Covalent Capture of Nitrous Oxide by N‐Heterocyclic Carbenes

Abstract

A good catch: N-Heterocyclic carbenes (NHCs) form stable adducts with nitrous oxide (N2O) under mild reaction conditions. The adducts display unique reactivity as evidenced by an alkylation reaction which leads to a rupture of the NN bond. In a recent publication, Ravishankara and et al. conclude that nitrous oxide will be “the dominant ozone-depleting substance emitted in the 21st century”.1, 2 N2O is also a greenhouse gas which is around 300 times more potent than CO2.3 The environmental impact of N2O should be regarded as a strong incentive to study the basic chemistry of this gas in more detail. In principle, N2O is a very interesting oxidizing agent.3, 4 It has a high oxidation potential and it is environmentally benign (side product: N2). However, N2O is kinetically very inert and this has hampered its utilization as an oxidant or as a building block for more complex molecules. It is thus not surprising that considerable efforts have been made to activate N2O chemically.3, 4 Transition-metal complexes have emerged as promising activation agents.5 Notwithstanding, N2O is a very poor ligand and structurally characterized N2O complexes have only been reported recently.6 The reaction of N2O with organic compounds typically requires elevated temperatures or pressures.3, 4 Olefins, for example, are converted into carbonyl compounds in the presence of N2O at 150–250 °C and pressures of more than 10 bar.4c The oxidation of PPh3 was found to occur at temperatures below 100 °C but supercritical N2O was employed (p>100 bar).7 Only highly reactive molecules such as triethylborane8 and certain silicon-containing compounds (e.g., silaethenes or disilenes)9 are oxidized by N2O under ambient conditions. An exciting recent finding was the fact that frustrated Lewis pairs (FLPs) are able to bind N2O.10 In contrast to the oxygen-transfer reactions mentioned above, N2O is bound intact between the Lewis acid (fluoroarylboranes) and the Lewis base (trialkylphosphines) as evidenced by crystallographic analyses. Herein we report that N-heterocyclic carbenes (NHCs) are also able to capture N2O to give stable adducts. The adducts display unique reactivity as evidenced by an alkylation reaction which results in rupture of the NN bond. N-heterocyclic carbenes such as the commercially available 1,3-dimesitylimidazol-2-ylidene (IMes) are highly Lewis-basic compounds,11 which are able to form adducts with the inert gas CO2.12 These findings prompted us to explore the reactivity of NHCs towards N2O. When a solution of IMes in THF (33 mM) was subjected to an atmosphere of N2O, the solution slowly became yellow along with the formation of a yellow precipitate (1). Isolation (yield: 90 %) and analysis of 1 by NMR spectroscopy, mass spectrometry, and elemental analysis suggested the formation of an N2O adduct (Scheme 1). This structure was confirmed by a crystallographic analysis (see below). In a related fashion, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) reacted with N2O to give the adduct 2. Purification of 2 required column chromatography on silica gel which reduced the yield of the isolated adduct to 41 %.13 The fact that 2 could be purified by chromatography was a first indication of its high stability. Synthesis of the N2O adducts 1 and 2. The adducts 1 and 2 are very soluble in polar organic solvents (e.g., CHCl3, CH2Cl2, or THF). Crystallization was achieved by slow evaporation of CH2Cl2/n-hexane (for 1) or Et2O/n-hexane (for 2) solutions. Single-crystal X-ray diffraction analyses were performed for both complexes14 and graphical representations of the molecular structures are depicted in Figure 1. Molecular structures of 1 (left) and 2 (right) in the crystal. Thermal ellipsoids shown at the 50 % probability level. In both adducts, a bent N2O group is connected through the N atom (N3) to the carbon atom (C1) of the heterocycle. Overall, the bond lengths and angles are similar for the two adducts (Table 1). The bonds of the C1 atom to the three adjacent nitrogen atoms (N1–N3) all have lengths of approximately 1.36 Å. With 1.25 Å, the N4-O1 bond is significantly shorter than the N3-N4 bond (1.333(2) and 1.352(4) Å for 1 and 2 respectively). This difference is in contrast to what has been observed for N2O adducts of FLPs, for which the NN bond [(1.25±0.01) Å)] is shorter than the NO bond [(1.33±0.01) Å].10 The degree of bending, in contrast, is similar for FLP/N2O adducts and for 1 and 2 (ca. 110°). The rather long bond between the two adjacent nitrogen atoms is reminiscent of what has been observed for imidazolylidene triazines, the coupling products of NHCs and azides. In these compounds, the length of the central NN bond is typically on the order of 1.33–1.37 Å.15 The plane defined by the bent N2O group is markedly inclined with respect to the plane defined by the heterocycle: the dihedral angle N1-C1-N3-N4 found for 1 is 25.4(2)° and that found for 2 is 39.5(5)°. 1 (exptl.) 2 (exptl.) 1 (theor.) 2 (theor.) C1-N1 [Å] 1.366(2) 1.354(4) 1.365 1.361 C1-N2 [Å] 1.360(2) 1.368(4) 1.361 1.357 C1-N3 [Å] 1.360(2) 1.358(4) 1.323 1.327 N3-N4 [Å] 1.333(2) 1.352(4) 1.352 1.346 N4-O1 [Å] 1.250(2) 1.250(4) 1.202 1.205 C1-N3-N4 [°] 109.6(1) 110.7(3) 110.9 110.5 N3-N4-O1 [°] 113.1(1) 112.9(3) 113.9 114.1 N4-N3-C1-N1 [°] 25.4(2) 39.5(5) 24.07 26.25 To gain further information about the electronic situation in NHC/N2O adducts, we performed density functional computations at the M06-2X/cc-pVTZ level of theory.16–18 Computational geometric data generally agrees well with the experimental data, although shorter N4-O1 (by ca. 0.05 Å) and C1-N3 (by ca. 0.03 Å) bond lengths are predicted. Additionally, the differences in dihedral angles (N4-N3-C1-N1) between 1 and 2, which measure the planarity of the N2O relative to the plane of the heterocycle, are not observed: both structures are predicted to be 24–27° out of plane. To assess the resonance picture depicted in Scheme 2, we computed Hirshfeld-I charges19 at the M06-2X/6-31G(d) level of theory as implemented in an in-house version of QChem.20, 21 Both structures 1 and 2 reveal the N3 atom (−0.57, −0.55) to be more negative than the O1 atom (−0.32), which is indicative of a significant contribution from the resonance structure B. Resonance structures of NHC/N2O adducts. The reactivity of adduct 1 was examined in a series of experiments. Solutions of 1 are stable towards water and air. This was evidenced by dissolving 1 in a mixture of [D8]THF and H2O (10:1) under air. No significant decomposition was observed by 1H NMR spectroscopy after three days. Crossover experiments were performed with THF solutions containing equal amounts of 1 and free carbene IPr, or 2 and free carbene IMes. After three days at room temperature, transfer of N2O was not observed by 1H NMR spectroscopy. These results imply a high stability of the adducts 1 and 2. At elevated temperatures, 1 was found to decompose into the urea 3: heating a toluene solution of 1 for 3 hours at 100 °C gave 3 in 60 % yield (Scheme 3). In situ NMR experiments in C2D2Cl4 at 130 °C revealed a clean and quantitative 1→3 transformation within 30 minutes. This type of reactivity is in line with what has been found for FLP/N2O adducts10 and for nitrosylimines,22 both of which decompose thermally with liberation of dinitrogen. Reactivity of the N2O adduct 1. The reactivity of 1 towards electrophiles was examined using methyliodide. When 1 was added to a mixture of toluene and MeI (1:1) a solution formed which slowly turned red. The speed of the transformation could be accelerated by placing the reaction flask next to a mercury lamp.23 After 5 hours, we were able to isolate the alkylation product 4 in 60 % yield (Scheme 3). Compound 4 was examined spectroscopically as well as by single-crystal X-ray analysis (Figure 2).14 The data reveal the formation of a salt composed of a guanidinium-type cation and a triiodide anion. The formation of this salt might proceed through an alkylation of the central N3 atom with subsequent liberation of nitrosyl iodine, which decomposes to give I2.24 A second methylation would give the cation along with the triiodide anion. However, a more detailed study is needed to corroborate this mechanistic hypothesis. The formation of 4 is significant because it demonstrates that NHC-activated N2O can undergo reactions which lead to a rupture of the NN bond. One should note that this type of reactivity if very rare for N2O.25 The fact that we observe alkylation at N3 also confirms the importance of the resonance structure B in agreement with the results of the computational study. Molecular structure of 4 in the crystal. Thermal ellipsoids shown at the 50 % probability level. In summary, we have demonstrated that N-heterocyclic carbenes are able to form stable adducts with N2O under ambient conditions. The adducts display a bent N2O group, which is bound to the heterocycle through the N atom. Alkylation of the N2O adduct 1 by MeI was shown to induce a rupture of the NN bond, a rarely observed reactivity pattern in N2O chemistry. Current efforts in our laboratory aim to provide a more detailed understanding of the chemistry of imidazolium-2-diazotates. Additional results will be reported in due course. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.