Oxidation-state and metal-ion dependent stereoisomerization in oxo molybdenum and tungsten complexes of a bulky alkoxy heteroscorpionate ligand

Article abstract of DOI:10.1039/b604751c

Monooxo Mo(V) complexes of a N₂O heteroscorpionate ligand designated (L10O) are found to exist as isolable cis and trans isomers. We have been able to trap the kinetically labile cis isomer and follow its isomerization to the thermodynamically

Full text of DOI:10.1039/b604751c

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www.rsc.org/dalton | Dalton Transactions
Oxidation-state and metal-ion dependent stereoisomerization in oxo
molybdenum and tungsten complexes of a bulky alkoxy heteroscorpionate
ligand
Justin T. Hoffman, Ba L. Tran and Carl J. Carrano*
Received 3rd April 2006, Accepted 17th May 2006
First published as an Advance Article on the web 25th May 2006
DOI: 10.1039/b604751c
Monooxo Mo(V) complexes of a N2O heteroscorpionate ligand designated (L10O) are found to exist as
isolable cis and trans isomers. We have been able to trap the kinetically labile cis isomer and follow its
isomerization to the thermodynamically more stable trans form. We have also followed the kinetics of
isomerization between the cis and trans isomers of the corresponding dioxo Mo(VI) and W(VI) species.
Here the trans is the labile isomer that spontaneously converts to the thermodynamically more stable
cis. It is observed that at 60 ◦C in DMSO the Mo(VI) complex isomerizes approximately 6.5 times faster
than the Mo(V) and nearly 5 times faster than the corresponding W(VI) analogs. The temperature
dependence to the kinetics of the Mo(V) and Mo(VI) isomerizations give activation parameters that are
similar for both oxidation states and consistent with those previously observed in [(L1O)MoOCl2]
suggesting a similar twist mechanism is operating in all cases. Thus there are oxidation state, metal ion
and donor atom dependent differences in isomeric stability that could have significant implications for
understanding the mechanisms of both enzymatic and nonenzymatic oxo atom transfer reactions
catalyzed by complexes of Mo, W and Re.
tungstoenzymes as well as a platform for Re based OAT
Introduction
catalysts.10–15 While in general one would suppose that a three-
Oxygen atom transfer (OAT) reactions between closed shell
fold symmetric, all nitrogen donor ligand would be a poor
molecules are an important class of transformations that are
substitute for the dithiolene and other sulfur donor atoms
catalyzed by a number of oxo metal complexes. Of particular
typical of molybdo and tungstoenzymes, they nevertheless provide
interest have been catalysts derived from Re, Mo and W. The
biomimetic attributes unmatched by any other system. Over the
first because of their extraordinary catalytic efficiency in carrying
last several years we and others have developed a series of poly-
out even difficult oxygen atom transfers such as the conversion of
functional, facially coordinating, tridentate ligands containing
−
ClO4 to Cl− and the latter because of their potential biological
two pyrazole groups, which also incorporate another coordinating
relevance.1,2 Thus there are a large number of mononuclear
moiety (thiolate, phenolate, alkoxide, carboxylate, etc.) to produce
molybdenum and tungsten containing enzymes that have the
various heteroscorpionate ligands with the same topology and
charge as TpR.16–18 Molybdenum complexes of these ligands of
the type [(L)MoOnClz] (where L = N2X ligand, Scheme 1) are
easily accessible and have been shown to exist in two isomeric
forms where the X group is either cis or trans relative to an oxo
group (Scheme 2).19–21 Thus in addition to the ability to change
donor atoms at will, a major advantage of the unsymmetrical
N2X ligands is the potential to explore the effects of geometry on
OAT reactions.
general function of catalyzing OAT to or from a physiological
donor/acceptor with the metal cycling between the +6 and +4
oxidation states. The Mo enzymes can be conveniently divided
into three major groups based on the structure about the metal
center, all of which include one or more pterin cofactors.3 Among
the three families, the DMSO reductase family is the most diverse,
where in addition to the ubiquitous pterin based ligands, the
metal center can be coordinated by endogenous ligation from a
serine alkoxide oxygen, a cysteinyl thiolate sulfur or a carboxylate
oxygen from aspartate.4 Although not explicitly invoked in the
mechanism of action of any molybdoenzyme, there is evidence
that conformational changes at the metal center could be occurring
during the redox cycle.5–7 Such processes have also been suggested
by detailed studies of the mechanism of OAT catalyzed by small
molecule complexes of Mo and Re.8,9
Tripodal pyrazolylborate (TpR) and pyrazolylmethane com-
plexes of molybdenum represent a uniquely successful and ex-
tensive model system for the pterin dependent molybdo- and
Scheme 1 Ligands referred to in this study.
Department of Chemistry and Biochemistry, San Diego State University,
San Diego, CA, 82182-1030, USA. E-mail: carrano@sciences.sdsu.edu;
Fax: 619 594 4634; Tel: 619 594 5929
We have previously isolated both the cis and trans isomers for
the Mo(V) complexes with phenoxy heteroscorpionate ligands
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isomer is asymmetric and shows two pyrazole peaks in a 1 : 1 ratio
in this area. Allowing a solution of the trans isomer to stand for
several days results in a complete loss of the peaks associated with
it and a concomitant increase in the peaks associated with the
symmetrical cis isomer. We have been able to crystallize and solve
the structure of the thermodynamically more stable cis isomer,
=
the structure of which is shown in Fig. 1. The two Mo O bonds
˚
are in the expected range near 1.7 A. The Mo–O bond from the
˚
˚
alkoxide ligand is over 0.2 A longer at 1.90 A, consistent with its
single bond character. The Mo–N bonds to the pyrazole groups
are somewhat longer than a “normal” Mo–N bond and average
˚
˚
2.36 A (expected ca. 2.2 A). This is due to the trans influence of the
oxo groups each of which occupies a positions trans to a pyrazole
nitrogen. Due to the relatively rapid isomerization of the trans to
the cis isomer we have as yet been unable to crystallize the former.
Scheme 2
LO− and L1O−.19,20 The two isomers differ significantly in their
physiochemical properties including EPR parameters and redox
potential. Mass spectrometry, DFT calculations and detailed
kinetic analysis suggest that the isomerization of [(L1O)MoOCl2]
proceeds through a “trigonal twist” mechanism, the rate and equi-
librium isomer ratio of which, are controlled by both electronic
and steric factors. This has lead Basu et al. to propose the so called
“serine gate hypothesis” where such a isomerization functions as
an electron transfer “gate” in the enzyme DMSO reductase.19,20
In this report we show for the first time, using a sterically
restricted alkoxy heteroscorpionate ligand, that both the monooxo
Mo(V) and dioxo Mo(VI) as well as the dioxo W(VI) complexes exist
as isolable cis and trans isomers but that both the nature of the
thermodynamically stable isomer, and the rate of isomerization,
are both oxidation-state and metal-ion dependent. A preliminary
account of portions of this work has already appeared.22
Fig. 1 ORTEP diagram with 40% thermal ellipsoids of the symmetric or
cis isomer of [(L10O)MoO2Cl] (2) showing selected atomic labeling.
In addition to the two isomers of the mononuclear complex
[(L10O)MoO2Cl] another product is isolated in variable yield. This
complex, which can be crystallized from hot DMSO solution,
has been determined by X-ray crystallography to be the l-oxo
dimer [(L10O)MoO2]2O. We believe this compound arises from
the hydrolysis of the mononuclear chloro complex driven by
the presence of hydroxide ion coming from reaction of the KH
with traces of water in the solvent. The isolated [(L10O)MoO2Cl]
appears not to be sensitive to the presence of water itself. The
X-ray structure of 4 (Fig. 2) shows that it too adopts the cis
Results and discussion
Mo(VI) complexes
[(L10O)MoO2Cl] was prepared using a synthetic methodology
previously employed in making other heteroscorpionate dioxo
Mo(VI) complexes.21 However NMR analysis of the crude reaction
product indicated the presence of two distinct species. Following
column chromatographic separation of crude [(L10O)MoO2Cl]
two fractions were isolated. The first fraction was determined
to be the thermodynamically stable, symmetrical, cis isomer,
while the second band contains the kinetically unstable trans
isomer. However, due to relatively rapid isomerization the best
purification we have been able to achieve thus far through column
chromatography is a product where trans predominates over cis
by a 4 : 1 ratio as determined by NMR spectroscopy. However,
through the method and conditions described in the experimental
section we have been able to isolate a product where the trans
predominates over the cis by better than 11 : 1 merely by saturating
the DMF solution which precipitates the less soluble trans product
almost exclusively. The two stereoisomers are easily distinguished
by NMR since the cis isomer is symmetric and thus shows only
a single pyrazole proton resonance near 6 ppm while the trans
=
stereochemistry. The M O group bond lengths are essentially
˚
the same as in the monomer (av. 1.70 A) as are the Mo–Npz
˚
bonds (av. 2.355 A). The only significant change is an increase
˚
in the M–Oalkoxide bond by about 0.05 A, probably a result of the
trans influence of the bridging oxide ion. Other bond lengths and
angles are unexceptional.
Mo(V) complexes
Purification of crude [(L10O)MoOCl2] by column chromatogra-
phy on silica gel using dichloromethane as an eluent produces an
initial green band of what has been identified as the cis isomer
=
followed by a purple band of the trans. The M O stretch in the
cis isomer appears at 960 cm−1 while the same vibration is found
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Fig. 2 ORTEP diagram with 40% thermal ellipsoids of the symmetric or
cis isomer of [(L10O)MoO2]2O (4) showing selected atomic labeling.
at 933 cm−1 in the trans. This difference is similar to that seen in
the cis/trans pair of the related complex [(L1O)MoOCl2].20 The
trans isomer has the expected d–d transition for the d1 Mo(V) ion
at 530 nm with a shoulder at 600 nm while the cis isomer has the
corresponding bands at lower energy ca. 620 and 710 nm. The
two isomers are also easily distinguished by EPR spectroscopy as
shown in Fig. 3. The isotropic solution EPR spectrum of each
isomer consists of a prominent 98Mo signal at g ≈ 2 and six
95,97Mo (I = 5/2; 25.38% abundance) hyperfine lines with the
trans isomer having the larger “g” value. The difference in the
resonance position of the main 98Mo signal between the cis and
trans isomers of [(L10O)MoOCl2] isomers of 26 G at X-band is
slightly smaller than the difference between the corresponding
isomers of the [(L1O)MoOCl2] phenolate complex of 32 G.
Fig. 4 ORTEP diagram with 40% thermal ellipsoids of the asymmetric
or cis isomer of [(L10O)MoOCl2] (5) showing selected atomic labeling.
Fig. 5 ORTEP diagram with 40% thermal ellipsoids of the symmetric
isomer or trans isomer of [(L10O)MoOCl2] (6) showing selected atomic
labeling.
likely the result of some slight compositional disorder (vide infra).
The fact that one of the pyrazole nitrogens is trans to an oxo
group while the other is not, results in a large difference in Mo–
˚
˚
N bond lengths (Mo–Ntrans = 2.34 A, Mo–Ncis = 2.18 A), again a
manifestation of the trans effect. The purple material, which can be
crystallized from either DMF or CH2Cl2, reflects the fact that the
equilibrium mixture after isomerization, while predominantly the
trans isomer (80%), contains about 20% of the cis as monitored
by square wave voltammetry. Thus the structure of the crystals
isolated from either solvent (only the data for the DMF solvate is
given here) shows evidence for a compositional disorder between
the oxo group and the chlorides engendered by the presence of
some of the minor cis isomer. This leads to a longer than expected
Fig. 3 EPR spectra of the cis and trans isomers of [(L10O)MoOCl2] in
dichloromethane solution.
The stereochemical assignments of the green and purple species
have been confirmed by complete X-ray structural analyses (Fig. 4
and 5). For the green or cis isomer there are two crystallographi-
cally independent but chemically similar molecules in the unit cell.
The data tabulated here represent the average of the two molecules.
˚
The M–Oalkoxide bond at 1.92 A is, as expected due to the decreased
charge on the metal, slightly longer that seen in the Mo(VI) analog.
˚
˚
=
=
The increase in the M O bond to 1.81 A is rather larger than what
Mo O bond (1.97 A) and shorter than expected Mo–Cl bonds
(2.29 A). This phenomenon, which had previously been dubbed
˚
would be expected based solely on charge considerations and is
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“bond stretch isomerization”, is now known to be a common effect
of solid solutions of mixed isomers.23 The pyrazole nitrogen atoms,
which are both now trans to a chloride rather than to an oxo, give
˚
rise to Mo–N bonds of 2.21 A, slightly longer than the Mo–N
˚
bond (2.18 A) seen in the pure cis isomer.
Mo(IV) complex
Unlike the situation with the Mo(V) and Mo(VI) complexes only
the trans isomer for the Mo(IV) complex could be isolated and was
crystallographically characterized. No evidence for the alternate
cis isomer was found suggesting that either or both that the trans
isomer was greatly favored thermodynamically over the cis in this
oxidation state and/or that the rate of isomerization between them
was fast.
In the unit cell of 7, there were two chemically distinct but struc-
turally similar molecules of trans-[(L10O)MoOCl(NC5H5)]. The
crystal structures of both complexes confirmed their distorted oc-
tahedral coordination geometry and trans conformation (Fig. 6).
Fig. 7 Cyclic voltammogram in DMF (scan rate 200 mV s−1) of a
thermodynamic mixture of the cis and trans isomers of [(L10O)MoOCl2]
using conditions as described in the text.
˚
˚
=
They yielded typical Mo O (∼1.70 A) and Mo–Cl (∼2.45 A)
bond lengths. Their Mo–X bond lengths are similar to those of
trans-[(L1O)MoOCl(NC5H5)]24 with the Mo–O bond length at
−570 mV vs. SCE in an approximately 20 : 80 ratio. The wave
at −570 mV corresponds to the Mo(V) → Mo(IV) reduction
potential of the predominant trans isomer while that at −800 mV
represents the minor cis. This was confirmed by recording the
voltammetry of the pure cis isomer (6) that shows a major wave at
−800 mV and just a trace of the species with the redox potential at
−570 mV. These alkoxide complexes of Mo(V) are approximately
100 mV more difficult to reduce than the cis/trans pair of the
related L1O phenolate heteroscorpionate ligand although the
difference in redox potential between the isomers of 230 mV is very
similar. The expected Mo(V) → Mo(VI) oxidation waves where not
observable in DMF.
˚
˚
2.017 A for the phenolate, and 2.013 A for alkoxide 7. In both
complexes the NPz–Mo–NPz deviated from the typical ligand “bite”
angle of 90◦, to ∼ 84◦. The Mo–Npyridine bond is slightly shorter for 7
˚
˚
(2.163 A) than the phenolate analog (2.177 A) while both Mo–NPz
˚
bond are shorter in the latter than in 7 with values of 2.163, 2.185 A
˚
and 2.208, 2.191 A, respectively. For both complexes, the pyrazole
˚
nitrogen trans to the chlorine as found to be approximately 0.02 A
further from the molybdenum center than the pyrazole nitrogen
trans to the pyridine adduct. Remaining bond lengths and angles
are unexceptional and were found to be very similar to previously
reported heteroscorpionate complexes.
A solution of the predominantly (4 : 1 as determined by
NMR) trans isomer of [(L10O)MoO2Cl] in CH2Cl2 showed a
quasireversible wave at −940 mV with an ipa/ipc of ca. 0.7
at 200 mV s−1. While Mo(VI) → Mo(V) reductions are often
completely irreversible due to chemical follow up reactions, the
partial reversibility of the reduction suggests that electron transfer
is competitive with such processes in this system. This is probably
due to stabilization of an initially formed dioxo Mo(V) species
by the bulky alkoxide ligand. However, cyclic voltammograms
obtained in DMF rather than CH2Cl2 show no evidence for a
return wave and are thus completely irreversible and similar to
what is seen with many other dioxo Mo(VI) complexes. Under the
same conditions used for the trans isomer a solution of the pure cis
isomer of [(L10O)MoO2Cl] in CH2Cl2 showed a quasireversible
wave at −1032 mV. The difference in redox potential between
the isomers of less than 100 mV is roughly half of the difference
between the corresponding isomers in the Mo(V) species. Thus
the stereochemical arrangement of the oxo groups vis a` vis other
ligands appears to be less important to the redox chemistry in
Mo(VI) than in Mo(V).
Fig.
6
ORTEP diagram with 40% thermal ellipsoids of trans-
[(L10O)MoOCl(NC5H5)] (7) showing selective atomic labeling.
As it has been difficult to isolate the oxo Mo(IV) complexes
of our heteroscorpionate ligands it was not surprising that they
proved to be very easy to oxidize. The MoIV/MoV redox couple
and electrochemical properties of trans-[(L10O)MoOCl(NC5H5)]
(7) are very similar to that of the phenolate analog.24 Thus the
MoIV/MoV redox couple of the former is found at 0 mV vs. +24 mV
Electrochemistry.
Cyclic voltammetry of 5 in DMF (Fig. 7) showed two closely
spaced quasireversible reduction waves centered at −800 and
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(0.1 M TBAPF6 in CH2Cl2, 200 mV s−1) in the latter and both are
fully reversible with ipa/ipc near unity.
studies on oxo-molybdenum complexes are very rare.25 Even more
rare is geometric isomerization in oxo-molybdenum complexes
where three sites are occupied by a facially coordinating ligand,
and to our knowledge the mechanisms of such isomerization
reactions have not been reported in detail. In general, there are
two major processes by which isomerization reactions in these
pseudo-octahedral complexes may proceed: the trigonal twist or
“turnstile” mechanism and the dissociative mechanism. In the case
of the twist mechanism, the geometric rearrangement originates
through a rotation about a pseudo-three-fold axis at the middle
of a trigonal face through a trigonal prismatic intermediate.
In contrast, the dissociative mechanism proceeds via geometric
rearrangements through breaking of metal–ligand bonds, followed
by reconstitution at a stereochemically different position relative to
the first.26 Within the dissociative mechanism, at least one type of
interaction may be immediately ruled out as an unlikely possibility
i.e. it is extremely improbable that the terminal oxo bonds (a
metal–ligand multiple bond) will dissociate in dry organic solvents.
Also unlikely is dissociation of the metal–alkoxide bond within the
tridentate ligand due to the strong nature of these bonds combined
with the high entropic penalty to be paid in the dissociation of any
of the chelate arms. Thus, the dissociative pathways remaining, if
this mechanism is operable, must involve the dissociation of either
a metal–halide or a metal–pyrazole bond. Metal–halide rupture
is viewed as unlikely since when the isomerizations are allowed
to occur in the presence of excess tetrabutylammonium bromide,
no evidence is seen in the ESI-MS spectrum for the incorporation
of bromide into the complexes. Such would be expected to occur
if a dissociative pathway via halide exchange were functioning.
This leaves dissociation of a metal–pyrazole bond, which can
be expected to be relatively weak particularly when trans to an
oxo group due to the kinetic trans effect, as the most likely
possibility for this mechanism. The activation parameters derived
from variable-temperature kinetic measurements however, reveal
a slightly negative entropy of activation, which if anything, is
suggestive of a more, rather than less, ordered transition state
(although the magnitude of the entropy of activation is too small to
definitely assign any mechanism). Although at present we favor the
trigonal-twist as the operative isomerization mechanism, clearly
further experimental and theoretical (detailed DFT calculations)
work is needed to clarify this point and is ongoing.
Kinetics of isomerization
We have already shown in the related [(L1O)MoOCl2] series that
the cis isomer spontaneously isomerizes to the trans, which process
is associated with distinctive changes in the optical spectra.20 The
same is true in this case. The changes in the optical spectra of the
isomer transformation in DMSO solution are provided in Fig. 8
and exhibit a tight isosbestic point at 625 nm. The rates of the
isomeric conversions have been followed spectrophotometrically
at 530 and 710 nm for 3–5 half-lives to ensure that the process had
neared completion. The reaction rates, which were quite slow, were
consequently measured over four different temperatures ranging
from 60 to 80 ◦C. The isomerization follows a first-order process,
indicating a unimolecular reaction. The first-order rate constants
obtained from appropriate fits to the data, were used to determine
the activation parameters. An enthalpy (23 2 kcal mol−1) and
the entropy (−10 1 cal mol−1 K−1) of activation were determined
from the Eyring relation.
Fig. 8 Changes in the optical spectrum of cis [(L10O)MoOCl2] in DMSO
at 60 ◦C as a function of time. The band near 500 nm is increasing while
that at 700 nm is decreasing.
While the cis and trans isomers of the Mo(VI) and W(VI)
complexes lack characteristic features in their UV-vis spectra,
they are diamagnetic, thereby allowing NMR spectroscopy to be
used to follow the isomerization process. The two isomers are
easily distinguished from each other by monitoring changes in the
pyrazole region of the proton NMR spectrum as a function of
time and temperature. Spectra were recorded in d6-DMSO so as
to make direct comparisons to the rate of isomerization in the
Mo(V) state (vide supra) feasible. Since the rate of isomerization
in the Mo(VI) state appeared to be considerably faster than that
in Mo(V), the rates were measured over the temperature range of
35–60 ◦C. This gave rise to an enthalpy of activation of 19 2 kcal
mol−1 and entropy of activation of −17 1 cal mol−1 K−1. At 60 ◦C
the rate of isomerization of the Mo(VI) complex was more than
6.5-fold faster (2.9(3) × 10−4 s−1) than that for the corresponding
Mo(V) complex at 4.4(1) × 10−5 s−1 and nearly 5 times faster than
the dioxo W(VI), 6.3(13) × 10−5 s−1.
Conclusion
Here we show for the first time, using a sterically restricted
alkoxy heteroscorpionate ligand, that the monooxo Mo(IV) and
(V) and dioxo Mo(VI) and W(VI) complexes exist as isolable cis and
trans isomers and that both the nature of the thermodynamically
stable isomer and the rates of isomerization are oxidation-state
and metal-ion dependent. Such information should be useful in
probing and understanding the mechanisms of both enzymatic
and nonenzymatic OAT transfer reactions.
Experimental
Mechanisms of isomerization
Preparation of compounds
In general, the mechanisms of geometric isomerization in coor-
dination complexes have been studied in detail, however, such
All reactions were performed under a dinitrogen or argon
atmosphere using standard Schlenk or drybox techniques.
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Acetonitrile, ether, dichloromethane and THF were freshly pu-
rified using an Innovative Technology solvent purification system.
All other solvents were dried with appropriate drying agents and
distilled prior to use. Silica gel (SilicAR Silica Gel 150, 60–200
Mesh 60–200 mesh) used in adsorption chromatography was
obtained from Mallinckrodt while the filtering agent Celite was
obtained from Aldrich. The purity of isolated compounds as well
as the progress of the reactions were monitored by thin-layer
chromatography. trans-[MoOCl(CNC(CH3)3)4]PF6 was prepared
as previously described.27
−30 ◦C with isopropyl ether. The total yield of the initial reaction
was 42.4% (32.8% trans and 9.6% cis). Symmetric isomer (cis): (2)
1
FTIR (KBr, cm−1): mMoO = 933, 905 (vs). H NMR (DMSO-d6)
d 7.66 (d, 4H, Ar–H), 7.29 (s, 1H, –CH–), 7.26 (dd, 4H, Ar–H),
7.20 (dd, 2H, Ar–H), 5.96 (s, 2H, Pz–H), 2.40 (s, 6H, Pz–CH3),
2.34 (s, 6H, Pz–CH3). 13C NMR (DMSO-d6) d 151.48, 141.93,
141.85, 127.92, 127.65, 125.54, 106.91, 87.67, 70.19, 13.95, 10.61.
Asymmetric isomer (trans): (3) FTIR (KBr, cm−1): mMoO = 927,
1
907 (vs). H NMR (DMSO-d6) d 7.98 (dd, 2H, Ar–H), 7.87 (dd,
2H, Ar–H), 7.33 (s, 1H, –CH–), 7.28 (dd, 2H, Ar–H), 7.25 (dd,
2H, Ar–H), 7.21 (m, 1H, Ar–H), 7.17 (m, 1H, Ar–H), 6.05 (s, 1H,
Pz–H), 5.90 (s, 1H, Pz–H), 2.40 (s, 3H, Pz–CH3), 2.38 (s, 3H, Pz–
CH3), 2.36 (s, 3H, Pz–CH3), 2.32 (s, 3H, Pz–CH3). Anal. Calc. for
C24H25ClMoN4O3: C, 52.52; H, 4.59; N, 10.21. Found C, 52.29; H,
4.74; N, 10.44%.
[L10OH] (1). To a dichloromethane solution (50 mL) of 3,5-
dimethylpyrazole (15.803 g, 0.164 mol) was added the phase
transfer reagent tetrabutylammonium hydrogensulfate (500 mg,
1.47 mmol), followed by 100 mL of 50% (w/v) NaOH. The
solution was refluxed for 54 h, and upon cooling the white
precipitate was filtered off. The solid was dissolved in hexane,
dried with MgSO4, filtered, reduced in volume and left at −20 ◦C
overnight to yield a white crystalline solid (11.2 g, 67%). To a THF
solution of bis(3,5-dimethylpyrazolyl)methane (6 g, 29.4 mmol) at
−78 ◦C was added 11.8 mL of BuLi (2.5 M in hexanes, 29.5
mmol). The solution was allowed to stir for 30 min, at which
time benzophenone (5.36 g, 29.4 mmol) was added. The reaction
mixture was allowed to stir and warm to room temperature
overnight, yielding a white precipitate in yellow colored solution.
The entire mixture was rotary evaporated, and then extracted
into dichloromethane. The extract was then brought to dryness
by rotary evaporation. The resulting solid was twice crystallized
from a minimum quantity of dichloromethane to yield the desired
ligand (6.1 g, 73%). 1H NMR (CD3Cl) d 7.98 (s, 1H, –OH), 7.31
(dd, 4H, Ar–H), 7.18 (dd, 2H, Ar–H), 7.16 (dd, 4H, Ar–H), 5.66
(s, 2H, Pz–H), 2.01 (s, 12H, Pz–CH3). 13C NMR (CDCl3) d 146.93,
143.92, 140.56, 127.61, 126.86, 126.25, 106.21, 81.74, 74.06, 13.17,
11.02. Anal. Calc. for C24H26N4O·0.25H2O: C, 73.72; H, 6.83; N,
14.33. Found: C, 73.70; H, 6.78; N, 14.36%.
[(L10O)MoO2]2O. When attempts were made to grow X-ray
quality crystals of 2 or 3 at room temperature rather than at
−30 ◦C, crystals of a third compound, which proved to be the
l-oxo Mo(VI) dimer, 4, were isolated in variable yield. FTIR
1
(KBr, cm−1): mMoO = 928, 903 (vs). H NMR (DMSO-d6) d 7.68
(dd, 8H, Ar–H), 7.20 (dd, 4H, Ar–H), 7.08 (dd, 8H, Ar–H), 6.98
(s, 2H, –CH–), 5.81 (s, 2H, Pz–H), 5.36 (s, 2H, Pz–H), 2.85 (s, 6H,
Pz–CH3), 2.36 (s, 6H, Pz–CH3), 2.16 (s, 6H, Pz–CH3), 0.73 (s, 6H,
Pz–CH3). Anal. Calc. for C48H50Mo2N8O7·2DMSO: C, 52.08; H,
5.21; N, 9.34. Found: C, 51.54; H, 5.35; N, 9.31%.
[(L10O)MoOCl2]. A mixture of 1 (394 mg, 1.02 mmol) and
NaOMe (54 mg, 1.01 mmol) was dissolved in 50 mL of THF
under inert atmosphere (N2) and stirred while cooling to −78 ◦C
in an acetone/dry ice bath. A solution of 40 mL THF and MoCl5
was also stirred and cooled to −78 ◦C under inert atmosphere.
The solution of MoCl5 was then added to the deprotonated ligand
solution via cannula transfer. The resulting yellow solution was
stirred overnight and allowed to warm to room temperature during
which time it became a dark brown. The solvent was removed by
rotary evaporation. The resulting solid was dissolved in CH2Cl2
and passed through a plug of Celite. After filtration the solvent
was once again removed. The solid was then dissolved in minimal
amount of CH2Cl2 and chromatographed on a silica gel column
(3 × 33 cm) using CH2Cl2 as the mobile phase. The first fraction
was green (137 mg, 10.3% yield) followed by a second purple
fraction (210 mg, 15.8% yield). The green fraction (cis isomer)
was crystallized by layering a CH2Cl2 solution with isopropyl ether
while the purple fraction was crystallized by dissolving the material
in minimal quantity of hot DMF and allowing it cool to room
temperature. The total yield of the reaction was 26.1% (15.8%
trans and 10.3% cis).
[(L10O)MoO2Cl]. A solution of KH (310 mg, 7.78 mmol)
and 1 (3.0 g, 7.78 mmol) in 10 mL DMF was stirred under
inert atmosphere until H2 gas evolution ceased. The solution was
then added dropwise over several minutes to a stirred solution of
MoO2Cl2 (1.55 g, 7.78 mmol) in 20 mL of DMF. The resulting
solution turned a pale yellow color, and was allowed to continue
stirring overnight. The solution was then cooled to −30 ◦C
for 2 h, and the resulting precipitate was collected via vacuum
filtration. The precipitate was washed with anhydrous diethyl ether
to remove remaining DMF, and then dissolved in CH2Cl2 and
passed through a plug of Celite to remove the residual KCl. The
solvent was then removed by reduced pressure to yield 1.4 g of
product that was predominantly the trans isomer (an 11 : 1 ratio,
Asymmetric isomer (cis): (5) FTIR (KBr, cm−1): mMoO = 960.
Uv-Vis, DMSO kmax (e/M−1 cm−1): 710 nm (13.1). Anal. Calc. for
C24H25Cl2MoN4O2: C, 50.43; H, 5.50; N, 11.76. Found: C, 49.79;
H, 5.48; N, 11.40%.
Symmetric isomer (trans): (6) FTIR (KBr, cm−1): mMoO = 933.
Uv-Vis, DMSO kmax (e/M−1 cm−1): 530 nm (20.3). Anal. Calc. for
C24H25Cl2MoN4O2: C, 50.72; H, 4.43; N, 9.86. Found: C, 50.51;
H, 5.09; N, 9.95%.
1
as determined by H NMR). Attempts to obtain crystals of this
isomer proved fruitless due to relatively rapid isomerization to
the thermodynamically more stable cis isomer and/or hydrolysis
to the l-oxo dimer (vide infra). The DMF mother liquor of the
first filtration was then heated in a water bath under dinitrogen
gas for several hours. Afterwards the solution was again cooled
to −30 ◦C overnight. The resulting precipitate was collected by
vacuum filtration to yield 410 mg of pure cis product (cis is
defined here as the symmetric product and trans as the asymmetric
product). X-Ray quality crystals of this isomer were produced
by layering a concentrated solution of the product in CH2Cl2 at
[(L10O)MoOCl(py)] (7). This complex could be prepared by
two different routes, either by direct synthesis using a labile Mo(IV)
precursor or by OAT from [(L10O)MoO2Cl].
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Method 1. A 10 mL solution of 1 (400 mg, 1.03 mmol) in
anhydrous pyridine was treated with KH (41.4 mg, 1.03 mmol)
and allowed to stir until gas evolution ceased. The pale yellow
solution was then added dropwise to a pyridine solution of trans-
[MoOCl(CNC(CH3)3)4]PF6 (646 mg, 1.03 mmol) and was allowed
to stir for eight hours at room temperature at which time the
solvent was removed under reduced pressure without heating.
The dark brown crude product was passed through a short silica
column (Mallinckrodt Silica Gel 150, 60–200 mesh, 10 × 3 cm)
using a 4 : 1 mixture of CH2Cl2–CH3CN as the eluent. The first
band is an as of yet unidentified brown material. The second
dark green band, the desired Mo(IV) complex, was collected and
dried.
Kinetics
The isomerization of the cis isomer to the trans isomer in the
paramagnetic Mo(V) complex [(L10O)MoOCl2], was monitored
via UV-vis spectroscopy in DMSO at four different temperatures
using a jacketed quartz cell attached to a circulating bath control-
ling the temperature to 0.2 ◦C. Rate constants were acquired
monitoring the reaction at 530 and 710 nm (approximately the
absorption maxima of the trans and cis isomers, respectively)
using the scanning kinetics program included with the instrument
software. Tight isosbestic points were maintained throughout the
reaction indicating the presence of only two absorbing species.
Due to a lack of a suitable chromophore the rate of isomerization
between the diamagnetic cis and trans isomers of [(L10O)MO2Cl],
where M = Mo or W, were monitored in DMSO-d6 by 1H
NMR utilizing an in-house kinetics program written by Dr Leroy
Lafferty.
To check for chloride ion dissociation as a potential pathway
for isomerization such isomerizations were allowed to proceed
in acetonitrile solution in the presence of a 10-fold excess of
tetraethylammonium bromide. Potential bromide incorporation
was monitored by ESI-MS experiments conducted on a Finnegan
LCQ Ion Trap mass spectrometer in the positive ion mode.
No evidence for bromide incorporation into the parent ion was
detected.
Method 2. To a solution of trans-[(L10O)MoO2Cl] (300 mg,
0.61 mmol) in 50 mL of anhydrous pyridine was added PPh3
(190 mg, 0.61 mmol). The resulting solution was purged with
dinitrogen, sealed, and stirred for 3 h. The solvent was then
removed under reduced pressure to yield a dark green–brown
solid. The crude product was passed through a short silica column
(Mallinckrodt Silica Gel 150, 60–200 mesh, 10 × 3 cm) using
a 4 : 1 mixture of CH2Cl2–CH3CN as the eluent. The desired
product was eluted from the column as a dark green band and
reduced to dryness without heat. The product was further◦purified
by hexane precipitation from a CH2Cl2 solution at −30 C. The
precipitate was collected via filtration and dried to yield trans-
[(L10O)MoOCl(NC5H5)] (105 mg 31.4%). X-Ray quality crystals
were produced by layering a concentrated CH2Cl2 solution with
Physical methods
Elemental analyses were performed on all compounds by Numega,
Inc., San Diego, CA. All samples were dried in vacuo prior to
analysis. IR spectra were recorded from KBr disks on either a
Perkin-Elmer 1600 Series FTIR spectrometer or a ThermoNicolet
FTIR Nexus 670 and are reported in wavenumbers. Solution
electronic absorption spectra were collected using a Cary 50
Bio UV-vis spectrophotometer. Cyclic voltammetric experiments
were conducted using a BAS CV 50 W (Bioanalytical Systems
Inc., West Lafayette, IN) voltammetric analyzer. All experiments
were done under argon at ambient temperature in solutions with
0.1 M tetrabutylammonium hexafluorophosphate as the support-
ing electrolyte. Cyclic voltammograms (CV) were obtained using
a three-electrode system consisting of Pt working, platinum wire
auxiliary, and SCE reference electrodes. Potentials are reported
vs. the Ag/AgCl (KCl saturated) couple. (197 mV vs. NHE).
The Fc/Fc+ couple was used as an internal standard to monitor
the reference electrode. Osteryoung square voltammograms were
obtained under similar conditions with a scan rate of 100 mV
s−1. EPR spectra were recorded in dichloromethane solution at
room temperature on a MicroNow 8300A X-band spectrometer
operating at ca. 9.36 GHz. Conditions: 3400 G field center, 500 G
sweep width, 1.6 G modulation amplitude, 30 s scan time.
◦
hexanes at −30 C. FTIR (KBr, cm−1): mMoO = 920 (vs). UV-
vis CH2Cl2 kmax(e/M−1 cm−1): 485 nm (3450), 690 nm (370). 13C
NMR (CDCl3) d 153.59, 142.21, 141.12, 128.92, 128.76, 128.59,
128.52, 128.23, 127.67, 127.49, 127.06, 126.64, 126.47, 126.32,
107.82, 107.26, 88.34, 71.40, 68.10, 14.70, 11.42. Anal. Calc. for
C30H32Cl3MoN5O2 C, 51.70; H, 4.62; N, 10.05. Found: C, 51.91;
H, 4.51; N, 10.30%.
[(L10O)WO2Cl]. L10OH (255 mg, 0.66 mmol) and NaOMe
(36 mg, 0.67 mmol) were stirred in 20 mL THF for 0.5 h.
The deprotonated ligand was then added dropwise via syringe
to a solution of WOCl4 (225 mg, 0.66 mmol) in 30 mL of
THF under nitrogen at room temperature. The tungsten solution
instantly decolorized. After stirring for 20 h, the solution was
filtered through a glass frit containing Celite to remove NaCl
and blue insoluble solids. Concentrating the filtrate to dryness
afforded a cream colored solid which was recrystallized from
dichloromethane–hexane (76%). NMR analysis indicated an ca.
2 : 1 ratio of trans to cis isomers in the sample as isolated.
Symmetric isomer (cis): FTIR (KBr, cm−1): mWO = 947, 903 (vs).
1H NMR (CDCl3) d 7.51–7.55 (m, 5H, Ar–H), 7.23–7.27 (m, 5H,
Ar–H), 6.76 (s, 1H, –CHC–), 5.82 (s, 2H, Pz–H), 2.61 (s, 6H, Pz–
CH3), 2.20 (s, 6H, Pz–CH3). 13C NMR (DMSO) d 157.78, 143.18,
128,83, 126.39, 108.19, 98.55, 88.69, 82.69, 28.51, 14.15, 11.51.
Asymmetric isomer (trans): 1H NMR (CDCl3) d 7.63–7.78 (m,
5H, Ar–H), 7.18–7.21 (m, 5H, Ar–H), 6.81 (s, 1H, –CH–), 5.85 (s,
1H, Pz–H), 5.78 (s, 1H, Pz–H), 2.65 (s, 3H, Pz–CH3), 2.58 (s, 3H,
Pz–CH3), 2.42 (s, 3H, Pz–CH3), 2.25 (s, 3H, Pz–CH3).
X-Ray crystallography
Crystal, data collection, and refinement parameters for
cis-[(L10O)MoO2Cl]
(2),
[(L10O)MoO2]2O
(4),
trans-
[(L10O)MoOCl2]·2DMF (5), cis-[(L10O)MoOCl2] (6) and
trans-[(L10O)MoOCl(py)] (7) are given in Table 1. Crystals were
attached to the end of glass fibers or nylon cryo loops (Hampton
Research) by Paratone oil and frozen in the cold stream of the
diffractometer. Data collection was initiated at 213 K for 2 and
Anal. Calc. for C24H25ClWN4O3 C, 45.27; H, 3.96; N, 8.80.
Found: C, 45.12; H, 3.80; N, 8.62%.
3828 | Dalton Trans., 2006, 3822–3830
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©

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Table 1 Summary of crystallographic data and parameters (refinement method: full-matrix least squares on F2)
2
4
5
6
7
Molecular formula
Mr
C24H25ClMoN4O3
548.87
213(2)
Monoclinic
P21/n
9.759(1)
15.518(1)
15.408(1)
90
94.171(1)
90
C25H27Cl2MoN4O3.5
606.35
200(2)
Orthorhombic
Pba2
9.5623(6)
29.6352(18)
9.4479(5)
90
90
90
C24H25Cl2MoN4O2
568.32
200(2)
Monoclinic
P21/c
19.148(1)
15.663(1)
17.534(1)
90
116.015(5)
90
C30H39Cl2MoN6O4
714.51
213(2)
Triclinic
¯
P1
8.751(1)
13.637(1)
14.864(1)
72.021(1)
73.220(1)
80.756(1)
2
1610.4(2)
0.618
1.474
C30H32N5O2Cl3Mo
696.90
200(2)
Triclinic
¯
P1
12.3441(11)
15.8510(14)
15.9522(15)
93.312(4)
91.687(4)
93.927(4)
4
3107.0(5)
0.716
1.490
T/K
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
Z
4
4
8
3
˚
V/A
2327.2(2)
0.713
1.567
2677.4(3)
0.754
1.504
4726.6(4)
0.811
1.597
l/mm−1
Dc/g cm−3
F(000)
1120
1236
2312
738
1424
Crystal dimens./mm
0.35 × 0.20 × 0.05
Mo-Ka (0.71073)
−12 → 11, −20 →
18, −20 → 20
1.87–28.19
16668
0.02 × 0.02 × 0.02
Mo-Ka (0.71073)
−12 → 12, −39 →
39, −12 → 10
2.24–22.84
66675
0.1 × 0.2 × 0.4
Mo-Ka (0.71073)
−19 → 19, −15 →
15, −17 → 17
1.18–20.85
32013
0.3 × 0.2 × 0.1
Mo-Ka (0.71073)
−11 → 11, −17 →
18, −18 → 17
1.49–28.20
11908
0.3 × 0.3 × 0.1
Mo-Ka (0.71073)
−16 → 16, −21 →
21, −21 → 20
1.28–28.46
57348
˚
Radiation (k/A)
h, k, l Ranges
h Range/◦
No. refl.
No. unique refl.
No. param.
Data/param. ratio
R(F)a
5482
398
13.77
0.0376
0.0924
1.080
4852
321
15.11
0.0583
0.1149
0.955
0.793, −0.690
4961
591
8.39
0.0450
0.1015
1.5
0.884, −0.707
7285
396
18.40
0.0805
0.1935
1.112
2.845, −1.515
15147
739
20.50
0.0507
0.1234
1.035
1.159, −0.748
Rw(F2)b
GOFwc
Dqmax,min/e A
−3
˚
1.199, −0.316
ꢀ
ꢀ
ꢀ
ꢀ
a R = [ |DF|/ |Fo|]. b Rw = [ w(DF)2/ wFo2]. c GOFw = Goodness of fit on F2.
◦
˚
Table 2 Selected bond lengths (A) and angles ( )
2
4
5
6
7
Mo–O1
Mo–O2
Mo–O3
Mo–N2
Mo–N4
Mo–Cl1
Mo–Cl2
Mo–X4
1.903(2)
1.956(4)
1.707(4)
1.699(4)
2.362(5)
2.352(5)
1.923(4)
1.812(4)
1.935(3)
1.974(5)
2.020(3)
1.697(2)
1.697(2)
1.717(2)
2.345(2)
2.343(2)
2.383(1)
2.187(5)
2.343(5)
2.372(2)
2.288(2)
2.219(4)
2.211(4)
2.321(2)
2.289(2)
2.212(3)
2.198(3)
2.438(1)
1.901(1)
2.157(3)
O1–Mo–O2
O1–Mo–O3
O1–Mo–N2
O1–Mo–N4
O1–Mo–Cl1
O1–Mo–Cl2
O2–Mo–O3
O2–Mo–N2
O2–Mo–N4
O2–Mo–Cl1
O2–Mo–Cl2
O3–Mo–N2
O3–Mo–N4
O3–Mo–Cl1
N2–Mo–N4
N2–Mo–Cl1
N2–Mo–Cl2
N4–Mo–Cl1
N4–Mo–Cl2
Cl1–Mo–Cl2
Mo1–O4–Mo2
100.13(9)
99.80(9)
77.49(7)
79.40(7)
156.29(5)
97.75(2)
96.52(2)
75.40(2)
77.29(2)
100.26(2)
167.70(16)
166.70(11)
84.36(2)
77.05(2)
91.28(1)
88.15(13)
79.96(1)
79.96(1)
93.71(1)
89.27(1)
92.65(1)
102.39(9)
91.41(10)
105.13(1)
164.08(9)
88.05(9)
94.87(7)
104.2(2)
167.05(2)
89.3(2)
92.82(2)
171.01(2)
100.54(12)
97.78(13)
90.52(2)
90.76(2)
94.53(13)
78.85(1)
80.25(1)
89.37(7)
97.08(12)
90.77(8)
166.67(8)
93.81(7)
76.03(7)
83.04(5)
87.70(18)
165.96(19)
78.57(17)
87.77(14)
78.43(18)
92.80(11)
166.49(15)
170.99(11)
84.07(13)
92.32(6)
79.86(14)
93.78(8)
169.28(10)
168.21(8)
92.45(11)
94.12(6)
83.82(10)
82.89(5)
160.63(13)
168.4(3)
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5 and at 200 K for 4, 6 and 7. Crystals were collected either
at UCSD (2 and 5 courtesy of Prof. Arnold L. Rheingold)
on a Bruker SMART APEX or at SDSU on a Bruker X8
APEX CCD diffractometer using Mo radiation and a graphite
monochrometer. The crystals of 4 were extremely small in two
dimensions giving weak diffraction data requiring unusually
long integration times (60 s). The systematic absences in the
diffraction data are consistent with the space groups P21/n for 2,
4 J. F. Stolz and P. Basu, ChemBioChem, 2002, 3, 198–206.
5 G. N. George, J. A. Mertens and W. H. Campbell, J. Am. Chem. Soc.,
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6 K. Heffron, C. Leger, R. A. Rothery, J. H. Weiner and F. A. Armstrong,
Biochemistry, 2001, 40, 3117.
7 K. Peariso, B. S. Chohan, C. J. Carrano and M. L. Kirk, Inorg. Chem.,
2003, 42, 6194–6203.
8 (a) L. D. McPherson, M. Drees, S. I. Khan, T. Straaner and M. M.
Abu-Omar, Inorg. Chem., 2004, 43, 4036–4050; (b) D. W. Lahti and
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¯
P21/c for 5 and P1 for 6 and 7. Compound 4 crystallized in the
acentric space group Pba2 and refined with a Flack parameter
of −0.08, indicative that the correct chirality had been found.
The structures were solved using either direct methods or via the
Patterson function, completed by subsequent difference Fourier
syntheses and refined by full-matrix least-squares procedures
on F2. All non-hydrogen atoms were refined with anisotropic
displacement coefficients, while hydrogens were treated as
idealized contributions using a riding model except where noted.
All software and sources of the scattering factors are contained
in the SHELXTL (6.0) program library (G. Sheldrick, Siemens
XRD, Madison WI). Selected bond distances and angles for the
complexes are shown in Table 2. Complete structural parameters
are provided in the supporting information.
9 H. Oku, N. Ueyama, M. Kondo and A. Nakamura, Inorg. Chem., 1994,
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15 S. B. Seymore and S. N. Brown, Inorg. Chem., 2000, 39, 325–332.
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18 A. Otero, J. Fernandez-Baeza, A. Antinolo, J. Tejeda and A. Lara-
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19 S. R. Davie, N. Rubie, B. S. Hammes, C. J. Carrano, M. L. Kirk and P.
Basu, Inorg. Chem., 2001, 40, 2632–2633.
CCDC reference numbers 246847–246849, 603447 and 603448.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b604751c
20 B. Kail, V. N. Nemykin, S. R. Davie, C. J. Carrano, B. S. Hammes and
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Acknowledgements
This work was supported in part by grants CHE-0313865 and
CHE-0202535 from the NSF. The NSF-MRI program grant CHE-
0320848 is acknowledged for support of the X-ray diffraction
facilities at San Diego State University. Prof. Arnold L. Rhein-
gold, Department of Chemistry and Biochemistry, University of
California San Diego is gratefully acknowledged for X-ray data
collection on compounds 2 and 5. We also thank Prof. Partha
Basu for many fruitful discussions that lead to the initiation and
continuation of this work.
22 J. T. Hoffman, S. Einwa¨chter, B. S. Chohan, P. Basu and C. J. Carrano,
Inorg.Chem., 2004, 43, 7573–7575.
23 (a) K. Yoon, G. Parkin and A. L. Rheingold, J. Am. Chem. Soc., 1991,
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24 J. T. Hoffman and C. J. Carrano, unpublished data.
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see: J. U. Mondahl, J. G. Zamora, M. D. Kinon and F. A. Schultz,
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see: A. K. Duhme, J. Chem. Soc., Dalton Trans., 1997, 773.
26 I. Ugi, D. Marquarding, H. Klusaecek, P. Gillespie and F. Ramirez,
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3830 | Dalton Trans., 2006, 3822–3830
This journal is
The Royal Society of Chemistry 2006
©