Synthesis and Characterization of New (η5-Cyclopentadienyl)dicarbonylruthenium(II) Amine Complexes: Their Application as Homogeneous Catalysts in Styrene Oxidation

Article abstract of DOI:10.1021/acs.organomet.5b00564

The water-soluble ruthenium(II) mononuclear complexes [CpRu(CO)₂NH₂R]BF₄ (Cp = η⁵-C₅H₅; R = C₆H₁₁ (1), C₆H₅ (2), CH₂C₆H₅ (3), CH(CH₃)C₆H₅ (4), CH₂(C₆H₄O)CH₃ (5), CH₂(C₆H₄)CN (6), C₆H₂(CH₃)₃ (7), CH₂CHCH₂ (8), CH(CH₃)₂ (9)) were synthesized from the reaction of the organometallic Lewis acid [CpRu(CO)₂]BF₄ with amine ligands at room temperature. These complexes are reported for the first time and have been fully characterized by IR, high-resolution mass spectrometry, ¹H and ¹³C NMR spectroscopy, and elemental analysis. Spectral data show that the amines are σ-bonded to the metal center via the nitrogen atom. The crystal structures of complexes 3 and 8 were determined by single-crystal X-ray crystallography. The 4-methoxybenzylamine, 4-aminomethylbenzonitrile and allylamine groups preferentially bind to the metal center via the amine nitrogen. The ruthenium complexes 1, 3-6, 9, and [CpRu(CO)₂NH₂CH₃]BF₄ (10) and the dinuclear complex [CpRu(CO)₂NH₂(CH₂)₆NH₂(CO)_2-RuCp][BF₄]₂ (11) demonstrated excellent catalytic activity in the oxidation of styrene using NaIO₄ as the co-oxidant with over 95% conversion and benzaldehyde yields, respectively, in some cases.

Full text of DOI:10.1021/acs.organomet.5b00564

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of New
(η5‑Cyclopentadienyl)dicarbonylruthenium(II) Amine Complexes:
Their Application as Homogeneous Catalysts in Styrene Oxidation
Eunice A. Nyawade, Holger B. Friedrich,* Bernard Omondi, and Philani Mpungose
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa
S
* Supporting Information
ABSTRACT: The water-soluble ruthenium(II) mononuclear
complexes [CpRu(CO)2NH2R]BF4 (Cp = η5-C5H5; R =
C6H11 (1), C6H5 (2), CH2C6H5 (3), CH(CH3)C6H5 (4),
CH2(C6H4O)CH3 (5), CH2(C6H4)CN (6), C6H2(CH3)3 (7),
CH2CHCH2 (8), CH(CH3)2 (9)) were synthesized from the
reaction of the organometallic Lewis acid [CpRu(CO)2]BF4
with amine ligands at room temperature. These complexes are
reported for the first time and have been fully characterized by
IR, high-resolution mass spectrometry, 1H and 13C NMR
spectroscopy, and elemental analysis. Spectral data show that the amines are σ-bonded to the metal center via the nitrogen atom.
The crystal structures of complexes 3 and 8 were determined by single-crystal X-ray crystallography. The 4-methoxybenzylamine,
4-aminomethylbenzonitrile and allylamine groups preferentially bind to the metal center via the amine nitrogen. The ruthenium
complexes 1, 3−6, 9, and [CpRu(CO)2 NH2 CH3]BF4 (10) and the dinuclear complex [CpRu-
(CO)2NH2(CH2)6NH2(CO)2‑RuCp][BF4]2 (11) demonstrated excellent catalytic activity in the oxidation of styrene using
NaIO4 as the co-oxidant with over 95% conversion and benzaldehyde yields, respectively, in some cases.
oxidation of the products may result in the formation of
carboxylic acids.
INTRODUCTION
■
The development of the organometallic chemistry of ruthenium
complexes is of great interest due to their multiple applications
in many different scientific fields, ranging from biomedicine to
catalysis.1−4 Ruthenium complexes containing the cyclo-
pentadienyl group have been the subject of investigation by
many research groups during the past few decades because of
their widespread applications in transition-metal-catalyzed
organic syntheses.5−11 Extensive studies have been carried out
on methods for specific catalytic oxidations of various organic
substrates such as alcohols, amines, amides, and hydrocarbons
using low-valent ruthenium complexes.12,13
Oxidation catalysis represents the core of a variety of useful
chemical processes for producing bulk and fine chemicals. One
such process is the oxidation of olefins, which can give a variety
of products depending on the conditions of the reaction. Olefin
oxidation, for example, can take place by epoxidation,
dihydroxylation, vinylic and allyllic oxidation, or CC double
bond cleavage (Scheme 1). The oxidative cleavage of a CC
double bond involves cleaving the double bond with inclusion
of oxygen into the two fragments, forming aldehydes and/or
ketones, depending on the extent of substitution. Over-
Oxidative cleavage of olefinic double bonds to carbonyl
compounds can be achieved by ozonolysis and stoichiometric
oxidation processes.14−16 It can be effected by conversion of
olefins into 1,2-diols, followed by cleavage with NaIO4 or other
co-oxidants,17 or by direct cleavage into a variety of
functionalized products, depending on the workup condi-
tions.18−20 The direct oxidative cleavage of olefinic double
bonds is largely achieved at the industrial level by the use of
ozone. The large-scale use of ozone for this application has
major drawbacks, because of safety issues and the need to use
expensive equipment.21 There are relatively few alternative
reactions that duplicate the direct cleavage of olefins without
formation of 1,2-diols.22,23 Stoichiometric amounts of some
transition-metal compounds have been successfully used in the
oxidative cleavage of olefinic double bonds. KMnO4, for
instance, oxidizes olefins to aldehydes24 or carboxylic acids25−27
in water as the solvent, but the organic substrates formed are
often poorly soluble in the aqueous reaction medium. In
addition, KMnO4 cannot be used in catalytic amounts due to
the irreversible precipitation of MnO2 after oxidation of the
substrate. On the other hand, chromyl chloride (CrO2Cl2)28
oxidizes styrene into benzaldehyde and phenylacetaldehyde29
but its use is discouraged due to the high toxicity of
chromium(VI) salts. Osmium tetroxide, OsO4, has been
Scheme 1. General Illustration of the Oxidative Cleavage of
Substituted Alkenes
Received: May 19, 2015
© XXXX American Chemical Society
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successfully used in stoichiometric amounts and as a catalyst in
the oxidation of olefins to diols and carboxylic acids, but the
main drawback in its use is its high toxicity.30,31
Simple ruthenium compounds, such as RuCl3·nH2O, RuO2,
and RuO4, have been successfully used as catalysts for the direct
oxidative cleavage of the olefinic double bonds to form
aldehydes and carboxylic acids. They form a variety of reactive,
high-valent oxo complexes with rich redox chemistry. RuCl3·
nH2O, for instance, has been used as a catalyst for the oxidative
cleavage of alkenes to acids.22,32,33 To achieve the cleavage,
NaIO4, NaOCl, and oxone (KHSO5) were used as co-oxidants
in different solvent systems. Most of the oxidative cleavage
reactions of the olefinic double bonds gave carboxylic acids, but
for the reaction where oxone was the primary oxidant with
NaHCO3, benzaldehyde was formed.22 In addition, RuO2·
2H2O and 2−6 equiv of NaIO4 in CCl4/H2O have been used to
cleave electron-poor cyclic enolic olefins (CC bond on the β-
position) to aldehydes in 35−95% yield.34
The use of catalytic amounts of ruthenium complexes in the
oxidative cleavage of olefinic double bonds improves selectivity
toward the cleavage products, while preventing side reactions
such as epoxidation, dihydroxylation, and allylic oxidation. The
coligand coordinated to the metal plays a major role in
improving the catalytic activity as well as the selectivity of the
catalyst. A few ruthenium complexes, as oxidation catalysts,
have demonstrated good selectivity, leading to the formation of
CC double bond cleavage products. For example, the
complex cis-[RuCl2(bipy)2]·2H2O was reported to be a good
catalyst in the chemoselective degradation of aromatic rings to
acids, with NaIO4 as the co-oxidant, and oxidative cleavage of
alkenes to acids, with IO(OH)5 as the co-oxidant, in a CCl4−
MeCN−H2O biphasic solvent system.35 The ruthenium
complex [cis-RuII(dmp)2(H2O)2]2+ (dmp = 2,9-dimethylphe-
nanthroline) demonstrated good catalytic activity in the
oxidative cleavage of terminal alkenes to aldehydes using
H2O2 as the oxidant in acetonitrile at 55 °C.36 Even though
ruthenium complexes are more efficient and selective catalysts
for the oxidative cleavage of alkenes to aldehydes than
ruthenium salts and oxides, little attention has been given to
their catalytic activity. We have studied (η5-cyclopentadienyl)-
iron and -ruthenium carbonyl complexes with nitrogen-
containing species in order to develop their chemistry and
also to study their catalytic and antimicrobial activities. The
synthesis and characterization of the (η5-cyclopentadienyl)-
dicarbonyliron and -ruthenium complexes of n-alkanamines,37
α,ω-diaminoalkanes,38 and N-heterocyclic ligands (DABCO,
HMTA, and methylimidazole) were reported.39,40
Figure 1. Ruthenium(II) complexes synthesized and catalytic activity
tested in the homogeneous oxidation of styrene.
CH2(C6H4O)CH3 (5), CH2(C6H4)CN (6), C6H2(CH3)3 (7),
CH2CHCH2 (8), CH(CH3)2 (9)) (Figure 1) were formed in
good yields from the reaction of the organometallic Lewis acid
[CpRu(CO)2]BF4 with the respective amines at room
temperature. The complexes form white, air-stable crystals
which are generally soluble in polar solvents such as water,
methanol, acetone, acetonitrile, and dimethyl sulfoxide and in
the chlorinated solvents chloroform and methylene chloride in
some cases but are insoluble in nonpolar solvents such as
hexane and diethyl ether. The elemental analysis and high-
resolution mass spectrometry results for the complexes
correlate with the calculated values; this is an indication that
the complexes were formed as predicted.
1
The H NMR data for the complexes were obtained and
peaks were assigned using 2D NMR, as well as D2O tests for
1
the amine protons. The H NMR spectra for the complexes in
CDCl3, CD3CN, and D2O showed a sharp singlet peak in the
region ca. 5.60−5.75 ppm, assignable to the five equivalent Cp
protons. This is an indication that the ligands in complexes 5, 6,
and 8, which have two possible coordination sites, bonded to
the metal center via only one of the sites. Coordination to both
sites would have shown either a single peak or two peaks in the
same region, ca. 5.60−5.75 ppm, but with peaks assignable to
10 protons for the Cp peaks in the 1H NMR spectrum.
Regioselective coordination of the metal to the NH2 is clearly
indicated by the downfield shift in the amine proton peaks, in
1
the H NMR spectra of compounds 5, 6, and 8, relative to
those of the uncoordinated ligands. For complexes 1, 8 and 9, a
1
singlet peak in the H NMR spectra obtained in CDCl3 was
observed at ca. 3.70, 3.79, and 3.78 ppm, respectively, and
assigned to the two amine protons. The amine proton signals in
the complexes were observed more downfield in comparison to
those of the free ligand peaks (approximately 2 ppm), probably
due to the deshielding by the metal center; this is an indication
of coordination of the amines to the metal center through the
N atom.37 Hydrogen bonding between the amine group
protons and the fluoride atoms of the tetrafluoroborate
counteranion can also contribute to the downfield shift of the
We now report the synthesis of nine new water-soluble
ruthenium(II) complex salts, [(η5-C5H5)Ru(CO)2NH2R]Y (Y
= BF4, CF3SO3; R = C6H11 (1), C6H5 (2), CH2C6H5 (3),
CH(CH3)C6H5 (4), CH2(C6H4O)CH3 (5), CH2(C6H4)CN
(6), C6H2(CH3)3 (7), CH2CHCH2 (8), CH(CH3)2 (9))
(Figure 1), their catalytic activity, and also the catalytic activity
of CpRu(CO)2 NH2 CH3 ]BF4 (10) and [CpRu-
(CO)2NH2(CH2)6NH2(CO)2RuCp][BF4]2 (11) in the oxida-
tion of styrene with NaIO4. To the best of our knowledge, these
ruthenium complexes and their catalytic activity in the
oxidation of alkenes have not been reported previously.
1
amine protons in the H NMR spectra. The amine proton
peaks for complexes 2, 3, and 5−7 were observed more
downfield than those for 1, 8, and 9 due to the electron-
withdrawing effect of the aromatic ring. It is worth noting that,
whereas the amine group proton peak for the free ligand in
complex 4 was observed as a singlet peak at ca. 1.59 ppm, two
singlet peaks were observed at ca. 4.28 and 4.01 ppm for the
1
complex in the H NMR spectrum in CD3CN. This is an
indication that the two NH2 protons are diastereotopic. The
reason only one NMR signal is observed for the NH2 protons
in the free amine is rapid inversion on the N center, which
cannot happen when the amine is coordinated to Ru. The H
NMR spectrum of complex 8 shows well-resolved characteristic
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The mononuclear
complexes [CpRu(CO)2NH2R]BF4 (Cp = η5-C5H5; R =
C6H11 (1), C6H5 (2), CH2C6H5 (3), CH(CH3)C6H5 (4),
1
B
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olefinic peaks; a multiplet peak assignable to the single proton
attached to the β-carbon atom was observed at ca. 5.88 ppm,
and two doublets were observed at ca. 5.42 and 5.27 ppm, each
integrating for one proton, assignable to the two nonequivalent
protons Ha and Hb, respectively, on the γ-carbon atom (Figure
2). These values are within the range reported for the
Molecular structure of complexes 3 and 8. Crystals for
complexes 3 and 8 suitable for single-crystal X-ray analysis were
obtained by layering with diethyl ether an acetonitrile solution
of each. Their molecular structures were determined by X-ray
diffraction, giving the molecular structures shown in Figures 3
Figure 2. Structure of the 3-aminopropyl-1-ene (allylamine)
ruthenium complex 8.
Figure 3. Molecular structure of complex 3 showing the atomic
numbering scheme. Displacement ellipsoids are drawn at the 50%
probability level, and H atoms are not shown for clarity.
analogous iron complexes.39 The protons on the α-carbon
atom exhibited a quartet at ca. 3.30 ppm, a 0.35 ppm shift
downfield from that reported for the iron complex analogue.39
The 13C NMR spectra of the complexes show two peaks at
ca. 87.8 and 195.7 ppm, corresponding to the five equivalent
Cp carbons and two identical carbonyls, respectively. Unlike
the rest of the complexes, the 13C spectrum of complex 4 shows
two Cp peaks at 87.97 and 87.95 ppm and two CO peaks at
195.99 and 195.59 ppm. This is a further indication that the Cp
and CO ligands are diastereostopic. The 13C NMR spectrum of
8 clearly show peaks corresponding to the allylic carbon atoms
at ca. 135.17 and 118.57 ppm for the β- and γ-carbon atoms,
respectively.
The IR spectra of complexes 1−9 showed ν(CO) absorption
bands in the expected region for terminal carbonyl groups for
amine-coordinated metal complexes.37,39−41 The ν(CO)
absorption bands were observed as two strong peaks, one in
the region ca. 2061−2049 cm−1 for the asymmetric CO
stretching vibrations and one at ca. 2016−1987 cm−1 for
symmetric CO stretching vibrations. It is worth noting that the
symmetric CO stretching frequency for the cyclohexylamine
complex 1 is lower than those observed for complexes 2−7,
which have the phenyl ring as a substituent of the amine group.
This can be attributed to the fact that the cyclohexyl group, a
strong electron donor, increases electron density at the
ruthenium center from the amine σ donation. The electrons
are thus passed to the CO π* orbital, and this is reflected in a
decreased ν(CO) stretching frequency, which corresponds to
weaker CO bonds. The symmetric ν(CO) stretching frequency
for complex 6 is greater than that for complex 5, probably
because the nitrile group substituent in the para position of the
phenyl ring in 6 is electron withdrawing, while the methoxy
group, a para substituent in the phenyl ring in complex 5, is
electron donating.
Figure 4. Molecular structure of complex 8 showing the atomic
numbering scheme. Displacement ellipsoids are drawn at the 50%
probability level, and H atoms are not shown for clarity.
and 4. Important bond distances and angles for both complexes
are given in Table 1. Complex 3 crystallizes with four
nonsymmetrically related molecules in its asymmetric unit:
two cationic molecules, [CpRu(CO)2C6H5CH2NH2]+, and two
−
counteranions, BF4 . However, complex 8 crystallizes with two
The IR spectra also show two characteristic absorption bands
in the regions ca. 3325−3270 and 3293−3264 cm−1, which
correspond to the NH2 asymmetric and symmetric peaks,
respectively. These bands are found at wavenumbers lower than
those for the uncoordinated ligands, due to coordination of the
amine functionality to the metal center and likely due to the
participation of the NH2 protons in hydrogen bonding.
molecules in its asymmetric unit: one cationic molecule,
[CpRu(CO)2CH2CHCH2NH2]+, and one counteranion.
Coordination around the ruthenium center is the same in
both complexes. The amine ligands coordinate to the
ruthenium centers of the CpRu(CO)2 moieties via the nitrogen
atoms by σ bonds. The Cp ligand and the two carbonyl ligands
C
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(Figure 5). At 60 °C, a homogeneous mixture is obtained and
more than 99% conversion and 85% benzaldehyde and 2%
Table 1. Selected Interatomic Distances (Å) and Angles
(deg) for Complexes 3 and 8
3
a
bond param
molecule 1
molecule 2
8
Cg−Ru
1.866
1.867
1.870
Ru−N
N−C
NC−C
CC
2.146(1)
1.488(2)
1.506(3)
2.138(2)
1.479(3)
1.505(3)
2.140(1)
1.486(2)
1.492(3)
1.316(3)
124.11
Cg−Ru−N
Ru−N−C
N−C−C
Cg−Ru−N−C
Ru−N−C−C
122.68
122.84
118.9(1)
112.3(2)
150.10
119.4(1)
113.0(2)
162.57
115.60(2)
113.1(1)
71.40
173.4(1)
176.1(1)
179.3(1)
a
Cg is the centroid of the Cp ring.
Figure 5. Effect of temperature on the conversion and yield for the
ruthenium complex catalyzed oxidation of styrene. Reaction
conditions: 0.478 mmol of styrene, 3 equiv of NaIO4, 2.5 mol % of
complex 1, CH3CN/H2O (3 mL/3 mL). The time to achieve the
highest conversion is given in hours.
occupy the remaining coordination sites resulting in distorted-
octahedral coordination geometries around the ruthenium
centers. Within these geometries the Cp ligands occupy three
coordination sites and the remaining three sites are occupied by
the two carbonyl ligands and the amine in what is normally
referred to as a “pseudo-octahedral three-legged piano stool”. In
the stool the Cp ligands occupy the apical position with the
carbonyl and amine ligands serving as the legs. Bond
parameters in the two complex salts in 3 and in 4 are similar.
The Ru−N bond lengths are comparable to similar Ru−N
bond lengths of previously reported (η5-cyclopentadienyl)-
dicarbonylruthenium(II) n-alkanamine complexes: 2.139(1) Å
for [CpRu(CO)2NH2CH2CH3]BF4, 2.1406(12) Å for [CpRu-
(CO)2NH2(CH2)2CH3]BF4, and 2.139(1) Å for [CpRu-
(CO)2NH2(CH2)3CH3]BF4.37 The coordinated allylamine
bond lengths (N−C, 1.484(8) Å; C−Callyl, 1.498(8) Å; C
C, 1.300(9) Å) in the iron complex CpFe-
(CO)2NH2CH2CHCH2]BF4 are comparable to those found
for complex 8. This implies that the metal center does not affect
the bond lengths within the allylamine ligand. As previously
observed for CpFe(CO)2NH2CH2CHCH2]BF4,39 the CC
bond distance in the allylamine coordinated to the ruthenium
center is slightly shorter than the formal CC double bond
(1.34 Å)40,42 but is comparable with the calculated distances for
a terminal CC bond: 1.315 Å for ethylene, 1.316 Å for
propene, and 1.321 Å for 2-methylpropene.43 The bond angles
of complex 3 and 8 are comparable to those of (η5-
cyclopentadienyl)dicarbonylruthenium(II) amine complexes:
Cg−Ru−N, 122.92−124.08 Å; Ru−N−C, 113.2−118.53 Å.37
In the crystal structures of complexes 3 and 8, the cations are
linked to the anions through a series of weak N−H···F and C−
H···O intermolecular interactions that stabilize the crystal
lattices.
styrene oxide yields are achieved in 3 h. The temperature of 60
°C was chosen for all catalytic reactions, since it gave a
homogeneous mixture and high conversion in a shorter time.
The oxidative cleavage of styrene with 3 equiv of NaIO4 in
the absence of the ruthenium complexes affords 32%
conversion and 21% benzaldehyde yield in 10 h, while in the
presence of 2.5 mol % of the ruthenium complexes, the
conversion and benzaldehyde yield rise to 99% and 85%,
respectively, in 3 h (Figure 6).
Figure 6. Effect of catalyst concentration on the oxidative cleavage of
the olefinic double bond in styrene. Reaction conditions: 0.478 mmol
of styrene, 3 equiv of NaIO4, x mol % of complex 1, CH3CN/H2O (1/
1; 6 mL) at 60 °C. The time taken to achieve the highest conversion is
given in hours.
Catalytic Oxidation of Styrene. Oxidative cleavage of the
olefinic carbon−carbon bond in styrene has been achieved in
very good conversions and benzaldehyde yields using sodium
periodate (NaIO4) as the co-oxidant and the (η5-
cyclopentadienyl)dicarbonylruthenium(II) complexes as cata-
lysts. Acetonitrile/water (1/1 v/v; 6 mL) was selected as the
solvent system, because of the need to have all the reagents and
catalysts in the same phase. Complex 1 was used as the model
catalyst for optimization studies. The solubility of the substrate
and oxidant and, hence, homogeneity of the system depends on
temperatures higher than ambient.
This is a clear indication that the ruthenium complexes
reported herein are effective catalysts for the oxidative cleavage
of the olefinic carbon−carbon bond in styrene (Scheme 2).
It was noted that the conversion, yield, and time needed for
the reaction to go to completion are proportional to the
amount of the co-oxidant used. Studies done on the effect of
co-oxidant concentration on the reaction progress indicate that
an excess of the co-oxidant is necessary for the reaction to give
100% conversion (Figure 7).
At 22 and 40 °C, more than 99% conversion and 90%
benzaldehyde yield is achieved in 18 and 17 h, respectively
This observation augments the step in the proposed
mechanism where the intermediate product, the metal diether,
D
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complex catalysts was compared with that of NaIO4. It was
observed that NaIO4 gave excellent conversion and yield under
the reaction conditions to afford benzaldehyde. Hydrogen
peroxide as an oxidant gave good conversion (86%) but low
yields of benzaldehyde (23%) and styrene glycol (9%) in 18 h
(Figure 8). The rest of the product was confirmed to be
Scheme 2. General Equation for the Oxidative Cleavage of
Styrene to Benzaldehyde
Figure 8. Oxidation products from different oxidants for the
ruthenium complex catalyzed oxidation of styrene. Reaction
conditions: 0.478 mmol of styrene, 3 equiv of oxidant, 2.5 mol % of
complex 1, CH3CN/H2O (3 mL/3 mL) at 60 °C. The time taken to
achieve the highest conversion is given in hours.
Figure 7. Effect of NaIO4 concentration on the percent conversion
and percent benzaldehyde yield for the ruthenium complex catalyzed
oxidation of styrene. Reaction conditions: 0.478 mmol of styrene, x
equiv of NaIO4, 2.5 mol % of complex 1, CH3CN/H2O (3 mL/3 mL)
at 60 °C. The time taken to achieve the highest conversion is given in
hours.
benzoic acid, an indication that overoxidation of benzaldehyde
had taken place. The styrene glycol is probably formed as an
intermediate product which is eventually oxidized to
benzaldehyde.44 The reaction with K2S2O8 as an oxidant, for
the oxidation of styrene, was noted to have stopped at 2 h,
giving a maximum of 40% conversion. NaIO4 was thus retained
as the best primary oxidant for the oxidative cleavage of the
olefinic C−C bond.
The effect of basic, acidic, and neutral conditions on the
catalytic oxidation of styrene was investigated and revealed that
the conversion of styrene is highest under neutral and acidic
conditions (Figure 9). This implies that a base is not suitable,
while an acid is not essential for the reaction to proceed to
completion. Thus, the environmentally more friendly neutral
conditions were selected for all other tests.
requires “oxygen” to form the benzaldehyde (Scheme 3).
Hence, 3 equiv of NaIO4 was selected as the optimum amount
of oxidant suitable for the process.
The use of hydrogen peroxide and K2S2O8 as co-oxidants in
the oxidation of styrene in the presence of the ruthenium
Scheme 3. Proposed Mechanism for the Ruthenium(II)
Complex Based Catalyzed Oxidation of Styrene44
Once 100% styrene conversion is achieved, the reaction
mixture turns from colorless to orange. The catalyst present in
Figure 9. Effect of acidic, basic, and neutral conditions on the
ruthenium complex catalyzed oxidation of styrene. Reaction
conditions: 0.478 mmol of styrene, 3 equiv of NaIO4, 2.5 mol % of
complex 1, CH3CN/H2O (1/1; 6 mL) at 60 °C. The time taken to
achieve the highest conversion is given in hours.
E
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the mixture at the end of the reaction was shown to have the
ability to catalyze the oxidation of fresh styrene to give 100%
conversion and more than 99% benzaldehyde yield. Further-
more, complete conversion was achieved within a shorter time
with the “reused” catalyst in comparison to that when fresh
catalyst was used. This may imply that the (η5-
cyclopentadienyl)dicarbonylruthenium(II) complex is con-
verted to a catalytically active intermediate which is not
deactivated at the end of the reaction. It is worth noting that,
when further catalytic tests were consecutively done on the
resulting mixture, the catalytic activity gradually decreased. This
may be an indication that the catalytic species present in the
mixture gradually deactivates/decomposes.
The catalytic activity of the dark orange oil obtained from the
reaction mixture, in this study, was tested. Gas chromatographic
analysis done on the mixture immediately after mixing showed
that the reaction was very fast, since the conversion of styrene
already was greater than 98%, with an 84% yield to
benzaldehyde determined after 1 h (Figure 10). This is a
clear indication that the dark orange compound is catalytic in
nature.
herein and those used by Goldstein and Drago. This peak may,
therefore, be assigned to a dioxoruthenium(VI) complex
species formed as an intermediate in the catalytic oxidation of
styrene. It is worth noting that the peak at 360 nm in the UV−
vis spectrum was absent in the spectra of the fresh catalyst and
the reaction mixtures sampled at 30 min intervals (Figure 11).
This implies that the dioxoruthenium species is highly active
and hence is only seen in solution once styrene is depleted
(Scheme 3). It is worth noting that the peaks observed by
Goldstein and Drago at 395 and 425 nm assignable to RuIII and
RuIVO, respectively, were absent in the UV−vis spectra of
the recovered catalyst and the reaction mixtures examined
(Figure 11). This could imply that, if the catalytic oxidation of
styrene involves the formation of RuIII and the postulated
RuIVO species, these are very rapidly oxidized to RuVIO
and hence are not detected.
The solid-state IR spectrum obtained for the recovered
catalyst, from complex 1, showed new peaks at 793, 769, and
728 cm−1 possibly assignable to the symmetric and asymmetric
stretches of RuO bonds of Ru(VI) dioxo species.46 No peaks
were observed in the carbonyl region (between 2056 and 1998
cm−1), an indication that the recovered catalyst does not
contain coordinated CO ligand. A 1H NMR spectrum obtained
for the recovered catalyst showed a signal at ca. δ 5.67 ppm
assignable to the five Cp protons and a peak at ca. δ 4.00 ppm
assignable to the amine nitrogen protons. Since the proposed
Ru(VI) dioxo species (d2) would be paramagnetic in nature, the
observed Cp and amine nitrogen proton signals in the 1H NMR
spectrum suggest that there is a diamagnetic species present.
Table 2 shows the results for the catalytic oxidation by 0.5
mol % of catalyst of the various ruthenium complexes under the
optimized conditions.
Table 2. Ruthenium Complex (0.5 mol %) Catalyzed
Oxidative Cleavage of the Styrene Olefinic Bond
Figure 10. Catalytic activity of the orange compound obtained from
the reaction mixture.
a
b
c
complex time (h)
yield (%) conversn (%)
TON TOF (h−1
)
The UV−vis spectrum obtained for the dark orange
compound recovered at the end of the reaction, with model
catalyst, showed a peak at ca. 360 nm (Figure 11). A similar
1
10
14
12
3
86
81
87
99
94
98
87
99
21
100
91
172
162
174
198
200
188
174
198
−
17
11
14
66
65
12
19
99
−
3
4
100
99
5
6
3
100
100
98
9
16
9
10
11
No
2
99
10
32
a
Reaction conditions: 0.478 mmol of styrene, 3 equiv of NaIO4,
CH3CN/H2O (1/1; 6 mL) at 60 °C. The time taken to achieve the
highest conversion is given in hours. Conversion.
b
c
All the complexes tested afforded more than 90% styrene
conversion and over 80% benzaldehyde yield. It is noteworthy
that the oxidation process in the presence of the ruthenium
dinuclear complex 11 has the highest turnover frequency
(TOF) of 99 h−1. This implies that the ruthenium center is
directly involved in the oxidation process, since the mole
percent in terms of ruthenium is twice as high as those for the
mononuclear complexes; probably the oxo species are formed
at both metal centers, thus doubling the catalysis rate. It is
interesting to note that the mononuclear complexes containing
4-methoxybenzylamine (5) and 4-aminomethylbenzonitrile
(6), with the ligand in 5 being more basic than that in 6,
demonstrated very good activities with almost equal TOFs of
Figure 11. Electronic spectra of samples obtained at different times in
the catalyzed reaction using 2.5 mol % of complex 1.
peak was observed by Goldstein and Drago, at 365 nm, and was
assigned to the RuVIO species.45 Their study was based on
the hydroxylation of methane by hydrogen peroxide as the co-
oxidant and cis-[Ru(dmp)2S2](PF6) (dmp = 2,9-dimethyl-1,10-
phenanthroline, S = Me3CN, H2O) as the catalyst.
The difference in peak positions may be attributed to the
different coordination environments of the complexes reported
F
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Article
66 and 65 h−1, respectively. In contrast, the TOF for the
benzylamine complex (3) was noted to be 11 h−1; the phenyl
ring increases the basicity of the amine, thus increasing the π-
back-bonding effect on the CO ligand and eventually
strengthening the M−CO bond. This would decrease the rate
at which the CO ligand is lost in the Ru(II) to Ru(VI)
oxidation step, hence decreasing the reaction rate. This
observation implies that the loss of CO, which is dependent
on the electronic effect of the ligand, and the oxidation of
Ru(II) to Ru(VI) steps may be rate determining in forming the
working catalyst in the oxidative cleavage of the styrene olefinic
double bond. It is worth noting that low TOFs, 12 and 14 h−1,
were observed for the complexes containing isopropylamine
(9) and α-methylbenzylamine (4), respectively, in comparison
to 19 h−1 observed for the methylamine-containing complex
(10). This may be due to, in addition to the aforementioned
factors, steric hindrance on the metal center caused by the
isopropylamine and α-methylbenzylamine groups, thus block-
ing the accessibility of the metal center by the co-oxidant and
the substrate.
The turnover frequencies observed in this study are
comparable to those reported by Kogan and co-workers,36
who reported 1 mol % of the complex [cis-RuII(dmp)2(H2O)2]-
(PF6)2 as catalyst, but 10 equiv of hydrogen peroxide as
oxidant, in a MeCN/H2O solvent mixture at 60 °C, in the
oxidation of styrene to benzaldehyde. They noted a TON of 92
and TOF of 15 h−1. Catalytic oxidation studies of styrene and
stilbene performed by Yang and co-workers using 3.5 mol % of
RuCl3 as a catalyst and oxone as oxidant in a basic medium at
60 °C revealed a TON of 21 and TOF of 42 h−1.22
clearly implies that the complexes synthesized in this study are
likely excellent and versatile catalysts for the oxidative cleavage
of olefinic carbon bonds to form aldehydes.
CONCLUSION
■
The (η5-cyclopentadienyl)dicarbonylruthenium(II) amine com-
plexes [CpRu(CO)2NH2R]BF4 have been successfully synthe-
sized. The amine ligands are linked to the ruthenium center by
a σ bond through the nitrogen atom. The 4-methoxybenzyl-
amine, 4-aminomethylbenzonitrile, and allylamine groups
preferentially bind to the metal center via the amine nitrogen.
Some of the complexes have exhibited excellent catalytic
activity in the oxidation of styrene with NaIO4 to give up to
99% benzaldehyde yield, an increase from 21% achieved in the
absence of catalyst. The “pseudo-octahedral three-legged piano
stool” structure exhibited by the complexes seems to favor the
formation of the cis-dioxoruthenium species required for the
formation of the important intermediate 2 + 3 metal diether
ring.23 The configuration displayed by the ruthenium
complexes favors the oxidative cleavage of the olefinic C−C
bond of styrene and thus the formation of benzaldehyde in high
yield. Bulky amine ligands seem to reduce the catalytic activity
of the complexes. The recovered catalyst is very active in the
oxidative cleavage of the olefin double bond of styrene.
EXPERIMENTAL SECTION
■
General Methods and Materials. Standard Schlenk techniques
were employed for all reactions. Chemical reagents and solvents were
obtained from the suppliers shown in parentheses. Reagent grade Et2O
(Merck) was distilled from sodium/benzophenone and stored over
molecular sieves; dichloromethane was distilled from phosphorus(V)
oxide. Silver tetrafluoroborate (Alfa Aesar), ruthenium trichloride
hydrate (DLD-Aldrich), dicyclopentadiene, cyclohexylamine, aniline,
benzylamine, α-methylbenzylamine, 4-methoxybenzylamine, trimethyl-
amine, allylamine, carbon monoxide, and iodine were used as supplied.
4-Aminomethylbenzonitrile was obtained from 4-aminomethylbenzo-
nitrile hydrochloride by neutralization using a stoichiometric amount
of aqueous sodium hydroxide. Nitrogen gas was dried over
phosphorus(V) oxide. Melting points were recorded on a Stuart
Scientific SMP3 melting point apparatus and are uncorrected.
Elemental analyses were performed on a Thermo-Scientific Flash
2000 CHNS/O analyzer. Solid-state infrared spectra were recorded
using an ATR PerkinElmer Spectrum 100 spectrophotometer between
4000 and 400 cm−1. NMR spectra were recorded on Bruker Topspin
400 and 600 MHz spectrometers. The deuterated solvents CDCl3
(Aldrich, 99.8%), C2D6SO and CD3CN (Merck), and D2O (Aldrich)
were used as purchased. Solutions for NMR spectroscopy were
prepared under nitrogen using nitrogen-saturated solvents. The high-
resolution mass spectra for compounds 3, 5, and 6 were recorded on a
Waters Synapt G2 instrument by injecting via the ESI probe into a
stream of methanol. The precursors Ru3(CO)12,47 [CpRu(CO)2]2 (Cp
= η5-C5H5),48 CpRu(CO)2I,49 [Cp(CO)2Ru]BF4,50 and [Cp-
The oxidation process catalyzed by 2.5 mol % of catalyst 1,
with the cyclohexylamine ligand, formed styrene oxide in
addition to benzaldehyde. The complexes 3−6, which contain a
phenyl ring in the amine ligand, form benzaldehyde in high
yield but styrene oxide is not observed. The expected
byproduct, HCHO, was found in all reactions.
Tests performed on the catalytic oxidation of stilbene, a
derivative of styrene, and 1-octene using NaIO4 as the co-
oxidant and 2.5 mol % complex 1 as a model catalyst resulted in
oxidative cleavage of the olefinic bond and formation of the
respective aldehydes in more than 99% yields (Table 3). This
a
Table 3. Catalytic Oxidation of Different Subtrates
51
(CO)2RuNCCH3]BF4 were prepared by literature methods. The
complexes in this work were synthesized by a modification of the
method reported for the synthesis of the (η5-cyclopentadienyl)-
dicarbonylruthenium(II) n-alkanamine complex salts.37 The ruthe-
nium complexes 10 and 11 used for the catalytic study in this work
were synthesized by previously reported methods.37,51
Synthesis of the Complex Salts [CpRu(CO)2NH2R]BF4. Cyclo-
hexylamine Complex Salt [CpRu(CO)2NH2C6H11]BF4 (1). CpRu-
(CO)2I (0.1638 g; 0.4692 mmol), silver tetrafluoroborate (0.1261 g;
0.6478 mmol), and a magnetic stirrer bar were placed in a Schlenk
tube wrapped in aluminum foil. The mixture was evacuated under
reduced pressure for 3 h. Dichloromethane (DCM; 20 mL) was added
and the mixture stirred for 11/2 h. A white precipitate and orange
mother liquor formed. The mother liquor was transferred into a clean
Schlenk tube by cannula filtration and a slight excess of cyclohexyl-
a
Reaction conditions: 0.478 mmol of substrate, 3 equiv of NaIO4, 2.5
b
mol % of complex 1, CH3CN/H2O (1/1; 6 mL) at 60 °C. The time
taken to achieve the highest conversion is given in hours.
G
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amine (0.0820 g; 0.8268 mmol) added while stirring. The mother
liquor immediately turned pale yellow. The mixture was stirred for 4 h
at room temperature, left to stand for /2 h, and then cannula-filtered
compound 2 to give a white solid. Yield: 0.0508 g, 57%. Mp: 174.9−
175.6 °C. Anal. Found (calcd) for C15H16BF4NO3Ru: C, 40.37
(40.38); H, 3.59 (3.61); N, 3.13 (3.14). HRMS: [C15H16NO3Ru]+ m/
z 360.0172 (360.0174). IR (solid state): νmax/cm−1 2053, 2001, and
1
under reduced pressure into a Schlenk tube. The solvent was
completely removed by evaporation under reduced pressure, leaving a
grayish white solid. Acetonitrile (2 mL) was added to the solid to
1
1974 (CO), 3306 and 3271 (NH2). H NMR (400 MHz, CDCl3): δ
5.44 (s, 5H, Cp), 4.22 (s, 2H, NH2), 7.25 (d, 2H, o-CH), 6.86 (d, 2H,
extract the complex salt. The solution was left to stand for /2 h and
m-CH), 3.64 (t, JHH = 7.04 Hz, 2H, NCH2), 3.75 (s, 3H, OCH3). 13
C
1
then filtered into a preweighed Schlenk tube. Diethyl ether (10 mL)
was added, and the mixture was shaken and left to stand for 2 h. A
white flaky precipitate formed, the mother liquor was filtered, and the
precipitate was dried under reduced pressure. Yield: 0.1428 g, 74%.
Mp: 188.8−189.2 °C. Anal. Found (calcd) for C13H18BF4NO2Ru: C,
NMR (400 MHz, CDCl3): δ 195.14 (CO), 87.84 (Cp), 130.52 (C-
CH2), 129.45 (o-CH), 114.55 (m-CH), 160.1 (C-p-OCH3), 56.29 (C-
NH2), 55.24 (OCH3).
4-Aminomethylbenzonitrile Complex Salt [CpRu-
(CO)2NH2CH2(C6H4)CN]BF4 (6). CpRu(CO)2I (0.160 g; 0.4583
mmol), AgBF4 (0.1181 g; 0.6067 mmol), and a stirrer bar were
placed in a Schlenk tube wrapped with aluminum foil and treated as
described for compound 1. Excess 4-aminomethylbenzonitrile (0.1175
g; 0.8893 mmol) was dissolved in tetrahydrofuran (10 mL) and added
to the filtrate, and the mixture was stirred for 4 h. The mother liquor
was filtered into a clean Schlenk tube by use of a cannula. The solvent
was completely removed by evaporation under reduced pressure,
38.15 (38.25); H, 4.52 (4.45); N, 3.39 (3.43). IR (solid state): νmax
/
cm−1 2056 and 1999 (CO); 2953 and 2917 (CH3); 3295 and 3264
(NH2). 1H NMR (400 MHz, CDCl3): δ 5.60 (s, 5H, Cp), 3.70 (s, 2H,
NH2), 2.00 (m, 1H, C-1), 1.76 (m, 2H, C-2,6), 1.59 (m, 2H, C-2,6;
2H C-4), 1.23 (m, 4H, C-3,5). 13C NMR (400 MHz, CDCl3): δ
195.94 (CO), 87.94 Cp), 59.90 (C-1), 33.71 (C-2,6), 24.59 (C-4),
24.23 (C-3,5).
Phenylamine Complex Salt [CpRu(CO)2NH2C6H5]BF4 (2). CpRu-
(CO)2I (0.1240 g; 0.3552 mmol) and a slight excess of silver
tetrafluoroborate (0.0892 g; 0.4582 mmol) were dried, dissolved in
DCM (15 mL), and treated as described for compound 1. Excess
phenylamine (0.0621 g; 0.6668 mmol) was added to the filtrate
obtained, and the mixture was stirred for 8 h at room temperature. The
rest of the procedure was performed as described for compound 1 to
give a white solid. Yield: 0.0845 g, 58%. Mp: 116.8−117.6 °C. Anal.
Found (calcd) for C13H12BF4NO2Ru: C, 38.92 (38.83); H, 3.16
(3.01); N, 3.51 (3.48). IR (solid state): νmax/cm−1 2050 and 1992
(CO); 3310 and 3274 (NH2). 1H NMR (400 MHz, CD3CN): δ 7.53−
7.37 (m, 5H, C6H5), 5.68 (s, 5H, Cp), 4.41 (s, 2H, NH2). 13C NMR
(400 MHz, CD3CN): δ 195.83 (CO), 128.44−128.04 (C-C6H5),
87.57 (Cp).
leaving a pale yellow oil. Acetonitrile (2 mL) was added to the oil, and
1
the solution was left to stand for
/ h and then filtered into a
2
preweighed Schlenk tube. Diethyl ether (10 mL) was added to the
filtrate, the mixture was shaken, and a white suspension appeared. The
mixture was left undisturbed for 12 h. Shiny white crystals formed. The
crystals were separated from the mother liquor and dried under
reduced pressure. Yield: 0.1208 g, 59%. Mp: 177.2−178.0 °C. Anal.
Found (calcd) for C15H13BF4N2O2Ru: C, 40.82 (40.84); H, 3.01
(2.97); N, 6.28 (6.35). HRMS: [C15H13N2O2Ru]+ m/z 355.0024
(355.0021). IR (solid state): νmax/cm−1 2061 and 2006 (CO); 2229
1
(C8−N); 3306 and 3271 (NH2). H NMR (400 MHz, DMSO): δ
7.90 (d, 2H, m-CH), 7.59 (d, 2H, o-CH), 5.81 (s, 5H, Cp), 5.37 (s,
2H, NH2), 3.78 (s, 2H, CH2). 13C NMR (400 MHz, DMSO): δ
196.64 (CO), 144.41 (C-CH2), 132.41 (m-CH), 129.29 (o-CH),
118.64 (CN), 110.61 (C-CN), 88.60 (Cp), 56.28 (C-7).
Benzylamine Complex Salt [CpRu(CO)2NH2CH2C6H5]BF4 (3).
CpRu(CO)2I (0.1760 g; 0.5042 mmol) and AgBF4 (0.1276 g;
0.6555 mmol) were placed in a Schlenk tube wrapped with aluminum
foil and treated as described for compound 1. Excess benzylamine
(0.0992 g; 0.9265 mmol) was added to the resulting orange filtrate,
and the mixture was treated as described for compound 2 to give a
white solid. Yield: 0.0928 g, 44%. Mp: 136.7−137.6 °C. Anal. Found
(calcd) for C14H14BF4NO2Ru: C, 40.37 (40.41); H, 3.34 (3.31); N,
3.35 (3.37). HRMS: [C14H14NO2Ru]+, m/z 330.0070 (330.0068). IR
(solid state): νmax/cm−1 2052 and 1986 (CO); 3325 and 3286 (NH2).
1H NMR (400 MHz, CD3CN): δ 7.41−7.33 (m, 5H, C6H5), 5.60 (s,
2,4,6-Trimethylaniline Complex Salt [CpRu(CO)2NH2(CH3)3C6H2]-
BF4 (7). CpRu(CO)2I (0.0728 g; 0.2085 mmol) and AgBF4 (0.0598 g;
0.3072 mmol) were placed in a Schlenk tube wrapped with aluminum
foil and treated as described for compound 1. Excess 2,4,6-
trimethylaniline (0.0246 g; 0.4164 mmol) was added to the resulting
orange filtrate, and the mixture was treated as described for compound
2 to give a white solid. Yield: 0.0571 g, 62%; The compound
decomposes at temperatures above 188 °C. Anal. Found (calcd) for
C16H18BF4NO2Ru: C, 43.24 (43.26); H, 4.05 (4.08); N, 3.17 (3.15).
IR (solid state): νmax/cm−1 2049, 1987, and 1953 (CO): 3270 (NH2).
1H NMR (400 MHz; CDCl3): δ 6.73 (s, 2H, CH), 5.68 (s, 5H, Cp),
5H, Cp), 4.04 (s, 2H, NH2), 3.78 (t, 2H, CH2). 13C NMR (400 MHz,
CD3CN): δ 195.57 (CO), 138.27 (C-CH2), 128.57−128.12 (C6H5),
87.73 (Cp), 57.27 (CH2).
5.42 (s, 2H, NH2), 2.29 (s, 3H, p-CH3), 2.19 (s, 6H, o-CH3). 13C
NMR (400 MHz, CDCl3): δ 194.87 (CO), 140.98 (C-NH2), 135.28
(C-p-CH3), 130.19 (CH), 126.63 (C-o-CH3), 88.08 (Cp), 20.56 (p-
CH3), 17.50 (o-CH3).
( )-α-Methylbenzylamine Complex Salt [CpRu(CO)2NH2(CH)-
CH3C6H5]BF4 (4). CpRu(CO)2I (0.0953 g; 0.2730 mmol) and a slight
excess of AgBF4 (0.0701 g; 0.3601 mmol) were placed in a Schlenk
tube wrapped with aluminum foil and treated as described for
compound 1. Excess ( )-α-methylbenzylamine (0.0658 g; 0.5430
mmol) was added to the resulting orange solution and the mixture
treated as described for compound 2 to give a white solid. Yield:
0.0673 g, 57%. Mp: 170.1−170.7 °C. Anal. Found (calcd) for
C15H16BF4NO2Ru: C, 41.79 (41.88); H, 3.78 (3.75); N, 3.25 (3.26)%.
IR (solid state): νmax/cm−1 2054 and 1995 (CO); 3301 and 3266
Allylamine Complex Salt [CpRu(CO)2NH2CH2CHCH2]BF4 (8). To a
solution of [CpRu(CO)2NCCH3]BF4 (0.1214 g; 0.3468 mmol) in
DCM (20 mL) was added allylamine (0.0349 g; 0.6111 mmol)
dropwise at room temperature, and the mixture was stirred for 12 h.
The resulting pale yellow solution was evaporated to dryness, and the
product was extracted with acetonitrile (5 mL). The mixture was
filtered, and the volume of filtrate was reduced to about 2 mL. Diethyl
ether (10 mL) was added to the extract, and a white precipitate
formed. Filtration followed by drying of the residue under reduced
pressure gave a white solid. Yield: 0.0874 g, 72%. The compound
decomposes at temperatures >120 °C. Anal. Found (calcd) for
C10H12BF4NO2Ru: C, 32.84 (32.81); H, 3.29 (3.30); N, 3.81 (3.83).
IR (solid state): νmax/cm−1 2059, 2016 (CO): 3315, 3280 cm−1(NH2).
1H NMR (400 MHz, CDCl3): δ 5.88 (m, 1H, C-β); 5.60 (s, 5H, Cp);
5.42 (d, 1Ha, C-γ trans); 5.27 (d, 1Hb, C-γcis); 3.79 (s, 2H, NH2); 3.30
(q, 2H, C-α). 13C NMR (400 MHz; CDCl3): δ 196.00 (CO), 135.17
(C-β), 118.57 (C-γ), 88.04 (Cp), 55.97 (C-α).
1
(NH2). H NMR (400 MHz, CD3CN): δ 7.45−7.33 (m, 5H, C6H5),
5.45 (s, 5H, Cp), 4.28(s, 1H, NH2), 4.01 (s, 1H, NH2), 3.66 (m, 1H,
CH), 1.49 (d, 3H, CH3). 13C NMR (400 MHz, CD3CN): δ 195.66
(CO), 195.59 (CO), 141.99 (C-CH), 128.74 (C-ortho), 127.03 (C-
para), 129.14 (C-meta), 87.97 (Cp), 87.95 (Cp), 61.91 (CH2), 23.99
(CH3).
4-Methoxybenzylamine Complex Salt [CpRu(CO)2NH2CH2(C6H4)-
OCH3]BF4 (5). CpRu(CO)2I (0.0700 g; 0.2005 mmol) and a slight
excess of AgBF4 (0.04893 g; 0.2513 mmol) were placed in a Schlenk
tube wrapped with aluminum foil and treated as described for
compound 1. Excess 4-methoxybenzylamine (0.1213 g; 0.8842 mmol)
was added to the resulting orange filtrate, and the mixture was stirred
for 6 h. The resulting pale yellow solution was treated as described for
Isopropylamine Complex Salt [CpRu(CO)2NH2CH(CH3)2]BF4 (9).
CpRu(CO)2I (0.0982 g; 0.2813 mmol) and AgBF4 (0.0602 g; 0.3092
mmol) were placed in a Schlenk tube wrapped with aluminum foil and
treated as described for compound 1. Excess isopropylamine (0.0325
H
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Article
g; 0.5498 mmol) was added to the resulting orange solution, and the
mixture was treated as described for compound 2. A white solid was
formed. Yield: 0.0792 g, 77%; Decomposes at temperatures above 188
°C. Anal. Found (calcd) for C10H14BF4NO2Ru: C, 32.64 (32.63); H,
3.81 (3.83); N, 3.79 (3.81). IR (solid state): νmax/cm−1 (CO) 2058,
2018 and 2002; 3331 and 3293 (NH2). 1H NMR (400 MHz, DMSO):
δ 5.80 (s, 5H, Cp), 4.80 (s, 2H, NH2), 2.58 (m, 1H, CH), 1.08 (d, 6H,
CH3). 13C NMR (400 MHz, DMSO): δ 196.00 (CO); 88.23 (Cp),
51.89 (C-2), 23.11 (CH3).
X-ray Crystal Structure Determination of Compounds 3 and
8. Crystals of compounds 3 and 8 suitable for single-crystal X-ray
diffraction studies were grown by the liquid diffusion method.
Solutions of each of the compounds 3 and 8 in acetonitrile were
layered with a 4-fold volume of diethyl ether and allowed to stand
undisturbed in the dark at room temperature for 24 h to give white
crystals. Crystals of compounds 3 and 8 were selected and glued onto
the tip of glass fibers separately. The crystals were then mounted under
a stream of cold nitrogen at 100(1) K and centered in the X-ray beam
using a video camera. The rest of the manipulations were done as per
the literature method.41 Crystal data and structure refinement
information for compounds 3 and 8 are summarized in Table S1 in
the Supporting Information.
AUTHOR INFORMATION
■
Corresponding Author
*H.B.F.: tel, +27 312603107; fax, +27 312603091; e-mail,
friedric@ukzn.ac.za.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the NRF
(South Africa), THRIP (grant Number TP 1208035643), and
UKZN (URF).
REFERENCES
■
(1) Ward, M. D. Coord. Chem. Rev. 2006, 250, 3128−3141.
(2) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239−
2246.
(3) Pandrala, M.; Li, F.; Feterl, M.; Mulyana, Y.; Warner, J. M.;
Wallace, L.; Keene, F. R.; Collins, J. G. Dalton Trans. 2013, 42, 4686−
4694.
(4) Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903−1912.
(5) Maitlis, P. M. J. Organomet. Chem. 1995, 500, 239−249.
(6) Halpern, J.; Trost, B. M. Proc. Natl. Acad. Sci. U. S. A. 2004, 101,
5347.
(7) Kureshy, R. I.; Khan, N. H. Polyhedron 1993, 12, 195−201.
(8) Comprehensive Organometallic Chemistry II: A Review of the
Literature; Abel, E. W., Stone, F. G. A., Wilkinson, A., Eds.; Pergamon
Press: Oxford, U.K., 1995; p 8177.
(9) Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Faltz, A. P.,
Yamamoto, H., Eds.; Springer: Berlin, 1999.
(10) Noyori, R.: Asymmetric Catalysis in Organic Synthesis; Wiley:
New York, 1994.
(11) Enzyme Catalysis in Organic Synthesis: A Comprehensive
Handbook, 2nd ed..; Drauz, K., Waldmann, H., Eds.; Wiley-VCH:
Weinheim, Germany, 2002; Vols. I−III.
(12) Murahashi, S.-I. Angew. Chem., Int. Ed. Engl. 1995, 34, 2443−
2465.
(13) Murahashi, S.-I.; Naota, T.: Comprehensive Organometallic
Chemistry II; Pergamon Press: Oxford, U.K., 1995; Vol. 12.
(14) Reddy, J. S.; Khire, U. R.; Ratnasamy, P.; Mitra, R. B. J. Chem.
Soc., Chem. Commun. 1992, 1234−1235.
(15) Hudlicky, M.: Oxidation in Organic Chemistry; American
Chemical Society: Washington, DC, 1990.
(16) Singh, F. V.; Milagre, H. M. S.; Eberlin, M. N.; Stefani, H. A.
Tetrahedron Lett. 2009, 50, 2312−2316.
(17) Shing, T. K. M.: Comprehensive Organic Synthesis; Pergamon
Press: Oxford, U.K., 1991; Vol. 7.
(18) Bailey, P. S. Chem. Rev. 1958, 58, 925−1010.
(19) Criegee, R. Angew. Chem., Int. Ed. Engl. 1975, 14, 745−752.
(20) Larock, R. C.: Comprehensive Organic Transformations; Wiley-
VCH: New York, 1999.
(21) Koike, K.; Inoue, G.; Fukuda, T. J. Chem. Eng. Jpn. 1999, 32,
295−299.
Catalytic Studies. In a typical experiment, a known amount of the
co-oxidant was placed in a dry Schlenk tube equipped with a magnetic
stirrer bar under an atmosphere of nitrogen. Deionized water (3 mL)
was added, and the mixture was stirred at a selected temperature until
the co-oxidant dissolved completely (where applicable). The (η5-
cyclopentadienyl)dicarbonylruthenium(II) amine complex and aceto-
nitrile (3 mL) were added, and the mixture was stirred until all the
ruthenium complex had dissolved. Styrene (0.478 mmol) and the
internal standard, benzophenone (0.478 mmol), were added to the
mixture in the Schlenk tube, and this mixture was stirred for 1 min and
then sampled for gas chromatographic (GC) analysis. The stirring was
continued and the progress of the reaction monitored hourly by GC.
The complex [CpRu(CO)2NH2C6H11]BF4 (1; 2.5 mol %) was used as
the model catalyst for optimization of the catalytic conditions. Its
catalytic activity was investigated at room temperature (22 °C) and 40
and 60 °C, with different co-oxidants at 60 °C, with different
concentrations of NaIO4 at 60 °C, in different solvent systems at 60
°C using the best co-oxidant, and in different reaction media (acidic
(H2SO4), basic (K2CO3 and N-methylmorpholine), or neutral). The
effect of varying the catalyst concentration on conversion and yield was
also investigated under the chosen conditions using 0.5, 1.5, and 2.5
mol % of complex 1. The catalytic activities of the ruthenium
complexes 1−6 and 9 were examined and compared under the
optimized conditions. The percentage yield of benzaldehyde was
calculated on the basis of the GC analysis with benzophenone as the
internal standard. The time taken to achieve the highest conversion
and benzaldehyde yield is given for each reaction, as presented in
Figures 5−10. The best reaction conditions chosen for catalytic testing
were 0.478 mmol of styrene, 3 equiv of NaIO4, CH3CN/H2O (1/1; 6
mL) at 60 °C; any changed parameter is stated for each reaction. The
time taken to achieve the highest conversion is given in hours.
(22) Yang, D.; Zhang, C. J. Org. Chem. 2001, 66, 4814−4818.
(23) Albarella, L.; Giordano, F.; Lasalvia, M.; Piccialli, V.; Sica, D.
Tetrahedron Lett. 1995, 36, 5267−5270.
ASSOCIATED CONTENT
■
S
* Supporting Information
(24) Lemieux, R. U.; Rudloff, E. V. Can. J. Chem. 1955, 33, 1701−
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00564.
1709.
(25) Wiberg, K. B.; Saegebarth, K. A. J. Am. Chem. Soc. 1957, 79,
2822−2824.
(26) Ogino, T.; Mochizuki, K. Chem. Lett. 1979, 8, 443−446.
(27) Viski, P.; Szeverenyi, Z.; Simandi, L. I. J. Org. Chem. 1986, 51,
3213−3214.
Crystal data and structure refinement details for
compounds 3 and 8 (CCDC 1053455 and 1053456)
(PDF)
(28) Fieser, L. F.; Szmuszkovicz, J. J. Am. Chem. Soc. 1948, 70, 3352−
3355.
Crystallographic data for compound 3 (CIF)
(29) Freeman, F.; Yamachika, N. J. J. Am. Chem. Soc. 1970, 92,
3730−3733.
Crystallographic data for compound 8 (CIF)
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DOI: 10.1021/acs.organomet.5b00564
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(30) Travis, B. R.; Narayan, R. S.; Borhan, B. J. Am. Chem. Soc. 2002,
124, 3824−3825.
(31) Sharpless, K. B.; Akashi, K. J. Am. Chem. Soc. 1976, 98, 1986−
1987.
(32) Wolfe, S.; Hasan, S. K.; Campbell, J. R. J. Chem. Soc. D 1970,
1420−1421.
(33) Carlsen, P. H. J.; Katsuki, T.; Martin, V. S.; Sharpless, K. B. J.
Org. Chem. 1981, 46, 3936−3938.
(34) Torii, S.; Inokuchi, T.; Kondo, K. J. Org. Chem. 1985, 50, 4980−
4982.
(35) Shoair, A. G. F.; Mohamed, R. H. Synth. Commun. 2006, 36,
59−64.
(36) Kogan, V.; Quintal, M. M.; Neumann, R. Org. Lett. 2005, 7,
5039−5042.
(37) Nyawade, E. A.; Friedrich, H. B.; Omondi, B. Inorg. Chim. Acta
2014, 415, 44−51.
(38) M’thiruaine, C. M.; Friedrich, H. B.; Changamu, E. O.; Bala, M.
D. Inorg. Chim. Acta 2011, 366, 105−115.
(39) M’thiruaine, C. M.; Friedrich, H. B.; Changamu, E. O.; Omondi,
B. Polyhedron 2012, 40, 81−92.
(40) M’thiruaine, C. M.; Friedrich, H. B.; Changamu, E. O.; Omondi,
B. J. Organomet. Chem. 2012, 717, 52−60.
(41) Nyawade, E. A.; Friedrich, H. B.; M’thiruaine, C. M.; Omondi,
B. J. Mol. Struct. 2013, 1048, 426−433.
(42) De Clercq, B.; Verpoort, F. Tetrahedron Lett. 2001, 42, 8959−
8963.
(43) Luke, B. T.; Collins, J. R.; Loew, G. H.; McLean, A. D. J. Am.
Chem. Soc. 1990, 112, 8686−8691.
(44) Lee, D. G.; Spitzer, U. A. J. Org. Chem. 1976, 41, 3644−3644.
(45) Goldstein, A. S.; Drago, R. S. J. Chem. Soc., Chem. Commun.
1991, 21−22.
(46) Aguirre, P.; Sariego, R.; Moya, S. A. J. Coord. Chem. 2001, 54,
401−413.
(47) Bruce, M. I.; Jensen, C. M.; Jones, N. L.; Suss-Fink, G.;
̈
Herrmann, G.; Dase, V.: Polynuclear Ruthenium Complexes. In
Inorganic Syntheses; Wiley: Hoboken, NJ, 2007; pp 216−220.
(48) Doherty, N. M.; Knox, S. A. R.; Morris, M. J.; Casey, C. P.;
Whiteker, G. T.: Tetracarbonylbis(η5-Cyclopentadienyl)-Diruthe-
nium. In Inorganic Syntheses; Wiley: Hoboken, NJ, 2007; pp 189−191.
(49) Haines, R. J.; Du Preez, A. L. J. Chem. Soc., Dalton Trans. 1972,
944−948.
(50) Reger, D. L.; Coleman, C. J. Organomet. Chem. 1977, 131, 153−
162.
(51) Nyawade, E. A. Ph.D. Thesis, University of KwaZulu-Natal,
2015; p 87.
J
DOI: 10.1021/acs.organomet.5b00564
Organometallics XXXX, XXX, XXX−XXX