Rhodium(I) complexes containing 9,10-phenanthrenequinone-N-substituted thiosemicarbazone ligands: Synthesis, structure, DFT study and catalytic diastereoselective nitroaldol reaction studies

Article abstract of DOI:10.1016/j.jorganchem.2015.06.004

Abstract New rhodium(I) complexes [Rh(CO)(L)] (1-3) were prepared by the reaction of [RhCl(CO)(PPh₃)₂] with 9,10-phenanthrenequinone-N-substituted thiosemicarbazones (HL_1-3) and characterized by elemental and spectral anal

Full text of DOI:10.1016/j.jorganchem.2015.06.004

Journal of Organometallic Chemistry 791 (2015) 244e251
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Rhodium(I) complexes containing 9,10-phenanthrenequinone-N-
substituted thiosemicarbazone ligands: Synthesis, structure, DFT
study and catalytic diastereoselective nitroaldol reaction studies
,
*
P. Anitha a, R. Manikandan a, P. Vijayan a, S. Anbuselvi b, P. Viswanathamurthi a
a Department of Chemistry, Periyar University, Salem 636011, Tamil Nadu, India
b Department of Chemistry, Sri Sarada College for Women (Autonomous), Salem 636016, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
New rhodium(I) complexes [Rh(CO)(L)] (1e3) were prepared by the reaction of [RhCl(CO)(PPh3)2] with
9,10-phenanthrenequinone-N-substituted thiosemicarbazones (HL1-3) and characterized by elemental
and spectral analyses (IR, UVeVis, 1H and 13C NMR, ESI-Mass). The structure of the complex, [Rh(CO)(L1)]
(1) has been established by single-crystal X-ray diffraction analysis. The structure has also been opti-
mized by DFT method using LANL2DZ basis set. The synthesized complexes used as catalysts for the
diastereoselective nitroaldol reaction of benzaldehyde with nitroethane in presence of ionic liquid at
room temperature. The optimized catalyst 2 was found efficient in the asymmetric Henry reaction be-
tween various aromatic aldehydes and nitroalkane. The reusability of the catalyst was checked up to five
catalytic runs. The result shows marginal loss in yield and the product retains the diastereoselectivity. It
is believed that this procedure provides an opportunity for facilely synthesize of large amounts of dia-
Received 5 January 2015
Received in revised form
29 May 2015
Accepted 3 June 2015
Available online 6 June 2015
Keywords:
ONS donor ligand
Rhodium(I) complexes
Crystal structure
Henry reaction
Reusability
stereomerically enriched b-nitroalcohol.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
activities of the resulting complexes [13]. With this background, we
have recently started to work on the ligands derived from 1,2-
naphthaquinone/9,10-phenanthrenequinone appended with thio-
semicarbazone/semicarbazone [14]. Among the transition metal
complexes, rhodium complexes have attracted a lot of attention in
the last few decades, mainly due to the metal's scope and versatility
as homogeneous and heterogeneous catalysts in a large variety of
industrial processes [15e18]. These processes include the produc-
tion of acetic acid via the Monsanto process [19,20] and the com-
A great number of Schiff base complexes of transition metal ions
have triggered wide interest because of their diverse spectra of
biological and catalytic properties. In particular, transition metal
complexes of typical Schiff bases, like semicarbazones, thio-
semicarbazones and hydrazones, present a wide range of biological
and catalytic applications [1e8]. Among them, thiosemicarbazones
are of much interest because of their simple preparation, excellent
complexation of not only transition but also non-transition p-ele-
ments, interesting structural characteristics of their complexes,
along with the possibility of their analytical application. This has
resulted in a large number of papers and several reviews [9e12]
that summarized various aspects of the chemistry of these com-
pounds, such as methods of their synthesis, spectral, magnetic,
stereochemical, structural and other characteristics. Hence, for the
past five years, our group has been mainly focusing on the study of
the variable coordination behavior of thiosemicarbazone ligands
towards transition metals, as well as the catalytic and biological
mercial production of L-dopa [21] for the treatment of Parkinson's
disease. The discovery of the catalytic properties of Wilkinson's
catalyst, viz. [RhCl(PPh3)3] naturally brought about a widespread
search for other rhodium complexes with catalytic activity [22,23].
The Henry or nitroaldol reaction is a powerful and atom-
economical carbonecarbon bond forming reaction that can be
used to create a new stereogenic center at the
b-position of nitro
functionality [24]. The resulting product of this reaction is a
b
-
nitroalcohol, which is a versatile intermediate in synthetic organic
chemistry [25]. The nitro group in the product can further be
conveniently converted into several other functionalities to give
synthetically and biologically important bi-functional compounds
[26]. In complicated synthetic ventures, the Henry reaction will
facilitate the combination of two molecular fragments, under mild
* Corresponding author.
E-mail address: viswanathamurthi72@gmail.com (P. Viswanathamurthi).
http://dx.doi.org/10.1016/j.jorganchem.2015.06.004
0022-328X/© 2015 Elsevier B.V. All rights reserved.

P. Anitha et al. / Journal of Organometallic Chemistry 791 (2015) 244e251
245
conditions with the formation of two asymmetric centers at the
new carbonecarbon juncture [27]. Moreover, the reaction also
turns out to be a key step in the preparation of some drugs [28,29].
Since Shibasaki's first report on the asymmetric version of nitro-
aldol reaction in 1992 [30], then the broad spectrum of transition
metal complexes including copper [31], zinc [32], cobalt [33],
chromium [34], palladium [35], rhodium [36], ruthenium [37] and
rare metals [38] have been developed to the catalytic version of the
nitroaldol reaction. However, to the best of our knowledge, no
report of rhodium complexes with ONS donor ligands applied to
nitroaldol reaction found in the literature.
obtain single crystal of the complex were unsuccessful. Yield: 80%;
MP: 215 ꢁC; Anal. calc. for C17H12N3O2RhS (%): C, 48.01; H, 2.84; N,
9.88; S, 7.54. Found (%): C, 48.27; H, 2.60; N, 9.63; S, 7.80. IR (KBr,
cmꢀ1): 1953 (C^O); 1627 (quinone C]O); 1584 (C]N); 757 (CeS).
UVeVis (l
max, nm): 421, 392, 318, 272. 1H NMR (400 MHz, CDCl3,
ppm): 8.88 (s, 1H, NHeCH3); 7.26e8.31 (m, 8H, AreH); 3.33 (s, 3H,
CH3). 13C NMR (100 MHz, CDCl3, ppm): 203.61 (C^O); 180.10
(quinone C]O); 171.08 (CeS); 160.12 (C]N); 127.20, 129.97, 131.41,
132.74, 132.89, 134.21, 134.33, 139.29, 139.93 (AreC); 30.32 (CH3).
ESI-Mass (m/z) ¼ 425.4 [Mþ].
Hence from the above aspects in mind, herein, we have reported
the synthesis, crystal structure, spectral characterization and DFT
study of new rhodium(I) complexes containing 9,10-
phenanthrenequinone N-substituted thiosemicarbazone ligands
with carbonyl as coligand. The new rhodium(I) complexes were
subjected to catalytic nitroaldol reaction of various aldehydes with
nitroalkane in ionic liquid medium.
2.2.3. Synthesis of [Rh(CO)(L3)] (3)
The synthesis of 3 was carried out by the same procedure as
described for 1 with HL3 and [RhCl(CO)(PPh3)2]. Our efforts to
obtain single crystal of the complex were unsuccessful. Yield: 76%;
MP: 229 ꢁC; Anal. calc. for C22H14N3O2RhS (%): C, 54.22; H, 2.90; N,
8.62; S, 6.58. Found (%): C, 54.13; H, 2.64; N, 8.50; S, 6.74. IR (KBr,
cmꢀ1): 1950 (C^O); 1615 (quinone C]O); 1581 (C]N); 740 (CeS).
UVeVis (l
max, nm): 444, 389, 337, 299. 1H NMR (400 MHz, CDCl3,
2. Experimental
ppm): 11.28 (s, 1H, NHeC6H5); 7.26e8.49 (m, 13H, AreH). 13C NMR
(100 MHz, CDCl3, ppm): 202.76 (C^O); 182.65 (quinone C]O);
170.88 (CeS); 161.12 (C]N); 124.26, 128.72, 130.06, 131.73, 131.80,
133.75, 133.84, 139.62, 139.90 (AreC). ESI-Mass (m/z) ¼ 487.2 [Mþ].
2.1. Materials and methods
All the reagents used were chemically pure and AR grade. The
solvents were purified and dried according to standard procedures.
The ligands, HL1-3 and starting complex [RhCl(CO)(PPh3)2] were
prepared according to literature procedures [14b,39]. Microanaly-
ses of carbon, hydrogen, nitrogen and sulfur were carried out using
Vario EL III Elemental analyzer at SAIF e Cochin India. The IR
spectra of the ligand and their complexes were recorded as KBr
2.3. Single crystal X-ray diffraction study
Crystal data were collected on an Oxford/Agilent Gemini
diffractometer. Structure was solved using the direct methods
program SHELXL [40]. All nonsolvent heavy atoms were located
using subsequent difference Fourier syntheses. The structure was
refined against F2 with the program SHELXL [41], in which all data
collected were used including negative intensities. All nonsolvent
heavy atoms were refined anisotropically. All nonsolvent hydrogen
atoms were idealized using the standard SHELXL idealization
methods.
pellets on
a Nicolet Avatar model spectrophotometer in
4000e400 cmꢀ1 range. Electronic spectra of the complexes have
been obtained in dichloromethane using a Shimadzu UV e 1650 PC
spectrophotometer in 800e200 nm range. 1H and 13C NMR spectra
were measured in Jeol GSX e 400 instrument using CDCl3 as the
solvent at room temperature with TMS as the internal standard.
The ESI-Mass spectra were recorded by LC-MS Q-ToF Micro
Analyzer (Shimadzu) in the SAIF, Panjab University, Chandigarh.
HPLC analyses were performed on a SHIMADZU LC-6AD using a
chiral column (Phenomenex Chiralpack) with HPLC grade n-hexane
and isopropanol as solvents. Melting points were checked on a
Technico micro heating table and were uncorrected.
2.4. Theoretical calculations
The gas phase geometry of complex 1 was fully optimized using
density functional theory (DFT) at the B3LYP level using LANL2DZ
basis set and employing the Windows versions of Gaussian 09 [42].
At the same level and basis set, calculations of natural electron
population, natural charge for each atom and frontier molecular
orbitals of the complex have been performed by natural bond
orbital (NBO) analysis on the gas phase optimized structure.
2.2. Synthesis of complexes
2.2.1. Synthesis of [Rh(CO)(L1)] (1)
Anethanolsolution(10 mL)containingHL1 (0.1 mmol)wasadded
to [RhCl(CO)(PPh3)2] (0.1 mmol) in ethanol (10 mL) and the resulting
red color solution was refluxed for 4 h. On cooling the contents to
room temperature, the red colored complex separated out. It was
filtered off and recrystallized from ethanol. Red colored crystals,
suitable for single crystal X-ray diffraction analysis was obtained by
slow evaporation of compound in ethanol/chloroform mixture.
Yield: 85%;MP: 224 ꢁC; Anal. calc. for C16H10N3O2RhS(%):C, 46.73; H,
2.45; N,10.22; S, 7.80. Found (%): C, 46.56; H, 2.31; N,10.40; S, 7.67. IR
(KBr, cmꢀ1): 1952 (C^O); 1625 (quinone C]O); 1578 (C]N); 746
2.5. Catalysis
A mixture of catalyst (3 mmol) and ionic liquid (0.10 mmol) in
2 mL of ethanol was stirred for 15 min. To this solution, nitroalkane
(3 mmol) and aldehyde (1 mmol) were then added. The reaction
mixture was stirred for 8 h at room temperature and then the
solvent was evaporated. The product was separated by repetitive
washing of the residue with petroleum ether and ethyl acetate (8:2)
mixture. After complete washing, the ionic liquid containing cata-
lyst was dried under vacuum and stored in a desiccator for its use in
subsequent catalytic runs. The product was then purified by column
chromatography to afford the corresponding adduct as the product.
The diastereoselectivity and enantiomeric excess of the products
were determined by HPLC analysis. The 1H NMR data of the prod-
ucts were compared with the literature [43].
(CeS). UVeVis (l
max, nm): 453, 393, 334, 297. 1H NMR (400 MHz,
CDCl3, ppm): 9.41 (s, 2H, NH2); 7.26e7.69 (m, 8H, AreH). 13C NMR
(100 MHz, CDCl3, ppm): 203.87 (C^O); 180.24 (quinone C]O);
171.82 (CeS); 159.96 (C]N); 125.60, 127.43, 129.58, 131.03, 131.18,
133.40, 133.56, 139.56, 139.89 (AreC). ESI-Mass (m/z) ¼ 411.4 [Mþ].
2.2.2. Synthesis of [Rh(CO)(L2)] (2)
2.5.1. 1-Phenyl-2-nitro-propan-1-ol
The synthesis of 2 was carried out by the same procedure as
described for 1 with HL2 and [RhCl(CO)(PPh3)2]. Our efforts to
The ee (87%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 95/5 n-hexane/i-PrOH, 1 mL/min,

246
P. Anitha et al. / Journal of Organometallic Chemistry 791 (2015) 244e251
tr (anti, major) ¼ 13.8 min, tr (anti, minor) ¼ 15.5 min, tr (syn,
major) ¼ 18.9 min, tr (syn, minor) ¼ 21.2 min. 1H NMR (300 MHz,
CDCl3, ppm): 7.23e7.47 (m, 5H); 5.28 (d, J ¼ 5.8 Hz, 0.67H) (anti);
5.04 (d, J ¼ 4.4 Hz, 0.33H) (syn); 4.24e4.32 (m, 1H); 2.76 (s, 0.67H)
(anti); 2.60 (s, 0.33H) (syn); 1.49 (d, J ¼ 2.6 Hz, 2.15H) (anti); 1.25 (d,
J ¼ 3.2 Hz, 0.85H) (syn).
2.5.8. 1-(4-Methoxyphenyl)-2-nitro-butan-1-ol
The ee (93%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 95/5 n-hexane/i-PrOH, 1 mL/min,
tr (anti, major) ¼ 20.2 min, tr (anti, minor) ¼ 21.3 min, tr (syn,
major) ¼ 28.6 min, tr (syn, minor) ¼ 31.0 min. 1H NMR (300 MHz,
CDCl3, ppm): 7.20e7.24 (m, 5H); 5.16 (d, J ¼ 2.6 Hz, 0.86H) (anti);
4.88 (d, J ¼ 2.6 Hz, 0.14H) (syn); 4.49e4.67 (m, 1H); 3.84 (s, 3H);
3.03 (s, 0.86H) (anti); 2.90 (s, 0.14H) (syn); 2.10e2.28 (m, 2H); 1.07
(t, J ¼ 3.4 Hz, 2.54H) (anti); 0.96 (t, J ¼ 4.2 Hz, 0.46H) (syn).
2.5.2. 1-(4-Methoxyphenyl)-2-nitro-propan-1-ol
The ee (78%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 90/10 n-hexane/i-PrOH, 1 mL/
min, tr (anti, major) ¼ 11.5 min, tr (anti, minor) ¼ 13.2 min, tr (syn,
major) ¼ 16.4 min, tr (syn, minor) ¼ 18.7 min. 1H NMR (300 MHz,
CDCl3, ppm): 7.44e7.70 (m, 2H); 6.86e7.02 (m, 2H); 5.42 (d,
J ¼ 5.8 Hz, 0.69H) (anti); 4.92 (d, J ¼ 5.8 Hz, 0.31H) (syn); 4.38e4.61
(m,1H); 3.88 (s, 3H); 2.80 (s, 0.76H) (anti); 2.42 (s, 0.24H) (syn); 1.58
(d, J ¼ 3.2 Hz, 2.14H) (anti); 1.22 (d, J ¼ 3.6 Hz, 0.86H) (syn).
2.5.9. 1-(4-Chlorophenyl)-2-nitro-butan-1-ol
The ee (81%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 95/5 n-hexane/i-PrOH, 1 mL/min,
tr (anti, major) ¼ 12.3 min, tr (anti, minor) ¼ 13.8 min, tr (syn,
major) ¼ 21.5 min, tr (syn, minor) ¼ 17.7 min. 1H NMR (300 MHz,
CDCl3, ppm): 7.18e7.30 (m, 5H); 5.14 (d, J ¼ 2.8 Hz, 0.79H) (anti);
4.96 (d, J ¼ 2.4 Hz, 0.21H) (syn); 4.44e4.54 (m, 1H); 3.08 (s, 0.79H)
(anti); 2.72 (s, 0.21H) (syn); 1.68e2.02 (m, 2H); 1.02 (t, J ¼ 3.2 Hz,
3H).
2.5.3. 1-(4-Chlorophenyl)-2-nitro-propan-1-ol
The ee (73%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 90/10 n-hexane/i-PrOH, 1 mL/
min, tr (anti, major) ¼ 15.2 min, tr (anti, minor) ¼ 16.9 min, tr (syn,
major) ¼ 24.0 min, tr (syn, minor) ¼ 21.7 min. 1H NMR (300 MHz,
CDCl3, ppm): 7.58e7.76 (m, 2H); 7.18e7.30 (m, 2H); 5.66 (d,
J ¼ 5.8 Hz, 0.68H) (anti); 5.30 (d, J ¼ 5.0 Hz, 0.32H) (syn); 4.68e4.82
(m, 1H); 2.90 (s, 0.71H) (anti); 2.64 (s, 0.29H) (syn); 1.58 (d,
J ¼ 5.8 Hz, 2.18H) (anti); 1.20 (d, J ¼ 5.8 Hz, 0.82H) (syn).
2.5.10. 1-Cyclohexyl-2-nitro-butan-1-ol
The ee (84%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 99/1 n-hexane/i-PrOH, 1 mL/min,
tr (anti, major) ¼ 44.3 min, tr (anti, minor) ¼ 48.2 min, tr (syn,
major) ¼ 60.0 min, tr (syn, minor) ¼ 56.4 min. 1H NMR (300 MHz,
CDCl3, ppm): 4.48e4.70 (m, 1H); 3.76 (d, J ¼ 4.2 Hz, 0.91H) (anti);
3.60 (d, J ¼ 4.6 Hz, 0.09H) (syn); 2.34 (s, 1H); 1.80e2.11 (m, 2H);
1.26e1.50 (m, 11H); 0.95 (t, J ¼ 4.8 Hz, 3H).
2.5.4. 1-(4-Nitrophenyl)-2-nitro-propan-1-ol
The ee (92%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 80/20 n-hexane/i-PrOH, 1 mL/
min, tr (anti, major) ¼ 16.0 min, tr (anti, minor) ¼ 17.7 min, tr (syn,
major) ¼ 20.2 min, tr (syn, minor) ¼ 22.7 min. 1H NMR (300 MHz,
CDCl3, ppm): 7.51e7.76 (m, 2H); 6.98e7.14 (m, 2H); 5.26 (d,
J ¼ 5.2 Hz, 0.65H) (anti); 4.99 (d, J ¼ 5.6 Hz, 0.35H) (syn); 4.31e4.52
(m, 1H); 2.94 (s, 0.75H) (anti); 2.50 (s, 0.25H) (syn); 1.40 (d,
J ¼ 2.6 Hz, 2.10H) (anti); 1.16 (d, J ¼ 3.2 Hz, 0.90H) (syn).
3. Results and discussion
The synthetic route of rhodium(I) complexes was shown in
Scheme 1. The isolated complexes were stable at room tempera-
ture, non-hygroscopic in nature and highly soluble in common
organic solvents such as dichloromethane, chloroform, benzene,
acetonitrile, ethanol, methanol, dimethylformamide and dime-
thylsulfoxide. All the complexes were structurally characterized by
elemental analyses, IR, electronic, NMR and ESI-Mass spectra. For
further confirmation, the structure of complex 1 was elucidated by
X-ray crystallographic analysis.
2.5.5. 1-(Naphthalen-1-yl)-2-nitro-propan-1-ol
The ee (83%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 98/2 n-hexane/i-PrOH, 1 mL/min,
tr (anti, major) ¼ 30.5 min, tr (anti, minor) ¼ 34.0 min, tr (syn,
major) ¼ 46.1 min, tr (syn, minor) ¼ 53.2 min. 1H NMR (300 MHz,
CDCl3, ppm): 8.26 (d, J ¼ 5.8 Hz, 1H); 7.68e7.82 (m, 2H); 7.26e7.38
(m, 4H); 5.90 (dd, J1 ¼ J2 ¼ 2.8 Hz, 1H); 5.14e5.20 (m, 1H); 2.86 (s,
1H); 1.56 (d, J ¼ 5.4 Hz, 2.52H) (anti); 1.24 (d, J ¼ 5.6 Hz, 0.48H)
(syn).
3.1. Spectroscopic studies
The IR spectra of the ligands and the corresponding rhodium(I)
complexes provided significant information about the metal-ligand
bonding.
A
strong vibration appeared at 1596e1598 and
(C]
(C]O) shifted to lower wave numbers
1630e1634 cmꢀ1 in the ligands corresponding to azomethine
n
2.5.6. 1-Cyclohexyl-2-nitro-propan-1-ol
N) and quinone carbonyl
n
1578e1584 and 1615e1627 cmꢀ1 in all the complexes indicating
the participation of azomethine nitrogen and quinone oxygen in
bonding [44] with rhodium atom. A sharp band was observed at
The ee (94%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 97/3 n-hexane/i-PrOH, 1 mL/min,
tr (anti, major) ¼ 15.7 min, tr (anti, minor) ¼ 19.0 min, tr (syn,
major) ¼ 25.3 min, tr (syn, minor) ¼ 17.5 min. 1H NMR (300 MHz,
CDCl3, ppm): 4.61e4.72 (m, 1H); 3.97 (d, J ¼ 4.8 Hz, 0.84H) (anti);
3.76 (d, J ¼ 4.4 Hz, 0.16H) (syn); 2.40 (s, 1H); 1.76e1.88 (m, 1H); 1.40
(d, J ¼ 6.8 Hz, 2.20H) (anti); 1.26 (d, J ¼ 5.2 Hz, 0.80H) (syn).
807e843 cmꢀ1, ascribed to
n(C]S) in the ligands, has completely
2.5.7. 1-Phenyl-2-nitro-butan-1-ol
The ee (81%) was determined by chiral HPLC analysis on a
Phenomenex Chiralpack column, 95/5 n-hexane/i-PrOH, 1 mL/min,
tr (anti, major) ¼ 26.7 min, tr (anti, minor) ¼ 22.5 min, tr (syn,
major) ¼ 31.2 min, tr (syn, minor) ¼ 28.5 min. 1H NMR (300 MHz,
CDCl3, ppm): 7.18e7.52 (m, 5H); 5.28 (d, J ¼ 2.8 Hz, 0.83H) (anti);
4.98 (d, J ¼ 2.4 Hz, 0.17H) (syn); 4.38e4.52 (m, 1H); 2.76 (s, 0.83H)
(anti); 2.48 (s, 0.17H) (syn), 1.48e1.68 (m, 2H); 0.90 (t, J ¼ 2.8 Hz,
2.48H) (anti); 0.70 (t, J ¼ 3.2 Hz, 0.52H) (syn).
Scheme 1. Synthetic route of rhodium(I) complexes.

P. Anitha et al. / Journal of Organometallic Chemistry 791 (2015) 244e251
247
disappeared in the spectra of all the new rhodium complexes and
the appearance of a new band at 740e757 cmꢀ1 due to
(CeS)
indicate the coordination of the sulfur atom after enolization fol-
n
lowed by deprotonation [45]. Further, the hydrazinic n(NeH) band
appeared at 3111e3148 cmꢀ1 in the free ligands disappeared in the
complexes, suggesting deprotonation of the eNH group [46]. All
the complexes displayed a medium to strong band in the region
1950e1953 cmꢀ1, which was attributed to the terminally coordi-
nated carbonyl group, n(C^O) [47]. Electronic spectra of the com-
plexes showed two very intense absorptions in the UV region
together with two intense absorptions in the visible region. The
absorptions in the UV region were assignable to transitions within
the ligand orbitals while those in the visible region were probably
due to metal-to-ligand charge-transfer transitions.
In the 1H NMR spectra, singlet that appeared for the hydrazinic
n
(NeH) proton of the free ligands at 14.41e14.81 ppm was absent in
the complexes, supporting the enolization and coordination of the
thiolate sulfur to the rhodium(I) ion. The terminal NH2 protons in
ligands HL1 were magnetically non-equivalent, have shown two
singlets at 9.07 and 9.36 ppm. These protons became equivalent
upon formation of rhodium complexes and appeared as a singlet at
9.41 ppm. Ligands HL2, HL3 and their corresponding complexes
showed singlet in the region 8.35e11.28 ppm was assigned to NH
methyl and NH phenyl protons. In the spectra of all the complexes,
the multiplet appeared around 7.26e8.49 ppm was assigned to
aromatic protons of ligands. Further, the methyl protons appeared
in the region 2.98e3.33 ppm. The 13C NMR spectra of the complexes
showed a peak at 202.76e203.87 ppm region was due to terminal
C^O carbon. The presence of peak at 180.24e182.65 ppm region
was assigned to quinone carbonyl (C]O) carbon. The azomethine
(C]N) carbon exhibited a peak in the region of 159.96e161.12 ppm.
In addition, the presence of peak in the region 170.88e171.82 ppm
was assigned to thiosemicarbazone CeS carbon. The appearance of
sharp singlet at 30.32 ppm was assigned to methyl carbon. The
aromatic carbons appeared in the region of 124.26e139.93 ppm.
The m/z value of the molecular ion peak for the complexes 1e3 was
obtained at 411.4, 425.4 and 487.2 [Mþ] respectively. The calculated
molecular mass correspond to these complexes was 411.2, 425.2
and 487.3. The obtained molecular masses were in good agreement
with that of the calculated molecular masses.
Fig. 1. ORTEP view of complex 1. Thermal ellipsoids were drawn at 30% probability
level. The hydrogen atoms were omitted for clarity.
Table 1
Crystal data and structure refinement parameters of the complex 1.
1
Empirical formula
Formula weight
C16H10N3RhO2S
411.24
Crystal dimensions (mm3)
Temperature (K)
Wavelength (Å)
0.32 ꢂ 0.25 ꢂ 0.16
293(2)
0.71073
Crystal system
Space group
Unit cell dimensions (Å,
Monoclinic
P21/c
a ¼ 10.6153(6)
b ¼ 5.1839(16)
c ¼ 35.6301(9)
1960.4(6)
ꢁ
)
Volume (Å3)
Z
4
Calculated density (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
1.393
0.987
816.0
)
Theta range for data collection (ꢁ)
Absorption correction
Refinement method
2.32 to 30.65
Multi-scan
Full-matrix least-squares on F2
6066/0/326
1.032
3.2. X-ray crystallography
Data/restraints/parameters
Goodness-of-fit on F2
Single crystals of 1 were obtained from slow evaporation of a
mixture of chloroform and ethanol at room temperature. A red
crystal of approximate dimensions 0.32 ꢂ 0.25 ꢂ 0.16 mm was
isolated and the single crystal X-ray diffraction experiment was
carried out at 293 K. From the unit cell dimensions, it was clear that
the crystal was monoclinic belonging to the P21/c space group.
Ellipsoidal plot of complex 1 labeled with atom numbering scheme
was shown in Fig. 1. Crystallographic data was given in Table 1. The
coordination geometry around the rhodium(I) ion was slightly
distorted square planar, where the basal plane was constructed of
quinone oxygen, the imine nitrogen and the thiolate sulfur atom of
the ligand in its mononegative tridentate ONS fashion and carbonyl.
The imine nitrogen atom was trans to the carbonyl group and the
quinone oxygen was trans to the thiolate sulfur atom. The bond
lengths were 1.940(18) [Rh(1)eN(1)], 2.163(13) [Rh(1)eO(1)],
2.339(6) [Rh(1)eS(1)] and 2.281(6) Å [Rh(1)eC(16)]. Comparison of
the bond lengths in the coordinated thiosemicarbazone ligand with
those in the uncoordinated ligand [14d] shows that upon coordi-
nation the CeS bond has undergone elongation, whereas the
adjacent CeN bond has undergone contraction. These changes in
bond lengths were consistent with the deprotonation of ligand
upon complexation. The tridentate ONS ligand coordinated
R indices [I > 2
s
(I)]
R1 ¼ 0.1006,
wR2 ¼ 0.2089
R1 ¼ 0.1465,
wR2 ¼ 0.2268
R indices (all data)
equatorially to the metal ion with the formation of two five
membered ring with the bite angles of 83.7(6)ꢁ [N(1)eRh(1)eO(1)]
and 82.7(6)ꢁ [N(1)eRh(1)eS(1)] were less than 90ꢁ while the an-
gles of 96.0(3)ꢁ [O(1)eRh(1)eC(16)] and 97.6(2)ꢁ [S(1)eRh(1)e
C(16)] were greater than 90ꢁ. This results showed significant
distortion of the {RhONS(CO)} core from the ideal square planar
geometry.
3.3. Computational
3.3.1. Geometry optimization
A DFT calculation was performed on the complex 1. The geom-
etry was optimized without symmetry constraints. The structural
parameters (distances and angles) of optimized geometry (Table 2)
obtained by DFT/B3LYP method have a good agreement with X-ray

248
P. Anitha et al. / Journal of Organometallic Chemistry 791 (2015) 244e251
structural data. The negligible difference between experimental
and theoretical data refers to the fact that the experimental data
belong to the solid state and the lattice with interactions in the
crystal structure, while the calculated values refer to a single
molecule in the gas phase.
3.3.2. Frontier molecular orbitals
The electronic, optical properties and chemical reactions of the
complexes mainly depend on frontier molecular orbitals [48].
Hence, the composition and energy levels of the HOMO and LUMO
orbitals may provide insight into the chemical behavior of such
species and those of 1 were depicted in Fig. 2. Positive and negative
regions were shown in red and green colors, respectively. Fig. 2
illustrate that the highest occupied molecular orbital was mainly
delocalized on metal with some contributions from ligands and the
lowest unoccupied molecular orbital was mainly delocalized on
ligands with minor contributions from metal. Since the first elec-
tron transfer occurs from the highest occupied molecular orbital
(HOMO) to lowest unoccupied molecular orbital (LUMO), it can be
inferred that the first electron transfer in 1 was related to MLCT.
Fig. 2. Calculated HOMO and LUMO for 1.
Table 3
Charge (a.u.) and electron configuration of the complex 1.
Atom
Charge
Electronic configuration
Rh
O
N
S
ꢀ0.495498
ꢀ0.430828
0.792792
[core]5s(0.45)4d(8.34)
[core]2s(1.68)2p(4.86)
[core]2s(1.32)2p(3.87)
[core]3s(1.74)3p(4.32)
[core]2s(1.23)2p(2.07)
ꢀ0.382028
3.3.3. Natural bond order analysis (NBO)
C
1.376631
The calculated atomic charge and electron configuration of
donor atoms and metal center were listed in Table 3. The calculated
formal charge on the central ion was quite smaller as compared to
the formal oxidation state of the metal (þ1). The charges on the
carbon and nitrogen atoms were positive, whereas the oxygen and
sulfur atoms were negatively charged. In a similar way, the calcu-
lated electronic configurations of the donor atoms with reference to
s and p orbitals were less than the expected values of valance or-
bitals, while the computed electron population in central ion was
more than the expected value for RhI with d8 electron configura-
tion. This confirms the electron transmission of donor atoms to-
wards the central metal.
3.3.4. Catalytic nitroaldol (Henry) reaction
In order to find optimal reaction conditions, the influence of
time, solvent and the catalyst concentration on the yield were
investigated. Our initial studies on the development of new rho-
dium(I) complexes in ionic liquid for nitroaldol condensation were
carried out with benzaldehyde and nitroethane as model substrates
using complex 1 as catalyst. The first set reactions were run with
constant concentration of catalyst (2.0 mmol) at various time in-
tervals in different solvents. The best conversion was observed in
ethanol and the least conversion was observed in toluene at 8 h
(Fig. 3). Next, we focused the effect of catalyst concentration on the
catalyst activity. The catalyst amount was varied from 0.5 to
Fig. 3. Optimization of solvent for catalytic nitroaldol reaction. Reaction conditions:
Benzaldehyde (1 mmol), nitroethane (3 mmol), catalyst (2
(0.1 mmol) was stirred at r.t for 8 h.
mmol) and ionic liquid
4.0 mmol in ethanol under similar reaction conditions. When the
catalyst amount was 3.0 mmol, an excellent yield was obtained. It
Table 2
Selective experimental (X-ray) and calculated (DFT) bond distances (Å) and bond angles (ꢁ) of the complex 1.
Interatomic distances (Å)
Bond angles (ꢁ)
Experimental
Experimental
Calculated
Calculated
RheN(1)
RheO(1)
RheS(1)
RheC(16)
C(15)eN(2)
C(15)eN(3)
1.940(18)
2.163(13)
2.339(6)
2.281(6)
1.35(3)
2.0536
2.0802
2.4054
1.8905
1.3453
1.3421
N(1)eRheO(1)
N(1)eRheS(1)
O(1)eRheC(16)
S(1)eRheC(16)
C(15)eS(1)eRh
N(2)eN(1)eRh
C(1)eO(1)eRh
C(14)eN(1)eRh
N(2)eC(15)eN(3)
O(1)eRheS(1)
N(1)eRheC(16)
83.7(6)
82.7(6)
96.0(3)
97.6(2)
97.6(8)
123.7(16)
109.0(12)
112.5(13)
116.6(17)
166.4(3)
179.7(6)
78.393
84.332
99.444
97.410
91.925
122.546
112.870
116.641
115.838
162.639
171.611
1.35(3)

P. Anitha et al. / Journal of Organometallic Chemistry 791 (2015) 244e251
249
in the yield of the products irrespective of the electron withdrawing
or electron donating substituents on the phenyl ring of aldehyde. The
products were mixtures of the b-nitroalkanol diastereoisomers (anti
and syn forms) with overall conversions upto 97% and predominance
of the anti isomer (anti/syn selectivity ¼ 89.6:10.4) (entries 1e4). The
substrates 1-naphthaldehyde and cyclohexane carboxaldehyde react
withnitroethaneexhibited good yields with high diastereoselectivity
(entries5and6). However, amongallthesubstrateusedhere,thebest
result in term of activity and diastereoselectivity was achieved with
4-nitro benzaldehyde (entry 4). Because of the good results obtained
with the addition of nitroethane to aldehydes, we decided to extend
the reaction to nitropropane. Diastereoselectivity was dramatically
improved when 1-nitropropane was used and affords the desired
product in good to highyields, accompanied by the predominant anti
diastereomer. Among the substrates, the unsubstituted benzalde-
hyde gave higher yield when compared to that of electron donating/
withdrawing substituent on benzaldehyde (entries 7e9). The sub-
strate cyclohexane carboxaldehyde react with nitropropane exhibi-
ted good yield with high diastereoselectivity (enty 10).
Recovery and reusability of catalysts was an important theme in
catalysis. This makes the catalysts to be used in many catalytic
cycles, there by more commercial for industrial catalysis. The
catalyst was recovered and recycled as follows. After completion of
the catalytic reaction, the solvent was completely evaporated under
reduced pressure and the products were extracted with petroleum
ether and ethyl acetate mixture (80:20). The extract was concen-
trated and purified by column chromatography. The recovered ionic
liquid containing the catalyst 2 was dried in vacuum and was used
for the subsequent catalytic runs without any further processing.
The recovered catalyst worked well up to five catalytic runs with
marginal loss in yield. During the catalytic recycle reactions the
diastereoselectivity of the product was retained (Table 5). In addi-
tion, the present catalytic system works at mild reaction conditions
with low reaction time, low catalyst loading and the efficiency in
terms of the yield of products was higher than the existing catalytic
systems [36,37,49e52].
Fig. 4. Optimization of catalyst loading in nitroaldol reaction. Reaction conditions:
Benzaldehyde (1 mmol), nitroethane (3 mmol) and ionic liquid (0.1 mmol) in ethanol
was stirred at r.t for 8 h.
was also observed that moderate yield was obtained even at very
low catalyst loading of 1.0 mmol (Fig. 4). It was noteworthy that the
reaction not able to proceed without a catalyst under similar re-
action conditions. In order to choose the best catalyst among the
synthesized complexes, the nitroaldol reaction of benzaldehyde
with nitroethane was carried out using the catalysts 1, 2 and 3
under the above optimized conditions. From the results, the com-
plex 2 was found to the best catalyst and showed the yield of 94%
(Fig. 5). The order of reactivity of complexes with respect to
different substitutions on thiosemicarbazone fragment of the li-
gands was given by, 2 > 1 > 3.
A proposed mechanism for nitroaldol reaction of benzaldehyde
with nitroethane in presence of rhodium complex 1 has been
shown in Scheme 2. The aldehyde was coordinated to the vacant
d orbital of the rhodium through the lone pair of the oxygen
forming a penta-coordinate transition state (I) thereby increasing
the electrophilicity of the carbonyl group. The coordination number
of the rhodium center may be extended to six from five on further
addition of active nucleophile, nitronate ion forming transition
state (II) where the nitronate ion attacks the activated aldehyde to
give the nitroaldol product. With respect to the stereo chemistry,
the attachment through the hydrogen side is more favorable
compared to that of highly steric phenyl group side. Hence, anti
attachment predominantly takes place and form anti product as a
major one. In order to confirm the intermediates formed during the
course of reaction, the ESI-Mass spectrum was recorded for the
reaction of benzaldehyde with nitroethane in presence of the
complex 1. It was observed that gradual replacement of main peak
associated with the complex 1, (m/z ¼ 411) by new peaks assigned
The above optimized reaction protocol for nitroaldol reaction was
further extended to various aromatic aldehydes using the complex 2
as catalyst in presence of [emim]BF4 as ionic liquid and the results
were summarized in Table 4. When nitroethane was used, all the
aldehydes used in the present studyshowed only marginal difference
to ([Rh(CO)(L1)(benzaldehyde)]þ) (I) (m/z
¼
517) and
[Rh(CO)(L1)(benzaldehyde) (nitroethane)]þ) (II) (m/z ¼ 591). These
peak displacements support the coordination of substrates with
metal and forming two intermediates.
4. Conclusion
In this study, new four coordinated rhodium(I) complexes con-
taining
semicarbazone derivatives have been synthesized and characterized.
The molecular structure of representative complex 1, was
9,10-phenanthrenequinone
appended
with
thio-
Fig. 5. Selection of suitable catalyst for catalytic nitroaldol reaction. Reaction condi-
tions: Benzaldehyde (1 mmol), nitroethane (3 mmol), catalyst (3
(0.1 mmol) in ethanol was stirred at r.t for 8 h.
mmol) and ionic liquid
a

250
P. Anitha et al. / Journal of Organometallic Chemistry 791 (2015) 244e251
Table 4
Nitroaldol (Henry) reaction of nitroethane and nitropropane with various aldehydes.a
Entry
R
R1
Yield (%)a
anti:syn (%)b
ee of anti (%)b
1
2
3
4
5
6
7
8
9
10
Ph
Me
Me
Me
Me
Me
Me
Et
Et
Et
Et
94
96
94
97
91
88
98
97
90
92
68.8:31.2
75.0:25.0
63.8:36.2
89.6:10.4
71.3:28.7
86.5:13.5
95.6:4.4
72.4:27.6
77.2:22.8
90.7:9.3
87
78
73
92
83
94
81
93
81
84
4-MeOeC6H4
4-CleC6H4
4-NO2eC6H4
1-Naphthyl
Cyclohexyl
Ph
4-MeOeC6H4
4-CleC6H4
Cyclohexyl
a
Determined by column chromatography.
Determined by chiral HPLC analysis.
b
Table 5
Catalytic results showed that the synthesized rhodium(I) complexes
were effective catalysts for the diastereoselective nitroaldol reaction,
leading to b-nitroalkanols with high yield and predominance of the
Recyclability test of the catalyst 2 using benzaldehyde and nitroethane as model
substrates in presence of ionic liquid [emim]BF4.
Run
Yield (%)a
anti:syn (%)b
ee of anti (%)b
anti diastereoisomer. The present ionic liquid mediated nitroaldol
protocolwasrecyclable(uptofivecycles)withnosignificantlossinits
performance. Optimized catalyst 2, promotes the diastereoselective
Henry reaction between various aromatic aldehyde and nitroalkane
substrates and gives the corresponding anti-selective adduct with up
to 98% yield and 95.6:4.4 anti/syn selectivity. A possible catalytic
mechanism was also suggested for the asymmetric Henry reactions.
Moreover, in comparison with other reported metal catalysts for the
Henry reaction, our catalytic system seems to be the best with respect
to activity, catalyst loading, time and recyclability.
1st
94
94
93
92
91
68.8:31.2
68.8:31.2
68.8:31.2
68.8:31.2
68.8:31.2
87
87
87
87
87
2nd
3rd
4th
5th
a
Determined by column chromatography.
Determined by chiral HPLC analysis.
b
determined by single crystal X-ray diffraction method and reveals a
distorted square planar geometry around the rhodium(I) ion. To
support the solid state structure, the geometric parameter of 1 was
calculated using the density functional theory at B3LYP level with
LANL2DZ basis set and compared with the experimental data.
Acknowledgment
The authors express their sincere thanks to Council of Scientific
and Industrial Research (CSIR), New Delhi, India [Grant No.
01(2437)/10/EMR-II] for financial support. One of the authors (PA)
thanks CSIR for the award of Senior Research Fellowship. The au-
thors wish to record grateful thanks to Dr. V. Jayamani, Department
of Chemistry, Sri Sarada College for Women, Salem-636016, Tamil
Nadu, India for discussion in DFT studies.
Supplementary material
CCDC 1035060 contains the supplementary crystallographic
data for the complex 1. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data request/cif.
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