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) Å ( 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.