Reversible ethylene dihydrogen mediated 11-vertex nido → closo → nido conversion in a metallathiaborane cluster

Article abstract of DOI:10.1021/ja078197j

The reaction of the unsaturated nido-rhodathiaborane [8,8-(PPh₃)₂-8,7-RhSB₉H₁₀] (1) with pyridine (py) generates the new nido-hydridorhodathiaborane [8,8,8-(PPh₃)(H)-9-(py)-8,7-RhSB₉H₉] (2) that reacts with excess ethene to give ethane and the novel ethene-ligated closo cluster [1,1-(PPh₃)(η²-C₂H₄)-3-(py)-1,2-RhSB₉H₈] (3). Reduction of 3 with dihydrogen regenerates 2, leading to a reaction cycle that opens new routes for potential catalytic hydrogenation of olefins. Copyright

Full text of DOI:10.1021/ja078197j

Published on Web 01/25/2008
Reversible Ethylene Dihydrogen Mediated 11-Vertex nido f closo f nido
Conversion in a Metallathiaborane Cluster
AÄ lvaro AÄ lvarez, Ramo´n Mac´ıas,* Mar´ıa Jose´ Fabra, Fernando J. Lahoz, and Luis A. Oro
Departamento de Qu´ımica Inorga´nica, Instituto UniVersitario de Cata´lisis Homoge´nea, Instituto de Ciencia de
Materiales de Arago´n, UniVersidad de Zaragoza-Consejo Superior de InVestigaciones Cient´ıficas,
50009-Zaragoza, Spain
Received October 26, 2007; E-mail: rmacias@unizar.es
Recently, we have focused our work on the chemistry of the
unsaturated 11-vertex nido-rhodathiaborane [8,8-(PPh3)2-nido-8,7-
RhSB9H10] (1)1 and the related N-heterocyclic-ligated species [8,8-
(η2-L2)-nido-8,7-RhSB9H10] (L2 ) bpy, Me2bpy, phen).2 These
compounds can add a Lewis base without change of the nido
structure, and additionally, they can undergo ligand substitution
reactions and nido to closo transformation, as well as exhibit other
interesting reactivity.3-6 With small ligands such as CO or CH3-
CN, the addition of the Lewis base takes place at the metal center;7,8
in contrast, the reaction of 1 with PPh2(CH2)3PPh2 (dppp) affords
[8,8,8-(η2-dppp)(H)-9-(η1-dppp)-nido-8,7-RhSB9H9] in which the
Lewis base has attacked the boron cage leading to a B-L linkage
and a hydride ligand.6 Herein we report a novel reaction cycle with
Figure 1. Molecular structure of 2.
potential catalytic applications that is driven by oxidation/reduction
chemistry of ethene and dihydrogen with rhodathiaboranes.
The reaction of 1 with a 4-fold excess of pyridine (py) leads to
the clean formation of a new hydrido-ligated species of formulation
[8,8,8-(PPh3)2(H)-9-(py)-nido-8,7-RhSB9H9] (2). This compound
exhibits an 11-vertex nido structure, which is shown in Figure 1.
The pair of electrons contributed by the boron-ligated pyridine
increases the skeletal electron pairs (sep) to 13, consistent with the
observed structure. This differs from the parent rhodathiaborane 1
that with 12 sep does not conform to the electron-counting rules.9
The 11B NMR spectrum of 2 exhibits nine resonances, and the
peak at the highest frequency corresponds to the pyridine-bound
boron atom at the nine position that does not bear a terminal
1
hydrogen atom. More informative is the H NMR spectrum that
shows the resonances of the eight boron-bound terminal hydrogen
atoms, the BHB bridging hydrogen atom, and the hydride ligand.
The latter appears at δH -12.46 as an apparent quartet that becomes
a doublet upon 31P decoupling. Interestingly, the 31P NMR spectrum
of 2 exhibits a temperature-dependent behavior that is consistent
with a PPh3 ligand that undergoes fast dissociation, whereas the
second phosphine remains bound to the Rh atom. The labilization
of metal-bound ligands by the presence of hydrides is documented
in organometallic and metallacarborane chemistry;10,11 however, we
are not aware of examples in the chemistry of either metallaboranes
or metallaheteroboranes; 2 is, therefore, a rare example of a labile
nido-hydridorhodathiaborane.
Compound 1 does not react with olefins; in contrast, reaction of
2 with excess ethene in dichloromethane at room temperature
affords ethane and the rhodathiaborane [1,1-(PPh3)(η2-C2H4)-3-(py)-
closo-1,2-RhSB9H8] (3) (Figure 2). Compound 3 is novel. We are
unaware of any previous crystallographic characterization of
metallaheteroboranes (included metallacarboranes) containing metal-
bound ethene ligands, although there are some olefin-ligated
metallacarboranes such as [1,3-{µ-(η2-3-CH2dCHCH2CH2)}-3-(H)-
3-(PPh3)-3,1,2-RhC2B9H10].12 Compound 3 crystallizes in the
orthorhombic system with eight molecules in the unit cell, arranged
Figure 2. Molecular structure of 3.
as enantiomeric pairs that are related by an inversion center; the
molecule exhibits the structure of an octadodecahedron as predicted
for a 12 skeletal electron pair cluster. The ethene CdC distance of
1.383(5) Å in 3 falls within the reported range of 1.126-1.508
Å,13 but it is slightly shorter than that found at 1.402(6) Å in the
organometallic compound [Rh(η5-Cp)(PPh3)(C2H4)],14 which ex-
hibits an analogous ligand distribution with the η5-Cp ring formally
replacing the {SB9H8(py)} fragment. The shorter CdC distance in
3 may indicate that the ethene exhibits lower π-back-bonding in 3
than in the organometallic analogue, suggesting that the thiaborane-
to-rhodium interaction leads to a lower electronic density at the
rhodium center than the η5-Cp-to-rhodium interaction.
The 11BNMR data for 3 revealed seven peaks rather than the
nine resonances that are expected for its unsymmetrical structure,
indicating that some of the signals are accidentally coincident. The
1H{11B} NMR spectrum indirectly resolves the 11B spectrum
showing the presence of eight BH terminal hydrogen atoms. The
1H signals of the ethene group show a temperature-dependent
behavior that is consistent with full rotation of the ligand about
the coordination bond [∆Gq(252 K) ) 11.8 kcal mol-1].15
9
2148
J. AM. CHEM. SOC. 2008, 130, 2148-2149
10.1021/ja078197j CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Figure 3. Plot of reaction time versus relative abundances of 3 and 2 based
on 1H NMR signal intensities (3 dark blue triangles, 2 magenta circles).
Scheme 1. Reaction of 3 with H2 in the Presence of One
Equivalent of PPh3
Acknowledgment. This work was supported by the Ministerio
de Educacio´n y Ciencia (MEC, Spain) (Factor´ıa de Cristalizacio´n,
CONSOLIDER INGENIO-2010, and Grant 2003-05412). R.M.
thanks the MEC-Universidad de Zaragoza for his Research Contract
in the framework of the “Ramo´n y Cajal” Program.
Supporting Information Available: Experimental procedures,
spectroscopic data for all the compounds reported, including X-ray data
for 2 and 3. This material is available free of charge via the Internet at
http://pubs.acs.org.
In CD2Cl2 at room temperature, the exposure of an equimolecular
mixture of 3 and free PPh3 to a hydrogen atmosphere regenerates
the nido-rhodathiaborane 2. After about 3 h, the closo f nido
transformation has reached 50% (Figure 3); 2 days later, the nido
compound 2 is at 80% of the reaction mixture. The reduction of
closo clusters furnishing open structures usually requires strong
reducing reagents, and the dihydrogen-promoted closo-to-nido
transformation of 3 to give 2 is without precedent (Scheme 1).
A possible mechanism for the reaction of 2 with ethene is the
substitution of the labile phosphine ligand in 2 to give an undetected
η2-alkene derivative [8,8,8-(PPh3)(η2-C2H4)(H)-9-(py)-nido-8,7-
RhSB9H9], which could undergo intramolecular ethene migratory
insertion into the rhodium-hydride bond to give an alkyl inter-
mediate. At this point, the second hydrogen atom necessary to
complete the reductive elimination of the alkane would come from
the boron cage (probably the BHB hydrogen atom). This second
hydrogen could either transfer directly to the alkyl ligand or migrate
first to the rhodium atom (Scheme 2). This mechanism resembles
the generally accepted hydride route for alkene hydrogenation by
organometallic compounds,16 with the particularity that the reaction
implies oxidation of a whole cluster rather than a single metal center.
Interestingly, the activation of dihydrogen by 3 regenerating the
nido-hydridorhodathiaborane 2 opens a door for catalytic hydro-
genation of olefins (Scheme 2). The easy preparation of the 11-
vertex nido precursor 1, the stability of these rhodathiaboranes, and
the facile functionalization via new N-heterocyclic ligands, phos-
phines, or heteroborane cages make this system attractive for
potential catalytic applications. We are presently exploring the
reactions of these compounds with other olefins and alkynes, and
we are evaluating their catalytic activity.
References
(1) Ferguson, G.; Jennings, M. C.; Lough, A. J.; Coughlan, S.; Spalding, T.
R.; Kennedy, J. D.; Fontaine, X. L. R.; Stibr, B. J. Chem. Soc., Chem.
Commun. 1990, 891-894.
(2) AÄ lvarez, AÄ .; Mac´ıas, R.; Fabra, M. J.; Mart´ın, M. L.; Lahoz, F. J.; Oro,
L. A. Inorg. Chem. 2007, 46, 6811-6826.
(3) Volkov, O.; Mac´ıas, R.; Rath, N. P.; Barton, L. Appl. Organomet. Chem.
2003, 17, 409-420.
(4) Volkov, O.; Mac´ıas, R.; Rath, N. P.; Barton, L. Inorg. Chem. 2002, 41,
5837-5843.
(5) Volkov, O.; Mac´ıas, R.; Rath, N. P.; Barton, L. J. Organomet. Chem.
2002, 657, 40-47.
(6) Mac´ıas, R.; Rath, N. P.; Barton, L. Organometallics 1999, 18, 3637-
3648. (e) Mac´ıas, R.; Rath, N. P.; Barton, L. Angew. Chem., Int. Ed. 1999,
38, 162-164.
(7) Coughlan, S.; Spalding, T. R.; Ferguson, G.; Gallagher, J. F.; Lough, A.
J.; Fontaine, X. L. R.; Kennedy, J. D.; Sˇt´ıbr, B. J. Chem. Soc., Dalton
Trans. 1992, 2865-2871.
(8) Adams, K. J.; McGrath, T. D.; Rosair, G. M.; Weller, A. S.; Welch, A.
J. J. Organomet. Chem. 1998, 550, 315-329.
(9) Wade, K. AdV. Inorg. Chem. Radiochem. 1976, 18, 1-66.
(10) Sola, E.; Navarro, J.; Lo´pez, J. A.; Lahoz, F. J.; Oro, L. A.; Werner, H.
Organometallics 1999, 18, 3534-3546.
(11) Paxson, T. E.; Hawthorne, M. F. J. Am. Chem. Soc. 1974, 96, 4674-
4676.
(12) Delaney, M. S.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem. 1981,
20, 1341-1347.
(13) CSD search with the M(η2-C2H4) fragment (M ) transition element).
(14) Caˆmpian, M. V.; Harris, J. L.; Jasim, N.; Perutz, R. N.; Marder, T. B.;
Whitwood, A. C. Organometallics 2006, 25, 5093-5104.
(15) Carmer, R.; Kline, J. B.; Roberts, J. D. J. Am. Chem. Soc. 1969, 91, 2519-
2524.
(16) Oro, L. A.; Carmona, D. In Handbook of Homogeneous Hydrogenation;
De Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, Germany,
2007; Vol. 1, pp 3-44.
JA078197J
9
J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2149