Chemistry, Structure, and Molecular Dynamics of the Tetrahydroboratotetracarbonylmolybdate(1-) Anion, MO(CO)4BH4-

The compound [(Ph3P)2N] [Mo(CO)4BH4] can be prepared in 25% yield by the reaction of [(Ph3P)2N] [Mo(CO)5I] 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 P1 (a=17.828 (8) A, b=9.714 (4) Å, c= 12.371 (5)Å, α=101.77 (1)° β= 115.36 (1)°=94.40 (1)°=1886.9 Å3, pobsd=1.33 g cm-3, pcalcd=1.34gcm-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) A. The ligational analogy between η3-allyl and BH4- is further strengthened by the results of this study. Boron-decoupled 1HNMR spectra reveal that bridge-terminal hydrogen interchange occurs within the tetrahydroborate ligand with ΔGC⧧=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; ΔG⧧ ≥ 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 differentiate rearrangement modes in covalent metal tetrahydroborates.