Substitution chemistry of MM quadruply bonded complexes (M = Mo or W) supported by the anion of 2-hydroxy-6-methylpyridine

Article abstract of DOI:10.1039/b618957a

The compounds M₂(mhp)₄, where M = Mo or W and mhp is the anion formed from deprotonation of 2-hydroxy-6-methylpyridine, are shown to react with carboxylic acids RCOOH to give an equilibrium mixture of products M₂(O₂CR)_n(mhp)_4-n where R = 2-thienyl and phenyl. The equilibrium can be moved in favor of M ₂(O₂CR)₄ by the addition of excess acid or by the favorable crystallization of these products. The latter provides a facile synthesis of the W₂(O₂CR)₄ compound where R = 9-anthracene. Reactions involving 2,4,6-triisopropyl benzoic acid, TiPBH, yield M₂(TiPB)₂(mhp)₂ compounds as thermodynamic products. Reactions involving Me₃OBF₄ (1 and 2 equiv.) yield the complexes Mo₂(mhp)₃(CH₃CN) ₂BF₄ and Mo₂(mhp)₂(CH ₃CN)₄(BF₄)₂, respectively. The latter compound has been structurally characterized and shown to have mirror symmetry with two cis mhp ligands: MoMo = 2.1242(5) A, Mo-O = 2.035(2) A, Mo-N(mhp) = 2.161(2) A, and Mo-N(CH₃CN) = 2.160(3) and 2.170(3) A. Reactions involving Mo₂(mhp)₃(CH ₃CN)₂²⁺ and Mo₂(mhp) ₂(CH₃CN)₄²⁺ with ⁿBu ₄NO₂CMe (1 and 2 equiv.) yield the complexes Mo ₂(mhp)₃(O₂CMe) and Mo₂(mhp) ₂(O₂CMe)₂ which are shown to be kinetically labile to ligand scrambling. Reactions between Mo₂(mhp) ₃(CH₃CN)₂⁺BF₄ ^- (2 equiv.) and [ⁿBu₄N⁺] ₂[O₂C-X-CO₂]^2- yielded dimers of dimers [Mo₂(mhp)₃]₂(-O₂C-X-CO ₂] where X = nothing, 2,5- or 3,4-thienyl and 1,4-C₆H ₄. Reactions between Mo₂(mhp)₂(CH ₃CN)₄²⁺(BF₄^-) ₂ and tetra-n-butylammonium oxalate and terephthalate yield compounds [Mo(mhp)₂bridge]_n which by MALDI-TOF MS are proposed to be a mixture of molecular squares (n = 4) and triangles (n = 3) along with minor products of [Mo₂(mhp)₃]₂(bridge) and Mo ₂(mhp)₄ that arise from ligand scrambling. The Royal Society of Chemistry.

Full text of DOI:10.1039/b618957a

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Substitution chemistry of MM quadruply bonded complexes (M = Mo or W)
supported by the anion of 2-hydroxy-6-methylpyridine†‡
Douglas J. Brown, Malcolm H. Chisholm* and Christopher W. Gribble
Received 2nd January 2007, Accepted 12th March 2007
First published as an Advance Article on the web 12th April 2007
DOI: 10.1039/b618957a
The compounds M2(mhp)4, where M = Mo or W and mhp is the anion formed from deprotonation of
2-hydroxy-6-methylpyridine, are shown to react with carboxylic acids RCOOH to give an equilibrium
mixture of products M2(O2CR)n(mhp)4−n where R = 2-thienyl and phenyl. The equilibrium can be
moved in favor of M2(O2CR)4 by the addition of excess acid or by the favorable crystallization of these
products. The latter provides a facile synthesis of the W2(O2CR)4 compound where R = 9-anthracene.
Reactions involving 2,4,6-triisopropyl benzoic acid, TiPBH, yield M2(TiPB)2(mhp)2 compounds as
thermodynamic products. Reactions involving Me3OBF4 (1 and 2 equiv.) yield the complexes
Mo2(mhp)3(CH3CN)2BF4 and Mo2(mhp)2(CH3CN)4(BF4)2, respectively. The latter compound has been
structurally characterized and shown to have mirror symmetry with two cis mhp ligands: MoMo =
˚
˚
˚
2.1242(5) A, Mo–O = 2.035(2) A, Mo–N(mhp) = 2.161(2) A, and Mo–N(CH3CN) = 2.160(3) and
2.170(3) A. Reactions involving Mo2(mhp)3(CH3CN)22+ and Mo2(mhp)2(CH3CN)42+ with nBu4NO2CMe
˚
(1 and 2 equiv.) yield the complexes Mo2(mhp)3(O2CMe) and Mo2(mhp)2(O2CMe)2 which are shown to
+
−
be kinetically labile to ligand scrambling. Reactions between Mo2(mhp)3(CH3CN)2 BF4 (2 equiv.) and
[nBu4N+]2[O2C–X–CO2]2− yielded dimers of dimers [Mo2(mhp)3]2(l-O2C–X–CO2] where X = nothing,
2,5- or 3,4-thienyl and 1,4-C6H4. Reactions between Mo2(mhp)2(CH3CN)42+(BF4−)2 and
tetra-n-butylammonium oxalate and terephthalate yield compounds [Mo(mhp)2bridge]n which by
MALDI-TOF MS are proposed to be a mixture of molecular squares (n = 4) and triangles (n = 3)
along with minor products of [Mo2(mhp)3]2(bridge) and Mo2(mhp)4 that arise from ligand scrambling.
and mhp represents the deprotonated anion.8
Introduction
D
One of the simplest and easiest routes into the chemistry of
MoMo quadruply bonded compounds is via the Mo2(O2CR)4
compounds which are readily prepared from Mo(CO)6 according
to the reaction shown in eqn (1).1
2M(CO)6 + 4mhpH −−−−−→ M2(mhp)4 + 12CO + 2H2
(2)
diglyme
This prompted us to investigate the substitution chemistry of
the M2(mhp)4 complexes and to attempt an alternate route to
W2(O2CR)4 compounds from that described above.
D
2Mo(CO)6 + 4RCOOH −−−−−→ Mo2(O2CR)4 + 12CO + 2H2
C
H Cl
4 2
6
Results and discussion
(1)
Regrettably the analogous reaction involving W(CO)6 does not
yield W2(O2CR)4 but rather products of further oxidation and
degradation of carboxylate ligand.2–4 Because of this, the chemistry
of the WW quadruple bond still remains relatively undeveloped
when compared to that of molybdenum.5 In fact, the synthesis of
W2(O2CR)4 compounds is considerably more tedious and typically
involves a reduction of WCl4 by Na/Hg or a similar reducing agent
in THF followed by a salt metathesis reaction with NaO2CR.6,7
Even then the yields of the highly reactive W2(O2CR)4 compounds
are relatively low.
Reactions involving carboxylic acids
The reactions between W2(mhp)4 and RCOOH, where R = Ph and
2-thienyl yield a mixture of W2(mhp)4−x(O2CR)x according to the
reaction shown in eqn (3), where x = 1–4.
◦
60−70
C
W2(mph)4 + 4RCOOH −−−−−−→
(3)
toluene
W2(mhp)4−x(O2CR)x + xmhpH + (4 − x)RCOOH
This reaction can be followed by both NMR spectroscopy
and mass spectrometry though NMR is somewhat ambiguous
due to the possibility of the existence of various isomers. As
shown in Fig. 1 with increasing equivalents of added RCOOH the
equilibrium can be driven in favor of the W2(O2CR)4 compound
although this is not a very satisfactory new general synthesis for
W2(O2CR)4 compounds. However, in the case of R = 9-anthracene
the compound W2(O2CR)4 is significantly less soluble than the
W2(mhp)4−x(O2CR)x compounds and so can be readily separated
from the equilibrium mixture. Indeed, in reaction (3) employing
≥4 equiv. of 9-anthracenecarboxylic acid, the synthesis of this
There is, however, one reaction involving the M(CO)6 com-
pounds (M = Mo or W) that does lead to MM quadruply bonded
compounds in high yields and that is the reaction shown in eqn
(2) where mhpH is the neutral ligand 2-hydroxy-6-methylpyridine
Department of Chemistry, The Ohio State University, 100 W. 18th Ave,
Columbus, Ohio, 43210, USA
† Electronic supplementary information (ESI) available: Full crystallo-
graphic data. See DOI: 10.1039/b618957a
‡ Based in part on the B.S. Honor Thesis of C. W. G.
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Fig. 2 UV-vis spectra recorded in THF solution.
of the intense absorption at 280 nm in the spectrum of the mhp
complex and indeed all mhp containing complexes. We assign
this to a ligand p to p* transition. At lower energy there are
intense absorptions due to M2d to ligand charge transfer. In the
mixed M2(mhp)4−x(TiPB)x compounds where x = 2 and 3, the M2d
to mhp and M2d to TiPBp* transitions overlap.
A comparison of the spectra of the complexes M2(mhp)2(TiPB)2
where M = Mo and W is shown in Fig. 3. In both metal complexes
the absorption at 280 nm is present which is consistent with our
suggestion that this is a ligand based transition involving mhp. The
lower energy bands are, however, quite different and the essential
difference is that the M2d to ligand p* transitions are red shifted
where M = W by ca. 0.5 eV, consistent with the higher energy of
the W2d orbitals.11
Fig. 1 MALDI-TOF MS of the products obtained from the reaction
between W2(mhp)4 and 4 equiv. PhCOOH (a) and W2(mhp)4 and 16 equiv.
PhCOOH (b).
compound is efficient and provides a notable improvement over
the more traditional route.
The reaction involving 2,4,6-triisopropyl benzoic acid proceeds
to give the compound W2(mhp)2(TiPB)2, where TiPB = 2,4,6-
triisopropyl benzoate. This intensely bright red colored complex
appears to be the thermodynamically favored product. No further
mhp substitution is observed, possibly due to the steric demands of
this bulky carboxylate ligand. The 1H NMR spectrum reveals the
presence of one isomer and even at 500 MHz there is no evidence
for diastereotopic isopropyl groups. Thus, we propose that the
complex adopts the trans geometry as we have found for a number
of other complexes of formula M2(TiPB)2(O2CR)2, where R =
thienyl, bithienyl and terthienyl.9,10
Reactions involving Mo2(mhp)4 proceed qualitatively similarly
and both the Mo2(mhp)2(TiPB)2 and Mo2(mhp)3(TiPB) complexes
have been isolated in pure form. The Mo2(mhp)3(TiPB) complex
has been prepared by the salt metathesis reaction between
Fig. 3 UV-vis spectra of M2(TiPB)2(mhp)2 in THF M = Mo (blue)
and M = W (brown).
[Mo2(mhp)3(CH3CN)2]+BF4 and [nBu4N][TiPB].
−
In comparing the spectra of the Mo2(mhp)2(TiPB)2 with that
of Mo(mhp)4 and Mo2(TiPB)4 we note that the Mo2d to mhp
absorption occurs at longer wavelength, 420 nm. Also for the
sterically demanding TiPB ligand the aromatic group commonly
twists significantly out of the CO2 plane with a dihedral angle
>50◦.9,10 This leads us to suggest that for Mo2(mhp)2(TiPB)2 the
absorption at 340 nm arises from Mo2d to TiPB MLCT.
Electronic absorption spectra of Mo2(mhp)4−x(TiPB)x (x = 0, 1, 2,
4) and W2(mhp)2(TiPB)2
A comparison of the electronic absorption spectra of the Mo2-
containing complexes is shown in Fig. 2. A notable difference
between the spectra of Mo2(mhp)4 and Mo2(TiPB)4 is the presence
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[M2(mhp)4−x(CH3N)2x]x+[BF4−]x, where x = 1, 2
from its preferential crystallization from solution but by 1H NMR
spectroscopy we only find evidence for one isomer in solution.
Electronic structure calculations employing density functional
The reaction between Mo2(mhp)4 and Meerwein’s reagent,
Et3O+BF4 in acetonitrile has previously been shown to give
−
2+
theory on cis-Mo2(mhp)2(CH3CN)4 with Cs and C2 symmetry
the monocation (x = 1) and this was structurally characterized
as a weakly associated dimer via intermolecular Mo2 · · · O(mhp)
bonds.12 In this work we have extended these studies to prepare the
dication, x = 2, by the use of ≥4 equiv. of Et3O+BF4−. However,
these reactions only proved successful for M = Mo. All attempts
to prepare related tungsten complexes failed to give the desired
complexes. The only isolable compound from these reactions was
W2(mhp)4. Also a reaction involving W2(mhp)3Cl2 (2 equiv.)13
and Zn (1 equiv.) in acetonitrile was undertaken in an attempt
were carried out (see Experimental) and these indicated that both
isomers in the gas phase had a very similar ground state energy
with that of the C2 isomer being slightly lower in the gas phase. It
may well be that the Cs structure, having a polar MoMo bond, is
favored in the polar solvent CH3CN.
Dimers of dimers: [Mo2(mhp)3]2(O2C–X–CO2)
The metathetic reaction involving [Mo2(mhp)3(CH3CN)2][BF4]
and the tetra-n-butyl salt of a dicarboxylic acid in acetonitrile
leads to the formation of linked Mo2 units as shown in eqn (4)
to prepare the salt [W2(mhp)3(CH3CN)2]2 [ZnCl4]2− but again the
+
only isolable product was W2(mhp)4. Furthermore, in an attempt
to introduce a substitutionally labile halide ligand reactions
involving W2(mhp)4 and each of BI3 and Me3SiI (1 equiv.) were
carried out. However, a product of the form [W2(mhp)3(l-I)]2
was not isolated and, although there was a reaction, only the
compound W2(mhp)4 was obtained as a crystalline product. These
reactions testify to the high formation tendency of W2(mhp)4 and
the kinetic lability of the mhp ligand toward ligand scrambling.
where X = nothing, 1,4-C6F4, 2,5- and 3,4-thienyl.
◦
25
C
2Mo2(mhp)3(CH3CN)+BF4 + [nBu4N]2O2C-X-CO2 −−−−−→
2
CH CN
3
[Mo2(mhp)3]2(O2C-X-CO2) + 2nBu4NBF4
(4)
The dimers of dimers formed in reaction (4) were precip-
itated from the solution as microcrystalline powders. These
were collected and washed with acetonitrile and hexanes.
These compounds were very sparingly soluble even in O-donor
solvents and DMSO was required to obtain electrochemical
data. Even this proved insufficient to obtain measurements on
the 2,5-thienyldicarboxylate bridged compound which is essen-
tially insoluble. The oxalate, perfluoroterephthalate and 3,4-
thienyldicarboxylate all showed two reversible oxidation waves by
cyclic voltammetry and differential pulse voltammetry separated
by 282, 240, and 200 mV, respectively. These values for the
perfluoroterephthalate are notably larger than those previously
reported with pivalate attendant ligands (65 mV).14 However, all
previous electrochemical data were recorded in THF and so a
direct comparison is not appropriate.
Molecular structure of cis-[Mo2(mhp)2(CH3CN)4][BF4]2
The molecular structure of the Mo2-containing dication is shown
in Fig. 4. The cis-arrangement of [Mo2(mhp)2(CH3CN)2]2+ was
expected based on the weak trans-influence of CH3CN ligands
and the MoMo distance is typical of that for MoMo quadruple
bonds.11 However, that one Mo atom was coordinated to two
oxygen atoms and the other to two nitrogen atoms of the mhp
ligands was unexpected. Given the known geometry of the O–
+
N bridging ligands in Mo2(mhp)4 and Mo2(mhp)3(CH3CN)2
this again testifies to the kinetic lability of the ligands at the
dinuclear center. The observed structure could therefore result
The compounds all show low energy electronic transitions in
the visible region of the spectrum associated with Mo2d to bridge
p* transitions in addition to Mo2d to mhp transitions at slightly
higher energy (see Experimental).
Because of the nature of the O–N mhp ligand a number of
isomers are possible for [Mo2(mhp)3]2(O2C–X–CO2) compounds.
The planar bridged structures are favored by M2d bridge p*
backbonding15,16 and assuming that the Mo2(mhp)3 framework
adopts the same geometry as that found for the monocation,
+
Mo2(mhp)3(CH3CN)2 then C2v and C2h isomers are possible as
shown in Fig. 5. Electronic structure calculations on the model
compounds, where the mhp methyl group was replaced by a
hydrogen atom in order to allow for a reduction in computational
time, indicated that the two structures were of very similar
energy. Consequently, the geometries were optimized for the C2h
structures. A frequency analysis of the optimized geometry of
the perfluoroterephthalate model compound revealed a transition
state with one imaginary frequency which related to rotation of
the C6-phenyl ring out of the plane of the carboxylate groups.
As in previous calculations performed on oxalate and perfluo-
roterephthalate bridged Mo4-containing compounds, the HOMO
and HOMO-1 are out-of-phase and in-phase combinations of
the two Mo2d combinations, respectively, with the later being
stabilized by its interaction with the LUMO of the bridge.14 The
LUMO is the out-of-phase combination of M4-bridge HOMO-1.14
Fig. 4 An ORTEP drawing of the Mo2(mhp)2(CH3CN)42+ cation showing
cis arrangement of mhp ligands and the atom number scheme. The
molecule has a crystallographically imposed mirror plane of symmetry.
˚
Selected bond distances (A): Mo–Mo = 2.1242(5), Mo–O = 2.035(2),
Mo–N(mhp) = 2.161(2), Mo–N(CH3CN) = 2.160(3) and 2.170(3).
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Fig. 5 Model structures employed for [Mo2(mhp)3]2(oxalate) (C1 and C2) and [Mo2(mhp)3]2(terephthalate) (D1 and D2) employed in the electronic
structure calculations.
The HOMO–LUMO electronic transition is fully allowed and rep-
resents the MLCT to the bridge.14 An indication of the magnitude
of the electronic coupling is seen in the energy separation between
the HOMO and the HOMO-1 and this is calculated to be 0.33 eV
and 0.15 eV for the oxalate and perfluoroterephthalate complexes,
respectively. These calculations closely resemble those earlier on
model compounds [(HCO2)3Mo2]2(O2C–X–CO2).15,16 Thus while
the oxalate bridged complex could be Class III in its mixed valence
singly oxidized state the other compounds under study are almost
surely Class II. Attempts to obtain epr data for the singly oxidized
compounds have been hindered by their sparing solubility and the
high dielectric constant of DMSO.
an ion corresponding to [Mo2(mhp)3]2(O2CC6F4CO2)+. The latter
is present as an impurity and probably arises from mhp ligand
exchange reactions (see later). It is also quite likely that polymeric
species [Mo2(mhp)2(O2CC6F4CO2)]n are present in the solid and
are not detectable by mass spectrometry.
Mo2(mhp)3(O2CMe) and Mo2(mhp)2(O2CMe)2
From the above it was inferred that mhp ligand scrambling is
kinetically facile. In order to check for this the reactions between
(i) [Mo2(mhp)3(CH3CN)2)[BF4] and nBu4NO2CMe (1 equiv.) and
(ii) Mo2(mhp)2(CH3CN)4(BF4)2 and nBu4NO2CMe (2 equiv.) were
carried out in acetonitrile solution. In each reaction the anticipated
products, namely Mo2(mhp)3(O2CMe) and Mo2(mhp)2(O2CMe)2
were formed in (i) and (ii), respectively. However, other species co-
existed in solution as evidenced from both 1H NMR spectroscopy
and mass spectrometry. Most notably were the species Mo2(mhp)4
and other Mo2(mhp)4−x(O2CMe)x compounds, where x = 1–3.
This clearly confirms the kinetic lability of the mhp ligands toward
ligand exchange.
Higher order assemblies: molecular squares and triangles
Cotton and coworkers have made extensive use of the
[Mo2(DAnif)2(CH3CN)4]2+ cation in the synthesis of molecular
squares and triangles by employing metathetic reactions involv-
ing linking groups such as dicarboxylates [DAnif = CH(N-p-
anisyl)2].17 Furthermore they have shown that molecular triangles
and squares in certain cases co-exist in a dynamic equilibrium.18
We were consequently interested to undertake related reactions
Concluding remarks
2+
involving the cis-templating Mo2(mhp)2(CH3CN)4 dication.
When the blue solution of Mo2(mhp)2(CH3CN)4(BF4)2 in
acetonitrile was allowed to react with (nBu4N)2(O2CC6F4CO2)
in the same solvent an immediate reaction occurred yielding
a dark red solution and a brick red precipitate. The latter
was completely insoluble in acetonitrile and other common
organic solvents. An elemental analysis suggested the product of
formula [Mo2(mhp)2(O2CC6F4CO2)] and by MALDI-TOF mass
spectrometry both triangles and square were seen together with
The potential utility of the easily prepared compound W2(mhp)4
in the synthesis of W2(O2CR)4 has been demonstrated for R =
9-anthracene. This is because the product is sparingly soluble in
certain solvents and so can readily be removed from dynamic
equilibria of W2(mhp)4−x(O2CR)4 compounds. This synthetic
procedure would seem likely to find only modest general utility
although under forcing conditions the equilibria can be driven in
favor of the W2(O2CR)4 compound. An exception to this is the
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formation of the M2(mhp)2(TiPB)2 compounds which appear to
be the thermodynamic products.
[nBu4N][PF6]–DMSO separated from the bulk solution by a Vycor
tip. The standard three electrode cell was utilized in a drybox with
an inert atmosphere. Ferrocene was added as an internal reference
and typically was found at +0.75 V under these conditions.
The [Mo2(mhp)3]2(O2C–X–CO2) compounds are similar to their
pivalate analogues but much less soluble in common organic
solvents. This too has limited the characterization of potential
molecular squares and triangles of formula [Mo2(mhp)2(O2C–X–
CO2)]n, where n = 3 or 4. Another problem that is common to mhp
and O2CBut ligands is ligand scrambling which complicates the
isolation and characterization of well defined discrete complexes.
This problem is not encountered when formamidinate ligands are
used as is well exemplified by the successful synthetic strategy
employed by Cotton and coworkers.17
Computational details
Density functional theory (DFT) calculations were performed
with the hybrid Becke-3 parameter exchange functional and
the Lee–Yang–Parr non-local correlation functional (B3LYP)19
implemented in the Gaussian03 (Revision B.04) program suite20
in conjunction with the 6–31G* basis set for all non-metal
atoms and the SDD basis set with effective core pseudo-potential
(ECP) for all metal atoms.21 The calculations were carried out on
simplified models in which each methyl substituent was replaced
with a hydrogen atom. All geometries were fully optimized at
the above levels using the default optimization criteria of the
program. Each stationary point was confirmed to be a minimum
on the potential energy surface (PES) by a vibrational frequency
analysis unless otherwise noted. Orbital analyses were completed
with GaussView.22 To gain insight into the electronic transitions
responsible for the observed UV-vis spectrum of all compounds,
time-dependent density functional theory (TD-DFT) calculations
were performed using the Gaussian program suite. All calculations
were run on an Itanium 2 Cluster running Linux version 2.4.18
located at the Ohio Supercomputer Center.
Experimental
All reactions were carried out under an inert atmosphere of
UHP-grade argon or nitrogen using standard Schlenk techniques
and under a dry oxygen-free nitrogen atmosphere using standard
glovebox techniques. All solvents were dried and degassed by
standard methods prior to use. THF was dried by refluxing over
sodium/benzophenone while toluene and hexanes were dried by
refluxing over sodium metal. Acetonitrile and dichloromethane
(DCM) was dried by refluxing over calcium hydride.
Starting materials Mo2(mhp)4,8 [Mo2(mhp)3(MeCN)2]+-
(BF4−),11 W2(mhp)4,8 and W2(mhp)3Cl213 were prepared according
to the literature procedures. 2-Hydroxy-6-methylpyridine, triethyl-
oxonium tetrafluoroborate (1.0 M in DCM), n-butylammonium
acetate, n-butylammonium hydroxide (1.0 M), oxalic acid,
tetrafluoroterephthalic acid, trimethylsilyl iodide, boron triiodide,
and zinc metal were purchased from commercial sources and
used as received. Other n-butylammonium salts were prepared
by the 1 : 1 reaction of the appropriate acid with a 1 M solution
of n-butylammonium hydroxide in methanol. After stirring for
1 h at room temperature, the mixture was stripped to dryness and
dried under vacuum at 50 ◦C for several hours.
X-Ray structure determination
All single crystals were coated with Paratone oil and mounted
on a quartz fiber or a nylon cryoloop affixed to a goniometer
head. Diffraction data were collected using a Nonius Kappa CCD
diffractometer. All work was done at 150 K using an Oxford
Cryosystem Cryostream Cooler. Data integration was done with
Denzo.23 Scaling and merging of data was done with Scalepack.
The structure was solved by the Direct or Patterson method and
the refinement were completed in SHELXL-97.24 A summary of
crystal data is given in Table 1.
Physical measurements
NMR spectra were recorded on a 400 MHz Bruker DPX
spectrometer. All H NMR chemical shifts are reported in ppm
[M2(mhp)4 compounds were prepared from M(CO)6 according
to published procedures.]8
1
relative to the protio impurity in MeCN-d3 at 1.53 ppm, THF-
d8 at 3.58 ppm, or DMSO-d6 at 2.50 ppm. UV-vis spectra were
recorded at room temperature using a Perkin-Elmer Lambda 900
spectrometer. Spectra in THF solution were obtained using an air-
tight 1 mm quartz cell. Microanalyses were carried out by Atlantic
Microlab, Inc.
CCDC reference number 632258.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b618957a
Preparation of [cis-Mo2(mhp)2(CH3CN)4]2+(BF4
)
−
2
Matrix-assisted laser desorption ionization (MALDI) mass
spectra were recorded at OSU’s Campus Chemical Instrument
Center (CCIC) using a Bruker Reflex III MALDI-TOF mass
spectrometer. Samples were run pure or mixed with dithrinol
matrix to aid in the flight of the molecule. All complexes were
kept in an inert atmosphere.
Cyclic voltammetric and differential pulse voltammetric data
were collected with the aid of a Princeton Applied Research (PAR)
173A potentiostat–galvanostat equipped with a PAR 176 current-
to-voltage converter with iR compensation capability. A single
compartment voltammetric cell was equipped with a platinum
working electrode, a platinum wire auxiliary electrode, and a
pseudo reference electrode consisting of a silver wire in 0.1 M
To a stirred solution of Mo2(mhp)4 (600 mg, 0.96 mmol) in 40 mL
of MeCN was added 5 mL of 1.0 M Et3OBF4. The solution
immediately turned purple and after stirring for 1.5 h at 70 ◦C all
the starting material appeared consumed. After stirring at room
temperature overnight, the solvent was reduced to approximately
5 mL. Approximately 10 mL of toluene was then added to force
the product to precipitate out of solution. The blue precipitate was
collected on a frit, washed three times with approximately 20 mL
of toluene and dried under vacuum. Overall yield: 575 mg (80%).
Microanalysis found: C 31.80; H 3.24; N 10.97; F 21.31%.
B2C20F8H24N6O2Mo2 requires: C 32.20; H 3.24; N 11.27; F 20.38%.
NMR (MeCN-d6): 1H (400 MHz) 1.57 (s, 3H), 6.68 (d, 1H), 7.16
(d, 1H), 7.65 (t, 1H).
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solution of (nBu4N)(O2CMe) (198 mg, 0.657 mmol) in approxi-
mately 10 mL of MeCN. The solution immediately turned from
red to orange. After stirring for 30 min, the orange precipitate was
washed three times with approximately 20 mL of MeCN and dried
under vacuum for 3 h. Overall yield: 320 mg (85%).
Table 1 Crystal data and structure refinement for [cis-Mo2(mhp)2-
(CH3CN)4]2+(BF4−)2 (1)
Empirical formula
Formula weight
Temperature/K
C22H24Mo2N6O2·2(C2H3N)·2(BF4)
828.06
150(2)
0.71073
˚
Wavelength/A
Microanalysis found:
C
42.35;
H
3.84;
N
8.81%.
Crystal system
Space group
Orthorhombic
P21nm
7.26150(10)
12.0539(2)
19.1113(3)
1672.9(4)
90
90
90
2
1.644
0.830
C20H21N3O5Mo2 requires: C 41.76; H 3.68; N 7.30%. NMR
1
(THF-d8): H (400 MHz) 1.78 (s, 3H), 1.94 (s, 6H), 2.35 (s, 3H),
˚
a/A
6.30 (d, 1H), 6.38 (d, 2H), 6.67 (d, 1H), 6.81 (d, 2H), 7.21 (t, 1H),
7.32 (t, 2H). MALDI-MS: 575.0 (100%, M+).
˚
b/A
˚
c/A
3
˚
V/A
a/◦
b/◦
c /◦
Z
Preparation of cis-Mo2(mhp)2(O2CCH3)2
To a stirred solution of [cis-Mo2(mhp)2(CH3CN)4]2+(BF4
)
−
2
Density (calculated)/Mg m−3
Absorption coefficient/mm−1
F(000)
(160 mg, 0.21 mmol) in 10 mL of CH3CN was added
(nBu4N)(O2CCH3) (160 mg, 0.53 mmol) in 10 mL of CH3CN. An
immediate reaction took place as observed by a color change to
yellow but no precipitate formed. After stirring for 30 min, the
solvent was reduced to a minimum to precipitate a yellow product
that was collected by filtration, and dried under vacuum. Overall
yield: 67 mg (59.4%).
824
Crystal size/mm3
0.04 × 0.08 × 0.38
h range for data collection/◦
Index ranges
3.28 to 27.49
−9 ≤ h ≤ 9, 0 ≤ k ≤ 15, 0 ≤ l ≤ 24
19161
Reflections collected
Independent reflections
Completeness to h = 27.49◦ (%)
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
3950 [R(int) = 0.0336]
Full-matrix least-squares on F2
3950/1/221
99.7
Microanalysis found:
C
36.51;
H
3.87;
N
5.08%.
C16H18N2O6Mo2 requires: C 36.52; H 3.45; N 5.32%. NMR
(THF-d8): 1H (400 MHz) 1.71 (s, 3H), 2.49 (s, 3H), 6.34 (d, 1H),
6.79 (d, 1H), 7.27 (t, 1H). MALDI-MS: 525.9 (100%, M+).
1.034
R1 = 0.0282, wR2 = 0.0671
R1 = 0.0324, wR2 = 0.0687
0.02(4)
0.544 and −0.381
Absolute structure parameter
Largest diff. peak and hole/e A
−3
˚
Attempted preparation of [Mo2(mhp)2(O2CCO2)]4
To a stirred solution of [cis-Mo2(mhp)2(CH3CN)4]2+(BF4
)
−
2
Preparation of [Mo2(mhp)3]2(O2CCO2)
(426 mg, 0.64 mmol) in 10 mL of CH3CN was added
(nBu4N)2(O2CCO2) (367 mg, 0.64 mmol) in 10 mL of CH3CN. An
immediate reaction took place with the formation of an apparent
red precipitate. After stirring for 30 min, the crude product could
not be collected by filtration and was re-evaluated to be an oil.
The oil was collected by separation for analysis. H NMR and
MALDI-TOF spectroscopy could only identify the product as a
mixture of species.
To a stirring solution of [Mo2(mhp)3(CH3CN)2]+(BF4−) (0.300 g,
0.438 mmol) in approximately 10 mL of MeCN was added a solu-
tion of (nBu4N)2(O2CCO2) (0.125 g, 0.219 mmol) in approximately
10 mL of MeCN. An immediate reaction took place with the
formation of a red precipitate. After stirring for 30 min, the red
precipitate was collected by filtration, washed several times with
CH3CN, and dried under vacuum. Overall yield: 205 mg (84%).
1
Microanalysis found:
C
37.16;
H
3.31;
N
6.57%.
C38H36N6O10Mo4 requires: C 40.73; H 3.24; N 7.50%. NMR
(DMSO-d6): H (400 MHz) 1.62 (s, 6H), 1.75 (s, 3H), 6.40 (d,
1H), 6.49 (d, 2H), 6.73 (d, 1H), 6.83 (d, 2H), 7.34 (t, 1H), 7.42 (t,
2H). MALDI-MS: 1120.9 (100%, M+).
Attempted preparation of [Mo2(mhp)2(O2CC6F4CO2)]4
1
−
To a stirred solution of [cis-Mo2(mhp)2(CH3CN)4]2+(BF4
)
2
(400 mg, 0.60 mmol) in 10 mL of CH3CN was added
(nBu4N)2(O2CC6F4CO2) (387 mg, 0.60 mmol) in 10 mL of
CH3CN. An immediate reaction took place with the formation
of a red precipitate. After stirring for 30 min, the crude product
was collected by filtration, washed several times with CH3CN, and
dried under vacuum. The reaction yielded 207 mg of a red solid
that was determined to be a mixture of pairs of dimers, molecular
triangles, and molecular squares by MALDI-TOF spectroscopy.
Preparation of [Mo2(mhp)3]2(O2CC6F4CO2)
The reaction was carried out under similar conditions out-
lined for above using 300 mg of [Mo2(mhp)3(CH3CN)2]+(BF4
)
−
(0.438 mmol) and 158 mg of (nBu4N)2(O2CCO2) (0.219 mmol).
171 mg of a purple precipitate was isolated. Overall yield: 171 mg
(78%).
Microanalysis found:
C
36.35;
H
1.98;
N
4.81%.
C80H48F16Mo8N8O24 requires: C 37.29; H 1.88; N 4.35%.
Microanalysis found: C 40.77; H 2.77; N 6.44; F 5.91%.
C44F4H36N6O10Mo4 requires: C 41.66; H 2.86; N 6.62; F 5.99%.
NMR (DMSO-d6): 1H (400 MHz) 1.78 (s, 3H), 1.94 (s, 6H), 2.35
(s, 3H), 6.30 (d, 1H), 6.38 (d, 2H), 6.67 (d, 1H), 6.81 (d, 2H), 7.21
(t, 1H), 7.32 (t, 2H). MALDI-MS: 1266.9 (100%, M+).
n+
Attempted preparation of [W2(mhp)4−n
]
Method A. To a solution of W2(mhp)4 (800 mg, 1.000 mmol)
in 25 mL of CH2C12 and 1 mL of MeCN was added 5 mL of 1.0 M
Et30BF4. The solution immediately turned reddish green and
within the first hour was green. After 2 h, stirring was stopped and
the solution was allowed to sit at room temperature overnight. No
green solid was isolated by cannula removal of the mother liquor.
Preparation of Mo2(mhp)3(O2CCH3)
+
To a stirring solution of Mo2(mhp)3(CH3CN)2 (BF4−) (450 mg,
0.657 mmol) in approximately 10 mL of MeCN was added a
1798 | Dalton Trans., 2007, 1793–1801
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The solution was split into two halves. Diethyl ether (∼25 mL)
was added to one half. This resulted in the precipitation of a green
solid. The solvent was then removed via cannula and the green
solid was dried under vacuum for 3 h. The yield was 440 mg of
an unknown material that could not be identified as the desired
complex. The second half was placed in a −20 ◦C refrigerator over
the weekend. No crystals formed.
in the oil bath for 3 days. After which, the reaction mixture was
cooled to room temperature and the solid product was collected
on a frit. The product was washed once with toluene (10 mL)
and three times with hexanes (15 mL), then dried under vacuum.
The 1H NMR spectrum displayed similar behavior to those above,
but the corresponding peaks for coordinated benzoate were much
more intense than before. MALDI-TOF: calculated MW for
W2C24N4O4H23: 799.37 (M+), calculated MW for W2C25N3O5H22:
812.3 (M+), calculated MW for W2C26N2O6H21: 825.23 (M+),
calculated MW for W2C27NO7H20: 838.16 (M+), calculated MW
for W2C28O8H19: 851.09 (M+), observed: 812.67, 825.68, 838.69,
and 851.70.
Method B. To a stirred solution of W2(mhp)4 (400 mg,
0.500 mmol) in approximately 40 mL of acetonitrile was added
iodotrimethylsilane (0.068 mL, 0.500 mmol). After stirring
overnight, the volume of the solvent was reduced to a minimum.
A red solid was collected by filtration and washed 2 × 10 mL of
hexanes. The red solid was characterized by 1H NMR as recovered
starting material, W2(mhp)4.
Reaction of W2(mhp)4 and 9-anthracenylcarboxylic acid
(Anth-9-CO2H)
Method C. To a stirred solution of W2(mhp)4 (400 mg,
0.500 mmol) in approximately 40 mL of acetonitrile was added
boron triiodide (0.068 mL, 0.500 mmol). After stirring overnight,
the solvent was stripped to dryness. The mixture yielded no
tractable product.
A Schlenk flask was charged with 0.278 g (1.25 mmol) of Anth-9-
CO2H, 0.250 g (0.313) of W2(mhp)4, and 15 mL of toluene. The
flask was placed in a 60 ◦C oil bath, and the reaction mixture was
stirred gently for 16 h. The solution turned from a red to a dark
purple color, within minutes. The solution was allowed to stir in the
oil bath for 3 days. After which, the reaction mixture was cooled
to room temperature and the solid product was collected on a frit.
The product was washed three times with toluene (15 mL total),
once with DMSO (5 mL), and three times with hexanes (15 mL),
and then dried under vacuum. 0.220 g of product was recovered
(66% yield). 1H NMR (DMSO, d, ppm): 6.92, 6.95, 6.95 (t), 7.28,
7.29, 7.32 (t), 7.96, 7.99 (d), 8.43 (s), 8.47, 8.50 (d). MALDI-TOF:
calculated MW for W2C60O8H36: 1251.57 (M+), observed 1252.2.
Method D. Approximately 40 mL of acetonitrile was added to
a reaction vessel containing W2(mhp)3Cl2 (400 mg, 0.500 mmol)
and excess Zn dust (7.2 g, 110 mmol). After stirring for 1 h, the
orange solution turned red. The mixture was allowed to continue
stirring overnight before workup. The mixture was then filtered
over Celite and the solvent of the filtrate stripped to dryness. The
red solid was characterized by 1H NMR as W2(mhp)4.
All experiments were conducted using standard glove box and
Schlenk techniques. Solvents were dried and degassed prior to use.
All reactions and subsequent workups were carried out under an
argon environment. All reagents were used as received.
Preparation of W2(mhp)2(TiPB)2
A Schlenk flask was charged with 0.248 g (1.0 mmol) of TiPBH,
0.200 g (0.25 mmol) of W2(mhp)4, and 10 mL of toluene. The
flask was placed in a 45 ◦C oil bath, and the reaction mixture
was stirred gently for 16 h. The solution turned from a red to
an intensely bright red color, within minutes. The solution was
allowed to stir in the oil bath for 3 days. After which, the reaction
mixture was cooled to room temperature and the solid product
was collected on a frit. The product was washed three times with
hexanes (15 mL), and then dried under vacuum. 0.143 g of product
was recovered: W2(mhp)2(TiPB)2.
1H NMR (DMSO, d, ppm): 0.99, 1.02 (d, CH3 of TiPB), 1.19,
1.22 (d, CH3 of TiPB), 2.22 (s, CH3 of mhp), 2.45–2.58 (m, CH
of TiPB), 2.77–2.89 (m, CH of TiPB), 6.46, 6.47, 6.49 (t, CH of
mhp), 6.95 (s, CH of TiPB), 7.08–7.10 (m, CH of mhp). MALDI-
TOF: calculated MW for W2C44O6N2H57: 1077.44 (M+), observed
1077.1.
Reaction of W2(mhp)4 and benzoic acid (Ph-CO2H) (4 equiv.)
A Schlenk flask was charged with 0.153 g (1.25 mmol) of benzoic
acid, 0.250 g (0.31 mmol) of W2(mhp)4, and 15 mL of toluene. The
flask was placed in a 70 ◦C oil bath, and the reaction mixture was
stirred gently for 24 h. The solution turned from a red to a dark
purple color, within minutes. The solution was allowed to stir in the
oil bath for 3 days. After which, the reaction mixture was cooled
to room temperature and the solid product was collected on a frit.
The product was washed once with toluene (10 mL) and three times
1
with hexanes (15 mL), then dried under vacuum. The H NMR
spectrum was complex with multiple multiplets appearing in the
aromatic range (6.0–8.0 ppm), which was indicative of a mixture
of products. MALDI-TOF: calculated MW for W2C24N4O4H23:
799.37 (M+), calculated MW for W2C25N3O5H22: 812.3 (M+),
calculated MW for W2C26N2O6H21: 825.23 (M+), calculated MW
for W2C27NO7H20: 838.16 (M+), calculated MW for W2C28O8H19:
851.09 (M+), observed: 799.85, 812.84, 825.84, 838.82, and
821.81.
Reaction of W2(mhp)4 and 2-thienylcarboxylic acid (th-2-CO2H)
(4 equiv.)
A Schlenk flask was charged with 0.128 g (1.0 mmol) of 2-
thienylcarboxylic acid, 0.200 g (0.25 mmol) of W2(mhp)4, and
15 mL of toluene. The flask was placed in a 50 ◦C oil bath, and the
reaction mixture was stirred gently. The solution turned from a red
to a dark purple color, within minutes. The solution was allowed
to stir in the oil bath for 3 days. After which, the reaction mixture
was cooled to room temperature and the product was collected on
a frit. The product was washed three times with toluene (15 mL
Reaction of W2(mhp)4 and benzoic acid (Ph-CO2H) (16 equiv.)
A Schlenk flask was charged with 2.345 g (19.2 mmol) of benzoic
acid, 1.000 g (1.25 mmol) of W2(mhp)4, and 50 mL of toluene.
The flask was placed in a 60 ◦C oil bath, and the reaction mixture
was stirred gently for 16 h. The solution turned from a red to a
dark purple color, within minutes. The solution was allowed to stir
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total) and once with hexanes (5 mL), then dried under vacuum.
The H NMR spectrum was complex with multiple multiplets
[nBu4N]2[3,4-th(CO2)2]
1
To a solution containing 0.028 g of 3,4-thienyldicarboxylic acid in
10 mL of methanol was added 0.320 mL of a 1 M solution of n-
butylammonium hydroxide in methanol. The solution was stirred
at room temperature for 1 h and then stripped to dryness at 50 ◦C
under vacuum.
appearing in the aromatic range (6.0–8.0 ppm), which was indica-
tive of a mixture of products. MALDI-TOF: Calculated MW for
W2C24N4O4H23: 799.37 (M+), calculated MW for W2C23N3O5SH20:
818.32 (M+), calculated MW for W2C22N2O6S2H17: 837.27 (M+),
calculated MW for W2C21NO6S3H16: 842.25 (M+), calculated MW
for W2C20O8S4H11: 875.17 (M+): observed: 799.55, 818.49, 837.45,
856.41, and 875.36.
[nBu4N]2[2,5-th(CO2)2]
To a solution containing 0.037 g of 2,5-thienyldicarboxylic acid in
5 mL of methanol was added 0.430 mL of a 1 M solution of n-
butylammonium hydroxide in methanol. The solution was stirred
at room temperature for 1 h and then stripped to dryness at 50 ◦C
under vacuum.
Mo2(mhp)2(TiPB)2
A Schlenk flask was charged with 0.395 g of TiPB (1.59 mmol,
FW = 248.36 g mol−1), 0.250 g of Mo2(mhp)4 (0.4 mmol, FW =
624 g mol−1), and 10 mL of toluene. The flask was placed in a
60 ◦C oil bath, and the reaction mixture was stirred for 24 h. The
solution turned from a rusty orange color to a bright yellow color
within a half hour at 60 ◦C. After 24 h at elevated temperature, the
reaction mixture was cooled to room temperature and the product
was collected on a frit. The product was washed three times with
hexane (15 mL total), then dried under vacuum. 0.294 g of a bright
[(mhp)3Mo2]2{l-3,4-th-(CO2)2}
A solution of 0.32 mmol of [Mo2(mhp)3(CH3CN)2][BF4] in 20 mL
of acetonitrile was combined with a solution of 0.16 mmol of
[nBu4N]2[3,4-th(CO2)2] in 10 mL of acetonitrile. A precipitate
formed and was collected on a frit. The product was washed
three times with acetonitrile (15 mL total) and once with hexanes
(5 mL), then dried under vacuum. 1H NMR (DMSO, d, ppm): 1.65
(s, CH3 of mhp), 1.76 (s, CH3 of mhp), 6.45, 6.48 (d, CH of mhp),
6.56, 6.59 (d, CH of mhp), 6.78–6.83 (m, CH of mhp), 7.348–7.513
(m, CH of mhp), 8.061, 8.067 (d, CH of thiophene). MALDI-TOF:
calculated MW for Mo4C42N6O10SH37, 1201.64 (M+).
1
yellow powder was recovered (62% yield). H NMR (DMSO, d,
ppm): 0.90, 0.92 (d, CH3 of TiPB), 1.16, 1.18 (d, CH3 of TiPB),
1.59 (s, CH3 on mhp), 2.53–2.60 (m, CH of TiPB), 2.76–3.90 (m,
CH of mhp), 6.55, 6.57 (d, H of mhp), 6.96 (s, H of TiPB), 7.05,
7.08 (d, H of mhp), 7.55, 7.58, 7.62 (t, H of mhp). MALDI-TOF:
calculated MW for Mo2C44N2O6H58, 901.83 (M+); found 902.13.
[(mhp)3Mo2]2{l-2,5-th-(CO2)2}
[Mo2(mhp)3(CH3CN)2][BF4]
A solution of 0.36 mmol of [Mo2(mhp)3(CH3CN)2][BF4] in 20 mL
of acetonitrile was combined with a solution of 0.18 mmol of
[nBu4N]2[2,5-th(CO2)2] in 10 mL acetonitrile. A precipitate formed
and was collected on a frit. The product was washed three times
with acetonitrile (15 mL total) and once with hexanes (5 mL), then
dried under vacuum.
1H NMR (DMSO, d, ppm): 1.66 (s, CH3 of mhp), 1.82 (s, CH3
of mhp), 6.41, 6.42 (d, H of mhp), 6.50, 6.52 (d, H of mhp), 6.74,
6.76 (d, H of mhp), 6.85, 6.87 (d, H of mhp), 7.16, 7.18 (d, H of
mhp), 7.23, 7.24 (d, H of mhp), 7.33, 7.35 (d, H of mhp), 7.40,
7.42, 7.44 (t, H of mhp), 7.64 (s, H of thiophene). MALDI-TOF:
calculated MW for Mo4C42N6O10SH37, 1201.64 (M+).
The [Mo2(mhp)3(CH3CN)2]+ cation was prepared as per the
literature.12
[nBu4N][2,4,6-triisopropylbenzoate]
To a solution of 0.500 g of 2,4,6-triisopropylbenzoic acid (approx-
imately 2 mmol, FW = 248.36 g mol−1) in 5 mL of methanol was
added 2.0 mL of a 1 M solution of n-butylammonium hydroxide
in methanol. The mixture was stirred at room temperature for 1 h
and then stripped to dryness under vacuum at 50 ◦C.
Mo2(mhp)3(TiPB)
Acknowledgements
A solution of 0.200 g of [Mo2(mhp)3(CH3CN)2][BF4] (2.9 mmol,
FW = 686.65 g mol−1) in 15 mL of acetonitrile was combined
with 0.143 g (2.9 mmol, FW = 490.36 g mol−1) of [nBu4N][2,4,6-
triisopropylbenzoate] in a 10 mL solution of acetonitrile. Upon
addition, a precipitate immediately formed and was collected on a
frit. The product was washed three times with acetonitrile (15 mL
total) and once with hexanes (5 mL), then dried under vacuum.
1H NMR (DMSO, d, ppm): 0.92, 0.95 (d, CH3 of TiPB), 1.19, 1.22
(d, CH3 of TiPB), 2.30 (s, CH3 of mhp), 2.34 (s, CH3 of mhp),
2.46–2.57 (m, CH of TiPB), 2.78–2.86 (m, CH of TiPB), 6.34, 6.37
(d, CH of mhp), 6.40, 6.43 (d, CH of mhp), 6.76, 6.79 (d, CH of
mhp), 6.88, 6.92 (d, CH of mhp), 6.91 (s, CH of TiPB), 7.07–7.18
(m, CH of mhp), 7.23 (m, CH of mhp), 7.26–7.27 (m, CH of mhp).
MALDI-TOF: calculated MW for Mo2C34O5N3H40, 762.57 (M+);
found 763.03.
We thank the National Science Foundation for financial support.
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