Reversible dioxygen binding and arene hydroxylation reactions: Kinetic and thermodynamic studies involving ligand electronic and structural variations

Article abstract of DOI:10.1016/j.ica.2012.01.042

Copper-dioxygen interactions are of intrinsic importance in a wide range of biological and industrial processes. Here, we present detailed kinetic/thermodynamic studies on the O₂-binding and arene hydroxylation reactions of a series of xylyl-bridged binuclear copper(I) complexes, where the effects of ligand electronic and structural elements on these reactions are investigated. Ligand 4-pyridyl substituents influence the reversible formation of side-on bound μ-η²:η²- peroxodicopper(II) complexes, with stronger donors leading to more rapid formation and greater thermodynamic stability of product complexes [Cu ^II₂(^RXYL)(O₂^2-)] ²⁺. An interaction of the latter with the xylyl π-system is indicated. Subsequent peroxo electrophilic attack on the arene leads to C-H activation and oxygenation with hydroxylated products [Cu^II ₂(^RXYLO^-)(^-OH)]²⁺ being formed. A related unsymmetrical binucleating ligand was also employed. Its corresponding O₂-adduct [Cu^II₂(UN)(O ₂^2-)]²⁺ is more stable, but primarily because the subsequent decay by hydroxylation is in a relative sense slower. The study emphasizes how ligand electronic effects can and do influence and tune copper(I)-dioxygen complex formation and subsequent reactivity.

Full text of DOI:10.1016/j.ica.2012.01.042

Inorganica Chimica Acta 389 (2012) 138–150
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Reversible dioxygen binding and arene hydroxylation reactions: Kinetic and
thermodynamic studies involving ligand electronic and structural variations
a
b
c
c
Kenneth D. Karlin a, , Christiana Xin Zhang , Arnold L. Rheingold , Benedikt Galliker , Susan Kaderli ,
⇑
Andreas D. Zuberbühler c,
⇑
a Department of Chemistry, The Johns Hopkins University, Baltimore, MD 21218, USA
b Department of Chemistry, University of Delaware, Newark, DE 19716, USA
c Department of Chemistry, University of Basel, 4056 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 2 February 2012
Copper–dioxygen interactions are of intrinsic importance in a wide range of biological and industrial pro-
cesses. Here, we present detailed kinetic/thermodynamic studies on the O2-binding and arene hydroxyl-
ation reactions of a series of xylyl-bridged binuclear copper(I) complexes, where the effects of ligand
electronic and structural elements on these reactions are investigated. Ligand 4-pyridyl substituents
Dedicated to Professor Jon Zubieta on the
occasion of his 65th birthday. All the best to
a special guy and wonderful colleague who
without hesitation or reservation
significantly influenced and aided the
launching of K.D.K.’s research career. One
could not be more grateful
influence the reversible formation of side-on bound
ger donors leading to more rapid formation and greater thermodynamic stability of product complexes
[CuII2(RXYL)(O22ꢀ)]2+. An interaction of the latter with the xylyl
-system is indicated. Subsequent peroxo
l- :g
g2 2-peroxodicopper(II) complexes, with stron-
p
electrophilic attack on the arene leads to C–H activation and oxygenation with hydroxylated products
[CuII2(RXYLO–)(ꢀOH)]2+ being formed. A related unsymmetrical binucleating ligand was also employed.
Its corresponding O2-adduct [CuII2(UN)(O22ꢀ)]2+ is more stable, but primarily because the subsequent
decay by hydroxylation is in a relative sense slower. The study emphasizes how ligand electronic effects
can and do influence and tune copper(I)–dioxygen complex formation and subsequent reactivity.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Copper–dioxygen complexes
Peroxo
Hydroxylation
O2-activation
Kinetics–thermodynamics
1. Introduction
try with a binuclear copper(I) complex [CuI2(H-XYL)]2+ which
quantitatively reacts with resulting in the hydroxylation of the 2-
Interests in copper(I)–dioxygen reactivity of coordination com-
plexes derive from the need to establish fundamental properties
and due to the occurrence of such chemistry in a variety of copper
proteins which are essential in O2-processing in aerobic organisms
[1]. Notable examples of the latter include hemocyanins [2] (diox-
ygen carriers in arthropods and mollusks) and enzymes involved in
O2-activation towards substrate oxidations such as tyrosinase (Tyr)
position of the arene spacer (Scheme 1) [6]. A Cu2O2 reactive inter-
mediate, [Cu2(XYL)(O22ꢀ)]2+ (Scheme 1) with a side-on bridging
peroxide moiety similar to that for oxy-hemocyanin and oxy-tyros-
inase was proven based on its UV–Vis spectroscopic features
(kmax = 360, 435 nm) and resonance Raman (rR) spectroscopic
properties (for R = NO2) [6b,c]. The reaction mechanism deduced
for this hydroxylation chemistry involves an electrophilic attack
of the arene by the side-on peroxo moiety. Theoretical studies sup-
ported this conclusion [6b], however this was first shown by sub-
stituent effect studies where for R = MeO, t-Bu, F, CN, and NO2 at
the 5-position of the arene (Scheme 1), the rate constants (k2) for
arene hydroxylation increased with R electron-donating ability
[6c,7]. Further support for the electrophilic mechanism came from
studies where a 2-Me substituted xylyl ligand was employed, as a
hydroxylation-induced methyl-migration (i.e. ‘‘N.I.H.’’ shift) oc-
curred [8]. As an interesting aside, a substituent effect on the diox-
ygen binding to [CuI2(R-XYL)]2+ where electron donating R-groups
increased the copper–dioxygen binding strength was rationalized
[3], dopamine b-monooxygenase (DbM) [4], peptidyl a-amidating
monooxygenase (PHM) [4] and particulate (membrane bound)
copper-dependent methane monooxygenase (pMMO) [1c,5]. The
monooxygenase Tyr employs a coupled binuclear copper site sim-
ilar to that in hemocyanin, where the reduced dicopper(I) site re-
acts with O2 giving a side-on (or
l- :g
g2 2) peroxodicopper(II)
species capable of phenol oxygenation, producing o-catechol or
o-quinone directly.
With aromatic ring hydroxylation via CuI/O2 chemistry as a goal
in Tyr modeling, we earlier on discovered and detailed the chemis-
by the suggestion that an interaction between the xylyl
and the peroxo group occurs [6b,7].
p-system
⇑
Corresponding authors. Tel.: +1 4015168027; fax: +1 4105167044 (K.D. Karlin).
E-mail address: karlin@jhu.edu (K.D. Karlin).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.01.042

K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
139
Scheme 1. Dicopper mediated hydroxylation of arenes as copper monooxygenase
models systems.
Fig. 1. Ligands used in this study of copper–dioxygen kinetics.
While a good number of other binuclear copper(I) complexes
with m-xylyl linker were since shown to also exhibit arene hydrox-
ylation chemistry [9], considerable attention has also focused on
dicopper systems effecting phenol o-hydroxylation. In this context,
elegant studies by Itoh and coworkers [3b] revealed that such
chemistry occurs efficiently, not only with certain pyridylalkyl-
amine chelates in synthetic models, but also with the enzyme
electronic and structural elements. Investigations of variations in
ligand electron-donating ability in CuI/O2 chemistry have in the
past been extremely revealing [7,12], also see the discussion be-
low. Here, this is achieved by incorporating various 4-pyridyl sub-
stituents into the XYL ligands. An unsymmetrical ligand analogue
UN (Fig. 1) is utilized to investigate the effect of ligand constraints
on the O2-interaction of the resulting dicopper(I) complex.
Tyr itself. In both cases, the reactive intermediate is a [CuII
2
(l-
g2
:g
2-O22ꢀ)]2+ species which attacks the phenol substrate as
2. Experimental
an electrophile in a manner very similar to that of our [Cu2(XYL)]2+
system (Scheme 1). Yet, the Tolman and Stack research groups
2.1. Material and methods
showed that CuII g2 2-O22ꢀ) are often in equilibrium with
(l- :g
2
an isomeric bis-l
-oxo dicopper(III) [CuIII2(O)2]2+ species (Scheme 2)
Reagents and solvents used were of commercially available re-
agent quality unless otherwise stated. Methylene chloride and
diethyl ether were purified by passing through an alumina column
under Ar. Preparation and handling of air-sensitive materials were
carried out under an argon atmosphere by using standard Schlenk
techniques. Deoxygenation of solvents and solutions was achieved
by bubbling with Ar for ꢁ1/2 h or by three freeze/pump/thaw cy-
cles. Solid samples were stored and transferred in an MBraun dry-
box filled with prepurified nitrogen. Samples for IR, NMR and UV
spectra were also prepared in the drybox.
[1d,e,10] and it has been shown that these groups have also shown
the latter to be capable of arene and o-phenol hydroxylation
[1d,e,9b,10b]. As a relevant aside for the studies described above
for the oxygenation of [CuI2(R-XYL)]2+ complexes, the side-on per-
oxo complex intermediate was readily detected and characterized,
while no hint of a related bis-l
-oxo species [CuIII2(NO2-XYL)(O)2]2+
was found experimentally; this and theoretical studies support
only the side-on peroxo–dicopper(II) species as the reacting elec-
trophile in the Cu2(XYL) hydroxylation system [6b]. Computational
studies on Tyr itself lead to a variety of conclusions. Thus, while a
1H NMR spectra were recorded in CD3NO2, or CDCl3 on a Bruker
AMX-300 NMR spectrometer. Chemical shifts are reported as d val-
ues downfield from an internal TMS reference. Elemental analyses
were performed by Desert Analytics, Tuczon, AZ. EI/CI and FAB
Mass spectra were obtained on a VG Instruments 70S Gas Chroma-
tography/Mass Spectrometer by Dr. J. Kachinzki, Mass Spectrome-
try Facility, Department of Chemistry, the Johns Hopkins
University. Infrared spectra were recorded as Nujol mulls on a
Mattson Galaxy 4030 series FT-IR spectrophotometer and cali-
brated with a polystyrene film. UV–Vis spectra were collected on
Hewlett-Packard Model 8453 diode array spectrophotometer with
HP Chemstation software.
bis-
chemical model studies leave the possibility of an unobservable
enzyme [CuII 2-O22ꢀ)]2+ to [CuIII2(O)2]2+ conversion in-
l-oxo-dicopper(III) entity has never been detected in Tyr, the
(l- :g
g2
2
duced by phenol/phenolate substrate copper coordinations
[10b,11].
To further elucidate fundamental aspects of the XYL type
dicopper(I)/O2 chemistry involving both O2-complex formation
and substrate hydroxylation, we have here extended our kinetic–
thermodynamic studies on the oxygenation reactions of a related
but new series of complexes focusing on the influence of ligand
2.2. Syntheses of ligands and copper(I) complexes
2.2.1. MeOXYL
To 4-methoxy-2-vinylpyridine (1.6 g, 0.012 mol) in a 15 mL
pressure tube (Aldrich) was added m-xylenediamine (0.35 g,
0.0025 mol) and MeOH (2 mL) along with five drops of glacial ace-
tic acid. The resulting mixture was stirred at 60 °C for 5 days. After
cooling to room temperature, the volatile components were re-
moved by rotary evaporation. The residual acetic acid was neutral-
ized with Na2CO3 aqueous solution. The resulting solution was
extracted with CH2Cl2 three times. The combined organic extracts
Scheme 2. Structures of well established dicopper O2-adducts.

140
K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
were dried over Na2SO4 and the volatile components were re-
moved by rotary evaporation. Purification of the crude product
on alumina gel using 5% MeOH–95% EtOAc gave 1.1 g (64%) of yel-
low oil. Rf = 0.4 (silica, 10% MeOH–90% EtOAc). 1H NMR (300 MHz,
CDCl3): d 8.27 (4H, d, J = 6.0 Hz), 7.16–7.07 (4H, m), 6.63–6.50 (8H,
m), 3.76 (12H, s), 3.67 (4H, s), 2.93 (16H, m). FAB-MS (m/z): 677
(M+H+).
1.08 0.10 B.M. UV–Vis (nm, CH2Cl2): 370
(e = 3450), 637
(e
= 150). IR (nujol, cmꢀ1): 3643 (br, OH), 840 (s, PF6). A 1H NMR
spectrum of 3b exhibits broad paramagnetically-shifted reso-
nances and the presence of CH2Cl2 solvate was indicated by a rel-
atively sharp peak at 5.32 ppm.
2.3. Stopped-flow kinetics experiments
2.2.2. Me2NXYL
Rapid kinetics were followed using a SFL-21 (2-mm light path/
2-mL syringes) low-temperature flow unit of a SF-3A stopped-flow
system (Hi-Tech Scientific) combined with a TIDAS-16 HQ/UV–Vis
512/16B diode array spectrometer (J & M, 507 diodes, 300–720 nm,
1.3 ms minimum sampling time) using flexible light guides con-
nected to a CLH-111 halogen lamp (ZEISS). The two glass coils, con-
taining Cu(I) and oxygen solutions, respectively, and the mixing
chamber, were immersed in an ethanol bath. This bath was placed
in a dewar, which was filled with liquid nitrogen for low-temper-
ature measurements. The ethanol bath was cooled by liquid nitro-
gen evaporation, and its temperature was measured by using a Pt
resistance thermometer and maintained to 0.1 °C by using a tem-
perature-controlled thyristor power unit (both Hi-Tech).
To 4-dimethylamino-2-vinylpyridine (3.2 g, 0.022 mol) in a
15 mL pressure tube (Aldrich) were added m-xylenediamine
(0.70 g, 0.0051 mol) and MeOH (10 mL) along with glacial acetic
acid (0.63 g, 0.011 mol). The resulting mixture was stirred at
60 °C for 5 days. After cooling to room temperature, the volatile
components were removed by rotary evaporation. The residual
acetic acid was neutralized with Na2CO3 aqueous solution. The
resulting solution was extracted with CH2Cl2 three times. The com-
bined organic extracts were dried over Na2SO4 and the volatile
components were removed by rotary evaporation. Purification of
the crude product on alumina gel using 3% MeOH–97% EtOAc gave
1.0 g (27%) of yellow oil. Rf = 0.19 (silica, 3% NH4OH, 97% MeOH). 1H
NMR (300 MHz, CDCl3): d 8.11 (4H, d, J = 6.0 Hz), 7.23–7.12 (4H,
m), 6.31–6.29 (8H, m), 3.71 (4H, s), 2.93–2.82 (40H, m). FAB-MS
(m/z): 729.6 (M+H+).
Data acquisition (up to 256 complete spectra, up to four differ-
ent time bases) was done based on the KINSPEC program (J & M). For
numerical analysis, all data were pretreated by factor analysis and
concentration profiles were calculated by numerical integration
using either Specfit (Spectrum Software Ass.) or Globfit (MATLAB).
The solvent CH2Cl2 (Uvasol, Merck) was dried with CaH2 (Merck)
and distilled under normal pressure immediately prior to use. For
[Cu2I(Me2NXYL)](PF6)2 seven series of Cu(I) concentrations were used
to carry out a total of 365 measurements between ꢀ90 and +20 °C. Of
these 257 were used for the final analysis. The concentrations of
Cu(I)solutionsusedwere:0.10, 0.17, 0.21, 0.37 mM(witha dioxygen
concentration of 1.90 mM); 0.13 mM ([O2] = 0.50 mM); 0.24 mM
([O2] = 0.24 mM) and 0.12 mM ([O2] = 0.13 mM). Reaction times
measured ranged from 0.02 to 158 s.
2.2.3. [CuI2(MeOXYL)](PF6)2 (1b)
Under an Ar atmosphere, 0.297 g (0.439 mmol) of MeOXYL in
20 mL of CH2Cl2 was deaerated for 20 min and then added to
0.320 g (0.859 mmol) of [Cu(CH3CN)4](PF6)28 in a 100 mL Schlenk
flask. The resulting solution was stirred for ½ h and then cooled
to ꢀ78 °C. Addition of 100 mL of deaerated diethyl ether precipi-
tated out light yellow powder. After warming to room tempera-
ture, the solvent was decanted and the solid was washed with
CH2Cl2/diethyl ether (v:v, 1:5) for five times. The solid was dried
under vacuum to afford 0.25 g (27%) of light yellow powder. 1H
NMR (300 MHz, CD3NO2): d 8.30 (4H, d), 7.43 (1H, s), 7.09–7.22
(3H, m), 6.85–6.90 (8H, m), 3.92 (12H, s), 3.72 (4H, s), 3.11(16H,
s,br). Anal. Calc. for C40H48N6O4F12P2Cu2: C, 43.92; H, 4.42; N,
7.68. Found: C, 43.92; H, 4.61; N: 7.97%.
I
For [Cu2 (UN)](PF6)2 nine series of Cu(I) concentrations were
used to carry out a total of 222 measurements between ꢀ90 and
ꢀ35 °C. Two hundred and eighteen of these were used for the final
analysis. For one series, a cutoff filter (480 nm) was used. The con-
centrations of Cu(I) solutions used varied between 0.14 and
1.08 mM. The dioxygen concentrations varied between 0.25 and
1.9 mM. Reaction times measured ranged from 172 to 227 s.
For [Cu2I(MeOXYL)](PF6)2, three series of Cu(I) concentrations
were used to carry out a total of 91 measurements between ꢀ91
and +20 °C. Seventy-two of these were used for the final analysis.
The concentrations of Cu(I) solutions used were 0.15, 0.30 and
0.41 mM. The dioxygen concentration used throughout was
1.9 mM. Reaction times measured ranged from 4 to 301 s.
2.2.4. [CuI2(Me2NXYL)](PF6)2ꢂ1.1CH2Cl2 (1c)
Under an Ar atmosphere, 0.213 g (0.292 mmol) of NMe2XYL in
15 mL of MeOH was deaerated for 20 min and then added to
0.210 g (0.563 mmol) of [Cu(CH3CN)4](PF6) in a 100 mL Schlenk
flask at 0 °C. The resulting mixture was stirred for 20 min and
70 mL of deaerated diethyl ether was added to precipitate out
yellow powder. After decanting the solvent, the solid was redis-
solved in 20 mL of deaerated CH2Cl2 at 0 °C and layered with
80 mL of diethyl ether to afford a yellow precipitate. The solvent
was decanted and the solid was dried under vacuum for 2 h
to yield 140 mg (44%) of yellow powder. 1H NMR (300 MHz,
CD3NO2): d 8.03 (4H, d), 7.48 (1H, s), 7.11 (3H, m), 6.5 (8H, s),
5.32 (2.2H, s), 3.57 (4H, s), 3.11–3.04 (40H, m). Anal. Calc. for
2.4. Electrochemistry
Cyclic voltammetry was carried out by using a Bioanalytical
Systems BAS-100B Electrochemistry Analyzer. The cell consisted
of a modification of a standard three-chamber design equipped
for handling air-sensitive solution by utillizing high-vacuum valve
seals. A platinum disk (BAS MF 2013) was used as the working
electrode. The reference electrode was Ag/AgNO3. The measure-
ments were performed at room temperature under an Ar atmo-
sphere in DMF solution containing 0.2 M tetrabutylammonium
hexafluorophophate and 10ꢀ4–10ꢀ3 M copper complex.
C
44H60N10F12P2Cu2ꢂ1.1CH2Cl2: C, 43.5; H, 5.04; N, 11.22. Found:
C, 43.2; H, 5.14; N, 11.22%.
2.2.5. [CuII
(
2
MeOXYLO–)(OH–)](PF6)2ꢂ0.5CH2Cl2 (3b)
In
a
100 mL Schlenk flask, [CuI2(MeOXYL)](PF6)2 (100 mg,
0.0914 mmol) was dissolved in 50 mL deaerated DMF under an
Ar atmosphere. The resulting yellow solution was cooled to 0 °C
and exposed to an atmosphere of dry oxygen overnight. The sol-
vent was removed on a high-vacuum rotary evaporator and the
green residue obtained was recrystallized from CH2Cl2/ether at
0 °C, giving 95 mg (48%) of green crystalline solid. Anal. Calc. for
2.5. Magnetic moment measurements
Room temperature solution magnetic moments of the complex
3b were determined using the Evans method [13]. NMR sample
tubes with screw caps and coaxial insert were purchased from
C
40H48N6Cu2O6P2F12ꢂ0.5CH2Cl2: C, 41.64; H, 4.23; N, 7.19. Found:
C, 41.81; H, 4.15; N, 7.20%. leff (Evans method, CD2Cl2):

K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
141
the Wilmad Glass Company. Solutions of 3b with known concen-
trations were placed in the NMR sample tube and the deuterated
reference solvent along with a small drop of undeuterated solvent
to facilitate the identification of the reference solvent peak in NMR
spectra were placed in the coaxial insert. The 1H NMR spectra were
taken on a Bruker AMX-300 NMR spectrometer. The chemical shift
difference of the solvent was used to calculate the molar magnetic
susceptibility of the complex based on the following equation:
peroxidep⁄
rorbital(to Cu) withincreased ligandN-donor strength;
this increases the effective negative charge on the peroxide moiety
and thus reduces the backbonding from the Cu to the peroxide p⁄
orbital. However, an increase in the amount of bis-l-oxo-dicop-
per(III) isomer relative to side-on peroxo-dicopper(II) species is ob-
served for R-MePY2 with R = H < MeO < Me2N. The stronger donor
ligands increase the stability of the CuIII(
l-O)2 isomer with its higher
oxidation state metal ion. In a related study [15], copper ion com-
plexes of so-called PYAN ligands (R-PYAN = N-[2-(4-R-pyridin-2-
cM ¼ ðꢀ3=4pÞðDn=nÞð1000=cÞ þ csMol ꢀ cD;
yl)-ethyl]-N,N0,N0-trimethyl-propane-1,3-diamine,
R = Cl-,
MeO- and Me2N-) were even more severely influenced with respect
to the side-on peroxo–dicopper(II)/bis- -oxo-dicopper(III) equilib-
H-,
where Dn is the paramagnetic shift of the solvent as the product of
the chemical shift difference Dd and n (frequency of the NMR spec-
trometer), c is the sample concentration, cSMol is the solvent suscep-
tibility and cD is the diamagnetic correction. cSMol can be calculated
as the product of cg (value obtained from the CRC hand book) and
the molecular weight of the metal complex. The value of cD is
obtained based on the molecular formulation of the complex and
Pascal’s constants. The solution magnetic moment is related to
l
rium. For R = Cl-, the Cu2O2 species generated consisted entirely of
the former isomer, while for Me2N-, nearly entirely the latter.
We also studied the kinetic–thermodynamic behavior for the
formation and stability of [{(R-TMPA)CuII}2(O22ꢀ)]2+ complexes
[12b]. In EtCN, a strongly coordinating solvent, the ligand elec-
tronic effects were dampened due to the competitive binding of
solvent and dioxygen to the copper centers, especially with weaker
electron-donating groups such as Me and tBu, and there was little
affect on kinetic and thermodynamic parameters for the formation
of both the superoxocopper(II) species [(R-TMPA)CuII(O2ꢀ)]+ inter-
mediate first formed (not shown) and the final product [{(R-
TMPA)CuII}2(O22ꢀ)]2+ species (Scheme 3). But with stronger MeO-
and Me2N- electron-donating groups, the rates of formation and
stability of the product complexes were increased, as O2-binding
to copper(I) is a redox process, thus favored when oxidation of
the metal ion is also preferred. Significant medium (solvent) effects
were also seen, with the non-coordinating solvents THF or acetone.
The rates of formation of both superoxo and peroxo complexes,
along with their thermodynamic stabilities were much enhanced
when stronger donor ligands such as MeO- or Me2N- 4-pyridyl
substituents were employed.
the magnetic susceptibility as follows:
l
= 2.84((cMT/n))1/2, where
n is the nuclearity.
2.6. Low-temperature UV–Vis spectroscopy
Low-temperature UV–Vis spectra were recorded on a Hewlett-
Packard HP8453 diode array spectrophotometer using a custom
manufactured vacuum dewar equipped with quartz windows. To
maintain and control low temperatures, a copper-tubing coil was
inserted into the methanol-filled UV–Vis dewar through which
cold methanol was circulated from an external source (Neslab End-
ocal refrigerated circulating bath). The temperature in the dewar
was monitored by using a resistance thermocouple probe (Omega
Model 651). The cuvette assembly consists of a quartz cell with
2 mm pathlength connected to a 14/20 female joint via a 10-cm
long glass tube. The apparatus and procedures have also been pre-
viously described [14].
A solution of the Cu(I)-complex 1b–d with a known concentra-
tion was prepared in a glovebox and transferred into a cuvette with
2 mm pathlength. Then the cuvette was brought out of the glove-
box and placed in the UV–Vis low-temperature (193 K) dewar.
The UV–Vis spectrum of the copper(I) complex was recorded after
20 min. Then excess O2 was introduced to the Cu(I) solution
through a syringe needle and spectrum of the O2-adduct was re-
corded and monitored over time to determine its stability.
As mentioned above, the complexes [{Cu(R-MePY2)}2(O2)]2+ pos-
sess a varying side-on-peroxo to bis-l-oxo dicopper isomer ratio. In
reactivity toward substituted anilines (Scheme 4), oxidative N-deal-
kylation takes place, but the mechanism of reaction shifts as one
goes from R = H- to R = Me2N- [12d]. The former complex, [{Cu(H-
MePY2)}2(O2)]2+, is a stronger one-electron oxidant and possesses
a lesser amount of CuIII
(l-O)2 species, the reaction appears to take
2
place by a predominantly proton-coupled electron-transfer (PCET)
or electron-transfer first/subsequent proton transfer (ETPT) mecha-
nism. Hydrogen-atom transfer (HAT, or ET/PT) chemistry occurs for
the [{Cu(Me2N-MePY2)}2(O2)]2+ species, a weaker oxidant with re-
spect to one-electron ET, but perhaps better able to accommodate
a H-atom transfer due to its possession of a more basic bridging
oxo group, because of the stronger ligand donors.
3. Results and discussion
3.1. Employing ligand 4-pyridyl substituents in CuI/O2 reactivity
studies
3.2. Ligand syntheses
The use of 4-pyridyl substituents has already proven very useful
in a number of studies involving copper–dioxygen complex forma-
tion and reactivity, demonstrating that they mediate changes in
the kinetics and thermodynamics of Cu2O2 complexes, the equilibria
involving different structural isomers of Cu2O2 species, and the sub-
sequent reactivity of such complexes [7,12]. In a notable study [12a],
we showed that for [{(R-TMPA)CuII}2(O22ꢀ)]2+, (Scheme 3), the ob-
served mO–O value shifts from 827 to 822 to 812 cmꢀ1 and mCu–O
(sym) shifts from 561 to 557 to 551 cmꢀ1, respectively, as R- varies
from H- to MeO- to Me2N-. Increasing the N-donor strength to the
The ligand UN was prepared following a literature-based proce-
dure [16], while MeOXYL and NMe2XYL were synthesized in a similar
manner through reactions of m-xylyldiamine with 4-substituted-
2-vinylpyridines (Scheme 5); the latter syntheses are published
[12a,c]. Also, see the Section 2 for details. As can be seen, the
substituents chosen for new studies are R = MeO- and Me2N-,
strong donors compared to R = H- (Fig. 1). Electron-withdrawing
groups were not included in these studies since we previously
observed that for mononuclear Cu(I) complexes of in our studies
on the Cl-MePY2 (R = Cl-) (MePY2 = N-methyl-N,N-bis[2-(2-pyridyl)
ethyl]amine), there was no reaction with dioxygen [12c].
copper decreases peroxide p⁄
the Cu–O and O–O bonds. A decrease in
r
donation to the copper, weakening
Cu–O of the bis- -oxo-dicop-
m
l
per(III) complex was also observed with increasing N-donor
strength for the R-MePY2 ligand system (Scheme 3). However, no
change was observed for mO–O of the side-on peroxo moiety which
is attributed to a compensating reduced charge donation from the
3.3. Synthesis of copper(I) complexes
The dicopper(I) complexes [CuI2(L)]2+ (1b–d, L = MeOXYL, Me2NXYL
and UN, Fig. 1) were prepared by the addition of two equiv of

142
K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
+
PY'
CuII
2+
+
Me
PY'
O
PY'
CuI
PY'
PY'
CuI
N
O2
2
N
2
N
CuII
O
N
PY'
PY'
PY'
PY'
PY'
[CuI(R-MePY2)]+
PY'
[{Cu(R-TMPA)}2(O2)]2+
-1,2-peroxodicopper(II)
[CuI(R-TMPA)(RCN)]+
O2
μ
PY'
Cu
2+
PY'
2+
PY'
N
PY'
N
O
Me
O
O
Me
N
N
Cu
N
Cu Cu
O
PY' =
R = H-, MeO-, Me2N-
Me
Me
PY'
PY'
R
PY'
PY'
[{Cu(R-MePY2)}2(O2)]2+ [{Cu(R-MePY2)}2(μ-O)2]2+
2
-
:
2-peroxodicopper(II)
bis- -oxo-dicopper(III)
μ
μ η
η
Scheme 3. Previously studied systems for copper(I)/O2 kinetics employing derivatized pyridyl ligands.
Scheme 4. Cu2O2 mediated oxidative N-dealkylation of dimethylanilines.
Fig. 2. Structure of the cationic portion of [CuI2 MeOXYL)]2+ (1b).
(
MeOH/diethyl ether was necessary for complete removal of MeCN
(derived from [CuI(MeCN)4]PF6) as a coordinating fourth ligand to
copper(I) ion with its tridentate N3 chelate. This procedure led to
the desired three-coordinate dicopper(I) complexes 1b–d (Fig. 1).
3.4. X-ray structure of [CuI2(MeOXYL)]2+ (1b)
Light yellow crystals suitable for X-ray crystallography were ob-
tained by slow diffusion of diethyl ether into an acetone solution of
1b. The structure includes a N,N-dimethyl formamide (DMF) sol-
vate molecule, possibly introduced as a solvent impurity. The cat-
ionic portion of the structure is shown in Fig. 2.
The structural parameters are similar to the previously reported
dicopper(I) structure with the parent ligand HXYL (Table 1) [6a]. As
shown in Fig. 1, each Cu(I) ion is three-coordinate with ligation
from two pyridine nitrogens and one tertiary amine nitrogen do-
nor. The chelating tridentate ligand causes considerable distortions
from ideal trigonal planar coordination. The geometry of the cop-
per centers can be best described as bent ‘‘T-shape’’ geometry.
The Namino–Cu–NPY (PY = 2-pyridyl) angles are acute within the
range of 92–100°, resulting in large NPY–Cu–NPY angles of 163.9°
and 166.5°. The Cu–NPY bond distances (1.902–1.929 Å) in complex
1b are typical for Cu–Nheterocyclic (heterocyclic = derivatives of
imidazole, pyrazole and pyridine) bond lengths in three-coordinate
copper(I)-compounds [6a,17], and these short distances are
approaching those found in purely linear two-coordinate
systems with similar N-donor ligands, Cu–N ꢁ 1.86–1.89 Å [18].
The Cu–Namine distances are much longer than those of Cu–NPY
Scheme 5. Outline of the synthetic procedures to generate RXYL binucleating
ligands.
[CuI(MeCN)4](PF6) to the CH2Cl2 solution of the appropriate ligand
RXYL under an Ar atmosphere. Precipitation with diethyl ether
followed by recrystallization affords light-yellow solids, which
were characterized by 1H NMR spectroscopy and elemental
analyses. Repeated recrystallization from CH2Cl2/diethyl ether or

K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
143
Table 1
Selected bond distances and angles for [CuI2(XYL)](PF6)2 (1a) and [CuI2
(
MeOXYL)](PF6)2 (1b) complexes.
[CuI2 MeOXYL)](PF6)2 (1b)
[CuI2(HXYL)](PF6)2 (1a)10
(
Interatomic distances (Å)
Cu1–N1
Cu1–N3
Cu2–N5
2.121(8)
1.924(8)
1.904(8)
Cu1–N2
Cu2–N4
Cu2–N6
1.937(9)
2.196(7)
1.918(8)
Cu1–N1
Cu1–N31.917(6)
Cu2–N51.929(8)
2.289(7)
Cu2–N4
Cu2–N6
Cu1–N2
2.287(6)
1.902(8)
1.923(6)
Interatomic angles (°)
N1–Cu1–N2
N2-Cu1–N3
102.5(3)
151.4(4)
104.4(3)
N1–Cu1–N3
N4-Cu2–N5
N5-Cu2–N6
100.4(3)
99.7(3)
150.8(3)
N1–Cu1–N292.2(3)
N2–Cu1–N3166.5(3)
N4–Cu2–N699.2(3)
N1–Cu1–N3
N4-Cu2–N5
N5-Cu2–N6
100.1(3)
95.5(3)
163.9(3)
N4-Cu2–N6
distances, possibly due to the
p back-donation (Cu(I) d electrons to
Table 2
the pyridine p⁄ orbitals. As shown in Table 1, the Cu–Namine
distances in complex [CuI2(MeOXYL)](PF6)2 (1b) are longer by
0.1–0.2 Å than the corresponding bond lengths found in the parent
complex [CuI2(XYL)](PF6)2 (1a) and the NPY–Cu–NPY angles are
ꢁ15° larger than those found in the parent complex. This may be
a ligand electronic effect, where greater electron density around
the copper(I) centers resulting from the electron-donating 4-
Cyclic voltammetry data for Cu(I)-complexes in DMF.a
Complex
E1/2 (V)
D
Ep (mV)
ipa/ipc
[CuI2(HXYL)]2+ (1a)
ꢀ0.21
ꢀ0.25
ꢀ0.28
ꢀ0.24
210
0.95
0.85
1.15
0.95
[CuI2
[CuI2
(
(
MeOXYL)]2+ (1b)
NMe2XYL)]2+ (1c)
170
300
90
[CuI2(UN)]2+ (1d)
a
Potentials are vs. Fc/Fc+, measured under the same electrochemical cell
MeO- groups disfavors the coordination of
r-type ligands such
conditions.
as amine nitrogens (i.e., giving longer Cu–Namine distances). This
results in larger NPY–Cu–NPY angles and more distortion from
trigonal planar geometry.
Besides T-shaped copper(I) complexes, three-coordinate Y-
shaped Cu(I)-complexes have also been described [19]. The bond
angle h3 and the ‘‘middle’’ bond distance d3 increase as the geom-
etry around the copper(I) distorts from the ideal trigonal planar
geometry to T-shape, whereas both h3 and d3 decreases when the
geometry moves from a trigonal planar to Y-shape geometry (see
diagram).
Fig. 3. Cyclic voltammogram of [CuI2 MeOXYL)](PF6)2 (1b) in DMF.
(
As shown in Table 2, small redox potential shifts are observed as
the ligands become more electron-donating; the E1/2 values be-
come more negative as the R group goes from H- to MeO- to
Me2N-. The observed ligand effect upon the redox potentials of
the corresponding Cu(I)-complexes correlates to the differences
in ligand basicity. The pKa values for pyridine, 4-MeO-pyridine
and 4-Me2N-pyridine are 5.21, 6.58 and 9.70, respectively. The
more basic ligand provides more electron density to the copper
center, making it easier to oxidize Cu(I) to Cu(II). The same corre-
lation between ligand basicity and the E1/2 values was observed
for the mononuclear copper(I) analogues with 4-pyridyl substi-
tuted MePY2 ligands. However, more negative E1/2 values
(ꢀ0.31 V for parent MePY2, ꢀ0.36 for MeO-MePY2 (with 4-MeO-
pyridyl groups) and ꢀ0.44 for Me2N–MePY2 (with 4-Me2N-pyridyl
groups)) are observed for the mononuclear copper(I) complexes
compared to those for the corresponding dicopper(I) complexes.
This may be due to the relative electron-donating capacities of
the substituents on the central amine nitrogens, i.e. for the binu-
clear Cu(I)-complexes with xylyl linker, benzyl (PhCH2) groups
are less electron-donating than Me- groups in the mononuclear
copper(I) complexes [CuI(R-MePY2)]+, resulting in more positive
redox potential for the binuclear complexes. However, a smaller
shift (40 mV) was observed for the analogous mononucleating
PhCH2PY2 complex [22] where a Me- group is replaced with a
PhCH2- group. Therefore, the observed more significant difference
3.5. Electrochemistry
The redox potentials for the binuclear copper(I) complexes were
measured by cyclic voltammetry (CV) under an Ar atmosphere in
N,N-dimethylformamide (DMF) solution containing 0.2 M
[Bu4N][PF6]. The potentials, listed in Table 2, are reported versus
the ferrocene/ferrocenium couple. All the Cu(I)-complexes exhibit
single quasi-reversible redox behavior with an ipa/ipc ratio close
to unity. Fig. 2 shows a typical CV scan here for [CuI2(MeOX-
YL)](PF6)2 (1b) in DMF.
As shown in Table 2 and Fig. 3, only a single redox process is ob-
served for these dicopper(I) complexes; no distinctive redox pro-
cesses occur for the two Cu centers. A similar behavior was
observed for analogous copper(I) complexes with either xylyl
[20] or alkyl [20] linkers connecting two PY2 (=bis(2-(2-pyri-
dyl)ethyl)amine) moieties. By contrast, differential pulse voltam-
metric measurements on bis(cyclam)dinickel and -dicopper
complexes possessing variable methylene or xylyl linkers indicate
that electrostatic effects and metal–metal distance as modified by
linker length can influence the occurrence or extent of separation
in CV experiments of the two metal-centered redox processes [21].

144
K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
in redox potentials for the binuclear Cu(I) complexes 1a–d and the
mononuclear Cu(I)-complexes [CuI(R-MePY2)]+ may arise from
some interaction between the two copper ions in the dicopper(I)
complexes, where a nearby positively charged center makes the
oxidation of one of the Cu(I) ions more difficult. In addition, rela-
tively bigger shifts resulting from modifications in ligand elec-
tron-donating ability are observed for the mononuclear copper(I)
complexes (130 mV shift from MePY2 to Me2N–MePY2 complexes)
compared to those for the binuclear copper(I) complexes
[CuI2(L)]2+ (1a–c) (70 mV shift from XYL to Me2NXYL complexes),
described here, again suggesting possible interaction between the
two copper centers in the binuclear complexes. In summary, it
seems that the effect of ligand electron-donating ability on the re-
dox potential of the metal ions in these binuclear complexes is re-
duced due to some (electrostatic) interaction between the two
copper(I) centers.
seems to be least efficient for [CuI2(Me2NXYL)]2+ (1c), where the
unreacted ligand (approximately 40%) as well as the hydroxylated
ligand were recovered after the oxygenation reaction. The lower
hydroxylation efficiency in 1c may arise from competition for an
inter- rather than intra-molecular O2-binding process; this has pre-
viously been suggested in the oxygenation reactions of a series of
analogous binuclear Cu(I)-complexes [7].
3.7. X-ray crystal structure of [CuII MeOXYLOꢀ)(ꢀOH)](PF6)2 (3b)
(
2
Green crystals suitable for X-ray crystallography were obtained
by slow diffusion of diethyl ether into a methylene chloride solu-
tion of 3b. Selected bond distances and angles are tabulated in Ta-
ble 3, along with the corresponding structural parameters for the
parent complex [CuII2(HXYLOꢀ)(ꢀOH)](PF6)2 (3a) previously re-
ported [6a] for comparison. An ORTEP view with atom labeling
scheme is shown in Fig. 4.
Each copper ion is coordinated with three nitrogen atoms from
the tridentate chelate along with the bridging-phenolate (O1) and
hydroxide oxygen (O2) atoms in a slightly distorted square pyra-
midal geometry. The structural parameters are very similar to
those of the parent complex, as shown in Table 3. The N3 and N5
pyridine nitrogen atoms occupy axial positions in an anti confor-
mation with respect to the Cu2O2 plane. These axially coordinated
Cu–NPY distances are 0.20 and 0.19 Å longer than those in the basal
plane. The basal Cu–NPY (N2 and N6) bond lengths average 1.99 Å,
which are ca. 0.2 Å shorter than that in the parent complex
[CuII2(XYLOꢀ)(ꢀOH)](PF6)2 (3a) [6a], possibly due to the greater
electron-donating abilities of the 4-MeO- compared to the unsub-
stituted pyridine nitrogen atoms.
A
small negative shift in the E1/2 value is observed for
[CuI2(UN)]2+ (1d) with the unsymmetrical ligand environment,
compared to that for [CuI2(HXYL)]2+ (1a). Following the arguments
made above, ligand constraints in UN may cause the copper ions to
be held further apart, making it easier to oxidize Cu(I) to Cu(II) in
complex.
Both ligand and structural properties are thus shown to affect
the redox potentials of the corresponding copper(I) complexes.
We previously observed that the difference in electrochemical
behavior of copper(I) complexes with pyridyl and/or quinolyl
ligands tended to correlate with variations in their O2 reactivity
[23]. The kinetic studies described below were carried out to
investigate both ligand electronic and structural effects on the
O2-interactions with the dicopper(I) complexes described here.
3.6. Isolation of the hydroxylation products
Similar to that in the parent complex, the Cu2O2 unit is essen-
tially planar and the phenolate ring is twisted with respect to the
planar Cu2O2 unit. The ligand orientation in these two complexes
show substantial differences from the phenolate- and hydroxy-
bridged binuclear Cu(II) complex with UN ligand, where the
unsymmetrical ligand environment enforces the Cu2O2 unit into
a butterfly shape [16] and the two axial nitrogens engage in a cis
conformation, in contrast to the anti conformation observed in
the two symmetrical complexes (3a and 3b). These structural dif-
ferences have been shown to decrease the antiferromagnetic cou-
pling between the two Cu(II) ions and to affect the O2-adduct
formation and subsequent hydroxylation reactions, based on
bench-top observations and kinetic measurements discussed
below.
As previously described [6a,c,16], the oxygenation of [CuI2(X-
YL)](PF6)2 and [CuI2(UN)](PF6)2 in DMF at room temperature gives
the corresponding phenoxo and hydroxo bridged dicopper(II) com-
plexes
[CuII2(XYLOꢀ)(ꢀOH)](PF6)2
(3a)
and
[CuII2(UN-
Oꢀ)(ꢀOH)](PF6)2 (3d). The stoichiometry of these reactions has
been established by manometric measurements for O2 uptake
(Cu/O2 = 2:1).10 Mass spectrometric analysis of the oxygenated
product of [CuI2(XYL)](PF6)2 (1a) prepared by using isotopically
pure 18O2 shows that both oxygen atoms in the [CuII2(XY-
LOꢀ)(ꢀOH)](PF6)2 complex were incorporated into the final
hydroxylation product.
Similarly, when the DMF solution of [CuI2(MeOXYL)](PF6)2 (1b)
was oxygenated at room temperature, a solution color change from
yellow to dark green was observed. Isolation of the green complex
yielded the hydroxylation product [CuII
(
MeOXYLOꢀ)(ꢀOH)](PF6)2
3.8. Bench-top oxygenation reactions in CH2Cl2 at low temperature
2
(3b) characterized by elemental analysis, X-ray crystallography,
UV–Vis and IR spectroscopies, as well as room-temperature mag-
netic moment measurements. This complex exhibits UV–Vis fea-
When a bench-top low-temperature (193 K) oxygenation of
[CuI2(HXYL)]2+ (1a) is carried out, no intermediate species
[CuII2(HXYL)(O22ꢀ)]2+ could be observed spectroscopically. How-
ever, under the same conditions, the O2-adducts [CuII2(L)(O22ꢀ)]2+
(2b–d, L = MeOXYL, Me2NXYL, UN) with characteristic UV–Vis
absorption features could be detected. The stability of
tures (kmax = 370 nm (e = 3450), 637 (e = 150)), which are similar
to those for complexes 3a and 3d [7,16]. The intense 370 nm
absorption is assigned as an OHꢀ ? Cu(II) ligand-to-metal
charge-transfer band and the weak absorption band at 637 nm is
consistent with a d-d transition. The IR spectrum of complex 3b
[CuII MeOXYL)(O22ꢀ)]2+ (2b) and [CuII Me2NXYL)(O22ꢀ)]2+ (2c) was
(
(
2
2
exhibits a
m
O–H at 3643 cmꢀ1. The room temperature magnetic mo-
still somewhat limited as indicated by the decrease in intensity
of their characteristic electronic absorptions over time (ꢁ5 min).
By contrast, the UN complex [CuII2(UN)(O22ꢀ)]2+ (2d) was much
more stable at 193 K (no intensity decrease over 1 h) [16]. These
bench-top observations manifest the ligand electronic or structural
effects upon copper(I)–dioxygen adducts. However, rapid kinetic
studies were needed to further understand, in a quantitative man-
ner, how the ligand environment affects the O2-interactions of
these dicopper(I) complexes (1b–d), followed by arene hydroxyl-
ation chemistry.
ment of complex 3b is 1.1
lB/Cu, comparable with 0.8 lB/Cu for 3a
[24], which indicates efficient antiferromagnetic coupling between
the two Cu(II) centers through the bridging phenoxo and hydroxo
ligands. Less efficient coupling is observed for the unsymmetrical
complex [CuII2(UN-Oꢀ)(OHꢀ)]2+ (3d) with a room temperature
magnetic moment of 1.5 lB/Cu [16], manifesting the effect of li-
gand constraints in a distorted structure, where less efficient over-
lap of orbitals on the two separate Cu ions occurs. However, under
synthetic conditions in CH2Cl2 solvent, the hydroxylation reaction

K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
145
Table 3
Selected bond distances and angles for 3c and 1c.
[CuI2
(
MeOXYLO–)(–OH))](PF6)2 (3c)
[CuI2(XYLO–)(ꢀOH))](PF6)2 (1c)
Interatomic distances (Å)
Cu1–N1
Cu1–N3
Cu2–N5
Cu1–O1
2.054(8)
2.180(8)
2.190(8)
1.969(6)
1.987(6)
Cu1–N2
Cu2–N4
Cu2–N6
Cu1–O2
Cu2–O2
1.980(8)
2.056(8)
2.003(8)
1.959(6)
1.968(6)
Cu1–N1
Cu1–N3
Cu2–N5
Cu1–O1
Cu2–O1
2.034(14)
2.258(13)
2.149(15)
1.979(10)
1.972(11)
Cu1–N2
Cu2–N4
Cu2–N6
Cu1–O2
Cu2–O2
2.006(16)
2.028(13)
2.027(14)
1.938(10)
1.962(10)
Cu2–O1
Interatomic angles (°)
O1–Cu1–O2
O1–Cu1–N2
O2–Cu1–N1
O2–Cu1–N3
N1–Cu1–N3
O1–Cu2–O2
O1–Cu2–N5
O2–Cu2–N4
O2–Cu2–N6
N4–Cu2–N6
Cu1–O1–Cu2
Cu1–O1-C17
75.6(3)
162.8(3)
159.7(3)
101.5(3)
96.6(3)
74.9(3)
98.3(3)
162.0(3)
92.7(3)
95.8(3)
104.2(3)
128.8(6)
O1–Cu1–N1
O1–Cu1–N3
O2–Cu1–N2
N1–Cu1–N2
N2–Cu1–N3
O1–Cu2–N4
O1–Cu2–N6
O2–Cu2–N5
N4–Cu2–N5
N5–Cu2–N6
Cu1–O2–Cu2
Cu2–O1-C7
93.0(3)
98.4(3)
92.7(3)
94.4(3)
96.2(3)
92.5(3)
161.1(3)
97.9(3)
96.7(3)
97.6(3)
105.3(3)
127.0(6)
O1–Cu1–O2
O1–Cu1–N2
O2–Cu1–N1
O2–Cu1–N3
N1–Cu1–N3
O1–Cu2–O2
O1–Cu2–N5
O2–Cu2–N4
O2–Cu2–N6
N4–Cu2–N6
Cu1–O1–Cu2
Cu1–O1-C17
76.7(4)
O1–Cu1–N1
O1–Cu1–N3
O2–Cu1–N2
N1–Cu1–N2
N2–Cu1–N3
O1–Cu2–N4
O1–Cu2–N6
O2–Cu2–N5
N4–Cu2–N5
N5–Cu2–N6
Cu1–O2–Cu2
Cu2–O1-C17
92.4(5)
97.6(5)
88.8(6)
96.5(6)
102.4(6)
93.9(5)
161.3(5)
98.9(5)
95.6(6)
97.6(6)
104.4(5)
127.1(10)
157.1(6)
161.5(5)
100.0(5)
96.1(5)
76.3(4)
98.3(5)
163.5(5)
91.4(5)
94.4(6)
102.5(5)
129.6(10)
Scheme 6. Kinetic scheme deduced for the [CuI2(RXYL)]2+ (1a-c) oxygenation
reactions employing PY0 donor ligands.
(2b) with the predominant charge-transfer absorption at
kmax = 363 nm (
= 2700), past proven to be a species with the
peroxodicopper(II) core. In a unimolecular process 2b subse-
quently decays and is transformed into [CuII MeOXYLO–)(ꢀOH)]2+
with 370 and 650 nm absorptions. The inset of Fig. 5 shows the
absorbance versus time trace at 363 nm, demonstrating a perfect
correlation between experimental data and the suggested mecha-
nism (Scheme 6).
e
= 14000) and the accompanying band 437 nm
(e
l- :g
g2
2-side-on
(
2
Fig. 4. ORTEP diagram of the cationic portion of [CuII
(
2
MeOXYLO–)(OH)]2+ (3b).
3.9. Stopped-flow kinetic studies
3.9.1. Overall findings
It is found that at low temperature, the formation of the side-on
peroxo species [CuII2(L)(O22ꢀ)]2+ (2) is complete, thus k being
ꢀ1
As mentioned in the Section 1, detailed kinetics studies for
the oxygenation reactions of [CuI(R-XYL)]2+ (R on the central arene
para to the position which is hydroxylated; R = –NO2, –H, –C(CH3)3,
–F) conform to Scheme 1, with the results providing corroborative
evidence for a hydroxylation reaction mechanism involving an
electrophilic peroxo species. Here, in our full investigation and
kinetic analysis, we find that the reactions of the dicopper(I)
complexes [CuI2(L)]2+ (1b–d, L = MeOXYL, Me2NXYL and UN, Fig. 1)
with O2 in dichloromethane follow the same mechanism found
for [CuI2(XYL)]2+ (1a) (Schemes 1 and 6). At low temperature,
[CuI2(L)]2+ (1a–d, L = XYL, MeOXYL, Me2NXYL and UN) binds dioxygen
irrelevant. However, at higher temperatures, k has to be taken
ꢀ1
into account since the intermediate 2 is then only partially formed.
Similar to the previously studied analogous systems [7,25], the
hydroxylation step (k2) is composed of a thermal and of a photo-
chemical temperature-independent term, and the latter dominates
below ꢁꢀ40 °C. Thus, data acquired only at higher temperatures,
where the photochemical terms could be accounted for and sub-
tracted out, were used for determination of the activation param-
eters for k2. An exceptional case was that for the oxygenation
reaction of [CuI2(UN)](PF6)2 (1d), where the photochemistry dom-
inated in the whole temperature range (183–238 K) studied, ini-
tially precluding the determination of the thermal parameters of
k2 for 1d. The problem could be solved however, as discussed
below.
reversibly (k1, kꢀ1) to form a
l- :g
g2 2-(side-on) peroxo species
[CuII2(L)(O22ꢀ)]2+ (2a–d), which subsequently hydroxylates the
arene (k2) to give [CuII2(LO–)(ꢀOH)]2+ (3a–d) products. Fig. 5 shows
the UV–Vis behavior, when, for example, [CuI2(MeOXYL)]2+ (1b) re-
acts with excess O2 at ꢀ50 °C. Upon rapid mixing of solution of O2
The variable-temperature stopped-flow data of the oxygenation
reactions of [CuI2(L)]2+ (1a–d, L = HXYL, MeOXYL, Me2NXYL and UN)
were analyzed by global analysis in the eigenvector space [23]
and 1b, there is very rapid formation of [CuII MeOXYL)(O22ꢀ)]2+
(
2

146
K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
20
40
60
Time (s)
80
100
120
140
300
400
500
600
700
800
Wavelength (nm)
Fig. 5. Time-dependent UV–Vis spectra for the oxygenation of [CuI2
curve, based on the model described in Scheme 6. See text for further details.
(
MeOXYL)]2+ (1b) at ꢀ50 °C. Inset: absorbance vs. time at 363 nm; (x) experimental data; (–) calculated
Table 4
Kinetic parameters for O2-interactions with dicopper(I) complexes 1a–d.
Parameters
k1 (Mꢀ1 sꢀ1
HXYL (1a)a
MeOXYL (1b)
Me2NXYL (1c)
UN (1d)
)
D
D
H– (kJ mol–1
)
8.2 0.1
6.9 0.3
2.3 0.2
12.3 0.2
S– (J Kꢀ1 molꢀ1
)
)
)
ꢀ146
1
ꢀ149
1
ꢀ156
1
ꢀ132.4 0.8
183 K
223 K
298 K
385
5
(6.9 0.2) ꢃ 102
(1.91 0.03) ꢃ 103
(6.5 0.3) ꢃ 103
(6.3 0.2) ꢃ 103
(1.38 0.02) ꢃ 102
(7.23 0.01) ꢃ 102
(5.2 0.2) ꢃ 103
(1.24 0.01) ꢃ 103
(1.01 0.01) ꢃ 104
(1.84 0.06) ꢃ 104
(5.1 0.1) ꢃ 103
k
D
D
(sꢀ1
)
ꢀ1
H– (kJ molꢀ1
)
70
50
1
6
78
76
1
5
71
60
2
8
67
63
1
6
S– (J Kꢀ1 molꢀ1
183 K
223 K
298 K
(1.5 0.4) ꢃ 10ꢀ5
(7.20 0.05) ꢃ 10ꢀ2
(1.3 0.2) ꢃ 103
(2.5 0.7) ꢃ 10ꢀ6
(2.9 0.3) ꢃ 10ꢀ2
(1.4 0.2) ꢃ 103
(3 1) ꢃ 10ꢀ5
(5.4 0.9) ꢃ 10ꢀ4
1.79 0.1
(1.3 0.2) ꢃ 10ꢀ1
(2.8 0.5) ꢃ 103
(2.2 0.5) ꢃ 104
k2 (sꢀ1
)
D
D
H– (kJ molꢀ1
)
50
1
48.9 0.6
54
2
57 1b
S– (J Kꢀ1 molꢀ1
ꢀ35
2
ꢀ47
2
ꢀ39
7
ꢀ26 6b
183 K
223 K
298 K
(3.4 0.2) ꢃ 10ꢀ4
(1.5 0.2) ꢃ 10ꢀ4
(2.1 0.6) ꢃ 10ꢀ5
(1.1 0.2) ꢃ 10ꢀ5 b
(1.04 0.06) ꢃ 10ꢀ2 b
31 8b
(1.49 0.03) ꢃ 10ꢀ1
(6.0 0.2) ꢃ 10ꢀ2
(1.4 0.1) ꢃ 10ꢀ2
172
8
61
3
26
4
a
Previously published results [7].
Data obtained using single wavelength (361 nm) interrogation for the kinetic study.
b
and the kinetic and thermodynamic parameters deduced are listed
in Tables 4 and 5. For the temperature-dependent reactions of
spectroscopically detectable
l
-peroxo dicopper(II) moiety. Similar
low or even negative activation enthalpies (i.e. consistent with a
multi-step process) are observed for the formation of the peroxodi-
copper(II) complexes with a series of binucleating ligands Nn hav-
ing the identical ligand set (i.e. the PY2 tridentate) linked instead
interest (k1, k and k2), plots of the kinetic and thermodynamic
ꢀ1
parameters log(k/T) or log(K/T) are all well behaved. Eyring plots
of k1, k and k2 for the MeOXYL complex are shown in Fig. 6.
ꢀ1
by alkyl chains [26]. In addition, the formation of the l-1,2-(end-
3.9.2. Reversible O2-binding to the dicopper(I) complexes [CuI2(L)]2+
(1)
on)-peroxodicopper(II) complexes with a series of mononucleating
tetradentate ligands exhibit negative activation enthalpies
[12b,23,27] as a result of a similar pre-equilibrium step (formation
of the initial CuI/O2 adduct, i.e. a superoxocopper(II) complex).
By contrast, Tolman, Zuberbühler and co-workers [9a,10a,28],
through detailed stopped-flow kinetic studies, demonstrated that
the formation of the O2-adducts of mononuclear or binuclear cop-
per(I) complexes containing substituted triazacyclononane (tacn)
tridentate ligands is associated with considerable high activation
As seen from Table 4, the binding of O2 (k1) to the dicopper(I)
complexes 1 to form the side-on peroxo species [CuII2(L)(O22ꢀ)]2+
(2) is characterized by very low activation enthalpies of less than
12 kJ molꢀ1 and large compensating activation entropies of ꢀ136
to ꢀ156 J Kꢀ1 molꢀ1. Such observations may be rationalized on
the basis of having reactive, easily oxidizable and coordinatively
unsaturated (three coordinate) cuprous species. An additional rea-
son for the observed low activation enthalpies may arise from the
composite nature for the formation of the peroxodicopper(II) spe-
cies [CuII2(L)]2+ (2) since it is unlikely that O2 would bind to both
copper ions simultaneously. Rather, as described in previous stud-
ies, we suggest that the dioxygen first binds to one copper moiety
to generate the superoxo intermediate (i.e. CuIꢂ ꢂ ꢂCuII–O2ꢀ) in a ra-
pid, left-lying equilibrium and only in a second step forms the
enthalpies (D D
H– around 40 kJ molꢀ1). These relatively large H–
values are comparable to those seen for the formation of Cu–O2
1:1 (i.e. superoxocopper(II) species) adducts in reactions of well-
studied mononuclear copper(I) complexes with TMPA (=tris(2-pyr-
idylmethyl)amine) and its analogs [12b,23]. Thus, it is concluded
that the formation of superoxo intermediates, which are not spec-
troscopically observable in these tacn derivative cases, are the

K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
147
Table 5
Equilibrium parameters for O2-interaction with dicopper(I) complexes 1a–d.
Parameters
HXYL (1a)
MeOXYL (1b)
Me2NXYL (1c)
UN (1d)
K1 (Mꢀ1) (=k1/k
)
ꢀ1
D
D
H° (kJ mol–1
)
ꢀ62
1
6
ꢀ71
2
5
ꢀ69
2
ꢀ55
1
6
S° (J Kꢀ1 molꢀ1
)
ꢀ196
ꢀ225
ꢀ222
9
ꢀ196
183 K
223 K
298 K
2.6 ꢃ 107
1.7 ꢃ 104
3.9
(2.8 0.8) ꢃ 108
(6.6 0.6) ꢃ 104
4.5 0.5
(3 1) ꢃ 108
(2.6 0.4) ꢃ 105
(4.0 0.1) ꢃ 102
(2.4 0.5) ꢃ 10-1
(7.5 0.8) ꢃ 104
6
1
k1
-23
k-1
k
-27
-31
-35
k2
0.0036
0.004
0.0044
0.0048
0.0052
0.0056
1/T (K-1
)
Fig. 6. Eyring plots for k1, kꢀ1, and k2 for [CuI2
4.1 ꢃ 10ꢀ4 M.
(
MeOXYL)]2+ (1b): k, Boltzmann constant; h, Planck constant; [O2] = 1.9 ꢃ 10ꢀ3 M; [1b] (
D
), 1.5 ꢃ 10ꢀ4 M; (O), 3.0 ꢃ 10ꢀ4 M; (1),
rate-determining steps in the oxygenation reactions of the corre-
sponding copper(I) complexes. However, a stopped-flow kinetic
study[12b] on the oxygenation reactions of a series of mononuclear
copper(I) complexes with 4-pyridyl substituted TMPA ligands in
propionitrile show that significant activation enthalpies (29–
32 kJ molꢀ1) associated with the formation of the superoxocop-
per(II) complexes involves the (rapid) dissociation of a coordinated
nitrile in a strongly left-lying equilibrium. In addition, when
weakly coordinating solvents such as tetrahydrofuran (THF) and
acetone are used as solvents, much faster formation of superoxo-
copper(II) species are observed, demonstrating the importance of
the solvent medium, where nitriles, which are excellent strong li-
gands for copper(I) centers [29], can compete with the O2-binding
process.
copper(I) centers with nearly identical coordination environments
(UN versus XYL) would exhibit different intrinsic reactivity
toward O2 (for the formation of superoxo intermediates), again
manifesting the composite nature of these oxygenation reactions.
In contrast to the observed very low activation enthalpies
(<12 kJ molꢀ1
) for k1, large activation enthalpies D
H– (67–
78 kJ molꢀ1) along with positive activation entropies are associ-
ated with the dissociation (kꢀ1) of O2 from Cu2O2 complexes.
Although not dramatic effects, a larger rate constant kꢀ1, as well
as a lower activation enthalpy (associated with kꢀ1) are observed
for the UN complex compared to that for XYL complexes (Ta-
ble 4), suggesting that ligand constraints in [CuII2(UN)(O22ꢀ)]2+
(2d) decreases its copper–dioxygen bonding strength, kꢀ1, repre-
senting the breaking of a Cu–O bond and release of O2. However,
no direct correlation between the ligand electron-donating abil-
More interestingly, for the oxygenation reactions of the
dicopper(I) complexes [CuI2(RXYL)]2+ (1a–c), the rate of dioxygen
binding (k1) increases (Table 4) as the ligand becomes more
electron-donating (i.e. XYL to MeOXYL to Me2NXYL), consistent with
oxygenation being accompanied by electron transfer from
copper(I) to O2 (giving a superoxo or peroxo anion). The activation
ity and the copper–dioxygen binding strength (D
H– for kꢀ1, Ta-
ble 4) is observed. In fact, the MeOXYL complex exhibits a higher
activation enthalpy kꢀ1) compared to that for the parent XYL li-
gand, which is in line with the notion that electron-donating li-
gand somehow increases the Cu–O bond strength in the peroxo
complex 2. Perhaps Cu(II) ? O (peroxide) back-bonding is not
insignificant in [CuII2(L)(O22ꢀ)]2+ (2) complex, as has been dis-
cussed with respect to the electronic structure and bonding in
enthalpy
D D
H– for k1 also exhibits a trend, where H– decreases as
the ligand electron-donating ability increases, again suggesting
more favorable dioxygen binding with strong electron-donating
ligands. Presumed ligand constraints (as seen in X-ray structure
of [CuII2(UN-O–)(ꢀOH)]2+ (3d), discussed above) are also shown to
affect Cu2-O2 binding, where a slower dioxygen binding (i.e.
l- :g
g2 2-(side-on)-peroxodicopper(II) species [30]. Nevertheless,
D
H– and k values are almost identical for the Me2NXYL (most
ꢀ1
electron-donating ligand) and HXYL complexes, exhibiting no ef-
fect of a more electron-donating ligand (Me2NXYL). As mentioned,
it was proposed that there is some interaction between the
smaller k1) and a higher activation enthalpy
D
H– (for k1) are
observed for the copper(I) complex [CuI2(UN)]2+ (1d) with the
unsymmetrical ligand UN (Fig. 1), compared to that for the parent
ligand XYL (Table 4). The observed differences suggest that for
these binuclear copper(I)-complexes, the formation of the super-
oxo intermediates (i.e. initial O2-adduct) is unlikely to be the
rate-determining step since it is hard to conceive that individual
dicopper-bound electrophilic peroxo group and the xylyl
tems in [CuII2(L)(O22ꢀ)]2+ (2) due to their close proximity. Per-
haps this can provide an explanation for the absence of
p sys-
a
correlation between the copper–dioxygen bond strength in the
peroxo complex 2 and the ligand electronic-donating ability.

148
K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
From k1 and kꢀ1, equilibrium constants k1 = k1/k and the cor-
peroxo species [CuII2(L)(O22-)]2+ (2). Ligand electron-donating abil-
ity appears to affect the binding of dioxygen (k1 and
H– for k1) to
ꢀ1
responding thermodynamic parameters
D
H° and
D
S° can be calcu-
D
lated; they are collected in Table 5. Here, again, the thermostability
the copper(I) complexes, where more electron-donating ligands fa-
vor O2-binding with smaller activation enthalpies. However, no di-
rect correlation between the stability of the resulting peroxo
complexes 2 with ligand electronic properties are observed, possi-
(D
H°) of the peroxo complex [CuII2(L)(O22ꢀ)]2+ (2) appears to corre-
late with the effect of ligand constraints, where the unsymmetrical
ligand UN affords a less stable peroxo complex compared to that
with XYL ligand, reflected by their K1 and
D
H° values. Ligand con-
bly due to some interaction between the Cu2O2 core and xylyl
orbitals (as previously mentioned).
p
straints or steric hindrance have well been demonstrated to influ-
ence the stability of copper–dioxygen adducts formed [27,31]. A
complex possessing a binucleating analogue (DO, Fig. 7) of TMPA
with –CH2OCH2– linked pyridyl groups exhibits significant stabil-
ity in acetone and in fact, the thermodynamic parameters could
not be obtained (i.e. since its formation is essentially complete at
all temperatures) [31a]. By contrast, an analogous ligand (D1,
Fig. 7) with –CH2CH2– linker leads to a strained and much less sta-
ble dioxygen adduct. In addition, in the oxygenation reactions of a
series of binuclear copper(I)-complexes (Nn, Fig. 7) containing the
same tridentate PY2 units [26], the length of the alkyl chain (i.e.
–(CH2)n–) connecting the two PY2 units appears to have a remark-
able effect on the equilibrium constants K1 for the formation of the
3.9.3. Arene hydroxylation step k2
As shown in Fig. 5, [CuII MeOXYL)(O22ꢀ)]2+ (2b) as well as the
(
2
other side-on peroxo species [CuII2(L)(O22ꢀ)]2+ (2) are not stable
(except the UN complex 2d, vide infra) even at low temperature,
as reflected by the disappearance of their UV–Vis features, and
their subsequent decay leads to arene hydroxylation (Schemes 1
and 6). However, bench-top observations mentioned above clearly
indicate that complex 2d is much more stable (i.e. longer-lived by
bench-top UV–Vis monitoring) than the other peroxo complexes
with XYL or substituted XYL ligands (MeOXYL and Me2NXYL). This
suggests that subsequent hydroxylation reaction is less favorable
with the UN ligand, which possesses an unsymmetrical and
strained ligand environment. A nearly ideal orientation of the
arene substrate and the peroxo core in [CuII2(R-XYL)(O22ꢀ)]2+ com-
plexes (Schemes 1 and 6) are proposed for these facile hydroxyl-
ation reactions [6b,27]. Thus, possible disruption of this desirable
orientation caused by ligand constraints of the unsymmetrical
UN ligand inhibits the occurrence of a fast arene hydroxylation
reaction which otherwise might be expected for 2d at low temper-
ature (<193 K). However, at higher temperatures, 2d is not stable
and it does decay, leading to the formation of the well-character-
ized hydroxylation product [CuII2(UN-O–)(ꢀOH)]2+ (3d) [16].
As mentioned, the experimental methods used for most of the
kinetic studies leads to dominating photochemistry relating to
the hydroxylation step (k2) for [CuII2(UN)(O22ꢀ)]2+ (2d); the diode
array spectrometer inputs rather intense light over the whole spec-
tral region. As copper–dioxygen complexes in general posssess
very strong peroxo-to-Cu(II) charge-transfer absorptions, it is not
too surprising that photochemical reactions would exist. In fact,
metal–dioxygen complex photochemistry in general has and does
attract general interest [33], and Cun(O2)]n+ complexes are candi-
dates for future studies. We found that the way around the photo-
chemistry issue for the UN system was to go to single-wavelength
studies. Thus, Fig. 8 shows data obtained via 361 nm monitoring of
the [CuI2(UN)]2+ (1d) + O2 reaction. From such data, valid kinetic
parameters were obtained for k2 (Table 4). Fig. 9 shows an Eyring
plot for k2 avoiding photochemistry and a comparison to the data
obtained when using the diode array spectrometer.
l- :g . The
g2 2-peroxodicopper(II) complexes [CuII(Nn)(O22ꢀ)]2+
[CuI2(N5)]2+ complex with the longest alkyl linker (–(CH2)5–) af-
fords the largest K1 value (i.e. most stable peroxo complex), which
is in accord with the view that the N5 complex forms the most
favorable Cu2O2 structure due to the relatively unconstrained nat-
ure of the ligand. This is consistent with a resonance Raman study
[32] of Cu2O2 complexes with Nn ligands, where the N5 complex
exhibits the lowest m(O–O) value (due to enhance back donation
form Cu to the peroxo r⁄ orbital within the planar Cu2O2 core),
closest to that for the unconstrained mononuclear Cu-MePY2 com-
plex, [{CuII(MePY2)}2( g2 2-O22ꢀ)]2+. Comparing to these Nn li-
l- :g
gands, the RXYL and UN ligands are more constrained because of
the bulky and less flexible xylyl groups connecting the PY2 triden-
tate moieties. Thus, the overall reaction enthalpies
DH° for the XYL
and UN systems are considerably less negative than those with Nn
ligands, ꢀ56 kJ molꢀ1 for UN, ꢀ62 to ꢀ71 kJ molꢀ1 for RXYL, ꢀ81 to
ꢀ84 kJ molꢀ1 for Nn, all forming dicopper complexes with the
[CuII g2 2-O22ꢀ)]2+ core.
(l- :g
2
The substituted XYL ligands (MeOXYL and Me2NXYL) afford more
stable peroxo complexes compared to that with the parent XYL li-
gand (i.e. more negative
DH° and bigger K1, Table 5). However, the
peroxo complexes with MeOXYL and Me2NXYL ligands exhibit nearly
identical thermostabilities (Table 5), again reflecting no direct cor-
relation between the stability of the peroxo species 2 and the li-
gand electron-donating ability, as said before possibly due to
interaction between the peroxo core and
group.
p
orbitals of xylyl linker
In summary, the thermodynamic parameters (
D
H° and S°)
D
In line with the benchtop spectroscopic findings for [CuI2(UN)]2+
(1d) oxygenation mentioned above, the k2 kinetic parameters re-
veal that indeed, the rate of the arene hydroxylation reaction,
[CuII2(UN)(O22ꢀ)]2+ (2d) ? [CuII2(UNO–)(ꢀOH)]2+ (3d), is less than
clearly indicate that the dioxygen binding to these dicopper(I)
complexes [CuI2(L)]2+ (1) is entirely driven by enthalpy, while large
negative entropies result in their room-temperature instability. Li-
gand constraints are shown to have a significant effect on the diox-
ygen binding (k1) as well as the stability (K1,
DH°) of the resulting
Fig. 8. Time-dependent spectra and 361 nm monitoring of the [CuI2(UN)]2+
(1d) + O2 reaction in CH2Cl2 at 183 K.
Fig. 7. Dinucleating ligands used in other CuI2/O2 kinetic studies.

K.D. Karlin et al. / Inorganica Chimica Acta 389 (2012) 138–150
149
the peroxo core and arene substrate present in the symmetrical li-
gand systems.
4. Conclusions
The effect of ligand electronic variations on the dioxygen bind-
ing and the arene hydroxylation reactions of the resulting Cu2O2
species has been achieved by incorporating 4-pyridyl substituents
in the extensively studied XYL system. The detailed kinetic/ther-
modynamic studies of the oxygenation reactions of the corre-
sponding copper(I) complexes with substituted XYL ligands
reveal that the more electron-donating ligand favors O2-binding
to form the peroxodicopper(II) complexes [CuII2(L)(O22ꢀ)]2+ (2).
However, no direct correlation between the ligand electron-donat-
ing ability and the stability of the peroxo complexes is observed,
which is rationalized by some interaction between the Cu2O2 core
Fig. 9. Eyring plots on data obtained for the hydroxylation reaction (k2) of
[CuII2(UN)(O22ꢀ)]2+ (2d) using either the diode array spectrometer (red) or single
wavelength monitoring (blue; lower straight line). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
and the xylyl p-orbitals. Less favorable arene hydroxylation of the
peroxo complexes with strong electron-donating ligands is consis-
tent with the proposed reaction mechanism involving electrophilic
attack of the peroxo moiety on the xylyl ring.
Ligand structural effects are also investigated utilizing the anal-
ogous but unsymmetrical UN ligand. The kinetic and thermody-
namic data show that the ligand constraints have significant
effect on the formation, stability and reactivity of the peroxo com-
plex. A significantly slower thermal arene hydroxylation reaction is
observed for the UN complex compared with the XYL complex,
suggesting disruption of the ideal positioning of the peroxo core to-
wards the arene substrate.
The present detailed systematic investigations provide a wealth
of new chemistry and information concerning how ligand elec-
tronic and structural elements affect copper–dioxygen interactions
as well as C–H activation, i.e. biomimetic hydroxylation mediated
by copper–dioxygen chemistry. Such fundamental studies help to
unravel the detailed nature of copper–dioxygen reactivity and pro-
vide a basis for a deeper understanding of relevant biological sys-
tems as well as potential practical applications for copper
mediated oxidative chemistry.
is found for the other RXYL dicopper complex analogues. This espe-
cially obvious if one compares k2 values and corresponding activa-
tion parameters with the hydroxylation reaction for the parent
complex [CuII2(HXYL)(O22ꢀ)]2+ (Table 4). For example, at 223 K, k2
is 15 times smaller for the UN system. While not a huge difference,
it is clearly sufficient to have allowed facile bench-top UV–Vis
spectroscopic monitoring of 2d at 183 K (vide supra) [16], while
[CuII2(HXYL)(O22ꢀ)]2+ (2a) cannot be detected. As discussed, the
slowed arene hydroxlyation rate for [CuII2(UN)(O22ꢀ)]2+ (2d)
undoubtedly reflects a less than ideal proximity or orientation of
the complex’s electrophilic peroxo group toward the arene
system.
p-
Ligand substituents also affect the arene hydroxylation reac-
tions of the peroxo complexes [CuII2(L)(O22ꢀ)]2+ (2). As seen in Ta-
ble 4, k2 decreases with increasing ligand electron-donating ability.
This is consistent with the proposed reaction mechanism involving
an electrophilic peroxo group. Strong electron-donating groups
provide more electron density to the peroxo core through bonding
to the copper centers, which should disfavor the electrophilic at-
tack of the peroxo species. The enhanced electrophilic nature of a
Acknowledgments
The authors acknowledge the research support from the USA
National Insitutes of Health Grant GM28962 (K.D.K.) and the Swiss
National Science Foundation (A.D.Z.).
l- :g
g2 2-peroxo group relative to an end-on peroxo-dicopper(II)
structure (see Scheme 2) is supported by theoretical studies
[30,34], where the peroxide has two antibonding interactions with
each of the two copper ions, resulting in a much less negative per-
Appendix A. Supplementary material
oxide due to extremely large
r-donor interactions between perox-
CCDC 867041 contains the supplementary crystallographic data
ide and copper. The electrophilic behavior of the side-on peroxo
moiety has also been demonstrated in a detailed study comparing
reactivity patterns of the side-on versus end-on peroxo dicop-
per(II) species [35]. However, theoretical calculations on the elec-
tronic structure of side-on peroxodicopper(II) moieties show that
the bonding interaction between the Cu dx2–y2 orbitals and the
for complex [CuII MeOXYLO–)(ꢀOH)](PF6)2 (3b). These data can be
(
2
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2012.01.042.
O2 r⁄ orbital facilitate the back-donation of electron-density
2ꢀ
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