Biguanide Complexes of Boron and Aluminum

In search of an alternative to the dominant industrial processes, facilitated mostly by transition-metal catalytic systems, current economic and environmental aspects are leading to a growing interest in the chemistry and application of main group element-based catalysts and materials. Mono- and dinuclear boron and aluminum complexes bearing a biguanide ligand (substituted 4,6-dimethoxypyrimidin-2-yl-guanidinate; L) were prepared by deprotonation of LH₂ with BH₃·Me₂S, BF₃·Et₂O, Me₃Al, or Me₂AlCl or, in some cases, by transmetallation reactions of the lithium complex. Complexes were characterized by multinuclear NMR and sc-XRD analyses. Two coordination isomers, where the central atoms can occur both in the four- and six-membered N,N'-chelate ring, were found for homobimetallic aluminum and boron biguanide complexes. A thorough study was performed to understand the formation and isomerization process, where thermally and kinetically conducted isomerization was explored. The relative Gibbs free energies of optimized structures and hypothetical arrangements were calculated and compared in order to support the described isomerization. The activity of selected complexes was tested in the ring-opening polymerization (ROP) of rac-lactide. While the boron complexes are inactive, dimethylaluminum species show moderate activity and good control over the polymer mass and dispersity after ⁱPrOH co-initiation. Furthermore, the species effectively promote hydroboration of unsaturated C-C bonds.