Only few examples for catalytic reactions of actinide complexes with oxygen containing substrates have been reported, and remain a challenge the field. This can be mainly attributed to the high oxophilicity of the early actinides, which lead to a decreased catalytic activity with oxygen containing molecules, due to the formation of unreactive actinide-oxo species. One possibility to overcome this challenge is by increasing the electron density of the highly electrophilic metal by coordination of highly nucleophilic imidazolin-2-iminato ligands. The imidazolin-2-iminato moiety, is regarded as a 2σ, 4π electron donor when coordinated to early transition metals, and is therefore considered as an electronic cyclopentadienyl analogue in these complexes, similarly to ketimido ligand systems. Therefore, it is interesting to explore the reactivity of actinide complexes bearing the imidazolin-2-iminato moiety in comparison to the analogous cyclopentadienyl actinide complexes, and addressing the substantial question whether the coordination of highly nucleophilic ligands leads to an increased catalytic activity towards oxygen containing molecules. In order to investigate the reactivity of these actinide complexes, we have carried out a systematic study with the bis(imidazolin-2-iminato) uranium (IV) complex [U(NImDipp)2(NMeEt)2] (III), it’s cyclopentadienyl analogue [Cp*2U(NMe2)2] (II), and the bis(ketimido) compound [Cp*2U(NCPhMe)2] (I), towards the catalytic ring opening polymerization (ROP) of ε-caprolactone. While [U(NImDipp)2(NMeEt)2] (III) displayed an extraordinary high catalytic activity of 7000 kg (PCL)·mol-1·h-1 at room temperature, [Cp*2U(NMe2)2] (II) and [Cp*2U(NCPhMe)2] (I) exhibited a significantly lower activity, even at elevated temperatures. Furthermore, the catalytic activity of the thorium complex [Cp*2Th(NImDipp)(Me)] (V) was investigated in the catalytic ROP of ε-caprolactone, and in the catalytic Tishchenko reaction, displaying an enhanced catalytic activity as compared to [Cp*2Th(Me)2] (IV).

Active catalysts for oxygen containing substrates: Actinide coordination complexes

249th National Meeting (2015)

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