Tert-butylborane: A bis (σ-B-H) ligand in ruthenium hydride chemistry

Article abstract of DOI:10.1016/j.jorganchem.2009.03.020

The reaction of tert-butylborane with the bis(dihydrogen) complex RuH₂(η²-H₂)₂ (PCy₃)₂ leads to the corresponding bis σ-borane complex which is the first example of a monoalkylborane ruthenium bis σ-complex. An alternative route involves the reaction of RuHCl(η²-H₂)(PCy₃)₂ with lithium tert-butylborohydride.

Full text of DOI:10.1016/j.jorganchem.2009.03.020

Journal of Organometallic Chemistry 694 (2009) 2839–2841
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Tert-butylborane: A bis (
r
-B–H) ligand in ruthenium hydride chemistry
*
*
Yann Gloaguen, Gilles Alcaraz , Laure Vendier, Sylviane Sabo-Etienne
CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, Université de Toulouse, UPS, INPT, F-31077 Toulouse, France
a r t i c l e i n f o
a b s t r a c t
The reaction of tert-butylborane with the bis(dihydrogen) complex RuH2(g
2-H2)2(PCy3)2 leads to the
Article history:
Received 5 February 2009
Received in revised form 11 March 2009
Accepted 12 March 2009
corresponding bis
-complex. An alternative route involves the reaction of RuHCl(
butylborohydride.
r
-borane complex which is the first example of a monoalkylborane ruthenium bis
r
g
2-H2)(PCy3)2 with lithium tert-
Available online 21 March 2009
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Ruthenium
Borane
Dihydrogen
Sigma-complexes
1. Introduction
of less-common borane reagents, we started to explore the coordi-
nation of monosubstitued boranes RBH2. The use of mesitylborane
Boranes are key compounds in a wide variety of metal mediated
organic transformations such as hydroboration, borylation [1–3],
boron transfer reactions [4] or dehydrogenation of ammonia bor-
ane [5,6]. Metal boron chemistry is thus an active field of investi-
gation dominated by reactivity studies with disubstituted
boranes HBRR0 (R,R0 = pinacol, catechol, alkyl) [7]. More recently,
this area has attracted considerable attention with the develop-
ment of hydrogen storage materials and the potential reversibility
of hydrogen-release reactions from amineborane compounds [8–
11]. Although the formation of hydrido boryl species in a final B–
H bond activation stage has been well documented [12,13] isolated
allowed us to isolate the first bis
H2BMes)(PCy3)2 (4) [23]. In this complex, the mesitylborane dis-
plays a unique coordination mode with two geminal -B–H bonds.
r : -
-borane complex RuH2(g2 g2
r
It is noteworthy that such a bonding mode is limited to mesitylbo-
rane with only two other complexes presenting a similar coordina-
tion: RuHCl(g2 2-H2BMes)(PCy3)2, the precursor to the first
:g
terminal borylene ruthenium complex ꢀ[24], and the cationic com-
plex [Cp*Ru(PiPr3)(BH2Mes)]+[B(C6F5)4] [25]. We now report the
reaction of tert-butylborane with a polyhydride ruthenium center
leading to the corresponding bis
example of a monoalkylborane bis
r
-borane complex as the first
r
-complex. In this communica-
r
-borane complexes remain quite rare since the first report in
tion, we mainly focus on spectroscopic and structural data in order
to evaluate the influence of the boron substituent on the coordina-
tion of the borane to the ruthenium center.
1996 by Hartwig et al. [14]. These complexes often represent key
intermediates in the boundary oxidative addition/reductive elimi-
nation steps of metal catalyzed reactions [15–18]. More evidence is
now in favour of
catalyzed borylation processes, thus supporting the recent metath-
esis mechanism termed -CAM for late transition metals ( -com-
r-B–H coordinated intermediates as observed in
2. Results
r
r
t
t
An ethereal solution of BuBH2 in situ generated from BuBH3Li
and Me3SiCl was reacted at room temperature with a stoichiome-
plex assisted metathesis) and involving metal induced dynamic
rearrangements of E–H bonds at constant oxidation state [19].
The situation can prove different in the presence of polyhydride
metal precursors. Depending first on the nature of the ligands coor-
dinated to the metal fragment and also on the Lewis acidity of the
boron atom, dihydroborate coordination resulting from an interac-
tion of the boron with a neighbouring hydride can be favoured
[15,20–22]. As part of our ongoing program on B–H bond activation
tric amount of RuH2(
yellow crystals of RuH2(g2
Complex 2 resulting from the substitution of two labile
ligands was fully characterized by multinuclear NMR and X-ray
diffraction crystallography. As previously reported in the case of
4, an alternative synthetic route could also be obtained by stoichi-
ometric reaction of RuHCl(
butylborohydride (Scheme 1).
NMR data are consistent with a bis-
1H NMR spectrum of 2 in C6D6 at 298 K, in the hydride region
g
2-H2)2(PCy3)2 (1) in toluene. After workup,
:g
2-H2BtBu)(PCy3)2 (2) were isolated.
-H2
r
g
2-H2)(PCy3)2 (3) with lithium tert-
* Corresponding authors.
E-mail addresses: gilles.alcaraz@lcc-toulouse.fr (G. Alcaraz), sylviane.sabo@lcc-
toulouse.fr (S. Sabo-Etienne).
r
coordination mode. The
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.03.020

2840
Cy3P
Y. Gloaguen et al. / Journal of Organometallic Chemistry 694 (2009) 2839–2841
Table 1
H
H
Comparison between selected geometrical parameters (distances in Å, angles in °)
from the X-ray structures of 2 and 4.
H3C
H3C
H3C
H
H
H
Ru
C
BH2
H
Cy3P
2
4
-2H2
Cy3P
Ru
CH3
CH3
H
H
1
Ru–B
Ru–P1
Ru–P2
B–C1
1.934(2)
2.3159(4)
2.3059(4)
1.582(3)
1.55(2)
1.53(3)
1.73(2)
1.69(2)
1.23(2)
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)
H
B
C
+Me3SiCl -Me3SiH
-LiCl
H
CH3
Cy3P
Cy3P
H
H
H3C
Ru–H3
Ru–H4
-LiCl
-H2
H
2
Ru
C
BH3 Li
H3C
H3C
Cl
Ruꢁ ꢁ ꢁH1
Ruꢁ ꢁ ꢁH2
B–H1
Cy3P
3
B–H2
1.25(2)
1.29(3)
123(2)
Scheme 1. Synthetic routes to the ruthenium tert-butylborane bis-
r complex (2).
H1–B–H2
P1–Ru–P2
P1–Ru–B
P2–Ru–B
Ru–B–C1
P1–Ru–B–P2
120.8(16)
151.691(16)
106.01(6)
101.79(6)
173.29(17)
174.6
150.87(3)
108.89(11)
100.19(11)
177.1(3)
178.3
consists of a sharp triplet and a broad singlet in a 1:1 integration
ratio at d ꢀ10.99 and d ꢀ6.48, respectively, the signals slightly
deshielded compared to the ones in RuH2(g2 2-H2BMes)(PCy3)2
:g
(4). The triplet (JP–H = 27.0 Hz) collapsed into a singlet upon phos-
phorus decoupling whereas the singlet sharpened upon boron
decoupling. T1 measurements on the hydride resonances are rem-
3. Conclusion
iniscent of the ones obtained in the case of the mesitylborane bis
r
complex 4 and rule out the presence of any (
g
2-H2) ligand in 2
Examples of
r-coordinated alkyl substituted boranes are
[23]. The integrals of the hydrides and the tert-butyl resonances
also confirm the presence of one tert-butyl group and four hydro-
gen atoms around the ruthenium. The 11B{1H} NMR spectrum
exhibits a broad signal at d 69 deshielded (Dd + 10) compared to
the resonance of 4, still in a region characteristic of a tricoordinated
boron atom [15].
scarce. Only three examples involving Mn or Ni and a very lim-
ited number of dialkylboranes R2BH (R = Cy, Me, Et) have been re-
ported so far [28,29]. The use of monosubstituted boranes (RBH2)
as a ligand is even more limited and restricted to mesitylborane
ligated in a bis-
[23–25]. In this communication, we report the first example of
a monoalkylborane bis- complex involving tert-butylborane li-
gated to a dihydride ruthenium fragment. NMR and X-ray data
tend to indicate that irrespective of the substituent on boron, al-
kyl in 2 or aryl in 4, the borane is coordinated to the metal in a
similar fashion. Computational studies are currently being under-
taken to analyze in more detail the tert-butylborane coordination
via the electronic structure of 2 and reactivity studies are in
progress.
r coordination mode to a ruthenium center
The X-ray structure of 2 was determined at 110 K (Fig. 1 and
Table 1). The Ru atom is in a pseudo-octahedral environment
with the two tricyclohexylphosphine ligands in axial position
and the equatorial plane occupied by four coplanar hydrogen
atoms (H1–4).
The Ru–B bond distance of 1.934(2) Å is shorter than the sum of
the covalent radii (2.09 Å) and very close to that of 4 (1.938(4) Å)
suggesting a similar interaction between the metal center and
the boron atom. With 1.582(3) Å, as expected for a Csp3–B, the B–
C1 bond distance is slightly longer than the one in 4 (1.543(5) Å)
but lies in the range quoted for tert-butylboranes (1.61 Å)
[26,27]. Interestingly, the Ru–H3 and Ru–H4 bond distances
(1.55(2) and 1.53(3) Å) are comparable to the ones in 4 (1.59(3)
and 1.61(3) Å). The B–H bond distances (1.23(2) and 1.25(3) Å)
are slightly elongated compared to free boranes (1.20 Å) and sim-
r
4. Experimental
4.1. Synthesis of RuH2(g2 2-H2BtBu)(PCy3)2 (2)
:g
All the manipulations were performed in a glove box under
an atmosphere of argon. The solvents were dried with a solvent
purification system MBraun SPS-800 Series. The NMR spectra
were recorded on Brucker AV-300 or AV-500 MHz spectrometers.
Me3SiCl (0.42 mL, 3.309 mmol) was added to an ethereal solu-
tion (4 mL) of lithium tert-butylborohydride (12.2 mg,
0.157 mmol) [30] at room temperature and stirred for 5 h. The
ilar to those found in 4. In these two complexes, the bis
nation mode is thus of the same nature and results from the
donation from (BH) to Ru and -back-donation from Ru into
the vacant p orbital on B.
r-coordi-
r
-
r
p
resulting suspension was transferred to
a toluene solution
(5 mL) of RuH2(
g
2-H2)2(PCy3)2 (1) (104.3 mg, 0.156 mmol) and
stirred for 3 h. After removal of the solvent and addition of pen-
tane, the suspension was filtrated over CeliteÒ and the solvent
evaporated. An impure yellow powder resulted from which suit-
able crystals of 2 were obtained after solubilisation in the min-
imum amount of pentane and standing at ꢀ35 °C. Yield: 73%
as estimated by 31P NMR integration on the crude yellow
powder.
NMR data for 2: 1H NMR (C6D6, 298 K, 300.131 MHz) d: ꢀ10.99
2
t
(t, 2H, JHP = 27.0 Hz, H3–4), ꢀ6.48 (br, 2H, H1–2), 1.40 (s, 9H, Bu),
1.00–2.50 (m, 66H, Cy). T1
(C7D8, 253 K, 500.33 MHz) d:
min
ꢀ11.01 (327 ms), ꢀ6.60 (160 ms). 31P{1H} (C6D6, 298 K,
121.495 MHz) d: 84.03 (s). 11B{1H} (C7D8, 293 K, 160.526 MHz) d:
69 (br). 13C{1H} (C6D6, 298 K, 75.467 MHz) d: 27.00 (CH2 Cy),
29.21 (CH3 tBu), 30.79 (CH2 Cy), 38.97 (CH Cy). The CIV (tBu) was
not observed.
Fig. 1. X-ray crystal structure of RuH2(g2 2-H2BtBu)(PCy3)2 (2).
:g

Y. Gloaguen et al. / Journal of Organometallic Chemistry 694 (2009) 2839–2841
2841
4.2. Crystal structure determination and refinement
Acknowledgements
Data were collected at low temperature (110 K) on a Bruker Kap-
pa Apex II diffractometer using a graphite-monochromated Mo K
We thank the CNRS and the ANR (Programme Blanc ANR-06-
BLAN-0060-01) for support.
a
radiation (k = 0.71073 Å) and equipped with an Oxford Cryosys-
tems Cryostream Cooler Device. The final unit cell parameters have
been obtained by means of a least-squares refinement performed
on a set of 9567 well measured reflections. The structures have
been solved by Direct Methods using SIR92 [31], and refined by
means of least-squares procedures on a F2 with the aid of the pro-
gram SHELXL97 [32] included in the software package WINGX version
1.63 [33]. The Atomic Scattering Factors were taken from Interna-
tional tables for X-ray crystallography [34]. All the hydrogen atoms
were located geometrically, and refined by using a riding model, ex-
cept for Hy1, Hy2, Hy3 and Hy4 which were found by calculating
Fourier difference maps of the electronic density observed at small
theta (<18°). The disorder on C2, C3 and C4 of the tert-butyl group
was treated using the option ‘Part’ in the SHELX97 refinement pro-
gram. All non-hydrogen atoms were anisotropically refined, and
in the last cycles of refinement a weighting scheme was used,
where weights were calculated from the following formula:
Appendix A. Supplementary material
CCDC 718699 contains the supplementary crystallographic data
for 2. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-
ta_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.jorganchem.2009.03.020.
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Empirical formula
Formula weight
Temperature (K)
C40H79BP2Ru
733.85
110
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0.71073
Triclinic, P1
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c
(°)
Volume (Å3)
Z, calculated density (g/cmꢀ3
)
)
Absorption coefficient (mmꢀ1
F(000)
796
Crystal size (mm)
Theta range for data collection (°)
Limiting indices
0.2 ꢃ 0.12 ꢃ 0.07
1.23–28.28
ꢀ14 6 h 6 8
ꢀ16 6 k 6 16
ꢀ22 6 l 6 22
60610/9858 [Rint = 0.0324]
98.6%
Semi-empirical from
equivalents
0.975 and 0.889
Reflections collected/unique
Completeness to theta = 28.28
Absorption correction
Maximum and minimum
transmission
Refinement method
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1.136
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C Tables 4.2.6.8 and 6.1.1.4,
Final R indices [I > 2
R indices (all data)
Largest difference peak and hole
(e Aꢀ3
r
(I)]
R1 = 0.0254, wR2 = 0.0660
R1 = 0.0313, wR2 = 0.0783
0.875 and ꢀ0.913
International Tables for Crystallography, Kluwer Academic Publishers,
Dordrecht, The Netherlands, 1995.
)
R1
=
R
||Fo| ꢀ |Fc||/R|Fo|.