Issue 5, 1986

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

Abstract

The complexes [FeIIL(CN)2xH2O 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 Mössbauer 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.

Article information

Article type
Paper

J. Chem. Soc., Dalton Trans., 1986, 991-995

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

S. M. Nelson, P. D. A. McIlroy, C. S. Stevenson, E. König, G. Ritter and J. Waigel, J. Chem. Soc., Dalton Trans., 1986, 991 DOI: 10.1039/DT9860000991

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