Synthesis, Electronic Properties and Reactivity of [B₁₂ X₁₁ (NO₂ )]^2- (X=F-I) Dianions

Nitro-functionalized undecahalogenated closo-dodecaborates [B₁₂ X₁₁ (NO₂ )]^2- were synthesized in high purities and characterized by NMR, IR, and Raman spectroscopy, single crystal X-diffraction, mass spectrometry, and gas-phase ion vibrational spectroscopy. The NO₂ substituent leads to an enhanced electronic and electrochemical stability compared to the parent perhalogenated [B₁₂ X₁₂ ]^2- (X=F-I) dianions evidenced by photoelectron spectroscopy, cyclic voltammetry, and quantum-chemical calculations. The stabilizing effect decreases from X=F to X=I. Thermogravimetric measurements of the salts indicate the loss of the nitric oxide radical (NO^. ). The homolytic NO^. elimination from the dianion under very soft collisional excitation in gas-phase ion experiments results in the formation of the radical [B₁₂ X₁₁ O]^2-. . Theoretical investigations suggest that the loss of NO^. proceeds via the rearrangement product [B₁₂ X₁₁ (ONO)]^2- . The O-bonded nitrosooxy structure is thermodynamically more stable than the N-bonded nitro structure and its formation by radical recombination of [B₁₂ X₁₁ O]^2-. and NO^. is demonstrated.