Copper dioxygen adducts: Formation of bis(μ-oxo)dicopper(III) versus (μ-1,2)peroxodicopper(II) complexes with small changes in one pyridyl-ligand substituent

Article abstract of DOI:10.1021/ic702437c

The preference for the formation of a particular Cu₂O ₂ isomer coming from (ligand)-Cu^I/O₂ reactivity can be regulated with the steric demands of a TMPA (tris(2-pyridylmethyl)amine) derived ligand possessing 6-pyridyl substituents on one of the three donor groups of the tripodal tetradentate ligand. When this substituent is an -XHR group (X = N or C) the traditional Cu^I/O₂ adduct forms a (μ-1,2)peroxodicopper(II) species (A). However, when the substituent is the slightly bulkier XR₂ moiety {aryl or NR₂ (R ≠ H)}, a bis(μ-oxo)dicopper(III) structure (C) is favored. The reactivity of one of the bis(μ-oxo)dicopper(III) species, [{(6tbp)Cu^III} ₂(O^2-)₂]²⁺ (7-O₂) (6tbp = (6-^tBu-phenyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine), was probed, and for the first time, exogenous toluene or ethylbenzene hydrocarbon oxygenation reactions were observed. Typical monooxygenase chemistry occurred: the benzaldehyde product includes an 18-0 atom for toluene/7-¹⁸O ₂ reactivity, and a H-atom abstraction by 7-O₂ is apparent from study of its reactions with ArOH substrates, as well as the determination of k_H/k_D ≈ 7 in the toluene oxygenation (i.e., PhCH₃ vs PhCD₃ substrates). Proposed courses of reaction are presented, including the possible involvement of PhCH₂OO ^? and its subsequent reaction with copper(I) complex, the latter derived from dynamic solution behavior of 7-O₂. External TMPA ligand exchange for copper in 7-O₂ and O-O bond (re)formation chemistry, along with the ability to protonate 7-O₂ and release of H ₂O₂ indicate the presence of an equilibrium between [{(6tbp)Cu^III}₂(O^2_)₂]²⁺ (7-O₂) and a (μ-1,2)peroxodicopper(II) form.

Full text of DOI:10.1021/ic702437c

Inorg. Chem. 2008, 47, 3787-3800
Copper Dioxygen Adducts: Formation of Bis(µ-oxo)dicopper(III) versus
(µ-1,2)Peroxodicopper(II) Complexes with Small Changes in One
Pyridyl-Ligand Substituent
Debabrata Maiti,† Julia S. Woertink,‡ Amy A. Narducci Sarjeant,† Edward I. Solomon,‡
and Kenneth D. Karlin*,†
Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218, and
Department of Chemistry, Stanford UniVersity, Stanford, California 94305
Received December 17, 2007
The preference for the formation of a particular Cu2O2 isomer coming from (ligand)-CuI/O2 reactivity can be regulated
with the steric demands of a TMPA (tris(2-pyridylmethyl)amine) derived ligand possessing 6-pyridyl substituents on
one of the three donor groups of the tripodal tetradentate ligand. When this substituent is an -XHR group (X )
N or C) the traditional CuI/O2 adduct forms a (µ-1,2)peroxodicopper(II) species (A). However, when the substituent
is the slightly bulkier XR2 moiety {aryl or NR2 (R * H)}, a bis(µ-oxo)dicopper(III) structure (C) is favored. The
reactivity of one of the bis(µ-oxo)dicopper(III) species, [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) (6tbp ) (6-tBu-phenyl-2-
pyridylmethyl)bis(2-pyridylmethyl)amine), was probed, and for the first time, exogenous toluene or ethylbenzene
hydrocarbon oxygenation reactions were observed. Typical monooxygenase chemistry occurred: the benzaldehyde
product includes an 18-O atom for toluene/7-18O2 reactivity, and a H-atom abstraction by 7-O2 is apparent from
study of its reactions with ArOH substrates, as well as the determination of kH/kD ≈ 7 in the toluene oxygenation
(i.e., PhCH3 vs PhCD3 substrates). Proposed courses of reaction are presented, including the possible involvement
of PhCH2OO• and its subsequent reaction with copper(I) complex, the latter derived from dynamic solution behavior
of 7-O2. External TMPA ligand exchange for copper in 7-O2 and O-O bond (re)formation chemistry, along with the
ability to protonate 7-O2 and release of H2O2 indicate the presence of an equilibrium between [{(6tbp)CuIII}2(O2-)2]2+
(7-O2) and a (µ-1,2)peroxodicopper(II) form.
Scheme 1
Introduction
In coordination chemistry, especially involving the use of
multidentate ligands, the nature of the metal bound chelate
is critical in dictating the metal complex chemistry, including
structure, spin-state (where appropriate), reactivity, etc.1–4
The ligand properties in question include the number of donor
atoms, their identity (nitrogen, oxygen, sulfur, etc.), the
overall ligand charge, the chelate ring size or sizes, the
* To whom correspondence should be addressed. E-mail: karlin@
jhu.edu.
†
The Johns Hopkins University.
Stanford University.
‡
overall ligand donor ability and steric interactions. With
respect to research in copper-dioxygen complex fundamen-
tal chemistry, with possible relevance to metallobiochemistry,
(1) Itoh, S.; Tachi, Y. Dalton Trans. 2006, 4531–4538.
(2) In Encyclopedia of Inorganic Chemistry, 2nd ed.; John Wiley & Sons
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it has been well demonstrated that copper-dioxygen complex
Structure/Reactivity and Synthetic Model Studies. In Concepts and
Models in Bioinorganic Chemistry; Metzler-Nolte, N., Kraatz, H.-B.,
formation, structure type, physical properties, and reactivity
Eds.; Wiley-VCH: New York, 2006; pp 363–395.
are dictated by ligand design and ligand nature.1,4–7
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10.1021/ic702437c CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 9, 2008 3787
Published on Web 04/09/2008

Maiti et al.
teranion,17,18 and steric or electronic effects can change the
structure from B to C or vice-versa, and in fact these two
forms are often in equilibrium (Scheme 1).6,19–24 25
Chart 1
In this report, we focus on new pyridylalkylamine tet-
radentate ligands and their copper-dioxygen chemistry, varied
in the nature of the substituent placed at the 6-position of
only one of the pyridyl arms of the parent TMPA ligand.
Here, what seem to be relatively subtle steric effects lead to
discrimination in the type of binuclear copper-dioxygen
complex formed. Specifically, when the 6-substituent first
atom X also has a hydrogen atom (Chart 2; X is a -CH-,
-CHR, -NHR), only the copper-dioxygen complex form
A, (µ-1,2)peroxodicopper(II), is generated, while if the
substituent is XR2 {aryl or NR2 (R * H)}, then only bis(µ-
oxo)dicopper(III) complexes C are formed. The ligand
complexes compared here, including examples from the
literature, are shown in Chart 2; they are divided into the
two groups being discussed. For four cases, this article
describes new previously uncharacterized ligand-copper(I)/
O2 reactivity, spectroscopic characterization of A and C, and
reactivity studies of bis(µ-oxo)dicopper(III) complex C
including substrate oxidation/oxygenation, ligand exchange,
and bond (re)formation chemistry.
Most often, copper(I) complexes possessing tetradentate
nitrogen-based ligands (alkylamino, pyridyl, imidazolyl,
mixed, or others) prefer to form (µ-1,2)peroxodicopper(II)
adducts (A), whereas tridentate ligands generally lead to
dioxygen adducts described as side-on (µ-η2:η2)peroxo
dicopper(II) complexes (B), and bidentate ligands most often
support bis(µ-oxo)dicopper(III) complexes (C) (Scheme
1).4–6 However, for a variety of tetradentate ligands, steric
effects can lead to changes from structure A to C. For
example, the dicopper(II) complex with TMPA (also TPA,
tris(2-pyridylmethyl)amine) [{(TMPA)CuII}2(µ-1,2-O22–)]2+
(A1), has an end-on structure (ν(O-O) ) 832 (∆[18O2]) -44)
cm-1, ν(Cu-O) ) 561 (∆[18O2]) -26) cm-1 in EtCN; ν(O-O)
) 827 (∆[18O2]) -44) cm-1, ν(Cu-O) ) 561 (∆[18O2]) -26)
cm-1 in Et2O),8,9 but with Me2TPA ((2-pyridylmethyl)bis(6-
methyl-2-pyridylmethyl)amine), Suzuki and co-workers
showed that the bis-µ-oxo “isomer” [{(Me2TPA)-
CuIII}2(O2-)2]2+ (C1) (ν(Cu-O) ) 590 (∆[18O2]) -26) cm-1)
results (Chart 1).6,10,11 When all three pyridyl arms have
6-phenyl groups12 or all are replaced by 2-quinolyl groups,13
the derived copper(I) complexes are inert to dioxygen. For
tridentate ligands, the nature of the solvent,14–16 the coun-
Experimental Section
Materials and Methods. Unless otherwise stated, all
solvents and chemicals used were of commercially available
analytical grade. Tetrahydrofuran (THF) and pentane were
used after they were passed through a 60 cm long column
of activated alumina (Innovative Technologies, Inc.) under
argon. Preparation and handling of air-sensitive compounds
were performed under an argon atmosphere using standard
Schlenk techniques or in an MBraun Labmaster 130 inert
atmosphere (<1 ppm O2, <1 ppm H2O) drybox filled with
nitrogen. Deoxygenation of solvents was achieved by either
repeated freeze/pump/thaw cycles or bubbling with argon
for 30-45 min. Elemental analyses were performed by
Desert Analytics (Tucson, AZ). 1H NMR spectra were
measured on a Bruker 400 MHz spectrometer. Chemical
shifts were reported as δ values relative to an internal
standard (Me4Si) and the residual solvent proton peak.
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683.
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104, 1013–1045.
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Solomon, E. I. J. Am. Chem. Soc. 1991, 113, 8671–8679.
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Young, J., V. G.; Cramer, C. J.; Que, J., L.; Tolman, W. B. J. Am.
Chem. Soc. 1996, 118, 11555–11574.
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A.; Watanabe, Y.; Kitagawa, T. J. Am. Chem. Soc. 2000, 122, 2124–
2125.
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Hedman, B.; Stack, T. D. P. J. Am. Chem. Soc. 2006, 128, 9268–
9269.
(11) Replacement of one pyridyl arm of TMPA by the more sterically
demanding quinolyl group affords a (µ-1,2) peroxodicopper(II) species,
whereas two quinolyl subunits in place of pyridyl donors of TMPA
produces an equilibrium mixture of (µ-1,2) peroxodicopper(II) and
bis(µ-oxo)dicopper(III) species. These results are to be reported
elsewhere.
(19) Hatcher, L. Q.; Vance, M. A.; Narducci Sarjeant, A. A.; Solomon,
E. I.; Karlin, K. D. Inorg. Chem. 2006, 45, 3004–3013.
(20) Que, L., Jr.; Tolman, W. B. Angew. Chem., Int. Ed. 2002, 41, 1114–
1137.
(21) Mahadevan, V.; Henson, M. J.; Solomon, E. I.; Stack, T. D. P. J. Am.
Chem. Soc. 2000, 122, 10249–10250.
(12) Chuang, C.-L.; Lim, K.; Chen, Q.; Zubieta, J.; Canary, J. W. Inorg.
Chem. 1995, 34, 2562–2568.
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(23) Henson, M. J.; Vance, M. A.; Zhang, C. X.; Liang, H.-C.; Karlin,
K. D.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 5186–5192.
(24) Obias, H. V.; Lin, Y.; Murthy, N. N.; Pidcock, E.; Solomon, E. I.;
Ralle, M.; Blackburn, N. J.; Neuhold, Y.-M.; Zuberbühler, A. D.;
Karlin, K. D. J. Am. Chem. Soc. 1998, 120, 12960–12961.
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(13) Karlin, K. D.; Wei, N.; Jung, B.; Kaderli, S.; Niklaus, P.; Zuberbühler,
A. D. J. Am. Chem. Soc. 1993, 115, 9506–9514.
(14) Liang, H. C.; Henson, M. J.; Hatcher, L. Q.; Vance, M. A.; Zhang,
C. X.; Lahti, D.; Kaderli, S.; Sommer, R. D.; Rheingold, A. L.;
Zuberbuhler, A. D.; Solomon, E. I.; Karlin, K. D. Inorg. Chem. 2004,
43, 4115–4117.
3788 Inorganic Chemistry, Vol. 47, No. 9, 2008

Copper Dioxygen Adducts
Chart 2
UV–vis spectra were recorded with either a Cary-50 Bio
spectrophotometer equipped with a fiber optic coupler
(Varian) and a fiber optic dip probe (Hellma 661.302 QX-
UV 2 mm for low-temperature) or a Hewlett-Packard Model
8453A diode array spectrophotometer equipped with a two-
window quartz H.S. Martin Dewar filled with cold MeOH
(25 to -85 °C) maintained and controlled by a Neslab VLT-
95 low temp circulator attached to the HP spectrophotometer.
Spectrophotometer cells used were made by Quark Glass
with column and pressure/vacuum side stopcock and 2 mm
path length. Dioxygen was dried by passing through a short
column of supported P4O10 (Aquasorb, Mallinckrodt) and was
bubbled into reaction solutions via an 18-gauge, 24 in. long
stainless steel syringe needle. IR spectra were collected using
a Mattson Instruments Galaxy series FT-IR (model 4030)
that was controlled by the PC program WinFIRST. The
copper(I) complex used for low-temperature UV–vis and all
monochromator and an Enhance (Mo) X-ray Source (λ )
0.71073Å) operated at 2 kW power (50 kV, 40 mA) and a
CCD detector. The frames were integrated with the Oxford
Diffraction CrysAlisRED software package. X-band electron
paramagnetic resonance (EPR) spectra were recorded on a
Bruker EMX CW-EPR spectrometer controlled with a Bruker
ER 041 XG microwave bridge operating at X-band (∼9.472
GHz). The low-temperature experiments were carried out
in THF solvent at 77 K using a N2(l) finger dewar.
Synthesis of LN(CH Ph) . To BrTMPA26 (2.4 g, 6.5 mmol)
()(6-bromo-2-pyridylmethyl)bis(2-pyridylmethyl)amine) in
a pressure tube were added (PhCH2)2NH.HCl (6.43 g, 32.5
mmol) (Aldrich), NaOH (0.78 g, 0.195 mmol), and water
(15 mL). The resulting mixture was stirred and heated to
433 K for 54 h and then cooled to RT (room temperature).
The resulting mixture was extracted with CH2Cl2 several
times; the combined organic layers were dried over Na2SO4,
and the volatile components were removed by rotary
evaporation. The mixture was chromatographed on alumina
(Activated Alumina, Chromatographic grade, 80–200 mesh)
using 2% (progressively brought to 5%) methanol in dichlo-
romethane as eluant. The product fraction was collected, and
the solvent was removed under reduced pressure to afford
2
2
-
reactivity studies reported below are the B(C6F5)4 salt
complexes unless otherwise stated. ESI mass spectra were
acquired using a Finnigan LCQDeca ion-trap mass spec-
trometer equipped with an electrospray ionization source
(Thermo Finnigan, San Jose, CA). Samples were dissolved
in CH3OH and introduced into the instrument at a rate of 10
µL/min using a syringe pump via a silica capillary line. The
heated capillary temperature was 250 °C, and the spray
voltage was 5kV. X-ray diffraction was performed at the
X-ray diffraction facility at the Johns Hopkins University.
The X-ray intensity data were measured on an Oxford
Diffraction Xcalibur3 system equipped with a graphite
1
the reddish-yellow oil LN(CH Ph) (1.95 g, 62% yield). H NMR
(CDCl3): δ 3.71 (s, 2H, CH2Py), 3.89 (s, 4H, 2CH2Py), 4.81
(s, 4H, 2CH2Ph) 6.33 (d, 1H), 6.76 (d, 1H), 7.08 (t, 2H),
7.21–7.35 (m, 11H), 7.52 (t, 2H), 7.63 (d, 2H), 8.49 (d, 2H).
2
2
(26) Chuang, C.-L.; dos Santos, O.; Xu, X.; Canary, J. W. Inorg. Chem.
1997, 36, 1967–1972.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3789

Maiti et al.
13C NMR (CDCl3): δ 50.90, 53.44, 60.23, 77.40, 104.01,
111.45, 121.78, 122.80, 126.88, 126.97, 128.18, 128.52,
136.36, 137.77, 138.76, 140.33, 148.91, 157.26, 158.19,
160.09. ESI-MS (486.50, M + H). Rf ) 0.37, alumina, 2%
MeOH + CH2Cl2.
2CH2Ph), 6.61 (d, 1H), 6.71 (d, 1H), 7.2–7.6 (m, 15H), 7.91
(t, 2H), 8.72 (d, 2H). Anal. Calcd for C62.9H51.8BCuF20N5O2.8:
C, 54.87; H, 3.79; N, 5.09. Found: C, 54.80; H, 3.81; N,
4.84.
I
Reaction of [(LN(CH Ph) )Cu ]B(C6F5)4 (10) with O2. Three
2
2
I +
Reaction of [(LCH OMe)CuI]+ (1) with O2. The synthesis
milliliters of an acetone solution of [(LN(CH Ph) )Cu ] (10)
(0.009 g, 0.007 mmol) was taken in a UV–vis cuvette
assembly under argon which was cooled to -80 °C, and an
initial spectrum was recorded. Dioxygen was bubbled for
30 s through the solution using a long syringe needle and
the new spectrum recorded (see Supporting Information,
Figure S1). The product was characterized as
2
2
2
and characterization of tetradentate ligand LCH OMe and
2
copper(I) complex, [LCH OMe)CuI]B(C6F5)4 (1) (Chart 2) were
2
previously reported.27 Crystallographically characterized
[LCH OMe)CuI]B(C6F5)4 (0.0049 g, 0.004 mmol) was dissolved
2
in 3.8 mL of THF and taken in a UV–vis cuvette inside the
dry box. Dioxygen was bubbled through -80 °C cold cuvette
solution for 30 s with a long syringe needle. The purple
III
2-
2+
[{(LN(CH Ph) )Cu }2(O )2] (10-O2), a bis-µ-oxo-dicop-
2
2
product was characterized as [{(LCH OMe)CuII}2(O22-)]2+ (1-
per(III) complex.
2
II
2+
N(CH2Ph)2
Synthesis of [{(LN(CH Ph) )Cu (Cl)}2] . [(L
)-
2
2
O2), a µ-1,2-peroxodicopper(II) complex.
CuI]B(C6F5)4 (0.055 g, 0.044 mmol) was dissolved in THF
(3.0 mL) in a 25 mL Schlenk flask. Upon removal of this
flask from glovebox to the benchtop, the addition of 0.3 mL
of CHCl3 resulted in a green copper complex. Pentane (35
mL) was added to precipitate the copper complex. The green
copper complex was further recrystallized from THF/pentane
I +
Synthesis of Copper(I) Complex [(LNH(CH ))Cu ] (2).
3
The generation and characterization of tetradentate ligand
28
LNH(CH ) (Chart 2) were previously reported. LNH(CH ) (0.055
3
3
29
g, 0.172 mmol) and [CuI(CH3CN)4]B(C6F5)4 (0.156 g,
0.172 mmol) were placed in a 25 mL Schlenk flask under
argon. THF (3 mL) was added under argon to the mixture
of solids to form a yellow solution. The resulting solution
was stirred under argon for 30 min. Pentane (15 mL) was
then added to the solution to precipitate a yellow solid. The
supernatant was decanted, and the solid was recrystallized
three times from THF/pentane under argon and dried under
vacuum (1 h), giving 0.129 g of yellow powder (71% yield).
1H NMR (CD3NO2): δ 1.76 (m, 4H, THF), 3.19 (s, 3H, CH3),
3.59 (m, 4H, THF), 4.10 (s, 2H, CH2), 4.24 (s, 4H, 2CH2),
7.0–7.8 (m, 5H), 8.1–8.8 (m, 6H). Anal. Calcd for
C47H29BCuF20N5O: C, 49.78; H, 2.58; N, 6.18. Found: C,
and
resulted
in
X-ray
quality
crystals
of
[{(LN(CH Ph) )Cu (Cl)}2] (0.042 g, 74%) (see Supporting
Information, Figure S3). As seen before,30 CHCl3 is the
chloride source for this complex; the copper(I) complex
reacts reductively with the chlorocarbon, oxidizing copper
and leading to a chloride-bound Cu(II) product. Infrared
(Nujol mull): 3490 cm-1 (w, br, H2O). EPR: g| ) 2.218, g⊥
) 2.042, A| ) 182 G, A⊥ ) 41.5 G. Anal. Calcd for
(C56H35BClCuF20N5O2)2: C, 51.75; H, 2.71; N, 5.39. Found:
C, 51.47; H, 2.55; N, 5.09.
II
2+
2
2
Reaction of [(LN(CH ) )Cu ]B(C6F5)4 (9) with O2. The
I
3 2
49.50; H, 2.59; N, 5.90.
synthesis and characterization of tetradentate ligand LN(CH )
I +
Reaction of [(LNH(CH ))Cu ] (2) with O2. A THF solution
3 2
3
and copper(I) complex, [(LN(CH ) )Cu ]B(C6F5)4 (9) were
I
(3.1 mL) of [(LNH(CH ))Cu ]B(C6F5)4 (2) (0.003 g, 0.003
I
3 2
3
previously reported.28 A 3.8 mL Et2O solution of
mmol) was taken in a UV–vis cuvette assembly under argon
and cooled to -80 °C, and an initial spectrum was recorded.
Dioxygen was bubbled for 20 s through the solution using a
long syringe needle, and a new spectrum recorded. Additional
[(LN(CH ) )Cu ] (9) (0.005 g, 0.005 mmol) was taken in a
I +
3 2
UV–vis cuvette assembly under argon and was cooled to
-80 °C, and an initial spectrum was recorded. Dioxygen
was bubbled for 30 s through the solution using a long
syringe needle, and the new spectrum was recorded (see
Supporting Information, Figure S2). The product was
bubbling did not change the spectrum further.
I +
Synthesis of Copper(I) Complex, [(LN(CH Ph) )Cu ]
2
2
(10). LN(CH Ph)
(0.050 g, 0.103 mmol) and
2
2
III
2-
2+
characterized as [{(LN(CH ) )Cu }2(O )2] (9-O2).
[CuI(CH3CN)4]B(C6F5)429 (0.093 g, 0.103 mmol) were placed
in a 25 mL Schlenk flask under argon. THF (2 mL) was
added with stirring to form a yellow solution under argon.
Pentane (30 mL) was then added to the solution to precipitate
a yellow solid. The supernatant was decanted, and the solid
was recrystallized three times from THF/pentane. The solid
obtained was washed with pentane and dried under vacuum
(1 h), giving 0.097 g of yellow powder (77% yield). 1H NMR
(DMSO-d6): δ 0.88 (t, 6H, 2CH3, C6H14), 1.27 (m, 8H, 4CH2,
C6H14), 2.10 (s, 2H, 0.3CH3COCH3), 3.36 (s, 5H, 2.5H2O),
3.73 (s, 2H, CH2Py), 4.01 (s, 4H, 2CH2Py), 4.93 (s, 4H,
3 2
Synthesis of [{(6tbp)CuII(Cl)}2]2+. In a 25 mL Schlenk
flask, [(6tbp)CuI]B(C6F5)4 (0.100 g, 0.086 mmol) was dis-
solved in 4 mL of THF under an argon atmosphere. The
resulting yellow solution was stirred at room temperature,
and CHCl3 (0.5 mL) was then added under argon, whereupon
the solution ceased to look transparent, and its color instantly
turned to green. After 2 h, 35 mL of pentane was added to
precipitate the copper complex, which was recrystallized
from THF/pentane. X-ray quality crystals of the dimer
complex [{(6tbp)CuII(Cl)}2]2+ were obtained (see Supporting
Information, Figure S3) from THF solution of the compound
by layering with pentane. After they were vacuum-dried, the
green crystals weighed 0.085 g (83%). Anal. Calcd for
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3790 Inorganic Chemistry, Vol. 47, No. 9, 2008

Copper Dioxygen Adducts
(C52H30BClCuF20N4)2: C, 52.02; H, 2.52; N, 4.67. Found:
C, 51.51; H, 2.80; N, 4.51. EPR: g| ) 2.219, g⊥ ) 2.041, A|
) 176 G, A⊥ ) 37.7 G.
S6, see below, for conditions) revealing the formation of
p-tolyl disulfide (∼70% yield, GC, GC-MS, authentic
commercial p-tolyl disulfide was used for comparison).
Reaction of Ethyl benzene with 7-O2. In a 10 mL
Schlenk flask, 4 mL of an ethylbenzene solution of
[(6tbp)CuI]B(C6F5)4 (0.030 g, 0.025 mmol) was prepared in
the glovebox. The flask was cooled to -80 °C. O2 was
bubbled for 10 s using a long syringe needle to form
[{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2 (excess O2 removed). The
resulting solution was warmed up to RT after 4 h, whereupon
rotary evaporation concentrated the solution to ∼0.4 mL.
The copper complex was precipitated out by 20 mL of
pentane and was washed two more times with 30 mL of
pentane. The collective pentane washes were dried over
Na2SO4, filtered, concentrated, and subjected to GC and GC-
MS analyses (see Figure S7, see below, for conditions),
which confirmed the formation of PhCOCH3 (authentic
commercial PhCOCH3 was used for comparison). After it
was washed with pentane, the copper product was further
purified by recrystallization from Et2O/pentane, giving
Reaction of [(6tbp)CuI]B(C6F5)4 (7) with O2. Crystal-
lographically characterized31 [(6tbp)CuI]B(C6F5)4 (7) (0.009
g, 0.008 mmol) was dissolved in 4.0 mL of Et2O and was
transferred in the UV–vis cuvette inside the MBraun drybox.
Outside on the benchtop, the cuvette was cooled to -80 °C,
and an initial spectrum was recorded. Dioxygen was bubbled
through the solution with a long syringe needle. This results
in an immediate color change from colorless to a yellowish-
brown (excess dioxygen removed) EPR silent complex. The
product is formulated as [{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2 (7-
O2), a bis-µ-oxo-dicopper(III) complex.
Reaction of [{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2 (7-O2)
with 1:1 PhCH3 and PhCD3. [(6tbp)CuI]B(C6F5)4 (7) (0.046
g, 0.039 mmol) was dissolved in a mixture of 0.477 g of
PhCH3 and 0.492 g of PhCD3 in the glovebox. After the
mixture was cooled to -80 °C, dioxygen was bubbled
through the solution for 10 s with a long needle, and
[{(6tbp)CuIII}2(O2-)2]2+ (7-O2) was formed; excess dioxygen
was removed carefully via evacuation and purging with
argon. After one hour, the solution was warmed to room
temperature and was transferred to a 20 mL vial connected
with an adaptor. The resulting solution was concentrated by
careful rotary evaporation until ∼ 0.3 mL of solution
remained. The copper complex was precipitated by addition
of 10 mL of pentane. Further washing was carried out two
more times with 15 mL of pentane, and the collective pentane
washes were dried over Na2SO4, filtered, concentrated, and
subjected to GC-MS analyses (see below, for conditions),
which confirmed the formation of PhCHO and PhCDO in
∼7:1 ratio (see Supporting Information Figure S5). (Note:
Authentic commercial PhCHO was used for comparison.)
Reaction of p-Thiocresol with [{(6tbp)CuIII}2(O2-)2]-
(B(C6F5)4)2 in Et2O. [(6tbp)CuI]B(C6F5)4 (7) (0.035 g, 0.030
mmol) was dissolved in a 10 mL Schlenk flask with 3 mL
of Et2O inside the glovebox. Removal of this flask from
glovebox to the benchtop and cooling to -80 °C, followed
by bubbling O2 (15 s) via a long syringe needle, was carried
out to generate [{(6tbp)CuIII}2(O2-)2]2+ (7-O2). Excess di-
oxygen was removed carefully via evacuation and purging
with argon. p-Thiocresol (0.005 g, 0.040 mmol) was added
to the solution of [{(6tbp)CuIII}2(O2-)2]2+; the resulting
mixture was bubbled with argon for 5 s to ensure thorough
mixing with the substrate at -80 °C. The reaction solution
was warmed up to RT after three hours, transferred to a 20
mL vial, connected with an adaptor, and concentrated by
rotary evaporation until ∼0.4 mL of solution remained. This
was transferred by syringe to a larger flask, and pentane (15
mL) was added. The pentane solution was decanted, and the
remaining solid washed three more times with pentane (30
mL), whereupon the pentane solutions were combined and
dried over Na2SO4. After filtration and concentration, the
pentane solution was analyzed by GC and GC-MS (Figure
0.026
g
(yield ∼85%) of product identified as
[(6tbp)2CuII (OH)2](B(C6F5)4)2 (7-OH, see Results and Dis-
2
cussion). The complex is EPR silent, and νOH ) 3580 cm-1
(see Supporting Information, Figure S8). Anal. Calcd for
[(6tbp)2CuII (OH)2](B(C6F5)4)2; C52H31BCuF20N4O: C, 52.83;
2
H, 2.64; N, 4.74. Found: C, 52.47; H, 2.68; N, 4.64.
Reaction of 7-O2 with Hydroquinone. In a 100 mL
Schlenk flask, an 11 mL Et2O solution of [(6tbp)CuI]B(C6F5)4
(7) (0.056 g, 0.048 mmol) was prepared in the dry box, and
this was removed to the benchtop and cooled to -80 °C.
Dioxygen was bubbled through the solution to form
[{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2 (7-O2) (excess O2 re-
moved). A 100 µL solution of hydroquinone (0.003 g, 0.027
mmol) was added, and the solution kept chilled for 3 h.
Pentane (40 mL) was added to the resulting solution, and
then removed from the resulting solid by decantation. Three
additional pentane washes were carried out. The volume of
the combined pentane solutions was reduced and dried over
Na2SO4, whereupon GC and GC-MS analysis (Supporting
Information Figure S9, see below, for conditions) confirmed
the formation of 1,4-benzoquinone (also, compared with
authentic commercial 1,4-benzoquinone) in ∼ 95% yield.
Reaction of 2,4,6-tBu3-phenol with [{(6tbp)CuIII}2-
(O2-)2](B(C6F5)4)2 (7-O2). A solution of 2,4,6-tBu3-phenol
(0.006 g, 0.023 mmol) was prepared in 100 µL of Et2O inside
the glovebox. [(6tbp)CuI]B(C6F5)4 (0.025 g, 0.022 mmol) was
dissolved in 5.0 mL of Et2O and taken into a Schlenk cuvette,
which was connected to a Schlenk line and cooled to -80
°C; the initial UV–vis spectrum was recorded. With a long
syringe needle, dioxygen was bubbled through the solution
for 10 s, and excess dioxygen was removed carefully via
evacuation and purging with argon. The 2,4,6-tBu3-phenol
solution was added, and the UV–vis spectrum recorded (yield
∼55%). The cold reaction solution was transferred to EPR
sample tubes at -80 °C and immediately frozen at 77 K.
The EPR spectra obtained confirmed formation of corre-
sponding 2,4,6-tBu3 phenoxyl radical and the presence of a
copper(II) complex.
(31) Maiti, D.; Lucas, H. R.; Sarjeant, A. A. N.; Karlin, K. D. J. Am. Chem.
Soc. 2007, 129, 6998–6999.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3791

Maiti et al.
Reaction of [{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2 with
Benzylamine. [(6tbp)CuI]B(C6F5)4 (7) (0.045 g, 0.039 mmol)
was dissolved in 5 mL of Et2O solution inside the dry box, and
this was taken into a 25 mL Schlenk flask. On the benchtop,
this flask was cooled to -80 °C, and dioxygen bubbling led to
[{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2, whereupon excess O2 was
removed. Benzylamine (0.004 g, 0.037 mmol) was added; the
resulting mixture was bubbled with argon for 5 s, and the
solution was kept at -80 °C for 3 h. The reaction solution was
washed two times with pentane (20 mL), and the resulting
supernatant was dried over Na2SO4. After the volume was
reduced, GC and GC-MS analysis (Supporting Information
Figure S10, see below, for conditions) confirmed the formation
of benzonitrile (also, compared with authentic commercial
benzonitrile) in ∼43% yield.
GC and GC-MS Analysis of the Reactions Above. All
GC/MS experiments were carried out and recorded using a
Shimadzu GC-17A/GCMS0QP5050 Gas Chromatograph/
Mass Spectrometer. GC experiments were carried out and
recorded using a Hewlett-Packard 5890 Series II Gas
Chromatograph. The GC conditions for the product analysis
were the following: injector port temp 250 °C; detector temp
250 °C; column temp, initial temp 80 °C, initial time 2 min,
final temp 250 °C, final time, 25 min; gradient rate 30 °C/
min; flow rate: 45 mL/min. The GC-MS conditions for the
product analysis were the following: injector port temp
220 °C; detector temp 280 °C; column temp, initial temp
100 °C, initial time 2 min, final temp 250 °C, final time 25
min; gradient rate 10 °C/min; flow rate: 16 mL/min;
ionization voltage: 1.5 kV.
Reaction of [{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2 (7-O2)
with TMPA. [(6tbp)CuI]B(C6F5)4 (0.007 g, 0.006 mmol) was
dissolved in 4.0 mL of Et2O under argon in a UV–vis cuvette
equipped with a glass stopper. This was cooled to -80 °C,
and an initial spectrum was recorded. O2 was bubbled to
the copper(I) solution to form [{(6tbp)CuIII}2(O2-)2]2+ (7-
O2). Once the formation of 7-O2 was complete, the cuvette
assembly was attached to a vacuum line, and excess dioxygen
was removed carefully via vacuum/argon cycles. While under
a hard stream of Ar, the glass stopper was replaced with a
rubber septum, and a TMPA (0.002 g, 0.007 mmol) solution
was transferred anaerobically to the cuvette. An immediate
solution color change occurred from yellowish brown to
purple. To ensure mixing of the solution, a long syringe
needle was inserted, and argon was purged through the
solution (kept at -80 °C) for 5 s. Further addition of TMPA
(0.002 g, 0.007 mmol) caused no change in spectrum
recorded. Thus the addition of TMPA causes generation of
species with absorption features at 525 nm, identical to that
observed for solutions from [(TMPA)CuI(CH3CN)]+/O2
reactivity. The percent conversion (∼90%) was calculated
on the basis of the extinction coefficient of the peak at
525 nm and comparison to an authentic sample of
[(TMPA)CuI(CH3CN)]+/O2.
Addition of HCl to [{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2
(7-O2). [(6tbp)CuI]B(C6F5)4 (7) (0.020 g, 0.017 mmol) was
dissolved in 5 mL of Et2O under argon in a Schlenk flask
equipped with a magnetic bar; the solution was cooled to
-80 °C. O2 was bubbled to the copper(I) solution to form
[{(6tbp)CuIII}2(O2-)2]2+, and excess O2 was removed care-
fully. HCl (34 µL) in Et2O (Aldrich, 2 M) was added
anaerobically to the flask using a plastic syringe; there was
an immediate color change from yellowish brown to green.
With a long needle, argon was purged through the solution
for 5 s, and the solution was stirred for 30 min. Distilled
water (15 mL) was added after the reaction solution was
warmed to room temperature. The resulting mixture was
stirred for 1 h at RT, and the aqueous layer was analyzed
for H2O2 using TiO(SO4) test reagent32–34 giving the result
that hydrogen peroxide was formed in ∼80% yield. The
Reaction of PhCH2OH and [(6tbp)CuI]+/O2. In a 10 mL
Schlenk flask, a 3 mL Et2O solution of [(6tbp)CuI]+ (7)
(0.040 g, 0.034 mmol) was prepared in the glovebox.
Following removal to the benchtop, O2 bubbling (10 s) at
-80 °C produced [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) (excess O2
was removed). PhCH2OH (0.004 g, 0.037 mmol) was added
to the copper solution, and the mixture was stirred for 2 h.
A copper complex was precipitated by addition of pentane
(20 mL). Washing was carried out as described above, and
the collective pentane washes were dried over Na2SO4,
filtered, concentrated, and subjected to GC and GC-MS
analyses, which confirmed the formation of PhCHO in ∼80%
yield.
Reaction of 9,10-Dihydroanthracene with [{(6tbp)-
CuIII}2(O2-)2]2+. [{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2 (excess O2
removed) was prepared (at -80 °C) in a 25 mL Schlenk
flask starting with [(6tbp)CuI]+ (7) (0.043 g, 0.037 mmol)
dissolved in 4 mL of Et2O. 9,10-Dihydroanthracene (0.006
g, 0.033 mmol) was added, and the mixture was kept cold
for 2 h. After the mixture was warmed to RT and left to sit
for 6 h, rotary evaporation led to a concentrated solution of
∼0.3 mL. The copper complex was precipitated out by
addition of 15 mL of pentane, and the mixture was washed
two more times with 25 mL of pentane. The collective
pentane washes were dried over Na2SO4, filtered, concen-
trated, and subjected to GC and GC-MS analyses (Supporting
Information Figure S11 and Figure S12), which confirmed
the formation of anthracene and anthraquinone in ∼10% and
∼15% yield, respectively.
Reaction of Xanthene with [{(6tbp)CuIII}2(O2-)2]2+. A
solution of xanthene (0.005 g, 0.027 mmol) was prepared in
50 µL of Et2O inside the glovebox. [(6tbp)CuI]B(C6F5)4
(0.030 g, 0.025 mmol) was dissolved in 2.0 mL of
Et2O and taken in
a
25 mL Schlenk flask.
[{(6tbp)CuIII}2(O2-)2](B(C6F5)4)2 (excess O2 removed) was
prepared at -80 °C, and the xanthene solution was added.
After 5 h, the solution was warmed to RT. The resulting
solution was concentrated to ∼0.2 mL, and the copper
complex was precipitated by addition of 20 mL pentane.
Additional pentane washes (2×, 15 mL each), drying over
Na2SO4, filtration, concentration, and subjection to GC and
GC-MS analyses (Figure S13), confirmed the formation of
xanthone in ∼33% yield.
(32) Chufan, E. E.; Mondal, B.; Gandhi, T.; Kim, E.; Rubie, N. D.; Moenne-
Loccoz, P.; Karlin, K. D. Inorg. Chem. 2007, 46, 6382–6394.
3792 Inorganic Chemistry, Vol. 47, No. 9, 2008

Copper Dioxygen Adducts
nonaqueous Et2O layer (above) was washed three times with
pentane (100 mL total), and the precipitate obtained was
recrystallized from THF/pentane. The X-ray quality crystals
obtained after vacuum drying (0.043 g, ∼85%) were identi-
fied as [{(6tbp)CuII(Cl)}2](B(C6F5)4)2 (Figure S4, Supporting
Information).
Copper(I) and Copper(II) Complexes. Mononuclear
copper(I) complexes not previously reported are
I +
I +
[(LNH(CH ))Cu ] (2) and [(LN(CH Ph) )Cu ] (10) (Chart 2)
which were synthesized under an argon atmosphere by
combination of equimolar mixtures of the respective tet-
3
2
2
29
radentate ligand and [CuI(CH3CN)4]B(C6F5)4 in THF.
Resonance Raman (rR) Spectroscopy. Resonance Ra-
man spectroscopic measurements were undertaken using a
Princeton Instruments ST-135 back-illuminated CCD detector
on a Spex 1877 CP triple monochromator with 1200, 1800,
and 2400 grooves/mm holographic spectrograph gratings.
The excitation was provided by Coherent 190C-K Kr+ and
Innova Sabre 25/7 Ar+ CW lasers. The spectral resolution
was <2 cm-1. Sample concentrations were approximately
0.4 mM in copper-dioxygen complexes and were run at 77
K in a liquid N2 finger Dewar (Wilmad). Samples were
prepared in dried solvents in standard low-grade NMR tubes.
In Baltimore, the precursor copper(I) sample solutions
prepared in the glovebox in NMR tubes under N2 were
cooled to -80 °C on the benchtop and then subjected to O2
bubbling via syringe. Samples employing 18O2 in 25 mL
break-seal flasks (ICON Services, NJ) were prepared as
previously described in detail.35
Isolation of solids was achieved by addition of pentane, and
this was followed by recrystallization (see Experimental
Section). In connection with ligands generated in our own
laboratories and related to the present study involving
dioxygen reactivity, copper(I) compounds [(LCH OMe)CuI]+
2
(1) (X-ray),27 [(BPQA)CuI]+ (6),41–43 [(6tbp)CuI]+ (7) (X-
ray),31 and [(LN(CH ) )Cu ] (9)28 have previously been
I +
3 2
reported.
It has always been very useful to generate and characterize
copper(II) complex analogues of the copper(I) ligand com-
pounds, for example, to deduce copper(II) coordination geom-
etries that may relate to copper-dioxygen complexes.30,44 The
following have previously been reported and character-
27
ized by X-ray crystallography: [{(LCH OMe)CuII(Cl)}2]2+,
2
27
28
+
[{(LCH OMe)CuII(Br)}2]2+, [(LNH(CH ))Cu (Cl)] , [(BPQA)
II
2
3
CuII(Cl)]+,41,42
[(6tbp)CuII(acetone)]2+,31
and
[(LN(CH ) )Cu (H2O)] .28 In the Supporting Information, we also
report the X-ray structures of [{(6tbp)CuII(Cl)}2](B(C6F5)4)2 and
II
2+
3 2
Results and Discussion
[{(LN(CH Ph) )Cu (Cl)}2](B(C6F5)4)2, generated by copper(I) ligand
complex exposure to chloroform,30 a very useful procedure with
which to readily obtain chloride copper(II) complexes which
typically crystallize.41,42
II
2
2
27
28
3 2
Ligand Synthesis. The syntheses of LCH OMe
,
LN(CH )
,
2
and 6tBP31 (Chart 2) were previously reported. LNH(CH )
(Chart 2) was obtained from oxidative N-dealkylation
chemistry of copper(II)-hydroperoxo species of LN(CH )
3
28
3 2
.
[(L)CuII (µ-1,2-O22-)]2+ (A) Complex Formation
2
Generation of LN(CH ) and LN(CH Ph) (Chart 2) involves
pressure tube reaction of BrTMPA with dimethylamine or
dibenzylamine, respectively, under basic conditions (see
Supporting Information). The ligands 6PhTPA ((6-phenyl-
2-pyridylmethyl)bis(2-pyridylmethyl)amine), by Canary and
co-workers12 and Que and co-workers,36 6MeTMPA by
Suzuki and co-workers,37,38 MAPA ((6-amino-2-pyridyl-
methyl)bis(2-pyridylmethyl)amine),39 MPPA ((6-tButylac-
etamido-2-pyridyl-methyl)bis(2-pyridylmethyl)amine) by Ma-
suda and co-workers,40 and BPQA ((2-quinolylmethyl)bis(2-
pyridylmethyl)amine) by Karlin and co-workers41–43 were
previously reported.
3 2
2
2
with Group I Ligand-Copper Complexes. As stated in
the Introduction, these ligand-copper complexes (Chart 2)
form (µ-1,2)-peroxodicopper(II) products (A, Charts 1 and
2), following oxygenation of ligand-copper(I) precursors in
THF solvent at -80 °C. For [(LCH OMe)CuI]B(C6F5)4 (1), we
2
previously reported that the colorless copper(I) solution
changes to the purple species that was formulated as
[{(LCH OMe)CuII}2(µ-1,2-O22-)]2+ (1-O2), {λmax ) 540 nm (ꢀ,
2
9550 M-1cm-1) and 610 nm, sh, (ꢀ, 6500 M-1cm-1)} (Figure
1a).27 These features closely resemble the distinctive pattern
known for the parent complex [{(TMPA)CuII}2(µ-1,2-
O22-)]2+ (A1) and other O2-adducts with similar structures.6
Confirmation, reported here for the first time, comes from
resonance Raman (rR) spectroscopic data (Figure 1b), where
the O-O (848 cm-1), symmetric Cu-O (550 cm-1), and
antisymmetric Cu-O (508 cm-1) stretching vibrations are
readily observable and confirmed by experiments carried out
using 18O2 to form the complexes. These data and corre-
sponding information for other complexes are given in
Table 1.
(33) Chaudhuri, P.; Hess, M.; Mueller, J.; Hildenbrand, K.; Bill, E.;
Weyhermueller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 9599–
9610.
(34) Eisenberg, G. M. Ind. Eng. Chem., Anal. Ed. 1943, 15, 327–328.
(35) Maiti, D.; Fry, H. C.; Woertink, J. S.; Vance, M. A.; Solomon, E. I.;
Karlin, K. D. J. Am. Chem. Soc. 2007, 129, 264–265.
(36) Jensen, M. P.; Que, E. L.; Shan, X. P.; Rybak-Akimova, E.; Que, L.
Dalton Trans. 2006, 3523–3527.
(37) Nagao, H.; Komeda, N.; Mukaida, M.; Suzuki, M.; Tanaka, K. Inorg.
Chem. 1996, 35, 6809–6815.
(38) Uozumi, K.; Hayashi, Y.; Suzuki, M.; Uehara, A. Chem. Lett. 1993,
963–966.
The absorption spectrum of [{(LCH OMe)CuII}2(µ-1,2-
2
O22-)]2+ (1-O2) shows a decreased peroxide πσ* f dσ
charge-transfer band energy as compared to that of [{(TM-
PA)CuII}2(µ-1,2-O22-)]2+ (A1) (1-O2, 540 nm; A1, 523 nm),8
(39) Wada, A.; Honda, Y.; Yamaguchi, S.; Nagatomo, S.; Kitagawa, T.;
Jitsukawa, K.; Masuda, H. Inorg. Chem. 2004, 43, 5725–5735.
(40) Yamaguchi, S.; Wada, A.; Funahashi, Y.; Nagatomo, S.; Kitagawa,
T.; Jitsukawa, K.; Masuda, H. Eur. J. Inorg. Chem. 2003, 4378–4386.
(41) Wei, N.; Murthy, N. N.; Chen, Q.; Zubieta, J.; Karlin, K. D Inorg.
Chem. 1994, 33, 1953–1965.
(42) Wei, N.; Murthy, N. N.; Karlin, K. D. Inorg. Chem. 1994, 33, 6093–
6100.
(44) Schatz, M.; Becker, M.; Thaler, F.; Hampel, F.; Schindler, S.; Jacobson,
R. R.; Tyeklár, Z.; Murthy, N. N.; Ghosh, P.; Chen, Q.; Zubieta, J.;
Karlin, K. D. Inorg. Chem. 2001, 40, 2312–2322.
(43) Lee, D.-H.; Wei, N.; Murthy, N. N.; Tyeklár, Z.; Karlin, K. D.; Kaderli,
S.; Jung, B.; Zuberbühler, A. D. J. Am. Chem. Soc. 1995, 117, 12498–
12513.
(45) Yamaguchi, S.; Nagatomo, S.; Kitagawa, T.; Funahashi, Y.; Ozawa,
T.; Jitsukawa, K.; Masuda, H. Inorg. Chem. 2003, 42, 6968–6970.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3793

Maiti et al.
angle relative to that in [{(TMPA)CuII}2(µ-1,2-O22-)]2+. As
this angle increases to greater than 120°, mechanical mixing
of the Cu-O and O-O modes would occur which could
result in a higher O-O stretching frequency even with a
small decrease in O-O bond strength.46
A THF solution of [(LNH(CH ))Cu ]B(C6F5)4 (2) at -80 °C
reacts with O2 leading to a reddish-brown solution with
prominent UV–vis features at λmax ) 498 nm (ꢀ, 7500 M-1
cm-1) and 635 nm (ꢀ, 1700 M-1 cm-1). Again, the UV–vis
charge-transfer pattern and rR data (vide infra) (Figure 2a
I
3
and b) support the formulation as [{(LNH(CH ))Cu }2(µ-1,2-
O22-)]2+ (2-O2). There is a considerable increase in the
peroxo πσ* f Cu dσ charge-transfer energy as compared to
the parent compound [{(TMPA)CuII}2(µ-1,2-O22-)]2+ (A1)
(Table 1). Masuda and co-workers39,40 have accounted for
such effects within a series of closely related ligand-
copper-dioxygen complexes possessing intramolecular H-
bonding interactions. For 2-O2, H-bonding from the N(H)CH3
II
3
Figure 1. (a) UV–vis spectra at -80 °C in THF, illustrating formation of
the 1-O2 (red spectrum) formed from [(LCH OMe)CuI]+ (1) (black spectrum)
group of LNH(CH ) to the peroxo moiety (see diagram below)
2
3
following bubbling with O2. (b) rR spectrum of [(LCH OMe)CuI]+ (1) (black),
2
would effect these changes. This H-bonding would result in
stabilization of the peroxo πσ* orbital leading to a larger dσ
(Cu) and πσ* (O22-) energy separation and thus an increase
in the πσ* f dσ charge-transfer energy.
1-16O2-(B(C6F5)4)2 (red), and 1-18O2-(B(C6F5)4)2 (blue) (λexcit ) 620 nm
at 77 K).
and the rR spectrum shows a lower (Cu-O)s stretch (1-O2,
550 cm-1; A1, 561 cm–1.8,23,27 These data indicate the Cu-O
bonds of 1-O2 are weaker than those of [{(TMPA)CuII}2(µ-
1,2-O22-)]2+ (A1). A possible explanation for this Cu-O
bond weakening is a square pyramidal distortion of the Cu(II)
The O-O and Cu-O stretching vibrations for 2-O2 occur
at 849 (∆(18O2) ) –47) and 544 (∆(18O2) ) –25) cm-1,
respectively (Table 1). The rR spectral behavior of 2-O2 can
also be interpreted in terms of the contribution of the
hydrogen bonding of the NH group introduced into the
pyridine 6-position of parent TMPA (diagram above). The
H-bonding interaction would decrease the lone-pair repulsion
in the peroxide, strengthening the O-O bond (higher O-O
stretch)47 while weakening the Cu-O bond, in comparison
to systems that do not have such H-bonding (Table 1).
We note that unlike the behavior found for other O2-
adducts with tetradentate N4 ligands possessing structure
A,6,9,43,48,49 the O2-binding and formation of 2-O2 appears
ligand field. The steric congestion in LCH OMe arising
2
from the presence of a 6-substituent at one of the pyridyl
arms of the TMPA parent chelate results in a square
pyramidal
distortion
of
the
ligand
in
27
[{(LCH OMe)CuII(Br)}2](B(C6F5)4)2. In a square pyramidal
complex, three of the chelate ligands would donate into Cu’s
2
2
2
dx -y orbital thus reducing the donation from the peroxo to
the Cu and the strength of this bond. The (Cu-O)as stretch
is also observed for this 1-O2 system (at 508 cm-1) which
further supports distortion of the complex such that it no
longer has a center of inversion.
to be irreversible. Warming, CO bubbling, or both did not
I +
lead to release of O2 and regeneration of [(LNH(CH ))Cu ]
3
While the πσ* f dσ charge transfer and (Cu-O)s stretch
are shifted down in energy; the O-O stretch has increased
in energy (1-O2, 848 cm-1; A1, 832 cm-1).8 The decreased
peroxo πσ* donation to Cu would result in more electron
density in the peroxo antibonding πσ* orbital, thus a weaker,
not stronger, O-O bond. The square pyramidal structural
distortion considered above would increase steric interactions
between ligands to the two Cu’s, potentially resulting in an
increased Cu-Cu distance. This could increase the Cu-O-O
(2).
(46) Brunold, T. C.; Tamura, N.; Kitajima, N.; Moro-Oka, Y.; Solomon,
E. I. J. Am. Chem. Soc. 1998, 120, 5674–5690.
(47) Root, D. E.; Mahroof-Tahir, M.; Karlin, K. D.; Solomon, E. I. Inorg.
Chem. 1998, 37, 4838–4848.
(48) Karlin, K. D.; Wei, N.; Jung, B.; Kaderli, S.; Zuberbühler, A. D. J. Am.
Chem. Soc. 1991, 113, 5868–5870.
(49) Tyeklár, Z.; Jacobson, R. R.; Wei, N.; Murthy, N. N.; Zubieta, J.;
Karlin, K. D. J. Am. Chem. Soc. 1993, 115, 2677–2689.
3794 Inorganic Chemistry, Vol. 47, No. 9, 2008

Copper Dioxygen Adducts
Table 1. Summary of Spectroscopic Features for Binuclear Copper-Dioxygen Adducts
µ-1,2-peroxodicopper(II) complexes
ν/cm-1
,
ν/cm-1
,
with type I ligands
solvent
UV–vis/nm (ꢀ/M-1 cm-1
)
16O-16O (18O-18O)
Cu-16O (Cu-18O)
ref
[{(TMPA)CuII}2(µ-1,2-O22-)]2+ (A1)
EtCN
Et2O
THF
525 (11 300), 615(58 00)
520 (19 200)
832 (788)
827 (783)
848 (801)
849 (802)
b
847 (800)
837 (792)
b
561 (535)
561 (535)
550 (524)
544 (519)
b
544 (518)
b
b
18
23
a
(parent complex)
[{(LCH OMe)CuII}2(µ-1,2-O22-)]2+ (1-O2)
540 (9550), 610 (6500)
498 (7500), 635 (1700)
537 (5500), 600 (4000)
503 (7500), 600 (1500)
517 (5600), 600 (1500)
535 (8600), 600
2
d
[{(LNH(CH ))Cu }2(µ-1,2-O22-)]2+ (2-O2)
THF
a
II
3
[{(6MeTMPA)CuII}2(µ-1,2-O22-)]2+ (3-O2)
[{(MAPA)CuII}2(µ-1,2-O22-)]2+ (4-O2)d
[{(MPPA)CuII}2(µ-1,2-O22-)]2+ (5-O2)d
[{(BPQA)CuII}2(µ-1,2-O22-)]2+(6-O2)
acetone
acetone
acetone
EtCN
27
39
40
41
bis(µ-oxo)dicopper(III) complexes
with type II ligands
solvent
UV–vis/nm (ꢀ/M-1 cm-1
)
ν/cm-1 (Cu-16O)
ν/cm-1 (Cu-18O)
ref
[{(6tbp)CuIII}2(O2-)2]2+ (7-O2)
CH2Cl2
acetone
Et2O
383 (Et2O, g7000)e
378
594
599
c
568
573
c
a
36
[{(6PhTPA)CuIII}2(O2-)2]2+ (8-O2)
[{(LN(CH ) )Cu }2(O2-)2]2+ (9-O2)
383 (18 900)c
390 (5300)
a, 28
III
3
2
a
[{(LN(CH Ph) )Cu }2(O2-)2]2+ (10-O2)
acetone
598
572
III
2
2
a This work. b Not reported. c Not detected. d these possess intramolecular H-bonding to the peroxo group, see text. e See text.
Bis(µ-oxo)dicopper(III) Complex [(L)CuIII2(O2-)2]2+
(C) Formation with Group II Ligand-Copper
Complexes. Reaction of O2 with Copper(I) Complexes
N(CH2Ph)2
of 6tBP, LN(CH ) , and L
. As previously described
3 2
in a qualitative manner, bubbling O2 through -80 °C
Et2O or THF solutions of [(6tbp)CuI]B(C6F5)4 (7) or
I
[(LN(CH ) )Cu ]B(C6F5)4 (9) (Chart 1) results in immediate
3 2
color changes from light to bright brownish-yellow and EPR
Figure 3. (a) UV–vis spectra at -80 °C in Et2O, illustrating formation of
7-O2 (red spectrum) from O2 reaction with [(6tbp)CuI]+ (7) (black
spectrum). A 7-O2 decomposition product (green spectrum at -80 °C) forms
in ∼20 min. (b) rR spectrum (νCu-O region) of 7-O2-(B(C6F5)4)2 with 16O2
(red) or 18O2 (blue) {CH2Cl2 solvent, λexcit ) 386 nm, 77 K}.
silent solutions of [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) {λmax
(Et2O) ) 383 nm (ꢀ g 7000 M-1 cm-1), Figure 3a}31 or
[{(LN(CH ) )Cu }2(O )2] (9-O2) {λmax ) 383 nm (ꢀ )
18900 M-1 cm-1) (Table 1).28,50 The structure assignment
as C (Scheme 1, Chart 2) for 7-O2 and 9-O2 (vide infra)
comes from the characteristic5,6 UV–vis spectra of this
structure, along with rR data given below. Formation of 7-O2
can be performed in a wide range of solvents, for example,
III
2-
2+
3 2
Figure 2. (a) UV–vis spectra at -80 °C in THF, illustrating formation of
µ-1,2-peroxodicopper(II) complex (2-O2) (red spectrum), formed from the
I
+
[(LNH(CH ))Cu ] (2) (black spectrum) reaction with O2, and possessing
3
I
H-bonding (see text). (b) rR spectrum of [(LNH(CH ))Cu ]+ (black), 2-16O2-
3
(B(C6F5)4)2 (red), and 2-18O2-(B(C6F5)4)2 (blue) (λexcit ) 501.7 nm at 77
K).
(50) See Supporting Information.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3795

Maiti et al.
acetone, CH2Cl2, Et2O, THF, propionitrile, toluene, and
ethylbenzene. However it is relatively unstable and decom-
poses within ∼ 20 min (Figure 3a) in all these solvents except
Et2O. Studies in the latter solvent allowed us to at least
provide a lower limit for the extinction coefficient (given
above).
For 7-O2, a strong peak at 594 cm-1 which shifts to 568
cm-1 upon isotopic substitution with 18O2 is observed in rR
spectra (Figure 3b). This can be assigned as a ν(Cu-O)
stretching vibration, consistent with the formulation of the
copper complex as having a bis(µ-oxo)dicopper(III) structure
(C).6 The data further show that there is a less strongly
enhanced band at 1188 cm-1 that is twice the frequency
compared to the strong ν(Cu-O) stretch (594 cm-1); with
O-18-labeling, this shifts to 1135 cm-1, which is nearly twice
that of the value for the ν(Cu-18O) stretch (Figure S14 in
Supporting Information). Thus, this band is assigned as the
overtone of the ν(Cu-O) mode. In addition, ν(Cu-N) modes
are observed (Figure 3b) from the equatorially bound
pyridine (ν(Cu-Npyridine) ) 460 cm-1) and equatorially bound
amine (ν(Cu-Namine) ) 495 cm-1) of 7-O2.23
I
Figure 4. rR spectrum of [(LN(CH Ph) )Cu ]B(C6F5)4 (black) and 10-O2-
(B(C6F5)4)2 formed using 16O2 (red) or 18O2 (blue) {acetone solvent, λexcit
) 380 nm, 77 K}.
2
2
Que and co-workers36 previously described UV–vis and
rR characterization (Table 1) of the bis(µ-oxo)dicopper(III)
species, [{(6-PhTPA)CuIII}2(O2-)2]2+ (8-O2) (Chart 2), which
is very closely related to our own system with 6tBP (but
without the tBu group), described above.
structures (A), most likely because copper(II) complexes with
overall pentacoordinate structures are stable and com-
mon.52,53 For symmetrically disposed, tripodal tetradentate
ligands, trigonal bipyramidal pentacoordiante copper(II)
complexes generally form.42,54 However, when steric inter-
actions become important, the presence of a 6-position
substituent (adjacent to the N-atom) of a pyridyl ring, for
example, the structure can distort. As detailed here, it takes
a 6-XR2 substituent and not just a 6-XHR substituent to effect
such a change. In the O2 adducts of the type discussed in
this report, the O-O bond breaks as elongated Cu-N bond
distances result from the presence of a 6-pyridyl substituent,30
leading to a stable essentially square-planar CuIII2(O2-)2
structure. As mentioned (Introduction), this has been proven
directly for the case of the copper-dioxygen adduct with
the ligand Me2TPA (Chart 1).10 Also, our interpretations are
consistent with ligand effect results observed and summarized
by Itoh and co-workers,1,55 that is, that bis(2-pyridylmethy-
l)amine tridentate ligands afford (bis-µ-oxo)dicopper(III)
complexes when copper(I) precursors are oxygenated at a
low temperature.
III
2-
2+
For [{(LN(CH ) )Cu }2(O )2] (9-O2), the characteristic
UV–vis spectrum obtained readily leads to its formula-
tion.6,28 The complex was found to be unstable for rR
interrogation at 77 K, perhaps because of photochemical
decomposition; therefore, we were unable to obtain rR data
for further confirmation. For this and other reasons,51 we
3 2
synthesized a close analogue LN(CH Ph) and a copper(I)
complex of this ligand (Chart 2). The O2-reactivity of
2
2
I
[(LN(CH Ph) )Cu ]B(C6F5)4 (10) is very similar to that for the
dimethyl analogue 9. Here, low-temperature UV–vis50 and
rR data (Figure 4) could be obtained (Table 1), thus
proving the bis(µ-oxo)dicopper(III) complex formulation,
2
2
III
2-
2+
[{(LN(CH Ph) )Cu }2(O )2] (10-O2).
2
2
Summary: Group I versus Group II (Chart 2)
Tetradentate Ligand-Copper Complexes and their O2
Adduct Structural Preferences. In this report, we have
elucidated the O2-adduct structures obtained with copper(I)
complexes with two ligands of Group I. From previous
literature, four further examples of TMPA-like ligands with
one pyridyl (or quinolyl) group possessing a 6-substituent
of the -XHR type (see Chart 2) are copper(I) complexes
which react with O2 to form (µ-1,2)peroxodicopper(II)
structures (A). Three new examples of Group II ligand-
copper complex O2-chemistry have been described here, all
forming a (bis-µ-oxo)dicopper(III) complex when the single
pyridyl group has a -XR2 substituent (Chart 2) {also see
Table 1}. Data for the closely related complex 8-O2 (Chart
2) are also provided in Table 1.
Exogenous Substrate Reactivity with the Bis(µ-oxo)-
dicopper(III) Complex [{(6tbp)CuIII}2(O2-)2]2+ (7-O2). We
recently reported quite interesting new chemistry when
generating hydroperoxo complexes starting with copper(II)
derivatives of either 6tBP or LN(CH ) (Chart 2).28,31 The
aryl hydroxylation chemistry for the former, or the
3
2
(52) Hathaway, B. J. Copper. In ComprehensiVe Coordination Chemistry;
Wilkinson, G., Ed.; Pergamon: New York, 1987; Vol. 5; pp 533–774.
(53) Kim, E.; Chufán, E. E.; Kamaraj, K.; Karlin, K. D. Chem. ReV. 2004,
104, 1077–1133.
As stated in the Introduction, tetradentate ligands with
nitrogen donors tend to form (µ-1,2)peroxodicopper(II)
(54) Karlin, K. D.; Hayes, J. C.; Shi, J.; Hutchinson, J. P.; Zubieta, J. Inorg.
Chem. 1982, 21, 4106–4108.
(51) The oxidative N-dealkylation chemistry of copper complexes with this
ligand will be reported elsewhere.
(55) Osako, T.; Ueno, Y.; Tachi, Y.; Itoh, S. Inorg. Chem. 2003, 42, 8087–
8097.
3796 Inorganic Chemistry, Vol. 47, No. 9, 2008

Copper Dioxygen Adducts
Scheme 2
g ≈ 2 radical signal, accompanied by absorptions typical
for copper(II) ion complexes (Figure 6). We suggest that
this latter only moderately stable (at -80 °C; UV–vis
criterion, Figure 5) species arises as a direct result of phenol
H-atom abstraction by 7-O2, possibly giving the binuclear
complex [{(6tbp)2CuIICuIII}(O2-)(OH-)]2+ (7-O2-H) (Scheme
3). However, the four-line EPR spectrum and lack of a half-
field (Ms ) 2 transition at g ≈ 4) transition suggests that a
mononuclear rather than binuclear species is present. In case
the spectrum represented a localized mixed-valent form of
7-O2-H, we hoped we might observe a delocalized structure
and seven-line spectrum by recording EPR spectra at higher
temperatures; however, the thermal instability of this EPR
active copper entity thwarted our efforts. Such an initial
mixed-valent dicopper product formed following CuIII2(O2-)2
abstraction of a substrate H-atom has been previously
hypothesized7,20,25,56,58–61 and suggested by computational
studies58,62,63 to have a reasonable stability; however, no
experimental evidence, spectroscopic or structural, has yet
been provided. It is interesting to note that Itoh and
co-workers60 have also speculated that such a binuclear
complex product, [{(ligand)2CuIICuIII}(O2-)(OH-)]2+,
might split into mononuclear species including
[(ligand)CuII-OH]1+, which would likely give rise to an EPR
signal like that which we observe. More studies are needed.
The bis(µ-oxo)dicopper(III) complex 7-O2 was also reacted
with 2,4-tBu2-phenol and 1,4-hydroquinone. With the former
substrate, the typically observed 2,4-tBu2-phenol oxidatively
coupled dimer forms (50% yield).31 With hydroquinone, 1,4-
benzoquinone could be isolated in high yield (∼95%)
(Scheme 2).
oxidative N-dealkylation chemistry in the latter system are
not effected by the related copper(I)-dioxygen adducts
described here, that is, bis-µ-oxodicopper(III) complexes
[{(6tbp)CuIII}2(O2-)2]2+ (7-O2) or [{(LN(CH ) )Cu }2(O )2]
(9-O2).28,31 While reactivity studies with bis(µ-oxo)dicop-
per(III) entities are reasonably well-known,5,7,56 we wished
to round out studies of these singly pyridyl 6-substituted
complexes. Here, we report such investigations with 7-O2,
as one example that we chose to study in some detail (vide
infra). For ligand complexes from our own laboratories (1,
2, 6, 9, and 10, Chart 2), reactivity studies with exogenous
substrates have not been examined, while as far as we are
aware (via searching the literature), the same is true for 2CuI/
O2 derived complexes for 3–5 and 8.
III
2- 2+
3 2
Reaction of 7-O2 with ArSH, PhCH2OH, and
ArCH2NH2 Substrates. After removal of excess dioxygen
from solutions of bis(µ-oxo)dicopper(III) complex 7-O2 at
-80 °C, 1–3 equiv of various substrates were added. After
workup, the organic products were characterized by GC and
GC-MS spectrometry (see Experimental Section), and the
results are summarized in Scheme 2. Reactions of thiocresol,
benzyl alcohol, benzylamine, and tetrahydrofuran produce
4-methylphenyl disulfide, benzaldehyde, benzonitrile, and
2-hydroxytetrahydrofuran (along with γ-butyrolactone), re-
spectively. [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) reacts with xan-
thene to give xanthone, and 9,10-dihydroanthracene leads
to the production of anthracene and anthraquinone. Unreacted
starting material substrates constitute the only other materials
isolated or detected in these reactions (Scheme 2). The
reactions with phenols, toluene, and ethylbenzene are
described below.
Reaction of 7-O2 with Phenolic Substrates. Upon
addition of an anaerobic solution of 2,4,6-tBu3-phenol to a
-80 °C solution of [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) (excess
O2 removed), there is an immediate change in its UV–vis
spectrum (Figure 5, yield ∼55%). The strong band at 383
nm from 7-O2 disappears, and a new broad peak appears at
∼360 nm (ꢀ ≈ 2150 M-1 cm-1) accompanied by sharp
feature at 405 nm (Figure 5, inset); the latter is character-
istic57 of the 2,4,6-tBu3 phenoxyl radical. Aliquots of this
-80 °C solution were transferred to EPR sample tubes and
immediately frozen at 77 K. EPR spectra exhibited the typical
Thus, as seen for many other CuIII2(O2-)2 complexes,5,7
7-O2 appears to effect hydrogen-atom abstraction reactions
and acts as an overall two-electron oxidant. This conclusion
is further supported by the more detailed study carried out
involving toluene as substrate.
Toluene and Ethylbenzene Oxygenation with
[{(6tbp)CuIII}2(O2-)2]2+ (7-O2). To our knowledge, neither
toluene nor ethylbenzene have previously been examined as
substrates for copper(I)/O2 derived oxidants. We find that
7-O2 oxidizes toluene or ethylbenzene (as solvents) to
benzaldehyde (∼33%) and acetophenone (∼35%), respec-
tively. The yields given are on a per dicopper basis. As will
be described here, the nature of these reactions are consistent
with that of monooxygenase-type chemistry where the result
of a 2 Cu(I)/O2 reaction would lead to transfer of one of the
two oxygen atoms of molecular oxygen (from a Cu2O2-
derived reactive species) to a substrate, leaving behind two
copper(II) ions (and oxide, hydroxide, or water also derived
(58) Henson, M. J.; Mukherjee, P.; Root, D. E.; Stack, T. D. P.; Solomon,
E. I. J. Am. Chem. Soc. 1999, 121, 10332–10345.
(59) Itoh, S.; Taki, M.; Nakao, H.; Holland, P. L.; Tolman, W. B., Jr.;
Fukuzumi, S. Angew. Chem., Int. Ed. 2000, 39, 398–400.
(60) Itoh, S.; Nakao, H.; Berreau, L. M.; Kondo, T.; Komatsu, M.;
Fukuzumi, S. J. Am. Chem. Soc. 1998, 120, 2890–2899.
(61) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. J. Am. Chem. Soc. 1996,
118, 11575–11586.
(56) Cole, A. P.; Mahadevan, V.; Mirica, L. M.; Ottenwaelder, X.; Stack,
T. D. P. Inorg. Chem. 2005, 44, 7345–7364.
(62) Spuhler, P.; Holthausen, M. C. Angew. Chem., Int. Ed. 2003, 42, 5961–
5965.
(57) Kajii, Y.; Fujita, M.; Hiratsuka, H.; Obi, K.; Mori, Y.; Tanaka, I. J.
Phys. Chem. 1987, 91, 2791–2794.
(63) Cramer, C. J.; Pak, Y. Theor. Chem. Acc. 2001, 105, 477–480.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3797

Maiti et al.
Figure 5. UV–vis spectra at -80 °C in Et2O, illustrating formation of the 7-O2 (red spectrum) from [(6tbp)CuI]+(7) (blue spectrum) reaction with dioxygen,
followed by addition of 2,4,6-tBu3-phenol to give a new species (green spectrum). The latter decomposes rapidly (within 5 min at -80 °C); the spectra in
black are observed with time, and these eventually give way to the final (at -80 °C) brown spectrum. The inset shows the expanded region containing the
2,4,6-tBu3 phenoxyl radical at ∼405 nm.
With this information in hand, we can suggest mechanisms
of reaction for the 7-O2 mediated oxygenation of toluene.
An initial H-atom abstraction from the benzylic toluene
methyl group gives a mixed-valent µ-hydroxo-µ-oxo dicop-
per species 7-O2-H; (Scheme 4), that species discussed
above in connection with reactions of 7-O2 with ArOH
substrates. {Note: A mixed-valent (µ-oxo)(µ-hydroxo) di-
metal(II,III) species has been previously proposed in the
oxidation of a supporting ligand mediated by a bis(µ-
oxo)dinickel(III) species.}66 Accompanying formation of this
could be a free benzyl radical; however we see no evidence
for this in that no coupled product, bibenzyl, could be
detected in the reaction product mixture. Thus, perhaps a
benzyl radical rebounds to give a µ-(OCH2Ph)-µ-OH product
7-OH-OR (Scheme 4, path A) or this forms directly in the
original 7-O2/toluene reaction. Continuing along Path A, to
account for the final products 7-OH and PhCHO, another
mol equiv of 7-O2 must react with 7-OH-OR in a process
which seems hard to imagine in detail, but which has been
discussed previously.7,20,25,56,58–61 We should point out that
absolutely no benzyl alcohol could be detected as a product,
which suggests this compound is not released from 7-OH-
OR, which would leave behind a mono-oxo-bridged dicop-
per(II) complex.
Another possible reaction pathway can involve the benzyl
peroxo radical (PhCH2OO•), formed from the reaction of
benzyl radical with molecular O2. The latter would be present
because of the dynamic behavior of 7-O2 (vide infra) via
reformation of an O-O bond, that is, the reverse reaction
of the copper(I) complex (7) oxygenation process. The
species PhCH2OO• can disproportionate via known alkylp-
eroxo radical chemistry to yield PhCHO and PhCH2OH in
1:1 ratio.67–69 However, this is likely not what occurs here
because PhCHO is the only observed product in 7-O2/toluene
reactivity (vide supra). Instead, we suggest the likely course
of reaction is a [(6tbp)CuI]+ (7) reduction of PhCH2OO•
giving the alkylperoxo species [(6tbp)CuII-OOCH2Ph]+;68,69
Figure 6. EPR spectrum following reaction of [{(6tbp)CuIII}2(O2-)2]2+
(7-O2) with 2,4,6-tBu3-phenol at 4 K in Et2O solvent. The products
include the 2,4,6-tBu3-phenoxyl radical (g
≈
2), along with
[{(6tbp)2CuIICuIII}(O2-)(OH-)]2+ (7-O2-H) or species derived from this
{EPR parameters g| ) 2.232, A| ) 175 G}, see text.
from O2).64,65 Thus the theoretical yield is 50% based on
the dicopper center, and thus our observed yields are really
67–70%. For toluene as substrate, a single dicopper(II)
product could also be isolated in 85% yield, the bis-µ-
hydroxide-bridged complex [{(6tbp)CuII}2(–OH)2]2+(7-OH).
Its formulation is supported by IR (νOH ) 3580 cm-1)50 and
EPR (silent) spectroscopies along with its elemental analysis
(see Experimental Section).
When 18O2 was employed to generate [{(6tbp)-
CuIII}2(18O2-)2]2+ (7-18O2) in toluene, workup revealed the
formation of PhCH18O, with ∼70% 18O incorporation.31
Thus, the O-atom in the benzaldehyde product is dioxygen
derived. Further insights came from a competitive intermo-
lecular substrate oxidation reaction, where 7-O2 was gener-
ated within a 1:1 solution mixture of PhCH3 and PhCD3.
The result was formation of PhCHO and PhCDO in a ∼7:1
ratio, as determined by GC and GC-MS spectrometry. Thus,
an apparent deuterium isotope effect (kH/kD) of ∼7 is
observed. In fact, kH/kD values vary from 1 to 25 for a wide
variety of substrate/bis(µ-oxo)dicopper(III) reactions.7,20,59,61,62
(64) Karlin, K. D.; Zuberbühler, A. D. Formation, Structure and Reactivity
of Copper Dioxygen Complexes. In Bioinorganic Catalysis: Second
Edition, ReVised and Expanded; Reedijk, J., Bouwman, E., Ed.; Marcel
Dekker, Inc.: New York, 1999; pp 469–534..
(65) Karlin, K. D.; Gultneh, Y.; Hayes, J. C.; Cruse, R. W.; McKown,
J. W.; Hutchinson, J. P.; Zubieta, J. J. Am. Chem. Soc. 1984, 106,
2121–2128.
(66) Cho, J.; Furutachi, H.; Fujinami, S.; Tosha, T.; Ohtsu, H.; Ikeda, O.;
Suzuki, A.; Nomura, M.; Uruga, T.; Tanida, H.; Kawai, T.; Tanaka,
K.; Kitagawa, T.; Suzuki, M. Inorg. Chem. 2006, 45, 2873–2885.
3798 Inorganic Chemistry, Vol. 47, No. 9, 2008

Copper Dioxygen Adducts
Scheme 3
Scheme 4
Scheme 5
Scheme 6
this would then eliminate PhCHO, while concomitantly
producing 0.5 equiv of [{(6tbp)CuII}2(-OH)2]2+ (7-OH)
(Scheme 4, path B).
(A1) compared to [{(6tbp)CuIII}2(O2-)2]2+ (7-O2). This likely
is the result of a lesser ligand electron-donating capability
of 6tBP or the steric hindrance coming from the 6-tert-
butylphenyl substituent which results in Cu-N bond elonga-
tion and geometric distortion from trigonal bipyramidal to
square pyramidal geometry.27,28,30,31
Reaction of [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) with
TMPA: Ligand Exchange Accompanied by O-O Bond
Formation. Evidence for dynamic behavior in the chemistry
of 7-O2 comes from observations of its reactivity with the
tripodal tetradentate ligand TMPA. When this (1.1 to excess)
is added to solutions of 7-O2 from which excess O2 has been
removed, [{(TMPA)CuII}2(µ-1,2-O22-)]2+ (A1) forms (Scheme
5). This transformation can be followed spectrophotometri-
cally, Figure 7; a brownish yellow solution of 7-O2 changes
to purple gives the A1, with its highly characteristic
spectrum. We have previously reported related (di)oxygen
transfer reactions.70
From the observed spectrum and known absorptivity of
[{(TMPA)CuII}2(µ-1,2-O22-)]2+ (A1)49 in Et2O solvent,23,71
one calculates that a 90% conversion of 7-O2 to A1 has
occurred (Scheme 5). The result indicates a greater overall
thermodynamic stability of [{(TMPA)CuII}2(µ-1,2-O22-)]2+
Protonation of [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) and
Generation of H2O2. When a solution of [{(6tbp)-
CuIII}2(O2-)2]2+ (7-O2) in Et2O at -80 °C is subjected to
acid by addition of excess (∼2 equiv) dry HCl in Et2O, an
immediate color change to green occurs, and [{(6tbp)CuII-
(Cl)}2]2+ forms in ∼85–90% yield. The identity of this
copper(II) product was confirmed by comparison to authentic
material (see discussion above and Supporting Information).
Hydrogen peroxide is also generated (80% yield); it was
identified and quantified using the titanium oxy-sulfate
method (see Experimental Section). This result further
demonstrates that a reversible equilibrium between bis(µ-
oxo)dicopper(III) and (µ-1,2)peroxodicopper(II) complex
forms exists. Here, protonation competes favorably with
peroxoide-Cu(II) binding resulting in the release of H2O2
(Scheme 6). We note here that Suzuki and co-workers10 were
even further able to show that O2 could be released from
the bis(µ-oxo)dicopper(III) complex formed using Me2TPA
(Chart 1), either by bubbling solutions with N2 or by adding
PPh3, the latter which binds strongly to the reformed
copper(I) complex.
Figure 7. Addition of TMPA to bis(µ-oxo)dicopper(III) complex 7-O2 at
-80 °C in Et2O (green spectrum) leads to immediate formation of
[{(TMPA)CuII}2(µ-1,2-O22-)]2+ (A1, red spectrum).
Inorganic Chemistry, Vol. 47, No. 9, 2008 3799

Maiti et al.
Summary/Conclusion
the lone-pair repulsions of the peroxide, leading to strength-
ening of the O-O bond with a shift of ν(O-O) to higher
energy (848 cm-1).
We have shown here that subtle steric demands lead to
structural changes in ligand-copper(I)/dioxygen complex
reactions when the ligand is a tripodal tetradentate analog
of TMPA (tris(2-pyridylmethyl)amine) where only one
pryidyl arm possesses a 6-substituent. When the 6-pyridyl
substituent is a -XHR group (X ) N or C), the traditional
copper/dioxygen adduct forms, a (µ-1,2)peroxodicopper(II)
species (A). However, when the substituent is the slightly
bulkier XR2 moiety {aryl or NR2 (R * H)}, a bis(µ-
oxo)dicopper(III) structure (C) is favored (Chart 2).
A -XHR group substitution results in a charge-transfer
band energy red shift and decreased ν(Cu-O)s stretches
compared to those of [{(TMPA)CuII}2(µ-1,2-O22-)]2+ (A1).
These changes are ascribed to geometric distortions which
lead to decreased overlap between dσ(Cu) and πσ* (O2
The bis(µ-oxo)dicopper(III) species substrate reactivity of
our complex [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) has been probed,
and for the first time (to our knowledge), we can demonstrate
exogenous toluene or ethylbenzene hydrocarbon oxygenation.
Typical monooxygenase type chemistry results, including
18O-labeling experiments, demonstrate that the oxygen atom
in the benzaldehyde product derived from toluene is derived
from molecular oxygen. A H-atom abstraction capability in
the chemistry of 7-O2 is apparent from its reactions with
ArOH substrates, as well as the determination of kH/kD ≈ 7
in the toluene oxygenation (i.e., demonstrated from competi-
tive reactions with PhCH3 vs PhCD3 substrates). Proposed
mechanisms of reaction were presented, including the
possible involvement of formation of PhCH2OO• and its
reaction with copper(I) complex that derives from dynamic
solution behavior of 7-O2. External TMPA ligand exchange
for copper in 7-O2 and O-O bond (re)formation chemistry,
along with the ability to protonate 7-O2 (which does not
possess an O-O bond) and effect release of H2O2,. indicate
an equilibrium between [{(6tbp)CuIII}2(O2-)2]2+ (7-O2) and
a (µ-1,2)peroxodicopper(II) form.
2-
)
ligand. There is also an accompanying shift in ν(O-O) to
higher energy, from 830 cm-1 for A1 to 848 cm-1 for
[{(LCH OMe)CuII}2(µ-1,2-O22-)]2+ (1-O2), which could result
2
from mechanical coupling of the Cu-O and O-O modes
in a structurally distorted binuclear complex. For the complex
II
[{(LNH(CH ))Cu }2(µ-1,2-O22-)]2+ (2-O2), a blue shift of the
prominent UV–vis charge-transfer band likely arises from
stabilization of the πσ* (O22-) orbital because of an intramo-
lecular H-bonding interaction, as previously observed by
Masuda and co-workers.39,40 The H-bonding also reduces
3
Acknowledgment. This work was supported by grants
from the National Institutes of Health (K.D.K., GM28962;
E.I.S, DK31450).
(67) Kim, C.; Dong, Y. H.; Que, L. J. Am. Chem. Soc. 1997, 119, 3635–
3636.
(68) Masarwa, A.; Rachmilovich-Calis, S.; Meyerstein, N.; Meyerstein, D.
Coord. Chem. ReV. 2005, 249, 1937–1943.
Supporting Information Available: X-ray structure, selected
bond lengths and angles, UV–vis, IR, and resonance Raman
spectroscopic details, and CIF files. This material is available free
of charge on the Internet at http://pubs.acs.org.
(69) Goldstein, S.; Meyerstein, D. Acc. Chem. Res. 1999, 32, 547–550.
(70) Sanyal, I.; Karlin, K. D.; Strange, R. W.; Blackburn, N. J. J. Am.
Chem. Soc. 1993, 115, 11259–11270.
(71) Zhang, C. X.; Kaderli, S.; Costas, M.; Kim, E.-i.; Neuhold, Y.-M.;
Karlin, K. D.; Zuberbühler, A. D. Inorg. Chem. 2003, 42, 1807–1824.
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