Electronic structure of bispidine Iron(IV) oxo complexes

The electronic structure, based on DFT calculations, of a range of (FeO)-O-IV complexes with two tetra- (L-1 and L-2) and two isomeric pentadentate bispidine ligands (L-3 and L-4) is discussed with special emphasis on the relative stability of the two possible spin states (S = 1, triplet, intermediate-spin, and S = 2, quintet, high-spin; bispidines are very rigid diazaadamantane-derived 3,7-diazabicyclo[3.3.1]nonane ligands with two tertiary amine and two or three pyridine donors, leading to cis-octahedral [(X)(L)Fe-IV=O](2+) complexes, where X = NCCH3, OH2, OH-, and pyridine, and where X = pyridine is tethered to the bispidine backbone in L-3, L-4). The two main structural effects are a strong trans influence, exerted by the oxo group in both the triplet and the quintet spin states, and a Jahn-Teller-type distortion in the plane perpendicular to the oxo group in the quintet state. Due to the ligand architecture the two sites for substrate coordination in complexes with the tetradentate ligands L-1 and L-2 are electronically very different, and with the pentadentate ligands L-3 and L-4, a single isomer is enforced in each case. Because of the rigidity of the bispidine ligands and the orientation of the Jahn-Teller axis, which is controlled by the sixth donor X, the Jahn-Teller-type distortion in the high-spin state of the two isomers is quite different. It is shown how this can be used as a design principle to tune the relative stability of the two spin states.