Magnetostructural correlation in isolated trinuclear iron(iii) oxo acetate complexes.

We elucidate the correlation between geometric structures and magnetic couplings in trinuclear iron(iii) oxo acetate complexes [Fe3O(OAc)6(Py)n]+ (n = 0, 1, 2, 3) when isolated and trapped as gaseous ions. Structural information arises from Infra Red-Multiple Photon Dissociation (IR-MPD) and Collision Induced Dissociation (CID) experiments in conjuction with Density Functional Theory (DFT) based calculations. We simulate the antiferromagnetic couplings between the FeIII (d5) centers by employing a Broken Symmetry approach within our DFT calculations, and we extract the associated antiferromagnetic coupling constants. Coordination of one, two or three axial pyridine ligands to the [Fe3O(OAc)6]+ subunit distorts the geometry of the triangular Fe3O core. The Fe-Ocentral bond lengths are enlarged or shortened depending on number of coordinated pyridine ligands. This significantly affects the antiferromagnetic coupling constants between the FeIII centers ranging from -62 cm-1 to -28 cm-1 in [Fe3O(OAc)6(Py)n]+ (n = 0, 1, 2, 3). A detailed analysis of the associated exchange couplings indicates a switching of magnetic ground states by pyridine coordination. The total spin ST in the ground states of [Fe3O(OAc)6(Py)n]+ raises from ST = 1/2 (n = 0) to 3/2 (n = 1) and 5/2 (n = 2). Coordination of the third pyridine ligand (n = 3) re-establishes a spin ground state of ST = 1/2. We thus identify a coordination controlled switching of magnetic ground states.