Influence of Hydrogen Bonds on the Stability of Some Cationic

Oct 11, 2010 - Mancha, Avenida Carlos III, s/n 45071 Toledo, Spain, ‡Departamento de Quımica Inorg´anica, Org´anica y. Bioquımica, Universidad d...
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Organometallics 2010, 29, 5834–5840 DOI: 10.1021/om100541g

Influence of Hydrogen Bonds on the Stability of Some Cationic Monoaminocarbene Tantalum Complexes Containing Tridentate Bis(phenolato) [OSO]-Type Ligands Rosa Fandos,† Jacob Fern andez-Gallardo,† Antonio Otero,*,‡ Ana Rodrı´ guez,§ and Marı´ a Jose Ruiz*,† †

Departamento de Quı´mica Inorg anica, Org anica y Bioquı´mica, Instituto de Nanociencia, Nanotecnologı´a y Materiales Moleculares (INAMOL), Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, Avenida Carlos III, s/n 45071 Toledo, Spain, ‡Departamento de Quı´mica Inorg anica, Org anica y Bioquı´mica, Universidad de Castilla-La Mancha, Facultad de Quı´micas, Campus de Ciudad Real, 13071 Ciudad Real, Spain, and §Departamento de Quı´mica Inorg anica, Org anica y Bioquı´mica, Universidad de Castilla-La Mancha ETS Ingenieros Industriales, Avenida Camilo Jos e Cela, 3, 13071 Ciudad Real, Spain Received June 1, 2010

A series of new cationic aminocarbene tantalum complexes have been synthesized and characterized. A significant contribution of the anion-cation interaction to the stability of the cationic moiety has been revealed. The complexes with a triflate or chloride counterion are very stable, even in air, while the corresponding tetraphenylborate derivatives are very unstable species. This stabilizing behavior can be explained by the establishment of hydrogen bonds between the electronegative atoms of the anion and the amine and hydroxyl protons of the cation. The presence of the hydrogen bonds has been detected by spectroscopic and structural techniques.

Introduction Carbenes are active species that have proven to be powerful tools in organic, organometallic, and material chemistry, and in this respect, the way that they interact with metal centers has grown to become a topic of intense investigation.1 In particular, N-heterocyclic carbenes (NHCs) have received a great deal of attention due to their strong σ-donor character, which means that they form remarkably stable metalcarbene bonds2 and offer the possibility of tuning the stereoelectronic properties by modification of the ring framework, thus making them versatile ligands for transition metal catalysts.3 In diaminocarbenes (DACs), including NHCs, the metalcarbon bond is seen as a simple bond because the metal π-back-donation is usually prevented by the more stabilizing π-donation from the amino groups.3 In addition, the σ-donor ability is increased in the acyclic diaminocarbenes, and recently the coordination behavior of some monoaminocarbenes has been studied, revealing a strong σ-donor/weak π-acceptor character and widening the possibilities of tuning by varying the nature of the second substituent.4 Regarding the latter, the *To whom correspondence should be addressed. E-mail: antonio.otero@ uclm.es. (1) Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009, 253, 862. (2) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913. (3) (a) Nolan, S. P., Ed. N-Heterocyclic Carbenes in Synthesis; WileyVCH: Weinheim, Germany, 2006. (b) Glorius, F., Ed. Topics in Organometallic Chemistry 21 (N-Heterocyclic Carbenes in Transition Metal Catalysis); Springer: Berlin, Germany, 2007. (4) (a) Sole, S.; Gornitzka, H.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. Science 2001, 292, 1901. (b) Catto€en, X.; Gornitzka, H.; Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 1342. pubs.acs.org/Organometallics

Published on Web 10/11/2010

fact that only one electron-active substituent is needed will enable numerous stable carbenes to become accessible, which in turn will open the way for new synthetic developments and applications in various fields. Moreover, aminocarbenes bind as two-electron donors to almost all metals in the periodic table,5 while, to date, the aminocarbene complexes of the most electropositive metals, including those of groups 4 and 5 and the s- and f-blocks, have been reported only as “curiosities”.6-8 Herein we report for the first time a systematic method for the synthesis of cationic monoaminocarbenes of tantalum with low levels of steric hindrance. Our complexes also contain tridentate [OSO]-type ligands9and different counterions (Scheme 1). The most important feature of this class of carbene complexes is that the stability of the carbene-metal bonds is very dependent on the ability of the anions to form H-bonds with the ligands.

Results and Discussion The monoaminocarbene triflate complexes [TaCp*{C(Me)N(H)Ar-κ1C}(OH)(κ3-tbop)][TfO]10 (3) and [TaCp*{C(Me)N(H)Ar-κ1C}(OH)(κ3-tbcp)][TfO] (4) were synthesized by (5) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (6) Arnold, P. L.; Mungur, S. A.; Blake, A. J.; Wilson, C. Angew. Chem., Int. Ed. 2003, 42, 5981, and references therein. (7) Galakov, M.; G omez, M.; Jimenez, G.; Royo, P.; Pellinghelli, M. A.; Tiripicchio, A. Organometallics 1995, 14, 1901. (8) Bo, C.; Fandos, R.; Feliz, M.; Hernandez, C.; Otero, A.; Rodrı´ guez, A.; Ruiz, M. J.; Pastor, C. Organometallics 2006, 25, 3336. (9) tbop: 2,20 -thiobis(6-tert-octylphenolato); tbcp: 2,20 -thiobis(4,6dichlorophenolato). (10) Ar = 2,6-dimethylphenyl (xylyl). r 2010 American Chemical Society

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Scheme 1

Table 1. Selected 1H NMR Data 3

Cp* CH3-CN CH3 (xylyl) Ta-OH CdNH

4

5

6

7

8

C6D6

CDCl3

C6D6

CDCl3

2.00 (s) 2.16 (s) 1.65 (s) 1.93 (s) 13.42 (s) 14.67 (s)

2.46 (s) 2.00 (s) 2.09 (s) 2.42 (s) 12.89 (s) 13.34 (s)

C6D6

CDCl3

CDCl3

1.93 (s) 1.62 (s) 2.18 (s) 2.20 (s) 11.84 (s) 12.70 (s)

2.32 (s) 1.94 (s) 2.06 (s)

2.46(s) 2.01 (s) 2.03 (s) 2.39 (s) 11.54 (s) 12.21 (s)

10.06 (s) 12.23 (s)

insertion of 2,6-dimethylphenyl isocyanide (xylyl-NC) into the methyl groups of the related neutral triflate complexes [TaCp*Me(κ3-tbop)(OTf)] (1) and [TaCp*Me(κ3-tbcp)(OTf)] (2), which were previously synthesized by us.11 These reactions require heating for 48 h in toluene at 100 C in the presence of one molar equivalent of water, and they probably proceed by initial displacement of the triflate ligand by the isocyanide, insertion into the C-Ta bond, and subsequent protonation of the generated cationic iminoacyl to form the carbene8 (see Scheme 1). The synthesis of the tetraphenylborate complexes was carried out by anion metathesis. Thus, complexes [TaCp*{C(Me)N(H)Ar-κ1C}(OH)(κ3-tbop)][BPh4] (5) and [TaCp*{C(Me)N(H)Ar-κ1C}(OH)(κ3-tbcp)][BPh4] (6) were isolated by precipitation from a mixture of 3 or 4, respectively, and sodium tetraphenylborate. These reactions were carried out in dichloromethane, and the compounds were isolated by precipitation at -40 C (see Scheme 1). Finally, complexes [TaCp*{C(Me)N(H)Ar-κ1C}(OH)(κ3tbop)]Cl (7) and [TaCp*{C(Me)N(H)Ar-κ1C}(OH)(κ3-tbcp)]Cl (8) were prepared by reaction of 5 or 6, respectively, with one (11) Fandos, R.; Fernandez-Gallardo, J.; L opez Solera, M. I.; Otero, A.; Rodrı´ guez, A.; Ruiz, M. J.; Terreros, P. Organometallics 2008, 27, 4803.

1.84 (s) 1.90 (s) 1.82 (s)

2.37 (s) 2.46 (s) 2.04 (s)

11.43 (br)

not detected

Table 2. Selected 13C NMR Data

Cp* CH3-CN CH3 (xylyl) Cipso-S Cipso-O CdN

3

4

5

6

7

8

C6D6

CDCl3

C6D6

CDCl3

C6D6

CDCl3

10.98 124.41 27.13 18.88 19.09 117.71 118.82 164.51 164.73 263.92

12.16 121.53 27.84 19.23 19.37 124.35 125.42 162.24 162.60 264.54

11.06 123.22 27.25 18.29

11.75 119.88 26.35 19.34

118.70 118.80 164.51 165.55 254.69

123.49 124.49 163.78 164.58 269.07

11.54 123.21 27.36 18.10 18.48 118.13 118.85 164.44 165.16 261.96

12.06 121.75 27.73 19.23 19.34 123.77 123.92 161.79 162.82 262.34

equivalent of HCl in dichloromethane. During this reaction the tetraphenylborate anion decomposed to form BPh3 and benzene.12 The NMR data for all complexes (Tables 1 and 2) are in full agreement with the presence of a monoaminocarbene ligand, with the carbene signals observed between 254 and 269 ppm in the 13C NMR spectra and the amino NH signal at around 14 ppm for the chlorides and around 12 ppm for the triflates. These data are consistent with those reported for (12) Fandos, R.; Gallego, B.; Otero, A.; Rodrı´ guez, A.; Ruiz, M. J.; Terreros, P.; Pastor, C. Organometallics 2007, 26, 2896.

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Figure 1. Thermal ellipsoid diagram (30% probability) of 3 (H atoms and tert-octyl chains are omitted for clarity). Hydrogen bonds are represented with dotted lines.

Figure 2. Thermal ellipsoid diagram (30% probability) of 4. The H atoms are omitted for clarity. Hydrogen bonds are represented with dotted lines.

similar aminocarbene cationic complexes.7,8 For the tetraphenylborate derivatives these signals are very broad and difficult to detect. Crystals of compounds 3, 4 3 C7H8, and 8 3 1/2CH2Cl2 suitable for X-ray diffraction studies were obtained, and the molecular structures were determined (Figures 1, 2, and 3, Table 3). All of these crystals are formed by the cationic units and the corresponding anions. The cations all have a similar structure, with the pentamethylcyclopentadienyl ring bound to the Ta atom in an almost symmetric η5-fashion, with the distance between the metal and the centroid of the ring ranging between 2.141 and 2.154 A˚. The Ta atom is also bound to the [OSO] ligands in a mer-κ3 bent mode through the oxygen atoms, which are located in the equatorial plane and are cis to each other, with the central sulfur atom occupying the position trans to the Cp* group.13 The Ta-O lengths of the (13) de Castro, I.; Galakhov, M. V.; G omez, M.; G omez-Sal, P.; Martı´ n, A.; Royo, P. J. Organomet. Chem. 1996, 514, 51.

Fandos et al.

Figure 3. Thermal ellipsoid diagram (30% probability) of 8. The H atoms are omitted for clarity. Hydrogen bonds are represented with dotted lines.

alkoxide bonds [2.004(3) and 2.047(8) A˚] are within the normal range for tantalum alkoxide complexes,13 and the Ta-S bond lengths [2.681(3)-2.697(3) A˚] are also comparable to that found previously by our group in similar ligands [2.641(7) A˚].14 As a result of the cis coordination mode, the [OSO] ligand is bent and the O-Ta-O and C-S-C angles are around 87 and 101, respectively. The coordination around Ta is completed by a hydroxyl ligand and the carbenic atoms of the aminocarbene ligand. As a result, all of the complexes can be described as pseudooctahedral with the Ta atom displaced slightly from the equatorial plane containing the oxygen atoms and the carbenic carbon (“umbrella effect”).15 Finally, in these complexes the Ta-OH distances [1.870(6)1.909(3) A˚] are similar to those found in other tantalum complexes with this type of ligand,16-18 and the Ccarbene-Ta bond lengths [2.28(1)-2.305(9) A˚] are comparable with those previously found in similar complexes7,8 and those published by Fryzuk et al. for NHC complexes of tantalum.19 Nevertheless, as expected due to the presence of a single electrondonating amino substituent, the C-N bonds are significantly shorter [1.28(1)-1.307(5) A˚] than those observed in the aforementioned tantalum NHCs (∼1.35 A˚). These data demonstrate that, despite the presence of a single amino substituent, the monoamino-methyl-carbene behaves as a strong σ-donor/ weak π-acceptor ligand. It is noteworthy that all of these carbene complexes are insoluble in toluene and pentane but soluble in acetonitrile and THF, as one would expect given their cationic nature. Nevertheless, while the triflate (3, 4) and chloride (7, 8) complexes are air-stable as solids and in solution for several days;as were the previously published examples7,8;the tetraphenylborate derivatives (5, 6) are unstable in solution (14) Fandos, R.; Hernandez, C.; L opez-Solera, I.; Otero, A.; Rodrı´ guez, A. M.; Ruiz, M. J.; Terreros, P. Organometallics 2000, 19, 5318. (15) Nardelli, J. Appl. Crystallogr. 1995, 28, 659. (16) Conde, A.; Fandos, R.; Otero, A.; Rodrı´ guez, A. Organometallics 2007, 26, 1568. (17) Conde, A.; Fandos, R.; Otero, A.; Rodrı´ guez, A. Organometallics 2009, 28, 5505. (18) Goux, J.; Le Gendre, P.; Richard, P.; Moise, C. Organometallics 2005, 24, 4902. (19) Spencer, L. P.; Beddie, C.; Hall, M. B.; Fryzuk, M. D. J. Am. Chem. Soc. 2006, 128, 12532.

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Table 3. Selected Bond Distances (A˚) and Angles (deg) for Compounds 3, 4 3 C7H8, and 8 3 1/2CH2Cl2 Ta(1)-O(1) Ta(1)-O(2) Ta(1)-O(3) Ta(1)-S(1) Ta(1)-C(29) N(1)-C(29) C(29)-C(30) N(1)-C(31) Ta(1)-Ct(1) Ct(1)-Ta(1)-S(1)a O(1)-Ta(1)-O(2) Ta(1)-C(29)-N(1) Ta(1)-O(1)-C(1) Ta(1)-O(2)-C(15) C(2)-S(1)-C(16) C(31)-N(1)-C(29) a

8 3 1/2CH2Cl2

4 3 C7H8

3 2.015(2) 2.004(3) 1.909(3) 2.688(1) 2.301(4) 1.307(5) 1.507 (6) 1.476(5) 2.154 177.51 90.0(1) 119.5(3) 129.8(2) 130.7(3) 102.0(2) 124.5(4)

Ta(1)-O(1) Ta(1)-O(2) Ta(1)-O(3) Ta(1)-S(1) Ta(1)-C(2) N(1)-C(2) C(2)-C(1) N(1)-C(3) Ta(1)-Ct(1) Ct(1)-Ta(1)-S(1) O(1)-Ta(1)-O(2) Ta(1)-C(2)-N(1) Ta(1)-O(1)-C(11) Ta(1)-O(2)-C(17) C(18)-S(1)-C(12) C(3)-N(1)-C(2)

2.047(8) 2.030(7) 1.895(7) 2.681(3) 2.28(1) 1.30(1) 1.48(1) 1.46(1) 2.141 178.03 86.8(3) 119.5(8) 127.9(8) 129.6(7) 101.8(5) 129(1)

Ta(1)-O(1) Ta(1)-O(2) Ta(1)-O(3) Ta(1)-S(1) Ta(1)-C(13) N(1)-C(13) C(13)-C(14) N(1)-C(15) Ta(1)-Ct(1) Ct(1)-Ta(1)-S(1) O(1)-Ta(1)-O(2) Ta(1)-C(2)-N(1) Ta(1)-O(1)-C(1) Ta(1)-O(2)-C(12) C(6)-S(1)-C(7) C(13)-N(1)-C(15)

2.030(6) 2.022(6) 1.870(6) 2.697(3) 2.305(9) 1.29(1) 1.49(5) 1.47(1) 2.145 178.12 87.2(2) 120.0(7) 128.6(5) 130.5(6) 101.7(4) 129.2(8)

Ct(1): centroid of Cp* ring.

Table 4. Hydrogen Bonds for Compound 3, 4 3 C7H8, and 8 3 1/2CH2Cl2 d(D-H) d(D 3 3 3 A) d(H 3 3 3 A) — DHA N1-H1 3 3 3 O3 O3 -H3A 3 3 3 O5 O3-H3A 3 3 3 O6 N1-H1 3 3 3 O6 N1-H1 3 3 3 O3 4 3 C7H8 N1-H1 3 3 3 O12 O3-H3 3 3 3 O11 8 3 1/2CH2Cl2 N1-H1 3 3 3 Cl5 N1-H1 3 3 3 O3 O3-H3A 3 3 3 Cl5 3

0.879 0.824 0.824 0.879 0.880 0.880 0.840 0.880 0.880 0.840

2.859(5) 2.728(4) 3.193(5) 2.907(5) 2.90(1) 2.61(3) 2.76(4) 3.188(6) 2.934(3) 2.859(3)

2.321 1.907 2.718 2.177 2.299 1.950 2.131 2.367 2.338 2.641

119.5 174.2 118.3 140.1 125.6 130.5 131.7 155.4 125.1 96.4

and as solids and they decompose in a few days at low temperature. Even so, these complexes are sufficiently stable to be characterized by spectroscopic and analytical methods. This surprising behavior cannot be explained in terms of the different natures of the ancillary alkoxide ligands and seems to be related with the presence or absence of hydrogen bonds between the carbene ligand and the anions. In the solid state, the proposed hydrogen bonds were unambiguously detected by X-ray diffraction studies for 3, 4 3 C7H8, and 8 3 1/2 CH2Cl2 (Table 4) as well as for the previously reported aminocarbenes.7,8 Unfortunately, however, the tetraphenylborate complexes were too unstable to be crystallized. The aforementioned hydrogen-bonding interaction of complexes 3, 4, 7, and 8 seems to be sustained in solution (on the NMR time scale at room temperature),20 as evidenced by the welldefined amino NH signal (see below) in the 1H NMR spectra as well as the chemically inequivalent methyl groups of the xylyl moiety in the 13C NMR spectra, indicating that the phenyl ring cannot rotate freely. This is probably due to the presence of the anions located close to the xylyl ring as a consequence of the aforementioned hydrogen-bonding interactions. This steric hindrance can be observed in the corresponding van der Waals diagrams for these complexes (Figure 4). These diagrams were obtained from X-ray data for compounds 3 and 4 but calculated by DFT methods for compounds 7 and 8. Nevertheless, in the borate-containing complexes 5 and 6, the methyl groups of the xylyl moiety exhibit only one signal in the 13C NMR spectra, suggesting that this group can freely rotate due to the (20) Bellachioma, G.; Ciancaleoni, G.; Zuccaccia, C.; Zuccaccia, D; Macchioni, A. Coord. Chem. Rev. 2008, 252, 2224.

Figure 4. van der Waals diagrams of compounds 3, 4, 7, and 8.

absence of hydrogen-bonding interactions in these complexes. Additionally, we can observe that the carbene signal in the 13 C NMR spectra of 5 in C6D6 appears at 254 ppm, while in the spectrum recorded in CDCl3 for 6 this signal appears at 269 ppm. If we take into account that the other carbene complexes where the presence of a H-bonding is proposed give rise to a carbene signal at around 263 ppm in both deuterated benzene and chloroform, we can infer that in a nonpolar solvent such as C6D6 the carbene moiety in 5 does not establish any interaction through the NH group, and as a consequence, there is a stronger π-donation from the nitrogen to the carbenic carbon atom, which is more electron-rich. Nevertheless, when the tetraphenylborate complex 6 is dissolved in CDCl3, the NH could form hydrogen bonds with the chlorine atoms of the solvent and the carbenic carbon signal appears at lower field. Finally, the H-bonding interaction is also manifested in the solid and liquid infrared spectra of complexes 3, 4, 7, and 8, where relatively sharp peaks corresponding to the H-bonded N-H or O-H functionalities are apparent (see Experimental Section).

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The results described here demonstrate that the presence of hydrogen bonds between the proton of the aminocarbene ligand and the anion (or another ligand in the case of neutral species) provides the complexes with exceptional stability and offers the possibility of tuning the reactivity by changing the nature of the anion. We are also currently exploring how to modulate the reactivity of these carbenes to prevent decomposition.

Experimental Section General Remarks. All compounds were prepared and handled with rigorous exclusion of air and moisture under a nitrogen atmosphere by using standard vacuum line and Schlenk techniques. All solvents were dried and distilled under nitrogen. The triflate derivatives [TaCp*Me(κ3-tbop)(TfO)] (1) and [TaCp*Me(κ3-tbcp)(TfO)] (2) were prepared by literature procedures,11 whereas the commercially available compounds 2,6dimethylphenyl isocyanide (xylyl-NC), sodium tetraphenylborate, and 1.0 M HCl in THF were used as received from Aldrich. Elemental analyses were performed with a Perkin-Elmer 2400 microanalyzer. IR spectra were recorded in the region 4000-650 cm-1 with a JASCO FT/IR-4100 Fourier Transform infrared spectrophotometer. 1H, 19F, and 13C NMR spectra were obtained on a 200 MHz Mercury Varian Fourier Transform spectrometer and a 400 MHz Bruker Ultrashield spectrometer. Trace amounts of protonated solvents were used as references, and chemical shifts are reported in units of parts per million relative to SiMe4. Synthesis of [TaCp*{C(Me)N(H)Ar-K1C}(OH)(K3-tbop)][TfO] (3). A yellow solution of [TaCp*Me(κ3-tbop)(TfO)] (1) (576 mg, 0.62 mmol) and 2,6-dimethylphenyl isocyanide (82 mg, 0.62 mmol) in toluene (10 mL) was heated to 100 C. One equivalent (11.2 μL, 0.62 mmol) of water was added, and the reaction mixture was heated at 100 C for 24 h to give a yellow-orange solution. The mixture was cooled to room temperature and a white solid precipitated. The solid was isolated by filtration, washed with pentane (5  4 mL), and dried under vacuum to afford 0.365 g (55% yield) of 3 as a white solid. Slow diffusion of pentane into a saturated solution of 3 in toluene at -30 C gave colorless crystals suitable for X-ray diffraction. 1H NMR (C6D6, 293 K): δ 0.64 (s, 9H, 3CH3 t-octyl), 0.62 (s, 9H, 3CH3 t-octyl), 1.11 (s, 3H, CH3 t-octyl), 1.19 (s, 6H, 2CH3 t-octyl), 1.21 (s, 3H, CH3 t-octyl), 1.55 (m, 4H, 2CH2 t-octyl), 1.62 (s, 3H, CH3-CdN), 1.93 (s, 15H, Cp*), 2.18 (s, 3H, CH3-xylyl), 2.20 (s, 3H, CH3-xylyl), 6.39 (d, 3JH-H = 8.50 Hz, 1H, ar), 6.58 (m, 1H, ar), 6.67 (m, 1H, ar), 6.74 (m, 1H, ar), 6.81 (m, 1H, ar), 6.93 (m, 1H, ar), 7.04 (m, 1H, ar), 7.51 (s, 1H, ar), 7.74 (s, 1H, ar), 11.84 (s, br, 1H, Ta-OH), 12.70 (s, br, 1H, CdNH). 1 H NMR (CDCl3, 293 K): δ 0.68 (s, 9H, 3CH3 t-octyl), 0.70 (s, 9H, 3CH3 t-octyl), 1.24 (s, 3H, CH3 t-octyl), 1.30 (s, 9H, 3CH3 t-octyl), 1.68 (m, 4H, 2CH2 t-octyl), 1.94 (s, 3H, CH3-CdN), 2.06 (s, 6H, CH3-xylyl), 2.32 (s, 15H, Cp*), 6.62 (m, 1H, ar), 7.03 (m, 1H, ar), 7.16 (m, 4H, ar), 7.35 (s, 1H, ar), 7.51 (s, 1H, ar), 10.06 (s, 1H, Ta-OH), 12.23 (s, 1H, CdNH). 13C{1H} NMR (C6D6, 293 K): δ 10.98 (Cp*), 18.88 (CH3xylyl), 19.09 (CH3xylyl), 27.13 (CH3-CdN), 30.59 (CMe2 t-octyl), 31.57 (CMe2 t-octyl), 31.61 (CMe3 t-octyl), 31.72 (CMe3 t-octyl), 31.77 (CMe3 t-octyl), 31.99 (CMe3 t-octyl), 32.07 (CMe2 t-octyl), 32.13 (CMe2 t-octyl), 37.87 (CMe2 t-octyl), 37.95 (CMe2 t-octyl), 56.63 (CH2 t-octyl), 56.72 (CH2 t-octyl), 117.71 (CipsoS), 118.82 (CipsoS), 120.63 (ar), 122.99 (ar), 124.41 (Cp*), 125.31 (ar), 128.62 (ar), 128.94 (ar), 129.66 (ar), 129.86 (ar), 130.03 (ar), 130.61 (ar), 133.25 (ar), 134.44 (ar), 137.83 (ar), 143.35 (Cipso t-octyl), 143.49 (Cipso t-octyl), 164.51 (CipsoO), 164.73 (CipsoO), 263.92 (CH3-CdN). 19F NMR (C6D6, 293 K): δ -78.21 (TfO). IR ν (cm-1): 1475 (s), 3176 (m, br), 3394 (w, br). Anal. Calcd for C49H69F3NO6S2Ta (1070.15): C, 54.99; H, 6.50; N, 1.31. Found: C, 54.88; H, 6.69; N, 1.44. Synthesis of [TaCp*{C(Me)N(H)Ar-K1C}(OH)(K3-tbcp)][TfO] (4). To a solution of [TaCp*Me(κ3-tbcp)(TfO)] (2) (350 mg,

Fandos et al. 0.40 mmol) in toluene (10 mL) was added one equivalent of 2,6-dimethylphenyl isocyanide (53 mg, 0.40 mmol), and the pale yellow solution was stirred for 24 h at 100 C to give an orange solution. One equivalent of H2O (7.2 μL, 0.40 mmol) was added, and the mixture was stirred at 100 C for an additional 24 h, which led to the formation of a white precipitate. The mixture was allowed to cool to room temperature, and the precipitate was filtered off, washed with pentane (2  10 mL), and dried under vacuum to afford 0.320 g (81% yield) of 4 as a white, microcrystalline powder. Slow cooling to -30 C of a saturated solution of 4 in toluene gave rise to crystals suitable for X-ray diffraction. 1H NMR (CDCl3, 293 K): δ 2.01 (d, 4JH-H = 0.83 Hz, 3H, CH3-CdN), 2.03 (s, 3H, CH3-xylyl), 2.39 (s, 3H, CH30 -xylyl), 2.46 (s, 15H, Cp*), 7.14-7.42 (m, 7H), 11.54 (s, 1H, Ta-OH), 12.21 (s, 1H, CdNH). 13C{1H} NMR (CDCl3, 293 K): δ 12.16 (Cp*), 19.23 (CH3-xylyl), 19.37 (CH3-xylyl), 27.84 (CH3-CdN), 121.53 (Cp*), 137.63 (ar), 133.39 (ar), 133.20 (ar), 132.96 (ar), 132.20 (ar), 130.63 (ar), 129.99 (ar), 129.76 (ar), 129.69 (ar), 129.60 (ar), 129.50 (ar), 127.61 (ar), 125.49 (ar), 125.42 (ar), 124.93 (ar), 124.35 (ar), 123.76 (ar), 123.49 (ar), 264.54 (CH3-CdN). 19F NMR (CDCl3, 293 K): δ -78.62 (TfO). IR ν (cm-1): 1456 (s), 2924 (w) 3056 (m), 3176 (m). Anal. Calcd for C33H33Cl4F3NO6S2Ta 3 C7H8 (1075.64): C, 44.66; H, 3.84; N, 1.30. Found: C, 44.51; H, 3.69; N, 1.45. Toluene signals in 1H NMR spectrum: δ 2.36 (s, 3H, CH3) and 7.06-7.14 (m), and in 13 C NMR spectrum: δ 21.65, 125.79, 128.42, 129.23, 138.06 (ar). Synthesis of [TaCp*{C(Me)N(H)Ar-K1C}(OH)(K3-tbop)][BPh4] (5). A yellow-orange solution of complex 3 (439 mg, 0.41 mmol) and NaBPh4 (140 mg, 0.41 mmol) in dichloromethane (8 mL) was stirred for 12 h at room temperature. The yellow solution was cooled to -30 C for 24 h, and a white precipitate in a yellow solution was obtained. The solid was filtered off, and the filtrate was concentrated under vacuum and cooled at -30 C to afford 0.322 g (63% yield) of 5 as a yellow microcrystalline solid. 1H NMR (C6D6, 293 K): δ 0.64 (s, 9H, 3CH3 t-octyl), 0.67 (s, 9H, 3CH3 t-octyl), 1.13 (s, 3H, CH3 t-octyl), 1.18 (s, 3H, 2CH3 t-octyl), 1.22 (s, 6H, CH3 t-octyl), δ 1.53 (m, 4H, 2CH2 t-octyl), 1.82 (s, 6H, CH3-xylyl), 1.84 (s, 15H, Cp*), 1.90 (s, 3H, CH3-CdN), 6.35 (d, 3 JH-H = 8.56 Hz, 1H, ar), 6.35 (m, 1H, ar), 6.57 (m, 1H, ar), 6.62 (m, 1H, ar), 6.73 (m, 1H, ar), 6.84 (m, 2H, ar), 6.95 (m, 1H, ar), 7.05 (m, 5H, ar), 7.21 (m, 12H, ar), 7.40 (s, 1H, ar), 7.55 (m, 3H, ar), 11.43 (s, br, 2H, Ta-OH and CdNH). 13C{1H} NMR (C6D6, 293 K): δ 11.06 (Cp*), 18.29 (CH3xylyl), 27.25 (CH3-CdN), 30.66 (CMe2 t-octyl), 30.99 (CMe2 t-octyl), 31.58 (CMe3 t-octyl), 31.62 (CMe3 t-octyl), 31.67 (CMe3 t-octyl), 31.80 (CMe3 t-octyl), 32.07 (CMe2 t-octyl), 32.12 (CMe2 t-octyl), 37.70 (CMe2 t-octyl), 37.87 (CMe2 t-octyl), 56.64 (CH2 t-octyl), 56.74 (CH2 t-octyl), 118.70 (CipsoS), 118.80 (CipsoS), 119.14 (ar), 122.58 (ar), 123.22 (Cp*), 123.50 (ar), 124.47 (ar), 125.85 (ar), 127.06 (ar), 128.20 (ar), 128.25 (ar), 128.59 (ar), 128.94 (ar), 129.52 (ar), 129.73 (ar), 132.51 (ar), 133.04 (ar), 133.14 (ar), 134.86 (ar), 134.94 (ar), 137.09 (ar-BPh4), 137.12 (ar-BPh4), 137.16 (ar-BPh4), 137.19 (ar-BPh4), 137.21 (ar-BPh4), 137.26 (ar-BPh4), 137.31 (ar-BPh4), 137.38 (ar-BPh4), 141.50 (Cipso t-octyl), 142.42 (Cipso t-octyl), 146.39 (CipsoB), 150.03 (CipsoB), 155.19 (CipsoB), 158.22 (CipsoB), 164.51 (CipsoO), 165.55 (CipsoO), 254.69 (CH3-CdN). IR ν (cm-1): 1484 (s), 3054 (m, br), 3470 (w, br). Anal. Calcd for C72H89BNO3STa (1240.31): C, 69.72; H, 7.23; N, 1.13. Found: C, 69.03; H, 7.02; N, 1.15. Synthesis of [TaCp*{C(Me)N(H)Ar-K1C}(OH)(K3-tbcp)][BPh4] (6). A colorless solution of complex 4 (243 mg, 0.25 mmol) and NaBPh4 (84 mg, 0.25 mmol) in dichloromethane (8 mL) was stirred for 12 h at room temperature. The pale yellow solution was cooled to -30 C for 24 h to give a white precipitate in a yellow solution. The solid was filtered off, and the filtrate was concentrated in vacuo and cooled to -30 C to afford 0.184 g (64% yield) of complex 6 as a yellow microcrystalline solid. 1H NMR (CDCl3, 293 K): δ 2.04 (s, 6H, CH3-xylyl), 2.37 (s, 15H, Cp*), 2.46 (s, 3H, CH3-CdN), 7.09-7.25 (m, 11H, ar), 7.29-7.48 (m, 11H, ar), 7.60-7.64 (m, 5H, ar). 13C{1H} NMR (CDCl3, 293 K) δ 11.75

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Table 5. Crystal Data and Structure Refinement for 3, 4 3 C7H8, and 8 3 1/2CH2Cl2 molecular formula fw temperature (K) wavelength (A˚) cryst syst space group a (A˚) b (A˚) c (A˚) β (deg) volume (A˚3) Z density(calcd) (g/cm3) absorp coeff (mm-1) F(000) cryst size (mm3) index ranges reflns collected indep reflns data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] largest diff peak and hole (e A˚-3)

3

4 3 C7H8

8 3 1/2CH2Cl2

C49H69F3NO6S2Ta 1070.12 180(2) 0.71073 monoclinic P21/c 18.876(2) 12.662(1) 21.644(2) 108.001(2) 4919.8(8) 4 1.445 2.377 2200 0.28  0.25  0.21 -24 e h e 25 -14 e k e 16 -28 e l e 28 37 984 11 955 [R(int) = 0.0663] 11 955/0/585 1.021 R1 = 0.0438 wR2 = 0.0809 1.094 and -0.799

C40H41Cl4F3NO6STa 1075.61 180(2) 0.71073 orthorhombic Pbca 27.569(3) 18.834(2) 17.903(2)

C32.5H34Cl6NO3STa 912.32 180(2) 0.71073 monoclinic P21/c 14.107(3) 17.260(3) 15.701(3) 107.246(3) 3651(1) 4 1.660 3.540 1804 0.17  0.12  0.09 -18 e h e 18 -22 e k e 22 -16 e l e 20 24 828 8530 [R(int) = 0.1262] 8530/274/428 0.905 R1 = 0.0658 wR2 = 0.1329 2.041 and -1.513

(Cp*), 19.34 (CH3-xylyl), 25.65 (CH3-CdN), 119.88 (Cp*), 121.39 (ar), 121.67 (ar), 121.77 (ar), 122.49 (ar), 123.49 (ar), 124.49 (ar), 124.77 (ar), 125.31 (ar), 127.18 (ar), 128.24 (ar), 129.06 (ar), 129.20 (ar), 129.27 (ar), 129.32 (ar), 129.39 (ar), 129.48 (ar), 130.18 (ar), 131.16 (ar), 131.58 (ar), 132.02 (ar), 132.15 (ar), 133.00 (ar), 137.38 (ar), 137.90 (ar), 163.78 (ar), 164.58 (ar), 269.07 (CH3-CdN). IR ν (cm-1): 1453 (s), 2912 (m), 2996 (m), 3038 (m), 3061 (m), 3217 (w). Anal. Calcd for C56H53BCl4NO3STa (1153.66): C, 58.30; H, 4.63; N, 1.21. Found: C, 58.53; H, 4.62; N, 1.22. Synthesis of [TaCp*{C(Me)N(H)Ar-K1C}(OH)(K3-tbop)]Cl (7). To a stirred solution of 5 (0.114 g, 0.09 mmol) in dichloromethane (5 mL) was added 1.0 M HCl in THF (92 μL, 0.09 mmol). The yellow-orange solution was stirred for 12 h at room temperature to afford a yellow solution, which was concentrated to yield a yellow solid. The solid was filtered off, washed with pentane (2 mL), and dried under vacuum to afford 0.062 g (71% yield) of 7 as a yellow, microcrystalline solid. 1H NMR (C6D6, 293 K): δ 0.66 (s, 18H, 3CH3 t-octyl), 1.12 (s, 3H, CH3 t-octyl), 1.15 (s, 6H, 2CH3 t-octyl), 1.26 (s, 3H, CH3 t-octyl), 1.54 (m, 4H, 2CH2 t-octyl), 1.65 (s, 3H, CH3-xylyl), 1.93 (s, 3H, CH3-xylyl), 2.00 (s, 15H, Cp*), 2.16 (s, 3H, CH3-CdN), 6.42 (m, 1H, ar), 6.91-7.06 (m, 5H, ar), 7.30 (s, 1H, ar), 7.49 (s, 1H, ar), 13.42 (s, 1H, Ta-OH), 14.67 (s, 1H, CdNH). 13C{1H} NMR (C6D6, 293 K): δ 11.54 (Cp*), 18.10 (CH3xylyl), 18.48 (CH3xylyl), 27.36 (CH3-CdN), 30.64 (CMe2 t-octyl), 31.01 (CMe2 t-octyl), 31.38 (CMe3 t-octyl), 31.55 (CMe3 t-octyl), 31.71 (CMe3 t-octyl), 31.78 (CMe3 t-octyl), 31.84 (CMe2 t-octyl), 32.00 (CMe2 t-octyl), 37.34 (CMe2 t-octyl), 37.95 (CMe2 t-octyl), 56.60 (CH2 t-octyl), 56.75 (CH2 t-octyl), 118.13 (CipsoS), 118.85 (CipsoS), 119.58 (ar), 121.08 (ar), 123.21 (Cp*), 124.18 (ar), 125.32 (ar), 125.75 (ar), 125.81 (ar), 127.11 (ar), 128.18 (ar), 128.95 (ar), 128.94 (ar), 129.52 (ar), 129.73 (ar), 132.51 (ar), 133.04 (ar), 133.14 (ar), 134.86 (ar), 130.64 (ar), 134.82 (ar), 135.97 (ar), 142.73 (Cipso t-octyl), 143.33 (Cipso t-octyl), 164.44 (CipsoO), 165.16 (CipsoO), 261.96 (CH3-CdN). IR ν (cm-1): 1485 (s), 2954 (m, br), 3427 (w, br). Anal. Calcd for C48H69ClNO3STa (956.53): C, 60.27; H, 7.27; N, 1.46. Found: C, 60.41; H, 7.16; N, 1.60. Synthesis of [TaCp*{C(Me)N(H)Ar-K1C}(OH)(K3-tbcp)]Cl (8). To a solution of 6 (0.183 g, 0.24 mmol) in dichloromethane (8 mL) was added 1.0 M HCl in THF (247 μL, 0.24 mmol), and the bright yellow solution was stirred for 12 h at room tempera-

9296(2) 8 1.537 2.739 4288 0.23  0.13  0.13 -32 e h e 32 -22 e k e 22 -21 e l e 20 59 226 8184 [R(int) = 0.1211] 8184/287/537 0.972 R1 = 0.0742 wR2 = 0.1831 1.172 and -0.694

ture to afford a pale yellow solution. Slow diffusion of Et2O at -30 C over this solution afforded pale yellow crystals, which were isolated by filtration, washed with Et2O (2 mL), and dried under vacuum to afford 0.061 g (45% yield) of 8 as a pale yellow crystalline solid suitable for X-ray analysis. 1H NMR (CDCl3, 293 K): δ 2.00 (s, 3H, CH3-CdN), 2.09 (s, 3H, CH3-xylyl), 2.42 (s, 3H, CH3-xylyl), 2.46 (s, 15H, Cp*), 7.04 (m, 1H, ar), 7.13 (m, 1H, ar), 7.19 (m, 2H, ar), 7.32 (m, 2H, ar), 7.46 (m, 1H, ar), 12.89 (s, 1H, Ta-OH), 13.34 (s, 1H, CdNH). 13C{1H} NMR (CDCl3, 293 K): δ 12.06 (Cp*), 19.23 (CH3-xylyl), 19.34 (CH3-xylyl), 27.73 (CH3-CdN), 121.75 (Cp*), 123.77 (ar), 123.92 (ar), 124.64 (ar), 125.11 (ar), 126.91 (ar), 128.24 (ar), 129.40 (ar), 129.44 (ar), 129.81 (ar), 130.63 (ar), 131.96 (ar), 132.72 (ar), 132.91 (ar), 133.03 (ar), 133.69 (ar), 137.59 (ar), 161.79 (ar), 162.82 (ar), 262.34 (CH3-CdN). IR ν (cm-1): 1455 (s), 2908 (m), 2998 (m), 3039 (m), 3062 (m). Anal. Calcd for C32H33Cl5NO3STa 3 CH2Cl2 (954.82): C, 41.51; H, 3.69; N, 1.47. Found: C, 42.81; H, 3.81; N, 1.50. Dichloromethane signals in 1 H NMR spectrum: δ 2.36 (s, 2H, CH2) and in 13C NMR spectrum: δ 54.00. Crystal Structure Determination for 3, 4 3 C7H8, and 8 3 1/2CH2Cl2. Data were collected on a Bruker X8 APEX II CCD-based diffractometer equipped with a graphite-monochromated Mo KR radiation source (λ = 0.71073 A˚). The crystal data, data collection, structural solution, and refinement parameters are summarized in Table 5. Data were integrated using SAINT,21 and an absorption correction was performed with the program SADABS.22 The structure was solved by direct methods using the SHELXTL package23 and refined by full-matrix least-squares methods based on F2. For compound 4 3 C7H8, the asymmetric unit contains one molecule of toluene solvent disordered over two positions. The toluene solvent molecules were refined with a set of restraints to aid in modeling the disorder and finally fixed, and their occupancies were refined initially and then fixed to a value of 50:50. In addition, the triflate counterion was found to be disordered, (21) SAINTþ NT ver. 6.04, SAX Area-Detector Integration Program; Bruker AXS: Madison, WI, 1997-2001. (22) Sheldrick, G. M. SADABS version 2.03, a Program for Empirical Absorption Correction; Universit€at G€ottingen, 1997-2001. (23) SHELXTL version 6.10, Structure Determination Package; Bruker AXS: Madison, WI, 2000.

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and this was modeled over two positions and refined with many restrictions. For compound 8 3 1/2CH2Cl2, the asymmetric unit contains a half-molecule of CH2Cl2 solvent disordered and was modeled over two positions. For all compounds the nonhydrogen atoms were refined with anisotropic displacement parameters, with the exception of the atoms of disordered toluene solvent for 4 3 C7H8. Hydrogen atoms were placed using a “riding model” and included in the refinement at calculated positions, except for H1 and H3A in 3, which are found in the Fourier map. Computational Details. Geometry optimizations of compounds 7 and 8 were carried out by DFT calculations using the program package Gaussian 0324 and the B3LYP25 combination of functionals. The SDD pseudopotential was employed for (24) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Pittsburgh, PA, 2004. (25) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623.

Fandos et al. the tantalum center, and the standard 6-31G(d) basis set was used for the atoms directly bonded to tantalum, the CH3-Cd N(H)xylyl group, and the proton of the OH group. The standard 6-31G basis set was used for the other atoms.

Acknowledgment. We gratefully acknowledge the Ministerio de Ciencia e Innovaci on of Spain for financial support (Grant Nos. CTQ2008-00318/BQU, CTQ200611845/BQU, Consolider Ingenio 2010 ORFEO CSD2007-00006) and also for a fellowship (J.F.-G., Grant No. AP2005-4738) and the Junta de Comunidades de Castilla-La Mancha, Spain (Grant No. PCI08-0010). Supporting Information Available: CIF files giving details of data collection, refinement, atom coordinates, anisotropic displacement parameters, and bond lengths and angles for complexes 3, 4, and 8. This material is available free of charge via the Internet at http://pubs.acs.org.