Electronic Fine-Tuning of Oxygen Atom Transfer Reactivity of cis-Dioxomolybdenum(VI) Complexes with Thiosemicarbazone Ligands

Article abstract of DOI:10.1002/ejic.201500059

A series of six cis-dioxomolybdenum(VI) complexes with thiosemicarbazone ligands was synthesized and characterized. The ligands were obtained by reacting ethyl thiosemicarbazide with salicylaldehydes substituted with a selection of electron-withdrawing an

Full text of DOI:10.1002/ejic.201500059

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DOI:10.1002/ejic.201500059
Electronic Fine-Tuning of Oxygen Atom Transfer
Reactivity of cis-Dioxomolybdenum(VI) Complexes with
Thiosemicarbazone Ligands
Aurélien Ducrot,[a] Bethany Scattergood,[a] Ben Coulson,[a]
Robin N. Perutz,[a] and Anne-K. Duhme-Klair*[a]
Keywords: Enzyme models / Metalloenzymes / Molybdenum / Oxygen atom transfer / Ligand effects
A series of six cis-dioxomolybdenum(VI) complexes with
thiosemicarbazone ligands was synthesized and charac-
terized. The ligands were obtained by reacting ethyl thio-
relationships between Hammett constants and key proper-
ties of the complexes, including the molybdenum–phenolate
bond lengths and the coordination shift of the imine proton
resonance. Compared with the unsubstituted catalyst, elec-
tron-withdrawing substituents increase the rate of oxygen
atom transfer from dimethyl sulfoxide to triphenylphosphine,
whereas electron-donating groups have the opposite effect.
The highest rate enhancement was achieved through the in-
troduction of a strongly electron-withdrawing NO2 substitu-
ent in the p-position of the phenolate donor.
semicarbazide with salicylaldehydes substituted with
a
selection of electron-withdrawing and electron-donating
groups. The crystal structures, IR, NMR spectroscopic data
and oxygen atom transfer activities of the complexes re-
vealed that the electronic effects of the substituents located
in the para-position of the phenolate donor are transmitted
through to the molybdenum center, as reflected by linear
tsc, 3[18,19]). Whereas tridentate ligands can occupy only
three of the four available coordination sites on the cis-
MoO2 center, tetradentate ligands, including tripodal (N3O,
Introduction
Nature uses the ability of molybdenum to switch between
oxidation states (VI) and (IV) to catalyze oxygen atom
transfer (OAT) reactions.[1] In molybdoenzymes, the two
electrons that are released during substrate oxidation are
rapidly transferred to spatially separated one-electron trans-
fer components, such as heme and/or Fe–S clusters. Finally,
the reducing equivalents generated are handed over to a
terminal electron acceptor.
4
[20]) or linear (salan[21,22]) ligands (5) form coordinatively
saturated complexes.
A range of structurally diverse dioxomolybdenum(VI)
complexes have been developed with the aim of catalyzing
biomimetic and industrially relevant OAT reactions.[2–6]
Whereas bidentate dithiolene ligands model the S,S donor
set found in the molybdopterin cofactor of the enzymes,[7,8]
bidentate ligands with N,S, N,N and N,O donor have also
been found to catalyze OAT. Examples include pyridine
thiolates,[9] iminopyrrolato,[10,11] diketiminate (NacNac),[12]
and β-ketiminato[13] ligands. In addition, dioxo-molybd-
enum complexes with ligands of higher denticity and mixed
donor sets have shown promise (Figure 1). Commonly
studied tridentate ligands include facially coordinating
hydrotris(pyrazolyl)borates (e.g., Tp*, 1[14–16]) and meridio-
nally coordinating Schiff base derivatives (e.g., ssp, 2[17] and
Figure 1. Schematic representation of the structures of selected bio-
mimetic cis-dioxomolybdenum(VI) complexes with multidentate
ligands (L = solvent molecule, * “linear” indicates donor atom con-
nectivity rather than 3D structure).
To assess the catalytic activity of functional model com-
plexes, the thermodynamically favorable OAT from sulf-
oxides to tertiary phosphines is often used as a benchmark
reaction. Biomimetic model complexes, however, tend not
to have additional redox centers incorporated, as found in
the molybdoenzymes, and hence both the oxidative and re-
ductive half-reaction take place at the molybdenum center
(Scheme 1).[23] The catalytic activity of these complexes is
[a] Department of Chemistry, University of York,
Heslington, York, YO10 5DD, United Kingdom
E-mail: anne.duhme-klair@york.ac.uk
http://www.york.ac.uk/chemistry/staff/academic/d-g/
a-kduhme-klair/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500059.
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therefore often limited by slow kinetics due to charge build-
Here we present the synthesis, characterization and cata-
up at the molybdenum center and the formation of inactive lytic OAT activity of a series of cis-dioxomolybdenum(VI)
MoV dimers as a result of the comproportionation of MoIV complexes with structurally related tridentate thiosemicarb-
and MoVI (Scheme 1).
azone ligands (Scheme 2). The complexes feature a selection
of EWGs and EDGs located in the para-position of the
phenolate donor that allow for a Hammett analysis of their
OAT reactivity.
Results and Discussion
Compound Design, Synthesis, and Characterization
To allow for a systematic Hammett analysis of the effects
of a p-aryl substituent on the electron density of the phen-
olate donor and hence the molybdenum center, the follow-
ing electron-donating and electron-withdrawing groups
were chosen (corresponding Hammett constants[31] σp given
in parenthesis): Me (–0.17), H (0), I (0.18), Br (0.23), OCF3
(0.35), and NO2 (0.78). The rather unusual trifluoro-
methoxy group was included in the series because of its in-
creasing use in the development of bioactive molecules.[32]
For ligands adjacent in the series (i.e., with similar
Hammett parameters), the properties discussed below are
not always significantly different if experimental errors are
taken into account. Therefore, discussions are based on ob-
served trends rather than on quantitative differences be-
tween compounds that have similar Hammett constants.
Six thiosemicarbazone ligands were prepared by heating
ethanolic solutions of the respective para-substituted 2-
hydroxy salicylaldehyde to reflux with an equimolar
amount of ethyl thiosemicarbazide (Scheme 2). The thio-
semicarbazones formed were isolated following partial re-
moval of the solvent. Evidence for the formation of the im-
ine bond was provided by the appearance of a singlet reso-
nance for the imine proton at 8.32–8.38 ppm and loss of
Scheme 1. (a) Simplified catalytic cycle for the biomimetic oxygen
atom transfer (OAT) from dimethyl sulfoxide (DMSO) to tri-
phenylphosphine (PPh3) to give dimethyl sulfide (DMS) and tri-
phenylphosphine oxide (OPPh3); (b) Comproportionation reaction
leading to the formation of a dinuclear, oxo-bridged MoV complex.
In this context, we are interested in the development of
chelate ligands that allow the electronic properties of their
donor atoms to be tuned so as to facilitate OAT. Thiosemi-
carbazone ligands attract much attention because they are
straightforward to synthesize and have scope for structural
diversity. The research area has recently been reviewed.[24]
In particular, thiosemicarbazones derived from salicyl-
aldehydes allow a systematic investigation of the effect of
electron-withdrawing groups (EWGs) or electron-donating
groups (EDGs) on the electronic properties of a coordi-
nated molybdenum center. It was found that these elec-
tronic trends are reflected in the spectroscopic properties of
the molybdenum complexes formed,[18,25] and in their cata-
lytic,[26] biological,[27,28] and pharmacological proper-
ties.[29,30]
1
the resonance due to the aldehyde proton in the H NMR
spectra ([D6]DMSO). Corroborating evidence was obtained
by 13C NMR and FTIR spectroscopy.
Subsequently, the six ligands H2LR were coordinated to a
cis-dioxo MoVI center and the resulting [MoO2(LR)MeOH]
complexes were isolated and characterized. Evidence of
1
complex formation was seen in the H NMR spectra with
the imine proton resonance shifting downfield by 0.14–
0.36 ppm ([D6]DMSO) and loss of the OH and one of the
NH resonances due to deprotonation upon coordination.
IR spectroscopic data also confirmed complex formation,
with v(C=N) shifting to lower frequencies upon molybd-
enum binding. Good quality mass spectra of the complexes
with well-resolved isotope patterns could be obtained by
using liquid injection field desorption/ionization
(LIFDI).[33]
Whereas the syntheses of H2LH, H2LMe [29] H2LBr,[34]
,
[MoO2(LMe)MeOH],[29] and [MoO2(LBr)MeOH][34] have
been reported previously, the availability of characterization
data varies depending on the selection of techniques and
conditions used in each case. Hence, for completion and for
Scheme 2. Synthesis of six thiosemicarbazone ligands and their cis-
dioxo MoVI complexes (hacac = acetylacetone).
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comparison purposes, full characterization data of the six azones act as meridionally coordinating dianionic ligands
ligands and complexes were obtained and are provided in with the sulfur, oxygen, and imine nitrogen donors contrib-
the Experimental Section. LIFDI and IR spectra can be uting to the coordination sphere. In addition, two oxo
found in the Supporting Information.
ligands and a molecule of MeOH coordinate to the MoVI
centers to form distorted octahedral complexes with config-
uration index OC-6-24.[35,36] Figure 2 shows the molecular
structures of the parent complex [MoO2(LH)MeOH] and
the most active catalyst of the series, [MoO2(LNO2)MeOH],
as representative examples; the other ORTEP plots are
shown in the Supplementary Information. Selected bond
lengths and angles (Table 1) agree well with those reported
for similar cis-dioxo MoVI complexes.[18,27,28,37–40]
X-ray Structures
Diffraction-quality crystals of all six [MoO2(LR)MeOH]
complexes were obtained and their crystal structures were
determined. Selected bond lengths and angles and crystallo-
graphic data are summarized in Tables 1 and 2, respectively.
The crystal structures confirmed that the thiosemicarb-
The crystals consist of racemic mixtures of OC-6-24-A-
and OC-6-24-C-configured[35,36] enantiomers. The com-
plexes crystallize in centrosymmetric space groups P21/c
{[MoO2(LMe)MeOH], [MoO2(LNO2)MeOH]} and P1
¯
{[MoO2(LH)MeOH], [MoO2(LBr)MeOH], [MoO2(LI)-
MeOH], [MoO2(LOCF3)MeOH]}, in which the asymmetric
unit contains one of the enantiomers and the second
enantiomer is symmetry generated through rotoinversion
axes and/or inversion centers, respectively.
As a consequence of the meridionally coordinating
O,N,S ligand, the two oxo ligands on the MoVI centers are
inequivalent: one is positioned trans to the N donor,
whereas the second is coordinated trans to the O donor of
methanol. This may be relevant because it was recently re-
ported that a trans-effect that weakens the Mo–oxo bond
of the catalytically active group can give rise to enhanced
OAT activity.[41] In the [MoO2(LR)MeOH] complexes, the
Mo–O1 bonds trans to the imine N donor tend to be
slightly longer than the Mo–O2 bonds in the trans-position
of the weakly coordinated methanol.
The phenolate donor (O3) is the donor atom that is
closest to the remote substituent R; therefore it is the one
that is most likely to be affected by differences in their elec-
tronic properties. The Hammett plot, shown in Figure 3,
correlates the electron-withdrawing/donating properties of
the substituent R in the p-position to the phenolate sub-
stituent of the aromatic ring with the Mo–O3 bond lengths.
EWGs, in particular the strongly electron-withdrawing NO2
group, lower the electron density of the aromatic ring,
which, in turn, reduces the electron density on the phen-
Figure 2. ORTEP plots (50% probability ellipsoids) of the molecu-
lar structure of [MoO2(LH)MeOH] (top) and [MoO2(LNO2)MeOH]
(bottom); only one enantiomer is shown; configuration index OC-
6-24, chirality symbol C.
Table 1. Selected bond lengths and angles for [MoO2(LR)MeOH], with R = Me, H, I, Br, OCF3, and NO2.
Bond [Å]
[MoO2(LMe)MeOH] [MoO2(LH)MeOH] [MoO2(LI)MeOH]
[MoO2(LBr)MeOH] [MoO2(LOCF3)MeOH] [MoO2(LNO2)MeOH]
Mo–O1
Mo–O2
Mo–O3
Mo–O4
Mo–S1
Mo–N1
C1–S1
1.7118(12)
1.6941(13)
1.9304(12)
2.3503(12)
2.4224(4)
1.7042(13)
1.7022(12)
1.9327(13)
2.3471(12)
2.4147(5)
1.703(2)
1.692(2)
1.932(2)
2.313(2)
2.4105(8)
2.296(2)
1.753(3)
1.7025(12)
1.6997(13)
1.9340(12)
2.3058(12)
2.4148(5)
1.7045(12)
1.6942(14)
1.9371(13)
2.3011(14)
2.4325(5)
1.7076(16)
1.6964(17)
1.9488(15)
2.3015(15)
2.4199(6)
2.2949(16)
1.746(2)
2.2755(13)
1.7493(16)
2.2803(14)
1.7489(17)
2.2908(14)
1.7493(17)
2.2880(14)
1.7379(17)
Angle [°]
O1–Mo–O2 105.61(6)
O1–Mo–N1 158.73(5)
105.46(6)
156.36(5)
76.20(4)
82.37(5)
105.67(11)
159.11(10)
76.29(7)
105.57(6)
159.16(6)
76.34(4)
82.77(5)
106.19(7)
159.41(6)
76.21(4)
82.35(5)
105.34(8)
159.90(7)
75.72(4)
82.26(6)
N1–Mo–S1
75.78(3)
O3–Mo–N1 83.38(5)
82.85(9)
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Table 2. Crystallographic data for [MoO2(LR)MeOH], with R = Me, H, I, Br, OCF3, and NO2.
[MoO2(LH)(MeOH)]
[MoO2(LMe)(MeOH)]
[MoO2(LBr)(MeOH)]
CCDC deposition number
Empirical formula
1042299
C11H15MoN3O4S
104301
C12H17MoN3O4S
1042300
C11H14BrMoN3O4S
+ 1 (CH4O)
492.20
Molecular weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
381.26
395.28
monoclinic
P21/c
7.55956(10)
18.9597(2)
10.72806(14)
90
108.7103(15)
90
1456.36(3)
4
1.803
1.063
800.0
triclinic
triclinic
¯
¯
P1
P1
7.4031(5)
9.4211(6)
10.8189(7)
87.104(5)
72.698(6)
72.365(6)
685.92(8)
2
7.5586(5)
9.4460(5)
12.3311(8)
78.820(5)
75.754(5)
85.855(5)
836.88(9)
2
α [°]
β [°]
γ [°]
Volume [Å3]
Z
ρcalcd. [g/cm3]
M [mm–1]
F(000)
1.846
1.125
384.0
1.953
3.322
488.0
2Θ range for data collection [°] 5.886 to 64.584
7.132 to 64.364
–11 Յ h Յ 10,
–27 Յ k Յ 26,
–16 Յ l Յ 15
13617
6.808 to 64.338
–11 Յ h Յ 10,
–13 Յ k Յ 13,
–18 Յ l Յ 17
15598
Index ranges
–10 Յ h Յ 11,
–13 Յ k Յ 14,
–16 Յ l Յ 15
12447
Reflections collected
Independent reflections
4459
4717
5410
[Rint = 0.0384, Rsigma = 0.0508] [Rint = 0.0258, Rsigma = 0.0308] [Rint = 0.0251, Rsigma = 0.0297]
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I Ն 2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole [eÅ–3]
4459/0/191
1.069
R1 = 0.0271, wR2 = 0.0567
R1 = 0.0318, wR2 = 0.0594
0.69/–0.78
4717/0/201
1.061
R1 = 0.0250, wR2 = 0.0549
R1 = 0.0297, wR2 = 0.0578
0.48/–0.67
5410/0/222
1.038
R1 = 0.0233, wR2 = 0.0502
R1 = 0.0285, wR2 = 0.0524
0.54/–0.63
[MoO2(LI)(MeOH)]
[MoO2(LOCF3)(MeOH)]
[MoO2(LNO2)(MeOH)]
CCDC deposition number
Empirical formula
104303
104304
C12H14F3MoN3O5S
1042302
C11H14IMoN3O4S
+ 1 (CH4O)
539.19
C11H14MoN4O6S
+ 1.5 (CH4O)
474.32
monoclinic
P21/c
7.61510(14)
13.4754(2)
17.8271(3)
90
101.3340(17)
90
1793.68(5)
4
1.756
0.894
964.0
Molecular weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
465.26
triclinic
triclinic
¯
¯
P1
P1
7.6024(5)
9.5206(6)
12.4608(9)
77.984(6)
75.848(6)
85.882(5)
855.17(10)
2
7.9658(4)
9.4849(3)
10.7998(4)
93.431(3)
91.398(3)
91.144(3)
814.09(5)
2
α [°]
β [°]
γ [°]
Volume [Å3]
Z
ρcalcd. [g/cm3]
μ [mm–1]
F(000)
2.094
2.721
524.0
1.898
0.996
464.0
2Θ range for data collection [°] 6.752 to 64.406
6.616 to 64.326
–11 Յ h Յ 11,
–13 Յ k Յ 13,
–16 Յ l Յ 15
15061
7.026 to 64.342
–10 Յ h Յ 11,
–19 Յ k Յ 19,
–26 Յ l Յ 26
22461
Index ranges
–11 Յ h Յ 11,
–13 Յ k Յ 12,
–16 Յ l Յ 18
9123
Reflections collected
Independent reflections
5387
5261
5876
[Rint = 0.0360, Rsigma = 0.0559] [Rint = 0.0370, Rsigma = 0.0385] [Rint = 0.0317, Rsigma = 0.0287]
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I Ն 2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole [eÅ–3]
5387/0/220
1.066
R1 = 0.0367, wR2 = 0.0856
R1 = 0.0461, wR2 = 0.0962
1.65/–1.37
5261/0/233
1.045
R1 = 0.0277, wR2 = 0.0607
R1 = 0.0334, wR2 = 0.0637
0.70/–0.72
5876/0/267
1.053
R1 = 0.0323, wR2 = 0.0795
R1 = 0.0417, wR2 = 0.0853
1.12/–0.78
olate donor in the p-position. Hence, the donor strength of trend is not reflected by the other molybdenum donor
the phenolate group is reduced and the Mo–O3 bond be- bonds, either because they are too far away from the remote
comes longer and weaker. As expected, the electron-donat- substituent or because of packing effects that may mask any
ing methyl group has the opposite effect. However, this electronic effects.
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Table 3. Selected 1H NMR chemical shifts and IR spectroscopic
data.
Compounds
1H NMR
IR
δ [ppm]
CHN
ν [cm–1]
˜
NH ν(MoO2)asym. ν(MoO2)sym.
H2LMe
8.33
8.44
7.49 896
–
–
934
–
930
–
937
–
[MoO2(LMe)MeOH] 8.47
H2LH
8.36
8.55
8.27
8.50
8.29
8.53
8.28
8.47
7.52 901
8.56
7.64 906
8.65
7.65 906
8.60
–
[MoO2(LH)MeOH]
H2LI
–
[MoO2(LI)MeOH]
H2LBr
–
[MoO2(LBr)MeOH]
H2LOCF3
942
–
–
[MoO2(LOCF3
MeOH]
)
8.59
8.37
7.69 908
937
H2LNO2
8.75
7.80 899
[MoO2(LNO2)MeOH] 8.73
941
Figure 3. Plot of the Hammett constant σp vs. Mo–O3 bond
lengths [Å].
IR and NMR Spectroscopic Investigations
1
Selected IR spectroscopic data and H NMR chemical
shifts are summarized in Table 3. To assess whether or not
the electronic properties of the remote substituent R are
transmitted through to the cis-dioxomolybenum core, we
examined the ν(MoO2) stretching frequencies. As can be
seen from the Hammett plot in Figure 4, there is an approx-
imate correlation between the Hammett constant for the
remote substituent R and the position of the IR band due
to the asymmetric ν(MoO2) stretching vibration for five of
the six complexes (R = Me, H, I, Br, and OCF3). A similar
observation was reported by Krebs et al. for cis-dioxo-
molybdenum complexes derived from (triphenylmethyl)-
thiosemicarbazones.[18] EWGs, such as CF3, result in a
band at higher frequency, whereas net EDGs, such as Me,
result in a band at lower frequency. The asymmetric
ν(MoO2) stretching frequency obtained for the nitro-substi-
tuted compound is lower than expected and does not fit the
predicted trend. Similarly, the symmetric stretching fre-
quencies of the six complexes do not fit the trend, poten-
tially due to crystal-packing effects in the solid state.
Given that the effects of the electron-donating or elec-
tron-withdrawing aromatic substituents R observed in the
solid state cannot be easily deconvoluted from crystal-pack-
ing effects, additional information was obtained from the
NMR spectra measured in solution ([D6]DMSO).
Figure 4. Plot of the Hammett constant σp vs. ν(MoO2)asym.
stretching frequency.
To determine the electronic substituent effects on the co-
ordinated thiosemicarbazone ligands, the coordination
shift, defined as Δδ = δcoordinated ligand – δfree ligand, was
plotted against the Hammett constant (Figure 5). This
relative approach eliminates the intrinsic effect that a
particular substituent has on the chemical shift of a reso-
nance. Hence, the coordination shift reflects the effect of
the substituents on the metal–ligand interactions.
Figure 5. Plot of the coordination shift of the imine proton reso-
nance [ppm] vs. the Hammett constant σp.
An examination of the Hammett plot shown in Figure 5
indicates that there is a linear relationship between the co-
ordination shift of the imine proton and the Hammett con- position of the aromatic ring is likely to influence the bond-
stant σp. Consequently, the remote substituent in the para- ing between the molybdenum and the ligand through the
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donor strength of the imine nitrogen. EWGs lead to more Table 4. Pseudo-first-order rate constants (kobs) and second-order
rate constants (k2) for the OAT reaction between PPh3 and DMSO
at room temperature ([catalyst] = 2, 3.5, 5, 7.5, 10 mm, [PPh3] =
127.5 mm, solvent: 50% CD2Cl2, 50% [D6]DMSO).
drastic deshielding of the imine proton, whereas the de-
shielding observed with the electron-donating methyl sub-
stituent is less pronounced.
[MoO2(LR)MeOH]
[a]
R =
Me
H
I
Br
OCF3
NO2
Catalytic Oxygen Atom Transfer Activity
kobs 107 [s–1
]
Catalytic oxygen atom transfer (OAT) between tri-
phenylphosphine and dimethyl sulfoxide was investigated at
room temperature by 31P NMR spectroscopy using an ex-
cess of PPh3 over catalyst. The decrease of the PPh3 concen-
tration was monitored over time and the reaction was found
2 mm
3.5 mm
5 mm
5.7(1)
7.2(1)
8.8(1) 9.9(1) 10.0(1) 50(1)
10.3(2) 12.6(2) 17(1)
14.5(3) 15.4(3) 21(1)
20.2(6) 24(1)
25(1)
2.4(1)
–0.17
15.8(4) 17(1)
90(2)
135(3)
200(3)
237(5)
24(2)
0.78
21(1)
33(2)
34(1)
22(1)
30(1)
38(2)
7.5 mm
10 mm
24(1)
38(1)
29(1)
2.8(1)
0
k2 104 [M–1s–1
]
3.3(5) 3.2(5) 3.5(1)
0.18 0.23 0.35
to be pseudo-first-order with respect to [PPh3]. The pseudo- σp
first-order rate constants (kobs) were determined by plotting
[a] Rate based only on the linear part of the graph (up to a reaction
time of 18 h).
ln([PPh3]t/[PPh3]0) vs. time followed by analysis by linear
regression. The results obtained at a catalyst concentration
of 5 mm are shown in Figure 6; the corresponding plots for
2, 3.5, 7.5 and 10 mm catalyst concentrations can be found
in the Supporting Information. The pseudo-first-order rate
constants obtained are listed in Table 4. Whereas the kobs
values obtained for the Me- and H-substituted catalysts are
similar, as are those obtained with I-, Br-, and OCF3-substi-
tuted catalysts, the NO2-substituted catalyst showed signifi-
cantly higher activity. For [MoO2(LNO2)MeOH], the
ln([PPh3]t/[PPh3]0) vs. time graph deviates from linearity at
longer reaction times, when the conversion of PPh3 into
OPPh3 exceeds approximately 50%. This may be caused by
catalyst deactivation, for example through formation of
oxo-bridged MoV dimers (Scheme 1), or a shift in rate-
determining step (see below).
negative charge during the rate-determining step in the
catalytic cycle. This observation is consistent with the
generally accepted OAT mechanism involving nucleophilic
attack of the phosphine lone pair on a vacant π* orbital of
one of the oxo ligands on the cis-MoVIO2 unit, leading to
the formation of the corresponding MoIVO complex and
phosphine oxide via a phosphine oxide-coordinated inter-
mediate.[42,43] Accordingly, if EWGs decrease the electron
density on the MoVIO2 center, the oxo ligand becomes more
susceptible to attack. In addition, if EWGs lower the energy
of the transition state by ameliorating negative charge
build-up, they give rise to faster reaction rates.
Figure 6. Plot of the decrease in ln([PPh3]t/[PPh3]0) over time
Figure 7. Plot of the relative rate constants logkR,obs/kH,obs vs. the
Hammett parameter σp for [cat] = 5 mm.
during the OAT reaction between PPh3 and DMSO ([PPh3]0
=
127.5 mm (25.5 equiv.), [cat] = 5 mm (1 equiv.) in 50% CD2Cl2/50%
[D6]DMSO at room temperature).
Similar observations have previously been made with
The log of the normalized rate constants (kR,obs/kH,obs other ligands, including other tridentate Schiff base ligands,
where kR,obs is the rate obtained with the respective p-sub- such as those obtained by condensation of substituted sali-
stituent R, and kH,obs is the rate obtained with the unsubsti- cylaldehydes with ortho-aminobenzenethiol,[17] substituted
tuted ligand; 5 mm catalyst concentration) plotted against thiophenolate,[44] substituted dithiolenes,[45] and salan-type
the Hammett constant σp is shown in Figure 7. It is evident ligands.[21,41] A particularly pronounced effect of the NO2
that the rate of PPh3 oxidation, as reflected by growth in substituent was also observed by Britovsek and Gibson et
kobs with the electron-withdrawing ability of the remote p- al. with dioxomolybdenum complexes with para-substituted
substituent on the catalyst, is indicative of a build-up of salan-type ligands.[21]
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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To determine the corresponding second-order rate con-
In addition, it was found that EWGs and EDGs have
stants, analogous OAT transfer experiments were carried opposite effects on the rate of the oxygen atom transfer
out by using a range of catalyst concentrations (2, 3.5, 5, from dimethyl sulfoxide to triphenylphosphine. EWGs re-
7.5, and 10 mm). For all six catalysts, the dependence of duce the electron density on the O and N donor atoms of
kobs on catalyst concentration was found to be linear, the ligand and hence the cis-dioxomolybdenum core,
indicating that the OAT reaction is also first order in thereby facilitating a nucleophilic attack by the phosphine
catalyst. Hence, the kinetic data can be interpreted as substrate. Hence, the reactivity of the cis-dioxomolybdenum
second order overall [Equation (1), Experimental Section]. unit in thiosemicarbazone complexes can be fine-tuned
From the slopes of the graphs shown in Figure 8, second- through ligand design, with electron-withdrawing substitu-
order rate constants (k2), which are independent of catalyst ents enhancing catalytic OAT activity.
concentration, were obtained (Table 4). The second-order
rate constants for the Me-, H-, I-, Br-, and OCF3-substi-
tuted catalysts are similar and increase slightly with increas-
ing electron-withdrawing effect of the substituent. Interest-
ingly, the second-order rate constant obtained for the NO2-
Experimental Section
substituted catalyst is significantly higher than expected
Materials and Instrumentation: 2-Hydroxy-5-nitrobenzal-
from the trend seen with the other five catalysts. For dehyde, 2-hydroxy-5-methylbenzaldehyde, 2-hydroxy-5-(tri-
[MoO2(LNO2)MeOH], the faster rate of the oxidative half fluoromethoxy)benzaldehyde, 5-iodosalicylaldehyde, 4-
reaction leads to a color change of the solution from ethyl-3-thiosemicarbazide, triphenylphosphine, sodium
yellow/orange to orange/brown at high conversion, presum- molybdate dihydrate, deuterated dichloromethane and
ably due to the resulting reduced Mo-species accumulating. DMSO were purchased from Aldrich. Salicylaldehyde was
It is conceivable that either the oxidative and reductive half- purchased from Fluka, 2-hydroxy-5-bromobenzaldehyde
reactions begin to compete in terms of kinetics, or the from Alfa Aesar, acetylacetone from BDH Chemicals. Sol-
dissociation of the oxidized product OPPh3 from the vents for syntheses were dried and stored over 3 Å molec-
catalyst becomes rate limiting. Absorption spectroscopic ular sieves. [MoO2(acac)2] was synthesized according to a
studies are underway to investigate these mechanistic reported procedure.[46] Triphenylphosphine was recrys-
1
aspects further.
tallized from hot ethanol prior to use. H and 13C{1H} de-
coupled NMR spectra were recorded with a Jeol ECS400
instrument (1H NMR 400 MHz, 13C NMR 100.6 MHz).
Assignment of resonances was confirmed by COSY,
DEPT135 and HSQC spectra. 31P{1H} decoupled NMR
spectra for kinetic studies were recorded with a Bruker 500
instrument (1H NMR 500 MHz, 31P NMR 202.4 MHz). IR
spectra were recorded in KBr with a Thermo Nicolet Ava-
tar 370 FTIR spectrophotometer in the region of 4000–
400 cm–1. Melting points were measured with a Stuart Sci-
entific SMP3 apparatus. Elemental analyses of compounds
were carried out with an Exeter CE-440 elemental analyzer.
Electron spray ionization mass spectrometry (ESI-MS) and
high-resolution mass spectra were recorded with a Bruker
microTOF electrospray mass spectrometer. LIFDI
measurements were carried out with a Waters GCT Premier
orthogonal time-of-flight mass spectrometer equipped with
a LIFDI probe from Linden GmbH. Crystallographic data
were collected with an Agilent SuperNova diffractometer
with Mo-Kα radiation (λ = 0.71073 Å) by using a EOS
CCD camera. The crystals were cooled with an Oxford In-
struments Cryojet. Face-indexed absorption corrections
were applied by using SCALE3 ABSPACK scaling.
OLEX2[47] was used for overall structure solution, refine-
Figure 8. Linear relationships between kobs and catalyst concentra-
tions used for the determination of the second-order rate constants
k2. The inset shows an expansion of the plots with smaller rate
constants.
Summary and Conclusions
From an analysis of the crystal structures, IR and NMR ment and preparation of computer graphics and publica-
spectroscopic data of six cis-dioxo MoVI complexes with tion data. Within OLEX2, the algorithms used for structure
thiosemicarbazone ligands that are derived from para-sub- solution were direct methods using SHELXS-97 and refine-
stituted salicylaldehydes, it is evident that the substituents ment by full-matrix least-squares used SHELXL-97 within
in the para-position of the phenolate donor significantly OLEX2. All non-hydrogen atoms were refined anisotropi-
affect the MoVI ligand interactions and that linear relation- cally. Hydrogen atoms were placed by using a riding model
ships exist between the Hammett constants and key proper- and included in the refinement at calculated positions, ex-
ties of the complexes.
cept for acidic hydrogen atoms, which were placed by using
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.eurjic.org
FULL PAPER
371.9638; found 371.9637. C10H12N3OIS: calcd. C 34.40, H 3.46,
N 12.03; found C 34.48, H 3.42, N 11.85.
the difference map method. The CCDC numbers given in
Table 2 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
1
H2LOCF3: Yield 48%; m.p. 161–162 °C. H NMR (400 MHz, [D6]-
DMSO, 25 °C): δ = 11.41 (s, 1 H, NH), 10.33 (s, 1 H, OH), 8.60
3
4
(t, JH,H = 6.0 Hz, 1 H, NH), 8.28 (s, 1 H, CH), 7.96 (d, JH,H
=
3
3.2 Hz, 1 H, CHAr), 7.16 (m, 1 H, CHAr), 6.89 (d, JH,H = 9.2 Hz,
1 H, CHAr), 3.55 (quin, JH,H = 6.9 Hz, 2 H, CH2), 1.1 (t, JH,H
3
3
General Procedure for the Synthesis of Thiosemicarbazones: A solu-
tion of 4-ethyl-3-thiosemicarbazide (3.36 mmol) and the appropri-
ate 5-substituted-2-hydroxybenzaldehyde (3.36 mmol) in ethanol
(40 mL) was heated to reflux overnight. After cooling to room tem-
perature, the solvent was partially evaporated and, once the prod-
uct started to precipitate, the mixture was cooled in an ice bath.
The product was collected by filtration, washed with cold ethanol
and dried.
=
7.1 Hz, 3 H, CH3) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C):
δ = 177.1, 155.8, 141.5, 137.5, 124.0, 122.4, 119.4, 117.7, 38.8,
15.2 ppm. MS-ESI: m/z calcd. [M + H+] 308.0675; found 308.0666;
m/z calcd. [M + Na+] 330.0495; found 330.0481. C11H12F3N3O2S
+ 0.4 CH3OH: calcd. C 42.77, H 4.28, N 13.13; found C 42.40, H
3.90, N 13.50.
1
H2LNO2: Yield 95%; m.p. 201–202 °C. H NMR (400 MHz, [D6]-
DMSO, 25 °C): δ = 11.57 (s, 1 H, OH), 11.52 (s, 1 H, NH), 8.81
H2LH: Yield 56%; m.p. 165–166 °C. 1H NMR (400 MHz, [D6]-
DMSO, 25 °C): δ = 11.37 (s, 1 H, NH), 9.91 (s, 1 H, OH), 8.47 (t,
4
3
(d, JH,H = 2.9 Hz, 1 H, CHAr), 8.75 (t, JH,H = 6.0 Hz, NH), 8.37
3
4
(s, 1 H, CH), 8.11 (dd, JH,H = 9.1, JH,H = 2.9 Hz, 1 H, CHAr),
3
3JH,H = 5.88 Hz, 1 H, NH), 8.36 (s, 1 H, CH), 7.94 (d, JH,H
=
7.05 (d, 3JH,H = 9.1 Hz, CHAr), 3.60 (m, 3JH,H = 7.0 Hz, CH2), 1.15
3
7.8 Hz, 1 H, CHAr), 7.21 (t, JH,H = 7.72 Hz, 1 H, CHAr), 6.86 (d,
(t, JH,H = 7.1 Hz, CH2) ppm. 13C NMR (100 MHz, [D6]DMSO,
3
3JH,H = 8.28 Hz, 1 H, CHAr), 6.83 (t, 3JH,H = 7.68 Hz, 1 H, CHAr),
25 °C): δ = 176.7, 161.9, 140.3, 136.4, 126.3, 122.0, 121.5, 116.5,
38.3; 14.6 ppm. MS-ESI: m/z calcd. [M + H+] 269.0703; found
269.0705; m/z calcd. [M + Na+] 291.0522; found 291.0525.
C10H12N4O3S1 + 0.55H2O + 0.45CH3OH: calcd. C, 42.89, H, 5.13,
N, 19.15; found C, 43.11, H, 4.72, N, 18.75.
3.58 (m, 2 H, CH2), 1.14 (t, JH,H = 7.12 Hz, 3 H, CH3) ppm. 13C
3
NMR (100 MHz, [D6]DMSO, 25 °C): δ = 176.5, 156.4, 139.0,
131.0, 126.6, 120.5, 119.2, 116.1, 38.3, 14.7 ppm. MS-ESI: m/z
calcd. [M + H+] 224.0852; found 224.0848; m/z calcd. [M + Na+]
246.0672; found 246.0661. C10H13N3OS (223.29): C 53.79, H 5.87,
N 18.82; found C 53.80, H 5.86, N 18.83.
General Procedure for the Synthesis of [MoO2(L)MeOH]: A solu-
tion of [MoO2(acac)2] (0.2 g, 0.61 mmol) and the appropriate thio-
semicarbazone ligand (0.61 mmol) in methanol (30 mL) was heated
to reflux overnight. After cooling to room temperature, the mixture
was concentrated until the product started to precipitate, then the
flask was cooled in an ice bath. The precipitated product was col-
lected by filtration, washed with cold methanol, and dried under
vacuum.
H2LMe [29]
:
Yield 46%; m.p. 168–169 °C. 1H NMR (400 MHz, [D6]-
DMSO, 25 °C): δ = 11.33 (s, 1 H, NH), 9.65 (s, 1 H, OH), 8.44 (t,
4
3JH,H = 5.9 Hz, 1 H, NH), 8.33 (s, 1 H, CH), 7.71 (d, JH,H
=
3
4
1.8 Hz, 1 H, CHAr), 7.02 (dd, JH,H = 8.3, JH,H = 2.3 Hz, 1 H,
3
3
CHAr), 6.76 (d, JH,H = 8.2 Hz, 1 H, CHAr), 3.59 (m, JH,H
=
3
7.1 Hz, 2 H, CH2), 2.23 (s, 3 H, CH3), 1.14 (t, JH,H = 7.1 Hz, 3
H, CH3) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C): δ = 176.4,
154.3, 139.4, 131.7, 127.8, 126.5, 120.0, 115.9, 38.3, 20.2, 14.7 ppm.
MS-ESI: m/z calcd. [M + H+] 238.1009; found 238.1005; m/z calcd.
[M + Na+] 260.0828; found 260.0818. C11H15N3OS (237.32): calcd.
C 55.67, H 6.37, N 17.71; found C 55.73, H 6.34, N 17.77. The
melting point and NMR resonances are consistent with those pre-
viously reported.[29]
1
[MoO2(LH)MeOH]: Yield 46%; m.p. 120–121 °C (dec.). H NMR
3
(400 MHz, [D6]DMSO, 25 °C): δ = 8.55 (s, 1 H, CH), 7.60 (dd, J
4
3
= 7.76, J = 1.68 Hz, 1 H, CHAr), 7.52 (t, J = 5.12 Hz, 1 H, NH),
3
3
4
7.40 (ddd, J = 9, J = 7.24, J = 1.72 Hz, 1 H, CHAr), 6.98 (ddd,
3J = 7.72, J = 1.12 Hz, 1 H, CHAr), 6.85 (d, J = 8.28 Hz, 1 H,
4
3
3
3
CHAr), 4.11 (q, 1 H, OH), 3.27 (qd, J = 7.20, J = 5.36 Hz, 2 H,
3
CH2), 3.16 (d, 3 H, CH3), 1.12 (t, J = 7.20 Hz, 3 H, CH3) ppm.
13C NMR (100 MHz, [D6]DMSO, 25 °C): δ = 158.6, 151.4, 133.4,
133.2, 121.1, 120.7, 117.9, 101.5, 48.6, 38.8, 14.5 ppm. MS-LIFDI:
m/z calcd. for 98Mo [M·+ – MeOH] 350.9575; found 350.9574. IR
1
H2LBr [34]
:
Yield 39%; m.p. 200–201 °C. H NMR (400 MHz, [D6]-
DMSO, 25 °C): δ = 11.42 (s, 1 H, NH), 10.26 (s, 1 H, OH), 8.65
3
4
(t, JH,H = 5.9 Hz, 1 H, NH), 8.29 (s, 1 H, CH), 8.16 (d, JH,H
=
3
4
(KBr): ν = 3365 (s), 2977, 1598 (s), 1555 (s), 1516 (s), 1476, 1438,
˜
2.6 Hz, 1 H, CHAr), 7.34 (dd, JH,H = 8.7, JH,H = 2.6 Hz, 1 H,
CHAr), 6.82 (d, JH,H = 8.7 Hz, 1 H, CHAr), 3.59 (dq, JH,H
1288, 930 (s, νMo=O), 901 (s, νMo=O), 726 (s) cm–1.
C11H15MoN3O4S: calcd. C 34.65, H 3.97, N 11.02; found C 34.84,
H 3.91, N 10.82.
3
3
=
7.0 Hz, 2 H, CH2), 1.14 (t, JH,H = 7.1 Hz, 3 H, CH3) ppm. 13C
NMR (100 MHz, [D6]DMSO, 25 °C): δ = 176.6, 155.6, 136.9,
133.2, 128.1, 123.0, 118.2, 111.0, 38.3, 14.7 ppm. MS-ESI: m/z
calcd. [M + H+] 301.9957; found 301.9948; m/z calcd. [M + Na+]
323.9777; found 323.9766. C10H12BrN3OS (302.19): calcd. C 39.75,
H 4.00, N 13.91; found C 39.99, H 4.05, N 13.81. The melting
point and NMR resonances are consistent with those previously
reported.[34]
3
[MoO2(LMe)MeOH]:[29] Yield 79%; m.p. 128–129 °C (dec.). 1H
NMR (400 MHz, [D6]DMSO, 25 °C): δ = 8.47 (s, 1 H, CH), 7.49
(t, 1 H, NH), 7.38 (d, 4J = 1.96 Hz, 1 H, CHAr), 7.21 (dd, 3J =
4
3
8.40, J = 2.04 Hz, 1 H, CHAr), 6.75 (d, J = 8.36 Hz, 1 H, CHAr),
3
3
4.11 (q, 1 H, OH), 3.27 (qd, J = 7.20, J = 5.40 Hz, 2 H, CH2),
3
3.17 (d, 3 H, CH3), 2.26 (s, 3 H, CH3), 1.12 (t, J = 7.20 Hz, 3 H,
CH3) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C): δ = 156.8,
151.4, 133.9, 133.1, 129.6, 120.8, 117.6, 48.6, 38.8, 20.0, 14.5 ppm.
MS-LIFDI: m/z calcd for 98Mo [M·+ – MeOH] 364.9731; found
H2LI: Yield 68%; m.p. 221–222 °C. 1H NMR (400 MHz, [D6]-
DMSO, 25 °C): δ = 11.39 (s, 1 H, NH), 10.22 (s, 1 H, OH), 8.61
3
4
(t, JH,H = 5.92 Hz, 1 H, NH), 8.27 (s, 1 H, CH), 8.24 (d, JH,H
=
364.9749. IR (KBr): ν = 3382 (s), 1560 (s), 1506 (s), 1475, 1286,
˜
3
4
2.28 Hz, 1 H, CHAr), 7.49 (dd, JH,H = 8.6, JH,H = 2.28 Hz, 1 H,
1020, 934 (s, νMo=O), 896 (s, νMo=O) cm–1. C12H17MoN3O4S: calcd.
C 36.46, H 4.33, N 10.63; found C 36.57, H 4.32, N 10.39. The IR
spectrum is consistent with that previously reported.[29]
3
3
CHAr), 6.70 (d, JH,H = 8.6 Hz, 1 H, CHAr), 3.59 (m, JH,H
=
7.04 Hz, 2 H, CH2), 1.15 (t, JH,H = 7.08 Hz, 3 H, CH3) ppm. 13C
NMR (100 MHz, [D6]DMSO, 25 °C): δ = 177.0, 156.6, 139.6,
137.6, 134.3, 123.9, 119.1, 82.5, 38.7, 15.2 ppm. MS-ESI: m/z calcd.
3
[MoO2(LBr)MeOH]:[34] Yield 99%; m.p. 109–110 °C (dec.). 1H
[M + H+] 349.9819; found 349.9829; m/z calcd. [M + Na+] NMR (400 MHz, [D6]DMSO, 25 °C): δ = 8.53 (s, 1 H, CH), 7.83
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Date: 01-04-15 15:30:09
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FULL PAPER
(d, 4J = 2.6 Hz, 1 H, CHAr), 7.65 (s, 1 H, NH), 7.51 (dd, 3J = 8.76,
ond-order rate constants k2 were obtained from the slope of the
linear plot of kobs vs. catalyst concentration.
3
4J = 2.6 Hz, 1 H, CHAr), 6.82 (d, J = 8.92 Hz, 1 H, CHAr), 4.11
3
3
(q, 1 H, OH), 3.28 (dq, J = 7.24, J = 5.40 Hz, 2 H, CH2), 3.17
(d, 3 H, CH3), 1.12 (t, 3J = 7.20 Hz, 3 H, CH3) ppm. 13C NMR
(100 MHz, [D6]DMSO, 25 °C): δ = 157.8, 149.9, 135.2, 134.9,
123.3, 120.2, 111.5, 48.6, 38.9, 14.4 ppm. MS-LIFDI: m/z calcd for
rate = k2 [cat][Ph3P]
rate = kobs [Ph3P]
(1)
(2)
(3)
ln([Ph3P]t/[Ph3P]0) = –kobs
t
98Mo, 79Br [M·+ – MeOH] 428.8680; found 428.8651. IR (KBr): ν
˜
= 3293.64 (s), 1592 (s), 1549 (s), 1523 (s), 1469 (s), 1328, 1266, 942
(s, νMo=O), 906 (s, νMo=O), 818 cm–1. C11H14BrMoN3O4S
0.85CH3OH: calcd. C 29.20, H 3.60, N 8.62; found C 29.06, H
3.44, N 8.48. The IR data and NMR resonances are consistent
with those previously reported.[34]
+
Acknowledgments
The authors thank the Engineering and Physical Sciences Research
Council (EPSRC) (EP/J019666/1), COST Action (CM1003; “Bio-
logical oxidation reactions - mechanisms and design of new cata-
lysts”) and the University of York, UK for financial support. Dr.
R. E. Douthwaite is thanked for helpful discussions. Experimental
support provided by K. Heaton (mass spectrometry), Dr. A. C.
Whitwood and N. Pridmore (X-ray crystallography) is gratefully
acknowledged.
[MoO2(LI)MeOH]: Yield 42%; m.p. 116–117 °C (dec.). 1H NMR
4
(400 MHz, [D6]DMSO, 25 °C): δ = 8.50 (s, 1 H, CH), 7.96 (d, J
3
4
= 2.28 Hz, 1 H, CHAr), 7.64 (s, 1 H, NH), 7.64 (dd, J = 8.64, J
= 2.28 Hz, 1 H, CHAr), 6.68 (d, 3J = 8.6 Hz, 1 H, CHAr), 3.27 (dq,
3
3J1 = 7.20, J2 = 5.40 Hz, 2 H, CH2), 3.16 (s, 3 H, CH3), 1.12 (t,
3J = 7.20 Hz, 3 H, CH3) ppm. 13C NMR (100 MHz, [D6]DMSO,
25 °C): δ = 158.3, 149.8, 140.9, 140.8, 123.9, 120.5, 82.6, 48.6, 38.9,
14.4 ppm. MS-LIFDI: m/z calcd. for 98Mo [M·+ – MeOH] 476.85;
[1] R. Hille, Chem. Rev. 1996, 96, 2757–2816.
[2] R. H. Holm, Chem. Rev. 1987, 87, 1401–1449.
[3] J. H. Enemark, J. J. A. Cooney, J.-J. Wang, R. H. Holm, Chem.
Rev. 2004, 104, 1175–1200.
[4] H. Sugimoto, H. Tsukube, Chem. Soc. Rev. 2008, 37, 2609–
2619.
found 476.85. IR (KBr): ν = 3297 (s), 1589 (s), 1543 (s), 1519, 1467,
˜
937 (s, νMo=O), 906 (s, νMo=O) cm–1. C11H14N3O4ISMo
+
0.5CH3OH: calcd. C 26.40, H 3.08, N 8.03; found C 26.13, H 3.14,
N 7.52.
[5] C. Schulzke, Eur. J. Inorg. Chem. 2011, 1189–1199.
[6] A. Majumdar, S. Sarkar, Coord. Chem. Rev. 2011, 255, 1039–
1054.
[7] J.-J. Wang, O. P. Kryatova, E. V. Rybak-Akimova, R. H. Holm,
Inorg. Chem. 2004, 43, 8092–8101.
[MoO2(LOCF3)MeOH]: Yield 35%; m.p. 108–109 °C (dec.). 1H
NMR (400 MHz, [D6]DMSO, 25 °C): δ = 7.81 (s, 1 H, CH), 6.92
4
3
4
(d, J = 2.7 Hz, 1 H, CHAr), 6.60 (dd, J = 8.70, J = 2.70 Hz, 1
H, CHAr), 6.17 (d, 3J = 8.70 Hz, 1 H, CHAr), 3.33 (br. s, NH), 1.73
(m, CH2), 0.34 (t, 3J = 7.10 Hz, 3 H, CH3) ppm. 13C NMR
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˜
= 3390, 1557, 1518, 1257 (s), 1214 (s), 1155, 937 (s, νMo=O), 908.5
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3
4
(d, J = 2.88 Hz, 1 H, CHAr), 8.21 (dd, J = 9.08, J = 2.92 Hz, 1
3
H, CHAr), 7.80 (s, 1 H, NH), 7.03 (d, J = 9.12 Hz, 1 H, CHAr),
4.11 (q, 1 H, OH), 3.30 (m, 2 H, CH2), 3.16 (d, 3 H, CH3), 1.14 (t,
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˜
1339 (s), 1297 (s), 941 (s, νMo=O), 899 (s, νMo=O) cm–1.
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Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
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Received: January 21, 2015
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I50059
/KAP1
Date: 01-04-15 15:30:09
Pages: 11
www.eurjic.org
FULL PAPER
Oxygen Atom Transfer
A. Ducrot, B. Scattergood, B. Coulson,
R. N. Perutz,
A.-K. Duhme-Klair* ........................ 1–11
Thiosemicarbazone ligands (H2LR) derived
from para-substituted salicylaldehydes al-
low the electronic properties and catalytic
activities of cis-dioxo MoVI complexes of
composition [MoO2(LR)MeOH] to be fine-
tuned by electron-donating or electron-
withdrawing groups R.
Electronic Fine-Tuning of Oxygen Atom
Transfer Reactivity of cis-Dioxomolybden-
um(VI) Complexes with Thiosemicarb-
azone Ligands
Keywords: Enzyme models / Metalloen-
zymes / Molybdenum / Oxygen atom trans-
fer / Ligand effects
Eur. J. Inorg. Chem. 0000, 0–0
11
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim