Synthesis of heteroleptic calcium amide complexes via manipulation of the Schlenk equilibrium

Abstract

Heteroleptic calcium β-diketiminate (BDI) complexes have been recognised as potent (pre)catalysts for a variety of transformations and have demonstrated valuable stoichiometric reactivity. Bulky ligands such as BDIs have been utilised to prevent unwanted dismutation and decomposition reactions of active heteroleptic species to their homoleptic congeners. Homoleptic bis-ligated alkaline earth metal (Ae) species of the type “L2Ae” have hitherto been often considered kinetic sinks and less valuable by-products. This work shows that through the use of the sterically demanding proligand HC(iPrCNDip)2H, Dip = 2,6-iPr2C6H3 (iPrDipBDIH), the homoleptic calcium complex [(iPrDipBDI)2Ca] can be destabilised to the degree that when treated with calcium bis(hexamethyldisilazide) (Ca(HMDS)2), a thermal dismutation of the ligands takes place to afford the heteroleptic complex [(iPrDipBDI)Ca(HMDS)]. DFT calculations support that this reaction is enthalpically unfavourable, an effect that is reduced by the destabilising influence of the bulk in [(iPrDipBDI)2Ca], but entropically made possible. This synthesis has been extended to a convenient one-pot regime from [(iPrDipBDI)K], CaI2 and Ca(HMDS)2, and the resultant calcium amide complex has been utilised as a precursor to prepare a thermally stable, donor-solvent-free molecular calcium hydride complex [{(iPrDipBDI)CaH}2].