Steric influence on the structure, magnetic properties, and reactivity of hexa- and octaisopropylmanganocene

The reaction of KCpni (Cp-ni = (C3H7)(n)C5H5-n; n = 3, 4) with MnCl2 in THF produces the metallocenes (Cp-3i)(2)Mn or (Cp-4i)(2)Mn in good yield. Solid-state magnetic susceptibility measurements indicate that orange-red (Cp-3i)(2)Mn is in high-spin/low-spin equilibrium at roam temperature. Tn the solid-state, it has a centrosymmetric sandwich structure with an average Mn-C bond length (2.130(6) Angstrom) typical of low-spin manganocenes. (Cp-3i)(2)Mn reduces tetracyanoethylene (TCNE) to generate the TCNE radical anion. Despite the presence of additional electron-donating isopropyl groups, pale yellow (Cp-4i)(2)Mn is entirely high-spin from room temperature to 10 K. The crystal structure of (Cp-4i)(2)Mn reveals a bent metallocene structure, with a centroid-Mn-centroid angle of 167.4 degrees and an average Mn-C distance of 2.42(2) Angstrom. This structure is consistent with its high-spin electron configuration; the longer Mn-C bonds provide relief from the steric strain that would be present in a low-spin complex. The reaction of (Cp-4i)(2)Mn with TCNE produces a tricyanovinylation product, C-5(i-Pr)(4)HC-(CN)=C(CN)(2), which was characterized by X-ray diffraction.