A series of dinuclear aluminum (AlPyr) complexes bridged by two pyrazole ligands were synthesized, and their catalytic activity toward ring-opening polymerization of ε-caprolactone (CL) was investigated. Different types of the Al-N-N-Al-N-N skeletal ring were found among these AlPyr complexes. The butterfly form, LAlMe, exerted the highest catalytic activity for CL polymerization. κ-CL coordination with both Al centers within the butterfly form LAlMe facilitates the initiation process. Generally speaking, the AlPyr complexes exhibited substantially higher catalytic activity for CL polymerization than literature examples such as β-diketiminate- or traiaza-bearing aluminum complexes. In fact, the AlPyr complexes can even carry out CL polymerization at room temperature.
Abstract.A series of titanium complexes bearing substituted diphenolate ligands (RCH(phenolate) 2 , where R = H, CH 3 ,2, was synthesized and studied as catalysts for the ring opening polymerization of L-lactide and ε-caprolactone. Ligands were designed to probe the role of chelate effect and steric effect in the catalytic performance. From the structure of triphenolate (with one extra coordination site than diphenolate ligand) Ti complex, TriOTiO i Pr 2 , we found no additional chelation to influence the catalytic activity of Ti complexes. It was found that bulky aryl groups in the diphenolate ligands decreased the rate of polymerization most. We conclude that steric effect is the most controlling factor in these polymerization reactions by using Ti complexes bearing diphenolate ligands as catalysts since it is responsible for the exclusion of needed space for incoming monomer by the bulky substituents on the catalyst.
In this study, aluminum complexes
bearing ferrocene-based and arylthiomethylphenolate
ligands were synthesized, and their catalytic activity for ε-caprolactone
(CL) polymerization was investigated. The catalytic activity of the
reduced form of Al complexes was higher than that of the oxidized
form. The CL polymerization rate of the reduced form fcO
2
AlMe (75 min, conversion =
100%) was higher than that of the oxidized form fc
ox
O
2
AlMe (4320 min, conversion = 45%), and the CL polymerization
rate of fc(OAlMe
2
)
2
(40 min, conversion = 100%) was higher
than that of fc
ox
(OAlMe
2
)
2
(60
min, conversion = 97%). Electron deficiency substituents on phenolate
decreased the catalytic activity of Al complexes bearing arylthiomethylphenolate
ligands. Density functional theory calculations revealed that thioether
coordination stabilized the transition state (TS1) and
that the oxidized form fc
ox
(OAlMe
2
)
2
exhibited weaker thioether coordination and higher activation
energy in TS1 compared with those of the reduced form fcO
2
AlMe. In addition,
our study determined that the thioether group is a suitable chelating
group for Al catalysts in CL polymerization due to its labile nature.
Trinuclear
aluminum complexes bearing bipyrazoles were synthesized,
and their catalytic activity for ε-caprolactone (CL) polymerization
was investigated. D
Bu
2
Al
3
Me
5
exhibited higher catalytic activity than
did the dinuclear aluminum complex L
Bu
2
Al
2
Me
4
(16 times as high
for CL polymerization; [CL]:[D
Bu
2
Al
3
Me
5
]:[BnOH] = 100:0.5:5, [D
Bu
2
Al
3
Me
5
] = 10 mM, conversion 93% after 18 min at room temperature).
Density functional theory calculations revealed a polymerization mechanism
in which CL first approached the central Al atom and then moved to
an external Al. The coordinated CL ring was opened because the repulsion
of two tert-butyl groups on the ligands pushed an
alkoxide initiator on an external Al to initiate CL. In these trinuclear
Al catalysts, the central Al plays a role in monomer capture and then
collaborates with the external Al to activate CL, accelerating polymerization.
The palladacycle complex [LsPdOAc]2 bearing 2‐phenyl benzothiazole was synthesized and characterized by NMR and X‐ray crystallography. [LsPdOAc]2 was used as a catalyst in the Suzuki–Miyaura cross coupling reaction of 4‐bromotoluene with phenylboronic acid, which resulted in a conversion of >90% with 5 mol% of the Pd complex within 10 min at 60°C.
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