The compound [(Ph3P)2N] [Mo(CO)4BH4] can be prepared in 25% yield by the reaction of [(Ph3P)2N| [Mo(CO)51] and [(Ph3P)2N][BH4] in anhydrous tetrahydrofuran. Mechanistic studies show that iodide ion acts as a catalyst in the synthesis. X-ray diffraction studies reveal that the compound crystallizes in the space group P1̄ (a = 17.828 (8) Å, b = 9.714 (4) Å, c = 12.371 (5)Å, α = 101.77(1)°, β = 115.36 (1)°, γ = 94.40 (1)°, V = 1886.9 Å3, ρobsd = 1.33 g cm-3, ρcalcd = 1.34 g cm-3 for Z = 2). Data were collected with Zr-filtered Mo Kα radiation to a 2θ limit of 45°. Standard Patterson, Fourier, and least-squares techniques resulted in final agreement factors: R = 8.3%, Rw = 8.1% for 3208 reflections with I > 3σ. The tetrahydroborate ligand is attached to the metal via two Mo-H-B bridge bonds with Mo-Hb = 2.02 (8) Å. The coordination about the central molybdenum atom is approximately octahedral, but two notable distortions occur in the equatorial plane: C(eq)-Mo-C(eq) = 84.5 (5)° and Hb-Mo-Hb = 59 (4)°. The geometry about the boron is virtually tetrahedral, with Mo-B = 2.41 (2) Å. The ligational analogy between μ3-allyl and BH4- is further strengthened by the results of this study. Boron-decoupled 1H NMR spectra reveal that bridge-terminal hydrogen interchange occurs within the tetrahydroborate ligand with ΔGcDagger = 10.0 ± 0.2 kcal/mol. As revealed by 13C NMR studies, this rearrangement process is not coupled to axial-equatorial CO exchange about the molybdenum coordination polyhedron; ΔCG‡ ≥ 18.6 kcal/mol for this process. This result places significant restrictions on operational BH4- rearrangement mechanisms. These are discussed in the light of a permutational analysis of differentiable rearrangement modes in covalent metal tetrahydroborates.
|Number of pages||9|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1977|
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