Preparation, crystal structures and properties of half-sandwich ruthenium complexes containing salicylbenzoxazole ligands

Article abstract of DOI:10.1080/00958972.2015.1109643

Three half-sandwich ruthenium complexes [Ru(p-cymene)LCl] containing salicylbenzoxazole ligands [LH = 2-(5-methyl-benzoxazol-2-yl)-4-methyl-phenol (2a), LH = 2-(5-methyl-benzoxazol-2-yl)-4-chloro-phenol (2b), and LH = 2-(5-methyl-benzoxazol-2-yl)-4-bromo-

Full text of DOI:10.1080/00958972.2015.1109643

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Preparation, crystal structures and properties of
half-sandwich ruthenium complexes containing
salicylbenzoxazole ligands
Ying Sun, Yuan-Chen Dai, Xue Wang, Jia-Ming Cheng & Wei-Guo Jia
To cite this article: Ying Sun, Yuan-Chen Dai, Xue Wang, Jia-Ming Cheng & Wei-Guo
Jia (2015): Preparation, crystal structures and properties of half-sandwich ruthenium
complexes containing salicylbenzoxazole ligands, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2015.1109643
To link to this article: http://dx.doi.org/10.1080/00958972.2015.1109643
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Date: 10 December 2015, At: 17:27

Journal of Coordination Chemistry, 2015
ꢁꢁp://ꢂx.ꢂꢃꢄ.ꢃꢅg/10.1080/00958972.2015.1109643
ꢀ
Preparation, crystal structures and properties of half-sandwich
ruthenium complexes containing salicylbenzoxazole ligands
Ying Sun, Yuan-Chen Dai, Xue Wang, Jia-Ming Cheng and Wei-Guo Jia
tꢀꢆ Kꢆꢇ lꢈbꢃꢅꢈꢁꢃꢅꢇ ꢃꢉ fꢊꢋcꢁꢄꢃꢋꢈꢌ mꢃꢌꢆcꢊꢌꢈꢅ sꢃꢌꢄꢂꢍ, Cꢃꢌꢌꢆgꢆ ꢃꢉ Cꢀꢆꢎꢄꢍꢁꢅꢇ ꢈꢋꢂ mꢈꢁꢆꢅꢄꢈꢌꢍ scꢄꢆꢋcꢆ, Cꢆꢋꢁꢆꢅ ꢉꢃꢅ nꢈꢋꢃ
scꢄꢆꢋcꢆ ꢈꢋꢂ tꢆcꢀꢋꢃꢌꢃgꢇ, mꢄꢋꢄꢍꢁꢅꢇ ꢃꢉ eꢂꢊcꢈꢁꢄꢃꢋ, aꢋꢀꢊꢄ lꢈbꢃꢅꢈꢁꢃꢅꢇ ꢃꢉ mꢃꢌꢆcꢊꢌꢈꢅ-Bꢈꢍꢆꢂ mꢈꢁꢆꢅꢄꢈꢌꢍ, aꢋꢀꢊꢄ nꢃꢅꢎꢈꢌ
uꢋꢄvꢆꢅꢍꢄꢁꢇ, Wꢊꢀꢊ, Cꢀꢄꢋꢈ
ABSTRACT
ARTICLE HISTORY
rꢆcꢆꢄvꢆꢂ 23 Jꢊꢌꢇ 2015
accꢆpꢁꢆꢂ 1 ocꢁꢃbꢆꢅ 2015
Three half-sandwich ruthenium complexes [Ru(p-cymene)LCl] containing
salicylbenzoxazole ligands [LH = 2-(5-methyl-benzoxazol-2-yl)-4-methyl-
phenol (2a), LH = 2-(5-methyl-benzoxazol-2-yl)-4-chloro-phenol (2b), and
LH = 2-(5-methyl-benzoxazol-2-yl)-4-bromo-phenol (2c)] were synthesized
and characterized. All half-sandwich ruthenium complexes were fully
KEYWORDS
hꢈꢌꢉ-ꢍꢈꢋꢂwꢄcꢀ; ꢅꢊꢁꢀꢆꢋꢄꢊꢎ;
ꢍꢈꢌꢄcꢇꢌbꢆꢋzꢃxꢈzꢃꢌꢆ; cꢇcꢌꢄc
vꢃꢌꢁꢈꢎꢎꢆꢁꢅꢇ
1
13
characterized by H and C NMR spectra, MS, elemental analyses, and UV–vis
as well as cyclic voltammetry (CV). The molecular structures of 2a, 2b, and
2
c were confirmed by single-crystal X-ray diffraction. Single-crystal X-ray
structures show that the synthesized ruthenium complexes are three-legged
piano-stools with a six-membered metallocycle formed by coordination of
the bidentate salicylbenzoxazole ligands to the metal centers. Data from
CV and UV–vis absorption of the ruthenium complexes indicated that by
changing the substituent on the para position of (donating or withdraw
group) the salicylbenzoxazole ligands, minor changes in redox and electronic
properties of the ruthenium complexes were observed.
CONTACT Wꢆꢄ-Gꢊꢃ Jꢄꢈ
wgjꢄꢈꢍꢇ@ꢎꢈꢄꢌ.ꢈꢀꢋꢊ.ꢆꢂꢊ.cꢋ
sꢊppꢌꢆꢎꢆꢋꢁꢈꢌ ꢂꢈꢁꢈ ꢉꢃꢅ ꢁꢀꢄꢍ ꢈꢅꢁꢄcꢌꢆ cꢈꢋ bꢆ ꢈccꢆꢍꢍꢆꢂ ꢀꢁꢁp://ꢂx.ꢂꢃꢄ.ꢃꢅg/10.1080/00958972.2015.1109643.
©
2015 tꢈꢇꢌꢃꢅ & fꢅꢈꢋcꢄꢍ

2
Y. SUN ꢀT AL.
1. Introduction
The coordination chemistry and reactivity of half-sandwich ruthenium complexes have been extensively
studied due to the fact that these complexes have a wide range of potential applications in metallo-
macrocycles, host–guest, anticancer drugs, biological activities, electrochemical sensors, and catalysts
in the past decades [1–7]. Various metal-organic frameworks including metallomacrocycles and cages
have been obtained using Ru(p-cymene) as a building block [8–10]. Half-sandwich ruthenium complexes
show high catalytic activities toward organic transformation, such as C–C bond formation via direct C–H
bond activation [11, 12], addition of arylboronic acid [13, 14], acceptorless dehydrogenative coupling
of alcohols [15], transfer hydrogenations of ketones [16] and water oxidation [17, 18]. Ruthenium com-
plexes as anti-cancer drugs have entered clinical trials [19]. Thus, synthesis of half-sandwich ruthenium
complexes and studying their chemical properties would be desirable.
In this work, the half-sandwich ruthenium complexes [Ru(p-cymene)LCl] bearing a chelating salicylben-
zoxazole ligand [LH = 2-(5-methyl-benzoxazol-2-yl)-4-methyl-phenol (2a), LH = 2-(5-methyl-benzoxazol-2-yl)-
4
-chloro-phenol (2b) and LH = 2-(5-methyl-benzoxazol-2-yl)-4-bromo-phenol (2c)] have been synthesized
and characterized. Solid state structures of the newly synthesized half-sandwich ruthenium complexes
2a–2c) were also elucidated by single-crystal X-ray diffraction. The results indicate that the ruthenium
(
complexes are three-legged piano-stools with a six-membered metallocycle formed by coordination of
the bidentate salicylbenzoxazole to ruthenium. The redox properties and UV–vis absorptions of ruthenium
complexes were also investigated, indicating that the substituent (donating or withdraw group) on the
salicylbenzoxazole ligands has minor influences on the ruthenium complexes.
2. Experimental
2
.1. Materials and measurements
All manipulations were carried out under pure nitrogen using standard Schlenk techniques. All solvents
were purified and degassed by standard procedures. The bidentate monoanionic [N,O] ligands were
1
13
synthesized according to procedures described [20]. H and C NMR spectra were recorded on 300 or
00 MHz NMR spectrometers at room temperature. Chemical shifts (δ) are in ppm relative to internal
5
1
13
TMS and are referenced to residual H and C solvent resonances. IR spectra were recorded on a Nicolet
AVATAR-360IR spectrometer. ꢀlemental analyses were performed on an ꢀlementar III vario ꢀI Analyzer.
Mass spectrometry was performed on a Bruker BIFLꢀX III MALDI-TOF-MS instrument. A CHI400c com-
puterace instrument was employed to obtain cyclic voltammograms in acetonitrile solutions of the
ruthenium complexes (2 mM) under nitrogen at room temperature (298 K) using 0.20 M tetrabutylam-
monium perchlorate (TBAP) as supporting electrolyte. A glassy carbon working electrode, a platinum
auxiliary electrode, and a Ag/AgCl reference electrode were used to obtain cyclic voltammograms. All
E1/2 values estimated from cyclic voltammetry (CV) were calculated as the average of the oxidative and
−1
reductive peak potentials (Epa + E )/2 at a scan rate of 100 mV s .
pc
2
2
.2. Synthesis
.2.1. Synthesis of 2-(5-methyl-benzoxazol-2-yl)-4-methyl-phenol (1a)
A solution of 5-methylsalicylaldehyde (1.36 g, 10 mmol), 2-amino-4-methylphenol (1.23 g, 10 mmol),
and 3–5 drops of acetic acid in 50 mL of methanol was refluxed with stirring for 3 h after bulk orange
precipitate formation under nitrogen. When the resulting mixture was cooled to room temperature,
PhI(OAc) (3.54 g, 11 mmol) was added and the mixture immediately turned black. After the mixture
2
was refluxed again for 1 h in open air, the solvent was evaporated to dryness and the black crude
product was chromatographed on silica gel with petroleum ether to give white powder (1.63 g, 68%).
1
Anal. Calcd for C H NO (239.09) : C, 75.30; H, 5.48; N, 5.85. Found : C, 75.40; H, 5.25; N, 5.60. H NMR
1
5
13
2
(
500 MHz, CDCl , ppm) : δ 11.33 (s, 1H, Ph-OH), 7.79 (s, 1H, Ph-H), 7.50 (t, 2H, Ph-H), 7.25 (s, 2H, Ph-H),
3

JOURNAL OF COORDINATION CHꢀMISTRY
3
1
3
7
1
.02 (d, 1H, Ph-H), 2.48 (s, 3H, –CH ), 2.36 (s, 3H, –CH ). C NMR (125 MHz, CDCl , ppm) : 163.04, 156.55,
3 3 3
47.31 140.23, 134.83, 134.33, 128.66, 126.83, 126.32, 119.12, 117.12, 110.24, 109.90, 21.48, 20.48.
.2.2. Synthesis of 2-(5-methyl-benzoxazol-2-yl)-4-chloro-phenol (1b)
2
A procedure similar to that used for preparation of 1a was employed. White powder, yield: 1.82 g (70%).
1
Anal. Calcd for C H ClNO (259.04) : C, 64.75; H, 3.88; N, 5.39. Found: C, 64.60; H, 4.05; N, 5.20. H NMR
1
4
10
2
(
7
500 MHz, CDCl , ppm): δ 11.50 (s, 1H, Ph-OH), 7.98 (d, 1H, Ph-H), 7.52 (t, 2H, Ph-H), 7.38 (d, 1H, Ph-H),
3
1
3
.22 (d, 1H, Ph-H), 7.07 (d, 1H, Ph-H), 2.50 (s, 3H, –CH ). C NMR (125 MHz, CDCl , ppm): 162.09, 157.53,
3 3
1
47.75, 140.30, 135.60, 133.55, 127.31, 126.67, 124.75, 119.70, 119.25, 112.07, 110.49, 21.87.
.2.3. Synthesis of 2-(5-methyl-benzoxazol-2-yl)-4-bromo-phenol (1c) [21]
2
A procedure similar to that used for preparation of 1a was employed. White powder, yield: 2.19 g (72%).
1
Anal. Calcd for C H BrNO (302.99): C, 55.29; H, 3.31; N, 4.61. Found: C, 55.41; H, 3.35; N, 4.53. H NMR
1
4
10
2
(
7
1
500 MHz, CDCl , ppm): δ 11.50 (s, 1H, Ph-OH), 8.08 (d, 1H, Ph-H), 7.48 (m, 3H, Ph-H), 7.19 (d, 1H, Ph-H),
3
13
.00 (t, 1H, Ph-H), 2.48 (s, 3H, –CH3). C NMR (125 MHz, CDCl , ppm): 161.57, 157.58, 147.35, 139.88,
3
35.96, 135.21, 129.23, 126.92, 119.30, 119.25, 112.28, 111.23, 110.10, 21.49.
2
.2.4. Synthesis of half-sandwich ruthenium complex 2a
[
Ru(p-cymene)(μ-Cl)Cl] (306 mg, 0.5 mmol), 1a (287 mg, 1.2 mmol), 193 mg (1.4 mmol) K CO and
2
2
3
3
0 mL CH CN as solvent were placed in a 100 mL Schlenk tube. The mixture was stirred and refluxed
3
for 3 h and then cooled to room temperature. The solvent was removed with a rotary evaporator; the
resulting solid was washed with ꢀt O. The product was recrystallized from ꢀt O/MeOH to give orange 2a
2
2
1
(
7
–
265 mg, 52%). H NMR (300 MHz, CDCl , ppm): δ 7.51 (d, 2H, Ph-H), 7.42 (s, 1H, Ph-H), 7.17 (d, 1H, Ph-H),
.07 (d, 2H, Ph-H), 5.54 (d, 1H, Ph-H), 5.47 (d, 1H, Ph-H), 5.42 (d, 1H, Ph-H), 5.35 (d, 1H, Ph-H), 2.77 (m, 1H,
CH(CH ) ), 2.52 (s, 3H, –CH ), 2.25 (s, 6H, –CH ), 1.19 (d, 3H, –CH(CH ) ), 1.10 (d, 3H, –CH(CH ) ). C NMR
3
1
3
3
2
3
3
3 2
3 2
(
125 MHz, CDCl , ppm): 168.23, 160.64, 148.24, 141.34, 135.70, 135.33, 126.85, 126.25, 123.89, 123.81,
3
1
18.86, 110.37, 110.26, 102.63, 97.54, 83.54, 81.95, 80.10, 79.78, 30.78, 22.71, 21.84, 21.77, 20.31, 18.90.
+
+
+
−1
MS (MALDI-TOF): Calcd for C H NO Ru 474.1007 (M ), found 474.4827 (M ). IR (KBr cm ): 3052(w),
25
26
2
2
969(w), 2912(w), 2861(w), 1626(m), 1556(s), 1485(m), 1339(m), 1250(w), 830(w).
2
.2.5. Synthesis of half-sandwich ruthenium complex 2b
Complex 2b was prepared by the same procedure as described above for 2a using [Ru(p-cymene)(μ-Cl)
Cl] (306 mg, 0.5 mmol), 1b (311 mg, 1.2 mmol) and 193 mg (1.4 mmol) K CO . Yield: (291 mg, 55%). H
1
2
2
3
NMR (300 MHz, CDCl , ppm): δ 7.70 (d, 1H, Ph-H), 7.47 (t, 2H, Ph-H), 7.21 (t, 2H, Ph-H), 7.03 (d, 1H, Ph-H),
3
5
.54 (d, 1H, Ph-H), 5.50 (d, 1H, Ph-H), 5.42 (d, 1H, Ph-H), 5.36 (d, 1H, Ph-H), 2.77 (m, 1H, –CH(CH ) ), 2.53
3 2
13
(
s, 3H, –CH ), 2.28 (s, 3H, –CH ), 1.20 (d, 3H, –CH(CH ) ), 1.11 (d, 3H, –CH(CH ) ). C NMR (125 MHz, CDCl ,
3 3 3 2 3 2 3
ppm): 168.51, 159.49, 148.19, 141.17, 135.70, 134.08, 126.86, 126.45, 125.64, 119.11, 118.94, 111.46,
+
+
1
(
1
10.52, 102.89, 97.85, 83.74 (MALDI-TOF): Calcd for C H ClNO Ru 494.0461 (M ), found 494.2528
24 23 2
+
−1
M ). IR (KBr cm ): 3041(w), 2971(w), 2915(w), 2869(w), 1600(m), 1551(m), 1518(m), 1458(s), 1396(m),
334(m), 1235(m), 1186(m), 1098(w), 1052(w), 819(m), 724(m), 658(w), 592(w), 533(w).
2
.2.6. Synthesis of half-sandwich ruthenium complex 2c
Complex 2c was prepared by the same procedure as described above for 2a using [Ru(p-cymene)(μ-
Cl)Cl] (306 mg, 0.5 mmol), 1c (193 mg, 1.2 mmol) and 193 mg (1.4 mmol) K CO . Yield: (332 mg 58%).
2
2
3
1
H NMR (500 MHz, CDCl , ppm): δ 7.84 (d, 1H, Ph-H), 7.47 (s, 1H, Ph-H), 7.43 (d, 1H, Ph-H), 7.26 (m, 1H,
3
Ph-H), 7.19 (d, 1H, Ph-H), 6.97 (d, 1H, Ph-H), 5.55 (d, 1H, Ph-H), 5.49 (d, 1H, Ph-H), 5.43 (d, 1H, Ph-H),5.35
(
d, 1H, Ph-H), 2.75 (m, 1H, –CH(CH ) ), 2.52 (s, 3H, –CH ), 2.27 (s, 3H, –CH ), 1.20 (d 3H, –CH(CH ) ), 1.11 (d,
H, –CH(CH ) ). C NMR (125 MHz, CDCl , ppm): 168.85, 159.32, 148.15, 141.12, 136.65, 135.69, 129.50,
3 2 3
3 3 3 3 3 2
1
3
3
1
2
26.86, 126.08, 118.91, 112.28, 110.51, 105.74, 102.86, 97.82, 83.72, 81.99, 80.21, 79.55, 30.82, 22.67,
+
+
+
1.85, 21.80, 18.93. MS (MALDI-TOF): Calcd for C H BrNO Ru 537.9956 (M ), found 538.1400 (M ). IR
24
23
2
−1
(
KBr cm ): 3070(w), 3041(w), 2968(w), 2959(w), 2919(w), 2866(w), 1597(m), 1511(m), 1521(m), 1462(s),

4
Y. SUN ꢀT AL.
Table 1. Cꢅꢇꢍꢁꢈꢌꢌꢃgꢅꢈpꢀꢄc ꢂꢈꢁꢈ ꢈꢋꢂ ꢍꢁꢅꢊcꢁꢊꢅꢆ ꢅꢆfiꢋꢆꢎꢆꢋꢁ pꢈꢅꢈꢎꢆꢁꢆꢅꢍ ꢉꢃꢅ 2a–2c.
2a
2b
2c
eꢎpꢄꢅꢄcꢈꢌ ꢉꢃꢅꢎꢊꢌꢈ
fꢃꢅꢎꢊꢌꢈ wꢆꢄgꢀꢁ
C h Cꢌno rꢊ
C h Cꢌ no rꢊ
C h BꢅCꢌno rꢊ
2
5
26
2
24 23
2
2
24 23
2
508.99
529.40
573.86
Cꢅꢇꢍꢁꢈꢌ ꢍꢇꢍꢁꢆꢎ, ꢍpꢈcꢆ gꢅꢃꢊp
mꢃꢋꢃcꢌꢄꢋꢄc, P21/c
18.263(4)
mꢃꢋꢃcꢌꢄꢋꢄc, P21/c
18.103(4)
tꢅꢄcꢌꢄꢋꢄc, P-1
7.3429(5)
10.3625(7)
14.5662(10)
90.1710(10)
98.2340(10)
93.9370(10)
1094.25(13), 2
1.742
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
17.428(4)
7.5956(18)
90.00
97.296(4)
90.00
2398.0(9), 4
1.410
0.785
17.336(4)
7.5709(17)
90.00
96.794(3)
90.00
2359.3(9), 4
1.490
0.911
3
Vꢃꢌꢊꢎꢆ (Å ), Z
−
3
D (ꢎg ꢎ )
c
−1
μ (mꢃ-Kα) (ꢎꢎ )
2.685
572
1.97–25.00
F(0 0 0)
1040
1072
θ ꢅꢈꢋgꢆ (°)
1.62–25.00
−21, 21; −20, 20; −8, 9
4191/1887 [0.1775]
25.00 (99.3%)
4191/0/268
0.938
1.63–25.00
−21, 21; −19, 20; −8, 8
4132/2697 [0.0865]
25.00 (99.4%)
4132/0/271
1.023
lꢄꢎꢄꢁꢄꢋg ꢄꢋꢂꢄcꢆꢍ
−8, 8, −12, 10, −16, 17
3837/3634 [0.0186]
25.00 (99.1%)
3837/0/319
1.058
rꢆflꢆcꢁꢄꢃꢋꢍ/ꢊꢋꢄqꢊꢆ [R(ꢄꢋꢁ)]
Cꢃꢎpꢌꢆꢁꢆꢋꢆꢍꢍ ꢁꢃ θ (°)
dꢈꢁꢈ/ꢅꢆꢍꢁꢅꢈꢄꢋꢁꢍ/pꢈꢅꢈꢎꢆꢁꢆꢅꢍ
2
Gꢃꢃꢂꢋꢆꢍꢍ ꢃꢉ fiꢁ ꢃꢋ F
ꢈ
R , wR [I > 2σ(I)]
R = 0.0823, wR = 0.1810
R = 0.0669, wR = 0.1550
R = 0.0247, wR = 0.0649
1
2
1
2
1
2
1
2
R , wR (ꢈꢌꢌ ꢂꢈꢁꢈ)
R = 0.1734, wR = 0.2107
R = 0.1074, wR = 0.1697
R = 0.0262, wR = 0.0660
1
2
1
2
1
2
1
2
ꢈ
R = Σ||F |−|F ||/Σ|F |; wR = [Σw(|F |−|F |) /Σw|F 2|2]1/2.
2
2 2
1
ꢃ
c
ꢃ
2
ꢃ
c
ꢃ
Table 2. sꢆꢌꢆcꢁꢆꢂ bꢃꢋꢂ ꢌꢆꢋgꢁꢀꢍ (Å) ꢈꢋꢂ ꢈꢋgꢌꢆꢍ (°) ꢉꢃꢅ 2a–2c.
2a
2b
2c
rꢊ1–n1
rꢊ1–o1
rꢊ1–Cꢌ1
n1-rꢊ1–o1
n1-rꢊ1–Cꢌ1
o1-rꢊ1–Cꢌ1
2.121(9)
2.052(7)
2.411(3)
86.2(3)
86.0(2)
85.8(2)
2.103(8)
2.065(6)
2.411(3)
85.6(3)
85.1(2)
86.0(2)
2.0981(19)
2.0794(17)
2.4079(7)
85.74(7)
82.73(6)
87.06(6)
1
6
393(m), 1311(m), 1256(m), 1232(m), 1186(m), 1085(w), 947(w), 895(w), 872(w), 849(w), 816(m), 708(m),
48(m), 589(w), 530(w).
2
.3. X-ray structure determination
Diffraction data of 2a, 2b, and 2c were collected on a Bruker AXS SMART APꢀX diffractometer equipped
with a CCD area detector using Mo Kα radiation (λ = 0.71073 Å). All the data were collected at 298 K and
2
the structures were solved by direct methods and subsequently refined on F using full-matrix least
squares techniques (SHꢀLXL) [22]. SADABS [23] absorption corrections were applied to the data, all
non-hydrogen atoms were refined anisotropically, and hydrogens were located at calculated positions.
All calculations were performed using the Bruker Smart program. A summary of the crystallographic
data and selected experimental information is given in Table 1 with selected bond distances and angles
given in Table 2.
3. Results and discussion
3
.1. Synthesis of ligands and half-sandwich ruthenium complexes
As outlined in Scheme 1, the salicylbenzoxazoles (1a–1c) were readily accessible from salicylaldehyde
in two steps [20], condensation of the corresponding salicylaldehyde with o-aminophenol and then

JOURNAL OF COORDINATION CHꢀMISTRY
5
ΟΗ
OH
Ο
OH
2
H N
3
EtOH, CH COOH, reflux
N
+
CH CN, PhI(OAc)
3
2
R
Ο
R
R = CH3 1a
R = Cl
1b
R = Br
1c
Scheme 1. sꢇꢋꢁꢀꢆꢍꢄꢍ ꢃꢉ ꢍꢈꢌꢄcꢇꢌbꢆꢋzꢃxꢈzꢃꢌꢆ ꢌꢄgꢈꢋꢂꢍ (1a–1c).
Cl
Ru
Ru Cl
CH
3
CN, reflux
CO
OH
R
N
+
Cl
Cl
O
2
2
O
K
2
3
N
O
Ru
Cl
R
R = CH3 2a
R = Cl
R = Br
2b
2c
Scheme 2. sꢇꢋꢁꢀꢆꢍꢄꢍ ꢃꢉ 2a–2c wꢄꢁꢀ ꢍꢈꢌꢄcꢇꢌbꢆꢋzꢃxꢈzꢃꢌꢆ ꢌꢄgꢈꢋꢂꢍ.
oxidation with PhI(OAc) in refluxing CH CN. The desired products were separated through column
2
3
chromatography on silica gel as white powders in moderate to good yields. A general synthetic route
for half-sandwich ruthenium complexes is shown in Scheme 2. The complexes 2a–2c were obtained
by the treatment of two equiv of the salicylbenzoxazole ligands with [Ru(p-cymene)(μ-Cl)Cl] in the
2
presence of K CO in CH CN under reflux for 3 h, affording orange crystals of 2a–2c in yields of 52–58%.
2
3
3
These complexes are stable toward air and moisture in the solid state.
These ruthenium complexes were characterized by IR and NMR spectroscopy and MS. These
1
half-sandwich ruthenium complexes have similar NMR spectra, so we use 2c for an example. The H
NMR spectrum of 2c show signals at δ 1.11, 1.20, and 2.75 ppm, which can be assigned to the isopropyl
groups, which exhibited signals at δ 5.35–5.55 ppm for the arene of p-cymene ring, respectively. Mass
spectra also provided further evidence for the formation of the half-sandwich ruthenium complexes,
with peaks at m/z = 474.1007 (for 2a), 494.2528 (for 2b), and 537.9956 (for 2c), corresponding to the
+
loss of the Cl anion [M–Cl] .
Crystals of 2a, 2b, and 2c suitable for X-ray crystallographic diffraction were obtained by slow dif-
fusion of diethyl ether into a concentrated solution of the ruthenium complexes in dichloromethane
or methanol solution. The crystallographic data for 2a, 2b, and 2c are summarized in Table 1, and
selected bond lengths and angles are given in Table 2. The molecular structures of 2a–2c are shown
in Figures 1–3.
As shown in Figures 1–3, the half-sandwich ruthenium complexes have remarkably similar structures.
ꢀach Ru is surrounded by one chloride, one nitrogen, and one oxygen from the ligand and one of the
p-cymene rings. All of the ruthenium centers have six-coordinate geometry, assuming that the p-cymene
ring is a three-coordinate ligand. Ruthenium of 2a, 2b, and 2c has a three-legged piano-stool confor-
mation with a six-membered metallocycle formed by coordination of the bidentate salicylbenzoxazole
ligands to the metal centers. The Ru–N distances (2.121(9) Å in 2a, 2.103(8) Å in 2b, and 2.0981(19) Å in
2c) are in the range of typical distances of reported ruthenium complexes containing pyridine or [N,O]
anionic bidentate ligands [9, 24, 25].

6
Y. SUN ꢀT AL.
Figure 1. Cꢃꢎpꢌꢆx 2a wꢄꢁꢀ ꢁꢀꢆꢅꢎꢈꢌ ꢆꢌꢌꢄpꢍꢃꢄꢂꢍ ꢂꢅꢈwꢋ ꢈꢁ ꢁꢀꢆ 30% ꢌꢆvꢆꢌ. aꢌꢌ ꢀꢇꢂꢅꢃgꢆꢋꢍ ꢈꢅꢆ ꢃꢎꢄꢁꢁꢆꢂ ꢉꢃꢅ cꢌꢈꢅꢄꢁꢇ.
Figure 2. Cꢃꢎpꢌꢆx 2b wꢄꢁꢀ ꢁꢀꢆꢅꢎꢈꢌ ꢆꢌꢌꢄpꢍꢃꢄꢂꢍ ꢂꢅꢈwꢋ ꢈꢁ ꢁꢀꢆ 30% ꢌꢆvꢆꢌ. aꢌꢌ ꢀꢇꢂꢅꢃgꢆꢋꢍ ꢈꢅꢆ ꢃꢎꢄꢁꢁꢆꢂ ꢉꢃꢅ cꢌꢈꢅꢄꢁꢇ.
3
.2. The CV and UV–vis absorbance of 2a–2c
The electrochemical properties of 2a–2c have been studied by CV in degassed CH CN solution under
3
nitrogen using 0.2 M TBAP as supporting electrolyte. A glassy carbon working electrode, a platinum
auxiliary electrode, and a Ag/AgCl reference electrode were used to obtain cyclic voltammograms. The
data for 2a–2c are collected in Table 3, and the cyclic voltammograms of the complexes are shown in
Figure 4. Complexes 2a–2c displayed a one-electron oxidation process Ru(II)/Ru(III) with quasi-reversible
half-potentials and similar to those of other half-sandwich ruthenium complexes [26]. The quasi-revers-
ible reduction couples (ꢀ1/2) for 2a–2c were −0.782, −0.796, and −0.783 V, respectively. On modifying

JOURNAL OF COORDINATION CHꢀMISTRY
7
Figure 3. Cꢃꢎpꢌꢆx 2c wꢄꢁꢀ ꢁꢀꢆꢅꢎꢈꢌ ꢆꢌꢌꢄpꢍꢃꢄꢂꢍ ꢂꢅꢈwꢋ ꢈꢁ ꢁꢀꢆ 30% ꢌꢆvꢆꢌ. aꢌꢌ ꢀꢇꢂꢅꢃgꢆꢋꢍ ꢈꢅꢆ ꢃꢎꢄꢁꢁꢆꢂ ꢉꢃꢅ cꢌꢈꢅꢄꢁꢇ.
Table 3. eꢌꢆcꢁꢅꢃcꢀꢆꢎꢄcꢈꢌ ꢈꢋꢂ uV/Vꢄꢍ ꢈbꢍꢃꢅpꢁꢄꢃꢋ ꢂꢈꢁꢈ ꢉꢃꢅ 2a–2c.
3
−1
−1
Complex
E /V
E /V
E1/2/V
Absorption λmax(nm) εmax (dm mol cm )
pc
pa
2a
2b
2c
−0.903
−0.911
−0.875
−0.660
−0.680
−0.691
−0.782
−0.796
−0.783
299 (11,812), 400 (5660)
298 (9950), 400 (5125)
299 (12,420), 400 (6254)
nꢃꢁꢆꢍ: mꢆꢈꢍꢊꢅꢆꢂ wꢄꢁꢀ ꢈ gꢌꢈꢍꢍꢇ cꢈꢅbꢃꢋ ꢆꢌꢆcꢁꢅꢃꢂꢆ ꢈꢁ 100 ꢎV ꢍ−1 ꢄꢋ Ch Cn cꢃꢋꢁꢈꢄꢋꢄꢋg 0.2 m nBꢊ Cꢌo ꢈꢋꢂ E = (Epꢈ + E )/2 ꢄꢋ
3
4
4
1/2
pc
vꢃꢌꢁꢍ ꢊꢍꢄꢋg ag/agCꢌ ꢈꢍ ꢅꢆꢉꢆꢅꢆꢋcꢆ. E1/2 = (Epꢈ + E )/2 ꢉꢃꢅ ꢁꢀꢆ qꢊꢈꢍꢄ-ꢅꢆvꢆꢅꢍꢄbꢌꢆ ꢅꢆꢂꢊcꢁꢄꢃꢋ cꢃꢊpꢌꢆꢍ; E ꢈꢋꢂ Epc ꢈꢅꢆ pꢆꢈk ꢈꢋꢃꢂꢄc
pc
pꢈ
ꢈꢋꢂ pꢆꢈk cꢈꢁꢀꢃꢂꢄc pꢃꢁꢆꢋꢁꢄꢈꢌꢍ, ꢅꢆꢍpꢆcꢁꢄvꢆꢌꢇ.
Figure 4. Cꢇcꢌꢄc vꢃꢌꢁꢈꢎꢎꢃgꢅꢈꢎꢍ ꢃꢉ 2a–2c.

8
Y. SUN ꢀT AL.
0
0
0
0
0
0
.25
.20
.15
.10
.05
.00
2
2
2
a
b
c
300
400
500
Wavelength/nm
Figure 5. tꢀꢆ uV–vꢄꢍ ꢈbꢍꢃꢅpꢁꢄꢃꢋ ꢃꢉ 2a–2c (1 × 10−5 ꢎꢃꢌ l ) ꢄꢋ Ch Cn.
−1
3
the para substituent on the ligand from –Me to –Br on the ligands, the oxidation potential increases
from −0.660 V (2a) to −0.691 V (2c). This slight variation is attributed to the electron-rich (–Me) and
−
−
electron-deficient (Cl and Br ) substituents on the salicylbenzoxazole ligands. It is apparent from the
cyclic voltammograms that the ꢀ1/2 values are sensitive to substituents on the salicylbenzoxazole ligands.
The absorption spectra of 2a–2c were measured at room temperature in CH CN. The UV/Vis absorp-
3
tion data are summarized in Table 3, and the corresponding electronic absorption spectra of ruthenium
complexes are depicted in Figure 5. All ruthenium complexes show intense absorptions at 299 and
4
3
−1
−1
4
00 nm with molar extinction coefficients (ε) in the order of 10 dm mol cm , which can be assigned
to charge transfer between salicylbenzoxazole ligands and ruthenium (MLCT). The salicylbenzoxazole
compounds have a strong absorption between 220 and 280 nm due to the π–π* transition. When com-
plexed to ruthenium, this absorption band was slightly red shifted for 2a–2c, as observed in ruthenium
complexes with [3] radialene ligands [27].
4. Conclusion
We have synthesized and characterized three half-sandwich salicylbenzoxazole ruthenium complexes.
A combination of spectroscopic studies and X-ray crystallographic confirmed the molecular structures
of 2a–2c. The redox properties and electronic absorptions of 2a–2c showed that the electron-rich or
electron-deficient substituents on the salicylbenzoxazole ligands have minor influences to the redox
and spectroscopic properties of the ruthenium complexes.
Supplementary material
The crystallographic data (excluding structure factors) for the structures in this article have been depos-
ited with the Cambridge Crystallographic Data Center, CCDC, 12 Union Road, Cambridge CB21ꢀZ, UK.
Copies of the data can be obtained free of charge on quoting the depository numbers CCDC–1063395
(2a), CCDC–1063396 (2b) and CCDC–1063397 (2c) (Fax: +44–1223-336–033; ꢀ-mail: deposit@ccdc.cam.
ac.uk; http://www.ccdc.cam.ac.uk).
Disclosure statement
No potential conflict of interest was reported by the authors.

JOURNAL OF COORDINATION CHꢀMISTRY
9
Funding
We acknowledge financial support from the National Nature Science Foundation of China [grant number 21102004], and
the Project-sponsored by SRF for ROCS and Special and ꢀxcellent Research Fund of Anhui Normal University.
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