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

The preparation of dizirconium complexes based on a novel bis(indenyl)-( E / Z )-stilbene platform was explored. Negishi coupling between the in situ -generated diorganozincates obtained from the respective o / m / p -( E / Z )-dibromostilbenes and the bromo-functionalized zirconocene (η 5 -Cp*)(η 5 -2-methyl-4-bromoindenyl)ZrCl 2 , or, alternatively, the preparation of bis(indene)stilbene pro-ligands { o / m / p -( E / Z )-BisIndSB}H 2 through Negishi coupling of the corresponding dibromostilbenes with 4-bromoindene and subsequent metallation/transmetallation with Cp*ZrCl 3 or Zr(NMe 2 ) 4 , 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 ( E )-complexes are more stable by 7-11 kcal mol -1 than their ( Z )-analogues. Thermal, uncontrolled isomerization of ( Z )- to ( E )-stilbene platform was observed experimentally for some systems, in the course of their synthesis, either from the ( Z )-dibromostilbene reagent or on the dinuclear complexes resulting from the Negishi coupling. Photoisomerization of the ( E )- and ( Z )-{BisIndSB}H 2 proligands and of complexes { o -( E )-BisIndSB}(Zr(NMe 2 ) 3 ) 2 and { m -( E )-BisIndSB}(ZrCl 2 Cp*) 2 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 ( Z )- and ( E )-isomers of a given type of complex under the reaction conditions investigated.