Tunable spin-crossover behavior in polymethylated bis(indenyl)chromium(II) complexes: The significance of benzo-ring substitution

The magnetic properties of monomelic bis(indenyl)chromium(II) complexes are sensitive to the relative orientation of the ligands (staggered, eclipsed, or gauche), regardless of whether such a conformation is imposed by sterically bulky substituents or is a consequence of crystal-packing forces. The staggered monomethylated compound (2-MeC₉H₆)₂Cr has previously been found to display a high-spin state in the solid state and solution, whereas the monomeric (1-MeC₉H₆)₂Cr is eclipsed in the solid state and exhibits spin-crossover behavior over a wide temperature range. The present work examines the synthesis, structures, and magnetic properties of polymethylated indenyl complexes containing two, three, and five substituents on the indenyl ligand. Methyl groups on the benzo portion appear to be critical in supporting a low-spin ground state; the order of the preference to exist as a low-spin complex is approximately (2,4,5,6,7-Me ₅C₉H₂)₂Cr > (2,4,7-Me ₃C₉H₄)₂Cr ≈(4,7-Me ₂C₉H₅)₂Cr > (1,2,3-Me ₃C₉H₆)₂Cr > (1-MeC ₉H₆)₂Cr ≥ (2-MeC₉H ₆)₂Cr. The (2,4,7-Me₃C₉H ₆)₂Cr complex crystallizes with two conformations in the unit cell: one with an eclipsed geometry and the other staggered. When the temperature is raised from 173 to 293 K, the staggered form undergoes approximately 3 times the bond length increase as the eclipsed form. The difference is attributed to the symmetry-driven preference of the staggered configuration for the high-spin state.