Response to steric constraint in azacryptate and related complexes of iron-(II) and -(III)*

Abstract

Iron(II) cryptates, where the Fe^II is in the high-spin S = 2 electronic configuration, even with six sp² N-donors, and iron(III) cryptates adopting the intermediate spin S =  state have been prepared. The structure of a high-spin iron(II) cryptate utilising six sp² N-donors shows long metal–ligand distances which effectively destabilise the low-spin ¹A_g configuration. Comparison is made between iron(III) cryptates, a less sterically constrained podand complex of the same donor set, and analogous N₄O^–₂ polychelates, which are respectively intermediate-, high- and low-spin or S =  ←→ S = ½ spin crossover systems. The Mössbauer spectra of the iron(III) cryptates and analogous podate are remarkably similar, despite their different spin states, suggesting covalency in the podate binding. The crystal structure of the podate supports this conclusion, as the iron–ligand donor distances are short for high-spin Fe^III.