Hydrogen Atom Transfer Oxidation by a Gold-Hydroxide Complex

Marta Lovisari and Aidan R. McDonald*

Article

Hydrogen Atom Transfer Oxidation by a Gold−Hydroxide Complex

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b03225

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sı Supporting Information

III

ABSTRACT: Au -oxygen adducts have been implicated as inter-

mediates in homogeneous and heterogeneous Au oxidation catalysis,

III

but their reactivity is under-explored. Complex 1, ([Au (OH)-

(

terpy)](ClO ) , (terpy = 2,2′:6′,2-terpyridine), readily oxidized

substrates bearing C−H and O−H bonds. Kinetic analysis revealed

that the oxidation occurred through a hydrogen atom transfer (HAT)

mechanism. Stable radicals were detected and quantiﬁed as products of

III

almost quantitative HAT oxidation of alcohols by 1. Our ﬁndings highlight the possible role of Au -oxygen adducts in oxidation

catalysis and the capability of late transition metal−oxygen adducts to perform proton coupled electron transfer.

INTRODUCTION

performing HAT of substrates containing very strong C−H

bonds. Given the dearth of insight into the active oxidants in

Au oxidation catalysis and the under explored reactivity of

heavy and late transition metal-hydroxides, we set out to

explore the oxidation reactivity properties of a Au −OH entity

of relevance to putative intermediates in Au oxidation catalysis.

■

Au has demonstrated great eﬃciency in the oxidation of

carbon monoxide and alcohols, the esteriﬁcation of ethylglycol,

and the hydrochlorination of acetylene,

increased relevance for a somewhat counterintuitive reactivity

of this soft metal in electrophilic oxidation. Discrete Au

−4

thus showing

III

complexes have proven to be excellent catalysts for various

classes of oxidation reactions, such as alkane oxygenation,

RESULTS AND DISCUSSION

■

III

8,9

[Au (OH)(terpy)](ClO ) (terpy = 2,2′:6′,2-terpyridine) (1,

oleﬁn epoxidation, oxidative cross-couplings, and alkyne

III

Scheme 1) was synthesized as reported. The identity and

hydroamination. Au −OH species have been proposed as

the active species in the oxidation of CO by Au supported on

1,12

Scheme 1. Reaction of 1 with 1,4-Cyclohexadiene (CHD)

metal oxides,

while in other cases the active oxidant has

III

been postulated to be a terminal Au −oxo (Au O) species,

2,11,13

both in homogeneous and heterogeneous oxidations.

III

Au O complex remains elusive precluding insight into its

reactivity properties. Given the expected high basicity of a

III

terminal Au O, its protonation might be favored to yield

III

Au −OH as the active oxidant. Bis-μ-oxo-Au species have

been shown to be adequate oxidants in oleﬁn hydroxylation

,14,15

purity of 1 was conﬁrmed by H nuclear magnetic resonance

and epoxidation.

Bochmann and co-workers have shown

III

(NMR) spectroscopy, attenuated total reﬂectance Fourier

transform infrared (ATR-FTIR) spectroscopy, and mass

how a Au −OH complex, supported by a cyclometalated

III

CNC ligand, allowed the isolation of Au −peroxide and

spectrometry (Figures S1 −S7). The H NMR spectra of 1 in

−

hydroperoxide complexes and showed OAT reactivity toward

III

-dimethyl sulfoxide (DMSO−D ) and D -N,N-dimethylfor-

6 7

phosphines. Despite the existence of stable Au −OH

6−19

mamide (DMF-D ) displayed comparable resonances, in

complexes,

little attention has been dedicated to their

agreement with those reported for similar complexes (Figures

capability to perform oxidation chemistry.

S1 −S5 ). The integrity of the hydroxide ligand in these

High-valent mononuclear metal−hydroxide species have

solvents was conﬁrmed by the presence of a signal at δ = 6.35

ppm in DMSO−D and δ = 6.65 ppm in DMF-D . This signal

been widely implicated as intermediates in catalysis by several

0−22

metallo-oxidases.

For most of these systems the rate-

determining step was a metal-based proton-coupled electron

23,24

Received: November 4, 2019

transfer (PCET) reaction,

in which the active high-valent

metal-based oxidant abstracted a hydrogen atom from a C−H

bond via a hydrogen atom transfer (HAT) mechanism (a class

5−27

of PCET).

high-valent hydroxides containing Mn,

centers have proven that these species are capable of

Biomimetic models of mononuclear mid- and

28−30

31,32

Fe,

and Cu

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

S21 ) and subsequently slowly decayed. After 1000 s a dark

Article

disappeared upon addition of a drop of D O and when the

complex 1 (0.15 mM, 25 °C, DMF, Figure S17). Pseudo-ﬁrst-

III

analogous [Au (OD)(terpy)](ClO ) was independently

order rate constants (k ) were obtained by exponential ﬁ tting

obs

synthesized (Figures S3, S9, and S10 ). 1 displayed a yellow

color in the solid state but yielded pink solutions once

dissolved in polar aprotic solvents such as DMF (λ_m_a_x= 545

nm) and DMSO (λ_m_a_x= 537 nm, Figure S8). Similar

of the plot of absorbance versus time up to 2800 s (Figure

S18). At the end of the reaction, which appeared to be very

slow, the characteristic absorption bands of anthracene were

observed (λ = 341, 360, and 379 nm in DMF). A second-order

solvatochromism has been observed in analogous d complexes

rate constant (k ) was determined from the slope of the plot of

34−36

−

1 −1

containing polypyridyl ligands.

The cyclic voltammogram

k_o_b_sversus [DHA] to be 0.020 M

(Figure S19 ). The same

−

of 1 showed one irreversible reduction peak E = −0.13 V

reaction was repeated with [D] -DHA, showing k = 0.006 M

red

−1

versus the ferrocene/ferrocenium (Fc/Fc ) standard (Figure

s , yielding a primary kinetic isotope eﬀect (KIE) of 3.3

S11 ). 1 was thus a weak oxidant which maintained its

hydroxide ligand in a variety of solvents.

(

Figure S19). This result suggested that the cleavage of the C−

H bond is involved in the rate-determining step. This KIE

value lies in the classical range (2−7) and in agreement for

what observed for other high-valent metal−oxygen adducts

(0.55 mM, DMF, 25 °C) was reacted with 1,4-

cyclohexadiene (CHD, 1000 equiv), resulting in a change in

its electronic absorption spectrum (Figure S12 ). An initial

growth of features at λ= 545 and 630 nm was observed within

(

M−O−X), including hydroxides, in contrast to high-valent

metal-oxo (MO) oxidants. For MO large KIEs (>10)

00 s, followed by a slower growth in the region between λ =

00−1000 nm over 2000 s, together with a drop in the

have been reported, which have been explained to stem from

tunneling eﬀects. The KIE value suggests that the rate-

limiting step is proton or H atom transfer.

intensity of the bands at λ < 380 nm. During the latter process

a dark purple precipitate was observed, which we attribute to

For both hydrocarbon substrates we can obtain little

mechanistic insight into the initial PCET reaction because

the products that we can identify represent two-electron, two-

proton PCET oxidized products, meaning that exploration of

the mechanism of the PCET is not possible. It is therefore also

37−39

colloidal Au formation.

We postulated that the initial

reaction was the oxidation of CHD by 1 to yield a reduced

form of 1 and benzene (Scheme 1).

To monitor the reaction by H NMR, the same reaction was

performed in DMSO−D on a diﬀerent scale (100 equiv CHD,

III

not possible to explore if Au 1 was solely responsible for all

Figures S13 −S14). We observed the growth in intensity of a

signal at δ = 7.36 ppm that was assigned to the C−H

PCET oxidation steps or if the reduced derivates of 1 (Au ,

Au ) also contribute to the oxidation of CHD and DHA.

resonances of benzene (Table S1). The yield of benzene was

determined to be 90 ± 15% with respect to [1] if each

molecule of 1 was responsible for one H atom transfer. To

oxidize CHD to benzene requires that two H atoms are

removed. The calculation above takes this into account. Given

the observation of colloidal gold at the end of the reaction, we

cannot exclude that 1, 2, or 3 electrons and protons might be

theoretically transferred to each equivalent of 1 which could

lower the estimated yield of benzene with respect to [1]. The

In order to gain further mechanistic insight, the reactivity of

toward substrates bearing weak O−H bonds was then

examined. When 4-methoxy-2,6-di-tert-butylphenol (4-CH O-

,6-DTBP, 100 equiv) was added to 1 (0.4 mM, DMF, 25 °C)

an immediate reaction was observed. The electronic absorption

spectrum displayed a sharp band at λ = 407 nm, alongside the

growth of features at λ = 545 and 630 nm (Figure 1). The

band at λ = 407 nm reached a maximum after 160 s (Figure

H NMR spectra also showed a signiﬁcant shift and

broadening of the residual signal attributed to H O (δ =

.34 ppm to δ = 3.55 ppm). Previous reports showed that

increased water content can shift H O resonances downﬁeld,

and notably a progressive peak shift is evident in DMSO−D

mixtures with variable water content. This observation

conﬁrmed that the water content during the reaction between

and CHD increased signiﬁcantly. A pseudo-ﬁrst-order

constant for the rate of benzene formation was determined

−3 −1

from the H NMR data to be 1.2 × 10

in DMSO−D

(

Figure S14). Combined with the electronic absorption

analysis, these results show that 1 was capable of oxidizing

CHD to yield benzene, presumably through a PCET oxidation.

(DMF, 25 °C, 0.55 mM) also reacted with 9,10-

dihydroanthracene (DHA, >200 equiv). As for the reaction

with CHD, the electronic absorption spectra showed a slow

increase of the bands at λ = 545 and 630 nm, followed by

indications of colloidal Au formation (Figures S15, S16 ).

Anthracene formation was conﬁrmed by gas chromatography

Figure 1. Electronic absorption spectra of the reaction of 1 (0.4 mM,

GC) with a measured yield of 65 ± 25% with respect to [1]

Table S1). Note that no oxygen-containing organic products

e.g., anthraquinone, anthrone) were idenitiﬁed by gas

DMF, black trace) with 4-CH O-2,6-DTBP (100 equiv) at 25 °C.

Legend: blue = 160 s; red = 1000 s. Inset: X-band EPR spectra of the

reaction mixture of 1 with 4-CH O-2,6-DTBP (blue trace) after 160 s

chromatography. This conﬁrmed that 1 was reacting with

DHA to yield anthracene, presumably as a result of PCET

oxidation.

and the independently synthesized 4-methoxy-2,6-di-tert-butylphe-

noxyl radical (gray trace). The spectra were acquired from a frozen

DMF solution and measured at 77 K with a 0.2 mW microwave power

and a 0.2 mT modulation amplitude. The spectra have been

normalized with respect to the microwave frequency used.

The rate of the reaction with DHA was measured by

monitoring the decay of the band at λ = 369 nm, assigned to

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

purple precipitate was observed to form in the reaction

mixture, as observed for the latter stages of CHD and DHA

oxidations.

Table 1. Kinetic Parameters for Radical Formation from the

27,45

Reaction between 1 and O−H Bonds

k₁(s⁻¹) K (M⁻¹

)

BDFE_O₋_H(kcal/mol)

An X-band EPR spectrum of the reaction mixture prepared

at 160 s (initial phase of the reaction) showed an isotropic

signal at g = 2.00 typical of an organic radical (Figure 1). We

postulated it to be the 4-methoxy-2,6-di-tert-butylphenoxyl

radical, which was conﬁrmed by independent synthesis of this

compound and comparison of its EPR spectra to that

measured for the reaction mixture (Figures 1 and S25, see

-CH O-TEMPOH

-oxo-TEMPOH

1.2

0.49

0.808

0.0305

0.0106

0.0014

−

1.5

0.016

0.067

0.025

65.0

65.6

66.5

−

77.1

TEMPOH

[

H]-4-CH O-2,6-DTBP

D]-4-CH O-2,6-DTBP

,4,6-TTBP

the SI for details). Likewise, the electronic absorption

spectrum of the independently synthesized radical displayed a

feature at λ = 407 nm (Figure S26 ). The yield of 4-methoxy-

It should be noted that for 4-oxo-TEMPOH the kobs versus [4-oxo-

TEMPOH] displayed behavior that was close to linear.^B^D^F^EO−H

values are not available for these substrates in DMF, and hence, we

25−27

have reported BDFE_O₋_Hvalues for CH CN.

Number in italics

,6-di-tert-butylphenoxyl radical was determined to be 80 ±

was estimated.

5% by EPR and 85 ± 15% by electronic absorption

spectroscopy (Table S1) when a one-electron reaction is

considered. Thus, the initial product of the reaction between 1

in DMSO−D . A H NMR of 4-CH O-2,6-DTBP showed

and 4-CH O-2,6-DTBP was 4-methoxy-2,6-di-tert-butylphe-

resonances at δ = 6.52 (O−H ), 6.62 (ArCH), 3.66 (OCH ),

and 1.36 ppm (C(C H ) ) (Figure S30 ). Upon reaction with 1

noxyl radical, presumably formed through PCET oxidation.

^A^p^l^o^t^o^f^t^h^e^a^b^s^o^r^b^aⁿ^c^e^a^t^λmax = 407 nm versus time

showed an initial phase (160 s, coinciding with the maximum

yield of radical) with an exponential growth and a second

phase showing its decay up to 400 s (Figure S24). Pseudo-ﬁrst-

in DMSO−D (Figures S32, S34 ), these resonances dis-

appeared together with the growth of new features at δ = 6.50,

.16, and 1.23 ppm. It should be noted that we were unable to

identify the 4-methoxy-2,6-di-tert-butylphenoxyl radical prod-

uct by H NMR. The signals at δ = 6.50 and 1.23 ppm have

order rate constants (k ) were obtained by exponential ﬁ tting

obs

of the plot of absorbance versus time up to 160 s (Figure S27).

The rate of this initial phase was shown to be dependent on

been attributed to 2,6-di-tert-butylquinone (2,6-DTBQ), and

that at δ = 3.16 ppm has been attributed to CH OH (Figures

[^s^u^b^s^t^r^a^t^e^]^,^aⁿ^d^t^h^e^p^l^o^t^o^f^t^h^e^kobs versus [substrate] exhibited

S31 −S34 ). Finally, as for CHD, we observed a shift and

broadening in the H O residual peak (δ = 3.33 ppm to δ =

a nonlinear correlation (Figure 2 ). This was consistent with a

.90 ppm). The k_o_b_sfor the rate of formation of 2,6-DTBQ

−

measured with this kinetic experiment was 0.0034 s (Figure

S35 ), which was one order of magnitude lower than the k₁

calculated for the formation of the radical (Table 1). The yield

of 2,6-DTBQ was quantiﬁed by GC as 80 ± 12% when a two-

electron oxidation was considered (Table S1). These

observations were consistent with the formation of the radical

occurring ﬁ rst followed by its further decay to 2,6-DTBQ and

CH OH (Figure S33).

We observed no diﬀerence in the k for the 4-methoxy-2,6-

di-tert-butylphenoxyl radical formation in the reaction between

and 4-CH O-2,6-DTBP in an aerobic versus an anaerobic

−

−1

atmosphere (0.0318 s versus 0.0305 s , respectively, Table

, Figures S36 −S39). This showed that O played no

signiﬁcant role in the oxidation of phenols by 1. For the

reaction of 1 with deuterated 4-CH O-2,6-DTBP a k = 0.0106

Figure 2. Plot of k_o_b_sagainst [4-CH O-2,6-DTBP] determined for the

−

reaction between 1 and 4-CH O-2,6-DTBP.

was determined, yielding a KIE of 2.9 (Figures S42 −S43),

which suggests that proton or H atom transfer was rate-limiting

in the oxidation of phenols by 1.

saturation kinetic model; thus, we employed the following

1 was then reacted with 2,4,6-tris-tert-butylphenol (2,4,6-

TTBP, DMF, 25 °C, 0.4 mM), where the electronic absorption

spectrum showed the formation of a band at λ = 402 nm

(maximum yield at 1500 s), together with the slower growth of

features at λ = 545 and 630 nm (Figures S48 −S49). An X-

band EPR spectrum after 1500 s showed a signal at g = 2.00

(Figure S50 ). The 2,4,6-tris-tert -butylphenoxyl radical was

synthesized independently (Figure S50 −S51, see the SI for

details) and was found to display the same EPR g values and

electronic absorption features, conﬁrming that the 2,4,6-tris-

tert-butylphenoxyl radical was the product. The yield of radical

was determined to be 88 ± 15% by EPR and 85 ± 20% by

electronic absorption spectroscopy, when a one-electron

kinetic equation to ﬁt the obtained results: k_o_b_s

k [substrate]/(K + [substrate]). A linearization of the data

−

Figure S28) was performed to determine k (0.0305 s ), the

maximum rate of the reaction, and K (0.016 M), the

[

(

substrate] at which half of the maximum rate was reached

Table 1).

The k_o_b_sof the reaction of 4-CH O-2,6-DTBP with diﬀerent

concentrations of 1 was measured in saturating conditions

350 equiv of substrate) in order to ensure a maximum rate

(

was achieved in the saturating region (Figure S29 ). The linear

correlation observed between k_o_b_sand [1] suggested that the

oxidation was ﬁrst order in [1] in the range of concentrations

explored.

reaction was considered (Table S1). The k for this reaction

−

The fate of the 4-methoxy-2,6-di-tert-butylphenoxyl radical,

was determined to be 0.0014 s (Table 1, Figures S52 −S53).

The ﬁnal product of this reaction was found to be 2,6-DTBQ,

after it reached its maximum yield, was explored with H NMR

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

and its yield was calculated by GC to be 120 ± 60% for a net

two-electron reaction (Table S1).

Complex 1 also reacted with 2,6-di-tert-butylphenol (2,6-

DTBP, >10 equiv), showing the growth of a band at λ = 426

nm and the features at λ = 545 and 630 nm (Figure S55). ESI-

MS analysis of the post reaction mixture showed the presence

of 3,3′,5,5′-tetra-tert-butyl-[1,1 ′-bis(cyclohexane)]-2,2′,5,5′-

tetraene-4,4′-dione (TTBTD, Figure S56). This product was

presumably formed by the coupling of two 2,6-di-tert-

butylphenoxyl radicals formed by PCET oxidation of 2,6-

DTBP (Figure S57). An electronic absorption spectrum of

pure TTBTD demonstrated that the feature at λ = 426 nm

belonged to TTBTD (Figure S58). H NMR spectroscopy

‡

Figure 3. Plot of ΔG against the BDFE

of the substrates. The

O−H

conﬁrmed the formation of TTBTD (10% yield for a two-

electron reaction, Table S1 ) and 2,6-DTBQ (10% yield for a

two-electron reaction, Figure S61 and Table S1). We deemed

the kinetics of the formation of TTBTD not related to the

initial PCET by 1 (Figure S66 ) and, thus, not comparable with

the formation of the radicals for the previous substrates, which

would not provide insight into the initial PCET reaction.

Similar results were obtained when 1 was reacted with 4-X-2,6-

‡

values of ΔG was determined from the calculated ﬁrst-order

constants k via the Eyring equation (slope = 0.30). BDFE values

are not available for these substrates in DMF; hence, we have plotted

BDFE_O₋_Hvalues for CH CN.

O−H

correlation was observed between the kinetic constants and the

pK or the E of the substrates, when available.

For the reaction of 1 with 4-CH O-2,6-DTBP we have

determined activation energy parameters from the reaction of

complex 1 with 4-CH O-2,6-DTBP (Figure S85 ). The

activation enthalpy was determined to be ΔH = 9.9(4)

kcal/mol, whereas the activation entropy value was determined

to be ΔS = −33(3) cal mol K . This value is close to those

reported for metal-based oxidants performing HAT.

Furthermore, these values are also similar to those reported

for other metal−hydroxide complexes which have been

determined to perform a HAT oxidation (Table 2). All these

45,51

1/2

DTBP (X = Br, NO ) excluding these substrates from our

kinetic analyses.

‡

Complex 1 reacted with 4-X-1-hydroxy-2,2,6,6-tetramethyl-

piperidines (4-X-TEMPOH, X = H, CH O, O). Upon reaction

‡

−1 −1

with 4-X-TEMPOH (>10 equiv, Figures S67 −S81 ), the

electronic absorption spectrum of 1 showed a drop of the

feature at λ = 369 nm that was complete within 150 s (for

TEMPOH), together with the increase in absorbance of the

features at λ = 545 and 630 nm over 1000 s, which was

common for all substrates. For 4-H-TEMPOH, an X-band

EPR spectrum of a frozen sample of the reaction mixture after

50 s showed a rhombic signal with g_a_v= 2.00 which was

consistent with previously reported spectra of (2,2,6,6-

tetramethyl-piperidin-1-yl)oxyl (TEMPO) (Figure S71).

The reactions with 4-CH O-TEMPOH and 4-oxo-TEMPOH

gave comparable radical signals in similar time frames (Figures

S75 −S79). The yield of 4-X-TEMPO radicals was calculated to

be 80 ± 10% for TEMPOH, 77 ± 15% for 4-CH O-

TEMPOH, and 75 ± 15% for 4-oxo-TEMPOH by EPR, all

considering a one-electron oxidation. We monitored the

change at λ = 369 nm and obtained pseudo-ﬁrst-order rate

constants (k ). As for the phenolic substrates, saturation

kinetic behavior was observed (Figures S69 −S81). The k

value was calculated to be 0.808 s for TEMPOH, 1.2 s for

-CH O-TEMPOH, and 0.49 s for 4-oxo-TEMPOH, all

approximately one order of magnitude higher than that

observed for 4-CH O-2,6-DTBP (Table 1). In summary, as

with 4-CH O-2,6-DTBP and 2,4,6-TTBP, the 4-X-TEMPOH

52,53

Table 2. Activation Energy Parameters for Metal−

Hydroxide Complexes in HAT Oxidation

‡

complex

ΔH (kcal/mol)

ΔS (cal/mol K)

9.9(4)

8.2

−33(3)

−25.5

III

Me2

[

Mn (OH)(S N (tren))]

III

Mn (OH)(dpaq)]

Mn (OH)(PY5)]

Fe (OH)(PY5)]

Cu (OH)(L)]

9.9(9)

9.3(5)

13.2(5)

5.1(1)

3.8(2)

4.9(1)

−35(3)

−36(5)

−26(5)

−31(3)

−38(3)

−27(1)

III

pipMe

Cu (OH)(

L)]

III

Cu (OH)( 2^L⁾^]

obs

L= 2,6-X-dipheylpyridine-dicarboxamidate; dpaq = 2-[bis(pyridin-2-

ylmethyl)]amino-N-quinolin-8-yl-acetamidate; PY5 = 2,6-bis-(bis(2-

pyridyl)methoxymethane)pyridine); S N (tren) = dimethylmercap-

to-tris(2-aminoethyl)amine). The parameters for 1 have been

calculated from the ﬁrst-order kinetic constants (k ), whereas those

cited are from second-order kinetic constants (k₂).

−

−1

−

family of substrates reacted with 1 to yield oxygen-based

radical products in high yields (>75%, calculated considering a

one-electron oxidation with respect to the starting complex)

which displayed pseudo-ﬁrst-order rates of reaction, indicating

PCET oxidation by 1.

hydroxide complexes in fact show a relatively large and

negative entropic contribution for a bimolecular reaction

within the same order of magnitude of that experimentally

identiﬁed for 1. This corroborates the mechanistic hypothesis

of HAT for 1 in the reaction with O−H bonds. The primary

KIE values calculated for the reaction of complex 1 with O−H

(2.9) and C−H bonds (3.3) further support this mechanistic

hypothesis. We exclude a hydride transfer mechanism based on

the weight of kinetic data indicating hydrogen atom transfer.

The Gibbs energies of activation were derived from the

‡

measured k values to determine a ΔG and were plotted

against the change in O−H bond dissociation free energy

(

BDFE ) to yield a slope = 0.30 (Figures 3 and S82 −S84),

O−H

which is close to the ideal value for HAT of 0.5 as predicted by

8,49

‡

Marcus theory.

This value falls in the range of of ΔG /

III

Δ(BDFE) slopes ascribed to HAT mechanisms (0.15−0.7)

These results conﬁrm that a Au −OH entity is a capable

oxidant of hydrocarbons and a plausible intermediate in Au

oxidation catalysis.

and indicates that HAT is the mechanism by which 1 oxidized

7,50

these substrates.

Furthermore, it should be noted that no

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

III

We postulate that the Au −OH complex 1 abstracted a H

1 we observed that the electronic absorption features

associated with Au appeared after the features associated

atom from hydrocarbons and alcohols, formally resulting in the

formation of a transient Au −OH adduct and a C- or O-based

radical (Scheme 2). The increased water content in all

with the HAT reaction products; thus, Au nanoparticles

III

formed after initial HAT by Au 1, possibly as a result of the

decay of a Au −OH product. Furthermore, no characteristic

Scheme 2. Mechanism of the HAT Oxidation of Alcohols by

superoxide resonances were observed in the EPR spectra of the

reaction mixtures prepared during the ﬁrst phase of the

reaction. Finally, k for the reaction of complex 1 with 4-

CH O-2,6-DTBP did not diﬀer when performed in aerobic or

anaerobic conditions (Table 1). These combined results show

that in this instance the active oxidant in HAT was the discrete

III

Au −OH complex 1, demonstrating that such an entity is a

plausible oxidant in Au-mediated oxidation catalysis.

CONCLUSION

In conclusion, the Au −OH complex 1 performed the

reactions from the transformation of the ancillary hydroxide

ligand of 1 into an aquo ligand after HAT (Scheme 2) adds

support to this postulate. We postulate that the product of the

■

III

oxidative activation of C−H and O−H bonds through a

HAT mechanism. Stable O-based radicals were detected as the

product of HAT of alcohols by 1 which suggested that a form

of PCET oxidation had occurred. Kinetic analysis conﬁrmed

the oxidation followed a HAT mechanism where protons and

electrons were transferred in a concerted manner from the

HAT step ([Au (H O)(terpy)] ) decays to yield Au

nanoparticles either by oxidizing substrate or via other routes.

We were not able to independently synthesize [Au (H O)-

(

terpy)] . To the best of our knowledge, only one

mononuclear Au complex has been isolated, suggesting

III

such species are unstable. The instability of Au has been

substrate to the Au −OH entity. These ﬁndings provide new

III

shown theoretically to be inﬂuenced by multiple factors such as

solvation energy, the ancillary ligand dissociation mechanism,

the charge of the intermediate, and the nature of the solvent

insights in the potential involvement of Au −OH species in

gold oxidation catalysis and demonstrate that soft and heavy

transition metal−oxygen adducts can also eﬀect PCET

37,58−61

III

and of the ancillary ligands.

The exact nature of the

oxidation. We continue to explore the role of Au −oxygen

decay of Au-products after HAT oxidation is therefore very

complex and needs further investigation. We conclude from

the experimental evidence available at this time that 1

facilitates HAT which was subsequently followed by decay of

adducts in Au oxidation catalysis.

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03225.

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the [Au (H O)(terpy)] product to H O and Au .

Saturation kinetics best described the relationship between

^kobs and [substrate], which would suggest an oxidant/substrate

interaction prior to irreversible HAT (Scheme 2). Such

Synthesis methods; reactivity protocols and results; EPR

analysis; and NMR analysis (PDF)

III

saturation behavior has been observed for Mn −OH and

III

Cu −OH complexes, where both hydrogen-bonding inter-

AUTHOR INFORMATION

Corresponding Author

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actions and coordination of substrate were implicated. The

former is supported for 1 by K = 0.016 M for proto-4-CH O-

Aidan R. McDonald − School of Chemistry, Trinity College

Dublin, The University of Dublin, Dublin 2, Ireland;

orcid.org/0000-0002-8930-3256;

,6-DTBP and K = 0.067 M for deutero-4-CH O-2,6-DTBP.

This diﬀerence suggests that the initial interaction step was

perturbed by substituting a hydrogen with a deuterium atom

and is thus plausibly a form of hydrogen bonding. Substrate

coordination at Au was eliminated as a plausible option,

because Au −phenol species would be expected to display

diagnostic resonances which were not observed in the H

Email: aidan.mcdonald@tcd.ie

III

Author

III

Marta Lovisari − School of Chemistry, Trinity College Dublin,

The University of Dublin, Dublin 2, Ireland

NMR and EPR spectra of the reaction of complex 1 with

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.9b03225

phenols (Figures S25, S34, S50). Furthermore, when an excess

III 18

of pyridine, a known coordinating ligand for Au , was added

to 1, its H NMR features were unchanged and a shift of the

Author Contributions

pyridine resonances was not observed (Figure S86).

Furthermore, when an excess of phenol (C H OH) was

M.L. performed all experimental work. M.L. prepared the

manuscript, and the manuscript was edited in collaboration

with A.M. All authors have given approval to the ﬁnal version

of the manuscript.

added to 1, the H NMR resonances of both the phenol and

the complex did not show a shift (Figure S87). We thus

concluded that the saturation behavior displayed by 1 was

most likely as a result of substrate hydrogen-bonding

Notes

III

The authors declare no competing ﬁnancial interest.

interactions with the Au −OH entity prior to HAT.

Many Au-based homogeneous oxidation catalysts have been

queried as to whether the true oxidant was a discrete Au cation

ACKNOWLEDGMENTS

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37,63

or Au nanoparticles formed upon reduction of the former.

This publication has emanated from research supported by the

European Research Council (ERC-2015-STG-678202). Re-

search in the McDonald lab is supported in part by a research

grant from the Science Foundation Ireland (SFI/15/RS-URF/

Au nanoparticles have demonstrated the ability to reduce

atmospheric dioxygen to superoxide, which in turn was

postulated to act as the oxidant. In the HAT oxidation by

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

307). We thank Prof. Robert Barklie for training on and use

of an EPR spectrometer and Dr. John O’Brien for NMR

support.

Calixto, J. C.; Behan, R. K.; Green, M. T. Iron(IV)hydroxide pK and

Article

(20) Yosca, T. H.; Rittle, J.; Krest, C. M.; Onderko, E. L.; Silakov, A.;

the Role of Thiolate Ligation in C-H Bond Activation by Cytochrome

P450. Science 2013, 342 (6160), 825−829.

(21) Holman, T. R.; Zhou, J.; Solomon, E. I. Spectroscopic and

REFERENCES

Functional Characterization of a Ligand Coordination Mutant of

Soybean Lipoxygenase-1: First Coordination Sphere Analogue of

Human 15-Lipoxygenase. J. Am. Chem. Soc. 1998, 120 (48), 12564−

■

1) Ciriminna, R.; Falletta, E.; Della Pina, C.; Teles, J. H.; Pagliaro,

M. Industrial Applications of Gold Catalysis. Angew. Chem., Int. Ed.

2572.

2) Ide, M. S.; Davis, R. J. The Important Role of Hydroxyl on

016, 55 (46), 14210−14217.

Oxidation Catalysis by Gold Nanoparticles. Acc. Chem. Res. 2014, 47

3), 825−833.

3) Villa, A.; Dimitratos, N.; Chan-Thaw, C. E.; Hammond, C.;

22) Goldsmith, C. R.; Stack, T. D. P. Hydrogen Atom Abstraction

by a Mononuclear Ferric Hydroxide Complex: Insights into the

Reactivity of Lipoxygenase. Inorg. Chem. 2006 , 45 (15), 6048 −6055.

(

(23) Mayer, J. M. Proton-Coupled Electron Transfer: A Reaction

(

Chemist ’s View. Annu. Rev. Phys. Chem. 2004, 55 (1), 363−390.

Prati, L.; Hutchings, G. J. Glycerol Oxidation Using Gold-Containing

(24) Darcy, J. W.; Koronkiewicz, B.; Parada, G. A.; Mayer, J. M. A

Catalysts. Acc. Chem. Res. 2015 , 48 (5), 1403 −1412.

Continuum of Proton-Coupled Electron Transfer Reactivity. Acc.

Chem. Res. 2018, 51 (10), 2391−2399.

(

4) Hutchings, G. J. Heterogeneous Gold Catalysis. ACS Cent. Sci.

García, C. J.; Rodenas, T.; Sabater, M. J. Enantioselective epoxidation

018, 4 (9), 1095−1101.

5) Shul ’pin, G. B.; Shilov, A. E.; Su

catalysed by gold complexes. Tetrahedron Lett. 2001, 42 (41), 7253−

256.

6) Corma, A.; Domínguez, I.; Domen

(25) Costentin, C.; Robert, M.; Saveant, J.-M. Concerted Proton-

ss-Fink, G. Alkane oxygenation

Electron Transfer Reactions in Water. Are the Driving Force and Rate

Constant Depending on pH When Water Acts as Proton Donor or

Acceptor? J. Am. Chem. Soc. 2007, 129 (18), 5870−5879.

(

ech, A.; Fornes, V.; Gomez-

́ ́ ́

26) Costentin, C.; Robert, M.; Saveant, J.-M.; Teillout, A.-L.

Concerted proton-coupled electron transfers in aquo/hydroxo/oxo

of olefins with molecular oxygen catalyzed by gold(III): A dual

metal complexes: Electrochemistry of [Os(II)(bpy) py(OH )] in

pathway for oxygen transfer. J. Catal. 2009, 265 (2), 238−244.

(

7) Cinellu, M. A.; Minghetti, G.; Cocco, F.; Stoccoro, S.; Zucca, A.;

water. Proc. Natl. Acad. Sci. U. S. A. 2009, 106 (29), 11829 −11836.

(27) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Thermochemistry of

Proton-Coupled Electron Transfer Reagents and its Implications.

Chem. Rev. 2010, 110 (12), 6961−7001.

Manassero, M. Reactions of Gold(III) Oxo Complexes with Cyclic

Alkenes. Angew. Chem., Int. Ed. 2005, 44 (42), 6892 −6895.

8) Zhang, G.; Cui, L.; Wang, Y.; Zhang, L. Homogeneous Gold-

Catalyzed Oxidative Carboheterofunctionalization of Alkenes. J. Am.

Chem. Soc. 2010, 132 (5), 1474 −1475.

(28) Zaragoza, J. P. T.; Siegler, M. A.; Goldberg, D. P. A Reactive

Manganese(IV)-Hydroxide Complex: A Missing Intermediate in

Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid

Compounds. J. Am. Chem. Soc. 2018 , 140 (12), 4380 −4390.

(29) Borovik, A. S. Role of metal-oxo complexes in the cleavage of

C-H bonds. Chem. Soc. Rev. 2011, 40 (4), 1870−1874.

(30) Wijeratne, G. B.; Corzine, B.; Day, V. W.; Jackson, T. A.

Saturation Kinetics in Phenolic O-H Bond Oxidation by a

Mononuclear Mn(III)-OH Complex Derived from Dioxygen. Inorg.

Chem. 2014, 53 (14), 7622−7634.

9) de Haro, T.; Nevado, C. Flexible Gold-Catalyzed Regioselective

Oxidative Difunctionalization of Unactivated Alkenes. Angew. Chem.,

Int. Ed. 2011, 50 (4), 906−910.

10) O ’Neill, J. A. T.; Rosair, G. M.; Lee, A.-L. Gold(III)-oxo

complexes as catalysts in intramolecular hydroamination. Catal. Sci.

Technol. 2012, 2 (9), 1818−1821.

11) Bond, G. C.; Thompson, D. T. Gold-catalysed oxidation of

carbon monoxide. Gold Bull. 2000, 33 (2), 41−50.

12) Sanchez-Castillo, M. A.; Couto, C.; Kim, W. B.; Dumesic, J. A.

(31) Mukherjee, J.; Lucas, R. L.; Zart, M. K.; Powell, D. R.; Day, V.

W.; Borovik, A. S. Synthesis, Structure, and Physical Properties for a

Series of Monomeric Iron(III) Hydroxo Complexes with Varying

Hydrogen-Bond Networks. Inorg. Chem. 2008, 47 (13), 5780−5786.

(32) Hill, E. A.; Weitz, A. C.; Onderko, E.; Romero-Rivera, A.; Guo,

Y.; Swart, M.; Bominaar, E. L.; Green, M. T.; Hendrich, M. P.; Lacy,

Gold-Nanotube Membranes for the Oxidation of CO at Gas-Water

Interfaces. Angew. Chem. 2004, 116 (9), 1160 −1162.

13) Guzman, J.; Gates, B. C. Catalysis by Supported Gold:

Correlation between Catalytic Activity for CO Oxidation and

Oxidation States of Gold. J. Am. Chem. Soc. 2004, 126 (9), 2672−

673.

D. C.; Borovik, A. S. Reactivity of an Fe -Oxo Complex with Protons

(

14) Cinellu, M. A.; Minghetti, G.; Pinna, M. V.; Stoccoro, S.; Zucca,

and Oxidants. J. Am. Chem. Soc. 2016, 138 (40), 13143−13146.

(33) Donoghue, P. J.; Tehranchi, J.; Cramer, C. J.; Sarangi, R.;

Solomon, E. I.; Tolman, W. B. Rapid C-H Bond Activation by a

Monocopper(III)-Hydroxide Complex. J. Am. Chem. Soc. 2011, 133

(44), 17602−17605.

A.; Manassero, M.; Sansoni, M. μ -Oxo and alkoxo complexes of

gold(III) with 6-alkyl-2,2 ′-bipyridines. Synthesis, characterization and

X-ray structures. J. Chem. Soc., Dalton Trans. 1998, 11, 1735−1742.

(

15) Agostina Cinellu, M.; Minghetti, G.; Pinna, M. V.; Stoccoro, S.;

Zucca, A.; Manassero, M. The first gold(III) dinuclear cyclo-

metallated derivatives with a single oxo bridge. Chem. Commun.

(34) Czerwinska, K.; Golec, M.; Skonieczna, M.; Palion-Gazda, J.;

Zygad ł o, D.; Szlapa-Kula, A.; Krompiec, S.; Machura, B.; Szurko, A.

Cytotoxic gold(III) complexes incorporating a 2,2 ′:6 ′,2 ″-terpyridine

ligand framework - the impact of the substituent in the 4 ′-position of a

terpy ring. Dalton Trans. 2017, 46 (10), 3381−3392.

(35) Mansour, M. A.; Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R.

Syntheses, Molecular Structures, and Spectroscopy of Gold(III)

Dithiolate Complexes. Inorg. Chem. 1998, 37 (18), 4625−4632.

(36) Camerel, F.; Ziessel, R.; Donnio, B.; Bourgogne, C.; Guillon,

D.; Schmutz, M.; Iacovita, C.; Bucher, J.-P. Formation of Gels and

Liquid Crystals Induced by Pt ···Pt and π -π * Interactions in

Luminescent σ -Alkynyl Platinum(II) Terpyridine Complexes.

Angew. Chem., Int. Ed. 2007 , 46 (15), 2659 −2662.

(37) Corma, A.; Garcia, H. Supported gold nanoparticles as catalysts

for organic reactions. Chem. Soc. Rev. 2008, 37 (9), 2096−2126.

(38) Sanna, G.; Pilo, M. I.; Minghetti, G.; Cinellu, M. A.; Spano, N.;

Seeber, R. Electrochemical properties of gold(III) complexes with

2,2 ′-bipyridine and oxygen ligands. Inorg. Chim. Acta 2000, 310 (1),

34−40.

16) Rosca, D.-A.; Wright, J. A.; Hughes, D. L.; Bochmann, M. Gold

998, No. 21, 2397−2398.

peroxide complexes and the conversion of hydroperoxides into gold

hydrides by successive oxygen-transfer reactions. Nat. Commun. 2013,

4, 2167.

(

17) Pitteri, B.; Marangoni, G.; Visentin, F.; Bobbo, T.; Bertolasi, V.;

Gilli, P. Equilibrium and kinetic studies of (2,2 ′:6 ′,2 ″-terpyridine)-

gold(III) complexes. Preparation and crystal structure of [Au(terpy)-

J. D.; Barnard, P. J.; Dutton, J. L. Facile Formation of Homoleptic

OH)][ClO ] . J. Chem. Soc., Dalton Trans. 1999, 5, 677−682.

(

18) Corbo, R.; Pell, T. P.; Stringer, B. D.; Hogan, C. F.; Wilson, D.

Au(III) Trications via Simultaneous Oxidation and Ligand Delivery

from [PhI(pyridine) ] . J. Am. Chem. Soc. 2014, 136 (35), 12415−

19) Beucher, H.; Merino, E.; Genoux, A.; Fox, T.; Nevado, C. κ 3-

2421.

NCC)Gold(III) Carboxylates: Evidence for Decarbonylation Pro-

cesses. Angew. Chem., Int. Ed. 2019, 58 (27), 9064−9067.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(58) Barakat, K. A.; Cundari, T. R.; Rabaa, H.; Omary, M. A.

Article

(

39) Maron,

A.; Czerwins

ka, K.; Machura, B.; Raposo, L.; Roma-

Structure and reactivity of a mononuclear gold(II) complex. Nat.

Chem. 2017, 9, 1249.

Rodrigues, C.; Fernandes, A. R.; Ma ł ecki, J. G.; Szlapa-Kula, A.; Kula,

S.; Krompiec, S. Spectroscopy, electrochemistry and antiproliferative

properties of Au(III), Pt(II) and Cu(II) complexes bearing modified

Disproportionation of Gold(II) Complexes. A Density Functional

Study of Ligand and Solvent Effects. J. Phys. Chem. B 2006, 110 (30),

14645−14651.

,2 ′:6 ′,2 ″-terpyridine ligands. Dalton Trans. 2018, 47 (18), 6444−

463.

40) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR Chemical

(59) Dang Nguyen Vo, K.; Kowandy, C.; Dupont, L.; Coqueret, X.

Shifts of Common Laboratory Solvents as Trace Impurities. J. Org.

Chem. 1997, 62 (21), 7512−7515.

41) Kang, E.; Park, H. R.; Yoon, J.; Yu, H.-Y.; Chang, S.-K.; Kim,

B.; Choi, K.; Ahn, S. A simple method to determine the water content

in organic solvents using the H NMR chemical shifts differences

between water and solvent. Microchem. J. 2018, 138, 395−400.

42) Mizuno, K.; Imafuji, S.; Ochi, T.; Ohta, T.; Maeda, S.

Hydration of the CH Groups in Dimethyl Sulfoxide Probed by NMR

and IR. J. Phys. Chem. B 2000, 104 (47), 11001 −11005.

43) Pirovano, P.; McDonald, A. R. Synthetic High-Valent M-O-X

Oxidants. Eur. J. Inorg. Chem. 2018, 2018 (5), 547−560.

44) Wittman, J. M.; Hayoun, R.; Kaminsky, W.; Coggins, M. K.;

Mayer, J. M. A C-C Bonded Phenoxyl Radical Dimer with a Zero

Bond Dissociation Free Energy. J. Am. Chem. Soc. 2013, 135 (35),

Evidence of chitosan-mediated reduction of Au(III) to Au(0)

•

nanoparticles under electron beam by using OH and e aq scavengers.

(

Chem. Commun. 2015, 51 (19), 4017−4020.

(60) Eustis, S.; Hsu, H.-Y.; El-Sayed, M. A. Gold Nanoparticle

Formation from Photochemical Reduction of Au3+ by Continuous

Excitation in Colloidal Solutions. A Proposed Molecular Mechanism.

J. Phys. Chem. B 2005, 109 (11), 4811−4815.

Bochmann, M. Electrochemistry of Au and Au pincer complexes:

(61) Dann, T.; Rosca, D.-A.; Wright, J. A.; Wildgoose, G. G.;

III

62) Dhar, D.; Tolman, W. B. Hydrogen Atom Abstraction from

determination of the Au -Au bond energy. Chem. Commun. 2013, 49

86), 10169−10171.

(

Hydrocarbons by a Copper(III)-Hydroxide Complex. J. Am. Chem.

Soc. 2015, 137 (3), 1322−1329.

(63) Pina, C. D.; Falletta, E.; Rossi, M. Update on selective oxidation

(

2956−12959.

using gold. Chem. Soc. Rev. 2012, 41 (1), 350−369.

45) Kundu, S.; Chernev, P.; Engelmann, X.; Chung, C. S.; Dau, H.;

Bill, E.; England, J.; Nam, W.; Ray, K. A cobalt(II) iminoiodane

complex and its scandium adduct: mechanistic promiscuity in

hydrogen atom abstraction reactions. Dalton Trans. 2016, 45 (37),

(

4538−14543.

46) Manner, V. W.; Markle, T. F.; Freudenthal, J. H.; Roth, J. P.;

Mayer, J. M. The first crystal structure of a monomeric phenoxyl

radical: 2,4,6-tri-tert-butylphenoxyl radical. Chem. Commun. 2008, 2,

47) Bedell, S. A.; Martell, A. E. Oxidation of 2,6-di-tert-butylphenol

56−258.

by molecular oxygen. 2. Catalysis by cobaltous polyamine chelates

through their (μ -peroxo)- and (μ -peroxo)(μ -hydroxo)dicobalt(III)

complexes. J. Am. Chem. Soc. 1985, 107 (26), 7909 −7913.

48) Roth, J. P.; Lovell, S.; Mayer, J. M. Intrinsic Barriers for

Electron and Hydrogen Atom Transfer Reactions of Biomimetic Iron

Complexes. J. Am. Chem. Soc. 2000 , 122 (23), 5486 −5498.

49) Mayer, J. M. Understanding Hydrogen Atom Transfer: From

Bond Strengths to Marcus Theory. Acc. Chem. Res. 2011, 44 (1), 36−

50) Hammes-Schiffer, S. Introduction: Proton-Coupled Electron

Transfer. Chem. Rev. 2010, 110 (12), 6937−6938.

51) Lee, J. Y.; Peterson, R. L.; Ohkubo, K.; Garcia-Bosch, I.; Himes,

Karlin, K. D. Mechanistic Insights into the Oxidation of Substituted

R. A.; Woertink, J.; Moore, C. D.; Solomon, E. I.; Fukuzumi, S.;

Phenols via Hydrogen Atom Abstraction by a Cupric-Superoxo

Complex. J. Am. Chem. Soc. 2014, 136 (28), 9925 −9937.

52) Mader, E. A.; Davidson, E. R.; Mayer, J. M. Large Ground-State

Entropy Changes for Hydrogen Atom Transfer Reactions of Iron

Complexes. J. Am. Chem. Soc. 2007, 129 (16), 5153−5166.

(

53) Mader, E. A.; Manner, V. W.; Markle, T. F.; Wu, A.; Franz, J.

A.; Mayer, J. M. Trends in Ground-State Entropies for Transition

Metal Based Hydrogen Atom Transfer Reactions. J. Am. Chem. Soc.

(

009, 131 (12), 4335−4345.

54) Dhar, D.; Yee, G. M.; Spaeth, A. D.; Boyce, D. W.; Zhang, H.;

Dereli, B.; Cramer, C. J.; Tolman, W. B. Perturbing the Copper(III)-

Hydroxide Unit through Ligand Structural Variation. J. Am. Chem.

Soc. 2016, 138 (1), 356−368.

55) Goldsmith, C. R.; Cole, A. P.; Stack, T. D. P. C-H Activation by

a Mononuclear Manganese(III) Hydroxide Complex: Synthesis and

Characterization of a Manganese-Lipoxygenase Mimic? J. Am. Chem.

Soc. 2005, 127 (27), 9904−9912.

56) Coggins, M. K.; Brines, L. M.; Kovacs, J. A. Synthesis and

III

Structural Characterization of a Series of Mn OR Complexes,

Including a Water-Soluble Mn OH That Promotes Aerobic Hydro-

III

gen-Atom Transfer. Inorg. Chem. 2013, 52 (21), 12383−12393.

(

57) Preiß, S.; Forster, C.; Otto, S.; Bauer, M.; Muller, P.;

Hinderberger, D.; Hashemi Haeri, H.; Carella, L.; Heinze, K.