Zhu H, Qu Z, Grimme S. Origin of the Ligand Ring-Size Effect on the Catalytic Activity of Cationic Calcium Hydride Dimers in the Hydrogenation of Unactivated 1-Alkenes.
ChemistryOpen 2022;
11:e202200240. [PMID:
36524742 PMCID:
PMC9756592 DOI:
10.1002/open.202200240]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2022] [Revised: 11/17/2022] [Indexed: 12/23/2022] Open
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.
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