Diborane(4) Azides: Surprisingly Stable Sources of Transient Iminoboranes

Abstract Herein we describe the first examples of isolable electron‐precise diboranes(4) that bear azide moieties: the acyclic 1,2‐diazido‐1,2‐bis(dimethylamino)diborane(4) and the cyclic 1,4‐diaryl‐2,3‐diazido‐1,4‐diaza‐2,3‐diborinines (aryl=mesityl, 2,6‐xylyl, 4‐tolyl). The reported examples are not only stable enough to be observed and isolated (putative transient diborane(4) azides previously reported by our group spontaneously decompose even below room temperature), but some of them are even robust enough to undergo controlled pyrolysis without explosive decomposition at temperatures well above 100 °C. In two cases, the controlled pyrolysis allows the isolation of complex diazaboretidines, which are the apparent dimerization products of endocyclic boryl‐iminoboranes.


X-ray Crystallographic Details
The crystal data of 2a, 2b, 2c and 3b were collected on a BRUKER D8 QUEST diffractometer with a CMOS area detector and multi-layer mirror monochromated MoK  radiation. The crystal data of 3a were collected on a BRUKER X8-APEX II diffractometer with a CCD area detector and multi-layer mirror monochromated MoK  radiation.

S23
The structures were solved using the intrinsic phasing method, [4] refined with the SHELXL program [5] and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized geometric positions and depicted as spheres of arbitrary radius. Ellipsoids are represented at the 50% probability level. Hydrogen atoms are omitted for clarity.

XDBN3 (2b)
The two azide moieties were found in two conformations each, which were modeled as two independent two-part disorders with refined ratios of 84: 16    One azide moiety was found to be in two conformations, which was modeled as a two-part disorder with a refined ratio of 0.51:0.49. The displacement parameters of atoms N101_1 > N103_2 of the disordered azide moiety were restrained to the same value with similarity restraint SIMU. The atomic displacement parameters of atoms N101_1 > N103_2 were restrained with the RIGU keyword in the ShelXL input ('enhanced rigid bond' restraint for all bonds in the connectivity list. Values of 0.003 for both parameters s1 and s2 were used). The 1-2 and 1-3 distances in N101 > N103 in residues 1 and 2 (disordered azide moiety) were restrained to the same values with SAME.

MDBN3dT 3a
One azide moiety was found to be in two conformations, which was modeled as a two-part disorder with a refined ratio of 0.66:0.34. The displacement parameters of atoms N101_1 > N103_2 of the disordered azide moiety were restrained to the same value with the similarity restraint SIMU. The 1-2 and 1-3 distances in N101 > N103 in residues 1 and 2 (disordered azide moiety) were restrained to the same values with SAME.  Goodness-of-fit on

Computational Details
Initially, we performed geometry optimization calculations on the diazidodiborane (4)  In order to characterize the optimized geometries as minimum energy structures, we performed Hessian calculations and thermochemical analyses also at the (U)B3LYP/6-31+G* level. All geometries were characterized as minimum energy structures in their respective potential energy surfaces by vibrational frequency analysis, as all Hessian eigenvalues are positive. We applied a scaling factor of 0.94 [9] on the computed vibrational frequencies as well as a Gaussian broadening fitting (band width at half-height: 50 cm -1 ) in order to compare them with the experimental IR spectra. Single-point calculations at the B3LYP/6-311++G** [7a, 10] level of theory starting from the optimized structures were obtained for generating molecular orbital diagrams for 1, 2a, 2a'' and 3a.
In order to address the presence of aromatic character in the cyclic diborane species, we performed nucleus-independent chemical shift (NICS) [11] calculations for 2a and benzene (5) S34 at the same level of theory (B3LYP/6-311++G**//B3LYP/6-31+G*) ( Figure S34). The out-ofplane zz component of NICS was obtained for several distances by placing ghost atoms up to 6 Å above and below the ring plane, with a step size of 0.1 Å. A negative value of the NICS (1) magnetic shielding, which is calculated for a distance of 1 Å from the ring plane, is indicative of aromaticity. By plotting the zz component of NICS as a function of the distance from the ring plane, we obtain the NICS-scan profile. The presence of a minimum in the NICS-scan curve also indicates aromatic character. [11c] All calculations were carried out with Gaussian 16, Revision B.01. [12] Pictures of molecular structures, orbitals and densities were visualized and generated with Chemcraft, [13] ADFView and CYLview.
In order to confirm the open-shell character of the singlet nitrene 1 2a nit , we performed single-point calculations using high-level complete active space self-consistent field (CASSCF) [14] and N-electron valence state second-order perturbation theory (NEVPT2) [ levels with cc-pVDZ [16] basis set were performed. We obtained the singlet-triplet gap for each of those levels and estimated the biradical character index ( ) in the spin-projected unrestricted Hartree-Fock (PUHF) formalism. [17] The index is defined by the weight of the doubly-excited configuration within a multiconfigurational self-consistent field (MCSCF) approach, and is given by the following expression: where T is the orbital overlap of the highest occupied natural orbital (HONO) and the lowest unoccupied natural orbital (LUNO), and is obtained by the occupation numbers, η, of the respective UHF natural orbitals: The index can vary from 0 (closed-shell system) to 1 (pure biradical state). The calculations were performed using the ORCA 4.1.1 software. [18] We also investigated the reaction mechanism starting from 2a that leads to the dimeric product 3a. We tried two distinct approaches: the first involved the formation of a transient nitrene species, while in the second the formation of the iminoborane seven-membered ring 2a'' is achieved directly after N2 liberation without a previous nitrene formation. Geometry optimizations, stability tests and Hessian calculations were performed at the B3LYP/6-31+G* for all transition states (TSs) and intermediates. Intrinsic reaction coordinate (IRC) [19] calculations were also performed in order to confirm the connectivity between transition states and their respective reactants and products. Single-point calculations at the B3LYP/6-311++G** level of theory starting from the optimized minimum and TS structures were performed in order to correct the electronic energies. We also took into account dispersion interactions by combining Grimme's empirical dispersion correction GD3 [20] with the B3LYP functional to all of the calculated minima and TSs. For computing the ΔG of solvation, we employed the solvation model based on density (SMD) [21] technique with mesitylene (ε = 2.2650) as solvent. The entropic contributions obtained within the ideal gas approximation at p = 1 atm for molecules in solution are overestimated, which affects especially associative/dissociative reaction steps. [22] In order to better describe the liquid state, the Gibbs free energies reported in this work incorporate zero-point energy, thermal and entropic contributions that were evaluated at 298.15 K and 176 atm (p = ρmesityleneRT; ρmesitylene = 0.8637 g cm -3 ), following the procedure described by Martin et al. [23] Finally, we compared the thermochemistry of the dimerization of two iminoborane rings 2a'' with the one that could hypothetically be achieved from the junction of two triplet nitrenes 3 2a nit leading to N2-bridged compounds. For that purpose, we performed the same calculations as the ones described in the previous paragraphs for the cisand trans-N2-bridged bis(azido diborane) compounds 4 and 4b.