Cis- Trans Interconversion in Ruthenium(II) Bipyridine Complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b01485

Most studies of ruthenium polypyridine complexes are devoted to their cis isomers. The fact that cis isomers are thermally more stable and thus easier to synthesize has prevented researchers from investigating the properties and applications of trans complexes. We present a study of thermal and photochemical cis-trans interconversion of the key complex [Ru(bpy)₂(PMe₃)(H₂O)]²⁺ (bpy = 2,2′-bipyridine, PMe₃ = trimethylphosphine), which results in specific synthetic applications of the trans species, potentially useful as a platform for designing highly efficient visible light activated caged compounds. We show, as a proof of concept, some examples of trans complexes bearing N-donor and P-donor ligands and their comparison with the cis isomers.

Full text of DOI:10.1021/acs.inorgchem.9b01485

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Cis−Trans Interconversion in Ruthenium(II) Bipyridine Complexes
́
Yeraldith Rojas Perez, Leonardo D. Slep,* and Roberto Etchenique*
́
Departamento de Química Inorganica, Analítica y Química Física, INQUIMAE, Facultad de Ciencias Exactas y Naturales,
́
Universidad de Buenos Aires, Pabellon 2, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina
S
* Supporting Information
ABSTRACT: Most studies of ruthenium polypyridine complexes are devoted
to their cis isomers. The fact that cis isomers are thermally more stable and
thus easier to synthesize has prevented researchers from investigating the
properties and applications of trans complexes. We present a study of thermal
and photochemical cis−trans interconversion of the key complex [Ru-
(bpy)2(PMe3)(H2O)]2+ (bpy = 2,2′-bipyridine, PMe3 = trimethylphosphine),
which results in specific synthetic applications of the trans species, potentially
useful as a platform for designing highly efficient visible light activated caged
compounds. We show, as a proof of concept, some examples of trans complexes bearing N-donor and P-donor ligands and their
comparison with the cis isomers.
INTRODUCTION
EXPERIMENTAL SECTION
■
■
Syntheses. All procedures were done under Argon.
Ruthenium bipyridine complexes have been profusely studied.
They present a rich and interesting photochemistry that arises
from a strong Ru(II) → π*bpy MLCT absorption band
followed by the thermal population of a nearby 3d−d
dissociative state after being irradiated. This state lasts long
enough to undergo photochemical reactions, usually expelling
monodentate ligands and leaving a [Ru(bpy)2(Solvento)]
species.1
These characteristics have been employed in photochemical
synthesis2 and also in the design of visible light activatable
caged compounds. The first caged compound based on
{Ru(bpy)2}2+ chemistry3 was cis-[Ru(bpy)2(4AP)2]2+, a
complex that delivers 4-aminopyridine (4AP), a blocker of
K+ channels,4 either in a linear regime under irradiation with
blue light or in a two-photon regime when NIR light is used.5
Many other phototriggers based on Ru-bpy complexes have
been subsequently reported, including caged glutamate,6 γ-
aminobutyric acid,7 nicotine,8 serotonine,9 dopamine,10 and
biomedically relevant nitriles.11 A similar approach, although
based on a different chemistry, has been employed to devise
nitric oxide deliverers.12
Relevant theoretical studies of this family of complexes have
also been reported,13 all of them devoted to the thermally
more favorable cis form, while just a few works report
calculated data of the trans isomers, for the diaquo trans-
[Ru(bpy)2(H2O)2]2+ 14 or the light harvesting related complex
trans-[Ru(dcbpy)2(NCS)2]15 (dcbpy = 4,4′-dicarboxy-2,2′-
bipyridine).
In this work we present a study of the photochemical
interconversion between cis- and trans-[Ru(bpy)2(PMe3)-
(H2O)]2+, showing that the trans aquo isomer can be regarded
as a useful platform to devise new highly efficient [Ru-
(bpy)2(PMe3)L]n+ phototriggers with a trans conformation.
cis-[Ru(bpy)2(PMe3)Cl)]Cl was obtained as described elsewhere.16
trans-[Ru(bpy)2(PMe3)(H2O)](CF3SO3)2 ([1](CF3SO3)2). A 100 mg
portion of cis-[Ru(bpy)2(PMe3)Cl)]Cl was dissolved in 3.0 mL of
distilled water. The solution was stirred with 500 mg of Dowex-22
anionic resin previously loaded with mesylate ion. Aquation of the
complex proceeds simultaneously with the ion exchange step, yielding
cis-[Ru(bpy)2(PMe3)(H2O)](CH3SO3)2. The obtained solution was
filtered and diluted to 20 mM, and 5 equiv of trifluoromethane-
sulfonic acid was added. The mixture was put into an ice bath and
irradiated with a 150 W metal halide lamp for 3.5 h. After 30 min of
irradiation the complex [1](CF3SO3)2 began to precipitate from the
mixture. It was filtered and washed with portions of 0.1 M KPF6 until
pH >4, further washed with tert-butyl alcohol, and vacuum-dried. An
intense red crystalline solid was obtained. Yield: 42%. εmax(460 nm) =
9.7 × 103 M−1 cm−1. 1H NMR (500 MHz, D2O/acetone-d6): δ (ppm)
9.30 (4H, d, 5.46 Hz); 8.47 (4H, d, 8.36 Hz); 8.19 (4H, t, 7.86 Hz);
7.73 (4H, t, 6.87 Hz); 0.574 (9H, d, 9.99 Hz) (see Figure S1) MS
(ESI+): m/z 639.0417 [[1] − H2O + CF3SO3]+; 254.0459 [1]2+;
245.0444 [[1] − H2O]2+; 216.0273 [[1] − PMe3]2+; 207.0218 [[1] −
H2O − PMe3]2+ (see Table S1, Figure S2).
cis-[Ru(bpy)2(PMe3)(H2O)](CF3SO3)2 ([2](CF3SO3)2). A 50 mg
portion of [1](CF3SO3)2 was dispersed in 3.0 mL of distilled water.
The mixture was heated to 60 °C under argon for 3 days and the
obtained solution lyophilized. In case the mesylate salt was needed, a
further ion exchange with Dowex-22 previously charged with
−
CH3SO3 ion was performed to yield [2](CH3SO3)2. εmax(444 nm)
1
= 6.7 × 103 M−1 cm−1. H NMR (300 MHz, acetone-d6): δ (ppm)
9.19 (1H, d, 5.79 Hz); 8.95 (1H, d, 5.50 Hz); 8.48 (1H, d, 8.16 Hz);
8.43 (1H, d, 8.08 Hz); 8.39 (1H, d, 8.25 Hz); 8.20 (1H, d, 8.25 Hz);
8.17 (1H, t, 8.25 Hz); 8.12 (1H, t, 8.25 Hz); 7.94 (1H, t, 7.83 Hz);
7.74 (4H, m); 7.46 (1H, s); 7.24 (1H, t, 6.58 Hz); 7.02 (1H, t, 6.76
Hz); 1.03 (9H, d, 8.89 Hz) (see Figure S3). MS (ESI+): m/z
639.0388 [[2] − H2O + CF3SO3]+; 245.0432 [[2] − H2O]2+;
Received: May 21, 2019
© XXXX American Chemical Society
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216.0262 = [[2] − PMe3]2+; 207.0208 [[2] − H2O − PMe3]2+ (see
Table S2, Figure S4).
calibrated laser source. Then, the quantum yield of photolysis was
adjusted as a parameter in order to fit the corresponding equations.
NMR spectra were obtained with Bruker AM-500, Bruker Advance
Neo 500, and Bruker Fourier 300 spectrometers. Internal standard
cis-[Ru(bpy)2(PMe3)(ImH)](PF6)2 ([3](PF6)2). A 90 mg portion of
[2](CH3SO3)2 was dissolved in 3 mL of distilled water. A 73 mg
portion of imidazole was added, and the mixture was stirred at 50 °C
for 24 h and then cooled to room temperature and precipitated with
600 μL of aqueous KPF6 0.5 M. An orange solid was obtained. It was
filtered, washed with water several times, and vacuum-dried. Further
purification was carried out by exchanging the PF6 ion by chloride
using Dowex-22 anionic resin and reprecipitating with KPF6. Yield:
1
NMR H (standard EtOH) measurements were performed to obtain
molar absorptivity values. All molar absorptivity values are given in
aqueous solution. Samples for mass spectrometric measurements were
prepared in LC-MS quality methanol and were performed by straight
injection of the samples into the spectrometer, equipped with an
ionization electrospray (ESI) source and using a high-resolution and
high-accuracy hybrid quadrupole (Q) and orthogonal time of flight
(Tof) mass spectrometer and Xevo G2S Q-TOF (Waters Corp.)
operating in the positive ion mode. Typically, the cations were
detected as the naked doubly charged gaseous species but also via a
series of cations with reduced charge states achieved via ion pairing. In
every case, only the observed m/z value of the most abundant
isotopologue ion of the multi-isotopic species forming multifaceted
isotope clusters is mentioned.
Theoretical Computations. Density functional theory (DFT)
was employed to fully optimize the ground and lowest energy triplet-
state geometries of cis- and trans-[Ru(bpy)2(PMe3)(H2O)]2+. The
calculations were performed with Gaussian 0917 using Becke’s three-
parameter hybrid functional with the correlation functional of Lee,
Yang, and Parr formalized as the B3LYP hybrid functional18 and the
effective core potential basis set LanL2DZ implemented in G09,17
which describes first-row elements using Dunning’s D95V basis set
and the Los Alamos ECP plus DZ basis for the heavier elements.19
This combination proved to be suitable for electronic structure
computations, geometry predictions, and spectral assignment in
coordination compounds related to those described in this
report.12c,20 Tight SCF convergence criteria and no symmetry
constraints were used along the geometry optimizations. The
polarizable continuum model (PCM) approximation was used all
along the computations to account for solvation effects in water.
Coordinated H2O might engage in specific solute−solvent inter-
actions. No improvement in the computed (TD)DFT electronic
spectra (vide infra) was observed upon inclusion of discrete explicit
water molecules in the computations. While this strategy proved
necessary in other situations,20a,21 it was avoided here to reduce the
computational cost. The nature of the stationary points obtained in
the optimization procedures was checked by means of vibrational
analyses. Electronic spectra for the different species were computed by
(TD)DFT, involving at least 100 excited states at the same level of
theory as that employed in the minimization step.
Energy profiles calculated for the electronic ground state as well as
for the low-lying triplet excited states provide a microscopic
description of the lowest energy reaction pathway. Taking into
account that the exchange of ligands in ruthenium octahedral
complexes usually proceeds via a dissociative pathway, the elongation
coordinate of the Ru−O bond in cis-/trans-[Ru(bpy)2(PMe3)-
(H2O)]2+ was explored. The pentacoordinated species cis-/trans-
[Ru(bpy)2(PMe3)]2+ derived from the complete dissociation of an
H2O molecule were fully optimized as both singlet and triplet species.
Transition state geometries associated with the cis- to trans-
[Ru(bpy)2(PMe3)]2+ interconnection were identified and optimized.
The synchronous transit-guided quasi-Newton (STQN) method22
implemented in G09 was used for this purpose. The vibrational
analyses of the species optimized in this way showed a single
imaginary frequency, consistent with a first-order saddle point on the
potential energy surface (PES). The reaction coordinate linking the
TS with the pentacoordinated cis-/trans-[Ru(bpy)2(PMe3)]2+ local
minima in both the ground and excited state’s PES was revealed by
Fukui’s intrinsic reaction coordinate (IRC) computations.23
1
45%. εmax(431 nm) = 6.0 × 103 M−1 cm−1. H NMR (500 MHz,
D2O/acetone-d6): δ (ppm) 9.23 (1H, d, 5.45 Hz); 8.80 (1H, d, 8.35
Hz); 8.54 (1H, d, 7.35 Hz); 8.41 (1H, d, 7.85 Hz); 8.36 (1H, d, 8.10
Hz); 8.33 (1H, d, 8.35 Hz); 8.20 (1H, t, 7.85 Hz); 8.09 (1H, t, 7.97
Hz); 7.91 (1H, t, 7.97 Hz); 7.84 (1H, t, 7.97 Hz); 7.73 (3H, m); 7.51
(2H, s); 7.30 (1H, t, 6.43 Hz); 7.17 (1H, t, 6.78 Hz); 7.02 (1H, t,
1.42 Hz); 6.76 (1H, t, 1.48 Hz); 0.99 (9H, d, 8.42 Hz) (see Figure
2+
S5). MS (ESI+): m/z 703.0878 [[3] + PF6]+; 279.0632 [3]
;
245.0444 [[3] − ImH]2+; 207.0221 [[3] − ImH − PMe3]2+ (see
Table S3, Figure S6).
trans-[Ru(bpy)2(PMe3)(ImH)](PF6)2 ([4](PF6)2). A 39 mg portion of
[1](CF3SO3)2 was dissolved in 2 mL of dry MeOH. A 21 mg portion
of imidazole was dissolved in 500 μL of dry MeOH and mixed with
the previous solution. This mixture was stirred at 40 °C for 40 min
and then cooled to 0 °C and precipitated by addition of 500 μL of
aqueous KPF6 0.5 M. The precipitate was filtered, washed two times
with KPF6 0.1 M and then with water, and dried under vacuum. Yield:
1
77%. εmax(464 nm) = 8.9 × 103 M−1 cm−1. H NMR (300 MHz,
acetone-d6): δ (ppm) 9.81 (4H, d, 5.63 Hz); 8.67 (4H, d, 8.22 Hz);
8.33 (4H, t, 7.79 Hz); 7.94 (4H, t, 6.92 Hz); 7.43 (1H, s), 7.00 (1H,
s); 6.47 (1H, s); 0.80 (9H, d, 9.30 Hz) (see Figure S7). MS (ESI+):
m/z 703.0875 [[4] + PF6]+; 279.0617 [4]2+; 245.0430 [[4] −
ImH]2+; 207.0207 [[4] − ImH − PMe3]2+ (see Table S4, Figure S8).
trans-[Ru(bpy)2(PMe3)(PPh3)](PF6)2 ([5](PF6)2). A 64 mg portion of
[1](CF3SO3)2 was dissolved in 2 mL of dry MeOH. A 206 mg
portion of PPh3 (PPh3 = triphenylphosphine) were dissolved in 2 mL
of dry MeOH and mixed with the previous solution. This mixture was
stirred at 50 °C for 3 h and then cooled to 0 °C. The excess PPh3 was
separated, and 5 equiv of KPF6 was added. The orange solid was
filtered, washed with MeOH and diethyl ether, and dried under
vacuum. Yield: 48%. εmax(440 nm) = 10.2 × 103 M−1 cm−1. 1H NMR
(300 MHz, acetone-d6): δ (ppm) 9.29 (4H, d, 5.65 Hz); 8.58 (4H, d,
8.00 Hz); 8.32 (4H, t, 8.00 Hz); 7.80 (4H, t, 5.63 Hz); 7.40 (3H, t,
7.55 Hz); 7.16 (6H, t, 7.32 Hz); 6.41 (6H, t, 8.46 Hz); 0.55 (9H, dd,
9.40 Hz) (see Figure S9). MS (ESI+): m/z 376.0892 [5]2+; 338.0668
[[5] − PMe3]2+; 245.0430 [[5] − PPh3]2+; 207.0206 [[5] − PMe3 −
PPh3]2+ (see Table S5, Figure S10).
trans-[Ru(bpy)2(PMe3)2](PF6)2 ([6](PF6)2). A 95 mg portion of
[1](CF3SO3)2 was dissolved in 2 mL of dry MeOH. A 400 μL portion
of a 1 M PMe3 solution in THF was added, yielding an immediate
color change of the solution. After 2 h of stirring at room temperature,
the reaction mixture was cooled to 0 °C and 500 μL of 0.5 M aqueous
solution of KPF6 was added. The orange precipitate was collected by
filtration and washed three times with a 1/1 water/MeOH mixture
and once with diethyl ether. Yield: 89%. εmax(445 nm) = 10.2 × 103
1
M−1 cm−1. H NMR (300 MHz, acetone-d6): δ (ppm) 9.66 (4H, d,
5.60 Hz); 8.82 (4H, d, 8.04 Hz); 8.40 (4H, t, 7.86 Hz); 7.97 (4H, t,
6.50 Hz); 0.65 (18H, t, 3.34 Hz) (see Figure S11). MS (ESI+): m/z
283.0651 [6]2+; 245.0450 [[6] − PMe3]2+; 207.0231 [6]− 2PMe3]2+
(see Table S6, Figure S12).
Spectroscopic Measurements and Photolysis. The optical
bench used for UV−vis measurements consisted of a 532 nm, 10 mW
DPSS laser or 405 nm, 45 mW laser diode, collimated and directed
toward a four-faceted cuvette, kept at 25 °C; the sample tested was
stirred. The absorbance was monitored perpendicularly to the laser
path using an OceanOptics PC2000 CCD spectrophotometer run by
OOIChem sofware.
RESULTS AND DISCUSSION
■
It is known that refluxing the complex [Ru(bpy)2Cl2] in water/
EtOH mixtures makes it possible to obtain the mono and bis
aqua complexes [Ru(bpy)2 Cl(H2 O)]+ and [Ru-
(bpy)2(H2O)2]2+, both in a cis configuration.1,2 Durham et
Quantum yield measurements of photouncaging were performed
by recording the spectra while the photoreaction occurred under a
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al. showed that trans-[Ru(bpy)2(H2O)2]2+ can be photochemi-
cally prepared24 by irradiation of a solution of the cis isomer in
the presence of a suitable anion that might induce the
fractional precipitation of the trans form, leaving the unreacted
cis precursor in solution. This trans species spontaneously
reverts to the cis form in solution. Although it can be used to
perform some further reactions,25 its lifetime in aqueous
solution at moderate temperatures is in the tens of minutes,
preventing its use in most syntheses.
If the electron-acceptor ligand PMe3 is introduced, the
lability of the resulting complex toward ligand exchange is
reduced. The same trend occurs regarding cis−trans thermal
isomerization. By irradiation of a solution of cis-[Ru-
(bpy)2(PMe3)(H2O)]2+ in the presence of triflate ions, a
precipitate of pure trans-[Ru(bpy)2(PMe3)(H2O)]2+ is ob-
tained. The characteristic four aromatic signals of the NMR
spectrum (Figure 1) unequivocally confirms the trans nature of
negligible. On the other hand, for low absorbances at the
irradiation wavelength and 1 cm path length, the differential
equation can be linearized. Under this condition
d[cis]
dt
= 2.3I0l(εt[trans] − ε [cis])
c
(2)
The integration of the latter equation yields a monoexponen-
tial of the general form
2.3I
V
[trans] = A(1 − e−kt) + B k =
0 (ε ϕ + εtϕ)
c t
c
(3)
where V is the reaction volume of the cuvette and ϕc and ϕt are
the quantum efficiencies of isomerization for the direct and
inverse photoreactions, respectively.
Due to the fact that the photoisomerization proceeds in both
cis to trans and trans to cis ways, irradiation of cis- or trans-
[Ru(bpy)2(PMe3)(H2O)]2+ solutions always yields an isomeric
mixture. In the photostationary steady state the [cis]/[trans]
ratio is given by the quotient ϕtεt/ϕcεc.
Figure 2 shows a typical photolysis experiment of trans-
[Ru(bpy)2(PMe3)(H2O)]2+ in a UV−vis cuvette using a 532
Figure 1. Structure and 1H NMR spectrum of trans-[Ru-
(bpy)2(PMe3)(H2O)]2+ in D2O.
this complex. Even when this species eventually undergoes
thermal trans−cis isomerization as do other {Ru(bpy)2}2+
complexes, it can be stored for long periods in solid form. It
is stable enough in aqueous solutions to perform proper
characterization, and most interestingly it can be warmed in
the presence of other ligands for synthetic purposes.
Photochemical Properties. A comparison between both
isomers shows a red-shifted MLCT band with a higher
absorptivity for the trans form (εmax = 9.7 × 103 M−1 cm−1 at
460 nm vs εmax = 6.7 × 103 M−1 cm−1 at 444 nm, see
Supporting Information, Figures S13 and S14), which suggests
a quality that makes it attractive for the design of a platform of
long-wavelength-active caged compounds. Irradiation of a
solution containing pure trans isomer shows that this form is
also photoactive, undergoing reverse isomerization to the cis
form. A simple mechanistic model can be used to elucidate the
main parameters that govern the photoactivity:
Figure 2. UV−vis spectra taken during photolysis of trans-[Ru-
(bpy)2(PMe3)(H2O)]2+ in water using a 532 nm laser.
nm laser. The presence of an isosbestic point indicates that just
two colored species are present, as expected. Full spectrum
analysis of the product using pure cis and trans complexes as
standards shows a [cis]/[trans] ratio of 2.08 in the photosta-
tionary state. As εt and εc can be measured from the pure
complexes, it is possible to determine that ϕt is 1.5 times
higher than ϕc. This result is at least striking, considering the
fact that a totally opposite trend has been reported when the
photosubstitution processes in the related cis-[Ru(bpy)2XY]n+
species have been studied.1 By fitting the complete photolysis
to eq 3, it is possible to obtain the values ϕt = 0.158 and ϕc =
0.105, respectively. It is important to note that full photolysis
of the trans isomer does not yield a pure cis species but a
photostationary state. Figure 3 shows the spectra of the pure
species and the different photostationary states that can be
established by changing the irradiation wavelength. Due to its
bathochromic shift, the trans species is less absorptive at short
wavelengths (405 nm) and a higher molar fraction of this
isomer is obtained. Conversely, the cis form predominates
under irradiation at 532 nm.
where φ1 and φ2 are the isomerization quantum yields and k
the kinetic constant of the trans/cis thermal isomerization.
Therefore
d[cis]
dt
= k[trans] + I0(1 − 10−ε l[trans]) − I0(1 − 10−ε l[cis]
(1)
)
t
c
Thermal Interconversion. The thermal decay of the trans
form to its cis isomer is another key feature in order to evaluate
synthetic potentiallity. The half-life in aqueous solution ranges
from 14 h at 30 °C to about 30 min at 60 °C, long enough to
obtain reasonably pure trans complexes by ligand exchange
(vide infra). An Eyring plot of the thermal decay yields ΔH⧧ =
where I0 is the irradiation power, εt and εc are the molar
absorptivities of the trans and cis forms at the irradiation
wavelength, respectively, and l is the optical path length. At T
≅ 25 °C and high irradiation intensities, k can be regarded as
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parameters are overall consistent with other reports involving
the same computational methodology. The same can be said
for the trans species, even though very few examples of
structurally characterized trans compounds based on the
{Ru(bpy)2}2+ core have been reported.24,26 Table S7 in the
Supporting Information summarizes relevant structural in-
formation for both species.
The electronic spectrum predicted by (TD)DFT for cis-
[Ru(bpy)2(PMe3)(H2O)]2+ shows very good agreement with
the experimental spectrum, as can be seen in Figure 5 (top).
Figure 3. (a) Absorption spectra of cis- and trans-[Ru(bpy)2(PMe3)-
(H2O)]2+ ([2] and [1]). (b) Photoconversion between isomers by
irradiation with different wavelengths.
118 kJ/mol and ΔS⧧ = 50 kJ/mol, suggesting a dissociative
path for the isomerization process. (see Figure S15 in the
Supporting Information)
In order to understand the differences and similarities of the
isomers of [Ru(bpy)2(PMe3)(H2O)]2+, several simulations
based in DFT were performed.
Ground-State DFT Analysis. DFT has been employed to
explore the electronic properties of the compounds described
in this work. The calculations lead to two stationary points
corresponding to the hexacoordinated cis and trans isomers,
with the latter lying ca. 39 kJ mol−1 higher in energy. This
value explains the impossibility of accessing the trans species by
thermal means and suggests that, in the absence of kinetic
restrictions, trans to cis interconversion is strongly favored. The
optimized geometry of cis-[Ru(bpy)2(PMe3)(H2O)]2+ (Figure
4) shows the presence of an Ru2+ ion in a distorted-octahedral
coordination environment. As is usual at this level of theory,
the Ru−N, Ru−O, and Ru−P bond lengths are consistently
longer than those observed experimentally in related
compounds, for which there are numerous examples. Structural
Figure 5. Experimental and computed electronic spectra for cis-
[Ru(bpy)2(PMe3)(H2O)]2+ ([2]) (top) and trans-[Ru(bpy)2(PMe3)-
(H2O)]2+ ([1]) (bottom).
The visible region of the spectrum is dominated by a broad
band that can be assigned as MLCT Ru(II)−π*bpy predicted at
432 nm, in excellent agreement with experimental observa-
tions. The same is true for the quality of the predicted
electronic spectrum for the trans species (Figure 5, bottom),
with visible transitions centered at 464 nm also of the MLCT
type. At a symmetry lower than octahedral, the degeneration of
the t2g orbitals is lifted. In the two cases discussed here, these
three orbitals become the molecular orbitals HOMO, H-1, and
H-2 of the molecule (see Supporting Information, Figures S16
and S17). These three orbitals are mostly Ru in character for
both cis-[Ru(bpy)2(PMe3)(H2O)]2+ (82%, 85%, and 79% Ru
for H, H-1, and H-2, respectively) and trans-[Ru-
(bpy)2(PMe3)(H2O)]2+ (82%, 76%, and 92% Ru for H, H-1,
and H-2, respectively), the remaining contributions arising
mostly from π orbitals of the bpy ligands. On the other hand,
the LUMO and LUMO+1 orbitals are bpy-centered (96% and
Figure 4. Optimized geometries (DFT) for cis-[Ru(bpy)2(PMe3)-
(H2O)]2+ ([2]) (left) and trans-[Ru(bpy)2(PMe3)(H2O)]2+ ([1])
(right.)
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Figure 6. Schematic representation of the reaction coordinate involved in the cis−trans interconversion over the ground and excited PES. All
energies are expressed in kJ mol−1.
94% bpy for L and L+1 in the cis species and 97% and 94% bpy
for L and L+1 in the trans species) and are π* in nature. In
both compounds the MLCT bands result from the convolution
of transitions involving the H-1 and H-2 donor orbitals while
the L and L+1 MOs behave as acceptors. Transitions involving
the HOMO are of very low intensity even when they are
symmetry allowed, because of the very poor overlap between
the metal and ligand orbitals. Consequently, the lowest energy
HOMO−LUMO excited MLCT state contributes only
marginally to the experimental absorption profile (see
Supporting Information, Tables S8−S11).
Excited-State Interconversion. On irradiation in the
MLCT transitions located in the visible region, solutions of cis-
[Ru(bpy)2(PMe3)(H2O)]2+ photoisomerize to yield trans-
[Ru(bpy)2(PMe3)(H2O)]2+. At prolonged irradiation times
the concentrations of both species become stationary, due to
the reversibility of the photoprocess and the simultaneous
occurrence of the back thermal reaction. The microscopic
sequence involves the following, as already described for the
photodetachment of monodentate ligands in Ru-polypyridine
complexes. (a) Absorption of light occurs in the visible region
1
to render an excited MLCT which relaxes to the lowest
Starting from the trans species, and always exploring
pathways over the ground-state potential energy surface, we
found that the thermal interconversion between the isomers
proceeds by a three-step mechanism. The most likely pathway
connecting the trans reagent with the cis product seems to
involve two intermediate local minima that correspond to
pentacoordinated (5C) species due to the dissociation of a
water molecule from the stable hexacoordinated (6C) species.
The first step involves the elongation of the Ru−O bond, a
process that occurs uphill, to allow for the formation of the first
5C intermediate, in which the arrangement of the ligands is
reminiscent of that of the 6C reagent. In fact, the Ru−N and
Ru−P bond lengths in 5C remain roughly unchanged in
comparison to trans-6C and the angle between vectors
perpendicular to the planes containing the bpy molecules
varies only from 32.0° in trans-6C to 33.3° in trans-5C (Figure
6). The theoretical dissociation energy associated with the
process is 67.8 kJ mol−1. The second step is associated with the
rotation of one bpy moiety around the Ru−N3 bond, inducing
a change in the relative angle between the bpy molecules,
ultimately bringing it to a value of 86.4°, in a conformation of
ligands similar to that of the 6C cis product (angle between
bpys of 89.4°). The latter is an activated step; the optimization
of the first-order saddle point in the trajectory between both
5C species leads to the transition state for the process, with a
bipyridine angle of 51.7° and a theoretical activation barrier of
36.6 kJ mol−1. The final step requires the downhill addition of
a water molecule to the 5C species to finally yield the more
stable 6C cis species.
energy HOMO−LUMO 1MLCT excited state. (b) Intersystem
3
crossing takes places which leads to a MLCT excited state,
with this state displaying geometric parameters similar to those
in the ground state (see Supporting information, Table S12,
3
Figure S18). (c) The photoactive d−d state is thermally
populated. The antibonding nature of the newly populated d
orbital, which is aligned with the O−Ru−P axis, induces the
3
elongation of the Ru−O bond, to yield a thermalized d−d
relaxed excited state stabilized 29.3 kJ mol−1 with respect to
the 3MLCT. The weakening of the Ru−O bond is followed by
(d) dissociation of a H2O molecule to render the 3d−d excited
cis-5C species. The separation of a water molecule from the
coordination sphere is slightly endothermic, involving 10.9 kJ
mol−1. Apart from the different coordination number, no
significant changes in the bpy conformation are associated with
the dissociation process (the bpy molecules lie at 87.1° in cis-
6C-3dd and 89.5° in cis-5C). The rotation of a bpy molecule
around the Ru−N3 bond up to 55.3° allows the conversion
between the cis- and trans-5C forms. The transformation
occurs over the lowest energy 3d−d PES and involves a shallow
barrier of only 3.7 kJ mol−1 to yield the slightly more stable
(2.9 kJ mol−1) trans-5C isomer. Addition of H2O to the
3
coordination sphere finally leads to the d−d excited state of
the trans-6C species. As is clear from Figure 6, the overall cis−
3
trans interconversion over the d−d PES involves barriers
significantly lower than those in the ground state.
Synthetic Applications. One of the central (and often
undesirable) aspects in the chemistry of monoaqua complexes
of Ru-bpy is the deprotonation of the coordinated water to
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Article
yield more inert hydroxo complexes at high pH values. The
pKa(cis) of cis-[Ru(bpy)2(PMe3)(H2O)]2+ is 11.05. We have
explored the deprotonation equilibrium in the analogous trans
complex, which turned to be less acidic (pKa(trans) = 12.15).
This apparently small pKa difference actually facilitates its use
in synthetic procedures, enlarging the pH window to perform
ligand change.
(bpy)2(PMe3)(PPh3)]2+ could not be synthesized, a compar-
ison of their properties was not possible. Another complex that
presents an interesting property is the symmetric trans-
[Ru(bpy)2(PMe3)2]2+, which is very stable both thermally
and photochemically. Once more, its cis isomer could not be
synthesized following the standard procedures.
Heating of trans-[Ru(bpy)2(PMe3)(H2O)]2+ in the presence
of an L ligand holding a donor N atom, at an appropriate
temperature, yields trans-[Ru(bpy)2(PMe3)L]2+. In this way,
we have been able to synthesize and isolate the complex trans-
[Ru(bpy)2(PMe3)ImH]2+ (ImH = imidazole), which presents
a higher quantum yield of photoaquation than its cis isomer.
Figure 7 shows a typical photolysis of an aqueous solution of
CONCLUSIONS
■
We have synthesized for the first time the trans isomers of the
[Ru(bpy)2(PMe3)(H2O)]2+ form. The reaction, which in-
volves an isomerization in the excited state reached by light
absorption, was rationalized by employing DFT calculations.
We showed that, although the trans isomer can revert to the cis
form, it is stable enough in solid form and even in aqueous
solutions, allowing ligand exchange and photorelease studies
and further synthetic uses in a direct, clean way.
Both cis- and trans-[Ru(bpy)2(PMe3)(H2O)]2+ undergo
photoisomerization under 1MLCT irradiation. In water
solution a photostationary state is reached, which mainly
depends on the irradiation wavelength, due to the red-shifted
absorption of the trans form. Interestingly, while among cis
isomers a red-shifted 1MLCT band is usually linked to a lower
photolysis quantum yield, the trans isomers present simulta-
1
neously a lower energy MLCT and a higher quantum yield.
This fact could be useful in devising efficient red-shifted
phototriggers. Complexes bearing two phosphines were also
synthesized, although it was not possible to produce their cis
isomers in a similar way. While the heteroleptic PMe3-PPh3
complex delivers PPh3 under visible irradiation, the complex
having two PMe3 groups did not present an appreciable
photochemistry.
Figure 7. UV−vis spectra of an aqueous solution of trans-
[Ru(bpy)2(PMe3)ImH]2+ ([4]) during photolysis using a 532 nm
laser. Inset: best fit of photoproduct vs irradiation time, for a quantum
yield φPD(trans) = 0.23.
trans-[Ru(bpy)2(PMe3)ImH]2+. The presence of a neat
isosbestic point indicates that the only colored product is the
aquo complex. Its cis/trans ratio will depend on the wavelength
used (vide supra).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01485.
The inset shows the fitting to a simple model,7 from which
the quantum yield of photolysis can be determined to be
φPD(trans) = 0.23, much higher than the corresponding QY for
the cis sibling φPD(cis) = 0.10. The visible region MLCT bands
of the trans species are red-shifted in comparison with their cis
analogues. Among cis complexes with similar structure, lower
MLCT energies correlate with lower photodissociation
quantum yields. However, this correlation cannot be extended
to the comparison between trans and cis isomers: a
systematically higher photodissociation efficiency was found
for the studied trans complexes, regardless of their red-shifted
absorption. The photolysis of the trans complexes yields pure
ligands in a similar way as for cis forms, with no detected side
reactions.
Mass spectrometric, NMR, and UV−vis characterization
of the compounds, complementary computational de-
tails and results, and time evolution of trans-[Ru-
(bpy)2(PMe3)(PPh3)]2+ irradiated at 525 nm as
monitored by NMR (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for L.D.S.: slep@qi.fcen.uba.ar.
*E-mail for R.E.: rober@qi.fcen.uba.ar.
We were also able to synthesize trans complexes even in
cases where we failed to synthesize the corresponding cis
forms. The reaction between trans-[Ru(bpy)2(PMe3)(H2O)]2+
and PPh3 yields trans-[Ru(bpy)2(PMe3)(PPh3)]2+ in pure
form. The corresponding complexes with cis-[Ru-
(bpy)2(PPh3)L]2+ structure have been widely studied.7b,27 It
is a well-established fact that in these complexes the irradiation
on the 1MLCT band is always followed by photorelease of the
ligand L, while the phosphine remains coordinated to the Ru
center. Conversely, photolysis of trans-[Ru(bpy)2(PMe3)-
(PPh3)]2+ yields free PPh3 and [Ru(bpy)2(PMe3)(H2O)]2+,
probably due to the higher basicity of PMe3 (see Figure S19 in
the Supporting Information). Unluckily, as cis-[Ru-
ORCID
Leonardo D. Slep: 0000-0001-6447-2216
Roberto Etchenique: 0000-0002-5485-8710
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the University of Buenos Aires, CONICET,
and ANPCyT for their financial assistance to conduct the
research. R.E. and L.D.S. are members of the scientific staff of
CONICET.
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Article
Complex in Aqueous Solution. Comments Inorg. Chem. 2016, 36,
343−354.
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