Bottleable Neutral Analogues of [B₂H₅]⁻ as Versatile and Strongly Binding η² Donor Ligands

Herein we report the discovery that two bottleable, neutral, base-stabilized diborane(5) compounds are able to bind strongly to a number of copper(I) complexes exclusively through their B−B bond. The resulting complexes represent the first known complexes containing unsupported, neutral σ_B−B diborane ligands. Single-crystal X-ray analyses of these complexes show that the X−Cu moiety (X=Cl, OTf, C₆F₅) lies opposite the bridging hydrogen atom of the diborane and is near perpendicular to the B−B bond, interacting almost equally with both boron atoms and causing a B−B bond elongation. DFT studies show that σ donation from and π backdonation to the pseudo-π-like B−B bond account for their formation. Astoundingly, these copper σ_B−B complexes are inert to ligand exchange with pyridine under either heating or photoirradiation.