When two R₂BH borane molecules engage the same transition-metal center, up to six product types (di(σ-borane), dihydride-diboryl, boryl-dihydrideborate, hydride-boryl-σ-borane, diboryl-dihydrogen, dihydride-diborane) are possible, but until 2015 only the first two had been isolated. The 2015 report of the first boryl-dihydrideborate complexes, Os(Bcat)(κ²-H₂Bcat)(CO)(P(i)Pr₃)₂ and its boryl-tetrahydrideborate relatives, left three linked questions open: the pathway of the thermally activated Bcat site exchange in this complex (proposed to proceed via hydride-boryl-σ-borane intermediates, but never confirmed experimentally or computationally), whether other metal fragments and boranes beyond catechol- and pinacol-borane support analogous boryl-dihydrideborate species, and whether such complexes are catalytically relevant.

Background

Catecholborane (HBcat) and pinacolborane (HBpin) are monohydroborane reagents whose B–H bonds are activated at transition metals in catalytic hydroboration and dehydrogenative borylation. The first step of B–H activation is coordination of the B–H bond to an unsaturated metal fragment, giving an M(η²-H–BR₂) σ-borane complex (a σ-complex in which the intact B–H bond donates into empty metal orbitals with back-donation into the B–H σ* orbital); oxidative addition then gives a boryl ligand (an X-type M–B bond, i.e., a one-electron covalent ligand–metal bond). A dihydrideborate ligand, by contrast, is an anionic [H₂BR₂]⁻ group bound to the metal through two bridging M–H–B hydrogen bonds, topologically reminiscent of diborane B₂H₆. When two R₂BH molecules interact with one metal center, the possible products were classified (Esteruelas, López, Mora, Oñate, Organometallics 2015, 34, 941) into six types: di(σ-borane) (I), dihydride-diboryl (II), boryl-dihydrideborate (III), hydride-boryl-σ-borane (IV), diboryl-dihydrogen (V), and dihydride-diborane (VI); before that work only types I and II had been isolated. The paper reported that the 16-electron fragment Os(CO)(P(i)Pr₃)₂ stabilizes the first type III species: the σ-borane complex OsH₂(η²-H–Bcat)(CO)(P(i)Pr₃)₂ reacts with a second HBcat molecule, releasing H₂, to afford octahedral Os(Bcat)(κ²-H₂Bcat)(CO)(P(i)Pr₃)₂ (complex 6 in the paper), in which an X-type catecholboryl ligand and a κ²-H,H-dihydrideborate ligand coexist. This complex can be viewed as a snapshot of full B–H oxidative addition of one R₂BH molecule combined with frustrated (arrested) B–H activation of a second. In solution complex 6 is fluxional: at room temperature the ¹¹B NMR spectrum shows a single broad resonance at 38 ppm and the OsHB hydrogens a single ¹H resonance, which at 183 K split into the separate resonances of the Os–Bcat boryl site and the Os(κ²-H₂Bcat) borate site — i.e., the two Bcat groups thermally exchange their positions. Borane (BH₃·THF) displaces HBcat from the borate to give the boryl-tetrahydrideborate Os(Bcat)(κ²-H₂BH₂)(CO)(P(i)Pr₃)₂, and the Bpin analogues are accessible from Os(BR₂)Cl(CO)(P(i)Pr₃)₂ and Na[BH₄]. The authors proposed that the site exchange proceeds via hydride-boryl-σ-borane intermediates in equilibrium with di(σ-borane) and dihydride-diboryl species (Scheme 3 of the paper), but offered no computational or labeling evidence; the concluding remarks only rank the nucleophilicity of the Os(CO)(P(i)Pr₃)₂ fragment between Ti(η⁵-C₅H₅)₂ and CpRh/CpIr fragments, which give other product types. No catalytic experiments were reported. Understanding these fluxional two-borane assemblies matters because the interconversions among σ-borane, boryl–hydride, boryl-dihydrideborate, and dihydride-diboryl structures are the elementary steps underlying metal-catalyzed hydroboration and dehydrogenative borylation.

Problem Statement

Establish the mechanistic basis and scope of the boryl-dihydrideborate (type III) motif formed by activation of two borane B–H bonds at a single transition-metal center, as instantiated in Os(Bcat)(κ²-H₂Bcat)(CO)(P(i)Pr₃)₂ (and its boryl-tetrahydrideborate relatives Os(Bcat)(κ²-H₂BH₂)(CO)(P(i)Pr₃)₂ and Os(Bpin)(κ²-H₂BH₂)(CO)(P(i)Pr₃)₂). Three linked questions, jointly constituting this objective: (i) Mechanism — identify, experimentally or computationally, the pathway of the thermally activated Bcat site exchange in Os(Bcat)(κ²-H₂Bcat)(CO)(P(i)Pr₃)₂, in particular whether it proceeds via hydride-boryl-σ-borane intermediates in equilibrium with di(σ-borane) and dihydride-diboryl species as proposed in the source paper, with activation barriers consistent with the observed variable-temperature NMR coalescence behavior (183–298 K in toluene-d₈); (ii) Generality — determine whether metal fragments other than Os(CO)(P(i)Pr₃)₂ can stabilize analogous boryl-dihydrideborate species as products of sequential or simultaneous B–H activation of two R₂BH molecules, and whether boranes beyond catecholborane and pinacolborane participate in such chemistry; (iii) Catalytic relevance — determine whether the boryl-dihydrideborate and boryl-tetrahydrideborate complexes themselves are catalytically competent, e.g., as (pre)catalysts or resting states in hydroboration, dehydrogenative borylation, or related B–H activation catalysis. All three strands are required for completion.

References

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