Supramolecular hydrogen-bonded networks formed from copper(II) carboxylate dimers.

The well-known copper carboxylate dimer, with four carboxylate ligands extending outwards towards the corners of a square, has been employed to generate a series of crystalline compounds. In particular, this work centres on the use of the 4-hydroxybenzoate anion (Hhba - ) and its deprotonated phenolate form 4-oxidobenzoate (hba 2- ) to obtain complexes with the general formula [Cu 2 (Hhba) 4-x (hba) x L 2-y ] x- , where L is an axial coligand (including solvent molecules), x = 0, 1 or 2, and y = 0 or 1. In some cases, short hydrogen bonds result in complexes which may be represented as [Cu 2 (Hhba) 2 (H 0.5 hba) 2 L 2 ] - . The main focus of the investigation is on the formation of a variety of extended networks through hydrogen bonding and, in some crystals, coordinate bonds when bridging coligands (L) are employed. Crystals of [Cu 2 (Hhba) 4 (dioxane) 2 ]·4(dioxane) consist of the expected Cu dimer with the Hhba - anions forming hydrogen bonds to 1,4-dioxane molecules which block network formation. In the case of crystals of composition [Et 4 N][Cu 2 (Hhba) 2 (H 0.5 hba) 2 (CH 3 OH)(H 2 O)]·2(dioxane), Li[Cu 2 (Hhba) 2 (H 0.5 hba) 2 (H 2 O) 2 ]·3(dioxane)·4H 2 O and [Cu 2 (Hhba) 2 (H 0.5 hba) 2 (H 0.5 DABCO) 2 ]·3CH 3 OH (DABCO is 1,4-diazabicyclo[2.2.2]octane), square-grid hydrogen-bonded networks are generated in which the complex serves as one type of 4-connecting node, whilst a second 4-connecting node is a hydrogen-bonding motif assembled from four phenol/phenolate groups. Another two-dimensional (2D) network based upon a related square-grid structure is formed in the case of [Et 4 N] 2 [Cu 2 (Hhba) 2 (hba) 2 (dioxane) 2 ][Cu 2 (Hhba) 4 (dioxane)(H 2 O)]·CH 3 OH. In [Cu 2 (Hhba) 4 (H 2 O) 2 ]·2(Et 4 NNO 3 ), a square-grid structure is again apparent, but, in this case, a pair of nitrate anions, along with four phenolic groups and a pair of water molecules, combine to form a second type of 4-connecting node. When 1,8-bis(dimethylamino)naphthalene (bdn, `proton sponge') is used as a base, another square-grid network is generated, i.e. [Hbdn] 2 [Cu 2 (Hhba) 2 (hba) 2 (H 2 O) 2 ]·3(dioxane)·H 2 O, but with only the copper dimer complex serving as a 4-connecting node. Complex three-dimensional networks are formed in [Cu 2 (Hhba) 4 (O-bipy)]·H 2 O and [Cu 2 (Hhba) 4 (O-bipy) 2 ]·2(dioxane), where the potentially bridging 4,4'-bipyridine N,N'-dioxide (O-bipy) ligand is employed. Rare cases of mixed carboxylate copper dimer complexes were obtained in the cases of [Cu 2 (Hhba) 3 (OAc)(dioxane)]·3.5(dioxane) and [Cu 2 (Hhba) 2 (OAc) 2 (DABCO) 2 ]·10(dioxane), with each structure possessing a 2D network structure. The final compound reported is a simple hydrogen-bonded chain of composition (H 0.5 DABCO)(H 1.5 hba), formed from the reaction of H 2 hba and DABCO.