Synthesis, characterization and crystal structures of half-sandwich ruthenium complexes with bidentate chiral Schiff-base ligands

Article abstract of DOI:10.1016/j.jorganchem.2020.121129

Condensation of salicylaldehyde or 5-nitrosalicylaldehyde with (S)-(+)-2-amino-3-methyl-1-butanol in refluxing ethanol afforded the chiral Schiff-base ligands (S)-N-(1-hydroxymethylisobutyl)salicylidene (HL1*) and (S)-N-1-hydroxymethylisobutyl-5-nitrosali

Full text of DOI:10.1016/j.jorganchem.2020.121129

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Synthesis, characterization and crystal structures of half-sandwich ruthenium
complexes with bidentate chiral Schiff-base ligands
Chong Chen, Jiao Ji, Chang-Jiu Wang, Ai-Quan Jia, Qian-Feng Zhang
PII:
S0022-328X(20)30030-9
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121129
JOM 121129
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 3 December 2019
Revised Date: 12 January 2020
Accepted Date: 18 January 2020
Please cite this article as: C. Chen, J. Ji, C.-J. Wang, A.-Q. Jia, Q.-F. Zhang, Synthesis, characterization
and crystal structures of half-sandwich ruthenium complexes with bidentate chiral Schiff-base ligands,
Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2020.121129.
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Synthesis, characterization and crystal structures of half-sandwich
ruthenium complexes with bidentate chiral Schiff-base ligands
Chong Chen, Jiao Ji, Chang-Jiu Wang, Ai-Quan Jia, and Qian-Feng Zhang*
Institute of Molecular Engineering and Applied Chemistry, Anhui University of Technology,
Ma’anshan, Anhui 243002, P R China
Condensation of salicylaldehyde or 5-nitrosalicylaldehyde with (S)-(+)-2-amino-3-
methyl-1-butanol in refluxing ethanol afforded the chiral Schiff-base ligands (S)-N-(1-
hydroxymethylisobutyl)salicylidene (HL1*) and (S)-N-1-hydroxymethylisobutyl-5-
nitrosalicylidene (HL2*), respectively. Similarly, reaction of 3,5-di-tert-
butylsalicylaldehyde or 3,5-dibromosalicylaldehyde with (R)-(+)-α-methylbenzylamine
gave the chiral Schiff base ligands (R)-N-(1-phenylethyl)-3,5-di-tert-butylsalicylidene
(HL3*) and (R)-N-(1-phenylethyl)-3,5-dibromosalicylidene (HL4*), respectively.
Treatment of 3,5-dibromosalicylaldehyde with (S)-(+)-2-amino-1-propanol, (S)-(+)-2-
amino-3-methyl-1-butanol or (S)-(-)-2-amino-3-phenyl-1-propanol in refluxing ethanol
afforded (S)-N-(1-hydroxymethylethyl)-3,5-dibromosalicylidene (HL5*), (S)-N-(1-
hydroxymethylisobutyl)-3,5-dibromosalicylidene
(HL6*),
and
(S)-N-(1-
hydroxymethylphenylethyl)-3,5-dibromosalicylidene (HL7*), respectively. Interaction of
[(η6-p-cymene)RuCl2]2 with two euivalents of the chiral Schiff-bases HL1*‒HL7* gave
the corresponding half-sandwich ruthenium(II) complexes [(η6-p-cymene)Ru(κ2-N,O-
L*)Cl] (1‒7). The molecular structures of HL1*‒HL7* and the complex 3 have been
determined by single-crystal X-ray crystallography.
Keywords: Half-sandwich ruthenium complex; Chiral Schiff base; Synthesis; Crystal
structure
___________________
* Corresponding author. E-mail: zhangqf@ahut.edu.cn, ahutchang@yahoo.com (Q.-F. Zhang).
1

1. Introduction
The significance of catalysis in asymmetric synthesis has led to the development of
numerous versatile chiral ligands [1−4]. Among them, chiral Schiff base ligands have
attracted great attention for their applications in organic catalysis and stereochemical
investigations [5,6]. It may be noted that bidentate chiral Schiff base ligands have various
coordination mode, such as [N,O] [7], [N,N] [8] and [N,P] [9] and are readily modifiable
to allow fine-tuning of steric and electronic properties for transition metal complexes,
notably for ruthenium complexes [10,11]. A number of half-sandwich ruthenium
complexes have been synthesized and applied to various organic transformations, such as
C−N bond formation from imines and ketones [12], direct alkylation through C−H bond
activation [13,14], olefin cyclopropanation [15] and transfer hydrogenation of ketones
[16,17]. Brunner and coworkers reported [(η6-p-cymene)Ru(LL*)(4-Mepy)](PF6) (LL* =
anion of (S)-(1-phenylethy1)salicylaldimine) [17] and [Ru(η6-C6H6)(L−L)Cl] (L−L = (S)-
N-(1-phenylethyl)salicylideneaminate) [18] and explored their stereochemistry and
crystal structures. Previously, we have reported a series of new nitrosyl and
triphenylphosphine ruthenium(II)/(III) complexes with three chiral N,O-chelate Schiff
base ligands [19], as our long-standing research interest in ruthenium complexes with the
potential catalytic properties, we disclose herein syntheses and structures of a series of
half-sandwich ruthenium complexes with bidentate chiral Schiff base ligands derived
from condensation of four different chiral amines (R)-(+)-α-methylbenzylamine, (S)-(+)-
2-amino-3-methyl-1-butanol, (S)-(+)-2-amino-1-propanol, or (S)-(-)-2-amino-3-phenyl-1-
propanol with salicylaldehyde or substituted salicylaldehyde in this paper.
2. Experimental
2.1. General considerations
All synthetic manipulations were carried out under dry nitrogen by standard
Schlenk techniques. Solvents were purified by standard procedures and distilled prior to
use.
Triethylamine,
salicylaldehyde,
5-nitrosalicylaldehyde,
3,5-di-tert-
2

butylsalicylaldehyde, 3,5-dibromosalicylaldehyde, (R)-(+)-α-methylbenzylamine, (S)-
(+)-2-amino-3-methyl-1-butanol, (S)-(+)-2-amino-1-propanol and (S)-(-)-2-amino-3-
phenyl-1-propanol were purchased from Alfa Aesar Ltd. and used without further
purification. The starting ruthenium complex [(η6-p-cymene)RuCl2]2 was prepared
according to the literature method [20]. NMR spectra were recorded on a Bruker ALX
1
400 Plus spectrometer operating at 400 MHz for H and chemical shifts (
δ
, ppm) were
reported with reference to SiMe4 (1H). Infrared spectra (KBr) were recorded on a Perkin-
Elmer 16 PC FT-IR spectrophotometer with use of pressed KBr pellets. Electronic
absorption spectra were obtained on a Shimadzu UV-2600 spectrophotometer. Elemental
analyses were carried out using a Perkin-Elmer 2400 CHN analyzer.
2.2. Synthesis of chiral Schiff base ligands (HL1*–HL7*)
The chiral Schiff base ligands were synthesized by dissolving the corresponding
salicyldehyde (2.0 mmol) in ethanol (20 mL), followd by slow addition of one equivalent
of (S)-(+)-2-amino-3-methyl-1-butanol, (R)-(+)-α-methylbenzylamine, (S)-(+)-2-amino-
1-propanol and (S)-(-)-2-amino-3-phenyl-1-propanol, respectively in ethanol (10 mL).
The mixture was heated and refluxed for 4 h. After cooled to room temperature, the
solvent was evaporated in vacuo and washed with cooled ethanol (5 mL × 3), and single
crystals of HL1*–HL7* were harvested from recrystallization of the seven chiral Schiff
base ligands in methanol/dither ether (v/v = 1:3) at room temperature.
1
HL1*. Yield: 336 mg, 81%. H NMR (400 MHz, DMSO-d6, ppm): δ 12.85 (s, 1H,
Ar−OH), 8.63 (s, 1H, CH=N), 7.62−6.92 (m, 4H, Ar−H), 4.26 (s, 1H, OH), 3.94 (d, J =
5.4 Hz, 2H, CH2), 3.78 (m, 1H, CH), 1.92 (m, 1H, CH), 0.89 (d, J = 7.0 Hz, 6H, CH3). IR
(KBr disc, cm-1): ν(O−H) 3263 (s), ν(C=N) 1619 (s), ν(CH3) 2962 (s), ν(C−O) 1266 (s).
Anal. calc. for C12H17NO2: C, 69.48; H, 8.20; N, 6.75%. Found: C, 69.45; H, 8.22; N,
6.76%.
1
HL2*. Yield: 415 mg, 82%. H NMR (400 MHz, DMSO-d6, ppm): δ 13.31 (s, 1H,
Ar−OH), 8.69 (s, 1H, CH=N), 8.47−7.51 (m, 3H, Ar−H), 4.24 (s, 1H, OH), 3.71 (d, J =
5.6 Hz, 2H, CH2), 3.66 (m, 1H, CH), 2.12 (m, 1H, CH), 0.90 (d, J = 6.8 Hz, 6H, CH3). IR
(KBr disc, cm-1): ν(O−H) 3335 (s), ν(CH3) 2965 (s), ν(C=N) 1629 (m), ν(C=C) 1536 (s),
3

1488 (s), ν(C−O) 1287 (s), ν(Ar−NO2) 1273 (s), 836 (s), 773 (s). Anal. calc. for
C12H16N2O4: C, 57.08; H, 6.34; N, 11.10%. Found: C, 57.07; H, 6.34; N, 11.12%.
1
HL3*. Yield: 594 mg, 88%. H NMR (400 MHz, CDCl3, ppm): δ 13.85 (s, 1H,
Ar−OH), 8.65 (s, 1H, CH=N), 7.47−7.12 (m, 7H, Ar−H), 2.57 (m, 1H, CH), 1.63 (d, J =
4.2 Hz, 3H, CH3), 1.50 (s, 9H, t-Bu), 1.34 (s, 9H, t-Bu). IR (KBr disc, cm-1): ν(CH3) 2963
(s), ν(CH2) 2906 (s), 2842 (s), ν(C=N) 1629 (s), ν(C=C) 1583 (s), 1467 (s), 1451 (s),
ν(C−O) 1250 (s). Anal. calc. for C23H31NO: C, 81.78; H, 9.19; N, 4.15%. Found: C, 81.77;
H, 9.18; N, 4.16%.
1
HL4*. Yield: 598 mg, 78%. H NMR (400 MHz, DMSO-d6, ppm): δ 13.57 (s, 1H,
Ar−OH), 8.54 (s, 1H, CH=N), 7.65−7.32 (m, 7H, Ar−H), 2.42 (m, 1H, CH), 1.56 (d, J =
4.2 Hz, 3H, CH3). IR (KBr disc, cm-1): ν(O−H) 3261 (br), ν(CH2) 2906 (s), 2842 (s),
ν(C=N) 1620 (s), ν(C=C) 1571 (s), 1475 (s), ν(C−O) 1266 (s) ν(Ar−Br) 615 (s), 558 (s).
Anal. calc. for C15H13NOBr2: C, 47.0; H, 3.39; N, 3.66%. Found: C, 47.06; H, 3.38; N,
3.67%.
1
HL5*. Yield: 555 mg, 82%. H NMR (400 MHz, DMSO-d6, ppm): δ 14.98 (s, 1H,
Ar−OH), 8.53 (s, 1H, CH=N), 7.84−7.70 (m, 2H, Ar−H), 4.24 (s, 1H, OH), 3.96 (d, J =
3.6 Hz, 2H, CH2), 3.24 (m, 1H, CH), 1.35 (d, J = 2.6 Hz, 3H, CH3). IR (KBr disc, cm-1):
ν(O−H) 3261 (br), ν(CH3) 2963 (s), ν(CH2) 2906 (s), 2842 (s), ν(C=N) 1618 (s), ν(C=C)
1586 (s), 1470 (s), ν(C−O) 1259 (s), ν(Ar−Br) 596 (s), 568 (s). Anal. calc. for
C10H11NO2Br2·0.25H2O: C, 35.17; H, 3.39; N, 4.10%. Found: C, 35.13; H, 3.36; N,
4.11%.
1
HL6*. Yield: 614 mg, 84%. H NMR (400 MHz, DMSO-d6, ppm): δ 15.08 (s, 1H,
Ar−OH), 8.61 (s, 1H, CH=N), 7.84−7.70 (m, 2H, Ar−H), 4.24 (s, 1H, OH), 3.96 (d, J =
4.5 Hz, 2H, CH2), 3.71 (m, 1H, CH), 1.94 (m, 1H, CH), 0.88 (d, J = 4.8 Hz, 6H, CH3). IR
(KBr disc, cm-1): ν(O−H) 3261 (s), ν(CH3) 2963 (s), ν(C=N) 1634 (s), ν(C=C) 1571 (s),
1475 (s), ν(C−O) 1249 (s), ν(Ar−Br) 612 (s), 577 (s). Anal. calc. for C12H15NO2Br2: C,
39.45; H, 4.11; N, 3.84%. Found: C, 39.47; H, 4.10; N, 3.85%.
1
HL7*. Yield: 702 mg, 85%. H NMR (400 MHz, DMSO-d6, ppm): δ 14.84 (s, 1H,
Ar−OH), 8.46 (s, 1H, CH=N), 7.82−7.23 (m, 7H, Ar−H), 4.26 (s, 1H, OH), 3.96 (d, J =
3.4 Hz, 2H, CH2O), 3.71 (m, 1H, CH), 3.52 (d, J = 5.2 Hz, 2H, CH2Ph). IR (KBr disc,
4

cm-1): ν(O−H) 3261 (s), ν(CH2) 2906 (s), 2842 (s), ν(C=N) 1616 (s), ν(C=C) 1571 (s),
1475 (s), ν(C−O) 1271 (s), ν(Ar−Br) 603 (s), 567 (s). Anal. calc. for C16H15NO2Br2: C,
46.49; H, 3.63; N, 3.39%. Found: C, 46.48; H, 3.63; N, 3.41%.
2.3. Synthesis of (S)-[(η6-p-cymene)Ru(κ2-N,O-L1*)Cl] (1)
To a solution of [(η6-p-cymene)RuCl2]2 (62 mg, 0.10 mmol) in tetrahydrofuran (20
mL) was added a solution of HL1* (42 mg, 0.20 mmol) and Et3N (22 mg, 0.20 mmol) in
tetrahydrofuran (5 mL), and then the mixture was heated at 90 °C with stirring overnight,
during which time there was a color change from yellow to orange. After removal of
solvent in vacuo, the resulting orange powder of 1 was washed with diethyl ether (5 mL ×
1
3) and dried in vacuo. Yield: 39 mg, 82% (based on Ru). H NMR (400 MHz, CDCl3,
ppm): δ 8.25 (s, 1H, CH=N), 7.06−7.28 (m, 4H, Ar−H), 5.24−5.52 (m, 4H, p-cymene),
4.26 (s, 1H, OH), 2.85 (d, J = 6.8 Hz, 2H, CH2), 2.29 (m, 1H, CH), 2.16 (m, 1H, CH),
2.03 (m, 1H, CH), 1.21 (m, 6H, CH3), 1.19 (d, J = 6.8 Hz, 6H, CH3), 1.12 (s, 1H, CH3).
IR (KBr disc, cm-1): ν(O−H) 3189 (s), ν(C=N) 1650 (s), ν(Ar−O) 1240 (s). Anal. calc. for
C22H29NO2ClRu: C, 55.49; H, 6.10; N, 2.94; O, 6.73%. Found: C, 55.48; H, 6.08; N,
2.95; O, 6.74%.
2.4. Synthesis of (S)-[(η6-p-cymene)Ru(κ2-N,O-L2*)Cl] (2)
A mixture of [(η6-p-cymene)RuCl2]2 (62 mg, 0.10 mmol), HL2* (51 mg, 0.20
mmol) and Et3N (22 mg, 0.20 mmol) in tetrahydrofuran (25 mL) was heated at reflux
with stirring overnight, during which time the color changed from orange to red-brown.
The solvent was removed in vacuo, and the residue was washed with diethyl ether (5 mL
× 3) and dissolved in dichloromethane (2 mL). n-Hexane (10 mL) was added to the
solution to precipitate the complex which was isolated and washed with diethyl ether (5
1
mL × 3) and finally dried in vacuo. Yield: 41 mg, 80% (based on Ru). H NMR (400
MHz, CDCl3, ppm): δ 8.32 (s, 1H, CH=N), 7.31−7.16 (m, 3H, Ar−H), 5.39−5.33 (m, 4H,
p-cymene), 4.02 (s, 1H, OH), 2.90 (d, J = 21.8 Hz, 2H, CH2), 2.26 (s, 1H, CH), 2.06 (m,
1H, CH), 2.03 (s, 1H, CH), 1.32 (m, 6H, CH3), 1.28 (s, 3H, CH3), 0.94 (d, J = 6.8 Hz, 6H,
5

CH3). IR (KBr disc, cm-1): ν(O−H) 3189 (s), ν(C=N) 1650 (s), ν(Ar−NO2) 1526 (s),
ν(Ar−O) 1261 (s). Anal. calc. for C22H28N2O4ClRu: C, 50.69; H, 5.38; N, 5.38; O,
12.29%. Found: C, 50.67; H, 5.38; N, 5.37; O, 12.28%.
2.5. Synthesis of (R)-[(η6-p-cymene)Ru(κ2-N,O-L3*)Cl] (3)
To a solution of [(η6-p-cymene)RuCl2]2 (62 mg, 0.10 mmol) in tetrahydrofuran (20
mL) was added a solution of HL3* (68 mg, 0.20 mmol) and Et3N (22 mg, 0.20 mmol) in
tetrahydrofuran (5 mL), and then the mixture was heated at 90 °C with stirring overnight,
during which there was a color change from yellow to orange. After removal of solvent in
vacuo, the residue was dissolved in dichloromethane (3 mL) and the solution was filtered.
The filtrate was layered with n-hexane (20 mL) at room temperature, and light red block-
shaped crystals of (S)-[(η6-p-cymene)Ru(κ2-N,O-L3*)Cl] (3) were obtained in five days.
1
Yield: 27 mg, 43% (based on Ru). H NMR (400 MHz, CDCl3, ppm): δ 8.16 (s, 1H,
CH=N), 7.23−7.16 (m, 2H, Ar−H), 6.52−5.94 (m, 5H, Ar−H), 5.22−5.00 (m, 4H, p-
cymene), 2.96 (s, 1H, CH), 2.21 (d, J = 29.4 Hz, 3H, CH3), 2.12−1.89 (m, 6H, CH3), 1.86
(s, 3H, CH3), 1.25 (d, J = 20.8 Hz, 18H, t-Bu). IR (KBr disc, cm-1): ν(O−H) 3253 (s),
ν(C=N) 1629 (s), ν(t-Bu) 1381 (s), 1362 (vs), ν(Ar−O) 1276 (s). Anal. calc. for
C33H44NOClRu: C, 65.22; H, 7.25; N, 2.31; O, 2.64%. Found: C, 65.20; H, 7.25; N, 2.33;
O, 2.65%.
2.6. Synthesis of (R)-[(η6-p-cymene)Ru(κ2-N,O-L4*)Cl] (4)
A mixture of [(η6-p-cymene)RuCl2]2 (62 mg, 0.10 mmol), HL4* (77 mg, 0.20 mmol)
and Et3N (22 mg, 0.20 mmol) in tetrahydrofuran (25 mL) was heated at reflux with
stirring overnight, during which time the color changed from orange to dark red. After
removal of solvent in vacuo, dichloromethane (10 mL) was added and the solution was
filtered. The filtrate was concentrated and the residue was washed with diethyl ether (5
mL × 2) and n-hexane (5 mL × 2). Recrystallization from dichloromethane/n-hexane (1:5)
1
afforded dark red solids of 4. Yield: 51 mg, 79% (based on Ru). H NMR (400 MHz,
CDCl3, ppm): δ 8.36 (s, 1H, CH=N), 7.26 (m, 5H, Ar−H), 7.15 (m, 2H, Ar−H), 5.47−5.19
6

(m, 4H, p-cymene), 2.36 (m, 1H, CH), 1.36 (m, 6H, CH3), 1.25 (d, J = 3.4 Hz, 3H, CH3),
1.23 (s, 1H, CH), 0.92 (d, J = 6.8 Hz, 3H, CH3). IR (KBr disc, cm-1): ν(C=N) 1659 (s),
ν(Ar−O) 1315 (s). Anal. calc. for C25H25NOClBr2Ru: C, 46.04; H, 3.84; N, 2.15; O,
2.46%. Found: C, 46.05; H, 3.84; N, 2.17; O, 2.45%.
2.7. Synthesis of (S)-[(η6-p-cymene)Ru(κ2-N,O-L5*)Cl] (5)
To a solution of [(η6-p-cymene)RuCl2]2 (62 mg, 0.10 mmol) in tetrahydrofuran (20
mL) was added a solution of HL5* (67 mg, 0.20 mmol) and Et3N (22 mg, 0.20 mmol) in
tetrahydrofuran (5 mL), and then the mixture was heated at 90 °C with stirring overnight,
during which there was a color change from orange to dark red. After removal of solvent
in vacuo, the residue was washed with diethyl ether (5 mL × 3). Recrystallization from
dichloromethane/n-hexane (1:5) gave dark red solids of 5. Yield: 46 mg, 76% (based on
Ru). 1H NMR (400 MHz, CDCl3, ppm): δ 8.29 (s, 1H, CH=N), 7.22−7.01 (m, 2H, Ar−H),
5.54−5.33 (m, 4H, p-cymene), 4.10 (s, 1H, OH), 3.88 (d, J = 6.6 Hz, 2H, CH2), 2.32 (m,
1H, CH), 1.35 (d, J = 6.9 Hz, 6H, CH3), 1.25 (d, J = 4.6 Hz, 3H, CH3), 1.23 (s, 1H, CH),
1.23−1.18 (m, 3H, CH3). IR (KBr disc, cm-1): ν(O−H) 3256 (s), ν(C=N) 1613 (s),
ν(Ar−O) 1314 (s). Anal. calc. for C20H23NO2ClBr2Ru: C, 39.64; H, 3.80; N, 2.31; O,
5.28%. Found: C, 39.63; H, 3.79; N, 2.33; O, 5.27%.
2.8. Synthesis of (S)-[(η6-p-cymene)Ru(κ2-N,O-L6*)Cl] (6)
To a solution of [(η6-p-cymene)RuCl2]2 (62 mg, 0.10 mmol) in tetrahydrofuran (20
mL) was added a solution of HL6* (73 mg, 0.20 mmol) and Et3N (22 mg, 0.20 mmol) in
tetrahydrofuran (5 mL), and then the mixture was heated at 90 °C with stirring overnight,
during which there was a color change from orange to dark red-brown. After removal of
solvent in vacuo, the residue was washed with diethyl ether (5 mL × 3) and n-hexane (3
mL × 3). The desired product 5 was obtained by recrystallization from
dichloromethane/n-hexane (1:5). Yield: 51 mg, 81% (based on Ru). 1H NMR (400 MHz,
CDCl3, ppm): δ 8.24 (s, 1H, CH=N), 7.26−7.09 (m, 2H, Ar−H), 5.69−5.43 (m, 4H, p-
cymene), 4.22 (s, 1H, OH), 3.23 (d, J = 3.5 Hz, 2H, CH2), 2.46 (m, 1H, CH), 2.31 (s, 1H,
7

CH), 2.17−2.09 (m, 6H, CH3), 1.19 (s, 3H, CH3), 1.18 (m, 1H, CH), 1.13 (d, J = 10.0 Hz,
6H, CH3). IR (KBr disc, cm-1): ν(O−H) 3223 (s), ν(C=N) 1606 (s), ν(Ar−O) 1256 (s).
Anal. calc. for C22H27NO2ClBr2Ru: C, 41.67; H, 4.26; N, 2.21; O, 5.05%. Found: C,
41.66; H, 4.26; N, 2.22; O, 5.06%.
2.9. Synthesis of (S)-[(η6-p-cymene)Ru(κ2-N,O-L7*)Cl] (7)
A mixture of [(η6-p-cymene)RuCl2]2 (62 mg, 0.10 mmol), HL7* (83 mg, 0.20 mmol)
and Et3N (22 mg, 0.20 mmol) in tetrahydrofuran (25 mL) was heated at reflux with
stirring overnight, during which time the color changed from orange to dark red. After
removal of solvent in vacuo, dichloromethane (5 mL) was added and the solution was
filtered. The filtrate was concentrated and the residue was washed with diethyl ether (5
mL × 2) and n-hexane (5 mL × 2). Recrystallization from dichloromethane/n-hexane (1:5)
afforded dark red solids of 7. Yield: 51 mg, 75% (based on Ru).
1H NMR (400 MHz,
CDCl3, ppm): δ 8.33 (s, 1H, CH=N), 7.33‒7.19 (m, 5H, Ar−H), 7.18 (m, 2H, Ar−H),
5.56−5.24 (m, 4H, p-cymene), 4.12 (s, 1H, OH), 3.20 (d, J = 3.2 Hz, 2H, CH2), 2.93 (s,
1H, CH), 2.38 (m, 1H, CH), 1.32−1.28 (m, 6H, CH3), 0.93 (m, 3H, CH3). IR (KBr disc,
cm-1): ν(O−H) 3230 (s), ν(C=N) 1649 (s), ν(Ar−O) 1236 (s). Anal. calc. for
C26H27NO2ClBr2Ru: C, 45.78; H, 3.96; N, 2.05; O, 4.70%. Found: C, 45.77; H, 3.96; N,
2.05; O, 4.71%.
2.10. X-Ray crystallography
A summary of crystallographic data and experimental details for ligands HL1*–
HL7* and complex 3 are summarized in Table 1. The structure of HL2* was included for
completion only. Intensity data were collected on a Bruker SMART APEX 2000 CCD
diffractometer using graphite-monochromated Mo-K radiation (λ = 0.71073 Å) at 296(2)
K. The collected frames were processed with the software SAINT [21]. The data was
corrected for absorption using the program SADABS [22]. Structures were solved by the
direct methods and refined by full-matrix least-squares on F2 using the SHELXTL
software package [23,24]. All non-hydrogen atoms were refined anisotropically. The
8

positions of all hydrogen atoms were generated geometrically (Csp3−H = 0.96 and Csp2−H
= 0.93 Å) and included in the structure factor calculations with assigned isotropic thermal
parameters but were not refined.
3. Results and discussion
As shown in Scheme 1, the chiral Schiff-base ligands HL1*–HL7* were
synthesized from the condensation of the corresponding salicylaldehyde and its
derivatives with four different chiral amines (S)-2-amino-3-methylbutan-1-ol, (R)-(+)-α-
methylbenzylamine, (S)-(+)-2-amino-1-propanol and (S)-(-)-2-amino-3-phenyl-1-
propanol, respectively, in moderate to high yields. Treatment of [(η6-p-cymene)RuCl2]2
with two equivalent chiral Schiff base ligands HL1*–HL7* in the presence of
triethylamine in tetrahydrofuran under reflux afforded the half-sandwich ruthenium
complexes (S)-[(η6-p-cymene)Ru(κ2-N,O-L1*)Cl] (1), (S)-[(η6-p-cymene)Ru(κ2-N,O-
L2*)Cl] (2), (R)-[(η6-p-cymene)Ru(κ2-N,O-L3*)Cl] (3), (R)-[(η6-p-cymene)Ru(κ2-N,O-
L4*)Cl] (4), (S)-[(η6-p-cymene)Ru(κ2-N,O-L5*)Cl] (5), (S)-[(η6-p-cymene)Ru(κ2-N,O-
L6*)Cl] (6) and (S)-[(η6-p-cymene)Ru(κ2-N,O-L7*)Cl] (7) (Scheme 2), respectively. The
two bridged chloro ligands in [(η6-p-cymene)RuCl2]2 were replaced by one monoanionic
L*– ligand without chiral inversion. The chiral Schiff base ligands L1*–L7* in
complexes 1–7 all serve as N,O-bidentate ligands, similar to those in the other reported
ruthenium complexs with chiral Schiff base ligands [(η6-p-cymene)Ru{2-O-(3,5-di-t-
Bu)C6H2CH=NCH(Me)Ph}Cl] [25], [Ru(Sal val)(PPh3)(bipy)] (Sal = salicylaldehyde,
val = L-valine) [26] and [(η6-p-cymene)Ru(3-H+)Cl] (3 = (S)-2-methoxy-2'-(2-
hydroxybenzylidenamino)-1,1'-binaphthyl) [27]. The half-sandwich ruthenium
complexes 1–7 are air and moisture stable, soluble in dichloromethane, chloroform,
acetone and alcohol.
IR spectra of chiral Schiff base ligands HL1*–HL7* and half-sandwich ruthenium
complexes 1–7 all showed the CH=N groups at around 1620 cm-1, which is characteristic
of the azomethine group absorptions [28]. The strong absorption at the region 1249–1287
cm-1 in HL1*–HL7* belong to the stretching vibration of C(Ar)–OH. Similar absorption
peaks of C(Ar)–O are observed in the spectra of complexes 1–7. The chiral ligands
9

HL4*–HL7* with dibromo substitutes show two medium peaks at 615 and 578 nm for
HL4*, 596 and 568 nm for HL5*, 603 and 567 nm for HL6* and 612 and 577 nm for
HL7*, which are attributed to the stretching vibration of C(Ar)–Br [29]. The aldimine (–
CH=N–) proton resonances of HL1*–HL7* are at the region of 8.46−8.53 ppm. The
chemical shift of Ar−H was found at 7.62–6.92 ppm for HL1* and 8.47–7.51 ppm for
HL2*, possibly due to the electron-withdrawing property of ‒NO2 group. Figs. 1 and 2
show the UV/Vis spectra of the chiral Schiff base ligands HL1*–HL7* and the half-
sandwich ruthenium(II) complexes 1–7 in dichloromethane, respectively. The bands at
around 285 nm of complexes 1–7 are assigned to π→π* transition of the azomethine
chromophore and there is a blue shift of about 40 nm compared with the free chiral Schiff
base ligands (325 nm), possibly due to the coordination influence [30]. The peaks in the
region of 365‒400 nm could be attributed to MLCT (metal-to-ligand charge transfer)
transition in complexes 1–7, in which complex 3 (365 nm) has a 20 nm blue shift
compared to complex 1 (385 nm), possibly due to the electron donating ability of the tert-
butyl groups. Conversely, complex 2 and 4–7 (about 395 nm) have a 10 nm red shift
compared to that of 1, due to the electron withdrawing ability of the nitro group and
bromo groups, respectively. This phenomenon could be ascribed to the lower
electronegativity and higher LUMO energy of L3*- and the opposite trend for L2*- and
L4*-‒L7*- [31].
The structures of chiral Schiff base ligands HL1*–HL7* and complex 3 have been
established by X-ray crystallography. The perspective views of the molecular structures
of ligands HL1*–HL7* and complex 3 are shown in Figs. 3−10, respectively, with atom
numbering scheme. The packing modes of the ligands (see Figs. S1−S7) clearly show the
intramolecular hydrogen bonds with the intramolecular O−H···N distances in HL1*–
HL7* being 1.69, 1.86, 1.72, 1.88, 1.82, 1.85 and 1.82 Å, respectively. Expect for HL3*,
the chiral ligands also have intermolecular hydrogen bonds and the distances of O−H···
O
are in the range of 1.74‒1.97 Å. The crystals of complex 3 belong to triclinic space group
Pī and both the ruthenium center and the C* of L3* adopt R-configuration ((RRu, RC)-3).
Selected bond lengths and bond angles for 3 are summarized in the caption of Fig. 10.
The configuration of ruthenium atom is obtained using a priority sequence: η6-p-
cymene > Cl > O (L–L) > N (L–L) [32] and the asymmetric carbon atom in the chiral
10

Schiff base ligand has the expected RC-configuration without chiral inversion. The bond
lengths of C=N is 1.303(7) Å, which is a little longer than that in the uncoordinated chiral
Schiff base ligand (1.265(4) Å). The molecular structure in the crystal clearly shows that
(RRu, RC)-3 consists of a p-cymene ligand, a terminal chlolo ligand and a bidentate N,O-
donor chelating ligand L3*-. The ruthenium center is in a slightly distorted octahedral
coordination environment and has six-coordinate geometry assuming that the p-cymene
ring serve as a three-coordinated ligand, which is common for half-sandwich “piano-
stool” structures [33–37]. The average of the Ru–C bond lengths (p-cymene ring) is
2.198(6) Å, which is about 0.036 Å longer than that in the η6-C6H6 complex (2.162(9) Å),
indicative of the higher Lewis basicity of the dialkyl substitution of the η6-p-cymene
ligand with respect to the η6-p-benzene ligand [18]. The Ru−O and Ru−N bond lengths
are 2.054(4) and 2.117(5) Å, respectively, similar to other half-sandwich ruthenium
complexes with N,O-chelated Schiff base ligands, such as (2.057(3) and 2.096(3) Å) [38].
The ligand L3*- coordinated with central ruthenium atom and formed a six-membered
planar ring (mean deviation, 0.0354 Å), which makes a dihedral angle of 5.428(11)° with
the phenyl ring of the salicylaldehyde ligand. The bite N−Ru−O angle is 88.21(17)°,
which is a little bigger than those in half-sandwich ruthenium complexes with (S)-N-(1-
phenylethyl)salicylideneamine ligands (83.2°−86.8°) [18, 39]. The 1-phenylethyl group at
the chiral carbon atom is almost perpendicular to the chelate plane (dihedral angles
78.44(23)o) and in a position for the stabilizing interaction with the π-bonded p-cymene
ligand, called “β-phenyl effect” [39]. The distances from the centroid of the cymene ring
(C0Ph) to the H and Me at the chiral carbon are 3.472 and 4.928 Å respectively, indicative
of a geometry that close to the cymene ring for C*−H [25].
In summary, a series of bidentate chiral Schiff base ligands and corresponding half-
sandwich ruthenium(II) complexes were synthesized. The molecular structures of
HL1*−HL7* and complex 3 have been determined by single-crystal X-ray
crystallography. The chiral Schiff base ligands have intramolecular O−H···N hydrogen
bonds in the range of 1.69‒1.88 Å, and intermolecular O−H···O in the range of 1.74‒1.97
Å. UV/Vis spectra of complexes 1–7 show a blue shift of about 40 nm compared with the
corresponding free chiral Schiff base ligands. The peak at 365 nm of complex 3 is
11

assigned to a MLCT transition, which has a blue shift compared with the other presented
ruthenium complexes, possibly due to the lower electronegativity and higher LUMO
energy of L3*- with tert-butyl groups [31]. In complex 3, the ruthenium atom adopts R-
configuration and the Rc-configuration possibly suggested that formation of the
ruthenium complexes proceeded without chiral inversion [19]. The 1-phenylethyl group
attached to the chiral carbon is in a position for the stabilizing interaction with the η6-p-
cymene ligand in complex 3, which is generally observed for half-sandwich ruthenium
complexes subject to “β-phenyl effect” [39]. Exploration of ruthenium complexes as
catalysts for asymmetric synthesis is underway in our laboratory.
Supplementary material
Crystallographic data for (S)-N-(1-hydroxymethylisobutyl)salicylidene (HL1*), (S)-
N-(1-hydroxymethylisobutyl)-5-nitrosalicylidene (HL2*), (R)-N-(1-phenylethyl)-3,5-di-tert-
butylsalicylidene (HL3*), (R)-N-(1-phenylethyl)-3,5-dibromosalicylidene) (HL4*), (S)-N-(1-
hydroxymethylethyl)-3,5-dibromosalicylidene (HL5*), (S)-N-(1-hydroxymethylisobutyl)-
3,5-dibromosalicylidene (HL6*), (S)-N-(1-hydroxymethylphenylethyl)-3,5-dibromosalicyli-
dene (HL7*) and (R)-[(η6-p-cymene)Ru(κ2-N,O-L3*)Cl] (3) have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1969259-
1969266, respectively. Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: (+44)1233-336-033; e-mail:
deposit@ccdc.cam.ac.uk].
Acknowledgements
This project was supported by the Natural Science Foundations of China
(21372007).
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16

Table 1. Crystallographic data and experimental details for HL1*–HL7* and (S)-[(η6-p-cymene)Ru(κ2-N,O-L3*)Cl] (3)
Complex
HL1*
HL2*
HL3*
HL4*
HL6*
HL7*
2(HL5*)·0.5H2O
3
Empirical formula C12H17NO2
C12H16N2O4
252.27
C23H31NO
337.49
C15H13NOBr
383.09
C20H23N2O4.5Br4 C12H15NO2Br2
C10H11NO2Br2
413.11
C33H43NOClRu
606.20
Triclinic
9.6395(16)
10.1718(17)
17.607(3)
83.990(2)
80.670(2)
63.684(2)
1525.9(4)
Pī
Formula weight
Crystal system
a (Å)
207.27
683.04
Triclinic
6.7964(15)
8.8071(19)
11.479(2)
109.685(2)
98.581(3)
101.767(2)
615.4(2)
P1
365.11
Rhombohedral
23.479(9)
23.479(9)
6.014(5)
Orthorhombic
7.542(3)
Orthorhombic
6.293(3)
9.885(5)
34.912(18)
Monoclinic
22.04(3)
Monoclinic
7.3140(10)
22.257(3)
8.7700(12)
Monoclinic
14.856(4)
7.020(2)
b (Å)
10.796(4)
32.777(11)
9.333(11)
15.541(19)
c (Å)
16.709(5)
α (°)
112.331(15)
93.030(2)
112.071(3)
β
(°)
120
γ (°)
V (Å3)
2871(3)
R3
2668.8(15)
P212121
8
2171.6(19)
P212121
4
2957(6)
C2
1425.7(3)
P21
1614.9(8)
P21
Space group
Z
9
8
1
4
4
2
Dcalc (g cm-3)
Temperature (K)
F(000)
µ(Mo-Kα) (mm-1)
Total refln
Independent refln
Rint
1.079
296(2)
1008
0.073
6088
3002
0.0329
1.256
296(2)
1072
1.032
296(2)
736
1.721
296(2)
1504
5.474
8923
6177
0.0583
1.843
1.691
296(2)
712
1.699
296(2)
816
1.319
296(2)
333
296(2)
634
0.095
16788
6054
0.062
13462
4837
6.570
5.676
8868
4648
0.0263
5.023
9938
5384
0.0405
0.626
3797
9586
3173
6713
0.1256
0.0340
0.0164
0.0383
17

R1a, wR2b (I > 2
R1, wR2 (all data) 0.1208, 0.1433
GoFc
σ
0.0481, 0.1106
(I))
0.0757, 0.1426
0.2904, 0.2184
0.894
0.0662, 0.1821
0.1298, 0.2240
1.023
0.0520, 0.1119
0.1404, 0.1431
0.904
0.0369, 0.0797
0.0550, 0.0851
0.984
0.0375, 0.0781
0.0766, 0.0890
1.004
0.0381, 0.0809
0.0582, 0.0891
1.009
0.0687, 0.1732
0.0978, 0.1995
0.893
1.055
-
Flack parameter
0.5(10)
2.0(10)
-1.5(10)
0.022(17)
0.028(16)
0.007(10)
0.001(9)
a R1 = Σ Fo– Fc /ΣFo.
2
2 2 1/2
b wR2 = [Σw(Fo  – Fc2)2/ΣwFo  ]
.
c GoF = [Σw(Fo – Fc)2/(Nobs – Nparam)]1/2.
18

Scheme 1. Syntheses of chiral Schiff bases HL1*–HL7*. Reagents and conditions: (i)
(S)-(+)-2-amino-3-methyl-1-butanol, ethanol, at reflux; (ii) (S)-(+)-2-amino-3-methyl-1-
butanol, ethanol, at reflux; (iii) (R)-(+)-α-methylbenzylamine, ethanol, at reflux; (iv) (R)-
(+)- -methylbenzylamine, ethanol, at reflux; (v) (S)-(+)-2-amino-1-propanol, ethanol, at
α
reflux; (vi) (S)-(+)-2-amino-3-methyl-1-butanol, ethanol, at reflux; (vii) (S)-(-)-2-amino-
3-phenyl-1-propanol, ethanol, at reflux.
19

Scheme 2. Syntheses of half-sandwich ruthenium(II) complexes 1–7.
20

Fig. 1. UV/Vis spectra of chiral Schiff bases HL1*–HL7* measured in dichloromethane.
Fig. 2. UV/Vis spectra of half-sandwich ruthenium(II) complexes 1–7 measured in dichloromethane.
21

Fig. 3. ORTEP diagram of (R)-N-(1-hydroxymethylisobuty)salicylidene (HL1*), showing the atom
labeling scheme and 40% thermal ellipsoids. Selected bond lengths (Å) and angles (deg): O(1)-C(1)
1.354(4), N(1)-C(7) 1.276(3), N(1)-C(8) 1.461(3).
Fig. 4. ORTEP diagram of (R)-N-(1-hydroxymethylisobuty)(5-nitrosalicylidene) (HL2*). Selected
bond lengths (Å) and angles (deg): O(1)-C(1) 1.271(7), N(1)-C(7) 1.275(7), N(1)-C(8) 1.450(8), O(5)-
C(13) 1.286(7), N(3)-C(19) 1.293(7), N(3)-C(20) 1.469(8).
22

Fig. 5. ORTEP diagram of (R)-N-(1-phenylethyl)(3,5-di-tert-butylsalicylidene) (HL3*), showing the
atom labeling scheme and 40% thermal ellipsoids. Selected bond lengths (Å) and angles (deg): O(1)-
C(1) 1.352(3), N(1)-C(7) 1.265(4), N(1)-C(8) 1.455(4).
Fig. 6. ORTEP diagram of (R)-N-(1-phenylethyl)(3,5-dibromosalicylidene) (HL4*), showing the atom
labeling scheme and 40% thermal ellipsoids. Selected bond lengths (Å) and angles (deg): O(1)-C(11)
1.355(10), N(1)-C(1) 1.487(9), N(1)-C(9) 1.242(11), O(1A)-C(11A) 1.330(11), N(1A)-C(1A)
1.477(9), N(1A)-C(9A) 1.269(11).
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Fig. 7. ORTEP diagram of (S)-N-(1-hydroxymethylethyl)(3,5-dibromosalicylidene) (HL5*), showing
the atom labeling scheme and 40% thermal ellipsoids. Selected bond lengths (Å) and angles (deg):
O(1)-C(2) 1.420(10), N(1)-C(1) 1.494(10), N(1)-C(4) 1.300(8), O(2A)-C(6A) 1.289(8), N(1A)-C(1A)
1.471(10), N(1A)-C(4A) 1.331(10).
Fig. 8. ORTEP diagram of (R)-N-(1-hydroxymethylisobutyl)(3,5-dibromosalicylidene) (HL6*),
showing the atom labeling scheme and 40% thermal ellipsoids, O–H N intermolecular hydrogen
bonds are depicted as dashed lines. Selected bond lengths (Å) and angles (deg): O(2)-C(8) 1.280(6),
24

N(1)-C(1) 1.473(6), N(1)-C(6) 1.288(6), O(2A)-C(8A) 1.275(6), N(1A)-C(1A) 1.452(7), N(1A)-C(6A)
1.278(7).
Fig. 9. ORTEP diagram of (S)-N-(1-hydroxymethylphenyl)(3,5-dibromosalicylidene) (HL7*),
showing the atom labeling scheme and 40% thermal ellipsoids, O–H N intermolecular hydrogen
bonds are depicted as dashed lines. Selected bond lengths (Å) and angles (deg): O(2)-C(12) 1.274(6),
N(1)-C(1) 1.463(7), N(1)-C(10) 1.305(6), O(2A)-C(12A) 1.280(6), N(1A)-C(1A) 1.489(7), N(1A)-
C(10A) 1.267(7).
25

Fig. 10. Molecular structure of half-sandwich ruthenium(II) complex 3. Thermal ellipsolids are shown
at the 40% probability level. Selected bond lengths (Å) and angles (deg): Ru(1)-Cav 2.198(6), Ru(1)-
O(1) 2.054(4), Ru(1)-N(1) 2.117(5), Ru(1)-Cl(1) 2.4190(16), O(1)-C(21) 1.289(6), N(1)-C(19)
1.303(7), N(1)-C(11) 1.482(7), C(19)-C(20) 1.418(8), C(20)-C(21) 1.418(8), O(1)-Ru(1)-N(1)
88.21(17), O(1)-Ru(1)-Cl(1) 85.75(13), N(1)-Ru(1)-Cl(1) 82.10(14), C(11)-N(1)-Ru(1) 119.8(4),
C(19)-N(1)-C(11) 117.4(5), C(19)-N(1)-Ru(1) 122.6(4), C(11)-N(1)-Ru(1) 119.8(4), C(21)-O(1)-
Ru(1) 130.8(4), O(1)-C(21)-C(20) 123.2(5), C(21)-C(20)-C(19) 124.0(5), N(1)-C(19)-C(20) 130.4(6).
26

Graphical Abstract
Synthesis, characterization and crystal structures of half-sandwich ruthenium
complexes with bidentate chiral Schiff-base ligands
Chong Chen, Jiao Ji, Chang-Jiu Wang, Ai-Quan Jia, and Qian-Feng Zhang*
A series of chiral Schiff base ligands and the corresponding half-sandwich ruthenium(II) complexes
were synthesized and characterized by single-crystal X-ray diffraction and their electronic absorption
spectra were also investigated.
27

Highlights
► Seven chiral N˄O ligands were characterized by single-crystal X-ray crystallography.
► The corresponding half-sandwich ruthenium(II) complexes were synthesized.
► The electronic absorption spectra of these ruthenium complexes were investigated.
1

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.