Synthesis of phosphonic acid derivatized bipyridine ligands and their ruthenium complexes

Article abstract of DOI:10.1021/ic4014976

Water-stable, surface-bound chromophores, catalysts, and assemblies are an essential element in dye-sensitized photoelectrosynthesis cells for the generation of solar fuels by water splitting and CO₂ reduction to CO, other oxygenates, or hydrocarbons. Phosphonic acid derivatives provide a basis for stable chemical binding on metal oxide surfaces. We report here the efficient synthesis of 4,4′-bis(diethylphosphonomethyl)-2,2′- bipyridine and 4,4′-bis(diethylphosphonate)-2,2′-bipyridine, as well as the mono-, bis-, and tris-substituted ruthenium complexes, [Ru(bpy) ₂(Pbpy)]²⁺, [Ru(bpy)(Pbpy)₂]²⁺, [Ru(Pbpy)₃]²⁺, [Ru(bpy)₂(CPbpy)]²⁺, [Ru(bpy)(CPbpy)₂]²⁺, and [Ru(CPbpy)₃] ²⁺ [bpy = 2,2′-bipyridine; Pbpy = 4,4′-bis(phosphonic acid)-2,2′-bipyridine; CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine].

Full text of DOI:10.1021/ic4014976

Article
pubs.acs.org/IC
Synthesis of Phosphonic Acid Derivatized Bipyridine Ligands and
Their Ruthenium Complexes
Michael R. Norris, Javier J. Concepcion, Christopher R. K. Glasson, Zhen Fang, Alexander M. Lapides,
Dennis L. Ashford, Joseph L. Templeton, and Thomas J. Meyer*
Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599, United States
*
S Supporting Information
ABSTRACT: Water-stable, surface-bound chromophores,
catalysts, and assemblies are an essential element in dye-
sensitized photoelectrosynthesis cells for the generation of
solar fuels by water splitting and CO reduction to CO, other
2
oxygenates, or hydrocarbons. Phosphonic acid derivatives
provide a basis for stable chemical binding on metal oxide
surfaces. We report here the efficient synthesis of 4,4′-
bis(diethylphosphonomethyl)-2,2′-bipyridine and 4,4′-bis-
(
diethylphosphonate)-2,2′-bipyridine, as well as the mono-,
bis-, and tris-substituted ruthenium complexes, [Ru-
2
+
2+
2+
(
(
(
(
bpy) (Pbpy)] , [Ru(bpy)(Pbpy) ] , [Ru(Pbpy) ] , [Ru-
2 2 3
2
+
2+
bpy) (CPbpy)] , [Ru(bpy)(CPbpy) ] , and [Ru-
2
2
2
+
CPbpy)3] [bpy = 2,2′-bipyridine; Pbpy = 4,4′-bis-
phosphonic acid)-2,2′-bipyridine; CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine].
INTRODUCTION
Key to the final requirement is the nature of the chromophore
anchoring links to the oxide surface.
■
Harnessing solar energy is one of the current grand challenges
of science. Solar cell designs based on the attachment of
ruthenium polypyridyl complexes to metal oxides such as TiO2
have given rise to a family of solar cells based on the
photosensitized injection of an electron from the ruthenium
complex into the conduction band of the metal oxide.
relatively long-lived excited states of ruthenium polypyridyl
complexes allow the complexes to act as efficient sensitizers for
electron injection upon light excitation.
solar cells (DSSCs) of this design generate electricity from
sunlight, utilizing ruthenium complexes and other photo-
sensitizers, with efficiencies approaching 12%.
One drawback of DSSCs is that they do not provide a way to
easily store the energy that is generated by sunlight. However,
in dye-sensitized photoelectrosynthesis cells (DSPECs), the
redox equivalents generated from photoexcitation and injection
are used in a two-compartment cell to oxidize water and reduce
In DSSCs, the ruthenium complexes are anchored to TiO2
via ester formation at the surface utilizing derivatives function-
4
alized with carboxylic acids. Carboxylic acid groups are stable
with respect to light and redox cycling in the acetonitrile
environment typically used in DSSC applications. In aqueous
environments, however, these anchoring groups quickly desorb
1
−7
The
from TiO by hydrolysis of the surface ester bonds. Phosphonic
2
acid derivatives have been shown to be far more stable than
8
−11
Dye-sensitized
18,19
these carboxylic acids.
hydroxamates,
Other functional moieties including
20,21
22
23
24
silanes, silatrane, and amides have also
been investigated as anchoring groups. A recent report has even
combined phosphonate and carboxylate linkers to stabilize a
1
2,13
25
dye on a metal oxide surface. Implementation of each of these
strategies has limitations. As noted above, carboxylates and
maleonates are not stable under operating conditions in water.
Siloxanes and amides present significant challenges in view of
the synthetic requirements associated with synthesizing the
derivatized ligands and subsequent complexes. Phosphonic
acids are appealing in offering relatively stable surface binding
+
14−17
H or CO into fuels.
The demands imposed by this
2
application compel stringent requirements on the chromo-
phore−catalyst combination. The chromophore should (1)
absorb light broadly in the visible and near-IR, (2) be capable of
either excited-state electron injection into the metal oxide
conduction band in photoanode applications or excited-state
hole injection for photocathodes, (3) have a redox potential
sufficient to drive water oxidation (>1.23 V vs NHE at pH 0) or
1
1,18,26,27
in water at pH ≤5.
Our work in chromophore−catalyst assemblies has been
primarily based on polypyridyl complexes of ruthenium.
Literature procedures exist for the functionalization of
polypyridyl ligands with phosphonic acids, but still easy access
to these ligands on large scales in only a few steps in high yield
water/CO reduction, and (4) have long-term stability at the
2
oxide interface over a wide pH range in aqueous solution, under
conditions of continuous solar illumination and redox cycling.
Received: June 13, 2013
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic4014976 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
remains a bottleneck to development in this area. We report
here modifications to the syntheses of phosphonic acid
derivatized bipyridine ligands at the 4 and 4′ positions, both
directly on the aromatic ring and with a methylene spacer.
These new procedures (i) have fewer number of synthetic
steps, (ii) give higher yields, and (iii) negate the need for
column chromatography. We have also developed or refined
argon at reflux for 4 h. After cooling, the reaction was added to ice
water (600 mL) and a white solid precipitated, which was filtered,
washed with water, and dried in a vacuum oven. The product was used
without further purification. Yield: 6.76 g (90%). Characterization data
1
1
match literature values.
,4′-Dihydroxymethyl-2,2′-bipyridine. This compound was pre-
4
11
pared as previously reported.
,4′-Bis(bromomethyl)-2,2′-bipyridine. This compound was syn-
4
II
11
the coordination chemistry of these ligands with Ru through
thesized by a modified literature procedure. A solution of 4,4′-
dihydroxymethyl-2,2′-bipyridine (2.34 g, 0.011 mol) in 48% HBr (60
mL) and concentrated sulfuric acid (20 mL) was heated at reflux
overnight and then allowed to cool to room temperature. The addition
of water (120 mL), followed by neutralization (pH 7) with a
concentrated aqueous sodium hydroxide solution, led to the
precipitation of a white solid. The solid was collected by filtration
and washed with water. Yield: 2.58 g (75%). Characterization data
the synthesis of mono-, bis-, and tris-substituted complexes
with both types of ligands. The ruthenium complexes
synthesized in this work are shown in Figure 1.
1
1
match literature values.
,4′-Bis(diethylphosphonomethyl)-2,2′-bipyridine. This ligand
was synthesized by a modified literature procedure that avoids the
4
3
1
use of column chromatography for purification. A solution of 4,4′-
bis(bromomethyl)-2,2′-bipyridine (2.58 g, 7.5 mmol) in triethylphos-
phite (6.6 mL, 37.7 mmol) was purged with argon for 15 min and then
heated at 80 °C for 12 h. The reaction mixture was allowed to cool to
room temperature and pentane (30 mL) was added, causing the
precipitation of an off-white solid. The product was collected by
vacuum filtration and washed with pentanes to remove any excess
1
triethylphosphite. Yield: 3.35 g (98%). H NMR (400 MHz, CDCl ):
3
δ 8.61 (d, 2H), 8.35 (t, 2H), 7.33 (dd, 2H), 4.09 (m, 8H), 3.25 (d,
4
H), 1.29 (t, 12H). 31P NMR (161 MHz, CDCl ): δ 24.4.
3
4
,4′-Dibromo-2,2′-bipyridine. Phosphorus oxybromide (52.8 g,
Figure 1. Structures of the six complexes synthesized and
characterized in this study. Complexes with the methylene spacer
are labeled “CP” followed by the number of functionalized bipyridines
and with the ring directly functionalized by “P” followed by the
number of phosphonate-substituted ligands.
0.184 mol) was added dropwise to a stirred solution of 4,4′-
dimethoxy-2,2′-bipyridine (5 g, 0.023 mol) in anhydrous N,N-
dimethylformamide (DMF; 80 mL) at 0 °C. Stirring was continued
at 0 °C for 1 h and then the mixture was heated to 105 °C for 18 h.
Stirring was ceased, and the reaction mixture was cooled to room
temperature. To this mixture was added water (160 mL), and the
solution was neutralized with NaHCO . The resulting precipitate was
3
isolated by filtration and washed with water. The product was used
without further purification. Yield: 5.88 g (81%).
EXPERIMENTAL SECTION
■
4
,4′-Dihydroxy-2,2′-bipyridine. This ligand was prepared by
Materials and Methods. Distilled water was further purified using
a Milli-Q Ultrapure water purification system. 4,4′-Dimethoxy-2,2′-
bipyridine, 2,2′-bipyrimidine (bpm), and ruthenium trichloride
trihydrate (RuCl ·3H O) were purchased from Aldrich and used as
32
modification of a literature procedure. A suspension of 4,4′-
dimethoxy-2,2′-bipyridine (4.00 g, 18.5 mmol) in a mixture of glacial
acetic acid (200 mL) and 48% hydrobromic acid (30 mL) was heated
at reflux for 15 h. The solvent was then removed, and the resultant
residue was dissolved in water and neutralized (pH 7) with a 70%
ammonium hydroxide solution. A white solid precipitated, which was
filtered and washed with water. This product was used without further
3
2
28
received. 4,4′-Dicarboxy-2,2′-bipyridine, 4,4′-dihydroxymethyl-2,2′-
11
29
bipyridine, dichlororuthenium(II) cyclooctadiene polymer, and
6
30
[
(η -C H )RuCl] Cl were prepared as described in the literature.
6 6 2 2
All other reagents were ACS grade and used without additional
purification. Microwave reactions were carried out in Microwave
Accelerated Reaction System, model MARS, 1200 W microwave oven
with HP 500 plus Teflon vessels.
UV/vis absorption spectra were recorded on an Agilent
Technologies model 8453 diode-array spectrophotometer. Electro-
chemical measurements were performed on a CH Instruments model
1
purification. Because of low solubility, H NMR characterization was
not possible. Yield: 1.65 g (47%).
4
,4′-Ditrifluoromethanesulfonate-2,2′-bipyridine. This ligand was
prepared in a manner similar to that of 2,2′:6′,2″-terpyridine-4′-
trifluoromethanesulfonate. Under an argon purge, 4,4′-dihydroxy-
,2′-bipyridine (0.88 g, 4.7 mmol) was dissolved in anhydrous pyridine
20 mL), and the reaction was cooled to 0 °C. Over 30 min,
3
3
2
(
6
60D potentiostat/galvanostat. Voltammetric measurements were
trifluoromethanesulfonic anhydride (1.6 mL, 2.6 g, 9.4 mmol) was
added, and the reaction was allowed to warm to room temperature.
After the reaction was stirred at room temperature for 16 h, it was
poured into ice water (150 mL) and stirred for 30 min. A white solid
made with a planar CHI104 3 mm glassy carbon working electrode
or FTO-coated glass slide, a platinum wire CHI115 counter electrode,
and a Ag/AgCl CHI111 reference electrode (3 M NaCl, 0.207 V vs
NHE) in buffered aqueous solutions with added 0.5 M KNO or 0.5 M
3
NaClO . The electrochemistry of the complexes was examined both in
precipitated, was filtered, washed with water, and dried in a vacuum
4
1
solution by dissolving them in a buffer (pH 1 and 7) and on a surface
by immersing a planar FTO slide in a 1 mM solution (0.01 M HNO3)
of the complex for 1 h and using the modified slide as the working
oven. Yield: 1.9 g (89%). H NMR (400 MHz, CDCl
1H), 8.40 (d, 1H), 7.30 (dd, 1H).
3
): δ 8.77 (d,
4,4′-Bis(diethylphosphonate)-2,2′-bipyridine. Using Schlenk tech-
niques to prevent air and moisture from entering the reaction vessel,
diethyl phosphite (2.6 g, 20 mmol), 4,4′-dibromo-2,2′-bipyridine, (2.8
g, 8.8 mmol), and tetrakis(triphenylphosphine)palladium(0) (0.946 g,
0.819 mmol) were added to a flask under argon. Anhydrous toluene
(86 mL) and triethylamine (2.6 mL) were then added, and the
reaction was heated to 110 °C for 4 h. The reaction mixture was
filtered hot, and the toluene was removed under vacuum.
Recrystallization of the resulting residue from refluxing hexanes gave
electrode in 0.1 M HClO . Potentials are reported versus NHE unless
4
1
31
otherwise noted. H and P NMR spectra were recorded on a Bruker
Avance 400 MHz or a Bruker 600 MHz spectrometer.
Ligand Synthesis and Characterization. 4,4′-Dicarboxyethyl
Ester 2,2′-Bipyridine. This compound was synthesized by a modified
11
literature procedure. Concentrated sulfuric acid (10 mL) was added
to a mixture of 4,4′-dicarboxy-2,2′-bipyridine (6.0 g, 0.025 mol) in
absolute ethanol (EtOH; 150 mL). The reaction was heated under
B
dx.doi.org/10.1021/ic4014976 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
light-yellow needles of pure 4,4′-bis(diethylphosphonate)-2,2′-bipyr-
then held at 150 °C for 20 min. After cooling to room temperature,
1
idine. Yield: 1.95 g (51%). H NMR (400 MHz, CDCl ): δ 8.81 (t,
EtOH was removed.
3
31
2+
2H), 8.75 (d, 2H), 7.70 (dd, 2H), 4.17 (m, 8H), 1.34 (t, 12H).
P
Hydrolysis. The isolated [Ru(NN)
2
(N′N′)] complexes were
NMR (161 MHz, CDCl ): δ 14.73.
hydrolyzed after isolation without any prior purification. [Ru-
3
2+
Metal Complex Synthesis and Characterization. cis-[Ru-
bpy) Cl ]. Dichlororuthenium(II) cyclooctadiene polymer (11.2 g,
(NN)
2
(N′N′)] (0.50 mmol) was dissolved in 30 mL of anhydrous
(
CH CN under an argon atmosphere. Trimethylsilyl bromide (1.02
2
2
3
4
0 mmol) and 2,2′-bipyridine (12.5 g, 80 mmol) were suspended in o-
equiv per −OEt group) was then added, and the reaction was heated
1
dichlorobenzene (100 mL). The reaction was heated to 190 °C under
argon for 2 h. The mixture was allowed to cool, and a dark solid
precipitated, which was filtered and washed with diethyl ether. This
product was used without further purification. Yield: 17.9 g (92%).
cis-[Ru(4,4′-(PO Et ) bpy) Cl ]. A suspension of RuCl ·3H O (500
at 70 °C for 48 h or until completion (as assessed by H NMR of an
aliquot of the reaction mixture). After cooling the reaction to room
temperature, anhydrous MeOH (1.0 mL) was added and an orange
solid precipitated from the reaction. The orange solid was filtered and
washed with cool CH
[Ru(bpy) (4,4′-(PO
pared as in Method A or Method C starting with cis-[Ru(bpy)
CN and Et
O.
2
3
2 2
2
2
3
2
3
2
mg, 1.4 mmol), 4,4′-bis(diethylphosphonate)-2,2′-bipyridine (1.2 g,
2
3
H )
2
bpy)](Cl)
2
(RuP). This complex was pre-
Cl
2.8 mmol), and zinc granules (327 mg, 5.0 mmol) was heated at reflux
2
]
2
and 4,4′-bis(diethylphosphonate)-2,2′-bipyridine. After hydrolysis of
the ester groups, the product was isolated as an orange solid and was
found to be pure by NMR. Yield: method A, 75%; method C, 85%.
in EtOH (200 mL) for 12 h. The reaction mixture was filtered hot, and
the solvent was removed from the filtrate under vacuum. The resulting
dark-purple solid was collected, washed with Et O, and dried. This
2
1
31
1
Both methods gave identical H and P NMR spectra. H NMR (400
product was used without further purification. Yield: 1.3 g (92%).
cis-[Ru(4,4′-(PO H ) bpy) Cl ]. In order to hydrolyze the ester
MHz, D O): δ 8.75 (d, 2H), 8.51 (d, 4H), 8.03 (t, 4H), 7.91 (m, 2H),
2
3
2 2
2
2
31
7
.79 (dd, 4H), 7.55 (dd, 2H), 7.35 (t, 4H). P NMR (161 MHz,
groups, cis-[Ru(4,4′-(PO Et ) bpy) Cl ] (300 mg, 0.29 mmol) was
3
2
2
2
2
D O): δ 6.78. Anal. Found (calcd) for C H Cl N O P Ru·5H O: C,
dissolved in anhydrous acetonitrile (30 mL) and bromotrimethylsilane
was added (8 equiv). The reaction was stirred at 70 °C for 48 h, after
which anhydrous methanol (MeOH; 3 mL) was added and the
reaction was stirred for 30 min. The solvent was then removed under
reduced pressure. The dark purple/black solid was collected and used
2
30 26
2
6
6
2
2
4
0.15 (40.46); H, 3.94 (4.07); N, 9.45 (9.44).
[
Ru(bpy)(4,4′-(PO H ) bpy) ](Cl) (RuP2). This complex was
3
2 2
2
2
prepared as in Method
C
starting with cis-[Ru(4,4′-
(
PO H ) bpy) Cl ] and 2,2′-bipyridine or as in Method B starting
3
2
2
2
2
with 4,4′-bis(diethylphosphonate)-2,2′-bipyridine. The pure product
without further purification. Yield: 223 mg (95%).
6
was isolated upon hydrolysis. Yield: method B, 80%; method C, 88%.
[
Ru(η -C H )(bpy)Cl]Cl. This complex was synthesized by mod-
6
6
1
31
1
34
Both methods gave identical H and P NMR spectra. H NMR (400
MHz, D O): δ 8.75 (d, 4H), 8.51 (d, 2H), 8.04 (t, 2H), 7.88 (m, 4H),
.74 (d, 2H), 7.56 (dd, 4H), 7.37 (t, 2H). P NMR (161 MHz, D O):
ification of a literature preparation. 2,2′-Bipyridine (1.00 g, 2.33
mmol) and [Ru(η -C H )Cl] Cl (583 mg, 1.17 mmol) were
6
2
6
6
2
2
31
7
2
suspended in MeOH (70 mL). The reaction was heated at reflux
under argon for 3 h. The reaction was then filtered hot to remove any
unreacted material, and the solvent was removed on a rotary
evaporator. A yellow-brown solid resulted that was collected under
diethyl ether and filtered. This product was used without further
purification. Yield: 1.49 g (94%).
δ 6.74. Anal. Found (calcd) for C H Cl N O P Ru·5H O·
3
0
28
2
6
12
4
2
2
CH OH: C, 34.19 (34.48); H, 4.17 (4.16); N, 7.77 (7.54).
3
[
Ru(4,4′-(PO H ) bpy) ](Cl) (RuP3). This complex was prepared as
3
2 2
3
2
in Method A starting with cis-[Ru(4,4′-(PO H ) bpy) Cl ] and
3
2
2
2
2
hydrolyzed 4,4′-bis(phosphonic acid)-2,2′-bipyridine, although better
results were found for an alternative synthesis as follows. A solution of
hydrolyzed 4,4′-bis(phosphonic acid)-2,2′-bipyridine (150 mg, 0.47
mmol) and RuCl ·3H O (31 mg, 0.12 mmol) in 1:1 (v/v) EtOH/H O
6
6
[
Ru(η -C H )(bpy)OTf]OTf. [Ru(η -C H )(bpy)Cl]Cl (1.50 g, 3.69
6 6 6 6
mmol) was suspended in CH Cl (50 mL) at room temperature, and
2
2
3
2
2
the solution was degassed with argon. After the addition of a needle to
vent HCl, trifluoromethanesulfonic acid (1.5 mL) was carefully added
(20 mL) was heated in a microwave reactor. The reaction was heated
at 160 °C for 15 min with a 5 min ramp to temperature. After the
reaction was allowed to cool, EtOH (30 mL) was added, and the
and the reaction was stirred for 2 h. Upon the addition of Et O (200
2
mL), a dark-green solid precipitated from solution. The solid was
product precipitated as a red microcrystalline solid that was collected
1
filtered and washed with Et O. Yield: 2.22 g (95%). H NMR (600
1
2
by filtration and washed with EtOH and Et O. Yield: 46 mg (94%). H
2
MHz, CD OD): δ 9.84 (d, 2H), 8.59 (d, 2H), 8.38 (t, 2H), 7.91 (m,
3
NMR (400 MHz, D O): δ 8.76 (d, 6H), 7.86 (dd, 6H), 7.58 (dd, 6H).
2
2
H), 6.38 (s, 6H).
General Procedure for the Complexes [Ru(NN) (N′N′)] .
31
P NMR (161 MHz, D O): δ 6.28. Anal. Found (calcd) for
2
2
+
2
C H Cl N O P Ru·H O: C, 31.90 (31.65); H, 3.19 (2.83); N, 7.37
30
30
2
6
18
6
2
Method A. A solution of the ligand [N′N′ = 4,4′-bis-
(
7.38).
(
(
d i e t h y l p h o s p h o n a t e ) - 2 , 2 ′ - b i p y r i d i n e , 4 , 4 ′ - b i s -
diethylphosphonomethyl)-2,2′-bipyridine, or 2,2′-bipyridine; 1.03
[Ru(bpy) (4,4′-(CH PO H ) bpy)](Cl) (RuCP). This complex was
2
2
3
2 2
2
prepared as in Method A or Method C starting with cis-[Ru(bpy) Cl ]
2
2
mmol] and cis-[Ru(NN) Cl ] [where NN
= 4,4′-bis-
2
2
and 4,4′-bis(diethylphosphonomethyl)-2,2′-bipyridine. After hydrol-
(
diethylphosphonate)-2,2′-bipyridine or 2,2′-bipyridine; 1.03 mmol]
ysis, the complex was run down a Sephadex LH-20 column, eluting
in 1:1 (v/v) EtOH/H O (40 mL) was heated at reflux under an
2
with H O. Yield: method A, 55%; method C, 68%. Both methods gave
2
atmosphere of argon. The reaction was monitored by UV/vis
absorption spectroscopy and stopped when spectra ceased to change
1
31
1
identical H and P NMR. H NMR (400 MHz, D O): δ 8.36 (d,
2
4
H), 8.27 (t, 2H), 7.87 (tdd, 4H), 7.77 (dd, 2H), 7.67 (dd, 2H), 7.51
(
4−12 h.). The solvent was then removed on a rotary evaporator, and
31
(d, 2H), 7.19 (m, 4H), 7.10 (dt, 2H), 3.03 (d, 4H). P NMR (161
the solid was collected and rinsed with Et O.
2
MHz, D O): δ 19.03. Anal. Found (calcd) for C H Cl N O P Ru·
2
32 30
2
6
9 2
6
Method B. A solution of [Ru(η -C H )(bpy)OTf]OTf (0.50
6
6
4H O: C, 42.91 (42.68); H, 4.55 (4.25); N, 9.36 (9.33).
2
mmol) and the ligand [N′N′ = 4,4′-bis(diethylphosphonate)-2,2′-
bipyridine, or 4,4′-bis(diethylphosphonomethyl)-2,2′-bipyridine; 1.00
mmol] in absolute EtOH (30 mL) was added to a Teflon vessel and
heated in a microwave. The temperature was ramped to 150 °C over 5
min and then held at 150 °C for 20 min. After cooling to room
temperature, EtOH was removed on a rotary evaporator.
[
Ru(bpy)(4,4′-(CH PO H ) bpy) ](Cl) (RuCP2). This complex was
2
3
2 2
2
2
prepared using Method
B
starting with 4, 4′-bis-
(
diethylphosphonomethyl)-2,2′-bipyridine. After hydrolysis, this com-
plex was run down a Sephadex LH-20 column, eluting with H O.
2
1
Yield: 62%. H NMR (400 MHz, D O): δ 8.36 (d, 2H), 8.28 (s, 4H),
2
7.87 (t, 2H), 7.77 (d, 2H), 7.62 (d, 2H), 7.54 (d, 2H), 7.20 (t, 2H),
7.10 (t, 4H), 3.09 (d, 8H). 31P NMR (161 MHz, D O): δ 17.35, 17.23.
Method C. A solution of the ligand [N′N′ = 4,4′-bis-
2
(
(
diethylphosphonate)-2,2′-bipyridine (1.03 mmol), 4,4′-bis-
Anal. Found (calcd) for C H Cl N O P Ru·4H O: C, 37.79
(37.51); H, 4.33 (4.07); N, 7.52 (7.72).
3
4
36
2
6
12
4
2
diethylphosphonomethyl)-2,2′-bipyridine (1.03 mmol), or 2,2′-
bipyridine (3.09 mmol)] and cis-[Ru(NN) Cl ] [where NN = 4,4′-
2
2
[Ru(4,4′-(CH PO H ) bpy) ]Cl (RuCP3). A solution of ligand 4,4′-
2
3
2 2
3
2
bis(diethylphosphonate)-2,2′-bipyridine or bipyridine; 1.03 mmol] in
absolute EtOH (30 mL) was added to a Teflon vessel and heated in a
microwave. The temperature was ramped to 150 °C over 5 min and
bis(diethylphosphonomethyl)-2,2′-bipyridine (500 mg, 1.5 mmol),
RuCl ·3H O (126 mg, 0.48 mmol), and zinc granules (65 mg, 1
3
2
mmol) was added to 1:1 (v/v) EtOH/H O (30 mL) and heated at
2
C
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Inorganic Chemistry
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reflux under dinitrogen overnight. The reaction was allowed to cool,
and the solvent was removed on a rotary evaporator. The product was
then hydrolyzed with trimethylsilyl bromide in CH CN. After
3
hydrolysis, the product was purified on a Sephadex LH-20 column,
eluting with water. Like fractions were combined, and the solvent was
1
removed to give a dark-orange powder. Yield: 434 mg (75%). H
NMR (600 MHz, D O): δ 8.27 (s, 6H), 7.61 (d, 6H), 7.10 (d, 6H),
2
3
.11 (d, 12H). 31P NMR (242 MHz, D O): δ 18.07. Anal. Found
2
(
calcd) for C H Cl N O P Ru·3H O·2CH OH: C, 34.17 (34.51);
36 42 2 6 18 6 2 3
H, 4.20 (4.27); N, 6.26 (6.35).
RESULTS
■
Ligand Synthesis. Synthesis of 4,4″-(CH PO Et ) bpy.
2
3
2 2
The route to this ligand that proved to be most successful
resulted in a significant improvement over two modified
11,31
literature procedures.
First, 4,4′-dimethyl-2,2′-bipyridine
was oxidized by allowing it to react with potassium dichromate
in concentrated sulfuric acid, followed by treatment with nitric
acid to give 4,4′-dicarboxy-2,2′-bipyridine. Esterification of the
carboxylic acid groups was accomplished in absolute EtOH by
the addition of a catalytic amount of sulfuric acid. After 12 h, a
white solid precipitated from the mixture. Isolation of the
precipitate, followed by dissolution in water and neutralization
with base gave the desired product as a white, crystalline solid.
Reduction of the ester groups was accomplished with sodium
borohydride in MeOH to give 4,4′-dihydroxymethyl-2,2′-
bipyridine. 4,4′-Dihydroxymethyl-2,2′-bipyridine was then
converted into 4,4′-dibromomethyl-2,2′-bipyridine in a mixture
of 48% HBr and sulfuric acid. Finally, 4,4′-bis-
1
31
Figure 2. H and P NMR spectra in CDCl at 22 °C of 4,4′-
3
bis(diethylphosphonomethyl)-2,2′-bipyridine showing the expected
aromatic and aliphatic resonances.
stirring for 1 h, the reaction was heated to 105 °C overnight.
The product was isolated by aqueous workup as pure 4,4′-
dibromo-2,2′-bipyridine. It must be noted, however, that better
results in the following step were achieved if the 4,4′-dibromo-
2,2′-bipyridine product was run down a plug of silica, which
likely removes a small amount of an inorganic impurity that is
(
diethylphosphonomethyl)-2,2′-bipyridine was synthesized by
heating 4,4′-dibromomethyl-2,2′-bipyridine in neat triethyl
phosphite. The product precipitated upon the addition of
excess pentane to give analytically pure 4,4′-bis(diethyl
phosphonomethyl)-2,2′-bipyridine in greater than 90% yield
without either high vacuum distillation of the excess triethyl
phosphite or column chromatography. The desired compound
was obtained in 68% overall yield from 4,4′-dimethyl-2,2′-
bipyridine.
1
not observable in the H NMR spectrum.
In both methods, the corresponding triflate- or bromo-
functionalized bipyridine compound was reacted with diethyl
phosphite using a tetrakis(triphenlyphosphine)palladium(0)
catalyst followed by recrystallization from hexanes to give
analytically pure 4,4′-bis(diethylphosphonate)-2,2′-bipyridine.
We have found the purity of the palladium catalyst to be crucial.
The use of pure tetrakis(triphenlyphosphine)palladium(0)
negates the use of excess triphenylphosphine in the cross-
coupling reaction, which not only improves the purity of the
product but also prevents the need for column chromatog-
raphy. The synthesis of these ligands is summarized in Scheme
1.
The ligand was found to be pure by H and 31P NMR
Figure 2). The H NMR spectrum consists of three resonances
1
1
(
in the aromatic region for the three ring protons. Of note is the
large doublet at 3.25 ppm due to the −CH − spacer between
2
2
the bipyridine and phosphonate groups that has a J of 24
HP
Hz. The ester groups of the phosphonates appear at 4.09 and
3
1
1
.29 ppm. The proton-decoupled P NMR spectrum shows
Recrystallization results in pure ligand as demonstrated by
H and P NMR spectra (Figure 3). There is no evidence for
1
31
only a singlet at 24.4 ppm.
Synthesis of 4,4′-(PO Et ) bpy. This ligand was synthesized
3
2 2
by two different routes from 4,4′-dimethoxy-2,2′-bipyridine. In
the first method, 4,4′-dimethoxy-2,2′-bipyridine was refluxed in
a mixture of acetic acid and 48% hydrobromic acid to form 4,4′-
dihydroxy-2,2′-bipyridine. A white crystalline solid precipitated
from the reaction mixture, which was filtered, dissolved in
water, and precipitated by neutralization of the solution with
ammonium hydroxide to yield clean 4,4′-dihydroxy-2,2′-
bipyridine. It was converted to 4,4′-ditrifluoromethanesulfo-
nate-2,2′-bipyridine by reaction with trifluoromethanesulfonic
anhydride in anhydrous pyridine at 0 °C following a procedure
Scheme 1. Synthesis of 4,4′-Bis(diethylphosphonate)-2,2′-
a
bipyridine by Two Routes
33
similar to the one reported for a related terpyridine derivative.
The addition of the reaction mixture to ice water precipitated
clean product in good yield.
In the second method, 4,4′-dimethoxy-2,2′-bipyridine was
converted directly to 4,4′-dibromo-2,2′-bipyridine. Phosphorus
oxybromide was carefully added to a solution of 4,4′-
dimethoxy-2,2′-bipyridine in anhydrous DMF at 0 °C. After
a
(i) HOAc, HBr (47%); (ii) TfOTf, pyridine (89%); (iii) DMF,
POBr (81%); (iv) P(O)H(OEt) , Pd(PPh ) , NEt , toluene (51%).
3
2
3
4
3
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bis(diethylphosphonomethyl)-2,2′-bipyridine] in absolute
2+
EtOH at 150 °C to give [Ru(bpy) (NN)] . Hydrolysis of
2
the ester groups was accomplished either by heating the crude
product with trimethylsilyl bromide under anhydrous con-
ditions in CH CN at 70 °C or by heating the crude product at
3
reflux in 4 M HCl. In both cases, complete hydrolysis took 48
h.
Synthesis of the Ruthenium Complexes with Two
2
+
Phosphonate-Derivatized Ligands ([Ru(N′N′)(NN) ] ,
2
Where N′N′ Is a Bidentate Ligand and NN Is 4,4′-
Bis(phosphonic acid)-2,2′-bipyridine or 4,4′-Bis-
(
methylphosphonic acid)-2,2′-bipyridine). For phosphonate-
derivatized bipyridine ligands, the straightforward procedure
used to prepare the parent cis-[Ru(bpy) Cl ] described above
2
2
was not successful in forming bis-ligand complexes. High-
temperature reactions either cause oligomerization through the
formation of P−O−P bonds or create synthetic problems
because of solubility complications.
Figure 3. 1H NMR (top) and 31P NMR (bottom) in CDCl at 22 °C
of 4,4′-bis(diethyl phosphonate)-2,2′-bipyridine showing aromatic and
aliphatic signals.
Two viable routes to the effective synthesis of these
complexes have been discovered. The first involves the reaction
3
6
of [(η -C H )Ru(N′N′)OTf]OTf with 2 equiv of 4,4′-bis-
6
6
(
(
diethylphosphonate)-2,2′-bipyridine or 4,4′-bis-
diethylphosphonomethyl)-2,2′-bipyridine in 1:1 (v/v)
EtOH/H O under refluxing conditions. The use of the triflate
excess triphenylphosphine or triphenylphosphine oxide in the
product. These are impurities that are seen when excess PPh is
2
3
complex allows for thermal removal of benzene at lower
temperatures conducive to the synthesis of phosphonate-
derivatized complexes. This procedure can also be carried out
in a microwave at 150 °C in the same solvent mixture but using
used in the synthesis, and both are difficult to separate from the
desired product by column chromatography. The H NMR
spectrum shows three aromatic peaks (a triplet at 8.81 ppm, a
doublet at 8.75 ppm, and a doublet of doublets at 7.70 ppm), as
well as the characteristic ethyl ester resonances from the
1
3
equiv of 4,4′-bis(diethylphosphonate)-2,2′-bipyridine or 4,4′-
bis(diethylphosphonomethyl)-2,2′-bipyridine.
phosphonates, with the −P−O−CH − group appearing as a
2
An alternate procedure involves the synthesis of [Ru(4,4′-
complex multiplet at 4.17 ppm because of the diastereotopic
nature of the protons and coupling to the phosphorus. The
(
PO Et ) bpy) Cl ], which can be used in many downstream
3 2 2 2 2
3
1
proton-decoupled P NMR spectrum consists of a singlet at
reactions analogous to reactions available with cis-[Ru-
(bpy) Cl ]. This complex can be prepared by refluxing
RuCl ·3H
1
4.7 ppm.
2
2
Although most reactions with ruthenium were carried out
3
2
O with 2 equiv of 4,4′-bis(diethylphosphonate)-
using the ligand as the phosphonate ester, the ligand can also be
hydrolyzed prior to complexation. Hydrolysis was achieved by
2,2′-bipyridine with zinc powder in EtOH. The reaction was
monitored by UV/vis absorption spectroscopy to detect any
2
+
stirring the ligand in anhydrous CH CN with trimethylsilyl
appearance of [Ru(4,4′-(PO Et ) bpy) ] . For 4,4′-bis-
3 2 2 3
3
bromide at room temperature. Hydrolyzed ligand [4,4′-
(diethylphosphonate)-2,2′-bipyridine, the addition of ligands
with electron-withdrawing groups decreases the rate of
substitution for a third ligand. Over 12 h, no [Ru(4,4′-
bis(phosphonic acid)-2,2′-bipyridine] is more reactive toward
coordination in EtOH/H O mixtures because of the electron-
2
2
+
donating nature of the deprotonated acid groups under these
(PO
(PO
3
Et
Et
2
)
)
2
2
bpy)
3
]
was formed and pure [Ru(4,4′-
] was isolated. This procedure was also
2
1
conditions relative to the electron-withdrawing esters. The H
3
2
bpy)
Cl
2
NMR spectrum of the 4,4′-bis(phosphonic acid)-2,2′-bipyr-
idine ligand (Figure S1 in the Supporting Information) is
similar to that of the nonhydrolyzed ligand (Figure 3).
Metal Complex Synthesis. Synthesis of the Ruthenium
Complexes with One Phosphonate-Derivatized Ligand
attempted with 4,4′-bis(diethylphosphonomethyl)-2,2′-bipyri-
dine, but because of the more electron-rich nature of these
ligands and the correspondingly facile substitution chemistry at
the metal, [Ru(4,4′-(CH
cleanly separated from [Ru(4,4′-(CH
Because of the difficulty in adding a third ligand to [Ru(4,4′-
(PO Et bpy) Cl ], which allowed for its clean preparation, it
proved necessary to carry out reactions of [Ru(4,4′-
(PO Et bpy) Cl ] with another ligand in a microwave reactor
PO
Et
)
bpy)
Cl
] could not be
) bpy) ] .
2 2 3
2
3
2
2
2
2
2
+
PO
2
Et
3
2+
[
Ru(bpy) (NN)] , Where NN is 4,4′-Bis(phosphonic acid)-
2
2
,2′-bipyridine or 4,4′-Bis(methylphosphonic acid)-2,2′-bi-
3
2
)
2
2
2
2+
pyridine. Essential to the synthesis of pure [Ru(bpy) (NN)]
2
analogues is the use of pure cis-[Ru(bpy) Cl ]. The reaction of
3
2
)
2
2
2
2
2
[
2
2
Ru(COD)Cl ] (COD = 1,5-cyclooctadiene) with 2 equiv of
in absolute EtOH at 150 °C with an excess of the to-be-added
bidentate ligand. This procedure is general and can be used to
add a variety of bidentate, polypyridyl ligands. The incorpo-
ration of bpm as the third ligand illustrates this strategy.
Additionally, bpm addition does not result in the formation of
dimers, with bpm acting as a bridge. Rather, clean conversion to
2
n
,2′-bipyridine in o-dichlorobenzene at 190 °C under argon for
2+
h results in pure cis-[Ru(bpy) Cl ], free of [Ru(bpy) ] . The
2
2
3
neutral product was isolated in >90% yield by the addition of
diethyl ether to the cooled reaction mixture.
To synthesize the complex with the phosphonate-derivatized
ligand, cis-[Ru(bpy) Cl ] was reacted either thermally with 1
2+
[Ru((PO Et ) bpy) (bpm)] occurs.
2
2
3
2 2
2
equiv of ligand [4,4′-bis(diethylphosphonate)-2,2′-bipyridine or
If a thermal reaction is preferable for the addition of a third
ligand, the ester groups of [Ru((PO Et ) bpy) Cl ] can be
4
,4′-bis(diethylphosphonomethyl)-2,2′-bipyridine] in refluxing
3
2 2
2
2
1
:1 (v/v) EtOH:H O or in a microwave reactor with 3 equiv of
hydrolyzed to give the more reactive [Ru((PO H ) bpy) Cl ],
2
3 2 2 2 2
ligand [4,4′-bis(diethylphosphonate)-2,2′-bipyridine or 4,4′-
with deesterification achieved by using trimethylsilyl bromide.
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Refluxing in 4 M HCl is effective, but yields for the addition of
the third ligand were low with this method. We suspect that
III
this may be due to partial oxidation of the metal to Ru during
hydrolysis, and it does not readily add a third ligand. After
hydrolysis, a third ligand is easily added in refluxing 1:1 (v/v)
EtOH/H O, and the resulting complex can be purified by
2
column chromatography.
Synthesis of the Ruthenium Complexes with Three
2
+
Phosphonate-Derivatized Ligands ([Ru(NN) ] , Where NN
3
Is 4,4′-Bis(phosphonic acid)-2,2′-bipyridine or 4,4′-Bis-
(
methylphosphonic acid)-2,2′-bipyridine). To make com-
plexes with three phosphonate-derivatized ligands, cis-[Ru-
4,4′-(PO H ) bpy) Cl ] was first prepared as previously
(
3
2 2
2
2
described. This intermediate was then reacted with hydrolyzed
4
,4′-bis(phosphonic acid)-2,2′-bipyridine in refluxing 1:1 (v/v)
EtOH/H O. This method was found to be more efficacious
2
than the direct synthesis of trisubstituted complexes using
unhydrolyzed 4,4′-bis(diethylphosphonate)-2,2′-bipyridine be-
cause of slow ligand substitution of the ester complexes as
noted above. An alternate preparative method that gave better
yields involved the reaction between RuCl ·3H O and
3
2
hydrolyzed 4,4′-bis(phosphonic acid)-2,2′-bipyridine in 1:1
(
v/v) EtOH/H O by the microwave technique at 160 °C.
2
Upon cooling and the addition of excess EtOH, the product
precipitated as a red microcrystals.
B e c a u s e t h e e s t e r d e r i v a t i v e o f 4 , 4 ′ - b i s -
(
diethylphosphonomethyl)-2,2′-bipyridine is much more re-
active than 4,4′-bis(diethylphosphonate)-2,2′-bipyridine, syn-
thesis of RuCP3 was achieved by a direct reaction between
RuCl ·3H O and 4,4′-bis(diethylphosphonomethyl)-2,2′-bipyr-
3
2
idine. Overnight reflux of RuCl ·3H O and 3.3 equiv of 4,4′-
3
2
Figure 4. UV/vis absorption spectra for the RuP series [A: RuP
bis(diethylphosphonomethyl)-2,2′-bipyridine in a 1:1 (v/v)
(black), RuP2 (red), and RuP3 (blue)] and RuCP series [B: RuCP
III
mixture of EtOH/H O with added zinc to reduce Ru gave
2
(black), RuCP2 (red), and RuCP3 (blue)] with an MLCT absorption
appearing at ∼460 nm for each complex.
RuCP3 in good yield. The crude product was isolated, and the
ligands were hydrolyzed in anhydrous CH CN with added
3
trimethylsilyl bromide. After hydrolysis, the product was
purified by column chromatography.
Spectroscopic and Electrochemical Characterization. UV/
in the cyclic voltammogram at lower potentials than the RuIII/II
wave. These waves are present from the first cycle and are likely
due to small amounts of impurity polypyridyl complexes where
a polypyridyl ligand has been substituted by water or anions
vis absorption measurements of RuP(1−3) and RuCP(1−3)
−5
were obtained in 0.1 M HClO containing a 1.0 × 10
M
4
−
−
complex (Figure 4). Typical of ruthenium polypyridyl
complexes, all six complexes have characteristic absorption
bands in the visible region of the spectrum due to metal-to-
ligand charge-transfer (MLCT) transitions. The band shape of
each complex is similar with maxima that vary slightly from 456
to 464 nm. For the RuP series (RuP, RuP2, and RuP3), the
low-energy edge of the MLCT absorption sharpens with each
consecutive addition of a phosphonate-derivatized ligand.
Similarly, the ligand-centered transition around 290 nm also
red shifts slightly. Further, with the consecutive addition of 4,4′-
bis(phosphonic acid)-2,2′-bipyridine ligands, the molar absorp-
−1
(Cl or NO ).
3
In voltammograms of the RuCP series anchored to FTO-
III/II
coated glass slides in 0.1 M HClO
, reversible Ru
waves
4
appear but with differences from the RuP series. For these
complexes, the trend of increasing the redox potentials with the
addition of phosphonic acid derivatized ligands is reversed with
III/II
E
1/2 for the Ru
couple for RuCP at 1.19 V and E1/2 for
III/II
RuCP3 at 1.15 V (vs NHE). E1/2 values for the Ru
couples
for the entire series also occur at much less positive potentials
than their RuP analogues with differences of 100−250 mV.
Spectroscopic and electrochemical data are summarized in
Table 1.
−
1
−1
tivity increases from 13300 M cm at 457 nm to 17300 M
−1
cm at 463 nm. For the RuCP series (RuCP, RuCP2, and
RuCP3), all of the absorption spectra are similar to one
another with extinction coefficients and MLCT and interligand
π → π* bands nearly constant across the series.
DISCUSSION
■
Synthesis of Ligands and Metal Complexes. Phos-
phonic acid substituents are desirable for DSPEC applications
because of their documented stability in anchoring ruthenium
polypyridyl complexes to metal oxide surfaces under conditions
of irradiation and potentials leading to electrochemical
Cyclic voltammograms of RuP, RuP2, and RuP3 anchored
to FTO are shown in Figure 5A, with square-wave voltammo-
grams shown in Figure 5B. Each complex displays a
III/II
35
characteristic Ru
reversible redox couple with E1/2 ranging
oxidation. Our focus in this manuscript is the improvement
from 1.29 V for RuP to 1.40 V (vs NHE) for RuP3. The peak-
to-peak separation is <60 mV, characteristic of a surface-bound
redox couple. Of note, RuP3 displays several additional waves
of synthetic routes to phosphonate-derivatized bipyridine
II
2+
ligands and their complexation with Ru . As [Ru(bpy) ]
3
analogues, they have proven to behave as effective photo-
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Figure 5. (A) Cyclic voltammagrams of RuP (black), RuP2 (red), and RuP3 (blue), (B) square-wave voltammagrams of RuP (black), RuP2 (red),
and RuP3 (blue), (C) cyclic voltammagrams of RuCP (black), RuCP2 (red), and RuCP3 (blue), and (D) square-wave voltammagrams of RuCP
−10
−2
−1
(
black), RuCP2 (red), and RuCP3 (blue) loaded on a planar FTO-coated glass slide (1 × 10 mol cm ) in 0.1 M HClO with a 100 mV s scan
4
rate at 298 ± 3 K.
Table 1. Summary of Spectroscopic and Electrochemical
Properties of the Ruthenium Complexes
In the syntheses, two options for functionalization of
bipyridine with phosphonate groups were available from the
literature: conversion of a −CH Br group on an aromatic ring
E1/2(RuIII/II) (V vs NHE)
2
to form a −CH − phosphonate ester or conversion of a
2
a
λabs max (nm) [ε
pH 1.0d
pH 1.0 pH 7.49 on FTO
heteroaryl−X bond to a phosphonate ester directly bound to
the ring through a cross-coupling reaction.
Literature procedures for the synthesis of phosphonate-
−1
−1
b
c
complex
(M cm )]
[
Ru(bpy) ]Cl2
453 [14700]
456 [11800]
1.26
1.21
1.26
1.12
3
[
Ru(bpy) (CPbpy)]
1.19
1.19
1.15
1.29
1.34
1.40
2
derivatized ligands with −CH − spacers are available through
2
Cl (RuCP)
2
4
,4′-(CH Br) -2,2′-bpy as the starting material. Its synthesis has
2
2
[
[
[
[
[
Ru(bpy)(CPbpy)2]
Cl (RuCP2)
460 [10700]
462 [11200]
457 [13300]
464 [14200]
463 [17300]
1.15
1.12
1.26
1.31
1.36
1.02
0.96
1.16
1.16
1.39
been reported from commercially available 4,4′-dimethyl-2,2′-
bipyridine by reaction with N-bromosuccinimide (NBS) and
2
Ru(CPbpy) ]Cl2
3
36
(
RuCP3)
azobis(isobutyronitrile) in CCl4. However, we have not been
able to produce significant yields with the published procedure.
Further, our overall yield for 4,4′-(CH PO Et ) -2,2′-bipyridine
Ru(bpy) (Pbpy)]Cl
2
2
2
(
RuP)
2
3
2 2
Ru(bpy)(Pbpy) ]Cl
RuP2)
2
through an alternate route to 4,4′-(CH Br) bpy is higher than
(
2
2
the reported yield for 4,4′-(CH Br) bpy by direct bromination
Ru(Pbpy) ]Cl2
2
2
3
(
RuP3)
using NBS. Another approach to 4,4′-(CH Br) bpy is to first
2
2
a
b
Spectra were obtained in 0.1 M HClO with a 1.0 × 10−5 M complex.
synthesize the trimethylsilyl adduct followed by reaction with
an electrophilic bromide source in the presence of cesium
4
Numbers from differential pulse voltammetry in 0.1 M HClO with a
4
c
37
1
mM complex. Numbers from differential pulse voltammetry in 0.1
fluoride. Although the preparation we report has more steps,
the overall yields are high, and relatively pure product is
obtained after straightforward recrystallization of the final
phosphonate-derivatized ligand.
M phosphate buffer (pH 7.49) with 0.5 M NaClO and a 1 mM
4
d
complex. 0.1 M HClO .
4
An important aspect of utilizing a dye molecule in DSPEC
applications is efficient excited-state electron injection into
sensitizers and have desirable properties as light-absorbing dyes
in DSPEC applications.
TiO . For this purpose, literature results point to the
2
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importance of having the phosphonate groups bound directly
to the aromatic ring. As reported by Meyer and co-workers,
bipyridines with phosphonates directly bound to the 4 and 4′
positions have nearly 40% higher incident photon-to-current
efficiencies (IPCEs) than phosphonate-derivatized bipyridines
with methylene spacers, at least when evaluated on the
11
nanosecond time scale.
Often phosphonate-derivatized bipyridines with methylene
spacers are used because of their accessibility and higher
synthetic yields. With our modified procedures, however, the
derivatives with phosphonates directly bound are more easily
accessible in high yields and with fewer steps than previously
reported. Either 4,4′-trifluoromethanesulfonate-2,2′-bipyridine
or 4,4′-dibromo-2,2′-bipyridine can be used in the coupling
step with good yield. In general, using 4,4′-dibromo-2,2′-
bipyridine gives overall better yields than the triflate
intermediate because it involves fewer synthetic steps.
However, it does require the use of a reagent, POBr , which
1
3
Figure 6. H NMR spectra in D O at 22 °C for RuP (top), RuP2
2
is more expensive and more difficult to handle than the
reagents in the triflate synthesis. 4,4′-Dibromo-2,2′-bipyridine
can also be purchased directly from suppliers at similar or lower
cost than the reagents required for the synthesis, which makes
the phosphonate derivative only one step away from a
commercially available reagent. This is perhaps the most
(
middle), and RuP3 (bottom) showing the aromatic region of the
spectra.
eight aromatic bipyridine protons at 8.51, 8.02, ∼7.75, and 7.32
ppm and three distinct resonances for the three aromatic
protons on the functionalized 4,4′-(PO H ) -bipyridine at 8.73,
3
2 2
economical route to 4,4′-(PO Et ) -2,2′-bipyridine.
3
2 2
∼7.85, and 7.55 ppm. Their chemical shifts remain relatively
constant as more phosphonate-derivatized ligands are added.
Further, all resonances are sharp and have well-defined splitting
patterns.
Our most significant finding in the synthesis of 4,4′-
PO Et ) -2,2′-bipyridine is the care required for the
(
3
2 2
palladium-catalyzed cross-coupling reaction in the final step.
Pure Pd(PPh ) and rigorously air-free conditions are necessary
3
4
Spectroscopic and Electrochemical Characterization
of Complexes. The UV/vis absorption spectra of the RuP and
RuCP complexes are relatively unremarkable. They all share
typical visible MLCT absorptions that are red-shifted from
[Ru(bpy)3]2+ by 4−10 nm (193−523 cm ). This is the
expected result given the electron-withdrawing substituent
effect of the phosphonate groups in lowering the π* levels on
4,4′-(PO H ) -2,2′-bipyridine with respect to unfunctionalized
in order to avoid excess PPh . In typical cross-coupling
3
reactions catalyzed by Pd(PPh ) or Pd(PPh ) Cl , an excess of
3
4
3 2
2
PPh is used to increase the turnover number of the palladium
3
−1
catalyst; however, this excess of PPh is not required for this
3
reaction. The addition of excess PPh is detrimental to the yield
3
and complicates isolation of pure 4,4′-(PO Et ) -2,2′-bipyridine
3
2 2
because at the end of the reaction the excess PPh and
3
3
2 2
triphenylphosphine oxide byproducts must be removed by
careful column chromatography. When the reaction is run
without excess PPh , 4,4′-(PO Et ) -2,2′-bipyridine can be
bipyridine. The same red shift of MLCT absorptions was also
observed for RuCP derivatives with the absorption maxima red-
shifted by 3−9 nm (145−430 cm ).
−1
3
3
2 2
III/II
recrystallized from hexanes in excellent yield in high purity. In
The redox potentials for the Ru
couples in the series
fact, the purity of 4,4′-(PO Et ) -2,2′-bipyridine synthesized in
RuP, RuP2, and RuP3 are more positive than those for the
3
2 2
1
III/II
2+
this manner is better (as determined by H NMR) than that
after using column chromatography for purification (after the
addition of excess PPh3).
Ru
couple in [Ru(bpy) ] , again because of the effect of
3
the phosphonic acid derivatized ligands upon stabilization of
the dπRuII levels at Ru . From known solution pK values at pH
II
a
The synthesis of ruthenium complexes with phosphonate-
derivatized ligands is challenging. We have discovered
discrepancies in literature reports for the synthesis, character-
ization, and properties of these complexes. For example, RuP2
has been reported to have a negligible IPCE in a DSSC
1, the phosphonic acid substituents not bound to the FTO
surface are likely protonated. Consistent with this observation,
the trend in the redox potentials from RuP to RuP2 and finally
to RuP3 becomes more positive as more electron-withdrawing
substituents are added to the bpy ligands. The same trend was
38
35
configuration in acetonitrile, in contrast to our results.
also observed in solution at pH 1.
In solution at pH 7, the potentials for the Ru
1
III/II
Moreover, the reported H NMR spectrum of RuP2 in D O/
couples are
2
NaOD consists of three unresolved multiplets, while λmax for its
MLCT transition was reported at 488 nm, values significantly
different from those reported here. The product synthesized by
our procedure has IPCE characteristics that are similar to those
all ∼100 mV less positive. Under these conditions, the
phosphonic acid substituents are completely deprotonated,
the ligands become electron-donating relative to bipyridine, and
the charge types of the couples are changed. For pH 7, there is
35
III/II
of analogues RuP and RuP3. For this complex, the low-
no difference in the Ru
although RuP3 has a Ru
couples for RuP and RuP2,
III/II
energy MLCT visible maximum occurs at λ = 464 nm, and
couple 230 mV more positive than
max
1
its H NMR spectrum shows seven clearly resolved resonances
RuP or RuP2.
The redox potentials for the RuIII/II couples in the RuCP
(Figure 6, middle). In addition, the phosphonate-derivatized
III/II
complexes are highly soluble in aqueous media without the
addition of NaOD(H).
series all occur at less positive potentials than that of the Ru
couple of [Ru(bpy)3]2+ under all experimental conditions,
which is opposite to the trend observed for the RuP series. The
trend in the redox potentials with added CP ligands is also
1
A comparison of H NMR spectra for RuP, RuP2, and RuP3
is shown in Figure 6. There are four clear resonances for the
H
dx.doi.org/10.1021/ic4014976 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
opposite to that found for the RuP series, with each additional
ligand causing a decrease in potential by ∼50−100 mV. These
lower potentials and the reversal of the trend with added
substituents can be attributed to the electron-donating nature
of the alkyl substituents on the RuCP complexes. The more
electron-rich ruthenium centers are easier to oxidize, and the
alkyl spacer between the functional group and aromatic ring
also lessens the inductive effects of the protonation state of the
phosphonic acids. These trends are in agreement with the σ-
donating and π-accepting nature of the substituents on the
bipyridine ligands affecting the ruthenium redox potentials as
U.S. Department of Energy, Grant DE-FG02-06ER15788. We
acknowledge funding by UVA EFRC: CCHF EFRC through
Grant DE-SC0001298 supporting C.R.K.G. D.L.A. acknowl-
edges support from a fellowship from the Department of
Energy Office of Science Graduate Fellowship Program (DOE
SCGF), made possible in part by the American Recovery and
Reinvestment Act of 2009, administered by ORISE-ORAU
under Contract DE-AC05-06OR23100.
REFERENCES
■
39
40
previously reported and originally reported by Lever.
Changing the number of phosphonate-derivatized ligands
around the metal center should also have little to no effect on
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(
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(
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CONCLUSIONS
̈
■
We describe here improved procedures and new synthetic
methods for the synthesis of phosphonate-derivatized poly-
pyridyl ligands. These include new column-chromatography-
free routes to 4,4′-(PO Et )-2,2′-bipyridine through a two-step,
(
(
(
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(
II
conversion to the corresponding Ru complexes. We have also
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added to a variety of complexes that have value as components
in the design and synthesis of more complex molecular
assemblies for DSPEC applications, for example. The key
feature is the aqueous solution stability of the resulting surface
linkages, which is an essential element in applications such as
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ASSOCIATED CONTENT
Supporting Information
NMR spectra of intermediates and additional voltammograms.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
*
S
(
19) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. J.
Phys. Chem. B 2004, 108, 14093.
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AUTHOR INFORMATION
Corresponding Author
E-mail: tjmeyer@email.unc.edu.
■
*
(
III; Batista, V. S.; Schmuttenmaer, C. A.; Brudvig, G. W.; Crabtree, R.
H. Energy Environ. Sci. 2010, 3, 917.
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ACKNOWLEDGMENTS
Funding from the UNC Energy Frontier Research Center
EFRC) “Center for Solar Fuels”, an EFRC funded by the U.S.
■
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