C-H Activation of Arene Substrates Mediated by Coordinative Unsaturation of Group 4 Metallocene-Iridium ELHB Hydrides

摘要

Low valent group 4 metal complexes have been recognized as useful organometallic reagents in synthetic chemistry such as oxidative couplings of carbon-carbon and carbon-heteroatom unsaturations. Besides, group 4 metal polyhydride derivatives, upon dihydrogen elimination, can also offer useful chemical transformations. Surprisingly, there are a few examples of early-late heterobimetallic (ELHB) hydrides comprising of group 4 metallocene and late transition metal units albeit a number of reports on homometallic hydrides. Therefore, it is highly challenging to develop coordinatively unsaturated ELHBs with the cooperativity in bond cleavage and formation reactions. Previously, we have reported synthesis and reactivity of zirconocene-iridium ELHB hydrides (L2ZrCl)(Cp'Ir)(μ-H)3 (1) (L = cyclopentadienyl derivative). 1 displayed smooth ligand elimination when treated with LiBEt3H or an alkyllithium base, leading to the formation of unprecedented square planar tetranuclear (L2Zr)2(Cp'Ir)2(μ-H)4 (2). The detailed NMR study for the formation of 2 revealed that the process involves C-H activation of arenes employed as solvents with proposed transient 30-electron intermediates (L2Zr)(Cp'Tr)(μ-H)2 (3) and bimolecular Cp ring C-H activation of 3 itself. However, no isolable intermediates from the zirconium-iridium system and arene substrates were obtained. From the bond disruption enthalpy difference of metallocene derivatives between D(Zr-X) and D(Hf-X) (X = hydrocarbyl, hydride, alkoxide, amide, halide), we decided to investigate C-H activation chemistry using hydride-bridged hafhocene-iridium counterparts. Here we report synthesis and reactivity of type of (L2HfO)(Cp'Ir)(μ-H)3 (4).