Stabilization of Calcium Hydride Complexes by Fine Tuning of Amidinate Ligands

A range
of symmetric amidinate ligands RAmAr (R is backbone
substituent, Ar is N substituent) have been investigated for their
ability to stabilize calcium hydride complexes of the type RAmArCaH. It was found that the precursors of the type RAmArCaN­(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: [RAmDIPPCaN­(SiMe3)2]2 (R = Me, p-Tol),
RAmDIPPCaN­(SiMe3)2·Et2O (R = Np, tBu), AdAmDIPPCaN­(SiMe3)2·THF, and AdAmDIPPCaN­(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 (RAmArCaH)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 (tBuAmDIPPCaH)3·2Et2O and (AdAmDIPPCaH)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.