Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
Nitrogen−Nitrogen Bond Formation via a Substrate-Bound Anion at a Mononuclear Nickel Platform Mikhail D. Kosobokov,† Aaron Sandleben,‡ Nicolas Vogt,‡ Axel Klein,‡ and David A. Vicic*,† †
Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States Institut für Anorganische Chemie, Department für Chemie, Universität zu Köln, Greinstraße 6, D-50939 Köln, Germany
‡
S Supporting Information *
ABSTRACT: The nickel-C4F8 fragment coordinates an aminoaryl N−H ketimine to form a stable complex, which upon treatment with base and oxidant leads to an N−N bondforming reaction and the release of indazole product. A key and previously unidentified intermediate in the formation of the indazole was a diimine complex of nickel bearing significant charge on the aryl ring that initially contained the amine substituent. The C4F8 coligand was key for the redox transformation and for stabilization of the intermediate for characterization.
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better understanding of how to control nitrogen− nitrogen bond-cleavage and bond-forming reactions at a metal center is required for advances in applications spanning the energy fields to the life sciences. Metal-mediated N−N bond cleavage represents an important step in nitrogen fixation chemistry, and much effort has been devoted to developing less energy intensive methods for ammonia production from N2.1−3 Fundamental studies on the reverse reaction, metal-mediated N−N bond formation, not only can provide complementary information related to the global nitrogen cycle as it relates to fertilizer production but also can lead to useful synthetic methodologies. Many natural products and pharmaceuticals contain N−N bonds,4,5 yet methods to effect N−N bond formation still often invoke highly reactive precursors or intermediates such as azides or nitrenes to proceed.5 Much interest has been focused on using copper for N−N bond-forming reactions.5−7 Copper-free and metal-catalyzed nitrogen−nitrogen bondforming reactions, however, are relatively rare.4,5,8−10 In early studies, a nickel-catalyzed N−N bond-coupling reaction was applied to the cyclotrimerizations of α-aminoketones.8 The first example of a formal reductive elimination of a nitrogen− nitrogen bond at nickel was recently reported,4 but the N−N elimination proceeded at a dinickel complex through a mixedvalent nickel(II)−nickel(III) intermediate that was accessed using a strong oxidant such as PhICl2. Because the further development of dinuclear metal complexes for such bondforming reactions can be challenging, we sought to understand how we could construct a monomeric nickel complex for new N−N bond-forming methodologies. As a starting point, we wished to provide proof in principle that nickel could mediate an intramolecular N−N coupling reaction. Chen and co-workers recently reported that aminoaryl N−H ketimines such as 1 react with copper salts under an aerobic environment to afford indazoles such as 2 (eq 1).11 The reaction described in eq 1 formally involves the loss of two electrons and two protons and © XXXX American Chemical Society
can be used to prepare indazoles bearing a variety of functional groups and chiral centers.11 Unfortunately, no metal-containing intermediates were isolated or studied, making an understanding of this reaction extremely limited. The conversion outlined in eq 1 from the aminoaryl N−H ketimine to an indazole is a two-electron oxidative process; therefore, a nickel promoter for the reaction should be stable under such conditions and possibly be able to shuttle between its higher valent states. The nickel-C4F8 fragment was identified as a promising platform for mediating the desired N−N bondforming reaction, given that it has recently been shown that the C4F8 fragment enables multiple oxidations beyond the commonly encountered nickel(II) state.12 For instance, the square-planar complex [(tbpy)Ni(C4F8)] (3, Chart 1, tbpy = 4,4′-di-tert-butyl-2,2′-dipyridyl) exhibits two one-electron oxidations at −0.02 and +1.16 V versus the ferrocene/ferrocenium couple in MeCN solvent (see the Supporting Information). The related terpyridine complex [(tpy)Ni(C4F8)] can likewise undergo chemical oxidation to the nickel(III) species [(tpy)Ni(C4F8)(MeCN)]+ (4, Chart 1), and this nickel(III) species exhibits an additional reversible oxidation at +1.34 V.12 Given the accessibility of two additional oxidations from the stable nickel(II) state upon ligation to a C4F8 fragment, we explored whether these oxidations could be coupled to a nitrogen−nitrogen bond-forming event in hopes of developing a new methodology with nickel. The coordination chemistry of aminoaryl N−H ketimines to nickel was initially explored in order to understand the Received: December 13, 2017
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DOI: 10.1021/acs.organomet.7b00887 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Chart 1. Electrochemical Potentials (vs Fc/Fc+ in MeCN) of Bipyridine and Terpyridine Nickel-C4F8 Complexes
(1.970(2) Å), presumably due to steric interactions. Interestingly, a weak contact between the amino-bound hydrogen and the oxygen in a cocrystallized THF molecule was also observed in the solid state (Figure 1). The electrochemical features of complex 6 in MeCN solution are highly dependent on the presence of base. In the absence of base, 6 exhibits a reversible one-electron oxidation centered at +0.11 V (vs the ferrocene/ferrocenium couple) and a second less reversible oxidation whose anodic peak potential is +1.14 V (Figure 2). Chemical oxidation of 6 in THF or MeCN using
stabilities of potential nickel intermediates in any N−N bondforming reactions. Reaction of the known [(MeCN)2Ni(C4F8)]13 with 2-(imino(phenyl)methyl)-N-methylaniline (5) led cleanly to the formation of complex 6 in 78% yield (eq 2). Complex 6 is
Figure 2. Cyclic voltammogram of 6 in MeCN (20 mM). Working and counter electrodes are platinum with a Ag pseudo reference electrode. Conditions: electrolyte, [Bu4N][BF4], 100 mM; scan rate, 100 mV s−1.
surprisingly stable and can be purified by column chromatography in the air. An X-ray crystal structure of 6 was obtained (Figure 1), which reveals a roughly square planar geometry around nickel. The imine nitrogen−nickel bond length (1.893(2) Å) is shorter than the amino nitrogen−nickel bond
[(NH4)2][Ce(NO3)6] allowed a study of the radical cationic complex 6•+ by EPR spectroscopy (spectra in the Supporting Information). Interestingly, the isotropic g values at 298 K of 2.136 are not consistent with the three g values from the rhombic spectra obtained at 120 K in glassy frozen solution and T-dependent experiments revealed a marked shift of the g values when the temperature was lowered. Obviously, the electron distribution within 6•+ changes slightly with T. We attribute these spectral features to slightly varying contributions of the two resonance forms [(L)NiIII(C4F8)]+ (A) ↔ [(L•+)NiII(C4F8)]+ (B). Overall, the g values and the large g anisotropy Δg clearly indicate the largest contribution coming from form A, a Ni(III) species, in agreement with related reports.13−16 When 6 was oxidized with (NH4)2[Ce(NO3)6] in THF in the presence of the spin trap N-tert-butyl-α-phenylnitrone (PBN), we observed the isotropic EPR signal of 6•+ alongside with a minor signal at g = 2.006. This isotropic three-line signal represents a PBN-trapped radical in line with an observed hyperfine splitting (HFS) aN of 14 G. aH(α) and further HFS could not be resolved. From this we can conclude that 6•+ is quite stable but cleaves a small amount of a reactive, presumably organic radical under these nonbasic conditions. Oxidative UV−vis spectroelectrochemical experiments were also consistent with clean reversible oxidation of 6 in the absence of base (see the Supporting Information). In the presence of a base such as pyridine, the CV shows an irreversible oxidation at +0.66 V whose peak, on the basis of its height, likely corresponds to an overall two-electron process (Figure 2).
Figure 1. ORTEP diagram of 6·THF. All hydrogens except those bound to nitrogen are omitted for clarity. Selected bond lengths (Å): Ni−N1 1.970(2); Ni−N2 1.893(2); Ni−C4 1.899(3); Ni−C1 1.902(3); O1−H1 2.41(3); N1−C17 1.454(3). Selected bond angles (deg): N2−Ni−C4 91.42(10); N2−Ni−C1 175.83(11); C4−Ni−C1 86.72(12); N2−Ni−N1 89.83(9); C4−Ni−N1 177.12(10); C1−Ni−N1 91.86(10). B
DOI: 10.1021/acs.organomet.7b00887 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
Additionally, 9 already has 2 equiv of fluoride built in to the system. Therefore, the ring-closing reaction described in eq 3 can proceed in good yield (75%) without the need for any CsF salt additives. When the reaction described in eq 3 is monitored by 19F NMR spectroscopy, the fate of the metal complex after the reaction is [(pyr)2Ni(C4F8)]17 (97% after 18 h at room temperature). Because conversion of aminoaryl N−H ketimines to indazoles ultimately involves proton transfers as well as electron transfers, reaction of complex 6 with a Brønsted base was studied. When 6 was stirred with sodium tert-butoxide in THF solvent in the presence of 18-crown-6, conversion to the deprotonated species 7 (eq 2) occurred in quantitative yield by 19F NMR spectroscopy. The NMR features, presented in Figure 3, reveal interesting details regarding the bonding of 6
Table 1. Exploration of Conditions for Nitrogen−Nitrogen Bond-Forming Reactions at Nickel
entry
oxidant (2 equiv)
base/solvent/additive
yield (%)
1 2 3 4 5 6 7 8 9 10 11
[Ag][PF6] [Ag][BF4] [Ag][BF4] AgOAc AgF AgOAc AgF [Fc][BF4] [Fc][BF4] [Fc][BF4] [Fc][BF4]
pyridine or MeCN pyridine or MeCN KOAc (2 equiv) pyridine pyridine pyridine MeCN MeCN MeCN pyridine/MeCN (1/1) pyridine pyridine/CsF (2 equiv)
