Inducing Nucleophilic Reactivity at Beryllium with an Aluminyl Ligand

The reactions of anionic aluminium or gallium nucleophiles {K[E(NON)]}2 (E = Al, 1; Ga, 2; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) with beryllocene (BeCp2) led to the displacement of one cyclopentadienyl ligand at beryllium and the formation of compounds containing Be–Al or Be–Ga bonds (NON)EBeCp (E = Al, 3; Ga, 4). The Be–Al bond in the beryllium–aluminyl complex [2.310(4) Å] is much shorter than that found in the small number of previous examples [2.368(2) to 2.432(6) Å], and quantum chemical calculations suggest the existence of a non-nuclear attractor (NNA) for the Be–Al interaction. This represents the first example of a NNA for a heteroatomic interaction in an isolated molecular complex. As a result of this unusual electronic structure and the similarity in the Pauling electronegativities of beryllium and aluminium, the charge at the beryllium center (+1.39) in 3 is calculated to be less positive than that of the aluminium center (+1.88). This calculated charge distribution suggests the possibility for nucleophilic behavior at beryllium and correlates with the observed reactivity of the beryllium–aluminyl complex with N,N′-diisopropylcarbodiimide—the electrophilic carbon center of the carbodiimide undergoes nucleophilic attack by beryllium, thereby yielding a beryllium–diaminocarbene complex.


Synthesis of (NON)GaBeCp (4)
To an ampoule fitted with a Teflon valve and equipped with a glass-coated stirrer bar was added a solid mixture of 2 (100 mg, 0.128 mmol) and BeCp 2 (18 mg, 0.130 mmol, 1.01 equ.). Toluene (7 mL) was condensed into the vessel in vacuo at -196 °C. The colourless solution was allowed to warm to room temperature and stirred overnight, leading to the formation of a colourless precipitate. The resulting suspension was filtered and volatiles were removed in vacuo. The waxy off-white solid was dried in vacuo for a further two hours at 60 °C to remove unreacted BeCp 2 . Subsequently, the solid

Synthesis of (NON)Al{(N i Pr) 2 C}BeCp (5)
To an ampoule fitted with a Teflon valve and equipped with a glass-coated stirrer bar was added 3 (30 mg, 0.0389 mmol). Toluene (1 mL) and CDI (0.02 mL) were condensed into the vessel in vacuo at -196 °C. The pale yellow solution was allowed to warm to room temperature and stirred overnight.
Subsequently, volatiles were removed in vacuo. The beige solid was dried in vacuo for a further two hours at 60 °C to remove unreacted CDI. Subsequently, the solid was extracted with hexane (

Synthesis of (NON)Ga{(N i Pr) 2 C(NH i Pr)} (6)
To an ampoule fitted with a Teflon valve and equipped with a glass-coated stirrer bar was added 4 (30 mg, 0.0389 mmol). Toluene (1 mL) and CDI (0.02 mL) were condensed into the vessel in vacuo at -196 °C. The pale orange solution was allowed to warm to room temperature and stirred overnight.
Subsequently, volatiles were removed in vacuo. The waxy orange solid was dried in vacuo for a further two hours at 60 °C to remove unreacted CDI. Subsequently, the solid was extracted with hexane (3 x 3 mL) and the solution concentrated to 3 mL. Storage overnight at -30 °C yielded a crop of pale orange crystals. Yield: 18 mg, 53%. Single crystals of 6 suitable for X-ray diffraction experiments were obtained by allowing a concentrated hexane solution to stand for two hours at room temperature. Once isolated as crystalline material, 6 is insoluble in C 6 D 6 and decomposes, with formation of a black/grey precipitate, upon heating. Therefore, NMR data of suitable quality could not be obtained for this compound. Anal. Calcd for C 63

Crystallographic Data
Single-crystal X-ray diffraction data for compound 3 was collected on a Stoe Stadi Vari. Single crystals were selected under a pre-dried argon stream in perfluorinated polyether (Fomblin YR 1800, Solvay Solexis) and mounted using the MiTeGen MicroLoop system at ambient temperature. X-ray diffraction data was collected using the monochromated Cu-Kα (λ = 1.54186 Å) radiation of a Stoe StadiVari diffractometer equipped with a Xenocs Microfocus Source and a Dectris Pilatus 300 K.
Evaluation, integration and reduction of the diffraction data was carried out using the X-AREA software suite. 15 Multi-scan absorption correction was applied with the LANA module of the X-AREA software suite. The structures were solved with dual-space methods (SHELXT-2018/2) and refined against F2 (SHELXL-2018/3) using the OLEX2 software package. [5][6][7] All atoms were located by Difference Fourier synthesis and non-hydrogen atoms refined anisotropically. Hydrogen atoms were refined using the "riding model" approach with isotropic displacement parameters 1. Cryosystems open flow N 2 cooling device. 3 Selected details of data collection are given in Table 1.
Data collected were processed using the CrysAlisPro package, including unit cell parameter refinement and inter-frame scaling (which was carried out using SCALE3 ABSPACK within CrysAlisPro). 4 Equivalent reflections were merged and diffraction patterns processed with the CrysAlisPro suite. 4 Structures were solved ab initio from the integrated intensities using SHELXT and refined on F2 using SHELXL with the graphical interface OLEX2. 5 Figure S10: Molecular structure of 4 in the solid state as determined by X-ray crystallography.
Thermal ellipsoids set at 50% probability; hydrogen atoms omitted, and selected substituents shown in wireframe format, for clarity. S13 Figure S11: Molecular structure of 6 in the solid state as determined by X-ray crystallography.
Thermal ellipsoids set at 50% probability; selected hydrogen atoms omitted, and selected substituents shown in wireframe format, for clarity.

S14 9 Be NMR Chemical Shifts -CpBeR Complexes
Complexes 3 and 4 have been characterized by a range of analytical techniques, including multinuclear NMR spectroscopy. Perhaps most informative are the 9 Be NMR shifts for these species.
In the case of both 3 and 4, 9 Be NMR resonances (-28.8 and -26.9 ppm, respectively) are extremely high-field shifted, as is common for cyclopentadienyl-beryllium species. This is generally ascribed to ring current effects and to high electron density at the cyclopentadienyl ring, given the covalency of   Figure S12: Plot of 9 Be NMR chemical shifts for a range of CpBeR species vs the Pauling electronegativity of the atom from each group which is bonded to beryllium.

Computational Details
The geometry optimizations (B3LYP D3BJ def2-TZVP def2/J) were performed with the ORCA (Revision 5.0.2) programme. [8][9][10][11][12] D3BJ dispersion corrections were used. 11,12 Full analytical frequency calculations were performed for the optimized structures to ensure the nature of the stationary points found (minima, no imaginary frequencies). The bonding and charge distribution in the complexes was examined by using Natural Atomic Orbital and Natural Bond Orbital analysis, in addition to Natural Population Analysis, as implemented in NBO7. 13 Topological and charge distribution analysis was carried out via QTAIM analysis (performed using the programme Multiwfn). 14 Figure S14). In the case of 4, the HOMO and HOMO-4 are of Be-Ga σ-bonding character ( Figure S16). In both 3 and 4 the LUMO comprises a Be-Al ( Figure S13) or Be-Ga ( Figure S15) in-phase combination of π-symmetry orbitals.