Molecular, electronic structure and spectroscopic properties of MM quadruply bonded units supported by trans-6-carboethoxy-2-carboxylatoazulene ligands

Article abstract of DOI:10.1039/b919282d

The reaction between M₂(TiPB)₄ (M = Mo, W) where TiPB = 2,4,6-triisopropylbenzoate and 6-carboethoxy-2-azulenecarboxylic acid (2 equiv.) in toluene leads to the formation of complexes M₂(TiPB) ₂(6-carboethoxy-2-

Full text of DOI:10.1039/b919282d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Molecular, electronic structure and spectroscopic properties of MM
quadruply bonded units supported by trans-6-carboethoxy-2-
carboxylatoazulene ligands†
Brian G. Alberding,a Mikhail V. Barybin,*b Malcolm H. Chisholm,*a Terry L. Gustafson,*a Carly R. Reed,a
Randall E. Robinson,b Nathan J. Patmore,a Namrata Singha and Claudia Turro*a
Received 16th September 2009, Accepted 24th November 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b919282d
The reaction between M2(TiPB)4 (M = Mo, W) where TiPB = 2,4,6-triisopropylbenzoate and
6-carboethoxy-2-azulenecarboxylic acid (2 equiv.) in toluene leads to the formation of
complexes M2(TiPB)2(6-carboethoxy-2-azulenecarboxylate)2. Compound I (M = Mo) is blue and
compound II (M = W) is green. Both are air sensitive, hydrocarbon soluble species that gave the
corresponding molecular ions in their mass spectra (MALDI-TOF). They show metal based oxidations
and ligand based reductions. Electronic structure calculations (DFT and time dependent DFT) indicate
that the two azulene carboxylate p systems are coupled by their interactions with the M2 d orbitals.
Their intense colors arise from M2 d to azulene p* electronic transitions. While compound I exhibits
weak emission at ~ 900 nm, no emission has been detected for II. Both I and II have been studied by fs
and ns transient absorption spectroscopy. The X-ray analysis of the molecular structure of II in the solid
state confirmed the paddlewheel nature of its W2(O2C)4 core and the trans orientation of the ligands.
energies and this interaction is stronger for M = W than for M =
Mo. Single electron oxidation of such linked dinuclear compounds
leads to metal based mixed valence species that have interesting
spectroscopic properties and can traverse the class II/III border.4,5
We recognized that the M2 center might also accommodate mixed
valence ligands in a trans oriented molecule upon reduction. Here
the key orbital interaction is shown in B below where the out-of-
phase p* ligand-based orbital interacts with the M2 d orbital.
Introduction
We have for some time been studying the properties of linked MM
quadruply bonded complexes (M = Mo or W) where the linker is
a dicarboxylate or a related unit capable of electronically coupling
the two M2 units by way of M2 d to bridge p conjugation.1-3 The
key orbital interactions are shown in A below. The out-of-phase
combination of the M2 d orbitals finds a symmetry match with
filled p orbitals involving the CO2 fragments and the in-phase
combination of the M2 d orbitals interacts with the p* orbital
involving the CO2 junctions.
The two mixed valence systems described by frameworks A
and B are complementary: oxidation in A and reduction in
B generate metal centered and ligand centered mixed valence
states, respectively. Based on our earlier studies of azulene-
2,6-dicarboxylate bridged compounds,6 we decided to use the
6-carboethoxy-2-azulene carboxylate ligand in this work. A pre-
liminary communication of some aspect of this work has been
published.7
Of these two orbital interactions, the back bonding, M2 (d) to
CO2 (p*) interaction is more important based on relative orbital
Results and discussion
Syntheses
aDepartment of Chemistry, The Ohio State University, 100 W. 18th Avenue,
Columbus, Ohio 43210
The complexes M2(TiPB)4,8,9 (M = Mo, W), where TiPB = 2,4,6-
triisopropyl benzoate were allowed to react with 6-carboethoxy-2-
azulene carboxylic acid (2 equiv.) in toluene at room temperature
under an Ar atmosphere. The M2(TiPB)4 compounds are yellow
(M = Mo) and orange (M = W) and the substitution of the
azulene carboxylate for TiPB was accompanied by a dramatic
bDepartment of Chemistry, The University of Kansas, 1251 Wescoe Hall
Drive, Lawrence, Kansas 66045-7582
† Electronic supplementary information (ESI) available: Transient absorp-
tion spectra (fs and ns), absorption, emission and excitation spectra for
the azulene carboxylic acid; DVP for the reduction of II]. See DOI:
10.1039/b919282d
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color change. The reactions were allowed to proceed at room
temperature for 3 days yielding finely divided microcrystalline
products of the title compounds I (M = Mo) and II (M = W)
which were blue and pale green, respectively. The trans substitution
is favored by the presence of the two bulky TiPB ligands as
well as by M2 (d) to azulene carboxylate (p*) back bonding.
Compounds I and II show molecular ions I+ and II+ by MALDI-
TOF mass spectrometry and their 1H NMR spectra are consistent
with expectations.
Molecular structure of II
A single crystal X-ray diffraction study of II confirmed the
trans substitution pattern. The molecular structure of II is
˚
illustrated in Fig. 1. The W–W distance of 2.208 A is typical
˚
of a W–W quadruple bond and the W–O distances 2.03 A are
Fig. 2 Frontier orbital energy diagram comparing I¢ and II¢¢. Gauss view
plots for molecular orbitals of I¢ are also shown.
as expected for a W2(O2C)4 paddlewheel core.10 Of particular
note are the interplanar angles between the azulenyl groups
and the corresponding carboxylate junctions which are 3 and
5◦. As can be seen from Fig. 1, this co-planarity facilitates the
extended Lp-M2d-Lp conjugation as predicted for the framework
B. In contrast the dihedral angle involving the TiPB aromatic
C-6 rings and their carboxylate junctions are 80 and 82◦. This
geometry allows relief of steric pressure at the dinuclear center
but removes the benzoid rings from conjugation with the M2
center. The full structural analysis of II was addressed in the earlier
communication.7 While we have not examined the molybdenum
complex I crystallographically, it is reasonable to expect that I is
structurally related to II.
its molybdenum counterpart in I¢ as has been seen in related
compounds.1 The LUMO and LUMO+1 are the in-phase and
out-of-phase combinations of an azulenecarboxylate p* orbital
as expected based on the simple orbital interaction diagram B.
The magnitude of the splitting of these two orbitals is a relative
measure of the extent of back bonding and mixing with the M2
d orbital. Fig. 2 shows the splitting. This is greater for II¢ than
for I¢ which is consistent with the relative orbital energies and
overlap with the M2 d orbitals: W2 > Mo2. In both I¢ and II¢
the HOMO-1 and HOMO-2 are isoenergetic and are the in-phase
and out-of-phase combinations of the azulene filled p orbitals of
highest energy. These do not mix with the MM based orbitals. For
the molybdenum complex these azulene p orbitals are much closer
in energy (~ 0.5 eV) to the HOMO in comparison to the tungsten
complex (~ 0.9 eV). The M2 p and s orbitals are located below the
HOMO-2.
Electrochemical studies
The compounds I and II have been studied by cyclic voltamme-
try and differential pulse voltammetry in THF solution. Both
complexes show reversible oxidation waves consistent with the
expectation of removal of an electron from the M2 d orbital:
I, 60 mV and II, -465 mV versus Cp2Fe0/+. The compounds
also show partially reversible reduction waves that we assign
to reduction of azulene carboxylate units. The DPV pattern for
compound I is given in Fig. 3. It seems that these are both two
electron reduction waves and that the first wave shows evidence
of a splitting quite possibly because of the coupling of the two
azulene carboxylate units via the Mo2 d orbital as predicted by the
calculations. The reduction waves for II were more complicated
and are shown in the supporting information.† Given the expected
greater coupling between the azulene carboxylate p* orbitals in
the tungsten compound, it is possible that each reduction wave
is split into two for II. Unfortunately, reduction of both Mo
and W systems lead to species that are kinetically labile towards
decomposition.
Fig. 1 Molecular structure of II in the solid-state. Thermal ellipsoids are
drawn at the 50% probability level.
Electronic structure calculations
Electronic structure calculations were carried out on the model
compounds I¢ and II¢ featuring formate ligands in place of the
two trans TiPB groups. Given the lack of M2 (d) to TiPB
arene (p) conjugation, the substitution of formate for TiPB
was reasonable for saving computational time and resources.
Calculations employed the Gaussian03 package as described in
the experimental section.
A frontier orbital energy diagram comparing I¢ and II¢ is
shown in Fig. 2. For both metals the HOMO is principally
the M2 d with some ligand contribution due to back bonding.
The tungsten d orbital in II¢ is notably higher in energy than
Electronic absorption spectra
The electronic absorption spectra of I and II in THF solution are
compared in Fig. 4. The lowest energy absorptions are assigned to
1980 | Dalton Trans., 2010, 39, 1979–1984
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Table 1 Calculated electronic transitions and their assignments for I¢
Wavelength/
nm
Oscillator
strength (f)
Assignment
672
632
381
366
350
0.858
0.018
0.011
0.009
0.201
M2 d to ligand p* (HOMO to
LUMO)
Ligand p to p* (HOMO - 2 to
LUMO + 1; HOMO - 1 to LUMO)
Ligand p to M2 d* (HOMO - 1 to
LUMO + 2)
M2 d to ligand p* (HOMO to
LUMO + 4)
Ligand p to p* (HOMO - 6 to
LUMO; HOMO - 4 to LUMO +
1; HOMO - 2 to LUMO + 3;
HOMO - 1 to LUMO + 4)
Fig. 3 DPV voltammogram exhibiting the oxidation and reduction waves
for I.
Table 2 Calculated electronic transitions and their assignments for II¢
Wavelength/nm Oscillator strength (f) Assignment
779
636
1.170
0.017
M2 d to ligand p* (HOMO to
LUMO)
Ligand p to p* (HOMO - 2
to LUMO + 1; HOMO - 1 to
LUMO)
403
358
0.009
0.500
M2 d to ligand p* + M2 d*
(HOMO to LUMO + 2;
HOMO to LUMO + 4)
M2 d to ligand p* (HOMO to
LUMO + 6)
Ligand p to p* (HOMO - 6
to LUMO; HOMO - 5 to
LUMO + 1)
M2 d to TiPB -CO2 (HOMO
to LUMO + 7)
347
0.135
Fig. 4 Electronic absorption spectra for I and II.
1MLCT involving the fully allowed HOMO to LUMO transition.
The molybdenum complex shows a broad absorption with a lmax
at 715 nm while the tungsten complex has a sharper and even
more intense absorption in the NIR at 1130 nm. In both cases
the absorptions occur to lower energy relative to the calculated
energies obtained by time dependent DFT. This is likely due to
the fact that the calculations consider model compounds and
constitute gas phase predictions. The THF solvent undoubtedly
plays a role in stabilizing the photoexcited state that places positive
charge on the metals. In addition, in the case of tungsten we
have previously noted that the energies of 1MLCT transitions are
overestimated in relation to their molybdenum analogues due to
the greater importance of spin–orbit coupling for the heavier 5d
element.11,12
Steady state emission spectra
1
Excitation of I in THF, into the MLCT transition at 785 nm,
results in a weak emission at room temperature in the near IR. At
low temperature in 2-methyl THF, the intensity of this emission is
significantly enhanced and is shown in Fig. 5.
The emission centered at 900 nm is relatively featureless and is
tentatively assigned to the fluorescence from the S1 state, 1MLCT.
The Stoke’s shift of ~ 2800 cm-1 is quite large and likely reflects the
change to a rigorously planar arrangement of the O2C-azulene-
CO2 framework in the excited state, S1. For comparison, a Stokes
shift of 5300 cm-1 was observed for Mo2(TiPB)2(O2C-terthienyl)2
where in the photoexcited state a planar quinoidal geometry is
expected.13 Fig. 5 illustrates that the emission tails significantly to
lower energy and it is possible that this represents vibronic features
associated with n (CO2) and n (CC) of the azulene carboxylate
that are not well resolved. In somewhat related systems of formula
Mo2(TiPB)2(O2CThn)2, where Th = thienyl and n = 1, 2 or 3, we
have observed emission from a T1 state that is assigned to MMdd*
at 1100 nm.13 However, in these compounds the triplet MMdd*
emissions show well resolved vibronic features arising from n
(MM). This is clearly not the case for compound I.
Time dependent calculations performed on I¢ and II¢ show that
for both compounds the lowest energy electronic transition is the
HOMO–LUMO transition. The calculated absorption bands and
their assignments for compound I¢ and II¢ are given in Tables 1
and 2 respectively.
1
The HOMO to LUMO transition is a fully allowed MLCT
with very high calculated oscillator strength: f = 1.17 for
II¢ and 0.86 for I¢. At higher energies the TD-DFT calcula-
tions predict weaker electronic transitions involving variously
mixed Lp/Md/Lp to Lp* orbital states. For II¢ the DFT predicts
a M2 d to TiPB CO2 p* transition at 347 nm with f = 0.13. This is
consistent with the observed spectrum shown in Fig. 4.
We have not been able to detect any emission from solutions of
II which is not surprising in view of the compound’s low energy
1MLCT. The energy gap law would predict a fast non-radiative
process to be likely.14
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Fig. 5 Steady state emission for I at 77 K in 2-methyl-THF (lexc
785 nm).
=
Fig. 6 a) Femtosecond broadband transient absorption spectra for I
in THF excited at 675 nm. b) Representative single wavelength kinetics
monitored at 550 nm. Inset: Early time dynamics.
Transient absorption spectroscopy
Both I and II have been studied by fs and ns transient absorption
spectroscopy and short lived and long lived excited states have been
observed. Compound I shows a transient absorption at ~ 485 nm
upon irradiation at 675 nm (which is into the 1MLCT). Monitoring
the kinetics at 550 nm indicates that the aforementioned band
decays in 7.61 ps to give rise to the triplet state absorption,
indicated by the bands with maxima at 434 nm and 543 nm.
Furthermore, a slower decay component with a lifetime of ~ 280 ps
is observed which is attributable to vibrational cooling of the triplet
state. The spectra and kinetic plots are shown in Fig. 6.
The kinetics of the triplet state for I have been monitored by ns
transient absorption spectroscopy. Upon excitation at 532 nm the
lifetime of T1 is ~ 260 ns. This is notably shorter than the T1
states that we have assigned to MMdd* which typically have t ~
50 to 100 ms for M = Mo.13 If our assignment of the emission at
900 nm to the S1 state is correct, then it is likely that the 3MLCT
gap law and vibronic coupling with n (CH) and n (CC) modes.
Compound II, when excited at 365 nm in THF, has a short lived
excited state, t = 4.5 ps with a broad absorption band at 470 nm,
and then decays to the triplet state. The absorption features for T1
are, for both compounds (see Supporting Information), similar to
the absorption of the azulene radical anion15 but for II the lifetime
is notably shorter than that for I. For II we can only estimate
it to be between 3 and 20 ns because of instrument limitations.
3
We can reliably assign this state to MLCT by comparison with
the spectrum for reduced azulene and since its lifetime is much
shorter than the 3MMdd* states that we have previously detected.13
Moreover its energy would likely be on the order of 5000 cm-1,
given that the S1 state may be estimated to be ~ 7000 cm-1 based
on the observed 1MLCT at ~ 9000 cm-1. The above photophysical
studies can be summarized in the form of a Jablonski diagram as
shown in Fig. 7.
3
state occurs at lower energy than the MMdd* and decays faster
by non-radiative processes due to a combination of the energy
Fig. 7 Jablonski diagram summarizing observed photophysical properties of I (blue) and II (red) in THF solution.
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A RG830 long pass filter was placed between the sample and the
detector.
Conclusions
The compounds I and II exhibit interesting spectroscopic and
electrochemical properties. The molybdenum complex I does not
show any phosphorescence from the 3MMdd* state which leads us
to believe that the 3MLCT state lies lower in energy. This is in con-
trast to other compounds of the form trans-Mo2(TiPB)2(O2CL)
where L = a p-conjugated ligand. The tungsten complex II is also
unusual as it gives a low energy intense absorption in the NIR at
1130 nm. The singly reduced species I- and II- are proved to be
kinetically labile which has hindered our characterization of these
systems featuring mixed valence ligands.16
Nanosecond transient absorption measurements were carried
out in 1 ¥ 1 cm square quartz cuvettes equipped with Kontes
stopcocks on a home-built instrument pumped by a frequency
doubled (532 nm) or tripled (355 nm) Spectra-Physics GCR-150
Nd:YAG laser (fwhm ~ 8ns, ~ 5 mJ per pulse). The signal from
the photomultiplier tube (Hamamatsu R928) was processed by a
Tektronics 400 MHz oscilloscope (TDS 380).17
The broadband femptosecond transient absorption experi-
ments for Mo2(TiPB)2(Azu)2 and W2(TiPB)2(Azu)2 were carried
out using laser and detection systems that which have been
previously described.18 The samples were excited at 675 nm
for Mo2(TiPB)2(Azu)2 and 365 nm for W2(TiPB)2(Azu)2 (with
excitation power ~ 1–2 mJ at the sample) and were prepared
with absorbance ~ 0.4–0.8 at the excitation wavelength. During
the measurements, the samples were kept in constant motion by
manual movement of an XYZ stage in the vertical and horizontal
directions. In order to ensure that no photodecomposition oc-
curred during data collection, absorption spectra were recorded
before and after the transient absorption measurements. The
measurements were repeated four times at each of the pump–
probe delay positions to confirm data reproducibility throughout
the experiment and the resulting spectra were corrected for the
chirp in the white-light super continuum.19
Experimental
General considerations
W2(TiPB)2(6-carboethoxy-2-azulenecarboxylate)2, II, was ob-
tained according to published procedures.7 All manipulations
were performed in a nitrogen-filled glovebox or by using standard
Schlenk-line techniques. Solvents were dried using standard pro-
cedures and degassed prior to use. 1H NMR spectra were recorded
on a 400 MHz Bruker DPX Avance spectrometer and referenced to
residual protio signals. Matrix assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectra were obtained on a
Bruker Microflex mass spectrometer provided by a grant from
the Ohio BioProducts Innovation Center. The spectrometer was
operated in a reflective, positive ion mode. Laser power was used
at the threshold level required to generate signal. The accelerating
voltage was set to 28 kV. Dithranol was used as the matrix
and prepared as a saturated solution in THF. Microanalysis
was performed by H. Kolbe Microanalytical Laboratory. UV-Vis
spectra were recorded using a Perkin Elmer Lambda 900 UV-Vis
spectrometer.
Kinetics were monitored using single probe wavelengths. In
these cases, the white-light continuum was generated from a
water cell having a 1 cm path length and the probe wavelengths
were isolated by a 10 nm bandwidth interference filter placed
between the sample and detector. The detector was a joule meter
(molectron) and the signals were acquired by a lock-in amplifier
(Stanford Research Systems) referenced to an optical chopper in
the pump pulse path. The kinetics were fit to an exponential decay
ꢀ
of the form, S(t) =
an exp(-t/tn) + y0, with amplitude, a,
lifetime, t, and offset, y0, using Origin 6.0. Error bars are reported
n
as the standard error of the exponential fit.
Electrochemistry
Cyclic voltammograms and differential pulse voltammograms
were collected using a Princeton Applied Research (PAR) 173A
potentiostat-galvanostat equipped with a PAR 176 current-to-
voltage converter. Electrochemical measurements were performed
Electronic structure calculations
The geometries of all the model compounds were optimized in
the gas-phase using density functional theory with the aid of the
Gaussian03 suite of programs. The B3LYP functional was used
along with the SDD energy consistent pseudopotentials for the
heavier elements, namely Mo and W, and the 6-31G* basis set
for C, H, and O. Optimizations were confirmed to be a minima on
the potential energy surface using harmonic vibrational frequency
analysis. Orbital analyses were performed using GaussView. All
orbital diagrams are shown at an isosurface value of 0.02.
under an inert atmosphere in a 0.1 M solution of Bun NPF6 in
4
THF inside a single compartment voltammetric cell equipped with
a platinum working electrode, a platinum wire auxiliary electrode,
and a pseudo-reference electrode consisting of a silver wire in 0.1 M
Bun NPF6/THF separated from the bulk solution by a Vycor
4
+
tip. The potentials are referenced internally to the FeCp2/FeCp2
couple by addition of a small amount of FeCp2 to the solutions of
the complexes.
Syntheses
Photophysical measurements
Mo2(TiPB)2(6-carboethoxy-2-azulenecarboxylate)2,
I. A
The steady-state NIR-luminescence measurements at room tem-
perature were carried out in 1 ¥ 1 cm square quartz cuvettes
equipped with Kontes stopcocks and the measurements at 77 K
were carried out in J. Young NMR tubes using an optical dewar
sample holder. The spectra were measured on a home-built
instrument utilizing a germanium detector. The samples were
excited at 405 nm or 785 nm (laser diode max power: 45.0 mW).
Schlenk tube was charged with Mo2(O2C-2,4,6-isopropyl-C6H2)4
(0.106 g, 0.09 mmol) and 6-carboethoxy-2-azulenecarboxylic
acid (0.04 g, 0.18 mmol), to which 10 mL of toluene was added.
The suspension was stirred for 4 days, during which time a
blue precipitate was formed. The precipitate was isolated by
centrifugation and washed with 2 ¥ 10 mL of toluene followed by
10 mL aliquot of hexane, before drying in vacuo to yield a blue
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solid. Yield: 0.08 g, 76%. Anal. Calculated for Mo2C60H68O12: C,
7 M. V. Barybin, M. H. Chisholm, N. J. Patmore, R. E. Robinson and N.
Singh, Chem. Commun., 2007, 3652.
8 F. A. Cotton, L. M. Daniels, E. A. Hillard and C. A. Murillo, Inorg.
Chem., 2002, 41, 1639.
61.43; H, 5.84. Found: C, 58.37; H, 6.04%. UV-Vis (lmax, values
1
of e ¥ 10-3 in parentheses): 715 (7627 M-1cm-1) nm. H NMR
(400 MHz; THF-d8): d 0.9 (d, 24H), 1.2 (d, 6H), 1.4 (t, 6H), 2.8
(m, 4H), 3.5 (m, 2H), 4.4 (q, 4H), 6.9 (s, 4H), 8.1 (s, 4H), 8.2 (d,
4H), 8.6 (d, 4H). MALDI-TOF: 1173.0 (100%, M+).
9 D. J. Santure, J. C. Huffman and A. P. Sattelberger, Inorg. Chem., 1985,
24, 371.
10 Multiple Bonds Between Metal Atoms, Third Edition, ed. F. A. Cotton,
C. A. Murillo and R. A. Walton, 2005.
11 M. H. Chisholm, P.-T. Chou, Y.-H. Chou, Y. Ghosh, T. L. Gustafson
and M.-L. Ho, Inorg. Chem., 2008, 47, 3415.
12 B. E. Bursten, M. H. Chisholm, R. J. H. Clark, S. Firth, C. M. Hadad,
A. M. MacIntosh, P. J. Wilson, P. M. Woodward and J. M. Zaleski,
J. Am. Chem. Soc., 2002, 124, 3050.
13 G. T. Burdzinski, M. H. Chisholm, P. T. Chou, Y. H. Chou, F. Feil,
J. C. Gallucci, Y. Ghosh, T. L. Gustafson, M. L. Ho, Y. Liu, R.
Ramnauth and C. Turro, Proc. Natl. Acad. Sci. U. S. A., 2008, 105,
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Acknowledgements
We thank the National Science Foundation and The Ohio State
University for financial support and the Ohio Super Computer
Center for computational resources.
14 R. Englman and J. Jortner, Mol. Phys., 1970, 18, 145.
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1984 | Dalton Trans., 2010, 39, 1979–1984
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©