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

Abstract

Recently, it was shown that the double Ca-H-Ca-bridged calcium hydride cation dimer [LCaH₂ 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, CH₂ =CH₂ and H₂ 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 H₂ 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.

Keywords: 1-alkene; calcium hydride complexes; homogenous catalysis; hydrogenation; isotope exchange.

Scheme 1

(A) Recently proposed ligand ring‐size effect for the catalytic activity of calcium hydride cation dimer catalysts; (B) subtle interplay of solvent, steric and dinuclear cooperative effects via competitive Ca−H−Ca bridges and terminal Ca−H bonds, as found in this DFT‐mechanistic work.

Figure 1

DFT‐computed free energy profile (in kcal mol⁻¹, at 298 K and 1 m) for the dimer‐to‐monomer equilibrium in THF solution of (A) the L4‐stabilized cation dimer 1²⁺  ⋅ THF; (B) the L5‐stabilized cation dimer 2²⁺ . All molecules shown in ball‐and‐stick model, crucial Ca, N, O and H atoms are highlighted as yellow‐green, blue, red and white balls, while most H atoms are omitted for clarity.

Figure 2

DFT‐computed free energy profile (in kcal mol⁻¹, at 298 K and 1 m) for the hydrogenation of unactivated CH₂=CHR (R=butyl or H) catalyzed by: (A) the L4‐stabilized cation dimer 1²⁺  ⋅ THF; (B) the L5‐stabilized cation dimer 2²⁺ . In both cases of catalysts, anti‐Markovnikov 1‐alkene addition to the terminal Ca−H bond of the respective cation monomer is the rate‐limiting step. In the ball‐and‐stick models, crucial Ca, N, O and H atoms are highlighted as yellow‐green, blue, red and white balls, while most H atoms are omitted for clarity.

Figure 3

Comparison of DFT‐computed free energy profiles (in kcal mol⁻¹, at 298 K and 1 m) for H₂ isotope exchange catalyzed by (A) the L4‐stabilized cation dimer 1²⁺  ⋅ THF used in our recent work; (B) the larger L5‐stabilized cation dimer 2²⁺ . In the ball‐and‐stick models, crucial Ca, N, O and H atoms are highlighted as yellow‐green, blue, red and white balls, while most H atoms are omitted for clarity.