Catalytic epoxidation using dioxidomolybdenum(VI) complexes with tridentate aminoalcohol phenol ligands

Article abstract of DOI:10.1016/j.ica.2018.10.012

Reaction of the tridentate aminoalcohol phenol ligands 2,4-di-tert-butyl-6-(((2 hydroxyethyl)(methyl)amino)methyl)phenol (H₂L¹) and 2,4-di-tert-butyl-6-(((1-hydroxybutan-2-yl)amino)methyl)phenol (H₂L²) with [MoO₂(acac)₂] in methanol solutions resulted in the formation of [MoO₂(L¹)(MeOH)] (1) and [MoO₂(L²)(MeOH)] (3), respectively. In contrast, the analogous reactions in acetonitrile afforded the dinuclear complexes [Mo₂O₂(μ-O)₂(L¹)₂] (2) and [Mo₂O₂(μ-O)₂(L²)₂] (4). The corresponding reactions with the potentially tetradentate ligand 3-((3,5-di-tert-butyl-2-hydroxybenzyl)(methyl)amino)propane-1,2-diol (H₃L³) led to the formation of the mononuclear complex [MoO₂(L³)(MeOH)] (5) in methanol while in acetonitrile solution a trinuclear structure [Mo₃O₃(μ-O)₃(L³)₃] (6) was obtained. In both cases, the ligand moiety L³ coordinated in a tridentate fashion. The catalytic activities of complexes 1–6 in epoxidation of five different olefins, S1-5, with tert-butyl hydroperoxide and hydrogen peroxide were studied. The catalytic activities were found to be moderate to good for the reaction of substrate cis-cyclooctene S1, while all complexes were less active in the epoxidation of the more challenging substrates S2-5. The molecular structures of 1, 2, 4 and 6 were determined by single crystal X-ray diffraction analyses.

Full text of DOI:10.1016/j.ica.2018.10.012

Accepted Manuscript
Research paper
Catalytic Epoxidation using Dioxidomolybdenum(VI) Complexes with Triden-
tate Aminoalcohol Phenol Ligands
Md. Kamal Hossain, Jörg A. Schachner, Matti Haukka, Nadia C. Mösch-Zanetti,
Ebbe Nordlander, Ari Lehtonen
PII:
S0020-1693(18)30919-8
https://doi.org/10.1016/j.ica.2018.10.012
ICA 18558
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
15 June 2018
8 October 2018
8 October 2018
Please cite this article as: d.K. Hossain, J.A. Schachner, M. Haukka, N.C. Mösch-Zanetti, E. Nordlander, A.
Lehtonen, Catalytic Epoxidation using Dioxidomolybdenum(VI) Complexes with Tridentate Aminoalcohol Phenol
Ligands, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.10.012
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Catalytic Epoxidation using Dioxidomolybdenum(VI) Complexes with Tridentate
Aminoalcohol Phenol Ligands
Md. Kamal Hossain,[a] Jörg A. Schachner,*[b] Matti Haukka,[c] Nadia C. Mösch-Zanetti,[b]
Ebbe Nordlander,*[a] and Ari Lehtonen*[d]
a. Chemical Physics, Department of Chemistry, Lund University, P.O. Box 124, SE-221
00 Lund, Sweden
b. Department of Chemistry, University of Graz, Schubertstraße 1, 8010 Graz, Austria
c. Department of Chemistry, P.O. Box 35, University of Jyväskylä, FI-40014 Jyväskylä,
Finland
d. Inorganic Materials Chemistry Research Group, Laboratory of Materials Chemistry and
Chemical Analysis, Department of Chemistry, University of Turku, 20014, Turku,
Finland
* Corresponding author
E-mail address:
Ari.Lehtonen@utu.fi (A. Lehtonen)
Ebbe.Nordlander@chemphys.lu.se (E. Nordlander)
Dedicated to Rabindranath Mukherjee on the occasion of his 65th birthday, and in
recognition of his outstanding contributions to coordination chemistry and bioinorganic
chemistry
Abstract
Reaction of the tridentate aminoalcohol phenol ligands 2,4-di-tert-butyl-6-(((2
hydroxyethyl)(methyl)amino)methyl)phenol
(H2L1)
and
2,4-di-tert-butyl-6-(((1-
hydroxybutan-2-yl)amino)methyl)phenol (H2L2) with [MoO2(acac)2] in methanol solutions
resulted in the formation of [MoO2(L1)(MeOH)] (1) and [MoO2(L2)(MeOH)] (3), respectively.
In contrast, the analogous reactions in acetonitrile afforded the dinuclear complexes [Mo2O2(μ-
O)2(L1)2] (2) and [Mo2O2(μ-O)2(L2)2] (4). The corresponding reactions with the potentially
tetradentate ligand 3-((3,5-di-tert-butyl-2-hydroxybenzyl)(methyl)amino)propane-1,2-diol
(H3L3) led to the formation of the mononuclear complex [MoO2(L3)(MeOH)] (5) in methanol
while in acetonitrile solution a trinuclear structure [Mo3O3(μ-O)3(L3)3] (6) was obtained. In
both cases, the ligand moiety L3 coordinated in a tridentate fashion. The catalytic activities of
complexes 1-6 in epoxidation of five different olefins S1-5 with tert-butyl hydroperoxide and
hydrogen peroxide were studied. The catalytic activities were found to be moderate to good for
the reaction of substrate cis-cyclooctene S1, while all complexes were less active in the
epoxidation of more challenging substrates S2-5. The molecular structures of 1, 2, 4 and 6 were
determined by single crystal X-ray diffraction analyses.
Keywords: Molybdenum complex, Tridentate ligand, Trinuclear structure, Epoxidation
1

1. Introduction
Complexes containing a cis-[MoO2]2+ moiety play crucial roles in catalytic oxidation
reactions such as alkene epoxidation [1] and oxotransfer reactions [2,3]. Molybdenum-
catalysed epoxidation reactions are of interest for the production of both bulk and fine
chemicals [1,4,5]. In a number of studies, dioxidomolybdenum(VI) complexes with chelating
phenolate ligands have been used as catalysts for catalytic oxidation reactions such as oxidative
bromination, oxotransfer form DMSO to PPh3 and alkene epoxidation [6].
Aminobisphenol ligands (with two coordinating phenolate moieties in the structure)
form a topical group of chelating phenols that can coordinate as tridentate or tripodal
tetradentate ligands (if an additional donor group is present) with various transition metals
[6a,7,8]. In general, the coordination of tridentate aminomonophenols (containing only one
coordinating phenol moiety together with other, e.g. aliphatic alcohol, moieties) to the
dioxidomolybdenum(VI) center leaves one vacant coordination site that can be filled either by
a solvent molecule or through dimerization [9,10]. The formation of monomeric solvent
adducts, dimers and, in some cases, oxido-bridged trinuclear species [11] depends on the
experimental conditions. A relatively limited number of aminomonophenolate alcoholate
complexes of transition metals [6b,12,17] have been reported in the literature compared with
reports on complexes of aminobisphenolate ligands.
In continuation of our research on Mo(VI) and W(VI) complexes with multidentate
aminobisphenolate ligands, [6e,13-15] we have investigated the catalytic performance of
MoO2(VI) complexes of several aminoalcohol phenol ligands. In the present study, we have
synthesized the three aminomonophenolate ligands containing different aminoalcohols as a
side-arm donor moieties, 2,4-di-tert-butyl-6-(((2-hydroxyethyl)(methyl)amino)methyl)phenol
(H2L1), 2,4-di-tert-butyl-6-(((1-hydroxybutan-2-yl)amino)methyl)phenol (H2L2) and 3-((3,5-
di-tert-butyl-2-hydroxybenzyl)(methyl)amino)propane-1,2-diol (H3L3) (Figure 1). In ligand
H3L3, the uncoordinated alcoholic hydroxyl group is in close vicinity to the molybdenum
centre and may form a hydrogen bond with incoming oxidants like tert-butyl hydroperoxide
(tBuOOH) or hydrogen peroxide (H2O2) in the first step of the catalytic epoxidation reaction.
The ligand H2L1 was previously used for the preparation of the bis(μ-alkoxido)diiron(III)
complexes [Fe(X1)L1]2 (X1 = acac-, Cl-) [16] and the monomeric oxidotungsten(VI) complexes
[WO(X2)(L1)] (X2 = ethylene glycolate, X = Cl-) [17]. The magnetic properties of these iron
complexes and the ring-opening metathesis polymerization activities of the tungsten complexes
were studied. The L-isomer of the ligand H2L2 was previously used for in situ catalytic
asymmetric hydrophosphonylation with Et2AlCl [18]. To our knowledge, there are no previous
publications on ligand (precursor) H3L3.
2

Figure 1. Aminomonophenolate ligands, H2L1, H2L2 and H3L3
In the present investigation, we report the syntheses, characterisations and solid-state
structures of new mono- (1, 3, 5), di- (2, 4) and trinuclear (6) Mo(VI) complexes with the
aforementioned ligands. The catalytic activities of complexes 1-6 in epoxidation of the five
olefins cis-cyclooctene S1, 1-octene S2, styrene S3, limonene S4 and α-terpineol S5 (vide infra,
Fig. 6) by either hydrogen peroxide or tert-butyl hydroperoxide were also investigated.
2. Experimental
2.1. Materials and physical measurements
Commercial grade chemicals were used without further purification and HPLC grade
solvents were used as received. All syntheses and manipulations were performed under ambient
atmospheric conditions with standard laboratory equipment. NMR spectra were recorded using
Bruker Optics 300 MHz or Varian Inova 500 MHz spectrometers with standard settings for 1H
and 13C nuclei, using deuterated chloroform and dimethyl sulfoxide as solvents, and referenced
to the residual signal of the solvent. Peaks are reported as singlet (s), doublet (d), doublet of
doublets (dd), triplet (t) and multiplet (m or unresolved), coupling constants are given in Hz.
IR spectra were recorded on a Bruker Vertex 70 spectrometer with wave number (cm-1) and
intensities (br = broad, vs = very strong, s = strong, m = medium, w = weak). Mass spectrometry
was performed on Waters ZQ 4000 and Waters QTof Xevo-G2 spectrometer using calibrant as
CsI. Results are denoted as cationic mass peaks; unit is the mass/charge ratio.
2.2. Preparation of the ligands
H2L1 was synthesized by a literature procedure [19].
Preparation of 2,4-di-tert-butyl-6-(((1-hydroxybutan-2-yl)amino)methyl)phenol (H2L2)
2,4-di-tert-butylphenol (4.12 g, 20 mmol), 2-amino-1-butanol (2.74 g, 20 mmol) and
paraformaldehyde (0.60 g, 20 mmol) were mixed and heated (110 – 120°C) in a round-
bottomed flask for two hours. The reaction mixture was cooled to room temperature and treated
with methanol to obtain white crystalline solid. Recrystallization from methanol produced solid
product in a 64 % yield (3.90 g). The synthesized aminophenol was characterized by IR and
3

NMR. Selected IR (cm-1) 2950m (N–H), 1114m (C–N), 766m (N–H wag). 1H NMR (300 MHz,
CDCl3) δ 10.21 (s, 1H, Ph-OH), 7.25 (d, J = 2.4 Hz, 1H, ArH), 6.84 (d, J = 2.4 Hz, 1H, ArH),
4.38 (d, J = 5.9 Hz, 1H, HNCH2), 4.31 (d, J = 5.9 Hz, 1H, HNCH2), 4.15 (dt, J = 14.2, 7.1 Hz,
1H, HNCH), 3.88 (q, J = 13.3 Hz, 2H, CH3CH2), 3.50-342 (m, 1H, CH2OH), 3.02 – 2.93 (m,
1H, CH2OH), 1.73 – 1.59 (m, 1H, CH2NH), 1.43 (s, 9H, C(CH3)3), 1.30 (s, 9H, C(CH3)3), 0.94
(t, J = 7.4 Hz, 3H, CH3CH2). 13C NMR (75 MHz, CDCl3) δ 153.96, 140.82, 136.01, 123.58,
123.32, 121.51 (Ar-C), 84.72 (CH), 69.72 (CH2), 65.02 (CH2), 34.88 (CH2), 34.16 [C(CH3)3],
31.69 [C(CH3)3], 29.63 (CH3), 26.29 (CH3), 10.77 (CH3).
Preparation of 3-((3,5-di-tert-butyl-2-hydroxybenzyl)(methyl)amino)propane-1,2-diol
(H3L3)
2,4-di-tert-butylphenol (4.12 g, 20 mmol), 3-methylamino-1,2-propanediol (2.74 g, 20
mmol) and paraformaldehyde (0.60 g, 20 mmol) were mixed and heated (110 – 120°C) in a
round-bottomed flask for two hours. The reaction mixture was cooled to the room temperature
and treated with methanol to obtain white crystalline solid. Recrystallization from methanol
produced solid product in a 59 % yield (3.80 g). The synthesized aminophenol was
characterized by IR and NMR. Selected FT-IR (cm-1) 2952m (N–H), 1102m (C–N). 1H NMR
(500 MHz, DMSO-d6) δ 7.09 (d, J = 2.4 Hz, 1H, ArH), 6.85 (d, J = 2.4 Hz, 1H, ArH), 3.72 –
3.68 (m, 1H, OHCH2CH), 3.66 (s, 2H, NCH2), 3.34 – 3.28 (m, 2H, NCH3CH2), 2.51 (dd, J =
5.9, 2.9 Hz, 1H, OHCH2), 2.42 (dd, J = 11.0, 5.4 Hz, 1H, OHCH2), 2.20 (s, 3H, NCH3), 1.35
(s, 9H, C(CH3)3), 1.23 (s, 9H, C(CH3)3). 13C NMR (126 MHz, DMSO-d6) δ 154.38, 139.85,
134.70, 123.55, 122.14, 122.07 (Ar-C), 69.24 (CH), 65.00 (CH2), 62.30 (CH2), 60.44 (CH2),
41.70 [C(CH3)3], 34.82 [C(CH3)3], 34.17 (CH3), 31.98 (CH3), 29.91 (CH3).
2.3 Preparation of complexes 1-6
[MoO2(L1)(MeOH)] 1. [MoO2(acac)2] 0.130 g (0.40 mmol) and ligand H2L1 0.117 g
(0.40 mmol) were dissolved in 5 mL of methanol. The yellow reaction mixture was stirred at
ambient temperature for 2 hours. The resulting yellow solution was allowed to evaporate
slowly at room temperature for a few days to obtain yellow crystals suitable for X-ray study.
Yield 60% (0.109 g). 1H NMR (500 MHz, DMSO-d6) δ 7.18 (d, J = 2.4 Hz, 1H, ArH), 6.97 (d,
J = 2.4 Hz, 1H, ArH), 4.66 (td, J = 11.8, 3.2 Hz, 1H, NCH2), 4.48 (dd, J = 11.0, 5.4 Hz, 1H,
NCH2CH2), 4.21 (d, J = 12.2 Hz, 1H, NCH2), 4.10 (q, J = 5.2 Hz, CH3OH), 3.27 – 3.23 (m,
2H, CH3NCH2), 3.17 (d, J = 4.9 Hz, CH3OH), 2.65 (dd, J = 11.5, 3.0 Hz, 1H, NCH2CH2), 2.46
(s, 3H, NCH3), 1.35 (s, 9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3). 13C NMR (75 MHz, DMSO-d6)
δ 158.70, 141.12, 136.82, 125.12, 123.86, 123.22 (Ar-C), 71.77 (CH2), 62.12 (CH2), 61.52
(CH2), 49.06 (CH3), 44.67 [C(CH3)3], 35.20 [C(CH3)3], 34.31 (CH3), 31.97 (CH3), 30.54 (CH3).
Selected IR resonances (cm-1) 3087 (O-H, MeOH) 929s (Mo=O), 901s (Mo=O). ESI-MS: m/z
= 487 [1·MeOH+H]+, 444 [1-MeOH+Na]+, 294 [H2L1+H]+.
[Mo2O2(μ-O)2(L1)2] 2. [MoO2(acac)2] 0.130 g (0.40 mmol) and ligand H2L1 0.117 g
(0.40 mmol) were dissolved in 5 mL of anhydrous acetonitrile. The orange reaction mixture
was stirred at ambient temperature for 4 hours. The resulting orange solution was left to
evaporate slowly at room temperature to obtain orange crystals suitable for X-ray study. Yield
4

1
65% (0.218 g). H NMR (500 MHz, DMSO-d6) δ 7.18 (d, J = 2.4 Hz, 2H, ArH), 6.98 (d, J =
2.4 Hz, 2H, ArH), 4.66 (td, J = 11.8, 3.2 Hz, 2H, NCH2), 4.47 (dd, J = 11.0, 5.4 Hz, 2H,
NCH2CH2), 4.20 (d, J = 12.2 Hz, 2H, NCH2), 3.27 – 3.24 (m, 4H, CH3NCH2), 2.65 (dd, J =
11.0, 5.4 Hz, 2H, NCH2CH2), 2.46 (s, 6H, NCH3), 1.35 (s, 18H, C(CH3)3), 1.25 (s, 18H,
C(CH3)3). 13C NMR (126 MHz, CDCl3) δ 157.97, 142.84, 137.54, 124.13, 123.47, 122.01(Ar-
C), 72.97 (CH2), 63.17 (CH2), 62.87 (CH2), 45.25 [C(CH3)3], 35.14 [C(CH3)3], 34.29 (CH3),
31.64 [C(CH3)3], 30.31 [C(CH3)3]. Selected IR resonances (cm-1) 913m (Mo=O), 877s (Mo–
O–Mo), 842m (Mo–O–Mo). ESI-MS: m/z = 863 [2+Na]+ , 841 [2+H]+, 294 [H2L1+H]+.
[MoO2(L2)(MeOH)] 3. [MoO2(acac)2] 0.130 g (0.40 mmol) and ligand H2L2 0.123 g
(0.40 mmol) were dissolved in 5 mL of methanol. The yellow reaction mixture was stirred at
room temperature with magnetic stirring for 2 hours. Subsequently the resulting yellow
solution was allowed to stand at room temperature for slow evaporation to grow yellow
microcrystals. Yield 55% (0.102 g). 1H NMR (500 MHz, DMSO-d6) δ 7.14 (d, J = 2.4 Hz, 1H,
ArH), 7.02 (d, J = 2.4 Hz, 1H, ArH), 5.07 (td, J = 11.0, 2.9 Hz, 1H, NCH2), 4.63 (dd, J = 11.0,
5.4 Hz, 1H, NCH2), 4.10 (q, J = 5.2 Hz, CH3OH), 3.97 (t, J = 10.6 Hz, 1H, NHCH), 3.78 –
3.68 (m, 2H, OCH2), 3.17 (d, J = 4.9 Hz, CH3OH), 2.92 – 2.86 (m, 1H, CH3CH2), 1.48 – 1.41
(m, 1H, CH3CH2), 1.35 (s, 9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3), 0.89 (t, J = 7.6 Hz, 3H,
CH3CH2). 13C NMR (126 MHz, DMSO-d6) δ 159.36, 140.56, 136.76, 125.70, 124.75, 122.86
(Ar-C), 78.52 (CH), 63.88 (CH2), 48.86 (CH2), 35.16 (CH2), 34.29 [C(CH3)3], 31.98 [C(CH3)3],
30.63 (CH3), 21.42 (CH3), 11.67(CH3). Selected IR resonances (cm-1) 3196 (O-H, MeOH)
953m (Mo=O), 920s (Mo=O). ESI-MS: m/z = 521 [3·MeOH+Na]+, 499 [3.MeOH+H]+, 458
[3-MeOH+Na]+, 308 [H2L2+H]+.
[Mo2O2(μ-O)2(L2)2] 4. [MoO2(acac)2] 0.130 g (0.40 mmol) and ligand H2L2 0.123 g
(0.40 mmol) were dissolved in 5 mL of anhydrous acetonitrile. The orange reaction mixture
was stirred at ambient temperature for 4 hours. X-ray quality orange crystals were obtained by
slow evaporation of the resulting solution at room temperature. Yield 75% (0.260 g). 1H NMR
(500 MHz, DMSO-d6) δ 7.15 (d, J = 2.4 Hz, 2H, ArH), 7.02 (d, J = 2.4 Hz, 2H, ArH), 5.06 (td,
J = 11.0, 2.9 Hz, 2H, NCH2), 4.63 (dd, J = 11.0, 5.4 Hz, 2H, NCH2), 3.97 (t, J = 10.6 Hz, 2H,
NHCH), 3.78 – 3.68 (m, 4H, OCH2), 2.93 – 2.86 (m, 2H, CH3CH2), 1.50 – 1.40 (m, 2H,
CH3CH2), 1.35 (s, 18H, C(CH3)3), 1.25 (s, 18H, C(CH3)3), 0.89 (t, J = 7.6 Hz, 6H, CH3CH2).
13C NMR (75 MHz, DMSO-d6) δ 159.39, 140.58, 136.76, 125.73, 124.78, 122.88 (Ar-C), 78.54
(CH), 63.88 (CH2), 48.86 (CH2), 40.76 (CH2), 35.18 [C(CH3)3], 34.31 [C(CH3)3], 32.00 (CH3),
30.64 (CH3), 21.44 (CH3). Selected IR resonances (cm-1) 919m (Mo=O), 868s (Mo–O–Mo),
818m (Mo–O–Mo). ESI-MS: m/z = 890 [4+Na]+, 867 [4+H]+, 308 [H2L2+H]+.
[MoO2(L3)(MeOH)] 5. [MoO2(acac)2] 0.163 g (0.50 mmol) and ligand H3L3 0.162 g
(0.50 mmol) were dissolved in 5 mL of methanol. The yellow reaction mixture was stirred at
ambient temperature for 2 hours. The resulting yellow solution was then left to evaporate
slowly at room temperature to afford yellow microcrystals of the complex. Yield 55% (0.137
g). 1H NMR (500 MHz, DMSO-d6) δ 7.18 (d, J = 2.4 Hz, 1H, ArH), 6.97 (d, J = 2.4 Hz, 1H,
ArH), 4.80 – 4.78 (m, 2H, NCH2), 4.20 (d, J = 12.2 Hz, 1H, OHCH2), 4.10 (q, J = 5.2 Hz,
CH3OH), 3.50 – 3.42 (m, 2H, NCH3CH2), 3.23 (d, J = 12.2 Hz, 1H, OHCH2), 3.17 (d, J = 4.9
5

Hz, CH3OH), 3.07 (t, J = 11.3 Hz, 1H, OHCH2), 2.73 (dd, J = 11.4, 3.3 Hz, 1H, OHCH2CH),
2.50 (s, 3H, NCH3), 1.35 (s, 9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3). 13C NMR (126 MHz, DMSO-
d6) δ 158.64, 141.10, 136.75, 125.09, 123.95, 123.20 (Ar-C), 82.97 (CH), 64.43 (CH2), 63.71
(CH2), 62.06 (CH2), 45.62 (CH3), 35.17 [C(CH3)3], 34.29 [C(CH3)3], 31.95 (CH3), 30.50 (CH3).
Selected IR resonances (cm-1) 3395 (O-H, MeOH) 934s (Mo=O), 915s (Mo=O). ESI-MS: m/z
= 506 [5+Na]+, 474 [5-MeOH+Na]+, 452 [5-MeOH+H]+, 324 [H3L3+H]+.
[{MoO2(L3)}3] 6. [MoO2(acac)2] 0.163 g (0.50 mmol) and ligand H3L3 0.162 g (0.50
mmol) were dissolved in 5 mL of anhydrous acetonitrile. The orange reaction mixture was
stirred at ambient temperature for 4 hours. The resulting orange solution was allowed to
evaporate slowly at room temperature to obtain crystalline material. Recrystallization from hot
chloroform solution yielded single crystal suitable for X-ray analysis. Yield 50% (0.396 g). 1H
NMR (500 MHz, DMSO-d6) δ 7.18 (d, J = 2.4 Hz, 3H, ArH), 6.97 (d, J = 2.4 Hz, 3H, ArH),
4.79 – 4.77 (m, 6H, NCH2), 4.19 (d, J = 12.2 Hz, 3H, OHCH2), 3.51 – 3.42 (m, 6H, NCH3CH2),
3.22 (d, J = 12.2 Hz, 3H, OHCH2), 3.07 (t, J = 11.3 Hz, 3H, OHCH2), 2.74 (dd, J = 11.4, 3.3
Hz, 3H, OHCH2CH), 2.50 (s, 9H, NCH3), 1.35 (s, 27H, C(CH3)3), 1.25 (s, 27H, C(CH3)3). 13C
NMR (126 MHz, DMSO-d6) δ 158.64, 141.10, 136.75, 125.09, 123.95, 123.21 (Ar-C), 82.94
(CH), 64.43 (CH2), 63.72 (CH2), 62.06 (CH2), 45.63 (CH3), 35.17 [C(CH3)3], 34.29 [C(CH3)3],
31.95 (CH3), 30.50 (CH3). Selected IR resonances (cm-1) 3410 (O-H, alcoholic OH), 967s
(Mo=O), 928s (Mo=O), 912s (Mo=O), 856s (Mo–O–Mo), 836s (Mo–O–Mo). ESI-MS: m/z =
1371 [Mo3O6(L3)3+Na]+, 923 [Mo2O4(L3)2+Na]+, 474 [MoO2(L3)+Na]+ , 324 [H3L3+H]+.
2.4. Catalysis studies
2.4.1. Epoxidation
Experiments were carried out in a Heidolph Parallel Synthesizer 1. In a typical
experiment, 2−3 mg of catalyst (1 mol%) was dissolved in 0.5 mL of CHCl3 and mixed with 1
equiv. of substrate. Then 50 μL of mesitylene were added as internal standard, and the reaction
mixtures were heated to 50 °C, whereupon the oxidant (3 equiv.) was added in one portion.
Aliquots for GC−MS (20 μL) were withdrawn with a calibrated Socorex Acura 825, 10−100
μL variable volume pipet at given time intervals, quenched with MnO2, and diluted with
HPLC˗grade ethyl acetate. The reaction products were analyzed by GC−MS (Agilent
Technologies 7890 GC System), and the epoxide produced from each reaction mixture was
quantified versus mesitylene as the internal standard.
2.5. X-ray structure determination
The crystals of 1, 2, 4 and 6 were immersed in cryo-oil, mounted in a MiTeGen loop,
and measured at a temperature of 120 K on a Bruker Kappa Apex II or on a Rigaku Oxford
Diffraction Supernova diffractometer using Mo K ( = 0.71073) radiation. The CrysAlisPro
[20] or Denzo-Scalepack [21] program packages were used for cell refinements and data
reductions. Multi-scan absorption corrections (SADABS [22] or CrysAlisPro [20]) were applied
to the intensities before structure solutions. The structures were solved by charge flipping
6

method using the SUPERFLIP [23] software or by using direct methods and SHELXT [24]
program. Structural refinements were carried out using SHELXL [24]. In 1 the coordinates of
the OH hydrogen atom were refined on a riding model (fixed distance) with Uiso = 1.5Ueq
parent oxygen. All other H-atoms were positioned geometrically and constrained to ride on
their parent atoms, with C-H = 0.95-1.00 Å, O-H = 0.84 Å and Uiso = 1.2-1.5Ueq (parent atom).
3. Results and discussion
3.1. Syntheses and spectroscopic characterization of complexes
All ligands were prepared by Mannich reactions, applying a solvent-free procedure
consisting of mixing stoichiometric amounts of 2,4-di-tert-butylphenol, paraformaldehyde and
the appropriate aminoalcohol, and heating the reaction mixture in an open vessel. The
mononuclear complexes [MoO2(L1)(MeOH)] (1) and [MoO2(L2)(MeOH)] (3) were prepared
by the treatment of [MoO2(acac)2] with one equivalent of ligand in methanol at ambient
temperature. The analogous reactions in acetonitrile solutions led to the formation of the
dinuclear complexes [Mo2O2(μ-O)2(L1)2] (2) and [Mo2O2(μ-O)2(L2)2] (4) (Scheme 1). The
complexes crystallised from the reaction mixtures upon slow evaporation of the solvents.
Complexes 1 and 3 are soluble in polar solvents such as acetonitrile, tetrahydrofuran,
chloroform, dichloromethane, while the dinuclear complexes 2 and 4 may be dissolved in hot
MeOH and DMSO but are practically insoluble in any other solvents.
Scheme 1. Formation of mononuclear and dinuclear Mo(VI) complexes with ligands H2L1 and
H2L2.
7

The coordinated MeOH in 1 and 3 evaporates slowly in open air and more rapidly in
vacuum to yield the dinuclear complexes 2 and 4, respectively. Similarly, when dissolved in
acetonitrile, the monomeric complexes are converted into their analogous dimers and
precipitate from the solutions as orange powders. Conversely, complexes 1 and 3 can be
obtained from complexes 2 and 4 by dissolution in hot MeOH solvent.
The ligand H3L3 reacts with [MoO2(acac)2] in methanol to form the monomeric
complex [MoO2(L3)(MeOH)] (5) with a dangling OH group as yellow crystals (Scheme 2).
Based on the observations discussed above, the analogous reaction in acetonitrile was expected
to lead to the formation of a dimeric complex. However, single crystal X-ray diffraction (vide
infra) showed that [Mo3O3(μ-O)3(L3)3] (6) is a trinuclear complex where the molybdenum and
oxygen atoms form a planar six-membered Mo3O3 ring (Scheme 2). When a methanol solution
of 6 was concentrated by slow evaporation at room temperature, the yellow solid that
precipitated was characterised by IR to be identical with mononuclear complex 5.
In conclusion, the formation of monomeric and dimeric complexes based on the ligands
in this study may be controlled by the choice of solvent. Specifically, the monomeric
complexes can be transformed to their dimeric counterparts by removal of the coordinated
methanol ligand, while dimers may be converted into monomers by dissolution in MeOH.
Previous work with oxidomolybdenum(VI) complexes with tri- and tetradentate
aminobisphenols as well as Schiff base ligands has shown similar reactivity [6h,10,25].
Scheme 2. Formation of mononuclear and trinuclear Mo(VI) complexes with ligand H3L3
8

The IR spectra for the mononuclear MoO2(VI) complexes (1, 3 and 5) exhibit two
strong νMo=O bands around 901-920 and 929-953 cm-1 corresponding to the antisymmetric and
symmetric stretching modes, respectively, for the cis-[MoO2]2+ moiety [26-28]. Broad bands at
~ 3087-3395 cm-1 are characteristic for coordinated or hydrogen bonded methanol molecules.
The μ-oxido-bridged dinuclear MoO(VI) complexes 2 and 4 show only a single vibrational
band at 913 and 919 cm-1, respectively, in their spectra instead of the characteristic doublets
for cis-[MoO2]2+ fragments. In addition, 2 and 4 display strong absorptions in the 877-818 cm-
1 range that are diagnostic for Mo=O····Mo bridges due to the weakened Mo=O bond [10]. The
trinuclear complex 6 showed three characteristic bands at 912, 928 and 967 cm-1 for νMo=O and
the antisymmetric oxygen bridges present strong bands at 836 and 856 cm-1.
1
13
The H and C NMR spectra in deuterated dimethyl sulfoxide solution showed the
expected resonances for aminophenolate alcoholate ligands. For all complexes, disappearance
of the phenolate OH signals in the 1H NMR spectra of the aminophenolate alcoholate ligands
confirm the deprotonation of the ligands upon complexation. The NMR spectra of all
complexes showed the expected resonances for the tridentate ligands. Complexes 1, 3 and 5
showed similar chemical shifts (as well as the resonances for free MeOH) as their dimeric or
trimeric counterparts, which indicates, that the complexes adopt comparable solution
structures, possibly due to the coordination of DMSO and formation of monomeric species;
such dissociation of the trimeric molybdenum-oxo species [{MoO2(L)}3] (H2L = 2,4-di-tert-
butyl-6-(((2-hydroxy-2-phenylethyl)amino)methyl)phenol), related to 6, has been
demonstrated previously [29].
In the 1H NMR spectra of complexes 1 and 2, the diastereotopic benzylic CH2-protons
exhibit two separate signals. One signal is seen at 4.66 ppm as a doublet of triplet (td, J = 11.8,
3.2 Hz) due to the long-range coupling to the N-CH2 protons as well as to the geminal proton,
and the second proton shows up as a doublet (d, J = 12.2 Hz) at ca. 4.20 ppm due to the geminal
coupling of a benzylic proton. The N-Me protons resonate at 2.46 ppm. Similarly, for
complexes 3 and 4, the benzylic protons show a doublet of triplet at 5.07 ppm with coupling
constants J = 11.0, 2.9 Hz for one proton while the other proton is seen at 4.63 ppm as a doublet
of doublets with coupling constants J = 11.0, 5.4 Hz. For complexes 5 and 6, the benzylic
protons are seen as a broad multiplet at ca. 4.78 ppm. For complexes 1, 3 and 5, the coordinated
methanol OH proton appears as a quartet at 4.10 ppm while the methyl protons are seen as a
doublet at 3.17 ppm. For 3 and 4, some uncharacterised minor peaks were seen in the spectra,
possibly due to slow decomposition in solution.
The metal complexes were characterized by mass spectrometry using electrospray
ionisation. In all mass spectra of the mono-, di- and trinuclear complexes there are ions of
significant intensity that can be attributed to the dissociated aminophenolate ligands. The
spectra of complexes 1, 3 and 5 contained the molecular peaks at m/z = 487, 521 and 506
because of the ions [1·MeOH+H]+, [3·MeOH+Na]+and [5+Na]+, respectively.
The dinuclear complexes 2 and 4 showed prominent peaks at m/z = 863 m/z = 890
corresponding to the species [2+Na]+ and [4+Na]+ as well as peaks for mononuclear species.
9

Similarly, the trinuclear 6 complex displayed a major molecular peak for [6+Na]+, (m/z = 1371)
and characteristic peaks for dimeric and monomeric structures. All complexes containing metal
ions displayed the predicted isotopic distributions with relative intensities and m/z values that
are consistent with the most abundant isotopic composition.
3.2. Crystal and molecular structures
The molecular structures of complexes 1, 2, 4 and 6·2CHCl3 were determined by single-
crystal X-ray diffraction analyses. Their molecular structures are shown in Figures 2-5. A
summary of the crystallographic data and final refinement details are given in Table 1. Selected
bond lengths and angles relevant to the coordination spheres of the molybdenum atoms are
listed in Table 2. The single crystals of complexes 1, 2 and 4 were isolated directly from the
reaction mixtures. Crystals of 6 isolated from acetonitrile were of rather low quality, but single
crystals suitable for X-ray analysis could be grown from chloroform, and two molecules of
chloroform were found in the asymmetric unit.
Table 1. Crystallographic data for compounds 1, 2, 4 and 6.
1
2
4
6
empirical formula
fw
C19H33MoNO5
451.40
C36H58Mo2N2O8
838.72
C38H62Mo2N2O8
866.77
C59H95Cl6Mo3N3O15
1586.89
temp (K)
(Å)
cryst syst
space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
120(2)
120(2)
120(2)
0.71073
120(2)
0.71073
0.71073
Monoclinic
P21/c
0.71073
Monoclinic
P21/c
Monoclinic
P21/n
Triclinic
P1
16.1436(8)
16.4390(8)
16.8147(8)
78.792(4)
62.009(5)
12.5882(2)
11.9673(2)
14.3727(3)
90
17.0417(3)
9.33570(10)
12.3744(2)
90
a = 8.6913(2)
b = 12.7016(3)
c = 18.4454(3)
90
106.1860(10)
102.1160(10)
92.7890(10)
γ (°)
V (Å3)
Z
90
90
90
70.992(5)
3720.9(4)
2
2079.38(7)
4
1.442
0.658
31955
6078
1.107
0.0414
0.0311
0.0787
1924.87(5)
2
1.447
0.701
28428
4969
1.103
0.0432
0.0338
0.0645
2033.84(7)
2
1.415
0.666
33774
5943
1.086
0.0392
0.0314
0.0637

calc (Mg/m3)
1.416
(K) (mm-1)
No. reflns.
Unique reflns.
GOOF (F2)
Rint
0.771
27872
17489
1.045
0.0446
0.0666
0.1468
R1a (I  2)
wR2b (I  2)
a R1 = ||Fo| – |Fc||/|Fo|. b wR2 = [[w(Fo2 – Fc2)2]/ [w(Fo ) ]]1/2.
2 2
10

Table 2. Selected bond lengths [Å] and angles [°] for complexes 1, 2, 4 and 6
4
6
[Å, °]
1
2
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(4)
Mo(1)-O(5)
Mo(1)-N(1)
O(1)-Mo(1)-O(2)
O(4)-Mo(1)-O(3)
O(3)-Mo(1)-N(1)
O(4)-Mo(1)-O(5)
N(1)-Mo(1)-O(5)
C(1)-O(1)-Mo(1)
C(19)-O(5)-Mo(1)
1.9363(12)
1.9666(11)
1.7132(12)
1.6974(13)
2.3935(12)
2.3745(13)
149.56(5)
105.53(6)
161.49(5)
170.75(5)
78.71(4)
1.9212(15)
1.9050(16)
1.6904(17)
1.7559(16)
2.4068(16)a
2.3717(19)
150.28(7)
106.80(8)
158.03(7)
176.30(7)b
82.04(6)c
1.9321(13)
1.9023(13)
1.7076(14)
1.7528(13)
2.3688(13)a
2.3010(15)
151.89(6)
108.92(6)
155.84(6)
172.43(6)b
78.97(5)c
1.899(3)
1.935(4)
1.695(4)
1.751(4)
2.287(4)d
2.354(4)
151.54(16)
104.55(19)
163.88(17)
168.59(16)
77.75(15)
140.3(3)
136.72(10)
119.01(10)
136.33(14)
128.99(11)
a A parameter for a Mo(1)-O(4)i where i = -x, -y+1, -z+1.
b O(3)-Mo(1)-O(4)i.
c N(1)-Mo(1)-O(4)i.
d A parameter for a Mo(1)-O(3)i where i = -x, -y, -z.
The molecular structures of all complexes reveal hexacoordinate metal centres in
distorted octahedral geometries. In complex 1, the fully deprotonated ligand (L1)2- is
coordinated to the cis-dioxidomolybdenum centre in a tridentate coordination mode through its
alkoxido oxygen, amine nitrogen and phenolate oxygen atoms. The remaining coordination site
trans to one oxido ligand is occupied by the oxygen of a coordinated methanol molecule. The
Mo•Osolvent bond trans to the oxido group is considerably longer than the Mo–Ophenolate bond
due to the strong trans influence of the oxido group. These features as well as the O=Mo=O
angles and the Mo=O bonds are similar to those found for other mononuclear cis-[MoO2]2+
complexes [9,30-33]. The coordinated MeOH molecule from the adjacent unit forms a
hydrogen bond to the oxido ligand O(2), with the observed O(5)-H(5) ···O(2)i (i = -x+1, -y+1,
-z+1) distance of 2.6976(16) Å being indicative of rather strong intermolecular hydrogen
bonding in the solid state. The overall structure of 1 can be compared with those obtained for
mononuclear dioxidomolybdenum(VI) complexes [MoO2(L)(ROH)], where
L
=
aroylhydrazone Schiff base ligand or ephedrine derivative ligand, R = Et or Me [11,34-36].
In the solid state, complexes 2 and 4 have a dinuclear structure, which consists of
doubly bridged units of [Mo2O2(μ-O)2(Ophenolate,Namine,Oalcoholate)2]. Each molybdenum(VI) ion
is bonded to three donor atoms Ophenolate,Namine,Oalcoholate of the tridentate ligands, two bridging
oxygen atoms and one terminal oxido ligand to yield a distorted octahedral coordination sphere.
Both complexes have a centrosymmetric structure with identical conformations. The Mo(VI)
ions with asymmetric di-μ-oxido-bridges form four-membered Mo–O–Mo–O rings, the Mo–O
distances being (1.7559(16) and 2.4068(16)) Å for 2 and (1.7528(13) and 2.3688(13)) Å for 4.
The long Mo···O distances indicate a rather weak coordination, which is seen in the reactivity
of the complexes. All Mo=O and Mo···O distances are rather similar to those observed for the
closely related dinuclear dioxidomolybdenum(VI) complexes [MoO2(L)]2, where H2L = an
11

aroylhydrazone ligand, an (R)-1,1'-binaphthyl-based dioxidoanionic pyridine ligand or a
tridentate Schiff base ligands [33,37-39]. In general, there are rather small differences between
the structural parameters around the Mo centre in monomeric 1 compared with the dimeric 2
and 4 (see Table 2).
The solid state structure of 6 consists of a trinuclear core where the molybdenum and
oxygen atoms form a planar six-membered Mo3O3 ring (Figure 5), so the complex unit in 6 has
a formal 3-fold axis. Each Mo(VI) ion is bonded to three donor atoms of the organic ligand as
well as two bridging and one terminal oxido ligand, resulting in a distorted octahedral
coordination sphere. One alcohol OH donor group of the ligand is not participating in the
coordination to the metal centre. The oxygen bridges are asymmetric, e.g. the Mo1-O1 distance
is 1.695(4) Å whereas the Mo1-O3 distance is 2.287(4) Å, which indicates a rather weak
intramolecular association of the monomeric species through a Mo=O→Mo coordination and
the Mo=O···Mo bridges are unsymmetrical in the dimeric and trimeric Mo complexes. The
terminal oxido groups are located in the Mo3O3 plane and the tridentate ligand is bonded to the
Mo(VI) centres in a meridional coordination mode, vertical to the Mo3O3 plane. The trimeric
structure is stabilized by three intramolecular H-bonds between the OH groups of the alcohol
side-arms and the alkoxide oxygen atoms of the adjacent Mo-centred units with O···O
distances of 2.747(5)-2.769(5) Å. The overall structure is closely related to [{MoO2(L)}3] (H2L
= 2,4-di-tert-butyl-6-(((2-hydroxy-2-phenylethyl)amino)methyl)phenol) reported previously
by us [29]. Typically, the dioxidomolybdenum(VI) complexes with ONO-type tridentate
ligands seem to complete the octahedral coordination sphere by a coordinating solvent
molecule, if present, or by oxido ligand from the adjacent molybdenum complex unit to form
a dinuclear oxo-bridged complex [10,40].
Figure 2. The molecular structure of [MoO2(L1)(MeOH)] (1). Hydrogen atoms are omitted for
the sake of clarity. Thermal ellipsoids are drawn at the 50% probability level.
12

Figure 3. The molecular structure of [Mo2O2(μ-O)2(L1)2] (2). Hydrogen atoms are omitted for
the sake of clarity. Thermal ellipsoids are drawn at the 50% probability level.
Figure 4. The molecular structure of [Mo2O2(μ-O)2(L2)2] (4). Hydrogen atoms are omitted for
the sake of clarity. Thermal ellipsoids are drawn at the 50% probability level.
13

Figure 5. The molecular structure of [Mo3O3(μ-O)3(L3)3] (6). Hydrogen atoms and solvent
molecules are omitted for the sake of clarity. Thermal ellipsoids are drawn at the 50%
probability level.
3.3. Catalytic epoxidation
Complexes 1-6 were tested as catalysts for epoxidation of the five olefinic substrates
cis-cyclooctene S1, 1-octene S2, styrene S3, limonene S4 and α-terpineol S5 (Figure 6), using
three equivalents of tert-butyl hydroperoxide (TBHP, 5.5 M in decane) or aqueous hydrogen
peroxide (H2O2, 30%) as oxidants and 1 mol% catalyst loadings. The reactions were run at 50
°C in CHCl3 solutions and conversions to epoxides were followed by GC–MS. In the case of
substrates S4 and S5, racemic mixtures of the D- and L-enantiomers were used. The results are
shown in Table 3.
Figure 6. Olefinic substrates used for epoxidation experiments.
14

With TBHP as oxidant, all six Mo complexes convert S1 quantitatively to its epoxide
with excellent selectivities within 24 h, reaching yields of >95% epoxide. Interestingly, there
was no significant difference in activity observed for monomeric complexes 1 and 3 with
respect to their dimeric counterparts 2 and 4. These four complexes also did not differ
significantly from each other in catalytic activity, which is probably due to their similar ligand
backbone. The third monomeric complex used in this study, complex 5, showed the highest
activity for cyclooctene epoxidation, reaching quantitative conversion after 4 h. In contrast,
trimeric complex 6, which contains 5 as a subunit (if the coordinated methanol molecule in 5
is ignored), gave full conversion of S1 only after 24 h reaction time. For S3 and S5 a very
similar reactivity profile of 6 compared to 5 was observed, hinting towards a certain stability
of the trimer in solution. If 6 was to dissociate in three molecules of 5, the resulting catalyst
loading would be 3 mol%, and should therefore display an enhanced catalytic activity. Since
this is not observed in the epoxidation with 1 mol% of 6, it is feasible that the trimeric complex
6 does not dissociate during catalysis.
For the more challenging substrates S2-S5 yields and selectivities were in general lower
for all six complexes 1-6. For the epoxidation of 1-octene S2, both yields of epoxide as well as
selectivity towards epoxide varied significantly. Yields of epoxide between 23% for 2 and 57%
for 5 were observed. Whereas 2 showed high selectivity (>99%) towards epoxide, the other
five complexes varied between 68 to 96% selectivity, with 2-octanone being the major side
product. For all six complexes, catalytic activity decreased after the first 4 h, with only little
conversion to epoxide in the next 20 h. For styrene (S3) a similar picture compared to other
published Mo complexes [6e,41] was observed, with low conversions and selectivity towards
epoxide. There are only few examples of catalysts with high selectivity [6e,6g]. For all six
complexes 1-6, a maximum selectivity towards styrene oxide of 20% was observed, with
phenyl acetaldehyde and benzaldehyde being the two main over-oxidation products observed.
Also yields of styrene oxide were low, ranging between only 6% for 3 and 17% for 1.
Finally, catalytic activities and selectivities for the substrates limonene S4 and α-
terpineol S5 are also low, with the over-oxidized ketones limonene oxide and carvone being
the major side products. For both substrates S4 and S5, a similar activity profile is observed,
with a maximum of epoxide yield observed after 4h, after which the yield of epoxide drops
again due to over-oxidation by the catalysts used. Overall, the catalytic activities of complexes
1-6 compare well to related dioxomolybdenum(VI) complexes investigated under similar
conditions [6e].
15

Table 3. Conversion of substrate (selectivity to epoxide) for complexes 1-6 with TBHP as
oxidant.
[%]
cyclooctene, S1
1-octene, S2
1
2
3
4
5
6
>95 (>99)
55 (85)
90 (19)
>95 (18)
>95 (26)
>95 (>99)
23 (>99)
65 (22)
>95 (35)
>95 (22)
>95 (>99)
52 (68)
94 (6)
>95 (30)
26 (26)
>95 (>99)
51 (74)
86 (11)
>95 (25)
>95 (23)
>95 (>99)[a]
>95 (>99)
31 (96)
74 (14)
>95 (59)
>95 (30)
60 (96)
71 (15)
>95 (27)
>95 (25)
styrene, S3
limonene, S4[b]
α-terpineol, S5[b]
[a] complete conversion after 4 h; [b] conversion of substrate (selectivity to epoxide) after 4h
In a second round of experiments, the more eco-friendly oxidant H2O2 was tested under
the same conditions as with TBHP. In this case, conversion of substrate to epoxide was only
observed with cyclooctene S1 (Table 4), but no conversions were observed for S2-5; this lack
of reactivity was also observed for similar molybdenum complexes [42].
Table 4. Conversion of substrate (selectivity to epoxide) for complexes 1-6 with H2O2 as
oxidant.
[%]
1
2
3
4
5
6
cyclooctene, S1 19 (72)
37 (80)
23 (82)
84 (86)
13 (55)
74 (75)
From the data in Table 4, it is evident that the dimeric complex 4 shows the highest
activity in cyclooctene (S1) epoxidation with H2O2 as oxidant. The reason for this enhanced
reactivity, especially when compared to dimeric complex 2, is not clear. While the selectivities
for epoxide are similar for these two dimeric complexes, the conversion rate is much higher for
4; this may be related to 4 being more prone to formation of (active) mononuclear
species/catalysts upon reaction with H2O2, thus effectively increasing the catalyst loading. The
monomeric complexes 1 and 3 show essentially similar catalytic activities, indicating that the
substitution pattern on the ligand backbone has only a small effect on catalytic activity for these
catalysts.
Finally, comparison with two previously published, structurally related ligands and
their respective Mo(VI) complexes is of interest (Figure 7). The Schiff-base ligand HL4,
functionalized with a pending ethyl-methoxy arm, [6f] coordinates to the dioxo-
molybdenum(VI) core in a bidentate, monoanionic fashion, resulting in the mononuclear
complex [MoO2(L4)2]. In addition, the corresponding, reduced Schiff-base ligand HL5
exhibiting an NH moiety and a pending ethyl-methoxy arm, [6g] coordinates to the dioxo-
molybdenum(VI) core in a bidentate, monoanionic fashion, however resulting in the dinuclear,
µ-oxo bridged complex {[MoO2(L5)]}2(µ-O). Both complexes [MoO2(L4)2] and
{[MoO2(L5)]}2(µ-O) showed excellent selectivities (>98 %) in epoxidation reactions including
for the challenging substrates S2-5 [6f,6g]. This is ascribed to the influence of the pending
methoxy substituent which can possibly form hydrogen bonds during the catalytic reaction and
which has been predicted to play an important role by DFT calculations [43]. The lack of such
a pending donor in the structurally related complexes 1 and 4 for example, may explain the
relatively low observed selectivities with substrates S2-5 (Table 3).
16

Figure 7. Comparison of structurally related ligands: H2L1 and HL4 [6f]; H2L2 and HL5 [6g].
4. Conclusions
Three mononuclear (1, 3, 5), two dinuclear (2, 4) and one trinuclear (6) molybdenum
complexes with tridentate aminoalcohol phenol ligands were synthesised and characterized.
All complexes are active and selective catalysts towards the epoxidation of cis-cyclooctene
using tert-butyl hydroperoxide (TBHP) and hydrogen peroxide as oxidants. In addition, all
studied complexes showed catalytic activity for more demanding substrates (1-octene, styrene,
limonene, α-terpineol) with TBHP as an oxidant, although the conversions and selectivities
were generally poor. Complex 6 appears to remain trimeric in solution, judging from the
different epoxidation activities in comparison to the analogous monomer 5. With H2O2 as
oxidant, the differences in epoxidation activity of S1 are more pronounced between the six
complexes. Complex 4 showed the highest activity with 72% yield of epoxide (conversion x
selectivity), whereas 5 gave the lowest yield of epoxide of 7%. The opposite trend was observed
for epoxidation with TBHP, where complex 5 was the most active catalyst of all six tested
complexes. Stability and solubility in the aqueous media of hydrogen peroxide might be
important factors that influence the observed differences in catalytic activity.
Acknowledgements
This paper is dedicated to Rabindranath Mukherjee, a friend, mentor, collaborator and
academic leader. We gratefully acknowledge financial support from the COST Action CM1003
Biological oxidation reactions - mechanisms and design of new catalysts. M.K.H. thanks the
European Commission for the award of an Erasmus Mundus predoctoral fellowship.
Appendix A. Supplementary material
Crystallographic data (excluding structure factors) for the structures in this paper have
been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road,
17

Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the
depository number CCDC 1847230-1847233 (Fax:+44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
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Highlights ICA_2018_809
 Six new Mo(VI) oxo complexes have been synthesized and characterized.
 All complexes are based on aminoalcohol phenolate ligands.
 Three complexes are mononuclear, two are dinuclear and one is trinuclear.
 All complexes catalyse epoxidation of olefins by H2O2 or tBuOOH.
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Graphical Abstract
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