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

The stepwise formation of two distinct bimetallic titanium/magnesium c omplexes during the reduction of TiCl3 (or TiCl4) by magnesium in tetr ahydrofuran (THF) has been identified. [TiMgCl2 . x THF] (1) is produc ed 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 lo cal environment in 2. The most striking feature is the existence of th e shortest Ti-Mg interatomic distance yet observed. The Ti-Mg bond len gth, 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 tra nsition metal-magnesium bonds. Overall, a dimeric model is compatible with the EXAFS-derived structural parameters and solubility characteri stics. The titanium atoms are linked through two bridging MgCl2Mg unit s and their coordination shells are completed by THF ligands. The stru cture of 2, and the existence of intermediate 1, can be used to ration alize the known catalytic activity of the TiCl3/Mg/THF system with res pect to ether cleavage.