Synthesis and Complexation of a New Chelating Bis(phosphite) Ligand, pinacop. Structural Characterization of (2, 4-Dimethylpentadienyl)M(pinacop) Complexes (M = Co, Rh)

A new chelating bis (phosphite) ligand, OCMe₂CMe₂OPOCMe₂CMe₂OPOCMe₂CMe₂O or “pinacop”, has been synthesized via condensation of pinacol with phosphorus trichloride. The addition of pinacop to the low-temperature reaction product of CoCl₂ and 2 equiv of potassium 2, 4-dimethylpentadienide yields (2, 4-dimethylpentadienyl)Co(pinacop) (1), which has been structurally characterized. 1 crystallizes in the monoclinic space group Cc with a = 13.964 (7) Å, b = 16.018 (5) Å, c = 13.775 (3) Å, β= 108.58 (3)°, V = 2921 (2) ų, and Z = 4. The pinacop ligand is oriented with one phosphorus atom eclipsing the open edge and the other phosphorus atom eclipsing the central carbon, C3, of the U-shaped η⁵-dimethylpentadienyl group. (2, 4-Dimethylpentadienyl)Rh(pinacop) (2) has been synthesized by reacting [(pinacop)RhCl]₂ with 2 equiv of potassium 2, 4-dimethylpentadienide and has been structurally characterized. 2 crystallizes in the monoclinic space group P2₁/n with a = 10.609 (3) Å, b = 25.241 (6) Å, c = 11.500 (1) Å, β = 104.36 (2)°, V = 2983 (1) ų, and Z = 4. The dimethylpentadienyl ligand in 2 is bonded to the Rh center in an η³fashion and adopts an anti-rotated (sickle shaped) geometry. The overall coordination geometry for 16 e⁻ 2 is square planar with Cl and C3 of the dimethylpentadienyl ligand and PI and P2 of the pinacop ligand occupying the four coordination sites. In solution, 2 exists as an equilibrium mixture of anti-η³-and syn-η³-2, 4-dimethylpentadienyl isomers. The anti isomer undergoes a fluxional process, which exchanges the ends of the dimethylpentadienyl ligand via an 18 e⁻ (η⁵-2, 4-dimethylpentadienyl)Rh(pinacop) intermediate.