Matrix Isolation Infrared Spectroscopic And Theoretical Studies On The Transition Metal Oxides And Dioxygen Complexes

Dioxygen binding and activation at transition metal centers are of major importance in a wide range of catalytic and biological processes. Transition metal oxides and dioxygen complexes are potential intermediates or products during oxidation of metal atoms. Spectroscopic studies on these species can provide detailed insights into their structures, bonding, and reaction mechanisms at molecular level. Compared with the extensive investigations on simple transition metal monoxides and dioxides, oxygen-rich species are less studied. Herein, we provide a laser ablation-matrix isolation spectroscopic study on a series of mononuclear transition metal oxides and dioxygen complexes. The vibrational spectra, structures and bonding of these newly formed species are studied by using infrared absorption spectroscopy and quantum chemical calculations. The periodic trends on the reaction mechanisms of transition metal atoms and dioxygen are discussed. The major results are as follows:(1) Most of early transition metal atoms react with O2 spontaneously to form the inserted dioxide molecules. Further sample annealing allows the primary formed metal dioxide molecules to react with additional dioxygen to form a series of oxygen-rich superoxide and peroxide complexes via ozonide complex intermediates. The ground state scandium and yttrium atoms react spontaneously with two dioxygen molecules to form the OM(η2-O3) (M=Sc,Y) intermediates, which convert to the more stable superoxo-peroxide M(η2-O2)2 complex isomers via visible light irradiation. In the lanthanum and O2 reaction, a (η2-02)La02 complex is observed to be the precursor for the formation of the OLa(η2-O3) complex. The OM(η2-O3) (M=Sc,Y) intermediates are also able to interact with additional dioxygen to give the trisuperoxide Sc(η2-O2)3 complex and the even oxygen-rich superoxo-bisozonide (η2-O2)M(η2-O3)2 complexes. It is found that the initially formed metal dioxide molecules of group IV metal atom reactions further react with two O2 molecules to form the OM(η2-O2)(η2-O3) (M=Ti, Zr, Hf) complexes. The OTi(η2-O2)(η2-O3) complex rearranges to a less stable OTi(η2-O2)(η1-O3) isomer with an end-on bonded O3 subunit under visible irradiation. Due to the larger radius of hafnium, the OHf(η2-O2)(η2-O3) complex either photochemically rearranges to the more stable Hf(η2-O2)3 isomer, a side-on bonded disuperoxo hafnium peroxide complex, or reacts with additional dioxygen to form the homoleptic hafnium tetrasuperoxide complex, Hf(η2-O2)4.For manganese, iron and iridium, the metal dioxide molecules primary formed from the reactions of metal atoms and O2 are found to react with dioxygen spontaneously to form the metal dioxide-dioxygen complexes. In the cases of Fe and Ir, both the side-on and end-on bonded complexes are formed, which are photochemically interconvertible. The metal dioxide-dioxygen complexes of Ir are also able to be rearranged to the iridium tetroxide isomer under visible light excitation, in which the iridium center is in an unusual+Ⅷoxidation state. For manganese, the manganese tetroxide MnO4 molecule is not able to be stabilized in solid argon matrix, however, it is found that the (η2-O2)MnO2 complex interacts with another weakly coordinated dioxygen to give the (η2-O2)MnO4 complex via visible light excitation, in which the manganese tetroxide is coordinated and stabilized by a side-on bonded O2 ligand.It is found that the ground state copper atom react with O2 to give the copper dioxygen complex Cu(O2) spontaneously on annealing. The Cu(02) complex is able to bind additional one or two dioxygen molecules to form the superoxo Cu(η2-O2)2 and Cu(η2-O2)(η1-O2)2 complexes, in which the copper centers are in +Ⅱand +Ⅲoxidation states. The Cu(η2-O2)(η2-O3) complex with side-on bonded O2 and O3 ligands can be produced via the reaction of Cu(O2) and O3.Besides the above-mentioned species, some transition metal oxides/dioxygen complexes with odd number of oxygen atoms are also produced from the reactions of transition metal monoxide molecules with O2. The peroxide (η2-O2)MO complexes are formed in the reactions of Group IV metal monoxides and O2. The (η2-O2)TiO complex can further be coordinated by another O2 to form the disuperoxo titanium monoxide complex, (η2-O2)2TiO. The ground state FeO molecule interacts with dioxygen to give the (O2)FeO complex, which isomerizes to the more stable iron trioxide molecule upon visible light excitation.(2) Besides the neutral molecules, transition metal dioxide anions including FeO2-, RhO2-,IrO2-, PtO2-, AuO2-, ZnO2- and CdO2- are also produced via electron capture by the corresponding neutral molecules during the condensation process. All of these dioxide anions are characterized to have linear or near linear geometries. Theoretical calculations indicate that single-reference methods are unreliable for the FeO2- anion, which is experimentally determined to be linear. Only the state-averaged multi-reference MRCI method, which incorporates both the dynamical and nondynamical correlations predicts that the anion has a linear doublet ground state, consistent with the experimental observations. The frequency shift of RhO2-, IrO2-, PtO2- and AuO2- anions upon electron attachment to the neutral molecules strongly depends on the nature of frontier molecular orbitals involved in the electron attachment process. The antisymmetric stretching vibrational frequencies of ZnO2- and CdO2- anions are significantly lower than those of the neutral dioxides while the symmetric ones are almost unchanged.