The redox series [Ru(bpy)2(L)]n, n = +3, +2, +1, 0, with L = bipyridine, click derived pyridyl-triazole or bis-triazole: A combined structural, electrochemical, spectroelectrochemical and DFT investigation

Article abstract of DOI:10.1039/c3dt52898g

The compounds [Ru(bpy)₂(L¹)](ClO₄) ₂ (1(ClO₄)₂), [Ru(bpy)₂(L ²)](ClO₄)₂ (2(ClO₄)₂), [Ru(bpy)₂(L³)](ClO₄)₂ (3(ClO ₄)₂), [Ru(bpy)₂(L⁴)](ClO ₄)₂ (4(ClO₄)₂), [Ru(bpy) ₂(L⁵)](ClO₄)₂ (5(ClO ₄)₂), and [Ru(bpy)₂(L⁶)](ClO ₄)₂6(ClO₄)₂ (bpy = 2,2′-bipyridine, L¹ = 1-(4-isopropyl-phenyl)-4-(2-pyridyl)-1,2, 3-triazole, L² = 1-(4-butoxy-phenyl)-4-(2-pyridyl)-1,2,3-triazole, L³ = 1-(2-trifluoromethyl-phenyl)-4-(2-pyridyl)-1,2,3-triazole, L⁴ = 4,4′-bis-{1-(2,6-diisopropyl-phenyl)}-1,2,3-triazole, L⁵ = 4,4′-bis-{(1-phenyl)}-1,2,3-triazole, L⁶ = 4,4′-bis-{1-(2-trifluoromethyl-phenyl)}-1,2,3-triazole) were synthesized from [Ru(bpy)₂(EtOH)₂](ClO₄)₂ and the corresponding click -derived pyridyl-triazole or bis-triazole ligands, and characterized by ¹H-NMR spectroscopy, elemental analysis, mass spectrometry and X-ray crystallography. Structural analysis showed a distorted octahedral coordination environment about the Ru(ii) centers, and shorter Ru-N(triazole) bond distances compared to Ru-N(pyridine) distances in complexes of mixed-donor ligands. All the complexes were subjected to cyclic voltammetric studies, and the results were compared to the well-known [Ru(bpy)₃]²⁺ compound. The oxidation and reduction potentials were found to be largely uninfluenced by ligand changes, with all the investigated complexes showing their oxidation and reduction steps at rather similar potentials. A combined UV-vis-NIR and EPR spectroelectrochemical investigation, together with DFT calculations, was used to determine the site of electron transfer in these complexes. These results provided insights into their electronic structures in the various investigated redox states, showed subtle differences in the spectroscopic signatures of these complexes despite their similar electrochemical properties, and provided clues to the unperturbed redox potentials in these complexes with respect to ligand substitutions. The reduced forms of the complexes display structured absorption bands in the NIR region. Additionally, we also present new synthetic routes for the ligands presented here using Cu-abnormal carbene catalysts.

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The redox series [Ru(bpy)2(L)]n, n = +3, +2, +1, 0,
with L = bipyridine, “click” derived pyridyl-triazole
or bis-triazole: a combined structural,
electrochemical, spectroelectrochemical and DFT
investigation†
Cite this: Dalton Trans., 2014, 43,
4437
Stephan Hohloch,a David Schweinfurth,a Michael G. Sommer,a Fritz Weisser,a
Naina Deibel,b Fabian Ehretb and Biprajit Sarkar*a
The compounds [Ru(bpy)2(L1)](ClO4)2 (1(ClO4)2), [Ru(bpy)2(L2)](ClO4)2 (2(ClO4)2), [Ru(bpy)2(L3)](ClO4)2
(3(ClO4)2), [Ru(bpy)2(L4)](ClO4)2 (4(ClO4)2), [Ru(bpy)2(L5)](ClO4)2 (5(ClO4)2), and [Ru(bpy)2(L6)](ClO4)2
6(ClO4)2 (bpy = 2,2’-bipyridine, L1 = 1-(4-isopropyl-phenyl)-4-(2-pyridyl)-1,2,3-triazole, L2 = 1-(4-
butoxy-phenyl)-4-(2-pyridyl)-1,2,3-triazole, L3
= 1-(2-trifluoromethyl-phenyl)-4-(2-pyridyl)-1,2,3-tri-
azole, L4 = 4,4’-bis-{1-(2,6-diisopropyl-phenyl)}-1,2,3-triazole, L5 = 4,4’-bis-{(1-phenyl)}-1,2,3-triazole,
L6 = 4,4’-bis-{1-(2-trifluoromethyl-phenyl)}-1,2,3-triazole) were synthesized from [Ru(bpy)2(EtOH)2]-
(ClO4)2 and the corresponding “click”-derived pyridyl-triazole or bis-triazole ligands, and characterized by
1H-NMR spectroscopy, elemental analysis, mass spectrometry and X-ray crystallography. Structural analy-
sis showed a distorted octahedral coordination environment about the Ru(II) centers, and shorter Ru–N-
(triazole) bond distances compared to Ru–N(pyridine) distances in complexes of mixed-donor ligands. All
the complexes were subjected to cyclic voltammetric studies, and the results were compared to the well-
known [Ru(bpy)3]2+ compound. The oxidation and reduction potentials were found to be largely uninfl-
uenced by ligand changes, with all the investigated complexes showing their oxidation and reduction
steps at rather similar potentials. A combined UV-vis-NIR and EPR spectroelectrochemical investigation,
together with DFT calculations, was used to determine the site of electron transfer in these complexes.
These results provided insights into their electronic structures in the various investigated redox states,
showed subtle differences in the spectroscopic signatures of these complexes despite their similar elec-
trochemical properties, and provided clues to the unperturbed redox potentials in these complexes with
respect to ligand substitutions. The reduced forms of the complexes display structured absorption bands
in the NIR region. Additionally, we also present new synthetic routes for the ligands presented here using
Cu-abnormal carbene catalysts.
Received 14th October 2013,
Accepted 21st November 2013
DOI: 10.1039/c3dt52898g
www.rsc.org/dalton
transfer properties,3a–h photochemistry,4 supramolecular
chemistry,5 magnetism,6 catalysis7 and anti-tumor agents.8 We
Introduction
Substituted 1,2,3-triazole ligands synthesized via the Cu(I) cata- have been interested in applying metal complexes of substi-
lyzed “click” reaction between alkynes and azides1 have found tuted 1,2,3-triazole ligands in various fields of chemistry.
a prominent place in the coordination chemists toolbox in Thus, we have investigated Pd(II) and Pt(II) complexes with
recent years.2 Metal complexes of these ligands have been bidentate pyridyl-triazole ligands for their electron transfer
investigated for various research objectives such as electron properties,9 Co(II) and Ni(II) complexes with tripodal triazole
ligands for their magnetic properties,10 Fe(II) and Co(II) com-
plexes of tripodal triazole ligands and Ni(II) complexes of
aInstitut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin,
Fabeckstraße 34-36, D-14195 Berlin, Germany. E-mail: biprajit.sarkar@fu-berlin.de
bInstitut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55,
pyridyl-triazole ligands as pre-catalysts for ethylene poly- and
oligomerisation,11 Cu(I) complexes of abnormal carbenes
derived from 1,2,3-triazoles as catalysts for the azide–alkyne
“click” cycloaddition reaction,12 and Pd(II) complexes of abnor-
mal carbenes for the Suzuki–Miyaura cross-coupling
reactions.13
D-70550 Stuttgart, Germany
†Electronic supplementary information (ESI) available. CCDC 846891, 846892,
892295 and 906187. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt52898g
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Since investigating the electronic and geometric structures
of metal complexes is an all important goal in coordination
chemistry,14 and since our group has been using a combined
electrochemical, spectroelectrochemical and theoretical
approach to address the electronic structures of various metal
complexes,15 we were interested in investigating a series of
metal complexes with a redox-active metal center where the
electronic properties of the “click” derived ligands would be
varied by choosing different azides for ligand synthesis. Ruthe-
nium complexes with “click” derived triazole ligands have
been investigated in this context. However, most of these
studies are restricted to cyclic voltammetry and DFT calcula-
tions.3f–h To the best of our knowledge, the only studies that
have applied a combined approach of various techniques
including spectroelectrochemistry for addressing issues
related to the electronic structures of such complexes are the
ones where diruthenium complexes with “click” derived
Fig. 1 ORTEP plot of L3. Ellipsoids are drawn at 50% probability. Hydro-
gen atoms and solvent molecules have been omitted for clarity.
ligands have been investigated for their mixed-valent proper- elsewhere. Single crystals of L3 were obtained by slow evapor-
ties.3d The electronic and electrochemical properties of the ation of a diethylether solution at room temperature. L3 crystal-
ruthenium complexes of 1,2,4-triazoles have been previously lizes in the tetragonal I41/a space group with 0.31 molecule of
investigated by Vos et al.3i,j
strongly disordered diethylether (Table S1†). The central N1vN3
In the following, we present the synthesis and characteriz- bond of the triazole rings in L3 and L4 7g are 1.303(2) and 1.305(2) Å,
ation of the complexes [Ru(bpy)2(L1)](ClO4)2 (1(ClO4)2), respectively (Table 2, Fig. 1), and these distances are typical of
[Ru(bpy)2(L2)](ClO4)2
(2(ClO4)2),
[Ru(bpy)2(L3)](ClO4)2 a NvN double bond. That bond in both the ligands is flanked
(3(ClO4)2), [Ru(bpy)2(L4)](ClO4)2 (4(ClO4)2), [Ru(bpy)2(L5)]- by N1–C3 and N3–N5 bonds which are longer than N1vN3
(ClO4)2 (5(ClO4)2), and [Ru(bpy)2(L6)](ClO4)2 6(ClO4)2 (bpy = (Table 2). Thus, the best localized description of the bonding
2,2′-bipyridine, L1 = 1-(4-isopropyl-phenyl)-4-(2-pyridyl)-1,2,3- situation in these triazole rings is that of a central NvN
triazole, L2
L3
L4 = 4,4′-bis-{1-(2,6-diisopropyl-phenyl)}-1,2,3-triazole, L5 = 4,4′-
=
1-(4-butoxy-phenyl)-4-(2-pyridyl)-1,2,3-triazole, double bond flanked by N–C and N–N single bonds. Similar
1-(2-trifluoromethyl-phenyl)-4-(2-pyridyl)-1,2,3-triazole, observations have been made by us and others previously.9,17
The pyridyl-N atom is anti to the triazole N atoms in L3. The
=
bis-{(1-phenyl)}-1,2,3-triazole, L6 = 4,4′-bis-{1-(2-trifluoromethyl- pyridyl and the triazole rings are almost coplanar with a di-
phenyl)}-1,2,3-triazole). The ligands L have been varied from hedral angle of only 2.87(7)°. In contrast, the 2-(trifluoro-
pyridyl-triazole to bis-triazoles with the electronic properties of methyl)-phenyl is largely twisted with respect to the triazole
the substituents on the triazole rings being different in each ring with a dihedral angle of 65.29(7)° (Fig. 1). The two triazole
case. Results obtained from 1H-NMR spectroscopy, elemental rings in L4 are anti to each other. The deviation from planarity
analysis, single crystal X-ray crystallography, cyclic voltamme- of those rings is minimal as seen from a dihedral angle of
try, UV-vis-NIR and EPR spectroelectrochemistry and DFT cal- 9.05(7)°. The two 2,6-(diisopropyl)-phenyl substituents are
culations are presented. The compounds are compared with almost perpendicular to the triazole rings with dihedral angles
the parent compound [Ru(bpy)3]2+. The data obtained from of 89.90(7)° and 88.45(7)°.7g
this combined approach are used to understand the properties
of these complexes and to rationalize the observed trends.
For the synthesis of the complexes, Ru(bpy)2Cl2 was first
activated with AgClO4 to generate [Ru(bpy)2(EtOH)2](ClO4)2
through extraction of the chloride ions and precipitation of
AgCl. The thus in situ generated [Ru(bpy)2(EtOH)2](ClO4)2 was
reacted with the ligands L1–L6 under reflux in ethanol to gene-
rate the complexes 1(ClO4)2–6(ClO4)2 respectively (Fig. 2).
Chromatography on alumina and subsequent precipitation
Results and discussion
Synthesis and crystal structure
The pyridyl-triazole ligands L1–L3 have been reported by us with NaClO4 delivered analytical pure complexes (see the
previously.9,11b In this work, we used a new synthetic route Experimental section). The purity of the complexes was
developed in our laboratory for synthesizing the ligands L1 demonstrated by 1H NMR spectroscopy, elemental analysis,
and L2.12 The new route takes advantage of the highly robust and mass spectrometry.
and oxidation resistant Cu(I)-abnormal carbene catalysts for
Crystals of 3(ClO4)2, 4(ClO4)2 and 6(ClO4)2·CH2Cl2 for single
catalyzing the “click” reaction. This procedure delivered L1 and crystal X-ray diffraction studies were obtained by slow
L2 in superior yields to those reported by us previously (see the diffusion of n-hexane into dichloromethane. 3(ClO4)2 and
Experimental section). The bis-triazole ligands L4–L6 were pre- 6(ClO4)2·CH2Cl2 crystallize in the triclinic P1 space group, and
ˉ
pared by modifications of literature procedures.3c,16 To the 4(ClO4)2 in the monoclinic P21/n space group (Table 1). The
best of our knowledge, the ligand L6 has not been reported bond lengths and bond angles within these three complexes
4438 | Dalton Trans., 2014, 43, 4437–4450
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Fig. 2 Complexes synthesized for the present study.
thus present us with the opportunity to compare the the two bpy ligands, and the pyridine and/or triazole N donors
complexes with pyridyl-triazole ligands with those of bis-tri- from the third chelating ligand (Fig. 3–5, Tables 3 and 4).
azoles. In all the three complexes, the ruthenium center is co-
The Ru–N bond distances to the four N atoms of the two
ordinated in distorted octahedral environment, the bpy rings are all in a similar range for all three complexes
a
distortion being caused by the chelating nature of the ligands, (Table 3), and match well with distances for similar complexes
as well as the presence of different kinds of donor atoms. The reported in the literature.18 Comparison of the Ru–N(triazole)
donor atoms in all cases are the four pyridine-N atoms from distances in the three complexes shows some similarities and
Table 1 Crystallographic details of complexes
3 (ClO4)2
4 (ClO4)2
6 (ClO4)2·CH2Cl2
Chemical formula
Mr
Crystal system, space group
C34H25N8F3Ru Cl2O8
902.59
Triclinic, P1
C48H52N10Ru Cl2O8
1068.97
Monoclinic, P2(1)/n
C38H26N10F6Ru Cl2O8 CH2Cl2
1121.58
Triclinic, P1
ˉ
ˉ
a, b, c (Å)
α, β, γ (°)
V (Å3)
10.6547(12), 11.6352(12), 14.5178(16)
89.532(5), 77.126(5), 87.806(4)
1753.2(3)
2
1.710
908
18.5069(12), 15.2755(11), 19.3531(13)
90, 116.969(4), 90
4876.2(6)
4
1.456
2208
11.199(2), 12.433(2), 16.087(3)
89.218(4), 87.674(4), 88.700(4)
2237.2(7)
2
1.665
Z
Density (g cm−3
F(000)
)
1124
Radiation type
Mo Kα
0.681
Cu Kα
4.142
Mo Kα
0.678
μ (mm−1
)
Crystal size
Meas. Refl.
Indep. Refl.
0.27 × 0.17 × 0.07
24 195
7226
5206
0.0565
0.0406, 0.0764, 1.005
0.523, −0.549
0.20 × 0.15 × 0.02
28 019
8249
5749
0.0692
0.0464, 0.1228, 1.013
1.152, −0.526
0.8 × 0.25 × 0.08
31 477
10 966
9419
0.0200
0.0821, 0.2396, 1.045
4.828, −2.990
Obsvd. [I > 2σ(I)] refl.
Rint
R [F2 > 2σ(F2)], wR(F2), S
Δρmax, Δρmin (e Å−3
)
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Table 2 Selected bond lengths of the ligands in Å
L3·0.31 Et2O
L4,7g
C1–C3
N1–N3
N1–C3
N3–N5
N5–C1
N2–C4
C3–C4
1.367(3)
1.303(2)
1.361(3)
1.353(2)
1.344(2)
1.335(3)
1.468(3)
1.366(2)
1.305(2)
1.362(2)
1.352(2)
1.350(2)
1.364(2)
1.454(2)
Fig. 5 ORTEP plot of 6(ClO4)2·CH2Cl2. Ellipsoids are drawn at 50%
probability. Hydrogen atoms, solvent molecule and counter ions have
been omitted for clarity.
Table 3 Selected bond lengths in the complexes in Å
3 (ClO4)2
4 (ClO4)2
6 (ClO4)2·CH2Cl2
N1–Ru1
N2–Ru1
N7–Ru1
N8–Ru1
N9–Ru1
N10–Ru1
2.090(3)
2.026(3)
2.055(3)
2.049(1)
2.051(3)
2.044(3)
2.065(3)
2.052(3)
2.070(3)
2.059(3)
2.062(3)
2.068(4)
2.054(4)
2.060(4)
2.057(4)
2.048(4)
2.049(4)
2.062(4)
Fig. 3 ORTEP plot of 3(ClO4)2. Ellipsoids are drawn at 50% probability.
Hydrogen atoms and counter ions have been omitted for clarity.
Table 4 Selected bond angles in the complexes in °
3 (ClO4)2
4 (ClO4)2
6 (ClO4)2·CH2Cl2
N1–Ru–N2
N2–Ru–N10
N10–Ru–N9
N9–Ru–N8
N8–Ru–N7
N2–Ru–N9
N2–Ru–N7
N9–Ru–N7
N8–Ru–N1
N10–Ru–N1
N7–Ru–N1
N8–Ru–N10
N1–Ru–N9
N2–Ru–N8
N10–Ru–N7
78.3(1)
85.4(1)
78.8(1)
94.0(1)
78.7(1)
94.4(1)
98.3(1)
101.7(1)
94.4(1)
95.4(1)
84.7(1)
97.5(1)
170.3(1)
172.4(2)
176.2(2)
76.9(1)
82.9(1)
79.0(1)
86.7(1)
78.4(1)
96.5(1)
98.7(1)
100.7(1)
100.1(1)
95.3(1)
85.1(1)
100.0(1)
171.9(1)
176.1(1)
178.4(2)
77.4(2)
89.9(2)
78.9(2)
90.3(2)
79.2(2)
95.9(1)
95.8(2)
94.9(2)
96.5(2)
99.5(2)
87.2(2)
95.8(2)
173.1(2)
172.3(2)
172.1(2)
other hand, for 4(ClO4)2 and 6(ClO4)2 that contain bis-triazoles
as the third ligand, all the Ru–N distances are almost identical
(Table 3). Thus, the shorter Ru–N(triazole) distance in 3(ClO4)2
is probably due to steric and/or packing effects in the crystal.
The bonding situation within the triazole rings in the com-
plexes remains the same as what has been discussed for the
free ligands above. Thus, in each case inside the triazole ring
we find a central NvN double bond that is flanked by an N–C
and an N–N single bond.9,17 The 2-(trifluoromethyl)-phenyl
Fig. 4 ORTEP plot of 4(ClO4)2. Ellipsoids are drawn at 50% probability.
Hydrogen atoms and counter ions have been omitted for clarity.
differences. Thus, in 3(ClO4)2, which contains the mixed donor
pyridyl-triazole ligand, the Ru1–N2 distance of 2.026(3) Å to the
triazole N donor is the shortest metal–ligand distance in that
complex (Table 3). This observation matches earlier reports on
metal complexes with mixed donor ligands such as L3.9 On the
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Table 5 Electrochemical potentials from cyclic voltammetrya
ring in 3(ClO4)2 is twisted with respect to the triazole ring with
a dihedral angle of 64.6(1)°. For 4(ClO4)2 and 6(ClO4)2, the sub-
stituents on the triazole rings are also out of plane as com-
pared to the triazole rings with dihedral angles of 77.4(2)° and
72.1(2)° for 4(ClO4)2 and 83.9(2)° and 86.2(2)° for 6(ClO4)2.
ox1
1/2
red1
1/2
red2
1/2
red3
E
E
E
E
1/2
12+
0.91
0.90
0.95
0.99
0.93
1.01
0.85
−1.77
−1.77
−1.76
−1.78
−1.79
−1.79
−1.76
−1.97
−1.97
−1.97
−2.00
−2.00
−1.99
−1.92
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
−2.16
22+
32+
42+
52+
Cyclic voltammetry
62+
[Ru(bpy)3]2+,b
The redox properties of the complexes were investigated in
both CH2Cl2–0.1 M Bu4NPF6 and CH3CN–0.1 M Bu4NPF6, and
these results were compared to those of the parent complex
[Ru(bpy)3]2+. The complexes 12+–62+ display a reversible one-
electron oxidation step in both CH2Cl2 and CH3CN. The
shape, position and reversibility of this peak seem to be the
same irrespective of the complex measured and the solvent
used (Fig. 6, 7, S1† and Table 5). The oxidation potentials of
these complexes fall in the same range as that of the parent
complex [Ru(bpy)3]2+.18 Thus, neither the substitution of one
or more of the pyridine rings in bpy nor the presence of
additional substituents in the triazole rings seems to have any
influence on the oxidation potentials of the investigated
a From measurements in CH3CN–0.1 M Bu4NPF6 at 295 K. b From ref.
18. n.o. = not observed.
complexes. However, the slightly higher oxidation potential for
42+ and 62+ seems to correlate with the higher dihedral angles
between the phenyl substituents and the triazole rings in that
molecule as compared to the other cases (see the discussion
on the crystal structure above). The oxidation step seems to be
totally localized on the ruthenium center (see DFT calculations
section), and remains insulated to ligand substitution.
The complex [Ru(bpy)3]2+ displays three one-electron
reduction steps in CH3CN.18 For complexes 12+–62+, only two
reduction processes are observed within the CH3CN solvent
window (Fig. 6, 7 and Table 5). Just like the oxidation step, the
reduction potentials for both the steps for all the investigated
complexes are in the same range, and are comparable to the
first two reduction processes of [Ru(bpy)3]2+. These results are
already an indication of the first two reduction steps in 12+–62+
being similar to those of [Ru(bpy)3]2+. As was seen for the oxi-
dation step, ligand substitution does not seem to have an
influence on the two observed reduction steps of these com-
plexes either. The third reduction observed in CH3CN for
[Ru(bpy)3]2+ is missing for 12+–62+ in the same solvent, and
this seems to be an effect of substituting one bpy in
[Ru(bpy)3]2+ with the ligands L1–L6 in the complexes 12+–62+
respectively. The redox potentials of the complexes reported
here are comparable to the analogous complexes of
[Ru(bpy)2]2+ with substituted 1,2,4-triazoles. Redox potentials
reported by Vos et al. against SCE,3j if converted against the
Fc/Fc+ standard, fit well with the redox potentials of the com-
plexes reported here with substituted 1,2,3-triazoles.
Fig. 6 Cyclic voltammogram of 12+–32+ in CH3CN/0.1 M Bu4NPF6 at
295 K. Ferrocene/ferrocenium was used as an internal standard.
Surprisingly, changing the solvent from CH3CN to CH2Cl2
has a large influence on the reversibility, as well as the appear-
ance of the reduction steps for the complexes 12+–62+
(Fig. S1†). Whereas for 42+ and 52+ the reduction processes in
CH2Cl2 are similar to that in CH3CN, the second reduction
step in CH2Cl2 is irreversible for 12+–32+, and it does not show
up within the CH2Cl2 solvent window for 62+. For comparison,
only two reduction steps are observed for [Ru(bpy)3]2+ in
CH2Cl2. The reason for the non-observance of certain
reduction processes at higher negative potentials in CH2Cl2 is
related to the smaller potential window of that solvent as com-
pared to CH3CN.19 However, even in cases where the same
number of reduction processes are observed in both solvents,
the reversibility of the reduction steps seems to be better in
CH3CN as compared to CH2Cl2. CH2Cl2 is known to react with the
Fig. 7 Cyclic voltammogram of complexes 42+–62+ in CH3CN/0.1 M
Bu4NPF6 at 295 K. Ferrocene/ferrocenium was used as an internal standard.
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reduced states of various metal complexes, and we believe that
such a phenomenon is responsible for the irreversible nature of
the second reduction step for some of the complexes in CH2Cl2.20
Reports on the substitution of one or more of the bpy
ligands in [Ru(bpy)3]2+ with an additional chelating nitrogen
donor ligand L are available in the literature. For instance,
complexes of the type [Ru(bpy)2(Q)]2+ (Q = bpy with substitu-
ents displaying a negative inductive (−I) effect, or other
π-accepting ligands) have been investigated with respect to
their electrochemical and spectroscopic properties.18 In those
complexes, the oxidation as well as the first reduction step
occurs at potentials different from those of [Ru(bpy)3]2+. Such
ligands are usually better π-acceptors than bpy. Hence, those
ligands are easier to reduce than bpy. Additionally, the lower
lying π* orbitals of Q as compared to bpy allow for their better
mixing with the Ru-based orbitals, thus influencing the metal
based oxidation potentials as well. Energetically, the π* orbi-
tals for the ligands L1–L6 are higher than bpy. Hence, they are
more difficult to reduce than bpy, and this explains the
absence of the third reduction process in 1–6 as compared to
[Ru(bpy)3]2+. The same energetically higher π* orbitals avoid
reasonable orbital mixing with the metal based d-orbitals and
are responsible for the insulation of the oxidation potentials
from the ligand substitution in 1–6.
UV-vis-NIR spectroelectrochemistry and TD-DFT calculations
As expected from the electrochemical properties, the UV-vis
signatures of the complexes 1–6 are very similar. For instance,
32+ shows absorptions at 240, 255, and 284 nm, between 310
and 380 nm with a maximum at 360, and between 390 and 590
with maxima at 411 and 438 nm (Fig. 8, S2† and Table 6). For
comparison, [Ru(bpy)3]2+ displays absorption bands at 243 and
286, in the range 310–360, and in the range 370–600 nm with
a maximum at 450 (Table 6).18 Structure based TD-DFT calcu-
lations were carried out to help in the assignment of the
bands observed in the UV-vis-NIR spectra of the complexes.
Calculations were performed on 32+ and 62+, for which experi-
mental structures are available and which can be used as
representative examples respectively for the complexes with
pyridyl-triazoles and bis-triazoles as ligands. Structural optim-
ization with the functional BP86 produced good agreement
between the bond lengths and bond angles of the experi-
mental and calculated structures (Tables S2 and S3†). These
calculations pointed out the presence of a predominantly
ruthenium (about 75% contribution) centered HOMO, and
bpy centered (about 70% contribution) LUMO for both 32+ and
62+ (Tables S4 and S5†). The bands at higher energies are
either transitions based on the ligands or have mixed origin
(Table 7). The transition at 284 is assigned to a d(Ru) → π*(tz)
MLCT transition (Table 7 and Fig. 9). The high energy of this
band is in keeping with the intuition that the triazole rings
should contribute strongly to higher lying unoccupied orbitals.
The bands between 310 and 360 nm are a mixture of d(Ru) →
π*(tz) and d(Ru) → π*(bpy) MLCT transitions. Metal centered
d → d transitions are likely to be hidden under these tran-
sitions.18 TD-DFT confirms the low energy bands with maxima
Fig. 8 Changes in the UV-vis-NIR spectrum of 32+ during the first oxi-
dation (top), the first reduction (middle) and the second reduction
(bottom). The inset shows an expansion of the NIR region for the one-
electron and two-electron reduced species.
at 411 and 438 nm as the d(Ru) → π*(bpy) 1MLCT transition
(Table 7 and Fig. 9). The one additional band observed for 32+
(and also for 12+ and 22+, Table 6) as compared to [Ru(bpy)3]2+
appears at 255 nm, and this band is assigned to a tz → bpy
LLCT transition. This particular band is missing in the com-
plexes 42+–62+ which contain the bis-triazole ligands L4–L6.
The origin of the transitions in 62+ which contain a bis-triazole
4442 | Dalton Trans., 2014, 43, 4437–4450
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Table 6 UV-vis-NIR data from spectroelectrochemistrya
λ/nm (ε/M−1 cm−1
)
12+
22+
32+
42+
52+
62+
242 (35 700), 254(32 700), 284 (77 300), 324 sh, 412 (12 300), 443 (12 600)
241 (35 600), 254 (31 700), 285 (91 000), 324 sh, 413 (13 400), 444 (13 600)
240 (35 600), 255 (sh), 284 (69 200), 360 sh, 411 (12 800), 438 (13 000)
236 (70 500), 285 (91 200), 330 sh, 408 sh, 439 (15 300)
242 (66 600), 285 (66 900), 330 sh, 402 sh, 439 (15 300)
239 (66 200), 279 (63 700), 296 (67 800), 330 sh, 397 sh, 433 (30 800)
13+
23+
33+
43+
53+
63+
244 (44 000), 255 sh, 286 (54 300), 314 sh, 414 (5200), 443 sh, 628 (830), 777 (700)
241 (43 900), 255 sh, 289 (59 300), 314 sh, 413 (6100), 444 sh, 690 (1200), 930 (1120)
238 (47 900), 250sh, 301 (39 700), 314 sh, 423 (4100), 438 sh, 680 (900), 700 (550)
240 (47 000), 255sh, 305 (40 700), 320 sh, 425 (4200), 440 sh, 650 (1100), 700 (550)
242 (34 200), 285 (38 500), 301 (37 300), 312 (34 900), 422 (4600), 577 (2200), 767 (1200)
239 (66 700), 284 (35 000), 301 (37 300), 315 sh, 402 sh, 427 (15 200), 720 (1500)
3+,b 210 (43 000), 240 (37 500), 255 sh, 282 (56 900), 333 (20 700), 361 (20 700), 406 sh, 444 (13 300), 486 (11 700), 520 sh, 873 (1300), 1004 (1300)
5+,b 211 (61 200), 241 (80 800), 287 (75 900), 328 (32 800), 350 sh, 438 (15 500), 486 (12 100), 525 sh, 889 (1600), 988 (1500)
6+,b 212 (134 600), 233 (142 500), 291 (137 400), 333 (59 700), 357 (58 800), 430 (31 200), 486 (28 200), 525 (sh), 869 (4000), 998 (4000)
3b
5b
6b
209 (46 300), 239 (37 900), 279 (42 400), 295 sh, 343 (37 100), 356 (37 600), 496 (19 300), 530 sh, 873 (2800), 1017 (2900)
212 (65 200), 240 (72 000), 291 (50 100), 342 (51 900), 357 (52 300), 462 sh, 492 (21 300), 533 sh, 888 (3900), 988 (3700)
212 (148 200), 232 (146 300), 294 (103 400), 342 (103 200), 357 (104 900), 460 (sh), 497 (42 200), 532 (sh). 869 (7900) 998 (7900)
a From OTTLE spectroelectrochemistry in CH2Cl2–0.1 M Bu4NPF6 at 295 K. b Measurements were carried out in CH3CN–0.1 M Bu4NPF6.
Table 7 Main TD-DFT calculated transitions of 32+ compared with experimental data
Main contributing
excitation (%)
Transition
wavelength (nm)
Oscillator
strength
Exp. transition
wavelength (nm)
Molar absorption
Compound
coefficient (M−1 cm−1
)
32+
HOMO−2 → LUMO (48)
HOMO−1 → LUMO+1 (23)
HOMO−1 → LUMO+1 (57)
HOMO → LUMO+7 (36)
HOMO → LUMO+6 (23)
HOMO−2 → LUMO+5 (26)
HOMO−2 → LUMO+7 (21)
HOMO−2 → LUMO+6 (43)
HOMO−1 → LUMO+10 (63)
Mixeda
426
0.135
438
13 000 M → L(bpy)
406
332
0.085
0.078
411
360
12 800 M → L(bpy)
sh M → L
322
0.061
316
280
272
266
262
260
0.077
0.122
0.230
0.444
0.248
0.228
284
69 200 M → L(tz)
Mixeda
Mixeda
HOMO−6 → LUMO+1 (33)
255
255
sh
sh L → L
a Mixed refers to transitions where many different starting and target orbitals contribute to the transition.
ligand is similar to those in 32+ (except for the band at the redox processes, UV-vis-NIR spectroelectrochemical
255 nm). The relevant frontier orbitals have been depicted in measurements were carried out on the complexes. Spectroelec-
Fig. S3,† and the main transitions are tabulated in Table S6.† trochemical studies on the reduced complexes were carried
The molar extinction coefficient for the long wavelength band out only on 32+, 52+, and 62+ as representatives of the two
of 62+ (Table 6) is approximately the double of those of the classes of ligands used in this study. On scanning back the
other complexes. This increase likely indicates better orbital redox potentials to the starting potential after each redox step,
overlap, leading to a larger transition probability for that tran- the spectra of the original compounds were recovered in quan-
sition in 62+. TD-DFT calculations delivered approximately the titative amounts. Hence these experiments provide proof for
same oscillator strengths for all complexes. However, we would the reversibility of the redox processes. On one-electron oxi-
like to note that such calculations do not always provide good dation, the MLCT bands for all complexes decrease in intensity
quantitative fits for molar extinction coefficients. Even though (Fig. 8 and Table 6) as would be expected for electron removal
the electrochemical properties of the complexes 1–6 are vir- from a metal centered orbital. Additionally, all one-electron
tually identical, small differences are observed in their UV-vis oxidized species display weak absorption features in the red to
spectra.
the NIR region. These weak bands are reminiscent of the weak
To shed light on the electronic structures of the various LMCT band(s) observed for [Ru(bpy)3]3+ (at 675 nm) and
redox forms of the complexes and to prove the reversibility of related species.21 Hence we assign these weak absorption
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d(Ru) → π*(bpy) MLCT transitions (Fig. 11, S5† and Table 9,
S8†). Furthermore, a weak, broad structured band is observed
in the NIR region on reducing the complexes with one-elec-
tron. This band gains in intensity on further reducing the com-
plexes with a second electron. Such a band with vibronic
coupling has previously been observed for [Ru(bpy)3]+. With
the support of TD-DFT calculations we assign the broad fea-
tures in the NIR region as a mixture of intra-ligand transitions
within (bpy)•−
, inter-ligand charge transfer (ILCT), from
(bpy)•− to L, and inter-valence charge transfer (IVCT) within
the formulation [RuII(bpy)•−(bpy)L]+ (Fig. 11, S5 and Table 9,
S8†). Further bands observed at higher energies are of intra-
ligand and mixed nature. In keeping with the NIR bands
observed for the two-electron reduced complexes, these
species can be formulated as [RuII(bpy)•−2L]+. Changes in the
UV-vis-NIR spectrum of 32+ during oxidation and reduction
have been plotted in wavenumbers in Fig. S6.†
Fig. 9 Relevant calculated frontier orbitals (canonical orbitals) for the
optical transitions observed in 32+. Orbitals are shown with an iso-value
of 0.06.
EPR spectroscopy and DFT calculations
bands to π (pyridyl-triazole) or π (bis-triazole) to partially filled
dπ orbitals of the Ru(III) center. This assignment is supported
by TD-DFT calculations on 33+ and 63+ which confirm the char-
acter of this transition as LMCT (Fig. 10, S4† and Table 8, S7†).
Further transitions in the range of 400–450 nm are also all
LMCT in character. Higher energy transitions are either intra-
ligand or mixed in character (Fig. 10, S4† and Table 8, S7†).
For 63+, certain higher energy transitions are also LMCT in
nature.
The one-electron reduced complexes display various bands
in the visible region that gain in intensity on reduction of the
complexes with a further electron (Fig. 8, S2† and Table 6).
These bands are assigned to MLCT transitions and are similar
to those observed for [Ru(bpy)3]+.22 TD-DFT calculations
confirm the assignment of these bands as d(Ru) → π*(tz) and
Both the one-electron oxidized and the one-electron reduced
forms of the complexes were investigated using EPR spec-
troscopy to determine the site of electron transfer in these
complexes and to unravel possible similarities and differences
in the EPR characteristics of these paramagnetic species. The
one-electron oxidized forms 13+–33+ of the complexes with the
pyridyl-triazole ligands are EPR-silent in fluid solutions at
295 K. At 110 K these complexes display a signal with axial
symmetry with g-values for instance of g∥ = 2.74 and g⊥ = 2.06
(Fig. 12 and Table 10). [Ru(bpy)3]3+ also displays an anisotropic
spectrum with an axial symmetry.23 However, in that case, g∥ <
g⊥, and the g-anisotropy is larger (Table 10). In contrast to 13+–
33+, the complexes 43+–63+ containing the bis-triazole ligands
are EPR silent down to 110 K. Thus, it is seen that the substi-
tution of a bpy ligand in [Ru(bpy)3]3+ with either L1–L3 ligands
containing pyridyl-triazole donors, or L4–L6 containing the bis-
triazole donors have a significant influence on their EPR pro-
perties. The symmetry of the electronic ground state of the
complexes 13+–33+ is inverted as compared to [Ru(bpy)3]3+ as
seen from the inversion of the g∥ and g⊥ values. For 43+–63+,
the inclusion of the symmetric bis-triazoles L4–L6 seems to
speed up the spin–lattice relaxation process, possibly through
the influence of close-lying states, thus leading to enormous
line broadening and EPR silence at 110 K. The EPR signatures,
or the absence of it in some cases, clearly point to species with
predominantly metal centered spin in all the cases. DFT calcu-
lations also predict a larger number of closely spaced states for
63+ as compared to 33+. Thus, the energy differences between
the orbital containing the unpaired electron and the next three
orbitals are 0.01, 0.06 and 0.11 eV for 63+, and the corres-
ponding values for 33+ are 0.06, 0.18 and 0.23 eV respectively.
The differences obtained from DFT calculations fit with the
experimentally observed faster relaxation for 63+ as compared
to 33+.
The one-electron reduced forms 11+–61+ were also probed by
EPR spectroscopy. All the complexes display strong signals in
fluid solutions at 295 K. The g-value in all cases is very close to
Fig. 10 Relevant calculated frontier orbitals (canonical orbitals) for the
optical transitions observed in 33+. Orbitals are shown with an iso-value
of 0.06.
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Table 8 Main TD-DFT calculated transitions of 33+ compared with experimental data
Main contributing
excitation (%)
Transition
wavelength (nm)
Oscillator
strength
Exp. transition
wavelength (nm)
Molar absorption
coefficient (M−1 cm−1
)
Compound
33+
HOMO−6(β) → LUMO(β) (73)
HOMO(β) → LUMO(β) (26)
HOMO−2(β) → LUMO(β) (35)
HOMO−7(β) → LUMO(β) (74)
HOMO−8(β) → LUMO(β) (68)
HOMO−7(β) → LUMO(β) (21)
HOMO−3(α) → LUMO(α) (49)
Mixeda
757
724
0.008
0.007
700
550 L(tz) → M
L(tz)/M → M
429
424
0.019
0.014
438
423
sh L(bpy) → M
4100 L → M
337
299
288
286
285
267
266
265
264
0.23
314
301
sh L/M → L
39 700
0.058
0.108
0.068
0.078
0.070
0.060
0.066
0.100
Mixeda
Mixeda
Mixeda
Mixeda
238
47 900
IL
Mixeda
Mixeda
HOMO(α) → LUMO+3(α) (30)
a Mixed refers to transitions where many different starting and target orbitals contribute to the transition.
that this signal does not arise from organic impurities. The
EPR data obtained for 1+–6+ match well with those of
[Ru(bpy)3]+.24 The appearance of an EPR signal in fluid solu-
tion at 295 K is a clear indication of a ligand centered spin. In
addition, comparison of the EPR, electrochemical and UV-vis
data to those of the parent tris-bpy-Ru(II) complex clearly estab-
lishes the reduction as being bpy centered.
In order to verify the conclusions drawn from the EPR
measurements, DFT calculations were carried out on selected
complexes. Geometry optimizations were performed using the
BP86 functional. A look at the spin densities by using the
Löwdin population analysis for 33+ and 63+ shows more than
90% spin density to be located at the ruthenium center
(Fig. 13, S8†). The absolute g-anisotropy is accurately repro-
duced by the DFT calculations (Δg of 0.7 from both experiment
and calculations). This result thus strongly corroborates the
large g-anisotropy observed for 13+–33+ and the “EPR-silence”
of 43+–63+ species at 110 K and confirms the formulation
Fig. 11 Relevant calculated frontier orbitals (canonical orbitals) for the
optical transitions observed in 3+. Orbitals are shown with an iso-value
of 0.06.
2.0 (Fig. 12, S7† and Table 11), and the peak-to-peak separ- [Ru+III(bpy)2(L)]3+ for these one-electron oxidized complexes.
ation is between 40 and 50 G. In the case of 3+, the hyperfine
In contrast to the one-electron oxidized complexes, for the
coupling to the nitrogen atoms was partially resolved. one-electron reduced forms 3+ and 6+ more than 90% spin
However, an appropriate simulation was not possible because density is found to be located at the bpy ligand(s). A similar
of the lack of a highly resolved spectrum (Fig. 12). Control situation has been observed for [Ru(bpy)3]+, where the spin
experiments with UV-vis-NIR spectroscopy were used to verify density is entirely located on a bpy ligand. The question of
Table 9 Main TD-DFT calculated transitions of 3+ compared with experimental data
Main contributing
excitation (%)
Transition
wavelength (nm)
Oscillator
strength
Exp. transition
wavelength (nm)
Molar absorption
coefficient (M−1 cm−1
)
3+
HOMO(α) → LUMO+6(α) (41)
HOMO(α) → LUMO+7(α) (23)
HOMO(α) → LUMO+6(α) (45)
HOMO(α) → LUMO+7(α) (22)
HOMO(α) → LUMO+9(α) (72)
HOMO(α) → LUMO+10(α) (92)
HOMO−2(β) → LUMO(β) (40)
HOMO−2(β) → LUMO+1(β) (22)
HOMO(α) → LUMO+11(α) (66)
HOMO−1(β) → LUMO+3(β) (51)
1088
1014
0.003
0.002
1004
1300 L → L
840
529
471
0.015
0.018
0.077
873
520
486
1300 IL
sh L → L
11 700 M → L(bpy)
444
394
0.064
0.058
444
406
13 300 IL
sh M → L(bpy)
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Fig. 13 Spin density plot for 33+ (top) and 3+ (bottom). Spin densities
are shown with an iso-value of 0.01.
Fig. 12 EPR spectra of in situ electrochemically generated 13+ at 110 K
(top) and 3+ at 295 K.
localized in one bpy ring.18,22–24 For the present case, DFT cal-
culations deliver a localized situation for 3+ and a delocalized
picture for 6+. However, we would like to note that DFT
methods are known to exaggerate delocalization, and hence in
the present case the reduced complex is likely best formulated
as [Ru(bpy)•−(bpy)L]+, with the spin density being localized at
one bpy ligand. The UV-vis and the EPR data also corroborate
this formulation. However, the possibility of electron hopping
between the two bpy rings cannot be ruled out.
Table 10 EPR data for the one-electron oxidized complexesa
g∥
g⊥
gav
Δg
13+
2.74
2.74
2.75
n.o.
n.o.
n.o.
1.14
2.06
2.06
2.07
n.o.
n.o.
n.o.
2.64
2.29
2.29
2.29
n.o.
n.o.
n.o.
2.14
0.68
0.68
0.68
n.o.
n.o.
n.o.
1.50
23+
33+
43+
53+
63+
[Ru(bpy)3]3+,b
a From measurements in CH3CN–0.1 M Bu4NPF6 at 295 K. b From ref.
23. n.o. = no EPR signal was observed down until 110 K.
Conclusions
Summarizing, six Ru(II) complexes have been presented here
with pyridyl-triazole and bis-triazole ligands. Structural analy-
sis reveals shorter Ru–N(triazole) bonds as compared to the
Ru–N(bpy) bonds in complexes with mixed pyridyl-triazole
donors. Comparison with the complexes containing bis-tri-
azoles and the parent complex [Ru(bpy)3]2+ clearly shows the
attenuated π-accepting capacity of pyridyl-triazole and bis-tri-
azoles as compared to bpy. All complexes display one revers-
ible oxidation and two reversible reduction steps, the
potentials of which are unperturbed by ligand substitution. In
comparison to [Ru(bpy)3]2+, a third reduction is not observed
for the present complexes in CH3CN proving the higher
reduction potentials for the pyridyl-triazole and bis-triazole
ligands as compared to bpy. The reduction steps of the com-
plexes are sensitive to solvent effects as seen from the compari-
son between electrochemical measurements in CH3CN and
CH2Cl2. Despite the invariant electrochemical responses,
Table 11 EPR data for the one-electron reduced complexesa
giso
1+
1.995
1.996
2.004
1.997
1.997
1.997
1.998
2+
3+
4+
5+
6+
[Ru(bpy)3]+,b
a From measurements in CH3CN–0.1 M Bu4NPF6 at 295 K. b From ref.
24.
localisation versus delocalisation of the spin density on the
bpy ligands has been extensively investigated for [Ru(bpy)3]+,
and the consensus is for a localized case with the spin density
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subtle changes as a function of ligand substitution are voltammetry was carried out in 0.1 M Bu4NPF6 solutions using
observed in the UV-vis-NIR and EPR spectra of the complexes a three-electrode configuration (a glassy-carbon working elec-
in the various investigated redox states. Thus, in UV-vis-NIR trode, a Pt counter electrode, and the Ag/AgCl reference) and a
spectra of the oxidized form, the position of the LMCT band is PAR 273 potentiostat and function generator or a Versastat
seen to vary with the change in the ligand. The reduced forms 4 potentiostat. The ferrocene/ferrocenium (Fc/Fc+) couple
of the complexes display structured absorptions in the NIR served as an internal reference. Elemental analyses were
region. The oxidized forms of the complexes display different performed using the Perkin-Elmer Analyzer 240 or an
characteristics depending on the nature of the chelating ELEMENTAR Vario EL III instrument.
ligand. The complexes [Ru(bpy)2(L)]3+ with the pyridyl-triazole
Synthesis of ligands
ligands show an inverse axial symmetry in their EPR spectrum
as compared to [Ru(bpy)3]3+; the complexes containing bis-tri- Ligands L1,9a L2,11b L3,9b L4,16 and L5,3c have previously been
azole ligands are “EPR silent” till 110 K. The one-electron reported in the literature.
reduced forms of all the complexes display bpy-centered spin
and can be best formulated as [Ru(bpy)•−(bpy)(L)]+. DFT calcu- following a new click procedure described by Hohloch et al.12a
lations of the spin density distribution for the one-electron oxi- 2-Pyridyl-acetylene (1 eq., 3 mmol) and the corresponding
In this particular work ligands L1 and L2 were synthesized
dized and the one-electron reduced states confirm the spin to azide (1 eq., 3 mmol) were mixed in a small vial and copper(I)-
be located on the ruthenium center for the oxidized complex, iodide (0.01 eq., 0.03 mmol, 6 mg), potassium tert-butoxide
and on bpy for the reduced complex. This contribution is one (0.02 eq., 0.06 mmol, 6 mg) and 3-methyl-1-(2-(methylthio)-
of the rare occasions where a combined structural, electro- phenyl)-4-phenyl-1,2,3-triazolium iodide (0.01 eq., 0.03 mmol,
chemical, UV-vis-NIR and EPR spectroelectrochemical 0.011 g) were added. After a short time the mixture evolved
approach, as well as DFT studies, has been used to elucidate heat and solidified. The crude products were purified by flash
the electronic structures of metal complexes containing “click” silica gel column chromatography using dichloromethane
derived triazole ligands. Our studies show that the electronic first, followed by dichloromethane–methanol 9 : 1, giving the
structures of metal complexes can be influenced by substitut- responding ligands as off-white powders in good yields of
ing a bpy ligand in [Ru(bpy)3]n+ with pyridyl-triazoles or bis- 90%.
triazoles. The electrochemical responses however remain
2-(1-(4-Isopropylphenyl)-1H-1,2,3-triazol-4-yl)pyridine (L1)
similar to [Ru(bpy)3]n+
.
1H NMR (250 MHz, CDCl3; 25 °C, TMS): δ (ppm) = 1.26 (d, J =
7 Hz, 6H); 2.97 (hept, J = 7 Hz, 1H); 7.22–7.24 (m, 1H);
7.25–7.28 (m, 2H); 7.67–7.70 (m, 2H); 7.76–7.79 (m, 1H);
8.20–8.24 (m, 1H); 8.54 (s, 1H) 8.57–8.60 (m, 1H); 13C NMR
(60 MHz, CDCl3, 25 °C, TMS): δ (ppm) = 23.8; 33.8; 119.9;
Experimental section
Materials and physical methods
Ru(bpy)2Cl2 was purchased from ABCR. All the reagents were 120.4; 127.7; 149.9 MS (ESI): found m/z 264.1381. calcd m/z
used as supplied. The solvents used for metal complex syn- 264.1375 [C16H16N4]; elemental analysis calcd (%) for
thesis were dried and distilled under argon and degassed by [C16H16N4] C 72.70, H 6.10, N 21.20; found C 72.16, H 6.19,
common techniques prior to use.
N 20.76.
The 1H NMR spectra were recorded on a Bruker AC 250 or a
Jeol ECS 400 spectrometer. Mass spectrometry was performed
2-(1-(4-Butoxyphenyl)-1H-1,2,3-triazol-4-yl)pyridine (L2)
on a Bruker Daltronics Mictrotof-Q mass spectrometer. Elec- 1H NMR (250 MHz, CDCl3; 25 °C, TMS): δ (ppm) = 0.94 (t, J =
tronic absorption studies were performed using on J&M 6.5 Hz, 3H); 1.49 (sex, J = 6.5 Hz, 2H); 1.79 (pent, J = 6.5 Hz,
TIDAS, Avantes spectrometer system and Shimadzu UV 3101 2H); 4.00 (t, J = 6.5 Hz, 2H); 6.97–7.02 (m, 2H); 7.20–7.25 (m,
PC spectrophotometers. Spectroelectrochemistry was per- 1H); 7.62–7.70 (m, 2H); 7.76–7.80 (m, 1H); 8.22 (d, J = 8 Hz,
formed using an optically transparent thin-layer electrode 1H), 8.48 (s, 1H); 8.59 (d, J = 4.25 Hz, 1H); 13C NMR (60 MHz,
(OTTLE) cell.25 Spectra were recorded by continuously scan- CDCl3, 25 °C, TMS): δ (ppm) = 13.7; 19.1; 31.1; 68.1; 115.3;
ning the potentials as shown in the cyclic voltammograms by 120.1; 120.4; 122.0; 122.9; 130.2; 136.9; 148.7; 149.4; 150.1;
using the diode-array spectrometer. Reversibility of the redox 159.5 MS (ESI): found m/z 294.1483 calcd m/z 294.1481
waves was tested by scanning back the potential to the starting [C17H18N4O]; elemental analysis calcd (%) for [C17H18N4O]
value. On doing this, the original spectra of the complexes C 69.37, H 6.16, N 19.03; found C 68.66, H 6.20, N 18.29.
were generated back quantitatively, demonstrating the reversi-
bility of the redox waves. EPR spectra in the X-band were reported by Fletcher et al.
The ligand L6 was synthesized using a standard procedure
recorded with a Bruker System EMX. Bulk-electrolysis was per-
Copper sulfate pentahydrate (0.4 eq., 1.2 mmol, 300 mg),
formed for generating the oxidized and reduced species for sodium ascorbate (0.8 eq., 2.4 mmol, 480 mg) and potassium
EPR measurements. Approximately 10−5 mmol solutions were carbonate (2 eq., 6 mmol, 830 mg) were mixed in a flask. After-
used. For the low temperature measurements, the samples wards 30 ml of tert-butanol was added, followed by the 2-(tri-
were electrolyzed at room temperature, and immediately fluoromethyl)-phenylazide (2 eq., 6 mmol, 1.13 g) and 1,4-bis-
frozen with liquid nitrogen in an EPR sample holder. Cyclic (trimethylsilyl)-1,3-butadiyne (1 eq., 3 mmol, 0.582 g). To the
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mixture were added 30 ml of water and 3 ml of pyridine. The Hz, 1H); 8.02 (d, J = 5.0 Hz, 1H); 8.11–8.23 (m, 6H); 8.29 (d, J =
mixture was capped and stirred at room temperature for up to 5.5 Hz, 1H); 8.46 (d, J = 8 Hz, 1H); 8.70–8.83 (m, 4H); 9.70 (s,
3 days. After the reaction, the mixture was diluted with 1H) MS (ESI): found m/z 354.0949 calcd m/z 354.0944
dichloromethane (150 ml) and extracted with 5% ammonia [C37H34N8Ru2+];
elemental
analysis
calcd
(%)
for
solution in water 5 times (each washing 50 ml). The organic [C37H34N8RuCl2O8] C 49.01, H 3.78, N 12.36; found C 48.84, H
layers were separated and dried over sodium sulfate (40 g) and 4.13, N 11.92.
the solvents were evaporated under reduced pressure. The
crude products were purified by silica flash column chromato- 135 mg); H NMR (250 MHz, acetone-d6; 25 °C, TMS): δ (ppm)
graphy using dichloromethane as an eluent giving the desired = 7.42–7.52 (m, 2H); 7.58–7.66 (m, 3H); 7.86–8.00 (m, 5H);
[Ru(bpy)2(L3)](ClO4)2 (3(ClO4)2). Yield: 77% (1.5 mmol,
1
product as a brownish powder in good yields up to 85%.
8.06–8.29 (m, 7H); 8.36 (d, J = 6.25 Hz, 1H); 8.71 (d, J = 8 Hz,
1H); 8.77–8.88 (m, 3H); 9.55 (s, 1H) 19F NMR (235 MHz,
acetone-d6, 25 °C): δ (ppm) = −60.1; MS (ESI): found m/z
352.0584 calcd m/z 352.0594 [C34H25N8F3Ru2+]; elemental ana-
1,1′-Bis(2-(trifluoromethyl)phenyl)-1H,1′H-4,4′-bi(1,2,3-
triazole) (L6)
1H NMR (250 MHz, CDCl3; 25 °C, TMS): δ (ppm) = 7.56–7.63 lysis calcd (%) for [C34H25N8F3RuCl2O8] C 45.24, H 2.79, N
(m, 2H); 7.67–7.81 (m, 4H); 7.87–7.93 (m, 2H); 8.40 (s, 2H) 13C 12.41; found C 44.98, H 2.82, N 12.18.
NMR (100 MHz, CDCl3, 25 °C, TMS): δ (ppm) = 121.3; 123.7;
[Ru(bpy)2(L4)](ClO4)2 (4(ClO4)2). Yield: 75% (0.15 mmol;
1
124.0; 126.4; 126.6; 126.7; 127.1; 127.4; 127.5; 127.6; 127.7; 160 mg); H NMR (400 MHz, acetone-d6; 25 °C, TMS): δ (ppm)
129.1; 130.8; 133.2; 134.7; 139.8; 19F NMR (376 MHz, CDCl3, = 0.60 (d, J = 6.8 Hz, 6H); 0.91 (d, J = 6.8 Hz, 6H); 1.08 (d, J =
25 °C): δ (ppm) = −59.0 MS (ESI): found m/z 447.0773 calcd 6.8 Hz, 6H); 1.24 (d, J = 6.8 Hz, 6H); 1.51 (hept, J = 6.8 Hz, 2H);
m/z 447.0769 [C18H10N6F6 + Na]; elemental analysis calcd (%) 2.61 (hept, J = 6.4 Hz, 2H); 7.26–7.32 (m, 2H); 7.36–7.42 (m,
for [C18H10N6F6] C 50.95, H 2.38, N 19.81; found C 50.91, 2H); 7.44–7.50 (m, 2H); 7.51–7.58 (m, 2H); 7.71–7.78 (m, 2H);
H 2.41, N 19.12.
8.04–8.13 (m, 2H); 8.21–8.27 (m, 2H); 8.29–8.35 (m, 4H);
8.70–8.76 (m, 2H); 8.82–8.88 (m, 2H); 9.16 (s, 2H) MS (ESI):
found m/z 435.1717 calcd m/z 435.1705 [C48H52N10Ru2+];
Synthesis of the ruthenium(II)-complexes
All complexes were synthesized following the general elemental analysis calcd (%) for [C48H52N10RuCl2O8] C 53.93,
procedure:
Ru(bpy)2Cl2 (1 eq., 0.2 mmol, 96 mg) and silver perchlorate
H 4.90, N 13.10; found C 54.40, H 5.20, N 12.85.
[Ru(bpy)2(L5)](ClO4)2 (5(ClO4)2). Yield: 74% (0.148 mmol;
1
(2 eq., 0.4 mmol, 86 mg) were refluxed in 10 ml ethanol under 133 mg); H NMR (250 MHz, acetone-d6; 25 °C, TMS): δ (ppm)
an argon atmosphere for 2 h. Afterwards the mixture was fil- = 7.50–7.61 (m, 10H); 7.72–7.77 (m, 4H); 8.15–8.22 (m, 6H);
tered into a solution of the corresponding ligand in 10 ml 8.42–8.46 (m, 2H); 8.75–8.80 (m, 4H); 9.47 (s, 2H) MS (ESI):
ethanol. The reaction was then heated under reflux overnight. found m/z 351.0762 calcd m/z 351.0766 [C36H28N10Ru2+];
The solvent was then evaporated until only 2 ml ethanol was elemental analysis calcd (%) for [C36H28N10RuCl2O8] C 48.01,
left. Sodium perchlorate (8 eq., 1.6 mmol, 225 mg) was added. H 3.13, N 15.55; found C 48.52, H 3.39, N 15.85.
After 5 minutes of heavy stirring, the mixture was diluted with
[Ru(bpy)2(L6)](ClO4)2 (6(ClO4)2). Yield: 86% (0.172 mmol;
1
100 ml water and the orange solids were filtered and dried at 178 mg); H NMR (250 MHz, acetone-d6; 25 °C, TMS): δ (ppm)
room temperature. The crude product was then purified by = 7.46–7.51 (m, 2H); 7.63–7.69 (m, 2H); 7.85–7.98 (m, 8H);
alumina column chromatography using dichloromethane– 8.09–8.14 (m, 2H); 8.18–8.22 (m, 2H); 8.24–8.30 (m, 2H);
methanol 99 : 1. All volatilities were removed and the orange 8.40–.43 (m, 2H); 8.72–8.76 (m, 2H); 8.79–8.83 (m, 2H); 9.25 (s,
product was once again precipitated from 5 ml acetone using a 2H); 19F NMR (376 MHz, acetone-d6, 25 °C): δ (ppm) = −60.0;
saturated sodium perchlorate–water (100 ml) solution and fil- MS (ESI): found m/z 937.0757 calcd m/z 937.0770
tered. The orange powder was air dried overnight to yield the [C38H26N10F6RuCl1O4+]; elemental analysis calcd (%) for
desired product in excellent yields.
[C38H26N10F6RuCl2O8] C 44.03, H 2.53, N 13.51; found C 43.78,
[Ru(bpy)2(L1)](ClO4)2 (1(ClO4)2). Yield: 85% (1.7 mmol, H 2.59, N 13.09.
1
149 mg); H NMR (250 MHz, acetone-d6; 25 °C, TMS): δ (ppm)
X-ray crystallography
= 1.23 (d, J = 7 Hz, 6H); 2.99 (hept, J = 6.75 Hz, 1H); 7.44–7.71
(m, 9H); 7.75 (d, J = 5 Hz, 1H); 8.07 (d, J = 5.5 Hz, 1H); X-ray quality crystals of L3 and L4 were obtained by slow evap-
8.14–8.26 (m, 7H); 8.34 (d, J = 5 Hz, 1H); 8.47 (d, J = 8 Hz, 1H); oration of a concentrated diethylether solution at room temp-
8.86–8.74 (m, 4H); 9.78 (s, 1H) MS (ESI): found m/z 339.0916 erature. A severely disordered diethyl ether molecule was
calcd m/z 339.0891 [C36H32N8Ru2+]; elemental analysis calcd found in L3. X-ray quality crystals of 3(ClO4)2, 4(ClO4)2 and 6
(%) for [C36H32N8RuCl2O8 + 1.5CH2Cl2] C 44.86, H 3.51, (ClO4)2 were obtained by slow diffusion of n-hexane into a con-
N 11.16; found C 44.75, H 2.99, N 12.10.
centrated solution of these complexes in dichloromethane at
[Ru(bpy)2(L2)](ClO4)2 (2(ClO4)2). Yield: 82% (1.65 mmol, 8 °C. Single crystal X-ray structural studies were performed on
1
148 mg); H NMR (250 MHz, acetone-d6; 25 °C, TMS): δ (ppm) a Bruker Kappa Apex II duo or a Bruker smart AXS diffracto-
= 0.94 (t, J = 6.5 Hz, 3H); 1.46 (d, J = 6.5 Hz, 2H); 1.75 (pent, J = meter. Data were collected at 100(2) K using graphite-mono-
6.5 Hz, 2H); 4.05 (t, J = 6.5 Hz, 2H); 7.07 (d, J = 9 Hz, 2H); chromated Mo Kα radiation (λα = 0.71073 Å). The strategy for
7.43–7.62 (m, 5H); 7.68 (d, J = 8.75 Hz, 2H); 7.91 (d, J = 5.25 the data collection was evaluated by using the CrysAlisPro CCD
4448 | Dalton Trans., 2014, 43, 4437–4450
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or Smart software. The data were collected by the standard
’phi-omega scan techniques, and were scaled and reduced
using CrysAlisPro RED software or Saint+ software. The struc-
tures were solved by direct methods using SHELXS-97 and
refined by full matrix least-squares with SHELXL-97,26 refining
on F2. CCDC 846891, 846892, 892295, and 906187 contain the
supplementary crystallographic files for this work.
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612; (b) S.-Q. Bai, S. Leelasubcharoen, X. Chen, L. L. Koh,
J.-L. Juo and T. S. A. Hor, Cryst. Growth Des., 2010, 10, 1715;
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DFT calculations
DFT calculations were done with the ORCA 2.9.1 program27
package using the BP86 and B3LYP functionals for the geome-
try optimization and single-point calculations respectively.28
The restricted and unrestricted DFT method was employed for
closed and open shell molecules respectively. All calculations
were run with empirical van der Waals correction.29 Conver-
gence criteria were set to default for the geometry-optimi-
zations (OPT) and tight for SCF calculations (TIGHTSCF).
Relativistic effects were included with the zeroth-order relati-
vistic approximation (ZORA).30 Triple-ζ-valence basis sets
(TZVPP-ZORA)31 were employed for all atoms. Calculations
were performed using the resolution of the identity approxi-
mation32 with matching auxiliary basis sets. Low-lying exci-
tation energies were calculated with time-dependent DFT
(TD-DFT). Solvent effects were taken into account with the con-
ductor-like screening model (COSMO)33 for all calculations.
Spin densities were calculated according to the Löwdin popu-
lation analysis.34 The contribution of molecular fragments to
molecular orbitals was analyzed using the MOAnalyzer tool.35
Molecular orbitals and spin densities were visualized using the
Molekel 5.4.0.8 program.36
Acknowledgements
We are grateful to the Fonds der chemischen Industrie, FCI
(doctoral stipend for DS) and the Carl-Zeiss Stiftung (doctoral
stipend for ND) for financial support. Prof. Dr W. Kaim is
kindly acknowledged for access to the EPR facilities at the
Institut für Anorganische Chemie, Universität Stuttgart.
6 (a) M. Ostermeier, M.-A. Berlin, R. A. Meudtner,
S. Demeshko, F. Meyer, C. Limberg and S. Hecht,
Chem.–Eur. J., 2010, 16, 10202; (b) P. M. Guha, H. Phan,
J. S. Kinyon, W. S. Brotherton, K. Sreenath, J. T. Simmons,
Z. Wang, R. J. Clark, N. S. Dalal, M. Shatruk and L. Zhu,
Inorg. Chem., 2012, 51, 3465.
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