Nitrogen (RN) contains a nitrogen atom that does not conform to the eight-electron rule, is usually extremely low in kinetic and thermodynamic stability, and has a lifetime in the nanosecond range at room temperature. Because of their "temper tantrums", azabine is also an important intermediate in synthetic chemistry. Similar to the related carbene (R2C) and equielectron nitrogen cation ([R2N]+), nitrogen is a hexavalent electron (6VE) compound with low coordination, electron deficiency, and non-bonding electron properties, which contribute to the realization of specific and violent reaction modes, such as the use of nitrogen bins to internalize benzene derivatives for backbone editing to construct related pyridines. However, due to the short lifetime of nitrogen bin at room temperature, its properties are mainly inferred by cryogenic trapping experiments, time-resolved spectroscopy and quantum chemical calculations. Over the years, chemists have made great efforts to control azabine, carbine, azabine cations, and their congeners, such as: 1) electron stabilization using donor atoms to compensate for electron deficiency; 2) The kinetically stable use of large steric hindrance substituents to shield the reaction center, and a large number of synthesis protocols to date can be used to prepare stable carbenees with various properties. Specifically: 1) the carbon-based bismuthine compound I reported by Dostál's group in 2010 and the phosphorus-substituted azine II and phosphorus bin III reported by Bertrand's group in 2012 and 2016, respectively (Fig. 1), which have electronically stable and singlet ground state characteristics, and are useful for simulating the bond activation of transition metal-driven small molecules by the main group elements; 2) In 2020, Holthausen and Schneider et al. generated a highly unstable platinum-azabin IV when irradiating a single crystal of the precursor azide on a diffractometer, and the palladium-azabin synthesized by the IV and the group was the only azine bin with a triplet ground state obtained experimentally; 3) In 2021, Prof. Beckmann successfully isolated the kinetically stable phosphorus ion [MSFluindMesP]+(V), which is also a Lewis superacid, by using a large steric hindrance and rigid MSFluind substituent; 4) In 2023, Neese and Cornella et al. used the substituent MSFluindtBu with tert-butyl group on the fluorene group to prepare a kinetically stable triplet bismuth bin MSFluindtBuBi(VI), which has a huge zero-field splitting and quenching response due to the large spin-orbit coupling, which in turn shows a sharp nuclear magnetic resonance (NMR) signal instead of an electron paramagnetic resonance (EPR) signal. 5) In the same year, Ye and Tan et al. introduced the cyclohexyl substituent MSFluind* on fluorenyl to synthesize a kinetically stable antimony bin MSFluind*Sb(VII), showing a broad NMR signal. Recently, the team also attempted to synthesize a similar phosphine bin, which was found to be a related singlet phosphine MSFluindtBuP(VIII), and that the significant electronic interaction of the phosphorus atom with one of the flanking fluorene groups broke the symmetry.
Recently, Emanuel Hupf and Jens Beckmann of the University of Bremen, Germany, and other researchers quantitatively generated stable aryl azine MSFluindN(2) by photolysis of aryl azide MSFluindN3(1), which can be stored at room temperature and argon atmosphere for more than 3 days without change. The ultra-long lifetime of this azabine allowed the investigators to fully characterize it by single crystal X-ray crystallography, EPR spectroscopy, and SQUID magnetometer and confirmed that it was a triple azine. At the same time, theoretical simulations show that in addition to the kinetic stability brought by the large steric hindrance MSFluind aryl substituent, the electron delocalization on the central aromatic ring also contributes to its electronic stability. The results were published in Science.
Figure 1. Carbene analogues of electronically and dynamically stable group 15 elements. Image source: Science
First, the authors prepared the azide precursor MSFluindN3 (1, yield: 70%) by a reaction between organolithium MSFluindLi(THF)2 and p-toluenesulfonyl azide (p-MeC6H4SO2N3), and the process could be synthesized at gram scale. According to thermogravimetric analysis, the azide MSFluindN3(1) is thermally stable in air (up to 150 °C) and has an absorption range of 250-300 nm. When the saturated benzene solution of MSFluindN3(1) was irradiated with UV-C at room temperature, MSFluindN(2) was generated, and the 1H-NMR spectroscopy showed that 1 had been completely consumed and the NMR silent 2 was quantitatively generated, and the paramagnetic red crystal 2·0.5 benzene (yield: 80%, Figure 2) was separated directly from the mother liquor at 80% yield by crystallization, but it was not sufficient for single crystal X-ray diffraction studies. It is worth mentioning that 2·2 CH2Cl2 single crystals suitable for X-ray diffraction analysis can be obtained by recrystallization of dichloromethane. In addition, thermodynamic simulations show that the formation of 2 is moderately exothermic at 298.15 K (ΔG°=-47.10 kJ mol-1), but the endothermic activity is weak (ΔH°=4.51 kJ mol-1). Unlike the previous study of arylazine, the authors found that 2 was stored in solid form at room temperature and argon atmosphere for ≥ 3 days and remained unchanged, but the bulk solid sample of 2 was heated at 140 °C for 18 h to undergo intramolecular C-H bond insertion and obtain isomers with 80% crude yield and 45% separation yield3. Similarly, when the benzene solution of 2 is heated at 80 °C for 48 h, the same main product 3 is formed, but the transformation is still incomplete. Mechanistically, C-H bond insertion is accompanied by N(I)→ N(III), which is mainly due to the strong interaction of N with one of the flanking fluorenyl groups, which in turn leads to the cleavage of spirocyclic aryl bonds and the formation of new C-N and N-H bonds and C=C double bonds after deprotonation of spirocyclic C-H bonds. In addition, computational chemistry has shown that the thermal rearrangement of 2 is an exothermic process (ΔH°=-183.9 kJ mol-1).
Figure 2. Synthesis and thermorearrangement of MSFluindN (2). Image source: Science
Given the thermal stability of 2, the authors characterized aryl nitrogen bins while using LiAlH4 to reduce1 and obtain a diamagnetic colorless solid aniline MSFluindNH2 (4, yield: 66%). By comparing the molecular structures of 2 (Fig. 3A) and 4 determined by single-crystal X-ray crystallography, the authors found that they had distinctly distinguishable structural features, such as the presence or absence of H atoms on N atoms, different C-N bond lengths (1.327(2) vs 1.382(2) Å), and different quinone type features (0.044 vs 0.013), indicating that the degree of electron delocalization between the nitrogen atom and the central aromatic ring was different. In addition, UV-vis and IR spectroscopy can also clearly distinguish between 2 and 4. Unlike the heavier triplet pnictoginidenes VI and VII, aryl azine 2 has EPR activity in benzene solution at room temperature, which confirms the triplet ground state (Figure 3B), while the g value of the EPR signal is 2.00145, which is coupled not only to nitrogen atoms (A1(14N) = 65 MHz, A2(14N) = 12 MHz), but also to the central ring para-hydrogen atom (A1(1H)=17 MHz), This indicates the presence of spin delocalization on the central aromatic ring. As shown in Figure 3C, the authors also measured the macroscopic magnetization of a 2•2 CH2Cl2 single crystal using a superconducting quantum interferometer (SQUID) magnetometer, where the magnetic susceptibility χM curve shows the typical temperature dependence of paramagnetic species. In addition, the C-value of the Curie constant changes from 0.159 for zero-field-cooled (ZFC) to 0.233 for field-cooled (FC) depending on the measurement mode.
Figure 3. Characterization of arylazine MSFluindN (2). Image source: Science
Next, the authors investigated the electrical properties of 2 by quantum chemical calculations, and the results showed that: 1) the energy of the open-shell and closed-shell singlet states was 76.4 kJ mol-1 and 134.1 kJ mol-1, respectively, which was similar to the reported value of phenylazabinine; 2) electron delocalization is also reflected in the occupancy number of natural orbitals based on the Fully Active Space Self-Consistent Field (CASSCF) and shows significant deviations from the ideal biorbital, single, or unfilled orbitals; 3) the detection of spin density revealed the spin density distribution of nitrogen (1.41 e) and central ring (0.52 e), further confirming the importance of spin delocalization (Fig. 3D); 4) The CASSCF occupancy number and spin density are in sharp contrast with those of Bismuth Bin VI and Antimonybin VII, which have significantly less electron delocalization and spin densities that are almost entirely present on Sb (1.97 e) and Bi atoms (2.03 e). 5) bond descriptors based on electron density (AIM, NCI, and IGMH) confirm that the nitrogen atom of 2 is essentially monocoordinated and does not participate in other intramolecular or intermolecular interactions; 6) The key parameters obtained by the AIM analysis (e.g., the electron density at the critical point of the N-C bond and the Laplace operator) indicate a strong interaction with multi-bond characteristics, and the delocalization index δ (N-C) is 1.43. Secondly, the analysis of the natural bond orbital (NBO) of 2 showed that α electrons had 3 nitrogen lone pairs and β electrons had 1 nitrogen lone pair. For N-C bond interactions, α electrons occupy one single bond orbital, while β electrons occupy two bonding orbitals. However, for α electrons, the second-order perturbation theory produces a strong N(pz)→C(pz) contribution (495.8 kJ mol-1), which in turn shows that the N-C bond interaction has a multi-bond characteristic, and the Wiberg bond index is 1.53. On the other hand, the authors analyzed the electronic structure by the electron locality indicator function (ELI-D), and the results showed that the spatial division and the change of the population number of different ELI-D spin operators reflected different α and β orbital occupation, which was consistent with the results of NBO analysis. Finally, the authors used time-dependent density functional theory (TD-DFT) calculations to study the deep red color of 2, and the results show that the simulated absorption spectra are consistent with the experimental spectra, and the differential density plot of the T0→Tn transitions shows that for the two functions, the fluorene group to the central N-aryl moiety is in most cases.
summary
In this paper, the authors synthesized a stable aryl azide MSFluindN(2) by photolysis of the aryl azide MSFluindN3(1), which can be stored at room temperature and argon atmosphere for more than 3 days without change. The ultra-long lifetime of the nitrogen bin allows for comprehensive characterization of single crystal X-ray crystallography and EPR spectroscopy, and the results confirm its triplet nitrogen properties, and theoretical simulations show that in addition to the kinetic stability brought by the large steric hindrance MSFluind aryl substituent, the electron delocalization on the central aromatic ring also contributes to the electronic stability of 2.
Synthesis of a stable crystalline nitrene
Marvin Janssen, Thomas Frederichs, Marian Olaru, Enno Lork, Emanuel Hupf, Jens Beckmann
Science, 2024, DOI: 10.1126/science.adp4963
(This article was contributed by pyridoxal)