Methylene-Linked Bis-NHC Half-Sandwich Ruthenium Complexes: Binding of Small Molecules and Catalysis toward Ketone Transfer Hydrogenation

Article abstract of DOI:10.1021/acs.organomet.1c00045

The complex [Cp*RuCl(COD)] reacts with LH2Cl2 (L = bis(3-methylimidazol-2-ylidene)) and LiBun in tetrahydrofuran at 65 °C furnishing the bis-carbene derivative [Cp*RuCl(L)] (2). This compound reacts with NaBPh4 in MeOH under dinitrogen to yield the labile dinitrogen-bridged complex [{Cp*Ru(L)}2(μ-N2)][BPh4]2 (4). The dinitrogen ligand in 4 is readily replaced by a series of donor molecules leading to the corresponding cationic complexes [Cp*Ru(X)(L)][BPh4] (X = MeCN 3, H2 6, C2H4 8a, CH2CHCOOMe 8b, CHPh 9). Attempts to recrystallize 4 from MeNO2/EtOH solutions led to the isolation of the nitrosyl derivative [Cp*Ru(NO)(L)][BPh4]2 (5), which was structurally characterized. The allenylidene complex [Cp*Ru═C═C═CPh2(L)][BPh4] (10) was also obtained, and it was prepared by reaction of 2 with HCCC(OH)Ph2 and NaBPh4 in MeOH at 60 °C. Complexes 3, 4, and 6 are efficient catalyst precursors for the transfer hydrogenation of a broad range of ketones. The dihydrogen complex 6 has proven particularly effective, reaching TOF values up to 455 h-1 at catalyst loadings of 0.1% mol, with a high functional group tolerance on the reduction of a broad scope of aryl and aliphatic ketones to yield the corresponding alcohols.

Full text of DOI:10.1021/acs.organomet.1c00045

pubs.acs.org/Organometallics
Article
Methylene-Linked Bis-NHC Half-Sandwich Ruthenium Complexes:
Binding of Small Molecules and Catalysis toward Ketone Transfer
Hydrogenation
José Manuel Botubol-Ares, Safa Cordón-Ouahhabi, Zakaria Moutaoukil, Isidro G. Collado,
Manuel Jiménez-Tenorio,* M. Carmen Puerta, and Pedro Valerga
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ABSTRACT: The complex [Cp*RuCl(COD)] reacts with LH2Cl2
(L = bis(3-methylimidazol-2-ylidene)) and LiBun in tetrahydrofuran
at 65 °C furnishing the bis-carbene derivative [Cp*RuCl(L)] (2).
This compound reacts with NaBPh4 in MeOH under dinitrogen to
yield the labile dinitrogen-bridged complex [{Cp*Ru(L)}2(μ-
N2)][BPh4]2 (4). The dinitrogen ligand in 4 is readily replaced
by a series of donor molecules leading to the corresponding cationic
complexes [Cp*Ru(X)(L)][BPh4] (X = MeCN 3, H2 6, C2H4 8a,
CH2CHCOOMe 8b, CHPh 9). Attempts to recrystallize 4 from
MeNO2/EtOH solutions led to the isolation of the nitrosyl
derivative [Cp*Ru(NO)(L)][BPh4]2 (5), which was structurally
characterized. The allenylidene complex [Cp*RuCC
CPh2(L)][BPh4] (10) was also obtained, and it was prepared by
reaction of 2 with HCCC(OH)Ph2 and NaBPh4 in MeOH at 60
°C. Complexes 3, 4, and 6 are efficient catalyst precursors for the transfer hydrogenation of a broad range of ketones. The
dihydrogen complex 6 has proven particularly effective, reaching TOF values up to 455 h−1 at catalyst loadings of 0.1% mol, with a
high functional group tolerance on the reduction of a broad scope of aryl and aliphatic ketones to yield the corresponding alcohols.
provide more robust complexes with different topological
properties, namely steric hindrance, bite angle, or fluxional
behavior.13 In contrast to their monodentate counterparts, the
chemistry of transition-metal complexes based on bis(NHC)
ligands have been far less explored.14 Ruthenium(II)−NHC
complexes have been recently used in a wide range of organic
reactions15 including TH reactions.1,11a,16−18 However, the
synthesis and catalytic application of their related hydrocarbon
chain-linked bis(NHC) complexes is scarce in the literature
and their potential synthetic applicability still needs to be
studied. To the best of our knowledge, only a few reports have
been described about the use of hydrocarbon chain-linked
bis(NHC) complexes of ruthenium in TH of ketones19 and
hydrogenation of olefins.20
INTRODUCTION
■
The catalytic transfer hydrogenation (TH) of carbonyl groups
to afford their corresponding alcohols is considered a useful
synthetic tool to produce valuable building blocks for the
pharmaceutical industry.1−3 This methodology has as an
advantage the use of alcohols,4 water,5 or formic acid6 instead
of molecular hydrogen or metal hydrides as sources of the
hydrogen atoms transferred in the reaction. Catalytic TH using
transition metal complexes has received a great deal of
attention due to its selectivity, efficiency, safety, broad scope,
and compatibility with Green Chemistry principles. Homoge-
neous complexes based on transition metals such as Ir,1,7 Ru,1,8
or Rh1,9 are often used as catalysts for TH.1 However, the
development of more efficient and selective transfer hydro-
genation catalysts are still in demand. Among them, ruthenium
complexes possess higher activity, selectivity, and cheaper cost
in comparison to those of rhodium or iridium.10
Following the recent works carried out in our research group
in the preparation of TpRu complexes bearing the methylene-
NHC ligands have been successfully used as ancillary ligands
in the design and synthesis of homogeneous catalysts.11 These
ligands are considered an alternative to phosphine ligands due
to their stronger sigma-donor properties that confer a greater
stability to the corresponding metallic complexes, and facilitate
the modulation of their stereoelectronic properties.12 Chela-
tion is a strategy used to stabilize the M−NHC bond and
Received: January 26, 2021
Published: March 9, 2021
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linked bis(NHC) ligand bis(3-methylimidazol-2-ylidene)-
methane (L),21 we focused our attention in the preparation
of homologous pentamethylcyclopentadienyl (Cp*) ruthe-
nium complexes with the same ligand in order to compare the
effect of the replacement of the supporting ligand Tp by Cp*
on the reactivity of the metal center. We have now found that
this modification enhances reactivity, and that several of the
new complexes prepared in this way are efficient catalyst
precursors for TH reactions of carbonyl groups.
have been reported. The average dihedral angle between the
plane defined by the atoms C(11)−Ru(1)−C(16) and the
imidazol rings of 35.65° is slightly higher than the same angle
observed for other pseudo-octahedral complexes containing
the same ligand L (range 19.7−32.1°).21,25−27 The Ru−C
distances for the NHC ligand are 2.038(3) and 2.045(2) Å,
typical for Ru−C σ-bonds, and indicative of an essentially
symmetrical arrangement of the imidazolylidene rings. The
Ru(1)−Cl(1) bond distance of 2.3766(9) Å is slightly shorter
than the Ru−Cl separations of 2.4436(11) and 2.389(2) Å
reported for the chloro complexes [TpRuCl(κ2-C,N-picoly-
l-iPrI)] (picolyl-iPrI = 3-isopropyl-1-(2-picolyl)imidazol-2-yli-
dene)23 and [(p-cymene)RuCl(κ2-C,N-picolyl-MeI)][PF6] (pi-
colyl-MeI = 3-methyl-1-(2-picolyl)imidazol-2-ylidene),18 but in
the range of 2.340−2.376 Å observed for Ru−Cl bond lengths
in the complexes [Cp*RuCl(NHC)] (NHC = 1,3-bis(2,6-
diisopropylphenyl)-4,5-imidazol-2-ylidene, 1,3-bis(mesityl)-
4,5-imidazol-2-ylidene, 1,3-bis(mesityl)-4,5-dihydroimidazol-
2-ylidene, 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-
2-ylidene).28
Since the transmetalation reaction of [Cp*RuCl(COD)]
with the silver bis-carbene in 1,2-dichloroethane takes place
with oxidation of the metal center to RuIII to yield 1, it was not
possible to use this synthetic approach for the preparation of
the RuII target compound [Cp*RuCl(L)]. Alternatively, we
used the direct reaction of [Cp*RuCl(COD)] with the bis-
carbene L generated in situ by deprotonation of the
bis(imidazolium) salt [LH2]Cl2 using an excess of LiBun in
tetrahydrofuran. In this fashion, the complex [Cp*RuCl(L)]
(2) (Chart 1) was isolated in the form of an extremely air-
sensitive yellow microcrystalline material.
RESULTS AND DISCUSSION
■
Synthesis of Complexes and Their Interaction with
Small Molecules. The transmetalation reaction of silver−
NHC complexes is a well-known synthetic procedure for the
introduction of NHC ligands into a system.12,13,22 We have
successfully used this methodology for the preparation of
Ru18,21,23 and Ni24 complexes. In particular, we prepared the
complex [TpRuCl(L)] by reaction of [TpRuCl(COD)] with
L·Ag2Cl2 in dichloroethane at 120 °C over 20 h.21 In an
attempt to synthesize the homologous complex bearing Cp*
instead of Tp, we carried out the reaction of [Cp*RuCl-
(COD)] with L·Ag2Cl2 in dichloroethane at 130 °C over 18 h.
A cherry red microcrystalline product was obtained from this
reaction. NMR spectroscopy showed that the compound was
paramagnetic. Recrystallization from dichloromethane/petro-
leum ether yielded dark cherry red crystals. The X-ray structure
analysis identified this product as the RuIII derivative
[Cp*RuCl(L)]Cl (1). An ORTEP view of the complex cation
[Cp*RuCl(L)]+ in 1 is shown in Figure 1, together with the
most relevant bond distances and angles.
Chart 1. Preparation of Complexes 1 and 2
Figure 1. ORTEP drawing (50% displacement ellipsoids, hydrogen
atoms omitted) of [Cp*RuCl(L)]+ in 1. Selected bond lengths (Å)
and angles (deg): Ru(1)−C(11) 2.038(3), Ru(1)−C(16) 2.045(2),
Ru(1)−Cl(1) 2.3766(9), Ru(1)−C(1) 2.284(3), Ru(1)−C(2)
2.230(3), Ru(1)−C(3) 2.183(3), Ru(1)−C(4) 2.222(3), Ru(1)−
C(5) 2.262(3); C(11)−Ru(1)−C(16) 81.9(1).
Compound 2 is extremely air-sensitive and turns cherry red
immediately upon exposure to atmospheric oxygen. This color
is consistent with the oxidation to the RuIII cation [Cp*RuCl-
(L)]+. It is very sparingly soluble in C6D6, and it reacts with
1
chlorinated solvents yielding oxidation products. Its H NMR
The complex cation has a typical three-legged piano stool
structure in which the ruthenium atom adopts a distorted
pseudo-octahedral geometry. The bis-carbene chelating bite
angle C(11)−Ru(1)−C(16) of 81.9(1)° is slightly smaller
than the typical values reported for this ligand, usually in the
range 83.2−87.8°,21,25−27 although values as small as 79.4°
spectrum in THF-d8 at room temperature consists of very
broad features. The resonances become sharper when the
temperature is raised. At 65 °C, the spectrum is reasonably
well-resolved, and shows one broad singlet for the Cp* ring
protons, one singlet for the protons of the methyl substituents
of the imidazolylidene rings, and two characteristic AB doublet
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signals with a coupling constant of 11.6 Hz corresponding to
the methylene bridge protons, which become diastereotopic
upon bidentate coordination of the ligand to the Ru atom. The
protons of the double bond of the imidazolylidene rings appear
as two broad singlets near 7 ppm. Similar NMR features have
been observed on the complex [TpRuCl(L)].21 The 13C{1H}
NMR signal of the equivalent carbene carbon atoms of 2 was
not observed. It is assumed that it is too broad even at high
temperature and it merges with the baseline. All the resonances
for the other carbon atoms of the ligands L and Cp* appear in
the expected positions. located as expected for Ru−NHC
compounds.12,13,17,23,25,26 The temperature dependence of the
NMR spectra is interpreted in terms of the rapid exchange of
the chloride ligand with the THF-d8 donor molecule (Chart
2).
Chart 2. Proposed Exchange in Solution of Chloride Ligand
with THF-d8
Figure 2. ORTEP drawing (30% displacement ellipsoids, hydrogen
atoms omitted) of [Cp*Ru(MeCN)(L)]+ in 3. Selected bond lengths
(Å) and angles (deg): Ru(1)−C(11) 2.029(4), Ru(1)−C(17)
2.035(4), Ru(1)−N(5) 2.051(3), Ru(1)−C(1) 2.224(5), Ru(1)−
C(2) 2.194(5), Ru(1)−C(3) 2.166(4), Ru(1)−C(4) 2.206(4),
Ru(1)−C(5) 2.204(4), C(20)−N(5) 1.134(4), C(20)−C(21)
1.464(5); C(11)−Ru(1)−C(17) 83.3(1); Ru(1)−N(5)−C(20)
179.4(3), N(5)−C(20)−C(21) 177.7(4), N(2)−C(14)−N(3)
110.6(3).
expected range for Ru−NHC compounds. Compound 3 is a
convenient and efficient catalyst precursor for hydrogen
transfer reactions to ketones, as discussed below.
In fact, the chloride ligand in 2 is easily replaced by a range
of donor molecules in the presence of a suitable chloride
scavenger. Thus, 2 reacts with acetonitrile and NaBPh4 in
MeOH at room temperature furnishing the complex [Cp*Ru-
(MeCN)(L)][BPh4] (3). Recrystallization from acetonitrile/
methanol yielded amber crystals suitable for X-ray structure
analysis. An ORTEP view of the complex cation [Cp*Ru-
(MeCN)(L)]+ in 3 is shown in Figure 2, together with the
most relevant bond distances and angles.
Complex 2 reacts with NaBPh4 in methanol under N2
yielding a yellow microcrystalline precipitate which exhibits a
strong band at 2050 cm−1 in its Raman spectrum, attributable
to ν(NN) in a bridging dinitrogen ligand attached to two Ru
atoms. As expected, this band is inactive and does not show on
the IR spectrum. This observation is consistent with the
formation of the binuclear complex [{Cp*Ru(L)}2(μ-N2)]-
[BPh4]2 (4). The value for the Raman ν(NN) band
compare well with data in the literature for bridging dinitrogen
complexes of ruthenium.23,29,30 The position of this band in 4
appears shifted ca. 50 cm−1 to lower wavenumbers with respect
to the ν(NN) in the homologous complex [{TpRu(L)}2(μ-
N2)][BAr′4]2 (2106 cm−1),21 consistent with a slight increase
of the relaxation of the coordinated NN in 4 in comparison
with the latter. At variance with [{TpRu(L)}2(μ-N2)][BAr′4]2,
which contains a dinitrogen ligand strongly bound to
ruthenium, compound 4 is very labile and the compound is
very reactive. Whereas the reaction of [{TpRu(L)}2(μ-
N2)][BAr′4]2 with CO or acetonitrile is very sluggish, 4 reacts
readily with MeCN furnishing complex 3. It also reacts with
chlorinated solvents such as dichloromethane and chloroform
yielding dark red solutions with display complex NMR spectra.
We found nitromethane as the best solvent to be used in this
system due to its polarity and poor coordinating abilities. Thus,
clean 1H and 13C{1H} NMR spectra were obtained in
nitromethane-d3. One singlet for Cp* and two AB doublet
signals for the methylene bridge protons are the most
characteristic features of the 1H NMR spectrum. The
equivalent carbene carbon atoms appear as one singlet at
185.1 ppm on the 13C{1H} NMR spectrum. We attempted
recrystallization of 4 in a mixture nitromethane/ethanol. Dark
orange crystals were obtained. These crystals showed a strong
band at 1813 cm−1 on the IR spectrum. This band was absent
in the compound prior to recrystallization. X-ray structure
The complex cation in 3 has a three-legged piano stool
structure. The bis-carbene chelating bite angle C(11)−Ru(1)−
C(12) of 85.3(2)° is within the expected range.21,25−27 The
average dihedral angle between the plane defined by the atoms
C(10)−Ru(1)−C(15) and the imidazol rings is 35.3°, still
above the average values for the same angle in other complexes
containing the same ligand L, but very similar to the value
found for compound 1. The Ru−C distances for the NHC
ligand of 2.029(4) and 2.035(4) Å are both of the same order
and consistent with Ru−C separations expected for σ-bonds.
The acetonitrile ligand is almost linearly assembled to
ruthenium. The Ru(1)−N(5) and N(5)−C(20) bond lengths
of 2.051(3) and 1.134(4) Å respectively compare well with the
values of 2.043(5) and 1.128(7) Å found in [Cp*Ru(MeCN)-
(κ2-C,N-picolyl-MeI)][BAr′4] (picolyl-MeI = 3-methyl-1-(2-
picolyl)imidazol-2-ylidene).17 All other dimensions in the
structure are in the expected ranges and are unexceptional.
Complex 3 is far more stable toward atmospheric oxygen than
2. It can be handled as a solid in the air without visible
decomposition or oxidation. It shows a medium intensity IR
band at 2252 cm−1 corresponding to ν(CN) in the
1
acetonitrile ligand. Its H NMR spectrum in acetonitrile-d3
shows the two characteristic AB doublet signals for the
methylene bridge protons as expected. In this case, the
13C{1H} NMR signal of the equivalent carbene carbon atoms
of 3 appear clearly on the spectrum at 193.0 ppm, i.e., in the
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analysis revealed that this product is the unexpected nitrosyl
complex [Cp*Ru(NO)(L)][BPh4]2 (5). An ORTEP view of
the complex cation [Cp*Ru(NO)(L)]2+ in 5 is shown in
Figure 3, together with the most relevant bond distances and
angles.
solution of 4 in nitromethane-d3 under dinitrogen over several
days. We were able to detect gradually increasing signals for
the nitrosyl compound 5, mixed with many other resonances
and broad features corresponding to unidentified species. We
did not detect the formation of any hydride containing species
in the overall process, which is not clean nor simple, and the
nitrosyl species 5 was the only fully characterized compound in
this most intriguing reaction sequence.
Compound 4 also acts as a catalyst precursor for ketone
hydrogen transfer reactions (vide infra). The dinitrogen ligand
in 4 is readily replaced by a number of donor molecules
furnishing the corresponding complexes. These reactions are
summarized in Scheme 1.
Scheme 1. Reactivity of the Bridging Dinitrogen Complexes
4
Figure 3. ORTEP drawing (30% displacement ellipsoids, hydrogen
atoms omitted) of one of the two identical molecules [Cp*Ru(NO)-
(L)]2+ in 5. Selected bond lengths (Å) and angles (deg): Ru(1)−
C(11) 2.056(4), Ru(1)−C(17) 2.057(3), Ru(1)−N(1) 1.769(3),
Ru(1)−C(1) 2.251(3), Ru(1)−C(2) 2.205(4), Ru(1)−C(3)
2.260(5), Ru(1)−C(4) 2.271(4), Ru(1)−C(5) 2.270(3), N(1)−
O(3) 1.140(4); C(11)−Ru(1)−C(17) 83.2(1), Ru(1)−N(1)−O(3)
169.6(3).
The crystal structure of 5 consists of two virtually identical
molecules [Cp*Ru(NO)(L)]2+ and two [BPh4]− anions per
metal cation. The crystal also contains one nitromethane
solvate molecule per ruthenium. The [Cp*Ru(NO)(L)]2+ ion
adopts a three-legged piano stool geometry, with almost
identical Ru(1)−C(11) and Ru(1)−C(17) distances for the
NHC ligand of 2.056(4) and 2.057(3) Å. The average dihedral
angle between the plane defined by the atoms C(11)−Ru(1)−
C(17) and the imidazol rings is 29.6°, smaller than the values
found for compounds 1 and 3, but within the range observed
for other complexes containing the L ligand. The nitrosyl
ligand is very slightly bent, with a Ru(1)−N(1)−O(3) angle of
169.6(3) Å. The Ru(1)−N(1) and N(1)−O(3) separations of
1.769(3) and 1.140(4) Å respectively compare well with the
values found for the nitrosyl complex [Ru(L1)(NO)Cl2] (L1 =
(N′-phenyl-N′-(pyridin-2-yl)picolinohydrazide)),31 and in the
RuII nitrosyl complexes with pyridine-functionalized N-
heterocyclic carbene ligands [Ru(L2)(NO)Cl3] (L2 = 3-tert-
butyl-1-(2-pyridyl)imidazol-2-ylidene, 3-n-butyl-1-(2-pyridyl)-
imidazol-2-ylidene, 3-tert-butyl-1-picolylimidazol-2-ylidene).32
The formation of the nitrosyl complex 5 at the expense of the
dinitrogen derivative 4 is completely unexpected. The crystals
of 5 were isolated in low yield based on ruthenium, and the
origin of the nitrosyl ligand is unclear. We can only speculate
that it comes from nitromethane, but we are unsure about the
chemical reactions leading to the ultimate products, as there
are no precedents for this. One reviewer tentatively suggested
the possibility of formation of the nitrosyl complex 5 by the
transfer hydrogenation between nitromethane and EtOH
during recrystallization. In fact, we assume that other metal-
containing products must be formed in the process, but we
failed to identify any other species. We monitored by NMR a
Thus, 4 reacts with H2 in nitromethane affording the labile
dihydrogen complex [Cp*Ru(H2)(L)][BPh4] (6). This
compound was isolated by reaction of 2 with NaBPh4 in
methanol under a dihydrogen atmosphere in the form of an off
white solid. All subsequent manipulations were performed
under an atmosphere of argon unless otherwise stated. The 1H
NMR spectrum of 6 in nitromethane-d3 under dihydrogen
shows a broad resonance at −7.81 ppm attributable to the
protons of the coordinated dihydrogen. As it is characteristic of
dihydrogen ligands,33 this resonance exhibits a short
longitudinal relaxation time T1 of 21.4 ms at 248 K (500
MHz). This was the shortest measured T1, as the minimum
value could not be determined due to freezing of the solvent.
In any case, the value is fully consistent with the presence of a
coordinated dihydrogen molecule. In general, dihydrogen
complexes stabilized with NHC-ligands are not very
common.34 We were not able to prepare the homologous
complex [TpRu(H2)(L)]+,21 an observation that emphasizes
the difference between the {Cp*Ru(L)} and {TpRu(L)}
moieties. The dihydrogen complex 6 has also been successfully
used as catalyst precursor for ketone hydrogen transfer
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reactions. The dihydrogen ligand in 6 is readily deprotonated
by strong bases yielding the corresponding neutral mono-
hydride complex. Thus, the reaction of 6 with KOBut in
tetrahydrofuran under dihydrogen afforded [Cp*RuH(L)] (7)
in high yield as a yellow microcrystalline, air-sensitive solid
(Chart 3).
ethylene in nitromethane solution. Alkylidene complexes can
be protonated by strong acids, and thus we prepared the
dicationic η2-benzyl derivative [Cp*Ru(κ2-P,N-PiPr2NHPy)-
(η2-CH2C6H5)][CF3SO3]2 by reaction of [Cp*RuCHPh(κ2-
P,N-iPr2PNHPy)][BAr′4] with an excess of trifluoromethane-
sulfonic acid in dichloromethane.35 Addition of an excess of
HBF4·OEt2 to 9 in nitromethane at room temperature led to
an immediate color change of the solution from dark red to
green. However, the 1H NMR did not show resonances for any
recognizable species. It seems that the products undergo
decomposition under these reaction conditions, and the system
was not further investigated.
Chart 3. Deprotonation of the Dihydrogen Complex 6
We have been studying in depth the reactivity of allenylidene
ligands bound to a ruthenium center supported by N-
heterocyclic carbene ligands in the general context of the
activation of propargyl alcohols by the Ru−NHC fragment.21
Thus, the reactivity toward the addition of nucleophiles to the
allenylidene ligand attached to the fragment {[TpRu(L)]+} has
been studied in detail, and it was compared to that of related
systems containing tertiary phosphines as ancillary ligands. We
found that nucleophilic additions to the allenylidene ligand
may take place alternatively at the Cα or Cβ atoms depending
on the R substituents present on the propargyl alcohol, and on
the nature of the incoming nucleophiles. We wanted to see the
effect of replacing Tp by the stronger donor Cp* ligand on the
reactivity of the allenylidene complex. Hence, we prepared the
allenylidene derivative [Cp*RuCCCPh2(L)][BPh4]
(10) by reaction of 2 with HCCC(OH)Ph2 and NaBPh4
in MeOH at 60 °C (Chart 4).
A medium band at 1780 cm−1 in the IR spectrum is
attributable to ν(RuH), whereas one singlet at −11.65 ppm in
1
the H NMR spectrum corresponds to the hydride proton.
Since 7 it is formed in strongly basic conditions, it is assumed
that this compound must be involved in the catalytic ketone
hydrogen transfer reactions described below, and it possibly
represents a key intermediate in the overall catalytic cycle.
Compound 4 also reacts with olefins CH2CHR (R = H,
COOMe) to yield the corresponding η2-adducts [Cp*Ru(η2-
CH2CHR)(L)][BPh4] (R = H 8a, COOMe 8b) (Scheme
1). These compounds were not isolated. They were generated
in solution either by bubbling ethylene or adding an excess of
methyl acrylate to a nitromethane-d3 solution of 4 under argon,
and were characterized in solution by NMR spectroscopy. The
resonances for the proton and carbon atoms of coordinated
ethylene in 8a appear at 2.14 and 47.2 ppm on the respective
1H and 13C{1H} NMR spectra. In the case of 8b, only one
diastereoisomer is observed as indicated by one single Cp*
Chart 4. Preparation of the Allenylidene Complex 10
1
signal in the H NMR spectrum, but the presence of the
COOMe substituent on the η2-alkene ligand renders the
imidazolylidene rings inequivalent. Thus, two resonances are
observed for the methyl substituents at the nitrogen, and four
separate resonances for the H and C atoms in the positions 3
and 4 of the NHC rings. Most notably, two carbene carbon
atom signals at 180.8 and 183.9 ppm are present on the
13C{1H} NMR spectrum. The two AB doublet signals for the
methylene bridge protons are maintained.
Compound 4 reacts with freshly prepared solutions of
phenyldiazomethane in tetrahydrofuran under argon yielding
the carbene complex [Cp*RuCHPh(L)][BPh4] (9)
(Scheme 1). This compound can be also prepared starting
from the dihydrogen derivative 6 in a similar fashion. We have
used a similar synthetic approach in the past for the
preparation of the carbene complexes [Cp*RuCHPh(κ2-
P,N-iPr2PNHPy)][BAr′4]35 (Py = C5H4N) and [TpRu
CHPh(Cl)(PMeiPr2)].36 The 1H NMR spectrum of com-
pound 9 is characterized by the presence of the carbene proton
resonance at 15.7 ppm. This resonance is correlated with a
signal at 287.2 ppm in the 13C{1H} spectrum as shown by a
gHSQC 2D NMR experiment. The carbene carbon atoms of
the L ligand appear at 184.0 ppm, i.e., in the expected range.
Compound 9 is remarkably stable, and unreactive toward
The allenylidene complex 10 shows a strong ν(CCC)
band in the IR spectrum at 1880 cm−1. The signals observed at
δ 274.9, 225.6, and 139.8 ppm in the 13C{1H} NMR spectrum
are respectively characteristic for the Cα, Cβ, and Cγ atoms of
the allenylidene ligand, whereas the resonance for the carbene
carbon atoms of the L ligand appear at 178.0 ppm. The signal
for the Cα atom in 10 is shifted upfield ca. 30 ppm compared
to the value observed for the homologous complex [TpRu
CCCPh2(L)][BPh4] (305.0 ppm).21 Likewise, the IR
ν(CCC) band for the allenylidene ligand in 10 appears
also shifted to lower wavenumbers compared to its TpRu
counterpart (1913 cm−1). The latter exhibits a rich reactivity
toward a variety of N- and S-donor nucleophiles such as
pyrazole, piperidine, 2-pyridinethiol or 1,3-benzenedithiol
leading to vinylcarbene complexes resulting from the addition
to the Cα atom of the allenylidene ligand.21 At variance with
this, complex 10 is apparently much less reactive toward
nucleophiles, and did not show any visible reaction with
pyrazole at 55 °C in acetone-d6 after 18 h.
Ketone Transfer Hydrogenation. The transfer hydro-
genation of ketones was explored using acetophenone (11) as
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model substrate, with 2-propanol as the hydrogen source in the
presence of 0.5 mol % of ruthenium complexes 3, 4, and 6 at
80 °C. A catalytic amount of KOiPr was initially used as base.
The results given in Table 1 showed that catalysts 3 and 4 led
Table 2. Screening of Substrates for Transfer
Hydrogenation of Carbonyl Groups Catalyzed by Complex
a
6
Table 1. Optimization of Reaction Conditions for Transfer
Hydrogenation of Acetophenone (11) with Ruthenium
a
Complexes
b
c
entry catalyst (mol %) base (mol %) t (h) yield (%) TOF (h−1
)
1
2
3
4
5
6
7
8
3 (0.5)
3 (0.5)
4 (0.5)
4 (0.5)
6 (0.5)
6 (0.5)
6 (0.5)
6 (0.5)
6 (0.5)
6 (0.1)
−
KOiPr (10)
KOiPr (10)
KOiPr (10)
KOiPr (10)
KOiPr (10)
KOH (10)
KOH (5)
KOH (5)
−
2
16
2
16
2
2
2
1
16
2
16
13
75
19
79
75
94
94
81
0
13
9.4
19
10
75
94
94
162
−
9
10
11
d
KOH (5)
KOH (10)
91
10
455
−
a
Unless noted otherwise, reactions were carried out with 2 mmol of
b
acetophenone in 2 mL of 2-propanol at 80 °C. Determined by GC-
MS. Turnover frequency (moles of ketone converted to alcohol per
mole of catalyst per hour). 4 mmol of acetophenone in 4 mL of 2-
c
d
propanol.
to the desired product ( )-11a in low yields and TOF values
after 2 h (Table 1, entries 1 and 3). With longer reaction times
(16 h), yields increased to 75% and 79%, respectively (Table 1,
entries 2 and 4). In contrast, complex 6 was found to be the
best catalyst toward transfer hydrogenation under the same
reaction conditions affording compound ( )-11a in 75% yield
after 2 h (Table 1, entry 5). When KOH was used as base
instead of KOiPr, the yield increased up to 94% in the presence
of complex 6 (Table 1, entry 6). Reduction of the amount of
KOH to 5 mol % gave an identical result (Table 1, entry 7).
These new conditions led to a higher catalytic activity after 1 h
affording ( )-11a in 81% yield with TOF value of 162 h−1
(Table 1, entry 8). However, the reaction did not take place in
the absence of base (Table 1, entry 9). Interestingly, when the
catalyst loading of complex 6 was reduced to 0.1 mol %,
compound ( )-11a was formed in a similar yield with a TOF
value of 455 h−1 after 2 h (Table 1, entry 10). Finally, we
confirmed that compound ( )-11a was only obtained in 10%
yield in the absence of catalyst (Table 1, entry 11).
With the optimized conditions in hand, the scope of the
transfer hydrogenation of ketones and aldehydes was
investigated. A variety of ketones were suitable for this
procedure giving the desired alcohols in high to good yields.
The bulkier 2-acetonaphthone (12) underwent transfer
hydrogenation to give alcohol ( )-12a in 88% yield (Table
2, entry 1). Para-substituted phenylketones or benzophenones
13−15 bearing electron-withdrawing groups such as trifluor-
omethyl or chloro groups were well tolerated to afford the
corresponding alcohols ( )-13a−15a in 71−99% yields
(Table 2, entries 2−4), whereas electron-donating substituents
on aryl group such as p-methoxy group led to alcohol ( )-16a
a
Unless otherwise noted, reactions were carried out with 2 mmol of
b
substrate in 2 mL of 2-propanol at 80 °C for 2 h. Determined by
GC-MS.
in 72% yield (Table 2, entry 5). Similarly, the transfer
hydrogenation was compatible with benzoazepin-5-one (17)
bearing 7-chloro substituent, providing the corresponding
alcohol ( )-17a in 66% yield (Table 2, entry 6). However, the
reaction was not compatible with the presence of a free
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hydroxyl group on the aromatic ring (Table 2, entry 7).
Aliphatic ketones were also well tolerated. Hex-4-en-2-one
(19) was transformed into alcohol ( )-19a in 75% yield
whereas (+)-dihydrocarvone (20) afforded stereoselectively
compound 20a in quantitative yield (Table 2, entries 8−9).
The absolute configuration of 20a was determined by NOE
effects.
On the other hand, the transfer hydrogenation of α,β-
unsaturated ketones 21−22 was performed to produce the
corresponding alkanols ( )-21a and 20a with both CC and
CO reduced in 90% and 99%, respectively (Table 2, entries
10−11).
We further extended our study to aldehydes 23−25.
Unfortunately, the reaction failed to give the corresponding
reduction products. The Guerbet-type alcohol 23a was
obtained in 83% yield from octanal (23) (Table 2, entry 12)
(Chart 5, a). The Guerbet reaction involves homocoupling of
picolyl-MeI)][PF6] (picolyl-MeI = 3-methyl-1-(2-picolyl)-
imidazol-2-ylidene) previously reported by our research
group.17 This compound had already shown an excellent
catalytic activity and wider scope compared to other previously
reported ruthenium40 or even iridium41 pyridyl−NHC
complexes. The hydride transfer is a key step in many catalytic
transformations, and further studies are in progress aimed to
extend the use of these new complexes as catalysts for reactions
of N-alkylation of amines with alcohols.
CONCLUSION
■
We have hereby reported the synthesis and characterization of
a series of novel pentamethylcyclopentadienylruthenium(II)
complexes bearing the methylene linked bis(NHC) ligand
bis(3-methylimidazol-2-ylidene)methane. The binuclear dini-
trogen-bridged compound 4 displays a rich reactivity toward a
range of small molecules. Compound 4 undergoes an
unprecedented degradation in nitromethane solution leading
to the nitrosyl derivative 5, which has been structurally
characterized. Compounds 3, 4, and 6 act as catalyst precursors
for the transfer hydrogenation of acetophenone. In particular,
the dihydrogen complex 6 has proven to be a very efficient
catalyst precursor, reaching a TOF of up to 455 h−1 at catalyst
loadings of 0.1% mol. Moreover, 6 exhibits high tolerance of
functional groups on the reduction of a broad scope of aryl and
aliphatic ketones giving rise the corresponding alcohols in high
yields. Additionally, the transfer hydrogenation of aldehydes
using 6 as a catalyst afforded Guerbet-type reaction products
and the β-alkylation of 2-propanol under mild conditions. The
study of further organic transformations mediated by these
novel complexes is currently in progress in our laboratory, and
will be reported in due course.
Chart 5. Formation of Products Derived from Guerbet-
Type Reactions
EXPERIMENTAL SECTION
■
All synthetic operations were performed under a dry dinitrogen or
argon atmosphere following conventional Schlenk techniques.
Tetrahydrofuran, diethyl ether, and petroleum ether (boiling point
range 40−60 °C) were obtained oxygen- and water-free from a
solvent purification apparatus. All other solvents (acetonitrile,
dichloromethane, toluene, nitromethane, methanol, 2-propanol)
were of anhydrous quality and used as received. All solvents were
deoxygenated immediately before use. [Cp*RuCl(COD)] was
prepared according to the literature.42 The imidazolium salt bis(3-
methylimidazolium)methane dichloride ([LH2]Cl2) was prepared
following suitable adaptations of published procedures.25 IR spectra
were taken in Nujol mulls on a FTIR spectrophotometer. Raman
spectrum was recorded at the Instituto de Ciencia de Materiales-CSIC
on a dispersive Raman microscope equipped with a He−Ne laser (λ =
532.14 nm) using a working power of 0.2 mW in order to avoid
overheating and alteration of the sample. Optical rotations were
determined with a digital polarimeter. NMR spectra were taken on
spectrometers operating at 400 or 500 MHz (1H frequency).
Chemical shifts are given in ppm from SiMe4 (1H and 13C{1H}).
1H and 13C{1H} NMR spectroscopic signal assignments were
confirmed by 1H-gCOSY, gHSQCAD (1H−13C), and gHMBCAD
(1H−13C) experiments. As a general feature, in the spectra of cationic
compounds the signals for the [BPh4]− anion are omitted. High
resolution mass spectroscopy (HRMS) was performed in a Q-TOF
mass spectrometer in the positive ion ESI mode. Silica gel (Merck)
was used for column chromatography. TLC was performed on Merck
Kiesegel 60 F254, 0.25 mm thick. GC-MS analyses were recorded in a
Bruker Scion GC-TQ gas chromatograph coupled to a Bruker TQ
primary alcohols to obtain β-alkylated dimer alcohols where
the new C−C bond is generated by an aldol condensation.37
Recently, Ir, Rh, and Ru complexes have been reported to
catalyze the Guerbet reaction and produce higher-order
alcohols in a more economical and environmentally friendly
manner.38 In most cases, these catalytic systems required
temperature higher than 100 °C. Compound 23a was also
prepared starting from octanol in 80% yield in a similar
fashion. Moreover, benzaldehyde derivative 24 performed a
cross-coupling with 2-propanol to afford the saturated dimer
24a in 56% yield, whereas 2-furfural (25) gave the unsaturated
dimer 25a as major product (Table 2, entries 13−14). These
findings indicate that complex 6 could be an efficient catalyst
precursor for β-alkylation of secondary alcohols and α-
alkylation of ketones by borrowing hydrogen reaction (Chart
5, b).39
In comparison with previously reported ruthenium(II)
complexes bearing chelating NHCs, our catalytic system
based on methylene-linked bis(NHC) ligands exhibits similar
efficiency in a wide scope of substrates even with lower catalyst
loadings allowing significant shortening of reaction times.17,18
In this sense, compound 6 seems to perform in ketone
hydrogen transfer somewhat less efficiently than the
picolylcarbene complex [Cp*Ru(MeCN)(κ2 -C,N-
mass spectrometer. Microanalyses were performed at the Servicio
́
́
Central de Ciencia y Tecnologia, Universidad de Cadiz.
[Cp*RuCl(L)]Cl 1. A Fisher-Porter vessel was charged with a
mixture of bis(3-methylimidazolium)methane dichloride (0.25 g, 1
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mmol) and Ag2O (0.28 g, 1.2 mmol). It was protected from the light
by wrapping with aluminum foil. Then, 1,2-dichloroethane (15 mL)
was added, and the mixture stirred at room temperature for 18 h.
After this time, a solution of [Cp*RuCl(COD)] (0.38 g, 1 mmol) in
1,2-dichloroethane (15 mL) was added. The resulting mixture was
stirred for further 18 h at 130 °C. A purple suspension was obtained.
It was filtered through Celite, and the Celite washed with two
portions of dichloromethane. The solvent of the filtered solution was
removed in vacuo. The resulting purple microcrystalline product was
washed with petroleum ether and dried in vacuo. Recrystallization
from dichloromethane/petroleum ether afforded crystals suitable for
X-ray structure analysis. Yield (based upon Ru): 0.21 g, 43%. Calcd
for C19H27N4Cl2Ru: C, 47.21; H, 5.63; N, 11.59. Found: C, 47.29; H,
5.71, N, 11.30. The product is paramagnetic.
CH3NO2: C, 71.52; H, 6.18; N, 7.36. Found: C, 71.50; H, 6.16, N,
1
7.27. IR: ν(NO) 1813 cm−1. NMR: H NMR (400 MHz, CD3NO2,
233 K) δ 2.12 (s, 15 H, C5(CH3)5), 2.89 (s, 6 H, NCH3), 5.74, 6.42
2
(d, JHH = 13.8 Hz, 1 H each, CHaHb), 7.53, 7.64 (s br, 1 H each,
CHCH); 13C{1H} NMR (101 MHz, CD3NO2, 233 K) δ 10.6
(C5(CH3)5), 39.0 (NCH3), 63.9 (CH2), 113.2 (C5(CH3)5), 127.0,
127.9 (CHCH), 159.9 (RuC).
[Cp*Ru(H2)(L)][BPh4] 6. Compound 6 was obtained following a
synthetic procedure similar to that for 4, but using an atmosphere of
dihydrogen instead of dinitrogen. To a mixture of 2 (1.4 g, 3.13
mmol) and NaBPh4 (1.6 g, 4.7 mmol), MeOH (20 mL) was added
under dihydrogen. An off white precipitate was formed. The mixture
was stirred at room temperature under dihydrogen for 12 h. At the
end of this time, the suspension was filtered. The white product was
washed thoroughly with ethanol and petroleum ether and dried in
vacuo. Subsequent manipulations in solution must be performed
under an atmosphere of dihydrogen. Yield: 1.46 g, 64%. Calcd for
C43H49N4BRu: C, 70.58; H, 6.47; N, 7.66. Found: C, 70.46; H, 6.36,
N, 7.45. NMR: 1H NMR (500 MHz, CD3NO2, 298 K, under
dihydrogen) δ −7.81 (s br, 2 H, (T1)min ≤ 21.4 ms, Ru(H2)), 1.89 (s,
15 H, C5(CH3)5), 3.86 (s, 6 H, NCH3), 5.66, 6.99 (d, 2JHH = 16.0 Hz,
[Cp*RuCl(L)] 2. A Schlenk tube was charged with bis(3-
methylimidazolium)methane dichloride (1.5 g, 6 mmol), deoxy-
genated and placed under dinitrogen. Deoxygenated tetrahydrofuran
(30 mL) was added. It was cooled to −80 °C and LiBun (7.5 mL of a
1.6 M solution, 12 mmol) was added via syringe. The mixture was
warmed to room temperature and stirred for 2 h. At the end of this
time, a solution of [Cp*RuCl(COD)] (1.77 g, 4.66 mmol) in
tetrahydrofuran (25 mL) was added. The resulting mixture was stirred
overnight at 65 °C. A brown-orange solution with a finely divided
white precipitate was obtained. The suspension was filtered through
Celite. The solvent was removed in vacuo, and the resulting yellow-
brown, very air-sensitive microcrystalline product was washed with
two portions of petroleum ether and dried in vacuo. Yield: 1.77 g,
85%. Calcd for C19H27N4ClRu: C, 50.94; H, 6.08; N, 12.51. Found:
C, 50.85; H, 5.99, N, 12.10. NMR: 1H NMR (500 MHz, THF-d8, 333
K) δ 1.77 (s, 15 H, C5(CH3)5), 3.93 (s, 6 H, NCH3), 5.26, 5.76 (d,
2JHH = 11.6 Hz, 1 H each, CHaHb), 6.95, 6.99 (s br, 2 H each, CH
CH); 13C{1H} NMR (125.7 MHz, THF-d8, 333 K) δ 10.3
(C5(CH3)5), 36.8 (NCH3), 61.8 (CH2), 83.3 (C5(CH3)5), 118.5,
121.4 (CHCH), RuC not observed.
3
1 H each, CHaHb), 7.20, 7.29 (d, JHH = 2.5 Hz, 2 H each, CH
CH); 13C{1H} NMR (125.7 MHz, CD3NO2, 298 K, under
dihydrogen) δ δ 11.3 (C5(CH3)5), 39.1 (NCH3), 62.2 (CH2), 93.9
(C5(CH3)5), 122.3, 124.0 (CHCH), 185.7 (RuC).
[Cp*RuH(L)] 7. A mixture of 6 (0.63 g, 0.86 mmol) and KOBut
(0.25 g, 2.2 mmol, excess) was placed under dihydrogen atmosphere.
Tetrahydrofuran (15 mL) was added. The mixture was stirred at
room temperature for 1 h. Then, the solvent was removed in vacuo.
The residue was extracted with toluene, and the solution filtered
through Celite. The solvent was removed in vacuo, and the yellow
crystalline solid washed with petroleum ether and dried in vacuo.
Yield: 0.29 g, 81%. Calcd for C19H28N4Ru: C, 55.19; H, 6.82; N,
13.55. Found: C, 55.05; H, 6.96, N, 13.15. IR: ν(RuH) 1780 cm−1.
1
NMR: H NMR (400 MHz, C6D6, 298 K) δ −11.65 (s, 1 H, RuH),
[Cp*Ru(MeCN)(L)][BPh4] 3. Compound 2 (0.45 g, ca. 1 mmol)
was dissolved in MeOH (15 mL) under dinitrogen. Acetonitrile (1
mL) and solid NaBPh4 (0.45 g, 1.3 mmol) was added. A yellow
precipitate was formed. The mixture was stirred at room temperature
for 1 h. The mixture was filtered, and the yellow solids washed with
ethanol and petroleum ether and dried in vacuo. The crude product
was dissolved in acetonitrile, and the solution filtered through Celite.
The solution was layered with MeOH. Slow diffusion afforded well-
formed amber crystals, suitable for X-ray analysis. Yield: 0.59 g, 77%.
Calcd for C45H50N5BRu: C, 69.94; H, 6.52; N, 9.06. Found: C, 70.02;
H, 6.54, N, 9.00. IR: ν(CN) 2252 cm−1. NMR: 1H NMR (500 MHz,
CD3CN, 298 K) δ 1.57 (s, 15 H, C5(CH3)5), 2.15 (s, 3 H, CH3CN),
3.77 (s, 6 H, NCH3), 5.31,5.80 (d, 2JHH = 13.5 Hz, 1 H each, CHaHb),
2.17 (s, 15 H, C5(CH3)5), 3.48 (s, 6 H, NCH3), 4.40, 5.07 (d, 2JHH
=
3
12.0 Hz, 1 H each, CHaHb), 6.13, 6.14 (d, JHH = 2.5 Hz, 2 H each,
CHCH); 13C{1H} NMR (101 MHz, C6D6, 298 K) δ 11.6
(C5(CH3)5), 37.6 (NCH3), 61.7 (CH2), 86.2 (C5(CH3)5), 116.5.1,
118.7 (CHCH), 207.5 (RuC).
[Cp*Ru(CH2CHR)(L)][BPh4] (R = H 8a, COOMe 8b).
Compounds 8a,b were generated in solution either by bubbling
ethylene (for 8a) or adding an excess of methyl acrylate (for 8b) to a
1
solution of 4 in CD3NO2 under argon. 8a: NMR: H NMR (500
MHz, CD3NO2, 298 K) δ 1.64 (s, 15 H, C5(CH3)5), 2.14 (s, 4 H,
C2H4), 3.95 (s, 6 H, NCH3), 5.93, 5.60 (d, 2JHH = 12.5 Hz, 1 H each,
CHaHb), 7.27 (br, 4 H, CHCH); 13C{1H} NMR (125.7 MHz,
CD3NO2, 298 K) δ 10.1 (C5(CH3)5), 39.0 (NCH3), 47.2 (C2H4),
61.7 (CH2), 94.3 (C5(CH3)5), 122.6, 125.3 (CHCH), 185.8 (RuC).
8b: NMR: 1H NMR (500 MHz, CD3NO2, 298 K) δ 1.69
3
7.10, 7.19 (d, JHH = 2.1 Hz, 2 H each, CHCH); 13C{1H} NMR
(125.7 MHz, CD3CN, 298 K) δ 10.8 (C5(CH3)5), 37.1 (NCH3), 62.3
(CH2), 86.93 (C5(CH3)5), 121.6, 126.6 (CHCH), 193.0 (RuC).
[{Cp*Ru(L)}2(μ-N2)][BPh4]2 4. To a mixture of 2 (1.77 g, 3.96
mmol) and NaBPh4 (1.9 g, 5.6 mmol), MeOH (20 mL) was added
under dinitrogen. A yellow precipitate was formed. The mixture was
stirred at room temperature for 12 h. At the end of this time, the
suspension was filtered, and the yellow product was washed
thoroughly with ethanol and petroleum ether and dried in vacuo.
Yield: 2 g, 68%. Calcd for C86H94N10B2Ru2: C, 69.25; H, 6.35; N,
9.39. Found: C, 69.11; H, 6.25, N, 9.10. Raman: ν(NN) 2050
cm−1. NMR: 1H NMR (500 MHz, CD3NO2, 298 K) δ 1.71 (s, 15 H,
C5(CH3)5), 3.32 (s, 12 H, NCH3), 5.99, 5.52 (d, 2JHH = 13.0 Hz, 2 H
3
(C5(CH3)5), 2.70 (d, JHHa = 8.5 Hz, 1 H, CHCHaHb), 2.76 (d,
3JHHb = 10.0 Hz, 1 H, CHCHaHb), 2.86 (s, 3 H, COOCH3), 3.02
3
3
(dd, JHHa = 8.5 Hz, JHHb = 10.0 Hz, 1 H, CHCHaHb), 3.97, 3.98
2
(s, 3 H each, NCH3), 5.65, 5.96 (d, JHH = 12.5 Hz, 1 H each,
CHaHb), 7.18, 7.22, 7.36 (d, 4 H, CHCH); 13C{1H} NMR (125.7
MHz, CD3NO2, 298 K) δ 10.3 (C5(CH3)5), 39.3, 39.6 (NCH3), 50.2
(CHCH2), 51.4 (COOCH3), 52.1 (CHCH2), 62.9 (CH2), 96.2
(C5(CH3)5), 122.5, 123.5, 125.2, 125.6 (CHCH), 177.2
(COOCH3), 180.8, 183.9 (RuC).
[Cp*RuCHPh(L)][BPh4] 9. To solid 4 (0.25 g, 0.17 mmol) or 6
(0.25 g, 0.34 mmol) under argon, tetrahydrofuran (10 mL) and
freshly prepared phenyldiazomethane (2 mmol in toluene, excess) was
added at room temperature. Dinitrogen evolution was observed. After
1 h, the solvent was removed in vacuo, and the residue washed with
diethyl ether and petroleum ether. Recrystallization from THF/
petroleum ether yielded a dark red microcrystalline solid, which was
filtered off, washed with petroleum ether, and dried in vacuo. Yield:
0.19 g, 70%. Calcd for C50H53N4BRu: C, 73.07; H, 6.50; N, 6.82.
Found: C, 73.15; H, 6.39, N, 6.85. NMR: 1H NMR (500 MHz,
CD3NO2, 298 K) δ 1.85 (s, 15 H, C5(CH3)5), 2.87 (s, 6 H, NCH3),
3
each, CHaHb), 7.05, 7.31 (d, JHH = 2.0 Hz, 4 H each, CHCH);
13C{1H} NMR (125.7 MHz, CD3NO2, 298 K) δ 11.0 (C5(CH3)5),
37.1 (NCH3), 63.1 (CH2), 92.1 (C5(CH3)5), 122.9, 124.2 (CH
CH), 185.1 (RuC).
[Cp*Ru(NO)(L)][BPh4]2·MeNO2 5. Compound 4 (0.3 g 0.2
mmol) was dissolved in MeNO2 and the solution filtered through
Celite. The solution was layered with EtOH. On standing at room
temperature for several days, well-formed dark orange crystals were
obtained. The crystals were filtered off, rinsed with petroleum ether
and dried in vacuo. Yield: 0.085 g, 37%. Calcd for C67H67N5B2ORu·
799
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6.29, 5.63 (d, 2JHH = 12.7 Hz, 1 H each, CHaHb), 7.39, 7.42 (d, 3JHH
=
imental details for the crystal structure determination
(Table S1) (PDF)
2.0 Hz, 2 H each, CHCH), 7.61, 7.41 (m, 5 H, C6H5), 15.7 (s, 1 H,
RuCHPh); 13C{1H} NMR (125.7 MHz, CD3NO2, 298 K) δ 10.5
(C5(CH3)5), 37.2 (NCH3), 63.9 (CH2), 102.9 (C5(CH3)5), 122.6,
123.9 (CHCH), 127.8, 129.6, 130.4, 153.9 (C6H5), 184.0 (RuC),
287.2 (RuCHPh).
Accession Codes
CCDC 1483010 and 2056271−2056272 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
[Cp*RuCCCPh2(L)][BPh4] 10. To a mixture of 2 (0.15 g,
0.33 mmol), NaBPh4 (0.3 g, excess) and HCCC(OH)Ph2 (0.1 g,
0.48 mmol) MeOH (15 mL) was added. An orange precipitate was
immediately formed. The mixture was stirred overnight at 60 °C.
During this time, the color of the mixture changed to dark red. The
mixture was cooled to room temperature and then to −20 °C. The
dark red crystalline precipitate was filtered off, washed with ethanol
and petroleum ether, and dried in vacuo. Yield: 0.22 g, 72%. Calcd for
C58H57N4BRu: C, 75.56; H, 6.23; N, 6.08. Found: C, 74.61; H, 6.29,
AUTHOR INFORMATION
Corresponding Author
■
Manuel Jiménez-Tenorio − Departamento de Ciencia de los
1
N, 6.05. IR: ν(CCC) 1879 cm−1. NMR: H NMR (500 MHz,
́
Materiales e Ingeniería Metalurgica y Química Inorgánica-
acetone-d6, 298 K) δ 1.88 (s, 15 H, C5(CH3)5), 3.46 (s, 6 H, NCH3),
INBIO, Facultad de Ciencias, Universidad de Cádiz, 11510
Puerto Real, Cádiz, Spain; orcid.org/0000-0003-4088-
4958; Email: manuel.tenorio@uca.es
5.62, 6.34 (d, 2JHH = 12.7 Hz, 1 H each, CHaHb), 7.22, 7.38 (d, 3JHH
=
3
2.0 Hz, 2 H each, CHCH), 7.28 (t, JHH = 8.1 Hz, 4 H), 7.54 (t,
3JHH = 7.4 Hz, 2 H), 7.69 (d, 3JHH = 8.1 Hz) (C6H5); 13C{1H} NMR
(125.7 MHz, acetone-d6, 298 K) δ 10.7 (C5(CH3)5), 37.6 (NCH3),
62.9 (CH2), 101.8 (C5(CH3)5), 122.2, 123.5 (CHCH), 128.7,
129.3, 129.6, 145.9 (C6H5), 139.8 (RuCCC), 178.0 (RuC),
225.6 (RuCCC), 274.9 (RuCCC).
Authors
José Manuel Botubol-Ares − Departamento de Química
Orgánica-INBIO, Facultad de Ciencias, Universidad de
Cádiz, 11510 Puerto Real, Cádiz, Spain; orcid.org/0000-
0002-2312-612X
Safa Cordón-Ouahhabi − Departamento de Química
Orgánica-INBIO, Facultad de Ciencias, Universidad de
Cádiz, 11510 Puerto Real, Cádiz, Spain
Zakaria Moutaoukil − Departamento de Química Orgánica-
INBIO, Facultad de Ciencias, Universidad de Cádiz, 11510
Puerto Real, Cádiz, Spain
Isidro G. Collado − Departamento de Química Orgánica-
INBIO, Facultad de Ciencias, Universidad de Cádiz, 11510
Puerto Real, Cádiz, Spain; orcid.org/0000-0002-8612-
0593
M. Carmen Puerta − Departamento de Ciencia de los
Materiales e Ingeniería Metalurgica y Química Inorgánica-
INBIO, Facultad de Ciencias, Universidad de Cádiz, 11510
Puerto Real, Cádiz, Spain
Pedro Valerga − Departamento de Ciencia de los Materiales e
Ingeniería Metalurgica y Química Inorgánica-INBIO,
Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto
Real, Cádiz, Spain
General Procedure for the Catalytic Transfer Hydrogena-
tion of Carbonyl Compounds. To a solution of the corresponding
ruthenium catalysts 3, 4, or 6 (0.005 mmol) and KOH (0.05 mmol)
in degassed isopropanol (1 mL) under argon, the corresponding
ketones 11−22 or aldehydes 23−25 (1 mmol) were added. The
reaction mixture was stirred at 80 °C for 2 h and then evaluated by
TLC and GC-MS. Solvent was evaporated under reduced pressure
and the crude was purified by silica gel column using petroleum ether
and ethyl acetate mixtures to afford compounds 11a−25a. Full
characterization data for each of the isolated organic products are
given in the Supporting Information.
Crystal Structure Analysis. Crystals suitable for X-ray structural
determination were mounted on glass fibers and then transferred to a
Bruker Smart CCD three-circle diffractometer with a sealed-tube
source and graphite-monochromated Mo Kα radiation (λ = 0.71073
́
́
Å) at the Servicio Central de Ciencia y Tecnologia de la Universidad
́
́
de Cadiz (1) or Servicios Tecnicos de Investigacion de la Universidad
de Alicante (3 and 5). In each case, three (3 and 5) at 298 K or four
at 100 K (1) sets of frames were recorded over a hemisphere of the
reciprocal space by ω scans with δ(ω) = 0.30° and exposure of 10 to
20 s per frame. In the case of 3 and 5, an additional run at ϕ = 0° of
100 frames was collected to improve redundancy. The diffraction
frames were integrated using the program SAINT43 and the integrated
intensities were corrected for Lorentz-polarization effects with
SADABS.44 An insignificant crystal decay correction was also applied.
The structure of 1 was solved by Patterson method, and direct
methods were used for 3 and 5. All structures were refined on F2 by
full-matrix least-squares (SHELX97)45 using all unique data. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed at idealized positions and refined as rigid atoms. ORTEP was
used for plotting.46 CCDC 1483010, 2056271−2056272 contain
supplementary 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.
́
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00045
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by “Programa de fomento e
impulso de la investigación y la transferencia” from University
of Cadiz (PR2017-047), and by Junta de Andalucía (PAIDI
FQM188 and FQM 295). We wish to thank Dr. Miguel Angel
Centeno, from the ICMSE-CSIC (Sevilla) for recording the
Raman spectra, and Rocío González Moya for efficient
assistance in laboratory experiments.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00045.
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