Nitric-oxide photorelease and photoinduced linkage isomerism on solid [Ru(NO)(terpy)(L)]BPh4 (L = glycolate dianion)

Article abstract of DOI:10.1039/b912162e

Following photoexcitation at 80 K, two phenomena were detected on solid samples of a single substance, the tetraphenylborate of the [Ru(NO)(terpy)(L)] ⁺ cation (terpy = 2,2′:6′,2″-terpyridine, L = glycolate(2-) [oxyacetate]): the formation of p

Full text of DOI:10.1039/b912162e

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Nitric-oxide photorelease and photoinduced linkage isomerism on solid
[Ru(NO)(terpy)(L)]BPh4 (L = glycolate dianion)†
Helene Giglmeier,a Tobias Kerscher,a Peter Klu¨fers,*a Dominik Schanielb and Theo Woike*b
Received 23rd June 2009, Accepted 28th August 2009
First published as an Advance Article on the web 7th September 2009
DOI: 10.1039/b912162e
1
2
trans-[RuCl(NO)(py)4](PF6)2· H2O.9 It should be noted that in
Following photoexcitation at 80 K, two phenomena were
detected on solid samples of
a
single substance, the
the case of solid-state PR, the nature of the photoproducts is
expected to be convoluted. Motivated by the observation that PR
may interfere with PLI in a solid, we started a more systematic
search for substances that show both PLI and PR and thus allow
the study of the connection or competition between both excitation
paths in more detail. As a result, we are presenting a preliminary
tetraphenylborate of the [Ru(NO)(terpy)(L)]+ cation (terpy =
2,2¢:6¢,2¢¢-terpyridine, L = glycolate(2-) [oxyacetate]): the
formation of photoinduced linkage isomers of the nitrosyl
ligand, and photorelease of nitric oxide. Populations of
about 9% and 4% were measured for the photoinduced
isonitrosyl (MS1) and the side-on nitrosyl (MS2) linkage
isomers, respectively. At room temperature, NO release in
the solid was observed as the only reaction.
6
study on a terpyridine-glycolate(2-)-coordinated {RuNO} -type
cation whose tetraphenylborate salt reacted on irradiation with
the formation of two photoinduced isomers as well as with nitric-
oxide release.‡
The title compound was prepared by the reaction of
[Ru(NO)(terpy)Cl2]PF6 with excess glycolic acid and slightly more
than an equimolar amount of sodium hydroxide to achieve
deprotonation of both the carboxylic and the hydroxy function
to a dianionic glycolate(2-) ligand. In agreement with the strong
p acidity of the nitrosyl ligand, the substitution reaction yielded
the product with the alkoxide function of the glycolate(2-) ligand
trans to NO and the carboxylate in the terpy plane. Crystals of the
dmso solvate of the tetraphenylborate of the nitrosyl-ruthenium
cation were obtained by the addition of sodium tetraphenylborate
to the aqueous reaction mixture and recrystallisation from dmso.
The crystal-structure analysis of this [Ru(NO)(terpy)L]BPh4·dmso
(1·dmso) showed disordered dmso molecules close to the nitrosyl
ligand. The [Ru(NO)(terpy)L]+ ions in crystals of 1·dmso ex-
hibit non-crystallographic Cs symmetry (Fig. 1). The apparent
octahedral coordination of the central metal is distorted from
its ideal position in two respects. First, the N–Ru–N angle in
the molecular mirror plane (N2–Ru1–N4 in Fig. 1) is about 10◦
6
The extensively studied photoreactivity of {RuNO} -type nitrosyl
complexes is dominated by two processes (cf. ref. 1 for the electron-
counting rules of the Enemark–Feltham notation). The first is the
photorelease of nitric oxide which is sometimes followed by the
recombination of NO and the metal centre.2 The second is
the photoinduced formation of nitrosyl linkage isomers.3–5 For
the linkage isomers, two forms that are energy-rich with respect
to the kN-bonded nitrosyl ground-state (GS) have been detected
for metal nitrosyls: the kO-bonded, isonitrosyl state (MS1),
and the kN,O-bonded, side-on nitrosyl state (MS2). Though a
relationship between both phenomena has been discussed,2,6 the
individual experimental setups used for the detection of either
phenomenon are different. Typically, a photorelease (PR) study
is conducted close to the experimental boundary conditions
of the attempted application, namely the medical usage of a,
hopefully, switchable NO donor under physiological conditions.
Accordingly, PR is investigated mostly in aqueous solution close
to or above room temperature. As a result of these conditions,
the final photoproducts are usually nitric oxide and the aqua-
ruthenium(III) analogue of the nitrosyl starting complex. In
contrast, photoinduced linkage isomerism (PLI) is detected mostly
in the solid state and at substantially lower temperatures. However,
recent studies show that the PLI phenomenon does not depend on
the existence of a cavity inside a crystal but may be observed
in solution as well.7,8 Conversely, PR may also occur in the
solid state and has been considered a reason for the decrease of
the achievable population of photoinduced linkage isomers on
irradiation with light in the spectral range below 458 nm in the
course of a typical PLI experiment on the pyridine (py) complex
aDepartment fu¨r Chemie und Biochemie, Ludwig-Maximilians-Universita¨t,
Butenandtstraße 5–13, 81377 Mu¨nchen, Germany. E-mail: kluef@cup.
uni-muenchen.de; Fax: +89218077407; Tel: +89218077404
Fig. 1 The molecular structure of the cations of 1 in crystals of the
tetraphenylborat◦e’s dmso solvate (50% probability ellipsoids). Distances
˚
(A) and angles ( ): from Ru1 to: N4 1.750(4), O2 1.939(2), N2 1.977(3),
bI. Physikalisches Institut, Universita¨t zu Ko¨ln, Zu¨lpicherstraße 77,
50937 Ko¨ln, Germany. E-mail: th.woike@uni-koeln.de; Fax: +2214705162;
Tel: +2214706355
O1 2.039(2), N1 2.082(3), N3 2.088(3); O4–N4 1.157(4); O4–N4–Ru1
167.1(3), N4–Ru1–N2 99.2(1), N4–Ru1–O1 89.8(1), N4–Ru1–O2 172.3(1);
chelate torsion: O2–C2–C1–O1 -3.1(5), N2–C12–C13–N3 -2.0(5),
N1–C7–C8–N2 1.4(5).
† CCDC reference number 736561. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b912162e
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larger than a right angle. Second, the nitrosyl-O atom is tilted
another 10◦, in the mirror plane, towards the carboxylato ligand.
In agreement with its embedding in the “soft” tetraphenylborate
counterions and disordered dmso solvent molecules, these tilts are
not caused by packing forces. In fact, the molecular parameters of
the cation are well reproduced by means of a DFT calculation on
the cation in vacuum. Also in agreement with this finding is the
structure of the related [Ru(NO)(terpy)(OH)Cl]+ ion in crystals of
the solvent-free hexafluoridophosphate, which shows essentially
the same direction and amount of distortion.10
Fig. 3. In order to investigate the release of nitric oxide without
the disturbance by PLI, we investigated the NO release at room
temperature where the lifetimes of MS1 and MS2 were too short
to be observed. In Fig. 3 the reduction of the GS n(NO) band
area during illumination with 442.5 nm radiation at T = 294 K is
ꢀ
shown as a function of exposure Q = Idt (I = intensity). Both
NO-release kinetics at low and room temperature needed to be
described with a sum of two exponentials (Fig. 3). The values
given in Fig. 3 show that NO release is faster and more efficient at
higher temperature and higher illumination energy.
Both metastable states, MS1 and MS2, were generated in 1 by
illumination in the blue-green spectral range at T = 80 K. Illumina-
tion in the spectral range of 532–442 nm resulted in the generation
of MS1 with a maximal population of 9.5% at 488 nm. By
subsequent irradiation of MS1 with l = 1064 nm, about half of the
MS1 isomer was transferred to MS2 resulting in a MS2 population
of 4% (the other half returned to the GS). The identification of
MS1 and MS2 was performed by infrared spectroscopy in the
spectral range of the n(NO) stretching vibration. As can be seen
from Fig. 2, the area of the GS band at 1861 cm-1 decreased
upon illumination (488 nm) and a novel band at a lower energy
of 1732 cm-1 (MS1) appeared. On continuing the illumination
with 1064 nm, the MS1 band decreased and a second new band
at 1550 cm-1 (MS2) appeared. At the same time, the GS band
increased slightly. After a sufficiently long period of illumination
with 1064 nm radiation, the MS1 and MS2 bands disappeared
completely, while the GS band area was not completely restored
(36% missing). Since MS1 and MS2 were completely erased, the
missing area was attributed to nitric oxide release. The MS1 and
MS2 populations as well as the amount of NO release were
determined from the areas of the n(NO) vibrations. Since the MS1
population was saturated at 9.5% after the first irradiation step of
Q = 151 J cm-2, the further decrease of the GS band area during
illumination with 488 nm was due to NO release as illustrated in
Fig. 3 NO release on irradiation with 442.5 nm light at room temperature
(T = 294 K) and with 488 nm light at T = 80 K over the total exposure Q.
The NO population is given as the area of the n(NO) stretch. The observed
data were fitted by: NO population = y0 + A1exp(-Q/Q1) + A2exp(-Q/Q2)
with y0 being the offset at Q = •, the amplitudes A1,2 and the Q1,2 values
which corresponds to a curve’s decline to e-1. The low-temperature fit
results were: y0 = 57.3(9)%, A1 = 33.1(5), Q1 = 1700(100) J cm-1; A2 =
9.5(5), Q2 = 91(10) J cm-1. The room-temperature values were: y0 = 24(2)%,
A1 = 28(2), Q1 = 500(60) J cm-1; A2 = 48(2), Q2 = 28(2) J cm-1.
The spectral range for PLI and PR is identical for 1. The
charge-transfer transition Ru(4d) → p*(NO) seems to induce both
processes. Hence, in light of the excitation paths discussed in ref. 11
we have two possible scenarios: first, a one-step process in which
after the excitation to the p*(NO) the NO ligand relaxes either
into MS2 by rotation of roughly 90◦ or the NO ligand is released.
Second, a two-step process in which first MS2 is reached (rotation
of NO by about 90◦, ª 1 eV above GS) from where, with a second
excitation at the same wavelength, NO release occurs. The release
via occupation of the MS1 state is very improbable since, up to now,
the existence of MS1 at room temperature could not be verified
using short laser pulses for excitation.7 At low temperature PLI
competes with PR. The population and transfer characteristics of
PLI in 1 show that it behaves as in other PLI compounds and
can thus be explained within the above-mentioned generalised
potential scheme.11 PR at low temperature might thus occur as
discussed above or via MS1.
The characteristics of the bonding situation were evaluated in a
preliminary DFT study on the GS of the cation of 1, some linkage
isomers, and the trans-carboxylato isomer. The latter isomer
was not expected in light of the trans-to-NO-ligand specificity
formulated above. In fact, the trans-carboxylato isomer that bears
Fig. 2 IR spectra in the n(NO) range (T = 80 K). The MS1 isomer was
populated by irradiation at 488 nm, and subsequently transferred to about
equal parts of GS and MS2 by irradiation at 1064 nm.
9114 | Dalton Trans., 2009, 9113–9116
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Table 1 Computational results for the GS, three MS2, the MS1 and the carboxylate-alkoxide-converted trans-carboxylate isomer. In the C1-symm.
MS2, the NO ligand is about perpendicular to the complex ion’s mirror plane. The other states are depicted in Fig. 4. For the scaling factor applied to
calculated frequencies see ref. 21
GS
MS2min
MS2max
C1-symm. MS2
MS1
174
1.81
1807
trans-COO
E/kJ mol-1
E/eV
0.961 ¥ n(NO)calc
n(NO)exp
0
0
116
1.20
1578
1550
140
1.45
1579
1734
122
1.27
1587
14
0.15
1867
1863
1861
the less p-donating group trans to the p-accepting nitrosyl ligand
is less stable, but moderately so, on the chosen level of theory
(Table 1). The calculation, the results of which are collected
in Table 1, predicted energetic and vibrational parameters. For
the NO stretch, experimental and calculated values reasonably
match for the GS and MS2 states. However, the experimental
value for MS1 indicates a more pronounced softening of the NO
stretch than predicted by the calculation. The methodological
background of this generally observed but yet unmastered problem
has been addressed in recent theoretical work.12,13
latter are Ru- and NO-centered whereas the ammine-centered
orbitals are more stable and show no significant contributions
to nitrosyl–ruthenium bonding. The same was found for the Ru-
terpy interaction in this work. Specifically, Ru–NO bonding is
characterised by the same types of interaction among the various
stable and metastable isomers, namely an NO-to-Ru s donor bond
and two Ru-to-NO p back bonds. The bonding situation in the title
cation resembles this latter picture. Fig. 4 shows, using the example
of MO 92 which contributes to one of the two p back bonds,
the largely unchanged shape of this orbital throughout the four
minimum structures. (The depicted minima were reached along a
full rotation of the nitrosyl ligand in the cation’s apparent mirror
plane. In addition, two symmetrically equivalent MS2 isomers
6
In terms of bonding, the comparison of 1 and the {RuNO}
moiety in the related [Ru(NO)(NH3)5]2+ ion showed the influence
of the glycolato(2-) ligand.4,14–17 Thus, the frontier orbitals of the
Fig. 4 Ball-and-stick drawings: the four minimum structures on the hypersurface of the electronic ground state which are reached, from the left to the
right, by a clockwise rotation of the nitrosyl ligand in the mirror plane of the cation of 1. The relative total energies (left scale, GS set to zero) refer to
the bar at the respective isomer’s label. Molecular orbital drawings: the occupied MOs (no. 92) which correspond mainly to the glycolate(2-)-assisted
Ru(dyz)-to-NO(p*) back bond (y to the right, z up). An orbital’s energy may be roughly assessed from its position with respect to the right scale.
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were found with the nitrosyl ligand perpendicular to the mirror
plane.) The almost constant orbital shape is a common feature
of the title cation and the ammine-nitrosyl complex. A marked
difference, however, is the contribution of the dianionic ligand’s
atomic orbitals to the Ru–NO back bond and the other frontier
orbitals. A bonding interaction between the anion and the formal
NO+ species would, moreover, be a reason for the peculiar tilt of
the nitrosyl towards the glycolate(2-) ligand in most of the states.
In conclusion, both significant excitation phenomena of a
metal nitrosyl complex—photoinduced linkage isomerism and
photorelease of nitric oxide—were observed on irradiation of solid
samples of 1. Remarkably, at a lower temperature, photorelease
was observed in the same samples that showed PLI. The title
compound thus appears to be a useful tool to investigate the
connection of PLI and PR, which may be either consecutive or
competing or non-related phenomena. The molecular structure
of the nitrosyl complex, both experimental and calculated, points
towards the usability of a hypothesis that is presently emerging
on a Nonius Kappa CCD diffractometer with graphite-monochromatised
˚
Mo Ka radiation (l = 0.71073 A). The structure was solved by direct
methods (SHELXS-97) and refined by full-matrix least-square calculations
on F2 (SHELXL-97). Anisotropic displacement parameters were refined
for all non-hydrogen atoms with the exception of solvent molecules.
Crystallographic data of [Ru(NO)(terpy)L]BPh4·dmso: C43H39BN4O5RuS,
Mr = 835.72 g mol-1, crystal size: 0.51 ¥ 0.09 ¥ 0.08 mm, T = 200(2) K,
˚
orthorhombic, Pbca, a = 11.3863(2), b = 23.9172(5), c = 28.1252(5) A,
3
-3
-1
˚
V = 7659.3(2) A , Z = 8, r = 1.449 g cm , m = 0.516 mm , 56 805
observed reflections, 8770 reflections in refinement, 493 parameters, R(F) =
0.0515, Rw(F2) = 0.1263, S = 1.003, shift/errormax = 0.001, max. and min.
-3
˚
residual electron density: 0.874 and -0.931 e A .† The infrared spectra
were detected with a Nicolet 5700 FTIR spectrometer. The fine powder was
mixed with KBr and pressed to a pellet. The KBr pellet was mounted on a
copper cold finger using silver paste for good thermal contact. The sample
was cooled to 80 K in a liquid nitrogen cryostat. CsI windows allowed the
irradiation of the sample with laser light and absorption measurements
down to 260 cm-1. The irradiation was performed by the monochromatic
light of an argon laser at 457.9 nm, 476.5 nm, 488 nm, or 496.5 nm
or a HeCd laser at 442.5 nm for the population of the metastable state
MS1. The transfer from MS1 into MS2 was performed with light of a
Nd:YAG laser at 1064 nm after MS1 had been previously generated up
to saturation with blue-green light. The electronic and structural ground
state of the cation of the title compound as well as the MS1 and MS2
in their diamagnetic electronic state were modelled by means of DFT
calculations on the B3LYP/SDD/6-31G(d,p) level of theory using the
SDD core potential for ruthenium. All stationary points were confirmed
by frequency analyses.
6
from DFT calculations on related {RuNO} -type amino-acid
complexes.18 These calculations show, formulated in a strongly
simplified way, that the ligand trans to the Ru–NO axis contributes
to the properties of the Ru–NO bonds both in the ground state and
the excited states, whereas the ligands of the equatorial plane show
significant influence on the activation barriers along the excitation
and decay paths. From the sparse data presently available, the
assumption that p-donating equatorial ligands lower the activation
barrier for an NO motion, i.e., that they stabilise the transition
states even more efficiently than the minima, possibly both in the
course of a PLI and a PR event, may be formulated as an entry to
future work.
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Acknowledgements
Financial support by DFG (WO618/8-1) and BMBF (FKZ 03 ¥
5510) is gratefully acknowledged (D. S. and T. W). The authors
thank Martina Preiner for contributing to this work during her
research course.
Notes and references
‡ K2[Ru(NO)Cl5] and [Ru(NO)Cl2(terpy)]PF6 were prepared according to
literature procedures.19,20 [Ru(NO)Cl2(terpy)]PF6 (200 mg, 0.344 mmol)
was suspended in 0.1 M NaOH (12 mL, 1.2 mmol). Glycolic acid (88 mg,
1.136 mmol) was added and the suspension was heated to 80 ◦C for 2 h
under stirring. After cooling, NaBPh4 (118 mg, 0.344 mmol) was added and
the resulting orange precipitate was filtered off, washed with water (50 mL)
and the resulting solid was dried in vacuo (210 mg, 0.277 mmol, 80% yield).
ESI HR-MS (positive ion, M = C17H13N4O4Ru): m/z calcd: 438.9980,
12 B. Delley, Z. Kristallogr., 2008, 223, 329–333.
13 O. V. Sizova, O. O. Lubimova, V. V. Sizov and N. V. Ivanova,
Z. Kristallogr., 2008, 223, 343–355.
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Chim. Acta, 2000, 300–302, 698–708.
15 S. I. Gorelsky and A. B. P. Lever, Int. J. Quantum Chem., 2000, 80,
636–645.
16 P. Coppens, I. Novozhilova and A. Kovalevsky, Chem. Rev., 2002, 102,
861–884.
1
found: 438.9979. 13C{ H} NMR (100.53 MHz, d6-dmso]: d ppm (the
subscripts a and b refer to the glycolate’s carboxyl and hydroxyl-bearing
carbon atom, respectively) 181.6 (Ca), 164.1–162.6 (4C, Cipso), 158.8 (terpy),
153.8 (terpy), 151.3 (2C, terpy), 144.4 (terpy), 142.8 (2C, terpy), 135.5 (8C,
17 G. F. Caramori and G. Frenking, Organometallics, 2007, 26, 5815–
5825.
18 A. Zangl, P. Klu¨fers, D. Schaniel and T. Woike, Dalton Trans., 2009,
1034–1045.
C
ortho) 133.0 (terpy), 129.2 (2C, terpy), 126.6 (terpy), 126.4 (2C, terpy), 125.3
(8C, Cmeta), 125.1 (2C, terpy), 121.5 (4C, Cpara), 72.0 (Cb). UV/Vis (dmso):
lmax nm (e/L mol-1 cm-1): 346 (10 995). The NMR assignments were
confirmed by DFT calculations. Crystals suitable for X-ray crystallography
were obtained by dissolving the product in dmso and slowly evaporating
the solvent. A suitable crystal was selected with the aid of a polarising
microscope, mounted on the tip of a glass fiber and investigated at 200 K
19 J. R. Durig, W. A. McAllister, J. N. Willis and E. E. Mercer, Spectrochim.
Acta, 1966, 22, 1091–1100.
20 T. Hirano, K. Ueda, M. Mukaida, H. Nagao and T. Oi, J. Chem. Soc.,
Dalton Trans., 2001, 2341–2345.
21 I. N. Levine, Quantum chemistry, Prentice Hall, Upper Saddle River,
NJ, 5th edn, 2000.
9116 | Dalton Trans., 2009, 9113–9116
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©