Phenyl tris(3-tert-butylpyrazolyl)borato complexes of lithium and thallium, [PhTp^But]M (M = Li, Tl): a novel structure for a monomeric tris(pyrazolyl)boratothallium complex and a study of its stereochemical nonrigidity by ¹H and ²⁰⁵Tl NMR spectroscopy

Abstract

The syntheses and structures of the phenyl substituted tris(3-tert-butylpyrazolyl)borato complexes, [PhTp^But]M (M = Li, Tl, H), are reported. In contrast to other monomeric [Tp^RR′]Tl derivatives, which exhibit symmetric tridentate coordination of the tris(pyrazolyl)borate ligand, [PhTp^But]Tl exhibits an unprecedented structure. Specifically, one of the tert-butylpyrazolyl groups is rotated by ca. 90° and the Tl interacts with the nitrogen attached directly to the boron via a p-orbital component of the aromatic π-system of the pyrazolyl nucleus. [PhTp^But]Tl is stereochemically nonrigid on the NMR spectroscopic timescale in solution at room temperature, but cooling to ca. –80 °C slows down the dynamic processes sufficiently to allow “axial” and “equatorial” isomers to be identified, with the descriptors denoting the position of the pyrazolyl group relative to the boat configuration of the six-membered [BN₄Tl] ring.

References

1. For recent reviews (a) S. Trofimenko, Chem. Rev., 1993, 93, 943; (b) G. Parkin, Adv. Inorg. Chem., 1995, 42, 291; (c) N. Kitajima and W. B. Tolman, Prog. Inorg. Chem., 1995, 43, 419; (d) I. Santos and N. Marques, New. J. Chem., 1995, 19, 551; (e) D. L. Reger, Coord. Chem. Rev., 1996, 147, 571; (f) M. Etienne, Coord. Chem. Rev., 1997, 156, 201; (g) P. K. Byers, A. J. Canty and R. T. Honeyman, Adv. Organomet. Chem., 1992, 34, 1.
2. For reviews of Tl[Tp^RR′] complexes, see: (a) C. Janiak, Main Group Met. Chem., 1998, 21, 33; (b) C. Janiak, Coord. Chem. Rev., 1997, 163, 107.
3. The abbreviations adopted here for tris(pyrazolyl)hydroborato ligands are based on those described by Trofimenko [ref. 1(a)]. Thus, the tris(pyrazolyl)hydroborato ligands are represented by the abbreviation Tp, with the 3- and 5-alkyl substituents listed respectively as superscripts. If the fourth substituent on boron is anything other than hydrogen, the substituent is listed as a prefix, e.g., [pzTp] and [RTp].
4. See, for example: J. L. Kisko, T. Hascall and G. Parkin, J. Am. Chem. Soc., 1998, 120, 10561.
5. S. Trofimenko, J. Am. Chem. Soc., 1967, 89, 6288.
6. K. Nidenzu and S. Trofimenko, Top. Curr. Chem., 1986, 131, 1.
7. Na[RTp](R = Prⁱ,^a Buⁿ,^b Ph,^b,cp-C₆H₄Br^c) have been prepared by reaction of R B(OH)₂ with Napz in the presence of pzH (a) D. L. Reger and M. E. Tarquini, Inorg. Chem., 1982, 21, 840; (b) ref. 5; (c) D. L. White and J. W. Faller, J. Am. Chem. Soc., 1982, 104, 1548.
8. [PhTp]Li has been prepared by reaction of Li[PhBH₃] with pyrazole. See: F. A. Cotton, C. A. Murillo and B. R. Stults, Inorg. Chim. Acta, 1977, 22, 75.
9. [pzH₂][PhTp] was obtained by the reaction of PhBCl₂ with excess pyrazole. See ref. 5.
10. The ferrocenyl [Fc =(C₅H₄)(C₅H₅)Fe] derivative, [FcTp]H has been obtained by reaction of FcBBr₂ with pzH in the presence of Et₃N. Subsequent reaction of [FcTp]H with TlOEt gives the thallium complex Tl[FcTp] F. Jäkle, K. Polborn and M. Wagner, Chem. Ber., 1996, 129, 603.
11. U. E. Bucher, T. F. Fässler, M. Hunziker, R. Nesper, H. Rüegger and L. M. Venanzi, Gazz. Chim. Ital., 1995, 125, 181.
12. For example, terminal alkyl, hydride, hydroxide and chalcogenide moieties have been stabilized with such ligation. See, for example, ref. 1(b) and (a) M. C. Kuchta and G. Parkin, J. Am. Chem. Soc., 1995, 117, 12651; (b) M. C. Kuchta and G. Parkin, Inorg. Chem., 1997, 36, 2492; (c) M. C. Kuchta and G. Parkin, J. Chem. Soc., Dalton Trans., 1998, 2279.
13. B. Singaram, T. E. Cole and H. C. Brown, Organometallics, 1984, 3, 774.
14. If the reaction is carried out in benzene solvent at room temperature, the phenylbis(3-tert-butylpyrazolyl)borato species [Ph(H)Bp^But]Li(Bu^tpzH) may be isolated. J. L. Kisko and G. Parkin, unpublished work.
15. (a) S. Trofimenko, J. Am. Chem. Soc., 1967, 89, 3170; (b) R. A. Kresinski, J. Chem. Soc., Dalton Trans., 1999, 401.
16. C. López, R. M. Claramunt, C. Foces-Foces, F. H. Cano and J. Elguero, Rev. Roum. Chim., 1994, 9, 795.
17. Other protonated [Tp^RR′]H derivatives have also been prepared by this method. See: J. Blackwell, C. Lehr, Y. Sun, W. E. Piers, S. D. Pearce-Batchilder, M. J. Zaworotko and V. G. Young, Jr., Can J. Chem., 1997, 75, 702 and footnote 5 therein.
18. For structures of other alkali metal [Tp^RR′]M derivatives, see, for example: (a) C. M. Dowling, D. Leslie, M. H. Chisholm and G. Parkin, Main Group Chem., 1995, 1, 29; (b) C. Lopez, R. M. Claramunt, D. Sanz, C. Foces Foces, F. H. Cano, E. Faure, E. Cayon and J. Elguero, Inorg. Chim. Acta, 1990, 176, 195; (c) G. G. Lobbia, P. Cecchi, R. Spagna, M. Colapietro, A. Pifferi and C. Pettinari, J. Organomet. Chem., 1995, 485, 45; (d) K. Weis and H. Vahrenkamp, Inorg. Chem., 1997, 36, 5589; (e) H. V. R. Dias and H.-J. Kim, Organometallics, 1996, 15, 5374; (f) H. V. R. Dias, W. C. Jin, H. J. Kim and H.-L. Lu, Inorg. Chem., 1996, 35, 2317; (g) H. V. R. Dias, H.-L. Lu, R. E. Ratcliff and S. G. Bott, Inorg. Chem., 1995, 34, 1975.
19. Although three coordination of lithium is less common than its ubiquitous tetrahedral coordination, it is nevertheless well precedented. For example, ca. 20% of the lithium complexes listed in the Cambridge Structural Database have a coordination number of three for lithium.
20. CSD Version 5.16, 1999, 3D Search and Research Using the Cambridge Structural Database, F. H. Allen and O. Kennard, Chem. Des. Automat. News, 1993, 8, 1, 31–37.
21. D. L. Reger, J. E. Collins, M. A. Matthews, A. L. Rheingold, L. M. Liable-Sands and I. A. Guzei, Inorg. Chem., 1997, 36, 6266.
22. D. C. Bradley, M. B. Hursthouse, J. Newton and N. P. C. Walker, J. Chem. Soc., Chem. Commun., 1984, 188.
23. For a compilation of data, see: R. Han, P. Ghosh, P. J. Desrosiers, S. Trofimenko and G. Parkin, J. Chem. Soc., Dalton Trans., 1997, 3713.
24. A. H. Cowley, R. L. Geerts, C. M. Nunn and S. Trofimenko, J. Organomet. Chem., 1989, 365, 19.
25. For other examples in which intraligand interactions influence the binding mode of poly(pyrazolyl)borate complexes, see: (a) Y. Sohrin, H. Kokusen and M. Matsui, Inorg. Chem., 1995, 34, 3928; (b) D. L. Reger, M. F. Huff, A. L. Rheingold and B. S. Haggerty, Inorg. Chem., 1992, 114, 579; (c) F. A. Cotton, B. A. Frenz and C. A. Murillo, J. Am. Chem. Soc., 1975, 97, 2118.
26. (a) C. Dowling, P. Ghosh and G. Parkin, Polyhedron, 1997, 16, 3469; (b) T. Fillebeen, T. Hascall and G. Parkin, Inorg. Chem., 1997, 36, 3787; (c) P. Ghosh, T. Hascall, C. Dowling and G. Parkin, J. Chem. Soc., Dalton Trans., 1998, 3355.
27. For example, only 7% of the structurally characterized lithium complexes listed in the Cambridge Structural Database are two coordinate.
28. Furthermore, it should be noted that the closest Tl ⋯ Tl separation in [PhTp^But]Tl is 6.68 Å, so that there is also no weak Tl ⋯ Tl interaction of the type that has been suggested for other [Tp^RR′]Tl complexes. See, for example: (a) G. Ferguson, M. C. Jennings, F. J. Lalor and C. Shanahan, Acta. Crystallogr., Sect. C, 1991, 47, 2079; (b) A. L. Rheingold, L. M. Liable-Sands and S. Trofimenko, J. Chem. Soc., Chem. Commun., 1997, 1691; (c) C. Janiak, S. Temizdemir and T. G. Scharmann, Z. Anorg. Allg. Chem., 1998, 624, 755.
29. The 2:3 ratio is comparable to the ratio of areas of the two ²⁰⁵Tl NMR spectroscopic signals (2:2.6 at –70 °C). In view of the potentially different relaxation times for the different thallium nuclei, the integral ratio is not expected to be an accurate measure of their relative concentrations.
30. The 2:1 ratio of pyrazolyl groups assumes that either the chemical shifts of the diasterotopic pyrazolyl groups are coincidentally the same, or a mechanism exists for their facile interconversion. Alternatively, B_2:1 could correspond to the equatorial isomer of Scenario I.
31. U. E. Bucher, A. Currao, R. Nesper, H. Rüegger, L. M. Venanzi and E. Younger, Inorg. Chem., 1995, 34, 66.
32. W. D. Jones and E. T. Hessell, Inorg. Chem., 1991, 30, 778.
33. (a) J. P. McNally, V. S. Leong and N. J. Cooper, in Experimental Organometallic Chemistry, ed. A. L. Wayda and M. Y. Darensbourg, American Chemical Society, Washington, DC, 1987, ch. 2, pp. 6–23; (b) B. J. Burger and J. E. Bercaw, in Experimental Organometallic Chemistry, eds. A. L. Wayda and M. Y. Darensbourg, American Chemical Society, Washington, DC, 1987, ch. 4, pp. 79–98; (c) D. F. Shriver and M. A. Drezdzon, The Manipulation of Air-Sensitive Compounds, Wiley-Interscience, New York, 2nd edn., 1986.
34. J. J. Dechter and J. I. Zink, J. Am. Chem. Soc., 1975, 97, 2937.
35. Specifically, the resonance frequencies of three solutions of Tl(NO₃) in H₂O (1.0, 0.5 and 0.25 M) were extrapolated to zero concentration.
36. G. M. Sheldrick, SHELXTL, An Integrated System for Solving, Refining and Displaying Crystal Structures from DiVraction Data, University of Göttingen, 1981.