Recurrent H-bond graph motifs between metal tris-ethylenediamine cations and uncoordinated oxalate anions: Fitting a three pin plug into a two pin socket

Abstract

The structures of four new compositionally related compounds are described; [Cu(en)₂(H₂O)₂][ox] (1), [Cu(en)₃][ox] (2), [Co(en)₃]₂[ox]₃(H₂O)_3.61 (3), and [Co(en₃)]₂[ox]₃(H₂O)₇ (4) [en = 1,2-ethylenediamine, ox = oxalate]. These materials all inhabit the broader structural landscape for compounds with a generic M_p(ox)_q(en)_r(H₂O)_s composition. Here the competing nature of the ligands; ethylenediamine, oxalate and water, results in complex solution chemistry. In addition the very different structure directing effects of each type of ligand yield a range of crystal architectures. In the present cases ethylenediamine displaces the oxalate dianion, which is non-coordinated in each of these compounds. Compound 3 has disordered water of crystallisation, and is a non-stiochoimetric hydrate, while compound 4 shows a correlated disorder in both ligand conformation and water site occupancies. The H-bond motifs linking pseudo 3-fold symmetric M(en)₃ⁿ⁺ cations to pseudo 2-fold symmetric ox²⁻ anions shows frequent occurrence of particular motifs; notably the R²₂(8) and R²₂(9) graphs and a tendency to form bifurcated hydrogen bonds. The oxalate geometric parameters of twist and C–C bond length in our compounds are correlated with data from related structures in the CSD.