Two nickelocenes and ferrocene in a rigid cis/trans chain

With the aim of studying the conformation of bridged paramagnetic metallocenes and the conformation-dependent electron spin delocalization therein, a trimetallic model compound was synthesized. First, a nickelocene bonded to cyclopentadienyl (Cp) anion by two SiMe₂ groups was made in three steps. Further reaction with solvated iron dichloride gave a trimetallic metallocene (6) that had the metal sequence NiFeNi. X-ray crystal analysis revealed that in one-half of 6 the arrangement of the metal atoms Ni and Fe with respect to the ligand bridge was trans; in the other half it was cis. The Cp ligands were neither staggered nor eclipsed, the metallocene axes were slightly bent, and bending was also observed about three vectors in the bridging ligand. The ¹H-, ¹³C-, and ²⁹Si-NMR spectra were in accord with two unpaired electrons per nickelocene, with both cis and trans arrangement of the metal atoms, and with bending of the ligand bridges. Large NMR signal shifts indicated transfer of spin density from the nickelocenes into the SiMe₂ bridges and the central ferrocene. The efficiency of the spin transfer was found to depend on the bending angle of the ligand bridge. MO calculations illustrated that spin delocalization in the cis half of 6 was less pronounced than in the trans half,