Ruthenium complexes of chelating amido-functionalized N-heterocyclic carbene ligands: Synthesis, structure and DFT studies

Article abstract of DOI:10.1007/s12039-011-0173-5

Synthesis, structure and density functional theory (DFT) studies of a series of new ruthenium complexes, [1-(R)-3-N-(benzylacetamido)imidazol-2- ylidene]RuCl(p-cymene) [R = Me (1c), i-Pr (2c), CH₂Ph (3c); p-cymene = 4-i-propyltoluene] supported over N/O-functionalized N-heterocyclic carbene (NHC) ligands are reported. In particular, the ruthenium (1-3)c complexes were synthesized from the respective silver complexes, [1-(R)-3-N-(benzylacetamido) imidazol-2-ylidene]₂Ag⁺Cl^- [R = Me (1b), i-Pr (2b), CH₂Ph (3b)] by the treatment with [Ru(p-cymene)Cl2]2 in 65-76% yields. The molecular structures of (1-3)c revealed the chelation of the N-heterocylic carbene ligand through the carbene center and an amido sidearm of the ligand in all of the three complexes. The density functional theory studies on the ruthenium (1-3)c complexes indicated strong binding of the NHC ligand to the metal center as was observed from the deeply buried NHC-Ru s-bonding molecular orbitals. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0173-5

c
J. Chem. Sci. Vol. 123, No. 6, November 2011, pp. 791–798. ꢀ Indian Academy of Sciences.
Ruthenium complexes of chelating amido-functionalized N-heterocyclic
carbene ligands: Synthesis, structure and DFT studies
SACHIN KUMARa, ANANTHA NARAYANANa, MITTA NAGESWAR RAOa,
MOBIN M SHAIKHb and PRASENJIT GHOSHa,∗
aDepartment of Chemistry and bNational Single Crystal X-ray Diffraction Facility, Indian Institute of
Technology Bombay, Powai, Mumbai 400076, India
e-mail: pghosh@chem.iitb.ac.in
Abstract. Synthesis, structure and density functional theory (DFT) studies of a series of new ruthenium comp-
lexes, [1-(R)-3-N-(benzylacetamido)imidazol-2-ylidene]RuCl(p-cymene) [R = Me (1c), i-Pr (2c), CH2Ph
(3c); p-cymene = 4-i-propyltoluene] supported over N/O-functionalized N-heterocyclic carbene (NHC) lig-
ands are reported. In particular, the ruthenium (1–3)c complexes were synthesized from the respective silver
complexes, [1-(R)-3-N-(benzylacetamido)imidazol-2-ylidene]2Ag+Cl− [R = Me (1b), i-Pr (2b), CH2Ph (3b)]
by the treatment with [Ru(p-cymene)Cl2]2 in 65–76% yields. The molecular structures of (1–3)c revealed the
chelation of the N-heterocylic carbene ligand through the carbene center and an amido sidearm of the ligand
in all of the three complexes. The density functional theory studies on the ruthenium (1–3)c complexes indi-
cated strong binding of the NHC ligand to the metal center as was observed from the deeply buried NHC-Ru
σ-bonding molecular orbitals.
Keywords. Carbenes; ruthenium; DFT; amido-functionalized NHC.
1. Introduction
increasingly popular as ligands for stabilizing transi-
tion metal complexes.11 Easy tunablity of the sterics
and the electronic environments through constant varia-
tion of the substituents on the nitrogen atom of the imi-
dazole ring, make these ligands versatile in stabilizing
a variety of transition metal complexes in varied oxi-
dation states. With our interests being in biomedical12
and catalytic applications13 of transition metal comp-
lexes of N-heterocyclic carbene ligands,14 we became
involved in synthesizing its ruthenium complexes for
their potential utility in chemical catalysis.
In this contribution, we report a series of ruthenium
complexes, [1-(R)-3-N-(benzylacetamido)imidazol-2-
ylidene]RuCl(p-cymene) [R = Me (1c), i-Pr (2c),
CH2Ph (3c); p-cymene = 4-i-propyltoluene], supp-
orted over N/O-functionalized N-heterocyclic carbene
ligands (figure 1). The Ru–NHC interaction in the (1–
3)c complexes have been investigated using density
functional theory (DFT) studies.
Ruthenium adds several new and interesting options
to the ever-growing world of transition metal medi-
ated organic methodologies, in constant demand for
meeting the everyday challenges of organic synthe-
sis.1 For instance, the ruthenium complexes display
selective C–H activation reactions of particular rele-
vance to synthesizing biaryl and heterocyclic frame-
works,2 and thereby providing useful alternatives to
the already overstretched palladium mediated carbon–
carbon cross-coupling reactions.3 As such, the C–H
activations, and more so the selective C–H activation
ones, fall among the difficult but much desired chall-
enges, currently confronting the world of synthesis
today.4 Apart from the elegant selective C–H activation
reactions, the ruthenium engages in a variety of other
synthetically important transformations namely, asymme-
tric transfer hydrogenation reactions,5 metathesis,6
olefin epoxidation reactions,7 hydroformylation reac-
tions,8 electrooxidation reactions,9 facial cycloaddition
of azides with terminal alkynes regioselectively10 etc.
to name a few.
2. Experimental
The phenomenal success of the N-heterocyclic car-
benes in homogeneous catalysis, has also made them
2.1 General procedures
All manipulations were carried out using a combina-
tion of a glove box and standard Schlenk techniques.
∗For correspondence
791

792
Sachin Kumar et al.
Ag2O (2.02 g, 8.72 mmol) in dichloromethane (ca.
40 mL) was stirred at room temperature for 4 h. The
reaction mixture was then filtered and the solvent
was removed under vacuum to give the product 1b
1
as off-white solid (2.31 g, 47%). H NMR (CDCl3,
400 MHz, 25◦C): δ 9.36 (br, 1H, NH), 7.36 (br,
1H, NCHCHN), 7.29 (br, 2H, C6 H5), 7.11 (m, 3H,
C6 H5), 6.94 (br, 1H, NCHCHN), 5.29 (br, CH2),
4.36 (s, 2H, CH2), 3.77 (s, 3H, CH3). 13C{1H} NMR
(CDCl3, 100 MHz, 25◦C): δ 182.3 (NCN-Ag), 167.2
(CO), 138.5 (ipso-C6H5), 128.4 (m-C6H5), 128.1
(o-C6H5), 126.9 (p-C6H5), 123.4 (NCHCHN), 121.6
(NCHCHN), 53.9 (COCH2), 43.2 (CH2NH), 38.8
(CH3). IR (KBr Pellet): 1676 (νCO) cm−1. Anal. calcd.
for C26H30AgN6O2Cl•CH2Cl2: C, 47.22; H, 4.70; N,
12.24. Found: C, 47.46; H, 5.03, N, 11.84.
Figure 1. Ruthenium (1–3)c complexes of chelating
amido-functionalized N-heterocyclic carbene ligands.
2.3 Synthesis of [1-(methyl)-3-N-(benzylacetamido)
imidazol-2-ylidene]Ru(p-cymene)Cl (1c)
A mixture of {[1-(methyl)-3-{N-(benzylacetamido)
imidazol-2-ylidene}]2Ag}+Cl− (1b) (0.302 g, 0.502 mmol)
and [Ru(p-cymene)Cl2]2 (0.307 g, 0.502 mmol) was
dissolved in dichloromethane (ca. 40 mL) and was
stirred at room temperature for 6 h, when the forma-
tion of an off-white AgCl precipitate was observed.
The reaction mixture was then filtered and the solvent
was removed under vacuum to yield the crude product
as a brown solid. The crude product 1c was purified
by column chromatography on a silica gel by elution
Solvents were purified and degassed by standard pro-
cedures. RuCl3•xH2O was purchased from SD-fine
Chemicals (India) and used without any further purifi-
cation. The 1a,15 (2–3)b16 and [Ru(p-cymene)Cl2]2
17
were synthesized according to reported literature pro-
1
cedures. H and 13C{1H} NMR spectra were recorded
1
on a Varian 400 MHz NMR spectrometer. H NMR
peaks are labelled as singlet (s), doublet (d), triplet
(t), and septet (sept). Infrared spectra were recorded
on a Perkin Elmer Spectrum One FT-IR spectro-
meter. Mass spectrometry measurements were done with a CHCl3/CH3OH (v/v 10:1) mixed medium to
1
on a Micromass Q-Tof spectrometer. The electronic
spectra were recorded in acetonitrile by using Jasco
V-570 UV/VIS/NIR Spectrophotometer. Cyclic volta-
mmetric measurements were carried out using a BAS
Epsilon electrochemistry system. Elemental Analy-
ses were carried out on Thermo Quest FLASH 1112
SERIES (CHNS) Elemental Analyzer. X-ray diffraction
data for (1–3)c were collected on an Oxford Diffraction
Excaliber-S diffractometer and crystal data collection
and refinement parameters are summarized in Support-
ing Information table S1. The structures were solved
using direct methods and standard difference map tech-
niques, and were refined by full-matrix least-squares
procedures on F2 with SHELXTL (Version 6.10).18
give the product 1c as brown solid (0.181 g, 72%). H
NMR (CDCl3, 400 MHz, 25◦C): δ 7.50 (d, 2H, 3JHH
8 Hz, o-C6 H5), 7.35 (m, 3H, m/p-C6 H5), 7.00 (d,
=
3
3
1H, JHH = 2 Hz, NCHCHN), 6.97 (d, 1H, JHH
=
2
2 Hz, NCHCHN), 5.31 (d, 1H, JHH = 15 Hz, CH2),
3
5.13 (d, 1H, JHH = 6 Hz, p-CH3C6 H4CH(CH3)2),
3
4.89 (d, 1H, JHH = 6 Hz, p-CH3C6 H4CH(CH3)2),
3
4.83 (d, 1H, JHH = 6 Hz, p-CH3C6 H4CH(CH3)2),
3
4.82 (d, 1H, JHH = 6 Hz, p-CH3C6 H4CH(CH3)2),
2
4.46 (s, 2H, CH2), 4.45 (d, 1H, JHH = 15 Hz,
3
CH2), 3.99 (s, 3H, CH3), 2.45 (sept, 1H, JHH
=
7 Hz, p-CH3C6H4CH(CH3)2), 1.98 (s, 3H, p-
3
CH3C6H4CH(CH3)2), 1.11 (d, 3H, JHH = 7 Hz,
3
p-CH3C6H4CH(CH3)2), 0.88 (d, 3H, JHH = 7 Hz,
p-CH3C6H4CH(CH3)2). 13C{1H} NMR (CDCl3,
100 MHz, 25◦C): δ 175.2 (NCN-Ru), 169.7 (CO),
143.6 (ipso-C6H5), 128.7 (m-C6H5), 128.0 (o-
C6H5), 126.1 (p-C6H5), 122.8 (NCHCHN), 122.0
(NCHCHN), 107.4 (ipso-p-CH3C6H4CH(CH3)2),
2.2 Synthesis of {[1-(methyl)-3-N-(benzylacetamido)
imidazol-2-ylidene]2Ag}+Cl− (1b)
100.1
(ipso-p-CH3C6H4CH(CH3)2),
88.6
(p-
A
mixture of 1-(methyl)-3-N-(benzylacetamido)
CH3C6H4CH(CH3)2), 85.2 (p-CH3C6H4CH(CH3)2),
imidazolium chloride (1a) (4.37 g, 16.4 mmol) and

Ruthenium complexes of chelating amido-functionalized N-heterocyclic carbene ligands
793
83.0
(p-CH3C6H4CH(CH3)2),
81.9
(p- cm−1. Anal. calcd. for C25H32RuN3OCl•0.5CH2Cl2: C,
CH3C6H4CH(CH3)2), 55.5 (COCH2), 55.4 (CH2NH), 53.78; H, 5.84; N, 7.38. Found: C, 53.54; H, 6.23; N,
37.8 (CH3), 31.5 (p-CH3C6H4CH(CH3)2), 24.9 (p- 6.87.
CH3C6H4CH(CH3)2), 20.1 (p-CH3C6H4CH(CH3)2),
18.7 (p-CH3C6H4CH(CH3)2). IR (KBr Pellet): 1686
(νCO) cm−1. Anal. calcd. for C23H28RuN3OCl: C,
55.36; H, 5.66; N, 8.42. Found: C, 55.35; H, 6.02; N,
8.34.
2.5 Synthesis of [1-(benzyl)-3-N-(benzylacetamido)
imidazol-2-ylidene]Ru(p-cymene)Cl (3c)
A mixture of {[1-(benzyl)-3-{N-(benzylacetamido)
imidazol-2-ylidene}]2Ag}+Cl− (3b) (0.421 g, 0.557 mmol)
and [Ru(p-cymene)Cl2]2 (0.170 g, 0.278 mmol) was
dissolved in dichloromethane (ca. 40 mL) and was
stirred at room temperature for 6 h, when the forma-
tion of an off-white AgCl precipitate was observed.
The reaction mixture was then filtered and the sol-
vent was removed under vacuum to yield the crude
product as a brown solid. The crude product 3c
was purified by column chromatography on a sil-
ica gel by elution with a CHCl3/CH3OH (v/v 10:1)
mixed medium to give the product 3c as brown solid
2.4 Synthesis of [1-(i-propyl)-3-N-(benzylacetamido)
imidazol-2-ylidene]Ru(p-cymene)Cl (2c)
A mixture of {[1-(i-propyl)-3-{N-(benzylacetamido)
imidazol-2-ylidene}]2Ag}+Cl− (2b) (0.331 g, 0.505 mmol)
and [Ru(p-cymene)Cl2]2 (0.308 g, 0.505 mmol) was
dissolved in dichloromethane (ca. 40 mL) and was
stirred at room temperature for 6 h, when the forma-
tion of an off-white AgCl precipitate was observed.
The reaction mixture was then filtered and the solvent
was removed under vacuum to yield the product 2c
as a brown solid. The crude product 2c was purified
by column chromatography on a silica gel by elution
with a CHCl3/CH3OH (v/v 10:1) mixed medium to
1
(0.208 g, 65%). H NMR (CDCl3, 400 MHz, 25◦C):
δ 7.50 (br, 1H, C6 H5), 7.48 (br, 1H, C6 H5), 7.41–
7.39 (br, 2H, C6 H5), 7.36–7.34 (br, 3H, C6 H5),
7.32 (br, 2H, C6 H5), 7.29 (br, 1H, C6 H5), 7.03 (br,
1H, NCHCHN), 6.91 (br, 1H, NCHCHN), 5.67
1
give the product 2c as brown solid (0.202 g, 76%). H
NMR (CDCl3, 400 MHz, 25◦C): δ 7.47 (d, 2H, 3JHH
=
2
2
7 Hz, o-C6 H5), 7.32 (br, 2H, m-C6 H5), 7.29 (br, 1H,
(d, 1H, JHH = 16 Hz, CH2), 5.55 (d, 1H, JHH
=
3
3
NCHCHN), 7.21 (t, 1 H, JHH = 7 Hz, p-C6 H5), 6.95
16 Hz, CH2), 5.29 (d, 1H, JHH = 16 Hz, CH2), 4.91
2
3
(s, 1H, NCHCHN), 5.23 (d, 1H, JHH = 15 Hz, CH2),
(d, 1H, JHH = 6 Hz, p-CH3C6 H4CH(CH3)2), 4.75
3
3
5.10 (d, 1H, JHH = 6 Hz, p-CH3C6 H4CH(CH3)2),
(d, 1H, JHH = 6 Hz, p-CH3C6 H4CH(CH3)2), 4.67
3
3
5.10 (sept, 1H, JHH = 7 Hz, CH(CH3)2), 4.84 (d, 1H,
(d, 1H, JHH = 6 Hz, p-CH3C6 H4CH(CH3)2), 4.48
3JHH = 6 Hz, p-CH3C6 H4CH(CH3)2), 4.81 (d, 1H,
3JHH = 6 Hz, p-CH3C6 H4CH(CH3)2), 4.70 (d, 1H,
3JHH = 6 Hz, p-CH3C6 H4CH(CH3)2), 4.43 (s, 2H,
(s, 2H, CH2), 4.44 (br, 1H, p-CH3C6 H4CH(CH3)2),
3
4.33 (d, 1H, JHH = 16 Hz, CH2), 2.46 (sept,
3
1H, JHH = 6 Hz, p-CH3C6H4CH(CH3)2), 1.98
2
CH2), 4.37 (d, 1H, JHH = 15 Hz, CH2), 2.45 (sept,
(s, 3H, p-CH3C6H4CH(CH3)2), 1.11 (d, 3H,
3JHH = 6 Hz, p-CH3C6H4CH(CH3)2), 0.88 (d, 3H,
3
1H, JHH = 6 Hz, p-CH3C6H4CH(CH3)2) 1.97 (s, 3H,
3
3JHH
=
6 Hz, p-CH3C6H4CH(CH3)2). 13C{1H}
p-CH3C6H4CH(CH3)2), 1.55 (d, 3H, JHH = 7 Hz,
3
NMR (CDCl3, 100 MHz, 25◦C): δ 175.9 (NCN-
Ru), 169.8 (CO), 143.0 (ipso-C6H5), 136.90
(ipso-C6H5), 128.9 (m-C6H5), 128.2 (m-C6H5),
128.1 (o-C6H5), 128.0 (o-C6H5), 127.9 (p-
C6H5), 127.1 (p-C6H5), 122.1 (NCHCHN), 121.9
(NCHCHN), 107.8 (ipso-p-CH3C6H4CH(CH3)2),
CH(CH3)2), 1.50 (d, 3H, JHH = 7 Hz, CH(CH3)2),
1.11 (d, 3H, 3JHH = 7 Hz, p-CH3C6H4CH(CH3)2), 0.86
(d, 3H, 3JHH = 7 Hz, p-CH3C6H4CH(CH3)2). 13C{1H}
NMR (CDCl3, 100 MHz, 25◦C): δ 175.4 (NCN-Ru),
169.9 (CO), 143.8 (ipso-C6H5), 128.8 (m-C6H5), 128.2
(o-C6H5), 126.3 (p-C6H5), 122.9 (NCHCHN), 122.2
(NCHCHN), 107.6 (ipso-p-CH3C6H4CH(CH3)2),
99.4
CH3C6H4CH(CH3)2), 84.1 (p-CH3C6H4CH(CH3)2),
83.5 (p-CH3C6H4CH(CH3)2), 82.3 (p-
(ipso-p-CH3C6H4CH(CH3)2),
87.6
(p-
100.3
CH3C6H4CH(CH3)2), 85.4 (p-CH3C6H4CH(CH3)2),
83.2 (p-CH3C6H4CH(CH3)2), 82.1 (p-
(ipso-p-CH3C6H4CH(CH3)2),
88.8
(p-
CH3C6H4CH(CH3)2), 55.2 (COCH2), 53.5 (CH2NH),
51.6 (CH2Ph), 31.1 (p-CH3C6H4CH(CH3)2), 24.4 (p-
CH3C6H4CH(CH3)2), 20.1 (p-CH3C6H4CH(CH3)2),
18.5 (p-CH3C6H4CH(CH3)2). Anal. calcd. for
C29H32RuN3OCl C, 60.56; H, 5.61; N, 7.31. Found: C,
60.35; H, 5.85; N, 7.60.
CH3C6H4CH(CH3)2), 56.7 (COCH2), 55.6 (CH2NH),
52.8 (CH(CH3)2), 52.5 (CH(CH3)2), 31.6 (p-
CH3C6H4CH(CH3)2), 25.1 (p-CH3C6H4CH(CH3)2),
23.4 (CH(CH3)2), 23.2 (CH(CH3)2), 20.3 (p-
CH3C6H4CH(CH3)2). IR (KBr Pellet): 1686 (νCO
)

794
Sachin Kumar et al.
3. Computational methods
(ii) π-back donation from [NHC]− ←[Ru(p-
cymene)Cl]+ fragments and
Density functional theory calculations were performed
on the ruthenium (1–3)c complexes using GAUSSIAN
(iii) repulsive polarization (r).
0319 suit of quantum chemical programs. The Becke The CDA calculations were performed using the pro-
three parameter exchange functional in conjunction gram AOMix,26 using the B3LYP/SDD, 6-31G(d) wave
with Lee–Yang–Parr correlation functional (B3LYP) function. Molecular orbital (MO) compositions and the
has been employed in the study.20 The SDD valence overlap populations were calculated using the AOMix
basis set was used for ruthenium atom,21 while all program. The analysis of the MO compositions in terms
other atoms are treated at the 6-31G(d) basis set.22 of occupied and unoccupied fragment orbitals (OFOs
All stationary points are characterized as minima by and UFOs respectively), construction of orbital inter-
evaluating Hessian indices on the respective poten- action diagrams, the CDA were performed using the
tial energy surfaces. A tight SCF convergence crite- AOMix-CDA.27
ria (10−8 a.u.) was used for all calculations. Natural
Bond Orbital (NBO) analysis was performed using the
NBO 3.123 program implemented in the GAUSSIAN 03
package.
4. Results and discussion
A series of ruthenium (1–3)c complexes was stabilized
over amido-functionalized N-heterocyclic carbene lig-
ands namely, [1-(R)-3-N-(benzylacetamido)imidazol-
2-ylidene] [R = Me, i-Pr and CH2Ph]. Specifically,
the ruthenium (1–3)c complexes were synthesized by
a commonly used carbene transfer route from their
respective silver (1–3)b complexes by treatment with
[Ru(p-cymene)Cl2]2 in 65–76% yield (scheme 1).
As expected, the 13C{1H} NMR spectrum of the
(1–3)c complexes showed the appearance of a slightly
Inspection of the metal–ligand donor–acceptor inter-
actions was carried out using the Charge Decompo-
sition Analysis (CDA)24 which is a valuable tool in
analysing the interactions between molecular fragments
on a quantitative basis, with an emphasis on the electron
donation.25 The orbital contributions in the ruthenium
(1–3)c complexes can be divided into three parts:
(i) σ-donation from the [NHC]− →[Ru(p-cymene)
Cl]+ fragments,
p
i
Scheme 1. Synthesis of Ru–NHC (1–3)c complexes

Ruthenium complexes of chelating amido-functionalized N-heterocyclic carbene ligands
795
upfield shifted (NCN–Ru) resonance at ca. 175.2– in close agreement with the sum of individual
175.9 ppm. In the 1H NMR spectrum, the amido covalent radii of Ru and C atoms (2.018 Å)29 and
(CONH) resonance was conspicuous by its absence, with other reported ruthenium N-heterocyclic car-
unlike in the silver (1–3)b complexes, for which it bene complexes, namely, [RuCl{2-(4,4-dimethyl-
appeared at ca. 9.12–9.36 ppm and thus suggestive of 4,5-dihydrooxazol-2-yl)-(3-mesityl)imidazolidene}
ligand chelation to the metal center after deprotona- (p-cymene)]PF6 [2.038(3) Å],29 [RuCl{2-(4(S)-t-
tion of the amido sidearm moiety. It is worth noting butyl-4,5-dihydrooxazol-2-yl)-(3-mesityl)imidazolidene}
that the deprotonation of the amido-sidearm resulting (p-cymene)]PF6 [2.03(2) Å],29 and [ethylene-bis
in the chelation to the ruthenium center occurred in (N-methylimidazolidene)RuCl(PF6)] [2.033(15) Å].30
the absence of any additional base. Interestingly, the Additionally, the Ru–Cl bond and Ru–Ccentroid bond
(CH2) moieties in the (1–3)c complexes were found to distances in (1c) [2.4256(14) Å and 1.706 Å], (2c)
be diastereotopic in nature as it appeared as two set of [2.4325(8) Å and 1.712 Å] and (3c) [2.4404(7) Å
doublets exhibiting two-bond geminal coupling (2JHH
of 14–16 Hz in the 1H NMR spectra.
)
and 1.719 Å] were comparable to the [RuCl
{2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-(3-mesityl)
The molecular structures of the ruthenium (1–3)c imidazolidene}(p-cymene)]PF6 [2.420(1) Å and 1.708 Å]. 29
complexes showed chelation of the N-heterocyclic car- Along the same line, the Ru–Namido bond distances
bene through an N-amido functionalized sidearm and in (1c) [2.125(5) Å], (2c) [2.126(2) Å] and (3c)
a carbene moiety to the metal center (figure 2 and [2.1074(16) Å] also match well with the other ruthe-
see Supporting Information figures S1–S2 and table nium complexes namely, [hydridotris(pyrazolyl)borate]
S1). The aryl group of the cymene moiety was seen Ru[P(OMe)3]2(NHPh) [2.1012(15) Å]31 and [2-(ben-
bound in a η6-fashion to the metal center in these comp- zoimidazol-2-yl)-6-(3,5-dimethylpyrazol-1-yl)pyridine}]
lexes. The (1–3)c complexes, having chelated amido- RuCl(PPh3)2 [2.088(17) Å].32
functionalized N-heteroyclic carbene ligands, closely
The electronic spectra of the complexes (1–3)c
in acetonitrile showed a strong absorption at 242–
resemble their non-chelated counterparts.28
The Ru-Ccarbene bond distances in (1c) [2.0172(19) 249 nm (2223–2371 M−1 cm−1) attributed to a ligand
Å], (2c) [2.033(5) Å] and (3c) [2.019(3) Å], were based transition along with two shoulders appear at
C23
C15
C22
C16
C17
C18
Ru1
Cl1
N1
C1
N3
N2
O1
Figure 2. ORTEP diagram of 1c with thermal ellipsoids drawn at 50%
probability level. Selected bond length (Å) and bond angles (◦): Ru1-
C1 2.0172(19), Ru1-N3 2.1074(16), Ru1-Cl1 2.4404(7), C1-Ru1-N3
84.31(7), C1-Ru1-Cl1 87.31(6), N1-C1-N2 104.02(16).

796
Sachin Kumar et al.
392–395 nm (314–373 M−1 cm−1) and 459–463 nm hybridized carbene carbon orbital with a sd hybridized
(249–286 M−1 cm−1) (see Supporting Information ruthenium orbital as observed from NBO analysis stud-
figures S3–S5 and table S2). Furthermore, the electro- ies (see Supporting Information tables S7–S8).
chemical behaviour of (1–3)c complexes as analysed
The Charge Decomposition Analysis (CDA) per-
by cyclic voltammetry studies showed two quasi- formed on the (1–3)c complexes provided insight about
reversible oxidation responses on the positive side of the relative extent of NHC ligand to metal σ-donation
SCE at 0.93–0.94 V, attributable to a Ru(II)–Ru(III) denoted by d and the metal to NHC ligand π-back
oxidation process,33 and at 1.41–1.44 V, attributable to donation denoted by b, occurring in these complexes
a Ru(III)–Ru(IV) oxidation process34 (see Supporting (see Supporting Information table S9). The high d/b
Information figures S6–S8 and table S3).
ratio observed for the 1c (5.63), 2c (5.96) and 3c
The NHC–Ru interaction in the (1–3)c complexes (5.28) complexes suggested the predominance of the
has been investigated using density functional theory σ-interaction in the carbene–Ru bond, and which is in
(DFT) studies by computing the minimum energy struc- agreement with the as such good σ-donation ability of
ture at the B3LYP level of theory using 6-31G(d)/SDD the N-heterocyclic carbene ligands. It is worth noting
basis set through geometry optimization using atomic that the σ-donation (d) and π-back donation (b) val-
coordinates adopted from the X-ray analysis and sub- ues obtained for (1–3)c represent the donation occurr-
sequently undertaking the single point calculation. The ing between a metal center of the [Ru(p-cymene)Cl]+
carbene–Ru interaction was probed by performing the and a functionalized NHC-ligand fragment. This par-
post-wave function analysis using the Natural Board ticular interaction is a composite of two interactions
Orbital (NBO) calculations on the (1–3)c complexes as namely a Ru–Ccarbene interaction and a Ru–Namido inter-
well as on the corresponding N-heterocyclic carbene action that arise out of chelation of the NHC ligand to
ligand and the metal ion fragments (see Supporting the metal center through an N–amido sidearm and a
Information tables S4–S12).
carbene moiety.
Both the natural and Mulliken charge analysis
The molecular orbital (MO) correlation diagram,
showed significant increase of electron density at the constructed from the individual fragment molecular
metal center in the (1–3)c complexes with subsequent orbitals (FMOs) of the free NHC ligand and the [Ru(p-
reduction of the same on the free NHC ligand fragment cymene)Cl]+ fragments using AOMix program,35 pro-
(see Supporting Information tables S4–S6). The Ru– vided useful insights on the nature of the Ru–NHC
NHC bond in (1–3)c involved an interaction of the sp2 interaction in the (1–3)c complexes (figure 3 and see
N
10
N
N
N
Ru
N
Cl
N
Ru
O
Cl
5
0
O
(1c)
HOMO-3
9%
6%
-5
HOMO-10
HOMO-12
HOMO-13
26%
19%
-10
-15
-20
4%
3%
HOMO-18
HOMO-6
HOMO-8
Figure 3. Simplified orbital interaction diagram showing the major contribution of NHC-ruthenium bond in 1c.

Ruthenium complexes of chelating amido-functionalized N-heterocyclic carbene ligands
797
Supporting Information figures S9–S10). Of particu- Computer Center are gratefully acknowledged. SK and
lar interest are the molecular orbitals representing the MNR thank Council of Scientific Industrial Research
Ru–NHC σ-interaction that comprized of the inter- (CSIR), New Delhi, for research fellowship.
action of the carbene lone pair with the ruthenium based
orbital namely, 1c (HOMO-18), 2c (HOMO-18) and
3c (HOMO-20). In this regard, it is worth noting that
these Ru–NHC σ-bonding molecular orbitals, by virtue
of being low lying, remain unaffected by the incom-
ing electrophiles or nucleophiles and thereby contribute
toward the stability of the ruthenium (1–3)c complexes.
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Electronic supplementary information
CCDC-676785 (1c), CCDC-697327 (2c) and CCDC-
685445 (3c) contain the supplementary crystallographic
data for this paper. The data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html or
from the Cambridge Crystallographic Data Center, 12
Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.)
+44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk.
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