Metallaboranes from Metal Carbonyl Compounds and Their Utilization as Catalysts for Alkyne Cyclotrimerization

The photolysis of [M₂ (CO)₁₀ ] (M=Re or Mn) with BH₃ ⋅thf at room temperature yields arachno-1 and 2, [(CO)₈ M₂ B₂ H₆ ] (1: M=Re, 2: M=Mn). Both the compounds show a butterfly structure with seven skeletal electron pairs and 42 valence electrons. This result presents a new method for general access to low-boron-content metal-boron compounds without the cyclopentadienyl ligand at the metal centers. This new synthetic route is superior to the existing procedures because it avoids the use of [LiBH₄ ] and metal polychlorides, for which the synthesis is very tedious. Compound 1 catalyzes the cyclotrimerization of a series of internal and terminal alkynes to yield mixtures of 1,3,5- and 1,2,4-substituted benzenes. The reactivity of 1 with alkynes demonstrates for the first time that the introduction of the [B₂ H₆ ] moiety into the [Re₂ (CO)₁₀ ] framework significantly enhances the catalytic activity. Note that [Re₂ (CO)₁₀ ] catalyzes the same set of alkynes under harsh conditions over a prolonged period of time. Quantum-chemical calculations using DFT methods are applied to afford further insight into the electronic structure, stability, and bonding of 1 and 2. All the compounds are characterized by IR and ¹ H, ¹¹ B, and ¹³ C NMR spectroscopy, and the geometry of 1 is established unambiguously through crystallographic analysis.