Tuning the electronic structure of Mo-Mo quadruple bonds by N for O for S substitution

Article abstract of DOI:10.1039/c2dt30234a

A series of quadruply bonded dimolybdenum compounds of form Mo ₂(EE′CCCPh)₄ (EE′ = {NPh}₂, Mo ₂NN; {NPh}O, Mo₂NO;{NPh}S, Mo₂NS; OO, Mo ₂OO) have been synthesised by ligand exchange reactions of Mo ₂(O₂CCH₃)₄ with the acid or alkali metal salt of {PhCCCEE′}^-. The compounds Mo₂NO, Mo₂NS and Mo₂OO were structurally characterised by single crystal X-ray crystallography. The structures show that Mo₂NO adopts a cis-2,2 arrangement of the ligands about the Mo₂⁴⁺ core, whereas Mo₂NS adopts the trans-2,2 arrangement. The influence of heteroatom substitution on the electronic structure of the compounds was investigated using cyclic voltammetry and UV-Vis spectroscopy. Simple N for O for S substitution in the bridging ligands significantly alters the electronic structure, lowering the energy of the Mo₂-δ HOMO and reducing the Mo₂^4+/5+ oxidation potential by up to 0.9 V. A different trend is found in the optoelectronic properties, with the energy of the Mo₂-δ-to-ligand-π* transition following the order Mo₂OO > Mo₂NO > Mo₂NN > Mo ₂NS. Electronic structure calculations employing density functional theory were used to rationalise these observations.

Full text of DOI:10.1039/c2dt30234a

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PAPER
Tuning the electronic structure of Mo–Mo quadruple bonds by N for O for S
substitution†
Jamie Hicks, Sam P. Ring and Nathan J. Patmore*
Received 1st February 2012, Accepted 21st March 2012
DOI: 10.1039/c2dt30234a
A series of quadruply bonded dimolybdenum compounds of form Mo2(EE′CCuCPh)4 (EE′ = {NPh}2,
Mo2NN; {NPh}O, Mo2NO;{NPh}S, Mo2NS; OO, Mo2OO) have been synthesised by ligand exchange
reactions of Mo2(O2CCH3)4 with the acid or alkali metal salt of {PhCuCCEE′}−. The compounds
Mo2NO, Mo2NS and Mo2OO were structurally characterised by single crystal X-ray crystallography.
The structures show that Mo2NO adopts a cis-2,2 arrangement of the ligands about the Mo24+ core,
whereas Mo2NS adopts the trans-2,2 arrangement. The influence of heteroatom substitution on the
electronic structure of the compounds was investigated using cyclic voltammetry and UV-Vis
spectroscopy. Simple N for O for S substitution in the bridging ligands significantly alters the electronic
structure, lowering the energy of the Mo2-δ HOMO and reducing the Mo24+/5+ oxidation potential by up
to 0.9 V. A different trend is found in the optoelectronic properties, with the energy of the Mo2-δ-to-
ligand-π* transition following the order Mo2OO > Mo2NO > Mo2NN > Mo2NS. Electronic structure
calculations employing density functional theory were used to rationalise these observations.
and have been extensively studied over the past couple of
Introduction
decades as strong electronic coupling is observed between the
dimetal units.7–15 The coupling is mediated by M2-δ to bridge-π
Quadruply bonded dimetal compounds of form M2(μ-L)4 (L =
bidentate, three atom, bridging ligand; M = Mo, W) have a
paddlewheel arrangement of the ligands about the dimetal core,
and two axial sites that can be used to coordinate exogenous
conjugation and is dependent on the nature of the bridging
ligand and metal employed.
More recent studies have shown that MM multiply bonded
paddlewheels have remarkably long lived excited state life-
times.16,17 Investigations by Chisholm and coworkers on the
photoexcited states of trans-[M2(TiPB)2(O2CR)2] (R = 2-thio-
phene18,19 or C6H4-4-CN; TiPB = 2,4,6-triisopropylbenzoate)20
4+
ligands.1 The M2 core has a σ2π4δ2 electronic configuration,
and a wide variety of bridging ligands, such as carboxylate and
formamidinate, have been employed to support the dimetal core.
The utility of the redox active quadruple bond was illustrated
by W2(hpp)4 (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]-
pyrimidine), which is the most easily ionised closed-shell mole-
cule known and a powerful reducing agent.2,3 In addition, Berry
and co-workers have shown that the ditungsten core of W2(2,2′-
dipyridylamide)4 is capable of undergoing a four-electron oxi-
dation process which, remarkably, is chemically reversible.4 The
redox potential of dimolybdenum paddlewheel compounds has
also been found to play an important role in their performance as
catalysts in radical addition and polymerisation reactions.5
Mixed-valence ‘dimers of dimers’ of the type [M2](μ-O2C-X-
CO2)[M2]+ (M = Mo, W), where X is a conjugated spacer, are
particularly suited for the study of electron transfer processes6
3
show emission from T1 states that are MMδδ* in nature when
3
M = Mo (τ = 77 or 93 μs), but MLCT in nature when M = W
(τ = <10 ns). The well-defined coordination environment about
the dimetal core also makes MM multiply bonded paddlewheel
compounds good candidates for incorporation into materials
with potentially interesting optoelectronic properties.21–27 This
was highlighted in a detailed study by Zhou and co-workers
which demonstrated that the shape and size of molecular archi-
4+
tectures formed using dicarboxylate ligands to bridge Mo2
units can be controlled by tuning the bridging angle and size of
the bridging dicarboxylate ligand.28
Changing the nature of the three atom ligand bridging the
M24+ core has been found to have a marked effect on the proper-
ties of this type of compound.29,30 The substitution of O for S in
[L3Mo2]2(μ-1,4-(EE′C)2-C6H4) (EE′ = OO, OS, SS; L = O2CBut
or N,N′-di-p-anisylformamidinate) results in a decrease of the
HOMO (M2-δ)–LUMO (bridge π*) energy gap, and a greater
mixing of the metal- and bridge-based orbitals which gives rise
to a significant increase in electronic coupling.31,32 Similar
reasoning was used to account for the increases in electronic
coupling observed when thiooxamidate as opposed to oxamidate
Department of Chemistry, The University of Sheffield, Sheffield, S3 7HF,
UK. E-mail: n.patmore@sheffield.ac.uk
†Electronic supplementary information (ESI) available: a diagram of the
second crystallographically independent molecule of Mo2NS. Calculated
atomic coordinates for all compounds and molecular orbital plots of the
ligand π* b2g combination for all compounds. CCDC reference numbers
859556 [Mo2NO(py)2·5py], 859557 [Mo2NS·4(toluene)] and 859558
[Mo2NS·4(toluene)]. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2dt30234a
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Scheme 2 Reaction conditions for the synthesis of compounds
Mo2NN, Mo2NO, Mo2NS and Mo2OO.
Scheme 1 Drawing of the complexes used in this study.
is used to bridge Mo2 quadruply bonded units.33 The M24+ brid-
ging ligand has also been shown to have an effect on the nature
3
of the excited states;18 the MLCT of trans-[W2(TiPB)2(L)2] is
localised on one ligand when L is an amidinate (L = N
(iPr)2CCuCPh), but delocalised over both ligands for a carboxy-
late (L = O2CC6H4-4-CN).20
Scheme 3 Possible regioisomers for Mo2NO and Mo2NS.
Despite the often dramatic effect that changing the bridging
atoms of the bridging ligand has on the electronic structure and
properties of quadruply bonded paddlewheel compounds, there
have been no systematic studies as to the origins of these effects.
Here we report the synthesis of a series of compounds of form
Mo2(EE′CCuCPh)4 (EE′ = {NPh}2, {NPh}O, {NPh}S, OO),
shown in Scheme 1. The effect of substitution of N for O for S
on the electronic structure of the dimetal core and conjugation
between the Mo2-δ and ligand-π orbitals was investigated using
UV-Vis spectroscopy, electrochemistry and X-ray diffraction
studies, which are correlated with the results from density func-
tional theory (DFT) calculations.
M+ by matrix assisted laser desorption/ionization time-of-flight
1
(MALDI-TOF) mass spectrometry. The H NMR spectra of the
compounds all display overlapping aromatic resonances associ-
ated with the ligand phenyl groups.
The yield of Mo2NN was consistently low, and MALDI-
TOF-MS analysis of the crude reaction mixture showed a
mixture of product and a molecular ion consistent with the for-
mation of a (PhCuCC{NPh}2)3Mo2(μ-OH)2Mo2({NPh}2CCu
CPh)3 by-product, presumably from reaction of the product with
adventitious H2O.34,35 However, the product could be extracted
cleanly from this mixture using warm hexane.
The synthesis of the dithiocarboxylate and monothiocarboxy-
late compounds Mo2(S2CCuCPh)4 and Mo2(OC(S)CuCPh)4
was also attempted. Reaction of Mo2(O2CCH3)4 with the alkali
salt of the ligand36 prepared in situ in THF at −78 °C resulted in
the formation of a green solution. Upon warming to room temp-
erature, the solutions turns a dark black colour above −10 °C,
and removal of the solvent yields a black oil in both instances.
No evidence of any dimolybdenum ions were found in the mass
spectra of these oils, and the thermal instability of the initial
green solution precluded further analysis.
Results and discussion
Synthesis
The compounds Mo2({NPh}2CCuCPh)4 (Mo2NN), Mo2({NPh}-
C(O)CuCPh)4 (Mo2NO), Mo2({NPh}C(S)CuCPh)4 (Mo2NS),
and Mo2(O2CCuCPh)4 (Mo2OO) were all prepared by ligand
substitution reactions with Mo2(O2CCH3)4. The reaction con-
ditions are summarised in Scheme 2, with some procedures
requiring use of the alkali metal salt or elevated temperatures to
ensure complete substitution of the acetate ligands in the di-
molybdenum starting material.
The solubility of the compounds varied significantly depend-
ing on the nature of the ligand employed; Mo2NN and Mo2NS
are soluble in most organic solvents, Mo2OO is soluble in weak
donor solvents such as THF, whilst Mo2NO is only soluble in
strong donor solvents such as DMSO or DMF. All compounds
gave satisfactory elemental analysis and show the molecular ions
For the compounds Mo2NO and Mo2NS there are 4 possible
regioisomers, depicted in Scheme 3. Isomerisation of dimolybde-
1
num paddlewheel compounds in solution can be studied by H
NMR spectroscopy,37 although in the case of Mo2NO and
Mo2NS overlap of the phenyl proton resonances precluded a
1
variable temperature H NMR study of potential isomeric forms
in solution. In the 13C{1H} NMR spectrum of Mo2NO and
6642 | Dalton Trans., 2012, 41, 6641–6650
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Mo2NS, only one N–C–E resonance was observed demonstrat-
ing that one isomeric form is present in solution at room temp-
erature. There is insufficient information in the NMR spectra to
assign which regioisomer is present in solution, however it is
reasonable to assume that the regioisomers observed in the solid-
state studies, vide infra, persist in solution.
ligands. The metal–metal distance for Mo2OO (2.1106(6) Å) is
also relatively long for a complex of form Mo2(O2CR)4(THF)2;
only Mo2(O2CCF3)4(THF)2 has a longer Mo–Mo bond distance
(2.1202(5) Å).38 The relatively long Mo–Mo bond lengths
observed for both compounds may be a result of significant
Mo2-δ to ligand-π* backbonding.
There have been only four structural studies of dimolybdenum
paddlewheel compounds containing bridging N−C−S− units,
using the ligands −SC(NMe)PMe2 (Mo–Mo = 2.083(1) Å),39
4,6-dimethyl-2-mercaptidopyrimidine (Mo–Mo = 2.083(2) Å),40
7-methyl-1,8-naphthyridine-2-thiolate (Mo–Mo = 2.131(2) Å)
Solid state structures
Despite numerous attempts, we were unable to obtain crystals of
Mo2NN suitable for X-ray diffraction studies. Crystals were
obtained for the compounds Mo2NO (Fig. 1), Mo2NS (Fig. 2)
and Mo2OO (Fig. 3). Selected bond lengths and angles for all
compounds are displayed in Table 1. The molecule Mo2NO lies
and 2-mercaptidoquinoline (Mo–Mo
= 2.089(1) Å). The
Mo–Mo bond lengths for the two independent molecules in the
crystal structure of Mo2NS are slightly different (2.1026(18) and
2.1171(16) Å), but fall within the range observed for other
Mo24+ cores with N–C–S− bridging ligands. The only significant
difference is the orientation of the phenyl ring on the ligand
backbone; the torsion angles with the N–C–S− moiety and
phenyl rings are 10.9(7)° and 72.2(7)°.
4+
about an inversion centre, and there are two independent Mo2
units present in the solid state structure of Mo2NS, which lie at
sites with crystallographically imposed 2-fold symmetry; a
drawing of the second molecule is given in the ESI†.
All compounds display the expected paddlewheel arrangement
of the ligands about the dimetal core. The solid-state structure of
Mo2NO reveals a cis-2,2 arrangement of the ligands, where as
Mo2NS adopts the trans-2,2 regioisomer.
The Mo–Mo bond distance for Mo2NO (2.1269(7) Å) is the
longest bond length found for a homoleptic dimolybdenum
quadruply bonded compound containing N–C–O− bridging
Fig. 2 Solid state structure of one of the crystallographically indepen-
dent cores of Mo2NS, with anisotropic displacement parameters drawn
at the 50% level and hydrogen atoms omitted for clarity. Atoms with a
prime (′), double prime (′′) and triple prime (′′′) character are generated
using the symmetry operations ‘1 − x, 1 − y, z’, ‘y, 1 − x, 2 − z’ and
‘1 − y, x, 2 − z’, respectively.
Fig. 1 Solid state structure of Mo2NO(py)2, with anisotropic displace-
ment parameters drawn at the 50% level and hydrogen atoms omitted for
clarity. Atoms with an additional prime (′) character are generated using
the symmetry operation 1 − x, −y, 1 − z.
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Table 1 Selected bond lengths (Å) and angles (°) for Mo2NO(py)2, Mo2NS, and Mo2OO(THF)2
Mo2NO(py)2
Mo2NS
Mo2OO(THF)2
Mo–Mo
Mo–Leq
Mo1–Mo1′ 2.1269(7)
Mo1–Mo1′′ 2.1026(18)
Mo2–Mo2′′′ 2.1171(16)
Mo1–N1 2.149(6)
Mo1–S1′′ 2.462(2)
Mo2–N2 2.144(6)
Mo2–S2′′ 2.457(2)
—
Mo1–Mo2 2.1106(6)
Mo1–N1 2.136(3)
Mo1–N2 2.141(3)
Mo1–O1 2.111(2)
Mo1–O2 2.114(2)
Mo1–N3 2.690(3)
O1′–C9–C6–C1 72.7(4)
O2′–C24′–C21′–C16′ 29.9(5)
Mo1–O1 2.107(4)
Mo1–O3 2.100(4)
Mo1–O5 2.101(4)
Mo1–O7 2.094(4)
Mo1–O9 2.540(6)
O1–C1–C4–C5 70.6(6)
O4–C10–C13–C14 1.3(6)
Mo2–O2 2.101(4)
Mo2–O4 2.092(4)
Mo2–O6 2.117(4)
Mo2–O8 2.114(4)
Mo2–O10 2.558(6)
O6–C19–C22–C23 80.3(6)
O8–C28–C31–C32 21.4(6)
Mo–Lax
Phenyl torsion
S1–C1–C4–C5 10.9(7)
S2–C16–C19–C20 72.2(7)
Fig. 4 Cyclic voltammograms of Mo2NN (in dichloromethane),
Mo2NO (in dimethylformamide), Mo2NS (in THF) and Mo2OO (in
THF) recorded in 0.1 M nBu4NPF6 solutions.
coordinated donor solvent molecules. The small change
observed in oxidation potential for Mo2NN in dimethylforma-
mide (−0.322 V) and dichloromethane solutions (−0.313 V)
suggests that the ligand N-phenyl groups protect the axial coordi-
nation site of the dimetal core from dimethylformamide coordi-
nation, accounting for the reversible nature of the Mo24+/5+ redox
process.
The trend in oxidation potentials (Mo2NN < Mo2NO <
Mo2NS < Mo2OO) correlate with ligand basicity; the more
basic the ligand the better the stabilisation of the Mo2 oxi-
Fig. 3 Solid state structure of Mo2OO(THF)2. Anisotropic displace-
ment parameters are drawn at the 50% level, with hydrogen atoms have
been omitted for clarity.
5+
dation state. Hence the lowest oxidation potential is observed for
Mo2NN, whilst Mo2OO has the highest. The range of the
Mo24+/Mo2 potentials (∼0.9 V) illustrates the dramatic effect
5+
Electrochemical studies
that the character of the ligand bridging atoms has on the elec-
tronic structure of these compounds.
The cyclic voltammograms of Mo2NN, Mo2NO, Mo2NS, and
Mo2OO are displayed in Fig. 4, with data summarised in
Table 3. All compounds display a single oxidation process
corresponding to the removal of an electron from the highest
Electronic absorption spectroscopy
occupied molecular orbital (HOMO), which is the Mo2-δ orbital.
4+/5+
The Mo2
redox process is reversible for Mo2NS in THF
The UV-Vis absorption spectra of the compounds are shown in
Fig. 5. All compounds show an intense absorption in the visible
region, which can be assigned to Mo2 δ → ligand π* (MLCT)
transitions. The energy of this transition is dependent on the
nature of the ligand. The highest energy MLCT transition is
observed for Mo2OO at 446 nm, with the same transition red
shifted for Mo2NO (496 nm), Mo2NN (520 nm) and Mo2NS
(526 nm). The compound Mo2NO also displays a weak
solution, and Mo2NN in dichloromethane and dimethylforma-
mide. However this process is irreversible for all compounds,
except Mo2NN, in dimethylformamide. In solutions containing
dimolybdenum paddlewheel compounds, donor solvents will
coordinate to the axial sites of the Mo2 core. The irreversible
nature of the oxidation in dimethylformamide is likely a result of
attack of the M–M bond in the Mo25+ ions formed by the axially
6644 | Dalton Trans., 2012, 41, 6641–6650
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transition at 605 nm. A more detailed analysis of these tran-
sitions can be obtained from the time-dependent density func-
tional theory calculations, discussed next.
present in the solid-state or at elevated temperatures in solution,
although no experimental evidence of multiple regioisomers was
observed. The solid state structure of Mo2NO it was found to
adopt the cis-2,2 regioisomer. As this is the only form observed
experimentally, subsequent discussion about Mo2NO′ will focus
on results from the cis-2,2 isomer calculations. For Mo2NS′ and
Mo2SO′, the results of the trans-2,2 isomer calculations will be
discussed.
In order to check that substitution of {NH} for {NPh} did not
affect the relative stability of the isomers, geometry optimisation
on the cis-2,2 and trans-2,2 forms of Mo2NO were performed;
the trans-2,2 isomer was still found to be the most stable.
Electronic structure calculations
The geometry and electronic structures of all the compounds
were studied by density functional theory calculations as
implemented in the Gaussian09 suite of programs.41 The model
compounds Mo2({NH}2CCuCPh)4 (Mo2NN′), Mo2({NH}C(O)-
CuCPh)4 (Mo2NO′) and Mo2({NH}C(S)CuCPh)4 (Mo2NS′),
in which {NPh} has been replaced by {NH}, were used to
reduce computational time. Whilst the compounds Mo2(SC(O)-
CuCPh)4 (Mo2SO) and Mo2(S2CCuCPh)4 (Mo2SS) could not
be isolated in this study due to their instability at room tempera-
ture, homoleptic dimolybdenum monothio- and dithiocarboxy-
late paddlewheel compounds are known.42 Calculations were
therefore also performed on Mo2SO and Mo2SS as it is informa-
tive to consider the effect of sulphur coordination on the elec-
tronic structure. Selected bond lengths for the optimised
structures are given in Table 2, and show excellent agreement
with the solid state structures obtained for Mo2NS, Mo2OO and
Mo2NO.
Electronic structure. The calculated frontier molecular orbital
energy levels and selected MO diagrams for Mo2OO are dis-
played in Fig. 7. The HOMO in all instances is the Mo2 δ, with
energies given in Table 3 ranging from −3.83 eV for Mo2NN to
−5.26 eV for Mo2SS. This large energy range of ∼1.4 eV shows
that substitution of N for O for S can result in dramatic changes
in the electronic structure of dimolybdenum paddlewheel com-
pounds. This effect is even greater than would be expected if Mo
was substituted by W, which would result in the M2-δ orbital
rising by ∼0.5 eV in energy.43 The electrochemical data is also
included in Table 3, and shows that the calculated trend in
HOMO energy is matched by experiment.
The LUMO for each compound is one of the ligand π* orbital
combinations, and included in Table 3 is the calculated HOMO–
LUMO gap. The effect of N for O for S substitution on the
HOMO–LUMO gap is less dramatic than observed for the
HOMO energy. Introduction of the more electronegative O for N
reduces the energy of the ligand π* orbitals. The effect of S for
Regioisomers. We first probed which regioisomers of
Mo2NO′, Mo2NS′ and Mo2SO are more stable in the gas phase
by optimising the structures of the 4,0, 3,1, trans-2,2, and cis-2,2
regioisomers (see Scheme 3). The results shown in Fig. 6 indi-
cate that the trans-2,2 regioisomer is the most stable in all
instances. For Mo2NO′ and Mo2SO, there are a number of
regioisomers close (<5 kJ mol−1) in energy to the trans-2,2
form. This suggests that more than one regioisomer could be
Fig. 5 UV-Vis spectra of Mo2NN, Mo2NO, Mo2NS and Mo2OO
recorded in DMF solutions at room temperature.
Fig. 6 Calculated stabilities of isomeric forms of Mo2NO′, Mo2NS′
and Mo2SO.
Table 2 Calculated bond lengths (Å)
Mo2NN′
Mo2NO′
Mo2NS′
Mo2OO
Mo2SO
Mo2SS
Mo–Mo
Mo–N
Mo–O
Mo–S
2.137
2.149
—
2.130
2.140
2.107
—
2.142
2.140
—
2.124
—
2.104
—
2.156
2.150
2.078
2.495
—
2.506
2.496
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Fig. 7 Calculated frontier molecular orbital energies and selected Gaussview plots of Mo2OO orbitals (drawn with an isosurface value of 0.03).
Table 3 Calculated MO energies, and a comparison of the experimental observed and calculated energy of the MLCT transition
HOMO–LUMO
λmax c obs (ε)/
λmax calc
assignment
Compound
HOMO/eVa
gap/eVa
ΔE(Lπ*)/eVa
E1/2(1)/Vb,c
nm (M−1 cm−1
)
λmaxa calc (f)/nm
Mo2NN
Mo2NO
−3.83
−4.31
2.62
2.77
0.46
0.52
−0.322 (−0.313d)
−0.200e
520 (5600)
605 (600)
496 (6000)
539 (1.564)
523 (0.545)
504 (0.778)
476 (0.216)
508 (1.300)
468 (0.138)
484 (1.560)
527 (1.297)
570 (1.064)
421 (0.302)
δ → Lπ*
δ → Lπ*
δ → Lπ*
δ → δ*
δ → Lπ*
δ → M2-π*
δ → Lπ*
δ → Lπ*
δ → Lπ*
S l.p. → Lπ*
Mo2NS
−4.68
2.79
0.55
0.092e (0.297f)
526 (6600)
Mo2OO
Mo2SO
Mo2SS
−4.82
−5.02
−5.27
2.95
2.74
2.56
0.49
0.60
0.45
0.282e (0.410e,f
—
—
)
446 (9100)
—
—
a Calculated values. b vs. Cp2Fe0/+ c Recorded in dimethylformamide solution. d Recorded in dichloromethane solution. e Irreversible oxidation wave.
.
f Recorded in THF solution.
N substitution is also to reduce the energy of the ligand π*,
however in this instance it is the longer and weaker C–S bond
that is responsible for the reduction in energy. These effects
combine to result in similar reduction in the ligand π* orbital
energies as the Mo2-δ orbitals energies are reduced.
observed. This can be rationalised by examining the MO dia-
grams which show the amount N–C–E π* character in the ligand
π* orbital follows the trend E = N < O < S. The bridging N–C–
E group is serving as an ‘alligator clip’,6,44 coupling metal and
ligand π orbitals. The large 3p orbitals of sulphur have the best
overlap, and hence the strongest metal–ligand coupling is
observed. This highlights that it is essential to consider metal
and E–C–E′ orbital overlap, and not just MO energy, when
choosing ligands to electronically couple dimetal units.
For Mo2SS, a smaller than expected separation of the ligand
π* orbital combinations is observed. The MO diagram of the b2g
ligand π* combination for Mo2SS, shown in the ESI†, reveals
additional bonding interactions between the diffuse S 3p orbitals
on adjacent ligands are present, which serve to stabilise the
orbital.
In complexes of the type M2(TiPB)2(L)2 (L = π-accepting car-
boxylate ligand) the separation between the in- and out-of-phase
π* combinations of ‘L’ can be used as a measure of the strength
of interaction between the O2CR π-systems and the Mo2-δ.43 In
homoleptic paddlewheel compounds having D4h symmetry, there
are three non-bonding ligand π* combinations of eu and a2g sym-
metry, and a b2g combination that has the correct symmetry to
interact with the Mo2-δ. Molecular orbital diagrams of the b2g
combination are given in the ESI†. The extent of separation
between these bonding and non-bonding combinations, ΔE-
(Lπ*), is therefore an indication of the extent of coupling
between the M2-δ and ligand π-systems, with values presented in
Table 3. Based on the HOMO–LUMO separation, the magnitude
of ΔE(Lπ*) would be expected to follow the order Mo2NN <
Mo2NO < Mo2NS. The reverse of this order is actually
Time-dependent DFT was used to calculate the absorption
spectra for each compound. Calculated transitions with signifi-
cant oscillator strength (f > 0.1) in the visible region are listed in
Table 3. In each instance, the main transition in the visible
region is the expected Mo2-δ to ligand π* transition, with the
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Materials and methods
calculated values closely matching the experimental data.
However, they do not predict the weak transition observed at
605 nm for Mo2NO. This peak could be due to a δ → δ* tran-
sition, which for Mo2NO′ is calculated to have appreciable oscil-
lator strength.
All experimental manipulations were performed under an inert
atmosphere using standard Schlenk-line and glovebox tech-
niques. THF was distilled over sodium wire, and methanol was
distilled over CaH2. All other solvents obtained from a “Grubbs”
solvent purification system. N,N′-Diphenyl carbodiimide
(PhNvCvNPh),45 PhCuC(O)CNHPh,46 and Mo2(O2-
Conclusion
47
CCH3)4 were synthesised according to literature procedures.
All other chemicals were obtained from commercial sources.
This investigation has probed how N for O for S substitution in
the E–C–E′ bridging group of dimolybdenum quadruply bonded
paddlewheel compounds influences their properties. For the
Mo2(EE′CCuCPh)4 compounds studied, this simple change was
shown to have a dramatic effect on the electronic structure of
Preparation of PhCuC(NPh)CN(H)Ph. This ligand was syn-
thesised by a modified literature procedure.48 Phenyl acetylene
(0.272 g, 2.66 mmol) was dissolved in THF (20 ml) and cooled
to −78 °C. n-Butyllithium (1.06 ml of a 2.5 M solution in
hexane, 2.66 mmol) was added slowly to the solution and stirred
for 20 min at −78 °C. A solution of freshly prepared and
degassed N,N′-diphenyl carbodiimide (0.486 g, 2.50 mmol) was
dissolved in THF (10 ml) and added dropwise. The pale yellow
solution was allowed to warm slowly to room temperature and
stirred for 2 h, producing a further colour change to orange. The
reaction was quenched by addition of MeOH (25 ml), and the
solvent removed to leave a yellow solid. The solid was purified
by recrystallisation from hot hexane to yield orange needle crys-
4+
dimolybdenum core. Electrochemical studies showed that Mo2
oxidation can be tuned over a range of nearly 0.9 V, with N for
O for S substitution, because of decreases in ligand basicity.
Density functional theory calculations indicate that this range
could be increased upon inclusion of mono- and di-thiocarboxy-
late ligands to the series.
Dicarboxylates are by far the most common ligands used in
the assembly of molecular architectures incorporating M24+ units
or other metal clusters. This study has shown that the optoelec-
tronic and redox properties of these assemblies may be tuned by
judicious selection of bridging ligand. Given that heteroatom
substitution has a dramatic effect on the ground-state properties
of these molecules, future studies will also investigate differ-
ences in the photoexcited states of these compounds.
1
tals (0.652 g, 88%). H NMR (CDCl3, 20 °C): δ 8.06 (s, 1H,
NH), 7.77–7.29 (m, 15H, ArH). 13C{1H} NMR (CDCl3, 20 °C):
80.6 (Ar–CuC); 92.7 (Ar–CuC); 120.7, 121.2, 123.3, 128.5,
128.7, 129.8, 132.2, 139.4 (Aryl C); 146.0 (NvC(C)N). mp
121 °C (lit.,48 122–123.5 °C). ESI-MS: calcd monoisotopic MW
for C21H16N2, 296.1; found m/z 297.1 (MH+, 100.0%).
Experimental
Preparation of Mo2({NPh}2CCuCPh)4, Mo2NN. A mixture
of sodium hydride (17 mg, 0.70 mmol) and PhCuC(NPh)
CNHPh (207 mg, 0.70 mmol) in THF (15 ml) were heated to
60 °C for 15 min, where upon an orange solution was formed.
The solution was then cooled to −78 °C and a solution of
Mo2(O2CCH3)4 (50 mg, 0.12 mmol) in THF (5 ml) was added
dropwise. After addition, the reaction mixture was heated to
55 °C, and stirred at this temperature for 72 h. The reaction was
then cooled to room temperature, and solvent removed in vacuo
to leave a dark red solid. The product was extracted from this
residue using warm (50 °C) hexane, and filtered before drying in
vacuo to yield a bright red solid (52 mg, 32% yield). Anal. calcd
for C84H60N8Mo2: C, 73.46; H, 4.40; N, 8.16. Found: C, 73.82;
H, 4.46; N, 8.09%. MALDI-TOF-MS: calcd monoisotopic MW
Physical measurements
Elemental analyses were carried out by the Microanalytical
Service of the Department of Chemistry at Sheffield with a
Perkin-Elmer 2400 analyzer. Electronic absorption spectra were
recorded using a Varian Cary 5000 UV-Vis-NIR spectropho-
tometer. Electrochemical measurements were carried out in nitro-
gen-purged 0.1 M [nBu4N][PF6] solutions using a standard
three-electrode system with a Pt microdisc working electrode, Pt
wire counter electrode, and Ag/AgCl reference electrodes. At the
end of every experiment ferrocene was added as an internal stan-
dard. ESI mass spectra were collected on a Waters Micromass
LCT operating in ESI mode. Matrix assisted laser desorption/ion-
isation time-of-flight (MALDI-TOF) mass spectrometry was per-
formed on a Bruker Reflex III (Bruker, Breman, Germany) mass
spectrometer operated in positive ion mode with a N2 laser.
Laser power was used at the threshold level required to generate
signal. Dithranol was used as the matrix and prepared as a satu-
rated 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. IR spectra were recorded as solid
samples with a Perkin-Elmer Spectrum RX I FT-IR spectrometer
equipped with a DuraSamplIR II diamond ATR probe and uni-
1
for C84H60N8Mo2, 1376.3; found m/z 1376.1 (M+, 100%). H
NMR (CDCl3, 20 °C): δ 7.55–7.02 (m, 60H, ArH). 13C{1H}
NMR(CDCl3, 20 °C): δ 83.3 (Ar–CuC); 92.6 (Ar–CuC);
120.6, 123.6, 125.46, 128.5, 128.4, 128.7, 131.9, 139.2 (aryl C);
149.9 (NvC(C)N). UV-Vis (DMF) [λmax, nm (ε, M−1 cm−1)]:
517 (5590).
Preparation of Mo2(OC{NPh}CuCPh)4, Mo2NO. Sodium
hydride (0.042 g, 1.8 mmol) and PhCuCC(O)NHPh (0.388 g,
1.8 mmol) were suspended in THF (20 ml) and heated to 60 °C
for 15 min. The orange solution that formed was then cooled to
−78 °C and a solution of Mo2(O2CCH3)4 (0.150 g, 0.35 mmol)
in THF (5 ml) was added dropwise. The mixture was stirred at
room temperature for 16 hours producing a solution with an
orange suspension. The precipitate was isolated by filtration and
1
versal press. H and 13C NMR spectra were collected at room
temperature on Bruker Avance 250, 400 or DRX500 spec-
trometers. Chemical shifts were assigned relative to the residual
solvent peak and are given to 0.01 ppm for 1H and 0.1 ppm
for 13C.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 6641–6650 | 6647

View Article Online
washed with THF (3 × 10 ml aliquots) before drying in vacuo to
yield the product as a bright orange solid (250 mg, 67% yield).
Crystals suitable for X-ray diffraction were grown by slow diffu-
sion of Et2O into a pyridine solution containing Mo2NO. Anal.
calcd for C60H40N4O4Mo2: C, 67.17; H, 3.76; N, 5.22. Found:
C, 67.37; H, 3.79; N, 5.13%. MALDI-TOF-MS: calcd monoiso-
topic MW for C60H40N4O4Mo2, 1076.1; found m/z 1075.9 (M+,
1441m, 1386s, 1220m, 986w, 942w. UV-Vis (DMF) [λmax, nm
(ε, M−1 cm−1)]: 265 (19 030), 441 (10 540).
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.49 The structures were solved by direct
methods (SHELXS-97)50 and refined by full-matrix least squares
on weighted F2 values for all reflections.51 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.
1
100%). H NMR (dmso-d6, 20 °C): δ 7.79–7.32 (m, 40H, ArH).
13C{1H} NMR (DMSO-d6, 20 °C): δ 83.5 (Ar–CuC); 87.0
(Ar–CuC); 120.7, 122.1, 124.6, 126.0, 128.3, 129.2, 132.3,
154.6 (aryl C); 174.7 (OvC(C)N). IR(cm−1): 2360w, 2340w,
1573s, 1488m, 1468s, 1438s, 1387s, 1259w, 1200m, 1098w,
1009w, 992w. UV-Vis (DMF) [λmax, nm (ε, M−1 cm−1)]: 265
(24 570), 492 (6010).
For Mo2NO(py)2·5py, pyridine solvate molecule located on
the inversion centre is delocalised over two positions with site
occupancy 0.5/0.5. Two other solvate molecules were disordered
over two positions with site occupancies of 0.76/0.24 and 0.52/
0.48. The axially coordinated THF molecules in Mo2OO(THF)2
are disordered and were refined isotropically over two positions,
both with site occupancies of 0.65/0.35. The toluene solvate
molecules in Mo2NS·4(toluene) are also disordered over two
positions with occupancies of 0.51/0.49 and 0.67/0.33, and
were refined isotropically. The residual electron density peak of
2.405 e Å−3 is located close (1.059 Å) to Mo2.
CCDC 859556 [Mo2NO(py)2·5py], 859557 [Mo2NS·4
(toluene)] and 859558 [Mo2NS·4(toluene)] contain the sup-
plementary crystallographic data for this paper. Experimental
data relating to the structure determinations of all complexes are
displayed in Table 4.
Preparation of Mo2(SC{NPh}CuCPh)4, Mo2NS. Phenyl
acetylene (0.192 ml, 1.75 mmol) was dissolved in THF and
cooled to −78 °C. n-Butyllithium (0.70 ml of a 2.5 M solution
in hexanes, 1.75 mmol) was added to the solution and stirred for
20 min at −78 °C. Phenyl isothiocyanate (0.201 ml, 1.75 mmol)
was added dropwise, then the solution was allowed to warm to
room temperature and stirred for 1 h producing a dark red sol-
ution. The solution was cooled to −78 °C and transferred to a
Schlenk flask containing a suspension of Mo2(O2CCH3)4
(0.150 g, 0.35 mmol) in THF (10 ml) also held at −78 °C. The
mixture was allowed to warm slowly to room temperature, and
stirred for 16 h. The solvent was removed in vacuo to leave a red
solid, which was redissolved in warm toluene (20 ml, 80 °C) and
filtered whilst hot. The filtrate was allowed to cool slowly to
room temperature yielding the product as red-orange crystals,
which were isolated by decantation and dried in vacuo (0.320 g,
81%). Anal. calcd for C60H40N4S4Mo2: C, 63.37; H, 3.55; N,
4.93; S, 11.28. Found: C, 63.82; H, 3.72; N, 4.87; S, 11.27%.
Computational details
MALDI-TOF-MS:
calcd
monoisotopic
MW
for
C60H40N4S4Mo2, 1140.0; found m/z 1140.2 (M+, 100%). 1H
NMR (DMSO-d6, 20 °C): δ 7.51–7.32 (m, 16H, ArH),
7.29–7.02 (m, 24H, ArH). 13C{1H} NMR (DMSO-d6, 20 °C): δ
88.5 (Ar–CuC); 95.8 (Ar–CuC); 120.5, 122.2, 123.2, 127.7,
128.7, 130.7, 131.8, 153.8 (aryl C); 174.7 (SvC(C)N). IR
(cm−1): 2359s, 2343m, 1589s, 1482m, 1421s, 1274m, 1766w,
1096s, 1070m, 1024m. UV-Vis (DMF) [λmax, nm (ε, M−1
cm−1)]: 286 (14 610), 520 (6680).
Molecular structure calculations were performed using density
functional theory as implemented in the Gaussian 09 software
package.41 The B3LYP functional52,53 and the 6-31G*(5d) basis
set54 were used for H, C, O, N and S, along with the SDD
energy consistent pseudopotentials for molybdenum.55 This
level of theory was chosen as it was recommended in a bench-
mark study probing the physical and electronic structure of
M2(O2CR)4
compounds.56
The
model
compounds
Mo2({NH}2CCuCPh)4 (Mo2NN′), Mo2({NH}C(O)CuCPh)4
(Mo2NO′) and Mo2({NH}C(S)CuCPh)4 (Mo2NS′), in which
{NPh} has been replaced by {NH}, were used. For Mo2NS′ and
Mo2SO results from calculations on the trans-2,2 regioisomer
are used in the discussion, and for Mo2NO the discussion is
based on computational results from the cis-2,2 regioisomer. The
structure of each compound was optimised in the gas phase in
Preparation of Mo2(O2CCuCPh)4, Mo2OO. Phenylpropiolic
acid (0.250 g, 1.71 mmol) and Mo2(O2CCH3)4 (0.100 g,
0.23 mmol) were refluxed in toluene (15 ml) for 16 h. The reac-
tion mixture was cooled to room temperature producing an
orange precipitate. This precipitate was isolated by filtration, and
washed with toluene (3 × 10 ml aliquots) before drying in
vacuo, yielding Mo2(O2CCuCPh)4 as a bright orange powder
(0.110 g, 60%). Crystals suitable for X-ray diffraction were
grown by slow diffusion of hexane into a THF solution contain-
ing Mo2OO. Anal. calcd for C36H20O8Mo2: C, 55.98; H, 2.61.
Found: C, 56.15; H, 2.73%. MALDI-TOF-MS: calcd monoisoto-
pic MW for C36H20O8Mo2, 775.9; found m/z 775.9 (M+, 100%).
1H NMR (DMSO-d6, 20 °C): δ 7.85–7.52 (m, 20H, ArH). 13C
{1H} NMR (DMSO-d6, 20 °C): δ 81.8 (Ar–CuC); 83.7 (Ar–
CuC); 118.9, 129.0, 130.9, 132.8 (aryl C); 183.4 (OvC(C)O).
IR(cm−1): 2360w, 2343w, 2210m, 2182w, 1492m, 1475s,
D
4h (Mo2NN′, Mo2OO, Mo2SS), D2d (Mo2NS′, Mo2SO) or C2h
(Mo2NO′) symmetry, and confirmed to be minima on the poten-
tial energy surface using harmonic vibrational frequency analy-
sis. Electronic absorption spectra were calculated using the time-
dependent DFT (TD-DFT) method.
Acknowledgements
The Royal Society are thanked for the award of a University
Research Fellowship (NJP). S. R. thanks the E-Futures Doctoral
6648 | Dalton Trans., 2012, 41, 6641–6650
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 4 Crystallographic data for Mo2NO(py)2·5py, Mo2NS·4(toluene) and Mo2OO(THF)2
Compound
Mo2NO(py)2·5py
Mo2NS·4(toluene)
Mo2OO(THF)2
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C95H75Mo2N11O4
1626.54
100(2) K
0.71073 Å
Monoclinic
C88H72Mo2N4S4
1505.62
150(2) K
0.71073 Å
Tetragonal
ˉ
I4
C44H36Mo2O10
916.61
150(2) K
0.71073 Å
Monoclinic
P21/n
P21/c
Unit cell dimensions
a = 13.501(4) Å; α = 90°
b = 20.289(6) Å; β = 90.104(8)°.
c = 14.126(4) Å; γ = 90°
3869(2) Å3
a = 26.8014(14) Å; α = 90°
b = 26.8014(14) Å; β = 90.104(8)°
c = 10.1004(6) Å; γ = 90°
7255.3(7) Å3
a = 18.8160(6) Å; α = 90°
b = 12.8292(4) Å; β = 90.032(2)°
c = 17.2278(5) Å: γ = 90°
4158.7(2) Å3
Volume
Z
2
4
4
Density (calculated)
Absorption coefficient
F(000)
1.396 Mg m−3
0.387 mm−1
1.378 Mg m−3
0.511 mm−1
1.464 Mg m−3
0.659 mm−1
1676
3104
1856
θ range for data collection
Index ranges
1.76 to 27.55°
−17 ≤ h ≤ 17,
−26 ≤ k ≤ 25,
−18 ≤ l ≤ 18
37 994
1.07 to 27.47°
1.08 to 27.50°
−24 ≤ h ≤ 24,
−16 ≤ k ≤ 16,
−22 ≤ l ≤ 22
53 498
−34 ≤ h ≤ 34,
−34 ≤ k ≤ 33,
−12 ≤ l ≤ 13
Reflections collected
44 580
Independent reflections
Completeness to θ
8876 [R(int) = 0.0464]
100.0%
8876/220/591
1.181
R1 = 0.0529, wR2 = 0.1280
R1 = 0.0708, wR2 = 0.1399
0.468 and −0.631
8266 [R(int) = 0.0845]
99.6%
8266/27/383
9550 [R(int) = 0.0396]
99.8%
9550/14/499
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
1.068
1.271
R1 = 0.0556, wR2 = 0.1276
R1 = 0.0908, wR2 = 0.1479
2.405 and −0.901
R1 = 0.0596, wR2 = 0.1406
R1 = 0.0668, wR2 = 0.1434
1.154 and −1.493 e Å−3
Largest diff. peak and hole (e Å−3
)
16 B. G. Alberding, M. H. Chisholm, Y.-H. Chou, J. C. Gallucci, Y. Ghosh,
T. L. Gustafson, N. J. Patmore, C. R. Reed and C. Turro, Inorg. Chem.,
2009, 48, 4394.
17 B. G. Alberding, M. H. Chisholm, T. L. Gustafson, Y. Liu, C. R. Reed
and C. Turro, J. Phys. Chem. A, 2010, 114, 12675.
Training Centre, a programme funded by the Engineering and
Physical Sciences Research Council (EPSRC), for support.
Dr Anthony Meijer is thanked for computational assistance and
Mr Harry Adams is thanked for crystallographic help.
18 B. G. Alberding, M. H. Chisholm, Y.-H. Chou, Y. Ghosh,
T. L. Gustafson, Y. Liu and C. Turro, Inorg. Chem., 2009, 48, 11187.
19 G. T. Burdzinski, M. H. Chisholm, P.-T. Chou, Y.-H. Chou, F. Feil,
J. C. Gallucci, Y. Ghosh, T. L. Gustafson, M.-L. Ho, Y. Liu, R. Ramnauth
and C. Turro, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 15247.
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21 F. A. Cotton, C. Lin and C. A. Murillo, Acc. Chem. Res., 2001, 34, 759.
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