Reductive O2 Binding at a Dihydride Complex Leading to Redox

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Reductive O2 Binding at a Dihydride Complex Leading to Redox Interconvertible μ‑1,2-Peroxo and μ‑1,2-Superoxo Dinickel(II) Intermediates Peng-Cheng Duan,† Dennis-Helmut Manz,† Sebastian Dechert,† Serhiy Demeshko,† and Franc Meyer*,†,‡ †

Institut für Anorganische Chemie and ‡International Center for Advanced Studies of Energy Conversion (ICASEC), Universität Göttingen, Tammannstr. 4, D-37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Dioxygen activation at nickel complexes is much less studied than for the biologically more relevant iron or copper systems but promises new reactivity patterns because of the distinct coordination chemistry of nickel. Here we report that a pyrazolate-based dinickel(II) dihydride complex [KL(Ni−H)2] (1a) smoothly reacts with O2 via reductive H2 elimination to give the μ-1,2-peroxo dinickel(II) complex [KLNi2(O2)] (2a) and, after treatment with dibenzo[18]-crown-6, the separated ion pair [K(DB18C6)][LNi2(O2)] (2b); these are the first μ-1,2-peroxo dinickel intermediates to be characterized by X-ray diffraction. In 2a, the K+ is found side-on associated with the peroxo unit, revealing a pronounced weaking of the O−O bond: d(O−O) = 1.482(2) Å in 2a versus 1.465(2) in 2b; ν̃(O− O) = 720 cm−1 in 2a versus 755 cm−1 in 2b. Reaction of 1a (or 2a/2b) with an excess of O2 cleanly leads to [LNi2(O2)] (3), which was shown by X-ray crystallography (d(O−O) = 1.326(2) Å), electron paramagnetic resonance and Raman spectroscopy (ν̃(O−O) = 1007 cm−1), magnetic measurements, and density functional theory calculations to feature two low-spin d8 nickel(II) ions and a genuine μ-1,2-superoxo ligand with the unpaired electron in the out-of-plane π*O−O orbital. These μ-1,2-superoxo and μ-1,2-peroxo species, all containing the O2-derived unit within the cleft of the dinickel(II) core, can be reversibly interconverted chemically and also electrochemically at very low potential (E1/2 = −1.22 V vs Fc/Fc+). Initial reactivity studies indicate that protonation of 2a, or reaction of 3 with TEMPO-H, ultimately gives the μ-hydroxo dinickel(II) complex [LNi2(μ−OH)] (4). This work provides an entire new series of closely related and unusually rugged Ni2/O2 intermediates, avoiding the use of unstable nickel(I) precursors but storing the redox equivalents for reductive O2-binding in nickel(II) hydride bonds.

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INTRODUCTION Transition metal mediated activation of dioxygen is important for the development of benign oxidation and oxygenation catalysts, and identification of the metal superoxo (O2−), peroxo (O 2 2− ), hydroperoxo (HO 2 −), and oxo (O 2− ) intermediates is considered crucial for mechanistically understanding those reactions and for steering catalytic transformations.1 Much work in the field has focused on Fe and Cu complexes, owing to the prevalence of these metals in the active sites of numerous metalloproteins that perform challenging hydrocarbon oxidations and oxygenations.2,3 Aerobic nickel-based oxidation chemistry in nature is limited to few enzymes such as nickel superoxide dismutase (Ni-SOD) and quercetine-2,4-dioxygenase (quercetinase),4 and Ni/O2 chemistry is generally less developed than for most other 3d transition metals. However, it is currently receiving increased attention because the distinct coordination chemistry of nickel, including its reluctance to generate high-valent metal oxo species, promises O2 activation trajectories that may differ from © XXXX American Chemical Society

those of more established mono- and dinuclear Mn, Fe, or Cu systems.5−7 Following pioneering work by Riordan and Brunold,8,9 the number of spectroscopically identified Nix/O2 intermediates is now growing rapidly,10−18 and evidence for the high reactivity of high-valent nickel−oxygen adducts is emerging.19 However, very few of these intermediates have so far been isolated and structurally characterized. Prominent examples include the sideon superoxo nickel(II) complex A12 and its reduced peroxo nickel(II) congener B,13 both stabilized by a bulky βdiketiminato ligand (Figure 1), and some complexes with tetraazamacrocyclic ligands, which were formulated as side-on peroxo nickel(III) species comprising [(12-tmc)NiIII(O2)]+ (C; 12-tmc = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane),14a [(13-tmc)NiIII(O2)]+,14b and a related complex with a pyridinophane ligand scaffold.17 In the latter cases using {N4} Received: February 6, 2018

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DOI: 10.1021/jacs.8b01468 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Scheme 1. Dinickel Dihydride Complex 1a and Proposed Reductive Small Molecule (XY) Activation with Concomitant Intramolecular H2 Elimination

of nitrogenase,23 and it represents a means of avoiding the prior formation of intermediates with highly reduced metal ions. Treating [KL(Ni-D)2] (1a-D) with phenylacetylene indeed led to rapid D2 evolution and incorporation of a twice reduced styrene-1,2-diyl unit bridging the two nickel(II) ions of the {LNi 2} core.22 We now demonstrate that dinickel(II) complexes 1a and 1b can be used for the reductive activation of O2, and we present the full characterization of novel μ-1,2superoxo and μ-1,2-peroxo intermediates including their redox interconversion and interaction with K+.

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Figure 1. Prominent examples of the few Ni/O2 intermediates that have been structurally characterized by X-ray diffraction (18C6 is [18]crown-6).12−14,17

RESULTS AND DISCUSSION Peroxo Dinickel(II) Complexes. Reacting a solution of 1a in THF with one equivalent of dry dioxygen at room temperature leads to an immediate color change from orange to red. 2H NMR spectroscopic monitoring of a reaction starting from the dideuteride isotopologue of 1a, namely 1a-D, reveals the disappearance of the resonance at −24.12 ppm characteristic for the Ni-D units and clean formation of D2 (Figure 2), in

macrocycles, the formation of either end-on superoxo nickel(II) or side-on peroxo nickel(III) species appears to delicately depend on the {N4} ligand ring size.14 Structural characterization of dinuclear Ni2/O2 intermediates is so far limited to the bis(superoxo) bridged high-spin nickel(II) complex D (Figure 1) and its corresponding bis(μ-oxo) dinickel(III) analogue.10 Since the metal-mediated activation of dioxygen usually requires the use of reduced metal ions, Ni/O2 chemistry is particularly challenging because accessing nickel(I) is often difficult,5,20 in contrast to iron(II) or copper(I) that offer convenient entries into reductive O2 binding. On the other hand, nickel(II) complexes are usually unreactive toward dioxygen and require the use of partially reduced forms of O2 (such as H2O2) or specific ligands that provide proper redox equivalents. In 2017, the latter strategy has been elegantly demonstrated for a β-diketiminato nickel(II) complex in which the metal ion serves as a Lewis acid to give a nickel(II) organoperoxide via electron shift from the redox-active coligand to O2,18 akin to the proposed O2 activation mechanism in quercetinase.21 We recently appended two β-diketiminate compartments to a central pyrazolate bridge and reported a set of unprecedented dinickel(II) dihydride complexes of that binucleating ligand scaffold.22 These complexes, including [KL(Ni−H)2] (1a, Scheme 1) and [K(DB18C6)][L(Ni−H)2] (1b; DB18C6 = dibenzo[18]crown-6), feature two terminal hydrides directed into a bimetallic cleft, and they were shown to smoothly undergo H2/D2 exchange without H/D scrambling. Mechanistic and density functional theory (DFT) studies indicated low barriers for intramolecular H2 elimination (followed by reductive D 2 binding during dihydride to dideuteride exchange), suggesting that 1a and 1b might be described as masked dinickel(I) synthons capable of reductively binding a variety of small molecules within the bimetallic pocket. Storing redox equivalents in two metal hydrides to couple H 2 elimination with substrate binding is reminiscent of the reductive N2 binding scenario proposed for the FeMo cofactor

Figure 2. 2H NMR spectrum (77 MHz) of a solution of 1a-D in THF (bottom) and after addition of O2 (top); solvent signals are marked with an asterisk.

accordance with the anticipated scenario sketched in Scheme 1. Electrospray ionization (ESI) mass spectrometry of the reaction mixture shows a dominant signal at m/z = 793.3 amu characteristic for the ion [KLNi2O2+H]+, which is shifted to 797.3 amu when using 18O2 (Figures S8 and S9). Single crystals of the product could be obtained by layering a THF reaction mixture with pentane and slow diffusion at −30 °C over several days. X-ray crystallography showed the crystals to contain both the μ-1,2-peroxo complex [KLNi2(O2)] (2a) and the μ-hydroxo complex [LNi2(OH)] (4) in 1:1 ratio; 4 is the degradation product of 2a and is obtained almost quantitatively if solutions of 2a are left at room temperature for prolonged times. The molecular structure of 2a is shown in Figure 3 (the structure of 4 is shown in the Supporting Information), and selected atom distances and bond angles are collected in Table 1. The two nickel ions in 2a are found tetracoordinated in square planar environment (sum of angles around Ni1 and Ni2 is 359.79° and 360.09°, respectively) and B


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also reminiscent of the K+···peroxo interaction observed for crystallographically characterized heterobimetallic peroxo nickel(II) complex B (Figure 1; d(O−O) = 1.468(2) Å; d(K+−O) = 2.695(2)/2.734(2) Å),13 but in 2a, the K+ is located even closer to the O−O unit. Thus, 2a may be best described as an unprecedented heterotrimetallic dinickel(II) peroxo complex. Raman spectroscopy of crystalline material of 2a (Figure 4a) shows an O2 isotope sensitive band at 720 cm−1 that shifts to

Figure 3. (a) Plot (30% probability thermal ellipsoids) of the molecular structure of 2a (hydrogen atoms omitted for clarity). (b) Front view of the molecular structure of 2a (hydrogen atoms and iPrgroups omitted for clarity).

hosting the O−O unit in a cis-μ-1,2 bridging mode within the bimetallic pocket. The Ni···Ni separation of 3.880(1) Å is significantly shorter by 0.279 Å than in the dinickel dihydride starting complex 1a (4.159(2) Å). The O−O bond length in 2a (1.482(4) Å) is in the range typical for metal peroxo complexes (1.4−1.5 Å),24 though somewhat on the long side, and the Ni− O−O−Ni torsion angle is 81.4(3)°, which reflects a situation strongly twisted away from a genuine cis-μ-1,2 arrangement (Figure 3b). Similar to what was found for the dihydride complex 1a,22 the potassium ion of 2a is encapsulated by the two flanking ligand aryl groups via cation−π interaction. Distances between K+ and the centroids of the aryl rings are 2.840(1) and 2.830(1) Å, which is in the typical range reported for cation−π bonding of K+ to aromatic systems.25 Rather close contacts result between the Lewis acidic K+ and the O−O unit with K−O1 and K−O2 distances of 2.515(2) and 2.545(2) Å, respectively. This is reminiscent of the situation observed for a recently reported cis-μ-1,2-peroxo dicopper(II) complex of a different pyrazolate-based ligand scaffold lacking peripheral aryl groups, which featured a Na+ cation in side-on interaction with the peroxo moiety (d(Na+−O) ≈ 2.3 Å).26 Side-on binding of K+ to the O2-derived ligand of bimetallic [LNi2(O2)]− in 2a is

Figure 4. Raman spectra (λex = 633 nm) of crystalline material of (a) 2a and (b) 2b prepared from 16O2 (black spectrum) and 18O2 (red spectrum), and difference spectra (insets).

680 cm−1 upon 18O labeling (Δ(18O2−16O2) = −40 cm−1, ν̃ (16O−16O)/ν̃(18O−18O) = 1.059, calculated 1.060 for an isolated harmonic O−O oscillator) and is hence assigned to the O−O stretch. Superconducting quantum interference device (SQUID) data confirm that solid 2a is diamagnetic in the temperature range from 2−295 K, in accordance with NMR spectra of 2a in THF-D8, which show resonances in the range

Table 1. Selected Metrical Parameters of 2a, 2b, and 3 complex Ni···Ni (Å) Ni−O (Å) Ni−N (Å) O−O (Å) K−O (Å) Ni−O−O (deg) Ni−O−O-Ni (deg)

2a

2b

3.8800(8) 1.861(3)

3.7812(7) 1.834(2)

1.835(4)−1.922(3) 1.482(4) 2.545(4) 2.515(4) 118.2(2) 118.0(2) 81.4(3)

1.843(2)−1.906(2) 1.465(2)

113.8(2) 89.9(2) C

3 3.8095(5) 1.828(2) 1.834(2) 1.831(2)−1.907(2) 1.326(2)

132.8(2) 131.0(2) 22.7(3) DOI: 10.1021/jacs.8b01468 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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also the Ni−O bonds are shorter (1.834(2) in 2b vs 1.861(3) in 2a), suggesting that polarization of the peroxide by the K+ cation leads to a decrease of the π*(peroxo) → nickel(II) σdonor interaction. Variable temperature (VT) 1H NMR and UV−vis spectra of THF solutions of 2b indicate some dynamics (Figures S15, S19, and S20), possibly involving changes in the Ni−O−O−Ni torsion or interactions with the [K(DB18C6)]+ cation; details of that process have not yet been elucidated. A Raman spectrum of crystalline 2b shows the O−O stretching at 755 cm−1 (ν̃(18O−18O) = 715 cm−1, Δ(18O2−16O2) = −40 cm−1 Figure 4b), which is shifted to higher energy compared to the O−O stretch in 2a. Both structural and vibrational data thus indicate that the peroxo O− O bond in [LNi2(O2)]− is significantly weakened by side-on interaction with a K+ ion, which reflects that Lewis acids can substantially alter the properties of metal-peroxo intermediates.26,28 O−O stretching frequencies for both 2a and 2b are lower than ν̃(16O−16O) = 778 cm−1 reported for the spectroscopically and computationally characterized trans-μ1,2-peroxo dinickel(II) complex [{Ni(tmc)}2(O2)]2+ that was proposed to feature five-coordinate nickel(II) ions with the peroxo ligand in axial position,9 but similar to ν̃ (16O−16O) = 742/752 cm−1 observed for [{Ni(pmbox)}2(O2)]2+ that is assumed to feature low-spin d8 nickel(II) ions in square planar environment.16 However, both those previously reported μ-1,2peroxo dinickel(II) precedents were not characterized crystallographically. O−O stretching frequencies for all known μ-1,2peroxo dinickel(II) complexes are clearly much lower than those of mononuclear nickel complexes with side-on bound peroxo ligand including the heterobimetallic [(nacnac)Ni(μ−η2:η2-O2)K(18C6)] (B; ν̃(O−O) = 829 cm−1).13 In the latter case B, however, it was not possible to remove the side-on bound K+ and to obtain spectral data for the bare [(nacnac)Ni(η2-O2)]− species for comparison. The Raman spectrum of 2b shows an additional weak O2 isotope sensitive band at 582 cm−1 that shifts to 558 cm−1 upon 18 O labeling and is tentatively assigned to the Ni−O mode (Δ(18O2−16O2) = −24 cm−1; ν̃(Ni−16O)/ν̃(Ni−18O) = 1.043, calculated 1.048 for a diatomic harmonic Ni−O oscillator). For comparison, the high-spin trans-μ-1,2-peroxo dinickel(II) complex [{Ni(tmc)}2(O2)]2+ showed a Ni−O mode at 479 cm−1 (Δ(18O2−16O2) = −23 cm−1).9 The rather high frequency for the Ni−O stretch in 2b indicates strong (i.e., covalent) nickel-peroxo bonding or low kinematic coupling between the Ni−O and O−O stretches because of the acute Ni−O−O angles (113.8°), in line with the relatively low O−O stretching frequency of 755 cm−1. In μ-1,2-peroxo bridged bimetallic complexes, the mechanical coupling between M−O and O−O modes is known to sensitively depend on the M−O−O bond angles.9,29 For [{Ni(tmc)}2(O2)]2+ with a trans-μ-1,2-peroxo bridge, the Ni−O−O angle predicted by DFT calculations was 120 °C, leading to energetically more separated ν̃(Ni−O) and ν̃(O−O) modes.9 Superoxo Dinickel(II) Complex. While the reaction of a THF solution of dihydride complex 1a with one equivalent of O2 at room temperature, or with an excess of O2 at low temperatures, cleanly leads to the peroxo complex 2a, a different product 3 is obtained when using an excess of dry O2 and conducting the reaction at room temperature. Starting from the dideuteride 1a-D, clean release of D2 is again observed. The positive ion ESI mass spectrum of a THF/ MeCN solution of 3 shows a dominant peak at m/z = 792.4 for

1.0−7.0 ppm typical for a diamagnetic species (Figures S1 and S13). The 1H NMR spectrum further reveals C2v symmetry on the NMR time scale at room temperature, suggesting fast toggling of the O−O unit within the bimetallic pocket. Structural, magnetic, and spectroscopic signatures clearly identify 2a as a μ-1,2-peroxo dinickel(II) complex with two low-spin d8 metal ions. This is in accordance with the expected redox balance considering H2 release from 1a (or D2 release from 1a-D) associated with two-electron reductive binding of O2 (Scheme 1). Addition of DB18C6 or [2,2,2]cryptand to solutions of 2a in THF results in an immediate color change to deep red. In case of DB18C6, layering of a THF solution with a mixture of diethyl ether and hexanes at −30 °C led to formation of crystals suitable for X-ray diffraction. The same product [K(DB18C6)][LNi2(O2)] (2b) can be obtained directly by treating the dihydride complex [K(DB18C6)][L(Ni−H)2] (1b) with dry dioxygen, which again is accompanied by the release of H2 (or D2, if starting from 1b-D). The molecular structure of the anion of 2b is shown in Figure 5, and selected atom distances and bond angles are listed in Table 1.

Figure 5. (a) Plot (30% probability thermal ellipsoids) of the molecular structure of the anion of 2b (hydrogen atoms omitted for clarity). (b) Front view of the anion of 2b (hydrogen atoms and iPrgroups omitted for clarity). Symmetry transformation used to generate equivalent atoms: (′) x, 3/2-y, 1-z.

The overall μ-1,2-peroxo dinickel(II) motif of 2b is similar to that of 2a, but DB18C6 chelates the potassium cation and removes it from the peroxo ligand of the [LNi2(O2)]− core, leading to an ionic compound. The Ni···Ni separation is slightly contracted to 3.7812(7) Å, and the Ni−O−O−Ni torsion is increased to 89.9(2)°, reflecting an essentially orthogonal situation (Figure 5b) (in between cis- and trans-μ-1,2peroxo).27 Notably, the peroxide O−O bond in 2b is significantly shorter than in 2a (1.465(2) vs 1.482(2) Å) and D


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Journal of the American Chemical Society the ion [LNi2O2+K]+ ([3+K]+), which shifts by four mass units to m/z = 796.3 amu when 3 is prepared with 18O2 (Figure 6) in agreement with 3 containing two O atoms deriving from O2.

{N2Ni2O2} core to become almost planar in 3 (Figure 7). Slightly longer O−O bonds have been reported for the mononuclear nickel(II) complex A with side-on bound superoxido ligand (1.347(2) Å)12 and for D containing a doubly μ-1,2-superoxo bridged dinickel(II) core (1.345(6)/ 1.338(3) Å).10 The Ni−O bond lengths of 1.834(2) Å and 1.828(2) Å in 3 are essentially identical to the ones of the peroxido complex 2b (1.834(2) Å) and similar to those of A, but much shorter than the Ni−O bonds in D (1.937(2) − 1.969(5) Å) because the latter contains six-coordinate high-spin nickel(II) ions.10 A Raman spectrum of crystalline 3 shows the O−O stretch around 1007 cm−1, shifting to about 951 cm−1 when using 18O2 (Δ(18O2−16O2) = −56 cm−1; ν̃ (16O−16O)/ν̃(18O−18O) = 1.059; Figure 8). This is in the range typical for superoxo

Figure 6. Positive ion ESI mass spectrum of a solution of 3 in THF/ MeCN. The inset shows the experimental and simulated isotopic distribution patterns for [LNi2O2+K]+ ([3+K]+) as well as the experimental pattern for [LNi218O2+K]+.

Single crystals of 3 suitable for X-ray diffraction were obtained from a hexane layered THF solution at −30 °C. The molecular structure of neutral 3, shown in Figure 7, features

Figure 8. Raman spectrum (λex = 633 nm) of crystalline material of 3 prepared from 16O2 (black spectrum) and partially 18O2 labeled (red spectrum), and difference spectrum (inset).

complexes30 and at significantly higher energy than the O−O stretches of peroxo complexes 2a and 2b (vide supra). ν(O−O) for 3, which contains a μ-1,2-superoxo ligand within the bimetallic pocket, is slightly lower than values reported for some high-spin nickel(II) superoxo complexes, namely the doubly μ-1,2-superoxo bridged D (1096 cm−1 (Δ(18O2−16O2) ≈ −52 cm−1)10 as well as mononuclear [Ni(14-tmc)(η1-O2)]+ and [Ni(13-tmc)(η1-O2)]+ proposed to feature an end-on superoxo ligand (1131 cm−1; Δ(18O2−16O2) ≈ −64 and 1130 cm−1; Δ(18O2−16O2) ≈ −60 cm−1, respectively).11,14b However, ν(O−O) for 3 is similar to the O−O stretching frequencies reported for structurally authenticated mononuclear A (971 cm−1; Δ(18O2−16O2) ≈ −52 cm−1) and for [(12tmc)NiIII(O2)]+ (C) and [(13-tmc)NiIII(O2)]+ (1002 cm−1; Δ(18O2−16O2) ≈ −60 and 1008 cm−1; Δ(18O2−16O2) ≈ −58 cm−1), all containing a side-on (η2) bound dioxygen unit.14 While nickel(II) superoxo character has been established for A,12 however, complex C and related ones have been formulated as nickel(III) peroxo species.14,17 Solutions of 3 display paramagnetically shifted 1H NMR resonances. The X-band EPR spectrum of a frozen THF solution of 3, recorded at 145 K, is shown in Figure 9 and has been simulated assuming two S = 1/2 subspectra. The major component (94%) is assigned to 3 and shows a rhombic spectrum with principal g values of 2.020, 2.058, and 2.119. The average g value (gav = 2.066) is in good agreement with results obtained from a SQUID measurement of a solid sample of 3,

Figure 7. (a) Plot (30% probability thermal ellipsoids) of the molecular structure of 3 (hydrogen atoms omitted for clarity). (b) Front view of the molecular structure of 3 (hydrogen atoms and iPrgroups omitted for clarity).

two tetracoordinated nickel ions in roughly square planar environment with d(Ni···Ni) = 3.8095(5) Å, similar to the situation in 2b. Major differences are the significantly shorter O−O bond characteristic for a superoxo ligand (1.326(2) Å in 3 vs 1.465(2) in 2b) and the much decreased Ni−O−O−Ni torsion (22.7(3)°), which causes the central six-membered E


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Figure 10. Spin density plot of 3 derived from Mulliken population analysis: Ni1 = 0.080426, Ni2 = 0.079847, O1 = 0.398559, O2 = 0.399162. Contour value: 0.03. Figure 9. X-band EPR spectrum of 3 in frozen THF at 154 K (9.41 GHz, microwave power 15 mW). The red line represents a powder simulation assuming two components with g = [2.020, 2.058, 2.119] (94%, major species) and g = 2.07 (6%, minor species).

which shows an almost constant χΜT value of 0.41 cm3 mol−1 K over a wide temperature range from 20−280 K (corresponding to μeff = 1.81 μB with gav = 2.10; Figure S33). A second minor component (6%) in the EPR spectrum has been modeled with an isotropic g value of 2.07. This species may possibly represent some residual “free” O2−,31 suggesting that KO2 is the side product formed during one-electron oxidation of peroxo complex 2a with O2 yielding superoxo complex 3. The g values of the major species assigned to 3 are closer to the free electron value than g values reported for mononuclear side-on superoxo complex A (g = 2.07, 2.12, 2.14)12 and the fivecoordinate end-on superoxo complexes [Ni(14-tmc)(η1-O2)] (g = 2.09, 2.21, 2.29)11 and [Ni(13-tmc)(η1-O2)] (g = 2.06, 2.21, 2.25),14b whose spectra also appear more axial. Complexes [(12-tmc)NiIII(η2-O2)]+ (C) and [(13-tmc)NiIII(η2-O2)]+, formulated as side-on peroxo nickel(III) species,14 feature g values of 2.06, 2.17, 2.22, and 2.07, 2.19, respectively, reflecting substantial nickel (dz2) character, in contrast to 3. The energy-minimized DFT calculated molecular structure of 3 having a doublet ground state (see Supporting Information for details) is in good agreement with that obtained by X-ray diffraction and shows that the singly occupied molecular orbital (SOMO) is the out-of-plane π*O−O orbital that is oriented roughly perpendicular to the plane of the central {N2Ni2O2} core. Mulliken population analysis indicates that the unpaired spin density is predominantly located on the O2-derived ligand (80%) with only minor contributions from the nickel ions (8% per Ni; see Figure 10). The combined structural, spectroscopic, magnetic, and computational analyses confirm that 3 is a genuine μ-1,2-superoxo dinickel(II) complex. Interconversion of Peroxo and Superoxo Dinickel(II) Complexes. The facile and clean oxidation of peroxo complexes 2a and 2b to give superoxo complex 3 in the presence of excess O2 is striking. A cyclic voltammogram (CV) of a THF solution of 3 was thus recorded and showed a wellbehaved reversible redox process interconverting the superoxo and peroxo species at very low potential (E1/2 = −1.22 V vs the Fc/Fc+ couple; Figure 11). Consequently, bulk reaction of 3 with elemental potassium in THF at rt results in a rapid color change from brown to red, generating 2a, which was identified by 1H NMR spectroscopy after workup. Conversely, treatment of THF solutions of 2a or 2b with dry O2 cleanly regenerates 3.

Figure 11. CV of 3 in THF at RT in the range from −2.9 V to +0.6 V at scan rate 200 mV/s, with NBu4PF6 (0.1 M) as supporting electrolyte; potentials versus Fc/Fc+. The inset shows the reversible process at E1/2 = −1.22 V at different scan rates.

Further (irreversible) reductive processes occur at even lower potential in the CV, while superoxo complex 3 is irreversibly oxidized at an anodic peak potential Eap = 0.28 V, presumably generating O2. Electronic absorption spectra of all dinickel complexes are shown in Figure 12. All complexes, including the dihydride complex 1a and the μ-hydroxo complex 4, show relatively

Figure 12. UV−vis absorption spectra of complex 1a, 2a, 2b, and 3 in THF solution. F


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Journal of the American Chemical Society Scheme 2. Overview of Reactions Studied in This Work

strong absorption in the range 350−400 nm (ε > 103 L mol−1 cm−1), which thus originates from the {LNi2} core. Compared to 1a, peroxo complexes 2a and 2b show an additional band around 510 nm (ε ≈ 1500 L mol−1 cm−1 for 2a; ε ≈ 2600 L mol−1 cm−1 for 2b); on the basis of comparison with literature data,9,16 this is assigned to a peroxo→nickel(II) charge transfer (CT) transition. Superoxo complex 3 shows two bands at 465 nm (ε ≈ 3920 L mol−1 cm−1) and 568 nm (ε ≈ 2100 L mol−1 cm−1); assigning transitions will be part of a comprehensive computational analysis of this new family of Ni 2 /O 2 intermediates. Variable temperature UV−vis absorption spectra of the peroxo and superoxo complexes can be found in the Supporting Information. Initial Reactivity Studies and Discussion. In view of the recently observed proton-induced, reversible interconversion of pyrazolate-based μ-1,2-peroxo and μ-1,1-hydroperoxo dicopper(II) complexes,32 THF solutions of peroxo complex 2a were treated with Brönstedt acids such as 2,6-lutidinium triflate ([HLut]OTf). However, these experiments so far only led to the diamagnetic green μ-hydroxo dinickel(II) complex [LNi2(μ−OH)] (4), without any detectable hydroperoxo intermediate. The same product 4 was obtained upon treating the superoxo complex 3 with TEMPO-H in THF at rt, which results in a color change from red to green within 20 min. 4 has been fully characterized (including single crystal X-ray diffractometry; see Supporting Information) and is readily identified by its characteristic 1H NMR resonance for the μhydroxo proton at δ = −7.25 ppm and a corresponding O−H stretching vibration at ν(O−H) = 3606 cm−1 in the IR spectrum, which shifts to ν(O−D) = 2633 cm−1 after exposure to D2O (Figures S36 and S37). While a pKa value for the elusive dinickel(II) hydroperoxo species is so far unknown, the very low reduction potential of the superoxo complex 3

suggests that the O−H bond dissociation free energy (BDFE) of a putative hydroperoxo complex is low and that 3 is a sluggish hydrogen atom abstractor. This likely contributes to the unusually high stability of 3. More detailed reactivity studies of 2a, 2b, and 3 will be performed in future work.

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SUMMARY AND CONCLUSIONS In summary, we here report the smooth reductive O2 binding in the cleft of a dinickel(II) core via elimination of H2 from the corresponding dinickel(II) dihydride complex. The resulting μ1,2-peroxo dinickel(II) complex could be fully characterized, both in “naked” form (2b) as well as with a K+ side-on bound to the bridging peroxo unit (2a), including the first X-ray crystallographic structure determinations for this type of Ni2/ O2 intermediate. Structural as well as spectroscopic (Raman) evidence shows a significant weakening of the peroxo O−O bond upon interaction with the Lewis acid K+. The μ-1,2peroxo dinickel(II) complex can be oxidized to the corresponding μ-1,2-superoxo dinickel(II) complex (3) at very low potential, and the two species can be reversibly interconverted both chemically and electrochemically. Remarkably, the peroxo complex is cleanly oxidized to the superoxo complex by an excess of O2 (likely generating KO2 as the other product). As also the μ-1,2-superoxo dinickel(II) complex could be structurally authenticated by single crystal X-ray diffraction, this work provides an entire new series of closely related and comprehensively characterized (including elemental analyses) Ni2/O2 intermediates, viz. the mutually interconvertible μ-1,2-superoxo (3) and μ-1,2-peroxo (2b) species as well as the K+ adduct of the latter (2a), all based on the same pyrazolate-based {LNi2} scaffold. The reactions investigated in this work are summarized in Scheme 2. G


Article


microscope, and a He:Ne-laser (633 nm) for excitation. All spectra were recorded at room temperature using solid samples. Magnetic Measurements. Temperature-dependent magnetic susceptibility measurements for 2a and 3 were carried out with a Quantum-Design MPMS-XL-5 SQUID magnetometer equipped with a 5 T magnet in the range from 295−2.0 K at a magnetic field of 0.5 T. The powdered samples were contained in a gelatin capsule (for 2a) or a Teflon bucket (for 3) and fixed in a nonmagnetic sample holder. Each raw data file for the measured magnetic moment was corrected for the diamagnetic contribution of the capsule/Teflon bucket according to Mdia = χg × m × H, with experimentally obtained gram susceptibilities of the capsule/Teflon bucket. The molar susceptibility data of the compounds were corrected for the diamagnetic contribution. Experimental data for 2a and 3 were modeled with the julX program40 using a fitting procedure to the spin Hamiltonian:

Using metal hydrides is an attractive strategy for storing the required electrons for reductive small molecule binding, as it circumvents the need for highly reduced metal ions. It is interesting to note that many transition metal hydride complexes, in particular for 4d and 5d metal ions such as PdII,33 PtIV,34 RhIII,35 or IrIII,36 react with dioxygen via insertion in the metal hydride bond to give hydroperoxo intermediates. However, this may involve a HX reductive elimination/O2 oxidative addition sequence.37 The present complex 1a (and 1b) with its two terminal Ni−H pointing into the bimetallic cleft undergoes reductive H2 elimination accompanied by O2 reduction, supporting the idea that dihydride complexes 1a and 1b can be viewed as masked versions of a reactive {LNiI2} synthon. Current efforts are directed at using these pyrazolatebased dihydride complexes for the activation of molecules that are otherwise difficult to reduce. It should also be noted that pyrazolate-based compartmental ligand scaffolds have previously been proven beneficial for the construction of highly preorganized bimetallic systems that enable cooperative binding of substrates within the bimetallic pocket and trapping of reactive intermediates.38 Further reactivity studies of the surprisingly rugged Ni2/O2 complexes 2a, 2b, and 3 as well as further computational studies to provide more detailed understanding of their electronic structures and of their unexpected stability are planned.

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Ĥ = gμB B ⃗ × S ⃗

(1)

For 2a, temperature-independent paramagnetism (TIP) and paramagnetic impurities (PI) were included according to χcalc = (1−PI) + PI + TIP (see Figures S13 and S33 for details). For 3, intermolecular interactions were considered in a mean field approach by using a Weiss temperature Θ = −1.7 K.41 The Weiss temperature, defined as Θ = zJinterS(S + 1)/3k) relates to intermolecular interactions zJinter, where Jinter is the interaction parameter between two nearest neighbor magnetic centers, k is the Boltzmann constant (0.695 cm−1 K−1), and z is the number of nearest neighbors. EPR Spectroscopy. EPR spectra were measured with a Bruker E500 ELEXSYS X-band spectrometer equipped with a standard cavity (ER4102ST, 9.45 GHz). The sample temperature was maintained constant with an Oxford instrument nitrogen flow cryostat (ESP910) and an Oxford temperature controller (ITC-4). The microwave frequency was measured with the built-in frequency counter, and the magnetic field was calibrated by using an NMR field probe (Bruker ER035M). EPR spectra were simulated using Easy-Spin.42 DFT Calculations. The ORCA package (version 3.0.3) was employed.43 A geometry optimization was performed based on the coordinates of 3 from the crystallographic structure determination (spin unrestricted DFT calculations, BP86 functional, def2-tzvp basis set,44 RI approximation using the auxiliary def2-tzvp/J basis set, D3 dispersion correction with zero damping,45 tight convergence, and optimization criteria). Selected geometric parameters are listed in Table S5. The calculated structure is in reasonable agreement with the solid state structure, except for the Ni−O−O−Ni torsion angle, which deviates by more than 10°. However, it can be assumed that this angle may be different in solution or in the gas-phase as well. A frequency calculation on the latter structure yielded no imaginary frequencies (Figure S46). The spin density calculation was carried out on the optimized coordinates by using the B3LYP functional and the RIJCOSX approximation with the same basis sets and dispersion correction as stated above. Single-Crystal X-ray Structure Determinations. Crystal data and details of the data collections are given in Table S2, selected bond lengths and angles in Table S3. X-ray data were collected on a STOE IPDS II diffractometer (graphite monochromated Mo−Kα radiation, λ = 0.71073 Å) by use of scans at −140 °C. The structures were solved by SHELXT46 and refined on F2 using all reflections with SHELXL2014/16.47 Nonhydrogen atoms were refined anisotropically. Most hydrogen atoms were placed in calculated positions and assigned to an isotropic displacement parameter of 1.2/1.5 Ueq(C). For the Ni− O(H)-Ni component (4) cocrystallized with 2a, the O-bound hydrogen atom was refined by applying a fixed isotropic displacement parameter of 0.08 Å2 and a DFIX restraint (d(O−H) = 0.82 Å). In case of 4, it was refined freely. The asymmetric unit of 2a consists of two crystallographically independent molecules, which have different chemical compositions. One iPr-group of the μ−OH bridged dinickel compound was found to be disordered about two positions (occupancy factors = 0.713(14)/0.287(14)). SADI, SAME, and RIGU restraints were applied to model the disorder. In case of 2b, the THF molecules coordinated to K+ as well as uncoordinated THF

EXPERIMENTAL SECTION

Materials and Methods. All experiments and manipulations were carried out under dry oxygen-free Argon using standard Schlenk techniques, or in a glovebox filled with dinitrogen (O2 < 0.5 ppm, H2O < 0.5 ppm). Solvents were dried by standard methods and freshly distilled prior use. THF, pentane, and hexane were dried over sodium in the presence of benzophenone; THF−d8 was also dried over sodium in the presence of benzophenone and stored over 3 Å molecular sieve. K was purchased as a dispersion in mineral oil and was washed repetitively with hexanes and then dried in vacuum prior to use. The starting materials [KL(Ni−H)2] (1a) and [K(DB18C6)][L(Ni−H)2] (1b) were prepared according to the literature procedure.22 TEMPO-H was prepared as described in literature.39 O2 gas was rigorously dried by storing it over concentrated H2SO4 for around 1 day. UV−vis spectra were recorded on an Agilent Cary 60 equipped with an Unisoku Cryostat (CoolSpek) and magnetic stirrer using quartz cuvettes with an attached tube and a screw cap with a septum. All UV−vis samples were prepared in a glovebox and transferred out of the glovebox prior to the measurement. Cyclic voltammetry (CV) experiments were performed with an Interface 1000B potentiostat using a three electrode setup consisting of a glassy carbon working electrode, a platinum wire counter electrode, and an Ag reference electrode and were analyzed by Gamry Framework software. CV experiments were performed in deoxygenated THF containing NBu4PF6 (0.1 M) as supporting electrolyte; ferrocene was used as an internal standard. Infrared spectra were recorded inside a glovebox on a Cary 630 FTIR spectrometer equipped with Dial Path Technology and analyzed by FTIR MicroLab software. ESI mass spectra were recorded on Bruker HCT ultra spectrometer. Elemental analyses were performed by the analytical laboratory of the Institute of Inorganic Chemistry at the University of Gö ttingen using an Elementar Vario EL III instrument. 1H and 13C NMR spectra were recorded on Bruker Avance 300 or 400 spectrometers. Chemical shifts are reported in parts per million relative to residual proton and carbon signals of the solvent THF (δH = 1.73 and 3.59 ppm; δC = 25.31 and 67.21 ppm). Raman Spectra. Raman spectra have been recorded using a HORIBA Scientific LabRAM HR 800 (400−1100 nm) spectrometer with open-electrode CCD detector and a confocal pinhole with user controlled variable aperture in combination with a free space optical H


Article


(LNi2(18O2)+2H)+ (2b-18O2). Anal. calcd (%) for C67H93N6KNi2O10, C 61.95, H 7.22, N 6.47; found, C 62.12, H 7.33, N 6.18. [LNi2(O2)] (3). A Schlenk flask was charged with complex 1a (30.5 mg, 0.04 mmol, 1 equiv) and THF (2 mL). The solution was degassed by the freeze−pump−thaw method, then exposed to excess dried dioxygen overnight while stirring. The color of the solution changed from orange to brown. The mixture was then evaporated, and the residue dissolved in THF and filtered. Black block-shaped crystals suitable for X-ray diffraction were obtained by layering hexanes on a solution of crude 3 in THF at −30 °C; yield 70%. 3-18O2. The synthesis of 3-18O2 was achieved by the same procedure as described above for 3 but using 18O2. ATR-IR (ν̃/cm−1) = 3058 (w), 2956 (m), 2924 (m), 2865 (m), 1553 (m), 1532 (s), 1461 (s), 1437 (s), 1394 (s), 1369 (s), 1313 (s), 1252 (s), 1234 (s), 1187 (s), 1176 (s), 1092 (m), 1032 (s), 982 (s), 936 (m), 916 (m), 870 (w), 797 (s), 759 (s), 743 (s), 714 (m), 588 (m), 565 (m). Raman (ν̃/cm−1) = 1007 (16O−16O), 951 (18O−18O). ESI-MS (THF/CH3CN 10:1): m/z (%) = 792.4 (100) (M+K)+ (3- 16 O2 ), 776.4 (20) (M+Na) + (3- 16 O 2 ), 754.3 (5) (M+H) + (3-16O2); 796.4 (100) (M+K)+ (3-18O2). UV−vis (THF): λmax = 312, 364, 465, 568 nm. Anal. calcd (%) for C39H53N6Ni2O2, C 62.02, H 7.07, N 11.13; found, C 62.05, H 7.18, N 11.03. [LNi2(μ−OH)] (4). Method A. Treatment of 2a (31.77 mg, 0.04 mmol, 1 equiv) in THF (2 mL) with [H-Lut]OTf (10.28 mg, 0.04 mmol, 1 equiv) resulted in an immediate color change from red to orange. The mixture was stirred for 1 h. After filtration, green X-ray quality crystals were grown by layering hexanes onto a THF solution of crude 4 at −30 °C for 2 days. Method B. Treatment of 3 (15.10 mg, 0.02 mmol, 1 equiv) in THF (1 mL) with TEMPO-H (3.14 mg, 0.02 mmol, 1 equiv) resulted in a color change from brown to red−brown within 20 min. The reaction mixture was stirred for 12 h at −30 °C. Green single crystals suitable for X-ray diffraction were obtained by layering hexanes onto a solution of the crude product in THF at −30 °C for 2 days. Method C. Treatment of 1a (30.56 mg, 0.04 mmol, 1 equiv) in THF (2 mL) with H2O resulted in an immediate color change from red to orange and gas evolution was observed. The mixture was stirred for 1 h. After filtration, X-ray quality crystals were grown by layering hexanes onto a THF solution of the crude product at −30 °C for 2 days. 1 H NMR (THF-d8, 400 MHz) = 6.94−6.98 (t, 4H, Ar), 6.80−6.82 (d, 2H, Ar), 5.48 (s, 1H, Pz), 4.60 (s, 2H, CHCCH3), 4.01 (s, 4H, CH2Pz), 3.32−3.37 (m, 4H, CH(CH3)2), 1.86 (s, 6H, CH3CCH), 1.61 (d, 12H, 2JH−H = 4 Hz, (CH3)2CH), 1.08 (s+d, 18H, CH3CCH +(CH3)2CH). 13C NMR (THF-d8, 100 MHz) = 21.36 (CH3), 23.73 (CH3), 25.89 (CH3), 29.20 (CH3), 55.14 (CH2Pz), 91.62 (4-Pz), 98.25 (CHCCH3), 125.33 (Ar), 126.14 (Ar), 142.56 (Ar), 145.40 (Ar), 159.36 (CHCCH3), 161.25 (CHCCH3). ATR-IR (ν/cm−1) = 3608 (m, OH), 3058 (w), 2955 (m), 2864 (m), 1553 (m), 1529 (vs), 1462 (s), 1436 (s), 1394 (vs), 1381 (vs), 1323 (m), 1314 (m), 1271 (m), 1251 (m), 1234 (m), 1196 (w), 1159 (w), 1104 (w), 1082 (w), 1060 (m), 1018 (m), 946 (m), 874 (m), 799 (s), 756 (vs), 732 (vs), 709 (w), 649 (m). ATR-IR (ν/cm−1) = 2633(OD). Anal. calcd (%) for C39H54N6Ni2O, C 63.28, H 7.35, N 11.35; found, C 63.18, H 7.23, N 11.49.

molecules were found to be disordered. The two coordinated and one uncoordinated THF molecules were disordered about a mirror plane and were refined at 1/2 occupancy. The second THF molecule was found to be disordered about two positions (occupancy factors = 0.505(6)/0.495(6)). DFIX and RIGU restraints were applied to model the disordered parts. In case of 3, one uncoordinated THF molecule was found to be disordered about a center of inversion with an additional positional disorder of the oxygen atom. The latter atom was refined at 1/4 occupancy and the carbon atoms at 1/2 occupancy. DFIX restraints were applied to model the disorder. Face-indexed absorption corrections were performed numerically with the program X-RED.48 [KLNi2(O2)] (2a). Method A. A Schlenk flask was charged with complex 1a (30.5 mg, 0.04 mmol, 1 equiv) and THF (2 mL). The solution was degassed by the freeze−pump−thaw method and then exposed to dried dioxygen (1.28 mg, 890 μL, 0.04 mmol, 1 equiv) for 30 min with stirring. The color changed from orange to red immediately. The mixture was then evaporated, and the residue was dissolved in THF (2 mL) and filtered. Suitable red block-shaped crystals for X-ray diffraction were obtained by layering hexanes on a solution of 2a in THF at −30 °C. Method B. Complex 3 (33 mg, 0.04 mmol, 1 equiv) and potassium (1.87 mg, 0.048 mmol, 1.2 equiv) were suspended in THF (2 mL) at room temperature, and a color change from brown to red occurred within 20 min. The mixture was stirred for 2 h, and then the solid components were filtered off, and the solution was evaporated to dryness. The product was identified by 1H NMR spectroscopy. 2a-18O2. The synthesis of 2a-18O2 was achieved by the same procedure A as described above for 2a but using 18O2. 1 H NMR (THF-d8, 400 MHz) = 6.91 (m, 6H, Ar), 5.99 (s, 1H, 4Pz), 4.53 (s, 2H, CHCCH3), 3.82 (s, 4H, CH2Pz), 3.69−3.79 (m, 4H, CH(CH3)2), 1.78 (s, 6H, CH3), 1.49 (d, 12H, 2JH−H = 8 Hz, CH(CH3)2), 1.08 (d, 12H, 2JH−H = 8 Hz, CH(CH3)2), 1.02 (s, 6H, CH3). 13C NMR (THF-d8, 100 MHz) = 161.1 (CHCCH3), 158.2 (CHCCH3), 152.8 (3,5-Pz), 151.8 (Ar), 145.3 (Ar), 124.5 (Ar), 123.2 (Ar), 97.6 (CHCCH3), 91.5 (4-Pz), 51.2 (CH2Pz), 28.8 (CH3), 24.8 (CH3), 23.1 (CH3), 22.0 (CH3). ATR-IR (ν̃/cm−1) = 3055 (w), 2958 (m), 2924 (m), 2864 (m), 1555 (m), 1527 (vs), 1460 (s), 1433 (vs), 1397 (vs), 1369 (m), 1315 (s), 1257 (vs), 1196 (w), 1055 (s), 1031 (vs), 1014 (vs), 859 (m), 799 (s), 774 (m), 757 (s), 732 (s), 683 (w), 623 (w), 589 (w), 548 (w). Raman (ν̃/cm−1) = 720 (16O−16O), 680 (18O−18O). ESI-MS (THF): m/z (%) = 793.3 (100) (M+H)+ (2a-16O2), 819.2 (100) (M+Na)+ (2a-18O2). UV−vis (THF): λmax = 272, 368, 378, 512 nm. Anal. calcd (%) for C39H53N6KNi2O2, C 58.97, H 6.72, N 10.58; found, C 59.32, H 6.82, N 10.37. [K(DB18C6)(thf)2][LNi2(O2)] (2b). Dibenzo[18]crown-6 (7.2 mg, 0.02 mmol, 1 equiv) was added into a solution of 2a (16 mg, 0.02 mmol, 1 equiv) in THF (2 mL) at room temperature, causing a color change from wine-red to cherry red. After stirring the resulting red solution for 2 hs, all volatiles were removed in vacuo. The red residue was washed twice with hexanes (10 mL). The crude powder was recrystallized by layering Et2O/hexane on a solution of 2b in THF at −30 °C to yield deep red block-shaped crystals. 2b-18O2. The synthesis of 2b-18O2 was achieved by the same procedure as described above for 2b but using 2a-18O2. 1 H NMR (THF-d8, 400 MHz) = 6.83−6.94 (m, Ar+DB18C6), 5.57 (s, 1H, 4H-Pz), 4.49 (br, 2H, CHCCCH3), 4.14 (DB18C6), 4.08 (DB18C6), 3.79 (br, 4H, CH2Pz), 1.40 (s, 12H, CH3), 1.05 (s, 12H, CH3). 13C {1H} NMR (THF-d8, 100 MHz) = 149.4, 144.5, 135.2, 126.2, 123.9, 121.9, 121.8, 113.5, 113.4, 113.2, 106.1, 97.7, 96.1, 92.6, 70.3, 69.2, 56.5, 56.1, 32.6, 29.0, 27.4, 26.4, 25.9, 24.0, 23.0, 21.2, 20.8, 14.5. ATR-IR (ν̃/cm−1) = 3058 (w), 2952 (m), 2925 (m), 2860 (m), 1663 (w), 1594 (m), 1547 (m), 1522 (s), 1502 (s), 1438 (s), 1402 (vs), 1320 (m), 1308 (m), 1281 (w), 1247 (vs), 1209 (s), 1123 (vs), 1099 (w), 1083 (w), 1055 (vs), 987 (w), 953 (w), 940 (s), 912 (w), 900 (w), 807 (w), 796 (m), 778 (m), 739 (s), 715 (s), 600 (m), 582 (w). Raman (ν̃/cm−1) = 755 (16O−16O), 715 (18O−18O). UV−vis (THF): λmax = 274, 380, 410, 520 nm. ESI-MS (THF/CH3CN 10:1): m/z (%) = 755.4 (15) (LNi2(O2)+2H)+ (2b-16O2), 759.4 (20)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b01468. NMR, UV-vis and IR spectra; ESI mass spectra; SQUID data; detailed crystallographic information; details about DFT calculations (PDF) Crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. I


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Franc Meyer: 0000-0002-8613-7862 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the China Scholarship Council (PhD fellowship to P.-C.D.) is gratefully acknowledged. We thank Dr. Claudia Stückl and Dr. Marie Bergner for recording the EPR spectra.

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