4-Nitrophenyl- and 4′-nitro-1,1′-biphenyl-4-carboxylates attached to Mo2 quadruple bonds: Ground versus excited state M 2δ-ligand conjugation

Article abstract of DOI:10.1039/c4dt01073f

From the reactions between Mo₂(TⁱPB)₄, where TⁱPB = 2,4,6-triisopropylbenzoate and two equivalents of the carboxylic acid LH (LH = 4-nitrobenzoic acid and 4′-nitro[1,1′- biphenyl]-4-carboxylic acid) the compound

Full text of DOI:10.1039/c4dt01073f

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4-Nitrophenyl- and 4’-nitro-1,1’-biphenyl-
4-carboxylates attached to Mo2 quadruple bonds:
ground versus excited state M2δ–ligand
conjugation†
Cite this: Dalton Trans., 2014, 43,
11397
Brian G. Alberding, Malcolm H. Chisholm,* Christopher B. Durr, Judith C. Gallucci,
Yagnaseni Ghosh and Thomas F. Spilker
From the reactions between Mo2(TiPB)4, where TiPB = 2,4,6-triisopropylbenzoate and two equivalents of
the carboxylic acid LH (LH = 4-nitrobenzoic acid and 4’-nitro[1,1’-biphenyl]-4-carboxylic acid) the com-
pounds trans-M2(TiPB)2L2 have been prepared: I (L = 4-nitrobenzoate and M = Mo), II (L = 4’-nitro-
1,1’-biphenylcarboxylate and M = Mo) and III (L = 4-nitrobenzoate and M2 = MoW). The compounds
have been characterized by 1H NMR, UV-Vis and steady state emission spectroscopy, ns and fs
transient absorption spectroscopy and cyclic voltammetry. These data are compared with predictions
based on electronic structure calculations on model compounds where TiPB is substituted for formate.
Together these data indicate stronger ground-state coupling of the Mo2δ and ligand π* systems in I
relative to II but this order is reversed in the photo excited S1 1MLCT state. Attempts to prepare the W2
containing analogs were unsuccessful.
Received 10th April 2014,
Accepted 4th June 2014
DOI: 10.1039/c4dt01073f
www.rsc.org/dalton
bithienyl and terthienyl and M = Mo or W.4 Here with increas-
ing number of rings the M2δ to thienyl carboxylate ligand
Introduction
The electronic coupling in 1,4-polyphenylenes is limited by the charge transfer bands were moved to lower energy and the
repulsive peri-CX to CX interactions. This produces dihedral solid state molecular structure of the bithienyl carboxylate
angles along the chain approaching 45°. Interestingly, the derivative revealed near coplanarity of the two trans-M2O2C–
solid-state structure of biphenyl1 has a near planar C6–C6 di- H2C4S–H3C4S units (dihedral angle of 8.1°) as shown in Fig. 1.
hedral angle of <1°, though the C6–C6 rings in the gas phase
These earlier findings prompted us to question the relative
are twisted and the structure of 4,4′-bitolyl has a C6–C6 di- degree of M2δ to ligand coupling that might exist for 1,4-
hedral angle of 34.9°.2 Similarly, in the crystal structure of linked C6 rings. To address this issue, we elected to compare
Mo2(O2C-4-biphenyl)4, the four unique dihedral angles associ- the influence of 4-nitrobenzoate and 4′-nitro-1,1′-biphenyl-4-
ated with the C6 rings are in the range of 32°–42°.3 It seems carboxylate ligands bound to the M2 center. These com-
4+
likely that the intermolecular C–H–C6 interactions in the solid- pounds were expected to show intense metal-to-ligand charge
state impose the planarity of the biphenyl molecule. 2,5- transfer transitions in the visible region of the spectrum.5
Coupled oligothiophenes also have repulsive C–H peri inter- These arise from M2δ to ligand π* transitions.
actions; however, these interactions are less than those in poly-
phenylenes and oxidation of a polythiophene chain is well
known to produce good hole transport/conduction along the
Results and discussion
Synthesis
chain. In previous work in this laboratory we prepared com-
pounds of the form trans-M2(TiPB)2(O2CR)2 where R = thienyl,
In order to address this question, we prepared and character-
ized compounds of the form trans-Mo2(TiPB)2L2, where L =
4-nitrobenzoate (I) and 4′-nitro[1,1′-biphenyl]-4-carboxylate (II)
The Ohio State University, 100 West 18th Avenue, Columbus, OH 43210, USA.
as shown in Fig. 2.
Related reactions involving W2(TiPB)4 did not yield related
E-mail: Chisholm@chemistry.ohio-state.edu; Fax: +1 614 292 0368;
Tel: +1 614 688 3525
quadruply bonded compounds. It appeared that the nitro
group oxidized the W2 center but as of yet no well character-
ized product from these reactions has been obtained. It was,
†Electronic supplementary information (ESI) available: Table S1, Fig. S1–S2 and
4+
crystallographic description. CCDC 994112. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4dt01073f
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The new compounds, shown in Fig. 2, are air-sensitive but
chemically persistent under inert atmospheres at room temp-
erature. They are soluble in donor solvents such as THF and
sparingly soluble in aromatic hydrocarbons such as benzene
and toluene. They gave molecular ions in their MALDI-TOF
mass spectra and 1H NMR data are consistent with expecta-
tions. The data are given in the Experimental.
Solid state molecular structure of I. The molecular structure
of the p-nitro benzoate complex is shown in Fig. 3. The mole-
cule has a crystallographically imposed center of inversion.
4+
The central Mo2(O2C)4 core is typical of Mo2 carboxylates.8
Each Mo atom is weakly ligated by THF along the Mo–Mo axis,
Mo–O(THF) = 2.566(2) Å. The C6 rings of the TiPB ligands are
twisted ∼90° from their CO2 planes while the two trans-4-nitro-
C6H4–CO2 units are nearly planar. The O2C–C6 dihedral angle
is 17° and this is clear evidence of Lπ–Mo2δ–Lπ conjugation.
Electronic structure calculations. In order to aid in the
interpretation of the spectroscopic and electrochemical data,
we performed electronic structure calculations on the model
compounds Mo2(O2CH)2(L2), I′, II′ and for comparison
Mo2(O2CH)2(O2C–Ph)2.
Fig. 1 (a) Molecular structure of Mo2(TiPB)2(BTh)2,4 (b) normalized
absorption spectra of trans-Mo2(TiPB)2(Thn)2 in THF where n = 1 (blue), 2
(red), 3 (green); λmax = 443 nm, 500 nm, and 518 nm, respectively.
however, possible to prepare the mixed metal compound
MoW(TiPB)2(O2C–C6H4–p-NO2)2 (III). The synthesis of these
complexes was accomplished by employing a ligand exchange
reaction with the homoleptic M2(TiPB)4 6,7 and two equivalents
of the respective carboxylic acid in toluene under argon as out-
lined in Scheme 1. They are isolated as microcrystalline pre-
cipitates from the reaction mixture.
Fig. 3 Molecular structure of I drawn at 50% probability. Hydrogens,
Scheme 1 Synthesis of I, II, and III.
solvent and disorder omitted for clarity.
Fig. 2 Molecular structures of compounds I, II, and III.
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For each compound, the calculations predict that the HOMO
is the Mo2δ orbital. In comparison to Mo2(O2CH)2(O2C–Ph)2,
the introduction of the 4-nitro group stabilizes the δ orbital
and this is most pronounced in the 4-nitrobenzoate, I′. For
both I′ and II′ the LUMO and LUMO+1 are the in-phase and
out-of-phase combinations of the ligand π* orbitals, respect-
ively. Only the out-of-phase can mix with the M2δ orbitals, and
it is this interaction that stabilizes the Mo2δ orbital. The
ligand π* orbitals are notably lower in energy in I′ and II′
relative to the simple benzoate and thus the HOMO–LUMO
gap is reduced by ∼0.5 eV.
The frontier molecular orbital energy level diagram comparing
I′ and II′ and the benzoate complex trans-Mo2(O2CH)2(O2CPh)2 is
given in Fig. 4. For compound II′, the C6–C6 dihedral angle is
calculated to be 37.2°. Gaussview plots of the LUMO orbitals
for the benzoate complex and I′ and II′ are shown in Fig. 5.
Electrochemical studies. Compounds I, II and III show
reversible oxidation waves due to the removal of an electron
from the δ orbital at +0.24, +0.06, and −0.09 V relative to the
ferrocene/ferrocenium couple, Cp2Fe0/+, in THF solution. It is
not surprising to find that it is easier to oxidize the (MoW)4+
Fig. 5 GaussView plots representing the lowest energy π* orbitals of
(a) Mo2(O2CH)2(O2CPh)2, (b) Mo2(O2CH)2(O2CPhNO2)2, (c) Mo2(O2-
CH)2(O2CBPhNO2)2 drawn at isovalue = 0.02.
and the measure of the splitting between the 1st and 2nd
reduction potentials, the ΔEy2 in mV can be used as an esti-
mate of the degree of electronic coupling. A separation of
∼250 mV is rather typical of a Class II ion on the Robin and
Day scheme with Kc ∼ 104.9 Electrochemical data for the com-
pounds are given in Table 1 and cyclic voltammetry traces can
be seen in ESI Fig. 1.†
Electronic absorption and steady state emission spectra.
Compound I is purple, II is orange-red and III blue. These
colors arise from Mo2δ to ligand charge transfer and the
absorption spectra of I, II, and III are shown in Fig. 6 along
with that of trans-Mo2(TiPB)2(O2CPh)2.10 The purpose of
showing the spectrum of the benzoate complex is to empha-
size that while the conjugation from one phenyl to the p-nitro-
biphenyl group in II is not very effective, there is some
influence of the terminal p-NO2–C6H4 unit. The relative minor
influence of the terminal p-NO2–C6H4 unit presumably arises
from the dihedral angle between the two phenyl rings in the
ground state. Based on biphenyl, this is expected to be ∼40°.
There is a much larger influence when comparing the benzo-
ate complex to I. In I, the p-nitro group imparts a much larger
influence shifting the absorption band to even lower energy
than II. Complex III is shifted lowest to due to the presence of
the W atom, which raises the energy of the δ orbital and sub-
sequently decreases the energy of the MLCT transition.
4+
4+
center relative to Mo2 by ∼0.3 V since related known W2
containing complexes are oxidized more easily by ∼0.5 V. It is
interesting to note that I is notably harder to oxidize than II.
This implies that the Mo2δ to Lπ* bonding is greater in I. It is
also interesting to note that compound I and its MoW ana-
logue III show evidence of two reduction waves separated by
∼250 mV whereas compound II shows evidence of only one at
∼−1.7 V. As we have noted before for compounds of the type
trans-Mo2(TiPB)2(L)2, where L = π acceptor, the anions formed
by electron transfer can be considered as mixed valence anions
Compounds I and II show weak fluorescence and phosphor-
escence. The latter is notably more intense at 77 K and occurs
Table 1 Electrochemical oxidation and reduction potentials of I, II, and
III in THF given in mV referenced to Fc/Fc+
Compound
Eox
Ered
I
II
III
238
58
−91
−1585, −1843
−1732
−1521, −1790
Fig. 4 Molecular orbital diagram for compounds I’ and II’ as compared
to Mo2(O2CH)2(O2C–Ph)2.
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Fig. 6 Electronic absorption spectra of I (purple), II (red), III (green),
and Mo2(TiPB)2(O2C–Ph)2 (orange); λmax = 563 nm, 506 nm, 708 nm,
and 428 nm respectively.
at ∼1100 nm with vibronic features ∼390 cm−1 corresponding
to the Mo–Mo stretching frequency.6 The phosphorescence
3
arises from the MoMoδδ* state and has been seen previously
for compounds of the form trans-Mo2(TiPB)2(O2CR)2. What is
more interesting is the fact that the fluorescence from com-
pound II has a significantly greater Stokes shift relative to I.
We attribute this to the fact that in the photoexcited state there
is notably greater planarity of the two phenyl rings due to the
resonance form of the 1,1-biphenylcarboxylate as shown below
(Scheme 2).
Fig. 7 (a) Steady state emission for I (blue) and II (red), (b) steady state
emission for III at room temperature (blue) and 77 K (red).
With irradiation into the MLCT band, compound III shows
weak fluorescence centered around 1000 nm attributed to
1
emission from the MLCT state. This emission sharpens and
blue shifts when cooled to 77 K. No emission from the triplet
state is observed, which we anticipate is 3MLCT. The steady
state emission spectra are shown in Fig. 7.
I and II. The lifetimes of these S1 and T1 states are comparable
to other carboxylate M2-containing complexes as shown in
Table 2.
Transient absorption spectra. Investigation of the excited
state dynamics in these compounds was carried out using
nanosecond and femtosecond transient absorption spectro-
scopy in THF solutions. The femtosecond TA spectra are
shown in Fig. 8 and the related nanosecond spectra in Fig. 9.
The lifetimes of the S1 MLCT states fall in the range of 8–14 ps
and the T1 states that are 3δδ* for compounds I and II are com-
parable, ∼75 μs. The T1 state of the MoW compound is notably
shorter, ∼100 ns. This state is not emissive and, given the low
Concluding remarks
The introduction of the nitro group in the 4-position in com-
pound I notably shifts the Mo2δ orbital to lower energy relative
to the simple benzoate complex and similarly lowers the
1
energy of the MLCT transition. The additional phenyl ring in
compound II mutes the influence of the terminal nitro group
in the ground state electronic structure presumably because of
the peri-CH–CH repulsive interactions favor a ground state geo-
metry in which the dihedral angle between the rings is close to
45°. In the calculated structure of II′ the C6–C6 dihedral angle
is 37.2°. This reduces the effect of extensive Mo2δ to ligand π*
conjugation. However, in the photoexcited state, this conju-
gation is enhanced and we observe a red shift of the fluore-
scence of II relative to I and a notably greater Stokes shift.
The oligothiophene units attached to M2-quadruply bonded
centers appear to be notably better as photon harvesters com-
pared to their oligophenylene counterparts.
3
3
energy S1 state, is surely MLCT. Note the energies of the δδ*
3
states follow the order W2 > MoW > Mo2, so the δδ* state for
III would be at a shorter wavelength relative to the 1100 nm for
Scheme 2 Proposed photoexcited state of the O2C-BPhNO2 ligand in
a quinoidal state.
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Fig. 8 fs-TA spectra for I (a), II (b), III (c) in THF.
Fig. 9 ns-TA spectra for I (a), II (b), III (c) in THF.
Table 2 Table of lifetimes and state assignments
Experimental
Measurements
Compounds
S1
T1
Ref.
a
14 ps (1MLCT)
8 ps (1MLCT)
12 ps (1MLCT)
6 ps (1MLCT)
500 fs (1MLCT)
12 ps (1MLCT)
4 ps (1MLCT)
79 μs (3δδ*)
NMR spectra were recorded on a 400 MHz Bruker DPX
Advance 400 spectrometer. All H NMR chemical shifts are in
I
II
1
a
a
74 μs (3δδ*)
III
∼100 ns (3MLCT)
40 μs (3δδ*)
ppm relative to the protio impurity in THF-d8 at 1.73 ppm.
Electronic absorption spectra at room temperature were
recorded using a Perkin-Elmer Lambda 900 spectrometer in
THF solution. A 10.00 mm IR quartz cell was employed.
Steady state emission spectra were recorded on a SPEX
Mo2(TiPB)4
6
6
12
12
MoW(TiPB)4
30 μs (3δδ*)
Mo2(TiPB)2(O2CTh3)2
MoW(TiPB)2(O2CTh3)2
76 μs (3δδ*)
165 ns (3MLCT)
a This work.
Fluoromax-2 spectrometer with
equipped with a 150 W Xe arc lamp as the source in the
visible. The steady state near-IR luminescence measurements in Kontes top cells with dimensions 10.00 × 10.00 mm2 or a
a 90° optical geometry
J-Young NMR tube.
were carried out on a home-built instrument utilizing a germa-
nium detector. Compounds I and II were excited at 405 nm
The cyclic voltammograms and differential pulse voltammo-
and compound III at 658 nm. Samples were prepared in THF grams of all the complexes were collected at a scan rate of
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100 mV s−1 and 5 mV s−1, respectively, using a Princeton The suspension was stirred at room temperature for three
Applied Research (PAR) 173A potentiostat–galvanostat equipped days, at the end of which a red precipitate had formed. This
with a PAR 176 current-to-voltage converter. Electrochemical was centrifuged and washed with toluene (2 × 10 ml) before
measurements were performed under an inert atmosphere in a being dried in vacuo to give 0.15 g (75% yield) of a red solid.
0.5 M solution of nBu4NPF6 in THF inside a single compartment Unreacted 4′-nitro[1,1′-biphenyl]-4-carboxylic acid was removed
voltammetric cell equipped with a platinum working electrode, by sublimation at 115 °C under dynamic vacuum. Microana-
a platinum wire auxiliary electrode, and a pseudoreference lysis found: C, 59.19; H, 5.26 C58H62Mo2N2O12, Requires: C,
electrode consisting of a silver wire in 0.5 M nBu4NPF6–THF 59.49; H, 5.34. NMR (THF-d8, 400 MHz): δH 8.51 (d, 4H, JHH
=
separated from the bulk solution by a Vycor tip. The potential 9 Hz), 8.38 (d, 4H, JHH = 9 Hz), 8.01 (d, 4H, JHH = 9 Hz), 8.01
+
values are referenced to the FeCp2/FeCp2 couple, obtained by (d, 4H, JHH = 9 Hz), 7.00 (s, 4H), 3.08 (m, 4H), 3.08 (m, 2H),
addition of a small amount of FeCp2 to the solution.
1.23 (d, 12H, JHH = 7 Hz), 1.05 (d, 24H, JHH = 7 Hz). UV-Vis (ε,
Nanosecond transient absorption experiments were carried M−1 cm−1): 305 nm (18 900), 506 nm (8,430). Calculated mono-
out on a home-built instrument pumped by a frequency isotopic MW for C58H62Mo2N2O12: 1171.0, Found: 1171.9 (M+).
doubled (532 nm) Nd:YAG laser (Spectra-Physics GCR-150 Nd:
Synthesis of MoW(TiPB)2(O2CPhNO2)2 (III). MoW(TiPB)4
YAG laser, fwhm ∼8 ns, ∼5 mJ per pulse), the signal was the (0.21 g, 0.17 mmol) was dissolved in dry toluene and added to
photomultiplier tube was processed by Tektronics 400 MHz 4-nitrobenzoic acid (0.05 g, 0.3 mmol). The suspension was
oscilloscope. Lifetime decays were fit to a single exponential S = A stirred at room temperature for three days, at the end of which
exp(−t/τ) + C, with amplitude (A), lifetime (τ) and offset (C), using blue precipitate had formed. This was centrifuged and washed
SigmaPlot 12.0. Errors reported are those of the exponential fit.11
with toluene (2 × 10 ml) before being dried in vacuo to give
Femtosecond transient absorption experiments were 0.13 g (69% yield) of an indigo blue solid. Unreacted 4-nitro-
carried out on a system described previously.12 Samples were benzoic acid was removed by sublimation at 110 °C under
prepared with an absorbance of ∼0.4–0.8 at the excitation dynamic vacuum. Microanalysis found: C, 49.86; H, 4.89;
wavelength. Compounds I and II were excited at 514 nm and C46H54MoWN2O12, Requires: C, 49.92; H, 4.92. NMR (THF-d8,
compound III at 675 nm with an excitation power of ∼1 μJ at 400 MHz): δH 8.49 (d, 4H, JHH = 9 Hz), 8.31 (d, 4H, JHH = 9 Hz),
the sample. The kinetics were fit to a single exponential decay 7.05 (s, 4H), 3.05 (m, 4H), 2.92 (m, 2H), 1.26 (d, 12H, JHH
=
of the form S = A exp(−t/τ) + C using SigmaPlot 12.0. Errors 7 Hz), 1.04 (d, 24H, JHH = 7 Hz). Calculated monoisotopic MW
+
reported are those of the exponential fit.
for C46H54MoWN2O12: 1108.0, Found: 1061.7 (M − NO2 ).
All reactions were carried out under one atmosphere of
oxygen-free UHP-grade argon using standard Schlenk tech-
niques or under a dry and oxygen-free nitrogen atmosphere
using standard glovebox techniques. All manipulations of the
studied compounds were performed in a nitrogen-filled glove-
box or by using standard Schlenk line techniques in an atmos-
phere of oxygen-free UHP-grade argon. All solvents were dried
over the appropriate drying agent, distilled prior to use, and
stored in reservoirs equipped with Kontes taps over activated
4 Å molecular sieves, under an argon atmosphere and
degassed prior to use. Mo2(TiPB)4 and MoW(TiPB)4 were syn-
thesized as described previously.6,13
Theoretical approaches
Electronic structure calculations on the model compounds
were performed by DFT with the aid of the Gaussian09 suite of
programs.14 The B3LYP exchange correlation functional was
used along with the 6-31G* basis set for C, H, N and O and the
SDD basis set and SDD energy consistent pseudopotentials for
Mo, and W.15,16 Geometry optimizations were performed in
appropriate symmetry and were confirmed as local minima on
the potential energy surfaces using frequency analysis. Orbital
analysis was performed using Gaussview.17
Synthesis of Mo2(TiPB)2(O2CPhNO2)2 (I). Mo2(TiPB)4 (0.30 g,
0.25 mmol) was dissolved in dry toluene and added to 4-nitro-
benzoic acid (0.081 g, 0.48 mmol). The suspension was stirred
at room temperature for three days, at the end of which purple
precipitate had formed. This was centrifuged and washed with
toluene (2 × 10 ml) before being dried in vacuo to give 0.2 g
(76% yield) of a pink solid. This solid was recrystallized from
tetrahydrofuran. Microanalysis found: C, 53.58; H, 5.05
C46H54Mo2N2O12, Requires: C, 54.23; H, 5.34. NMR (THF-d8,
400 MHz): δH 8.61 (d, 4H, JHH = 9 Hz), 8.59 (d, 4H, JHH = 9 Hz),
Acknowledgements
We thank the National Science Foundation for funding on
grant nos. 0957191 and 1266298. We also thank the Ohio
Supercomputer Center for computational resources, the Ohio
State University Center for Chemical and Biophysical
Dynamics for use of laser systems, and Professor Claudia
Turro for use of instrumentation.
7.06 (s, 4H), 3.05 (m, 4H), 2.91 (m, 2H), 1.26 (d, 12H, JHH
=
7 Hz), 1.07 (d, 24H, JHH = 7 Hz). UV-Vis (ε, M−1 cm−1): 255 nm
(42 300), 563 nm (17 300). Calculated monoisotopic MW for
C46H54Mo2N2O12: 1154.63, Found: 1154.4 (M+).
Notes and references
1 J. Trotter, Acta Crystallogr., 1961, 14, 1135–1140.
2 G. Casalone, C. Mariani, A. Mugnoli and M. Simonetta,
Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem.,
1969, 25, 1741–1750.
Synthesis of Mo2(TiPB)2(O2CBPhNO2)2 (II). Mo2(TiPB)4
(0.20 g, 0.17 mmol) was dissolved in dry toluene and added to
4′-nitro[1,1′-biphenyl]-4-carboxylic acid (0.074 g, 0.30 mmol).
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