Oxygen atom transfer catalysis by dioxidomolybdenum(VI) complexes of pyridyl aminophenolate ligands

Article abstract of DOI:10.1016/j.poly.2021.115234

A series of new cationic dioxidomolybdenum(VI) complexes [MoO₂(Lⁿ)]PF₆ (2–5) with the tripodal tetradentate pyridyl aminophenolate ligands HL²-HL⁵ have been synthesized and characterized. Ligands HL

Full text of DOI:10.1016/j.poly.2021.115234

Journal Pre-proofs
Oxygen Atom Transfer Catalysis by Dioxidomolybdenum(VI) Complexes of
Pyridyl Aminophenolate Ligands
Md. Kamal Hossain, Jörg A. Schachner, Matti Haukka, Michael G.
Richmond, Nadia C. Mösch-Zanetti, Ari Lehtonen, Ebbe Nordlander
PII:
S0277-5387(21)00216-3
DOI:
Reference:
https://doi.org/10.1016/j.poly.2021.115234
POLY 115234
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
8 February 2021
18 April 2021
19 April 2021
Please cite this article as: Md. Kamal Hossain, J.A. Schachner, M. Haukka, M.G. Richmond, N.C. Mösch-
Zanetti, A. Lehtonen, E. Nordlander, Oxygen Atom Transfer Catalysis by Dioxidomolybdenum(VI) Complexes
of Pyridyl Aminophenolate Ligands, Polyhedron (2021), doi: https://doi.org/10.1016/j.poly.2021.115234
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Oxygen Atom Transfer Catalysis by Dioxidomolybdenum(VI) Complexes of
Pyridyl Aminophenolate Ligands
Md. Kamal Hossain*[a,b], Jörg A. Schachner* , Matti Haukka , Michael G.
[c]
[d]
[e]
[c]
[f]
[a]
Richmond, Nadia C. Mösch-Zanetti , Ari Lehtonen and Ebbe Nordlander*
a. Chemical Physics, Department of Chemistry, Lund University, P.O. Box 124, SE-
2100 Lund, Sweden
2
b. Department of Chemistry, Jahangirnagar University, Savar, Dhaka-1342,
Bangladesh
c. Institute of Chemistry, Inorganic Chemistry, University of Graz, Schubertstrasse
1, 8010 Graz, Austria
d. Department of Chemistry, P.O. Box 35, University of Jyväskylä, FI-40014
Jyväskylä, Finland
e. Department of Chemistry, University of North Texas, 1155 Union, Circle#305070,
Denton, TX 76203, USA
f. Inorganic Materials Chemistry Research Group, Laboratory of Materials
Chemistry and Chemical Analysis, Department of Chemistry, University of Turku,
2
0014, Turku, Finland
Corresponding author
E-mail address: Ebbe.Nordlander@chemphys.lu.se (E. Nordlander)
*
1

ABSTRACT
A series of new cationic dioxidomolybdenum(VI) complexes [MoO (L )]PF (2-5) with
n
2
6
2
5
the tripodal tetradentate pyridyl aminophenolate ligands HL -HL have been synthesized
2
4
and characterized. Ligands HL -HL carry substituents in the 4-position of the phenolate
5
ring, viz. Cl, Br and NO , respectively, whereas the ligand HL , N-(2-hydroxy-3,5-di-tert-
2
butylbenzyl)-N,N-bis(2-pyridylmethyl)amine,
is
a
derivative
of
3,5-di-tert-
butylsalicylaldehyde. X-ray crystal structures of complexes 2, 3 and 5 reveal that they
have a distorted octahedral geometry with the bonding parameters around the metal
centres being practically similar. Stoichiometric oxygen atom transfer (OAT) properties
3
1
of 5 with PPh were investigated using UV-Vis, P NMR and mass spectroscopy. In a
3
5
CH Cl solution, a dimeric Mo(V) complex [(µ-O){MoO(L )} ](PF ) 6 was formed while
in methanol solution an air-sensitive Mo(IV) complex [MoO(OCH )(L )] 7 was obtained.
2
2
2
6 2
5
3
The solid-state structure of the µ-oxo bridged dimer 6 was determined by X-ray
diffraction. Complex 7 is unstable under ambient conditions and capable of reducing
DMSO, thus showing reactivity analogous to that of DMSO reductases. Similarly, the
OAT reactions of complexes 2-4 also resulted in the formation of dimeric Mo(V) and
unsaturated monomeric Mo(IV) complexes that are analogous to complexes 6 and 7.
Catalytic OAT at 25 ºC could also be observed, using complexes 1-5 as catalysts for
oxidation of PPh in deuterated dimethylsulfoxide (DMSO-d ), which functioned both as
3
6
a solvent and oxidant. All complexes were also tested as catalysts for sulfoxidation of
methyl-p-tolylsulfide and epoxidation of various alkene substrates with tert-butyl
hydroperoxide (TBHP) as an oxidant. Complex 1 did not exhibit any sulfoxidation
activity under the conditions used, while 2-5 catalyzed the sulfoxidation of methyl-p-
tolylsulfide. Only complexes 2 and 3, with ligands containing halide substituents
exhibited good to moderate activity for epoxidation of all alkene substrates studied, and,
in general, good activity for all molybdenum(VI) catalysts was only exhibited when cis-
cyclooctene was used as a substrate. No complex catalysed epoxidation of cis-
cyclooctene when an aqueous solution of H O was used as potential oxidant.
2
2
Keywords: Molybdenum • Tripodal tetradentate ligand • Epoxidation • Oxygen atom
transfer
2

1. Introduction
Mononuclear molybdenum enzymes are widely distributed in nature, and they
are generally involved in oxygen atom transfer reactions to or from biological oxygen
acceptors/donors.[1-3] These enzymes also contribute to the carbon, nitrogen, and sulfur
cycles and their active sites contain an organic cofactor called molybdopterin which
consists of a molybdenum atom and one or two organic pterin derivatives.[4,5] The
mononuclear molybdenum enzymes can be divided into two subcategories -
[
3]
hydroxylases and oxotransferases - under the Hille classification. The latter class can
be subdivided into two families based on their structures, i.e. the sulfite oxidase (SO)
and DMSO reductase (DMSOR) families.[3]
A number of complexes have been studied as structural and/or functional models
for molybdenum-containing oxotransferase enzymes that catalyse oxygen atom transfer
(
OAT) reactions.[1,2,6-13] The OAT reactions between phosphines and oxygen donors
have been studied as models for biological reactions. The generally accepted
mechanism for oxygen atom transfer to a phosphine proceeds via bonding between the
phosphine and one of the oxo ligands which leads to a transient intermediate, and
concomitant bridged electron transfer to the molybdenum centre.[14,15] This general type
of mechanism – for both oxidative and reductive OAT - has been studied and is strongly
supported by numerous computational studies.[16-19] Oxidomolybdenum species
containing N-, O- and S-donor ligands have been studied extensively as catalysts for
OAT reactions.[20-24]
Sulfoxides are important synthons for the synthesis of natural and artificial
compounds and are used in medicinal chemistry.[25-27] Similarly, epoxides are valuable
intermediates and targets in laboratory synthesis, and are important feedstocks in
pharmaceutical, agrochemical and chemical industry.[28-31] Molybdenum-based catalysts
are effective for the epoxidation of unfunctionalized olefins. Particularly,
oxidomolybdenum complexes also have potential applications in the field of asymmetric
epoxidation of chiral alkenes.[32,33] We are interested in the development of new,
relatively inexpensive molybdenum(VI) based catalysts for catalytic oxidation reactions
that are also significant in the chemical industry, such as sulfoxidation and expoxidation
3

reactions,.[34-39] We have also investigated the reactivities of molybdenum oxo
complexes as bioinspired models for molybdenum-containing OAT enzymes.[1,40]
There are previous reports on metal complexes with phenols carrying di-(2-
picolyl)amine substituents at the ortho positions. For example, the tripodal ligands HL1-
5
[41]
HL (Figure 1) are known to readily form coordination compounds with Mg(II), Zn(II),
Fe(III),[42] V(V),[43] Eu(III) and Yb(III).[44] The catalytic activities of complexes based on
these ligands have been studied, e.g. the catechol dioxygenase and proton reduction
properties of iron complexes, ring-opening polymerization of ɛ-caprolactone by zinc
complexes, and catalytic activities for ethylene polymerization of vanadium complexes.
We have previously reported the dioxidomolybdenum(VI) complex 1 of the
1
[1]
pyridylaminophenolate ligand HL but its catalytic oxidation chemistry was not studied.
In this contribution, we present the syntheses and characterizations of four MoO (VI)
.
2
complexes (2-5) containing related pyridyl aminophenolate ligands with different
electron-donating and -withdrawing groups on the ligand backbone (Figure 1 and
Scheme 1), and explore the catalytic applications of 1-5 in oxygen atom transfer,
sulfoxidation and epoxidation reactions with various substrates. A dinuclear oxido-
bridged molybdenum(V) complex 6 and a mononuclear MoO(IV) complex 7 were
isolated as products in the OAT reactions of 5 with PPh3.
OH
OH
OH
N
N
N
tBu
N
N
N
tBu
X
N
N
N
X = Cl, Br, NO2
HL -HL4
HL1
2
HL5
Figure 1. Structures of the tripodal ligands employed in the study.
4

2. Results and discussion
2.1. Synthesis and characterization of complexes
1
5
The tripodal tetradentate pyridylaminophenolate ligands HL -HL were prepared
in a straightforward manner by reductive amination, a known procedure starting from di-
2-picolyl)amine and the corresponding salicylaldehyde.[42,45,46] Complex 1 was prepared
following a published procedure,[1] and the other complexes were synthesised
(
2
5
analogously. The reaction of [MoO Cl ] with the tripodal ligands HL -HL in the
2
2
presence of NaOMe and KPF in methanol results in the formation of cationic
6
n
+
complexes of the type [MoO (L )] (Scheme 1). In each case, the reaction mixture was
2
diluted with diethyl ether and hexane, filtered through a pad of Celite and evaporated to
dryness. Crystallisation from methanol gave pure products in 61-75% yields. Complexes
2
5
2
-5 can also be prepared by the reaction of HL -HL with other molybdenum precursors,
such as [MoO (acac) ] or [MoO Cl (dmso) ], followed by the above synthetic protocol.
2
2
2
2
2
O
O
OH
R1
N
Mo
N
R1
O
N
NaOMe
[
MoO Cl ]
+
2 2
PF6
N
R2
R2
MeOH, KPF6
N
N
1
2
R = R = H
1
2
1
2
R = H, R = Cl
R = R = H, 1
1
2
1
2
R = H, R = Br
R = H, R = Cl, 2
1
2
1
2
R = H, R = NO
R = H, R = Br, 3
2
1
2
1
2
R = R = tBu
R = H, R = NO , 4
2
1
2
R = R = tBu, 5
Scheme 1. Synthetic route of dioxidomolybdenum(VI) complexes 1–5.
1
5
The tripodal ligands HL -HL can coordinate to the cis-dioxidomolybdenum
moiety to form two possible configurational isomers, i.e. the methylpyridyl groups may
be coordinated cis or trans to each other. One isomer (isomer A in Figure 2; X =H) has
these two pyridyl side-arms donor located in trans positions to the phenolate and oxido
5

ligands whereas in another isomer (B), the phenolate moiety is trans to one oxido
1
+ [1]
ligand. Previous studies on Mo and Fe complexes, for example [MoO (L )] ,
2
1
[46,47]
4 [48]
[
FeCl (L )],
and [FeCl (L )] have shown that such ligands tend to coordinate with
2
2
the pyridyl arms cis to each other (Figure 2, isomer A), a feature that was confirmed by
DFT calculations.[1] The structures of the two isomers based on complex 1 were
optimized and these optimized structures are shown in Figure 2. Isomer A was
computed to be 9.3 kcal/mol more stable than isomer B. Complex 1 is a known
compound, but prepared during this work to evaluate the catalytic efficiency.
+
O
O
+
O
O
N
Mo
N
N
Mo
O
N
N
X
O
N
X
B
A
Figure 2. Two possible configurational isomers for the studied complexes with the DFT-
optimized structures of each isomer of complex 1.
The solubility in common solvents varies with the electron-withdrawing and
electron-releasing substituents on the phenyl rings. Complexes 2-4 are readily soluble
in methanol and acetonitrile whereas they are less soluble in chlorinated solvents
(
CHCl , CH Cl ). Complex 5 is soluble in both polar and non-polar organic solvents due
3 2 2
6

1
to the presence of the tert-butyl substituents. All complexes were characterized by H,
13C, 31P and 19F NMR, IR spectroscopy and mass spectrometry. The structures of
complexes 2, 3 and 5 were also verified by single-crystal X-ray crystallography (vide
infra). As anticipated, complexes 2-5 have relatively similar NMR and IR spectra. The
infrared spectra of all studied complexes showed two strong bands around 880-905 and
9
10-950 cm-1 which are assigned to asymmetric and symmetric M=O stretches,
respectively.[49-52] The H NMR spectra were recorded in CD OD solutions while the
1
3
chemical shifts for the phenolate ring protons as well as for the two inequivalent pyridine
rings were seen around 6.5–9.3 ppm as 11 sharp signals for 2-4 and 10 resonances for
5
(cf. ref 1). This indicates that the two pyridyl side-arm donors are located in cis
position, so isomer A (Figure 2) is the prevalent species in solution. The chemical shifts
for the two tert-butyl groups of 5 are within the expected region. The singlets of the six
N-CH methylene protons exhibit separated resonances within the range of 4-6 ppm
2
which is consistent for all three pendant arms of the ligand being inequivalent. The NMR
spectra match well with the solid state structures (vide infra).
When the reaction of 5 with PPh in dichloromethane was studied under N
3
2
atmosphere, the solution turned red and then the colour changed to dark purple. After
1
half an hour, the solution was filtered and the dark purple solid was collected. The H
NMR spectrum of the complex showed rather complicated and broad signals indicated
the formation of a paramagnetic Mo(V) complex. The UV-vis spectrum is consistent with
the dimeric complex prepared previously by the reaction of 1 under similar conditions.[1]
5
The formation of the dark purple Mo(V) dimer [(µ-O){MoO(L )} ](PF ) 6 was verified by
2
6 2
the observation of a peak in its electrospray high resolution mass spectrum that
matches the expected empirical formula (see Figure S1 and Scheme 2). Single crystal
X-ray crystallographic analysis revealed that 6 is a dinuclear oxo-bridged Mo(V)
complex (see Figure S2 and crystallographic data is summarized in Table S1).
When the reaction of 5 with 1.5 equivalents of PPh was studied in a methanol
3
solution, the yellow colour of the solution turned red within fifteen minutes. The reaction
led to the formation of OPPh3 (as identified by 31P NMR) and a reduced
5
oxidomolybdenum(IV) complex. The red complex was identified as [MoO(OCH )(L )] 7
3
+
through a high resolution mass spectrum, which shows a molecular ion peak [M] with a
7

characteristic molybdenum pattern (see Figure S3). Complex 7 is comparatively stable
under the mother liquor but it decomposes upon exposure to air. The red complex
reduces biological oxygen donors such as dimethyl sulfoxide (DMSO) albeit in a slow
reaction, as evidenced by the reformation of 5 seen by UV-Vis spectroscopy. This
phenomenon is also closely related with our previous studies.[1] Unfortunately, all
attempts to isolate the desired complex 7 were unsuccessful, leading only to the
formation of small amounts of the dark purple Mo(V) oxo dimer 6 (vide supra).
N
tBu
tBu
O
N
O
O
5
+ PPh3
Mo
(PF6)2
Mo
CH Cl
N
O
2
2
O
N
N
N
tBu
tBu
6
5
Scheme 2. Synthesis of Mo(V) dimer [(µ-O){MoO(L )} ](PF ) 6
2
6 2
The analogous oxygen atom transfer reactions to PPh were studied under
3
equivalent conditions for complexes 2-4 in both dichloromethane and methanol. The
results were analogous to the above-mentioned reactions; in the potentially coordinating
solvent methanol, red unstable Mo(IV) complexes of the general formula
[
MoO(OCH )(L)] (L=pyridine aminophenolate ligand) could be identified, while in the
3
2+
usually non-coordinating solvent dichloromethane, purple [(µ-O){MoO(L)} ] complexes
2
were formed. The Mo(V) dimers and unstable Mo(IV)O(OCH ) complexes were
3
1
characterized by the H NMR spectra and UV-vis spectroscopy, respectively, and the
spectra correspond to those for 6 and 7.
Electrospray ionization (ESI) mass spectrometry was used for the
characterization of complexes 2-7. The mass spectrum of 2 shows a molecular ion [M]+
2
+
peak at m/z = 468 and a ligand molecular ion at [HL +H] at m/z = 340. Similarly,
complexes 3 and 4 exhibit peaks at m/z = 512 and m/z = 479, respectively, which can
8

+
be assigned to the molecular ion [M] peak. The ligand molecular ions were observed at
3
+
4
+
[
HL +H] at m/z = 384 and [HL +H] at m/z = 351, accordingly. Complex 5 is identified
+
5
+
as molecular ion [M] peak at m/z = 546 and the corresponding ligand peak [HL +H] at
m/z = 418. In addition, in solution 5 shows a Mo(V) dimeric complex as molecular ion
5
+
[
(µ-O){MoO(L )} +NaOMe] at m/z = 1126 due to the samples injected as
2
dichloromethane solutions, preceded and followed by methanol. The characteristic
molecular ion peaks for complexes 6 and 7 are at m/z = 1073 and m/z = 561,
respectively.
2.2. Description of the X-ray crystal structures
Single crystals of 2, 3 and 5 suitable for crystallographic analysis were obtained
from concentrated methanol solutions at −4 °C and their solid state structures were
determined by X-ray diffraction. The crystallographic data together with the collection
and refinement parameters for complexes are summarized in Table 1. Selected bond
distances and angles are listed in Table 2. In all structures, there is one cationic
-
complex and one PF anion in the asymmetric unit. The molecular structure of 2, 3 and
6
5
are illustrated in Figures 3, 4 and 5, respectively. All complexes have a distorted
octahedral geometry in which the two cis-positioned oxido ligands bind strongly to the
Mo(VI) ion, while the geometrical parameters are very similar. The two pyridyl rings are
almost perpendicularly aligned relative to each other in complexes 2, 3 and 5; the N(1)-
Mo(2)-N(3) angle is 84.35(10)° for 2, 84.15(9)° for 3 and 83.38(7)° for 5, respectively. In
the three complexes, the three oxygen donor atoms are in the fac arrangement of a
distorted octahedron where all three donor groups are about 105º-108º apart. The
structures confirm that two nitrogen atoms in the pyridine rings of the chelating amine
bis-pyridine phenolate ligand are located in cis positions with the N(1)-Mo(1)-N(2) and
N(3)-Mo(1)-N(2) bond angles are 70.87(10)° and 71.98(9)° for 2, 70.86(9)° and
71.70(8)° for 3 and 71.32(7)° and 71.91(6)° for 5, respectively. One pyridyl group is
coordinated trans to the aryloxide group and the other one is trans to the oxido ligand
O3 while the central amine nitrogen N2 of the ligand backbone is coordinated trans to
the terminal oxido ligand O2. The bond distances of the oxido ligands O2 and O3 and
9

the O(2)–Mo(1)–O(3) angle of complexes 2, 3 and 5 are within the expected range and
comparable with previously reported six-coordinate dioxidomolybdenum complexes.[1,21]
The Mo(1)–N(3) bond distance, 2.298(3) Å for 2, 2.302(2) Å for 3 and 2.3120(18) Å for
5, respectively, that is slightly longer than the Mo(1)–N1 bond distance, 2.183(3) Å for 2,
2.181(2) Å for 3 and 2.192(2) Å for 5, respectively due to the strong trans influence of
the oxido ligand compared to the phenolate moiety. The bond distances Mo(1)–N1 and
Mo(1)–N(3) are seemingly shorter than the Mo(1)–N(2) bond distance (2.322(3) Å for 2,
2.332(2) Å for 3 and 2.3116(18) Å for 5, respectively) indicating that the central nitrogen
atom of the ligand backbone is coordinated more weakly to the Mo centre than the
pyridine N-donors. All these Mo–N distances are shorter than in previously reported
2−
[
MO (L–O )] complexes with Mo–N bond lengths >2.30 Å, where L–O
2
is the
2
2
corresponding tripodal ligand with one pyridyl and two phenolate rings.[53]
(a)
10

(b)
11

(c)
2
+
Figure 3. Mercury diagrams of the molecular structures of (a) [MoO (L )] (2) (b)
2
3
+
2 +
[
MoO (L )] (3) (c) [MoO (L5 )] (5) showing parts of the numbering schemes. Thermal
2 2
ellipsoids are plotted at the 30% probability level. Hydrogen atoms and the
hexafluorophosphate anions are omitted for clarity of presentation.
Single crystals of 6 were grown from acetonitrile solution with excess of NaBPh4
in order to ascertain the dimeric framework. Unfortunately, the very thin needle-shaped
crystals gave weak diffraction patterns with both Mo-Kα radiation leading to the high R-
values because of the lack of the good quality data set. However, the dimeric nature of
6
could be proven (see Figure S2).
12

Table 1. Crystallographic data for complexes 2, 3 and 5.
2
3
5
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C
19
H
17ClF
6
MoN
3
O
3
P
C
19
H
17BrF
6
MoN
3
O
3
P
C
27
H
34
F
6
3 3
MoN O P
611.71
170(2)
0.71073
Triclinic
P1
8.3565(3)
8.6310(2)
15.4995(5)
91.456
656.17
170(2)
0.71073
Triclinic
P1
8.5034(3)
8.6577(2)
15.3716(5)
90.956
689.48
120(2)
0.71073
Monoclinic
P2 /n
17.8055(6)
12.1599(3)
13.9951(4)
90
1
β (°)
γ (°)
Volume (Å3)
Z
96.2870(10)
95.183(2)
1105.93(6)
2
96.3750(10)
95.868(2)
1118.31(6)
2
97.2110(10)
90
3006.16(15)
2
Density (calculated) (Mg
1.837
1.949
1.523
−
3
m )
−
1
μ (Mo Kα) (mm )
F(000)
0.865
608
2.522
644
0.560
1408
Crystal size (mm)
θ range (°)
Limiting indices
0.361x 0.128 x 0.069
2.371 to 30.095
-11 ≤ h ≥ 11
0.212 x 0.194 x 0.148
2.366 to 35.145
-13 ≤ h≥ 13
0.375 x 0.262 x 0.260
2.591 to 30.074
-25 ≤ h ≥ 24
-
-
12 ≤ k ≥ 12
21 ≤ l ≥ 21
-13 ≤ k ≥ 13
-24 ≤ l ≥ 24
-17 ≤ k ≥ 12
-19 ≤ l ≥ 19
Reflections collected
23338
29459
34997
Independent reflections
6431 (0.0267)
9715 (0.0413)
8733 (0.0343)
int
(R )
Completeness to theta =
6.000°
Absorption correction
99.7 %
98.6 %
99.5 %
2
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Max. and min. transmission 0.7460 and 0.6948
0.7469 and 0.6017
Full-matrix least-squares
0.7460 and 0.6226
Full-matrix least-
Refinement method
Full-matrix least
2
2
2
squares on F
on F
squares on F
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I > 2σ(I)]
6
1
431 / 56 / 302
.053
9715 / 56 / 302
1.065
8733 / 0 / 376
1.078
R
1
= 0.0482
wR = 0.1253
= 0.0567
wR = 0.1334
.674 and -0.913
R
1
= 0.0517
wR = 0.1316
= 0.0715
wR = 0.1469
1.636 and -1.427
R
1
= 0.0415
wR = 0.0985
= 0.0508
wR = 0.1054
0.970 and -1.066
2
2
2
R indices (all data)
R
1
R
1
R
1
2
2
2
Largest difference in peak
1
−
3
and hole (e Å )
13

Table 2. Selected bond lengths [Å] and angles [°] for complexes 2, 3 and 5
5
[
Å, °]
2
3
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-N(1)
Mo(1)-N(2)
Mo(1)-N(3)
1.927(2)
1.704(3)
1.700(3)
2.183(3)
2.322(3)
2.298(3)
1.913(2)
1.699(2)
1.700(2)
2.181(2)
2.332(2)
2.302(2)
1.9091(17)
1.7083(17)
1.7058(18)
2.192(2)
2.3116(18)
2.3120(18)
O(1)-Mo(1)-O(2)
O(1)-Mo(1)-O(3)
O(2)–Mo(1)–O(3)
N(1)-Mo(1)-N(2)
N(3)-Mo(1)-N(1)
N(3)-Mo(1)-N(2)
105.68(14) 106.14(10) 107.25(8)
100.23(12) 100.62(11) 102.01(9)
107.17(14) 106.83(13) 107.00(9)
70.87(10)
80.77(10)
89.26(12)
70.86(9)
84.15(9)
71.70(8)
71.32(7)
83.38(7)
71.91(6)
Mo(1)–O(1)–C(19) 131.43(19) 132.10(17) 134.58(13)
14

3. Catalysis studies
3.1. Catalytic oxygen atom transfer
Catalytic oxygen atom transfer (OAT) properties of complexes 1-5 to the non-
biological substrate triphenylphosphine were investigated (Scheme 3) in deuterated
dimethyl sulfoxide (DMSO-d ) while the reaction course was followed by 31P NMR.
6
Preliminary experiments showed that reactions are very fast with catalyst loadings of 1-
1
0 mol- % at 65 ºC. Under such conditions, PPh was fully oxidized into OPPh (as
3
3
seen by 31P NMR at around 27 ppm) within 1-2 min with all studied complexes. To
further probe the catalytic profiles of the catalysts, the reactivity screening was carried
out with low catalyst loadings (1 mol %) at 25 ºC. Complex 2 exhibited the fastest
reaction rate with a half-life of approximately 5 minutes compared to complexes 1 and 3,
which showed the half-lives of approximately 15 minutes. The sterically more hindered
complex 5 showed a lower rate of reaction (half-life approx. 1h) under similar conditions.
Under these experimental conditions, PPh was formed in a 98% yield with catalyst 2
3
after 1 h while with catalysts 1, 3 and 5, 3 hrs were required to reach a 95% yield. The
calculated turn-over frequencies (TOFs) for complexes 1, 2, 3 and 5 were of the same
-1
-1
-1
-1
order of magnitude, viz. 208 h , 296 h , 240 h and 104 h , respectively, after an
induction period of around 15 minutes whereas 4 showed the lowest activity (TOF = 40
-1
h ). The data obtained in the OAT experiments is summarized in Table 3.
O
[Mo-catalyst]
PPh3
+
S
S
OPPh3
+
Scheme 3
15

Table 3. Catalytic OAT of triphenylphosphine by dimethylsulfoxide at 25 °C with 1 mol-
of catalysts.
%
Catalyst
Yield (%)a
TON b
52
TOF (h-1) c
208
1
2
3
4
95
98d
98
51
74
296
60
240
10
40
5
92
26
104
a
1
b
Yield of the product measured by H NMR after 3 hours. TON calculated after 15 min of reaction as (mol
−
1
c
−1
−1
product) (mol catalyst) . TOF calculated after 15 min of reaction as (mol product) (mol catalyst) (t/h) .
d
1
Yield of the product measured by H NMR after 1 hour.
3.2. Catalytic sulfoxidation
The oxidation of methyl-p-tolyl sulfide with two equivalents of tert-butyl
hydroperoxide as the oxygen atom source was studied using dioxidomolybdenum
complexes 1-5 as catalysts (Scheme 4). The reactions were run under atmospheric
1
conditions in deuterated methanol solution and the reaction course was monitored by H
NMR. In control experiments that were run in the absence of catalysts no sulfoxide was
obtained. Complex 1 was found to be inactive as a sulfoxidation catalyst under the
conditions employed. In other experiments, the catalytic reactions started immediately
without any noticeable induction times. After a one-hour reaction, the highest yield
(97%) of a sulfoxide product was observed with catalyst 4 and the lowest yield (60%)
was obtained for 3. Moreover, the sulfoxidations were found to be selective, without any
sign of the formation of sulfone. All values obtained in the sulfoxidation experiments are
listed in Table 4.
O
[
Mo] = 2, 3, 4, 5
S
S
tBuOOH
CD OD
3
Scheme 4
16

Table 4. Catalytic sulfoxidation of methyl-p-tolylsulfide by tert-BuOOH at 25 °C.
Catalyst
Yield (%)a
TONb
-
TOF (h-1) c
1
2
3
4
5
-
-
70
60
97
91
30
23
77
59
124
92
308
236
a
Reaction conditions: 1 mol % of catalysts (1-5), 2.0 equivalents of tert-BuOOH at 25 °C. Yield of the
1
b
product measured by H NMR after an hour. TON calculated after 15 min of reaction as (mol product)
−
1
c
−1
−1
(
mol catalyst) . TOF calculated after 15 min of reaction as (mol product) (mol catalyst) (t/h) .
3.3. Catalytic Epoxidation
Complexes 1-5 were tested as catalysts for the epoxidation of five olefinic
substrates, viz. cis-cyclooctene S1, 1-octene S2, styrene S3, limonene S4 and α-
terpineol S5 (Figure 4). In the case of S4 and S5, the racemic mixtures of the D- and L-
enantiomers were used. The reactions were carried out with three equivalents of tert-
butylhydroperoxide (TBHP) as an oxidant and 1 mol % catalyst loadings. The reactions
were run at 50 °C in CHCl solutions and conversions to epoxides were followed by
3
GC–MS.
Figure 4. Olefinic substrates used for epoxidation experiments. In the cases of S4 and
S5, racemic mixtures of the R- and S-isomers were used.
17

As expected, all five molybdenum complexes did show catalytic activity with S1,
albeit with highly different activities (Table 5). Whereas the two halogenated complexes
2
and 3 showed complete conversion of cyclooctene to epoxide within seven and four
hours respectively, complexes 1, 4 and 5 only showed low to moderate activity with
cyclooctene S1 (Table 5). For the more challenging substrates S2-S5, again only
complexes 2 and 3 showed some catalytic activity (Table 5). For 1-octene S2,
complexes 1, 4 and 5 showed no conversion of substrate, for substrates S3-S5,
complexes 1, 4 and 5 displayed conversion of substrate, but low selectivity to epoxide
due to over-oxidation despite the relatively low excess of oxidant. For example, in
styrene epoxidation phenyl acetaldehyde and benzaldehyde were formed; for limonene
and α-terpineol formation of the respective ketones was observed in GC-MS
spectrometry (Table 5).
Table 5. Conversion of substrate (selectivity to epoxide) for complexes 1-5.
[
%]
1
2
3
4
5
cyclooctene, S1
48 (81)
>95 (99)[a] >95 (99)[b]
59 (97)
no conv.
unsel.
unsel.
unsel.
86 (93)
no conv.
unsel.
unsel.
unsel.
1-octene, S2 no conv.
33 (92)
64 (22)
>95 (35)
70 (22)
29 (97)
59 (63)
>95 (47)
88 (53)
styrene, S3
unsel.
unsel.
unsel.
limonene, S4
α-terpineol, S5
General conditions: 1 mol % catalyst, 0.5 ml CHCl
3
, 50 °C, 3 equiv. TBHP, conversion of substrate
(selectivity to epoxide) after 24h; no conv. = no substrate conversion, unsel. = selectivity to epoxide <10
%
; [a] max. yield of epoxide reached after 7 h; [b] max. yield of epoxide reached after 4 h
From the observed catalytic activities in olefin epoxidation, it can be concluded that
complexes 1-5 are very sensitive to the substituents present in the ligand backbone.
The presence of halide substituents like Cl in complex 2 and Br in complex 3 results in
epoxidation catalysts with reasonable activity. The absence of substituents, as in 1, or
t
the presence of Bu-groups (5) or a NO -group (4) do not result in active epoxidation
2
catalysts.
18

Using Hammett  constants as qualitative indicators of the relative electron-
p
withdrawing/electron-donating properties of the different substituents, it may be noted
that the most strongly electron-withdrawing substituent (NO , complex 4) leads to the
2
best performance in sulfoxidation of methyl-p-tolylsulfide (Table 4) but the worst
performance (with the exception of non-substituted HL1, complex 1) in catalytic
epoxidations (Table 5). It may also be noted that the chloro-substituted complex 2 and
the bromo-substituted complex 3 show the same catalytic activities, within experimental
error of detection, with chlorine and bromine possessing identical  constants. These
p
two complexes with weakly electron-withdrawing substituents exhibit the best catalytic
performances in epoxidation but are the two worst (bar complex 1) catalysts for
sulfoxidation. The mediocre performance of complex 4 in phosphine oxidation runs
counter to earlier observations where the presence of a strongly electron-withdrawing
substituent (e.g. -NO ) in para-position on a molybdenum-coordinating phenolate leads
2
to increased rate of phosphine oxidation.[54,55] This suggests that it is not simply the
electronic influence of the substituent that is responsible for catalyst activity in the
present study. Other factors, like catalyst stability in the presence of a large amount of
oxidant, may also play a vital role. The poor catalytic performance by complex 1 is
puzzling and difficult to rationalize.
4. Conclusion
Five tripodal tetradentate pyridyl aminophenolate ligands, with varying electronic
and steric properties depending on the nature of the substituents on the phenolate ring
were used to prepare a series of monomeric molybdenum(VI) complexes of the general
n
formula [MoO (L )]PF (1-5). In addition, the dimeric molybdenum(V) complex [(µ-
2
6
5
O){MoO(L –O)} ](PF ) (6) was isolated and characterized as a disproportionation
2
6 2
5
product formed upon oxotransfer from [MoO (L )]PF (5) In the catalytic oxygen atom
2
6
transfer reaction (OAT) from DMSO to PPh , complex 2 exhibited significant activity
3
compared with 1 and 3, whereas 5 showed very poor conversion, which is attributed to
the steric hindrance exerted by the tert-butyl substituents of the ligand. Compound 4
with a nitro substituent in the phenolate showed the lowest activity in the OAT
19

experiment. The major difference of structure related activity was observed in the
sulfoxidation reaction, where catalyst 4 shows the highest conversion while 1 was found
to be completely inactive. In the epoxidation of cis-cyclooctene, the halogenated
catalysts 2 and 3 showed full conversion to epoxide while complexes 1, 4 and 5 showed
low to moderate activity. For the more demanding olefinic substrates, only complexes 2
and 3 showed some activity while 1, 4 and 5 were practically inactive.
5. Experimental section
5.1. General procedures and materials
Unless otherwise noted, all experiments involving metal complexes were carried
out under atmospheric conditions with standard laboratory equipment. Commercial
grade reagents and solvents were purchased from commercial suppliers (Fisher
1
5
Scientific and Sigma-Aldrich) and used without further purification. The ligands HL -HL
were synthesized according to published procedures.[42,45,46] The H, 13C, 31P and 19
1
F
NMR spectra were recorded on a Varian Inova 500 MHz instrument spectrometer. NMR
chemical shifts are reported in ppm using deuterated methanol or chloroform as
solvents, and referenced to the residual solvent peak as internal standard; 31P NMR
chemical shifts are referenced to external H PO . Peaks are reported as singlet (s),
3
4
doublet (d), doublet of doublets (dd), triplet (t), heptet (hept) and multiplet (m or
unresolved), coupling constants are given in Hz. The IR spectra were measured with a
Bruker Optics, Vertex 70 spectrophotometer with a diamond ATR setup; resonances are
-1
listed with wave numbers (cm ) and intensities (br = broad, vs = very strong, s = strong,
m = medium, w = weak). Mass spectrometry was performed on Waters ZQ 4000 or
Waters QTof Xevo-G2 spectrometers. The samples were injected as MeCN and MeOH
solutions for 6 and 7 and measured on a Waters QTof Xevo-G2 spectrometer. Results
are denoted as cationic mass peaks; unit is the mass/charge ratio. Gas chromatography
mass spectroscopy (GC-MS) measurements were performed with an Agilent 7890 A
gas chromatograph (column type, Agilent 19091J–433), coupled to an Agilent 5975 C
mass spectrometer.
20

5.2. Preparation of complexes 2-5
General procedure: The complexes were prepared by the reaction of [MoO Cl ]
2
2
n
(
1.0 mmol) with the respective ligand HL (1.0 mmol), KPF (1.20 mmol) and NaOMe
6
(1.25 mmol) in 20 mL of methanol. The reaction mixture was stirred for 4 h and placed
in a freezer overnight to yield a precipitate which was removed by filtration. Diethyl ether
10 mL) and hexane (10 mL) were added to the filtered solution to precipitate more
(
white solid, which was also removed by filtration through a Celite pad. The yellow
solution was kept at −4 ºC overnight. Removal of the solvent at reduced pressure
afforded the relevant product which was dried under vacuum. The pure products were
recrystallized from concentrated methanol solutions at −4 °C and isolated after a week.
2
1
[
MoO (L )]PF , 2, orange-red crystals, 0.40 g (65%). H NMR (500 MHz, CD OD) δ 9.24
2 6 3
(d, J = 5.2 Hz, 1H), 8.79 (d, J = 5.2 Hz, 1H), 8.29 (t, J = 7.7 Hz, 1H), 7.91 (d, J = 5.2 Hz,
1
1
1
H), 7.86 (d, J = 5.2 Hz, 1H), 7.72 (d, J = 5.2 Hz, 1H), 7.55 (t, J = 7.7 Hz, 1H), 7.46 (s,
H), 7.12 (t, J = 7.7 Hz, 2H), 6.52 (d, J = 5.2 Hz, 1H), 5.32 – 5.24 (m, 2H), 4.93 (d, J =
1
0.9 Hz, 1H), 4.51 (dd, J = 14.3, 9.0 Hz, 2H), 4.22 (d, J = 17.9 Hz, 1H). H NMR (500
MHz, CDCl ) δ 9.16 (d, J = 5.2 Hz, 1H), 8.66 (d, J = 5.2 Hz, 1H), 8.15 (t, J = 7.7 Hz,
3
1
=
=
1
1
1
7
9
H), 7.95 (d, J = 5.2 Hz, 1H), 7.78 (t, J = 7.7 Hz, 1H), 7.56 (t, J = 7.7 Hz, 1H), 7.42 (t, J
7.7 Hz, 1H), 7.35 (s, 1H), 7.17 (d, J = 5.2 Hz, 1H), 7.10 (d, J = 5.2 Hz, 1H), 6.52 (d, J
5.2 Hz, 1H), 5.26 (t, J = 16.6 Hz, 2H), 5.05 (d, J = 14.0 Hz, 1H), 4.48 (dd, J = 32.1,
13
5.4 Hz, 3H). C NMR (126 MHz, CD OD) δ 157.87, 156.47, 154.66, 153.52, 151.82,
3
50.04, 144.04, 142.44, 141.65, 130.39, 127.72, 126.90, 125.02, 123.98, 122.20,
18.89 (Ar-C), 66.54, 60.44 (CH2). 31P NMR (202 MHz, CDCl ) δ -144.22 (hept, J =
3
19
-1
13.4 Hz). F NMR (470 MHz, CD OD) δ -74.06 (s), -75.57 (s). Selected FT-IR (cm )
3
+
2
+
38s (Mo=O), 917s (Mo=O). ESI-MS: m/z = 468 [2] , 340 [HL +H] .
3
1
[
MoO (L )]PF , 3, red crystals, 0.46 g (71%). H NMR (500 MHz, CD OD) δ 9.27 (d, J =
2 6 3
5.2 Hz, 1H), 8.82 (d, J = 5.2 Hz, 1H), 8.32 (t, J = 7.7 Hz, 1H), 7.94 (d, J = 5.2 Hz, 1H),
7.90 (t, J = 7.7 Hz, 1H), 7.76 (t, J = 7.7 Hz, 1H), 7.63 (s, 1H), 7.58 (t, J = 7.7 Hz, 1H),
7.29 (d, J = 5.2 Hz, 1H), 7.16 (d, J = 7.9 Hz, 1H), 6.50 (d, J = 8.6 Hz, 1H), 5.31 (dd, J =
21

1
1
1
1
7
9
4.3, 9.0 Hz, 2H), 4.94 (d, J = 15.4 Hz, 1H), 4.54 (dd, J = 14.3, 9.0 Hz, 2H), 4.25 (d, J =
13
7.9 Hz, 1H). C NMR (126 MHz, CD OD) δ 158.47, 157.66, 156.23, 154.68, 153.45,
3
50.04, 144.05, 141.70, 133.39, 133.24, 126.96, 125.02, 124.41, 122.16, 119.27,
15.01 (Ar-C), 66.56, 60.45 (CH2). 31P NMR (202 MHz, CDCl ) δ -144.22 (hept, J =
3
19
-1
13.4 Hz). F NMR (470 MHz, CD OD) δ -74.09 (s), -75.59 (s). Selected FT-IR (cm )
3
+
3
+
36s (Mo=O), 916s (Mo=O). ESI-MS: m/z = 512 [3] , 384 [HL +H] .
4
1
[
MoO (L )]PF , 4, red crystals, 0.38 g (61%). H NMR (500 MHz, CD OD) δ 9.26 (d, J =
2 6 3
5
8
7
1
1
.2 Hz, 1H), 9.13 (d, J = 5.2 Hz, 1H), 8.88 (d, J = 5.2 Hz, 1H), 8.78 (d, J = 5.2 Hz, 1H),
.61 (t, J = 7.7 Hz, 1H), 8.42 (s, 1H), 8.31 (t, J = 7.7 Hz, 1H), 7.96 (t, J = 7.7 Hz, 1H),
.75 (t, J = 7.7 Hz, 1H), 7.15 (d, J = 7.9 Hz, 1H), 6.72 (d, J = 8.6 Hz, 1H), 5.34 (t, J =
6.6 Hz, 2H), 5.07 (d, J = 15.4 Hz, 1H), 4.98 (d, J = 15.4 Hz, 1H), 4.62 (d, J = 15.4 Hz,
31
H), 4.39 (d, J = 15.4 Hz, 1H). P NMR (202 MHz, CDCl ) δ -144.22 (hept, J = 713.4
3
19
-1
Hz). F NMR (470 MHz, CD OD) δ -73.86 (s), -75.36 (s). Selected FT-IR (cm ) 949s
3
+
4
+
(
Mo=O). ESI-MS: m/z = 479 [4] , 351 [HL +H] .
5
1
[
MoO (L )]PF , 5, deep red crystals, 0.52 g (75%). H NMR (500 MHz, CDCl ) δ 9.18 (d,
2
6
3
J = 5.2 Hz, 1H), 8.70 (d, J = 5.2 Hz, 1H), 8.16 (td, J = 7.8, 1.5 Hz, 1H), 7.91 (d, J = 5.2
Hz, 1H), 7.78 (td, J = 7.8, 1.5 Hz, 1H), 7.59 (t, J = 7.7 Hz, 1H), 7.40 (t, J = 7.7 Hz, 1H),
7
.25 (d, J = 5.2 Hz, 1H), 7.23 (d, J = 5.2 Hz, 1H), 7.13 (d, J = 5.2 Hz, 1H), 5.37 (d, J =
5.4 Hz, 1H), 5.12 (dd, J = 14.3, 9.0 Hz, 2H), 4.65 (d, J = 15.4 Hz, 1H), 4.50 (d, J = 15.4
1
Hz, 1H), 4.36 (d, J = 15.4 Hz, 1H), 1.31 (s, 9H), 1.20 (s, 9H). 13C NMR (126 MHz,
CDCl ) δ 155.56, 155.52, 155.11, 152.54, 150.31, 147.01, 143.79, 141.62, 138.02,
3
126.90, 126.10, 125.63, 125.58, 124.64, 122.53, 121.27 (Ar-C), 67.43, 62.71, 60.30
31
(
CH ), 34.97, 34.63 [C(CH ) ], 31.37, 29.97 (CH ). P NMR (202 MHz, CDCl ) δ -
2 3 3 3 3
1
44.22 (hept, J = 713.4 Hz). 19F NMR (470 MHz, CDCl ) δ -71.04 (s), -72.56 (s).
3
Selected FT-IR (cm ) 921s (Mo=O), 907s (Mo=O). UV-vis in MeCN:  , nm (, M–
-
1
max
1
–1
+
5
+
cm ): 209 (49166), 275 (11023), 402 (3163). ESI-MS: m/z = 546 [4] , 418 [HL +H] ,
5
+
[
(µ-O){MoO(L –O)} +NaOMe] at m/z = 1126.
2
22

5
Preparation of complex [(µ-O){MoO(L )} ](PF ) , 6. A solution of PPh (0.038 mmol) in 2
2
6 2
3
mL of dichloromethane was added to the solution of 5 (0.073 mmol) in 5 mL of
dichloromethane under inert atmosphere. The colour of the solution changed from
yellow to dark purple within 5 min. After 15 min, a purple solid started to precipitate and
5
5
mL diethyl ether was added. The product [(µ-O){MoO(L )} ](PF ) 6 was isolated by
2
6 2
filtration as a dark purple solid (0.025 mmol, 35% yield). UV-vis in MeCN: max, nm (,
–1
–1
M cm ): 210 (89723), 265 (29615), 404 (6568), 570 (9111). HRMS: m/z = 1073.3405
+
+
[
6] , 1114.3145 [6+MeCN] .
5
Preparation of complex [MoO(OCH )(L )], 7. Complex 5 (0.02 mmol) was dissolved in 5
3
mL of methanol and PPh (0.03 mmol) was added under nitrogen atmosphere. A red
3
colour appeared immediately upon addition of the PPh to the complex solution. After
3
stirring for 15 min, the UV-Vis and mass data were recorded for 7. UV-vis in MeOH:
–1
–1
max, nm (, M cm ): 227 (20102), 257 (9107), 274 (9080), 393 (1478). HRMS: m/z =
+
5
61.1899 [7] . Finally, the solution was dried under vacuum leading to the isolation of a
small quantities of solid product as 6 (vide supra).
5.3. Catalytic oxygen atom transfer reactions
The OAT reactions of PPh (0.2 mmol) catalysed by complexes 1-5 (0.002 mmol)
3
were run in DMSO-d (0.5 mL) at 25 ºC. The reaction progress was monitored by 31P
6
NMR using a fifteen-minute interval, measuring the disappearance of a resonance at -6
31
1
ppm for PPh and the appearance of a signal for OPPh at approx. 27 ppm. The P{ H}
3
3
NMR spectra were externally referenced to 85% H PO .
3
4
5.4. Sulfoxidation reactions
In these experiments, complexes 1-5 were tested as catalysts for the oxidation of
methyl-p-tolyl sulfide at room temperature in CD OD solutions using 1:2 molar ratios of
3
substrate/tert-BuOOH (0.19 M: 0.38 M) and 10μL of 1,2-dichloroethane was added as
an internal standard in a 5 mm NMR tube. The complete selectivity towards the
23

1
sulfoxide was monitored by H NMR spectroscopy at fifteen-minute intervals. The
reaction rates were estimated on the basis of the integrated intensities of substrate and
product spectra. In the sulfoxidation test, the sulfide methyl singlet at 2.27 was turned to
the sulfoxide methyl singlet at 2.75 ppm.
5.5. Epoxidation of olefins
A Heidolph Parallel Synthesizer 1 was used. In a typical epoxidation experiment
2
-3 mg of the respective catalyst was dissolved in 0.5 mL of CHCl and the substrate
3
was added. Mesitylene was used as internal standard. After the experiment temperature
was reached tert-butylhydroperoxide was added to start the reaction and aliquots for
GC-MS measurements were withdrawn at given time intervals. The GC samples were
quenched with MnO , diluted with ethyl acetate and measured. All yields obtained by
2
GC have an esd of ±5 %.
5.6. DFT Modelling Details
The reported calculations were performed with the hybrid meta exchange-
correlation functional M06,[56] as implemented by the Gaussian 09 program package.[57]
The Mo atom was described by Stuttgart-Dresden effective core potentials (ECP) and
an SDD basis set,[58] while a 6-31G(d’) basis set was employed for the remaining
atoms.[59] All computations were performed using an ultrafine grid and Grimmes’
dispersion correction.[60]
The reported geometries for the isomers of complex 1 (species A and B) represent fully
optimized ground states (positive eigenvalues). The computed frequencies were used to
make zero-point and thermal corrections to the electronic energies; the reported free
energies are quoted in kcal/mol relative to the specified standard. The geometry-
optimized structures presented here have been drawn with the JIMP2 molecular
visualization and manipulation program.[61]
24

5.7. Crystallography
X-ray crystallographic measurements were carried out at 170 K (2 and 3) or at
20 K (5) according to the ω-scan method on a Nonius KappaCCD diffractometer using
1
Mo-Kα radiation (λ= 0.71073 Å). The Denzo Scalepack program packages were used
for cell refinements and data reductions. SADABS absorption correction[62] was applied
for the data of all compounds. The structures were solved by direct methods, using
SHELXS-97[63] or SUPERFLIP[64] programs with the WinGX[65] graphical user interface.
Structural refinements were carried out using SHELX-97.[66]
CRediT authorship contribution statement
Md. Kamal Hossain: Investigation, Writing - original draft, Writing - review &
editing. Jörg A. Schachner: Investigation, Writing - original draft, Matti Haukka:
Investigation. Michael G. Richmond: Investigation, Writing - original draft, Nadia C.
Mösch-Zanetti: Supervision, Writing – original draft Ari Lehtonen: Supervision, Writing
-
original draft, Ebbe Nordlander: Conceptualization, Supervision, Writing - review &
editing, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests that
could have appeared to influence the work reported in this paper.
Acknowledgements
We gratefully acknowledge financial support from the COST Action CM1003 Biological
oxidation reactions-mechanisms and design of new catalysts. MGR thanks the Robert
A. Welch Foundation (Grant B-1093) for funding. The DFT calculations were performed
at UNT through CASCaM, which is an NSF-supported facility (CHE-1531468). M.K.H.
thanks the European Commission for an Erasmus Mundus predoctoral scholarship and
Dr. Arun Kumar Raha for experimental assistance.
25

Appendix A. Supplementary data
CCDC 2044570-2044573 contains the supplementary crystallographic data for
compounds 2, 3, 5 and 6 These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with
or
from
the
Cambridge
1
this article can be found, in the online version.
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