σ-borane and dihydroborate complexes of ruthenium

Article abstract of DOI:10.1021/ja017429q

Room temperature reaction of the bis(dihydrogen) complex RuH₂(H₂)₂(PCy₃)₂ (1) with excess pinacol borane (HBpin) generates the novel complex RuH[(μ-H)₂Bpin](σ-HBpin)(PCy₃)₂ (2) by loss of dihydrogen. Complex 2 was characterized spectroscopically and by X-ray crystallography. It contains two pinacolborane moieties coordinated in a different fashion, one as a dihydroborate (B-H distances : 1.58(3) and 1.47(3) A) and the other as a σ-borane (B-H distance: 1.35(3) A). In addition, reaction of 1 with one equiv of HBpin yields total conversion to a new complex tentatively formulated as RuH[(μ-H)₂Bpin](H₂)(PCy₃)₂ (3) on the basis of NMR data. In the presence of excess HBpin, 3 is converted to 2. Furthermore, under an atmosphere of dihydrogen, a C₇D₈ solution of 2 rapidly converts to 3 and finally regenerates 1 over a much longer period. Thus, complex 3 is an intermediate in the formation of 2 from 1. In these processes the borane is eliminated as HBpin later hydrolyzed to BpinOBpin. Selective hydroboration of ethylene (3 bar) into C₂H₅Bpin is achieved using 1 or 2 as catalyst precursors in toluene, whereas in THF, competitive formation of the vinylborane C₂H₃Bpin (56% under 20 bar of C₂H₄) can be favored. Copyright

Full text of DOI:10.1021/ja017429q

Published on Web 04/24/2002
σ-Borane and Dihydroborate Complexes of Ruthenium
Virginia Montiel-Palma, Mar´ıa Lumbierres, Bruno Donnadieu, Sylviane Sabo-Etienne,* and
Bruno Chaudret
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France
Received October 30, 2001
Much attention is being paid to σ-complexes, due to their
fundamental importance and the role they might play during the
oxidative addition and reductive elimination steps occurring in a
wide variety of catalytic processes.1 The chemistry of dihydrogen
and silane complexes has blossomed in the last 15 years.1,2 A very
good amount of knowledge of their properties has been obtained,
thanks in particular to the improvement of NMR and X-ray analyses.
In such a context, it is remarkable that only very few σ-borane
compounds have been isolated. Activation of a boron hydride by a
transition metal complex leads to a versatile chemistry due to the
different bonding modes that can be adopted. The use of neutral
borane reagents such as the base adduct BH3‚PR3 or the three-
coordinated borane HBR2 proved to be the best candidates to isolate
true σ-borane complexes of groups 4 to 7.1,3-6 The titanocene
catecholborane complexes reported by Hartwig et al. are perharps
Figure 1. ORTEP drawing of compound 2. Selected bond lengths (Å):
the most representative examples, and they are active catalysts for
Ru-B(1), 2.188 (5); Ru-B(2), 2.157 (5); Ru-P(1), 2.3673 (13); Ru-P(2),
2.3571 (13); Ru-H(1), 1.48 (3); Ru-H(2), 1.58 (3); Ru-H(3), 1.49 (3);
the hydroboration of vinylarenes.5
Our group has developed an excellent entry to a wide range of
dihydrogen and silane complexes from the bis(dihydrogen) complex
RuH2(H2)2(PCy3)2 (1).7 We now report the synthesis, structure, and
preliminary catalytic activity of ruthenium complexes coordinating
either one or two pinacolborane ligands bonded in different modes,
that, σ-coordination and dihydridoborate ligation.
Ru-H(4), 1.55 (3); B(1)-H(3), 1.58 (3); B(1)-H(4), 1.47 (3); B(2)-H(2),
1.35 (3). Selected bond angles (deg): P(1)-Ru-P(2), 160.45 (3); B(1)-
Ru-B(2), 113.68 (17); Ru-H(2)-B(2), 94.4 (20); H(4)-Ru-B(2), 153.6
(12); H(2)-Ru-B(2), 38.6 (11); B(2)-Ru-H(3), 67.5 (12); H(3)-Ru-
H(1), 173.1 (19); H(4)-Ru-H(2), 166.3 (16); H(3)-Ru-H(4), 88.0 (17);
H(1)-Ru-H(4), 85.3 (17); H(1)-Ru-H(2), 81.2 (17).
Scheme 1
Room-temperature reaction of (1)7c with excess pinacolborane
(HBpin) proceeds rapidly with gas evolution, and after workup,
the new complex RuH[(µ-H)2Bpin](σ-HBpin)(PCy3)2 (2) was
isolated in very high yield and characterized by NMR and X-ray
diffraction (Scheme 1).8,9 The formulation as a σ-borane(dihydro-
borate) complex is supported by the following observations. The
molecular structure is shown in Figure 1. The ruthenium atom is
in a pseudo-octahedral environment with the phosphines in axial
positions. The four coordination sites in the equatorial plane are
occupied by one hydride H(1), one σ-B(2)-H(2) bond of a
pinacolborane ligand, and the two hydrogens H(3) and H(4) of a
dihydridopinacolborate ligand. The Ru-B(2) distance of 2.157 (5)
Å is significantly shorter that the Ru-B(1) distance (2.188(5) Å),
but they are both longer than the sum of the covalent radii of Ru
and B (2.13 Å). This tendency contrasts with what is generally
observed for group 8 boryl complexes showing M-B distances
shorter (Ru, Os, Co, Rh, Ir, Pt) or similar (Fe) to the sum of the
covalent radii.10 Keeping in mind the uncertainties in hydride
positions, the σ-B(2)-H(2) distance, 1.35 (3) Å, appears shorter
than the B(1)-H(3) and B(1)-H(4) distances which are 1.58 (3)
and 1.47 (3) Å respectively, although the differences are not
significant at the 3σ level.11 Similar σ-B-H distances were reported
for σ-borane complexes of three-coordinated boron.5b,6 Further
evidence of a σ-borane coordination is given by the close
resemblance between the X-ray structure of 2 and that of the
previously reported tris(trimethylphosphine) complex RuH[(µ-
H)2BH2](PMe3)3.12 In 2 the σ-borane ligand acts as a 2-electron
donor, playing a similar role as PMe3 in the latter complex.
1
The room-temperature H NMR spectrum of 2 in the hydride
region is featureless. However, at 233 K three well-resolved
resonances are observed in a 2:1:1 ratio (Figure 2). The broad singlet
at δ -11.4 is attributed to the two bridging hydrides of the
dihydroborate ligand, the sharp triplet at δ -8.03, to the terminal
hydride coupled to the two phosphorus (2JP-H ) 25 Hz), and the
broad singlet at δ -7.13, to the σ-bonded B-H proton. Upon 11
B
decoupling, the two broad signals sharpened. A dihydrogen
formulation can be ruled out on the basis of T1min values (around
100 ms at 300 MHz for the three signals). The 11B NMR spectrum
shows at 293 K, a very broad signal at δ 37.3 (w1/2 ) 640 Hz),
downfield of that of pinacolborane (δ 28.4). We were unable to
1
measure any coupling constant between H and 11B.
When performing the reaction with only one equiv of pinacol-
borane, conversion of 1 to the new complex 3 is observed (Scheme
1
1, Figure 2). The integrations of the H NMR signals at δ -8.83
and δ 1.25 are in agreement with a species bearing one pinacolboron
ligand and five hydrogens around the metal.8 The high-field signal
remains broad at all temperatures, indicative of a fast exchange.
* To whom correspondence should be addressed. E-mail: sabo@lcc-toulouse.fr.
9
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J. AM. CHEM. SOC. 2002, 124, 5624-5625
10.1021/ja017429q CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. This work is supported by the CNRS.
Supporting Information Available: Synthesis and characterization
data for 2, 3, and B2pin2O (PDF). X-ray structural data for 2 and
B2pin2O (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
References
(1) Kubas, G. J. In Metal Dihydrogen and σ-Bond Complexes; Fackler, J. P.,
Ed.; Kluwer Academic/Plenum Publishers: New York, 2001.
Figure 2. 1H NMR spectra at variable temperature, in the hydride region
(300 MHz, C7D8) showing a mixture of 2 and 3.
(2) For dihydrogen complexes: (a) Kubas, G. J. Acc. Chem. Res. 1988, 21,
120. (b) Crabtree, R. H. Acc. Chem. Res. 1990, 23, 95. (c) Jessop, P. G.;
Morris, R. H. Coord. Chem. ReV. 1992, 121, 155. (d) Heinekey, D. M.;
Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913. (e) Crabtree, R. H. Angew.
Chem., Int. Ed. Engl. 1993, 32, 789. (f) Esteruelas, M. A.; Oro, L. A.
Chem. ReV. 1998, 98, 577. For silane complexes: (g) Corey, J. Y.;
Braddock-Wilking, J. Chem. ReV. 1999, 99, 175. (h) Schubert, U. AdV.
Organomet. Chem. 1990, 30, 151.
The T1min value of 40 ms at 253 K (300 MHz) is in agreement
with the presence of a dihydrogen ligand. The 11B NMR spectrum
shows as for 2, a very broad signal at δ 35.1 (w1/2 ) 509 Hz).
Conversion of 3 to 2 is obtained by increasing the pinacolborane
concentration. Therefore 3 can be formulated as Ru“H3Bpin”(H2)-
(PCy3)2. Two modes of coordination of the boron ligand can be
proposed leading to a dihydride(σ-borane) (A) or hydrido(dihy-
droborate) (B) structure involving in each case a σ-dihydrogen
ligand.13,14 All attempts to isolate 3 have failed.
(3) Smith, M. R., III. Prog. Inorg. Chem. 1999, 48, 505.
(4) Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.; Katoh, K.; Uruichi,
M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704 and references therein.
(5) (a) Hartwig, J. F.; Muhoro, C. N.; He, X.; Eisenstein, O.; Bosque, R.;
Maseras, F. J. Am. Chem. Soc. 1996, 118, 10936. (b) Muhoro, C. N.; He,
X.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 5033. (c) Hartwig, J. F.;
Muhoro, C. N. Organometallics 2000, 19, 30.
(6) Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9435.
(7) (a) Sabo-Etienne, S.; Chaudret, B. Coord. Chem. ReV. 1998, 178-180,
381. (b) Borowski, A. F.; Donnadieu, B.; Daran, J. C.; Sabo-Etienne, S.;
Chaudret, B. Chem. Commun. 2000, 543. Ibid. 1697. (c) Borowski, A.
F.; Sabo-Etienne, S.; Christ, M. L.; Donnadieu, B.; Chaudret, B.
Organometallics 1996, 15, 1427. (d) 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. (e) Atheaux, I.; Donnadieu, B.; Rodriguez,
V.; Sabo-Etienne, S.; Chaudret, B.; Hussein, K.; Barthelat, J. C. J. Am.
Chem. Soc. 2000, 122, 5664. (f) Toner, A. J.; Grundemann, S.; Clot, E.;
Limbach, H.-H.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B. J. Am.
Chem. Soc. 2000, 122, 6777.
Remarkably, 3 was also generated upon heating at 80 °C for 4
h, a mixture of 1 and B2pin2 (1.3 equiv), as a result of B-B bond
breaking.
A C7D8 solution of 2 in a NMR tube was kept under dihydrogen
atmosphere, and the reaction was monitored by 1H, 31P, 11B NMR.
Total conversion into 3 and HBpin was first observed. After several
hours, conversion of 3 into 1 was finally achieved, and HBpin was
gradually hydrolyzed into B2pin2O due to traces of water.15 3 is
thus an intermediate in the formation of 2 and in the reverse reaction
leading back to 1. Although none of the two suggested structures
A or B can be ruled out, all these observations and specifically the
slow conversion of 3 into 1 are in favor of the dihydrogen-
(dihydroborate) structure B, RuH[(µ-H)2Bpin](H2)(PCy3)2. This is
another example of the ruthenium ability to accommodate hydrogens
around its coordination sphere, either as terminal or bridging
hydrides or as dihydrogen ligands.
(8) 2: 1H NMR (C7D8, 233 K, 300.13 MHz) -11.4 (br, 2H, Ru[(µ-H)2Bpin],
2
-8.03 (t, 1H, RuH, JPH ) 25 Hz), -7.13 (br, 1H, RuHBpin), 1.24 (s,
24H, Bpin), 1.28-2.25 (m, 66H, PCy3). 13C{1H} NMR (C7D8, 293 K,
75.47 MHz) 24.9 (s, CH3), 82.3 (s, C). 31P{1H} NMR (C7D8, 293 K, 121.49
MHz) 66.3 (s). 11B{1H} NMR (C7D8, 293 K, 96.29 MHz) 37.3 (br, w1/2
640 Hz). Anal. Calcd for C48H94O4B2P2Ru: C, 62.67; H, 10.30. Found:
C, 62.45; H, 10.24. 3: 1H NMR (C7D8, 293 K, 300.13 MHz) -8.83 (br,
5H), 1.25 (s, 12H, Bpin), 1.34-2.17 (m, 66H, PCy3). 13C{1H} NMR (C7D8,
293 K, 75.47 MHz) 25.1 (s, CH3), 81, 7 (s, C). 31P{1H} NMR (C7D8, 293
K, 121.49 MHz) 72.2 (s). 11B{1H} NMR (C7D8, 293 K, 96.29 MHz) 35.1
(br, w1/2 509 Hz).
(9) Crystal data for 2: pale yellow crystal, T ) 160 K, triclinic, P1h, a )
12.118 (5) Å, b ) 13.945 (5) Å, c ) 15. 406 (5) Å, R ) 93.277 (5)°, â
) 102.539 (5)°, γ ) 97.880 (5)°, Z ) 2, R1 ) 0.0405, GOF ) 0.929.
(10) (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) Braunschweig, H.; Kollann, C.; Klinkhammer,
K. W. Eur. J. Inorg. Chem. 1999, 1523. (c) Rickard, C. E. F.; Roper, W.
R.; Williamson, A.; Wright, L. J. Organometallics 2000, 19, 4344.
(11) (a) Rhodes, L. F.; Venanzi, L. M.; Sorato, C.; Albinati, A. Inorg. Chem.
1986, 25, 3337. (b) Pangan, L. N.; Kawano, Y.; Shimoi, M. Organo-
metallics 2000, 19, 5575.
We have previously demonstrated that 1 serves as an efficient
catalyst precursor for hydrosilylation or dehydrogenative silylation
of ethylene leading to the formation of vinylsilanes.16 The activity
of 1 as catalyst precursor for hydroboration was thus examined.
Preliminary experiments show that in toluene, selective hydro-
boration of ethylene (3 bar) by HBpin (100 equiv) into C2H5Bpin
is achieved (100% yield by GC), whereas in THF a competitive
reaction leads to the formation of the vinylborane C2H3Bpin in 23%
yield (eq 1).17 The boron products were characterized by NMR and
GC-MS.
(12) Statler, J. A.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, M. B. J.
Chem. Soc. Dalton Trans. 1984, 1731.
(13) We have already reported the characterization of an analogous complex
RuH[(µ-H)2BBN](PCy3)2 resulting from the reaction of 1 with HBBN.
See: Rodriguez, A.; Sabo-Etienne, S.; Chaudret, B. Anal. Quim. Int. Ed.
1996, 131. We have also published an analogous dihydrogen(silane)
complex. In that case, the X-ray structure showed the two phosphines in
a cis position. See: Hussein, K.; Marsden, C. J.; Barthelat, J. C.;
Rodriguez, V.; Conejero, S.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret,
B. Chem. Commun. 1999, 543.
(14) The competition between three structural extremes in the bonding of
dihydroborate ligands (σ-borane, dihydroborate, or hydrido(boryl)) has
been discussed in niobium complexes. See Hartwig, J. F.; De Gala, S. R.
J. Am. Chem. Soc. 1994, 116, 3661.
(15) B2pin2O was fully characterized by NMR, MS, and X-ray data. See
Supporting Information.
(16) (a) Christ, M. L.; Sabo-Etienne, S.; Chaudret, B. Organometallics 1995,
14, 1082. (b) Delpech, F.; Mansas, J.; Leuser, H.; Sabo-Etienne, S.;
Chaudret, B. Organometallics 2000, 19, 5750.
Remarkably, the same results were obtained when 2 was used
as the catalyst precursor. Using 20 bar of ethylene favors the
formation of vinylborane, a versatile reagent for organic synthesis.
Indeed, 1000 equiv of HBpin were converted in 15 min at room
temperature into 56% of C2H3Bpin and 44% of C2H5Bpin. The role
in the catalytic cycle of 2, bearing a σ-borane, a dihydridoborate,
and a hydride coordinated to the same metal, needs further
investigations. Theoretical studies and the results with other olefins
and boranes will be reported in due course.
(17) For transition-metal catalyzed addition of boron compounds to unsaturated
substrates, see references in: Coapes, R. B.; Souza, F. E. S.; Fox, M. A.;
Batsano, A. S.; Goeta, A. E.; Yufit, D. S.; Leech, M. A.; Howard, J. A.
K.; Scott, A. J.; Clegg, W.; Marder, T. B. J. Chem. Soc., Dalton Trans.
2001, 1201. Dehydrogenative borylation between ethylene and benzo-
1,3,2-diazaborolane (HBOp) was selectively observed with 58% conver-
sion into CHdCH(BOp). See: Motry, D. H.; Brazil, A. G.; Smith, M.
R., III. J. Am. Chem. Soc. 1997, 119, 2743.
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