reactivity of a hydride niobocene, [Nb(η5-C5H4SiMe3)2(H)(CO)] (1), with a series of α,β-unsaturated carboxylic acids. A series of saturated carboxylato-containing niobocene complexes has been isolated as a result of the intramolecular hydrogenation of the CC bond. Additionally, the X-ray molecular structure of the hydride carbonyl niobocene complex 1 has been determined. DFT studies of the reaction mechanisms show that the CC hydrogenation implies the coordination of the CC bond to the Nb center and its subsequent insertion into the Nb−H

INTRODUCTION There have already been a large number of contributions given to the chemistry of hydride metallocenes of group 5 metals.1 For the past few years, some of us have focused our attention on studies on the reactivity of hydride−niobocene derivatives, namely, the trihydride complex [Nb(η5-C5H4SiMe3)2(H)3] and the 18-electron [Nb(η5-C5H4SiMe3)2(H)(L)] species, where L = CO, isocyanides, phosphines, or phosphites.1 In particular, the reactivity of the trihydride complex toward Lewis acids, such as B(C6F5)3 and boranes,2 has shown the basic character of the hydride ligands. Indeed the basic nature of the hydrides in niobocene trihydride complexes has been demonstrated by the study of dihydrogen-bond formation with perfluorinated alcohols, showing medium-strength NbH··HOR dihydrogen bonds of about 5 kcal mol−1.3 In the presence of stronger acids, such as aromatic carboxylic acids (benzoic and terphthalic carboxylic acids) dihydrogen adducts have not been detected. Instead a direct proton transfer took place to form a dihydrogen [Nb(η5-C5H4SiMe3) 2(η 2-H2) 2]+ intermediate, which easily evolves to give the final thermodynamically stable corresponding carboxylates.4 The other research area we have focused our attention on involves insertion reactions of unsaturated molecules into Nb−hydride bonds. Some years ago we published5 a paper concerning the preparation of a styrene-containing niobocene hydride, [Nb(η5C5 H4 SiMe3 )2 (H)(η 2-CH 2CH-Ph)], and a mechanistic study on the insertion of the coordinated alkene into the © 2012 American Chemical Society

Received: June 8, 2012 Published: July 3, 2012 5177

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with other types of carboxylic acids, namely, β,δ-unsaturated acids, do not work. The results commented on above clearly indicate a reaction pathway different from that previously observed with saturated carboxylic acids. Structural Characterization of Complexes. All the complexes described in this work were spectroscopically characterized. Infrared and especially NMR spectroscopy have proved to be useful for the characterization of the carboxylato complexes.4,8 The presence of the ancillary ligand CO was confirmed in the IR spectra by means of a ν(CO) band at ca. 1915 cm−1, which is a typical value for this ligand in Nb(III) derivatives.6 Additionally, in the carboxylato-containing complexes the CO2− stretching frequencies are usually the most prominent feature of their IR spectra. The usual approach has been to relate the values of Δν [the separation between νasym(CO2−) and νsym(CO2−)] with the mono- or bidentate character of the ligands.9 For example, the IR spectrum of complex 2 shows a Δν value of 248 cm−1, which is consistent with the presence of a monodentate carboxylato ligand. Additionally, the corresponding bands of the remaining carboxylic functional group COOH appear at 3550−2500 cm−1 in the IR spectrum of complex 2 (see Experimental Section). Furthermore, the 1H and 13C{1H} NMR spectra of 2 confirm the existence of the remaining COOH unit showing a broad singlet centered at ca. δ 11.80 and 176.4 ppm, corresponding to the O−H and C(O)OH moieties, respectively. Moreover, the absence of a resonance in the typical region for the olefinic functional group, corresponding to the CHCH group, and the existence of two signals as a multiplet at ca. δ 2 ppm and a singlet at ca. δ 30 ppm in the 1H and 13C{1H} NMR spectra, respectively, confirm the presence of two −CH2− groups in the hydrogenated carboxylato ligand (κ1-O-OOC-CH2-CH 2COOH). In addition, the 13C{1H}NMR spectrum of 2 shows additional characteristic signals of the COO group of the carboxylato and the carbonyl ligands at δ 180.8 and 254.8 ppm, respectively. Similar spectroscopic data have been found for complex 3 (see Experimental Section). Thermal treatment performed on the transformation of a monodentate to a bidentate carboxylato moiety by carbon monoxide elimination from the Nb coordination sphere has been carried out for complexes 2 and 3. All attempts to remove the CO ligand, for instance by heating in toluene under reflux for several hours for 2, were unsuccessful, but in the case of complex 3 the κ2-carboxylatoniobocene complex [Nb(η5C5H4SiMe3)2(κ2-O-O′-OOC(C2H4)(COOMe))] (4), which evolves from a κ1-carboxylatoniobocene derivative with the subsequent elimination of carbon monoxide, was isolated (see Scheme 2). Complex 4 was spectroscopically characterized (see Experimental Section). Furthermore, the reaction of 4 with CO at room temperature in 3 h gives rise to complex 3; see Scheme 2. CO elimination is

bond followed by intramolecular proton transfer of one O−H proton to the other carbon atom.

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RESULTS AND DISCUSION Syntheses of Complexes. Following our previous studies of the reactions of [Nb(η5-C5H4SiMe3)2(H)3] with different types of saturated carboxylic acids,4 in the current work we studied the reactivity of [Nb(η5-C5H4SiMe3)2(H)(CO)] (1) toward different classes of α,β-unsaturated carboxylic acids, RCHCH-COOH. With this aim in mind, both isomers of cisand trans-butenedicarboxylic acid, HOOC-CHCH-COOH (maleic and fumaric acid, respectively), and trans-monomethylfumarate ester, MeOOC-CHCH-COOH, were initially selected. In all our previous studies4 we observed that monocarboxylic benzoic acid, bicarboxylic terphthalic acid, and tricarboxylic 1,3,5-phenyltricarboxylic acid react with a Nb−H bond through an initial proton transfer process, in which the COOH unit behaves as a Bronsted acid (see for example Scheme 1). Scheme 1. Reactivity of [Nb(η5-C5H4SiMe3)2(H)3] with Benzoic Acid

Interestingly, in the case of the aforementioned α,βunsaturated carboxylic acids, different results have been obtained on their interactions with complex 1. In fact, the reaction of 1 with the aforementioned acids in 1:1 molar ratio gives rise to a series of new alkylcarboxylato-niobocene complexes, [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (κ 1 -O-OOC-CH 2 -CH 2 R)(CO)] (R = COOH (2); R = COOMe (3)); see eq 1.

These alkylcarboxylato-niobocene complexes were isolated as green air-sensitive solids after appropriate workup (see Experimental Section). The preparation of 3 requires heating of the THF solution at 65 °C, while the reaction of 1 with maleic or fumaric acid to give 2 takes place at 30 °C. It is noteworthy to emphasize that in the reaction of 1 with maleic or fumaric acid only one carboxylic moiety is evolved in the process independently from the molar ratios, 1:1 or 2:1, of niobocene to carboxylic acid employed. Furthermore, in all cases no evolution of H2 was observed. Similar processes of 1 Scheme 2

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Experimental and Theoretical Studies of the Reaction Mechanism. Herein we present our experimental and DFT theoretical studies concerning the mechanistic studies for the hydrogenation of unsaturated carboxylic acids and esters with the [Nb(η5-C5H4SiMe3)2(H)(CO)] (1) complex. We have to mention that the experimental analysis by means of 1H NMR was not successful in observing any intermediates formed in the reaction; only complex 1 and final carboxylato complexes were detected. In order to gain insight into the mechanism of hydrogenation, the deuterated isotopomer [Nb(η5-C5H4SiMe3)2(D)(CO)] (1-d1) has been synthesized,14 and the reaction of 1-d1 with monomethyl fumarate ester was monitored by 1H NMR. No intermediates have been detected in the reaction, but as final carboxylato-containing complexes we have observed the isotopomer complexes [Nb(η 5 -C 5 H 4SiMe3 ) 2(κ1 -O-OOCCHD-CH2-COOMe)(CO)] and [Nb(η5-C5H4SiMe3)2(κ1-OOOC-CH2-CHD-COOMe)(CO)] (3-d1), where an equal distribution of deuterium in the former olefinic α and β carbon atoms took place. The formation of complexes 3-d1 agrees very well with the proposed mechanism (see below). At the first stage of the reaction an adduct 1a can be formed between the carboxylic acid and complex 1. In order to explain the above reported distribution of isotopomers 3-d1, 50% versus 50%, a fast H/D exchange between Nb−D and HCOO− moieties would be assumed at this stage. Analysis of geometric data of complex 1, obtained from calculations showed good agreement with the X-ray data. (Comparison of some representative geometric parameters obtained from X-ray analysis and DFT calculations is shown in the Supporting Information.) The concrete reaction we present in detail in the following is the reduction of fumaric acid; however, other substrates, such as trans-monomethylfumarate ester, have been analyzed by calculations. On the basis of our results, we can conclude that the mechanism is general for the analyzed unsaturated acids. The overall energy profile in THF for the selected reaction of fumaric acid is shown in Scheme 3. The starting point of the reaction is the adduct formed between fumaric acid and the hydride complex 1a, the structure of which is shown in Figure 2. The d(H−H) distance in 1a (1.527 Å) agrees with the formation of a strong dihydrogen bond between the hydride and the acidic proton of fumaric acid.15 The binding energy of the carboxylic proton donor is 10.3 kcal mol−1, whereas the Gibbs energy is positive by 2.3 kcal/mol; these values suggest that formation of adduct 1a may be feasible, though not too stable.16 It has been recognized that dihydrogen bond formation is the first, incipient step of the proton transfer reaction yielding cationic dihydrogen complexes, [M(η2H2)]+.17 Starting from complex 1 the transition state for the protonation of the hydride ligand by the acidic proton of

a reversible process and led us to consider the carboxylato ligand in complex 3 as a hemilabile ligand.10 X-ray Diffraction Study of [Nb(η5-C5H4SiMe3)2(H)(CO)] (1). Suitable crystals of 1 for an X-ray diffraction study were obtained. The X-ray molecular structure is depicted in Figure 1, together with the atomic numbering scheme. Selected bond distances and angles are given in Table 1.

Figure 1. ORTEP drawing of [Nb(η5-C5H4SiMe3)2(H)(CO)] (1). Hydrogen atoms are omitted for clarity, and thermal ellipsoids are shown at 30% probability.

The structure consists of discrete molecules separated by van der Waals distances. The niobium atom shows a geometry that is typical for pseudotetrahedral niobocene(III) derivatives. Both cyclopentadienyl rings are bound to the metal in an η5-mode, adopting the typical bent metallocene conformation. The environment of the niobium atom is completed by a hydride ligand and a carbonyl ligand bonded through the carbon atom to the niobium as a monodentate ligand. Bond distances and angles are within the normal range found for this class of complexes.11 The Cent(1)−Nb(1)−Cent(2) angle of 143.5° is typical of bent niobocene derivatives and similar to those reported for other similar Nb(III) complexes.12 The angle between the CO ligand and the hydride atom, C(17)−Nb(1)−H(1), is 86.7(10) °. The Nb(1)−C(17) bond distance of 2.043(4) Å is shorter than a usual Nb−C single bond (ca. 2.2 Å),13 and the C(17)− O(1) distance of 1.163(4) Å is longer than that of a free CO (ca. 1.13 Å), which indicates a substantial back-donation from the metal to the coordinated CO ligand. This fact is in agreement with the linearity of the Nb(1)−C(17)−O(1) bond angle of 178.2(3)°. Table 1. Selected Bond Lengths and Angles for Complex 1 bond lengths (Å) Nb(1)−H(1) Nb(1)−C(17) C(17)−O(1) Nb(1)−Cp(1)a Nb(1)−Cp(2)a

bond angles (deg) 1.68(3) 2.043(4) 1.163(4) 2.365(4) 2.384(4)

C(17)−Nb(1)−H(1) Nb(1)−C(17)−O(1) Cent(1)−Nb(1)−Cent(2)b Cent(1)−Nb(1)−C(17)b Cent(2)−Nb(1)−C(17)b

86.7(10) 178.2(3) 143.5 105.6 103.6

a

Refers to the average bond distance between Nb(1) and the carbon atoms of the cyclopentadienyl ring. bCent(1) and Cent(2) are the centroids of C(1)−C(5) and C(9)−C(13), respectively. 5179

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Scheme 3. Energy Profile [(ΔEsolv,), {ΔGsolv}; kcal mol−1] in THF for the Mechanism of the Hydrogenation of Fumaric Acid Mediated by [Nb(η5-C5H4SiMe3)2(H)(CO)] (1)

Figure 2. Optimized geometries for the initial step of substrate coordination to the metal center. The Me groups of the SiMe3 groups and the hydrogens of the Cp rings are omitted for clarity.

between the initial η5 and the final η2-coordination mode), as two of the five Nb−C bonds are already lengthened; d(Nb−C) = 2.654 and 2.660 Å are longer than the Nb−C distances in 1, which are in the range 2.345−2.444 Å. In complex 1b, the Cp ring is no longer coordinated in a η5-fashion but as an η2-ligand, as seen in the three elongated Nb−C distances (d(Nb−C) = 3.219, 3.377, and 3.742 Å); the Nb−C distances for the two carbon atoms that remain bonded are 2.373 and 2.576 Å, respectively. The C−C double bond of the substrate is activated, which can be noticed in the lengthening of the double bond compared to 1a (d(C−C) = 1.335 Å in 1a and 1.438 Å in 1b). The barrier from 1a for the slippage− coordination step is 35.1 kcal mol−1, whereas the π-complex 1b lies 20.1 kcal/mol over the initial intermediate. In the next step of the reaction, the coordinated double bond is inserted into the Nb−H bond and the corresponding alkyl intermediate 1c (see Figure 3) is formed. The insertion takes place via TS-2, which is shown also in Figure 3. In the transition state the Nb−H is already elongated (d(Nb−H) = 1.822 Å), whereas the C−H bond is not yet formed (d(C−H) = 1.549 Å). As seen in Figure 3, in the alkyl complex 1c the C5H4SiMe3 moiety is coordinated again to the metal center as a η5-ligand. The transition state for the insertion (TS2) lies 35.0 kcal/mol over 1a, whereas the alkyl intermediate is 15.6 kcal

fumaric acid pointing to the hydride and the corresponding product with the η2-H2 ligand were obtained by calculations. The protonation has a low energy barrier (4.8 kcal/mol); however, the reverse deprotonation (the protonation step is reversible) is much easier (the barrier was calculated to be 3.3 kcal/mol). Thus it is consistent with the experimental fact that no H2 release was found in this system. We have explored the possibility of an inner-sphere mechanism for the hydrogenation reaction involving the initial coordination of the double bond to the Nb center. Giving the saturated nature of the hydride carbonylniobocene complex 1, this coordination step should entail decoordination of one ligand. Cp ring-slippage has already been considered in innersphere hydrogenation mechanisms catalyzed by half-sandwich ruthenium carbonyl complexes.18 We were able to locate the transition state (TS1, Figure 2), which corresponds to simultaneous coordination and ring-slippage. A change in the Cp-coordination mode from η5 to η2 takes place along this process that leads to the π-complex 1b (Figure 2). The intrinsic reaction coordinate (IRC) calculations from TS1 in both directions backward and forward along with subsequent geometry optimizations give rise to structures 1a and 1b, respectively. When looking at the structures, it can be seen that in TS1 the Cp ring is coordinated in a η3-manner (a transition 5180

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fumaric acid were found to be much higher (46.9 and 49.4 kcal/mol, respectively) than that for the insertion, this pathway could be discarded. According to the reaction energy profile for the most favorable pathway, we cannot distinguish one concrete ratedetermining step, as the transition states for the coordination/ ring-slippage step and the insertion reaction step are at almost the same energy level (ΔEsolv values of 35.1 vs 35.0 kcal/mol). These structures can be found at high energy level, as the ringslippage is a very energy demanding process; however, for this particular catalytic system, this series of mechanistic steps was found to be the most favorable one. As we stated earlier above, we have carried out mechanistic investigations for not only fumaric acid but also a series of substrates. An analogous mechanism was obtained for the reduction of trans-monomethylfumarate ester. The corresponding overall barriers were calculated to be in the same range as those obtained with fumaric acid (for details see the Supporting Information). Hydrogenation Mechanism. The mechanism we have theoretically characterized for the hydrogenation of α,βunsaturated acids by the hydride niobocene complex 1 is depicted in Scheme 4. It consists of three steps: Cp ringslippage−π-coordination of the CC bond, followed by CC insertion into the Nb−H bond, and finally proton migration from the oxygen to the carbon of the acidic moiety. The initial intermediate 1a is a dihydrogen-bonded adduct, which is not productive for the hydrogenation. From this intermediate the coordination of the CC bond to the Nb center would occur in order for the reaction to proceed. The CC coordination to an 18-electron species must be accompanied by the slippage process of one of the cyclopentadienyl rings, formally η5 to η2. Such slippage is similar to what has been proposed for inner-sphere imine hydrogenations catalyzed by ruthenium carbonyl hydrides.18 However this slippage of a cyclopentadienyl ring in niobocenes is rare, and as far as we are aware, only one example has been described.19 After the coordination, an insertion step of the coordinated CC into the Nb−H bond leads to an alkyl intermediate, and in the final step an intramolecular proton transfer from the carboxylic group to the α carbon of the alkyl moiety gives rise to the final thermodynamically stable carboxylato-niobocenes 2 and 3. This mechanism is similar to that proposed from DFT calculations for the selective CC reduction in acidic aqueous media of α,β-unsaturated aldehydes catalyzed by water-soluble ruthenium(II) complexes.20 In the ruthenium system the active species is also the monohydride and the CC hydrogenation takes place through the insertion of one carbon atom into the Ru−H bond and subsequent protonation of the other carbon by the hydroxonium ions present in the acidic solution. As far as we are aware, it is the first time that such a mechanism has been found operative with an early transition metal complex. In the niobium system in the absence of external protons the acidic proton of the substrate is playing the same role in an intramolecular pathway. It is noteworthy that the reaction of 1 with trans-dimethylfumarate ester, MeOOC-CHCHCOOMe, does not work. The barriers are considerably higher in the niobium case due to both the difficulties in coordinating the substrate to a saturated complex and the less acidic character of the incoming proton. The strong binding of the carboxylate final product to the metal affords a thermodynamical driving force for the hydrogenation but hampers setting up a catalytic version of this reaction.

Figure 3. Optimized structure for the transition state and product of the insertion of the coordinated C−C into the Nb−H bond. The Me groups of the SiMe3 groups and the hydrogens of the Cp rings are omitted for clarity.

mol−1 more stable in energy than the dihydrogen-bonded adduct. In order to complete the reduction, a second hydrogen has to be transferred to the other carbon atom of the double bond. Since the initial complex is a monohydride (there is no second hydride ligand) and the solvent is aprotic (THF), the only reasonable candidates are the protons of one of the carboxylic groups in the coordinated fumaric acid. The protonation takes place via TS3 (see Figure 4), in which a five-membered ring is

Figure 4. Optimized structure for the proton transfer TS and the final product of the reaction. The Me groups of the SiMe3 groups and the hydrogens of the Cp rings are omitted for clarity.

formed, whereas the proton transfer from the other carboxylic group would involve a four-membered ring with a higher energy barrier. The proton in TS3 is found between the carbon and the oxygen atom (d(C−H) = 1.281 Å and d(O−H) = 1.394 Å), and the relative energy barrier for the proton transfer was calculated to be 24.0 kcal mol−1. The final product of the reaction is the alkylcarboxylato-niobocene complex 2 (see Figure 4), in which the carboxylate group is strongly coordinated to the metal center (d(Nb−O) = 2.167 Å), whereas the other oxygen forms a weak interaction (d(Nb−O) = 3.259 Å) with the saturated complex. This alkylcarboxylatoniobocene complex is very stable in energy (40.5 kcal mol−1 below 1a), giving the thermodynamic driving force to the hydrogenation. This high stability also accounts for the reaction not being catalytic, and the initial hydride complex cannot be recovered. We have to mention that an alternative mechanism for the hydrogenation of the coordinated unsaturated acid was also considered, which involves the protonation of the double bond by one of the protons of fumaric acid preceding the insertion step. However, as the corresponding energy barriers for the protonation of the double bond by either of the two protons of 5181

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Scheme 4. Proposed Pathway for the Formation of 2 and 3

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CONCLUDING REMARKS This work provides evidence that α,β-unsaturated carboxylic acids can be hydrogenated by a niobocene hydride complex, namely, [Nb(η5-C5H4SiMe3)2H(CO)]. Complex 1 reacts with a set of α,β-unsaturated carboxylic acids, yielding the corresponding carboxylato-containing niobocene derivatives that result from the hydrogenation of the CC bond and the subsequent coordination of the carboxylato moiety to the Nb center. From the theoretical calculations exploring several mechanistic pathways we propose a plausible mechanism for the reduction of α,β-unsaturated carboxylic acids. It was shown that the reaction takes place in three well-defined steps: (1) coordination of the carboxylic acid to the metal center accompanied by the slippage of the Cp moiety, (2) insertion of the C−C double bond to the Nb−H bond, (3) intramolecular protonation of the other carbon atom in the alkyl intermediate by one of the carboxylic groups in the substrate. The latter step leads to the experimentally detected κ1-carboxylatoniobocene complex, which represents an energetically very stable complex that explains why the initial hydride cannot be recovered from this latter complex.

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Table 2. Crystal Data and Structure Refinement for Complex 1 empirical formula Mw temperature (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (calcd) (g/cm3) abs coeff (mm−1) F(000) cryst size (mm3) index ranges

reflns collected indep reflns obsd reflns no. of data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)]

EXPERIMENTAL SECTION

General Procedures. All reactions were carried out using standard Schlenk techniques. Oxygen and water were excluded through the use of vacuum lines supplied with purified N2. Hexane was distilled from sodium/potassium alloy. THF was distilled from sodium benzophenone. All solvents were deoxygenated prior to use. Deuterated solvents were dried over 4 Å molecular sieves and degassed prior to use. [Nb(η5-C5H4SiMe3)2H(CO)] (1)21 was prepared as described in the literature. Fumaric acid, maleic acid, and monomethyl fumarate were used as supplied by Aldrich. 1H and 13C{1H} NMR spectra were recorded on a Varian Innova 500 MHz spectrometer at room

largest diff peak hole (e Å−3)

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C17H27NbOSi2 396.48 200(2) 0.71073 monoclinic P21/n 12.132(1) 12.819(1) 12.794(1) 90 97.575(3) 90 1972.3(4) 4 1.335 0.729 824 0.28 × 0.22 × 0.17 −14 ≤ h ≤ 14 −15 ≤ k ≤ 15 −15 ≤ l ≤ 15 12 410 3472 [R(int) = 0.0886] 2629 3472/0/200 0.961 R1 = 0.0365 wR2 = 0.0871 0.361 and −1.059

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temperature unless stated otherwise. 1H and 13C{1H} NMR chemical shifts (δ values) are given in ppm relative to the solvent signal (1H, 13 C) or standard resonances. IR spectra were recorded on a PerkinElmer 883 spectrophotometer as Nujol/polyethylene mulls. Microanalyses were carried out with a Perkin-Elmer 2400 microanalyzer. Synthesis of [Nb(η5-C5H4SiMe3)2(κ1-O-OOC(C2H4)COOH))(CO)] (2). To a solution of 1 (0.20 g, 0.51 mmol) in anhydrous THF (50 mL) was added an equimolar quantity of fumaric acid (0.06 g, 0.51 mmol) (or maleic acid). The mixture was stirred at 30 °C for 18 h. During this time the solution changed color from violet to greenbrown. The solvent was evaporated under vacuum to dryness. The green-brown, oily residue was extracted with cool THF. After filtration, the resulting solution was evaporated under vacuum to afford a green solid, which corresponds to complex 2. Yield: 90%. Complex 2 is soluble in polar solvents such as THF and acetone and slightly soluble in apolar solvents such as hexane and toluene. IR (Nujol/polyethylene): ν (cm−1) 3550−2500 (O−H), 1915 (CO), 1741 (COOH), 1624 (COO−)asym, 1376 (COO−)sym. 1H NMR (500 MHz, C6D6): δ (ppm) 0.08 (s, 18H, SiMe3), 2.19, 2.45 (m, 2H, CH2), 4.77, 5.03, 5.13, 5.41 (m, 2H, exact assignment not possible, C5H4SiMe3), 11.8 (s, 1H, COOH). 13C{1H} NMR (125 MHz,C6D6): δ (ppm) −0.1 (SiMe3), 30.9, 32.4 (CH2), 95.0 (C1, C5H4SiMe3), 97.2, 102.1, 106.7, 106.9 (C2−5, exact assignment not possible, C5H4SiMe3), 176.4 (COOH), 180.8 (COO), 254.8 (CO). Anal. Calcd for C21H31NbO5Si2: C, 49.21; H, 6.10. Found: C, 49.18; H, 6.14. Synthesis of [Nb(η5-C5H4SiMe3)2(κ1-O-OOC(C2H4)COOCH3)(CO)] (3). To a solution of 1 (0.20 g, 0.51 mmol) in anhydrous THF (50 mL) was added an equimolar quantity of monomethyl fumarate (0.07 g, 0.51 mmol). The mixture was stirred at 65 °C for 3 h. During this time the solution changed color from violet to greenbrown. The solvent was evaporated under vacuum to dryness. The green-brown, oily residue was washed with cool hexane to give complex 3 as a green solid. Yield: 92%. Complex 3 is soluble in polar solvents such as THF or acetone, and slightly soluble in apolar solvents such as hexane and toluene. IR (Nujol/polyethylene): ν (cm −1 ); 1914 (CO), 1724 (COOMe), 1645 (COO−)asym, 1304 (COO−)sym. 1H NMR (500 MHz, C6D6): δ (ppm) 0.12 (s, 18H, SiMe3), 2.30, 2.41 (m, 2H, CH2), 3.31 (s, 3H, CH3), 4.78, 5.01, 5.14, 5.41 (m, 2H, exact assignment not possible, C5H4SiMe3). 13C{1H} NMR (125 MHz, C6D6): δ (ppm) −0.04 (SiMe3), 30.3, 32.5 (CH2), 51.1 (CH3), 95.0 (C1, C5H4SiMe3), 97.5, 102.2, 106.7, 106.9 (C2−5, exact assignment not possible, C5H4SiMe3), 173.6 (COOMe), 180.4 (COO), 254.7 (CO). Anal. Calcd for C22H33NbO5Si2: C, 50.18; H, 6.32. Found: C, 50.26; H, 6.28. Synthesis of [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (κ 2 -O-O′-OOC(C 2 H 4 )COOCH3)] (4). A solution of 3 (0.20 g, 0.40 mmol) in anhydrous THF (50 mL) was stirred at 65 °C for 6 h. After this time, the solvent was evaporated under vacuum to dryness to give a green solid, which corresponds to complex 4. Yield: 95%. IR (Nujol/polyethylene): ν(cm−1) 1741 (COOMe)asym, 1682 (COOMe)sym, 1644 (COO¯)asym, 1553 (COO¯)sym. 1H NMR (500 MHz, C6D6): δ (ppm) 0.01 (s, 18H, SiMe3), 1.75, 1.88 (m, 2H, CH2), 3.25 (s, 3H, CH3), 4.22, 5.76 (m, 4H, exact assignment not possible, C5H4SiMe3). 13C{1H} NMR (125 MHz, C6D6): δ (ppm) 0.5 (SiMe3), 27.8, 31.8 (CH2), 51.1 (CH3), 93.0, (C1, C5H4SiMe3), 104.7, 107.4 (C2−5, exact assignment not possible, C5H4SiMe3), 172.4 (COOMe), 188.5 (COO). Anal. Calcd for C21H33NbO4Si2: C, 50.59; H, 6.67. Found: C, 50.48; H, 6.71. Computational Details. The geometry optimizations have been carried out by DFT calculations with the program package Gaussian0922 and the M0623 combination of functionals. The SDD24 pseudopotential was employed for the niobium center, and the standard 6-31G(d)25 basis set was used for the other atoms. The effect of the bulk solvent (THF) was estimated by the application of the polarizable continuum model26 as implemented in Gaussian 09 [ε(THF) = 7.4257]. In order to consider the effect of the basis set, for one profile the energies have been calculated by using the same SDD pseudopotential for the metal center and the extended 6-311++G(d,p) basis set for the other atoms (see Supporting Information). ΔEsolv energies given in the text correspond to those including the effect of the bulk solvent, which was obtained by adding the contribution of the

Gibbs energy of solvation to the gas phase total energies (no ZPE corrections added). ΔGsolv values are calculated according to the following scheme:27 ΔGsolv = Esolv + (Ggas − Egas). In the case of the transition states, normal coordinate analysis has been used to calculate the imaginary frequencies, and for each transition structure we calculated the intrinsic reaction coordinate routes toward the corresponding minima. If the IRC calculations failed to reach the energy minima on the potential energy surface, we performed geometry optimizations from the final phase of the IRC path. X-ray Structure Determination for Complex 1. Crystals were obtained from THF solution and were mounted at low temperature in inert oil on a glass fiber. Data were collected on a Bruker X8 APPEX II CCD-based diffractometer, equipped with a graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å). The crystal data, data collection, structural solution, and refinement parameters for the complexes are summarized in Table 2. Data were integrated using SAINT,28 and an absorption correction was performed with the program SADABS.29 The structures were solved by direct methods using SHELXTL30 and refined by full-matrix leastsquares methods based on F2. All non-hydrogen atoms were refined with anisotropic thermal parameters. All H atoms were computed and refined with an overall isotropic temperature factor using a riding model.

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ASSOCIATED CONTENT

S Supporting Information *

CIF file giving details of data collection, refinement, atom coordinates, anisotropic displacement parameters, bond lengths and angles, geometric parameters from DFT calculations, and Cartesian coordinates and energies (in hartree values) are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministerio de Ciencia e Innovación (MICINN), Spain (Grant Nos. CTQ2011-23336 CTQ2008-00318, CTQ2009-09214, Consolider-Ingenio 2010 ORFEO CSD2007-00006, and a Juan de la Cierva contract to G.K.), and the Junta de Comunidades de Castilla-La Mancha, Spain (Grant Nos. PCI08-0010 and PCIO8-0032). Thanks are given to the Consejo Superior de ́ Investigaciones Cientificas (CSIC) of Spain for the award of a license for the use of the Cambridge Crystallographic Data Base (CSD).

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DEDICATION Dedicated to the memory of Prof. Juan J. Berzas, a good friend and excellent colleague recently deceased. REFERENCES

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