Dinuclear group IV metal complexes based on a bis(indenyl)-(/)-stilbene platform: a potential prototype of "photoswitchable" catalysts for olefin polymerization.

The preparation of dizirconium complexes based on a novel bis(indenyl)-(/)-stilbene platform was explored. Negishi coupling between the -generated diorganozincates obtained from the respective //-(/)-dibromostilbenes and the bromo-functionalized zirconocene (η-Cp*)(η-2-methyl-4-bromoindenyl)ZrCl, or, alternatively, the preparation of bis(indene)stilbene pro-ligands {//-(/)-BisIndSB}H through Negishi coupling of the corresponding dibromostilbenes with 4-bromoindene and subsequent metallation/transmetallation with Cp*ZrCl or Zr(NMe), allowed the preparation of a series of dinuclear complexes. These were analyzed by NMR spectroscopy and some of them by iASAP-mass spectrometry and by X-ray diffraction studies. Experimental results were compared with DFT modelling of the targeted dinuclear complexes evidencing that the ()-complexes are more stable by 7-11 kcal mol than their ()-analogues. Thermal, uncontrolled isomerization of ()- to ()-stilbene platform was observed experimentally for some systems, in the course of their synthesis, either from the ()-dibromostilbene reagent or on the dinuclear complexes resulting from the Negishi coupling. Photoisomerization of the ()- and ()-{BisIndSB}H proligands and of complexes {-()-BisIndSB}(Zr(NMe)) and {-()-BisIndSB}(ZrClCp*) was investigated under a variety of conditions. It proved effective for the proligands but induced decomposition of the dizirconium complexes. Time-dependent DFT (TD-DFT) computations were performed to identify unambiguously the nature of the observed absorption bands and account for decomposition of the complexes. Preliminary ethylene/1-hexene homo- and copolymerization investigations did not evidence putative cooperativity phenomena within these dinuclear systems nor significantly differentiated behavior between the ()- and ()-isomers of a given type of complex under the reaction conditions investigated.