Thermally Stable Chromium(III) η 3-Allyl Complexes. Direct Structural Comparison of Neutral Chromium(II) and Cationic Chromium(III) η 3-Allyl Redox Isomers

The first thermally stable chromium(III) η 3 -allyl complexes have been prepared. Oxidation of neutral cyclopentadienylchromium(II) dicarbonyl η 3 -allyl, η 3 -crotyl, and η 3 -cyclohexenyl complexes 1−3, respectively, with NOPF6, provides the corresponding cationic chromium(III) analogues 4−6. The structure of the cationic complex 4 was confirmed in the solid state by X-ray crystallography and compared to the crystal structure of neutral Cr(II) complex 1, which bears an identical ligand set. The cationic chromium(III) allyl complexes are thermally stable, but disproportionate at high concentration in donor solvents or in the presence of donor ligands: substitution of the carbonyl ligands with concomitant retention of the η 3 -allyl moiety is unsuccessful under a range of reaction conditions. The reaction between cationic allyl complex 4 and the N-heterocyclic carbene, 1,3-bis(2,4,6,-trimethylphenyl)imidazole-2-ylidene (IMes), leads to the formation of IMesHPF6 and intractable chromium-containing products, while the reaction between the neutral complex 1 and IMes results exclusively in nucleophilic alkylation of the allyl ligand. Treatment of complex 4 with tertiary phosphines provides low yields of diamagnetic bis(phosphine)chromium(II) complexes.