Stabilization of Calcium Hydride Complexes by Fine Tuning of Amidinate Ligands

A range of symmetric amidinate ligands RAm^Ar (R is backbone substituent, Ar is N substituent) have been investigated for their ability to stabilize calcium hydride complexes of the type RAm^ArCaH. It was found that the precursors of the type RAm^ArCaN­(SiMe₃)₂ are only stable toward ligand exchange for Ar = DIPP (2,6-diisopropylphenyl). The size of the backbone substituent R determines aggregation and solvation. The following complexes could be obtained: [RAm^DIPPCaN­(SiMe₃)₂]₂ (R = Me, p-Tol), RAm^DIPPCaN­(SiMe₃)₂·Et₂O (R = Np, tBu), AdAm^DIPPCaN­(SiMe₃)₂·THF, and AdAm^DIPPCaN­(SiMe₃)₂. Reaction of these heteroleptic calcium amide complexes with PhSiH₃ gave only for larger backbone substituents (R = tBu, Ad) access to the dimeric calcium hydride complexes (RAm^ArCaH)₂. (N,aryl)-coordination of the amidinate ligand seems crucial for the stability of these complexes, and the aryl···Ca interaction is found to be strong (17 kcal/mol). Addition of polar solvents led to a new type of trimeric calcium hydride complex exemplified by the crystal structures of (tBuAm^DIPPCaH)₃·2Et₂O and (AdAm^DIPPCaH)₃·2THF. The overall conclusion of this work is that minor changes in sterics (tBu vs Ad) or coordinated solvent (THF vs Et₂O) can have large consequences for product formation and stability.