Simultaneous occurrence of three different valence tautomers in meso -vinylruthenium-modified zinc porphyrin radical cations

Article abstract of DOI:10.1021/ja400673c

The mixed-valent radical cation of a styrylruthenium-modified meso-tetraarylzinc porphyrin forms a mixture of three different valence tautomers (VTs) in CH₂Cl₂ or 1,2-C₂H ₄Cl₂ solutions. One of these

Full text of DOI:10.1021/ja400673c

Communication
pubs.acs.org/JACS
Simultaneous Occurrence of Three Different Valence Tautomers in
meso-Vinylruthenium-Modified Zinc Porphyrin Radical Cations
Jing Chen,† Evelyn Wuttke,† Walther Polit,† Thomas Exner,‡ and Rainer F. Winter*,†
†Fachbereich Chemie, Universitat Konstanz, Universitatsstraße 10, D-78457 Konstanz, Germany
̈
̈
‡Pharmazeutisches Institut, Eberhard-Karls Universitat Tubingen, Auf der Morgenstelle 8, D-72076 Tubingen, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: The mixed-valent radical cation of a
styrylruthenium-modified meso-tetraarylzinc porphyrin
forms a mixture of three different valence tautomers
(VTs) in CH2Cl2 or 1,2-C2H4Cl2 solutions. One of these
VTs has the charge and spin delocalized over the
porphyrin and the styrylruthenium moieties, while the
other two display charge and spin localization on just one
of the different redox sites. The relative amounts of the
three different VTs were determined by EPR and IR
spectroscopies at variable temperatures, while delocaliza-
tion in the ground state was confirmed by DFT
calculations.
Figure 1. Potential hypersurface for an asymmetric MV system with
unequal redox sites of similar redox potentials.
alence tautomers (VTs) are isomers that differ with
Vrespect to the electron distribution and redox-state
assignment between two different redox-active entities of the
same molecule.1,2 They constitute bistable systems whose
optical or magnetic properties can be controlled by exterior
physical stimuli such as temperature, pressure, light, applied
magnetic field, or changes in their environment.2,3 This makes
them promising candidates for switchable materials with
possible use in information storage. The vast majority of VT
systems derive from the combination of a redox-active metal
ion and a redox-active, “non-innocent” ligand,4,5 which are
oxidized or reduced at similar potentials. A particularly rich
chemistry has evolved around the family of catecholate,
semiquinonate, and quinone redox systems, where many of
the conceivable permutations arising from the isoelectric
replacement of oxolate (RO−) by thiolate (RS−) or amide
(R2N−) donors have been realized.1b,6−9 Other examples are
transition metal complexes of porphyrins, some with relevance
to the active sites of metalloenzymes.10 Less common are VTs
that are based on mixed-valent (MV) systems with non-
identical redox sites of similar intrinsic redox potentials. Here,
the necessary condition of a double minimum ground-state
hypersurface with a small free-energy gap ΔG0 between the
individual minima and an energy barrier of sufficient height to
allow for the coexistence of two isomeric forms requires a
delicate balance of the electronic coupling parameter HAB and
the free-energy difference ΔG0, which is only rarely achieved.11
Increasing HAB aids in delocalizing the charge over the available
redox sites while at the same time diminishing the energy gap
between the VTs from its thermodynamic value ΔG0 in the
diabatic case (HAB = 0) to ΔG1 (Figure 1).11 The few reported
examples include pyrazine-bridged triruthenium clusters with
either two slightly different triruthenium subunits or an
asymmetric bridge,11,12 a ferrocenylvinyl-tris(chlorophenyl)-
methyl radical,13 ferrocenyl- or triarylamine-substituted dipro-
tonated anthraquinones or pyrylium salts,14 and biferrocenyl-
bridged bis(ethynyl)diiron, iron/ruthenium, and iron/osmium
complexes.15
Vinyl-substituted ruthenium complexes constitute redox-
active metal−organic π-systems whose HOMO receives major
contributions from the alkenyl ligand.16 Oxidized styrylruthe-
nium complexes featuring another conjugated organic or
metal−organic redox-system with similar redox potential
usually adopt a delocalized electronic structure.17 We now
observed that the one-electron-oxidized meso-tetraarylzinc
porphyrin styrylruthenium conjugates 1 and 2 of Chart 1
form an equilibrated mixture of three VTs where the positive
charge is either localized on the zinc porphyrin (Zn-TPP•+-
StyRu) or the styrylruthenium site (Zn-TPP-StyRu•+) or
delocalized over both ([Zn-TPP-StyRu]•+) (Zn-TPP =
5,10,15-tris(3,5-ditbutylphenyl) zinc porphyrin, StyRu =
−C6H4−CHCH−Ru(X)(CO)(PiPr3)2 (X = Cl or
OOCC6H4CN-4-κ2O).
The monoethynylated zinc tetraphenylporphyrin (Zn-TPP,
L) was synthesized according to known literature procedures.18
Complex 1 was obtained in 65% yield by the regio- and
stereoselective insertion of the ethynyl function of L into the
metal−hydride bond of RuClH(CO)(PiPr3)2. NMR spectros-
copy shows the presence of a trans-disubstituted vinyl group as
i
well as the signals of six equivalent Pr groups in a 1:1 integral
ratio with the signals of the Zn-TPP, the characteristic 13C
Received: January 21, 2013
Published: February 20, 2013
© 2013 American Chemical Society
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L•+ displays a sharp isotropic EPR signal at g = 1.999 in fluid
CH2Cl2 solution or at g = 2.000 in frozen solutions or as a solid
(SI Figure 4), which is characteristic of porphyrin-based radical
cations.19 No 14N or 1H hyperfine splittings were resolved. The
same kind of signal (g = 2.002) was also observed for solid 1•+,
classifying it as an entirely Zn-TPP centered paramagnetic
species under these conditions (see SI Figure 5).
Chart 1. Structures of Zn-TPP (L) and Vinylruthenium-
Modified Zn-TPP Complexes 1 and 2
The situation changes dramatically, however, in CH2Cl2 or
1,2-C2H4Cl2 solution, where separate signals coexist in a T-
dependent equilibrium (Figure 3). The sharp isotropic signal at
signals of the vinyl group and the Ru(CO) ligand, and a sharp
singlet in the 31P NMR spectrum (see the Supporting
Information (SI)). Treatment of complex 1 with 1 equiv of
sodium 4-cyanobenzoate produced complex 2 in a yield of 85%
(see SI).
In the NBu4PF6/CH2Cl2 (0.1 M) electrolyte, L undergoes
two reversible one-electron oxidations at half-wave potentials of
0.351 and 0.678 V and one reversible reduction at −1.866 V
(Figure 2a). Considering the innocent character of the Zn2+
Figure 3. EPR spectra of radical cation 1•+ in 1,2-C2H4Cl2 at various
temperatures. Left: overlay of experimental (black lines) and simulated
spectra (red lines). Right: subspectrum of the [Zn-TPP-StyRu]•+ VT
after subtraction of the contributions of the other VTs.
g = 2.001 is readily assigned as belonging to the Zn-TPP•+-
StyRu VT. The second signal, a resolved triplet at g = 2.029
with A(31P) = 15.6 G, shows the typical signature of a
styrylruthenium-type radical cation, indicating that the second
species is the Zn-TPP-StyRu•+ isomer. As T is lowered, the
relative intensity of the triplet signal increases with respect to
the singlet for Zn-TPP•+-StyRu.
In keeping with findings on other oxidized aryl-substituted
vinylruthenium complexes,16 the triplet signal of Zn-TPP-
StyRu•+ collapses into a broad isotropic signal with no resolved
hyperfine splitting on cooling below −40 °C or on freezing.
Spectral changes with temperature are fully reversible as long as
the temperature is maintained below 50 °C, where the sample
gradually decomposes. While the radical cation of benzoate
complex 2 behaves in an overall similar manner to 1•+, there are
three differences: (i) at comparable temperature, the
contribution of the Zn-TPP-StyRu•+ VT is predictably higher
than for 1•+ (cf. the lowering of the half-wave potential of the
StyRu-based redox couple as the 16 valence electron (VE)
count in 1 changes to 18 VE in 2); (ii) even at higher T in
solution, the signal of the Zn-TPP-StyRu•+ VT does not exhibit
resolved hyperfine splitting; and (iii) signals of both VTs are
already present in solid samples of 2•+ (SI Figures 6 and 7).
The equilibrium indicated by EPR spectroscopy also leaves
its marks in IR spectroscopy. Spectra of radical cations 1•+ and
2•+ in CH2Cl2 or CH2Cl2/NBu4PF6 display two separate
Ru(CO) bands located at 1918 and 1964 cm−1 for 1•+ or at
1912 and 1969 cm−1 for 2•+ (Figure 4a and SI Figure 8). The
band at the lower energy is only slightly blue-shifted with
Figure 2. Cyclic voltammograms (CH2Cl2, 0.1 M NBu4PF6, rt) of Zn-
TPP L (a) and of complexes 1 (b) and 2 (c).
cation, all redox processes involve the porphyrin π-system.
Under the same conditions, complexes 1 and 2 show one
additional redox couple close to the first oxidation of the zinc
porphyrin, which is obviously due to the styrylruthenium
moiety (Figure 2b,c). Redox potentials are 0.248, 0.374, 0.740,
and −1.966 V (peak potential of irreversible process) for 1 and
0.145, 0.329, 0.714, and −1.897 V for 2. Considering the
potentials of the first anodic process of L and those of the
styrylruthenium complexes RuCl(CHCHPh)(CO)(PiPr3)2
(RuST, 0.270 V) and Ru(CHCHPh)(OOCC6H4CN-4-κ2O)-
(CO)(PiPr3)2 (0.182 V; SI Figure 3), the intrinsic energy gap
between the two VTs is estimated as −0.081 and −0.169 V,
corresponding to −7.8 or −16.3 kJ/mol in favor of the Zn-
TPP-StyRu•+ form.
respect to that in the neutral complexes 1 (ν
̃
(CO) = 1912
The radical cations L•+, 1•+, and 2•+ were generated from
cm−1) and 2 (ν(CO) = 1906 cm−1) and is in the regime
̃
−
their neutral precursors by oxidation with the SbF6 salt of the
expected of a vinylruthenium complex undergoing electron
transfer from an appended oxidizable unit with little
acetylferrocenium ion as a stoichiometric one-electron oxidant.
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Communication
VT with the unpaired spin and the positive charge (hole)
shared between the Zn-TPP and the StyRu moieties.
The T-dependent equilibrium between the three VTs of 1•+
was investigated by full digital simulation of the EPR spectra
utilizing the EPR parameters extracted for L•+ and RuST
•+
(Figure 3 and SI Figures 14 and 15) as the initial input
parameters. Difference spectra obtained by subtracting
appropriately weighed subspectra of the Zn-TPP•+-StyRu and
the Zn-TPP-StyRu•+ VTs with optimized A(31P), A(99/101Ru),
and g-values result in a broadened isotropic peak at g = 2.007
for [Zn-TPP-StyRu]•+. Figure 3 and SI Figure 14 show that the
added subspectra fit the experimental ones very well (for
simulation parameters see the SI). Relative amounts of the
three species as calculated from the integration of the
appropriately weighed subspectra reveal that the relative
proportion of the delocalized VT increases at the expense of
those of the two more localized ones as T is lowered (SI Table
3). The same trend is observed from T-dependent solution IR
spectra at selected temperatures (see Figure 4 and SI Figures 16
and 17 and Table 4). We point out that the integrals for the
deconvoluted IR bands do not necessarily match the relative
concentrations as the oscillator strengths of the individual
species may differ from each other. Direct comparison with the
EPR results is thus not possible. Of note is a large broadening
of the Ru(CO) band of [Zn-TPP-StyRu]•+ on cooling.
Paralleling the increase of the IR band assigned to the [Zn-
TPP-StyRu]•+ form, the broad electronic NIR transition also
intensifies (SI Figure 16). This further supports the notion that
both kinds of bands belong to the same species.
We assume that the extent of charge and spin (de)-
localization in 1•+ and 2•+ is triggered by conformational
changes, i.e., rotation of the meso-styrylruthenium substituent
with respect to the porphyrin or of the vinylruthenium moiety
with respect to the styryl plane. IR spectra on samples of solid
1•+, i.e., under conditions where rotation is blocked, only
showed the Ru(CO) band of Zn-TPP•+-StyRu from 0 to 50 °C
(SI Figure 18). Quantum chemical calculations were under-
taken in order to investigate this point further. Structure
optimization of neutral Zn-TPP-StyRu resulted in an
interplanar angle of 63.2° between the meso-styryl and the
porphyrin planes (SI Figure 19). This is at the low end of
values of similar meso-tetraarylporphyrins where interplanar
angles are usually in the range of 65−88°. Even with this
relative large deviation from coplanarity, the HOMO of the
neutral complex and the unpaired spin density of its associated
radical cation are fully delocalized over the porphyrin and the
styrylruthenium moieties (SI Figures 20 and 21). Changes in
the frontier orbital compositions of neutral 1 and spin density
distributions in 1•+ as a function of the torsion around the
porphyrin Cmeso−styryl Cipso and the styryl−vinyl bonds were
mapped for torsional angles varying in 30° steps between 0 and
360°. The softness of these degrees of freedom shown by our
calculations demonstrates the accessibility of many different
structures at finite temperature. No additional local minima
could be identified with these rough scans. We note, however,
that the unpaired spin becomes localized on the porphyrin ring
if any of the two torsion angles adopts a value of 90° (SI
Figures 22 and 23). Reoptimization of these structures resulted
in exactly the same delocalized structure as the first
optimization. Due to limitations in the theoretical description
of the system (approximations in the DFT functionals, no
explicit solvents, neglect of finite temperature effects), no
Figure 4. IR spectra of 1•+ at various temperatures (solid black line)
and deconvolution (green, purple and blue lines for individual peaks,
red line for their sum).
conjugation to the {Ru(CHCHR)(CO)} moiety.20 It is thus
assigned to the Zn-TPP•+-StyRu VT. The band at the higher
energy falls close to that of other donor-substituted
styrylruthenium radical cations such as [RuCl(CH
̃
CHC6H4-OMe-4)(CO)(PiPr3)2]•+ (ν(CO) = 1966 cm−1) and
hence is ascribed to Zn-TPP-StyRu•+. For comparison, the
Ru(CO) band of both oxidized RuCl(CHCHPh)(CO)-
(PiPr3)2 and [Ru(CHCHPh)-(OOCC6H4CN-4-κ2O)(CO)-
(PiPr3)2]•+ appears at 1976 cm−1 (SI Figure 10). Further
oxidation to the dications induces only a minor additional blue
shift of the Ru(CO) band of the Zn-TPP-StyRu•+ form to
1972 cm−1 for 12+ and 22+ (SI Figure 8 and Table 1 for a more
comprehensive list of vibrations). In further agreement with the
EPR data, the relative proportions of the Zn-TPP-StyRu•+
isomer are larger for 2•+ (SI Figure 8). While solid samples of
radical cations 1•+ and 2•+ exhibit an IR band at 1267 and 1276
cm−1, respectively, the intensity of this band is notably lower for
2•+ (SI Figures 11 and 12). Such a band has been established as
diagnostic of a porphyrin-based radical cation.21
A closer look at the solution EPR spectra of 1•+ recorded
between −110 to +50 °C reveals the presence of a third signal
at a g value close to that of the Zn-TPP•+-StyRu VT (Figure 3
and SI Figure 14). The only slight displacement from ge and the
broadening of the signal point to a dominant organic character
of the underlying species and an increased metal contribution
to the SOMO with respect to Zn-TPP•+-StyRu. Spectral
deconvolution clearly indicates the presence of a third Ru(CO)
band at 1932 or 1946 cm−1 for solutions of 1•+ or 2•+,
respectively, in the region between the two resolved Ru(CO)
bands assigned to Zn-TPP-StyRu•+ and Zn-TPP•+-StyRu (see
Figure 4b−d). We also observe extremely broad electronic
bands in the near-infrared (NIR) that protrude well into the
mid-IR with peaks at ca. 6000 and 4900 cm−1 for 1•+ or 5850
and 4150 cm−1 for 2•+ that are absent in the spectra of solid
1•+, i.e., under conditions where only the Zn-TPP•+-StyRu
form is present (SI Figure 13). Ru(CO) bands of such energy
and low-energy electronic transitions of that kind are typical of
paramagnetic, MV styrylruthenium complexes where the charge
is delocalized onto a secondary redox-active, π-conjugated
entity such as [StyRu-NAn2]•+ (An = anisyl, C6H4OMe-4).17b
We therefore assign this third species as the [Zn-TPP-StyRu]•+
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̈
In conclusion, radical cations 1•+ and 2•+, in CH2Cl2
solution, form a mixture of three thermally equilibrating,
interconverting valence tautomeric species, two of which, Zn-
TPP•+-StyRu and Zn-TPP-StyRu•+, have their charge and spin
localized on one of the two different, yet nearly degenerate,
redox sites while the third one displays extensive charge and
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ASSOCIATED CONTENT
* Supporting Information
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■
S
(18) Zeitouny, J.; Aurisicchio, C.; Bonifazi, D.; De Zorzi, R.;
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Full experimental details and characterization data for 1−3, and
results of quantum chemical calculations on 1, including
supplemental Figures 1−23 and Tables 1−4. This material is
available free of charge via the Internet at http://pubs.acs.org.
(20) Pevny, F.; Winter, R. F.; Sarkar, B.; Zal
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AUTHOR INFORMATION
■
(21) Shimomura, E. T.; Phillippi, M. A.; Goff, H. M.; Scholz, W. F.;
Reed, C. A. J. Am. Chem. Soc. 1981, 103, 6778.
Corresponding Author
rainer.winter@uni-konstanz.de
(22) Hoekstra, R. M.; Telo, J. O. P.; Wu, Q.; Stephenson, R. M.;
Nelsen, S. F.; Zink, J. I. J. Am. Chem. Soc. 2010, 132, 8825.
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Yufit, D. S.; Howard, J. A. K.; Manca, G.; Halet, J.-F.; Hartl, F.; Low, P.
J. J. Am. Chem. Soc. 2011, 133, 18433. (b) Low, P. J. Coord. Chem. Rev.
2012, DOI: 10.1016/j.ccr.2012.08.008. (c) Costuas, K.; Cardor, O.; Le
Stang, S.; Paul, F.; Monari, A.; Evangelisti, S.; Toupet, L.; Lapinte, P.;
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support of this
work by the Deutsche Forschungsgemeinschaft (Wi1262/9-1).
We also thank Malte Drescher and Martin Spitzbarth for
helpful discussions and kind assistance with the simulation of
the EPR spectra and Karin Hauser and Benjamin Heck for their
assistance with the recording of variable-T IR spectra on solid
1•+.
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