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Scheme 4 Proposed reaction mechanism and relative free energies, DG298 (kcal molꢀ1), for the activation of H2 by [(IMe)(PPh3)(Py)RhSB9H8] (4/40).
Following this isonido2nido structural lability, the DFT- ligands such as carbenes may induce further examples of cage
calculations predict that the nido-isomer is capable of forming a non-rigidity in metallaheteroboranes that can be reactive versus
complex with an entering dihydrogen molecule. The transition state inactive bonds, resulting in abundant opportunities for research
TS2 from the nido-cluster, 40, entails the perpendicular approach of into new ways of bond activation.
H2 to the rhodium centre and the subsequent rotation to form a
We gratefully acknowledge the Spanish Ministry of Science and
side-on bonded intermediate, INT 1, and it has a free energy barrier Innovation (CTQ2009-10132, CSD2009-00050, and CSD2006-0015,
of 19.4 kcal molꢀ1 (Scheme 4). This intermediate is comparable CTQ2012-35665) for financial support. B.C. thanks the ‘‘Diputacion
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with well-characterized mononuclear dihydrogen complexes,2b General de Aragon’’ for a pre-doctoral scholarship.
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and it exhibits an elongated H–H distance at 0.809 Å.
To our knowledge, there are no examples of dihydrogen-
ligated polyhedral boron-containing compounds. Therefore, the
DFT-calculated complex, INT 1, is a good theoretical model of a
Notes and references
1 (a) G. J. Kubas, Metal Dihydrogen and s-Bond Complexes: Structure,
Theory and Reactivity, Kluwer Academic/Plenum, New York, 2001;
H2 molecule in the coordination environment furnished by a
metallaheteroborane. From this unstable Z2-(H2)-ligated rhodathia-
borane, the H–H bond is heterolytically cleaved by proton transfer
to the adjacent B(9)–B(10) edge, passing over the transition state
TS3 to form a hydridorhodathiaborane, which should be one of the
two conformers of compound 5 that have been identified in situ
using NMR spectroscopy.
In a NMR tube at room temperature, the exposure of a CH2Cl2
solution of 4 and ethylene to a dihydrogen atmosphere affords
ethane (Fig. S7 in ESI†). Under catalytic conditions, the carbene-
ligated rhodathiaborane exhibited activity in the hydrogenation
and isomerisation of 1-hexene, reaching a conversion of 69% in
5 hours (see Table S1 in ESI†).
In summary, the carbene-ligated clusters 4 and 5 exhibit an
unprecedented isonido2nido equilibrium sustained by H2. The
response of 4 to the addition of dihydrogen can be regarded as a
form of metal–ligand cooperation,4 which is triggered by a
structural change of the cluster, leading to vacant coordination
sites at the metal centre. The subsequent binding of H2 results in
the heterolytic splitting of the H–H bond along the Rh(8)–B(9)
edge to a hydride ligand and a proton that is transferred to the
B(9)–B(10) edge. The system is active in the catalytic hydrogena-
tion of ethylene and 1-hexene.
(b) R. Noyori and T. Ohkuma, Angew. Chem., Int. Ed., 2001, 40,
40–73; (c) Handbook of Homogeneous Hydrogenation, Wiley-VCH,
Germany, 2007.
2 (a) G. S. McGrady and G. Guilera, Chem. Soc. Rev., 2003, 32, 383–392;
(b) G. J. Kubas, Chem. Rev., 2007, 107, 4152–4205; (c) R. H. Crabtree,
Acc. Chem. Res., 1990, 23, 95–101.
3 G. J. Kubas, Science, 2006, 314, 1096–1097.
4 J. I. van der Vlugt and J. N. H. Reek, Angew. Chem., Int. Ed., 2009, 48,
8832–8846.
5 (a) T. Ikariya, K. Murata and R. Noyori, Org. Biomol. Chem., 2006, 4,
393–406; (b) T. Ikariya and A. J. Blacker, Acc. Chem. Res., 2007, 40,
1300–1308; (c) C. Gunanathan and D. Milstein, Acc. Chem. Res., 2011,
44, 588–602; (d) M. Findlater, W. H. Bernskoetter and
M. Brookhart, J. Am. Chem. Soc., 2010, 132, 4534–4535;
(e) R. Hartmann and P. Chen, Angew. Chem., Int. Ed., 2001, 40,
3581–3585; ( f ) V. Miranda-Soto, D. B. Grotjahn, A. L. Cooksy,
J. A. Golen, C. E. Moore and A. L. Rheingold, Angew. Chem., Int.
Ed., 2011, 50, 631–635; (g) Z. M. Heiden and T. B. Rauchfuss, J. Am.
Chem. Soc., 2009, 131, 3593–3600; (h) A. Friedrich, M. Drees,
J. r. Schmedt auf der Gu¨nne and S. Schneider, J. Am. Chem. Soc.,
2009, 131, 17552–17553; (i) J. M. Camara and T. B. Rauchfuss, J. Am.
Chem. Soc., 2011, 133, 8098–8101.
6 (a) R. N. Grimes, Carboranes, Academic Press, Elsevier Inc., Amsterdam,
2nd edn, 2011; (b) Boron Science: New Technologies and Applications,
ed. N. S. Hosmane, CRC Press, Boca Raton, FL, 2012.
7 G. Ferguson, M. C. Jennings, A. J. Lough, S. Coughlan,
T. R. Spalding, J. D. Kennedy, X. L. R. Fontaine and B. Stibr,
J. Chem. Soc., Chem. Commun., 1990, 891–894.
8 K. Wade, Adv. Inorg. Chem. Radiochem., 1976, 18, 1–66.
9 J. D. Kennedy, in The Borane, Carborane, Carbocation Continuum,
ed. J. Casanova, Wiley, New York, 1998, pp. 85–116.
Given the tailorability of these 11-vertex clusters by alter-
ation of their exo-polyhedral units, the use of strong trans-effect
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10 A. Alvarez, R. Macıas, M. J. Fabra, F. J. Lahoz and L. A. Oro, J. Am.
Chem. Soc., 2008, 130, 2148–2149.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 9863--9865 9865