Selective palladation of a large (32 ring atom) macrocyclic ligand at a bis(N-heterocyclic carbene) coordination pocket through transmetallation of the corresponding mercury(ii) derivative

Abstract

Treatment of N²,N⁶-bis(6-acrylamidopyridin-2-yl)pyridine-2,6-dicarboxamide with benzimidazole gives the acyclic aza-Michael addition product N²,N⁶-bis(6-(3-(1H-benzo[d]imidazol-1-yl)propanamido)pyridin-2-yl)pyridine-2,6-dicarboxamide (2). The macrocycle N¹,N⁷-pyridine-2,6-dimethyl-N²,N⁶-bis(6-(3-(1H-benzo[d]imidazol-1-yl)propanamido)pyridin-2-yl)pyridine-2,6-dicarboxamide dibromide ([H₂L²]Br₂) is formed through the double alkylation of 2 with 2,6-bis(bromomethyl)pyridine. The imidazole analogues of 2 and [H₂L²]Br₂ (1 and [H₂L¹]Br₂, respectively) have also been prepared. Mercuration of the two benzimidazolium groups in [H₂L²]Br₂ with mercury(II) acetate in the presence of [N(CH₃)₄]₂[HgBr₄] proceeds to give [HgL²][HgBr₄] in good yield. The ability of [HgL²][HgBr₄] to readily partake in transmetallation reactions is demonstrated by the reaction that occurs with PdCl₂(COD) to form [PdClL²][PF₆]. The structures of 2, [HgL²][HgBr₄] and [PdClL²][PF₆] have been determined.