Mo2 paddlewheel complexes functionalized with a single MLCT, S1 infrared-active carboxylate reporter ligand: Preparation and studies of ground and photoexcited states

Article abstract of DOI:10.1021/ic402775s

From the reactions between Mo₂(DAniF)₃pivalate (DAniF = N,N′-di(p-anisyl)formamidinate) and the carboxylic acids LH, the title compounds Mo₂(DAniF)₃L have been prepared and characterized: compounds I (L = O₂CC≡CPh), II (L = O ₂CC₄H₂SC≡CH), and III (L = O ₂CC₆H₄-p-CN). The new compounds have been characterized in their ground states by spectroscopy (¹H NMR, ultraviolet-visible absorption, near-infrared absorption, and steady state emission), cyclic voltammetry, and density functional theory calculations. The compounds show strong metal Mo₂ to ligand L δ-π* transitions in their visible spectra. The nature of the S₁ ¹MLCT and T₁ states has been probed by time-resolved (femtosecond and nanosecond) transient absorption and infrared spectroscopy. The observed shifts of the C≡C and C≡N vibrational modes are found to be consistent with the negative charge being localized on the single L in the S₁ states, while the T₁ states are ³Mo ₂ δδ*. The present results are compared to earlier studies of the photoexcited states of trans-Mo₂(2,4,6- triisopropylbenzoate)₂L₂ compounds that have been assigned as either localized or delocalized.

Full text of DOI:10.1021/ic402775s

Article
pubs.acs.org/IC
Mo2 Paddlewheel Complexes Functionalized with a Single MLCT, S1
Infrared-Active Carboxylate Reporter Ligand: Preparation and
Studies of Ground and Photoexcited States
Samantha E. Brown-Xu, Malcolm H. Chisholm,* Christopher B. Durr, Sharlene A. Lewis,
Thomas F. Spilker, and Philip J. Young
Department of Chemistry and Biochemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210, United States
S
* Supporting Information
ABSTRACT: From the reactions between Mo2(DAniF)3pivalate (DAniF =
N,N′-di(p-anisyl)formamidinate) and the carboxylic acids LH, the title
compounds Mo2(DAniF)3L have been prepared and characterized: compounds
I (L = O2CCCPh), II (L = O2CC4H2SCCH), and III (L = O2CC6H4-p-
CN). The new compounds have been characterized in their ground states by
spectroscopy (1H NMR, ultraviolet−visible absorption, near-infrared absorption,
and steady state emission), cyclic voltammetry, and density functional theory
calculations. The compounds show strong metal Mo2 to ligand L δ−π*
transitions in their visible spectra. The nature of the S1 1MLCT and T1 states has
been probed by time-resolved (femtosecond and nanosecond) transient
absorption and infrared spectroscopy. The observed shifts of the CC and
CN vibrational modes are found to be consistent with the negative charge
being localized on the single L in the S1 states, while the T1 states are 3Mo2 δδ*.
The present results are compared to earlier studies of the photoexcited states of
trans-Mo2(2,4,6-triisopropylbenzoate)2L2 compounds that have been assigned as either localized or delocalized.
complexes, respectively.12 We sought to answer these questions
INTRODUCTION
■
by incorporating ligands containing IR-active CX groups (X
Photoinduced metal-to-ligand charge transfer (MLCT) is
= C or N) and measuring the shift in ν(CX) in the excited
states and by performing calculations on the neutral complex
and on a singly reduced species in which the formal negative
charge is delocalized over both ligands. A more obvious and
practical comparison lies in examining the excited states of
molecules having only one π-acceptor ligand, and we herein
report studies prompted by this consideration.
fundamental to life in seemingly innumerable ways.1 Of
particular current interest are applications in the areas of
photovoltaics, photocatalysis, and photoluminescence.2−6 Car-
boxylate ligands attached to metal-to-metal (MM) quadruply
bonded centers exhibit strong MM-to-ligand δ−π* transitions
that can be tuned to span the entire visible region of the
spectrum and well into the near-infrared (NIR) region (400−
1200 nm) by the appropriate selection of MM (MM = Mo2,
MoW, or W2) and carboxylate.7,8 Interestingly, the photo-
excited states have relatively long S1 lifetimes, 1−20 ps, before
intersystem crossing leads to T1 states that for Mo2 complexes
are typically 3Mo2 δδ* but for W2 complexes are 3MLCT. This
has allowed us to probe both the S1 and T1 states, where S1 and
T1 are the lowest-energy singlet and triplet excited states,
respectively, by time-resolved femtosecond and nanosecond
spectroscopies and to monitor how charge delocalization
changes with time. We have previously examined compounds
of the general formula trans-M2L2L2′, where L′ is 2,4,6-
triisopropylbenzoate (Ti-PrB) and L is a π-acceptor ligand, or
where L′ is acetate and L is i-PrNC(CCR)Ni-Pr.9−11 In
these complexes, the lowest-energy singlet state is 1MLCT and
involves the π-acceptor ligands L. This immediately raises the
question of whether the negative charge is localized on one
ligand or delocalized over both. These can be viewed as
valence-trapped or delocalized excited-state mixed valence
RESULTS AND DISCUSSION
■
Syntheses. Compounds of the form trans-Mo2(Ti-PrB)2L2
have previously been prepared from the reactions between
Mo2(Ti-PrB)4 and 2 equiv of the carboxylic acids LH. Cis−
trans isomerization is not known to occur in these
compounds,13 and preparation of cis-carboxylates requires a
different methodology.14 Therefore these trans-substituted
complexes appear to represent a thermodynamic product as a
result of both relief of strain, which is present in the sterically
crowded Mo2(Ti-PrB)4 molecule, and of π-back bonding to the
planar conjugated carboxylate ligands, as this geometry
facilitates the ligand π-to-π coupling though the M2 center. A
prime example of this is seen in the structure of the trans-
azulene carboxylate ester complex whose structure is seen in
Received: November 5, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic402775s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1.13 Here the planar structure of the two trans ligands is
a result of the W2-to-ligand δ−π* conjugation. In previous
and other characterization data are summarized in the
Experimental Section.
Single Crystal and Molecular Structure of III (L =
O2CC6H4-p-CN). A view of the p-cyanobenzoate derivative
III in its molecular form as seen in the solid state is shown in
Figure 2. This confirms the expected paddle-wheel structure
and the presence of a single carboxylate ligand. The dihedral
angle between the O2C and C6H4 plane is ∼18°, which allows
for the Mo2-to-ligand δ−π* conjugation. The metric parame-
ters are given in the Supporting Information (Table S1) and are
20
4+
typical for a quadruply bonded Mo2 unit.
Electronic Structure Calculations. To aid in the
interpretation of the spectroscopic data, vide infra, we have
carried out calculations on the model compounds Mo2(HNC-
(H)NH)3L. The substitution of NH for N-p-anisole reduces
computational time, and the results of the calculations reflect
the trends expected as a function of the carboxylate ligands L.
The model compounds of I, II, and III are denoted as I′, II′,
and III′, respectively. The HOMO in each compound is
principally the Mo2 δ orbital but with significant mixing of the
ligands. In comparison to tetracarboxylate complexes the Mo2 δ
orbital is raised in energy by ∼1 eV. The amidinate ligands are
higher in energy because of the less electronegative nature of
nitrogen relative to oxygen. This results in greater mixing of the
filled NCN π-orbitals with the Mo2 δ orbital. A similar effect is
also seen on the Mo2 δ* orbital, which again is notably higher
in energy because of interaction with the NCN nonbonding
orbital. Below the HOMO-1, which is a filled NCN π
combination, are the Mo2 π and L π orbitals that are close in
energy. In all cases, the LUMO is principally carboxylate ligand
π* in character, and the LUMO+1 is an orbital which has both
Mo2 σ* and 5s character. The LUMO+2 is the Mo2 δ* orbital.
The HOMO−LUMO gap follows the order of ligands such that
the gap is largest for L = O2CCCPh and smallest for L =
O2CC6H4CN. Thus the lowest-energy electronic transition is
primarily a fully allowed Mo2-to-ligand δ−π* 1MLCT transition
that readily accounts for the colors of compounds I (orange), II
(orange-red), and III (purple).
Figure 1. Molecular representation of trans-W2(Ti-PrB)2L2, where L =
6-carboethoxy-2-azulene carboxylate in the solid state.19 Thermal
ellipsoids are drawn at the 50% probability level. Hydrogens have been
removed for clarity. Black = tungsten; scarlet = oxygen; gray = carbon.
work, we observed that if 1 equiv of a carboxylic acid is allowed
to react with Mo2(Ti-PrB)4, only the bis-substituted product is
obtained.15 The preparation of a related M2(Ti-PrB)3L complex
is thus not favored on either thermodynamic or kinetic
grounds, as the carboxylate ligands are labile and prone to
ligand scrambling.16,17 In the present case, we have employed as
the starting material a complex with one labile carboxylate and
three kinetically persistent formamidinate ligands,
Mo2(DAniF)3(piv), where DAniF = N,N′-di(p-anisyl)-
formamidinate and piv = pivalate. A similar compound,
Mo2(DAniF)3(O2CCH3), was employed by Cotton, Murillo,
and co-workers in reactions with dicarboxylic acids leading to
linked dinuclear compounds.18 The reaction leading to the
compounds is shown in eq 1 below (THF = tetrahydrofuran).
25°C
Mo2(DAniF)3(piv) + LH ⎯⎯⎯⎯→ Mo2(DAniF)3L + pivH
THF
(1)
The frontier molecular orbital energies for the model
compounds are shown in Figure 3 along with the GaussView
isosurface contour plots for the cyanobenzoate. The LUMOs of
the three compounds are shown in Figure 4 to more clearly
represent the nature of the L π* orbitals.
Electronic Absorption and Steady State Emission
Spectra. The ultraviolet−visible (UV−vis) spectra of
The new compounds I, II, and III, where L = O2CCCPh,
O2CC4H2SCCH, and O2CC6H4-p-CN, respectively, are
orange (I), orange-red (II), and purple (III) crystalline solids
that are soluble in toluene and polar solvents such as THF,
dimethyl sulfoxide (DMSO), and CH2Cl2. They show
molecular ions in the matrix-assisted laser desorption time-of-
1
flight mass spectrometry (MALDI-TOF MS), and H NMR
Figure 2. Two views of the molecular structure of compound III in the solid state. Thermal ellipsoids are drawn at the 50% probability level.
Hydrogens have been removed for clarity. Teal = molybdenum; blue = nitrogen; scarlet = oxygen; gray = carbon.
B
dx.doi.org/10.1021/ic402775s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Frontier orbital energy diagram comparing I′, II′, and III′. GaussView 5.0.8 isosurface contour plots for the molecular orbitals of III′ are
also shown.
Figure 4. GaussView 5.0.8 contour plots of the LUMO of model compounds I′, II′, and III′.
Mo2(DAniF)3(piv) and compounds I, II, and III are shown in
Figure 5, and the calculated λmax from time-dependent density
functional theory (TD-DFT) calculations are compared with
the observed λmax values for compounds I, II, and III in Table
1. The calculated values pertain to molecules with O2C-ring
Table 1. λmax of Calculated and Observed MLCT Absorption
Band for Compounds I, II, and III
compound
calculated λmax, nm
observed λmax, nm
I
516
558
568
444
488
498
II
III
dihedrals of 0°, which thus maximizes the degree of Mo2-to-
ligand δ−π* overlap and lowers the energy of the HOMO−
LUMO gap to a minimum. In solution at room temperature
there will be a Boltzmann distribution of O2C−Ar dihedral
angles with a weighted average of 0°. It is thus not surprising
that the calculated λmax are somewhat red-shifted when
compared to the observed.
Note that the intensities of these MLCT transitions are
notably less than those seen for compounds of the form trans-
Mo2(Ti-PrB)2L2. It is reasonable to assume that, to a first
approximation, the intensity of the MLCT correlates inversely
with the energy gap of the interacting orbitals but is directly
proportional to the degree of overlap of the same.21,22
Therefore in these formamidinate complexes the mixing of
the NCN π system with the Mo2 δ orbital decreases the
interaction between the Mo2 δ and the CO2 π system relative to
compounds of the form Mo2(O2CR)4. This reduced orbital
Figure 5. Absorption spectra of Mo2(DAniF)3(piv) (black) and I
(orange), II (red), and III (purple) in THF. An expanded view of the
MLCT absorption bands is presented in the inset.
C
dx.doi.org/10.1021/ic402775s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
overlap results in the observed decrease in intensity in the
MLCT transition.
how charge distribution changes in the excited states. Ground-
state IR spectra can be found in the Supporting Information
(Figure S2) as well as in the TRIR spectra. The spectra of
Mo2(DAniF)3(piv) (Figure 7) provide a benchmark for
compounds I, II, and III and are described first.
The complexes show very weak singlet emission at room
temperature but emit strongly from the T1 states. All of the new
compounds show phosphorescence at ∼860 nm (Supporting
Information, Figure S1), as does Mo2(DAniF)3(piv), which we
3
assign to emission from the Mo2 δδ* state due to the ligand
independence of the emission. This is notably higher in energy
relative to that seen in Mo2 tetracarboxylates as a result of the
greater mixing of the Mo2 δ and δ* orbitals with the filled NCN
π orbitals. DFT calculations indicate that the lowest-energy
triplet state is 3Mo2 δδ* in nature. The fluorescence spectra for
compounds I, II, and III (Figure 6) can be readily assigned to
Figure 7. Femtosecond TRIR spectra of Mo2(DAniF)3(piv) in THF at
room temperature (RT), λex = 400 nm. The ground-state IR spectrum
is shown in the dashed line.
The principal features in the spectra of Mo2(DAniF)3(piv)
occur from 1600 to 1450 cm−1, where the νas(CO2) and
νas(CN2) vibrational modes in the ground and excited states are
found. Initially, bleaches of the νas(CN2) and νas(CO2) modes
are seen at 1540 and 1500 cm−1, respectively, along with a
transient absorption at 1490 cm−1 that is assignable to νas(CO2)
in the 1MLCT (S2) state . A weak signal assignable to νas(CN2)
is observed at higher energy relative to its position in the
ground state. Within 5 ps, the νas(CO2) band begins to shift to
higher energy, and a new band appears at ∼1525 cm−1. These
Figure 6. Fluorescence spectra of Mo2(DAniF)3(piv) (black) and
compounds I (orange), II (red), and III (purple) in THF at room
temperature.
1
emission from the MLCT state; however, for the pivalate
complex we present evidence, vide infra, that the S1 state is
1
features are evidence of the formation of the Mo2 δδ* state
23
1
likely Mo2 δδ*, as was seen in Mo2(piv)4.
1
from the MLCT state, which was observed previously in
Cyclic Voltammetric Studies. In THF, all three
Mo2(piv)4.23 Finally, over 70 ps, a νas(CN2) band at 1560 cm−1
grows in while νas(CO2) shifts to slightly higher energy. These
compounds show a quasi-reversible oxidation wave between
0/+
−0.15 and −0.29 V relative to the FeCp2 couple assignable
3
longer-lived features can reliably be associated with the Mo2
to the removal of an electron from the Mo2 δ orbital. The E1/2
values are notably shifted to lower potential when compared
with their trans-Mo2(Ti-PrB)2L2 counterparts, as expected on
the basis of the calculations. Oxidation of the formamidinate
ligands occurs at higher potentials (∼0.60 V). No reversible
reduction waves were assigned within the solvent window,
though for compound II an irreversible wave appears. The
electrochemical data are summarized in Table 2.
1
δδ* state formed following intersystem crossing from Mo2
δδ*.
Interestingly, in these tris-formamidinate complexes it is the
νas(CN2) stretch that is shifted to higher energy rather than the
νas(CO2) stretch, as was seen for Mo2(piv)4.23 Removal of an
electron from a δ orbital will reduce the degree of back-bonding
to the ligands and may contribute to the above observations. In
any event the calculations on the T1 state of this molecule
predict this relative positioning of the vibrations in the triplet
state. Similar long-lived features are seen for the other
compounds characteristic of a 3Mo2 δδ* T1 state, though
bands due to a 1Mo2 δδ* state are not observed for I, II, or III
Time-Resolved Spectroscopic Studies. Time-Resolved
Infrared Spectroscopy. The four compounds have been
examined by femtosecond time-resolved infrared (TRIR)
spectroscopy in the window of 2300−1350 cm−1 to learn
1
since the S1 state is MLCT. In these complexes, however, the
Table 2. Summary of Electrochemical Data for Compounds
main focus is examining the magnitude of the shift of ν(CX)
a
I, II, and III
1
from the ground state to the MLCT state in which electron
ox
density resides on the one carboxylate ligand.
compound
E
, V
Ered, V
−2.66
1/2
For compound I, where L = O2CCCPh, the femtosecond
TRIR spectra (Figure 8) in the higher-energy region display
weak features due to the ν(CC) in the S1 state at ∼1980
cm−1. When compared to the ground-state IR spectrum
(Supporting Information, Figure S2) this reflects a substantial
shift to lower energy (Δν(CC) = −245 cm−1) and is in
I
−0.15, 0.68
−0.29
−0.18, 0.61
II
III
a
+/0
Potentials are referenced to the FeCp2 couple. Data obtained in
THF at room temperature.
D
dx.doi.org/10.1021/ic402775s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
Again, following S1 decay (τ = 4.5 ps), the Mo2 δδ* state is
formed as denoted by the characteristic features from 1500 to
1600 cm−1.
The TRIR spectra for the cyanobenzoate complex III are
shown in Figure 10. The higher-energy region displays the
Figure 8. Femtosecond TRIR spectra of I in THF at RT, λex = 400
nm. The ground-state IR spectrum is shown in the dashed line.
agreement with previously studied compounds which were
assigned as having localized 1MLCT states. Namely, the
compounds trans-W2(Ti-PrB)2(O2CCCC6H4CH3)2 and
trans-Mo2(O2CMe)2[(PriN)2CCCPh]2 both showed a
Figure 10. Femtosecond TRIR spectra of compound III in THF at
RT, λex = 568 nm. The ground-state IR spectrum is shown in the
dashed line.
1
prominent CC stretch in their MLCT states shifted −240
cm−1 from the ground-state vibration.11,24 In contrast, the
compound trans-Mo2(Ti-PrB)2(O2CCC-9-anthracene)2 gave
a Δν(CC) of only −90 cm−1 and in this case was assigned as
bleach of the ground state ν(CN) at 2232 cm−1 and the
appearance of the singlet excited state ν(CN) at 2148 cm−1,
shifted 84 cm−1 from the ground state. The change in the
CN vibration is again greater than that observed for the
corresponding complex with two cyanobenzoate ligands, trans-
Mo2(Ti-PrB)2(O2CC6H4CN)2,11 consistent with the pre-
1
a delocalized MLCT state with the negative charge shared by
both trans ligands.24 For I, the 1MLCT state decays (τ = 2 ps)
3
to form the Mo2 δδ* state that shows evidence of ν(CC)
shifted to a slightly higher energy relative to the ground state.
The TRIR spectra for compound II (Figure 9) show the
appearance of a weak absorption band at 1970 cm−1 because of
1
vious assignment of a delocalized MLCT state. In the present
case, however, there is a new band visible at 1655 cm−1 that
appears to be at a higher energy relative to ground-state ligand
modes. This vibration we assign to ν(CC) of a ring mode
associated with the quinoidal nature of the excited state as
depicted below.
These features disappear after singlet state decay (τ = 5 ps)
in accordance with formation of a metal-based T1 state. The
values of ν(CX) in the present compounds are compared
with those in the related trans-Mo2L2L2′ compounds in Table 3
along with the values of Δν(CX) and those calculated for the
anions.
Transient Absorption Spectroscopy. The compounds
have been examined in THF solutions by both femtosecond
and nanosecond transient absorption (TA) spectroscopy, which
has allowed us to correlate the lifetimes of their excited states
with those estimated from the TRIR studies. The lifetimes for
compounds I and II agree well, and the TA spectra can be
found in the Supporting Information (Figures S4, S5, S8, and
S9). However, the TA spectra of Mo2(DAniF)3(piv) and
compound III warrant discussion.
The TA spectra for the pivalate complex in the region 350−
650 nm are shown in Figure 11. At ∼350 nm we see the
expected bleach associated with the 1MLCT. At short times less
than 5 ps, we observe the appearance of an absorption band at
Figure 9. Femtosecond TRIR spectra of compound II in THF at RT,
λex = 515 nm. The ground-state IR spectrum is shown in the dashed
line.
1
ν(CC) in the MLCT state. This is shifted ∼130 cm−1 to
lower energy relative to the ground state. This shift is twice the
magnitude of Δν(C≡C) of the related compound trans-
Mo2(Ti-PrB)2(O2CC4H2SCCH)2 that has two conjugated
carboxylate ligands and was assigned as a delocalized 1MLCT.25
E
dx.doi.org/10.1021/ic402775s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Summary of ν(CX) and Δν(CX) Values
experimental, cm−1
1MLCT state
ν(CC)*
calculated, cm−1
shift in anion
Δν(CC)
ground state
shift
compound
ν(CC)
Δν(CC)
I
2220
1975
240
a
b
Mo2(O2CMe)2[(PriN)2CCC-Ph]2
2200
2155, 1959
2113
45, 241
141
c
Mo2(Ti-PrB)2(O2CCC-9-An)2
2204
91
101
II
2100
1970
130
155
d
Mo2(Ti-PrB)2(O2CC4H2SCCH)2
2099
2033
66
57
ν(CN)
ν(CN)*
Δν(CN)
Δν(CN)
III
2230
2145
85
112
67
e
Mo2(Ti-PrB)2(O2CPhCN)2
2230
2157
73
a
b
c
1
Calculation on the anion returned with a negative frequency. Values from ref 11; localized MLCT. Values from ref 17; An = anthracenyl;
d
e
1
1
1
delocalized MLCT. Values from ref 18; delocalized MLCT. Values from ref 11; delocalized MLCT.
1
the MLCT state (τ = 3 ps), consistent with the decay of the
transient seen at 365 nm. There is also clearly a band at 430 nm
that shows a different kinetic decay. We propose that this arises
from an S1→Sm absorption that differs from the S1→Sn
absorption (n > m) at 365 nm. Within 5 ps this transient
decays to leave the longer-lived features associated with the T1
state. From the nanosecond TA spectra shown in the
Supporting Information (Figure S10) the lifetime of the T1
3Mo2 δδ* state is estimated to be ∼75 μs.
CONCLUSIONS
■
Studies of Mo2 complexes with a single IR-active group (CX)
provide a way to experimentally evaluate charge distribution in
excited states. Monofunctionalized complexes possess fully
localized excited states, which can be used to model electron
delocalization in higher-order systems. The shift of ν(CX) in
Figure 11. Femtosecond TA spectra of Mo2(DAniF)3(piv) in THF at
RT, λex = 350 nm.
1
460 nm. This early transient is assigned to the MLCT state
1
the MLCT states of I, II, and III are roughly twice the
that decays to the 1Mo2 δδ* state. A feature at 440 nm, because
of the decay of the 1Mo2 δδ* state into the long-lived 3Mo2 δδ*
state, grows in over 70 ps. This lifetime is comparable to that
seen for the homoleptic compound Mo2(piv)4.23 The isosbestic
point observed at 410 nm indicates direct conversion from the
singlet to the triplet δδ* state. The nanosecond TA spectrum is
given in the Supporting Information (Figure S7) from which
we can reasonably estimate the lifetime of the T1 3Mo2 δδ*
state to be ∼70 μs.
The femtosecond TA spectra for the para-cyanobenzoate
complex III are shown in Figure 12. The bleach of the 1MLCT
occurs from 450 to 525 nm. At lower energy, namely ∼660 nm,
there is a negative feature that is not reconcilable with the
1MLCT absorption but rather with stimulated emission from
magnitude of those seen for related trans-Mo2(Ti-
PrB)2(O2CR)2 compounds that have been assigned as
1
delocalized MLCT states. This supports our earlier assign-
ments of delocalized and localized excited states. The presence
of the three formamidinate ligands raises the energy of the Mo2
δ orbital relative to that of the tetracarboxylate complexes. This
is clearly seen in the electrochemistry and also in the emission
from the 3Mo2 δδ* state, which moves to higher energy. Finally,
we note here that the time-resolved spectra for the complex
1
Mo2(DAniF)3(piv) indicate that the S1 is Mo2 δδ* as was
found for Mo2(piv)4. Interestingly, internal conversion between
1
1
the MLCT state and the Mo2 δδ* state, which are close in
energy, is sufficiently slow that the former can be detected by
femtosecond spectroscopies.
EXPERIMENTAL SECTION
■
General Procedures. The Mo2 complexes were prepared
under an inert atmosphere (argon or nitrogen) using standard
glovebox and Schlenk line techniques. Solvents were dried
using standard methods and degassed before use if needed.
Phenylpropiolic acid and p-cyanobenzoic acid were purchased
26
from Sigma Aldrich. Mo2(piv)4 and 5-ethynylthiophene-2-
carboxylic acid27 were prepared by reported procedures.
Mo2(DAniF)3(piv) was prepared using an adapted procedure
in which Mo2(piv)4 was used instead of Mo2(O2CCH3)4.18 The
1H NMR spectra were recorded on a 400 MHz Bruker DPX
1
Advance 400 spectrometer. All H NMR chemical shifts are in
ppm relative to the protio impurity in THF-d8 at 3.58 ppm or
DMSO-d6 at 2.50 ppm.
Figure 12. Femtosecond TA spectra of compound III in THF at RT,
λex = 568 nm.
F
dx.doi.org/10.1021/ic402775s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Electronic Absorption Spectra. UV−vis-NIR electronic
spectra were measured in THF at room temperature with a
Perkin-Elmer Lambda 900 spectrometer using 1 cm2 quartz
cuvettes sealed with Kontes taps.
samples were prepared in a 1 cm quartz cuvette with Kontes
top with absorbance in the range of 0.1−0.3. Excitation power
at the sample was 100 mW.38 All samples were excited at a
wavelength of 355 nm.
Electrochemical Studies. Cyclic voltammograms were
collected at a scan rate of 100 mV s−1 with a Pine Research
Instrumentation Wavenow potentiostat−galvanostat. The
measurements were performed under a nitrogen atmosphere
in a 0.1 M THF solution of the supporting electrolyte tetra-n-
butylammonium hexafluorophosphate using platinum working
and auxiliary electrodes patterned on ceramic and a silver wire
pseudo reference electrode (Pine Research Instrumentation).
Kinetics for the time-resolved data was fit to a sum of
exponentials S(t) = ∑i Ai exp(−1/τi) + C, where Ai is the
amplitude, τI is the lifetime, and C is an offset, in Igor Pro 6.0 or
SigmaPlot 12.0. Standard errors of the exponential fits are given
as the error bars of the lifetimes.
Steady State Emission Spectra. The steady-state
luminescence spectra between 350 and 800 nm were acquired
on a SPEX Fluoromax-2 spectrofluorometer. The spectra were
measured in THF at room temperature and in 2-methylte-
trahydrofuran at 77 K. Samples were excited into their MLCT
bands. The steady-state near-IR luminescence spectra were
taken on a home-built instrument utilizing a germanium
detector. All measurements were recorded in 2-methyltetrahy-
drofuran with an excitation wavelength of 405 nm.
+/0
The potentials are referenced to the FeCp2 couple.
Mass Spectrometry. The MALDI-TOF mass spectrometry
experiments were performed on a Bruker Microflex mass
spectrometer. Dithranol was employed as the matrix.
Electronic Structure Calculations. Utilizing the Gaus-
sian09 suite of programs, DFT was used for the gas-phase
optimization of the model complexes.28,29 The B3LYP30,31
functional was used together with the SDD energy-consistent
pseudopotentials and the SDD energy-consistent basis set for
molybdenum, and the 6-31G* basis set was used for H, C, N, S,
and O atoms. Vibrational frequency analysis was used to ensure
that the optimized structures represented global minima on the
potential energy surface. GaussView 5.0.8 isosurface contour
plots are shown with an isovalue of 0.02.31 Electronic
absorption spectra were calculated using the TD-DFT method
with the same functional and basis set used for DFT
calculations.
Preparation of Mo2(DAniF)3(O2CCCPh), I. THF (ca. 5
mL) was added to a mixture of Mo2(DAniF)3(piv) (0.151 g,
0.143 mmol) and phenylpropiolic acid (0.0235g, 0.161 mmol).
An orange solution was immediately formed. This was stirred at
room temperature for 1 week. The solution was concentrated,
and hexanes were added to precipitate the orange product. The
mixture was washed once with ethanol and once with hexanes.
The residue was dried under vacuum to yield an orange
1
powder. Yield: 35%. H NMR (THF-d8, 400 MHz): 8.61 (s,
2H), 8.41 (s, 1H), 7.66 (d, 2H), 7.42 (d, 2H), 6.68 (d, 8H),
6.59 (d, 8H), 6.46 (d, 4H), 6.39 (d, 4H), 3.78 (s, 12H), 3.72 (s,
6H). MALDI-TOF: Calculated: 1102.9 Found: 1103.8. UV−vis
(THF, nm): 444, 264.
Crystallographic Information. Single crystals of III were
isolated as prismatic dark-purple crystals and handled under a
pool of fluorinated oil. Examination of the diffraction pattern
was done on a Nonius Kappa CCD diffractometer with Mo Kα
radiation. All work was done at 150 K using an Oxford
Cryosystems Cryostream Cooler. Data integration was done
with Denzo, and scaling and merging of the data was done with
SADABS.32 The structures were solved by the direct methods
program in SHELXS-13.33 Full-matrix least-squares refinements
based on F2 were performed in SHELXL-13,33 as incorporated
in the WinGX package.34 For each methyl group, the hydrogen
atoms were added at calculated positions using a riding model
with U(H) = 1.5Ueq (bonded carbon atom). The rest of the
hydrogen atoms were included in the model at calculated
positions using a riding model with U(H) = 1.2Ueq (bonded
atom). Neutral atom scattering factors were used and include
terms for anomalous dispersion.35
Preparation of Mo2(DAniF)3(O2CC4H2SCCH), II. THF
(ca. 5 mL) was added to a mixture of Mo2(DAniF)3(piv)
(0.131 g, 0.124 mmol) and 5-ethynylthiophene-2-carboxylic
acid (0.028 g, 0.184 mmol). A red-orange solution was
immediately formed. This was stirred at room temperature
for 1 week. The solution was concentrated and hexanes were
added to precipitate the red-orange product. The mixture was
washed once with ethanol and once with hexanes. The residue
was dried under vacuum to yield a red-orange powder. Yield:
1
51%. H NMR (DMSO-d6, 400 MHz): 8.55 (s, 1H), 8.43 (s,
2H) 7.65 (d, 2H), 7.42 (d, 2H), 6.68 (d, 8H), 6.59 (d, 8H),
6.46 (d, 4H), 6.39 (d, 4H), 3.66 (s, 12H), 3.61 (s, 6H).
MALDI-TOF: Calculated: 1108.9 Found: 1107.3. UV−vis
(THF, nm): 488, 289.
Time-Resolved Measurements. Femtosecond TA36 and
TRIR37 spectroscopy experiments were performed with a
Ti:sapphire and regenerative amplifier combination (1 kHz,
300 fs full width at half-maximum) that has been previously
described. In the femtosecond TA experiment, samples were
prepared with absorbances of ∼0.3−0.8 in a 1.0 mm quartz
cuvette with Kontes top. Excitation power at the sample was 1−
2 μJ. Spectra collected underwent wavelength calibration and
group velocity dispersion corrections. Mo2(DAniF)3(piv) and
compounds I, II, and III were excited at 350, 514, 514, and 568
nm, respectively. In the femtosecond TRIR experiment,
samples were prepared in THF with an absorbance of ∼1 at
λmax. The solutions were sealed in a PerkinElmer semi-
demountable cell with a 0.1 mm Teflon spacer between 4
mm CaF2 windows. Mo2(DAniF)3(piv) and compounds I, II,
and III were excited at 400, 400, 515, and 568 nm, respectively,
with a laser power of ∼1 μJ. In the nanosecond TA experiment,
Preparation of Mo2(DAniF)3(O2CC6H4-p-CN), III. THF
(ca. 5 mL) was added to mixture of Mo2(DAniF)3(piv) (0.1664
g, 0.157 mmol) and p-cyanobenzoic acid (0.0249 g, 0.169
mmol). A red-purple solution was immediately formed. This
was stirred at room temperature for 4 days. The solution was
concentrated and hexanes (ca. 5 mL) and ethanol (ca. 5 mL)
were added to precipitate the purple product. The mixture was
washed once with ethanol and once with hexanes. The residue
was dried under vacuum to yield a purple powder. Yield: 61%.
1H NMR (THF-d8, 400 MHz): 8.53 (s, 2H), 8.52 (s, 1H), 8.41
(d, 2H), 7.83 (d, 2H), 6.67 (d, 8H), 6.62 (d, 8H), 6.47 (d, 4H),
6.36 (d, 4H), 3.70 (s, 12H), 3.65 (s, 6H). MALDI-TOF:
Calculated: 1103.9 Found: 1102.6. UV−vis (THF, nm): 498,
297.
G
dx.doi.org/10.1021/ic402775s | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(19) Alberding, B. G.; Barybin, M. V.; Chisholm, M. H.; Gustafson,
T. L.; Reed, C. R.; Robinson, R. E.; Patmore, N. J.; Singh, N.; Turro,
C. Dalton Trans. 2010, 39, 1979−1984.
ASSOCIATED CONTENT
■
S
* Supporting Information
Table of crystallographic information, NIR emission spectra,
ground-state IR spectra, kinetic data for time-resolved IR
spectroscopy measurements, femtosecond and nanosecond
transient absorption spectra and associated kinetic traces.
This material is available free of charge via the Internet at
http://pubs.acs.org. CCDC 969746 contains the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif.
(20) Multiple Bonds between Metal Atoms, 3rd ed.; Cotton, F. A.;
Murillo, C. A.; Walton, Richard A., eds.; Springer Science and Business
Media, Inc.: New York, 2005.
(21) Mulliken, R. S. J. Am. Chem. Soc. 1952, 74, 811−824.
(22) Turro, N. J.; Ramamurthy, V.; Gallucci, J. C. Principles of
Molecular Photochemistry: An Introduction; University Science Books:
Sausalito, CA, 2009.
(23) Alberding, B. G.; Chisholm, M. H.; Gustafson, T. L. Inorg. Chem.
2012, 51, 491−498.
(24) Alberding, B. G.; Brown-Xu, S. E.; Chisholm, M. H.; Gallucci, J.
C.; Gustafson, T. L.; Naseri, V.; Reed, C. R.; Turro, C. Dalton Trans.
2012, 41, 12270−12281.
AUTHOR INFORMATION
■
(25) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Spilker, T. F. J.
Am. Chem. Soc. 2013, 135, 8254−8259.
Corresponding Author
*E-mail: chisholm@chemistry.ohio-state.edu.
(26) Brignole, A. B.; Cotton, F. A. Inorg. Synth. 1972, 13, 81−89.
(27) Suzuki, T.; Ota, Y.; Kasuya, Y.; Mutsuga, M.; Kawamura, Y.;
Tsumoto, H.; Nakagawa, H.; Finn, M. G.; Miyata, N. Angew. Chem.,
Int. Ed. 2010, 49, 6817−6820.
Notes
The authors declare no competing financial interest.
(28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, N. J.; Klene, M.; Knox, J. E.; 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.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation for financial
support from Grant No. 0957191 and the Ohio Super
Computer Center for computational resources. We also
acknowledge Professor Terry L. Gustafson and the Ohio
State University Center for Chemical and Biophysical
Dynamics for use of the laser systems and Professor Claudia
Turro for use of instrumentation. S.E. Brown-Xu acknowledges
support from the NSF GRFP.
REFERENCES
■
(1) Vlce
̌
k, A., Jr. Coord. Chem. Rev. 1998, 177, 219−256.
(2) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.;
Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.;
Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125,
12971−12979.
(29) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(30) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett.
1989, 157, 200−206.
(31) Dennington, R., II; Keith, T.; Millam, J.; Eppinnett, K.; Hovell,
W. L.; Gilliland, R. GaussView; Semichem Inc: Shawnee Mission, KS,
2003.
(3) Chisholm, M. H.; Epstein, A. J.; Gallucci, J. C.; Feil, F.; Pirkle, W.
Angew. Chem., Int. Ed. 2005, 44, 6537−6540.
(4) 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,
15247−15252.
(32) Sheldrick, G. M. SADABS, v2008/1 semi-empirical absorption and
beam correction program; University of Gottingen: Germany.
(33) Sheldrick, G. Acta Crystallogr. 2008, A64, 112−122.
(34) Farrugia, L. J. Appl. Crystallogr. 1999, 32, 837−838.
(35) Wilson, A. J. C.; Geist, V. International Tables for Crystallography,
Vol. C; Kluwer Academic Publishers: Dordrecht, Netherlands, 1992.
(36) Burdzinski, G.; Hackett, J. C.; Wang, J.; Gustafson, T. L.; Hadad,
C. M.; Platz, M. S. J. Am. Chem. Soc. 2006, 128, 13402−13411.
(37) Wang, J.; Burdzinski, G.; Kubicki, J.; Platz, M. S. J. Am. Chem.
Soc. 2008, 130, 11195−11209.
(5) Takeda, H.; Ishitani, O. Coord. Chem. Rev. 2010, 254, 346−354.
(6) Wong, W.-Y.; Ho, C.-L. Acc. Chem. Res. 2010, 43, 1246−1256.
(7) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Lewis, S. A.;
Naseri, V.; Spilker, T. F. Chem. Sci. 2013, 4, 2105−2116.
(8) Chisholm, M. H.; Gustafson, T. L.; Turro, C. Acc. Chem. Res.
2013, 46, 529−538.
(9) Brown, D. J.; Chisholm, M. H.; Gallucci, J. C. Dalton Trans. 2008,
1615−1624.
(10) Alberding, B. G.; Chisholm, M. H.; Gustafson, T. L.; Liu, Y.;
Reed, C. R.; Turro, C. J. Phys. Chem. A 2010, 114, 12675−12681.
(11) Alberding, B. G.; Chisholm, M. H.; Gallucci, J. C.; Ghosh, Y.;
Gustafson, T. L. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 8152−8156.
(12) Plummer, E. A.; Zink, J. I. Inorg. Chem. 2006, 45, 6556−6558.
(13) Barybin, M. V.; Chisholm, M. H.; Patmore, N. J.; Robinson, R.
E.; Singh, N. Chem. Commun. 2007, 3652−3654.
(38) Byrnes, M. J.; Chisholm, M. H.; Gallucci, J. A.; Liu, Y.;
Ramnauth, R.; Turro, C. J. Am. Chem. Soc. 2005, 127, 17343−17352.
(14) Cotton, F. A.; Liu, C. Y.; Murillo, C. A. Inorg. Chem. 2004, 43,
2267−2276.
(15) Chisholm, M. H.; Epstein, A. J.; Gallucci, J. C.; Feil, F.; Pirkle,
W. Angew. Chem., Int. Ed. 2005, 44, 6537−6540.
(16) Cayton, R. H.; Chacon, S. T.; Chisholm, M. H.; Folting, K.
Polyhedron 1993, 12, 415−422.
(17) Chen, H.; Cotton, F. A. Polyhedron 1995, 14, 2221−2224.
́
(18) Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Villagran, D.; Wang, X.
J. Am. Chem. Soc. 2003, 125, 13564−13575.
H
dx.doi.org/10.1021/ic402775s | Inorg. Chem. XXXX, XXX, XXX−XXX