C-scorpionate Au(III) complexes as pre-catalysts for industrially significant toluene oxidation and benzaldehyde esterification reactions

Article abstract of DOI:10.1016/j.ica.2020.119881

The new Au(III) complex [AuCl₂(Tpms)] (1) and the previously reported [AuCl₂(Tpm)]Cl (2), bearing the potentially tridentate ligands tris(1-pyrazolyl)methanesulfonate (SO₃C(C₃H₃N₂)₃^-, Tpms) or hydrotris(1-pyrazolyl)methane (HC(C₃H₃N₂)₃, Tpm), respectively, were synthesized in water at room temperature and characterized using NMR and IR spectroscopy. The molecular structure of 1 was authenticated by single crystal X-ray diffraction analysis. The catalytic performance of the Au(III) complexes was tested, for the first time, in toluene and benzyl alcohol oxidation reactions. The oxidative esterification of benzaldehyde, by-product of toluene oxidation, was further explored. In order to optimize the catalytic systems, the influence of parameters such as temperature, reaction time, amount of pre-catalyst and the presence of additives was evaluated. In the peroxidative (by H₂O₂ or t-BuOOH) oxidation reactions, a maximum total yield (benzylic alcohol and benzaldehyde) of 8percent for toluene oxidation with pre-catalyst 1 (6 h, 80 °C, H₂O₂ 30percent aq. sol.) and a maximum total yield (benzaldehyde and benzoic acid) of 43percent for benzyl alcohol oxidation with pre-catalyst 2 (24 h, 80 °C, t-BuOOH 70percent aq. sol.) with a selectivity of 72percent for benzaldehyde, were obtained. The esterification of benzaldehyde yielded, in the presence of 1, a maximum of 27percent and 48percent of methyl benzoate, at room temperature and 80 °C, respectively, and with a selectivity of 78percent for methyl benzoate.

Full text of DOI:10.1016/j.ica.2020.119881

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Research paper
C-scorpionate Au(III) complexes as pre-catalysts for industrially significant
toluene oxidation and benzaldehyde esterification reactions
Hugo M. Lapa, M. Fátima C. Guedes da Silva, Armando J.L. Pombeiro,
Elisabete C.B.A. Alegria, Luísa M.D.R.S. Martins
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Reference:
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https://doi.org/10.1016/j.ica.2020.119881
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Please cite this article as: H.M. Lapa, M.F.C. da Silva, A.J.L. Pombeiro, E.C.B. Alegria, L.M.D. Martins, C-
scorpionate Au(III) complexes as pre-catalysts for industrially significant toluene oxidation and benzaldehyde
esterification reactions, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119881
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C-scorpionate Au(III) complexes as pre-catalysts for industrially significant toluene
oxidation and benzaldehyde esterification reactions
Hugo M. Lapaa,b, M. Fátima C. Guedes da Silvaa, Armando J. L. Pombeiroa, Elisabete C.B.A.
Alegriaa,b,* and Luísa M.D.R.S. Martinsa,*
a
Centro de Química Estrutural and Departamento de Engenharia Química, Instituto Superior
Técnico, Universidade de Lisboa, Portugal
b
Área Departamental de Engenharia Química, ISEL, Instituto Politécnico de Lisboa, Portugal
* Corresponding authors:
Elisabete C.B.A. Alegria ebastos@deq.isel.ipl.pt,
Luísa M.D.R.S. Martins luisammartins@tecnico.ulisboa.pt

Graphical Abstract
Highlights
► A new C-scorpionate Au(III) was synthesized in water and fully characterized
► Au(III) complexes were tested in toluene and benzyl alcohol oxidation reactions
► C-scorpionate Au(III) complexes act as active catalysts for peroxidative esterification of
benzaldehyde
► High selectivity in oxidation and esterification reactions

Abstract
The new Au(III) complex [AuCl2(Tpms)] (1) and the previously reported [AuCl2(Tpm)]Cl (2),
-
bearing the potentially tridentate ligands tris(1-pyrazolyl)methanesulfonate (SO3C(C3H3N2)3 ,
Tpms) or hydrotris(1-pyrazolyl)methane (HC(C3H3N2)3, Tpm), respectively, were synthesized in
water at room temperature and characterized using NMR and IR spectroscopy. The molecular
structure of 1 was authenticated by single crystal X-ray diffraction analysis. The catalytic
performance of the Au(III) complexes was tested, for the first time, in toluene and benzyl alcohol
oxidation reactions. The oxidative esterification of benzaldehyde, by-product of toluene oxidation,
was further explored. In order to optimize the catalytic systems, the influence of parameters such
as temperature, reaction time, amount of pre-catalyst and the presence of additives was evaluated.
In the peroxidative (by H2O2 or t-BuOOH) oxidation reactions, a maximum total yield (benzylic
alcohol and benzaldehyde) of 8% for toluene oxidation with pre-catalyst 1 (6 h, 80 C, H2O2 30%
aq. sol.) and a maximum total yield (benzaldehyde and benzoic acid) of 43% for benzyl alcohol
oxidation with pre-catalyst 2 (24 h, 80 °C, t-BuOOH 70% aq. sol.) with a selectivity of 72% for
benzaldehyde, were obtained. The esterification of benzaldehyde yielded, in the presence of 1, a
maximum of 27% and 48% of methyl benzoate, at room temperature and 80 C, respectively, and
with a selectivity of 78% for methyl benzoate.
Keywords: Gold(III) complex; C-scorpionate; toluene oxidation; oxidative esterification; value-
added product.
1. Introduction
The research on gold complexes has recently experienced exponential growth due to their
successful use as catalysts for several reactions such as hydrogenation, low-temperature carbon
monoxide oxidation, alkane oxidation, C-C coupling, and cyclization, among others [1–5].
However, gold(III) complexes are still underdeveloped when compared to gold(I) complexes. Being
a planar coordination center, Au(III) typically has a coordination number of 4, allowing the use of
various ligands to form different complexes. The main uses of such complexes are in cancer
treatment [4,6] (due to similarities to cisplatin), in C-C coupling reactions and, recently, also in the
oxidation [3,7] or hydrocarboxylation of alkanes [8]. After their discovery in 1966 by Trofimenko

[9], B-scorpionates, and later C-scorpionates, have been coordinated to a huge number of transition
metals in very different ways [10–12]. C-scorpionates is the general designation for a class of
compounds of general formula [R(4-n)C(R’pz)n] (pz = pyrazol-1-yl; n = 2 or 3) including the
tris(pyrazol-1-yl)methane compounds and derivates (n =3). This nomenclature reflects the versatile
coordinating behavior of tris(pyrazol-1-yl)methanes when binding to a metal center, resembling a
scorpion‘s attack to its pray. C-scorpionates interesting derivatives can be formed either by adding
substituents to the pyrazolyl rings or the methine carbon leading to different complexes with
characteristic stereochemical and electronic properties. They are isoelectronic with cyclopentadienyl
ligands and can present related coordination chemistries. Moreover, they can easily interchange their
coordination modes between tri- and bi-dentate providing structural and chemical versatility to their
metal complexes. Some of such complexes have been used as catalysts in several reactions, in
particularly oxidations [10,11]. C-scorpionate complexes e.g., of V, Fe, Co, Cu, Re or Au can act as
catalysts for alkane oxidation reactions, a field that remains underexplored due to the difficulty to
activate the C-H bond in such unreactive compounds [10,11].
Regarding gold C-scorpionate complexes, to the best of our knowledge,
[AuMe2(Tpm)]NO3.H2O [13], and [AuCl2{2-RC(R′pz)3}]Cl (R = R′ = H, R = CH2OH, R′ = H and
R = H, R′ = 3,5-Me2, pz = pyrazol-1-yl) [3], are the only ones reported. The most recent ones,
promptly obtained in water at room temperature, were used as efficient catalysts for the peroxidative
homo- and heterogeneous oxidation of cyclohexane [3] but were not tested for other catalytic
reactions. No other metal complexes using C-scorpionates were tested until now for the oxidation
reactions of this work. Thus, there is an opportunity window to test C-scorpionate Au(III) complexes
in other important oxidation reactions and to continue the development of this underexplored field
of research involving new gold(III) complexes.
Toluene, being a VOC (volatile organic compound) poses an environmental and health hazard. In
2015, 10 kt were emitted to the atmosphere by USA industries [14], while worldwide is estimated
that 10 Mt of toluene was emitted [15]. In 2017/18 Australia reported emissions of 1.9 kt of toluene
[16]. Oxidation allows to convert toluene in chemical products of added value (Scheme 1) like
benzaldehyde used in dyes, perfumes, pharmaceuticals or as an intermediate to form benzoates (e.g.,
methyl benzoate) [17,18]. The industrial process to obtain benzaldehyde from toluene is performed
in very harsh conditions, pressure and with low conversion [17]. Moreover, benzyl alcohol, a by-
product of toluene oxidation, is used as solvent of inks, waxes, shellacs and epoxy resins or as a
precursor to various esters and ethers to the soap, perfume and flavor industries [19,20]. Oxidation

of benzyl alcohol allows increasing the production of benzaldehyde since this is the main product
of the reaction. The oxidative esterification (Scheme 1) allows the formation of esters in a single
step from the aldehyde without the formation of the corresponding acid. Methyl benzoate can be
obtained by oxidative esterification of benzaldehyde with methanol. It has been used as a solvent
for resin and cellulose, air care products, biocides, perfumery, cosmetic and food industry [21–23].
Scheme 1. Valorization reactions for conversion of hazardous toluene.
Thus, herein we report the synthesis and full characterization of a new C-scorpionate gold(III)
complex, [AuCl2(Tpms)] (1), and its application, and of its analogue [AuCl2(Tpm)]Cl (2) [3], as pre-
catalysts for the industrially important selective oxidations of toluene or benzyl alcohol as well as
for the esterification of benzaldehyde to methyl benzoate.
2. Experimental
2.1. Materials and methods
Pyrazole (Aldrich), HAuCl4·3H2O (Alfa Aesar), methanol (Acros), acetonitrile (Riedel-de-
Haën), tetrahydrofuran (Riedel-de-Haën), hydrogen peroxide (30%) (Fluka), t-butyl hydroperoxide
(70% aq.) (Alfa Aesar), toluene 98% (Aldrich), benzyl alcohol (99.8%) (Sigma-Aldrich),
benzaldehyde (98%) (Alfa Aesar), cycloheptanone (99%) (Sigma-Aldrich), nitric acid (65%)
(Riedel-de-Haën), nitromethane (Aldrich), diethyl ether (Riedel-de Haën), 2 M n-butyllithium
(Aldrich), sulfur trioxide trimethylamine complex (Aldrich), sodium carbonate (Panreac),

chloroform (Riedel-de-Haën), celite (Fulka), activated carbon (Panreac), sodium sulfate (Acros),
tetrabutylammonium bromide (Acros) were used as received from the supplier, without further
purification or drying. Infrared spectra (4000–400 cm−1) were recorded with a Bruker Vertex 70
spectrophotometer in KBr pellets and far infrared spectra (400–200 cm−1) in CsI pellets
(wavenumbers are in cm−1). 1H and 13C NMR spectra were recorded with a Bruker Avance III 400
MHz (UltraShieldTM Magnet) spectrometer at ambient temperature. δ values are in ppm.
Abbreviations: s, singlet; b, broad; d, doublet; t, triplet; dd, doublet of doublet; C, H, S and N
elemental analyses were carried out by the Analytical Services of the Instituto Superior Técnico.
Gas Chromatographic (GC) measurements were carried out using an Agilent 7820A series gas
chromatograph with FID detector and a capillary column (DB-WAX, column length: 30 m; internal
diameter: 0,32 nm). Helium was used as carrier gas. UV-Vis measurements were carried out using
an Agilent Cary 60 UV-Vis spectrophotometer with double beam and Xenon Flash Lamp (80 Hz)
as light source at room temperature using quartz cuvettes with a path length of 1 cm.
2.2. Catalyst precursor preparation
The hydrotris(pyrazol-1-yl)methane (Tpm), lithium tris(pyrazol-1-yl)methanessulfonate (LiTpms)
and the complex [AuCl2(Tpm)]Cl (2) were obtained according to the published methods [3,24,25].
Synthesis of the C-scorpionate gold(III) complex [AuCl2(Tpms)] (1)
LiTpms (0.372 g, 1.3 mmol) was added to distilled water (5 mL) under continuous stirring at
room temperature. The gold precursor HAuCl4•3H2O (0.500 g, 1.3 mmol) was dissolved in distilled
water (5 mL) and added dropwise in a stoichiometric proportion of 1:1 to the ligand solution. A
green-yellow solid was obtained, after a few drops added of the gold precursor and was let to stirred
for two hours at room temperature. The suspension was filtered by gravity and the solution was
concentrated in the rotary evaporator to collect more complex that was still soluble in water. The
solid was washed with ethanol and diethyl ether and dried under vacuum. 67% yield was obtained.
[AuCl2(Tpms)] (1) : IR (KBr pellet, cm−1): 3136 [ν(CH)], 1610, 1512, and 1286 [ν(N=C), ν(C=C),
ν(C–N), HC(pz)3], 1323 [ν(S-C)]. Far-IR (CsI pellet, cm−1): 353 [νasym(Au–Cl)], 326 [νsym(Au-Cl)].
1H NMR [δH (400.15 MHz, DMSO-d6, 25 °C)]: 8.13 [d, 3H, 3JHH= 2.9, H(3), HC(pz)3], 7.40 [s br,
3H, H(5), HC(pz)3], 6.32 [dd, 3JHH= 3.1, 3H, H(4),HC(pz)3]. 13C NMR [δC (400.15 MHz, DMSO-
-
d6, 25 °C)]: 96.34 [s, SO3 C(pz)3], 105.87 [s, C(4), HC(pz)3], 132.28 [s, C(5), HC(pz)3], 138.35 [s,

C(3), HC(pz)3]. Elemental analysis found (calculated): C, 21.35 (21.40); H, <2 (1.62); N, 14.76
(14.98); S, 5.73 (5.71). The C-scorpionate gold(III) complex is soluble in MeCN, DMSO, acetone
and MeOH, but, as expected from its synthetic procedure, poorly soluble in water (less than 1x10-5
mol/L).
2.3. X-ray crystallographic data
The measurements were performed on a Bruker AXS-KAPPA APEX II PHOTON 100 using
Mo Kα radiation of 0.71073 Å) from a fine-focus sealed X-ray source. Full spheres of data were
collected using  and  scans of 0.5º per frame. Cell parameters were retrieved using Bruker
SMART [26] software and refined using Bruker SAINT [26] on all the observed reflections.
Absorption corrections were applied using the SADABS [27]. The structures were solved by direct
methods using SIR97 package [28] and refined with SHELXL-2018/3 [29]. Calculations were
performed using the WinGX System-Version 2018-3 [30]. C-bonded hydrogen atoms were included
in the refinement in calculated positions using the riding-model approximation with the Uiso(H)
defined as 1.2Ueq of the parent methylene atoms. Least square refinements with anisotropic thermal
motion parameters for all the non-hydrogen atoms were employed. Crystallographic data are given
in Table 1. The structure has been deposited to the Cambridge Crystallographic Data Centre
(CCDC 1985561).
Table 1. Crystal data and structure refinement details for 1.
1
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C10H9AuCl2N6O3S
561.15
orthorhombic
P 212121
8.2896(4)
10.8130(6)
34.6191(17)
90
b (Å)
c (Å)
α, β ,γ (º)
V (Å3)
3103.1(3)

Z
Dcalc (g/cm3)
8
2.402
F000
2112
µ (mm–1)
9.983
Reflections measured
Obs / Unique reflections
Number of parameters
Rint
25963
6363 / 4897
416
0.0769
0.0596
0.0931
1.102
R(F) (I  2σ)
wR (F2) (all data)
GOF (F2)
Max. and min. residual density
2.031-1.226
2.4. Catalytic studies
2.4.1 Peroxidative oxidation of toluene
In typical conditions, the reactions were carried out in a glass tube at atmospheric pressure and
the reaction mixtures for toluene oxidation were prepared as follows: 10 µmol of pre-catalyst, 5
mmol of toluene, 10 mmol of H2O2 (30% aq. sol.) or t-BuOOH (70 % aq. sol.) were added to 3 mL
of MeCN. The mixture was stirred for 24 h at 80 °C. When HNO3 was used as an additive, the
additive/pre-catalyst molar ratios 25-70 were used. For the product analysis, 90 μL of
cycloheptanone (internal standard) and 10 mL of diethyl ether (to extract the substrate and the
organic products from the reaction mixture) were added. The resulting mixture was stirred for a
maximum of 5 min and then a sample was taken from the organic phase and analyzed by GC using
the internal standard method.
The stability of complex 1 was evaluated by comparison of the UV-vis spectra (6.8x10-4 mol/L)
of HAuCl4.3H2O and of 1 before the reaction and the spectrum of the mixture after the catalytic
oxidation reaction.

2.4.2. Peroxidative oxidation of benzyl alcohol
In typical conditions, the reaction mixtures of benzyl alcohol were prepared as follows: 10 µmol
of pre-catalyst, 5 mmol of benzyl alcohol, 10 mmol of t-BuOOH (70% aq. sol.) and 50 µL of
nitromethane (internal standard) were added to 3 mL of MeCN. The reaction mixture was stirred for
24 h, at 80 °C and a sample was taken at 5 h and 24 h of reaction time and analyzed by GC using
the internal standard method.
2.4.3. Peroxidative esterification of benzaldehyde
In typical conditions, the reaction mixtures of benzaldehyde esterification were prepared as
follows: 5 µmol of pre-catalyst, 5 mmol of benzyl alcohol or benzaldehyde, 10 mmol of t-BuOOH
(70% aq. sol.) and 50 µL of nitromethane (internal standard) were added to 1.5 mL of MeCN and
1.5 mL of MeOH. The reaction mixture was stirred for 24 h, at 80 °C and a sample was taken at 5 h
and 24 h of reaction time and analyzed by GC using the internal standard method.
3. Results and Discussion
3.1. Syntheses and characterization of the gold(III) complexes
The C-scorpionates hydrotris(1-pyrazolyl)methane [HC(C3H3N2)3, Tpm] and lithium
tris(pyrazol-1-yl)methanesulfonate [LiSO3C(C3H3N2)3, LiTpms] were synthesised by following
known procedures [24,25] and their characteristic features were confirmed by NMR spectroscopy.
Reactions of HAuCl4•3H2O with Tpm or the C-functionalized LiTpms derivative were performed
in water at room temperature, and led to the new C-scorpionate gold(III) complex, [AuCl2(Tpms)]
(1), and the previously reported complex [AuCl2(Tpm)]Cl (2) [3] (Fig. 1). As far as we know,
complex 1 is the first example of a gold complex bearing the tris(pyrazol-1-yl)methanessulfonate
ligand.

1
2
Fig. 1 Structures of C-scorpionates Au(III) complexes [AuCl2(Tpms)] (1) and [AuCl2(Tpm)]Cl
(2).
Compound 1 was analysed by IR and NMR spectroscopy, elemental analysis as well as by single
1
13
crystal X-ray diffraction. The room temperature H and C NMR spectra (in DMSO-d6 , Fig. S1-
S3) display only three resonances for the pyrazolyl protons and carbons, respectively, that is a single
resonance for each type of proton or carbon in each of the position (3, 4 or 5, Fig. 1) of the pyrazolyl
ring, suggesting that, in solution, there is an interchange of the uncoordinated pyrazolyl group, which
is not distinguishable from the coordinated pyrazolyl ones. The same spectroscopic behaviour was
found previously for this type of compounds [3]. The IR spectra of 1 and 2 (Fig. S4-S8) show typical
ν(C=C) and ν(C=N) bands of coordinated pyrazolyl groups at the usual range of 1612-1510 cm-1
[3]. The presence of the chloro ligands is confirmed by the strong and broad ν(Au-Cl) at 353 cm-1
and 326 cm-1. Additionally, 1 exhibit a band in the 1372-1335 cm-1 range attributed to ν(S=O) of the
sulfonate group.
X-ray quality crystals of 1 were obtained by slow evaporation of an acetone solution of the
complex. The compound crystallized in the orthorhombic P 212121 space group (Table 1), the
asymmetric unit containing two independent complex molecules with similar bond parameters (Fig.
2 and legend). The metal cations exhibit almost perfect square planar geometry constructed from
two chloride anions and two N-atoms from two pyrazolyl rings (maximum deviation of 0.051 Å
from the N2Cl2 least-square plane pertaining to N4 and N6 atoms), therefore leaving coordinatively
free a pyrazolyl ring and the sulfonate moiety [31]. The AuN and AuCl bond distances (see Fig.
2 legend) are in the range of those already reported [32–39]. The angle between the least-square
planes of the coordinated pyrazolyl rings differ slightly in the two molecules (27.85 for Au1 and
25.79 for Au2), a similar situation occurring for the angle between the least-square plane of the free

pz moiety and the one constructed from the coordinated N- and Cl-atoms (83.16 for Au1 and 84.42
for Au2). The shortest AuAu distance assumes the value of 5.944(1) Å. In each molecule short
intramolecular interactions between the Au(III) and one of the Osulfonate atoms could be found
(average AuO distance of 2.88(1) Å, see Fig. S9) which, being shorter than the sum of the van der
Waals radii of gold and oxygen (2.32 and 1.50 Å, respectively) [40] can extend the geometry of the
complexes to distorted square-pyramids. The other nearest atom to Au1 is N8, and to Au2 is N2
(Fig. S9) but only in the latter case the obtained distance of 3.41(2) Å may suggest a further extension
of the coordination number. The structure of 1 is stabilized by interactions (Fig. S10) involving
intramolecular ClH and intermolecular OH contacts.
Fig. 2 Ellipsoid plot of 1, drawn at 30% probability level. Selected bond distances (Å) and
angles (o): Cl1Au1 2.259(5), Cl2Au1 2.246(5), Cl3Au2 2.252(5), Cl4Au2 2.249(5),
Au1N6 2.045(14), Au1N4 2.064(13), Au2N12 2.032(13), Au2N10 2.032(14);
N6Au1N4 90.5(5), N6Au1Cl2 90.3(4), N4Au1Cl2 177.2(4), N6Au1Cl1 177.4(4),
N4Au1Cl1 90.3(4), Cl2Au1Cl1 89.0(2), N12Au2N10 89.2(5), N12Au2Cl4
177.4(4), N10Au2Cl4 91.2(4), N12Au2Cl3 91.4(4), N10Au2Cl3 177.6(4),
Cl4Au2Cl3 88.3(2).

3.2.
Catalytic studies
The C-scorpionate gold(III) complexes [AuCl2(Tpms)] (1) and [AuCl2(Tpm)]Cl (2) and, for
comparative purposes, the precursor HAuCl4•3H2O were screened for i) the oxidation of toluene to
the corresponding oxidized products, benzaldehyde and benzyl alcohol, ii) oxidation of benzyl
alcohol to benzaldehyde and benzoic acid and finally iii) for the oxidative esterification of
benzaldehyde to methyl benzoate (Schemes 2-4). We chose these reactions and experimental
conditions due to our longstanding interest in oxidation reactions and previous experience. One class
of raw materials is alkyl aromatics; toluene, for example, the simplest member of this class, can be
oxidized to benzyl alcohol, benzaldehyde, benzoic acid, and benzyl benzoate as this sequential
oxidation is rapid at this temperature [41–43].
Peroxidative oxidation of toluene by 1 and 2
Scheme 2. Oxidation reaction of toluene to benzyl alcohol and benzaldehyde
The peroxidative oxidations of toluene (Scheme 2, Table 2) were undertaken in acetonitrile at
80 C for 24 h and using t-BuOOH (aq. 70%, 2 eq.) or H2O2 (aq. 30%, 2 eq.) as oxidants, in the
presence of the C-scorpionate gold(III) complex 1 or 2. Under the above-mentioned reaction
conditions, toluene is oxidized to yield predominantly benzaldehyde, along with some amount of
benzyl alcohol. For longer reaction periods, residual amounts of benzyl benzoate and benzoic acid,
among other oxidation products, were detected (by GC-MS). Both complexes show similar catalytic
performance, although at different reaction conditions, achieving an overall yield of 6% with
selectivity to benzaldehyde of over 85% (Table 2, entries 4 and 13 for complexes 1 and 2,
respectively). The previously recognized promoting effect of an acidic medium on the peroxidative
oxidation of alkanes catalysed by other C-scorpionates complexes [44, 45] is also observed for the
oxidation of toluene in the presence of 1 (entry 1 vs. 2 ) and 2 (entry 12 vs. 13). The presence of the
acid additive also resulted in an improvement for selectivity to benzaldehyde.

Table 2. Total yield (benzaldehyde and benzyl alcohol) in the oxidation of toluene by t-BuOOH
(aq. 70%) or H2O2 (aq. 30%) and 1 or 2 as catalyst precursors.a
Catalyst
Entry
Oxidant
Co-catalyst
(nacid/ncat)
-
Total
Yieldb
1.1
3.5
2.9
6.1
2.1
6.7
3.4
4.0
3.3
11.4i
< 0.2
4.5
6.3
5.9
0.3
5.9
3.4
4.7
2.0
-
precursor
Selectivityc TONd
1
2
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
t-BuOOH , 70%
73
83
70
87
43
83
75
78
51
36j
-
4
14
10
26
5
HNO3 (25)
HNO3 (50)
HNO3 (70)
HNO3 (70)
HNO3 (70)
HNO3 (70)
HNO3 (70)
-
3
4
5e
1
6f
17
25
9
7g
8h
9
8
10
11k
12
13
14
15e
16f
17g
18h
19
20
21l
22
t-BuOOH , 70% HNO3 (25)
20
-
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
H2O2, 30%
t-BuOOH , 70%
H2O2, 30%
H2O2, 30%
H2O2, 30%
HNO3 (70)
-
77
84
84
52
79
72
78
66
-
17
27
25
0.6
24
24
9
HNO3 (25)
HNO3 (50)
HNO3 (25)
2
HNO3 (25)
HNO3 (25)
HNO3 (25)
-
-
-
-
6
-
-
HAuCl4.3H2O
2.9
6.8
97
81
14
27
HAuCl4.3H2O
a Reaction conditions: toluene (5.0 mmol), catalyst precursor 1 or 2 (10 mol), t-BuOOH (70% aq,
2 eq.) or H2O2 (30% aq, 2 eq.) (10 mmol), MeCN (3 mL), 80 C, 24 h. b Molar yield (%) based on
c
the substrate i.e. moles of products (benzaldehyde and benzyl alcohol) determined by GC. Molar
selectivity (%) moles of benzaldehyde per total mmoles. dTurnover number (moles of
benzaldehyde per mol of Au pre-catalyst). e Reaction performed at 50 C. f Reaction performed at
100 ºC. gReaction performed with 5 µmol of pre-catalyst. hReaction performed with 20 µmol of pre-

catalyst. iBased on the substrate i.e. moles of products (benzaldehyde, benzyl alcohol and benzoic
acid) determined by GC. jMolar selectivity (%) moles of benzaldehyde per moles of (benzaldehyde
k
l
+ benzyl alcohol + benzoic acid). Reaction performed in the presence of Ph2NH. Reaction
performed for 6 hours.
The temperature plays an important role in the oxidation of toluene in the presence of 1 and 2,
conceivably due to the high energy normally involved in the activation of little reactive substrates
such as hydrocarbons. The total product yield (benzaldehyde + benzyl alcohol) increases as the
temperature rises from 50 to 80 C. For higher temperatures (100 C) the yield remains almost
unchanged, even in some cases decreasing, probably due to the oxidant decomposition (Fig. 3). In
the reaction carried out at room temperature, the detected products are residual. Therefore, the
temperature of 80 C was considered the most adequate, combining yield, selectivity, and energy
efficiency.
Fig. 3 Effect of temperature on total yield (a) and selectivity (b) of the major products of oxidation
of toluene (benzaldehyde + benzyl alcohol) catalyzed by 1 and 2. Reaction conditions: toluene (5.0
mmol), catalyst precursor 1 or 2 (10 mol), H2O2 (30% aq), MeCN (3 mL), nacid/ncat = 70 for 1 and
nacid/ncat = 25 for 2), 24 h.
The influence of the amount of pre-catalyst (1 and 2) on the oxidation of toluene was
investigated. For this study, half (5 µmol, entries 7 and 17, for 1 and 2, respectively) and twice (20
µmol, entries 8 and 18, for 1 and 2, respectively) of the amount previously used were tested.
According to the results presented in Table 2, it can be concluded that for both catalyst precursors,
1 and 2, the best performance occurs when 10 µmol is used, both in terms of yield and selectivity.

The oxidant/substrate molar ratio was varied between 1 and 4 and from the results obtained it
is observed that the excess of oxidant (2:1) promotes the oxidation reaction as well as selectivity, to
a certain extent. The excessive use of oxidant, namely for the ratios 3:1 and 4:1, results in a decrease
in yield and selectivity for benzaldehyde possibly due to overoxidation.
In this reaction, comparable results were obtained with the salt HAuCl4.3H2O (Table 2, entry
22) what could suggest that similar catalytic active species are formed under the reaction conditions.
The reaction kinetic profile over a 24-hour reaction period was investigated at 80 °C (Fig. 4).
In the presence of 1, the reaction starts slowly, with a total yield of 2.6% but already with a selectivity
of 72% indicating that the benzyl alcohol formed during this initial period was readily oxidized to
the main product benzaldehyde. After that time, a marked growth in the yield to 8.1% was reached
after 6 hours of reaction, where after the total yield starts to decrease, however keeping the selectivity
for the aldehyde always above 75% (Figure 4a).
In the case of 2, a maximum yield of 8.1% is achieved after 10 hours of reaction, then decreasing
slightly in the next two hours and remaining constant until completing 24 hours reaction. The
selectivity of the reaction has remained relatively constant over time with only slight fluctuations
(Figure 4b).
The oxidation reaction carried out in the presence of the salt HAuCl4.3H2O for 6 hours resulted
in a much more modest total yield of 2.6 (Table 2, entry 21) compared to those achieved in the
presence of 1 or 2, for the same period.
Figure 4. Yield of the major products of oxidation of toluene (benzaldehyde + benzyl alcohol) and
selectivity for benzaldehyde, catalyzed by 1 (a) and 2 (b) along time (2-24 h). Reaction conditions:

(a) toluene (5.0 mmol), catalyst precursor 1 (10 mol), H2O2 (30% aq), MeCN (3 mL), 80
C, nacid/ncat = 70, 2-24 h. (b) toluene (5.0 mmol), catalyst precursor 2 (10 mol), H2O2 (30% aq),
MeCN (3 mL), 80 C, nacid/ncat = 25, 2-24 h.
Our catalytic systems have been benchmarked to other Au-based catalytic systems reported
previously, and we can essentially highlight their selectivity for one of the toluene oxidation
products, benzaldehyde. Kesavan et al. reported the activation of the methyl C-H bond in toluene
and other derivatives by several Au-Pd alloy nanoparticles [46] using molecular oxygen as the
oxidant (10 bar) and carrying out the reactions in the absence of solvent at 160 C having obtained
a maximum conversion of 5.3% and a selectivity of 8.3% for benzaldehyde, for an Au:Pd molar
ratio of 1:2. By varying the Au:Pd ratio, it was possible to improve selectivity up to 63%, but in this
case the conversion did not go beyond 1.5% [46]. This catalytic system is less selective than ours,
always obtaining four oxidation products, benzyl alcohol, benzaldehyde, benzoic acid and benzyl
benzoate, showing that the sequential oxidation is rapid at very high temperatures like the one used
in this case.
Peneau et al. applied supported gold–platinum nanoalloys Au-Pt/TiO2 and Au-Pt/C for the
oxidation of toluene using t-BuOOH (aq. 70%) as the oxidant at 80 C and achieved conversions
of 12.1 and 10.8% and selectivities for benzaldehyde of 1.6 and 1.7%, respectively. Benzoic acid
was the major product obtained for both sol-immobilisation materials [47] resulting from the
overoxidation of the benzaldehyde formed from benzyl alcohol oxidation. Saima et al. applied TiO2-
supported Au and Au-Pd catalysts 1%Au/TiO2 and 1%Au-Pd/TiO2 for the oxidation of toluene and
achieved conversions of 3.3 and 4.4% and selectivities for benzaldehyde of 47 and 37%, respectively
[48]. In all cases mentioned above the Au metal centre is indicated as playing the key role in the
catalysis.
The stability of complex 1 in the reaction conditions was evaluated by comparison of its UV-
Vis spectra (Fig. S11) before and after the reaction and the one of HAuCl4.3H2O. The spectrum of
HAuCl4.3H2O is completely different from the spectra of complex 1 (before in acetonitrile and after
the oxidation reaction), with a characteristic absorption band at 380 nm, suggesting the non-
decomposition of 1 into the Au salt.

The mechanism of this reaction should initially involve the formation of oxygen-based radicals
HOO● and HO●, as suggested by the oxidation reaction carried out in the presence of the oxygen-
radical trap diphenyl amine Ph2NH resulting in an almost complete reaction inhibition (Table 2,
entry 11), upon oxidation or reduction of the oxidant H2O2 involving the Au3+/Au2+ redox couple.
The HO/HOO radicals can abstract an H atom from the substrate (CyCH3) to form the benzyl

radical CyCH2 which then reacts with dioxygen, leading to the CyCH2OO peroxyl radical. The
peroxyl radical may abstract hydrogen from CyCH3 and give rise to CyCH2OOH, whereas CyCHO
and CyCH2OH can be produced by metal-assisted reduction of CyCH2OO• (for CyCHO) or through
CyCH2OO• dismutation leading to CyCHO and CyCH2OH (plus O2).
Au3+ + H2O2 → HOO + H+ + Au2+
Au2+ + H2O2 → HO + Au3+ + HO−
(1)
(2)
(3)
(4)
(5)
(6)
(7)

HO + CyCH3 → H2O + CyCH2

CyCH2 + O2 → CyCH2OO
CyCH2OO + CyCH3 → CyCH2OOH + CyCH2
CyCH2OO + Au2 → Au3+ + CyCHO + HO−
2 CyCH2OO → CyCH2OH + CyCHO + O2

Peroxidative oxidation of benzyl alcohol by 1 and 2
Scheme 3. Oxidation reaction of benzyl alcohol to benzaldehyde and benzoic acid

Benzyl alcohol is one of the products obtained in the oxidation of toluene and as such the study
of the catalytic activity of Au(III) scorpionate complexes was extended to the peroxidative oxidation
of this substrate.
In this study, several reaction parameters were explored, namely the amount and type of oxidant,
reaction time, reaction temperature and pre-catalyst amount, and a total yield (benzaldehyde and
benzoic acid) above 45% after 24 hours of reaction, using t-BuOOH (aq. 70%, 2 eq.) as the oxidizing
agent, could be achieved (Scheme 3). Regarding these tests it was possible to conclude that an excess
of oxidant relatively to substrate (2:1) favours the reaction, for e.g. the total yield increasing from
14.2 to 43.1% when the stoichiometry is changed from 1:1 to 2:1 (entries 17 and 19, Table 3,
respectively), however decreasing the selectivity for benzaldehyde from 92 to 72%. In fact, the
selectivity does not diverge much over the first six hours nevertheless the oxidant to substrate molar
ratio is 1:1, 2:1 or 4:1. However, for longer periods, selectivity remains constant for the 1:1 molar
ratio but decreased for the cases in which an excess of oxidant is used, conceivably due to the
oxidation of benzyl alcohol to benzaldehyde and sequential oxidation to benzoic acid.
Table 3. Total yield (benzaldehyde and benzoic acid) in the oxidation of benzyl alcohol by
t-BuOOH (aq. 70%) and 1 or 2 as catalyst precursors. a
Catalyst
Entry
n(oxi)/n(subs) Reaction
Time (h)
precursor
Total Yieldb Selectivityc TONd
1
2
1:1
1:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
6
24
6
6.2
12.1
11.9
29.3
6.3
96
93
93
77
94
87
95
95
95
75
94
30
56
3
56
4
24
6
113
29
5e
6e
7f
24
6
6.4
28
6.7
31
1
8f
24
6
10.6
9.1
50
9g
10g
11h
86
24
6
33.2
12.0
248
28

12h
13
2:1
3:1
3:1
2:1
1:1
1:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
3:1
3:1
2:1
2:1
24
6
34.9
6.2
71
96
79
86
92
92
>99
72
>99
>99
96
95
95
63
95
84
93
79
97
32
61
36
14
24
6
31.2
3.1
122
13
15i
16
6
4.9
23
17
24
6
14.2
11.2
43.1
4.1
66
18
56
19
24
6
154
20
20e
21e
22f
23f
24g
25g
26h
27h
28
24
6
4.5
22
7.1
34
2
24
6
9.0
43
10.1
45.6
8.7
96
24
6
288
21
24
6
25.0
11.1
31.3
11.8
75.8
53
52
29
24
24
24
123
-
30
-
31
HAuCl4.3
H2O
122
a Reaction conditions: benzyl alcohol (5.0 mmol), catalyst precursor 1 or 2 (10 mol), t-BuOOH (aq.
70%, 2 eq.), MeCN (3 mL), 80 C. b Molar yield (%) based on the substrate i.e. moles of products
(benzaldehyde and benzoic acid) determined by GC. cMolar selectivity (%) moles of benzaldehyde
d
per total moles. Turnover number (moles of benzaldehyde per mol of Au pre-catalyst). e Reaction
f
g
performed at r.t. Reaction performed at 50 C. Reaction performed with 5 µmol of pre-catalyst.
hReaction performed with 20 µmol of pre-catalyst. iReaction performed in the presence of Ph2NH.
In this reaction, the effect of temperature was also studied, and it was observed that carrying out
the reaction at 80 C resulted in a greater conversion of benzyl alcohol. At room temperature the
conversion did not exceed 6.4 and 4.5% in the presence of 1 and 2, respectively, even after 24 hours
of reaction, however, it should be noted that at this temperature the selectivity is total for

benzaldehyde in the presence of 2. At 50 ºC the conversion went up for both catalytic systems,
doubling in some cases the initial value and maintaining the high selectivity for benzaldehyde.
Despite the higher yield obtained in the presence of HAuCl4.3H2O, the selectivity for the desired
product (benzaldehyde) is much lower than those achieved with compounds 1 and 2, suggesting the
formation in high amount of the unwanted benzoic acid.
Catalyst 1
Catalyst 2
Catalyst 2
Catalyst 1
50
40
30
20
10
0
50
40
30
20
10
0
6 h
6 h
24 h
24 h
r.t.
50
Temperature(C)
80
r.t.
50
Temperature(C)
80
Figure 5. Effect of temperature on total yield of the major products of oxidation of benzyl
alcohol (benzaldehyde + benzoic acid) catalyzed by 1 and 2. Reaction conditions: benzyl
alcohol (5.0 mmol), t-BuOOH (aq. 70%, 2 eq.), catalyst precursor 1 or 2 (10 mol), MeCN (3
mL).
Different amounts of catalyst precursor were tested (5, 10 and 20 mol) and it was found that,
for complex 1 the use of the different amounts of pre-catalyst does not have much influence on the
reaction yield being so possible to decrease the amount used to 5 mol. For complex 2, similar
conversions were obtained with 5 mol of pre-catalyst, significantly improving the TON of the
reaction, however with lower selectivity and for that reason most of the tests were performed in the
presence of 10 mol of pre-catalyst.
Choudhary et al. [49] reported the solvent-free oxidation of benzyl alcohol to benzaldehyde by
t-BuOOH (at 95 °C) over nanogold supported on TiO2. The TiO2-supported nanogold catalyst
prepared by the homogeneous deposition-precipitation (HDP) showed conversions up to 70% and
selectivity for benzaldehyde of ca. 60%, after 2 h with an Au loading of 0.02 mmolg-1). Higher gold
loading resulted in higher yield but lower selectivity for benzaldehyde. The activity and selectivity
(up to 90 and 89%, respectively, after 300 min and at 125 °C) of Al2O3 supported Au nanoparticles
on the liquid-phase oxidation of benzyl alcohol to benzaldehyde by t-BuOOH in the absence of
solvent was also reported by Ndolomingo et al. [50]

Peroxidative esterification of benzaldehyde by 1 and 2
Scheme 4. Oxidative esterification reaction of benzaldehyde to methyl benzoate and benzoic
acid
Benzyl benzoate resulting from the esterification of benzyl alcohol with benzoic acid was
detected as a minor by-product in our previous studies related to the oxidation of benzyl alcohol.
As proposed by Adnan et al. [51] studies carried out in the presence of Au nanoparticles, show that
esterification reaction competes with the formation of benzoic acid.
We studied the esterification of benzaldehyde starting from benzyl alcohol (cascade reaction)
using 5 mmol of benzyl alcohol, 10 mol of pre-catalyst, 1.5 mL of acetonitrile (as solvent), 1.5 mL
of methanol and 10 mmol of oxidant (t-BuOOH aq. 70%, 2 eq. or H2O2 aq. 30%, 2 eq.) for 3, 6 and
24 hours (Scheme 4).
The cascade reaction starts with the oxidation of benzyl alcohol to benzaldehyde followed by
the oxidation or esterification reaction of the latter to benzoic acid or methyl benzoate (“in situ”),
respectively (Table 4). The reaction was performed at 80 C and at room temperature being the
conversion residual at the latter, requiring heating.
In order to optimize the reaction conditions to produce methyl benzoate, the effect of the
presence and the amount of solvent (acetonitrile) was studied, using an equivalent volume of solvent
(acetonitrile, 1.5 mL) and esterifying agent (methanol, 1.5 mL) or using exclusively the esterifying
agent (methanol, 3 mL). For both cases the use of methanol as solvent and esterification agent
appears to be beneficial for the reaction increasing the production of methyl benzoate over 24 hours
(entries 6 and 16, Table 4).

Increasing the amount of oxidant leads to an important increase of the total product yield, mainly
in the reaction catalyzed by 1 (entry 7 vs. entry 1, Table 4), with similar selectivity for the
esterification product, suggesting the formation of the oxidation product, the benzoic acid.
Table 4. Total yield in the peroxidative esterification of benzaldehyde (starting from benzyl
alcohol) using 1 or 2 as catalyst precursors. a
Entry
n(oxi)/n(subs)
Reaction
Total Yieldb Selectivityc TONd
Time (h)
1
2
2:1
2:1
2:1
2:1
2:1
2:1
4:1
4:1
2:1
2:1
2:1
2:1
2:1
2:1
4:1
4:1
6
24
6
0.9
2.1
68
64
43
62
74
63
61
34
90
34
>99
>99
78
61
65
53
3
7
3e
0.6
1
4e
24
6
0.9
3
1
5f
1.8
6
6f
24
6
6.5
20
18
28
7
7
16.7
1.6
8
24
6
11
12
13e
14e
15f
16f
17
18
1.9
24
6
15.2
<0.1
0.2
26
0.3
1
24
6
1.6
6
2
24
6
11.1
3.5
34
11
44
24
16.7
a Reaction conditions: benzyl alcohol (5.0 mmol), catalyst precursor 1 or 2 (10 mol), t-BuOOH (aq.
70%, 2 eq.), MeCN (1.5 mL), MeOH (1.5 mL), 80 ºC. b Molar yield (%) based on the substrate i.e.
c
moles of products (methyl benzoate and benzoic acid) determined by GC. Molar selectivity (%)
moles of methyl benzoate per total moles. dTurnover number (moles of methyl benzoate per mol of
Au pre-catalyst). eReaction performed at r.t. f 3 mL methanol.
This behavior may suggest that the production of benzaldehyde is dominant, thus becoming the
limiting step, in the benzyl alcohol oxidation and that the esterification reaction passes through

benzaldehyde and not benzoic acid. Considering that the formation of methyl benzoate is, in this
case, a two-step process, and being the formation of benzaldehyde the slow step we decide to explore
the esterification reaction directly from benzaldehyde (Table 5) [51].
As in the previous study, the effect of the presence and the amount of solvent (acetonitrile) and
esterifying agent (methanol) was explored. Using an equivalent volume of solvent (acetonitrile, 1.5
mL) and esterifying agent (methanol, 1.5 mL) resulted in lower conversion and selectivity for the
benzoate (Table 5, entries 1-4 and 13-16, for 1 and 2, respectively), being this reduction more evident
in the presence of 1.
The production of benzoate seems to occur preferentially in the first hours, for e.g. in the case
of 1 reaching already a total yield of 48.2% and high selectivity of 78% for this product after the
first 3 hours (Table 5, entry 5). For longer periods, the formation of benzoic acid is favored, in the
case of pre-catalyst 1 and after 24 hours only the oxidation product was detected (Table 5, entry 4).
Table 5. Total yield in the peroxidative esterification of benzaldehyde (starting from
benzaldehyde) using 1 or 2 as catalyst precursors. a
Entry
n(oxi)/n(subs) Reaction
Time (h)
Total Yieldb Selectivityc TONd
1e
2e
3
2:1
2:1
2:1
2:1
2:1
0.5:1
1:1
3:1
2:1
2:1
2:1
2:1
2:1
2:1
6
24
6
20.6
40.9
52
36
68
-
52
74
51.8
176
-
f
4
24
3
5
48.2
20.6
24.4
39.9
32.2
26.7
26.8
16.0
24.4
34.9
78
78
82
74
70
73
78
73
74
46
189
80
6
3
1
7
3
100
147
226
48
8
3
9g
10h
11i
12j
13e
14e
3
3
3
104
21
3
6
80
24
90

15
16
2:1
2:1
2:1
0.5:1
1:1
3:1
1:1
1:1
1:1
2:1
2:1
2:1
2:1
6
24
3
32.8
47.4
54.1
19.3
38.2
17.4
28.6
24.5
24.1
6.8
72
49
53
72
79
77
77
81
82
10
69
67
80
117
115
143
70
152
67
221
49
99
-
17
18
3
19
2
3
20
3
21g
22h
23i
24i
25
3
3
3
-
3
-
3
21.0
26.8
25.1
-
26i
27
3
90
99
HAuCl4.3H2O
HAuCl4.3H2O
3
a Reaction conditions: benzaldehyde (5.0 mmol), catalyst precursor 1 or 2 (10 mol), t-BuOOH (aq.
70%, 2 eq.), MeOH (3 mL), 80 C. b Molar yield (%) based on the substrate i.e. moles of products
c
(methyl benzoate and benzoic acid) determined by GC. Molar selectivity (%) moles of methyl
benzoate per moles of per total moles. dTurnover number (moles of methyl benzoate per mol of Au
pre-catalyst). eMeCN (1.5 mL), MeOH (1.5 mL), fthe solid product was analysed and mainly benzoic
g
h
i
acid was found. 5 µmol of pre-catalyst. 20 µmol of pre-catalyst. Reaction performed at r.t.
jReaction performed in the presence of Ph2NH.
The effect of the amount of oxidant was explored and e.g. for pre-catalyst 2, lower
oxidant:substrate molar ratio (0.5:1 and 1:1) resulted in lower total yields (19.3 and 38.2%,
respectively, entries 18 and 19 , Table 5, Fig. 6), though higher selectivities for methyl benzoate of
72 and 79%, suggesting, as expected, a greater extension of the esterification reaction over the
oxidation reaction, while an excess of oxidant (2:1) resulted in an increase in total yield (54%, entry
17, Table 5, Fig. 6), accompanied by a decrease in selectivity (53%, entry 17, Table 5, Fig. 6),
indicative of a greater formation of benzoic acid. In the case of pre-catalyst 1, although verified, the
effect was not as pronounced (Fig. 6). The results obtained when the reaction is performed with
HAuCl4.3H2O are lower at 80 C and similar at room temperature and at 80 C (Table 5 entries 26
and 27). In both cases the achieved TON is lower than those obtained when the reaction is performed
under the same conditions in the presence of complex 1 or 2 (Table 5 entries 5, 11, 19 and 23).

Fig. 6 Yield of the major products of the peroxidative oxidation and esterification of
benzaldehyde and selectivity for the esterification product (methyl benzoate) relatively to
oxidation product (benzoic acid), catalyzed by 2. Reaction conditions: benzaldehyde (5.0
mmol), catalyst precursor 2 (10 µmol), t-BuOOH (aq. 70%), MeOH (3 mL), 80 ºC, 3 h.
Suzuki et al reported [52] the efficient aerobic oxidative esterification of methacrolein in
methanol to methyl methacrylate by supported gold-nickel oxides (AuNiOx) nanoparticles with
conversions up to 63% and selectivity to the ester of 97% after 2 hours reaction at 60 ºC. Besides
that, Hashmi et al. reported [53] that mononuclear gold compounds (HAuCl4) can also be active
catalysts in the oxidative esterification of aldehydes, using tert-butyl hydroperoxide (t-BuOOH) and
pyridine as prefered oxidant and performing the reactions during 12 hours at 75 C afford isolated
yields up to 90%.
The mechanism for the peroxidative esterification of aldehydes had been a topic of discussion
and two types of mechanisms have been proposed: a coordinative mechanism [54–56] and a
radicalar mechanism [57,58]. In our case, a radical mechanism is expected to occur, taking into
account the result obtained when added the radical trap Ph2NH to the reaction medium (Table 5,
entry 12), involving the metal-assisted generation of t-BuOO• and t-BuO• radicals upon oxidation
or reduction of the oxidant (t-BuOOH), respectively, with the gold cycling between oxidation states
+II and +III. The produced oxygen-based radicals are believed to behave as hydrogen atoms
abstractors from the aldehydes [53].

4. Conclusions
A neutral C-scorpionate gold(III) complex was synthesised for the first time (previously only
cationic C-scorpionate Au(III) compounds were reported). This compound was easily synthesised
from the commercially available metallic compound in water and its structure confirmed by single
crystal X-ray diffraction, contributing to expand the still underexplored coordination chemistry of
gold(III).
A comparison between the catalytic activities of both neutral and cationic compounds was
performed using oxidation reactions that have never been carried out in the presence of Au(III)
compounds as pre-catalysts.
The possibility of applying such type of complexes as homogeneous catalyst precursors for
oxidation, under mild conditions, of toluene and benzyl alcohol and for the esterification of
benzaldehyde has also been opened. Moreover, the use of mild conditions for reactions that typically
use harsh ones is encouraging for pursuing the search for gold complexes and their application in
further reactions following the green chemistry principles.
1
13
Appendix A. Supplementary data: Fig. S1. H NRM spectrum for complex 1; Fig. S2. C
NRM spectrum for complex 1; Fig. S3. 1H NRM spectra for complex 2; Fig. S4. IR spectrum
for complex 1; Fig. S5. FAR-IR spectrum for complex 1; Fig. S6. IR spectrum for complex 2;
Fig. S7. IR spectrum for complex 2; Fig. S8. FAR-IR spectrum for complex 2.; Fig. S9. Shortest
intra- and intermolecular interactions involving the Au metal cations (values in Å) in 1.; Fig.
S10. Non-covalent interactions in the molecules of 1 [d(DA) in Å,  DHA in ]:
C6H6Cl1 3.22(2), 110; C6H6O5 3.01(2), 134; C8H8O4 3.39(2), 153; C9H9Cl2
3.20(2), 106; C11H11O5 3.02(2), 112 C16H16Cl4 3.21(2), 110; C19H19Cl3
3.22(2), 107; C19H19O1 3.14(2), 137. Fig. S11. UV-vis spectra of complex 1, before in
acetonitrile and after reaction, and of HAuCl4.3H2O (6.8x10-4 mol/L).
Acknowledgments
This research was funded by Fundação para a Ciência e Tecnologia (FCT), project
UIDB/00100/2020 of Centro de Química Estrutural, and grants numbers Acção Integrada Luso-
Alemã A13/16-DAAD/CRUP, UID/NAN/50024/2019, PTDC/QUI-QIN/29778/2017 and
UID/MULTI/04046/2019, and from Instituto Politécnico de Lisboa IPL/2019/STREAM_ISEL
Project.

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