Stabilization of Calcium Hydride Complexes by Fine Tuning of Amidinate Ligands

Abstract

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.

Imine hydrogenation with simple alkaline earth metal catalysts

“…The calcium hydride pathway A was further confirmed by stoichiometric reactions with a recently introduced well-defined calcium hydride complex 16 (3, Fig. 6).…”

“…The synthesis and structure of complex 3 ( Fig. 6) have been reported previously 16 . Stoichiometric conversions of 3 with imine and subsequently with H 2 (6 bar) were performed in a high-pressure NMR tube and were monitored by 1 H NMR (see Supplementary Methods).…”

“…Figure 1a shows a working hypothesis for styrene hydrogenation with a dibenzylcalcium catalyst (CaBn 2 ) 7 . The first step is the generation of a calcium hydride species, for which ample precedence exists [13][14][15][16][17] . Further reaction with H 2 may cause precipitation of insoluble (CaH 2 ) n , but catalyst loss is partly prevented by aggregation to soluble but undefined Ca x Bn y H z species.…”

A Simple Route to Calcium and Strontium Hydride Clusters

“…Thespectator ligand Ls hould be bulky,m ultidentate and strongly bound to Scheme 1. [13] While lessons learned now allow for access to ag rowing number of calcium hydride complexes,the heavier and much more challenging strontium and barium hydride complexes have hitherto remained elusive.I ncreasing the size of the metal does not only result in faster ligand exchange [14] but also makes it increasingly difficult to prevent Schlenk equilibria by steric hindrance.Additionally,the inherently higher reactivity of heavier Group 2m etal complexes poses problematic decomposition pathways.O ur attempts to control ligand exchange by use of extraordinarily large ligands has so far failed. prevent formation of homoleptic species through the Schlenk equilibrium:2LAeH Q L 2 Ae + AeH 2 fl.…”

Dogma-breaking catalysis

“…The simulations also supported the proposed stepwise mechanism for the catalytic cycle. Finally, Bauer et al used a previously reported calcium hydride complex 11 as a model of the proposed catalytic hydride intermediate, and found that it reacts with an aldimine and hydrogen gas in a way that is consistent with the proposed catalytic cycle.…”