Pentadlenyl–Metal–Phosphine Chemistry. 6. Syntheses, Structures, and Solution Dynamics of (η⁵-Pentadlenyl)[trls(phosphine)]manganese Complexes

Reaction of MnBr₂ with potassium pentadienide–tetrahydrofuran and the electron-rich chelating tris(phosphine) ligands (Me₂PCH₂)₃CMe or (Et₂PCH₂CH₂)₂PPh produces (η⁵-pentadienyl)Mn-[(Me₂PCH₂)₃CMe] (1) or (η⁵-pentadienyl)Mn[(Et₂PCH₂CH₂)₂PPh] (2) in high yield. Substitution of potassium 2,4-dimethylpentadienide–tetrahydrofuran for potassium pentadienide–tetrahydrofuran in the above reaction system leads to the syntheses of the 2,4-dimethylpentadienyl analogues of 1 and 2, namely, (η⁵-2,4-dimethylpentadienyl)Mn[(Me₂PCH₂)₃CMe] (3) and (η⁵-2,4-dimethylpentadienyl)Mn-[(Et₂PCH₂CH₂)₂PPh] (4). Single-crystal X-ray diffraction studies of 2 and 3 have been carried out. 2 crystallizes in the orthorhombic space group P2₁2₁2₁ with a = 16.69 (1) Å, b = 17.06 (1) Å, c = 8.513 (3) Å, V = 2423 (4) ų, and Z = 4. 3 also crystallizes in space group P2₁2₁2₁ with a ₌14.788 (3) Å, b = 9.709 (3) Å, c ₌14.761 (4) Å, V = 2119 (2) ų, and Z = 4. Both complexes assume approximate octahedral geometries with C1, C3, and C5 of the pentadienyl ligands and the three phosphorus atoms of the tris-(phosphine) ligands occupying the six coordination sites. In 2, the chelating tris(phosphine) ligand is oriented unsymmetrically with one of the terminal phosphine groups situated beneath the open mouth of the pentadienyl ligand and the other terminal phosphine group under an edge of the pentadienyl ligand. This unsymmetrical orientation of the tris(phosphine) ligand also manifests itself in the room-temperature NMR spectra of 2. However, as the temperature is raised, the rate of rotation of the pentadienyl group with respect to the MnP₃ fragment increases, exchanging the ends of the tris(phosphine) ligand and the ends of the pentadienyl ligand. Line-shape simulations of the variable-temperature ³¹P NMR spectra have enabled us to calculate rotational barriers (ΔG*’s) of 18.3 ± 0.5 kcal/mol for 2 and 17.3 ± 0.2 kcal/mol for 4. For compounds 1 and 3, rotation is rapid at room temperature but can be slowed by cooling the solutions. Again, line-shape simulations of the variable-temperature ³¹P NMR spectra have yielded rotational barriers (ΔG*’s) of 11.4 ± 0.6 kcal/mol and 10.9 ± 0.2 kcal/mol for 1 and 3, respectively.