Molybdenum(VI) dioxo and oxo-imido complexes of fluorinated β-ketiminato ligands and their use in OAT reactions

Article abstract of DOI:10.1021/ic201681u

Substitution of a methyl by a trifluoromethyl moiety in well-known β-ketimines afforded the ligands (Ar)NC(Me)CH₂CO(CF₃) (HL^H, Ar = C₆H₅; HL^Me, A r= 2,6-Me₂C₆H₃

Full text of DOI:10.1021/ic201681u

Article
pubs.acs.org/IC
Molybdenum(VI) Dioxo and Oxo-Imido Complexes of Fluorinated
β-Ketiminato Ligands and Their Use in OAT Reactions
Manuel Volpe and Nadia C. Mosch-Zanetti*
̈
Institut fur Chemie, Bereich Anorganische Chemie, Karl-Franzens-Universitat, Graz Stremayrgasse 16, A-8010 Graz, Austria
̈
̈
S
* Supporting Information
ABSTRACT: Substitution of a methyl by a trifluoromethyl
moiety in well-known β-ketimines afforded the ligands
(Ar)NC(Me)CH2CO(CF3) (HLH, Ar = C6H5; HLMe, A r=
2,6-Me2C6H3; HLiPr, Ar = 2,6-iPr2C6H3). Subsequent complex-
ation to the [MoO2]2+ core leads to the formation of novel
complexes of general formula [MoO2(LR)2] (R = H, 1; R =
Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido
complex [MoO(NtBu)(LMe)2] (4) has also been synthesized.
Complexes 1−4 were investigated in oxygen atom transfer
(OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced MoIV complexes
[MoO(PMe3)(LR)2] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo(NtBu)(PMe3)(LMe)2] (8), were isolated. All complexes have
been characterized by NMR spectroscopy, and 1−4 also by cyclic voltammetry. A positive shift of the MoVI−MoV reduction wave
upon fluorination was observed. Furthermore, molecular structures of complexes 2, 4, 5, and 8 have been determined via single
crystal X-ray diffraction analysis. Complex 8 represents a rare example of a MoIV phosphino-imido complex. Kinetic
measurements by UV−vis spectroscopy of the OAT reactions from complexes 1−4 to PMe3 showed them to be more efficient
than previously reported nonfluorinated ones, with ligand L′ = (Ar)NC(Me)CH2CO(CH3) [MoO2(L′)2] (9) and [MoO(NtBu)-
(L′)2] (10), respectively. Thermodynamic activation parameters ΔH‡ and ΔS‡ of the OAT reactions for complexes 2 and 4 have
been determined. The activation enthalpy for the reaction employing 2 is significantly smaller (12.3 kJ/mol) compared to the
reaction with the nonfluorinated complex 9 (60.8 kJ/mol). The change of the entropic term ΔS‡ is small. The reaction of the
oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.
For a better understanding of mechanistic aspects of OAT
reactions in general, we developed Schiff-base Mo(VI)
complexes [MoO(X)(L)2] (X = O, NtBu; L = bidentate Schiff
base ligands) and investigated their reactivity in OAT.13 The
imido functionality was introduced in an attempt to simulate
the presence of a second doubly bonded nonoxygen atom as
found in xanthine oxidase. Although an imido group is not a
close substitute for a sulfide group, its introduction allows the
differentiation of the doubly bonded moieties at molybdenum.
In addition, oxo-sulfido species are significantly more
challenging to prepare, no convenient starting materials
are available, and they usually display a high tendency for
dimerization under formation of a disulfide bridge
[(L)2OMoS−SMoO(L)2]. Rare examples showed that high
steric demand works against it allowing the isolation of
[(Tp)MOS(OAr)] (Tp = tris(pyrazolyl)borate).14,15 Although
the imido ligand is not found in the active sites of the enzyme
families, such compounds represent examples of monooxo
molybdenum(VI) compounds related to xanthine oxidase and
DMSO reductase, which are limited in the literature.16−19 We
found them to be stable toward dimerization, presumably due
to the steric demand at the tert-butyl imido nitrogen, and to be
INTRODUCTION
■
Oxygen atom transfer (OAT) belongs to the fundamental
reactions that have both industrial and biological relevance: in
industry, it is used to afford epoxides from readily available
olefins,1 while living organisms utilize OAT in metabolic
processes such as the degradation of xanthine to uric acid.2−4
Both processes are mediated by molybdenum. Synthetic
catalysts are often molybdenum dioxo complexes due the
readily availability of the [MoO2Cl2] starting material.5 In
biological systems, OAT is catalyzed by various molybdoen-
zymes which modulate the reactivity on the basis of the
structure of their active centers: according to this, these
enzymes can be classified into three different families, xanthine
oxidase, sulfite oxidase, and DMSO reductase families. The
structures of several of these enzymes have been characterized
by X-ray crystallography.3,6−10 In all cases the mononuclear
molybdenum center is coordinated by at least one terminal oxo
group and by one or two pterin based ligands. Furthermore,
depending on the family a second oxo group (sulfite oxidase), a
sulfido group (xanthine oxidase), or a nondoubly bonded group
(DMSO reductase) is found in the active site. Due to this
structural diversity, function−structure relationships are com-
plex and as yet not fully understood. For this reason, much
biomimetic chemistry has been devoted to this class of
enzymes.5,11,12
Received: August 3, 2011
Published: January 23, 2012
© 2012 American Chemical Society
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active in the OAT reaction to trimethyl phosphine.
Comparison of the oxo-imido to corresponding dioxo
compounds allowed us to investigate the influence of the
second doubly bonded ligand at molybdenum. The oxygen
atom transfer is influenced by the imido and the β-ketiminato
ligand with a fine interplay of steric and electronic effects.13
However, we observed a small influence of various aromatic
substitutents at the nitrogen donor of the ketiminato ligand.
Variation of the substituent at the imido nitrogen would be
desirable, but other than with the NtBu group, molybdenum
oxo-imido compounds are synthetically challenging.20−24 Only
recently a synthetic procedure for the NtBu imido precursor has
been reported from which some coordination chemistry was
developed.25−30 Therefore, we decided to investigate ligand
backbone substitution and introduced a CF3 functionality
(Figure 1) which removes electronic density at the metal center
Figure 2. Numbering scheme of the here described complexes 1−8
and the previously published 9 and 10.
time. In principle, there are two possible regioisomers (Figure 3).
Following our procedure, the reaction affords purely the CF3CO
Figure 3. Possible isomers with respect to the position of CF3.
Figure 1. General scheme of ligands presented in this work.
isomer, whereas when more polar solvents are used, both
isomers CF3CO and CF3CN are observed, especially in the
case of HLH.41
Preparation of Molybdenum(VI) Complexes. The
complexes [MoO2(LR)2] (R = H, 1; R = Me, 2; R = iPr, 3) can
be synthesized from [MoO2Cl2] by addition of the respective ligand
and NEt3 in toluene. All compounds were obtained as yellow to
orange crystalline solids by recrystallization from pentane/toluene.
They are well soluble in polar solvents such as THF or MeCN as
well as in toluene, but insoluble in aliphatic solvents.
and should make the electron donation capability of the imido
group more pronounced.31
The aniline derived trifluoromethyl substituted ligand HLH
(phenylimino-1,1,1-trifluoropentan-2-one) employed in this
study has been known for some time,32 whereas the methyl
and iso-propyl substituted versions have just very recently been
published. They were mainly used as ancillary ligands in group 4
and 5 complexes suitable for ethylene polymerizations.33−36
t
Furthermore, norbornene and Bu-metacrylate have been coupled
in the presence of a NiII derivative of such Schiff bases,37,38 and
the literature reports a single example of a half-sandwich CrIII com-
plex, also an olefin polymerization catalyst.39 Incorporation of this
fluorinated β-ketiminato motive in a more elaborated, macrocyclic
framework has also been reported for VO2+ and CuII complexes.40
In all of the reported cases, catalyst performance has been im-
proved. No molybdenum complexes containing the fluorinated
HLR ligands (R = H, Me, iPr) were known before the present work.
This work describes the preparation of dioxo [MoO2(LR)2]
(1−3) and the oxo-imido Mo(VI) [MoO(NtBu)(LMe)2] (4)
complexes with 2,6-disubstituted phenylimino-1,1,1-trifluoro-
pentan-2-ones HLR (R = H, Me, iPr), the isolation of reduced
compounds [MoO(PMe3)(LR)2] (5−7) and [Mo(NtBu)-
(PMe3)(LMe)2] (8), as well as a comparison to the previously
published nonfluorinated analogues [MoO2(L′)2] (9)42 and
[MoO(NtBu)(L′)2] (10) (Figure 2).13 Kinetic investigations
provide an interesting insight into the influence of electronic
factors on the rate of the OAT reaction.
+2NEt3
R
R
[MoO Cl ] + 2HL ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ [MoO (L ) ]
2
2
2
2
−2NEt3HCl
R = H, 1
= Me, 2
i
= Pr, 3
(1)
NMR spectroscopy displayed for complexes 1−3 one set of
ligand resonances indicating single isomers in solution (i.e., no
cis−trans N,N or N,O distribution is observed).42 Particularly
1
indicative in this regard are the H NMR spectra in the region
at around 5 ppm, where only a single signal, assignable to the γ-
H in the ligand backbone (Δδ = 0.2−0.3 ppm downfield with
respect to the free ligand), can be seen. Only one singlet in this
region points to a symmetric coordination mode around the
metal center. Further confirmation is given by the 19F NMR
spectra, which show one resonance for the two equivalent CF3
groups quite distinct from the free ligand (Δδ ≈ 2 ppm).
In order to vary not only the sterics around the Mo center,
but also the electronics, one representative oxo-imido complex
[MoO(NtBu)(LMe)2] (4) was prepared. Its synthesis was carried out
from the chloro precursor [MoO(NtBu)Cl2(dme)] and HLMe afford-
ing the product as a single isomer (eq 2). The compound is obtained
in moderate yield as a yellow powder, is extremely sensitive to
hydrolysis, and is readily soluble in most common organic solvents.
Quite obviously, in this case the equivalency of the γ-H and
CF3 resonances is lost, the former appearing as two signals at
5.63 and 5.53 ppm in the 1H NMR spectrum and the latter as a
RESULTS AND DISCUSSION
■
Ligand Synthesis. Ligands (Ar)NC(Me)CH2CO(CF3)
(HLH, Ar = C6H5; HLMe, Ar = 2,6-Me2C6H3; HLiPr, Ar =
2,6-iPr2C6H3) employed in this study have been synthesized by
a modified literature procedure by Schiff base condensation of
1,1,1-trifluoropentane-2,4-dione with the respective ani-
lines.32−36 We have found that azeotropic removal of water is
not necessary and the syntheses can be carried out in an one-
pot reaction in benzene with molecular sieves. In this way,
much higher yields up to 70% can be achieved in 3 h reaction
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pair of singlets at −73.24 and −72.36 ppm in the 19F NMR
ligand is also evident in the 31P NMR spectra of the complexes,
where a single signal, intermediate in position between those of
PMe3 and OPMe3 (δ = −1.28 ppm (5), δ = −7.10 ppm (6), δ =
−1.24 ppm (7)), can be detected. Mass spectrometry, even when
very mild ionization techniques are used, fails to provide the
correct molecular peak, the highest mass corresponding instead to
the [MoO(L)2] fragment.
spectrum, respectively.
[MoO(NtBu)Cl2(dme)] + 2HLMe
+NEt
3
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [MoO(NtBu)(LMe)2]
−2NEt HCl
3
4
(2)
−dme
The OAT reaction between the oxo-imido derivative 4 and
PMe3 leads to reduced complex 8 as shown in eq 4.
During an attempted crystallization of 1, a single crystal of the
mixed oxo anilido derivative [MoO(N(2,6-Me2C6H3))(LMe)2]
could be picked out of the vessel (Figure S1 and Table S1 in
Supporting Information). The formation of this quite unusual
derivative is probably due to adventitious traces of water which
hydrolyzed the Schiff base ligand, decomposing it into its aniline
component, which then proceeded to react with a MoO
moiety. Although a rational synthesis of [MoO(NR)]2+
compounds with various R groups would be highly desirable,20−22,24
several attempts to synthesize this compound in a rational way
(i.e., reaction with 2,6-dimethylaniline, a respective isocyanate,43,44
or sulfinylamine)45 have not been successful.
A solution of the yellow complex 4 turns very rapidly to
purple upon addition of 2 equiv of PMe3. After workup and
removal of the phosphine oxide by sublimation, compound
[Mo(NtBu)(PMe3)(LMe)2] (8) can be recovered as a violet
powder which is moderately soluble in aromatic solvents and THF
and completely insoluble in aliphatic ones. The 1H NMR and 19F
NMR spectra of 8, again, point to the existence of a single
isomer in solution. The 31P NMR spectrum shows a single signal
at 2.82 ppm for the coordinated phosphine, whose protons can be
Oxygen Atom Transfer Reactivity. Complexes 1−3
undergo OAT reaction to PMe3 affording the corresponding
Mo(IV) monooxo derivatives 5−7 according to eq 3, with
elimination of 1 equiv of OPMe3.
1
observed as a doublet at 0.51 ppm in the H NMR spectrum.
Experiments to investigate the kinetics of OAT reactions of
1−4 by UV−vis spectroscopy were investigated under pseudo-
first-order conditions in PMe3. All reactions were performed in
toluene at 20 °C with a defined excess of PMe3 (50−150 equiv
of phosphine). The reaction progress was monitored by the
appearance of a peak in the visible region of the electronic
spectrum (1−3 λmax = 480 nm, 4 λmax = 517 nm). These
absorptions are identical to those found in the isolated
compounds 5−8. As a representative example, UV−vis data is
shown for the reduction of 2 at different PMe3 concentrations
allowing the determination of the second-order rate constant.
For UV−vis data on complex 4, see Supporting Information.
The individual rate constants kobs were obtained by fitting the
data to a first-order rate law. Clean transformation from the
MoVI to MoIV species was confirmed throughout the kinetic
experiments by the appearance of an isosbestic point. Second-
order rate constants k2 were deducted from the linear plot at
the various PMe3 concentrations (Figure 4).
The reaction color changes quickly from orange to green upon
addition of the phosphine. After removal of the solvent and
subsequent removal of the OPMe3 byproduct by sublimation
(0.05 mmHg, 70 °C) deep green powders were obtained. They
1
are very soluble in aromatic and polar solvents. The H NMR
spectra of derivatives 5−7 in benzene-d6 show two sets of
resonances for the coordinated ligands as expected for the change
of chemical environment upon PMe3 coordination for the two
ketiminato ligands. Furthermore, the spectra exhibit a high-field
doublet (δ ≈ 0.5 ppm), which integrates for 9 protons, consistent
with a coordinated PMe3 ligand. Coordination of the phosphorus
Figure 4. Left: UV−vis spectroscopy at 480 nm of the reaction of 2 with 150 equiv of PMe3, superimposed with a nonlinear fit of the data. In the
inset, plot of the actual spectra (Δt = 10 s). Right: determination of the second-order rate constant.
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The results for the determination of the second-order rate
constants of complexes 1−4 are summarized in Table 1. In the
sinks for the complex, thus leaving the oxo atoms more
susceptible to nucleophilic attacks.
Experiments in which a CF3 moiety was inserted in the complex
periphery to examine the influence of the electron withdrawing
effect have also been recently published by Basu et al. on a
tris(pyrazolyl)borate Mo phenolate complex.46 In this case a 2-fold
increase in the reaction rate was observed in going from the H to
CF3 substituted ligand, in contrast to our 92-fold increase, consistent
with the shorter distance of the CF3 group from the metal oxo core.
Furthermore, we determined the thermodynamic activation
parameters ΔH‡ and ΔS‡ for complex 2 and compared them to
those of the nonfluorinated 9 by running kinetic experiments
at different temperatures (k2 = 3.79(5) × 10−1 at 293.15 K;
k2 = 4.74(6) × 10−1 at 303.15 K and k2 = 5.62(1) × 10−1 at
313.15 K). In the present case, the activation enthalpy ΔH‡ amounts
to 12.3(6) kJ/mol, a significant decrease when compared to the
value for corresponding nonfluorinated derivative (60.8 kJ/mol
for 9).42 The activation enthropy ΔS‡, on the other hand, lies still in
the same negative range (−208 vs −112 J/mol) as expected for an
associative process.
A summary of all reaction rates is given in Table 2. The 92-fold
increase in reaction rate between 2 and 9 versus the 15-fold
increase in the oxo-imido complexes (4 to 10) is interesting. As the
change in sterics going from the CF3 to the CH3 derivatives is
identical in both systems (dioxo and oxo-imido), the significantly
smaller increase of the rate ratio in the case of the oxo-imido
complexes can most likely be attributed to the electron-donating
capacity of the imido group. The steric demand of the imido group
decreases the overall rate of OAT. However, in the absence of an
electronic effect of the imido group we would also expect an
increase of the rate ratio of approximately 100 in the oxo-imido
compounds going from the CH3 to the CF3 derivative. The fact
that an increase of only 15 is observed can thus be attributed to a
significant electron-donation. Similarly, this is evidenced by
comparison of the two dioxo/oxo-imido couples in the CF3 and
CH3 case (Table 2). The difference in increase of the ratio in the
CF3 derivatives is with 14 much higher than the 2-fold increase in
the CH3 case. Electron donation of the imido group in the electron
depleted system decreases the rate much more profoundly.
Electrochemical Investigations. To further probe the
influence of the backbone fluorination, cyclic voltammetry
experiments were performed on all of the three derivatives, and
the results are displayed in Figure 5.
Table 1. Second-Order Rate Constants k2 for the OAT
Reaction for 1−4
−1
complex
k2 (M−1 s )
[MoO2(LH)2] (1)
9(1)
[MoO2(LMe)2] (2)
[MoO2(LiPr)2] (3)
[MoO(NtBu)(LMe)2] (4)
3.79(5) × 10−1
2.56(3) × 10−3
2.76(1) × 10−2
case of 1, the reaction is completed in a few seconds (in this
case a full spectrum per measurement could not be acquired
and the reaction was monitored at a single wavelength).
The reaction rate in going from the unsubstituted derivative
i
1 to the Pr one (3) decreases which can be explained with an
increase in the steric protection around the oxo moieties, as
already experienced in the series of nonfluorinated derivatives.
A general increase in the OAT reaction rate is observed
compared to the nonfluorinated corresponding derivatives: for
complex 2, the increase is 92-fold (4.1(2) × 10−3 M−1 s−1 for 9,
Table 2).42 The second-order rate constant which has been
Table 2. Second-Order Rate Constants k2 for the Fluorinated
and Nonfluorinated Derivatives (M−1 s−1) Showing the Ratio
of Rate Increase
CH3
derivative
ratio k2(CF3)/
k2(CH3)
CF3 derivative
[MoO2(LMe)2]: 2 vs 9
[MoO(NtBu)(LMe)2]: 4 vs 10 2.76(1) × 10−2 1.83(1) × 10−3
ratio k2(dioxo)/k2(oxo-imido) 14
3.79(5) × 10−1 4.1(2) × 10−3
92
15
2
found for derivative 4, 2.76(1) × 10−2 M−1 s−1, is 15 times
bigger than the corresponding value for the methylated
derivative (1.83(1) × 10−3 M−1 s−1, Table 2).13 The rate
increase of the oxo-imido complex 4 versus 10 is not as evident
as in the dioxo case. The importance of the CF3 contribution
can also be observed in the fact that, whereas 3 reacts with
PMe3, albeit slowly, no reaction at all is observed when the CF3
group is substituted by a CH3.42 This supports the mechanistic
idea of a nucleophilic attack of the incoming phosphine onto
the oxo atom of the complex: the CF3 groups act as electron
Figure 5. Left: comparison of cyclic voltammograms of [MoO2(LMe)2] (2) (solid line) and nonfluorinated analogue 9 (dotted line). Right:
comparison of cyclic voltammograms of the dioxo derivatives 1−3. Conditions: MeCN, [NBu4][PF6] (0.2 M), 100 mV s−1, vs Fc/Fc+, only the
molybdenum couple is shown. Full voltammogram is shown in Figure S3.
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Upon fluorination of the ligand backbone, all of the dioxo
complexes show a single reversible wave assignable to the
Mo(VI)/Mo(V) couple, sensibly shifted to less negative
potentials with respect to the methyl substituted analogue
(e.g., −1.179 for 2 vs −1.590 V for 9) supporting once more
the notion of facilitated nucleophilic attack. On the other hand,
the degree of alkylation of the arene rings does not influence
the reduction potential at all: the three derivatives exhibit
virtually the same voltammetric behavior. No +I inductive effect
contribution seems to be operating in these systems. This renders
the substitution by a CF3 group described in this work even more
important for the investigation of the electronic effects.
Complex 2 crystallized from a cold (−30 °C) saturated
MeCN solution in the form of small orange blocks, while
crystals of 4 could be obtained by vapor diffusion of pentane in
a THF solution of the complex as small yellow blocks. Selected
bond distances and angles are reported in Table 3 and
crystallographic data in Table 5.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[MoO2(LMe)2] (2) and [MoO(NtBu)(LMe)2] (4)
[MoO2(LMe)2]
[MoO(NtBu)(LMe)2]
(2)
(4)
Mo1−O1
Mo1−O2
1.7150(12)
1.7050(11)
1.7131(12)
Such a behavior is also observed in the cyclic voltammetry
trace of 4 in MeCN (Figure 6): the oxo-imido derivative shows
Mo1−N3
Mo1−N1
Mo1−N2
Mo1−O3
1.7411(15)
2.1798(13)
2.1965(13)
2.1458(12)
2.1646(13)
175.92(13)
104.31(7)
2.1578(12)
2.1792(13)
2.1374(11)
2.1028(11)
Mo1−O4
Mo1−N3−C27
O1−Mo1−E (E = O2, N3)
O2−Mo1−O3
O2−Mo1−N3
103.88(6)
166.47(5)
166.44(3)
Both compounds display the same trans-N,N arrangement of
t
the bidentate β-ketiminato ligands, with the oxo or the Bu-
imido groups completing the octahedral coordination sphere
around the molybdenum atom. In the case of 4, the NtBu group
coordinates essentially linearly to the metal atom (the Mo1−
N3−C27 angle being 175.9(1)o), as reported for several other
MoVI tBu-imido complexes of similar structure. Quite
surprisingly, neither the MoO or the MoN bond distances
in both complexes seem to be affected by the electron-
withdrawing properties of the introduced CF3 group (i.e., for
the Mo=NtBu in the nonfluorinated analogue of 4,13 the
reported value is 1.734(2) Å, while in a salpen analogue26 it is
at 1.739(1) Å). No significant elongation could be similarly
found for the C−O moiety of the ligand, more closely bound to
the electronegative group.
Figure 6. Comparison of cyclic voltammetram of [MoO(NtBu)(LMe)2]
(4) and the nonfluorinated analogue 10. Conditions: MeCN, [NBu4][PF6]
−1
(0.2 M), 100 mV s , vs Fc/Fc+. Only the molybdenum couple is shown.
a quasireversible redox wave at −1.83 V, similar in shape to the
one exhibited by the corresponding nonfluorinated compound 10.
Molecular Structures of the Compounds. Complex 2, 4, 5,
and 8 were subjected to a single crystal X-ray diffraction study
(Figure 7).
Complex 5 crystallized from a saturated toluene solution
at −30 °C as deep green blocks (which were found to contain
a molecule of toluene per molecule of complex) in the
Figure 7. ORTEP drawings of the crystal structures of [MoO2(LMe)2] (2) (right) and [MoO(NtBu)(LMe)2] (4) (left). Thermal ellipsoids are drawn
at 30% probability level, and hydrogen atoms are omitted for clarity.
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monoclinic space group P21/c. Selected bond lengths and
angles are reported in Table 4 and crystallographic data in
separation, at 2.5051(4) Å, is a bit longer than that reported for
[MoO(acac)2(PMe3)] (2.468(2) Å).48 Comparable values can
be found throughout the literature.49 The two N atoms of the
β-ketiminato ligands are trans to each other, forming in this way
one axis of the coordination octahedron, but the two bond
lengths are not equivalent, measuring at 2.2012(9) and
2.1500(10) Å, respectively. At the same time the reverse
behavior is observed for the Mo−O bond distances, where
Mo1−O3 (2.1067(9) Å) is significantly shorter than the Mo1−
O2 one (2.1272(9) A), which is trans to the oxo-ligand, as it is
observed in the [MoO(acac)2(PMe3)] structure.48
Single crystals suitable for X-ray diffraction analysis of
complex 8 were obtained from a saturated benzene-d6 solution
(a molecular view is given in Figure 8), and its structure in the
solid state determined by X-ray diffraction. Table 4 contains
selected bond lengths and angles for this derivative. Complex 8
crystallizes in the noncentrosymmetric monoclinic space group
P21 as a single isomer, but the similar steric properties of the
NtBu and PMe3 ligands (cone angles 126° and 131° as
calculated from the X-ray data50) induce the presence of a
crystallographic disorder in which the two groups exchange
with each other in a nearly 1:1 ratio. A search in the CCDC
database revealed that not many MoIV imido phosphine adducts
are present in the literature, most of those featuring bulkier
and more protecting NAr groups. Moreover, the present
derivative is the first reported example of a MoIV imido
phosphine deriving from an OAT reaction. Scarcity of
derivatives notwithstanding, two examples can be used for
comparison, namely [Mo(NtBu)(PMe3)(tBu3SiO)2]51and
[(η5-MeCp)Mo(NtBu)(PMe3)]+.52 As in the oxo analogue, the
Mo in complex 8 is octahedrally coordinated by two bidentate
ketiminato ligands in a trans N,N configuration, leaving two
Table 4. Selected Bond Distances (Å) and Angles (deg) for
[MoO(PMe3)(LH)2] (5) and [Mo(NtBu)(PMe3)(LMe)2] (8)
[Mo(NtBu)(PMe3)(LMe)2]
[MoO(PMe3) (LH)2] (5)
(8)
Mo1−O1
1.6833(9)
Mo1−N3
1.703(3)
Mo1−P1
Mo1−N1
Mo1−N2
Mo1−O2
2.5051(4)
2.1500(10)
2.2012(9)
2.1272(9)
2.1067(9)
2.5408(10)
2.2363(10)
2.2403(11)
Mo1−O3
Mo1−O1
2.1236(9)
Mo1−O2
2.1224(12)
O1−Mo1−O2
O1−Mo1−N3
O−Mo1−P1
167.76(4)
163.99(2)
173.32(12)
166.49(3)
Table 5. In the structure, the Mo atom sits in a (distorted)
octahedral coordination environment constituted by a terminal
oxo atom, the two N,O bidentate β-ketiminato ligands and a
phosphorus atom of a PMe3 molecule, similarly to the
arrangement found in the nonfluorinated complex [MoO-
(PMe3)L2].42 Unfortunately, for this previously reported
nonfluorinated derivative, the quality of the X-ray diffraction
data was not good enough to allow for a meaningful
comparison.
The Mo1−O1 bond distance (1.6833(9) Å) is in the normal
range for Mo(IV) monooxo complexes,47 while the Mo1−P1
Table 5. Crystallographic Data and Structure Refinement Details for Complexes 2, 4, 5, and 8
2
4
5•C7H8
8
empirical formula
Mr, g/mol
C26H26F6MoN2O4
640.43
C30H35F6MoN3O3
695.55
C32H35F6MoN2O3P
736.53
C33H44F6MoN3O3P
755.62
cryst description
cryst syst
shard, orange
monoclinic
C2/c
block, yellow
monoclinic
P21/n
block, green
monoclinic
P21/c
block, purple
monoclinic
P21
space group
a, Å
31.6359(19))
12.8657(8)
13.9803(8)
90.00
9.7563(5)
25.5215(13)
13.4956(7)
90.00
11.9011(8)
20.1516(13)
13.5855(10)
90.00
11.6011(6)
12.9908(7)
11.6276(6)
90.00
b, Å
c, Å
α, deg
β, deg
109.019(2)
90.00
110.223(2)
90.00
95.499(2)
90.00
101.049(2)
90.00
γ, deg
V, Å3
5379.6(6)
8
3153.2(3)
4
3243.2(4)
4
1719.88(16)
2
Z
T, K
100(2)
100(2)
100(2)
100(2)
Dc, g/cm3
1.581
1.465
1.508
1.459
μ(Mo Kα), mm−1
0.564
0.486
0.524
0.494
F(000)
2592
1424
1504
780
reflns collected
unique reflns
reflns with I ≥ 2σ(I)
R(int), R(σ)
no. params/restraints
final R1, wR2 (I ≥ 2σ)
R indices (all data)
GOF on F2
larg diff peak and hole, e/Å3
CCDC deposition numbers
82 813
123 069
96 367
48 378
7754
10253
15579
8845
6911
8501
13189
8620
0.0317, 0.0170
358/0
0.0331, 0.0381
425/0
0.0279, 0.0224
440/3
0.0287, 0.0196
510/18
0.0251, 0.0637
0.0306, 0.0685
1.085
0.0282, 0.0635
0.0428, 0.0795
1.178
0.0324, 0.0722
0.0436, 0.0805
1.076
0.0167, 0.0411
0.0179, 0.0417
1.032
0.562, −0.458
837153
0.606, −0.685
837155
1.125, −0.776
837154
0.502, −0.370
837156
1445
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Inorganic Chemistry
Article
Figure 8. Thermal ellipsoid plot (30% probability level) of [MoO(PMe3)(LH)2] (5) (left) and [Mo(NtBu)(PMe3)(LMe)2] (8) (right). Hydrogen
atoms and one molecule of cocrystallized toluene (for 5) not shown for clarity.
residual sites for the NtBu and PMe3 groups arranged in a
EXPERIMENTAL SECTION
■
mutually cis disposition. The imido group is once more almost
General. All manipulations were carried out under dry argon using
standard Schlenk line or glovebox techniques. All solvents were dried
by a solvent purification system from Innovative Technology Inc. and
linear, although less so than in the MoVI analogue 4 (Mo−N−C
angle 169.5(4)o for 8 vs 175.9(1) for 4), as in the two reported
flushed with argon prior to use. [Mo(NtBu)2Cl2(dme)] was prepared
according to literature procedures.53 All other chemicals mentioned
were used as purchased from commercial sources. Samples for mass
spectrometry were measured on an Agilent Technologies 5795C inert
XL MSD spectrometer and all NMR spectra on a Bruker Avance 300
MHz spectrometer. Spectra were obtained at 25 °C unless otherwise
noted. UV−vis spectra were recorded on a Varian Cary 50
spectrophotometer which was connected to a Hellma fiber optics
all-quartz immersion probe.
UV−Vis Kinetic Experiments. In a typical experiment a stock
solution was prepared, with 40 mg of the required complex dissolved
in 20 mL of toluene (passed over alox). A 5-mL aliquot was then
transferred in a 3-necked Schlenk flask and kept at the required
temperature by means of a thermostat. Under a stream of argon gas
the fiber optic probe was immersed in the solution and connected to
the spectrophotometer. Via a syringe, the required amount of PMe3
was added, and enough toluene was introduced so as to keep the total
final volume of the solution the same throughout all the kinetic
experiments. The measurement was started immediately after the
addition.
examples, in contrast to aromatic imido complexes where the
same group is almost invariably bent (av 160°). The Mo−P
bond distance, at 2.541(1) Å, is sensibly longer than the two
previously reported complexes (2.3202(8) and 2.443(5) Å,
respectively) while a contraction is noticed in the MoN bond
distance (1.703(3) vs 1.833(3) Å) indicating a greater degree of
M→ N π-donation, in agreement with the (almost) linear
coordination mode observed. No sensible variation is noted
going from the oxo derivative 5 to 8, suggesting little, if any at
all, electronic contribution from the MoX (X = O, NR)
moiety to the MoP fragment.
CONCLUSIONS
■
A new set of molybdenum dioxo and oxo-imido complexes,
featuring the 2,6-disubstituted phenylimino-1,1,1-trifluoropen-
tan-2-one ligand system, has been synthesized and charac-
terized by spectroscopic means and by X-ray crystallography.
The complexes were investigated in OAT reactions and the
resulting reduced species isolated. The impact of the CF3
electron withdrawing moiety was evaluated with kinetic
measurements by UV−vis spectroscopy as well as by cyclic
voltammetry. The insertion of such a moiety in the complex
backbone increases the rate of the OAT reaction of a factor
ranging between 14 and 92, depending on the heteroatom
present on molybdenum and on the steric contraints imposed
by the ketiminate ligand in contrast to a previous system.46 The
fluorinated group depletes the metal atom of electron density,
thus making the bound oxo moiety much more susceptible to
nucleophilic attack from the incoming oxygen atom acceptor.
Moreover, the introduction of the electronegative group has
allowed us to better differentiate electronic from steric effects
and resolve the influences coming from the second doubly
bonded atom. The dramatic increase in kinetic rate is, however,
not reflected in a MoO bond length elongation in the solid
state structures. Moreover, the first solid state characterization
of a reduced imido phosphine MoIV complex deriving from an
OAT reaction has been reported.
Crystallography. Data were collected on a SMART APEX II
diffractometer equipped with a CCD detector using graphite
monochromated Mo Kα radiation (λ = 0.710 73 Å) and an Oxford
cryostat (100(2) K); reduction, integration, and cell refinement were
performed with SAINT.54 The data sets were corrected for absorption
and Lp effects with SADABS.55 Structures were solved by direct
methods (SHELX-9756), and refinement was carried on (full matrix
least-squares on F2) using the WINGX suite of programs.57 All
hydrogens atoms were added on calculated positions and refined
isotropically. Graphical plots of the structures were made using
ORTEP-3.58 Crystallographic data (excluding structure factors) for the
structures of compounds 2, 4, 5, 8 reported in this paper and
[MoO{N(2,6-Me2C6H3)}(LMe)2] reported in the Supporting In-
formation have been deposited with the Cambridge Crystallographic
Data Center (CCDC 837153−837156 and 837220). Copies of the
data can be obtained free of charge on application to the Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. [fax (interna-
tional) +44-1223/336-033; e-mail deposit@ccdc.cam.ac.uk].
General Procedure for the Synthesis of the ligands HLR. The
ligands were prepared according to a modification of the literature
procedure35 by condensation of 1,1,1-trifluoropentan-2,4-dione with
the appropriate aromatic amine in benzene in the presence of catalytic
1446
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Inorganic Chemistry
Article
amounts of p-toluenesulfonic acid-monohydrate over molecular sieves.
The mixture was refluxed until TLC showed no further changes, at
which point it was filtered and the solvent was removed in vacuo,
affording a red oil. The oil was taken up in a minimum amount of
CH2Cl2, washed with diluted H2SO4, saturated NaHCO3, and H2O
until neutrality; the organic layer was collected, dried over MgSO4, and
evaporated to dryness once more, prior to dissolution in petroleum
ether, from which the product precipitated as colorless to yellow
crystals upon cooling at −30 °C overnight.
6.8 Hz), 1.11 (d, 6H, CH3−CH−CAr, JH−H = 6.8 Hz), 1.22 (d, 6H,
CH3−CH−CAr overlapping with s, 6H, CH3−C), 1.58 (d, 6H, CH3−
CH−CAr, JH−H = 6.6 Hz), 2.67 (sept, 2H, CH3−CH−CAr, JH−H
=
6.80 Hz), 3.59 (sept, 2H, CH3−CH−CAr, JH−H = 6.7 Hz), 5.69 (s, 2H, H ),
γ
6.85−7.07 (m, 6H, HAr). 19F NMR (282 MHz, C6D6, δ, ppm): −74.01
(s, CF3). 13C NMR (75 MHz, C6D6, δ, ppm): δ 24.13 (s, CH3−CN),
24.61 (s, CH3−CH), 24.89 (s, CH3−CH), 25.16 (s, CH3−CH), 25.2
(s, CH3−CH), 28.03 (s, CH3−CH), 28.27 (s, CH3−CH), 99.03 (s, C ),
γ
119.81 (q, CF3, JC−F = 282 Hz), 148.97 (Cipso), 124.07, 124.4, 125.26
1
HLMe: Yield = 48.2%. H NMR (300 MHz, C6D6): δ 1.05 (s, 3H,
(s, CAr), 140.82 (s, C-iPr), 141.07 (s, C-iPr), 162.80 (q, CO, JC−F
=
CH3CN), 1.69 (s, 6H, CH3CAr), 5.43 (s, 1H, γ-H), 6.67−6.86
(m, 3H, HAr), 12.12 (br s, 1H, NH) ppm. 19F NMR (282 MHz,
C6D6): δ −76.15 (s, CF3) ppm.
34.3 Hz), 177.15 (s, CH3−CN). HR-MS Calcd for C34H42F6MoN2O4:
748.2117. Found: 748.2167.
Synthesis of [MoO(NtBu)(LMe)2] (4). [MoO(NtBu)Cl2(dme)]
(0.110 g, 0.32 mmol) was dissolved in 10 mL of toluene, and a
solution of HLMe (0.400 g, 0.64 mmol) and 0.25 mL of NEt3 in 5 mL
of toluene was added dropwise, with constant stirring. No evident
color change was observed, and the reaction was left to stir overnight.
The dark suspension (white solid visible) was then filtered over Celite
to afford a dark orange solution. Evaporation of the toluene left a dark
oily solid, which turned to a yellowish solid upon addition of 5 mL of
pentane. The yellow powder was then filtered and dried in vacuo
(0.150 g, 65%). Crystals suitable for X-ray diffraction were grown from
1
HLiPr: Yield = 43.6%. H NMR (300 MHz, C6D6): δ 0.89 (d, 6H,
CH3−CH−CAr, JH−H = 6.8 Hz), 0.95 (d, 6H, CH3−CH−CAr, JH−H
=
6.9 Hz), 1.21 (s, 3H, CH3CN), 2.76 (sept, 2H, CH3−CH−CAr,
JH−H = 6.8 Hz), 5.51 (s, 1H, γ-H), 6.90−7.10 (m, 3H, HAr), 12.61
(br s, 1H, NH) ppm. 19F NMR (282 MHz, C6D6): δ −76.07 (s, CF3)
ppm. 13C NMR (75 MHz, C6D6): δ 18.64 (s, CH3CN), 22.10 (s,
CH3−CH−CAr), 23.89 (s, CH3−CH−CAr), 28.55 (s, CH3−CH−CAr),
89.48 (s, γ-C), 118.27 (q, CF3, JC−F = 288.77 Hz), 123.77 (s, CAr),
129.07 (s, CAr), 132.22 (s, CAr), 145.39 (s, CAr), 169.94 (s, CN),
177.03 (q, CN, JC−F = 32,84 Hz) ppm.
1
a toluene/pentane solution of the complex at room temperature. H
1
HLH: Yield = 70.5%. H NMR (300 MHz, C6D6): δ 1.25 (s, 3H,
NMR (300 MHz, C6D6, δ, ppm): 0.84 (s, 9H, tBu), 0.96 (s, 3H, Me),
1.22 (s, 3H, Me), 2.04 (s, 3H, Me), 2.27 (s, 3H, Me), 2.30 (s, 3H, Me),
2.42 (s, 3H, Me), 5.53 (s, 1H, H ), 5.63 (s, 1H, H ), 7.04−6.69 (m, 6H,
CH3CN), 5.35 (s, 1H, γ-H), 6.42−6.89 (m, 5H, HAr), 12.69 (br s,
1H, NH) ppm. 19F NMR (282 MHz, C6D6): δ −76.25 (s, CF3) ppm.
13C NMR (75 MHz, C6D6): δ 19.09 (s, CH3CN), 90.81 (s, γ-C),
118.05 (q, CF3, JC−F = 288.82 Hz), 124.63 (s, CAr), 126.55 (s, CAr),
128.93 (s, CAr), 136.88 (s, CAr), 167.04 (s, CN), 176.55 (q, CO,
JC−F = 32,82 Hz) ppm.
γ
γ
aromatic). 19F NMR (282 MHz, C6D6, δ, ppm): −73.24 (s, CF3),
−72.36 (s, CF3). 13C NMR (75 MHz, C6D6, δ, ppm): 18.2 (Me), 18.55
(Me), 19.0 (Me), 20.6 (Me), 23.2 (Me), 23.6 (Me), 28.7 (CMe3), 73.8
(CMe3), 97.7 (C ), 99.7 (C ), 119.9, 120.2 (overlapping q, JCF = 287.6
γ
γ
General Procedure for the Synthesis of [MoO2(LR)2]. In a
typical experiment, [MoO2Cl2] (1 equiv) was suspended in toluene
(10 mL), and a solution of the appropriate HLR (2 equiv) and NEt3
(2 equiv) in the same solvent (5 mL) was added dropwise. The reaction
mixture immediately changed color to a deep red, whereby all of the
suspended starting material dissolved. The mixture was left stirring for
12 h at room temperature and then filtered over Celite. The orange
solution was then evaporated to dryness causing the formation of a
yellow-orange solid, which was washed with 3 × 5 mL pentane and
dried in vacuo, to afford [MoO2(LR)2].
and 284.0 Hz, CF3), 126.6, 126.8, 128.95, 128.97, 129.4, 129.5 130.76,
130.9, 132.2, 133.6, 135.4, (CAr), 150.1 (Cipso), 154.4 (Cipso), 164.4,
164.7 (overlapping quartets JCF = 33.2 and 32.7 Hz, CO), 175.2 (C
N), 175.5 (CN). EI-MS: 697.5 ([M]+, 12%), 626.3 ([M − NtBu]+,
12%), 146.2 ([ArNC(CH3)]+, 100%). Anal. Calcd for
C30H35F6MoN3O3: C, 51.80; H, 5.07; N, 6.04%. Found: C, 51.44;
H, 5.03; N, 6.41%.
General Procedure for the Synthesis of the Phosphine MoIV
Adducts [MoO(PMe3)(LR)2]. In a typical experiment [MoO2L2]
(1 equiv) was dissolved in 10 mL of toluene, and PMe3 (0.1 mL) was
added quickly via a syringe. The solution immediately turned deep
green, and it was left stirring for 2 h at rt. The solvent was then
removed in vacuo to afford a green powder, which was washed with
3 × 5 mL cold pentane and subjected to sublimation (0.05 mmHg,
70 °C) to remove residual OPMe3. The products were isolated as
green powders.
[MoO2(LH)2] (1). The compound was prepared according to the
general procedure, using 0.60 g of [MoO2Cl2] (3.0 mmol) and 1.40 g
of HLH (6.1 mmol). Isolated yield after workup: 0.82 g (46%).
1H NMR (300 MHz, C6D6, δ, ppm): 1.05 (s, 3H, Me), 6.43−7.12
(m, 10H, HAr), 5.68 (s, 1H, H ). 19F NMR (282 MHz, C6D6, δ, ppm):
γ
−73.76 (s, CF3). 13C NMR (75 MHz, C6D6, δ, ppm): 24.2 (s, CH3−
CN), 100.56 (s, γ-C), 119.86 (q, CF3, JC−F = 282.2 Hz), 123.6, 124.0,
126.1, 129.0 (s, CAr), 152.49 (Cipso), 164.25 (q, CO, JC−F = 35 Hz),
167.3 (s, CH3-CN) ppm. EI-MS: 586.2 ([M]+, 15%), 358.0 ([M −
L]+, 12%), 118.0 ([ArNC(CH3)]+, 100%). Anal. Calcd for
C22H18F6MoN2O4: C, 45.22; H, 3.10; N, 4.79%. Found: C, 45.10;
H, 3.35; N, 4.63%.
[MoO(PMe3)(LH)2] (5). The compound was prepared according to
the general procedure, using 0.40 g of complex 1 (0.68 mmol).
Isolated yield after workup: 0.35 g (79%). 1H NMR (300 MHz, C6D6,
δ, ppm): 0.55 (d, 9H, PMe3, JHP = 8.4 Hz), 1.27 (s, 3H, CH3−C), 1.97
(s, 3H, CH3−C), 5.49 (s, 1H, H ), 5.90 (s, 1H, H ), 6.77−7.06 (m,
γ
γ
10H, HAr). 19F NMR (282 MHz, C6D6, δ, ppm): δ −73.33 (s, CF3),
−71.96 (s, CF3). 31P NMR (121 MHz, C6D6, δ, ppm): δ −1.28 (m,
PMe3, JPH = 8.4 Hz) ppm. 13C NMR (75 MHz, C6D6, δ, ppm): 15.45
(d, PMe3, JC−P = 22.2 Hz), 24.14 (s, CH3−CN), 24.2 (s, CH3−CN),
[MoO2(LMe)2] (2). The compound was prepared according to the
general procedure, using 0.15 g of [MoO2Cl2] (0.75 mmol) and 0.40 g
of HLMe (1.6 mmol). Crystals suitable for X-ray diffraction analysis
were grown from a cold solution (−30 °C) of the complex in MeCN.
Isolated yield after workup: 0.39 g (81%). 1H NMR (300 MHz, C6D6,
δ, ppm): 1.06 (s, 3H, Me) 1.82 (s, 3H, Mering), 2.43 (s, 3H, Mering),
96.45 (C ), 97.21 (C ), 121.10, 122.99 (CAr), 123.75 (q, CF3, JC−F
=
γ
γ
281.5 Hz), 124.17 (s, CAr), 124.45 (q, CF3, JC−F = 283,9 Hz), 124.97,
125.36, 125.71, 126.20, 128.95, 129.39, 129.8 (CAr), 152.7 (Cipso),
156.5 (Cipso), 160.75 (q, CO, JC−F = 31.5 Hz), 163.50 (q, CO,
JC−F = 31.7 Hz), 165.53 (s, CH3−CN), 171.8 (s, CH3−CN). HR-MS
Calcd for C22H18F6MoN2O3 ([M − PMe3]+): 564.0290. Found:
564.0321.
5.68 (s, 1H, H ), 6.73−6.85 (m, 6H, HAr). 19F NMR (282 MHz, C6D6,
γ
δ, ppm): −73.81. 13C NMR (75 MHz, C6D6, δ, ppm): 17.33 (Me),
17.99 (Me), 23.2 (Me), 99.2 (C ), 119.4 (q, J = 282.0, CF3), 126.82,
γ
128.52, 129.27, 130.14, 130.43 (CAr), 150.5 (ipso C), 163.2 (q,
J = 34.2, CO), 176.18 (CN). EI-MS: 642.2 ([M]+, 15%), 386.0
([M − L]+, 7%), 146.0 ([ArNC(CH3)]+, 100%). Anal. Calcd for
C26H26F6MoN2O4: C, 48.76; H, 4.09; N, 4.37%. Found: C, 48.59;
H, 4.32; N, 4.25%.
[MoO(PMe3)(LMe)2] (6). The compound was prepared according to
the general procedure, using 0.27 g of complex 2 (0.42 mmol).
Isolated yield after workup: 0.24 g (81%). 1H NMR (300 MHz, C6D6,
δ, ppm): 0.58 (d, 9H, JPH = 7.84 Hz, PMe3), 1.19 (s, 3H, Me), 1.84
(s, 3H, Me), 2.20 (s, 3H, Me), 2.27 (s, 3H, Me), 2.25 (s, 3H, Me), 2.31
(s, 3H, Me), 5.48 (s, 1H, H ), 5.82 (s, 1H, H ), 6.82 (m, 6H, aromatic).
[MoO2(LiPr)2] (3). The compound was prepared according to the
general procedure, using 0.14 g of [MoO2Cl2] (0.7 mmol) and 0.44 g
γ
γ
19F NMR (282 MHz, C6D6, δ, ppm): −72.44 (s, CF3), −71.27
(s, CF3). 31P NMR (121 MHz, C6D6, δ, ppm): −7.1. 13C NMR (75 MHz,
1
of HLiPr (1.4 mmol). Isolated yield after workup: 0.16 g (28%). H
NMR (300 MHz, C6D6, δ, ppm): 0.72 (d, 6H, CH3−CH−CAr, JH−H
=
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Inorganic Chemistry
Article
C6D6, δ, ppm): 16.30 (d, JPC = 20.9 Hz, PMe3), 18.13 (Me), 18.24
(Me), 19.74 (Me), 19.77 (Me), 21.05 (Me), 23.57 (Me), 24.19 (Me),
96.50 (C ), 98.74 (C ), 119.4 (q, J = 282.0 Hz, CF3), 123.1 (q, J =
280 Hz, CF3), 125.57, 126.19, 128.86, 129.35, 129.60, 130.73, 132.47,
133.33 (CAr), 150.6 (Cipso), 154.64 (Cipso), 160.2 (q, J = 34.2, CO),
164.3 (q, J = 34.0 Hz), 172.60 (CN), 173.65 (CN). HR-MS
Calcd for C26H26F6MoN2O3 ([M − PMe3]+): 620.0916. Found:
620.0925.
(5) Enemark, J. H.; Cooney, J. J. A.; Wang, J.-J.; Holm, R. H. Chem.
Rev. 2004, 104, 1175−1200.
(6) Johnson, M. K.; Rees, D. C.; Adams, M. W. W. Chem. Rev. 1996,
96, 2817−2840.
γ
γ
(7) Bray, R. C.; Adams, B.; Smith, A. T.; Bennett, B.; Bailey, S.
Biochemistry 2000, 39, 11258−11269.
(8) Li, H.-K.; Temple, C.; Rajagopalan, K. V.; Schindelin, H. J. Am.
Chem. Soc. 2000, 122, 7673−7680.
[MoO(PMe3)(LiPr)2] (7). The compound was prepared according to
the general procedure, using 0.42 g of complex 3 (0.55 mmol).
Isolated yield after workup: 0.40 g (89%). 1H NMR (300 MHz, C6D6,
δ, ppm): 0.55 (d, 9H, PMe3, JHP = 8.1 Hz), 0.92 (d, 3H, JHH = 6.2 Hz,
Me), 0.96 (d, 3H, JHH = 6.2 Hz, Me), 1.01 (d, 3H, JHH = 6.2 Hz, Me),
1.12 (d, 3H, JHH = 6.2 Hz, Me), 1.14 (d, 3H, JHH = 6.2 Hz, Me), 1.34
(d, 3H, JHH = 6.2 Hz, Me), 1.46 (d, 3H, JHH = 6.2 Hz, Me), 1.49 (d,
3H, JHH = 6.2 Hz, Me), 1.58 (s, 3 H, Me), 2.01 (s, 3 H, Me), 3.14 (sept,
1H, JHH = 6.2 Hz, CH), 3.26 (sept, 1H, JHH = 6.2 Hz, CH), 3.36 (sept,
1H, JHH = 6.2 Hz, CH), 3.70 (sept, 1H, JHH = 6.2 Hz, CH), 5.77
(s, 1H, H ), 5.92 (s, 1H, H ), 7.06 (m, 6 H, aromatic). 19F NMR (282
(9) Schrader, N.; Fischer, K.; Theis, K.; Mendel, R.; Schwarz, G.;
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γ
γ
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MHz, C6D6, δ, ppm): −73.34 (s, CF3), −71.92 (s, CF3). 31P NMR
(121 MHz, C6D6, δ, ppm): −1.24. 13C NMR (75 MHz, C6D6, δ,
ppm): 16.16 (d, JP−C = 22 Hz, PMe3), 22.09 (MeiPr), 23.25 (MeiPr),
25.95 (MeiPr), 23.55 (MeiPr), 22.52 (MeiPr), 25.52 (MeiPr), 25.62
(MeiPr), 23.09 (MeiPr), 20.04 (Me), 28.52 (Me), 26.86 (CH), 27.07
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(CH), 28.19 (CH), 29.67 (CH), 98.06, 98.15 (C ). HR-MS Calcd for
γ
C34H42F6MoN2O3 ([M − PMe3]+): 732.2168. Found: 732.2167.
Synthesis of [Mo(NtBu)(PMe3)(LMe)2] (8). [MoO(NtBu)(LMe)2]
(4) (0.25 g, 0.36 mmol) was dissolved in 10 mL of toluene, and PMe3
(0.1 mL, 2.6 equiv) was added quickly via a syringe. The solution
immediately turned deep purple, and it was left stirring for 6 h at
room temperature. The solvent was then removed in vacuo to afford a
violet powder, which was washed with 3 × 5 mL cold pentane and
subjected to sublimation (0.05 mmHg, 70 °C) to remove residual
OPMe3 (yield 0.22 g, 82%).
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t
PMe3), 1.08 (s, 9H, Bu), 1.8 (s, 3H, Me), 1.24 (s, 3H, Me), 2.09
(s, 3H, Me), 2.15 (s, 3H, Me), 2.24 (s, 3H, Me), 2.44 (s, 3H, Me), 5.36
(s, 1H, H ), 5.96 (s, 1H, H ), 6.73−7.03 (m, 6H, aromatic). 19F NMR
γ
γ
(282 MHz, C6D6, δ, ppm): −72.23 (s, CF3), −69.97 (s, CF3). 31P (121
MHz, C6D6, δ, ppm): 2.82 (PMe3). 13C NMR (75 MHz, C6D6, δ,
ppm): 17.60 (d, PMe3, JCP = 6.7 Hz), 17.29 (Me), 18.57 (Me), 22.14
(Me), 23.27 (Me), 23.42 (Me), 29.62 (CMe3), 23.94 (Me), 72.25
(CMe3), 96.96 (C ), 97.97 (C ), 116.03 (CF3, q, JCF = 282.3 Hz),
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γ
γ
120.12 (CF3, q, JCF = 284.7 Hz), 125.32, 125.72, 128.68, 128.71,
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129.98, 130.05, 133.88, 134.47, 135.46 (C ), 154.05 (Cipso), 154.20
γ
(Cipso), 158.40 (q, COCF3, JCF = 31.24 Hz), 164.27 (q, COCF3, JCF
=
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30.47 Hz), 167.94 (CN), 170.60 (CN). HR-MS Calcd for
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ASSOCIATED CONTENT
* Supporting Information
CIF data. Additional figures and table. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
(31) Thrasher, J. S.; Strauss, S. H. Inorganic fluorine chemistry: toward
the 21st century (Washington, DC, 1994).
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AUTHOR INFORMATION
Corresponding Author
*E-mail: nadia.moesch@uni-graz.at.
■
(34) Long, Y.-Y.; Ye, W.-P.; Tao, P.; Li, Y.-S. Dalton Trans. 2011, 40,
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(35) Yu, S.-M.; Tritschler, U.; Gottker-Schnetmann, I.; Mecking, S.
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ACKNOWLEDGMENTS
Financial support by the Austrian Science Foundation FWF
P19309-N19 is gratefully acknowledged.
■
(36) Tang, L.-M.; Wu, J.-Q.; Duan, Y.-Q.; Pan, L.; Li, Y.-G.; Li, Y.-S.
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