Vanadium(III)–catalyzed copolymerization of ethylene with norbornene: Microstructure at tetrad level and reactivity ratios

Synthesis, Structure and 1,3-Butadiene Polymerization Behavior of Vanadium(III) Phosphine Complexes

“…The complexes adopt slightly distorted trigonal-bipyramidal geometry, where the main distortion comes from the deviation (by up to 13 • ) of the P-V-P angles from 180 • . As previously discussed [30], such a deviation is mainly due to crystal packing forces (essentially C-H···Cl [38], C-H···π and π-π intermolecular interactions, the latter two arising in structures where phenylphosphines are present), as denoted by the more regular conformation as obtained by gas-phase calculations, which provide a maximum 6 • deviation from a P-V-P straight angle. As a general observation, the V-P bond lengths, ranging on average from 2.520(1) to 2.570(1) Å (2.537 to 2.587 Å from UM06 calculations) in the compounds here investigated, appear to be much longer than the Mt-P (Mt = metal) ones as found in tetrahedral MtCl 2 (PR n Ph 3-n ) 2 complexes, suggesting weaker bonds with scarce or null π-character for the vanadium complexes.…”

“…Single crystals, suitable for X-ray structure determination, were obtained for 1b, whose structure was previously reported [25,27,30], 1c and 2a from cold pentane. In contrast, the reaction in toluene with phosphine of type PR n Ph 3-n (n = 2, R = Et, t Bu, Cy) and PR 3 (R = n Pr, t Bu) gave an oily product, difficult to recover.…”

“…Dusty solids were instead successfully obtained by performing the reaction in THF and adding the ligand dropwise; single crystals, suitable for X-ray structure determination, were obtained only for 1g. Due to their low stability in air, all crystals required low temperature to minimize crystal decay during X-ray data collection (130 K for 1b [30], and 1c, 150 K for 1g and 100 K for 2a, according to their different stability). A list of selected bond distances and angles for 1b [30], 1c, 1g and 2a as obtained by X-ray investigation is reported in Table 1, while a view of the molecular structures of 1c, 1g and 2a is reported in Figure 1.…”

“…Due to their low stability in air, all crystals required low temperature to minimize crystal decay during X-ray data collection (130 K for 1b [30], and 1c, 150 K for 1g and 100 K for 2a, according to their different stability). A list of selected bond distances and angles for 1b [30], 1c, 1g and 2a as obtained by X-ray investigation is reported in Table 1, while a view of the molecular structures of 1c, 1g and 2a is reported in Figure 1. DFT gas-phase geometry optimizations, starting from the experimental structures, have been performed on the four complexes in order to discern the effect of crystal packing, if any, on the experimental geometries.…”

“…By extending the polymerization runs up to 24 h, it was possible to reach complete monomer consumption ( Table 2, entry 2), meaning that the catalysts are not deactivated with time, as it occurs in the polymerization of ethylene where high concentration of reoxidant (typically Cl 3 CCOOEt or chlorinated hydrocarbons) is required to keep the catalyst active [30,40,41]. This can be ascribed to the higher nucleophilicity of 1,3-butadiene with respect to a simple olefin, but mainly to the higher stability of the vanadium-π-allyl bond compared to the vanadium-alkyl bond of the intermediate species involved in the polymerization of 1,3-butadiene and ethylene, respectively [42].…”