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

Abstract

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 (η⁵-Cp*)(η⁵-2-methyl-4-bromoindenyl)ZrCl₂, or, alternatively, the preparation of bis(indene)stilbene pro-ligands {o/m/p-(E/Z)-BisIndSB}H₂ through Negishi coupling of the corresponding dibromostilbenes with 4-bromoindene and subsequent metallation/transmetallation with Cp*ZrCl₃ or Zr(NMe₂)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}H₂ proligands and of complexes {o-(E)-BisIndSB}(Zr(NMe₂)₃)₂ and {m-(E)-BisIndSB}(ZrCl₂Cp*)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.