Synthesis, structure, and optical and redox properties of chlorophyll derivatives directly coordinating ruthenium bisbipyridine at the peripheral β-diketonate moiety

Article abstract of DOI:10.1021/ic400509q

The diketonate group of the peripheral position in chlorophyll derivatives 1 and 2 coordinated ruthenium bisbipyridine to give direct linkages 3-5 of the chlorin ring with the Ru(II) complex. Zinc metalation of the central position in the chlorin ring of free base 3 afforded the Ru-Zn binuclear complex 3-Zn. Because the diketonate group at the C3 position of chlorophyll derivatives coordinated to bulky Ru(bpy)₂²⁺, the plane of the diketonate group was twisted from the chlorin π ring in synthetic 3-5 and 3-Zn to lead to a partial deconjugation and a slight blue shift of the longest wavelength electronic absorption band in dichloromethane. A broad metal-to-ligand charge-transfer absorption band derived from the Ru complex was observed around 500 nm, in addition to visible absorption bands from the chlorophyll moiety. Chlorophyll derivatives 3-5 and 3-Zn directly coordinating the ruthenium complex were less fluorescent in dichloromethane than chlorophyll-diketonate ligands 1, 2, and 1-Zn due to the heavy atom effect of the ruthenium in a molecule. The coordination to the ruthenium complex moiety at the peripheral position shifted the electrochemical reduction of the chlorin part in acetonitrile to a negative potential, and the coordination to zinc at the central position decreased the redox potentials. Chemical modification of the bipyridine and diketonate ligands of the ruthenium complexes greatly affected the redox potentials of Ru(II)/(III) and/or Ru(II)/(I) but minimally the redox properties of the chlorin moiety. Substitution with electron-donating groups shifted the former to a negative potential but only barely shifted the latter. The zinc metalation caused no apparent shifts for the redox potentials of the Ru center.

Full text of DOI:10.1021/ic400509q

Article
pubs.acs.org/IC
Synthesis, Structure, and Optical and Redox Properties of
Chlorophyll Derivatives Directly Coordinating Ruthenium
Bisbipyridine at the Peripheral β‑Diketonate Moiety
Yusuke Kinoshita, Youhei Yamamoto, and Hitoshi Tamiaki*
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
*
S Supporting Information
ABSTRACT: The diketonate group of the peripheral position
in chlorophyll derivatives 1 and 2 coordinated ruthenium
bisbipyridine to give direct linkages 3−5 of the chlorin ring
with the Ru(II) complex. Zinc metalation of the central
position in the chlorin ring of free base 3 afforded the Ru−Zn
binuclear complex 3-Zn. Because the diketonate group at the
C3 position of chlorophyll derivatives coordinated to bulky
Ru(bpy)22 , the plane of the diketonate group was twisted
from the chlorin π ring in synthetic 3−5 and 3-Zn to lead to a
partial deconjugation and a slight blue shift of the longest
wavelength electronic absorption band in dichloromethane. A
broad metal-to-ligand charge-transfer absorption band derived
from the Ru complex was observed around 500 nm, in addition
+
to visible absorption bands from the chlorophyll moiety. Chlorophyll derivatives 3−5 and 3-Zn directly coordinating the
ruthenium complex were less fluorescent in dichloromethane than chlorophyll−diketonate ligands 1, 2, and 1-Zn due to the
heavy atom effect of the ruthenium in a molecule. The coordination to the ruthenium complex moiety at the peripheral position
shifted the electrochemical reduction of the chlorin part in acetonitrile to a negative potential, and the coordination to zinc at the
central position decreased the redox potentials. Chemical modification of the bipyridine and diketonate ligands of the ruthenium
complexes greatly affected the redox potentials of Ru(II)/(III) and/or Ru(II)/(I) but minimally the redox properties of the
chlorin moiety. Substitution with electron-donating groups shifted the former to a negative potential but only barely shifted the
latter. The zinc metalation caused no apparent shifts for the redox potentials of the Ru center.
1
3
INTRODUCTION
three substituents R −R in the diketonate group and produces
the desired electron environment for any metal centers. The
chelate effect of bidentate diketonate ligands further stabilizes
the metal complexes.
■
β-Diketonates are such interesting functional groups that they
1
−3
have been investigated from various aspects. In coordination
chemistry, β-diketonates are effective for the ligand of metal
complexes (Figure 1, left). Negatively charged oxygen atoms of
the β-diketonates tightly coordinate a cationic metal through
their strong σ-donation to stabilize the metal complex. The
electron-donating ability of the oxygen sites is tunable by the
When a diketonate ligand coordinated to bis(4,4′-dicarboxy-
2
,2′-bipyridine)ruthenium (Figure 1, right), the resulting
complex gave the metal-to-ligand charge-transfer (MLCT)
absorption band in a wide visible region and acted as an
4,5
efficient sensitizer of a solar cell. The conversion efficiency
reached 6% and was comparable to that of a Ru complex
possessing two thiocyanate ligands. Moreover, in the case of R
1
=
CF , the ruthenium complexes widely absorbed light from the
3
visible to the near infrared (NIR) region and were effective
dyes for solar cells, whose conversion efficiencies were
3
enhanced by an increase of the alkyl chain length in the R
group.
6
−9
On the other hand, chlorophylls (Chls) play important roles
in light-harvesting, energy-transferring, and charge-separating
10−12
processes in photosynthesis.
Chls show characteristic
absorption spectra in the ultraviolet (UV), visible, and NIR
Figure 1. Chemical structures of metal complexes possessing a β-
diketonate.
Received: February 27, 2013
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic400509q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. Chemical structures of Chl-a, peripheral Mg complex of methyl pheophorbide-a enol, and β-diketonated chlorophyll derivatives 1 and 2.
3
9
regions, of which the two intense bands are called Qy and Soret
bands in the longer and shorter wavelengths, respectively. Chls
have several kinds of substituents at the peripheral positions of
the chlorin ring. These substituents greatly affect the electronic
absorption bands. Chls usually have a magnesium (Mg)
dication at the central position of the chlorin ring (Chl-a in
Figure 2). Additionally, a Chl possessing a zinc (Zn) dication
was identified as a photosynthetically active pigment in specific
as well as the crystal structure of 1. Here, we used synthetic
molecules 1 and 2 as the β-diketonate ligand and prepared their
Chl derivatives, which coordinated Ru complexes bearing two
bipyridines. The synthetic Chl derivatives possessing Ru
complexes at the peripheral position were isolated and fully
identified, and one of them was further zinc-metalated at the
central position to give the binuclear Chl complex. In addition,
their optical properties and redox potentials in a solution were
measured, and their substitution effect was investigated.
1
3
purple bacteria. Central metals are requisite for the
construction of photosynthetic apparatuses, including light-
harvesting antennas and reaction centers. In these apparatuses,
the four-coordinated metals in naturally occurring Chls are
bound with several functional groups at the axial position to
EXPERIMENTAL SECTION
■
General. Electronic absorption was measured in CH Cl at room
temperature on a Hitachi U-3500 spectrophotometer. Fluorescence
quantum yields in air-saturated CH Cl2 were determined using a
Hamamatsu Photonics C9920-02 spectrometer. H NMR and
2
2
1
4,15
form five-coordinated Chl−peptide
and Chl−Chl com-
1
6
2
plexes. Metalation of chlorin rings has been readily achieved,
and various synthetic metallochlorophylls have proven to be
1
19
F
NMR spectra were recorded in CDCl at room temperature on a
3
1
7,18
useful as models of natural Chls.
JEOL ECA-600 (600 and 565 MHz) spectrometer; CHCl (δ = 7.26
3
H
Coordination to the peripheral position of synthetic
porphyrins has attracted some attention in the preparation of
ppm) was used as an internal reference and CF COOH (δ = −79.0
3
F
1 1
ppm) as an external reference. Peaks were assigned by H− H COSY
and NOESY spectra. Electrospray ionization (ESI) high-resolution
mass (HRMS) data were obtained with a Bruker micrOTOF-II. Flash
column chromatography (FCC) was carried out on silica gel (Merck
silica gel 60, 230−400 mesh). Electrochemical measurements were
carried out by a BAS CV-50W voltammeric analyzer with a
conventional three-electrode system.
19−21
supramolecular assemblies.
Several porphyrins coordinat-
ing a bipyridineruthenium complex at the peripheral position
are available for important complexes possessing photophysical,
redox-active, electron-transferring, and light-harvesting proper-
2
2−25
ties.
The porphyrin−Ru conjugates were useful for dye
26−29 30,31
sensitizers of solar cells,
molecular switches,
The peripheral coordination of a chlorin
to a metal ion has been primarily studied in the field of
and
Methyl 3-devinyl-3-(4,4,4-trifluoro-1,3-dioxo-butan-1-yl)-pyro-
3
2,33
3
9
molecular motors.
pheophorbide-a enol (1), methyl 3-devinyl-3-(4,4,5,5,5-penta-
39
fluoro-1,3-dioxo-pentan-1-yl)-pyropheophorbide-a enol (2), methyl
3
4,35
40
photodynamic therapy (PDT).
Pyropheophorbide-a, one
3-acetyl-3-devinyl-pyropheophorbide-a (6), bis(2,2′-bipyridine)-
4
1
of the Chl-a derivatives, was connected with a potassium
tetrachloroplatinate at the terminal position of the C17
substituent, and the resulting chlorin−Pt conjugate showed a
dichlororuthenium(II) [Ru(bpy) Cl ], bis(4,4′-dimethyl-2,2′-
2 2
41,42
bipyridine)dichlororuthenium(II) [Ru(dmb) Cl ],
and [Ru-
were prepared according
to reported procedures. Commercially available CH Cl , MeOH, and
2
2
4
3,44
(
bpy) (CF COCHCOC H )]PF (7)
2 3 6 5 6
36
2
2
greater cytotoxic effect than the original cisplatin. A similar
chlorin−Gd conjugate was prepared to use as an in vivo PDT
and a fluorescent imaging agent based on the chlorophyll
moiety, as well as a magnetic resonance imaging agent based on
EtOH (Nacalai Tesque, extra pure reagents) were used as reaction
solvents. For measurements of electronic absorption spectra, CH Cl
2
2
was purchased from Nacalai Tesque as a grade of reagent prepared
specially for spectroscopy and used without further purification.
Synthesis of [Ru(bpy) (1−H)]PF (3). Ru(bpy) Cl (96.5 mg,
37
the gadolinium moiety. All of the isolated chlorin−metal
linkages investigated so far have been indirect conjugates of
chlorin π ring systems with metal complexes. A Chl derivative
directly conjugating Mg at the peripheral position of the chlorin
π ring was prepared, as shown in the center drawing of Figure
2
6
2
2
0.20 mmol) was dissolved in EtOH (10 mL) and CH Cl (90 mL)
2
2
with stirring. Compound 1 (100 mg, 0.15 mmol) was added to the
solution, and the mixture was refluxed overnight under nitrogen in the
dark. The reaction mixture was cooled to room temperature and added
3
8
to MeOH saturated with NH PF6 (100 mL) with stirring. The
4
2.
The chlorin−Mg conjugate was identified in a solution by
solvents were evaporated in vacuo, and the resulting residue was
both NMR and IR spectroscopy; the complex could not be
obtained in the solid state due to its instability even in a
solution.
dissolved in CH Cl . The solution was washed with water twice, dried
2
2
over Na SO , and filtered. After evaporation of the solution in vacuo,
2
4
the residue was purified by FCC using 1.5% MeOH−CH Cl as
2
2
Recently, we synthesized Chl derivatives 1 and 2, which have
a trifluoro- or pentafluoro-β-diketonate group at the C3
position (Figure 2), and we reported their optical properties,
eluents, and recrystallization from CH Cl and hexane gave a pure
2
2
sample of 3 (67 mg, 0.055 mmol, 36%): VIS (CH Cl ) λ 687
2
2
max
(relative intensity, 71%), 627 (11), 550 (23), 516 (31), 420 (92), 382
B
dx.doi.org/10.1021/ic400509q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 1. Synthetic Scheme of Chlorophyll Derivatives 1 and 2, Their Ruthenium Complexes 3−5, and 1/3-Zn
a
Conditions: (i) HBr/AcOH, H O, CH N /Et O, Pr NRuO −Me(O)N(CH CH ) O/CH Cl ; (ii) RCOOC H −NaOCH /CH OH−CHCl ;
2
2
2
2
4
4
2
2
2
2
2
2
5
3
3
3
(
iii) Ru(bpy) Cl or Ru(dmb) Cl /C H OH−CHCl , NH PF /CH OH; (iv) Zn(OAc) ·2H O/CH OH−CH Cl .
2
2
2
2
2
5
3
4
6
3
2
2
3
2
2
1
1
nm (100). H NMR (CDCl ) δ (1/1): 9.51/9.50 (1H, s, 10-H), 9.46
(
=
8
18-CH ), 1.71 (3H, t, J = 7 Hz, 8 -CH ), −0.05, −2.14 (each 1H, s,
3
3
3
19
1H, s, 5-H), 9.18/9.15 (1H, d, J = 5 Hz, 6-H of bpy-I), 9.01 (1H, d, J
5 Hz, 6-H of bpy-II), 8.66/8.63 (1H, d, J = 8 Hz, 3-H of bpy-II),
NH × 2). F NMR (CDCl ) δ: −74.4 (6F, d, J = 713 Hz, PF ), −75.3
3
6
3
+
(s, 3 -CF ). HRMS (ESI) m/z: [M − PF ] calcd for
3
6
.63/8.62 (1H, s, 20-H), 8.54 (1H, d, J = 8 Hz, 3-H of bpy-I), 8.44/
.42 (1H, d, J = 8 Hz, 3′-H of bpy-I), 8.33 (1H, d, J = 8 Hz, 3′-H of
C H F N O Ru, 1129.3521; found, 1129.3520.
60
58
3
8
5
8
Synthesis of [Ru(bpy) (2−H)]PF (5). Similar to the synthesis of
3, metalation of 2 (22.5 mg, 0.034 mmol) with Ru(bpy) Cl (29.9 mg,
0.062 mmol) gave ruthenium complex 5 (34.2 mg, 0.027 mmol, 79%)
after FCC (CH Cl /CH OH = 98.5:1.5): VIS (CH Cl ) λ 687
(relative intensity, 68%), 626 (11), 549 (23), 516 (32), 422 (89), 382
2
6
bpy-II), 8.27/8.26 (1H, t, J = 8 Hz, 4-H of bpy-II), 8.22, (1H, t, J = 8
Hz, 4-H of bpy-I), 7.92/7.89 (1H, d, J = 5 Hz, 6′-H of bpy-I), 7.89/
2 2
7
5
7
5
6
1
1
3
.88 (1H, d, J = 5 Hz, 6′-H of bpy-II), 7.83/7.82 (1H, dd, J = 5, 8 Hz,
-H of bpy-II), 7.758/7.752 (2H, t, J = 8 Hz, 4′-H of bpy-I and II),
.69/7.68 (1H, dd, J = 5, 8 Hz, 5-H of bpy-I), 7.21 /7.21 (1H, dd, J =
2
2
3
2
2
max
1
nm (100). H NMR (CDCl ) δ (1/1): 9.598/9.596 (1H, s, 10-H),
5
1
3
, 8 Hz, 5′-H of bpy-II), 7.15/7.14 (1H, dd, J = 5, 8 Hz, 5′-H of bpy-I),
.962/6.958 (1H, s, 3 -CH), 5.271/5.269, 5.124/5.121 (each 1H, J =
9 Hz, 13 -CH ), 4.49/4.48 (1H, dq, J = 1, 7 Hz, 18-H), 4.30 (1H, m,
7-H), 3.70−3.65 (2H, m, 8-CH ), 3.63/3.62 (3H, s, 12-CH ), 3.58/
.57 (3H, s, 17 -COOCH ), 3.05 (3H, s, 7-CH ), 3.02/2.99 (3H, s, 2-
CH ), 2.70−2.64, 2.58−2.50, 2.29−2.20 (1H+1H+2H, m, 17-
CH CH ), 1.77/1.73 (3H, d, J = 7 Hz, 18-CH ), 1.682/1.676 (3H,
9.49/9.48 (1H, s, 5-H), 9.23/9.21 (1H, d, J = 5 Hz, 6-H of bpy-I), 9.00
(1H, d, J = 5 Hz, 6-H of bpy-II), 8.66/8.63 (1H, d, J = 8 Hz, 3-H of
bpy-II), 8.63/8.62 (1H, s, 20-H), 8.51 (1H, d, J = 8 Hz, 3-H of bpy-I),
8.45/8.43 (1H, d, J = 8 Hz, 3′-H of bpy-I), 8.33 (1H, d, J = 8 Hz, 3′-H
of bpy-II), 8.28/8.25 (1H, t, J = 8 Hz, 4-H of bpy-II), 8.21, (1H, t, J =
8 Hz, 4-H of bpy-I), 8.03/8.01 (1H, d, J = 6 Hz, 6′-H of bpy-I), 7.95/
7.94 (1H, d, J = 5 Hz, 6′-H of bpy-II), 7.83−7.81 (2H, m, 4′-H of bpy-
I and II), 7.79 (1H, dd, J = 5, 8 Hz, 5-H of bpy-II), 7.69/7.68 (1H, dd,
J = 5, 8 Hz, 5-H of bpy-I), 7.27 (1H, m, 5′-H of bpy-II), 7.19 (1H, m,
1
1
2
2
3
2
3
3
3
2
2
3
1
19
t, J = 8 Hz, 8 -CH ) −0.13, −2.18 (each 1H, s, NH × 2). F NMR
3
3
(
CDCl ) δ: −74.4 (6F, d, J = 713 Hz, PF ), −75.3 (3F, s, 3 -CF ).
3
6
3
+
1
HRMS (ESI) m/z : [M − PF ] calcd for C H F N O Ru,
5′-H of bpy-I), 7.02/7.01 (1H, s, 3 -CH), 5.28, 5.13 (each 1H, J = 19
6
56 50
3
8
5
1
1
073.2894; found, 1073.2848 .
Synthesis of [Ru(dmb) (1−H)]PF (4). Similar to the synthesis of
Hz, 13 -CH ), 4.50 (1H, dq, J = 1, 7 Hz, 18-H), 4.33 (1H, br, 17-H),
2
2
6
3.72 (2H, t, J = 8 Hz, 8-CH ), 3.70 (3H, s, 12-CH ), 3.59/3.58 (3H, s,
2
3
2
3
, peripheral metalation of 1 (23.3 mg, 0.035 mmol) with
17 -COOCH ), 3.06 (3H, s, 7-CH ), 3.00/2.97 (3H, s, 2-CH ), 2.73−
3
3
3
Ru(dmb) Cl2 (21.9 mg, 0.041 mmol) gave ruthenium complex 4
18.9 mg, 0.015 mmol, 42%) after FCC (CH Cl /MeOH = 99:1): VIS
2.68, 2.58−2.51, 2.32−2.24 (1H+1H+2H, m, 17-CH CH ), 1.78/1.76
2
2
2
1
(
(
(
(
(3H, d, J = 7 Hz, 18-CH ), 1.72 (3H, t, J = 8 Hz, 8 -CH ), −0.09,
2
2
3
3
19
CH Cl ) λ 686 (relative intensity, 70%), 625 (12), 548 (24), 515
−2.13 (each 1H, s, NH × 2). F NMR (CDCl ) δ (1/1): −74.4 (6F,
2
2
max
3
1
4
3
34), 417 (98), 384 nm (100). H NMR (CDCl ) δ (1/1): 9.57/9.56
1H, s, 10-H), 9.46/9.45 (1H, s, 5-H), 9.00/8.97 (1H, d, J = 6 Hz, 6-H
of dmb-I), 8.81 (1H, d, J = 6 Hz, 6-H of dmb-II), 8.63/8.62 (1H, s, 20-
H), 8.47/8.45 (1H, s, 3-H of dmb-II), 8.34 (1H, s, 3-H of dmb-I),
d, J = 715 Hz, PF ), −83.7/−84.1 (3F, br, 3 -CF ), −125.4 (2F, br, 3 -
3
6
3
+
CF ). HRMS (ESI) m/z: [M − PF ] calcd for C H F N O Ru,
2
6
57 50
5
8
5
1123.2862; found, 1123.2800.
Synthesis of Zinc Methyl 3-devinyl-3-(4,4,4-trifluoro-1,3-
dioxo-butan-1-yl)-pyropheophorbide-a enol (1-Zn). Enol com-
pound 1 (6.0 mg, 0.0091 mmol) was dissolved in CH Cl (50 mL), to
8
7
.25/8.24 (1H, s, 3′-H of dmb-I), 8.16 (1H, s, 3′-H of dmb-II), 7.72/
.69 (1H, d, J = 6 Hz, 6′-H of dmb-I), 7.69 (1H, d, J = 6 Hz, 6′-H of
2
2
dmb-II), 7.59/7.58 (1H, d, J = 6 Hz, 5-H of dmb-II), 7.46/7.45 (1H, d,
which MeOH saturated with Zn(OAc) ·2H O was then added. The
2 2
J = 6 Hz, 5-H of dmb-I), 7.04 (1H, d, J = 6 Hz, 5′-H of dmb-II), 6.96/
solution was stirred at room temperature for 2.5 h under nitrogen in
the dark. The reaction mixture was washed with water, aq 4%
NaHCO and water, dried over Na SO , and filtered. The solvent was
1
6
5
7
3
.94 (1H, d, J = 6 Hz, 5′-H of dmb-I), 6.90/6.89 (1H, s, 3 -CH), 5.30/
1
.29, 5.144/5.142 (each 1H, J = 19 Hz, 13 -CH ), 4.50 (1H, dq, J = 3,
2
3
2
4
Hz, 18-H), 4.32 (1H, m, 17-H), 3.71 (2H, q, J = 7 Hz, 8-CH ),
evaporated in vacuo, and the resulting residue was recrystallized from
2
2
.699/3.697 (3H, s, 12-CH ), 3.59/3.58 (3H, s, 17 -COOCH ), 3.038
CH Cl and hexane to give 1-Zn (6.2 mg, 0.0085 mmol, 94%): VIS
3
3
2
2
(
2
2
3H, s, 7-CH ), 3.035/3.011 (3H, s, 2-CH ), 2.80/2.78, 2.77, 2.53,
(THF) λ 685 (relative intensity, 80%), 433 (96), 388 (100), 319
3
3
max
1
.45 (each 3H, 4,4′-CH of dmb-I and II), 2.73−2.63, 2.60−2.53,
nm (55). H NMR (CDCl −1% pyridine-d ) δ: 9.75 (1H, s, 5-H),
3
3
5
1
.33−2.21 (1H+1H+2H, m, 17-CH CH ), 1.78/1.75 (3H, d, J = 7 Hz,
9.40 (1H, s, 10-H), 8.40 (1H, s, 20-H), 6.85 (1H, s, 3 -CH), 5.20, 5.09
2
2
C
dx.doi.org/10.1021/ic400509q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
Figure 3. Partial H NMR spectrum of 3 in CDCl in >6.5 ppm.
3
1
(
each 1H, d, J = 19 Hz, 13 -CH ), 4.38 (1H, dq, J = 2, 8 Hz, 18-H),
mmol) with Ru(dmb) Cl (30.4 mg, 0.056 mmol) gave 8 (16.3 mg,
2
2
2
4
1
2
.21 (1H, dt, J = 8, 2 Hz, 17-H), 3.68 (3H, s, 12-CH ), 3.56 (3H, s,
0.020 mmol, 35%): VIS (CH Cl ) λ 491 (relative intensity, 100%),
425 (61), 365 nm (66). H NMR (CDCl ) δ: 8.49 (1H, d, J = 5 Hz, 6-
3
3
2 2 max
2
1
7 -COOCH ), 3.43 (3H, s, 2-CH ), 3.36 (2H, q, J = 8 Hz, 8-CH ),
3
3
2
.84 (3H, s, 7-CH ), 2.60−2.52, 2.40−2.33, 2.28−2.20, 2.00−1.99
H of dmb-I), 8.40 (1H, d, J = 5 Hz, 6-H of dmb-II), 8.25 (1H, s, 3-H
of dmb-I), 8.20 (1H, s, 3-H of dmb-II), 8.13 (1H, s, 3′-H of dmb-I),
8.05 (1H, s, 3′-H of dmb-II), 7.58 (3H, m, 2-, 6-H of Ph, 6′-H of dmb-
I), 7.51 (1H, d, J = 6 Hz, 6′-H of dmb-II), 7.46 (1H, t, J = 8 Hz, 4-H of
Ph), 7.36 (1H, d, J = 5 Hz, 5-H of dmb-I), 7.31 (3H, m, 3-, 5-H of Ph,
5-H of dmb-II), 6.98 (1H, d, J = 6 Hz, 5′-H of dmb-I), 6.92 (1H, d, J =
3
(
each 1H, m, 17-CH CH ), 1.68 (3H, d, J = 8 Hz, 18-CH ), 1.49 (3H,
2
2
3
1
19
t, J = 8 Hz, 8 -CH ) [the peak of OH could not be observed].
NMR (CDCl ) δ: −78.3 (3F, s, 3 -CF ). HRMS (ESI) m/z: M calcd
for C H F N O Zn, 722.1695; found, 722.1694.
F
3
3
+
3
3
36
33
3
4
5
Synthesis of [Ru(bpy) (1-Zn−H)]PF (3-Zn). Similar to the
2
6
6
Hz, 5′-H of dmb-II), 6.40 (1H, s, COCH), 2.68 (3H, s, 4-CH of
synthesis of 1-Zn, zinc metalation of [Ru(bpy) (1−H)]PF (3, 6.0 mg,
3
2
6
dmb-I), 2.66 (3H, s, 4-CH of dmb-II), 2.49 (3H, s, 4′-CH of dmb-I),
0
.0049 mmol) gave the ruthenium−zinc binuclear complex 3-Zn (6.2
3
3
19
2
.45 (3H, s, 4′-CH of dmb-II). F NMR (CDCl ) δ: −74.1 (6F, d, J
mg, 0.0048 mmol, 99%): VIS (CH Cl ) λ 678 (relative intensity,
3
3
2
2
max
+
1
= 713 Hz, PF ), −75.4 (3F, s, CF ). HRMS (ESI) m/z: [M − PF ]
calcd for C H F N O Ru, 685.1359; found, 685.1359.
8
9%), 627 (18), 567 (17), 428 (100), 386 nm (87). H NMR
6
3
6
(
9
CDCl −1% pyridine-d ) δ (1/1): 9.599/9.596 (1H, s, 10-H), 9.319/
34 30
3
4
2
3
5
.315 (1H, s, 5-H), 9.20/9.16 (1H, d, J = 5 Hz, 6-H of bpy-I), 8.98
(
1H, d, J = 6 Hz, 6-H of bpy-II), 8.66/8.63 (1H, d, J = 8 Hz, 3-H of
bpy-II), 8.48/8.47 (1H, d, J = 8 Hz, 3-H of bpy-I), 8.41/8.40 (1H, d, J
8 Hz, 3′-H of bpy-I), 8.39/8.38 (1H, s, 20-H), 8.29 (1H, d, J = 8 Hz,
′-H of bpy-II), 8.22/8.20 (1H, t, J = 8 Hz, 4-H of bpy-II), 8.16 (1H, t,
J = 8 Hz, 4-H of bpy-I), 7.93/7.90 (1H, d, J = 6 Hz, 6′-H of bpy-I),
.85/7.84 (1H, d, J = 6 Hz, 6′-H of bpy-II), 7.83−7.73 (3H, m, 5-H of
RESULTS AND DISCUSSION
■
Synthesis of Chlorophyll Derivatives Coordinating
Ruthenium Bisbipyridine Complex. Preparation of Chl
=
3
39
derivatives 1 and 2 was preliminarily reported, and their
synthesis was described briefly. Methyl pyropheophorbide-a
7
40
bpy-II and 4′-H of bpy-I and II), 7.65−7.61 (1H, m, 5-H of bpy-I),
was prepared by modifying Chl-a, and the 3-vinyl group was
45
7
6
4
8
.21 (1H, m, 5′-H of bpy-II), 7.18−7.13 (1H, m, 5′-H of bpy-I), 6.92/
hydrated and oxidized to the 3-acetyl group in 6 (Scheme 1).
1
1
.91 (1H, s, 3 -CH), 5.16, 5.04, (each 1H, J = 19 Hz, 13 -CH ), 4.39/
2
Claisen condensation of 3-acetyl-chlorin 6 with ethyl perfluoro-
carboxylates gave Chl derivatives 1 and 2 possessing a β-
diketonate moiety at the C3 position.
.38 (1H, m, 18-H), 4.19/4.18 (1H, m, 17-H), 3.74 (2H, q, J = 7 Hz,
2
-CH ), 3.677/3.6675 (3H, s, 12-CH ), 3.531/3.528 (3H, s, 17 -
2
3
COOCH ), 3.016/3.014 (3H, s, 7-CH ), 2.98/2.95 (3H, s, 2-CH ),
2
1
41
3
3
3
C3-trifluoroacetylacetonated chlorin 1 and Ru(bpy) Cl
2
2
.55−2.49, 2.38−2.31, 2.24−2.17 (1H+1H+2H, m, 17-CH CH ),
2
2
1
were dissolved in ethanol and dichloromethane, and the
solution was refluxed overnight under nitrogen in the dark. The
reaction mixture was cooled to room temperature and added to
methanol saturated with ammonium hexafluorophosphate
.70/1.68 (3H, t, J = 7 Hz, 8 -CH ), 1.66/1.64 (3H, d, J = 7 Hz, 18-
3
19
CH3). F NMR (CDCl ) δ (1/1): −74.4 (6F, d, J = 711 Hz, PF ),
3
6
3
+
−
75.14/−75.15 (3F, s, 3 -CF ). HRMS (ESI) m/z: [M − PF ] calcd
3
6
for C H F N O RuZn, 1135.2029; found, 1135.2029.
56
48
3
8
5
Synthesis of [Ru(dmb) (CF COCHCOC H )]PF (8). Similar to
(NH PF ) to give ruthenium(II) complex 3, containing the
4 6
2
3
6
5
6
−
the synthesis of 3, metalation of CF COCH COC H (16.3 mg, 0.075
anionic species of 1, two bpy molecules, and PF6 in 36% yield.
3
2
6
5
D
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The resulting chlorin−Ru conjugate 3 was so stable that it
could be purified by FCC on silica gel and characterized by its
1
19
H/ F NMR and HRMS spectra.
1
1
1
The H NMR spectra of 3 in CDCl (1D, 2D H− H COSY,
3
and NOESY) showed 20 protons in a low magnetic field (> 6.5
ppm), which were assigned to three meso (5, 10, 20-H) and
2
one diketonated proton (3 -H) derived from the chlorin part
and 16 aromatic protons of two bpy ligands (Figure 3). The
detailed assignment of bis(2,2′-bipyridine) moiety was
1
mentioned in the Experimental Section. H NMR spectra of
3
in CDCl at 20 °C showed that each proton signal at the C10
3
and C20 positions was split into two 1:1 signals (see Figure 3);
the proton signal at C5 was not split by accidentally
overlapping. When they were heated, their split signals
coalesced to become relatively broad singlet signals in
CDCl CDCl at 120 °C (see Figure S1). The changes indicate
2
2
that each split signal peaking at the C10 and C20 positions at
room temperature should be ascribable to the two rotational
1
isomers (about 1:1) around the C3−C3 bond; the more
sterically crowded C3 substituent relative to the diketonate
group is discussed later. A similar interconversion was reported
in a Chl derivative possessing a bulky group at the C3
position. On the other hand, the proton signal at the C32
46
position was split into two 1:1 singlet peaks even at 120 °C
2
(
Figure S1). This result shows that the split of the C3 −H was
due to the two 1:1 Δ/Λ-isomers of the ruthenium complex
which were not isomerized thermally; the diastereomeric
isomers could not be separated by conventional HPLC. Thus,
Figure 4. (A) Observed ESI-HRMS peaks of 3 and (B) calculated
−
peaks of 3 lacking PF6
.
3
was a mixture of rotamers and stereoisomers, which could not
be separated by FCC. ESI-HRMS of 3 was measured at a
Scheme 2. Synthesis of Reference Ru(II) Complexes 7 and
8
positive mode to show the parent peak as the cationic species of
a
3
lacking PF6− at m/z = 1073.284, and the observed isotopic
pattern was in good agreement with the calculated distribution
(
Figure 4).
41,42
Instead of bpy, C4,4′-dimethylated bpy (dmb)
was used
as the ligand of the Ru center. Similar to the preparation of 3,
the reaction of 1 with Ru(dmb) Cl afforded 4 in 42% yield
2
2
after purification by FCC. Its structure was confirmed by its
1
19
1
H/ F NMR and MS spectra. In the H NMR spectra of 4, two
sets of the proton signal of the dimethyl group at C4 and C4′
positions in bipyridine were observed around 3 ppm (Figure
S2). Similar to the ESI-HRMS of 3, that of 4 showed the parent
−
peak as the cationic species of 4 lacking PF6 at m/z =
1129.3521. Moreover, complexation of chlorin 2 possessing a
pentafluorinated β-diketonate group at the C3 position with
Ru(bpy) Cl gave 5 in 79% yield after FCC purification and
characterized by H NMR (Figure S3) and ESI-MS. The
a
Conditions: (i) Ru(bpy)Cl or Ru(dmb) Cl /C H OH, NH PF /
3
2
2
2
2
2
2
5
4
6
1
CH OH.
HRMS peak of 5 at m/z = 1123.2800 corresponded to the
−
cationic species of 5 lacking PF .
6
Optical Properties of Chlorophyll Derivatives Coordi-
nating Ruthenium Bisbipyridine Complex. Figure 5A
shows UV−vis absorption spectra of 1, 3, 6, and 7 in CH Cl .
1
7,18
Zinc metalation of 3 by a standard procedure
quantitatively afforded binuclear complex 3-Zn after recrystal-
lization. In the H NMR spectrum of 3-Zn in CDCl and 1%
pyridine, two NH signals disappeared in a high field region (<0
ppm, Figure S3), indicating that a zinc ion was introduced at
the central position of the chlorin ring. The ESI-HRMS peak of
-Zn at m/z = 1135.2029 supports the cationic species of 3-Zn
lacking PF . As reference complexes of 3(-Zn) and 4, Ru
2
2
1
3
The Qx/Qy maxima of C3-trifluoroacetylacetonated chlorin 1
were located at 554/692 nm and were red-shifted compared
with those of 3-acetyl-chlorin 6 due to the extension of π-
39
conjugation at the C3-substitution (see also Table 1). When
2
+
3
the β-diketonate group of 1 coordinated to Ru(bpy)2 , its Qx/
Qy maxima were hypsochromically shifted to 551/687 nm in 3.
The blue shifts can be explained by the following steric effect.
The crystal structure of 1 showed that the diketonate group at
the C3 position was twisted at an angle of 25.8° from the plane
of the chlorin π ring due to the steric repulsion from the two
neighbors, the 2-methyl group and 5-hydrogen atom. Complex-
−
6
complexes 7 and 8 possessing a phenyl group instead of a
chlorin moiety were prepared by reaction of trifluoroacetyl-
acetonated benzene with Ru(bpy)Cl and Ru(dmb) Cl ,
2
2
2
4
3,44
respectively (Scheme 2).
Zinc complex 1-Zn was
synthesized as the additional reference of 3-Zn.
E
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Inorganic Chemistry
Article
Figure 5. UV−vis absorption spectra of (A) 1 (black solid line), 3 (red), 6 (black broken), and 7 (blue); (B) 1 (black), 4 (red), and 8 (blue); (C) 2
black) and 5 (red); and (D) 1-Zn (black) and 3-Zn (red). The spectra of chlorophyll derivatives were normalized at Qy maxima.
(
Table 1. Electronic Absorption Data of 1−6 and 1/3-Zn in
wavelengths than those of ligand 1. The blue shifts were
ascribed to the twist of the diketonate group from the chlorin
π-system by its coordination to the bulky ruthenium moiety;
the estimated dihedral angle increased from 34 to 39°. In
addition, the Qy maxima of 4 showed a slightly hypsochromical
shift compared with those of compound 3 possessing a
CH Cl
2
2
absorption maximum (nm)
Qx Qy
(0,1) (0,0) (0,1) (0,0) dihedral angle /°
a
compound
Soret
2+
Ru(bpy)2 moiety (Figure 5B). The substitution of 4,4′-H in
1
2
3
4
5
6
382, 421
381, 418
382, 421
384, 416
382, 422
385, 417
386, 430
387, 428
520
520
516
516
516
514
532
554
557
551
548
549
547
579
564
632
633
627
624
626
622
692
695
687
685
687
683
682
678
34
34
37
39
37
bpy with CH in dmb would enhance the dihedral angle and
3
slightly reduce the π-conjugation of the chlorin with the C3
†
substituent. Considering that the methylation hardly increased
the steric repulsion around the C3 position, the plane of the
diketonate group would have been twisted from the chlorin
ring by an electronic effect. This interpretation was supported
by the molecular modeling which showed that the dihedral
b
1
-Zn
-Zn
−
629
32
36
b
3
−
a
1
2
1
2
The dihedral angles of C4−C3−C3 −C3 were estimated by
molecular models using MM+/PM3 calculations.
observed.
angle (39°) of C4−C3−C3 −C3 in 4 was slightly larger than
47 b
Not apparently
that in 3 (37°).
The Qx/Qy absorption bands of compound 5 coordinating
the ruthenium complex at the diketonate group possessing a
C F group were hypsochromically shifted from those of ligand
ation of a Ru(bpy)22+ moiety with the diketonate group made
the C3 substituent more bulky. The larger steric hindrance
around the C3 position increased the dihedral angle of C4−
2
5
2
(Figure 5C). The plane of the diketonate group in 5 was
twisted from the chlorin ring slightly more than that in 2 (34°
→ 37°), similarly as in 1 → 3. The absorption spectra of 3 and
5 were almost identical, and no remote substitution effect of
1
2
C3−C3 −C3 (27.2°, observed in crystallographic analysis)
47
and 34° (estimated by molecular modeling) in 1 to 37°
estimated) in 3. The enhancement in the twist between the
CF to C F was observed in chlorin-specific absorption bands.
(
3
2 5
This consistency was supported by the same estimated dihedral
angles of 3 and 5 (37°). On the other hand, the terminal
two moieties in 3 led to their lower conjugation to give blue
shifts of Q absorption maxima.† Based on the UV−vis
spectrum of reference compound 7 (blue line in Figure 5A),
the absorbance at around 500 nm in 3 was primarily due to a
MLCT absorption band. In a Soret region, 6 apparently gave
two bands at 385 (minor) and 417 nm (major). More split and
broadened Soret bands were visible in 1. Ru-complexation of 1
to 3 afforded similar Soret maxima at 382 and 421 nm, which
were comparably intense.
substitution of CF with C F at the diketonate group in chlorin
3
2 3
ligand molecules moved the Qy band to a longer wavelength:
92 (1) → 695 nm (2). Because the estimated dihedral angles
6
of 1 and 2 were the same (34°), a remote and electronic
substitution effect on the Qy bands operated in ligands 1 and 2.
Zinc metalation at the central position of 1 and 3 would shift
the Qx/Qy maxima to a longer/shorter wavelength, respec-
The Qx/Qy absorption maxima of compound 4 coordinating
tively (Table 1), which was consistent with the observation
Ru(dmb)22 at the C3 diketonate moiety were shifted to longer
+
reported previously.
17,18
The Qx/Qy bands of ruthenium−zinc
F
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a
Table 2. Redox E1/2 and Peak Potentials E (in mV) of 1−5, 7, 8, and 3-Zn
p
E1/2ox (ΔEp)
E1/2red (ΔEp)
Chl
Chl
compound
first
second
Ru(II)/(III)
first
second
Ru(I)/(II)
1
2
3
4
5
7
8
533 (Epa)
538 (Epa)
∼500 (Epa)
870 (Epa)
843 (Epa)
842 (Epa)
835 (Epa)
915 (Epa)
−1516 (63)
−
−1780 (91)
b
533 (104)
419 (83)
476 (91)
441 (70)
348 (66)
544 (125)
−1320 (56)
−1332 (62)
−1457 (60)
−1477 (63)
−1988 (65)
−2079 (82)
577 (E )
pa
c
b
−
−
b
−
−1983 (82)
−1988 (84)
c
3-Zn
246 (70)
−
−1511 (52)
−1677 (117)
a
Cyclic voltammograms were recorded in CH CN/0.1 M Bu NClO with a scan rate of 10 mV/s at ≈1 mM sample solutions. The redox potential
3
4
4
values are cited, based on the oxidation potential of ferrocene as a standard. E and E were the anodic and cathodic peak potentials, respectively.
ΔE = (Epa + E )/2 and ΔE = E − E . Reversible peaks could not be apparently observed. E for a chlorin part could not be detected due to
the overlap with the E for Ru(II)/(III).
pa
pc
b
c
1/2
pc
p
pa
pc
pa
pa
binuclear chlorin 3-Zn were observed at a lower wavelength
than those of 1-Zn. The blue-shifted value of the Qy maxima (4
nm for 1-Zn → 3-Zn) was nearly identical to that by the
coordination of 1 with Ru(bpy)2 to 3 (5 nm). This result was
supported by the molecular modeling which showed that the
dihedral angle in 3-Zn (36°) increased from that of 1-Zn (32°).
Therefore, the diketonate group directly conjugated to the
chlorin ring was twisted by the ruthenium complexation, and
the Qy bands were hypsochromically shifted.
chlorin 1 easily reduced the electrochemical chlorin part in 3,
mainly due to the presence of the cation (Ru ) near the
chlorin π-system.
Comparing the voltammograms of 3 and 4, no change was
apparently observed in the redox properties of the chlorin
moieties, but redox potentials of Ru(II)/(III) and Ru(II)/(I)
were shifted to about 0.1 V on the negative side. Especially,
reversible oxidation of Ru(II)/(III) first occurred in 4, and then
sequential oxidation of the chlorin moiety took place. The
2+
2
+
The fluorescence emission quantum yields of Chl derivatives
, 4, 5, and 3-Zn coordinating the ruthenium complex at the
substitution of 4,4′-H with electron-donating CH groups in
3
3
bpy affected predominantly one-electron oxidation and
reduction of the Ru center but minimally the redox in the
chlorin part. This minimal electrochemical shift was consistent
with the lack of change in electron absorption bands of chlorin
diketonate group were less than 1% and much smaller than
those of chlorin ligands; these values were ∼20% for 1, 2, and 6
and ∼10% for 1-Zn. The quenching was ascribed to the heavy
atom effect of the ruthenium center partially conjugated with
the fluorescent chlorin π-system through the diketonate group.
Electrochemical Properties of Chlorophyll Derivatives
Coordinating Ruthenium Bisbipyridine Complex. The
cyclic voltammograms of 1−5, 7, 8, and 3-Zn were measured in
acetonitrile containing 0.1 M tetrabutylammonim perchlorate
ox
chromophores in 3 and 4 (vide supra). The E1/2 for Ru(II)/
(III) of 8 was shifted by 0.1 V to the negative side from that of
7. This shift was almost the same as that from 3 to 4. Moreover,
the observed shift value (0.1 V) was consistent with that
reported (∼0.1 V) for the electrochemical oxidation of Ru(II)
5
2
to Ru(III) in the complexes possessing (bpy)
The oxidation potential of Ru(II)/(III) in 5 was negatively
shifted by about 0.05 V from that in 2. Substitution of CF with
group on the diketonate
or (dmb) .
2
2
50
(
TBAP) as a supporting electrolyte (see Figures S4−S10),
and the data relative to ferrocene/ferrocenium are summarized
in Table 2. During sweeping of the applied potential from 0 to
3
53
a less-electron-withdrawing C F
2 5
ox
1
.2 V, two oxidation (anodic) peaks of 1 were observed, but the
ligand decreased E1/2 of Ru(II)/(III). The substitution effect
is in good agreement with that in the aforementioned
dimethylation. Introduction of electron-donating and elec-
tron-withdrawing groups to the ligands on Ru complexes
decreased and increased, respectively, the redox potentials of
the Ru center, Ru(II)/(III), and Ru(II)/(I).
reverse sweep resulted in no apparent reduction (cathodic)
peaks (Figure S4A). The electrochemical oxidation of 1 was
irreversible, which was consistent with previous reports for Chl
5
1
derivatives. When sweeping the potential from 0 to −2.4 V,
two anodic peaks were observed, and the reverse sweep
afforded two cathodic peaks. Thus, compound 1 gave two
After zinc metalation at the central position of a chlorin, the
moiety is easily oxidized and difficult to reduce in comparison
red
reversible and sequential one-electron reductions at E1/2
=
54
−
1.52 (ΔE = 0.06 V) and = −1.78 V (0.09 V) (see Figure
with the corresponding free base. The cyclic voltammogram
of ruthenium−zinc binuclear chlorin 3-Zn showed several
reversible redox potentials (Figure S9). The redox potentials of
the chlorin moiety were shifted to negative sides (∼0.2 V) by
zinc metalation of the chlorin ring (3 → 3-Zn), but almost no
shifts of the redox potentials of Ru(II)/(III) and (II)/(I) were
observed. The above results indicate that the chlorin moiety did
not electrochemically interact with the ruthenium center in a
complex.
p
S4B).
In the positively applied-potential side of the cyclic
voltammogram of chlorin−ruthenium conjugate 3, two
irreversible oxidations of chlorin and one reversible oxidation
of Ru(II) to Ru(III) were observed (Figure S6A). The
ox
oxidation potential E1/2 of Ru(II)/(III) was 0.53 V, which
was larger than the first anodic peak potential E of the chlorin
pa
moiety and smaller than the second E . In the negatively
pa
applied potential, three reversible reductions were observed,
CONCLUSION
which included the two sequential one-electron reductions of
■
red
chlorin in 3, followed by Ru(II)/(I). The two E1/2 of chlorin
A ruthenium bisbipyridine complex was successfully coordi-
nated with the diketonate group at the peripheral C3 position
of Chl derivatives 1 and 2, and the chlorin−ruthenium
were positively shifted by about 0.2−0.3 V from the values in 1.
2
2
+
The coordination with Ru(bpy) at the peripheral position of
G
dx.doi.org/10.1021/ic400509q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
conjugates 3 and 5 were isolated after purification. The
dimethyl substituent at the bpy of 3 as in 4 and the zinc
complex of the central position of the chlorin ring of 3 as in 3-
Zn were obtained as chemically stable compounds. The
synthetic chlorin−ruthenium conjugates had the electronic
absorption bands characteristic of chlorin and ruthenium
moieties. The apparent interaction between the two moieties
in a complex was not observed in a solution. The fluorescence
emission derived from the chlorin moiety was largely quenched
by its coordination to the ruthenium moiety due to its internal
heavy atom effect in the close situation. The redox properties of
chlorin−ruthenium conjugates were affected largely by the
substituents close to redox centers but minimally by those far
from them. For example, the oxidation of Ru(II)/(III) in
Ru(bpy)22 of 3 occurred at slightly more positive potential
than that of the chlorin moiety, while that of Ru(II)/(III) in
Ru(dmb)22 occurred before that of the chlorin moiety. In the
zinc chlorin−ruthenium conjugate 3-Zn, the chlorin moiety was
obviously oxidized first, followed by the ruthenium center.
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+
(
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(
ASSOCIATED CONTENT
Supporting Information
H NMR spectra and cyclic voltammograms have been
̈
er, W.;
■
Noy, D.; Scherz, A.; Scheer, H. J. Am. Chem. Soc. 1998, 120, 3675−
*
S
1
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(
Peripherally Metalated Porphyrin Derivatives: Synthetic Approaches
and Properties. In Handbook of Porphyrin Science; Kadish, K. M.,
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AUTHOR INFORMATION
Corresponding Author
E-mail: tamiaki@fc.ritsumei.ac.jp (H.T.), Fax: +81-77-561-
659.
■
3
, Chapter 14, pp 429−483.
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*
(
2
Notes
(
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
(
This work was partially supported by a Grant-in-Aid for
Scientific Research (A) (No. 22245030) from the Japan Society
for the Promotion of Science (JSPS).
Rev. 1997, 165, 239−266.
(
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