Reductive Binding and Ligand-Based Redox Transformations of

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Reductive Binding and Ligand-Based Redox Transformations of Nitrosobenzene at a Dinickel(II) Core Eleonora Ferretti, Sebastian Dechert, and Franc Meyer* Institut für Anorganische Chemie, Universität Göttingen, Tammanstrasse 4, D-37077 Göttingen, Germany

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S Supporting Information *

ABSTRACT: The metal-mediated activation of PhNO represents an important starting point for understanding the reactivity patterns of nitrosoarenes in biological systems and catalysis. Here we report that the pyrazole-based dinickel(II) dihydride complex [KL(NiH)2] (1) reacts with PhNO to eliminate dihydrogen concomitant with binding of the doubly reduced substrate in μκ(O):κ(N) mode in the bimetallic pocket of [KLNi2(PhNO)] (2). The addition of [2,2,2]cryptand leads to the ionic complex [K(crypt)][LNi2(PhNO)] (3). Structural and spectroscopic analyses evidence that interaction with the Lewis acidic K+ in 2 causes significant elongation and weakening of the substrate’s N− O bond [dN−O = 1.487(12) Å in 2 vs 1.374(4) Å in 3]. Complex 2 (or 3) reacts with [FeCp*2][PF6] to give LNi2(PhNO) (4), which is shown by electron paramagnetic resonance and IR spectroscopies and density functional theory calculations to feature two low-spin d8 nickel(II) ions and a bridging (PhNO)•− radical anion ligand, with the out-of-plane π*(NO) being the singly occupied molecular orbital. Cyclic voltammetry and UV− vis spectroelectrochemical experiments show that 4 and the anion of 3 can be reversibly interconverted at very low potential (E1/2 = −1.53 V vs Fc/Fc+). Protonation of 2 leads to the N-phenylhydroxylamine complex [LNi2(ONHPh)] (5) with a long N−O bond of 1.464(2) Å, and titration studies suggest a pKa of around 23−25 in tetrahydrofuran. This allows one to derive a bond dissociation energy of 62−65 kcal mol−1 for the N−H bond of 5. Accordingly, 5 readily reacts with the phenoxy radical 2,4,6-tBu3C6H2O• to yield 4. This work demonstrates the reductive binding of PhNO without prior formation of unstable nickel(I) species and the redox noninnocence of the PhNO ligand in the less common μ-κ(O):κ(N) bridging mode. Thermodynamic data for H-atom-abstraction chemistry at the activated PhNO may be valuable for understanding the reactivity patterns of the transient but biologically relevant nitroxyl (HNO) ligand.

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neutral (PhNO)0, singly reduced (PhNO)•−, and doubly reduced (PhNO)2−.9 Characteristic trends are observed for the N−O bond length and the N−O stretching frequency νNO, which reflect the degree of electron density in the out-of-plane π*(NO) orbital that represents the lowest unoccupied molecular orbital (LUMO) of (PhNO)0. However, it was noted that monodentate metal-coordinated arylnitroxide radicals often exhibit inherent instability in solution.10 Nowadays, the number of PhNO adducts that have been isolated and structurally characterized is growing, and various ligand scaffolds have been employed to stabilize such complexes with PhNO in its different redox forms. Despite the use of nickel in catalytic reactions involving nitroarenes; however, only a few Ni-containing nitrosoarene complexes have been synthesized so far,11−13 in addition to a number of nitrosoarene compounds of other metals such as Fe,14,15 Rh,16,17 Os,18,19 Pt,20 and Ru.21−24 Notable examples relevant to the present work involve the mononuclear side-on (PhNO)•−NiII complex (A) and its reduced (PhNO)2−NiII

INTRODUCTION The coordination chemistry of nitrosoarenes and N-arylhydroxylamines is of considerable interest because of its relevance in biological systems and its exploitation in diverse stoichiometric and catalytic transformations involving Cnitroso compounds.1,2 For example, nitrosobenzene (PhNO) binds to hemoglobin substantially more strongly than dioxygen (O2),3 and it may serve as a stable analogue of highly reactive nitroxyl (HNO); the latter is endogenously produced and involved in several important biological transformations connected to nitric oxide (NO).4−6 Besides the significance of PhNO interaction with metal ions in living organisms, nitrosoarene metal complexes show fascinating structural diversity as well as potential synthetic utility in, e.g., Cucatalyzed allylic CH amination reactions7 or Ni-catalyzed reductive aminocarbonylation of aryl halides with nitroarenes.8 This has stimulated studies aimed at identifying well-defined nitrosoarene complexes and elucidating their electronic structures related to reactivity. Wieghardt et al. have established the redox noninnocence of nitrosoarene ligands in transition-metal complexes and have defined spectroscopic and metric signatures for assigning their oxidation states, viz., © XXXX American Chemical Society

Received: January 26, 2019

A

DOI: 10.1021/acs.inorgchem.9b00256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry congener (B), which are both supported by a bulky βdiketiminato ligand and can be interconverted chemically and electrochemically (Figure 1).12 In the bimetallic nitrosoarene

eliminate H2 when substrates such as phenylacetylene28 or O229 were added, thereby accommodating the 2-fold-reduced substrates within the bimetallic cleft. 1 and related dihydride complexes can thus be viewed as masked dinickel(I) synthons capable of the reductive bimetallic activation of small molecules. In previous work, this led to the isolation and comprehensive characterization of μ-1,2-superoxo- and μ-1,2peroxodinickel(II) intermediates, and it was demonstrated that the Lewis acid K+ originating from the starting complex 1 may significantly affect the electronic structure of these intermediates.29 Upon reaction of 1 with NO, two NO molecules are coupled within the bimetallic cleft to generate a new type of cis-hyponitrite complex with a N,O-bridging cis-[N2O2]2− moiety.30 In the present work, we have now studied the reactivity of complex 1 toward PhNO, which has allowed for the isolation of a series of adducts with reduced, PhNOderived moieties that are enforced to adopt N,O-bridging modes within the dinickel cleft. The effect of interaction with K+ is elucidated, and redox as well as proton-induced interconversions of the PhNO-derived species are also presented.

Figure 1. Relevant examples of reductively activated PhNO bound to mononuclear metal complex fragments supported by bulky βdiketiminato ligands (18C6 = [18]crown-6; Ar = 2,6-Me2C6H3; Ar′ = 3,5-Me2C6H3).

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RESULTS AND DISCUSSION Reductive Binding of PhNO. The treatment of a tetrahydrofuran (THF) solution of 1 with 1 equiv of PhNO at 0 °C leads to an immediate color change from orange to black and the release of H2 (Scheme 1). An electrospray ionization mass spectrometry spectrum of the reaction mixture showed essentially two major peaks at m/z 867.3 and 851.3 assigned to the cations [LNi 2 (PhNO)+K] + and [LNi2(PhNO)+Na]+, respectively (Figure S5), indicating the binding of one PhNO substrate molecule and concomitant loss of H2, as was observed for the reductive binding of other small substrates within the bimetallic cleft of 1.28,29 Single crystals of the resulting complex [KLNi2(PhNO)] (2) suitable for X-ray diffraction were obtained within a few days by diffusion of diethyl ether and pentane into a THF/ acetonitrile (MeCN) solution of the product at −30 °C. The molecular structure of 2 is represented in Figure 2, and selected bond distances and angles are listed in Table 1. Complex 2 in solid state features a Ni···Ni separation of 3.9217(8) Å, which is significantly shorter than in the dinickel dihydride starting material 1 [4.1584(7) Å/4.1636(7) Å].28 The two Ni ions are found in a square-planar environment, and the PhNO moiety bridges them in a μ-κ(O):κ(N) binding mode. The two flanking aryl groups of the β-diketiminato subunits are oriented toward one side of the pyrazolate-based dinickel core, whereas the substrate phenyl group is located on the other side and is coplanar with the CAr−N−O plane. While steric crowding may contribute to this arrangement, it mainly reflects that the (PhNO)2− dianion prefers to bind facially through its highest occupied molecular orbital (HOMO), which is the π*(NO) orbital perpendicular to the Ph−N−O plane.9 In the case of 2, the NO moiety of the PhNO ligand is slightly tilted with respect to the bimetallic scaffold [torsion angle Ni1−O1−N4−Ni1′ of 74.4(9)°], causing the Ni1′bound N4 atom to lie below the plane defined by the pyrazolate and the two Ni ions. Similar to what was found for the starting complex 1, the K+ cation is encapsulated between the two aryl groups of the β-diketiminato units. The distance between the K+ cation and the aryl groups is 2.9173(5) Å, which is in the typical range of cation−π interactions involving the K+ and aromatic systems.31 Furthermore, a relatively close

adduct [(nacnac)Ni]2(ArNO) (C; nacnac = [ArNC(Me)]2C and Ar = 2,6-iPr2C6H3), the substrate is bound in a μ-η2:η2 fashion, which increases the degree of reductive activation of the N−O bond in comparison with the mononuclear analogues.13 In a similar but distinct chemistry, the η2nitrosoarene ligand in D was found to be much less activated with a short N−O bond, which was interpreted in terms of modest back-bonding from the {(nacnac)CuI} fragment. Binding of a second {(nacnac)CuI} fragment to D is labile and reversible in solution.13 On the other end of the scale, the dicobalt complex E represents a limiting case for PhNO reduction because the N−O bond is fully cleaved and the product features a diamond core with Co−(Ph)N−Co and Co−O−Co bridging units.25 Analogous to related metal/O2 chemistry, reductive binding of nitrosoarenes usually occurs only when the metal is able to provide proper reducing equivalents. For Ni, however, this is rather challenging because of the difficulties associated with isolating the required nickel(I) species that are, in general, highly reactive.26 Redox-active ligands that serve as electron reservoirs,27 or nickel hydride complexes that reductively eliminate dihydrogen (H2) upon substrate binding, represent intriguing alternatives. Recently, we reported a set of dinickel(II) dihydride complexes based on a compartmental ligand that provide two β-diketiminato subunits spanned by a central pyrazolate.28 These complexes, including [KL(NiH)2] (1; Scheme 1), feature the two hydrides in close proximity within the bimetallic pocket and were shown to smoothly Scheme 1. Reductive Binding of PhNO Mediated by the Dinickel(II) Dihydride Complex 1 with Concomitant H2 Elimination

B


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Inorganic Chemistry

Figure 2. Plots (50% probability thermal ellipsoids) of the molecular structure of 2: (a) top view (H atoms omitted for clarity); (b) front view (H atoms and isopropyl groups omitted for clarity). The PhNO moiety is disordered over two positions (only one of them is shown).

Figure 3. Plots (50% probability thermal ellipsoids) of the molecular structure of the anion of 3: (a) top view (H atoms omitted for clarity); (b) front view (H atoms and isopropyl groups omitted for clarity).

contact between the Lewis acidic K+ and the bridging N−O group, particularly with the O atom of the reduced PhNO substrate, results in K···N and K···O distances of 2.909(11) and 2.570(10) Å, respectively; the latter is similar to the K···O contacts of 2.515(4) and 2.545(4) Å observed for the related dinickel(II) peroxo complex [LNi2(O2)K],29 but both the K··· N and K···O distances in 2 are significantly shorter that those in the mononickel(II) complex B [3.082(3) and 2.716(2) Å, respectively].12 It should be noted that twinning of the crystals of 2 and disorder of the whole PhNO moiety lead to some uncertainty with respect to the atom distances of the core region of the complex, which likely explains why the experimental values show some deviation from the density functional theory (DFT)-calculated ones (Table S5). The addition of [2.2.2]cryptand to a suspension of 2 in THF leads to an immediate color change to deep green, and diffusion of Et2O into the resulting solution at −30 °C gave single crystals of the new compound [LNi2(PhNO)][K(crypt)] (3) suitable for X-ray diffraction analysis. The molecular structure of the anion of 3 is shown in Figure 3, and selected structural parameters are included in Table 1. The

main structural features found in complex 2 are retained in 3, but the K+ cation is now encapsulated in the cavity of the cryptand and thus removed from the [LNi2(PhNO)]− core, resulting in an ionic compound with a slightly contracted Ni··· Ni separation of 3.8266(7) Å [torsion angle Ni1−O1−N7− Ni2 is 73.6(3)°]. A comparison of the bound PhNO moieties reveals that the N−O bond is dramatically shorter in 3 compared to 2 [dN−O = 1.374(4) vs 1.487(12) Å], indicating that interaction with the K+ cation polarizes the substrate and induces a significant weakening of the N−O bond in 2. Binding of K+ also perturbs coordination to the Ni ions, reflected by very similar Ni− OPhNO and Ni−NPhNO bond lengths in 3 [1.865(3) vs 1.895(3) Å] that become noticeably different in the presence of K+ [1.798(10) vs 1.992(9) Å in 2; Table 1]. These trends are qualitatively reproduced in the DFT-optimized structures of 2 and 3 (see Tables S5 and S6). For both cases 2 and 3, a comparison with the N−O bond lengths of free nitrosoarene compounds (1.13−1.29 Å)4 indicates a significant reduction of the NO bond order.

Table 1. Selected Metrical Parameters for Compounds 2, 3, and 5

Ni···Ni (Å) Ni−OPhNO (Å) Ni−NPhNO (Å) N−O (Å) Ni−Nligand (Å) K−O (Å) K−N (Å) Ni−O−N (deg) Ni−O−N−Ni (deg)

2

3

5

3.9217(8) 1.798(10) 1.992(9) 1.487(12) 1.862(5)−1.931(5) 2.570(10) 2.909(11) 119.9(7) 120.1(7) 74.4(9)

3.8266(7) 1.865(3) 1.895(3) 1.374(4) 1.842(3)−1.917(3)

3.8866(6) 1.870(1) 1.945(2) 1.464(2) 1.841(2)−1.914(2)

119.8(2) 121.9(2) 73.6(3)

119.3(1) 117.4(1) 77.2(1)

C


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Inorganic Chemistry

much lower energy [765 cm−1; Δν̃(15NO−14NO) = −14 cm−1];12 while ligand backbone fluorination in B may have an influence, the reasons for these large variations in the N−O stretching frequencies remain unclear. Comparing the N−O stretches of 2 and 3 was expected to reveal the effect of interaction of the substrate with the K+ ion, similar to the substantial shift of the O−O stretch observed upon K+ binding to the μ-1,2-peroxodinickel(II) complex [LNi2(O2)]−.29 Unfortunately, the IR spectra of crystalline 3 and 3-15NO (Figure 4b) do not reveal any salient isotope-sensitive band, suggesting that the intensity of the ν̃(N−O) band of 3 is weak. This is in accordance with the DFT-calculated IR spectrum of 3, which predicts a relatively weak band for the N−O stretch at 1007 cm−1 (991 cm−1 for 3-15NO; Figure S34 and Table S9); the higher value compared to the experimental and DFTcalculated N−O stretches for 2 reflects the weakening of the N−O bond upon interaction with the K+ cation in 2. UV−vis spectra of THF solutions of 2 and 3 are essentially identical (Figures S6 and S9), suggesting that the K+ cation in 2 is largely dissociated from the complex anion in that solvent or that it does not exert any influence on the main chromophores of the complex. The 1H NMR spectra of 2 recorded in toluene-d6, and of 3 recorded in THF-d6, show resonances in the range 0.5−11.5 ppm typical for diamagnetic species (Figures S1 and S11); the spectral patterns reflect the C1 symmetry of the complexes. Variable-temperature 1H NMR spectra evidence that rotation of the PhNO substrate’s phenyl group around the N−C bond is slow on the NMR time scale. This is most evident for the two o-H nuclei, which resonate at low fields and show decoalescence at low temperatures. Exchange rate constants determined from line-shape analysis32 (Tables S1 and S2 and Figure S18) gave activation parameters ΔH⧧ = 63 ± 2 kJ·mol−1, ΔS⧧ = 35 ± 9 J·mol−1·K−1, and ΔG⧧298 = 52 ± 1 kJ·mol−1 for rotation of the phenyl group in 3. These values are similar to the rotational barrier measured for a μ-κ(C):κ(C′)-styrene-1,2-diyldinickel(II) complex based on the L3− scaffold and for other PhNO metal complexes.28,33,34 Furthermore, the activation parameters are of similar magnitude to those of the free organonitroso ligand,35 suggesting that reduction and coordination of the PhNO group in the clamp of the two Ni ions does not significantly affect the barrier for rotation of the phenyl ring. In the case of 2, in addition to decoalescence at low temperatures, the 1H NMR resonances of the two o-H nuclei show a temperaturedependent shift, which is likely caused by the K+ binding equilibrium. The structural and spectroscopic properties of 2 and 3 indicate that the [LNi2(PhNO)]− core is best described as having a (PhNO)2− moiety that is N,O-bridging two low-spin d8 NiII ions. This is consistent with the view of the dihydride complex 1 as a dinickel(I) surrogate that eliminates H2 concomitant with two-electron reductive binding of PhNO within the bimetallic cleft, as was previously reported for the related substrate O2.29 Oxidation of the (PhNO) 2− Moiety. The cyclic voltammogram (CV) of a THF solution of 2 recorded under strictly inert conditions shows a reversible oxidation at very low potential (E1/2 = −1.53 V vs Fc/Fc+; Figure 5). A further irreversible oxidation occurs at an anodic peak potential Ep = +0.02 V, presumably generating PhNO. Chemical reversibility of the redox process at E1/2 = −1.53 V was confirmed by UV− vis spectroelectrochemistry of a THF solution of 2 at room temperature (Figure 6). Application of a steady oxidizing

While the N−O distance in 3 is similar to dN−O = 1.38 Å recently reported for the mononickel complex B that features a twice-reduced PhNO substrate (Figure 1),12 the N−O bond in the trimetallic complex 2 is unusually long and even longer than dN−O = 1.44 Å found for the μ-η2:η2-PhNO in C.13 The geometric and spectroscopic features of relevant PhNO adducts are compiled in Table S4. IR spectroscopy of the crystalline material of 2 shows an isotope-sensitive feature at 907 cm−1, which shifts to 894 cm−1 in the corresponding 15NO-labeled compound, 2-15NO [Figure 4a; Δν̃(15NO−14NO) = −13 cm−1; ν̃(15N−O)/ν̃(14N−O) =

Figure 4. IR spectra of crystalline (a) 2 and (b) 3 prepared from PhNO (black line) and Ph15NO (red line) and the difference spectrum of the NO stretching vibrations for 2 (inset).

0.986, calculated as 0.982 for an isolated harmonic N−O oscillator]. These values are in good agreement with the DFTcalculated ones [936 cm−1; Δν̃(15NO−14NO) = −15 cm−1; Figure S33 and Table S9]. ν̃(14N−O) for 2 is much lower than the value reported for free PhNO (1506 cm−1)4 but in the range of the N−O stretching of hydroxylamines (895 cm−1 for H2N−OH), confirming that the PhNO substrate in 2 is doubly reduced, essentially resulting in a N−O single bond. The dinickel complex C has a reported N−O stretch for the sideon-bridging PhNO at 915 cm−1 [Δν̃(15NO−14NO) = −19 cm−1],13 similar to and just slightly higher than that of 2, in line with the very long N−O bonds in both compounds. Somewhat surprisingly, and contrasting the trends in the N− O bond lengths, the mononickel complex B with a η2(PhNO)2− unit was reported to have a ν̃NO vibrational band at D


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Inorganic Chemistry

4-15NO, recorded at 153 K, indicate slightly rhombic patterns with small g anisotropy (g = 2.051, 2.036, and 2.024 for 4 and g = 2.054, 2.042, and 2.022 for 4-15NO) but pronounced anisotropy of the hyperfine coupling [A(14N) = 15, 30, and 83 MHz; A(15N) = 14, 20, and 80 MHz]. The closely spaced g values near the free-electron value (ge = 2.0023) in combination with the labeling experiment indicate that the radical is mostly ligand-centered at the bound PhNO substrate and that the redox event at E1/2 = −1.53 V vs Fc/Fc+ can be assigned to interconversion of dinickel(II)-bridging (PhNO)2− and (PhNO)•− species. The energy-minimized DFT-calculated structure of 4 (BP86/def2-tzvp) supports this interpretation and reveals that the singly occupied molecular orbital (SOMO) is the π*(NO) orbital that is perpendicularly oriented to the plane of the central {Ni1−Npz−Npz−Ni2− N−O−} core. A Mulliken population analysis indicates that the unpaired spin density is predominantly located on the bound PhNO (69%), in particular on its N atom (32%), in accordance with the large 14N/15N hyperfine coupling, whereas only a minor part is found on the Ni ions (10% on the Obound Ni1 and 17% on the N-bound Ni2; Figure 8). The IR spectra of solid 4 and 4-15NO suggest the N−O stretch of 4 to lie around 1155 cm−1 [Δν̃(15NO−14NO) = −20 cm−1; ν̃(15N−O)/ν̃(14N−O) = 0.982; Figure S21]. This value is in good agreement with the DFT-calculated N−O stretch for 4 at 1108 cm−1 (1092 cm−1 for 4-15NO; Table S9 and Figure S35). A shift of νNO to higher energy by around 200 cm−1 upon oxidation of 2 to 4 supports that oxidation occurs at the PhNO ligand. DFT-calculated N−O stretching frequencies for 3 and 4 (see the Supporting Information) differ by around 100 cm−1; the lower value reflects that the N−O bond in 2 is further weakened by interaction with the K+ cation. The findings for 2−4 are in line with the results for mononuclear nickel(II) complexes A and B (Figure 1 and Table S3), which were reported to exhibit redox noninnocence of the side-on-bound PhNO with a mostly ligand-centered radical in the oxidized species A,12 as is now also shown for the μ-κ(O):κ(N) PhNO substrate binding mode. Protonation of the Reduced PhNO Complexes. The redox couple (PhNO)2−/(PhNO)•− is reminiscent of the more familiar peroxo (O22−)/superoxo (O2•−) pair, so that similar reactivity patterns might be anticipated. For instance, reversible acid/base interconversion between pyrazolate-based μ1,2peroxo- and μ1,1-hydroperoxodicopper(II) complexes was recently authenticated,36 and H-atom abstraction (HAA) from the latter was reported to give the corresponding μ1,2superoxodicopper(II) species.37 In contrast, protonation of the peroxodinickel(II) complex so far only led to isolation of the diamagnetic μ-hydroxodinickel(II) complex [LNi2(μ-OH)] without any detectable intermediate.29 The dinickel(II) complexes 2 or 3 featuring the doubly reduced (PhNO)2− ligand were now found to undergo clean protonation with Brönsted acids such as 2,6-lutidinium triflate ([HLut][OTf]) to give the diamagnetic product LNi2(μκ(O):κ(N)-ONHPh) (5). Single crystals suitable for X-ray diffraction were grown by diffusion of hexanes into a concentrated THF solution of 5 at room temperature within a few days. The molecular structure of 5 is depicted in Figure 9, and selected structural parameters are included in Table 1. Both Ni ions are again found in a slightly distorted square-planar geometry, and the PhNOderived ligand retains its μ1,2 binding mode at a Ni···Ni distance of 3.88 Å, i.e., between the Ni···Ni distances of 2 and

Figure 5. CV of complex 2 in THF at room temperature at a scan rate of 100 mV·s−1 with NBu4PF6 (0.1 M) as the supporting electrolyte; potentials vs Fc/Fc+. The inset shows the reversible process centered at E1/2= −1.53 V at different scan rates.

Figure 6. Electrochemical oxidation of complex 2 in THF at room temperature monitored via UV−vis spectroscopy.

potential of −1.0 V leads to full conversion to a new species 4, reflected by the vanishing of the broad absorption bands at 611 and 721 nm characteristic for 2, while the band around 374 nm that is seen for all nickel(II) complexes of L3− slightly increases in intensity. Subsequent application of a rereducing potential of −2.0 V fully restores the features of the original spectrum of complex 2. Bulk oxidation of 2 was performed with decamethylferrocenium hexafluorophosphate ([FeCp*2]PF6), and the resulting solution was analyzed by electron paramagnetic resonance (EPR) spectroscopy (Figure 7). The X-band EPR spectrum of 4 in THF at room temperature shows an isotropic three-line signal centered at giso = 2.040 characteristic for an S = 1/2 system with strong coupling to a 14N nucleus [Aiso(14N) = 40 MHz]. The spectrum of the corresponding 15NO-labeled complex, 4-15NO, expectedly shows two lines with giso = 2.039 and Aiso(15N) = 51 MHz; the ratio Aiso(14N)/Aiso(15N) = 0.78 is close to the absolute value expected from the gyromagnetic ratios (0.71). EPR spectra of frozen THF solutions of 4 and E


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Inorganic Chemistry

Figure 7. X-band EPR spectra (9.41 GHz; microwave power 15 mW) of 4 in THF at room temperature (a) and at 153 K (b) and of 4-15NO at room temperature (c) and at 153 K (d). The red line represents simulation of the experimental data with g = 2.040 and A(14N) = 40 MHz (a), g = 2.051, 2.036, and 2.024 and A(14N) = 15, 30, and 83 MHz (b), g = 2.039 and A(15N) = 51 MHz (c), g = 2.054, 2.042, and 2.022 and A(15N) = 14, 20, and 80 MHz (d). The asterisk (*) in part a indicates a minor impurity around g = 2.0.

the description of 5 as a low-spin dinickel(II) complex having a singly deprotonated (viz., O-deprotonated) phenylhydroxylamine within the bimetallic cleft. The presence of a Ph(H)NO unit was further confirmed by IR spectroscopy of solid 5 and 5-15NO, which shows the 14N− H stretching at 3147 cm−1 and the expected isotopic shift for the 15N-labeled compound [Δν̃(15NH−14NH) = −9 cm−1; ν̃(15N−H)/ν̃(14N−H) = 0.997, calculated 0.997 for an isolated harmonic N−H oscillator; Figure 10]. The calculated IR spectrum for the DFT-optimized structure of 5 shows the 14 N−H/15N−H stretch at 3257/3250 cm−1 and predicts the relatively weak 14N−O stretch to occur around 818 cm−1 [with Δν̃(15NO−14NO) = −13 cm−1; Figure S36 and Table S9], in line with the N−O single-bond character. However, no isotope-sensitive feature could be observed in that range of the experimental IR spectra of 5 and 5-15NO (Figure S23). N−H Bond Dissociation Energy (BDE) of the Ph(H)NO Complex 5 and Radical Reactivity. The Ph(H)NO unit in 5 could be readily deprotonated in a THF solution using Verkade’s base (pKa = 26.6 in THF),38 and UV−vis monitoring evidenced the formation of 3 (Figure S24). In contrast, no reaction occurs when the weaker base 1,5,7triazabicyclo[4.4.0]dec-5-ene (pKa = 21.1 in THF)39 was used, even in a large excess. This allows one to bracket the pKa value of 5, which we estimate to be in the range of 23−25. In

Figure 8. DFT-calculated spin-density plot of complex 4 (see Experimental Section for details). Mulliken spin population: Ni1, 0.099; Ni2, 0.173; O, 0.173; N, 0.321; C(PhNO), 0.197.

3. The H atom could be located at N7 of the PhNO group, which is also reflected in long Ni−N7 [1.945(2) Å] and N7− O1 [1.464(2) Å] bonds that approach the values found for 2; the Ni1−O1 bond length in 5 [1.870(1) Å] is essentially unchanged from that in 3 [1.864(3) Å] but clearly shorter than that in 2. Interaction of the reduced (PhNO)2− ligand with K+ (in 2) or N-protonation (in 5) thus appears to have similar but distinct effects on the structure of the {Ni1−O−N(Ph)−Ni2} fragment. Furthermore, the N−O bond length of 5 is very similar to that of hydroxylamines (1.47 Å), in accordance with F


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Inorganic Chemistry

Scheme 2. Interconversions of the Reductively Bound PhNO Substrate in the Cleft of the {LNi2}− Scaffold, Studied in This Worka

a

The lower right triangle defined the thermodynamic scheme for deriving the BDE of the N−H bond in 5. Figure 9. Plots (50% probability thermal ellipsoids) of the molecular structure of 5; (a) top view (most H atoms omitted for clarity); (b) front view (most H atoms and isopropyl groups omitted for clarity).

characteristic EPR spectrum (Figure S25), while treatment of 5 with an excess (ca. 5 equiv) of TEMPOH [the BDE(O−H) of TEMPOH is 70.6 kcal·mol−1 in MeCN]41 gave an equilibrium mixture of 4 and TEMPO• (Figure S22). Experimental data for the hydroxylamine N−H BDE is rare. Warren et al. reported that complex A (Figure 1) featuring a radical-anion PhNO ligand bound to a single NiII ion undergoes clean HAA reactivity with 1,4-cyclohexadiene [CHD; BDE(C−H) = 76 kcal·mol−1]45 in benzene at 65 °C. Accompanying DFT studies indicated that a single HAA from CHD by A to give the corresponding nickel(II) complex with a bound η2-ON(H)Ph ligand is uphill with ΔG = +10.9 kcal· mol−1,12 implicating a BDE of the N−H bond of nickel-bound hydroxylamine that is similar to the value obtained for the present μ1,2-Ph(H)NO ligand. The obtained BDE(N−H) for 5 is lower than the values estimated for cobalt and iridium complexes containing amido functions [BDE(N−H) > 70 kcal· mol−1]46,47 but higher than that of a iron imido complex (BDE ≤ 43 kcal·mol−1 in THF) and a [P3SiFeN−N(Me)H]+ hydrazido compound (BDE = 49 kcal·mol−1 in THF).48

Figure 10. IR spectra of solid 5 (black line) and 5-15NO (red line) in the range 3300−2600 cm−1 and the difference spectrum of the NH stretching vibrations (inset).

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CONCLUSIONS This report details the reductive binding and ligand-based redox transformations of the archetypical nitrosoarene, PhNO, in μ-κ(O):κ(N)-bridging mode within the bimetallic cleft of a preorganized dinickel(II) scaffold. Reductive binding of PhNO to give a twice-reduced (PhNO)2− ligand is achieved via H2 elimination from a dinickel(II) dihydride complex,28 circumventing the usually required metal reduction prior to substrate activation and highlighting the usefulness of metal hydrides as masked synthons for reduced metal species. The resulting [LNi2(μ-PhNO)]− complex has been characterized structurally and spectroscopically, both with (2) and without (3) K+, as a K+ cation associated with the NO unit of the coordinated nitrosoarene ligand. Similar to what was already observed for the O−O bond of the related μ-1,2-peroxodinickel(II)

combination with the redox potential E0 = −1.53 V40 versus Fc/Fc+ for the conversion of 2 to 4, we were thus able to set up a thermodynamic scheme (shown as part of Scheme 2) and to derive a BDE in the range of 62−65 kcal·mol−1 for the N−H bond of the bridging Ph(H)NO unit in complex 5, according to eq 1, where CH,sol is a constant associated with the H+/H• standard reduction potential in a given solvent (CH,THF = 66 kcal·mol−1).41−43 BDEsol(N−H) = 1.37pK a + 23.06E° + C H,sol

(1)

In accordance with the derived BDE(N−H) for 5, the reaction of 5 with 1 equiv of the phenoxyl radical 2,4,6-tBu3C6H2O• [the BDE(O−H) of 2,4,6-tBu3C6H2OH is 81.2 kcal·mol−1]44 readily produced 4 identified by its G


Inorganic Chemistry complex,29 interaction with the Lewis acid K+, which is hosted between the aryl flaps of the bimetallic cleft close to the substrate’s NO unit in 2, significantly affects the N−O bond, reflected by a significant bond elongation. It is well recognized that the binding of Lewis acidic alkali-metal ions to, e.g., diatomic ligands derived from reduced O2 or N2, may induce significant changes in the bonding situation at both monometallic49 or bimetallic50 sites. Subsequent transformations of the coordinated μ-κ(O):κ(N)-(PhNO) 2− ligand are summarized in Scheme 2: Protonation gives 5 with a bridging O-deprotonated phenylhydroxylamine. HAA from 5 affords 4, which is best described as having a bridging (PhNO)•− radical ligand. Interconversion of the one-electron redox couple 3/4 occurs at very low potential, highlighting the redox noninnocence of the reduced nitrosoarene substrate. The present ligand-based transformations closely parallel a recent report for reduced side-on-bound η2-PhNO at a single Ni ion (A/B in Figure 1),12 which indicates that these fundamental properties are inherent to the PhNO substrate and largely independent of the coordination mode. However, redox interconversion of 3 and 4 occurs at much lower potential (E1/2 = −1.53 V vs Fc/Fc+) than in the case of A/B (E1/2 = −0.89 V vs Fc/Fc+), which may be due to the trianionic nature of the pyrazolate-based supporting ligand in the present system. In combination with the experimentally bracketed pKa value of 5, this allowed for an estimation of the BDE of the N−H bond of the bridging Ph(H)NO unit, which is rather low in the range of 62−65 kcal·mol−1. These findings may be relevant not only in view of the many metal-catalyzed reactions involving nitrosoarenes but also for an understanding of biological transformations of the related substrates hydroxylamine and highly reactive HNO.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Dr. A. C. Stückl for recording EPR spectra. This work has been supported by the University of Göttingen.

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Reductive Binding and Ligand-Based Redox Transformations of

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