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 RCCR, 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 Delta H = 8 kcal/mol and Delta S = 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)(mu-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 H-1 and B-11 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 + 2H(2), 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.