DINUCLEAR PARAMAGNETIC METALLOCENES BRIDGED BY SILYL GROUPS - SYNTHESIS AND INTRAMOLECULAR INTERACTIONS

The dilithium salt of the dro-4,4,8,8-tetra-methyl-4,8-disila-s-indace nediyl dianion (L2-, 2) was allowed to react with the solvated metal h alides MX2 (M = Ni, Co, Cr) in the presence of the cyclopentadienyl (C p) 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 4C oCo. Similarly, the reaction of 2 with [CpVCl(PEt3)]2 gave the trans-v anadium 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 CpFeL-NiCp (3FeNi) was synthesized from CpFeL-, Cp- and solvated NiBr2. The molecules were ch aracterized by mass spectrometry, elemental analyses, cyclic voltammet ry, H-1-, C-13-, and Si-29-NMR spectroscopy and solid-state magnetic m easurements. Cyclic voltammetry showed up to six electron transfers pe r molecule. A metal-dependent splitting of the half-wave potentials of up to 355 mV indicated rather strong electrostatic interaction betwee n the metallocene units. The NMR results established unpaired spin on the ligands. Its distribution within the bridging ligand was correlate d with the molecular orbital splitting and the magnetic interaction. A ntiferromagnetic interaction was found for 3NiNi, 3CrCr, and 3VV with J = -11.6, -2.56, and -1.34 cm-1, respectively (H = -J . S(A) . S(B)). A temperature-dependent folding of the bridging ligand was deduced fr om the temperature behavior of the H-1-NMR signal shifts.