Local structure analysis around the Nd center in a ternary catalyst comprising Nd(vers)₃, Al(ⁱBu)₃ and Al(ⁱBu)₂Cl by XAFS

Abstract

Lanthanide-based catalysts are highly active for isoprene polymerization in hexane. In this paper, a ternary catalyst consisting of neodymium neodecanoate {Nd(vers)3}, Al((i)Bu)3 and Al((i)Bu)2Cl was studied by using X-ray-absorption fine-structure (XAFS) technique. A sealed and moisture-proof liquid sample cell with adjustable thickness was designed for Nd LIII-edge XAFS measurements. Based on the XAFS data analysis, detailed structure changes around the Nd center were obtained. It was found that the Nd(vers)3 molecules formed an oligomer structure in hexane solution with two Nd-O subshells (5O @ 2.39 Å and 5O @ 2.54 Å) around the Nd center. The alkylation process by adding Al((i)Bu)3 to the hexane solution of Nd(vers)3 partially destroyed the aggregation degree of Nd(vers)3 molecules in hexane solution. Al((i)Bu)3 ligands were bonded to the Nd center by Nd-C bonding. With the Nd : Al ratio increasing from 1 : 2.5 to 1 : 10, the O neighbors around Nd decreased from 4 to 2 but with an unchanged Nd-O bond length of 2.38 Å, and the C neighbors around Nd were kept at ca. 4 with Nd-C bond lengths in the range of 2.57-2.58 Å. The Nd-O bonds can be further replaced by Nd-C bonds during the aging process. The chlorination process by adding Al((i)Bu)2Cl to the mixture solution of Nd(vers)3 and Al((i)Bu)3 restrained intensively the agglomeration of Nd(vers)3 molecules in hexane solution. Al((i)Bu)2Cl ligands were bonded to the Nd center by Nd-Cl bonds. There were about 3-4 C neighbors at 2.58 Å, 2 Cl neighbors at 2.87 Å, and 2 Al next-neighbors at 3.14 Å around the Nd center. After allowing the ternary catalyst to stand for 5 days, the coordination numbers of Nd-C and Nd-Cl were all stabilized to 3 without bond length changes, and partial single Cl(-) anions were also bonded to the Nd center. All these structural details and their change tendency demonstrate that the decrease of aggregation degree of Nd(vers)3 molecules in hexane solution can improve the catalytic activity of the ternary lanthanide-based catalyst system.

In situ X-ray absorption fine structure study on the polymerization of isoprene assisted by Nd-based ternary catalysts

“…X-ray absorption spectra of Nd L III -edge were collected at beamline 4B9A of the Beijing Synchrotron Radiation Facility (BSRF). A moistureproof and adjustable-thickness liquid sample-cell 41 was used for the in situ XAFS measurements. The Nd-L III edge XAFS spectra of all the samples were collected in transmission mode at room temperature.…”

“…Some distinct spectral changes can be found from the X-ray-absorption nearedge structure (XANES) spectra and the corresponding Fourier transform (FT) spectra of the three different activation steps although the quantitative structural information were not provided. In our previous work, 41 in situ XAFS technique was used to detect the liquid structures of the Nd-based ternary catalyst containing Nd(vers) 3 , Al( i Bu) 3 , and chlorodiisobutyl aluminium {Al( i Bu) 2 Cl} in hexane solvent. The quantitative structural evolution around Nd center was obtained at the three activation steps.…”

Effect of Ancillary Ligands as a Part of Counteranion in Neodymium-Catalyzed Isoprene Polymerization

“…The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.organomet.0c00112. Details of crystallographic parameters and IR spectra of neodymium complexes, and 1 H and 13 C NMR spectra of obtained polyisoprene (PDF)…”

“…At the same time, halogens are usually located between two metal centers, bridging them in a μ 2 -manner, and they often make cluster complexes, resulting in catalytically inert species or a broad polydispersity index (PDI) polymer. Recent X-ray absorption fine structure analysis revealed that multiple halogens are located on the active lanthanide species, 13 and some cationic lanthanide clusters bearing chloride bridges have been isolated. 14,15 Some single-component chlorolanthanide catalyst systems show controlled behavior on diene polymerization and give a narrow PDI polymer, thus enabling more precise estimations of active species.…”

Synthesis of stereodiblock polyisoprene consisting of cis-1,4 and trans-1,4 sequences by using a neodymium catalyst: change of the stereospecificity triggered by an aluminum compound

“…The addition of chloride would prevent the deactivation by coordinating to the neodymium center. 25,26 The synthesis of stereodiblock polymers by the sequential addition of Bu 2 Mg and MMAO/ t BuCl or dMMAO/Me 2 SiCl 2 was performed ( Table 2). In both systems, a polymer with a narrow molecular weight distribution was obtained in high yield.…”