Half-sandwich rhodium complexes with phenylene-based SCS ligands: Synthesis, characterization and catalytic activities for transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.poly.2020.114978

A series of half-sandwich rhodium complexes with tridentate phenylene-based bis(thione) (SCS) ligand have been synthesized and characterized. Both half-sandwich rhodium complexes and phenylene-based bis(thione) compounds were fully characterized by ¹H and ¹³C NMR spectra, mass spectrometry and single-crystal X-ray diffraction method. The catalytic activities of half-sandwich rhodium complexes toward the transfer hydrogenation of ketones to their corresponding alcohols were explored using 2-propanol as hydrogen source and solvent. And the half-sandwich rhodium complexes exhibited high catalytic activity for transfer hydrogenation of ketones with a broad functional group tolerance.

Full text of DOI:10.1016/j.poly.2020.114978

Polyhedron 195 (2021) 114978
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Half-sandwich rhodium complexes with phenylene-based SCS ligands:
Synthesis, characterization and catalytic activities for transfer
hydrogenation of ketones
a
Wei-Guo Jia a,b, , Li-Li Gao , Xue-Ting Zhi a,b, Xiao-Dong Li a,b, Zhi-Bao Wang a, Ying Sun a
⇑
a The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials (State Key Laboratory Cultivation Base), College
of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, China
b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of half-sandwich rhodium complexes with tridentate phenylene-based bis(thione) (SCS) ligand
have been synthesized and characterized. Both half-sandwich rhodium complexes and phenylene-based
bis(thione) compounds were fully characterized by 1H and 13C NMR spectra, mass spectrometry and sin-
gle-crystal X-ray diffraction method. The catalytic activities of half-sandwich rhodium complexes toward
the transfer hydrogenation of ketones to their corresponding alcohols were explored using 2-propanol as
hydrogen source and solvent. And the half-sandwich rhodium complexes exhibited high catalytic activity
for transfer hydrogenation of ketones with a broad functional group tolerance.
Received 17 September 2020
Accepted 11 December 2020
Available online 24 December 2020
Keywords:
Rhodium
Thione
Complex
Structure
Catalyst
Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction
transition metal complexes bearing imidazolin-2-thiones moieties
have been synthesized and investigated [23–29]. The half-sand-
Half-sandwich rhodium complexes have been extensively stud-
ied in metallomacrocycles, host–guest chemistry, and catalysts
over the past decades [1–4]. Various interlocked structures includ-
ing metalla catenanes [5–7], molecular Borromean rings (BRs) [8–
10], and Solomon links [11–12] have been successively obtained by
coordination-driven self-assembly using Cp*Rh as organometallic
fragment. The half-sandwich rhodium(II) complexes were efficient
catalysis for CAC or C-heteroatom bond formation by direct CAH
bond activation reaction through C-Rh bond intermediate [13–
18]. Various aromatic or heteroaromatic molecules have been
obtained by half-sandwich rhodium complexes catalyzed CAH
bond activation. The rhodium complexes were potential candidate
for transfer hydrogen reaction of ketones [19–22]. It is indicated
that the catalytic activities of the half-sandwich rhodium com-
plexes depend on the steric and electronic properties of the ligands
employed.
wich iridium complexes with methylene bridged imidazoline-2-
chalcogenone ligands were used catalysts for olefin polymerization
[30]. We recently reported the palladium complexes with pheny-
lene-bridged bis(thione) ligands as efficient catalyst towards
reduction of quinolines and nitro arene reduction [31–32]. Inspired
by this work, we synthesized the half-sandwich rhodium com-
plexes (4a-4d) with methoxy-tagged phenylene-based bis(thione)
(SCS) ligand (3a-3d) (Scheme 1) and explored their catalytic per-
formance for transfer hydrogenation of ketones to their corre-
sponding alcohols using 2-propanol as hydrogen source and
solvent. The results demonstrated that half-sandwich rhodium
complexes exhibited high catalytic activities for the transfer
hydrogenation process. The molecular structure of 4a was also
determined by X-ray crystallography.
The half-sandwich rhodium complexes with N-, O-, S-, P-donor
atoms or C atom of N-heterocyclic carbene (NHC) fragment usually
adopt three-legged piano-stool configuration. Due to the versatile
coordination properties of the imidazoline-2-chalcogenone, many
2. Experimental
2.1. Materials and physical measurements
All manipulations or operations were carried out under a pure
nitrogen atmosphere (99.99%) using standard Schlenk techniques.
Solvents were purified and degassed by standard procedures
[33]. The 1,10-(5-methoxy-1,3-phenylene)bis(1H-imidazole) (1)
⇑
Corresponding author.
E-mail address: wgjiasy@mail.ahnu.edu.cn (W.-G. Jia).
https://doi.org/10.1016/j.poly.2020.114978
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

Wei-Guo Jia, Li-Li Gao, Xue-Ting Zhi et al.
Polyhedron 195 (2021) 114978
Scheme 1. Synthesis of phenylene-bridged bis(thione) ligand (3a-3d).
was synthesized according to literature [34]. 1H and 13C NMR were
recorded on a 400 MHz NMR spectrometer at room temperature.
Chemical shifts (d) are given in ppm relative to internal TMS
(Tetramethylsilane) and are internally referenced to residual 1H
and 13C solvent resonances. The following abbreviations were used
for the assignment of the signals and their multiplicities: s (sin-
glet), d (doublet), t (triplet), q (quartet), m (multiplet). The given
coupling constants J are listed as the average of the experimental
findings. The IR (KBr pellet) spectrum was recorded (400–
4000 cmꢀ1 region) on a Bruker TENSOR 27 FT-IR spectrometer.
Electrospray ionization mass spectra (ESI-MS) were recorded on
Bruker micrOTOF-Q II 10,280 mass spectrometer. GC–MS analyses
were carried out with a Thermo Scientific TRACE 1300 & ISQ QD
system.
(1.1000 g, 8.0 mmol). The mixtures were stirred at 110 °C for
10 h. The white solids formed were filtered and washed with ethyl
acetate and diethyl ether to afford salts 2c (yield:1.01 g, 98%). 1H
NMR (400 MHz, DMSO d6): d 10.40 (t, 2H), 8.64 (d, J = 7.6 Hz,
2H), 8.17 (d, J = 1.6 Hz, 2H), 8.12 (m, 1H), 7.71 (s, 2H), 4.31 (t,
4H), 4.00 (s, 3H), 1.93 (m, 4H), 1.34 (m, 4H), 0.93 (t, 6H) 13C NMR
(100 MHz, DMSO d6): d 161.2, 136.5, 135.9, 123.5, 121.0, 108.3,
107.0, 57.0, 49.3, 31.0, 18.8, 13.3.
2.2.4. Syntheses of 1,3-bis(3-allylimidazolium)-5-methoxybenzene
bromide (2d)
The reagents used were 1,10-(5-methoxy-1,3-phenylene)bis
(1H-imidazole) (1) (0.4800 g, 2.0 mmol) and 3-bromoprop-1-ene
(0.9600 g, 8 mmol). The mixtures were stirred at 110 °C for 10 h.
The white solids formed were filtered and washed with ethyl acet-
ate and diethyl ether to afford salts 2d (yield:0.93 g, 96%). 1H NMR
(400 MHz, DMSO d6): d 10.31 (t, 2H), 8.66–8.62 (m, 2H), 8.11 (t,
J = 1.6 Hz, 3H), 7.71 (s, 2H), 6.17 (m, 2H), 5.45 (t, 4H), 5.00 (d,
J = 5.2 Hz, 4H), 4.00 (s, 3H). 13C NMR (100 MHz, DMSO d6): d
161.1, 136.5, 136.0, 131.1, 123.4, 121.2, 121.0, 108.5, 107.1, 57.0,
51.5.
2.2. Syntheses of compounds 2a-2d
Phenylene-bridged imidazolium salts 2a-2d were synthesized
by dissolving 1 (1 eq.) in DMF (5 mL) and corresponding alkyl
halide (3.0 eq.). The reaction mixtures were stirred at 110 °C for
10 h. The white solids formed were filtered and washed with ethyl
acetate and diethyl ether to afford salts 2a-2d.
2.3. Syntheses of phenylene-bridged bis(thione) ligands
2.2.1. Syntheses of 1,3-bis(3-methylimidazolium)-5-methoxybenzene
iodide (2a)
A 100 mL round-bottomed flask fitted with a reflux condenser
was charged with imidazolium salt (1.5 mmol), sulfur powder
(0.1440 g, 4.5 mmol), K2CO3 (0.8300 g, 6 mmol) and 30 mL metha-
nol as solvent. The mixture was refluxed for 24 h and then the sol-
vent removed in vacuo. The remaining solid was extracted with
2 ꢁ 30 mL CH2Cl2, then filtered and rotary evaporated to give cor-
responding product.
The reagents used were 1,10-(5-methoxy-1,3-phenylene)bis
(1H-imidazole) (1) (0.4800 g, 2.0 mmol) and iodomethane
(1.1400 g, 8.0 mmol). The mixtures were stirred at 110 °C for
10 h. The white solids formed were filtered and washed with ethyl
acetate and diethyl ether to afford salts 2a (yield: 1.0100 g, 97%).
1H NMR (400 MHz, DMSO d6): d 9.97 (s, 2H), 8.41 (d, 2H,
J = 1.2 Hz, 2H), 8.04 (d, J = 1.2 Hz, 2H), 7.91 (s, 1H), 7.65 (d,
J = 1.6 Hz, 2H), 3.99 (d, J = 4 Hz, 9H).13C NMR (100 MHz, DMSO d6):
d 161.1, 136.5, 136.4, 124.6, 120.8, 108.5, 107.1, 56.9, 36.5.
2.3.1. Syntheses of 1,3-bis(30-methylimidazole-20-thione)-5-
methoxybenzene (3a)
The reagents used were 2a (0.7900 g, 1.5 mmol) and sulfur
powder (0.1443 g, 4.5 mmol), K2CO3 (0.8300 g, 6 mmol). The mix-
ture was allowed to reflux for 24 h and then the solvent was
removed with a rotary evaporator. The reminding solid was
extracted with 2 ꢁ 30 mL CH2Cl2 which was then filtered and
rotary evaporated to give product (0.4536 g, 91%). 1H NMR
(400 MHz, CDCl3): d 7.32 (t, J = 2.0, 1.6 Hz, 1H), 7.20 (d,
J = 2.0 Hz, 2H), 6.86 (d, J = 2.4 Hz, 2H), 6.77 (d, J = 2.4 Hz, 2H),
3.75 (s, 3H), 3.54 (s, 6H). 13C NMR (100 MHz, CDCl3): d 162.7,
159.8, 139.1, 118.8, 117.4, 115.0, 111.3, 55.8, 35.2.
2.2.2. Syntheses of 1,3-bis(3-ethylimidazolium)-5-methoxybenzene
bromide (2b)
The reagents used were 1,10-(5-methoxy-1,3-phenylene)bis
(1H-imidazole) (1) (0.4800 g, 2.0 mmol) and bromoethane
(0.8730 g, 8.0 mmol). The mixtures were stirred at 110 °C for
10 h. The white solids formed were filtered and washed with ethyl
acetate and diethyl ether to afford salts 2b (yield: 0.89 g, 97%). 1H
NMR (400 MHz, DMSO d6): d 10.39 (d, 2H), 8.65 (m, 2H), 8.18 (s,
2H), 8.13 (s, 1H), 7.70 (d, J = 2.0 Hz, 2H), 4.34 (q, 4H), 4.00 (s,
3H), 1.56 (t, 6H). 13C NMR (100 MHz, DMSO d6): d 161.2, 136.5,
135.7, 123.2, 120.9, 108.1, 106.7, 57.1, 45.0, 14.8.
2.3.2. Syntheses of 1,3-bis(30-ethylimidazole-20-thione)-5-
methoxybenzene (3b)
2.2.3. Syntheses of 1,3-bis(3-butylimidazolium)-5-methoxybenzene
bromide (2c).
The reagents used were 2b (0.6870 g, 1.5 mmol) and sulfur
powder (0.1443 g, 4.5 mmol), K2CO3 (0.8300 g, 6 mmol). The mix-
ture was allowed to reflux for 24 h and then the solvent was
removed with a rotary evaporator. The reminding solid was
The reagents used were 1,10-(5-methoxy-1,3-phenylene)bis
(1H-imidazole) (1) (0.4800 g, 2.0 mmol) and 1-bromobutane
2

Wei-Guo Jia, Li-Li Gao, Xue-Ting Zhi et al.
Polyhedron 195 (2021) 114978
extracted with 2 ꢁ 30 mL CH2Cl2 which was then filtered and
rotary evaporated to give product (0.4852 g, 90%). 1H NMR
(400 MHz, CDCl3): d 7.33 (t, J = 2.0, 1.6 Hz, 1H), 7.21 (d,
J = 1.6 Hz, 2H), 6.88 (d, J = 2.4 Hz, 2H), 6.79 (d, J = 2.4 Hz, 2H),
4.6 (q, J = 7.2, 4H), 3.78 (s, 3H), 1.32 (t, J = 7.2 Hz, 6H). 13C NMR
(100 MHz, CDCl3): d 162.0, 159.8, 139.0, 117.8, 117.0, 115.5,
111.6, 55.8, 42.9, 13.9.
2.4.2. Syntheses of [1,3-bis(30-ethylimidazole-20-thione-
methoxybenzene-j-C] pentamethyl-g
chloride (4b)
j
-S)-5-
5-cyclopentadienyl rhodium(III)
The reagents used were 3b (0.0541 g, 0.15 mmol) and
[Cp*Rh( -Cl)Cl]2 (0.0464 g, 0.075 mmol). The mixture was kept
l
at room temperature stirring for 0.5 h. The reaction mixture
was concentrated to 2 mL and 10 mL of Et2O was added to pre-
cipitate a dark-red solid. The solid was collected by filtration,
washed with Et2O, and dried in vacuo (85.5 mg, 90%). 1H
NMR (400 MHz, CD3OD): d 7.82 (d, J = 2.4 Hz, 2H),7.44 (d,
J = 2.4 Hz, 2H), 6.98 (s, 2H) 4.51–4.42 (m, 2H), 4.33–4.24 (m,
2H), 3.89 (s, 3H), 1.50 (t, J = 7.2 Hz, 6H), 1.33 (s, 15H). 13C
NMR (100 MHz, CD3OD): d 160.2, 157.8, 142.3, 120.3, 120.0,
108.8, 99.6, 99.6, 56.4, 44.7, 15.1, 8.6. IR (KBr cmꢀ1): 3423
(vs), 3171(m), 3037(s), 2970(s), 2911(m), 1601(s), 1572(m),
1469(s), 1454(s), 1435(s), 1408(s), 1377(m), 1343(m), 1320(m),
1277(s), 1225(m), 1207(s), 1107(w), 1073(s), 1040(w), 1018
(m), 991(w), 953(w), 871(m), 845(s), 766(m), 725(m), 668(m),
526(w). ESI-MS m/z: Calcd for [C27H34ON4S2Rh]+ ([M - Cl]+)
597.1224; found: 597.1253; Calcd for [C27H35ON4S2Rh]+
([M + H - Cl]+) 598.1256; found: 598.1250.
2.3.3. Syntheses of 1,3-bis(30-butylimidazole-20-thione)-5-methoxy-
benzene (3c)
The reagents used were 2c (0.7711 g, 1.5 mmol) and sulfur pow-
der (0.1443 g, 4.5 mmol), K2CO3 (0.83 g, 6 mmol). The mixture was
allowed to reflux for 24 h and then the solvent was removed with a
rotary evaporator. The reminding yellow solid was extracted with
2 ꢁ 30 mL CH2Cl2 which was then filtered and rotary evaporated to
give product (0.5639 g, 90%). 1H NMR (400 MHz, CDCl3): d 7.36 (t,
J = 2.0 Hz, 1H), 7.26 (t, J = 2.0 Hz, 2H), 6.91 (d, J = 2.4 Hz, 2H), 6.78
(d, J = 2.4 Hz, 2H), 4.04 (t, J = 7.2 Hz, 4H), 3.82 (s, 3H), 1.76 (m, 4H),
1.37 (m, 4H), 0.93 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3): d
162.4, 160.0, 139.2, 117.8, 117.7, 115.8, 111.8, 55.9, 47.8, 30.7, 19.8,
13.7.
2.3.4. Syntheses of 1,3-bis(30-allylimidazole-20-thione)-5-
methoxybenzene (3d)
2.4.3. Syntheses of [1,3-bis(30-butylimidazole-20-thione-
methoxybenzene-j-C] pentamethyl-g
j
-S)-5-
5-cyclopentadienyl rhodium(III)
The reagents used were 2d (0.7230 g, 1.5 mmol) and sulfur
powder (0.1443 g, 4.5 mmol), K2CO3 (0.83 g, 6 mmol). The mixture
was allowed to reflux for 24 h and then the solvent was removed
with a rotary evaporator. The reminding yellow solid was extracted
with 2 ꢁ 30 mL CH2Cl2 which was then filtered and rotary evapo-
rated to give product (0.5424 g, 94%). 1H NMR (400 MHz, CDCl3): d
7.39 (t, J = 2.4 Hz, 1H), 7.28 (t, J = 2.0 Hz, 2H), 6.94 (d, J = 2.4 Hz, 2H),
6.79 (d, J = 2.4 Hz, 2H), 5.98–5.88 (m, 2H), 5.31–5.24 (m, 4H), 4.70
(d, J = 6.0 Hz, 4H), 3.83 (s, 3H). 13C NMR (100 MHz, CDCl3): d 162.7,
160.0, 139.1, 131.5, 119.6, 117.9, 117.4, 115.5, 111.7, 55.9, 50.2.
chloride (4c)
The reagents used were 3c (0.0541 g, 0.13 mmol) and [Cp*Rh(l-
Cl)Cl]2 (0.0402 g, 0.065 mmol). The mixture was kept at room tem-
perature stirring for 0.5 h. The reaction mixture was concentrated
to 2 mL and 10 mL of Et2O was added to precipitate a dark-red
solid. The solid was collected by filtration, washed with Et2O, and
dried in vacuo (84.2 mg, 95%). 1H NMR (400 MHz, CD3OD): d
7.81 (d, J = 2.4 Hz, 2H), 7.42 (d, J = 2.4 Hz,2H), 6.99 (s, 2H), 4.53–
4.46 (m, 2H), 4.21–4.14 (m, 2H), 3.89 (s, 3H), 1.92–1.82 (m, 4H),
1.49–1.40 (m, 4H), 1.32 (s, 15H), 1.02 (t, J = 7.2 Hz, 6H). 13C NMR
(100 MHz, CD3OD): d 158.8, 156.6, 141.0, 119.6, 118.5, 107.5,
98.3, 98.2, 55.1, 31.3, 19.3, 12.6, 7.3. IR (KBr cmꢀ1): 3426(vs),
3150(m), 3077(s), 2980(m), 1612(s), 1572(w), 1507(vw), 1463(s),
14035(s), 1318(m), 1265(m), 1225(w), 1200(m), 1075(m), 1025
(w), 984(m), 938(m), 870(w), 847(w), 726(m), 677(w), 530(w).
ESI-MS m/z: Calcd for [C31H42ON4S2Rh]+ ([M – Cl]+) 653.1850;
found: 653.1887; Calcd for [C31H43ON4S2Rh]+ ([M + H – Cl]+)
654.1882; found: 654.1878.
2.4. Syntheses of half-sandwich rhodium complexes
To a solution of corresponding phenylene-bridged bis(thione)
ligands (2 equiv.) in MeOH (15 mL), [Cp*Rh(l-Cl)Cl]2 (1 equiv.)
were added and the mixture was kept at room temperature stirring
for 0.5 h. The reaction mixture was concentrated to 2 mL and
10 mL of Et2O was added to precipitate a dark-red solid. The solid
was collected by filtration, washed with Et2O, and dried in vacuo.
2.4.4. Syntheses of [1,3-bis(30-allylimidazole-20-thione-
methoxybenzene-j-C] pentamethyl-g
chloride (4d)
j
-S)-5-
2.4.1. Syntheses of [1,3-bis(30-methylimidazole-20-thione-
j
-S)-5-
5-cyclopentadienyl rhodium(III)
methoxybenzene-j-C] pentamethyl-
g
5-cyclopentadienyl rhodium(III)
chloride (4a)
The reagents used were 3d (0.0461 g, 0.12 mmol) and [Cp*Rh
The reagents used were 3a (0.0465 g, 0.14 mmol) and [Cp*Rh(
l-
(l-Cl)Cl]2 (0.0401 g, 0.06 mmol). The mixture was kept at room
Cl)Cl]2 (0.0442 g, 0.07 mmol). The mixture was kept at room tem-
perature stirring for 0.5 h. The reaction mixture was concentrated
to 2 mL and 10 mL of Et2O was added to precipitate a dark-red
solid. The solid was collected by filtration, washed with Et2O, and
dried in vacuo (79.6 mg, 95%). 1H NMR (400 MHz, CD3OD): d
7.79 (d, J = 2.4 Hz, 2H), 7.36 (d, J = 2.4 Hz, 2H), 6.97 (s, 2H), 3.88
(d, J = 1.6 Hz, 9H), 1.33 (s, 15H). 13C NMR (100 MHz, CD3OD): d
160.2, 158.6, 142.4, 132.8, 121.9, 119.6, 108.7, 99.6, 99.5, 56.4,
35.8, 8.6. IR (KBr cmꢀ1): 3440(vs), 3062(s), 3006(s), 1602(s), 1576
(m), 1471(s), 1391(s), 1354(m), 1307(s), 1274(m), 1260(m), 1233
(m), 1214(s), 1194(w), 1100(w), 1073(s), 1022(s), 982(w), 870
(m), 847(s), 832(m), 773(w), 747(vw), 728(s), 717(m), 701(w),
675(s), 640(vw), 618(vw), 531(w). ESI-MS m/z: Calcd for
[C25H30ON4S2Rh]+ ([M - Cl]+) 569.0911; found: 569.0941; Calcd
for [C25H31ON4S2Rh]+ ([M + H - Cl]+) 570.0943; found: 570.0951.
temperature stirring for 0.5 h. The reaction mixture was concen-
trated to 2 mL and 10 mL of Et2O was added to precipitate a
dark-red solid. The solid was collected by filtration, washed with
Et2O, and dried in vacuo (70.5 mg, 92%). 1H NMR (400 MHz,
CD3OD): d 7.85 (d, J = 2.4 Hz, 2H), 7.39 (d, J = 2.4 Hz,2H), 7.00
(s, 2H), 6.11–6.026 (m, 2H), 5.39 (m, 4H), 5.07–5.01 (m, 2H),
4.91 (m, 2H), 3.90 (s, 3H), 1.34 (s, 15H). 13C NMR (100 MHz,
CD3OD): d 160.2, 158.3, 142.3, 132.3, 120.8, 120.7, 120.1, 108.9,
99.7, 99.6,56.4, 51.6, 8.7. IR (KBr cmꢀ1): 3423(vs), 3159(m),
3075(s), 2976(m), 2909(m), 1602(s), 1575(w), 1505(vw), 1460
(s), 1403(s), 1317(m), 1263(m), 1229(w), 1202(m), 1071(m),
1020(w), 988(m), 934(m), 868(w), 846(w), 726(m), 678(w), 529
(w). ESI-MS m/z: Calcd for [C29H34ON4S2Rh]+ ([M
-
Cl]+)
621.1224; found: 621.1251; Calcd for [C29H35ON4S2Rh]+
([M + H - Cl]+) 622.1265; found: 622.1241.
3

Wei-Guo Jia, Li-Li Gao, Xue-Ting Zhi et al.
Polyhedron 195 (2021) 114978
2.5. General procedure for the reduction of ketone compounds with
half-sandwich rhodium complex
formation through direct CAH bond activation in the absence of
base. The downfield shift was observed for the doublet at 6.76
and 6.86 ppm (3a) to 6.97 and 7.35 ppm (4a) for imidazole protons
of thione ligand in half-sandwich rhodium complex. Furthermore,
13C NMR spectrum of 4a showed characteristic resonance for the
C of NHC = S at d = 160.17 and 158.59 ppm, significantly shifted
upfield compare with the Cimidazole resonance of 3a
(d = 162.66 ppm). It is in agreement with the values already
reported rhodium complex [30,37–38]. Further support for the for-
mation of half-sandwich rhodium complex 4a was provided by
mass spectrometry, for which m/z [M - Cl]+ 569.0951 (Calcd for
[C25H30ON4S2Rh]+ [M - Cl]+ 569.0911) was observed for complex
4a. It should be noted that the [M + H - Cl]+ (m/z: 570.0951, Calcd
for [C25H31ON4S2Rh]+ ([M + H - Cl]+) 570.0943) peak was also
detected in complex 4a.
A mixture of ketones (0.1 mmol), NaOH (0.4 mmol, 4 equiv.),
and half-sandwich rhodium complex (0.5 mol%) in 2-propanol
(1.0 mL) was heated to 80 °C in the presence of 4 Å molecular sieve
for 3 h. After complete reaction (monitored by TLC), the reaction
mixture was cooled to room temperature. The solvent was
removed in vacuo, extracted with ethyl acetate and water
(3 ꢁ 5 mL). The organic layers were analyzed by GC–MS chro-
matography using n-dodecane as an internal standard.
2.6. X-ray crystallography for 4a
Diffraction data of 4a was collected on a Bruker AXS SMART
APEX diffractometer, equipped with a CCD area detector using
3.2. Molecular structure
Mo Ka radiation (k = 0.71073 Å). All the data were collected at
296 K and the structures were solved by direct methods and sub-
sequently refined on F2 by using full-matrix least-squares tech-
niques (SHELXL) [35], SADABS absorption corrections were
applied to the data [36], all non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were located at calculated
positions. All calculation was performed using the Bruker Smart
program.
Single crystals of 4a suitable for X-ray diffraction studies were
obtained by slow diffusion of diethyl ether into a concentrated
solution of the complex in MeOH solvent. The crystallographic data
for 4a is summarized in Table S1, and selected bond lengths and
angles are listed in Fig. 1 caption.
Half-sandwich complex 4a crystallized in triclinic system with
ꢀ
Pı space group. The crystal structure confirms the formation of
the half-sandwich rhodium complex [Cp*Rh(L-H)](Cl), with one
3. Result and discussions
Cl- counter anion. As shown in Fig. 1, rhodium center adopts a
3.1. Synthesis of half-sandwich rhodium complexes
The phenylene-based bis(thione) compounds (3a-3d) were syn-
thesized via a one-step reaction between 1,10-(5-methoxy-1,3-phe-
nylene)bis(1H-imidazole) dihalide and sulfur powder with K2CO3
giving high yields as previously reported (Scheme 1) [31]. The
half-sandwich rhodium complexes, formulated as [Cp*Rh(L-H)]Cl
(4a-4d), were achieved by reacting [Cp*RhCl2]2 with two equiva-
lents of the corresponding phenylene-based bis(thione) ligands
(3a-3d) through direct CAH bond activation of the phenyl ring in
CH3OH at room temperature, as shown in Scheme 2. These half-
sandwich rhodium complexes (4a-4d) were air and moisture
stable solids. The half-sandwich rhodium complexes were moder-
ately soluble in MeCN and MeOH solvents. They were isolated by
recrystallization and fully characterized by IR, NMR spectroscopy,
mass spectrometry, and X-ray crystallography.
Fig. 1. Molecular structure of complex 4a. Hydrogen atoms in both structure and
the non-coordinating chloride anion are not shown. Selected lengths (Å) and angles
(°): Rh(1)-C(1) 2.054(3); Rh(1)-S(1) 2.3598(8); Rh(1)-S(2) 2.3760(9); C(1)-Rh(1)-S
(1) 86.50(9); C(1)-Rh(1)-S(2) 86.98(9); S(1)-Rh(1)-S(2) 88.50(3).
The proton signal of the 2-position on the benzene ring
observed in 3a at d = 7.32 ppm was absent in the 1H NMR of
half-sandwich rhodium complex 4a indicating C-Rh bond
Scheme 2. Synthesis of half-sandwich rhodium complexes with phenylene-based bis(thione) ligands.
4

Wei-Guo Jia, Li-Li Gao, Xue-Ting Zhi et al.
Polyhedron 195 (2021) 114978
Table 1
Table 2
Optimization of reaction conditions for transfer hydrogenation of ketones with half-
Screening of substrates for transfer hydrogenation of ketones catalyzed by
sandwich rhodium complexes.a,b
complex 4a.a,b
aEntry
Catalyst (mol%)
Base
Temperature (°C)
Yield (%)b
1
2
3
4
5
6
7
8
4a (1)
4b (1)
4c (1)
4d (1)
4a (1)
4a (1)
t-BuOK
t-BuOK
t-BuOK
t-BuOK
NaOH
t-BuONa
KOH
NaOH
KOH
NaOH
NaOH
NaOH
80
80
80
80
80
80
80
80
80
70
60
80
90
33
87
47
99
92
98
87
77
58
42
NR
4a (1)
4a (0.5)
4a (0.5)
4a (0.5)
4a (0.5)
9
10
11
12
a: Reaction conditions: acetophenone (0.1 mmol), base (4 equiv), 2-propanol
(1 mL), 80 °C; n-dodecane as an internal standard, 3 h. b: Determined by GC–MS.
classic half-sandwich geometry in which the Cp* ring binds typi-
cally as a three-coordinate ligand. Two sulfur and one carbon atom
from the SCS ligand bind to Rh forming the three-legged piano
stool complex. The SCS ligand acted as a tridentate ligand and coor-
dinated to the rhodium(III) center in a facial manner. And Rh is out
of SCN plane with dihedral angle RhASACAN is 41.96°. The Rh-C
distance (2.054(3) Å) [39–41] and the Rh-S distances (2.3598(8)
Å and 2.3760(9) Å) [42–44] are in the range of distances of
reported rhodium complexes bearing tridentate SCS ligands.
3.3. Catalytic transfer hydrogenation of ketones
Since rhodium complexes are popular as transfer hydrogenation
reaction catalysts [45–47], we put the half-sandwich rhodium
complexes 4a-4d to test for the transfer hydrogenation (TH) of ace-
tophenone in the presence of base potassium t-butoxide in reflux-
ing 2-propanol under a nitrogen atmosphere. The results are
summarized in Table 1. Complex 4a showed the best reactivity
for the reaction with 1 mol % loading in 3 h (Table 1, entry 1).
The choice of base was crucial for the reaction efficiency. We found
NaOH to be the most ideal base for the TH reaction of acetophe-
none (Table 1, entries 5–9). When the catalyst loading of 4a was
reduced to 0.5 mol %, lower yield was observed. Lower yields were
also obtained when the reaction temperature decreased (Table 1,
entries 8–11). No desired product was obtained in the absence of
the rhodium catalyst (Table 1, entry 12). Thus, the optimal reaction
conditions for the transfer hydrogenation of acetophenone are
0.5 mol % 4a catalyst, with NaOH as base for 3 h in refluxing 2-
propanol.
To evaluate the general applicability and substrate scope of this
catalytic system, various ketones were subjected to the optimized
conditions. Ketones with different substituent groups on the phe-
nyl ring were tolerated under the standard conditions in excellent
yields. For 4-nitroacetophenone, only the nitro group was reduced
(Table 2, entry 2). It is indicated that the -COMe group was unaf-
fected under the same reaction condition (Fig. S30 and Fig. S31).
This highly selectivity reduction result was also found in other
Rh, Pd and Ir-based nanocatalysts catalytic system [48–50]. How-
ever, the 80% yields were observed for 4-methylpropiophenone
(Table 2, entry 6). For 2-hydroxy-2-methylpropiophenone, leading
to the corresponding product in moderate yield (74%, Table 2, entry
a: Reaction conditions: ketones (0.1 mmol), catalyst (0.5 mol%), base (4 equiv),
2-propanol (1 mL), 80 °C, 3 h, n-dodecane as an internal standard. b: Deter-
mined by GC–MS.
7). The benzophenone was reduced to alcohol almost quantitative
yield (Table 2, entry 9). Notably, sterically crowded substrate
fluoren-9-one was also efficiently reduced to the corresponding
alcohol (Table 2, entry 10). The reactions of other aliphatic com-
pounds including 4-tert-butylcyclohexanone and 5,5-dimethyl-
1,3-cyclohexanedione were also performed. All reactions worked
well to produce the corresponding products in excellent yields
(Table 2, entries 11 and 12).
4. Conclusions
In summary, we have reported the straightforward preparation
of a class of half-sandwich rhodium complexes with tridentate
phenylene-based bis(thione) (SCS) ligand. The Rh-C bond forma-
tion was achieved through the direct CAH bond activation of the
phenyl ring in phenylene-based bis(thione) ligand. A combination
5

Wei-Guo Jia, Li-Li Gao, Xue-Ting Zhi et al.
Polyhedron 195 (2021) 114978
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The authors declare that they have no known competing finan-
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