Bioinspired Mo, W and V complexes bearing a highly hydroxyl-functionalized Schiff base ligand

Article abstract of DOI:10.1016/j.ica.2020.119414

A series of bioinspired dioxidomolybdenum(VI), dioxidotungsten(VI) and oxidovanadium(V) complexes [MoO₂(H₂L^Saltris)], [WO₂(H₂L^Saltris)] and [VO(HL^Saltris)]₂ were prepared by the reaction of a hydroxyl-rich Schiff base proligand N-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)-3,5-di-tert-butylsalicylaldimine (H₄L^Saltris) with metal precursors in methanol solutions. Molybdenum and tungsten complexes crystallize as mononuclear molecules, whereas the vanadium complex forms dinuclear units. From the complexes, [VO(HL^Saltris)]₂ shows activity in the oxidation of 4-tert-butylcatechol and 3,5-di-tert-butylcatechol, mimicking the action of the dicopper enzyme catechol oxidase.

Full text of DOI:10.1016/j.ica.2020.119414

Journal Pre-proofs
Research paper
Bioinspired Mo, W and V complexes bearing a highly hydroxyl-functionalized
Schiff base ligand
Pasi Salonen, Anssi Peuronen, Ari Lehtonen
PII:
S0020-1693(19)31573-7
https://doi.org/10.1016/j.ica.2020.119414
ICA 119414
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
16 October 2019
3 January 2020
4 January 2020
Please cite this article as: P. Salonen, A. Peuronen, A. Lehtonen, Bioinspired Mo, W and V complexes bearing a
highly hydroxyl-functionalized Schiff base ligand, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/
j.ica.2020.119414
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1
Bioinspired Mo, W and V complexes bearing a highly hydroxyl-functionalized Schiff base
ligand
Pasi Salonena, Anssi Peuronena,b, Ari Lehtonena,*
* Corresponding author
a Group of Inorganic Materials Chemistry, Department of Chemistry, University of
Turku, FI-20014 Turku, Finland.
b Laboratory of Inorganic Chemistry, Department of Chemistry, University of Jyväskylä,
FI-40014 Jyväskylä, Finland.
Graphical abstract
Highlights




Mo, W and V form complexes with a highly hydroxyl- functionalized Schiff base ligand
Molecular structures were characterized
V complex shows catechol oxidase mimetic activity
Catechol oxidase activity of V complex was mechanistically assessed
Abstract
A series of bioinspired dioxidomolybdenum(VI), dioxidotungsten(VI) and oxidovana-
dium(V) complexes
[MoO2(H2LSaltris)], [WO2(H2LSaltris)] and [VO(HLSaltris)]2 were
prepared by the reaction of a hydroxyl-rich Schiff base proligand N-(1,3-dihydroxy-2-
(hydroxymethyl)propan-2-yl)-3,5-di-tert-butylsalicylaldimine (H4LSaltris) with metal
precursors in methanol solutions. Molybdenum and tungsten complexes crystallize as
mononuclear molecules, whereas the vanadium complex forms dinuclear units. From
the complexes, [VO(HLSaltris)]2 shows activity in the oxidation of 4-tert-butylcatechol and
3,5-di-tert-butylcatechol, mimicking the action of the dicopper enzyme catechol
oxidase.*
Introduction
The easy exploitation of aerial dioxygen in the context of oxidation catalysis represents
one of the most desired goals in synthetic organic chemistry because of its ready
availability, low cost and environmental compatibility.[1] Nature has perfected the use
of the relatively inert dioxygen molecule in a wide variety of metabolic oxidation
reactions through metalloenzymes. Efforts to mimic various metalloenzyme active sites
by (bio)inorganic chemists can be divided into biomimetic (structural) and bioinspired
(functional) modelling.[2,3] A central idea in coordination chemistry is to use ligands
List of abbreviations: 3,5-DTBC (3,5-di-tert-butylcatechol), 3,5-DTBSQ (3,5-di-tert-butylsemiquinone),
3,5-DTBQ (3,5-di-tert-butyl-1,2-benzoquinone), 4-TBC (4-tert-butylcatechol), 4-TBQ (4-tert-butyl-1,2-
benzoquinone), OAT (oxygen atom transfer).

2
with diverse electronic and steric features to modulate the properties of central metals
in various model complexes, and thus achieve some properties related to natural
enzyme active sites.
The enzymes from the molybdenum oxotransferase superfamily catalyze oxygen atom
transfer (OAT) reactions, whereby an oxygen atom is transferred from a high-valent,
catalytically active (di)oxidomolybdenum species to a suitable donor/acceptor
molecule.[4,5] The superfamily can be divided into three subfamilies, namely xanthine
oxidase, sulfite oxidase and DMSO reductase.[6] The active sites of the eponymous
enzymes DMSO reductase and sulfite oxidase consist of a monooxido- and dioxido-
molybdenum(VI) centers, respectively.[6] They, together with many other molyb-
doenzymes have been structurally and functionally modelled by a number of
monooxido- and dioxidomolybdenum(VI)- and tungsten(VI) complexes, which find
applications in OAT catalysis research.[7]
Vanadates and other vanadium-containing compounds have been studied for decades
due to their biological relevance and catalytic activity in a variety of oxidation
reactions.[8–11] Especially, the bioinorganic chemistry of vanadium, highlighted by the
three vanadium haloperoxidases and V-nitrogenase, as well as the anti-diabetic effects
of many vanadium compounds have motivated synthetic chemists to prepare model
compounds for use in oxidation catalysis, as well as in structural studies, and in
medicinal research.[12–14] Catechol oxidase is an dioxygen-activating dicopper
enzyme[15–17] which catalyzes the oxidation of catechols to the corresponding o-
benzoquinones, producing water as a by-product. A number of vanadium(IV) and
vanadium(V) complexes have also been discovered to efficiently catalyze the same
reaction.[18–23]
Although compounds that display catechol oxidase mimetic activities are usually
reserved for of Cu coordination model compounds[24,25], recently many other metals
e.g. Co, Mn, Fe, Ni and Zn, as substitutes for copper, have also been used to reach
moderate to high conversions of various catechols to o-benzoquinones.[26–28] There
are also some examples on the catechol oxidase mimicking activity of molecular
vanadium(IV) and vanadium(V) complexes.[18–23] It is well established that various
catechols readily form redox non-innocent complexes with vanadium, where the
vanadium center may be assigned +III, +IV and +V oxidation states.[29–33] There are
also some reports for catecholate-containing molybdenum and even tungsten
complexes.[34] In the case of vanadium(V) complexation of catechols may lead to a
reduction in the metal center (V(V) → V(IV)), sometimes accompanied by cleavage of the
oxido ligand.[30,31,33]
The mechanism(s) of vanadium-catalyzed oxidation of catechols are still not fully
understood. For example, many vanadium systems apparently demonstrate catechol
oxidase- and catechol dioxygenase-like activities simultaneously, leading to a number
of oxidation products ranging from intra- and extradiol cleavage products to
autoxidation products.[35,36] Furthermore, there is no definitive consensus on the

3
nature of the active catalyst, with some proponents in support of a “common catalyst
hypothesis”, which states that a specific vanadium-containing, catalytically active
species is always formed in-situ by vanadium-leaching regardless of used vanadium
catalyst precursor.[36] This is evidenced by very similar product distributions forming
by the action of very different precatalysts, as well as from indicative spectroscopic and
spectrometric signatures.
In light of available data[37], the common catalyst has been suggested to be [VO(3,5-
DTBC)(3,5-DTBSQ)], which is formed during catalytic turnover conditions from its
catalytic resting-state [VO(3,5-DTBC)(3,5-DTBSQ)]2, also known as Pierpont’s
complex.[29] The Pierpont’s complex itself is derived from H2O2-assisted leaching of
virtually any vanadium catalyst precursor, provided that 3,5-DTBC is present, in a so-
called autoxidation-product-initiated reaction pathway.[36,38] However, there are also
some reports of discrete oxidovanadium(V) catalysts where the supposed active
catalyst, allegedly not formed from metal leaching, has been identified using mass
spectrometry.[20,23]
Herein we describe the synthesis and characterization of dioxidomolybdenum(VI) and -
tungsten(VI) complexes (1, 2) as well as the oxidovanadium(V) complex (3) and their
application as bioinspired synthetic analogues to molybdenum oxotransferases and
catechol oxidase. The syntheses of 1–3 involve complexation to a bulky Schiff base
proligand N-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)-3,5-di-tert-butylsalicylald-
imine (H4LSaltris) featuring several alcohol side-arms. The presence of non-coordinating
hydrogen bond/donor acceptor moieties such as alcohols or amides in the outer
coordination spheres of various complexes has been associated with heightened
catalytic activities of the complexes in some oxidation reactions.[39,40] However, only 3
showed catechol oxidase-like activities, forming 3,5-di-tert-butyl-1,2-benzoquinone
(3,5-DTBQ) and 4-tert-butyl-1,2-benzoquinone (4-TBQ), respectively. Moreover,
mechanistic studies with regard to vanadium-catalyzed catechol oxidation were
performed to further elaborate on the nature of the active catalyst.
Experimental
All syntheses, experiments and manipulations were run in an ambient atmosphere.
[MoO2(acac)2] (acac = acetylacetonato) and [W(eg)3] (eg = ethane-1,2-diolato = ethylene
glycolato) were prepared according to known methods.[41,42] Other reagents and
solvents were purchased from commercial sources and used as received. IR spectra
were recorded using a Bruker VERTEX 71 FTIR spectrophotometer equipped with a RT-
DLaDTGS detector. 64 scans were performed in the ATR mode for individual
measurements using a Harrick VideoMVP accessory. In this setup, the sample is pressed
against a diamond anvil. All data were recorded in transmittance mode with a
resolution of 4 cm–1. The UV-Vis measurements were performed in MeOH, MeCN and
CHCl3 using a ∅ 1 cm quartz cuvette on an Agilent CARY60 spectrophotometer. High-
resolution mass spectra were recorded on a Bruker Daltonics MicroTOF-Q II
electrospray ionization time-of-flight (ESI-TOF) mass spectrometer using both negative

4
and positive polarization as well as in MSMS mode. Full UV-Vis, NMR, IR and ESI-MS
1
13
19
51
spectra are given in the supplementary material. The H, C, F and V spectra were
recorded on either Bruker Avance III 600 MHz (1H: 600.16 MHz, 13C: 150.91 MHz)
equipped with a CryoProbe Prodigy triple resonance inverse (TCI) probe or Bruker
13
19
51
Avance 500 MHz (1H: 500.08 MHz C: 125.75 MHz, F: 470.55 MHz, V: 131.54 MHz)
1
13
equipped with a broad-band observe probe (Bruker BBO-5 mm-Zgrad). All H and C
NMR spectra are reported in ppm downfield relative to tetramethylsilane (TMS, δ = 0.00
ppm) and referenced to residual solvent signals CHCl3-H (1H: δ 7.26, 13C: δ 77.16),
DMSO-H6 (1H: δ 2.50, C: δ 39.52), acetone-H6 (1H: δ 2.05, C: δ 206.26, 29.84) if TMS
13
13
is not present.[43] The 0 ppm fluorine and vanadium reference frequencies were
1
calculated from the TMS H frequency using the unified chemical shift scale by IUPAC
(Ξ(19F, CCl3F) = 94.094011) and (Ξ(51V, VOCl3) = 26.302948), respectively.[44]

5
Syntheses
N-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)-3,5-di-tert-butylsalicylald-
imine (H4LSaltris). The proligand was synthesized according to a published procedure
with slight modifications.[28] Detailed synthetic procedure is disclosed in the ESI. Yield:
1
3.08 g (91 %). H NMR (DMSO-d6, 298 K, 600 MHz, TMS) δ 14.90 (1H, d, J = 2.2 Hz,
ArOH,), 8.55 (1H, d, J = 2.2 Hz, Ar—CH=N—CR3), 7.27 (1H, d, J = 2.5 Hz, ArH), 7.22 (1H,
d, J = 2.5 Hz, ArH), 4.70 (3H, t, J = 5.4 Hz, NC—(CH2OH)3), 3.62 (6H, d, J = 5.4 Hz, NC—
13
(CH2OH)3), 1.37 (9H, s, t-Bu), 1.27 (9H, s, t-Bu). C NMR (DMSO-d6, 298 K, 600 MHz,
TMS) δ 165.37, 160.21, 138.12, 136.18, 126.57, 125.98, 117.49, 66.80, 61.41, 34.55,
33.78, 31.32, 29.29. IR/cm–1 ATR mode: ν 3500–3000vb (ROH, ArOH, R(CH2OH)3),
2958–2868s (C–H, t-Bu), 1620s (RCH=N), 1593–1361m (arom. C=C), 1252m (C—O,
ArOH), 1049vs, 1038vs (C—O, R(CH2OH)3), 882m, 644s.
[MoO2(H2LSaltris)], 1. 337 mg (1.0 mmol) H4LSaltris and 326 mg (1.0 mmol)
[MoO2(acac)2] were dissolved in 15 mL MeOH in a 50 mL Erlenmeyer flask equipped
with a magnetic stir-bar. The clear yellow reaction mixture was stirred at RT for 72
hours, after which it was transferred to a –25 ℃ freezer. In a week, single crystals
suitable for XRD had formed and were subsequently isolated by Büchner filtration,
washed with small amounts of ice-cold MeOH, and air-dried to afford the analytically
pure target complex. Yield: 340 mg (73 %). 1H NMR (DMSO-d6, 298 K, 600 MHz, TMS) δ
8.49 (1H, s, Ar—CH=N—CR3), 7.47 (1H, d, J = 2.5 Hz, ArH), 7.44 (1H, d, J = 2.5 Hz, ArH),
4.95 (2H, t, J = 5.5 Hz, NRC—(CH2OH)2), 4.43 (2H, s, NR2C—(CH2O–)), 3.73 (2H, dd, J1 =
11.3 Hz, J2 = 5.5 Hz, NRC—(CH2OH)2), 3.63 (2H, dd, J1 = 11.3 Hz, J2 = 5.5 Hz, NRC—
13
(CH2OH)2), 1.37 (9H, s, t-Bu), 1.29 (9H, s, t-Bu). C NMR (DMSO-d6, 298 K, 600 MHz,
TMS) δ 164.72, 156.92, 140.49, 137.92, 129.20, 128.55, 120.60, 76.65, 73.69, 61.28,
48.61, 34.89, 33.97, 31.27, 29.62. IR/cm–1 ATR mode: ν 3500–3000vb (ROH,
R(CH2OH)2), 2956–2866s (C–H, t-Bu), 1620s (RCH=N), 1562–1361m (arom. C=C),
1038vs (C—O, R(CH2OH)3), 1253m (C—O, ArOH), 1045s (C—O, R(CH2OH)2 and
R(CH2O–)), 916vs νs(MoO2)[45–48] 880vs νas(MoO2)[45–48], 761m, 558s ν(Mo—N)[49].
UV-Vis (MeCN; λmax/ nm (ε/ M–1 cm–1) 355 (3180). ESI-HRMS+ [M+Na]+ calculated m/z =
488.0945, found m/z = 488.0900. ESI-HRMS– [M–H]– calculated m/z = 464.0980, found
m/z = 464.1056.
1 could be also synthesized using equimolar amounts of H4LSaltris and Na2MoO4 ∙ 2 H2O.
However, excess glacial acetic acid was required, as well as overnight refluxing. Yield
using this methodology was also slightly reduced (277 mg, 60 %).
[WO2(H2LSaltris)], 2. 337 mg (1.0 mmol) H4LSaltris and 330 mg (1.0 mmol) Na2WO4 ∙ 2
H2O were dissolved in 15 mL MeOH in a 100 mL Erlenmeyer flask equipped with a
magnetic stir-bar. The solution was treated with 1 mL glacial acetic acid, heated to boil
in an oil-bath for two hours, then left to stir for 72 hours at RT. The pale reaction
mixture was concentrated to ca. half of the original volume, filtered and transferred to
+5 ℃ refrigerator. Upon ca. two weeks of storage, pale yellow microcrystals suitable for
single crystal XRD had deposited. They were subsequently isolated by Büchner

6
filtration, washed with small amounts of ice-cold MeOH and air-dried to afford the
1
analytically pure target compound. Yield: 258 mg (47 %). H NMR (DMSO-d6, 298 K,
600 MHz, TMS) δ 8.46 (1H, s, Ar—CH=N—CR3), 7.53 (1H, d, J = 2.4 Hz, ArH), 7.47 (1H, d,
J = 2.4 Hz, ArH), 4.99 (2H, t, J = 5.4 Hz, NRC—(CH2OH)2), 4.58 (2H, s, NR2C—(CH2O–)),
3.72 (2H, dd, J1 = 11.1 Hz, J2 = 3.4 Hz, NRC—(CH2OH)2), 3.63 (2H, dd, J1 = 11.1 Hz, J2 =
13
3.4 Hz, NRC—(CH2OH)2), 1.38 (9H, s, t-Bu), 1.30 (9H, s, t-Bu). C NMR (DMSO-d6, 298
K, 600 MHz, TMS) δ 165.51, 155.81, 141.03, 138.36, 129.37, 129.06, 121.22, 76.76,
73.72, 61.17, 48.61, 34.85, 33.97, 31.26, 29.60. IR/cm–1 ATR mode: ν 3500–3000vb
(ROH, R(CH2OH)2), 2958–2850s (C–H, t-Bu), 1621s (RCH=N), 1566–1362m (arom. C=C),
1300m (C—O, ArOH), 1048vs (C—O, R(CH2OH)2 and R(CH2O–)), 893vs νs(WO2)[45–48]
860vs νas(WO2)[45–48], 764m, 561s ν(W—N)[49]. UV-Vis (MeOH; λmax/ nm (ε/ M–1 cm–
1) 365 (2440). ESI-HRMS+ [M+Na]+ calculated m/z = 574.1399, found m/z = 574.1435.
ESI-HRMS– [M–H]– calculated m/z = 550.1424, found m/z = 550.2204.
2 could be also synthesized using equimolar amounts of H4LSaltris and [W(eg)3]. The
isolated yield of a 1 mmol scale synthesis was very low, only 40 mg (9 %).
[VO(HLSaltris)]2, 3. 337 mg (1.0 mmol) H4LSaltris and 265 mg (1.0 mmol) [VO(acac)2] were
dissolved in 15 mL MeOH in a 100 mL Erlenmeyer flask equipped with a magnetic stir-
bar. Upon ca. 15 minutes of stirring at RT, an umber-colored solid precipitated out of
solution. The solid was collected by Büchner filtration, washed with ca. 30 mL RT MeOH,
and air-dried to afford the target compound in a pure non-crystalline form. Umber-
colored single crystals suitable for x-ray diffraction were obtained from hot acetonitrile
solution upon slow cooling. Yield: 357 mg (89 %). 1H NMR (acetone-d6, 298 K, 600 MHz,
TMS) δ 9.08 (1H, s, Ar—CH=N—CR3), 7.66 (1H, d, J = 2.5 Hz, ArH), 7.53 (1H, d, J = 2.5 Hz,
ArH), 5.25 (1H, d, J = 14.3 Hz, NR2C—(CHAHBO–)A), 5.13 (2H, m, NR2C—(CHAHBO–)A and
NR2C—(CHAHBO–)B), 4.70 (1H, d, J = 8.9 Hz, NR2C—(CHAHBO–)B), 4.58 (1H, t, J = 5.4 Hz,
NR2C—(CH2OH)C), 4.04 (2H, dd, J1 = 5.6 Hz, J2 = 3.2 Hz, NR2C—(CH2OH)C), 1.51 (9H, s,
13
t-Bu), 1.35 (9H, s, t-Bu). C NMR (acetone-d6, 298 K, 600 MHz, TMS) δ 165.46, 160.98,
141.26, 137.84, 131.29, 129.74, 121.38, 85.88, 81.87, 79.65, 64.51, 35.99, 34.84, 31.70,
30.45. 51V NMR (acetone-d6, 298 K, 600 MHz, VOCl3) δ –562. IR/cm–1 ATR mode: ν
3571w (ROH), 2954–2864s (C–H, t-Bu), 1623s (RCH=N), 1556–1360m (arom. C=C),
1303m (C—O, ArOH), 1075s, 1030s (C—O, R(CH2OH)2 and R(CH2O–)), 893vs
ν(V=O)[19,22,50]. 853m, 764m, 552s. UV-Vis (MeCN; λmax/ nm (ε/ M–1 cm–1) 520 (550).
ESI-HRMS+ [M+Na]+ calculated m/z = 825.2718, found m/z = 825.2599. ESI-HRMS– [M +
Cl]– calculated m/z = 837.2508, found m/z = 837.2273.
3 could be also synthesized using equimolar amounts of H4LSaltris and VOSO4 ∙ 5 H2O,
with two equivalents of Et3N added as a base. Similarly to the synthesis involving
[VO(acac)2], an umber solid was quickly precipitated out of solution, however, with a
slightly lowered yield; a 1.0 mmol scale synthesis yielded 277 mg of the target
compound, corresponding to a yield of 69 %.

7
X-ray crystallography
Single crystal X-ray data were collected using Rigaku Oxford diffraction SuperNova
diffractometer equipped with micro-focus single-source (Mo-Kα radiation, λ = 0.71073
Å) and Eos detector. Data collection and processing were done using CrysAlisPro[51]
software. Crystal structures were solved and refined within Olex2[52] GUI using
SHELXS[52] and SHELXL[53] programs, respectively. Atoms heavier than H were
located from the difference density map and refined anisotropically. All C—H atoms
were calculated to their ideal positions and refined using a riding model with the
isotropic displacement parameters fixed to values corresponding to 1.2–1.5 times the
Uaniso of the respective host atom. O—H hydrogen atoms that participate in hydrogen
bond interactions were located from the difference density map and refined
isotropically without restrictions in displacement parameters. The single crystal X-ray
data for 1–3 are given in ESI table S1. Crystallographic data for the compounds reported
in this paper were deposited with the Cambridge Crystallographic Data Centre, CCDC,
12 Union Road, Cambridge CB21EZ, UK. These data can be obtained free of charge on
quoting
the
depository
number
CCDC
1959437–1959439
(E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk). The powder x-ray diffraction
measurements were performed using a Huber G670 detector (Cu-Kα radiation, λ =
1.5406 Å). For each individual measurement, the exposure time was set to 30 min and
with a total of 10 scans.
Cyclic voltammetry
The cyclic voltammetry electrochemical measurements of 1–3 (c = 1 × 10–3 M) were
performed using a standard three-electrode setup using an Autolab PGSTAT101
potentiostat. The CVs were recorded at RT using Ø 1 mm platinum and glassy carbon
(GC) working electrodes. Before use both working electrodes were polished using 6, 3, 1
and 0.25 µm diamond paste, and rinsed with quartz distilled water and technical
ethanol. The quasi-reference electrode was an Ag/AgCl wire, and it was calibrated
against the ferrocene redox-couple (Fc/Fc+) (E1/2(Fc/Fc+) = 0.55 V).[54] A coiled
platinum wire was employed as the counter electrode. Measurements were performed
using 50, 100 and 200 mV s–1 scanning rates in dry electrochemical grade acetonitrile
with 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) as the supporting
electrolyte. Prior to every measurement, the sample solutions were bubbled with dry N2
for 10 minutes. The electrochemical window was –2.5 – 2.5 V for GC working electrode,
and –0.8 – 2.3 V for Pt electrode, respectively.
Oxygen atom transfer reactions
All complexes were preliminarily assessed in the formal oxygen atom transfer (OAT)
reactions of tris(4-fluorophenyl)phosphine (P(p-C6H4F)3) to the corresponding tris(4-
fluorophenyl)phosphine oxide (P(p-C6H4F)3O).[55] In the OAT experiments, 500 µL of 5
× 10–3 M P(p-C6H4F)3 in DMSO-H6 solution and 500 µL of 5 × 10–4 M 1–3 in DMSO-H6
solution were combined in an screw-capped scintillation vial and maintained for 24

8
hours at 60 °C in a thermal oven. Additionally, a control reaction involving 1000 µL 5 ×
10–3 M P(p-C6H4F)3 in DMSO-H6 solution was also run. In all cases, the conversion was
assessed by 19F NMR and after the 24-hour period. All measurements were run in
duplicate and the results obtained are given as the average of the two reactions.
Oxidation of 4-TBC and 3,5-DTBC
The oxidations of 4-TBC and 3,5-DTBC to 4-TBQ and 3,5-DTBQ were monitored by in-
situ UV-Vis spectroscopy on an Agilent Cary 60 UV-Vis spectrophotometer and a ∅ 1.00
cm quartz cuvette. All measurements were run in duplicate and conducted at room
temperature (25 ℃) under ambient atmosphere. The reaction kinetics were assessed
using the method of initial rates. For this, a 1.00 × 10–5 M solution of precatalyst 3 was
prepared in CHCl3. The substrate concentration was varied 1000, 2000, 3000, 4000,
6000, 8000 and 10000-fold relative to the catalyst to evaluate the substrate dependence
of the reactions for 4-TBC. In the case of 3,5-DTBC, substrate concentration ratios of
12500, 15000 and 17500 were additionally used. The reactions were initiated directly
in the quartz cuvette by combining 1.500 mL precatalyst solutions and 1.500 mL of 1.75
× 10–1 M – 1.00 × 10–2 M substrate solutions. Accordingly, final concentrations of [cat] =
5.00 × 10–6 M and [S] = 8.75 × 10–2 M – 5.00 × 10–3 M, respectively, were obtained during
catalysis. The progress of the reaction was monitored by following the increase in the
absorbance at ca. 388 nm due to 4-TBQ and 3,5-DTBC (ε(4-TBQ) = 1150 M–1 cm–1 and
ε(3,5-DTBQ) = 2200 M–1 cm–1)[56,57] for 10 minutes at every one-minute interval.
Absorbance vs. time graphs were plotted, and initial reaction rates at every
concentration were determined directly utilizing the on-board Cary WinUV Kinetics
software. Finally, the initial reaction rates and the substrate concentrations were fitted,
using Origin 2015 software, to the Michaelis–Menten equation (1) using non-linear
regression analysis, in order to obtain the maximum reaction rate Vmax, Michaelis
constant KM ([S] at ½Vmax) and turnover frequency kcat values. The turnover frequency
was calculated using the formula (2), where [ET] is the catalyst concentration.
푉
×[S]
max
푣 =
푘cat
(1)
(2)
퐾
+[푆]
M
푉
max
=
[퐸 ]
푇
Iodometric assay
Hydrogen peroxide was determined from the catechol oxidation reaction mixture based
on I3–, which has a characteristic UV-Vis absorbance at ca. 353 nm in water. A 100 mL
Erlenmeyer flask was charged with ca. 1 g 4-tert-butylcatechol and dissolved in 30 mL
CHCl3. 1 mol-%, or ca. 48 mg 3 was added to the reaction mixture, which was
subsequently stirred for several days at RT. Afterwards, the reaction mixture was
extracted with 25 mL distilled water. The aqueous phase was adjusted to a pH ~ 2 with
dilute aqueous H2SO4, and a portion of it was treated with ca. 0.32 M KI (aq) solution

9
and was allowed to react overnight. I3– was observed by UV-Vis spectroscopy at ca. 353
nm, whereas no such signal was observed for the bare KI solution, which was allowed to
react with atmospheric oxygen overnight. Additionally, to further confirm the presence
of hydrogen peroxide, a portion of the KI solution was treated with excess amounts of
30 % aqueous H2O2, and the formation of a 353 nm band was again observed (ESI
Figure S57)
Mechanistic catechol oxidation investigations
The UV-Vis spectroscopic investigations were performed in 1 cm quartz cuvettes by
combining 1.500 mL 0.100 M 4-TBC chloroform solution containing 0.010 M BHT,
DMSO or H2O2, and 1.500 mL chloroform solutions consisting of 1 × 10–4 M 3. For H2O2
tests, 0.020 and 0.030 M solutions were additionally used. For control reactions, 0.100
M 4-TBC reacts with 0.030 M H2O2 in the absence of 3 (control 1), and 0.100 M 4-TBC
reacts with 1 × 10–4 M 3 in the absence of H2O2 (control 2). The initial reaction rates
were determined using the on-board Cary WinUV Kinetics software as described above.
All measurements were repeated once, and the results given in figure 7 represent
average values obtained from the two measurements.
For 51V NMR and ESI-HRMS studies, 6 mg of 3 was suspended in 650 µL CDCl3 and
treated with 100 equivalents of 3,5-DTBC (166 mg) or 4-TBC (124 mg) to reflect the
stoichiometry of the kinetic catechol oxidation experiments. Prior to measurements the
solutions were filtered through a small cotton plug to remove any undissolved solids.
51V NMR measurements were performed immediately after filtration. The same CDCl3
solutions were diluted in MS grade acetonitrile, and subsequently analyzed by ESI-
HRMS immediately after the 51V NMR measurements.
To determine the product distribution in the oxidation of 3,5-DTBC, 500 mg of 3,5-DTBC
and 18 mg of 3 (1 mol-%) were allowed to react in 30 mL boiling chloroform under
ambient atmosphere overnight for ca. 16 hours in a 50 mL round-bottomed flask
equipped with a magnetic stir-bar and a reflux condenser. Afterwards, the reaction
mixture was analyzed by TLC on silica plates with technical DCM. Three components
with Rf-values ca. 0.85, 0.58 and 0.33 were detected. The compounds were isolated by
1
13
column chromatography on silica gel (technical DCM), and identified with H, C NMR
and positive mode ESI-HRMS according to published data (ESI).[35]
Results and discussion
Syntheses and characterization of complexes
The ligand precursor H4LSaltris was synthesized according to a slightly modified
procedure.[28] We were also interested in the corresponding H5LSaltris amine analogue,
but our attempts to obtain it through reductive amination from H4LSaltris proved
unsuccessful. However, Mannich reaction did provide the target compound, although
unselectively, and with trace yields only. Accordingly, complexations were not tested
with H5LSaltris. The Mannich reaction including the product distribution is presented the

10
electronic supplementary information (Scheme S1). The spectral data of all products are
also listed in the ESI.
H4LSaltris reacts with equimolar amounts of molybdenum precursors [MoO2(acac)2] or
Na2MoO4 ∙ 2 H2O in methanol to afford cis-[MoO2(H2LSaltris)] (1) as a yellow crystalline
solid, whereas the use of [MoO2(acac)2] provides slightly better yields. Likewise, the
tungsten precursor [W(eg)3][42] gave cis-[WO2(H2LSaltris)] (2) in methanol as pale
crystals, although the (isolated) yield was very low. However, Na2WO4 ∙ 2 H2O can be
used to obtain 2 in a moderate yield. Vanadium precursor [VO(acac)2] rapidly reacts
with H4LSaltris in methanol at RT to form an umber-colored precipitate in a high yield.
The solid can be crystallized from hot acetonitrile to yield analytically pure 3 with a
formula [VO(HLSaltris)]2. The schematic representation of the complex syntheses is given
in scheme 1.
Scheme 1. Detailed description of the syntheses of 1–3.
All complexes were fully characterized by means of high-resolution electrospray
ionization time-of-flight mass spectrometry in positive and negative ionization modes,
also utilizing MSMS. Characteristic to all complexes in the positive ionization mode is
the presence of multiple singly charged sodium adducts of the type [M – nH + (n+1)Na]+,
where n = 1, 2, 3. In these species the protons in the trisbase alcohol arms of the
complexes are sequentially substituted by sodium ions. However, singly protonated
adducts of the type [M + H]+ are generally not observed. Other common adducts in the
positive ionization are singly charged oligomerized adducts of general type [nM+Na]+,
where n = 1, 2, 3. These are most likely formed by electrostatic interactions, and not

11
representative of genuine species observable outside ionizing conditions. In the
negative ionization mode, the most characteristic ionization adduct is the [M–H]– from
the loss of a proton. Other common adducts are chlorido adducts of the type [M+Cl]–,
and dealkylation adducts [M–R]–, where R represent a generic alkyl or alcohol group.
Similarly to what is observed in the positive ionization mode, electrostatic
oligomerization is evident in the negative mode also.
The UV-Vis spectra of H4LSaltris, 1, 2 and to a certain extent 3, are rather similar, and
contain three clear electronic absorptions. All UV-Vis spectra are presented in ESI figure
14, and the spectral data has been tabulated in Table 1. By comparing the UV-Vis
spectrum of H4LSaltris to those of 1–3 it may be concluded that all signals below ca. 400
nm in the case of 1 and 2, or below ca. 460 nm in the case of 3 are primarily of
intraligand origin. While all electronic absorptions in the proligand are visible in the UV-
Vis spectra of the complexes, a noticeable redshift is observed in all signals, consistent
with coordination. All high-intensity (ε ≳ 2000–20000 M–1 cm–1) electronic absorptions
below ca. 240 nm, and the second, lower energy (ε ~ 1150–17700 M–1 cm–1)
absorptions at ca. 260–287 nm in all compounds are attributed to π → π* transitions in
the aromatic ring and due to extended conjugation in the salicylaldimine chromophore.
The third band at ca. 328–363 for H4LSaltris, 1 and 2 is assigned to the n → π* transitions
(ε ~ 40– 3200 M–1 cm–1) in the aldimine functionality.[58] 3 also contains this
transition, however, it is eclipsed by what are clearly multiple overlapping absorptions
in the general area between ca. 320–460 nm. These multiple signals have been assigned
as LMCT transitions, most likely attributable to alkoxido to vanadium(V) transitions.[59]
A very low-intensity LMCT at ca. 520 nm is assigned to phenolato → V(V) (pπ → d), with
an ε ~ 550 M–1 cm–1.[59] The pπ → d transitions in 1 and 2, although they must exist, are
too weak to be appropriately assigned. Likewise, the alkoxido to metal LMCT bands are
much weaker in 1 and 2 compared to 3.
In the IR spectra of all compounds, a broad signal at 3100–3500 corresponds to the
phenolic and/or alcoholic OH groups while a strong stretching vibration of the imine
group occurs at around 1622 cm–1. The complexation of Mo, W and V to H4LSaltris can be
substantiated from corresponding metal oxido stretching vibrations.[29] Specifically,
the characteristic symmetric and asymmetric ν(MO2) (M = Mo, W ) stretches are found
at 930 and 889 cm–1 for 1, and at 946 and 893 cm–1 for 2, respectively.[47] These are in
line with what has been reported for similar dioxidomolybdenum complexes.[60] For 3,
featuring only a single oxido ligand, a characteristic ν(V=O) stretching vibration is
found at 952 cm–1, which is consistent with experimental values obtained for other
oxidovanadium(V) complexes bearing similar ligands.[49,59] Furthermore, a low energy
signal at 553 cm–1 can be seen, which is assigned as ν(V—N) stretch.[61] This signal is
also found in 1 at 557 cm–1 and is tentatively assigned to ν(Mo—N), since similar
stretching vibrations are known in dioxidomolybdenum(VI) complexes bearing variable
bidentate cysteamine-type ligands.[62] The signal seen in the IR spectrum of 2 at 562
cm–1 is cautiously assigned to ν(W—N) stretches by virtue of similarity to 1. It should be
emphasized, that the strong signal in question is missing in the IR spectrum of the

12
proligand. IR spectral data for the most characteristic signals in all compounds in
presented in table 1.
Table 1. IR and UV-Vis data for H4LSaltris and 1–3. See text for further information
IR (cm–1)
ν(M=O)
–
930
946
952
Compound
ν(O—H)
ν(C=N)
1622
1620
1621
1623
νasymm.(M=O) ν(M—N)
H4LSaltris
1
2
3
3100–3500
3100–3500
3100–3500
3570
–
–
889
893
–
557
562
553
Compound
UV-Vis
λmax (nm)
225
262
ε (M–1 cm–1)
transition
π → π*
π → π*
n → π*
π → π*
π → π*
n → π*
π → π*
π → π*
n → π*
π → π*
π → π*
n → π*
pπ → d
H4LSaltris
≳ 2000
1150
328
230
260
355
230
274
363
240
400
1
2
3
≥ 20000
16700
3200
≥ 20000
12500
2500
≥ 20000
17700
~7600–700
550
287
320–460
520
For H4LSaltris the reported 1H NMR data[28] significantly differs from that obtained by us,
possibly due to the some difficulties in the measurements in the previous report. In any
1
13
case, the H and C NMR data obtained by us are in good agreement with the expected
structure of the ligand precursor and as such, we find re-publication of good NMR data
1
warranted. Striking features in the H NMR spectrum of H4LSaltris, very useful for
structure determination, are the proton signals corresponding to the phenol, aldimine
1
carbon, and tris-alcohol sidearms. The H NMR data for H4LSaltris and 1–3 is given in
table 2.
The signal for the phenolic proton is found at 14.90 ppm, which is significantly
downfield for what would generally be expected for phenols or salicylaldehydes, at
around 10 ppm or below. Schiff bases derived from salicylaldehydes are known to exist
as tautomers, shifting between their enolimine (OH) and ketoenamine (NH) forms,
through what is known as RAHB (resonance assisted intramolecular hydrogen
bonding).[63,64] The significant downfield resonance for the phenol proton in H4LSaltris

13
can be attributed to moderate de-shielding effect caused by the intramolecular
hydrogen bonding to the aldimine nitrogen (ArO—H---N=R1R2).[64,65] Furthermore,
with similar molecules the 13C chemical shift of the ipso carbon atom connected to
phenol OH is known to be sensitive to the relative tautomeric population between OH
and NH forms, with lower average values of ca. 155 ppm, and higher average values of
ca. 180 ppm generally being expected for OH, and NH forms, respectively.[64,65] In
H4LSaltris the corresponding 13C NMR signal is found at 160.21 ppm in DMSO-d6,
indicative of tautomeric equilibrium favoring the OH form, a feature that is also
common for similar compounds.[64] A small J-coupling (2.2 Hz) between the phenol-
apparent and aldimine carbon proton can be seen in the 1H NMR spectrum in DMSO-d6.
3
In light of tautomeric equilibrium, this coupling has been shown to arise from JNH,αH
vicinal coupling between NH and CH of the ketoenamine form.[64] Small J-couplings of
this type have been encountered before for similar compounds.[64] Other information,
particularly useful for determining coordination mode of H4LSaltris to various metals, are
the proton signals of the tris-alcohol sidearms. In H4LSaltris the three alcohol protons are
3
clearly visible at δ 4.70, as a triplet, with a JHH = 5.4 Hz coupled to the six vicinal
methylene protons, appearing as a duplet, at δ 3.62 (3JHH = 5.4 Hz) when relatively dry
DMSO-d6 is used.
The coordination mode of H4LSaltris to Mo, W and V was unequivocally determined by
NMR in addition to single-crystal XRD. Quite unsurprisingly, the corresponding cis-
dioxidomolybdenum(VI) and -tungsten(VI) complexes cis-[MO2(H2LSaltris)], where M = Mo
or W, share analogous structures in DMSO solution as well. H4LSaltris, as a potentially
pentadentate ligand, coordinates to both metal centers in tridentate, dianionic manner
via the phenolato oxygen, aldimino nitrogen and one alcoholato sidearm.[66,67] The
1
coordination mode is clearly evident from H NMR in dry DMSO-d6. For example, the
phenol proton at δ 14.90 is missing, and the aromatic protons have been shifted
downfield, relative to H4LSaltris, for ca. 0.21 ppm and 0.26 ppm for 1 and 2, respectively.
Curiously, the signal corresponding to aldimine CH proton is shifted upfield very slightly
(< 0.10 ppm) in both complexes relative to H4LSaltris. These effects might be explained by
inductive effects: Upon coordination of the aldimine nitrogen, its electron density is
withdrawn towards the electron-deficient Mo(VI) and W(VI) centers, manifesting as a
slight shielding effect for the aldimine CH proton.
As was disclosed earlier, the three alcohol protons in H4LSaltris are clearly visible as a
triplet, however, in the complexes these same triplets only correspond to two protons.
In 1, the triplet corresponding to two free sidearm alcohol protons is found slightly
downfield relative to H4LSaltris at δ 4.95 ppm, (3JHH =5.5 Hz), whereas in 2 it is located at δ
4.99 ppm. In both complexes, the four methylene protons corresponding to the two
uncoordinated alcohol sidearms appear as duplets of duplets, at δ 3.73 and 3.64 for 1,
and δ 3.72 and 3.63 ppm for 2, respectively. Accordingly, the methylene CH2 for the
coordinated alcoholato sidearm is found, as a singlet, at δ 4.43 for 1 and δ 3.58 for 2.

14
Table 2. 1H NMR spectral data of H4LSaltris, 1–3 (δ in ppm). Multiplicity: s = singlet, d
= duplet, t = triplet, m = multiplet (unresolved), b = broad.
Compound ArOH CH=N
Arom. H NC(CH2OH)3 NC(CH2OH)3
tBu
H4LSaltris
14.90
(d, 1H)
–
8.55
7.27, 7.22
4.70 (t, 3H)
4.95 (t, 2H)
3.61 (d, 6H)
1.37, 1.27
(s, 9H)
(d, 1H) (d, 1H)
8.49
(s, 1H) (d, 1H)
7.47, 7.44
3.73; 3.64
(dd, 2H); (dd, (s, 9H)
2H)
1.37, 1.29
1
–
–
8.46
(s, 1H) (d, 1H)
7.53, 7.47
4.99 (bt, 2H)
4.58 (t, 1H)
3.72; 3.64
(dd, 2H); (dd, (s, 9H)
2H)
5.25 (d, 1H)
5.13 (m, 2H)
4.70 (d, 1H)
4.04 (dd, 2H)
1.38, 1.30
2
3
9.08
(s, 1H) (d, 1H)
7.66, 7.53
1.51, 1.35
(s, 9H)
13
The most distinguishable feature of the C NMR spectra for 1 and 2 relative to that of
H4LSaltris are the methylene CH2 carbons in the alcohol sidearms. In 1 and 2 these
become inequivalent due to expected slightly different electronic environments and
steric perturbations caused by coordination to the metals. Except for the quaternary
carbon connecting to the alcoholato arms and aldimino nitrogen, which is upshifted in
both complexes for ca. 18 ppm, the alcohol and alcoholato methylene protons have been
downshifted for ca. 15 ppm in both 1 and 2, consistent with coordination and changes
in electronic environment.
In the oxidovanadium(V) complex 3, H4LSaltris coordinates in a tetradentate, trianionic
fashion via phenolato, aldimino and two alcoholato groups, in stark contrast to 1 and 2.
In principle, 1H NMR spectrum of 3 agrees well with a trigonal bipyramidal coordination
mode, which is common for vanadium(V) complexes. In the absence of single crystal
XRD and 51V NMR measurements, comparable 5-coordinate solution-state structure has
also been proposed for several very similar vanadium(V) complexes.[59] However, in
our case, single crystal XRD reveals that 3 is dinuclear in the solid state (see discussion
below), with both vanadium centers adopting a distorted octahedral coordination mode.
Furthermore, 51V NMR shows only one signal, which supports the dinuclear
configuration in solution state as well, since both vanadium centers are chemically
equivalent. Generally speaking, 5-coordinate vanadium complexes may isomerize to 6-
coordinate species via, for example, solvent ligand exchange, and vice versa.
Furthermore, 6-coordinate vanadium complexes may have multiple isomers if a solvent
molecule is coordinated. These dynamics are frequently observed as multiple signals in
the 51V NMR, although the choice of NMR solvent may or may not greatly influence this.
51
The single V NMR signal supports the notion that 3 is 6-coordinate, and is rigid i.e.
does not contain any coordinated solvent molecules in the solution state.
In 3, the aldimine carbon proton and aromatic protons have been moderately shifted
downfield for ca. 0.34 and 0.27 ppm, respectively, relative to H4LSaltris. Similarly to 1 and
2, where the tris-alcohol sidearms become inequivalent relative to the free ligand, in 3
these changes are much more pronounced. For example, the methylene CH2 protons for

15
the two alcoholato sidearms, have been shifted downfield by 0.79 – 1.35 ppm. They also
appear separately as diastereotopic duplets, due to formation of rigid 5- and 6-
membered ring structures, respectively. The single free alcohol sidearm, on the other
hand, appears 0.59 ppm downshifted for OH, and 0.13 ppm for the CH2 protons,
respectively.
The 13C NMR spectrum of 3 is very similar to those of 1 and 2. Most striking features are
the collapse of the equivalent (in H4LSaltris) tris-alcohol carbons to inequivalent carbons
in 3, with approx. 15 ppm shift downfield observed for the two coordinating alcoholato
methylene protons. Strikingly, the 51V NMR spectrum of 3 shows only one signal at ca. –
51
562 ppm. The simple nature of the V NMR spectrum points towards that 3 is present,
in solution, as a one stable species, and that there is probably little if any solution
dynamics commonly encountered with vanadium compounds. The 51V NMR observation
is well in line with single-crystal XRD results, in that the complex unit is symmetric, and
both oxidovanadium(V) centers adopt rigid six-coordinate distorted octahedral
51
configurations, making the vanadium centers chemically equivalent. The obtained V
NMR spectrum is closely similar to those of other related oxidovanadium(V)
complexes.[49] Furthermore, the 51V NMR signal is present in the expected general
area† attributable to oxidovanadium systems bearing redox-innocent O4N type
ligands.[33,68]
Description of crystal and molecular structures
Good quality single crystals of 1–3 could be grown thus enabling their structural
characterization by single crystal X-ray diffraction methods. 1 and 2 crystallized directly
from the reaction mixtures, i.e. from methanol as brightly yellow needle-like crystals (1)
and pale yellow needle-like crystals (2). 3, on the other hand, deposited from the
methanol reaction mixture in a non-crystalline form. Umber-colored block-like crystals
could be obtained from boiling acetonitrile upon slow cooling. All three complexes
crystallize in the monoclinic crystal system and have the same space group (P21/c).
The X-ray structure of 1 (Figures 1 and 2) reveals that the reaction between
molybdenum precursor and H4LSaltris has involved the deprotonation of two proligand
OH groups – the phenolic and one of the three alcoholic OH moieties – and coordination
of the respective O atoms to the dioxidomolybdenum(VI) center. The rather distorted
octahedral coordination sphere of the Mo(VI) ion is fulfilled by the aldimino N donor and
one of the alcoholic OH groups of an adjacent complex unit (for selected bond
parameters, see Figure 1). A CSD survey[69] of dioxidomolybdenum(VI) complexes with
similar trisbase salicylaldimine-type ligands reveals that in such complexes only one of
the three available alcoholic groups tends to coordinate to the central metal ion,
similarly to 1. Furthermore, the remaining sixth coordination site in the previously
reported complexes is occupied by a solvent molecule (H2O, MeOH, DMF or DMSO),
which leads to discrete molecular complexes. In the crystal structure of 1, however, the
† V(V) complexes bearing a redox-innocent O5N1 donor set generally appear between ca. –600 to –400
ppm according to ref. [33] and at lowest ca. –650 ppm according to ref. [68] (δ ppm vs. VOCl3).

16
distinguishable complex units couple together via Mo—Oalcohol bonds thus creating a
polymeric 1D chain along the crystallographic b-axis. The polymeric system is further
stabilized by intrapolymer Mo=O···H—O [d(O2···O6’) = 2.65 Å] hydrogen bonds (HB) as
well as interpolymer HBs occurring via Mo—OH···OHCH2CN [d(O5’···O6’’) = 2.63 Å]
contacts. It is noteworthy that, albeit being polymeric in the solid state, 1 dissolves
readily in DMSO which can be attributed to the degradation of the polymeric chains into
the respective monomeric complex units (see NMR analysis above). This confirms the
lability of the Mo—Oalcohol bonds and the discrete nature of the complex upon
dissolution.

17
Figure 1. Presentation of the asymmetric unit of 1 with bonds to the neighboring
complex units shown in dashed lines. All C—H hydrogen atoms are omitted for clarity.
Principal ellipsoids are presented at the 50 % probability level. Relevant bond lengths
(Å): Mo1—O1 = 1.9300(14), Mo1—O2 = 1.7237(12), Mo1—O3 = 1.6942(13), Mo1—O4
= 1.9060(13), Mo1—O5′ = 2.4123(13) and Mo1—N8 = 2.2912(14). Symmetry
operations: (′) = 1-x, ½+y, ½-z; (′′) = 1-x, -½+y, ½-z.
Figure 2. Partial view of the 1D polymeric structure of 1 (top) and illustration of
packing of the discrete polymers (alternating blue and grey colors, below) viewed along
the crystallographic b-axis. Top: C—H hydrogen atoms and tert-butyl groups omitted.
Below: All hydrogen atoms omitted.

18
In contrast to 1, the respective tungsten analogue 2 crystallizes as monomeric units in
which the WO2 unit is bound to a dianionic tridentate H2LSaltris ligand in agreement with
the NMR analyses. The remaining coordination site, which is trans to O3, has been taken
up by a water molecule (Figure 3). Although there is one example of tungsten complex
bearing a very similar ligand, the [WO2(sapd)] (sapd = salicylidene propanediol)[70], to
the best of our knowledge, 2 is the first report of a tungsten complex characterized by
XRD that bears a Saltris-type Schiff base ligand. It should be noted, that the solid state
structure of 2 closely resembles some known Mo-complexes‡ such bearing Saltris-like
ligands.[58,71] In 2, two of the three ligand alcoholic OH groups remain uncoordinated
and are thus available for hydrogen bonding. In case of 2, this leads to an intricate
hydrogen bonding network in the crystal lattice in which the monomeric units build
into 2D sheets, terminated by the aliphatic regions of the molecules (ESI, Figure S59).
Figure 3. Illustration of the asymmetric unit of structure of 2. All C—H hydrogen atoms
are omitted for clarity. Principal ellipsoids are presented at the 50 % probability level.
Relevant bond lengths (Å): W1—O1 = 1.928(4), W1—O2 = 1.764(4), W1—O3 =
1.715(4), W1—O4 = 1.902(4), W1—O7 = 2.309(4) and W1—N8 = 2.283(4).
X-ray structure of 3 shows a formation of a dinuclear vanadium complex consisting of
two oxidovanadium(V) centers and two ligands wherein each ligand is coordinated in
tetradentate trianionic fashion (Figure 4). The crystal structure is solved with half of the
[VO(HLSaltris)]2 dimer constituting the asymmetric unit while the complete
centrosymmetric complex is generated via an inversion center between the V atoms.
Also, an acetonitrile solvent molecule is located in the asymmetric unit. The crystallo-
graphically unique complex moiety comprises of oxidovanadium(V) ion chelated in ONO
fashion by phenolato (O1), aldimino (N8) and alcoholato (O3) groups of HLSaltris. The
dimer is upheld by alcoholato atoms O3 and O3′, which act as bridging donors between
the V(V) centers, and alcoholato atoms O5 and O5′ which coordinate to neighboring V1′
and V1, respectively. This bonding scheme yields an approximately planar (maximum
‡ Solid state structure of 2 is very similar to MoO2 complexes 1–5 from ref. [58] and 1–3 from ref. [71].

19
deviation from planarity is ca. 17°) V2O6 system to which two alcoholato and two
aldimino groups coordinate perpendicularly. The V—O distances in the V1—O3/O3′—
V1′ bridges show high degree of asymmetry (ca. 0.4 Å). Structurally, 3 is very similar to
some other reported dimeric vanadium complexes bearing analogous ligands.[49,66,72]
Figure 4. Presentation of the [VO(HLSaltris)]2 complex found in the crystal structure of 3.
All C—H hydrogen atoms are omitted for clarity. Principal ellipsoids are presented at
the 50 % probability level. Relevant bond lengths (Å): V1—O1 = 1.8646(12), V1—O2 =
1.6009(13), V1—O3 = 1.9028(12), V1—O3′ = 2.3066(12), V1—O5′ = 1.7981(12) and
V1—N8 = 2.1323(15). Symmetry operations: (′) = -x, 1-y, 1-z.
X-ray powder diffraction analysis
Bulk samples of 1–3 were analyzed by powder X-ray diffraction (PXRD). The unit cell
parameters of complexes 2 and 3 were determined by Pawley analysis[73] within the
HighScore Plus 4.7 program using the cell parameters of corresponding single crystal
structures as the starting point of the least-squares refinements. The variable
parameters in the refinements were zero-offset, unit cell and peak profile parameters.
The fitted diffraction graphs for 2 and 3 are found in ESI figures S61–62 whereas the
refined unit cells and the resulting R-factors and goodness-of-fit values are presented in
ESI table S2. For 2, due to instrument limitations, we could not record one of the major
low angle peaks which resides at ca. 4.5° according to the simulated single crystal
pattern.
In case of complex 1, the experimental PXRD pattern shows some similarities with the
pattern simulated from the respective single crystal structure but it is in general
distinctively different (ESI figure S60). We suspect that this may arise from a
degradation of the polymeric structure of 1 while preparing the sample of PXRD
analysis by grinding of the crystalline bulk material. In contrast, the unit cell parameters
of bulk materials of 2 and 3, derived from the Pawley analysis, are in good agreement
with the corresponding single crystal structures and thus demonstrate their structural
similarity. The refined unit cells (PXRD) show ca. 2 % increase in unit cell volume which

20
is reasonable considering the thermal expansion due to the differences in data
recording temperature between the single crystal and powder data (120 vs. 298 K)

21
Cyclic voltammetry
To gain insight into the reactivity of the complexes, and specifically to study the ability
of 1–3 to undergo reversible redox-reactions relevant regarding catechol oxidation,
electrochemical investigations were undertaken. The cyclic voltammograms of 1–3 are
presented in the electronic supplementary material, while the electrochemical data has
been tabulated in table 3. 3 has a reversible electrochemical response with E = –415
½
mV with Pt working electrode, and E = –422 mV using GC working electrode,
½
respectively. This response has been assigned as the V(V)/V(IV) redox-couple, since it is
also observed in the same potential area with some other structurally similar§ Schiff
base oxidovanadium(V) complexes.[49,59]. With higher scan rates, slight quasi-
reversibility is observed, although E does not change significantly. In the positive
½
potential range three irreversible oxidation responses occur at E = ca. +900 mV,
½
+1.496 and +1.683 V (Pt WE), that are most likely ligand-centered, since they are also
varyingly present in 1 and 2. For example, in some unrelated complexes phenoxyl
radical formation has been suggested to occur at approx. +650–750 mV.[74]
Accordingly, we tentatively assign the ~ +900 mV redox-response to PhO → PhO•+, and
subsequent oxidation events at higher potentials to alkoxy-centered oxidations i.e. RO →
RO•+. The cyclic voltammograms of 1 and 2 are remarkably featureless in comparison to
those of 3. All redox-processes present in 1 and 2 are irreversible, with the one-electron
reductions e.g. Mo(VI) → Mo(V) and W(VI) → W(V) tentatively assigned as –0.896 (Pt)
and –1.296 V (GC), respectively.[66] Similar ligand-centered irreversible oxidation
events are present in 1 and 2 than what is found in 3.
Table 3. Electrochemistry data obtained for 1–3.
Complex ligand-centered ligand-centered ligand-centered metal-centered
e.g. RO → RO•+
e.g. RO → RO•+
Pt GC
+2.159 +1.691 +1.717
e.g. PhO → PhO•+ M(os)/M(os+1)
Pt
GC
Pt
GC
Pt
GC
1
2
3
–
–
+0.942
–
–0.986
irrev.
–1.296
irrev.
–
–
+1.653 +1.684 +0.934 +0.959
–
+1.683 1.682
+1.496 +1.484 +0.896 +0.917 –0.415 –0.422
ps. rev. ps. rev.
The values are given in V vs. Ag/AgCl calibrated against Fc/Fc+ redox-couple.[54] GC
and Pt represent glassy carbon and platinum working-electrodes (WE), respectively.
Catechol oxidase mimetic activity
In various catechol oxidase studies 3,5-DTBC is the substrate of choice[19,20,28], owing
to its relatively easy oxidation due to its low redox potential.[75] As such, the catechol
oxidase mimetic activity of all complexes was studied in the aerial oxidation of 3,5-
DTBC as well as 4-tert-butylcatechol (4-TBC) and pyrocatechol in chloroform,
§ Structurally related V(V) complexes such as 3 and 6–8 from ref [49] and 1, 2, 4 and 5 from ref. [59]
display a V(IV)/V(V) redox couple at E values –470 to –400 mV vs. Ag/AgCl in DMF.
½

22
acetonitrile and methanol. 4-TBC and pyrocatechol were chosen because they are more
resistant to autoxidation compared to 3,5-DTBC.
Preliminary investigations indicated that 3,5-DTBC and 4-TBC readily reacted to form
the corresponding o-benzoquinones, namely 3,5-di-tert-butyl-1,2-benzoquinone (3,5-
DTBQ) and 4-tert-butyl-1,2-benzoquinone (4-TBQ), respectively, when 3 was used as
the catalyst in CHCl3, MeOH and MeCN. However, 1 and 2 showed no noticeable catechol
oxidase-like activity in the same time frame, although UV-Vis and ESI-HRMS
measurements confirmed that the catechols coordinate to the metal centers (ESI
Figures S35–S46). Furthermore, pyrocatechol was observed not to react in any solvent
with any of the catalysts. With 3, catechol coordination was also evident from the fact
that all of the reaction solutions turned blue after the addition of the catechols, a
characteristic color for vanadium-catecholato species[31], with also typical vanadium-
catecholato LMCT bands observable in the UV-Vis spectra at ca. 650 and 830 nm (figure
5).[31,33,76] Furthermore, the solutions eventually turned green and finally yellow-
brown due to the formation of o-benzoquinones. UV-Vis spectroscopic measurements
confirmed the UV bands of 3,5-DTBQ and 4-TBQ, that appear at approx. 400 nm.[56,57]
Figure 5. The formation characteristic vanadium-catecholato LMCT bands at approx.
650 and 830 nm during 3,5-DTBC oxidation by 3. The reaction was monitored for 1
hour and spectra were recorded at every 5-minute interval.
From the kinetics data it can be concluded that the oxidation of 4-TBC and 3,5-DTBC
catalyzed by 3 follows typical Michaelis–Menten like kinetics (figure 6). At low substrate
concentrations, the measured absorbance increases linearly as a function of time,
indicative of a first order reaction with respect to the substrate. However, upon

23
significant excess (> 10 000-fold) of the substrate relative to catalyst the reaction rate
becomes pseudo zero-order. At this regime subsequent substrate additions will not
speed up the reaction, as the catalyst becomes saturated. The kinetic parameters Vmax
(maximum reaction rate), Michaelis-constant KM ([S] at ½Vmax) and kcat (turnover
frequency) are given in table 4. From the kinetics data it can be seen that 3 compares
well with other oxidovanadium systems, with its catalytic activity falling well within the
range of that of others (table 4).[18–21] However, the most remarkable aspect is the
fact that 3 offers a comparable catechol oxidase-like activity to the structurally very
similar dicobalt complex [Co(H2LSaltris)(OAc)]2.[28]
Figure 6. Michaelis–Menten plots of the oxidations of 4-TBC and 3,5-DTBC by 3 in
CHCl3.
While the natural catechol oxidase catalyzes the oxidation of catechols to corresponding
o-benzoquinones, producing water[15] as the sole by-product, a number of model
transition metal complexes produce hydrogen peroxide as well.[28,77–81] Recently, a
dicobalt complex [Co(H2LSaltris)(OAc)]2 structurally strikingly similar (lacking oxido
ligands) to 3 was assessed in the oxidation of 3,5-DTBC.[28] Mechanistic investigations
involving MS measurements strongly hint towards a single and double 3,5-DTBSQ
adducts [(Co(III)(HLSaltris)Co(II)(H2LSaltris)(3,5-DTBSQ)] and [Co(II)(H2LSaltris)(3,5-
DTBSQ)]2. Some studies highlight the importance of metal–metal distance, which may
give insights into the overall catalytic mechanisms of any given catechol oxidase model
system.[78,79] In light of the obvious structural similarities between the dinuclear
cobalt(III) complex and our dinuclear oxidovanadium(V) complex, as well as the fact that
there are only a limited[20,23] mechanistic investigations on catechol oxidation
involving modern oxidovanadium systems, we conducted mechanistic investigations of
51
our own using UV-Vis, V NMR and ESI-HRMS spectrometry. The detailed descriptions
of the experiments are given in supplementary material.

24
Table 4. Catechol oxidation results for the present [VO(HLSaltris)]2 as well as some other
selected vanadium-based catechol oxidase mimetic systems.
Compound
Vmax (M min–
)
kcat (h–1)
164∓32a
94∓4b
KM (M)
Ref.
1
3
3
This
work
This
work
[18]
[21]
[21]
[19]
[28]
[20]
[23]
0.05727
0.01363
1.37 × 10–5
7.84 × 10–6
[VO(L1)]2 SO4
1439bc
12a
–
–
[VO(OMe)(L2)]
[VO(OMe)(L3)]
[VO(OMe)(MeOH)(L4)]
[Co(H2LSaltris)(OAc)]2
[VO2(L5)]
0.00115
0.00107
0.00557
0.0087
0.00079
0.00103
1.01 × 10–5
1.04 × 10–5
7.66 × 10–6
1.33 × 10–5
5.73 × 10–5
13a
3a
80a
2063a
[VO(L6)]
a
395a
6.58 × 10–3
c
b
= For oxidation of 3,5-DTBC. = For oxidation of 4-TBC. = In the presence of
stoichiometric amounts of base (Et3N).
The initial reaction rate of the oxidation of 4-TBC by 0.1 mol-% of 3 was measured by
UV-Vis spectroscopy in the presence of DMSO (hydroxyl radical quencher), BHT
(butyrated hydroxytoluene, radical inhibitor), and varying concentrations of H2O2 in
chloroform (figure 7). The addition of DMSO or BHT (entries 3 and 4) have little effect
on the initial reaction rate when compared to the effects of 3 alone (entry 2), so it may
be concluded that hydroxyl radicals or any other radicals most likely do not significantly
contribute to the overall oxidation mechanism. However, hydrogen peroxide seems to
accelerate the oxidation to some extent in the presence of 3, with reaction rate
increasing approx. linearly with increasing H2O2 concentration (entries 5–7). However,
and importantly, even in relatively high concentrations H2O2 does not significantly
oxidize 4-TBC in the absence of 3 (entry 1), so the reaction rate enhancement most
probably does not come from the direct substrate oxidation by H2O2. Indeed, it has been

25
previously shown that H2O2 may assist the dissociation, or leaching, of vanadium from
vanadium-containing species, which then rapidly convert to catalytically active
catecholato-bearing vanadium species e.g. [VO(3,5-DTBC)(3,5-DTBSQ)] and [V(3,5-
DTBC)3] in the presence of very large excess of catechol, 3,5-DTBQ and H2O2 relative to
vanadium.[36,38] In a separate experiment, an iodometric assay was done to confirm
the formation of hydrogen peroxide in our system.
Figure 7. Initial reaction rate of 4-TBC (0.1 M) oxidation over the period of 20 minutes
in the presence of 3 (1 × 10–4 M) and/or several additives. See text for further
information. Control 1: 0.1 M 4-TBC + 0.030 M H2O2 in the absence of 3. Control 2: 0.1 M
4-TBC + 3 in the absence of H2O2. All measurements run in duplicate.
51V NMR experiments were undertaken to determine qualitatively the nature of
vanadium-containing species formed during catechol oxidation, and to gain some
insight into the catalytically active vanadium species. In the experiments, 100
equivalents of 3,5-DTBC and 4-TBC were reacted with 3 in CDCl3 at RT, and the 51V NMR
spectra were recorded immediately. The 51V NMR spectra obtained from these
51
experiments drastically differed from the original V NMR spectra of 3 (figure 8). In
both cases, the original signal corresponding to 3 at ca. –562 ppm vanished completely,
which is a clear indication of dynamic and very rapid changes to the first coordination
sphere of 3. New, very broad signals were observed at ca. +1221 and +1185 with 4-TBC
and at ca. +1391 with 3,5-DTBC viz. significantly downfield relative to VOCl3.

26
51
Figure 8. V NMR spectra of 1) 3 in acetone-d6., 2) 3 + 100 eq. 3,5-DTBC in CDCl3. and
3) 3 + 100 eq. 4-TBC in CDCl3.
51
These prominent changes, and the absence of V NMR signals at approx. –400 – (–600)
ppm, the expected region for oxidovanadium(V) complexes bearing a redox-innocent
NO5 donor set, indicate that there are most likely no vanadium species bearing HLSaltris
left.[33,68] In fact, the significantly downfield signals are almost certainly related to
diamagnetic catecholato-bearing vanadium species with at least two or even three
catechol ligands, since many reported non-innocent monocatecholato oxido-
vanadium(V) systems with mixed ON donors appear upfield, and at best only slightly
51
downfield of VOCl3.[33] It should be noted, that the V NMR spectra did not drastically
change over a period of two days. Furthermore, when 4-TBC was used a signal was
observed at –438 ppm, only slightly downfield of 3. This species has been tentatively
assigned to [(VO(HLSaltris)(O-4-TBC)], where 4-TBC coordinates in a singly anionic
monodentate fashion. These assignments are in line with observations from ESI-MS
studies (see below), where bis- and tris(catecholato) oxidovanadium and non-
oxidovanadium species are detected.
51
To supplement the rather cautious assignments done in the V NMR section above, the
same NMR samples were analyzed by ESI-HRMS in acetonitrile. Some reported
mechanistic studies investigating catechol oxidation by oxidovanadium complexes
frequently involve ESI-MS only in the positive mode.[20,23] However, as per the
findings by Finke and co-workers related to the nature of the true active catalyst,

27
negative mode MS studies might also reveal important information, and should always
be run.[36] Accordingly, we report both positive and negative MS results.
In the case of 3 + 3,5-DTBC, and in the negative mode, the main ionization product was
[V(3,5-DTBC)3]– at m/z = 711.3884 (calcd. m/z = 711.3835). Other species in the
negative mode are [VO(3,5-DTBC)2]– at m/z = 507.2336 (calcd. m/z = 507.2321). A very
low-intensity signal can be observed for [VO(HLSaltris)(3,5-DTBC)]– at m/z = 622.2976
(calcd. m/z = 622.2954). In the positive mode the only clearly detected species is [3,5-
DTBQ + Na]+ at m/z = 243.1400 (calcd. m/z = 243.1356). However, there exists a very
low-intensity (ca. 2 %) signal at m/z = 646.2889. We have tentatively assigned this to
the species [VO(H2LSaltris)(O-3,5-DTBC) + Na]+, which has a calculated m/z = 646.2919.
There are no traces of the intact 3 in either positive or negative polarization. From this
data, the detection of [VO(3,5-DTBC)2]– or, perhaps [V(IV)O(3,5-DTBC)(3,5-DTBSQ)]–, is
significant, as it has been linked to the Pierpont’s complex [V(V)O(3,5-DTBC)(3,5-
DTBSQ)]2, which is the dimerized resting state of the supposed active catalyst [VO(3,5-
DTBC)(3,5-DTBSQ)].[38] Pierpont’s complex, having a calculated m/z = 1014.4631, is
not detected, however. The MS spectra are presented in the ESI (figures S42–43).
With 3 + 4-TBC, a somewhat similar speciation is observed, however, the main species
in the negative ionization corresponds to [VO(HLSaltris)(O-4-TBC)]– at m/z = 566.2312
(calcd. m/z = 566.2328). The tris catecholato species [V(4-TBC)3]– is also present at m/z
= 543.1966 (calcd. m/z = 543.1957) with low intensity. These findings may suggest that
in the presence of 4-TBC, which reacts slower than 3,5-DTBC, 3 is somewhat more
inclined to form monocatecholato adducts. This would also give some credibility to the
51V NMR assignment at –438 ppm (see above). Similarly to the above case, the
[VO(H2LSaltris)(O-4-TBC) + Na]+ low-intensity adduct is tentatively assigned to the m/z =
590.2240 observed in the positive mode (calcd. m/z = 590.2293). [3 + Na]+ is also
detected in the positive ionization mode with m/z = 825.2625 (calcd. m/z = 825.2706)
unlike previously in the presence of 3,5-DTBC. The MS spectra are presented in the ESI
(figures S44–S45).
After overnight reaction of 3,5-DTBC in the presence of 1 mol-% 3 three compounds
were isolated (table 5 and ESI). The product distribution resembles that found in many
3,5-DTBC oxidation reactions catalyzed by a variety of different vanadium pre-
catalysts.[35,36,38] For instance, the 7-membered 3,5-di-tert-butyl-1-oxacyclohepta-
3,5-diene-2,7-dione (4) is generally obtained with the highest yield of 40–57 % of the
reaction products. While our isolated yield falls slightly short of 40 %, it is still the main
product at 31 % yield. The second to most abundant product is 3,5-di-tert-butyl-1,2-
benzoquinone (5) with a mean yield of 9–25 % and agrees with our observations (yield
= 23 %). Additionally, the dimeric 4',6,6',8-tetra-tert-butyl-3H-spiro[benzo[b][1,4]di-
oxine-2,2'-pyran]-3-one (6) represents one of the low-yielding products (10–18 %), a
fact which is also observed by us (18 % yield). However, we were unable to isolate, nor
detect, the lactones 3,5-di-tert-butyl-2-pyrone (7) or 3,5-di-tert-butyl-5-(carboxymeth-
yl)-2-furanone (8) which generally, and in combination, represent at most 20 % of the

28
total product yield. Accordingly, they might represent the “missing” 28 %, as the
combined isolated yield of our reaction amounts to only 72 %. TLC was however used to
establish that no 3,5-DTBC was present, and hence we estimate a minimum 95 %
conversion. It is important to emphasize that the formation of the products 4, 6, 7 and 8,
which represent the dioxygenase intra- and extradiol cleavage products, have been
linked[36,38] to the vanadium species [V(3,5-DTBC)3]– (detected with ESI-HRMS, see
above), which is itself intimately related to [VO(3,5-DTBC)(3,5-DTBSQ)] and the
Table 5. The product distribution obtained by reacting 3,5-DTBC with air in boiling
chloroform in the presence of 1 mol-% 3.
Isolated product yield (%)
Estimated conversiona (%)
≥95
4
31
5
23
6
18
7
ndb
8
ndb
a As determined by TLC. b Not detected. Reaction conditions: 500 mg 3,5-DTBC, 18 mg
“common catalyst hypothesis”.[36]

29
3, 30 mL CHCl3, reflux 16 h.
The mechanistic investigations described above provide compelling evidence in support
of the notion that neither 3, nor a close structural derivative thereof, is the active
catechol oxidation catalyst. Consequently, a catalytic cycle resembling the one described
for the closely similar dicobalt system cannot be credibly proposed.[28] Instead, the
results obtained from UV-Vis, 51V NMR, and from the ESI-HRMS and product
distribution experiments in particular, confer a rather different state of affairs. The
combined results clearly indicate that dioxygenase activity is presented during catalysis
as well, and that the system presented herein confers very similar behavior to many
other vanadium precatalysts. These results are decisively in favor of the “common
catalyst hypothesis”.[36]
Oxygen atom transfer activity
cis-Dioxidomolybdenum(VI) and –tungsten(VI) complexes are well known to catalyze
oxygen atom transfer (OAT) to suitable organic substrates such as, but not limited to,
benzoin and triphenylphosphine.[82–86] The propensity of Mo/W compounds to
catalyze this particular reaction is not at all surprising considering the role of these
metals in enzymes such as DMSO reductase that catalyzes similar reactions.[86] As a
result, oxidovanadium complexes have generally received much less attention in this
regard. We chose tris(4-fluorophenyl)phosphine as the model compound, due to the
19
ease of monitoring of the reaction by the very sensitive and quantitative F NMR.[55]
Quite unexpectedly, however, the complexes were found to not have any significant OAT
activity, even at moderate catalyst loadings of 10 mol-% (ESI figure S58).
Conclusions
Mononuclear complexes [MO2(H2LSaltris)] (M = Mo (1), W (2)), as well as a dinuclear
complex [VO(HLSaltris)]2 (3) with a hydroxyl-rich Schiff base proligand N-(1,3-
dihydroxy-2-(hydroxymethyl)propan-2-yl)-3,5-di-tert-butylsalicylaldimine (H4LSaltris),
were prepared by the reaction with Mo, W and V metal precursors in methanol
solutions. The dinuclear vanadium complex shows moderate activity in the oxidation of
4-TBC and 3,5-DTBC, mimicking the action of the dicopper enzyme catechol oxidase.
Strikingly, the activity of 3 in catechol oxidation was observed to be similar to that of a
structurally very similar dicobalt complex [Co(H2LSaltris)(OAc)]2, whose mechanism of
catalysis has been established. However, a set of experiments investigating the catechol
oxidation mechanism by 3, prompted by similarity to [Co(H2LSaltris)(OAc)]2, strongly hint
that 3 is not the catalytically active species in the solution. Rather, the presented
mechanistic studies reinforce a strong argument in the literature which suggests that a
catecholato-bearing vanadium species is formed in-situ during catalytic turnover
conditions in the presence of 3,5-DTBC, is in fact the real active catalyst. It is further
proposed, that such a complex is formed in virtually every scenario involving vanadium-
based precatalysts and 3,5-DTBC, a fact clearly demonstrated by the [VO(HLSaltris)]2
system presented herein. Furthermore, and quite unexpectedly, the complexes were