Cyclic alkyl(amino)iminates (CAAIs) as strong 2 sigma,4 pi-electron donor ligands for the stabilisation of boranes and diboranes(4): a synthetic and computational study

Singly and doubly cyclic alkyl(amino)iminate (CAAI)-substituted boranes and diboranes(4) were synthesised by halosilane elimination between a silylimine and halo(di)borane precursors. B-11 NMR-spectroscopic studies show that the CAAI ligand is a much stronger electron donor than amino ligands. X-ray crystallographic analyses reveal that the degree of B-N-CAAI double bonding increases with the electron-withdrawing capacity of the other substituents at boron. The C-N-B bond angle displays a great flexibility, ranging from 131 degrees to near-linear 176 degrees, the narrowest angles being observed for NMe2-substituted derivatives and the widest angles for highly sterically demanding substituents. Density functional theory (DFT) calculations on the electronic structures of the anionic CAAI ligand compared to unsaturated and saturated N-heterocyclic iminate (NHI) ligands show that the former is the best sigma donor of the three but less pi-donating than the unsaturated NHI. Nevertheless, the linear (CAAI)BH2 complex displays somewhat stronger C-N and N-B pi bonding than the corresponding ((S)NHI)BH2 complexes.