Cyclometalated ru complexes of type [RuII(N^N) 2(C^N)]z: Physicochemical response to substituents installed on the anionic ligand

Article abstract of DOI:10.1021/ic100063c

The electrochemical and photophysical properties of a series ofRu(II) complexes related to [Ru(dcbpyH₂)₂(ppy)]¹⁺ (1; dcbpy-H₂ = 4,4′-dicarboxy-2,2′-bipyridine; ppy = 2-phenylpyridine) were examined to elucidate

Full text of DOI:10.1021/ic100063c

4960 Inorg. Chem. 2010, 49, 4960–4971
DOI: 10.1021/ic100063c
Cyclometalated Ru Complexes of Type [RuII(N∧N)2(C∧N)]z: Physicochemical
Response to Substituents Installed on the Anionic Ligand
Paolo G. Bomben, Bryan D. Koivisto, and Curtis P. Berlinguette*
Department of Chemistry and The Institute for Sustainable Energy, Environment and Economy, University of
Calgary, 2500 University Drive N.W., Calgary, Canada T2N 1N4
Received January 11, 2010
The electrochemical and photophysical properties of a series of Ru(II) complexes related to [Ru(dcbpyH2)2(ppy)]1þ (1;dcbpy-
H2 = 4,40-dicarboxy-2,20-bipyridine; ppy = 2-phenylpyridine) were examined to elucidate the effect of modifying the anionic
fragment of the C∧N ligand with conjugated substituents (R). Included in this study is a family of compounds (2-5) consisting
of one or two -NO2 groups installed meta, ortho, and para to the organometallic bond. A suite of compounds with electron-
donating and withdrawing groups (e.g., R = -F (6), -phenyl (7), -4-pyridine (8), -thiophene-2-carbaldehyde (9)) were
also evaluated. Deprotonated forms of select compounds were isolated as tetrabutylammonium salts to benefit solution studies.
All complexes were structurally characterized by a combination of mass spectrometry, 1H and 13C NMR spectroscopy, and/or
elemental analysis. The electronic absorption spectra for all of the compounds reveal three broad bands over the 350-700 nm
range. The maximum wavelength of the lowest energy absorbance bands for complexes modified with electron-withdrawing
groups are hypsochromically shifted up to 45 nm relative to 1; the weakly emitting compounds (i.e., 1, 3, 6-9) display a
hypsochromic shift of up to 63 nm compared to 1. Emission was not observed in cases where the -NO2 group was positioned
meta to the Ru-C bond. The sensitivity of the oxidation potentials to the nature, number, and position of the electron-
withdrawing/-donating substituents for the entire set of compounds reflect a highest occupied molecular orbital (HOMO)
character extended over the metal, the anionic portion of the C∧N ligand, and, in the case of 7-9, the conjugated R group. The
reduction potentials indicate that the lowest unoccupied molecular orbital (LUMO) is localized to the C∧N ligand where R =
-NO2, and on the dcbpyH2 ligands for all other compounds. This assessment was corroborated by time-dependent density
functional theory (TD-DFT) studies.
Introduction
chemical stability, luminescence behavior, and excited-state
reactivity.6-12 The development of cyclometalated Ru(II)
congeners, however, has received far less attention in the
context of photochemical studies.13-23 This observation is
The design and study of polypyridyl Ru(II) complexes is
fueled by the vast utility of this class of compounds in a
myriad of applications (e.g., solar cells,1 artificial photosyn-
thetic schemes,2,3 sensors4,5). The versatility of these com-
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r
pubs.acs.org/IC
Published on Web 05/17/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4961
presumably because the requisite C-H activation step ren-
ders the synthesis of cyclometalated systems more onerous
and/or because luminescence is usually compromised when
the dative Ru-N bond is replaced by a Ru-C bond.21,24
Notwithstanding, the accessibility of these compounds
has increased because of the development of new synthetic
methodologies, and it has been shown that the strongly
σ-donating cyclometalating ligand promotes a unique set
of photophysical properties.15,17,25-28 A notable recent
advance is the recognition by van Koten et al., that tridentate
cyclometalated Ru(II) chromophores can generate reason-
able power conversion efficiencies in the dye-sensitized
solar cell (DSSC1).29 This finding was followed up by a
Figure 1. Frontier molecular orbitals of 1 illustrating our motivation to
install substituents on the phenyl ring of the C∧N ligand to achieve
optimal interaction with the HOMO. (Color scheme: Ru = purple;
N = blue; O = red; C = gray; H atoms not shown.).
€
report by Gratzel et al., documenting a cell power conver-
sion efficiency (η) of 10.1% using the C∧N complex, [Ru-
(dcbpyH2)2(L6)]þ (dcbpyH2=4,40-dicarboxy-2,20-bipyridine;
HL6=2-(2,4-difluorophenyl)pyridine).30 This complex, which
represents the first “champion” (i.e., η > 10%) dye devoid of
NCS- groups, provides the imperative to study these com-
pounds in further detail.
The highest occupied molecular orbital (HOMO) of cyclo-
metalated complexes of type [Ru(N∧N∧N)(N∧N∧C)]þ and
[Ru(N∧N)2(C∧N)]þ is typically extended over the metal
and, to a lesser extent, the anionic portion of the cyclometalat-
ing ligand.15,31-36 The lowest-unoccupied molecular orbital
(LUMO) typically resides on the neutral polypyridyl ligands
along with low-lying excited states delocalized over the pyridyl
portion(s) of the cyclometalating ligand. This scenario leads to
a series of broad absorption bands in the visible region of the
spectrum that arise primarily from mixed-metal/ligand-to-
ligand charge-transfer transitions. These electronic transitions
have important implications in light-harvesting applications
because it offers the opportunity to manipulate the redox
behavior and optical properties by selectively tuning the
LUMO and HOMO energy levels (Figure 1).35
(Figure 2 and Chart 1). This study is designed to gain a
specific understanding of how conjugated substituents inter-
act with the HOMO of 1. Our curiosity is driven by the notion
that expanding the orbital character of the HOMO over the
conjugated substituents should produce a superior absorp-
tion envelope for light-harvesting applications. The isolation
and study of the series 2-5 confirm that the presence of the
-NO2 groups can, indeed, increase the broadness and
intensity of the absorption bands relative to 1 and 6. Because
this group is not ideal for applications that require excited-
state reactivity, we replaced the quenching -NO2 groups
with aromatic substituents to form 7-9. It is found that the
thiophene-based substituent of 9 is a particularly effective
means of improving the optical properties relative to 1. The
polypyridyl ligand of choice in this study is dcbpyH2 because
the utility of these metal complexes as sensitizers often
requires a mode for binding to a semiconducting material
(e.g., TiO2 in the DSSC). The electrochemical and photo-
physical properties of these complexes are detailed herein, as
well as the identification of an efficient preparative route for
isolating cyclometalated Ru(II) complexes.
Taking these observations into account, we have set out to
examine how the photophysical and electrochemical proper-
ties of [Ru(dcbpyH2)2(L1)]þ (1; HL1 = 2-phenylpyridine)
are affected when various substituents (e.g., -NO2, -F,
-phenyl, -4-pyridine, -thiophene-2-carbaldehyde) are in-
stalled on the phenyl ring of the cyclometalating ligand
Experimental Section
Preparation of Compounds. All manipulations were per-
formed using solvents passed through an MBraun solvent
purification system prior to use; chloroform (CHCl3) and tetra-
hydrofuran (THF) solvents were of analytical grade (without
stabilizer). All reagents were purchased from Aldrich, except for
RuCl3 (Pressure Chemical Company), bpy and dcbpyH2 (Alfa
Aesar). Ligands HL1 (2-phenylpyridine) and HL6 (2-(2,4-
difluorophenyl)pyridine) and phenylboronic acid were used as
supplied from Aldrich. Purification by column chromatography
was carried out using silica (Silicycle: Ultrapure Flash Silica),
basic alumina (Fluka), or Sephadex LH-20 (Pharmacia). Ana-
lytical thin-layer chromatography (TLC) was performed on
aluminum-backed sheets precoated with silica 60 F254 adsor-
bent (0.25 mm thick; Merck, Germany) or with plastic-backed
sheets precoated with basic alumina 200 F254 adsorbent (0.25
mm thick, Selecto Scientific: Georgia, U.S.A.) and visualized
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Klink, G. P. M.; van Koten, G. Chem. Commun. 2007, 1907–1909.
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under UV light. H NMR chemical shifts (δ) are reported in
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parts per million (ppm) from low to high field and referenced to
residual non-deuterated solvent. Standard abbreviations indi-
cating multiplicity are used as follows: s = singlet; d = doublet;
t = triplet; m = multiplet. A labeling scheme for the 1H NMR
assignments of selected ligands are provided in Supporting
Information.
2-(3-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-
pyridine:37 A THF solution containing 2.51 g (10.7 mmol) of
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4962 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bomben et al.
Figure 2. Cyclometalated Ru(II) complexes 1-9 under investigation in this study.
Chart 1. Designation of Compoundsa
The reaction was then quenched with MeOH (5 mL), preab-
sorbed on silica, and the solvent was removed in vacuo.
The sample was purified using chromatography [SiO2: (DCM/
EtOAc 9:1): Rf = 0.57] to afford 2.2 g (73%) of a colorless oil.
1H NMR (400 MHz, CDCl3): δ = 8.67 (ddd, 3J = 5 Hz, 4J = 2
Hz, 5J = 1 Hz, 1H, Ha), 8.37 (s, 1H, He), 8.11 (ddd, 3J = 8 Hz,
4J = 2 Hz, 5J = 1 Hz, 1H, Hf), 7.84 (dt, 3J = 8 Hz, 4J = 1 Hz,
1H, Hh), 7.77 (dt, 3J = 8 Hz, 4J = 1 Hz, 1H, Hd), 7.71 (dt, 3J = 7
Hz, 4J = 2 Hz, 1H, Hc), 7.47 (t, 3J = 8 Hz, 1H, Hg), 7.19 (ddd,
3J = 7 Hz, 4J = 5 Hz, 5J = 1 Hz, 1H, Hb), 1.34 (s, 12H, HMethyl);
13C NMR (100 MHz, CDCl3): δ = 157.7, 149.8, 139.0,
136.8, 135.5, 133.4, 130.0, 128.4, 122.2, 120.9, 84.1, 25.1; HRMS
(EI): m/z = 281.1598 [(M)þ] (calcd for C17H20BNO2þ: m/z =
281.1587).
2-(2-Nitrophenyl)pyridine (HL2). An alternative method to a
previously reported synthesis is provided.39 A THF (30 mL)
solution containing 500 mg (2.48 mmol) of 1-bromo-2-nitro-
benzene was charged with 5.0 mL (2.5 mmol) of a 0.5 M THF
solution containing 2-pyridylzinc bromide. This mixture was
stirred for 5 min prior to the addition of 143 mg (0.124 mmol) of
Pd(PPh3)4, and then heated at reflux for 15 h. The resultant
solution was filtered to obtain a brown filtrate that formed a
brown/orange solid upon removal of solvent in vacuo. Purifica-
tion by column chromatography [SiO2: (hexanes/EtOAc 5:2)]
afforded 265 mg (53.5%) of the product as a yellow oil that
solidified upon standing overnight at 4 °C. 1H NMR (CDCl3): δ
8.66 (ddd, 3J = 5 Hz, 4J = 1 Hz, 5J = 1 Hz, 1H, Ha), 7.90 (dd,
3J = 8 Hz, 4J = 1 Hz, 1H, Hh), 7.80 (td, 3J = 8 Hz, 4J = 2 Hz,
1H, Hc), 7.65 (m, 2H, He, Hf), 7.55 (ddd, 3J = 9 Hz, 7 Hz, 4J = 2
Hz, 1H, Hg3), 7.48 (dt, 3J = 8 Hz, 4J = 1 Hz, 5J = 1 Hz, 1H, Hd),
4
7.33 (ddd, J = 8 Hz, 5 Hz, J = 1 Hz, 1H, Hb). EI-MS: m/z
199.9 (calcd for C11H8N2O2þ: m/z 200.0).
2-(3-Nitrophenyl)pyridine (HL3). An alternative method to
a previously reported synthesis is provided.40 A THF/EtOH
(20 mL) solution (THF:EtOH, 3:1 v:v) containing 501 mg (3.00
mmol) of 3-nitrophenylboronic acid, 966 mg (7.00 mmol) of
K2CO3 and 173 mg (0.150 mmol) of Pd(PPh3)4 was charged with
0.26 mL (2.7 mmol) of 2-bromopyridine and then heated at
reflux for 65 h. The suspension was then filtered to obtain an
orange filtrate that afforded a dark red-orange oil upon removal
of solvent in vacuo. Purification by column chromatography
[SiO2: (hexanes/EtOAc/CHCl3 5:2:3)] yields a light yellow solid
that was recrystallized from EtOH affording 465 mg (87.2%) of
the product as a pale yellow crystalline solid. 1H NMR (CDCl3):
δ 8.87 (dd, 4J = 2 Hz, 2 Hz, 1H, Hh), 8.75 (ddd, 3J = 8 Hz, 4J =
2 Hz, 5J = 1 Hz, 1H, Ha), 8.38 (ddd, 3J = 8 Hz, 4J = 2 Hz, 1 Hz,
a dcbpyH2 = 4,40-dicarboxy-2,20-bipyridine (dcbpyH and dcbpy in-
dicate monoanionic and dianionic forms of dcbpyH2, respectively);
HL1 = 2-phenylpyridine; HL2 = 2-(2-nitrophenyl)pyridine; HL3 =
2-(3-nitrophenyl)pyridine; HL4 = 2-(4-nitrophenyl)pyridine; HL5 =
2-(2,4-dinitrophenyl)pyridine; HL6 = 2-(2,4-difluorophenyl)pyridine;
HL7 = 2-(biphenyl-3-yl)pyridine; HL8 = 2,40-(1,3-phenylene)dipyri-
dine; HL9 = 5-(3-(pyridin-2-yl)phenyl)thiophene-2-carbaldehyde.
2-(3-bromophenyl)pyridine38 was cooled to -78 °C. To this
solution was added 1 mL portions of n-BuLi (1.6 M in hexanes,
5.1 mL, 13 mmol) resulting in a dark green solution. After
45 min of stirring at -78 °C, 3.0 g (16 mmol) of 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane was added and stirred
at -78 °C for 30 min. The dry ice bath was then removed, and
the reaction was left to warm to room temperature overnight.
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Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4963
1H, Hg), 8.28 (ddd, 3J = 8 Hz, 4J = 2 Hz, 1 Hz, 1H, He), 7.83 (m,
2H, Hc, Hd), 7.66 (dd, 3J = 8 Hz, 8 Hz, 1H, Hf), 7.34 (ddd, 1H,
Hb). EI-MS: m/z 200 (calcd for C11H8N2O2þ: m/z 200.0).
2-(4-Nitrophenyl)pyridine (HL4). An alternative to a pre-
viously reported synthesis is provided.41 A THF (25 mL) solu-
tion containing 503 mg (2.49 mmol) of 4-bromo-2-nitrobenzene
was charged with 5.0 mL (2.5 mmol) of a 0.5 M THF solution of
2-pyridylzinc bromide and stirred for 5 min prior to the addition
of 143 mg (0.124 mmol) of Pd(PPh3)4. The solution was heated
at reflux for 16 h, and then filtered to obtain an orange/brown
filtrate. The organic layer was washed with H2O (3 ꢀ 15 mL),
collected, and the solvent removed in vacuo resulting in a white
solid that was purified by column chromatography [SiO2:
(hexanes/EtOAc/CHCl3 5:2:3)] to afford 276 mg (55.4%) of
the product as a white solid. 1H NMR (CDCl3): δ 8.76 (dt, 3J =
5 Hz, 4J = 1 Hz, 5J = 1 Hz, 1H, Ha), 8.34 (dt, 3J = 9 Hz, 4J = 2
Hz, 5J = 2 Hz, 2H, Hf), 8.19 (dt, 3J = 9 Hz, 4J = 2 Hz, 5J = 2
Hz, 2H, He), 7.87-7.82 (m, 2H, Hc, Hd), 7.35 (ddd, 3J = 7 Hz, 5
Hz, 4J = 2 Hz, 1H, Hb). EI-MS: m/z 199.9 (calcd for
C11H8N2O2þ: m/z 200.0).
removal of the solvent in vacuo resulted in an oil that was purified
using column chromatography [SiO2: using a gradient DCM/
EtOAc 8:2 to remove impurities and then increasing to the more
polar DCM/EtOAc/MeOH 8:1:1] to afford 200 mg (73%) of the
product as a colorless oil. 1H NMR (400 MHz, CDCl3): δ = 8.63
(d, 3J = 5 Hz, 1H, Ha), 8.58 (d, 3J = 6 Hz, 2H, Hj), 8.23 (s, 1H,
He), 7.94 (d, 3J = 8 Hz, 1H, Hf), 7.70-7.64 (m, 2H, Hd, Hc), 7.57
3
(d, J = 8 Hz, 1H, Hh), 7.50-7.45 (m, 3H, Hi, Hg), 7.16 (ddd,
3J = 7 Hz, 4J = 5 Hz, 5J = 1 Hz, 1H, Hb). 13C NMR (100 MHz,
CDCl3): δ = 156.5, 150.1, 149.6, 148.1, 140.1, 138.4, 136.8, 129.4,
127.4, 127.3, 125.5, 122.4, 121.6, 120.5. HRMS (EI): m/z =
232.1007 [(M)þ] (calcd for C16H12N2þ: m/z = 232.1000).
5-(3-(Pyridin-2-yl)phenyl)thiophene-2-carbaldehyde (HL9).
A sparged mixture of THF (45 mL) and H2O (5 mL) containing
1.10 g (3.91 mmol) of 2-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)phenyl)pyridine37 and 747 mg (3.91 mmol) of 5-bromo-
thiophene-2-carbaldehyde was charged with 2.70 g (19.6 mmol)
of K2CO3 and 316 mg (0.273 mmol) of Pd(PPh3)4. The mix-
ture was heated at reflux for 14 h under inert atmosphere, then
cooled and poured into H2O. The product was extracted
with diethyl ether, washed with brine and dried with MgSO4.
Filtration and removal of the solvent in vacuo resulted in an
oil that was purified by column chromatography [SiO2: (DCM/
EtOAc 9:1): Rf = 0.57] to afford 820 mg (79.0%) of the product,
an oil that solidified as a yellow solid upon standing. 1H NMR
(400 MHz, CDCl3): δ = 9.87 (s, 1H, Hk), 8.70 (ddd, 3J = 5 Hz,
4J = 2 Hz, 5J = 1 Hz, 1H, Ha), 8.31 (t, 4J = 2 Hz, 1H, He), 7.95
(ddd, 3J = 8 Hz, 4J = 2 Hz, 5J = 1 Hz, 1H, Hf), 7.83-7.75 (m,
3H, Hc, Hh, Hj), 7.67 (dt, 3J = 8 Hz, 4J = 1 Hz, 1H, Hd), 7.50 (t,
3J = 8 Hz, 1H, Hg5), 7.47 (d, 3J = 8 Hz, 1H, Hi), 7.25 (ddd, 3J = 7
2-(2,4-Dinitrophenyl)pyridine (HL5). A THF (25 mL) solu-
tion containing 1.012 mg (4.097 mmol) of 1-bromo-2,4-dinitro-
benzene was charged with 8.3 mL (4.2 mmol) of a 0.5 M THF
solution containing 2-pyridylzinc bromide and stirred for 5 min
prior to the addition of 240 mg (0.208 mmol) of Pd(PPh3)4. The
solution was heated at reflux for 17 h, and then filtered to obtain
a brown filtrate. Subsequent solvent removal in vacuo left a dark
orange/brown oil that was purified by column chromatography
[SiO2: (hexanes/EtOAc/CHCl3 5:2:3)] to afford 604 mg (60.2%)
of the product as a yellow oil that solidified upon standing
overnight at 4 °C. 1H NMR (CDCl3): δ 8.74 (d, 4J = 2 Hz, 1H,
Hg), 8.70 (ddd, 3J = 5 Hz, 4J = 1 Hz, 5J = 1 Hz, 1H, Ha), 8.51
(dd, 3J = 8 Hz, 4J = 2 Hz, 1H, Hf), 7.87 (m, 2H, Hc, He), 7.55
(dt, 3J = 8 Hz, 4J = 1 Hz, 5J = 1 Hz, 1H, Hd), 7.41 (ddd, 3J = 8
Hz, 5 Hz, 4J = 1 Hz, 1H, Hb). 13C NMR (CDCl3): 153.4, 150.4,
149.5, 147.7, 140.8, 137.4, 132.7, 126.8, 124.2, 123.0, 120.2.
HRMS (EI): m/z 245.0444 [(Mþ)] (calcd for C11H7N3O4þ: m/z
245.0437).
4
Hz, J = 5 Hz, J = 1 Hz, 1H, Hb); 13C NMR (100 MHz,
CDCl3): δ = 183.0, 156.6, 154.2, 150.0, 142.8, 140.6, 137.5,
137.1, 133.8, 129.8, 128.0, 127.0, 125.2, 124.7, 122.8, 120.8;
HRMS (EI): m/z = 265.0548 [(M)þ] (calcd for C16H11NOSþ:
m/z = 265.0561).
General Synthetic Procedure for Precursors P1-P9. The addi-
tion of HL1-HL9 to the Ru precursor, [(η6-p-cymene)RuCl2]2,
leads to a mixture of [Ru(CH3CN)4(L#)]PF6 and [Ru(p-
cymene)(CH3CN)2(L#)]PF6 (denoted P#; Scheme 1). While it
is possible to isolate each of these cyclometalated Ru species,
they both serve as satisfactory precursors to the target metal
complexes 1-9. We therefore did not attempt to isolate each of
these intermediates; while experimental data for most mixtures
is not provided, representative characterization data for P1 and
P5 is included. General synthetic protocol: A flask containing
0.33 mmol of [(η6-p-cymene)RuCl2]2, 0.66 mmol of HL#, 0.66
mmol of NaOH and 1.32 mmol of KPF6 in 5 mL of MeCN was
stirred at 45 °C for 2 days (Scheme 1). The resultant reaction
mixture was filtered to remove the suspended solid. The solvent
was then removed in vacuo, and reconstituted in a minimum
volume of a 9:1 CH2Cl2/MeCN solvent mixture and purified
by column chromatography [basic Al2O3: (CH2Cl2/MeCN 9:1)].
The yellow/orange band was collected, and the solvent was
removed. The crude product was precipitated using CH2Cl2/
diethyl ether, filtered, and washed with diethyl ether to afford
the desired mixture P#. The mixture was verified by 1H NMR
spectroscopy, and used as prepared in subsequent reactions
without further separation or purification.
2-(Biphenyl-3-yl)pyridine (HL7). A sparged mixture of THF
(45 mL) and H2O (5 mL) containing 0.69 g (3.0 mmol) of 2-(3-
bromophenyl)pyridine38 and 400 mg (3.28 mmol) of phenyl-
boronic acid was charged with 2.27 g (16.4 mmol) of K2CO3 and
265 mg (0.229 mmol) of Pd(PPh3)4. The suspension was heated
at reflux for 14 h under an inert atmosphere, then cooled and
poured into H2O. The product was extracted with diethyl ether,
washed with brine, and then dried with MgSO4. Filtration and
removal of the solvent in vacuo resulted in an oil that was
purified by column chromatography [basic Al2O3: DCM/hex-
ane 1:1; Rf = 0.78] affording 650 mg (95.2%) of the product as a
colorless oil. 1H NMR (400 MHz, CDCl3): δ = 8.78 (ddd, 3J =
5 Hz, 4J = 2 Hz, 5J = 1 Hz, 1H, Ha), 8.32 (t, 4J = 2 Hz, 1H, He),
8.03 (dt, 3J = 8 Hz, 4J = 2 Hz, 5J = 1 Hz, 1H, Hf), 7.85-7.67
(m, 5H, Hd, Hh, Hc, Hi), 7.59 (t, 3J = 8 Hz, 1H, Hg), 7.51 (t, 3J =
7 Hz, 2H, Hj), 7.42 (t, 3J = 7 Hz, 1H, Hk), 7.28 (ddd, 3J = 7 Hz,
4J = 5 Hz, 5J = 1 Hz, 1H, Hb); 13C NMR (100 MHz, CDCl3):
δ =157.1, 149.4, 141.7, 140.9, 139.6, 136.9, 129.2, 128.8, 127.8,
127.4, 127.2, 125.8, 122.2, 120.6; HRMS (EI): m/z = 231.1042
[(M)þ] (calcd for C17H13Nþ: m/z = 231.1048).
2,40-(1,3-Phenylene)dipyridine (HL8). A sparged mixture of
THF (45 mL) and H2O (5 mL) containing 330 mg (1.17 mmol) of
2-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine37
and 251 mg (1.29 mmol) of 4-bromopyridine was charged with
811 mg (5.88 mmol) of K2CO3 and 95 mg (0.082 mmol) of
Pd(PPh3)4. The suspension was heated at reflux for 14 h under
inert atmosphere. The reaction mixture was then cooled and
poured into H2O. The product was extracted with diethyl ether,
washed with brine, and then dried with MgSO4. Filtration and
[Ru(p-cymene)(CH3CN)2(L1)]PF6 (P1). An MeCN (5 mL)
suspension containing 101 mg (0.653 mmol) of HL1, 26 mg
(0.65 mmol) of crushed NaOH, 241 mg (1.31 mmol) of KPF6
and 199 mg (0.325 mmol) of [(η6-p-cymene)RuCl2]2 was stirred
at 45 °C for 2 days. The reaction mixture was then cooled and
filtered to remove suspended solid, followed by the removal of
solvent in vacuo to yield an orange/brown solid. The solid was
purified by column chromatography [basic Al2O3: (CH2Cl2/
MeCN 9:1)]. The yellow fraction that was collected and recon-
stituted in a minimum volume of CH2Cl2, and was then drawn
out of solution with diethyl ether. The resultant solid was
(41) Walla, P.; Kappe, C. O. Chem. Commun. 2004, 564–565.

4964 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bomben et al.
Scheme 1. General Synthetic Scheme Illustrating the Initial Cyclometalation of [(η6-p-cymene)RuCl2]2 with HL1-HL9 to Form a Mixture of
a
Intermediates P1-P9 (Denoted by Dashed Box), which Render Complexes 1-9 upon Coordination with 2 equiv of dcbpyH2
a Protonation levels vary for certain complexes; see Chart 1 for precise formulations.
filtered, washed with diethyl ether (3 ꢀ 10 mL), and dried to
afford 280 mg (69.9%) of the product as a yellow/green solid. 1H
NMR (CD3CN): δ 9.20 (td, 3J = 6 Hz, 4J = 1 Hz, 1H), 8.14 (dd,
3J = 8 Hz, 4J = 1 Hz, 1H), 7.91 (m, 2H), 7.76 (dd, 3J = 8 Hz,
4J = 1 Hz, 1H), 7.25 (m, 2H), 7.15 (dt, 3J = 8 Hz, 4J = 1 Hz, 1H)
5.93 (d, 3J = 6 Hz, 2H), 5.65 (dd, 3J = 6 Hz, 4J = 1 Hz, 1H), 5.41
(dd, 3J = 6 Hz, 4J = 1 Hz, 1H), 2.35 (septet, 3J = 7 Hz, 1H), 2.12
(s, 3H), 2.04 (s, 3H), 1.99 (s, 3H), 0.93 (d, 3J = 7 Hz, 3H), 0.91 (d,
3J = 7 Hz, 3H). 13C NMR (CD3CN): 176.0, 166.6, 156.8, 145.5,
141.1, 139.7, 130.7, 125.4, 124.8, 123.7, 120.5, 105.0, 103.3, 93.4,
89.3, 85.7, 31.9, 22.5, 22.3, 18.9, 3.9.
with diethyl ether. The solid was collected by filtration, washed
with diethyl ether (3 ꢀ 10 mL), and dried in air.
[Ru(dcbpyH2)2(L1)]PF6 (1). Yield: 55 mg (61%) of the pro-
duct as a dark purple solid. Characterization data matches the
previously reported synthesis.35
[Ru(dcbpyH2)2(L2)]PF6 (2). Yield: 93 mg (77%) of the pro-
duct as a dark purple solid. 1H NMR (CD3OD with a drop of
NaOD): δ 9.11 (s, 1H), 9.04 (s, 1H), 8.98 (s, 2H), 8.22 (dd, 3J = 6
Hz, 4J = 1 Hz, 1H), 7.94 (m, 4H), 7.75 (m, 5H), 7.58 (d, 3J = 8
Hz, 1H), 7.07 (m, 2H), 6.95 (t, 3J = 8 Hz, 1H), 6.61 (dd, 3J = 8
Hz, 4J = 1 Hz, 1H). ESI-MS: m/z 789.02 (calcd for RuC35H23-
N6O10 789.05). Anal. Calcd. for RuC35H23N6O10PF6: C, 45.03;
H, 2.48; N, 9.00. Found: C, 45.43, H, 2.77, N, 9.37.
[Ru(CH3CN)4(L5)]PF6 (P5). An MeCN (5 mL) suspension
containing 161 mg (0.657 mmol) of HL5, 26 mg (0.65 mmol)
of crushed NaOH, 241 mg (1.30 mmol) of KPF6 and 200 mg
(0.327 mmol) of [(η6-p-cymene)RuCl2]2 was refluxed for 2 days
and then filtered to yield a deep purple solution. Solvent was
removed in vacuo, and the resultant solid was reconstituted in a
minimum volume of 9:1 CH2Cl2/MeCN and purified by column
chromatography [basic Al2O3: (CH2Cl2/MeCN 9:1)]. After the
solvent was removed in vacuo, the residual solid was dissolved in
a minimal volume of MeCN and drawn out of solution by the
slow addition of diethyl ether. The solid was isolated and
washed with diethyl ether (3 ꢀ 10 mL) to afford 144 mg
[Ru(dcbpyH2)2(L3)]PF6 (3). Yield: 178 mg (87.4%) of the
1
product as a dark purple solid. H NMR (CD3OD): δ 9.04 (s,
1H), 8.96 (s, 1H), 8.92 (s, 1H), 8.90 (s, 1H), 8.68 (d, 4J = 2 Hz,
1H), 8.27 (d, 3J = 8 Hz, 1H), 7.99 (d, 3J = 6 Hz, 1H), 7.92-7.88
(m, 2H), 7.85-7.75 (m, 3H), 7.70 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H),
7.68-7.56 (m, 3H), 7.50 (d, 3J = 6 Hz, 1H), 7.10 (ddd, 3J = 7
4
3
Hz, 6 Hz, J = 1 Hz 1H), 6.74 (d, J = 8 Hz, 1H). HRMS
(MALDI-TOF): m/z 789.0492 [Mþ] (calcd for C35H23N6O10-
Ruþ: m/z 789.0523). Anal. Calcd. for RuC35H23N6O10PF6 þ
7H2O: C, 39.67; H, 3.52; N, 7.93. Found: C, 39.74; H, 3.19;
N, 7.92.
1
(34.2%) of a pink solid product. H NMR (CD3CN): δ 9.13
(ddd, 3J = 6 Hz, 4J = 2 Hz, 5J = 1 Hz, 1H), 8.90 (d, 4J = 2 Hz,
1H), 7.92 (d, 4J = 2 Hz, 1H), 7.83 (ddd, 3J = 9 Hz, 8 Hz, 4J = 2
Hz, 1H), 7.52 (dt, 3J = 8 Hz, 4J = 1 Hz, 1H), 7.38 (ddd, 3J = 7
Hz, 6 Hz, 4J = 1 Hz, 1H), 2.57 (s, 3H), 2.02 (s, 6H), 1.97 (s, 3H).
13C NMR (CD3CN): 197.2, 162.4, 155.0, 148.6, 145.8, 142.4,
138.1, 134.2, 125.3, 125.2, 123.1, 123.0, 112.2, 4.5, 4.0.
[Ru(dcbpyH2)2(L4)]PF6 (4). Yield: 132 mg (99.0%) of the
product as a dark purple solid. 1H NMR (CD3OD with a drop of
NaOD): δ 9.03 (s, 1H), 8.96 (s, 1H), 8.95 (s, 1H), 8.90 (s, 1H),
8.26 (d, 3J = 8 Hz, 1H), 8.08 (d, 3J = 9 Hz, 1H), 8.03 (d, 3J = 6
Hz, 1H), 7.76 (m, 10H), 7.23 (d, 4J = 2 Hz, 1H), 7.13 (dd, 3J = 8
Hz, 4J = 1 Hz, 1H). 13C NMR (CD3OD with a drop of NaOD):
195.8, 171.2, 171.1, 171.0, 170.9, 170.7, 166.8, 159.3, 158.5,
158.3, 157.0, 155.4, 153.8, 151.9, 151.3, 151.2, 150.3, 148.7,
147.9, 146.6, 145.7, 145.6, 137.7, 129.6, 127.8, 127.4, 126.9,
126.8, 125.5, 124.2, 124.1, 124.0, 123.8, 122.2, 117.5. HRMS
(ESI): m/z = 789.05200 [Mþ] (calcd for [RuC35H23N6O10]þ:
General Synthetic Procedure for 1-4, 6. A degassed aqueous
MeOH solution (5 mL; H2O/MeOH, 1:4 v:v) containing 0.20
mmol of dcbpyH2 and 0.40 mmol of crushed NaOH was stirred
for 40 min prior to the addition of 0.10 mmol of P#. The solution
was brought to reflux for 3 h, cooled to room temperature,
followed by the removal of solvent in vacuo. The resultant dark
purple solid was reconstituted in H2O, followed by the dropwise
addition of 0.2 M HPF6 until the formation of a precipitate was
observed. The isolated precipitate was washed with H2O (3 ꢀ 10
mL), reconstituted in DMF, and then drawn out of solution
m/z = 783.05196). Anal. Calcd. for RuC35H23N6O10PF6
DMF þ 2H2O: C, 50.84; H, 3.82; N, 10.92. Found: C, 50.63;
þ
H,3.91; N, 10.83.
[Ru(dcbpyH2)2(L5)]PF6 (5). A degassed aqueous metha-
nol solution (5 mL; H2O/MeOH, 1:4 v:v) containing 22 mg

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4965
(0.092 mmol) of dcbpyH2 and 7 mg (0.2 mmol) of crushed
NaOH was stirred for 40 min prior to the addition of 30 mg
(0.046 mmol) of P5. The light purple solution was brought to
reflux for 3 h and then cooled to room temperature. Removal of
solvent in vacuo yielded a dark purple solid that was purified by
column chromatography [SiO2: (12 cm ꢀ2 cm)(MeOH/CHCl3
3:1)]. The purple band was collected followed by the removal of
solvent in vacuo. The resultant solid was dissolved in a minimum
volume of MeOH and drawn out of solution with acetone. The
solid was collected by filtration and washed with diethyl ether
(3 ꢀ 10 mL). The solid was reconstituted in H2O, followed by the
dropwise addition of 0.2 M HPF6 until the formation of a
precipitate was observed. The solid was collected by filtration,
washed with H2O (3 ꢀ 10 mL), and then reconstituted in DMF
and drawn out of solution with diethyl ether. The solid was
isolated, washed with diethyl ether (3 ꢀ 10 mL), and dried to
3J = 6 Hz, 4J = 1 Hz, 1H), 7.87 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H),
7.85 (dd, 3J = 6 Hz, 4J = 1 Hz, 1H), 7.82 (dd, 3J = 6 Hz, 4J = 1
Hz, 1H), 7.78 (ddd, 3J = 9 Hz, 8 Hz, 4J = 2 Hz, 1H), 7.75 (dd,
3J = 5 Hz, 4J = 2 Hz, 2H), 7.69 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H),
7.66 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H), 7.61 (m, 2H), 7.24 (dd, 3J =
8 Hz, 4J = 2 Hz, 1H), 7.03 (ddd, 3J = 7 Hz, 6 Hz, 4J = 1 Hz,
1H), 6.63 (d, 3J = 8 Hz, 1H). 13C NMR (CD3OD with a drop of
NaOD): 198.9, 171.4, 171.3, 171.1, 171.0, 168.4, 159.4, 158.7,
158.3, 156.9, 155.3, 151.8, 151.4, 151.2, 151.0, 150.4, 150.1,
148.0, 147.7, 146.4, 145.3, 145.1, 137.5, 137.4, 131.5, 127.8,
127.6, 127.1, 126.7, 126.6, 124.1, 124.0, 123.7, 123.6, 123.0,
122.4, 120.7. HRMS (MALDI-TOF): m/z = 821.0962 [Mþ]
(calcd for [RuC40H28N6O8]þ: m/z = 821.0939). Anal. Calcd. for
RuC40H28N6O8 þ 3H2O: C, 53.87; H, 3.84; N, 9.43. Found: C,
53.61; H, 3.26; N, 9.42.
[Ru(dcbpyH2)(dcbpyH)(L9)] (9). A degassed methanol (250
mL) solution charged with 75 mg (0.11 mmol) of P9, 53 mg
(0.22 mmol) of dcbpyH2 and 18 mg (0.44 mmol) of crushed
NaOH was heated at reflux for 3 h. The solvent was then
removed in vacuo, and the resultant solid was purified by
column chromatography [SiO2: (MeOH/KNO3(aq, sat)/1%
acetic acid 5:1:1)]. The isolated solid was reconstituted in H2O
and acidified with 0.2 M HNO3 until the formation of a
precipitate was observed. The solid was isolated, washed with
H2O and diethyl ether, and dried in vacuo to afford 65 mg (70%)
of the product as a purple/red solid. 1H NMR (CD3OD with a
drop of NaOD): δ 9.77 (s, 1H), 9.03 (s, 1H), 8.95 (d, 4J = 1 Hz,
1H), 8.91 (d, 4J = 1 Hz, 1H), 8.89 (d, 4J = 1 Hz, 1H), 8.22 (m,
2H), 8.15 (dd, 3J = 6 Hz, 4J = 1 Hz, 1H), 7.92 (dd, 3J = 6 Hz,
4J = 1 Hz, 1H), 7.88-7.75 (m, 3J = 6 Hz, 4J = 1 Hz, 5H), 7.69
(dd, 3J = 6 Hz, 4J = 2 Hz, 1H), 7.66 (dd, 3J = 6 Hz, 4J = 2 Hz,
1H), 7.62 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H), 7.60 (ddd, 3J = 7 Hz,
4J = 2 Hz, 5J = 1 Hz, 1H), 7.55(d, 3J = 4 Hz, 1H), 7.19 (dd,
3J = 8 Hz, 4J = 2 Hz, 1H), 7.03 (ddd, 3J = 7 Hz, 6 Hz, 4J = 1
Hz, 1H), 6.58 (d, 3J = 8 Hz, 1H). HRMS (MALDI-TOF): m/z
854.0488 [Mþ] (calcd for C40H26N5O9SRuþ: m/z 854.0500).
Anal. Calcd. for RuC40H25N5O9S þ 3H2O: C, 52.98; H, 3.45;
N, 7.72. Found: C, 53.05; H, 3.21; N, 7.70.
1
afford 25 mg (56%) of the product as a dark purple solid. H
NMR (CD3OD with a drop of NaOD): δ 9.05 (s, 1H), 8.98 (s,
2H), 8.94 (s, 1H), 8.02 (d, 3J = 6 Hz, 1H), 7.75 (m, 11H), 7.47 (d,
4J = 2 Hz, 1H), 7.19 (ddd, 3J = 7 Hz, 6 Hz, 4J = 1 Hz, 1H). 13
C
NMR (D2O with a drop of NaOD and MeOD): 202.5, 172.7,
172.6, 172.4, 172.2, 169.5, 161.1, 158.8, 157.9, 157.8, 156.3,
155.6, 152.8, 151.9, 151.6, 150.5, 148.5, 145.8, 145.6, 144.6,
144.1, 144.0, 142.2, 137.3, 132.1, 127.0, 126.9, 126.5, 126.4,
126.2, 124.3, 123.7, 123.6, 123.5, 112.5. ESI-MS: m/z 833.95
[Mþ] (calcd for RuC35H22N7O12 834.04). Anal. Calcd. for
RuC35H23N6O10PF6 þ 2CH3OH: C, 42.62; H, 2.90; N, 9.40.
Found: C, 43.02; H,3.18; N, 9.66.
[Ru(dcbpyH2)2(L6)]PF6 (6). Yield: 109 mg (87.5%) of the
product as a dark purple solid. All characterization data
matches the previously reported synthesis.30
[Ru(dcbpyH2)(dcbpyH)(L7)] (7). A degassed aqueous metha-
nol (15 mL) solution (H2O/MeOH, 1:4 v:v) containing 194 mg
(0.794 mmol) of dcbpyH2 and 64 mg (1.6 mmol) of crushed
NaOH was stirred for 40 min prior to the addition of 261 mg
(0.401 mmol) of P7. The reaction mixture was brought to reflux
for 3 h generating a color change from yellow/green to dark
purple. Insoluble particulates were removed from the solution
by filtration, followed by the removal of solvent from the fil-
trate in vacuo. Further purification by column chromato-
graphy [SiO2: (MeOH/CHCl3 3:1) followed by Sephadex LH-
20: (D2O)] yielded the deprotonated product as a sodium salt.
The solid was reconstituted in H2O, followed by the addition of
0.2 M HNO3 until the formation of a precipitate was observed
and placed in the refrigerator overnight. The solid was isolated
by filtration, washed with acid (pH 3) and dried in vacuo to
afford 51 mg (14%) of the dark purple product. 1H NMR
(CD3OD with a drop of NaOD): δ 9.03 (s, 1H), 8.95 (s, 1H),
8.91 (s, 1H), 8.88 (s, 1H), 8.22 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H),
8.19 (d, 3J = 8 Hz, 1H), 8.11 (d, 3J = 2 Hz, 1H), 7.94 (dd, 3J = 6
Hz, 4J = 1 Hz, 1H), 7.87 (dd, 3J = 6 Hz, 4J = 1 Hz, 1H), 7.86
(dd, 3J = 6 Hz, 4J = 2 Hz, 1H), 7.83 (dd, 3J = 6 Hz, 4J = 1 Hz,
1H), 7.73 (ddd, 3J = 9 Hz, 7 Hz, 4J = 2 Hz, 1H), 7.68 (dd, 3J = 6
Hz, 4J = 2 Hz, 1H), 7.65-7.60 (m, 4H), 7.56 (ddd, 3J = 6 Hz,
4J = 2 Hz, 5J = 1 Hz, 1H), 7.37 (t, 3J = 7 Hz, 2H), 7.23 (t, 3J = 7
Hz, 1H), 7.12 (dd, 3J = 8 Hz, 4J = 2 Hz, 1H), 6.98 (ddd, 3J = 7
Hz, 6 Hz, 4J = 1 Hz, 1H), 6.51 (d, 3J = 8 Hz, 1H). Anal. Calcd.
for RuC41H27N5O8 þ 2H2O: C, 57.61; H, 3.66; N, 8.19. Found:
C, 57.29; H, 3.72; N, 8.04.
General Synthetic Procedure for 1a, 7a-9a. An aqueous
(20 mL) suspension containing 0.112 mmol of 1 was titrated
with 0.2 M tetrabutylammonium hydroxide (NBu4OH) to reach
pH 7. Solid impurities were removed by filtration, followed by
the removal of solvent in vacuo. The resultant solid was
reconstituted in MeOH and then drawn out of solution using
a 1:1 mixture of diethyl ether/petroleum ether and purified by
column chromatography [Sephadex LH-20: (H2O)]. Solvent
was removed from the dark purple fraction in vacuo leaving a
solid that was dried overnight under reduced pressure to afford
the product.
(Bu4N)3[Ru(dcbpy)(dcbpyH)(L1)]PF6 (1a). Yield: 73 mg (40%)
of the product as a dark purple solid. 1HNMR(CD3OD): δ9.01 (s,
1H), 8.94 (s, 1H), 8.89 (s, 1H), 8.87 (s, 1H), 8.14 (dd, 3J = 6 Hz,
4J = 1 Hz, 1H), 8.06 (d, 3J = 8 Hz, 1H), 7.91 (d, 3J = 6 Hz, 4J =
1 Hz, 1H), 7.88-7.84 (m, 3H), 7.79 (dd, 3J= 6Hz, 4J = 1 Hz, 1H),
7.71 (ddd, 3J = 9 Hz, 8 Hz, 4J = 2 Hz, 1H), 7.67 (dd, 3J = 6 Hz,
4J= 2 Hz, 1H), 7.64 (dd, 3J=6Hz, 4J= 2 Hz, 1H), 7.60 (dd, 3J=
6 Hz, 4J = 2 Hz, 1H), 7.55 (ddd, 3J = 6 Hz, 4J = 2 Hz, 5J = 1 Hz,
1H), 6.96 (ddd, 3J = 7 Hz, 6 Hz, 4J = 1 Hz, 1H), 6.90 (ddd, 3J =
7 Hz, 7 Hz, 4J = 1 Hz, 1H), 6.81 (ddd, 3J = 7 Hz, 7 Hz, 4J = 1 Hz,
1H), 6.40 (dd, 3J = 7 Hz, 4J = 1 Hz, 1H), 3.22 (m, 24H), 1.64 (m,
24H), 1.38 (sextet, 3J = 7 Hz, 24H), 0.98 (t, 3J = 7 Hz, 36H). ESI-
MS: m/z = 744.2 [(M - 3Bu4Nþ þ 4Hþ)þ] (calcd for RuC35-
H24N5O8þ: m/z = 744.1). Anal. Calcd. for RuC83H129N8O8-
PF6 þ 4H2O: C, 59.16; H, 7.91; N, 6.81. Found: C, 59.42; H,
8.19; N, 6.65.
[Ru(dcbpyH)2(L8-H)] (8). A degassed H2O/MeOH (15 mL)
solution (H2O/MeOH, 1:4 v:v) containing 218 mg (0.892 mmol)
of dcbpyH2 and 72 mg (1.8 mmol) of crushed NaOH was stirred
for 40 min prior to the addition of 287 mg (0.440 mmol) of P8.
The solution was brought to reflux for 3 h leading to a color
change from yellow to dark purple. A reaction workup analo-
gous to that of 7 affords 26 mg (6%) of a dark purple product.
1H NMR (CD3OD with a drop of NaOD): δ 9.04 (s, 1H), 8.95
(s, 1H), 8.91 (s, 1H), 8.89 (s, 1H), 8.49 (dd, 3J = 5 Hz, 4J = 2 Hz,
2H), 8.28 (m, 2H), 8.17 (dd, 3J = 6 Hz, 4J = 1 Hz, 1H), 7.93 (dd,
(Bu4N)4[Ru(dcbpy)2(L3)]PF6 (3a). A suspension containing
100 mg (0.107 mmol) of 1 in 20 mL of H2O was titrated with
0.2 M NBu4OH to reach pH 7 and then filtered. Removal of
solvent in vacuo yielded an oily solid that was reconstituted in

4966 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bomben et al.
MeOH and brought out of solution with a 1:1 mixture of diethyl
ether/petroleum ether. The isolated solid was dried overnight
under vacuum to afford 94 mg (46%) of a dark purple solid
product. 1H NMR (CD3OD): δ 9.05 (s, 1H), 8.96 (s, 1H), 8.93 (s,
1H), 8.90 (s, 1H), 8.68 (d, 4J = 2 Hz, 1H), 8.26 (d, 3J = 8 Hz,
1H), 7.99 (d, 3J = 6 Hz, 1H), 7.92-7.88 (m, 2H), 7.84-7.76 (m,
3H), 7.71 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H), 7.68-7.60 (m, 3H),
7.56 (d, 3J = 6 Hz, 1H), 7.10 (ddd, 3J = 7 Hz, 6 Hz, 4J = 1 Hz
1H), 6.75 (d, 3J = 8 Hz, 1H), 3.23 (m, 32H), 1.65 (m, 32H), 1.40
1H), 7.66 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H), 7.63 (dd, 3J = 6 Hz,
4J = 2 Hz, 1H), 7.61 (ddd, 3J = 7 Hz, 4J = 2 Hz, 5J = 1 Hz, 1H),
7.57(d, 3J = 4 Hz, 1H), 7.19 (dd, 3J = 8 Hz, 4J = 2 Hz, 1H), 7.04
(ddd, 3J = 7 Hz, 6 Hz, 4J = 1 Hz, 1H), 6.59 (d, 3J = 8 Hz, 1H),
3.21 (m, 24H), 1.63 (m, 24H), 1.38 (sextet, 3J = 7 Hz, 24H), 0.98
3
(t, J = 7 Hz, 36H). 13C NMR (CD3OD): 200.7, 184.8, 168.0,
159.3, 158.7, 158.3, 156.8, 155.3, 151.5, 151.3, 150.3, 148.0,
141.7, 140.6, 137.6, 137.3, 127.9, 127.5, 127.2, 127.0, 126.8,
124.3, 124.2, 124.1, 124.0, 123.8, 123.7, 122.1, 120.7, 59.6, 24.9,
20.9, 14.1. HRMS (ESI): m/z = 854.04918 [(M - 3Bu4Nþ
þ
(sextet, 3J = 7 Hz, 32H), 1.00 (t, 3J = 7 Hz, 48H). ESI-MS: m/z =
4Hþ)þ] (calcd for RuC40H26N5O9Sþ: m/z = 854.04947). Anal.
Calcd. for RuC88H146N8O17S þ 8H2O: C, 61.40; H, 8.55; N,
6.51. Found: C, 61.07; H, 7.89; N, 6.44.
þ
789.1 [(M - 3Bu4Nþ þ 4Hþ)þ] (calcd for RuC35H23N6O10
:
m/z = 789.1). Anal. Calcd. for RuC99H171N10O14PF6 þ 4H2O:
C, 60.31; H, 8.74; N, 7.10. Found: C, 60.40; H, 8.47; N, 7.39.
(Bu4N)3[Ru(dcbpy)(dcbpyH)(L6)]PF6 (6a). A suspension con-
taining 100 mg (0.108 mmol) of 6 in 20 mL of H2O was titrated
using 0.2 M NBu4OH to reach pH 7. A reaction workup
identical to that of 3a affords 80 mg (45%) of the product as a
1
Physical Methods. Routine H and 13C NMR spectra were
recorded at 400 and 100 MHz, respectively, on a Bruker AV 400
instrument at ambient temperature unless otherwise stated.
Elemental analysis (EA), electrospray ionization mass spectro-
metry (ESI-MS), matrix-assisted laser desorption/ionization
mass spectrometry (MALDI-TOF), and electron impact (EI)
mass spectrometry data were collected at the University of
Calgary. Electrochemical measurements were performed under
anaerobic conditions with a Princeton Applied Research Versa-
Stat 3 potentiostat using dry solvents, Pt working and counter
electrodes, a Ag pseudoreference electrode, and 0.1 M NBu4BF4
supporting electrolyte. Electronic spectroscopic data were
collected on MeOH solutions using a Cary 5000 UV-vis
spectrophotometer (Varian). Steady-state emission spectra were
obtained at room temperature using an Edinburgh Instruments
FLS920 Spectrometer equipped with a Xe900 450W steady
state xenon arc lamp, TMS300-X excitation monochromator,
TMS300-M emission monochromator, Hamamatsu R2658P
PMT detector and corrected for detector response. Lifetime
measurements were obtained at room temperature using an
Edinburgh Instruments FLS920 Spectrometer equipped with
Fianium SC400 Super Continuum White Light Source, Hama-
matsu R3809U-50 Multi Channel Plate detector and data were
analyzed with Edinbrugh Instruments F900 software. Curve
fitting of the data was performed using a nonlinear least-squares
procedure in the F900 software.
1
dark purple solid. H NMR (CD3OD): δ 9.03 (s, 1H), 8.96 (s,
1H), 8.92 (s, 1H), 8.91 (s, 1H), 8.31 (d, 3J = 8 Hz, 1H), 8.09 (d,
3J = 6 Hz, 1H), 7.89-7.85 (m, 2H), 7.80-7.62 (m, 7H), 7.00 (t,
3J = 6 Hz, 1H), 6.38 (ddd, 3J = 13 Hz, 9 Hz, 4J = 2 Hz, 1H),
3
4
5.90 (dd, J = 6 Hz, J = 1 Hz, 1H), 3.23 (m, 24H), 1.64 (m,
24H), 1.38 (sextet, 3J = 7 Hz, 24H), 0.98 (t, 3J = 7 Hz, 36H).
ESI-MS: m/z = 780.1 [(M - 3Bu4Nþ þ 4Hþ)þ] (calcd for
RuC35H22F2N5O8þ: m/z = 780.1). Anal. Calcd. for RuC83H127
-
F8N8O8P: C, 60.46; H, 7.76; N, 6.80. Found: C, 60.78; H, 8.26;
N, 6.75.
(Bu4N)3[Ru(dcbpy)2(L7)] (7a). Yield: 48 mg (62%) of the
1
product as a dark purple solid. H NMR (CD3OD): δ 9.03 (s,
1H), 8.95 (s, 1H), 8.91 (s, 1H), 8.89 (s, 1H), 8.22 (m, 2H), 8.13 (d,
4J = 2 Hz, 1H), 7.94 (d, 3J = 6 Hz, 1H), 7.87 (m, 2H), 7.83 (d,
3J = 6 Hz, 1H), 7.74 (ddd, 3J = 8 Hz, 7 Hz, 4J = 2 Hz, 1H), 7.68
(dd, 3J = 6 Hz, 4J = 2 Hz, 1H), 7.66-7.61 (m, 4H), 7.57 (d, 3J =
6 Hz, 1H), 7.38 (t, 3J = 7 Hz, 2H), 7.24 (t, 3J = 7 Hz, 1H), 7.12
(dd, 3J = 8 Hz, 4J = 2 Hz, 1H), 6.99 (ddd, 3J = 7 Hz, 6 Hz, 4J =
1 Hz, 1H), 6.53 (d, 3J = 8 Hz, 1H), 3.20 (m, 24H), 1.62 (m, 24H),
1.37 (sextet, 3J = 7 Hz, 24H), 0.97 (t, 3J = 7 Hz, 36H). 13C NMR
(CD3OD): 193.3, 171.3, 171.1, 171.0, 168.8, 159.4, 158.7, 158.3,
157.0, 155.4, 151.3, 151.1, 151.0, 150.2, 147.9, 147.4, 146.4,
145.3, 145.0, 143.5, 137.3, 136.9, 135.6, 129.9, 128.5, 127.8,
127.6, 127.4, 127.0, 126.6, 124.0, 123.9, 123.8, 123.7, 123.6,
123.5, 120.4, 59.7, 24.9, 20.8, 14.1. ESI-MS: m/z = 820.1
[(M - 3Bu4Nþ þ 4Hþ)þ] (calcd for RuC41H28N5O8þ: m/z =
820.1). Anal. Calcd. for RuC89H148N8O16 þ 8H2O: C, 63.36; H,
8.84; N, 6.64. Found: C, 63.57; H, 8.64; N, 6.59.
DFT Calculations. Density functional theory (DFT) calcula-
tions were carried out using B3LYP42-45 (Becke’s three-para-
meter exchange functional (B3) and the Lee-Yang-Parr
correlation functional (LYP)) and the LanL2DZ basis set.46-49
All geometries were fully optimized in the ground states (closed-
shell singlet S0). Time-dependent density functional theory (TD-
DFT) calculations were performed with IEFPCM solvation
model (MeCN)50 using a spin-restricted formalism to examine
low-energy excitations at the ground-state geometry (output files
are provided as Supporting Information). All calculations were
carried out with the Gaussian 03W software package.51
(Bu4N)3[Ru(dcbpy)2(L8)] (8a). Yield: 36 mg (84%) of the
1
product as a dark purple solid. H NMR (CD3OD): δ 9.05 (s,
1H), 8.96 (d, 4J = 1 Hz, 1H), 8.92 (d, 4J = 1 Hz, 1H), 8.89 (d,
4J = 1 Hz, 1H), 8.49 (d, 3J = 6 Hz, 2H), 8.28 (m, 2H), 8.17 (dd,
3J = 6 Hz, 4J = 1 Hz, 1H), 7.94 (dd, 3J = 6 Hz, 4J = 1 Hz, 1H),
3
7.89-7.82 (m, 3H), 7.80-7.75 (m, 3H), 7.70 (dd, J = 6 Hz,
Results
4J = 2 Hz, 1H), 7.66 (dd, 3J = 6 Hz, 4J = 2 Hz, 1H), 7.61 (m,
2H), 7.25 (dd, 3J = 8 Hz, 4J = 2 Hz, 1H), 7.03 (ddd, 3J = 7 Hz, 6
Hz, 4J = 1 Hz, 1H), 6.63 (d, 3J = 8 Hz, 1H), 3.22 (m, 24H), 1.63
(m, 24H), 1.38 (sextet, 3J = 7 Hz, 24H), 0.98 (t, 3J = 7 Hz, 36H).
13C NMR (CD3OD): 199.1, 171.1, 170.9, 170.8, 168.4, 159.3,
158.7, 158.3, 156.9, 155.3, 151.8, 151.4, 151.1, 151.0, 150.4,
150.2, 148.1, 146.6, 145.5, 137.5, 131.4, 127.8, 127.6, 127.1,
126.7, 124.1, 124.0, 123.8, 123.7, 123.0, 122.4, 120.7, 59.6,
24.9, 20.8, 14.1. ESI-MS: m/z = 821.2 [(M - 3Bu4Nþ þ 4Hþ)þ]
(calcd for RuC40H27N6O8þ: m/z = 821.1). Anal. Calcd. for
RuC88H147N9O16 þ 8H2O: C, 62.61; H, 8.78; N, 7.47. Found: C,
62.43; H, 8.72; N, 7.26.
Synthesis and Structural Characterization. Cyclometa-
lated Ru(II) complexes of the form [Ru(dcbpyH2)2-
(C∧N)]þ can be accessed using one of these two general
routes: (i) initial coordination of the dcbpyH2 ligands to
(42) Becke, A. D. Phys. Rev. A: Gen. Phys. 1988, 38, 3098–3100.
(43) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(44) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter 1988,
37, 785–789.
(45) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200–206.
(Bu4N)3[Ru(dcbpy)2(L9)] (9a). Yield: 45 mg (59%) of the
product as a dark purple solid. 1H NMR (CD3OD): δ 9.80
(s, 1H), 9.04 (s, 1H), 8.96 (d, 4J = 1 Hz, 1H), 8.92 (d, 4J = 1 Hz,
1H), 8.89 (d, 4J = 1 Hz, 1H), 8.24 (m, 2H), 8.15 (dd, 3J = 6 Hz,
4J = 1 Hz, 1H), 7.93 (dd, 3J = 6 Hz, 4J = 1 Hz, 1H), 7.89-7.76
(m, 3J = 6 Hz, 4J = 1 Hz, 5H), 7.70 (dd, 3J = 6 Hz, 4J = 2 Hz,
(46) Dunning, T. H., Jr.; Hay, P. J. Mod. Theor. Chem. 1977, 3, 1–27.
(47) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283.
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(49) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298.
(50) The solvent model was not used for 7 and 8.
(51) Frisch, M. J.; et al. Gaussian 03, Revision c02; Gaussian Inc.:
Wallingford, CT, 2004.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4967
the Ru center followed by cyclometalation; or (ii) initial
activation of the C-H bond to furnish a cyclometalated
Ru site with subsequent coordination of the dcbpyH2
ligands. While the first method is typically employed to
isolate compounds of type [Ru(N∧N)2(C∧N)]þ,16-19,21,30
the latter approach has been shown to converge on a
number of heteroleptic Ru(II)26,27,34,52-54 and Ir55-58
complexes. The syntheses described in this study follow
this latter approach because it generates high yields
under mild reaction conditions and eliminates the occur-
rence of undesirable isomerization products when Ru-
(dcbpyH2)2Cl2 is utilized as a synthon.
rather than the respective NO3- salts. Compound 8 exists
as the pyridinium salt as a consequence of the acidifica-
tion process.
The structural identities of the compounds were ver-
ified by a combination of elemental analysis, MALDI-
1
TOF, ESI-MS, H NMR, and/or 13C NMR spectro-
scopy. Representative H NMR spectra are provided in
1
the Supporting Information. The signature upfield doub-
let observed at 6.39 ppm in the 1H NMR spectrum of 1,
emanating from the proton adjacent to the Ru-C bond,
did not always serve as a useful spectroscopic handle
because it is, in certain cases, drawn downfield to overlap
with other aromatic signals. Because of inherent solubi-
lity issues, compounds 1, 3, and 6-9 were deprotonated
by adjusting the pH to 7 using NBu4OH;61 these hygro-
scopic compounds are all soluble in polar organic sol-
vents. 1H NMR spectroscopy and elemental analysis were
used to establish that 3a is fully deprotonated, and that
1a, 6a, 7a, 8a, and 9a all contain three NBu4þ counterions.
This disparity is ascribed to the stronger electron-with-
drawing character of the -NO2 group lowering the pKa of
the acid modalities. Diastereomers of complexes 7a-9a
were not observed because poor solubility precluded the
collection of NMR data in solvents (e.g., hexanes,
chloroform) that restrict proton exchange. Thermogravi-
metric and elemental analysis data collectively confirm
the hygroscopic nature of the deprotonated complexes
(no less than 5 H2O molecules are present in solid samples
of 7a-9a).61
The installation of various substituents (e.g., -NO2,
-F, -4-phenyl, -4-pyridine, -thiophene-2-carbaldehyde)
on the cyclometalating 2-phenylpyridine ligand was
achieved in moderate to high yields using standard Ne-
gishi or Suzuki cross-coupling reaction conditions. Not-
ing that [(η6-C6H6)RuCl2]2 has been documented to be a
useful precursor in cyclometalation reactions,27 we car-
ried out the requisite C-H activation of the cyclometa-
lating ligands using [(η6-p-cymene)RuCl2]2.59,60 The
reaction workup can be carried out in ambient condi-
tions, which contrasts with the anaerobic conditions
required for reactions involving [(η6-C6H6)RuCl2]2. For
all of the ligands used in this study, the reaction generated
a solution containing two products, [(η2-p-cymene)Ru-
(MeCN)2(L#)]PF6 and [Ru(MeCN)4(L#)]PF6. Monitor-
ing the reaction by 1H NMR spectroscopy indicates that
[(η2-p-cymene)Ru(MeCN)2(L#)]PF6 is formed initially,
and is progressively converted to [Ru(MeCN)4(L#)]PF6
over the course of the reaction.27 Both of these species
afford the respective target metal complex (i.e., 1-9)
when combined with 2 equiv of dcbpyH2; however, great-
er emphasis was placed on ensuring that [(η6-p-cymene)-
RuCl2]2 was completely consumed rather than separating
the two intermediate species. We therefore do not offer
the synthesis and characterization details for most of
these precursors; however, the data for P1 and P5 is
included as a representative sample. Complexes 1-9 were
obtained in reasonably high yields after refluxing the
corresponding precursor mixtures P1-P9 with NaOH
and dcbpyH2 in degassed (aqueous) methanol. The fully
protonated forms of complexes 1-6 were isolated by the
dropwise addition of HPF6 to the reaction mixture until
the products precipitated out of solution; acidification of
7, 8, and 9 with HNO3 produced zwitterionic compounds
Electrochemical Properties. The electrochemical beha-
vior for 1-6 in DMF was determined by cyclic voltam-
metry. Solubility issues precluded the collection of
satisfactory voltammograms for 1 and 7-9; thus, depro-
tonated forms of select compounds (i.e., 1a, 3a, 6a-9a)
were prepared to facilitate a comprehensive comparison
of the series. Relevant redox couples are collected in
Table 1.
The voltammograms for all of the complexes reveal a
reversible one-electron metal-based oxidation process
and a series of reduction processes. Compound 1a ex-
hibits a single reversible wave ascribed to the one-electron
reduction of a dcbpy ligand. For the -NO2 series 2-5, the
cathodic sweep leads to the reduction of the -NO2 group
of the C∧N ligand. Compound 5, which contains two
-NO2 groups, exhibits a reduction potential about 0.23 V
lower than those containing a single -NO2 group. The
reduction of 6, which contains two strong electron-with-
drawing -F groups, involves the dcbpyH2 groups, there-
by demonstrating that the -NO2 group exhibits superior
electron-withdrawing character. Inspection of 2-4 indi-
cates the complexes are most easily oxidized when the
-NO2 groups are meta to the organometallic bond: the
additional -NO2 group for 5 leads to an oxidation
potential that is anodically shifted by an additional
∼100 mV relative to 2-4. The placement of the aromatic
substituents para to the organometallic bond reveals
progressively higher oxidation potentials for 7a-9a, re-
spectively, thus reflecting the relative electron-withdraw-
ing character of the substituents. Compounds 7a-9a
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A. D.; Le Lagadec, R. Eur. J. Inorg. Chem. 2008, 4866–4869.
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Lagadec, R. Inorg. Chim. Acta 2006, 359, 883–887.
(54) Ryabov, A. D.; Le Lagadec, R.; Estevez, H.; Toscano, R. A.;
Hernandez, S.; Alexandrova, L.; Kurova, V. S.; Fischer, A.; Sirlin, C.;
Pfeffer, M. Inorg. Chem. 2005, 44, 1626–1634.
(55) Duan, L. L.; Fischer, A.; Xu, Y. H.; Sun, L. C. J. Am. Chem. Soc.
2009, 131, 10397–10399.
(56) Lowry, M. S.; Bernhard, S. Chem.;Eur. J. 2006, 12, 7970–7977.
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Chem. 2009, 48, 6501–6508.
(58) Lee, T.-C.; Hung, J.-Y.; Chi, Y.; Cheng, Y.-M.; Lee, G.-H.; Chou,
P.-T.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C. Adv. Funct. Mater. 2009, 19,
2639–2647.
(59) Kuznetsov, V. F.; Abdur-Rashid, K.; Lough, A. J.; Gusev, D. G.
J. Am. Chem. Soc. 2006, 128, 14388–14396.
(60) Steenwinkel, P.; James, S. L.; Gossage, R. A.; Grove, D. M.; Kooijman,
H.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. Organometallics 1998, 17, 4680–
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4968 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bomben et al.
Table 1. Electrochemical and Electronic Spectroscopy Data for Cyclometalated Ru(II) Compounds
UV-vis dataa
E1/2 (V vs NHE)b
compound
λ
max1 (nm)
λ
max2 (nm)
λmax3 (nm)
λ
em (nm)a
Eox1
Ered1
E(Sþ/S*)(V vs NHE)c
f
[Ru(dcbpyH2)2(L1)]PF6 (1)
[Ru(dcbpyH2)2(L2)]PF6 (2)
[Ru(dcbpyH2)2(L3)]PF6 (3)
[Ru(dcbpyH2)2(L4)]PF6 (4)
575
503
491 (0.8)
494 (1.8)
413
827
-
794
-
-
803
810
781
770
802
794
788
-
-
-
554 (0.9)
545 (1.9)
557 (1.9)
532 (1.2)
557 (1.6)
563 (2.0)
530 (2.0)
545 (2.0)
563 (1.6)d
556 (1.6)d
550 (1.7)d
405 (1.1)
405 (2.2)
401 (1.7)
379 (1.2)
404 (1.7)
409 (2.3)
393 (1.9)
401 (2.3)
408 (1.8)
382 (2.4)
412 (2.8)
þ1.09
þ1.12
þ1.08
þ1.21
þ1.06
þ0.64
þ0.80
þ0.76
þ0.61
þ0.65
þ0.68
-0.96g
-0.95g
-0.94g
-0.72g
-1.01g
-1.47
-1.12
-1.46
-1.50
-1.49
-1.45
-
-0.67
-
-
-0.70
-1.10
-0.99
-1.04
-1.12
-1.12
-1.08
e
-
-
e
[Ru(dcbpyH2)2(L5)]PF6 (5)
[Ru(dcbpyH2)2(L6)]PF6 (6)30
(Bu4N)3[Ru(dcbpy)(dcbpyH)(L1)]PF6 (1a)
(Bu4N)4[Ru(dcbpy)2(L3)]PF6 (3a)
(Bu4N)3[Ru(dcbpy)(dcbpyH)(L6)]PF6 (6a)
(Bu4N)3[Ru(dcbpy)2(L7)] (7a)
(Bu4N)3[Ru(dcbpy)2(L8)] (8a)
(Bu4N)3[Ru(dcbpy)2(L9)] (9a)
489 (1.1)
496 (1.6)
e
-
485 (1.5)
497 (1.3)
494 (1.4)
492 (2.2)
a Recorded in MeOH; ε values indicated in parentheses with units of ꢀ104 M-1 cm-1
.
b Data collected using 0.1 M NBu4BF4 DMF solutions
at 200 mV/s and referenced to a [Fc]/[Fc]þ internal standard followed by conversion to NHE ([Fc]/[Fc]þ vs NHE = 0.69 V).62 c Calculated using
E(Sþ/S*) = E(Sþ/S) - E(0-0), where E(0-0) is obtained from the higher energy side of corrected emission band where the intensity is ca. 10% of the
maximum.30 d λmax1 = 577, 564, and 564 nm for 7-9, respectively. e Observed as shoulder of λmax1. f Could not be accurately measured because of poor
solubility. g (Quasi-)irreversible; Ep,c reported.
Figure 3. Electronic absorption spectra for (a) the -NO2 series 2-5, and (b) 7a-9a, which contain aromatic substituents. The spectra for benchmark
complexes 1 and 1a are included in the respective plots.63
exhibit a single reversible reduction wave at comparable
potentials consistent with the reduction of the dcbpy
ligands.
Electronic and Fluorescence Spectroscopy. The spectral
profile for each compound studied contains three broad
absorption bands in the visible region centered at roughly
exhibit intermediate λmax1 values. The λmax3 values adhere
to this same trend. Data for 3 reveals a lower λmax1 value
relative to the compounds where the -NO2 group is
situated meta to the organometallic bond. This trend is
consistent with enhanced π-electron conjugation for 3
leading to a lower-energy HOMO level relative to the
cross-conjugated systems 2 and 4. The molar extinction
coefficients (ε) are found to range from 1 ꢀ 104-2 ꢀ 104
M-1cm-1 over the series. The ε value of 0.9 ꢀ 104
M-1cm-1 for the signal at λmax1 for 2 is less than half
that of 3 or 4 (ε = 1.9 ꢀ 10-4 M-1 cm-1). The diminished
intensity of this band is presumably a consequence of the
loss of conjugation in 2 arising from steric interactions
between the H atom on the adjacent pyridyl ring and the
-NO2 group. Compound 4 is not subject to this steric
encumbrance; thus, the -NO2 group can reside in the
plane of the aromatic rings of the C∧N ligand.
The UV-vis data for the deprotonated analogues
7a-9a reveal that the lowest-energy transitions occur at
563, 556, and 550 nm, respectively. The relative intensities
of the three maxima in the visible region indicate that
the ε value for λmax1 and λmax3 increases for 7a-9a,
respectively. A hypsochromic shift of the λmax1 and
λmax2 values is observed with progressively stronger elec-
tron-withdrawing groups (EWGs). This trend reflects a
diminished level of π-backbonding to dcbpyH2 as the
540, 490, and 400 nm (maxima are denoted as λmax1
,
λ
max2, and λmax3, respectively, in Table 1) along with
intense intraligand (π-π*) transitions below 350 nm.
Representative UV-vis absorption spectra are depicted
in Figure 3. The two low-energy bands are superimposed
in the cases of 3a, 4, and 5. The bands in the visible region
for all of the compounds are ascribed to a collection of
mixed-metal/ligand to ligand charge-transfer transitions
arising from a HOMO involving both the metal and the
anionic portion of the C∧N ligand. Electrochemical mea-
surements indicate that significant electron density is
situated on the metal; thus, these transitions are, for the
sake of brevity, defined herein as metal-to-ligand charge-
transfer (MLCT).34
The maxima for the lowest-energy MLCT bands are
found over the 532-575 nm range for the series 1-5.
There is a direct correlation between the number of -NO2
groups present and λmax values, namely, the λmax1 values
are found at the lowest energy for 1 and the highest energy
for 5, while the monosubstituted nitro derivatives 2-4

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4969
Figure 4. Energy leveldiagramsdepictingthe dominanttransitions thatcomprise the lowest-energyabsorption bandλmax1 for 3, 7, and 9. (Transitions that
are predicted to contribute less than 10% to the absorbance band are omitted. Color scheme: Ru = magenta; O = red; N = blue; C = gray; H atoms not
shown.).
aromatic moiety of the C∧N ligand becomes more elec-
tron withdrawing. The increasing ε for λmax3 is the result
of an increasing number of transitions to the C∧N ligand
and enhanced conjugation between the adjacent aromatic
rings (vide infra). The λmax3 band is more intense with
increasing electron-withdrawing character of the aro-
matic substituent. Consequently, 9a exhibits the greatest
absorption envelope in the visible region of any of the
compounds studied in this work.
Fluorescence data indicates that emission is quenched
in cases where cross-conjugation is present in the -NO2
series, that is, where -NO2 groups are installed at either
one or both -meta positions relative to the Ru-C bond.
Complexes containing -F groups or substituents in-
stalled para to the Ru-C bond are all weakly emissive
when excited at wavelengths corresponding to λmax1, and
display lifetimes over the 2-40 ns range.
TD-DFT Calculations. TD-DFT calculations were car-
ried out on geometry-optimized structures of the metal
complexes using a B3LYP/LanL2DZ level of theory to
qualitatively assess the frontier molecular orbitals. The
low level of symmetry within these systems leads to a
relatively complicated orbital structure and an increase in
the number of allowed electronic transitions (hence the
broader absorption profiles). The predicted low-energy
transitions are blue-shifted approximately 5-9 nm com-
pared to the experimental values listed in Table 1. All
predicted UV-vis spectra and transitions at λ > 400 nm
are available in the Supporting Information.
An evaluation of the electronic transitions by TD-DFT
indicates that the low-energy excitation bands arise from
a set of mixed-metal/ligand-to-ligand charge-transfer
transitions from the predominantly metal-based dxz,
dyz, dxy orbitals to a set of π* orbitals with exclusively
dcbpyH2 character. Although numerous transitions com-
prise each of the three absorption bands in the visible
region for all of the complexes, a reduction of the TD-
DFT results reveals some overarching trends. The lowest-
energy band λmax1, for instance, involves excited states
localized primarily to the π* system of the dcbpyH2 ligand
cis to the anionic fragment of the C∧N ligand. Absorption
band λmax2 arises from the population of the π* orbitals of
the dcbpyH2 ligand trans to the phenyl ring bound to the
Ru site, while the higher energy band at about 400 nm
(λmax3) involves a more significant participation of the π*
orbitals belonging to the C∧N ligand. Anomalous beha-
vior is observed for 3 because the low-lying empty orbitals
contain more C∧N character.
The dominant allowed transitions that comprise the
lowest-energy absorption band, λmax1, are depicted sche-
matically for 3, 7, and 9 in Figure 4. The dyz, dxy and dxz
orbitals comprise the three highest occupied molecular
orbitals for compounds 1-7 and 9; however, they are not
degenerate because of the C1 symmetry. For complex 8,

4970 Inorganic Chemistry, Vol. 49, No. 11, 2010
Bomben et al.
the HOMO-1 orbital is situated solely on the pendant
pyridine moiety, and the dxz and dxy orbitals are found at
the HOMO-2 and HOMO-3 levels. In general, the
HOMO is a linear combination of the metal dyz orbital
and the anionic phenyl ring of the C∧N ligand with empty
low-lying orbitals extended over the π* system of the
dcbpyH2 ligands. For 2, 4, and 5, however, the lowest
energy excited states are localized to the phenyl ring and
the -NO2 group(s) of the C∧N ligand. For 3, where the
-NO2 group is para to the organometallic bond, the
lowest unoccupied levels involve both the dcbpyH2
ligands and the nitrophenyl scaffold; the HOMO is a
mixture of the dyz orbital and nitrophenyl moiety.
ficiently large driving force for dye regeneration.1,30,67,68
For this reason, electron-withdrawing substituents on the
C∧N ligand are desirable to avoid raising the energy of
the HOMO level above that of the redox couple of the
electrolyte; that is, Eox > ∼0.5 V versus NHE. This line of
reasoning prompted us to study how the oxidation po-
tential of 7 is affected when the phenyl substituent is
replaced with the -4-pyridyl group of 8. Compound 9
was prepared in this same vein by utilizing an aldehyde to
withdraw electron density from the metal; this compound
also provides a platform for examining how the torsional
strain between adjacent aromatic rings affects the optical
properties.
The addition of aromatic substituents to the C∧N
ligand results in an expansion of the HOMO to include
the phenyl, 4-pyridine, and thiophene-2-carbaldehyde
moieties in 7-9, respectively. There is an enhancement
of orbital character over the aromatic scaffold of 9
compared to 7 and 8 because of the relatively small
dihedral angle between the phenyl group of the C∧N
ligand and the thiophene substituent. Electronic transi-
tions comprising the low energy excitation MLCT band
for 7-9 occur from the metal dxz, dyz, dxy orbitals to
LUMOs localized to the dcbpyH2 ligands; the C∧N π*
orbitals are found to be 1.4, 1.4, and 0.7 eV higher in
energy than the LUMO in 7-9, respectively.
Discussion. Compounds 2-9 were designed to gain
insight into how the nature, position, and number of
substituents affect the electrochemical and optical prop-
erties of cyclometalated chromophores related to 1. The
central aim of this study is to exploit the mixed-metal/
ligand character of the HOMO for 1 by placing conju-
gated substituents on the anionic fragment of the biden-
tate C∧N ligand while holding the balance of the structure
at parity. This investigation is part of our larger effort to
develop chromophores for sensitization applications;
thus, the remaining coordination sphere for each of the
compounds is occupied by two polypyridyl dcbpyH2
ligands, a common linker for binding metal complexes
to semiconducting surfaces in energy conversion schemes
such as the DSSC.
The series of compounds selected for this study include
complexes where the C∧N ligand of 1 contains -NO2
groups positioned about the phenyl ring (i.e., 2-5). These
nitro-substituted derivatives were evaluated to verify that
optimal electronic coupling is achieved by placing the
-NO2 group para to the organometallic bond. We set out
to demonstrate that by extending the HOMO and optical
cross-section of the molecule large ε values could be
produced as a result of the increased charge separation
distance in the excited state;64-66 thus, we furnished the
C∧N ligand of 1 with aromatic substituents to form 7-9.
For the purposes of generating dyes for the DSSC where
I-/I3- is used as the electrolyte, the HOMO energy level
must be appropriately positioned to accommodate a suf-
The C∧N ligands HL2-HL9 are all readily accessible
through carbon-carbon cross-coupling procedures. The
general synthetic protocol for isolating the target com-
plexes 1-9 was achieved using an initial cyclometalation
step to form a cyclometalated intermediate(s) followed by
coordination of the polypyridyl dcbpyH2 ligands. This
strategy offers numerous advantages to the established
method of synthesizing derivatives of 1. For instance, the
more common approach of binding the C∧N ligand to cis-
Ru(dcbpyH2)2Cl2 requires that the metal precursor be
kept in the dark to avoid isomerization to the trans form.
This route also generates homoleptic byproducts that
ultimately lower the yields of reactions involving cis-
Ru(dcbpyH2)2Cl2. Not only are said obstacles overcome
with the synthetic route described herein, but our proto-
col also provides a more efficient pathway for isolating
Ru dyes containing a combination of bidentate polypyr-
idyl and C∧N ligands.
The electrochemical properties of the entire series ex-
hibit an exquisite sensitivity to substituents on the C∧N
ligand. The reversibility of the oxidation wave indicates
that the metal is the primary redox-active component;34
however, the Eox values are particularly responsive to
substituents on the phenyl ring arising from the direct
interaction with the HOMO that is extended over the
ligand. For instance, the presence of one -NO2 group
increases the Eox value by 0.16 V relative to 1a. The
highest Eox value is observed for 3 among the complexes
containing a single -NO2 group because the substituent
is situated para to the organometallic bond. The (quasi-)
irreversibility of the first reduction waves for 2-5 is
consistent with reduction of the -NO2 groups; however,
the first reduction process for 3 may also involve the
dcbpyH2 ligands. This possibility is supported by a quasi-
reversible reduction wave, TD-DFT calculations and
the observation of a weak emission signal for 3a. Our
data also shows that a single -NO2 group increases
the Eox value more than the presence of two -F substit-
uents.
Despite the observation that the LUMO is localized
primarily to the -NO2 group(s), these compounds pro-
vide some promising insight in the context of chromo-
phore design for light-harvesting applications; namely,
the extension of the π-system of the C∧N ligand clearly
leads to a significant increase in ε values. This feature is
(62) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298,
97–102.
(63) Note that the intensity of 1 may be diminished because of poor
solubility.
(67) Hagberg, D. P.; Marinado, T.; Karlsson, K. M.; Nonomura, K.; Qin,
P.; Boschloo, G.; Brinck, T.; Hagfeldt, A.; Sun, L. J. Org. Chem. 2007, 72,
9550–9556.
(68) Koops, S. E.; O’Regan, B. C.; Barnes, P. R. F.; Durrant, J. R. J. Am.
Chem. Soc. 2009, 131, 4808–4818.
(64) Brunschwig, B. S.; Creutz, C.; Sutin, N. Coord. Chem. Rev. 1998, 177,
61–79.
(65) Karki, L.; Lu, H. P.; Hupp, J. T. J. Phys. Chem. 1996, 100, 15637–
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Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4971
reflected by broad absorption bands with ε values that
can be 2-fold greater than that of 1. Compound 3, for
instance, exhibits the most intense profile of the entire
series. This observation emphasizes the importance of
delocalizing the ground-state orbital character as a means
of increasing the optical cross-section of the dye. On this
basis, conjugated substituents were installed para to the
Ru-C bond in pursuit of complexes with significantly
higher intensity absorption bands relative to 1. The poor
solubility of compounds 7-9 led to the preparation of
deprotonated versions 7a-9a to extract accurate ε values.
The profiles for 7a-9a reveal similar profiles to the -NO2
series, but the intensity of the band centered at about 410
nm (i.e., λmax3) increases as the electron-withdrawing
nature of the aromatic moiety increases. Consequently,
9a exhibits the most intense and broad absorption profile
in the visible region of any protonated or deprotonated
complex evaluated in this study.
Although 7a contains an additional aromatic ring
relative to 1, torsional strain between the two phenyl
rings inhibits conjugation resulting in little improvement
in the light-harvesting properties. The Eox value is also
slightly lower than that of 1a; thus, 8a was prepared as a
means of increasing the oxidation potential. While this
approach proved successful, the oxidation potential is
still significantly lower than that of 6 (and 6a), and the
absorption profile was not markedly better than that
of 1a because of the adjacent six-membered rings. The
installation of a thiophene group, however, proved to be a
viable strategy for enhancing conjugation on the basis
that the spectral profile of 9a is more intense than that of
1a. The presence of the aldehyde at the 2-position of the
thiophene also imposes electron-withdrawing character
on the substituent, which is reflected by the higher Eox
value relative to 8a.
electrolyte is I-/I3-): the Eox value is higher than that of
1a; the spectral profile is significantly more intense than
that of 1a and 6a; and the orbital structure is poised for
electron-injection and dye regeneration. Results from this
study demonstrate that expansion of the HOMO is a valid
strategy for producing compounds that may exceed the
performance of benchmark complexes 1 and 6 reported
by Gratzel et al.30 Studies are currently underway to build
€
on these developments and to examine their feasibility in
solar cell devices.
Conclusions
The electrochemical and photophysical properties of a
series of cyclometalated Ru(II) complexes related to 1 were
examined to elucidate the effect of modifying the anionic
fragment of the C∧N ligand with conjugated substituents.
It is shown that the installation of substituents para to the
organometallic bond imparts the greatest influence on the
redox behavior and optical properties. The electron-with-
drawing character of the substituents are reflected by sub-
stantial changes in Eox values, a consequence of extending the
HOMO beyond the phenyl ring of the C∧N ligand. Expan-
sion of the HOMOoverthe substituent is most effective in the
case where torsional strain is reduced (i.e., 9a). In effect, this
strategy produces a larger spectral envelope for 9a relative to
1a. The presence of an EWG on the thiophene also maintains
a higher oxidation potential relative to 1a. These collective
observations provide guiding principles that are critical to the
evolution of cyclometatated Ru(II) chromophores.
Acknowledgment. This work was financially supported
by the Canadian Natural Science and Engineering
Research Council (NSERC), Canada Research Chairs,
Canada Foundation for Innovation, Alberta Ingenuity,
Canada School for Sustainable Energy, and The Institute
for Sustainable Energy, Environment & Economy. P.G.B.
acknowledges Alberta Ingenuity for financial support.
The -NO2 compounds 2-5 are likely not ideal for
sensitizing semiconducting materials because the excited-
states are, in large part, quenched by the strongly electron-
withdrawing substituents. The aromatic substituents of 7a
and 8a did not produce superior photophysical properties
despite the extension of the π system owing to the steric
encumbrance of the adjacent six-membered rings. Com-
pound 9a appears to be a particularly promising candi-
date for the DSSC (where the anode is TiO2 and the
Supporting Information Available: Labeling scheme for ligand
1H NMR assignments, 1H NMR spectra of 1a, 6a, 7a, 9a,
UV-vis of the fully protonated aromatic series, calculated
UV-vis spectra, and a summary of the TD-DFT for all com-
pounds. This material is available free of charge via the Internet
at http://pubs.acs.org.