[TI(MGCL)2.XTHF]Q - A REAGENT FOR THE MCMURRY REACTION AND A NOVEL INORGANIC GRIGNARD COMPLEX

The stepwise formation of two distinct bimetallic titanium/magnesium complexes during the reduction of TiCl3 (or TiCl4) by magnesium in tetrahydrofuran (THF) has been identified. [TiMgCl2 . x THF] (1) is produced in a first stage, but reacts further with excess Mg to give [Ti(MgC.)2 . xTHF] (2). The reaction is reversible in the presence of excess TiCl 3. X-ray absorption spectroscopy has been used to identify the local environment in 2. The most striking feature is the existence of the shortest Ti-Mg interatomic distance yet observed. The Ti-Mg bond length, 2.72(1) angstrom, is close to the sum of the Pauling single bond metallic radii (2.69 angstrom) and the complex is termed an inorganic Grignard reagent by analogy with other complexes containing direct transition metal-magnesium bonds. Overall, a dimeric model is compatible with the EXAFS-derived structural parameters and solubility characteristics. The titanium atoms are linked through two bridging MgCl2Mg units and their coordination shells are completed by THF ligands. The structure of 2, and the existence of intermediate 1, can be used to rationalize the known catalytic activity of the TiCl3/Mg/THF system with respect to ether cleavage.