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Density Functional Theory Studies of [Fe(O) 2 L] 2+ : What is the Role of the Spectator Ligand L with Different Coordination Numbers?
Author(s) -
Tong Glenna So Ming,
Che ChiMing
Publication year - 2010
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201000656
Subject(s) - chemistry , trigonal bipyramidal molecular geometry , density functional theory , coordination geometry , unpaired electron , crystallography , ligand (biochemistry) , geometry , pentagonal bipyramidal molecular geometry , homo/lumo , ion , molecular geometry , bipyramid , coordination number , electronic structure , electron configuration , computational chemistry , molecule , crystal structure , hydrogen bond , biochemistry , receptor , mathematics , organic chemistry
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+ ( 1 2+ ) 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 1 2+ . With a TBP geometry, 1 2+ adopts a [Fe V (O) 2 (qpy) + · ] 2+ formalism where a d 3 quartet Fe V 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 Fe V ion in 1 2+ also formally has three unpaired electrons (a d 3 quartet) with the fourth unpaired electron localized on a single oxido ligand to give a quintet state. The unoccupied orbital of 1 2+ 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, 1 2+ with PBP geometry is proposed to be a more reactive oxidant than 1 2+ with TBP geometry. On the other hand, 1 2+ 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.