Frontier orbitals of photosubstitutionally active ruthenium complexes: An experimental study of the spectator ligands' electronic properties influence on photoreactivity

Article abstract of DOI:10.1039/c7dt01540b

The synthesis and characterization of [Ru(tpy)(R₂bpy)(L)](X)_n complexes (tpy = 2,2′:6′,2′′-terpyridine, R₂bpy = 4,4′-dimethyl-2,2′-bipyridine (dmbpy), or 4,4′-bis(trifluoromethyl)-2,2′-bipyridine (tfmbpy), X = Cl^- or PF₆^-, and n = 1 or 2) are described. The dmbpy and tfmbpy bidentate ligands allow for investigating the effects of electron-donating and electron-withdrawing ligands, respectively, on the frontier orbital energetics as well as the photoreactivity of these ruthenium polypyridyl complexes for five prototypical monodentate ligands L = Cl^-, H₂O, CH₃CN, 2-(methylthio)ethanol (Hmte), or pyridine. According to spectroscopic and electrochemical studies, the dmbpy analogues displayed a singlet metal-to-ligand charge transfer (¹MLCT) transition at higher energy than the tfmbpy analogues. The shift of the ¹MLCT to higher energy results from the lowest unoccupied molecular orbital (LUMO) for the dmbpy analogues being tpy-based, whereas for the tfmbpy analogues orbital inversion occurs resulting in a tfmbpy-based LUMO. The energy level of the highest occupied molecular orbital (HOMO) was considerably affected by the nature of the monodentate ligand. Visible light irradiation of the complexes demonstrated that the tfmbpy analogue increased the rate and quantum yields of photosubstitution reactions, compared to the dmbpy analogue, suggesting that the electron-withdrawing substituents allowed better thermal accessibility of the triplet metal-centered (³MC) state from the photochemically generated triplet metal-to-ligand charge transfer (³MLCT) excited state. A correlation between the photolability of the monodentate ligands and the electrochemical reversibility of the metal-based oxidation is also reported.

Full text of DOI:10.1039/c7dt01540b

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Frontier orbitals of photosubstitutionally active
ruthenium complexes: an experimental study
of the spectator ligands’ electronic properties
influence on photoreactivity†
Cite this: DOI: 10.1039/c7dt01540b
Hyo Jin Jang,a Samantha L. Hopkins, a Maxime A. Sieglerb and
a
Sylvestre Bonnet
*
The synthesis and characterization of [Ru(tpy)(R2bpy)(L)](X)n complexes (tpy = 2,2’:6’,2’’-terpyridine, R2bpy =
4,4’-dimethyl-2,2’-bipyridine (dmbpy), or 4,4’-bis(trifluoromethyl)-2,2’-bipyridine (tfmbpy), X = Cl− or
PF6−, and n = 1 or 2) are described. The dmbpy and tfmbpy bidentate ligands allow for investigating the
effects of electron-donating and electron-withdrawing ligands, respectively, on the frontier orbital
energetics as well as the photoreactivity of these ruthenium polypyridyl complexes for five prototypical
monodentate ligands L = Cl−, H2O, CH3CN, 2-(methylthio)ethanol (Hmte), or pyridine. According to
spectroscopic and electrochemical studies, the dmbpy analogues displayed a singlet metal-to-ligand
1
charge transfer (1MLCT) transition at higher energy than the tfmbpy analogues. The shift of the MLCT to
higher energy results from the lowest unoccupied molecular orbital (LUMO) for the dmbpy analogues
being tpy-based, whereas for the tfmbpy analogues orbital inversion occurs resulting in a tfmbpy-based
LUMO. The energy level of the highest occupied molecular orbital (HOMO) was considerably affected by
the nature of the monodentate ligand. Visible light irradiation of the complexes demonstrated that the
tfmbpy analogue increased the rate and quantum yields of photosubstitution reactions, compared to the
dmbpy analogue, suggesting that the electron-withdrawing substituents allowed better thermal accessi-
bility of the triplet metal-centered (3MC) state from the photochemically generated triplet metal-to-
ligand charge transfer (3MLCT) excited state. A correlation between the photolability of the monodentate
ligands and the electrochemical reversibility of the metal-based oxidation is also reported.
Received 27th April 2017,
Accepted 12th July 2017
DOI: 10.1039/c7dt01540b
rsc.li/dalton
Over the past several decades, ruthenium photochemistry has ruthenium polypyridyl complexes. Upon visible light
been studied for various applications including dye-sensitized irradiation, Ru-based complexes display low energy metal-to-
solar cells,1 energy storage,2 photochemical synthesis,3 water ligand charge transfer (1MLCT) transitions, which are the
oxidation catalysis,4 or nitric oxide delivery to biological result of an electron being excited from the Ru(dπ)-based
targets.5 Additionally, ruthenium polypyridyl complexes have highest occupied molecular orbital (HOMO) to a ligand(π*)-
received significant attention as light-activated anti-cancer based lowest unoccupied molecular orbital (LUMO). The
drugs because they can undergo ligand photosubstitution, 1MLCT excited state then undergoes rapid intersystem crossing
3
which sometimes results in an “activated” ruthenium species to the MLCT excited state, from which triplet metal-centered
that is more cytotoxic than the non-irradiated precursor.6–12 (3MC) states are thermally accessible. Such 3MC states are
Photosubstitution reactions occur under visible light ligand field excited states where an electron is excited from the
irradiation because of the unique excited state properties of t2g to eg orbital, resulting in ligand dissociation due to the
anti-bonding character of the eg orbital.13,14 A number of
studies have been performed that have increased our under-
aLeiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2300 RA Leiden,
The Netherlands. E-mail: bonnet@chem.leidenuniv.nl
standing of the relationship between the photoreactivity of
ruthenium polypyridyl complexes and the energy of their
3MLCT and 3MC states15–21 or the topology of their triplet
hypersurface.22 These studies have demonstrated the critical
role of steric hindrance on the 3MLCT–3MC energy gap,23–28
showed that extended conjugation of the bidentate ligand
affects the population of 3MC state via competition of low-
bSmall Molecule X-ray Facility, Department of Chemistry, Johns Hopkins University,
Baltimore, Maryland 21218, USA
†Electronic supplementary information (ESI) available: Ligand synthesis and
characterization, complex/photoproduct characterization and crystal structures.
CCDC 1544943–1544950. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c7dt01540b
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1
lying bpy-based π–π* excited state,12,25 or explained why show the full characterization of the tfmbpy ligand by H, 13C,
thioether ligands are more prone to photosubstitution compared 19F{1H} NMR spectroscopy, and electrospray mass spec-
to amines.29 Electrochemical studies of [Ru(tpy)(N,N)(L)]n+ trometry (ES-MS).
complexes demonstrated that electron-withdrawing bidentate
Synthesis of [Ru(tpy)(R2bpy)(L)]n+ complexes
ligands tended to decrease the energy gap between the frontier
orbitals (HOMO and LUMO).30 However, one question remains
As illustrated in Scheme 1, the dmbpy and tfmbpy ligands
poorly addressed, what is the effect of electron-withdrawing
were coordinated to [Ru(tpy)Cl3] to afford complexes [1]Cl and
spectator ligands that have no additional steric hindrance on
[2]Cl (see Fig. 1 for complex numbers).27,36,37 Both complexes
the photosubstitution quantum yields?
had a typical dark violet colour. The 1H NMR spectrum of [2]Cl
In this work we aimed at answering this question by study-
showed that the protons were de-shielded compared to [1]Cl.
As expected, the chemical shift of the alpha proton of the
bipyridine (i.e. B6, see Scheme 1) was noticeably shifted down-
ing two series of complexes having the general formula
[Ru(tpy)(R2bpy)(L)]n+ (tpy = 2,2′:6′,2″-terpyridine, L = monodentate
ligand) with two electronically different bidentate ligands:
R2bpy = 4,4′-dimethyl-2,2′-bipyridine (dmbpy) and R2bpy = 4,4′-
bis(trifluoromethyl)-2,2′-bipyridine (tfmbpy). Methyl and tri-
fluoromethyl groups are typical isosteres as the hydrogen and
fluorine atoms have comparable sizes. The van der Waals
radius of fluorine (1.35 Å) is the next smallest after hydrogen
(1.20 Å). However, CH3 and CF3 groups have opposite elec-
field because of the electron-withdrawing trifluoromethyl sub-
stituents.38 The non-equivalence of the two trifluoromethyl
groups was confirmed by two chemical shifts observed in the
19F{1H} NMR spectrum, as well as separate B7 and B7′ quartets
by 13C NMR (see spectra in Fig. S6–S10†).
From the chlorido complexes, the aqua ([3](PF6)2 and
[4](PF6)2), acetonitrile ([5](PF6)2 and [6](PF6)2), thioether ([7](PF6)2
and [8](PF6)2), and pyridine ([9](PF6)2 and [10](PF6)2) com-
plexes were synthesized using modified literature pro-
tronic properties because of the large differences in electro-
negativity and inductive effects of F vs. H:31 dmbpy is electron-
donating,
whereas
tfmbpy
is
electron-withdrawing.
cedures.25,27,30 Generally, the syntheses started from the chlor-
ido complex, which was refluxed under argon with AgPF6 and
an excess of the target monodentate ligand (see Experimental
section). The difference between the dmbpy and tfmbpy ana-
logues was not very significant except for the syntheses of the
aqua complexes. Complex [1]Cl fully converted into [3](PF6)2
after refluxing the mixture of [1]Cl and AgPF6 in acetone/water
for 3 h, whereas [2]Cl showed minimal conversion to [4](PF6)2
after 16 h under the same reaction conditions. Therefore, a
microwave reaction was performed, where acetone was
excluded to prevent the potential competition with H2O for
coordination to ruthenium. To remove the starting material,
Importantly, these substituents were placed in 4 and 4′ posi-
tion on the bipyridine spectator ligand, so that both series of
complexes have rigorously identical steric requirements. The
monodentate ligands L were also systematically varied as their
electronic properties strongly influence the photosubstitution
reaction efficiency: L was Cl−, H2O, CH3CN, 2-(methylthio)
ethanol (Hmte), or pyridine. The synthesis of ten [Ru(tpy)
(R2bpy)(L)]n+ complexes is reported, as well as the study of
their electrochemical, spectroscopic, and photochemical
properties.
Results and discussion
Ligand synthesis
Unlike dmbpy, which is commercially available, 4,4′-bis(tri-
fluoromethyl)-bpy was synthesized using a modified method
from Liao et al. based on the nickel catalyst NiCl2·6H2O
(Scheme S1†).32 Our modifications included the change of the
solvent from DMF to THF, adding twice as much nickel cata-
lyst as reported, and also one equivalent of 2,2′-bpy per mol
catalyst. Iodine and acetic acid were not necessary. As a result,
the ligand was obtained with much higher yield (56%) com-
pared to the original protocol (10%). The yield was also
improved compared to that reported by Chan et al. where
30 mol% of Ni(PPh3)2Cl2, a typical catalyst of Negishi coupling
reaction, was used.33 The improvement in yield was attributed
to two effects. First, the initial formation of [Ni(bpy)Cl2], which
according to Liao et al. facilitates the catalytic cycle,32 was veri-
Scheme 1 Synthesis and atom numbering for [Ru(tpy)(R2bpy)(Cl)]Cl,
where R2bpy = dmbpy ([1]Cl) or tfmbpy ([2]Cl).
1
fied by H NMR spectroscopy: a paramagnetic bpy-containing
species34 was observed instead of free bpy ligand (Fig. S1†).
Furthermore, THF being a stronger ligand than DMF, it may
stabilize the catalyst by coordinating to nickel throughout the
Fig. 1 The synthesized and characterized [Ru(tpy)(R2bpy)(L)]n+ com-
plexes in this work. n = 1 when L = Cl−, and n = 2 when L = H2O,
catalytic cycle.35 The Experimental section and Fig. S2–S5† CH3CN, Hmte, or py.
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dichloromethane extraction was used, yielding [4](PF6)2 in the Experimental section. The displacement ellipsoid plots of the
aqueous phase (Fig. S13†). tfmbpy analogue crystal structures are given in Fig. 2, and the
1H NMR spectroscopy of all the complexes in acetone-d6, dmbpy analogues are shown in Fig. S34† along with the crystal
displayed a trend in the chemical shifts of the alpha proton on data in Table S1.† As expected, the complexes display octa-
the bipyridine ligand (B6). For both the dmbpy and tfmbpy hedral geometry without significant distortion of the bpy
series, the B6 proton became gradually less de-shielded follow- (Fig. 2 and S34† and Table 1). Comparing a dmbpy complex to
ing the series Cl− > Hmte ≈ CH3CN ≈ H2O > pyridine (Fig. S20 its tfmbpy analogue (i.e. [1]+ vs. [2]+, or [5]2+ vs. [6]2+), the bond
and S21†). The particularly upfield-shifted B6 proton of the distances from the Ru1 atom to the bidentate ligand were
pyridine complexes resulted from the shielding cone of the co- longer for the dmbpy complexes than their tfmbpy analogues.
ordinated pyridine ligand, whereas the large downfield shift However, the Ru1–L bond distances were not significantly
for the chloride analogues was interpreted as a consequence of affected by the property of the bidentate ligands, whereas sig-
the inductive effect of the chloride.
nificant variation occurred depending on the heteroatom
Single crystals of [1]Cl·C2H6O, [2](PF6)·C3H6O, [5] bound to ruthenium (Table 1).
(PF6)2·0.70CH3CN, [6](PF6)2·CH3CN, [7](PF6)2·C3H6O, [8](PF6)2,
[9](PF6)2·C7H8 and [10](PF6)2·C3H6O suitable for X-ray structure
determination were obtained by slow vapor diffusion. Specific
Electrochemical properties
To gain insight into the frontier orbitals of these complexes,
solvent conditions for crystallization are described in the
the electrochemical properties of the complexes were studied
using cyclic voltammetry (CV). As summarized in Table 2 and
Table 2 Redox potentials and ipa/ipc of [1]Cl, [2](PF6), and [5](PF6)2–[10]
(PF6)2 as measured by cyclic voltammetrya
[complex]{(n+1)/n}+
[complex]{n/(n−1)}+
Complex
E1/2 (V)
ipa/ipc
E1/2 (V)
ipa/ipc
[1]+
0.41
0.60
0.93
1.12
0.98b
1.16b
0.86
1.04
0.42
0.92
0.54
0.72
1.01
1.03
0.96
0.86
—
—
0.88
0.87
−1.80
−1.45
−1.62
−1.33
−1.57
−1.33
−1.58
−1.33
−1.81
−1.67
−1.22
−1.83
0.94
0.96
1.13
1.07
0.60
1.09
1.03
1.00
[2]+
[5]2+
[6]2+
[7]2+
[8]2+
[9]2+
[10]2+
[Ru(tpy)(bpy)(Cl)]+ c
[Ru(tpy)2]2+ d
[Ru(tfmbpy)3]2+
[Ru(dmbpy)3]2+ e
a Measurement conditions: 1 mM of the complexes in 0.1 M Bu4NPF6/
Fig. 2 Displacement ellipsoid plot (50% probability level) of the crystal
structures of [2](PF6), [6](PF6)2, [8](PF6)2, and [10](PF6)2. The counter-
anions, disorder, hydrogen atoms, and the uncoordinated lattice solvent
molecules were omitted for clarity.
CH3CN, scan rate 100 mV s−1. n = 1 for [1]+ and [2]+, and n = 2 for the
. .
other complexes. The potentials are referenced to Fc0/+ b Epa c Ref. 40.
d Ref. 41. e Ref. 42.
Table 1 Selected bond lengths (Å) and angles (°) in the crystal structures of [1]Cl·C2H6O, [2](PF6)·C3H6O, [5](PF6)2·0.70CH3CN, [6](PF6)2·CH3CN,
[7](PF6)2·C3H6O, [8](PF6)2, [9](PF6)2·C7H8 and [10](PF6)2·C3H6O
[1]
Cl·C2H6O
[2]
[5]
[6]
[7]
[8]
[9]
[10]
(PF6) ·C3H6O (PF6)2·0.70MeCNb (PF6)2·CH3CN (PF6)2·C3H6O (PF6)2
(PF6)2·C7H8 (PF6)2·C3H6O
Ru1–N1
Ru1–N2
Ru1–N3
Ru1–N4
2.062(4)
1.961(3)
2.072(4)
2.086(3)
2.047(3)
2.3983(9)
2.062(3)
1.962(3)
2.071(3)
2.070(3)
2.026(3)
2.4050(9)
2.076(2)
1.974(2)
2.073(2)
2.093(6)
2.044(7)
2.032(2)
159.08(9)
79.4(6)
2.084(5)
1.983(5)
2.074(4)
2.076(5)
2.033(5)
2.036(5)
159.04(18)
78.84(18)
−0.6(6)
2.070(2)
1.963(2)
2.070(2)
2.099(2)
2.070(2)
2.3757(7)
159.46(9)
78.33(9)
2.3(3)
2.078(3)
1.963(4)
2.074(3)
2.092(4)
2.061(3)
2.374(2)
2.085(2)
1.961(2)
2.055(2)
2.089(2)
2.056(2)
2.100(2)
2.072(3)
1.974(3)
2.081(3)
2.077(3)
2.057(3)
2.103(3)
158.19(12)
78.15(11)
−7(5)
Ru1–N5
Ru1–La
N1–Ru1–N3
N4–Ru1–N5
Ru1–N4–C20–C21 −6.3(4)
Ru1–N5–C21–C20 3.7(5)
158.95(15) 159.18(11)
78.46(13)
158.51(15) 159.49(8)
78.79(11)
6.9(4)
−4.8(4)
78.16(13)
−3.3(5)
−2.0(5)
78.34(8)
2.9(3)
1.8(3)
−4.8(5)
4.5(5)
0.8(6)
−1.1(3)
−7(3)
a L = Cl1 for [1]Cl and [2](PF6); N6 for [5](PF6)2, [6](PF6)2, [9](PF6)2, and [10](PF6)2; or S1 for [7](PF6)2 and [8](PF6)2. See Fig. S34. b A crystal structure
of [5](PF6)2 was previously reported39 with comparable parameters as the data in this table.
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Fig. 4 HOMO and LUMO orbital energy diagram derived from cyclic
voltammetry data (Table 2). The orange lines represent the 4,4’-
dimethyl-2,2’-bipyridine analogues [1]+, [5]2+, [7]2+, [9]2+, and the black
lines represent the 4,4’-bis(trifluoromethyl)-2,2’-bipyridine analogues
[2]+, [6]2+, [8]2+, [10]2+, with the monodentate ligands specified on the
x-axis. The red lines represent the E1/2 of the first metal-based oxidation
and ligand-based reduction of the references. a For both Hmte com-
plexes, E1/2 was calculated from the electrochemical data measured
with a scan rate of 1000 mV s−1, and used in this chart, instead of the Epa
in Table 2. b The potentials were previously reported.40–42
Fig. 3 Cyclic voltammograms of
1 mM [1]Cl, [2](PF6), or [5](PF6)2
through [10](PF6)2 in 0.1 M Bu4NPF6/CH3CN (ν = 100 mV s−1). The
orange lines represent the 4,4’-dimethyl-2,2’-bipyridine analogues and
the black lines represent the 4,4’-bis(trifluoromethyl)-2,2’-bipyridine
analogues. Dashed lines show the second ligand-based reductions of
the complexes.
Fig. 3, the dmbpy and tfmbpy complexes showed clear differ-
ences in oxidation and reduction potentials. Irrespective of the
monodentate ligand, the half-wave potential of the Ru-based between the dmbpy and tfmbpy complexes was present for the
oxidation for the tfmbpy complex was 100–200 mV higher than other monodentate ligands due to the significant overlap of
that of the dmbpy complex. The shift to higher potential for the Epa of the first ligand-based reduction.
the tfmbpy analogues was expected as the electron-withdraw-
In addition to the electronic effects observed for the dmbpy
ing substituents decreased the electron density of the metal and tfmbpy analogues on the LUMO, the monodentate ligands
center, making the RuII to RuIII oxidation more difficult. influenced the RuIII/RuII oxidation potentials, hence the
Similarly, the half-wave potential of the ligand-based reduction HOMO energy level in these complexes. The HOMO of the
for the tfmbpy complex was more negative than that of the chloride complexes appeared at lower potentials (∼0.5 V vs.
dmbpy complexes. This result is consistent with the trifluoro- Fc0/+) due to the π-donor character of the Cl− ligand, while the
methyl groups stabilizing the extra charge added on the difference among the CH3CN, Hmte, and pyridine complexes
reduced ligand, which makes reduction more favourable. In was found negligible. The stabilization of the HOMO, for
Fig. 4, an energy diagram derived from Table 2 clearly shows example, from [1]+ to [5]2+ was greater than the stabilization of
that the stabilization of the LUMO from the dmbpy to the the HOMO from [1]+ to [2]+, meaning that the monodentate
tfmbpy complexes was greater than that of the HOMO. For ligands affected the HOMO more than the LUMO. Also of
ruthenium polypyridyl complexes, the LUMO is usually more interest are the metal-based oxidations for the Hmte com-
ligand-based than the HOMO. Here, comparison of the plexes [7]2+ and [8]2+, which were the only ones to show ir-
LUMOs for [Ru(tpy)(bpy)(Cl)]+ (−1.81 V vs. Fc0/+),40 [Ru(tpy)2]2+ reversible character. Such deviation was ascribed to the character
(−1.67 V vs. Fc0/+),41 and [Ru(tfmbpy)3]2+ (−1.22 V vs. Fc0/+
)
of the sulphur atom whose affinity for RuIII is weaker than for
(Fig. S35†) suggested that the LUMO of the dmbpy complexes RuII. Hence, the Ru–S bond was likely to dissociate upon oxi-
was tpy-based, whereas that of the tfmbpy complexes was dation of the metal center, and thus the Epc of the original
mostly localized on the tfmbpy ligand.
sulphur-binding complex was not observed. Though it is poss-
In addition to the energy of the frontier orbitals, the elec- ible that the oxygen atom on Hmte, which is a hard base com-
tronic effects also influenced the lability of the monodentate pared to sulphur, binds to Ru, the question whether the S–O
ligand. As previously reported, the cyclic voltammogram of [1]+ linkage isomerization occurred like in the ruthenium-sulfoxide
showed that the Cl− ligand was replaced by CH3CN after the complexes, is unclear from these results.43 When the scan rate
first ligand-based reduction.30 This was indicated by oxidation was increased to 2000 mV s−1, the cathodic peak for the
occurring at about −1.65 V during anodic scanning. In con- reduction of RuIII could be detected, i.e. the electrochemical
trast, the ligand substitution was not observed in the CV scan RuIII/RuII couple became more reversible (Fig. S36 and S37†).
of [2]+. The inertness of the complex was previously attributed In this respect, the irreversibility at standard scan rates seems
to the localization of the extra charge in electron-deficient to be an indication of the increased lability of the Hmte ligand
tfmbpy rather than in the Ru–Cl bond. Unfortunately, it was when the metal center becomes electron-deficient. This result
difficult to discern whether such differences in ligand lability will be revisited in relation to the photosubstitution results.
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Electronic absorption properties
In order to investigate how the changes in the frontier orbital
1
energy levels are reflected in the MLCT transitions, the elec-
tronic absorption spectra of the Ru complexes were measured
in acetonitrile (except for the aqua complexes, which were
measured in acetone). Each of the spectra revealed intense
MLCT-based transitions between 450–530 nm, summarized in
Table 3 and Fig. 5 (see Fig. S38† for the spectra of the aqua
complexes). Overall, the wavelength of the 1MLCT transition
(λambasx) for the tfmbpy complexes occurred at lower energy than
the dmbpy analogues. Thus, the tfmbpy ligand decreased the
HOMO–LUMO gap, compared to dmbpy. The spectra of the
dmbpy complexes were analogous to those of the [Ru(tpy)(bpy)
(L)]n+ complexes in that the weak absorptions (band I and II)
1
were observed at lower energy than the dominant MLCT tran-
sition.40 The bands were not as resolved for the tfmbpy com-
1
plexes. When the energy of the dominant MLCT transitions,
which was calculated from the λambasx, was plotted against
ΔEHOMO–LUMO, the potential difference between the E1/2 of the
HOMO and LUMO (Fig. 6), the eight complexes generally fit a
linear trend (R2 = 0.9845). Nevertheless, the R2 slightly
improved (0.9937 and 0.9898) when two separate trends were
obtained for the respective series of complexes. This diver-
gence between the dmbpy and tfmbpy series presumably
results from the different nature of the 1MLCT transition,
likely due to the change in the character of the LUMO.
Fig. 5 Electronic absorption spectra of (a) [Ru(tpy)(dmbpy)(L)]n+, and (b)
[Ru(tpy)(tfmbpy)(L)]n+. Three of the 1MLCT transitions were labelled in
the spectrum of [9]2+ as an example.
Photochemistry
end of the irradiation only showed the starting complex
The impact of the bidentate and monodentate ligands on (Fig. S41 and S42†). The inertness of the Cl− ligand is attribu-
light-induced ligand substitution was studied in acetonitrile as ted to the strong π-donor character of the ligand, in addition
illustrated in Scheme 2. The Cl−, pyridine, or Hmte analogues to a particularly low energy 1MLCT, which theoretically extends
3
3
were irradiated in CH3CN by 490 nm light for 30 minutes at the energy gap between the MLCT and MC states, and thus
25 °C. The reactions were monitored by electronic absorption lowers photosubstitution rates. In the case of the photoreac-
spectroscopy, and the end product was analysed using tions of [9]2+ and [10] , a slow shift of the λ
towards higher
2+
abs
max
1H NMR spectroscopy and electrospray mass spectrometry energy wavelengths was observed upon visible light irradiation,
(ES-MS).
and isosbestic points were observed at 465 nm and 467 nm on
The monodentate photosubstitution rate was considerably each of the electronic absorption spectra (Fig. S43 and S44†).
affected by the choice of coordinated monodentate ligand ES-MS confirmed that the shift resulted from the replacement
rather than by that of the bidentate ligand. The most unreac- of the pyridyl ligand by CH3CN, resulting in complexes [5]2+
tive analogue among the three was the chlorido complexes [1]+ and [6]2+, respectively (Fig. S45 and S46†). Due to the low con-
and [2]+: no photoconversion was observed according to elec- version of the photoreaction, the photoproducts were difficult
tronic absorption spectra (Fig. S39 and S40†). ES-MS at the to discern by 1H NMR spectroscopy (Fig. S47 and S48†).
1
−1
Table 3 Wavelength of MLCT transition (λ ), molar absorptivity (ε (M cm−1)), and quantum yields of ligand photosubstitution (ΦPS) of [Ru(tpy)
abs
max
(R2bpy)(L)](PF6)n complexesa
[Ru(tpy)(dmbpy)(L)](PF6)n
[Ru(tpy)(tfmbpy)(L)](PF6)n
ε (M−1 cm−1
)
ΦPS
λ
(nm)
ε (M−1 cm−1
)
ΦPS
abs
abs
L
λ
(nm)
max
max
Cl−
506
456
455
470
488
10 500
10 100
7000
9100
9600
—
—
522
467
468
481
494
14 100
10 900
8700
12 400
6700
—
—
MeCN
Hmte
Py
0.011
5.1 × 10−5
—
0.038
6.5 × 10−5
—
H2Ob
a Measured in CH3CN at 25 °C unless otherwise specified. b Measured in acetone.
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1
abs
max
Fig. 6 Plot of the MLCT transition energy calculated from λ
versus
the difference between the Ru-based oxidation potential and the first
ligand-based reduction potential (ΔEHOMO–LUMO) obtained from cyclic
voltammetry. The orange lines represent the 4,4’-dimethyl-2,2’-bipyri-
dine analogues and the black lines represent the 4,4’-bis(trifluoro-
methyl)-2,2’-bipyridine analogues.
Fig. 7 UV-vis spectra of (a) [7](PF6)2 irradiated by 490 nm light (photon
flux = 1.08 × 10−7 mol s−1) in CH3CN for 30 min, [Ru]tot = 1.6 × 10−4
M. Dashed line shows the spectrum expected for full conversion into
[5]2+, black line shows the initial spectrum, and red line shows the spec-
trum obtained after 30 min irradiation.
of the starting complexes (Fig. S52–S55†). The tfmbpy complex
was noticeably more reactive than the dmbpy analogue
(Fig. S51†), with ΦPS of 0.038 for [8]2+ vs. 0.011 for [7]2+
(Table 3). These results demonstrate that the electron-with-
drawing tfmbpy ligand considerably improved the efficiency of
the photosubstitution reaction. Such increased reactivity was
not observed in the ground states, as in complexes [7]2+–[10]2+
were all stable at room temperature over 12 h in the dark
(Fig. S56–S59†).
Scheme 2 Scheme of visible light-induced ligand exchange reaction.
In ruthenium polypyridyl complexes thermal population of
the 3MC state from the photochemically generated 3MLCT-
based excited state is usually claimed to determine whether
photosubstitution or 3MLCT-based phosphorescence will
occur. Both processes compete with each other, so that a
higher emission quantum yield is usually associated with a
lower photosubstitution efficiency. Looking at the particular
case of the thioether monodentate ligand (Hmte), the electron-
withdrawing properties of tfmbpy are expected to lower the
However, the reaction kinetics showed first-order behaviour,
and the rate constant (kΦ ) of the tfmbpy complex was approxi-
PS
mately twice larger than that of the dmbpy analogue. The
quantum yields of ligand photosubstitution (ΦPS) were esti-
mated using eqn (1),27
ϕ ꢀ ϕPS ꢁ ð1 ꢂ 10ꢂA
Þ
e
kϕ
¼
ð1Þ
PS
nRuðtotÞ
where Φ is photon flux (mol s−1), Ae is absorbance at the ligand field splitting parameter of [8]2+ and thus the energy
3
3
irradiation wavelength, and nRu(tot) is the total number of level of its MC state, compared to [7]2+. If the MLCT remains
moles of Ru complexes (mol) (Table S2†). For complexes [9]2+ at the same energy, complex [8]2+ would have a smaller
and [10]2+ ΦPS was found lower than 10−4 for both complexes 3MC–3MLCT gap and therefore a higher photosubstitution
(Table 3).
quantum yield, which was supported by the ΦPS for [8]2+ being
Unlike the chloride and pyridine analogues, the Hmte four times higher than for [7]2+. This interpretation also
ligand in complexes [7]2+ and [8]2+ were readily substituted by suggests that the phosphorescence of [8]2+ should be weaker
3
CH3CN upon visible light irradiation. As shown in Fig. 7 and than that of [7]2+. The MLCT-based phosphorescence for this
S50,† the selectivity of the photoreaction was indicated by family of complexes is typically very weak, with emission
clear isosbestic points in the electronic absorption spectra at quantum yields (Φem) in the range of 10−4 to 10−5, but it can
475 nm for [7]2+, and 497 and 266 nm for [8]2+, despite negli- be measured (Table S3†). Unexpectedly, we found that [8]2+
gible shift of the absorption maxima λambasx. Within 10 min, the displayed a slightly more intense (Φem = 1.1 × 10−4) and red-
spectra of [8]2+ became time-independent, suggesting quick shifted emission (699 nm) compared to [7]2+ (640 nm, Φem
=
conversion to the CH3CN complex (Fig. S49b†). In addition, 3.5 × 10−5). These facts are consistent with the electrochemical
ES-MS and H NMR spectra at the final time point supported results suggesting that the LUMO is terpyridine-based for [7]2+,
1
the displacement of the Hmte ligand and the full consumption but tfmbpy-based for [8]2+. Due to the electron-withdrawing
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effect of the trifluoromethyl groups, putting an electron in the Hmte ligand, which presumably elongates the Ru–S bond due
tfmbpy(π*)-based orbital must contribute to stabilizing the to polarization effects.44 Both factors may contribute to enhan-
3
3
3MLCT state of [8]2+, which should red shift the emission cing the rate of the MLCT → MC transition, and indeed the
maximum for this complex and lower the photosubstitution photosubstitution quantum yield for this complex is higher
3
3
quantum yield. In other words, [8]2+ has both a lower MLCT than that of [7]2+ (in the same solvent). In the MLCT state of
state and a lower 3MC state compared to [7]2+, which makes [7]2+ an electron is promoted to the tpy(π*)-based orbital that
any comparison of the relative values of phosphorescence and lies in a plane perpendicular to the Ru–S bond (Fig. 8a), which
3
photosubstitution quantum yields difficult.
may lower the coupling with the MC excited state, and hence
On the other hand, in the MLCT state of [8]2+ the electron the rate of the 3MLCT → MC transition. The fact that for [7]2+
promoted in the π*(tfmbpy) orbital lies first in a plane that both the emission and the photosubstitution quantum yields
contains the Ru–S bond (Fig. 8b), and second trans to the are lower than for [8]2+ also suggests that non-radiative decay
processes that do not go via the 3MC states may also play a role
3
3
in the deactivation mechanism of the 3MLCT state of these
ruthenium complexes. Overall, the electronic effects of the
tfmbpy ligand seem not only to change the relative energy
levels of the 3MLCT and 3MC states (Fig. 8), but they also
change the geography and the geometry of the electronic
3
density distribution in the MLCT excited states, and thus the
3
electronic rearrangements necessary to perform the MLCT →
3MC transition.
In the discussion on the irreversibility of the Ru oxidation
for the [Ru(tpy)(R2bpy)(Hmte)]2+ complexes, we mentioned
that the degree of reversibility of the oxidation peak was a
measure of ligand lability in the ground state. It seems that it
is also correlated to the variation of ligand lability in the
excited state. Indeed, we observe that reversible metal-based
oxidation is synonymous with photochemical inertness of the
monodentate ligand. For instance, the metal-based redox reac-
tion of the chlorido complexes [1]+ and [2]+ was highly revers-
ible (ipc/ipa ≈ 1.0), and they were found extremely unreactive
upon light irradiation. In contrast, the Epc of the Hmte ana-
logues [7]2+ and [8]2+ were not observed, i.e. the oxidation
peaks were highly irreversible, but they displayed increased
photosubstitution of Hmte, compared to the chloride com-
plexes. The pyridine complexes displayed quasi-reversible be-
haviour and very slow photoconversion. This correlation is
reasonable as the electrochemical oxidation and the photo-
chemical transition to 3MLCT states share great similarities, as
in the 3MLCT state an electron is excited from the metal-based
HOMO to a ligand-based LUMO orbital, thereby formally
increasing the oxidation state of the metal in the excited state
to RuIII. In this respect, reversibility of the Ru-based oxidation
as measured by CV is a useful way to predict the lability of the
monodentate ligand upon light irradiation. This correlation
mostly depends on the monodentate ligand and not to be
strongly affected by the electronic property of the bipyridine
ligand.
Fig. 8 Triplet potential energy curves showing interconversion of a ter-
Conclusion
pyridine- or bipyridine-based 3MLCT state (blue) in (a) [7]2+ or (b) [8]2+
,
respectively, into the 3MC state (black) leading to Ru–S bond breakage.
The HOMOs of the 3MLCT and 3MC states are also calculated at the
PBE0/TZP/COSMO level and shown for [7]2+ and [8]2+. Black dots
represent transition states on the triplet hypersurface. Y-Axis (potential
energy), ground states, 1MLCT states, and photoproducts, are not shown
for clarity.
This work represents one of the few examples of a series of
ruthenium polypyridyl complexes investigating solely the effect
of the electronic properties of the bidentate spectator ligand
on photosubstitution reactions. Focusing on complexes of the
type [Ru(tpy)(R2bpy)(L)]n+, our work clearly demonstrates that
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the monodentate ligand (L) more strongly affected the Ru(dπ)- and dried with MgSO4. After filtration, the solvent was
based HOMO orbitals than the ligand(π*)-based LUMO: the removed. The crude product was purified by silica gel column
more π-donating Cl analogue displayed 0.5 V more positive chromatography using hexane/DCM (6 : 1) (Rf = 0.28). The
RuIII/RuII redox couple compared to the more π-accepting solvent was evaporated to yield a white solid (247 mg, 56%).
ligands CH3CN, or pyridine. By contrast, the electron-with- 1H NMR (400 MHz, CDCl3) agreed with literature.46 1H NMR
drawing tfmbpy ligand stabilized both the HOMO and LUMO, (500 MHz, CD3OD): δ 8.95 (d, J = 5.0 Hz, 1H, H6), 8.73 (s, 1H,
3
which changed the electronic distribution in the MLCT state, H3), 7.77 ppm (dd, J = 5.1, 1.1 Hz, 1H, H5). 13C NMR
2
compared to complexes based on dmbpy. The dmbpy com- (500 MHz, CD3OD): δ 157.3 (s, C2), 152.1 (s, C6), 140.6 (q, JCF
1
3
plexes displayed typical Ru(dπ) → tpy(π*)-based MLCT states, = 34.0 Hz, C4), 124.4 (q, JCF = 272.3 Hz, C7), 121.3 (q, JCF
=
while the tfmbpy analogues displayed Ru(dπ) → tfmbpy(π*)- 3.5 Hz, C5), 117.7 ppm (q, JCF = 3.8 Hz, C3). 19F{1H} NMR
3
based MLCT states. Such changes have counter-intuitive (500 MHz, CD3OD): δ −66.5 ppm. MS: m/z 293.0, [L + H]+
effects on the photochemical properties of the tfmbpy com- (calc.: 293.1).
1
plexes. On the one hand, the MLCT absorption band is red-
[Ru(tpy)(dmbpy)Cl]Cl ([1]Cl). [Ru(tpy)Cl3] (500 mg,
3
shifted by 8–16 nm and the weak MLCT phosphorescence is 1.1 mmol), 4,4′-dmbpy (210 mg, 1.1 mmol), Et3N (250 μL,
red-shifted by 52–59 nm. On the other hand, for the Hmte and 1.8 mmol), and LiCl (50 mg, 1.2 mmol) were added and stirred
pyridine adducts, which were the only ones to be photosubsti- in EtOH/H2O (3 : 1) mixture (100 mL), and the reaction was
tutionally reactive in acetonitrile, the tfmbpy complexes dis- heated at reflux for 5 h under Ar. A dark violet solution was
played up to a 4 times higher photosubstitution quantum hot filtered over Celite to remove insoluble starting material
yield, compared to the dmbpy analogue, although both the and by-products. The filtrate was rotary evaporated and puri-
3MLCT and 3MC states are predicted to be stabilized by the fied by deactivated alumina column chromatography using 5%
electron-withdrawing group. These results open interesting MeOH/DCM. The dark violet fraction was collected, and evap-
1
questions, such as whether it is possible to play on the elec- orated (465 mg, 69%). H NMR (500 MHz, CD3OD): δ 10.00 (d,
tron donating and accepting properties of the spectator J = 5.7 Hz, 1H, B6), 8.68 (s, 1H, B3), 8.63 (d, J = 8.2 Hz, 2H,
ligands to design ruthenium complexes that can photosubsti- T3′), 8.52 (d, J = 8.0 Hz, 2H, T6), 8.40 (s, 1H, B3′), 8.08 (t, J = 8.1
tute ligands at higher quantum efficiencies using lower energy Hz, 1H, T4′), 7.89 (td, J = 7.8, 1.5 Hz, 2H, T5), 7.86 (dd, J = 5.8,
light.
1.8 Hz, 1H, B5), 7.72 (d, J = 5.5 Hz, 2H, T3), 7.33 (ddd, J = 7.2,
5.5, 1.3 Hz, 2H, T4), 7.11 (d, J = 6.0 Hz, 1H, B6′), 6.87 (dd, J =
6.1, 1.8 Hz, 1H, B5′), 2.81 (s, 3H, B7), 2.37 ppm (s, 3H, B7′). 13
C
NMR (500 MHz, CD3OD): δ 160.3 (s, T2), 159.7 (s, T2′), 159.2
(s, B2), 157.2 (s, B2′), 153.2 (s, T3), 152.7 (s, B6), 151.9 (s, B6′),
150.5, (s, B4), 149.6 (s, B4′), 138.2 (s, T5), 135.0 (s, T4′), 129.0
Experimental section
General specifications
All reagents for which the synthesis is not given were commer- (s, B5), 128.5 (s, T4), 128.4 (s, B5′), 125.4 (s, B3), 125.3 (s, B3′),
cially available from Sigma-Aldrich, J. T. Baker, Alfa Aesar, or 124.7 (s, T6), 123.6 (s, T3′), 21.4 (s, B7), 20.8 ppm (s, B7′). MS:
Actu-All Chemicals, and they were used as received from the m/z 554.1, [M − Cl]+ (calc.: 554.1). Crystal growing: vapor
suppliers. The ruthenium precursor [Ru(tpy)Cl3] was prepared diffusion of diethyl ether into a solution of [1]Cl in ethanol.
following literature method.45 Microwave reaction was per-
[Ru(tpy)(tfmbpy)Cl]Cl ([2]Cl). [Ru(tpy)Cl3] (370 mg,
formed using a Biotage Initiator. 1H, 13C, and 19F{1H} NMR 0.85 mmol), 4,4′-tfmbpy (3) (250 mg, 0.85 mmol), Et3N
spectra were recorded on a Bruker DPX-300, AV-400, AV-500, or (188 μL, 1.35 mmol), and LiCl (39 mg, 0.93 mmol) were added
AV-600 spectrometer, all at room temperature. Electronic and stirred in EtOH/H2O (3 : 1) mixture (140 mL), and the reac-
absorption spectroscopy was recorded using a Varian Cary 50 tion was heated at 100 °C for 4 h under Ar. A dark violet solu-
UV-vis spectrometer. Electrospray mass spectra were obtained tion was hot filtered over Celite to remove insoluble starting
by Dionex UltiMate 3000 HPLC.
material and by-products. The filtrate was rotary evaporated
4,4′-Bis(trifluoromethyl)-2,2′-bipyridine. Nickel(II) chloride and purified by deactivated alumina column chromatography
hexahydrate (71 mg, 0.30 mmol) was added to dry THF using 5% MeOH/DCM. The solvent was evaporated to yield
(50 mL) and the solution was heated to 40 °C to dissolve the dark violet solid. It was redissolved in minimal amount of
solid. To the pale yellow solution, 2,2′-bpy (47 mg, 0.30 mmol) MeOH, and precipitated with ether (417 mg, 71%). The PF6
was added, which turned green. Then 2-bromo-4-trifluoro- salt of the complex, [2]PF6, was made by dissolving [2]Cl in a
methylpyridine (679 mg, 3.0 mmol), Zn (245 mg, 3.75 mmol), minimal amount of MeOH and stirring with a KPF6 saturated
and LiCl (127 mg, 3.0 mmol) were added, and the reaction was aqueous solution for 3 h. The solution was filtered and
heated at 60 °C for 5 h under Ar. When the reaction was com- washed with water and ether. 1H NMR (300 MHz, CD3OD):
plete as monitored by TLC (silica gel, hexane/DCM (6 : 1)), δ 10.49 (d, J = 6.0 Hz, 1H, B6), 9.35 (s, 1H, B3), 9.08 (s, 1H, B3′),
∼5 mL of 1 M HCl was added to consume the remaining Zn, 8.71 (d, J = 8.1 Hz, 2H, T3′), 8.57 (d, J = 8.1 Hz, 2H, T6), 8.33
followed by the addition of ammonium hydroxide (25% NH3) (dd, J = 6.0, 1.8 Hz, 1H, B5), 8.25 (t, J = 8.1 Hz, 1H, T4′), 7.97
until the mixture became alkaline. THF was rotary evaporated, (td, J = 7.9, 1.5 Hz, 2H, T5), 7.80 (d, J = 6.0 Hz, 1H, B6′), 7.68
and DCM and water were added for extraction (∼70 mL × 3). (d, J = 5.2 Hz, 2H, T3), 7.44–7.26 ppm (m, 3H, B5′ + T4). 13C
The organic phase was washed with water, extracted again, NMR (500 MHz, CD3OD): δ 161.4 (s, B2), 159.9 (s, T2), 158.78
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(s, T2′), 158.7 (s, B2′), 155.0 (s, B6), 154.6 (s, B6′), 153.7 (s, T3), for an additional 18 hours, which resulted in an orange sus-
2
2
139.0 (s, T5), 138.8 (q, JCF = 35.2 Hz, B4), 137.6 (q, JCF = 35.2 pension. The suspension was filtered over Celite, and washed
Hz, B4′), 136.8 (s, T4′), 128.7 (s, T4), 125.1 (s, T6), 125.1 (q, 1JCF with acetonitrile. The solvent was rotary evaporated. The solid
3
= 73.2 Hz, B7), 124.2 (q, JCF = 3.8 Hz, B5), 124.1 (s, T3′), 123.5 was dissolved in minimal amount of acetonitrile, then precipi-
3
1
3
1
(q, JCF = 3.7 Hz, B5′), 122.9 (q, J = 72.7 Hz, B7′), 122.0 (q, J = tated with ether, and filtered (47 mg, 67%). H NMR spectrum
3.8 Hz, B3), 121.6 ppm (q, 3J = 3.8 Hz, B3′). 19F{1H} NMR in CD3CN corresponded to literature.30 1H NMR (300 MHz,
(500 MHz, CD3OD): δ −65.6, −66.1 ppm. MS: m/z 662.0, [M − (CD3)2CO): δ 9.69 (d, J = 5.7 Hz, 1H, B6), 8.85 (d, J = 8.1 Hz, 2H,
Cl]+ (calc.: 662.0). Crystal growing: vapor diffusion of diethyl T3′), 8.79 (s, 1H, B3), 8.72 (d, J = 8.1 Hz, 2H, T6), 8.54 (s, 1H,
ether into a solution of [2](PF6) in acetone. Elemental analysis B3′), 8.43 (t, J = 8.1 Hz, 1H, T4′), 8.14 (td, J = 7.9, 1.5 Hz, 2H,
calcd (%) for C27H17Cl2F6N5Ru·1.5 H2O: C 44.76 H 2.78 N 9.67; T5), 7.97 (dd, J = 5.4, 1.5 Hz, 2H, T3), 7.92 (dd, J = 5.9, 1.7 Hz,
found: C 44.89 H 2.79 N 9.67.
1H, B5), 7.52 (ddd, J = 7.4, 5.6, 1.3 Hz, 2H, T4), 7.39 (d, J =
[Ru(tpy)(dmbpy)(OH2)](PF6)2 ([3](PF6)2). The complex [1]Cl 5.8 Hz, 1H, B6′), 7.06 (d, J = 5.6 Hz, 1H, B5′), 2.79 (s, 3H, B7),
(25 mg, 0.0424 mmol) was dissolved in 16 mL of acetone/water 2.40 (s, 3H, B7′), 2.30 ppm (s, 3H, CH3-CN). MS: m/z 279.8
(3 : 5), and AgPF6 (23.6 mg, 0.0933 mmol) was added. The reac- [M − 2PF6]2+ (calc.: 280.1); 705.2 [M − PF6]+ (calc.: 705.1).
tion was refluxed at 80 °C for 3 h under Ar. It was filtered over Crystal growing: vapor diffusion of toluene into a solution of
Celite and the solvent was rotary evaporated. It was dissolved [5](PF6)2 in acetonitrile.
in minimal amount of acetone and precipitated with ether
[Ru(tpy)(tfmbpy)(NCCH3)](PF6)2 ([6](PF6)2). The complex
(24 mg, 69%). 1H NMR (300 MHz, (CD3)2CO): δ 9.39 (d, J = 5.7 Hz, [2]Cl (25 mg, 0.036 mmol) was dissolved in 1 : 1 mixture of
1H, B6), 8.78–8.65 (m, 3H, T3′ + B3), 8.57 (d, J = 8.1 Hz, 2H, acetonitrile and water (50 mL), and AgPF6 (20 mg, 0.079 mmol)
T6), 8.38 (s, 1H, B3′), 8.20 (t, J = 8.1 Hz, 1H, T4′), 8.07–7.92 (m, was added. The reaction was refluxed at 82 °C under Ar for
3H, T5 + B5), 7.73 (d, J = 5.4 Hz, 2H, T3), 7.37 (t, J = 6.7 Hz, 2H, 17 h. The orange solution was filtered over Celite, and the
T4), 7.14 (d, J = 5.9 Hz, 1H, B6′), 6.85 (d, J = 6.0 Hz, 1H, B5′), solvent was rotary evaporated. The solid was dissolved in
2.77 (s, 3H, B7), 2.30 ppm (s, 3H, B7′). MS: m/z 268.4 [M − minimal amount of acetonitrile, then precipitated with ether,
2PF6]2+ (calc.: 268.6); 259.5 [M − 2PF6 − H2O]2+ (calc.: 259.6); and filtered (25 mg, 73%). 1H NMR (300 MHz, (CD3)2CO):
538.2 [Ru–O–Ru dimer + Na]2+ (calc.: 538.1).
δ 10.24 (d, J = 5.9 Hz, 1H, B6), 9.51 (s, 1H, B3), 9.27 (s, 1H, B3′),
[Ru(tpy)(tfmbpy)(OH2)](PF6)2 ([4](PF6)2). The complex [2]Cl 8.90 (d, J = 8.1 Hz, 2H, T3′), 8.73 (d, J = 8.1 Hz, 2H, T6), 8.53 (t,
(30 mg, 0.0434 mmol), and AgPF6 (33 mg, 0.130 mmol) were J = 8.1 Hz, 1H, T4′), 8.39 (dd, J = 6.0, 1.8 Hz, 1H, B5), 8.17 (td,
added to 5 mL of water in a microwave vial. The vial was J = 7.9, 1.5 Hz, 2H, T5), 8.07 (d, J = 6.0 Hz, 1H, B6′), 8.03 (dd, J =
purged with Ar for 20 min. The mixture was heated by micro- 5.5, 1.4 Hz, 2H, T3), 7.57 (d, J = 6.0, 1.9 Hz, 1H, B5′), 7.49 (ddd,
wave at 140 °C for 10 min. Some of the starting complex did J = 7.2, 5.5, 1.3 Hz, 2H, T4), 2.33 ppm (s, 3H, CH3-CN). MS: m/z
not react and suspended in the red solution, so the reaction 333.7 [M − 2PF6]2+ (calc.: 334.0); 813.2 [M − PF6]+ (calc.: 813.0).
was heated again at 140 °C for another 10 min. DCM and more Crystal growing: vapor diffusion of diethyl ether into a solution
water were added in order to extract the product. The product of [6](PF6)2 in acetonitrile. Elemental analysis calcd (%) for
dissolved in the water phase and the starting complex dis- C29H20F18N6P2Ru·0.5 H2O·0.5 C3H6O·0.5 CH3CN: C 37.24 H 2.53
solved in organic phase. Approximately 170 mL total was used N 8.96; found: C 37.16 H 2.55 N 8.97.
for each phase for extraction. The water phase was filtered over
[Ru(tpy)(dmbpy)(Hmte)](PF6)2 ([7](PF6)2). The complex
Celite to filter away insoluble by-products. The water was rotary [3](PF6)2 (28 mg, 0.034 mmol) was dissolved in 16 mL of mixture
evaporated to yield a dark red solid (15 mg, 36%). 1H NMR of acetone and water (3 : 5), and 2-(methylthio)ethanol (30 μL,
(300 MHz, (CD3)2CO): δ 10.15 (d, J = 5.9 Hz, 1H, B6), 9.54 (s, 0.34 mmol) was added. The reaction was refluxed at 80 °C
1H, B3), 9.22 (s, 1H, B3′), 8.91 (d, J = 8.2 Hz, 2H, T3′), 8.75 (d, under Ar for 21 h. The orange solution was rotary evaporated,
J = 8.0 Hz, 2H, T6), 8.54 (d, J = 6.0 Hz, 1H, B5), 8.48 (t, J = redissolved in minimal amount of acetone, and then precipi-
8.1 Hz, 1H, T4′), 8.15 (t, J = 7.9 Hz, 2H, T5), 8.11 (d, J = 6.1 Hz, tated with ether. It was filtered to yield orange solid (19 mg,
1
1H, B6′), 8.06 (dd, J = 5.5 Hz, 2H, T3), 7.53–7.42 ppm (m, 3H, 62%). H NMR (300 MHz, (CD3)2CO): δ 9.72 (d, J = 5.8 Hz, 1H,
B5′ + T4), 6.25 (s, 2H, OH2). MS: m/z 322.3 [M − 2PF6]2+ (calc.: B6), 8.89 (d, J = 8.2 Hz, 2H, T3′), 8.80 (s, 1H, B3), 8.74 (d, J =
322.5); 313.2 [M − 2PF6 − H2O]2+ (calc.: 313.5); 646.1 [Ru–O– 8.1 Hz, 2H, T6), 8.57 (s, 1H, B3′), 8.47 (t, J = 8.1 Hz, 1H, T4′),
Ru dimer + Na]2+ (calc.: 646.1).
8.17 (td, J = 7.9, 1.5 Hz, 2H, T5), 8.01 (d, J = 5.0 Hz, 2H, T3),
[Ru(tpy)(dmbpy)(NCCH3)](PF6)2 ([5](PF6)2). The complex was 7.96 (dd, J = 5.9, 1.7 Hz, 1H, B5), 7.54 (ddd, J = 7.4, 5.6, 1.3 Hz,
synthesized using a modified literature procedure.30 Complex 2H, T4), 7.33 (d, J = 5.8 Hz, 1H, B6′), 7.11 (dd, J = 5.5, 1.6 Hz,
[1]Cl (49 mg, 0.083 mmol) was dissolved in 30 mL of aceto- 1H, B5′), 3.53 (t, 5.7 Hz, 2H, –S–CH2–CH2–), 2.78 (s, 3H, B7),
nitrile, and the reaction was refluxed at 82 °C under Ar. 2.40 (s, 3H, B7′), 1.97 (t, 5.7 Hz, 2H, –S–CH2–CH2–), 1.50 ppm
According to TLC (aluminum oxide, 5% MeOH/DCM), no (s, 3H, CH3–S–). MS: m/z 305.3 [M − 2PF6]2+ (calc.: 305.6);
product was observed after 18 h. To the mixture, 2.2 equi- 756.2 [M − PF6]+ (calc.: 756.1). Crystal growing: vapor diffusion
valents (46 mg, 0.182 mmol) of AgPF6 was added and refluxed of diethyl ether into a solution of [7](PF6)2 in acetone.
for an additional 3 h, resulting in a yellow compound on TLC Elemental analysis calcd (%) for C30H31F12N5OP2RuS·2
along with starting material. To assist conversion to the aqua, H2O·0.75 C3H6O·0.25 PF6: C 38.11 H 3.92 N 6.89; found:
20 mL of distilled water was added to the reaction and refluxed C 38.19 H 3.92 N 6.97.
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[Ru(tpy)(tfmbpy)(Hmte)](PF6)2 ([8](PF6)2). The complex [2]Cl J = 8.1 Hz, 2H, T3′), 8.84 (d, J = 8.3 Hz, 2H, T6), 8.47 (t, J =
(27 mg, 0.0383 mmol) was dissolved in 16 mL of mixture of 8.1 Hz, 1H, T4′), 8.35 (d, J = 6.1 Hz, 1H, B5), 8.27–8.17 (m, 4H,
acetone and water (3 : 5), and 2-(methylthio)ethanol (33 μL, T5 + T3), 8.11 (d, J = 5.8 Hz, 3H, B6′ + P2), 7.96 (t, J = 7.6 Hz,
0.38 mmol), and AgPF6 (21 mg, 0.084 mmol) were added. The 1H, P4), 7.64–7.50 (m, 3H, B5′ + T4), 7.41 ppm (t, J = 6.7 Hz,
reaction was refluxed at 80 °C for 22 h. The reaction mixture 2H, P3). MS: m/z 352.8 [M − 2PF6]2+ (calc.: 353.0). Crystal
was filtered over Celite and the solvent was rotary evaporated. growing: vapor diffusion of toluene into
a solution of
According to 1H NMR, there seemed to be more than one [10](PF6)2 in acetone. Elemental analysis calcd (%) for
species. Therefore, the crude product was dissolved in 16 mL C32H22F12N6P2Ru·1.5 H2O·0.25 C3H6O·0.25 PF6·0.25 C5H5N:
of acetone/water (3 : 5), the same amount of AgPF6 and 100 μL C 39.34 H 2.75 N 8.51; found: C 39.40 H 2.72 N 8.54.
of 2-(methylthio)ethanol were added, and the reaction
[Ru(tfmbpy)3]Cl2. The complex was synthesized using litera-
was refluxed at 80 °C under Ar for 24 h. The orange solution ture method.48 1H NMR (400 MHz, (CD3)2CO): δ 9.44 (d, J = 1.9
was again filtered over Celite, and the solvent was removed Hz, 1H, H3), 8.54 (d, J = 5.9 Hz, 1H, H6), 7.89 ppm (dd, J = 5.9,
(32 mg, 82%). 1H NMR (400 MHz, (CD3)2CO): δ 10.36 (d, J = 1.8 Hz, 1H, H5). MS: m/z 489.4 [M − 2Cl]2+ (calc.: 489.0);
5.9 Hz, 1H, B6), 9.53 (s, 1H, B3), 9.33 (s, 1H, B3′), 8.99 (d, J = 1013.0 [M − Cl]+ (calc.: 1013.0).
8.2 Hz, 2H, T3′), 8.81 (dd, J = 8.1, 1.2 Hz, 2H, T6), 8.57 (t, J =
8.1 Hz, 1H, T4′), 8.52 (dd, J = 6.1, 1.9 Hz, 1H, B5), 8.19 (td, J =
7.9, 1.5 Hz, 2H, T5), 8.10 (dd, J = 5.5, 1.6 Hz, 2H, T3), 7.99 (d,
J = 5.9 Hz, 1H, B6′), 7.63 (dd, J = 5.9, 1.9 Hz, 1H, B5′),
7.50 (ddd, J = 7.7, 5.5, 1.3 Hz, 2H, T4), 4.83 (t, J = 5.5 Hz, 1H,
–OH), 3.53 (td, J = 5.5 Hz, 2H, –CH2–OH), 2.10 (t, J = 5.7 Hz,
2H, –S–CH2–), 1.52 ppm (s, 3H, CH3–S–). MS: m/z 359.1 [M −
2PF6]2+ (calc.: 359.5); 864.2 [M − PF6]+ (calc.: 864.0). Crystal
growing: vapor diffusion of diethyl ether into a solution of
[8](PF6)2 in ethanol. Elemental analysis calcd (%) for
C30H25F18N5OP2RuS: C 35.73 H 2.50 N 6.94; found: C 35.78 H
2.52 N 6.83.
Electrochemistry
Cyclic voltammetry experiments were performed using a three-
electrode cell setup, which included a platinum working elec-
trode, a silver wire pseudo reference electrode, and platinum
wire auxiliary electrode. The supporting electrolyte was 0.1 M
tetrabutylammonium hexafluorophosphate (Bu4NPF6) in
CH3CN. The final complex concentration was 1 mM. Prior to
measurement, the solvent was purged with argon and was
measured at room temperature using an Autolab PGSTAT10
potentiostat and GPES 4.9 program by Eco Chemie. Each CV
experiment was referenced to the ferrocene oxidation
potential.
[Ru(tpy)(dmbpy)(py)](PF6)2 ([9](PF6)2). The complex [3](PF6)2
(41 mg, 0.05 mmol) was dissolved in 20 mL of ethanol/water
(1 : 1), and excess amount of pyridine (1 mL) was added. The
reaction was refluxed at 80 °C under Ar for 24 h. Ethanol was
Emission quantum yields
rotary evaporated, and then saturated KPF6 aqueous solution The quantum yield of 3MLCT emission was determined in a
was added. After the mixture was stirred, the dark orange pre- custom-built setup. All optical parts were connected using
cipitate was filtered and washed with water and ether. optical fibers from Avantes (Apeldoorn, The Netherlands), with
1
According to H NMR in (CD3)2CO, the product still had pyri- a diameter of 200–600 μm. The samples were measured in a
dine. The crude product was dissolved in minimal amount of 1 cm path length QS cuvette. For each measurement, 3 mL of
1
nm
450
abs
acetone, precipitated with ether, and filtered (23 mg, 51%). H sample consisting of the compound in acetonitrile (λ
=
NMR spectrum in CD3CN corresponded to literature.47 1H 0.1), was placed in a CUV-UV/VIS-TC temperature-controlled
NMR (300 MHz, (CD3)2CO): δ 8.83 (m, 3H, B3 + T3′), 8.76 (d, cuvette holder from Avantes. The sample was deoxygenated
J = 5.9 Hz, 1H, B6), 8.73 (d, J = 8.1 Hz, 2H, T6), 8.58 (s, 1H, B3′), using nitrogen gas for 10 min while also equilibrating to
8.34 (t, J = 8.1 Hz, 1H, T4′), 8.17 (td, J = 7.9, 1.5 Hz, 2H, T5), 293 K. Emission spectroscopy was performed with a 450 nm
8.12 (d, J = 5.4, 1.6 Hz, 2H, T3), 8.02 (dd, J = 6.5, 1.6 Hz, 2H, fiber-coupled laser (Laser system LRD-0450 from Laserglow,
P2), 7.89 (tt, J = 7.7, 1.6 Hz, 1H, P4), 7.84 (d, J = 5.8 Hz, 1H, Toronto, Canada), which was set to 50 mW at the cuvette
B5), 7.59 (ddd, J = 7.4, 5.6, 1.3 Hz, 2H, T4), 7.41 (d, J = 5.8 Hz, (4 mm beam diameter; 0.4 W cm−2) at a 90° angle with respect
1H, B6′), 7.34 (t, J = 6.5 Hz, 2H, P3), 7.06 (dd, J = 5.5, 1.6 Hz, to the spectrometer. The excitation power was measured using
1H, B5′), 2.77 (s, 3H, B7), 2.41 ppm (s, 3H, B7′). MS: m/z 298.9 a S310C thermal sensor connected to a PM100USB power
[M − 2PF6]2+ (calc.: 299.1); 743.2 [M − PF6]+ (calc.: 743.1). meter (Thorlabs). The emission spectrum was visualized from
Crystal growing: vapor diffusion of toluene into a solution of 500–800 nm with an Avantes 2048L StarLine UV-Vis spectro-
[9](PF6)2 in acetone.
meter as detector. No difference in the electronic absorption
[Ru(tpy)(tfmbpy)(py)](PF6)2 ([10](PF6)2). The complex spectrum (measured using a Varian Cary 50 UV-vis spectro-
[4](PF6)2 (20 mg, 0.022 mmol) was dissolved in 20 mL of ethanol/ meter) was found due to exposure to the 450 nm laser,
water (1 : 1), and excess amount of pyridine (1 mL) was added. showing that emission is from the starting compound. All
The reaction was refluxed at 80 °C under Ar for 24 h. After the emission spectra were recorded using Avasoft software from
solvent was rotary evaporated, the orange solid was dissolved Avantes and further processed using Microsoft Office Excel
in minimal amount of acetone, precipitated with ether, and fil- 2010 and Origin Pro software.
1
tered (19.5 mg, 90%). H NMR (300 MHz, (CD3)2CO): δ 9.60 (s,
The quantum yield of emission from the 3MLCT excited
1H, B3), 9.36 (s, 1H, B3′), 9.34 (d, J = 4.8 Hz, 1H, B6), 8.96 (d, state was calculated using the relative method with [Ru(bpy)3]
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Cl2 as the standard (0.06 in deoxygenated CH3CN), according 10 A. N. Hidayatullah, E. Wachter, D. K. Heidary, S. Parkin
to eqn (2):
and E. C. Glazer, Inorg. Chem., 2014, 53, 10030–10032.
11 B. Siewert, V. H. S. van Rixel, E. J. van Rooden,
S. L. Hopkins, M. J. B. Moester, F. Ariese, M. A. Siegler and
S. Bonnet, Chem. – Eur. J., 2016, 22, 10960–10968.
12 L. N. Lameijer, S. L. Hopkins, T. G. Brevé, S. H. C. Askes
and S. Bonnet, Chem. – Eur. J., 2016, 22, 18484–18491.
13 P. S. Wagenknecht and P. C. Ford, Coord. Chem. Rev., 2011,
255, 591–616.
450
std
Esample
A
3MLCT
sample
3MLCT
std
Φ
¼ Φ
ꢁ
ꢁ
ð2Þ
450
A
Estd
sample
where Φ3MLCT is the quantum yield of 3MLCT emission, A450 is
the absorbance at 450 nm (generally 0.1 for a 1 cm path
length), E is the integrated area of the MLCT emission band,
and sample and std denote the sample and standard
([Ru(bpy)3]Cl2), respectively.
3
14 S. Bonnet and J.-P. Collin, Chem. Soc. Rev., 2008, 37, 1207–
1217.
15 P. A. Scattergood, U. Khushnood, A. Tariq, D. J. Cooke,
C. R. Rice and P. I. P. Elliott, Inorg. Chem., 2016, 55, 7787–
7796.
16 Q. Sun, B. Dereka, E. Vauthey, L. M. Lawson Daku and
A. Hauser, Chem. Sci., 2017, 8, 223–230.
17 Q. Sun, S. Mosquera-Vazquez, L. M. Lawson Daku,
L. Guénée, H. A. Goodwin, E. Vauthey and A. Hauser, J. Am.
Chem. Soc., 2013, 135, 13660–13663.
18 Q. Sun, S. Mosquera-Vazquez, Y. Suffren, J. Hankache,
N. Amstutz, L. M. Lawson Daku, E. Vauthey and A. Hauser,
Coord. Chem. Rev., 2015, 282–283, 87–99.
Photochemistry
Photosubstitution experiments were performed using the
setup described in literature,27 where a 490 nm LED lamp was
fixed on top of a 1 cm quartz cuvette, and the temperature was
controlled using a Peltier controller to maintain 25 °C during
the 30 min irradiation. The beginning concentration of the
solutions was determined such that the absorbance at the
irradiation wavelength (λirr = 490 nm) was approximately 0.5.
The photon flux of the 490 nm LED lamp was determined to
be approximately 1.0 × 10−7 Einstein s−1 by ferrioxalate actino-
metry.49 The specific photon flux and reaction conditions are
referenced in each experiment. The concentrations were calcu-
lated using the two-wavelength method.27
19 C. E. Welby, G. K. Armitage, H. Bartley, A. Wilkinson,
A. Sinopoli, B. S. Uppal, C. R. Rice and P. I. P. Elliott, Chem.
– Eur. J., 2014, 20, 8467–8476.
20 C. E. Welby, C. R. Rice and P. I. P. Elliott, Angew. Chem., Int.
Ed., 2013, 52, 10826–10829.
21 J. K. White, R. H. Schmehl and C. Turro, Inorg. Chim. Acta,
2017, 454, 7–20.
22 A. J. Göttle, F. Alary, M. Boggio-Pasqua, I. M. Dixon,
J.-L. Heully, A. Bahreman, S. H. C. Askes and S. Bonnet,
Inorg. Chem., 2016, 55, 4448–4456.
23 S. Bonnet, J.-P. Collin, J.-P. Sauvage and E. Schofield, Inorg.
Chem., 2004, 43, 8346–8354.
Acknowledgements
The European Research Council is kindly acknowledged for a
Starting Grant to S. B. This work was also supported by the
Dutch Organization for Scientific Research (NWO) via a VIDI
grant to S. B. Prof. E. Bouwman is kindly acknowledged for
support and input.
24 J. D. Knoll, B. A. Albani and C. Turro, Chem. Commun.,
2015, 51, 8777–8780.
25 J. D. Knoll, B. A. Albani, C. B. Durr and C. Turro, J. Phys.
Chem. A, 2014, 118, 10603–10610.
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