Density Functional Theory Study of trans-Dioxo Complexes of Iron, Ruthenium, and Osmium with Saturated Amine Ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), and Detection of [Fe(qpy)(O)2]n+ (n=1, 2) by High-Resolution ESI Mass Spectrometry

Density functional theory (DFT) calculations on trans‐dioxo metal complexes containing saturated amine ligands, trans‐[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), were performed with different types of density functionals (DFs): 1) pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta‐GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta‐GGAs, a singlet d2 ground state for trans‐[Fe(O)2(NH3)2(NMeH2)2]2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at λ≈509 and 515 nm in the respective ground‐state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand‐to‐metal charge‐transfer (LMCT) transition with PW91, but a π(FeO)→π*(FeO) transition with B3LYP, in which π and π* are the bonding and antibonding combinations between the dπ(Fe) and pπ(O2−) orbitals. The FeVI/V reduction potential of trans‐[Fe(O)2(NH3)2(NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The [Fe(qpy)(O)2]n+ (qpy=2,2′:6′,2′′:6′′,2′′′:6′′′,2′′′′‐quinquepyridine; n=1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high‐resolution ESI‐MS spectroscopy. The first FeVI dioxo complex, trans‐[Fe(O)2(NH3)2(NMeH2)2]2+ has been generated in silico and was detected to have a reduction potential of +1.30 V (vs. NHE). [Fe(qpy)O2]2+ was also detected in the gas phase by high‐resolution ESI‐MS spectroscopy. The picture shows the model complex and the MO diagram of its singlet d2 ground state. qpy=2,2′:6′,2′′:6′′,2′′′:6′′′,2′′′′‐quinquepyridine