Spin Equilibria in Monomeric Manganocenes: Solid-State Magnetic and EXAFS Studies

Abstract

Magnetic susceptibility measurements and X-ray data confirm that tert-butyl-substituted manganocenes [(Me3C)nC5H5−n]2Mn (n = 1, 2) follow the trend previously observed with the methylated manganocenes; that is, electron-donating groups attached to the Cp ring stabilize the low-spin (LS) electronic ground state relative to Cp2Mn and exhibit higher spin-crossover (SCO) temperatures. However, introducing three CMe3 groups on each ring gives a temperature-invariant high-spin (HS) state manganocene. The origin of the high-spin state in [1,2,4-(Me3C)3C5H2]2Mn is due to the significant bulk of the [1,2,4-(Me3C)3C5H2]− ligand, which is sufficient to generate severe inter-ring steric strain that prevents the realization of the low-spin state. Interestingly, the spin transition in [1,3-(Me3C)2C5H3]2Mn is accompanied by a phase transition resulting in a significant irreversible hysteresis (ΔTc = 16 K). This structural transition was also observed by extended X-ray absorption fine-structure (EXAFS) measurements. Magnetic susceptibility studies and X-ray diffraction data on SiMe3-substituted manganocenes [(Me3Si)nC5H5−n]2Mn (n = 1, 2, 3) show high-spin configurations in these cases. Although tetra- and hexasubstituted manganocenes are high-spin at all accessible temperatures, the disubstituted manganocenes exhibit a small low-spin admixture at low temperature. In this respect it behaves similarily to [(Me3C)(Me3Si)C5H3]2Mn, which has a constant low-spin admixture up to 90 K and then gradually converts to high-spin. Thermal spin-trapping can be observed for [(Me3C)(Me3Si)C5H3]2Mn on rapid cooling.