Pentadienyl-Metal-Phosphine Chemistry: 10:¹Comparison of the Reactions of (77⁵-Pentadienyl)Mn[(Me₂PCH₂)3CMe] and (i7⁵-2,4-Dimethylpentadienyl)Re(PMe₂Ph)₃with H⁺: Isolation of a Protonated Manganese Complex Containing an Agostic C-H-M Interaction

(n⁵-Pentadienyl)Mn[(Me₂PCH₂)₃CMe] (1) and (?7⁵-2,4-dimethylpentadienyl)Re(PMe₂Ph)₃(4) react with acids to generate protonated complexes. In the cationic manganese product, 2, the added hydrogen resides in an agostic (semibridging) position between the metal center and Cl of the pentadienyl ligand, while the protonated rhenium complex, 5, contains a normal (terminal) metal hydride. The crystal structure of (r/⁵-2,4-dimethylpentadienyl)Re(H)(PMe₂Ph)₃⁺BF4^_*OC₄H₈(5a) has been determined by X-ray diffraction. The complex crystallizes in the monoclinic space group PIJn with a = 10.267 (2) A, b = 18.487 (5) A, c = 19.843 (4) A, = 98.83 (2)°, V = 3721 (2) A³, and Z = 4. The hydride ligand resides beneath the “backbone” of the dimethylpentadienyl ligand between phosphorus atoms P1 and P2. Experiments involving D⁺have probed the mechanisms of the protonation reactions. In the manganese system, reaction with D⁺at -130 °C leads to deuterium incorporation exclusively in the semibridging position, indicating initial endo attack either at the metal center or directly at the semibridging site. In the rhenium system, initial D⁺attack appears to occur at the metal center from the open (“mouth”) side of the 2,4-dimethylpentadienyl ligand. This mouth deuteride then rapidly exchanges with the hydrogens on pentadienyl carbons Cl and C5 before moving to its thermodynamic position under the pentadienyl backbone. The cationic manganese complex, (C5H₇-At-H)Mn[(Me₂PCH₂)₃CMe]⁺(2), undergoes three fluxional processes in solution, as evidenced by variable-temperature NMR spectroscopy. The first process, which involves the intermediacy of a 16e“ r?⁴-pentadiene-metal complex, exchanges the three hydrogens in the agostic methyl group (Cl). The second process involves a ?7⁵-pentadienyl-metal-hydride intermediate and exchanges the agostic methyl group hydrogens with the hydrogens bonded to C5. The third fluxional process exchanges the three phosphorus atoms in the phosphine ligand via pentadienyl ligand rotation. Line shape simulations of the variable-temperature NMR spectra yield free energies of activation (AG* ‘s) of 9.5 ± 0.4, 16.5 ± 0.4, and 11.4 ± 0.5 kcal, respectively, for these processes. 2 reacts with additional (Me₂PCH₂)₃CMe to release cis-l,3-pentadiene and produce Mn-[(Me₂PCH₂)₃CMe]₂⁺(3). Cation 5 also undergoes dynamic processes in solution. The hydride ligand can migrate to either end of the 2,4-dimethylpentadienyl ligand, causing exchange of the hydride with the four hydrogens on Cl and C5 of the pentadienyl chain (AG* = 17.3 ± 0.2 kcal). The 2,4-dimethylpentadienyl ligand can also rotate with respect to the MP₃fragment, resulting in exchange of the three phosphorus atoms (AC* = 16.9 ± 0.3 kcal).