Comparative Study of Aluminum Complexes Bearing N,N'-Diaryldioxalamidate and N,N'-Diaryldithiooxalamidate Ligands in Ring-Opening Polymerization of e-Caprolactone

A series of Al catalysts with N,N'-diaryloxalamidate and N,N'-diaryldithiooxalamidate ligands were synthesized, and their e- caprolactone polymerization rates were determined in this study. Of the Al complexes bearing N,N'-diaryldithiooxalamidate, [(PhNCS)-N-H](2)Al2Me4 exhibited the highest catalytic activity (conv. of poly-e-caprolactone = 94%; [e-caprolactone]:[[(PhNCS)-N-H](2)Al2Me4]:[benzyl alcohol] = 100:0.5:2 with [e-caprolactone] = 2 M at 25 degrees C after 20 min), and of the Al complexes bearing N,N'-diaryldioxalamidate, [(PhNCS)-N-H](2)Al2Me4 exhibited the highest catalytic activity (conv. of poly-e-caprolactone = 94%; [e- caprolactone]:[(OAl2Me4)-Al-H]:[benzyl alcohol] = 100:0.5:2 with [e-caprolactone] = 2 M at 25 degrees C after 30 min). The Al complexes bearing N,N'- diaryldithiooxalamidate demonstrated a greater catalytic activity (1.4-7.3-fold) than Al complexes bearing N,N'-diaryldioxalamidate. Density functional theory calculations revealed that [(PhNCS)-N-iPr](2)Al-2(OMe)(4) with a weaker Al-OMe bond and less repulsion of the highest transition state TS1 revealed the lower potential energy of TS1 compared to [(PhNCS)-N-iPr](2)Al-2(OMe)(4) and further exhibited a higher polymerization rate than [(PhNCS)-N-iPr](2)Al-2(OMe)(4).