Synthesis, X-ray Crystal Structure, and Phosphine-Exchange Reactions of the Hafnium(III)-Hafnium(III) Dimer Hf₂Cl₆(PEt₃)₄

Reduction of HfCl₄ with sodium-potassium alloy in the presence of PEt₃ yields the dark green hafnium(III)-hafnium(III) dimer Hf₂Cl₆(PEt₃)₄. The X-ray crystal structure shows that molecules of Hf₂Cl₆(PEt₃)₄ consist of two octahedral hafnium centers that are connected by a hafnium-hafnium bond and two bridging chloride ligands. The two independent molecules in the unit cell have slightly different structures due to differences in the relative orientations of the ethyl groups of the PEt₃ ligands. In both molecules, the PEt₃ groups are trans to the bridging chloride ligands. Average distances and angles for the two independent molecules: Hf-Hf = 3.097 (1) and 3.118 (1) Å, Hf-P = 2.748 (2) and 2.761 (2) Å, Hf-Cl_t = 2.417 (2) and 2.415 (2) Å, Hf-Cl_b = 2.533 (2) and 2.538 (2) Å, Hf-Cl_b-Hf = 75.38 (5) and 75.78 (4)°, Cl_b-Hf-Cl_b = 104.62 (5) and 104.22 (5)°, Cl,-Hf-Cl_t = 165.75 (6) and 165.33 (6)°, and P-Hf-P = 92.67 (5) and 98.95 (6)°. The triethylphosphine complex undergoes exchange with trimethylphosphine to afford successively the complexes Hf₂Cl₆(PEt₃)₃(PMe₃), Hf₂Cl₆(PEt₃)₂(PMe₃)₂, Hf₂-Cl₆(PEt3)(PMe₃)₃, and Hf₂Cl₆(PMe₃)₄; two of the three possible isomers of Hf₂Cl₆(PEt₃)₂(PMe₃)₂ are formed. The sequential replacement of PEt₃ groups by PMe₃ was followed by ³¹P{¹H} NMR spectroscopy. Interestingly, the J_PP coupling constants for phosphorus nuclei attached to different hafnium centers are <2 Hz, while the J_PP coupling constants for adjacent phosphines are small at 3–4 Hz. Crystal data for Hf₂Cl₆(PEt₃)₄ at−75 °C: monoclinic, space group P2₁/c, with a = 10.256 (4) Å,b= 18.787 (8) Å, c = 20.275 (6) Å, β = 94.52 (3)°, V = 3894 (4) ų, Z = 4, R_F = 0.028, and R_wF = 0.035 for 326 variables and 5081 data with I > 2.58σ(I).