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–OtBu] 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–OtBu, 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(OtBu)­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

[(Me₃P)₃Co(Bcat)₃]: Equilibrium Oxidative Addition of a B–B Bond and Interconversion between the <i>fac</i>-Tris-Boryl and the <i>mer</i>-Tris-Boryl Complex

The tris-boryl complex [(Me3P)3Co­(Bcat)3], obtained by oxidative addition of B2cat2 to [(Me3P)4Co-Me] via the mono-boryl complex [(Me3P)4Co-Bcat], exhibits unique structural and electronic properties. While it is formed in an equilibrium reaction (ΔG ≈ 0 kJ mol–1) from the intermediate mono-boryl complex [(Me3P)4Co-Bcat], it is isolated in the solid state, depending on the temperature of crystallization, as the fac or mer isomer. Further studies reveal that the fac and the mer isomers of [(Me3P)3Co­(Bcat)3] are comparable in Gibbs energy and their interconversion is thermodynamically feasible. The mer isomer, comprising an unexpected trans B–Co–B motif, was further studied by means of an AIM analysis of the computed (DFT) and experimental electron densities, giving evidence for appreciable B···B interactions between two of the boryl ligands. The complex mer-[(Me3P)3Co­(Bcat)3], featuring three distinct boryl ligands, may, as a limiting case, be described as a diborane(4) complex of [(Me3P)3Co-Bcat]. DFT computations suggest that those B···B interactions are crucial for the stability of the trans B–Co–B complex mer-[(Me3P)3Co­(Bcat)3]

[(Me₃P)₃Co(Bcat)₃]: Equilibrium Oxidative Addition of a B–B Bond and Interconversion between the <i>fac</i>-Tris-Boryl and the <i>mer</i>-Tris-Boryl Complex

The tris-boryl complex [(Me3P)3Co­(Bcat)3], obtained by oxidative addition of B2cat2 to [(Me3P)4Co-Me] via the mono-boryl complex [(Me3P)4Co-Bcat], exhibits unique structural and electronic properties. While it is formed in an equilibrium reaction (ΔG ≈ 0 kJ mol–1) from the intermediate mono-boryl complex [(Me3P)4Co-Bcat], it is isolated in the solid state, depending on the temperature of crystallization, as the fac or mer isomer. Further studies reveal that the fac and the mer isomers of [(Me3P)3Co­(Bcat)3] are comparable in Gibbs energy and their interconversion is thermodynamically feasible. The mer isomer, comprising an unexpected trans B–Co–B motif, was further studied by means of an AIM analysis of the computed (DFT) and experimental electron densities, giving evidence for appreciable B···B interactions between two of the boryl ligands. The complex mer-[(Me3P)3Co­(Bcat)3], featuring three distinct boryl ligands, may, as a limiting case, be described as a diborane(4) complex of [(Me3P)3Co-Bcat]. DFT computations suggest that those B···B interactions are crucial for the stability of the trans B–Co–B complex mer-[(Me3P)3Co­(Bcat)3]