Coordination modes of boranes in polyhydride ruthenium complexes: σ-borane versus dihydridoborate

Article abstract of DOI:10.1021/om050276l

The bis(dihydrogen) complex RuH₂(η²-H ₂)₂(PCy₃)₂ (1) reacts at room temperature with 1 equiv of either HBpin or HBcat to produce the σ-borane complexes RuH₂(η²-HBpin)(η²-H ₂)-(PCy₃)₂ (2Bpin) and RuH₂(η ²-HBcat)(η²-H₂)(PCy₃) ₂ (2Bcat), respectively, by substitution of one σ-H₂ ligand by one σ-B-H. In contrast, when using the 9-BBN reagent, the dihydridoborate complex RuH[(μ-H)₂BBN](η²-H ₂)(PCy₃)₂ (2BBN) is formed. The coordination modes of the borane ligands have been ascertained by NMR spectroscopy, X-ray diffraction, and theoretical studies (DFT/B3LYP). The results indicate that the dialkoxyborane ligands (HBpin and HBcat) are not acidic enough to stabilize a true symmetrical dihydridoborate coordination mode. They thus lead to σ-borane complexes presenting a small H/BH cis interaction between the boron atom and the adjacent hydride. The σ-H₂ ligand in 2Bpin is located by X-ray diffraction at 90 K and found to be perpendicular to the equatorial plane. DFT calculations lead to the optimization of the two degenerate isomers RuH₂[η²-HB(OCH₂) ₂]-(η²-H₂)(PMe₃)₂ (5Bpin_a) (analogous to 2Bpin) and RuH[(μ-H)₂B(OCH ₂)₂](η²-H₂)(PMe ₃)₂ (5Bpin_b), demonstrating that σ-H₂ rotation and σ-borane versus dihydridoborate ligation are intimately correlated. In contrast, the 9-BBN reagent is a strong Lewis acid and leads to a dihydridoborate complex. The theoretical study on RuH[(μ-H) ₂Bpin](η²-HBpin)(PCy₃)₂ (3Bpin) shows that the bonding is also dependent on the hydride basicity: the RuH[(μ-H)₂B(OCH₂)₂](PMe₃) ₂ fragment used as a model for RuH[(μ-H)₂Bpin](PCy ₃)₂ is not basic enough to contain a second ligand bound in a dihydridoborate mode, despite the stabilization that should be gained from the resulting symmetrical structure.

Full text of DOI:10.1021/om050276l

Organometallics 2005, 24, 2935-2943
2935
Coordination Modes of Boranes in Polyhydride
Ruthenium Complexes: σ-Borane versus
Dihydridoborate
Se´bastien Lachaize,† Khaled Essalah,‡,§ Virginia Montiel-Palma,†,|
Laure Vendier,† Bruno Chaudret,† Jean-Claude Barthelat,‡ and
Sylviane Sabo-Etienne*,†
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse
Cedex 04, France, and Laboratoire de Physique Quantique, IRSAMC/UMR 5626, Universite´
Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 04, France
Received April 11, 2005
The bis(dihydrogen) complex RuH2(η2-H2)2(PCy3)2 (1) reacts at room temperature with 1
equiv of either HBpin or HBcat to produce the σ-borane complexes RuH2(η2-HBpin)(η2-H2)-
(PCy3)2 (2Bpin) and RuH2(η2-HBcat)(η2-H2)(PCy3)2 (2Bcat), respectively, by substitution of
one σ-H2 ligand by one σ-B-H. In contrast, when using the 9-BBN reagent, the dihydri-
doborate complex RuH[(µ-Η)2BBN](η2-H2)(PCy3)2 (2BBN) is formed. The coordination modes
of the borane ligands have been ascertained by NMR spectroscopy, X-ray diffraction, and
theoretical studies (DFT/B3LYP). The results indicate that the dialkoxyborane ligands
(HBpin and HBcat) are not acidic enough to stabilize a “true” symmetrical dihydridoborate
coordination mode. They thus lead to σ-borane complexes presenting a small H/BH cis
interaction between the boron atom and the adjacent hydride. The σ-H2 ligand in 2Bpin is
located by X-ray diffraction at 90 K and found to be perpendicular to the equatorial plane.
DFT calculations lead to the optimization of the two degenerate isomers RuH2[η2-HB(OCH2)2]-
(η2-H2)(PMe3)2 (5Bpin•a) (analogous to 2Bpin) and RuH[(µ-H)2B(OCH2)2](η2-H2)(PMe3)2
(5Bpin•b), demonstrating that σ-H2 rotation and σ-borane versus dihydridoborate ligation
are intimately correlated. In contrast, the 9-BBN reagent is a strong Lewis acid and leads
to a dihydridoborate complex. The theoretical study on RuH[(µ-Η)2Bpin](η2-HBpin)(PCy3)2
(3Bpin) shows that the bonding is also dependent on the hydride basicity: the RuH[(µ-
H)2B(OCH2)2](PMe3)2 fragment used as a model for RuH[(µ-Η)2Bpin](PCy3)2 is not basic
enough to contain a second ligand bound in a dihydridoborate mode, despite the stabilization
that should be gained from the resulting symmetrical structure.
Introduction
The past decade has seen major conceptual advances
in the field of borane activation.1 Landmark findings
have been disclosed both in fundamental aspects of
coordination chemistry and in catalysis.2-4 Several
reports have shown that boron-containing moieties can
adopt several geometries when bound to both early-5
and late-transition-metal centers6 that also support
hydride ligands (see Figure 1). Better knowledge has
Figure 1. Possible structures resulting from the coordina-
tion of HBR2 on the LnMH fragment.
been gained on the properties of transition-metal boryl
compounds,6a and isolation of a few σ-borane complexes
has been achieved following the first report in 1996 by
Hartwig et al.7-11 These two classes of compounds play
* To whom correspondence should be addressed. E-mail:
sabo@lcc-toulouse.fr.
† LCC-CNRS.
(6) (a) 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. (b) Baker, R. T.; Ovenall, D. W.; Calabrese,
J. C.; Westcott, S. A.; Taylor, N. J.; Williams, I. D.; Marder, T. B. J.
Am. Chem. Soc. 1990, 112, 9399. (c) Westcott, S. A.; Marder, T. B.;
Baker, R. T.; Calabrese, J. C.; Harlow, R. L.; Lam, K. C.; Lin, Z.
Polyhedron 2004, 23, 2665
‡ IRSAMC.
§ Permanent address: IPEST, 2070 La Marsa, Tunisia.
| Permanent address: CIQ, Universidad Auto´noma del Estado de
Morelos, Cuernavaca, Morelos 62210, Me´xico.
(1) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer
Academic/Plenum Publishers: New York, 2001.
(2) Burgess, K.; Ohlmeyer, M. J. Chem. Rev. 1991, 91, 1179.
(3) Smith, M. R., III. Prog. Inorg. Chem. 1999, 48, 505.
(4) Braunschweig, H.; Colling, M. Coord. Chem. Rev. 2001, 223, 1.
(5) (a) Hartwig, J. F.; De Gala, S. R. J. Am. Chem. Soc. 1994, 116,
3661. (b) Lantero, D. R.; Motry, D. H.; Ward, D. L.; Smith, M. R., III.
J. Am. Chem. Soc. 1994, 116, 10811. (c) Lantero, D. R.; Miller, S. L.;
Cho, J. Y.; Ward, D. L.; Smith, M. R., III. Organometallics 1999, 18,
235. (d) Liu, D.; Lam, K.-C.; Lin, Z. J. Organomet. Chem. 2003, 680,
148. (e) Pandey, K. K. Inorg. Chem. 2001, 40, 5092.
(7) Hartwig, J. F.; Muhoro, C. N.; He, X.; Eisenstein, O.; Bosque,
R.; Maseras, F. J. Am. Chem. Soc. 1996, 118, 10936.
(8) Muhoro, C. N.; Hartwig, J. F. Angew. Chem., Int. Ed. 1997, 36,
1510.
(9) Muhoro, C. N.; He, X.; Hartwig, J. F. J. Am. Chem. Soc. 1999,
121, 5033.
(10) Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9435.
(11) Montiel-Palma, V.; Lumbierres, M.; Donnadieu, B.; Sabo-
Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624.
10.1021/om050276l CCC: $30.25 © 2005 American Chemical Society
Publication on Web 05/12/2005

2936 Organometallics, Vol. 24, No. 12, 2005
Lachaize et al.
bonded in different modes, σ-borane and dihydridobo-
rate,11 whereas when using an excess of the 9-BBN
dimer a bis(dihydridoborate) complex was isolated.23 In
this article, we describe a new family of σ-borane and
dihydridoborate complexes prepared by stoichiometric
reactions of HBpin, HBcat, and 9-BBN with 1. We focus
on structural and theoretical studies (DFT/B3LYP) that
show subtle differences between σ-borane and dihydri-
doborate coordination modes. These differences are not
only dependent on the substituents on the boron atom,
i.e., the Lewis acidity of the borane, but also on the
behavior of the ancillary ligands. An understanding of
how borane coordination to a metal center is controlled
could be useful in understanding and improving known
catalytic processes and, hopefully, in designing new
ones.
Figure 2. (a) Proposed σ-bond metathesis step in CpM-
(RH)(CO)n[HB(OR)2] (R ) CH2; M ) Fe, n ) 2; M ) W,
n ) 1). (b) Representation of the two possible mesomers of
Cp2Ti(η2-HBcat)(η2-HSiH2Ph).
a central role in key events in catalysis, such as B-H
oxidative addition and reductive elimination steps. New
applications overcoming the well-known hydroboration
process have been disclosed. Borane activation by
transition-metal complexes now holds promise as a way
to selectively functionalize alkanes and arenes. Ish-
iyama and Miyaura have reviewed the most recent
developments in that field.12-18 The formation of an
intermediate with a strong but reactive C-B bond is
indeed a method of choice to functionalize alkanes.19,20
Hartwig et al. have recently proposed a mechanism for
alkane borylation in which a σ-borane ligand is directly
implied in a σ-bond metathesis step (see Figure 2a).21
In this case, the Lewis acidity of the boron atom plays
a major role in the interaction with the adjacent alkyl
ligand. A separate work shows that the mixed borane-
silane complex Cp2Ti(η2-HBcat)(η2-HSiH2Ph), present-
ing an interaction between the Si-H bond and the boron
atom,9 can be seen as a model for C-H bond activation
assisted by a σ-borane. A dihydridoborate silyl species
appears to be a possible resonance form (see Figure 2b).
Lin et al. have shown by theoretical calculations that
the same behavior could be expected with CH4, the
Lewis acidity of the boron atom playing again a key
role.22
Experimental Section
General Considerations. All reactions were performed
using standard Schlenk or drybox techniques under argon.
Solvents were dried and distilled prior to use. All reagents
were purchased from Aldrich, except RuCl3‚3H2O, which came
from Johnson Matthey Ltd, and were used without further
purification, except HBpin and HBcat, which were purified by
trap-to-trap techniques. NMR solvents were dried using ap-
propriate methods and degassed prior to use. NMR samples
of sensitive compounds were all prepared under an argon
atmosphere, using NMR tubes fitted with Teflon septa. NMR
spectra were recorded on Bruker AC 200(with 1H at 200.13
MHz and 31P at 81.015 MHz), DPX 300 (with 1H at 300.13
MHz, 31P at 121.49 MHz, and 13C at 75.46 MHz), and AMX
400 (with 1H at 400.13 MHz, 31P at 161.98 MHz,13C at 100.71
MHz, 29Si at 79.50 MHz, 11B at 128.38 MHz, and H at 61.42
2
MHz) spectrometers. Crystal data were collected at low
temperature on an Xcalibur Oxford Diffraction diffractometer,
equipped with an Oxford Cryosystems cryostream cooler device
and using graphite-monochromated Mo KR radiation (λ )
0.710 73 Å). The complex RuH2(η2-H2)2(PCy3)2 (1) was prepared
according to a published procedure,24 and the synthesis of
3Bpin has already been reported.11
Synthesis of RuH2(η2-HBpin)(η2-H2)(PCy3)2 (2Bpin).
Method a. Addition of HBpin (13.3 µL, 0.09 mmol) to a stirred
suspension of 1 (47.2 mg, 0.07 mmol) in 0.4 mL of THF led to
gas evolution and complete dissolution after 2 min. The beige
solution was left at room temperature in the glovebox for
crystallization. After 24 h, a mixture of colorless crystals in
small quantity with a large amount of starting material was
deposited in the bottom of the Schlenk tube. Unfortunately,
it was not possible to separate them efficiently.
Method b. A sample of 3Bpin dissolved in C7D8 was placed
under an atmosphere of dihydrogen. The NMR spectra run
after 5 min showed quantitative conversion to 2Bpin.
Method c. A 20 mg portion of 1 (0.03 mmol) and 10 mg of
B2pin2 (0.04 mmol) were placed in an NMR tube, and 0.5 mL
of C7D8 was added. The tube was heated to 80 °C for 5 h. By
the last two methods, NMR spectra were consistent with the
formation of 2Bpin and HBpin (which later was hydrolyzed
Our group has recently reported that the bis(dihy-
drogen) complex RuH2(η2-H2)2(PCy3)2 (1) reacts with
excess HBpin to give a complex with two borane ligands
(12) Ishiyama, T.; Miyaura, N. J. Organomet. Chem. 2003, 680, 3.
(13) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N.
R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390.
(14) Lawrence, J. D.; Takahashi, M.; Bae, C.; Hartwig, J. F. J. Am.
Chem. Soc. 2004, 126, 15334.
(15) Cho, J.-Y.; Iverson, C. N.; Smith, M. R., III. J. Am. Chem. Soc.
2000, 122, 12868.
(16) Cho, J.-Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E., Jr.; Smith,
M. R., III. Science 2002, 295, 305.
(17) Shimada, S.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B.
Angew. Chem., Int. Ed. 2001, 40, 2168.
(18) Lam, W. H.; Lam, K. C.; Lin, Z.; Shimada, S.; Perutz, R. N.;
Marder, T. B. Dalton 2004, 1556.
1
(19) Jones, W. D. Science 2000, 287, 1942.
to (Bpin)2O). Data for 2Bpin are as follows. H NMR (C7D8,
(20) The reaction is thermodynamically favored, as shown by
calculations. B-C (468 kJ/mol) and B-H (464 kJ/mol) bonds are
stronger by 87 kJ/mol than B-B (434 kJ/mol) and C-H (409 kJ/mol)
bonds that are broken in the reaction between B2pin2 and an alkane
(calculations were performed with CH4). The reaction with HBpin
producing an H-H bond (434 kJ/mol) is also exothermic by 29
kJ/mol. This clearly shows why boron-containing compounds are key
reagents in alkane functionalization. See: Wan, X.; Wang, X.; Luo,
Y.; Takami, S.; Kubo, M.; Miyamoto, A. Organometallics 2002, 21, 3703.
(21) Webster, C. E.; Fan, Y.; Hall, M. B.; Kunz, D.; Hartwig, J. F.
J. Am. Chem. Soc. 2003, 125, 858.
293 K, 300.13 MHz): δ -8.83 (br, 5H, RuH5), 1.25 (s, 12H,
Bpin), 1.34-2.17 (m, 66H, PCy3). T1min (C7D8, 253 K, 300.13
MHz): 40 ms for the hydride resonance. 13C{1H} NMR (C7D8,
293 K, 75.47 MHz): δ 25.1 (s, CH3), 81.7 (s, BOC). 31P{1H}
(23) Essalah, K.; Barthelat, J. C.; Montiel-Palma, V.; Lachaize, S.;
Donnadieu, B.; Chaudret, B.; Sabo-Etienne, S. J. Organomet. Chem.
2003, 680, 182.
(24) Borowski, A. F.; Sabo-Etienne, S.; Christ, M. L.; Donnadieu,
B.; Chaudret, B. Organometallics 1996, 15, 1427.
(22) Liu, D.; Lam, K. C.; Lin, Z. Organometallics 2003, 22, 2827.

Coordination Modes of Boranes
Organometallics, Vol. 24, No. 12, 2005 2937
NMR (C7D8, 293 K, 121.49 MHz): δ 72.2 (s). 11B{1H} NMR
(C7D8, 293 K, 96.29 MHz): δ 35.1 (br, w1/2 ) 509 Hz).
Scheme 1. Reactions of 1 with Neutral Boranes:
(a) 1 Equiv of HBpin in THF; (b) 1.1 Equiv of
HBcat in Pentane; (c) 0.5 Equiv of (9-BBN)2 in
THF, Benzene, or Toluene
Synthesis of RuH2(η2-HBcat)(η2-H2)(PCy3)2 (2Bcat). Ad-
dition at room temperature of HBcat (11.5 µL, 0.107 mmol) to
a stirred suspension of 1 (57.5 mg, 0.086 mmol) in 1.1 mL of
pentane resulted in immediate dihydrogen evolution and
complete dissolution of the solid. A red-orange solid precipi-
tated immediately in very small quantities and was identified
as the complex RuH(η6-C6H4O2BO2C6H4)(PCy3)2 (4). The fil-
tered orange solution was kept at -36 °C until pale yellow
crystals were formed (after one night). The solution was
filtered off, and the crystals were dried under vacuum (32%
1
yield). Data for 2Bcat are as follows. H NMR (C7D8, 293 K,
300.13 MHz): δ -8.48 (br, 5H, RuH5), 6.95, 6.77 (m, 4H, Bcat),
1.0-2.2 (m, 66H, PCy3). T1min (C7D8, 198 K, 300.13 MHz): 47
ms for the hydride resonance. 13C{1H} NMR (C7D8, 293 K,
75.47 MHz): δ 152.5 (s, BOC), 120.4, 109.9 (s, Bcat). 31P{1H}
NMR (C7D8, 293 K, 121.49 MHz): δ 70.2 (s). 11B{1H} NMR
(C7D8, 293 K, 96.29 MHz): δ 35.8 (br, w1/2 ) 350 Hz). Anal.
Calcd for C42H75O2BP2Ru: C, 64.17; H, 9.63. Found: C, 64.21;
H, 10.31.
Synthesis of RuH[(µ-H)2BBN](η2-H2)(PCy3)2 (2BBN).
Addition of [9-BBN]2 (13.8 mg, 0.056 mmol) dissolved in 1.2
mL of THF to a stirred suspension of 1 (53.4 mg, 0.080 mmol)
in 0.8 mL of THF resulted in very slow but total dissolution
in about 1 h. The orange solution was kept at room temper-
ature for crystallization over 48 h. Yellow crystals of 2BBN
were obtained together with colorless crystals of 9-BBN dimer.
The solution was filtered off, and 2BBN crystals were sepa-
rated manually from the others, as far as possible (less than
25% yield). Purification of the mixture using different solvents
was unsuccessful. Data for 2BBN are as follows. 1H NMR
(C7D8, 300.13 MHz): 293 K, δ -9.3 (vbr), 2.5-1.1 (m, C8H14
and PCy3); 253 K, δ -4.9 (br, 2H, Ru-µ-H2B), -11.48 (br t,
3H, RuH(H2)). T1 (C7D8, 253 K, 300.13 MHz): 40 ms (Ru-µ-
H2B), 34 ms (RuH(H2)). 31P{1H} NMR (C7D8, 121.49 MHz): 293
K, δ 66.0 (br); 253 K, δ 65.8 (s).
critical points on the potential energy surface was fully
optimized with the gradient method available in Gaussian 98.
Calculations of harmonic vibrational frequencies were per-
formed to determine the nature of each critical point. The NBO
analysis of our complexes was not relevant. Even when
appropriate methods were used to consider 3c-2e bonds and/
or resonance in the Lewis structure, the highly delocalized
electron density always prevented the localization of one to
two valence electrons in the calculated natural orbitals. The
analysis and comparison of their occupancies were then
meaningless.
Synthesis of RuH(η6-C6H4O2BO2C6H4)(PCy3)2 (4). Com-
pound 4 was obtained by decomposition of 2Bcat in the
presence of an excess of HBcat.25 Alternatively, 4 was isolated
as an orange-red solid in small quantities during the synthesis
Results and Discussion
1
Synthesis and Characterization of RuH2(η2-HB-
pin)(η2-H2)(PCy3)2 (2Bpin). The stoichiometric reac-
tion of 1 with HBpin in toluene-d8 was followed by NMR
spectroscopy. Substitution of one labile σ-H2 ligand by
one B-H bond results in an equilibrium between 1 and
RuH2(η2-HBpin)(η2-H2)(PCy3)2 (2Bpin) at room temper-
ature (see Scheme 1a). When using a large excess of
HBpin, a new equilibrium is established between 2Bpin
and RuH[(µ-Η)2Bpin](η2-HBpin)(PCy3)2 (3Bpin), syn-
thesized and characterized previously by X-ray diffrac-
tion.11 These successive equilibria prevented any isola-
tion of pure 2Bpin in high yield. Nevertheless, NMR
experiments confirm its formulation without, however,
giving insights into the borane coordination mode. The
1H NMR spectrum displays in the high-field region only
one broad signal at δ -8.83, corresponding to the five
hydrides in fast exchange. No decoalescence was ob-
served, even at 183 K. The T1min value of 40 ms at 253
K (300 MHz) is in agreement with the presence of a σ-H2
ligand. Fast exchange let us anticipate that low energy
barriers and low energy differences should be encoun-
tered between at least two of the three possible isomers
that can be found in polyhydride species, as shown in
Figure 1. The 11B{1H} NMR spectrum shows a broad
signal (δ 35.1, w1/2 ) 509 Hz) slightly shifted to low field
compared to free HBpin (δ 28.1), in the region generally
associated with σ-borane or boryl complexes.
of 2Bcat. H NMR (C7D8, 293 K, 300.13 MHz): δ -11.46 (t,
2JPH ) 43.2 Hz, 1H, RuH), 6.98, 7.31 (br, non coordinated ring),
5.07, 5.21 (br, coordinated ring), 1.1-2.1 (m, PCy3). 31P{1H}
NMR (C7D8, 293 K, 121.49 MHz): δ 57.7 (s). 11B{1H} NMR
(C7D8, 293 K, 96.29 MHz): δ 15.7 (s).
Computational Details. DFT calculations were performed
with the Gaussian 98 series of programs26 using the nonlocal
hybrid functional denoted as B3LYP.27,28 For ruthenium, the
core electrons were represented by a relativistic small-core
pseudopotential using the Durand-Barthelat method.29 The
16 electrons corresponding to the 4s, 4p, 4d, and 5s atomic
orbitals were described by a (7s, 6p, 6d) primitive set of
Gaussian functions contracted to (5s, 5p, 3d). Standard
pseudopotentials developed in Toulouse were used to describe
the atomic cores of all other non-hydrogen atoms (C, B, O, and
P).30 A double plus polarization valence basis set was employed
for each atom (d-type function exponents were 0.80, 0.60, 0.85,
and 0.45, respectively). For hydrogen, a standard primitive (4s)
basis contracted to (2s) was used. A p-type polarization
function (exponent 0.9) was added for the hydrogen atoms
directly bound to ruthenium. The geometry of the various
(25) Rodriguez, A.; Sabo-Etienne, S.; Chaudret, B. Anal. Quim. Int.
Ed. 1996, 131.
(26) Frisch, M. J., et al. Gaussian 98, revision A.11; Gaussian, Inc.:
Pittsburgh, PA, 2001.
(27) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(28) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(29) Durand, P.; Barthelat, J. C. Theor. Chim. Acta 1975, 38, 283.
(30) Bouteiller, Y.; Mijoule, C.; Nizam, M.; Barthelat, J. C.; Daudey,
J. P.; Pelissier, M.; Silvi, B. Mol. Phys. 1988, 65, 295.

2938 Organometallics, Vol. 24, No. 12, 2005
Table 1. Crystal Data for the Compounds 2Bpin, 2Bcat, and 2BBN
Lachaize et al.
2Bpin
2Bcat
2BBN
formula
C
42H83BO2P2Ru
C42H75BO2P2Ru
785.84
C44H85BP2Ru
formula wt
793.90
787.94
cryst syst
monoclinic
P21/c
monoclinic
P21/c
orthorhombic
Pnma
4; 1.101
0.423
1712
23.6220(13)
20.4907(12)
9.8244(5)
90
4755.3(5)
100(2)
space group
Z; calcd density, mg/m3
abs coeff, mm-1
F(000)
4; 1.240
4; 1.268
0.477
0.492
1720
1688
a, Å
10.9533(7)
15.9247(9)
24.7405(13)
99.786(5)
4252.6(4)
90(2)
23.827(4)
9.8024(7)
19.0158(18)
112.01(2)
4117.8(8)
100(2)
b, Å
c, Å
â, deg
V, Å3
temp, K
no. of data/restraints/params
goodness of fit on F2
R1 (I > 2σ(I))
wR2 (I > 2σ(I))
R1 (all data)
wR2 (all data)
largest diff peak and hole, eÅ-3
12 972/0/457
1.102
13 559/0/453
0.927
4982/0/245
1.112
0.0270
0.0306
0.0378
0.0585
0.0625
0.0980
0.0292
0.0504
0.0391
0.0596
0.0667
0.0989
0.697 and -0.842
1.048 and -0.525
0.355 and -1.810
Table 2. Selected Bond Lengths (Å) and Angles
(deg) for 2Bpin (X-ray) and the Four Optimized
Isomers of 5Bpin (DFT/B3LYP)
The formulation of 2Bpin as a σ-borane complex is
finally ascertained by single-crystal X-ray diffraction.
A first measurement at 180 K allowed the localization
of all the heavy atoms and the hydrides, except those
of the σ-H2 ligand. However, a maximum of electron
density accounting over one electron was found at the
middle of the expected location for a σ-H2 moiety,
behavior which could be in agreement with a free
rotating dihydrogen. A new X-ray diffraction experiment
was then carried out at 90 K. The high quality of the
data and the measurement at very low temperature, to
prevent dihydrogen rotation as much as possible, al-
lowed the location of all the hydrogen atoms around the
ruthenium. Remarkably, the σ-H2 ligand is found to be
perpendicular to the equatorial plane (see Figure 3 and
Tables 1 and 2). The ruthenium atom is in a pseudooc-
tahedral environment with the phosphines in axial
positions. The dihydrogen ligand is only slightly acti-
vated with a H-H distance of 0.79(3) Å. Moreover, its
orientation is quite unusual for a polyhydride complex,
where a stabilizing H/H2 cis interaction resulting from
5Bpin
2Bpin
a
b
c
d
Ru-P1
Ru-P2
Ru-B
Ru-H1
Ru-H2
Ru-H3
Ru-H4
Ru-H5
B-H1
2.3525(3) 2.353 2.353 2.340 2.343
2.3600(3) 2.353 2.354 2.340 2.342
2.173(2)
1.67(2)
1.57(2)
1.56(2)
1.71(1)
1.70(3)
1.30(2)
1.89(2)
0.79(3)
1.406(2)
1.406(2)
2.162 2.207 2.276 2.252
1.764 1.775 1.750 1.745
1.615 1.678 1.784 1.768
1.637 1.636 1.744 1.758
1.788 1.720 1.631 1.634
1.787 1.726 1.631 1.632
1.362 1.372 1.320 1.324
2.003 1.497 2.224 2.465
0.858 0.906 0.878 0.872
1.413 1.424 1.399 1.400
1.413 1.423 1.399 1.400
B‚‚‚H2
H4-H5
B-O1
B-O2
Ru-H1-B
Ru-B-H1
Ru-H2-B
B-Ru-H3
B-Ru-H4
P1-Ru-P2
O1-B-O2
H1-Ru-H2
H2-Ru-H3
H4-Ru-H5
93.28(6)
50.1(8)
86.5
54.6
72.5
88.1
53.5
87.9
94.6
50.0
68.2
93.4
50.7
61.7
83.1
77.2(6)
146.5(8)
123.8(6)
143.4 130.7 93.9
126.4 156.3 164.2 160.8
157.66(1) 164.8 170.7 166.7 162.6
109.2(1)
94.7(10)
88.4(11)
26.6(10)
109.5 109.2 110.9 110.8
100.9 81.1 100.4 109.4
81.3
27.8
88.1
30.5
28.8
81.4
28.6
84.9
Cent(O1,O2)-B-Ru 170.0
165.3 173.2 162.6 161.3
∆E (kJ/mol)
2.5 0.0 18.4 23.0
an in-plane orientation is generally expected.31 In our
case, the perpendicular orientation seems to be pre-
ferred to avoid back-donation competition between the
σ-borane and the σ-dihydrogen ligands. The B-H1 bond
distance (1.30(2) Å) represents a normal elongation for
a σ-borane complex by comparison to the calculated
B-H bond distance of 1.17 Å in a free dialkoxyborane.32
The coordination mode adopted by the borane is clearly
of a σ-borane mode, with a very small Lewis acid/base
interaction between the hydride H(2) and the boron
atom, as reflected by the long distance of 1.89(2) Å.
Moreover, the structural data on the σ-borane dihydri-
doborate complex 3Bpin give structural parameters for
comparison between dihydridoborate and σ-borane
Figure 3. Structure of 2Bpin (thermal ellipsoids set at
50% probability; protons omitted for clarity). In the framed
view, the cyclohexyl groups are omitted for clarity.
(31) Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O. Chem. Rev.
2000, 100, 601.
(32) Rablen, P. R.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 4648.

Coordination Modes of Boranes
Organometallics, Vol. 24, No. 12, 2005 2939
coordination modes.11 The most convincing criterion is
related to the orientation of the Bpin groups. The Bpin
group in a σ-borane ligation is clearly not pointing
toward the metal, as it should be in a dihydridoborate
or a dihydride boryl species. The angle between the
middle of [O,O], B, and Ru (170.0°) in 2Bpin is close to
the σ-borane corresponding angle (171.5°) and signifi-
cantly different from the dihydridoborate angle (177.1°)
in 3Bpin. The Ru-B bond length (2.173(2) Å) is
intermediate to the Ru-B distances in 3Bpin
(2.188(5) and 2.157(5) Å). Discussion on the parameters
involving the different types of hydrogen atoms, hydride
or σ-coordinated (dihydrogen or borane), will be found
in the theoretical section.
Table 3. Selected Bond Lengths (Å) and Angles
(deg) for 2Bcat (X-ray) and the Four Optimized
Isomers of 5Bcat (DFT/B3LYP)
5Bcat
2Bcat
a
b
c
d
Ru-P1
Ru-P2
Ru-B
Ru-H1
Ru-H2
Ru-H3
Ru-H4
Ru-H5
B-H1
2.3573(5) 2.357 2.358 2.345 2.348
2.3335(5) 2.357 2.358 2.345 2.348
2.124(2)
1.71(2)
1.63(2)
1.58(2)
1.74(2)
1.71(2)
1.24(2)
1.60(2)
0.77(3)
1.443(2)
1.444(2)
2.135 2.198 2.231 2.208
1.777 1.791 1.752 1.749
1.615 1.693 1.792 1.766
1.634 1.629 1.752 1.758
1.789 1.713 1.632 1.633
1.789 1.719 1.629 1.632
1.359 1.354 1.320 1.325
1.958 1.451 2.201 2.461
0.856 0.909 0.872 0.872
1.437 1.447 1.422 1.424
1.437 1.447 1.422 1.424
B‚‚‚H2
H4-H5
B-O1
Synthesis and Characterization of RuH2(η2-HB-
cat)(η2-H2)(PCy3)2 (2Bcat). The mixture of 1 with 1.1
equiv of HBcat in pentane affords colorless crystals of
RuH2(η2-HBcat)(η2-H2)(PCy3)2 (2Bcat) isolated in 32%
yield (see Scheme 1b). The rather low yield can be
explained by the formation of the zwitterionic complex
RuH(η6-C6H4O2BO2C6H4)(PCy3)2 (4) as a result of bo-
rane decomposition. We have reported the characteriza-
tion of 4 in a previous paper.25 Marder et al. have
proposed a decomposition mechanism based on the
presence of free phosphine.33 NMR experiments reveal
a behavior similar to that of 2Bpin, as expected with
borane reagents of the same nature. Indeed, the hy-
drides are all in fast exchange even at 183 K, as shown
B-O2
Ru-H1-B
Ru-B-H1
Ru-H2-B
B-Ru-H3
B-Ru-H4
P1-Ru-P2
O1-B-O2
H1-Ru-H2
H2-Ru-H3
H4-Ru-H5
90.6(7)
53.7(8)
82.3(7)
136.1(6)
130.5(7)
84.9
56.0
72.3
87.5
54.5
88.4
92.1
51.6
67.0
90.7
52.4
60.3
82.4
142.9 130.3 93.5
127.0 157.4 163.0 159.7
155.67(2) 164.1 169.8 166.7 162.2
106.4(2)
83.5(8)
88.0(8)
25.7(9)
107.4 107.3 108.5 108.3
100.7 79.3 101.3 111.3
81.2
27.7
89.0
30.7
28.5
82.1
28.7
85.3
Cent(O1,O2)-B-Ru 170.3
165.5 173.5 163.1 162.3
∆E (kJ/mol)
4.6 0.0 24.0 25.0
1
as the main compound (see Scheme 1c). The reaction
could also be performed in benzene or toluene, but was
faster in THF thanks to the 9-BBN‚THF adduct forma-
tion.34 The successive reaction of 2BBN with an excess
of 9-BBN affords the 16-electron bis(dihydridoborate)
complex Ru[(µ-Η)2BBN]2(PCy3) and the 9-BBN‚PCy3
adduct, as previously reported.23 At 298 K, the 1H NMR
spectrum of 2BBN shows fast-exchanging hydrides
characterized by one broad peak at δ -9.3. Decoales-
cence is observed when decreasing the temperature, and
two peaks appear at high field at 253 K. The broad
singlet at δ -4.90 is assigned to the two bridging
hydrides (B-H-Ru), whereas the terminal hydride and
the σ-dihydrogen ligands, still in fast exchange even at
193 K, resonate as a broad pseudo-triplet at δ -11.48.
The presence of the σ-dihydrogen is confirmed by T1
measurements. At 253 K, the values are 40 and 34 ms,
respectively (the minimum value could not be mea-
sured). The low relaxation time for the bridging hydrides
is indicative of a slow exchange process, still remaining
between all the hydrides at that temperature. We were
not able to detect any boron signal for 2BBN by 11B-
{1H} NMR. The decoalescence phenomenon with a 2:3
ratio of the hydride signals and the well-known strong
Lewis acidity of 9-BBN support a dihydridoborate
coordination mode in 2BBN.
by the single broad resonance at -8.48 ppm in the H
NMR spectrum. The integration is in agreement with
one Bcat group in the coordination sphere of the metal,
and the T1min value of 47 ms at 198 K (300 MHz)
indicates the presence of a σ-dihydrogen ligand. The 11B-
{1H} NMR spectrum shows a broad signal (δ 35.8, w1/2
) 353 Hz) shifted to low field compared to free HBcat
(δ 28.8).
The σ-borane formulation for 2Bcat is deduced from
the X-ray structure determined at 100 K (see Tables 1
and 3 and the Supporting Information). The data are
of good quality, but the electronic density is quite
delocalized around the ruthenium center. Even if the
hydrides have been located on electron density maps,
analysis of their positions must be done with caution,
especially regarding the orientation of the dihydrogen
ligand H(4)-H(5), which could still be rotating. How-
ever, the overall structure compares well with that of
2Bpin. The Bcat orientation is a key parameter to
confirm the adoption of a dissymmetrical coordination
mode: i.e., a σ-borane. Indeed, the angle Cent(O1,O2)-
B-Ru is 170.3°, with Cent(O1,O2) in the equatorial
plane. The Ru-B bond length of 2.124(2) Å is signifi-
cantly shorter than in 2Bpin. The better π-acceptor
character of H-Bcat compared to H-Bpin is certainly
responsible for a stronger Ru-B bond in that case. This
trend has already been described in the literature.3
Synthesis and Characterization of RuH[(µ-Η)2-
BBN](η2-H2)(PCy3)2 (2BBN). The addition of 0.5 equiv
of (9-BBN)2 dimer to a solution of 1 in THF results in
slow formation of RuH[(µ-Η)2BBN](η2-H2)(PCy3)2 (2BBN)
This formulation is confirmed by X-ray analysis of a
yellow monocrystal of 2BBN at 100 K (see Figure 4 and
Table 1). The ruthenium is in a pseudo-octahedral
environment with the phosphines in axial positions. The
cyclohexyl groups of the phosphines are oriented in
order to avoid any steric hindrance with the BBN. The
adopted coordination mode is thus the result of elec-
tronic effects. As expected for a dihydridoborate ligand,5a
the Ru-B bond is significantly longer than in 2Bpin
(33) (a) Dai, C. Y.; Robins, E. G.; Scott, A. J.; Clegg, W.; Yufit, D.
S.; Howard, J. A. K.; Marder, T. B. J. Chem. Soc., Chem. Commun.
1998, 1983. (b) Westcott, S. A.; Taylor, N. J.; Marder, T. B.; Baker, R.
T.; Jones, N. J.; Calabrese, J. C. J. Chem. Soc., Chem. Commun. 1991,
304. (c) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T.;
Calabrese, J. C. Inorg. Chem. 1993, 32, 2175.
(34) Brown, H. C.; Kukarni, S. U. J. Org. Chem. 1977, 42, 4169.

2940 Organometallics, Vol. 24, No. 12, 2005
Lachaize et al.
Figure 4. Structure of 2BBN (thermal ellipsoids set at
50% probability; protons omitted for clarity). The disorder
on a carbon in the BBN group has been resolved as two
possible conformations. Selected bond lengths (Å) and
angles (deg): Ru-P(1), 2.3502(5); Ru-B, 2.346(4); B-C(1),
1.608(3); B-H(1), 1.31(4); B-H(2), 1.34(5); P(1)-Ru-P(1′),
165.98(3); H(1)-Ru-H(2), 69(2); Cent(C1,C1′)-B-Ru, 177.9.
Figure 5. Fourier difference map of the experimental
electronic density at small angles (θ ) 16.5°) in the
equatorial plane of 2BBN (heavy-atom contributions have
been subtracted). The atoms are localized according to the
restricted density.
and 2Bcat, and its value of 2.346(4) Å is high enough
to exclude a dihydride boryl coordination mode.6a More-
over, the orientation of the BBN (the angle Cent(C1,-
C1′)-B-Ru is 177.9°) agrees with a dihydridoborate
coordination. This angle slightly deviates from 180°, as
a result of the dissymmetry induced by the terminal
hydride and the σ-H2, both trans to the dihydridoborate.
Unfortunately, the highly delocalized electronic density
precluded any good location in that region of the
equatorial plane trans to the dihydridoborate, and the
H(3), H(4), and H(5) positions should be considered as
artifacts (see below). To solve that problem, we calcu-
lated a Fourier difference map of the electronic density
observed at small angles in the equatorial plane (see
Figure 5 and the Supporting Information).35,36 After
subtraction of the heavy-atom contributions, the result-
ing map allows a more accurate localization of the
hydrogen atoms, and the elongated dihydrogen H(4)-
H(3) now clearly appears. A small dissymmetry is also
revealed in the dihydridoborate, in agreement with the
previous observation. Even when looking at small
angles, the electronic density is still very delocalized,
as a result of an H/H2 cis interaction between H(5) and
H(4)-H(3). Eisenstein et al. have proved that kind of
interaction to be favored by the σ-donor and π-donor
abilities of a ligand such as iodine in RuHI(η2-H2)(PCy3)2
or the carboxylate in RuH(η2-O2CCH2CH3)(η2-H2)-
(PCy3)2, for example.31,37,24,38 The dihydridoborate be-
haves in a similar manner. This interaction is also
responsible for the fast exchange of the three hydrogen
atoms, as observed by NMR spectroscopy.
key to understanding why the σ-H2 ligand is preferen-
tially out of the equatorial plane in 2Bpin and 2Bcat
structures. The free lone pairs of the oxygen atoms
interact with the sp2-hybridized “vacant” p orbital of the
boron atom in the two complexes (at some point, the
effect is reduced in the Bcat analogue because of
structural constraints and competing interaction with
the arene ring). The interaction with the adjacent
hydride H(2) is disfavored, and consequently a dihydri-
doborate mode and a H/H2 cis interaction are also
1
unfavorable. According to H NMR data, the rotation
of the σ-H2 group has a low energy barrier in these
complexes. What happens when the σ-H2 ligand is in
the equatorial plane? A theoretical study (DFT/B3LYP)
of 2Bpin and 2Bcat was thus performed to address this
question. RuH5[B(OCH2)2](PMe3)2 (5Bpin) and RuH5-
(Bcat)(PMe3)2 (5Bcat) were used as models for 2Bpin
and 2Bcat, respectively. For each one, four isomers have
been optimized as local minima on the singlet spin state
potential energy surface: 5Bpin•a and 5Bcat•a cor-
responding to the X-ray structures, 5Bpin•b and
5Bcat•b with a dihydridoborate coordination mode,
and finally, isomers with the σ-H2 adjacent to the boron,
either in the equatorial plane (5Bpin•c and 5Bcat•c)
or perpendicular to it (5Bpin•d and 5Bcat•d). The
geometries of the 5Bpin isomers are depicted in Figure
6, and selected parameters are listed in Tables 2 and 3
for 5Bpin and 5Bcat, respectively.
The isomers RuH2[η2-HB(OCH2)2](η2-H2)(PMe3)2
(5Bpin•a) and RuH[(µ-H)2B(OCH2)2](η2-H2)(PMe3)2
(5Bpin•b) are accidentally degenerate, as their energy
difference ∆E is only 2.5 kJ/mol. The same remark can
be made for 5Bpin•c and 5Bpin•d. The optimized
structural parameters of 5Bpin•a fit well with those
determined by X-ray diffraction for 2Bpin and are in
agreement with a σ-borane formulation. The angles
implying the hydrides are not so well reproduced, but
the trends are well represented. The difference between
the calculated and experimental P(1)-Ru-P(2) angles
Theoretical Studies of 2Bpin and 2Bcat: The
Role of σ-H2. The last remark about 2BBN gives us a
(35) La Placa, S. J.; Ibers, J. A. Acta Crystallogr. 1965, 18, 511.
(36) Lantero, D. R.; Ward, D. L.; Smith, M. R., III. J. Am. Chem.
Soc. 1997, 119, 9699.
(37) Chaudret, B.; Chung, G.; Eisenstein, O.; Jackson, S. A.; Lahoz,
F. J.; Lopez, J. A. J. Am. Chem. Soc. 1991, 113, 2314.
(38) Heyn, R. H.; Macgregor, S. A.; Nadasdi, T. T.; Ogasawara, M.;
Eisenstein, O.; Caulton, K. G. Inorg. Chim. Acta 1997, 259, 5.

Coordination Modes of Boranes
Organometallics, Vol. 24, No. 12, 2005 2941
the boron atom returns to an sp2 hybridization and the
B-O bonds are reinforced. We can then conclude that
the energy difference between 5bpin•b and 5Bpin•c,
for example (18.4 kJ/mol), gives us an idea of the
stabilization energy due to a H/BH interaction in our
system. However, this energy difference remains small,
in agreement with fast-exchanging hydrides. When we
now compare 5Bpin•c and 5Bpin•d, the rotation of
the σ-H2 ligand let us appreciate the structural influence
of the back-donation from the metal to the σ*(B-H).
While the Ru-B bond is shortened by -0.024 Å in
5Bpin•d, thanks to a better bonding, the Ru-H(1) and
B-H(1) bonds change by no more than 0.005 Å. This
structural deformation related to the bond activation
process has already been described for silane activation
and alkane agostic interactions.39-41
A similar analysis can be done for 5Bcat isomers,
thanks to their great similitude with 5Bpin. This is not
surprising, since HBcat and HB(OCH2)2 are of the same
nature. The only subtle differences are the steric
constraints due to the arene ring and the electron-
withdrawing character of the latter. A slightly higher
Lewis acidity for HBcat is expected, as the oxygen lone
pairs are less willing to interact with the boron “vacant”
p orbital. Unfortunately, the energy differences are
again too small to discuss such a subtle difference in
Lewis acidity. For the same reason, 5Bcat•a and
5Bcat•b are degenerate, like 5Bcat•c and 5Bcat•d
obviously are.
Our attempts to reproduce a similar theoretical study
on the 2BBN complex led to an unexpected result. The
calculations gave, as expected, a dihydridoborate struc-
ture5d but also the same wrong positions for H3, H4,
and H5 deriving from the X-ray data before analysis at
small angles. This comforts our idea that these hydrides
are delocalized, as a result of a strong H/H2 cis interac-
tion. The potential energy surface is then certainly too
flat to support an arrested structure, with a terminal
hydride and a σ-H2 ligand in the equatorial plane, as a
local minimum. No isomer with an out-of-plane σ-H2
could be optimized. It is indeed noteworthy that this
theoretical study of 2BBN leads to the same conclusion
about proton delocalization as that deduced from NMR
and X-ray data. The frontier between a Ru(IV) and
Ru(II) species appears not to be so well delimited.
Theoretical Study of 3Bpin. The complex RuH-
[(µ-Η)2Bpin](η2-HBpin)(PCy3)2 (3Bpin) was previously
characterized by X-ray and NMR studies.11 It was the
first complex displaying two boron-attached ligands
coordinated to the same metal center with different
coordination modes, a σ-borane and a dihydridoborate.
The geometry of RuH[(µ-Η)2B(OCH2)2][η2-HB(OCH2)2]-
(PMe3)2 (6Bpin•a) optimized at the DFT/B3LYP level
provides a good agreement with the 3Bpin X-ray
structure (see Figure 7 and Table 4). The hydrides H(3)
and H(4) are slightly different, for the same reasons
invoked in the case of 5Bpin•b. No bis(σ-borane)
isomer was found on the potential energy surface. Since
H-Bpin is similar to H-H in terms of π-acceptor ability,
it leads to an important competition for back-donation
that results in the formation of a strong Lewis acid/base
Figure 6. Representation of the four optimized isomers
of 5Bpin (DFT/B3LYP).
is the result of using PMe3 as a model for PCy3. The
simple rotation of the σ-H2 ligand between 5Bpin•a
and 5Bpin•b has a dramatic effect in the coordination
mode of the borane attached ligand going from σ-borane
to dihydridoborate. The latter isomer is now very similar
to 2BBN, although the hydride delocalization does not
seem as important. The slight dissymmetry in the
dihydridoborate (the B-H distances are 1.372 and 1.497
Å) is certainly induced by the overall dissymmetry in
the equatorial plane. As expected, this change in the
borane coordination mode is accompanied by an elonga-
tion of the Ru-B bond of +0.045 Å. The B(OCH2)2 group
tends to be closer to the H(1)-B-H(2) bisecting plane,
as shown by the Cent(O1,O2)-B-Ru angle of 173.2°
(versus 165.3° in 5Bpin•a). This is a way to compen-
sate back-donation competition between the H-H and
B-H antibonding orbitals. The dihydridoborate mode
5Bpin•b favors a H/H2 cis interaction. Actually, the
“dihydridoborate” formulation should be replaced by
“acid/base Lewis adduct” as a better reflection of the
dissymmetric situation. Such a term highlights the
Lewis acidity of the borane, a key factor in these
systems.
The isomers 5Bpin•c and 5Bpin•d are typically
σ-borane complexes without any stabilizing interaction
between the boron atom and the adjacent hydride.
Indeed, their Cent(O1,O2)-B-Ru angles are the small-
est ones of the series. The Ru-B bonds of 2.276 and
2.252 Å, respectively, are longer than in 5Bpin•a, as
a result of a weaker coordination to the metal center.
This explains why the B-O bonds (1.399 and 1.400 Å)
tend to go back to their initial length of 1.372 Å
calculated for HB(OCH2)2. As coordination is weakened,
(39) Schubert, U. Adv. Organomet. Chem. 1990, 30, 151.
(40) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789.
(41) Crabtree, R. H.; Holt, E. M.; Lavin, M. E.; Morehouse, S. M.
Inorg. Chem. 1985, 24, 1986.

2942 Organometallics, Vol. 24, No. 12, 2005
Lachaize et al.
min is required to regenerate 1 from 2Bpin under 1 bar
of H2. Interestingly, the lability of the σ ligands can be
used to catalyze the conversion of HBpin into DBpin
under 1 bar of D2.42
Conclusion
Combination of NMR and X-ray data with theoretical
studies is invaluable in establishing the coordination
mode of borane reagents to a metal fragment, a problem
that is difficult to solve in highly fluxional systems.
Caution should always be taken to avoid any conclusion
based on artifacts that can be present on such delocal-
ized species. We have fully characterized three new
borane complexes resulting from the stoichiometric
reaction of 1 with HBpin, HBcat, and 9-BBN. The
coordination mode adopted by the borane simulta-
neously reflects the Lewis acidity of the borane reagent
and the Lewis basicity of the polyhydride ruthenium
fragment. The dialkoxyborane ligands (HBpin and HB-
cat) are not acidic enough to stabilize a “true” sym-
metrical dihydridoborate mode. They thus lead to
σ-borane complexes presenting a small H/BH cis inter-
action between the boron atom and the adjacent hydride
H(2). Such a H/BH cis interaction obviously depends on
the Lewis basicity of H(2) but also reflects the subtle
change in the boron acidity, as shown by the theoretical
study. When the in-plane position of the σ-H2 ligand
imposes a competition for back-donation, the H/BH cis
interaction is reinforced, resulting in an unsymmetrical
dihydridoborate mode also described as a strong Lewis
acid/base adduct. Remarkably, σ-H2 rotation and σ-bo-
rane versus dihydridoborate ligation are intimately
correlated. The 9-BBN reagent is, in contrast, a strong
Lewis acid and leads to a dihydridoborate complex. This
ligand induces such a high delocalization of the trans
hydrides that only an in-depth study of the experimental
electron density allowed us to differentiate the terminal
hydrides from the σ-H2 ligand. Finally, one can notice
that the symmetry of the borane coordination mode is
dependent on the symmetry of the coordination sphere
of the metal, and the inverse is true. However, the
theoretical study of 3Bpin shows that the bonding is
also dependent on the hydride basicity. The RuH[(µ-
H)2B(OCH2)2](PMe3)2 fragment is not basic enough to
form a second dihydridoborate ligand, despite the
stabilization that should be gained from the resulting
symmetrical structure. This study highlights the role
of the borane Lewis acidity on their behavior as ligand
and especially on the H/BH cis interaction in our
systems. The role of the ancillary ligands is also crucial,
since they determine the hydride basicity and the
overall symmetry of the coordination sphere of the
metal.
Figure 7. Representation of the two optimized isomers
of 6Bpin (DFT/B3LYP).
Table 4. Selected Bond Lengths (Å) and Angles
(deg) for 3Bpin (X-ray) and the Two Optimized
Isomers of 6Bpin (DFT/B3LYP)
6Bpin
3Bpin
a
b
Ru-P1
Ru-P2
Ru-B1
Ru-B2
Ru-H1
Ru-H2
Ru-H3
Ru-H4
B2-H2
B2‚‚‚H3
B1-H3
B1-H4
B1-O11
B1-O12
B2-O21
B2-O22
2.367(2)
2.357(2)
2.188(5)
2.157(5)
1.48(3)
1.58(3)
1.49(3)
1.55(3)
1.35(3)
2.09(3)
1.58(3)
1.47(3)
1.415(5)
1.392(6)
1.383(5)
1.396(5)
2.364
2.365
2.234
2.159
1.636
1.685
1.762
1.709
1.394
2.487
1.389
1.426
1.423
1.424
1.402
1.402
2.414
2.413
2.205
2.195
1.703
1.708
1.715
1.709
1.447
3.678
1.439
1.436
1.426
1.417
1.418
1.427
B1-Ru-B2
113.7(2)
47(2)
116.4
51.3
179.1
51.0
Ru-B2-H2
Ru-B1-H3
43(2)
52.0
51.0
Ru-B1-H4
45(2)
49.9
50.8
B1-Ru-H3
46(2)
38.4
41.2
B1-Ru-H4
42(2)
39.6
41.3
B2-Ru-H2
39(2)
40.2
40.7
B2-Ru-H3
67(2)
78.0
139.7
179.3
108.4
108.4
81.3
P1-Ru-P2
160.45(3)
109.2(4)
110.0(3)
81(2)
169.0
109.3
110.3
73.9
O11-B1-O12
O21-B2-O22
H1-Ru-H2
H1-Ru-H4
Cent(O11,O12)-B1-Ru
Cent(O21,O22)-B2-Ru
85(2)
89.8
176.4
168.1
97.2
177.7
177.7
177.1
171.5
∆E (kJ/mol)
0.0
16.0
adduct, assimilated to a dihydridoborate. In contrast,
we could optimize the geometry of the bis(dihydrido-
borate) isomer Ru[(µ-Η)2B(OCH2)2]2(PMe3)2 (6Bpin•b).
The ruthenium center is in an octahedral environment,
and the Ru-B bonds (2.205 and 2.195 Å) are very
similar to that in 5Bpin•b (2.207 Å). In comparison to
the latter, the ligands display a symmetrical dihydri-
doborate arrangement. This isomer is, however, at
significantly higher energy than 6Bpin•a (∆E ) 16.0
kJ/mol). The exo isomer 6Bpin•a is thus preferred, in
agreement with the X-ray structure of 3Bpin.
These results on borane activation are reminiscent
of the σ-ligand substitution mechanism some of us
already proposed in silane chemistry,43 in which the
same kind of cis interaction between hydrides and
silicon (SISHA interaction) plays a key role. Keeping
It is noteworthy that borane substitution by H2 in
3Bpin is fast and quantitative, leading to 2Bpin. In
contrast, substitution of the σ-borane ligand in 2Bpin
by H2, leading back to 1, is a much lower process: 15
min to go from 3Bpin to 2Bpin, whereas around 1000
(42) Callaghan, P. L.; Fernandez-Pacheco, R.; Jasim, N.; Lachaize,
S.; Marder, T. B.; Perutz, R.; Rivalta, E.; Sabo-Etienne, S. Chem.
Commun. 2004, 242.
(43) Atheaux, I.; Delpech, F.; Donnadieu, B.; Sabo-Etienne, S.;
Chaudret, B.; Hussein, K.; Barthelat, J. C.; Braun, T.; Duckett, S. B.;
Perutz, R. N. Organometallics 2002, 21, 5347.

Coordination Modes of Boranes
Organometallics, Vol. 24, No. 12, 2005 2943
in mind the problem of alkane and arene functional-
ization, our borane complexes could serve as models for
a σ-bond metathesis step between Ru-H and B-H
bonds. Theoretical investigations on the corresponding
transition states are in progress. Efforts to establish the
significance of mechanisms involving σ-bonds and weak
secondary interactions in catalysis are currently under-
way.
French Ministry “Education Nationale, Enseignement
Supe´rieur, Recherche” for his AMN grant. We also
thank Dr. Jean-Claude Daran for assistance with the
X-ray studies.
Supporting Information Available: Tables and figures
giving complete crystallographic details for RuH2(η2-HBpin)-
(η2-H2)(PCy3)2 (2Bpin), RuH2(η2-HBcat)(η2-H2)(PCy3)2 (2Bcat),
and RuH[(µ-Η)2BBN](η2-H2)(PCy3)2 (2BBN) and optimized
geometries of all species and calculated energies. This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work is supported by the
CNRS. We thank the CINES (Montpellier, France) for
a generous allocation of computer time. S.L. thanks the
OM050276L