One-electron reduction of 3-Cp-3,1,2-closo-CoC2B9H11 followed by heating to reflux in DME (b.p. 85 °C) induces isomerisation to 4-Cp-4,1,2-closo-CoC2B9H11.</p

Man, Wing Y.

Rosair, Georgina M.

Welch, Alan J.

Dalton Transactions

English

One-electron reduction of 3-Cp-3,1,2-closo-CoC&lt;inf&gt;2&lt;/inf&gt;B&lt;inf&gt;9&lt;/inf&gt;H&lt;inf&gt;11&lt;/inf&gt; followed by heating to reflux in DME (b.p. 85 °C) induces isomerisation to 4-Cp-4,1,2-closo-CoC&lt;inf&gt;2&lt;/inf&gt;B&lt;inf&gt;9&lt;/inf&gt;H&lt;inf&gt;11&lt;/inf&gt;, a compound previously only synthesised at much higher temperatures (&gt;380 °C). The 4,1,2-isomer has been thoroughly characterised both spectroscopically and crystallographically.</p

Heriot Watt Pure

Dalton Transactions  COMMUNICATION This journal is © The Royal Society of Chemistry 2015 Dalton Trans., 2015, 44, 1-3 | 1  Please do not adjust margins Please do not adjust margins Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.  E-mail: a.j.welch@hw.ac.uk † Dedicated to Professor M. Frederick Hawthorne in celebration of 50 years of metallacarboranes (see ref. 1). Electronic Supplementary Information (ESI) available: Summary of crystal data for the eight crystallographic studies of CpCoC2B9H11.  For 4-Cp-4,1,2-closo-CoC2B9H11; CCDC 1405004.  See DOI: 10.1039/x0xx00000x Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ Reduction-induced facile isomerisation of metallacarboranes: synthesis and crystallographic characterisation of 4-Cp-4,1,2-closo-CoC2B9H11 † Wing. Y. Man, Georgina M. Rosair and Alan J. Welch*One-electron reduction of 3-Cp-3,1,2-closo-CoC2B9H11 followed by heating to reflux in DME (bp 85 °C) induces isomerisation to 4-Cp-4,1,2-closo-CoC2B9H11, a compound previously only synthesised at much higher temperatures (>380 °C).  The 4,1,2- isomer has been thoroughly characterised both spectroscopically and crystallographically. It is exactly 50 years since Hawthorne’s first synthesis of metallacarboranes, specifically the icosahedral sandwich anions [Fe(C2B9H11)2]n– (n = 1, 2).1  Throughout the intervening period metallacarborane chemistry has been very heavily dominated by the icosahedron (reflecting the exceptional stability of closo-C2B10 and closo-CB11 carboranes) with thousands of icosahedral closo-MC2B9 species having been synthesised and characterised.2 Cyclopentadienyl cobaltacarboranes can be considered as the archetypal metallacarboranes since {CpCo} is a simple and readily-available fragment isolobal with {BH}, and in fact CpCoC2B9H11 is known for seven of the nine possible isomers shown in Fig. 1, specifically the 3,1,2-, 2,1,7-, 2,1,12-, 4,1,2-, 2,1,8-, 2,1,9- and 9,1,7- isomers.  3-Cp-3,1,2-closo-CoC2B9H11,3 2-Cp-2,1,7-closo-CoC2B9H11 4 and 2-Cp-2,1,12-closo-CoC2B9H11 5 are prepared by metallation of the [7,8-nido-C2B9H11]2–, [7,9-nido-C2B9H11]2– and [2,9-nido-C2B9H11]2– anions, respectively (which, in turn, are afforded by deboronation of commercially-available 1,2-, 1,7- and 1,12-closo-C2B10H12).  Gas-phase thermolysis of 3-Cp-3,1,2-closo-CoC2B9H11 leads to varying amounts of all the other six known isomers of CpCoC2B9H11 dependent on the temperature employed.6  The two “missing” isomers, 8-Cp-8,1,2-closo-CoC2B9H11 and 9-Cp-9,1,2-closo-CoC2B9H11, both have adjacent cage C atoms (neither of which are directly bound to the metal) and so these isomers would be most unlikely to result from high-temperature thermolysis  Fig. 1  The nine isomers of an icosahedral MC2B9 metallacarborane.  reactions since thermolysis classically causes C atom separation.  However, derivatives of these isomers with the cage C atoms tethered together (via a trimethylene unit) could be obtained by gas-phase thermolysis of the tethered 3,1,2- precursor.6 Clearly the syntheses of 4-Cp-4,1,2-closo-CoC2B9H11, 8-Cp-8,1,2-closo-CoC2B9H11 and 9-Cp-9,1,2-closo-CoC2B9H11 (the three isomers of CpCoC2B9H11 apart from 3-Cp-3,1,2-closo-CoC2B9H11 that have adjacent cage C atoms, by conventional chemistry) represent significant challenges.  As part of our interest in this area we recently prepared 8-Cp-8,1,2-closo-CoC2B9H11 (and its 2,1,8- analogue) by a low-temperature M3C1C2M2C7C1M2C1C12M4C1C2M2C1C8M2C9C1M9C1C7M8C2 C1M9C1C23,1,2-MC2B9 2,1,7-MC2B9 2,1,12-MC2B92,1,9-MC2B92,1,8-MC2B94,1,2-MC2B98,1,2-MC2B9 9,1,2-MC2B99,1,7-MC2B9COMMUNICATION Dalton Transactions 2 | Dalton Trans., 2015, 44, 1-3 This journal is © The Royal Society of Chemistry 2015 Please do not adjust margins Please do not adjust margins  Fig. 2  Perspective view of 4-Cp-4,1,2-closo-CoC2B9H11.  Selected interatomic distances (Å): Co4–C1, 2.0115(19); Co4–B3, 2.023(2); Co4–B8, 2.079(2); Co4–B9, 2.102(2); Co4–B5, 2.079(2); Co4–Cp, 2.0655(18)-2.0857(18); C1–C2, 1.657(3). route, specifically decapitation of the 13-vertex metallacarborane 4-Cp-4,1,8-closo-CoC2B10H12 followed by oxidative closure of the 12-vertex dianion thereby produced.7  Both species were characterised spectroscopically and crystallographically, taking to five the number of isomers of CpCoC2B9H11 to have been structurally characterised, previously the 3,1,2-,8 2,1,7- 4a,b and 2,1,12- 4a isomers and now the 8,1,2- 7 and 2,1,8- isomers.7  We now report the synthesis by conventional chemistry of another of these “challenging” isomers. Instead of high-temperature thermolysis, an alternative way to isomerise metallacarboranes is by 1-e reduction, sometimes in conjunction with mild heating,9 and Hanusa and Todd used this approach to prepare both the 2,1,7- and 2,1,12- isomers of CpCoC2B9H11 from 3-Cp-3,1,2-closo-CoC2B9H11.9a  In repeating this synthesis we have now discovered that a small amount of the compound 4-Cp-4,1,2-closo-CoC2B9H11,‡ which we have fully characterised both spectroscopically and crystallographically,§ is also afforded.  This represents only the second synthesis of this compound and the first at relatively low temperature. Treatment of 3-Cp-3,1,2-closo-CoC2B9H11 with 1.1 equivalents of sodium naphthalenide in DME followed by heating to reflux (85 °C) for 56 hrs and subsequent aerial oxidation resulted in partial isomerisation to a mixture of the 2,1,8-, 8,1,2-, 2,1,7- and 4,1,2- isomers, separated initially by column chromatography and ultimately by thin-layer chromatography.  All products were identified by a combination of 1H and 11B NMR spectroscopies against authentic samples (2,1,8-isomer,7 8,1,2-isomer 7 and 2,1,7-isomer 4b).  In the 1H spectrum of 4-Cp-4,1,2-closo-CoC2B9H11 are observed a sharp integral-5 singlet at  5.44 ppm assigned to the Cp protons and two broad integral-1 resonances at  3.36 and 2.80 ppm assigned to the CcageH atoms.  The 11B NMR  Fig. 3  Relative energies (kcal mol–1) of the nine isomers of CpCoC2B9H11  (data taken from ref. 17). spectrum consists of eight resonances between  9.1 and –17.5 ppm with integrals in the ratio 1:1:1:2:1:1:1:1 from high frequency to low frequency, confirming the asymmetric nature of the species. The precise nature of the compound was established by a crystallographic study (Fig. 2) as part of which the identities of the cage C atoms were unambiguously determined by both the Vertex-to-Centroid Distance 10 and Boron-Hydrogen Distance 11 methods.  Key interatomic distances appear in the legend to Fig. 2.  The Cp ligand is essentially parallel to the least-squares planes through atoms C1,B3,B8,B9,B5 [dihedral angle 0.37(8)°] and the plane through atoms C2,B6,B10,B12,B7 [dihedral angle 1.51(8)°]. 4-Cp-4,1,2-closo-CoC2B9H11 is the sixth of nine possible isomers of CpCoC2B9H11 to be structurally studied leaving only the 9,1,2-, 2,1,9- and 9,1,7- isomers remaining.  There are two polymorphic forms of each of the 3,1,2- and 2,1,7- isomers, taking to eight the number of crystallographic studies of CpCoC2B9H11 and, somewhat surprisingly, none of these show isomorphism (see ESI). There are nine examples of 4,1,2-MC2B9 compounds in the Cambridge Structural Database,12 but only one of these is the result of direct metallation of a 2,7-nido-C2B9 anion.13  In every other case the initial metallation is of a 7,8-nido-C2B9 anion followed by isomerisation of the 3,1,2-MC2B9 species thus formed (sometimes only transiently) into the 4,1,2-MC2B9 final product.  Although there is evidence that the relief of steric crowding plays a part in several of these 3,1,2- to 4,1,2- isomerisations,14 this is not obvious in every case.15  Moreover, in examples where the cage C atoms are not tethered together it is not at all clear why the more common 3,1,2- to 2,1,8- isomerisation 16 is not observed.  On the other hand a common feature of many of the 3,1,2-MC2B9 species which isomerise to 4,1,2-MC2B9 is that they have a relatively electron-rich metal 3,1,22,1,72,1,122,1,92,1,84,1,28,1,29,1,29,1,70.04.08.012.016.020.0Relative energy / kcal mol-1Dalton Transactions  COMMUNICATION This journal is © The Royal Society of Chemistry 2015 Dalton Trans., 2015, 44, 1-3 | 3  Please do not adjust margins Please do not adjust margins centre (NiII or PdII), perhaps suggesting an analogy with the reduction-induced 3,1,2- to 4,1,2- isomerisation of CpCoC2B9H11 observed herein. In Fig. 3 are shown the relative energies of the nine isomers of CpCoC2B9H11 from DFT calculations.17  They fall into two distinct groups, those with the cage C atoms adjacent (3,1,2-, 4,1,2-, 9,1,2- and 8,1,2-) at relatively high energy and those with the cage C atoms separated (2,1,9-, 2,1,7-, 2,1,8-, 9,1,7- and 2,1,12-) at lower energy.  This work has demonstrated a low-temperature synthesis of the second-least thermodynamically stable isomer. Conclusions In conclusion we have shown that 1-e reduction of 3-Cp-3,1,2-closo-CoC2B9H11 followed by relatively mild heating affords (a small amount of) the isomer 4-Cp-4,1,2-closo-CoC2B9H11, which we have fully characterised including a crystallographic study.  This takes to six the number of isomers of CpCoC2B9H11 to be isolated by low-temperature routes, and experiments targeting the remaining isomers yet to be afforded by conventional chemistry (9,1,2-, 2,1,9- and 9,1,7-) are currently in hand.  Access to a complete set of isomers of this archetypal metallacarborane will afford a unique opportunity for detailed comparative study which we believe will be of fundamental interest. Acknowledgement We thank the EPSRC for support (project EP/I031545/1). Notes and references ‡ Experimental procedure: to a freshly prepared solution of sodium naphthalenide (1.1 eq, 0.64 mmol) was added a solution of 3-Cp-3,1,2-closo-CoC2B9H11 (0.15 g, 0.58 mmol) in dry, degassed DME (12 ml).  The reagents were heated to reflux for 56 h, oxidised using a water aspirator and the solvent removed in vacuo.  Purification of the crude residue using column chromatography in an eluent system of 30:70 dichloromethane:petroleum ether gave naphthalene (Rf = 0.76), two yellow bands, yellow1 (Rf = 0.46) and yellow2 (Rf = 0.28), and unreacted cobaltacarborane starting material (Rf = 0.14).  Using 1H and 11B NMR spectroscopies, yellow1 was identified as 2-Cp-2,1,8-closo-CoC2B9H11 with a trace of 8-Cp-8,1,2-closo-CoC2B9H11 whilst re-purification of yellow2 using preparative TLC in an eluent system of 20:80 ethyl acetate:petroleum ether gave 2-Cp-2,1,7-closo-CoC2B9H11 (Rf = 0.18) and 4-Cp-4,1,2-closo-CoC2B9H11 (Rf = 0.10) in trace amounts.  For 4-Cp-4,1,2-closo-CoC2B9H11: 1H NMR (CDCl3, 298 K); δ 5.44 (s, 5H, C5H5), 3.36 (s, 1H, CcageH), 2.80 (s, 1H, CcageH).  11B NMR (CDCl3, 298 K); δ 9.1 (1B), 0.8 (1B), -1.9 (1B), -5.7 (2B), -7.3 (1B), -12.0 (1B), -15.3 (1B), -17.5 (1B).  EIMS; m/z 256.1(M+). § Crystal data: C7H16B9Co, M = 256.42, monoclinic, P21/c, a = 11.6409(12), b = 6.6488(6), c = 16.1299(15) Å,  = 93.823(5)º, V = 1245.6(2) Å3, Z = 4, Dc = 1.367 Mg m−3,  = 1.336 mm−1, F(000) = 520.  Data to max = 32.00º collected at 100(2) K on a Bruker X8 diffractometer using Mo-K  radiation.  4297 independent reflections out of 29748 measured, Rint = 0.0421.  S = 1.102 for all data, and R1 = 0.0377, wR2 = 0.0803 for 3596 data with I>2 (I).  Max. and min. e-density 0.872 and -0.573 eÅ−3, respectively.  1 M. F. Hawthorne, D. C. Young and P. A. Wegner, J. Am. Chem. Soc., 1965, 87, 1818. 2 R. N. Grimes, Carboranes, Academic Press, Oxford, UK, 2nd edn, 2011. 3 M. F. Hawthorne, D. C. Young, T. D. Andrews, D. V. Howe, R. L. Pilling, A. D. Pitts, M. Reintjes, L. F. Warren and P. A. Wegner, J. Am. Chem. Soc., 1968, 90, 879. 4 (a) M. E. Lopez, D. Ellis, P. R. Murray, G. M. Rosair, A. J. Welch and L. J. Yellowlees, Collect. Czech. Chem. Commun., 2010, 75, 853; (b) W. Y. Man, G. M. Rosair and A. J. Welch, Acta Cryst., 2015, E71, m141. 5 D. C. Busby and M. F. Hawthorne, Inorg. Chem., 1982, 21, 4101. 6 M. K. Kaloustian, R. J. Wiersema and M. F. Hawthorne, J. Am. Chem. Soc., 1972, 94, 6679. 7 W. Y. Man, S. Zlatogorsky, H. Tricas, D. Ellis, G. M. Rosair and A. J. Welch, Angew. Chem. Int. Ed., 2014, 53, 12222. 8 (a) D. E. Smith and A. J. Welch, Organometallics, 1986, 5, 760; (b) J. G. Planas, C. Viñas, F. Teixidor, M. E. Light and M. B. Hursthouse, CrystEngComm, 2007, 9, 888. 9 e.g. (a) T. P. Hanusa and L. J. Todd, Polyhedron, 1985, 4, 2063, (b) G. G. Thiripuranathar, W. Y. Man, C. Palmero, A. P. Y. Chan, B. T. Leube, D. Ellis, D. McKay, S. A. Macgregor, L. Jourdan, G. M. Rosair and A. J. Welch, Dalton Trans., 2015, 44, 5628. 10 A. McAnaw, G. Scott, L. Elrick, G. M. Rosair and A. J. Welch, Dalton Trans., 2013, 42, 645. 11 A. McAnaw, M. E. Lopez, D. Ellis, G. M. Rosair and A. J. Welch, Dalton Trans., 2014, 43, 5095. 12 C. R. Groom and F. H. Allen, Angew. Chem. Int. Ed., 2014, 53, 662.  For this study we used the CSD version 5.36. 13 M. A. Fox, J. A. K. Howard, A. K. Hughes, J. M. Malget and D. S. Yufit, J. Chem. Soc. Dalton Trans., 2001, 2263. 14 (a) M. R. Churchill and K. Gold, J. Am. Chem. Soc., 1970, 92, 1180; (b) T. E. Paxson, M. K. Kaloustian, G. M. Tom, R. J. Wiersema and M. F. Hawthorne, J. Am. Chem. Soc., 1972, 94, 4882; (c) N. Carr, D. F. Mullica, E. L. Sappenfield and F. G. A. Stone, Inorg. Chem., 1994, 33, 1666; (d) R. M. Garrioch, P. Kuballa, K. S. Low, G. M. Rosair and A. J. Welch, J. Organomet. Chem., 1999, 575, 57; (e) S. Robertson, D. Ellis, G. M. Rosair and A. J. Welch, Appl. Organomet. Chem., 2003, 17, 518; (f) S. Robertson, R. M. Garrioch, D. 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Reduction-induced facile isomerisation of metallacarboranes:synthesis and crystallographic characterisation of 4-Cp-4,1,2-closo-CoC2B9H11

Wing Y. Man

Georgina M. Rosair

Alan J. Welch

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Reduction-induced facile isomerisation of metallacarboranes: synthesis and crystallographic characterisation of 4-Cp-4,1,2-closo-CoC2B9H11

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Reduction-induced facile isomerisation of metallacarboranes:synthesis and crystallographic characterisation of 4-Cp-4,1,2-closo-CoC<sub>2</sub>B<sub>9</sub>H<sub>11 </sub>

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