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 (RAmCaH)-Ca-Ar. It was found that the precursors of the type (RAmCaN)-Ca-Ar(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: [(RAmCaN)-Ca-DIPP(SiMe3)(2)](2) (R = Me, p-Tol), (RAmCaN)-Ca-DIPP(SiMe3)(2)center dot Et2O (R = Np, tBu), AdAm(DIPP)CaN(SiMe3)(2)center dot THF, and AdAm(DIPP)CaN(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 ((RAmCaH)-Ca-Ar)(2). (N,aryl)-coordination of the amidinate ligand seems crucial for the stability of these complexes, and the aryl center dot center dot center dot 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(DIPP)CaH)(3)center dot 2Et(2)O and (AdAm(DIPP)CaH)(3)center dot 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.