Pentadienyl-Metal-Phosphine Chemistry. 9. The 2,4-Dimethylpentadfenyl-Iron-Trimethylphosphine Reaction System

The reaction of FeCl₂(PMe₃)₂with potassium 2,4-dimethylpentadienide-tetrahydrofuran (K⁺2,4-Me₂pd“) can be directed into three separate manifolds by varying the ratio of K⁺2,4-Me₂pd^-to iron. Use of a 2:1 K⁺2,4-Me₂pd^-:Fe ratio leads to the production of (η⁵-2,4-Me₂pd)(ή³-2,4-Me₂pd)Fe(PMe₃) (1) while a 1:1 K⁺2,4-Me₂pd^-:Fe ratio yields (η⁴-isopropenyltrimethylenemethyl)Fe(PMe₃)₃(2) via a reaction sequence involving activation of a C-H bond in a 2,4-Me₂pd methyl group. When the ratio of K⁺2,4-Me₂pd^-:Fe is reduced to 1:2, (ή⁵-2,4-Me₂pd)Fe(PMe₃)₃⁺FeCl₃(PMe₃)^_(3) is produced. Single-crystal X-ray diffraction studies of both 2 and 3 have been carried out. Compound 2 crystallizes in the monoclinic space group P2₁/c with a = 8.878 (1) Å, b = 14.737 (4) Å, c = 16.689 (3) Å, β= 102.25 (1)°, V = 2129.5 (8) ų, and Z = 4. The complex assumes an approximate octahedral geometry with Cl, C3, and C6 of the isopropenyltrimethylenemethyl ligand (iTMM) and the three phosphine phosphorus atoms occupying the six coordination sites. The iTMM ligand exhibits an umbrella shape with C2 bent out of the C1/C3/C6 plane. Compound 3 crystallizes in the monoclinic space group C2/c with a = 18.442 (5) Å, b =18.433 (5) Å, c = 17.875 (5) Å, β= 91.87 (2)°, V =6073 (3) ų, and Z = 8. The coordination geometry of the cation is approximately octahedral with Cl, C3, and C5-of the 2,4-Me₂pd ligand and P1, P2, and P3 of the PMe₃ligands occupying the six coordination sites. The anion of 3, FeCl₃(PMe₃)^-, adopts an ethane-like geometry with the three chloro ligands on iron and the three methyl groups on phosphorus arranged in a staggered orientation. The reaction of 1 with HO₃SCF₃and 2 equiv of PMe₃in diethyl ether produces (ή⁵-2,4-Me2pd)Fe(PMe₃)₃⁺0₃SCF₃^-(4). In solution, the ή⁵-2,4-Me₂pd ligand in 4 rotates with respect to the Fe(PMe₃)₃fragment. Line-shape simulations of the variable-temperature³¹P{¹H} NMR spectra have enabled us to calculate a AG* of 11.5 ± 0.5 kcal for this process.