Cyclopentadienyl in Cp2Hf Complexes Is No Longer a Spectator Ligand: Reactions of a Hafnacyclocumulene Complex with Ketones

Abstract

The interaction of five-membered hafnacyclocumulene complex 1Hf with ketones was investigated. The reaction outcome depends on the conditions and the ketone substituents. Under UV irradiation, insertion can be halted at one or two ketone molecules inserted into Hf–C bonds, affording hafnadihydrofuran 2Ph or the nine-membered dioxahafnacyclocumulene 3Fl. Under thermal conditions, the reaction of 1Hf with ketones revealed, for the first time, ketone insertion into one or two Cp–Hf bonds of the metallocene moiety. In the reaction with benzophenone, the corresponding insertion product 4Phb was isolated, and its structure was confirmed by SC-XRD. Products resulting from the insertion of benzophenone or fluorenone into the Cp–Hf bonds of the metallocene fragment of the hafnacyclocumulene were also characterized. A ketone insertion mechanism into the Cp–Hf bonds was proposed based on quantum chemical calculations at the DLPNO–CCSD(T)/Def2-TZVP//r2-SCAN-3c/Def2-TZVP level of theory. The calculation results revealed that the ketone undergoes insertion via η1-type of Cp–Hf bond. Furthermore, benzophenone and fluorenone also insert into the Cp–Hf bonds of hafnocene diphenolates by the same manner. The results obtained allow understanding the general deactivation mechanism in metallocene catalytic processes under interaction with carbonyl compounds.