Copper(I)-dioxygen reactivity of [(L)CuI]+ (L = tris(2-pyridylmethyl)amine): Kinetic/thermodynamic and spectroscopic studies concerning the formation of Cu-O2 and Cu2-O2 adducts as a function of solvent medium and 4-pyridyl ligand substituent variations

Inorg. Chem. 2003, 42, 1807−1824

Copper(I)−Dioxygen Reactivity of [(L)Cu^I]⁺(L )

Tris(2-pyridylmethyl)amine): Kinetic/Thermodynamic and Spectroscopic

Studies Concerning the Formation of Cu−O and Cu₂−O Adducts as a

2

Function of Solvent Medium and 4-Pyridyl Ligand Substituent Variations

Christiana Xin Zhang,^†Susan Kaderli,^‡Miguel Costas,^‡Eun-il Kim,^†Yorck-Michael Neuhold,^‡

,†

,‡

Kenneth D. Karlin,* and Andreas D. Zuberbu1hler*

Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218, and

Department of Chemistry, UniVersity of Basel, CH-4056 Basel, Switzerland

Received September 18, 2002

The kinetic and thermodynamic behavior of O -binding to Cu(I) complexes can provide fundamental understanding

2

of copper(I)/dioxygen chemistry, which is of interest in chemical and biological systems. Here we report stopped-

I

flow kinetic investigations of the oxygenation reactions of a series of tetradentate copper(I) complexes [(L^R)Cu-

(MeCN)]⁺(1^R, R ) H, Me, tBu, MeO, Me₂N) in propionitrile (EtCN), tetrahydrofuran (THF), and acetone. The

syntheses of 4-pyridyl substituted tris(2-pyridylmethyl)amine ligands (L^R) and copper(I) complexes are detailed.

I

Variations of ligand electronic properties are manifested in the electrochemistry of 1^Rand ν(CO) of [(L^R)Cu−CO]⁺

complexes. The kinetic studies in EtCN and THF show that the O -reactions of 1^Rfollow the reaction mechanism

2

established for oxygenation of 1^Hin EtCN (J. Am. Chem. Soc. 1993, 115, 9506), involving reversible formation

(k₁/k_-1) of [(L^R)Cu (O ^-)]⁺(2^R), which further reacts (k₂/k_-2) with 1^Rto form the 2:1 Cu O complex [{(L^R)Cu }₂(O ^2-)]²⁺

II

2

(3^R). In EtCN, the rate constants for formation of 2^R(k₁) are not dramatically affected by the ligand electronic

variations. For R ) Me and tBu, the kinetic and thermodynamic parameters are very similar to those of the parent

complex (1^H); e.g., k₁is in the range 1.2 × 10⁴to 3.1 × 10⁴M^-1s^-1at 183 K. With the stronger donors R ) MeO

and Me₂N, more significant effects were observed, with the expected increase in thermodynamic stability of resultant

2^Rand 3^Rcomplexes, and decreased dissociation rates. The modest ligand electronic effects manifested in EtCN

are due to the competitive binding of solvent and dioxygen to the copper centers. In THF, a weakly coordinating

solvent, the formation rate for 2^His much faster (g100 times) than that in EtCN, and the thermodynamic stabilities

of both the 1:1 (K ) and 2:1 (â ) K K ) copper−dioxygen species are much higher than those in EtCN (e.g., for

1

1 2

2^H, ∆H° (K ) ) −41 kJ mol^-1in THF versus −29.8 kJ mol^-1in EtCN; for 3^H, ∆H° (â) ) −94 kJ mol^-1in THF

1

versus −77 kJ mol^-1in EtCN). In addition, a more significant ligand electronic effect is seen for the oxygenation

reactions of 1^MeOin THF compared to that in EtCN; the thermal stability of superoxo- and peroxocopper complexes

are considerably enhanced using L^MeOcompared to L^H. In acetone as solvent, a different reaction mechanism

I

involving dimeric copper(I) species [(L^R)₂Cu₂]²⁺is proposed for the oxygenation reactions, supported by kinetic

analyses, electrical conductivity measurements, and variable-temperature NMR spectroscopic studies. The present

study is the first systematic study investigating both solvent medium and ligand electronic effects in reactions

forming copper−dioxygen adducts.

Introduction

tive and the extensive occurrence of copper-containing

proteins which process dioxygen.^2,8,9X-ray structures of

Much of the inspiration for the current research in synthetic

copper-O₂chemistry^1-7comes from a bioinorganic perspec-

(1) Kitajima, N.; Moro-oka, Y. Chem. ReV. 1994, 94, 737-757.

(2) Karlin, K. D.; Zuberbu¨hler, A. D. In Bioinorganic Catalysis: Second

Edition, ReVised and Expanded; Reedijk, J., Bouwman, E., Eds.;

Marcel Dekker: New York, 1999; pp 469-534.

* To whom correspondence should be addressed.

^†Johns Hopkins University.

^‡University of Basel.

(3) Schindler, S. Eur. J. Inorg. Chem. 2000, 2311-2326.

Published on Web 02/12/2003

Inorganic Chemistry, Vol. 42, No. 6, 2003 1807

Zhang et al.

blood-dioxygen carrier hemocyanins (i.e., in arthropods and

mollusks)^8,10,11reveal the presence of side-on µ-η²:η²-

peroxodicopper(II) moieties where each copper ion binds

three nitrogen ligands from histidine imidazole groups.

Spectroscopic comparisons suggest that this binding mode

occurs also in tyrosinases (phenol f o-quinone)^8,12and

catechol oxidase (o-catechol f o-quinone).^12-14Cu-O₂(1:

1) adducts are suggested to be involved in the activities of

dopamine â-hydroxylase^15,16and amine oxidases,^17-21and

Cu₃-O₂adducts form in multicopper oxidases (MCOs)

reducing O₂to water.⁹Low-resolution X-ray structures of

O₂-adducts have been reported for copper amine oxidase,¹⁷

laccase (an MCO)²²and for the heme-copper active site in

cytochrome c oxidase.²³

Recent advances in copper-dioxygen chemistry of syn-

thetic complexes^1-5,7have shown that the ligand (i.e., its

denticity, chelate ring size, donor type, substituents on or

near N-donors) dramatically influences Cu₂-O₂structure and

reactivity. Generally, tetradentate N-donor ligands give the

end-on µ-1,2-peroxo coordination,^24-27while tridentate or

bidentate N-donor ligands yield side-on µ-η²:η²-peroxodi-

copper(II) complexes, or related bis-µ-oxo-dicopper(III)

species;^28-36these normally are in rapid equilibrium.^28,32,37

Other well-characterized synthetically derived complexes

possessing intact O-O bonds are known, including mono-

-

or dicopper(II) species with a superoxo (O₂),^38-40hydro-

peroxo (HOO^-),^41-45or ROO^-moiety.^46-48Several interest-

ing µ₄-peroxo tetracopper(II) complexes have also been

structurally characterized.^49,50

In fact, the chemistry of [(L^H)Cu^I(MeCN)]⁺(1) (Figure

1; L^H≡ TMPA ≡ tris(2-pyridylmethyl)amine)⁵¹has been

quite prominent in copper-dioxygen reactivity studies. The

product of low-temperature (i.e., 193 K) O₂-reaction of 1 is

[{(L^H)Cu^II}₂(O₂^2-)]²⁺(3) (Figure 1), whose X-ray structure

(the first in Cu₂-O₂chemistry),^24,25spectroscopic/electronic

properties,⁵²and kinetics/thermodynamics^53,54of formation

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2313.

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V. G., Jr.; Que, L., Jr.; Zuberbu¨hler, A. D.; Tolman, W. B. Science

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(51) In our previous papers, i.e., refs 2, 7, 24, 25, 52-55, 59, 74, 85, and

108, L^His called TMPA (or tmpa); it is also more commonly referred

to as TPA (or tpa).

(52) Baldwin, M. J.; Ross, P. K.; Pate, J. E.; Tyekla´r, Z.; Karlin, K. D.;

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1808 Inorganic Chemistry, Vol. 42, No. 6, 2003

Copper(I)-Dioxygen ReactiWity of [(L)Cu^I]⁺

Chart 1

Figure 1. Previously deduced kinetic scheme for the reaction of [(L^H)-

Cu^I(MeCN)]⁺(1) with dioxygen in EtCN as solvent.

have been elucidated. Complex 3 has a bound dioxygen

ligand coordinated as a µ-1,2-peroxodicopper(II) moiety, λ_max

) 525 nm (ꢀ ∼ 10 500), and ν_O-O) 831 cm^-1(resonance

Raman spectroscopy). A cupric superoxo intermediate

electron-withdrawing groups on the porphyrin ring periphery

diminish the dioxygen affinity by increasing the dioxygen

dissociation rate constant. Equally, increasing donor strength

of the axial base significantly increases the dioxygen binding

affinity to ferrous porphyrin complexes.^69-73

[(L^H)Cu^II(O₂)]⁺(2) (λ_max) 410 nm) forms during the

-

reaction proceeding to 3 (Figure 1).

In contrast to Fe(II) and Co(II) systems, no detailed studies

on how ligand electronic effects influence the O₂-reactions

with copper(I) complexes have been reported. Here we use

the well-defined L^H(≡TMPA) system as a starting point to

systematically vary the ligand electronic properties; a 4-py-

ridyl substituent should exert no or minimal steric effects

upon the resulting chemistry (Chart 1). The present stopped-

flow kinetic investigations show that there can be significant

ligand (L^R) electronic and solvent effects on the oxygenation

reactions of the resulting copper(I) complexes. Ligand

electronic effects are also manifested in the electrochemistry

and ν(CO) of [LCu^I-CO]⁺complexes. The results are

significant in terms of developing a fundamental understand-

ing of copper(I)/dioxygen chemistry, which may contribute

to bioinorganic problems as well as to chemical applications.

In the present study, we describe the use of modified L^H

tetradentate ligands; rather than elucidate ligand structural

and/or steric variations (i.e., quinolyl-for-pyridyl,^53,54change

in chelate ring size,⁵⁵pyridyl 6-methyl substituents,^56,57or

binucleating versions^58,59), 4-pyridyl substituents are incor-

porated for the purpose of investigating kinetics and ther-

modynamics of (L^H)Cu^I/O₂reaction, as influenced by purely

electronic effects.

Ligand electronic properties have been shown to signifi-

cantly influence the dioxygen affinities of synthetic iron(II)

and cobalt(II) dioxygen carriers.^60-63Using a series of linear

polyamine ligands, Martell and co-workers have reported a

linear correlation between the dioxygen binding constants

and the logarithms of the total basicity of the ligand donor

group.^64,65With a series of cobalt(II) Schiff base complexes,

it has also been shown that log(K_O2) for the formation of

the 1:1 Co-O₂complex is linearly related to the Co^III/Co^II

reduction potentials.⁶⁶Similar results have been found for

the heme systems. For hemes, Traylor et al.^67,68reported that

Results and Discussion

Ligand Syntheses. Electronic effects for (ligand)Cu^I/O₂

reactivity studies were carried out by utilizing tripodal

tetradentate L^R(see Introduction) ligands.

The ligand L^Mewas synthesized by following a literature-

(54) Karlin, K. D.; Wei, N.; Jung, B.; Kaderli, S.; Niklaus, P.; Zuberbu¨hler,

A. D. J. Am. Chem. Soc. 1993, 115, 9506-9514.

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Karlin, K. D. Inorg. Chem. 2001, 40, 2312-2322.

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

2125.

based procedure,⁷⁴while L^tBu, L^MeO, and L^{Me N}were prepared

2

in a similar manner as shown in Scheme 1. Further details

are given in the Experimental Section. As can be seen, the

substituents chosen for these studies range from Me to Me₂N,

which are electron-donating groups. Synthetic difficulties

(57) Hayashi, H.; Uozumi, K.; Fujinami, S.; Nagatomo, S.; Shiren, K.;

Furutachi, H.; Suzuki, M.; Uehara, A.; Kitagawa, T. Chem. Lett. 2002,

416-417.

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392-400.

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S.; Jung, B.; Zuberbu¨hler, A. D. J. Am. Chem. Soc. 1995, 117, 12498-

12513.

(67) Traylor, T. G.; White, D. K.; Campbell, D. H.; Berzinis, A. P. J. Am.

Chem. Soc. 1981, 103, 4932-4936.

(68) Traylor, T. G.; Traylor, P. S. In ActiVe Oxygen: ActiVe Oxygen in

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J. F., Eds.; Chapman & Hall: New York, 1995; pp 84-187.

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Chem. Commun. 1983, 962-964.

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R. M.; Gibson, O. H. Inorg. Chem. 1983, 22, 1427-1432.

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1-79.

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Martell, A. E., Sawyer, D. T., Ed.; Plenum: New York, 1988.

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8, 251.

Inorganic Chemistry, Vol. 42, No. 6, 2003 1809

Zhang et al.

Scheme 1

4-coordinate formulation [(L^{Me N})Cu^I]⁺without incorporating

MeCN as its fifth ligand⁷⁶even when prepared and isolated

in MeCN as solvent. This is in contrast to the analogous

complexes with L^R(R ) H, Me, tBu, and MeO) which prefer

pentacoordination geometry when prepared in MeCN. This

may possibly be rationalized by ligand electronic effects,

2

encountered were the cause for not including any examples

of electron-withdrawing groups, although this would have

been of interest.⁷⁵A 4-NO₂group was used as a precursor

for introducing desired R groups onto the 4-pyridyl posi-

tions; however, we found that it was labile and the 4-NO₂

group did not survive the later steps required for ligand

preparation.

where a more electron-rich ligand (L^{Me N}) can provide more

2

electron density to the copper(I) center which then disfavors

the coordination of an additional RCN ligand.⁷⁷The coor-

dinated nitrile ligand in these complexes with BArF^-as anion

can in general be removed by recrystallization from a

noncoordinating solvent such as diethyl ether.

Electrochemistry. The electrochemical behavior of the

copper(I) complexes was measured by cyclic voltammetry

(CV) under Ar in MeCN solution. The data are given in

Table 1. The copper(I) complexes [(L^R)Cu^I(MeCN)]⁺display

single quasireversible redox behavior with i_pa/i_pcvarying from

0.91 to 1.05. Peak separations were all less than 110 mV at

a scan rate of 100 mV/s. The Cu^II/Cu^Ireduction potentials

measured for the copper(I) complexes are reported relative

to the ferrocene-ferrocenium couple which was used as an

external reference.

Syntheses of Copper(I) Complexes. Copper(I) complexes

[(L^R)Cu^I(MeCN)]ClO₄used for the kinetic studies in EtCN

and acetone as solvents were synthesized by mixing 1 equiv

each of ligand L^Rand [Cu(MeCN)₄]ClO₄under an Ar

atmosphere in acetonitrile (MeCN) solution. Due to the low

solubilities of these complexes in THF, the corresponding

copper(I) complexes with BArF^-(tetrakis(3,5-bis-trifluoro-

methylphenyl)borate) as counteranion were prepared by

metathesis reactions between [(L^R)Cu^I(MeCN)](ClO₄) and

NaBArF (see Experimental Section). Interestingly, the iso-

lated Cu(I)-complex with L^{Me N}ligand readily adopts a

2

(75) It should be noted that in 4-pyridyl substituted MePY2 tridentate

ligands, {MePY2 ≡ N-(methyl)-N,N-bis(2-pyridylethylamine)}, we

found that the Cl-substituted ligand copper(I) complex did not even

react with O₂. 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.

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(77) Storhoff, B. N.; Lewis, H. C., Jr. Coord. Chem. ReV. 1977, 23, 1-29.

1810 Inorganic Chemistry, Vol. 42, No. 6, 2003

Copper(I)-Dioxygen ReactiWity of [(L)Cu^I]⁺

Table 1. Cyclic Voltammetry Data for Cu(I) Complexes in CH₃CN^a

complex

E

_1/2(V)

∆E_p(mV)

i_pa/i_pc

[(L^H)Cu^I(MeCN)]⁺(1^H)

[(L^tBu)Cu^I(MeCN)]⁺(1^tBu

-0.40

-0.46

-0.49

-0.70

79

84

76

0.91

1.01

1.05

0.98

)

[(L^MeO)Cu^I(MeCN)]⁺(1^MeO

)

[(L^{Me N})Cu^I]⁺(1^{Me N}

)

106

2

^aPotentials are versus Fc/Fc⁺, measured under the same elctrochemical

cell conditions.

Table 2. Carbonyl Stretching Frequencies for [(L^R)Cu^I(CO)]⁺

Complexes

^tBu

1^H

1

1^MeO

1^{Me N}

2

ν(CO) (cm^-1

)

2092

2089

2087

2079

The data shown in Table 1 reveal that, with 4-substituted

pyridyl ligands L^R, the E_1/2values for the (L^R)Cu^II/(L^R)Cu^I

redox couple become more negative compared to that for

the complex with the parent ligand L^H, resulting in more

thermodynamically stable Cu(II) complexes. In other words,

as the R group becomes more electron-donating (i.e., R )

OMe vs tBu vs H), more negative values for the Cu^II/Cu^I

reduction potentials are observed making it easier to oxidize

Cu(I) to Cu(II). The observed ligand effect upon E_1/2

correlates to the difference in ligand basicity. The pK_avalues

for pyridine, 4-Me-pyridine (similar to 4-tBu-pyridine),

4-MeO-pyridine, and 4-Me₂N-pyridine are 5.21, 6.03, 6.58,

and 9.70, respectively.^78,79We previously observed that the

difference in electrochemical behavior of copper(I) com-

plexes with pyridyl and/or quinolyl ligands did relate to

variations in their O₂reactivity. However, in those cases,

steric effects probably dominate.⁵⁴

Figure 2. Time-dependent UV-vis spectra for the oxygenation of [(L^MeO)-

Cu^I(RCN)]⁺(1^MeO) at 179 K. Inset: absorbance vs time at 413 nm.

ligands. The copper CO complexes with neutral ligands in

general exhibit stronger CO bonding with higher ν(CO)

frequencies in the range 2080-2102 cm^-1.⁸⁰Ligand con-

straints, which affect the geometry around the copper center

for CO binding, and electronic properties of the donor

groups⁸¹contribute to the differences in ν(CO) values

observed for the CO adducts of the proteins and model

complexes.

Kinetics of Oxygenation Reactions. The kinetic scheme^53,54

of the reaction of [(L^H)Cu(EtCN)](ClO₄) (1^H) with dioxygen

in EtCN is outlined in Figure 1 (Introduction). However,

when the chemistry was carried out with the parent complex

in acetone, the stopped-flow kinetics revealed that a super-

-

oxocopper(II) species [(L^H)Cu^II(O₂)]⁺(2^H) with λ_max) 420

CO Stretching Frequencies for the Carbonyl Com-

plexes. The carbonyl complexes [(L^R)Cu^I(CO)]⁺were ob-

tained by bubbling the MeCN solutions of the corresponding

copper(I) species [(L^R)Cu^I(MeCN)]ClO₄(1^R) with CO, and

their in situ IR spectra indicated the presence of CO

stretching bands in the range 2079-2092 cm^-1. The Cu/CO

1:1 stoichiometry is indicated by our previous studies with

[(L^H)Cu^I(CO)]⁺,²⁵by the single observed IR-band and the

known tendency for Cu(I) complexes with N₃or N₄ligands.⁸⁰

The carbonyl stretching frequencies generally decrease with

increasing electron density on the ligand due to the π-back-

bonding interaction effected through the metal center that

weakens the CO bond. Here, the observed ν(CO) shift (Table

2) is consistent with this picture, where the most electron-

nm formed within the stopped-flow mixing time (even at

183 K), and the peroxo complex [{(L^H)Cu^II}₂(O₂^2-)]²⁺(3^H)

exhibited reasonable room temperature (rt) stability.^59,82This

shows that solvent medium is very important in LCu^I/O₂

reactivity. To further elucidate the fundamental aspects of

this copper-dioxygen interaction, we carried out the present

extended kinetic study on the oxygenation reactions of

mononuclear copper(I) complexes [(L^R)Cu^I(MeCN)]⁺with

4-pyridyl substituted ligand analogues L^R(R ) Me, tBu,

MeO) (Chart 1) in different media (EtCN, THF, and acetone).

1. Oxygenation Reactions of Copper(I) Complexes

[(L^R)Cu(MeCN)](ClO₄) in EtCN. The interaction of

[(L^R)Cu^I(RCN)]⁺(R ) Me or Et) with dioxygen in EtCN

follows the same basic mechanism which has been found in

the analogous reaction of [(L^H)Cu(RCN)]⁺(Figure 1). At

low temperature, [(L^R)Cu(RCN)]⁺reacts in a reversible

fashion with a dioxygen molecule to form a 1:1 Cu-O₂

species which further reacts reversibly with [(L^R)Cu(RCN)]⁺

ion to form the dinuclear 2:1 Cu₂O₂adduct. Figure 2 shows

the UV-vis behavior when, for example, [(L^MeO)Cu(RCN)]⁺

(1^MeO) reacts with excess O₂at 179 K. Upon mixing of

solutions of O₂and 1^MeO, there is a fast formation of

donating ligand L^{Me N}exhibits the lowest ν(CO) frequency.

2

The CO adducts of copper-containing proteins with tris-

(imidazole) coordination core for the copper centers (e.g.,

in hemocyanins, amine oxidases, peptidylglycine monooxy-

genase, and cytochrome c oxidase) have been reported to

exhibit ν(CO) in the range 2043-2063 cm^-1.⁸⁰However, in

synthetic complexes, comparable CO stretching frequencies

(2059-2067 cm^-1) have been observed only with non-

chelating imidazole ligands or anionic tris(pyrazolyl)borate

-

species [(L^MeO)Cu^II(O₂)]⁺(2^MeO) with λ_maxat 413 and 598

(78) Schofield, K. In Hetero-aromatic Nitrogen Compounds; Butter-

worths: London, 1967; p 146.

(79) Ho¨fle, G.; Steglich, W.; Vorbru¨ggen, H. Angew. Chem., Int. Ed. Engl.

1978, 17, 569.

(80) Rondelez, Y.; Se´ne`que, O.; Rager, M.-N.; Duprat, A. F.; Reinaud, O.

Chem. Eur. J. 2000, 6, 4218-4226.

(81) Hirsch, J.; George, S. D.; Solomon, E. I.; Hedman, B.; Hodgson, K.

O.; Burstyn, J. N. Inorg. Chem. 2001, 40, 2439-2441.

(82) Detailed numerical analysis was not possible for this “parent” system

(R ) H) because of recurring deviations from ideal behavior expected

for reasonably simple mechanistic schemes.

Inorganic Chemistry, Vol. 42, No. 6, 2003 1811

Zhang et al.

Table 3. Kinetic Parameters for O₂-Interaction with [(L^R)Cu^I(RCN)]⁺(in EtCN) and [(L^R)Cu^I]⁺(in THF)^a

EtCN

THF^dR) H

parameter

R ) H^b

R ) H^c

R ) Me

k₁(M^-1s^-1

29.1 ( 0.5

R ) tBu

R ) MeO

)

∆H^{q e}

∆S^{q f}

183 K

223 K

298 K

32 ( 4

14 ( 18

31.6 ( 0.5

31.8 ( 0.4

30.5 ( 0.6

10 ( 3

0 ( 3

19 ( 2

8 ( 3

(1.8 ( 0.1) × 10⁴

(9 ( 4) × 10⁵

8 × 10⁷

(1.18 ( 0.01) × 10⁴

(5.0 ( 0.3) × 10⁵

(5.8 ( 0.8) × 10⁷

(1.95 ( 0.02) × 10⁴

(7.3 ( 0.4) × 10⁵

(5.0 ( 0.7) × 10⁷

(3.11 ( 0.02) × 10⁴

(1.61 ( 0.08) × 10⁶

(1.6 ( 0.2) × 10⁸

(1.93 ( 0.04) × 10⁴

(8.5 ( 0.5) × 10⁵

(7 ( 1) × 10⁷

g2 × 10⁶

k_-1(s^-1

57.8 ( 0.6

)

∆H^q

66 ( 4

61.5 ( 0.5

118 ( 3

(1.59 ( 0.01) × 10

(2.7 ( 0.2) × 10⁴

(1.5 ( 0.2) × 10⁸

59.4 ( 0.5

62.2 ( 0.7

∆S^q

137 ( 18

8 ( 1

90 ( 3

107 ( 3

100 ( 3

183 K

223 K

298 K

5.57 ( 0.08

(1.63 ( 0.02) × 10¹

(2.2 ( 0.1) × 10⁴

(9 ( 1) × 10⁷

1.11 ( 0.06

(3 ( 1) × 10⁴

(6.2 ( 0.4) × 10³

(2.1 ( 0.1) × 10³

(1.3 ( 0.2) × 10⁷

2 × 10⁸

(2.1 ( 0.3) × 10⁷

k₂(M^-1s^-1

)

∆H^q

14 ( 1

-78 ( 2

22.6 ( 0.1

19.3 ( 0.2

-51.6 ( 0.8

16.67 ( 0.08

20.59 ( 0.06

17.5 ( 0.1

∆S^q

-38.6 ( 0.6

-61.4 ( 0.4

-46.7 ( 0.3

-50 ( 1

183 K

223 K

298 K

(3.2 ( 0.2) × 10⁴

(2.48 ( 0.02) × 10⁵

(1.8 ( 0.1) × 10⁶

(1.34 ( 0.02) × 10⁴

(2.33 ( 0.02) × 10⁵

(6.7 ( 0.2) × 10⁶

(2.45 ( 0.05) × 10⁴

(2.89 ( 0.04) × 10⁵

(5.3 ( 0.2) × 10⁶

(4.10 ( 0.04) × 10⁴

(3.57 ( 0.02) × 10⁵

(4.58 ( 0.06) × 10⁶

(1.85 ( 0.01) × 10⁴

(2.56 ( 0.01) × 10⁵

(5.58 ( 0.06) × 10⁶

(1.0 ( 0.2) × 10⁵

(9.53 ( 0.08) × 10⁵

(1.36 ( 0.03) × 10⁷

k_-2(s^-1

67.4 ( 0.5

)

∆H^q

61 ( 3

19 ( 10

69.8 ( 0.6

66.4 ( 0.6

69 ( 1

40 ( 5

70 ( 4

∆S^q

51 ( 3

40 ( 2

38 ( 3

32 ( 14

183 K

223 K

298 K

(1.5 ( 0.8) × 10^-4

(2.9 ( 0.4) × 10^-1

(1.2 ( 0.3) × 10³

(2.0 ( 0.2) × 10^-5

(9.1 ( 0.4) × 10^-2

(1.6 ( 0.1) × 10³

(2.6 ( 0.2) × 10^-5

(8.9 ( 0.3) × 10^-2

(1.12 ( 0.07) × 10³

(4.1 ( 0.5) × 10^-5

(1.26 ( 0.06) × 10^-1

(1.38 ( 0.09) × 10³

(8 ( 2) × 10^-6

(3.4 ( 0.4) × 10^-2

(5.5 ( 0.4) × 10²

2 × 10^-6

(9 ( 4) × 10^-3

(1.6 ( 0.4) × 10²

k_on()K₁× k₂) (M^-2s^-1

)

∆H^q

-20 ( 2

-7.3 ( 0.2

-9.5 ( 0.3

-10.9 ( 0.2

-11.2 ( 0.4

-24 ( 2

∆S^q

-201 ( 5

-147 ( 1

-141 ( 1

-150 ( 1

-139 ( 2

-162 ( 9

183 K

223 K

298 K

(6.2 ( 0.5) × 10⁷

(6.4 ( 0.4) × 10⁶

(6 ( 1) × 10⁵

(9.9 ( 0.2) × 10⁶

(5.1 ( 0.1) × 10⁶

(2.6 ( 0.1) × 10⁶

(8.6 ( 0.2) × 10⁷

(3.40 ( 0.08) × 10⁷

(1.24 ( 0.07) × 10⁷

(7.8 ( 0.1) × 10⁷

(2.62 ( 0.05) × 10⁷

(7.9 ( 0.4) × 10⁶

(3.2 ( 0.2) × 10⁸

(1.04 ( 0.02) × 10⁸

(3.0 ( 0.2) × 10⁷

(7 ( 3) × 10¹⁰

(5.3 ( 0.5) × 10⁹

(2.9 ( 0.7) × 10⁸

^aConstants k₁and k₂are determined experimentally, while k_-1()k₁/K₁) and k_on()k₂× K₁) are calculated. ^bPreviously published data. ^cRecent data

collected on a new instrument with faster sampling time. While activation parameters show some differences between old and new measurements, actual rate

constants measured, i.e., those with low associated errors, are rather close. ^dNo RCN involved in this solvent. ^eUnits for ∆H^‡values: kJ mol^{-1 f}Units for

.

∆S^‡values: J K^-1mol^-1

.

nm; this quickly decays (∼20 s) and is transformed into

[{(L^MeO)Cu^II}₂(O₂^2-)]²⁺(3^MeO) with 525 and 595 nm absorp-

tions. The inset of Figure 2 shows the absorbance versus

and co-workers structurally characterized a side-on super-

-

oxocopper(II) complex, Cu^II(O₂)[HB(3-tBu-5-iPrPz)₃].³⁸

However, terminal end-on coordination is well-known for

oxy-hemoglobin or oxy-myoglobin and Schiff base co-

balt(III)-superoxo complexes.^61,84No definitive structural

-

time trace at 413 nm. The formation of [(L^MeO)Cu^II(O₂)]⁺

(2^MeO) can only be followed below 203 K because at higher

temperature it occurs faster than the stopped-flow instru-

mental limit (1.3 ms). Above 233 K, the formation of

[{(L^MeO)Cu^II}₂(O₂^2-)]²⁺(3^MeO) is incomplete, and its ir-

reversible decay to secondary products becomes significant.

The variable-temperature stopped-flow data of the oxygen-

ation reactions of [(L^R)Cu^I(RCN)]⁺were analyzed by global

analysis in the eigenvector space,⁵⁴and the kinetic and

thermodynamic parameters deduced are listed in Tables 3

and 4.⁸³Unfortunately, quantitative data for the oxygenation

-

information for the 1:1 Cu-O₂adduct [(L^R)Cu^II(O₂)]⁺(2^R)

described here is available, but it is suggested to have an

end-on coordination, following arguments previously pub-

lished.^53,85

As shown in Table 3, the rate constants for formation of

-

[(L^R)Cu^II(O₂)]⁺(2^R) (k₁) fall into a small range between

1.18 × 10⁴and 3.11 × 10⁴M^-1s^-1at 183 K with ligand L^R

having different substituent groups (R ) H, Me, tBu, MeO);

i.e., the dioxygen association rates are hardly affected by

these variations in ligand electronic environment. The

O₂-binding processes are accompanied by significant activa-

tion enthalpies (29.1-31.8 kJ mol^-1) and small positive

activation entropies (0-19 J K^-1mol^-1). The insensitivity

of the O₂-binding rate constant (k₁) toward variations of

substituents in L^Rin R groups may be rationalized by the

importance of the accompanying dissociation of a coordi-

nated EtCN from the Cu(I)-complex during the dioxygen

binding process leading to 2^R. The same phenomenon has

reaction of 1^{Me N}cannot be included; its low solubility in

2

EtCN at reduced temperatures (even with BArF^-as coun-

teranion) prevented accurate concentration measurements and

full kinetic analysis. However, qualitative observations will

be discussed in the following sections.

(a) Formation and Dissociation of the 1:1 Adduct

-

[(L^R)Cu^II(O₂)]⁺(2^R). As mentioned in the Introduction,

Cu-O₂(1:1) adducts are of interest since they are the

primary products of the oxygenation reactions of Cu(I)-com-

plexes. With a sterically hindered tridentate ligand, Kitajima

(84) Bakac, A. Prog. Inorg. Chem. 1995, 43, 267-351.

(85) Wei, N.; Murthy, N. N.; Chen, Q.; Zubieta, J.; Karlin, K. D. Inorg.

Chem. 1994, 33, 1953-1965.

(83) For Eyring and/or van’t Hoff plots related to data summarized in tables

on kinetic and thermodynamic parameters, see Supporting Information.

1812 Inorganic Chemistry, Vol. 42, No. 6, 2003

Copper(I)-Dioxygen ReactiWity of [(L)Cu^I]⁺

Table 4. Thermodynamic Parameters for O₂-Interaction with [(L^R)Cu^I(RCN)]⁺(in EtCN) and [(L^R)Cu^I]⁺(in THF)^a

EtCN

THF^d

parameter

R ) H^b

R ) H^c

R) Me

K₁(M^-1

R) t-Bu

R) MeO

R ) H

)

∆H° ^e

∆S° ^f

183 K

223 K

298 K

-34 ( 1

-29.8 ( 0.2

-28.8 ( 0.2

-27.6 ( 0.2

-31.8 ( 0.4

-41 ( 2

-123 ( 4

-108 ( 1

-90 ( 1

-88 ( 1

-93 ( 2

-112 ( 9

(1.9 ( 0.1) × 10³

(2.7 ( 0.2) × 10¹

(3.4 ( 0.8) × 10^-1

(7.42 ( 0.04) × 10²

(2.20 ( 0.04) × 10¹

(3.8 ( 0.2) × 10^-1

(3.50 ( 0.04) × 10³

(1.18 ( 0.02) × 10²

2.4 ( 0.1

(1.91 ( 0.02) × 10³

(7.4 ( 0.1) × 10¹

1.73 ( 0.08

(1.73 ( 0.08) × 10⁴

(4.09 ( 0.06) × 10²

5.5 ( 0.3

(7 ( 3) × 10⁵

(5.5 ( 0.5) × 10³

(2.1 ( 0.5) × 10¹

K₂(M^-1

)

∆H°

-47 ( 3

-47.2 ( 0.6

-48.1 ( 0.4

-49.7 ( 0.6

-49 ( 1

-53 ( 4

∆S°

-97 ( 10

-89 ( 2

-91 ( 2

-99 ( 3

-87 ( 5

-82 ( 14

183 K

223 K

298 K

(2.2 ( 0.7) × 10⁸

(8.6 ( 1.2) × 10⁵

(1.5 ( 0.4) × 10³

(6.7 ( 0.7) × 10⁸

(2.6 ( 0.1) × 10⁶

(4.2 ( 0.3) × 10³

(9.4 ( 0.6) × 10⁸

(1.0 ( 0.1) × 10⁹

(2.8 ( 0.1) × 10⁶

(3.3 ( 0.2) × 10³

(2.4 ( 0.7) × 10⁹

(7.6 ( 0.9) × 10⁶

(1.02 ( 0.08) × 10⁴

5 × 10¹⁰

(3.24 ( 0.09) × 10⁶

(1.1 ( 0.5) × 10⁸

(9 ( 2) × 10⁴

(4.7 ( 0.3) × 10³

â ) K₁K₂(M^-2

-76.9 ( 0.4

)

∆H°

∆S°

183 K

223 K

298 K

-81 ( 3

-77.1 ( 0.6

-77.3 ( 0.6

-81 ( 1

-94 ( 3

-220 ( 11

-197 ( 2

-181 ( 2

-188 ( 2

-179 ( 4

-194 ( 11

(4.3 ( 1.5) × 10¹¹

(2.3 ( 0.4) × 10⁷

(5 ( 1) × 10²

(5.0 ( 0.5) × 10¹¹

(5.6 ( 0.2) × 10⁷

(1.6 ( 0.1) × 10³

(3.3 ( 0.2) × 10¹²

(3.81 ( 0.09) × 10⁸

(1.11 ( 0.04) × 10⁴

(1.9 ( 0.2) × 10¹²

(2.08 ( 0.08) × 10⁸

(5.7 ( 0.3) × 10³

(4 ( 1) × 10¹³

(3.1 ( 0.4) × 10⁹

(5.6 ( 0.3) × 10⁴

4 × 10¹⁶

(6 ( 3) × 10¹¹

(1.9 ( 0.1) × 10⁶

^aConstants K₁and â ()K₁K₂) are determined experimentally, while K₂()â/K₁) is calculated. ^bPreviously published data. ^cRecent data collected on a

new instrument with faster sampling time. See footnote to Table 3. ^dNo RCN involved in this solvent. ^eUnits for ∆H° values: kJ mol^{-1 f}Units for ∆S°

values: J K^-1mol^-1

.

been observed in the aqueous oxygenation reactions of some

six-coordinate cobalt(II) complexes^62,86with very similar rate

constants (k₁) for dioxygen binding to form the 1:1 Co-O₂

adducts. In addition, relatively high activation enthalpies

along with positive activation entropies were found, indica-

tive of a dissociative process (ligand/solvent dissociation is

the rate-determining step). In contrast, when five-coordinate

cobalt(II) complexes possessing a vacant coordination site

were used in more recent O₂-binding studies, much smaller

activation enthalpies (5-15 kJ mol^-1) and negative activation

entropies were observed.^87,88

coordination for superoxocopper(II) (2^R) and the structurally

characterized peroxodicopper(II) (3^R) species. Nevertheless,

actual rate constants k_-2are 4-5 orders of magnitude smaller

than corresponding values of k_-1due to significantly less

favorable activation entropies. This may reflect differing

influences of solvent on the Cu-O-O (2^R) versus Cu-O-

O-Cu (3^R) copper-oxygen cleavage and/or the fact that this

dissociation process for 2^R(k_-1) involves the release of a

dioxygen molecule.

The overall reaction to form the peroxodicopper(II) species

[{(L^R)Cu^II}₂(O₂^2-)]²⁺(3) is accompanied by strongly nega-

tive reaction enthalpies, which indicate strong complex

formation at low temperature, along with the usual compen-

sating large negative entropies. The thermodynamic stability

for the 2:1 adduct shows relatively small differences with

∆H° in the range -77 to -81 kJ mol^-1. Yet, we note that

the equilibrium constant for the formation of 3^R(â ) K₁K₂)

increases on going from the L^Hto L^Me,tButo L^MeOcomplex

(Table 4), by a total of 2 orders of magnitude.

Large activation enthalpies along with positive activation

entropies are associated with the dissociation (k_-1) of O₂from

-

Cu-O₂adducts [(L^R)Cu^II(O₂)]⁺(2). The rate constants k_-1

(Table 3) reveal a small but significant ligand effect (1 order

of magnitude decrease from 2^Hto 2^MeO), which thus translates

-

into a similarly small effect (i.e., factor of ∼10) in the O₂

binding ability (K₁, Table 4). As the R group becomes more

electron-donating, the rate constant k_-1decreases and K₁

increases, consistent with oxygenation being accompanied

by electron transfer from copper(I) to O₂(giving superoxide

anion).

(c) Observations for the Oxygenation of 1^{Me N}. The low

2

solubility of [(L^{Me N})Cu^I(EtCN)]⁺(1^{Me N}) in EtCN leads to

low reproducibility of data, and no complete kinetic analysis

could be obtained. However, qualitative observations showed

a remarkably different UV-vis behavior for the oxygenation

2

(b) Formation and Dissociation of the 2:1 Adduct

[{(L^R)Cu^II}₂(O₂^2-)]²⁺(3^R). As shown in Table 3, the

reaction of complex 1^{Me N}(Figure S3)⁸⁹compared to those

2

association rate constants k₂are similar for 3^H, 3^Me, 3^tBu

,

of the other Cu(I) complexes 1^R. The formation of the

and 3^MeO. This again suggests the importance of EtCN

competition in these oxygenation reactions. The activation

enthalpies for k_-2pertaining to Cu-O bond splitting are also

very similar among all Cu₂O₂adducts. Interestingly, these

activation enthalpies match the values observed for the

related process k_-1. This may reflect similar terminal

-

superoxocopper(II) species [(L^{Me N})Cu^II(O₂)]⁺(2^{Me N}) is

complete within the mixing time of the stopped-flow

instrument (1.3 ms), and very little if any Cu(I)-complex

2

1^{Me N}is present, as indicated by the lack of a 340 nm

2

absorption. The transformation of the superoxo species into

the peroxo species [{(L^{Me N})Cu^II}₂(O₂^2-)]²⁺(3^{Me N}) is ex-

ceedingly slow at low temperature but increases dramatically

with increasing temperature (Figure S3).⁸⁹The slower

2

(86) Fallab, S.; Mitchell, P. R. AdV. Inorg. Bioinorg. Mech. 1984, 3, 311-

377.

(87) Rybak-Akimova, E. V.; Otto, W.; Deardorf, P.; Roesner, R.; Busch,

D. H. Inorg. Chem. 1997, 36, 2746.

formation of the peroxo species 3^{Me N}does not represent a

2

(88) Rybak-Akimova, E. V.; Masarwa, M.; Marek, K.; Warburton, P. R.;

Busch, D. H. Chem. Commun. 1996, 1451-1452.

(89) See Supporting Information.

Inorganic Chemistry, Vol. 42, No. 6, 2003 1813

Zhang et al.

smaller rate constant (k₂); it is only due to the much lower

concentration of 1^{Me N}, since the formation of species 3^{Me N}

is proportional to the Cu(I) concentration (Figure 1, second

equation). These results thus clearly reveal a significant Me₂N

2

substituent effect; i.e., species 2^{Me N}has a much higher

2

thermodynamic stability (i.e., K₁) compared to the other

superoxo species 2^R.

(d) Overall Behavior in RCN. Ligand electronic effects

are manifested in the [(L^R)Cu^I(MeCN)]⁺(1^R) redox be-

havior (i.e., electrochemistry), CO-binding ability and in

kinetics and thermodynamics of the oxygenation reactions.

Relatively smaller influences for ligands with weak electron-

donating abilities (L^tBuand L^Me) are due to the competitive

binding of EtCN and O₂to the copper centers, which is

superimposed on the observation of intrinsic electronic

effects. However, strong electron-donating groups such as

MeO and Me₂N result in the observation of more significant

effects on the O₂-interactions of the corresponding Cu(I)-

complexes.

As noted in the Introduction, M^III/M^IIreduction potentials

and O₂-binding are known to relate for iron(II) and cobalt-

(II) complexes;^5,60,61linear correlations between Co^III/Co^II

reduction potentials and log(K_O2) have been observed for

several series of Co(II)-complexes with different ligand

systems.^66,90-92Among these, the relationship between

∆E_1/2and ∆K_O2between complexes in a given series is

quite variable, with effects seen which are greater than and

smaller than what we observed for the copper systems

reported here.

As we have seen, ligand electronic effects on the oxy-

genation reactions of [(L^R)Cu^I(MeCN)]⁺(1^R) are mostly

overshadowed when the reaction is carried out in EtCN, a

solvent which is also a very good ligand to Cu(I) ions.

Therefore, the weakly coordinating solvents THF and acetone

were used for further kinetic studies, as described in

following paragraphs.

2. Oxygenation Reaction of Copper Complexes [(L^R)-

Cu^I]BArF in THF. (a) Comparison of [(L^H)Cu^I]⁺/O₂

Interactions in THF and EtCN. Figure 3a shows the time-

dependent UV-vis spectral change for the oxygenation

reaction of [(L^H)Cu^I]⁺in THF. The kinetic analysis shows

that the behavior observed can be described by the same

reaction mechanism as that determined in EtCN (cf. Figure

1). However, the reaction rates and thermodynamic stabilities

of the O₂-adducts are significantly different from those in

EtCN.

Figure 3. (a) Time-dependent UV-vis spectra for the oxygenation of

[(L^H)Cu^I]⁺(1^H) in THF at 183 K. The spectrum of [(L^H)Cu^I]⁺(1^H) is also

presented here for comparison. (b) Absorbance vs time at 422 nm

([(L^H)Cu^II(O₂^-)]⁺, 2^H) and 522 nm ([{(L^H)Cu^II}₂(O₂^2-)]²⁺, 3^H) for the

oxygenation of [(L^H)Cu^I]⁺(1^H) in THF at 183 K.

in THF could not be calculated, and k₁must be at least

∼100 times faster than that in EtCN (1.2 × 10⁴M^-1s^-1at

183 K).

However, from the temperature dependence of the spectral

changes, thermodynamic parameters could be extracted,

Table 4.⁸³The superoxocopper(II) complex 2^Hformed in

THF is enthalpically more favored than that in EtCN (∆H°

) -41 kJ mol^-1in THF vs -29.8 kJ mol^-1in EtCN, Table

4). At 183 K, the equilibrium constant for formation of

species 2^His 2 orders of magnitude larger than that in

EtCN. This is consistent with the observation that the

UV-vis spectra for the oxygenation reaction (Figure 3a)

lack the characteristic absorption feature for [(L^H)Cu^I]⁺

(1^H) at 346 nm. At higher temperatures, the superoxocop-

The rate of formation of the superoxocopper(II) complex

-

[(L^H)Cu^II(O₂)]⁺(2^H, λ_max) 422 nm) in THF is faster than

-

per(II) species [(L^H)Cu^II(O₂)]⁺(2^H) in THF is destabilized

the stopped-flow instrumental limit of 1.3 ms (k g 2 ×

10⁶M^-1s^-1) even at 183 K, and only the subsequent decay

of 2^His observed, Figure 3a,b. Therefore, the rate con-

stants for formation (k₁) and dissociation (k_-1) of species 2^H

by unfavorable entropic contributions, and spectral fea-

tures of 1^Hbecome apparent, similar to those observed in

EtCN.

Data pertaining to kinetics and thermodynamics of forma-

tion of the Cu/O₂2:1 species [{(L^H)Cu^II}₂(O₂^2-)]²⁺(3^H) in

THF are given in Tables 3 and 4.⁸³At 183 K, the formation

of peroxodicopper(II) species 3^Hin THF (Figure 3b, 522

nm) is about 2 orders of magnitude slower than that in EtCN

even though the rate constant k₂for THF is even somewhat

(90) Harris, W. R.; McLendon, G. L.; Martell, A. E.; Bess, R. C.; Mason,

M. Inorg. Chem. 1980, 19, 21-26.

(91) Kasuga, K.; Iida, Y.; Yamamoto, Y. Inorg. Chim. Acta 1984, 84, 113-

116.

(92) Zanello, P.; Cini, R.; Cinquantini, A.; Orioli, P. L. J. Chem. Soc.,

Dalton. Trans. 1983, 2159-2166.

1814 Inorganic Chemistry, Vol. 42, No. 6, 2003

Copper(I)-Dioxygen ReactiWity of [(L)Cu^I]⁺

faster (at 183 K, 1.34 × 10⁴in EtCN and 1.0 × 10⁵in THF).

The rate of formation of 3^His proportional to the Cu^I

concentration (Figure 1, second equation). However, there

is very little [(L^H)Cu^I]⁺(1^H) present because of the large

equilibrium constant for formation (K₁) of the superoxocop-

per(II) species 2^Hin THF, as discussed already for the case

of the reaction of [(L^{Me N})Cu^I]⁺(1^{Me N}) in EtCN. At higher

temperatures, k_-1and thus the amount of 1^Hbecome more

significant, and the formation rates of 2^Hin THF and EtCN

become comparable. The dissociation (k_-2) of the Cu₂O₂

adduct 2^His also affected by solvent medium as reflected

by a smaller k_-2in THF compared to that in EtCN in the

temperature range that the rate constants could be determined

(179-260 K).

2

The solvent medium has a remarkable effect on the

thermodynamic stability of the 2:1 adduct 3^Has indicated

by a more negative reaction enthalpy in THF compared to

that in EtCN, ∆H° ) -94 kJ mol^-1in THF versus -77 kJ

mol^-1in EtCN (Table 4). The overall formation constant

(â) for species 3^Hin THF is 5 orders of magnitude larger

than that in EtCN at 183 K and almost 4 orders of magnitude

even at rt (Table 4).

In summary, the thermodynamic stabilities of both the 1:1

species [(L^H)Cu^II(O₂)]⁺(2^H) and the dinuclear adduct

-

[{(L^H)Cu^II}₂(O₂^2-)]²⁺(3^H) are dramatically affected by the

reaction medium. In THF, both the superoxo and peroxo

species are more stable than in EtCN by several orders of

magnitude, the latter being a binding competitor of O₂to

the copper centers. THF, although it can presumably

coordinate to the metal in [(L^R)Cu^I]⁺(1), is a weak ligand

and does little to inhibit O₂-binding to copper(I).

Figure 4. (a) Time-dependent UV-vis spectra for the oxygenation of

[(L^MeO)Cu^I]⁺(1^MeO) in THF at 183 K. (b) Absorbance vs time at 420 nm

([(L^MeO)Cu^II(O₂^-)]⁺, 2^MeO).

(b) Oxygenation Reaction of [(L^MeO)Cu^I]⁺in THF. As

shown in Figure 4a, at 183 K the UV-vis behavior for the

oxygenation reaction of [(L^MeO)Cu^I]⁺(1^MeO) in THF shows

remarkable differences compared to that for the unsubstituted

analogue [(L^H)Cu^I]⁺(1^H), Figure 3a. The formation of the

-

superoxocopper(II) complex [(L^H)Cu^II(O₂)]⁺(2^H) is com-

plete within the mixing time of the stopped-flow instrument

(Figure 3b), while some superoxo species 2^MeOis still

forming following the mixing (Figure 4b). This relatively

slower formation of species 2^MeOcan be rationalized by the

existence of a dimeric Cu(I) species ([(L^MeO)₂Cu^I₂]²⁺(see

description in the acetone section which follows), where

disruption of the dimer structure upon dioxygen binding

slows down the formation of the superoxo species. More

strikingly, the transformation of the superoxo into the peroxo

species is much slower for 1^MeOthan for 1^H(Figure 4b).

The concentration of the copper(I) species [(L^MeO)Cu^I]⁺

(1^MeO) must be minute due to a very large equilibrium

constant for formation of the superoxocopper(II) complex

2^MeOat 183 K. These results are indicative of a clear ligand

substituent electronic effect (L^Hvs L^MeO) seen in THF as

solvent medium, precluding kinetic analysis at the lowest

temperature.

Figure 5. Time-dependent UV-vis spectra for the oxygenation of

[(L^MeO)Cu^I]⁺in THF at 223 K.

for the formation of the peroxo complex 3^MeO(cf. Figure

1). Also, the thermodynamic stability of [{(L^MeO)Cu^II}₂-

(O₂^2-)]²⁺(3^MeO) is much higher than that of [{(L^H)Cu^II}₂-

(O₂^2-)]²⁺(3^H). Spectral examination (from initial spectra

observed after mixing) shows that ∼80% of 3^MeOis ob-

tained even at 293 K while only ∼20% of 3^His formed.

The more electron-donating ligand (L^MeO) significantly

increases the thermodynamic stability of the corresponding

peroxo species, again manifesting a considerable ligand

electronic effect.

At higher temperatures, the rate of formation of the per-

oxo species 3^MeOstrongly increases; compare Figure 5 to

Figure 4a. No doubt this is again due to the shift in the

equilibrium between 1^MeOand 2^MeO, making 1^MeOavailable

Inorganic Chemistry, Vol. 42, No. 6, 2003 1815

Zhang et al.

Figure 7. Absorbance vs time at 424 nm, showing the inverse [O₂]-

dependence of the decay of [(L^MeO)Cu^II(O₂^-)]⁺(2^MeO) in THF at 223 K.

Figure 6. Eyring plots for k_obs[O₂] pertaining to the oxygenation of

[(L^MeO)Cu^I]⁺in THF based on the alternative kinetic model. [(L^MeO)Cu^I]⁺/

[O₂]: (+), 2.768 × 10^-4M/3.3 × 10^-3M; (0), 1.064 × 10^-4M/3.3 ×

10^-3M; (4), 3.412 × 10^-4M/3.3 × 10^-3M; (O), 2.073 × 10^-4M/6.93 ×

10^-4M.

Table 5. Calculated k₂/K₁Values for Oxygenation Reactions of 1^Hand

1^MeOin Both EtCN and THF Solvents

in THF

in EtCN

parameter

1^H

58.5

1^MeO

1^H

1^MeO

On the basis of the general kinetic scheme shown in Figure

1, the formation rate of the peroxodicopper(II) species 3^MeO

is expressed by

k₂/K₁(s^-1

)

∆H^‡(kJ mol^-1

)

65.0 ( 0.4 52.5

66.9 ( 1.7 69.4

3.2 × 10^-318.1

52.38

46.3

1.07

∆S^‡(J K^-1mol^-1

183 K

)

62

0.14

d[3^MeO]/dt ) k₂[1^MeO][2^MeO

]

(1)

223 K

298 K

1.73 × 10²8.4

1.26 × 10⁴6.27 × 10²

6.48 × 10⁵7.7 × 10⁴

1.73 × 10⁷1.01 × 10⁶

As the concentration of 1^MeOis too low to be experimentally

determined, we can neglect k_-2in the rate expression, and

we have to represent 1^MeOas a function of the concentration

of 2^MeO, where [1^MeO] ) [2^MeO]/(K₁‚[O₂]). Thus, the rate of

formation of 3^MeOcan be rewritten as

Chemically, direct formation of the µ-peroxo spe-

cies 3^MeOfrom 2 2^MeOvia a bis-superoxodicopper(II) species,

-

[(L)₂Cu^II(O₂)₂]²⁺, followed by elimination of O₂, would

2

-

be viable. Several bis-µ-(O₂) metal complexes have been

recently characterized in detail; in fact, dioxygen evolution

was observed in the decomposition processes of these bis-

µ-superoxo species.^93,94Intramolecular expulsion of O₂would

not yield an inverse [O₂] dependence, however, and has to

be ruled out for the (L^MeO)-Cu system.

d[3^MeO]/dt ) (k₂/K₁)‚([2^MeO]²/[O₂])

(2)

As the equilibrium constant K₁could not be evaluated, a

simplified expression was used to analyze the data.

For comparison, k₂/K₁values for the oxygenation of 1^H

in EtCN and THF as well as 1^MeOin EtCN were calculated

and tabulated in Table 5. The values of k₂/K₁will reflect the

relative influences of ligand or solvent on the formation (k₂)-

of Cu₂O₂adduct (3^R) and the thermodynamic stability (K₁)

of Cu-O₂species (2^R). In THF, k₂/K₁(183 K) for L^His ∼44

times bigger than that for L^MeO. This suggests that in THF

the ligand electronic properties have a more significant effect

than in EtCN where the corresponding factor is reduced to

18. If we assume that k₂for different substituents is nearly

the same in a given solvent (cf. Table 3 for EtCN), there

k_obs

2(2^MeO

)

8 3^MeO+ O₂

On the basis of this expression

d[3^MeO]/dt ) k_obs[2^{MeO 2}

]

(3)

(4)

and along with eq 2

(k₂/K₁) ) k_obs[O₂]

would be a 44-fold increase in K₁on going form L^Hto L^MeO

.

is derived. The Eyring plot for k_obs[O₂] (i.e., k₂/K₁) is shown

in Figure 6.

Note that the activation enthalpies for L^Hand L^MeOare almost

identical in EtCN. By contrast, in THF as solvent, the

activation enthalpy (k₂/K₁) for L^MeOis ∼6 kJ mol^-1higher

than that for L^H, possibly attributed to a more favorable

The reaction mechanism to form µ-peroxo species

[{(L^MeO)Cu^II}₂(O₂^2-)]²⁺(3^MeO) does not in fact involve an

elementary step between two molecules of 2^MeO(k_obs, eq 3).

In line with the Eyring plot, Figure 7 reveals that the decay

of the superoxo complex 2^MeOoccurs faster with air than

with pure dioxygen. In order for 3^MeOto form, some 2^MeO

must lose O₂giving rise to 1^MeO, which can react with 2^MeO

to give the peroxo product.

(93) Reinaud, O. M.; Yap, G. P. A.; Rheingold, A. L.; Theopold, K. H.

Angew. Chem., Int. Ed. Engl. 1995, 34, 2051-2052.

(94) Shiren, K.; Ogo, S.; Fujinami, S.; Hayashi, H.; Suzuki, M.; Uehara,

A.; Watanabe, Y.; Moro-oka, Y. J. Am. Chem. Soc. 2000, 122, 254-

262.

1816 Inorganic Chemistry, Vol. 42, No. 6, 2003

Copper(I)-Dioxygen ReactiWity of [(L)Cu^I]⁺

(negative) reaction enthalpy (K₁) for the formation of the

superoxo species. As the temperature increases, the difference

in k₂/K₁values decreases due to entropic contributions.

Again, these comparisons (of k₂/K₁) indicate that ligand

electronic effects manifest themselves more strongly in THF

than in EtCN, and we base this on competitive binding of

EtCN to copper(I) centers.

The values of k₂/K₁for the same ligand (L^Hor L^MeO) can

also be compared in different solvents. As shown in Table

5, the values in THF are in general smaller than those in

EtCN, again manifesting a solvent effect. As indicated in

Table 3, the rate constant k₂for L^Hin THF is comparable

with that in EtCN. Thus, the significantly smaller value of

k₂/K₁in THF (at 183 K, 0.14 in THF vs 18.1 in EtCN, Table

5) is a result of a pronounced increase in the thermodynamic

stability (K₁) of the superoxo species in THF (as given in

Table 4). A smaller k₂/K₁value was observed in THF

compared with that in EtCN, for L^MeOalso. Assuming the

same expected trend for k₂, this again reflects the significantly

increased thermodynamic stability of 2^MeOin the less

coordinating solvent.

were recorded at variable temperature (over ∼100 K range,

Figures S9-S11),⁹⁷exhibiting very different behavior in

the two solvents. In EtCN-d₅, spectra at all temperatures

(185-297 K, Figure S9)⁹⁷contain a single peak (∼4 ppm)

corresponding to the hydrogen atoms from three symmetry-

equivalent methylene groups connecting the amine nitrogen

and the pyridine rings. A monomeric formulation, [(L^R)Cu^I-

(MeCN)]⁺(1^R), is indicated, consistent with X-ray struc-

tures^25,76and the conductivity data (vide supra). The broad-

ened resonances near room temperature can be rationalized

by the occurrence of the fluxional processes (i.e., involving

possible dissociation and reassociation of the pyridine

nitrogens) which are well-known in copper(I) complexes with

nitrogen-containing ligands.^98-101By contrast, two sets of

methylene signals in a ratio of 2:1 are observed in acetone-

d₆at low temperatures (Figure S10),⁹⁷indicating two different

chemical environments for the three methylene groups. This

may be rationalized by the adoption of a dimeric structure

(see diagram), where each copper(I) ion is ligated with the

two pyridyl nitrogens and one tertiary amine nitrogen atom

of a single L^Hligand, with the fourth coordination atom from

the pyridyl nitrogen of the second adjacent L^Hligand,

resulting in two chemically inequivalent methylene groups.

Similar splittings and temperature-dependent behavior for

3. Oxygenation Reaction of [(L^R)Cu^I]ClO₄in Acetone.

A previous study⁵⁹showed that when the oxygenation

reaction of the parent complex [(L^H)Cu^I]⁺(1^H) was carried

out in acetone, a weakly coordinating solvent, the super-

oxocopper(II) species formed within the mixing time of the

stopped-flow instrument and the peroxodicopper(II) complex

[{(L^H)Cu^II}₂(O₂^2-)]²⁺(3^H) exhibited enhanced rt stability

compared to that generated in EtCN. But the current more

detailed kinetic studies (using various L^R) show that the

reaction mechanism described in Figure 1 cannot be applied

for the oxygenation reaction in acetone as solvent. To analyze

the kinetic results, the nature of the starting Cu(I)-complexes

was studied using electrical conductivity measurements and

1

the methylene and 4-methoxy (CH₃O) H-resonances are

observed for 1^MeOin acetone-d₆(Figure S11).⁹⁷The proposed

dimeric structure (see diagram) is strongly suggested from

the X-ray structure of the Cu(I)-complex with a similar ligand

BPIA (bis((2-pyridyl)methyl)(1-methylimidazol-2-yl)methyl-

((2-pyridyl)methyl)amine).⁸⁵The dimerization of Cu(I)-

complexes is furthermore well-precedented, especially when

using tetradentate or tridentate nitrogen-containing ligands,

such as tris(alkyl) substituted pyrazolylborates,^98,101,102a tris-

(imidazolylmethoxymethane) ligand,⁹⁶tripodal tetradentates

with oxazoline,¹⁰³and others.^104-107Either Cu(I)-Cu(I)

interactions,^104-107or aromatic π-stacking of ligands^108,109

(i.e., pyridine or imidazole rings) appear to facilitate the

formation or stabilization of such dimers. For the copper(I)

complex with BPIA ligand, a π-π interaction between two

pyridine rings of the two ligands in the dimer complex

1

variable-temperature H NMR spectroscopy. The results

show that they exist as dimers [(L^R)₂Cu^I₂]²⁺in acetone.

Evidence for Cu(I) Dimers in Acetone Solution. Evi-

dence for the dimeric nature of the Cu(I) complexes in

acetone comes from electrical conductivity measurements

and variable-temperature NMR spectroscopic studies.

(a) Conductivity Measurements. Electrical conductivity

measurements^95,96of the Cu(I) complexes were carried out

in both MeCN and acetone. Onsager plots⁹⁵for [(L^R)₂Cu^I₂]²⁺

complexes along with other typical but established copper-

containing 1:1 and 2:1 electrolytes (see Experimental Sec-

tion) are given in the Supporting Information, Figures

S7-S8. The copper(I) complexes of L^Rin acetone fall clearly

within the family of linear plots for 2:1 electrolytes,

consistent with their dimer formulations. In contrast, when

conductivity measurements were carried out in MeCN, the

results⁸⁹indicate that the Cu(I) complexes are monomeric

1:1 electrolytes, suggesting that this strongly coordinating

solvent breaks up the dimeric structure.

(97) See Supporting Information.

(98) Mealli, C.; Arcus, C. A.; Wilkinson, J. L.; Marks, T. J.; Ibers, J. A. J.

Am. Chem. Soc. 1976, 98, 711-718.

(99) Jacobson, R. R. Ph.D. Thesis, State University of New York at Albany,

1989.

(100) Coggin, D. K.; Gonzalez, J. A.; Kook, A. M.; Stanbury, D. M.;

Wilson, L. J. Inorg. Chem. 1991, 30, 1115-1125.

(101) Carrier, S. M.; Ruggiero, C. E.; Houser, R. P.; Tolman, W. B. Inorg.

Chem. 1993, 32, 4889-4899.

(102) Hu, Z.; Williams, R. D.; Tran, D.; Spiro, T. G.; Gorun, S. M. J. Am.

Chem. Soc. 2000, 122, 3556-3557.

(103) Sorrell, T. N.; Pigge, F. C.; White, P. S. Inorg. Chim. Acta 1993,

210, 87-90.

(104) Drew, M. G. B.; Lavery, A.; McKee, V.; Nelson, S. M. J. Chem.

Soc., Dalton Trans. 1985, 1771-1774.

(105) Gagne, R. R.; Kreh, R. P.; Dodge, J. A.; Marsh, R. E.; McCool, M.

Inorg. Chem. 1982, 21, 254-261.

(b) Variable-Temperature NMR Studies. The ¹H NMR

spectra of the Cu(I)-complexes in EtCN-d₅or acetone-d₆

(106) Singh, K.; Long, J. R.; Stavropoulos, P. Inorg. Chem. 1998, 37,

1073-1079.

(107) Lee, S. W.; Trogler, W. C. Inorg. Chem. 1990, 29, 1659-1662.

(108) Wei, N.; Murthy, N. N.; Tyekla´r, Z.; Karlin, K. D. Inorg. Chem.

1994, 33, 1177-1183.

(95) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81-122.

(96) Sorrell, T. N.; Borovik, A. S. J. Am. Chem. Soc. 1987, 109, 4255-

4260.

(109) Bonnefous, C.; Bellec, N.; Thummel, R. P. Chem. Commun. 1999,

1243-1244.

Inorganic Chemistry, Vol. 42, No. 6, 2003 1817

Zhang et al.

Scheme 2

Figure 8. Time-dependent UV-vis spectra for the oxygenation of

[(L^tBu)₂Cu^I₂]²⁺in acetone at 201 K. See text for further explanations.

of the initial Cu(I) species, [(L^tBu)₂Cu^I₂]²⁺, the superoxo

observed in the solid state structure is suggested to be the

possible driving force for the dimerization. We suggest that

the same phenomenon occurs with ligand complex 1^Rin

acetone.

-

species A thus is formulated as [(L^tBu)₂Cu^ICu^II(O₂)]²⁺. The

assignment is further corroborated by the calculated species

spectra based on kinetic analysis. The experimental data

could be described assuming a second-order transformation

of superoxo complex A into an intermediate B, followed by

the production of the normal peroxodicopper(II) species 3^tBu

(C). As observed for the oxygenation of [(L^MeO)Cu^I]⁺(1^MeO

)

in THF, the second-order transformation of A into B also is

accompanied by an inverse [O₂] dependence (Figure S12),⁸⁹

indicating that 2 molecules of A cannot interact directly, but

need fully reduced [(L^tBu)₂Cu^I₂]²⁺present in a steady state

equilibrium.

On the basis of these results, we propose a reaction mecha-

nism, shown in Scheme 2, which involves an initial binding

of dioxygen to the Cu(I) dimer at one of its two copper(I)

centers to form a mixed-valence superoxocopper(II)copper-

-

(I) species [(L^tBu)₂Cu^ICu^II(O₂)]²⁺(A). This reacts with a

Dynamic processes for [(L^R)Cu^I₂]²⁺(1^R) in acetone solu-

tion are indicated by the resonance broadening observed with

increasing temperature (Figures S9-S11).⁹⁷In part, because

limiting (i.e., sharp) spectra were not yet observed up to room

temperature, we have not attempted to analyze or determine

structures or mechanisms involved in the dynamic behavior.

These have been discussed elsewhere for other related

systems.^101,102

Kinetics in Acetone. With the substituted series

[(L^R)Cu^I]⁺in hand, we carried out kinetic studies to inves-

tigate ligand electronic effects. We describe here the kinetic

behavior for only the reaction of [(L^tBu)Cu^I]⁺(1^tBu) with

dioxygen, since it could be analyzed in detail. In fact, the

oxygenation reactions of [(L^H)Cu^I]⁺(1^H), [(L^Me)Cu^I]⁺(1^Me),

and [(L^MeO)Cu^I]⁺(1^MeO) were examined as well, qualitatively,

and it was found that their overall behavior was similar, but

even more complicated.⁸²

second Cu(I) dimer molecule to form a µ-peroxotetracopper

species [{(L^tBu)₂Cu^ICu^II}₂(O₂^2-)]⁴⁺(B). Finally, intermediate

B rearranges to the (normal) binuclear peroxodicopper(II)

species C through an obligate intermediate P′ (vide infra).

On the basis of the kinetics analysis, spectra for species

A, B, and C can be calculated, and these are shown in Figure

9. The spectrum of the Cu(I)-complex, [(L^tBu)₂Cu^I₂]²⁺, is also

provided in this plot for comparison. Consistent with the

proposed chemistry and kinetic model, the superoxo species

A has absorption bands at both 340 nm (characteristic of

the Cu(I) moiety) and 420 nm, assigned as the superoxo-

-

to-Cu^IIcharge-transfer band of [(L^tBu)₂Cu^ICu^II(O₂)]²⁺. The

absorptivitity at 420 nm (ꢀ 4900 M^-1cm^-1) is in fact close

-

to that observed for [(L^tBu)Cu^II(O₂)]⁺(2^tBu) in EtCN. The

-

ratio of the Cu(I) and O₂-complex moieties in species A,

which is calculated by dividing the absorbance at 340 and

The stopped-flow kinetic behavior for the oxygenation

reaction of Cu(I)-complex with L^tBuin acetone (201 K, Figure

8) is very different from that in EtCN. A key observation is

420 nm with the corresponding extinction coefficients, is ∼1:

-

1, as expected for this binuclear Cu^I‚‚‚Cu^II-O₂species

(Scheme 2). The calculated spectra for the peroxo species B

and C show the same absorptivities at 525 and 610 nm,

which are the peroxo-to-Cu(II) charge-transfer bands, while

B possesses significant absorption at 340 nm in addition,

corresponding to the two Cu(I) moieties in the proposed

tetranuclear complex Cu^I‚‚‚Cu^II-(O₂^2-)-Cu^II‚‚‚Cu^I. These

spectra and the kinetic analysis convincingly support the

formulations of A, B, and C, and thus the reaction mecha-

that a superoxo type species A with characteristic λ_max

)

420 nm is generated within the mixing time of the instrument

(∼1 ms), even at the lowest temperature (183 K). Complex

A is not a pure superoxo species, however, since the typical

Cu(I) complex absorption (λ_max) 340 nm) is retained at

constant, i.e., concentration- and temperature-independent,

proportion of roughly 50%. In line with the dimeric nature

1818 Inorganic Chemistry, Vol. 42, No. 6, 2003

Copper(I)-Dioxygen ReactiWity of [(L)Cu^I]⁺

Figure 9. Calculated spectra for species A, B, and C and reference to the dicopper(I) complex starting compound, for the kinetics of reaction of O₂with

[(L^tBu)₂Cu^I₂]²⁺, according to Scheme 2.

Table 6. Kinetic Parameters for O₂-Interaction with [(L^tBu)₂Cu^I₂]²⁺in

Acetone

(I) complexes were investigated from the kinetic and ther-

modynamic aspects, utilizing a series of 4-pyridyl substituted

parameter

k′₂/K′₁(s^-1

)

k′_-1(s^-1

35 ( 3

)

k′₃(s^-1

)

tetradentate tripodal ligands in different solvents. We found

that modification in ligand electron-donating ability can

significantly affect both kinetic and thermodynamic param-

eters of the oxygenation reactions. The solvent also plays a

crucial role in these reactions.

∆H^q(kJ mol^-1) 67.5 ( 0.6

54.5 ( 0.5

∆S^q(J K^-1

112 ( 3

mol^-1

183 K

-95 ( 13

1 ( 2

)

(1.4 ( 0.1) × 10^-1(5 ( 2) × 10^-3

(1.15 ( 0.08) × 10^-3

(1.9 ( 0.1) × 10³

223 K

298 K

(5.1 ( 0.1) × 10²(3.8 ( 0.3) × 10^-1(8.7 ( 0.1) × 10^-1

(6.5 ( 0.6) × 10⁶(6 ( 3) × 10¹

In EtCN, a strongly coordinating solvent, the ligand

electronic effect is reduced due to the competitive binding

of solvent and dioxygen to the copper centers. Thus, with

weak electron-donating groups, such as Me and tBu, the

kinetic and thermodynamic parameters for the formation of

nism described in Scheme 2. In fact, no alternative mecha-

nism (i.e., without dimeric A and/or tetrameric B) could be

found to reasonably fit the data.

In analogy with eqs 1-2 described in the section on

kinetics in THF solvent, the formation rate for species B

can be expressed by

-

both the superoxocopper(II) species [(L^R)Cu^II(O₂)]⁺(2^R)

and the peroxodicopper(II) species [{(L^R)Cu^II}₂(O₂^2-)]²⁺(3^R)

are very similar to those for the parent complexes (i.e., R )

H). But with relatively stronger electron-donating groups,

utilizing ligands with MeO and Me₂N substituents, more

significant effects were observed. These fall along the lines

expected, as O₂-binding to cuprous ion is a redox process,

favored by more reducing metal ion complexes.

d[B]/dt ) k′₂[{(L^tBu)₂Cu^I₂}²⁺][A] ) k′₂[A]²/(K′₁‚[O₂]) (5)

The Eyring plots for k′₂/K′₁and k′_-1are given in the

Supporting Information (Figure S13), obtained from data

taken from 201 to 243 K for k′₂/K′₁and from 207 to 231 K

for k′_-1. The results are tabulated in Table 6.

According to the observed rate law, the transformation of

species B to the normal peroxodicopper(II) species C is not

[O₂]-dependent, although the reaction stoichiometry requires

one molecule of dioxygen. This indicates that transformation

of B into C consists of at least two steps (Scheme 2). Species

B first rearranges (k′₃, Figure S13, 219-243 K) to a more

reactive form P′, which might be simply an isomer of B or

may have one of its copper(I) moieties split off from B. This

rearrangement is the rate-determining step and exhibits no

[O₂] dependence. The following step(s) leading to complete

transformation into species C are fast; neither intermediates

are observed, nor are there deviations from the first-order

rate law for this transformation.

When THF or acetone is used as the solvent, the formation

-

rate for [(L^H)Cu^II(O₂)]⁺(2^H) is much faster than that in

EtCN, suggesting the crucial role of the dissociation of the

coordinated nitrile ligand in the O₂-binding process of the

copper(I) complex in EtCN. For the same reason, the

thermodynamic stabilities of Cu-O₂species (both 1:1 and

2:1 adducts) are much higher than those in EtCN. In addition,

a more significant ligand electronic effect compared to that

in EtCN was observed, where the presence of electron-

donating ligand L^MeOresulted in a pronounced increase in

the thermal stability of the superoxo and peroxo species

compared to that with ligand L^H.

When the oxygenation reactions are carried out in acetone,

a different reaction mechanism involving a dimeric copper-

(I) species which exists in this solvent is proposed to describe

the oxygenation reactions and supported by kinetic analyses.

Conclusion

The effects of systematic variation in ligand electronic

properties on the O₂-interactions of the corresponding copper-

Inorganic Chemistry, Vol. 42, No. 6, 2003 1819

Zhang et al.

δ 8.42 (1 H, d, J ) 6.0 Hz), 7.29 (1 H, d, J ) 3.0 Hz), 7.19 (1H,

dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 4.78 (2 H, s), 4.34 (1H, s, br), 1.31

(9 H, s).

The present study affords the first systematic investigation

of both solvent medium and ligand electronic effects. Such

studies are critical in the further evaluation of copper-

dioxygen reactivity in biological systems, and in the design

of reagents or catalysts for copper mediated oxidative

chemistry.

2-Chloromethyl-4-tert-butylpyridine (13). To a stirred solution

of 2-hydroxymethyl-4-tert-butylpyridine (0.50 g, 0.0030 mol) in

CH₂Cl₂(20 mL) was added thionyl chloride (0.54 g, 0.0046 mol)

slowly. After addition, the resulting mixture was stirred at rt

overnight. The reaction was quenched with Na₂CO₃aqueous

solution, and the organic layer was collected. The aqueous layer

was extracted with CH₂Cl₂, and the combined organic phase was

dried over Na₂SO₄. Removal of volatile components gave 0.41 g

Experimental Section

Material and Methods. Reagents and solvents used were of

commercially available reagent quality unless otherwise stated.

EtCN, MeCN, and heptane used for complex synthesis were distilled

from CaH₂under Ar. Diethyl ether and THF were distilled from

sodium/benzophenone under Ar. Acetone was distilled from Drierite

under Ar.

1

(74%) of a brown liquid. R_f) 0.88 (alumina, EtOAc). H NMR

(300 MHz, CDCl₃): δ 8.48 (1 H, d, J ) 6.0 Hz), 7.45 (1 H, d, J

) 3.0 Hz), 7.24 (1H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 4.68 (2 H, s),

1.36 (9 H, s).

¹H NMR spectra were measured in CDCl₃or CD₃NO₂on a

Bruker AMX-300 NMR spectrometer. Chemical shifts are reported

as δ values downfield from an internal TMS reference. Elemental

analyses were performed by Desert Analytics, Tuczon, AZ. Mass

spectra were performed by the Mass Spectrometry Facility at The

Johns Hopkins University.

2-Phthalimidomethyl-4-tert-butylpyridine (17). To a stirred

solution of 2-chloromethyl-4-tert-butylpyridine (2.47 g, 0.0133

mol) in DMF (13 mL) was added 2.47 g (0.0348 mol) of potas-

sium phthalimide. The mixture was stirred at ∼358 K for 25 h and

then cooled to rt, whereupon water (20 mL) was added and the

resulting solution was extracted with CHCl₃(2 × 10 mL). The

organic extracts were combined and concentrated by rotary

evaporation. Recrystallization from hot EtOH gave 2.6 g (81%)

Syntheses of Ligands and Complexes. 1. L^tBu. 4-tert-Butyl-

2-picoline (4). To a stirred solution of 4-tert-butylpyridine (30.5

g, 0.226 mol) in freshly distilled diethyl ether (250 mL) at 195 K

under Ar was added a 1.4 M diethyl ether solution of MeLi (162

mL, 0.227 mol) dropwise over 1 h. The mixture was then warmed

to rt and allowed to stir overnight yielding a bright red solution.

Water (∼5 mL) was added slowly to destroy residual MeLi and

form LiOH as a precipitate. The resulting mixture was filtered, and

the filtrate was concentrated by rotary evaporation to give a yellow

oil. Purification of the crude product by vacuum distillation gave

1

of a crystalline solid. H NMR (300 MHz, CDCl₃): δ 8.40 (1 H,

d, J ) 6.0 Hz), 7.89 (2 H, m), 7.73 (2H, m), 7.28 (1 H, d, J ) 3.0

Hz), 7.14 (1H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 4.99 (2 H, s), 1.29 (9

H, s).

2-Amino-4-tert-butylpyridine bis hydrochloride (21). To 2-

phthalimidomethyl-4-tert-butylpyridine (2.40 g, 8.15 mmol) were

added 12 M HCl (10 mL) and glacial acetic acid (10 mL). The

resulting mixture was refluxed for 48 h, during which 3 aliquots

of 12 M HCl (3 mL each time) were added. The reaction mixture

was allowed to cool to rt, and a white crystalline solid was formed

and removed by filtration. The filtrate was washed with 12 M HCl

(3 × 3 mL) and then dried on a high-vacuum rotary evaporator.

The resulting solid was redissolved in MeOH and layered with

1

24.3 g (72%) of a pale yellow oil. H NMR (300 MHz, CDCl₃):

δ 8.37 (1 H, d, J ) 6 Hz), 7.12 (1H, s), 7.06 (1 H, d, J ) 3 Hz),

2.53 (3 H, s), 1.28 (9 H, s).

4-tert-Butyl-2-picoline N-oxide (5). To 4-tert-butyl-2-picoline

(24.3 g, 0.163 mol) in a 500 mL round-bottom flask was added

glacial acetic acid (100 mL) along with 30% H₂O₂(20 mL). The

reaction mixture was then heated to 353 K and stirred for 12 h.

The next day, more 30% H₂O₂(20 mL) was added, and the heating

was maintained for a further 12 h. After removal of the volatile

components on a high-vacuum rotary evaporator, CH₂Cl₂was added

to the remaining liquid, and the residual acetic acid was neutralized

with Na₂CO₃. The mixture was filtered, and the filtrate was

1

diethyl ether to yield 1.2 g (61%) of an off-white solid. H NMR

(300 MHz, CD₃OD): δ 8.79 (1H, d), 8.27 (1H, d), 8.10 (1H, dd),

4.5 (2H, s), 1.46 (9H, s).

L^tBu: (Tris[{(4-tert-butyl)-2-pyridyl}methyl]amine) (24). To

a stirred solution of 2-aminomethyl-4-tert-butylpyridine bis(hydro-

chloride) salt (0.26 g, 0.0011 mol) and 2-chloromethyl-4-tert-

butylpyridine (0.41 g, 0.0023 mol) in water (10 mL) was added a

1 M NaOH solution (10 mL, 0.010 mol). The reaction mixture was

allowed to stir at rt overnight (the pH of the solution dropped to

7.0). The resulting solution was extracted with CH₂Cl₂several times.

The combined organic layers were dried over Na₂SO₄, and the

volatile components were removed. Purification of the crude product

on an alumina gel column using EtOAc as eluting solvent gave

0.30 g (60%) of a light yellow solid. ¹H NMR (300 MHz, CDCl₃):

δ 8.41 (3H, d, J ) 6.0 Hz), 7.70 (3H, s), 7.12 (3H, dd, J ) 6.0 Hz,

J′ ) 3.0 Hz), 3.89 (6H, s), 1.30 (27H, s). MS(FAB): m/z 459 (M

+ H⁺).

1

concentrated to yield 22 g (82%) of a thick, pale, orange oil. H

NMR (300 MHz, CDCl₃): δ 8.18 (1H, d, J ) 6 Hz), 7.21 (1H, d,

J ) 3 Hz), 7.12 (1H, dd, J ) 6 Hz, J′ ) 3 Hz), 2.53 (3H, s), 1.31

(9H, s).

2-Hydroxymethyl-4-tert-butylpyridine (9). To 4-tert-butyl-2-

picoline N-oxide (22 g, 0.133 mol) was added an excess of acetic

anhydride (100 mL). The resulting mixture was heated to 383 K

and stirred for 8 h. Then, after cooling to rt, the volatile compo-

nents were removed by using a high-vacuum rotary evaporator.

Purification of the crude product by vacuum distillation gave 9.1 g

(20%) of 2-acetoxymethyl-4-tert-butylpyridine as a pale yellow oil.

¹H NMR (300 MHz, CDCl₃): δ 8.51 (1H, d, J ) 6.0 Hz), 7.33

(1H, s), 7.26 (1H, d, J ) 3.0 Hz), 5.21 (2H, s), 2.17 (3H, s), 1.32

(9H, s).

To 2-acetoxymethyl-4-tert-butylpyridine (9.1 g, 0.044 mol) was

added NaOH (5.0 g, 0.13 mol) along with 50 mL of water. The re-

sulting mixture was refluxed for 30 h, then cooled to rt, and ex-

tracted with ether (4 × 50 mL). The combined organic extracts

were dried over Na₂SO₄, and the volatile components were removed

to give 7.0 g (98%) of an orange oil. ¹H NMR (300 MHz, CDCl₃):

[(L^tBu)Cu^I(MeCN)](ClO₄) (1^tBu). To 0.16 g (0.35 mmol) of L^tBu

and 0.11 g (0.33 mmol) of [Cu(MeCN)₄](ClO₄) in a 100 mL

Schlenk flask was added 5 mL of deaerated CH₃CN under Ar. The

resulting mixture was stirred for 30 min, and diethyl ether (50 mL)

was added until a slight cloudiness developed. The solution was

filtered through a coarse porosity frit, and 50 mL of diethyl ether

was added to the filtrate to precipitate the complex. The supernatant

was decanted, and the solid was washed with diethyl ether (2 ×

50 mL). Drying in vacuo yielded 0.17 g (80%) of a bright yellow

1

solid. H NMR (300 MHz, CD₃NO₂): δ 7.56-7.30 (9H, s, br),

1820 Inorganic Chemistry, Vol. 42, No. 6, 2003

Copper(I)-Dioxygen ReactiWity of [(L)Cu^I]⁺

2.08 (3H, s), 1.32-1.23 (27 H, s). Anal. Calcd for C₃₂H₄₅N₅-

CuClO₄: C, 57.99; H, 6.84; N, 10.57. Found: C, 58.14; H, 6.41;

N, 9.85.

2-Phthalimidomethyl-4-methoxypyridine (18). To a stirred

solution of 2-chloromethyl-4-methoxy-pyridine (5.5 g, 0.035 mol)

in DMF (60 mL) was added 6.6 g (0.035 mol) of potassium

phthalimide. The mixture was stirred at ∼358 K for 72 h and then

cooled to rt, and DMF was removed on a high-vacuum rotary

evaporator. The remaining solid was redissolved in H₂O (100 mL),

and the aqueous solution was extracted with CH₂Cl₂several times.

The organic extracts were dried over Na₂SO₄, and the volatile

components were removed in vacuo. The resulting brown solid was

recrystallized from hot EtOH to yield 6.6 g (71%) of a white

crystalline solid. R_f) 0.10 (silica, 40% EtOAc-60% hexane). ¹H

NMR (300 MHz, CDCl₃): δ 8.33 (1 H, d, J ) 6.0 Hz), 7.87 (2 H,

m), 7.73 (2H, m), 6.78 (1 H, d, J ) 3.0 Hz), 6.70 (1H, dd, J ) 6.0

Hz, J′ ) 3.0 Hz), 4.95 (2 H, s), 3.81 (3 H, s).

2. L^MeO. 4-Nitro-2-picoline N-oxide (6). To a mixture of

2-picoline N-oxide (5.2 g, 0.046 mol) and 18 M H₂SO₄(18 mL)

was added fuming HNO₃(13.8 mL) dropwise at 273 K. The

resulting mixture was then heated to 373-378 K and stirred for 2

h. After cooling to rt, it was neutralized with Na₂CO₃and extracted

with CH₂Cl₂several times. The combined organic extracts were

dried over Na₂SO₄, and the volatile component was removed under

vacuum to give 6.8 g (95%) of a pale yellow solid. R_f) 0.56 (silica,

10% MeOH-90% CH₂Cl₂). ¹H NMR (300 MHz, CDCl₃): δ 8.31

(1H, d, J ) 7.2 Hz), 8.14 (1H, d, J ) 2.7 Hz), 8.00 (1H, dd, J )

7.2 Hz, J′ ) 2.7 Hz), 2.57 (3H, s).

2-Aminomethyl-4-methoxypyridine (22). To a stirred solution

of 2-phthalimidomethyl-4-methoxy-pyridine (6.6 g, 0.025 mol) in

MeOH (100 mL) was added 1.8 mL (0.057 mol) of hydrazine

slowly. The resulting mixture was refluxed for 3 h, and after cooling

to rt, the volatile components were removed. The remaining solid

was redissolved in 1 M NaOH aqueous solution (200 mL), and the

solution was extracted with CH₂Cl₂several times. The organic

extracts were dried over Na₂SO₄, and the volatile component was

removed in vacuo to give 3.3 g (quantitative yield) of a light yellow

liquid. ¹H NMR (300 MHz, CDCl₃): δ 8.35 (1 H, d, J ) 6.0 Hz),

6.81 (1 H, d, J ) 3.0 Hz), 6.68 (1H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz),

3.92 (2 H, s), 3.84 (3 H, s), 1.77 (1 H, b, br).

L^MeO: (Tris[{(4-methoxy)-2-pyridyl}methyl]amine) (25). To

a stirred solution of 2-aminomethyl-4-methoxy-pyridine (0.98 g,

0.0071mol) in CH₂Cl₂(20 mL) were added 2-chloromethyl-4-

methoxy-pyridine (2.35 g, 0.0149 mol) and 1 M NaOH solution

(15 mL, 0.015 mol). The reaction mixture was stirred at 323-333

K for 96 h (the pH of the solution dropped to 7.0). The mixture

was cooled to rt, the organic layer was separated, and the aqueous

layer was extracted with CH₂Cl₂several times. The combined

organic layers were dried over Na₂SO₄, and the volatile components

were removed. Purification of the crude product on silica gel using

3% MeOH-97% CH₂Cl₂gave 0.95 g (35%) of a light yellow solid.

¹H NMR (300 MHz, CDCl₃): δ 8.33 (3H, d, J ) 6.0 Hz), 7.17

(3H, d, J ) 3.0 Hz), 6.66 (3H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 3.85

(6H, s), 3.84 (9H, s). High-resolution MS: m/z calcd for C₂₁H₂₄N₄O₃

380.1848, found 380.1846.

4-Methoxy-2-picoline N-oxide (7). To a solution of 4-nitro-2-

picoline N-oxide (7.2 g, 0.047 mol) in MeOH (450 mL) was added

sodium methoxide prepared from mixing 1.1 g (0.048 mol) of

sodium and 150 mL of MeOH at 333 K. The resulting mixture

was stirred at 333 K for 20 min and then cooled to rt, whereupon

the volatile components were removed. The remaining residue was

redissolved in H₂O (200 mL) and extracted with CH₂Cl₂several

times. The combined organic layers were dried over Na₂SO₄, and

the volatile components were removed to give 5.5 g (84%) of light

1

yellow liquid. R_f) 0.35 (silica, 10% MeOH-90% CH₂Cl₂). H

NMR (300 MHz, CDCl₃): δ 8.16 (1H, d, J ) 6.0 Hz), 6.78 (1H,

d, J ) 3.0 Hz), 6.71 (1H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 3.84 (3H,

s), 2.57 (3H, s).

2-Hydroxymethyl-4-methoxypyridine (10). To 4-methoxy-2-

picoline N-oxide (5.4 g, 0.039 mol) was added acetic anhy-

dride (15 mL). The resulting mixture was heated to 383 K and

stirred for 4 h. After the mixture was cooled to rt, it was neutra-

lized with a Na₂CO₃aqueous solution and extracted with CH₂Cl₂

several times. The combined organic extracts were dried over

Na₂SO₄, and the volatile components were removed. Purification

of the crude product on silica gel using 4% MeOH-96% CH₂Cl₂

gave 4.2 g (60%) of 2-acetoxymethyl-4-methoxypyridine as a

pale brown liquid. R_f) 0.50 (silica, 5% MeOH-95% CH₂Cl₂).

¹H NMR (300 MHz, CDCl₃): δ 8.46 (1H, d, J ) 6.0 Hz), 6.92

(1H, s), 6.80 (1H, d, J ) 3.0 Hz), 5.22 (2H, s), 3.91 (3H, s), 2.22

(3H, s).

A solution of 2-acetoxymethyl-4-methoxy-pyridine (4.2 g, 0.023

mol) in 2 N HCl aqueous solution (30 mL) was stirred at 343 K

for 2 h. After the mixture was cooled to rt, it was neutralized with

Na₂CO₃and extracted with CH₂Cl₂several times. The combined

organic extracts were dried over Na₂SO₄, and the volatile compo-

nents were removed. Purification of the crude product on silica

gel using 5% MeOH-95% CH₂Cl₂gave 1.9 g (60%) of a white

[(L^MeO)Cu^I(MeCN)](ClO₄)‚0.1MeCN (1^MeO-(ClO₄)). In a 100

mL Schlenk flask equipped with a stir bar was added L^MeO(0.20

g, 0.52 mmol) and [Cu(MeCN)₄](ClO₄) (0.17 g, 0.52 mmol). To

this mixture was added 10 mL of deaerated CH₃CN under an Ar

atmosphere. The resulting bright yellow solution was stirred for

30 min. Diethyl ether (50 mL) was added to the solution until a

slight cloudiness developed. The solution was filtered through a

coarse frit, and 40 mL of diethyl ether was added to the filtrate to

precipitate the complex. The supernatant was decanted, and the solid

was washed with diethyl ether (2 × 50 mL) and dried in vacuo to

1

solid. R_f) 0.13 (silica, 5% MeOH-95% CH₂Cl₂). H NMR (300

MHz, CDCl₃): δ 8.35 (1H, d, J ) 6.0 Hz), 6.79 (1H, d, J ) 6.0

Hz), 6.72 (1H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 4.71(2H, s), 3.86

(3H, s).

1

yield 0.25 g (81%) of a bright yellow solid. H NMR (300 MHz,

2-Chloromethyl-4-methoxypyridine (14). To a stirred solution

of 2-hydroxymethyl-4-methoxy-pyridine (5.0 g, 0.036 mol) in

CH₂Cl₂(150 mL) was added thionyl chloride (5.1 g, 0.043 mol)

slowly. After addition, the resulting mixture was stirred at rt for 1

h. The reaction was quenched with a Na₂CO₃aqueous solution,

and the organic layer was collected. The aqueous layer was extracted

with CH₂Cl₂, and the combined organic phase was dried over

Na₂SO₄. Removal of volatile components gave 1.6 g (99%) of

CD₃NO₂): δ 7.04 (9H, s, br), 3.84 (9H, s), 2.00 (3.3H, s). Anal.

Calcd for C₂₇H₂₇N₅CuClO₇‚0.1MeCN: C, 47.26; H, 4.66; N, 11.98.

Found: C, 47.34; H, 4.67; N, 12.01.

[Cu^I(L^MeO)(MeCN)]BArF (1^MeO-(BArF)). To [(L^MeO)Cu^I-

(MeCN)](ClO₄) (0.11 g, 0.19 mmol) in a 100 mL Schlenk flask

equipped with a stir bar was added a solution of NaBArF (0.15 g,

0.17 mmol) in 30 mL diethyl ether under Ar. The resulting solution

was stirred at rt for 1 h and filtered through a coarse frit. The filtrate

was collected, and the volatiles were removed under vacuum to

yield 0.18 g (78%) of a yellow solid. ¹H NMR (300 MHz,

CD₃NO₂): δ 7.84 (9H, s, br), 7.67 (4H, s), 3.91 (9H, s), 2.03 (3H,

1

brown liquid. R_f) 0.35 (silica, 50% EtOAc-50% hexane). H

NMR (300 MHz, CDCl₃): δ 8.44 (1 H, d, J ) 6.0 Hz), 7.05 (1 H,

d, J ) 3.0 Hz), 6.81 (1H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 4.93 (2 H,

s), 3.93 (3 H, s).

Inorganic Chemistry, Vol. 42, No. 6, 2003 1821

Zhang et al.

s). Anal. Calcd for C₅₃H₃₆N₄O₃CuBF₂₄: C, 48.7; H, 2.78; N, 4.29.

Found: C, 47.74; H, 2.63; N, 4.45.

dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 7.90 (2H, dd, J ) 6.0 Hz, J′ )

3.0 Hz), 7.30 (1 H, s), 7.20 (1H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 5.0

(2 H, s).

3. L^{Me N}. 4-Chloro-2-picoline N-oxide (8). To 4-nitro-2-picoline

2

N-oxide (4.0 g, 0.026 mol) in a pressure tube was added 12 M

HCl aqueous solution (20 mL). After stirring at 433 K for 18 h,

the reaction mixture was cooled to rt and neutralized with

Na₂CO₃. The resulting aqueous solution was extracted with

CH₂Cl₂several times. The combined organic layers were dried over

Na₂SO₄, and the volatile components were removed by rotary

evaporation to yield 3.3 g (87%) of pale yellow liquid. R_f) 0.5

(silica, 10% MeOH-90% CH₂Cl₂). H NMR (300 MHz, CDCl₃):

δ 8.17 (1H, d, J ) 6.0 Hz), 7.28 (1H, s), 7.15 (1H, dd, J ) 6.0 Hz,

J′ ) 3.0 Hz), 2.54 (3H, s).

2-Aminomethyl-4-dimethylaminopyridine (23). To 2-phthal-

imidomethyl-4-chloropyridine (3.6 g, 0.013 mol) in a pressure tube

were added (CH₃)₂NH‚HCl (6.5 g, 0.080 mmol), NaOH (3.2 g,

0.080 mmol), and water (10 mL). The resulting mixture was stirred

and heated to 423 K for 24 h. Following cooling to rt, water was

removed by using a high-vacuum rotary evaporator. The remaining

solid (20) was redissolved in MeOH (100 mL). To this solution

was syringed in hydrazine (1.5 mL), and the resulting solution was

refluxed for 3 h. The mixture was cooled to rt, and the volatile

components were removed by rotary evaporation to yield brown

solid which was redissolved in water (200 mL). The aqueous

solution was extracted with CH₂Cl₂several times, and the combined

organic layers were dried over Na₂SO₄. Removal of the volatile

components gave 2.2 g (quantitative yield) of light yellow liquid.

¹H NMR (300 MHz, CDCl₃): δ 8.16 (1H, d, J ) 6.0 Hz), 6.48

(1H, d, J ) 3.0 Hz), 6.38 (1H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 3.84

(2H, s), 3.00 (2H, s), 2.17 (2H, s, br).

2-Hydroxymethyl-4-dimethylaminopyridine (12). To 2-hy-

droxymethyl-4-chloropyridine (2.3 g, 0.016 mol) in a pressure tube

were added (CH₃)₂NH‚HCl (6.5 g, 0.080 mmol), NaOH (3.0 g,

0.075 mmol), and water (10 mL). The resulting mixture was stirred

and heated to 423 K for 48 h and then cooled to rt, and water was

removed by using a high-vacuum rotary evaporator. The remaining

solid was extracted with CH₂Cl₂several times. The combined

organic layers were dried over Na₂SO₄, and the volatile components

were removed by rotary evaporation to yield 2.0 g (82%) of a light

brown solid. ¹H NMR (300 MHz, CDCl₃): δ 8.12 (1H, d, J ) 6.0

Hz), 6.50 (1H, s), 6.48 (1H, m), 4.69 (2H, s), 4.44 (1H, s, br), 3.10

(6H, s).

1

To 4-chloro-2-picoline N-oxide (3.3 g, 0.023 mmol) was added

acetic anhydride (40 mL). After refluxing at 383 K for 3 h, the

reaction mixture was cooled to rt, and the volatile components were

removed by using a high-vacuum rotary evaporator. Water (100

mL) was added to the remaining residue, and the resulting solution

was neutralized with Na₂CO₃, which was then extracted with

CH₂Cl₂several times. The combined organic layers were dried over

Na₂SO₄, and the volatile components were removed by rotary

evaporation. Purification of the crude product on silica gel using

3% MeOH-97% CH₂Cl₂gave 3.5 g (82%) of 2-acetoxymethyl-

4-chloropyridine as a brown liquid. R_f) 0.2 (silica, 3% MeOH-

1

97% CH₂Cl₂). H NMR (300 MHz, CDCl₃): δ 8.49 (1H, d, J )

6.0 Hz), 7.37 (1H, d, J ) 3.0 Hz), 7.26 (1H, dd, J ) 6.0 Hz, J′ )

3.0 Hz), 5.21 (2H, s), 2.20 (3H, s).

2-Hydroxymethyl-4-chloropyridine (11). To 2-acetoxymethyl-

4-chloropyridine (3.5 g, 0.018 mol) was added 2 N HCl (50 mL).

After stirring at 353 K for 3 h, the reaction mixture was cooled to

rt and neutralized with Na₂CO₃. The resulting aqueous solution was

extracted with CH₂Cl₂several times. The combined organic layers

were dried over Na₂SO₄, and the volatile components were removed

by rotary evaporation to give a brown liquid. Purification of the

crude product on silica gel using 5% MeOH-95% CH₂Cl₂gave

2.3 g (85%) of a white solid. R_f) 0.19 (silica, 5% MeOH-95%

2-Chloromethyl-4-dimethylaminopyridine (16). To a stirred

solution of 2-hydroxymethyl-4-dimethylamino-pyridine (2.7 g,

0.018 mmol) in CH₂Cl₂(150 mL) was added thionyl chloride (2.8

g, 0.023 mmol) slowly at rt. After addition, the resulting mixture

was stirred at rt for 2 h. The reaction was then quenched with

Na₂CO₃aqueous solution, and the organic layer was collected. The

aqueous layer was extracted with CH₂Cl₂several times, and the

combined organic layers were dried over Na₂SO₄followed by

removal of volatile components. Purification of the crude product

on alumina gel using CH₂Cl₂gave 3.0 g (99%) of a brown solid.

R_f) 0.15 (alumina, CH₂Cl₂). ¹H NMR (300 MHz, CDCl₃): δ 8.19

(1H, d, J ) 6.0 Hz), 6.66 (1H, d, J ) 3.0 Hz), 6.44 (1H, dd, J )

6.0 Hz, J′ ) 3.0 Hz), 4.56 (2H, s), 3.03 (6H, s).

1

CH₂Cl₂). H NMR (300 MHz, CDCl₃): δ 8.46 (1H, d, J ) 6.0

Hz), 7.32 (1H, d, J ) 3.0 Hz), 7.23 (1H, dd, J ) 6.0 Hz, J′ ) 3.0

Hz), 4.76 (2H, s), 3.56 (1H, s, br).

2-Chloromethyl-4-chloropyridine (15). To a stirred solution of

2-hydroxymethyl-4-chloropyridine (3.2 g, 0.022 mmol) in CH₂Cl₂

(200 mL) was added thionyl chloride (3.4 g, 0.029 mmol) slowly.

After addition, the resulting mixture was stirred at rt for 12 h. The

reaction was then quenched with Na₂CO₃aqueous solution, and

the organic layer was collected. The aqueous layer was extracted

with CH₂Cl₂several times, and the combined organic layers were

dried over Na₂SO₄. Removal of volatile components gave 3.1 g

(85%) of a brown liquid. R_f) 0.71 (silica, 10% MeOH-90%

L^{Me N}: (Tris[(4-dimethylamino-2-pyridyl)methyl]amine) (26).

2

To a stirred solution of 2-chloromethyl-4-dimethylamino-pyridine

(1.96 g. 0.0115 mol) and 2-aminomethyl-4-dimethylamino-pyridine

(0.76 g, 0.005 mol) in CH₂Cl₂(10 mL) was added 1 N NaOH

solution (15 mL). After stirring at 323-333 K for 4 days, the

reaction mixture was cooled to rt, and the organic layer was

separated. The aqueous layer was extracted with CH₂Cl₂several

times, and the combined organic layers were dried over Na₂SO₄.

The volatile components were removed by rotary evaporation, and

the remaining residue was washed with CH₃CN to yield 0.87 g

(41%) of a white solid after drying in vacuo. ¹H NMR (300 MHz,

CDCl₃): δ 8.14 (3H, d, J ) 6.0 Hz), 6.98 (3H, d, J ) 3.0 Hz),

6.35 (3H, dd, J ) 6.0 Hz, J′ ) 3.0 Hz), 3.82 (6 H, s), 3.01 (18H,

s). High-resolution MS, m/z calcd for C₂₄H₃₃N₇419.2797, found

419.2802.

1

CH₂Cl₂). H NMR (300 MHz, CDCl₃): δ 8.46 (1H, d, J ) 6.0

Hz), 7.52 (1H, d, J ) 3.0 Hz), 7.27 (1H, dd, J ) 6.0 Hz, J′ ) 3.0

Hz), 4.66 (2 H, s).

2-Phthalimidomethyl-4-chloropyridine (19). To a stirred solu-

tion of 2-chloromethyl-4-chloro-pyridine (3.2 g, 0.020 mol) in DMF

(60 mL) was added 3.7 g (0.035 mol) of potassium phthalimide.

The mixture was stirred at ∼358 K for 48 h and then cooled to rt,

and DMF was removed on a high-vacuum rotary evaporator. The

remaining solid was redissolved in H₂O (100 mL), and the aqueous

solution was extracted with CH₂Cl₂several times. The organic

extracts were dried over Na₂SO₄, and the volatile components were

removed by rotary evaporation. The resulting brown solid was

recrystallized from hot EtOH to yield 3.6 g (68%) of white

[(L^{Me N})Cu^I]ClO₄(1^{Me N}-(ClO₄)). In a 100 mL Schlenk flask

2

1

equipped with a stir bar was added L^{Me N}(0.2 g, 0.47 mmol) and

2

crystalline solid. R_f) 0.50 (SiO₂, 50% EtOAc-50% hexane). H

NMR (300 MHz, CDCl₃): δ 8.42 (1 H, d, J ) 6.0 Hz), 8.02 (2 H,

[Cu(CH₃CN)₄](ClO₄) (0.15 g, 0.45 mmol). To this mixture was

1822 Inorganic Chemistry, Vol. 42, No. 6, 2003

Copper(I)-Dioxygen ReactiWity of [(L)Cu^I]⁺

added 10 mL of deaerated CH₃CN under an Ar atmosphere. The

resulting bright yellow mixture was stirred for 30 min. Diethyl ether

(50 mL) was added to the solution to precipitate the complex at

273 K. The supernatant was decanted, and the solid was washed

with diethyl ether and dried in vacuo to yield 0.22 g (75%) of a

yellow powder. ¹H NMR (300 MHz, CD₃NO₂): δ 7.40-6.4 (9 H,

s, br), 2.98 (18 H, s). Anal. Calcd for C₂₄H₃₃N₇CuClO₄: C, 49.48;

H, 5.71; N, 16.83. Found: C, 49.09; H, 5.54; N, 17.47.

) OMe). These are close to those of typical 2:1 electrolytes such

as [(L^H)Cu^II(H₂O)](ClO₄)₂⁹⁹(b ) 1477) and [Cu^II(XYL-O^-)(OH^-)]-

110

(ClO₄)₂(b ) 1659), while they are very different from that of

111

an 1:1 electrolyte [(L^H)Cu^II(Cl^-)]ClO₄(b ) 864).

IR Spectra for CO-Adducts of the Copper(I) Complexes

[(L^R)Cu^I(MeCN)]⁺. IR samples were prepared in a drybox by

dissolving ∼35 mg of [(L^R)Cu^I(MeCN)](ClO₄) in ∼2 mL of

deaerated CH₃CN in a 10 mL Schlenk flask. Small amounts of the

stock solution were transferred to a solution IR cell, and the spectra

of [(L^R)Cu^I(MeCN)]⁺were recorded as a control. The remaining

stock solutions were bubbled with CO for ∼10 s, and the resulting

solutions were transferred to a solution cell in a glovebox with a

gastight syringe. The spectra were taken of the carbonylated species

in order to observe the specific CO stretching frequency.

Stopped-Flow Experiments. Rapid kinetics were followed using

a SFL-21 (2-mm light path/2-mL syringes) low-temperature flow

unit of a SF-3A stopped-flow system (Hi-Tech Scientific) combined

with a TIDAS-16 HQ/UV-vis 512/16B diode array spectrometer

(J & M, 507 diodes, 300-720 nm, 1.3 ms minimum sampling time)

using flexible light guides connected to a CLH-111 halogen lamp

(ZEISS). The two glass coils, containing Cu(I) and dioxygen

solutions, respectively, and the mixing chamber were immersed in

an ethanol bath. This bath was placed in a Dewar, which was filled

with liquid nitrogen for low-temperature measurements. The ethanol

bath was cooled by liquid nitrogen evaporation, and its temperature

was measured by using a Pt resistance thermocouple and maintained

to 0.1 K by using a temperature-controlled thyristor power unit

(both Hi-Tech). Concentrations of dioxygen and complex are given

for rt throughout the text. For the numerical analyses, values

adjusted for temperature-dependent solvent contraction were used.

Dry and CO₂-free air was used instead of pure O₂in order to vary

the concentration of the latter.

Data acquisition (up to 256 complete spectra; up to 4 different

time bases) was performed by using the Kinspec program (J &

M). For numerical analysis, all data were pretreated by factor

analysis, and concentration profiles were calculated by numerical

integration using either Specfit (Spectrum Software Ass.) or Globfit

(MATLAB).

The solvents EtCN and THF were dried as follows: EtCN (p.a.,

Merck) was predried with phosphorus pentoxide, then distilled under

normal pressure, again dried using CaH₂, and redistilled immediately

prior to use. THF (puriss, Merck) was dried using sodium and

distilled immediately prior to use. Acetone (Uvasol, Merck) was

used without further purification.

[(L^{Me N})Cu^I]BArF (1^{Me N}-(BArF)). To [(L^{Me N})Cu^I](ClO₄) (0.12,

2

0.19 mmol) was added a solution of NaBArF (0.15 g, 0.17 mmol)

1

in diethyl ether (30 mL). The resulting solution was stirred for /₂

h and filtered through a coarse frit. The volatile component was

removed under vacuum, and the solid was washed with pentane

and dried under vacuum to afford 0.16 g (70%) of yellow solid.

¹H NMR (300 MHz, CD₃NO₂): δ 7.84 (8 H, s), 7.67 (4 H, s),

2.85 (s, 9H). Anal. Calcd for C₅₆H₄₅N₇CuBF₂₄: C, 49.96; H, 3.37;

N, 7.28. Found: C, 49.39; H, 3.37; N, 7.09.

4. L^H. [Cu^I(L^H)]BArF‚Et₂O (1^H-(BArF)). To [(L^H)Cu^I(MeCN)]-

(ClO₄) (0.150 g, 0.303 mmol) in a 100 mL Schlenk flask equipped

with a stir bar was added a solution of NaBArF (0.268 g, 0.302

mmol) in 30 mL of diethyl ether under Ar. The resulting solution

was stirred at rt for 1 h and filtered through a coarse frit. The filtrate

was collected, and the volatiles were removed under vacuum to

1

yield 0.300 g (78.9%) of a yellow solid. H NMR (300 MHz,

CD₃NO₂): δ 7.85 (9H, s, br), 7.37 (4H, s), 3.41 (4H, q), 2.07 (3H,

s), 1.14 (6H, t). Anal. Calcd for for C₅₄H₄₀N₄OCuBF₂₄: C, 50.23;

H, 3.12; N, 4.34. Found: C, 50.68; H, 3.19; N, 4.61.

5. L^Me. [(L^Me)Cu^I(MeCN)]ClO₄(1^Me-(ClO₄)). In a 100 mL

Schlenk flask equipped with a stir bar was added L^Me(0.402 g,

1.21 mmol) and [Cu(CH₃CN)₄](ClO₄) (0.355 g, 1.08 mmol). To

this mixture was added 10 mL of deaerated CH₃CN under an Ar

atmosphere. The resulting bright yellow mixture was stirred for 30

min. Diethyl ether (50 mL) was added to the solution until the

solution became cloudy. The resulting mixture was filtered, and

the filtrate was collected in a 100 mL Schlenk flask. An additional

50 mL of diethyl ether was added to precipitate the complex. The

supernatant was decanted, and the solid was washed with diethyl

ether and dried in vacuo to yield 0.470 g (81%) of a yellow powder.

¹H NMR (300 MHz, CD₃NO₂): δ 7.40-6.4 (9 H, s, br), 2.98 (18

H, s). Anal. Calcd for C₂₃H₂₇N₅CuClO₄: C, 51.49; H, 5.07; N,

13.05. Found: C, 51.32; H, 4.90; N, 12.93.

Electrochemistry. Cyclic voltammetry was carried out using a

Bioanalytical Systems BAS-100B electrochemistry analyzer. The

cell consists of a modification of a standard three-chamber design

equipped for handling air-sensitive solutions by utilizing high-

vacuum valve seals. A platinum disk (BAS MF 2013) was used as

the working electrode. The reference electrode was Ag/AgNO₃. The

measurements were performed at rt under an Ar atmosphere in

MeCN solution containing 0.1 M tetrabutylammonium hexafluo-

rophosphate and 10^-4-10^-3M copper complex.

The following provides information for the kinetic studies.

[(L^H)Cu^I]⁺(EtCN solvent): three series, 0.17, 0.47, and 0.52 mM;

198 measurements made (179-267 K); 182 measurements used

for final analysis; [O₂] ) 4.4 mM; reaction time range 0.1-74 s.

[(L^Me)Cu^I]⁺(EtCN solvent): three series, 0.939, 0.399, and 0.269

mM; 204 measurements made (179-273 K); 202 measurements

used for final analysis; [O₂] ) 4.4 mM; reaction time range

0.1-195 s. [(L^tBu)Cu^I]⁺(EtCN solvent): three series, 0.877, 0.502,

and 0.202 mM; 210 measurements made (179-271 K); all 210

measurements used for final analysis; [O₂] ) 4.4 mM; reaction

time range 0.03-293 s. [(L^MeO)Cu^I]⁺(EtCN solvent): three series,

0.49, 0.36, and 0.16 mM; 219 measurements made (179-297 K);

204 measurements used for final analysis; [O₂] ) 4.4 mM; reaction

time range 0.02-501 s. [(L^H)Cu^I]⁺(THF solvent): three series,

0.26, 0.39, and 0.52 mM; 145 measurements made (183-303 K);

120 measurements used for final analysis; [O₂] ) 3.3 mM; reaction

Conductivity Measurements. Electrical conductivity measure-

ments were carried out in acetone using a Barnstead Sybron

Corporation model PM-70CB conductivity bridge with a Yellow

Springs Instrument Co. Inc. 3403 cell. The cell constant, k, was

determined by using a standard aqueous KCl solution. The specific

conductance, Λ, was calculated according to eq 6

Λ ) 1000k(CR)^-1

(6)

where R is measured resistance and C is the concentration of the

sample. Studies with varying concentrations of [(L^R)₂Cu^I₂]²⁺(R )

H, tBu, and OMe) gave Onsager plots with straight lines having

slope b ) 1422 (R ) H), b ) 1494 (R ) tBu), and b ) 1390 (R

(110) Mahroof-Tahir, M. Thesis, The Johns Hopkins University, 1992.

(111) Wei, N. Ph.D. Thesis, State University of New York at Albany, 1994.

Inorganic Chemistry, Vol. 42, No. 6, 2003 1823

Zhang et al.

time range 0.2-751 s. [(L^MeO)Cu^I]⁺(THF solvent): four series,

0.34, 0.28, and 0.11 mM (all at [O₂] ) 3.3 mM) and 0.21 mM

copper(I) complex at [O₂] ) 0.69 mM; 158 total measurements

made (175-295 K); 111 measurements used for final analysis;

reaction time range 0.05-248 s. [(L^Me)Cu^I]⁺(acetone solvent): one

series, 0.64 mM; 47 measurements made (183-293 K); [O₂] )

5.11 mM; reaction time range 0.03-600 s. The complexity of

the data prevented numerical analysis. [(L^tBu)Cu^I]⁺(acetone sol-

vent): two series, 0.88 mM ([O₂] ) 5.11 mM) and 0.46 mM

([O₂] ) 0.46 mM); 109 total measurements made (182-283 K);

54 measurements used for final analysis; reaction time range

1-142 s.

Acknowledgment. We are grateful to the National

Institutes of Health (K.D.K, GM28962) and Swiss National

Science Foundation (A.D.Z.) for support of this research.

Supporting Information Available: Eyring plots, van’t Hoff

plots, time-dependent UV-vis spectra, Onsager plots of conductiv-

ity data, variable-temperature NMR spectra, and an [O₂]-dependence

on the kinetics of reaction, all material pertaining to the data and

discussion in the main text. This material is available free of charge

via the Internet at http://pubs.acs.org.

IC0205684

1824 Inorganic Chemistry, Vol. 42, No. 6, 2003

Article Doi

Copper(I)-dioxygen reactivity of [(L)CuI]+ (L = tris(2-pyridylmethyl)amine): Kinetic/thermodynamic and spectroscopic studies concerning the formation of Cu-O2 and Cu2-O2 adducts as a function of solvent medium and 4-pyridyl ligand substituent variations

DOI: 10.1021/ic0205684

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Article abstract of DOI:10.1021/ic0205684

Full text of DOI:10.1021/ic0205684

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