Density Functional Theory Studies of [Fe(O) 2 L] 2+ : What is the Role of the Spectator Ligand L with Different Coordination Numbers?

Abstract

Density functional theory (DFT) studies were carried out on [Fe(O)2(L)]n+ [L = qpy (1), simple amines (2), and tpy (3); qpy = 2,2′:6′,2″:6″,2″′:6″′,2″″-quinquepyridine and tpy = terpyridine; n = 1 or 2] to study how the coordination number of the spectator ligand L affects the geometries and electronic structures of the complexes. It was found that qpy can act as both a tridentate and pentadentate ligand resulting in [Fe(O)2(qpy)]2+ (12+) having a trigonal bipyramidal (TBP) geometry in the former case, and a pentagonal bipyramidal (PBP) geometry in the latter case. The difference in coordination geometries has a significant impact on the electronic structures of 12+. With a TBP geometry, 12+ adopts a [FeV(O)2(qpy)+·]2+ formalism where a d3 quartet FeV ion ferromagnetically and antiferromagnetically couples to the qpy cation radical to give close-lying triplet and quintet states (within ca. 0.2 eV). With a PBP geometry, the FeV ion in 12+ also formally has three unpaired electrons (a d3 quartet) with the fourth unpaired electron localized on a single oxido ligand to give a quintet state. The unoccupied orbital of 12+ in PBP geometry is lower lying in energy and has higher oxido character than when the complex has TBP geometry. Thus, based on the MO energies and oxido character of the unoccupied orbital, 12+ with PBP geometry is proposed to be a more reactive oxidant than 12+ with TBP geometry. On the other hand, 12+ with TBP geometry has a similar electronic structure to heme Cpd I, and it is possible that these two compounds have similar oxygen atom transfer reaction mechanisms. By varying the ligand coordination number using different spectator ligands L, the dioxido-iron complex [Fe(O)2(L)]2+ can change from a high-spin triplet when L = tpy, to a low-spin singlet when L = simple amines, to a quasi-degenerate triplet and quintet state when L = qpy.