Hydrogen bonding and electron transfer between dimetal paddlewheel compounds containing pendant 2-pyridone functional groups

Article abstract of DOI:10.1021/ic401555g

The compounds M₂(TiPB)₃(HDON) (TiPB = 2,4,6-triisopropylbenzoic acid; H₂DON = 2,7-dihdroxy-1,8- napthyridine; M = Mo (1a) or W (1b)) and Mo₂(TiPB)₂(O ₂CCH₂Cl)(HDON) (1c) which contain a pendant 2-pyridone functional group have been prepared. These compounds are capable of forming self-complementary hydrogen bonds, resulting in the formation of dimers of dimers ([1a-c]₂) in CH₂Cl₂ solutions. Electrochemical studies reveal two successive one-electron redox processes for [1a-c]₂ in CH₂Cl₂ solutions that correspond to successive oxidations of the dimetal core, indicating stabilization of the mixed-valence state. Only small changes in the value of K_c are observed upon changing the ancillary ligand or metal, implying that proton coupled mixed valency is responsible for the stabilization. Dimethylsulfoxide (DMSO) disrupts the hydrogen bonding interactions in these compounds, and a single oxidation process is observed in DMSO which shifts to lower potential as the number of HDON ligands increases. Further substitution of carboxylate ligands with HDON leads to the formation of Mo₂(TiPB) ₂(HDON)₂ (2) and Mo₂(HDON)₄ (3), which adopt trans-1,1 and cis-2,2 regioisomers in the solid-state. ¹H NMR spectroscopy indicates that there are at least two regioisomers present in solution for both compounds. The lowest energy transition in the electronic absorption spectra of these compounds corresponds to a M₂-δ → HDON-π* transition. The electrochemical, spectroscopic and structural results were rationalized with the aid of density functional theory (DFT) calculations.

Full text of DOI:10.1021/ic401555g

Article
pubs.acs.org/IC
Hydrogen Bonding and Electron Transfer between Dimetal
Paddlewheel Compounds Containing Pendant 2‑Pyridone Functional
Groups
Luke A. Wilkinson, Laura McNeill, Paul A. Scattergood, and Nathan J. Patmore*
Department of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K.
S
* Supporting Information
ABSTRACT: The compounds M2(TiPB)3(HDON) (TiPB =
2,4,6-triisopropylbenzoic acid; H2DON = 2,7-dihdroxy-1,8-
napthyridine; M = Mo (1a) or W (1b)) and
Mo2(TiPB)2(O2CCH2Cl)(HDON) (1c) which contain a
pendant 2-pyridone functional group have been prepared.
These compounds are capable of forming self-complementary
hydrogen bonds, resulting in the formation of “dimers of
dimers” ([1a−c]2) in CH2Cl2 solutions. Electrochemical
studies reveal two successive one-electron redox processes for
[1a−c]2 in CH2Cl2 solutions that correspond to successive
oxidations of the dimetal core, indicating stabilization of the
mixed-valence state. Only small changes in the value of Kc are observed upon changing the ancillary ligand or metal, implying that
proton coupled mixed valency is responsible for the stabilization. Dimethylsulfoxide (DMSO) disrupts the hydrogen bonding
interactions in these compounds, and a single oxidation process is observed in DMSO which shifts to lower potential as the
number of HDON ligands increases. Further substitution of carboxylate ligands with HDON leads to the formation of
1
Mo2(TiPB)2(HDON)2 (2) and Mo2(HDON)4 (3), which adopt trans-1,1 and cis-2,2 regioisomers in the solid-state. H NMR
spectroscopy indicates that there are at least two regioisomers present in solution for both compounds. The lowest energy
transition in the electronic absorption spectra of these compounds corresponds to a M2-δ → HDON-π* transition. The
electrochemical, spectroscopic and structural results were rationalized with the aid of density functional theory (DFT)
calculations.
Recent investigations by Chisholm and co-workers have also
used transient absorption and time-resolved infrared spectros-
copy to elucidate the nature of the singlet and triplet states in a
family of mixed ligand compounds of form trans-
M2(TiPB)2(L)2 (M = Mo, W; L = conjugated carboxylate or
amidinate bridging ligand).25,27,28 Interestingly, studies on these
compounds have revealed that in the metal-to-ligand charge-
transfer (MLCT) states, the single electron on the ligands (L)
is fully delocalized over both ligands.29,30 Delocalization is a
result of strong M2δ-Lπ conjugation, a property which has been
employed extensively to study ground state electron transfer in
“dimers of dimers”, which consist of two dimetal units bridged
by a conjugated ligand.
The redox, photophysical, and electron transport properties
of MM quadruple bonds have made them attractive targets for
incorporation into functional materials.31−34 Dicarboxylate
bridges are often employed as linkers to generate molecular
triangles,35,36 and squares.37,38 Recent work by Zhou and co-
workers has also demonstrated that the shape and size of
molecular architectures incorporating quadruple bonds can be
INTRODUCTION
■
Dimetal paddlewheel compounds have four ligands bridging the
dimetal core in the equatorial sites and two axial sites which are
available for coordination by donor ligands or solvent.1 The
well-defined coordination environment makes them good
candidates as building blocks for the synthesis of metal−
organic frameworks,2 having potential applications in areas such
as gas sorption,3 catalysis,4−7 magnetic materials,8−10 or as
building blocks in coordination polymers for use in molecular
electronic devices.11−16 These materials typically employ rigid
organic bridging ligands that rely on metal−ligand interactions
to propagate one-, two-, or three-dimensional architectures.
Metal−metal quadruply bonded paddle compounds are
redox active, with the dimetal core readily losing one or more
electrons.17−19 The redox potential can be tuned by changing
the nature of the bridging ligand,20 and recent studies from
Berry and co-workers have also demonstrated, using a series of
MM···M′ compounds, that axially coordinated metal ions can

be used to influence the redox behavior of the quadruple
bond.21−23 Quadruply bonded dimetal compounds also have
interesting photophysical properties.24,25 For example, the
compounds M2(TiPB)4 (M = Mo, W; TiPB = 2,4,6-
triisopropylbenzoate) show emission from a long-lived (Mo, τ
Received: June 19, 2013
Published: August 8, 2013
26
3
= 43 μs; W, τ = 1.6 μs) metal-based T1 state, the M2δδ*.
© 2013 American Chemical Society
9683
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691

Inorganic Chemistry
Article
controlled by varying the bridging angle and size of the
dicarboxylate linker.39
are needed to append four H2DON ligands. Compounds with
one, M2(TiPB)3(HDON) (M = Mo, 1a; M = W, 1b), two,
Mo2(TiPB)2(HDON)2 (2), and four, Mo2(HDON)4 (3),
appended HDON ligands were synthesized. Reaction of 1a
There is significant interest in developing metal complexes
containing pendant hydrogen bond donor−acceptor groups, as
the hydrogen bond is directional and can self-assemble to form
a variety of architectures.40−42 However, there have been
relatively few examples of using hydrogen bonding as a tool to
generate self-assembled architectures containing dimetal
paddlewheel building blocks.43,44 We have recently reported
the synthesis of Mo2(TiPB)3(HDON) (H2DON = 2,7-
dihydroxy-1,8-naphthyridine) and Mo2(TiPB)3(HDOP)
(H2DOP = 3,6-dihydroxypyridazine) which form hydrogen
bonded “dimers of dimers” in CH2Cl2.45 Electrochemical
studies demonstrated that the mixed-valence states of these
dimers are stabilized upon oxidation by one-electron. The
absence of an intervalence-charge transfer band in the NIR
region of their electronic absorption spectra indicate that these
are rare examples of proton-coupled mixed valency, in which
electron transfer is dependent on the proton coordinate.46
In this investigation we report the synthesis of a family of
compounds of form Mo2(TiPB)4−n(HDON)n (n = 1, 2, or 4),
which contain a pendant 2-pyridone functional group that can
act as hydrogen bond donor−acceptor, and are novel building
blocks for hydrogen bonded assemblies. Changes in the energy
of the M2-δ orbitals which act as the donor and acceptor have a
dramatic impact on the extent of electronic coupling in
covalently bound M2-bridge-M2+ systems, and provide valuable
insight into possible superexchange mechanisms.47 Therefore
we will also investigate the effect that changing the metal (M =
Mo or W) or ancillary ligand (O2CR) has on the stability of
mixed valence hydrogen-bonded “dimers of dimers”.
with
1 equiv of chloroacetic acid generated
Mo2(TiPB)2(O2CCH2Cl)(HDON), 1c, and drawings of all
the ligands and compounds used in this study are shown in
Scheme 1.
Ditopic ligands such as H2DON are capable of coordinating
two dimetal units to form “dimers of dimers” or other
oligomers. The use of the bulky TiPB ancillary ligands was
essential in this study as it prevents ligand scrambling or
coordination of another dimetal unit to form mixtures of
unwanted oligomeric species.48 Compounds [1a−c] are soluble
in most organic solvents, whereas compound 2 is soluble in
donor solvents such as tetrahydrofuran (THF), and compound
3 is only sparingly soluble in strong donor solvents such as
dimethylsulfoxide (DMSO). Matrix-assisted laser desorption/
ionization time-of-flight (MALDI-TOF) mass spectrometry of
all compounds showed the expected molecular ions corre-
sponding to the products.
X-ray Crystallography. Despite extensive efforts, we have
been unable to isolate crystals of 1a−c suitable for X-ray
diffraction from coordinating or noncoordinating solvents. We
were however able to obtain crystals of 2(DMSO)2·4DMSO
and 3(DMSO)2·2DMSO from DMSO solutions. The crystal
structures of these compounds are shown in Figures 1 and 2,
with selected bond lengths and angles in Table 1.
RESULTS AND DISCUSSION
■
Synthesis. The series of dimetal paddlewheel compounds
containing pendant 2-pyridone functional groups used in this
study were synthesized by reaction of M2(TiPB)4 (M = Mo, W;
TiPB = 2,4,6-triisopropylbenzoate; Scheme 1) or
Mo2(O2CCH3)4 with H2DON (2,7-dihydroxy-1,8-naphthyr-
idine; Scheme 1). The number of pendant hydrogen bond
donor−acceptor ligands was controlled by the reagent
stoichiometry and reaction conditions; elevated temperatures
Scheme 1. Ligands and Compounds Employed in This Study
Figure 1. Molecular structure of 2(DMSO)2 with anisotropic
displacement parameters drawn at the 40% level and all hydrogen
atoms (except those associated with the lactam functional groups)
omitted for clarity. The molecule sits on an inversion center, and
atoms with an additional prime (′) character are generated from the
corresponding atoms without the prime label by an inversion
operation.
Both compounds sit on a crystallographic inversion center
and have the expected paddlewheel arrangement of the ligands
about the dimetal core. The Mo1−Mo1′ bond lengths of
2.1042(6) Å (2(DMSO)2) and 2.1026(15) Å (3(DMSO)2) are
consistent with a formal metal−metal quadruple bond.1 The
Mo−O bond lengths of the axially coordinated DMSO
molecules are similar to those found in other
9684
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691

Inorganic Chemistry
Article
Scheme 2. Different Possible Regioisomers for 2 and 3
DMSO disrupts the hydrogen bonding to form a monomer.45
1
The same behavior was observed for [1a]2, and the H NMR
spectra of [1a]2 in CD2Cl2 at 5.0, 2.5, and 0.5 mM
concentrations are shown in the Supporting Information. No
new peaks are observed upon dilution, and there is no change
in the chemical shift or broadening observed for the NH proton
at δ 12.29. In addition, there are no changes observed in the 1H
NMR spectrum upon cooling to 203 K. This indicates that the
association constant for the monomer−dimer equilibrium
(Scheme 3) must be relatively high, and that there will only
Figure 2. Molecular structure of 3(DMSO)2 with anisotropic
displacement parameters drawn at the 40% level and all hydrogen
atoms (except those associated with the lactam functional groups)
omitted for clarity. The molecule sits on an inversion center, and
atoms with an additional prime (′) character are generated from the
corresponding atoms without the prime label by an inversion
operation.
Mo2(O2CR)4(DMSO)2 compounds.49 The DMSO molecules
also form hydrogen bonds with the N−H protons of the
HDON ligand. The CO bond lengths in the free amide
functional groups of the HDON ligand are relatively short for
2(DMSO)2 and 3(DMSO)2 (<1.26 Å). This is consistent with
the compounds adopting the lactam tautomer in the solid-
state.50
Scheme 3
As well as potential tautomeric isomers, paddlewheel
compounds containing bridging ligands with inequivalent
binding atoms can adopt a number of different regioisomers,
as shown in Scheme 2.20,51−53 The solid state structure of
2(DMSO)2 shows that the ligands adopt a trans-1,1 isomeric
form in the solid state, whereas for 3(DMSO)2 the ligands
adopt a cis-2,2 arrangement.
Solution Behavior. The structure of 2-pyridone in the gas-
phase, solution, and solid-state has been extensively studied as it
is a good model for understanding hydrogen bonding in DNA
base pairs.54,55 It undergoes lactim/lactam tautomerism and can
dimerize to form a number of cyclic dimers. We have previously
shown using diffusion-ordered 1H NMR spectroscopy that
Mo2(TiPB)3(HDOP) (H2DOP = 3,6-dihydroxypyridazine)
forms the hydrogen bonded “dimer of dimers”
[Mo2(TiPB)3(HDOP)]2 in CD2Cl2 solutions, but addition of
be minor concentration effects in the concentration range used
for spectroscopic and electrochemical measurements. In
addition, the single peak indicates that the dimer adopts the
lactam−lactam tautomer exclusively in solution.
DMSO is known to disrupt hydrogen bonding, and in
DMSO or CH2Cl2/DMSO solvent mixtures the monomer 1a is
generated. The 1H NMR spectra of [1a]2 in CD2Cl2 and 1a in a
CD2Cl2/DMSO-d6 (4:1) mixture are compared in Figure 3.
The NH proton resonance shifts to δ 10.30 for 1a, and there is
no shift of this peak or evidence of a lactim tautomer forming
for 1a in CD2Cl2/DMSO-d6 (4:1) solution at low temperature
(203 K).
The IR spectra of [1a]2 and 1a were recorded in CH2Cl2 and
CH2Cl2/DMSO (20:1) solutions respectively, and are shown in
Table 1. Selected Bond Lengths (Å), Angles (deg), and Torsion Angles (deg) for 2(DMSO)2 and 3(DMSO)2
2(DMSO)2
3(DMSO)2
Mo1−Mo1′
Mo1−O1
Mo1′−O2
Mo1−O3
Mo1′−N1
Mo1−O5
C24−O4
N2···O5′
2.1042(6)
2.122(3)
2.117(3)
2.088(3)
2.204(3)
2.542(3)
1.243(5)
2.809(5)
170.91(7)
0.91(11)
2.47(12)
Mo1−Mo1′
Mo1−O1
Mo1−O3
Mo1−N1
Mo1−N3
Mo1−O5
C8−O2
C16−O4
N2···O5
2.1026(15)
2.095(5)
2.084(5)
2.184(7)
2.203(6)
2.535(6)
1.255(13)
1.235(11)
2.775(10)
2.778(9)
168.54(14)
1.9(2)
Mo1′−Mo1−O5
O1−Mo1−Mo1′−O2
O3−Mo1−Mo1′−N1
N4···O5
Mo1′−Mo1−O5
N1−Mo1−Mo1′−O1′
O3−Mo1−Mo1′−N3′
1.2(2)
9685
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691

Inorganic Chemistry
Article
mixed valence state when comparing similar systems, several
groups have pointed out that care needs to be taken when
comparing values in different systems as it is an equilibrium
constant which can be affected by a variety of factors.57,58
However, conclusions can be drawn about trends in the
magnitude of Kc in similar systems employing the same
+
electrolyte. For M2-B-M2 compounds, strong coupling is
observed with relatively modest Kc values by comparison to M-
B-M+ systems, as any charge is more diffuse in dimetal
compounds.59
The voltammograms for [1a]2, [1b]2, and [1c]2 are shown in
Figure 4, and the data are summarized in Table 2. All of the
1
Figure 3. Low-field region of the H NMR spectra of [1a]2 (2.5 mM
in CD2Cl2; bottom) and 1a (5.0 mM in DMSO-d6/CD2Cl2 (4:1)
mixture; top).
the Supporting Information. A weak band appears at 2800 cm−1
for [1a]2, which could be assigned to the NH stretch of the
HDON ligand, and for 1a a weak broad band appears around
3040 cm−1. The ν(CO) stretch of the lactam tautomer for
[1a]2 and 1a appears at 1644 and 1672 cm−1, respectively.
Once again there is no obvious evidence of tautomerism in
1
solution in agreement with the H NMR spectroscopy data.
1
The H NMR spectrum for 2 in DMSO-d6 indicates that
there are two species in solution. Two resonances are observed
at δ 10.23 and 10.16 in the low-field region of 2 in the ratio 1.7:
1, and two sets of HDON and aromatic TiPB resonances can
also be distinguished in the same ratios. Based on the solution
behavior of 1a, and the similar chemical shift of the low-field
resonances, these peaks are likely to correspond to different
regioisomers (see Scheme 2) rather than tautomers. The most
abundant species is assigned to the trans-1,1 regioisomer, as
observed in the solid-state. Based on the density functional
theory (DFT) calculations, vide infra, we tentatively assign the
less abundant regioisomer as the trans-2,0.
Figure 4. Cyclic voltammograms of [1a]2 (red), [1b]2 (black), and
[1c]2 (green) in 0.1 M NBu4PF6/CH2Cl2 solutions.
Table 2. Electrochemical and Spectroscopic Data for All
Compounds
M2-δ →
HDON-
E
(1)/
1/2 a
E
(2)/ ΔE1/2
/
π* λmax
/
1/2 a
b
c
compound
solvent
CH2Cl2
V
V
mV
140
Kc
nm
[1a]2
−0.182
−0.131
−0.042
−0.516
0.330
233
111
164
1a
CH2Cl2/
DMSO
430
573
1
d
d
Evidence of regioisomerism is also seen in the H NMR
[1b]2
CH2Cl2
−0.637
−0.716
121
131
spectrum of 3. Three resonances are observed at δ = 10.28,
10.16, and 9.94 in a ratio of 1.0: 1.3: 1.7. Again, we assume that
these resonances correspond to the presence of at least two
different regioisomers for 3 in solution, although only one
regioisomer was observed in the solid-state.
Electrochemistry. The cyclic voltammogram of compound
[1a]2 in CH2Cl2 has been previously reported,45 and shows two
successive one-electron redox processes separated by 140 mV.
The successive one-electron oxidations indicate that the mixed
valence state is stabilized with respect to its neutral and doubly
oxidized forms. The comproportionation constant (Kc; Scheme
4)56 for [1a]2 is 233.
e
1b
CH2Cl2/
DMSO
[1c]2
CH2Cl2
0.199
1c
CH2Cl2/
DMSO
−0.026
421
448
461
d
d
2
CH2Cl2/
DMSO
−0.131
−0.250
3
DMSO
a
b
Potential vs Fc/Fc+. Obtained from the equation Kc = e(ΔE1/2
25.69). All spectra obtained in DMSO solutions at room temperature.
Obtained by addition of 100 μL of DMSO to an electrochemical cell
containing 4 mL of a 0.1 M NBu4PF6/CH2Cl2 solution. Irreversible.
/
c
d
e
The magnitude of Kc is often correlated to the extent of
electronic coupling in mixed valence compounds, and large
values (>106) are often indicative of Class III (fully delocalized)
behavior in mixed valence compounds of form M-B-M+.
Although Kc is a useful guide to the extent of stabilization of the
dimers show two redox processes corresponding to successive
oxidations of the Mo2 dimetal cores. Previous studies on
4+
[1a]2 indicated that stabilization of the mixed-valence state is
likely to be due to proton-coupled mixed valency, in which
electron transfer between the dimetal units is dependent on the
proton coordinates of the hydrogen bonds, as opposed to
electronic coupling via a superexchange mechanism involving
direct overlap of the donor, bridge, and acceptor orbitals.45
Comparison of the Kc values for [1a]2 (Kc = 233) and [1b]2
(Kc = 111) support this assignment. The Kc values for Mo2-B-
Scheme 4
+
+
Mo2 and W2-B-W2 systems with covalent bridges differ by
9686
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691

Inorganic Chemistry
Article
many orders of magnitude as the W2-δ orbitals are ∼0.5 eV
higher in energy than for molybdenum, and they overlap more
effectively with the bridge π-orbitals.47,60 For example, for
[M2(O2CtBu)3](μ-2,5-dihydroxyterephthalate)+ Kc is 21 for M
= Mo, but increases to 2600 for M = W.61 Therefore, while
there is an effect on changing Mo ([1a]2) to W ([1b]2), the
magnitude of change in Kc is not consistent with electronic
coupling being the primary mechanism by which the mixed
valence state is stabilized.
small amount for 3, which has 4 HDON ligands. This implies
that as well as acting as a π-acceptor, the HDON ligand is also
acting as π-donor.
UV/vis Spectroscopy. The UV/vis spectra of all
compounds in DMSO are shown in Figure 6, with data
The addition of the electron withdrawing chloroacetate
group in [1c]2 results in a small reduction in Kc by comparison
to [1a]2. While the change of metal and ancillary ligand has a
significant effect on the redox potential of the dimetal core, it
will not significantly affect the properties of the bridging
ligands, and hence only a small effect is seen on the value of Kc.
Proton-coupled electron transfer can occur via sequential
electron (ET) and proton (PT) transfer, or stepwise
mechanisms (ET-PT or PT-ET), and elucidating a mechanism
is often complicated.62,63 The subtle effect that metal or
ancillary ligand changes have on the extent of stabilization of
the mixed valence states of [1a−c]2 makes it difficult to draw
any conclusions about possible mechanisms for proton-coupled
mixed valency in these systems. However, we anticipate that
addition of electron-donating and -withdrawing substituents to
the bridging ligands will have a more substantial impact and
permit proposal of a possible mechanism in future work.
Addition of a small amount of DMSO to the electrochemical
cell containing compounds [1a−c]2 results in the formation of
the monomers 1a−c, as evidenced in their cyclic voltammo-
grams (Figure S3, Supporting Information) which now display
a single oxidation process. As anticipated, the electron
withdrawing chloroacetate group makes 1c the most difficult
compound to oxidize, whereas the tungsten compound 1b has
the lowest oxidation potential.
Figure 6. UV/vis spectra of 1a (red), 1b (black), 1c (green), 2 (blue),
and 3 (purple) recorded in DMSO at room temperature.
provided in Table 2. All Mo2 compounds display intense
absorptions around 330 nm which result from a combination of
the Mo2-δ → TiPB-π* and HDON-π → HDON-π* transitions.
For 1b, the W2-δ → TiPB-π* MLCT transition is shifted to
lower energy because of the increase in energy of the W2-δ
orbital by comparison to molybdenum. Two additional
transitions are observed at 446 and 510 nm which are due to
the two different TiPB environments in the molecule.
The lowest energy transition in each instance is assigned as
the M2-δ → HDON-π* MLCT transition. This transition
slightly decreases in energy as the number of HDON ligands in
the complex increases.
The cyclic voltammograms of 1a, 2, and 3 are compared in
Figure 5. Despite evidence for the presence of different
Theoretical Studies. To help rationalize the solution
behavior of these compounds, DFT calculations have been
performed using Gaussian09 at the M06/SDD and 6-
311G(d,p) level of theory using the model compounds
Mo2(O2CH)3(HDON) (1a′), W2(O2CH)3(HDON) (1b′),
trans-1,1-Mo2(O2CH)2(HDON)2 (2′), and cis-2,2-
Mo2(O2CH)2(HDON)2 (3′). Formate groups have been
used in place of TiPB to reduce computational time.
The relative stabilities of the three other regioisomers for 2′
were calculated, and found to be just 0.8 (trans-2,0), 1.9 (cis-
1,1), and 2.2 (cis-2,0) kJ mol−1 less stable than the trans-1,1
1
regioisomer. This is consistent with the H NMR data for 2
which indicated the presence of two regioisomers in solution,
tentatively assigned as the trans-1,1, and trans-2,0 regioisomers
based upon the solid-state structure and these calculations. The
spectroscopic and electrochemical data indicate that regioiso-
merism does not significantly influence the electronic structure
of the dimetal core, so subsequent discussion will focus on the
trans-1,1 and cis-2,2 regioisomers of 2′ and 3′ observed in the
solid-state.
Figure 5. Cyclic voltammograms of 1a (red), 2 (blue), and 3 (black)
in 0.1 M NBu4PF6 solutions of CH2Cl2/DMSO (1a) or DMSO (2 and
3).
1
regioisomers in the H NMR spectra of 2 and 3, only a single
oxidation is observed for these species indicating that the
regioisomers have similar electronic properties. The HDON
ligand is likely to be a better π-acceptor ligand than HTiPB, in
which case the oxidation potential of the Mo−Mo quadruple
bond is expected to increase as 1, 2, and 4 HDON ligands are
The geometries for all compounds were optimized using a
PCM solvation model in DMSO without symmetry constraints;
for each compound the lactam tautomer was calculated to be
the most stable form. Calculated bond lengths are given in
Table 3. The calculated Mo−Mo and W−W bond lengths
(∼2.10 and 2.20 Å) are consistent with those found for
quadruply bonded M2(O2CR)4 compounds,1 and there is
5+
added to the dimetal core. In fact, the Mo24+/Mo2 potentials
of all compounds are very similar, and it actually decreases by a
9687
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691

Inorganic Chemistry
Article
a
Table 3. Calculated Bond Lengths (Å) for 1a′, 1b′, 2′, and 3′
1a′ (M = Mo)
1b′ (M = W)
2′ (M = Mo)
3′ (M = Mo)
M−M
2.109
2.211
2.089
2.143a
2.148a
1.220
2.203
2.191
2.080
2.140
2.140
1.220
2.103 (2.1042(6))
2.222 (2.204(3))
2.097 (2.088(3))
2.148a (2.120(4))a
2.095 (2.1026(15))
2.238a (2.194(9))a
2.098a (2.090(7))a
M−N(HDOP)
M−O(HDOP)
M−O(cis‑TiPB)
M−O(trans‑TiPB)
CO(HDON)
1.220 (1.243(5))
1.220a (1.245(17))a
a
Experimentally observed values for 2(DMSO)2 and 3(DMSO)2 have been included in parentheses.
excellent agreement between the calculated and observed bond
lengths for 2′ and 3′
dimolybdenum compounds. The calculations are consistent
with the electrochemical data, in which 3 has a lower oxidation
potential than 1a and 2.
The frontier MO energy level diagrams calculated for 1a′,
1b′, 2′, and 3′ are shown in Figure 7, and selected frontier
CONCLUSIONS
■
A series of compounds with 1, 2, or 4 HDON ligands has been
synthesized. The pendant 2-pyridone functional groups can
form self-complementary double hydrogen bond interactions,
and compounds 1a−c dimerize in CH2Cl2 solutions. Cyclic
voltammetry shows stabilization of the mixed valence state in
these compounds, and the modest effect observed on the values
of Kc by changing the metal or ancillary ligands is consistent
with proton-coupled mixed valency being responsible for the
electron self-exchange, as opposed to a mechanism involving
overlap of donor, bridge, and acceptor orbitals.
These compounds are rare models for the study of electron
transfer processes through hydrogen-bond assemblies relevant
to biological processes, such as interstrand proton-coupled
electron transfer in DNA. In addition, the controlled
substitution chemistry and well-defined coordination geometry
about the dimetal core makes these compounds potential
building blocks for the development of functional hydrogen
bonded assemblies.
Figure 7. Calculated frontier MO energy level diagrams for 1a′, 1b′,
2′, and 3′.
EXPERIMENTAL SECTION
■
Physical Methods. MALDI-TOF spectrometry data was collected
on a Bruker Reflex III (Bruker, Breman, Germany) mass spectrometer
operated in linear, positive ion mode with an N2 laser. Laser power was
used at the threshold required to produce a signal. The matrix used for
each experiment was dithranol, and this was integrated into the analyte
by dissolving a mixture of dithranol and the analyte in DMSO, THF,
or CH2Cl2. Electron absorption spectroscopy data was obtained using
a Varian Cary 5000 UV−vis−NIR spectrophotometer, in a cell with a
0.5 mm path length with an analyte concentration of approximately 1
mM. Infrared experiments were carried out on a Perkin-Elmer
Spectrum Two FT-IR spectrometer using a solution cell with KBr
plates. In each experiment the analyte was dissolved in either CH2Cl2,
or a solvent mixture of DMSO (5% v/v) in CH2Cl2. Electrochemical
studies were carried out in an N2-purged solution containing the
electrolyte NBu4PF6 (0.1 M) and sample (5 mM concentration). In
each case, a standard three-electrode setup was implemented with a
platinum disk working electrode, a platinum wire as a counter
electrode, and a Ag/AgCl electrode as a pseudoreference. After each
experiment, a small amount of ferrocene was added as an internal
reference, and all data reported herein is reported versus the Fc/Fc+
couple. All potentials are reported at a scan rate of 100 mV/s.
Figure 8. Gaussview plots of selected orbitals for 1a′.
Materials. All experimental procedures were performed under an
inert atmosphere of argon and using standard Schlenk-line and
glovebox techniques. All solvents were obtained from a “Grubbs”
solvent purification system with the exception of DMSO and 1,2-
dichlorobenzene which were dried via vacuum distillation over CaH2
and then degassed with argon. Mo2(OAc)4,64 M2TiPB4 (M = Mo,17
W26) and H2DON65 were synthesized according to previously
published methods. H2DON was ground to a fine powder prior to
molecular orbital diagrams for 1a′ are shown in Figure 8. The
HOMO in each instance is the M2-δ, and the LUMO is the M2-
δ* when M = Mo and HDON-π* when M = W. The proximity
of both the HDON-π and -π* orbitals to the M2-δ orbitals
means the HDON ligand could potentially act as either a π-
donor or π-acceptor ligand, and the compound with four
HDON ligands (3′) has the highest HOMO energy of all the
9688
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691

Inorganic Chemistry
Article
each reaction. All other compounds were obtained from commercial
sources and used without further purification.
around one-third of the original volume in vacuo, and the product
precipitated by addition of hexane (30 mL). The solid was isolated by
centrifugation and dried in vacuo to yield the product as an orange
powder (0.080 g, 0.08 mmol, 80%). 1H NMR (400 MHz, (CD3)2SO)
two regioisomers can be determined (trans-1,1 and trans-2,0), which
exist in a 1.7: 1 ratio. trans-1,1: δ 10.23 (s, 2 H, NH), 8.00−7.85 (m, 2
H, aromatic HDON), 7.04 (s, 2 H, aromatic TiPB), 6.98 (d, JHH = 9.0
Hz, 1 H, aromatic HDON), 2.94−2.57 (m, 6 H, CH(CH3)2), 1.23−
0.72 (m, 36 H CH(CH3)2). trans-2,0: δ 10.16 (s, 2 H, NH), 8.00−7.85
(m, 2 H, aromatic HDON), 7.12 (d, JHH = 9.0 Hz, 1 H, aromatic
HDON), 7.04 (s, 2 H, aromatic TiPB), 2.94−2.57 (m, 6 H,
CH(CH3)2), 1.23−0.72 (m, 36 H, CH(CH3)2). IR (CH2Cl2/DMSO
(10:1 v/v)): νmax cm−1; 3110 (br, w), 3504 (w), 2968 (s), 2950 (m),
2876 (m), 2823 (w) 1715 (w), 1672 (s), 1617 (s), 1615 (s), 1534 (w),
1508 (s), 1466 (m), 1419 (s). UV−vis (DMSO); λmax nm (ε, M−1
cm−1) 332 (32874), 346 (sh) (23418), 448 (4398). MALDI-TOF-MS
calcd. monoisotopic MW for Mo2C48H56O8N4, 1012.2, found m/z
1012.8 (M+). Anal. Calcd for Mo2C48H56O8N4: C, 57.14; H, 5.59; N,
5.55. Found: C, 56.78; H, 5.88; N, 5.11.
Synthesis of Mo2(TiPB)3HDON, 1a. The synthesis of this
compound has been previously reported,45 but we include here a
1
summary of the IR, UV−vis and H NMR data in different solvents.
1H NMR (400 MHz, CD2Cl2) δ 12.29 (s, 1 H, N-H), 7.78 (d, JHH
=
9.0 Hz, 1 H, aromatic HDON), 7.67 (d, JHH = 9.0 Hz, 1 H, aromatic
HDON), 7.27 (s, 2 H, aromatic TiPB), 7.11 (d, JHH = 9.0 Hz, 1 H,
aromatic HDON), 6.98 (s, 4 H, aromatic TiPB), 6.66 (d, JHH = 9.0 Hz,
1 H, aromatic HDON), 3.66 (septet, JHH = 7.0 Hz, 2 H, CH(CH3)2),
3.04, (septet, JHH = 7.0 Hz, 1 H, CH(CH3)2), 2.83 (septet, JHH = 7.0
Hz, 6 H, CH(CH3)2), 1.38 (d, JHH = 7.0 Hz, 6 H, CH(CH3)2), 1.36
(d, JHH = 7.0 Hz, 12 H, CH(CH3)2), 1.23 (d, JHH = 7.0 Hz, 12 H,
CH(CH3)2), 1.02 (d, JHH = 7.0 Hz, 12 H, CH(CH3)2), 0.94 (d, JHH
=
1
7.0 Hz, 12 H, CH(CH3)2). H NMR (400 MHz, (CD3)2SO) δ 10.34
(s, 1 H, N-H), 7.95 (d, JHH = 9.0 Hz, 1 H, aromatic HDON), 7.91 (d,
JHH = 9.0 Hz, 1 H, aromatic HDON), 7.25 (s, 2 H, aromatic TiPB),
7.04 (s, 4 H, aromatic TiPB), 7.01 (d JHH = 9.0 Hz, 1 H, aromatic
HDON), 6.23 (d, JHH = 9.0 Hz, 1 H, aromatic HDON), 3.70 (septet,
JHH = 7.0 Hz, 1 H, CH(CH3)2), 3.00 (septet, JHH = 7.0 Hz, 1 H,
CH(CH3)2), 2.85 (m, 7 H, CH(CH3)2), 1.13 (m, 53 H, CH(CH3)2).
IR (CH2Cl2/DMSO (20:1 v/v)): νmax cm−1; 3108 (br, w), 3038 (w),
2966 (s), 2949 (m), 2874 (m), 2815 (w), 1617 (s), 1616 (s), 1591
(w), 1534 (sh, w), 1506 (s), 1509 (sh, m), 1465 (m), 1406 (s). UV−
vis (DMSO); λmax nm (ε, M−1 cm−1) 321 (sh) (10074), 334 (12488),
350 (11865), 430 (4152).
Synthesis of Mo2HDON4, 3. Mo2(OAc)4 (0.200 g, 0.46 mmol)
and H2DON (0.313 g, 1.93 mmol) were placed in a Schlenk tube, and
1,2-dichlorobenzene (30 mL) was added. The reaction was heated to
reflux and left for 18 h. Upon cooling to room temperature the
product precipitated as a red-brown precipitate, which was isolated by
filtration, washed with hexane (2 × 30 mL), and dried in vacuo
1
(0.366g, 0.44 mmol, 95%). H NMR (400 MHz, (CD3)2SO) δ 10.29,
10.16, 9.94 (three singlets, 4 H total, NH), 7.94−7.65 (m, 8 H,
aromatic HDON), 7.10−6.90 (m, 4 H, aromatic HDON), 6.25−6.05
(m, 4 H aromatic HDON). UV−vis (DMSO); λmax nm (ε, M−1 cm−1)
321 (39850), 331 (39150), 346 (35550), 461 (9750). MALDI-TOF-
MS calcd. monoisotopic MW for Mo2C32H20O8N8, 840.00, found m/z
839.8 (M+).
X-ray Crystallography. Data were collected and measured on a
Bruker Smart CCD area detector with Oxford Cryosystems low
temperature system. After integration of the raw data and merging of
equivalent reflections, an empirical absorption correction was applied
(SADABS) based on comparison of multiple symmetry-equivalent
measurements.67 The structures were solved by direct methods
(SHELXS-97)68 and refined by full-matrix least-squares on weighted
F2 values for all reflections.69 All hydrogens were included in the
models at calculated positions using a riding model with U(H) = 1.5 ×
Ueq (bonded carbon atom) for methyl and hydrogens and U(H) = 1.2
× Ueq (bonded carbon atom) for methine, methylene, and aromatic
hydrogens.
For 2(DMSO)2·4DMSO, one of the isopropyl groups on the TiPB
ligand was disordered and modeled in two positions isotropically with
0.58/0.42 site occupancies. One dimethyl sulfoxide molecule (S3) was
disordered over two positions (0.61/0.39 site occupancies); only the
associated sulfur atoms were refined anisotropically. Another dimethyl
sulfoxide molecule (S2) was disordered about two positions along the
crystallographic C2 axis and another about a center of inversion (S4);
both molecules had equal disorder (0.5 occupancy) about both
positions and the atoms refined isotropically. The disorder associated
with the center of inversion for S4 resulted in one of the carbon atoms
(C32) and an oxygen atom (O8) to be disordered over the same site,
which was modeled using the EXYZ constraint and that each atom has
0.5 site occupancy. The residual electron density peak of 2.620 e Å−3 is
associated with the disorder observed in these solvate molecules.
The DMSO molecules in 3(DMSO)2·2DMSO were disordered
over two positions, and were modeled with site occupancies of 0.59/
0.41 (S1) and 0.54/0.44 (S2). Experimental data relating to the
structure determination of both compounds is shown in Table 4. The
supplementary crystallographic data for these compounds are
contained in CCDC 945118 (2) and 945119 (3).
Synthesis of W2(TiPB)3HDON, 1b. A Schlenk tube was charged
with W2(TiPB)4 (0.148 g, 0.11 mmol) and 2,7-dihydroxy-1,8-
naphthyridine (H2DON) (0.016 g, 0.10 mmol). Dry toluene (30
mL) was added via cannula and the reaction was stirred for 48 h,
whereupon a dark brown solution formed. The solvent was removed in
vacuo, and the resulting solid washed with dry hexane (15 mL) to yield
1
a dark green powder (0.118 g, 0.093 mmol, 93%). H NMR (400
MHz, CD2Cl2) δ 12.67 (s, 1 H, N−H), 7.82 (m, 1 H, aromatic
HDON), 7.27 (s, 2 H, aromatic TiPB), 7.17 (m, 2 H, aromatic
HDON), 6.96 (s, 4 H, aromatic TiPB), 6.82 (m, 1 H, aromatic
HDON), 3.60 (m, 2.5 H CH(CH3)2), 2.87, (m, 2.5 H, CH(CH3)2),
2.66 (septet, JHH = 7.0 Hz, 4 H, CH(CH3)2), 1.34 (d, JHH = 7.0 Hz, 13
H, CH(CH3)2), 1.22 (d, JHH = 7.0 Hz, 15 H, CH(CH3)2), 0.98 (d, JHH
= 7.0 Hz, 13 H, CH(CH3)2), 0.92 (d, JHH = 7.0 Hz, 13 H, CH(CH3)2).
UV−vis (DMSO); λmax nm (ε, M−1 cm−1) 300 (sh) (7650), 331
(11000), 346 (8660), 442 (6900), 507 (6680), 573 (6370). MALDI-
TOF-MS calcd. monoisotopic MW for W2C56H74N2O8, 1270.45,
found m/z 1270.40 (M+). The compound is too air-sensitive to obtain
accurate elemental analysis, which has been encountered in previous
studies on related ditungsten compounds.66
Synthesis of Mo2 (TiPB)2 (HDON)(O2 CCH2 Cl), 1c.
Mo2(TiPB)3HDON (0.109 g, 0.1 mmol) and finely ground
monochloroacetic acid (0.009 g, 0.1 mmol) were placed in a flame-
dried Schlenk tube. Toluene (20 mL) was added, and the reaction was
stirred for 24 h to afford a light orange precipitate. The solution was
concentrated to around 10% volume in vacuo, and hexane (15 mL)
was added. The resulting precipitate was isolated via centrifugation,
and dried in vacuo to obtain a light orange powder (0.080 g, 0.085
1
mmol. 85%). H NMR (400 MHz, (CD3)SO) δ 9.92 (s, 1 H, N-H),
7.94 (d, JHH = 9.0 Hz, 1 H, aromatic HDON), 7.93 (d, JHH = 9.0 Hz, 1
H, aromatic HDON), 7.08 (d, JHH = 9.0 Hz, 1 H, aromatic HDON),
7.05 (s, 4 H, aromatic TiPB), 6.19 (dd, JHH = 9.0 Hz, JHH = 2 Hz, 1 H,
aromatic HDON), 4.96 (s, 2 H, ClH2CCOO), 2.90 (septet, JHH = 7.0
Hz, 3 H, CH(CH3)2), 2.85−2.72 (m, 3 H, CH(CH3)2) 1.21 (d, JHH
=
7.0 Hz, 24 H, CH(CH3)2), 1.15 (d, JHH = 7.0 Hz, 12 H, CH(CH3)2).
UV−vis (DMSO); λmax nm (ε, M−1 cm−1) 320 (sh) (29600), 331
(35100), 349 (30000), 421 (7190). MALDI-TOF-MS calcd.
monoisotopic MW for Mo2C42H53N2O8Cl, 944.2 found m/z 944.3.
(M+). Anal. Calcd for Mo2C42H53N2O8Cl: C, 53.59; H, 5.68; N, 2.98;
Cl, 3.77. Found: C, 53.04; H, 5.51; N, 3.28; Cl, 3.75%.
Theoretical Methods. Molecular structure calculations were
performed using DFT as implemented in the Gaussian 09 software
package.70 The M06 functional71 and the 6-311G(d,p) basis set72−74
were used for H, C, O, and N, along with the SDD energy consistent
pseudopotentials75 for molybdenum and tungsten. The same func-
tional/basis set combinations have been successfully employed in a
Synthesis of Mo2(TiPB)2(HDON)2, 2. A Schlenk tube was charged
with Mo2(TiPB)4 (0.1193 g, 0.1 mmol) and H2DON (0.032 g, 0.2
mmol). Toluene (30 mL) was added via cannula, and the mixture
stirred at 60 °C for 48 h. The resulting solution was concentrated to
9689
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691

Inorganic Chemistry
Article
crystallographic help. The University of Sheffield is thanked for
a PhD studentship (L.A.W.).
Table 4. Crystal Data for 2(DMSO)2·4DMSO and
3(DMSO)2·2DMSO
2(DMSO)2·4DMSO
3(DMSO)2·2DMSO
REFERENCES
■
empirical formula
formula weight
temperature (K)
wavelength (Å)
C60H92Mo2N4O14S6
1477.62
C40H44Mo2N8O12S4
1148.95
(1) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Multiple Bonds
Between Metal Atoms, 3rd ed.; Springer Science and Business Media,
Inc: New York, 2005.
150(2)
150(2)
0.71073
0.71073
(2) Special Issue: Metal-Organic Frameworks; Zhou, H.-C.; Long, J. R.;
Yaghi, O. M., Eds.; Chem. Rev., 2012; 112, 673−1268.
crystal system, space group monoclinic, C2/c
monoclinic, P21/n
15.2138(11)
9.9307(7)
15.6547(11)
90
̆
(3) Dinca, M.; Long, J. R. Angew. Chem., Int. Ed. 2008, 47, 6766.
a (Å)
33.0716(7)
12.4762(3)
17.4131(4)
90
(4) Berry, J. F. Dalton Trans. 2012, 41, 700.
b (Å)
(5) Davies, H. M. L.; Manning, J. R. Nature 2008, 451, 417.
(6) Doyle, M. P.; Duffy, R.; Ratnikov, M.; Zhou, L. Chem. Rev. 2010,
110, 704.
(7) (a) Kumar, D. K.; Filatov, A. S.; Napier, M.; Sun, J.; Dikarev, E.
V.; Petrukhina, M. A. Inorg. Chem. 2012, 51, 4855. (b) Villalobos, L.;
Cao, Z.; Fanwick, P. E.; Ren, T. Dalton Trans. 2012, 41, 644.
(8) Fishman, R. S.; Satoshi, O.; Miller, J. S. Phys. Rev. B 2009, 80,
140416/1.
(9) Mikuriya, M.; Yoshioka, D.; Borta, A.; Luneau, D.; Matoga, D.;
Szklarzewicz, J.; Handa, M. New J. Chem. 2011, 35, 1226.
(10) Motokawa, N.; Matsunaga, S.; Takaishi, S.; Miyasaka, H.;
Yamashita, M.; Dunbar, K. R. J. Am. Chem. Soc. 2010, 132, 11943.
(11) Cummings, S. P.; Cao, Z.; Fanwick, P. E.; Kharlamova, A.; Ren,
T. Inorg. Chem. 2012, 51, 7561.
(12) (a) Olea, D.; Gonzal
Pablo, P. J.; Torres, M. R.; Gom
Zamora, F. Chem. Commun. 2007, 1591. (b) Amo-Ochoa, P.; Jimen
Aparicio, R.; Torres, M. R.; Urbanos, F. A.; Gallego, A.; Gomez-García,
C. J. Eur. J. Inorg. Chem. 2010, 4924.
c (Å)
α (deg)
β (deg)
92.4240(10)
90
106.240(4)
90
γ (deg)
volume (Å3)
7178.4(3)
4, 1.367
2270.8(3)
2, 1.680
Z, Calculated density
(g cm−3
)
F(000)
3088
1168
θ range for data collection 1.74−27.54
2.20−25.07
(deg)
limiting indices
−41 ≤ h ≤ 42,
−16 ≤ k ≤ 16,
−22 ≤ l ≤ 22
45689/8174
[Rint = 0.0515]
−17 ≤ h ≤ 18,
−11 ≤ k ≤ 11,
−18 ≤ l ≤ 17
19305/4024
[Rint = 0.1632]
reflections collected/
unique
́
ez-Prieto, R.; Priego, J. L.; Barral, M. C.; de
ez-Herrero, J.; Jimenez-Aparicio, R.;
ez-
́
́
completeness
to θ = 27.54; 98.80% to θ = 25.07; 99.90%
́
data/restraints/parameters 8174/24/372
goodness-of-fit on F2
1.058
4024/91/358
0.956
́
(13) Berry, J. F.; Cotton, F. A.; Murillo, C. A. Organometallics 2004,
23, 2503.
(14) Ismayilov, R. H.; Wang, W.-Z.; Wang, R.-R.; Yeh, C.-Y.; Lee, G.-
H.; Peng, S.-M. Chem. Commun. 2007, 1121.
(15) Georgiev, V. P.; Mohan, P. J.; DeBrincat, D.; E., M. J. Coord.
Chem. Rev. 2013, 257, 290.
final R indices [I > 2σ(I)] R1 = 0.0604,
wR2 = 0.1582
R1 = 0.0629,
wR2 = 0.1148
R1 = 0.1659,
wR2 = 0.1507
0.394 and −0.464
R indices (all data)
R1 = 0.0791,
wR2 = 0.1729
2.620 and −1.242
largest diff. peak and hole
(e Å−3
)
(16) Blum, A. S.; Ren, T.; Parish, D. A.; Trammell, S. A.; Moore, M.
H.; Kushmerick, J. G.; Xu, G.-L.; Deschamps, J. R.; Pollack, S. K.;
Shashidhar, R. J. Am. Chem. Soc. 2005, 127, 10010.
(17) Cotton, F. A.; Daniels, L. M.; Hillard, E. A.; Murillo, C. A. Inorg.
Chem. 2002, 41, 1639.
(18) Chisholm, M. H.; D’Acchioli, J. S.; Pate, B. D.; Patmore, N. J.;
Dalal, N. S.; Zipse, D. J. Inorg. Chem. 2005, 44, 1061.
(19) Nippe, M.; Goodman, S. M.; Fry, C. G.; Berry, J. F. J. Am. Chem.
Soc. 2011, 133, 2856.
study on related quadruply bonded compounds which have weak
metal−ligand interactions.76
Geometry optimizations of the model compounds
Mo2(O2CH)3(HDON) (1a′), W2(O2CH)3(HDON) (1b′),
Mo2(O2CH)2(HDON)2 (2′), and Mo2(HDON)4 (3′) were per-
formed without symmetry constraints. All optimizations were
performed in a DMSO solvent cavity using the polarizable continuum
model, as implemented in Gaussian 09, and the structures were
confirmed to be minima on the potential energy surface using
harmonic vibrational frequency analysis.
(20) Hicks, J.; Ring, S. P.; Patmore, N. J. Dalton Trans. 2012, 41,
6641.
(21) Nippe, M.; Bill, E.; Berry, J. F. Inorg. Chem. 2011, 7650.
(22) Nippe, M.; Turov, Y.; Berry, J. F. Inorg. Chem. 2011, 50, 10592.
(23) Dolinar, B. S.; Berry, J. F. Inorg. Chem. 2013, 52, 4658.
(24) Hopkins, M. D.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 2468.
(25) Alberding, B. G.; Chisholm, M. H.; Gustafson, T. L. Inorg. Chem.
2012, 51, 491.
(26) 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.
ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR spectra of [1a]2 at different concentrations, IR spectra
of 1a and [1a]2, cyclic voltammograms of 1a, 1b, and 1c, and
final calculated atomic coordinates for all compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(27) Alberding, B. G.; Chisholm, M. H.; Ghosh, Y.; Gustafson, T. L.;
Liu, Y.; Turro, C. Inorg. Chem. 2009, 48, 8536.
AUTHOR INFORMATION
Corresponding Author
*E-mail: n.patmore@sheffield.ac.uk.
■
(28) Alberding, B. G.; Chisholm, M. H.; Chou, Y.-H.; Ghosh, Y.;
Gustafson, T. L.; Liu, Y.; Turro, C. Inorg. Chem. 2009, 48, 11187.
(29) Alberding, B. G.; Chisholm, M. H.; Gallucci, J. C.; Ghosh, Y.;
Gustafson, T. L. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 8152.
(30) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Spilker, T. F. J.
Am. Chem. Soc. 2013, 135, 8254.
Notes
The authors declare no competing financial interest.
(31) Cotton, F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res. 2001, 34,
759.
(32) Cotton, F. A.; Lin, C.; Murillo, C. A. Proc. Natl. Acad. Sci. U.S.A.
2002, 99, 4810.
(33) Chisholm, M. H.; Macintosh, A. M. Chem. Rev. 2005, 105, 2949.
ACKNOWLEDGMENTS
■
The Royal Society is gratefully acknowledged for a University
Research Fellowship (N.J.P.). Dr. Anthony Meijer and Mr.
Harry Adams are thanked for computational assistance and
9690
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691

Inorganic Chemistry
Article
(34) Koberl, M.; Cokoja, M.; Herrmann, W. A.; Kuhn, F. E. Dalton
Trans. 2011, 40, 6834.
(35) Cotton, F. A.; Lin, C.; Murillo, C. A. Inorg. Chem. 2001, 40, 575.
(36) Chisholm, M. H.; Patmore, N. J.; Reed, C. R.; Singh, N. Inorg.
Chem. 2010, 49, 7116.
N.; Millam, J. M.; 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 C.01; Gaussian, Inc.: Wallingford, CT, 2009.
(71) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215.
(72) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639.
(73) Wachters, J. H. J. Chem. Phys. 1970, 52, 1033.
(74) Hay, P. J. J. Chem. Phys. 1977, 66, 4377.
(75) Andrae, D.; Haeussermann, U.; Dolg, M.; Preuss, H. Theor.
Chim. Acta 1990, 77, 123.
́
(76) Carrasco, M.; Faust, M.; Peloso, R.; Rodríguez, A.; Lopez-
Serrano, J.; Alvarez, E.; Maya, C.; Power, P. P.; Carmona, E. Chem.
Commun. 2012, 48, 3954.
(37) Cotton, F. A.; Daniels, L. M.; Lin, C.; Murillo, C. A. J. Am.
Chem. Soc. 1999, 121, 4538.
(38) Koberl, M.; Cokoja, M.; Bechlars, B.; Herdtweck, E.; Kuhn, F. E.
̈
̈
Dalton Trans. 2011, 40, 11490.
(39) Li, J.-R.; Yakovenko, A. A.; Lu, W.; Timmons, D. J.; Zhuang, W.;
Yuan, D.; Zhou, H.-C. J. Am. Chem. Soc. 2010, 132, 17599.
(40) Brammer, L. Dalton Trans. 2003, 3145.
(41) Lynam, J. M. Dalton Trans. 2008, 4067.
(42) Cowley, M. J.; Lynam, J. M.; Whitwood, A. C. Dalton Trans.
2007, 4427.
(43) Pogozhev, D.; Baudron, S. A.; Hosseini, M. W. Dalton Trans.
2011, 40, 7403.
(44) Bera, J. K.; Vo, T.-T.; Walton, R. A.; Dunbar, K. R. Polyhedron
2003, 22, 3009.
(45) Wilkinson, L. A.; McNeill, L.; Meijer, A. J. H. M.; Patmore, N. J.
J. Am. Chem. Soc. 2013, 135, 1723.
(46) Goeltz, J. C.; Kubiak, C. P. J. Am. Chem. Soc. 2010, 132, 17390.
(47) Chisholm, M. H.; Patmore, N. J. Acc. Chem. Res. 2007, 40, 19.
(48) Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.; Lobkovsky, E.
B. J. Am. Chem. Soc. 1991, 113, 8709.
(49) Alberding, B. G.; Chisholm, M. H.; Lear, B. J.; Naseri, V.; Reed,
C. R. Dalton Trans. 2011, 40, 10658.
(50) Forlani, L.; Cristoni, G.; Boga, C.; Todesco, P. E.; Vecchio, E.
D.; Monari, M. ARKIVOC 2002, 11, 198.
(51) Kadish, K. M.; Wang, L.-L.; Thuriere, A.; Van Caemelbecke, E.;
Bear, J. L. Inorg. Chem. 2003, 42, 834.
(52) Dequeant, M. Q.; Ren, T. J. Cluster Sci. 2006, 17, 479.
(53) Majumdar, M.; Patra, S. K.; Kannan, M.; Dunbar, K. R.; Bera, J.
K. Inorg. Chem. 2008, 47, 2212.
(54) Yang, M.; Szyc, L.; Dreyer, J.; Nibbering, E. T. J.; Elsaesser, T. J.
Phys. Chem. A 2010, 114, 12195.
(55) Matsuda, Y.; Ebata, T.; Mikami, N. J. Chem. Phys. 1999, 110,
8397.
(56) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278.
(57) D’Alessandro, D. M.; Keene, F. R. Dalton Trans. 2004, 3950.
(58) Barriere, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U.
T.; Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262.
(59) Chisholm, M. H.; Clark, R. J. H.; Gallucci, J.; Hadad, C. M.;
Patmore, N. J. J. Am. Chem. Soc. 2004, 126, 8303.
(60) Chisholm, M. H. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 2563.
(61) Chisholm, M. H.; Feil, F.; Hadad, C. M.; Patmore, N. J. J. Am.
Chem. Soc. 2005, 127, 18150.
(62) Mayer, J. M. Annu. Rev. Phys. Chem. 2004, 55, 363.
(63) Huynh, M. H. V.; Meyer, T. J. Chem. Rev. 2007, 107, 5004.
(64) Brignole, A. B.; Cotton, F. A. Inorg. Synth. 1972, 13, 81.
(65) Newkome, G. R.; Garbis, S. J.; Majestic, V. K.; Fronczek, F. R.;
Chiari, G. J. Org. Chem. 1981, 46, 833.
(66) Byrnes, M. J.; Chisholm, M. H.; Clark, R. J. H.; Gallucci, J. C.;
Hadad, C. M.; Patmore, N. J. Inorg. Chem. 2004, 43, 6334.
(67) Sheldrick, G. M. A Program for Absorption Correction with the
Siemens SMART System; University of Gottingen: Gottingen,
̈
̈
Germany, 1996.
(68) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(69) SHELXTL program system, version 5.1; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 1998.
(70) 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,
9691
dx.doi.org/10.1021/ic401555g | Inorg. Chem. 2013, 52, 9683−9691