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The reductive coupling of dinitrogen

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Science  22 Mar 2019:
Vol. 363, Issue 6433, pp. 1329-1332
DOI: 10.1126/science.aav9593

Boron brings nitrogen together

Whereas carbon is prone to making chains, nitrogen usually sticks to itself just once in the particularly stable form of N2. Légaré et al. now show that boron can coax two N2 molecules together under reductive conditions below room temperature. Two borylene units sandwiched the resulting N4 chain between them.

Science, this issue p. 1329

Abstract

The coupling of two or more molecules of dinitrogen (N2) occurs naturally under the radiative conditions present in the ionosphere and may be achieved synthetically under ultrahigh pressure or plasma conditions. However, the comparatively low N–N single-bond enthalpy generally renders the catenation of the strongly triple-bonded N2 diatomic unfavorable and the decomposition of nitrogen chains a common reaction motif. Here, we report the surprising organoboron-mediated catenation of two N2 molecules under near-ambient conditions to form a complex in which a [N4]2– chain bridges two boron centers. The reaction entails reductive coupling of two hypovalent-boron-bound N2 units in a single step. Both this complex and a derivative protonated at both ends of the chain were characterized crystallographically.

The propensity of carbon to form molecular chains is unrivalled across the periodic table. In stark contrast, chain formation is heavily disfavored for nitrogen—the next element on the periodic table. With its disproportionately strong homonuclear triple bond (as compared with N–N single and double bonds), dinitrogen (N2) is the thermodynamic end point of nitrogen chemistry (1). Catenated nitrogen compounds tend to be metastable with respect to decomposition and release of N2, rendering them high-energy-density materials with applications in gas-generating formulations for airbag technologies, propulsion, and explosives (2). Recent work has demonstrated the existence of different phases of nitrogen at ultrahigh pressures (36), ultimately leading to the identification of the crystalline allotrope Nn, which consists of a covalently bonded network of nitrogen atoms in a cubic gauche structure (7). Further computational and experimental studies have identified phases of catenated nitrogen in alloys with other elements at ultrahigh pressures and often high temperatures (810). Additionally, under plasma conditions (1113) and in the Earth’s ionosphere (14), N4+, N5+, N5, and other ions can be formed from N2, processes that have no parallels under ambient conditions (15). More recently, the detection of neutral tetranitrogen (N4) from the reionization of N4+, with lifetimes on the order of microseconds, has attracted considerable attention (16, 17).

In synthetic chemistry, in which extreme temperature and pressure conditions are usually unfeasible, the chemistry of nitrogen catenae is dominated by molecular species, with N2 once again being both the start and end point of all nitrogen-containing molecules. Indeed, the near-totality of anthropogenic and biological nitrogen chemistry arises from the initial transition-metal mediated cleavage and reduction of N2 to ammonia (18) and ends with the regeneration of N2 through the denitrification of nitrogen products (19). Remarkably, even those compounds that contain longer chains of nitrogen atoms that are generally only kinetically stable, being energetic materials and prone to loss of N2 (20, 21), are produced from NH3, which is generated from N2 cleavage (Fig. 1). This is true of commercially relevant products (1)—such as hydrazine and azides—of transition metal–stabilized nitrogen chains (22), and of metal-free cyclic and linear polynitrogen compounds (21, 23, 2428). To this day, the formation of nitrogen chains directly from N2 remains elusive under ambient conditions.

Fig. 1 Schematic representation of the current commercial preparation of common nitrogen compounds via NH3 production, versus the direct catenation of N2 reported here.

Natural and anthropogenic reactions of N2 are generally transition metal–mediated. We have recently demonstrated, however, that hypovalent boron species are capable of binding and reducing N2 in the absence of transition metals (29). The coordination and functionalization of N2 by the transient dicoordinate-borylene [(CAAC)BDur] [CAAC, 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene; Dur, 2,3,5,6-tetramethylphenyl] (30) was facilitated by boron-to-N2 π-backbonding, which is usually the preserve of transition metal chemistry (31) but was enabled in this case by the electron-rich, hypovalent nature of the borylene center (32). Here, we report a surprising extension of this work, in which borylene chemistry transcends transition metal mimickry by mediating the spontaneous catenation of two N2 molecules into an N4 chain, capped by two boron centers, under near-ambient conditions.

The end-on bridging (μ2) coordination mode of N2 between two metal centers is very common in transition metal complexes (33, 34) and was also the bonding motif that we observed in the boron-based fixation of N2. Indeed, the reduction of [(CAAC)BBr2Dur] by KC8 under an atmosphere of N2 affords {[(CAAC)DurB]22-N2)}, putatively through the generation of the reactive borylene [(CAAC)BDur]. Neither the likely monoborylene intermediate [(CAAC)BDur(N2)] nor any other coordination modes were observed under these conditions. We were therefore interested in studying the possibility of stabilizing such intermediates by disfavoring the formation of end-on μ2-bridging complexes by increasing the steric repulsion of the ancillary groups at the boron centers.

To this end, we synthesized the dihaloborane precursor [(CAAC)BCl2Tip] (Tip, 2,4,6-triisopropylphenyl) using standard methods and successfully reduced it in good yields to the stable [(CAAC)BClTip] radical (1), which is a one-electron reduction step away from the hypothetical base-stabilized borylene [(CAAC)BTip] (2-Tip) (35). Further reduction of 1 by KC8 in toluene under argon affords the C–H activation product 3, likely from the intramolecular insertion of transient borylene 2-Tip into a C–H bond of the CAAC ligand. This process has precedent with other CAAC-stabilized arylborylenes, and its regioselectivity is consistent with previous reports (2931). The formation of 3 thus strongly supports the formation of the transient borylene 2-Tip from the reduction of 1.

When we reduced 1 using KC8 (10 equivalents) in toluene under 4 atm of N2, we obtained a turquoise-colored solution after filtration. In this reaction, from a paramagnetic, 11B nuclear magnetic resonance (NMR)–silent reactant (1), we obtained a diamagnetic product featuring a broad 11B NMR resonance at δ = 27.9 parts per million (ppm) (5). After we isolated this product as dark blue crystals in a moderate yield (31%), analysis of the mother liquor revealed that it still contained 5 as the main product, which we were not able to further separate from the small amounts of 3 present as a contaminant. The conversion of 1 to 5 thus appears to be selective, but its quantitative isolation from the minor side-product remains difficult. The isolation of crystalline 5 allowed us to elucidate its solid-state structure by means of x-ray diffraction analysis, revealing an unusual bonding motif for nitrogen (Fig. 2). Instead of featuring N2 in the common end-on bridging motif, 5 is best described as a bis-borylene complex of a [N4K2] moiety, {[(CAAC)TipB]22-N4K2)}, which arises from the direct catenation of two N2 molecules mediated by the hypovalent boron centers. The room-temperature 1H NMR spectrum of 5 showed broad resonances. At low temperature, however, a sharpening of the signals allowed us to discern spectra consistent with a centrosymmetric molecule. Although the line-broadening could be attributed to fluxional processes such as hindered rotations in the bulky substituents, no decoalescence of the resonances was observed at temperatures as low as –70°C, at which the signals are considerably sharper. This broadening could possibly be attributed to the influence of a triplet excited state, which is consistent with computational studies that predict a low singlet-triplet gap.

Fig. 2 Reactivity of 1 with KC8 and N2 toward the catenation of nitrogen, including the solid-state structures of 3, 5, and 6, in which hydrogen atoms are omitted for clarity.

Similarly, the solid-state structure of 5 reveals a centrosymmetric complex in which the (NCB)N4(BCN) core is entirely planar, suggesting extensive π delocalization. This conclusion is confirmed by the bond lengths within this planar core, which are all indicative of partial double bonds. The B–N bond lengths 1.442(2) Å, where the number in parentheses is the estimated standard deviation on the last digit of the measurement suggest a degree of multiple bonding between boron and nitrogen and lie between those in the previously described N2 complexes {[(CAAC)DurB]22-N2)} [1.423(4) and 1.403(5) Å] and {[(CAAC)DurB]22-N2K2)} [1.484(4) Å] (29). The short B–CAAC bonds [1.526(3) Å] suggest B-to-CAAC backbonding typical of borylene complexes (2934). All the N–N bond distances in the N4K2 moiety of 5 are similar [1.349(2) Å for N1-N2 and N3-N4 and 1.335(2) Å for N2-N3] and lie between the ranges of N–N double and single bonds {as exemplified by azobenzene [d(N=N); 1.247(2) Å] (36) and 1,2-diphenylhydrazine [d(N–N); 1.394(7) Å]} (37), further confirming the delocalized nature of the bonding in 5.

The dipotassium complex 5 is strongly basic and could be selectively protonated. Upon addition of an excess of degassed water to a turquoise toluene solution of 5, the mixture immediately turned dark blue. The resulting product was 11B NMR–silent, but protonation could be easily confirmed with the appearance of a sharp N–H vibrational band in the infrared spectrum (3315 cm–1). Through subsequent filtration and slow evaporation, we successfully isolated dark blue crystals of {[(CAAC)TipB]22-N4H2)} (6) in good yield (75%). Contrary to 5, 6 displays paramagnetic behavior at room temperature, which can be attributed to a diradical structure. Variable-temperature electron paramagnetic resonance (EPR) spectroscopy of a toluene solution of 6 showed a dependence of the EPR signal intensity on the temperature. Measurements in the 230 to 290 K range are consistent with a thermally excited triplet state for 6, and the results can be fitted to a small singlet-triplet gap of 0.5 kcal mol–1 according to the Bleaney-Bowers equation (supplementary materials).

Single-crystal x-ray diffraction analysis of 6 showed that the tetrazene core is preserved but that the structural parameters substantially differ from those of 5, as would be expected from its paramagnetic nature. The B–N bonds [1.432(2) Å] are closer in length to those of formal, covalent single bonds. Additionally, the terminal N–N bond lengths lie within the range of single bonds [1.371(1) Å], and a conventional double bond exists between the central N2 and N3 atoms [1.272(2) Å]. This localized bonding motif is reminiscent of known organic tetrazenes and of the parent N4H4, which possess central double bonds in their most stable isomers (24, 38). Relatively short B–CAAC bonds [1.534(2) Å], as well as coplanarity between the CAAC π frameworks and the trigonal planar boron atoms, were also found in 6, these features being in accord with known B–CAAC radicals (29, 32, 35).

Using the same methods described above with 15N2 as a substrate, the 15N isotopologues of 5 and 6 (5-15N and 6-15N, respectively) could be prepared. In both cases, high-resolution mass spectrometry (HRMS) analysis was consistent with the quantitative enrichment of 5 and 6. In the case of 5-15N, 1H- and 11B-decoupled 15N (15N{1H,11B}) NMR spectroscopy gives two symmetrical multiplets at δ = 13.2 and 69.3 ppm of equal intensity, which is consistent with a symmetrical N4 chain. The multiplicity of the resonances is consistent with an AA′XX′ spin system, and the 15N–15N spin-spin coupling was confirmed with a 15N,15N{11B} correlation spectroscopy experiment. Thus, the preparation and characterization of 5-15N and 6-15N not only confirm the identification of a N4 unit in 5 and 6 but also provide a simple entry into isotopically labeled 15N4 chains—which are otherwise difficult to obtain—directly from the coupling of 15N2 gas.

Despite the apparent differences of 5 and 6 in terms of magnetism and bonding detailed above, a quantum-chemical assessment revealed far-reaching similarities between both species. In both cases, the computed strong delocalization throughout the π-orbital framework is consistent with the crystallographic bond distances. Furthermore, the electronic structures of both systems are dominated by strong, nondynamic correlation effects arising from small highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energy gaps (detailed discussion of the bonding situation is provided in the supplementary materials). State-averaged CASSCF(2,2)/DLPNO-NEVPT2/def2-TZVPP calculations based on closed-shell singlet M06L-D3/6-311G(d,p) geometries of 5 and 6 resulted in antiferromagnetically coupled biradicaloid (singlet) ground states, which lie 3.5 kcal mol–1 and 0.9 kcal mol–1 below the corresponding triplet biradicals, respectively. These values are in agreement with the variable-temperature NMR (VT-NMR) characterization of 5 (significant line-broadening at room temperature) and with the VT-EPR results for 6 (singlet ground state favored by 0.5 kcal mol–1), as well as its paramagnetic character at room temperature. The larger singlet-triplet gap computed for 5 is consistent with a drastically lower thermal population of the triplet state, which provides a convincing rationale for the relatively well-resolved NMR spectra of this biradicaloid species.

The strong π delocalization in 6 is reflected in the computed Wiberg bond index (WBI) pattern and atomic charges resulting from natural population analysis (NPA) (supplementary materials). With reference to the neutral constituents, the NPA reveals a net electron transfer of 0.39 e from each borylene fragment onto the central N4H2 moiety. A WBI of 1.66 indicates a central N=N double bond, whereas significantly smaller values were calculated for the terminal N–N bonds (1.15), closely resembling the data obtained for the parent tetrazene H2N–N=N–NH2 (N=N, 1.81; N–N, 1.11). Compared with the bare borylene [(CAAC)BTip] (2-Tip), the B–CAAC double-bond character in 6 is more delocalized across the N–C–B groups, which also carry most of the biradical spin density (fig. S7). These results suggest the Lewis structure representation shown in Fig. 2, illustrating the π bond pattern in 6. The bonding situation in 5 is similar, yet π delocalization is even more pronounced, resulting in a weaker central N=N bond (1.31), which is consistent with a smaller computed net electron transfer of 0.18 e onto the N4K2 fragment (supplementary materials).

We propose that the formation of 5 proceeds through the initial coordination of N2 to the transient borylene [(CAAC)BTip] (2-Tip) to form a 1:1 borylene-N2 adduct (4-Tip). An analogous intermediate without steric protection (4-Dur) was postulated as an intermediate in the previously described end-on coordination of N2 to [(CAAC)BDur]. Whereas in the case of the less bulky 4-Dur the reaction with a second equivalent of borylene 2-Dur is rapid (k1-Dur > k2-Dur) (Fig. 3), the corresponding process is considerably hindered by steric repulsion in the case of the bulkier 4-Tip. Under these conditions, the reduction of 4-Tip by excess KC8 can become the predominant mechanism, leading to a nitrene-like radical, which readily dimerizes to 5 (k2-Tip > k1-Tip) (Fig. 3). This last step is supported by precedent in the reductive dimerization of organic azides (39) and diazo compounds (40) mediated by a magnesium(I) reducing agent.

Fig. 3 Proposed mechanism for the fixation of N2 by 2-Dur and the catenation of N2 by 2-Tip.

Whereas borylenes have been shown to bind N2 in a manner akin to transition metals, forming end-on bridging (and potentially also terminal) N2 complexes, they retain a distinct main-group character to their reactivity, as highlighted by the transformation of intermediate 4-Tip, which is more reminiscent of the reductive coupling of organic diazo compounds (40). Terminal N2 complexes of transition metals are not known to catenate N2, even under reductive conditions, presumably because reduction preferentially occurs at the metal center rather than at the N2 ligand. These results underscore the potential of low-valent main group elements to unlock distinct reactivity pathways.

Supplementary Materials

www.sciencemag.org/content/363/6433/1329/suppl/DC1

Materials and Methods

Figs. S1 to S57

Tables S1 to S7

References (4171)

References and Notes

Acknowledgments: We warmly thank R. D. Dewhurst for the concept of Fig. 1, K. Hammond and M. Müller for the synthesis of CAAC precursors, and C. Mahler for mass spectrometry. Quantum-chemical calculations were performed at the Center for Scientific Computing (CSC) Frankfurt on the FUCHS and LOEWE-CSC high-performance computer clusters. Funding: Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged. M.-A.L. thanks the Natural Sciences and Engineering Research Council of Canada for a postdoctoral fellowship. G.B.-C. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship. Author contributions: Experiments were designed by M.-A.L. and performed by M.R., M.-A.L., and G.B.-C. Data analysis was done by M.-A.L., G.B.-C., and M.R. X-ray crystallography was performed by G.B.-C. and M.A. The EPR investigation was led by I.K., and the NMR experiments were designed by R.B. Computational investigations were performed by J.I.S. and led by M.C.H. The project was overseen by H.B. and coordinated by M.-A.L. Competing interests: The authors declare no competing interests. Data and materials availability: Experimental and computational details, as well as NMR, HRMS, ultraviolet-visible, and infrared spectroscopy data are available as supplementary materials, and crystallographic data are available free of charge from the Cambridge Crystallographic Data Centre under reference numbers CCDC 1876961–1876969.
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