One-electron oxidation of paramagnetic chromium(II) alkyl complexes with alkyl halides: synthesis and structure of five-coordinate chromium(III) complexes

Abstract

The reaction of square-planar, high-spin CrR[N(SiMe₂CH₂PPh₂)₂] (R = Me, CH₂SiMe₃) with alkyl halides (MeI, CF₃CH₂I, MeBr, PhCH₂Cl) generates one-electron oxidation products Cr(R)X[N(SiMe₂CH₂PPh₂)₂], unusual examples of five-coordinate chromium(III) complexes. Cr(Me)Br[N(SiMe₂CH₂PPh₂)₂] and Cr(CH₂SiMe₃)Cl[N(SiMe₂CH₂PPh₂)₂] have been structurally characterized. Alkylation of the latter complex with LiCH₂SiMe₃ gave a five-coordinate Cr(III) dialkyl complex Cr(CH₂SiMe₃)₂[N(SiMe₂CH₂PPh₂)₂], which was structurally characterized as well. Attempts to isolate sterically unencumbered Cr(III) dialkyl (e.g., dimethyl) complexes resulted in decomposition. Addition of an excess of PhCH₂Cl to {[(Ph₂PCH₂SiMe₂)₂N]Cr}₂(µ-Cl)₂ resulted in halide-transfer to form CrCl₂(THF)[N(SiMe₂CH₂PPh₂)₂] in low yield. Reaction of the low-spin CrCp[N(SiMe₂CH₂PPh₂)₂] complex with PhCH₂Cl, however, gave both Cr(Cp)(CH₂Ph)[N(SiMe₂CH₂PPh₂)₂] and Cr(Cp)Cl[N(SiMe₂CH₂PPh₂)₂]. The five-coordinate Cr(III) alkyl halide complexes do not polymerize ethylene at 60 °C and 1 atm; the dialkyl complex Cr(CH₂SiMe₃)₂[N(SiMe₂CH₂PPh₂)₂] does catalyze polyethylene formation but is quickly deactivated. A discussion comparing the structural distortions observed in these five-coordinate high-spin d³ Cr(III) complexes with those observed in the analogous low-spin d⁶ Ir(III) complexes is presented.