Stabilization of Calcium Hydride Complexes by Fine Tuning of Amidinate Ligands

A range of symmetric amidinate ligands RAm<sup>Ar</sup> (R is backbone substituent, Ar is N substituent) have been investigated for their ability to stabilize calcium hydride complexes of the type RAm<sup>Ar</sup>CaH. It was found that the precursors of the type RAm<sup>Ar</sup>CaN­(SiMe<sub>3</sub>)<sub>2</sub> 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<sup>DIPP</sup>CaN­(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> (R = Me, <i>p</i>-Tol), RAm<sup>DIPP</sup>CaN­(SiMe<sub>3</sub>)<sub>2</sub>·Et<sub>2</sub>O (R = Np, <i>t</i>Bu), AdAm<sup>DIPP</sup>CaN­(SiMe<sub>3</sub>)<sub>2</sub>·THF, and AdAm<sup>DIPP</sup>CaN­(SiMe<sub>3</sub>)<sub>2</sub>. Reaction of these heteroleptic calcium amide complexes with PhSiH<sub>3</sub> gave only for larger backbone substituents (R = <i>t</i>Bu, Ad) access to the dimeric calcium hydride complexes (RAm<sup>Ar</sup>CaH)<sub>2</sub>. (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 (<i>t</i>BuAm<sup>DIPP</sup>CaH)<sub>3</sub>·2Et<sub>2</sub>O and (AdAm<sup>DIPP</sup>CaH)<sub>3</sub>·2THF. The overall conclusion of this work is that minor changes in sterics (<i>t</i>Bu vs Ad) or coordinated solvent (THF vs Et<sub>2</sub>O) can have large consequences for product formation and stability.