First cycloruthenation of 2-alkenylpyridines: synthesis, characterization and properties

Article abstract of DOI:10.1039/d0ra09833g

Several cyclometalated ruthenium complexes 1-5 with 2-alkenylpyridines as C,N-chelating ligands were synthesized and then characterized by NMR, MS, IR and UV-Vis spectra. According to the single crystal of complex 2, it is evident that carbon from vinyl group is successfully bonded to Ru(ii) center. Moreover, the Ru-N bond trans to the Ru-C bond is elongated (2.127(5) ?), which is consistent with the strong trans effect of the carbon atom compared to that of the nitrogen atom. With different electron-donating groups linked to vinyl, these complexes exhibited regular changes in MLCT absorption bands, which were identified by UV-Vis and CV spectra in combination with DFT and TD-DFT. Interestingly, protonated intermediate species of these complexes in acidic solutions were tracked by the absorption changes and MS spectra, which displayed a possible protonation process of these complexes with the cleavage of Ru-C σ bonds.

Full text of DOI:10.1039/d0ra09833g

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First cycloruthenation of 2-alkenylpyridines:
synthesis, characterization and properties†
Cite this: RSC Adv., 2021, 11, 4138
Yuhao Wu,‡ Xianlong Su,‡ Chaoyi Xie, Rongrong Hu, Xianghong Li, *a
a
a
a
b
b
a
a
Qiang Zhao,
Guoli Zheng and Junkun Yan
Several cyclometalated ruthenium complexes 1–5 with 2-alkenylpyridines as C,N-chelating ligands were
synthesized and then characterized by NMR, MS, IR and UV-Vis spectra. According to the single crystal
of complex 2, it is evident that carbon from vinyl group is successfully bonded to Ru(II) center. Moreover,
the Ru–N bond trans to the Ru–C bond is elongated (2.127(5) A˚ ), which is consistent with the strong
trans effect of the carbon atom compared to that of the nitrogen atom. With different electron-donating
groups linked to vinyl, these complexes exhibited regular changes in MLCT absorption bands, which
were identified by UV-Vis and CV spectra in combination with DFT and TD-DFT. Interestingly,
protonated intermediate species of these complexes in acidic solutions were tracked by the absorption
changes and MS spectra, which displayed a possible protonation process of these complexes with the
cleavage of Ru–C s bonds.
Received 19th November 2020
Accepted 5th January 2021
DOI: 10.1039/d0ra09833g
rsc.li/rsc-advances
15
16
17
cells, chemosensors, and PDT sensitizers by personalized
Introduction
modications of cyclometalated ligands such as expanding p-
13,15
16b,17
Cyclometalated complexes have attracted considerable interest systems,
for a wide variety of applications, since the rst cyclometalated of ligand frameworks used in these complexes have been based on
platinum complex was reported in 1965. There are now many 2-arylpyridine derivatives
examples of reactions involving metalation of ligands by transition rare examples with 2-phenylimidazole, and N,N-dimethylbenzy-
and graing photosensitive groups.
However, most
1
2
11–17
18
and N-heterocyclic carbenes with
19
14b,20
metals such as Ir(III), Rh(III), Pt(IV), Ru(II), and Os(II). It is noted that lamine.
To the best of our knowledge, few cycloruthenations of
the metalated ligand should contain at least one common donor 2-vinylpyridine and derivatives have been reported.
3
atom (N, P, O, S) and one carbon donor atom. The carbon donor
Therefore, our interest in the design of new organic ligands
4
5
4,6
atoms usually involve alkyl, aryl, or benzyl. In comparison with for cyclometalated ruthenium complexes has led us to synthesize
aryl carbons, the metalation of alkenyl carbons is much less a series of 2-alkenylpyridines. Herein, we investigated in details
7
common. Up to now, some transition metals such as rhodium,
a series of ruthenium(II) metalations at alkenyl carbons involving 2-
8
9
platinum and iridium have been reported by metalations at alkenylpyridine and related ligands. The synthetic route is shown
alkenyl carbons involving 2-vinylpyridines.
in Scheme 1. Aer being characterized by NMR, MS, FT-IR and
Cyclometalated ruthenium complexes have been investigated elemental analysis, photophysical properties of these complexes
for their signicantly improved absorptions in the visible region were investigated by UV-Vis and FL spectra in combination with CV
2+
10,11
especially when compared with the classic Ru(bpy)
3
.
To date, spectra and theoretical calculations. To the best of our knowledge,
many cycloruthenated complexes have been synthesized and used deprotonations of ligands are necessary for the formation of
1
2
13
as energy acceptors, electron transfer sensitizers in dyads, redox cyclometalated ruthenium complexes. However, there have been
14
mediators in oxidase chemistry, sensitizers for dye sensitized solar rare examples on the protonation of cycloruthenated complexes
despite that. Therefore, these complexes incubated in acidic
solutions were investigated in details. Then, possible acid–base
Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education, equilibria for these complexes were depicted.
a
Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for
Nationalities, Wuhan 430074, China. E-mail: lixhchem@mail.scuec.edu.cn
b
Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of
Experimental sections
Advanced Materials (IAM), Nanjing University of Posts & Telecommunications (NUPT),
Nanjing 210023, China
Electronic supplementary information (ESI) available: Additional spectral data
Materials
†
Ethyltriphenylphosphonium bromide, benzyltriphenylphosphonium
bromide, triphenyl-(4-uorobenzyl)phosphonium bromide, tri-
phenyl-(4-N,N-dimethylbenzyl)phosphonium bromide, and tri-
phenyl-(thiophen-2-ylmethyl)phosphonium bromide were prepared
associated with this article can be found. CCDC 2032477 containing the
supplementary crystallographic data for complex 2. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/d0ra09833g
‡
These authors contributed equally.
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Scheme 1 The reactions of 2-alkenylpyridine and related ligands with ruthenium(II).
21
23
0
by the methods described in literatures and used directly in the Gaussian 09 package. The structures in the ground state (S ) were
synthesis of 2-vinylpyridine derivatives. All solvents and reagents fully optimized by the Becke's three-parameter exchange func-
were commercially available and used without further purication tional along with the Lee–Yang–Parr correlation functional
unless specied.
(B3LYP) and plus polarization function 6-31G* basis set. Calcula-
tions of electronic absorption spectra were computed with the
time-dependent density functional theory (TD-DFT) method at the
B3LYP/6-31G* level based on the optimized ground-state struc-
tures. For all the calculations, the associated basis set was applied
to describe Ru with the “double-z” quality LANL2DZ basis set,
whereas, for all other atoms, the 6-31G(d) basis set was employed.
The contours of the HOMOs and LUMOs orbitals were plotted.
Physical measurements
NMR spectra were recorded on a Bruker Avance-400 spectrom-
eter. Mass spectra were obtained on a Bruker autoex speed MALDI-
TOF spectrometer by using CHCA (a-cyano-4-hydroxycinnamic acid)
as matrix and elemental analyses were determined by a VarioEL III
O-Element Analyzer system. Absorption spectra were recorded using
a Shimadzu UV-2550 spectrophotometer. Fluorescence spectra were
recorded on a Varian Cary Eclipse spectrophotometer in a 4.5 mL
Synthesis of ligands
(
1 cm in diameter) cuvette with 2 mL solution at 77 K. All pH
(E)-2-(Prop-1-en-1-yl)pyridine. In a three-necked ask, 2-pyr-
measurements were made with a Model pHS-3B pH meter idylaldehyde (45.6 mg, 0.43 mmol) and anhydrous BuOK
t
(
Shanghai, China). Cyclic voltammogram spectra of 1–5 were (98.7 mg, 0.88 mmol) were dried in vacuum for 30 min and
collected in MeCN with [n-Bu N]PF
as the supporting electrolyte at followed by adding 20 mL of dry THF. Then, ethyl-
00 mV s , using the standard three-electrode setup with a plat- triphenylphosphonium bromide (160.2 mg, 0.43 mmol) dis-
inum disk as working electrode, platinum wire as counter electrode, solved in 5 mL of dry THF was added dropwise to the above
and AgNO
/Ag as reference electrode. Herein, potentials reported solution with stirring under N atmosphere. The mixture was
4
6
ꢀ
1
1
3
2
+
relative vs. ferrocenium/ferrocene (Fc /Fc) used as an internal stan- stirred at ambient temperature for another 3 h and then poured
dard. The single-crystal X-ray data were collected on a Bruker P4 into ice-water (40 mL). The mixture was extracted with
˚
dichloromethane and concentrated in vacuum. Aer puried by
scan mode. The crystal of 2 was kept at a steady T ¼ 293(2) K during silica gel column chromatography with CH Cl /petroleum ether
(v/v ¼ 2 : 1) as eluent, orange oil was obtained with yield of 61%
diffractometer with Mo Ka radiation (l ¼ 0.71073 A) using the u-
2
2
2
2a
data collection. The structure of 2 was solved with the Olex2.solve
1
22b
solution program by using Olex2 1.3-dev as the graphical inter- (0.031 g). The HNMR was in good agreements with the ref. 9a.
face. The model was rened with ShelXL 2018/3 (ref. 22c) using full
matrix least squares minimization on F .
(E)-2-Styrylpyridine. The procedure was the same as that of (E)-
2-(prop-1-en-1-yl)pyridine. Yield: 0.052 g (67%). H NMR (CDCl ,
3
2
1
4
00 MHz), d (ppm): 8.61 (d, J ¼ 4.4 Hz, 1H), 7.48 (td, J ¼ 8.0 Hz, J ¼
2
.0 Hz 1H), 7.27–7.25 (m, 5H), 7.19 (d, J ¼ 8.0 Hz, 1H), 7.12–7.09
Preparation of solutions
(m, 1H), 6.87 (d, J ¼ 12.4 Hz, 1H), 6.73 (d, J ¼ 12.4 Hz, 1H).
Deionized water was used throughout all experiments. Stock
solutions of 2-alkenylpyridine ligands (1.0 mM) and complexes
(
E)-2-(2-(Thiophen-2-yl)vinyl)pyridine. Yield: 0.046 g (58%).
1
H NMR (CDCl , 400 MHz), d (ppm): 8.73 (d, J ¼ 4.0 Hz, 1H),
3
1–5 were prepared in acetonitrile and ethanol, respectively.
7
(
.68 (td, J ¼ 1.6 Hz, J ¼ 7.8 Hz, 1H), 7.35 (d, J ¼ 7.8 Hz, 1H), 7.29
d, J ¼ 4.0 Hz, 1H), 7.25 (d, J ¼ 3.6 Hz, 1H), 7.19 (dd, J ¼ 7.8 Hz, J
4.8 Hz, 1H), 7.00 (dd, J ¼ 7.2 Hz, J ¼ 3.6 Hz, 1H), 6.92 (d, J ¼
2.4 Hz, 1H), 6.51 (d, J ¼ 12.4 Hz, 1H).
E)-2-(4-Fluorostyryl)pyridine. Yield: 0.053 g (62%). H NMR
CD
OD, 400 MHz), d (ppm): 8.51 (d, J ¼ 5.2 Hz, 1H), 7.66–7.63
Then, solutions of 2-alkenylpyridine ligands were diluted to 20
mM with acetonitrile, while complexes were diluted to 20 mM
with ethanol. For complexes 1–5, their buffered solutions were
prepared in ethanol and Briton–Robinson buffer at volume ratio
of 1 : 2 and then incubated for 4 h.
¼
1
1
(
(
(
3
m, 1H), 7.28–7.21 (m, 4H), 7.01–6.96 (m, 2H), 6.89 (d, J ¼
Theoretical calculations
1
2.1 Hz, 1H), 6.70 (d, J ¼ 12.1 Hz, 1H).
All the density functional theory (DFT) and time-dependent density
(E)-N,N-Dimethyl-4-(2-(pyridin-2-yl)vinyl)aniline.
Yield:
1
functional theory (TD-DFT) calculations were performed with the 0.044 g (46%). H NMR (CDCl
3
, 400 MHz), d (ppm): 8.59 (s, 1H),
©
2021 The Author(s). Published by the Royal Society of Chemistry
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1
7
¼
.64–7.49 (m, 4H), 7.37 (d, J ¼ 7.6 Hz, 1H), 7.09 (s, 1H), 7.03 (d, J
4. Yield: 0.049 g (25%). HNMR (CD CN, 400 MHz), d (ppm):
3
16.0 Hz, 1H), 6.75 (d, J ¼ 8.4 Hz, 2H), 3.02 (s, 6H).
8.84 (d, J ¼ 5.2 Hz, 1H), 8.47–8.42 (dd, J ¼ 8.0 Hz, 2H), 8.19 (d, J ¼
8
(
6
.4 Hz, 1H), 8.02–7.87 (m, 4H), 7.74 (dd, J ¼ 5.2 Hz, 2H), 7.62–7.51
m, 3H), 7.47–7.35 (m, 5H), 7.14 (s, 1H), 7.01 (t, J ¼ 7.2 Hz, 1H),
Preparation of complexes
13
.71–6.68 (m, 1H), 6.60 (t, J ¼ 8.8 Hz, 2H), 6.43 (m, 2H); CNMR
A suspension of [Ru(cymene)Cl ] (0.16 g, 0.26 mmol), triethyl- (d -DMSO, 125 MHz), d (ppm): 198.87, 169.18, 159.22, 157.39,
2
2
6
amine (0.05 mL), 2-vinylpyridine derivative (0.50 mmol) and 156.88, 156.56, 155.55, 155.03, 150.69, 149.92, 149.57, 148.91,
KPF
5
6
(0.182 g, 0.99 mmol) in 10 mL of acetonitrile was stirred at 148.02, 137.31, 135.85, 135.72, 134.77, 134.42, 129.06, 127.83,
ꢁ
0 C for 24 h under argon. Then, the solvent was removed 127.33, 126.95, 126.64, 125.31, 125.25, 124.32, 123.91, 123.30,
under vacuum. The orange red solid was obtained and added 123.22, 120.66, 119.18, 114.31, 114.15. MS (Maldi-TOF, CHCA): m/z
0
ꢀ
into the methanol solution (10 mL) containing 2,2 -bipyridine calculated for 612.09 (M-PF6 ), found: 612.16. C H F N PRu calc.
33
25 7 5
(
0.169 g, 1.08 mmol). Aer the solution was reuxed for 2 h C, 52.38; H, 3.33; N, 9.26; found: C, 52.05; H, 3.84; N, 9.73.
1
under argon, the solvent was removed by evaporation. The
3
5. Yield: 0.051 g (25%). H NMR (CD CN, 400 MHz), d (ppm):
crude compound was puried directly by column chromatog- 8.79 (d, J ¼ 5.6 Hz, 1H), 8.46 (dd, J ¼ 8.0 Hz, 2H), 8.20 (d, J ¼
raphy on silica gel using CH CN/CH Cl as eluent to afford the 8.0 Hz, 1H), 8.07 (d, J ¼ 8.0 Hz, 1H), 7.97 (td, J ¼ 7.2 Hz, J ¼
3
2
2
product as black solid.
. Yield: 0.079 g (45%). HNMR (CD
.48 (dd, J ¼ 5.6 Hz, J ¼ 0.8 Hz, 1H), 8.45 (d, J ¼ 8.0 Hz, 1H), 8.39 1H), 7.40–7.29 (m, 5H), 7.11 (s, 1H), 7.03 (t, J ¼ 6.4 Hz, 1H), 6.62
1.6 Hz, 1H), 7.89–7.83 (m, 2H), 7.73 (d, J ¼ 5.6 Hz, 1H), 7.67–7.62
1
1
3
CN, 400 MHz), d (ppm): (m, 2H), 7.59 (d, J ¼ 5.6 Hz, 1H), 7.49 (td, J ¼ 8.0 Hz, J ¼ 1.6 Hz,
8
(
d, J ¼ 8.0 Hz, 1H), 8.35 (d, J ¼ 8.0 Hz, 1H), 8.30 (d, J ¼ 8.0 Hz, (t, J ¼ 6.4 Hz, 1H), 6.46 (d, J ¼ 8.0 Hz, 2H), 6.29 (d, J ¼ 8.0 Hz,
1
3
1
7
7
1
1
H), 7.99 (td, J ¼ 8.0 Hz, J ¼ 1.6 Hz, 1H), 7.91–7.85 (m, 2H), 7.82– 2H), 2.76 (s, 6H); CNMR (d -DMSO, 125 MHz), d (ppm):
6
.77 (m, 1H), 7.74 (dd, J ¼ 5.6 Hz, 2H), 7.68 (d, J ¼ 5.6 Hz, 1H), 221.47, 167.33, 155.37, 154.75, 154.53, 153.51, 152.88, 148.51,
.47 (td, J ¼ 8.0 Hz, J ¼ 1.6 Hz, 1H), 7.42–7.37 (m, 2H), 7.34 (m, 147.78, 147.20, 146.76, 146.48, 137.75, 135.11, 133.54, 133.30,
H), 7.28–7.26 (m, 2H), 7.18–7.14 (m, 1H), 7.04 (d, J ¼ 1.2 Hz, 132.16, 132.10, 125.76, 125.41, 125.04, 124.58, 124.40, 123.68,
H), 6.60 (td, J ¼ 7.2 Hz, J ¼ 1.2 Hz, 1H), 1.84 (d, J ¼ 1.2 Hz, 3H). 122.11, 121.71, 121.25, 121.11, 117.98, 116.03, 29.49, 28.16. MS
1
3
ꢀ
CNMR (CD
3
CN, 100 MHz), d (ppm): 215.54, 169.35, 157.54, (Maldi-TOF, CHCA): m/z calculated for 637.17 (M-PF6 ), found:
1
1
1
2
56.99, 156.79, 154.96, 154.71, 150.63, 149.99, 149.35, 148.68, 637.12. C H F N PRu calc. C, 53.78; H, 4.00; N, 10.75; found:
36.34, 134.94, 134.78, 133.67, 133.54, 126.87, 126.77, 126.37, C, 53.55; H, 4.44; N, 10.93.
26.18, 125.86, 123.28, 122.95, 122.74, 122.60, 118.56, 117.30,
35 31 6 6
7.66. MS (Maldi-TOF, CHCA): m/z calculated for 532.11 (M-
Results and discussion
ꢀ
PF6 ), found: 532.06. C H F N PRu calc. C, 49.71; H, 3.58; N,
28
24 6 5
Synthesis and characterization
10.35; found C, 50.06; H, 3.35; N, 10.43.
1
2
. Yield: 0.058 g (30%). H NMR (CD
3
CN, 400 MHz), d (ppm): The synthetic route of complexes 1–5 is presented in Scheme 1.
8
1
1
8
1
.89 (d, J ¼ 8.0 Hz, 1H), 8.47 (dd, J ¼ 8.0 Hz, 2H), 8.19 (d, J ¼ 8.0 Hz, Herein, 2-vinylpyridine derivatives involving different electron-
H), 7.99–7.86 (m, 4H), 7.76 (d, J ¼ 5.4 Hz, 1H), 7.71 (d, J ¼ 5.4 Hz, withdrawing groups were rstly prepared by Wittig reactions
H), 7.57–7.44 (m, 5H),7.39–7.35 (m, 3H), 7.13 (s, 1H), 6.98 (td, J ¼ between their corresponding phosphonium bromides and 2-pyr-
.0 Hz, J ¼ 1.6 Hz, 1H), 6.85–6.83 (m, 3H), 6.69 (td, J ¼ 8.0 Hz, J ¼ idylaldehyde in the presence of t-BuOK with the moderate yields (46–
13
1
.6 Hz, 1H), 6.43 (dd, J ¼ 1.6 Hz, 2H). CNMR (CD
3
CN, 100 MHz), 67%), which were characterized by HNMR spectra (Fig. S1–S4†).
24
d (ppm): 200.14, 169.49, 157.50, 156.94, 156.55, 155.53, 155.05, Then, trimethylamine was used as a deprotonating agent to get
1
1
1
1
5
3
51.59, 150.85, 150.07, 149.62, 148.91, 136.54, 135.24, 135.13, their corresponding cyclometalated ruthenium complexes without
33.95, 133.55, 129.74, 128.42, 128.17, 127.15, 126.91, 126.41, purifying intermediates. And the yields were still satised in the
26.08, 126.01, 125.67, 124.33, 123.46, 123.09, 122.35, 122.31, range from 25 to 45%. All these complexes were characterized by
20.07, 119.65, 118.34. MS (Maldi-TOF, CHCA): m/z calculated for NMR (Fig S5–S14†), MS (Fig. S5–S9† inset), IR (Fig. S15†) and
ꢀ
94.12 (M-PF
6
), found: 594.09. C33
H
26
F
6
N
5
PRu calc. C, 53.66; H, elemental analysis spectra. It should be noted that the peaks attrib-
13
.55; N, 9.48; found C, 52.96; H, 3.35; N, 10.02.
uted to Ru–C in CNMR spectra of these complexes were clearly
CN, 400 MHz): 8.69 (d, J observed in the range from 198 to 222 ppm, which are in accordance
5.6 Hz, 1H), 8.44 (dd, J ¼ 8.0 Hz, 2H), 8.28 (dd, J ¼ 8.0 Hz, 2H), with those of [RuL(bpy) ]PF indicating successful cyloruthenations
1
3
. Yield: 0.052 g (27%). H NMR (CD
3
¼
2
6
11–17
7
7
6
.99 (t, J ¼ 8.0 Hz, 1H), 7.89 (dd, J ¼ 8.0 Hz, 2H), 7.77 (t, J ¼ of these 2-vinylpyridine derivatives.
Moreover, all the MS spectra
.2 Hz, 1H), 7.67–7.65 (m, 3H), 7.54–7.28 (m, 7H), 7.13 (t, J ¼ of these complexes showed the parent peaks with the loss of one
.8 Hz, 1H), 7.02 (d, J ¼ 5.2 Hz, 1H), 6.65 (d, J ¼ 5.6 Hz, 2H), 6.09 counter ion, which are in good agreements with the structures.
1
3
(
d, J ¼ 3.6 Hz, 1H); CNMR (CD
3
CN, 100 MHz), d (ppm): 207.76,
A single-crystal of 2 suitable for X-ray analysis was obtained by
1
1
1
1
68.92, 157.52, 156.96, 156.84, 155.56, 155.16, 154.20, 150.88, slow diffusion of petroleum ether into a solution of 2 in methanol.
50.27, 149.61, 148.87, 136.80, 135.32, 135.21, 134.18, 134.05, Then, complex 2 was further identied by X-ray crystallographic
29.38, 127.28, 127.12, 126.58, 126.15, 126.02, 123.97, 123.52, analysis. The structure of 2 is shown in Fig. 1. Further details of the
23.20, 122.82, 122.63, 122.52, 120.30, 118.29. MS (Maldi-TOF, crystal data, data collection, structure solution, and renement
ꢀ
CHCA): m/z calculated for 600.03 (M-PF6 ), found: 600.10. C31- parameters were summarized in Table S1.† Selected bond distances
24 6 5
H F N PRuS calc. C, 50.00; H, 3.25; N, 9.40. Found: C, 49.75; H, and angles were collected in Table 1. Herein, 2 adopts a triclinic,
3.64; N, 9.07.
distorted octahedral coordination around the Ru(II) center with two
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involving 2-vinylpyridine derivatives were successful. In
comparison with complex 1, the MLCT absorptions attributed
to Ru(II) / bpy CT transitions are blue-shied when methyl
linked to vinyl was substituted by aryl such as 2-thienyl and
phenyl derivatives. However, it should be noted that molar
extinction coefficients of these complexes increased when aryl
groups were linked to vinyl, as shown in Fig. 2 and Table 2. In
addition, when electron donating or withdrawing groups were
linked to phenyl in vinylphenyl parts, the absorption properties
were also altered. As shown in Fig. 2 inset, Ru(II) / bpy CT tran-
sitions of complex 2 are centered at 527 nm while those of complex
4
with electron withdrawing group are centered at 524 nm.
However, for complex 5 with N,N-dimethyl, the maximum
absorption peak is centered at 538 nm. The results may be due to
the different electron distributions on complexes induced by these
groups. Nevertheless, there were no strong emissive spectra
observed upon excited at ambient temperature. Then, the emis-
Fig. 1 ORTEP representation of the X-ray structure of complex 2.
Herein, CH OH is omitted for clarity.
3
chelating bipyridines and one (N,C)-bidentate ligand. The C(29)– sion spectra at 77 K were investigated. As shown in Fig. S17,† the
Ru(1)–N(4) deviates slightly from linearity with the corresponding maximum emission peaks of complexes 1–5 are centered at 774,
ꢁ
angle of 172.0(3) . Along this line, the Ru(1)–C(29) bond was 710, 707, 708 and 726 nm, respectively. The results are in agree-
˚
ments with the regular changes of absorptions.
measured to be 2.037(7) A, in the range of Ru–C distances observed
25
for similar complexes. The Ru(1)–N(4) bond trans to the Ru(1)–
˚
C(29) bond is too elongated (2.127(5) A) with respect to the Ru–N
Electrochemical behaviors
distances observed for bipyridine N(1) and N(2) atoms (2.047(5) and
˚
The electrochemical behaviors of complexes 1–5 have been
studied via cyclic voltammetry (CV) (Fig. S18†) and the
summarized data are listed in Table 3. In the positive charge
2
.056(6) A, respectively). The result is consistent with the strong
trans effect of the carbon atom compared with that of the nitrogen
atom. However, it should be noted that the Ru(1)–N(5) bond is also
2
+/3+
˚
region, oxidation peaks attributed to Ru
and anionic C,N-
somewhat elongated (2.067(5) A).
Photophysical properties
The spectroscopic properties of complexes 1–5 were investi-
gated in details. As shown in Fig. 2 and Table 2, all these
complexes display several absorption bands in UV-Vis spectral
regions. According to the absorption spectra of these 2-alke-
nylpyridines in Fig. S16,† the bands between 300 and 450 nm
can be assigned as LC transitions of 2-vinylpyridine derivatives
mixed with LMCT transitions. The bands occurring between 450
and 800 nm are assigned as MLCT transitions. The rst intense
absorption band was observed at about 490 nm for these
complexes, which is assigned as Ru(II) / cyclometalated
ligands CT. The second intense absorption band was observed
around 530 nm, which is assigned as Ru(II) / bpy CT transi-
2
+
tions and noticeably red-shied relative to Ru(bpy)
3
deriva-
tives. Compared with absorptions of 2-alkenylpyridines, this
11,25
red-shi can be attributed to the formation of Ru–C bond,
Fig. 2 Absorption spectra of complexes 1–5 (20 mM) in ethanol
which clearly indicated that ruthenations at alkenyl carbons solution. Inset: normalized absorption spectra of complexes.
ꢁ
Table 1 Selected bond lengths [ A˚ ] and angles [ ] for 2
Ru(1)–N(1)
Ru(1)–N(4)
C(28)–C(29)
C(5)–N(1)–C(1)
C(6)–N(2)–Ru(1)
C(15)–N(3)–Ru(1)
C(28)–C(29)–Ru(1)
2.047(5)
2.127(5)
1.345(10)
117.6(6)
115.4(5)
116.5(5)
114.4(6)
Ru(1)–N(2)
Ru(1)–N(5)
C(29)–C(30)
C(1)–N(1)–Ru(1)
C(10)–N(2)–Ru(1)
C(16)–N(4)–Ru(1)
C(30)–C(29)–Ru(1)
2.056(6)
2.067(5)
1.490(10)
126.4(5)
126.6(5)
115.2(5)
124.6(5)
Ru(1)–N(3)
Ru(1)–C(29)
2.056(6)
2.037(7)
172.0(3)
116.0(5)
125.7(5)
115.5(5)
120.4(7)
C(29)–Ru(1)–N(4)
C(5)–N(1)–Ru(1)
C(11)–N(3)–Ru(1)
C(27)–N(5)–Ru(1)
C(28)–C(29)–C(30)
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Table 2 Photophysical and electrochemical properties of complexes 1–5
a
4
ꢀ1
ꢀ1
b
c
c
d
2+/3+
Complex
l
abs nm, 3/10 M cm
l
em /nm
HOMO/LUMO [eV]
Band gap /eV
E
ox /V for Ru
p
(DE /mV)
1
2
3
4
5
245 (2.65), 287 (3.54), 295 (3.65), 368
1.10), 401 (sh, 0.96), 490 (0.60), 552 (0.65)
246 (3.16), 288 (4.64), 295 (5.12), 360
1.32), 399 (1.18), 490 (0.88), 527 (0.87)
244 (2.92), 286 (4.11), 294 (4.56), 344
1.77), 408 (sh, 0.93), 495 (0.77), 519 (0.78)
241 (3.34), 282 (4.58), 292 (4.72), 356
1.37), 395 (1.22), 484 (0.82), 524 (0.83)
246 (0.75), 288 (4.96), 297 (5.47), 360
702, 774
654, 710
707
ꢀ7.32/ꢀ4.69
ꢀ7.30/ꢀ4.67
ꢀ7.22/ꢀ4.70
ꢀ7.35/ꢀ4.72
ꢀ6.63/ꢀ4.54
2.63
2.63
2.52
2.63
2.09
0.64 (68)
0.70 (63)
0.74 (99)
0.72 (69)
0.67 (25)
(
(
(
653, 708
726
(
(
(
2.70), 406 (sh, 1.55), 494 (1.01), 538
0.93), 685 (sh, 0.085)
a
b
c
d
Absorptions measured in ethanol. Emissions measured in 2-methyltetrafuran glasses at 77 K. DFT/B3LPY calculated values. Recorded in
+
MeCN with ferrocenium/ferrocene (Fc /Fc) as an internal reference and converted to NHE by addition of 490 mV.
16d,26
chelating ligands were observed. For complexes 1–5, the charge of these complexes.
In these complexes, vinyl is an
oxidation for anionic C,N-chelating ligands occurred at 0.99, ideal electron delocalized channel. When it is used as a bridge
0
.94, 0.89, 0.97 and 0.92 V, respectively. As expected, the peaks between a pyridyl group and a substituted phenyl, the effect of
2
+/3+
attributed to Ru
redox couples appeared at 0.64, 0.70, 0.74, electron donor strengths of the substituents is obvious. It can be
0
.72 and 0.67 V, respectively, which are substantially more seen in Table 2 that cyclometalated ligand in complex 5 renders
2
+
11,26
2+/3+
negative than that of [Ru(bpy) ] .
Moreover, Ru
(25–99 mV) indicate that the processes are a uoride atom linked to phenyl shis the oxidation potential to
reversible (at least semi-reversible). It should be noticed that a bit more positive value. A similar trend for complex 1 and 2
redox the Ru(II) metal center easier to oxidize. The ligand involving
3
couples with DE
p
2
+/3+
these cathodic shis of Ru
redox processes are in agree- was observed because methyl in complex 1 is a stronger electron
ment with an increase in the donor strength of cyclometalated donor than phenyl in complex 2. These results are consistent
ligands induced by anion binding and a decrease in the overall with those cyclometalated ruthenium complexes involving 2-
Table 3 Calculated molecular orbital distributions and energy levels of complexes 1–5
HOMOꢀ2
HOMOꢀ1
HOMO
LUMO
LUMO+1
LUMO+2
1
2
3
4
5
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1
1,26b
arylpyridines.
In particular, a new reversible oxidation peak gaps between the HOMO and LUMO are calculated to be 2.63,
+
at 0.56 V (versus NHE) (0.07 V versus Ag /Ag) was observed for 2.63, 2.52, 2.63 and 2.09 eV, respectively. For vinylphenyl based
complex 5, which can be assigned as N,N-dimethylphenyl cyclometalated complexes 2, 4 and 5, the decrease of the
group. It indicated that there may be a possible intramolecular HOMO–LUMO gap resulted in an obvious red shi in the
27
27
electron transfer from N,N-dimethylaminophenyl to Ru(III).
absorption spectrum of complex 5.
2 2
TD-FDT calculations on optimized geometries in CH Cl
were then employed to model their corresponding absorption
spectra. As suggested by TD-DFT calculations (see Table 4), the
Theoretical calculations
DFT calculations were carried out on complexes 1–5 to deter- lowest singlet transitions responsible for the measured low-
mine their electronic structures. Herein, calculated molecular energy absorption bands of these complexes are assigned as
orbital distributions and energy levels of complexes 1–5 are S3 states, which are mainly composed of HOMOꢀ1 / LUMO or
listed in Table 3. Obviously, the HOMOs of complexes 1–4 are HOMOꢀ1 / LUMO+1 transitions. According to the orbital
primarily localized on ruthenium center and 2-vinylpyridine distributions, these absorption bands are mainly assigned as
parts of the cyclometalated ligands. However, the HOMO of ½dpðRuÞ/p ꢂ MLCT transitions and ½p /p* ꢂ ligand-to-
*
*
^
^
^
NN
CN
NN
complex 5 is mainly delocalized on the double bond of vinyl and ligand charge-transfer (LLCT) transitions. For complexes 2, 4
N,N-dimethylphenyl ring in the cyclometalated ligand with less and 5, TD-DFT shows that the absorption bands corresponding
distributions on Ru(II) and pyridine ring. It is clear that a set of to S3 states are predicted at 523, 522 and 540 nm, respectively. It
p orbitals associated with 2-vinylpyridine parts of the cyclo- is consistent with the trend given by their experimental
metalated ligands is located at slightly lower energy than absorption spectra, in which their peaks occurred at 527, 524
vinylphenyl rings that comprise HOMO orbitals. In addition, all and 538 nm, respectively.
HOMOꢀ1 and HOMOꢀ2 orbitals primarily reside on Ru(II)
center. And for complex 3, the thienyl ring also makes a partial
pH sensitivity
contribution to comprise the HOMOꢀ2. As anticipated, all
LUMOs of these complexes are associated with one or two Considering the deprotonation of C–H in C,N-chelating ligands
10a,28,29a
neutral bipyridyl ligands. Herein, the corresponding energy for synthesizing cyclometalated ruthenium complexes,
Table 4 Absorptions of complexes 1–5 in dichloromethane solutions from TDDFT calculations
Complex
States
S1
l (nm)
E (eV)
Oscillator
0.0061
Main congurations
CI coeff.
Assignment
1
614
2.02
HOMO / LUMO
0.62
0.25
0.62
0.64
0.50
0.54
0.46
0.54
0.55
0.41
0.65
0.50
0.51
0.44
0.51
0.52
0.66
0.46
0.44
0.45
0.39
0.49
0.49
0.65
0.55
0.49
0.49
0.59
0.57
0.45
0.58
0.55
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
MLCT/LLCT
HOMO / LUMO+1
HOMO / LUMO+1
HOMOꢀ1 / LUMO
HOMOꢀ2 / LUMO
HOMOꢀ1 / LUMO+1
HOMOꢀ2 / LUMO+1
HOMO / LUMO+1
HOMO / LUMO
HOMO / LUMO+1
HOMOꢀ1 / LUMO
HOMOꢀ3 / LUMO+1
HOMOꢀ2 / LUMO
HOMO / LUMO
S2
S3
S4
S5
S6
S1
S2
605
535
506
503
462
600
596
2.0470
2.3135
2.4496
2.46
2.68
2.06
0.0002
0.0310
0.0022
0.084
0.067
0.0002
0.0045
2
3
2.0788
S3
S4
S6
S1
523
490
456
599
2.3687
2.53
2.71
0.0150
0.056
0.072
2.07
0.0004
HOMO / LUMO+1
HOMO / LUMO
S2
S3
S4
594
519
489
2.0854
2.3888
2.54
0.0036
0.0094
0.054
HOMOꢀ1 / LUMO
HOMOꢀ1 / LUMO+1
HOMOꢀ2 / LUMO+1
HOMOꢀ2 / LUMO
HOMO / LUMO+2
HOMO / LUMO
S6
456
2.72
0.059
4
5
S1
S2
S3
S4
S5
S6
S1
S2
S3
S4
S6
598
596
522
490
489
456
637
635
540
535
495
2.07
0.0005
0.0040
0.0158
0.0285
0.071
2.0790
2.3729
2.5257
2.53
2.72
1.94
1.9511
2.2943
2.31
HOMO / LUMO
HOMOꢀ1 / LUMO
HOMOꢀ2 / LUMO+1
HOMOꢀ1 / LUMO+1
HOMOꢀ2 / LUMO
HOMO / LUMO
HOMO / LUMO+1
HOMOꢀ1 / LUMO+1
HOMOꢀ2 / LUMO
HOMOꢀ2 / LUMO+1
0.072
0.0026
0.0006
0.0050
0.011
2.50
0.1002
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are almost unchanged for complexes 1–5, which indicates the
presence of Ru(II) / bpy CT originated from cyclometaled
ruthenium complexes. Upon decreasing pH, the intensities of
absorption bands around 520 nm are gradually decreasing and
new peaks are centered at around 485 nm for these ve
complexes, indicating the cleavages of Ru–C bonds. Obviously,
+
the protonation process involved an attack of H to the anionic
carbon atom, which weaken the Ru–C bond and resulted in
great changes in MLCT absorptions. It is interesting that the
newly formed band at 485 nm for complex 5 increased slightly
in the intensities upon decreasing pH from 5.00 to 1.40
(
Fig. 3b), which could be ascribed to the concurrent combina-
31
tion of one proton with N,N-dimethylamine.
To better understanding the protonation process of these
ve cyclometalated complexes, absorption changes of

+
+
Ru(bpy)
2
(ppy) (Hppy ¼ 2-phenylpyrdine), Ru(bpy)
2
(dfppy)
(thpy)
+
(
(
Hdfppy ¼ 2-(2,4-diuorophenyl)pyrdine) and Ru(bpy)
2
Hthpy ¼ 2-(2-thienyl)pyridine) shared from our previous
29b
work are shown in Fig. S20.† It should be noted that the
+
+
absorptions of complexes Ru(bpy) (ppy) , Ru(bpy) (dfppy) and
2
2
+
Ru(bpy)
2
(thpy) in acids are all centered at 450 nm and their
(H
However, absorptions of complexes 1–
in acids are around at 485 nm, which demonstrated that there
may be different species in given acidic solutions rather than
corresponding products have been identied as [Ru(bpy)
2
2
-
+
+
25,29
2 2
O) ] or [Ru(bpy) X] .
5
+
+
[
Ru(bpy) (H O) ] or [Ru(bpy) X] . Then, MS spectra of these
2 2 2 2
complexes were tracked aer being incubated in methanol/HAc
solution for 4 h. As shown in Fig. S21,† there is an evident peak
occurring at m/z 563.18 in the MS spectrum of complex 1, which
+
+ +
Fig. 3 Absorption spectral changes of complexes 5 (20 mM) after could be attributed to the formation of [MH –CH
being incubated for 4 h in ethanol/Briton–Robinson buffer solutions
OH–H ]
3
(
calc. m/z 563.63). For complexes 2–5 (Fig. S22–S25†), the peaks
(
v/v ¼ 1 : 2). (a) pH ¼ 11.92–5.52; (b) pH ¼ 5.00–1.40, respectively.
+
+ +
3
attributed to [MH –CH OH–H ] occurred at m/z 625.36 (calc.
6
6
Arrows show spectral changes upon decreasing pH. Inset: changes of
absorption intensity at different pHs.
25.71), 631.34 (calc. 631.64), 643.37 (calc. 643.69), 668.44 (calc.
68.77), respectively. These peaks demonstrated that the pro-
nated species could have been formed as 2-alkenylpyridines
attached to Ru(II) centers through N atoms when protons
attacked carbonions and resulted in the cleavage of Ru–C
bonds. Therefore, the different absorptions of complexes 1–5 in
acidic solutions could be clearly explained by the presence of
their protonated species. However, when these complexes were
the presence of acids may lead to the intrusion of protons to
carbanion, which could result in cleavages of Ru–C bonds and
25,29
absorption spectral changes.
However, there are only a few
reports on the protonation of cyclometalated ruthenium
25,29,30
complexes.
Herein, UV-Vis absorption spectra of
complexes 1–5 as a function of pH were then investigated. As
shown in Fig. S19† and 3, in both alkaline and neutral envi-
ronments, the intensities of absorption bands around 520 nm
incubated in HAc/CH
3
OH for a long enough time, complexes 1–
4
were decomposed by losing C,N-chelating ligands and then
Scheme 2 The possible acid–base equilibria of complexes 1–5.
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Table 5 pK
metaleted ruthenium complexes
b
values of complexes 1–5 and three classical cyclo- synthesized and characterized. Their structures were identied
by NMR, MS, FT-IR and elemental analysis spectra. Then, their
photophysical properties were primarily studied by CV, UV-Vis
and FL spectra combined with theoretical calculations.
Furthermore, the inuence of acids on these new cyclo-
Complex
pK
b
Complex
pK
b
1
2
3
4
5
3.14 ꢃ 0.02
2.28 ꢃ 0.03
2.53 ꢃ 0.02
2.16 ꢃ 0.02
2.86 ꢃ 0.02
+
Ru(bpy)
Ru(bpy)
Ru(bpy)
2
2
2
(ppy)
(thpy)
(dfppy)
4.02 ꢃ 0.03 metalated ruthenium complexes was investigated by UV-Vis
+
4.99 ꢃ 0.03
spectra. The regular changes in absorptions and different
absorption bands from cyclometalated 2-arylpyridine ruthe-
nium complexes could be attributed to the cleavage of Ru–C
bonds without release of C,N-chelating ligands, which were
further identied and conrmed by the MS spectra. Then,
+
2.44 ꢃ 0.02
5.53 ꢃ 0.06
+
+
afforded [Ru(bpy)
complex 5, the species [MH –CH OH–H ] was still observed proposed.
2
CHCA–H ] (Fig. S26–S29†). However, for possible acid–base equilibria for these complexes were
+
+ +
3
(
Fig. S30†). In addition, MS spectra of these complexes incu-
+
bated in HCl/CH OH were also investigated, [Ru(bpy) Cl] and
3
2
Conflicts of interest
+
+
2
[Ru(bpy) CHCA–H ] were found as the primary species for all
complexes (Fig. S31–S35†). It should be noted that free 2-alke- There are no conicts to declare.
nylpyridines have been isolated and then characterized by
1
HNMR spectra for all these complexes in these acidic solutions
Acknowledgements
3 3
including HAc/CH OH and HCl/CH OH. It can be seen that
these ve cyclometalated complexes are thermally unstable in We are grateful for National Science Foundation of China
strong acidic conditions. These complexes nally decomposed (21971258), and thanks to the help of Analytical & Measuring
and gave off free C,N-ligands. Furthermore, the C,N-ligand of Center, School of Pharmaceutical Sciences, South-Central
2 2 3
complex 5 was chosen to mix with Ru(bpy) Cl in CH
OH. As University for Nationalities.
shown in Fig. S36,† some fragments that are the same as
complex 5 in acid were observed aer being treated by NaOH or
Et N. According to these results of MS and UV-Vis spectra, the
3
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