Homoleptic allyl complexes of chromium with trimethylsilylated ligands

Reaction of two equivalents of K[1-(SiMe₃)C₃ H₄], K[1,3-(SiMe₃)₂C₃ H₃], or K[1,1′,3-(SiMe₃)₃ C₃H₂] with CrCl₂ in THF at -78 °C produces the red complexes {[1-(SiMe₃)C₃ H₄]₂Cr}₂ (1), [1,3-(SiMe₃) ₂C₃H₃]₂Cr (2) and [1,1′,3-(SiMe₃)₃C₃H₂] ₂Cr (3), respectively. They are thermally stable compounds, remarkably so for the monomeric 2 and 3, which possess formal 12-electron counts. Single crystal X-ray structures confirm the dimeric nature of {[1-(SiMe₃)C₃H₄]₂Cr} ₂, which is constructed around a Cr₂ core with Cr-Cr′=1.9784(7) Å. In the monomeric complexes, the allyl ligands are bound in a trihapto manner to the metals, with Cr-C distances of 2.193(2)-2.257(2) Å in 2 and 2.223(5)-2.319(5) Å in 3. The allyl ligands adopt staggered conformations, with a 10.4° angle between the C₃ planes in (2) and parallel ligands in (3). The trimethylsilyl groups in both complexes are in a syn, anti conformation. The monomeric complexes are high-spin, with four unpaired electrons. The steric shielding provided to the metal by the trimethylsilyl groups is probably responsible for the lack of reactivity of 2 with bulky donors such as PPh₃, although it forms a monoadduct with the more sterically compact PMe₃. Density functional theory calculations were performed on (C₃H₅)₂Cr, and comparisons made with the structures of the trimethylsilylated derivatives.