|M.Sc Student||Adam Brill|
|Subject||Synthesis of Benzimidazolin-2-iminato Actinide|
Complexes: Application in Catalysis of Aldehyde
|Department||Department of Chemistry||Supervisor|
A new class of mono(benzimidazolin-2-iminato) actinide complexes was investigated with respect to their reactivity toward the catalytic Tishchenko homocoupling reaction of aromatic and aliphatic aldehydes, resulting in symmetric ester products. In this endeavor, symmetric and unsymmetric NRBenzImi (benzimidazolin-2-iminato) thorium complexes demonstrated high catalytic activity towards the homocoupling reaction in the presence of the majority of aldehyde substrates. The symmetric and unsymmetric thorium NRBenzImi (benzimidazolin-2-iminato) thorium species showed minor differences in their reactivity towards aldehyde homocoupling, due most likely to the differences in their respective coordination spheres, for which the unsymmetric complex exhibits a smaller ligand cone angle than the symmetric complex. The smaller cone angle plausibly leaves the thorium center of this unsymmetric complex open to the donation of lone pair electrons from the heteroatomic substituents of various aldehyde substrates, which may lower the reactivity of the complex by increasing the electron density of the thorium center through σ electron donation. In contrast to the aforementioned thorium complexes, symmetric NRBenzImi (benzimidazolin-2-iminato) uranium complexes showed selective reactivity toward the esterification of aliphatic aldehydes, and was almost entirely inactive towards the coupling of most aromatic aldehydes. The one exception observed in this trend was the propensity of the uranium complex to successfully produce symmetric esters of the ortho, meta and para nitrobenzaldehyde isomers. A model to account for the lack of the reactivity by the uranium complex in the case of most aromatic aldehydes was proposed. In this model, two molar equivalents of aldehyde are speculated to insert into the U-N(TMS)2 bond of the uranium complex, which results in an aromatic acetal moiety that plausibly donates electrons to the uranium center by way of π-coordination. In this model, the coordination may be favorable, since uranium possesses 5f electron, which may be utilized in π backdonation to the aromatic moiety of the resulting complex, resulting in a strong coordinate bond. In the presence of strong resonance withdrawing substituents, such is the case with nitrobenzaldehyde; this strong coordination may be weakened, thus decreasing the electron density of the uranium center, which then allows for the esterification of the aldehyde. A plausible mechanism and catalytic cycle for the Tishchenko reaction is proposed, which can be compared and contrasted with the proposed mechanism of previously conducted Tishchenko aldehyde reactions, wherein NRImi (Imidazolin-2-iminato) thorium complexes were employed. These mechanisms are similar in that the rate-determining step (RDS) for both mechanisms involves the hydride transfer from the acetal carbon of the catalytic species to the carbonyl carbon of an incoming aldehyde subunit, which is justified by kinetic isotope experiments demonstrated in this work. Additional supporting evidence for the proposed mechanism was obtained by way of the addition of stoichiometric quantities of benzaldehyde to the symmetric thorium complex, which implied that two of the Th-N(TMS)2 bonds are activated in the formation of the catalytically active species. The proposed mechanism was considered to be applicable to the Tishchenko reaction as conducted by the symmetric NRBenzImi (benzimidazolin-2-iminato) uranium and unsymmetric NRBenzImi (benzimidazolin-2-iminato) thorium complexes.