DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1998, 2965–2967 2965 Macropolyhedral boron-containing cluster chemistry. The [S2B18H19]2 anion, and the reversible dismantling and regeneration of an apical boron cluster site with cluster connectivity six Tomás¡ Jelínek,a,b Ivana Cisar¡ová,c Bohumil S¡ tíbr,b John D. Kennedy a and Mark Thornton-Pett a a The School of Chemistry of the University of Leeds, Leeds, UK LS2 9JT b The Institute of Inorganic Chemistry of the Academy of Sciences of the Czech Republic, 25068 R¡ ez¡-by-Prague, The Czech Republic c The Faculty of Natural Sciences of Charles’ University, Hlavova 2030, 12842 Prague 2, The Czech Republic Received 24th July 1998, Accepted 24th July 1998 The double-cluster [S2B18H19]2 anion, from the [syn- B18H21]2 anion and elemental sulfur, has an eleven-vertex arachno-type subcluster with a {B7} hexagonal planar-based pyramidal feature that reversibly disassembles and reassembles upon protonation followed by deprotonation.There is contemporary continuing interest in the generation of new geometries in boron-containing cluster chemistry.1 In the development of this context we have described the nineteenvertex macropolyhedral thiaborane cluster compound [S2B17- H17(SMe2)] (compound 1, schematic IA), obtained from the heat-induced autofusion of [SB8H10(SMe2)].2 Compound 1 has an unusual cluster structure in that one of its subclusters, formally arachno ten-vertex (IB), has an apical boron atom with the high cluster connectivity of six (vertex BH in IB).This open arachno-type subcluster geometry IB is quite diVerent from the conventional 3 arachno ten-vertex geometry as exhibited, for example, by the arachno [6-SB9H12]2 anion (IC).4 From this isolated result it has not been clear whether the six-connectivity {BH} component of this new cluster shape IB is (a) inherently stable, and has not yet been seen in an isolated single-cluster compound because the necessary synthetic route is not yet discovered, or (b) it is a kinetic artefact trapped by the constraints of the reaction coordinate that generates the intercluster linkage.2 Of relevance here, we have found now a second example of this type of high cluster connectivity.It occurs in an eleven-vertex arachno-type subcluster in the twenty-vertex [S2B18H19]2 macropolyhedral thiaborane monoanion (compound 2, [N(PPh3)2]1 salt). The route to anion 2 involves a completely diVerent synthetic strategy to that for compound 1, and, furthermore, it appears that the observed six-connectivity feature can reversibly disassemble and reassemble via a simple protonation–deprotonation sequence.S S SMe2 S B H H IA IB H H neutral 1 S IC syn-B18H22 (360 mg, 1.64 mmol) in thf (25 ml) was deprotonated with excess NaH, elemental sulfur (400 mg) was added, the mixture stirred for 12 h, and then heated at reflux for 3 h. Water was added, the thf was evaporated (water pump), and the solution was filtered and then precipitated with a slight excess of [N(PPh3)2]Cl.The pale yellow precipitate, the [N(PPh3)2]1 salt of the yellow anion [S2B18H19]2 (compound 2), was purified by column chromatography (silica gel, CHCl3) (RF 0.25 by analytical TLC on silufol with CH2Cl2 as liquid phase), yield 48%. The other principal product was the well recognised [nido-7- SB10H11]2 anion. Compound 2 is characterised by single-crystal X-ray diVraction analysis (Fig. 1 and schematic IIA) * and NMR spectroscopy.† It is seen to consist of a conventionally shaped3 nido eleven-vertex {B10S} subcluster that is fused, with two boron atoms in common, with a {B10S} unit of a previously unrecognised eleven-vertex arachno-type. The shape of this latter unit (IIB), formally based on an (as yet hypothetical) [SB10H13]2 anion, resembles a conventional ten-vertex arachno cluster (e.g. IC) except that an additional boron atom is incorporated in the open face, thus generating an eleven-vertex cluster and a cluster connectivity of six for the adjacent apical boron atom (vertex BH in IIB).This geometry of type IIB is Fig. 1 Crystallographically determined molecular structure of the [S2B18H19]2 anion 2. Selected interatomic distances (pm) are: from B(1) to B(2) 185.2(11), to B(3) 1.851(11), to B(4) 1.952(12), to B(5) 1.869(11), to B(6) 1.883(11), and to B(7) 1.913(11); B(2)–B(3) is 1.770(12), B(3)– B(4) 1.781(11), B(4)–B(5) 1.735(10), B(5)–B(6) 1.709(11), B(6)–B(7) 1.734(11) and B(7)–B(2) is 1.787(12); from S(11) to B(8) 1.964(10), to B(9) 1.945(10), and to B(10) 1.940(9).There was 50 : 50 {S(99)}: {BH(119)H(109,119)} crystallographic disorder within the conventionally structured (primed numbering) subcluster.2966 J. Chem. Soc., Dalton Trans., 1998, 2965–2967 previously unobserved. It is quite diVerent from the more conventional 5 eleven-vertex arachno geometries of structures IIC and IID.The essential planar-based hexagonal pyramidal {B7} unit is an unprecedented feature of both 1 and 2, and does not figure in classical 3 borane building-block philosophies. The 11B and 1H NMR chemical shifts † of species 2 are tentatively but reasonably assigned and are consistent with the molecular formulation. The two common boron atoms B(5) and B(6) do not have BH(exo) bondings, and, in accord with this, two singlets are discernible in the 11B spectrum, at d(11B) = 23.9 and 214.8.An interesting feature of the 11B spectrum consists of four unusually high-field resonances between d(11B) 247 and 250 which arise from the hexagonal basal B(2), B(3), B(4) and B(7) positions of the novel eleven-vertex subcluster of geometrical type IIB. Upon reprotonation with concentrated H2SO4, monitoring by 11B NMR spectroscopy shows that (a) these four very high-field resonances are lost, (b) a completely new, more compact, spectrum is obtained,‡ and (c) there is now only one singlet in the 11B NMR spectrum, at d(11B) = 213.1. We propose that these features result from the loss of the hexagonal pyramidal feature and the generation of a neutral conjugate acid [S2B18H20] (compound 3) which consists of two more conventional (though mutually diVerent) eleven-vertex arachno subclusters IIC and IID joined as in schematic III.There is a precedent for both cluster types in thiaborane cluster chemistry,5 and the NMR assignments‡ are not inconsistent with this formulation.In this scheme, the common vertex, designated B in IIA and BH in III (see also Scheme 1), is protonated on conversion of anion 2 to neutral 3. Concomitant with this, there is a diamond–square–diamond (DSD) intracluster flexing (bold lines in Scheme 1), and the twenty-vertex macropolyhedral unit thereby acquires a somewhat reduced inter-subcluster intimacy. Interestingly, deprotonation of neutral 3 with tmnda (N,N,N9,N9-tetramethylnaphthalene-1,8-diamine) does not immediately regenerate the starting anion 2, but immediately and quantitatively generates a species 4 which exhibits a 1:2:2:2:2:2:1:2:2:2 relative intensity pattern in its 11B NMR spectrum.§ This may result from the conversion of neutral 3 to a fluxional anion 4.Such a fluxionality could for example arise from an arachno(IIC)/arachno(IID)�arachno(IID)/ arachno(IIC) interconversion as in Scheme 1: clusters of types S B S IIB IIC H S B S IIA anion 2 H H H S IID IIC and IID are very closely related, and deprotonation of the right-hand subcluster of structure III and adjustment of the two open-face connectivities to the vertex designated BH would readily achieve interconversion of the two subcluster types.Over 3 h at 294–297 K in CDCl3 solution, this fluxional species 4 reverts quantitatively to the original anion 2 with its unusual hexagonal pyramidal feature. This reversion entails an unusual movement of hydrogen (presumably as a proton) from an open-face bridging position to an apical site, and a reversal of the original DSD process (Scheme 1).In sum, the overall protonation–deprotonation sequence appears to result in the deconstruction and the reconstruction of the very unusual hexagonal pyramidal feature. This second incidence of a {BH} vertex wi a cluster connectivity to six other boron atoms in the context of an open cluster, but now one which can apparently be readily disassembled and reassembled, reinforces ideas 2 that the hexagonal pyramidal {B7} unit may be of fundamental significance and can therefore be used as a building block in future borane-based architecture.We are currently attempting to elucidate further the two intermediates 3 and 4 associated with this fundamental cluster disassembly–reassembly sequence, as well as attempting to devise entries into other systems that may exhibit this type of feature. Acknowledgements Contribution no. 72 from the R¡ ez¡-Leeds Anglo–Czech Polyhedral Collaboration (ACPC).We thank the EPSRC (Grant nos. F78323, J56929 and K05818) and the Grant Agency of the Academy of Sciences of the Czech Republic (Grant no. A 403 2701) for support, the Royal Society (London), the Czech Academy of Sciences, and Borax Research (now Borax Europe Ltd.) for assistance with reciprocal visits, and Simon A. Barrett for kind assistance with NMR spectroscopy. Scheme 1 B S H S H B S H S H B S H S H H H protonation DSD regeneration DSD deprotonation anion 2 fluxionality neutral 3 H anion 4 H S BH S H H H III neutral 3J.Chem. Soc., Dalton Trans., 1998, 2965–2967 2967 Notes and references * Crystals of [N(PPh3)2][S2B18H19], C36H49B18NP2S2, M = 816.40, from CHCl3–OEt2, triclinic, space group P1� , a = 1092.28(8), b = 1421.90(12), c = 1532.04(10) pm, a = 70.977(6), b = 88.202(6), g = 80.321(6)8, U = 2216.8(3) Å3, Z = 2, T = 200(2) K, 5851 independent reflections collected on a Stoe STADI4 diVractometer in the range 3.05 < q < 608 were used in calculations after Lorentz-polarisation and absorption corrections (m = 1.97 mm21, based on azimuthal y-scans).The borane anion possesses a pseudo-mirror plane [passing through atoms B(1), B(9), S(11), B(19), B(39) and B(109)] which causes a 50 : 50 disorder {S(99)} : {B(119)H(109,119)} within the eleven-vertex subcluster. Thus the 9- and 10-positions of this subcluster were refined as 50 :50 B:S atoms. Final wR2 = 0.1232 for all unique data, conventional R = 0.0446 for F values of 4941 reflections with Fo 2 > 2s(Fo 2).CCDC reference number 186/1101. See http://www.rsc.org/suppdata/dt/1998/2965/ for crystallographic files in .cif format. † 11B and 1H NMR data for anion 2 [formally the nido-99-thiaundecaborano-( 79,89 : 5,6)-iso-(116kc·VII Ò)-arachno-11-thiaundecaboranate- (12) anion]; CD3CN, 294–297 K (Note: [PPh4]1 salt, not [N(PPh3)2]1 salt) {ordered as: tentative assignment d(11B) relative to X 32.083971 MHz [d(1H) of directly attached hydrogen]}: BH(29) 116.5 [13.81], BH(49) 13.0 [13.43], BH(109) 11.9 [12.83], BH(1) 10.9 [13.91], B(5) ca. 23.9 [conjuncto position], BH(59) ca. 23.9 [12.74], BH(9) 28.6 [13.13], B(6) 214.8 [conjuncto position], BH(8) 215.3 [12.33], BH(10) 215.8 [12.28], BH(119) 217.6 [11.13], BH(69) 221.1 [11.51], BH(39) 223.2 [20.38], BH(19) 223.6 [11.805], BH(7) ca. 247.4 [10.52], BH(4) 247.4 [10.11], BH(2) 248.7 [10.37], BH(3) 249.7 [10.15], with m-H(7,8), (4,10) and (109,119) at d(1H) 21.45, 21.70 and 21.79 respectively; assignments by homo- and hetero-nuclear 11B and 1H NMR experiments. ‡ 11B and 1H NMR data for neutral 3 [formally m-(79,8)-arachno-99- thiadecaborano-(69 : 7)-nido-10-thiaundecaborane], CDCl3, 294–297 K {ordered, assigned and referenced as above}: BH(3) 115.9 [13.69], BH(29) 115.3 [14.55], BH(5) 111.1 [13.57], BH(49) 1 9.1 [13.65], BH(11) 17.3 [13.39], BH(8) 15.6 [13.13], BH(6) 21.9 [13.09], BH(59) 25.1 [12.58], BH(8) 25.95 [12.60], BH(109) 28.5 [12.41], BH(1) and BH(69) both ca. 211.6 [12.98 and 12.79], B(69) 213.1 [conjuncto position], BH(2) and BH(39) both ca. 219.5 [12.47 and 11.61], BH(7) 222.2 [11.08], BH(4) 224.2 [11.54], BH(19) 234.9 [11.52], and m-H(79,89), (8,9) and (59,109) at d(1H) 11.00, 20.88 and 21.14 respectively. § d(11B) NMR values (and relative intensities) for the fluxional anion 4, which exhibits time-average two-fold symmetry (see Scheme 1): CDCl3, 294–297 K: ca. 14.8 (2BH), ca. 14.8 (1B), 12.3 (2BH), 25.3 (2BH), 26.8 (2BH), 27.9 (2BH), 210.0 (1BH), 211.1 (2BH), 238.0 (2BH) and 241.6 (2BH); all had doublet structures arising from couplings 1J(11B–1H) in the range ca. 135 to ca. 165 Hz, except for the resonance of relative intensity 1B at d(11B) ca. 14.8; 1H-{11B} NMR work was precluded because of the relatively rapid reversion of 4 to regenerate 2. 1 See, for example, J. Bould, J. D. Kennedy and M. Thornton-Pett, J. Chem. Soc., Dalton Trans., 1992, 563; J.D. Kennedy and B. S. tíbr, in Current Topics in the Chemistry of Boron, ed. G. W. Kabalka, Royal Society of Chemistry, Cambridge, 1994, pp. 285–292; B. S. tíbr, J. D. Kennedy, E. Drdáková and M. Thornton-Pett, J. Chem. Soc., Dalton Trans., 1994, 229; J. D. Kennedy, in The Borane–Carborane–Carbocation Continuum, ed. J. Casanova, Wiley, New York, 1998, ch. 3, pp. 85–116. 2 P. Kaur, J. Holub, N. P. Rath, J. Bould, L. Barton, B. S. tíbr and J. D. Kennedy, Chem. Commun., 1996, 273. 3 R. E. Williams, Adv. Inorg. Chem. Radiochem., 1976, 18, 64; K. Wade, Adv. Inorg. Chem. Radiochem., 1976, 18, 1. 4 K. Nestor, X. L. R. Fontaine, N. N. Greenwood, J. D. Kennedy and M. Thornton-Pett, J. Chem. Soc., Dalton Trans., 1991, 2657; see also B. S. tíbr, J. Holub, T. Jelínek, X. L. R. Fontaine, J. Fusek, J. D. Kennedy and M. Thornton-Pett, J. Chem. Soc., Dalton Trans., 1996, 1741. 5 J. Holub, A. E. Wille, B. S. tíbr, P. J. Carroll and L. G. Sneddon, Inorg. Chem., 1994, 33, 4920; T. Jelínek, J. D. Kennedy, B. S.tíbr and M. Thornton-Pett, Angew. Chem., Int. Ed. Engl., 1994, 33, 1599; J. Chem. Soc., Chem. Commun., 1995, 1665; P. Kaur, J. D. Kennedy, M. Thornton-Pett, T. Jelínek and B. S. tíbr, J. Chem. Soc., Dalton Trans., 1996, 1775. Communication 8/0579