Rare configuration of tautomeric benzimidazolecarboxylate ligands in cadmium(II) and copper(II) coordination polymers

Two Cd(HBimc)-based isomers, [Cd(HBimcN)(HBimcT)(H2O)]·3.5H2O·EtOH (1a·3.5H2O·EtOH, H2Bimc=1H-benzimidazole-5-carboxylic acid) and [Cd(HBimcN)(HBimcT)(H2O)] (1b), and two Cu(HMBimc)-based coordination polymers, [Cu(HMBimcN)2(H2O)]·1/2H2O (2·1/2H2O, H2MBimc=2-methyl-1H-benzimidazole-5-carboxylic acid) and [Cu(HMBimcT)2]·2THF·H2O (3·2THF·H2O), were self-assembled from Cd(ClO4)2·6H2O/H2Bimc and Cu(ClO4)2·6H2O/H2MBimc systems, respectively. Compound 1a adopts a ladder-like chain structure, comprised of a hydrogen-bond-stabilized Cd2(HBimcN)2-metallocyclic stair and a 1D straight –(Cd–HBimcT)n– edge, whereas compound 1b exhibits a 2D (4,4)-rhombus layered structure, intercrossed by 1D –(Cd–HBimcN)n– chains and –(Cd–HBimcT)n– chains. Compound 2 shows a 1D double-stranded wave-like chain from two single-stranded wave-like –(Cu–HMBimcN)n– chains and compound 3 adopts a 2D (4,4)-topological layer structure, intercrossed by subunits of 1D –(Cu–HMBimcT)n– chains. Interestingly, a pair of tautomeric HBimc building blocks—normal (N or HBimcN) and tautomer (T or HBimcT)—is simultaneously included in the structures of 1a and 1b, whilst the N- and T-configured HMBimc building blocks are present as separate entities in Cu species, 2 and 3, respectively. The existence of only a tautomer (T) mode of the benzimidazolecarboxylate-based ligand in a Cu(II) network is observed for the first time.

A pair of tautomeric HBimc building blocks (normal (N) and tautomer (T)) is found simultaneously in two Cd(II) networks, whereas, the normal and tautomer modes of HMBimc are present as separate entities in two Cu(II) frameworks. The isolation of a Cu(II) network with only a tautomer (T) mode of the benzimidazolecarboxylate-based ligand is achieved for the first time.Figure optionsDownload as PowerPoint slideHighlights
► Benzimidazolecarboxylates could exhibit normal (N) and tautomer (T) configurations.
► A pair of N- and T-modes of HBimc is found simultaneously in two Cd(II) networks.
► The N- and T-modes of HMBimc are separately present in two Cu(II) frameworks.
► Only a T mode of HMBimc that existed in the structure is observed for the first time.