Metal-dependent formation of mononuclear complexes and M2L2 mesocates with Schiff-base ligands

The coordination chemistry of the tetradentate pyridylN-donor ligands r-1-hydroxy-t-3,t-5-bis{[(2-pyridinyl)methylene]amino}cyclohexane (DDOP) and r-1-hydroxy-t-3,t-5-bis{[(E)-(6-methyl-2-pyridinyl)methylene]amino}cyclohexane(DDMOP) has been investigated with the d10 metal centres silver(I) and cadmium(II). These conformationally flexible ligands offer two coordination modes; either a mononucleating tetradentate coordination pocket or a bridging bis(bidentate) chelate. Hence, reaction with CdII produced six-coordinate mononuclear complexes whose compositions were determined by X-ray crystallography as [Cd(DDOP)(NO3)2] (1) and [Cd(DDMOP)(Cl)2]·MeOH (2), while AgI led to the four-coordinate dinuclear mesocate structures [{Ag(DDOP)}2](CF3SO3)2 (3) and [{Ag(DDMOP)}2](CF3SO3)2·0.5MeOH (4). It is thought that the mononuclear, octahedral complexes in the case of zinc(II) and cadmium(II) result from the energetic stabilisation gained through two extra coordinate bonds, while in the case of silver(I) the lower propensity of the metal centre for six-coordination causes the ligand to retain its energetically preferred dinucleating conformation and to form the mesocates.