NOTE pubs.acs.org/Organometallics
Novel 11-Vertex, 11-Skeletal Electron Pair Tantalaborane of Unusual Shape Shubhankar Kumar Bose and Sundargopal Ghosh* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
bS Supporting Information ABSTRACT: This work reports the synthesis and structural characterization of a hydrogen-rich tantalaborane cluster, [(Cp*Ta)(Cp*TaCl)B9H16] (2, Cp* = η5-C5Me5), obtained from the thermolysis of [BH3 3 thf] and an in situ generated intermediate, produced from the reaction of [Cp*TaCl4], 1, and [LiBH4 3 thf]. Cluster 2 has an 11-vertex nido-cage geometry. The core geometry of 2 can be derived from an icosahedron by removing the apical five-coordinate vertex. The title cluster is a rare example of an electronically unsaturated metallaborane containing four TaHB and three BHB bonds.
’ INTRODUCTION Metallaborane compounds, in particular the earlier transition metals, present interesting challenges to the well-established cluster electron-counting rules.1 4 Although the electron-counting rules suggest a wide variety of target metallaborane compositions, they provided no direct information on their synthesis. The answer to overcoming the inherent instability of the M B bond network relative to a mixture of boranes and metal complexes is to lower all free energy barriers in the preparative pathway. The successful approach of Fehlner utilized the formation of metal polyborohydrides either from metathesis of metal halogen bonds with metal borohydrides or by M X, B H bond metathesis with neutral boranes.5 The H2 elimination from the metal polyborohydride leads to metallaboranes in preference to borane elimination that produces metal hydrides. This approach has been successful for metallaboranes containing a variety of group 6 9 metals having a pentamethylcyclopentadienyl ligand.6 In the same way, we applied the above approach, with some modifications, to group 5 metals and discovered a number of interesting metallaborane compounds having novel cage geometries and unusual structural features of interest.7,8 Interestingly, these results as well as the existence of higher nuclearity closorhenaboranes9 and hydrogen-rich nido-ruthenaboranes,10 led us to revisit the Ta system that yielded hypoelectronic tantalaborane [(Cp*Ta)(Cp*TaCl)B9H16], 2. Reported here is the synthesis and structural characterization of tantalaborane 2. ’ RESULTS AND DISCUSSION As previously reported, the reaction of 1 with monoborane reagents (LiBH4 and BH3 3 thf) resulted in [(Cp*Ta)2(B2H6)2], [(Cp*TaCl)2B5H11], and [(Cp*Ta)2B5H11] (Scheme 1).7 Minor product 2, with an Rf higher than [(Cp*TaCl)2B5H11], was enhanced by using excess [BH3 3 thf] and reduced temperature r 2011 American Chemical Society
and time. Although compound 2 is produced in a mixture, these compounds can be separated by preparative thin-layer chromatography (TLC), allowing the characterization of pure materials. The FAB mass spectrum gave a molecular ion peak corresponding to C20H46B9Cl1Ta2, while the IR spectrum displayed bands at 2506 and 2484 cm 1, characteristic of terminal B H stretches. The 11B NMR spectrum of 2 displays seven signals in the ratio 1:1:1:2:1:2:1, distributed over an unusually large chemical shift range of ca. 100 ppm. Besides the BH terminal protons, three B H B and four Ta H B protons with an equal intensity were observed. Furthermore, 1H and 13C NMR spectra imply two inequivalent Cp* ligands. The variable-temperature 1H{11B} and 11B{1H} NMR study revealed no fluxional behavior associated with Ta H B or B H B bonding. In order to confirm the spectroscopic assignments and determine the crystal structure of 2, an X-ray analysis was undertaken. Crystals suitable for X-ray diffraction studies were grown by cooling a concentrated hexane solution to 10 °C. The crystal structure of 2 corresponds to discrete molecules of [(Cp*Ta)(Cp*TaCl)B9H16] separated by normal van der Waals distances. As shown in Figure 1, the molecular structure of 2 can be viewed as a nido structure based on a closo icosahedron. The core geometry of 2 displays an open structure such that the overall structure may be described as bowl-like geometry, an unanticipated form in metallaborane chemistry. The Ta1 Ta2 separation in 2 (3.2556(16) Å) is significantly longer than that found in [(Cp*Ta)2B5H11] (2.9261 Å).7a This may be due to the position of the tantalum atoms in the opencage cluster. The average Ta B bond length of 2.396 Å is about 0.1 Å longer than the other tantalaboranes.7 The boron boron distances about the open face of the cage of 2 (B4 B6, B6 B9, Received: June 14, 2011 Published: August 11, 2011 4788
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Scheme 1. Synthesis of 2 and Other Tantalaborane Clusters
Figure 1. Molecular structure and labeling diagram for 2. Thermal ellipsoids are shown at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ta1 Ta2 3.2556(16), Ta1 B1 2.408(15), Ta2 B2 2.310(15), Ta2 B7 2.484(16), B4 B5 1.79(2), B4 B6 1.94(2), B6 B7 1.79(2), B6 B9 1.78(3), B7 B9 1.78(3), Ta1 Cl1 2.473(4); Ta1 B3 Ta2 85.3(5), B3 Ta2 B7 84.3(5), B3 B5 B7 135.2(13), Ta2 B7 B9 110.8(11), B5 B6 B9 112.9(12).
and B9 B8) range from 1.79(3) to 1.94(2) Å (average 1.836 Å) and are similar to those of an 11-vertex ruthenaborane, [exoBH2(Cp*Ru)2B9H14]10b (average 1.852 Å). Although all of the hydrogen atoms were not located in the X-ray diffraction study, evidence for their presence has been unequivocally supported by 1 H NMR spectroscopy.
The 1H and 11B NMR spectra are consistent with the solid-state structure of 2. The 11B NMR spectrum exhibits seven resonances, at δ = 74.0, 39.6, 26.4, 18.7, 17.0, 15.3, and 33.3 ppm, in the ratio 1:1:1:2:1:2:1. The 1H NMR of 2 is somewhat unusual, but it was unraveled with a 1H{11B}/11B{1H} HSQC experiment. The HSQC experiment showed two of the four Ta H B protons to be coupled exclusively to a pair of equivalent boron atoms appearing at δ = 18.7 ppm and thus can be assigned to the open face boron atoms B4 and B8 (Figure 1). The other two Ta H B protons are coupled to the unique boron atom B1 (δ = 17.0 ppm), whereas the three B H B protons are associated to a pair of equivalent boron atoms appearing at δ = 15.3 ppm and can be assigned to the open face boron atoms B6 and B9. Metallaborane 2 is the new example of a polyhedral boroncontaining compound that is similarly anomalous to other 11vertex metallaboranes and may therefore shed additional light on the structure. A set of three metallaborane types, I III, containing a M2B9 framework of group 6 10 metals is shown in Chart 1. They are [(Cp*Re)2B9H9],11 I, [exo-BH2(Cp*M)2B9H14] (M = Ru, Re),10b II, and [(μ-PPh2)(PPh3)2Pt2B9H6Cl(OiPr)2] or [(μ-PPh2)(PPh3)2Pt2B9H6(OiPr)3], III.12 The observed geometry of I is different from that of the canonical deltahedra of [B11H11]2 ; however this can be related to the canonical ones by diamond square diamond (dsd) rearrangements.13 The isostructural metallaborane type II with an exopolyhedral BH2 can be generated from a 13-vertex docosahedron by removal of two vertices of connectivity five and six. The diplatinaundecarborane cluster type III, on the other hand, exhibits a nido 11-vertex {Pt2B9} polyhedral skeleton with two Pt atoms in neighboring positions of the open Pt2B3 face.12 The molecular structure of these compounds reveal that they have different geometries despite having the same number of borane fragments. Cluster 2 may thus be considered as the next member of this series containing a M2B9 framework. The structure of 2 is not routine and presents an interesting problem in the context of known 11-atom clusters. As shown at 4789
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Chart 1. Eleven-Vertex Metallaboranesa
a
Bridging hydrogen atoms are not shown for clarity.
Figure 2. Generation of the observed geometry of 2 from an icosahedron by removing the apical 5-connect vertex followed by one dsd rearrangement (bold lines) and finally removal of two connections (dashed lines).
Figure 2, cluster 2 exhibits a shape that can be derived from a 12vertex icosahedron by removing one apical five-connected vertex and performing one dsd rearrangement, thereby increasing the connectivity of one of the tantalum sites (Ta2) to six and decreasing the connectivities of the two boron sites correspondingly.14,15 As a result, it has a nido geometry with 11 skeletal electron pairs (sep) rather than the prescribed 13 sep. Therefore, it is hypoelectronic,16 i.e., fewer valence electrons than apparently are required by its geometric structure. Alternatively, the observed geometry of 2 may also be generated from an octadecahedron by performing two dsd rearrangements followed by removal of four edges (Figure S1, Supporting Information). Compound 2 is particularly significant, as it demonstrates systematic and large deviations from the cluster shape/electron count principle, which has proven satisfactory for understanding the structures of a majority of borane and transition metal clusters.17 Cluster 2 shows that the formal sep can vary extensively from the canonical number, and, as it decreases, the cluster shape deviates from that of a most spherical deltahedron. The low yield of 2 and lack of information concerning the other reaction product(s) make it difficult to study the mechanism of formation of this unusual compound. However, the observed structure provides some hints about how 2 might form. Note that 2 contains a {(Cp*Ta)2B5Cl} core, which is similar to that of known tantalaborane [(Cp*TaCl)2B5H11].7a Thus, formation of 2 could be based on the cluster expansion by borane addition external to the cluster framework of [(Cp*TaCl)2B5H11].
’ CONCLUSION The structural characterization of 2 provides the first example of an open-cage hypoelectronic 11-vertex tantalaborane that possesses a nido geometry based on icosahedron geometry.
Detailed investigation of the analogous group 6 8 metal chloride systems, reported to date, revealed only a limited number of cluster parallels. This example demonstrates that an earlier transition metal fragment creates electronic requirements that are articulated in structural features and not formerly observed for metallaboranes of groups 6 9.
’ EXPERIMENTAL SECTION General Procedures and Instrumentation. All the operations were conducted under an Ar/N2 atmosphere by using standard Schlenk techniques or an inert-atmosphere glovebox. Solvents were distilled prior to use under argon. [Cp*TaCl4], [BH3 3 thf], and [LiBH4 3 thf], (Aldrich) were used as received. The external reference [Bu4N(B3H8)] for the 11B NMR was synthesized with the literature method.18 Thin-layer chromatography was carried on 250 mm diameter aluminum-supported silica gel TLC plates (Merck TLC plates). NMR spectra were recorded on a 400 and 500 MHz Bruker FT-NMR spectrometer. Residual solvent protons were used as reference (δ, ppm, CDCl3, 7.26), while a sealed tube containing [Bu4N(B3H8)] in [D6]benzene (δB, ppm, 30.07) was used as an external reference for the 11B NMR. Infrared spectra were recorded on a Nicolet 6700 FT spectrometer. Microanalyses for C and H were performed on a Perkin Elimer Instruments Series II model 2400. Mass spectra were obtained on a Jeol SX 102/Da-600 mass spectrometer with argon/xenon (6 kV, 10 mÅ) as FAB gas. Synthetic Procedure for 2. In a flame-dried Schlenk tube, [Cp*TaCl4] (0.12 g, 0.26 mmol) was suspended in toluene (10 mL) and cooled to 70 °C, [LiBH4 3 thf] (0.8 mL, 1.56 mmol) was added via syringe, and the reaction mixture was allowed to warm slowly over 30 min to room temperature and left stirring for an additional hour. The solvent was evaporated under vacuum, the residue was extracted into hexane, and filtration afforded an extremely air- and moisture-sensitive deep blue intermediate. The filtrate was concentrated and a toluene solution (12 mL) of the intermediate was pyrolyzed in the presence of excess 4790
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Organometallics [BH3 3 thf] (2.6 mL, 2.6 mmol) at 75 °C for 15 h. The solvent was evaporated, and the residue was extracted into hexane and passed through Celite. After removal of solvent, the residue was subjected to chromatographic workup using silica gel TLC plates. Elution with a hexane/CH2Cl2 (80:20 v/v) yielded red 2 (0.011 g, 5%), Rf = 0.44; [(Cp*Ta)2(B2H6)2] (0.043 g, 24%), Rf = 0.75; [(Cp*Ta)2B5H11] (0.05 g, 27%), Rf = 0.72; and [(Cp*TaCl)2B5H11] (0.024 g, 12%), Rf = 0.14. 11B NMR (22 °C, 128 MHz, CDCl3): δ 74.0 (d, 1B), 39.6 (d, 1B), 26.4 (br, 1B), 18.7 (br, 2B), 17.0 (br, 1B), 15.3 (br, 2B), 33.3 (d, 1B). 1H NMR (22 °C, 400 MHz, CDCl3): δ 6.35 (partially collapsed quartet (pcq), 1BHt), 4.97 (pcq, 2BHt), 4.62 (pcq, 2BHt), 4.54 (pcq, 1BHt), 4.18 (pcq, 1BHt), 4.08 (pcq, 1BHt), 3.96 (pcq, 1BHt), 2.14 (s, 15H; Cp*), 1.91 (s, 15H; Cp*), 0.54 (br, 1 B H B), 1.16 (br, 1 B H B), 1.25 (br, 1 B H B), 2.54 (br, 1 Ta H B), 4.17 (br, 1 Ta H B), 4.69 (br, 1 Ta H B), 6.93 (br, 1 Ta H B). 13C NMR (22 °C, 100 MHz, CDCl3): δ 115.0 (s; C5(CH3)5), 111.4 (s; C5(CH3)5), 14.3 (s; C5(CH3)5), 11.7 (s; C5(CH3)5). IR (hexane) ν/cm 1: 2506w, 2484w (BHt). MS (FAB) P+(max): m/z (%): 781. Anal. (%) calcd for C20H46B9Cl1Ta2: C, 30.75; H, 5.93. Found: C, 31.51; H, 6.21. X-ray Structure Determination. The crystal data for 2 were collected and integrated using a Bruker AXS kappa apex2 CCD diffractometer, with graphite-monochromated Mo KR (λ = 0.71073 Å) radiation at 173 K. The structures were solved by heavy atom methods using SHELXS-97 or SIR9219 and refined using SHELXL-97 (Sheldrick, G. M., University of G€ottingen).20,21 The molecule 2 is disorder in two position with 90% and 10% occupancy. For the 10% component only tantalum and chlorine atoms could be located and refined. The electron densities of the other atoms were too weak to be located and refined. Crystal data for 2: C20H46B9ClTa2, Mr = 781.21 g/mol, monoclinic, space group P21/n, a = 12.1533(4) Å, b = 16.3984(6) Å, c = 14.3437(5) Å, β = 100.4750(10)°, V = 2810.98(17) Å3, Z = 4, Fcalcd = 1.846 g/cm3, final R indices [I > 2σ(I)] R1 = 0.0548, wR2 = 0.0929, index ranges 12 e h e 13, 18 e k e 18, 14 e l e 16, θ range for data collection 1.90 24.11°, crystal size 0.18 0.15 0.13 mm3, reflections collected 16 668, independent reflections 4410, Rint = 0.0634, goodness-of-fit on F2 1.143.
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(4) (a) Fehlner, T. P. In Inorganometallic Chemistry; Plenum: New York, 1992. (b) Weller, A. S.; Aldridge, S.; Fehlner, T. P. Inorg. Chim. Acta 1999, 289, 85–94. (c) Fehlner, T. P. J. Chem. Soc., Dalton Trans. 1998, 1525–1532. (5) Fehlner, T. P. Proc. Indian Natl. Sci. Acad. A 2002, 68, 579–596. (6) Fehlner, T. P. Organometallics 2000, 19, 2643–2651. (7) (a) Bose, S. K.; Geetharani, K.; Varghese, B.; Mobin, Sk. M.; Ghosh, S. Chem.—Eur. J. 2008, 14, 9058–9064. (b) Bose, S. K.; Geetharani, K.; Ramkumar, V.; Mobin, Sk. M.; Ghosh, S. Chem.—Eur. J. 2009, 15, 13483–13490. (8) (a) Bose, S. K.; Geetharani, K.; Varghese, B.; Ghosh, S. Inorg. Chem. 2010, 49, 6375–6377. (b) Bose, S. K.; Geetharani, K.; Varghese, B.; Ghosh, S. Inorg. Chem. 2011, 50, 2445–2449. (9) Le Guennic, B.; Jiao, H.; Kahlal, S.; Saillard, J.-Y.; Halet, J.-F.; Ghosh, S.; Beatty, A. M.; Rheingold, A. L.; Fehlner, T. P. J. Am. Chem. Soc. 2004, 126, 3203–3217. (10) (a) Ghosh, S.; Beatty, A. M.; Fehlner, T. P. Angew. Chem., Int. Ed. 2003, 42, 4678–4680. (b) Ghosh, S.; Noll, B. C.; Fehlner, T. P. Angew. Chem., Int. Ed. 2005, 44, 2916–2918. (11) Ghosh, S.; Shang, M.; Li, Y.; Fehlner, T. P. Angew. Chem., Int. Ed. 2001, 40, 1125–1128. (12) Dou, J.; Wu, L.; Guo, Q.; Li, D.; Wang, D. Eur. J. Inorg. Chem. 2005, 63–65. (13) Williams, R. E. Inorg. Chem. 1971, 10, 210–214. (14) Johnston, R. L.; Mingos, D. M. P.; Sherwood, P. New J. Chem. 1991, 15, 831–841. (15) Fehlner, T. P. Adv. Chem. 2002, 822, 49. (16) Corbett, J. D. Struct. Bonding (Berlin) 1997, 87, 157–193. (17) Kennedy, J. D. In The Borane, Carborane, Carbocation Continuum; Casanova, J., Ed.; Wiley: New York, 1998; p 85. (18) Ryschkewitsch, G. E.; Nainan, K. C. Inorg. Synth. 1974, 15, 113–114. (19) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26, 343–350. (20) Sheldrick, G. M. SHELXS-97; University of G€ottingen: Germany, 1997. (21) Sheldrick, G. M. SHELXL-97; University of G€ottingen: Germany, 1997.
’ ASSOCIATED CONTENT
bS
Supporting Information. CIF file for 2. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Generous support of the Indo-French Centre for the Promotion of Advanced Research (IFCPAR-CEFIPRA), No. 4405-1, New Delhi, is gratefully acknowledged. S.K.B. thanks the University Grants Commission (UGC), India, for a Senior Research Fellowship. We thank V. Ramkumar for X-ray crystallography analysis. We would like to thank SAIF, IIT Madras for HSQC and variable-temperature NMR. ’ REFERENCES (1) Wade, K. Inorg. Nucl. Chem. Lett. 1972, 8, 559–563. (2) Wade, K. Adv. Inorg. Chem. Radiochem. 1976, 18, 1–66. (3) (a) Mingos, D. M. P. Nat. (London) Phys. Sci. 1972, 236, 99–102. (b) Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry; Prentice Hall: New York, 1990. 4791
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