Isolation and Reactivity of an Antiaromatic s‐Block Metal Compound

Abstract The concepts of aromaticity and antiaromaticity have a long history, and countless demonstrations of these phenomena have been made with molecules based on elements from the p, d, and f blocks of the periodic table. In contrast, the limited oxidation‐state flexibility of the s‐block metals has long stood in the way of their participation in sophisticated π‐bonding arrangements, and truly antiaromatic systems containing s‐block metals are altogether absent or remain poorly defined. Using spectroscopic, structural, and computational techniques, we present herein the synthesis and authentication of a heterocyclic compound containing the alkaline earth metal beryllium that exhibits significant antiaromaticity, and detail its chemical reduction and Lewis‐base‐coordination chemistry.


Crystallographic Details
The crystal data of 1 and 3 were collected on a BRUKER X8-APEX II diffractometer with a CCD area detector using Mo-Kα radiation and those of 2 on a RIGAKU OXFORD DIFFRACTION SYNERGY-S equipped with a Hybrid Pixel Array Detector HyPix6000 using Cu-K radiation.
Both diffractometers are equipped with multi-layer mirror monochromators and a FR-591 rotating anode source or a microfocus seal-tube, respectively. The structures were solved using intrinsic phasing methods, [4] refined with the SHELXL program [5] and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were assigned to idealized geometric positions and included in structure-factor calculations.
The atomic displacement parameters of atoms belonging to the disordered moiety were restrained with similarity restraint SIMU, rigid-body restraint RIGU and isotropic restraint ISOR.

Computational Details
Geometry optimizations at the B3LYP-D3(BJ)/def2-SVP [6][7][8][9][10][11][12] level of theory were performed for 1, 2, 3 and for a model compound where the Li(OEt2) moieties of 2 were removed, thus leading to the naked dianion [(CAAC)BeC4Ph4] 2-. Stability tests were then conducted on the singledeterminant restricted closed-shell wavefunctions of the optimized geometries at singlet multiplicities, following a previously described protocol. [13,14] While no instability was observed system using high-level complete active space self-consistent field (CASSCF) [15] and Nelectron valence state second-order perturbation theory (NEVPT2) [16][17][18] calculations with def2-SVP and the minimally-augmented minimally augmented diffuse (ma-def2-SVP) basis sets. [19] Given the large size of the system, the CASSCF calculations were performed using the resolution-of-the-identity approximation for Coulomb integrals (RI-J) [20,21] in combination with the numerical chain-of-sphere integration for the HF exchange integrals (COSX). [22,23] In turn, the NEVPT2 calculations were carried out using the DLPNO-NEVPT2 approximation. [24] The biradical character , [25][26][27] which can vary from 0 (closed-shell system) to 1 (pure biradical state), was obtained for [(CAAC)BeC4Ph4] 2using the natural orbital occupancy numbers (NOON) [28] of a CASSCF(4,3) calculation. [29] Additionally, NEVPT2 calculations were performed, using CASSCF(4,3) and CASSCF(6,6) wavefunctions as reference, for both the singlet and triplet multiplicities for an accurate determination of the adiabatic singlet-triplet gap The vertical singlettriplet gap of 2 was also obtained for comparison. The index [25][26][27] was calculated using the weight of the doubly-excited configuration in a CASSCF(4,3) approach, and is given by the following expression: where is the orbital overlap of the highest occupied natural orbital (HONO) and the lowest unoccupied natural orbital (LUNO), and is obtained by collecting the occupation numbers (ON) of the proper natural orbitals: The nature of the Be-CAAC bond was investigated using the energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV) method. [30,31] These calculations were performed using a truncated model of 1 where the phenyl substituents of the BeC4 ring, the methyl groups at CAAC and the diisopropyl groups at the Dip group were replaced by hydrogen atoms. The chosen level of theory was B3LYP-D3(BJ)/TZ2P. Two distinct scenarios were tested, depending on the choice of the fragments. In the first scenario, the BeC4H4 and the CAAC fragments were calculated in their closed-shell singlet states, and their interaction gives rise to a donor-acceptor bonding scheme as a result of σ-donation of the CAAC to the beryllium atom. In the second scenario, the BeC4H4 fragment was calculated with an additional negative charge, whereas the CAAC bore a positive charge. The bond situation in this case would then be an electron-sharing bond between Be and CAAC. The more appropriate choice of fragments is the one in which the orbital interaction energy has the value closest to zero. [31] In order to describe the beryllium systems obtained herein in terms of their (non/anti)aromaticity, nucleus-independent chemical shift (NICS) calculations [32,33] were performed through the gauge-independent atomic orbital (GIAO) method. [34][35][36] These quantities were calculated by placing ghost atoms at the ring centroid and at several distances along the axis perpendicular to the ring plane, with a step size of 0.1 Å. For each one of these points, the zz-component of the magnetic shielding tensor was obtained, and the plot of these values with respect to the distance to the ring centroid gives the NICSzz-scan curve. [37,38] [39,40] obtained herein were calculated at the B3LYP/6-311++G** level of theory from structures optimized at the B3LYP/6-31+G* level. The zero-point-energy-corrected electronic energies were used for estimating the ASEs. Calculations using the CBS-QB3 method [41,42] of selected systems were used to validate the DFT protocol. All DFT and NICS calculations were performed with the Gaussian 16, Revision B.01 software. [43] CASSCF and NEVPT2 calculations were performed with the Orca 4.1.1 software. [44] The EDA-NOCV calculations S21 were performed with ADF 2019. [45] Pictures of molecular structures, orbitals and densities were visualized and generated with Chemcraft, [46] ADFView and Gaussview.
The anisotropy of the induced current density (ACID) method [47,48] was applied to assess the antiaromaticity of CAAC-stabilized BeC4 rings in comparison to C5H5 + and C4H4BH. NMR properties were calculated using the continuous set of gauge transformations (CSGT) method. [36,49,50] ACID plots were obtained using the ACID software package.     (not shown) lies in between the other two. MO-2 remains nearly unchanged because X does not contribute. The computed shapes of the orbitals are given in Figure 3 of the main text.