Ge=B π-Bonding: Synthesis and Reversible [2+2]Cycloaddition of Germaborenes

Phosphine-stabilized germaborenes featuring an unprecedented Ge=B double bond with short B...Ge contacts of 1.886(2) (4) and 1.895(3)angstrom (5) were synthesized starting from an intramolecular germylene-phosphine Lewis pair (1). After oxidative addition of boron trihalides BX3 (X=Cl, Br), the addition products were reduced with magnesium and catalytic amounts of anthracene to give the borylene derivatives in yields of 78% (4) and 57% (5). These halide-substituted germaborenes were characterized by single-crystal structure analysis, and the electronic structures were studied by quantum-chemical calculations. According to an NBO NRT analysis, the dominating Lewis structure contains a Ge=B double bond. The germaborenes undergo a reversible, photochemically initiated [2+2] cycloaddition with the phenyl moiety of a terphenyl substituent at room temperature, forming a complex heterocyclic structure with Ge-IV in a strongly distorted coordination environment.