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) to obtain complexes with the general formula [Cu(Hhba)(hba)L], 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(Hhba)(Hhba)L]. 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(Hhba)(dioxane)]·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 [EtN][Cu(Hhba)(Hhba)(CHOH)(HO)]·2(dioxane), Li[Cu(Hhba)(Hhba)(HO)]·3(dioxane)·4HO and [Cu(Hhba)(Hhba)(HDABCO)]·3CHOH (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 [EtN][Cu(Hhba)(hba)(dioxane)][Cu(Hhba)(dioxane)(HO)]·CHOH. In [Cu(Hhba)(HO)]·2(EtNNO), 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][Cu(Hhba)(hba)(HO)]·3(dioxane)·HO, but with only the copper dimer complex serving as a 4-connecting node. Complex three-dimensional networks are formed in [Cu(Hhba)(O-bipy)]·HO and [Cu(Hhba)(O-bipy)]·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(Hhba)(OAc)(dioxane)]·3.5(dioxane) and [Cu(Hhba)(OAc)(DABCO)]·10(dioxane), with each structure possessing a 2D network structure. The final compound reported is a simple hydrogen-bonded chain of composition (HDABCO)(Hhba), formed from the reaction of Hhba and DABCO.