Phosphinoborane and sulfidoborohydride as chelating ligands in polyhydride ruthenium complexes: Agostic s-borane versus dihydroborate coordination

Full text of DOI:10.1002/anie.200806178

Communications
DOI: 10.1002/anie.200806178
À
B H Agostic Interactions
Phosphinoborane and Sulfidoborohydride as Chelating Ligands in
Polyhydride Ruthenium Complexes: Agostic s-Borane versus
Dihydroborate Coordination**
Yann Gloaguen, Gilles Alcaraz,* Anne-Frꢀdꢀrique Pꢀcharman, Eric Clot, Laure Vendier, and
Sylviane Sabo-Etienne*
À
The chemistry of s-borane complexes remains at an early
stage of development by comparison to the analogous s-
dihydrogen and s-silane families.[1–5] However, since the
isolation of the first s-borane metal complex in 1996,[6]
olefinic C H bond.[23] The corresponding complex [Cr{HBN-
=
(SiMe3)2CH CHCMe3}(CO)4] was isolated and characterized
as a formally chromium(0) olefin complex with a weak
B H coordination to the metal center. We now report the
À
À
important findings dealing with B H bond activation have
synthesis of two new boron-containing compounds, the
phosphinomethyl(amino)borane Ph2PCH2BHNiPr2 (1) and
the lithium sulfido(methyl)borohydride Li[MeSCH2BH2Me]
(2), as well as their coordination to the two ruthenium
precursors [RuH2(h2-H2)2(PCy3)2] (3) and [RuHCl(h2-H2)-
(PCy3)2] (4). The strategy is based on the one we recently used
to isolate the bis s-borane complex [RuH2(h2:h2-H2BMes)-
(PCy3)2] upon reaction of (3) with mesitylborane or the chloro
complex (4) with lithium mesitylborohydride.[17] Our study
been regularly disclosed.[7–9] The s-borane complexes are
often invoked as intermediates in the formation of the
corresponding hydridoboryl complexes and the microscopic
reverse reductive elimination. More and more compelling
À
evidence for s-B H coordination is found in catalyzed
borylation processes,[10] which supports the recent metathesis
mechanism termed s-CAM for late-transition-metals (s-
complex-assisted metathesis, which involves metal-induced
À
dynamic rearrangements of E H bonds at constant oxidation
includes the isolation of the first d-agostic ruthenium complex
2
state).[11] Moreover, s-borane complexes and the related s-
amine–borane species are now found to be relevant to the
catalytic dehydrogenation of amine–boranes.[12–16] As part of
containing an h -B H bond that involves a trivalent boron
À
atom and illustrates the competition between s-coordination
and dihydroborate ligation.
À
our research on B H bond activation of less-common borane
The addition of 1 to the bis(dihydrogen) complex 3 in
toluene at room temperature led to the substitution of the two
reagents,[17,18] we have decided to develop a new class of
borane-attached ligands to study hydrogen-transfer
reactions and compare the reactivity of true s-borane
complexes with agostic species.[19,20] The synthesis of
bidentate and potentially hemilabile L ~ BHR (L = P,
S and R = amino, alkyl) borane-containing ligands
was challenging as this class of compounds remains
unexplored in particular by comparison to the
ambiphilic borane derivatives.[21,22] Braunschweig
Scheme 1. Synthesis of the dihydride phosphinomethyl(amino)borane complex
5.
et al. very recently reported access to a vinylborane
by insertion of a borylene chromium complex into an
dihydrogen ligands by 1 (Scheme 1). The new complex
[RuH2{h2-H-B(NiPr2)CH2PPh2}(PCy3)2] (5) was fully charac-
terized by multinuclear NMR spectroscopy and single-crystal
X-ray diffraction. The 31P{1H} NMR spectrum of 5 displays an
AM2 pattern with a triplet at d = 2.58 ppm and a doublet at
d = 65.22 ppm corresponding to the PPh2 group cis to two
PCy3 ligands, as indicated by the small JPP coupling constant of
15 Hz. The 11B{1H} NMR spectrum exhibits a broad signal
centered at d = 35.7 ppm slightly more upfield than that of 1
(d = 38.5 ppm). The 1H NMR spectrum of 5 in C7D8 at 258 K
has, in the hydride region, a broad singlet, a multiplet (tdd),
and a broad signal in a 1:1:1 integration ratio at d = À7.29,
À11.52, and À18.38 ppm, respectively, in agreement with a
[*] Y. Gloaguen, Dr. G. Alcaraz, A.-F. Pꢀcharman, Dr. L. Vendier,
Dr. S. Sabo-Etienne
CNRS, LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, 31077 Toulouse (France)
and
Universitꢀ de Toulouse, UPS, INPT
31077 Toulouse (France)
E-mail: alcaraz@lcc-toulouse.fr
sabo@lcc-toulouse.fr
Homepage: http://www.lcc-toulouse.fr/lcc/spip.php?article433
Dr. E. Clot
Universitꢀ Montpellier 2, Institut Charles Gerhardt
CNRS 5253, cc 1501
Place Eugꢁne Bataillon, 34095 Montpellier (France)
ruthenium center surrounded by three hydrogen atoms. The
singlet at d = À7.29 ppm was the only signal to sharpen upon
boron decoupling, whereas the two other signals collapsed
into doublets (JHH = 8.5 Hz) upon phosphorus decoupling.
[**] We thank the CNRS and the ANR (programme blanc ANR-06-BLAN-
0060-01) for support (Y.G., G.A., L.V., S.S.E.)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200806178.
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Chemie
Hydride disposition around the ruthenium was supported by
selective phosphorus-decoupled 1H NMR, NOESY and
EXSY experiments. The X-ray structure of 5 was determined
at 110 K (Figure 1 and Table 1). The Ru atom is in a distorted
octahedral environment with the PCy3 units in pseudo axial
positions and bent away from the phosphinoborane ligand to
reduce steric repulsion (P1-Ru-P3: 146.55(7)8). The coordi-
nation sites in the equatorial plane are occupied by three
coplanar hydrogen atoms H1, H2, H3, and one phosphorus
an agostic species, the Ru···H3 distance is elongated com-
À
pared to the two other ruthenium–hydride distances. The Ru
B distance (2.7574(80) ꢀ), much longer than the sum of the
covalent radii (2.09 ꢀ), excludes a boryl formulation. It is also
À
longer than those observed in ruthenium s-B H com-
plexes,[24,25] but shorter than in the ruthenium complex
[RuH{HBBNCH2PMe2}(PMe3)3] resulting from HBBN (9-
À
borabicyclo[3.3.1]nonane) insertion into a Ru C bond of a
metalated phosphine (2.994(1) ꢀ).[26] The B-C1-P2 angle of
103.2(4)8 in 5 is in agreement with a constrained d-agostic
coordination mode.[27] We noted that the substitution of the
two dihydrogen ligands from 3 proceeds in a step-by-step
manner as monitored by NMR spectroscopy. The ruthenium
dihydrido(dihydrogen) intermediate 6 with a pendant borane
ligand (Scheme 1) could be identified before evolution of the
second dihydrogen and final formation of 5. Interestingly,
pressurization of a C6D6 solution of 5 with dihydrogen (3 bar)
leads after 50 min to the complete disappearance of the
agostic complex and formation of 6, with traces of 3.
À
atom P2. The B H3 bond length (1.23(7) ꢀ) is in the range
observed for ruthenium s-borane complexes,[17,24,25] and
comparable to that reported by Braunschweig et al. for the
vinyl(amino)borane chromium complex.[23] As expected for
We have also designed a second type of L ~ BHR ligand
which, compared to 1, has marked electrophilic character at
the boron center. The reaction of the new lithium borohy-
dride salt 2 with the chloro complex 4 (Scheme 2) gave a
complex formulated as a hydrido(dihydroborate) complex
[RuH{(m-H)2BMeCH2SMe}(PCy3)2] (7) which was isolated in
31% yield and fully characterized by NMR spectroscopy and
single-crystal X-ray diffraction.
Figure 1. X-ray crystal structure of 5.
Scheme 2. Synthesis of the hydrido(dihydroborate) complex 7.
Table 1: Selected geometrical parameters (distances in ꢂ, angles in 8) for
the experimental and calculated structures for 5 and 7.
The 31P{1H} NMR spectrum of 7 in C7D8 at 298 K consists
of an AB pattern at d = 67.79 ppm and d = 63.90 ppm with a
JPP value of 17.2 Hz, typical of a cis arrangement of the
phosphine groups. The 11B{1H} NMR spectrum exhibits a
signal centered at d = 19.6 ppm which is upfield of that of 5.
Parameter
5
Parameter
7
Exp
Calcd
Exp
2.266(8)
2.3103(16)
2.3242(15)
2.4071(16)
1.85(5)
1.70(7)
1.65(8)
1.15(6)
1.38(7)
Calcd
2.250
Ru-B
2.7574(80)
2.3397(17)
2.3016(18)
2.3234(17)
1.77(7)
1.69(6)
1.92(7)
1.23(7)
2.858 Ru-B
Ru-P1
Ru-P2
Ru-P3
Ru-H1
Ru-H2
Ru···H3
B-H3
2.369 Ru-P1
2.372 Ru-P2
2.371 Ru-S
1.615 Ru-H1
1.559 Ru-H2
1.952 Ru-H3
1.241 B-H1
B-H2
2.370
2.378
2.412
1.877
1.770
1.581
1.313
1.366
1
The H NMR spectrum of 7 displays at set of three hydride
resonances in a 1:1:1 integration ratio: a broad signal at d =
À4.30 ppm (H1), a broad doublet at d = À8.99 ppm (H2), and
a doublet of doublet at d = À14.36 ppm (H3). The disposition
of the hydrogen atoms around the ruthenium was ascertained
by selective phosphorus and/or boron-decoupled 1H NMR
spectroscopy, TOCSY and NOE experiments at variable
temperature. The X-ray structure of 7 was determined at
110 K (Figure 2 and Table 1).
The ruthenium atom is also in a distorted octahedral
environment but in contrast to 5, the PCy3 units are in a cis
position with a P1-Ru-P2 angle of 105.18(6)8. The sulfur atom
is trans to one PCy3 ligand and occupies an apical position.
The three hydrogen atoms H1–H3 are coplanar and located in
P1-Ru-P2 105.46(6)
P1-Ru-P3 146.55(7)
P2-Ru-P3 104.77(6)
105.0
P1-Ru-P2 105.18(6)
106.7
146.3
105.0
105.6
125.6
P1-Ru-S
P2-Ru-S
S-C1-B
Ru-H2-B
Ru-H1-B
Ru-S-C1
C1-B-H1
C1-B-H2
90.29(6)
164.26(6)
101.4(4)
94(4)
91.0
161.0
101.1
90.8
87.8
88.5
P2-C1-B
Ru-H3-B
103.2(4)
120(5)
95(4)
Ru-P2-C1 100.1(2)
C1-B-H3 122(3)
98.0
121.1
89.00(19)
114(3)
110.4
106.3
À
the equatorial plane. The Ru B distance of 2.266(8) ꢀ is
100(3)
longer than those reported for s-borane complexes, but
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firmed the description of the system as a phosphinoborane
6
II
À
ligand in interaction with a d Ru center. The s(B H) bond,
slightly more polarized toward H in the complex than in the
free ligand (57% vs. 54.4%), interacts with the antibonding
À1
À
s*(Ru H) orbital (143.8 kJmol ) in a typical s-donation
process. However, no significant back-donation from Ru to B
could be identified by the NBO procedure. The main
contributor to the stabilization of the vacancy at boron is
p donation from the nitrogen lone pair (367.0 kJmolÀ1), a
second-order perturbation energy interaction approximately
À
three-times larger than the s donation from B H to Ru. This
stabilization from the nitrogen lone pair is already present in
the free ligand (359 kJmolÀ1) and complexation has not
significantly altered it.
All attempts to locate an isomer with two hydrides and a
À
s(B H) coordination, as formally depicted for BS in Scheme 3
(L = S), failed: the hydride cis to B always formed the second
À
B H bond of a borohydride ligand. The lowest minimum, B’S
Figure 2. X-ray crystal structure of 7.
(Figure S2 in Supporting Information), displays calculated
geometrical parameters for the heavy atoms in excellent
agreement with the X-ray data for 7. The two other isomers
are respectively at 50.3 kJmolÀ1 (AS) and 17.0 kJmolÀ1 (CS).
Moreover, the calculations allowed more secure location of
the hydrogen atoms in B’S, and, in particular, the hydride and
the H atoms bonded to B. The calculated geometry around
the boron is indicative of a borohydride character with two
similar to that of the dihydroborate metallacycle [Ru{(m-
H)2BPhCMe2 CHMeCH2}(PMe3)3] (2.243(3) ꢀ).[26]
Despite the different synthetic procedures, complexes 5
and 7 could be considered formally as adducts between the
L ~ BHR ligand and the {RuH2(PCy3)2} fragment. Thus, there
are potentially three different isomers of general formulation
[RuH2(LCH2BHR)(PCy3)2] as depicted in Scheme 3. To
estimate the relative energies between these three isomers,
ONIOM(B3PW91/HF) calculations have been performed.
The geometries of the three complexes AL, BL, and CL (L = P
or S) are given in the Supporting Information.
À
B H bond lengths of similar magnitudes (1.313 ꢀ and
1.366 ꢀ). The NBO analysis confirmed the geometrical
analysis with the formation of a borohydride species. The
À
dissymmetry in the B H bond lengths originates from the
À
different nature of the ligand trans to the coordinated B H
bonds.[28]
The relative energies obtained for the complexes AL, BL,
and CL and the NBO analysis illustrate the subtle influence of
the substituent on B to trigger the mode of coordination. For
L = PPh2, the amino group on B is a strong p-base that
provides enough stabilization to boron. The preferred isomer
is thus the one that puts the weakest ligands (PPh2 and
s(BH)) trans to the strongest ones, the hydrides as in 5. For
L = SMe, the boron atom is strongly electrophilic and seeks
electron density upon coordination. This is best achieved with
the isomer B’S, where hydride transfer to boron affords the
borohydride complex 7.
Scheme 3. Different isomers resulting from the interaction of L~BHR
with {RuH2P2} (P=PCy3).
In conclusion, we report herein the first example of an
2
À
In the case of the phosphinoborane ligand (L = P) the
most stable isomer is AP, in agreement with the X-ray
structure of 5. The two other isomers are less stable by
31.5 kJmolÀ1 (BP) and 7.9 kJmolÀ1 (CP). The calculated
geometrical parameters for AP are given in Table 1 and are
agostic ruthenium complex containing a h -B H bond
involving a trivalent boron. The hemilability of the ligand is
illustrated by the reversible loss of dihydrogen from the
complex. When using a more electrophilic boron reagent, a
dihydroborate complex is stabilized. Further work is in
progress to design new L ~ BHR as potentially hemilabile
ligands and to study their impact in stoichiometric and
catalytic reactions.
À
in very good agreement with the X-ray data for 5. The Ru P
bond distances are computed to be slightly longer than the
experimental ones, but the P-Ru-P angles are faithfully
À
reproduced. The interaction of B H with Ru is best described
À
as a d-agostic interaction with an elongated B H bond
(1.241 ꢀ in AP, 1.204 ꢀ in free Ph2PCH2BHNiPr2) and a short
Ru···H contact (1.952 ꢀ). The HBNiPr2 moiety is almost
planar with H-B-N-C dihedral angles of 0.18 and 175.68. The
interaction with Ru is essentially through the hydrogen atom
bonded to B (Ru-H3-B = 125.68). An NBO analysis con-
Experimental Section
1: Ph2PCH2Li (0.849 g, 4.118 mmol) was added to an ethereal solution
(40 mL) of iPr2NBHCl (0.730 g, 4.951 mmol) at À788C. The suspen-
sion was stirred for 30 min at this temperature and for 17 h at room
temperature. After filtration over celite, removal of the solvent and
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Chemie
trap-to-trap distillation, Ph2PCH2BHNiPR2 (1) was isolated as a
colorless oil (1.012 g) contaminated by 10% of Ph2PMe. Selected
NMR data (CDCl3, 298 K): 31P{1H} NMR (121.494 MHz): d =
À16.96 ppm. 11B{1H} NMR (128.377 MHz): d = 38.5 ppm. 1H NMR
(400.132 MHz): d = 0.95 and 1.04 (d, 2 ꢁ 6H, 3JHH = 6.70 Hz, CH3 iPr),
The calculations have been performed with the Gaussian03
package (see Supporting Information). ONIOM(B3PW9/HF) calcu-
lations have been carried out on the experimental systems.[29] In the
high-level layer, treated with the hybrid functional B3PW91,[30,31] the
PCy3 ligands were modeled as PMe3, the PPh2 ligand as PH2, the iPr
groups as H. These groups were explicitly incorporated in the low-
level layer and were treated at the HF level. All the other groups on
the complex were explicitly incorporated in the high-level layer. For
the calculations at the B3PW91 level, the Ru and P atoms were
described with the pseudo-potentials from Dolg et al. and the
associated basis sets,[32,33] augmented by a polarization function.[34,35]
The other atoms were treated with a 6-31G(d,p) basis set.[36] For the
calculations at the HF level, the Ru and P atoms were described by
the pseudo-potentials from Hay and Wadt and the associated basis
sets.[37,38] The other atoms were treated with a 4–31G basis set.[39] The
NBO analysis[40] was performed on the B3PW91 electronic density
obtained on the ONIOM geometry with the basis set described for the
high-level layer.
3
1.73 (m, 2H, BCH2P), 3.17 and 3.90 (h, 2 ꢁ 1H, JHH = 6.70 Hz, CH
iPr), 4.60 (br, BH), 7.20–7.50 ppm (m, 10H, CH Ph). 13C{1H} NMR
(100.613 MHz): d = 17.82 (br, BCH2P), 22.15 (s, CH3 iPr), 26.80 (s,
CH3 iPr), 45.15 (s, CH iPr), 49.08 ppm (s, CH iPr).
2: Methyllithium 1.6m in diethyl ether (1.28 mL, 2.048 mmol) was
added to a cooled (À788C) ethereal solution (20 mL) of MeSCH2B-
(OiPr)2 (0.390 g, 2.051 mmol). The resulting solution was stirred for
2.5 h at this temperature before warming to room temperature. It was
then transferred to a cooled (À788C) ethereal suspension (20 mL) of
LiEtOAlH3 (2.051 mmol) and stirred for 2 h before warming to room
temperature. After workup, MeSCH2BH2MeLi (2) was isolated as a
white powder (0.200 g) contaminated by traces of LiBH4 and
LiMeBH3. 2 was used without any further purification. 11B NMR
([D8]THF, 298 K, 128.377 MHz): d = À23.6 ppm (t, 1JBH = 72 Hz,
BH2). 1H NMR ([D8]THF, 298 K, 400.132 MHz): d = À0.43 (s, 3H,
BCH3), 0.55 (q, 2H, 1JBH = 72 Hz, BH2), 1.56 (br, 2H, CH2), 1.90 ppm
(s, 3H, SCH3). 13C{1H} NMR ([D8]THF, 298 K, 100.623 MHz): d =
Received: December 18, 2008
Published online: March 12, 2009
1
Keywords: agostic interactions · bifunctional ligands · boron ·
hydrides · ruthenium
3.35 (q, JCB = 44.0 Hz, BCH3), 19.39 (m, 3H, SCH3), 32.73 ppm (q,
.
1JBC = 41.9 Hz, CH2).
5: A toluene (10 mL) solution of 1 (93.7 mg) was added to a
toluene (10 mL) solution of [RuH2(H2)2(PCy3)2] (3; 200.5 mg,
0.300 mmol) and stirred for 48 h at room temperature. After
workup, 5 was isolated as a white solid (54.8 mg, 18%). 31P{1H}
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2
NMR (C6D6, 298 K, 121.50 MHz): d = 65.22 (d, JPP = 15 Hz, Cy3P),
2.58 ppm (t, 2JPP = 15 Hz, Ph2P). 11B{1H} NMR (C7D8, 298 K,
160.52 MHz): d = 35.7 ppm (s). 1H NMR (C7D8, 258 K,
2
500.33 MHz): d = À18.38 (br, 1H, 2JP1,3ÀH2 = 25.10 Hz, JP2H2
=
9.8 Hz, JH1H2 = 8.5 Hz, H2), À11.52 (tdd, 1H, 2JP1,3ÀH1 = 30.9 Hz,
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2JP2H1 = 70.6 Hz, 2JH1H2 = 8.5 Hz, H1), À7.29 (s, br, 1H, BH), 1.18
and 1.38 (d, 2 ꢁ 6H, 3JHH = 6.65 Hz, CH3 iPr), 3.14 (d, 2H, JPH
=
2
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9.70 Hz, CH2P), 3.20 and 4.21 ppm (h, 2 ꢁ 1H, 3JHH = 6.65 Hz, CH
iPr). 13C{1H} NMR (C7D8, 278 K, 125.808 MHz): d = 56.63 (s, CH iPr),
45.12 (s, CH iPr), 33.88 (br, BCH2P), 24.39 (s, CH3 iPr), 21.78 ppm (s,
CH3 iPr).
6: Selected NMR data (C7D8, 298 K): 31P{1H} NMR
(202.547 MHz): d = 67.22 (d, 2JPP = 16.3 Hz, Cy3P), 39.52 ppm (t,
2JPP = 16.3 Hz, Ph2P). 11B{1H} NMR (160.526 MHz): d = 40.2 ppm
1
2
(br). H NMR (400.130 MHz): d = À8.53 (td, 4H, JHP1,2 = 13.91 Hz,
2JHP3 = 13.51 Hz , RuH2(H2)), 0.93 and 1.10 (d, 2 ꢁ 6H, 3JHH = 6.71 Hz,
CH3 iPr), 1.10–2.30 (m, CH2P + Cy), 3.05 and 3.93 (h, 2 ꢁ 1H, 3JHH
=
6.71 Hz, CH iPr), 4.92 ppm (br, BH). T1min (258 K, 500.33 MHz): d =
À8.46 (57 ms).
7: A diethyl ether solution (6 mL) of 2 (70.6 mg, 0.736 mmol) was
added to a diethyl ether solution (8 mL) of [RuH(H2)Cl(PCy3)2] (4;
386.9 mg, 0.552 mmol) and stirred for 15 min at room temperature.
After workup, 7 was isolated as a beige solid in 31% yield. 31P{1H}
NMR (C7D8, 298 K, 202.537 MHz): d = 67.79 (d, 2JP-P = 17.2 Hz,
2
Cy3P1) , 63.90 ppm (d, JP-P = 17.2 Hz, Cy3P2). 11B{1H} NMR (C7D8,
298 K, 160.526 MHz): d = 19.6 ppm (br). 1H NMR (C7D8, 298 K,
500.330 MHz): d = À14.36 (dd, 1H, 2JP-H1 = 25 and 30 Hz, H3), À8.99
(br d, 1H, JP1-H2 = 40.6 Hz, H2), À4.30 (br s, 1H, H1), 0.65 (s, 3H, B
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2
3
CH3), 2.42 (br s, 3H, SCH3), 2.69 (dd, 1H, , JHa-Hb = 13.76 Hz, , JHa-
H3 = 9.11 Hz, Ha), 3.50 ppm (dd, 1H, 2JHb-Ha = 13.76 Hz, JHb-H2
=
3
6.71 Hz, Hb). 13C{1H} NMR (C7D8, 298 K, 125.808 MHz): d = 9.74
(br s, BCH3), 31.29 (s, SCH3), 52.77 ppm (br s, BCH2S). Elemental
analysis (%) calcd for C39H77BP2RuS: C 62.30; H 10.32; found: C
62.26; H 10.41.
CCDC 713433 (5) and CCDC 713434 (7) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Angew. Chem. Int. Ed. 2009, 48, 2964 –2968
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
À
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À
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À
À
ligand, the hydride. The s(B H) to s*(Ru H) 2nd-order
perturbation energy amounts to 288 kJmolÀ1, a value signifi-
À
À
cantly lower than the s(B H) to s*(Ru P) 2nd-order perturba-
tion energy of 442.7 kJmolÀ1 for the donation of the B H bond
À
trans to the phosphine. As a matter of fact, the larger donation
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