Terminal versus Bridging Boryl Coordination in N-Heterocyclic Carbene Copper(I) Boryl Complexes: Syntheses, Structures, and Dynamic Behavior.

The B-B bond activation of the diborane(4) derivatives B2cat2 with the copper(I) alkoxido complex [(SIDipp)Cu-O tBu] delivers, depending on the solvent, either the linear boryl complex [(SIDipp)Cu-Bcat] from PhMe or the μ-boryl complex [((SIDipp)Cu)2Bcat][cat2B] from THF. The relevant conversion of the linear boryl complex to the μ-boryl complex occurs in the polar solvent via formal boryl anion abstraction by the Lewis acid catB-O tBu, concomitantly formed during the B-B activation. With Lewis acids such as BPh3 or [CPh3][BArF] (reversible), boryl abstraction from the linear complexes [(SIDipp)Cu-Bcat] or [(SIDipp)Cu-Bdmab] occurs and results in the μ-boryl complexes [((SIDipp)Cu)2Bcat/dmab][Ph3B-Bcat/dmab] and [((SIDipp)Cu)2Bcat][BArF]. The formation of [((SIDipp)Cu)2Bcat][cat2B] is generally accompanied by the concomitant formation of the μ-hydrido complex [((SIDipp)Cu)2H][cat2B]. The spiroborate [cat2B]- is formed from the initially formed Lewis acid/base adduct [catB-B(O tBu)cat]- presumably in a process that involves the glass surface of the reaction vessel. All complexes are thoroughly characterized structurally as well as spectroscopically, in particular with respect to the dynamic behavior of the μ-boryl complexes in solution.