DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 1929–1935 1929 Phenyl tris(3-tert-butylpyrazolyl)borato complexes of lithium and thallium, [PhTpBut]M (M 5 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 Jennifer L. Kisko, Tony Hascall, Clare Kimblin and Gerard Parkin * Department of Chemistry, Columbia University, New York, NY 10027, USA Received 9th March 1999, Accepted 19th April 1999 The syntheses and structures of the phenyl substituted tris(3-tert-butylpyrazolyl)borato complexes, [PhTpBut]M (M = Li, Tl, H), are reported.In contrast to other monomeric [TpRR9]Tl derivatives, which exhibit symmetric tridentate coordination of the tris(pyrazolyl)borate ligand, [PhTpBut]Tl exhibits an unprecedented structure. Specifically, one of the tert-butylpyrazolyl groups is rotated by ca. 908 and the Tl interacts with the nitrogen attached directly to the boron via a p-orbital component of the aromatic p-system of the pyrazolyl nucleus.[PhTpBut]Tl is stereochemically nonrigid on the NMR spectroscopic timescale in solution at room temperature, but cooling to ca. 280 8C slows down the dynamic processes suYciently 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. Introduction The poly(pyrazolyl)borato ligand system is one of the most widely used in modern coordination chemistry.1 Alkali metal and thallium 2 derivatives, in particular, have played a prominent role in the development of this chemistry by virtue of their use as ligand transfer agents.Other than their use as reagents, however, these complexes have received relatively little attention. To date, the majority of studies using tris(pyrazolyl)- borato ligands have concentrated on [TpRR9] derivatives in which the fourth site on boron is a hydride, with comparatively fewer studies having been reported for those with an alkyl or aryl substituent, [RTpRR9].3 We are particularly interested in applying such [RTpRR9] ligands to situations in which the potential reactivity associated with the B–H bond may prove problematic. 4 In this paper, we report the syntheses and structures of the phenyl substituted tris(3-tert-butylpyrazolyl)borato complexes, [PhTpBut]M (M = Li, Tl, H).Significantly, [PhTpBut]Tl exhibits an unprecedented type of structure for monomeric tris(pyrazolyl)borato thallium derivatives.Results and discussion Syntheses of [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H Tris(pyrazolyl)borato ligands with alkyl or aryl substituents on boron, i.e. [RTpRR9], have been known for more than 30 years 5 and have been synthesized by a variety of methods, as illustrated in Scheme 1.6–10 However, the majority of these [RTpRR9] ligands are derivatives of unsubstituted pyrazole, with relatively few examples incorporating bulky substituents in the 3-positions.For example, methyl is the bulkiest pyrazolyl substituent reported in the literature to have been incorporated into the [RTpRR9] ligand system; specifically, Na[MeTpMe] was obtained via the reaction of MeB(OPri)2 with a mixture of Na[pzMe] and HpzMe.11 In view of the important role that bulky substituents in the 3-positions of [TpRR9] ligands has played in allowing isolation of certain reactive M–X functionalities,12 we are particularly interested in the construction of related boronsubstituted counterparts, [RTpRR9], with the notion that a more resistant ligand system will be obtained upon elimination of possible degradation reactions involving the B–H bond.Signifi- cantly, the phenyl tris(3-tert-butylpyrazolyl)borato ligand [PhTpBut] may be constructed by heating a mixture of Li[Ph- BH3] 13 and 3-tert-butylpyrazole (ca. 3 equivalents) at ca. 220 8C (Scheme 2).14 Subsequent metathesis of [PhTpBut]Li with Tl(O2CMe) yields the thallium derivative, [PhTpBut]Tl, from which the acid form [PhTpBut]H may be obtained by treatment with H2S (Scheme 2). In contrast to the hydrated derivatives [TpRR9]H?n(H2O) that have been prepared by other methods,15,16 the use of H2S permits the synthesis of an anhydrous material.17 Molecular structures of [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H The molecular structures of [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H, as determined by single crystal X-ray diVraction (Fig. 1–3), exhibit several interesting features. For example, the lithium complex is notable because there are no other structurally characterized [RTpRR9]Li (or even [TpRR9]Li) complexes listed in the Cambridge Structural Database,18 and it exists as a discrete mononuclear species with an uncommon trigonal coordination environment.19 The average Li–N bond length of Scheme 11930 J. Chem. Soc., Dalton Trans., 1999, 1929–1935 1.96 Å (Table 1) is comparable to, though marginally shorter than, the mean value of 2.08 Å for all complexes with Li–N bonds listed in the Cambridge Structural Database.20 It is also shorter than the mean Li–N bond length of 2.06 Å in the tris(3,5-dimethylpyrazolyl)methane derivative, [HC(pzMe2)3]- Li(h3-H3BH).21 Structurally characterized [TpRR9]H derivatives are also rare, and the only examples of which we are aware are the tetrakis- (pyrazolyl) complexes [pzTp]H?(H2O)16 and [(pzMe2)TpMe2]H,22 the latter of which was obtained as a decomposition product in the reaction of Cp2TiCl2 with [TpMe2]H. The structure of [PhTpBut]H, as illustrated in Fig. 3, diVers from that of the lithium derivative [PhTpBut]Li in that the pyrazolyl groups of [PhTpBut]H are not symmetrically disposed with C3v symmetry. The [PhTpBut]H hydrogen atom was both located and refined, and is localized on a single pyrazolyl nitrogen atom [N(12)] with a bond length of 0.84 Å; with a distance of 2.00 Å to the closest nitrogen atom [N(22)] on an adjacent pyrazolyl group, it is evident the N–H group does not participate in a significant hydrogen bonding interaction.In contrast to the paucity of structurally characterized [RTpRR9]Li and [RTpRR9]H complexes, structurally characterized thallium derivatives are numerous.23 Nevertheless, the structure of [PhTpBut]Tl (Fig. 2) is unique amongst [RTpRR9]Tl Fig. 1 Molecular structure of [PhTpBut]Li.Scheme 2 derivatives. Specifically, whereas all other monomeric [RTpRR9]- Tl complexes possess trigonally coordinated thallium, similar to that of the lithium derivative [PhTpBut]Li, the thallium center of [PhTpBut]Tl is principally coordinated to only two of the pyrazolyl groups: the third pyrazolyl group is eVectively rotated by ca. 908 about the B–N bond. Despite the fact that one of the pyrazolyl groups is orthogonal to its position in other [RTpRR9]Tl derivatives, it is evident that there is still an interaction with the thallium center.The interaction is, however, of a fundamentally diVerent nature to those in other [RTpRR9]Tl derivatives for two reasons: (i) the Tl interacts with the nitrogen atom attached directly to the boron, and (ii) the interaction is with the nitrogen p-orbital component of the aromatic psystem of the pyrazolyl nucleus. As a consequence, the unique Tl–N bond length [2.833(2) Å] is distinctly greater than those for the conventional bonds [2.528(3) Å and 2.585(3) Å], which are comparable to the average value in the counterpart without the phenyl substituent, [TpBut]Tl [2.59(1) Å].24 While examples of bidentate [h2-TpRR9]MX derivatives are known, as summarized in Table 2, the two coordinate nature of monomeric [PhTpBut]Tl is of particular interest because such a coordination motif is unknown for thallium in [TpRR9]Tl complexes.Since all other monomeric [TpRR9]Tl complexes with a hydrogen substituent on boron adopt a common trigonal (C3v) h3-coordination geometry, the unusual structure of [PhTpBut]Tl Fig. 2 Molecular structure of [PhTpBut]Tl. Table 1 Selected bond lengths (Å) and angles (8) for [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H [PhTpBut]Li [PhTpBut]Tl [PhTpBut]H M–N(12) M–N(22) M–N(3X)a B–N(11) B–N(21) B–N(31) B–C(41) N(12)–M–N(22) N(22)–M–N(3X)a N(12)–M–N(3X)a N(11)–B–N(21) N(21)–B–N(31) N(11)–B–N(31) C(41)–B–N(11) C(41)–B–N(21) C(41)–B–N(31) 1.977(3) 1.934(3) 1.979(3) 1.572(2) 1.571(2) 1.568(2) 1.615(2) 96.1(2) 97.6(2) 98.1(2) 109.3(2) 109.4(2) 104.5(2) 112.7(2) 107.0(2) 113.8(2) 2.585(3) 2.528(3) 2.833(2) 1.560(4) 1.554(4) 1.558(4) 1.610(4) 71.58(8) 69.35(7) 66.09(8) 110.1(2) 107.7(2) 104.9(2) 110.6(2) 111.1(2) 112.3(2) ——— 1.572(4) 1.554(3) 1.540(4) 1.603(4) ——— 107.6(2) 107.6(2) 106.0(2) 109.8(2) 111.6(2) 114.0(2) a X = 2 for [PhTpBut]Li and [PhTpBut]H; X = 1 for [PhTpBut]Tl.J.Chem. Soc., Dalton Trans., 1999, 1929–1935 1931 is most likely a consequence of the increased steric demands of the phenyl group.Thus, steric interactions between the 5-H substituents and the phenyl group would be expected to destabilize symmetric h3-coordination.25 Rotation about the B–N bond relieves such non-bonding interactions, but at the expense of the Tl–N interaction. The observed structure is, therefore, a compromise of maximizing Tl–N interactions and minimizing steric interactions with the phenyl substituent.Evidently, for [PhTpBut]Tl, the steric interactions must be the dominant component. In contrast, the conventional h3-coordination geometry of the lithium complex [PhTpBut]Li is presumably a result of the Li–N interactions providing the dominant influence. Such a diVerence between [PhTpBut]Tl and [PhTpBut]Li is not unreasonable in view of the fact that twocoordinate thallium complexes, and especially [BpRR9]Tl derivatives, 26 are well known. In contrast, two coordination is not common for lithium.27 The novel coordination geometry of [PhTpBut]Tl has a specific feature in common with polymeric {[FcTp]Tl}x,10 namely that one of the pyrazolyl groups in {[FcTp]Tl}x is also orthogonal to its conventional position; however, in this case the 2-N coordinates to the thallium center of another molecule, thereby resulting in a polymeric structure (Fig. 4). The Fig. 3 Molecular structure of [PhTpBut]H. Fig. 4 Schematic representation of the polymeric nature of the ferrocynyl derivative {[FcTp]Tl}x.“intermolecular” Tl–N bond length of 2.780(5) Å in {[FcTp]Tl}x is only slightly greater than the intramolecular bond lengths [2.638(5) and 2.676(5) Å], but is distinctly shorter than the value of 3.21 Å for the intramolecular interaction involving 1-N from the orthogonal pyrazolyl group (Fig. 4). The latter value for {[FcTp]Tl}x is significantly greater than the corresponding bond length in [PhTpBut]Tl [2.833(2) Å], which therefore provides a good indication that the unconventional interaction between thallium and the orthogonal pyrazolyl group in [PhTpBut]Tl is structurally significant. It is also important to note that for [PhTpBut]Tl the closest intermolecular Tl ? ? ? N interaction is 5.67 Å; thus, the orthogonal pyrazolyl group of [PhTpBut]Tl exhibits no significant interaction with adjacent molecules, and the strictly monomeric nature is presumably a consequence of the bulky tert-butyl substituents.28 Stereochemical nonrigidity of [PhTpBut]Tl Whereas the asymmetric nature of [PhTpBut]Tl in the solid state is certain, the question remains as to how the ligand coordinates in solution.At room temperature, the 1H NMR spectrum of [PhTpBut]Tl reveals only one set of resonances attributable to the tert-butylpyrazolyl moiety. Such an observation is consistent with at least two common possibilities for the solution structure: (i) [PhTpBut]Tl exists as a static symmetric C3v tridentate structure, or (ii) [PhTpBut]Tl exists with the asymmetric structure shown in Fig. 2, but the molecule is stereochemically nonrigid on the NMR spectroscopic timescale. In order to Table 2 Representative examples of structurally characterized complexes with bidentate [RTpRR9] ligands Compound Ref. [h2-TpBut]AlEt2 [h2-TpMe2]Ga(Me)X (X = Me, Cl) [h2-pzTp][pzTp]GaMe [h2-TpMe2][TpMe2]Sn [h2-TpMe2]V(O)Cl2 [h2-TpMe2]Rh(CNR)2 (R = 2,6-Me2C6H3, CH2CMe3) [h2-TpMe2]Rh(CO)(PMe3) [h2-Tp]2Pd [h2-Tp]Pt(Me)CO {[h2-TpMe2]PtMe2}{PPN} [h2-pzTp]Cu(PPh3)2 [h2-TpPh2]2Zn [h2-pzTp]2Pb [h2-pzTp]HgSMe [h2-Tp][Tp]2Yb T1 T2 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T1 M.H. Chisholm, N. W. Eilerts and J. C. HuVman, Inorg. Chem., 1996, 35, 445. T2 D. L. Reger and Y. Ding, Organometallics, 1993, 12, 4485. T3 A. H. Cowley, R. L. Geerts, C. M. Nunn and C. J. Carrano, J. Organomet. Chem., 1988, 341, C27. T4 E. Kime-Hunt, K. Spartalian, M. DeRusha, C. M. Nunn and C. J. Carrano, Inorg. Chem., 1989, 28, 4392.T5 W. D. Jones and E. T. Hessel, Inorg. Chem., 1991, 30, 778. T6 V. Chauby, C. S. Le Berre, P. Kalck, J.-C, Daran and G. Commenges, Inorg. Chem., 1996, 35, 6354. T7 A. J. Canty, N. J. Minchin, L. M. Engelhardt, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1986, 645. T8 P. E. Rush and J. D. Oliver, J. Chem. Soc., Chem. Commun., 1974, 996. T9 D. D. Wick and K. I. Goldberg, J. Am. Chem. Soc., 1997, 119, 10235. T10 P. Cecchi, B. Bovio, G. G.Lobbia, C. Pettinari and D. Leonesi, Polyhedron, 1995, 14, 2441. T11 F. Hartmann, W. Klaüi, A. Kremer-Aach, D. Mootz, A. Strerath and H. Z. Wunderlich, Z. Anorg. Allg. Chem., 1993, 619, 2071. T12 D. L. Reger, M. F. HuV, A. L. Rheingold and B. S. Haggerty, Inorg. Chem., 1992, 114, 579. T13 S. Aime, G. Digilio, R. Gobetto, P. Cecchi, G. G. Lobbia and M. Camalli, Polyhedron, 1994, 13, 2695. T14 M. V. R. Stainer and J. Takats, Inorg. Chem., 1982, 21, 4050.1932 J. Chem. Soc., Dalton Trans., 1999, 1929–1935 address these possibilities, variable temperature 1H NMR spectroscopic studies were conducted.Significantly, upon lowering the temperature to 280 8C, decoalescence was observed (Fig. 5), giving rise to three sets of resonances in the ratio ca. 2 : 2 : 1 (as most clearly seen by examination of the tert-butyl region). This pattern, however, is not consistent with the solid state structure of [PhTpBut]Tl, which would result in three sets of resonances in the ratio 1:1:1 corresponding to the three inequivalent tert-butylpyrazolyl groups (note that the two coordinated pyrazolyl groups of a static structure are not chemically equivalent, but are diastereotopically related).Alternatively, if the chemical shifts of the diastereotopically related pyrazolyl groups are coincidentally the same, or a mechanism exists which allows for their facile interconversion, then a 2 : 1 pattern of pyrazolyl groups would be expected. It is, therefore, evident that the experimentally observed 2:2:1 pattern is not consistent with the presence of a single chemical species at low temperature; the presence of a second species is thus required to generate the observed low temperature 1H NMR spectrum.Excellent support for the presence of a second species is provided by 205Tl NMR spectroscopy: specifically, at low temperature (ca. 270 8C), the 205Tl NMR spectrum exhibits two resonances (Fig. 6). Furthermore, these two signals coalesce upon warming to room temperature, clearly indicating that the two species are isomers of each other.The 2:2:1 distribution of pyrazolyl groups in the 1H NMR spectrum can, therefore, be rationalized in terms of two species, A3:0 and B2:1 (Scheme 3), which possess equivalent and inequivalent (in the ratio 2 : 1) sets of pyrazolyl groups, respectively. Thus, if the ratio of A3:0 to B2:1 is 2 : 3,29 the composite 1H NMR spectrum would show Fig. 5 Variable temperature 1H NMR spectra of [PhTpBut]Tl in toluene (* = residual protio solvent).pyrazolyl groups in the ratio 6 : [6:3], i.e. 2:2:1. Assuming this hypothesis to be correct, the important issue is concerned with identifying the nature of A3:0 and B2:1. At least two possible scenarios emerge. (a) Scenario I. A3:0 and B2:1 correspond to isomers related by interchange of the uncoordinated pyrazolyl and phenyl substituents (Scheme 3). These isomers may be described as “axial” and “equatorial”, with the descriptors denoting the position of the pyrazolyl group relative to the boat configuration of the six-membered [BN4Tl] ring.Of these two structures, the axial isomer may be identified as A3:0 and the equatorial isomer as B2:1. The rationale for these assignments is based on the notion that the axial isomer is more likely to exhibit equivalent pyrazolyl groups on the NMR spectroscopic timescale than is the equatorial isomer. Specifically, rotation about the B–N bond of the axial isomer allows access to a symmetric tridentate species, thereby enabling facile exchange of all three pyrazolyl groups. In contrast, rotation about the B–N bond in the equatorial isomer (B2:1) does not result in chemical exchange of the pyrazolyl groups, which therefore retain a 2 : 1 pattern.For this scenario, the interchange of axial (A3:0) and equatorial (B2:1) isomers corresponds to flipping the boat configuration of the six-membered [BN4Tl] ring.(b) Scenario II. A second possibility is that A3:0 corresponds to a symmetric h3-structure, and B2:1 corresponds to the h2- axial isomer as observed in the solid state.30 The principal diVerence between the structure corresponding to B2:1 in Scenario II and that of A3:0 in Scenario I is that in Scenario I it is highly fluxional. Of these two scenarios, we consider the first to be the more reasonable since it makes no assumptions concerning coincidental chemical shifts of the diastereotopic pyrazolyl groups for B2:1.Thus, as summarized in Scheme 3, the proposed identity of B2:1 is the equatorial bidentate isomer, while A3:0 is the axial isomer in rapid equilibrium with the tridentate species. Precedent for this suggestion is provided by Venanzi’s study of stereochemical nonrigidity within complexes of the types [TpRR9]Rh(CO)2 and [TpRR9]Rh(L2) (L2 = cyclooctadiene, nor- Fig. 6 Variable temperature 205Tl NMR spectra of [PhTpBut]Tl in toluene.J.Chem. Soc., Dalton Trans., 1999, 1929–1935 1933 bornadiene), each of which exhibits magnetically equivalent pyrazolyl groups at room temperature.31 IR spectroscopic studies on the carbonyl complexes demonstrated that the solutions consisted of three diVerent species: (i) two fourcoordinate complexes with bidentate [TpRR9] ligands, diVering according to whether the uncoordinated pyrazolyl group was in an axial or equatorial position, and (ii) a five-coordinate complex with tridentate [TpRR9] coordination.The relative amounts of these three species depended upon both the nature of the [TpRR9] ligand and the solvent. In one specific example, [Tp32Pri,4-Br]Rh(CO)2 was observed to decoalesce to a “symmetric” and asymmetric species. IR spectroscopy demonstrated that both species were four coordinate, so that the “symmetric” Scheme 3 species was proposed to be stereochemically nonrigid and bidentate rather than a complex with tridentate coordination. As a further illustration that magnetic equivalence of the three pyrazolyl groups in [h2-TpRR9]MX complexes does not require species with tridentate coordination to be present in detectable concentrations, Jones reported that the equivalence of the pyrazolyl groups in [h2-TpMe2]Rh(CNR) (R = 2,6-Me2C6H3, CH2CMe3) is a consequence of facile chemical exchange of coordinated and uncoordinated pyrazolyl groups, with a symmetric tridentate species having no detectable concentration as determined by IR spectroscopy.32 Conclusions In summary, the phenyl substituted tris(3-tert-butylpyrazolyl)- borato complexes, [PhTpBut]M (M = Li, Tl, H) have been synthesized and structurally characterized by X-ray diVraction.In contrast to other monomeric [TpRR9]Tl derivatives, which exhibit symmetric tridentate coordination of the tris(pyrazolyl)- borate ligand, [PhTpBut]Tl exhibits an unprecedented structure in which one of the tert-butylpyrazolyl groups is rotated by ca. 908 and the Tl interacts with the nitrogen attached directly to the boron via a p-orbital component of the aromatic p-system of the pyrazolyl nucleus. [PhTpBut]Tl is stereochemically nonrigid on the NMR spectroscopic timescale in solution at room temperature, equilibrating between isomers which diVer according to whether the uncoordinated pyrazolyl group is located axially or equatorially relative to the six-membered [BN4Tl] ring. Experimental General considerations All manipulations were performed using a combination of glovebox, high-vacuum or Schlenk techniques.33 Solvents were purified and degassed by standard procedures.Commercially available reagents were not further purified. All glassware was oven dried prior to use. NMR spectra were recorded on Bruker Avance 300wb DRX, Bruker Avance 400 DRX, and Bruker Avance 500 DMX spectrometers. 1H and 13C chemical shifts are reported in ppm relative to SiMe4 (d = 0) and were referenced internally with respect to the protio solvent impurity or the 13C resonances, respectively; 205Tl NMR spectra were recorded on a Bruker Avance 300wb DRX (173.393 Hz) instrument and are referenced relative to aqueous TlNO3 (extrapolated to infinite dilution; d = 0.00),34,35 using an external solution of aqueous Tl(O2CMe) as calibrant.All coupling constants are reported in Hz. C, H, and N elemental analyses were measured using a Perkin-Elmer 2400 CHN Elemental Analyzer. Synthesis of [PhTpBut]Li A rapidly stirred mixture of Li[PhBH3] (5.82 g, 59.5 mmol) and HpzBut(22.2 g, 0.18 mol) was heated at ca. 220 8C under N2 until evolution of H2 ceased (ca. 1 h). The product was allowed to cool to room temperature and extracted into pentane (ca. 200 mL). The mixture was filtered and the volatile components removed in vacuo giving [PhTpBut]Li as a white solid (27.0 g, 98%). The product may be crystallized from pentane at 218 8C over a period of several days (Calc. for [PhTpBut]Li: C, 69.8; H, 8.3; N, 18.1.Found: C, 69.6; H, 9.5; N, 18.9). NMR spectroscopic data are listed in Table 3. Synthesis of [PhTpBut]Tl A mixture of [PhTpBut]Li (9.5 g, 20 mmol) and Tl(O2CMe) (8.1 g, 31 mmol) in THF (50 mL) was stirred at room temperature for 1 day. After this period, the mixture was filtered and the volatile components were removed in vacuo giving [PhTpBut]Tl as an oV-white solid (5.97 g, 45%). Pure [PhTpBut]Tl was1934 J. Chem. Soc., Dalton Trans., 1999, 1929–1935 Table 3 NMR spectroscopic data for [ThTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H Assignment [PhTpBut]Li a [PhTpBut]Tl [PhTpBut]H 1H (C6D6) C(CH3)3 C3N2H2 C6H5 NH 13C (C6D6) C(CH3)3 C(CH3)3 C3N2H2 BC6H5 1.31, s 5.91, d, 3JH–H = 2 7.96, br 7.24–7.55 — 31.0, q, 1JC–H = 126 32.1, s 100.3, d, 1JC–H = 173 136.5, d, 1JC–H = 181 161.8, s 128.0, d, 1JC–H = 158 (3C) 135.6, 1JC–H = 159 (2C) (Cipso not observed) 1.32, s 6.04, d, 3JH–H = 2 7.66, br 7.12–7.20 — 31.7, q, 1JC–H = 126 32.4, s 101.4, d, 1JC–H = 172 137.2, d, 1JC–H = 184 163.7, s 128.6, d, 1JC–H = 162 (3C) b 135.4, d, 1JC–H = 157 (2C) 148.5, br (Cipso) 1.24, s 5.94, d, 3JH–H = 2 7.29, d, 3JH–H = 2 7.20–7.25 Not observed 30.6, q, 1JC–H = 126 32.0, s 101.5, d, 1JC–H = 174 136.6, 1JC–H = 186 161.1, s 127.7, d, 1JC–H = 156 (3C) b 133.2, d, 1JC–H = 156 (2C) (Cipso not observed) a The NMR signals in the presence of additional tert-butylpyrazole are perturbed from the values listed here, presumably due to the formation of an adduct. b An HMQC experiment indicates that the meta and para resonances are coincident with each other.obtained as colorless crystals by repeated crystallization from pentane at 215 8C (Calc. for [PhTpBut]Tl: C, 49.0; H, 5.8; N, 12.7. Found: C, 49.3; H, 5.7; N, 13.4). NMR spectroscopic data are listed in Table 3. Synthesis of [PhTpBut]H A solution of [PhTpBut]Tl (220 mg, 0.33 mmol) in benzene was treated with H2S resulting in the formation of a black precipitate. The mixture was filtered and the volatile components were removed in vacuo giving [PhTpBut]H as a white powder, from which colorless crystals were obtained by crystallization from pentane (ca. 1 mL) at 215 8C (66 mg, 43%) (Calc. for [PhTpBut]H: C, 70.7; H, 8.6; N, 18.3. Found: C, 71.0; H, 8.4; N, 17.4). NMR spectroscopic data are listed in Table 3. Crystal structure determinations Crystal data, data collection and refinement parameters for [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H are summarized in Table 4.X-Ray diVraction data were collected on a Bruker P4 diVractometer equipped with a SMART CCD detector. The structures were solved using direct methods and standard Table 4 Crystal, intensity collection and refinement data. [PhTpBut]Li [PhTpBut]Tl [PhTpBut]H Lattice Formula Formula weight Space group a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 ZT /K l/Å rcalc/g cm23 m(Mo-Ka)/mm21 q max/8 No. of data No. of parameters R1 a wR2 a S Monoclinic C27H38BN6Li 464.38 C2/c (no. 15) 18.6329(10) 16.1563(9) 18.7233(10) 90.454(1) 5636.3(5) 8 203 0.71073 1.095 0.065 28.3 6413 326 0.0518 0.1185 1.045 Monoclinic C27H38BN6Tl 661.81 P21/c (no. 14) 15.2499(7) 13.8902(7) 15.3951(7) 117.055(1) 2904.2(2) 4 203 0.71073 1.514 5.586 28.3 6600 326 0.0279 0.0585 1.044 Triclinic C27H39BN6 458.45 P1� (no. 2) 11.6639(8) 12.1658(8) 12.2480(8) 114.003(1) 101.409(1) 109.440(1) 1380.7(2) 2 173 0.71073 1.103 0.066 28.3 5727 321 0.0740 0.1610 1.070 a R1 = S|Fo| 2 |Fc|/S|Fo| for [I > 2s(I)]; wR2 = {S[w(Fo 2 2 Fc 2)2]/S [w(Fo 2)2]}1/2 for [I > 2s(I)].diVerence map techniques, and were refined by full-matrix least-squares procedures using SHELXTL.36 Hydrogen atoms on carbon were included in calculated positions. Systematic absences for [PhTpBut]Li were consistent with Cc (no. 9) and C2/c (no. 15), of which a satisfactory solution was obtained in the centrosymmetric alternative C2/c (no. 15). Systematic absences for [PhTpBut]Tl were consistent uniquely with P21/c (no. 14). Systematic absences for [PhTpBut]H were consistent with P1 (no. 1) and P1� (no. 2), of which a satisfactory solution was obtained in the centrosymmetric alternative, P1� (no. 2). CCDC reference number 186/1440. See http://www.rsc.org/suppdata/dt/1999/1929/ for crystallographic files in .cif format. Acknowledgements We thank the National Science Foundation (CHE 96-10497) for support of this research. Drs. JeVrey B. Bonanno and Peter Desrosiers are thanked for helpful suggestions concerning the variable temperature NMR studies, and Drs.Jun Ho Shin and Brian Bridgewater are thanked for helpful assistance. The referees are thanked for their helpful comments. Notes and 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; ( 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[TpRR9] 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,c p-C6H4Brc) have been prepared by reaction of RB(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.J. Chem. Soc., Dalton Trans., 1999, 1929–1935 1935 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)BpBut]Li(ButpzH) 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 [TpRR9]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 [TpRR9]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. PiVeri 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. RatcliV 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, pp. 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. HuV, 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 [PhTpBut]Tl is 6.68 Åthat there is also no weak Tl ? ? ? Tl interaction of the type that has been suggested for other [TpRR9]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 270 8C). In view of the potentially diVerent relaxation times for the diVerent 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, B2:1 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. Paper 9/01875A