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Organometallics 2001, 20, 2114-2117
Notes Increasing the Lipophilic Character of Tetraphenylborate Anions through Silyl Substituents Joep van den Broeke,† Martin Lutz,‡ Huub Kooijman,‡ Anthony L. Spek,‡,⊥ Berth-Jan Deelman,*,§ and Gerard van Koten† Department of Metal-Mediated Synthesis, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, Department of Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, and ATOFINA Vlissingen B.V., P.O. Box 70, 4380 AB Vlissingen, The Netherlands Received October 30, 2000 Summary: Trimethylsilyl- and dimethyl(1H,1H,2H,2Hperfluorooctyl)silyl substituents have been introduced in the para position of tetraphenylborate anions, resulting in enhanced solubility of their complexes n-Bu4N[B(C6H4SiMe2R-4)4] (R ) Me, CH2CH2C6F13) and [Rh(dppe)2][B(C6H4SiMe2R-4)4] (dppe ) 1,2-bis(diphenylphosphino)ethane, R ) Me, CH2CH2C6F13) in apolar organic solvents.
such as super critical carbon dioxide.5a,b Herein we report the preparation of large and weakly coordinating trimethylsilyl- and perfluoroalkylsilyl-substituted tetraphenylborate anions, which are more lipophilic than [BPh4]-, but in a chemical sense very similar to this anion.
Introduction
Starting from p-bromo(trimethylsilyl)benzene and p-bromo[dimethyl(1H,1H,2H,2H-perfluorooctyl)silyl]benzene, 1a and 1b were prepared. Ion exchange with n-Bu4NF and [Rh(dppe)2]Cl yielded the corresponding complexes 3 and 4 (Scheme 1). To assess the influence of the silyl substitution on the tetraphenylborate, single crystals of 2a and 4a were obtained. Their molecular structures in the solid state were determined by singlecrystal X-ray diffraction (Figures 1 and 2, respectively).6
Weakly coordinating anions are of interest for a range of applications that require generation and stabilization of highly reactive cationic species.1,2 A lot of research has been directed at reducing the nucleophilicity of such anions by increasing their steric requirements, as in [B(C6F4SiR3)4]- 3 and [MeB(2-C6F4C6F5)3]-,4 or by using polyhedral carboranes such as [CB11H12]-, [CB11H6Cl6]-, and [CB11HF11]-.1a,b However, relatively simple tetraphenylborates such as [B{3,5-C6H3(CF3)2}4]- (TFPB) and [B(C6F5)4]- remain the most widely used,1a because of their easy synthesis. Another interesting implication of the use of TFPB and [B(C6F4SiR3)4]- is their lipophilic behavior.3,5 Especially TFPB is known for increasing the solubility of transition metal complexes in apolar media
Results and Discussion
†
Debye Institute. Bijvoet Center. To whom correspondence concerning crystallographic data should be addressed. § ATOFINA, currently based at the Debye Institute. Fax: +31 302523615. E-mail:
[email protected]. (1) Recent reviews: (a) Lupinetti, A. J.; Strauss, S. H. Chemtracts 1998, 11, 565. (b) Reed, C. A. Acc. Chem. Res. 1998, 32, 1025. (c) Strauss, S. H. Chem. Rev. 1993, 93, 927. (2) Selected references: (a) Lancaster, S. J.; Walker, D. A.; ThorntonPett, M.; Bochmann, M. Chem. Commun. 1999, 1533. (b) Xie, Z.; Manning, J.; Reed, R. W.; Mathur, R.; Boyd, P. D. W.; Benesi, A.; Reed, C. A. J. Am. Chem. Soc. 1996, 118, 2922. (c) Giardello, M. A.; Eisen, M. S.; Stern, S. L.; Marks, T. J. J. Am. Chem. Soc. 1993, 115, 3326. (d) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920. (3) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 842. (4) Chen, Y.-X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 2582. (5) (a) Kainz, S.; Brinkmann, A.; Leitner, W.; Pfaltz, A. J. Am. Chem. Soc. 1999, 121, 6421. (b) Burk, M. J.; Feng, S.; Gross, M. F.; Tumas, W. J. Am. Chem. Soc. 1995, 117, 8277. (c) Iwamoto, H.; Sonoda, T.; Kobayashi, H. Tetrahedron Lett. 1983, 24, 4703. ‡
⊥
Figure 1. Molecular structure of 2a with ellipsoids at 30% probability level. H atoms were omitted. Selected bond lengths (Å) and angles (deg): B-C(1) ) 1.658(4), B-C(10) ) 1.643(5), B-C(19) ) 1.647(4), B-C(28) ) 1.655(4), NaO(1) ) 2.685(3), Na-O(2) ) 2.712(3), Na-O(3) ) 2.712(3), Na-O(4) ) 2.704(4), Na-O(5) ) 2.822(3), Na-O(6) ) 2.786(3), Na-O(7) )2.268(5), Na-O(8) ) 2.284(4), C(1)B-C(10) ) 106.6(3), C(1)-B-C(19) ) 111.9(2), C(1)-BC(28) ) 108.8(2), C(10)-B-(C19) ) 110.9(2), C(10)-BC(28) ) 111.1(2), C(19)-B-C(28) ) 107.6 (3).
10.1021/om0009177 CCC: $20.00 © 2001 American Chemical Society Publication on Web 04/19/2001
Notes
Organometallics, Vol. 20, No. 10, 2001 2115 Scheme 1. Synthesis of the Various p-Silyl-Substituted Tetraaryl Borate Complexes
The structural similarity of the borate anions in 2a and 4a with those in [Na(18-crown-6)(H2O)2][BPh4],7a [Na(benzo-15-crown-5)2][BPh4],7b and [Rh(HOCH(CH2PPh2)2)2][BPh4]8 indicates that the influence of the silyl substitution on the geometry of the borate anion is not significant, and its weakly coordinating character is at least comparable to [BPh4]-. This is further supported by 11B NMR spectroscopy in solution; the chemical shifts measured in methanol-d4 for all compounds described herein hardly deviate from that measured for sodium tetraphenylborate.9 A difference between fluorous borate 1b and nonfluorous 1a was observed in the IR spectra of these compounds. The spectrum of 1a shows the presence of coordinated water resulting from the aqueous workup, whereas 1b is isolated as a solvent-free salt. The absence of coordinated H2O in 1b is confirmed by elemental analysis and is remarkable, as sodium borates are usually obtained free of solvent only after thorough drying.10 The lack of solvent coordination in 1b could be caused by the presence of stabilizing Na‚‚‚F-C interactions11,12 between the Na+ cation and
Figure 2. Molecular structure of 4a with ellipsoids at 30% probability level. H atoms were omitted. Selected bond lengths (Å) and angles (deg): B-C(53) ) 1.637(3), B-C(62) ) 1.637(3), B-C(71) ) 1.662(3), B-C(80) ) 1.653(3), RhP(1) ) 2.2979(6), Rh-P(2) ) 2.2997(6), Rh-P(3) ) 2.2833(6), Rh-P(4) ) 2.2956(6), Rh-C(57) ) 6.260(2), C(53)-BC(62) ) 109.03(16), C(53)-B-C(71) ) 108.89(17), C(53)B-C(80) ) 108.02(16), C(62)-B-C(71) ) 108.75(16), C(62)B-C(80) ) 110.75(17), C(71)-B-C(80) ) 111.35(16), P(1)Rh-P(2) ) 83.74(2), P(1)-Rh-P(3) ) 163.09(2), P(1)-RhP(4) ) 98.68(2), P(2)-Rh-P(3) ) 99.07(2), P(3)-Rh-P(4) ) 83.54(2).
the perfluoroalkyl tail, thereby eliminating the need for polar solvent (i.e., H2O) coordination. IR data were inconclusive in this respect, as no changes in the C-F stretching region (1200-1350 cm-1) of the IR spectrum of 1b relative to that of 3b were observed.11a,13 Attempts to detect these interactions either by 19F NMR spectroscopy in solution or by determination of a molecular structure in the solid state were hampered by the low solubility of 1b in apolar solvents as well as the impossibility to obtain 1b in single crystalline form. The solubility of 1, 3, and 4 in various solvents was explored, which revealed that the introduction of per(6) Crystal data for 2a: C56H92BNaO8Si4, monoclinic, spacegroup P21/c (no. 14), with a ) 13.540(2), b ) 20.606(3), c ) 27.693(3) Å, β ) 118.86(2)°, V ) 6767.1(16) Å3, Z ) 4, Dcalcd ) 1.020 g cm-3. All data, where relevant, are given without disordered solvent contribution; 44 964 reflections measured (Nonius KappaCCD diffractometer, 150 K, Mo KR radiation), 12 222 unique. The structure was solved by automated direct methods (SHELXS86) and refined on F2 (SHELXL97-2, 631 parameters). A disordered solvent area (792 Å, 187 e, probably THF) was incorporated in the model using PLATON/ SQUEEZE. Several of the SiMe3 groups and one of the phenyl rings of the anion display conformational disorder. The available data did not allow for the refinement of a disorder model. wR2 ) 0.2304, R1 ) 0.0793 for 9530 I > 2σ(I), S ) 1.069, -0.48 < ∆F < 0.96 e Å-3. Crystal data for 4a: C88H100BP4RhSi4, monoclinic, spacegroup P21 (no. 4), with a ) 14.7259(2), b ) 20.1809(2), c ) 16.4196(2) Å, β ) 96.5775(4)°, V ) 4847.49(10) Å3, Z ) 2, Dcalcd ) 1.033 g cm-3. All data, where relevant, are given without disordered solvent contribution; 96 384 reflections measured (as for 2a), 22 082 unique. The structure was solved by automated direct methods (SHELXS97) and refined on F2 (SHELXL97-2, 992 parameters). A disordered solvent area (1001 Å, 376 e) was incorporated in the model using PLATON/SQUEEZE. A two-site disorder model was introduced to describe the rotational disorder of the phenyl moieties around C3-P3; the occupancy of the major component refined to 0.537(9). wR2 ) 0.0880, R1 ) 0.0348 for 19688I > 2σ(I), S ) 1.039, -0.36 < ∆F < 0.39 e Å-3. (7) (a) Seik Weng Ng. Z. Kristallogr.-New Cryst. Struct. 1998, 213, 159. (b) Owen, J. D. J. Chem. Soc., Dalton Trans. 1980, 1066. (8) Scherer, J.; Huttner, G.; Buchner, M.; Bakos, J. J. Organomet. Chem. 1996, 520, 45. (9) The 11B NMR spectrum of NaBPh4 in CD3OD shows a resonance at δ -2.76, which is similar to the chemical shifts of the 11B resonances found for 1b (δ -2.77), 2a (δ -2.60), 3a (δ -2.79), 3b (δ -2.79), 4a (δ -2.80), and 4b (δ -2.80). (10) (a) Wittig, G.; Raff, P. Liebigs Ann. Chem. 1950, 573, 195. (b) Nishida, H.; Takada, N.; Yoshimura, M.; Sonoda, T.; Kobayashi, H. Bull. Chem. Soc. Jpn. 1984, 57, 2600. (c) Bahr, S. R.; Boudjouk, P. J. Org. Chem. 1992, 57, 5545. (11) (a) For a Na...F-C interaction, see: Samuels, J. A.; Lobkovsky, E. B.; Streib, W. E.; Folting, K.; Huffman, J. C.; Zwanziger, J. W.; Caulton, K. G. J. Am. Chem. Soc. 1993, 115, 5093. (b) For a Zr‚‚‚F-C interaction, see: Karl, J.; Erker, G.; Fro¨hlich, R. J. Am. Chem. Soc. 1997, 119, 11165. (12) Another possible interaction of Na with the borate anion would be η6-coordination with a phenyl group. This is a well-known coordination mode of [BPh4]- toward metal ions (ref 1c and Aresta, M.; Quaranta, E.; Tommasi, I. New J. Chem. 1997, 21, 595.). However, the ease with which 1b, compared to 1a, is obtained free of water suggests involvement of the perfluoroalkyl groups.
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Organometallics, Vol. 20, No. 10, 2001
Notes
Table 1. Solubility of the Borate Complexes in Hexanesa compound [n-Bu4N][BPh4]c 3ac 3bd [Rh(dppe)2][BPh4]c 4a 4b
µmol/Lb
mg/Lb
