Synthesis, characterization and catalytic activity of copper(ii) complexes containing a redox-active benzoxazole iminosemiquinone ligand

Article abstract of DOI:10.1039/c3dt00004d

A tridentate benzoxazole-containing aminophenol ligand HL^BAP was synthesized and complexed with Cu^II. The resulting Cu^II complexes were characterized by X-ray, IR, UV-vis-NIR spectroscopies, and magnetic susceptibility studies, demonstrating that the ligand is oxidized to the o-iminosemiquinone form [L^BIS]^- in the isolated complexes. L^BISCu^IICl exhibits a distorted tetrahedral geometry, while L^BISCu^IIOAc is square pyramidal. In both solid state structures the ligand is coordinated to Cu^IIvia the benzoxazole, as well as the nitrogen and oxygen atoms from the o-iminosemiquinone moiety. The chloride, or acetate group occupies the fourth and/or fifth positions in L^BISCu^IICl and L ^BISCu^IIOAc, respectively. Magnetic susceptibility measurements indicate that both complexes are diamagnetic due to antiferromagnetic coupling between the d⁹ Cu^II centre and iminosemiquinone ligand radical. Electrochemical studies of the complexes demonstrate both a quasi-reversible reduction and oxidation process for the Cu complexes. While L^BISCu^IIX (X = Cl) is EPR-silent, chemical oxidation affords a species with an EPR signal consistent with ligand oxidation to form a d⁹ Cu^II iminoquinone species. In addition, chemical reduction results in a Cu^II centre most likely bound to an amidophenoxide. Mild and efficient oxidation of alcohol substrates to the corresponding aldehydes was achieved with molecular oxygen as the oxidant and L^BISCu^IIX-Cs₂CO₃ as the catalyst. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt00004d

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Synthesis, characterization and catalytic activity of
copper(II) complexes containing a redox-active
benzoxazole iminosemiquinone ligand†
Cite this: Dalton Trans., 2013, 42, 6829
S. Esmael Balaghi,a Elham Safaei,*a Linus Chiang,b Edwin W. Y. Wong,b Didier Savard,b
Ryan M. Clarkeb and Tim Storr*b
A tridentate benzoxazole-containing aminophenol ligand HLBAP was synthesized and complexed with
CuII. The resulting CuII complexes were characterized by X-ray, IR, UV-vis-NIR spectroscopies, and magnetic
susceptibility studies, demonstrating that the ligand is oxidized to the o-iminosemiquinone form [L ]
BIS − in
the isolated complexes. LBISCuIICl exhibits a distorted tetrahedral geometry, while LBISCuIIOAc is square
pyramidal. In both solid state structures the ligand is coordinated to CuII via the benzoxazole, as well as
the nitrogen and oxygen atoms from the o-iminosemiquinone moiety. The chloride, or acetate group
occupies the fourth and/or fifth positions in LBISCuIICl and LBISCuIIOAc, respectively. Magnetic suscepti-
bility measurements indicate that both complexes are diamagnetic due to antiferromagnetic coupling
between the d9 CuII centre and iminosemiquinone ligand radical. Electrochemical studies of the com-
plexes demonstrate both a quasi-reversible reduction and oxidation process for the Cu complexes. While
L
BISCuIIX (X = Cl) is EPR-silent, chemical oxidation affords a species with an EPR signal consistent with
Received 1st January 2013,
ligand oxidation to form a d9 CuII iminoquinone species. In addition, chemical reduction results in a CuII
centre most likely bound to an amidophenoxide. Mild and efficient oxidation of alcohol substrates to the
corresponding aldehydes was achieved with molecular oxygen as the oxidant and LBISCuIIX–Cs2CO3 as
the catalyst.
Accepted 20th February 2013
DOI: 10.1039/c3dt00004d
www.rsc.org/dalton
coordinated to a CuII center. This enzyme has inspired
researchers to design and study small-molecule models
Introduction
Small molecule complexes employing redox active transition- capable of efficient alcohol oxidation under mild
metal ions and pro-radical ligands provide significant insight conditions.15–21 Copper complexes employing non-innocent
into the complexity of a number of metalloenzyme pro- catecholate and semiquinone ligands have also been identified
cesses.1,2 Researchers have harnessed the redox activity of tyro- as exhibiting GOase-like reactivity (Scheme 1).22
sine ligands to develop catalysts for multi-electron
catalysis.3–14 The metalloenzyme galactose oxidase (GOase) is
an important example, which catalyzes the oxidation of
D-galactose and a wide range of primary alcohols with conco-
mitant reduction of dioxygen to hydrogen peroxide.2,15 The
GOase active site contains a cysteine-modified phenol moiety
Tyr 272 (Y272) and a phenol group of tyrosine 495 (Y495
)
aInstitute for Advanced Studies in Basic Sciences (IASBS), 45137-66731 Zanjan, Iran.
E-mail: safaei@iasbs.ac.ir
bDepartment of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby,
BC V5A 1S6, Canada. E-mail: tim_storr@sfu.ca
†Electronic supplementary information (ESI) available: X-ray crystallographic
data in CIF format, DFT-computed metrical parameters, cyclic voltammograms,
and 1H NMR spectra. CCDC 899392 (LBISCuIICl) and 899393 (LBISCuIIOAc).
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt00004d
Scheme 1 Redox states of GOase.22
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6829–6839 | 6829

View Article Online
Paper
Dalton Transactions
Pt counter electrode with 0.1 M NBu4ClO4 solutions in CH2Cl2.
Ferrocene was used as an internal standard. Mass spectra
(positive ion) were obtained on an Agilent 6210 TOF ESI-MS
instrument. The magnetic measurements were obtained
with the use of a Quantum Design SQUID magnetometer
MPMS-XL between 1.8 and 290 K with a dc applied field of
1000 Oe. Measurements were performed on polycrystalline
samples of 23.86 and 35.21 mg for LBISCuIICl and
LBISCuIIOAc, respectively. The data sets were corrected for the
diamagnetic response in the samples and sample holders. All
EPR spectra were collected using a Bruker EMXplus spec-
trometer operating with a premiumX X-band (∼9.5 GHz)
microwave bridge. Low temperature measurements of frozen
solutions used a Bruker helium temperature-control system
and a continuous flow cryostat. Samples for X-band measure-
ments were placed in 4 mm outer-diameter sample tubes with
sample volumes of ∼300 μL. EPR spectra were simulated with
EasySpin 3.1.7 software.58
Scheme 2 The structure of HLBAP
.
Significant efforts have been made towards the synthesis of
CuII complexes containing redox active ligands.23–25 These
complexes have provided a platform to study both the mag-
netic interaction between the d9 CuII center and the radical
ligand Tyr 272 (Y272), and the role of the radical ligand in cata-
lysis. One of the most common classes of redox active ligands
are based on o-aminophenols. Metal complexes of this ligand
have been studied in the amidophenoxide, iminosemiquinone,
and iminoquinone redox states, and certain compounds have
shown promise as catalysts.26–31 Specifically, copper complexes
of this ligand act as catalysts for alcohol oxidation, closely fol-
lowing the reactivity of GOase. Thus, these metal complexes
can serve as models of metalloenzymes such as
GOase.28,29,32–34 In addition, metal complexes of phenoxazines
and benzoxazoles have proven very useful as catalysts for allylic
substitutions, Heck reactions, hydrosilylations, cyclopropana-
tions and Diels–Alder additions.35–56 Herein, we report the syn-
thesis, characterization, magnetic and redox properties of
LBISCuIIOAc and LBISCuIICl complexes of the newly synthesized
Synthesis
HLBAP. To a solution of 3,5-DTBQ (0.44 g, 2 mmol) in aceto-
nitrile (9 mL) was added 2-aminobenzyl amine (0.122 g,
1 mmol), and the reaction stirred for 4 days at room tempera-
ture in the presence of air. The solvent was removed in vacuo,
and the green material formed was dissolved in methanol–
dichloromethane (1 : 1). After evaporation of the solvent, the
brown precipitate was washed with ethanol and dried under
vacuum. X-ray quality colorless crystals were grown from a 1 : 1
solvent mixture of dichloromethane–ethanol. Yield: 0.225 g
(40%). Anal. calcd (found) for C35H48N2O2: C 79.81 (79.67),
H 8.80 (8.61), N 5.32 (5.39). νmax(KBr)/cm−1: 3415 (O–H), 3258
(N–H), 2959 (C–H), 1595 (CvC), 1474 (CvN), 1047 (C–N), 740
[N2O]-donor benzoxazole ligand (HLBAP
addition, we describe the mild and efficient oxidation of alco-
hols to aldehydes with oxygen as the oxidant and LBISCuIIX–
Cs2CO3 as the catalyst-base.
, Scheme 2). In
1
(vC–H bending). H NMR (400 MHz, CDCl3, 298 K): δ 1.32 (s,
9H), 1.43 (s, 9H), 1.50 (s, 9H), 1.61 (s, 9H), 5.33 (s, 1H), 6.34 (s,
1H), 6.69 (d, 1H), 6.94 (t, 1H), 7.13 (d, 1H), 7.31 (s, 1H), 7.34 (t,
1H), 7.62 (d, 1H), 8.20 (t, 1H), 9.66 (s, 1H). 13C{H} NMR
(100 MHz, CDCl3): δ 29.60, 30.08, 30.96, 31.67, 31.89, 34.44,
34.55, 35.13, 110.52, 113.76, 114.40, 117.80, 119.53, 122.36,
Experimental section
Materials and methods
All chemicals and solvents were purchased from Merck and 122.64, 126.68, 128.49, 132.38, 133.55, 135.46, 141.58, 142.32,
used without further purification. Manipulations were per- 145.40, 147.78, 147.83, 149.56, 162.34. m.p. 139–141 °C.
formed under aerobic conditions using chemicals and solvents
LBISCuIIOAc. HLBAP (0.525 g, 1 mmol) and triethylamine
as received. 3,5-DTBQ (3,5-di-tert-butylcyclohexa-3,5-diene-1,2- (0.28 mL, 2 mmol) were dissolved in a 2 : 1 acetonitrile–
dione) was synthesized according to a modified literature pro- dichloromethane mixture (45 mL), and then Cu(OAc)2 (0.199 g,
cedure.57 Electronic spectra were obtained on a Cary 5000 1 mmol) was added. The reaction mixture was stirred for
spectrophotometer with a custom-designed immersible fiber- 4 hours at room temperature in the presence of air. X-ray
optic quartz probe with variable path length (1 and 10 mm; quality blue crystals were grown from a 1 : 1 solvent mixture of
Hellma, Inc.). Constant temperatures were maintained by a dry dichloromethane–methanol. Yield: 0.70 g (96%). Anal. calcd
ice/acetone bath. Solvent contraction was accounted for in all (found) for C38H52CuN3O4·CH3CN: C 68.65 (68.34), H 7.32
variable-temperature studies. IR spectra were recorded in the (7.25), N 4.33 (4.38). νmax(KBr)/cm−1: 2959 (C–H), 1617 (CvC),
solid state on a FT-IR Bruker Vector 22 spectrophotometer in 1475 (CvN), 1027 (C–N), 802 (vC–H bending). 1H NMR
the 400–4000 cm−1 range. NMR spectra were recorded at (400 MHz, CDCl3, 298 K): δ 1.68 (s, 10H), 1.57 (s, 8H), 1.56 (s,
400 MHz on a Bruker DRX spectrometer in CDCl3 solution. 10H), 1.43 (s, 9H) 2.25 (m, 2H), 2.66 (s, 3H), 3.88 (d, 2H), 5.37
The chemical shifts were referred to TMS using the residual (t, 2H), 6.76 (s, 1H), 7.26 (d, 1H), 8.15 (s, 1H), 9.43 (d, 1H),
signals from the solvent. Cyclic voltammetry (CV) was per- 10.15 (s, 1H).
formed on a PAR-263A potentiometer, equipped with a Ag wire
reference electrode, a glassy carbon working electrode, and a (0.28 mL, 2 mmol) were dissolved in a 2 : 1 acetonitrile–
LBISCuIICl. HLBAP (0.525 g, 1 mmol) and triethylamine
6830 | Dalton Trans., 2013, 42, 6829–6839
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
dichloromethane mixture (45 mL), and then CuCl2·2H2O X-ray analysis
(0.17 g, 1 mmol) was added. The reaction mixture was stirred
Single crystal X-ray crystallographic analysis was performed on
a Bruker SMART diffractometer equipped with an APEX II CCD
area detector fixed at a distance of 6.0 cm from the crystal and
a Mo Kα fine focus sealed tube (λ = 0.71073 nm) operated at
1.5 kW (50 kV, 30 mA) and filtered with a graphite monochro-
mator. Temperature was regulated using an Oxford Cryo-
systems Cryostream. Suitable crystals were mounted on glass
fibers using epoxy as adhesive. The structures were solved
using direct methods (SIR92)67 and refined by least-squares
procedures using CRYSTALS.68 Hydrogen atoms were placed in
idealized geometric positions and linked to their respective
atoms using a riding model during refinement. The isotropic
temperature factor of each hydrogen atom was initially set to
1.2 times that of the atom it is bonded to and then the temp-
erature factors of groups of similar hydrogen atoms were
linked during refinement. The tert-butyl groups in LBISCuIIOAc
were disordered and modeled accordingly (see CIF file for
details). Due to twinning issues, the data for HLBAP could not
be refined satisfactorily and so the resulting structure was only
for 4 hours at room temperature in the presence of air. X-ray
quality blue crystals were grown from a 1 : 1 solvent mixture of
dichloromethane–methanol. Yield: 0.50 g (72%). Anal. calcd
(found) for C35H44ClCuN2O2: C 66.84 (66.16), H 6.21 (6.13),
N 11.14 (10.80). νmax(KBr)/cm−1: 2960 (C–H), 1627 (CvC), 1473
1
(CvN), 1031 (C–N), 807 (vC–H bending). H NMR (400 MHz,
CDCl3, 298 K): δ 1.15 (s, 9H), 1.43 (s, 9H), 1.51 (s, 19H), 7.10 (s,
1H), 7.38 (s, 1H), 7.62 (q, 2H), 7.92 (d, 1H), 8.25 (s, 2H), 8.32
(s, 1H).
Spectrophotometric chemical redox titrations
Oxidation: under a nitrogen atmosphere at 195 K a 150 μL
aliquot of a CH2Cl2 solution of LBISCuIICl (31 mM) was added
to 3.0 mL CH2Cl2. Monitored by UV-vis-NIR, a solution of
[N(C6H3Br2)3˙]+SbF6− in CH2Cl2 (2.3 mM) was added in 250 μL
increments resulting in clean conversion to [LBIQCuIICl]+.
Reduction: under a nitrogen atmosphere at 195 K a 100 μL
aliquot of a CH2Cl2 solution of LBISCuCl (31 mM) was added to
3.0 mL of PhMe. Monitored by UV-vis-NIR, a solution of CoCp2
in 30 : 1 PhMe : CH2Cl2 (10 mM) was added in 100 μL incre-
ments resulting in clean conversion to [LBAPCuCl]−.
used to determine connectivity. Diagrams of LBISCuIICl, LBIS
-
CuIIOAc, and HLBAP were generated by ORTEP-3 for Windows
(v. 2.00)69 and rendered using POV-Ray (v. 3.6.1).70 For the dis-
ordered atoms in LBISCuIIOAc only the atoms with the higher
occupancy are shown. The acetonitrile solvent molecule in
LBISCuIIOAc has not been shown.
Catalytic activity of LBISCuCl and LBISCuIIOAc complexes in
alcohol oxidation
Alcohol oxidation experiments were carried out in an oxygen
atmosphere at room temperature. In a typical experiment,
alcohol (1 mmol), LBISCuIICl (0.024 g, 4 mol%) or LBISCuIIOAc
(0.026 g, 4 mol%) and Cs2CO3 (0.650 g, 2 mmol) were stirred
in 5 mL of oxygen saturated toluene in a 15 mL two-neck
round-bottom flask equipped with an oxygen balloon. The
solution was stirred for 6–36 hours, after which the mixture
was passed through a syringe filter and was diluted with
toluene (2 mL). The progress of the reaction was monitored by
gas chromatography. To study the effect of the ligand, a 15 mL
two-neck round-bottom flask was charged with CuCl2 or
Cu(OAc)2 and Cs2CO3 (0.650 g, 2 mmol) in 5 mL of oxygen
saturated toluene. The flask was equipped with an oxygen
balloon, and the progress of the reaction was monitored by gas
chromatography. To check for the formation of carboxylic acid
during the oxidation reaction, the reaction mixture was neu-
tralized with 5% HCl to remove any excess base. After passing
through a syringe filter, the sample was dried with sodium
sulfate and monitored by gas chromatography.
Results and discussion
Synthesis and structural characterization of ligand HLBAP
The ligand HLBAP was synthesized via the condensation of
2-aminobenzylamine with two equivalents of 3,5-DTBQ. HLBAP
was characterized by 1H and 13C NMR spectroscopy (ESI
Fig. S1 and S2†), IR, elemental analysis, mass spectrometry
and X-ray crystallography. The free ligand HLBAP exhibits
characteristic IR bands at 1511–1542 cm−1 (νCvN stretch), and
3416 cm−1 (νO–H stretch). Single crystals of HLBAP suitable for
X-ray crystallography were obtained by recrystallization from
dichloromethane/n-hexane. An ORTEP view of the HLBAP
ligand is shown in Fig. 1. X-ray analysis shows that the benzox-
azole heterocycle forms during the synthesis of the ligand. In
Calculations
Geometry optimizations were performed using the Gaussian
09 program (Revision A.02),59 the B3LYP60,61 functional, and
the 6-31G(d) basis set on all atoms. Broken-symmetry62–64 (BS)
DFT calculations were performed with the same functional
and basis sets. Frequency calculations at the same level of
theory confirmed that the optimized structures were located at
a minimum on the potential energy surface. Single point cal-
culations were performed using the B3LYP60,61 functional and
the TZVP basis set of Ahlrichs65,66 on all atoms.
Fig. 1 Molecular structure of HLBAP. H atoms have been omitted for clarity.
Thermal ellipsoids are set at 50% probability.71
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6829–6839 | 6831

View Article Online
Paper
Dalton Transactions
Table 2 Comparison of selected bond lengths (Å) in LBISCuIIOAc and LBISCuIICl
addition, a redox-active aminophenolate function is present.
Due to the high R-value for the structure of HLBAP we were
unable to determine the aminophenolate ring bond lengths.
LBISCuCl
LBISCuOAc
1
However, a H NMR titration of HLBAP using NaOD is consist-
Cu1–N1
Cu1–N2
Cu1–O1
Cu1–Cl1
C1–C2
C1–O1
C2–C3
C3–C4
C4–C5
C5–C6
C1–C6
C14–C15
C15–C16
C16–C17
C17–C18
C18–C19
C14–C19
1.949(2)
1.962(8)
2.006(2)
2.202(2)
1.449(3)
1.273(2)
1.368(3)
1.444(3)
1.361(3)
1.425(3)
1.460(3)
1.396(3)
1.394(3)
1.422(3)
1.388(3)
1.391(3)
1.386(3)
Cu1–N1
Cu1–N2
Cu1–O1
Cu1–O101
Cu1–C100
C22–C23
C22–O1
C23–C24
C24–C25
C25–C26
C26–C27
C22–C27
C8–C9
1.929(0)
1.957(1)
1.965(8)
1.908(8)
2.695(3)
1.442(3)
1.275(3)
1.362(3)
1.435(3)
1.359(3)
1.414(3)
1.456(3)
1.386(3)
1.383(3)
1.411(3)
1.384(3)
1.379(3)
1.381(3)
ent with phenolate deprotonation as the pH is raised (ESI
Fig.S3†). In addition, the 1H NMR data is consistent with a
recent report on a similar ligand system.54
Synthesis and structural characterization of LBISCuIIX
complexes
Treating HLBAP with CuCl2 or Cu(OAc)2 in the presence of 2
equivalents of Et3N under aerobic conditions in acetonitrile at
298 K afforded a microcrystalline precipitate of LBISCuIICl and
LBISCuIIOAc, respectively. Elemental analytical data of the com-
plexes corresponds to the calculated values for a 1 : 1 metal :
ligand ratio. In the IR spectra of the complexes, the strong and
sharp νO–H stretch of the ligand disappears, and the νCvN
band is shifted to lower energy by 10–20 cm−1 as compared to
the νCvN band of the free ligand. The monomeric complexes
isolated herein differ from the dimeric CuII complex of a
similar aminophenol ligand recently reported.72 The increased
steric bulk of the [LBIS]− ligand used in this work likely restricts
dimerization.
C9–C10
C10–C11
C11–C12
C12–C13
C8–C13
X-ray analysis
Blue crystals of the two copper complexes for X-ray analysis were
obtained from a methanol–dichloromethane (1 : 2) mixture.
Crystallographic data for both complexes are shown in Table 1.
Selected bond distances and angles are listed in Table 2.
Fig. 2 Molecular structure of LBISCuIICl. H atoms have been omitted for clarity.
Crystal structure of LBISCuIICl
Thermal ellipsoids are set at 50% probability.
The solid state structure of LBISCuIICl displays a distorted tetra-
hedral geometry (Fig. 2). The dihedral angle between the
planes defined by N[1]–Cu[1]–O[1] and N[2]–Cu[1]–Cl[1] is
50.6° indicating a significant twist in the structure. The CuII
ion is four-coordinate, bound to the tridentate [LBIS −
] ligand,
and one chloride ion. The coordination sphere Cu–N distance
is 1.949 Å for Cu[1]–N[1] and 1.963 Å for Cu[1]–N[2], compar-
able with those reported in the literature.73,74 The Cu[1]–Cl[1]
and Cu[1]–O[1] bonds are 2.202 Å and 2.006 Å respectively. Of
immediate interest is the relatively long Cu[1]–O[1] bond.
Further analysis of the ring bond lengths for the aminopheno-
late function allowed for the assignment of the iminosemi-
Table 1 Crystallographic data for LBISCuIIOAc, and LBISCuIICl
LBISCuIIOAc
LBISCuIICl
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C
39H50CuN3O4
C35H44ClCuN2O2
623.74
Monoclinic
P21/n1
16.4069(18)
12.3090(14)
17.1757(19)
90
quinone redox-state for the [LBIS −
ligand (Table 2). The
]
688.39
Monoclinic
P21/c
12.5963(12)
16.3407(16)
18.3053(18)
90
relatively short C[2]–C[3] and C[4]–C[5] bond lengths (1.36 Å
average) in comparison to the longer C[1]–C[2] and C[5]–C[6]
bond lengths (1.44 Å average), and the short C[1]–O[1] bond
length (1.273 Å) are further evidence for iminosemiquinone
formation. The metrical parameters for LBISCuIICl also match
other reports of transition metal iminosemiquinone
complexes.75–77
β (°)
98.1790(10)
90
3729.5(6)
4
110.1470(10)
90
3256.4(6)
4
γ (°)
V (Å3)
Z
T (K)
298
1.226
0.627
44 491
6148
0.0417
0.0440
1.0332
150
1.272
0.785
35 679
7055
0.0413
0.0624
1.1308
ρcalcd (g cm−3
)
Crystal structure of LBISCuIIOAc
μ (mm−1
Reflections collected
Significant reflections
R [I > 2.5σ(I)]
Rw [I > 2.5σ(I)]
Goodness of fit
)
The structure of LBISCuIIOAc (Fig. 3) is composed of a copper
center ligated by the [LBIS −
ligand and a bidentate acetate
]
molecule. The distortion parameter τ = [(Φ1 − Φ2)/60], in
which Φ1 and Φ2 are the two largest L–M–L angles of the
6832 | Dalton Trans., 2013, 42, 6829–6839
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
with the iminosemiquinone π singly occupied molecular
orbital.
Electrochemistry
The electrochemical properties of the HLBAP ligand and its
copper complexes were examined by cyclic voltammetry (CV) in
CH2Cl2 at low temperature (233 K). The ligand HLBAP displays
a series of irreversible oxidations (ESI Fig. S6†) under the
experimental conditions. In comparison, both Cu complexes
exhibit two quasi-reversible one-electron redox processes in
the range studied (Fig. 4 and Table 3).
The redox process at negative potentials occurs at essen-
tially the same value for the two Cu complexes (Fig. 4). The Epc
− Epa peak separation is slightly larger for the OAc derivative,
and could indicate more significant rearrangement for this
derivative. This redox process has been assigned as a ligand-
based iminosemiquinone/amidophenoxide81 redox couple on
the basis of additional experiments (vide infra) (Scheme 3).
Fig. 3 Molecular structure of LBISCuIIOAc. H atoms have been omitted for
clarity. Thermal ellipsoids are set at 50% probability.
Cu coordination sphere78 for this complex is 0.16. Comparing
this τ value with corresponding values for an ideal trigonal
bipyramid (τ = 1) and square pyramid (τ = 0) indicates that this
complex exhibits a distorted square based pyramidal (SP)
structure. In this geometry for LBISCuIIOAc one nitrogen N[2]
of the coordinated benzoxazole ligand, one oxygen atom O[1]
and an imine nitrogen N[1] of the iminosemiquinone and the
O[101] oxygen atom from the acetate moiety are nearly co-
planar, occupying the basal positions of the pyramid whereas
the other oxygen atom O[100] occupies the apical position. The
observed basal bond lengths (Cu[1]–N[1], 1.929 Å; Cu[1]–N[2],
1.957 Å; Cu[1]–O[101], 1.909 Å; Cu[1]–O[1], 1.966 Å) are in the
range of those reported for other square based pyramidal (SP)
CuII complexes containing semiquinone ligands79,80 while the
apical bond length of Cu[1]–O[100] (2.695 Å) is elongated. The
twist in the basal plane (dihedral angle between N[1]–Cu[1]–
O[1] and N[2]–Cu[1]–O[101] planes) is 37.7°, which is slightly
less in comparison to the LBISCuIICl analogue. The apical
oxygen may play a role in reducing the twist in the LBISCuIIOAc
derivative. The relatively short C[23]–C[24] and C[25]–C[26]
bond lengths (1.36 Å average) in comparison to the longer
C[22]–C[23] and C[26]–C[27] bond lengths (1.43 Å average),
and the short C[22]–O[1] bond length (1.275 Å) are further evi-
Fig. 4 Cyclic voltammograms of LBISCuIIOAc (black line) and LBISCuIICl (red line).
Conditions: 1 mM complex, 0.1 M NBu4ClO4, scan rate 100 mV s−1, CH2Cl2,
233 K.
dence for iminosemiquinone formation.75 Overall, the [LBIS −
bond lengths in both the Cu derivatives are very similar. The
]
iminosemiquinone assignment for [LBIS −
] results in a possible
Table 3 Redox potentials of LBISCuIIOAc and LBISCuIICl versus Fc+/Fc.a Peak to
magnetic interaction with the d9 CuII metal centre.
peak separation in brackets
Compound
E11/2/V
E21/2/V
Magnetic susceptibility measurements
Magnetic susceptibility data for polycrystalline samples of LBIS
-
L
L
BISCuIICl
−0.57 (0.17)
−0.58 (0.22)
0.55 (0.16)
0.28 (0.23)
CuIIOAc and LBISCuIICl complexes was collected in the temp-
erature range 1.8–290 K with an applied magnetic field of
1000 Oe. The effective magnetic moment per molecule (μeff) for
these complexes remains near zero for all temperatures studied,
confirming the diamagnetic nature of the complexes. This obser-
BISCuIIOAc
a Peak to peak difference for the Fc+/Fc couple at 233 K is 0.080 V.
1
vation is consistent with the H NMR characterization; as the
spectra of both CuII complexes contain relatively sharp reson-
ances (ESI Fig. S4 and S5†). This result supports the antiferro-
magnetic coupling between the CuII and iminosemiquinone
ligand radical. The significant twist in the structures (vide
Scheme 3 Schematic of the possible ligand oxidation states of the LBISCuIIX
supra) likely facilitates overlap of the magnetic Cu dx2−y2 orbital complexes.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6829–6839 | 6833

View Article Online
Paper
Dalton Transactions
Fig. 6 Electronic spectra of 3 mM toluene : CH2Cl2 (ca. 30 : 1) solutions of LBIS
-
Fig. 5 Electronic spectra of 3 mM CH2Cl2 solutions of LBISCuIICl (black) and
[LBIQCuIICl]+ (red) and spectra during incremental oxidation (grey) with
[N(C6H3Br2)3˙]+SbF6− at 195 K.
CuIICl (black) and [LBAPCuIICl]− (red) and spectra during incremental reduction
(grey) with cobaltocene at 195 K.
Interestingly, the redox processes at positive potentials differ Table 4 Electronic absorption properties of the complexesa
by ca. 0.27 V for the two Cu complexes, likely due to the
Complex
νmax, cm−1 (εmax, M−1 cm−1
)
different electron-donating effects of Cl− and OAc− anionic
ligands. On the basis of EPR experiments (vide infra), the
couple at positive potentials has been assigned to a ligand-
based iminosemiquinone/iminoquinone redox process. Due
to the similar properties of LBISCuIIOAc and LBISCuIICl com-
plexes we chose to perform further experiments on LBISCuIICl.
LBISCuIICl
24 400 (sh, 7300), 23 200 (8800), 20 600 (8100), 16 900
(5100), 11 300 (2700)
[LBIQCuIICl]+ 24 400 (sh, 4000), 23 200 (4400), 20 600 (4600), 16 900
(2000), 11 300 (1000)
[LBAPCuIICl]− 22 600 (8000), 19 900 (12 300), 17 900 (sh, 7900), 15 700
(sh, 2700), 11 400 (1400)
a For measurements see Experimental section.
Electronic spectroscopy
The electronic spectrum of LBISCuIICl in CH2Cl2 exhibits a
series of bands in the visible region (Fig. 5). An oxidation titra-
reduction process is ligand-based, likely affording a CuII ami-
dophenoxide complex.29 The oxidized complex [LBIQCuIICl]+
also displays an EPR signal, but the signal suggests significant
distortion from axial symmetry in comparison to the reduced
analogue.83–86 Fitting the data affords a rhombic spin system
with three different g values as displayed in Table 5. In
addition, the A3 value is decreased substantially to 330 MHz
(ca. 118 × 10−4 cm−1), which suggests significant tetrahedral
distortion for this species.83–86 We tentatively assign the oxi-
dized species as a distorted CuII iminoquinone species on the
basis of this preliminary analysis. Thus, both redox processes
observed for LBISCuIICl are ligand based.
− 82
tion with [N(C6H3Br2)3˙]+SbF6
from 0→1 equivalent at low
temperature (195 K) exhibits an isosbestic point at 26 300 cm−1
upon conversion to the iminoquinone form [LBIQCuIICl]+.
Upon oxidation, the electronic transitions decrease in intensity
in the range studied. In addition, a reduction titration of LBIS
-
CuIICl with cobaltocene from 0→1 equivalent at low tempera-
ture (195 K) was performed. Due to the limited solubility of
cobaltocene in CH2Cl2, a toluene : CH2Cl2 mixture (30 : 1) was
used as the solvent. Slight differences in the intensities of the
spectral bands for LBISCuIICl in the two different solvents were
evident (Fig. 5 and 6). Upon reduction, isosbestic points at
25 400 and 20 300 cm−1 were observed, as well as a new promi-
nent band at 19 900 cm−1 (Table 4). Samples of both the
oxidized and reduced forms of LBISCuIICl prepared by the
redox titrations described in this section were transferred to
EPR tubes for further analysis.
Theoretical analysis
The solid state magnetic data, in combination with the EPR
results, demonstrate that the complex LBISCuIICl exhibits a dia-
magnetic ground state. Thus the complex can be described as
one of the following: (i) a d8 CuIII amidophenoxide complex,
(ii) a CuII complex antiferromagnetically coupled to an imino-
EPR analysis
A frozen CH2Cl2 solution of the neutral species LBISCuIICl is semiquinone ligand radical, or (iii) a CuI complex bound to
EPR silent at 20 K (Fig. 7b), consistent with a diamagnetic elec- an iminoquinone. The CuII iminosemiquinone electronic
tronic ground state (S = 0) measured in the solid state. In con- structure is most likely on the basis of the solid state X-ray
trast, the reduced complex [LBAPCuIICl]− displays a typical data. We calculated the relative energies of the triplet and
pattern for an axially-symmetric d9 CuII complex with g1
=
broken symmetry (BS) electronic states for LBISCuIICl. BS calcu-
g2 = g⊥ = 2.045 and g3 = g = 2.206 (Fig. 7c, Table 5). The EPR lations provide a localized solution for the ground-state wave
k
spectrum of [LBAPCuIICl]− provides further evidence that the function.62–64,87 The BS solution was determined to be lower
6834 | Dalton Trans., 2013, 42, 6829–6839
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
in energy by 1.12 kcal mol−1 in comparison to the triplet, results in substantial overlap between the Cu dx2−y2 and
matching the experimental data. The closed-shell singlet solu- iminosemiquinone π magnetic orbitals (Fig. 8). The calculated
tion (S = 0) is found to be higher in energy by 5.49 kcal mol−1 magnetic orbital overlap is 0.49.92–94 The computed exchange
in comparison to the BS solution. The exchange coupling J was coupling is likely an underestimation of the singlet–triplet
calculated using the Yamaguchi formula (eqn (1)), which is splitting, as no evidence for population of the triplet state was
applicable for systems from the strong to the weak exchange observed in the variable temperature magnetic susceptibility
limit.88,89 The computed exchange coupling for this complex is experiment.95,96
−316 cm−1, corresponding to a significant antiferromagnetic
(S = 0) exchange interaction for this system. The corresponding
orbital transformation (COT) was used to determine the mag-
netic orbitals for the LBISCuIICl complex.90,91 The large distortion
EHS ꢀ EBS
J ¼ ꢀ
ð1Þ
ˆ2
ˆ2
kS lHS ꢀ kS lBS
The optimized geometry (BS solution) is in good agreement
in the solid state structure (also predicted by calculations) with the X-ray data. A comparison of relevant experimental
and theoretical bond lengths is shown in Table 6. The imino-
semiquinone bonding pattern observed in the X-ray data for
LBISCuIICl is predicted by the DFT calculations. This includes
the ring bond lengths as well as the relatively short C(1)–O(1) and
N(1)–C(6) bonds. The bond lengths are similar to other iminose-
miquinone metal complexes reported in the literature.76,77 The
Cu coordination sphere metrical parameters are also well pre-
dicted, a notable exception being a deviation of 0.076 Å between
the experimental and calculated Cu(1)–O(1) bond length.
Catalytic activity of LBISCuX complexes
We were interested in studying the catalytic activity of LBIS
-
CuIIX for the oxidation of a large number of structurally
diverse alcohols to the corresponding aldehydes. Our initial
Fig. 8 Magnetic orbitals for LBISCuIICl showing the Cu dx2−y2 and semiquinone
π antiferromagnetic interaction.
Table 6 Comparison of experimental and calculated (in parentheses) bond
lengths for the LBISCuIICl complexa
Bond
Bond length (Å)
Bond
Bond length (Å)
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–Cl(1)
Cu(1)–N(2)
C(1)–O(1)
2.006 (1.930)
1.949 (1.955)
2.202 (2.214)
1.963 (1.951)
1.273 (1.285)
1.340 (1.355)
C(1)–C(6)
C(5)–C(6)
C(1)–C(2)
C(2)–C(3)
C(4)–C(5)
C(3)–C(4)
1.460 (1.464)
1.425 (1.420)
1.449 (1.442)
1.368 (1.378)
1.361 (1.379)
1.444 (1.436)
Fig. 7 EPR spectrum of (a) oxidized [LBIQCuIICl]+ in CH2Cl2, experimental
(black), simulation (red); (b) neutral LBISCuIICl in CH2Cl2, experimental (black);
and (c) reduced [LBAPCuIICl]− in toluene, experimental (black), simulation (red).
Conditions: frequency = (a) 9.381 GHz, (b) 9.381 GHz, (c) 9.381 GHz; power =
2 mW; modulation frequency = 100 kHz; amplitude = 0.6 mT; T = 20 K; concen-
tration = 3 mM.
N(1)–C(6)
a See Experimental section for calculation details.
Table 5 EPR parameters for [LBIQCuIICl]+ and [LBAPCuIICl]−a
Compound
g1
g2
g3
A1 b (Cu)
A2 b (Cu)
A3 b (Cu)
A⊥ b (N)
A
b (N)
k
[LBAPCuIICl]−
[LBIQCuIICl]+
2.045
2.043
2.045
2.118
2.206
2.263
39
103
39
220
499
330
39
—
39
—
a See Experimental section for details. [LBAPCuIICl]− in toluene, [LBIQCuIICl]+ in CH2Cl2. All measurements at 20 K. b Values in MHz.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6829–6839 | 6835

View Article Online
Paper
Dalton Transactions
Table 7 The effect of various bases and solvents for the aerobic oxidation of benzyl alcohol by LBISCuX
LBISCuIIOAca
LBISCuIIClb
Time (h)
Entry
Base
Solvent
Time (h)
Conversion (%)
Conversion (%)
1
2
3
4
5
6
7
4 equiv. K3PO4
4 equiv. K2CO3
4 equiv. Et3N
1 equiv. Cs2CO3
2 equiv. Cs2CO3
3 equiv. Cs2CO3
2 equiv. Cs2CO3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
CH3CN
24
24
24
24
8
8
24
31
19
7
72
91
94
35
24
24
24
24
4
4
24
26
37
5
63
100
100
35
a 298 K, 4 mmol% LBISCuIIOAc. b 333 K, 4 mmol% LBISCuIICl.
studies centered on determining the optimal conditions (base
and solvent) for the conversion of a representative alcohol
(benzyl alcohol) to the corresponding aldehyde. Both LBIS
-
CuIICl and LBISCuIIOAc were examined under a number of
different conditions (Table 7) and it was found that LBIS
CuIIOAc exhibited superior catalytic performance at 298 K. The
superior catalytic activity of LBISCuIIOAc in comparison to LBIS
-
Scheme 4 Catalysis conditions relevant to Table 8.
-
CuIICl could be due to the acetate ligand improving solvation of
the complex in toluene, and/or the ability of this ligand to act
as a base during catalysis. For all subsequent catalysis studies
LBISCuIIOAc was tested at 298 K, while LBISCuIICl was tested at
333 K. The catalytic studies were completed with one-half the
catalyst concentration (on a per Cu basis) in comparison to our
recently reported studies on a related dimeric complex.72
characterized. Both metal complexes exhibit an oxidized
iminosemiquinone chelated to the metal ion to afford LBIS-
CuIIOAc and LBISCuIICl, which were characterized by a variety
of methods. X-ray crystallography revealed that the bond
lengths are consistent with a CuII centre and an iminosemiqui-
none ligand. The isolation of a CuII complex containing a tri-
dentate redox active ligand that can be both oxidized and
reduced, and in addition, an exchangeable co-ligand, provided
an interesting platform for reactivity studies.
It was found that benzyl alcohol conversion was highest
when Cs2CO3 was used in comparison to other bases (Table 7).
For both complexes LBISCuIIX, maximum conversion of benzyl
alcohol was achieved with 2 equivalents of Cs2CO3 in toluene
(Table 7, entries 4–6); but no significant increase was observed
when 3 equivalents was used. Toluene was chosen as a solvent
based on previous work with similar systems.97,98 Changing to
the more polar solvent CH3CN, and utilizing 2 equiv. of
Cs2CO3 afforded poor results (Table 7, entry 7). Using the opti-
mized conditions (Scheme 4) determined from Table 7 we
screened a variety of alcohol substrates as detailed in Table 8.
Under the conditions tested, the alcohols in Table 8 could be
oxidized to the corresponding aldehydes with good conversions
(>80% entries 1–14). No over-oxidation of the aldehyde to the
carboxylic acid was observed by GC analysis. Testing of CuII
salts under our reaction conditions (4 mmol% CuCl2/2 equiv.
Cs2CO3 and 4 mmol% Cu(OAc)2/2 equiv. Cs2CO3) demonstrates
aldehyde formation, but in comparatively low yield (Table 9).
Addition of ligand HLBAP (Table 9, entries 3 and 4) improved
the yield in the case of Cu(OAc)2 but not to the level when pre-
formed LBISCuIIOAc is used (Table 7, entry 5). These results
highlight that the pre-formed complex is necessary to afford
high conversion of the tested alcohol substrates.
Variable temperature magnetic susceptibility measure-
1
ments in combination with H NMR demonstrated both com-
plexes are diamagnetic due to strong antiferromagnetic
coupling between the Cu dx2−y2 orbital and the iminosemi-
quinone π singly occupied molecular orbital. EPR studies on
the LBISCuIICl analogue were in agreement with an S =
0 ground state. Theoretical calculations on this complex also
predict that the antiferromagnetically coupled electronic state
is lowest in energy, with a computed exchange coupling of
−316 cm−1. Cyclic voltammetry measurements showed two
quasi-reversible redox events for both Cu complexes,
suggesting that the LBISCuIIX complexes can be both oxidized
and reduced. Chemical redox titrations and EPR analysis of
the reduced [LBAPCuIICl]− and oxidized [LBIQCuIICl]+ species
suggested that the redox events are ligand-based. Preliminary
reactivity studies show that both Cu complexes particularly the
acetate complex are competent catalysts for the oxidation of
alcohols in the presence of O2. Based on our results, both com-
plexes are capable of catalyzing the oxidation of a large
number of alcohols to the corresponding aldehyde. Of note,
the acetate complex generally performed better than the chlo-
ride analog. It may be possible to improve the catalytic activity
Conclusion
A new benzoxazole-based aminophenol ligand HLBAP and of these complexes through modifications to the ligand frame-
two corresponding CuII complexes were synthesized and work to fine tune the electronics around the metal center. This
6836 | Dalton Trans., 2013, 42, 6829–6839
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Table 8 LBISCuIIX-catalyzed aerobic oxidation of different alcohols
under Grant No. G2013IASBS127. T. S. acknowledges an NSERC
Discovery Grant. Westgrid and Compute Canada are thanked for
access to computational resources. R. M. C. acknowledges
NSERC for a graduate fellowship.
L
BISCuIIOAca
LBISCuIIClb
Time
(h)
Conversion
(%)
Time
(h)
Conversion
(%)
Entry
1
Substrate
8
6
6
91
94
81
4
24
6
94
References
2
3
60
85
1 J. Stubbe and W. A. van der Donk, Chem. Rev., 1998, 98,
705–762.
2 J. W. Whittaker, Chem. Rev., 2003, 103, 2347–2364.
3 K. J. Blackmore, N. Lal, J. W. Ziller and A. F. Heyduk, J. Am.
Chem. Soc., 2008, 130, 2728–2729.
4
8
70
8
96
5
6
6
6
95
98
6
6
81
83
4 T. Büttner, J. Geier, G. Frison, J. Harmer, C. Calle,
A. Schweiger, H. Schönberg and H. Grützmacher, Science,
2005, 307, 235–238.
5 P. J. Chirik, Inorg. Chem., 2011, 50, 9737–9740.
6 P. J. Chirik and K. Wieghardt, Science, 2010, 327, 794–795.
7 M. R. Haneline and A. F. Heyduk, J. Am. Chem. Soc., 2006,
128, 8410–8411.
7
8
10
6
83
90
8
6
80
94
8 C. C. Hojilla Atienza, C. Milsmann, E. Lobkovsky and
P. J. Chirik, Angew. Chem., Int. Ed., 2011, 50, 8143–8147.
9 M. Königsmann, N. Donati, D. Stein, H. Schönberg,
J. Harmer, A. Sreekanth and H. Grützmacher, Angew.
Chem., Int. Ed., 2007, 46, 3567–3570.
9
8
97
94
10
6
87
84
10
10
11
12
24
8
71
95
6
99
10 P. Maire, M. Königsmann, A. Sreekanth, J. Harmer,
A. Schweiger and H. Grützmacher, J. Am. Chem. Soc., 2006,
128, 6578–6580.
11 W. R. McNamara, Z. Han, P. J. Alperin, W. W. Brennessel,
P. L. Holland and R. Eisenberg, J. Am. Chem. Soc., 2011,
133, 15368–15371.
12 Y. Miyazato, T. Wada and K. Tanaka, Bull. Chem. Soc. Jpn.,
2006, 79, 745–747.
13 M. R. Ringenberg, S. L. Kokatam, Z. M. Heiden and
T. B. Rauchfuss, J. Am. Chem. Soc., 2008, 130, 788–789.
14 S. K. Russell, E. Lobkovsky and P. J. Chirik, J. Am. Chem.
Soc., 2011, 133, 8858–8861.
—
—
13
14
36
8
87
98
—
—
n-C8H17OH
8
96
a 298 K, 4 mmol% LBISCuIIOAc. b 333 K, 4 mmol% LBISCuIIOCl.
Table 9 Blank tests with HLBAP and copper saltsa
Temp.
(K)
Time Conversion
15 L. Benisvy, A. J. Blake, D. Collison, E. S. Davies,
C. D. Garner, E. J. L. McInnes, J. McMaster, G. Whittaker
and C. Wilson, Chem. Commun., 2001, 1824–1825.
16 P. Chaudhuri, M. Hess, U. Flörke and K. Wieghardt, Angew.
Chem., Int. Ed., 1998, 37, 2217–2220.
17 P. Chaudhuri, M. Hess, J. Müller, K. Hildenbrand, E. Bill,
T. Weyhermüller and K. Wieghardt, J. Am. Chem. Soc., 1999,
121, 9599–9610.
Entry Catalytic system
Solvent (h)
(%)
1
2
3
4
CuCl2/Cs2CO3
Cu(OAc)2/Cs2CO3
CuCl2/Cs2CO3/HLBAP
Cu(OAc)2/Cs2CO3/
HLBAP
333
298
333
298
Toluene 24
Toluene 24
28
24
8
Toluene
Toluene
8
8
50
5
HLBAP/Cs2CO3
298
Toluene
8
5
a Reaction conditions: 4 mmol% CuCl2/Cu(OAc)2/HLBAP and 2 equiv.
Cs2CO3.
18 J. A. Halfen, B. A. Jazdzewski, S. Mahapatra, L. M. Berreau,
E. C. Wilkinson, L. Que and W. B. Tolman, J. Am. Chem.
Soc., 1997, 119, 8217–8227.
will be particularly important for the future development of
bioinspired catalysts.
19 F. Michel, F. Thomas, S. Hamman, C. Philouze, E. Saint-
Aman and J.-L. Pierre, Eur. J. Inorg. Chem., 2006, 3684–3696.
20 R. C. Pratt and T. D. P. Stack, J. Am. Chem. Soc., 2003, 125,
8716–8717.
21 R. C. Pratt and T. D. P. Stack, Inorg. Chem., 2005, 44,
2367–2375.
Acknowledgements
E. S. gratefully acknowledges the support by the Institute 22 B. A. Jazdzewski and W. B. Tolman, Coord. Chem. Rev.,
for Advanced Studies in Basic Sciences (IASBS) Research Council
2000, 200–202, 633–685.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6829–6839 | 6837

View Article Online
Paper
Dalton Transactions
23 A. Ghosh, T. Wondimagegn and A. B. J. Parusel, J. Am. 48 P. Cao, C. Deng, Y. Y. Zhou, X. L. Sun, J. C. Zheng, Z. Xie
Chem. Soc., 2000, 122, 5100–5104. and Y. Tang, Angew. Chem., Int. Ed., 2010, 49, 4463–4466.
24 W. Kaim, M. Wanner, A. Knödler and S. Záliš, Inorg. Chim. 49 Y.-J. Sun, N. Li, Z.-B. Zheng, L. Liu, Y.-B. Yu, Z.-H. Qin and
Acta, 2002, 337, 163–172. B. Fu, Adv. Synth. Catal., 2009, 351, 3113–3117.
25 V. Kasack, W. Kaim, H. Binder, J. Jordanov and E. Roth, 50 M. Schönleber, R. Hilgraf and A. Pfaltz, Adv. Synth. Catal.,
Inorg. Chem., 1995, 34, 1924–1933. 2008, 350, 2033–2038.
26 P. Chaudhuri, R. Wagner, U. Pieper, B. Biswas and 51 R. Rasappan, D. Laventine and O. Reiser, Coord. Chem.
T. Weyhermuller, Dalton Trans., 2008, 1286–1288. Rev., 2008, 252, 702–714.
27 R. Hübner, B. Sarkar, J. Fiedler, S. Záliš and W. Kaim, 52 H.-M. Li, J.-G. Su, Z.-L. Lu, H.-K. Fun and
Eur. J. Inorg. Chem., 2012, 3569–3576.
28 K. S. Min, T. Weyhermüller, E. Bothe and K. Wieghardt,
Inorg. Chem., 2004, 43, 2922–2931.
S. Chantrapromma, Inorg. Chem. Commun., 2008, 11,
235–238.
53 M. Gómez, G. Muller and M. Rocamora, Coord. Chem. Rev.,
1999, 193–195, 769–835.
29 C. Mukherjee, U. Pieper, E. Bothe, V. Bachler, E. Bill,
T. Weyhermüller and P. Chaudhuri, Inorg. Chem., 2008, 47, 54 O. Iasco, G. Novitchi, E. Jeanneau, J. B. Tommasino,
8943–8956.
N. Roques and D. Luneau, Inorg. Chem., 2012, 51,
2588–2596.
30 C. Nazari Verani, S. Gallert, E. Bill, T. Weyhermuller,
K. Wieghardt and P. Chaudhuri, Chem. Commun., 1999, 55 C. J. Richards and A. J. Locke, Tetrahedron: Asymmetry,
1747–1748. 1998, 9, 2377–2407.
31 A. V. Piskunov, I. N. Meshcheryakova, E. V. Baranov, G. 56 A. Togni and L. M. Venanzi, Angew. Chem., Int. Ed. Engl.,
K. Fukin, V. K. Cherkasov and G. A. Abakumov, Russ. Chem.
Bull., 2010, 59, 361–370.
32 V. Bachler, P. Chaudhuri and K. Wieghardt, Chem.–Eur. J.,
2001, 7, 404–415.
1994, 33, 497–526.
57 T. Khomenko, O. Salomatina, S. Kurbakova, I. Il’ina,
K. Volcho, N. Komarova, D. Korchagina, N. Salakhutdinov
and A. Tolstikov, Russ. J. Org. Chem., 2006, 42,
1653–1661.
33 P. Chaudhuri, C. N. Verani, E. Bill, E. Bothe,
T. Weyhermüller and K. Wieghardt, J. Am. Chem. Soc., 2001, 58 S. Stoll and A. Schweiger, J. Magn. Reson., 2006, 178, 42–55.
123, 2213–2223.
34 G. Speier, J. Csihony, A. M. Whalen and C. G. Pierpont,
Inorg. Chem., 1996, 35, 3519–3524.
35 A. R. Silva, H. Albuquerque, S. Borges, R. Siegel, L. Mafra,
A. P. Carvalho and J. Pires, Microporous Mesoporous Mater.,
2012, 158, 26–38.
36 J.-H. Chen, S.-H. Liao, X.-L. Sun, Q. Shen and Y. Tang,
Tetrahedron, 2012, 68, 5042–5045.
37 A. Bisai and V. K. Singh, Tetrahedron, 2012, 68, 3480–3486.
38 W. Yang and D.-M. Du, Eur. J. Org. Chem., 2011, 1552–1556.
39 L. Wen, Q. Shen, X. Wan and L. Lu, J. Org. Chem., 2011, 76,
2282–2285.
40 A. R. Silva, H. Albuquerque, A. Fontes, S. Borges,
Â. Martins, A. P. Carvalho and J. Pires, Ind. Eng. Chem. Res.,
2011, 50, 11495–11501.
41 M. Rostami, A. Khosropour, V. Mirkhani, M. Moghadam,
S. Tangestaninejad and I. Mohammadpoor-Baltork, Appl.
Catal., A, 2011, 397, 27–34.
42 Y. Motoyama, T. Nishikata and H. Nagashima, Chem.–Asian
J., 2011, 6, 78–82.
59 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas,
D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 09,
Revision A.02, Gaussian, Inc., Wallingford, CT, 2009.
43 D. Didier, C. Magnier-Bouvier and E. Schulz, Adv. Synth.
Catal., 2011, 353, 1087–1095.
60 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
44 F. Riobé and N. Avarvari, Coord. Chem. Rev., 2010, 254, 61 P. J. Stephens, F. J. Devlin, C. F. Chabalowski and
1523–1533. M. J. Frisch, J. Phys. Chem., 1994, 98, 11623–11627.
45 J.-M. Lee, J. Kim, Y. Shin, C.-E. Yeom, J. E. Lee, T. Hyeon 62 L. Noodleman, J. Chem. Phys., 1981, 74, 5737–5743.
and B. Moon Kim, Tetrahedron: Asymmetry, 2010, 21, 63 L. Noodleman and D. A. Case, Adv. Inorg. Chem., 1992, 38,
285–291.
46 K. Lang, J. Park and S. Hong, J. Org. Chem., 2010, 75, 64 L. Noodleman and E. R. Davidson, Chem. Phys., 1986, 109,
6424–6435. 131–143.
47 G. Chollet, D. Didier and E. Schulz, Catal. Commun., 2010, 65 A. Schafer, H. Horn and R. Ahlrichs, J. Chem. Phys., 1992,
423.
11, 351–355.
97, 2571–2577.
6838 | Dalton Trans., 2013, 42, 6829–6839
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
66 A. Schafer, C. Huber and R. Ahlrichs, J. Chem. Phys., 1994,
100, 5829–5835.
Class of 1,4-Dioxene Based DNA Cleaving Agents, University
of Cincinnati, Cincinnati, 2002.
67 A. Altomare, G. Cascarano, C. Giacovazzo and 83 F. G. Herring, D. J. Patmore and A. Storr, J. Chem. Soc.,
A. Guagliardi, J. Appl. Crystallogr., 1993, 26, 343–350. Dalton Trans., 1975, 711–717.
68 P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout 84 A. A. Gewirth, S. L. Cohen, H. J. Schugar and E. I. Solomon,
and D. J. Watkin, J. Appl. Crystallogr., 2003, 36, 1487–1487.
69 L. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
Inorg. Chem., 1987, 26, 1133–1146.
85 S.-k. Kang, H.-s. Kim and Y.-i. Kim, Bull. Korean Chem. Soc.,
2004, 25, 1959–1962.
86 S. Ghosh, J. Cirera, M. A. Vance, T. Ono, K. Fujisawa and
E. I. Solomon, J. Am. Chem. Soc., 2008, 130, 16262–16273.
70 Persistence of VisionTM Raytracer, Persistence of Vision Pty.
Ltd., Williamstown, Victoria, Australia, 2004.
71 The X-ray data for HLBAP could not be fully solved. Selected
data: empirical formula, C70.5H93N4O4Cl; formula weight, 87 J. Li, L. Noodleman and D. A. Case, in Inorganic Electronic
ˉ
569.22; crystal system, triclinic; space group, P1; a(Å),
14.2265(6); b(Å), 16.3701(7); c(Å), 16.7959(8); α (°), 74.1570
(10); β (°), 88.5650(10); γ (°), 65.7200(10); V (Å3), 3413.1(3);
Z, 1; T (K), 298.
Structure and Spectroscopy, ed. E. I. Solomon and A. B. P.
Lever, John Wiley
pp. 661–724.
88 T. Soda, Y. Kitagawa, T. Onishi, Y. Takano, Y. Shigeta,
H. Nagao, Y. Yoshioka and K. Yamaguchi, Chem. Phys. Lett.,
2000, 319, 223–230.
& Sons, New York, 1999, vol. I,
72 S. E. Balaghi, E. Safaei, M. Rafiee and M. H. Kowsari, Poly-
hedron, 2012, 47, 94–103.
73 B. J. Hathaway and D. E. Billing, Coord. Chem. Rev., 1970, 5, 89 K. Yamaguchi, Y. Takahara and T. Fueno, in Applied
143–207.
Quantum Chemistry, ed. V. H. Smith, Reidel, Dordrecht,
1986.
90 D. Herebian, K. E. Wieghardt and F. Neese, J. Am. Chem.
Soc., 2003, 125, 10997–11005.
74 R. W. Saalfrank, M. Decker, F. Hampel, K. Peters and
H. G. von Schnering, Chem. Ber., 1997, 130, 1309–1313.
75 S. N. Brown, Inorg. Chem., 2012, 51, 1251–1260.
76 A. V. Piskunov, I. N. Mescheryakova, G. K. Fukin, 91 F. Neese, J. Phys. Chem. Solids, 2004, 65, 781–785.
V. K. Cherkasov and G. A. Abakumov, New J. Chem., 2010, 92 S. Gorelsky and E. Solomon, Theor. Chem. Acc., 2007, 119,
34, 1746–1750.
57–65.
77 A. M. Reynolds, E. A. Lewis, N. W. Aboelella and 93 S. I. Gorelsky, AOMix: Program for Molecular Orbital Analysis,
W. B. Tolman, Chem. Commun., 2005, 2014–2016.
78 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and
http://www.sg-chem.net/, University of Ottawa, Canada,
2007.
G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 94 S. I. Gorelsky and A. B. P. Lever, J. Organomet. Chem., 2001,
1349–1356. 635, 187–196.
79 P. J. Alonso, M. Fraile, I. Garcı and C. Sa, Langmuir, 2000, 95 L. K. Thompson, F. W. Hartstock, P. Robichaud and
16, 5607–5612. W. Hanson, Can. J. Chem., 1984, 62, 2755–2762.
80 G. Desimoni, G. Faita, M. Toscanini and M. Boiocchi, 96 A. Yatani, M. Fujii, Y. Nakao, S. Kashino, M. Kinoshita,
Chem.–Eur. J., 2009, 15, 9674–9677.
W. Mori and S. Suzuki, Inorg. Chim. Acta, 2001, 316,
81 A. I. Poddel’sky, V. K. Cherkasov and G. a. Abakumov,
Coord. Chem. Rev., 2009, 253, 291–324.
82 A. Bjorn, A Mechanistic Investigation of the Photochemical
127–131.
97 J. M. Hoover and S. S. Stahl, J. Am. Chem. Soc., 2011, 133,
16901–16910.
and Thermal Activation of 2,2- and 2,3-Diaryl and 2,2,3- 98 L. Liang, G. Rao, H.-L. Sun and J.-L. Zhang, Adv. Synth.
Triaryl-2,3-dihydro-phenanthro[9,10-b]-1,4-dioxins,
a
New
Catal., 2010, 352, 2371–2377.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6829–6839 | 6839