2-thienylcarboxylato and 2-thienylthiocarboxylato ligands bonded to MM quadruple bonds (M = Mo or W): A comparison of ground state, spectroscopic and photoexcited state properties

Article abstract of DOI:10.1021/ic901607u

The compounds M₂(TiPB)₂(OSC-2-Th)₂ have been prepared from the reactions between M₂(TiPB)₄ and Th-2-COSH (2 equiv) in toluene solution, where M = Mo (Mo₂ThCOS) or W (W₂ThCOS), Ti

Full text of DOI:10.1021/ic901607u

Inorg. Chem. 2009, 48, 11187–11195 11187
DOI: 10.1021/ic901607u
2-Thienylcarboxylato and 2-Thienylthiocarboxylato Ligands Bonded to MM
Quadruple Bonds (M = Mo or W): A Comparison of Ground State, Spectroscopic
and Photoexcited State Properties
Brian G. Alberding, Malcolm H. Chisholm,* Yi-Hsuan Chou, Yagnaseni Ghosh, Terry L. Gustafson,* Yao Liu, and
Claudia Turro*
Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210
Received August 11, 2009
The compounds M2(TiPB)2(OSC-2-Th)2 have been prepared from the reactions between M2(TiPB)4 and Th-2-COSH
(2 equiv) in toluene solution, where M = Mo (Mo2ThCOS) or W (W2ThCOS), TiPB = 2,4,6-triisopropylbenzoate and
Th = thienyl. The molybdenum and tungsten compounds are pink and blue, air-sensitive, ether soluble solids that show
Mþ ions in the mass spectrometer and metal and ligand based reversible oxidation and reduction waves, respectively,
by cyclic voltammetry. Electronic structure calculations on the model compounds M2(O2CH)2(OSC-2-Th)2 indicate
that the highest occupied molecular orbital (HOMO) is principally M2δ and the lowest unoccupied molecular orbital
(LUMO) is thienylthiocarboxylate π* but with significant metal-sulfur mixing. The intense visible absorptions arise
1
from MLCT, M2δ to thienylthiocarboxylate. The photoexcited states of these molecules have been studied by
transient absorption spectroscopy and steady state emission. These properties are compared with those of previously
reported thienylcarboxylate compounds, M2(TiPB)2(O2C-2-Th)2, where M = Mo (Mo2ThCO2) or W (W2ThCO2).
Introduction
leading in certain cases to reductive cleavage reactions.3 In
our studies of linked MM quadruple bonds we have shown
that the electronic coupling by thioterephthalate bridges
[SOCC6H4COS]2- is much larger in comparison to ter-
ephthalate [O2CC6H4CO2]2-.4 The principal origin of this
phenomenon lies in the lower energy of the C-S π* orbital
which affords greater M2δ to bridge π* back-bonding.
However, there is also a secondary reason which arises from
the weaker C-S π bond and the higher energy of the sulfur 3p
π atomic orbital relative to its oxygen counterpart. This leads
to a greater mixing of the S 3p orbital with the out-of-phase
combination of the filled M2δ orbitals which is the highest
occupied molecular orbital (HOMO) of the bridged com-
plexes. We were naturally led to question what effect the
substitution of a sulfur for an oxygen would have at a trans-
disubstituted compound of the type M2(TiPB)2(O2C-L)2
shown in Figure 1, where TiPB = 2,4,6-triisopropylbenzo-
ate.
The classification of ligands and metals as being hard or
soft is perhaps most useful at the extreme left and right-hand
side of the d-block elements.1 Thus, the group 4 and 5
elements can easily be seen as hard metals and oxophilic
while the group 10, 11, and 12 metals are soft having
relatively little affinity toward oxygen but a greater affinity
toward sulfur. In the middle of the d-block the classification
is less clear-cut and may rest with the specific oxidation state
of the metal and the nature of the ancillary ligands. Certainly,
this could be claimed for molybdenum and tungsten which in
their high oxidation states of þ5 and þ6 can be claimed to be
oxophilic and hard but in their lower oxidation states of 0 and
þ2 can be considered soft with a high affinity for tertiary
phosphines and other soft π-acceptor ligands. In our studies
of the chemistry of MM (M = Mo or W) multiply bonded
complexes of molybdenum and tungsten, we have compared
the influence of alkoxide and thiolate ligands on the electro-
nic structure and reactivity of the triply bonded ethane-like
dimers X3MtMX3.2 Here the stronger π-donating proper-
ties of the alkoxide labilize the MtM bond toward reactions
with π-acceptor ligands such as CO, alkynes, and nitriles
Here the bulky TiPBligands are trans and their C6 rings are
twisted out of the -O2C plane removing M2δ to arene
π-conjunction. (O2C-L)π-M2δ-(O2C-L)π conjugation arises
principally from the interaction of the out-of-phase carbox-
ylate π* combination shown in Figure 2 and is maximized
when the aromatic rings are coplanar with the carboxylate
planes. This is indeed seen in the solid state structures where
*To whom correspondence should be addressed. chisholm@chemistry.
ohio-state.edu (M.H.C.), gustafson@chemistry.ohio-state.edu (T.L.G.),
turro@chemistry.ohio-state.edu (C.T.).
(1) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533.
(2) Chisholm, M. H.; Davidson, E. R.; Huffman, J. C.; Quinlan, K. B. J.
Am. Chem. Soc. 2001, 123, 9652.
(3) Chisholm, M. H. Chem. Rec. 2001, 1, 12.
(4) Chisholm, M. H.; Patmore, N. J. Dalton Trans. 2006, 3164.
r
2009 American Chemical Society
Published on Web 11/04/2009
pubs.acs.org/IC

11188 Inorganic Chemistry, Vol. 48, No. 23, 2009
Alberding et al.
carboxylate analogue, Mo2ThCO2,6 as depicted in the
drawing in Supporting Information, Figure S1.
Electronic Absorption Spectroscopy. The UV-vis elec-
tronic absorption spectra for the compounds are shown in
Figure 3. Two notable features are immediately obvious
in relation to the lowest energy 1MLCT bands.
(1) The spectra of the ditungsten compounds,
W2ThCOS and W2ThCO2, are red-shifted compared to
their dimolybdenum counterparts, Mo2ThCOS and
Mo2ThCO2, respectively. This is consistent with the
relative energies of the M2δ orbitals and is in-line with
both electrochemical measurements and computations
1
(vide infra). (2) The MLCT bands of Mo2ThCOS and
W2ThCOS are red-shifted compared to Mo2ThCO2 and
W2ThCO2, respectively. This indicates that the substitu-
tion of S for O in the -O2C moiety has a pronounced
influence in lowering the energy of the 1MLCT transition,
the M2δ to thienyl π* transition. At higher energies
and λmax ∼ 400 nm for W2ThCOS and W2ThCO2
and ∼320 nm for Mo2ThCOS and Mo2ThCO2 compounds
there is a weaker absorption due to M2δ to -O2C π* of the
TiPB ligands. This transition is similarly seen in the parent
M2(TiPB)4 compounds, albeit, that they are more red-
shifted because of greater coplanarity of the TiPB ligands
with the dimetal core in the parent M2(TiPB)4 compounds.8
Inall casesthe1MMδto δ* transition, which is known to be
weak with, ε ∼100 M-1cm-1, and is expected to occur
around λmax ∼ 460 nm, is masked by the intense 1MLCT.
Finally, it can be noted that there are absorptions at <300
nm which we assign to aromatic ring (TiPB) π to π*
Figure 1. Generic structure of a trans-M2(TiPB)2(O2C-L)2 compound,
where M = Mo or W.
L = 6-azulene,5 2-thiophene6 and 2,20-bithiophene.7 By sym-
metry the non-bonding carboxylate lone pair orbitals can
interact with the MMδ*, but the energy difference makes this
interaction of little significance. Here we report the prepara-
tion of the new compounds trans-M2(TiPB)2(OSC-2-Th)2,
where M=Mo(Mo2ThCOS) or W(W2ThCOS), Th=thienyl,
and compare their properties with the previously reported
M2(TiPB)2(O2C-2-Th)2 compounds, where M=Mo (Mo2-
ThCO2)7 or W(W2ThCO2).6
1
1
transitions, LC. The relative intensities of the MLCT
bands follow the order Mo2 < W2 and O2C < OSC as
expected for MLCT bands which to a first order approx-
imation correlate with the energy difference between the
two states. In Figure 3, the spectra are drawn to a normal-
ized intensity scale such that all 1MLCT bands have
equivalent intensities. That the room temperature solution
1MLCT absorption of the tungsten compounds, W2ThCO2
and W2ThCOS, show vibronic features is also worthy of
note. On lowering the temperature, these become more
pronounced, as shown for W2ThCO2 in Supporting Infor-
mation, Figure S2. This most likely reflects the greater
degree of M2δ to ligand π* back-bonding and the higher
barrier to rotation about the C-C bond of the O2C/OSC-
Th ring. At low temperature the Boltzmann distribution of
rotamers about the C-C bond of the O2C/OSC-Th ring
planes favor the planar geometry. A more detailed discus-
sion of vibronic features and the nesting of the potential
energy surfaces in 1MLCT transitions is given in a recent
paper.9
Results and Discussion
Syntheses. The new ligand Th-2-COSH and the com-
pounds Mo2ThCOS and W2ThCOS, were prepared ac-
cording to the reactions shown in Scheme 1. Details are
described in the Experimental Section. Mo2ThCOS is
pink and W2ThCOS is blue. Both compounds are air
and moisture-sensitive. They are sparingly soluble in
hexane and toluene but appreciably soluble in tetrahy-
drofuran (THF). They showed molecular ions Mþ by
matrix assisted laser desorption/ionization time-of-flight
(MALDI-TOF) mass spectrometry and gave satisfactory
1
elemental analysis. The H NMR spectra are consistent
with the trans-substitution pattern (for cis-substitution,
we would expect the isopropylmethyl groups of TiPB
ligands to reveal diastereotopism). Compounds Mo2Th-
CO2 and W2ThCO2 were prepared according to reported
procedures.6
Solid-State Molecular Structure. Single crystals of
Mo2ThCOS grown from THF solutions were examined,
and a partial solution was obtained. The refinement was
complicated by a disorder of the sulfur atoms both within
the thienyl ring and the -OSC moiety, and a satisfactory
solution was not obtained. This notwithstanding, the
gross structural features are akin to that of its thienyl
Luminescence Studies. Both Mo2ThCO2 and Mo2Th-
COS show evidence of weak fluorescence from their
1MLCT states at ∼560 and ∼620 nm, respectively, and
W2ThCO2 also shows weak 1MLCT emission at ∼700 nm,
as shown in Figure 4. Because of the limits of our
instrumentation (one detector cuts off at >800 nm) we
are not able to detect any emission from the 1MLCT state
of the thienylthiocarboxylate of tungsten, W2ThCOS.
(5) Barybin, M. V.; Chisholm, M. H.; Patmore, N. J.; Robinson, R. E.;
Singh, N. Chem. Commun. 2007, 3652.
(6) Chisholm, M. H.; Epstein, A. J.; Gallucci, J. C.; Feil, F.; Pirkle, W.
Angew. Chem., Int. Ed. 2005, 44, 6537.
(7) Burdzinski, G. T.; Chisholm, M. H.; Chou, P. T.; Chou, Y. H.; Feil,
F.; Gallucci, J. C.; Ghosh, Y.; Gustafson, T. L.; Ho, M. L.; Liu, Y.;
Ramnauth, R.; Turro, C. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 15247.
(8) Alberding, B. G.; Chisholm, M. H.; Chou, Y.-H.; Gallucci, J. C.;
Ghosh, Y.; Gustafson, T. L.; Patmore, N. J.; Reed, C. R.; Turro, C. Inorg.
Chem. 2009, 48, 4394.
(9) Lear, B. J., Chisholm, M. H. Inorg. Chem., published ASAP October 27,
2009 (ic900995u).

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11189
Figure 2. Orbital interactions between filled M2-δ and empty carboxylate π* and filled carboxylate π orbitals.
Figure 3. UV-vis absorption spectra of M2(TiPB)2(OXC-2-Th)2com-
pounds; orange for Mo2ThCO2; pink for Mo2ThCOS; blue for
W2ThCO2; and green for W2ThCOS.
Figure 4. Emission spectra of M2(TiPB)2(OXC-2-Th)2 compounds at
room temperature; orange for Mo2ThCO2 with λex = 450 nm; pink for
Mo2ThCOS with λex = 550 nm; blue for W2ThCO2 with λex = 560 nm.
recent report from our lab has shown, that, the 3WWδδ*
based emission has its λmax at ∼820 nm and exhibits
prominent vibronic features of ∼390 cm-1 attributable
toa υ(WW) stretching. Onthe basis ofthe energyofλmax of
triplet emission for W2ThCO2 and absence of aforemen-
tionedvibronicspacingsinthe emissionpeak, weassignthe
emissive state to be a 3MLCT state.
Scheme 1. Synthesis of Th-2-COSH Ligand and M2(TiPB)2(OSC-2-
Th)2 Compounds
For W2ThCOS we see very weak near-infrared (NIR)
emission only at low temperature (77 K), centered at
∼1090 nm, which we assign to be arising from the 3MLCT
state, using the same arguments as used for W2ThCO2.
The NIR emission spectra for the three emissive com-
pounds are shown in Figure 5.
At this point it is worth noting that although the
substitution of S for O leads to a significant red shift for
the 1MLCT absorption, the triplet emission for the
tungsten compounds, W2ThCOS and W2ThCO2, occurs
at a similar energy. The Stoke’s shift for the compounds
W2ThCOS and W2ThCO2 for singlet absorption and
triplet emission are 4600 and 6650 cm-1, respectively. A
similar effect is seen for the molybdenum compounds
where the Stokes shift of the 1MLCT absorption and S1
emissive states are 2500 and 4680 cm-1 for the -OSC and
-O2C containing compounds, Mo2ThCOS and Mo2Th-
CO2, respectively.
For all four compounds mentioned, excitation spectra for
the singlet emission indicated that the emissions arise from
the compounds and not from any impurities. With a NIR
detector that operates in the range 800-1600 nm, we
observed strong room temperature phosphorescence for
both Mo2ThCO2 and Mo2ThCOS; at ∼1100 nm and at 77
K these show vibronic features due to υ(Mo-Mo) ∼ 400
cm-1 as shown in Supporting Information, Figure S3 for
Mo2ThCOS. On the basis of earlier reports on related
compounds,7,8,10 we assign these emissions as arising
from the 3MMδδ* state. A triplet emission at ∼1070 nm
is also observed for the tungsten compound, W2ThCO2. A
Hence, we see that energy and character of the photo-
excited states depend on the metal center (Mo or W) as
well as on the heteroatom X in the co-ordinating -OXC
(10) Chisholm, M. H.; Chou, P.-T.; Chou, Y.-H.; Ghosh, Y.; Gustafson,
T. L.; Ho, M.-L. Inorg. Chem. 2008, 47, 3415.

11190 Inorganic Chemistry, Vol. 48, No. 23, 2009
Alberding et al.
Figure 5. NIR emission spectra for M2(TiPB)2(OXC-2-Th)2 com-
pounds at room temperature; where orange for Mo2ThCO2 with λex
Figure 6. Gaussview plots of frontier orbitals drawn with iso-surface
value = 0.02 for M2(O2CH)2(XOC-2-Th)2compounds, where, M = Mo
or W and X = O or S.
=
405 nm; pink for Mo2ThCOS with λex = 532 nm; and blue for W2ThCO2
with λex = 675 nm.
functionality. This trend is reflected in the energies of
the LUMOs as obtained from electronic structure cal-
culations (vide infra). Additionally, Mo2ThCOS clearly
shows two reduction waves separated by 320 mV.
Table 1. Oxidation and Reduction Potentials for M2(TiPB)2(OXC-2-Th)2
Compounds, Where M = Mo or W and X = O or S, in THF, versus
Cp2Fe0/þ Couple
Electronic Structure Calculations. To obtain a more
quantitative insight into the bonding, spectroscopies, and
electrochemical data of these compounds, we carried out
electronic structure calculations employing density func-
tional theory (DFT).11-14 In these calculations formate
ligands were substituted for the TiPB ligands, and the
model compounds were thus trans-M2(O2CH)2(OXC-2-
Th)2, where M = Mo or W and X = O or S. Because of
the large dihedral angle involving the C6 ring and the
-O2C groups of the TiPB plane, there is relatively little
M2δ to TiPB back-bonding, so these model compounds,
with formates as ancillary groups, allow us to interrogate
the nature of the M2δ-thienylcarboxylate and thienylthio-
carboxylate bonding. Calculations were carried out on six
possible geometries of the model compounds, trans-M2-
(O2CH)2(OXC-2-Th)2. These are depicted in Supporting
Information Figure S4, which shows six different ways of
trans-orientation of the ThCOS- ligands about the MM
quadruple bond. The highest energy geometry was Cs
while the lowest energy geometry was C2h and the energy
difference between these two conformers is only 0.22 kJ/
mol. These small energy differences between the various
ground state geometries imply there will be very little
difference in the nature and energy of the frontier orbitals
among the various conformers. That is why the lowest
energy geometry of C2h was chosen when further calcula-
tions were performed.
oxidation
potential (V)
reduction
potential 1 (V)
compounds
Mo2(TiPB)2(OSC-2-Th)2
W2(TiPB)2(OSC-2-Th)2
Mo2(TiPB)2(O2C-2-Th)2
W2(TiPB)2(O2C-2-Th)2
0.16
-2.38
-2.25
-2.88
-2.81
-0.32
0.056
-0.56
functionality. Mo2ThCOS and Mo2ThCO2 indicate the
1
3
presence of two emissive states, MLCT and MMδδ*.
The two ditungsten compounds, W2ThCOS and
W2ThCO2, also indicate the presence of weakly emitting
photoexcited states, a higher energy 1MLCT and a lower
energy 3MLCT state.
Electrochemical Studies. The electrochemical proper-
ties of the four compounds were examined in THF by
both cyclic voltammetry and differential pulse voltam-
metry. A summary of redox potentials relative to the
Cp2Fe0/þ couple is given in Table 1. All compounds show
reversible oxidation waves that can readily be assigned to
the removal of an electron from the M2δ orbital. That it is
easier to oxidize the W2- relative to the Mo2-containing
compounds is consistent with the expected relative M2δ
orbital energies. This is also seen in the parent M2(TiPB)4
compounds.8 However, that it is easier to oxidize the
carboxylate compounds, Mo2ThCO2 and W2ThCO2, by
>200 mV relative to the thiocarboxylate compounds,
Mo2ThCOS and W2ThCOS, respectively, is perhaps less
expected and certainly noteworthy. It implies that the
substitution of S for an O, in the -O2C functionality,
stabilizes the M2δ orbital, and this is supported by
electronic structure calculations (vide infra). The com-
pounds also show reduction waves associated with reduc-
tion of the thienyl ligands. This is equivalent to putting an
electron in the empty thienyl ligand based π* orbital, the
lowest unoccupied molecular orbital (LUMO). The first
reduction becomes easier on going from Mo2ThCO2 to
Mo2ThCOS and from W2ThCO2 to W2ThCOS. This
implies that the thienyl ligand based π* orbital gets
stabilized upon substitution of an O with S in the -O2C
Gaussview15 plots of the respective HOMO, HOMO-
1, LUMO, and LUMOþ1 orbitals are shown in Figure 6
and these, together with the frontier molecular orbital
(11) Hohenberg, P.; Kohn, W. Phys. Rev. 1964, 164.
(12) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 137, 1697.
(13) Parr, R. G.; Yang, W. Density-functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989; p 333 ff.
(14) The Challenge of d and f Electrons: Theory and Computation; Salahub,
D. R., Zerner, M. C., Eds.; ACS Symposium Series 394; Developed from a
Symposium at the 3rd Chemical Congress of North America Toronto, Canada,
June 5-11, 1988; American Chemical Society: Washington, DC, 1989; p 405 ff.
(15) Dennington, R., II; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W.
L.; Gilliland, R. GaussView; Semichem Inc: Shawnee Mission, KS, 2003.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11191
Table 2. Lifetimes of Singlet and Triplet Excited States Determined from fs and
ns TA Spectroscopic Measurements
compounds
τ
1MLCT (ps)
τ3MMδδ* (μs)
τ3MLCT (ns)
Mo2(TiPB)2(OSC-2-Th)2 1.99 ( 0.06a 51b
Mo2(TiPB)2(O2C-2-Th)2 4.1c 77c
W2(TiPB)2(OSC-2-Th)2
W2(TiPB)2(O2C-2-Th)2
0.43 ( 0.01d
0.59 ( 0.01f
3-10e
3-10e
a λex = 514 nm, kinetics monitored at 370 nm. b λex = 355 nm, kinetics
monitored at 600 nm. c Inserted from reference 7. d λex = 675 nm, kinetics
f
monitored at 400 nm. e Instrument response limited. λex = 675 nm,
kinetics monitored at 375 nm.
molybdenum and tungsten compounds when one O in
the -O2C functionality is replaced with S. This stabiliza-
tion effect on the thienyl π* based orbitals was again
reflected in the reduction potentials of the compounds.
The calculations thus support the observed trends seen in
the metal based oxidation and the thienyl based reduction
1
potentials and also in the trends seen for the MLCT
bands. These subtle differences in energies of the frontier
orbitals between molybdenum and tungsten arise because
of the relative orbital energies of the M2δ to -OSC π
system. In the case of molybdenum the enhanced back-
bonding, M2δ to -OSC π*, is more than compensated by
the S 3p to Mo2 δ filled-filled interaction with the net
result that the Mo2δ is slightly raised in energy relative to
the -O2C complex. However, for tungsten, the higher
energy of the W2δ orbital is further removed in energy
from the filled S 3pπ orbital and closer to the -OSCπ*
orbital. Here the greater W2δ to -OSC π* backbonding
leads to a stabilization of the W2δ relative to the -Ο2C
analogue.
Figure 7. Orbital energy level diagram for M2(O2CH)2(OXC-2-Th)2
model compounds, where, M = Mo or W and X = O or S.
energy level diagram shown in Figure 7, reveal some very
interesting features.
The HOMO is in all cases the M2δ orbital. The HOMO-
1 and HOMO-2 are MMπ bonding orbitals for both
tungsten complexes, whereas, for Mo2ThCOS, the
HOMO-1 and HOMO-3 are in- and out-of-phase combi-
nations of filled thienylthiocarboxylate π orbitals and the
HOMO-2 is a MMπ orbital. For Mo2ThCO2, the
HOMO-1 and HOMO-2 are MMπ bonding orbitals
(see Supporting Information, Figure S5 for additional
frontier orbitals for the M2(O2CH)2(COX-2-Th)2 com-
pounds).
An examination of the energy levels of the frontier
molecular orbitals, as depicted in Figure 7, reveals that
the substitution of S for O in the -O2C functionality
causes the energy levels of the HOMO, HOMO-1, and
HOMO-2 to remain essentially the same in the molybde-
num compounds. However, in the tungsten compounds
this substitution leads to lowering of the energies and
consequent stabilization of the aforementioned orbitals.
This stabilization of the HOMO was reflected in the
experimentally observed oxidation potentials of the com-
pounds.
For the tungsten compounds, the LUMO and the
LUMOþ1 are respectively the in-phase and out-of-phase
combinations of the lowest energy π* orbitals of the
thienylcarboxylate and thiocarboxylato ligands and the
energy splitting of these is a measure of the W2δ to ligand
π* interaction. As can be seen in Figure 6, the LUMOþ1
contains some contribution from the M2δ.
For molybdenum, the LUMO is the M2δ* based
orbital for Mo2ThCO2 but ligand π* based for Mo2Th-
COS, with some admixture from Mo2δ*. The LUMOþ1
is mostly Mo2δ* for the thiocarboxylate and ligand π* for
the carboxylate.
Transient Absorption Spectroscopy. All four com-
pounds have been examined in THF solutions by both
femtosecond (fs) and nanosecond (ns) transient absorp-
tion (TA) spectroscopy. The compounds were excited
into their 1MLCT bands, and the consequent TA spectra
were monitored. The data are summarized in Table 2. All
four compounds showed transients with short lifetimes
ranging from 400 fs to 4 ps, which we assign to decay of
1
the MLCT based S1 state. It should be noted that the
samples of type M2ThCOS contain a mixture of isomers,
as mentioned above, some of which lack a center of
inversion. While there is some evidence suggesting the
presence of a center of inversion can have a marked effect
on the observed dynamics compared to the absence of a
center of inversion, the results obtained herein are gen-
erally similar (in terms of the number of transient species
and decay components observed) to those of homoleptic
complexes reported previously8 which do possess inver-
sion symmetry. Therefore, in these cases, the observed
dynamics is an average over all species but there is little
effect between the various isomers. The fs TA spectra and
the corresponding decay kinetics for Mo2ThCOS,
W2ThCOS, and W2ThCO2 are depicted in Figure 8, while
the ns TA spectrum for Mo2ThCOS is shown in Figure 9.
The time-resolved spectroscopic studies on Mo2ThCO2
have been recently reported,7 and the singlet and triplet
lifetimes observed for Mo2ThCO2 are compared with the
rest of the compounds.
One common feature present in the fs TA spectra for
each of the studied complexes is the presence of excited
state absorption bands belonging to two distinct excited
Going back to Figure 7, we see that the energies of the
LUMO and LUMOþ1 orbitals are lowered for both

11192 Inorganic Chemistry, Vol. 48, No. 23, 2009
Alberding et al.
Figure 8. Femtosecond TAspectra (left) and correspondingdecay kinetics(right) for compounds (a) Mo2ThCOS, withλex = 514 nm; (b) W2ThCOS, with
ex = 675 nm, and (c) W2ThCO2 with λex = 514 nm in THF at room temperature.
λ
states. Those bands which are present immediately upon
excitation into the MLCT absorption and those which
observed, with lifetime ∼11.5 ps, and is attributed to
vibrational cooling within the triplet state.
1
persist on the nanosecond time scale or longer are in-
dicative of these states. This is most clearly evident for
Mo2ThCOS, as shown in Figure 8, panel a, where bands
1a and 2a are observed directly after excitation but decay
to give rise to band 3a. Specifically, bands 1a and 2a decay
with lifetimes of ∼1.35 and 1.99 ps respectively while
band 3a is observed to concurrently grow in on a similar
time scale of ∼3.0 ps. This result is interpreted as the
direct conversion between the singlet and triplet mani-
folds. Additional kinetics of band 2a should also be
noted. Here, both the singlet and triplet states have a
positive absorption as indicated by the long time offset in
the kinetic trace. A second component to the kinetics is
The fs TA studies on compound Mo2ThCO2 have been
previously reported, and it was found that it possessed a
short-lived singlet excited state, with lifetime of ∼4 ps.
The nature of the S1 excited state was determined to be
1MLCT. This agrees very well with our observations for
Mo2ThCOS, and indicates the similarity between these
two. The lifetime of S1 state is found to decrease on going
from Mo2ThCO2 to Mo2ThCOS.
In the cases of W2ThCOS and W2ThCO2, the excited
state absorption bands belonging to the singlet and triplet
states are much more convoluted but still distinctly pre-
sent. Initially for W2ThCOS (Figure 8, panel b), a broad
absorption band, 1b, and ground state bleach 2b are

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11193
energy emission for each of the complexes in the NIR. For
the Mo2 containing complexes, the triplet state lifetimes
are too long (see below) to be estimated in the fs TA
experiment. However, Figure 8a shows that the TA signal
does not return to the baseline, consistent with the pre-
sence of a long-lived state. The long-lived T1 states
3
correspond to the MMδδ* states, which decay radia-
tively by emitting at ∼1100 nm. These emissions were
solvent independent and showed vibronic features at
77 K, corresponding to υ(MoMo) stretching mode
(See Supporting Information, Figure S3).
On the basis of the observation from the combined TA
studies, it can be concluded that both W2ThCOS and
W2ThCO2 possess triplet excited states, which evolve
upon direct intersystem crossing from the singlet excited
state. The short lifetimes of these T1 states (3 -20 ns) and
the energy of emission from the T1 states unequivocally
points to them being 3MLCT based. As can be seen from
Table 2, the T1 lifetimes for tungsten are notably shorter
than for molybdenum even though they could not be
accurately determined. The lifetimes of triplet states also
increase on going from Mo2ThCOS to Mo2ThCO2. This
is consistent with the introduction of the heavier element,
W versus Mo and S versus O, which introduces greater
spin-orbit coupling. Also, the lower energy of T1 states
with the heavier elements enhances non-radiative decay.
Figure 9. Nanosecond TA spectrum for the compound Mo2(OSC-2-
Th)2(TiPB)2.
observed. Band 1b decays into a more broadened absorp-
tion band 3b that covers the same region of the spectrum
and is persistent throughout the experiment. Monitoring
the kinetics at 500 nm shows an initial decay of 0.43 ps and
a much longer component of 2.5 ((1.1) ns. Similarly,
W2ThCO2 initially possesses the broad absorption band
1c which decays into the narrower, only slightly blue-
shifted band 3c. Monitoring the kinetics at 375 nm shows
again two components, an initial decay of 0.59 ps and a
longer component of 1.0 ((0.5) ns. For both of the
tungsten compounds, the short and long-lived compo-
nents are attributed to the singlet state lifetime and partial
ground state recovery from the triplet state, respectively.
Additionally, the recovery of the MLCT bleach with
lifetimes similar to the aforementioned long components
supports the assignment as the process converting the
triplet state to the ground state. Overall, changing the
metal center leads to a significant reduction in both the
singlet and triplet state lifetimes.
ns TA also revealed the presence of longer lived tran-
sients, which are assigned to triplet states. The ns TA
spectra for the dimolybdenum compound Mo2ThCOS is
shown in Figure 9 while that for the recently reported
Mo2ThCO27 is shown in Supporting Information, Figure
S6. It is worthy of note that the TA spectra corresponding
to the two molybdenum compounds are very similar, and
the triplet excited states possess comparable lifetimes, in
the range of ∼50-70 μs. Lifetimes of the triplet excited
states of the ditungsten compounds, W2ThCOS and
W2ThCO2, could not be accurately determined from their
ns TA measurements because of instrument limita-
tions (fwhm ∼ 8 ns). However fs TA measurements have
revealed the presence of a long-lived excited state in both
W2ThCOS and W2ThCO2 with lifetimes of 1-2.5 ns (vide
supra).
Concluding Remarks
The effect of substitution of S for O in the carboxylate
linker has a pronounced effect on the electro-optical proper-
ties of these compounds. Notably the 1MLCT is red-shifted
and the redox potentials are also affected as a result of
stabilization of the M2δ orbital. The latter effect is offset
for molybdenum because the S 3p and Mo2 δ orbitals are
close in energy leading to a filled-filled orbital interaction
which compensates for the enhanced back-bonding. The
replacement of an O by S in the -O2C moiety also decreases
the lifetimes of the S1 states, presumably because of enhanced
spin-orbit coupling which facilitates intersystem crossing. A
notable difference between molybdenum and tungsten is seen
in the nature of T1 states that we assign to 3MoMoδδ* and
3WLCT. For tungsten, the 3WLCT states have lifetimes
between 3 and 20 ns, which are notably shorter than the
1.6 μs for the 3WWδδ* of W2(TiPB)4.8
Experimental Section
Measurements. NMR spectra were recorded on a 400 MHz
Bruker DPX Advance400 spectrometer. All 1H NMR chemical
shifts are in ppm relative to the protio impurity in THF-d8 at
3.58 ppm.
Electronic 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.
The two excited states observed in the fs TA experi-
ments are also consistent with the emissive nature of each
of the complexes. Mo2ThCO2 is dual emissive and the
luminescent states are correlated with the singlet and
triplet states observed in the TA experiments. However,
in Mo2ThCOS, the -COS functionality provides better
coupling to the metal center, reducing the singlet state
lifetime to ∼2 ps, and, consequently, the MLCT fluores-
cence is weaker. Similarly, W2ThCO2 exhibits very weak
fluorescence and has a longer singlet state lifetime (∼0.59
ps) than W2ThCOS (∼0.43 ps), which possesses only one
emissive state.
The cyclic voltammogram and differential pulse voltammo-
gram of all the complexes were collected at a scan rate of 100 mV
s-1 and 5 mV s-1, respectively, using a Princeton Applied
Research (PAR) 173A potentiostat-galvanostat equipped with
a PAR 176 current-to-voltage converter. Electrochemical mea-
surements were performed under an inert atmosphere in a 0.5 M
solution of nBu4NPF6 in THF inside a single compartment
voltammetric cell equipped with a platinum working electrode,
a platinum wire auxiliary electrode, and a pseudoreference
n
The dynamics observed in the triplet state bands and in
the ns TA experiments can also be correlated to the lower
electrode consisting of a silver wire in 0.5 M Bu4NPF6/THF
separated from the bulk solution by a Vycor tip. The potential

11194 Inorganic Chemistry, Vol. 48, No. 23, 2009
Alberding et al.
values are referenced to the FeCp2/FeCp2þ couple, obtained by
addition of a small amount of FeCp2 to the solution.
Synthesis of 2-Thiophenecarbothioic Acid (C5H4OS2 = Th-
COSH). 2-Thenoyl chloride (0.15 mL, 1.4 mmol) was dissolved
in dry benzene and then added dropwise to a suspension of
thioacetamide (0.13 g, 1.7 mmol) in benzene. This was then
stirred at 35 °C for 3 h after which an equal volume of 10% KOH
(w/w) was added to it. This was allowed to stir for another
30 min. After separating the two layers, dil. HCl acid was added
to the aqueous layer. The organic droplets were extracted with
CH2Cl2 and then evaporated. The oily liquid was used in situ for
the next step.
Nanosecond TA measurements were carried out in 1 ꢀ 1 cm
square quartz cuvettes equipped with Kontes stopcocks. Nano-
second TA spectra were measured on a home-built instru-
ment pumped by a frequency doubled (532 nm) or tripled
(355 nm) Spectra-Physics GCR-150 Nd:YAG laser (fwhm
∼ 8 ns, ∼ 5 mJ per pulse). The signal from the photomultiplier
tube (Hamamatsu R928) was processed by a Tektronics
400 MHz oscilloscope (TDS 380).16
The steady-state NIR-luminescence measurements at 77 K
were carried out in J. Young NMR tubes. The spectra were
measured on a home-built instrument utilizing a germanium
detector. The sample was excited at 658 nm (laser diode max
power: 65mW), and a RG830 long pass filter was placed
between the sample and the detector.
Synthesis of Mo2(TiPB)2(OSC-2-Th)2. Mo2(TiPB)4 (0.62 g,
0.5 mmol) was dissolved in dry toluene and then added to the
ThCOSH acid (0.14 g, 1 mmol) in situ. The suspension was
stirred at room temperature for 3 days, at the end of which an
intense pink precipitate had formed. This was centrifuged and
washed with toluene (2 ꢀ 10 mL) before being dried in vacuo to
give 350 mg (72% yield) of a pink solid. Microanalysis found: C
51.81, H 5.34 C42H52Mo2O6S4 requires: C, 51.84; H, 5.39. NMR
(THF-d8): δH (400 MHz) 8.63 (d, 2H, JHH = 4 Hz), 7.69 (d, 2H,
JHH =4 Hz), 7.21 (d, 2H, JHH =4 Hz), 7.05 (s, 4H), 3.20 (m,
4H), 2.97 (m, 2H), 1.28 (d, 12H, JHH = 7 Hz), 1.14 (d, 24H,
JHH = 7 Hz) ppm. MALDI-TOF: Calculated monoisotopic
MW for C42H52Mo2O6S4: 973.0. Found: 970.91 (Mþ).
UV-vis-NIR (in THF, 293 K, values of ε (M-1 cm-1) are given
in parentheses): 545 (∼10000), 510 (∼11000), 290 (∼20000) nm.
Crystallographic Data. Space group: P21/c. Unit cell dimensions:
The femtosecond TA experiments were carried out using laser
and detection systems that have been previously described.17
The samples were prepared with absorbance ∼0.4-0.8 at the
excitation wavelength and were excited at 675 nm for W2-
(TiPB)2(O2C-2-Th)2 and 514 nm for Mo2(TiPB)2(OSC-2-Th)2
and W2(TiPB)2(OSC-2-Th)2 (with excitation power ∼ 1-2 μJ at
the sample). During the measurements, the samples were kept in
constant motion by manual movement of an XYZ stage in the
vertical and horizontal directions. To ensure that no photode-
composition occurred during data collection, absorption spec-
tra were recorded before and after the TA measurements. The
measurements were repeated four times at each of the pump-p-
robe delay positions to confirm data reproducibility throughout
the experiment, and the resulting spectra were corrected for the
chirp in the white-light super continuum.18 The kinetics were fit
ꢀ
˚
ꢀ
˚
ꢀ
˚
a = 10.936(1)A, b = 14.124(1)A, c=17.096(2)A, β = 97.318(3)°.
Synthesis of W2(TiPB)2(OSC-2-Th)2. W2(TiPB)4 (0.15 g,
0.12 mmol) was dissolved in dry toluene and then added to the
ThCOSH acid (0.288 g, 0.20 mmol) in situ. The suspension was
stirred at room temperature for 5 days, at the end of which a blue
precipitate had formed. This was centrifuged and washed with
toluene (2 ꢀ 10 mL) before being dried in vacuo to give 103 mg
(75% yield) of a blue solid. Microanalysis found: C 43.84,
H 4.49 C42H52W2O6S4 requires: C, 43.91; H, 4.56. NMR
(Benze ne-d6): δH (400 MHz) 7.60 (d, 2H, JHH = 4 Hz), 6.96
(s, 4H), 6.76 (d, 2H, JHH = 4 Hz), 6.68 (d, 2H, JHH = 4 Hz), 3.11
(m, 4H), 2.76 (m, 2H), 1.30 (d, 12H, JHH = 7 Hz), 1.19 (d, 24H,
JHH = 7 Hz) ppm. MALDI-TOF: Calculated monoisotopic
MW for C42H52W2O6S4: 1148.8. Found: 1148.0 (Mþ). UV-
vis-NIR (in THF, 293 K, values of ε (M-1 cm-1) are given in
parentheses): 720 (∼23000), 665 (∼17000), 390 (∼7000), 300
(∼13000) nm.
in general to a sum of exponential decay terms of the form,
P
S(t) = i Ai exp(-t/τi) þ C or to an exponential rise to maxi-
mum of the form S(t) = Ai(1 - exp(-t/τ)) þ C, with amplitude,
Ai, lifetime, τ, and offset, C, using Origin 6.0. Error bars for the
lifetimes are reported as the standard error of the exponential fit.
Microanalysis was performed by H. Kolbe Microanalytisches
Laboratorium, Germany.
MALDI-TOF was performed on a Bruker Reflex III (Bruker,
Bremen, Germany) mass spectrometer operated in a linear,
positive ion mode with an N2 laser. Dithranol was used as the
matrix and prepared as a saturated solution in THF. Allotments
of matrix and sample were thoroughly mixed together; 0.5 mL of
this was spotted on the target plate and allowed to dry.
All reactions were carried out under one atmosphere of
oxygen-free UHP-grade argon using standard Schlenck techni-
ques 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 glovebox or by
using standard Schlenck line techniques in an atmosphere of
oxygen-free UHP-grade argon. All solvents were dried over the
appropriate drying agent, distilled prior to use, and stored in
Theoretical Approaches. Electronic structure calculations on
the model compounds were performed by DFT11-14 with the aid
of the Gaussian0321 suite of programs. The B3LYP22,23 ex-
change correlation functional was used along with the 6-31G*
basis set for C, H, and O, 6- 31þG (2d) basis set for S, and
the SDD energy consistent pseudopotentials for molybdenum
and tungsten. Geometry optimizations were performed in
˚
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
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Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
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reservoirs equipped with Kontes taps over activated 4 A molec-
ular sieves, under an argon atmosphere and degassed prior to
use.
19
20
The starting materials Mo2(TiPB)4 and W2(TiPB)4 were
prepared according to published procedures. The compounds
M2(TiPB)2(ThCO2)2, where M = Mo or W were also synthe-
sized according to reported procedures.6
(16) Byrnes, M. J.; Chisholm, M. H.; Gallucci, J. A.; Liu, Y.; Ramnauth,
R.; Turro, C. J. Am. Chem. Soc. 2005, 127, 17343.
(17) Burdzinski, G.; Hackett, J. C.; Wang, J.; Gustafson, T. L.; Hadad, C.
M.; Platz, M. S. J. Am. Chem. Soc. 2006, 128, 13402.
(18) Nakayama, T.; Amijima, Y.; Ibuki, K.; Hamanoue, K. Rev. Sci.
Instrum. 1997, 68, 4364.
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Chem. Soc. 2000, 122, 416.
(22) Becke, A. D. Phys. Rev. A: Gen. Phys. 1988, 38, 3098.
(20) Santure, D. J.; Huffman, J. C.; Sattelberger, A. P. Inorg. Chem. 1985,
24, 371.
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Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11195
appropriate symmetry and were confirmed as local minima on
the potential energy surfaces using frequency analysis. Orbital
analyses were performed using Gaussview.15
computational resources. We also acknowledge Dr. Judith
Gallucci for examining the crystals of Mo2ThCOS.
Supporting Information Available: Listings of preliminary
ORTEP plot for Mo2ThCOS, variable temperature absorption
spectra for W2ThCO2, NIR emission spectra for Mo2ThCOS, ns
TA spectra for Mo2ThCO2, and Gaussview plots of frontier
orbitals for model compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the National Science Founda-
tion for support of this work through NSF CHE0515835,
and we also thank the Institute of Materials Research at The
Ohio State University for partial support of this work. The
Ohio Supercomputing Center is acknowledged for providing