Synthesis and structural characterization of tris[3-trifluoromethyl-5-(2-thienyl)pyrazolyl]hydroborato thallium, Tl[Tp^CF₃,Tn]: a monovalent thallium complex with a highly solvent dependent J_Tl–F coupling constant, ranging from 0 to 850 Hz

Abstract

The tris[3-trifluoromethyl-5-(2-thienyl)pyrazolyl]hydroborato thallium complex, Tl[Tp^CF₃,Tn], has been prepared via the reaction of 3-trifluoromethyl-5-(2-thienyl)pyrazole with KBH₄ followed by metathesis with TlNO₃. Both ¹⁹F and ²⁰³Tl NMR spectroscopies reveal the presence of an exceptionally large 850 Hz four-bond ⁴J_Tl–F coupling constant for Tl[Tp^CF₃,Tn] in chloroform at room temperature; however, the observed coupling constant is highly solvent dependent and is reduced to 0 Hz in methanol, acetonitrile and dimethyl sulfoxide. The molecular structure of Tl[Tp^CF₃,Tn] has been determined by X-ray diffraction: Tl[Tp^CF₃,Tn] is triclinic, P1̄ (no. 2), a = 8.328(1), b = 11.432(2), c = 16.088(3) Å, α = 70.58(2), β = 82.90(2), γ = 77.54(2)°, U = 1408(1) ų, Z = 2.

References

1. For recent reviews see; (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.
2. The abbreviations adopted here for tris(pyrazolyl)hydroborato ligands are based on those described by Trofimenko [ref. 1(a)]. Thus, tris(pyrazolyl)hydroborato ligands are represented by the abbreviation [Tp^RR′] with the 3- and 5-alkyl substituents listed respectively as superscripts.
3. For a review of monomeric alkyl, hydride and hydroxide complexes of the s- and p-block metals, see ref. 1(b).
4. M. C. Kuchta, J. B. Bonanno and G. Parkin, J. Am. Chem. Soc., 1996, 118, 10 914.
5. H. V. R. Dias, W. Jin, H.-J. Kim and H.-L. Lu, Inorg. Chem., 1996, 35, 2317.
6. (a) O. Renn, L. M. Venanzi, A. Marteletti and V. Gramlich, Helv. Chim. Acta, 1995, 78, 993; (b) H. V. R. Dias, H.-L. Lu, R. E. Ratcliff and S. G. Bott, Inorg. Chem., 1995, 34, 1975.
7. C. K. Ghosh, J. K. Hoyano, R. Krentz and W. A. G. Graham, J. Am. Chem. Soc., 1989, 111, 5480; U. E. Bucher, A. Currao, R. Nesper, H. Rüegger, L. M. Venanzi and E. Younger, Inorg. Chem., 1995, 34, 66.
8. H. V. R. Dias and H.-J. Kim, Organometallics, 1996, 15, 5374.
9. As an illustration of the ability of R_f substituents to have a pronounced effect on the stability and reactivity of {[Tp^R_f,R]M} complexes, as compared with their alkylated counterparts, Dias has successfully used the [Tp^(CF₃)₂] ligand to isolate a series of carbonyl derivatives of Cu, Ag and Au. See: H. V. R. Dias and W. Jin, J. Am. Chem. Soc., 1995, 117, 11 381; Inorg. Chem., 1996, 35, 3687; H. V. R. Dias and H.-L. Lu, Inorg. Chem., 1995, 34, 5380.
10. The Tl[Tp^RR′] derivatives with bulkier substituents (in both 3- and 5-positions) typically show a marked twisting of the pyrazolyl planes with respect to the Tl ⋯ B axis. For example, see: C. Dowling, D. Leslie, M. H. Chisholm and G. Parkin, Main Group Chemistry, 1995, 1, 29.
11. r_cov(Tl)= 1.57 Å, r_cov(N)= 0.74 Å. See: L. Pauling, The Nature of The Chemical Bond, Cornell University Press, Ithaca, 3rd edn., 1960.
12. A. Haaland, Angew. Chem., Int. Ed. Engl., 1989, 28, 992.
13. Furthermore, since the thienyl group is less sterically demanding than trifluoromethyl, as judged by studies on the 3-thienyl derivative Tl[Tp^Tn],¹³ such an observation is not easily rationalized as a steric effect in which increased repulsion between the substituents in the 5-position and the B-H moiety would modify the bite of the ligand such that a longer Tl-N interaction would result. See: J. C. R. Calabrese, P. J. Domaille, S. Trofimenko and G. J. Long, Inorg. Chem., 1991, 30, 2975.
14. H. V. R. Dias, H.-J. Kim, H.-L. Lu, K. Rajeshwar, N. R. de Tacconi, A. Derecskei-Kovacs and D. S. Marynick, Organometallics, 1996, 15, 2994.
15. The van der Waals radii of Tl and F are 1.96 Å and 1.47 Å, respectively. See: A. Bondi, J. Phys. Chem., 1964, 68, 441.
16. For reference, the Tl-F bond length in (TPP)TlF (TPP = tetraphenylporphyrinato) is 2.441(6)Å. See: A. G. Coutsolelos, M. Orfanopoulas and D. L. Ward, Polyhedron, 1991, 10, 885.
17. For a review of secondary bonding (i.e. interactions with bond lengths that are intermediate between the sum of covalent and van der Waals radii), see: N. W. Alcock, Adv. Inorg. Chem. Radiochem., 1972, 15, 1.
18. W. A. W. A. Bakar, J. L. Davidson, W. E. Lindsell, K. J. McCullough and K. W. Muir, J. Chem. Soc., Dalton Trans., 1989, 991; W. A. W. A. Bakar, J. L. Davidson, W. E. Lindsell and K. J. McCullough, J. Organomet. Chem., 1987, 322, C1.
19. J. A. Samuels, E. B. Lobkovsky, W. E. Streib, K. Folting, J. C. Huffman, J. W. Zwanziger and K. G. Caulton, J. Am. Chem. Soc., 1993, 115, 5093; J. A. Samuels, J. W. Zwanziger, E. B. Lobkovsky and K. G. Caulton, Inorg. Chem., 1992, 31, 4046.
20. H. W. Roesky, M. Scholz, M. Noltemeyer and F. T. Edelmann, Inorg. Chem., 1989, 28, 3829.
21. K. Henrick, M. McPartlin, R. W. Matthews, G. B. Deacon and R. J. Phillips, J. Organomet. Chem., 1980, 193, 13.
22. K. Henrick, M. McPartlin, G. B. Deacon and R. J. Phillips, J. Organomet. Chem., 1981, 204, 287.
23. S. Tachiyashiki, H. Nakayama, R. Kuroda, S. Sato and Y. Saito, Acta Crystallogr., Sect. B, 1975, 31, 1483.
24. R. Usón, J. Forniés, M. Tomás, R. Garde and P. J. Alonso, J. Am. Chem. Soc., 1995, 117, 1837.
25. H. Rothfuss, K. Folting and K. G. Caulton, Inorg. Chim. Acta, 1993, 212, 165.
26. D. Labahn, E. Pohl, R. Herbst-Irmer, D. Stalke, H. W. Roesky and G. M. Sheldrick, Chem. Ber., 1991, 124, 1127.
27. H. Luth and M. R. Truter, J. Chem. Soc. A, 1970, 1287.
28. E. J. Fernández, P. G. Jones, A. Laguna and A. Mendiá, Inorg. Chim. Acta, 1994, 215, 229.
29. Thallium exists as two naturally occuring spin ½ isotopes: ²⁰³Tl (29.5%) and ²⁰⁵Tl (70.5%). Owing to the similarity of their gyromagnetic ratios, the difference in ²⁰³Tl and ²⁰⁵Tl coupling constants is frequently not discernible.
30. The ²⁰³Tl NMR studies were carried out in preference to ²⁰⁵Tl NMR studies due to the unavailability of a suitable probe for the latter nucleus.
31. J. F. Hinton, Magn. Reson. Chem., 1987, 25, 659; J. F. Hinton, K. R. Metz and R. W. Briggs, Prog. Nucl. Magn. Reson. Spectrosc., 1988, 20, 423; J. F. Hinton and K. R. Metz, NMR Newly Accessible Nucl., 1983, 2, 367; J. F. Hinton, K. R. Metz and R. W. Briggs, Annu. Rep. NMR Spectrosc., 1982, 13, 211.
32. V. P. Tarasov and S. I. Bakum, J. Magn. Reson., 1975, 18, 64.
33. G. B. Deacon, R. M. Slade and D. G. Vince, J. Fluorine Chem., 1978, 11, 57.
34. For other J_Tl-F coupling constant data, see: W. Kitching, D. Praeger, C. J. Moore, D. Doddrell and W. Adcock, J. Organomet. Chem., 1974, 70, 339; D. E. Fenton, D. G. Gillies, A. G. Massey and E. W. Randall, Nature (London), 1964, 201, 818.
35. J. A. Samuels, E. B. Lobkovsky, W. E. Streib, K. Folting, J. C. Huffman, J. W. Zwanziger and K. G. Caulton, J. Am. Chem. Soc., 1993, 115, 5093; J. A. Samuels, J. W. Zwanziger, E. B. Lobkovsky and K. G. Caulton, Inorg. Chem., 1992, 31, 4046.
36. For [Tl₂Zr{OCH(CF₃)₂}₆] it was argued that since the observed J_Tl-F coupling constant is an average of six ‘close’¹J_Tl-F and 30 ‘distant’J′_Tl-F coupling constants, a value of 2298 Hz can be estimated for ¹J_Tl-F assuming the ‘distant’J′_Tl-F coupling constants to be close to zero.
37. It is also worth noting that the ⁴J_Tl-F coupling constant of 850 Hz in Tl[Tp^CF₃,Tn] is significantly greater than ⁴J_Ag-F(1.4 Hz) in the related silver complex, [Tp^(CF₃)₂]AgPPh₃. See: H. V. R. Dias, W. Jin, H.-J. Kim and H.-L. Lu, Inorg. Chem., 1996, 35, 2317.
38. A singlet is also observed at ca. –70 °C in the ¹⁹F NMR spectrum recorded in CD₃OD. The resonance is, however, broadened (Δν_½≈ 140 Hz) compared to that of the room temperature spectrum (Δν_½≈ 7 Hz).
39. For example, the decrease in J_Tl-C coupling constants for Tl[Tp^Me₂] with increasing temperature has been attributed to a dynamic process;^39a likewise, solutions of the cryptand N(CH₂CH₂OCH₂CH₂OCH₂CH₂)₃N in the presence of Tl⁺ exhibit J_Tl-H coupling at low temperatures, but the coupling disappears at ca. 33 °C.^39b Furthermore, the doublet observed at –50 °C in the ¹⁹F spectrum of [CpMo(SC₆F₅)₂(CO)₂Tl] broadens at 20 °C, a result that has also been interpreted as due to dissociation of Tl⁺.¹⁸ (a) D. Sanz, R. M. Claramunt, J. Glaser, S. Trofimenko and J. Elguero, Magn. Reson. Chem., 1996, 34, 843; (b) J. M. Lehn, J. P. Sauvage and B. Dietrich, J. Am. Chem. Soc., 1970, 92, 2916.
40. J. J. Dechter and J. I. Zink, J. Am. Chem. Soc., 1975, 97, 2937.
41. For an η²-complex exhibiting rapid exchange, the observed J_Tl-F coupling constant is the weighted average of two contributions, involving the co-ordinated and unco-ordinated pyrazolyl groups. Assuming the two co-ordinated pyrazolyl groups to exhibit a coupling constant of 850 Hz, and the unco-ordinated pyrazolyl group to exhibit a coupling constant of 0 Hz, such a complex would exhibit an apparent coupling constant of ⅔× 850 = 566 Hz. To the extent that the η³-complex may be present in solution, or J_Tl-F coupling to the unco-ordinated pyrazolyl group is non-zero, the observed coupling would be expected to be greater than 566 Hz..
42. C. López, D. Sanz, R. M. Claramunt, S. Trofimenko and J. Elguero, J. Organomet. Chem., 1995, 503, 265.
43. Note that the ⁴J_Tl-H coupling of 7 Hz in CDCl₃ is slightly greater than that in C₆D₅CD₃(Table 1), and is consistent with the solvent dependence of the ⁴J_Tl-F coupling constant (Table 2).
44. S. P. Singh, S. Sehgal, L. S. Tarar and S. N. Dhawan, Indian J. Chem., Sect. B, 1990, 29, 310.
45. B. J. Evans, J. T. Doi and W. K. Musker, J. Org. Chem., 1990, 55, 2337.
46. G. M. Sheldrick, SHELXTL, An Integrated System for Solving, Refining and Displaying Crystal Structures from Diffraction Data, University of Göttingen, 1981.