Reactions of the coordinatively unsaturated group 6 precursors [M(CO)3(PCy3)2] (M = Cr, Mo, W; PCy3 = tricyclohexylphosphine) with tricoordinate dihydroboranes R–BH2 were reported in 2019 to give the first bis(σ)-borane complexes of group 6 metals for chromium and tungsten, while molybdenum behaves anomalously: with durylborane (DurBH2) it forms an η6-arene complex in which a phosphine traps the boron center, and only with the π-donor aminoborane (Me3Si)2NBH2 does it give a bis(σ)-borane, with concomitant CO loss. The mechanistic origin of this divergence — for which candidate explanations include the enhanced lability of 4d metals and a promoting role of the amido π-donor — has not been established in the subsequent literature (2019–2026), which treats these complexes as synthetic milestones without mechanistic analysis, extension to other dihydroboranes, or catalytic application. The problem is to provide direct experimental and/or computational mechanistic evidence that explains the observed metal- and borane-dependent product distribution, thereby making dihydroborane coordination at group 6 centers predictable.

Background

A σ-complex is a coordination compound in which an X–H or X–Y σ bond donates its bonding electron pair to a transition metal, usually binding side-on (η2); σ-complexes of H2 and silanes are classical objects of study, and they are the entry point to bond oxidative addition (formal cleavage of the σ bond with transfer of both fragments to the metal). σ-Borane complexes, in which a B–H bond of a borane binds η2 to a metal, are rarer: because boron is electron-deficient, the metal can back-donate into the empty p/sp3 orbital at boron, so σ-boranes occupy a continuum between pure 3-center-2-electron σ-coordination and full B–H oxidative addition. A bis(σ)-borane complex carries two such η2-B–H ligands at the same metal. The standard classification tool is QTAIM (quantum theory of atoms in molecules), an analysis of the computed electron density that locates bond critical points: a ring critical point in the M–H–B triangle indicates an 'elongated' or 'stretched' σ-borane with advanced B–H activation, while its absence indicates a 'pure' σ-complex. Wiberg bond indices (computed bond orders) quantify the same trend (higher M–H and lower B–H order = more activation).

Until 2019, σ-borane complexes of the group 6 metals (Cr, Mo, W) were limited to tetracoordinate boranes of the type BH3·L (borane Lewis-base adducts); no complex of a tricoordinate dihydroborane R–BH2 was known for these metals, even though the 16-electron (coordinatively unsaturated, two electrons short of an 18-electron shell) precursors [M(CO)3(PCy3)2] (PCy3 = tricyclohexylphosphine) bind H2 and silanes readily. Lenczyk, Roy, Ghosh, Schwarzmann, Phukan, and Braunschweig (Chem. Eur. J. 2019) closed this gap: [M(CO)3(PCy3)2] reacts with the dihydroboranes DurBH2 (Dur = 2,3,5,6-tetramethylphenyl, 'duryl') and (Me3Si)2NBH2 (bis(trimethylsilyl)aminoborane) to give the first group 6 bis(σ)-borane complexes fac-[M(CO)3(PCy3){η2-(H2BR)}] for M = Cr and W (compounds 1–4). QTAIM (no ring critical point in the M–H–B rings) and Wiberg indices (M–H bond orders 0.233–0.337, B–H orders 0.585–0.651) establish these as pure σ-boranes with largely closed-shell metal–boron interactions.

The striking anomaly is molybdenum. With DurBH2, [Mo(CO)3(PCy3)2] gives not a bis(σ)-borane but the arene complex [Mo(CO)3(η6-Dur)BH2·PCy3] (5), in which the duryl ring binds η6 (face-on, through its π system) to Mo and the boron center is trapped by a phosphine (a P→B donor-acceptor adduct); with (Me3Si)2NBH2, molybdenum does form a bis(σ)-borane, cis,trans-[Mo(CO)2(PCy3)2{η2-(H2BN(SiMe3)2)}] (6), but only with loss of one CO ligand and a different auxiliary-ligand arrangement than in 1–4. The paper reports no direct mechanistic evidence for the Cr/Mo/W divergence; candidate explanations include the enhanced lability of 4d (versus 3d and 5d) metal complexes and a promoting role of the π-donor N(SiMe3)2 substituent for B–H σ-coordination over adduct formation at molybdenum.

Problem Statement

Establish, with direct experimental and/or computational mechanistic evidence, the factors that control the divergent product distribution in reactions of the 16-electron group 6 precursors [M(CO)3(PCy3)2] (M = Cr, Mo, W) with tricoordinate dihydroboranes R–BH2, as observed by Lenczyk et al. (2019):

(1) Why does molybdenum, alone of the three metals, react with DurBH2 to give the η6-arene complex [Mo(CO)3(η6-Dur)BH2·PCy3] (5) — with apparent phosphine-to-boron transfer — rather than the bis(σ)-borane complexes formed by chromium and tungsten? In particular, is the enhanced lability of 4d metal complexes (faster ligand dissociation and rearrangement at Mo) responsible, or do other factors govern, such as the relative binding strengths of η6-arene versus η2-B–H coordination, phosphine or CO dissociation, or kinetic versus thermodynamic control?

(2) Does the π-donor N(SiMe3)2 substituent of the aminoborane in fact promote B–H σ-coordination over P→B/borane-adduct formation at molybdenum, and through what electronic or steric mechanism does it operate — noting that complex 6 forms only with concomitant loss of one CO ligand?

A complete answer must provide a coherent mechanistic account that reproduces all six observed products and their metal/borane dependence, including the reference behavior of Cr and W; parts (1) and (2) are mandatory components of that account, and the behavior of Cr and W must be explained mechanistically (why they give the bis(σ)-borane complexes 1–4) rather than merely reported as comparison data. Deriving a general predictive rule for dihydroboranes R–BH2 beyond DurBH2 and (Me3Si)2NBH2 is a bonus, not a requirement of a complete answer. A demonstration of catalytic turnover by the bis(σ)-borane complexes, which the source proposes as a further direction, is a natural continuation but does not by itself answer the mechanistic question.

References

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