Bimetallic Ethylene Tetramerization Catalysts Derived from Chiral DPPDME Ligands: Syntheses, Structural Characterizations, and Catalytic Performance of [(DPPDME)CrCl₃]₂ (DPPDME = S,S- and R,R-chiraphos and meso-achiraphos)

Chromium-based ethylene tetramerization catalyst precursors of the type {(DPPME)CrCl₂(μ-Cl)}₂ with a chiraphos ligand backbone [DPPME = {Ar(R₁R₂)}₂PCH(CH₃)CH(CH₃)P{Ar(R₁R₂)}₂] were prepared and characterized, in which substituents on the aryl phosphine varied with electron-donating and -withdrawing groups (S,S-ArR₁ = H, R₂ = H 1; R,R-ArR₁ = H, R₂ = H 2; meso-ArR₁ = H, R₂ = H 3; S,S-ArR₁ = Me, R₂ = H 4; S,S-ArR₁ = OMe, R₂ = H 5; S,S-ArR₁ = H, R₂ = Me 6; S,S-ArR₁ = H, R₂ = OMe 7; S,S-ArR₁ = H, R₂ = F 8). Depending on the coordination of chiraphos or achiraphos ligands, the corresponding bimetallic chromium complexes were prepared from the reaction of CrCl₃(THF)₃. S,S- and R,R-Chiraphos-based chromium metal complexes, 9 and 10, were crystallographically characterized to be dimeric, bridged by two chlorine ligands, whereas a meso achiraphos ligand resulted in a monomeric chromium complex, 17, when crystallized in acetonitrile. The observed average Cr−P bond distances of 9 [2.491(1) Å] and 10 [2.489(2) Å] were shorter than those found in 17 [2.512(1) Å]. The bite angles of 9 and 10 [79.45(6)° and 79.91(8)°] were also smaller than those of 17 [82.06(5)°]. Isolated chiraphos-based Cr(III) precursors, 9−16, exhibited enhanced ethylene tetramerization activities in the range 274−2,256 kg/g Cr/h when 30 bar of ethylene was engaged in the presence of MMAO-12A. Further, methyl substitution at the para-position of the phenyl group on the S,S-chiraphos ligand produced the expected chromium dimer, [CrCl₂(μ-Cl){(P,P)-κ²-(S,S-DPPDME-6)}]₂ (14), and exhibited higher catalytic performance, reaching a 1-octene activity of 2256 kg/g Cr/h with 51.1% selectivity at 45 °C. Further, temperature control at 25 °C increased 1-octene selectivity to 77.4% at the expense of reduced activity of 580 kg/g Cr/h. DFT calculations on the seven-membered metallacycle intermediates suggested stronger Cr−P bonding with a S,S-/R,R-chiraphos ligand backbone, and this interaction can be related to the enhanced catalytic performance.