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

Iron(II) cryptates, where the Fe-II is in the high-spin S = 2 electronic configuration, even with six sp(2) N-donors, and iron(III) cryptates adopting the intermediate spin S = 3/2 state have been prepared. The structure of a high-spin iron(II) cryptate utilising six sp(2) N-donors shows long metal-ligand distances which effectively destabilise the low-spin (1)A(g) configuration. Comparison is made between iron(III) cryptates, a less sterically constrained podand complex of the same donor set, and analogous N4O2- polychelates, which are respectively intermediate-, high- and low-spin or S = 5/2 <-> S = 1/2 spin crossover systems. The Mossbauer spectra of the iron(III) cryptates and analogous pedate are remarkably similar, despite their different spin states, suggesting covalency in the pedate binding. The crystal structure of the pedate supports this conclusion, as the iron-ligand donor distances are short for high-spin Fe-III.