Aldehyde coordination in template reactions: Crystal structure of a possible reaction intermediate

• Chelating aldehydes and template reaction mechanisms
• Hirshfeld surfaces and bonding interactions
• Zn(II) coordination and substrate activation

The aldehyde complex [Zn(Im4al)2(NO3)2], where Im4al = imidazole-4-aldehyde (N(1)H tautomer), crystallises from methanol as a monoclinic species, space group C2/c, a 15.890(3), b 6.8920(14), c 14.077(3) Å, β 119.67(3) °. Its structure solution shows that the aldehyde–O donors are situated relatively distant (2.57(2) Å) from the metal ion centre, creating uncertainty as to whether the metal should be described as 4- or 6-coordinate. The seemingly very weak interaction of the aldehyde–O atoms with the Zn(II) indicated by the calculation of the Hirshfeld surface for the molecule in its lattice is consistent with the fact that this metal ion does not favour hydration of the aldehyde in aqueous solution. It may indicate that the role of the metal ion in template reactions involving the aldehyde and amines is to poise the aldehyde–C between its “unactivated”, and probably quite differently solvated, free state and a second state which is not sufficiently activated to favour addition of solvent water (or methanol) but which is activated sufficiently to undergo rapid addition of an amine nucleophile.

The crystal structure of potentially chelating imidazole-4-aldehyde when bound to zinc(II) shows that the aldehyde is bound through imidazole-N only, indicating that activation of the aldehyde in template reactions may not be due to direct polarisation of the carbonyl group.Figure optionsDownload as PowerPoint slide