Heterolysis of Dihydrogen by Nucleophilic Calcium Alkyls

Abstract

β-Diketiminato (BDI) calcium alkyl derivatives undergo hydrogenolysis with H2 to regenerate [(BDI)CaH]2, allowing the catalytic hydrogenation of a wide range of 1-alkenes and norbornene under very mild conditions (2 bar H2, 298 K). The reactions are deduced to take place with the retention of the dimeric structures of the calcium hydrido-alkyl and alkyl intermediates via a well-defined sequence of Ca−H/C=C insertion and Ca−C hydrogenation events. This latter deduction is strongly supported by DFT calculations (B3PW91) performed on the 1-hexene/H2 system, which also indicates that the hydrogenation transition states display features which discriminate them from a classical σ-bond metathesis mechanism. In particular, NBO analysis identifies a strong second order interaction between the filled α-methylene sp3 orbital of the n-hexyl chain and the σ* orbital of the H2 molecule, signifying that the H−H bond is broken by what is effectively the nucleophilic displacement of hydride by the organic substituent.