Synthesis and Characterization of the First Enol-Germenolate and its Keto-Germenolate Tautomer

The enol-tautomer of a germenolate - [(tBuMe2Si)2Ge=C(OLi·Et2O)(1 Ad)]2 (8) (1 Ad=1 adamantyl), and its keto-tautomer (tBuMe2Si)2Ge–C(=O···Li·15-crown-5)(1 Ad) (9), were synthesized, isolated, and characterized by X-ray crystallography, NMR, ssNMR, and UV/Vis spectroscopy. Enol-germenolate 8 is the first reported enol-tautomer of a germenolate. 8 has a nearly planar geometry around the Ge atom (Σ∡(Ge)=355.1°) and a short Ge=C bond (1.885 Å). In contrast, its keto-tautomer 9 has pyramidal geometry around the Ge (Σ∡(Ge) = 299.0°) and a significantly longer Ge–C bond (2.062 Å). This is the first example of a heavy Group 14 enolate, which can be isolated as both the keto- and the enol-tautomer, depending on the coordinated solvent. In THF solution, the precursor germenolate exists mainly as the keto-tautomer (tBuMe2Si)2Ge–C(=O···Li·nTHF)(1 Ad) (10). When isolated in the solid state, 10 becomes the enol-tautomer [(tBuMe2Si)2Ge=C(OLi·THF)(1 Ad)]2 (12), as determined by NMR, ssNMR, and UV/Vis spectroscopy. Our results show that solution NMR, ssNMR, or UV/Vis spectroscopy, combined with routine DFT calculations, can be a reliable tool for assigning the structures of heavier Group 14 enolates and for characterizing their keto-enol (or intermediate) tautomeric form in solution and in the solid state - even without crystallographic data.