A Computational Determination of the Lowest Energy Electronic and Geometric States of First Row Transition Metal Dioxygen Dications

The lowest energy geometric structures and electronic spin states of first row transition metal (TM) dioxygen dication molecules ([TM–O2]2+; TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) have been determined at the B3LYP/LANL2DZ level of theory (along with an extra -type polarization function added to the O atoms). In order to further verify the spin states, CASSCF(6 + , 9) energy points were determined ( = number of TM electrons). It has been found that with the exception of [Sc–O2]2+, [V–O2]2+, [Co–O2]2+, and [Ni–O2]2+, all [TM–O2]2+ molecules take on a high-spin state. [Sc–O2]2+ adopts a trigonal structure, while [Ti–O2]2+-[Mn–O2]2+ are essentially linear and [Fe–O2]2+-[Zn–O2]2+ are bent. It is further noted that the O–O bond decreases from 130.0？pm to 118.1？pm as the TM changes from Sc to Zn. However, the TM–O2 bond lengths fluctuate between values of 182.2？pm for [Ni–O2]2+ and 232.2？pm for [Zn–O2]2+. 1. Introduction Transition metals (TMs) bound to an O2 molecule are the chemically active site in many industrial [1–5] and biological [1, 2, 6–10] molecules. Considering the TM–O2 bond to lie along the -axis of a Cartesian coordinate system, in TM–O2 complexes, bonds can form by the interactions of TM , , and atomic orbitals with suitable orbitals on the O2 ligand in order to make ( ) or ( and ) bonds (Scheme 1). While there have been numerous studies of neutral TM–O2 [11, 12] complexes, in many TM–O2 containing molecules of biological and industrial importance, the TM is bound to a porphyrin ring in the 2？ oxidation state. Therefore, [TM–O2]2+ molecules are the simplest approximations to this important class of TM–O2 containing molecules. Scheme 1 Probably the most studied TM–O2 bond is the Fe–O2 bond due to its role in heme-containing proteins [13–16]. This bond has been described as low-spin Fe(II) accepting an electron pair from excited singlet-state O2 forming the bond, while donating an electron pair to oxygen to form a bond. An alternative view involves Fe(III) interacting with O2？. In a third possibility, triplet Fe(II) couples with triplet O2 to form a closed shell singlet. Computational results vary depending on the extent of electron correlation and basis set size. It is clear, however, that the overall bonding scheme involves some degree of both O2 Fe and Fe O2？？ and electron transfer. Further complicating matters, the number of near degenerate TM -orbitals leads to the possibility of many low lying spin states. In addition, there are a number of different ways that the O2 unit can be arranged relative to the TM. Herein, we seek to