Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of -caprolactone: aluminum thioamidate and thioureidate systems
In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for -caprolactone (CL) polymerization was evaluated. S Pr -Al exhibited a higher catalytic activity than O Pr -Al (3.2 times as high for CL polymerizatio...
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Veröffentlicht in: | Dalton transactions : an international journal of inorganic chemistry 2023-11, Vol.52 (46), p.17132-17147 |
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Zusammenfassung: | In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for -caprolactone (CL) polymerization was evaluated.
S
Pr
-Al
exhibited a higher catalytic activity than
O
Pr
-Al
(3.2 times as high for CL polymerization; [CL] : [
S
Pr
-Al
] : [BnOH] = 100 : 0.5 : 2; [
S
Pr
-Al
] = 10 mM, conv. = 93% after 14 min at 25 °C), and
US
Cl
-Al
exhibited a higher catalytic activity than
U
Cl
-Al
(4.6 times as high for CL polymerization; [CL] : [
US
Cl
-Al
] : [BnOH] = 100 : 0.5 : 2; [
US
Cl
-Al
] = 10 mM, conv. = 90% after 15 min at 25 °C). Regardless of whether aluminum amidates or ureidates were present, thioligands improved the polymerization rate of aluminum catalysts. Density functional theory calculations revealed that the eight-membered ring
[S
Pr
-AlOMe
2
]
2
decomposed into the four-membered ring
S
Pr
-AlOMe
2
. However,
[O
Pr
-AlOMe
2
]
2
did not decompose because of its strong bridging Al-O bond. The overall activation energy required for CL polymerization was lower when using
[S
Pr
-AlOMe
2
]
2
(18.1 kcal mol
−1
) as a catalyst than when using
[O
Pr
-AlOMe
2
]
2
(23.9 kcal mol
−1
). This is because the
TS2a
transition state of
S
Pr
-AlOMe
2
had a more open coordination geometry with a small N-Al-S angle (72.91°) than did
TS3c
of
[O
Pr
-AlOMe
2
]
2
, the crowded highest-energy transition state of
[O
Pr
-AlOMe
2
]
2
with a larger N-Al-O angle (99.63°).
S-Al
system exhibited a higher level of catalytic activity (1.8-5.5 times) than did the
O-Al
system. Furthermore, the
US-Al
system exhibited a higher level of catalytic activity (1.7 - 4.6 times) than did the
U-Al
system. |
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ISSN: | 1477-9226 1477-9234 |
DOI: | 10.1039/d3dt03198e |