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

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 Cu₂O₂isomer coming from (ligand)-Cu^I/O₂reactivity 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/O₂adduct forms a (µ-1,2)peroxodicopper(II) species (A). However, when the substituent

is the slightly bulkier XR₂moiety {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-O atom for toluene/7-¹⁸O₂reactivity, and a H-atom abstraction by 7-O₂is 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., PhCH₃vs PhCD₃substrates). 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-O₂and O-O bond (re)formation chemistry, along with the

ability to protonate 7-O₂and release of H₂O₂indicate the presence of an equilibrium between [{(6tbp)Cu^III}₂(O^2-)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

Ltd.: New York, 2005.

(3) Lee, Y.; Karlin, K. D. Highlights of Copper Protein Active-Site

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

(4) Hatcher, L. Q.; Karlin, K. D. AdV. Inorg. Chem. 2006, 58, 131–184.

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,^17,18and 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)/

O₂reactivity, 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).^4–6However, 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)Cu^II}₂(µ-1,2-O₂^2–)]²⁺

(A1), has an end-on structure (ν_(O-O)) 832 (∆[¹⁸O₂]) -44)

cm^-1, ν_(Cu-O)) 561 (∆[¹⁸O₂]) -26) cm^-1in EtCN; ν_(O-O)

) 827 (∆[¹⁸O₂]) -44) cm^-1, ν_(Cu-O)) 561 (∆[¹⁸O₂]) -26)

cm^-1in Et₂O),^8,9but with Me₂TPA ((2-pyridylmethyl)bis(6-

methyl-2-pyridylmethyl)amine), Suzuki and co-workers

showed that the bis-µ-oxo “isomer” [{(Me₂TPA)-

Cu^III}₂(O^2-)₂]²⁺(C1) (ν_(Cu-O)) 590 (∆[¹⁸O₂]) -26) cm^-1)

results (Chart 1).^6,10,11When all three pyridyl arms have

6-phenyl groups¹²or 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,^14–16the 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.

(5) Quant Hatcher, L.; Karlin, K. D. J. Biol. Inorg. Chem. 2004, 9, 669–

683.

(6) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. ReV. 2004,

104, 1013–1045.

(7) Lewis, E. A.; Tolman, W. B. Chem. ReV. 2004, 104, 1047–1076.

(8) Baldwin, M. J.; Ross, P. K.; Pate, J. E.; Tyeklár, Z.; Karlin, K. D.;

Solomon, E. I. J. Am. Chem. Soc. 1991, 113, 8671–8679.

(9) Jacobson, R. R.; Tyeklár, Z.; Karlin, K. D.; Liu, S.; Zubieta, J. J. Am.

Chem. Soc. 1988, 110, 3690–3692.

(15) Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38, 2161–

2168.

(16) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Wang, X.;

Young, J., V. G.; Cramer, C. J.; Que, J., L.; Tolman, W. B. J. Am.

Chem. Soc. 1996, 118, 11555–11574.

(10) Hayashi, H.; Fujinami, S.; Nagatomo, S.; Ogo, S.; Suzuki, M.; Uehara,

A.; Watanabe, Y.; Kitagawa, T. J. Am. Chem. Soc. 2000, 122, 2124–

2125.

(17) Stack, T. D. P. Dalton Trans. 2003, 1881–1889.

(18) Ottenwaelder, X.; Rudd, D. J.; Corbett, M. C.; Hodgson, K. O.;

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.

(22) Siegbahn, P. E. M. J. Biol. Inorg. Chem. 2003, 8, 577–585.

(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.

(25) Itoh, S.; Fukuzumi, S. Bull. Chem. Soc. Jpn. 2002, 75, 2081–2095.

(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 N_2(l)ﬁnger dewar.

Synthesis of L^{N(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 CH₂Cl₂several

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

-

reactivity studies reported below are the B(C₆F₅)₄salt

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 L^{N(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

(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). R_f) 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 C_62.9H_51.8BCuF₂₀N₅O_2.8:

C, 54.87; H, 3.79; N, 5.09. Found: C, 54.80; H, 3.81; N,

4.84.

I

Reaction of [(L^{N(CH Ph)})Cu ]B(C₆F₅)₄(10) with O₂. Three

2

I +

Reaction of [(L^{CH OMe})Cu^I]⁺(1) with O₂. The synthesis

milliliters of an acetone solution of [(L^{N(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

and characterization of tetradentate ligand L^{CH OMe}and

2

copper(I) complex, [L^{CH OMe})Cu^I]B(C₆F₅)₄(1) (Chart 2) were

2

previously reported.²⁷Crystallographically characterized

[L^{CH OMe})Cu^I]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+

[{(L^{N(CH Ph)})Cu }₂(O )₂] (10-O₂), a bis-µ-oxo-dicop-

2

product was characterized as [{(L^{CH OMe})Cu^II}₂(O₂^2-)]²⁺(1-

per(III) complex.

2

II

2+

N(CH₂Ph)₂

Synthesis of [{(L^{N(CH Ph)})Cu (Cl)}₂] . [(L

)-

2

O₂), a µ-1,2-peroxodicopper(II) complex.

Cu^I]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 CHCl₃resulted 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 [(L^{NH(CH )})Cu ] (2).

3

The generation and characterization of tetradentate ligand

28

L^{NH(CH )}(Chart 2) were previously reported. L^{NH(CH )}(0.055

3

29

g, 0.172 mmol) and [Cu^I(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

[{(L^{N(CH Ph)})Cu (Cl)}₂] (0.042 g, 74%) (see Supporting

Information, Figure S3). As seen before,³⁰CHCl₃is 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

Reaction of [(L^{N(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 L^{N(CH )}

I +

Reaction of [(L^{NH(CH )})Cu ] (2) with O₂. A THF solution

3 2

3

and copper(I) complex, [(L^{N(CH )})Cu ]B(C₆F₅)₄(9) were

I

(3.1 mL) of [(L^{NH(CH )})Cu ]B(C₆F₅)₄(2) (0.003 g, 0.003

I

3 2

3

previously reported.²⁸A 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

[(L^{N(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, [(L^{N(CH Ph)})Cu ]

2

(10). L^{N(CH Ph)}

(0.050 g, 0.103 mmol) and

2

III

2-

2+

characterized as [{(L^{N(CH )})Cu }₂(O )₂] (9-O₂).

[Cu^I(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)Cu^II(Cl)}₂]²⁺. In a 25 mL Schlenk

ﬂask, [(6tbp)Cu^I]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)Cu^II(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

(27) Maiti, D.; Woertink, J. S.; Vance, M. A.; Milligan, A. E.; Narducci

Sarjeant, A. A.; Solomon, E. I.; Karlin, K. D. J. Am. Chem. Soc. 2007,

129, 8882–8892.

(28) Maiti, D.; Narducci Sarjeant, A. A.; Karlin, K. D. J. Am. Chem. Soc.

2007, 129, 6720–6721.

(30) Lucchese, B.; Humphreys, K. J.; Lee, D.-H.; Incarvito, C. D.; Sommer,

R. D.; Rheingold, A. L.; Karlin, K. D. Inorg. Chem. 2004, 43, 5987–

5998.

(29) Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin, K. D.

Inorg. Chem. 2002, 41, 2209–2212.

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)Cu^I]B(C₆F₅)₄(0.030 g, 0.025 mmol) was prepared in

the glovebox. The ﬂask was cooled to -80 °C. O₂was

bubbled for 10 s using a long syringe needle to form

[{(6tbp)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂(excess O₂removed). 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 PhCOCH₃was used for comparison). After it

was washed with pentane, the copper product was further

puriﬁed by recrystallization from Et₂O/pentane, giving

Reaction of [(6tbp)Cu^I]B(C₆F₅)₄(7) with O₂. Crystal-

lographically characterized³¹[(6tbp)Cu^I]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)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂(7-

O₂), a bis-µ-oxo-dicopper(III) complex.

Reaction of [{(6tbp)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂(7-O₂)

with 1:1 PhCH₃and PhCD₃. [(6tbp)Cu^I]B(C₆F₅)₄(7) (0.046

g, 0.039 mmol) was dissolved in a mixture of 0.477 g of

PhCH₃and 0.492 g of PhCD₃in 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)Cu^III}₂(O^2-)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)Cu^III}₂(O^2-)₂]-

(B(C₆F₅)₄)₂in Et₂O. [(6tbp)Cu^I]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)Cu^III}₂(O^2-)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)Cu^III}₂(O^2-)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)₂Cu^II(OH)₂](B(C₆F₅)₄)₂(7-OH, see Results and Dis-

2

cussion). The complex is EPR silent, and ν_OH) 3580 cm^-1

(see Supporting Information, Figure S8). Anal. Calcd for

[(6tbp)₂Cu^II(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-O₂with Hydroquinone. In a 100 mL

Schlenk ﬂask, an 11 mL Et₂O solution of [(6tbp)Cu^I]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)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂(7-O₂) (excess O₂re-

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)Cu^III}₂-

(O^2-)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)Cu^I]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-^tBu₃phenoxyl 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)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂with

Benzylamine. [(6tbp)Cu^I]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)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂, whereupon excess O₂was

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)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂(7-O₂)

with TMPA. [(6tbp)Cu^I]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. O₂was bubbled to

the copper(I) solution to form [{(6tbp)Cu^III}₂(O^2-)₂]²⁺(7-

O₂). Once the formation of 7-O₂was 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)Cu^I(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)Cu^I(CH₃CN)]⁺/O₂.

Addition of HCl to [{(6tbp)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂

(7-O₂). [(6tbp)Cu^I]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. O₂was bubbled to the copper(I) solution to form

[{(6tbp)Cu^III}₂(O^2-)₂]²⁺, and excess O₂was 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 H₂O₂using TiO(SO₄) test reagent^32–34giving the result

that hydrogen peroxide was formed in ∼80% yield. The

Reaction of PhCH₂OH and [(6tbp)Cu^I]⁺/O₂. In a 10 mL

Schlenk ﬂask, a 3 mL Et₂O solution of [(6tbp)Cu^I]⁺(7)

(0.040 g, 0.034 mmol) was prepared in the glovebox.

Following removal to the benchtop, O₂bubbling (10 s) at

-80 °C produced [{(6tbp)Cu^III}₂(O^2-)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)-

Cu^III}₂(O^2-)₂]²⁺. [{(6tbp)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂(excess O₂

removed) was prepared (at -80 °C) in a 25 mL Schlenk

ﬂask starting with [(6tbp)Cu^I]⁺(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)Cu^III}₂(O^2-)₂]²⁺. A

solution of xanthene (0.005 g, 0.027 mmol) was prepared in

50 µL of Et₂O inside the glovebox. [(6tbp)Cu^I]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)Cu^III}₂(O^2-)₂](B(C₆F₅)₄)₂(excess O₂removed) 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)Cu^II(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 +

[(L^{NH(CH )})Cu ] (2) and [(L^{N(CH Ph)})Cu ] (10) (Chart 2)

which were synthesized under an argon atmosphere by

combination of equimolar mixtures of the respective tet-

3

2

29

radentate ligand and [Cu^I(CH₃CN)₄]B(C₆F₅)₄in 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 N₂ﬁnger 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 N₂were

cooled to -80 °C on the benchtop and then subjected to O₂

bubbling via syringe. Samples employing ¹⁸O₂in 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 [(L^{CH OMe})Cu^I]⁺

2

(1) (X-ray),²⁷[(BPQA)Cu^I]⁺(6),^41–43[(6tbp)Cu^I]⁺(7) (X-

ray),³¹and [(L^{N(CH )})Cu ] (9)²⁸have 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.^30,44The

following have previously been reported and character-

27

ized by X-ray crystallography: [{(L^{CH OMe})Cu^II(Cl)}₂]²⁺,

2

27

28

+

[{(L^{CH OMe})Cu^II(Br)}₂]²⁺, [(L^{NH(CH )})Cu (Cl)] , [(BPQA)

II

2

3

Cu^II(Cl)]⁺,^41,42

[(6tbp)Cu^II(acetone)]²⁺,³¹

and

[(L^{N(CH )})Cu (H₂O)] .²⁸In the Supporting Information, we also

report the X-ray structures of [{(6tbp)Cu^II(Cl)}₂](B(C₆F₅)₄)₂and

II

2+

3 2

Results and Discussion

[{(L^{N(CH Ph)})Cu (Cl)}₂](B(C₆F₅)₄)₂, generated by copper(I) ligand

complex exposure to chloroform,³⁰a very useful procedure with

which to readily obtain chloride copper(II) complexes which

typically crystallize.^41,42

II

2

27

28

3 2

Ligand Synthesis. The syntheses of L^{CH OMe}

,

L^{N(CH )}

,

2

and 6tBP³¹(Chart 2) were previously reported. L^{NH(CH )}

(Chart 2) was obtained from oxidative N-dealkylation

chemistry of copper(II)-hydroperoxo species of L^{N(CH )}

3

28

3 2

.

[(L)Cu^II(µ-1,2-O₂^2-)]²⁺(A) Complex Formation

2

Generation of L^{N(CH )}and L^{N(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-workers¹²and Que and co-workers,³⁶6MeTMPA by

Suzuki and co-workers,^37,38MAPA ((6-amino-2-pyridyl-

methyl)bis(2-pyridylmethyl)amine),³⁹MPPA ((6-^tButylac-

etamido-2-pyridyl-methyl)bis(2-pyridylmethyl)amine) by Ma-

suda and co-workers,⁴⁰and BPQA ((2-quinolylmethyl)bis(2-

pyridylmethyl)amine) by Karlin and co-workers^41–43were

previously reported.

3 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 [(L^{CH OMe})Cu^I]B(C₆F₅)₄(1), we

2

previously reported that the colorless copper(I) solution

changes to the purple species that was formulated as

[{(L^{CH OMe})Cu^II}₂(µ-1,2-O₂^2-)]²⁺(1-O₂), {λ_max) 540 nm (ꢀ,

2

9550 M^-1cm^-1) and 610 nm, sh, (ꢀ, 6500 M^-1cm^-1)} (Figure

1a).²⁷These features closely resemble the distinctive pattern

known for the parent complex [{(TMPA)Cu^II}₂(µ-1,2-

O₂^2-)]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 ¹⁸O₂to 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 [{(L^{CH OMe})Cu^II}₂(µ-1,2-

2

O₂^2-)]²⁺(1-O₂) shows a decreased peroxide π_σ* f d_σ

charge-transfer band energy as compared to that of [{(TM-

PA)Cu^II}₂(µ-1,2-O₂^2-)]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)Cu^II}₂(µ-1,2-O₂^2-)]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 [(L^{NH(CH )})Cu ]B(C₆F₅)₄(2) at -80 °C

reacts with O₂leading to a reddish-brown solution with

prominent UV–vis features at λ_max) 498 nm (ꢀ, 7500 M^-1

cm^-1) and 635 nm (ꢀ, 1700 M^-1cm^-1). Again, the UV–vis

charge-transfer pattern and rR data (vide infra) (Figure 2a

I

3

and b) support the formulation as [{(L^{NH(CH )})Cu }₂(µ-1,2-

O₂^2-)]²⁺(2-O₂). There is a considerable increase in the

peroxo π_σ* f Cu d_σcharge-transfer energy as compared to

the parent compound [{(TMPA)Cu^II}₂(µ-1,2-O₂^2-)]²⁺(A1)

(Table 1). Masuda and co-workers^39,40have 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 [(L^{CH OMe})Cu^I]⁺(1) (black spectrum)

group of L^{NH(CH )}to the peroxo moiety (see diagram below)

2

3

following bubbling with O₂. (b) rR spectrum of [(L^{CH OMe})Cu^I]⁺(1) (black),

2

would effect these changes. This H-bonding would result in

stabilization of the peroxo π_σ* orbital leading to a larger d_σ

(Cu) and π_σ* (O₂^2-) energy separation and thus an increase

in the π_σ* f d_σcharge-transfer energy.

1-¹⁶O₂-(B(C₆F₅)₄)₂(red), and 1-¹⁸O₂-(B(C₆F₅)₄)₂(blue) (λ_excit) 620 nm

at 77 K).

and the rR spectrum shows a lower (Cu-O)_sstretch (1-O₂,

550 cm^-1; A1, 561 cm^–1.^8,23,27These data indicate the Cu-O

bonds of 1-O₂are weaker than those of [{(TMPA)Cu^II}₂(µ-

1,2-O₂^2-)]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-O₂occur

at 849 (∆(¹⁸O₂) ) –47) and 544 (∆(¹⁸O₂) ) –25) cm^-1,

respectively (Table 1). The rR spectral behavior of 2-O₂can

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)⁴⁷while 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 N₄ligands possessing structure

A,^6,9,43,48,49the O₂-binding and formation of 2-O₂appears

ligand ﬁeld. The steric congestion in L^{CH 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

[{(L^{CH OMe})Cu^II(Br)}₂](B(C₆F₅)₄)₂. In a square pyramidal

complex, three of the chelate ligands would donate into Cu’s

2

d_{x -y}orbital thus reducing the donation from the peroxo to

the Cu and the strength of this bond. The (Cu-O)_asstretch

is also observed for this 1-O₂system (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 O₂and regeneration of [(L^{NH(CH )})Cu ]

3

While the π_σ* f d_σcharge transfer and (Cu-O)_sstretch

are shifted down in energy; the O-O stretch has increased

in energy (1-O₂, 848 cm^-1; A1, 832 cm^-1).⁸The decreased

peroxo π_σ* donation to Cu would result in more electron

density in the peroxo antibonding π_σ* 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^-1cm^-1

)

¹⁶O-¹⁶O (¹⁸O-¹⁸O)

Cu-¹⁶O (Cu-¹⁸O)

ref

[{(TMPA)Cu^II}₂(µ-1,2-O₂^2-)]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)

550 (524)

544 (519)

b

544 (518)

b

18

23

a

(parent complex)

[{(L^{CH OMe})Cu^II}₂(µ-1,2-O₂^2-)]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

[{(L^{NH(CH )})Cu }₂(µ-1,2-O₂^2-)]²⁺(2-O₂)

THF

a

II

3

[{(6MeTMPA)Cu^II}₂(µ-1,2-O₂^2-)]²⁺(3-O₂)

[{(MAPA)Cu^II}₂(µ-1,2-O₂^2-)]²⁺(4-O₂)^d

[{(MPPA)Cu^II}₂(µ-1,2-O₂^2-)]²⁺(5-O₂)^d

[{(BPQA)Cu^II}₂(µ-1,2-O₂^2-)]²⁺(6-O₂)

acetone

EtCN

27

39

40

41

bis(µ-oxo)dicopper(III) complexes

with type II ligands

solvent

UV–vis/nm (ꢀ/M^-1cm^-1

)

ν/cm^-1(Cu-¹⁶O)

ν/cm^-1(Cu-¹⁸O)

ref

[{(6tbp)Cu^III}₂(O^2-)₂]²⁺(7-O₂)

CH₂Cl₂

acetone

Et₂O

383 (Et₂O, g7000)^e

378

594

599

c

568

573

c

a

36

[{(6PhTPA)Cu^III}₂(O^2-)₂]²⁺(8-O₂)

[{(L^{N(CH )})Cu }₂(O^2-)₂]²⁺(9-O₂)

383 (18 900)^c

390 (5300)

a, 28

III

3

2

a

[{(L^{N(CH Ph)})Cu }₂(O^2-)₂]²⁺(10-O₂)

acetone

598

572

III

2

^aThis work. ^bNot reported. ^cNot detected. ^dthese possess intramolecular H-bonding to the peroxo group, see text. ^eSee text.

Bis(µ-oxo)dicopper(III) Complex [(L)Cu^III₂(O^2-)₂]²⁺

(C) Formation with Group II Ligand-Copper

Complexes. Reaction of O₂with Copper(I) Complexes

N(CH₂Ph)₂

of 6tBP, L^{N(CH )}, and L

. As previously described

3 2

in a qualitative manner, bubbling O₂through -80 °C

Et₂O or THF solutions of [(6tbp)Cu^I]B(C₆F₅)₄(7) or

I

[(L^{N(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 O₂reaction with [(6tbp)Cu^I]⁺(7) (black

spectrum). A 7-O₂decomposition product (green spectrum at -80 °C) forms

in ∼20 min. (b) rR spectrum (ν_Cu-Oregion) of 7-O₂-(B(C₆F₅)₄)₂with ¹⁶O₂

(red) or ¹⁸O₂(blue) {CH₂Cl₂solvent, λ_excit) 386 nm, 77 K}.

silent solutions of [{(6tbp)Cu^III}₂(O^2-)₂]²⁺(7-O₂) {λ_max

(Et₂O) ) 383 nm (ꢀ g 7000 M^-1cm^-1), Figure 3a}³¹or

[{(L^{N(CH )})Cu }₂(O )₂] (9-O₂) {λ_max) 383 nm (ꢀ )

18900 M^-1cm^-1) (Table 1).^28,50The structure assignment

as C (Scheme 1, Chart 2) for 7-O₂and 9-O₂(vide infra)

comes from the characteristic^5,6UV–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

+

[(L^{NH(CH )})Cu ] (2) (black spectrum) reaction with O₂, and possessing

3

I

H-bonding (see text). (b) rR spectrum of [(L^{NH(CH )})Cu ]⁺(black), 2-¹⁶O₂-

3

(B(C₆F₅)₄)₂(red), and 2-¹⁸O₂-(B(C₆F₅)₄)₂(blue) (λ_excit) 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^-1which shifts to 568

cm^-1upon isotopic substitution with ¹⁸O₂is 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).⁶The data further show that there is a less strongly

enhanced band at 1188 cm^-1that 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-N_pyridine) ) 460 cm^-1) and equatorially bound

amine (ν(Cu-N_amine) ) 495 cm^-1) of 7-O₂.²³

I

Figure 4. rR spectrum of [(L^{N(CH Ph)})Cu ]B(C₆F₅)₄(black) and 10-O₂-

(B(C₆F₅)₄)₂formed using ¹⁶O₂(red) or ¹⁸O₂(blue) {acetone solvent, λ_excit

) 380 nm, 77 K}.

2

Que and co-workers³⁶previously described UV–vis and

rR characterization (Table 1) of the bis(µ-oxo)dicopper(III)

species, [{(6-PhTPA)Cu^III}₂(O^2-)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.^52,53For symmetrically disposed, tripodal tetradentate

ligands, trigonal bipyramidal pentacoordiante copper(II)

complexes generally form.^42,54However, 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-XR₂substituent and not just a 6-XHR substituent to effect

such a change. In the O₂adducts 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 Cu^III₂(O^2-)₂

structure. As mentioned (Introduction), this has been proven

directly for the case of the copper-dioxygen adduct with

the ligand Me₂TPA (Chart 1).¹⁰Also, our interpretations are

consistent with ligand effect results observed and summarized

by Itoh and co-workers,^1,55that 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 [{(L^{N(CH )})Cu }₂(O )₂] (9-O₂), the characteristic

UV–vis spectrum obtained readily leads to its formula-

tion.^6,28The 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,⁵¹we

3 2

synthesized a close analogue L^{N(CH Ph)}and a copper(I)

complex of this ligand (Chart 2). The O₂-reactivity of

2

I

[(L^{N(CH Ph)})Cu ]B(C₆F₅)₄(10) is very similar to that for the

dimethyl analogue 9. Here, low-temperature UV–vis⁵⁰and

rR data (Figure 4) could be obtained (Table 1), thus

proving the bis(µ-oxo)dicopper(III) complex formulation,

2

III

2-

2+

[{(L^{N(CH Ph)})Cu }₂(O )₂] (10-O₂).

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 O₂to 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 -XR₂substituent (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)Cu^III}₂(O^2-)₂]²⁺(7-O₂). We

recently reported quite interesting new chemistry when

generating hydroperoxo complexes starting with copper(II)

derivatives of either 6tBP or L^{N(CH )}(Chart 2).^28,31The

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)₂Cu^IICu^III}(O^2-)(OH^-)]²⁺(7-O₂-H) (Scheme

3). However, the four-line EPR spectrum and lack of a half-

ﬁeld (M_s) 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 Cu^III₂(O^2-)₂

abstraction of a substrate H-atom has been previously

hypothesized^{7,20,25,56,58–61}and suggested by computational

studies^58,62,63to have a reasonable stability; however, no

experimental evidence, spectroscopic or structural, has yet

been provided. It is interesting to note that Itoh and

co-workers⁶⁰have also speculated that such a binuclear

complex product, [{(ligand)₂Cu^IICu^III}(O^2-)(OH^-)]²⁺,

might split into mononuclear species including

[(ligand)Cu^II-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-O₂was 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).³¹With 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)Cu^III}₂(O^2-)₂]²⁺(7-O₂) or [{(L^{N(CH )})Cu }₂(O )₂]

(9-O₂).^28,31While reactivity studies with bis(µ-oxo)dicop-

per(III) entities are reasonably well-known,^5,7,56we 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 2Cu^I/

O₂derived complexes for 3–5 and 8.

III

2- 2+

3 2

Reaction of 7-O₂with ArSH, PhCH₂OH, and

ArCH₂NH₂Substrates. After removal of excess dioxygen

from solutions of bis(µ-oxo)dicopper(III) complex 7-O₂at

-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)Cu^III}₂(O^2-)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-O₂with Phenolic Substrates. Upon

addition of an anaerobic solution of 2,4,6-^tBu₃-phenol to a

-80 °C solution of [{(6tbp)Cu^III}₂(O^2-)₂]²⁺(7-O₂) (excess

O₂removed), there is an immediate change in its UV–vis

spectrum (Figure 5, yield ∼55%). The strong band at 383

nm from 7-O₂disappears, and a new broad peak appears at

∼360 nm (ꢀ ≈ 2150 M^-1cm^-1) accompanied by sharp

feature at 405 nm (Figure 5, inset); the latter is character-

istic⁵⁷of the 2,4,6-^tBu₃phenoxyl 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 Cu^III₂(O^2-)₂complexes,^5,7

7-O₂appears 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)Cu^III}₂(O^2-)₂]²⁺(7-O₂). To our knowledge, neither

toluene nor ethylbenzene have previously been examined as

substrates for copper(I)/O₂derived oxidants. We ﬁnd that

7-O₂oxidizes 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)/O₂reaction 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)Cu^I]⁺(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-^tBu₃phenoxyl radical at ∼405 nm.

With this information in hand, we can suggest mechanisms

of reaction for the 7-O₂mediated 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-O₂with 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.}⁶⁶Accompanying 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-O₂must 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.^67–69However, 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)Cu^I]⁺(7) reduction of PhCH₂OO^•

giving the alkylperoxo species [(6tbp)Cu^II-OOCH₂Ph]⁺;^68,69

Figure 6. EPR spectrum following reaction of [{(6tbp)Cu^III}₂(O^2-)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)₂Cu^IICu^III}(O^2-)(OH^-)]²⁺(7-O₂-H) or species derived from this

{EPR parameters g_|) 2.232, A_|) 175 G}, see text.

from O₂).^64,65Thus 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)Cu^II}₂(^–OH)₂]²⁺(7-OH).

Its formulation is supported by IR (ν_OH) 3580 cm^-1)⁵⁰and

EPR (silent) spectroscopies along with its elemental analysis

(see Experimental Section).

When ¹⁸O₂was employed to generate [{(6tbp)-

Cu^III}₂(¹⁸O^2-)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-O₂was gener-

ated within a 1:1 solution mixture of PhCH₃and 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 (k_H/k_D) of ∼7 is

observed. In fact, k_H/k_Dvalues 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)Cu^II}₂(^-OH)₂]²⁺(7-OH)

(Scheme 4, path B).

(A1) compared to [{(6tbp)Cu^III}₂(O^2-)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)Cu^III}₂(O^2-)₂]²⁺(7-O₂) with

TMPA: Ligand Exchange Accompanied by O-O Bond

Formation. Evidence for dynamic behavior in the chemistry

of 7-O₂comes from observations of its reactivity with the

tripodal tetradentate ligand TMPA. When this (1.1 to excess)

is added to solutions of 7-O₂from which excess O₂has been

removed, [{(TMPA)Cu^II}₂(µ-1,2-O₂^2-)]²⁺(A1) forms (Scheme

5). This transformation can be followed spectrophotometri-

cally, Figure 7; a brownish yellow solution of 7-O₂changes

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)Cu^II}₂(µ-1,2-O₂^2-)]²⁺(A1)⁴⁹in Et₂O solvent,^23,71

one calculates that a 90% conversion of 7-O₂to A1 has

occurred (Scheme 5). The result indicates a greater overall

thermodynamic stability of [{(TMPA)Cu^II}₂(µ-1,2-O₂^2-)]²⁺

Protonation of [{(6tbp)Cu^III}₂(O^2-)₂]²⁺(7-O₂) and

Generation of H₂O₂. When a solution of [{(6tbp)-

Cu^III}₂(O^2-)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)Cu^II-

(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-workers¹⁰were

even further able to show that O₂could be released from

the bis(µ-oxo)dicopper(III) complex formed using Me₂TPA

(Chart 1), either by bubbling solutions with N₂or 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-O₂at

-80 °C in Et₂O (green spectrum) leads to immediate formation of

[{(TMPA)Cu^II}₂(µ-1,2-O₂^2-)]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 XR₂moiety {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)_sstretches

compared to those of [{(TMPA)Cu^II}₂(µ-1,2-O₂^2-)]²⁺(A1).

These changes are ascribed to geometric distortions which

lead to decreased overlap between d_σ(Cu) and π_σ* (O₂

The bis(µ-oxo)dicopper(III) species substrate reactivity of

our complex [{(6tbp)Cu^III}₂(O^2-)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-O₂is apparent from its reactions with

ArOH substrates, as well as the determination of k_H/k_D≈ 7

in the toluene oxygenation (i.e., demonstrated from competi-

tive reactions with PhCH₃vs PhCD₃substrates). 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-O₂and 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)Cu^III}₂(O^2-)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^-1for A1 to 848 cm^-1for

[{(L^{CH OMe})Cu^II}₂(µ-1,2-O₂^2-)]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

[{(L^{NH(CH )})Cu }₂(µ-1,2-O₂^2-)]²⁺(2-O₂), a blue shift of the

prominent UV–vis charge-transfer band likely arises from

stabilization of the π_σ* (O₂^2-) orbital because of an intramo-

lecular H-bonding interaction, as previously observed by

Masuda and co-workers.^39,40The 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.

IC702437C

3800 Inorganic Chemistry, Vol. 47, No. 9, 2008

Article Doi

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

DOI: 10.1021/ic702437c

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