Asymmetric bis(pyrazolyl)hydroborato ligands via direct synthesis: structural characterization of thallium and zinc complexes †

Abstract

Asymmetrically substituted bis(pyrazolyl)hydroborato ligands, in which the two pyrazolyl groups possess different substituents, have been synthesized by the direct reaction of a ca. 1∶1 molar mixture of the respective pyrazoles with LiBH₄.

References

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4. Specifically, attempts to synthesize the borane adduct of pyrazole, 4-nitropyrazole and diphenylpyrazole, resulted in elimination of H₂ and the formation of pyrazolyl bridged dimers, [H₂B(pz^RR′)₂BH₂]. See ref. 3.
5. For example, [H₂B(pz)(pz^But₂)]M (M = Li and Tl) are synthesized as follows. A solution of LiBH₄ in THF (35 mL of 2 M, 70.0 mmol), diluted with toluene (ca.35 mL), was added to a mixture of pyrazole (4.8 g, 70.5 mmol) and 3,5-di-tert-butylpyrazole (13.2 g, 73.2 mmol) and stirred overnight at room temperature. The solvent was removed in vacuo, toluene (ca. 40 mL) was added, and the resulting mixture was refluxed for one day. The solution was concentrated to ca. 20 mL, at which point the mixture became turbid and white solid started to precipitate. Pentane (ca. 50 mL) was added to complete precipitation and the mixture was filtered. The filtrate was allowed to stand at room temperature overnight, over which period colorless crystals of [H₂B(pz)(pz^But₂)]Li were deposited (5.5 g, 30%). [H₂B(pz)(pz^But₂)]Li was treated with TlOAc (8.2 g, 31.1 mmol) and THF (ca. 60 mL) and the mixture was stirred overnight at room temperature. The mixture was filtered and the residue was further extracted with pentane (ca. 50 mL). The THF and pentane extracts were combined and the solvent removed in vacuo to give [H₂B(pz)(pz^But₂)]Tl as a white solid which was washed with pentane (7.4 g, 77%). [H₂B(pz^Me₂)(pz^But₂)]Tl is synthesized similarly, with the exception that the lithium derivative was converted to the potassium complex prior to metathesis with TlOAc.
6. [H₂B(pz)(pz^But₂)]Tl is monoclinic, P2₁/c(no. 14), C₁₄H₂₄BN₄Tl, M= 463.55, a= 10.2116(5), b= 26.000(1), c= 14.1494(7)Å, β= 106.872(1)°, U= 3596.7(3)ų, Z= 8, T= 293 K, µ= 8.977 mm^–1, 8222 independent reflections, R1 = 0.0362 [I > 2σ(I)]. [H₂B(pz)-(pz^But₂)]ZnI(Hpz^But,Pri) is monoclinic, P2₁/n(no. 14), C₂₄H₄₂BIN₆Zn, M= 617.72, a= 15.4692(8), b= 12.2119(6), c= 16.4996(8)Å, β= 110.910(1)°, U= 2911.6(3)ų, Z= 4, T= 203 K, µ=1.926 mm^–1, 6641 independent reflections, R₁= 0.0321 [I > 2σ(I)]. [H₂B(pz^Trip)-(pz^But₂)]ZnI is triclinic P1̄(no. 2), C₃₄H₃₆BIN₄Zn, M= 703.75, a= 9.9469(5), b= 11.9592(5), c= 13.9733(6)Å, a= 90.085(1), β= 96.387(1), γ= 103.157(1)°, U= 1607.9(1)ų, Z= 2, T= 203 K, µ= 1.752 mm^–1, 6874 independent reflections, R₁= 0.0327 [I > 2σ(I)]. [H₂B(pz^Me₂)(pz^But₂)]ZnI(Hpz^But₂) is monoclinic, P2₁/n(no. 14), C₂₇H₄₈BIN₆Zn, M= 659.79, a= 17.701(1), b= 9.5135(6), c= 21.543(2)Å, β= 113.781(2)°, U= 3319.7(4)ų, Z= 4, T= 293 K, µ= 1.693 mm^–1, 5797 independent reflections, R₁= 0.0444 [I > 2σ(I)]. CCDC number 186/1142.
7. H. Kokusen, Y. Sohrin, M. Matsui, Y. Hata and H. Hasegawa, J. Chem. Soc., Dalton Trans., 1996, 195.
8. M. V. Capparelli and G. Agrifoglio, J. Crystallogr. Spectrosc. Res., 1992, 22, 651.