Dinuclear Paramagnetic Metallocenes Bridged by Silyl Groups — Synthesis and Intramolecular Interactions

The dilithium salt of the 3a,4,7a,8-tetrahydro-4,4,8,8-tetra-methyl-4,8-disila-s-indacenediyl dianion (L2−, 2) was allowed to react with the solvated metal halides MX2 (M = Ni, Co, Cr) in the presence of the cyclopentadienyl (Cp) anion to give trans-CpMLMCp (3NiNi, 3CoCo, and 3CrCr) in yields up to 80%. The only cis isomers which could be detected were 4NiNi and 4CoCo. Similarly, the reaction of 2 with [CpVCl(PEt3)]2 gave the trans-vanadium analogue 3VV while the successive reaction of 2 with CrCl3 (THF)3 and PEt3 yielded the bridged half-sandwich (Et3P)Cl2CrLCrCl2(PEt3) (5CrCr). The mixed-metal dinuclear metallocene CpFelNiCp (3FeNi) was synthesized from CpFel−, Cp− and solvated NiBr2. The molecules were characterized by mass spectrometry, elemental analyses, cyclic voltammetry, 1H, 13C, and 29SiNMR spectroscopy and solid-state magnetic measurements. Cyclic voltammetry showed up to six electron transfers per molecule. A metal-dependent splitting of the half-wave potentials of up to 355 mV indicated rather strong electrostatic interaction between the metallocene units. The NMR results established unpaired spin on the ligands. Its distribution within the bridging ligand was correlated with the molecular orbital splitting and the magnetic interaction. Antiferromagnetic interaction was found for 3NiNi, 3CrCr, and 3VV with J = −11.6, −2.56, and −1.34 cm−1, respectively (H = -J · SA · SB). A temperature-dependent folding of the bridging ligand was deduced from the temperature behavior of the 1HNMR signal shifts.