Quadridentate versus quinquedentate co-ordination of some N5 and N3O2 macrocyclic ligands and an unusual thermally controlled quintet ⇌ singlet spin transition in an iron(II) complex

The complexes [FeIIL(CN)2]·xH2O of the macrocyclic Schiff-base ligands L1–L3 derived from the condensation of 2,6-diacetylpyridine with 3,6-diazaoctane- 1,8-diamine (L1), 3,7-diazanonane1,9-diamine (L2), and 3,6-dioxaoctane-1,8-diamine (L3) have been prepared and their magnetic susceptibilities and Mossbauer spectra studied over the temperature range 80–300 K. The complexes of the N5 macrocycles L1 and L2 have low-spin (S= 0) ground states and are assigned six-co-ordinate structures in which the macrocycle acts as a quadridentate ligand with one secondary amine group unco-ordinated. This contrasts with the situation in all previously characterised complexes of these macrocycles (containing other metal ions and/or other axial ligands) where all five potential donor atoms are co-ordinated in a pentagonal planar arrangement. The exceptional structure of the present complexes is attributed to the ligand-field stabilization energy associated with the low-spin, approximately octahedral configuration generated by the strong field cyanide ligands. The magnetic properties of [FeL3(CN)2]·H2O exhibit a complex variation with temperature. The thermodynamically stable forms are high-spin (S= 2) at ambient temperature and low-spin (S= 0) below 150 K while between 150 and 200 K the stable form appears to contain approximately equal numbers of high-spin and low-spin molecules. The structure of the low-spin form is believed to be six-co-ordinate, with one ether oxygen of the macrocycle unco-ordinated while the high-spin form may have either a six- or a seven-co-ordinate structure.