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

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 structurally characterized. Alkylation of the latter complex with LiCH2SiMe3 gave a five-coordinate Cr(III) dialkyl complex Cr(CH2SiMe3)(2)[N(SiMe2CH2PPh2)(2)](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)(2)N]Cr}(2)(mu-Cl)(2) resulted in halide-transfer to form CrCl2(TNF)[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(SiMe2CH2CH2PPh2)(2)]. The five five-coordinate Cr(III) alkyl halide complexes do not polymerize ethylene at 60 degrees 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 d(3) Cr(III) complexes with those observed in the analogous low-spin d(6) Ir(III) complexes is presented.