Response to steric constraint in azacryptate and related complexes of iron-(II) and -(III)*
Iron(II) cryptates, where the FeII 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 1Ag 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 FeIII.