Gas-phase and solution n.m.r. spectral studies
of beryllium borohydride, Be(BH4)2, indicate that it
contains magnetically equivalent BH4 groups which
undergo rapid internal hydrogen exchange; previously
postulated triangular boron-beryllium-boron molecular
configurations are inconsistent with the n.m.r. data

NC DOCKS at The University of North Carolina at Greensboro

Walsh, Jerome "Jerry"

English

The University of North Carolina at Greensboro

224 J.C.S. CHEM. COMM., 1977 Gas-phase Nuclear Magnetic Resonance Spectroscopic Study of the Molecular Structure of Beryllium Borohydride, Be(BH,), By DONALD F. GAINES,* JERRY L. WALSH, and DAVID F. HILLENBRAND (Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706) Summary Gas-phase and solution n.m.r. spectral studies of beryllium borohydride, Be(BH,),, indicate that i t  contains magnetically equivalent BH, groups which undergo rapid internal hydrogen exchange ; previously postulated triangular boron-beryllium-boron molecular configurations are inconsistent with the n.m.r. data. THE gas-phase structure of beryllium borohydride, Be- (BH4)2, has been a subject of some controversy. As many as nine different structures have been proposed for the gas-phase monomer1 with varying degrees of supporting evidence based primarily on vibrational,le5 electron diffraction,6-10 and dipole moment5 ,l1 studies. Several theoretical studies have also been undertaken.l2-l6 The solid-state structure of Be(BH,), has been shown to be a helical polymer by a single-crystal X-ray diffraction study. l 7  In spite of these efforts, unambiguous experimental evidence defining the gas-phase structure of Be(BH,), has not been reported. We report here the results of ambient gas-phase and solution l1B and lH n.m.r. spectral studies of Be(BH,),. Beryllium borohydride was prepared by the method of Schlesinger, Brown, and Hydel* and was handled using standard high-vacuum techniques. llB and lH n.m.r. spectra (86.7 and 270 MHz, respectively) of Be(BH,), were obtained on a Bruker WH-270 F.T.n.m.r. spectrometer. For the gas-phase spectra, the boron probe was modified to B2H6 and/or argon] to effect collision narrowing of the spectral lines. Solution-phase spectra were obtained using [2H,]- benzene and [2H,]dichloromethane solvents. The gas-phase llB and lH n.m.r. spectra of Be(BH,),, shown in Figure 1, indicate the presence of a single species. The llB spectrum consists of a 1 : 4: 6: 4 : 1 quintet that  arises from equivalent boron atoms, each of which is coupled to four equivalent hydrogen nuclei [J(llB-H) = 87 Hz]. The lH spectrum is a 1 : 1 : 1 : 1 quartet indicating that all hydrogen atoms are equivalent and are coupled to a single llB (SO%, 1 = 3/2) nucleus [small resonances due to coupling to loB (20%, 1 = 3) are also present]. The llB spectrum shows the presence of a small amount of di- borane(6) [5S.O p.p.m. downfield from the Be(BH,), signal], a decomposition product which accumulates during the course of the spectral acquisition (ca. 12 h). No line-shape features of the llB or lH spectra require the invocation of gBe (I = 3/2) coupling, nor have such couplings been identified in the spectra of alkyl beryllium hydrides.lg The solution spectra exhibit the same patterns and the same coupling constants as are observed for the gas-phase spectra (11B: 6 f36.2 p.p.m. (BF,-OEt, = 0), J = S6 Hz, in [zH,]benzene; 1H: 6 +O-4 (Me,Si = 0), J = 86 Hz in [,H,] benzene }. H-B-H-Be-H-€3-H /H\ / \ \H/ \H/ fl fl FIGURE 1. Gas-phase n.m.r. spectra of Be(BH,),. (a) llB spectrum a t  86.6MHz. (b) IH spectrum at  100MHz. eliminate borosilicate glass, and the samples were contained in 5mm quartz tubes in order to eliminate background boron signals. 1H spectra were also obtained on a Varian XL-100 F.T.n.m.r. spectrometer using a 12 mm 0.d. Pyrex sample tube, The lH spectra showed significant probe backgrounds unique to each instrument. Spectra were obtained a t  ambient temperature (ca. 23 "C) on saturated-vapour samples containing partial pressures of ca, 6 Torr of Be(BH,), and 2-5 atm of inert gas [(CD,),Si FIGURE 2 The n.m.r. spectra can be interpreted only in terms of a Be(BH,), structure containing magnetically equivalent borohydride groups. Within each borohydride group rapid internal hydrogen exchange takes place on the n.m.r. time scale. None of the previously postulated (see refs. 2, 3, and 6) or remotely possible triangular Be(BH,), structures are consistent with these spectra, regardless of the mechanism of hydrogen exchange. However, an exchange mechanism Downloaded by University of North Carolina at Greensboro on 02 March 2011Published on 01 January 1977 on http://pubs.rsc.org | doi:10.1039/C39770000224View OnlineD. F. Gaines*, J. L. Walsh and D. F. Hillenbrand, "Gas-Phase Nuclear Magnetic Resonance Spectroscopic Study of the  Molecular Structure of Beryllium Borohydride, Be(BH4)2" J. C. S.. Chem. Commun., 224, (1977).  Made available courtesy of Royal Society of Chemistry: http://www.rsc.org/  ***Reprinted with permission. No further reproduction is authorized without written permission from the Royal Society of Chemistry.  This version of the document is not the version of record. Figures and/or pictures may be missing from this format of the document.*** J.C.S. CHEM. COMM., 1977 225 involving rapid transformation from one linear B-Be-B configuration to another, such as that depicted in Figure 2, is consistent with the observed n.m.r. spectral data. All uncontroversial Be-BH, interactions [solid, polymeric Be(BH,),,17 (MeBeBH,),,20,21 and solid B,HloBeBH,22] indicate the presence of double hydrogen bridges between beryllium and the borohydride boron. Theoretical studies have favoured either double or triple hydrogen bridges and a linear B-Be-B framework.l4-16 Most recent i.r. spectral studies suggest either double or triple hydrogen bridges.l s 3  Thus, gas-phase monomeric Be( BH,) is highly fluxional on the n.m.r. time scale and, based on the present as well as previous studies,1~9~12~15~16 undoubtedly has a linear B-Be-B framework in which local configurational interchange results in rapid hydrogen exchange within each borohydride group. We thank the National Science Foundation for financial support. (Received, 20th December 1976; Corn. 1384.) J. W. Nibler. J .  Amer. Chem. SOC., 1972, 94, 3349. T. H. Cook and G. L. Morgan, J .  Amer. Chem. SOC., 1969,91, 774. T. H. Cook and G. L. Morgan, J .  Amer. Chem. SOC., 1970, 92, 6493. W. C. Price, H. C. Longuet-Higgins, B. Rice, and T. F. Young, J .  Chem. Phys., 1949, 17, 217. J. W. Nibler and J. McNabb, Chem. Comm., 1969, 134. A. Almenningen, G. Gundersen, and A. Haaland, Actu Chem. 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Allamandola and J .  W. Nibler, J .  Amer. Chem. Soc., 1976, 98, 2096. a2 D. F. Gaines and J.  L. Walsh, J.C.S. Chem. Comm., 1976, 482. Downloaded by University of North Carolina at Greensboro on 02 March 2011Published on 01 January 1977 on http://pubs.rsc.org | doi:10.1039/C39770000224View Online

Gas-phase Nuclear Magnetic Resonance Spectroscopic Study of the Molecular Structure of Beryllium Borohydride, Be(BH<sub>4</sub>)<sub>2</sub>

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Gas-phase Nuclear Magnetic Resonance Spectroscopic Study of the Molecular Structure of Beryllium Borohydride, Be(BH<sub>4</sub>)<sub>2</sub>

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