Unambiguous characterization of a photoreactive ligand-loss intermediate

Article abstract of DOI:10.1002/anie.201304219

Hanging in there: Unprecedented quantitative formation of a long-lived intermediate 2 that contains a monodentate N N ligand has been observed and the species characterized. Photoreactive ligand loss from a ruthenium(II) tris(chelate) complex 1 leads to 2, which has a half-life of 14 hours. Bn=benzyl. Copyright

Full text of DOI:10.1002/anie.201304219

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DOI: 10.1002/anie.201304219
Photochemistry
Unambiguous Characterization of a Photoreactive Ligand-Loss
Intermediate**
Christine E. Welby, Craig R. Rice, and Paul I. P. Elliott*
The photophysics and photochemistry of the complex [Ru-
(bpy)3]2+ (bpy = 2,2’-bipyridyl) and its many N^N trischelate
analogues have been the subject of enormous interest over
the past four decades.[1] The interest stems primarily from
their attractive photophysical and electrochemical properties,
with potential applications in light harvesting, solar energy
conversion, and artificial photosynthesis. One drawback
however of [Ru(N^N)3]2+ type complexes can be photo-
chemical ligand loss or isomerization reactions.[2] Whilst
ordinarily an inconvenient side-reaction, such ligand loss
pathways have been exploited, for example, in the develop-
ment of photodynamic anticancer agents.[3] In these examples,
ligand loss is often sterically promoted through inclusion of
substituents adjacent to the N-donor atoms of the ligand that
is ejected.[4]
ligands to form a coordinatively unsaturated species of the
form [Ru(k2-N^N)2(k1-N^N)]2+, which is subsequently trap-
ped by a solvent molecule. Isomerism or dissociation of the
monodentate N^N ligand may then occur. More recently,
computational calculations by Alary et al. suggest that the
initially formed species as a result of 3MC state population in
[Ru(bpy)3]2+ and related complexes is the four-coordinate
[Ru(k2-bpy)(k1-bpy)2]2+ in which two ligands dechelate
[6]
À
through elongation of two mutually trans Ru N bonds.
Tachiyashiki and co-workers[7] reported HPLC and elec-
trospray mass spectrometry detection of the species [Ru-
(bpy)2(3,3’-dmbpy)(NCMe)]2+ (3,3’-dmbpy = 3,3’-dimethyl-
2,2’-bipyridyl). Signals for the methyl groups were also
observed by 1H NMR spectroscopy that are suggestive of
the formation of this intermediate in which the 3,3’-dmbpy
ligand is coordinated in a monodentate fashion. Here,
dechelation is presumably facilitated by the steric repulsion
between the methyl groups of the dmpby ligand. The
relatively long lifetime of the intermediate species then
presumably results from inhibition of rechelation for the same
reason. To the best of our knowledge, there have been no
other reports of the directly observed intermediates for
photochemical ligand loss from [Ru(bpy)3]2+ analogue com-
plexes.
The dominant features in the visible region of their optical
absorption spectra are the characteristic metal-to-ligand
charge-transfer (MLCT) bands. Absorption at wavelengths
in these bands involves excitation of a metal d-orbital-
centered electron to vacant p* orbitals on the N^N chelate
ligands. Rapid intersystem crossing (ISC) then converts these
1
3
initially formed MLCT states into MLCT states. It is these
latter triplet states that are primarily responsible for phos-
phorescent emission exhibited by these complexes. In them-
3
selves, these MLCT states are inert toward ligand loss and
Herein we present the first unambiguous observation and
characterization by NMR spectroscopy of a metastable
ligand-loss intermediate with a monodentate N^N ligand
for a non-sterically promoted [Ru(N^N)3]2+ type complex.
Furthermore, this intermediate is observed to form quantita-
tively, reverting to the starting material with a long lifetime.
Additionally we report a novel concomitant rearrangement in
which the two remaining bidentate ligands adopt a square-
coplanar geometry.
isomerization reactions. Instead, it is higher-lying metal-
centered (MC) states, characteristic of population of the
metal–ligand antibonding ds* orbitals, that are associated for
ligand dechelation and ligand dissociation pathways and
quenching of luminescent emission. Population of 3MC states
can be effected by absorption of high-energy light in the UV
or two-photon absorption.[5] If close enough in energy,
3
however, MC states can undergo efficient thermal popula-
3
tion from MLCT states.
We have previously reported[8] the synthesis and photo-
Population of 3MC states in [Ru(N^N)3]2+ type complexes
was long thought to result in dechelation of one of the N^N
physical investigation of the series of complexes [Ru(bpy)3Àn-
(btz)n]2+ (n = 0 to 3, btz = 1,1’-dibenzyl-4,4’-bi-1,2,3-triazolyl).
Sequential replacement of bpy by btz results in destabilization
of the bpy p*-based LUMO in these complexes and a resultant
blue-shift in the 1MLCT bands. We reasoned that this
destabilization might promote photochemical reactivity
[*] Dr. C. E. Welby, Prof. C. R. Rice, Dr. P. I. P. Elliott
Department of Chemical & Biological Sciences
University of Huddersfield
3
Queensgate, Huddersfield, HD1 3DH (UK)
E-mail: p.i.elliott@hud.ac.uk
Homepage: http://www.hud.ac.uk
through enhanced MC state population. Indeed, we report
herein the unprecedented observation and characterization of
the quantitatively formed ligand-loss intermediate trans-[Ru-
(k2-bpy)(k2-btz)(k1-btz)(NCMe)]2+ (2) from [Ru(bpy)-
(btz)2]2+ (1) and its subsequent conversion to trans-[Ru(bpy)-
(btz)(NCMe)2]2+ (3; Scheme 1).
Upon examination of NMR spectroscopy samples of 1 as
the hexafluorophosphate salt in [D3]acetonitrile that had
been left in the laboratory in ambient daylight, numerous new
signals were observed. On closer repeat examination of fresh
[**] The authors thank the Leverhulme Trust for funding this research.
As a member of the UK HPC Materials Chemistry Consortium,
P.I.P.E. acknowledges the EPSRC (grant no EP/F067496) and the
UK’s national supercomputing service HECToR as well as the
Huddersfield Centre for High Performance Computing for compu-
tational resources utilized in this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304219.
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ligand mandate a coplanar
arrangement of these two
ligands with presumably
two acetonitrile ligands
occupying the remaining
trans coordination sites.
Samples of 3 in CH3CN
were examined by electro-
spray mass spectrometry.
Ions with m/z 328.1, 307.6,
and 287.1 were observed
corresponding to the dicat-
ions
3,
[Ru(bpy)(btz)-
(NCMe)]2+, and [Ru(bpy)-
(btz)]2+, respectively, con-
firming the formation of
a species containing two sol-
vent ligands. Each signal
Scheme 1. Photochemical conversion of complex 1 into 3 with ejection of btz via monodentate btz-containing
intermediate 2.
exhibited the expected isotope pattern for a mononuclear
ruthenium complex.
After prolonged illumination of concentrated solutions of
1 in acetonitrile, free btz was observed to precipitate. The
liquor from these samples was decanted, the solvent removed
and redissolved in CH3CN. Crystals of X-ray diffraction
quality were then obtained by slow vapor diffusion of
diisopropylether. The molecular structure of the cation 3 is
depicted in Figure 2 (see the Experimental Section and
Figure 1. Representative 1H NMR spectra following the photochemical
conversion of 1 into 3 via intermediate 2 in [D3]acetonitrile at room
&
^
*
temperature in ambient daylight ( 2, 3, free btz).
samples of 1, similarly left in daylight, clean conversion of
1 into a new complex 3 along with one equivalent of free btz is
observed to occur. The process was observed to proceed
through the intermediate species 2. Representative spectra
for this conversion are presented in Figure 1. Samples in
which 1 had converted into 2 and which were subsequently
left in the dark were observed to revert to the starting
material. Samples that had been left to undergo full con-
version into 3, a process that therefore appears to require
a second photon, were observed to be unchanged after similar
storage in the dark. This indicates that re-coordination of
ejected btz does not occur or is at least very slow.
Figure 2. ORTEP of the structure of the cation [Ru(bpy)(btz)-
(MeCN)2]2+ (hydrogen atoms and counterions removed for clarity,
ellipsoids set at 50% probability). Selected bond lengths [ꢀ] and
angles [8]: Ru–N1 2.051(1), Ru–N2 2.053(1), Ru–N3 2.078(1), Ru–N6
2.097(1), Ru–N9 2.018(2), Ru–N10 2.021(2); N1-Ru-N2 79.04(6), N3-
Ru-N6 76.67(5), N1-Ru-N6 177.38(6), N2-Ru-N3 179.51(6), N9-Ru-N1)
178.45(6)).
Along with the signals for free btz, the 1H NMR spectrum
of a fully converted sample containing 3 exhibits four
resonances for the bpy ligand, which indicates that it retains
the magnetic equivalence of the two pyridyl rings. The
remaining coordinated btz ligand gives rise to a singlet
resonance for the two triazole ring protons at d 8.50 and
a further singlet for the benzyl substituent methylene protons
at d 5.91.
Supporting Information). The complex adopts a distorted
octahedral geometry with two trans acetonitrile ligands and
coplanar bpy and btz ligands, in agreement with NMR data.
For complexes of the type [Ru(bpy)2(N^N)]2+, the
resultant products from photochemical N^N ligand ejection
have the form cis-[Ru(bpy)2(solvent)2]2+ rather than the trans
geometry as observed here. Density functional calculations
were performed to determine optimized geometries for the
cis and trans isomers of complexes 3 and [Ru(bpy)2-
(NCMe)2]2+ (Supporting Information). Cis-[Ru(bpy)2-
The observed magnetic equivalence of the two triazole
rings combined with the symmetrical environment for the bpy
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(NCMe)2]2+ is calculated to be some 40 kJmolÀ1 more stable
than its trans isomer, whereas trans-3 is 7.76 kJmolÀ1 more
stable than its corresponding cis isomer. In the case of
[Ru(bpy)2(NCMe)2]2+, the 6/6’-position CH groups result in
a severe steric clash between the two bpy ligands, giving rise
to a significant deviation from planarity of the two ligands
observed crystallographically[9] and is reproduced in our
calculations. The absence of H-substituents adjacent to the
coordinated N-atoms of the btz ligand in 3 means that similar
steric congestion does not occur, allowing for the observed
coplanarity and formation of the trans isomer.
The definitive stereochemical characterization of the
product 3 enabled straightforward elucidation of the struc-
tural nature of the intermediate 2. 1H NMR spectra of
samples in which 1 had converted into 2 again show four
resonances for the bpy ligand and a singlet resonance for the
two triazole ring protons of the k2-btz
stereochemistry of the intermediate. The rearrangement of
the bidentate ligands to give a coplanar arrangement will
inhibit the re-coordination of the pendant triazole of the k1-
btz ligand. Furthermore, the 1H NMR resonances for the
phenyl protons of the btz ligand in complexes such as 1 are
generally ill-resolved and overlapping. 1H NMR spectra for 2,
however, show clearly resolved and well-separated signals for
the ortho- and meta-positions for one of the k1-btz phenyl
rings. This could be indicative of some intramolecular p-
stacking interaction that may further stabilize 2 to conversion
back into 1 and indeed ultimately to subsequent ligand loss to
yield 3. No further evidence from nOe data could be obtained
to corroborate this however.
We propose a tentative mechanism for the formation of 2
upon photoexcitation of 1 (Scheme 2). Visible excitation of
1 yields a 1MLCT state, which rapidly undergoes ISC to yield
ligand. Rather than appearing as
a singlet, the methylene protons of
this k2-btz ligand give rise to a pair of
roofed geminal doublets centered at
d 5.84. The data confirm the copla-
narity of the bpy and k2-btz ligands as
exhibited by 3, but show that the two
remaining mutually trans ligands are
different from each other. Further
signals for a second btz ligand in
which the two triazole rings are
magnetically inequivalent are also
Scheme 2. Proposed mechanism for the formation of intermediate 2 after photoexcitation of 1.
observed; two singlet resonances
are observed d 7.03 and 7.58 for the
triazole ring protons along with two
further singlet resonances for the methylene protons at d 5.06
and 5.40. Owing to the stereochemical arrangement of the two
bidentate ligands, this third ligand must therefore be coordi-
nated in a monodentate fashion with the sixth coordination
site trans to it occupied by a solvent molecule. We therefore
assign the structure of 2 as trans-[Ru(bpy)(k2-btz)(k1-btz)-
(NCMe)]2+. This is further supported by electrospray mass
spectrometry data on partially converted samples of 1 in
CH3CN, which show signals along with that for 1 for an ion
with m/z 445.1, in agreement with the proposed structure of 2.
It was found that the photochemical conversion process
could be dramatically accelerated if conducted by placing the
NMR sample between the tubes of a domestic 23 W 1450
lumen fluorescent bulb. Cooling through the use of an electric
fan allowed maintenance of the sample temperature at 408C.
In this simple experimental set-up and with periodic NMR
spectroscopic interrogation, near-complete conversion of
1 into 2 occurs in 30 min, with subsequent formation of 3
occurring over a period of 2 days. Samples in which 1 had
converted into 2 were transferred and left in the bore of the
spectrometer in the dark at 408C and were monitored every
20 min. Complex 2 is observed to revert to 1 in a first-order
process with an approximate half-life of 13.75 h. Whilst
monitoring these backward reactions, concentrations of 3
were observed to remain constant.
the corresponding 3MLCT state. Owing to the destabilization
of this 3 MLCT state relative to the 3MC state when compared
to those states of [Ru(bpy)3]2+, efficient thermal interconver-
sion to the 3MC state occurs. This results in the formation of
the bis(dechelation) photoproduct [Ru(bpy)(k1-btz)2]2+
(A),[6,8] which is trapped by a solvent molecule to form the
species [Ru(bpy)(k1-btz)2(NCMe)]2+ (B). The incoming sol-
vent ligand deflects the adjacent pendant triazole ring of btz
ligand which then re-coordinates to establish the coplanar
bpy/btz arrangement. In the process, the second btz ligand is
induced to migrate to the coordination site trans to the new
solvent ligand, thereby forming 2.
In summary, we have reported the unprecedented quan-
titative formation and unambiguous characterization of
a long-lived photochemical ligand-loss intermediate contain-
ing a monodentate N^N ligand. Further, the process involves
a novel concomitant rearrangement of the other two chelate
ligands such that they become coplanar. These results there-
fore point the way to numerous possible applications of this
new class of complex. These include the possibility of utilizing
complexes of this type for the photogeneration of supra-
molecular synthons, as components in light-activated molec-
ular machines and switches, and as potential new photo-
dynamic anticancer drugs.
This extremely long life-time of the intermediate 2 with
respect to reversion to 1 is partially explained by the
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Experimental Section
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¯
Mr = 945.67, triclinic P1, a = 9.2947(4), b = 11.8646(5), c =
17.2001(7) ꢀ, a = 96.455(1), b = 99.541(1), g = 93.188(1)8, V=
1853.35(13) ꢀ3, T= 150(2) K, Z = 2, Rint = 0.0315 Final R1 value =
0.0329 Final wR(F2) value = 0.0734, Final R1 value (all data) =
0.0429, Final wR(F2) value (all data) = 0.0782, Goodness of fit =
1.028, largest peak and hole 0.650/À0.616 (eꢀÀ3).
CCDC 933143 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Received: May 16, 2013
Published online: August 22, 2013
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Keywords: monodentate ligands · photochemistry ·
reactive intermediates · ruthenium · triazoles
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