Synthesis, structure and characterization of dimolybdaheteroboranes

Chalcogen-stabilized dimolybdaboranes 3-5 (3: [(Cp^∗Mo)₂B₄H₅Se(Ph)], 4: [(Cp^∗Mo)₂B₄H₃Se₂(SeCH₂Ph)] and 5: [(Cp^∗Mo)₂B₃H₆(BSR)(μ-η¹-SR)] (R = 2,6-(^tBu)₂-C₆H₂OH)) have been isolated from the mild pyrolysis of dichalcogenide ligands, RE-E‘R (R = Ph: E = S, E‘ = Se; R = CH₂Ph, [2,6-(^tBu)₂-C₆H₂OH]: E = E‘ = Se, S) and [(Cp^∗Mo)₂B₄H₈], 2, an intermediate generated from the reaction of [Cp^∗MoCl₄] (1) (Cp^∗ = η⁵-C₅Me₅), with [LiBH₄.thf]. The geometry of [(Cp^∗Mo)₂B₄H₅Se(Ph)] is similar to that of [(Cp^∗Mo)₂B₅H₉], in which one BH₃ unit on the open face is replaced by a triple bridged selenium atom. All the compounds have been characterized in solution by ¹H, ¹¹B, ¹³C NMR and IR spectroscopy and elemental analysis. The structural types were unequivocally established by X-ray crystallographic analysis of compounds 3-5.     

Metal fragment exchange from a molybdaborane to a tungstaborane

Reaction of the molybdaborane arachno-2-[Mo(η-C₅H₅)(η⁵:η¹-C₅H₄)B₄H₇] (I) with the electron-rich molecule [W(PMe₃)₃H₆] at 60 °C for 12 h in toluene gives the novel tungstaborane nido-2-W(PMe₃)₃H₂B₄H₇[Mo(η-C₅H₅)(η⁵:η¹-C₅H₄)H₂] (II) in 60% yield. The reaction is almost quantitative when followed by NMR. This is a rare example of metal fragment exchange within a metallaborane cage. The molybdenum atom is retained in the molecule via a σ-bond between the substituted cyclopentadienyl ring and a basal boron atom in the metallaborane cluster.     

The relevance of boranes and metallaboranes to the structures of p-block element nanoparticles

Recent d-block element metallaborane chemistry, in which metal identity is varied with constant ancillary ligand, demonstrates how the rising energy of the d orbitals as one moves to earlier metals gives rise to non spherical cluster shapes that permit low formal cluster electron counts. In essence, the separation of frontier orbitals from “nonbonding” orbitals required by the isolobal analogy breaks down and the resulting mixing generates additional high-lying empty orbitals concurrently with shape change. A very similar mechanism explains recent p-block cluster chemistry albeit with variation in extent of external cluster ligation as the variable and separation of external lone pair orbitals from cluster bonding as the problem. Sensible, novel explanations of the shape/electron count relationships can be discovered for large group 13 clusters by recognizing the perturbation in cluster orbital energies when stabilization by ligand interactions is removed. These observations are pertinent to an understanding of large p-block clusters with internal atoms often referred to as nanoparticles.     

Reactions of nido-1,2-(Cp*RuH)2B3H7 with RCCR′ (R, R′=H, Ph; Me, Me) to yield novel metallacarboranes

Addition of the internal alkyne, 2-butyne, to nido-1,2-(Cp*RuH)₂B₃H₇ (1) at ambient temperature produces nido-1,2-(Cp*Ru)₂(μ-H)(μ-BH₂)-4,5-Me₂-4,5-C₂B₂H₄ (2), nido-1,2-(Cp*RuH)₂-4,5-Me₂-4,5-C₂B₂H₄ (3), and nido-1,2-(Cp*RuH)₂-4-Et-4,5-C₂B₂H₅ (4), in parallel paths. On heating, 2, which contains a novel exo-polyhedral borane ligand, is converted into closo-1,2-(Cp*RuH)₂-4,5-Me₂-4,5-C₂B₃H₃ (5) and nido-1,6-(Cp*Ru)₂-4,5-Me₂-4,5-C₂B₂H₆ (6) the latter being a framework isomer of 3. Heating 2 with 2-butyne generates nido-1,2-(Cp*RuH)₂-3-{CMeCMeB(CMeCHMe)₂}-4,5-Me₂-4,5-C₂B₂H₃ (7) in which the exo-polyhedral borane is triply hydroborated to generate a boron bound ---CMeCMeB(CMeCHMe)₂ cluster substituent. Along with 3, 4, 5, 6, and 7, the reaction of 1 with 2-butyne at 85 °C gives closo-1,7-(Cp*Ru)₂-2,3,4,5-Me₄-6-(CHMeCH₂Me)-2,3,4,5-C₄B (8). Reaction of 1 with the terminal alkyne, phenylacetylene, at ambient temperature permits the isolation of nido-1,2-(Cp*Ru)₂(μ-H)(μ-CHCH₂Ph)B₃H₆ (9) and nido-1,2-(Cp*Ru)₂(μ-H)(μ-BH₂)-3-(CH₂)₂Ph-4-Ph-4,5-C₂B₂H₄ (11). The former contains a Ru---B edge-bridging alkylidene fragment generated by hydrometallation on the cluster framework whereas the latter contains an exo-polyhedral borane like that of 2. Thermolysis of 11 results in loss of hydrogen and the formation of closo-1,2-(Cp*RuH)₂-3-(CH₂)₂Ph-4-Ph-4,5-C₂B₃H₃ (12).