High-spin and spin-crossover behavior in monomethylated bis(indenyl)chromium(II) complexes

Previous work on bis(indenyl)chromium(II) complexes substituted with bulky groups (i-Pr, t-Bu, SiMe ₃) found that their spin state (S = 1 or 2) depended on the symmetry of the molecules. Complexes with inversion symmetry (staggered rings) were high-spin; lower symmetry compounds with twisted (gauche) ligands were low spin. The present work explores the effect of methyl group substitution on the indenyl ligand, which leads to complexes possessing either staggered or eclipsed conformations. The monosubstituted compounds [(1 or 2)-MeC ₉H ₆] ₂Cr are prepared from the substituted alkali metal indenides and CrCl ₂ in THF. X-ray diffraction results indicate that (2-MeC ₉H ₆) ₂Cr exists in a staggered conformation, with Cr-C (av) = 2.308(7) Å, and is a high-spin species in the solid state and solution. In contrast, the monomeric (1-MeC ₉H ₉) ₂Cr is eclipsed in the solid state, where it exhibits spin-crossover behavior over a wide temperature range; the average Cr-C distances vary with temperature, from 2.179(9) Å at 105 K to 2.262(10) Å at 298 K. An attempt to form (4-MeC ₉H ₆) ₂Cr produced the dimeric, thermally unstable complex (η ⁵-indenyl′) ₂(μ,η ³-indenyr) Cr ₂(μ-Cl) instead. Correlations between the structure and magnetic properties in bis(indenyl)chromium(II) complexes have been made with density functional theory calculations, which indicate that an eclipsed ligand conformation supports a high spin state, but not to the extent that the staggered form does.