Probing the Noninnocent π-Bonding Influence of N-Carboxyamidoquinolate Ligands on the Light Harvesting and Redox Properties of Ruthenium Polypyridyl Complexes

Article abstract of DOI:10.1021/acs.inorgchem.5b02834

Electronic and photophysical characterization is presented for a series of bis-heteroleptic [Ru(bpy)₂(R-CAQN)]⁺ complexes where CAQN is a bidentate N-(carboxyaryl)amidoquinolate ligand and the aryl substituent R = p-tolyl, p-fluorobenzene, p-trifluoromethylbenzene, 3,5-bis(trifluoromethyl)benzene, or 4-methoxy-2,3,5,6-tetrafluorobenzene. Characterized by a strong noninnocent Ru(dπ)-CAQN(π) bonding interaction, density functional theory (DFT) analysis is used to estimate the contribution of both atomic Ru(dπ) and ligand CAQN(π) manifolds to the frontier molecular orbitals of these complexes. UV-vis absorption and emission studies are presented where the noninnocent Ru(dπ)-CAQN(π) bonding scheme plays a major role in defining complex electronic and photophysical properties. Oxidation potentials are tuned over a range of 0.92 V with respect to the [Ru(bpy)₃]²⁺ reference system, hereafter referred to as 1²⁺, by varying the degree of R-CAQN fluorination while maintaining consistently strong and panchromatic visible absorption properties. Electron paramagnetic resonance (EPR) spectroscopy is employed to experimentally map delocalization of the unpaired electron/electron-hole within the delocalized Ru(dπ)-CAQN(π) singly occupied valence molecular orbital of the one-electron oxidized complexes. EPR data is complemented experimentally by UV-vis-NIR spectroelectrochemistry, and computationally by molecular orbital Mulliken contributions and spin-density analysis. It is ultimately demonstrated that the CAQN ligand framework provides a simple yet broad synthetic platform in the design of redox-active transition metal chromophores with a range of electronic and spectroscopic characteristics hinting at the diversity and potential of these complexes toward photochemical and catalytic applications.

Full text of DOI:10.1021/acs.inorgchem.5b02834

Article
pubs.acs.org/IC
Probing the Noninnocent π‑Bonding Influence of
N‑Carboxyamidoquinolate Ligands on the Light Harvesting and
Redox Properties of Ruthenium Polypyridyl Complexes
Ken T. Ngo,† Nicholas A. Lee,† Sashari D. Pinnace,† David J. Szalda,§ Ralph T. Weber,‡
and Jonathan Rochford*,†
†Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston, Massachusetts 02125, United
States
‡EPR Division, Bruker BioSpin Corporation, 44 Manning Road, Billerica, Massachusetts 01821, United States
§Chemistry Department, Baruch College, New York, New York 10010, United States
S
* Supporting Information
ABSTRACT: Electronic and photophysical characterization is
presented for a series of bis-heteroleptic [Ru(bpy)2(R-CAQN)]+
complexes where CAQN is a bidentate N-(carboxyaryl)-
amidoquinolate ligand and the aryl substituent R = p-tolyl, p-
fluorobenzene, p-trifluoromethylbenzene, 3,5-bis-
(trifluoromethyl)benzene, or 4-methoxy-2,3,5,6-tetrafluoroben-
zene. Characterized by a strong noninnocent Ru(dπ)−CAQN-
(π) bonding interaction, density functional theory (DFT)
analysis is used to estimate the contribution of both atomic
Ru(dπ) and ligand CAQN(π) manifolds to the frontier
molecular orbitals of these complexes. UV−vis absorption and
emission studies are presented where the noninnocent Ru(dπ)−
CAQN(π) bonding scheme plays a major role in defining complex electronic and photophysical properties. Oxidation potentials
are tuned over a range of 0.92 V with respect to the [Ru(bpy)3]2+ reference system, hereafter referred to as 12+, by varying the
degree of R-CAQN fluorination while maintaining consistently strong and panchromatic visible absorption properties. Electron
paramagnetic resonance (EPR) spectroscopy is employed to experimentally map delocalization of the unpaired electron/
electron−hole within the delocalized Ru(dπ)−CAQN(π) singly occupied valence molecular orbital of the one-electron oxidized
complexes. EPR data is complemented experimentally by UV−vis−NIR spectroelectrochemistry, and computationally by
molecular orbital Mulliken contributions and spin-density analysis. It is ultimately demonstrated that the CAQN ligand
framework provides a simple yet broad synthetic platform in the design of redox-active transition metal chromophores with a
range of electronic and spectroscopic characteristics hinting at the diversity and potential of these complexes toward
photochemical and catalytic applications.
applications.7−12 Motivation for studying the CAQN ligand
stems from an interest in evaluating the transition metal
INTRODUCTION
■
Metal complexes incorporating noninnocent ligands (NILs) are
bonding properties of asymmetric NIL frameworks, building
growing increasingly important as a strategy to allow for unique
upon the more common cyclometalated phenylpyridine (ppy),
opportunities in engineering molecular redox properties.1−3
oxyquinolate (OQN), and β-ketoiminate systems.8,13,14 To the
Often used interchangeably with the term redox-active,
best of our knowledge there only exists a single prior report of a
noninnocent ligands derive their label from the fact that they
ruthenium CAQN complex where Ru(η6-p-cymene) (Ph-
participate in extensive π-overlap with metal based atomic
orbitals resulting in highly delocalized molecular orbitals.4,5
CAQN)Cl was utilized as a catalyst for the synthesis of
isoquinolones via oxidative annulation of N-quinolin-8-yl-
Thus, one may consider NILs to be a subset of the redox-active
ligand family, having significant consequences on the electronic
benzamides with alkynes.15 Furthermore, only a handful of
structural and catalysis based reports exist for CAQN type
structure of the complex as opposed to just displaying redox
activity.6 The purpose of this study is to investigate the
ligands with alternative (Ti, Fe, Co, Ni, Cu, Pd, Ir) transition
metal centers.15−23 A family of bis-heteroleptic complexes
noninnocent nature of the N-(carboxyaryl)amidoquinolate
(CAQN) ligand system at the d6 Ru(II) center. Ru(II)
containing a series of CAQN ligands at the ruthenium(II)
polypyridyl complexes are well-established at providing an
ideal platform for metal d(π)−ligand(π) covalent mixing and
have a broad interest for photophysical and catalytic
Received: December 9, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Structures of [Ru(bpy)2(R-CAQN)]+ complexes 2+−6+ investigated here alongside the 12+ reference compound.
Scheme 1. (a) Synthetic Procedure Employed in the Synthesis of the N-(Carboxyaryl)-8-aminoquinoline (R-CAQN·H) Ligands
and (b) Synthesis of the [Ru(bpy)2(R-CAQN)]+ Complexes
bis(2,2′-bipyridyl) core are presented here where varying
degrees of fluorination are introduced on the carboxy aryl
ring system in order to provide a means of tuning complex
oxidation potentials while maintaining favorable photophysical
properties (Figure 1). An in-depth spectroscopic and electro-
chemical analysis is presented, supported by computational
studies, to give a detailed picture of complex photophysical and
electronic properties.
resulting in analytically pure dark reddish-brown solids.
Curiously, efforts to synthesize the perfluorinated [Ru-
(bpy)2(F5Ph-CAQN)](PF6) complex from ligand L7 consis-
tently resulted in quantitative formation of a complex whose
molecular ion (m/z = 763.1) was 12 mass units higher than
anticipated. A combination of 1H and 19F NMR studies
confirmed the p-fluorine atom was replaced by a methoxy
group, presumably from the methanol solvent, yielding complex
6+. Although unexpected, etherification has been recently
reported at the m-trifluoromethylphenyl ring of a CAQN ligand
catalyzed by Cu(II).25 Direct synthesis of 6+ confirmed the
complex regiochemistry following synthesis of the N-(4-
methoxy-2,3,5,6-tetrafluorophenyl)-8-carboxyaminoquinoline
ligand precursor L6. Attempts to isolate the perfluorinated
ruthenium complex by changing a variety of reaction conditions
(low temperature, aprotic solvents etc.) were in vain.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. All amido
ligand precursors were prepared by a simple one-pot
condensation reaction between 8-aminoquinoline and the
appropriate benzoyl chloride similar to a reported literature
procedure (Scheme 1).24 The deprotonated amido CAQN
ligands are easily bound to the ruthenium(II) bis(2,2-bipydidyl)
core in methanol solution with a stoichiometric quantity of base
such as tetrabutyl ammonium hydroxide. [Ru(bpy)2(R-
CAQN)](PF6) salts were isolated via aqueous KPF6 mediated
metathesis followed by acetone/diethyl ether recrystallization
Single crystals of the tolyl-CAQN complex 2(PF6)·0.125H2O
suitable for X-ray diffraction were isolated by slow evaporation
of methanol from an aqueous methanol solution. The complex
crystallizes as a racemic mixture of Δ and Λ enantiomers with
B
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
one molecule of water of crystallization in the unit cell. The two
2+ cations in the asymmetric unit are approximately related by a
noncrystallographic inversion center with the quinoline rings of
each cation slightly overlapped with a dihedral angle of
3.76(12)° and shortest C···C contacts of ∼3.4 Å. An ORTEP
drawing of one of the cations of 2(PF6)·0.125H2O is presented
in Figure 2. Most notable for both cations of the unit cell is π-
these complexes. For brevity, a detailed comparison of 12+
relative to just the [Ru(bpy)2(tolyl-CAQN)]+ complex 2+ is
presented in Figure 3. The bonding scheme for 12+ is described
according to pseudo-Oh symmetry in Figure 3a and consists
primarily of M−L σ-bonding with negligible π-bonding
between the metal center and the ligand set.28 The nonbonding
filled HOMO and degenerate (HOMO − 1, HOMO − 2) d
atomic orbitals, and virtual LUMO and degenerate (LUMO +
1, LUMO + 2) unoccupied bpy(π*) levels, are distinctive for
this classical coordination complex and are responsible for the
typical singlet metal-to-ligand charge-transfer (1MLCT) elec-
tronic transitions observed for so many ruthenium polypyridyl
systems.29,30 In contrast, introduction of the π-donating tolyl-
CAQN anion at the ruthenium center significantly perturbs the
classical M−L σ-bonding scenario. First, a reduction in
symmetry from pseudo-Oh to pseudo-Cs breaks down any energy
level degeneracy.31 Electron donation from both the tolyl-
CAQN anion and a filled Ru(d) atomic orbital gives rise to
covalent Ru(dπ)−CAQN(π) molecular orbital overlap. The
resultant bonding (HOMO − 3) and antibonding (HOMO)
levels are distinct from 12+ in that they each contain a
significant mixture of metal and ligand contributions.
Furthermore, due to destabilization of the HOMO level, a
narrowing of the HOMO−LUMO band gap is observed where
the LUMO is still predominantly bpy(π*) in character. As a
result of this Ru(dπ)−CAQN(π) mixing the HOMO →
LUMO charge-transfer excitation is now more accurately
described as a singlet (metal−ligand)-to-ligand charge-transfer
(1MLLCT) transition and is accessible with lower energy
radiation.
Figure 2. Ortep diagram of 2(PF6)·0.125H2O illustrating π-stacking of
the tolyl-CAQN aryl substituent with an adjacent bpy ligand.
−
Hydrogen atoms, the PF6 anion, and water of solvation have been
removed for clarity.
stacking of the N-carboxytolyl substituent of the CAQN ligand
with the adjacent bpy ligand at the ruthenium center; plane-to-
plane ring distance = 3.538 Å, and a plane-to-plane dihedral
angle = 11.4(2)° and 12.6(2)° for either enantiomer. Similar π-
stacking has been previously observed for [Ru(bpy)2(2-aryl-
phenanthroline)]+ complexes and has been shown to improve
the stability of excited state ligand field states.26 Figure 2 shows
clearly that the tolyl group is rotated with respect to the amide
bond (dihedral angle = 46.58°) to facilitate the observed π-
stacking, however, not to the extent where the electronic
influence of the aryl ring is silenced (vide infra). It should be
noted however that observation of such π-stacking in the solid
state does not rule out rotation of the carboxy−aryl bond at
room temperature in solution phase, which may even promote
electronic communication with the quinoline ring.
Computational Analysis. Density functional theory
analysis was carried out on complexes 12+−6+ to inform
qualitatively and quantitatively on the relative extent of
ruthenium(dπ)−CAQN(π) mixing in the frontier orbitals of
each complex. Qualitatively, the bonding versus antibonding
character and delocalization of ruthenium(dπ)−CAQN(π)
overlap can be ascertained by visual inspection of the resulting
molecular orbital surfaces. From a quantitative perspective, the
influence of ruthenium(dπ)−CAQN(π) mixing on relative
frontier orbital energies is determined by investigation of
Mulliken molecular orbital contributions and the contribution
of these frontier orbitals to the visible electronic transitions of
each complex analyzed by time-dependent density functional
theory (TDDFT). It should be appreciated that the extent of
metal−ligand mixing inferred by Mulliken molecular orbital
contributions can vary depending upon the choice of basis
set.27 As such, any quantitative DFT MO contribution analysis
here presented is not deemed absolute but merely used to aid
interpretation of spectroscopic and electrochemical behavior of
The molecular orbital surfaces presented in Figure 3b for the
HOMO and HOMO − 3 levels of 2+ show clear evidence of
the π-antibonding and π-bonding Ru(dπ)−CAQN(π) charac-
ter, respectively, at the Ru−Namido interface. This Ru(dπ)−
CAQN(π) interaction is noteworthy as the character of these
frontier orbitals contributes significantly to the enhanced light
harvesting of 2+ as determined by TDDFT analysis. The
TDDFT absorption spectrum of 12+ is characterized by a single
1MLCT absorption band derived from an isoenergetic mixture
of HOMO − 1/HOMO − 2 → LUMO/LUMO + 1/LUMO +
1
2 MLCT electronic transitions (Table SI-1). Complex 2+ on
the other hand displays a much broader TDDFT absorption
profile stretching from the UV across the entire visible range
due to a diverse set of underlying electronic transitions. The
dominant electronic transitions observed for 2+ are a mixture of
1MLCT and 1MLLCT character. The calculated TDDFT
absorption spectrum shows excellent qualitative agreement with
experiment as illustrated in Figure 3c. A more detailed
discussion of spectral assignments is discussed below for all
complexes. Table 1 summarizes the percentage Mulliken
contribution of Ru(d) and ligand(π) orbitals to the relevant
HOMO and HOMO − 3 energy levels for complexes 2+−6+ to
illustrate the noninnocent Ru(dπ)−CAQN(π) bonding inter-
action across the series. A summary of frontier orbital surfaces
and corresponding energies for complexes 12+−6+ is presented
in Figure 4.
UV−Vis Electronic Absorption Spectra and TDDFT.
The photophysical properties of 12+ have been studied on many
occasions and are well-established.30,32,33 As briefly discussed
above, 12+ displays a characteristic absorption maximum at 450
nm (ε = 14 600 M−1 cm−1) due to a series of unresolved
1MLCT electronic transitions (Figure 3a, Table SI-1). Each of
C
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. (a) Molecular orbital correlation diagrams and (b) select frontier molecular orbital surfaces (isofactor = 0.04) for 12+ and 2+. (c) An
overlay of experimental and computational (TDDFT) UV−vis spectra is also included for both complexes with quantitative TDDFT assignments to
illustrate the influence of the π-donating noninnocent CAQN ligand on the observed electronic transitions.
the ruthenium CAQN complexes 2+−6+ display very similar
spectra where replacement of one of the bpy ligands on 12+
with a noninnocent CAQN ligand gives rise to enhanced light
harvesting due to a broader and red-shifted absorption profile.
The degree of fluorination at the N-carboxyaryl ring substituent
has limited influence on the light harvesting properties across
the series with just a slight blue shift of the lowest energy
absorption maximum observed with increased fluorination/
decreased CAQN donor strength (Figure 5 and Table 2).
The broad absorption profile observed for 2+−6+ is due to a
1 1
range of both MLCT and MLLCT electronic transitions
stretching across the UV−vis spectrum consistent with their
reduced pseudo-Cs symmetry relative to 12+. The high energy
absorption bands ranging from 334 to 353 nm and the low
energy absorption maxima ranging from 471 to 497 nm across
the series are highly characteristic of the Ru(dπ)−CAQN(π)
bonding in these complexes. According to TDDFT analysis,
both of these absorption bands are 1MLLCT [Ru(dπ)-
CAQN(π)] → CAQN(π*) in character originating from the
D
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Mulliken Percentage Population Analysis of the
HOMO and HOMO − 3 Levels for Complexes 2+−6+
a
Determined by DFT Analysis
Ru d(π) R-CAQN(π) bpy (total) energy (eV)
12+
2+
HOMO
88
28
56
28
56
29
55
31
53
36
49
12
4
9
4
9
4
9
4
9
5
8
−6.070
−4.948
−6.089
−4.986
−6.111
−5.012
−6.132
−5.114
−6.182
−5.221
−6.240
HOMO
68
35
68
35
67
36
65
38
59
43
HOMO − 3
HOMO
3+
4+
5+
6+
HOMO − 3
HOMO
HOMO − 3
HOMO
HOMO − 3
HOMO
HOMO − 3
a
Group contributions to the HOMO of 12+ are included for
comparison.
Figure 5. Overlay of UV−vis absorption spectra for [Ru(bpy)2(R-
CAQN)]+ complexes recorded in acetonitrile with reference to 12+.
bonding (HOMO − 3) and antibonding (HOMO) Ru(dπ)−
CAQN(π) orbitals, respectively, and terminating at the vacant
CAQN(π*) antibonding orbital (LUMO + 2). Thus, a major
portion of the UV−vis absorption spectra for complexes 2+−6+
is derived entirely from the noninnocent Ru(dπ)−CAQN(π)
data for all complexes is provided in the Supporting
Information.
Corrected low temperature 3MLCT phosphorescence
emission spectra of all dyes were recorded in a 77 K frozen
ethanol/methanol (4:1) glass and are presented in Figure 6. As
anticipated, due to CAQN destabilization of the HOMO level,
a narrower S0 ← T1 energy gap (E0) is observed ranging from
15 800 to 16 000 cm−1 relative to 17 900 cm−1 observed for 12+.
Apart from complex 5+, emission maxima shift to lower energy
with increasing donor strength (decreased fluorination) of the
CAQN ligand concomitant with an increased broadening and
full width half maxima (Δν1/2 ranges from 2200 to 2600 cm−1)
due to reduced vibrational spacing, e.g., ℏωM ∼ 685 cm−1 for 6+.
Complex 5+ shows the lowest energy and broadest emission
1
bonding interaction. Of course traditional MLCT electronic
transitions of Ru(d) → bpy(π*) and Ru(d) → CAQN(π*)
character are also observed via TDDFT analysis for complexes
2+−6+ giving rise to intermediate visible absorption bands,
which are observed experimentally in the range 405−452 nm.
The lowest energy absorption band tails out to ∼700 nm for all
Ru-CAQN complex experimental spectra. This low energy
absorption is attributed to weak oscillator strength HOMO →
LUMO and HOMO → LUMO + 1 electronic transitions which
are characterized as [Ru(dπ)-CAQN(π)] → bpy(π*) charge-
transfer transitions by TDDFT. A detailed database of TDDFT
Figure 4. Plot of frontier molecular orbital energy levels (eV) for complexes 12+−6+ as calculated by DFT/B3LYP/6-31g(d,p) (C,H,N,O,F) and
LANL08 (Ru) in an acetonitrile polarizable continuum model. Electron occupancy is removed for clarity, and HOMO−LUMO levels are highlighted
in blue and red, respectively. Select frontier molecular orbital surfaces (isofactor = 0.04) are included for comparison.
E
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
as their red-shifted electronic spectra, all Ru-CAQN complexes
show a strong cathodic shift of their first oxidation potential
relative to 12+. The metal based Ru(III/II) couple of 13+/2+
occurs at +1.29 V versus SCE. Replacing one of the bpy ligands
with the strongest electron donating ligand tolyl-CAQN gives
rise to a reversible first oxidation of 22+/+ at +0.37 V versus SCE
representing a potential shift of ΔE = −0.92 V. The least
electron donating F4(MeO)Ph-CAQN complex 6+ displays a
reversible oxidation at +0.66 V versus SCE representing a
potential shift of ΔE = −0.63 V relative to the 13+/2+ couple.
While a cathodic shift of oxidation potentials relative to 12+ is
primarily attributed to the π-donating influence of the R-
CAQN ligands, one cannot rule out an additional Coulombic
contribution due to the reduced charge of complexes 2+−6+.
Unambiguous, however, is the 310 mV range of oxidation
potentials observed for the [Ru(bpy)2(R-CAQN)]2+/+ redox
couple induced by the degree of fluorination at the N-
carboxyaryl substituent. Cyclic voltammograms of 12+, 2+, and
6+ are presented in Figure 7 for comparison. Remaining Ru−
CAQN complexes also display highly reversible first oxidations
across a range of ∼0.92 V relative to 1+ and 2+ (Table 3).
Table 2. UV−Vis Electronic Absorption and
Phosphorescence Emission Data
a
b
abs λmax (nm) (ε × 104 M−1 cm−1
)
em λmax (nm)
12+ 243 (2.49), 286 (7.79), 430 (sh), 450 (1.46)
581, 630, 680 (sh)
683, 739 (sh)
2+
3+
4+
5+
6+
250 (4.23), 295 (4.29,) 352 (0.99),
405 (sh, 0.82), 452 (1.09), 497 (1.17)
251 (4.53), 295 (4.98), 352 (1.06),
412 (sh, 0.91), 451 (1.20), 495 (1.27)
681, 737 (sh)
673, 728 (sh)
697, 758 (sh)
249 (4.41), 295 (5.12), 351 (1.08),
405 (sh, 0.95), 448 (1.27), 491 (1.29)
248 (4.13), 294 (4.87), 353 (1.05),
412 (sh, 0.99), 446 (1.26), 485 (1.26)
246 (4.93), 294 (5.13), 334 (1.09), 445 (1.33), 662, 707 (sh)
471 (1.27)
a
b
Recorded at room temperature in acetonitrile. recorded at 77 K in
an ethanol:methanol (4:1) frozen glass.
Figure 6. Overlay of corrected emission spectra recorded in a frozen
EtOH/MeOH (4:1) glass at 77 K.
band with the least vibrational structure which is inconsistent
with the remainder of the series. This is likely due to both
trifluoromethyl substituents of the 3,5-bis(trifluoromethyl)-
benzene-CAQN ligand having limited electron withdrawing
influence at the meta positions of the aryl ring. All dyes display
a much weaker photoluminescence relative to 12+ consistent
with the energy-gap law [knr ∝ exp(−E0)] where the rate
constant for nonradiative decay (knr) increases exponentially
with a decreasing S0 ← T1 energy, thus precluding accurate
determination of phosphorescence quantum yields.34 In fact, all
Ru-CAQN complexes 2+−6+ are nonemissive at room
temperature. This behavior is consistent with increased
3MLLCT vibrational coupling due to significant Ru(dπ)−
CAQN(π) mixing at the HOMO level (Table 1) and is
reminiscent of the isoelectronic [Ru(bpy)2(ppy)]+ cyclo-
metalated class of complexes.35 While short excited state
lifetimes and nonemissive behavior at room temperature may
suggest poor applicability as a photosensitizer, there is
precedent for related ruthenium complexes performing well
in dye sensitized solar cells due to kinetically competitive
reduction of the short-lived excited state by excess iodide ions
in the redox mediator electrolyte.36
Figure 7. Cyclic voltammograms of 12+, 2+, and 6+ recorded at room
temperature in acetonitrile (0.1 M Bu4NPF6) at a glassy carbon
working electrode with scan rate of 50 mV s−1.
Unlike the 13+/2+ redox couple that according to X-ray
crystallography, DFT, and EPR analysis is predominantly metal
based,9,37 oxidation of Ru-CAQN complexes can potentially be
described by two canonical resonance structures with Ru(III)
or CAQN• radical character. This is implied by the hybrid
character of the HOMO level and is a result of the noninnocent
Table 3. Electrochemical Data for Complexes 12+−6+
Recorded in Acetonitrile (0.1 M Bu4NPF6) at a Glassy
Carbon Working Electrode with a Scan Rate of 50 mV s−1
E (V vs SCE)
a
12+
2+
3+
4+
5+
6+
1.29
0.37
0.39
0.47
0.46
0.66
−1.33
−1.46
−1.46
−1.45
−1.51
−1.44
−1.52
−1.79
−1.76
−1.76
−1.79
−1.74
−1.76
−2.29
−2.29
−2.23
−2.31
−2.28
−2.31
a
a
a
a
a
a
a
a
a
a
1.21
1.23
1.26
1.23
1.39
Electrochemistry. The electrochemical properties of all
dyes were investigated by cyclic voltammetry. Consistent with
destabilization of the HOMO level as predicted by DFT, as well
a
Irreversible.
F
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Structural Comparison of the [Ru(bpy)3]3+/2+ and [Ru(bpy)2(R-CAQN)]2+/+ Redox Couples Demonstrating
Noninnocence of the CAQN Ligand via Resonance between a Ru(III) Center and a CAQN• Radical
Ru(dπ)−CAQN(π) bonding interaction (Scheme 2). The
hybrid nature of the HOMO is further corroborated by
spectroelectrochemical UV−vis−NIR and EPR studies dis-
cussed below.
isotropic g-factor ⟨g⟩ summarized in Table 4. A maximum
isotropic g-factor is observed at ⟨g⟩ = 2.140 for 13+ which
a
Table 4. EPR Data of Complexes 13+−62+
In contrast to 12+, all Ru-CAQN complexes also show an
easily accessible irreversible second oxidation event occurring in
the range from +1.21 V (23+/2+) to +1.39 V (63+/2+) versus
SCE. This oxidation event most likely involves formation of the
Ru(IV)-CAQN• species. Scanning in the negative direction, 12+
displays three sequential and reversible one-electron reduction
events which are well-established as being bpy(π*)0/•− based.30
In fact, upon one-electron reduction of 12+ to 1+, it can be
considered formally isoelectronic to complexes 2+−6+. Indeed
all Ru-CAQN complexes display two reversible bpy(π*)0/•−
based reductions only slightly shifted relative to the second and
third reductions of 12+. A third irreversible reduction event is
also observed for all Ru-CAQN complexes. Whether this
reduction event occurs on the CAQN or bpy ligands is unclear,
however it is similar to a fourth irreversible reduction observed
for 12+ at −2.31 V versus SCE.
gx
gy
gz
Δg (gx − gz)
⟨g⟩
41
13+
22+
32+
42+
52+
62+
2.640
2.118
2.119
2.126
2.141
2.149
2.640
2.067
2.068
2.077
2.089
2.089
1.140
1.959
1.958
1.956
1.948
1.943
1.500
0.159
0.161
0.169
0.192
0.206
2.140
2.048
2.048
2.053
2.059
2.060
a
All data was recorded at 120 K in 0.1 M Bu4NPF6 acetonitrile
electrolyte following room temperature controlled potential electrol-
ysis.
decreases with increasing Ru(II)-CAQN• contribution to a
minimum value of ⟨g⟩ = 2.048 for 22+. An example of the EPR
spectrum observed for 22+ is presented in Figure 8.
Spectroelectrochemistry: Electron Paramagnetic Res-
onance. The combination of controlled potential electrolysis
with EPR spectroscopy is a very powerful tool to probe the
frontier orbital character of transition metal complexes.38 EPR
spectroscopy of paramagnetic transition metal complexes is
capable of quantifying spin-density at the metal center via axial
or rhombic g-anisotropy or by deviation of the isotropic g-factor
⟨g⟩ from that of the free electron g-factor (ge = 2.0023).39 The
in situ generated one-electron oxidized [Ru(bpy)2(R-
CAQN)]2+ complexes are hypothesized to have a singly
occupied valence molecular orbital where the electron−hole
is delocalized across the Ru(dπ)−CAQN(π) manifold due to
the noninnocent nature of this system (Scheme 2). The one-
electron oxidized species are EPR silent at room temperature
due to a large Ru spin−orbital coupling constant (ξ ∼ 1000
cm−1) giving rise to fast relaxation times.40 As such, controlled
potential electrolysis was first completed at room temperature,
and electrolyte solutions were subsequently transferred to an
EPR tube under an inert argon atmosphere prior to freezing in
liquid nitrogen. When recorded in a frozen glass, at first glance
all [Ru(bpy)(R-CAQN)]2+ EPR spectra appear to display axial
g-anisotropy (gx = gy ≠ gz); however, the spectra fit best to a
rhombic anisotropy (gx ≠ gy ≠ gz). The Ru(III) centered radical
in 13+ displays axial g-anisotropy and deviates most from the
free electron g-factor (ge = 2.0023) consistent with a maximum
influence of Ru spin−orbital coupling. Increasing ligand
contribution from the Ru(II)-CAQN• resonance form (Scheme
2) decreases the Ru(III) contribution thus narrowing the g-
factor splitting. This trend is best identified following the
Figure 8. X-band EPR spectroelectrochemical data for complex 22+
recorded at 120 K following room temperature electrolysis at +0.55 V
versus SCE in 0.1 M Bu4NPF6 acetonitrile electrolyte.
The g-anisotropy value (Δg = gx − gz) is also strongly
influenced by the spin contribution of heavy atoms and
provides a means to distinguish between metal and ligand
centered radical character.42,43 With the trend in Δg across the
series 22+−62+, it is possible to identify the predominant
resonance form contributing to the one-electron oxidized
species (Scheme 2). The F4(MeO)Ph−CAQN complex 62+
displays the largest g-anisotropy of Δg = 0.206 from the R-
CAQN series consistent with the weakest donating strength of
G
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
this ligand due to its extensive fluorination. In contrast, the
electron donating tolyl substituent gives rise to the lowest g-
anisotropy of the series with Δg = 0.159 implying the greatest
Ru(II)-CAQN• character for 22+. Relative to our prior EPR
studies of isoelectronic oxyquinolate [Ru(bpy)(R-OQN)]2+
complexes,8,9 the range of g anisotropies here observed is
quite narrow, and the values are also consistently low in value
relative to that of the metal centered Ru(d5) radical in 13+ (Δg
= 1.500).41 Thus, while rhombic anisotropy confirms the
Ru(III) contribution to the singly occupied HOMO(α) level,
the low g-anisotropies observed imply a majority contribution
from the Ru(II)-CAQN• resonance form across the 22+−62+
series. This trend observed for fluorination versus hole
delocalization across the Ru(dπ)-CAQN(π) framework is
further corroborated by computational Mulliken spin-density
analysis as illustrated in Figure 9. The calculated Mulliken spin-
changes were observed for the paramagnetic complexes 22+−
62+ across the entire UV−vis−NIR spectral range. An example
of electrochromic transformation of 2+ to 22+ under controlled
potential electrolysis conditions is presented in Figure 10 with
an accompanying molecular orbital diagram for both redox
states.
Isosbestic points are observed at 304 and 327 nm confirming
a clean transformation from 2+ to 22+. Concurrent with the loss
of major visible absorptions for the parent 2+ complex is the
growth of strong vis−NIR absorption bands at 392, 577, and
927 nm for 22+. The lowest energy absorption band at 927 nm
(10 787 cm−1) is particularly impressive stretching across a 400
nm range with a full width half-maximum of Δν1/2 = 2010 cm−1
and a molar extinction coefficient of ε = 20 750 M−1 cm−1. The
NIR absorption of 22+ consists of an almost equal mixture of
two electronic transitions: (i) metal−ligand π−π* [HOMO(β)
→ LUMO(β)] and (ii) metal-to-(metal−ligand) charge-
transfer [MMLCT; HOMO−2(β) → LUMO(β)]. The latter
MMLCT transition can be anticipated on the basis of a major
resonance structure, in this case Ru(II)-CAQN•, originating
from the primarily metal based HOMO − 2(β) orbital and
populating the hybrid LUMO(β) energy level. Although some
structural reorganization is observed in the DFT optimized
geometry of 22+, most obvious in the reduced bpy/tolyl π-
stacking, close inspection of the MO surfaces in Figure 9 allows
assignment of the antibonding Ru(dπ)−CAQN(π) LUMO(β)
orbital as the “electron−hole” of the paramagnetic complex
seeing how it correlates to the HOMO surface of the
diamagnetic 2+ redox state. Also noteworthy is destabilization
of the bonding Ru(dπ)−CAQN(π) HOMO(β) orbital of 22+,
which correlates to the HOMO − 3 level of 2+. This
destabilization upon one-electron oxidation is most likely due
to reduced π-donation of the CAQN• ligand. Consistent with
EPR and Mulliken spin-density analysis, where increased
fluorination increases contribution from the Ru(III)-CAQN
metalloradical resonance form, the oscillator strength of this
NIR transition decreases accordingly until for 62+ it is barely
visible with a molar extinction coefficient decreased by 2 orders
of magnitude (ε = 500 M−1 cm−1, Figure 11).
Figure 9. Mulliken spin-density analysis illustrating hole-delocalization
onto the R-CAQN ligands of complexes 22+−62+, relative to the spin-
localized 13+ metallo-radical. Spin-density on the bpy ligands is
presented as an average value.
density is located entirely on Ru(III) for 13+ which drops
dramatically to 37% for 22+ with the remaining 63% located on
the tolyl-CAQN ligand. This implies a majority Ru(II)-(tolyl-
CAQN•) contribution according to Scheme 2, consistent with
EPR analysis. A minimum calculated R-CAQN based Mulliken
spin-density of 59% is observed for the least electron donating
F4(MeO)Ph−CAQN ligand complex 62+ consistent with it
displaying the highest isotropic g-factor (⟨g⟩ = 2.060) and g-
anisotropy (Δg = 0.206) of the Ru(R-CAQN) series.
Spectroelectrochemistry: Electronic Absorption Spec-
troscopy. Transition metal(dπ)−NIL(π) systems have a
tendency to show strong NIR electronic transitions when
containing NIL radical character. This is especially true of o-
semiquinone, o-iminosemiquinone, and o-diiminosemiquinone
complexes.44 Previous spectroelectrochemical UV−vis−NIR
studies of [Ru(bpy)(R-OQN)]2+ complexes showed a strong
NIR electronic transition whose maximum absorption and
oscillator strength increased with increasing NIL radical
character.9 On the basis of this premise and consistent with
computational, electrochemical, and EPR data presented above,
a similar trend was anticipated for the isoelectronic [Ru(bpy)-
(R-CAQN)]2+ series here under investigation. Sure enough,
upon one-electron oxidation, strong in situ electrochromic
COMPARISON WITH RELATED NONINNOCENT
LIGANDS
■
Finally, it is worth considering the electronic properties of
[Ru(bpy)2(R-CAQN)]+ complexes against related ruthenium
polypyridyl noninnocent ligand (NIL) complexes to gauge
where R-CAQN lies on the spectrum of noninnocent versus
classical redox-active metal−ligand combinations. For this
purpose 2+ with the simple tolyl-CAQN ligand is used, and
as above, 12+ is used as the innocent redox-active reference. The
isolectronic 8-oxyquinolate (OQN) ligand previously studied
by our group8 is also included as well as the popular o-
semiquinone [(O,O)SQ], o-aminosemiquinone [(NH,O)SQ],
and o-diaminosemiquinone [(NH,NH)SQ] series of ligands
(Figure 12).3,45
Table 5 summarizes data for the first oxidation potential of
all complexes as well as the EPR determined isotropic g-factors
and DFT calculated Mulliken spin-densities. Note that all three
[Ru(bpy)2(NIL)]+ semiquinone complexes are paramagnetic
and EPR active in their native 1+ redox states. Although all of
the noninnocent ligands compared in Table 5 (excluding bpy of
course) carry a formal charge of −1 there is a distinct difference
in their oxidation potentials which occur over a relatively large
range of 0.84 V. A simple Coulombic explanation is insufficient
H
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 10. (Top) Overlay of UV−vis−NIR absorption spectra for the spectroelectrochemical conversion of [Ru(bpy)2(tolyl-CAQN)]+ to its one-
electron oxidized derivative [Ru(bpy)2(tolyl-CAQN)]2+. Evolution of new experimental absorption maxima and isosbestic points (304 and 327 nm)
is highlighted alongside majority TDDFT assignments. All spectra were recorded in 0.1 M Bu4NPF6 acetonitrile electrolyte at room temperature
under controlled potential electrolysis conditions at an applied bias of +0.55 V versus SCE. (Bottom) Molecular orbital correlation diagram for 2+
and 22+ (left = α orbitals, right = β orbitals) including select frontier molecular orbital surfaces (isofactor = 0.04).
to explain this range of redox potentials. This is best
understood in terms of the so-called noninnocent ligand effect
which is essentially a variance in the extent of metal−ligand π-
bonding (vide supra). More specifically, trends in the observed
first oxidation redox couples can be rationalized by the relative
energy alignment of NIL frontier π-orbitals with the bonding
Ru atomic d-orbitals in both redox states of the complex, i.e.,
[Ru(bpy)2(NIL)]n+, where n = +2 and +1. All three
semiquinone complexes have been previously characterized
by Lever and co-workers who established that the radical charge
is primarily located on the NIL fragment.45 This is consistent
with their low isotropic g-factors (1.997−2.000) characteristic
of an organic radical and the high spin-density calculated on the
NIL fragment (84−86%). Both o-aminosemiquinone [(NH,O)-
SQ] and o-diaminosemiquinone [(NH,NH)SQ] complexes
appear to have high lying SOMO levels due to their ease of
oxidation at +0.05 and −0.45 V versus SCE, respectively. This
is consistent with each of their N atoms carrying significant
radical character46 and is in stark contrast to the o-semiquinone
[(O,O)SQ] complex which is oxidized at +0.56 V versus SCE,
slightly positive of the tolyl-CAQN (1+) and OQN complexes.
The tolyl-CAQN redox potential (12+/+) occurs intermediate
between the (O,O)SQ and (NH,O)SQ complexes which is
consistent with partial Ru(III) character evident by its rhombic
EPR spectrum, increased g-factor (2.048), and 37% calculated
Mulliken spin-density at the metal center. In comparison, the
[Ru(bpy)2(OQN)]2+ complex carries a greater percentage of
Ru(III) character in its SOMO level consistent with it having
the highest g-factor (2.120) of the NIL series here compared.
Thus, the R-CAQN class of ligands can be considered its own
class of NIL alongside its OQN and o-semiquinone counter-
parts, at least when coordinated to Ru(II), with the advantage
of having a greater scope for synthetic variation and fine-tuning
of its properties as described above for the 2+−6+ series of
complexes.
CONCLUSIONS
■
In summary, a series of bis-heteroleptic ruthenium(II) bis-
bipyridyl N-(carboxyaryl)amidoquinolate complexes are pre-
sented with varying degrees of N-carboxyaryl fluorination. X-ray
crystallography confirms a favorable π-stacking of the N-
carboxyaryl substituent of the CAQN ligand with the adjacent
I
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
absorption spectra are dominated by 1MLLCT absorption
bands with major contributions from [Ru(dπ)-CAQN(π)] →
CAQN(π*) type transitions (Figure 3). While fluorination of
the N-carboxyaryl substituent does not have a strong influence
on the absorption spectra, the first oxidation potential is tuned
over a range of 0.92 V with respect to 12+. EPR and Mulliken
spin-density analyses of the one-electron oxidized complexes
confirm that an increased contribution of the CAQN ligand to
the HOMO level occurs with decreased fluorination.
Combination of ruthenium based redox stability with the
strong CAQN light harvesting properties is a highly attractive
feature of the noninnocent Ru(dπ)−CAQN(π) system, which
also offers a great scope for synthetic variation and electronic
tuning relative to related OQN and o-semiquinone systems.
Future studies will take advantage of these properties for
application in photovoltaic and catalysis applications.
EXPERIMENTAL SECTION
■
Figure 11. Overlay of UV−vis absorption spectra for the one-electron
oxidized derivatives 13+−62+. All spectra were recorded in 0.1 M
Bu4NPF6 acetonitrile electrolyte at room temperature following
controlled potential electrolysis.
Physical Measurements. UV−vis−NIR absorption spectra were
recorded on an Agilent 8453 diode array spectrophotometer in
spectrophotometric grade acetonitrile. NMR spectra were recorded on
an Agilent spectrometer operated at 399.80 MHz for 1H, 100.54 MHz
for 13C, and 376.15 MHz for 19F. Deuterated solvents d-chloroform,
d6-acetone, and d6-dimethyl sulfoxide were used as received from
1
Aldrich and their residual H solvent signals (δ = 7.26, 2.05, and 2.50
1
ppm respectively) used as internal references for reporting the H
chemical shift (δ).47 Likewise, the 13C signal of d6-dimethyl sulfoxide
(δ = 39.52 ppm) was used as an internal reference for reporting 13C
NMR spectra. Trifluoroethanol (δ (CDCl3) = −77.00 ppm) was used
as an external standard for referencing 19F chemical shifts in deuterated
d6-acetone (δ = −77.44 ppm) and d6-dimethyl sulfoxide (δ = −75.07
ppm). LC−MS of ligands L5−L7 was performed on an Agilent 2100
system using atmospheric pressure chemical ionization (APCI) mode.
Mobile phases consisted of methanol and water both containing 0.05%
trifluoroacetic acid. A linear gradient was used to increase from 25:75
v/v methanol/water to 100% methanol over 7.0 min at a flow rate of
0.7 mL/min with a C18 (5.0 μm, 6.0 mm × 50 mm) column. UV
detection of the eluent was conducted at 210, 254, and 365 nm. ESI-
MS was carried out on a Thermo Finnigan mass spectrometer.
Elemental analysis was conducted with a PerkinElmer 2400 instru-
ment. Cyclic voltammetry was carried out on a CH Instruments 620D
potentiostat. A standard three electrode cell was used under an
atmosphere of argon with 0.1 M Bu4NPF6 in spectrophotometric
grade acetonitrile as supporting electrolyte. Glassy carbon (3 mm
diameter) and Pt wire were used as working and counter electrodes,
respectively. A nonaqueous reference electrode was used to minimize
ohmic potential drop at the solvent interface. This consisted of a Ag
wire in 0.1 M Bu4NPF6 acetonitrile supporting electrolyte isolated by a
vycor frit and was calibrated using the ferricenium/ferrocene redox
couple as a pseudoreference (+0.45 V vs SCE).48 Redox potentials (E)
were determined from cyclic voltammetry as (Epa + Epc)/2, where Epa
and Epc are the anodic and cathodic peak potentials, respectively.
Where E could not be calculated due to irreversible behavior, Epc or
Epa are reported accordingly. For controlled potential electrolysis
experiments, Pt gauze electrodes were used as both working and
counter electrodes with the counter electrode isolated via a fine
porosity vycor frit. UV−vis−NIR spectroelectrochemical experiments
were conducted with a custom spectroelectrochemical flow cell whose
design is based upon a literature description.49 EPR spectra were
recorded on a Bruker Elexys E500 spectrometer at 120 K with an X-
band microwave source.
Figure 12. Structures of the previously reported [Ru(bpy)2(NIL)]+
complexes where the noninnocent ligand (NIL) = 8-oxyquinolate
(OQN), o-semiquinone [(O,O)SQ], o-aminosemiquinone [(NH,O)-
SQ], and o-diaminosemiquinone [(NH,NH)SQ].
Table 5. Summary of Electronic Properties for a Series of
Isoelectronic [Ru(bpy)2(NIL)]2+ Complexes (Where NIL =
Bidentate Noninnocent Ligand) Including 13+, 22+, and
Related Semiquinone Complexes
a
EOx
⟨g⟩
Ru/NIL Mulliken spin-density
b
b
b
c
b
12+
+1.29
+0.37
+0.52
+0.56
+0.05
−0.47
2.140
2.048
2.120
2.000
2.000
1.997
100/0
b
2+
37/63
OQN8
46/54
b
(O,O)SQ45
(NH,O)SQ45
(NH,NH)SQ45
16/86
d
c
d
17/84
c
d
12/88
a
All redox potentials were recorded in 0.1 M Bu4NPF6 acetonitrile
b
electrolyte and are reported vs SCE. Data obtained for one-electron
c
d
oxidized species Recorded at 100 K in dichloroethane. Calculated
using the B3LYP exchange-corellation functional with the TZVP (Ru)
and DGDZVP (C,H,N,O) basis sets.46
bpy ligand at the ruthenium center. Spectroelectrochemical
EPR and UV−vis−NIR spectroscopies show complementary
results to DFT analysis supporting the presence of a
noninnocent Ru(dπ)−CAQN(π) bonding interaction, which
is responsible for enhanced light harvesting and redox tunability
of these complexes. The noninnocent Ru(dπ)−CAQN(π)
interaction imparts a strong influence on the complex electronic
structure where both the UV and visible regions of their
Computational Details. All calculations were carried out using
density functional theory (DFT) with the B3LYP functional and
acetonitrile polarizable continuum model (PCM)50 as implemented in
the Gaussian 09 B.01 program package.51 The LANL08 relativistic
effective core potential (RECP) basis set was used for Ru52 and 6-
31G(d,p)53,54 for C, H, N, O, F. A vibrational frequency analysis was
carried out in order to confirm the minimum-energy geometry and
J
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
determine the zero-point energy for each species; i.e., geometry
optimization and frequency calculations were performed for both the
native 12+−6+ complexes and their one-electron oxidized derivatives
13+−62+. Electronic transitions (N = 60) were calculated in acetonitrile
with the PCM optimized geometry using time-dependent density
functional theory (TDDFT)55 at the same level of theory. Successful
implementation of these DFT/TDDFT parameters has proven
successful in the past for ruthenium polypyridyl complexes.56
Collection and Reduction of X-ray Data. A crystal (0.50 × 0.20 ×
0.13 mm3) of 2(PF6)·0.125H2O was mounted on the end of a glass
fiber and transferred to Bruker Kappa Apex II diffractometer for the
collection of diffraction data. Diffraction data indicated monoclinic
symmetry and systematic absences consistent with space group P2/n.
Crystal data and information about the data collection are provided in
the Supporting Information.
Determination and Refinement of the Structure. The structure of
2(PF6)·0.125H2O was solved by direct methods.57 There are two
molecules of 2(PF6)·0.125H2O in the symmetry unit. A disordered
model was used for the hexafluorophosphate ions, and the occupancy
factor of the water of crystallization was refined to 0.129(8). In the
least-squares refinement,57 anisotropic temperature parameters were
used for all the non-hydrogen atoms except for the fluorine and
oxygen atoms of the disordered hexafluorophosphate and water with
occupancy factors less than 0.5. Hydrogen atoms were placed at
calculated positions and allowed to “ride” on the atom to which they
were attached except for the hydrogen atoms on the water of
crystallization which were not included. The isotropic thermal
parameter for the hydrogen atoms were determined from the atom
to which they were attached. The data was corrected using the
multiscan method (SADABS).58
N-(Carboxy-2,3,5,6-tetrafluoro-4-methoxyphenyl)-8-aminoqui-
1
noline (L6). H NMR δ[(CD3)2SO]: 4.13 (t, 3H, J = 1.5 Hz), 7.64−
7.68 (m, 2H), 7.80 (dd, 1H, J = 1.2, 8.0 Hz), 8.46 (dd, 1H, J = 1.6, 8.0
Hz), 8.70 (dd, 1H, J = 0.8, 8.0 Hz), 8.94 (dd, 1H, J = 1.6, 4.0 Hz),
11.03 (s, 1H) ppm. 13C NMR δ[(CD3)2SO]: 45.60, 117.89, 122.35,
123.54, 126.87, 127.95, 133.75, 136.65, 138.37, 149.28, 156.36 ppm.
19F NMR δ[(CD3)2SO]: −143.36 (m), −157.46 (m) ppm. LC−MS
(m/z): calcd (M + 1) 351.1; obsd 351.1.
N-(Carboxy-perfluorophenyl)-8-aminoquinoline (L7). 1H NMR
δ[(CD3)2SO]: 7.65−7.69 (m, 2H), 7.82 (dd, 1H, J = 1.2, 8.0 Hz), 8.46
(dd, 1H, J = 2.0, 8.0 Hz), 8.70 (dd, 1H, J = 1.2, 8.0 Hz), 8.94 (d, 1H, J
= 4.5 Hz), 11.20 (s, 1H) ppm. 13C NMR δ[(CD3)2SO]: 118.21,
122.38, 123.82, 126.86, 128.00, 133.66, 136.68, 138.47, 149.36, 155.75
ppm. 19F NMR δ[(CD3)2SO]: −141.87 (m), −153.09 (t), −161.77
(m) ppm. LC−MS (m/z): calcd (M + 1) 339.1; obsd 339.1.
General Synthetic Method for [Ru(bpy)2(R-CAQN)](PF6) Com-
plexes. A 50 mL flask was charged with 10 mL of methanol and the
solution purged with argon for 10 min. The flask was charged with
0.10 mmol of Ru(bpy)2Cl2, 0.11 mmol of the appropriately substituted
N-(arylcarboxy)-8-aminoquinoline ligand and 1.1 mL of 0.1 M
aqueous tetrabutylammonium hydroxide. Maintaining an argon
atmosphere the purple suspension was allowed to reflux with stirring
for 5 h resulting in a deep reddish/brown solution. The methanol was
then removed on a rotary evaporator resulting in a crude aqueous
solution of the [Ru(bpy)2(R-CAQN)]Cl salt. Additional water was
added (5 mL), and any unreacted ligand was removed by gravity
filtration. To the deep red homogeneous filtrate was added dropwise 1
M aqueous KPF6 until a dark red precipitate developed. The solid was
isolated by vacuum filtration on a medium porosity sintered funnel.
Recrystallization from acetone and diethyl ether gave analytically pure
product in 70−80% yield.
Synthetic Procedures. Materials. 8-Aminoquinoline, toluoyl
chloride, 4-methoxy-2,3,5,6-tetrafluorobenzoyl chloride, triethylamine,
tetrabutylammoniumhydroxide (0.1 M in methanol), potassium
hexafluorophosphate, and spectroscopic grade acetonitrile were
purchased from Aldrich and used as received. 4-Fluorobenzoyl
chloride, 4-trifluoromethylbenzoyl chloride, 3,5-bis(trifluoromethyl)-
benzoyl chloride, and pentafluorobenzoyl chloride were purchased
from Oakwood Chemicals and used as received. Reagent grade
dichloromethane, methanol, acetone, and diethyl ether were purchased
from Pharmco Aaper and used as received. Tetrabutyl ammonium
hexafluorophosphate (Aldrich) was recrystallized thrice from hot
ethanol and dried under vacuum prior to use. The complexes
[Ru(bpy)2(Tolyl-CAQN)](PF6) (2+). 1H NMR δ[(CD3)2CO]: 2.14 (s,
3H), 6.52 (d, 2H, J = 7.6 Hz), 6.59 (s, 2H), 7.02−7.06 (m, 1H), 7.21−
7.25 (m, 1H), 7.32−7.40 (m, 2H), 7.49 (d, 2H, J = 5.2 Hz), 7.54−7.57
(m, 1H), 7.61−7.71 (m, 3H), 7.75 (d, 1H, J = 5.2 Hz), 7.98 (ddd, 1H,
J = 0.4, 1.6, 8.0 Hz), 8.03 (ddd, 1H, J = 0.4, 1.6, 8.0 Hz), 8.11−8.21
(m, 5H), 8.53−8.59 (m, 3H), 9.27 (d, 1H, J = 5.2 Hz) ppm. 19F NMR
−
δ[(CD3)2CO]: −72.39 (d, J = 707 Hz) ppm. ESI-MS [m/z (−PF6 )]:
calcd 675.1; obsd 675.1. Anal. Calcd for C37H29F6N6OPRu: C, 54.21;
H, 3.57; N, 10.25. Found: C, 54.11; H, 3.70; N, 10.11.
1
[Ru(bpy)2(F-CAQN)](PF6) (3+). H NMR δ[(CD3)2CO]: 6.48 (dd,
2H, J = 8.8, 8.8 Hz), 6.77 (s, 2H), 7.08 (ddd, 1H, J = 1.6, 3.2, 6.4 Hz),
7.24 (dd, 1H, J = 3.2, 4.8 Hz), 7.33−7.41 (m, 2H), 7.51−7.59 (m,
3H), 7.61−7.73 (m, 3H), 7.77 (d, 1H, J = 6.0 Hz), 8.00 (ddd, 1H, J =
0.8, 1.6, 8.4 Hz), 8.05 (ddd, 1H, J = 0.8, 1.6, 8.4 Hz), 8.12−8.23 (m,
5H), 8.57−8.59 (m, 3H), 9.27 (d, 1H, J = 5.4 Hz) ppm. 19F NMR
δ[(CD3)2CO]: −72.40 (d, J = 707 Hz), −113.68 (s) ppm. ESI-MS
[m/z (−PF6−)]: calcd 679.1; obsd 679.1. Anal. Calcd for
C36H26F7N6OPRu: C, 52.50; H, 3.18; N, 10.20. Found: C, 52.26; H,
3.28; N, 10.08.
59
60
Ru(bpy)2Cl2 and [Ru(bpy)3](PF6)2 were prepared according to
the literature. Ligands N-(carboxy-4-tolyl)-8-aminoquinoline (L2),24
N-(carboxy-4-fluorophenyl)-8-aminoquinoline (L3),24 and N-(car-
boxy-4-trifluoromethylphenyl)-8-aminoquinoline (L4)61 were synthe-
sized as described below, and their analytical data was consistent with
those reported previously.
General Procedure for Synthesis of N-Carboxyaryl-8-amino-
quinoline (R-CAQN·H) Ligands. To a 5 mL solution of dry CH2Cl2
under an argon atmosphere was added sequentially 8-aminoquinoline
(0.432 g, 3 mmol) and Et3N (0.43 mL, 3.1 mmol). The appropriate
benzoyl chloride (3 mmol) was then added dropwise if liquid or
directly if a solid. The resulting mixture was stirred at room
temperature for 1 h maintaining an argon atmosphere. The dark
brown solution was then transferred to a separating funnel and washed
with water, sat. NaHCO3, and brine. The organic layer was dried over
MgSO4, and the solvent was removed under reduced pressure.
Typically a pale brown solid or oil was recovered. In all cases
analytically pure compound was obtained in 70−80% yield by
recrystallization from a concentrated dichloromethane solution by
addition of excess hexanes.
1
[Ru(bpy)2(CF3Ph-CAQN)](PF6) (4+). H NMR δ[(CD3)2CO]: 6.88
(s, 2H), 7.01−7.05 (m, 3H), 7.25 (dd, 1H, J = 4.8, 8.0 Hz), 7.36−7.40
(m, 1H), 7.42 (dd, 1H, J = 1.2, 8.0 Hz), 7.51 (ddd, 1H, J = 0.8, 1.2, 6.4
Hz), 7.55−7.59 (m, 2H), 7.62−7.67 (m, 2H), 7.72 (dd, 1H, J = 1.2,
5.2 Hz), 7.74 (ddd, 1H, J = 0.8, 1.2, 5.6 Hz), 8.01 (ddd, 1H, J = 0.8,
1.2, 12 Hz), 8.07 (ddd, 1H, J = 0.4, 1.2, 12 Hz), 8.15−8.19 (m, 3H),
8.25 (dd, 1H, J = 1.6, 8.4 Hz), 8.51 (dd, 1H, J = 1.2, 8.0 Hz), 8.56−
8.61 (m, 3H), 9.23 (ddd, 1H, J = 0.8, 1.6, 6.0 Hz) ppm. 19F NMR
δ[(CD3)2CO]: −62.85 (s), −72.43 (d, J = 711 Hz) ppm. ESI-MS [m/
z
(−PF6−)]: calcd 729.1; obsd 729.1. Anal. Calcd for
C37H26F9N6OPRu: C, 50.87; H, 3.00; N, 9.62. Found: C, 50.62; H,
3.06; N, 9.48.
1
[Ru(bpy)2(CF3)2Ph-CAQN](PF6) (5+). H NMR δ[(CD3)2CO]: 7.00
N-(Carboxy-3,5-bis(trifluoromethyl)phenyl)-8-aminoquinoline
1
(L5). H NMR δ[(CD3)2SO]: 7.65−7.70 (m, 2H), 7.83 (dd, 1H, J =
(ddd, 1H, J = 0.8, 1.6, 13.6 Hz), 7.28 (dd, 1H, J = 5.2, 8.4 Hz), 7.30−
7.45 (m, 3H), 7.48 (dd, 1H, J = 1.2, 8.0 Hz), 7.51 (ddd, 1H, J = 0.8,
1.6, 5.6 Hz), 7.56−7.63 (m, 3H), 7.65−7.71 (m, 3H), 7.75 (dd, 1H, J
= 1.6, 4.8 Hz), 8.02 (ddd, 1H, J = 0.8, 1.6, 16.0 Hz), 8.10 (ddd, 1H, J =
0.8, 1.6, 16.0 Hz), 8.13−8.20 (m, 2H), 8.23 (d, 1H, J = 7.2 Hz), 8.29
(dd, 1H, J = 1.6, 8.4 Hz), 8.47 (dd, 1H, J = 1.6, 8.4 Hz), 8.58−8.64 (m,
3H), 9.33 (ddd, 1H, J = 0.8, 1.6, 16.0 Hz) ppm. 19F NMR
1.2, 8.0 Hz), 8.42 (s, 1H), 8.47 (dd, 1H, J = 1.6, 8.0 Hz), 8.55 (dd, 1H,
J = 1.20, 8.0 Hz), 8.65 (s, 2H), 8.99 (dd, 1H, J = 1.6, 4.0 Hz), 10.94 (s,
1H) ppm. 13C NMR δ[(CD3)2SO]: 109.56, 119.64, 121.75, 122.27,
123.82, 125.36, 126.77, 128.05, 128.41, 130.49, 130.82, 133.86, 136.67,
137.16, 139.48, 144.80, 149.51, 162.69 ppm. 19F NMR δ[(CD3)2SO]:
−61.00 (s) ppm. LC−MS (m/z): calcd (M + 1) 385.1; obsd 385.1.
K
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
δ[(CD3)2CO]: −62.97 (s), −72.42 (d, J = 707 Hz) ppm. ESI-MS [m/
(12) Weisser, F.; Plebst, S.; Hohloch, S.; van der Meer, M.; Manck,
z
(−PF6−)]: calcd 797.1; obsd 797.1. Anal. Calcd for
S.; Fuhrer, F.; Radtke, V.; Leichnitz, D.; Sarkar, B. Inorg. Chem. 2015,
̈
C38H25F12N6OPRu: C, 48.47; H, 2.68; N, 8.92. Found: C, 48.24; H,
54, 4621−4635.
2.88; N, 8.81.
(13) Robson, K. C. D.; Bomben, P. G.; Berlinguette, C. P. Dalton
Trans. 2012, 41, 7814−7829.
1
{Ru(bpy)2[F4(MeO)Ph-CAQN]}(PF6) (6+). H NMR δ[(CD3)2CO]:
3.95 (t, 3H, J = 1.5 Hz), 7.20 (ddd, 1H, J = 1.2, 5.6, 7.2 Hz), 7.28 (dd,
1H, J = 4.8, 8.0 Hz), 7.39 (ddd, 1H, J = 1.2, 6.4, 7.2 Hz), 7.54 (ddd,
1H, J = 1.2, 6.6, 7.6 Hz), 7.59 (dd, 1H, J = 1.2, 8.0 Hz), 7.64−7.70 (m,
4H), 7.75 (dd, 1H, J = 1.2, 4.8 Hz), 7.84 (ddd, 1H, J = 0.8, 1.6, 7.2
Hz), 8.01−8.09 (m, 2H), 8.14 (ddd, 1H, J = 0.8, 1.2, 5.6 Hz), 8.19
(ddd, 1H, J = 0.8, 1.2, 5.6 Hz), 8.30 (dd, 1H, J = 1.2, 8.0 Hz), 8.46 (d,
1H, J = 8.0 Hz), 8.57−8.61 (m, 2H), 8.69 (d, 1H, J = 8.0 Hz), 9.01 (m,
1H), 9.45 (s, 1H) ppm. 19F NMR δ[(CD3)2CO]: −72.43 (d, J = 707
Hz), −143.96 (m), −147.14 (m), 157.43 (m), 158.29 (m) ppm. ESI-
(14) Radwan, Y. K.; Maity, A.; Teets, T. S. Inorg. Chem. 2015, 54,
7122−7131.
(15) Allu, S.; Swamy, K. C. K. J. Org. Chem. 2014, 79, 3963−3972.
(16) Begum, R. A.; Day, V. W.; Kumar, M.; Gonzalez, J.; Jackson, T.
A.; Bowman-James, K. Inorg. Chim. Acta 2014, 417, 287−293.
(17) Harrop, T. C.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem.
2005, 44, 9527−9533.
(18) Zhang, S.-Y.; Li, Q.; He, G.; Nack, W. A.; Chen, G. J. Am. Chem.
Soc. 2015, 137, 531−539.
−
MS [m/z (−PF6 )]: calcd 763.1; obsd 763.1. Anal. Calcd for
(19) Liu, S.; Sun, W.-H.; Zeng, Y.; Wang, D.; Zhang, W.; Li, Y.
Organometallics 2010, 29, 2459−2464.
C37H25F10N6O2PRu: C, 48.96; H, 2.78; N, 9.26. Found: C, 48.70; H,
2.85; N, 9.18.
(20) Wang, K.; Shen, M.; Sun, W.-H. Dalton Trans. 2009, 4085−
4095.
ASSOCIATED CONTENT
* Supporting Information
(21) Lee, J. C., Jr.; Peris, E.; Rheingold, A. L.; Crabtree, R. H. J. Am.
Chem. Soc. 1994, 116, 11014−11019.
́
(22) Gregolinski, J.; Hikita, M.; Sakamoto, T.; Sugimoto, H.;
Tsukube, H.; Miyake, H. Inorg. Chem. 2016, 55, 633−643.
(23) Begum, R. A.; Day, V. W.; Kumar, M.; Gonzalez, J.; Jackson, T.
A.; Bowman-James, K. Inorg. Chim. Acta 2014, 417, 287−293.
(24) Lee, J. C.; Peris, E.; Rheingold, A. L.; Crabtree, R. H. J. Am.
Chem. Soc. 1994, 116, 11014−11019.
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02834.
NMR and MS spectra, crystallographic parameters,
TDDFT data and Cartesian coordinates, and complete
ref 51 (PDF)
(25) Roane, J.; Daugulis, O. Org. Lett. 2013, 15, 5842−5845.
(26) Wu, F.; Riesgo, E.; Pavalova, A.; Kipp, R. A.; Schmehl, R. H.;
Thummel, R. P. Inorg. Chem. 1999, 38, 5620−5628.
Crystallographic data (CIF)
(27) Philips, J. J.; Hudspeth, M. A.; Browne, P. M., Jr.; Peralta, J. E.
Chem. Phys. Lett. 2010, 495, 146−150.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jonathan.rochford@umb.edu.
■
(28) Daul, C.; Baerends, E. J.; Vernooijs, P. Inorg. Chem. 1994, 33,
3538−3543.
(29) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967−3977.
(30) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Vonzelewsky, A. Coord. Chem. Rev. 1988, 84, 85−277.
(31) The actual point groups of 12+ and 2+ are D3 and C1,
respectively, if the peripheral ligand structures are considered. The
pseudo-Oh and pseudo-Cs point groups are used here for simplicity.
(32) Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C.
V.; McCusker, J. K. Science 1997, 275, 54−57.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
(33) O’Donnell, R. M.; Johansson, P. G.; Abrahamsson, M.; Meyer,
G. J. Inorg. Chem. 2013, 52, 6839−6848.
ACKNOWLEDGMENTS
■
We thank Dr. E. Fujita of Brookhaven National Laboratory for
the use of the Bruker Kappa Apex II diffractometer for X-ray
data collection.
(34) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952−957.
(35) Muro-Small, M. L.; Yarnell, J. E.; McCusker, C. E.; Castellano,
F. N. Eur. J. Inorg. Chem. 2012, 2012, 4004−4011.
(36) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.;
Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc.
̈
REFERENCES
■
1993, 115, 6382−6390.
(1) Broere, D. L. J.; Plessius, R.; van der Vlugt, J. I. Chem. Soc. Rev.
2015, 44, 6886−6915.
(37) Biner, M.; Buergi, H. B.; Ludi, A.; Roehr, C. J. Am. Chem. Soc.
1992, 114, 5197−5203.
(2) Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev. 2013, 42, 1440−
(38) Kaim, W.; Fiedler, J. Chem. Soc. Rev. 2009, 38, 3373−3382.
(39) Mondal, P.; Ray, R.; Das, A.; Lahiri, G. K. Inorg. Chem. 2015, 54,
3012−3021.
1459.
(3) Boyer, J. L.; Rochford, J.; Tsai, M. K.; Muckerman, J. T.; Fujita, E.
Coord. Chem. Rev. 2010, 254, 309−330.
(4) Jørgensen, C. K. Coord. Chem. Rev. 1966, 1, 164−178.
(5) Karen, P. Angew. Chem., Int. Ed. 2015, 54, 4716−4726.
(6) Kaim, W. Eur. J. Inorg. Chem. 2012, 2012, 343−348.
(7) Ingram, J. D.; Costa, P. J.; Adams, H.; Ward, M. D.; Fel
Thomas, J. A. Inorg. Chem. 2012, 51, 10483−10494.
(8) Zhao, H. C.; Fu, B.-L.; Schweinfurth, D.; Harney, J. P.; Sarkar, B.;
Tsai, M.-K.; Rochford, J. Eur. J. Inorg. Chem. 2013, 2013, 4410−4420.
(9) Bellinger-Buckley, S.; Chang, T.-C.; Bag, S.; Schweinfurth, D.;
Zhou, W.; Torok, B.; Sarkar, B.; Tsai, M.-K.; Rochford, J. Inorg. Chem.
2014, 53, 5556−5567.
(10) Mandal, A.; Grupp, A.; Schwederski, B.; Kaim, W.; Lahiri, G. K.
Inorg. Chem. 2015, 54, 10049−10057.
(11) Laine, T. M.; Karkas, M. D.; Liao, R.-Z.; Siegbahn, P. E. M.;
Akermark, B. Chem. - Eur. J. 2015, 21, 10039−10048.
(40) Goodman, B. A.; Raynor, J. B. Electron Spin Resonance of
Transition Metal Complexes; Academic Press: New York, 1970; Vol. 13,
pp 135−362.
(41) DeSimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92,
2343−2352.
́
ix, V.;
(42) Patra, S.; Sarkar, B.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Inorg.
Chem. 2003, 42, 6469−6473.
(43) Weisser, F.; Huebner, R.; Schweinfurth, D.; Sarkar, B. Chem. -
Eur. J. 2011, 17, 5727−5736.
(44) Sarkar, B.; Schweinfurth, D.; Deibel, N.; Weisser, F. Coord.
Chem. Rev. 2015, 293−294, 250−262.
(45) Masui, H.; Lever, A. B. P.; Auburn, P. R. Inorg. Chem. 1991, 30,
2402−2410.
(46) Lever, A. B. P. Can. J. Chem. 2009, 87, 1451−1459.
L
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(47) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;
Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I.
Organometallics 2010, 29, 2176−2179.
(48) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877−910.
(49) Fajer, J.; Fujita, I.; Davis, M. S.; Forman, A.; Hanson, L. K.;
Smith, K. M. Adv. Chem. Ser. 1982, 201, 489−513.
(50) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,
2999−3093.
(51) Frisch, M. J.; et al. Gaussian 09, Revision A.1; Gaussian Inc.:
Wallingford, CT, 2009.
(52) Roy, L. E.; Hay, P. J.; Martin, R. L. J. Chem. Theory Comput.
2008, 4, 1029−1031.
(53) Harihara, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213−
222.
(54) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; Defrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654−3665.
(55) Scalmani, G.; Frisch, M. J.; Mennucci, B.; Tomasi, J.; Cammi, R.;
Barone, V. J. Chem. Phys. 2006, 124, 94107.
(56) Hu, K.; Severin, H. A.; Koivisto, B. D.; Robson, K. C. D.; Schott,
E.; Arratia-Perez, R.; Meyer, G. J.; Berlinguette, C. P. J. Phys. Chem. C
2014, 118, 17079−17089.
(57) Sheldrick, G. M. SHELXL Version 5; Siemens Analytical
Instruments, Inc.: Madison, WI, 1994.
(58) Sheldrick, G. M. SADABS Version 2007/2; Bruker AXS Inc.
(59) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17,
3334−3341.
(60) Browne, W. R.; Passaniti, P.; Gandolfi, M. T.; Ballardini, R.;
Henry, W.; Guckian, A.; O’Boyle, N.; McGarvey, J. J.; Vos, J. G. Inorg.
Chim. Acta 2007, 360, 1183−1190.
(61) Rouffet, M.; de Oliveira, C. A. F.; Udi, Y.; Agrawal, A.; Sagi, I.;
McCammon, J. A.; Cohen, S. M. J. Am. Chem. Soc. 2010, 132, 8232−
8233.
M
DOI: 10.1021/acs.inorgchem.5b02834
Inorg. Chem. XXXX, XXX, XXX−XXX