Metallaborane Reactivity. A Stoichiometric Mechanism for the Insertion of Two Alkynes into an Iridaborane Framework via a Disposable Molybdenum Chaperone

Building on earlier work that showed the formation of [1-Cp*-2,2,2-(CO)3-2-THF-nido-1,2-IrMoB4H8], 2, from the reaction of [1-Cp*-arachno-1-IrB4H10], 1, with (arene)Mo(CO)3, the stoichiometric
mechanism for the generation of [1-Cp*-5,6,7,8-(R)4-nido-1,5,6,7,8-IrC4B3H3], 8, from the reaction of 2 with
RC⋮CR, R = Me, Ph, has been identified. For R = Me, the major product in solution is [1-Cp*-5,6,7,8-(CH3)4-closo-1,5,6,7,8-IrC4B3H3Mo(CO)3], 7, which is in equilibrium with 8. The equilibrium 8 + Mo(THF)3(CO)3 ⇄ 7 + 3THF is characterized by ΔH = 8 kcal/mol and ΔS = 34 cal/mol K. Density functional theory
calculations carried out on 7 indicate that the Mo(CO)3 moiety is weakly bound to the cluster mainly through
Mo−C rather than Mo−B interactions. Under alkyne deficient conditions, the product [1-Cp*-2,2,2-(CO)3(μ-CO)-3,4-(CH3)2-closo-1,2,3,4-IrMoC2B3H3], 6, can be isolated. Solid-state structures of 1 and 2 have
been reported previously, and those of 6, 7, and 8, R = Me, Ph, are reported here. The evolution of products
with time was monitored by 1H and 11B NMR and showed the formation and decay of two additional species
which have been identified as the structural isomers [1-Cp*-7,7,7-(CO)3-7-THF-2,3-(CH3)2-nido-1,7,2,3-IrMoC2B3H5], 4, and [5-Cp*-7,7,7-(CO)3-7-THF-2,3-(CH3)2-nido-5,7,2,3-IrMoC2B3H5], 5, with the metals
nonadjacent in 4 and adjacent in 5. Circumstantial evidence suggests that 4 is the precursor to 5 and 5 is
the precursor to both 6 and 7. Cluster 2 also is a catalyst or catalyst precursor for the isomerization of
olefins, namely, hex-1-ene to cis-hex-2-ene and tetramethyl allene to 2,4-dimethylpenta-1,3-diene. These
novel results also establish that [1-Cp*-2,2,2-(CO)3-2-(alkyne)-nido-1,2-IrMoB4H8], 3, forms from 2 and
constitutes a logical precursor to 4. The entire process, 1 + 2alkyne = 8 + BH3 + 2H2, which is promoted
by (arene)Mo(CO)3, constitutes an explicit example of a transition-metal-facilitated process analogous to
metal-facilitated organic transformations observed in organometallic chemistry.