CuII-mediated controllable creation of tertiary and quaternary carbon centers: designed assembly and structures of a new class of copper complexes supported by in situ generated substituted 1-pyridineimidazo[1,5-a]pyridine ligands

Abstract

Hydrothermal reaction of CuCl₂·2H₂O, 3-(pyridin-2-yl)imidazo[1,5-a]pyridine (HPIP) and formaldehyde, yielded a novel mixed-valence Cu^I/Cu^II complex [L1Cu₂Cl₂][Cu₂Cl₄]·2EtOH (1·2EtOH; L1 = tetrakis(3-(pyridin-2-yl)imidazo[1,5-a]pyridin-1-yl)methane). The in situ generated quaternary carbon-containing ligand L1 was derived from Cu^II-mediated quadruple coupling of HPIP and formaldehyde, where the three bonds of formaldehyde were totally cleaved. 1·2EtOH can also be prepared by replacing formaldehyde by formic acid, N,N-dimethylformamide, and glyoxal, respectively. When acetic acid, which usually serves as a radical reaction inhibitor, was introduced into the reaction systems, two Cu^I complexes [(L2)₂Cu₃](CuCl₂)₃·2EtOH (2·2EtOH) and L3Cu₂Cl₂ (3) (L2 = tris(3-(pyridin-2-yl)imidazo[1,5-a]pyridin-1-yl)methane, and L3 = 1,1,2,2-tetrakis(3-(pyridin-2-yl)imidazo[1,5-a]pyridin-1-yl)ethane) were afforded. The tertiary carbon-bearing ligands L2 and L3 were generated in situ via Cu^II-mediated controllable cleavage of two bonds of formaldehyde, N,N-dimethylformamide and glyoxal, respectively. The plausible mechanisms for the formation of the ligands L1–L3 were proposed.