Reactivity of Copper(I) Complexes with Tripodal Ligands towards O2: Structures of a Precursor [L3CuI(NCCH3)](BF4), L3 = Tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and of its Oxidation Product [L3CuII(-OH)2CuIIL3](BF4)2 with Strong Antiferromagneti

(1)

Article

Reference

Reactivity of Copper(I) Complexes with Tripodal Ligands towards O2:

Structures of a Precursor [L3CuI(NCCH3)](BF4), L3 = Tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and of its

Oxidation Product [L3CuII(-OH)2CuIIL3](BF4)2 with Strong Antiferromagnetic Spin-Spin Coupling

KAIM, Wolfgang, et al .

Abstract

The molecular structure of the highly oxygen-sensitive complex [L3CuI(NCCH3)](BF4) (1) reveals approximately symmetrical coordination by the fac-tridentate (tripodal) ligand L3 = tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and a rather short CuI-N(acetonitrile) distance of 1.865(5) Å. In CH2Cl2 at -78 °C the colourless compound reacts with O2 to yield a labile purple intermediate (max 517 nm) - presumably a peroxodicopper(II) complex - which decomposes at -30 °C. No such intermediate was observed on reaction of the CuI complex of bis(2-pyridylmethyl)benzylamine with O2 at -80 °C. However, an EPR spectrum with g = 2.17 and g = 2.03 without 63,65Cu hyperfine splitting was observed at low temperatures. Exposure of the precursor 1 to air under ambient conditions yields dinuclear [L3CuII(-OH)2CuIIL3](BF4)2 (2) which exhibits an EPR detectable dissociation into monomers in CH2Cl2 solution. The structure of the hexakis(dichloromethane) solvate of 2 with Cu-Cu and Cu-O distances of 3.055 and 1.94Å, respectively, is typical for dihydroxo-bridged dicopper compounds with square-pyramidal CuII configuration ( = 0.03), [...]

KAIM, Wolfgang, et al . Reactivity of Copper(I) Complexes with Tripodal Ligands towards O2:

Structures of a Precursor [L3CuI(NCCH3)](BF4), L3 =

Tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and of its Oxidation Product

[L3CuII(-OH)2CuIIL3](BF4)2 with Strong Antiferromagnetic Spin-Spin Coupling. Zeitschrift für anorganische und allgemeine Chemie , 2005, vol. 631, no. 13-14, p. 2568-2574

DOI : 10.1002/zaac.200500053

Available at:

http://archive-ouverte.unige.ch/unige:3316

Disclaimer: layout of this document may differ from the published version.

1 / 1

(2)

Structures of a Precursor [L

³

Cu

^I

(NCCH

₃

)](BF

₄

), L

³

ⴝ Tris(3-isopropyl-4,5- trimethylenepyrazolyl)methane and of its Oxidation Product [L

³

Cu

^II

(µ- OH)

₂

Cu

^II

L

³

](BF

₄

)

₂

with Strong Antiferromagnetic Spin-Spin Coupling

Wolfgang Kaimâ,*, Christoph Titzeâ, Thilo Schurrâ, Monika Siegerâ, Max Lawson^b, Jeanne Jordanov^b, Darıó Rojas^c, Ana M. Garcıá^c, and Jorge Manzur^c,*

aStuttgart, Institut für Anorganische Chemie der Universität

bGrenoble/France, Centre d’Etudes Nucle´aires

cSantiago/Chile, Departamento de Ciencia de Materiales, Facultad de Ciencias Fisicas y Matematicas, Universidad de Chile Received February 4th, 2005.

Dedicated to Professor Herbert W. Roesky on the Occasion of his 70. Birthday

Abstract. The molecular structure of the highly oxygen-sensitive complex [L³Cu^I(NCCH₃)](BF₄) (1) reveals approximately symmetrical coordination by the fac-tridentate (tripodal) ligand L³ ⫽ tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and a rather short Cu^I-N(acetonitrile) distance of 1.865(5) A˚ . In CH₂Cl₂ at

⫺78°C the colourless compound reacts with O₂to yield a labile purple intermediate (λ_max517 nm) ⫺presumably a peroxodicopper(II) complex⫺which decomposes at⫺30°C. No such intermediate was observed on reaction of the Cu^Icomplex of bis(2-pyridylmethyl)benzylamine with O₂ at⫺80°C. However, an EPR spectrum withg_储⫽2.17 andg_Ⲛ⫽2.03 without^63,65Cu hyperfine splitting was observed at low temperatures. Exposure of the precursor 1to air under ambient conditions yields dinuclear [L³Cu^II(µ-OH)_2-

Introduction

The reaction of copper(I) complexes with dioxygen, O₂, has been intensely researched in connection with the essential O₂binding role of Cu^Iin biology [1⫺7] and, more recently, in connection with the copper-based utilization of O₂ for synthetic purposes such as stoichiometric and catalytic ver- sions of C-H activation reactions [8, 9]. Reversibly O₂trans- porting hemocyanin as well as C-H bond activating enzymes such as tyrosinase or dopamine-β-monooxygenase have been investigated as proteins and with regard to small molecular model complexes. Mononuclear and dinuclear electron transfer intermediates involving superoxo (O₂^·⫺) or peroxo (O₂^2⫺) complexes of copper(II) have been identified spectroscopically and, in part, structurally [1⫺7]. Sterically hindered ligands proved to be advantageous to stabilize these intermediates, however, electronic and charge effects must also be considered.

* Prof. Dr. Wolfgang Kaim Institut für Anorganische Chemie Universität Stuttgart

Pfaffenwaldring 55 D-70550 Stuttgart Fax:⫹49 711 685-4165

e-mail: [email protected]

Cu^IIL³](BF₄)₂ (2) which exhibits an EPR detectable dissociation into monomers in CH₂Cl₂solution. The structure of the hexakis- (dichloromethane) solvate of2 with Cu⫺Cu and Cu-O distances of 3.055 and 1.94 A˚ , respectively, is typical for dihydroxo-bridged dicopper compounds with square-pyramidal Cu^II configuration (τ⫽0.03), adopting an antiarrangement. In agreement with the relatively wide Cu-O-Cu angles of 103.5°an analysis of the temperature dependence of the magnetic susceptibility revealed a rather strong (J⫽ ⫺633 cm^⫺1) antiparallel spin-spin coupling. The effect is ascribed to the steric bulk of the ligand L³.

Keywords:Copper; Crystal structures; Magnetism; Tripodal ligands

Following reports of the successful use of tris(3-organo- pyrazolyl)borato(1-) complexes of copper in stabilizing oxy- genated intermediates [1a, 3⫺6] we have chosen a related neutral tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane ligand L³[10a] in corresponding O₂reaction studies using [L³CuÎ(NCCH₃)](BF₄) 1 as a precursor. We also describe the structure and magnetism of the final reaction product [L³CuÎI(µ-OH)₂CuÎIL³](BF₄)₂(2) in reference to the numer- ous magneto-structural correlations reported for complexes with the [Cu(µ-OH)₂Cu]^2⫹core [11⫺13, 14a]. Whereas tris- (pyrazolyl)methane ligands have rarely been used in copper/

dioxygen chemistry [1a, 10, 14], the corresponding product complexes with bis- and tris(2-pyridylmethyl)amine ligands are well established [1a, b, 6c-e, 7c, 13]. We therefore describe EPR and UV-VIS spectroscopic results of low temperature reactions between the copper(I) complexes with both L³ and bis(2-pyridylmethyl)benzylamine with dioxygen.

Results and Discussion

Characterization of [L³Cu^I(NCCH₃)](BF₄) (1) The ligand L³ [10] reacts with [Cu(NCCH₃)₄](BF₄) under argon to yield a very air-sensitive colourless complex [L³Cu^I(NCCH₃)](BF₄) (1).

(3)

Reactivity of Copper(I) Complexes with Tripodal Ligands towards O2

L³⫹[Cu(NCCH3)4](BF4)씮[L³Cu(NCCH3)](BF4)⫹3 CH3CN (1) Other such complexes with substituted tris(pyrazolyl)- borato(1-) or tris(pyrazolyl)methane ligands were found to be more tolerant towards O₂[5, 6, 14]. In addition to the crystallographic characterization (Tabs. 1, 2, Fig. 1) the¹H- NMR spectra support the composition of the compound, with slightly shifted resonances for the coordinated ligands [10a]. However, the C⬅N stretching band could not be detected by IR vibrational spectroscopy, in agreement with previous observations for coordinated nitrile ligands (van- ishing dipole moment change) [15]. Electrochemical oxidation of1in CH₂Cl₂/0.1 M Bu₄NPF₆occurred irreversibly at ⫹0.81 V vs. ferrocenium/ferrocene. This anodically shifted peak potential in comparison to that of analogous tris(pyrazolyl)borato(1-) complexes [5d] reflects the different charge (cationic vs. neutral).

Table 1Crystallographic data of compounds1and2·6 CH2Cl2

1 2·6 CH₂Cl₂

formula C₂₈H₄₀BCuF₄N₄ C₆₂H₈₂B₂Cl₁₂Cu₂F₈N₁₂O₂

mol mass 611.01 1753.5

crystal size /mm 0.1⫻0.1⫻0.1 0.4⫻0.3⫻0.1 crystal system monoclinic triclinic

space group P21/n P1

a/A˚ 13.433(4) 12.202(4)

b/A˚ 14.3089(4) 12.392(6)

c/A˚ 17.115(4) 16.732(5)

α/° 90 86.83(1)

β/° 104.61(2) 69.62(1)

γ/° 90 60.60(1)

V/A˚3 3201(2) 2045.4(13)

Z 4 1

ρcalc/g cm^⫺3 1.585 1.424

hklrange ⫺10 <h<⫹16 ⫺16 <h<⫹9

⫺5 <k<⫹17 ⫺16 <k<⫹14

⫺21 <l<⫹20 ⫺22 <l<⫹20

2θrange /° 3.5⫺52 3.8⫺56

µ(Mo K_α) /cm^⫺1 9.15 9.77

reflection measured 6562 9902

indep. reflections 6298 9902

parameters 388 901

R1 0.0764 0.0684

wR2 0.1998 0.2128

The structure of the molecule1in the crystal (Fig. 1, Tab.

2) reveals an approximately symmetrical coordination of distorted tetrahedral copper(I) by the tripodal ligand L³. The Cu^I-N(acetonitrile) distance is rather short at

Z. Anorg. Allg. Chem.2005,631, 2568⫺2574 zaac.wiley-vch.de _2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2569 Table 2Selected bond lengths/A˚ and angles/°for compound1in the crystal.

Cu1-N1 2.086(5) Cu1-N7 1.865(5)

Cu1-N5 2.098(5) N7-C2 1.134(7)

Cu1-N3 2.072(5) C2-C3 1.462(8)

N1-Cu1-N5 92.7(2) N7-Cu1-N5 119.6(2)

N5-Cu1-N3 85.2(2) N7-Cu1-N3 125.8(2)

N3-Cu1-N1 87.1(2) Cu1-N7-C2 172.6(5)

N7-Cu1-N1 133.0(2) N7-C2-C3 178.5(7)

Figure 1 Molecular structure of the cation in the crystal of [L³Cu(NCCH3)](BF4) (1).

1.865(5) A˚ [14b, 16], reflecting the good access of small molecules to the metal.

Reaction of copper(I) complexes with O2at low temperature

Addition of dry O₂ to a cooled (⫺78°C) solution of1 in CH₂Cl₂resulted in the slow development of an intense purple colour (λmax⫽517 nm). Such purple colours and corresponding absorption maxima between 500 and 600 nm have been observed previously for solution-generated and isolated intermediates from reactions between O₂ and copper(I) model complexes [3⫺6], including a tris(3,5-dimethylpyrazolyl)methane compound [14a], or oxy-hemocyanin [3], they are assigned to ligand-to-metal charge transfer (LMCT) transitions of µ-peroxodicopper(II) species resulting from electron transfer between O₂^2⫺and Cu^II[2, 3].

Intense bands around 340 or 400 nm which are usually found for bis(side-on) coordinated peroxodicopper(II) species [1, 4⫺6, 14a] or for bis(µ-oxo)-bridged dicopper(III) systems [1a], respectively, were not observed; according to established correlations [1a] these data would be more com- patible with an end-on coordination despite the merely tridentate nature of the ligand L³. The related complex with

(4)

tris(3,5-dimethylpyrazolyl)methane showed bands at 518 and 332 nm [14a].

Attempts to reverse the O₂addition reaction by bubbling argon through the system in dilute solutions were unsuc- cessful as evident from the remaining colour, indicating an irreversible O₂ binding process in contrast to the reactivity of the hemocyanin proteins. A copper(II) EPR signal at g_储⫽2.292 (A_储⫽16.0 mT,^63,65Cu with I⫽3/2) and g_Ⲛ⫽ 2.063 was observed in frozen solution, suggesting some de- composition to monomers [17]. Removal of the solvent at

⫺60°C gave a purple residue which rapidly decomposed at

⫺30°C to a green material as reported before for related systems [13, 18, 19]. The irreversibility of O₂ binding and the thermal lability distinguish the compound described here from related tris(pyrazolyl)borato(1-) complexes [4, 5]

or from the analogue with tris(3,5-dimethylpyrazolyl)methane [14a].

The successful use of tris(2-pyridylalkyl)amine ligands in copper/dioxygen chemistry [6c-g, 18] has prompted us to study the solely tridentate but also established [13] ligand bis(2-pyridylmethyl)benzylamine in a similar experiment.

The final dicopper(II) product with bridging hydroxide ligands has been characterized with respect to structure and magnetism [13]. While single crystals of a copper(I) complex with bis(2-pyridylmethyl)benzylamine could not be obtained, such solutions obtained from the ligand and [Cu(NCCH₃)₄](BF₄) in CH₂Cl₂reacted rapidly with dry O₂ even at ⫺90°C without showing typical peroxo-to-copper(II) LMCT transitions. The only absorption observed was that of the final bis(hydroxo)bridged product with a ligand-field band at 610 nm (see below). We attribute this high reactivity to a lack of steric shielding or electronic sta- bilization by the bis(2-pyridylmethyl)benzylamine ligand.

However, EPR experiments showed the presence of a new signal at low temperatures (Fig. 2) different from the famil- iar trace spectra resulting from small amounts of mononuclear Cu^II.

Figure 2X band EPR spectrum obtained from the reaction of a bis(2-pyridylmethyl)benzylamine-copper(I) solution in CH₂Cl₂ with O₂at⫺90°C.

This EPR signal has a diminishedg anisotropy ofg_储⫽ 2.17 and g_Ⲛ ⫽ 2.03 without detectable ^63,65Cu hyperfine splitting, suggesting considerable localization of spin on the ligand rather than on the metal(s) [2, 20, 21]. Tentatively one could attribute the signal to a metastable species formu- lated as either a superoxo complex [19] of dicopper(I) (see (6b)) or a valence-isomeric dicopper(I,II) mixed-valent

complex ofµ-peroxide (see (6c)), as suggested by a 1:3 stoi- chiometry between O₂ and Cu^I compound. Although superoxide [7c] may be considered too oxidizing for copper(I) as evident from the function of Cu,Zn-superoxide dismutase enzymes [22] we have shown that the the formally related anion radicals of azo ligands (RN)₂(RNZO) can accommodate copper(I) in stable and even structurally characterized dicopper(I)-radical complexes [23]. While the g anisotropies are smaller in these cases as evident from high-field EPR studies [24] they confirm the small metal hyperfine splitting < 2 mT in such a situation.

Formation, structure and magnetism of [L³Cu^II(µ- OH)₂Cu^IIL³](BF₄)₂(2)

Reaction of the precursor1 with air at room temperature yields a dinuclear compound [L³Cu(µ-OH)₂CuL³](BF₄)₂ (2).

4 [L³Cu(NCCH₃)](BF₄)⫹O₂⫹2 H₂O씮

2 [L³Cu(µ-OH)₂CuL³](BF₄)₂⫹4 CH₃CN (2) Such species were frequently encountered as final products from the reaction of copper(I) complexes with dioxygen [13, 14a, 18, 19]. The presence of hydroxo ligands is evident from a relatively narrow IR vibrational band at 3675 cm^⫺1 , the tris(3,5-dimethylpyrazolyl)methane analogue [14a] shows 3650 cm^⫺1. Furthermore, the absorption maxima at 663 nm (ε ⫽ 130 M^⫺1 cm^⫺1, ligand-field tran- sition) and 326 nm (ε ⫽ 3700 M^⫺1 cm^⫺1) agree with reported data for corresponding tris(3,5-dimethylpyrazolyl)methane (641 and 325 nm [14a]), tris(pyrazolyl)borato(1-) [5a] and other such systems [1a, 18], the corresponding complex with di(2-pyridylmethyl)benzylamine has λ_max⫽610 nm. The broadening of the¹H-NMR spectrum and the observation of a copper(II)-type EPR signal with g_储 ⫽ 2.298 (A_储 ⫽ 14.2 mT) and g_Ⲛ ⫽ 2.055 suggests dissociation (3) of the EPR-silent solid (4 K) in solution to yield [L³Cu(OH)(BF₄)] with unknown metal coordination.

Quantitative EPR spectroscopy experiments according to AasaandVanngard[25] yielded about 40 % presence of the monomer in frozen dichloromethane solution.

[L³CuÎI(µ-OH)2CuÎIL³](BF4)2v2 [L³CuÎI(OH)(BF4)] (3) The dissociation of such bis-hydroxo bridged dimers into mononuclear copper(II) complexes explains the ready con- version betweensynandantiisomers (cf. below).

Single crystals of the dinuclear compound were obtained as the hexakis(dichloromethane) solvate 2·6 CH₂Cl₂, whereas the dried powder analyzed as the bis(dichloromethane) solvate. The large number of partially labile solvent molecules in the crystal is held responsible for the relatively poor structure quality (Tabs. 1,3) which, however, reveals sufficient information to fit the compound into the vast number of bis(µ-hydroxo)dicopper(II) complexes [11⫺14, 19].

(5)

The molecular structure of2 in the hexakis(dichloromethane) solvate crystal (Tab. 3, Fig. 3) shows an essentially planar [Cu₂O₂] core with the metals in square pyramidal (sqp) configuration. Theantiarrangement (Fig. 4) [14a] in contrast to possiblesynstructures [5a, 13] is observed here which also causes relatively [11⫺13] wide Cu-O-Cu angles at 103.5° and rather long Cu-O and Cu⫺Cu distances at about 1.94 A˚ and at 3.055(1) A˚, respectively.

Table 3Selected bond lengths/A˚ and angles/°for compound2in the crystal of2·6 CH2Cl2.

Cu1-O1 1.947(7) Cu2-N21 2.017(7)

Cu1-O2 1.944(8) Cu2-N31 2.021(9)

Cu2-O1 1.942(8) Cu1-N41 2.022(8)

Cu2-O2 1.949(8) Cu1-N51 2.029(11)

Cu1⫺Cu2 3.0554(12) Cu1-N61 2.339(10)

Cu2-N11 2.356(9)

O1-Cu1-O2 76.5(3) N41-Cu1-N21 119.6(2)

O1-Cu2-O2 76.5(3) O2-Cu2-N11 104.9(3)

O1-Cu1-N41 170.8(4) O2-Cu2-N21 96.5(3)

O1-Cu1-N51 96.2(3) O2-Cu2-N31 170.0(4)

O1-Cu1-N61 104.0(3) N41-Cu1-N61 84.8(4)

O2-Cu1-N41 99.4(4) N41-Cu1-N51 86.5(4)

O2-Cu1-N51 169.0(4) N51-Cu1-N61 86.6(4)

O2-Cu1-N61 103.1(4) N21-Cu2-N11 87.7(3)

O1-Cu2-N11 103.0(4) N21-Cu2-N31 86.6(4)

O1-Cu2-N21 168.4(3) N31-Cu2-N11 84.7(3)

O1-Cu2-N31 98.8(3) Cu1-O2-Cu2 103.4(4)

N41-Cu1-N11 133.0(2) Cu1-O1-Cu2 103.6(3)

Figure 3 Molecular structure of the dication in the crystal of [L³Cu(µ-OH)₂CuL³](BF₄)₂· 6 CH₂Cl₂.

Theτvalue typically used to assess the square-pyramidal or trigonal-bipyramidal characteristics of five-coordinated arrangements [26] is very low for2 atτ ⫽0.03, indicating essential square-pyramidal configuration.

τ⫽(β-α)/60

β,α: basal angles;τ⫽0 for square-pyramidal,τ⫽1 for trigonal- bipyramidal configuration

Since the hydroxo oxygen atoms participate to make up the basal plane in2 (Fig. 4), the tripodal ligand L³ has to coordinate in a strongly unsymmetrical 2⫹1 fashion with two rather short Cu-N bonds at about 2.02 A˚ and one long

Z. Anorg. Allg. Chem.2005,631, 2568⫺2574 zaac.wiley-vch.de _2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2571 Figure 4 Structure of the metal ion coordination spheres in the cation of [L³Cu(µ-OH)2CuL³](BF4)2· 6 CH2Cl2.

bond at ca. 2.35 A˚ as a result of the Jahn-Teller distortion.

Similar effects have been noted previously with unsymmetrical [13] or symmetrical [14a, 27] tripodal ligands.

In comparison to analogous compounds [LCu(µ- OH)₂CuL] such as 3 [14a], 4 [4a] and5 [5a] with tris(3,5- dimethylpyrazolyl)methane (3), tris(3,5-dimethylpyrazolyl)borato(1-) (4) and tris(3,5-diisopropylpyrazolyl)borato(1-) ligands L (5), respectively, the latter exhibits a syn configuration in the crystallized form isolated. The bonds to the apical nitrogen atoms are shorter in3(2.270(6) A˚ ) or 4(2.277(7) A˚ ) than in2at 2.356(9) A˚ or in5at 2.371(6) A˚ . On the other hand, the Cu⫺Cu distances are more similar for2 at 3.055(1) A˚ and4 (3.059(2) A˚ [4a]) than in 3 with 3.021(2) A˚ [14a] or in5 with 2.937(2) A˚ [5a]. This effect is related to similar angles Cu-O-Cu (ca. 103.4°⫺104.0°) and O-Cu-O (76.0°⫺76.5°) in theanti-configurated compounds 2 and 4 whereas syn-configurated 5 is distinguished by wider angles O-Cu-O at 79.2° and reduced angles Cu-O- Cu at 100.8° [5a]. The tris(3,5-dimethylpyrazolyl)methane analogue 3 [14a] adopts an intermediate position with 77.7(2)°and 102.4(2)°.

The dissociation (3) of bis-hydroxo bridged dimers into mononuclear copper(II) complexes explains the ready con- version between syn and anti isomers and their isolation depending on the solvent of crystallization, as was observed for compounds with di(2-pyridylmethyl)benzylamine [13].

The structural characteristics of2are reflected by the results from a temperature dependence analysis of the magnetic susceptibility (Fig. 5).

The classical analysis [11⫺14] using the Bleaney-Bowers equation [28] (Fig. 5) yields a very good fit with the exchange coupling parameter J ⫽ ⫺633 cm^⫺1, signifying strong antiferromagnetic coupling in agreement with the absence of EPR signals for the solid even at 4 K. This result is in agreement with reports [13] from related systems and with general guidelines as given in a paper byRuiz et al.

[12a]; the wide Cu-O-Cu angle, the rather long Cu-O bonds and the largely undistorted [Cu₂O₂] core of 2 all favour

(6)

Figure 5 χT vs T plot for [L³Cu(µ-OH)2CuL³](BF4)2· 6 CH2Cl2: experimental data (o) and best fit (see text for details).

strong antiparallel spin-spin coupling. While the O-H vec- tors could not be determined in the solvate structure due to the insufficient crystal quality, it may be assumed due to steric requirements that they do not point significantly out of plane, which would also favor very negative J values [12a], relative to what might be estimated applying tra- ditional correlations betweenJ and the Cu-O-Cu angle or the Cu⫺Cu distance [11, 14a].

Summary and conclusion

The use of the tripodal ligand L³for copper complexes in- volved in the reactivity towards dioxygen has led to the isolation and structural characterization of a copper(I) precursor [L³Cu(NCCH₃)](BF₄) and of a dinuclear copper(II) end product [L³Cu(µ-OH)₂CuL³](BF₄)₂ with strong antiferromagnetic exchange coupling. A purple compound was observed as the intermediate product of an irreversible reaction between the precursor and dry O₂at⫺78°C in CH₂Cl₂ however, it could not be characterized in detail because of its high lability. With the less sterically protecting bis(2-pyridylmethyl)benzylamine tripodal ligand such an intermediate could not even be detected at ⫺80°C, however, a new kind of EPR signal was observed which was tentatively at- tributed to a superoxo complex of copper(I) (6b). Combin- ing these observations, including the formation of dihydroxo-bridged final products such as 2 (7), with other literature results [1a, 7a, 18, 29, 30] leads to reaction schemes (4)-(7) for different O₂/Cu ratios:

O2⫹1 LCuÎ ———씮LCuÎI(µ-O2앫⫺) [29] (4) O2⫹2 LCuÎ ———씮(LCuÎI)2(µ-O22⫺) (5a) or

O₂⫹2 LCuÎ ———씮(LCuÎII)₂(µ-O^2⫺)₂[1a, 30] (5b) O2⫹3 LCuÎ ———씮(LCuÎI)2(LCuÎII)(µ3-O^2⫺)2[7a, 30] (6a) or

O2⫹3 LCuÎ ———씮(LCuÎ)2(µ-O2앫⫺)⫹LCuÎI (6b) or

O2⫹3 LCu^I ———씮(LCu^1.5)2(µ-O22⫺)⫹LCu^II (6c)

O2⫹4 LCu^I ———씮2 (LCu^II)2(µ-O^2⫺) [18] (7a) or

⫹2 H^⫹

O2⫹4 LCuÎ ———씮(LCuÎI)2(µ-OH^⫺)2⫹2 LCuÎI (7b) or

⫹2 H2O

O₂⫹4 LCu^I ———씮2 (LCu^II)₂(µ-OH^⫺)₂[18] (7c) The lack of charge in both bis(2-pyridylmethyl)benzylamine and tris(pyrazolyl)methane as compared to tris(pyrazolyl)borato ligands is held responsible for this increased lability. In addition, the presence of benzylic/allylic functions in L³ or (2-pyridylmethyl)amine ligands may render these systems susceptible towards CH activation reactions because of possible self-attack.Nevertheless, this study has once again [1a, 4⫺7] revealed that several factors determine the binding and activation of O₂ by copper compounds.

New ligands will thus be tested in our laboratories in the directions of stabilizing intermediates or enhancing oxy- genating reactivity.

Experimental Section Instrumentation

EPR spectra were recorded in the X band on a Bruker System ESP 300 equipped with a Bruker ER035M gaussmeter and a HP 5350B microwave counter.¹H-NMR spectra were taken on a Bruker AC 250 spectrometer, infrared spectra were obtained using Perkin Elmer 684 and 283 instruments. UV/Vis/NIR absorption spectra were recorded on Shimadzu UV160 and Bruins Instruments Omega 10 spectrophotometers. Cyclic voltammetry was carried out at 100 mV/s scan rate in dichloromethane/0.1 M Bu4NPF6using a three-electrode configuration (glassy carbon electrode, Pt counter electrode, Ag/AgCl reference) and a PAR 273 potentiostat and function generator. The ferrocene/ferrocenium couple served as in- ternal reference. Susceptibility measurements were carried out using previously described set-up and methodology [31]. The fitting was performed using the standard Bleaney-Bowers approach [28]

for dicopper(II) complexes [11⫺14].

Syntheses

The ligand L³ ⫽ tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane was obtained as described [10]. Products isolated from the reaction of bis(2-pyridylmethyl)benzylamine with copper precur- sors were reported [13].

[L³Cu(NCCH3)](BF4) (1)

A solution containing 102 mg (0.326 mmol) [Cu(NCCH₃)₄](BF₄) [32] in 30 ml CH₃OH was slowly added dropwise under argon to a solution of 150 mg (0.326 mmol) L³in 15 ml methanol. Following reflux for 2 hours the solvent was removed and the residue recrys- tallized from methanol. After two days at ⫺15°C the colourless product was obtained in 58 % yield (123 mg). Mp 198°C. Calc. for C₃₀H₄₃BCuF₄N₇(652.1) C, 55.26; H, 6.65; N, 15.04 %. Found. C, 55.11; H, 6.76; N, 14.89 %.

1H-NMR(CDCl₃):δ⫽1.20 (d,J⫽7 Hz, 18 H, CH(CH₃)₂), 2.35 (s, 3H, CH3CN), 2.55 (m, 12H, CH2), 2.96 (m, 6H, CH2, 3H, CH(CH3)2), 7.81 (s,

(7)

1H, CH).IR(CH₂Cl₂):ν˜⫽2964 w, 2926 w, 2869 w, 2360 w, 1564 w, 1506 w, 1484 s, 1398 w, 1385 w, 1364 w, 1336 w, 1314 w, 1062 br, 1037 vs, 928 w, 859 w, 850 w, 824 w cm^⫺1.

Low-temperature reactions with O₂

Dry O2was slowly added to a cooled (⫺78°C) solution of 100 mg (0.153 mmol)1 in 40 ml dichloromethane. The initially colourless solution turned pink and then dark purple within 30 minutes (λmax⫽517 nm). Attempts to reverse the reaction by bubbling argon through the system in more dilute solutions were not successful, as evident from the remaining colour. Removal of the solvent at ⫺60°C gave a purple residue which rapidly decomposed at

⫺30°C to a green material. The thermal lability thus precluded further characterization.

In an analogous experiment, dry O2was slowly added to a cooled (⫺90°C) solution of a 1:1 molar mixture of [Cu(NCCH3)4](BF4) [32] and bis(2-pyridylmethyl)benzylamine [13]. Before the initially yellowish solution turned blue at higher temperatures an axial EPR signal withg_储⫽2.17 andg_Ⲛ⫽2.03 was observed.

[L³Cu(µ-OH)2CuL³](BF4)2·2 CH2Cl2(2·2 CH₂Cl₂)

A solution containing 100 mg (0.153 mmol)1in 20 ml CH2Cl2was stirred under open atmosphere for 5 minutes. After two days in air dark blue single crystals had formed which were collected and then carefully dried (42 mg, 22 %). Calc. for C56H82B2Cu2F8N12O2·2 CH2Cl2(1425.93): C, 48.85; H, 6.08; N, 11.79 %. Found: C, 48.67;

H, 6.08; N, 11.66 %.

UV/Vis (CH₂Cl₂):λ_max (ε)⫽ 326 (3700), 663 (130) nm (M^⫺1 cm^⫺1).IR (KBr):ν˜⫽3674 w, 2966 vs, 2867 s, 1564 w, 1485 vs, 1451 s, 1400 s, 1312 w, 1250 vs, 1060 vs, 924 s, 862 s, 736 s, 520 w, 487 w cm^⫺1.

Crystallography

Single crystals were obtained from methanol at ⫺15°C (1) and from the reaction mixture (2). The latter crystallized with six dichloromethane molecules per formula unit. The X-ray data were collected at 188(2) K on a Siemens P4 diffractometer, using graph- ite monochromated Mo-Kα radiation (λ ⫽0.71073 A˚ ) and employing Wyckoff scans. Further details are given in Table 1. All structures were solved by the Patterson method using the SHELXTL package while refinement was carried out with SHELXL97 employing full-matrix least-squares methods on F² with F₀² ⱖ ⫺2σ(F02) [33]. All non-hydrogen atoms were refined anisotropically, hydrogen atoms were introduced using appropriate riding models.

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, CCDC-262408 and 262409. Copies of the information may be obtained free of charge from: The Direc- tor, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:

(⫹44)1223-336-033; e-mail: [email protected]; www: http://

www.ccdc.cam.ac.uk).

This work was supported by the Deutsche Forschungsgemeinschaft and FONDECYT (projects No. 1020101 and 7020101). We also thank Dr. W. Schwarz for crystallographic data collection.

References

[1] a) L. M. Mirica, X. Ottenwaelder, T. D. P. Stack,Chem. Rev.

2004, 104, 1013; b) S. Schindler, Eur. J. Inorg. Chem.2000,

Z. Anorg. Allg. Chem.2005,631, 2568⫺2574 zaac.wiley-vch.de _2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2573 2311; c) J. P. Klinman,Chem. Rev.1996,96, 2541; d) P. Gamez, I. A. Koval, J. Reedijk,Dalton Trans.2004, 4079; e) C. Gerde- mann, C. Eicken, B. Krebs,Acc. Chem. Res.2002,35, 183; f) T. Osako, K. Ohkubo, M. Taki, Y. Tachi, S. Fukuzumi, S.

Itoh,J. Am. Chem. Soc.2003,125, 11027; g) S. T. Prigge, B.

A. Eipper, R. E. Mains, L. M. Amzel,Science2004,304, 864.

[2] a) J. E. Pate, R. W. Cruse, K. D. Karlin, E. I. Solomon, J.

Am. Chem. Soc.1987, 109, 2624; b) N. C. Eickman, R. S.

Himmelwright, E. I. Solomon,Proc. Natl. Acad. Sci. U.S.A.

1979,76, 2094; c) E. I. Solomon, U. M. Sundaram, T. E. Ma- chonkin, Chem. Rev. 1996, 96, 2563; d) E. I. Solomon, P.

Chen, M. Metz, S.-K. Lee, A. E. Palmer,Angew. Chem.2001, 113, 4702;Angew. Chem. Int. Ed.2001,40, 4570.

[3] a) N. Kitajima,Adv. Inorg. Chem.1992,39, 1; b) N. Kitajima, Y. Moro-oka,Chem. Rev.1993,94, 737; c) H. Decker, F. Tuc- zek,TIBS2000,25, 392.

[4] a) N. Kitajima, T. Koda, S. Hashimoto, T. Kitagawa, Y. Moro- oka,J. Am. Chem. Soc.1991,113, 5664; b) N. Kitajima, K.

Fujisawa, Y. Moro-oka, K. Toriumi,J. Am. Chem. Soc.1989, 111, 8975; c) N. Kitajima, Y. Moro-oka,Chem. Rev.1994,94, 737; d) M. Suzuki, H. Furutachi, H. Otawa, Coord. Chem.

Rev.2000,200⫺202, 105.

[5] a) N. Kitajima, K. Fujisawa, C. Fujimoto, Y. Moro-oka, S.

Hashimoto, T. Kitagawa, K. Toriumi, K. Tatsumi, A. Naka- mura,J. Am. Chem. Soc.1992,114, 1277; b) Y. Moro-oka, K.

Fujisawa, N. Kitajima,Pure Appl. Chem.1995,67, 241; c) S.

M. Carrier, C. E. Ruggiero, R. P. Houser, W. B. Tolman,Inorg.

Chem.1993,32, 4889.

[6] a) N. Kitajima, T. Koda, Y. Moro-oka,Chem. Lett.1988, 347;

b) N. Kitajima, T. Koda, S. Hashimoto, T. Kitagawa, Y. Moro- oka, J. Chem. Soc., Chem. Commun. 1988, 151; c) C. X.

Zhang, S. Kaderli, M. Costas, E. Kim, Y. Neuhold, K. D.

Karlin, A. D. Zuberbühler,Inorg. Chem.2003,42, 1807; d) W.

B. Tolman,Acc. Chem. Res.1997,30, 227; e) C. X. Zhang, H.

Liang, E.-L. Kim, J. Shearer, M.-E. Helton, E. Kim, S. Kad- erli, C. D. Incarvito, A. D. Zuberbühler, A. L. Rheingold, K.

D. Karlin,J. Am. Chem. Soc.2003,125, 634; f) Z. Hu, R. D.

Williams, D. Tran, T. G. Spiro, S. M. Gorun,J. Am. Chem.

Soc.2000,122, 3550; g) K. D. Karlin, S. Kaderli, A. D. Zub- erbühler,Acc. Chem. Res.1997,30, 139.

[7] a) M. Taki, S. Teremae, S. Nagatomo, Y. Tachi, T. Kitagawa, S. Itoh, S. Fukuzumi,J. Am. Chem. Soc.2002,124, 6367; b) Y. Tachi, K. Aita, S. Teramae, F. Tani, Y. Naruta, S. Fuku- zumi, S. Itoh,Inorg. Chem.2004, 43, 4558; c) M. Schatz, V.

Raab, S. P. Foxon, G. Brehm, S. Schneider, M. Reiher, M. C.

Holthausen, J. Sundermeyer, S. Schindler,Angew. Chem.2004, 116, 4460;Angew. Chem. Int. Ed.2004,43, 4360; d) H. Börzel, P. Comba, K. S. Hagen, C. Katsichtis, H. Pritzkow,Chem. Eur.

J.2000,6, 914.

[8] a) K. D. Karlin, A. D. Zuberbühler, inBioinorganic Catalysis, Second Edition, Revised and Expanded, J. Reedijk, E. Bouw- man, Eds., Marcel Dekker, New York, 1999, p 469; b) M. A.

Kopf, K. D. Karlin, in Biomimetic Oxidations, B. Meunier, Ed., Imperial College Press, London, 2000, Chapter 7, p 309.

[9] a) B. Schönecker, T. Zheldakova, Y. Liu, M. Kötteritzsch, W.

Günther, H. Görls, Angew. Chem. 2003, 115, 3361; Angew.

Chem. Int. Ed.2003,42, 3240; b) G. Battaini, E. Monzani, A.

Perotti, C. Para, L. Casella, L. Santagostini, M. Gullotti, R.

Dillinger, C. Näther, F. Tuczek,J. Am. Chem. Soc.2003,125, 4185.

[10] For tris(pyrazolyl)methane ligands see a) C. Titze, J. Herm- ann, H. Vahrenkamp, Chem. Ber. 1995, 1095; b) S. Trofi- menko,J. Am. Chem. Soc.1970,92, 5118; c) T. Astley, M. J.

(8)

Gulbis, A. Hitchmann, E. R. T. Tiekink,J. Chem. Soc., Dalton Trans.1993, 509; d) D. L. Reger. J. E. Collins, S. M. Myers, A. L. Rheingold, L. M. Liable-Sands,Inorg. Chem.1996,35, 4904.

[11] a) W. H. Crawford, H. W. Richardson, J. R. Wasson, D. J.

Hodgson, W. E. Hatfield,Inorg. Chem.1976, 15, 2107; b) L.

K. Thompson, S. K. Mandal, S. J. Tandon, J. N. Budson, M.K. Park,Inorg. Chem.1996,35, 3117; c) O. Kahn,Molecu- lar Magnetism, VCH, Weinheim, 1993.

[12] a) E. Ruiz, P. Alemany, S. Alvarez, J. Cano, Inorg. Chem., 1997,36, 3683; b) E. Ruiz, P. Alemany, S. Alvarez, J. Cano,J.

Am. Chem. Soc.1997,119, 1297.

[13] D. Rojas, A. M. Garcia, A. Vega, Y. Moreno, D. Venegas- Yazigi, M. T. Garland, J. Manzur,Inorg. Chem.2004,43, 6324.

[14] a) M. Cvetkovic, S. R. Batten, B. Moubaraki, K. S. Murray, L. Spiccia,Inorg. Chim. Acta2001,324, 131; b) D. L. Reger, J.

E. Collins, A. Rheingold, L. M. Laible-Sands,Organometallics 1996,15, 2029.

[15] a) Z. Tyeklar, R. R. Jacobson, N. Wie, N. N. Murthy, J. Zubi- eta, K. D. Karlin,J. Am. Chem. Soc.1993,115, 2677; b) K.

D. Karlin, M. S. Haka, R. W. Cruse, G. J. Meyer, A. Farooq, Y. Gultneh, C. Hayes, J. Zubieta,J. Am. Chem. Soc.1988,110, 1196; c) B. N. Storhoff, H. C. Lewis,Coord. Chem. Rev.1977, 23, 1; d) J. Weidlein, U. Müller, K. Dehnicke, Schwingungs- spektroskopie, Thieme, Stuttgart, 1982.

[16] a) W. E. Lynch, D. M. Kurtz, Jr., S. Wang, R. A. Scott,J. Am.

Chem. Soc.1994,116, 11030; b) S. Schindler, D. J. Szalda, C.

Creutz,Inorg. Chem.1992,31, 2255.

[17] E. Asato, S. Hashimoto, N. Matsumoto, S. Kida, J. Chem.

Soc., Dalton Trans.1990, 1741.

[18] I. Sanyal, M. Nahroof-Tahir, M. S. Nasir, P. Ghosh, B. I. Co- hen, Y. Gultneh, R. W. Cruse, A. Farooq, K. D. Karlin, S.

Liu, J. Zubieta,Inorg. Chem.1999,31, 4322.

[19] A. N. Vedernikov, M. Pink, K. G. Caulton,Inorg. Chem.2004, 43, 4300.

[20] W. Kaim,Coord. Chem. Rev.1987,76, 187.

[21] W. Kaim,Dalton Trans.2003, 761.

[22] P. J. Hart, M. M. Balbirnie, N. L. Ogihara, A. M. Nersissian, M. S. Weiss, J. S. Valentine, D. Eisenberg,Biochemistry1999, 38, 2167.

[23] a) M. Moscherosch, J. S. Field, W. Kaim, S. Kohlmann, M.

Krejcik,J. Chem. Soc., Dalton Trans.1993, 211; b) N. Doslik, T. Sixt, W. Kaim, Angew. Chem. 1998, 110, 2521; Angew.

Chem. Int. Ed.1998,37, 2403.

[24] A.-L. Barra, L.-C. Brunel, F. Baumann, M. Schwach, M. Mo- scherosch, W. Kaim,J. Chem. Soc., Dalton Trans.1999, 3855.

[25] R. Aasa, T. Vanngard,J. Magn. Res.1975,19, 308.

[26] a) A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn, G. C.

Verschoor, J. Chem. Soc., Dalton Trans. 1984, 1349; b) A.

Schmuck, K. Seppelt,Chem. Ber.1989,122, 803.

[27] J. Rall, M. Wanner, M. Albrecht, F. M. Hornung, W. Kaim, Chem. Eur. J.1999,5, 2802.

[28] B. Bleaney, K. D. Bowers,Proc. R. Soc. London1952,A214, 451.

[29] K. Fujisawa, M. Tanaka, Y. Moro-oka, N. Kitajima,J. Am.

Chem. Soc.1994,116, 12079.

[30] a) A. P. Cole, D. E. Root, P. Mukherjee, E. I. Solomon, T. D.

P. Stack,Science1996,273, 1848. b) L. M. Mirica, M. Vance, D. J. Rudd, B. Hedman, K. O. Hodgson, E. I. Solomon, T. D.

P. Stack,Science2005,308, 1890.

[31] E. Waldhör, W. Kaim, M. Lawson, J. Jordanov,Inorg. Chem.

1997,36, 3248.

[32] H. J. Gyslins, G. J. Kubas,Inorg. Synth., Vol. 19, Wiley, New York, 1979, S. 92.

[33] a) G. M. Sheldrick,SHELXTL, Bruker Analytical X-Ray Sys- tems, Madison, Wisconsin, USA,1998; b) G. M. Sheldrick, SHELXL-97: A Program for Crystal Structure Determination (Ed.: Universität Göttingen), Göttingen, Germany,1997.