Bifunctional water activation for catalytic hydration of organonitriles

Article abstract of DOI:10.1021/om300297y

Treatment of [Rh(COD)(μ-Cl)]₂ with excess ^tBuOK and subsequent addition of 2 equiv of PIN?HBr in THF afforded [Rh(COD)(κC₂-PIN)Br] (1) (PIN = 1-isopropyl-3-(5,7-dimethyl-1, 8-naphthyrid-2-yl)imidazol-2-ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of 1 confirms ligand coordination to "Rh(COD)Br" through the carbene carbon featuring an unbound naphthyridine. Compound 1 is shown to be an excellent catalyst for the hydration of a wide variety of organonitriles at ambient temperature, providing the corresponding organoamides. In general, smaller substrates gave higher yields compared with sterically bulky nitriles. A turnover frequency of 20 000 h^-1 was achieved for the acrylonitrile. A similar Rh(I) catalyst without the naphthyridine appendage turned out to be inactive. DFT studies are undertaken to gain insight on the hydration mechanism. A 1:1 catalyst-water adduct was identified, which indicates that the naphthyridine group steers the catalytically relevant water molecule to the active metal site via double hydrogen-bonding interactions, providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as the key steps in the catalytic cycle.

Full text of DOI:10.1021/om300297y

Article
pubs.acs.org/Organometallics
Bifunctional Water Activation for Catalytic Hydration
of Organonitriles†
Prosenjit Daw, Arup Sinha, S. M. Wahidur Rahaman, Shrabani Dinda, and Jitendra K. Bera*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
S
* Supporting Information
ABSTRACT: Treatment of [Rh(COD)(μ-Cl)]2 with excess tBuOK and
subsequent addition of 2 equiv of PIN·HBr in THF afforded [Rh(COD)(κC2-
PIN)Br] (1) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazol-2-
ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of 1 confirms ligand
coordination to “Rh(COD)Br” through the carbene carbon featuring an unbound
naphthyridine. Compound 1 is shown to be an excellent catalyst for the hydration
of a wide variety of organonitriles at ambient temperature, providing the
corresponding organoamides. In general, smaller substrates gave higher yields
compared with sterically bulky nitriles. A turnover frequency of 20 000 h−1 was
achieved for the acrylonitrile. A similar Rh(I) catalyst without the naphthyridine
appendage turned out to be inactive. DFT studies are undertaken to gain insight on
the hydration mechanism. A 1:1 catalyst−water adduct was identified, which
indicates that the naphthyridine group steers the catalytically relevant water
molecule to the active metal site via double hydrogen-bonding interactions,
providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent
cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction
between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as
the key steps in the catalytic cycle.
design strategies of enzymes to develop organometallic catalysts?3
Incorporation of secondary H-bond interactions in a synthetic
system possesses a significant challenge since a combination of
INTRODUCTION
■
Metal-mediated reactions that utilize water as reagent are of
fundamental interests.1 Several metalloenzymes catalyze the
catalytic functionalities, a Lewis acid center (metal ion) and a
Lewis base (proton-acceptor group), would normally involve in a
nonproductive interaction. Such self-inhibition is precluded in a
metalloenzyme owing to the orchestrated assembly of the metal
ion and the H-bond functionalities on the protein backbone. The
present work seeks to develop an organometallic catalyst that
utilizes a ligand scaffold to engage in secondary interactions with
water and thus facilitates the hydration of nitriles.
Our approach toward water activation relies on a bifunctional
mechanism. We have recently demonstrated the multifaceted
coordination of naphthyridine (NP)-functionalized N-hetero-
cyclic carbene (NHC) ligands with different metal ions.4 The
use of an appropriate low-valent metal precursor results in a
metal−NHC complex with unbound naphthyridine. It is
anticipated that the naphthyridine nitrogens would involve in
H-bond interactions with metal-bound water mimicking the
enzyme active site, as illustrated in Scheme 2. Toward this
effort, the [Rh(COD)(κC2-PIN)Br] (1) (PIN = 1-isopropyl-3-
(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazol-2-ylidene) is syn-
thesized and its catalytic activity for the hydration of
organonitriles is evaluated. To gain insight into the microscopic
hydrolysis of amide, carboxylic ester, and phosphate ester, and
the hydration of nitrile and carbondioxide as well. The active site
of one such hydrolytic enzyme carboxypeptidase involves a
divalent zinc attached to the protein backbone by three amino acid
residues (two histidines and an acidic side chain), and the metal
ion is poised to act as a Lewis acid toward the amide carbonyl and
the water molecule. A glutamate carboxylate interacts with the
metal-bound water via a hydrogen-bond (H-bond) interaction and
thus promotes the nucleophilic attack of the polarized water (or
hydroxide) to the carbonyl carbon of the substrate (Scheme 1).2
Scheme 1. Active-Site Coordination Environment in a
Hydrolytic Metalloenzyme
Cooperative interactions between various components contribute
greatly to the enzyme activity and selectivity. Can we apply the
Received: April 12, 2012
Published: April 27, 2012
© 2012 American Chemical Society
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of them are able to hydrate RCN under catalytic conditions.9
Most of the catalysts are effective only at high temperature
(80 °C or higher),7a although a few are reported to work under
mild conditions.9a,c,i,m,o We are aware of two RhI compounds
that exhibit outstanding nitrile-hydration activity. The [Rh-
(OMe)(COD)]2/PCy3 (Cy = cyclohexyl) system by Saito and
co-workers9a hydrates a number of organonitriles at ambient
temperature with very high conversions and excellent functional
group tolerance. The other example is a water-soluble
[Rh(COD)Cl]2/P(m-C6H4SO3Na)3 by Djoman and Ajjou for
which the catalytic efficiency is dependent on the pH of the
media.9h
Scheme 2. Bifunctional Activation and Simultaneous
Nucleophilic Attack of the Water Oxygen to the Nitrile
Carbon in a Synthetic Complex
We have evaluated the catalytic utility of 1 for the hydra-
tion of organonitriles. An isopropanol solution of the catalyst
t
(0.005 mmol) was treated with benzonitrile (1 mmol), BuOK
steps involved in the catalytic cycle, DFT calculations are
undertaken. The prospect of a bifunctional mechanism in
promoting the catalytic hydration of organonitrile is examined
in this work.
(0.1 mmol), and water (1 mL) under an atmosphere of
nitrogen. Room-temperature stirring for 2 h provided
benzamide in a near quantitative amount as monitored by
GC-MS. Subsequent purification by silica gel column chro-
matography with petroleum ether/ethyl acetate (1:1 v/v)
afforded a 95% isolated yield (entry 1, Table 1), and the
product was characterized by NMR. Although complete con-
version occurred within 2 h, the reaction was continued for an
additional 4 h to check for possible overhydration. No trace of
benzoic acid or ester was found. This is significant given that
the acid/base-catalyzed processes often produce a considerable
amount of acid as side product.
RESULTS AND DISCUSSION
■
The NP-substituted NHC-ligand precursor (PIN·HBr) was
obtained by the direct linkage between 2-imidazolyl-1,8-
naphthyridine and isopropyl bromide.5 Reaction of [Rh-
t
(COD)(μ-Cl)]2 with excess BuOK and subsequent treatment
of 2 equiv of PIN·HBr in THF provided a 1:1 metal-to-ligand
complex [Rh(COD)(κC2-PIN)Br] (1) in good yield (81%).
The molecular structure of 1 is shown in Figure 1, which
i
Protic solvents MeOH, EtOH, and PrOH afforded higher
conversions (75−95%; 2 h, RT) compared with aprotic
solvents dimethylacetamide (DMA), tetrahydrofuran (THF),
and dimethyl sulfoxide (DMSO) (35−45%; 2 h, RT). Inter-
estingly, different catalyst loadings have the least effect on the
i
benzamide yield when PrOH is employed as the solvent.
Among different types of base used in this reaction, tBuOK and
NaOH were found to be equally effective, but Na2CO3 gave a
lower conversion. Since a high isolated yield (95%) of
benzamide was obtained using 0.005 mmol of catalyst 1, 0.1
mmol of tBuOK, and 1 mL of H2O for 1 mmol of benzonitrile
in iPrOH (3 mL) solvent, in 2 hours and at room temperature,
we employed the same reaction conditions for other substrates.
For less reactive substrates, however, an increase in catalyst
loading and higher water content lead to marginal improve-
ment in yields.
The presence of the catalytic amount of tBuOK was found to
be essential for achieving the full potential of the catalyst. In the
absence of the base, only 15% conversion of benzonitrile to the
corresponding amide was observed after 24 h of reaction time.
An identical reaction without the catalyst 1 afforded only
benzoic acid quantitatively.
Subsequently, we employed several nitriles to evaluate the
scope of the catalyst 1, and the results are collected in Table 1.
Among the substituted benzonitriles, those containing electron-
withdrawing groups get hydrated efficiently with good yields.
Aromatic nitriles with electron-donating groups, however,
afforded lower conversions. Nitro- and chloro-substituted
benzonitriles provided yields in the range of 85−90% (entries
2−4), whereas p-tolunitrile afforded a lesser yield (60%) even
on a higher catalyst loading (entry 5). Potentially coordinating
groups on the aromatic moiety poison the catalyst, and hence,
very little conversion was observed for p-N,N-dimethylamino-
benzonitrile (entry 6). The 2-cyanonaphthalene afforded a
moderate yield around 70%, but 9-cyanoanthracene gave a very
poor yield (<10%) (entries 7 and 8). The activity of the aryl
Figure 1. ORTEP diagram (50% probability thermal ellipsoids) of
[Rh(COD)(κC2-PIN)Br] (1) with important atoms labeled. Hydro-
gen atoms, except H29, are omitted for the sake of clarity.
reveals the coordination of “Rh(COD)Br” to the carbene
carbon. The Rh atom binds to the C2 carbene center (C33)
while the N atoms of NP remain unbound and disposed away
from the metal, resulting in a square-planar geometry around
rhodium. The Rh1−C33 distance is 2.016(6) Å. The 1H NMR
of 1 exhibits four sets of multiplets for COD protons. The
NMR signal for the H29 proton undergoes a considerable
downfield shift (Δδ = 1.64 ppm), which is credited to its
preagostic interaction with the rhodium.6 The carbene carbon
(C33) resonates at δ 182 ppm in 13C NMR.
Hydration of organonitrile is a synthetically important
process for the preparation of organoamides. Several of these,
for example, acrylamide and nicotinamide, have broad applica-
tions in chemical and pharmaceutical industries.7 Although
metal-mediated hydration of nitriles with selective formation of
the metal-bound carboxamide is well-documented,8 only a few
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a b c d e
, , , ,
Catalyst 1 was found to be highly effective for the acrylonitrile
(entry 14). A gradual decrease in the product yield was observed
on going from acrylonitrile to 2-methylacrylonitrile to cinnamoni-
trile (entries 14−16) owing primarily to the increased steric bulk
of the substrate. A similar trend was also observed for aliphatic
Table 1. Catalytic Hydration of Organonitriles by 1
t
nitriles in which BuCN afforded the lowest conversion (entries
17−20).9g The 2-methoxyacetonitrile was readily hydrated with
excellent yield (entry 21).
Encouraged by the brilliant activity of the catalyst 1 for the
hydration of acrylonitrile to the corresponding amide, a pre-
cursor for industrially important polyacrylamide,7a we inves-
tigated this particular reaction in detail. The catalyst loading
and the reaction time were varied, and a turnover frequency
(mol of amide/(mol of catalyst·hr)) up to 20 000 was achieved.
Compound 1 performs catalytic hydration of a wide variety
of organonitriles to the corresponding amides under mild
conditions (25 °C, 2−6 h). It strongly competes with the
Saito’s RhI catalyst,9a which is the most efficient, works at
ambient temperature, but usually takes a longer reaction time
(25 °C, 24−72 h). Catalyst 1 is found to be generally more
effective for smaller substrates but gave poor yields for sterically
bulky nitriles. For acrylonitrile, a TOF value of 20 000 h−1 was
estimated, which compares well with a RuII complex bearing
phosphinopyridine ligand (vide infra).11 Other substrates
exhibit TOF values from a high of about 95 (entries 1, 9, 11)
to more moderate values about 20−30 (entries 2−4, 7, 15−19, 21)
to a low value of nearly 3 (entry 20) per hour. The common
midrange is comparable to the TOF values reported for acetonitrile
hydration by Trogler’s [Pt(H)(H2O)(PMe3)2](OH) catalyst9i
(22 h−1 at 25 °C) and Ajjou’s water-soluble [Rh(COD)Cl]2/
P(m-C6H4SO3Na)3 catalyst (31 h−1 at 25 °C).9h Catalyst 1
affords selectively the monohydration product for dinitrile
substrates, which appears to be a unique example.9a,e,10
It is widely accepted that the metal coordination of the nitrile
increases its susceptibility for nucleophilic attack by water.
Further, the presence of a proton acceptor in the ligand scaffold
dramatically accelerates the rate of hydration.12 Oshiki, Takai,
and co-workers reported the remarkable hydration activity of a
ruthenium complex cis-Ru(acac)2(Ph2Ppy)2 (acac = acetylacet-
onate; Ph2Ppy = 2-diphenylphosphinopyridine) attributed to
the H-bond interaction of water with the pendant pyridinyl unit
of the Ph2Ppy ligand.11 In another example, Lau, Lin, and co-
workers have credited the hydration rate enhancement catalyzed
by an indenyl ruthenium hydride complex (η5-C9H7)Ru(dppm)H
(dppm = bis(diphenylphosphino)methane) to a Ru−H···H−OH
dihydrogen-bond interaction.13 In agreement with this proposal,
the corresponding chloro complex (η5-C9H7)Ru(dppm)Cl was
found to be inactive.
a
Reaction conditions: 1 mmol of organic nitrile, 1 mL of distilled
i
t
water, 3 mL of PrOH, 0.5 mol % catalyst, 10 mg of BuOK, 25 °C.
b
Reaction conditions: 1 mmol of organic nitrile, 1.5 mL of distilled
c
t
water, 3 mL of iPrOH, 2 mol % catalyst, 10 mg of BuOK. 48% acid.
d
e
Isolated yield unless mentioned otherwise. Determined by GC.
To understand the role of a basic functional group promoting
the hydration reaction, we synthesized [Rh(COD)(κC2-Me2-
NHC)Br] (Me2-NHC = 1,3-dimethylimidazol-2-ylidene),
which is devoid of the pendant naphthyridine group.14 It
showed sluggish activity for benzonitrile under identical
conditions (nitrile/catalyst/tBuOK/water = 1/0.005/0.1/40−
nitriles decreases on going from benzene to naphthalene to
anthracene. This reduction in yield is clearly due to the
increased steric bulk of the substrate. Nicotinamide was pro-
duced efficiently with 96% yield (entry 9) from 3-cyanopyr-
idine. In contrast, overhydration occurs for 2-cyanopyridine,
leading to a 1:1 mixture of acid and amide (entry 10). This is
the only case among all the nitriles studied where overhydration
was observed. Interestingly, 2-furanonitrile was readily hydrated
to amide (yield: 90%) with no trace of the corresponding acid
(entry 11).
Hydration of dinitriles is a matter of significance as there are
equal possibilities of mono- and double hydration.9a,e,10 Only
the monohydration product was observed for 1,4-dicyanoben-
zene and 1,4-phenyldiacetonitrile (entries 12 and 13).
i
50; RT, 24 h) in PrOH (3 mL), providing a meager 30%
overall conversion with more than 80% overhydration product
benzoic acid.
On the basis of this observation, we have proposed a
mechanism in which the unbound naphthyridine acts as a
stimulant for the nitrile hydration catalyzed by 1. The initial
approach of the water to the active metal site is steered by the
naphthyridine nitrogens via H-bond interactions. Bifunctional
activation and simultaneous nucleophilic attack of the
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Scheme 3. Proposed Mechanism for the Hydration of Organonitrile Catalyzed by 1
hydroxide to the nitrile carbon result in the metal-coordinated
amido species. Finally, proton transfer from the protonated
naphthyridine to the amido group affords the product and com-
pletes the catalytic cycle with regeneration of the active catalytic
species (Scheme 3).
To garner support for the proposed mechanism, DFT
calculations at the B3LYP level of theory were carried out
where acetonitrile is considered as the model substrate. To reduce
the computational cost, the isopropyl group at the imidazolyl
nitrogen is replaced by the methyl. Important bond parameters
for the intermediates and the transition state (TS) are provided in
Figure 2. The computed reaction profile along with the calculated
free energies (kcal/mol) is presented in Figure 3.
active metal site and thus lowers the entropic barrier for the
catalysis.
Release of one of the N···H interactions leads to a single H-
bonded species C, which is slightly higher in energy (3.3 kcal/
mol) compared to the double H-bonded species B (Figure 2).
The different N···H separation distances (N1−H1 = 1.796 Å,
N8−H2 = 2.936 Å) in C indicate that the predominant H-bond
interaction involves the proximal nitrogen N1 at this stage. The
following features in C favor the subsequent hydration process:
(1) the metal ion coordination (Rh−O = 2.569 Å) and H-bond
interaction polarize the water molecule and (2) the secondary
interactions orient the oxygen lone pair for nucleophilic attack
to the substrate. Nucleophilic attack of the polarized water to
the nitrile carbon forms intermediate D. Examination of the
metrical parameters reveals that D is an enol form of the amido
species coordinated to the Rh (Rh−N = 2.023 Å, N−C = 1.254 Å,
C−O = 1.408 Å, O−H = 0.968 Å), leaving a proton at the N1
nitrogen (N1−H1 = 1.075 Å). The corresponding transition
state TS-CD involves a proton migration from the water to the
naphthyridine nitrogen N1 and a complementary interaction
between the hydroxide and the nitrile carbon, leading to a
significant bending of the acetonitrile from linearity (Figure 4).
This results in considerable shortening of C−O (from 3.370 Å
in C to 1.549 Å) and Rh−N distances (from 2.284 Å in C to
2.044 Å), and lengthening of C−N (from 1.158 Å in C to 1.227 Å)
and O−H1 distances (from 0.984 Å in C to 1.317 Å) (see
Figure 2). The C−C−N angle of acetonitrile bends in the
direction of the hydroxide by 132°. The normal modes in TS-
CD displays multicomponent cooperativity involving the
naphthyridine nitrogens, the rhodium, the nitrile to be
hydrolyzed, and the water molecule that performs the
hydrolysis (Figure 4). As a consequence, the water is
The initial event involves the dissociation of the labile
15
t
bromide from 1, assisted by the BuOK, to generate a vacant
site at the catalyst for nitrile coordination, leading to a model
square-planar species A. Unlike 1, the naphthyridine nitrogens
in A are disposed to the side of the metal. This particular con-
figuration is relevant since it puts the water molecule toward
the metal-coordinated acetonitrile in the next step (vide infra).
An alternate geometry with a naphthyridine configuration
similar to 1 (i.e., disposed away from the metal) was found to
be 3.8 kcal/mol higher in energy. A hydrated species B is
computed in which the water molecule is double H-bonded to
the naphthyridine nitrogens (N···H distances are 2.161 and
2.282 Å). The acceptor···acceptor (AA) motif of the
naphthyridine unit has been amply demonstrated in Zimmerman’s
quadruply H-bonded heterocomplexes.16 The Rh···O distance
(3.719 Å) is long and does not appear to have a role for the initial
water-adduct formation. Rather, the naphthyridine appendage
steers the catalytically relevant water molecule in the vicinity of the
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Figure 2. Optimized structures, important bond parameters (Å), and relative energies (kcal/mol) of the intermediates and the transition state.
Figure 4. Normal modes of the TS-CD shown by arrows.
Figure 3. Computed reaction profile for nitrile hydration catalyzed by 1.
Energies are shown in kcal/mol.
active site via metal coordination. The TS-CD has a single
bifunctionally activated, leading to the nucleophilic attack of the
hydroxide to the acetonitrile, the latter being brought in the
imaginary frequency of −606 cm−1 with a free energy of
activation of 20.0 kcal/mol.
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concentrated benzene solution of the compound. Yield: 0.041 g
(88%). H NMR (500 MHz, CDCl3, 292 K): δ 10.36 (1H, d, J = 8.6
The enol form of the amido unit in D tautomerizes to the
keto form, resulting in the intermediate E (Rh−N = 2.049 Å,
N−C = 1.347 Å, C−O = 1.247 Å, N(amido)−H = 1.014 Å). In
the next step, the naphthyridine proton either can be directly
transferred to the amido nitrogen to form the amide
coordinated species G or can be transferred to the amido
oxygen to give intermediate F, featuring the enol form of the
amide. Subsequent tautomerization of F to G is a downhill
process by 4 kcal/mol. We have also examined the possibility of
a proton transfer from the naphthyridine nitrogen to the metal.
However, all attempts to identify a Rh(III)-H species failed.
The end product invariably was either F or G, indicating a facile
proton transfer to the amido group (O or N, respectively).
Finally, the dissociation of the amide and concomitant
coordination of nitrile complete the catalytic cycle.
1
Hz, NP), 8.62 (1H, d, J = 8.6 Hz, NP), 8.19 (1H, d, J = 1.5 Hz, Im),
7.27 (1H, s, NP), 7.05 (1H, d, J = 2 Hz, Im), 6.17 (1H, septet, J = 6.6
Hz, methyne), 3.06−2.97 (2H, br, COD), 2.77 (6H, 2s, MeNP), 2.34−
2.28 (2H, br, COD), 2.07−2.02 (2H, br, COD), 1.92−1.84 (3H, br,
COD), 1.72−1.66 (3H, br, COD), 1.60 (6H, d, J = 6.9 Hz, MeiPr). 13
C
NMR (125.7 MHz, CDCl3, 294 K): 181.9 (d, 1JRh−C = 66 Hz,
NCNIm), 163.5 (NCNNP), 147.5 (NCCNP), 145.7 (NCCNP), 134.6
(CCCNP), 128.7 (CHNP), 125.5 (CHNP), 123.5(CHIm), 120.3
(CCCNP), 117.2 (CHNP), 116.7 (CHIm), 97.5 (CHcod), 70.2
(CHcod), 54.2 (CHMe2), 29.7 ((CH2)cod), 29.1 ((CH2)cod), 28.1
((CH2)cod), 25.5 ((CH2)cod), 23.4 (CH3), 18.9 (CH3), 18.4 (CH3),
14.1 (CH3). ESI-MS, m/z: 491 [Rh(PIN)(CH3CN)Br + H]+. Anal.
Calcd for C24H30N4BrRh: C, 51.72; H, 5.43; N, 10.05. Found: C,
51.87; H, 5.52; N, 10.17.
Hydration of Nitrile. Organic nitrile (1 mmol) and distilled water
i
(1 mL) were sequentially added to a 3 mL PrOH solution of the
CONCLUSION
■
catalyst 1 (0.5 mol%) under an inert atmosphere. After stirring at
t
The present work demonstrates that a naphthyridine group
suitably placed on the ligand scaffold enhances the hydration
activity of a Rh(I) catalyst. The absence of a basic appendage
on the NHC ligand leads to lower activity and poor selectivity.
The pendant naphthyridine paves the way for a water molecule
to the active metal site using double H-bonded interactions.
DFT calculations reveal multicomponent cooperativity involv-
ing proton movement from the water to the naphthyridine
nitrogen and a complementary interaction between the
hydroxide and the nitrile carbon. Bifunctional water activation
and cooperative proton migration during the catalytic cycle
open up new possibilities for organometallic catalysis. Efforts
are on to develop mild, highly efficient, and selective hydration
and hydrolytic catalysts based on this bifunctional design
strategy.
room temperature for 30 min, 10 mg of BuOK was added to the
reaction mixture, and the stirring was continued for the time
mentioned in Table 1. The progress of the reaction was monitored
by GC-MS. After the reaction was over, the reaction mixture was
dissolved in MeOH and the solution was evaporated under vacuo. The
residue was purified by column chromatography on silica gel
(petroleum ether/EtOAc) to give the desired amide. Yields are
calculated based on isolated products, except for low-boiling substrates
for which GC yields are reported.
X-ray Data Collections and Refinement. Single-crystal X-ray
structural studies were performed on a CCD Bruker SMART APEX
diffractometer equipped with an Oxford Instruments low-temperature
attachment. Data were collected at 100(2) K using graphite-
monochromated Mo Kα radiation (λα = 0.71073 Å). The frames
were indexed, integrated, and scaled using the SMART and SAINT
Table 2. Crystallographic Data and Pertinent Refinement
EXPERIMENTAL SECTION
General Procedures. All reactions with metal complexes were
carried out under an atmosphere of purified nitrogen using standard
Parameters for 1
■
empirical formula
fw
C60H72Br2N8Rh2
1270.90
1
Schlenk-vessel and vacuum line techniques. H NMR spectra were
cryst syst
space group
a (Å)
triclinic
obtained on JEOL JNM-LA 500 MHz spectrometers. 1H NMR
chemical shifts were referenced to the residual hydrogen signal of the
deuterated solvents. 13C NMR spectra were recorded at 125 MHz.
Elemental analyses were performed on a Thermoquest EA1110
CHNS/O analyzer. The GC-MS experiment was performed on an
Agilent 7890A GC and 5975C MS system. The crystallized
compounds were powdered, washed several times with dry diethyl
ether, and dried in a vacuum for at least 48 h prior to elemental
analyses. ESI-MS were recorded on a Waters Micromass Quattro
Micro triple-quadrupole mass spectrometer.
P1
̅
11.640(2)
13.661(2)
17.730(3)
90.036(3)
97.222(3)
91.916(3)
2795.3(8)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
Materials. Solvents were dried by conventional methods, distilled
ρcalcd (g cm−3)
1.510
under nitrogen, and deoxygenated prior to use.17 RhCl3·xH2O was
μ (mm−1
)
2.066
t
purchased from Arora Matthey, India. BuOK and organonitriles were
F(000)
1296
purchased from Sigma-Aldrich. The compound [Rh(COD)(μ-Cl)]2
was synthesized following the literature procedures.18
reflns
Synthesis of Compound 1. Bromo-(η4-1,5-cyclooctadiene)-[1-
isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazol-2-ylidene]-
rhodium(I) (1): [Rh(COD)(μ-Cl)]2 (0.04 g, 0.081 mmol) and
tBuOK (0.020 g, 0.178 mmol) were stirred in THF at room
temperature for 30 min. The resulting mixture was added slowly to
a suspension of 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)-
imidazolium bromide (0.058 g, 0.167 mmol) in THF at −78 °C.
The mixture was allowed to attain room temperature and then refluxed
for 3 h. The bright yellow mixture was filtered through a small pad of
celite. The filtrate was evaporated to dryness and dissolved in a
minimum amount of dichloromethane. A 15 mL portion of hexane was
added with stirring to induce precipitation. The yellow solid was
washed with hexane and dried under vacuum. Crystals suitable for
X-ray diffraction were grown by layering petroleum ether over a
collected
18 824
independent
observed [I > 2σ (I)]
9391
7148
no. of variables
647
GOF
1.014
Rint
0.0426
a
final R indices [I > 2σ(I)]
R1 = 0.0539
wR2 = 0.1347
R1 = 0.0735
wR2 = 0.1471
a
R indices (all data)
a
2
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo| with Fo > 2σ(Fo ). wR2 = [∑w(|Fo | −
2 2
|Fc2|)2/∑|Fo | ]1/2
.
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Organometallics
Article
software package,19 and the data were corrected for absorption
using the SADABS program.20 The structures were solved and refined
using the SHELX suite of programs,21 while additional crystallographic
calculations were performed by the programs PLATON.22
Figures were drawn using ORTEP32.23 The hydrogen atoms were
included into geometrically calculated positions in the final stages of
the refinement and were refined according to the “riding model”.
Hydrogen atoms of the coordinated water molecule were not included
in the structure. All non-hydrogen atoms, except C5 and C54, were
refined with anisotropic thermal parameters. Anisotropic treatment of
these two atoms resulted in nonpositive definite displacement tensors
and was, therefore, subjected to isotropic refinement. Crystallographic
data and pertinent refinement parameters for 1 are presented in
Table 2. CCDC-855342 contains the supplementary crystallographic
data for compound 1. This data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
Chem. Res. 2000, 33, 225−233. (d) Kukushkin, V. Y.; Pombeiro, A. J.
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ASSOCIATED CONTENT
* Supporting Information
■
2010, 363, 1092−1096. (g) Jiang, X.-b.; Minnaard, A. J.; Feringa, B. L.;
de Vries, J. G. J. Org. Chem. 2004, 69, 2327−2331. (h) Djoman, M. C.
K. B.; Ajjou, A. N. Tetrahedron Lett. 2000, 41, 4845−4849. (i) Jensen,
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J. Org. Chem. 1992, 57, 2521−2523. (m) Kopylovich, M. N.;
S
Important bond distances and bong angles for compound 1 and
atomic coordinates of the intermediates A−G and the
transition state TS-CD are included in the Supporting
Information. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jbera@iitk.ac.in.
Kukushkin, V. Y.; Haukka, M.; Frausto da Silva, J. J. R.; Pombeiro,
́
■
A. J. L. Inorg. Chem. 2002, 41, 4798−4804. (n) Mitsudome, T.;
Mikami, Y.; Mori, H.; Arita, S.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K.
Chem. Commun. 2009, 3258−3260. (o) Chin, J.; Kim, I. H. Angew.
Chem., Int. Ed. Engl. 1990, 29, 523−525.
Notes
The authors declare no competing financial interest.
(10) Breuilles, P.; Leclerc, R.; Uguen, D. Tetrahedron Lett. 1994, 35,
1401−1404.
ACKNOWLEDGMENTS
■
(11) Oshiki, T.; Yamashita, H.; Sawada, K.; Utsunomiya, M.;
Takahashi, K.; Takai, K. Organometallics 2005, 24, 6287−6290.
(12) (a) Knapp, S. M. M.; Sherbow, T. J.; Juliette, J. J.; Tyler, D. R.
This work is financially supported by the Department of
Science and Technology (DST), India, and the Indo-French
Centre for the Promotion of Advanced Research (IFCPAR).
J.K.B. thanks DST for the Swarnajayanti fellowship. P.D. thanks
CSIR for the SPM fellowship.
́
Organometallics 2012, 31, 2941. (b) García-Alvarez, R.; Díez, J.;
Crochet, P.; Cadierno, V. Organometallics 2010, 29, 3955−3965.
(c) Leung, C. W.; Zheng, W.; Zhou, Z.; Lin, Z.; Lau, C. P.
̌
Organometallics 2008, 27, 4957−4969. (d) Smejkal, T.; Breit, B.
Organometallics 2007, 26, 2461−2464. (e) Leung, C. W.; Zheng, W.;
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Organometallics 2007, 26, 1924−1933. (f) Breno, K. L.; Pluth, M. D.;
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DEDICATION
■
†Dedicated to Professor N. Sathyamurthy.
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