Electrochemiluminescent ruthenium(II) N-heterocyclic carbene complexes: A combined experimental and theoretical study

Article abstract of DOI:10.1021/ic400263r

A series of four Ru(II) complexes of the form [Ru(bpy)₂(C ^aN)]²⁺ (where C ^aN is a bidentate pyridine-functionalized imidazolylidene- or benzimidazolylidene-based N-heterocyclic carbene (NHC) ligand and bpy is 2,

Full text of DOI:10.1021/ic400263r

Article
pubs.acs.org/IC
Electrochemiluminescent Ruthenium(II) N‑Heterocyclic Carbene
Complexes: a Combined Experimental and Theoretical Study
Gregory J. Barbante,†,‡ Paul S. Francis,‡ Conor F. Hogan,*,† Peyman R. Kheradmand,†
David J. D. Wilson,† and Peter J. Barnard*,†
†Department of Chemistry, La Trobe Institute for Molecular Science, La Trobe University, Victoria 3086, Australia
‡Centre for Chemistry and Biotechnology, Faculty of Science, Engineering and Built Environment, Deakin University, Geelong,
Victoria 3216, Australia
S
* Supporting Information
ABSTRACT: A series of four Ru(II) complexes of the form
[Ru(bpy)2(C∧N)]2+ (where C∧N is a bidentate pyridine-
functionalized imidazolylidene- or benzimidazolylidene-based
N-heterocyclic carbene (NHC) ligand and bpy is 2,2′-
bipyridine) have been synthesized using a Ag(I) trans-
metalation protocol from the Ru(II) precursor compound,
Ru(bpy)2Cl2. The synthesized azolium salts and Ru(II)
1
complexes were characterized by elemental analysis, H and
13C NMR spectroscopy, cyclic voltammetry, and electronic
absorption and emission spectroscopy. The molecular
structures for two benzimidazolium salts and three Ru(II)
complexes were determined by single crystal X-ray diffraction. The complexes display photoluminescence within the range 611−
629 nm, with the emission wavelength of the benzimidazolylidene containing structures, slightly blue-shifted relative to the
imidazolylidene containing complexes. All complexes exhibited a reversible, one-electron oxidation, which is assigned to the
Ru2+/3+ redox couple. When compared to [Ru(bpy)3]2+, complexes of imidazolylidene containing ligands were oxidized at more
negative potentials, while those of the benzimidazolylidene containing ligands were oxidized at more positive potentials. All four
complexes exhibited moderately intense electrochemiluminescence (ECL) with the obtained ECL spectra closely resembling the
photoluminescence spectra. The ability to predictably fine-tune the highest occupied molecular orbital (HOMO) level of the
Ru(II) complexes via the flexible synthetic strategy offered by NHCs is valuable for the design of ECL-based multiplexed
detection strategies.
pyridyl substituted imidazolylidene- and benzimidazolylidene-
based NHC ligands for application as sensitizers in dye-
sensitized solar cells (Figure 1, iii).24,27 These researchers
demonstrated that modification of the NHC ligand type and its
substituents allowed for tuning of the electronic properties,
such that the highest occupied molecular orbital (HOMO) was
stabilized while the lowest unoccupied molecular orbital
(LUMO) was maintained at a favorable level for charge
injection into the conduction band of the TiO2 electrode.
Apart from photoluminescence, electrochemiluminescence
or electrogenerated chemiluminescence (ECL), where certain
species emit light following heterogeneous electron-transfer
reactions, is an area of growing importance.28−31 Typically, in
ECL reactions the excited state is populated when an electron is
transferred from a powerful reductant derived from either the
reduction of the metal complex itself (annihilation ECL) or a
sacrificial reagent (coreactant ECL) to the π* orbital of a ligand
of the oxidized complex. We are interested in the synthesis and
development of new ECL-based sensing materials with varying
INTRODUCTION
■
For several years metal complexes of N-heterocyclic carbene
(NHC) ligands have been the subject of intense research
interest for a variety of applications. Although the greatest focus
has been on the development of NHC-metal complexes as
homogeneous catalysts,1−4 there is growing interest in potential
applications related to the medicinal and luminescent proper-
ties of NHC-metal complexes.5−8 Surprisingly, NHC com-
plexes of relatively few metals have been evaluated for
luminescence properties. Metals that have been studied include
Pt(II),9−14 Ir(III),15−17 Re(I),18,19 and Au(I),20−22 while the
luminescent properties of Ru(II)-NHC complexes have also
occasionally been investigated.23−27 In an early study,23 the
photophysical properties of homoleptic Ru(II) analogues of
[Ru(bpy)3]2+ and [Ru(terpy)2]2+ with pyridine functionalized
bidentate and tridentate NHC ligands (Figure 1, i and ii) were
evaluated. The [Ru(terpy)2]2+ analogue displayed strong
emission with λmax of 532 nm; however, an NHC analogue of
[Ru(bpy)3]2+ was nonemissive.23 More recently, homo- and
heteroleptic25 and proton-sensitive26 Ru(II) complexes of
pincer-type tridentate NHC ligands have been reported. Li
and co-workers have synthesized Ru(II)-NHC complexes with
Received: February 1, 2013
© XXXX American Chemical Society
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Figure 1. Structures of known Ru(II) complexes of pyridyl-functionalized NHC ligands.
emission and redox characteristics for their potential application
in multiplexed sensing. Such applications require fine control
over often opposing electrochemical, spectroscopic, and
physical properties. As demonstrated previously,32 the elec-
tronic properties of tris-diimine Ru(II) complexes can be
modulated by varying substituents on one of the bipyridine
ligands, thus allowing the LUMO energy levels to be tuned
while keeping the HOMO energy relatively constant. However,
tuning of the HOMO level is also desirable, particularly for
applications involving selective excitation on the basis of
oxidation potential.33,34 While such control is readily achieved
with Ir(III) complexes because the HOMO is delocalized over
the metal and ligand,35,36 this is not true for Ru(II) complexes
of diimine ligands, where the HOMO is typically metal based.
In the search for new luminescent and ECL active materials
we have become interested in NHCs. Because of their excellent
framework flexibility, NHC-based ligands are often more easily
prepared than diimine-based systems. Additionally, the
electronic properties of NHC-based ligands can be readily
tuned through the choice of the azole precursor, that is,
benzimidazole, imidazole, imidazoline, and triazole, as well as
the wing-tip substituents.
Scheme 1. Synthesis of Azolium Salts I·I, II·Br, III·I, and
a
IV·Br
a
R-X = methyl iodide or benzyl bromide.
In this paper we report the synthesis of four Ru(II)
complexes incorporating two bipyridine ligands and either a
pyridine-functionalized imidazolylidene or benzimidazolylidene
NHC ligand via a Ag(I) transmetalation protocol. Electro-
chemical and spectroscopic studies show that the choice of
NHC (either imidazolylidene or benzimidazolylidene) has a
subtle yet distinct and predictable influence on the electron
density at the metal center. The luminescence and electro-
chemiluminescence properties of the complexes have also been
investigated. Theoretical studies provide insight into the
bonding and electronic structure of these new complexes. To
our knowledge, despite their prevalence in other areas of
chemistry, NHCs have not previously been utilized in the
development of ECL active metal complexes.
Formation of the desired [Ru(bpy)2(C∧N)]2+ complexes
(where C∧N is a bidentate NHC ligand derived from the
azolium salts I·I, II·Br, III·I and IV·Br) was achieved using a
Ag(I) transmetalation protocol and the ruthenium precursor
compound, Ru(bpy)2Cl2 (Scheme 2). Our initial attempts to
carry out this reaction at ∼60 °C in solvents commonly used
for NHC/Ag(I) transmetalation reactions (e.g., CH2Cl2 and
CH3OH)42 were unsuccessful, with only starting materials
isolated. This is consistent with previous attempts for similar
systems.43 We reasoned that, although Ru(bpy)2Cl2 should be
suitable for the Ag(I) transmetalation protocol, the lack of
reactivity was most likely the result of the slow ligand
substitution rates for the low-spin 4d6 Ru(II) complex.
Therefore, in an effort to increase the reaction rate, a
considerably higher reaction temperature (110 °C) was used
in combination with the high boiling solvent, ethylene glycol.
This approach led to the successful formation of the desired
Ru(II)-NHC complexes, for both the imidazolylidene- and the
benzimidazolylidene-based NHC ligands. The combination of
solvent ethylene glycol with RuCl3·XH2O and high temper-
atures (140−190 °C) has been utilized previously for the
formation of NHC analogues of [Ru(bpy)3]2+ and [Ru-
(terpy)2]2+ and related compounds.23,25,44 The Ag2O trans-
metalation approach to Ru(II)-NHC complexes has also been
successfully used in conjunction with the Ru(II) precursor
compound [RuCl2(cymene)]2.24,43,45
RESULTS AND DISCUSSION
■
Synthesis. The azolium salt precursor, 2-pyridyl substituted
azoles, 1-(2-pyridyl)imidazole and 1-(2-pyridyl)benzimidazole
were prepared via an Ullmann-type Cu(I) coupling reaction
between 2-bromopyridine and the chosen azole.37 Formation of
the desired azolium salts I·I, II·Br, III·I, and IV·Br (Scheme 1)
was achieved by heating the 1-(2-pyridyl)azoles with the
appropriate alkyl halide (methyl iodide or benzyl bromide) in
acetonitrile. The methyl substituted azolium cations I+ and III+
have been described previously, and NHC ligands derived from
them have been utilized in the synthesis of various metal-NHC
complexes.22,23,38−41
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are invariant for these complexes the chemical shift difference
between the imidazolylidene vs benzimidazolylidene-based
NHC ligands may result from greater π-back-donation for the
Ru(II)-benzimidazolylidene complexes. An increase in π-back-
donation of electron density from the metal to the p-orbital of
the carbene carbon could be expected to result in a decrease in
the π-overlap from the electron rich nitrogen atoms adjacent to
the carbene and a corresponding downfield shift in the carbene
resonance. For complexes 2·(PF6)2 and 4·(PF6)2, where the
NHC is substituted with a benzyl group, the protons of the
methylene linker are nonequivalent and an AX pattern is
observed. The methylene protons are enantiotopic and the
nonequivalence appears to result from the chirality associated
with the octahedral tris(bidentate) complexes (a racemic
mixture of the enantiomeric Δ and Λ forms is expected).
Selected geometric parameters for the crystal structures of
III·I, IV·Br, and the Ru(II) complexes: 2·(PF6)2, 3·(PF6)2, and
4·(PF6)2 are collated in Table 1. The X-ray crystal structures of
Scheme 2. Synthesis of Ruthenium(II) Complexes 1·(PF6)2−
a
4·(PF6)2
Table 1. Selected Bond Distances (Å) and Bond Angles
(deg) from X-ray Structures of Precursor Salts III·I and
IV·Br, and Ruthenium(II) Complexes 2·(PF6)2, 3·(PF6)2,
and 4·(PF6)2
III·I
IV·Br
2·(PF6)2
3·(PF6)2
4·(PF6)2
C6−N2
C6−N3
1.340(4)
1.319(4)
1.343(3)
1.317(3)
1.371(5)
1.395(5)
2.004(4)
2.079(4)
2.058(4)
2.056(4)
2.112(3)
2.057(3)
103.8(3)
1.393(3)
1.340(3)
1.974(3)
2.070(2)
2.066(2)
2.069(2)
2.119(2)
2.061(2)
105.3(2)
1.394(4)
1.348(4)
1.983(3)
2.070(3)
2.067(3)
2.068(3)
2.115(3)
2.059(3)
105.1(3)
C6−Ru1
N1−Ru1
N4−Ru1
N5−Ru1
N6−Ru1
N7−Ru1
a
Reagents and conditions: (a) 1. azolium salt, Ag2O in ethyleneglycol,
70°C, 4 h; 2. Ru(bpy)2Cl2 110°C, 17 h; 3. aq. KPF6.
The dicationic complexes were obtained as their hexafluor-
ophosphate salts in moderate yields (24−58%) after recrystal-
lization. In the case of the methyl functionalized azolium salts I+
and III+, it was necessary to exchange the I− anion for PF6− via
a metathasis reaction with potassium hexafluorophosphate prior
to carrying out the Ru(II) complexation reaction. Anion
exchange was required because if I− was present in the reaction
mixture, an insoluble black powder formed, and the desired
compound could not be isolated.
N2−C6−
110.1(2)
110.6(2)
N3
the benzimidazolium salts III·I and IV·Br are illustrated in
Figure 2. For compound III·I a short C6−H···I1 distance of
2.884(2) Å, indicative of a hydrogen bond, is found between
the weakly acidic proton on C6 and the iodide counterion.
Similarly for IV·Br a hydrogen bonding interaction is observed
between the proton on C6 and a bromide counterion
(2.5360(2) Å), in addition to a short contact between the
counterion and a benzylic proton (3.1499(3) Å).
Single crystals of the ruthenium complexes 2·(PF6)2,
3·(PF6)2, and 4·(PF6)2 were grown by slow evaporation of
methanol solutions of each compound. The X-ray crystal
structures of the Ru(II) cations 22+, 32+, and 42+ are shown in
Figure 3. All the molecular structures display a distorted
octahedral coordination geometry for the Ru(II) centers with
two chelating bpy ligands and the bidentate NHC-pyridine
unit. In the case of 42+, disorder was observed in the benzyl
substituent on the NHC ligand. Slight differences were
observed between the Ru1−C6 bond lengths for 22+
(imidazolylidene NHC) (2.004(4) Å) and 32+ and 42+
(benzimidazolylidene NHC) (1.974(3) Å and 1.983(3) Å,
respectively). The Ru−Ccarbene bond distance for the
imidazolylidene-based complex 22+ is similar to or shorter
than those reported previously for related Ru(II) complexes
(e.g., 2.014−2.116 Å,47 2.033(3) Å and 2.054(6) Å,43 2.004(5)
Å24). Comparable Ru(II) complexes with benzimidazolylidene-
based ligands are rare, although for one related complex, the
Ru−Ccarbene bond distance is 1.943 Å.41 A strong trans influence
Characterization. The structure of the azolium salts and
1
the [Ru(bpy)2(C∧N)]2+ complexes were confirmed by H and
13C NMR spectroscopy and in the case of the III·I, IV·Br,
2·(PF6)2, 3·(PF6)2, and 4·(PF6)2 by X-ray crystallography. The
1
azolium salts gave relatively simple H NMR spectra, all with a
characteristic downfield resonance for the strongly deshielded
pro-carbenic proton, which occurred at 10.00, 10.25, 10.44, and
10.81 ppm for I·I, II·Br, III·I, and IV·Br, respectively. For the
benzyl-substituted salts II·Br and IV·Br, the methylene linker
protons appear as singlets at 5.52 and 5.95 ppm, respectively.
1
The predicted number of signals were obtained in the H and
13C spectra for the synthesized [Ru(bpy)2(C∧N)]2+ complexes,
consistent with the low symmetry structures (point group C1).
As expected, upon coordination of the NHC group, the signal
for the azolium salt, pro-carbenic proton was absent from the
1H NMR spectra and a characteristic downfield chemical shift
was observed for the carbenic carbon atom, occurring at 192.4,
194.1, 205.8, and 207.5 ppm for the complexes 1·(PF6)2−
4·(PF6)2 respectively. It is interesting to note the relatively large
difference in the chemical shift for Ccarbene between the
imidazolylidene and the benzimidazolylidene-based NHC
ligands (13.4 ppm). As the metal ion, coligands and anion
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Figure 2. ORTEP46 representations of the X-ray crystal structures of the benzimidazolium salts (a) III·I and (b) IV·Br. Thermal ellipsoids are shown
at 40% probability.
Figure 3. ORTEP46 representations of the X-ray crystal structures of the Ru(II) cations: (a) 22+, (b) 32+, and (c) 42+. The benzyl substituent is
disordered, and the site occupancy factors for the disordered atoms are 0.58 and 0.42. The hexafluorophosphate anions, hydrogen atoms, and
solvents of crystallization have been omitted for clarity. Thermal ellipsoids are shown at 40% probability.
from the carbene is apparent, with a significant elongation of
the Ru1−N6 bond (2.112(3), 2.119(2), and 2.115(3) Å for 22+,
32+, and 42+, respectively) when compared to the average of the
other Ru1−Npyridyl bond distances (2.064, 2.068, and 2.068 Å
for 22+, 32+, and 42+, respectively). The trans influence exerted
by NHCs is well-known and has been described for other
Ru(II)-NHC complexes.24,41,43,47 Comparison of the N2−C6−
N3 angle between the benzimidazolium salts (110.1(2)° and
110.6(2)° for III·I and IV·Br, respectively) and that of the
Ru(II)-benzimidazolylidene complexes (105.3(2)° and
105.1(3)° for 32+ and 42+, respectively) shows that upon
formation and coordination of the carbene this angle becomes
more acute.
of the enantiomeric Δ and Λ forms of the Ru(II) complexes.
For 2·(PF6)2 the structure was solved in the chiral space group
P21 and the asymmetric unit contains two independent
molecules, these being the Δ and Λ forms of the octahedral
complex.
Absorption Spectroscopy. The UV−visible absorbance
spectra for compounds 1·(PF6)2−4·(PF6)2 are similar to the
expected profile for ruthenium polypyridyl complexes such as
[Ru(bpy)3]2+ (see Supporting Information, Figure S1). The
visible region is dominated by broad bands of moderate
intensity, which are metal to ligand charge transfer (MLCT) in
nature, (i.e., d → π* transitions). The UV region on the other
hand is characterized by spin-allowed π−π* ligand-centered
(LC) transitions. The position of the MLCT band for each
complex in acetonitrile is dependent on the identity of the
auxiliary NHC ligand; for 12+ and 22+ it is similar to that of
Compounds 3·(PF6)2 and 4·(PF6)2 crystallized in the
centrosymmetric space groups C2/c and P21/c, respectively,
indicating that the crystals are composed of a racemic mixture
D
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[Ru(bpy)3]2+, while for 32+ and 42+ (benzimidazolylidene) the
MLCT band is slightly blue-shifted (see Table 2). This suggests
Table 2. Spectroscopic Properties of Ru(II) Complexes
a
1·(PF6)2−4·(PF6)2
photoluminescence
b
c
d
λmax/nm; (ε/M−1 cm−1
)
λmax/nm
ϕp
[Ru(bpy)3]
(PF6)2
207, 240, 251, 285, 322, 344,
450(14800)
620
0.095
1·(PF6)2
2·(PF6)2
3·(PF6)2
4·(PF6)2
240(30600), 256(21200,sh),
289(53400), 365(8000),
422(8900), 460(7000)
622
629
611
619
0.010
240(38100), 256(2 6200,sh),
289(70300), 365(11300),
422(12200), 460(10800)
0.021
0.007
0.004
248(27600), 255(26000),
291(54000), 360(7400),
408(9800), 445(7900)
248(27600), 255(25000),
291(49300) 360(7400),
408(9800), 445(7100)
b
a
1 × 10−5 M in acetonitrile. λmax is the position of the peak or
shoulder in the absorbance or emission profile and ε is the molar
c
absorptivity. Emission corrected for variation in detector sensitivity
d
with wavelength. Relative quantum yield (tris(2,2′-bipyridine)-
ruthenium(II) = 0.095) (acetonitrile, RT, deaerated).48
that the benzimidazolylidene NHC moiety in 32+ and 42+ exerts
a moderate stabilizing influence on the energy of the metal
based HOMO in these two complexes relative to the same MO
in [Ru(bpy)3]2+. There is also a small difference in the positions
of the highest energy bands at wavelengths below 260 nm,
which are shifted to slightly higher energy for 12+ and 22+
relative to 32+ and 42+, consistent with a slight destabilization of
the LUMO in the former.
Photoluminescence. The characteristic orange photo-
luminescence from Ru(II) diimine complexes results from the
excitation of an electron from the metal-based d(πM) orbitals to
ligand-based π* antibonding orbitals, a metal-to-ligand charge-
transfer (MLCT), followed by intersystem crossing to the
lowest triplet state, from which the emission occurs. For each of
the complexes 12+−42+, intense luminescence results from the
decay of this excited state. In Figure 4(a) the corrected
photoluminescence spectra in deaerated CH3CN at room
temperature are shown for 1·(PF6)2−4·(PF6)2.
The emission color varies by a relatively small amount
depending on the nature of the substituents. The data
presented in Table 2 shows that there is a range of 18 nm
(469 cm−1) between the shortest and the longest wavelength-
emitting complexes. The two complexes containing the
imidazolylidene-based NHC ligands emit at wavelengths longer
than [Ru(bpy)3]2+, whereas the two benzimidazolylidene
containing complexes emit at wavelengths shorter than the
[Ru(bpy)3]2+ standard. The values of the photoluminescent
quantum yield (Φp) were all lower than that of [Ru(bpy)3]2+,
with 22+ having the highest value at 2.1% (Table 2).
Electrochemistry. The redox properties of 1·(PF6)2−
4·(PF6)2 were studied using cyclic voltammetry and the results
are summarized in Table 3. Figure 5 shows the cyclic
voltammetric response for 4·(PF6)2 dissolved in acetonitrile.
Similar electrochemical behavior is observed for complexes
1·(PF6)2−3·(PF6)2 (Supporting Information, Figure S2).
Several of the voltammetric features observed are typical of
those commonly displayed by ruthenium polypyridyl com-
plexes, such as [Ru(bpy)3]2+.49 When scanned anodically each
Figure 4. (a) Photoluminescence and (b) electrochemiluminescence
spectra of Ru(II) complexes: 1·(PF6)2 (green), 2·(PF6)2 (red),
3·(PF6)2 (blue), and 4·(PF6)2 (black) in acetonitrile. The ECL was
generated using a glassy carbon electrode in a 1 mM solution of the
complex containing 0.1 M [Bu4N][PF6] supporting electrolyte and 10
mM tripropylamine (TPA) as the coreactant.
complex exhibited a reversible, one-electron oxidation process,
which can be assigned to the Ru2+/3+ redox couple (Figure 5).
The data in Table 3 indicates that the E°′ values associated with
the Ru2+/3+ redox couple for complexes 12+ and 22+
(imidazolylidene NHC) were both substantially more negative
than that for [Ru(bpy)3]2+. In contrast, complexes 32+ and 42+
(benzimidazolylidene NHC) gave E°′ values that were more
positive than that for [Ru(bpy)3]2+ (Table 3). The lowering of
the redox potential for 12+ and 22+ is caused by the influence of
the imidazolylidene containing ligands on the metal core,
consistent with the strong σ-donating properties of the NHC
relative to bipyridine as noted in a number of previous
studies.23,25,43,50 Significantly however, this trend is not
followed by the Ru(II)-benzimidazolylidene complexes which
are more difficult to oxidize than [Ru(bpy)3]2+, and suggests
that the phenyl ring of the benzimidazolylidene-based NHC
exerts a significant electron withdrawing influence. When
examined in the context of the previously described 13C
NMR Ccarbene chemical shifts, the electrochemical results
support the notion that the benzimidazolylidene is a stronger
π-accepting ligand than imidazolylidene. That is, benzimidazo-
lylidenes accept electron density from the metal center (π-back-
bonding), thereby increasing the oxidation potential of the
E
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a
Table 3. Electrochemical and Electrochemiluminescence Properties of 1·(PF6)2−4·(PF6)2
b
RuII/RuIII couple E°/V
bpy reductions E°/V
NHC reductions Ep/V ΔE /V 106 D/cm2 s−1 ECL λmax rel. ECL intensity
c
[Ru(bpy)3](PF6)2
1·(PF6)2
2·(PF6)2
3·(PF6)2
4·(PF6)2
0.89
0.85
0.84
0.90
0.92
−1.75, −1.94, −2.18
−1.78, −1.99
−1.78, −1.98
−1.75, −1.95
−1.76, −1.96
2.64
2.62
2.63
2.66
2.68
5.8
620
628
633
613
608
100
68
95
7
−2.56, −2.77
−2.58, −2.79
−2.56, −2.77
4.4
5.83
7.24
5.1
−2.60, −2.80
52
a
b
c
1 mM/0.1 M (Bu4N)PF6 in acetonitrile. Electrochemical HOMO−LUMO gap. Annihilation between [Ru(L)3]3+ and [Ru(L)3]+.
(not shown) are observed at approximately −1 V, which result
from the oxidation of the products formed during the
irreversible reductive processes.
Electrochemiluminescence. All four complexes exhibited
intense to moderately intense electrochemiluminescence
(ECL). To quantify the ability of these complexes to produce
ECL, their relative annihilation ECL intensities were measured
from potential step experiments where the oxidized and
reduced forms were generated sequentially at the working
electrode according to
[Ru(bpy)2(L)]2+ → [Ru(bpy)2(L)]3+ + e−
(1)
[Ru(bpy)2(L)]2+ + e− → [Ru(bpy)2(L)]+
(2)
[Ru(bpy)2(L)]3+ + [Ru(bpy)2(L)]+
→ [Ru(bpy)2(L)]2+* + [Ru(bpy)2(L)]2+
Figure 5. Cyclic voltammetric response for 4·(PF6)2 (1 mM) dissolved
in acetonitrile containing 0.1 M [Bu4N][PF6]. Scan rate = 0.1 V s−1,
glassy carbon disk working electrode (⌀ = 3 mm). The red and blue
traces show the responses when the cathodic switching potential was
made sufficiently negative to encompass the third and fourth reduction
processes respectively.
(3)
[Ru(bpy)2(L)]2+* → [Ru(bpy)2(L)]2+ + hν
(4)
Here L is the bidentate C∧N imidazolylidene- or
benzimidazolylidene-based NHC ligand. For all of the
complexes studied, the ECL annihilation experiments resulted
in emission from the electrode surface that was easily visible
with the naked eye for 1 mM solutions. As shown by the data in
Table 3, compounds 1·(PF6)2, 2·(PF6)2, and 4·(PF6)2 produced
ECL intensities of a slightly lower magnitude to the benchmark
ECL emitter, [Ru(bpy)3]2+. As the electrochemical properties
of the complexes appear uniformly favorable (in terms of
reversibility and redox power), the lower ECL intensities must
stem from the relatively low luminescent quantum yields (see
Table 3) compared with [Ru(bpy)3]2+. Normalized ECL
emission spectra for 1·(PF6)2−4·(PF6)2 (1 mM, acetonitrile)
using 10 mM TPA as the coreactant are shown in Figure 4b,
and the data is summarized in Table 3. The fact that the
emission profiles are essentially identical regardless of whether
optical or electrochemical excitation is employed indicates that
the same excited state is attained in both types of experiment.
Theoretical Studies. The mPW1PW9151,52/SDD,53,54
TZVP55 with acetonitrile SCRF solvent optimized geometries
metal complex. These observations point to a destabilization of
the HOMO by the NHC ligand in 12+ and 22+ but a net
stabilization of the HOMO in 32+ and 42+, which may result
from a greater level of π back-bonding associated with the
benzimidazolylidene NHC ligand. These combined effects
result in a smaller HOMO−LUMO gap for 12+ and 22+
compared with 32+ and 42+, consistent with the luminescence
data presented in Figure 4.
When scanned cathodically, the first reductive processes for
all complexes (12+−42+) are two reversible, one-electron waves
which may be confidently assigned to the stepwise reduction of
the two bipyridine ligands on the basis of the theoretical
calculations presented later, which indicate that the HOMOs of
the bipyridine ligands are lower in energy than that of the NHC
ligands. As was the case for the Ru2+/3+ redox couple, the
potentials of the bpy reductions are influenced by the nature of
the NHC ligand, albeit to a lesser degree. For 12+ and 22+, the
bpy-centered reduction potentials are more negative than those
of [Ru(bpy)3]2+, whereas the reductions associated with 32+
and 42+ (Table 3) do not differ significantly from this complex.
These results show that the electron donating imidazolylidene
ligands moderately destabilize the bipyridine based LUMO, but
that the benzimidazolylidenes have little or no effect on this
molecular orbital. In each case the LUMO is centered on the
bpy ligand.
of the ruthenium complexes 12+−42+ (omitting PF6 counter-
−
ions) are in excellent agreement with the X-ray structures
(Table 1), with differences between calculated and exper-
imental Ru−N and Ru−C bond distances all less than 0.03 Å
(average deviation of 0.02 Å).
The mPW1PW91 calculated bond distances and Wiberg
bond indices (WBI) for Ru−N and Ru−C reflect the difference
between the imidazolylidene (12+and 22+) and benzimidazoly-
lidene (32+and 42+) containing complexes (see Table 4). In
12+and 22+, the Ru−C6 bond distance is slightly longer than
those for 32+and 42+, and this small difference is reflected in the
WBI values (indicative of bond order). In contrast, there is no
appreciable difference in Ru−N5 and Ru−N6 bond distances
When scanned to more negative potentials, two irreversible
reductive processes are observed which are assigned to
reduction of the NHC ligand (see Figure 5, red and blue
traces). On the reverse (positive-going) scan after these more
negative potential excursions, a series of overlapping features
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The HOMO of the bipyridine ligand (−6.9 eV) is significantly
lower lying than the carbon σ lone-pair orbitals of the
imidazolylidene (−5.6 eV) and benzimidazolylidene (−5.8
eV) ligands, with the R substituent having little effect on the
energies of valence orbitals. That is, the NHC σ-donor orbitals
are more accessible than those of bipyridine. The relative
positions of the bipyridine and NHC ligand reductions in the
electrochemistry is consistent with this analysis, in that the
HOMO of the bipyridine ligand is lower lying than those of the
NHC ligands. The energy levels indicate that the σ-donor
strength of the ligands follows the trend of imidazolylidene >
benzimidazolylidene > bipyridine. It has previously been noted
that NHCs have stronger σ-donating properties than
bipyridine,23,25,43,50 which is rationalized by the analysis
presented here.
Table 4. mPW1PW91 Calculated Bond Distances and
Wiberg Bond Indices (WBI) for Complexes 12+ to 42+
a
Ru−C6
bond (Å)
Ru−N6
bond (Å)
Ru−N5
bond (Å) WBI
WBI
WBI
12+
22+
32+
42+
2.018
2.014
1.993
2.005
0.72
0.72
0.76
0.76
2.152
2.138
2.137
2.146
0.31
0.31
0.31
0.31
2.086
2.078
2.075
2.074
0.44
0.44
0.44
0.44
a
mPW1PW91/SDD,TZVP optimized geometries, SCRF acetonitrile
solvent. Atom labeling shown in Figure 3.
and WBI values between the imidazolylidene (12+and 22+) and
benzimidazolylidene (32+and 42+) containing complexes.
The well-known trans influence24,41,43,47 from the NHC
group is replicated in the theoretical results, with calculated
Ru1−N6 bond distances (2.138, 2.137, and 2.146 Å for 22+, 32+,
and 42+, respectively) significantly longer that the average of the
other Ru1−Npyridyl bond distances (2.087, 2.097, 2.082 Å for
22+, 32+, and 42+, respectively).
To understand the bonding and properties of the complexes,
the separate ligands are discussed first.56 Table 5 shows the
HOMOs and LUMOs of the separate ligands. Note that the
geometries were not optimized separately but were retained at
the structure of the optimized metal−ligand complex, since
optimization of free ligands led to nonplanar ligand geometries.
The HOMOs of the ligands correspond to σ lone-pair orbitals
on the carbon of the NHC and nitrogen of the pyridine ring.
Representative plots of the valence MOs of 22+ are given in
Figure 6. The MO plots for 12+, 32+, and 42+ are provided as
Supporting Information (Figures S3−S5), along with a
summary of contributions from metal and ligand fragments
to the HOMO and LUMO (Supporting Information, Figure
S6). These confirm that the HOMO, HOMO-1, and HOMO-2
are largely metal based and, in accord with the interpretation of
the electrochemical data presented in Table 3, that the LUMO
and LUMO+1 are associated with the bipyridine ligands, while
the LUMO+2 is associated with the NHC ligand.
Calculated energies of valence orbitals of complexes 12+−42+
and the reference compound [Ru(bpy)3]2+ are presented in
Table 6. The energy of the HOMOs of 32+ and 42+ are lower
a
Table 5. DFT Valence Orbitals (mPW1PW91/SDD,TZVP) of the Ligands in 12+−42+
a
Orbital energies are given below in eV. SCRF solvent calculation with acetonitrile solvent. Geometries of ligands are taken from 12+−42+ as listed.
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(ILCT) transitions. For 12+and 42+ an intense peak is predicted
at about 271 nm, which is best described as ILCT, centered on
the bpy ligands. In 32+and 42+ there is an additional peak at 269
nm, which is a heavily mixed ML-IL charge transfer transition.
It is suggested that this additional transition contributes to the
observed differences in the absorption bands of 12+and 22+
compared to 32+and 42+ in this region of the spectrum. A peak
at about 400 nm is associated with a MLCT transition,
dominated by HOMO-2 to LUMO/LUMO+1 transitions,
where the LUMO and LUMO+1 are predominantly centered
on the bipyridine ligands.
Finally, an analysis of natural bonding orbitals (NBO)
provides evidence of the increased π back-bonding in the
benzimidazolylidene complexes. Second-order perturbation
analysis of the Fock matrix indicates greater stabilization energy
is obtained by donation from the Ru atom to the NHC ligand
in the case of the benzimidazolylidene complexes (32+and 42+).
Figure 6. Valence molecular orbitals of 22+ (mPW1PW91/SDD,TZVP
in acetonitrile PCM-SCRF).
than those for 12+ and 22+, and are in closer agreement to the
energy of the HOMO of [Ru(bpy)3]2+. It is suggested that the
benzimidazolylidene exerts a greater stabilizing influence on the
metal-based HOMO than does the imidazolylidene. The blue-
shifted MLCT bands from the absorption spectra of 32+ and 42+
are consistent with the benzimidazolylidene exerting a
stabilizing influence on the metal-based HOMO. The energy
of the LUMO is higher for 12+ and 22+ compared to 32+ and
42+, although the variation is much smaller than with the
HOMO energies. There is a consistent trend reproduced by
several density functionals in that the HOMO−LUMO energy
gap of 12+−22+ is less than that of 32+−42+ (Table 6). The larger
HOMO−LUMO gap for 32+ and 42+ (consistent with the
analysis of luminescence spectra) predominantly arises from the
lower HOMO energies of 32+ and 42+.
CONCLUSIONS
■
A series of [Ru(bpy)2(C∧N)]2+ complexes (where C∧N is a
pyridine-functionalized imidazolylidene- or benzimidazolyli-
dene-based NHC ligand) were synthesized using a Ag(I)
transmetalation protocol. These complexes display moderate to
intense electrochemiluminescence (ECL) and represent the
first examples of ECL active metal-NHC complexes reported.
As the metal ion, co-ligands, and anion are invariant for this
series of compounds, they represent a useful system for probing
the influence of the imidazolylidene versus benzimidazolylidene
NHC on the electronic properties of the complex ions. The
electrochemical, spectroscopic, and theoretical studies reveal
that the chosen NHC group (imidazolylidene vs benzimidazo-
lylidene) have a subtle yet distinct and predictable influence on
the electron density at the metal center. The maximum
emission wavelength for both the photoluminescence and the
electrochemiluminescence spectra shows a red-shift for
complexes 1·(PF6)2 and 2·(PF6)2 relative to [Ru(bpy)3]2+
while 3·(PF6)2 and 4·(PF6)2 are blue-shifted relative to this
complex. In conjunction with the electrochemical findings,
these data suggest that the red-shift in emission color for
1·(PF6)2 and 2·(PF6)2 occurs as a result of a narrowing of the
HOMO−LUMO gap caused by destabilization of the HOMO.
This is confirmed by the DFT calculations which show that the
HOMO is significantly higher in energy for the two
imidazolylidene containing complexes. The electrochemical
data also suggests that the imidazolylidene NHC also has a
slight destabilizing influence on the LUMO in 1·(PF6)2 and
2·(PF6)2, which would be expected to have the opposite effect
on the emission energy. However, the DFT calculations show
that this effect is small relative to the HOMO destabilization.
The blue-shift in the emission maxima observed for 3·(PF6)2
and 4·(PF6)2 is also in accord with the electrochemical results
For complexes 12+ and 22+ the destabilization of the HOMO
relative to [Ru(bpy)3]2+ dominates relative to the smaller
LUMO destabilization, resulting in a net contraction of the
HOMO−LUMO gap and a red shift in the luminescence for
these complexes. The blue shift observed in the luminescence
spectra of complexes 32+ and 42+ relative to [Ru(bpy)3]2+ is
primarily due to a stabilization of the HOMO. Figure S7
(Supporting Information) which compares the variance from
[Ru(bpy)3]2+ in the HOMO and LUMO energies (calculated
using M06 functional) for each complex, illustrates these trends
most clearly. The results of the theoretical studies are also in
excellent agreement with the electrochemical data presented in
Table 3 where, for example, the HOMO stabilization in 32+ and
42+ is manifested as a positive shift in oxidation potential.
Calculation of electronic excitations with TD-DFT
(mPW1PW91) supports the above discussion of the spectro-
scopic and electrochemical results, with transitions in the visible
region best described as MLCT, and transitions in the UV
region of the spectrum dominated by intraligand charge transfer
a
Table 6. DFT Calculated HOMO, LUMO, and HOMO-LUMO Energies (eV)
mPW1PW91
LUMO
B3LYP
LUMO
M06
HOMO
ΔE
HOMO
ΔE
HOMO
LUMO
ΔE
[Ru(bpy)3]2+
−6.50
−6.36
−6.37
−6.47
−6.49
−2.71
−2.64
−2.65
−2.67
−2.71
3.79
3.73
3.72
3.80
3.79
−6.21
−6.09
−6.09
−6.19
−6.21
−2.84
−2.77
−2.78
−2.80
−2.83
3.37
3.32
3.31
3.39
3.38
−6.29
−6.23
−6.25
−6.32
−6.36
−2.67
−2.62
−2.65
−2.65
−2.69
3.62
3.61
3.60
3.67
3.67
12+
22+
32+
42+
a
SDD,TZVP basis set and effective core potential with acetonitrile PCM SCRF.
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Table 7. Crystal Refinement Data
III·I
C13H12N3I
IV·Br
2·(PF6)2
3·(PF6)2
4·(PF6)2
empirical formula
formula weight
temperature/K
crystal system
space group
a/Å
(C19H16N3Br)2 (C2H3N) C35H29N7Ru(PF6)2
C33H27N7ORu(PF6)2
928.63
C50H34N7ORu(PF6)2
1019.75
337.16
773.57
938.66
173
173
173
173
173
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21
monoclinic
C2/c
monoclinic
P21/c
14.7507(3)
12.1244(3)
7.18753(15)
90
14.0030(2)
17.7221(2)
19.8032(2)
90
12.5569(3)
14.8590(3)
20.3832(4)
90
40.444(8)
10.140(2)
19.104(4)
90
9.79520(10)
18.7887(3)
22.5522(3)
90
b/Å
c/Å
α/deg
β/deg
92.997(2)
90
133.8030(10)
90
104.3169(19)
90
113.84(3)
90
106.6800(10)
90
γ/deg
volume/Å3
1283.69(5)
4
3546.85(7)
4
3685.03(12)
4
7166(2)
3975.84(9)
4
Z
8
ρcalc mg/mm3
m/mm−1
1.745
1.449
1.692
1.721
1.704
19.443
3.206
0.612
0.63
0.576
F(000)
656
1576
1880
3712
2052
crystal size/mm3
wavelength/Å
θ range collected
index ranges
0.07 × 0.07 × 0.02
Cu−Kα 1.54184
6 to 147.64°
−18 ≤ h ≤ 18
−14 ≤ k ≤ 14
−6 ≤ l ≤ 8
4795
0.1 × 0.05 × 0.05
Cu−Kα 1.54184
7.94 to 140.12°
−15 ≤ h ≤ 17
−12 ≤ k ≤ 21
−24 ≤ l ≤ 21
14158
0.18 × 0.18 × 0.18
Mo−Kα 0.71073
5.86 to 52.04°
−13 ≤ h ≤ 15
−18 ≤ k ≤ 18
−24 ≤ l ≤ 25
22847
0.18 × 0.18 × 0.08
Mo−Kα 0.71073
6.22 to 52.04°
−49 ≤ h ≤ 49
−12 ≤ k ≤ 12
−23 ≤ l ≤ 23
47151
0.15 × 0.07 × 0.05
Mo−Kα 0.71073
5.32 to 54.2°
−12 ≤ h ≤ 12
−23 ≤ k ≤ 24
−28 ≤ l ≤ 28
43041
reflections collected
independent reflections
data/restraints/parameters
goodness-of-fit on F2
2513 [R(int) = 0.0229] 6701 [R(int) = 0.0201] 12928 [R(int) = 0.0203] 7064 [R(int) = 0.0224] 8736 [R(int) = 0.0264]
2513/0/155
1.033
6701/0/443
1.036
12928/1/1027
1.023
7064/0/516
1.061
8736/20/564
1.033
final R indexes [I > 2σ (I)] R1 = 0.0292
R1 = 0.0324
wR2 = 0.0889
R1 = 0.0355
wR2 = 0.0917
0.912/−0.828
R1 = 0.0321
wR2 = 0.0730
R1 = 0.0377
wR2 = 0.0767
0.540/−0.339
R1 = 0.0345
wR2 = 0.0876
R1 = 0.0382
wR2 = 0.0909
1.207/−0.765
R1 = 0.0478
wR2 = 0.1292
R1 = 0.0557
wR2 = 0.1362
1.363/−0.941
wR2 = 0.0757
final R indexes [all data]
R1 = 0.0308
wR2 = 0.0770
largest diff. peak/hole/e Å−3 0.827/−0.611
Australia. All compounds were prepared in air unless otherwise
specified. Mass sprectra were obtained using a Bruker Esquire6000
mass spectrometer fitted with an Agilent electrospray (ESI) ion source.
UV−visible spectra were recorded using a Varian Cary 50 Bio UV−
visible spectrophotometer using quartz cuvettes (1 cm). Fluorescence
spectra were recorded on a Varian Cary Eclipse spectrofluorimeter (5
nm band-pass, 1 nm data interval, PMT voltage: 600 V) using quartz
cuvettes (1 cm).
which show that the HOMO is stabilized when compared to
[Ru(bpy)3]2+. The influence of the benzimidazolylidene-based
NHC ligand on the properties of 3·(PF6)2 and 4·(PF6)2 can be
rationalized as being an effect related to π back-bonding, where
back-donation of electron density from the metal to the ligand,
mitigates the strong sigma donating properties commonly
found for NHCs. The complexes exhibit moderately intense
photoluminescence and compare well with [Ru(bpy)3]2+ in
terms of their electrochemiluminescence intensity. The ability
to predictably fine-tune the HOMO levels using this flexible
synthetic strategy is valuable for the design of ECL-based
multiplexed detection strategies. Given the framework flexibility
displayed by NHCs, and the decreased influence that the
wingtip substituent (methyl and benzyl) have on the redox and
luminescent properties of the complexes, we believe that
luminescent Ru(II)-NHC complexes offer significant scope for
the development of luminescent probes for biological imaging
applications and for sensor development.
Photoluminescence quantum yields (Φ) were determined from
2
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞
⎟
⎠
η
Gradx
x
⎜
⎜
⎟
⎟
Φ = Φ
x
ref
η2
Grad
ref
(5)
ref
where ref denotes the reference complex [Ru(bpy)3][PF6]2 (0.095 in
CH3CN)48 and x denotes a sample complex, Φ is the quantum yield,
Grad is the gradient of the absorbance vs integrated emission intensity
graph (at least five different concentrations were used with absorbance
below 0.1 and achieving r2 close to 1), and η is the refractive index of
the solvent. Quantum yield determinations were conducted at room
temperature (21 3 °C). All solutions were thoroughly degassed with
nitrogen in septum sealed quartz cells prior to measurements.
Electrochemistry. Electrochemical experiments were performed
using an AUTOLAB type II electrochemical station potentiostat (MEP
Instruments, North Ryde, NSW, Australia) with General Purpose
Electrochemical Systems (GPES) software (version 4.9). For electro-
chemical characterization a conventional three-electrode configuration,
consisting of a 3 mm diameter glassy carbon working electrode
shrouded in Teflon (CH Instruments, Austin, TX, U.S.A.), a 1 cm2
platinum gauze auxiliary electrode and a silver wire quasi reference
electrode. The Ru(II) complexes were prepared at a concentration of 1
mM in freshly distilled acetonitrile, with 0.1 M [Bu4N][PF6] as the
EXPERIMENTAL SECTION
■
General Procedures. All reagents were purchased from Sigma-
Aldrich or Alfa Aesar and were of analytical grade or higher and were
used without further purification unless otherwise stated. Dry CH3CN
and tetrahydrofuran (THF) were distilled from CaH2 and sodium
benzophenone ketyl under nitrogen, respectively. NMR spectra were
1
recorded using a Bruker ARX-300 (300.14 MHz for H, 75.48 MHz
for 13C) NMR spectrometer and were referenced to solvent
resonances. Microanalyses were performed by the Microanalytical
Laboratory at the ANU Research School of Chemistry, Canberra,
I
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supporting electrolyte. Prior to each experiment, the working electrode
was polished using 0.3 μm and then 0.05 μm alumina with water on a
felt pad, rinsed with Milli-Q water followed by a final rinse with freshly
distilled acetonitrile and dried with a stream of argon. Scan rates
ranging from 0.01 to 0.5 V s−1 were used in cyclic voltammetry to
evaluate diffusion coefficients (D) using the Randles Sevci̧ k equation.
Potentials were referenced to the ferrocene/ferrocenium couple
measured in situ (1 mM) in each case.
orbital analysis was carried out with the AOMix program71 and NBO
5.9.72
Synthesis. 1-(2-Pyridyl)imidazole. This compound was prepared
using a modified literature procedure.37 A mixture of imidazole (3.4 g,
50 mmol), 2-bromopyridine (4.9 mL, 50 mmol), potassium tert-
butoxide (7.8 g, 70 mmol), CuI (0.48 g, 2.5 mmol), benzotriazole (0.6
g, 5 mmol), and DMSO (50 mL) was heated at 110 °C for 14 h under
N2. Water (50 mL) was added, and the mixture was extracted with
ethyl acetate (3 × 100 mL). The organic extracts were washed with
brine (100 mL), dried with Na2SO4, and the solvent removed under
reduced pressure. After purification on silica, with ethyl acetate/hexane
(3:1 v/v) as the eluent, the product was obtained as a pale orange
crystalline solid (Yield: 4.1 g, 60%). 1H NMR (d6-DMSO): δ = 8.49 (s,
1H), 8.45−8.47 (m, 1H), 7.92−7.99 (m, 2H), 7.76 (d, J = 7.5 Hz,
1H), 7.31 (dd, J = 4.4, 3.9 Hz, 1H), 7.09 (s, 1H). 13C NMR (d6-
DMSO): δ = 148.9, 148.7, 139.8, 135.0, 130.1, 122.4, 116.6, 112.8.
1-(2-Pyridyl)benzimidazole. This compound was prepared as
described for 1-(2-pyridyl)imidazole, from 2-bromopyridine (9.8 mL,
100 mmol), benzimidazole (11.8 g, 100 mmol), potassium tert-
butoxide (15.6 g, 140 mmol), CuI (0.96 g, 5 mmol) and benzotriazole
(1.2 g, 10 mmol) and DMSO (100 mL). After purification on silica,
with ethyl acetate/hexane (1:1, v/v) as the eluent, the product was
obtained as a brown oil (Yield: 7.6 g, 39%). 1H NMR (d6-DMSO): δ =
8.90 (s, 1H), 8.59−8.62 (m, 1H), 8.26 (d, J = 7.2 Hz, 1H), 8.01−8.04
(m, 1H), 7.90 (d, J = 8.4 Hz, 1H), 7.72−7.75 (m, 1H), 7.29−7.43 (m,
3H). 13C NMR (d6-DMSO): δ = 150.0, 149.1, 144.5, 142.2, 139.7,
132.0, 124.0, 123.1, 122.1, 119.9, 114.7, 113.9.
Electrochemiluminescence (ECL). Relative ECL efficiencies
(ΦECL) were evaluated, using a 3 mm diameter glassy carbon
electrode, by comparison of the ECL spectra with that of the standard
(1 mM [Ru(bpy)3]2+ and 0.1 M [Bu4N][PF6] in acetonitrile) = 100%.
Annihilation ECL was generated using chronoamperommetry where
the cathodic and anodic potentials were stepped for 1.0 s and
underwent 10 cycles. An overpotential of 0.1 V was used to generate
the 3+ and 1+ forms of the Ru(II) complexes in the annihilation
reactions. Co-reactant ECL were generated using a 3 mm diameter
glassy carbon electrode in a 1 mM solution of the complex containing
0.1 M [Bu4N][PF6] supporting electrolyte and 10 mM tripropylamine
(TPA) as the coreactant. ECL spectra were obtained using an Ocean
Optics CCD, model QE6500, UV/vis fiber optic (length 1.00 m) with
a HR 4000 Breakout box trigger in conjunction with a PGstat 12
AUTOLAB potentiostat. The electrochemical cell was encased in a
custom-built light-tight faraday cage.
X-ray Crystallography. Single crystals of benzimidazolium salts
III·I and IV·Br were grown by slow evaporation of acetonitrile
solutions of each compound. Single crystals of the ruthenium
complexes 2·(PF6)2, 3·(PF6)2, and 4·(PF6)2 suitable for X-ray
diffraction studies were grown by slow evaporation of methanol
solutions of each compound. Crystallographic refinement data for all
structures determined are given in Table 7. For all samples, crystals
were removed from the crystallization vial and immediately coated
with paratone oil on a glass slide. A suitable crystal was mounted in
paratone oil on a glass fiber and cooled rapidly to 173 K in a stream of
cold N2 using an Oxford low temperature device. Diffraction data were
measured using an Oxford Gemini diffractometer mounted with Mo−
Kα λ = 0.71073 Å and Cu−Kα λ = 1.54184. Data were reduced and
corrected for absorption using the CrysAlis Pro program.57 The
SHELXL97 program58 was used to solve the structures with Direct
Methods, with refinement by the Full-Matrix Least-Squares refinement
techniques on F2. The non-hydrogen atoms were refined anisotropi-
cally and hydrogen atoms were placed geometrically and refined using
the riding model. Coordinates and anisotropic thermal parameters of
all non-hydrogen atoms were refined. All calculations were carried out
using the program Olex2.59 Images were generated by using ORTEP-
3.46 Further XRD details are provided in the Supporting Information.
CCDC 922496−922500 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif
Theoretical Calculations. Density functional theory (DFT)
calculations were carried out within the Gaussian 09 suite of
programs.60 Ground state geometries were optimized in the absence
of solvent with B3LYP61−63 and mPW1PW9151,52 functionals in
conjunction with the 6-31+G(d) basis set64−66 for nonmetal atoms
and the LANL2DZ basis set and core potential for ruthenium.53,67
Only mPW1PW91 geometry results are presented since it has been
shown previously that this functional yields reliable results.32,68 Single-
point energy calculations were carried out at the 6-31+G(d)/
LANL2DZ optimized geometries using the SDD basis and core
potential (MWB)53,54 for ruthenium and the TZVP basis set55 for all
other atoms. The polarizable continuum model (PCM)69 self-
consistent reaction field (SCRF) was used to model solvent effects
at the gas-phase optimized geometries with a solvent of acetonitrile,
consistent with the experimental system. Frontier MO energies were
calculated using DFT MOs with mPW91PW91, B3LYP, BP86 and
M06. Excitation energies to singlet and triplet excited states were
investigated with TD-DFT70 with 40 states calculated. An SCF
convergence criteria of 10−8 a.u. was employed throughout. Molecular
I·I. A mixture of 1-(2-pyridyl)imidazole (1 g, 6.9 mmol) and CH3I
(1 g, 7 mmol) in CH3CN (80 mL) was refluxed for 19 h under N2.
The solvent was removed under reduced pressure, and the crude
product recrystallized from CH3CN and ether. The product was
obtained as a white crystalline solid (Yield: 1.41 g, 71%). 1H NMR (d6-
DMSO): δ = 10.00 (s, 1H), 8.62 (m, 1H), 8.46 (dd, J = 2.1, 1.8 Hz,
1H), 8.19 (dd, J = 6.3, 5.7 Hz, 1H), 7.93−7.99 (m, 2H), 7.59 (dd, J =
4.2, 2.1 Hz, 1H), 3.94 (s, 3H). 13C NMR (d6-DMSO): δ = 149.3,
146.4, 140.7, 135.6, 125.3, 124.9, 119.1, 114.2, 36.5. ESI-MS: m/z =
160.1 (I+) calcd. For C9H10N3 = 160.09.
I·PF6. Aqueous solutions of I·I (0.2 g, 7 mmol) and KPF6 (0.15 g, 8
mmol) were mixed and left to stand for 0.5 h. The white crystalline
precipitate of I·PF6 was collected by vacuum filtration (Yield: 0.2 g,
1
93%). H NMR (d6-DMSO): δ = 10.00 (s, 1H), 8.60−8.62 (m, 1H),
8.45 (dd, J = 2.1, 1.8 Hz, 1H), 8.18 (dd, J = 6.3, 5.7 Hz, 1H), 7.92−
7.97 (m, 2H), 7.61 (dd, J = 4.2, 2.1 Hz, 1H), 3.94 (s, 3H). ESI-MS: m/
z = 160.1 (I+) calcd. For C9H10N3 = 160.09.
II·Br. This compound was prepared as described for I·I, from 1-(2-
pyridyl)imidazole (1.0 g, 7 mmol) and benzylbromide (0.8 mL, 7
mmol). The crude product was obtained as a brown oil and was
purified by trituration with acetone (Yield: 1.66 g, 72%). 1H NMR (d6-
DMSO): δ = 10.25 (s, 1H), 8.61−8.63 (m, 1H), 8.51 (dd, J = 2.1, 1.8
Hz, 1H), 8.18 (dd, J = 6.3, 6 Hz, 1H), 8.02 (d, J = 6 Hz, 2H), 7.61 (dd,
J = 4.8, 4.2 Hz, 1H), 7.47−7.51 (m, 2H), 7.36−7.44 (m, 3H), 5.52 (s,
2H, CH2).13C NMR (d6-DMSO): δ = 149.3, 146.5, 140.6, 135.2,
134.5, 129.1, 128.9, 128.6, 125.3, 123.6, 119.9, 114.4, 52.6. Found: C,
53.83; H, 4.62; N, 12.57%. C15H14N3Br.H2O requires C, 53.91; H,
4.83; N, 12.57%. ESI-MS: m/z = 236.1 (II+) calcd. For C15H14N3 =
236.12.
III·I. This compound was prepared as described for I·I, from 1-(2-
pyridyl)benzimidazole (1.5 g, 7.7 mmol) and CH3I (1.1 g, 7.7 mmol).
The product was obtained as a white crystalline solid (Yield: 1.5 g,
1
58%). H NMR (d6-DMSO): δ = 10.44 (s, 1H), 8.74−8.76 (m, 1H),
8.41−8.45 (m, 1H), 8.25 (dd, J = 6.3, 5.7 Hz, 1H), 8.08−8.12 (m,
1H), 8.01 (d, J = 8.1 Hz, 1H), 7.66−7.79 (m, 3H), 4.17 (s, 3H). 13C
NMR (d6-DMSO): δ = 149.6, 147.4, 142.8, 140.7, 132.4, 129.4, 127.8,
127.2, 125.1, 116.9, 115.8, 114.0, 33.9. ESI-MS: m/z = 210.1 (III+)
calcd. For C13H12N3 = 210.10.
III·PF6. This compound was prepared as described for I·PF6 from
III·I (0.2 g, 0.6 mmol) and KPF6 (0.13 g, 0.7 mmol) (Yield: 0.2 g,
1
94%). H NMR (d6-DMSO): δ = 10.42 (s, 1H), 8.74−8.76 (m, 1H),
8.42−8.45 (m, 1H), 8.23 (dd, J = 6.3, 5.7 Hz, 1H), 8.08−8.12 (m,
J
dx.doi.org/10.1021/ic400263r | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1H), 7.98 (d, J = 8.1 Hz, 1H), 7.66−7.79 (m, 3H), 4.17 (s, 3H). ESI-
MS: m/z = 210.1 (III+) calcd. For C13H12N3 = 210.10.
J = 7.5 Hz, 2H), 5.52 (d, J = 17.4 Hz, 1H), 4.92 (d, J = 17.7 Hz, 1H).
13C NMR (d6-DMSO): δ = 207.5, 156.1, 155.8, 154.4, 151.3, 150.7,
150.3, 148.2, 140.1, 139.2, 138.1, 137.7, 137.3, 136.5, 135.6, 131.5,
128.3, 128.1, 127.5, 127.4, 127.1, 124.8, 124.6, 124.4, 124.2, 123.7,
122.8, 113.6, 112.5, 111.0, 49.0. Found: C, 46.93; H, 2.98; N, 9.73%.
C39H31F12N7P2Ru.·H2O requires C, 46.53; H, 3.30; N, 9.74%. ESI-MS:
m/z = 844.1 ([4·PF6]+) calcd. For C39H31F6N7PRu = 844.13.
IV·Br. This compound was prepared as described for I·I, from 1-(2-
pyridyl)benzimidazole (1 g, 5 mmol) and benzyl bromide (0.61 mL, 5
mmol). The product was obtained as a white crystalline solid (Yield:
1.47 g, 80%). 1H NMR (d6-DMSO): δ = 10.81 (s, 1H), 8.75−8.77 (m,
1H), 8.44−8.47 (m, 1H), 8.27 (dd, J = 6.3, 5.7 Hz, 1H), 8.12 (d, J =
8.1 Hz, 1H), 7.97 (d, J = 4.5 Hz, 1H), 7.64−7.74 (m, 3H), 7.61 (d, J =
1.2 Hz, 2H), 7.30−7.43 (m, 3H), 5.95 (s, 2H). 13C NMR (d6-DMSO):
δ = 149.4, 147.5, 142.8, 140.6, 133.6, 131.3, 129.8, 129.0, 128.9, 128.8,
128.6, 127.8, 127.3, 125.2, 117.2, 116.3, 114.4, 50.5. Found: C, 62.64;
H, 4.60; N, 11.67%. C19H16N3Br requires C, 62.31; H, 4.40; N,
11.47%. ESI-MS: m/z = 286.1 (IV+) calcd. For C19H16N3 = 286.13.
1·(PF6)2. A mixture of I·PF6 (0.2 g, 0.66 mmol) and Ag2O (0.23 g,
0.94 mmol) in ethylene glycol (10 mL) was heated in the dark at 70
°C for 4 h under N2. Ru(bpy)2Cl2 (0.32 g, 0.66 mmol) was added and
the temperature increased to 110 °C; this temperature was maintained
for 17 h. The hot reaction mixture was filtered through Celite, and
water (100 mL) and KPF6 (0.78 g, 4.2 mmol) were added to the
filtrate. After 1 h the orange precipitate was collected and recrystallized
from methanol giving the product as a dark orange solid (Yield: 0.33 g,
58%). 1H NMR (d6-DMSO): δ = 8.67−8.79 (m, 4H), 8.52 (d, J = 2.1
Hz, 1H), 8.02−8.26 (m, 6H), 7.93 (d, J = 4.8 Hz, 1H), 7.75 (d, J = 5.1
Hz, 1H), 7.61 (dd, J = 6.6, 5.1 Hz, 2 H), 7.38−7.56 (m, 6 H), 7.24
(dd, J = 6.3, 6 Hz, 1H), 2.99 (s, 3H). 13C NMR(d6-DMSO): δ = 192.4,
156.5, 156.3, 155.2, 155.0, 153.8, 151.3, 151.1, 150.3, 148.9, 139.8,
138.8, 137.7, 137.5, 137.4, 128.2, 128.0, 127.7, 127.4, 126.3, 124.5,
124.3, 124.2, 123.2, 117.4, 112.4, 35.3. Found: C, 40.50; H, 3.07; N,
11.42%. C29H25F12N7P2Ru requires C, 40.38; H, 2.92; N, 11.37%. ESI-
MS: m/z = 718.1 ([1·PF6]+) calcd. For C29H25F6N7PRu = 718.09.
2·(PF6)2. This compound was prepared as described for 1·(PF6)2
from II·Br (0.218 g, 0.69 mmol), Ag2O (0.22 g, 0.94 mmol) and
Ru(bpy)2Cl2 (0.24 g, 0.49 mmol). The product was obtained as an
orange solid after recrystallization from a mixture of methanol and
water. (Yield: 0.28 g, 33%). 1H NMR (d6-DMSO): δ = 8.70−8.75 (m,
3H), 8.35 (d, J = 8.4 Hz, 1H), 8.06−8.20 (m, 6H), 8.05 (d, J = 5.7 Hz,
1H), 8.01 (t, J = 7.2 Hz, 1H), 7.82 (t, J = 7.5 Hz, 1H), 7.72 (d, J = 2.4
Hz, 1H), 7.63 (d, J = 5.1 Hz, 1H), 7.44−7.54 (m, 4H), 7.37 (d, J = 4.8
Hz, 1H), 7.32 (t, J = 6.3 Hz, 1H), 7.25 (t, J = 6.3 Hz, 1H), 7.05 (t, J =
7.5 Hz, 1H), 6.83 (t, J = 7.8 Hz, 2H), 6.03 (d, J = 7.2 Hz, 2H), 5.10 (d,
J = 16.8 Hz, 1H), 4.57 (d, J = 16.8 Hz, 1H). 13C NMR (d6-DMSO): δ
= 194.1, 156.4, 156.2, 155.9, 155.2, 154.9, 153.9, 151.0, 150.5, 150.4,
148.5, 139.8, 138.8, 137.8, 137.2, 137.0, 136.9, 128.3, 128.2, 128.0,
127.3, 127.2, 127.1, 126.1, 124.5, 124.3, 124.2, 123.9, 123.7, 123.3,
118.3, 112.7, 51.8. Found: C, 44.95; H, 3.04; N, 10.40%.
C35H29F12N7P2Ru requires C, 44.79; H, 3.11; N, 10.45%. ESI-MS:
m/z = 794.1 ([2·PF6]+) calcd. For C35H29F6N7PRu = 794.12.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data for III·I, IV·Br, 2·(PF6)2, 3·(PF6)2,
and 4·(PF6)2 in CIF format and additional X-ray crystallo-
graphic details for 2·(PF6)2, 3·(PF6)2, and 4·(PF6)2. UV−visible
absorption spectra and cyclic voltammetric response for
1·(PF6)2−4·(PF6)2. Additional theoretical study results, includ-
ing variance in HOMO and LUMO energies from those of
[Ru(bpy)3]2+ for complexes 1·(PF6)2−4·(PF6)2 calculated
using the M06 functional, frontier molecular orbitals of 12+,
32+, 42+ and fragment contributions to the HOMO and LUMO
of complexes 12+−42+. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: p.barnard@latrobe.edu.au (P.J.B.), c.hogan@latrobe.
edu.au (C.F.H.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Australian Research Council for ARC LIEF
funding (LE100100109) for the X-ray diffractometer used in
this work and for ARC Discovery grant funding (DP1094179).
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