Origin of the Ligand Ring‐Size Effect on the Catalytic Activity of Cationic Calcium Hydride Dimers in the Hydrogenation of Unactivated 1‐Alkenes

Recently, it was shown that the double Ca−H−Ca‐bridged calcium hydride cation dimer [LCaH(2)CaL](2+) when stabilized by a larger macrocyclic N,N’,N’’,N’’’,N’’’’‐pentadentate ligand showed evidently higher activity than when stabilized by a smaller N,N’,N’’,N’’’‐tetradentate ligand in the catalytic hydrogenation of unactivated 1‐alkenes. In this DFT‐mechanistic work, the origin of the observed ring‐size effect is examined in detail using 1‐hexene, CH(2)=CH(2) and H(2) as substrates. It is shown that, at room temperature, both the N,N’,N’’,N’’’,N’’’’‐stabilized dimer and the monomer are not coordinated by THF in solution, while the corresponding N,N’,N’’,N’’’‐stabilized structures are coordinated by one THF molecule mimicking the fifth N‐coordination. Catalytic 1‐alkene hydrogenation may occur via anti‐Markovnikov addition over the terminal Ca−H bonds of transient monomers, followed by faster Ca−C bond hydrogenolysis. The higher catalytic activity of the larger N,N’,N’’,N’’’,N’’’’‐stabilized dimer is due to not only easier formation of but also due to the higher reactivity of the catalytic monomeric species. In contrast, despite unfavorable THF‐coordination in solution, the smaller N,N’,N’’,N’’’‐stabilized dimer shows a 3.2 kcal mol(−1) lower barrier via a dinuclear cooperative Ca−H−Ca bridge for H(2) isotope exchange than the large N,N’,N’’,N’’’,N’’’’‐stabilized dimer, mainly due to less steric hindrance. The observed ring‐size effect can be understood mainly by a subtle interplay of solvent, steric and cooperative effects that can be resolved in detail by state‐of‐the‐art quantum chemistry calculations.