Double addition of H2 to transition metal-borane complexes: A 'hydride shuttle' process between boron and transition metal centres

Article abstract of DOI:10.1039/c0cc02245d

The addition of H₂ across a transition metal-borane bond is reported for the first time providing a mechanism for recharging borane functional groups to borohydride.

Full text of DOI:10.1039/c0cc02245d

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Double addition of H2 to transition metal–borane complexes: a
‘hydride shuttle’ process between boron and transition metal centreswz
Nikolaos Tsoureas, Yu-Ying Kuo, Mairi F. Haddow and Gareth R. Owen*y
Received 30th June 2010, Accepted 23rd September 2010
DOI: 10.1039/c0cc02245d
The addition of H2 across a transition metal–borane bond is
reported for the first time providing a mechanism for recharging
borane functional groups to borohydride.
The synthesis of the first authenticated transition metal–
borane complexes, the so-called ‘metallaboratranes’,1 in 1999
enriched the fields of organometallic, coordination and transi-
tion metal–boron chemistry.2 Since this time, a wide range
of diverse transition metal–borane complexes have been
Scheme 1 Reaction of [Rh{k3-NNH-HB(azaindolyl)3}(NBD)] (1) to
prepared. Such complexes are of particular interest due to
form [Rh{k4-NNBN-B(azaindolyl)3}(nortricyclyl)] (2).
the sigma acceptor (Z-class) interaction between the boron
and metal centre.2,3 Over the course of the past few years we
We have previously reported the reactivity of [Rh{k3-NNH-
HB(azaindolyl)3}(NBD)] (1) (where NBD = Z2-norbornadiene)
have carried out a number of investigations aimed at utilizing
to form the rhodium–borane complex, [Rh{k4-NNBN-B-
this class of compounds as ‘hydride shuttles’; probing a ‘BH
(azaindolyl)3}(nortricyclyl)] (2) (Scheme 1).4b In this transforma-
cooperation effect’ involving reversible migration of a hydride
species between boron and transition metal centres.4 Control
tion, the hydride migrates from the boron atom into the former
over hydride migration could have wide ranging scope,
norbornadiene ligand followed by a double bond rearrangement
however, little is known regarding such reactivity. In parti-
to form the nortricyclyl moiety. The reactivity of complex 2 with
cular, little is known concerning addition reactions across
trialkylphosphines was explored further. Complex 2 readily
these bonds. Parkin et al. have previously reported 1,2-addition
reacts with a slight excess of PMe3 to give complex 3
reactions of halogens to this bond in a number of complexes.5
(Scheme 2; see ESIz for details of synthesis and structural
While contrasting evidence was found by Hill et al. who
characterisation). When the reaction was repeated using bulkier
trialkylphosphines [such as PtBu3, PCyp3 (Cyp = cyclopentyl),
subjected a platinum–borane complex to oxidants such as
Cl2, Br2, I2 and CH3I and found retention of the Pt–B bond.6
PCy3] no significant conversion of the starting material was
The reactivity of transition metal–borane bonds remains
observed even at elevated temperatures or prolonged periods
elusive and to date there have been no reports of its reactivity
of time.
towards H2. A clear insight of such reactivity could have
Complex 2 was dissolved in toluene-d8 and one equivalent
potential applications in processes such as hydrogen storage
of tricyclopentylphosphine was added. The mixture was placed
technologies, catalysis and contemporary organometallic
under 2.5 bar of H2 at 85 1C (Scheme 3). After 16 h, the
11B{1H} spectrum showed the conversion of complex 2 into
transformations.
Sabo-Etienne et al. have recently reported a ruthenium
two new species; a major species (ca. 95%, as determined by
complex which exhibits reversible hydrogen release.7 In this
complex, a reversible transformation was observed between
borylene and bis(s-borane) functionalities. Weller et al. have
also shown reversible hydrogen addition to a rhodium com-
plex containing a s-bound carborane.8 Herein, we report for
the first time, the successful demonstration of H2 addition to a
transition metal–borane complex and its utilization in the
catalytic hydrogenation of olefins.
Scheme 2 Addition of trialkylphosphines to complex 2.
The School of Chemistry, University of Bristol, Bristol, UK.
E-mail: gareth.owen@bristol.ac.uk; Fax: +44 (0)117 925 1295;
Tel: +44 (0)117 928 7652
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Full experi-
mental details outlining the synthesis and characterisation of 3–6 and
further details and structural data. CCDC 783205–783209. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc02245d
y Royal Society Dorothy Hodgkin Research Fellow.
Scheme 3 Reaction of 2 with H2 and trialkylphosphines to give 4
t
(R = Cyp) and 5 (R = Bu).
c
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relative integration) as a singlet at ꢀ0.6 ppm (4) and a small
quantity of a second species with a signal centred at 3.5 ppm
(ca. 5%). The identity of the minor species is described below
(complex 6). Complex 4 was isolated as a spectroscopically
and analytically pure microcrystalline solid (see ESIz). The
spectroscopic data were consistent with the formation of
[Rh{k3-NNH-HB(azaindolyl)3}(H)2(PCyp3)] (4) (Scheme 3).
The 11B{1H} NMR spectrum (s, Dn1/2 = 20 Hz) confirmed
that the rhodium–boron bond have been lost and the corres-
ponding boron–coupled experiment confirmed the formation
of a new borohydride functionality (1JBH = 75 Hz). The
chemical shift and coupling constant found in 4 are similar
to those found for complex 1 (cf. ꢀ1.6 ppm; 1JBH = 78 Hz).4b
The proton NMR spectrum was notable for the presence of
three hydridic protons; one at ꢀ4.25 ppm (1 : 1 : 1 : 1 quartet,
1JBH = 73 Hz)z integrating for one hydrogen atom (BH) and
two overlapping signals centred at ꢀ16.30 ppm as broad
multiplet signals integrating for two hydrogen atoms.
Fig. 2 Crystal structure of 6 (solvent and hydrogen atoms, with the
exception of H1, have been removed for clarity, ellipsoids drawn at
50% probability level). Selected bond lengths (A): Rh1–N14 2.027(3),
Rh1–N2 2.100(3), Rh1–N4 2.024(3), Rh1–B1 2.171(4), Rh1–Rh2
2.8803(4), Rh2–N13 2.046(3), Rh2–N8 2.095(3), Rh2–N10 2.095(3),
Rh2–B1 2.139(5).8**zz
The analogous reaction was carried out with PtBu3 to form
[Rh{k3-NNH-HB(azaindolyl)3}(H)2(PtBu3)] (5). In this case, a
larger quantity (up to 25%) of a second species was observed
in the reaction mixtures. This species was the same as that
found in the reaction involving PCyp3 (i.e. complex 6). The
identity of 5 was further confirmed by structural characterisa-
tion (Fig. 1). The molecular structure revealed an octahedral
rhodium(III) centre bound by Tai [Tai = HB(azaindolyl)3],
coordinating with a k3-NNH coordination mode, one hydride
ligand located cis to the BH group and one trans to the
BH group. A PtBu3 ligand is additionally coordinated cis to
the BH group. The presence of two terminal hydride ligands
as the major species, although some decomposition was also
apparent in the mixture. Isolation and structural characteri-
sation of 6 revealed its identity as the unexpected product
[(Rh{k3-NNB-B(azaindolyl)3})2(m-H)(m-azaindolyl)] (Fig. 2).
Single crystals of 6 were obtained by allowing a diethyl ether
solution to stand at ꢀ30 1C overnight. The molecular structure
of 6 consists of a dimeric species where the rhodium centres
adopt pseudo square based pyramidal geometries with coordi-
nation spheres comprising of two nitrogen atoms and the
boron atom of the former Tai ligand (with a k3-NNB coordi-
nation mode). An azaindolyl unit bridges the two rhodium
centres via its two nitrogen atomsww and an additional hydride
ligand bridges the two metal centres.8 The 1H NMR spectrum
of 6 gave some signals which were broad at ambient tempera-
tures indicating fluxional behaviour within the complex. At
ꢀ60 1C, the signals sharpened revealing several azaindole ring
environments with a total integration for 35 protons. A signal
integrating for one proton was also observed at ꢀ22.4 ppm
1
was confirmed by the H NMR spectrum which revealed two
distinct chemical environments with the corresponding signals
at ꢀ17.2 ppm and ꢀ16.92 ppm.
The higher proportion of the unknown species 6 in the
reaction involving the bulkier phosphine PtBu3 suggested that
it might not contain a phosphine ligand. Therefore, in a
control experiment, 2 was placed under the same conditions
as those shown in Scheme 3 in the absence of a phosphine
ligand. The 11B{1H} NMR spectrum of the reaction mixture
indicated the complete conversion of 2 and the formation of 6
1
(t, JRhH = 23.5 Hz) confirming the presence of the bridging
hydride. The hydride signal remained as a broad triplet at
lower temperatures even though two boron chemical environ-
ments were observed in the 11B NMR spectrum between the
temperatures ꢀ60 1C to 80 1C.
The formation of 6 suggests that a highly-reactive unsaturated
rhodium species, such as [(Rh{k3-NNB-B(azaindolyl)3}(m-H)]2
(A*) (Scheme 4), is formed during the hydrogenation reaction
pathway. This species can either be captured by a phosphine
ligand to form complex 4 or 5 (following subsequent reaction
with H2) or undergo decomposition in the absence of a tri-
alkylphosphine ligand. The presence of a bridging azaindolyl
unit in 6 suggests that the unsaturated species (A*) reacts with
free azaindole which is generated by an unknown decompo-
sition pathway involving B–N bond fission (Scheme 4).
Indeed repeating the same reaction in the presence of
0.5 equivalents of 7-azaindole resulted in complex 6 as the
only boron-containing species observed in the 11B NMR
spectrum (see ESIz). The involvement of such an intermediate
was also confirmed by the inability of 3 to react with H2 under
the same conditions.
Fig. 1 Crystal structure of 5 (solvent and hydrogen atoms, with the
exception of H1, H2 and H3, have been removed for clarity, ellipsoids
drawn at 50% probability level) Selected bond lengths (A): Rh1–N4
2.1159(7), Rh1–N2 2.1896(8), Rh1–P1 2.2928(3), N1–B1 1.5336(13),
N3–B1 1.5386(12), N5–B1 1.5323(14).8**
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 484–486 485

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is eliminated from the complex during the catalytic cycle.
Further investigations are currently being carried out to
explore the borane recharge mechanism and the role of borane
and phosphine ligands within the catalytic cycle.
The authors would like to thank the Royal Society (G.R.O.)
and Leverhulme Trust (N.T.) for funding, Johnson Matthey
for the loan of the rhodium salts, Dr C. P. Butts for his help
with the NMR studies and the EPSRC National Crystallo-
graphy Service (University of Southampton) for the data
collection for complex 6.
Notes and references
z Further coupling is apparent in this signal, however the signal was
too broad to resolve the coupling constants.
Scheme 4 Reaction of 2 with H2 (A* is a proposed intermediate, see
text for details).
** Crystal data for complex 5: C33H45BN6PRhꢁ0.5(C5H12),
M = 706.51, monoclinic, a = 11.6549(8) A, b = 12.9893(9) A,
c = 22.6123(15) A, a = 90.001, b = 90.483(3)1, g = 90.001, V =
3423.1(4) A3, T = 100(2) K, space group P21/c, Z = 4, 109 624
reflections measured, 21 767 independent reflections (Rint = 0.0298).
The final R1 values were 0.0298 (I 4 2s(I)). The final wR(F2) values
were 0.0743 (I 4 2s(I)). The final R1 values were 0.0402 (all data). The
final wR(F2) values were 0.0807 (all data). Crystal data for complex 6:
C57H56B2N14O2Rh2, M = 1196.60, triclinic, a = 11.3780(3) A,
b = 11.9021(3) A, c = 20.7067(6) A, a = 74.070(2)1, b =
78.342(2)1, g = 80.084(2)1, V = 2620.47(12) A3, T = 100(2) K, space
ꢀ
group P1, Z = 2, 53 963 reflections measured, 12 055 independent
reflections (Rint = 0.0941). The final R1 values were 0.0504 (I 4 2s(I)).
The final wR(F2) values were 0.1015 (I 4 2s(I)). The final R1 values
were 0.0972 (all data). The final wR(F2) values were 0.1164 (all data).
ww The structure reveals that the bridging azaindolyl unit occupies the
two possible bridging orientations with equal disorder.
8 Full details regarding the collection parameters and refinement for
complexes 3, 5 and 6 (CCDC 783205–783209) are presented in the
ESI.z
Scheme 5 Suggested pathway involving the transformation of 1 to
complexes 4 and 5 (some substituents at boron and rhodium have been
removed for clarity).
The complexes presented herein represent the first examples
exhibiting reactivity of a transition metal–borane bond with
H2. The conversion of complex 1 to complexes 4 and 5
involves a stepwise transfer of hydrogen atoms. A suggested
mechanism is shown in Scheme 5, in which the first hydrogen
originates from boron. Addition of hydrogen gas to the system
allows for elimination of tricyclo[2.2.1.02,6]heptane and the
boron atom is recharged with a hydride ligand.
zz Complex 6 was also crystallised as a second polymorph.
1 A. F. Hill, G. R. Owen, A. J. P. White and D. J. Williams, Angew.
Chem., Int. Ed., 1999, 38, 2759.
2 (a) I. Kuzu, I. Krummenacher, F. Armbruster and F. Breher, Dalton
Trans., 2008, 5836; (b) F.-G. Fontaine, J. Boudreau and
M.-H. Thibault, Eur. J. Inorg. Chem., 2008, 5439; (c) G. R. Owen,
Transition Met. Chem., 2010, 35, 221; (d) H. Braunschweig,
C. Kollann and D. Rais, Angew. Chem., Int. Ed., 2006, 45, 5254;
(e) J. I. van der Vulgt, Angew. Chem., Int. Ed., 2010, 49, 252;
(f) H. Braunschweig, R. D. Dewhurst and A. Schneider, Chem. Rev.,
2010, 110, 3924.
This reactivity prompted us to investigate the potential of
our complexes as hydrogenation catalysts (Table 1). Preliminary
investigations reveal that under 1 mol% of 2, the substrates
styrene and cyclooctene are fully converted to ethyl benzene
and cyclooctane, respectively, at 2.5 bar H2 at 85 1C over a
period of 18 h. At lower catalyst loading (0.1 mol%), these
substrates were reduced to their corresponding alkanes with
85% and 43% conversions. When the phosphine complex 4
was employed as the catalyst, these conversions were reduced
to 50% and 7%. This result suggests that the phosphine ligand
3 (a) R. B. King, Adv. Chem., 1967, 62, 203; (b) M. L. H. Green,
J. Organomet. Chem., 1995, 500, 127.
4 (a) G. R. Owen, N. Tsoureas, A. Hamilton and A. G. Orpen, Dalton
Trans., 2008, 6039; (b) N. Tsoureas, T. Bevis, C. P. Butts,
A. Hamilton and G. R. Owen, Organometallics, 2009, 28, 5222;
(c) N. Tsoureas, M. F. Haddow, A. Hamilton and G. R. Owen,
Chem. Commun., 2009, 2538; (d) G. C. Rudolf, A. Hamilton,
A. G. Orpen and G. R. Owen, Chem. Commun., 2009, 553;
(e) G. R. Owen, P. H. Gould, J. P. H. Charmant, A. Hamilton
and S. Saithong, Dalton Trans., 2010, 39, 392; (f) G. R. Owen,
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39, 49.
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1008; (b) J. S. Figueroa, J. G. Melnick and G. Parkin, Inorg. Chem.,
2006, 45, 7056; (c) K. Pang, S. M. Quan and G. Parkin, Chem.
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6 I. R. Crossely, A. F. Hill and A. C. Willis, Organometallics, 2008,
27, 312.
Table 1 Hydrogenation of olefins using complexes 2 and 4a
Cat. loading
(mol%)
Conversiond
(%)
Complex
Substrate
2
2
4
2
2
4
1.0b
0.1c
0.1c
1.0b
0.1c
0.1c
Styrene
Styrene
Styrene
Cyclooctene
Cyclooctene
Cyclooctene
499
85
50
499
43
7 (a) G. Alcaraz, U. Helmstedt, E. Clot, L. Vendier and S. Sabo-
Etienne, J. Am. Chem. Soc., 2008, 130, 12878; (b) G. Alcaraz,
M. Grellier and S. Sabo-Etienne, Acc. Chem. Res., 2009, 42, 1640;
(c) H. Braunschweig and R. D. Dewhurst, Angew. Chem., Int. Ed.,
2009, 48, 1893.
7
a
b
2.5 bar H2, 85 1C. 2 mmol of olefin, 0.02 mmol of 2 or 4, C6D6
(2 mL). 7 mmol of olefin, 0.007 mmol of 2 or 4, C6D6 (1 mL).
c
d
Conversion measured by NMR integration relative to internal
standard after 18 h.
8 E. Molinos, S. K. Brayshaw, G. Kociok-Kohn and A. S. Weller,
¨
Dalton Trans., 2007, 4829.
c
486 Chem. Commun., 2011, 47, 484–486
This journal is The Royal Society of Chemistry 2011