Dioxygen activation with molybdenum complexes bearing amide-functionalized iminophenolate ligands

Article abstract of DOI:10.3390/molecules24091814

Two novel iminophenolate ligands with amidopropyl side chains (HL2 and HL3) on the imine functionality have been synthesized in order to prepare dioxidomolybdenum(VI) complexes of the general structure [MoO₂L2] featuring pendant internal hydrogen bond donors. For reasons of comparison, a previously published complex featuring n-butyl side chains (L1) was included in the investigation. Three complexes (1-3) obtained using these ligands (HL1-HL3) were able to activate dioxygen in an in situ approach: The intermediate molybdenum(IV) species [MoO(PMe₃)L2] is first generated by treatment with an excess of PMe₃. Subsequent reaction with dioxygen leads to oxido peroxido complexes of the structure [MoO(O₂)L2]. For the complex employing the ligand with the n-butyl side chain, the isolation of the oxidomolybdenum(IV) phosphino complex [MoO(PMe₃)(L1) ₂] (4) was successful, whereas the respective Mo(IV) species employing the ligands with the amidopropyl side chains were found to be not stable enough to be isolated. The three oxido peroxido complexes of the structure [MoO(O₂)L2] (9-11) were systematically compared to assess the influence of internal hydrogen bonds on the geometry as well as the catalytic activity in aerobic oxidation. All complexes were characterized by spectroscopic means. Furthermore, molecular structures were determined by single-crystal X-ray diffraction analyses of HL3, 1-3, 9-11 together with three polynuclear products {[MoO(L2) ₂]₂ (μ-O)} (7), {[MoO(L2)] ₄ (μ-O) ₆} (8) and [C₉H₁₃N_2O]₄ [Mo₈O₂6] 6OPMe₃ (12) which were obtained during the synthesis of reduced complexes of the type [MoO(PMe3)L2] (4-6).

Full text of DOI:10.3390/molecules24091814

molecules
Article
Dioxygen Activation with Molybdenum
Complexes Bearing Amide-Functionalized
Iminophenolate Ligands
Niklas Zwettler, Madeleine A. Ehweiner, Jörg A. Schachner, Antoine Dupé, Ferdinand Belaj
and Nadia C. Mösch-Zanetti *
Institute of Chemistry, Inorganic Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria;
niklas.zwettler@uni-graz.at (N.Z.); madeleine.ehweiner@uni-graz.at (M.A.E.);
joerg.schachner@uni-graz.at (J.A.S.); antoine.dupe@uni-graz.at (A.D.); ferdinand.belaj@uni-graz.at (F.B.)
* Correspondence: nadia.moesch@uni-graz.at; Tel.: +43-316-380-5286
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Academic Editor: Antonella Dalla Cort
Received: 17 April 2019; Accepted: 9 May 2019; Published: 10 May 2019
Abstract: Two novel iminophenolate ligands with amidopropyl side chains (HL2 and HL3) on the
imine functionality have been synthesized in order to prepare dioxidomolybdenum(VI) complexes of
the general structure [MoO2
comparison, a previously published complex featuring n-butyl side chains (L1) was included in the
investigation. Three complexes ( ) obtained using these ligands (HL1 HL3) were able to activate
dioxygen in an in situ approach: The intermediate molybdenum(IV) species [MoO(PMe3) 2] is first
generated by treatment with an excess of PMe3. Subsequent reaction with dioxygen leads to oxido
peroxido complexes of the structure [MoO(O2) 2]. For the complex employing the ligand with the
n-butyl side chain, the isolation of the oxidomolybdenum(IV) phosphino complex [MoO(PMe3)(L1)2]
) was successful, whereas the respective Mo(IV) species employing the ligands with the amidopropyl
side chains were found to be not stable enough to be isolated. The three oxido peroxido complexes of
the structure [MoO(O2) 2] ( 11) were systematically compared to assess the influence of internal
L2] featuring pendant internal hydrogen bond donors. For reasons of
1
–3
–
L
L
(4
L
9–
hydrogen bonds on the geometry as well as the catalytic activity in aerobic oxidation. All complexes
were characterized by spectroscopic means. Furthermore, molecular structures were determined by
single-crystal X-ray diffraction analyses of HL3
,
1–3
,
9
–11 together with three polynuclear products
{[MoO(L2)2]2( -O)} ( ), {[MoO(L2)]4( -O)6} ( ) and [C9H13N2O]4[Mo8O26]
obtained during the synthesis of reduced complexes of the type [MoO(PMe3)L2] (4–6).
µ
7
µ
8
·
6OPMe3 (12) which were
Keywords: molybdenum; iminophenolate ligands; dioxygen activation; peroxido complexes
1. Introduction
Molybdenum is an earth-abundant transition metal with crucial biological relevance for most
organisms including humans [
oxygen atom transfer (OAT) enzymes [
dioxidomolybdenum(VI) complexes over the past decades, both in biomimetic chemistry as well as
in oxygenation catalysis [ 13]. A major drawback in virtually all reported catalytic systems is,
however, the necessity of a terminal oxidant [13 21]. Thus, the possibility to use environmentally
benign and cheap dioxygen (or even air) would be highly desirable [22]. Several transition metals,
including molybdenum, are known to activate dioxygen [23 32]. In our group, molybdenum(IV)
complexes based on -ketiminate and iminophenolate ligand systems were investigated regarding
their usability in dioxygen activation [33 36]. As a result, we were able to demonstrate the catalytic
aerobic oxidation of phosphanes and determine the influence of steric and electronic properties of the
1
–
3]. The molybdenum(VI) dioxido motif, which is widespread in
1,4,5], sparked the intensive investigation of mononuclear
2,6–
–
–
β
–
Molecules 2019, 24, 1814; doi:10.3390/molecules24091814
www.mdpi.com/journal/molecules

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ligands. In this context, we reported the unusual electronic properties of the first imido supported
molybdenum peroxido complex [35]. A common disadvantage of the investigated systems is, however,
the rather high stability of molybdenum(VI) oxido and peroxido complexes, which is why we are
currently investigating intra- and intermolecular additives to render these moieties more reactive. Very
recently, we were able to increase the reactivity of a terminal molybdenum oxido group by the addition
of a strong Lewis acid, leading to a reduction of the Mo=O bond with hydrosilanes [37].
Inspired by biomimetic modeling approaches focusing on the importance of the second
coordination sphere, e.g., hydrogen bonding [23,24,38–43], we started to investigate the influence of
internal hydrogen bond donors on dioxygen activation. Previous results demonstrated a beneficial effect
of such side-arm functionalized iminophenolate ligands on catalytic activity and selectivity [16]. First
attempts to introduce an amide side-arm into an iminophenolate ligand system unexpectedly resulted
in stepwise C–C and C–N coupling upon coordination at the electron poor dioxidomolybdenum(VI)
center (Figure 1) [44]. This unusual coupling behavior is presumably caused by the two electron
withdrawing moieties adjacent to the methylene carbon. We thus reasoned that an elongation of the
side chain by one methylene group should allow for the isolation of the desired complexes (Figure 1).
Figure 1. Iminophenolate ligands with amide functionalities featuring a C1 and C2 bridge, respectively,
and their behavior upon coordination to [MoO2]2+
.
Herein, we report on the synthesis and coordination chemistry of novel iminophenolate ligands
with amidopropyl functionalities, which were successfully coordinated to dioxidomolybdenum
centers to form mononuclear complexes bearing pendant amide functionalities as hydrogen bond
donors. The synthesis of such mononuclear complexes has not always been achieved since basic
conditions during the synthetic procedure often result in deprotonation of the N–H leading to
unidentified polymeric compounds [45]. The resulting compounds were investigated with regard to
the reduction of the metal center and subsequent dioxygen activation. For comparison, a previously
reported dioxidomolybdenum complex featuring n-butyl side chains was also investigated [16]. All
three dioxidomolybdenum complexes were able to activate dioxygen in an in situ approach via
molybdenum(IV) species. The formed oxido peroxido complexes were fully characterized including
single-crystal X-ray diffraction analysis. The dioxido complexes were furthermore found to be active
precatalysts in aerobic oxidation.
2. Results and Discussion
2.1. Ligand Synthesis
Ligand HL1 featuring n-butyl side chains was synthesized according to a previously published
procedure (Scheme 1) [46]. The novel amide-functionalized ligands HL2 and HL3 are accessible
starting with commercially available β-alanine in a four-step procedure as shown in Scheme 1. In the

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first step, the amino group is protected with a carboxybenzyl (Cbz) group according to published
procedures [47]. In the second step, the carboxylic acid moiety is chlorinated using ethyl chloroformate,
and subsequent reaction with a primary amine yields the tert-butyl and phenyl amide derivatives,
respectively. Deprotection with Pd/C, H2 (R = tBu) or HBr (33% in acetic acid; R = Ph) gives the desired
free aminoamides, which were subsequently condensed with 3,5-di-tert-butyl-2-hydroxybenzaldehyde
to yield the desired Schiff base ligands HL2 (R = tBu, 56% overall yield) and HL3 (R = Ph, 48%
overall yield).
Scheme 1. Synthesis of HL1
–
HL3; (a) benzyl chloroformate (1 equiv), NaOH 1 M, 0 ◦C to 25 ◦C, 16 h;
10 ◦C, 15 min; ii. R-NH2 (1 equiv in 10 mL
(b) i. ethyl chloroformate (1 equiv), Et3N (1 equiv), toluene,
−
CHCl3), toluene,
10 ◦C to 110 ◦C, 1 h; (c) Pd/C (15 w%), H2 (5 atm), MeOH, 25 ◦C, 16 h; (d) i. HBr (33%
−
in AcOH, 20 mL), 25 ◦C, 16 h; ii. NaOH 2 M; (e) 3,5-di-tert-butyl-2-hydroxybenzaldehyde (1 equiv),
MeOH, 60 ◦C, 16 h.
Analytical data for the previously reported ligand HL1 match those in literature [46]. All three
ligands HL1–
HL3 were characterized by 1H- and 13C-NMR and IR spectroscopy as well as EI-MS; for
HL3, the molecular structure was determined via single-crystal X-ray diffraction analysis. Similar
to the related amidoethyl (C1) analogues [44], HL2 and HL3 (amidopropyl, C2) feature a significant
difference in the shift of the amide NH proton depending on the R’ substituent (4.86 ppm in HL2 and
6.89 ppm in HL3, tert-butyl amide vs. phenyl amide, C6D6).
2.2. Synthesis of Dioxidomolybdenum(VI) Complexes
For the synthesis of molybdenum(VI) complexes of the type [MoO2
using [MoO2(acac)2] [48], [MoO2Cl2] or [MoO2Br2(DMSO)2] [49] as metal source were investigated.
Alternative to the literature [16], complex [MoO2(L1)2] ( ) was synthesized via a reaction of two equiv
of HL1 with the metal precursor [MoO2Br2(DMSO)2] in the presence of excess NEt3 in good yield.
Complex [MoO2(L2)2] ( ) was synthesized following a similar procedure using [MoO2Cl2] and two
equiv HL2 in the presence of excess NEt3. The compound was also accessible in good yield by reacting
two equiv of HL2 with [MoO2(acac)2]. Complex [MoO2(L3)2] ( ) was synthesized in good yield
L2], synthetic procedures
1
2
3
via reaction of [MoO2Cl2] with two equiv of HL3. Synthesis attempts starting from [MoO2(acac)2]
resulted in the formation of the monosubstituted complex [MoO2(acac)(L3)] (3acac) instead. This
statement is supported by 1H-NMR spectroscopy featuring resonances for one acetylacetonate and one
iminophenolate ligand, as well as single-crystal X-ray diffraction analysis (vide infra). The formation
of such a monosubstituted complex was previously observed with a phenyl-imine based ligand [36].
All complex syntheses are summarized in Scheme 2.

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Scheme 2. Synthesis of complexes 1–3.
Complexes
trans positions) and an asymmetric isomer (N,O) in solution, with approximate isomeric ratios of 2:1
, C6D6) and 3:1 ( and , C6D6). The N,N isomer is electronically favored while large groups at N
lead to the O,O isomer as observed in a system with a iminophenolate ligand with a tert-butyl group
at nitrogen [34]. However, in solid state, complexes exhibit exclusively the N,N arrangement
(vide infra), which is in good agreement with previous observations [16 36]. Complexes and
1–3 are in a dynamic equilibrium between a symmetric (N,N with respect to the oxido
(1
2
3
1–3
,
2
3
feature the desired dangling arms with intramolecular amide hydrogen bond donors, confirmed by
the broad singlet resonances in the 1H-NMR spectra. In addition, 1H-NMR spectroscopy reveals a
pronounced downfield shift of the resonances for the amide proton in comparison to the free ligand
(e.g., 6.12 vs. 4.86 ppm,
2 vs. HL2, C6D6), presumably indicating hydrogen bonding to an oxido ligand
in solution [50 53]. The bidentate coordination mode of L2 and L3 is in agreement with previous
–
observations for iminophenolate complexes with or without additional donor atoms, whereas related
aminophenolate ligands coordinate in a facial tridentate fashion due to their higher coordinative
flexibility [15,16,18].
As expected, complexes
common laboratory solvents including aliphatic hydrocarbons, complex
hydrocarbons, benzene and toluene, but shows poor solubility in MeCN and MeOH. Complex
well soluble in CH2Cl2, but exhibits only limited solubility in other standard laboratory solvents. The
1
–3
are sensitive to moisture. Whereas complex
is well soluble in chlorinated
is
1 is well soluble in most
2
3
composition as well as the (mononuclear) structure of complexes 1–3 was confirmed by spectroscopic
measurements (1H- and 13C-NMR, IR), mass spectrometry, elemental analysis and single-crystal X-ray
diffraction analysis (vide infra).
2.3. Reduction of Complexes 1–3 and Activation of Dioxygen
Reaction of [MoO2(L1)2] (
coordinated Mo(IV) oxido compound [MoO(PMe3)(L1)2] (
1H-NMR resonances for
, also compound features signals for two species, reflected by two distinct
sets of resonances for two ligands each. Additionally, two distinct resonances for coordinated PMe3
are observed in the 31P{1H}-NMR spectrum. The isomeric ratio for compound
is approximately
4:1, with the major isomer probably adopting the O,O conformation, which is in accordance with
1
) with five equiv of PMe3 resulted in the formation of the phosphane
4) in good yield (Scheme 3). Similar to the
1
4
4

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previous reports [36]. Addition of five equiv of PMe3 to complexes
2
or
3
6
led to a similar mixture
1
of isomers of compounds [MoO(PMe3)(L2)2] (
spectroscopy revealed for also two and for
data point to the actual formation of and
purification attempts led to the formation of several new, presumably polynuclear species together
with free ligand. Nevertheless, during such attempts with , two types of single crystals were obtained
revealing the formation of a -oxido bridged molybdenum(V) dimer, {[MoO(L2)2]2( -O)} ( ), which
is similar to previous observations [36], as well as a tetranuclear -oxido bridged molybdenum(VI)
cluster, {[MoO(L2)]4( -O)6} ( ), as depicted in Figure 2. We attribute the increased instability and
the resulting decomposition to the additional acidic functionalities in the ligand side chains. The
tetranuclear structure of has not been described before in molybdenum coordination chemistry; only
5) and [MoO(PMe3)(L3)2] (
). H- and 31P{1H}-NMR
5
6
at least three isomers in solution. Although these
5
6, we were unable to isolate them in pure form. Any
5
µ
µ
7
µ
µ
8
8
a tungsten derivative is known [54]. In contrast, the structural motifs of
common [36,55–57].
7 and 12 (vide infra) are quite
Figure 2. Polynuclear compounds
7 and 8 forming during reduction of 2 with PMe3, and 12 forming
during reduction of 3 with PMe3 under O2 atmosphere.
Complex
4 is very well soluble in most common organic solvents including aliphatic hydrocarbons.
The compound is highly sensitive to moisture and air and tends to slowly decompose in solution.
The composition of
Compound
conditions. Compounds
were only characterized via single-crystal X-ray diffraction analysis.
4
was additionally confirmed via FT-IR spectroscopy and elemental analysis.
1
5
was characterized in situ via H-, 13C- and 31P{1H}-NMR spectroscopy under inert
7
and 8 were only obtained in small quantities of single crystals and thus
Scheme 3. Oxygen atom transfer (OAT) reactivity of complexes
1–3 and activation of O2 to form oxido
peroxido compounds 9–11. For clarity, only the major isomer is depicted for all complexes.
The phosphane coordinated complex
quick color change from red-brown to orange-red, to form the desired oxido peroxido compound
[MoO(O2)(L1)2] ( ) in excellent yield. Alternatively, is also accessible in a one-pot reaction via the
4 cleanly reacts with dioxygen, which is indicated by a
9
9

Molecules 2019, 24, 1814
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addition of three equiv of PMe3 to a toluene solution of
1
and subsequent stirring under O2 atmosphere
for 18 h, which is similar to previous reports [34
species from the corresponding molybdenum dioxido complex was also investigated for complexes
and (Scheme 3). For the synthesis of [MoO(O2)(L2)2] (10), the same protocol as described above could
,36]. Such a direct formation of the oxido peroxido
2
3
be applied. A noteworthy difference, however, was the work-up procedure, since removal of residual
OPMe3 via an extraction protocol using benzene or alkanes resulted in the isolation of a 1:1 mixture of 10
with OPMe3. This mixture was subsequently identified via single-crystal X-ray diffraction as 10·OPMe3
in which OPMe3 is hydrogen-bonded to the amide (vide infra). Nonetheless, recrystallization of the
crude reaction product from the coordinating solvent MeCN yielded microcrystalline 10 in good yield
as well as single crystals of this OPMe3-free compound. For the formation of [MoO(O2)(L3)2] (11), an
analogous reaction at room temperature was examined. However, only a slow reaction progress as
well as side product formation (presumably a dimeric compound) was observed. An increase of the
reaction temperature to 50 ◦C led to the formation of the desired compound and additionally allowed
for a shorter reaction (6 h). Similar to 10, after work-up, a 1:1 mixture of 11 and OPMe3 was observed.
In contrast to 10, however, recrystallization from MeCN did not remove the OPMe3, presumably
because of the more acidic amide N-H groups and thus stronger hydrogen bonds. Compound 11
was thus isolated as 11·OPMe3 in fair yield. Slow evaporation of a concentrated MeCN solution of
11·OPMe3 led to single crystals of two distinct adducts of 11
as a small quantity of crystals, which could be identified as a hydrolysis/decomposition product of
the formula [C9H13N2O]4[Mo8O26] 6OPMe3 (12, Figure 2). The sensitivity of 11 towards hydrolysis
is noteworthy, since it is in contrast to previously investigated molybdenum(VI) oxido peroxido
(
11·OPMe3 and 11·2OPMe3) as well
·
complexes [35,36]. Complex 9 exists as a mixture of two isomers in solution, with an isomeric ratio
of >10:1, as determined by 1H-NMR spectroscopy; for complexes 10 and 11, only a single isomer is
observed. All three complexes were found to adopt the O,O isomer in solid state via single-crystal
X-ray diffraction analysis and, based on spectroscopic evidence, the isomeric conformation is the same
in solution, which is in agreement with previous observations [14
1H-NMR resonances for the amide protons exhibit a slight upfield shift in comparison to [MoO2(L2)2]
), which is in contrast to the solid state structures (vide infra), where no hydrogen bond is observed
in but a weak interaction to the peroxido group in 10. This finding suggests generally flexible and
weak interactions in our complexes. The amide resonances in 11 are shifted downfield in comparison
to , which is probably caused by hydrogen bonding to OPMe3. Complexes 11 are well soluble in
most common laboratory solvents and moderately soluble in alkanes. Whereas complexes and 10 are
,33,34,36]. In compound 10, the
(2
2
3
9–
9
stable to air and only slightly sensitive to moisture, complex 11 tends to decompose within days upon
exposure to moisture.
2.4. Molecular Structures
Molecular structures of the ligand HL3, molybdenum(VI) dioxido complexes
molybdenum(VI) oxido peroxido complexes 10
10·OPMe3, 11·OPMe3, 11·2OPMe3 and the side
products and 12 were determined by single-crystal X-ray diffraction analysis. Molecular views of
are given in Figure 3, molecular views of 10 and 11(OPMe3) in Figure 4 and the intermolecular
1–3, as well as
9
,
,
7, 8
1
–3
9,
hydrogen bonding in the phosphane oxide adducts of 10·OPMe3 and 11·2OPMe3 in Figure 5. The
molecular views of the ligand HL3, the monosubstituted complex 3acac as well as the polynuclear
compounds
lengths and angles for complexes
in Table 2. Full crystallographic details such as structure refinement as well as experimental details are
provided within the Supplementary Materials.
7
,
8
and 12 are depicted in Figures S2–S6 in the Supplementary Materials. Selected bond
are provided in Table 1 and for complexes
1–3
9,
10 and 11·2OPMe3

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Figure 3. Molecular views (50% probability level) of the dioxidomolybdenum(VI) complexes
[MoO2(L1)2]15
, top left), [MoO2(L2)2] ( , top right) and [MoO2(L3)2] ( , bottom). H atoms, except
amide protons, as well as solvent molecules are omitted for clarity reasons.
(1
2
3
In complexes 1–3, the molybdenum atoms are coordinated in a distorted octahedral fashion by
two bidentate ligands and two oxido ligands, with the iminophenolate ligands adopting the N,N
isomeric form with respect to the oxido trans positions. Hydrogen bonding from the pendant amide
functionalities to the oxido ligands is not observed. The bond lengths to all ligands are very similar in
complexes
1–3; only the Mo–N bonds are slightly shorter in complex 3. The Mo=O bond lengths are
within expected ranges (Table 1) [58].
Table 1. Selected bond lengths (Å) and angles (◦) for the dioxidomolybdenum(VI) complexes 1–3.
1
2
3
Mo1–O1
Mo1–O2
Mo1–O11
Mo1–O21
Mo1–N17
1.705(3)
1.712(3)
1.952(3)
1.955(3)
2.351(2)
2.341(4)
106.80(16)
160.48(11)
73.30(12)
1.7078(10)
-
1.9571(10)
-
2.3548(12)
-
107.62(7)
157.63(6)
73.42(6)
1.708(3)
-
1.950(2)
-
2.333(3)
-
108.0(2)
159.49(16)
74.78(16)
Mo1–N27
O1–Mo1–O2
O11-Mo1-O21
N17-Mo1-N27

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Figure 4. Molecular views (50% probability level) of the oxido peroxido molybdenum(VI) complexes
[MoO(O2)(L1)2] ( , left), [MoO(O2)(L2)2] (10, middle) and [MoO(O2)(L3)2] (11, right). H atoms, except
amide protons, as well as solvent molecules and OPMe3 (in 11) are omitted for clarity reasons.
9
In compounds
ligands, an oxido ligand and a
9
–
11, the molybdenum centers are coordinated by two bidentate iminophenolate
2 side-on coordinated peroxido ligand in a distorted octahedral fashion.
η
All three compounds adopt the O,O isomeric form in solid state, with the phenolate oxygen atoms trans
to the oxido and peroxido ligand, respectively. The change of the isomeric form, with respect to the
parent complexes 1–3, is reflected by the N17-Mo1-N27 and O11-Mo-O21 bond angles. Additionally,
the Mo–N bonds are shortened by approx. 0.15 Å on average, whereas the Mo–O (phenolate) bonds are
slightly elongated, caused by the trans influence of the oxido (and peroxido) ligands. The bonds to the
bidentate ligands as well as the Mo=O bonds are of similar length in all oxido peroxido complexes in
the observed modifications. Comparison of the Mo–(O–O) bond lengths of all three complexes shows
a rather small influence of the ligand. The bond lengths in 10 follow the expected trend in comparison
to 9, namely a shortening of the Mo–(O–O) bonds as well as a slight elongation of the (O–O) bond due
to the electron richer tert-butyl system. The shorter (O–O) bond in complex 11 could then be explained
by the rather electron withdrawing character of the phenyl system.
Table 2. Selected bond lengths (Å) and angles (◦) for the oxido peroxido molybdenum(VI) complexes
10 and 11·2OPMe3.
9,
9
10
11·2OPMe3
Mo1–O1
Mo1–O2
Mo1-O3
Mo1–O11
Mo1–O21
Mo1–N17
Mo1–N27
O2–O3
O1–Mo1–O2
O1–Mo1–O3
O2–Mo1–O3
O11-Mo1-O21
N17-Mo1-N27
1.6817(18)
1.9573(19)
1.9469(16)
2.0353(10)
2.0113(10)
2.1670(14)
2.2053(14)
1.431(3)
101.97(9)
99.60(8)
43.01(7)
80.51(4)
159.62(5)
1.6959(13)
1.9398(13)
1.9651(14)
2.0256(13)
2.0283(12)
2.1542(15)
2.1818(15)
1.4426(18)
100.50(6)
98.18(6)
1.698(3)
1.961(4)
1.945(3)
2.014(3)
2.036(3)
2.167(4)
2.189(4)
1.406(4)
99.89(15)
98.13(14)
42.21(13)
82.21(12)
161.32(13)
43.35(5)
81.56(5)
161.89(6)

Molecules 2019, 24, 1814
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Figure 5. Molecular views (50% probability level) of the hydrogen bonds in the oxido peroxido complex
adducts {[MoO(O2)(L2)2]2(OPMe3)2} (10·OPMe3, top) and {[MoO(O2)(L3)2](OPMe3)2} (11·2OPMe3,
bottom). H atoms, except amide protons, as well as solvent molecules are omitted for clarity reasons.
The amido hydrogen atoms of compounds 10 and 11 are in principle involved in hydrogen
bonding. Since the latter was only isolable with additional OPMe3 molecules, hydrogen bonding to
the oxygen atom of the phosphine occurred exclusively (Figure 5, bottom). In compound 10, a weak
intramolecular hydrogen bond from the amide H to one O atom of the peroxido moiety is formed
(Table S18: N29-H29-O3, D-H···A: d(D-H) 0.88 Å, d(H···A) 2.277(7) Å, d(A-D) 3.0718(19) Å, <(DHA)
150.1(12)◦). However, comparison of the O–O bond length in 10 to that of
9 (without hydrogen bond)
reveals little influence (Table 2). While the pending substituent is able to rotate into the right position
or distance to the peroxido ligand, the flexibility seems to be too high for effective hydrogen bonding in
the desired fashion. However, hydrogen bonding might facilitate reactivity by bringing the substrate
in close proximity to the peroxido group.
2.5. Catalysis
The catalytic aerobic oxidation of trimethyl phosphane as a widespread model reaction using
complexes 1–3, according to Equation (1), was investigated. Used conditions were 1 mol% catalyst
under 1 atm of dry O2 gas in C6D6 in 100 mL Schlenk flasks. Conversions were determined after
24 h by 31P{1H}-NMR spectroscopy. Although PMe3 is easily oxidized without any metal in presence
of water and air, the stability of PMe3 against dry O2 is high as blank experiments under identical
conditions but without catalysts led to a conversion of PMe3 <5%.

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(1)
The highest conversion of trimethyl phosphane was observed for catalyst
2
, bearing tert-butyl
amide functionalities, with a phosphane oxide yield of 53% accompanied by 4% of the previously
observed side product methyl dimethylphosphinate (Table 3) [36]. Catalyst , without hydrogen bond
1
donors, selectively oxidized 46% of PMe3 to OPMe3 within 24 h. Both values are comparable to other
molybdenum dioxido systems [36]. This is in line with the formation of a weak hydrogen interaction,
which is not sufficient to increase the reactivity. Interestingly, complex 3, equipped with a phenyl
amide group, led only to 25% conversion (23% OPMe3). This finding is corroborated by the solid-state
structures where the OPMe3 molecule is retained by hydrogen bonding thereby decreasing catalytic
reactivity. Another possibility is the formation of polynuclear compounds during the reaction, as
observed in the synthesis of the peroxido complex 11 (vide supra). With more challenging substrates
such as cyclooctene or also SMe2 no conversion was observed.
Table 3. Yield after 24 h in the aerobic oxidation of PMe3.
Product
-
1
2
3
OPMe3
OP(OMe)Me2
<5%
-
46%
-
53%
4%
23%
2%
Conditions: 1 mol% catalyst, rt, C6D6, excess O2.
3. Experimental Section
3.1. General
Unless specified otherwise, experiments were performed under inert conditions using standard
Schlenk equipment. Chemicals were purchased from commercial sources and used as received. No
further purification or drying operations have been performed. The metal precursors [MoO2(acac)2] [48
]
and [MoO2Br2(DMSO)2] [59] as well as the compound 3-(((benzyloxy)carbonyl)amino)propanoic
acid [47] were synthesized according to published procedures. Solvents were purified via a Pure–Solv
MD–4–EN solvent purification system from Innovative Technology, Inc. (Oldham, UK). Methanol was
refluxed over activated magnesium for at least 24 h and then distilled prior to use. The 1H-, 13C-, 31P-
and HSQC-NMR spectra were recorded on a Bruker Avance III instrument at 300/75/121 MHz (Bruker,
Billerica, MA, USA). Peaks are denoted as singlet (s), broad singlet (bs), doublet (d), doublet of doublets
(dd), doublet of triplets (dt), triplet (t), triplet of doublets (td) and multiplet (m). Used solvents and
peak assignment are mentioned at the specific data sets. Electron impact mass spectroscopy (EI-MS)
measurements have been performed with an Agilent 5973 MSD mass spectrometer with push rod
(Agilent Technologies Inc., Santa Clara, CA, USA). Peaks are denoted as cationic mass peaks, and
the unit is the according ion’s mass/charge ratio. Gas chromatography mass spectroscopy (GC–MS)
measurements have been performed with an Agilent 7890 A gas chromatograph (column type, Agilent
19091J-433, Agilent Technologies Inc., Santa Clara, CA, USA), coupled to an Agilent 5975 C mass
spectrometer. Samples for infrared spectroscopy were measured on a Bruker Optics ALPHA FT-IR
spectrometer (Bruker, Billerica, MA, USA). IR bands are reported with wavenumber (cm−1) and
intensities (s, strong; m, medium; w, weak). All elemental analyses were measured at the University of
Technology of Graz, Institute of Inorganic Chemistry using an Elementar vario MICRO cube analyzer
(Elementar, Langenselbold, Germany).

Molecules 2019, 24, 1814
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3.2. X–ray Diffraction Analyses
Single-crystal X-ray diffraction analyses were measured on a BRUKER–AXS SMART APEX II
diffractometer equipped with a CCD detector (Bruker-AXS, Karlsruhe, Germany). All measurements
were performed using monochromatized Mo Kα radiation from an Incoatec microfocus sealed
tube at 100 K (Tables S1 and S2). Absorption corrections were performed semi–empirical from
equivalents. Structures were solved by direct methods (SHELXS-97) [60] and refined by full–matrix
least–squares techniques against F2 (SHELXL-2014/6) [61]. CCDC numbers 1865601–1865613 contain
the supplementary crystallographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Full
experimental details for single-crystal X-ray diffraction analyses of all compounds are provided in the
Supplementary Materials.
3.3. Ligand Syntheses
All ligands are stable towards air but slightly sensitive towards moisture. They can be stored in a
desiccator over P2O5 for several weeks without decomposition.
3.3.1. Synthesis of 2,4-Di-tert-butyl-6-((butylimino)methyl)phenol (HL1)
Ligand HL1 was synthesized according to literature procedure [46]. Analytical data are consistent
with literature, 1H- and 13C-NMR shifts in ◦C6D6 as well as IR absorption bands are given for comparison
reasons. 1H-NMR (300 MHz, C6D6, 25 C)
δ: 14.33 (s, 1H, OH), 7.82 (s, 1H, CH=N), 7.58 (d, 1H,
ArH), 7.00 (d, 1H, ArH), 3.15 (td, 2H, CH2), 1.67 (s, 9H, tBu), 1.43–1.33 (m, 2H, CH2), 1.35 (s, 9H, tBu),
1.26–1.14 (m, 2H, CH2), 0.79 (t, 3H, CH3) ppm; 13C-NMR (75 MHz, C6D6, 25 ◦C)
: 165.89 (C=N), 159.00
(Ar-O), 140.03, 137.17, 126.78, 126.25, 118.72 (Ar), 59.40 (CH2), 35.48, 34.33 (q-tBu), 33.25 (CH2), 31.81,
29.83 (tBu), 20.63 (CH2), 13.98 (CH3) ppm. IR (ATR, cm−1
: 3316 (w, OH + NH), 2955 (w, C-H), 1668
δ
)
δ
(m, C=O), 1629 (m, C=N), 1600 (s), 1543 (s), 1439 (s), 1310 (m), 1249 (m), 828 (m), 755 (s), 693 (s), 503
(m); IR (ATR, cm−1) : 3437 (br w, OH), 2955 (s, C-H), 1632 (s, C=N), 1466 (s), 1440 (s), 1361 (m), 1273
ῦ:
(s) 1251 (s), 1173 (s), 827 (m), 772 (m).
3.3.2. Synthesis of N-(tert-Butyl)-3-((3,5-di-tert-butyl-2-hydroxybenzylidene)amino)-
propanamide (HL2)
For the synthesis of HL2, a three-step procedure was applied. In the first step, 1 equiv of
3-(((benzyloxy)carbonyl)amino)propanoic acid (10.00 g, 44.8 mmol) and 1 equiv Et3N (6.2 mL,
44.8 mmol) were dissolved in 100 mL of dry toluene and subsequently cooled to
bath). Subsequently, 1 equiv of ethyl chloroformate (4.4 mL, 44.8 mmol) was added dropwise over
30 min while keeping the temperature below
10 ◦C. After stirring of the reaction solution at 12 ◦C
for 30 min, 1 equiv of tert-butyl amine (4.7 mL, 44.8 mmol) in 10 mL dry dichloromethane was added
dropwise over 15 min while keeping the temperature below
10 ◦C. Subsequently, the reaction mixture
was stirred for 30 min at
12 ◦C, warmed to room temperature and finally stirred under reflux for
−
12 ◦C (ice/NaCl
−
−
−
−
10 min. The reaction was then stirred at room temperature overnight. The subsequent work-up was
performed at ambient conditions. After addition of 200 mL chloroform, the reaction mixture was
washed two times with saturated aqueous NaHCO3 solution and deionized water (100 mL each). The
organic phase was dried over MgSO4 and evaporated in vacuo. The crude product was recrystallized
from ethyl acetate to yield benzyl (3-oxo-3-(phenylamino)propyl)carbamate as a white crystalline solid
(68%, 8.53 g).
In the second step, 1 equiv of benzyl (3-oxo-3-(phenylamino)propyl)carbamate (4.67 g, 17.0 mmol)
was dissolved in 100 mL of MeOH. Subsequently 15 w% Pd/C (0.70 g) were added and the reaction
mixture stirred under an H2 atmosphere (5 atm) overnight at room temperature. Filtration and
evaporation of the filtrate in vacuo gave 3-amino-N-(tert-butyl)propanamide as light yellow oil (94%,
2.28 g). In the final step, 1.1 equiv of 3-amino-N-(tert-butyl)propanamide (1.00 g, 6.9 mmol) were

Molecules 2019, 24, 1814
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added to a solution of 1 equiv of 3,5-di-tert-butyl-2-hydroxybenzaldehyde (1.48 g, 6.3 mmol) in 5 mL of
MeOH. The resulting deep yellow solution was subsequently stirred at 50 ◦C for 3 h. Evaporation of
all volatiles in vacuo led to a deep yellow oil that solidified over the course of several days. Washing of
the crude product twice with 5 mL of pentane gave HL2 as bright yellow solid (88%, 1.99 g, overall
1
yield 56%). H-NMR (300 MHz, C6D6, 25 ◦C)
δ
: 14.06 (s, 1H, OH), 7.90 (s, 1H, CH=N), 7.56 (d, 1H,
ArH). 6.96 (d, 1H, ArH), 4.86 (bs, 1H, NH), 3.54 (t, 2H, CH2), 1.93 (t, 2H, CH2), 1.66 (s, 9H, tBu), 1.30 (s,
9H, tBu), 1.21 (s, 9H, tBu) ppm; 13C-NMR (75 MHz, C6D6, 25 ◦C)
δ: 169.57 (C=O), 167.33 (C=N), 158.72
(Ar-O), 140.23, 136.92, 126.93, 126.58, 118.62 (Ar), 55.81 (CH2), 50.99 (q-tBu), 38.68 (CH2), 35.42, 34.29
(q-tBu), 31.74, 29.81, 28.74 (tBu) ppm; EI-MS: m/z 360.4 ([M]+); IR (ATR, cm−1) : 3322 (w, OH + NH),
ῦ:
2952 (m, C-H), 1639 (s, C=O), 1629 (s, C=N), 1541 (s), 1438 (s), 1390 (m), 1276 (m).
3.3.3. Synthesis of 3-((3,5-Di-tert-butyl-2-hydroxybenzylidene)amino)-N-phenylpropan-amide (HL3
)
For the synthesis of HL3, a two-step procedure was applied. In the first step,
3-amino-N-phenylpropanamide was synthesized following a published procedure (yield: 58%) [62].
In the second step, 1 equiv of 3-amino-N-phenylpropanamide (1.45 g, 8.8 mmol) was added to a
solution of 1 equiv of 3,5-di-tert-butyl-2-hydroxybenzaldehyde (2.07 g, 8.8 mmol) in 20 mL of MeOH.
The resulting yellow solution was stirred at reflux temperature overnight and subsequently evaporated
in vacuo. The crude product was washed thoroughly with pentane to give HL3 as pale yellow solid
(84%, 2.83 g, 48% overall yield). Crystals suitable for single-crystal X-ray diffraction analysis were
obtained via crystallization from a concentrated C6D6 solution of HL3 at 25 ◦C. 1H-NMR (300 MHz,
◦
C6D6, 25 C)
δ: 14.02 (s, 1H, OH), 7.91 (s, 1H, CH=N), 7.56 (d, 1H, ArH). 7.56–7.50 (m, 2H, ArH),
7.12–7.04 (m, 2H, ArH), 6.95 (d, 1H, ArH), 6.89–6.82 (m, 1H, ArH), 6.69 (bs, 1H, NH), 3.54 (t, 2H, CH2),
2.01 (t, 2H, CH2), 1.65 (s, 9H, tBu), 1.30 (s, 9H, tBu), 1.21 (s, 9H, tBu) ppm; 13C-NMR (75 MHz, C6D6,
25 ◦C)
δ: 168.69 (C=O), 167.80 (C=N), 158.75 (Ar-O), 140.34, 138.64, 137.00, 129.11, 127.14, 126.68, 124.25,
120.10, 118.57 (Ar), 55.18 38.23 (CH2), 35.43, 34.29 (q-tBu), 31.71, 29.79 (tBu) ppm; EI-MS: m/z 380.4
([M]+); IR (ATR, cm−1) : 3316 (w, OH + NH), 2955 (w, C-H), 1668 (m, C=O), 1629 (m, C=N), 1600 (s),
ῦ:
1543 (s), 1439 (s), 1310 (m), 1249 (m), 828 (m), 755 (s), 693 (s), 503 (m).
3.4. Complex Syntheses
All complexes except
in a desiccator over P2O5 for several weeks without decomposition. Complex
4
–
6
are stable towards air but sensitive towards moisture and can be stored
is sensitive towards air
4
and moisture and can be stored in an N2-filled glovebox for several weeks without decomposition;
compounds 5 and 6 could not be isolated due to substantial decomposition during work-up.
3.4.1. Alternative Synthesis of [MoO2(L1)2] (1)
For the synthesis of
1, 1 equiv of [MoO2Br2(DMSO)2] (610 mg, 1.37 mmol) was suspended in
dry toluene (10 mL). Subsequently, 2 equiv of HL1 (800 mg, 2.74 mmol) as well as 2.5 equiv of NEt3
(48 µ
L, 3.43 mmol) were added and the deep orange reaction mixture stirred at 60 ◦C for 18 h. The
orange reaction mixture was subsequently concentrated in vacuo and stored at –25 ◦C for 24 h. The
precipitate was filtered off and washed with little dry toluene and subsequently twice with dry pentane
to give
1 as a yellow microcrystalline powder (57%, 550 mg). Crystals suitable for single-crystal X-ray
diffraction analysis were obtained via crystallization from the residual filtrate at 5 ◦C (1·NEt3
·
HBr).
Analytical data is consistent with the literature [16], 1H- and 13C-NMR shifts in C6D6 as well as IR
data is given for comparison reasons. H-NMR (300 MHz, C6D6, 25 ◦C, major isomer)
1
δ
: 7.85 (s, 2H,
CH=N), 7.72 (d, 2H, ArH), 7.12 (d, 2H, ArH), 3.50–3.29 (m, 4H, CH2), 1.66 (s, 18H, tBu), 1.66–1.16 (m,
4H, CH2), 1.30 (s, 18H, tBu), 1.05–0.98 (m, 4H, CH2), 0.69 (t, 6H, CH3) ppm; 13C-NMR (75 MHz, C6D6,
25 ◦C, major isomer, 1 Ar concealed by solvent)
δ: 167.15 (CH=N), 161.15 (Ar–O), 142.04, 139.60, 129.64,
122.16 (Ar), 60.32 (CH2), 35.60, 34.40 (q-tBu), 33.68 (CH2), 31.61, 30.35 (tBu), 20.68 (CH2), 13.84 (CH3)
ppm; IR (ATR, cm−1): 2957 (m, C-H), 1628 (s, C=N), 1439 (m), 1270 (s), 1249 (s), 917 (m, Mo=O), 904
ῦ:
(s, Mo=O), 844 (s), 754 (m), 552 (s, Mo-O), 431 (m).

Molecules 2019, 24, 1814
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3.4.2. Synthesis of [MoO2(L2)2] (2)
For the synthesis of 2, 2 equiv of HL2 (150 mg, 0.42 mmol) as well as 1 equiv of [MoO2(acac)2]
(68 mg, 0.21 mmol) were dissolved in dry MeOH (5 mL). The deep orange reaction mixture was heated
to 50 ◦C and stirred for 18 h, whereupon a bright yellow precipitate formed. The mixture was cooled
to 5 ◦C for 2 h and the precipitate subsequently filtered off and washed with a small portion of cold
dry MeOH as well as cold dry pentane. Drying in vacuo gave 2 as a bright yellow microcrystalline
solid (65%, 116 mg). Crystals suitable for single-crystal X-ray diffraction analysis were obtained via
recrystallization from a concentrated MeCN solution at
25 ◦C, major isomer (symm.))
−
35 ◦C (2·2MeCN). 1H-NMR (300 MHz, C6D6,
δ
: 8.02 (s, 2H, CH=N), 7.67 (d, 2H, ArH), 7.12 (d, 2H, ArH), 6.12 (bs,
2H, NH), 3.70–3.60 (m, 2H, CH2), 3.57–3.42 (m, 2H, CH2), 3.12–2.99 (m, 2H, CH2), 2.23–2.12 (m, 2H,
CH2), 1.56 (s, 18H, tBu), 1.26 (s, 18H, tBu), 1.04 (s, 18H, tBu) ppm; 1H-NMR (300 MHz, C6D6, 25 ◦C,
minor isomer (asymm.))
δ: 8.22 (s, 1H, CH=N), 7.80 (s, 1H, CH=N), 7.63–7.61 (m, 2H, ArH), 7.01
(“d”, 2H, ArH), 6.05 (bs, 1H, NH), 5.80 (bs, 1H, NH), 4.47–4.38 (m, 1H, CH2), 3.98–3.85 (m, 1H, CH2),
3.57–3.42 (m, 1H, CH2), 3.38–3.26 (m, 1H, CH2), 2.56–2.40 (m, 2H, CH2), 2.18–2.07 (m, 1H, CH2), 1.64
(s, 9H, tBu), 1.30 (s, 9H, tBu), 1.22 (s, 9H, tBu), 1.21 (s, 9H, tBu), 1.15 (s, 9H, tBu), 1.04 (s, 9H, tBu),
ppm; 13C-NMR (75 MHz, C6D6, 25 ◦C, major isomer (symm.))
δ: 169.42 (C=O), 168.29 (CH=N), 159.13
(Ar-O), 142.87, 139.64, 130.17, 129.60, 121.69 (Ar), 58.89 (CH2), 50.80 (q-tBu), 40.31 (CH2), 35.60, 34.40
(q-tBu) 31.50, 30.65, 28.52 (tBu) ppm; IR (ATR, cm−1) : 3375 (w, NH), 2961 (m, C-H), 1676 (m, C=O),
ῦ:
1628 (C=N, s), 1524 (m), 1442 (m), 1361 (m), 1248 (s), 912 (sh, s, Mo=O), 898 (s, Mo=O), 842 (s), 752
(m), 549 (s, Mo-O), 483 (m), 436 (m); EI-MS (70 eV) m/z: 848.7 [M]+, 832.7 [M
C44H70MoN4O6·0.5CH3OH: C, 61.93; H, 8.41; N, 6.49. Found: C, 61.61, H, 8.01; N, 6.73%.
−
O]+; Anal. calcd. for
3.4.3. Synthesis of [MoO2(L3)2] (3)
For the synthesis of 3, 2 equiv of HL3 (765 mg, 2.01 mmol) were suspended in a small portion of
dry MeOH and mixed with 2.5 equiv of NEt3 (0.35 mL, 2.52 mmol). Subsequently, a solution of 1 equiv
[MoO2Cl2] (200 mg, 1.01 mmol) in the same solvent (5 mL) was added dropwise to the mixture. The
orange-red reaction mixture was stirred for 18 h whereupon yellow solids precipitated. The mixture
was stored at
−
25 ◦C overnight and the precipitate subsequently filtered off and washed with small
portions of dry MeOH and n-pentane. Removal of all volatiles in vacuo yielded
3
as a bright yellow
solid (66%, 638 mg). Crystals suitable for single-crystal X-ray diffraction analysis were obtained via
recrystallization from a concentrated MeOH solution at
25 ◦C, major isomer (symm.))
−
35 ◦C (3·4MeOH). 1H-NMR (300 MHz, C6D6,
δ
: 8.09 (s, 2H, NH), 7.91 (s, 2H, CH=N), 7.61 (d, 2H, ArH), 7.43–7.36 (m,
4H, ArH), 6.94–6.67 (m, 8H, ArH), 3.73–3.61 (m, 2H, CH2), 3.48–3.37 (m, 2H, CH2), 3.13–2.97 (m, 2H,
CH2), 2.35–2.24 (m, 2H, CH2), 1.57 (s, 18H, tBu), 1.12 (s, 18H, tBu) ppm. 13C-NMR (HSQC 300/75 MHz,
C6D6, 25 ◦C, major isomer (symm.), q-C obscured)
δ: 168.72 (CH=N), 130.89, 128.64, 128.51, 123.69,
119.38 (Ar), 59.26, 40.29 (CH2), 31.02, 30.42 (tBu); IR (ATR, cm−1): : 3320 (m, NH), 2952 (m, C-H), 1666
(s, C=O), 1619 (s, C=N), 1601 (s), 1542 (s), 1442 (s), 1270 (m), 1176 (m), 929 (w), 914 (sh, s, Mo=O), 900
(s, Mo=O), 846 (s), 752 (s), 692 (m), 549 (m, Mo-O), 496 (m), 432 (w); Anal. calcd. for C48H62MoN4O6:
C, 65.00; H, 7.05; N, 6.32. Found: C, 64.98, H, 7.39; N, 6.32%.
ῦ:
3.4.4. Synthesis of [MoO(PMe3)(L1)2] (4)
For the synthesis of
Schlenk tube under N2 atmosphere. Subsequently 5 equiv of PMe3 (95
4
, 1 equiv of 1 (125 mg, 0.18 mmol) was dissolved in dry toluene (5 mL) in a
L, 0.90 mmol) were added,
µ
whereupon the reaction mixture changed color to reddish brown. The reaction mixture was stirred
for 18 h at room temperature. Subsequently all volatiles were evaporated in vacuo, cold dry heptane
(3 mL) was added to the crude dark residue and insoluble OPMe3 was removed via filtration over a
glass frit packed with Celite. Evaporation of all volatiles gave
1H-NMR (300 MHz, C6D6, 25 ◦C, major isomer)
: 8.04 (s, 1H, CH=N), 7.90 (s,1H, CH=N), 7.50 (d, 1H,
ArH), 7.40 (d, 1H, ArH), 7.18 (d, 1H, ArH), 7.03 (d, 1H, ArH), 4.22–3.86 (m, 3H, CH2), 3.77–3.63 (m, 1H,
4 as a red-brown solid (80%, 110 mg).
δ

Molecules 2019, 24, 1814
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CH2), 2.59–2.42 (m, 1H, CH2), 2.22–1.87 (m, 3H, CH2), 1.45–1.20 (m, 4H, CH2), 1.36 (s, 27H, tBu), 1.29
(s, 9H, tBu), 1.01–0.82 (m, 6H, CH3), 0.92 (d, 9H, PMe3) ppm; 13C-NMR (75 MHz, C6D6, 25 ◦C, major
isomer, 1 Ar obscured) δ: 168.10, 165.93 (CH=N), 162.38, 162.24 (Ar–O), 140.24, 138.53, 136.44, 136.26,
131.03, 129.90, 129.24, 121.25, 121.05 (Ar), 35.64, 35.48 (q-tBu), 35.28 (CH2), 34.26, 34.01 (q-tBu) 33.07
(CH2) 31.89, 31.79, 30.12, 29.99 (tBu), 21.42, 20.53, 18.61, 17.70 (CH2), 16.84, 16.59 (CH3), 14.25 (d, PMe3)
ppm; 31P{1H}-NMR (121 MHz, C6D6, 25 ◦C, major isomer):
−
3.04 ppm; IR (ATR, cm−1) : 2952 (s,
ῦ:
C-H), 1611 (s, C=N), 1433 (s), 1308 (m), 1254 (s), 952 (m), 911 (s, Mo=O), 836 (s), 745 (m), 524 (w, Mo-O),
443 (m); Anal. calcd for C41H69MoN2O3P: C, 64.38; H, 9.09; N, 3.66 found: C, 64.32; H, 8.83; N, 3.57%.
3.4.5. Characterization of [MoO(PMe3)(L2)2] (5)
For the characterization of 5, 5 equiv of PMe3 (10 µL, 0.10 mmol) were added to a solution of
1 equiv of 2 (20 mg, 0.02 mmol) in dry C6D6 (0.5 mL), whereupon the solution turned reddish brown.
1
NMR spectra (1H, 13C, 31P) were recorded after 18 h. H-NMR (300 MHz, C6D6, 25 ◦C, major isomer)
δ
: 8.19 (s, 1H, CH=N), 8.08 (s, 1H, CH=N), 7.45 (d, 1H, ArH), 7.38 (d, 1H, ArH), 7.18 (d, 1H, ArH), 7.00
(d, 1H, ArH), 6.86 (br s, 1H, NH), 5.71 (br s, 1H, NH), 4.57–4.50 (m, 2H, CH2), 4.38–4.13 (m, 2H, CH2),
3.46–3.31 (m, 1H, CH2), 3.28–3.11 (m, 1H, CH2), 3.07–2.92 (m, 1H, CH2), 2.71–2.60 (m, 1H, CH2), 1.33 (s,
9H, tBu), 1.32 (s, 9H, tBu), 1.31 (s, 9H, tBu), 1.29 (s, 9H, tBu), 1.28 (s, 9H, tBu), 1.21 (s, 9H, tBu), 0.81 (d,
9H, coord. PMe3) ppm; 13C-NMR (75 MHz, C6D6, 25 ◦C, major isomer)
δ: 170.24, 169.82 (C=O), 169.33
(C=N), 165.78 (Ar-O), 163.48 (C=N), 161.42 (Ar-O), 139.57, 139.54, 138.33, 137.04, 136.93, 131.69, 130.17,
129.61, 121.18, 121.10 (Ar), 69.16, 64.34 (CH2), 50.94, 50.91 (q-tBu), 39.95, 39.14 (CH2), 35.51, 35.36, 34.27,
33.95 (q-tBu), 31.80, 31.69, 29.95, 29.84, 28.96, 28.76 (tBu), 16.51 (d, coord. PMe3) ppm; 31P{1H}-NMR
(121 MHz, C6D6, 25 ◦C, major isomer): −5.32 ppm.
3.4.6. Synthesis of [MoO(O2)(L1)2] (9)
For the synthesis of 9, 1 equiv of 4 (30 mg, 0.04 mmol) was dissolved in dry toluene (3 mL) in
a Schlenk tube under O2 atmosphere (1.5 atm). Subsequently, the reaction mixture was stirred for
18 h at room temperature whereupon the color gradually changed to dark orange. After removal of
all volatiles in vacuo, 3 mL of cold dry heptane were added to the crude dark orange residue and
insoluble OPMe3 was removed via filtration over a glass frit packed with Celite. Evaporation in vacuo
gave
9 as an orange solid (90%, 25 mg). Alternatively, 9 is accessible directly from 1. Thus, 1 equiv of 1
(120 mg, 0.17 mmol) was dissolved in dry toluene (5 mL). After addition of 3 equiv of PMe3 in toluene
(0.51 mmol, 0.5 mL 1 M solution), the reaction solution was stirred under an O2 atmosphere (1.5 atm)
for 18 h at room temperature. The reaction mixture was subsequently evaporated to dryness; cold
dry heptane (5 mL) was added and residual OPMe3 was removed via filtration through a glass frit
packed with Celite. The filtrate was subsequently stored at
−
35 ◦C overnight, whereupon residual
traces of precipitated. After removal of the precipitate via filtration and evaporation of the filtrate,
1
9
was obtained as an orange solid (84%, 103 mg). Crystals suitable for single-crystal X-ray diffraction
analysis were obtained via slow evaporation of a concentrated benzene solution at room temperature
1
(9). H-NMR (300 MHz, C6D6, 25 ◦C, major isomer)
δ: 8.05 (s, 1H, CH=N), 8.02 (s,1H, CH=N), 7.67 (d,
1H, ArH), 7.51 (d, 1H, ArH), 7.09 (d, 1H, ArH), 7.07 (d, 1H, ArH), 5.08–4.99 (m, 1H, CH2), 4.88–4.79 (m,
1H, CH2), 3.80–3.71 (m, 2H, CH2), 2.31–1.82 (m, 4H, CH2), 1.49–1.08 (m, 4H, CH2), 1.34 (s, 9H, tBu),
1.29 (s, 18H, tBu), 1.19 (s, 9H, tBu), 0.92–0.63 (m, 6H, CH3) ppm; 13C-NMR (75 MHz, C6D6, 25 ◦C, major
isomer) δ: 167.81, 167.29 (CH=N), 162.37, 160.66 (Ar–O), 139.70, 139.61, 139.26, 138.97, 132.01, 129.16,
128.89, 122.04, 121.88, 66.70, 62.29 (CH2), 35.07, 34.95, 34.16 (q-tBu), 34.12 (CH2), 34.03 (q-tBu), 31.79
(CH2), 31.63, 31.50, 29.75, 29.63(tBu), 20.71, 20.68 (CH2), 18.50, 14.05 (CH3) ppm; IR (ATR, cm−1) : 2954
ῦ:
(s, C-H), 1613 (s, C=N), 1438 (m), 1413 (m), 1253 (s), 927 (s, O-O), 915 (s, Mo=O), 841 (s), 750 (w), 540 (w,
Mo-O); Anal. calcd for C38H60MoN2O5: C, 63.32; H, 8.39; N, 3.89 Found: C, 63.81; H, 8.19; N, 3.82%.

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3.4.7. Synthesis of [MoO(O2)(L2)2] (10)
For the synthesis of 10, 3 equiv of PMe3 (18
µL, 0.18 mmol) were added to a solution of 1 equiv
of
2 (50 mg, 0.06 mmol) in dry toluene (3 mL), containing 3 Å molecular sieves, and subsequently
stirred under an O2 atmosphere (1.5 atm) for 18 h at room temperature. The reaction mixture was
◦
subsequently cooled to 5 C, filtered and evaporated in vacuo. The resulting orange residue was
dissolved in a minimum amount of dry acetonitrile, layered with dry heptane and stored in a freezer
at
35 ◦C for 24 h. The orange precipitate was filtered off and dried to afford 10 as an orange solid
−
(55%, 28 mg). Single crystals suitable for X-ray diffraction analysis were obtained from concentrated
1
MeCN (10) or benzene (10·OPMe3) solutions at
−
35 ◦C and room temperature, respectively. H-NMR
(300 MHz, C6D6, 25 ◦C)
δ
: 8.53 (s, 1H, CH=N), 8.38 (s, 1H, CH=N), 7.60 (d, 1H, ArH), 7.45 (d, 1H, ArH),
7.09 (d, 1H, ArH), 7.02 (d, 1H, ArH), 5.96 (bs, 1H, NH), 5.50 (dt, 1H, CH2), 5.40 (bs, 1H, NH), 5.10 (dt,
1H, CH2), 4.49 (td, 1H, CH2), 4.26 (td, 1H, CH2), 3.26–3.01 (m, 2H, CH2), 2.61–2.49 (m, 2H, CH2), 1.33
(s, 9H, tBu), 1.21 (2s, 9H, tBu), 1.21 (s, 9H, tBu), 1.20 (s, 9H, tBu), 1.11 (s, 9H, tBu), 1.05 (s, 9H, tBu), ppm;
13C-NMR (75 MHz, C6D6, 25 ◦C)
δ
: 170.11, 169.74 (C=O), 169.33, 169.06 (CH=N), 161.09, 160.51 (Ar-O),
140.52, 139.45, 139.35, 138.96, 132.55, 130.98, 129.99, 129.46, 121.61, 121.27 (Ar), 63.21, 59.97 (CH2), 51.19,
51.05 (q-tBu), 39.80, 39.23 (CH2), 34.96, 34.79, 34.13 (2 ) (q-tBu), 31.52, 31.39, 29.72, 29.46, 28.66, 28.49
ῦ:
×
(tBu) ppm; IR (ATR, cm−1) : 3272 (w, NH), 2958 (m, C-H), 1666 (m, C=O), 1611 (C=N, s), 1556 (m),
1254 (s), 931 (m, O-O), 907 (s, Mo=O), 840 (s), 541 (s, Mo-O), 463 (m); EI-MS (100 V) m/z: 848.7 [M
O]+,
832.7 [M
O2]+; Anal. Calcd. for C44H70MoN4O7: C, 61.24; H, 8.18; N, 6.49. Found: C, 60.14; H, 7.66;
N, 6.36%.
−
−
3.4.8. Synthesis of [MoO(O2)(L3)2] (11)
For the synthesis of 11, 3 equiv of PMe3 (49
µL, 0.48 mmol) were added to a suspension of 1 equiv
of
3 (140 mg, 0.16 mmol) in dry toluene (5 mL), containing 3 Å molecular sieves, and the mixture
◦
subsequently stirred under an O2 atmosphere (1.5 atm) at 50 C for 6 h, whereupon the solution
gradually turned red. Subsequently, all volatiles were removed in vacuo. The resulting dark residue
was dissolved in a minimum amount of dry acetonitrile and recrystallized at
as a red crystalline solid (36%, 58 mg). Single crystals suitable for X-ray diffraction analysis were
obtained from a saturated MeCN solution at
C6D6, 25 ◦C)
−
35 ◦C to afford 11·OPMe3
1
−
35 ◦C (11·OPMe3, 11·2OPMe3). H-NMR (300 MHz,
δ
: 9.29 (bs, 2H, NH), 8.56 (s, 1H, CH=N), 8.47 (s, 1H, CH=N), 7.92–7.89 (m, 4H, Ph), 7.59
(d, 1H, ArH), 7.43 (d, 1H, ArH), 7.11–7.07 (m, 4H, Ph), 7.03 (d, 1H, ArH), 6.99 (d, 1H, ArH), 6.90–6.79
(m, 2H, Ph), 5.59–5.43 (m, 1H, CH2), 5.26–5.09 (m, 1H, CH2), 4.60–4.40 (m, 2H, CH2), 3.51–2.99 (m, 4H,
CH2), 1.26 (s, 9H, tBu), 1.17 (s, 9H, tBu), 1.14 (s, 9H, tBu), 1.09 (s, 9H, tBu), 0.90 (d, 9H, OPMe3) ppm;
13C-NMR (75 MHz, C6D6, 25 ◦C)
δ
: 170.10, 169.50, 169.35, 169.13 (C=O, CH=N), 161.74, 160.14 (Ar-O),
140.44, 139.75, 139.73, 139.70, 139.52, 138.99, 132.66, 131.09, 129.60, 129.32 (Ar), 129.00 (Ph), 123.78 (Ph),
121.95, 121.73 (Ar), 120.18 (Ph), 62.63, 58.65, 39.39, 38.89 (CH2), 34.96, 35.83, 34.10 (2 ) (q-tBu), 31.44,
×
31.33, 29.65, 29.48 (tBu), 17.65 (d, OPMe3) ppm; 31P{1H}-NMR (121 MHz, C6D6, 25 ◦C): 36.19 (OPMe3)
ppm; IR (ATR, cm−1) : 3325 (br m, NH), 2957(m, C-H), 1653 (m, C=O), 1612(C=N, s), 1536 (m), 1255
ῦ:
(s), 1170 (m), 937 (m, O-O), 904 (s, Mo=O), 836 (s), 809 (m), 535 (w, Mo-O), 463 (m); EI-MS (100 V) m/z:
848.7 [M + H2O]+; Anal. Calcd. for C48H62MoN4O7
59.77; H, 6.41; N, 5.31%.
·
OPMe3 H2O: C, 60.47; H, 7.26; N, 5.54. Found: C,
·
4. Conclusions
The reported synthesis of iminophenolate ligands equipped with pendant amidopropyl
functionalities (HL2 and HL3) allowed for the successful synthesis and isolation of mononuclear
dioxidomolybdenum(VI) complexes [MoO2L2] (2 and 3). The here presented ligand design was
successful in terms of an undesired cyclization of the ligand moieties, as previously observed with
shorter amide substituents. More precisely, this cyclization reaction was prevented by introducing an
additional methylene group between the imine and the amide functionality of the ligands.

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The resulting complexes
2
and
3
together with complex
1
featuring n-butyl side chains were reduced
with PMe3 and subsequently exposed to molecular oxygen allowing the isolation of oxido peroxido
complexes of the structure [MoO(O2) 2] ( 11). Thus, all complexes are capable of activating dioxygen
in the desired fashion. The isolation of the reduced molybdenum(IV) intermediate was successful for
the n-butyl ligand system, [MoO(PMe3)(L1)2] ( ), while increased sensitivity of complexes and
L
9–
4
5
6
prevented their isolation in pure form. Instead, three polynuclear secondary products could be
identified via single-crystal X-ray diffraction analysis.
Complexes 1–3 were found to be moderately active catalysts in the aerobic oxidation of PMe3
while cyclooctene and dimethyl sulfide were not oxidized. Although formation of a weak hydrogen
bond to the peroxido moiety was observed, it has no beneficial influence on the reactivity. Nevertheless,
the here presented investigation reveals that hydrogen bonds may preferentially be formed to the
product OPMe3 rather than to the peroxido group thereby decreasing their expected beneficial effect.
Therefore, systems that are more reactive will have to consider less flexible substituents leading to
stronger hydrogen bonds directed towards the peroxido oxygen atoms.
Supplementary Materials: The supplementary materials are available online.
Author Contributions: Conceptualization, N.C.M.-Z.; synthetic experiments, N.Z., M.A.E. and A.D.; catalysis,
J.A.S.; X-ray analysis, F.B.; writing—original draft preparation, N.Z.; writing—review and editing, M.A.E. and
J.A.S.; supervision, N.C.M.-Z.
Funding: This research was funded by the Austrian Science Fund (FWF, grant number P26264).
Conflicts of Interest: The authors declare no conflict of interest.
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