Influence of ion pairing on styrene hydrogenation using a cationic η6-arene β-diketiminato-ruthenium complex

Article abstract of DOI:10.1021/om900634s

A series of salts composed of the coordinatively unsaturated ruthenium β-diketiminato cation [(η⁶C₆H₆) Ru((ArNCMe)₂CH)]⁺ (Ar = 2,6-dimethylphenyl) and different anions, i.e., OTf^- (1), BF

Full text of DOI:10.1021/om900634s

6432 Organometallics 2009, 28, 6432–6441
DOI: 10.1021/om900634s
Influence of Ion Pairing on Styrene Hydrogenation Using a
Cationic η6-Arene β-Diketiminato-Ruthenium Complex
†
,†
‡
‡
ꢀ
Aitor Moreno, Paul S. Pregosin,* Gabor Laurenczy, Andrew D. Phillips, and
Paul J. Dyson*,‡
†
€
Laboratorium fu€r Anorganische Chemie, ETHZ HCI Honggerberg, CH-8093 Zurich, Switzerland, and
Institut des Sciences et Ingenierie Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015
€
‡
ꢀ
ꢀ ꢀ
Lausanne, Switzerland
Received July 20, 2009
A series of salts composed of the coordinatively unsaturated ruthenium β-diketiminato cation [(η6-
C6H6)Ru((ArNCMe)2CH)]þ (Ar = 2,6-dimethylphenyl) and different anions, i.e., OTf- (1), BF4
(2), PF6- (3), BPh4- (4), and BArF- (B((3,5-CF3)2C6H3)4-) (5), have been prepared and character-
ized. The solid state structures of 1, 2, and 5 have also been established using single-crystal X-ray
diffraction. Both solution and solid state data reveal the presence of anion-cation interactions, the
extent of which depends on the nature of the anion, which have been further rationalized via
computed charge density profiles using DFT energy optimized models. The catalytic activity of 1-5
in the hydrogenation of styrene was found to be highly dependent on the nature of the counteranion,
as inferred from investigations based on high-pressure solution NMR, pulsed gradient spin-echo
(PGSE) NMR diffusion, and Overhauser NMR spectroscopy. A good correlation between catalytic
activity and the extent and nature of ion pairing was found, and the structure of the active catalytic
species is proposed.
-
Introduction
transfer of a hydride and proton from an appropriate hydro-
gen donor to the substrate.
Since the pioneering and paradigm-changing discovery
of the catalytic properties of Wilkinson’s catalyst, RhCl-
(PPh3)3, irrespective of the metal employed, phosphine liga-
nds overwhelmingly dominate ligand choice in homoge-
neous hydrogenation catalysis. Tolmann elegantly classified
the electronic and steric properties of monophosphine
ligands in his seminal review article, which ultimately led to
rational ligand design in the field of homogeneous catalysis.1
In addition, bisphosphine ligands have received considerable
attention, in regard to parameters such as bite angles and
chirality, leading to new design concepts.
Catalysts based on newer hybrid bidentate ligands such as
phosphino-amino and carbene-amino systems also demon-
strate high activity and good selectivity in hydrogenation
reactions. However, very few hydrogenation catalysts are
known where the supporting ligand chelates through two or
more nitrogen centers,2 with notable examples including
the application of 2-aminomethyl-1-ethylpyrrolidine and
1,2-diphenyl-ethylenediamine ligands.3 Chelating diamino
ligands find widespread use in catalytic transfer hydrogena-
tion reactions whereby both the metal and ligand assist in the
β-Diketiminates as supporting ligands for hydrogenation
catalysts have received only minor attention despite their
attractiveness in stabilizing electronically unsaturated metal
centers.4 Budzelaar and co-workers found that 14-electron
olefin-substituted β-diketiminato-rhodium complexes can
hydrogenate a variety of alkene-based substrates at low H2
pressures, albeit with very low turnover numbers.5 Further
studies with rhodium and iridium β-diketiminato complexes
revealed that di- and multihydride6,7-containing species are
formed readily in the presence of hydrogen and appear to be
important intermediates in hydrogenation and dehydro-
genation processes. We recently reported ruthenium com-
plexes with 2,6-dimethylphenyl-R,R-dimethyl-substituted
β-diketiminato ligands (Chart 1).8 The compounds exist in
either the neutral form (type a), with a Ru-coordinated
chloride ligand, or the cationic form (type b), prepared via
the removal of the chloride ligand.
Notably, the cationic type b complex, as its triflate salt,
undergoes thermoreversible addition reactions across the
metal center and the β-carbon of the β-diketiminato ligand
(4) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002,
102, 3031–3066.
(5) Budzelaar, P. H. M.; Moonen, N. N. P.; de Gelder, R.; Smits,
J. M. M.; Gall, A. W. Eur. J. Inorg. Chem. 2000, 753–769.
(6) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Chem. Com-
mun. 2004, 764–765.
*Corresponding authors. E-mail: pregosin@ethz.ch; paul.dyson@
epfl.ch.
(1) Tolman, C. A. Chem. Rev. 2002, 77, 313–348.
(2) Togni, A. L.; Venanzi, L. M. Angew. Chem., Int. Ed. Engl. 1994,
33, 497–526.
(7) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Organometallics
(3) Jones, M. D.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Lewis,
D. W.; Rouzaud, J.; Kenneth, J. R.; Harris, K. D. M. Angew. Chem., Int.
Ed. 2003, 42, 4326–4331.
2005, 4367–4373.
(8) Phillips, A. D.; Laurenczy, G.; Scopelliti, R.; Dyson, P. J.
Organometallics 2007, 26, 1120–1122.
r
pubs.acs.org/Organometallics
Published on Web 10/20/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 22, 2009 6433
Chart 1
Scheme 1. General Synthesis of Cationic β-Diketiminato-Ruthenium Complexes with Different Anionsa
a Ar corresponds to 2,6-dimethylphenyl. A single-pot synthesis is possible only in the case of the triflate salt, 1.
with reagentscontaining unsaturated carbon-carbon bonds.
Adducts with both ethylene, c, and acetylene, d, have been
isolated and characterized. Moreover, it has been shown that
coordinatively unsaturated cationic complexes of type b are
highly active toward heterolytic cleavage of H2, forming a
species containing a coordinated monohydride species e,
supported by a β-diimine ligand,8 which readily reverts back
to the β-diketiminato complex when the H2 atmosphere is
removed.
The observation of such cooperative reactivity indicates
that the cationic type b complexes could be effective hydro-
genation catalysts.8 We therefore prepared a series of catio-
nic ruthenium β-diketiminato complexes with different
anions, ranging from the strongly coordinating triflate anion
to the weakly interacting trifluoromethylated tetrakis-phen-
yl borate (BArF-) anion, and compared their activity in the
hydrogenation of styrene. Herein we report on the outcome
of these studies.
phosphorus-centered heterocycles.9 However, clean quanti-
tative yielding reactions are observed when the correspond-
ing Cl-ligated precursor is combined with the sodium salt of
the desired anion, and the resulting complexes 2-5 are
removed from the insoluble NaCl byproduct (Scheme 1).
The 1H NMR spectra of 1-5 in CD2Cl2 confirm the C2v
symmetry of the ruthenium cations, as only one single
resonance for the ortho methyl groups attached to the
flanking aryl groups of the β-diketiminato ligand is ob-
served. The 1H NMR spectra also indicate that the counter-
ions interact to some extent with the cations, as some modest
chemical shift variations among the different salts are ob-
served. In particular, an increased amount of shielding for
the arene protons is observed in complexes 4 and 5 that
-
contain the BPh4 and BArF- anions. Moreover, slight
variations in the frequency of the resonances of the β-
position protons of the β-diketiminato ligand are observed.
However, these minor differences still indicate that specific
and well-defined cation-anion interactions are present in
solution.
Results and Discussion
Characterization in the Solid State. All of the complexes
were isolated as fine microcrystalline powders, and using
liquid-vapor diffusion methods (see Experimental Section)
1, 2, and 5 were crystallized with dimensions suitable for
structural determination using X-ray diffraction analysis.
Data treatment and relevant parameters are given in the
Experimental Section and Supporting Information.
For 1, 2, and 5 no significant structural variation of the
cation is observed to within experimental errors. Neverthe-
less, upon examination of the crystal packing for the salts, a
As previously reported,8 the cationic ruthenium η6 ben-
zene-β-diketiminato species, 1, with a triflate counterion is
conveniently prepared in a one-step reaction from the η6-
C6H6-dichlororuthenium dimer, β-diketiminato-lithium
conjugate, and sodium or potassium triflate. However, the
direct exchange of the chloride group with other anions, i.e.,
BF4-, PF6-, BPh4-, and BArF-, cannot be performed as a
single-pot reaction due to the strong basicity of the β-
diketiminato-lithium conjugates. In the case of PF6-, for
example, the above-mentioned procedure yielded significant
quantities of PF3. It is known that β-diketiminato-lithium
complexes react readily with PCl3 to form a variety of
(9) Burford, N.; D’eon, M.; Ragogna, P., J.; McDonald, R.; Ferguson,
M. J. Inorg. Chem. 2004, 43, 734–738.

6434 Organometallics, Vol. 28, No. 22, 2009
Moreno et al.
Table 1. Selected 1H and 13C NMR Data of the η6-C6H6 and the
2,6-Dimethylphenyl, r,r0-Dimethyl β-Diketiminate Ligands in
1-5a
δ(1H)
β-CH
δ(13C)
β-CH
δ(13C)
δ(1H)
δ(13C)
anion
OTf-
R-CCH3
η6-C6H6
η6-C6H6
1
2
3
4
5
6.63
6.64
6.67
6.60
6.65
105.57
105.52
105.71
105.64
105.94
163.92
163.89
164.02
163.96
164.24
5.17
5.16
5.14
4.91
5.08
84.13
84.11
84.43
83.85
83.96
-
BF4-
PF6
BPh4
-
BArF-
a Spectra recorded in CD2Cl2 at 25 °C.
Table 2. Comparison of Key Intermolecular Distances between the
Various Components of the Cation and Anion in Complexes 1, 2,
and 5a
cation
component
anion
component
interaction
˚
distance (A)
anion
1
1
1
2
2
2
2
2
5
5
OTf-
OTf-
OTf-
η6-C6H6
R-CH3
o-CH3
η6-C6H6
R-CH3
o-CH3
m-CH
p-CH
η6-C6H6
R-CH3
O3SCF3
O3SCF3
O3SCF3
BF4
BF4
BF4
BF4
BF4
CF3
CF3
2.397, 2.684
2.659
2.653
2.404, 2.485, 2.580, 2.607
2.428, 2.439, 2.657, 2.713
2.633
2.603
2.485
2.520, 2.619
2.645, 2.655, 2.666
Figure 1. ORTEP packing diagram of 1 showing the interac-
tions between the benzene group of the cation and the oxygen
and fluorine atoms of the triflate OTf- anion. The thermal
ellipsoids of non-hydrogen atoms were drawn at the 50%
probability level. The chloroform solvate is omitted for clarity.
-
BF4-
BF4-
BF4-
BF4-
BF4
BArF-
BArF-
arrange to form an overall D2d molecular symmetry instead
of S4. However, the latter is calculated to be lower in energy
by a very small amount, only 0.52 kcal mol-1, and in the case
of BPh4-, 0.78 kcal mol-1. Analogous with the OTf- and
BF4- anions, another set of interactions is formed with the
hydrogen atoms of the η6-C6H6 ring, which are also relativity
short (Table 2), although somewhat less extensive than those
formed with the OTf- and BF4- anions.
a The interacting atoms are highlighted in bold.
clear distinction between the two classes of anions is appar-
-
ent. The OTf- and BF4 containing salts pack in a prefer-
ential arrangement that favors the formation of a maximum
number of van der Waals type interactions between the
fluorine and oxygen atoms of the anions and hydrogen
atoms of the η6-C6H6 ring; see Table 2 and Figure 1. In the
case of complex 2, the fluorine atoms of the BF4- anion form
short interactions with the hydrogen atoms of the η6-C6H6
ligand. The anions in the crystal packing of 1 and 2 appear to
help orientate the cations into a position in which the η6-
benzene rings form π-π stacking interactions, albeit at long
distances (Figure 1). The interaction between an oxygen
center of a triflate group and a hydrogen atom associated
with an η6-benzene is quite common.10 Another series of
cation-anion interactions is observed between the fluorine
atoms of OTf- and the 2,6-dimethyl hydrogens of the
flanking aryl group, and interactions between the R-CH3 of
the β-diketiminato ligand and the trifluoromethyl group of
the triflate moiety are also observed. In the case of BF4-, the
closest interactions are between the η6-C6H6 ring and the
fluorine atoms; however, benzene-benzene stacking is not
as pronounced as for 1. The most important anion-cation
interactions are summarized in Table 2.
A computed charge density profile of the cationic compo-
nent of 2 (Figure 3), obtained through a DFT energy
optimized model with a constrained C2v symmetry, indicates
that the highest areas of positive charge on the molecule are
primarily associated with the hydrogen atoms of the η6-C6H6
ring. Secondary sites include the methyl groups of the
flanking aryls and associated R-CH3 groups attached to
-
the β-diketaminato ligand. For OTf-, BF4-, and PF6 all
of the outer atoms are highly electronegative. In the case of
BArF-, the fluorine atoms of the CF3 groups represent the
highest regions of electronegativity, but for BPh4-, the outer
edges of the phenyl groups are significantly less electroneg-
ative than the other counterions. Therefore, this anion is
expected to interact with the higher regions of electronega-
tivity on the cation, i.e., the R-CH3 and flanking aryl protons.
The electrostatic maps reveal that OTf-, BF4-, and PF6- are
small charge-localized anions and therefore prefer to interact
with specific electropositive areas of the cation, whereas the
-
larger, charge-diffuse anions, BPh4 and BArF-, interact
In the structure containing the larger and weaker coordi-
nating BArF- counterion, a different packing arrangement
is observed, whereby the backbone of the β-diketiminate
ligand slots between two 3,5-(CF3)2C6H3 aryl groups
(Figure 2). Accordingly, hydrogen bonding between the R-
CH3 and CF3 group is present. A similar packing arrange-
ment has been reported previously.11 In order to maximize
cation-anion packing, the four aryl groups of BArF-
with a larger region of the cation, i.e., flanking aryls and the
R-CH3 groups. The individual intermolecular interactions
-
between the cation and BPh4 and BArF- may be weaker
than those of OTf-, BF4-, and PF6-, but a greater total of
interactions in the former results in stronger ion pairing for
these types of anions. Thus, although the calculated cation
and anion models represent isolated gas phase molecules, a
good overall correlation with the crystal-packing arrange-
ment is observed. Moreover, the anion-benzene interaction
is also maintained in solution, as indicated by 1H,19F
HOESY data (see below).
(10) Hayashida, T.; Kondo, H.; Terasawa, J.; Kirchner, K.; Sunada,
Y.; Nagashima, H. J. Organomet. Chem. 2007, 692, 382–394.
(11) Menendez, C.; Morales, D.; Perez, J.; Riera, V.; Miguel, D.
Organometallics 2001, 20, 2775–2781.

Article
Organometallics, Vol. 28, No. 22, 2009 6435
Figure 2. ORTEP packing diagram of 5 showing the interactions between the η6-benzene ring and R-CH3 groups of the cation with the
fluorine atoms (shaded in gray) of the BArF- anions. The thermal ellipsoids of non-hydrogen atoms are shown at the 50% probability
level.
Table 3. Turnover Frequencies for Styrene Hydrogenation for
Complexes 1-5a
TOF (mol substrate
mol catalyst h-1
3
complex
anion
)
3
1
OTf-
OTf-
838
783
759
737
149
287
1b
2
-
BF4-
3
4
5
PF6
BPh4
-
BArF-
a Complexes are listed in order of decreasing activity. Conditions:
40 atm of H2, 80 °C, dry and degassed THF (2 mL), styrene (1 g,
9.60 mmol), 0.01% mmol catalyst. b Performed with the addition of
100 mg of Hg.
larly useful in hydrogenations performed by cationic
Ir-PHOX complexes.15 For this ruthenium β-diketaminato
complex, in contrast, the catalytic activity is dramati-
cally reduced in the presence of these weakly coordinating
anions.
Further control experiments were undertaken including
a Hg poisoning experiment to establish the contribution
from heterogeneous catalysts generated in situ under the
reductive conditions of the hydrogenation reaction.16
Since only a small decrease in the conversion of styrene
was observed in the presence of Hg, homogeneous cata-
lysis presumably dominates. It is also worth noting that
the postcatalysis solutions reveal no signs of decomposi-
tion. The choice of solvent for the hydrogenation reactions
appears to be very critical. For all of the above-described
hydrogenations, dry and degassed THF was used; how-
ever, when chloroform is used, TOFs for styrene hydro-
Figure 3. Visual representations of the electrostatic potential
for the cation [(η6-C6H6)Ru((2,6-(CH3)2C6H3)NC(CH3))2CH]þ
(top left) and the relevant anions, OTf- (top right), BPh4
-
(bottom left), and BArF- (bottom right). The areas of greatest
electronegativity are represented in red, and the lowest electro-
negative areas are shown in blue.
Styrene Hydrogenation Studies Using 1-5. Complexes
1-5 were screened as (pre)catalysts for the hydrogenation
of styrene under conditions similar to those reported for
a related chelating bisphosphine complex.12 All hydro-
genation reactions were performed under an inert
atmosphere, to avoid deactivation of the catalyst by
reaction with O2, and the obtained results are listed in
Table 3.
genation by complex
1 are significantly reduced.
Moreover, if 10% of THF is added to a solution 1 in
chloroform, some activity is regained, suggesting a direct
(coordination) role of the THF in the catalytic cycle; see
complex 6 in Scheme 2.
A comparison of the catalytic activity of salts 1-5 reveals
a clear difference between the smaller potentially coordinat-
-
ing anions such as OTf-,13 BF4-, and PF6 and the larger
High-pressure (sapphire) 1H NMR spectroscopy17 was
used to study the rate of styrene hydrogenation for 1 (X =
OTf-) and 5 (X = BArF-) over 45 min. Conditions for the
apparently weakly coordinating borates, BPh4- andBArF-.14
In most situations, BArF- is employed to help accelerate
catalytic-based reactions and has been shown to be particu-
(15) Smidt, S. P.; Zimmermann, N.; Studer, M.; Pfaltz, A. Chem.-
Eur. J. 2004, 10, 4685–4693.
(16) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855–859.
(17) de Vries, J. G.; Elsevier, C. J., Eds. Handbook of Homogeneous
Hydrogenation; Wiley-VCH: Weinheim, D.E., 2007.
(12) Daguenet, C.; Scopelliti, R.; Dyson, P. J. Organometallics 2004,
23, 4849–4857.
(13) Lawrance, G. A. Chem. Rev. 1986, 86, 17–33.
(14) Strauss, S. H. Chem. Rev. 1993, 93, 927–942.

6436 Organometallics, Vol. 28, No. 22, 2009
Moreno et al.
1
Figure 4. Graph of styrene hydrogenation by 1 and 5 established in situ using high-pressure H NMR spectroscopy. Zero time
corresponds to when the sample reached a temperature of 80 °C, typically within a 5 min period.
Scheme 2. Formation of the Hydride Species 10 from 1 and Subsequent Generation of the Proposed Active Catalytic Species 6 and Final
Stable Species (7 and 8) at the End of the Reaction Following Release of the H2 Pressure
catalytic hydrogenation of styrene were approximated as
close as possible to those in the reactor; however, a higher
pressure of 100 bar was used in place of 40 bar, to ensure
effective mass transfer of H2 into the solution due to less
efficient mixing in the narrow sapphire NMR tube. The rate
of conversion of styrene to ethylbenzene differs signifi-
cantly for the two tested systems (Figure 4) and is consistent
with the TOF numbers obtained in the batch reactor.
Complex 1, with the triflate counterion, is significantly
more active than 5, with the BArF- anion, suggesting that
the anion continues to exert an influence with respect to the
active catalytic species. Interestingly, no induction period
was observed from the time measurements started, although
during the sample heating period of approximately 5 min, 1
undergoes conversion to the active catalytic species, which
overall is relativity fast and tends to exclude the possibility of
heterogeneous (nanoparticle) catalysts generated from the
complexes.
reaction involving the β-hydrogen position of the β-diimine
ligand. As the temperature was raised to 80 °C over a 15 min
period, the signal corresponding to the η6-C6H6 arene
slowly disappears; however, there is no evidence for a
resonance corresponding to free benzene or a deturated
analogue. This observation suggests that the benzene ring
is removed by (partial) hydrogenation, albeit in a stoichio-
metric fashion.
1
Moreover, the residual signals associated with the H8-
THF solvent reveal a set of secondary peaks shifted to
slightly higher field (þ0.35 ppm) that are consistent with
THF coordination to a metal center via the O atom. There-
fore, it is proposed that the active catalytic species consists
of a β-diimino ruthenium hydride species 6 supported by a
THF ligand (Scheme 2). Electrospray ionization mass
spectrometry (ESI-MS) was used to analyze the solution
after depressurization, revealing the presence of species
similar to the cation in 1, but in which the benzene has been
replaced by styrene (7 in Scheme 2) or ethylbenzene (8 in
Scheme 2). This observation suggests that the proposed
THF-coordinated intermediate is unstable and reverts to a
η6-coordinated arene species in the absence of a pressurized
hydrogen environment, which is as expected for a weakly
coordinated ligand.
A high-pressure 1H NMR experiment was also performed
using 1 in combination with D2 and d8-styrene in d8-THF.
Initially, prior to heating the sample, the formation of the
deuterated monohydride β-diimide species was observed,
i.e., complex e in Chart 1. Interestingly, a signal for HD
was observed, suggesting the occurrence of a scrambling

Article
Table 4. D (10-10 m2 s-1) and rH (A) Values for 1-5 in d8-THF
Organometallics, Vol. 28, No. 22, 2009 6437
a
˚
concn
(mM) fragment
Dc/
Da
corr
salt anion
D
rH rH
TOFb
788
1
2
3
4
5
OTf-
3.7
2.3
2.8
3.1
6.4
cation
anion
8.21 0.92 5.8
8.97 5.3
6.5
6.1
-
BF4
cation
anion
8.57 0.89 5.5
9.60 5.0
6.3
5.8
759
737
149
287
-
PF6
cation
anion
8.64 0.87 5.5
9.94 4.8
6.2
5.6
-
BPh4
cation
anion
7.52 1.00 6.3
7.55 6.3
6.9
6.9
BArF-
cation
anion
7.46 1.13 6.4
6.67 7.1
7.0
7.6
a For the calculation of rH, the viscosity of the nondeuterated solvent
at 299 K was used (η(d8-THF) = 0.46 10-3 kg s-1 m-1). rH was
corr
calculated using the Stokes-Einstein equation, while rH
lated using a semiempirical estimation of c (rvdW of THF was estimated
was calcu-
using Bondi’s group increments25).21,48 b In units of moles of substrate
per moles of catalyst per h-1
.
PGSE Diffusion and HOESY Studies. The recent litera-
ture15,18-20 suggests that, when salts are employed as cata-
lysts, the choice of counterions can be important, not only in
terms of determining the structure of the salt but in terms of
its reactivity. While the anions may be “innocent”, it has
become clear that ion-pairing effects frequently play a sig-
nificant role. Although measuring the extent of the ion
pairing in solution is not a trivial matter, recent NMR
studies, combining both pulsed gradient spin-echo
(PGSE) NMR diffusion21 and Overhauser NMR spectros-
copy, have begun to reveal how ions interact. Assuming that
both of the charged species in question contain NMR-active
spins, e.g., 1H and 19F, inspection of the magnitudes of the
NMR diffusion constants provides a direct estimate of the
extent of the ion pairing. For 100% ion pairing (and in the
absence of, for example, hydrogen-bonding or encapsulation
effects), the diffusion constants (D-values) for the anion and
cation will be identical within the experimental error.
Figure 5. 1H,19F HOESY spectrum of 3 in d8-THF (14 mM).
There are strong contacts to the 12 equivalent ortho methyl
protons of the aryl rings and the protons of the η6-benzene
ring. This is consistent with a selective position for the anion
remote from the N,N-chelate ring, but close to the η6-benzene
ring.
transition metal salts in methanol solution give much smaller
rH values (and thus faster translation due to less association)
-
for these two anions.24 For the BPh4 salt, the ratio D-
(anion)/D(cation) is unity, suggesting 100% ion pairing for
this salt. As a consequence of this ion pairing, this salt is
effectively much larger and the calculated rH value is ca.
In an effort to estimate the possible role of the anions in the
catalysis described herein we have carried out PGSE and
1H,19F HOESY NMR studies on the ruthenium complexes
1-5; see Table 4. In addition to the experimental D-values,
we show a ratio, D(cation)/D(anion), as well as hydrody-
namic radii, rH (calculated from the Stokes-Einstein equa-
tion and its proposed modifications).21
-
˚
6.9 A. In the BArF analogue, the ratio is >1, which is
certainly partially a reflection of the fact that this anion is
quite large. The estimated corrected rH value for this anion is
˚
˚
about 6.6 A; however the calculated rH value of ca. 7.6 A
suggests more ion pairing than usually encountered for this
1
anion in THF. The H, 19F HOESY NMR spectra of 1-3
For salts 1-5 there are considerable differences between
the experimental D-values for the cations and the anions. For
the OTf-, BF4-, and the PF6- salts, the D(cation)/D(anion)
ratio is <1, but quite substantial and indicative of a con-
siderable amount of ion pairing. This result is also observed
for a number of different salts in THF solution.22-24 For
share many similar features. There are strong contacts to (a)
the 12 equivalent ortho methyl protons of the aryl rings (see
the spectrum for 3 in Figure 5) and (b) the protons of the η6-
complexed benzene ring. These features are consistent with a
selective position for the anion in which it is somewhat
remote from the N,N-chelate ring, but close to the benzene
-
-
comparison it is worth noting that many BF4 and PF6
-
ring, in keeping with the computational study. The BPh4
salt 4 clearly reveals modest contacts from all three of
its protons to the ortho methyl protons of the aryl
rings. However, the BArF- anion in 5 shows no significant
(18) Semeril, D.; Bruneau, C.; Dixneuf, P. H. Adv. Synth. Catal. 2002,
344, 585–595.
(19) Fagnou, K.; Lautens, M. Angew. Chem., Int. Ed. 2002, 41, 26–47.
(20) Faller, J. W.; Fontaine, P. P. Organometallics 2005, 24, 4132–
4138.
(21) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M. C.;
Roberts, J. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448–1464.
(22) Fernandez, I.; Martinez-Viviente, E.; Breher, F.; Pregosin, P. S.
Chem.-Eur. J. 2005, 11, 1495–1506.
(23) Fernandez, I.; Martinez-Viviente, E.; Pregosin, P. S. Inorg.
Chem. 2005, 44, 5509–5513.
1
contacts (from either the 19F or the H spins) to the cation.
-
If the BPh4 contacts are maintained during the catalytic
cycle, they may reflect the observed TOF value for this salt
(24) Pregosin, P. S.; Kumar, P. G. A.; Fernandez, I. Chem. Rev. 2005,
105, 2977–2998.

6438 Organometallics, Vol. 28, No. 22, 2009
Moreno et al.
˚
either by (a) steric hindrance due to ion pairing or (b) the
occupation of the coordination sites critical to catalytic activity.
Given the ion pairing found for the BArF- analogue, steric
effects may be responsible for the reduced catalytic activity of
this salt.
stored over 4 A molecular sieves. d8-THF was dried and distilled
over 1:2 sodium-potassium alloy. Both solvents were degassed
by repeated freeze-thaw cycles prior to sample preparation.
Chemicals shifts for 1H and 13C spectra were referenced to
residual solvent peaks and external references for Et2O-BF3
for 11B spectra, 85% H3PO4:H2O for 31P spectra, and CF3Cl for
19F spectra. Coupling constants were determined by iterative
spectral simulation using the program gNMR.33 ATR FT-IR
was performed with a Perkin-Elmer Spectrum One instrument,
using freshly ground samples pressed on top of a diamond anvil
window. Sample preparation and spectral recording (in air)
were performed within 2 min. Elemental microanalyses were
obtained using a CE Instruments EA-1110 or Exeter analytical
CE-440 elemental analyzer. Mass spectra were recorded using
either solution or nanoelectrospray ESI techniques using a
ThermoFinnigan LCQ DECA XP Plus (quadrupole ion trap)
with the following conditions: solvent THF; flow rate 5 μL
min-1; spray voltage 5 kV; capillary temperature 100 °C;
capillary voltage 20 V.
Concluding Remarks. The electronically unsaturated ca-
tionic ruthenium complexes 1-5, supported by the sterically
demanding 2,6-dimethylphenyl R,R0-dimethyl β-diketami-
nate ligand, are good catalyst precursors for the hydrogena-
tion of styrene to ethylbenzene. The active catalyst appears
to be a homogeneous species, in which the benzene ring has
been lost with the ruthenium center stabilized by weakly
coordinating THF solvent molecules. The catalytic activity
of the salts is additionally influenced to a significant extent
by the nature of the counterion. A combination of structural,
computational, and NMR studies suggest that the observed
anion effects are due to ion-pairing phenomena and poten-
tially, but probably to a much lesser extent, direct anion
coordination. Indeed, the classic weakly coordinating anions
Synthesis of 1-5. To 0.125 g of μ2-((η6-C6H6)RuCl2)2 in a 50
mL round-bottom flask was added 10 mL of CH2Cl2, forming
an orange suspension. While stirring, a solution of 2 equiv of β-
diketiminato-lithium complex, Li((2,6-CH3)2C6H3)NC(CH3))2,
in 2 mL of CH2Cl2 was added slowly dropwise, and instantly the
solution changed to a purple color then slowly after 10 to 20 min
to orange-brown. The reaction flask was capped and the mixture
was stirred for 14 h. Afterward the suspension was filtered
through a frit containing at least 1 cm pad of Celite. The Celite
was washed with CH2Cl2 until the filtrate was colorless. The
volume of the filtrate was reduced, under vacuum, to 2 mL, and
25 mL of n-pentane was added to precipitate a purple-colored
microcrystalline powder. The solution was decanted from the
solid and dried under vacuum for several hours. The purple solid
was dissolved with 25 mL of dry CH2Cl2 and transferred to a
flask containing the sodium salt of the desired anion, i.e.,
NaO3SCF3 (1), NaBF4 (2), NaPF6 (3), NaBPh4 (4), or NaBArF
(5). After stirring for 1 h, the mixture was filtered through a 1 cm
pad of Celite. The resulting orange-brown solution was reduced
to a volume of approximately 2 mL, and n-pentane was added to
precipitate a brown solid. The solid was washed several times
with n-pentane and dried under vacuum overnight.
-
(BPh4 and BArF-), often used to facilitate catalytic re-
actions,26-28 suppress activity more than the other anions
included in this study (OTf-, BF4-, and PF6-). Thus, not
only does this study demonstrate how careful the choice of
the anion is required to optimize the catalytic activity, but at
a molecular level, it also provides a rationalization for the
observed anion effects.
Experimental Section
General Procedures. Synthesis of the N-protonated 2,6-di-
methylphenyl R,R0-dimethyl β-diketaminate was performed un-
der conditions typical for organic synthesis, whereas other
operations were carried out under a N2 atmosphere with stan-
dard Schlenk techniques. Synthesis and manipulations of all
β-diketaminato-ruthenium complexes and reagents were per-
formed in a drybox with a N2 atmosphere containing less than
1 ppm of O2 and H2O and equipped with a vacuum outlet. All
glassware was predried, and the flasks underwent several purge/
refill cycles before the introduction of solvents or reagents. All
solvents were dried by passage through aluminum oxide col-
umns or, in some cases, degassed by passage through a copper
column (Innovative Technologies) and then stored in Schlenk
Complex 1. Yield: 0.278 g (88% based on ((η6-C6H6)RuCl2)2).
Anal. Found [Calcd] C: 53.07 [52.98], H: 4.93 [4.78], N: 4.42
[4.30]. 1H NMR (25 °C, 400.1 MHz, CD2Cl2): δ (ppm) 2.140 (s,
12H, Ar o-CH3), 2.153 (s, 6H, R-CH3), 5.165 (s, 6H, η6-C6H6),
6.634 (s, 1H, β-CH), 7.362 (m, 3JHH = 1.86 Hz, 2H, Ar p-CH),
7.422 (m, 3JHH = 1.86 Hz, 4H, m-CH). 13C NMR (25 °C, 100.1
MHz, CD2Cl2): δ (ppm) 19.08 (s, Ar o-CH3), 23.28 (s, R-CH3),
84.13 (s, η6-C6H6), 105.57 (s, β-CH), 121.43 (q, 1JCF = 322 Hz,
SO3CF3-), 128.03 (s, Ar p-CH), 129.56 (s, Ar m-CH), 129.98 (s,
Ar o-C), 158.69 (s, Ar i-C), 163.92 (s, R-CCH3). 19F NMR
(25 °C, 188.1 MHz, CD2Cl2): δ (ppm) -79.2 (s, 1JFC = 322 Hz,
CF3SO3-). UV-vis (25 °C, CH2Cl2), λ (nm) [ε (L mol-1 cm-1)]:
434 [5455], 291 [11 981]. ESI-MS (25 °C, CH2Cl2) (m/z): positive
mode 485.200 [parent Mþ, 100%, calcd 485.153], negative mode
149.200 [parent M-, 100%, calcd 148.952]. FT-IR (25 °C, solid):
˚
flasks equipped with Teflon stopcocks over 3 or 4 A molecular
sieves. Celite (545 grade, Merck Co.) was dried at 160 °C for two
days. The ruthenium benzene precursor [(η6-C6H6)RuCl2]2,29
the unsolvated Li((2,6-CH3)2C6H3)NC(CH3))2CH complex,30
and anhydrous Na[B(3,5-(CF3)2C6H3)4]31,32 were prepared ac-
cording to literature procedures. All other reagents and gases
(technical grade) were purchased from commercial sources and
used as received. NMR spectra were recorded using either a
Bruker Avance 200 or 400 MHz instrument. 1H (COSY, NOE)
and 13C (HMBC and HSQC) one- and two-dimensional spectra
were used to assign molecular connectivity and conformation in
solution. High-purity CD2Cl2 was distilled over CaH2 and
ν(cm-1
) 1592.78(w); 1553.50(w); 1518.35(w); 1507.88(w);
1468.96(w); 1436.95(w); 1340.53(w); 1265.82(vs, S-O);13
1221.21(m); 1178.10(w); 1147.33(s, C-F);13 1087.72(w); 1097.08-
(w, C-F);13 1028.57(s); 981.17(w); 963.67(w); 867.34(w); 846.72-
(m); 776.00(m); 752.27(w); 711.83(w); 667.72(w).
(25) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.
(26) Chen, M. C.; Roberts, J. A. S.; Marks, T. J. J. Am. Chem. Soc.
2004, 126, 4605–4625.
(27) Leatherman, M. D.; Svejda, S. A.; Johnson, L. K.; Brookhart,
M. J. Am. Chem. Soc. 2003, 125, 3068–3081.
(28) Janka, M.; He, W.; Frontier, A. J.; Eisenberg, R. J. Am. Chem.
Soc. 2004, 126, 6864–6865.
(29) Bennett, M. A.; Smith, A. K. Dalton Trans. 1974, 233–241.
(30) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead, M.
M.; Roesky, H. W.; Power, P. P. Dalton Trans. 2001, 3465–3469.
(31) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,
3920–3922.
Complex 2. Yield: 0.296 g (89% based on ((η6-C6H6)RuCl2)2).
Anal. Found [Calcd] C: 56.62 [56.75], H: 5.63 [5.47], N: 4.74
[4.90]. 1H NMR (25 °C, 400.1 MHz, CD2Cl2): δ (ppm) 2.142 (s,
12H, Ar o-CH3), 2.151 (s, 6H, R-CH3), 5.162 (s, 6H, η6-C6H6),
3
6.627 (s, 1H, β-CH), 7.323 (m, JHH = 1.83 Hz, 2H, Ar p-
CH), 7.384 (m, 3JHH = 1.83 Hz, 4H, m-CH). 13C NMR (25 °C,
(32) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579–
3581.
(33) Budzelaar, P. H. M. gNMR 5.6: A NMR Simulation Program;
Ivorysoft: Nijmegen, Netherlands, 2006.

Article
Organometallics, Vol. 28, No. 22, 2009 6439
100.1 MHz, CD2Cl2): δ (ppm) 19.06 (s, Ar o-CH3), 23.27 (s, R-
CH3), 84.11 (s, η6-C6H6 CH), 105.52 (s, β-CH), 128.00 (s, Ar
p-CH), 129.55 (s, Ar m-CH), 130.01 (s, Ar o-C), 158.67 (s, Ar i-
C), 163.89 (s, R-CCH3). 19F NMR (25 °C, 188.2 MHz, CD2Cl2):
δ (ppm) -153.42 (s, BF4). 11B NMR (25 °C, 128.4 MHz, CD2-
Cl2): δ (ppm) -1.62 (s, BF4). ESI-MS (25 °C, CH2Cl2) (m/z):
positive mode 485.200 [parent Mþ, 100%, calcd 485.153],
negative mode 87.267 [parent M-, 100%, calcd 87.003]. FT-
IR (25 °C, solid): ν(cm-1): 3055.16(w), 2983.25(w), 1643.49(w),
1579.13(w), 1553.60(w), 1508.22(w), 1478.69(w), 1468.40(w),
1438.18(w), 1426.37(w), 1377.60(w), 1342.56(w), 1298.53(w),
1262.79(w), 1239.94(w), 1203.58(w), 1178.67(w), 1147.57(w),
1132.70(w), 1096.27(w), 1065.33(w, B-F),34 1031.83(w),
999.36(w), 982.86(w), 912.44(w), 866.88(w), 839.21(w), 771.25-
(w, B-F),34 748.26(w), 732.29(s), 703.19(s).
s, 8H, BArF o-CH). 13C NMR (25 C, 100.1 MHz, CD2Cl2): δ
(ppm) 19.02 (s, Ar o-CH3), 23.28 (s, R-CH3), 83.96 (s, η6-C6H6),
1
105.94 (s, β-CH), 118.04 (s, BArF, p-CH), 125.14 (m, JCF
=
2
271 Hz, BArF m-CCF3), 129.35 (q, JCF = 79 Hz, BArF
m-CCF3), 128.28 (s, Ar p-CH), 129.65 (s, Ar m-CH), 129.74 (s,
Ar o-C), 135.35 (s, BArF, o-CH), 158.69 (s, Ar i-C), 162.30 (m,
1JBC = 49.5 Hz, BArF i-C), 164.24 (s, R-CCH3). 19F NMR (25
1
°C, 188.2 MHz, CD2Cl2): δ (ppm) -62.95 (s, JFC = 271 Hz,
BArF m-CF3). 11B NMR (25 °C, 128.4 MHz, CD2Cl2): δ (ppm)
-6.61 (s, 1JBC = 49.5 Hz, B(ArF)4). ESI-MS (25 °C, CH2Cl2)
(m/z): positive mode 485.317 [parent Mþ, 100%, calcd 485.153],
negative mode 863.467 [parent M-, 100% calcd 863.068]. FT-IR
(25 °C, solid): 1610.25(w), 1555.14(w), 1467.84(w), 1439.69(w),
1352.78(m), 1338.48(w, sh), 1276.81(s), 1157.84(m, sh), 1116.18(s),
887.09(m), 869.87(w), 838.25(m), 816.19(w), 781.97(w), 772.85(m),
744.12(w), 715.78(m), 710.11(m), 681.82(m), 669.04(m).
Complex 3. Yield: 0.290 g (92% based on ((η6-C6H6)RuCl2)2).
Anal. Found [Calcd] C: 53.07 [51.51], H: 4.92 [4.96], N: 4.40
[4.45]. 1H NMR (25 °C, 400.1 MHz, CD2Cl2): δ (ppm) 2.176 (s,
12H, Ar o-CH3), 2.176 (s, 6H, R-CH3), 5.178 (s, 6H, η6-C6H6),
6.668 (s, 1H, β-CH), 7.363 (m, 3JHH = 1.83 Hz, 2H, Ar p-CH),
7.424 (m, 3JHH = 1.83 Hz, 4H, m-CH). 13C NMR (25 °C, 100.1
MHz, CD2Cl2): δ (ppm) 19.17 (s, Ar o-CH3), 23.23 (s, R-CH3),
84.43 (s, η6-C6H6), 105.71 (s, β-CH), 127.69 (s, Ar p-CH), 129.34
(s, Ar m-CH), 130.14 (s, Ar o-C), 158.72 (s, Ar i-C), 164.02 (s,
R-CCH3). 19F NMR (25 °C, 188.2 MHz, CD2Cl2): δ (ppm)
-73.62 (d, 1JPF = 711 Hz, PF6). 31P NMR (25 °C, 161.9 MHz,
CD2Cl2): δ (ppm) -144.4 (sept, 1JPF = 711 Hz, PF6). ESI-MS
(25 °C, CH2Cl2) (m/z): positive mode 485.200 [parent Mþ,
100%, calcd 485.153], negative mode 144.641 [parent M-,
100%, calcd 144.964]. FT-IR (25 °C, solid): ν(cm-1): 3095(w);
2921(w); 1554(m); 1516(w); 1506(w); 1469(m); 1437(m);
1382(m); 1351(m); 1299(m); 1265(w); 1265(w); 1241(w);
1205(w); 1178(w); 1097(w); 1088(w); 1025(w); 983(w); 831(s,
P-F);34 769(s, P-F);34 740(m); 712(m); 667(w).
Crystallographic Details. Suitable single crystals were re-
moved from the sample vial under a flow of N2 and manipulated
in a perfluoropolyalkyl ether oil matrix (F06206K, ABCR
Company) in a specially constructed Dewar partially filled with
liquid nitrogen. The crystals were mounted to the end of a glass
fiber (diameter at least 0.1 mm) attached to a metal pin fixed to a
goniometer head, which was placed in the Euler cradle, while
maintaining a cold blanket of N2 gas. For all structures, an
Oxford-Kuma KM-4 diffractometer setup with a Sapphire
CCD area detector was used as the collecting instrument. Both
instruments utilize a graphite-monochromated Mo KR radia-
˚
tion source with λ = 0.71073 A. The crystals were kept under a
140 or 100 K gaseous flow of N2 during the collection procedure.
The unit cell and orientation matrix was determined by indexing
reflections measured from the entire data set using CrysAlis
RED.35 All data sets are composed on collecting reflections
using an optimized scanning strategy utilizing the program
CrysAlis CCD.36 After data integration with CrysAlis RED35
a multiscan absorption correction based on a semiempirical
method was applied using the SCALE3 ABSPACK scaling
algorithm contained within the CrysAlis CCD suite.36 Space
group determination was performed with the XPREP pro-
gram.37 A structure solution based on the direct-method algo-
rithm was employed with SHELXS-97.38 Afterward, aniso-
tropic refinement of all non-hydrogen atoms was completed
on the basis of a least-squares full-matrix method against F2
data using SHELXL-97.38 Hydrogen atoms were added through
geometrically calculated positions and refined as a riding model
whereby the thermal parameter is a scaled value of the connect-
Complex 4. Yield: 0.290 g (83% based on ((η6-C6H6)RuCl2)2).
Anal. Found [Calcd] C: 76.09 [76.20], H: 6.46 [6.39], N: 3.71
[3.48]. 1H NMR (25 °C, 400.1 MHz, CD2Cl2): δ (ppm) 2.100 (s,
12H, Ar o-CH3), 2.131 (s, 6H, R-CH3), 4.906 (s, 6H, η6-C6H6),
3
6.600 (s, 1H, β-CH), 6.872 (m, JHH = 6.84 Hz, 4H, BPh4
p-CH), 6.999 (m, 3JHH = 6.84 Hz, 8H, BPh4 m-CH), 7.285 (br s,
8H, BPh4 o-CH), 7.369 (m, JHH = 1.81 Hz, 2H, Ar p-CH),
3
3
7.386 (m, JHH = 1.81 Hz, 4H, m-CH). 13C NMR (25 °C,
100.1 MHz, CD2Cl2): δ (ppm) 19.12 (s, Ar o-CH3), 23.29 (s,
R-CH3), 83.85 (s, η6-C6H6), 105.64 (s, β-CH), 122.33 (s, BPh4
p-CH), 126.17 (s, JBC = 2.8 Hz, BPh4 m-CH), 128.15 (s, Ar
3
-
p-CH), 129.61 (s, Ar m-CH), 129.78 (s, Ar o-C), 136.46 (s,
2JBC = 1.4 Hz, BPh4, o-CH), 158.59 (s, Ar i-C), 163.96 (s, R-
CH3), 164.50 (m, 1JBC = 49.9 Hz, BPh4 i-C). 11B NMR (25 °C,
ing atom. In complexes 2 and 5, the counterion (BF4 and
BArF-) featured positional disorder of the fluorine atoms. The
disorder was treated by splitting the affected atoms over two
positions and allowing the total occupancy of the disordered
1
128.4 MHz, CD2Cl2): δ (ppm) -6.62 (s, JBC = 49.9 Hz,
BPh4-). ESI-MS (25 °C, CH2Cl2) (m/z): positive mode
485.467 [parent Mþ, 100%, calcd 485.153], negative mode
319.533 [parent M-, 100%, calcd 319.166]. FT-IR (25 °C, solid):
3055.16(m), 303.174(w), 2983.25(w), 2996.03(w), 2916.66(w),
1642.40(w), 1579.13(w), 1553.60(w), 1477.59(w), 1468.40(w),
1435.94(w), 1426.37(m), 1377.60(w), 1342.56(m), 1298.53(w),
1262.79(w), 1239.94(w), 1178.67(w), 1147.57(w), 1132.02(w),
1096.27(w), 1065.33(w), 1031.83(w), 982.86(w), 913.16(w),
866.88(w), 839.21(m), 771.25(m), 748.26(m), 732.29(s), 703.19(s).
Complex 5. Yield: 0.579 g (86% based on ((η6-C6H6)RuCl2)2).
Anal. Found [Calcd] C: 51.46 [52.58], H: 3.02 [3.22], N: 1.78
[2.08], note: residual traces of CH2Cl2 could not be removed
groups to freely refine (details described in the CIFs). A small
2
number of reflections in some cases were removed when Δ(Fo
-
2
Fc )/σ exceeded 10.0. In the case of 5, the absolute structure was
determined (a Flack parameter equal to 0.03(2)) by refinement
against the inverted twin matrix. Important data for all struc-
tures are given in the Supporting Information, Table S1. Draw-
ings in the paper were produced with the program XP,38
and CIF data formatting was performed using the program
CIFTAB38 and enCIFer.39
In Silico Studies. Computational modeling of the cations and
anion described in this paper were performed using the Gaussian
1
under vacuum or washing with n-pentane. H NMR (25 °C,
(35) CrysAlis Pro RED, 1.71; Oxford Diffraction Ltd.: Abingdon,
Oxfordshire, U.K., 2006.
(36) CrysAlis Pro CCD, 1.71; Oxford Diffraction Ltd.: Abingdon,
Oxfordshire, U.K., 2006.
(37) Sheldrick, G. M. XPREP, A Reciprocal Space Exploration
Program, 2.06; Bruker-AXS: Madison, WI, 2003.
(38) Sheldrick, G. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112–122.
400.1 MHz, CD2Cl2): δ (ppm) 2.108 (s, 12H, Ar o-CH3), 2.147
(s, 6H, R-CH3), 5.080 (s, 6H, η6-C6H6), 6.651 (s, 1H, β-CH),
7.317 (m, JHH = 1.73 Hz, 3H, Ar p-CH), 7.366 (m, JHH =
1.73 Hz, 6H, Ar m-CH), 7.556 (br s, 4H, BArF p-CH), 7.718 (br
3
3
(34) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds Part A: Theory and Applications in Inorganic
Chemistry, 6th ed.; Wiley: New York, 2009.
(39) Allen, F. H.; Johnson, O.; Shields, G. P.; Smith, B. R.; Towler,
M. J. Appl. Crystallogr. 2004, 37, 335–338.

6440 Organometallics, Vol. 28, No. 22, 2009
Moreno et al.
1
03 program40 on a SGI Altix ICE 8200EX cluster, employing
density functional theory with the three-parameter hybrid
method developed by Becke (B3LYP).41 For all nonmetal
atoms, the contracted 6-31G(d,p)42 basis set was selected, which
included diffuse functions. For the ruthenium center, the dou-
ble-ζ basis set Lanl2dz was used in conjunction with a pseudo-
potential representing the core set of electrons.43 All structures
were geometrically optimized to an energy minimum. The
position on the local potential energy surface was confirmed
through vibration analysis using second derivatives, where no
imaginary frequencies were observed. The electrostatic potential
maps, as shown in Figure 3, were generated from the calculated
electron density and potential using Gaussview.44
Catalytic Evaluation Studies. The experiments were con-
ducted in a custom in-house built multicell steel autoclave
reactor. For each cell, a 10 mL volume glass liner was inserted
into an autoclave block, along with a Teflon-coated stir bar. A
stock solution (4.8 ꢀ 10-3 mol mL-1) of each catalyst was
prepared using dry and degassed tetrahydrofuran. The catalytic
reaction mixture consisted of 2.00 mL of stock solution (9.6 ꢀ
10-3 mmol) with 1.00 mL of styrene (9.602 mmol) (Acros,
stabilized, degassed and dried with molecular sieves) and
0.10 mL of degassed and dried n-octane (8.7 ꢀ 10-1 mmol).
All preparations and autoclave loading were performed in a N2-
filled drybox. After sealing the autoclave, the cells were purged
with high-purity H2 for at least three cycles. The autoclave was
then heated to 80 °C under 10 bar of H2. The reactor was
pressurized and stirred at 40 bar of H2, which continued for 1 h
at 80 °C. The reaction was quenched by depressurizing and rapid
cooling to ambient temperature. The autoclave was opened in
air, small aliquots from each cell were taken, and the sample was
diluted with toluene. The reaction mixture was analyzed with
GC with the following conditions. A Varian CP-3380 gas
chromatographic analyzer was used with a CP-Sil 5CB normal
phase (low polarity) 25 m ꢀ 0.25 mm column. The reported
TON values are averages from three separate runs. High-
pressure NMR studies were performed by preparing solutions
identical to those used in the autoclave experiments, except for
the use of d8-THF. The solution was added to a 10 mm medium-
pressure sapphire NMR tube equipped with a stainless steel
head with lockable gas-inlet valve, under a N2-filled atmo-
sphere. Subsequently, the NMR was pressurized with 100 atm
of H2, causing in all cases an instant color change.
was monitored by H NMR spectroscopy, and recording of the
spectra was initiated when the probehead had reached a tempera-
ture of 80 °C, which typically occurred within a period of 5 min.
PGSE NMR Studies. The PGSE measurements were carried
out without spinning and in the absence of external airflow. The
sample was dissolved in 0.7 mL of d8-THF, with a concentration
range of 2.0-6.4 mM. The sample temperature was calibrated,
before the PGSE measurements, by introducing a thermocouple
inside the bore of the magnet. All the PGSE diffusion measure-
ments were performed using the standard stimulated echo pulse
sequence on a 400 MHz Bruker Avance spectrometer equipped
with a microprocessor-controlled gradient unit and an inverse
multinuclear probe with an actively shielded Z-gradient coil.
The shape of the gradient pulse was rectangular, its duration δ
was 1.75 ms, and its strength varied automatically in the course
of the experiments. The calibration of the gradients was carried
out via a diffusion measurement of HDO in D2O, which
afforded a slope of 1.976 ꢀ 10-3. The data obtained were used
to calculate the D values of the samples, according to the
literature.22,45-47
In the 1H-PGSE experiments, the diffusion delay, Δ, was set
to 117.75 and 167.75 ms, respectively. The number of scans was
16 per increment with a recovery delay of 10 to 30 s. Typical
experimental times were 2-4 h. For 19F, Δ was set to 117.75 and
167.75 ms, respectively. Sixteen scans were taken with a recovery
delay of 10 to 25 s and a total experimental time of ca. 2-4 h. All
the spectra were acquired using 32K points and processed with a
line broadening of 1 Hz (1H) and 2 Hz (19F). The slopes of the
lines, m, were obtained by plotting their decrease in signal
intensity vs G2 using a standard linear regression algorithm.
Normally, 15-20 points were used for regression analysis, and
all of the data leading to the reported D-values afforded
lines whose correlation coefficients were >0.999. The grad-
ient strength was incremented in 3-4% steps from 3 to 72%.
A measurement of 1H and 19F T1 was carried out before
each diffusion experiment, and the recovery delay set to
5 times T1. We estimate the experimental error in D-values
at (2%. The hydrodynamic radii, rH, were estimated using
the Stokes-Einstein equation (c = 6) or by introducing the
semiempirical estimation of the c factor, which can be derived from
the microfriction theory proposed by Wirtz and co-workers,48 in
which c is expressed as a function of the solute to solvent ratio
of radii.
Warning: Sapphire NMR tubes when filled with pressurized gas
represent a potential explosion hazard; moreover, hydrogen gas is
highly flammable. Protective Plexiglas shields were placed around
the samples at times when not residing inside in the NMR magnet.
The NMR tube was weighed to ensure gas transfer and to verify
that the tube was sealed. The progression of the catalytic reaction
The 1H-19F HOESY NMR measurements were acquired
using the standard four-pulse sequence on a 400 MHz Bruker
Avance spectrometer equipped with a doubly tuned (1H, 19F)
TXI probe. A mixing time of 800 ms was used. The number
of scans was 8-16, and the number of increments in the
F1 dimension is 512. The delay between the increments
was set to 2.5-4.5 s. The concentrations of the samples were
10-14.1 mM.
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Acknowledgment. This research was supported by
grants from the European Commission Marie Curie
Action (A.D.P., MEIF-CT-2005-025287, CARCAS),
the Swiss National Science Foundation, ETH Zurich
(A.M., P.S.P.), and the EPFL (G.L., A.D.P., P.J.D.).
Computational resources were provided by the Irish
Center for High-End Computing (ICHEC). Additional
support and sponsorship (A.M., P.S.P.) was provided by
COST Action D24 “Sustainable Chemical Processes:
Stereoselective Transition Metal-Catalyzed Reactions”.
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Article
Organometallics, Vol. 28, No. 22, 2009 6441
The authors wish to thank Rosario Scopelliti (EPFL) for
assistance with the crystallographic data collections.
additional data are found in the included CIFs. An ORTEP plot
of 2 and atomic coordinates for the computational models are
also included as well as ESP maps for the BF4- and PF6- anions.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Supporting Information Available: A table of single-crystal
X-ray diffraction data for complexes 1, 2, and 5 is given, and