Mesitylborane as a bis(σ-B-H) ligand: An unprecedented bonding mode to a metal center

Article abstract of DOI:10.1021/ja0733538

The new borane complex RuH₂(η²:η²-H₂BMes)(PCy₃)₂ (2) can be isolated in good yield by reaction of H₂BMes with RuH₂(η²-H₂)₂(PCy₃)₂ (1) or alternatively, by reacting the chloro dihydrogen complex RuHCl(η²-H₂)(PCy₃)₂ (3) with lithium mesitylborohydride. 2 has been fully characterized by NMR and X-ray diffraction crystallography. A DFT/B3PW91 analysis shows that in 2, the H₂BMes group is coordinated to the {RuH₂(PCy₃)₂} fragment through two geminal σ-B-H bonds. This coordination involves σ-donation to the ruthenium and π back-bonding from the ruthenium to the vacant p orbital of the boron. These two geminal σ-B-H bonds are responsible for the short Ru-B distance (1.938 (4) A by X-ray, 1.957 A by DFT). Copyright

Full text of DOI:10.1021/ja0733538

Published on Web 06/27/2007
Mesitylborane as a Bis(σ-B-H) Ligand: An Unprecedented Bonding Mode to
a Metal Center
Gilles Alcaraz,*,† Eric Clot,*,‡ Ulrike Helmstedt,‡ Laure Vendier,† and Sylviane Sabo-Etienne,*,†
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France, and
Institut Charles Gerhardt, UniVersite´ Montpellier 2, CNRS, case courrier 1501, Place Euge`ne Bataillon,
34000 Montpellier, France
Received May 11, 2007; E-mail: sabo@lcc-toulouse.fr
Borane compounds are key reagents for a wide variety of metal-
mediated organic transformations. One important class of such
reactions is the catalyzed hydroboration of olefins, and more
recently important findings have been disclosed concerning direct
alkane or arene borylation.1-3 These reactions are generally
dominated by the use of disubstituted boranes of general formulation
HBRR′ (R, R′ ) alkyl, pinacol, catechol). Coordination of the
borane compound to the metal center is an important step often
preceding the activation process.4-6 Although often invoked as
intermediates, few examples of σ-borane complexes have been
isolated.4,7-15 The final activation stage leads to oxidative addition
with formation of the corresponding hydrido boryl species.16,17
The situation can prove more complicated in the presence of
polyhydride metal precursors. In that case, a dihydroborate coor-
dination can be favored depending in particular on the Lewis acidity
of the borane.12 In this context, we became interested in exploratory
chemistry of monosubstituted boranes H2BR. To the best of our
Figure 1. X-ray crystal structure of RuH2(η2:η2-H2BMes)(PCy3)2 2.
knowledge, even though mesitylborane (H2BMes) has been avail-
able since the 1990s,18 no investigation has been carried out on the
Scheme 1. Synthesis of 2 via Two Different Pathways
coordination properties of this potential ligand. Herein we report
its reaction with a polyhydride ruthenium center leading to the
isolation of a complex which represents the first example of one
borane bound to a metal center through two σ-B-H bonds.
Room-temperature reaction of RuH2(η2-H2)2(PCy3)2 (1) in toluene
with a stoichiometric amount of H2BMes proceeds with rapid gas
evolution. After workup, a yellow powder analyzed as RuH2(η2:
η2-H2BMes)(PCy3)2 (2) was isolated (74% yield) and fully char-
acterized by NMR and X-ray diffraction crystallography (Scheme
1).19 An alternative synthesis of complex 2 was also developed by
reacting the chloro dihydrogen complex RuHCl(η2-H2)(PCy3)2 (3)
with lithium mesitylborohydride. 2 was again isolated in very good
the equatorial plane are occupied by four coplanar hydrogen atoms
H1, H2, H3, and H4. The Ru-B distance of 1.938 (4) Å is the
shortest Ru-B bond ever reported, much shorter than the sum of
the covalent radii (2.09 Å), thus suggesting an interaction between
the metal center and the boron atom.
While X-ray data have become more and more reliable for
hydrogen location, computational studies have proven a valuable
complementary tool in securely locating the positions of the H
atoms.12,21 In addition, calculations offer the advantage to analyze
in detail the mesitylborane coordination mode. 2 has been optimized
at the B3PW91 level (see Supporting Information). The optimized
geometry is in excellent agreement with the experimental structure
(see Table 1). The Ru-P and B-C bonds are computed slightly
too long but the other geometrical parameters are reproduced
accurately. All the experimental and computational data are
yield (71%).
1
The H NMR spectrum of 2 in C7D8 exhibits at 296 K in the
hydride region a broad singlet and a triplet in a 1:1 integration
ratio at δ -5.90 and δ -11.05, respectively. Upon phosphorus
decoupling the triplet (JP-H ) 25.2 Hz) collapsed into a singlet,
whereas the singlet sharpened upon boron decoupling. T1 measure-
ments on the hydride resonances rule out the presence of any (η2-
H2) ligand in 2. The integrations of the hydride and the mesityl
resonances are in agreement with a species bearing one mesityl
group and four hydrogens around the ruthenium. The 11B{1H} NMR
spectrum shows a broad signal centered at δ 58 in a region
characteristic of a three-coordinated boron atom and downfield of
that of mesitylborane in C7D8 (δ 22).
The X-ray structure of 2 was determined at 110 K (Figure 1 and
Table 1).20 The Ru atom is in a pseudo-octahedral environment
indicative of a borane adduct to a RuII dihydrido fragment.
with the phosphines in axial positions. The coordination sites in
The geometry of the free borane has also been optimized at the
B3PW91 level and the B-H bonds are computed to be 1.197 Å,
while the B-C bond is 1.535 Å. Upon coordination there is thus
† Laboratoire de Chimie de Coordination du CNRS.
‡ Institut Charles Gerhardt.
9
8704
J. AM. CHEM. SOC. 2007, 129, 8704-8705
10.1021/ja0733538 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Comparison between Selected Geometrical Parameters
(Distances in Å, Angles in Degrees) for the Experimental and
Calculated Structures for 2. Calculated Geometrical Parameters
for H2BMes Are Also Given.
dihydroborane substrate and we are currently exploring both the
coordination and the catalytic activity of this class of compounds.
Acknowledgment. We thank the CNRS and the ANR (Pro-
gramme blanc ANR-06-BLAN-0060-01) for support (G.A., L.V.,
S.S.E.) and the German Academic Exchange Service (DAAD) for
a postdoctoral fellowship (U.H.).
exptl
calcd
H2BMes
Ru-B
1.938(4)
2.3186(9)
2.2952(9)
1.543(5)
1.61(3)
1.59(3)
1.73(3)
1.77(3)
1.24(3)
1.957
2.362
2.338
1.554
1.614
1.608
1.778
1.788
1.324
1.315
Ru-P1
Ru-P2
B-C1
Supporting Information Available: X-ray crystallographic files
for 2 (CIF); computational details and Cartesian coordinates for the
structures calculated. This material is available free of charge via the
Internet at http://pubs.acs.org.
1.535
Ru-H3
Ru-H4
Ru‚‚‚H1
Ru‚‚‚H2
B-H1
1.197
1.197
References
B-H2
1.29(3)
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P1-Ru-P2
P1-Ru-B
P2-Ru-B
Ru-B-C1
P1-Ru-B-P2
123(2)
122.7
152.1
107.7
100.1
177.4
178.4
116.8
150.87(3)
108.89(11)
100.19(11)
177.1(3)
178.3
(2) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D.; Fan, Y.; Webster,
C. E.; Hall, M. B. J. Am. Chem. Soc. 2005, 127, 2538-2552.
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a significant increase of both bond distances (B-H ) 1.315/1.324
Å, B-C ) 1.554 Å, 2) that could be interpreted as the result of a
synergetic transfer of electron density: σ-donation from σ(BH) to
Ru and π-back-donation from Ru into the vacant p orbital on B.
These bonding interactions are reminiscent of the classical Dewar-
Chatt-Duncanson model for bonding of alkene or H2 to transition-
metal complexes.4 To evaluate the reaction energy of the transfor-
mation shown in Scheme 1, the bis(dihydrogen) complex 1 has
also been optimized at the B3PW91 level. The computed reaction
energies are clearly indicative of an irreversible exothermic reaction
(∆E ) -18.5 kJ mol-1, ∆G ) -43.2 kJ mol-1). This is in
agreement with the experimental observations where, in contrast
to the reversible process observed in the case of the σ-pinacolborane
complex RuH2(η2-HBpin)(η2-H2)(PCy3)2,12 no reaction was ob-
served for 2 after prolonged saturation of the solution with H2, as
(7) Hartwig, J. F.; Muhoro, C. N.; He, X.; Eisenstein, O.; Bosque, R.; Maseras,
F. J. Am. Chem. Soc. 1996, 118, 10936-10937.
(8) Muhoro, C. N.; Hartwig, J. F. Angew. Chem., Int. Ed. 1997, 36, 1510-
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(9) Muhoro, C. N.; He, X.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 5033-
5046.
(10) Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9435-9443.
(11) Montiel-Palma, V.; Lumbierres, M.; Donnadieu, B.; Sabo-Etienne, S.;
Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624-5625.
(12) Lachaize, S.; Essalah, K.; Montiel-Palma, V.; Vendier, L.; Chaudret, B.;
Barthelat, J.-C.; Sabo-Etienne, S. Organometallics 2005, 24, 2935-2943.
(13) Crestani, M. G.; Mun˜oz-Herna´ndez, M.; Are´valo, A.; Acosta-Ram´ırez,
A.; Garc´ıa, J. J. J. Am. Chem. Soc. 2005, 127, 18066-18073.
(14) Hartwig, J. F.; De Gala, S. R. J. Am. Chem. Soc. 1994, 116, 3661-3662.
(15) We only consider σ-borane complexes resulting from the coordination of
a neutral HBRR′ substrate, excluding compounds obtained from the
coordination of the base adducts HBRR′.L. For this type of compounds,
see for example: Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.;
Katoh, K.; Uruichi, M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704-
11715.
(16) Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C.
R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem.
ReV. 1998, 98, 2685-2722.
1
monitored by H and 31P NMR.
The difference in bonding strength for the σ-H-H vs σ-B-H
bonds has been rationalized with an NBO analysis of the electronic
structure of 1 and 2.22 While the σ-donating NBOs have nearly the
same population in 1 (1.70 e, σ(H2)) and 2 (1.74 e, σ(BH)), the π
back-bonding situation differs significantly between the two
compounds. The “vacant” p atomic orbital at boron (LP*(B))
exhibits a significant increase of population upon coordination (0.51
e, 2; 0.15, free H2BMes), whereas the population of σ*(H2) in 1
only increases by 0.14 e upon coordination. The NBO analysis thus
shows that even though σ-donation for two B-H bonds is
comparable to σ-donation for two H-H bonds, the Lewis acidity
of boron renders the borane strongly accepting toward Ru, thus
stabilizing 2 with respect to 1.
In summary, we report here an unprecedented coordination mode
of a borane to a metal center. Mesitylborane is able to substitute
the two σ-dihydrogen ligands in 1. The experimental and theoretical
data on RuH2(η2:η2-H2BMes)(PCy3)2 (2) are in favor of a {RuH2-
(PCy3)2} fragment coordinated by one H2BMes molecule through
two geminal σ-B-H bonds.23 This coordination involves σ-donation
to the ruthenium and π back-bonding from the ruthenium to the
vacant p orbital of the boron. These two geminal σ-B-H bonds
are responsible for the short Ru-B distance.24 The chemistry of
σ-complexes (dihydrogen and silanes) has been extensively devel-
oped over 25 years now, but the knowledge on the properties of
the σ-borane class still remains at an early stage.25 Definitively,
the presence of an accepting p orbital on boron plays a crucial role
in this area. Our results highlight the unique properties of a
(17) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006,
45, 5254-5274.
(18) Smith, K.; Pelter, A.; Jin, Z. Angew. Chem., Int. Ed. Engl. 1994, 33, 851-
853.
(19) A toluene solution (2 mL) of MesBH2 (32.8 mg, 0.248 mmol) was added
to a toluene suspension (4 mL) of 1 (170 mg, 0.242 mmol) and stirred at
room temperature for 17 h. After removal of the solvent and addition of
pentane, the precipitate was separated from the supernatant and dried under
vacuum leading to 2 (148.5 mg, 74 %). Selected NMR (C7D8) data for 2:
1H NMR (293K, 500.33 MHz) δ: -11.26 (t, 2H, 2JPH ) 25.3 Hz, RuH),
-6.10 (br, 2H, Ru(σ-BH)), 1.20-2.20 (m, 66H, Cy), 2.04 (s, 3H, p-CH3,
Mes), 2.90 (s, 6H, o-CH3, Mes), 6.74 (s, 2H, CH Mes). T1min (263 K,
500.33 MHz) δ: -11.13 (326 ms) -6.05 (138 ms). 31P{1H} NMR (293
K, 202.54 MHz) δ: 83.78 (s). 11B{1H} NMR (293 K, 160.52 MHz) δ:
58 (br). Anal. Calcd for C45H81BP2Ru: C, 67.90; H, 10.26. Found: C,
68.26; H, 10.16.
(20) Crystals are triclinic, space group P1h, a ) 10.2779(6) Å, b ) 12.5838(7)
Å, c ) 18.0772(10) Å, R ) 84.103(4)°, â ) 84.059(5)°, γ ) 68.424(5)°,
V ) 2157.2(2) Å3, Z ) 2.16678 data were collected (8787 unique), R )
0.0408, GOF ) 0.959.
(21) Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O. Chem. ReV. 2000, 100,
601-636.
(22) In 2 the NPA charge on Ru is -0.731, while in 1 it is -0.953 pointing
out to a more electron-rich metal center in the bis-dihydrogen complex.
(23) As noted by a reviewer, our findings weigh in to the argument that alkanes
such as methane might be favored to bind to metal complexes via multiple
C-H interactions.
(24) A similar case was found in a ruthenium complex where SiH4 was trapped
by two RuH2(PR3)2 units. The multiple σ-interactions were responsible
of the very short Ru-Si distances. See: Atheaux, I.; Donnadieu, B.;
Rodriguez, V.; Sabo-Etienne, S.; Chaudret, B.; Hussein, K.; Barthelat,
J.-C. J. Am. Chem. Soc. 2000, 122, 5664-5665. See also: Delpech, F.;
Sabo-Etienne, S.; Daran, J. C.; Chaudret, B.; Hussein, K.; Marsden, C.
J.; Barthelat, J. C. J. Am. Chem. Soc. 1999, 121, 6668-6682. Nikonov,
G. I. Angew. Chem., Int. Ed. 2001, 40, 3353-3355.
(25) The first σ-borane complex Cp2Ti(η2-HBcat)2 to be characterized in 1996
remains the only one with two σ-B-H bonds (see ref 7).
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