Monosubstituted borane ruthenium complexes RuH2(η 2:η2-H2BR)(PR′3) 2: A general approach to the geminal bis(σ-B-H) coordination mode

Article abstract of DOI:10.1021/om400610k

A series of borane bis(σ-B-H) ruthenium complexes RuH ₂(η²:η²-H₂BR) (PR′₃)₂ (R = alkyl, aryl; R′ = Cy, Cyp, ⁱPr) has been prepared by using two synthetic strategies. The first one is based on a simple substitution reaction by adding the corresponding monosubstituted H₂BR borane to the bis(dihydrogen) ruthenium complex RuH₂(η²-H₂)₂(PCy ₃)₂. The second one, more general, results from the reaction of the chloro complex RuHCl(H₂)(PR′₃) ₂ (R′ = Cy, Cyp, ⁱPr) with the corresponding lithium monosubstituted borohydrides RBH₃Li (R = Mes, ^tBu, Me, C₄H₃S, Ph). All the complexes have been characterized by multinuclear NMR, IR, and X-ray diffraction studies. DFT calculations have been used to better define the bonding mode of the borane ligand to the metal center as well as to establish the thermodynamic cycle that delineates the coordination process. The ^tBu species displays a dynamic behavior evidencing an equilibrium between a borohydride and a σ-borane formulation. The thienyl case illustrates the competition between sulfur coordination and a bis(σ-B-H) coordination mode.

Full text of DOI:10.1021/om400610k

Article
pubs.acs.org/Organometallics
Monosubstituted Borane Ruthenium Complexes
RuH2(η2:η2‑H2BR)(PR′3)2: A General Approach to the Geminal
Bis(σ-B−H) Coordination Mode
Yann Gloaguen,†,‡ Gaetan Benac-Lestrille,†,‡ Laure Vendier,†,‡ Ulrike Helmstedt,§,⊥ Eric Clot,*,§
́
̈
Gilles Alcaraz,*,†,‡ and Sylviane Sabo-Etienne*,†,‡
†LCC (Laboratoire de Chimie de Coordination), CNRS, BP44099, 205 Route de Narbonne, F-31077 Toulouse Cedex 4, France
‡Universite
́
de Toulouse, UPS, INPT, F-31077 Toulouse, France
§Institut Charles Gerhardt, CNRS 5253, Universite
́
Montpellier 2, cc 1501, Place Eugen
̀
e Bataillon, 34095 Montpellier, France
S
* Supporting Information
ABSTRACT: A series of borane bis(σ-B−H) ruthenium com-
plexes RuH2(η2:η2-H2BR)(PR′3)2 (R = alkyl, aryl; R′ = Cy,
i
Cyp, Pr) has been prepared by using two synthetic strategies.
The first one is based on a simple substitution reaction by adding
the corresponding monosubstituted H2BR borane to the
bis(dihydrogen) ruthenium complex RuH2(η2-H2)2(PCy3)2.
The second one, more general, results from the reaction of the
chloro complex RuHCl(H2)(PR′3)2 (R′ = Cy, Cyp, iPr) with the
corresponding lithium monosubstituted borohydrides RBH3Li
t
(R = Mes, Bu, Me, C4H3S, Ph). All the complexes have been
characterized by multinuclear NMR, IR, and X-ray diffraction
studies. DFT calculations have been used to better define the
bonding mode of the borane ligand to the metal center as well
as to establish the thermodynamic cycle that delineates the coordination process. The Bu species displays a dynamic behavior
evidencing an equilibrium between a borohydride and a σ-borane formulation. The thienyl case illustrates the competition
between sulfur coordination and a bis(σ-B−H) coordination mode.
t
aminoboranes, for which the empty p orbital on boron is
strongly stabilized by donation from the adjacent nitrogen lone
pair, alkyl or arylboranes present a marked Lewis acid character.
When using conventional disubstituted boranes (HBpin, HBcat,
9-BBN), earlier studies on the resulting bis(tricyclohexylphos-
phine) ruthenium complexes showed that the Lewis acid pro-
perties of the borane influenced its coordination mode. Three
limit structures could be observed featuring a σ-ligand, a σ-ligand
with additional interaction between the boron and a neighboring
terminal hydride, or a borohydride unit.15−18 Promoting the
bis(σ-B−H) coordination mode from a ligated borohydride
motif remains an isolated case, nicely illustrated by Stradiotto et al.
via chlorine abstraction from [Cp*Ru(PiPr3)(κ2-H2BMesCl)].19
To evaluate the scope of the bis(σ-B−H) coordination mode
and better define its properties, we selected various Lewis acidic
monosubstituted alkyl and arylboranes and built a series of
ruthenium complexes stabilized by different phosphines. In this
article, we focus on spectroscopic and structural data in order to
evaluate the level of the B−H activation with the help of
theoretical calculations.
INTRODUCTION
■
We recently disclosed the bis(σ-B−H) coordination mode
between a monosubstituted borane (RBH2) and a metal center
from the addition of RBH2 to a polyhydride metal precursor.
The prototypical example of this new family of complexes,
RuH2(η2:η2-H2BMes)(PCy3)2 (1), resulted from the coordina-
tion of one molecule of mesitylborane to the [RuH2(PCy3)2]
ruthenium fragment through two geminal σ-B−H bonds. The
borane bonding mode involves a four-center−four-electron
interaction between the ruthenium center and the “H−B(sp2)−
H” moiety of mesitylborane.1,2 Related complexes incorporating
monosubstituted boranes (RBH2) to a hydrido metal fragment
are rare and mostly limited to the weak Lewis acid amino-
boranes R2−nHnN−BH2 (n = 0−2).3−10 Their recent develop-
ment derives from metal-catalyzed dehydrogenation of the
corresponding amine-boranes R3−nNHn−BH3 (n = 1−3).
Understanding and controlling the dehydrogenation process
of this class of compounds is still attracting a lot of interest, as
they are considered as potential hydrogen source and storage
materials.3−12
The σ-borane coordination mode is reminiscent of the
classical model of Chatt, Dewar, and Duncanson for the olefin−
metal interaction with an additional contribution due to the
presence of an empty p orbital on boron.13,14 In contrast to
Received: June 25, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Geminal Bis(σ-B−H) Borane Ruthenium Complexes
Scheme 2. Redistribution of Monosubstituted Boranes
DFT/B3PW91 level of the PCy3 series (1−5, see Computa-
tional Details).
RESULTS AND DISCUSSION
■
Synthesis of Borane Bis(σ-B−H) Ruthenium Com-
Spectroscopic Characterization of the Bis(σ-B−H)
Ruthenium Complexes. σ-B−H complexes are often
considered as intermediates in the microreversible process of
oxidative addition of the B−H bond to a metal center. In the
case of a ruthenium polyhydride fragment, the presence of both
terminal hydrides and Ru−H−B bridging hydrogens makes
these systems highly dynamic, often preventing an unambig-
uous formulation only on the basis of multinuclear NMR
data.28 The situation is even more complicated in the case of
the coordination of bidentate boranes containing an L−BH
(L = P, S) moiety.29,30
The solid-state infrared spectra for complexes 1−7 present a
similar profile in the frequency region between 1780 and 1985
cm−1. The first set of broad shape and medium to weak
intensity absorption bands is assigned to the terminal hydrides
Ru−H with vibration frequencies observed between 1900 and
1985 cm−1. The second set of weaker intensity but also of
broad shape is located at lower frequencies, in the 1780 to
1864 cm−1 region, and corresponds to the Ru−H−B vibrations
(Table 1). The frequency range observed for the Ru−H and
Ru−H−B vibrations in 1−7 clearly differs from the B−H
stretching frequencies of the corresponding lithium borohy-
drides (2100 < νBH < 2250 cm−1)27 or monosubstituted borane
dimers31 that are observed at even higher frequencies (2500 <
νBH < 2605 cm−1) when reported (Table 1). For the five
complexes 1−5 optimized at the B3PW91 level, the calculated
IR spectra featured four bands in the range 1750−2000 cm−1
(Table 1). The two higher frequencies correspond essentially to
Ru−H modes, while the two remaining modes of weaker
intensities correspond to Ru−H−B vibrations. Comparison
between the experimental and calculated values reveals that the
agreement is overall satisfactory.
plexes RuH2(η2:η2-H2BR)(PR′3)2 (R = alkyl, aryl; R′ = Cy,
i
Cyp, Pr). In the previously reported communications, we
showed that treatment of RuH2(η2-H2)2(PCy3)2 with mono-
t
substituted boranes RBH2 (R = Mes, Bu) was the most direct
and straightforward method to give the corresponding com-
plexes RuH2(η2:η2-H2BR)(PCy3)2 (R = Mes (1), Bu (2)) in
t
high yield by simple dihydrogen exchange reaction (Scheme 1).1,20
However, this method suffers from some limitations, intrinsic
to the stability of the precursors. With less sterically demanding
phosphine ligands than tricyclohexylphosphine, the ruthenium
dihydride bis(dihydrogen) precursors are more difficult to handle
and less stable, particularly with respect to loss of H2.21−23 In
addition, this synthetic strategy is restricted to the use of stable
monosubstituted boranes, which exist in a very limited number
due to their propensity to rearrange into borane and the
corresponding disubstituted borane (Scheme 2).24,25 In order
to develop a viable synthetic approach to a variety of borane
bis(σ-B−H) ruthenium complexes, we explored a pathway that
combines hydride transfer, loss of dihydrogen, and salt
elimination in a one-pot procedure1 from stable and storable
monosubstituted lithium borohydrides and chlorohydrido-
(dihydrogen) ruthenium complexes (Scheme 1).
i
RuHCl(H2)(PR′3)2 (R′ = Cy, Cyp, Pr) precursors were
obtained in high yield on a multigram scale from [Ru(COD)Cl2]n
according to known procedures.26 The lithium monosubsti-
t
tuted borohydrides RBH3Li (R = Mes, Bu, Me, C4H3S, Ph)
were also obtained in fairly good quantity by treatment of their
respective boronate precursors with LiAlH4.27 The target
borane bis(σ-B−H) ruthenium complexes could be obtained
pure and in high yield from the stoichiometric reaction of the
aforementioned precursors emphasizing the viability of this
synthetic approach. Complexes 3−7 were fully characterized by
IR and NMR spectroscopy and by X-ray diffraction
crystallography (vide infra). The borane coordination mode
was also studied by conducting a full optimization at the
Homogeneous NMR data were collected, and a similar trend
for the hydride, boron, and phosphorus chemical shifts was
observed for all the complexes (Table 2). The 11B{1H} NMR
spectra of 1−7 show a broad signal between δ +54 and +69.
B
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Table 1. Experimental and Computed (in italics) IR Vibrational Frequencies (cm−1) for the Modes Involving the Hydrides and
a
the Hydrogen Atoms Bonded to Boron
RuH2(H2BR)(PR′3)2
(RBH2)2
νBH
RBH3Li
νBH
R
R′
νRu−H
νRu−H−B
1
Mes
Cy
1948 (m br)
1936 (m br)
1965 (140)
1925 (80)
1829 (w br)
2501 (m br)
2236 and 2203 (s br)
1893 (23)
1824 (24)
1780 (w br)
1863 (24)
1777 (41)
1785 (w br)
1851 (25)
1772 (24)
1858 (w br)
1805 (w br)
1876 (26)
1793 (16)
1864 (w br)
1801 (w br)
1911 (7)
2501 (150)
2
3
4
tBu
Me
Ph
Cy
Cy
Cy
1948 (w br)
1964 (98)
260531
2155 (s br)
2524 (150)
1938 (76)
1889 (w br)
1964 (102)
1924 (84)
n.r.
2221, 2185, and 2142 (s br)
2299, 2249 (m, br), and 2108 (s, br)
1980 (m s)
1917 (w br)
1962 (126)
1924 (85)
251231
2523 (150)
n.r.
5
C4H3S
Cy
1985 (m br)
1915 (w br)
1974 (125)
1934 (103)
1930 (m br)
1970 (w br)
1897 (m s)
2258, 2225, and 2200 (m br)
1815 (5)
6
7
Mes
Mes
iPr
1847 (w br)
1833 (m br)
Cyp
a
The computed values in italics have been scaled by 0.9572,32 and the intensities are given in parentheses.
boron chemical shifts with respect to the prototypical com-
Table 2. Selected NMR Chemical Shifts (δ) for Complexes
1−7
plex 1. In the case of RuH2(η2:η2-H2BMes)(PiPr3)2 (6), better
1
resolution of the signals in the hydride zone of the H NMR
1H
spectrum was observed, and the resonance at δ −11.24
appeared as a triplet of doublets. Upon selective proton
decoupling at δ −6.04, the signal collapsed into a sharp triplet,
indicating the presence of a small JHH coupling constant (3 Hz)
between the terminal hydrides and the hydrogen attached to
boron (Supporting Information, Figure S2).
complex
RuH2
RuH2B
31P{1H}
11B{1H}
1
2
3
4
5
6
7
−11.26 (t)
−10.99 (t)
−11.06 (t)
−11.11 (td)
−11.95 (td)
−11.24 (td)
−11.08 (t)
−6.10 (br)
−6.48 (br)
−6.22 (br)
−5.60 (br)
−5.80 (br)
−6.04 (br)
−6.18 (br)
83.78
84.03
81.48
81.60
80.27
96.84
88.77
58
69
66
61
54
57
57
The case of the thienylborane complex 5 is particularly
interesting, as it highlights the stabilization brought by the bis
σ-B−H ligation. The presence of a potential coordinating sulfur
atom in the thienyl ring could influence the linkage of the
thienylborane at the metal, as reported in the case of the
reaction of MeSCH2BH2MeLi with RuHCl(H2)(PCy3)2,
leading to the formation of the hydroborate complex
[RuH{(μ-H)2(BMeCH2SMe)}(PCy3)2] (8).29 Variable-tem-
In the case of complexes incorporating PCy3 at ruthenium
(1−5), the small variation in the chemical shift is illustrative of
the electronic effects induced by the substituent at boron as
observed in the case of free boranes.33 With alkyl substituents
only exerting a positive inductive effect at boron, one can
observe more deshielded signals (δ 69 (2), 66 (3)). The
introduction of aryl substituents that supply electron density to
the boron by a positive resonance results in more shielded
signals (δ 61 (4), 58 (1), 54 (5)) compared to the alkyl
analogues. The 31P{1H} NMR spectra display a sharp singlet
found in a narrow range for the PCy3 complexes (δ +80 to δ
1
perature H NMR spectra of 5 showed no change on cooling
to 180 K, excluding the presence in solution of any borohydride
linkage potentially leading to 5′ (similar to 8) or to an agostic
species (5″) resulting from sulfur coordination and one B−H
bond of the BH2 moiety (Scheme 3).
DFT calculations were carried out to probe the relative
energy of 5′ and 5″ with respect to 5. Figure 1 displays the
geometry of various optimized isomers, together with their
relative Gibbs energy. All of them are higher in energy with
respect to 5. A structure featuring both σ(B−H) coordination
and S coordination (5″ in Scheme 3) could not be located on
the potential energy surface. Instead, two isomers featuring
either S coordination (5a, Figure 1) or σ(B−H) coordination
(5b, Figure 1) were optimized. Their geometry is typical of 16-
electron Ru-dihydride complexes with one hydride ligand trans
to a vacant site and the other one trans to the weakest ligand,
here S or σ(B−H). The energies of 5a and 5b relative to 5 are
similar (ΔG = 72.5 kJ·mol−1, 5a; ΔG = 72.1 kJ·mol−1, 5b) and
1
+84). All the H NMR spectra exhibit at 298 K in the hydride
region a characteristic set of two signals in a 1:1 ratio: a
shielded triplet around δ −11 that collapses upon phosphorus
decoupling (2JPH ≈ 25 Hz) corresponding to the terminal
hydrides, and a broad signal close to δ −6 that sharpens upon
boron decoupling and thus assigned to the hydrogen atoms
attached to boron (Supporting Information, Figure S1). No
direct 1JBH coupling constant could be measured. In the case of
coordinated mesitylborane in complexes 6 and 7, the nature of
the phosphines has no significant influence on the hydride and
C
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Scheme 3. Possible Coordination Modes of Thienylborane at Ruthenium
Scheme 4. Equilibrium between the Bis(σ-B−H) and the
Borohydride Ruthenium Complexes 2 and 2′
doublet at δ −9.25, and a sharp triplet at δ −14.13 in a 1:2:1
integration ratio, respectively (Supporting Information, Figure S4).
The two broad signals that sharpened upon boron decoupling
and collapsed into singlets upon 31P decoupling are assigned to
the bridging hydrogens Hb (δ −4.41) and Hc (δ −9.25) in 2′.
1
The sharp triplet that collapsed into a singlet in the H{11B}-
{31P δ77} NMR spectrum is attributed to Ha (δ −14.13)
(Supporting Information, Figure S5). No coupling constant
between 11B and 1H could be measured, but a HMQC
11B{1H}/1H experiment recorded at 273 K clearly showed the
correlation of the hydrides of 2 and 2′ with a boron atom
resonating at δ +69 (2) and at δ +39 (2′) (Supporting
Information, Figures S6 and S7). The observation at this
temperature of a high-field-shifted signal (Δδ +30) is in
Figure 1. Optimized geometry and relative Gibbs energy (kJ·mol−1) of
the optimized isomers for H2BC4H3S interacting with [Ru-
(H)2(PCy3)2]. Most H atoms are omitted for clarity.
much too high for either complex to be observed. The optimized
structure 5c (Figure 1) is similar to the bonding pattern shown
in 5′ (Scheme 3). 5c features a borohydride coordinated to
[RuH(PCy3)2]+ through two B−H bonds and a Ru···S inter-
action (Ru···S = 2.806 Å). Another possibility would have been
to put the two phosphine ligands in the equatorial plane, thus
leading to structure 5d with no Ru···S interaction trans to the
hydride (Ru···S = 3.873 Å). The loss of Ru···S coordination has
been compensated by the optimal position of the hydride trans
to a vacant site, and 5d has a Gibbs energy similar to that of 5c
(ΔG = 35.2 kJ·mol−1 and 38.5 kJ·mol−1, respectively). Finally,
5e, the closest isomer to 5 (ΔG = 17.8 kJ·mol−1, Figure 1)
displays a borohydride interacting with Ru through its three
hydrogen atoms. The B−H bond (1.303 Å) in pseudotrans
position with respect to Ru−H is the least activated. The two
other B−H bonds (1.343 and 1.397 Å) are activated to different
extents as a result of different Ru−P bond lengths.
1
agreement with a borohydride formulation for 2′. A 2D H-
EXSY experiment carried out at different temperatures
illustrates the dynamic behavior of this system. At 273 K, the
presence in the spectrum of cross-peaks between all the
hydrides of 2 and 2′ indicates the two species are
interconverting. At 233 K, only the hydrides (Ha, Hb, and Hc)
in 2′ are exchanging, and no other cross-peak in the hydride
zone is observed (Supporting Information, Figure S8). This
corroborates the borohydride character of 2′ since borohydride
complexes are known to undergo hydrogen exchange processes
with low activation barriers.34,35
The geometries of 2 and 2′ were optimized, and the resulting
structures are shown in Figure 2. The calculated geometry for 2
is in excellent agreement with experiment (vide infra Table 3).
The borohydride ligand in 2′ presents two similar hydrogen
Deeper investigation of the properties of the tert-butylborane
complex 2 evidenced a dynamic behavior undetected during
our previous study.20 The 31P{1H} NMR spectrum recorded at
298 K of a deuterated toluene solution prepared by solubilizing
crystals of 2 exhibits a persistent set of two singlets at δ +84.03
and +77.3 in approximately a 3:1 ratio indicating the presence
of two phosphorus-containing species. This ratio appeared
temperature dependent (Supporting Information, Figure S3)
and illustrates the existence of an equilibrium characterized by
an equilibrium constant Keq of 3 at 298 K, leading to a ΔrG°
1
value close to zero (vide infra). In the hydride zone of the H
NMR spectrum at 298 K, only the well-resolved signals in a 1:1
ratio corresponding to 2 could be clearly seen. At lower
temperatures, three new resonances attributed to the
borohydride complex 2′ were observed (Scheme 4). At 233
K, the new signals are a broad signal at δ −4.41, a broad
Figure 2. Optimized geometry for 2 and 2′. Most H atoms are omitted
for clarity.
D
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a
Table 3. Comparison between Selected Distances (Å) and Angles (deg) for the X-ray and DFT/B3PW91 Calculated Structures
of Complexes 1−5 and Selected Distances (Å) and Angles (deg) for the X-ray Structures of Complexes 6 and 7
b
11 exp/calc
220 exp/calc
3 exp/calc
4 exp/calc
5 exp/calc
6 exp
7
exp
Ru−B
1.938(4)
1.957
1.934(2)
1.945
1.947(6)
1.936
1.923(8)
1.942
1.960(6)
1.946
1.959(2)
1.961(4)/1.953(5)
2.3151(9)/2.3047(10)
2.3079(9)/2.3070(9)
1.559(5)/1.549(6)
1.64(5)/1.84(5)
Ru−P1
2.3186(9)
2.362
2.3159(4)
2.356
2.2866(12)
2.326
2.3057(18)
2.338
2.3297(12)
2.337
2.3052(3)
2.3052(3)
1.548(3)
1.72(2)
Ru−P2
2.2952(9)
2.338
2.3059(4)
2.346
2.3448(12)
2.370
2.3183(18)
2.363
2.3364(12)
2.366
B−C1
1.543(5)
1.554
1.582(3)
1.590
1.537(8)
1.569
1.541(10)
1.551
1.518(8)
1.537
Ru···Hy1
Ru···Hy2
Ru−Hy3
Ru−Hy4
B−Hy1
B−Hy2
P1−Ru−P2
Ru−B−C1
∑B
1.73(3)
1.778
1.69(2)
1.777
1.79(4)
1.809
1.76(6)
1.806
1.56(4)
1.826
1.77(3)
1.788
1.73(2)
1.793
1.81(4)
1.785
1.80(6)
1.784
1.90(5)
1.795
1.68(2)
1.71(4)/1.67(4)
1.61(3)
1.608
1.53(3)
1.607
1.34(4)
1.606
1.42(6)
1.605
1.55(4)
1.611
1.54(3)
1.51(4)/1.54(4)
c
c
1.59(3)
1.614
1.55(2)
1.612
1.62(4)
1.615
1.71(6)
1.614
1.39(4)
1.600
1.53(2)
1.5894 /1.7259
1.24(3)
1.324
1.25(2)
1.334
1.18(4)
1.322
1.38(6)
1.318
1.09(4)
1.307
1.26(2)
1.27(4)/1.32(5)
1.19(4)/1.27(4)
152.61(3)/150.68(4)
177.8(3)/178.5(3)
359.95/359.25
1.29(3)
1.315
1.23(2)
1.328
1.33(4)
1.333
1.32(6)
1.328
1.36(5)
1.321
1.28(2)
150.87(3)
152.1
151.691(16)
149.51
173.29(17)
178.63
359.35
149.23(4)
153.73
168.1(4)
173.40
356.22
154.18(7)
153.8
152.93(5)
153.79
175.2(4)
174.84
359.23
158.335(18)
179.36(15)
360.00
177.1(3)
177.4
173.6(6)
176.7
359.72
358.84
a
b
The computed values for the selected distances and angles of 1−5 are in italics. 7 displays two independent molecules in the asymmetric unit.
Hy4 could not be properly refined.
c
atoms Hc (Ru−Hc = 1.768 and 1.770 Å; B−Hc = 1.366 and
1.384 Å) each in pseudotrans position with respect to the
phosphorus atoms. The third one, Hb, is pseudotrans to the
hydride Ha and, consequently, is associated with a longer Ru···
Hb distance of 1.916 Å and to a shorter B−Hb bond of 1.312 Å.
Experimentally, the van’t Hoff plot of the equilibrium
1
constant obtained by integration from both the H and the
31P NMR spectra recorded over the temperature range 298−
233 K provided a set of thermodynamic parameters ΔH° and
Figure 3. Atom numbering in complexes 1−7.
ΔS° (ΔH°(1H) = −12.4
2.5 kJ·mol−1, ΔH°(31P) = −8.0
0.8 kJ·mol−1, ΔS°(1H) = −54 10 J·mol−1.K−1, ΔS°(31P) = −36
3 J·mol−1·K−1 with 95% confidence limits) (Supporting
are given in Figure 4 (see also Figure S10 for 4 and Figure S11
for 7 in the Supporting Information). Selected geometrical
parameters are provided in Table 3 as well as those previously
published for 1 and 2.1,20 The atom numbering follows the one
given in Figure 3. It can be seen that homogeneous data have
been obtained for this series of bis(σ-B−H) ruthenium
complexes. The Ru atom is in a pseudo-octahedral environment
with the phosphines (P1 and P2) in axial positions. Four
coplanar hydrogen atoms (Hy1, Hy2, Hy3, and Hy4) occupy
the equatorial coordination sites of the ruthenium. The
interaction between the ruthenium and the boron atom is
illustrated by a Ru−B distance that is shorter than the sum of
the covalent radii (2.09 Å), being between 1.923(8) and
1.961(4) Å (Table 3). A similar distance (1.9212(2) Å) was
1
Information, Figure S9). Broadness of the signals in the H
NMR spectra might explain the differences obtained from the
two techniques. The rather high negative entropy value might
result from the change in degrees of freedom from 2 to 2′. DFT
calculations were first performed without any dispersion
corrections. The enthalpy value sets 2′ at 0.5 kJ·mol−1 above
2, but the computed Gibbs energy difference is in good
agreement with the experiment (ΔG°(calc) = −5.6 kJ·mol−1 vs
ΔG°(exp) = −2.7 to −3.7 kJ·mol−1). When D3 dispersion
corrections are included, the enthalpy of 2′ drops 2.2 kJ·mol−1
below that of 2, in better agreement with the experiment.
However, the Gibbs free energy difference is now ΔrG° = +5.8
kJ·mol−1. Overall, with or without dispersion corrections, the
two complexes 2 and 2′ are very close in energy, and the
difference, with 2 being ca. 5 kJ mol−1 more or less stable than
2′, falls within the error range of the calculations.
observed in the cationic ruthenium complex [Cp*Ru(η2:η2-
36
H2BMes)(PiPr3)]+BArF −
.
In 1−7, the P1−Ru−P2 angle lies
4
between 149.23° and 158.33°, indicating that the phosphines
are bending away from the borane ligand. In this series, the
P1−Ru−P2 angles are indeed smaller than in the PCyp3
bis(dihydrogen) complex RuH2(η2-H2)2(PCyp3)2 (168.64(2)°
by X-ray, 168.9(1)° by neutron),22 which remains the only
Structural Characterization of the Bis(σ-B−H) Ruthe-
nium Complexes. The X-ray crystal structures of compounds
3−7 were recorded at 110 K. Views of complexes 3, 5, and 6
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Figure 4. X-ray structures of complexes 3, 5, and 6. Ellipsoids are set at the 30% level. The hydrogen atoms not associated with the metal are omitted
for clarity.
Table 4. Computed X−H Bond Distances (X = H, B; Å) for the Free and Coordinated Ligands in the (Dihydrogen) and Borane
Complexes and Reaction Energy and Energy Components (kJ·mol−1) Associated with the Substitution of Two H2 Molecules by
a
H2BR
Ru(H)2(H2BR)(PCy3)2
Ru(H)2(H2)2(PCy3)2
1
2
3
4
5
X−H (free)
X−H (coord)
0.743
0.887
0.878
0
1.197
1.323
1.316
−7.1
1.200
1.333
1.328
−12.1
40.5
1.200
1.334
1.322
−29.3
31.8
1.200
1.328
1.318
−21.3
32.2
1.197
1.307
1.321
−14.6
32.2
ΔEsmd
ΔEdist(Ru)
ΔEdist(L)
22.2
38.9
57.3
36.8
38.9
43.9
34.7
35.1
ΔEint(Ru−L)
ΔEbond(Ru−L)
−223.6
−144.6
−227.4
−151.7
−236.2
−156.8
−249.1
−173.5
−232.8
−165.5
−226.1
−158.8
a
See Scheme 5 for definition of energy terms.
bis(dihydrogen) complex characterized by unambiguous
structural data.37,21 In the borane series, the larger value is
observed with the less hindered phosphine PiPr3 in 6 (P1−Ru−
P2 = 158.332(18)°). The quality of the data enabled the secure
location of the hydrogen atoms around the ruthenium from
difference Fourier electron density maps. They were freely
refined except for Hy4 in complex 7, which displays two
independent molecules in the asymmetric unit. In all the
complexes, sp2-hybridization of the boron atom is ascertained
by its trigonal planar environment, as evidenced by the sum of
the H−B−C1 and H−B−H angles (∑B, Table 3). The
deviation from planarity (360 − ∑B) is around 1° except for 3,
which is notably more distorted (ca. 4°), with the Ru−B−C1
angle diverging from linearity by 11.9(4)°.
bonds upon coordination by ca. 0.12 Å. Table 4 also reports the
reaction energy ΔEsmd associated with the substitution of two
H2 molecules by H2BR (Scheme 1). For all the borane
considered, the transformation is exothermic and the reactivity
increases in the following order: H2BMes < H2BtBu <
H2BC4H3S < H2BPh < H2BMe. In order to better understand
the origin of this trend, the coordination process has been
modeled as a sequence of steps as indicated in Scheme 5.
Scheme 5. Thermodynamic Cycle Associated with the
Coordination Process of Ligand L (L = bis-H2, H2BR) to the
Ru Fragment [Ru(H)2(PCy3)2]
Computational Studies of the Bis-σ Coordination. The
first bis-σ adduct 1 was obtained through reaction of the borane
H2BMes with the bis(dihydrogen) complex Ru-
(H)2(H2)2(PCy3)2 (Scheme 1). Even though an alternative
experimental procedure has been developed, this reaction is
important, as it sets the scene for a better understanding of the
bonding pattern associated with the formation of the bis-σ
adducts. Conceptually, the transformation is the substitution of
two coordinated σ(H−H) bonds by two coordinated σ(B−H)
ones. In the framework of the Dewar−Chatt−Duncanson
model, the interaction of these σ-bonds with the metal center is
the result of the synergetic σ-donation from the ligand to the
metal and of the π-back-donation from the metal into the σ*
orbital (H−H or B−H). Structurally, this results in an
elongation of the coordinated σ-bonds. Table 4 shows a
comparison between the B−H bond length in the free borane
H2BR (R = Mes, tBu, Me, Ph, C4H3S) and in the corresponding
bis-σ adducts 1−5. There is a clear elongation of the B−H
Even though the substitution of the two H2 ligands by H2BR
is likely to occur stepwise, simultaneous coordination of the two
B−H bonds, or the two H2 ligands, to the [Ru(H)2(PCy3)2]
fragment affords easier comparison. In order for the Ru−L
bond to be formed (L = (H2)2, H2BR), the metal fragment
[Ru(H)2(PCy3)2] has to distort from its optimized geometry
[Ru]opt to its geometry in the final complex, [Ru]dist, and this is
associated with a distortion energy ΔEdist(Ru) (see Scheme 5).
The same applies for the ligand L, where ΔEdist(L) is the energy
needed to bring the optimal geometry of free L, [L]opt, to the
geometry it adopts once coordinated to Ru, [L]dist. The
interaction energy ΔEint(Ru−L) corresponds to the energy
gained upon formation of the final complex between the two
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distorted fragments [Ru]dist and [L]dist. The coordination
energy ΔEbond(Ru−L) is thus obtained as the sum of the
contributions from distortions and interaction (eq 1).
Table 5. Expression of Selected NLMOs as a Linear
Combination of the NBOs
a
σ-donation
π-back-donation
b
ΔEbond(Ru−L) = ΔEdist(Ru) + ΔEdist(L) + ΔEint(Ru−L)
[Ru](H2)2 σ(H−H) = 0.920 σ(H−H) + dxy = 0.961 dxy + 0.21 σ*(H−H)
0.356 σ*(Ru−H)
(1)
dyz = 0.972 dyz + 0.201 σ*(H−
H)
The distortion energy of the transition metal fragment
[Ru(H)2(PCy3)2] is directly influenced by the size of the ligand
L. Coordination of a small ligand like H2 (and even two H2)
requires less distortion than coordination of bulky ligands like
H2BMes and H2BtBu (Table 4). It is interesting to note the
impact of the ortho CH3 groups in H2BMes with an increase of
6.7 kJ·mol−1 of ΔEdist(Ru) when compared to H2BPh. The
impact of H2BMe, H2BPh, and H2BC4H3S on ΔEdist(Ru) is
identical, as these three ligands exert the same steric pressure.
The distortion of Ru(H)2(PCy3)2 is essentially associated with
a change in the P−Ru−P angle. In the optimal geometry of
Ru(H)2(PCy3)2, the P−Ru−P angle is 173.1°. Coordination of
two H2 molecules induces a bending of the phosphine ligands
away from the two H2 ligands, and the P−Ru−P angle is
reduced to 163.6°. The bending is more pronounced for
H2BMe, H2BPh, and H2BC4H3S with a P−Ru−P angle of
153.8°, thus corresponding to larger ΔEdist(Ru) values (Table 4).
Finally, the larger distortion energies ΔEdist(Ru) for H2BMes
and H2BtBu correspond to even smaller P−Ru−P angles
(152.1° and 149.5°, respectively).
1
2
3
4
5
σ(B−H) = 0.930 σ(B−H) + dxy = 0.978 dxy + 0.12(σ1*(B−
0.33 σ*(Ru−H)
H) + σ2*(B−H))
d2 = 0.907 dyz + 0.376 LP*(B)
σ(B−H) = 0.928 σ(B−H) + dxy = 0.976 dxy + 0.129(σ1*(B−
0.334 σ*(Ru−H)
H) + σ2*(B−H))
d2 = 0.894 dyz + 0.366 LP*(B)
σ(B−H) = 0.927 σ(B−H) + dxy = 0.978 dxy + 0.118(σ1*(B−
0.336 σ*(Ru−H)
H) + σ2*(B−H))
d2 = 0.870 dyz + 0.390 LP*(B)
σ(B−H) = 0.929 σ(B−H) + dxy = 0.975 dxy + 0.132(σ1*(B−
0.329 σ*(Ru−H)
H) + σ2*(B−H))
d2 = 0.906 dyz + 0.378 LP*(B)
σ(B−H) = 0.933 σ(B−H) + dxy = 0.977 dxy + 0.124(σ1*(B−
0.320 σ*(Ru−H)
H) + σ2*(B−H))
d2 = 0.904 dyz + 0.379 LP*(B)
a
For σ-donation from σ(X−H) (X = H, B) into σ*(Ru−H), two
slightly different expressions were obtained, but the average of the
coefficients is given below. See text for definition of d2 and LP*(B).
b
[Ru](H2)2 = RuH2(H2)2(PCy3)2.
A natural bonding orbital (NBO) analysis of the electronic
structure of [Ru]opt has been carried out, and Figure 5 shows
Figure 6. Shape of selected NLMOs of 3.
σ*(B−H) because of the geometry imposed by H2BR. Table 5
gives the composition of the NLMOs associated with σ-
donation from σ(X−H) (with X = H, B) and back-donation
into σ*(X−H) for the various complexes, and Figure 6 displays
the corresponding NLMOs in the case of H2BPh. From the
values in Table 5, one can conclude that σ-donation from H2 is
slightly larger than from any borane as the weight of the parent
NBO is lower. In the same spirit, back-donation from dxy and
dyz in the bis(dihydrogen) complex is stronger than back-
donation from dxy in the borane complexes. Yet, the interaction
energy values ΔEint(Ru−L) are more negative for 1−5, and the
substitution of H2 by borane is exothermic. This extra gain of
stability originates from the π-acid properties at boron with a
pseudovacant orbital centered on boron and perpendicular to
the BH2 plane, LP*(B). The two isoenergetic d orbitals dyz and
dxz, can mix to transform into two equivalent d orbitals, d2 and
d3, with d2 having the correct symmetry to interact with the
vacant orbital at boron and d3 remaining essentially non-
bonding (Figure 6). The interaction between d2 and LP*(B) is
significant, as illustrated by the expression of the resulting
Figure 5. Shape and energy (eV) of the frontier NLMO of the
Ru(H)2(PCy3)2 fragment.
the shape and energy of selected natural localized molecular
orbitals (NLMOs). The metal-centered frontier NLMOs for
the d6 fragment Ru(H)2(PCy3)2 are composed of three d lone
pairs and two vacant orbitals σi*(Ru−H) (i = 1, 2) essentially
developed trans to the hydride ligands. The set of pseudo
nonbonding occupied NLMOs on Ru is split in two because
back-donation into σ*(P−C) orbitals lowers the energy of the
d orbital perpendicular to the RuH2 plane (dxz and dyz).
The coordination of H2BR to Ru(H)2(PCy3)2 proceeds
through σ-donation from σ(B−H) into σi*(Ru−H) and π-back-
donation from the equatorial dxy lone pair into σ*(B−H). The
essential difference with the bis(dihydrogen) complex is that
only one d lone pair (dxy) is used for back-donation into
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400, or 500 spectrometers operating at 300.13, 400.13, or 500.33
NLMO d2 (Table 5, Figure 6). This strong interaction is
responsible for the short Ru−B distance observed experimen-
tally. Back-donation into σ*(B−H) is efficient from dxy, as the
borane ligand lies in the equatorial plane. From the expression
of the NLMOs and from the values of ΔEint(Ru−L) (Table 4),
there are two distinct groups: aromatic and aliphatic boranes.
The former experiences weaker back-donation from the metal
center because the vacancy at boron is destabilized by con-
jugation with the aromatic ring. Such an interaction is clearly
visible in the shape of the NLMO (LP*(B) in Figure 6). For
the aliphatic cases, the only source of stabilization of the
vacancy at boron in free H2BR is through hyperconjugation.
This is effective in H2BtBu and H2BMe, where one C−X bond
1
MHz, respectively, for H; 121.5, 161.97, or 202.5 MHz, respectively,
for 31P; 75.48, 100.62, or 125.81 MHz, respectively, for 13C; and 96.29,
1
128.38, or 160.52 MHz, respectively, for 11B. H and 13C chemical
shifts are reported in ppm referenced internally to residual protio-
solvent, while 31P chemical shifts are relative to 85% H3PO4 and those
of 11B are relative to BF3·OEt2 external references. Chemical shifts are
quoted in δ (ppm), and coupling constants in hertz. The following
abbreviations are used: br, broad; s, singlet; d, doublet; t, triplet; m,
multiplet. Infrared spectra were recorded on a Bruker Alpha FT-IR
spectrometer equipped with a Platinum single reflection ATR module.
Elemental analyses were performed by the “in-house” service of the
Laboratoire de Chimie de Coordination, Toulouse
RuHCl(η2-H2)(PCyp3)2. A mixture of [Ru(COD)Cl2]n (1.00 g, 3.6
mmol), tricyclopentylphosphine (1.73 mL, 7.2 mmol), triethylamine
(0.50 mL, 3.6 mmol), and 2-propanol (20 mL) was heated at 85 °C
overnight in a Fischer−Porter bottle pressurized under dihydrogen
(2.5 bar). After cooling at −40 °C, the suspension was filtered.
The brown solid was washed with cold ether and dried under vacuum
(1.84 g, 84%).
t
(X = H, for Me; X = C, for Bu) is aligned with the p OA on
boron. This C−X bond is elongated with respect to the two
t
other ones (1.106 Å vs 1.094 Å, Me; 1.559 Å vs 1.534 Å, Bu)
and presents a reduced B−C−X angle (104.0° vs 115° Me;
t
98.4° vs 114.5°, Bu). This hyperconjugation is not strong
1
31P{1H} NMR (C6D6, 298 K, 162 MHz): δ 54.84 (s, PCyp3). H
enough to compete with back-donation from d2, and the NBO
analysis even found a bond, constructed on d2 and LP*(B),
between Ru and B. This illustrates the greater π-acidity of the
aliphatic boranes.
NMR (C6D6, 298 K, 400 MHz): −16.43 (s, 3H, RuH(H2)), 1.5−2.4
(m, 54H, Cyp). 13C{1H} NMR (C6D6, 298 K, 100 MHz): 26.45 (vt,
app
J
= 4.0 Hz, CH2 Cyp), 30.33 (vt, appJPC < 2 Hz, CH2 Cyp), 36.71
PC
app
(vt,
J
= 11 Hz, PCH Cyp).
PC
RuH2(η2:η2-H2BMe)(PCy3)2 (3). An ethereal solution (2 mL) of
MeBH3Li (35.1 mg, 0.419 mmol) was added to a suspension of
RuHCl(η2-H2)(PCy3)2 (293.4 mg, 0.419 mmol) in ether (4 mL) and
stirred at room temperature for 5 min. After removal of the solvent
under vacuum, pentane was added, affording a suspension that was
filtered over activated Celite. The solvent was then removed under
vacuum, leading to RuH2(η2:η2-BH2Me)(PCy3)2 (3) (259.3 mg, 90%)
CONCLUSION
■
In summary, we have shown that the geminal bis(σ-B−H)
coordination mode between a monosubstituted borane (RBH2)
and a [RuH2(PR′3)2] ruthenium fragment could be obtained
with different phosphines and a variety of boron substituents.
The corresponding complexes RuH2(η2:η2-H2BR)(PR′3)2 were
conveniently synthesized in a one-pot procedure by the use of
lithium monosubstituted borohydrides RBH3Li and the chloro
complexes RuHCl(H2)(PR′3)2, thus enabling an easy access to
this class of compounds. Although the obtained complexes pre-
sent similar spectroscopic and structural data, complex 2 exhibits
original behavior in solution; it is involved in an equilibrium
with the κ3H-tert-butylborohydride form 2′ that is very close in
energy. In contrast, the presence at boron of a thienyl group
with a sulfur atom likely to coordinate to the metal center does
not favor an agostic species such as 5″ and neither a boro-
hydride linkage as in 2′ or as previously reported.29 In our systems,
borane coordination to the metal center proceeds predom-
inantly through σ-donation from σ(B−H) into σi*(Ru−H) and
π-back-donation from the equatorial dxy lone pair into σ*(B−H),
the borane ligand being forced to lie in the equatorial plane for
better orbital overlap. NBO analysis well evidenced the greater
π-acidity of the aliphatic boranes. The geminal bis(σ-B−H)
coordination mode represents a first level in the B−H bond
activation of monosubstituted boranes, and future work will
address reactivity studies toward more pronounced degrees of
B−H activation.
1
as an orange-brown powder. H NMR (C6D6, 400,13 MHz): −11.06
2
(t, 2H, JPH = 25.6 Hz, RuH2), −6.22 (br, 2H, RuH2B), 0.82 (s, 3H,
Me), 1.20−2.50 (m, 66H, Cy). T1 min (tol-d8, 253 K, 500.33 MHz):
−11.06 (344 ms), −6.22 (173 ms). 13C{1H} NMR (C6D6, 100.62
MHz): 11.66 (CH3), 27.01, 28.05, 30.70 (CH2, PCy3), 38.81 (CH,
PCy3). 31P{1H} NMR (C6D6, 161.99 MHz): 81.48 (s). 11B{1H} NMR
(C6D6, 128.38 MHz): 66.4 (br). Anal. Calcd for C37H73BP2Ru: C,
64.24; H, 10.64. Found: C, 63.62; H, 11.46.
RuH2(η2:η2-H2BPh)(PCy3)2 (4). Toluene (5 mL) was added to a
mixture of RuHCl(η2-H2)(PCyp3)2 (72.2 mg, 0,103 mmol) and
lithium phenylborohydride, PhBH3Li (12.0 mg, 0.122 mmol). The
solution was stirred at room temperature for 17 h and filtrated over
Celite. A minimum amount of pentane was added, and the suspension
was stirred. After removal of the pentane supernatant, the yellow solid
was dried under vacuum. Compound 4 was isolated as a yellow
1
powder in 71% yield (55 mg). H NMR (C6D6, 298 K, 400 MHz):
2
−11.11 (td, 2H, JPH = 25.2 Hz, JHH = 3.5 Hz RuH2), −5.6 (brs, 2H,
RuH2B), 1.3−2.3 (m, 66H, Cy), 7.28−7.36 (m, 3H, CH Ph), 8.17 (m,
2H, CH Ph). 13C{1H} NMR (C6D6, 298 K, 100.62 MHz): 27.0, 27.95,
30.71 (CH2 Cy), 38.80 (m, CH Cy), 127.92, 129.87, 135.82 (s, CH Ph),
the ipso-CIVB was not observed. 31P{1H} NMR (C6D6, 298 K, 161.97
MHz): 81.60 (s, PCy3). 11B{1H} NMR (C6D6, 298 K, 128.38 MHz):
61.4 (brs). Anal. Calcd for C42H75BP2Ru: C, 66.91; H, 10.03. Found:
C, 67.75; H, 10.00.
RuH2(η2:η2-H2BC4H3S)(PCy3)2 (5). Ether (5 mL) was added to a
mixture of 2-thienylborohydride (2-C4H3SBH3Li) (34 mg, 0.286
mmol) and RuHCl(η2-H2)(PCy3)2 (200.0 mg, 0.286 mmol), and the
resulting yellow solution was stirred for 1 h at room temperature. After
removal of the solvent under vacuum and addition of toluene (8 mL),
the resulting suspension was filtered over Celite. The solvent was
removed under vacuum, affording a yellow solid. This solid was
washed with a minimum amount of pentane and dried under vacuum
to afford pure RuH2(η2:η2-H2BC4H3S)(PCy3)2, which was isolated as
an orange-yellow solid (175 mg, 81%).
EXPERIMENTAL SECTION
■
General Procedures. All experiments were performed under an
atmosphere of dry argon using standard Schlenk and glovebox techni-
ques. Unless stated, all chemicals were purchased from Aldrich and
used without further purification. [RuH2(η2-H2)2(PCy3)2],38 [RuHCl-
(η2-H2) (PCy3)2],26 [RuHCl(η2-H2)(PiPr3)2],26 and lithium borohy-
drides (RBH3Li)27 were prepared according to literature procedures.
THF was freshly distilled under argon from Na/benzophenone. All
other solvents were purified and dried through an activated alumina
purification system (MBraun SPS-800). NMR solvents were dried
using appropriate methods and degassed prior to use. NMR samples of
sensitive compounds were prepared under an argon atmosphere.
Nuclear magnetic resonance spectra were recorded on Bruker AV 300,
1H NMR (THF-d8, 400.13 MHz): −11.95 (td, 2H, 2JPH = 25.2 Hz,
JHH = 2.4 Hz, RuH2), −5.80 (br, 2H, RuH2B), 1.20−2.10 (m, 66H,
3
3
Cy), 7.10 (dd, 1H, JHH = 2.4 Hz, JHH = 4.5 Hz, CH4 thienyl), 7.51
(d, 1H, 3JHH = 2.4 Hz, CH3 thienyl), 7.63 (d, 1H, 3JHH = 4.5 Hz, CH5
H
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thienyl). T1 min (tol-d8, 253 K, 500.33 MHz): −11.55 (357 ms), −5.46
(157 ms). 13C{1H} NMR (THF-d8, 100.62 MHz): 26.73, 27.70, 30.37
(CH2 PCy3), 38.48 (m, CH PCy3), 127.78 (s, C4H thienyl), 131.31
(s, C5H thienyl), 136.60 (s, C3H thienyl), 148.37 (br, ispo-C2B thienyl).
31P{1H} NMR (THF-d8, 161.97 MHz): 80.27 (s). 11B{1H} NMR
(THF-d8, 128.38 MHz): 54 (br). Anal. Calcd for C40H73BP2RuS: C,
63.22; H, 9.68. Found: C, 63.20; H, 9.39.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gilles.alcaraz@lcc-toulouse.fr, eric.clot@univ-montp2.
fr, sylviane.sabo@lcc-toulouse.fr.
Present Address
⊥Leibniz-Institut fur Oberflachenmodifizierung e.V. Permoser-
RuH2(η2:η2-H2BMes)(PiPr3)2 (6). Ether (15 mL) was added to a
mixture of RuHCl(η2-H2)(PiPr3)2 (300 mg, 0.652 mmol) and lithium
mesitylborohydride, MesBH3Li (91.3 mg, 0.652 mmol), and the
solution was stirred at room temperature for 15 min. The resulting
suspension was filtrated over Celite, and the solvent removed under
vacuum, affording a yellow solid. Pentane was added at −25 °C.
Addition of cold pentane (−25 °C) afforded a precipitate that was
separated from the supernatant. After drying under vacuum, 6 was
isolated as a yellow solid (210 mg, 58% yield). 1H NMR (C6D6, 298 K,
̈
̈
strasse 15, D-04318 Leipzig, Germany.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the CNRS (G.A. and S.S.E.), the ANR (programme
HyBoCat, ANR-09-BLAN-0184), and the French Ministry of
Research (Y.G.) for support, and the DGA for the salary of
G.B.-L. Johnson Matthey is gratefully acknowledged for a
generous gift of RuCl3·nH2O.
2
300 MHz): −11.24 (td, 2H, JPH = 25.4 Hz, JHH = 3.0 Hz, RuH2),
−6.04 (brs, 2H, RuH2B), 1.19 (m, 36, JHH = 6.8 Hz, CH3 iPr), 2.01
3
(m, 6H, 3JHH = 6.8 Hz, CH iPr), 2.17 (s, 3H, p-CH3 Mes), 2.96 (s, 6H,
o-CH3 Mes), 6.87 (s, 2H, CH Mes). 13C{1H} NMR (C6D6, 298 K,
100.62 MHz): 20.40 (s, CH3 iPr), 21.10 (s, p-CH3 Mes), 23.06 (s,
i
o-CH3 Mes), 28.31 (m, Hz, CH Pr), 128.89 (s, CH Mes), 139.66 (s,
p-CIV Mes), 144.76 (s, o-CIV Mes), the ipso-CIVB was not observed.
31P{1H} NMR (C6D6, 298 K, 121.5 MHz): 96.84 (s). 11B{1H} NMR
(C6D6, 298 K, 128.38 MHz): 57 (brs). Anal. Calcd for C27H57BP2Ru:
C, 58.37; H, 10.34. Found: C, 58.57; H, 10.71.
REFERENCES
■
(1) Alcaraz, G.; Clot, E.; Helmstedt, U.; Vendier, L.; Sabo-Etienne, S.
J. Am. Chem. Soc. 2007, 129, 8704−8705.
(2) Alcaraz, G.; Grellier, M.; Sabo-Etienne, S. Acc. Chem. Res. 2009,
42, 1640−1649.
RuH2(η2:η2-H2BMes)(PCyp3)2 (7). Ether (15 mL) was added to a
mixture of RuHCl(η2-H2)(PCyp3)2 (400 mg, 0,649 mmol) and lithium
mesitylborohydride, MesBH3Li (113.6 mg, 0.649 mmol). The resulting
solution was stirred at room temperature for 15 min. Then, the solvent
was evaporated under vacuum, and toluene (10 mL) was added. The
resulting suspension was filtered over Celite, and the solvent removed
under vacuum. Pentane was added and removed again under vacuum.
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1
After drying, 7 was isolated as a yellow solid (58% yield, 268 mg). H
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2
NMR (C6D6, 298 K, 400 MHz): −11.08 (t, 2H, JPH = 25.4 Hz,
RuH2), −6.18 (brs, 2H, RuH2B), 1.5−2.1 (m, Cyp), 2.15 (s, 3H, p-
CH3 Mes), 2.97 (s, 6H, o-CH3 Mes), 6.87 (s, 2H, m-H Mes). 13C{1H}
NMR (C6D6, 298 K, 100.62 MHz): 21.22 (s, p-CH3 Mes), 23.20 (s, o-
CH3 Mes), 26.34, 30.46 (CH2 Cyp), 41.14 (CH Cyp), 129.04 (s, CH
Mes), 139.83 (s, p-CIV Mes), 144.78 (s, o-CIV Mes), the ipso-CIVB was
not observed. 31P{1H} NMR (C6D6, 298 K, 161.97 MHz): 88.77 (s).
11B{1H} (C6D6, 298 K, 128.38 MHz): 57.5 (brs). Anal. Calcd for
C39H69BP2Ru: C, 65.81; H, 9.77. Found: C, 65.01; H, 10.23.
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Computational Details. All the calculations have been performed
with the Gaussian09 package at the B3PW91 level of hybrid density
functional theory.39−41 For the optimization of geometry, the ruthenium
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augmented by an f polarization function.43 The phosphorus and sulfur
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ASSOCIATED CONTENT
■
S
* Supporting Information
NMR data; computational details, Cartesian coordinates, and
energy of the optimized structures; and X-ray data. The five
crystal structures have been deposited at the Cambridge
Crystallographic Data Centre and allocated the deposition
numbers CCDC 946541−946545. This material is available
free of charge via the Internet at http://pubs.acs.org.
I
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