Synthesis, Structure, and Protonation Studies of Cp*MH₃(dppe) (M = Mo, W). Pseudo-Trigonal-Prismatic vs Pseudo-Octahedral Structures for Half-Sandwich Group 6 M(IV) Derivatives

The compounds Cp*MH₃(dppe) (M = Mo, 1; W, 2) are accessible in good yields from reacting the corresponding compound Cp*MCl₄ and LiAlH₄ in toluene/Et₂O followed by methanolysis. The X-ray structure of 1 shows a pseudo-trigonal-prismatic geometry which is unprecedented for half-sandwich CpML₅-type compounds. Protonation with HBF₄·Et₂O in Et₂O at low temperature affords [Cp*MH₄(dppe)]⁺BF₄^- salts (M = Mo, 3; W, 4). While 3 spontaneously decomposes, even at low temperatures, in coordinating solvents and CH₂Cl₂, 4 is stable at room temperature in MeCN. An X-ray structure of 4 is consistent with a classical tetrahydrido species with a distorted pseudo-pentagonal-bipyramidal structure. The low-temperature NMR properties, J_HD ≤ 1 Hz for 4-d₃, and the T_1(min) value for the hydride resonance are also consistent with this structural assignment. Decomposition of 3 in MeCN at room temperature selectively affords [Cp*MoH₂(MeCN)(dppe)]⁺BF₄^-, 5. The NMR properties of this complex indicate a fluxional structure with inequivalent H and P nuclei and are consistent with a pseudo-trigonal-prismatic structure analogous to that of the precursor 1. Further protonation of 5 in MeCN or direct protonation of 1 with excess acid in MeCN affords two isomers of complex [Cp*MoH(MeCN)₂(dppe)](BF₄)₂, 6 and 7. Thermal treatment in MeCN slowly converts 7 into 6 initially, but both isomers further transform into a third isomer, 8, upon prolonged heating. The structure of 6 has been elucidated by X-ray crystallography and consists of a highly distorted pseudo-octahedral geometry with relative trans MeCN ligands. The structures of 7 and 8 and mechanisms of the interconversions between the various isomeric structures are proposed on the basis of the solution NMR studies.