Ruthenium agostic (Phosphinoaryl)borane complexes: Multinuclear solid-state and solution NMR, X-ray, and DFT studies

Article abstract of DOI:10.1021/ja203828r

The reactivity of the (o-phosphinophenyl)(amino)borane compound HB(N ⁱPr₂)C₆H₄(o-PPh₂) prepared from Li(C₆H₄)PPh₂ and HBCl(N ⁱPr₂) toward the bis(dihydrogen) complex RuH ₂(H₂)₂(PCy₃)₂ (1) was studied by a combination of DFT, X-ray, and multinuclear NMR techniques including solid-state NMR, a technique rarely employed in organometallic chemistry. The study showed that the complex RuH₂{HB(N ⁱPr₂)C₆H₄(o-PPh₂)} (PCy₃)₂ (3), isolated in excellent yield as yellow crystals and characterized by X-ray diffraction, led in solution to PCy ₃ dissociation and formation of an unsaturated 16-electron complex RuH₂{HB(NⁱPr₂)C₆H ₄(o-PPh₂)}(PCy₃) (4), with a hydride trans to a vacant site. In both cases, the (phosphinoaryl)(amino)borane acts as a bifunctional ligand through the phosphine moiety and a Ru-H-B interaction, thus featuring an agostic interaction.

Full text of DOI:10.1021/ja203828r

ARTICLE
pubs.acs.org/JACS
Ruthenium Agostic (Phosphinoaryl)borane Complexes: Multinuclear
Solid-State and Solution NMR, X-ray, and DFT Studies
Yann Gloaguen,†,‡ Gilles Alcaraz,*,†,‡ Alban S. Petit,§ Eric Clot,*,§ Yannick Coppel,†,‡ Laure Vendier,†,‡ and
Sylviane Sabo-Etienne*,†,‡
†CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, F-31077 Toulouse, France
‡Universitꢀe de Toulouse, UPS, INPT, F-31077 Toulouse, France
§Institut Charles Gerhardt, CNRS 5253, Universitꢀe Montpellier 2, cc 1501, Place Eugꢁene Bataillon, 34095 Montpellier, France
S Supporting Information
b
ABSTRACT: The reactivity of the (o-phosphinophenyl)(amino)borane
compound HB(NiPr2)C6H4(o-PPh2) prepared from Li(C6H4)PPh2 and
HBCl(NiPr2) toward the bis(dihydrogen) complex RuH2(H2)2(PCy3)2
(1) was studied by a combination of DFT, X-ray, and multinuclear NMR
techniques including solid-state NMR, a technique rarely employed in
organometallic chemistry. The study showed that the complex RuH2-
{HB(NiPr2)C6H4(o-PPh2)}(PCy3)2 (3), isolated in excellent yield as
yellow crystals and characterized by X-ray diffraction, led in solution to
PCy3 dissociation and formation of an unsaturated 16-electron complex RuH2{HB(NiPr2)C6H4(o-PPh2)}(PCy3) (4), with a
hydride trans to a vacant site. In both cases, the (phosphinoaryl)(amino)borane acts as a bifunctional ligand through the phosphine
moiety and a RuꢀHꢀB interaction, thus featuring an agostic interaction.
’ INTRODUCTION
The ability to accurately describe the bonding nature of
ligands coordinated to a metal center has been facilitated by
the increasingly sophisticated techniques available to chemists. In
recent years, X-ray diffraction and DFT data in combination with
multinuclear NMR studies enhanced understanding of a mol-
ecule’s structure in the solid state and in solution. In the specific
Figure 1. Various MꢀHꢀB coordination modes.
By comparison to the dihydrogen family, examples of “true” σ-
borane complexes are still limited,2 an occurrence that might be
accounted for by the more challenging synthetic procedures that
are required and by the tendency of dihydroborate ligation,
especially when working with hydride systems.7 Very recently, by
adding monosubstituted boranes of general formula H2BR to the
bis(dihydrogen) complex RuH2(η2-H2)2(PCy3)2 (1), we have
discovered a new type of bonding in which two geminal σ-BꢀH
bonds are coordinated to the metal center.8 A series of complexes
of general formula RuH2(η2:η2-H2BR)(PCy3)2 was obtained
with R = alkyl, aryl, amino.9 The stabilization through two
geminal BꢀH bonds allowed us to isolate the complex resulting
from the first elementary step of dehydrogenation of ammonia
borane, thus trapping the elusive aminoborane H2NꢀBH2.9b Pre-
serving the tricoordinated mode at boron as a prerequisite,10 we also
launched a research program aimed at designing new bifunctional
ligands to monitor the impact of the additional functional groups on
the BꢀH activation mode.11 Our first attempt allowed us to prepare
the (diphenylphosphinomethyl)(diisopropylamino)borane, which
field of σ-complexes in which a σ bond of a ligand is coordinated
to the metal center in a 3-center-2-electron mode, spectacular
achievements have been made since the first report from Kubas
et al. in 1984 of the coordination of dihydrogen to a metal center,
without HꢀH bond breaking, leading to the unambiguous
characterization of a σ-dihydrogen complex.1,2 Seminal work
was also conducted by Brookhart and Green in the early 1980s
with the grounding of the concept of an agostic bond, that is to
say, a ligand attached to a metal center via a 3-center-2-electron
MꢀHꢀC interaction mode and a linker, thus making a distinct
contrast with the corresponding σ-alkane complexes in which the
alkane is bonded to the metal through the σ-CꢀH bond alone.3
The recent example of the methane complex by Brookhart et al. is
the prototypical example of a “true” σ-alkane complex.4
As regards boron chemistry, the use of tricoordinated borane
compounds was a fruitful strategy to prepare σ-borane com-
plexes as exemplified by the pioneering work of Hartwig et al. in
1996.5 In this class of compounds, the HBR2 borane is coordi-
nated to the metal center via a σ-BꢀH bond. This class of
compound is given the title “true” σ-borane to discriminate it
from the Shimoi-type phosphine or amineꢀborane adducts
(Figure 1).6
Received: May 5, 2011
Published: September 09, 2011
r
2011 American Chemical Society
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Scheme 1. Reaction of 1 with HB(NiPr2)C6H4(o-PPh2)
Figure 2. 31P{1H}NMR spectrum in C7D8 solution (202.537 MHz) of
the crude reaction mixture at 298 K.
upon reaction with the bis(dihydrogen) complex 1 led to the
isolation of the γ-BꢀH agostic complex RuH2{HB(NiPr2)CH2-
PPh2}(PCy3)2 (2) resulting from the substitution of the two
dihydrogen ligands and coordination of the borane compound
through both phosphorus ligation and RuꢀHꢀB interaction.12 At
this point, we want to emphasize our awareness that the term
“agostic” should normally be reserved for a 3-center-2-electron
MꢀHꢀC bond.3a The MꢀHꢀB interactions generally reported in
the literature are common examples of 3-center-2-electron
bonds, but involve a tetrahedral Bsp3 atom, and are not encom-
passed by the term “agostic”. However, in our systems as well as
in the chromium complex reported by Braunschweig et al.,13 in
which the boron is kept trigonal, we believe that such a term
better defines the interactions than the term “bridging hydrogen”
currently employed for MꢀHꢀB bonds. We now describe our
results concerning the synthesis and coordination at ruthenium,
of a new (phosphinoaryl)borane compound HꢀB(NiPr2)C6H4-
(o-PPh2). Our study shows not only the tremendous influence
the linker can have on the coordination, but also the benefits one
can gain by combining solid-state and solution NMR techniques,
at a moment when solid-state NMR is very much underutilized in
organometallic chemistry.14 Our detailed analysis by multinuc-
lear solution and solid-state NMR, X-ray, and DFT data allows us
to evidence a dynamic process and better define the nature of the
“agostic” BꢀH bond.
Figure 3. 31P{1H}NMR spectrum in C7D8 solution (202.537 MHz) of
the crude reaction mixture at 198 K in the 40ꢀ80 ppm region.
parameters (see below).15 Indeed, the hydrogen at boron could be
located with a BꢀH distance of 1.076(10) Å, an interesting
parameter to monitor the BꢀH bond activation upon coordination.
By analogy with the reaction leading to the synthesis of RuH2-
{HꢀB(NiPr2)CH2PPh2}(PCy3)2 (2),12 we postulated that addi-
tion of 1 equiv of the new (phosphinoaryl)borane compound to
the bis(dihydrogen) complex RuH2(η2-H2)2(PCy3)2 (1) should
produce the new complex RuH2{HꢀB(NiPr2)C6H4(o-PPh2)}-
(PCy3)2 (3), corresponding to the substitution of the two dihydro-
gen ligands in 1 by the (phosphinoaryl)borane. However, we were
quite surprised that NMR monitoring of the crude mixture led to
clean spectra but not spectra accordant with the postulated species 3.
Indeed, as shown in Figure 2, the 31P{1H} NMR spectrum in C7D8
at room temperature shows two species: an AB pattern at δ 72.1 and
’ RESULTS AND DISCUSSION
Synthesis and Properties of the (Phosphinophenyl)(amino)-
borane Complexes 3 and 4. The (phosphinophenyl)(amino)-
borane compound HꢀB(NiPr2)C6H4(o-PPh2) was isolated in good
yield by addition of (o-PPh2)C6H4Li to HBCl(NiPr2). It was fully
characterized by multinuclear NMR and X-ray diffraction. The most
important parameters are the singlet at ꢀ10.91 ppm and the broad
signal at 38.4 ppm in the 31P{1H} and 11B{1H} NMR spectra, re-
spectively. The X-ray diffraction structure determined at 110 K
excludes any interaction between the boron and the phosphorus with
2
δ 64.3 with a JPP coupling constant of 218 Hz, indicative of two
different phosphorus in trans position, together with a singlet at δ 9.8
consistent with the formation of free PCy3 in the mixture.
The 1H NMR spectrum shows three signals of equal intensity
in the hydride region. At 218 K, the two more shielded signals
appear as a doublet of doublet of doublets at δ ꢀ25.69 and
δ ꢀ12.67, whereas a broad signal is still observed at δ ꢀ6.41, but
which sharpens upon 11B decoupling. Selective irradiation led to
complete assignment of the signals, and particularly a double
selective irradiation 1H{11B}bb{31P}64.9 allows the measurement
of a 2JPPh2BH coupling constant of 5 Hz between the two ends of
the (phosphinoaryl)borane ligand (P and H3, see Scheme 1).
a P B distance higher than 3 Å (see Supporting Information,
3 3 3
Figure S1). Location of hydrogen atoms by X-ray diffraction should
be taken cautiously, but a great deal of progress has been made over
the past decade, and in combination with DFT calculations, one can
use the values involving the located hydrogens as, at least, additional
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The 11B{1H} NMR spectrum shows one broad resonance at δ 55
shifted to low field by comparison to the free (phosphinoaryl)-
borane (Δδ = 17). These data are in agreement with the struc-
ture proposed for complex 4 in Scheme 1, and corresponding
to an unsaturated 16-electron species RuH2{HꢀB(NiPr2)C6H4-
(o-PPh2)}(PCy3).
Upon lowering the temperature below 238 K, the appearance
of a broad ABM pattern in the 31P{1H} NMR spectrum was
identified and is shown in Figure 3: AB resonances at 50.5 and
66.1 ppm with a large JPP value of 233 Hz, and M resonance at
59.7 ppm (vide infra).
We monitored several crude reactions, and a postulated
dynamic process consisting of phosphine dissociation from 3
and formation of 4 (vide infra) was consistently reproduced. By
adding 10 equiv of PCy3 to the reaction mixture, the postulated
equilibrium could not be displaced, and the spectrum displayed
in Figure 2 was observed yet again. In contrast, by using a very
large excess of PCy3, we could not detect any signal correspond-
ing to an organometallic species, probably as a result of a
decomposition pathway. However, when using ca. 40 equiv of
PCy3, NMR monitoring in the presence of an external reference
showed at 195 K the decrease of the AB signal assigned to 4.
Complex 4 is the only organometallic species detected in
solution at room temperature. We do need to lower the
temperature bellow 238 K to start to detect 3 in solution as
depicted in Figure 3 (spectrum run at 198 K, see also Figure
S3). It was difficult to properly integrate the signals corre-
sponding to 3 and 4, but an equilibrium constant Keq of ca. 3.7
could be estimated at 195 K, leading to a ΔrG° value close to zero
(ca. ꢀ2 kJ molꢀ1 at 195 K; see Supporting Information Figure
S2). One can see from the 31P{1H} NMR variable temperature
spectra (Supporting Information Figure S3) that when 3 could
be detected, the two PCy3 ligands of 3 appeared non equivalent
and only one exchanged, with no average between free PCy3
and 4.
At 238 K, we were able to grow a few crystals, and the X-ray
structure was determined at 110 K. The structure depicted in
Figure 4 corresponds to the complex RuH2{HꢀB(NiPr2)C6H4-
(o-PPh2)}(PCy3)2 (3), with ruthenium in a pseudo octahedral
environment and two PCy3 in trans positions, bent away from
the (phosphinoaryl)borane ligand.
As can be seen from Table 1, the main geometrical parameters
are essentially the same as those previously reported for
complex 2 with the methylene spacer in place of the phenyl.12
The main difference is a shorter RuꢀB distance in 3 (2.503(3) Å).
Comments on the parameters involving the hydrogen atoms will
be found in the Computational Studies.
Figure 4. X-ray structure of 3. Ellipsoids are shown at the 30% level.
Hydrogens, except the hydrides, have been omitted for clarity.
Table 1. Selected Geometrical Parameters for the Experi-
mental and Calculated (DFT/B3PW91) Structures of 3
exp
calcd
RuꢀP1
RuꢀP2
RuꢀP3
2.3350(6)
2.3674(6)
2.3265(6)
2.503(3)
1.419(3)
1.600(4)
1.22(3)
2.369
2.412
2.375
2.551
1.429
1.598
1.275
1.800
Ru
B
3 3 3
BꢀN
BꢀC8
BꢀHy3
Ru Hy3
1.79(3)
3 3 3
P1ꢀRuꢀP2
P1ꢀRuꢀP3
P2ꢀRuꢀP3
P1ꢀRuꢀB
102.95(2)
107.91(2)
144.37(2)
74.03(6)
103.1
106.8
145.6
74.2
at δ 57, thus in the same region as the signal observed for 4. To
validate our simulation, we treated free (phosphinoaryl)borane
in a similar way (Figure 6) and obtained an identical value both in
solution and in solid states (δ 38).
Upon dissolving the crystals of 3, we observed the same
31P{1H} NMR spectrum featured in Figure 2. Consequently, it
was thought pertinent to run a solid-state NMR spectrum of the
orange powder obtained after addition of the (phosphinoaryl)-
borane to 1 and immediate evaporation to dryness. The solid-
state 31P{1H} NMR spectrum, recorded at room temperature
and shown in Figure 5, displays exactly the same ABM pattern
detected in solution at low temperature as shown in Figure 3
(AB resonances at 48.5 and 65.5 ppm with a large JPP value of
233 Hz, and M resonance at 57.9 ppm). This is in agreement with
a complex with two PCy3 ligands (AB resonances) and one
phosphinoarylborane ligand (M resonance).
Remarkably, when C7D8 is added to the same sample in the
same rotor and the 31P{1H} and 11B{1H} NMR spectra are run
at room temperature in the same solid-state NMR AVANCE
WB machine, the data indicate the formation of 4 as the sole
ruthenium complex. Blank experiments with free PCy3 were also
performed to monitor the change of chemical shift in solution
(δ 9.69) versus solid state (δ 7.6). Finally, the variable-tempera-
ture 1H solution NMR spectra below 258 K showed, in addition
to 4, a new set of signals in agreement with the formulation of
3: at 238 K, the hydride trans to the BH bond resonates as a
pseudo doublet of triplets at δ ꢀ14.99 with 2JPH values of 15 and
20 Hz, consistent with its cis position to the three phosphorus
The 11B{1H} solid-state NMR spectrum shows one complex
signal depicted in Figure 6. Treatment of the signal and simula-
tion with the program Dmfit allowed us to extract a chemical shift
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Figure 5. 1Hꢀ P CP/MAS NMR spectrum in the solid state (161.768
31
MHz) of 3 at 298 K in the 40ꢀ80 ppm region.
2
entities; the hydride at δ ꢀ9.53 displays a large JPH value of
70 Hz, consistent with its trans position to the PPh2 group; and
the hydrogen connected to the boron resonates as a broad singlet
at δ ꢀ8.11, the only signal sharpening upon 11B decoupling.
Separation of 4 from free PCy3 proved difficult. However, upon
dissolution of 3 in the presence of 1 equiv of BH3 THF and
3
immediate evaporation to dryness, BH3 PCy3 was formed,
3
allowing, after workup, the isolation of 4 as a brown powder
contaminated by less than 10% of the tricyclohexylphosphineꢀ
borane adduct. Multinuclear NMR spectra of the powder, both in
solution and in solid state, were in perfect agreement with the
data described above.
Figure 6. MAS 11B{1H} NMR spectra in the solid state (128.212 MHz)
at 298 K. Top: The free (phosphinoaryl)borane HB(NiPr2)C6H4-
(o-PPh2). Bottom: RuH2{HB(NiPr2)C6H4(o-PPh2)}(PCy3)2 (3).
Computational Studies. Density functional theory (DFT)
calculations have been carried out to shed more light on the
nature of the BꢀH agostic interaction in 3 and 4, and on the
energetic of the transformation 3 f 4 + PCy3. The actual
experimental systems 3, 4, and PCy3 have been computed with
the hybrid functional B3PW91. The calculated structure for 3 is
in good agreement with the experimental data (Table 1). Albeit
calculated as being longer than the experimental value (2.551 vs
Table 2. Composition of the NLMO σnlmo(BꢀH) and
LPnlmo(Ru) Resulting from the Delocalization of the Parent
NBO (σ(BꢀH) and LP(Ru)) into the Accepting NBO σ*-
(RuꢀH) and LP*(B), Respectively
σnlmo(BH)
LPnlmo(Ru)
2
3
4
0.9586 σ(BH) + 0.2391 σ*(RuH) 0.9569 LP(Ru) + 0.1017 LP*(B)
0.9407 σ(BH) + 0.2901 σ*(RuH) 0.9570 LP(Ru) + 0.1553 LP*(B)
0.9297 σ(BH) + 0.3226 σ*(RuH) 0.9356 LP(Ru) + 0.2385 LP*(B)
2.503(3) Å), the Ru B distance in 3 clearly indicates a
3 3 3
stronger interaction of BꢀH with Ru as compared to that
observed in 2 (2.7574(80) Å).12 The calculated structure for 2
is in good agreement with experiment, and the Ru B distance
significant extent, leading to a geometry of interaction similar
to that observed in 3, but in contrast to that observed in 2.
However, in the present systems, the steric strain introduced by
the tether in the phosphino∼borane ligand strongly influences
the magnitude of the σ-donation. The population of the σ(BH)
NBO is higher in 2 as compared to 3 (1.844 and 1.792 electron,
respectively) as a result of less efficient σ-donation in the former.
In addition, the more flexible nature of the hemilabile ligand in 3
allows for a better back-donation from Ru into the formally
vacant p atomic orbital (AO) on boron. This electron transfer
from Ru to B competes with π-donation from the nitrogen lone
pair. The significantly longer BꢀN distance in 3 as compared to 2
(1.429 and 1.409 Å, respectively) indicates that π-donation from
N is less efficient in the former as a result of increased back-
donation from Ru. The latter interaction specifically shortens the
3 3 3
computed at 2.783 Å is consistent with a weaker BꢀH agostic
interaction. This trend is further confirmed by the bond distances
involving the hydrogen atom bonded to boron. The BꢀH bond
is calculated as being longer in 3 than in 2 (1.275 and 1.244 Å,
respectively); on the other hand, the Ru H distance is
3 3 3
calculated as being shorter in 3 than in 2 (1.800 and 1.891 Å,
respectively). In both complexes, the BꢀH bond lies in the
equatorial plane, but in the case of 2, the BꢀH bond interacts
mainly through the hydrogen atom, whereas in 3, both atoms, H
and B, participate to the agostic interaction. The BꢀH bond in 2
and 3 is elongated with respect to the calculated value in the free
ligand (1.203 Å, CH2; 1.202 Å, phenyl) essentially as a result of σ-
donation from σ(BꢀH) to Ru. A Natural Bond Orbital (NBO)
analysis16 indicated that, in both complexes, the σ(BH) bond is
more developed on the hydrogen atom (56.9%, 2; 55.5%, 3), and,
therefore, the σ-donation induces a larger participation by the
hydrogen atom. In the case of an agostic interaction with a less
polar CꢀH bond, the interaction involves both atoms to a
Ru B distance. Overall, both σ-donation from σ(BH) into
3 3 3
σ*(RuH) and back-donation from a nonbonding d orbital on Ru
(LP(Ru)) into the vacant AO on B (LP*(B)) are stronger in 3 as
a result of the more flexible nature of the ligand. This is nicely
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Figure 7. Optimized geometries for the 16-electron complexes 3-P, 4, 40, and 400 and their relative energies (kcal molꢀ1). Most H atoms omitted for
clarity.
illustrated by the composition of the Natural Localized Molec-
ular Orbitals (NLMO) corresponding to σ(BH) and LP(Ru)
(Table 2).
the ruthenium lone pair involved in the back-donation (Table 2).
In addition, one methyl group of the NiPr2 group occupies the
vacant site trans to the hydride, creating a very weak agostic
To probe the energy landscape associated with the coordina-
tion of HꢀB(NiPr2)C6H4(o-PPh2) to the RuH2(PCy3)2 fragment,
alternate isomerswere optimized (SupportingInformation). There
is a clear energetic preference for the configuration observed
experimentally for 3 with apical PCy3 and the hydrides and the
hemilabile ligand in the equatorial plane. In the solid-state struc-
ture of 3, the RuꢀP bond distances for the PCy3 ligands are
not equivalent (2.3265(6) and 2.3674(6) Å). To enhance the
accepting capacity of the boron atom with respect to Ru, while
keeping efficient donation from N, the phenyl tether expands
toward one PCy3 group, and the corresponding RuꢀP bond is
longer. This asymmetry is reproduced in the calculated structure
for 3 (2.375 and 2.412 Å). The PCy3 ligand “cis” to the phenyl
tether is thus expected to be more labile. Optimization of a 16-
electron complex, 3-P, with a starting geometry similar to that of
3 deprived of the labile PCy3 group yielded a local minimum
interaction (RuꢀC = 2.734 Å and Ru H = 2.111 Å).3a,17 BꢀH
3 3 3
activation in 4 would result in dihydrogen hydridoꢀboryl
complexes with either H (40) or B (400) trans to the vacant site
(Figure 7). These two isomers have been computed and are less
stable than 4 by 13.7 and 18.6 kcal molꢀ1, respectively. By
optimizing the relative positions of the various ligands in terms of
stereoelectronic properties, the 16-electron complex 4 is com-
puted to be more stable than 3 by ꢀ2.3 kcal molꢀ1. Correction
for basis set superposition effect (BSSE) has been taken into
account using the counterpoise correction of Boys and
Bernardi.18 The BSSE-corrected relative energy between 3 and
4 is ꢀ6.9 kcal molꢀ1 in favor of 4. So, even without any entropy
correction associated with phosphine dissociation, the 16-electron
complex 4 is more stable than the 18-electron complex 3.
Not surprisingly for a dissociation process, the Gibbs free energy
correction in the gas phase for the transformation 3 f 4 + PCy3
amounts to ꢀ21 kcal molꢀ1. There is no doubt that this value is
overestimated by comparison to the NMR experimental determi-
nation. Following Ziegler,19 when only ca. 50% of this correction is
considered in solution, the Gibbs free energy difference between 3
and 4 is ca. ꢀ15 kcal molꢀ1 in favor of 4. As complex 4 is already
more stable than 3 electronically, entropy effects can only con-
tribute to enhance its stability. This explains why the 16-electron
complex 4 is the only compound observed at room temperature,
and also why large excess of PCy3 is necessary at low T to observe 3.
featuring the phenyl tether weakly coordinated to Ru (Ru C =
3 3 3
2.357 and 2.468 Å, see Figure 7).
This interaction introduces various geometrical modifications.
The BꢀH bond is now perpendicular to the equatorial plane and
is significantly more activated than in the 18-electron complex 3
(BꢀH = 1.298 Å, Ru H = 1.752 Å, Ru B = 2.237 Å). In
3 3 3
3 3 3
addition, the PCy3ꢀRuꢀPPh2 angle has increased from 106.8°
in 3 to 118.7° in 3-P. The stronger BꢀH agostic interaction, the
weak phenyl coordination, and the diminished trans influence of
the hydride trans to PPh2 are all stabilizing factors compensating
for loss of RuꢀPCy3 bonding. Consequently, 3-P + PCy3 is only
6.3 kcal molꢀ1 less stable than 3. Further increase of the
PꢀRuꢀP angle leads to a 16-electron complex more stable than
3-P by 8.6 kcal molꢀ1, with a geometry in agreement with the
experimental observations of 4 (Figure 7). The calculated
complex 4 features trans phosphines, a hydride trans to a vacant
site, and an agostic BꢀH bond trans to the remaining hydride.
The BꢀH bond is significantly activated (BꢀH = 1.313 Å,
Ru H = 1.771 Å, Ru B = 2.256 Å).
’ CONCLUSIONS
By preparing a new phosphinoborane compound with a
different spacer between the phosphine and the borane groups,
phenyl versus methylene, we have demonstrated that the very
recent examples of agostic borane complexes are more than mere
curiosity, as it is possible to tune the BꢀH bond activation. In the
case of the methylene spacer, the 18-electron complex 2 is stable
in solution, whereas for the phenyl spacer, complex 3 undergoes a
dissociative process, leading to free PCy3 and formation of the
corresponding 16-electron species 4. Combination of various
techniques is a prerequisite if one wants to better define the
bonding nature in particular in complex systems. Here, we
combine the widely routinized techniques of solution multi-
nuclear NMR/DFT/X-ray with the more original tool of solid-
state NMR. Together, the methods have enhanced understand-
ing of the dissociative process. Computational studies have been
very efficient for improving the definition of the BꢀH activation
3 3 3
3 3 3
The NBO analysis of the electronic structure of 4 indicated
that the σ(BH) bond is less populated than in the 18-electron
complex 3 (1.792 vs 1.742 electrons) as a result of stronger σ-
donation in 4. This is further confirmed by the expression of the
NLMO for σ(BH) (Table 2) where the contribution of the
accepting σ*(RuH) orbital has increased from 3 to 4. Addition-
ally, the stronger agostic interaction in 4 results from stronger
back-donation from Ru to boron as illustrated by the lengthening
of the BꢀN bond (1.433 Å) and the expression for the NLMO of
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level and to complement the structural data. We hope that this
approach will encourage a large community to design strategies
involving both solution and solid-state NMR techniques for
improved understanding at a molecular level.
RuH2{HꢀB(NiPr2)C6H4(o-PPh2)}(PCy3)2 (3). A toluene (2 mL)
solution of HꢀB(NiPr2)C6H4(o-PPh2) (400 mg, 1.072 mmol) was
added to a toluene suspension (4 mL) of RuH2(η2-H2)2(PCy3)2 (1)
(715.9 mg, 1.072 mmol) and stirred at room temperature for 1 min.
After removal of the solvent, some pentane was added and then
evaporated. This operation was repeated five times successively. After
being dried under vacuum, a yellow orange powder was isolated in 95%
yield. Suitable crystals of 3 were obtained after solubilization in a
minimum amount of pentane and standing at ꢀ35 °C. Data for 3:
31P{1H} NMR (Tol-d8, 198 K, 202.54 MHz) δ 50.5 (d, 2JPP = 233 Hz,
PCy3), 59.7 (br, Ph2P), 66.1 (d, 2JPP = 233 Hz, PCy3). 31P{1H} NMR
(solid state, 298 K, 202.54 MHz) δ 48.7 (d, 2JPP = 225 Hz, PCy3), 57.9
(br, Ph2P), 65.7 (d, 2JPP = 223 Hz, PCy3). 11B{1H} (solid state, 298 K,
MHz) δ 51 (after simulation). Anal. Calcd for C60H97BNP3Ru: C,
69.48; H, 9.43; N, 1.35. Found: C, 69.66; H, 9.76; N, 1.11.
’ EXPERIMENTAL SECTION
All experiments were performed under a dry argon atmosphere using
either Schlenck tube or glovebox techniques. Diethyl ether, THF,
pentane, and toluene were obtained either from a solvent purification
system MBraun SPS-800 Series or from distillation techniques under
argon (refluxing over Na/benzophenone for diethylether and THF, over
Na for toluene, and over CaH2 for pentane). Deuterated NMR solvents
were dried over molecular sieves (and over Na for THF-d8), degassed by
freezeꢀpumpꢀthaw cycles, and stored under argon. The compounds
(o-PPh2)C6H4Li20 and RuH2(η2-H2)2(PCy3)2 (1)21 were prepared in
accordance with published methods. NMR samples of sensitive com-
pounds were all prepared in the glovebox, using NMR tubes fitted with
Teflon septa. NMR spectra were obtained on Bruker AV 400 (1H 400.13
MHz, 13C 100.62 MHz, 11B 128.38 MHz, 31P 161.98 MHz) or AV 500
(1H 500.33 MHz, 31P 202.55 MHz, 11B 160.53 MHz) spectrometers.
Linear predictions of the FID from 11B{1H} spectra were made to extract
the boron chemical shifts where necessary. Solid-state NMR experiments
were recorded on a Bruker Avance 400 spectrometer equipped with a 4
mm probe. All MAS experiments were performed atambient temperature.
No corrections were made with respect to sample heating under MAS
conditions. For 11B MAS single pulse experiments, small flip angles of 1 μs
for selective excitation of the central transition were used with recycle
delay of 10 s. 31P-CP/MAS spectra were recorded with a recycle delay of 5
s and a contact time of 3 ms. All of the 11B and 31P NMR spectra were
recorded under high-power proton decoupling conditions. 11B spectrum
simulations were carried out using Dmfit program with the “Q mas 1/2”
RuH2{HꢀB(NiPr2)C6H4(o-PPh2)}(PCy3) (4). 48.2 μL of a 1 M
solution of BH3 THF in THF (0.048 mmol) was added via syringe on a
3
benzene solution of 3 (50 mg in 2 mL, 0.0482 mmol). All volatiles were
immediately evaporated after addition and coevaporated with pentane.
The crude was then dissolved in a minimum of pentane and kept at
ꢀ35 °C overnight. A white precipitate could be separated, correspond-
ing to the PCy3 BH3 adduct. Solvent from the filtrate was then
3
evaporated, and a brown powder was dried, containing mostly 4
contaminated with traces of the PCy3 BH3 adduct. Data for 4: 31P{1H}
3
NMR (Tol-d8, 298 K, 202.54 MHz) δ 72.1 (d, 2JPP = 218 Hz, Ph2P),
64.3 (d, 2JPP = 218 Hz, PCy3). 11B{1H} NMR (Tol-d8, 298 K, 160.53
MHz) δ 55 (br). 1H NMR (Tol-d8, 218 K, 500.33 MHz) δ ꢀ25.69 (br,
1H, H2), ꢀ12.67 (ddd, 1H, 2JP1H = 22.5 Hz, 2JP2H = 20.0 Hz, JH1H2
=
i
5.0 Hz, H1), ꢀ6.41 (br, 1H, JP2H =5.0Hz, H3), ꢀ0.4 (br, 2 ꢁ 6H, CH3 Pr),
0.90ꢀ2.30 (m, 33H, Cy), 3.55 (br, 2 ꢁ 1H, CH iPr), 6.9ꢀ7.5 (m, 10H,
Ph), 7.66 (t, 2H, 3JP1H = 10.0 Hz, Ph), 7.86 (t, 2H, 3JP1H = 10.0 Hz, Ph).
T1 min (C7D8, 500.33 MHz) δ ꢀ25.69 (298 K, 330 ms), ꢀ12.67 (313 K,
352 ms), ꢀ6.41 (258 K, 138 ms). 13C{1H} NMR (C6d6, 298 K, 100.62
MHz) δ 21.0 (br), 23.0 (br), 24.5 (s), 24.9 (s), 26.8 (s), 27.0 (s), 27.3
(s), 27.9 (s), 28.1 (dd), 37.8 (s), 38.0 (s), 48.2 (br), 51.9 (s), 125.7 (d,
JPC = 5.0 Hz, CHar), 127.2 (d, JPC = 9.1 Hz, CHar), 128.0 (pt, JPC = 7.0
Hz, CHar), 128.3 (d, JPC = 6.0 Hz, CHar), 129.1 (s, CHar), 129.3 (s,
CHar), 129.4 (s, CHar), 129.7 (s, CHar), 133.1 (d, JPC = 10.1 Hz, CHar),
134.3 (d, JPC = 11.1 Hz, CHar), 134.9 (d, JPC = 12.1 Hz, CHar), 138.9
model. All chemical shifts for 1H and 13C are relative to TMS. 31P and 11
B
chemical shifts were referenced to external 85% H3PO4 and BF3 Et2O
3
samples, respectively. Mass experiments were performed by the Mass
Service of the University of Toulouse (Universitꢀe Paul Sabatier). DCI
technique (with CH4) was used on a TSQ 7000 Thermo Electron
apparatus. Elemental analyses were performed by the “in-house” service
of the LCC, Toulouse. X-ray structures were recorded on a Bruker Kappa
Apex II diffractometer using a graphite-monochromated Mo Kα radiation
(λ = 0.71073 Å) and equipped with an Oxford Cryosystems Cryostream
Cooler Device. Crystal data were collected at 110 K. All of the calculations
have been performed with the Gaussian 09 package, at the B3PW91 level.
See the Supporting Information for more details.
(d, JPC = 24.2 Hz, CHar), 143.5 (d, JPC = 55.3 Hz, CHar), 146.2 (d, JPC
=
46.3 Hz, CHar), 158.8 (br, CIVB).
’ ASSOCIATED CONTENT
HꢀB(NiPr2)C6H4(o-PPh2). (o-PPh2)C6H4Li (0.512 g, 1.908 mmol)
was added to an ethereal solution (20 mL) of HBCl(NiPr2) (0.360 g,
2.442 mmol) at ꢀ78 °C. The suspension was stirred for 30 min at this
temperature and for 2 h at room temperature. Volatiles and solvent were
then evaporated, and the crude mixture was dissolved in toluene. After
filtration over Celite and removal of the solvent, a gum was washed with
cooled pentane (ꢀ35 °C, 3 ꢁ 3 mL). HꢀB(NiPr2)C6H4(o-PPh2) was
dried under vacuum and isolated as a white powder (78% yield). Data:
31P{1H} NMR (C6D6, 298 K, 161.99 MHz) δ ꢀ10.91. 11B{1H} NMR
(C6D6, 298 K, 128.38 MHz) δ 38 (br). 1H NMR (C6D6, 298 K, 400.13
MHz) δ 0.82 and 1.38 (d, 2 ꢁ 6H, 3JHH = 6.4 Hz, CH3 iPr), 3.12 and
3.87 (h, 2 ꢁ 1H, 3JHH = 6.4 Hz, CH iPr), 5.95 (br, BH), 7.10ꢀ7.55 (m,
14H, CHar). 13C{1H} NMR (C6D6, 298 K, 100.62 MHz) δ 21.5 and
S
Supporting Information. NMR data; computational de-
b
tails, Cartesian coordinates, and energy of the optimized struc-
tures; and X-ray data. The two crystal structures have been
deposited at the Cambridge Crystallographic Data Centre and
allocated the deposition numbers CCDC 817345 and 817346.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
gilles.alcaraz@lcc-toulouse.fr; clot@univ-montp2.fr; sylviane.sabo@
lcc-toulouse.fr
26.8 (s, CH3 iPr), 44.5 and 50.2 (s, CH Pr), 127.4 (s, 1CHar), 128.0
i
(d, JPC = 11.2 Hz, 1CHar), 128.3 (s, 2CHar), 128.5 (d, JPC = 6.4 Hz,
2CHar), 130.2 (d, JPC = 14.6 Hz, 1CHar), 132.8 (s, 1CHar), 134.6 (d, JPC
= 19.4 Hz, 2CHar), 138.4 (d, JPC = 12.8 Hz, 2CIVPh), 140.6 (d, JPC = 7.2
Hz, 1CIVPhB), 150.6 (br, 1CIVB). Anal. Calcd for C24H29BNP: C,
77.22; H, 7.83; N, 3.75. Found: C, 77.02; H, 7.82; N, 3.60. m/z = 373.28
(th), 374.22 (exp).
’ ACKNOWLEDGMENT
This work was supported by the CNRS and the ANR
(Programme HyBoCat, ANR-09-BLAN-0184).
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’ REFERENCES
(1) Kubas, G. J. Chem. Rev. 2007, 107, 4152–4205.
(2) Alcaraz, G.; Sabo-Etienne, S. Coord. Chem. Rev. 2008, 252,
2395–2409.
(3) (a) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad.
Sci. U.S.A. 2007, 104, 6908–6914. (b) Lein, M. Coord. Chem. Rev. 2009,
253, 625–634.
(4) Bernskoetter, W. H.; Schauer, C. K.; Goldberg, K. I.; Brookhart,
M. Science 2009, 326, 553–556.
(5) Hartwig, J. F.; Muhoro, C. N.; He, X.; Eisenstein, O.; Bosque, R.;
Maseras, F. J. Am. Chem. Soc. 1996, 118, 10936–10937.
(6) Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.; Katoh, K.;
Uruichi, M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704–11712.
(7) Lachaize, S.; Essalah, K.; Montiel-Palma, V.; Vendier, L.; Chaudret,
B.; Barthelat, J. C.; Sabo-Etienne, S. Organometallics 2005, 24, 2935–2943.
(8) Alcaraz, G.; Clot, E.; Helmstedt, U.; Vendier, L.; Sabo-Etienne, S.
J. Am. Chem. Soc. 2007, 129, 8704–8705.
(9) (a) Gloaguen, Y.; Alcaraz, G.; Vendier, L.; Sabo-Etienne, S.
J. Organomet. Chem. 2009, 694, 2839–2841. (b) Alcaraz, G.; Vendier, L.;
Clot, E.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2010, 49, 918–920.
(c) Alcaraz, G.; Chaplin, A. B.; Stevens, C. J.; Clot, E.; Vendier, L.; Weller,
A. S.; Sabo-Etienne, S. Organometallics 2010, 29, 5591–5595.
(10) Alcaraz, G.; Grellier, M.; Sabo-Etienne, S. Acc. Chem. Res. 2009,
42, 1640–1649.
(11) Crabtree, R. H. New J. Chem. 2011, 35, 18–23.
(12) Gloaguen, Y.; Alcaraz, G.; Pꢀecharman, A.-F.; Clot, E.; Vendier,
L.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2009, 48, 2964–2968.
(13) Braunschweig, H.; Dewhurst, R. D.; Herbst, T.; Radacki, K.
Angew. Chem., Int. Ed. 2008, 47, 5978–5980.
(14) (a) Duer, M. J. Solid-State NMR Spectroscopy Principles and
Applications; Blackwell Science Ltd.: Cambridge, MA, 2002. (b) Rubio,
ꢀ
ꢁ
M.; Suꢀarez, A. s.; del Rio, D.; Galindo, A. n.; Alvarez, E.; Pizzano, A.
Organometallics 2008, 28, 547–560. (c) Walaszek, B.; Adamczyk, A.;
Pery, T.; Yeping, X.; Gutmann, T.; Amadeu, N. d. S.; Ulrich, S.; Breitzke,
H.; Vieth, H. M.; Sabo-Etienne, S.; Chaudret, B.; Limbach, H.-H.;
Buntkowsky, G. J. Am. Chem. Soc. 2008, 130, 17502–17508. (d) Chierotti,
M. R.; Gobetto, R. Eur. J. Inorg. Chem. 2009, 2009, 2581–2597. (e) Thibault,
M.-H.; Lucier, B. E. G.; Schurko, R. W.; Fontaine, F.-G. Dalton Trans.
2009, 7701–7716. (f) Rossini, A. J.; Mills, R. W.; Briscoe, G. A.; Norton,
E. L.; Geier, S. J.; Hung, I.; Zheng, S.; Autschbach, J.; Schurko, R. W. J. Am.
Chem. Soc. 2009, 131, 3317–3330. (g) Lucier, B. E. G.; Tang, J. A.; Schurko,
R. W.; Bowmaker, G. A.; Healy, P. C.; Hanna, J. V. J. Phys. Chem. C 2010,
114, 7949–7962.
(15) Balcells, D.; Clot, E.; Eisenstein, O. Chem. Rev. 2010, 110, 749–
823.
(16) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural
Bond Orbital Donor-Acceptor Perspective; Cambridge University Press:
Cambridge, U.K., 2005.
(17) Toner, A.; Matthes, J.; Gr€undemann, S.; Limbach, H.-H.;
Chaudret, B.; Clot, E.; Sabo-Etienne, S. Proc. Natl. Acad. Sci. U.S.A.
2007, 104, 6945–6950.
(18) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553–566.
(19) Cooper, J.; Ziegler, T. Inorg. Chem. 2002, 41, 6614–6622.
(20) Higginson, B. R.; McAuliffe, C. A.; Venanzi, L. M. Inorg. Chim.
Acta 1971, 5, 37–40.
(21) Borowski, A. F.; Sabo-Etienne, S.; Christ, M. L.; Donnadieu, B.;
Chaudret, B. Organometallics 1996, 15, 1427–1434.
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