Replacement of an oxo by an imido group in oxotransferase model compounds: Influence on the oxygen atom transfer

Article abstract of DOI:10.1021/ic101159g

Treatment of [MoO(N-t-Bu)Cl₂(dme)] (dme = dimethoxyethane) with 2 equiv of the potassium salts of Schiff base ligands of the type KArNC(CH ₃)CHC(CH₃)O afforded oxo imido molybdenum(VI) compounds [MoO(N-t-Bu)L₂] {1, with Ar = phenyl (L_Ph), 2 with Ar = 2-tolyl (L_MePh), 3 with Ar = 2,6-dimethylphenyl (L_Me2Ph) and 4 with Ar = 2,6-diisopropylphenyl (L_iPr2Ph)}. We have also prepared related bisimido complexes [Mo(N-t-Bu)₂L₂ (5 with L = L_Ph, 6 with L = L_MePh, and 7 with L = L _Me2Ph) by treatment of [Mo(N-t-Bu)₂Cl₂(dme)] with 2 equiv of the potassium salt of the respective ligand. 1, 3, 5, and 6 were characterized via single crystal X-ray diffraction. The oxo imido complexes exhibit oxygen atom transfer (OAT) reactivity toward trimethyl phosphine. Kinetic data were obtained for 1 and 3 by UV/vis spectroscopy revealing decreased OAT reactivity in comparison to related dioxo complexes with the same Schiff base ligands and decreased reactivity of 1 versus 3. Cyclic voltammetry was used to probe the electronic situation at the molybdenum center showing reversible reduction waves for 3 and [MoO₂(L_Me2Ph) ₂] at comparable potentials while 1 exhibits a significant lower potential. Density functional theory (DFT) calculations showed a higher electron density on oxygen in the oxo imido complexes.

Full text of DOI:10.1021/ic101159g

8914 Inorg. Chem. 2010, 49, 8914–8921
DOI: 10.1021/ic101159g
Replacement of an Oxo by an Imido Group in Oxotransferase Model Compounds:
Influence on the Oxygen Atom Transfer
,†
†
†
†
†
†
€
Nadia C. Mosch-Zanetti,* Dietmar Wurm, Manuel Volpe, Ganna Lyashenko, Bastian Harum, Ferdinand Belaj, and
Judith Baumgartner‡
†
€
Institut fu€r Chemie, Karl-Franzens-Universitat Graz, Schubertstrasse, 1 A-8010 Graz, Austria, and
‡
€
Institut fu€r Anorganische Chemie, Technische Universitat Graz, Stremayrgasse, 8010 Graz, Austria
Received June 9, 2010
Treatment of [MoO(N-t-Bu)Cl2(dme)] (dme = dimethoxyethane) with 2 equiv of the potassium salts of Schiff base
ligands of the type KArNC(CH3)CHC(CH3)O afforded oxo imido molybdenum(VI) compounds [MoO(N-t-Bu)L2]
{1, with Ar = phenyl (LPh), 2 with Ar = 2-tolyl (LMePh), 3 with Ar = 2,6-dimethylphenyl (LMe2Ph) and 4 with Ar = 2,6-
diisopropylphenyl (LiPr2Ph)}. We have also prepared related bisimido complexes [Mo(N-t-Bu)2L2 (5 with L=LPh, 6 with
L = LMePh, and 7 with L = LMe2Ph) by treatment of [Mo(N-t-Bu)2Cl2(dme)] with 2 equiv of the potassium salt of the
respective ligand. 1, 3, 5, and 6 were characterized via single crystal X-ray diffraction. The oxo imido complexes exhibit
oxygen atom transfer (OAT) reactivity toward trimethyl phosphine. Kinetic data were obtained for 1 and 3 by UV/vis
spectroscopy revealing decreased OAT reactivity in comparison to related dioxo complexes with the same Schiff base
ligands and decreased reactivity of 1 versus 3. Cyclic voltammetry was used to probe the electronic situation at the
molybdenum center showing reversible reduction waves for 3 and [MoO2(LMe2Ph)2] at comparable potentials while 1
exhibits a significant lower potential. Density functional theory (DFT) calculations showed a higher electron density on
oxygen in the oxo imido complexes.
Introduction
ferable one whereas the other one takes the role of a spectator
ligand.5,6 Only a few complexes containing the interesting
[MoOS]2þ core have been prepared although they seem to be
quite prone to dimerization, forming a Mo-S-S-Mo unit.7-12
For this reason, we aim at the preparation of molybdenum
compounds [MoOX]2þ where the additional doubly bonded
unit is represented by an imido group. The imido function-
ality provides better steric stabilization of the metal than an
oxo ligand preventing dimerization. Electronically, the
ModO bond should be weakened as imido groups are better
π donors; however, steric demand is increased.41 Comparison
of oxygen atom transfer (OAT) reactivity of various oxo
imido to analogous dioxo complexes will give insights into
steric and electronic influences on the OAT activity within
mixed non-dioxo systems. The obtained information could
Molybdenum is found in a large class of enzymes capable
of transferring anoxygenatomfromor to a substrate referred
to as mononuclear molybdoenzymes or oxotransferases.1,2
They can be classified into three different families (xanthine
oxidase (XO), sulfite oxidase (SO), and DMSO reductase
(DMSOR) families) according to the structure of their
oxidized active center. They all contain in their oxidized state
a mononuclear molybdenum(VI) atom with at least one oxo
ligand, but the XO family possesses a MoOS, the SO family
an MoO2, and the DMSOR family a MoO(OR) core. For
this reason model chemistry for these enzymes has focused on
molybdenum compounds that contain a metal core with at
least one oxo group and an additional doubly bonded
chalcogen atom.3,4 Most model compounds, both functional
as well as structural, possess a [MoO2]2þ core; however, a com-
pound having next to the oxo a non-oxygen group would be
ꢀ
(6) Rappe, A. K.; Goddard, W. A., III J. Am. Chem. Soc. 1982, 104, 448.
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Nordlander, E.; Hedman, B.; Hodgson, K. O.; Holm, R. H. Inorg. Chem.
1999, 38, 4104–4114.
ꢀ
desirable as early studies by Rappe and Goddard suggest that
in dioxo systems one oxygen atom represents the actual trans-
(8) Smith, P. D.; Slizys, D. A.; George, G. N.; Young, C. G. J. Am. Chem.
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(9) Partyka, D. V.; Staples, R. J.; Holm, R. H. Inorg. Chem. 2003, 42,
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Chem. 2005, 44, 4506–4514.
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Inorg. Chem. 2005, 44, 6927–6929.
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*To whom correspondence should be addressed. E-mail: nadia.moesch@
uni-graz.at.
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r
pubs.acs.org/IC
Published on Web 09/10/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8915
Figure 2. Four possible isomers in oxo imido compounds with two
general bidentate O,N ligands.
Figure 1. Molybdenum dioxo and monooxo complex with Schiff base
ligands; the asterisk denotes the γ-position referred to in the text.
1
The compounds can be conveniently characterized by H
be relevant for a better understanding of the role of the oxo
ligand in biological non-dioxo systems.
and 13C NMR spectroscopy. In particular, the resonances
of the γ-protons in the backbone (see Figure 1) represent an
easy tool for determining the ligand’s coordination to the
metal core. Thus, in the 1H NMR spectra resonances for the
γ-protons in the free ligands can be found around 5.0 ppm,
whereas upon coordination resonances shift toward lower
fields (∼5.2 ppm).
Whereas dioxo molybdenum(VI) complexes have been
prepared in a very wide variety,3,4,13-15 oxo imido Mo(VI)
complexes are significantly rarer. Early work describes molyb-
denum oxo imido complexes mainly as byproduct,16-20 and
only recently an interesting starting material has been devel-
oped from which coordination chemistry evolved.21-26
Recently, we reported molybdenum dioxo complexes with
Schiff base ligands as shown in Figure 1 that are active
catalyst in the OAT from dimethylsulfoxide to trimethylphos-
phine. This well-behaved system allowed the isolation of the
reduced monooxo compound after the transfer giving in-
sights into the mechanism which proceeds via a mononuclear
species with a coordinated trimethylphosphine molecule.27,28
Here, we report analogous complexes in which the oxo
groups are replaced by one or two t-butyl imido functional-
ities. Oxygen atom transfer reactivity of the oxo imido com-
plexes toward trimethyl phosphine was investigated and
compared to data obtained with the dioxo system.
Well-defined starting materials for the preparation of
oxo imido molybdenum compounds are scarce. A straight-
forward method of preparation for the tert-butyl imido
derivative [MoO(N-t-Bu)Cl2(dme)] has been described in
the literature. Its synthesis involves an exchange reaction
between the dioxo compound [MoO2Cl2(dme)] and the
bisimido compound [Mo(N-t-Bu)2Cl2(dme)] under pro-
longed refluxing conditions (3 days). Purification occurs
by recrystallization from toluene where the target com-
pound [MoO(N-t-Bu)Cl2(dme)] crystallizes selectively.
Stoichiometry and strict oxygen- and moisture-free reaction
conditions are extremely crucial, otherwise the product
does not crystallize and only impure material is obtained.21
The reaction of [MoO(N-t-Bu)Cl2(dme)] with 2 equiv of
the potassium salts of the ligands in cold toluene under
elimination of potassium chloride affords oxo imido molyb-
denum(VI) compounds of the type [MoO(N-t-Bu)L2]
(eq 1). Crude mixtures contain some unidentified bypro-
ducts. Analytically pure material can be obtained by crys-
tallization from pentane. For this reason and also because
of a pronounced solubility in pentane isolated yields are
moderate. Crystals of 1 and 3 suitable for X-ray diffrac-
tion analysis were obtained from concentrated pentane
solution upon cooling to -30 ꢀC for few days (vide infra).
Results and Discussion
Synthesis of the Compounds. The employed β-ketiminate
ligands can easily be prepared by condensation of acetyl-
acetone and the corresponding aromatic amines in a ratio
1:1 with catalytic amounts of concentrated sulfuric acid.29
(13) McCleverty, J. A.; Ward, M. D. Compr. Coord. Chem. II 2004, 2,
743–757.
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Weinheim, 2006; pp 237-257.
(16) Murdzek, J. S.; Schrock, R. R. Organometallics 1987, 6, 1373–1374.
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2003, 4457–4465.
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A. C. Inorg. Chem. 2004, 43, 4072–4079.
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2005, 4902–4906.
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2006, 45, 4556–4561.
Proton and carbon NMR spectra of 1, 3, and 4 show
two sets of sharp resonances for the signals of two types of
ligands consistent with the expected oxo imido com-
pounds. Particularly indicative are the two resonances
of equal intensity of the two inequivalent γ-protons in the
region of 5 ppm in the 1H NMR spectra and in the region
of 100 ppm in the 13C NMR spectra. Compound 2 can
also be obtained in analytically pure form; however, it
behaved differently in solution. The 1H NMR spectrum is
complex showing eight sets of resonances assignable to
(26) Cross, W. B.; Anderson, J. C.; Wilson, C. S. Dalton Trans. 2009,
1201–1205.
€
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€
Mosch-Zanetti et al.
8916 Inorganic Chemistry, Vol. 49, No. 19, 2010
Figure 3. Molecular structures of [MoO(N-t-Bu)(LPh)2] (1, left) and [MoO(N-t-Bu)(LMe2Ph)2] (3, right). Thermal ellipsoids are drawn at 50% probability
level and hydrogen atoms are omitted for clarity.
the ligand; two at a time showing equal intensity. In contrast,
both sterically less as well as more demanding ligands
form a single isomer in solution.
All compounds 1 to 7 are extremely well soluble in all
organic solvents including apolar ones such as pentane
which hampers isolation in high yields. They are rather
sensitive toward moisture so that even in the glovebox
they can only be stored for a limited amount of time. Within
weeks they decompose to blue intractable products.
Molecular Structures. Single crystals suitable for X-ray
diffraction analyses were obtained from compounds 1, 3,
5, and 6. Molecular structures of compounds 1 and 3 are
shown in Figure 3 and those of 5 and 6 in Figure 4.
Selected bond lengths and angles are given in Tables 1 and
2 and crystallographic data and structure refinement in
Table 3.
In principle, four isomers as shown in Figure 2 can be
envisioned explaining the observed NMR spectra. An-
other possibility for the found isomers could be hindered
rotation about the nitrogen-aryl axis. Thus, different
positions of the methyl group in compound 2 is a source
of further asymmetry leading possibly to the observed
isomer distribution. We found a similar behavior of this
ligand in related molybdenum dioxo and rhenium oxo
complexes.28,34
Bisimido complexes [Mo(N-t-Bu)2L2] (L = LPh 5, L =
LMePh 6, L=LMe2Ph 7) were synthesized for comparative
studies. They were obtained by the reaction of [Mo(N-t-
Bu)2Cl2(dme)]30 and the potassium salt of the respective
ligand (eq 2). The reactions proceeded similar to those of
1 to 4 and again isolated yields were moderate because of
the high solubility in pentane solution.
Structural analysis shows all four compounds consisting
of a hexa-coordinate molybdenum(VI) atom in a distorted
octahedral geometry. The metal atom is surrounded by an
imido and an oxo or two imido groups and two bidentate
Schiff-base ligands. The two nitrogen atoms of the latter
are found in trans position to each other in all cases
independent of the steric bulk imposed by the ligands.
The substitution of one oxo ligand with one imido group
results in a slight elongation of the residual ModO bond
length, which goes from 1.710(6) (average in structurally
þ 2 KL
- 2 KCl
½MoðN-t-BuÞ2Cl2ðdmeÞꢀ
½MoðN-t-BuÞ L ꢀ ð2Þ
f
2
2
˚
where L = LPh 5; LMePh 6; and LMe2Ph 7.
related dioxo complexes) to 1.7259(15) A in 1 and
The analogous reaction employing the bulky ligand
LiPr2Ph did not yield in any product, but only a mixture of
unidentified species was formed. Presumably, the steric
bulk of two t-butyl groups on the Mo atom and two
i-propyl groups on each ligand is too high to form the
expected compound. NMR spectroscopy of complexes 5
and 7 shows them to be of symmetric nature as only one
set of resonances for the ligands and only one for both
t-butyl groups is detected in each case. This is consistent
with a structure where the two nitrogen atoms of the
ligands are trans to each other which was confirmed by
X-ray diffraction analysis (vide infra). However again,
ligand LMePh gave the analytically pure compound [Mo-
(N-t-Bu)2(LMePh)2] (6) that behaved differently in solu-
tion. The 1H NMR spectrum of 6 in benzene-d6 showed
sets of resonances for three isomers which corresponds to
the number of possible isomers in this type of bisimido
complexes.
1.723(4) in 3. In compounds 1 and 3 the imido groups
are linear with molybdenum-nitrogen-carbon angles of
175.63(15)ꢀ in 1 and 172.5(5)ꢀ in 3. This is consistent with
˚
short molybdenum-nitrogen(imido) bonds of 1.7418(19) A
˚
and 1.734(6) A, respectively, and with the formulation of
a triple bond.32,33 The two corresponding angles in the
bisimido compound 5 are Mo1-N5-C51 175.12(11)ꢀ
and Mo1-N6-C61 158.15(12)ꢀ. The more acute angle
is close to those found in bent imido groups where angles
<150ꢀ are typical and where the lone pair is located at
nitrogen.32,33 This is apparently due to electronic reasons
as in the sterically more demanding bisimido compound
6, both angles are in good agreement with linear imido
(31) Robin, T.; Montilla, F.; Galindo, A.; Ruiz, C.; Hartmann, J.
Polyhedron 1999, 18, 1485–1490.
(32) Barrie, P.; Coffey, A.; Forster, G. D.; Hogarth, G. J. Chem. Soc.,
Dalton Trans. 1999, 4519–4528.
(33) Chen, T.; Soarenee, K. R.; Wu, Z.; Diminnie, J. B.; Xue, Z. Inorg.
Chim. Acta 2003, 345, 113–120.
€
(34) Schrockeneder, A.; Traar, P.; Raber, G.; Baumgartner, J.; Belaj, F.;
(30) Fox, H.; Yap, K. B.; Robbins, J.; Cai, S.; Schrock, R. R. Inorg. Chem.
1992, 31, 2287–2289.
€
Mosch-Zanetti, N. C. Inorg. Chem. 2009, 48, 11608–11614.

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8917
Figure 4. Molecular structures of [Mo(N-t-Bu)2(LPh)2] (5, left) and [Mo(N-t-Bu)2(LMePh)2] (6, right). Thermal ellipsoids are drawn at 50% probability
level and hydrogen atoms are omitted for clarity.
˚
˚
Table 2. Selected Bond Lengths (A) and Angles (deg) of Compounds 5 and 6
Table 1. Selected Bond Lengths (A) and Angles (deg) of Compounds 1 and 3
1
3
5
6
Mo1-N1
Mo1-O1
Mo1-N14
Mo1-N34
Mo1-O12
Mo1-O32
1.7418(19)
1.7259(15)
2.1303(17)
2.1728(17)
2.1578(16)
2.1474(15)
Mo1-N3
Mo1-O1
Mo1-N1
Mo1-N2
Mo1-O2
Mo1-O3
1.734(6)
1.723(4)
2.144(6)
2.176(6)
2.112(4)
2.148(4)
Mo1-N5
Mo1-N6
Mo1-N14
Mo1-N34
Mo1-O12
Mo1-O32
1.7512(14)
1.7622(14)
2.1779(13)
2.1788(14)
2.1720(12)
2.1819(12)
1.755(2)
1.751(2)
2.177(2)
2.179(2)
2.1821(19)
2.1661(18)
N1-Mo1-O1
104.25(8)
97.47(7)
93.29(7)
168.25(7)
91.18(7)
98.08(7)
92.35(7)
87.30(7)
163.32(7)
162.68(7)
82.80(6)
85.94(6)
83.93(6)
80.29(6)
77.10(6)
175.63(15)
N3-Mo1-O1
N3-Mo1-N1
N3-Mo1-N2
N3-Mo1-O2
N3-Mo1-O3
O1-Mo1-N1
O1-Mo1-N2
O1-Mo1-O2
O1-Mo1-O3
N1-Mo1-N2
N1-Mo1-O2
N1-Mo1-O3
N2-Mo1-O2
N2-Mo1-O3
O2-Mo1-O3
Mo1-N3-C27
104.3(2)
97.7(2)
94.3(2)
167.5(2)
90.1(2)
95.2(2)
N5-Mo1-N6
N5-Mo1-N14
N5-Mo1-N34
N5-Mo1-O12
N5-Mo1-O32
N6-Mo1-N14
N6-Mo1-N34
N6-Mo1-O12
N6-Mo1-O32
N34-Mo1-N14
N14-Mo1-O12
N14-Mo1-O32
N34-Mo1-O12
N34-Mo1-O32
O12-Mo1-O32
Mo1-N5-C51
Mo1-N6-C61
104.89(7)
97.83(6)
94.51(6)
165.47(6)
89.72(6)
92.85(6)
97.70(6)
89.60(6)
165.24(6)
161.15(5)
82.13(5)
82.86(5)
82.35(5)
83.01(5)
75.84(5)
175.12(11)
158.15(12)
106.75(11)
98.14(9)
91.77(9)
164.07(9)
90.20(9)
91.36(8)
97.65(9)
88.98(10)
162.92(9)
164.13(8)
83.62(7)
84.01(7)
83.50(7)
83.59(7)
74.19(8)
169.80(19)
175.12(18)
N1-Mo1-N14
N1-Mo1-N34
N1-Mo1-O12
N1-Mo1-O32
O1-Mo1-N14
O1-Mo1-N34
O1-Mo1-O12
O1-Mo1-O32
N14-Mo1-N34
N14-Mo1-O12
N14-Mo1-O32
N34-Mo1-O12
N34-Mo1-O32
O12-Mo1-O32
Mo1-N1-C1
90.9(2)
88.20(19)
163.57(19)
164.70(18)
81.18(19)
90.44(19)
85.02(18)
80.09(19)
77.41(17)
172.5(5)
groups (169.80(19) and 175.12(18)ꢀ). All other bond
lengths and angles in the four compounds exhibit very
similar features (Table 1 and 2).
compound 2 is again explainable by isomers. To unambi-
guously identify the nature of the reduced species, a
toluene solution of compound 3 was treated with 2 equiv
of trimethyl phosphine. After workup green-brown com-
pound [Mo(N-t-Bu)(PMe3)(LMe2)2] (8) was isolated in
moderate yield (eq 3).
Oxygen Atom Transfer Properties. Oxygen atom trans-
fer properties of 1-3 to the non-biological substrate
trimethylphosphine were investigated. Thus, a solution
of the corresponding compound was treated with an
excess (3 equiv) of PMe3 in benzene-d6. Reactivity was
immediately apparent as in all cases a color change from
orange to green-brown occurred and no precipitate was
observed. After 2 h at room temperature, 31P NMR
spectra were recorded revealing resonances for the phos-
phorus atom of excess trimethylphosphine at -61.1 ppm,
for formed trimethylphosphine oxide at 32.0 ppm and
additional resonances: 10.3 ppm (1), 7.7, 8.4, and 9.6 ppm
(2), and 6.5 ppm (3). We believe the new resonances are
assignable to reduced molybdenum imido compounds of
the type [Mo(N-t-Bu)(PMe3)L2] with a coordinated tri-
methyl phosphine molecule. This is in good agreement
with our previous results employing the related dioxo
compound where we also found phosphine adducts after
OAT.28 The occurrence of three resonances employing
Compound 8 exhibits a single resonance in the 31P
NMR spectrum at 6.5 ppm which is identical to that
found in the above-described NMR experiment. Proton
and carbon NMR spectra are also consistent with a
structure shown in eq 3 so that we are confident that the
mononuclear compound 8 is formed. Molybdenum(IV)
imido compounds are relatively rare, and only recently

€
Mosch-Zanetti et al.
8918 Inorganic Chemistry, Vol. 49, No. 19, 2010
Table 3. Crystallographic Data and Structure Refinement of Compound [MoO(N-t-Bu)(LPh)2] (1), [MoO(N-t-Bu)(LMe2Ph)2] (3), [Mo(N-t-Bu)2(LPh)2] (5), and
[Mo(N-t-Bu)2(LMePh)2] (6)
1
3
5
6
empirical formula
Fw
temperature, K
wavelength, A
cryst syst
C
531.49
95
0.71069
monoclinic
P21/n
26H33MoN3O3
C30H43MoN3O3
589.61
100(2)
0.71073
triclinic
P1
C30H42MoN4O2
586.62
95
0.71069
triclinic
P1
C32H46MoN4O2
614.67
95
0.71069
monoclinic
P2/c
˚
space group
unit cell dimens
˚
a, A
10.960(2)
20.887(2)
11.6976(18)
9.992(2)
11.645(2)
13.350(3)
89.34(3)
85.90(3)
73.58(3)
1486.1(5)
2
1.318
9.6105(16)
11.4339(18)
14.540(2)
84.994(13)
79.468(13)
70.033(12)
1475.8(4)
2
1.320
0.476
616
0.34 ꢁ 0.24 ꢁ 0.20
2.60 to 30.00ꢀ
-13 e h e 13
-16 e k e 15
-13 e l e 20
9521
20.584(3)
9.0773(13)
19.137(3)
˚
b, A
˚
c, A
R, deg
β, deg
y. deg
106.305(17)
117.604(11)
3
˚
volume, A
2570.1(7)
4
1.374
0.541
1104
0.22 ꢁ 0.20 ꢁ 0.16
2.66 to 26.00ꢀ
-2 e h e 13
-25 e k e 2
-14 e l e 14
7210
3168.7(9)
4
1.288
0.447
1296
0.30 ꢁ 0.20 ꢁ 0.16
2.51 to 28.00ꢀ
-27 e h e 27
-1 e k e 11
-19 e l e 25
9279
Z
density (calcd), Mg/m3
absorp coeff, mm-1
F(000)
0.475
620
cryst size, mm3
θ range for data collection
index ranges
0.32 ꢁ 0.26 ꢁ 0.16
1.82 to 24.50ꢀ
-11 e h e 11
-13 e k e 13
-15 e l e 15
10238
4899 [R(int) = 0.0719]
98.9%
0.9279 and 0.8629
1.119
R1 = 0.0732
wR2 = 0.1553
R1 = 0.0951
wR2 = 0.1659
0.971 and -1.305 755728
no. of reflns collected
no. of indep reflns
completeness to θmax
max. and min transmn
goodness-of-fit on F2
final R indices [I > 2σ(I)]
4395 [R(int) = 0.0206]
99.9%
7945 [R(int) = 0.0241]
99.7%
6621 [R(int) = 0.0229]
99.9%
1.032
1.074
1.075
R1 = 0.0416
wR2 = 0.0871
R1 = 0.0516,
wR2 = 0.0916
1.092 and -0.463 755509
R1 = 0.0276
wR2 = 0.0609
R1 = 0.0353
wR2 = 0.0647
0.407 and -0.340 755511
R1 = 0.0301
wR2 = 0.0741
R1 = 0.0340
wR2 = 0.0764
0.461 and -0.439 755510
R indices (all data)
3
˚
larges diff peak and hole, e/ A
CCDC dep. number
were phosphine containing Mo(IV) imido complexes
reported in the literature.35,36
E1/2 = -1.60 V for 3 and -1.59 V (vs Fc/Fcþ) for
[MoO2(LMe2Ph)2] (see Supporting Information). Despite
the better donor capabilities of an imido group versus an
oxo group,41 in the two compounds no relevant electronic
difference seems to be discernible. Thus, the slower kine-
tics of the oxo imido compound can be attributed to a
steric influence of the tert-butyl substituents. To deter-
mine the extent of the steric effect on the transfer, kinetics
of the sterically less demanding oxo imido compound 1
were investigated. Quite surprisingly, compound 1 exhi-
bits slower transfer kinetics compared to 3 by a factor of
2. This is probably due to the electron-donating capability
of the two methyl groups of LMe2Ph in compound 3 as
compared to the unsubstituted ligand in 1. This leads to a
higher electron density on the oxo atom in 3 pointing to a
significant contribution of the second resonance structure
in ModO T M;Oh. This is supported by cyclic voltam-
metry that shows a significant difference in the reduction
potential between 1 and 3. Whereas 3 has a reversible
reduction wave (Mo(VI)/Mo(V)) at E1/2 = -1.60 V, 1
presents an irreversible wave at -1.82 V giving evidence
for the trend of 1 being harder to reduce. The absence of
electron-donating substituents in 1 decreases the reaction
rate enough to counter balance the increase expected by
the larger coordination pocket. Thus, substitution on the
aromatic ring within the Schiff base ligand more strongly
affects the electronic rather than the steric influence on
the transfer reaction.
Kinetic studies of the oxygen atom transfer reaction
shown in eq 3 with compound 1 and 3 were carried out by
UV/vis spectroscopy. The reaction was performed under
pseudo-first order conditions with excesses of trimethyl-
phosphine over molybdenum compounds in the range of
100 to 200 equiv. UV/vis spectra were recorded every 30 s
between 400 and 800 nm (see Supporting Information).
The second-order rate constants, evaluated by plotting
ln R versus time (R = (At - A¥)/(A0 - A¥), A = absor-
bance) at several [PMe3]/[Mo] ratios, were found to be
9.34ꢁ10-4 L mol-1 s-1 for 1 and 1.83ꢁ10-3 L mol-1 s-1
for 3. The data of compound 3 can be compared to those
of the analogous dioxo system [MoO2(LMe2Ph)2] (4.1 ꢁ
10-3 L mol-1 s-1), which is faster by a factor of 2.28
Steric hindrance of the tert-butylimido group in 3 is signi-
ficantly higher than that of an oxo group, and the increase
in sterics around the Mo center should disfavor the
entrance of the incoming phosphine, that is regarded as
the first step in the associative mechanism leading to
phosphine oxide formation with concomitant reduction
of the metal. The electronic effect induced by the sub-
stitution of an oxo by an N-t-Bu group was investigated by
cyclic voltammetry. Both compounds 3and [MoO2(LMe2Ph)2]
show a reversible reduction wave (Mo(VI)/Mo(V)) at
(35) Watanabe, D.; Gondo, S.; Seino, H.; Mizobe, Y. Organometallics
2007, 26, 4909–4920.
(36) Peterson, E.; Khalimon, A. Y.; Simionescu, R.; Kuzmina, L. G.;
Howard, J. A. K.; Nikonov, G. I. J. Am. Chem. Soc. 2009, 131, 908–909.
To aid our understanding of relative influences of steric
and electronic effects on the OAT, density functional theory

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8919
phosphine led to the isolation of the reduced molybdenum-
(IV) imido compound 8 after oxygen atom transfer. Kinetics
of this transfer employing 1 and 3 were investigated by UV/
vis spectroscopy under pseudo-first order conditions reveal-
ing a decreased rate in comparison to the related dioxo com-
plex [MoO2(LMe2Ph)2]. Keeping constant the oxo imido environ-
ment showed compound 1 to be slower than 3. Furthermore,
cyclic voltammetry revealed essentially identical reduction
potential for 3 and [MoO2(LMe2Ph)2] while a significant lower
potential was found for 1. The electronic influence of the
N-t-Bu group was additionally investigated by DFT calcula-
tions revealing higher electron density on oxygen. Consider-
ing just the electronic effect of the imido group, a faster
transfer reaction was expected.41 The present findings show
the steric effect of the t-Bu imido group to be dominating.
However, comparison of the two slower oxo imido com-
plexes 1 and 3 has revealed a profound electronic influence of
the substitution pattern at the Schiff base ligand. Because of
the overruling steric effects of the t-Bu substituent on the
imido ligand, the spectator oxo effect cannot be investigated
by the presented system. Complexes featuring imido groups
with different electronic and steric effects are thus in need,
which we are currently investigating.
Figure 5. HOMO of [MoO(N-t-Bu)(LMe2Ph)2] (3, left) and [MoO2-
(LMe2Ph)2] (right).
Experimental Section
General Procedures. 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 innova-
tive technology inc. and flushed with argon prior to use. Ligands,29
[Mo(N-t-Bu)2Cl2(dme)]30 and [MoO2Cl2]31 were prepared accord-
ing to literature procedures. 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. Elemental
analyses were performed by the Analytisches-Chemisches Labo-
Figure 6. HOMO of [Mo(N-t-Bu)(LMe2Ph)2(PMe3)] (8).
(DFT) calculations were performed on the oxo imido
complex [MoO(N-t-Bu)(LMe2Ph)2] (3) and the OAT pro-
duct [Mo(N-t-Bu)(PMe3)(LMe2Ph)2] (8). In addition, the
related dioxo complex [MoO2(LMe2Ph)2] was included in
the theoretical study for comparison. Figure 5 shows
the highest occupied molecular orbitals (HOMO) of the
dioxo as well as of the oxo imido complex. Whereas the
HOMO of the dioxo complex was found to be localized
essentially on the Schiff base ligands, the one in 3 has a
significant p contribution on the oxo atom. The HOMO
on complex 8 shows to have no contribution on the
phosphorus atom (Figure 6). These findings are consis-
tent with the expected electron-donating capability of an
aliphatic imido group which results in an elongation of
the molybdenum oxo bond (as evidenced by the long
bond found by X-ray crystallography) and a higher
electron density on the oxo atom. Comparison to these
theoretical data to cyclic voltammetry points to a push-
pull interplay between oxo and imido which cannot be
resolved experimentally.
€
ratorium des Instituts fur Anorganische Chemie der Tech-
nischen Universitat Graz, Austria. IR spectra were recorded
€
on a Perkin-Elmer FT-IR Spectrometer 1725X as nujol mull
between KBr plates. UV/vis spectra were recorded on a Varian
Cary 50 spectrophotometer which was connected to a Hellma
fiber optics all-quartz immersion probe.
X-ray diffraction data were collected on a Stoe four-circle
(1, 5, and 6) or on a Bruker AXS-SMART APEX CCD (3)
diffractometer using graphite-monochromated MoKa radia-
˚
tion (0.71073 A). The data for all compounds were reduced,
corrected for Lorentz and polarization effects and for absorp-
tion using SAINT42 and SADABS43 programs, respectively.
The structures were solved by direct methods (SHELXS-97)44
and refined by full-matrix least-squares method.45 If not noted
otherwise, all non-hydrogen atoms were refined anisotropically;
hydrogen atoms were located in calculated positions to corre-
spond to standard bond lengths and angles.
Gas-phase single point calculations of the different complexes
were done using the B3LYP37,38 DFT method as implemented in
TURBOMOLE program.39 All atoms except Mo were treated
with a standard triple-ζ quality basis (def2-TZVP). The effective
core potential ecp-28-mwb was used for Mo with the corre-
sponding triple-ζ quality basis set ecp-28-mwb-TZVP. Input
geometries were created from crystal structures, wherever possible,
Conclusion
A series of molybdenum oxo imido and bisimido com-
plexes of the type [MoO(N-t-Bu)L2] (1-4) and [Mo(N-t-
Bu)2L2] (5-7) that contain L = β-ketiminate ligands were
synthesized. They exhibit octahedral structures with the two
nitrogen atoms of the ligand being trans to each other
elucidated by X-ray diffraction analyses. Comparison to
the related dioxo compound28 reveals an increase in the bond
lengths of the molybdenum oxo bonds upon substitution of
the second oxo by the imido groups. NMR studies of 1, 3, 4,
5, and 7 in benzene-d6 confirm the structures found by X-ray
diffraction in solution. In contrast, compounds 2 and 6 with
ortho tolyl substitutents at the ligand were found to be
mixtures of isomers in solution. Reaction of 3 with trimethyl
(37) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(38) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B. 1988, 37, 785–789.
(39) TURBOMOLE, V6.0.2; University of Karlsruhe and Forschungszen-
trum Karlsruhe GmbH: Karlsruhe, Germany, 2009; Since 2007, freely available
from: http://www.turbomole.com.

€
Mosch-Zanetti et al.
8920 Inorganic Chemistry, Vol. 49, No. 19, 2010
or were obtained by replacing an oxo atom with a preoptimized
PMe3 model. Molecular orbitals were visualized using the
Gabedit software.40
5.11 (s, 1H, γ-H), 5.15 (s, 1H, γ-H), 6.83-7.50 (m, H-Ar) ppm.
Mass (EI, m/z, (%)): 561 (15) [M]þ, 490 (15) [M - NtBu]þ. Anal.
Calcd. for C28H37MoN3O3: C, 60.10; H, 6.66; N, 7.51%. Found:
C, 59.60; H, 6.36; N, 7.89%.
General Procedure for the Preparation of the Oxo Imido and
Bis Imido Molybdenum Complexes. All ligands were transformed
into their potassium salt prior to complexation to molybdenum
according to the following general procedure: approximately 2 g of
the ligand and an approximate 3-fold excess of KH were placed in a
Schlenk flask equipped with a bubbler, and 30 mL of THF were
added. The corresponding suspension was stirred overnight at
room temperature during which the evolution of hydrogen was
apparent. The suspension was filtered over Celite, and the solvent
was removed in vacuo. The thus obtained salt was used without
further purification.
Synthesis of [MoO(N-t-Bu)(LMe2Ph)2] (3). The compound was
prepared following the general procedure employing 0.200 g
(0.58 mmol) of [MoO(N-t-Bu)Cl2(dme)] and 0.2785 g (1.15
mmol) of KLMe2Ph giving 0.118 g (35%) of 3. 1H NMR
(benzene-d6): δ 0.99 (s, 9H, (H3C)3C), 1.26 (s, 3H, H3C), 1.44
(s, 3H, H3C), 1.73 (s, 3H, H3C), 1.94 (s, 3H, H3C), 2.33 (s, 3H,
H3C), 2.40 (s, 3H, H3C), 2.49 (s, 3H, H3C), 2.69 (s, 3H, H3C),
5.10 (s, 1H, γ-H), 5.12 (s, 1H, γ-H), 6.90-7.11 (m, 6H, H-Ar)
ppm. 13C NMR (benzene-d6): δ=18.9 (CH3), 19.5 (2 CH3), 21.1
(CH3), 23.3 (CH3), 23.5 (CH3), 25.6 (CH3), 25.8 (CH3), 29.3
((CH3)3C), 71.9 (CMe3), 101.8 (γ-C), 103.3 (γ-C), 125.7, 125.8,
128.0, 128.8, 129.1, 129.5, 129.2, 132.78, 132.82, 134.1, 151.5,
156.1, 169.1 (CdN), 170.4 (CdN), 182.7 (CdO), 184.2 (CdO)
ppm. Mass (EI, m/z, (%)): 589 (10) [M]þ, 518 (10) [M - NtBu]þ.
Anal. Calcd. for C30H41MoN3O3: C, 61.32; H, 7.03; N, 7.15%.
Found: C, 61.21; H, 6.99; N, 7.00%.
The potassium salt of the respective ligand (2 equiv) was
suspended in 10 mL of cold toluene (approximately -20 ꢀC). A
cold solution of [MoO(N-t-Bu)Cl2(dme)] or [Mo(N-t-Bu)2Cl2-
(dme)] in 10 mL of toluene was added dropwise. The yellow to
orange mixtures were left to stir at room temperature for 2 h
followed by centrifugation at 13000 rpm for 4 min. The toluene
solution was syringed out and the volatiles were removed in
vacuo. The thus obtained yellow to orange solids were dissolved
in pentane and crystallized by slow evaporation of the solvent.
The yellow to orange products were filtered off and washed with
small amounts of cold pentane. Crystals suitable for X-ray diffrac-
tion analysis were obtained by recrystallization from pentane.
Synthesis of Oxo Imido Complexes. Synthesis of [MoO(N-t-Bu)-
(LPh)2] (1). The compound was prepared following the general
procedure employing 0.200 g (0.58 mmol) of [MoO(N-t-Bu)Cl2-
(dme)] and 0.247 g (1.16 mmol) of KLPh giving 0.117 g (38%) of 1.
1H NMR (benzene-d6): δ 1.05 (s, 9H, (H3C)3C), 1.41 (s, 3H, H3C),
1.51 (s, 3H, H3C), 1.97 (s, 3H, H3C), 1.98 (s, 3H, H3C), 5.06 (s, 1H,
γ-H), 5.23 (s, 1H, γ-H), 6.80-7.45 (m, 10H, H-Ar) ppm. 13C NMR
(benzene-d6):δ=23.96 (CH3), 24.08(CH3), 26.2 (CH3), 27.2 (CH3),
29.0 ((CH3)3C), 71.2 (CMe3), 101.6 (γ-C), 102.6 (γ-C), 125.2, 125.3,
125.6, 126.4, 128.8, 155.1, 157.3, 168.8 (CdN), 169.3 (CdN), 182.9
(CdO), 185.0 (CdO) ppm. Mass (EI, m/z, (%)): 533 (30%) [M]þ,
462 (20%) [M - NtBu)]þ. Anal. Calcd. for C26H33MoN3O3: C,
58.76; H, 6.26; N, 7.91%. Found: C, 58.61; H, 6.31; 7.68%.
Synthesis of [MoO(N-t-Bu)(LMePh)2] (2). The compound was
prepared following the general procedure employing 0.200 g
(0.58 mmol) of [MoO(N-t-Bu)Cl2(dme)] and 0.263 g (1.15
mmol) of KLMePh giving 0.115 g (35%) of 2. Proton NMR
spectroscopy revealed four isomers in solution: isomer A (40%):
1H NMR (benzene-d6): δ 1.06 (s, 9H, (H3C)3C), 1.32 (s, 3H,
H3C), 1.44 (s, 3H, H3C), 1.81 (s, 3H, H3C), 1.88 (s, 3H, H3C),
2.40 (s, 3H, H3C), 2.60 (s, 3H, H3C), 5.07 (s, 1H, γ-H), 5.17 (s,
1H, γ-H), 6.83-7.50 (m, H-Ar) ppm; isomer B (24%): 1H NMR
(benzene-d6): δ 1.00 (s, 9H, (H3C)3C), 1.34 (s, 3H, H3C), 1.40 (s,
3H, H3C), 1.77 (s, 3H, H3C), 1.98 (s, 3H, H3C), 2.20 (s, 3H,
H3C), 2.44 (s, 3H, H3C), 5.07 (s, 1H, γ-H), 5.13 (s, 1H, γ-H),
6.83-7.50 (m, H-Ar) ppm; isomer C (20%): 1H NMR (benzene-
d6): δ 1.03 (s, 9H, (H3C)3C), 1.30 (s, 3H, H3C), 1.46 (s, 3H, H3C),
1.89 (s, 3H, H3C), 1.92 (s, 3H, H3C), 2.40 (s, 3H, H3C), 2.44 (s,
3H, H3C), 5.04 (s, 1H, γ-H), 5.17 (s, 1H, γ-H), 6.83-7.50 (m, H-
Ar) ppm; isomer D (16%): 1H NMR (benzene-d6): δ 0.97 (s, 9H,
(H3C)3C), 1.32 (s, 3H, H3C), 1.47 (s, 3H, H3C), 1.90 (s, 3H,
H3C), 1.99 (s, 3H, H3C), 2.23 (s, 3H, H3C), 2.57 (s, 3H, H3C),
Synthesis of [MoO(N-t-Bu)(Li-Pr2Ph)2] (4). The compound
was prepared following the general procedure employing
0.200 g (0.58 mmol) of [MoO(N-t-Bu)Cl2(dme)] and 0.3451 g
1
(1.16 mmol) of KLi-Pr2Ph giving 0.114 g (28%) of 4. H NMR
(benzene-d6): δ 0.95 (s, 9H, (H3C)3C), 1.11 (d, 3H, CH(CH3)2),
1.17 (d, 3H, CH(CH3)2), 1.18 (d, 3H, CH(CH3)2), 1.20 (d, 3H,
CH(CH3)2), 1.38 (d, 3H, CH(CH3)2), 1.38 (s, 3H, H3C), 1.43 (d,
3H, CH(CH3)2), 1.51 (s, 3H, H3C), 1.67 (d, 3H, CH(CH3)2), 1.72
(d, 3H, CH(CH3)2), 1.72 (s, 3H, H3C), 1.89 (s, 3H, H3C), 3.06
(hept, 1H, CHMe2), 3.81 (hept, 1H, CHMe2), 4.02 (hept, 1H,
CHMe2), 4.08 (hept, 1H, CHMe2), 5.08 (s, 2H, γ-H), 7.08 (m,
3H, H-Ar) 7.19 (s, 3H, H-Ar) ppm. 13C NMR (benzene-d6): δ
23.8, 24.62, 24.65, 24.75, 25.5, 25.6, 25.8, 25.9, 26.0, 26.2, 26.8,
27.5, 27.6, 27.7, 28.3, 28,4, 30.7 ((CH3)3C), 72.9 (CMe3), 101.3
(γ-C), 103.6 (γ-C), 123.8, 124.0, 124.9, 125.5, 126.5, 126.7, 141.2,
143.0, 143.5, 144.0, 150.0, 155.5, 170.6 (CdN), 172.5 (CdN),
182.0 (CdO), 183.7 (CdO) ppm. Anal. Calcd. for C38H57Mo-
N3O3: C, 65.22; H, 8.21; N, 6.00%. Found: C, 65.31; H, 8.10; N,
6.22%.
Synthesis of [Mo(N-t-Bu)2(LPh)2] (5). The compound was
prepared following the general procedure employing 0.399 g
(1 mmol) of [Mo(N-t-Bu)2Cl2(dme)] and 0.427 g (2 mmol) of
KLPh giving 0.126 g (22%) of 5. 1H NMR (benzene-d6): δ 1.03 (s,
18H, (H3C)3C), 1.51 (s, 6H, H3C), 2.06 (s, 6H, H3C), 5.10 (s, 2H,
γ-H), 6.90-7.14 (m, 8H, H-Ar), 7.78 (d, 2H, H-Ar) ppm; 13C
NMR (benzene-d6): δ 23.7 (CH3), 27.0 (CH3), 29.0 ((CH3)3C),
69.6 (CMe3), 100.8 (γ-C), 124.6, 125.1, 127.0, 128.0, 128.4,
158.1, 167.7 (CdN), 182.7 (CdO). Mass (EI, m/z, (%)): 588
(12) [M]þ, 517 (5) [M - NtBu)]þ. Anal. Calcd. for C30H42Mo-
N4O2: C, 61.42; H, 7.22; N, 9.55%. Found: C, 61.34; H, 7.20; N,
9.51%.
Synthesis of [Mo(N-t-Bu)2(LMePh)2] (6). The compound was
prepared following the general procedure employing 0.399 g
(1 mmol) of [Mo(N-t-Bu)2Cl2(dme)] and 0.455 g (2 mmol) of
KLMePh giving 0.122 g (20%) of 6. NMR spectroscopy revealed
1
three isomers in solution: isomer A H NMR (benzene-d6) δ
(43%): 1.08 (s, 18H, (H3C)3C), 1.41 (s, 6H, H3C), 1.90 (s, 6H,
H3C), 2.58 (s, 6H, H3C), 5.07 (s, 2H, γ-H), 6.9 - 7.8 (m,
overlapping Ar); isomer B (46%): 1.02 (s, 9H, (H3C)3C), 1.03
(s, 9H, (H3C)3C), 1.40 (s, 3H, H3C), 1.44 (s, 3H, H3C), 1.93 (s,
3H, H3C), 2.01 (s, 3H, H3C), 2.29 (s, 3H, H3C), 2.59 (s, 3H,
H3C), 5.08 (s, 1H, γ-H), 5.09 (s, 1H, γ-H); 6.9 - 7.8 (m,
overlapping Ar); isomer C (11%): 0.99 (s, 18H, (H3C)3C), 1.43
(s, 6H, H3C), 2.03 (s, 6H, H3C), 2.28 (s, 6H, H3C), 5.12 (s, 1H,
γ-H), 6.9 - 7.8 (m, overlapping Ar). Anal. Calcd. for C32H46-
MoN4O2: C, 62.53; H, 7.54; N, 9.11%. Found: C, 62.35; H, 7.16;
N, 9.66%;
(40) Allouche, A. R. Gabedit, Graphical User Interface for computational
chemistry packages; available for free from: http://gabedit.sourceforge.net/.
(41) Hogarth, G.; Konidaris, P. C.; Saunders, G. C. J. Organomet. Chem.
1991, 406, 153–157.
(42) SAINTPLUS, Software Reference Manual, Version 6.45; Bruker-
AXS: Madison, WI, 1997-2003.
(43) Sheldrick, G. M. SADABS, Version 2.1; Bruker-AXS: Madison, WI,
1998; Acta Crystallogr., Sect. A 64, 112-122, 2008.
(44) Sheldrick, G. M. SHELXS-97, Program for Structure Solution;
Bruker-AXS: Madison, WI, 1990; Acta Crystallogr., Sect. A A46, 467-473.
(45) Sheldrick, G. M. Program for the Refinement of Crystal Structures;
Synthesis of [Mo(N-t-Bu)2(LMe2Ph)2] (7).The compound was pre-
pared following the general procedure employing 0.399 g (1 mmol)
€
€
University of Gottingen: Gottingen, Germany, 1997.

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8921
of [Mo(N-t-Bu)2Cl2(dme)] and 0.483 g (2 mmol) of KLMe2Ph giving
0.127 g (20%) of 7. 1H NMR (benzene-d6): δ 1.05 (s, 18H,
(H3C)3C), 1.36 (s, 6H, H3C), 1.90 (s, 6H, H3C), 2.46 (s, 6H,
H3C), 2.59 (s, 6H, H3C), 5.09 (s, 2H, γ-H), 6.83-7.30 (m, 6H, H-
Ar) ppm. 13C NMR (benzene-d6): δ 19.7 (CH3), 22.9 (CH3), 23.2
(CH3), 25.7(CH3), 30.2 ((CH3)3C), 71.2 (CMe3), 102.0 (γ-C), 125.4,
129.0, 129.1, 131.7, 134.8, 154.6, 169.0 (CdN), 181.8 (CdO) ppm.
Mass (EI, m/z, (%)): 644 (3) [M]þ. Anal. Calcd. for C34H50Mo-
N4O2: C, 63.54; H, 7.84; N, 8.72%. Found: C, 63.54; H, 7.82; N,
8.15%.
(benzene-d6): δ = 18.2 (PMe3), 20.0 (CH3), 20.7 (2 CH3), 22.8
(CH3), 23.1 (CH3), 23.4 (CH3), 24.1 (CH3), 25.4 (CH3), 26.2
((CH3)3C), 70.7 (CMe3), 100.0 (γ-C), 100.6(γ-C), 124.2, 124.7,
129.0, 129.2, 129.7, 130.4,134.2, 134.8,154.8, 156.0, 166.1 (CdN),
166.7 (CdN), 175.1 (CdO), 181.6 (CdO) ppm. 31P NMR
(benzene-d6): δ=6.5 (PMe3).
General Procedure for Kinetic Experiments. In a typical
experiment 30 mg of complex 3 were dissolved in 5 mL of
toluene (passed over alox) in a 3-necked Schlenk flask. Under
a stream of Argon the fiber optic probe was immersed in the
solution. Via a syringe, the required amount of PMe3 was added
and enough toluene was introduced as to keep the final volume
of the solution the same throughout all the kinetic experiments.
The measurement was started immediately after the addition.
Synthesis of [Mo(N-t-Bu)(LMe2Ph)2(PMe3)] (8). Compound
[MoO(N-t-Bu)(LMe2Ph)2] (3) (0.100 g, 0.170 mmol) was dis-
solved in 10 mL of toluene. Trimethyl phosphine (25 mg, 0.34
mmol) was added by syringe at room temperature upon which a
color change from orange to deep brown occurred. The mixture
was stirred at room temperature for 24 h, and subsequently the
solvent was removed in vacuo. Washing of the brown powder
with pentane gave the product in moderate yield (47 mg, 43%).
1H NMR (benzene-d6): δ 0.62 (d, 9H, PMe3 JPH=6.86 Hz), 0.98
(s, 3H, H3C), 1.16 (s, 9H, (H3C)3C), 1.39 (s, 3H, H3C), 1.78 (s,
3H, H3C), 1.82 (s, 3H, H3C), 2.29 (s, 3H, H3C), 2.31 (s, 3H,
H3C), 2.40 (s, 3H, H3C), 2.61 (s, 3H, H3C), 4.84 (s, 1H, γ-H),
5.31 (s, 1H, γ-H), 6.80-7.12 (m, 6H, H-Ar) ppm. 13C NMR
Acknowledgment. Financial support by the Austrian Science
Foundation FWF P19309-N19 is gratefully acknowledged.
Supporting Information Available: Crystallographic data in
cif format. Figures of UV/vis spectra for the reduction of 3
kinetic plots and data for the reduction of 1 and 3. Cyclic voltam-
mograms of 1 and 3. EI mass spectra of 1 and 3. This material is
available free of charge via the Internet at http://pubs.acs.org.