A Planar Five‐Membered Aromatic Ring Stabilized by Only Two π‐Electrons

Abstract Many chemicals known today are partially or fully aromatic, since a ring framework experiences additional stabilization through the delocalization of π‐electrons. While aromatic rings with equal numbers of π‐electrons and ring atoms such as benzene are particularly stable, those with the minimally required two π‐electrons are very rare and yet remain limited to three‐ and four‐membered rings if not stabilized in the coordination sphere of heavy metals. Here we report the facile synthesis of a dipotassium cyclopentagallene, a unique example of a five‐membered aromatic ring stabilized by only two π‐electrons. Single‐crystal X‐ray diffraction revealed a planar Ga5 ring with almost equal gallium–gallium bond lengths, which together with computational and spectroscopic data confirm its aromatic character. Our results prove that aromatic stabilization goes far beyond what has previously been assumed as minimum π‐electron count in a five‐atom ring fragment.

. a) Simulated UV-Vis spectrum of the free pentacyclogallane dianion 1 2derived from TD-DFT calculations (PBE-GD3BJ/def2TZVP); oscillator strengths shown as green lines. b) Electronic transitions associated with selected absorption events of the calculated UV-Vis spectrum along with the respective frontier molecular orbitals.

Characterization
The NMR spectra were recorded with Bruker Fourier 300 and Bruker Avance 400 spectrometers (T = 285 K or 298 K) with δ referenced to external tetramethylsilane standard ( 1 H, 13 C). 1 H and 13 C NMR spectra were calibrated by using the solvent residual signal (C6D12: δ( 1 H) = 1.38 ppm) and the solvent signal (C6D12: δ( 13 C) = 26.4 ppm), respectively. For the assignment of the multiplicities, the following abbreviations are used: s = singlet, m = multiplet. Elemental analysis was performed with a Vario EL Cube (Elementar Analysensysteme GmbH). For GC-MS analysis, the liquid sample was injected via aTriPlus™ RSH autosampler (Thermo Fisher Scientific, Bremen, Germany) on a TRACE™ 1310 gas chromatograph (Thermo Fisher Scientific, Bremen, Germany). The SSL injector in split mode (1:10 split flow) at a temperature of 300 °C, and a gas flow of 1 mL/min. Helium 5.0 was used as carrier gas with an additional moisture and oxygen trap (the vendor specifies a gas quality of 6.0 after passage), using a flow rate of 1 mL/min on a ZBSemiVolatiles column (5% phenyl, 95% dimethylpolysiloxane; 30 m length, 0.25 mm inner diameter, 0.25 μm-film thickness, 10 m pre-column, Phenomenex, Aschaffenburg, Germany). The initial oven temperature of 40 °C was held for 2 min, and then increased to 320 °C using a rate of 10 °C/min. This temperature was kept for 5 min. A Q Exactive GC mass spectrometer (Thermo Fisher Scientific, Bremen, Germany) was used in the experiment. The resolution was set to 120,000 (FWHM) throughout analysis the mass range was set to 50-650 u for measurements. Automated gain control (AGC target) was set to 1 × 10 6 , and maximum inject time was set to "auto". Auxiliary temperatures were set to 280 °C for both transfer lines 1 and 2. MS transfer line temperature was set to 250 °C and the temperature of the electron ionization source was set to 300 °C. EI was performed at 70 eV energy, a filament delay was set to 5.4 min. Nitrogen for supply of the C-Trap and HCD cell of the S5 GC Orbitrap had a minimum purity of 99.999% (Linde AG, Munich, Germany), and was further dried, using a moisture filter (the vendor specifies a gas quality of 6.0 after passage; Thermo, Bremen, Germany). IR spectra (given in cm -1 ) were recorded with an Agilent Cary 630 FT-IR spectrometer using a diamond ATR unit inside a nitrogen-filled glovebox. UV-Vis spectra (wavelength given in nm) were recorded with a Specord S 600 device. Melting points were measured in grease-sealed glass capillaries under dinitrogen atmosphere using an apotec Schmelzpunktbestimmer (Kleinfeld Labortechnik) apparatus and are uncorrected.

Synthetic Procedures
To a Schlenk vessel containing 100 mg (0.205 mmol) of the gallium(I) β-diketiminate 2, 110 mg (0.406 mmol) of KCH(TMS)2•thf 3 and a glass stirring bar, 5 mL of n-hexane were added and the mixture was stirred at r.t. for one hour, which led to formation of a deep-green solution. This solution was kept at 5 °C for one day to precipitate the potassium β-diketiminate 4 from the mixture. The respective colourless crystals were filtered off and the green solution was concentrated and stored at 5 °C for a week, which afforded 6 mg (0.004 mmol, 10 % yield) of 1 as deep-green crystals ( Figure S1).

Crystallographic Details
The single-crystal X-ray intensity data for compound 1 were collected on a Nonius Kappa-CCD diffractometer equipped with an Apex-II CCD detector, using Mo-Kα radiation, at T = 120(2) K. Multiscan absorption correction was applied to the intensity data. [4] The crystal structure was solved with SHELXT-2018/3 [5] and refined by full matrix least-squares methods on F 2 with SHELXL-2018/3, [6] using the Olex 1.2 environment. [7] For the refinement of the disordered CH(SiMe3)2 groups and THF ligands, restraints on the interatomic distances and anisotropic displacement parameters were used (SADI, SIMU, and ISOR instructions in SHELXL) [6] . The site occupancy factors of the disordered atoms were refined freely. Crystallographic data and details on structure refinement for compound 1 are given in Table S1.   Table S1. Crystallographic data and details on structure refinement for compound 1.

Hirshfeld Surface Analysis
In order to investigate the intermolecular interactions in the crystal structure (in this particular case interactions between the anionic and cationic fragment), Hirshfeld surface analysis was conducted using the program Crystal Explorer 17.5. [8,9] Figure S4 displays the Hirshfeld surface of the dianionic 1 2-fragment. The red surface area located around the Ga5 ring indicates strong interactions, which were identified as the expected Ga-K interactions. Figure S5 shows exclusively the contribution of  Table S2). This type of interactions can be seen as slightly red spots on the Hirshfeld surface in Figure S4 and is due to H•••H contacts between the Si(CH3)3 groups.

Computational Details
All geometry optimizations and frequency calculations were performed with the Gaussian16 program package [10] in conjunction with the PBE functional [11] and def2-TZVP basis sets [12] . To consider London dispersion, the D3 version of Grimme's dispersion with the Becke-Johnson function was used. [13] The stability of the wavefunction was tested and is given. The absence of imaginary frequencies confirmed stationary points as minima. The global minimum structure of the 1 2dianion corresponds to a singlet ground state and features a Ga5 core that deviates slightly from planarity. To obtain insight into the electronic absorption spectra, the 18 lowest singlet excited states were calculated using time-dependent DFT (TDDFT), the PBE functional [11] and def2-TZVP basis sets [12[ . Solvent effects (n-hexane: ε = 1.8819) were taken into account by the solute electron density (SMD) model. [14,15] The absorption spectra were simulated with GaussSum 3.0 [16] using a half-bandwidth of NICS calculations [17,18] were performed according to the following scheme on geometry optimized structures (level of theory: PBE/def2TZVP, tight convergence criterion). The probes for the NICSZZ S11 height scan in Z direction were aligned in 0.1 Å distance intervals orthogonal to the averaged Ga5 ring plane in 1 2calculating the NMR shielding tensors with the Gauge-Independent Atomic Orbital (GIAO) method (level of theory: PBE/def2TZVP). Extraction of the anisotropic NMRZZ component of the shielding tensors provides the "out-of-plane" shielding of the induced ring current. NICS XY scans and several NICS XY maps were calculated at heights starting from 1.5 Å above and parallel aligned to the ring plane with a rectangular grid size of 10 Å × 10 Å in intervals of 0.5 Å resulting in 400 probes. [19] Based on these scans, the critical height with least influence of σ-electrons but sufficient contribution of π-electrons was determined (for 1 2-:1.7-2.0 Å) as suggested in the literature. [20] The obtained 2D NICSZZ heat maps were compared to three different cyclic carbocationic scaffolds, namely trimethylcyclopropenyl cation, tetramethylcyclobutadienyl dication and pentamethylcyclopentadienyl trication (optimized at the PBE/def2TZVP level of theory with zero imaginary frequencies).
The density of delocalized electrons which induce a ring current was calculated and visualized using the ACID method at the level of theory indicated for each plot. [21,22] The geometry-optimized structure of   to a diatropic ring current (negative NICSZZ values) with increasing distances to the ring plane (graph on the right). The NICSZZ valley at distances between 1.7 Å and 2.0 Å represents a moderate diatropic ring current of the π-electron system, which represents the π-aromatic character of the 2π-system in 1.    The scale bar on the right shows the colour code of the respective NICSZZ values in ppm and the coordinates are in Å. The non-planar feature in the optimized geometry of the tetramethylcyclobutadienyl dication is in agreement with literature, [23,24] and was similarly found for the pentamethylcyclopentadienyl trication.