J. Chem. Soc., Dalton Trans., 1998, 3355–3357 3355 DALTON COMMUNICATION Asymmetric bis(pyrazolyl)hydroborato ligands via direct synthesis: structural characterization of thallium and zinc complexes † Prasenjit Ghosh,a Tony Hascall,a Conor Dowling b and Gerard Parkin *a a Department of Chemistry, Columbia University, New York, NY 10027, USA b Elf Atochem, North America, 900 First Ave., PO Box 61536, King of Prussia, PA 19406, USA Received 10th August 1998, Accepted 1st September 1998 Asymmetrically substituted bis(pyrazolyl)hydroborato ligands, i.e.[H2B(pzRR9)(pzBut 2)]2, 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. While Trofimenko’s poly(pyrazolyl)hydroborato ligand system has emerged as one of the most popular in modern coordination chemistry, the majority of studies have concentrated on the use of tris(pyrazolyl)hydroborato derivatives.1 Indeed, tris(pyrazolyl)hydroborato ligands have been subject to a large variety of modifications, which include the incorporation of sterically demanding (e.g.tert-butyl), electron withdrawing (e.g. CF3), and optically active substituents.1 By comparison, bis(pyrazolyl)hydroborato ligands have received much less attention, although we have recently used such ligands as a framework for constructing tridentate [NNO] and [NNS] donor ligands.2 In this paper, we report the syntheses of asymmetric bis(pyrazolyl)hydroborato ligands in which the two pyrazolyl groups incorporate diVerent substituents, i.e.[H2B(pzRR9)- (pzBut 2)]2. Since the first report in 1982, asymmetrically substituted bis(pyrazolyl)hydroborato ligands have been restricted to derivatives in which one of the pyrazolyl groups is methyl substituted in both the 3- and 5-positions, i.e. [H2B(pzMe2)- (pzR9R0)]2.3 The principal reason for this restriction is due to the fact that the synthetic methods employed to incorporate two diVerent pyrazolyl groups on boron have to date required the use of a pyrazole–borane reagent, (HpzRR9)(BH3), of which only the 3,5-dimethylpyrazole adduct could be synthesized (Scheme 1).4 It is, therefore, significant that we have discovered that a variety of asymmetrically substituted bis(pyrazolyl)- hydroborato ligands may be constructed straightforwardly, by the direct reaction of LiBH4 with a ca. 1 : 1 molar mixture of two diVerent pyrazoles, as illustrated in Scheme 2.For example, a mixture of pyrazole and 3,5-di-tert-butylpyrazole reacts with LiBH4 to give [H2B(pz)(pzBut 2)]Li.5 Metathesis of the latter complex with TlOAc yields its thallium derivative [H2B(pz)- (pzBut 2)]Tl, which has been structurally characterized by X-ray diVraction (Fig. 1).6 Subsequent treatment of [H2B(pz)- (pzBut 2)]Tl with ZnI2 gives [H2B(pz)(pzBut 2)]ZnI, which has been structurally characterized as the 3-tert-butyl-5-isopropylpyrazole adduct, [H2B(pz)(pzBut 2)]ZnI(HpzBut,Pri) (Fig. 2). In addition to [H2B(pz)(pzBut 2)]2, the synthetic method is also Fig. 1 Molecular structure of [H2B(pz)(pzBut 2)]Tl (only one of the crystallographically independent molecules is shown). Selected bond lengths (Å) and angles (8): Tl(1)–N(12) 2.634(4), Tl(1)–N(22) 2.677(3), N(12)–Tl(1)–N(22) 72.87(10), Tl(2)–N(112) 2.658(4), Tl(2)–N(122) 2.680(3), N(112)–Tl(2)–N(122) 75.20(11). Scheme 13356 J. Chem. Soc., Dalton Trans., 1998, 3355–3357 Table 1 Comparison of M–N bond lengths for asymmetric bis(pyrazolyl)hydroborato complexes {[H2B(pz1)(pz2)]M} [H2B(pz)(pzBut 2)]Tl [H2B(pzTrip)(pzBut 2)]ZnI [H2B(pz)(pzBut 2)]ZnI(HpzBut,Pri) c [H2B(pzMe2)(pzBut 2)]ZnI(HpzBut 2) d [H2B(pz)(pzMe2)]2Ni [H2B(pzMe2)(pzPh2)]2Zn d(M–Npz1)/Å a 2.634(4), 2.658(4) b 1.994(2) 2.017(2) 1.978(4) 1.883(3) 1.982(2), 1.982(2) d(M–Npz2)/Å a 2.677(3), 2.680(3) b 1.994(2) 2.011(2) 2.017(3) 1.894(2) 2.019(2), 2.010(2) |Dd(M–N)|/Å 0.032 0.000 0.006 0.039 0.011 0.033 Ref.This work This work This work This work 78 a pz1 and pz2 are the first and second pyrazolyl groups, respectively, in the compound formula. b Values for two crystallographically independent molecules. c d(Zn–pzBut,PriH) = 2.038(2) Å. e d(Zn–pzBut 2H) = 2.090(3) Å. applicable for [H2B(pzMe2)(pzBut 2)]2 and [H2B(pzTrip)(pzBut 2)]2 (Trip = triptycyl, 9,10-o-benzeno-9,10-dihydroanthracenyl) ligands, which have been structurally characterized as the zinc iodide derivatives, [H2B(pzMe2)(pzBut 2)]ZnI(HpzBut 2) (Fig. 3) and [H2B(pzTrip)(pzBut 2)]ZnI (Fig. 4). Other than the complexes described above, structurally authenticated asymmetric bis(pyrazolyl)hydroborato complexes are limited to the nickel and zinc complexes, [H2B(pz)- (pzMe2)]2Ni7 and [H2B(pzMe2)(pzPh2)]2Zn,8 respectively. For comparison, the M–N bond lengths for all structurally characterized asymmetric bis(pyrazolyl)hydroborato ligands are summarized in Table 1.Interestingly, despite substantial diVerences in the size of the pyrazolyl substituents, there is little variation in the two M–N bond lengths for a given complex, i.e. Dd(M–N) ª 0. Fig. 2 Molecular structure of [H2B(pz)(pzBut 2)]ZnI(HpzBut,Pri). Selected bond lengths (Å) and angles (8): Zn–N(22) 2.011(2), Zn–N(12) 2.017(2), Zn–N(31) 2.038(2), Zn–I 2.6150(3), N(22)–Zn–N(12) 98.71(8), N(22)–Zn–N(31) 117.83(8), N(12)–Zn–N(31) 113.43(8), N(22)–Zn–I 127.46(6), N(12)–Zn–I 100.62(6), N(31)–Zn–I 97.95(5).Scheme 2 In summary, a new method for the synthesis of asymmetrically substituted bis(pyrazolyl)hydroborato ligands is provided by the reactions of LiBH4 with a ca. 1 : 1 molar mixture of two diVerent pyrazoles. Ligands which have been constructed using this method include [H2B(pz)(pzBut 2)]2, [H2B(pzMe2)(pzBut 2)]2 and [H2B(pzTrip)(pzBut 2)]2. As such, this method is more convenient and potentially more general than previously reported procedures for the syntheses of asymmetrically substituted bis(pyrazolyl)hydroborato ligands.Acknowledgements We thank the National Science Foundation (CHE 96-10497) and ELF Atochem North America, Inc., for support of this research. G. P. is the recipient of a Presidential Faculty Fellowship Award (1992–1997). Fig. 3 Molecular structure of [H2B(pzMe2)(pzBut 2)]ZnI(HpzBut 2). Selected bond lengths (Å) and angles (8): Zn–N(22) 1.978(4), Zn– N(12) 2.017(3), Zn–N(32) 2.090(3), Zn–I 2.5864(6), N(22)–Zn–N(12) 105.4(2), N(22)–Zn–N(32) 101.8(2), N(12)–Zn–N(32) 115.31(13), N(22)– Zn–I 113.44(10), N(12)–Zn–I 116.31(10), N(32)–Zn–I 103.94(10).Fig. 4 Molecular structure of [H2B(pzTrip)(pzBut 2)]ZnI. Selected bond lengths (Å) and angles (8): Zn–N(22) 1.994(2), Zn–N(12) 1.994(2), Zn–H(1) 2.41(3), Zn–I 2.4651(3), N(22)–Zn–N(12) 99.92(8), N(22)– Zn–H(1) 72.4(7), N(12)–Zn–H(1) 71.2(7), N(22)–Zn–I 130.08(6), N(12)–Zn–I 129.91(6), H(1)–Zn–I 116.6(7).J. Chem.Soc., Dalton Trans., 1998, 3355–3357 3357 Notes and references † Supplementary data available: experimental details and NMR data. For direct electronic access see http://www.rsc.org/suppdata/dt/1998/ 3355/, otherwise available from BLDSC (No. SUP 57435, 9 pp.) or the RSC Library. See Instructions for Authors, 1998, Issue 1 (http:// www.rsc.org/dalton). 1 For recent reviews, see: S. Trofimenko, Chem. Rev., 1993, 93, 943; G. Parkin, Adv. Inorg. Chem., 1995, 42, 291; N. Kitajima and W.B. Tolman, Prog. Inorg. Chem., 1995, 43, 419; I. Santos and N. Marques, New. J. Chem., 1995, 19, 551; D. L. Reger, Coord. Chem. Rev., 1996, 147, 571; M. Etienne, Coord. Chem. Rev., 1997, 156, 201; P. K. Byers, A. J. Canty and R. T. Honeyman, Adv. Organomet. Chem., 1992, 34, 1. 2 P. Ghosh and G. Parkin, Chem. Commun., 1998, 413; C. Dowling and G. Parkin, Polyhedron, 1996, 15, 2463; P. Ghosh and G. Parkin, J. Chem. Soc., Dalton Trans., 1998, 2281. 3 E. Frauendorfer and G. Agrifoglio, Inorg.Chem., 1982, 21, 4122; G. Agrifoglio, Inorg. Chim. Acta, 1992, 197, 159. 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(pzRR9)2BH2]. See ref. 3. 5 For example, [H2B(pz)(pzBut 2)]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)(pzBut 2)]Li were deposited (5.5 g, 30%).[H2B(pz)(pzBut 2)]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)(pzBut 2)]Tl as a white solid which was washed with pentane (7.4 g, 77%).[H2B(pzMe2)(pzBut 2)]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)(pzBut 2)]Tl is monoclinic, P21/c (no. 14), C14H24BN4Tl, M = 463.55, a = 10.2116(5), b = 26.000(1), c = 14.1494(7) Å, b = 106.872(1)8, U = 3596.7(3) Å3, Z = 8, T = 293 K, m = 8.977 mm21, 8222 independent reflections, R1 = 0.0362 [I > 2s(I)]. [H2B(pz)- (pzBut 2)]ZnI(HpzBut,Pri) is monoclinic, P21/n (no. 14), C24H42BIN6Zn, M = 617.72, a = 15.4692(8), b = 12.2119(6), c = 16.4996(8) Å, b = 110.910(1)8, U = 2911.6(3) Å3, Z = 4, T = 203 K, m = 1.926 mm21, 6641 independent reflections, R1 = 0.0321 [I > 2s(I)]. [H2B(pzTrip)- (pzBut 2)]ZnI is triclinic P1� (no. 2), C34H36BIN4Zn, M = 703.75, a = 9.9469(5), b = 11.9592(5), c = 13.9733(6) Å, a = 90.085(1), b = 96.387(1), g = 103.157(1)8, U = 1607.9(1) Å3, Z = 2, T = 203 K, m = 1.752 mm21, 6874 independent reflections, R1 = 0.0327 [I > 2s(I )]. [H2B(pzMe2)(pzBut 2)]ZnI(HpzBut 2) is monoclinic, P21/n (no. 14), C27H48BIN6Zn, M = 659.79, a = 17.701(1), b = 9.5135(6), c = 21.543(2) Å, b = 113.781(2)8, U = 3319.7(4) Å3, Z = 4, T = 293 K, m = 1.693 mm21, 5797 independent reflections, R1 = 0.0444 [I > 2s(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. Communication 8/0629