Unusual nonoctahedral geometry with molybdenum oxoimido complexes containing η2-pyrazolate ligands

Article abstract of DOI:10.1021/ic201308g

The preparation and oxygen-atom-transfer (OAT) reactivity of oxoimido complexes [MoO(N-t-Bu)(t-Bu₂-4-Rpz)₂] [where R = H (1), Br (2), and Me (3); t-Bu₂pz = 3,5-di-tert-butylpyrazolate] are reported. The reaction of the potassium salt of the respective pyrazolate ligands and the molybdenum oxoimido precursor, [MoO(N-t-Bu)Cl₂(dme)] (dme = dimethoxyethane), in toluene afforded complexes 1-3 in good yields. The complexes were fully characterized by ¹H and ¹³C NMR and IR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray crystallography. The solid-state structures reveal that, in each case, the molybdenum center is coordinated by one oxo, one N-t-Bu group, and two sterically demanding pyrazolate ligands via their two adjacent nitrogen atoms in an η² fashion. Coordination around the metal center is severely distorted from octahedral and might be seen as closely approaching a distorted trigonal-prismatic geometry, which is relevant to the active site of dimethyl sulfoxide reductase in its oxidized form. The potential utility of all of the complexes 1-3 for OAT reactivity toward PMe₃ at room temperature is examined, and plausible mechanistic pathways are explored by density functional theory calculations. Furthermore, the complexes reported here open a new and convenient entry into mixed oxoimidomolybdenum complexes.

Full text of DOI:10.1021/ic201308g

Article
pubs.acs.org/IC
Unusual Nonoctahedral Geometry with Molybdenum Oxoimido
2
Complexes Containing η -Pyrazolate Ligands
T. Arumuganathan, Manuel Volpe, Bastian Harum, Dietmar Wurm, Ferdinand Belaj,
and Nadia C. Mosch-Zanetti*
̈
̈
̈
Institut fur Chemie, Anorganische Chemie, Karl-Franzens-Universitat Graz, Stremayrgasse 16, A-8010 Graz, Austria
*
S Supporting Information
ABSTRACT: The preparation and oxygen-atom-transfer (OAT) reac-
tivity of oxoimido complexes [MoO(N-t-Bu)(t-Bu -4-Rpz) ] [where R =
2
2
H (1), Br (2), and Me (3); t-Bu pz = 3,5-di-tert-butylpyrazolate] are
2
reported. The reaction of the potassium salt of the respective pyrazolate
ligands and the molybdenum oxoimido precursor, [MoO(N-t-Bu)-
Cl (dme)] (dme = dimethoxyethane), in toluene afforded complexes
2
1
1
−3 in good yields. The complexes were fully characterized by H and
C NMR and IR spectroscopy, mass spectrometry, elemental analysis,
1
3
and single-crystal X-ray crystallography. The solid-state structures reveal
that, in each case, the molybdenum center is coordinated by one oxo, one
N-t-Bu group, and two sterically demanding pyrazolate ligands via their
2
two adjacent nitrogen atoms in an η fashion. Coordination around the
metal center is severely distorted from octahedral and might be seen as closely approaching a distorted trigonal-prismatic
geometry, which is relevant to the active site of dimethyl sulfoxide reductase in its oxidized form. The potential utility of all of the
complexes 1−3 for OAT reactivity toward PMe at room temperature is examined, and plausible mechanistic pathways are
3
explored by density functional theory calculations. Furthermore, the complexes reported here open a new and convenient entry
into mixed oxoimidomolybdenum complexes.
INTRODUCTION
and, hence, prevention of in situ dimerization. Whereas
■
6
7
molybdenum diimido and dioxo complexes are well
documented in the literature, only a few mixed oxoimido
The oxygen-atom-transfer (OAT) reaction is a fundamental
process both in industrial applications and in natural systems.
Its technological importance arises principally from the
possibility of converting olefins (internal or terminal ones) to
their corresponding epoxides, valuable starting materials for
5
,8
complexes are known until now.
Furthermore, our group has been active in exploring changes
in the OAT reactivity by modification of the geometry imposed
around the metal center: recently, we found that asymmetric
molybdenum dioxo complexes exhibit significantly higher OAT
1
added-value synthetic products. In biology, OAT is important
in various degradation pathways of metabolites. A large group
of enzymes is commonly referred to as oxotransferases or
hydroxylases depending on whether the substrate is trans-
formed by a primary OAT or whether a water molecule is
involved in the substrate transformation. The common feature
of such enzymes is the presence, at the core of the enzyme
itself, of a molybdenum(VI) oxo unit, eventually complemented
by an additional doubly bonded atom (S or O) and one or two
dithiolene residues belonging to a pterin ligand. Depending on
the arrangement around the metal atom, such enzymes can be
classified in three major families, also differing among
themselves in the geometry adopted, ranging from octahedral
to trigonal-prismatic extremes: (i) xanthine oxidase, (ii) sulfite
7a
activity than their symmetric analogues. We attributed this to
different trans effects in the two systems and ruled out the
9
involvement of a spectator oxo effect. A series of trigonal-
prismatic molybdenum(VI) dioxo complexes has also been
2
reported, which featured the quite unusual η -pyrazolate anion
10
as the ancillary ligand.
1
Usually, pyrazolate ligands display η -bonding to a single
11
metal ion or bridge between two metal centers. Besides these
2
common bonding modes, the unusual η -pyrazolate coordina-
tion where both nitrogen atoms coordinate to the same metal
atom has been demonstrated in the past decade in coordination
2
2
−4
chemistry. The first η -pyrazolate ligation in a transition-metal
oxidase, and (iii) dimethyl sulfoxide (DMSO) reductase.
12
complex was reported by Erker and co-workers. Subse-
quently, it was found that sterically demanding pyrazolate
Previous contributions from our group have focused on the
introduction of a heteroatom in coordination complexes of
oxomolybdenum(VI) in order to explore the influence of such a
2
ligands coordinate predominantly η to electrophilic transition-
5
ligation in OAT reactions. Although not found in biological
systems, the advantage of introducing an imido functionality
rather than a sulfur group is due to its better steric stabilization
Received: June 17, 2011
Published: December 9, 2011
©
2011 American Chemical Society
150
dx.doi.org/10.1021/ic201308g | Inorg. Chem. 2012, 51, 150−156

Inorganic Chemistry
Article
1
3
10,14
5
metal centers, as shown by Winter et al. and us,
and to
NMR (C
6
D
6
): δ 0.979 (s, 18H, t-BuH), 1.16 (d, 3H, CCH
3
), 4.07 (q,
3 3
CH). 13C NMR (C D ): δ 15.3 (CCH ), 27.5 (C(CH ) ), 44.2
1
6
6
3
lanthanoids, as shown by Deacon and co-workers.
We have published molybdenum cis-dioxo complexes
(
C(CH ) , 49.6 (CCH ), 211.0 (CO).
3
3
3
Synthesis of 3,5-Di-tert-butyl-4-methylpyrazole (t-Bu -4-
2
2
2
2
[
MoO Cl(η -t-Bu -pz)] and [MoO (η -t-Bu -pz) ] with η -
2 2 2 2 2
10
MepzH). This ligand was prepared following the reported procedure
1
8
coordinate pyrazolate ligands. The small metallacycle formed
by the two nitrogen atoms of the pyrazolate and the metal leads
to distorted trigonal-prismatic geometries. Molybdenum oxo
complexes in a trigonal-prismatic coordination environment are
biologically relevant because of their resemblance to the active
for t-Bu pzH except that 2,2,4,6,6-pentamethyl-3,5-heptanedione was
2
used instead of 2,2,6,6-tetramethyl-3,5-heptanedione to yield 5.4 g
(95%) of the product as a white solid. H NMR (C
1
D
6
): δ 1.35 (s,
6
1
3
18H, t-BuH), 2.20 (s, 3H, ring CH
C D ): δ 11.7 (ring CCH ), 26.2 (C(CH ) ), 29.8 (C(CH ) ), 32.5
), 10.35 (br, 1H, NH). C NMR
3
(
6
6
3
3
3
3 3
4d
2
(ring CCH ), 107.7 (CC(CH ) ).
Synthesis of Potassium 3,5-Di-tert-butyl-4-methylpyrazo-
late (t-Bu -4-MepzK). This ligand was prepared following the
3 3 3
site of DMSO reductases. In addition, we found [MoO Cl(η -
t-Bu -pz)] and [MoO (η -t-Bu -pz) ] to be convenient starting
materials for the synthesis of molybdenum coordination
compounds by offering easier purification of the products.
The pyrazolate ligand can be easily exchanged by any other
ligand and the formed pyrazole conveniently removed by
washing or sublimation. A more conventional method employ-
ing [MoO Cl ] leads most often to lower yields.
In the present paper, we have decided to combine the
heteroatom ligation with a nonoctahedral geometry, preparing
molybdenum oxoimido complexes that contain sterically
demanding pyrazolate ligands and investigating their OAT
2
2
2
2
2
2
2
14c
reported procedure except that t-Bu -4-MepzH was used instead
7
f,16
2
of t-Bu -4-BrpzH to yield 0.93 g (31%) of the product as a white solid.
2
1
H NMR (CDCl ): δ 1.35 (s, 18H, t-BuH), 2.22 (s, 3H, CH ). MS: m/
3
3
+
z 194 (M − K).
General Procedure for the Synthesis of Molybdenum
Oxoimido Complexes 1−3. To a stirred solution of 0.052 g
2
2
(0.15 mmol) of [MoO(N-t-Bu)Cl (dme)] in dry toluene (3 mL) were
2
added solutions of 0.066 g (0.30 mmol) of t-Bu pzK for the
preparation of complex 1, 0.089 g (0.30 mmol) of t-Bu -4-BrpzK for
complex 2, and 0.070 g (0.30 mmol) of t-Bu -4-MepzK for complex 3
in 2 mL of toluene. The reaction mixture was allowed to stir for 12 h
followed by filtration through a pad of Celite (2 cm). The filtrate was
evaporated to dryness, affording complexes 1−3 as pale-yellow solids.
The product was extracted into dry pentane, and the volatiles were
removed under vacuum, giving yellow microcrystalline solids. They
were dissolved in a minimum volume of dry pentane and stored at
2
2
2
properties. Here, we report the synthesis, structural character-
ization, and OAT behavior of [MoO(N-t-Bu)(η -t-Bu -4-
Rpz) ] [where R = H (1), Br (2), and Me (3)].
2
2
2
EXPERIMENTAL SECTION
■
−
25 °C overnight, giving yellow crystals that proved to be suitable for
General Procedures. All syntheses were performed under an
atmosphere of dry argon using glovebox or Schlenk techniques. The
starting materials [MoO Cl (dme)], [Mo(N-t-Bu) Cl (dme)],
MoO(N-t-Bu)Cl (dme)], 3,5-di-tert-butylpyrazole (t-Bu pzH),
,5-di-tert-butyl-4-bromopyrazole (t-Bu -4-BrpzH),
were prepared according to published
procedures. 3,5-Di-tert-butyl-4-methylpyrazole (t-Bu -4-MepzH)
X-ray diffraction analysis.
[MoO(N-t-Bu)(t-Bu pz) ] (1). Yield: 0.069 g (85%). 1H NMR
1
7
6h
18
2
2
2
2
2
2
8
c
(C D ): δ 1.31 (s, 36H, t-BuH), 1.41 (s, 9H, NC(CH ) ), 6.22 (s, 2H,
[
3
6
6
3
3
2
2
1
3
1
0a
13g
ring H). C NMR (C D ): δ 29.6 (NC(CH ) ), 30.3 (C(CH ) ), 32.0
6 6 3 3 3 3
t-Bu pzK,
2
2
1
0a
(C(CH ) ), 73.8 (NC(CH ) ), 104.3 (ring CH), 160.6 (CC(CH ) ).
and t-Bu -4-BrpzK
3
3
3
3
3 3
2
−
1
+
1
9
IR (cm ): 913 (s, MoO). MS(EI): m/z 543 (12%; [M] ), 528
2
+
(
8
60%; [M − (CH )] ). Anal. Calcd for C H N OMo: C, 57.65; H,
and the corresponding potassium salt (t-Bu -4-MepzK) were prepared
3
26 47
5
2
1
4c
.76; N, 12.93. Found: C, 57.30; H, 8.70; N, 12.84.
MoO(N-t-Bu)(t-Bu -4-Brpz) ] (2). Yield: 0.085 g (81%). H NMR
with slight modifications with respect to the literature procedure. All
other chemicals mentioned were used as purchased from commercial
sources. All of the solvents were dried by a Pure Solv MD-4-EN
solvent purification system from Innovative Technology, Inc., flushed
with argon, and further passed through an activated aluminum oxide
column prior to use. All deuterated solvents for NMR studies were
purchased from Deutero GmbH and dried over molecular sieves.
NMR spectra were recorded on a Bruker Avance 300 MHz
spectrometer. Chemical shift values are given in parts per million
1
[
2
2
1
3
(
(
(
C D ): δ 1.28 (s, 9H, NC(CH ) ), 1.41 (s, 36H, t-BuH). C NMR
6 6 3 3
C D ): δ 28.6 (C(CH ) ), 29.4 (NC(CH ) ), 33.1 (C(CH ) ), 74.6
6
6
3
3
3
3
3 3
−
1
NC(CH ) ), 94.7 (ring CBr), 156.0 (CC(CH ) ). IR (cm ): 909 (s,
3
3
3 3
+
+
MoO). MS(EI): m/z 701 (10%; [M] ), 686 (45%; [M − (CH )] ).
3
Anal. Calcd for C H N Br OMo: C, 44.65; H, 6.50; N, 10.02.
26 45
5
2
Found: C, 45.14; H, 6.53; N, 10.07.
MoO(N-t-Bu)(t-Bu -4-Mepz) ] (3). Yield: 0.068 g (80%). H NMR
1
[
2
2
(
C D ): δ 1.34 (s, 36H, t-Bu), 1.37 (s, 9H, NC(CH ) ), 2.17 (s, 6H,
(
ppm). Spectra were obtained at 25 °C unless otherwise noted. IR
6
6
3
3
13
ring CH3). C NMR (C D ): δ 12.5 (ring CH ), 29.9 (C(CH ) ),
spectra were recorded with a Bruker Alpha FT-IR spectrometer.
Electron impact mass spectra were measured on an Agilent 5973 direct
probe. Elemental analyses were carried out using a Heraeus Vario
Elementar automatic analyzer at the Institute of Inorganic Chemistry,
Graz University of Technology, Graz, Austria.
6
6
3
3 3
3
0.1 (NC(CH ) ), 33.2 (C(CH ) ), 73.6 (NC(CH ) ), 115.6 (ring
3 3 3 3 3 3
CCH
)), 156.6 (CC(CH )
3 3
). IR: 905 (s, MoO). MS(EI): m/z 571
3
+
+
(24%; [M] ), 556 (100%; [M − (CH
)] ). Anal. Calcd for
3
C
H
51
N
OMo: C, 59.03; H, 9.04; N, 12.30. Found: C, 58.70; H,
28
5
OAT reactions were carried out by reacting freshly prepared
8.85; N, 12.37.
complexes 1−3 and PMe in a 1:4 ratio in 1 mL of dry benzene
6 6
Synthesis of Ligands. The diketone 2,2,4,6,6-pentamethyl-3,5-
X-ray Crystallographic Determination. Single crystals of the
complexes were analyzed on a four-cycle Stoe diffractometer with
graphite-monochromated Mo Kα radiation (0.710 73 Å) at 100 K. The
3
(
C D ).
20
heptanedione was prepared with a slight modification from the
structures were solved by direct methods (SHELXS-97) and refined
19
2
20
literature procedure, as described in the following:
by full-matrix least-squares techniques against F (SHELXL-97). The
non-hydrogen atoms were refined with anisotropic displacement
parameters without any constraints. In complex 1, the hydrogen atoms
H14 and H24 were put at the external bisector of the C−C−C angle at
a C−H distance of 0.95 Å, but the individual isotropic displacement
parameters were left free to refine. The hydrogen atoms of the methyl
groups were refined with common isotropic displacement parameters
for the hydrogen atoms of the same tert-butyl group and idealized
geometries with tetrahedral angles, enabling rotation around the C−C
bond, and C−H distances of 0.98 Å.
To a suspension of sodium (1.7 g, 74 mmol) in 30 mL of benzene
was added slowly at 5 °C 2,2,6,6-tetramethyl-3,5-heptanedione (11.48
g, 62.3 mmol). The reaction mixture was allowed to warm to room
temperature and then stirred at 50 °C for 24 h. Subsequently, the
reaction flask was placed in a refrigerator (8 °C) for 1 day, during
which a colorless crystalline material precipitated. To this mixture was
added dropwise at 5 °C methyl iodide (19.87 g, 140 mmol). The
resulting solution was heated to 80 °C for 15 h. The formed yellow
precipitate was filtered off, the colorless filtrate was collected in a
Schlenk flask, and the solvent was removed at atmospheric pressure.
The product 2,2,4,6,6-pentamethyl-3,5-heptanedione was collected by
Computational Details. The OAT reaction between complexes
1−3 and PMe was calculated using the hybrid density functional B3-
3
1
21
22
distillation at 130 °C in vacuo (1.0 mbar). Yield: 7.4 g (60%). H
LYP, as implemented in the TURBOMOLE program. All
1
51
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Inorganic Chemistry
Article
geometries were first optimized in the gas phase with the standard
sents a convenient leaving group because the formed side
2
3
double-ζ quality basis set def2-SVP for all atoms except
molybdenum, which was treated with an effective core potential and
its corresponding basis set (ecp-28-mwb-SVP). Further optimizations
were performed by employing the COSMO solvation model
product, t-Bu pzH, can easily be removed by washing with
2
pentane or by sublimation. We observe a smaller tendency to
reductive degradation, which is sometimes observed by other,
more conventional methods for the introduction of ligands (the
addition of KL or LH and a base to metal halogenides).
Furthermore, an additional benefit of using pyrazolate
complexes 1−3 as molybdenum sources is their high solubility
in pentane, heptane, etc., whereas the normally used
(
benzene: rsolv = 2.60 Å, ε = 2.28) with the larger basis sets ecp-28-
r
23b,24
mwb-TZVP for molybdenum and with def2-TZVP
for all other
atoms. The nature of all stationary points was confirmed by vibrational
analysis. Input geometries were derived from crystal structures by
replacing the bulky tert-butyl groups of the pyrazolates by hydrogen
atoms and the tert-butyl group of the imido functionality by methyl to
save computational costs.
dihalogenide precursor [MoO(N-t-Bu)Cl (dme)] is insoluble
2
in the aforementioned solvents.
Complexes 1−3 were fully characterized by H and 13C
NMR and IR spectroscopy as well as by electron-impact mass
spectrometry. Elemental analyses agree well with the expected
formula. Furthermore, single crystals suitable for X-ray
diffraction analysis were obtained for all compounds, allowing
determination of their molecular structures (vide infra).
1
RESULTS AND DISCUSSION
■
Three pyrazole ligands were prepared according to literature
1
0a,18,19
procedures.
The sterically demanding tert-butyl groups
in the 3 and 5 positions have to be present in all molecules in
2
order to enforce η -coordination to the molybdenum atom and
1
thus decrease the tendency for the formation of polynuclear
complexes. Variation within the ligand system then occurred in
the 4 position, ranging from the unsubstituted ligand (HL1) to
one with an electron-withdrawing bromide (HL2) and one
with an electron-donating methyl group (HL3). Potassium salts
of all ligands were obtained by the addition of excess potassium
hydride as white solids. They serve as excellent pyrazolate
transfer reagents to the molybdenum atom.
The H NMR spectra of complexes 1−3 at room
temperature show a singlet between 1.31 and 1.41 ppm
integrating for 36 protons assignable to the protons of the four
t-Bu groups. In static structures, the t-Bu groups should give
rise to four resonances. Thus, the occurrence of one singlet
points to a dynamic behavior in solution, presumably by
rotation of the heterocycle (thereby exchanging the two
nitrogen atoms) as well as exchange of the two cycles
1
Synthesis of the Oxoimido Complexes. Treatment of
themselves. However, low-temperature H NMR spectroscopy
the [MoO(N-t-Bu)Cl (dme)] precursor with the potassium
performed with complex 1 in CDCl down to −30 °C did not
2
3
resolve the singlet. Similarly, the 13C NMR spectra display one
salts t-Bu -4-RpzK (KL1−KL3) in a 1:2 stoichiometric ratio in
2
dry toluene led to the formation of complexes of the type
resonance for the four carbon atoms (CC(CH ) ) of the t-Bu
3
3
[
(
MoO(N-t-Bu)(t-Bu -4-Rpz) ] [R = H (1), Br (2), and Me
3)] in good yields (Scheme 1). All compounds were isolated
groups.
2
2
The reaction of [MoO(N-t-Bu)Cl (dme)] with only 1 equiv
2
of t-Bu -pzK in toluene led to a mixture of more than two
compounds, as evidenced by H NMR spectroscopy. We were
2
1
Scheme 1. Synthesis of Oxoimido Complexes 1−3
not able to separate a single compound because of their similar
solubility behavior. However, just once, during a purification
attempt by crystallization, a single crystal suitable for X-ray
crystallography could be picked out of the mixture (see the
Supporting Information). Analysis revealed an unexpected
molybdenum compound, [Mo(N-t-Bu)Cl (t-Bu pz) ], contain-
2
2
2
2
ing one imido, two chloro, and two η -pyrazolate ligands,
whereas an oxo group is absent. Its formation is unclear, but
reactions in which the oxo group is replaced by two chloro
25
ligands are known in the literature. For example, [MoO-
NPh)(Et dtc) ] (Et dtc = diethyldithiocarbamate) in the
as pale-yellow crystalline solids by the evaporation of toluene
and purified by recrystallization from pentane at −25 °C.
The large amount of tert-butyl groups in the pyrazolate
complexes 1−3 renders them readily soluble in common
organic solvents such as pentane, toluene, dichloromethane,
and acetonitrile. In the solid state, all complexes are found to be
stable in an argon atmosphere, but they readily decompose in
the presence of moisture, leading to the formation of blue
solids. In a pentane solution, decomposition occurs after 1
week, as is apparent by precipitation of the free ligands as
colorless crystalline solids. For example, in a crystallization
attempt of complex 3 in pentane, the obtained colorless crystals
(
2
2
2
presence of gaseous HCl leads to the formation of [Mo(NPh)-
25b
Cl (Et dtc) ·CHCl ]. Although several attempts were made
2
2
2
3
to reproduce the compound rationally, we were not able to
isolate this material in bulk. However, we feel that the
information is important because it demonstrates the sensitivity
of the systems toward any protic conditions, justifying the
rigorous anhydrous and nonprotic conditions needed for the
preparation of compounds 1−3.
Molecular Structures of the Oxoimido Complexes.
The solid-state structures of complexes 1−3 were determined
by single-crystal X-ray diffraction analyses. Crystals suitable for
the analyses were obtained by recrystallization from pentane at
proved to be the ligand t-Bu -4-MepzH (see the Supporting
2
Information). Thus, complexes 1−3 are extremely sensitive
toward substitution reactions. This represents a drawback
concerning the stability; however, it is an advantage if they are
used as starting materials for the preparation of further
complexes. We have previously used the dioxo complexes
−
25 °C. Complexes 1−3 exhibit similar structural features. For
this reason, only a molecular view of complex 1 is shown in
Figure 1, whereas those of 2 and 3 can be found in the
Supporting Information (Figures S1 and S2). Crystal data of all
compounds are given in Table 1 and selected bond lengths and
angles in Table 2. In the unit cells of compounds 2 and 3, two
independent molecules are found with very similar structural
2
containing pyrazolate ligands [MoO (η -t-Bu pz) ] as starting
2
2
2
7
f,16
materials with significant success.
The heterocycle repre-
1
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Inorganic Chemistry
Article
Table 2. Selected Bond Lengths (Å) and Angles (deg) of 1−
3
1
2 (both molecules) 3 (both molecules)
Mo1O1
1.7057(11) 1.701(4)/1.706(4)
1.7027(17)/
.7017(17)
1
Mo1N1
Mo1−N11
Mo1−N12
Mo1−N21
Mo1−N22
1.7375(13) 1.723(4)/1.722(4)
2.0336(13) 2.053(4)/2.050(4)
2.4962(13) 2.309(4)/2.314(4)
2.0738(13) 2.070(5)/2.075(4)
2.1836(13) 2.206(4)/2.192(4)
1.726(2)/1.728(2)
2.050(2)/2.069(2)
2.310(2)/2.303(2)
2.073(2)/2.050(2)
2.189(2)/2.187(2)
O1−Mo1−N1
104.89(6)
33.65(5)
37.71(5)
130.34(5)
85.03(5)
103.3(2)/103.6(2)
103.35(9)/
1
03.48(9)
Figure 1. ORTEP drawing of complex 1 with an atom labeling scheme
N11−Mo1−
36.18(16)/
36.12(15)
36.30(7)/36.30(7)
N12
(
ellipsoids at the 50% probability level). All hydrogen atoms have been
N21−Mo1−
37.08(16)/
37.16(16)
37.09(8)/37.18(8)
omitted for clarity.
N22
N11−Mo1−
127.33(17)/
128.13(17)
128.25(8)/
128.64(8)
Table 1. Crystallographic Data for 1−3
N21
N12−Mo1−
86.09(16)/
85.02(16)
86.22(8)/85.07(8)
1
2
3
N22
empirical
formula
C H MoN O
C H Br MoN O
C H MoN O
2
6
47
5
26 45
2
5
28 51
5
Mo1−N1−C1
152.30(11) 150.4(4)/151.2(4)
151.51(18)/
1
51.73(18)
fw
541.63
699.43
569.68
cryst syst
space group
a (Å)
triclinic
triclinic
triclinic
coordinate with highly distorted geometries. This is best
described by the largest angles between two ligands: e.g., O1−
Mo1−N22 varying between of 133.43(5) and 135.87(17)° and
N1−Mo1−N12 between 138.19(5) and 142.22(18)°. These
bond angles are severely deviating from those found in an
octahedral environment and are closer to those in a trigonal-
prismatic geometry. Anyway, no matter how the geometries of
the compounds are described, they both feature the absence of
a ligand trans to the oxo functionality in comparison to an
octahedral structure, in which the presence of the trans ligand
has a strong effect. The occurrence of a (distorted) trigonal-
prismatic geometry has biological significance because it was
found that trigonal-prismatic transition states play a crucial role
P1
̅
P1
̅
P1
̅
9.6391(14)
12.251(2)
14.203(3)
65.037(12)
72.924(12)
76.939(12)
1443.4(5)
2
11.7782(6)
15.5201(8)
17.9310(9)
104.523(2)
90.810(2)
100.914(3)
3109.1(3)
4
11.8054(4)
15.3574(6)
17.8372(6)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
104.4582(16)
90.4678(14)
100.7104(17)
3071.85(19)
4
ρcalc (g/cm3)
1.246
1.494
1.232
cryst size
0.26 × 0.22 ×
0.18
0.36 × 0.30 × 0.04 0.20 × 0.20 ×
3
(
mm )
0.16
F(000)
576
1424
1216
26
in biological OAT reactions. The small N−Mo−N bite angles
μ(Mo Kα)
0.479
3.018
0.454
−
1
2
(mm
)
[between 36.18(16)° and 37.08(16)°] of the η -coordinate
T (K)
θ range (deg)
95
100
100
pyrazolate ligands assist the central metal ion to attain what
could be seen as a trigonal-prismatic geometry. The Mo−N
distances of the heterocycles are similar, albeit not equal. In a
given complex, the differences in the Mo−N(pz) distances are
in one of the two ligands significantly larger than in the other.
2.72−30.00
1.76−26.00
26100
2.02−26.00
24726
reflns collected 9344
indep reflns
8412
12055
12014
R(int)
0.0233
0.0406
0.0223
2
data/param/
restraints
8412/330/0
12055/671/0
12014/699/0
This type of bonding variation is common for η -pyrazolate
10,13−15
ligands and is often reported in the literature.
a
b
R1 /wR2 , [I > 0.0285/0.0655
σ(I)]
0.0468/0.1159
0.0702/0.1255
0.0313/0.0743
0.0451/0.0809
Presumably, the pyrazolate anion not being fully aromatic
[see, for example, N11−C15 1.3703(18) Å vs N12−C13
1.3332(18) Å] leads to different electron densities at the two
nitrogen atoms. Depending on the difference of the Mo−N
2
a
b
R1 /wR2 , (all 0.0332/0.0678
data)
2
GOF (F )
1.056
1.018
1.030
2
CCDC dept
number
828736
828737
828738
bond lengths, the heterocycle is described as being η -, slipped-
2
1
13
η - or η -coordinate. For example, in compound 1, the Mo1−
N21 bond length is 2.0336(13) Å and the corresponding
Mo1−N22 is 2.4962(13) Å. This represents the largest
difference within a given ligand of all three complexes and,
thus, this particular ligand coordination is better described as
a
R1 = ∑∥F | − |F ∥/∑|F |. wR2 = [∑w(F − F ) /∑w(F ) ]1/2.
b
2
o
2
c
2
2 2
o
c
o
o
features. For this reason, only one of each is discussed in the
following section.
All three compounds were found to exhibit rare geometries
because of the η -coordinate pyrazolate ligands. The
molybdenum atoms in complexes 1−3 are coordinated by an
oxo, an imido, and two η -coordinate pyrazolate ligands. Taking
1
η . The smallest difference in the Mo−N distances is found in
2
the same complex 1 in the second pz ligand [Mo1−N11
2.1836(13) Å and Mo1−N12 2.0738(13) Å], which is
2
2
indicative for a true η coordination. This demonstrates the
the middle point of the N−N bonds in the pyrazolate ligands as
the coordinating site, they exhibit distorted tetrahedral
geometries. When the four nitrogen atoms are considered as
the sites of coordination instead, the complexes are six-
variability of the ligand system and is consistent with the
dynamic behavior found in solution. Nevertheless, the steric
demand of the 3,5-t-Bu groups prevents bridging modes
11
preferably observed with less bulky pz ligands.
1
53
dx.doi.org/10.1021/ic201308g | Inorg. Chem. 2012, 51, 150−156

Inorganic Chemistry
Article
Figure 2. Energy profile for OAT from complexes 1−3 to PMe . Energies are given in kJ/mol relative to the reactants at the B3LYP def2-SVP level
3
of theory.
The MoO bond distances of Mo1−O1 are in the range of
.701(4)−1.705(11) Å for complexes 1−3, which is normal for
reacted with 4 equiv of trimethylphosphine in 1.0 mL of dry
1
benzene-d . Upon the addition of phosphine, the color of the
6
5,8
molybdenum oxoimido systems. Mo−N bond lengths of the
solution changed from yellow to deep green, pointing to
reduction of the compounds. The samples were subjected to
imido groups (MoN) are varying in the range of
3
1
1
.7057(11)−1.7375(13) Å for complexes 1−3. These values
P NMR spectroscopy at room temperature, immediately after
their preparation. The three 31P NMR spectra reveal, next to
are consistent with those of reported similar molybdenum
5
,8
oxoimido compounds. Complexes containing a metal−imido
bond can be classified into two different groups according to
the geometry of the M−N−C bond angle: linear (180−160°)
the resonance for excess PMe (−62.6 ppm), one at 32.4 ppm
3
for OPMe and additional resonances at 2.96 (1), 12.02 (2),
3
and 26.22 (3) ppm, respectively, characteristic for the
6f,8d,27
and bent (150−130°).
60−150° belong to the semibent geometry class. The Mo1−
N1−C1 bond angles deviate significantly from linearity
152.30(11)° for 1, 150.4(4)° for 2, and 151.51(18)° for 3].
The bond angles in the range of
formation of PMe -coordinated molybdenum(IV) adducts of
3
28
1
the type [Mo(N-t-Bu)(pz) (PMe )]. This is in line with our
2
3
previous observation in oxoimido complexes where after OAT
[
monophosphine adducts [Mo(N-t-Bu)(L) (PMe )] (L = β-
2
3
5
These values are bordering on a bent geometry, and the imido
ketiminate) are formed. On the other hand, in dioxo
pyrazolate complexes, the OAT leads to [MoO(t-
Bu pz) (PR ) ] (R = Me, Et) with two coordinated
2
ligand in complexes 1−3 is better described as a sp -hybridized
four-electron donor. In oxoimido complexes, linear bonding
2
2
3 2
8
10
modes are far more common than bent ones. An example of
phosphines. Thus, the formation of a bis(phosphine) adduct
in the present complexes cannot be ruled out, in particular
considering the similar 31P NMR shifts of 1 (2.96 ppm) and
6g
the latter is represented by complex [Mo(NPh) (S CNEt ) ],
2
2
2 2
which was found to exhibit one linear and one bent imido
group. Its mixed oxoimido analogue [MoO(NPh)(Et dtc) ] is
10
2
2
[MoO(t-Bu pz) (PMe ) ] (0.96 ppm). The downfield shift
2
2
3 2
25b
not structurally characterized.
We have recently reported
in going from the oxo to the imido derivative is consistent with
oxoimido complexes with ketiminate ligands that exhibit linear
imido groups. Thus, complexes 1−3 represent rare examples
what we have already observed in the β-ketiminate complexes
5
7c
([MoX(L) (PMe )]; X = O, −5.7 ppm, or X = N-t-Bu, +6.5
2
3
5
of structurally characterized molybdenum oxoimido species
with a bent imido moiety. Presumably, the steric hindrance of
two t-Bu groups in the 3 and 5 positions leads to the observed
metal imido angle. Such steric hindrance, in the case of the t-Bu
group C35−C38, also seems to “protect” an otherwise empty
coordination site on the molybdenum atom, with the C36 atom
pointing directly toward the metal center. However, in complex
ppm). Moreover, the longer distance between C36 and Mo1
in the solid state for complex 1 might, in principle, allow for
formation of the bis(phosphine) derivative. Unfortunately,
kinetic data of the OAT of the compounds allowing a
meaningful comparison to the dioxo system [MoO (t-
2
Bu pz) ] could, however, not be obtained. All attempts were
2
2
hampered by concomitant decomposition of the system.
Nonetheless, these investigations demonstrate the capability
of 1−3 for OAT activity.
1, this distance is significantly longer (4.145 Å) than those in
complexes 2 and 3 (3.847 and 3.840 Å), respectively (vide
infra).
Theoretical Calculations. In order to gain insight into the
influence of the substituents on the OAT reactivity given the
absence of kinetic data, theoretical calculations were performed
on the complexes. Two mechanisms (A and B) for the OAT
reaction between compounds 1−3 (abbreviated as [Mo′O] in
OAT Studies. Complexes 1−3 were designed to study the
OAT reactivity to phosphines. A comparison of their reactivity
10
to that of the previously investigated dioxo system would
provide valuable information on the influence of the second
group next to the oxygen atom that is actually transferred
Figure 2) and PMe were calculated with density functional
3
2
1,22
(
spectator oxo effect). Thus, oxoimido compounds 1−3 were
theory (DFT) at the B3-LYP level of theory
in order to
1
54
dx.doi.org/10.1021/ic201308g | Inorg. Chem. 2012, 51, 150−156

Inorganic Chemistry
Article
investigate the electronic influence of their different sub-
stituents. The COSMO solvation model was employed to take
solvent effects into account.
ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic data in CIF format, ORTEP drawings,
additional crystallographic data, and X-ray crystallographic
determination. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
*
S
An approaching molecule PMe can either directly attack the
3
oxo group to form a coordinated phosphine oxide intermediate
species [Mo′(OPMe )] (mechanism A) or coordinate to the
3
molybdenum atom, forming [Mo′O(PMe )] (mechanism B).
3
The coordination to molybdenum is energetically a barrier-less
process where the compounds gain 6−17 kJ/mol depending on
the substituent in the 4 position of the heterocycle. Direct OAT
AUTHOR INFORMATION
Corresponding Author
E-mail: nadia.moesch@uni-graz.at. Tel: (+43) 316 3805286.
Fax: (+43) 316-380-9835.
■
*
from 1−3 to PMe requires 100−110 k/mol (mechanism A),
3
whereas the attack of a second molecule PMe on the oxo
3
group of the phosphine adduct species ([Mo′O(PMe )]
ACKNOWLEDGMENTS
Financial support by the Austrian Science Foundation FWF
P19309-N19) is gratefully acknowledged.
3
■
(
according to mechanism B) requires only 47−58 kJ/mol.
According to mechanism A, the coordinated phosphine oxide
molecule in [Mo′(OPMe )] can be replaced by PMe , in either
3
3
an associative or a dissociative process, forming the reduced
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