Copper-dioxygen adducts and the side-on peroxo dicopper(II)/bis(μ-oxo) dicopper(III) equilibrium: Significant ligand electronic effects

Article abstract of DOI:10.1021/ic052185m

The variation of ligand para substituents on pyridyl donor groups of tridentale amine copper(I) complexes was carried out in order to probe electronic effects on the equilibrium between μ-η²:η²-(side-on)-peroxo [Cu₂^II(O₂^2-)]²⁺ and bis(μ-oxo) [Cu₂^III(O^2-)₂] species formed upon reaction with O₂. [Cu^I(R-PYAN)(MeCN)_n]B(C₆F₅) ₄ (R-PYAN = N-[2-(4-R-pyridin-2-yl)-ethyl]-N,N′,N′-trimethyl-propane-1,3- diamine, R = NMe₂, OMe, H, and Cl) (1^R) vary over a narrow range in their Cu^II/Cu^I redox potentials (E_1/2 vs Fe(cp)₂^+/0 = -0.40 V for 1^NMe2, -0.38 V for 1^OMe, -0.33 V for 1^H, and -0.32 V for 1^Cl) and in C-O stretching frequencies of their carbonyl adducts, 1^R-CO: ν(C-O) = 2080, 2086, 2088, and 2090 cm^-1 for R = NMe₂, OMe, H, and Cl, respectively. However, within this range of electronic properties for 1^R, dioxygen reactivity is significantly affected. The reaction of 1^Cl or 1^H with O₂ at -78°C in CH₂Cl₂ gives UV-vis and resonance Raman spectra indicative of a μ-η²:η²-(side-on)-peroxo dicopper(II) adduct (2^R). Compound 1^OMe reacts with O₂, yielding equilibrium mixtures of side-on peroxo (2^OMe) and bis(μ-oxo) (3^OMe) species. Oxygenation of 1^NMe2 leads to the sole generation of the bis(μ-oxo) dicopper(III) complex (3^NMe2). A solvent effect was also observed; in acetone or THF, increased ratios of bis(μ-oxo) relative to side-on peroxo complex are observed. Thus, the equilibrium between a dicopper side-on peroxo and bis(μ-oxo) species can be tuned by ligand design-specifically, more electron donating ligands favor the formation of the latter isomer, and the peroxo/bis-(μ-oxo) equilibrium can be shifted from one extreme to the other within the same ligand system. Observations concerning the reactivity of the dioxygen adducts 2^H and 3^NMe2 toward external substrates are also presented.

Full text of DOI:10.1021/ic052185m

Inorg. Chem. 2006, 45, 3004−3013
Copper−Dioxygen Adducts and the Side-on Peroxo Dicopper(II)/
Bis( -oxo) Dicopper(III) Equilibrium: Significant Ligand Electronic
µ
Effects
Lanying Q. Hatcher,† Michael A. Vance,‡ 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 23, 2005
The variation of ligand para substituents on pyridyl donor groups of tridentate amine copper(I) complexes was
η2
:
η
2-(side-on)-peroxo [CuII2(O2 -)]2
2
+
carried out in order to probe electronic effects on the equilibrium between
and bis(
N-[2-(4-R-pyridin-2-yl)-ethyl]-N,N
µ
-
µ
-oxo) [CuIII2(O2-)2] species formed upon reaction with O2. [CuI(R-PYAN)(MeCN)n]B(C6F5)4 (R-PYAN
)
′
,N′
-trimethyl-propane-1,3-diamine, R
)
NMe2, OMe, H, and Cl) (1R) vary over a
narrow range in their CuII/CuI redox potentials (E1 vs Fe(cp)2 ) −0.40 V for 1NMe2
,
−
0.38 V for 1OMe
O stretching frequencies of their carbonyl adducts, 1R
CO: (C O)
NMe2, OMe, H, and Cl, respectively. However, within this range of electronic
78 C in CH2Cl2
2-(side-on)-peroxo dicopper(II) adduct (2R).
Compound 1OMe reacts with O2, yielding equilibrium mixtures of side-on peroxo (2OMe) and bis( -oxo) (3OMe) species.
Oxygenation of 1NMe2 leads to the sole generation of the bis( -oxo) dicopper(III) complex (3NMe2). A solvent effect
was also observed; in acetone or THF, increased ratios of bis( -oxo) relative to side-on peroxo complex are
observed. Thus, the equilibrium between a dicopper side-on peroxo and bis( -oxo) species can be tuned by ligand
design specifically, more electron donating ligands favor the formation of the latter isomer, and the peroxo/bis-
-oxo) equilibrium can be shifted from one extreme to the other within the same ligand system. Observations
concerning the reactivity of the dioxygen adducts 2H and 3NMe2 toward external substrates are also presented.
,
−
0.33 V
) 2080,
+/
0
/
2
for 1H, and
−
0.32 V for 1Cl) and in C
−
−
ν
−
1
2086, 2088, and 2090 cm- for R
)
properties for 1R, dioxygen reactivity is significantly affected. The reaction of 1Cl or 1H with O2 at
gives UV vis and resonance Raman spectra indicative of a
−
°
−
µ- :η
η2
µ
µ
µ
µ
s
(
µ
Introduction
activation include hemocyanins (Hcs), tyrosinase (Tyr), and
catechol oxidase (CO). Arthropodal and molluskan Hcs are
blood O2 carriers, Tyr is a widespread o-phenol hydroxylase,
while CO converts catechols to quinones.3-6 In these proteins,
two copper(I) ions reduce O2 to peroxide while generating
a µ-η2:η2-(side-on)-peroxo dicopper(II) structure (Figure 1).9
However, extensive biomimetic inorganic model studies
have shown that Cu(I) can activate O2 and bind (O22-) in
several different isoelectronic structural compositions.4,5,10-12
Ubiquitous copper ions in the active sites of proteins/
enzymes mediate a broad scope of chemical processes
including electron transfer, reversible dioxygen binding and
activation, and nitrogen oxide transformations.1-8 Biological
binuclear copper centers involved with dioxygen binding and
* Author to whom correspondence should be addressed. E-mail:
karlin@jhu.edu.
† The Johns Hopkins University.
‡ Stanford University.
(1) Bioinorganic Chemistry of Copper; Karlin, K. D., Tyekla´r, Z., Eds.;
Chapman and Hall: New York, 1993.
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683.
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3004 Inorganic Chemistry, Vol. 45, No. 7, 2006
10.1021/ic052185m CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/02/2006

Cu2O2 Adducts and the [CuII2(O2)]2+/CuIII2(O)2]2+ Equilibrium
Chart 1
lished distinctive spectroscopic and structural properties
(Figure 1).4,5,19,28 This equilibrium is especially intriguing
to study because of the reversible O-O bond making/
breaking process critical in dioxygen activation and oxidation
chemistries employing O2 or ROOH and the possible
relevance to biological systems.
Figure 1. Equilibrium between side-on peroxo and bis(µ-oxo) dicopper
cores and their foremost associated physical properties.
It has been well established that the coordinating ligand
has a large effect on the resulting structure of the 2CuI/O2
adduct.5,29 In general, tetradentate amine ligands tend to favor
the formation of end-on peroxo dicopper(II) compounds,13,18
while tridentate30 and bidentate 31 ligands can generate both
side-on peroxo and/or bis(µ-oxo) dicopper complexes.32,33
Within the peroxo and bis(µ-oxo) realm, ligand denticity,34
sterics,26,35 and nitrogen donor type (alkyl vs pyridyl, or 2°
vs 3° amine)36,37 play important roles in the equilibrium
between the isomeric Cu2O2 adducts.38 To complicate matters
further, changes in reaction conditions such as concentra-
tion,39 temperature,25 solvent,27 or a change in counterion26
can induce a shift in the equilibrium.5
With our own research program in copper-O2 chemistry,
we have also been interested in identifying and sorting out
the principal features that dictate the resulting structure of
Cu2O2 adducts. To these ends, we have synthesized a series
of ligands that alter the electronic environment about the
copper centers by adding electron-donating or electron-
withdrawing groups to the para position of a pyridyl nitrogen
donor. Previous studies on an analogous ligand system,
R-MePY2 (bis[2-(4-R-2-pyridn-2-yl)ethyl]methylamine) (Chart
1), showed a marginal increase in bis(µ-oxo) formation
While nature selectively chooses the side-on peroxo binding
mode (Figure 1), copper(I) complexes can also bind peroxide
in a µ(1,2-O22-) “end-on” fashion to generate a CuII (µ-1,2-
2
(O22-) species such as the well studied [{CuII(TMPA)}2(O22-)]
(TMPA ) tris(2-pyridyl)methylamine) complex.13-18 An-
other feasible Cu2O2 structure is related to the µ-η2:η2 side-
on form but comprises a fully cleaved O-O bond producing
a formal CuIII2(µ-O2-)2 species, first discovered by Tolman
and co-workers and now well established in many different
ligand systems (Figure 1).5,6,19 Theoretical calculations predict
that the free energies of dicopper(III) bis(µ-oxo) species are
nearly identical to those of the side-on peroxo dicopper(II)
isomer (∆G° ) 0.3-12.7 kcal/mol), and the barrier for
interconversion is very small.20-24 Thermodynamic param-
eters were also experimentally obtained and demonstrate the
energetic similarity: ∆H° ) -0.6 to -3.8 kcal/mol, and
∆S° ) -2 to -20 cal/mol.K.25-27 Hence, with many ligand
systems, Cu2O2 complexes coexist in a dynamic equilibrium
between the dicopper(II) side-on peroxo and the dicopper-
(III) bis-µ-oxo isomeric forms but which have now estab-
(13) Jacobson, R. R.; Tyekla´r, Z.; Karlin, K. D.; Liu, S.; Zubieta, J. J. Am.
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Inorganic Chemistry, Vol. 45, No. 7, 2006 3005

Hatcher et al.
reference, and J values are reported in hertz. Infrared spectra were
recorded on a Bruker vector 22 infrared spectrometer.
relative to side-on peroxo when adding electron-donating
substituents, R ) NMe2 and OMe, compared to R ) H.40
However, the Cu2O2 adducts were always a mixture of both
Low-temperature UV-vis spectra were taken either with a
Hewlett-Packard model 8453 diode array spectrophotometer equipped
with a custom-made quartz Dewar filled with cold (-78 °C)
methanol (maintained and controlled by a Neslab ULT-95 low-
temperature recirculating unit), or using a -78 °C methanol bath
(maintained by a CC-100 immersion cooler and Agitainor, Neslab)
and taking measurements with 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). Air-
sensitive solutions were prepared in a nitrogen atmosphere in a
glovebox (MBraun) and carried out in 2 mm Schlenk cuvettes
(Quark) or custom-made Schlenk tubes designed for the fiber-optic
dip probe (Chemglass, part no. JHU-0407-271-MS).
Resonance Raman (rR) spectroscopy was performed with 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. Excitation was
provided by a Coherent I90C-K Kr+ ion laser. The spectral
resolution was <2 cm-1. Sample concentrations were ∼1 mM in
Cu2O2. For resonance enhancement at 364 and 407 nm, samples
were run at 77 K in a liquid N2 finger Dewar (Wilmad) and were
hand-spun to minimize sample decomposition during scan collec-
tion. To observe the peroxide stretching frequency, excitation was
at 531 nm and samples were run at 193 K. Isotopic substitution
was achieved by oxygenating with 18O2 (Icon, Summit, NJ).
Cyclic voltammetry measurements were undertaken in freshly
distilled dimethylformamide (DMF) with a BAS 100B electro-
chemical analyzer with a glassy carbon working electrode and a
platinum wire auxiliary electrode. Potentials were recorded versus
a saturated calomel reference electrode. E1/2 values are reported
CuII (O2) and CuIII2(O)2 in equilibrium, and the peroxide
2
O-O bond strength remained unaffected.41 We also previ-
ously reported on the copper(I) complex of H-PYAN
(N-[2-(pyridin-2-yl)ethyl]-N,N′,N′-trimethyl-propane-1,3-di-
amine) (Chart 1) which reacts with O2 to generate a mixture
of side-on peroxo and bis(µ-oxo) dicopper compounds, with
the equilibrium being highly solvent dependent.27 In THF,
the resulting dioxygen adduct was roughly a 50/50 mixture
of side-on peroxo dicopper(II) and bis(µ-oxo) dicopper(III),
providing a unique system to study ligand electronic effects
on that midway equilibrium. The ligands, R-PYAN (N-[2-
(4-R-pyridin-2-yl)-ethyl]-N,N′,N′-trimethyl-propane-1,3-di-
amine) where R ) NMe2, OMe, H, and Cl, have now been
synthesized in order to isolate and study solely the electronic
effects on the peroxo/oxo equilibrium without altering any
other aspects (e.g., sterics, denticity, solvent, etc.). The
copper(I) complexes of R-PYAN have been studied with
respect to redox potential, CO binding, and dioxygen
reactivity. UV-visible and resonance Raman (rR) spectro-
scopy have been employed to determine the ligand-electronic
effects on the µ-η2:η2-(side-on)-peroxo dicopper(II) and bis-
µ-oxo dicopper(III) equilibrium. To compare and contrast,
the ligand system R-MePY2 (Chart 1) and its CuI/O2
reactivity are also discussed.
Experimental Section
+/0
+/0
General Considerations and Instrumentation. Elemental
analyses were performed by Desert Analytics, Tucson, AZ. Mass
spectrometry was conducted at the mass spectrometry (MS) facility
at either the Johns Hopkins University or at the Ohio State
University. CI spectra were acquired at the Johns Hopkins
University MS facility using a VG70S double-focusing magnetic
sector mass spectrometer (VG Analytical, Manchester, UK, now
Micromass/Waters) equipped with a Xe gas FAB gun (7.5 kV at 1
mA) and an off-axis electron multiplier. Samples were introduced
into the CI source (block temperature ) 200 °C) using a heated
direct insertion probe using a deep quartz cup at a rate of 0.5 °C/s.
Methane reagent gas was used with an electron energy of 70 eV.
High-boiling perfluorokerosene was used as the reference mass for
accurate mass analyses. For accurate mass analyses, the matrix
contained 10% PEG mass calibrant. Data were acquired using a
MSS Data system (MasCom, Bremen, Germany) for subsequent
processing. High-resolution electrospray ionization (ESI) mass
spectrometry analyses were performed at the Ohio State University
(MS) facility with a 3-T Finnigan FTMS-2000 Fourier transform
mass spectrometer. Samples were sprayed from a commercial
Analytical electrospray ionization source, and then focused into the
FTMS cell using a home-built set of ion optics. X-ray diffraction
was performed at the X-ray diffraction facility at the Johns Hopkins
University with an Xcalibur3 diffractometer.42 1H NMR and 13C
NMR spectra were measured on a Varian 400 MHz spectrometer,
chemical shifts are reported in ppm downfield from an internal TMS
versus the Fe(cp)2 couple according to Fe(cp)2 ) 0.45 V vs
SCE in DMF with [Bu4N][PF6].43 Scans were run at 50-300 mV/s
under an argon atmosphere using ca. 0.1 M [Bu4N][PF6] as the
supporting electrolyte.
GC analyses were carried out on a HP-5890 Series II gas
chromatograph using an Rtx-5 (crossbonded 5% diphenyl 95%
dimethyl polysiloxane) 30 m × 0.32 mm I.D. × 0.25 µm film
thickness and analyzed with a flame ionization detector connected
to peak simple chromatography data system from SRI. GC
conditions. Benzyl alcohol: inject/detector temp, 250 °C; oven
temp, 110 °C; column head pressure, 115 kPa; purge vent, 5 mL/
min; split vent, 55 mL/min; total flow, 65 mL/min. Dihydroan-
thracene: inject/detector temp, 350 °C; oven temp, 120 °C for 2
min then ramped at 30 °C/min until temperature reached 210 °C;
column head pressure, 120 kPa; purge vent, 5 mL/min; split vent,
55 mL/min; total flow 61 mL/min. Dimethylaniline: inject/detector
temp, 250 °C; oven temp, 100 °C; column head pressure, 120 kPa;
purge vent, 5 mL/min; split vent, 55 mL/min; total flow, 67 mL/
min. Thioanisole: inject/detector temp, 250 °C; oven temp, 120
°C for 2 min then ramped at 15 °C/min until temperature reached
145 °C; column head pressure, 120 kPa; purge vent, 5 mL/min;
split vent, 55 mL/min; total flow, 67 mL/min.
Synthesis of Ligands and Copper Complexes. Reagents and
solvents used were of commercially available reagent quality unless
otherwise stated. Methylene chloride was purified by being passed
(42) Crystals suitable for diffraction were obtained by layering pentane
over a saturated CH2Cl2 solution of 1Cl kept under argon and by
dissolving 1NMe2 into a mixture of CH3NO2/THF/Et2O and then layered
with pentane. Residual O2 in the glovebox oxidized the complex, and
crystals of 4NMe2 were obtained.
(40) Zhang, C. X.; Liang, H.-C.; Kim, E.-i.; Shearer, J.; Helton, M. E.;
Kim, E.; Kaderli, S.; Incarvito, C. D.; Zuberbu¨hler, A. D.; Rheingold,
A. L.; Karlin, K. D. J. Am Chem. Soc. 2003, 125, 634-635.
(41) 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.
(43) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
3006 Inorganic Chemistry, Vol. 45, No. 7, 2006

Cu2O2 Adducts and the [CuII2(O2)]2+/CuIII2(O)2]2+ Equilibrium
through a double alumina column solvent purification system by
Innovative Technologies, Inc. Tetrahydrofuran was purified by
distillation from sodium/benzophenone under argon. Acetone was
distilled from Drierite under argon. HPLC grade pentane was
purchased from Fisher Scientific. Air-sensitive compounds were
synthesized and handled under an argon atmosphere using standard
Schlenk techniques and stored in an MBraun drybox filled with
N2. Deoxygenation of solvents was achieved either by bubbling
argon through the solution for 30-45 min or by applying three
freeze-pump-thaw cycles. 4-R-2-vinylpyridine (R ) NMe2, OMe,
Cl),40 H-PYAN,27 [Cu(MeCN)4][B(C6F5)4]44 and [Cu(H-PYAN)-
45.7, 42.3, 36.2, 25.8. High-resolution ESI-MS Calcd (M + H+):
m/z ) 256.15750, Found: m/z ) 256.15659.
Synthesis of [Cu(NMe2-PYAN)][B(C6F5)4]. Under an Ar at-
mosphere using air-free glassware, a solution of NMe2-PYAN (190
mg, 7.16 × 10-4 mol) in 6 mL of deoxygenated CH2Cl2 was added
to solid [Cu(MeCN)4][B(C6F5)4] (645 mg, 7.11 × 10-4 mol). The
resulting peachy-pink solution was allowed to stir for 30 min at
room temperature and then was gravity filtered through a medium-
porosity frit under argon. The small amount of orange solid collected
was unreactive toward O2 and discarded. To the pink filtrate was
added deoxygenated pentane (∼80 mL) and left to stand at -78
°C under argon for 2 h, after which an oil separated. The supernatant
solution was removed by cannula, and the oil was dried in vacuo
to yield 537 mg (75%) of light pink powder. Anal. Calcd for
(C39H28BCuF20N4): C, 46.52; H, 2.80; N, 5.56. Found: C, 46.31;
H, 2.41; N, 5.79.
Synthesis of [Cu(OMe-PYAN)(MeCN)][B(C6F5)4]. Under an
argon atmosphere using air-free glassware, a solution of OMe-
PYAN (125 mg, 4.97 × 10-4 mol) in 8 mL of deoxygenated CH2-
Cl2 was added to solid [Cu(MeCN)4][B(C6F5)4] (645 mg, 7.11 ×
10-4 mol). The resulting olive-colored solution was allowed to stir
for 30 min at room temperature and then gravity filtered through a
medium-porosity frit under argon. The small amount of orange solid
was unreactive toward O2 and discarded. To the dark yellow filtrate
was added deoxygenated pentane (∼80 mL) and left to stand at
-78 °C under argon for 2 h, after which oil separated. The colorless
supernatant solution was removed by cannula, and the oil was dried
in vacuo to yield 411 mg (78%) of light green powder. Anal. Calcd
for (C40H28BCuF20N4O): C, 46.42; H, 2.73; N, 5.41. Found: C,
47.04; H, 2.68; N, 5.36.
Synthesis of [Cu(Cl-PYAN)(MeCN)][B(C6F5)4]. Under an
argon atmosphere using air-free glassware, a solution of Cl-PYAN
(6.7 mg, 2.62 × 10-4 mol) in ∼10 mL of deoxygenated CH2Cl2
was added to solid [Cu(MeCN)4][B(C6F5)4] (237 mg, 2.61-4 mol).
The resulting clear, bright, yellow solution was allowed to stir for
30 min at room temperature, and then deoxygenated pentane (∼70
mL) was added with stirring at -78 °C under argon until a
precipitate formed. The reaction mixture was filtered under argon
through a coarse-porosity frit, and the solid was dried in vacuo to
yield 233 mg (82%) light yellow solid. Anal. Calcd for (C39H25-
BClCuF20N4): C, 45.07; H, 2.42; N, 5.39. Found: C, 45.54; H,
2.47; N, 5.24.
27
(MeCN)]B(C6F5)4 were synthesized as previously reported.
N-[2-(4-(Dimethylamino)-pyridin-2-yl)-ethyl]-N,N′,N′-trimeth-
yl-propane-1,3-diamine (NMe2-PYAN). In a 50-mL round-bottom
flask, N,N,N′-trimethyl-1,3-propanediamine (0.964 g, 0.0083 mol)
and 4-(dimethylamino)-2-vinylpyridine (1.23 g, 0.0083 mol) were
added along with 5 mL of MeOH and a few drops of glacial acetic
acid. The reaction mixture was refluxed for 3 days at 100 °C. The
crude reaction mixture was collected after rotary evaporation and
purified by column chromatography using 2% MeOH/EtOAc on
alumina to give 0.920 g (42% yield) of the pure oil. (Rf ) 0.2 in
2% MeOH/EtOAc on alumina, or 0.2 in 98% MeOH/2% NH4OH
on silica). 1H NMR (CDCl3): δ 8.13 (1 H, py-6, d (J ) 2.8)), 6.37
(1 H, py-3, d (J ) 1.2)), 6.33 (1 H, py-5, dd(J ) 3)), 2.97 (6 H, s),
2.81 (2 H, m), 2.76 (2 H, m), 2.43 (2 H, t (J ) 3.8)), 2.30 (3 H, s),
2.25 (2 H, t (J ) 3.6)), 2.19 (6 H, s), 1.65 (2H, quint (J ) 3.8)).
13C NMR (CDCl3): δ 160.6, 154.9, 149.4, 105.7, 104.7, 58.1, 58.0,
55.8, 45.7, 42.4, 39.6, 34.3, 25.9. EI-MS m/z ) 265.2 (M + H+).
N-[2-(4-Methoxy-pyridin-2-yl)-ethyl]-N,N′,N′-trimethyl-pro-
pane-1,3-diamine (OMe-PYAN). In a 50-mL round-bottom flask,
N,N,N′-trimethyl-1,3-propanediamine (153 mg, 0.00132 mol) and
a slight excess of 4-methoxy-2-vinylpyridine (200 mg, 0.00148 mol)
were added along with 5 mL of MeOH and a few drops of glacial
acetic acid. The reaction mixture was refluxed for 3 days at 70-
90 °C. The crude reaction mixture was rotary evaporated and
purified by column chromatography using 3% MeOH/EtOAc on
alumina to give 155 mg (46% yield) of the pure oil. (Rf ) 0.2 in
3% MeOH/EtOAc on alumina, or 0.25 in 98% MeOH/2% NH4-
1
OH on silica). H NMR (CDCl3): δ 8.32 (1 H, py-6, d (J ) 3)),
6.69 (1 H, py-3, d (J ) 1.2)), 6.64 (1 H, py-5, dd (J ) 3, 1.2)),
3.82 (3 H, s), 2.86 (2 H, m), 2.75 (2 H, m), 2.42 (2 H, t (J ) 3.8)),
2.30 (3 H, s), 2.24 (2 H, t (J ) 3.6)), 2.195 (6 H, s), 1.64 (2H,
quint (J ) 3.8)). 13C NMR (CDCl3): δ 166.1, 162.4, 150.6, 109.2,
107.6, 58.1, 57.6, 55.8, 55.2, 45.7, 42.3, 36.2, 25.8. EI-MS m/z )
252.2 (M + H+).
Synthesis of [Cu(R-PYAN)(CO)]B(C6F5)4 Complexes. CO
adducts were generated in situ by bubbling CO through an air-free
CH2Cl2 solution of the corresponding Cu(I) complex.
Synthesis of [{Cu(R-PYAN)}2(OH)2][B(C6F5)4]2 Complexes.
The dicopper(II) dihydroxy compounds were prepared by adding
O2 to a CH2Cl2 solution of [Cu(R-PYAN)(MeCN)n]B(C6F5)4 at -78
°C and left at -80 °C overnight. The solutions were then warmed
to room temperature, and the copper(II) decomposition product was
precipitated by the addition of pentane. Anal. Calcd for [{Cu(NMe2-
PYAN)}2(OH)2][B(C6F5)4]2‚CH2Cl2‚C5H12 (C84H72B2Cl2Cu2F40-
N8O2): C, 45.75; H, 3.29; N, 5.08. Found: C, 45.35; H, 3.20; N,
5.00. Anal. Calcd for [{Cu(OMe-PYAN)}2(OH)2][B(C6F5)4]2‚CH2-
Cl2‚C5H12 (C82H66B2Cl2Cu2F40N6O4): C, 45.20; H, 3.05; N, 3.86.
Found: C, 44.78; H, 2.43; N, 3.58. Anal. Calcd for [{Cu(H-
PYAN)}2(OH)2][B(C6F5)4]2‚CH2Cl2‚C5H12 (C80H62B2Cl2Cu2F40-
N6O2): C, 45.35; H, 2.95; N, 3.97. Found: C, 45.57; H, 3.20; N,
4.18. Anal. Calcd for [{Cu(Cl-PYAN)}2(OH)2][B(C6F5)4]2‚C5H12
(C79H58B2Cl2Cu2F40N6O2): C, 45.12; H, 2.78; N, 4.00. Found: C,
45.43; H, 3.34; N, 4.08.
N-[2-(4-Chloro-pyridin-2-yl)-ethyl]-N,N′,N′-trimethyl-propane-
1,3-diamine (Cl-PYAN): In a 50-mL round-bottom flask, N,N,N′-
trimethyl-1,3-propanediamine (250 mg, 0.00215 mol) and a slight
excess of 4-chloro-2-vinylpyridine (322 mg, 0.00231 mol) were
added along with 5 mL of MeOH and a few drops of glacial acetic
acid. The reaction mixture was refluxed for 3 days at 70-90 °C.
The crude reaction mixture was rotavapped and purified by column
chromatography using 3% MeOH/EtOAc on alumina to give 245
mg (44% yield) of the pure oil. (Rf ) 0.5-0.3, 3% MeOH/EtOAc
1
on alumina, or 0.3-0.2 in 98% MeOH/2% NH4OH on silica). H
NMR (CDCl3): δ 8.41 (1 H, py-6, d (J ) 2.8)), 7.21 (1 H, py-3,
d (J ) 1)), 7.13 (1 H, py-5, dd (J ) 2.8, 1)), 2.92 (2 H, m), 2.76
(2 H, m), 2.43 (2 H, t (J ) 3.8)), 2.24 (3 H, s), 2.24 (2 H, t (J )
3.6)), 2.205 (6 H, s), 1.63 (2H, quint (J ) 3.6)). 13C NMR
(CDCl3): δ 166.1, 162.4, 150.6, 109.2, 107.6, 58.1, 57.6, 55.8, 55.2,
Exogenous Substrate Oxidations. The general procedure for
substrate oxidations is as follows. The Cu(I) solutions (2 mL, ∼(3-
(44) Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin, K.
D. Inorg. Chem. 2002, 41, 2209-2212.
Inorganic Chemistry, Vol. 45, No. 7, 2006 3007

Hatcher et al.
Table 1. Selected Bond Angles and Distances for
[Cu(Cl-PYAN)(MeCN)]B(C6F5)4 (1Cl) and
[Cu(H-PYAN)(MeCN)]B(C6F5)4 (1H)
anglea (deg)
1Cl
1H b
N(1)-Cu(1)-N(3)
N(1)-Cu(1)-N(2)
N(2)-Cu(1)-N(3)
N(1)-Cu(1)-N(4)
N(2)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
107.62 (6)
101.00 (6)
103.45 (6)
119.91 (7)
117.18 (7)
106.29 (6)
109.77 (6)
101.56 (7)
102.20 (6)
119.87 (7)
117.18 (7)
104.90 (7)
distance (Å)
1Cl
1H b
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-N(3)
Cu(1)-N(4)
2.0297 (15)
2.0997 (15)
2.1405 (16)
1.9268 (17)
2.0399 (17)
2.1145 (17)
2.1527 (16)
1.9437 (18)
a According to atom labels depicted in Figure 2. b From ref 27.
Table 2. Copper(I) Complex Reduction Potentials and Carbonyl
Stretching Frequencies
Figure 2. ORTEP diagram (at 50% probability) of the cationic portion
[CuI(Cl-PYAN)(MeCN)]B(C6F5)4 (1Cl).
E1/2a vs Fe(cp)2+/0 (V)b
ν(C-O) (cm-1
)
4) × 10-5 M) were prepared in the drybox and then brought out
and handled in Schlenk tubes. The solutions were cooled to -78
°C with an acetone/dry ice bath. Dry O2 gas was bubbled through
the solutions for a few minutes, and enough time was allowed to
generate full formation of the peroxo or oxo adduct. Excess O2
was then purged by three vacuum/Ar purge cycles and bubbling
argon through the solutions for at least 60 s. Then, 1 equiv of the
internal standard (decane) and 10 equiv of substrate were added in
CH2Cl2 solutions. Argon was bubbled again to remove any
dissolved O2 in the substrate/internal standard, and the reaction was
allowed to proceed under argon for 20-24 h. Finally, pentane was
added to the green solutions to precipitate the copper complex and
analyze the supernatant organic substrates by GC. Yields reported
are an average of five or six runs.
1NMe2
-0.40
-0.38
-0.33
-0.32
1NMe2-CO
2080
2086
2088
2090
1OMe
1H
1OMe-CO
1H-CO
1Cl
1Cl-CO
+/0
a In N,N-dimethylformamide. b According to Fe(cp)2
) 0.45 V vs
SCE.45
The copper(I) complex 1Cl has remarkable structural
resemblance to that of the parent compound 1H. A compari-
son of the structural parameters for 1Cl and 1H is listed in
Table 1. The virtually superimposable geometry (along with
both bond distances and angles) around the copper(I) ion is
a clear indication that the ligand modification by the 4-R-
pyridyl substituent does not affect the steric environment
around the copper center. Although the exact ligand geometry
of the CuII O2 adducts is unknown, the similarities of the
Results and Discussion
2
crystal structures of 1Cl and 1H suggest that the R-PYAN
ligand series will dominantly induce an electronic effect in
its CuI/O2 reactivity.
Synthesis and Physical Properties of 1R. The new ligands
R-PYAN (N-[2-(4-R-pyridin-2-yl)-ethyl]-N,N′,N′-trimethyl-
propane-1,3-diamine, where R ) NMe2, OMe, and Cl) were
Electrochemistry. The redox potentials of the CuII/CuI
couple for the complexes was determined by cyclic volta-
mmetry in DMF solvent (Supporting Information) to assess
the electronic influence induced by the 4-R-pyridyl substitu-
1
synthesized and characterized by H and 13C NMR spec-
troscopy and mass spectrometry. The corresponding copper-
(I) complexes were synthesized by the anaerobic addition
of the ligand to [CuI(CH3CN)4]B(C6F5)4 dissolved in dichlo-
romethane. Upon isolation (see Experimental Section), the
[CuI(R-PYAN)(CH3CN)n]B(C6F5)4 complexes (1R) (n ) 1
for R ) Cl, H, and OMe; n ) 0 for R ) NMe2) were
characterized by elemental analysis, and [Cu(Cl-PYAN)-
(MeCN)]B(C6F5)4 (1Cl) was recrystallized to yield crystals
suitable for X-ray diffraction.
The structure of the cationic portion of 1Cl is shown in
Figure 2 and depicts a distorted tetrahedral copper center,
with three N donors from the PYAN ligand and a fourth N
donor from acetonitrile, similar to other Cu(I) complexes of
tridentate pyridylalkylamine ligands including Cu(H-PYAN)-
(MeCN)]B(C6F5)4 (1H).27 [Cu(OMe-PYAN)(MeCN)]B(C6F5)4
(1OMe) also binds acetonitrile, while the complex with the
most electron donating ligand [Cu(NMe2-PYAN)]B(C6F5)4
(1NMe2) is stable as a tridentate and does not seem to require
an additional nitrogen donor; unlike the others, it is readily
isolated without a strong MeCN ligand, even in the presence
of MeCN in solution.
ent. The E1/2 values for 1NMe2, 1OMe, 1H, and 1Cl are -0.40,
-0.38, -0.33, and -0.32 V vs Fe(cp)2
+/0
in DMF,45
respectively (Table 2), indicating that, as expected, the more
electron rich copper(I) complexes with better R-donors are
easier to oxidize than those with electron-deficient substit-
uents. These values are similar to those of the complexes
containing the ligand series R-MePY2, where the E1/2 values
are -0.44, -0.36, -0.31, and -0.27 V vs Fe(cp)2+/0 in DMF
for R ) NMe2, OMe, H, and Cl, respectively.40 However,
the overall range of redox potentials of 0.08 V for the series
1R is considerably less than that of 0.17 V observed for the
CuII/CuI couple for the ligands R-MePY2. This result is
perhaps expected, since the ligand system R-MePY2 has
double the substituent effects versus R-PYAN; the former
ligand contains two pyridyl nitrogen (Npy) donors versus the
one Npy donor in R-PYAN (Chart 1).
(45) Converted value according to Fc+/0 E1/2 ) 0.45 V vs SCE in DMF
(Connelly, N. G, Geiger, W. E. Chem. ReV. 1996, 96, 877-910).
3008 Inorganic Chemistry, Vol. 45, No. 7, 2006

Cu2O2 Adducts and the [CuII2(O2)]2+/CuIII2(O)2]2+ Equilibrium
Scheme 1
Figure 3. Solution (CH2Cl2) C-O stretching frequencies of 1R -CO.
Carbon Monoxide Adducts. The study of carbon mon-
oxide binding to metal centers is a useful approach to probe
bonding and structure, as well as having been used to
stabilize reactive copper(I) complexes for X-ray crystal-
lographic analysis.46 The diatomic CO molecule is also often
used as a surrogate for O2-binding studies. It is also common
to use CO to probe the relative amount of back-bonding a
given metal complex bestows on the bound CO; the more
electron-rich a metal center, the more back-bonding occurs,
and this is manifested by a lowered carbonyl stretching
frequency. For this latter purpose, the carbonyl adducts 1R-
CO were synthesized in situ by bubbling CO(g) through a
methylene chloride solution of 1R and solution IR spectra
were recorded (Figure 3). As expected, the complex with
the most electron donating ligand, 1NMe2-CO, possesses the
lowest carbonyl stretching frequency, 2080 cm-1. The next
most electron donating copper(I) complex, 1OMe-CO, has a
slightly stronger carbon monoxide bond with ν(C-O) )
2086 cm-1. The parent and chloro-substituted analogues do
not differ much from 1OMe-CO as ν(C-O) ) 2088 and 2090
cm-1 for 1H-CO and 1Cl-CO, respectively. Thus, the
substituted ligands impose only a minor change in the
ligand-CuI-CO carbonyl stretching frequency, suggesting
that the difference in back-bonding as a result of the varying
R substituent is quite small, although measurable. The CO
stretching frequencies are similar to those reported for [Cu-
(R-MePY2)(CO)]+ where ν(C-O) ranges from 2075 to 2093
cm-1; however, notice the range in values is again larger in
this latter case.40 As described above, this is probably a result
of the R-MePY2 ligand system having essentially two times
the substituent effect than the R-PYAN system.
As examined by low-temperature UV-vis spectroscopy,
the dioxygen reactivity with 1R is considerably different than
that of the CuI complexes of R-MePY2, whereby the R
substituent in R-PYAN induces a substantial effect on the
resulting dioxygen intermediate. For example, in THF at -78
°C, 1H reacts with O2 forming an approximately equal
mixture of side-on peroxo [{CuII(H-PYAN)}2(O2)]2+ (2H) and
bis(µ-oxo) [{CuIII(H-PYAN)}2(O)2]2+ (3H) adducts according
to the intensities of the LMCT band at 360 nm corresponding
to 2H and the one at 395 nm corresponding to 3Hsassuming
that the ꢀ values for the pure side-on peroxo and bis(µ-oxo)
dicopper complexes fall within the range for previously
reported systems (Scheme 1 and Figure 4).24,27,35,47-49
However, the copper(I) complex containing an electron-
withdrawing ligand substituent, 1Cl, reacts with O2 to form
predominantly the side-on peroxo adduct, [{CuII(Cl-PYAN)}2-
(O2)]2+ (2Cl) with the bis(µ-oxo) isomer, [{CuIII(Cl-PYAN)}2-
(O)2]2+ (3Cl), being a minor product (Figure 4, top). On the
other hand, the complex with the electron-donating methoxy
substituent, 1OMe, preferentially binds O2 in a bis(µ-oxo)
dicopper(III) structure, [{CuIII(OMe-PYAN)}2(O)2]2+ (3OMe),
with very little side-on peroxo [{CuII(OMe-PYAN)}2(O2)]2+
(2OMe) in equilibrium. Finally, on the basis of these UV-
vis spectroscopic interrogations, 1NMe2 favors exclusively the
bis(µ-oxo) [{CuIII(NMe2-PYAN)}2(O)2]2+ (3NMe2) adduct
(Figure 4, top). Hence, the one pyridyl substituent on
R-PYAN imposes a dramatic electronic (due to substituent)
effect on dioxygen reactivity such that the equilibrium
between the dioxygen isomers is shifted from mostly a side-
on peroxo species 2Cl to a bis(µ-oxo) structure 3NMe (Figure
5).
Dioxygen Reactivity of [Cu(R-PYAN)(MeCN)n]B-
(C6F5)4 (1R). According to the physical structures, redox
potentials, and ν(C-O) of [LCuI(CO)]+ of 1R and in
comparison to the copper(I) complexes of R-MePY2, one
might think the dioxygen reactivity would follow a very
similar pattern. Indeed, both sets of copper(I) complexes react
with dioxygen as outlined in Scheme 1, where the low-
temperature stable dioxygen adducts, 2R and 3R, can be in
equilibrium; upon warming, these transform to the more
stable dihydroxy-bridged dicopper(II) complexes, 4R (vide
infra).
The effect of solvent is known to be a factor in the
equilibrium between the isomeric side-on peroxo and bis-
(µ-oxo) dicopper cores, whereby coordinating solvents bias
the equilibrium toward the bis(µ-oxo) structure.25-27 To
catalog the solvent effects on the dioxygen reactivity of [Cu-
(R-PYAN)(MeCN)n]B(C6F5)4 (1R), oxygenation reactions
were also carried out in methylene chloride (Figure 4,
(47) 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.
(48) Mahadevan, V.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack,
T. D. P. J. Am. Chem. Soc. 1999, 121, 5583-5584.
(46) Karlin, K. D.; Tyekla´r, Z.; Farooq, A.; Haka, M. S.; Ghosh, P.; Cruse,
R. W.; Gultneh, Y.; Hayes, J. C.; Toscano, P. J.; Zubieta, J. Inorg.
Chem. 1992, 31, 1436-1451.
(49) According to the calculated spectra derived from kinetics analyses,
the ꢀ values for pure 2H and 3H are 21 000 (λmax ) 360) and 25 000
M-1 cm-1 (λmax ) 400), respectively. See ref 29.
Inorganic Chemistry, Vol. 45, No. 7, 2006 3009

Hatcher et al.
Figure 6. rR spectra of 3R with 16O2 (solid lines) and 18O2 (dashed lines)
for R ) NMe2 (red), OMe (orange), H (green), and Cl (blue).
Table 3. Resonance Raman Stretching Frequencies (cm-1) of Peroxo
(2R) and bis(µ-oxo) (3R) in THF-d8
side-on peroxo dicopper(II)
bis(µ-oxo) dicopper(III)
ν(Cu2O2)c
Figure 4. Absorbance spectra of the oxygenation reaction of 1R in THF
(top) and CH2Cl2 (bottom) at -78 °C. All at a similar concentration of
∼0.4 mM in THF or ∼0.5 mM in CH2Cl2.
ν(O-O)a
16O2 18O2
∆ν
ν(Cu-Cu)b
16O2 18O2 ∆ν
d
e
d
e
d
d
f
2NMe2
2OMe
2H
3NMe2
3OMe
2H
591
585
587
588
558
561
562
564
e
f
f
f
287
282
278
714 678 -36
726 685 -41
2Cl
3Cl
b
c
a λex ) 514 nm, T ) 195 K. λex ) 364 nm, T ) 77 K. λex ) 407 nm,
T ) 77 K. d No 2NMe2 features observed nor expected (UV-vis criterion).
e Attempts to obtain ν(O-O) for 2OMe were unsuccessful due to sample
instability at 195 K. f Absolute isotope shifts are difficult to evaluate due
to Fermi shifting in the 16O2 data.
for R ) NMe2 since 3NMe2 is the sole oxygenation product
formed in either THF or CH2Cl2; also 2Cl is less solvent
dependent, as mostly 2Cl is produced in either solvent. Hence,
in CH2Cl2, the equilibrium between a dicopper(II) side-on
peroxo and a dicopper(III) bis(µ-oxo) species can be tailored
to either limit by way of the electron donating/withdrawing
R substituent in the ligand system R-PYAN. The low-
temperature oxygenation reactions of 1R were also carried
out in acetone and produced similar results to those observed
in THF (see Supporting Information).
Figure 5. Shift in equilibrium from [CuII2(O2)]2+ to [CuIII2(O)2]2+ by ligand
electronic effects.
Resonance Raman Spectroscopy. Resonance Raman
spectroscopy supports the copper-dioxygen structural as-
signments derived from the UV-vis absorption data. In THF-
d8 with 407 nm excitation, [{Cu(R-PYAN)}2(16O2)]2+ pro-
duces a characteristic dicopper(III) bis(µ-oxo) spectrum with
a resonance-enhanced peak at ∼587 cm-1 which shifts by
∼25 cm-1 when 18O2 is used as the dioxygen source (Figure
6).24,50 The core bis(µ-oxo) (Cu2O2) stretching frequency for
3R shifts as R is varied; ν(Cu216O2) ) 588, 587, and 585
cm-1, for R ) Cl, H and OMe, respectively (Table 3). The
16O2-derived peaks are complicated by Fermi interactions
bottom). In the noncoordinating CH2Cl2 solvent, 1Cl and 1H
react with O2 at -78 °C to yield predominantly the side-on
peroxo species, 2Cl and 2H, respectively. The dioxygen adduct
of the electron-donating complex 1OMe which favors the bis-
µ-oxo structure in THF, generates mostly the side-on peroxo
structure in CH2Cl2 with a minor percentage of the bis-µ-
oxo species in equilibrium (i.e., [2OMe] > [3OMe]). The
reaction of 1NMe2 with O2 does not change even in the
noncoordinating solvent and gives exclusively the high-valent
dicopper(III) isomer, 3NMe2, signifying that pure electronic
effects, and not a solvent effect, influences the full formation
of the bis(µ-oxo) species. Solvent plays a secondary role,
specifically in the equilibria between 2R f 3R, where R )
Cl, H, and OMe, but the electronic effect takes precedence
(50) Holland, P. L.; Cramer, C. J.; Wilkinson, E. C.; Mahapatra, S.; Rodgers,
K. R.; Itoh, S.; Taki, M.; Fukuzumi, S.; Que Jr., L.; Tolman, W. B. J.
Am. Chem. Soc. 2000, 122, 792-802.
3010 Inorganic Chemistry, Vol. 45, No. 7, 2006

Cu2O2 Adducts and the [CuII2(O2)]2+/CuIII2(O)2]2+ Equilibrium
Table 4. Exogenous Substrate Reactivity of 2H and 3NMe2
2H
3NMe2
benzyl alcohol
f benzaldehyde
85%
96%
dihydroanthracene
f anthracene
f anthrone
81%
28%
14%
20%
a
a
f anthraquinone
dimethylaniline
f methylaniline {+ CH2(O)}
64%
52%
58%
58%
phenylmethyl sulfide
f phenylmethyl sulfoxide
a Negligible amounts detected.
However, the R-PYAN system is significantly different in
that, unlike the partial equilibrium shift observed in the Cu2O2
complexes of R-MePY2, the R-PYAN ligand induces a
nearly complete shift from a side-on peroxo dicopper(II) to
a bis(µ-oxo) dicopper(III) species. This difference in behavior
between the two ligand systems might be explained by the
second alkyl nitrogen (Nalkyl) donor present in R-PYAN (R-
MePY2 contains two pyridyl donors (Npy) and only one Nalkyl
donor) (Chart 1). Alkyl nitrogen donors have been shown
to support high-valent dicopper(III) bis(µ-oxo) cores by way
of σ donation to the Cu ion.19,24 This shift from side-on
peroxo to mostly bis(µ-oxo) (in the present R-PYAN system)
is observed in conjunction with a weakening of the O-O
bond. Additional studies are under way to elucidate the
fundamental differences in ligand-copper(II) bonding for
tridentate amine systems.
Figure 7. rR spectra of 2R depicting the “Cu-Cu” motion for R ) NMe2
(red), OMe (orange), H (green), and Cl (blue).
with peaks in the ∼580-600 cm-1 region. The 18O2-related
peaks are shifted out of this region and show a trend of
decreasing values with better R donor, shifting from 564 to
562, 561, and 558 cm-1 as R changes from Cl to H, OMe,
and NMe2, respectively (Table 3). Alternatively, excitation
at 364 nm in a THF-d8 solution enhances the side-on peroxo
Cu-Cu stretching vibration, ν(Cu-Cu). As R becomes more
electron donating (Figure 7), ν(Cu-Cu) shifts to higher
frequency, from 278 to 282 to 287 cm-1 for 2Cl, 2H, and 2OMe
,
respectively.
While the core stretching frequencies for 2R and 3R are
only modestly affected by the R substituent on the pyridyl
nitrogen donor, the O-O stretches of 2Cl and 2H (in acetone
at 514 nm excitation) reveal a significant shift in frequency
from ν(O-O) ) 726 cm-1 for 2Cl to 714 cm-1 for 2H (see
Supporting Information).27 (Note that ν(O-O) could not be
obtained for R ) MeO and NMe2 due to their dominant
conversion to the bis(µ-oxo) species and the relatively weak
enhancement of ν(O-O) compared to ν(Cu-Cu).) Typical
O-O stretching frequencies lie in the 730-760 cm-1 range,
and only one other example of side-on peroxo complex with
a lower ν(O-O) value than 714 cm-1 (for 2H)27 exists.5,25
The unusually low O-O stretching frequency of 2H reflects
the propensity of that O-O bond to cleave.24 Indeed, the
bond becomes weaker in 2H relative to 2Cl until it is fully
cleaved in 3NMe2. Thus, we observe for the first time ligand
electronic effects leading to a weakening of ν(O-O) and
correlating to corresponding shift in equilibrium from side-
on peroxo to bis(µ-oxo).
Reactivity of 2H and 3NMe2 toward External Substrates.
The ability to synthesize discrete [CuII (O22-)]2+ and
2
[CuIII2(O2-)2]2+ compounds is desirable for the study of the
reactivity patterns of each individual isomeric core. Thus,
the reactivity of [{Cu(H-PYAN)}2(O2)]2+ (2H) as a pre-
dominate side-on peroxo dicopper(II) complex and [{Cu-
(NMe2-PYAN)}2(O)2]2 (3NMe2) as a bis(µ-oxo) dicopper(III)
species toward exogenously added substrates (10-fold excess)
was examined (Table 4). Benzyl alcohol, dimethylaniline,
and thioanisole were all oxidized/oxygenated, with the
product yields not varying too much between 2H or 3NMe2
(Table 4). However, the reaction toward dihydroanthracene
varies significantly with the isomers 2H and 3NMe2, the former
producing an 81% yield of anthracene, with only 28%
observed for 3NMe2. However, other over-oxidized products
were found, anthrone (14% yield) and anthraquinone (20%
yield). While such behavior is known by other strong
inorganic metal oxidants,51,52 the reaction is unprecedented
for dicopper-dioxygen species. When 18O2 was used as the
dioxygen source, according to the intensities of the relevant
mass spectrometric peaks, C14H818O2 (anthraquinone) is
observed in 42% yield, and the mixed-labeled and 16O2
products, C14H816O18O and C14H816O2, are observed in smaller
quantities of 31% and 27%, respectively. The low yield of
18O2 incorporation may indicate that the over-oxidation may
be occurring during the workup process and not from the
These results are in contrast to those observed for copper
complexes of R-MePY2, where increased electron donation
from the ligand led to a more limited increase in bis(µ-oxo)
formation, and had virtually no effect on ν(Cu-Cu) or ν-
(O-O).41 This was attributed to opposing effects, whereby
the added electron donation to the copper from the R-MePY2
ligand resulted in a more negatively charged peroxide that
hindered back-bonding into the peroxide σ* orbital. Ther-
modynamic stabilization of the bis(µ-oxo) relative to the side-
on peroxo with better R donors led to varying mixtures
(51) Lam, W., W. Y.; Yiu, S.-M.; Douglas, T. Y. Y.; Lau, T.-C.; Yip,
W.-P.; Che, C.-M. Inorg. Chem. 2003, 42, 8011-8018.
(52) Wang, K. M.; James M. J. Am. Chem. Soc. 1997, 119, 1470-1471.
CuII (O2) and CuIII2(O)2 coexisting in equilibrium but in all
2
cases favoring the side-on peroxo dicopper(II) isomer.
Inorganic Chemistry, Vol. 45, No. 7, 2006 3011

Hatcher et al.
Table 5. Spectroscopic and Physical Properties of 4R
UV-vis [nm (M-1 cm-1)] ν(O-H) (cm-1
)
µ-eff [B.M./CuII]
4NMe2
4OMe
4H
670 (380)
680 (300)
670 (380)
670 (360)
3651
3655
3653
3649
1.2
1.6
1.3
1.2
4Cl
The thermal transformation product 4NMe2 is also charac-
terized by its CuII d-d transitions at λmax ) 670 nm (380
M-1 cm-1) and a sharp O-H infrared stretch at 3651 cm-1
(Table 5). The corresponding products 4OMe, 4H, and 4Cl have
similar spectroscopic properties to those of crystalline 4NMe2
,
with d-d transitions at 680 (300 M-1 cm-1), 670 (380 M-1
cm-1), and 670 nm (360 M-1 cm-1), and sharp O-H
stretches at 3655, 3653, and 3649 cm-1, for 4OMe, 4H, and
4Cl, respectively. The room-temperature magnetic properties
of 4R were determined by the Evans method56 and values of
1.2, 1.6, 1.3, and 1.2 µB per CuII ion were found for the
complexes with R ) NMe2, OMe, H, and Cl, respectively.
These are diminished µeff values relative to that for typical
d9 CuII complexes (µB g1.73, but usually >2.0), but are
typical for a dihydroxy bridged antiferromagnetically coupled
dicopper(II) core. A summary of the physical properties of
4R is presented in Table 5.47
Figure 8. Cationic portion of [{Cu(NMe2-PYAN)}2(OH)2][B(C6F5)4]2
(4NMe2) at the 50% probability level.
actual dicopper-dioxygen adducts; further studies are needed.
Based on yields, differential reactivity of the different
dicopper cores (2H and 3NMe2) does not seem apparent.
However, preliminary observations show that the side-on
peroxo 2H and bis(µ-oxo) 3NMe2 react with thioanisole at
considerably different rates: the reaction by 2H is over in
less than 1 h, but oxidation by 3NMe2 takes ∼20 h (under
similar reaction conditions/concentrations). Further, the
oxidation of benzyl alcohol takes twice as long when 3NMe2
is used as the oxidant compared to 2H, and most interestingly,
the oxidation of dimethylaniline by 2H and 3NMe2 appears to
occur via different kinetic mechanisms.53 While these are
currently only general observations, they indicate that rich,
mechanistic insights may be obtained in future studies on
the oxidative chemistry of the distinctive side-on peroxo (2H)
and bis(µ-oxo) (3NMe2) cores.
Summary and Conclusions
This study further evaluates the factors affecting the
equilibrium between a µ-η2:η2-(side-on)-peroxo dicopper-
(II) species and its bis(µ-oxo) dicopper(III) complex isomeric
form. We have showed that seemingly small changes in the
electronic environment around the copper(I) center can
drastically influence the structure of the resulting Cu2O2
adduct. While the pyridyl substituent only marginally affects
the CuII/CuI redox couple and the back-bonding to CO in
[CuI(R-PYAN)(CO)]+, it is capable of shifting the equilib-
rium from a dicopper complex that is mostly the side-on
peroxo isomer to the bis(µ-oxo) extreme for R ) NMe2. A
significant electronic effect is observed; O-O bond weaken-
ing occurs with a ligand which is a better donor; the O-O
bond stretch decreases from 726 cm-1 in 2Cl to 714 cm-1 in
2H. It is noteworthy that, while the complexes of R-MePY2
exhibit larger differences in CO stretches and CuII/CuI
reduction potentials, the dioxygen reactivity is only moder-
ately affected. One might have expected a larger electronic
effect from the copper(I) complexes of R-MePY2 rather than
with those with R-PYAN due to the double substituent effect
arising from the second pyridyl arm. One key difference
possibly leading to the variations between the two systems
is the nature of the nitrogen donors of the chelating ligand.
Alkylamine ligands are better σ donors to the copper ion
and are able to stabilize a high-valent dicopper(III) core.24
R-PYAN contains two alkylamine ligand donors, versus just
one alkylamine nitrogen in R-MePY2. It may be this feature
that apparently allows the R-PYAN system of copper-
dioxygen adducts to lie close to the midpoint (i.e., 1:1 ratio
of concentrations) of the peroxo/bis(µ-oxo) equilibrium and
Bis(µ-OH) Dicopper(II) Compounds (4R). In the absence
of external substrates, or upon warming, the dioxygen
intermediates 2R and 3R transform to stable bis(µ-OH)
dicopper(II) compounds (4R) analogous to the decomposition
products of [{Cu(R-MePY2)}2(O2)]2+ and many other di-
copper-dioxygen intermediates.40,47 The 4R thermal decom-
position products were characterized by UV-vis and IR
spectroscopy with [{Cu(NMe2-PYAN)}2(OH)2][B(C6F5)4]2
(4NMe2) also being analyzed by X-ray crystallography (Figure
8).
The dihydroxy structure shows that the pyridyl- and
central-amine donors bind equatorially and the peripheral
amine coordinates axially, at least in this structure. Whether
or not this same ligand wrapping likely occurs around the
CuII centers in 2R and 3R will be assessed in a future report.
The structure of 4NMe2 contains a center of symmetry, and
the copper(II) centers adopt a distorted square pyramidal
geometry where the axial ligands are anti with respect to
one another. According to Addison and Reedijk’s description
for five-coordinate centric molecules,54 the central copper
atoms deviate from perfect square pyramidal form with τ )
0.19, a distortion larger than the CuII (µ-OH)2 analogue
2
supported by the ligand NMe2-MePY2.55 We surmise that
this may a consequence of the asymmetry of the NMe2-
PYAN ligand.
(53) The overall reaction order is different for 2H and 3NMe2
.
(54) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G.
(55) Zhang, C. X. Ph.D. Dissertation, Johns Hopkins University, 2001.
(56) Evans, D. F. J. Chem. Soc. 1959, 2003.
C. J. Chem. Soc., Dalton Trans. 1984, 1349-1356.
3012 Inorganic Chemistry, Vol. 45, No. 7, 2006

Cu2O2 Adducts and the [CuII2(O2)]2+/CuIII2(O)2]2+ Equilibrium
to truly probe how small electronic modifications can cause
very clearly measurable effects on the copper-dioxygen
chemistry which must be arising only from the R variations
of the R-PYAN pyridyl donor. Indeed, ligand electronic
effects can be a significant contributions to the side-on
peroxo/bis(µ-oxo) equilibrium.
detailed mechanistic insights in order better understand the
oxidative chemistry of (side-on)-peroxo dicopper(II) versus
bis(µ-oxo) dicopper(III) species.
Acknowledgment. K.D.K (GM28962) and E.I.S. (DK31-
450) are grateful to the National Institutes of Health for
financial support of this research. We also thank Dr. Liviu
M. Mirica for his assistance in resonance Raman sample
preparation.
Finally, both dicopper cores can oxidize external substrates
in reasonable yields, allowing for a further investigation of
the reactivity patterns of each CuII (O2) and CuIII2(O)2 isomers
2
Supporting Information Available: X-ray crystallographic data
in cif format and supplemental figures. This material is available
free of charge via the Internet at http://pubs.acs.org.
without introducing drastic changes in the supporting ligand.
The preliminary overview reactivity studies presented here
qualitatively show differential oxidative reactivity according
to reaction kinetics. Future studies will focus on uncovering
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Inorganic Chemistry, Vol. 45, No. 7, 2006 3013