Synthesis and Reactivity of Tantalum Complexes Supported by the

Organometallics 2009, 28, 5505–5513 DOI: 10.1021/om900643c

5505

Synthesis and Reactivity of Tantalum Complexes Supported by the Pincer Ligand 2,6-Pyridinedicarboxylate. Preparation of an Unprecedented Water-Soluble Iminoacyl Complex Ana Conde,† Rosa Fandos,*,† Antonio Otero,*,‡ and Ana Rodrı´ guez§ †

Facultad de Ciencias del Medio Ambiente, Departamento de Quı´mica Inorg anica, Org anica y Bioquı´mica, Universidad de Castilla-La Mancha, Avenida Carlos III, s/n, 45071 Toledo, Spain, ‡Facultad de Quı´micas, Campus de Ciudad Real, Departamento de Quı´mica Inorg anica, Org anica y Bioquı´mica, Universidad de Castilla-La Mancha, Avenida Camilo Jos e Cela, 10, 13071 Ciudad Real, Spain, and §ETS Ingenieros Industriales, Universidad de Castilla-La Mancha, Campus de Ciudad Real, Avenida Camilo Jos e Cela, 3, 13071 Ciudad Real, Spain Received July 21, 2009

The tantalum complex [TaCp*Me(OTf)((OOC)2py-κ3-O,N,O)] (1) has been synthesized by reaction of [TaCp*Me3(OTf)] (Cp* = η5-C5Me5, OTf = trifluoromethylsulfate) with 2,6-pyridinedicarboxylic acid. Complex 1 reacts with HOTf to yield the new tantalum carboxylate derivative [TaCp*(OTf)2((OOC)2py-κ3-O,N,O)] (2) or with MeOH to render the corresponding methoxide compound [TaCp*(OMe)(OTf)((OOC)2py-κ3-O,N,O)] (3). The reaction of 2 with H2O gives the hydroxo complex [TaCp*(OH)(OTf)((OOC)2py-κ3-O,N,O)] (4). Moreover, compound 1 reacts with t BuNC to yield the cationic iminoacyl-containing derivative [TaCp*{C(Me)dNtBu-κ2-C,N}((OOC)2py-κ3-O,N,O)](OTf) (5), which is hygroscopic and reacts with water to render the corresponding water-soluble adduct (5 3 H2O). The reaction of 1 with B(C6F5)3 in a 1:1 or 1:2 molar ratio renders the corresponding adducts [TaCp*Me(OTf){(O(dO 3 B(C6F5)3C))(OOC)py-κ3-O,N,O}] (6) or [TaCp*Me(OTf){(O(dO 3 B(C6F5)3C))2py-κ3-N,O,O}] (7), respectively. The molecular structures of complexes 1 3 C7H8, 3, 4, 5 3 H2O, and 6 3 CH2Cl2 have been established by X-ray diffraction methods.

Introduction Oxygen donor ligands such as alkoxides, aryloxides,1 and carboxylates2 are extremely versatile ligands because appropriate substitution patterns allow substantial modification of the steric and electronic properties of the metal center. Among them, carboxylate ligands are interesting because they are ubiquitous in biological materials and common in systems of catalytic utility. In addition, they offer a wide variety of coordination modes that make them useful in the construction of molecular architectures.3 In addition, the development

of polydentate ligand frameworks containing anionic oxygen or nitrogen donors has allowed many of the recent advances in early transition metal organometallic chemistry.4-8

*Corresponding authors. E-mail: [email protected]. (1) (a) Rothwell, I. P. Acc. Chem. Res. 1988, 21, 153. (b) Rothwell, P. Chem. Commun. 1997, 1331. (2) Dang, Y. Coord. Chem. Rev. 1994, 135, 93. (3) Oldhan, C. Carboxylates, Squarates and Related Species. In Comprehensive Coordination Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford, 1987. (4) Floriani, C.; Floriani-Moro, R. Adv. Organomet. Chem. 2001, 47, 167. (5) (a) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421. (b) Katsuki, T. Adv. Synth. Catal. 2002, 344, 131. (c) Katsuki, T. Chem. Soc. Rev. 2004, 33, 437. (6) (a) Hustad, P. D.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 11578. (b) Reinartz, S.; Mason, A. F.; Lobkovsky, E. B.; Coates, G. W. Organometallics 2003, 22, 2542. (c) Furuyama, R.; Saito, J.; Ishii, S.; Mitani, M.; Matsui, S.; Tohi, Y.; Makio, H.; Matsukawa, N.; Tanaka, H.; Fujita, T. J. Mol. Catal. A: Chem. 2003, 200, 31. (d) Pennington, D. A.; Coles, S. J.; Hursthouse, M. B.; Bochmann, M.; Lancaster, S. J. Chem. Commun. 2005, 3150.

(7) (a) Balsells, J.; Carroll, P. J.; Walsh, P. J. Inorg. Chem. 2001, 40, 5568. (b) Tshuva, E. T.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2001, 20, 3017. (c) Tshuva, E. T.; Groysman, S.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2002, 21, 662. (d) Toupance, T.; Dubberley, S. R.; Rees, N. H.; Tyrrell, B. R.; Mountford, P. Organometallics 2002, 21, 1367. (e) Groysman, S.; Goldberg, I.; Kol, M.; Genizi, E.; Goldschmidt, Z. Organometallics 2003, 22, 3013. (f) Owiny, D.; Parkin, S.; Ladipo, F. T. J. Organomet. Chem. 2003, 678, 134. (g) Capacchione, C.; De Carlo, F.; Zannoni, C.; Okuda, J.; Proto, A. Macromolecules 2004, 37, 8918. (h) Knight, P. D.; Munslow, I.; O'Shaughnessy, P. N.; Scott, P. Chem. Commun. 2004, 894. (i) Cuomo, C.; Strianese, M.; Cuenca, T.; Sanz, M.; Grassi, A. Macromolecules 2004, 37, 7469. (j) Segal, S.; Goldberg, I.; Kol, M. Organometallics 2005, 24, 200. (k) Capacchione, C.; Manivannan, R.; Barone, M.; Beckerle, K.; Centore, R.; Oliva, L.; Proto, A.; Tuzi, A.; Spaniol, T. P.; Okuda, J. Organometallics 2005, 24, 2971. (l) Boyd, C. L.; Toupance, T.; Tyrrell, B. R.; Ward, B. D.; Wilson, C. R.; Cowley, A. R.; Mountford, P. Organometallics 2005, 24, 309. (8) (a) Edema, J. J. H.; Libbers, R.; Ridder, A. M.; Kellogg, R. M.; van Bolhuis, F.; Kooijman, H.; Spek, A. L. J. Chem. Soc., Chem. Commun. 1993, 625. (b) Mack, H.; Eisen, M. S. J. Chem. Soc., Dalton. Trans. 1998, 917. (c) Manickam, G.; Sundararajan, G. Tetrahedron: Asymmetry 1999, 10, 2913. (d) Kemmitt, T.; Al-Salim, N. I.; Ginsford, G. J.; Henderson, W. Aust. J. Chem. 1999, 52, 915. (e) Gauvin, R. M.; Osborn, J. A.; Kress, J. Organometallics 2000, 19, 2944. (f) Manivannan, R.; Sundararajan, G.; Kaminsky, W. J. Mol. Catal. A 2000, 160, 85. (g) Shao, P.; Gendron, R. A. L.; Berg, D. J.; Bushnell, G. W. Organometallics 2000, 19, 509. (h) Shao, P.; Gendron, R. A. L.; Berg, D. J. Can. J. Chem. 2000, 78, 255. (i) Kim, Y.; Han, Y.; Do, Y. J. Organomet. Chem. 2001, 634, 19. (j) Manivannan, R.; Sundararajan, G. Macromolecules 2002, 35, 7883.

r 2009 American Chemical Society

Published on Web 08/25/2009

pubs.acs.org/Organometallics

5506

Organometallics, Vol. 28, No. 18, 2009

Conde et al.

Scheme 1

The success of these supporting ligands can be attributed to the hard nature of the O and N donor atoms, the diversity and relative ease of manipulation of ligand geometry, chirality, and coordination number, and the “tunability” of the associated steric factors. Moreover, multidentate ancillary ligands can help increase the stability of the complexes, preventing dissociation processes that sometimes produce degradation of catalysts over time. In the last years we have carried out studies based on the synthesis of several families of early transition metal complexes with dicarboxylate pincer ligands, finding out that the bonding of the ONO ligands to the metal center is remarkably stable.9 Now we report the synthesis of a carboxylate alkyl complex and its reactivity with different acidic species or toward unsaturated organic molecules.

Figure 1. ORTEP drawing of [TaCp*Me(OTf)((OOC)2py-κ3O,N,O)] (1 3 C7H8).

Table 1. Selected Bond Lengths [A˚] and Bond Angles [deg] for 1 3 C7H8 and 2 1 3 C7H8

2

Results and Discussion Bond Lengths

A general way to achieve the synthesis of carboxylate derivatives of early transition metals is the reaction of metal alkyl complexes with carboxylic acids to yield the corresponding alkanes and the carboxylate complexes. Following this procedure, the tantalum complex [TaCp*Me3(OTf)] reacts with 2,6-pyridinedicarboxylic acid in 1:1 ratio according to Scheme 1 to render complex 1. It was isolated as a yellow solid, soluble in CH2Cl2 and toluene, less soluble in diethyl ether or pentane. In this field, we have previously described an analogous reaction using the titanium complex [TiCp*Me3] as starting material.9b Complex 1 has been characterized by the usual analytical and spectroscopic techniques. The 1H NMR spectrum of compound 1 exhibits two singlet signals at 0.17 and 2.39 ppm assigned to the methyl group bonded to the tantalum center and to the Cp* group, respectively. The aromatic protons of the pyridinic ring appear as multiplet resonances at 8.39 and 8.49 ppm. The 13 C NMR is in agreement with a symmetric coordination mode of the dicarboxylate ligand pointing to a trans disposition of the methyl group and the triflate ligand. For instance, the resonance for the carbon atoms of the carboxylate moieties appears at 166.4 ppm. The ligand coordination mode has been confirmed by an X-ray diffraction study. An ORTEP drawing of 1 3 C7H8 is shown in Figure 1, and some selected bond distances and angles are summarized in Table 1. The coordination geometry around the metal can be described as pseudo-octahedral. The tantalum atom is bonded to the cyclopentadienyl ring in a η5-mode. On the other hand, the carboxylate group is bonded to the metal as a “pincer” ligand, through two oxygen atoms that are placed in the equatorial plane, and to the nitrogen of the pyridinic moiety, which is in trans-position to the Cp* group. In (9) (a) Fandos, R.; Gallego, B.; Otero, A.; Rodrı´ guez, A.; Ruiz, M. J.; Terreros, P.; Pastor, C. Dalton Trans. 2006, 2683. (b) Conde, A.; Fandos, R.; Otero, A.; Rodríguez, A. Organometallics 2008, 27, 6090.

Ta(1)-O(2) Ta(1)-O(1) Ta(1)-N(1) Ta(1)-O(5) Ta(1)-C(8)

2.011(4) 2.04(1) 2.171(5) 2.216(5) 2.194(7)

Ta(1)-O(3) Ta(1)-O(1) Ta(1)-N(1) Ta(1)-O(5) Ta(1)-O(8)

1.977(7) 1.992(7) 2.146(8) 2.065(7) 2.104(7)

Bond Angles O(2)-Ta(1)-O(1) O(2)-Ta(1)-C(8) O(1)-Ta(1)-C(8) C(1)-O(2)-Ta(1) C(7)-O(1)-Ta(1)

144.3(3) 88.1(3) 95.7(4) 123.9(4) 122.1(9)

O(3)-Ta(1)-O(1) O(3)-Ta(1)-O(8) O(1)-Ta(1)-O(8) C(1)-O(1)-Ta(1) C(7)-O(3)-Ta(1)

147.3(3) 86.2(3) 85.4(3) 125.0(7) 124.9(7)

addition, the methyl group and the triflate ligands are in the equatorial plane, in trans-position to each other. The coordination environment around the tantalum center closely resembles that in [TaCp*Cl2((OCH2)2py-κ3-O,N,O)]10 and [TaCp*Cl2((OOC)2py-κ3-O,N,O)].9a The Ta(1)-O(1) and Ta(1)-O(2) bond distances (2.04(1) and 2.011(4) A˚, respectively) are rather short for tantalum carboxylate complexes.11 The Ta-N bond length (2.171(5) A˚) is comparable to that found in anionic nitrogen ligands12 or in [TaCp*Cl2((OOC)2py-κ3-O,N,O)] (2.159(4) A˚). It is interesting to notice that the Ta(1)-O(5) bond length, 2.216(5) A˚, is considerably longer than that reported for covalent triflate-tantalum complexes, while the Ta(1)-C(8) bond length (2.194(7) A˚) is much shorter than the Ta-C distances generally found in tantalum alkyl complexes.13 This is in agreement with that expected according to the trans-influence series. (10) Fandos, R.; L opez-Solera, I.; Otero, A.; Rodrı´ guez, A.; Ruiz, M. J.; Terreros, P. Organometallics 2004, 23, 5030. (11) (a) Bayot, D.; Tinant, B.; Devillers, M. Inorg. Chem. 2004, 43, 5999. (b) Bayot, D.; Tinant, B.; Devillers, M. Inorg. Chem. 2005, 44, 1562. (12) Khin, M.; Tin, T.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 1999, 38, 998. (13) (a) Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21, 5549. (b) Bo, C.; Fandos, R.; Feliz, M.; Hernandez, C.; Otero, A.; Rodríguez, A. M.; Ruiz, M. J.; Pastor, C. Organometallics 2006, 25, 3336. (c) Conde, A.; Fandos, R.; Otero, A.; Rodriguez, A. Organometallics 2007, 26, 1568.

Article


Figure 2. ORTEP drawing of [TaCp*(OTf)2((OOC)2py-κ3-O, N,O)] (2).

Figure 3. ORTEP drawing of [TaCp*(OMe)(OTf)((OOC)2pyκ3-O,N,O)] (3).

Scheme 3

Scheme 2

It is also interesting to point out that the reaction of [TaCp*Me4] with 2,6- pyridinedicarboxylic acid in 1:1 ratio does not yield the corresponding dicarboxylate dimethyl derivative but the dinuclear oxo complex [TaCp*(μ-O)((OOC)2py-κ3-O,N,O)]2.9a Complex 1 reacts slowly with HOTf in toluene, at 60 °C, to yield the corresponding bis-triflate derivative 2 (Scheme 2). Complex 2 was isolated as a yellow solid, soluble in CH2Cl2, THF, or toluene and less soluble in pentane or Et2O. Complex 2 has been characterized by the usual analytical and spectroscopic techniques as well as by X-ray diffraction methods. The NMR data indicate that both carboxylate groups in the pincer ligand are in the same chemical environment. The 13 C NMR spectrum shows a signal at 164.9 ppm assigned to the carbon atoms of the equivalent carboxylate moieties. The molecular structure of compound 2 has been established by an X-ray diffraction study. An ORTEP drawing of 2 is depicted in Figure 2, and selected bond distances and angles are given in Table 1. The coordination geometry is analogous to that of complex 1 3 C7H8. Bond distances from the tantalum atom to the Cp* ligand and to the carboxylate moieties are comparable to that found in compound 1. The Ta(1)-O(5) and Ta(1)-O(8) bond distances (2.065(7) and 2.104(7) A˚, respectively) are much shorter than the Ta(1)-O(5) distance found in 1 (2.216(5) A˚) and somewhat shorter than that found in [TaCp*(OTf)2((OCH2)2py-κ3-O, N,O)] (2.129(4) A˚).13 Complex 1 reacts slowly with an excess of MeOH in toluene, at 60 °C, to yield the corresponding methoxide derivative 3 (Scheme 3). Complex 3 was isolated as a pale yellow solid, soluble in CH2Cl2 and THF and less soluble in toluene or Et2O. The 1H NMR of 3 in CDCl3 shows a singlet signal at 2.40 ppm due to the Cp* ligand. In addition, the methoxide group gives rise to a singlet signal at 3.55 ppm and the

5507

Table 2. Selected Bond Lengths [A˚] and Bond Angles [deg] for 3 and 4 3

4 Bond Distances

O(1)-Ta(1) O(3)-Ta(1) O(5)-Ta(1) O(11)-Ta(1) N(1)-Ta(1)

2.048(4) 2.041(4) 1.905(5) 2.163(4) 2.174(5)

O(3)-Ta(1)-O(1) O(5)-Ta(1)-O(3) O(5)-Ta(1)-O(1) C(1)-O(1)-Ta(1) C(7)-O(3)-Ta(1)

144.6(2) 92.4(2) 91.4(2) 123.0(4) 123.7(4)

Ta(1)-O(2) Ta(1)-O(4) Ta(1)-O(1) Ta(1)-O(6) Ta(1)-N(1)

2.027(5) 2.056(5) 1.897(5) 2.170(5) 2.170(5)

Bond Angles O(2)-Ta(1)-O(4) O(1)-Ta(1)-O(2) O(1)-Ta(1)-O(4) C(1)-O(2)-Ta(1) C(7)-O(4)-Ta(1)

144.6(2) 94.5(2) 89.6(2) 123.2(4) 123.3(4)

aromatic protons appear as multiplet signals at 8.33 and 8.43 ppm. The molecular structure of 3 has also been established by an X-ray diffraction study. The structure is closely related to that of complex 1 3 C7H8 (Figure 3, Table 2). In addition, the O(5)-Ta(1) bond length (1.905(5) A˚) is within the range expected for alkoxide derivatives of tantalum, while the O(11)-Ta(1) bond distance (2.163(4) A˚) is intermediate between the tantalum-triflate bond length in complex 1 3 C7H8 and that found in compound 2. Complex 2 reacts with water in a 1:1 molar ratio, in toluene, at room temperature, to yield a pale yellow solid that was identified as a mixture of the corresponding hydroxide derivative 4 along with small amounts of the starting material and an unidentified complex (ca. 5% and 7%, respectively, according to the integral of the Cp* group in the 1H NMR). Although so far it has not been possible to obtain 4 as an analytically pure compound, it has been characterized by spectroscopic techniques as well as by X-ray diffraction methods.

5508


Conde et al. Scheme 5

Scheme 6

Figure 4. (a) ORTEP drawing of [TaCp*(OH)(OTf)((OOC)2py-κ3-O,N,O)] (4). (b) Hydrogen bonding in compound 4. Scheme 4

Its IR spectrum shows a characteristic band for OH group at νh 3390 cm-1. The NMR data indicate also that both carboxylate groups in the pincer ligand are in the same chemical environment. For example, in the 13C NMR spectrum the signal corresponding to the carbon atoms of the equivalent carboxylate moieties appears at 166.6 ppm. Additionally, in the 1H NMR spectrum it was not possible to find the resonance corresponding to the OH unit. As was mentioned above, the molecular structure of 4 was established by means of the appropriate X-ray diffraction study (Figure 4a, Table 2). The geometry around the tantalum atoms is, as in complexes 2 and 3, pseudo-octahedral. The hydroxide group and the triflate ligand are in the equatorial plane, trans to each other. The intramolecular Ta-N, Ta-C, and Ta-O bond lengths are very similar to those found in complex 3. In the crystal two molecules are packed together via hydrogen bonds involving the coordinated hydroxide group and the carboxylate moieties (Figure 4b).

In order to study the reactivity of the Ta-Me bond with unsaturated organic molecules, we have carried out the reaction of of tBu-NtC with complex 1. As it is well known, the insertion of unsaturated reagents into M-C bonds of transition metal complexes is, due to its implications in homogeneous catalysis and in organic synthesis, one of the most important chemical reactions in organometallic chemistry. Among such reactions, the insertion of isocyanides has been the subject of extensive studies.14 The insertion of t Bu-NtC into the tantalum-carbon bond of complex 1 leads to the synthesis of the iminoacyl derivative 5 as the only product. It was isolated, after the appropriate workup, in 77% yield as a white crystalline solid. This compound is soluble in dichloromethane and THF and insoluble in toluene and pentane (Scheme 5) and was characterized by the most commonly used spectroscopic techniques. The 1H NMR spectrum in CD2Cl2 shows three singlet signals at 1.61, 1.93, and 3.08 ppm assigned to the tBu group, the Cp* ligand, and the methyl moieties, respectively. In addition, the aromatic protons appear as multiplet signals at 8.28 and 8.39 ppm. The 13C NMR spectrum exhibits only one signal (at 167.9 ppm) for the carboxylate carbon atoms. This means that the molecule has a mirror plane containing the N and C atoms of the iminoacyl fragment and bisecting the carboxylate pincer group. Moreover, the spectrum shows a singlet at 233.2 ppm for the iminoacyl carbon atom, which is within the range expected for κ2-C,N iminoacyl ligands. In order to test the stability of complex 5, we have studied the reaction of compound 5 with water on an NMR experiment scale. In this way, the addition of water to a solution of 5 in CDCl3 in a ca. 1:8 molar ratio gives rise to a white precipitate that is water-soluble and has been characterized by the usual spectroscopic and analytical techniques and also by X-ray diffraction methods as the corresponding water adduct 5 3 H2O (Scheme 6). In contrast to the symmetric disposition of the pincer ligand observed for 5, the 13C NMR spectrum of complex 5 3 H2O in D2O shows that in solution the carboxylate groups are in a different chemical environment. The resonances for the carbon atoms of the nonequivalent carboxylate moieties appear at 170.1 and 171.3 ppm, respectively. Additionally, (14) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059.

Article


5509

Figure 5. (a) ORTEP drawing of [TaCp*{η2-C(Me)dNtBu}((OOC)2py-κ3-O,N,O)(H2O)](OTf) (5 3 H2O). (b) Hydrogen bonding supramolecular arrangement in 5 3 H2O.

the resonance for the iminoacyl carbon atom appears at 230.9 ppm. Its IR spectrum exhibits a broad band at νh 3155 cm-1 corresponding to the ν(OH) from the coordinated water molecule. Complex 5 3 H2O constitutes an unusual iminoacyl tantalum compound since it is soluble in water. This behavior as far as we are aware has no precedents in tantalum chemistry. Figure 5a represents an ORTEP diagram of the molecular structure of the water adduct 5 3 H2O, while selected bond distances and angles are listed in Table 3. The molecular structure of 5 3 H2O is best described as pentagonal bipyramid with the Cp* ligand and the water molecule placed in the apical positions. The iminoacyl group is κ2-C, N coordinated to the metal with the nitrogen and the carbon atoms placed in the equatorial plane. The dicarboxylate behaves as a κ3-O,N,O trihapto ligand with the oxygen atoms trans to each other and the nitrogen atom of the pyridinic ring in trans-position to the CdN bond of the iminoacyl moiety. The structural parameters of the tantalum-iminoacyl unit are similar to previously reported ones.15 The Ta(1)N(1), 2.209(5) A˚, is longer than that found in any of the (15) Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2008, 27, 6123.

Table 3. Selected Bond Lengths [A˚] and Bond Angles [deg] for 5 3 H2O Bond Distances Ta(1)-O(1) Ta(1)-O(3) Ta(1)-O(5) Ta(1)-N(1) Ta(1)-N(2) Ta(1)-C(8) N(2)-C(8)

2.095(5) 2.136(5) 2.227(5) 2.209(5) 2.132(5) 2.134(7) 1.26(1) Bond Angles

O(1)-Ta(1)-N(2) O(1)-Ta(1)-O(3) N(2)-Ta(1)-O(3) C(8)-Ta(1)-O(3) C(6)-N(1)-C(2) C(6)-N(1)-Ta(1)

84.5(2) 140.1(2) 118.0(2) 85.5(3) 121.7(6) 119.6(5)

complexes reported in this paper. The Ta-O(5), 2.227(5) A˚, is similar to those previously reported for other aqua-tantalum complexes.13b An interesting issue in the crystal structure of 5 3 H2O is the formation of intermolecular hydrogen bonds. There is much current interest in the formation of complex structures by self-assembly from simple molecular building blocks.

5510


Conde et al.

Scheme 7

However, supramolecular organometallic chemistry is less developed than in organic or coordination chemistry, in part because many metal-carbon bonds are incompatible with the functional groups such as OH or NH commonly used in self-assembly through hydrogen bonding.16 The water molecule hydrogen bonds intermolecularly to one carboxylate oxygen atom of a second molecule and to the triflate anion, yielding a supramolecular linear arrangement along the b axis in the unit cell (Figure 5b). The ability of 5 3 H2O to form hydrogen bonds could also be responsible for its solubility in water.13b Furthermore, a well-known procedure to synthesize olefin polymerization catalysts is the reaction of a metal complex with different Lewis acids to form zwitterionic or cationic metal complexes. In some instances, activation occurs by Lewis acid attachment at a site removed from the substrate trajectory. In this way, the partial positive charge at the metal center is less pronounced than in related cationic counterparts. Moreover, a variety of Lewis acids can be used to modulate the steric encumbrance around the active site and the electron density at the metal center.17 In this field, we have studied the reactivity of the alkyl-containing complex 1 toward the Lewis acid B(C6F5)3. Thus, complex 1 reacts with B(C6F5)3 in a 1:1 molar ratio, in toluene, to yield the corresponding carboxylate adduct 6 (Scheme 7). Complex 6 was isolated as a yellow solid, soluble in CH2Cl2, less soluble in toluene, and insoluble in pentane. In the process, the abstraction of the methyl unit by B(C6F5)3 does not take place, but alternatively the Lewis acid interacts with the oxygen atom of a carboxylic moiety, giving rise to the corresponding adduct 6. This complex has been characterized by the usual analytical and spectroscopic techniques as well as by an X-ray diffraction study. The 13C NMR spectrum displays two resonances at 167.4 and 169.8 ppm corresponding to the carbon atoms of two nonequivalent carboxylic moieties.

(16) (a) Comprehensive Supramolecular Chemistry, Vol. 6, Solid State Supramolecular Chemistry: Crystal Engineering; Atwood, J. L., Ed.; Pergamon: New York, 1996. (b) Haiduc, I.; Edelmann, F. T. Supramolecular Organometallic Chemistry; Wiley-VCH: Weinheim, 1999. (c) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853. (d) Braga, D.; Maini, L.; Polito, M.; Scaccianoce, L.; Cojazzi G.; Grepioni, F. Coord. Chem. Rev. 2001, 216-217, 225. (17) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (b) Komon, Z. J. A.; Bu, X.; Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 12379. (c) Komon, Z. A. J.; Bazan, G. C.; Fang, C.; Bu, X. Inorg. Chim. Acta 2003, 345, 95. (d) Wasilke, J.-C.; Komon, Z. J. A.; Bu, X.; Bazan, G. C. Organometallics 2004, 23, 4174.

Figure 6. ORTEP drawing of [TaCp*Me(OTf){(O(dO 3 B(C6F5)3C))(OOC)py-κ3-O,N,O}] (6 3 CH2Cl2).

Table 4. Selected Bond Lengths [A˚] and Bond Angles [deg] for 6 3 CH2Cl2 Bond Distances Ta(1)-O(1) Ta(1)-O(3) Ta(1)-O(5) Ta(1)-C(18) Ta(1)-N(1) N(1)-C(2) N(1)-C(6) O(1)-C(1) O(2)-C(1) O(3)-C(7) O(4)-C(7) O(4)-B(1)

1.997(5) 2.143(5) 2.124(5) 2.160(7) 2.164(6) 1.328(9) 1.341(9) 1.325(9) 1.206(9) 1.245(8) 1.253(9) 1.57(1) Bond Angles

O(1)-Ta(1)-O(5) O(1)-Ta(1)-O(3) O(5)-Ta(1)-O(3) O(1)-Ta(1)-C(18) O(5)-Ta(1)-C(18) O(3)-Ta(1)-C(18) O(1)-Ta(1)-O(5)

87.2(2) 144.8(2) 81.5(2) 93.0(3) 153.6(2) 83.2(2) 87.2(2)

An ORTEP diagram of the molecule is shown in Figure 6, and the most important bond distances and angles are summarized in Table 4. The general features of the molecular structure in terms of distances and angles between the metal and the pyridinedicarboxylate ligand, as well as the coordination parameters of the Cp* to the tantalum center, are similar to those of complex 1 3 C7H8, with the methyl and triflate groups in a trans-position to each other in a pseudo-octahedral geometry. However, it is interesting to analyze the structural changes in the molecule due to the coordination of the boron atom to the carboxylate group. In this way, the Ta(1)-O(1) bond distance (1.977(5) A˚) is significantly shorter than the Ta(1)-O(3) bond length (2.143(5) A˚). On the other hand, while O(3)-C(7) and O(4)-C(7) bond lengths are quite close [1.245(8) and 1.253(9) A˚], those around C(1) are rather asymmetric [O(1)-C(1), 1.325(9) A˚; O(2)-C(1), 1.206(9) A˚]. The structural parameters indicate an important contribution of the zwitterionic form B (see Scheme 7) to the bonding in 6. Therefore, it can be seen that the coordination of the B(C6F5)3 moiety to the carboxylate group in the pincer ligand creates a partial positive charge at the metal center. In addition, the O(4)-B(1) bond distance (1.57(1) A˚) is similar

Article

Organometallics, Vol. 28, No. 18, 2009 Scheme 8

to the values previously reported in other tantalum derivatives where a O-B interaction was present.13c,18 Complex 1 reacts with B(C6F5)3 in a 1:2 molar ratio, in C6D6, to yield the corresponding carboxylate adduct 7, for which we propose that two B(C6F5)3 units are interacting with both carboxylate moieties (Scheme 8). Complex 7 has been characterized by the usual spectroscopic techniques, but all attempts to crystallize it have been so far unsuccessful, rendering compound 6 as the only crystalline solid. Also, complex 7 reacts with 1 in C6D6 to yield 6. Its 13C NMR spectrum shows a resonance at 170.0 ppm corresponding to the carbon atoms of both equivalent carboxylates. In summary, in this paper we report the synthesis of several monocyclopentadienyl complexes of tantalum with a dicarboxylate ligand that is bonded to the metal center in a “pincer” fashion. We have synthesized the methyl-triflate complex [TaCp*Me(OTf)((OOC)2py-κ3-O,N,O)] (1), and we have studied the reactivity of the Me-Ta bond with acidic reagents such as triflic acid and MeOH to give complexes 2 and 3, respectively, resulting in a methane elimination, as well as with tBuCN to give the iminoacyl-derivative 5 and the unprecedented water-soluble adduct 5 3 H2O. Finally, we have also explored the reactivity of complex 1 with B(C6F5)3, giving rise to two adducts, 6 and 7, in which the Lewis acid is interacting with the carboxylate units of the pincer ligand.

Experimental Section General Procedures. The preparation and handling of described compounds were performed under a nitrogen atmosphere using standard vacuum line and Schlenk techniques. All solvents were dried and distilled under a nitrogen atmosphere. The compound [TaCp*Me3(OTf)] was prepared by literature procedures.19 The commercially available compounds (HOOC)2py, tBuCN, B(C6F5)3, and HOTf were used as received from Aldrich. MeOH was dried and distilled under a nitrogen atmosphere, and water was distilled and deoxygenated. 1 H and 13C NMR spectra were recorded on a 400 MHz Avance Bruker Fourier transform spectrometer. Trace amounts of protonated solvents were used as references, and chemical shifts are reported in units of parts per million relative to SiMe4. IR spectra were recorded in the region 4000-400 cm-1 with a Nicolet Magna-IR 550 spectrophotometer. Elemental microanalyses were carried out in the SIDI (Universidad Aut onoma de Madrid). Synthesis of [TaCp*Me(OTf)((OOC)2py-K3-O,N,O)] (1). To a solution of [TaCp*Me3(OTf)] (0.734 g, 1.44 mmol) in 8 mL of ether was added (HOOC)2py (0.240 g, 1.44 mmol) at room temperature. The mixture was stirred for 1 h, and then the solvent was removed under vacuum. The residue was extracted with toluene at 90 °C to give, after cooling the solution at -20 °C for 20 h, a yellow crystalline solid, which was identified as complex 1. Yield: 0.794 g, 85%. IR (KBr, νh cm-1): 1743 (vs), 1724 (vs), 1473 (w), 1431 (w), 1382 (w), 1326 (m), 1300 (s), 1231 (18) S anchez-Nieves, J.; Royo, P.; Mosquera, M. E. G. Eur. J. Inorg. Chem. 2006, 1, 127. (19) Glassman, T. E.; Liu, A. H.; Schrock, R. R. Inorg. Chem. 1991, 30, 4723.

5511

(m), 1198 (s), 1126 (m), 1081 (m), 999 (s), 916 (m), 786 (w), 757 (m), 681 (w), 635 (m), 596 (w), 510 (w), 459 (w). 1H NMR (CDCl3, rt, 400 MHz): δ 0.17 (s, 3 H, Me), 2.39 (s, 15 H, Cp*), 8.39 (m, 2 H, Ar), 8.49 (m, 1 H, Ar). 13C{1H} NMR (CDCl3): δ 11.3 (s, Cp*), 53.7 (s, Me), 129.1 (s, Cp*), 129.6 (s, Ar), 144.5 (s, Ar), 146.2 (s, Aripso), 166.4 (s, OOC). Anal. Calcd for C19H21F3NO7STa 3 1/2 C7H8: C, 39.08 ; H, 3.64; N, 2.02; S, 4.63. Found: C, 38.91; H, 3.65; N, 2.10; S, 4.51. Synthesis of [TaCp*(OTf)2((OOC)2py-K3-O,N,O)] (2). To a solution of [TaCp*Me(OTf)((OOC)2py-κ3-O,N,O)] (1) (0.395 g, 0.612 mmol) in 5 mL of toluene was added triflic acid (0.054 mL). The mixture was stirred at 60 °C for 15 h, and after, the resulting yellow solution was filtered and cooled to -20 °C to give 2 as a yellow crystalline compound. Yield: (0.300 g, 63%). IR (KBr, νh cm-1): 1763 (w), 1741 (m), 1480 (w), 1433 (w), 1356 (s), 1295 (m), 1236 (w), 1183 (s), 1162 (m), 1146 (m), 1109 (m), 1080 (s), 1022 (w), 991 (m), 950 (s), 912 (s), 848 (w), 767 (w), 755 (m), 679 (w), 626 (s), 592 (m), 540 (w), 527 (w). 1H NMR (C6D6, rt, 400 MHz): δ 2.07 (s, 15 H, Cp*), 6.66 (m, 1 H, Ar), 7.40 (m, 2 H, Ar). 13C{1H} NMR (C6D6): δ 10.6 (s, Cp*), 128.1 (s, Cp*), 134.1 (s, Ar), 146.1 (s, Ar), 146.3 (s, Aripso), 164.9 (s, OOC). Anal. Calcd for C19H18NF6O10S2Ta: C, 29.28; H, 2.32; N, 1.79; S, 8.22. Found: C, 28.83; H, 2.58; N, 1.84; S, 8.08. Synthesis of [TaCp*(OMe)(OTf)((OOC)2py-K3-O,N,O)] (3). To a solution of [TaCp*Me(OTf)((OOC)2py-κ3-O,N,O)] (1) (0.050 g, 0.076 mmol) in toluene was added an excess of MeOH (50 μL). The mixture was stirred at 50 °C for 3 days. After that the solution was left at room temperature for 5 h to yield pale yellow crystals of 3 (0.035 g, 68%). IR (KBr, νh cm-1): 1718 (s), 1302 (s), 1236 (w), 1193 (s), 1141 (m), 1093 (m), 1078 (m), 1025 (w), 997 (s), 919 (m), 847 (w), 774 (w), 751 (m), 681 (w), 630 (s), 596 (w). 1H NMR (CDCl3, rt, 400 MHz): δ 2.40 (s, 15 H, Cp*), 3.55 (s, 3 H, OMe), 8.33 (m, 2 H, Ar), 8.43 (m, 1 H, Ar). 13C{1H} NMR (CDCl3): δ 11.0 (s, Cp*), 62.4 (s, OMe), 128.0 (s, Cp*), 129.6 (s, Ar), 145.0 (s, Ar), 146.7 (s, Aripso), 166.2 (s, OOC). Anal. Calcd for C19H21NF3O8STa: C, 34.50; H, 3.20; N, 2.11; S, 4.84. Found: C, 34.41; H, 3.14; N, 2.22; S, 4.97. Synthesis of [TaCp*(OH)(OTf)((OOC)2py-K3-O,N,O)] (4). To a solution of [TaCp*(OTf)2((OOC)2py-κ3-O,N,O)] (2) (0.048 g, 0.061 mmol) in toluene was added water (1.11 μL, 0.061 mmol). The solution was left at room temperature for 3 days, yielding pale yellow crystals of 4 (0.015 g). IR (KBr, νh cm-1): 3390 (br), 1732 (m), 1692 (s), 1475 (w), 1379 (w), 1312 (m), 1232 (m), 1205 (s), 1187 (s), 1082 (m), 1022 (s), 916 (m), 837 (w), 770 (m), 752 (m), 734 (m), 656 (m), 631 (vs), 589 (m). 1H NMR (CDCl3, rt, 400 MHz): δ 2.44 (s, 15 H, Cp*), 8.30 (m, 2 H, Ar), 8.42 (m, 1 H, Ar). 13C{1H} NMR (CDCl3): δ 11.0 (s, Cp*), 127.7 (s, Cp*), 130.0 (s, Ar), 145.1 (s, Ar), 146.7 (s, Aripso), 166.7 (s, OOC). Synthesis of [TaCp*{C(Me)dNtBu-K2-C,N}((OOC)2py-K3O,N,O)](OTf) (5). To a solution of [TaCp*Me(OTf)((OOC)2py-κ3-O,N,O)] (1) (0.294 g, 0.455 mmol) in dichloromethane was added tBuNC (0.051 mL), and the mixture was stirred overnight. After this time, the solvent was evaporated to dryness and washed with ether to give 5 as a white solid. Yield: (0.254 g, 77%). 1H NMR (CD2Cl2, rt, 400 MHz): δ 1.61 (s, 9 H, tBu), 1.93 (s, 15 H, Cp*), 3.08 (s, 3 H, C-Me), 8.28 (m, 2 H, Ar), 8.39 (m, 1 H, Ar). 13C{1H} NMR (CD2Cl2): δ 11.2 (s, Cp*), 19.0 (s, CMe), 29.8 (s, tBu), 66.5 (s, tBu), 122.1 (s, Cp*), 127.0 (s, Ar), 143.9 (s, Ar), 148.5 (s, Aripso), 167.9 (s, OOC), 233.2 (s, CdN). To a solution of 5 (0.051 g, 0.069 mmol) in CDCl3 was added water (10 μL), and the tube was exposed to ultrasound, yielding a white solid that after filtration was characterized as 5 3 H2O (yield: 0.039 g, 73%). Crystals of complex 5 3 H2O suitable for X-ray diffraction were obtained by crystallization of 5 from CH2Cl2/H2O/Et2O. IR (KBr, νh cm-1): 3155 (br), 1705 (vs), 1675 (vs), 1437 (w), 1355 (m), 1334 (m), 1284 (s), 1243 (s), 1223 (s), 1190 (m), 1165 (s), 1077 (w), 1029 (m), 926 (w), 767 (w), 746 (w), 686 (w), 638 (m), 518 (w). 1H NMR (D2O, rt, 400 MHz): δ 1.47 (s, 9 H, tBu), 1.82 (s, 15 H, Cp*), 2.98 (s, 3 H, C-Me), 8.35

5512


Conde et al.

Table 5. Crystal Data and Structure Refinement for 1 3 C7H8, 2, 3, 4, 5 3 H2O, and 6 3 CH2Cl2 1 3 C7H8 empirical formula fw temperature (K) wavelength (A˚) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) volume (A˚3) Z density(calc) (g/cm3) absorp coeff (mm-1) F(000) cryst size (mm3) index ranges reflns collected indep reflns data/restr/params goodness-of-fit on F2 final R indices [I > 2σ(I)]

C23H26F3NO7STa 698.44 180(2) 0.71073 triclinic P1 8.416(1) 9.859(2) 16.002(3) 72.875(2) 81.617(3) 85.181(2) 1254.2(4) 2 1.812 4.529 682 0.36 0.17 0.17 -9 e h e 10 -11 e k e 11 -18 e l e 17 8340 4334 [R(int) = 0.0286] 4334/53/306 0.989 R1 = 0.0374 wR2 = 0.0919

2 C19H18F6NO10S2Ta 779.41 290(2) 0.71073 monoclinic P21/n 12.623(6) 15.732(8) 12.847(6) 96.797(7) 2533(2) 4 2.043 4.602 1512 0.26 0.20 0.19 -14 e h e 14 -15 e k e 18 -14 e l e 15 12 634 4393 [R(int) = 0.0730] 4393/0/299 1.005 R1 = 0.0521 wR2 = 0.1232

3

4



(br, 2 H, Ar), 8.56 (m, 1 H, Ar). 13C{1H} NMR (D2O): δ 10.5 (s, Cp*), 18.1 (s, C-Me), 29.2 (s, tBu), 66.0 (s, tBu), 122.9 (s, Cp*), 128.8 (s, Ar), 145.7 (s, Ar), 145.7 (s, Aripso), 147.1 (s, Aripso), 170.1 (s, OOC), 171.3 (s, OOC), 230.9 (s, CdN). Anal. Calcd for C24H30F3N2O7STa 3 2H2O: C, 37.70; H, 4.47; N, 3.66 ; S, 4.19. Found: C, 37.28 ; H, 4.21 ; N, 3.48 ; S, 4.04. Synthesis of [TaCp*Me(OTf){(O(dO 3 B(C6F5)3C))(OOC)py-K3-O,N,O}] (6). To a solution of [TaCp*Me(OTf)((OOC)2py-κ3-O,N,O)] (1) (0.214 g, 0.331 mmol) in toluene (8 mL) was added at room temperature a solution of B(C6F5)3 (0.169 g, 0.331 mmol) in toluene (4 mL), and the mixture was stirred for 3 h. The resulting suspension was filtered, and the residue was extracted with dichloromethane, which was partially evaporated under vacuum. The resulting solution was cooled to -30 °C to give 6 as yellow crystals. Yield: (0.095 g, 25%). IR (KBr, νh cm-1): 1683 (m), 1516 (m), 1462 (s), 1335 (m), 1283 (w), 1239 (w), 1203 (w), 1175 (m), 1084 (m), 971 (s), 876 (w), 808 (w), 750 (m), 679 (m), 629 (s), 604 (m). 1H NMR (CD2Cl2, rt, 400 MHz): δ 0.32 (s, 3 H, Me), 2.36 (s, 15 H, Cp*), 8.43 (m, 2 H, Ar), 8.58 (m, 1 H, Ar). 13C{1H} NMR (CD2Cl2): δ 10.9 (s, Cp*), 128.1 (s, Cp*), 128.9 (s, Ar), 129.9 (s, Ar), 130.5 (s, Ar), 145.4 (s, Aripso), 145.8 (s, Aripso), 167.4 (s, OOC), 169.8 (s, OOC). Anal. Calcd for C36H21BF19NO7STa 3 CH2Cl2: C, 35.57; H, 1.85; N, 1.12 ; S, 2.53. Found: C, 35.50; H, 2.01; N, 1.19; S, 2.53. Characterization of [TaCp*Me(OTf){(O(dO 3 B(C6F5)3C))2py-K3-N,O,O}] (7). To a solution of [TaCp*Me(OTf)((OOC)2pyκ3-O,N,O)] (1) (0.040 g, 0.062 mmol) in C6D6 was added a solution of B(C6F5)3 (0.063 g, 0.123 mmol) in C6D6. The resulting compound 7 was characterized by 1H and 13C NMR. 1 H NMR (C6D6, rt, 400 MHz): δ 0.32 (s, 3 H, Me), 1.63 (s, 15 H, Cp*), 6.75 (m, 1 H, Ar), 7.49 (m, 2 H, Ar). 13C{1H} NMR (C6D6): δ 9.36 (s, Cp*), 64.04 (s, Me), 130.65 (s, Ar), 132.00 (s, Cp*), 144.83 (s, Ar), 146.78 (s, Aripso), 170.63 (s, OOC). X-ray Crystallography. A summary of crystal data collection and refinement parameters for all compounds is given in Table 5. Single crystals of 1 3 C7H8, 3, 4, 5 3 H2O, and 6 3 CH2Cl2 were mounted on a glass fiber and transferred to a Bruker X8 APEX II CCD-based diffractometer equipped with a graphite-monochromated Mo KR radiation source (λ=0.71073 A˚). The highly

5 3 H2O

6 3 CH2Cl2

C23H34N2O6TaCF3SO3 764.54 180(2) 0.71073 monoclinic P21/n 12.986(1) 8.9104(8) 24.942(2)

C38H23BCl2F18NO7STa 1242.29 180(2) 0.71073 monoclinic P21/n 11.247(1) 11.809(3) 32.316(2)

90.049(1)

97.912(2)

2886.0(4) 4 1.760 3.952 1520 0.43 0.40 0.36 -17 e h e 16 -11 e k e 10 -32 e l e 32 21 585 7027 [R(int) = 0.0820] 7027/2/373 1.095 R1 = 0.0503 wR2 = 0.1333

4251(1) 4 1.941 2.890 2416 0.33 0.21 0.19 -13 e h e 13 -14 e k e 11 -38 e l e 38 26 902 7479 [R(int) = 0.0909] 7479/4/657 0.981 R1 = 0.0438 wR2 = 0.0667

redundant data sets were integrated using SAINT20 and corrected for Lorentz and polarization effects. The absorption correction was based on fitting a function to the empirical transmission surface as sampled by multiple equivalent measurements with the program SADABS.21 The software package SHELXTL version 6.1022 was used for space group determination, structure solution, and refinement by full-matrix least-squares methods based on F2. A successful solution by the direct methods provided most non-hydrogen atoms from the E-map. The remaining non-hydrogen atoms were located in an alternating series of least-squares cycles and difference Fourier maps. All non-hydrogen atoms were refined with anisotropic displacement coefficients unless specified otherwise. Hydrogen atoms were placed using a “riding model” and included in the refinement at calculated positions. In the case of 1 3 C7H8, the molecule and solvent is highly disordered and was refined with soft restraints and constraints, and several atoms were refined with isotropic displacement parameters. The toluene solvent has been refined as a benzene molecule. For 2 and 4 the Cp* ligands are disordered and were modeled over two static sites, and the occupancies of the disorder components were refined initially and then fixed to a value of 50:50 and refined with isotropic displacement parameters. In addition, for 4 there is the C6D6 solvent in the asymmetric unit. After elaborate attempts to include discrete solvent molecular entities in the refinement, it was found advantageous to squeeze the solvents with the squeeze option in the Platon program (van der Sluis and Spek, 1990; Spek, 2001). Platon calculated the upper limit of volume that can be occupied by the solvent to be 174 A˚3, or 14% of the unit cell volume. The program calculated 48 electrons in the unit cell for the diffuse species. This approximately corresponds to one molecule of C6D6 per Ta complex in the asymmetric unit. The (20) SAINTþ v7.12a, Area-Detector Integration Program; Bruker-Nonius AXS: Madison, WI, 2004. (21) Sheldrick, G. M. SADABS version 2004/1, Program for Empirical Absorption Correction; University of G€ottingen: G€ottingen, Germany, 2004. (22) SHELXTL-NT version 6.12, Structure Determination Package; Bruker-Nonius AXS: Madison, WI, 2001.

Article


5513

derived quantities (Mr, F(000), μ, and Dx) in the crystal data do not contain the contribution from this disordered solvent. Compound 5 crystallizes with one molecule of H2O. Compound 6 crystallizes with one molecule of CH2Cl2 disordered over two positions.

Consolider-Ingenio 2010 ORFEO CSD2007-00006), and the Junta de Comunidades de Castilla-La Mancha (Grant PCI08-0010)

Acknowledgment. We gratefully acknowledge financial support from the Ministerio de Ciencia e Innovaci on (MICINN), Spain (Grants CTQ2008-00318/BQU and

Supporting Information Available: X-ray diffraction data (in CIF format) for compounds 1 3 C7H8, 2, 3, 4, 5 3 H2O, and 6 3 CH2Cl2. This material is available free of charge via the Internet at http://pubs.acs.org.

Synthesis and Reactivity of Tantalum Complexes Supported by the

Recommend Documents