Steric Influence on the Structure, Magnetic Properties, and Reactivity of Hexa- and Octaisopropylmanganocene

The reaction of KCp ni (Cp ni = (C 3 H 7 ) n C 5 H 5 - n; n = 3, 4) with MnCl 2 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 room temperature. In the solid-state, it has a centrosymmetric sandwich structure with an average Mn−C bond length (2.130(6) Å) 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° and an average Mn−C distance of 2.42(2) Å. 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.