An Isolable Three‐Coordinate Germanone and Its Reactivity

Abstract A rare three‐coordinate germanone [IPrN]2Ge=O (IPrN=bis(2,6‐diisopropylphenyl)imidazolin‐2‐imino) was successfully isolated. The germanone has a rather high thermal stability in arene solvent, and no detectable change was observed at 80 °C for at least one week. However, high thermal stability of [IPrN]2Ge=O does not prevent its reactivity toward small molecules. Structural analysis and initial reactivity studies revealed the highly polarized nature of the terminal Ge=O bond. Besides, the addition of phenylacetylene, as well as O‐atom transfer with 2,6‐dimethylphenyl isocyanide make it a mimic of nucleophilic transition‐metal oxides. Mechanism for O‐atom transfer reaction was investigated via DFT calculations, which revealed that the reaction proceeds via a [2+2] cycloaddition intermediate.


Assignment of the Ge=O peak
There are two relevant peaks around 900 cm -1 that can be assigned to the Ge=O bond. DFT calculations indicated that Ge=O stretching is at 907 cm -1 while there is another notable peak at 861 cm -1 that mainly belongs to the C=N moiety of the IPrN groups. We emphasize that the N-Ge π-interactions weakens the degree of Ge=O π-bonding in 2. However, the phenyl rings in (Eind)2Ge=O are not perpendicular to the Ge=O bond and as such also have a weakening effect on the Ge=O bond. Therefore, the Ge=O π-bonding in 2 may not be significantly different than that in (Eind)2Ge=O, which can explain the similar IR frequency. This is backed up by the fact that the measured Ge=O bond length is almost identical in 2 (1.6494(10) Å) and in (Eind)2Ge=O (1.6468(5) Å). Additionally, the detailed orbital analysis shows clear Ge=O π and π* orbitals in 2, which are barely coupled with the N of the twisted IPrN groups.

Synthesis of compound 3
To the solution of 2 (40 mg, 45 μmol) in C6D6 (0.4 mL) in a J. Young PTFE tube, pinacolborane (HBpin) (7 μL, 45 μmol, 1.0 eq.) was added at room temperature. After 45 min, the completion of the reaction was confirmed by 1 H NMR. The solvent was removed in vacuo to afford 3 as an orange solid (45 mg, 98%).

Synthesis of compound 8
To the solution of 2 (40 mg, 45 μmol) in C6D6 (0.4 mL) in a J. Young PTFE tube, 2,6dimethylphenyl isocyanate (7 μL, 45 μmol, 1.0 eq.) was added at room temperature. After 30 min, the completion of the reaction was confirmed by 1 H NMR. The solvent was removed in vacuo to afford 8 as an orange solid (44 mg, 90%  Figure S31. 1 H NMR spectrum of compound 8. The respective resonances of IPrNH, resulting from minor complex decomposition in solution, are marked with asterisks * and residual 2,6-dimethylphenyl isocyanate with an S. Figure S32. 13 C NMR spectrum of compound 8.

Reaction of 2 and methanol (MeOH)
To the solution of 2 (40 mg, 45 μmol) in C6D6 (0.4 mL) in a J. Young PTFE tube, methanol (MeOH) (2 μL, 45 μmol, 1.0 eq.) was added at room temperature. The color rapidly changed from orange to yellow. The product was confirmed by 1 H NMR spectrum as the free ligand IPrNH.

General Information
The X-ray intensity data of 2 was collected on an X-ray single crystal diffractometer equipped with a CMOS detector , an IMS microsource with MoKα radiation (λ = 0.71073 Å) and a Helios mirror optic by using the APEX III software package. [S3] The measurement was performed on single crystals coated with the perfluorinated ether Fomblin® Y. The crystal was fixed on the top of a microsampler, transferred to the diffractometer and frozen under a stream of cold nitrogen. A matrix scan was used to determine the initial lattice parameters. Reflections were merged and corrected for Lorenz and polarization effects, scan speed, and background using SAINT. [S4] Absorption corrections, including odd and even ordered spherical harmonics were performed using SADABS. [S4] Space group assignments were based upon systematic absences, E statistics, and successful refinement of the structures. Structures were solved by direct methods with the aid of successive difference Fourier maps, and were refined against all data using the APEX III software in conjunction with SHELXL-2014 [S5] and SHELXLE. [S6] All H atoms were placed in calculated positions and refined using a riding model, with methylene and aromatic C-H distances of 0.99 and 0.95 Å, respectively, and Uiso(H) = 1.2· Ueq(C). Full-matrix least-squares refinements were carried out by minimizing w(Fo 2 -Fc 2 ) 2 with SHELXL-97 [S7] weighting scheme. Neutral atom scattering factors for all atoms and anomalous dispersion corrections for the non-hydrogen atoms were taken from International Tables for Crystallography. [S8] The image of the crystal structure was generated by Mercury. [S9] The CCDC number 2053427 (2)

Computational Section
DFT calculations were performed at the ωB97X-D/def2-TZVPP//B97-D/def2-SVP level of theory. [S10-S13] Stationary points on the potential energy surface (PES) were characterized by harmonic vibrational frequency calculations. Electronic structure analysis was carried out at the same level of theory as the geometry optimization. All calculations were carried out using GAUSSIAN 09 program. [S14] Figure S34. DFT-derived mechanism of the reaction of 2 and XylNC.     Table S6. Cartesian coordinates of transition state (+20.5 kcal/mol) in Figure S34 in Å.   Table S12. Cartesian coordinates of 8 in Figure S34 in Å.