Synthesis, characterization, and reactivity of metal-metal bonded complexes with cobalt, iron, and manganese

Abstract

Metal-metal bonding is important in dirhodium catalysts that mediate carbene insertions into C-H bonds, cyclopropanations, aziridinations, and ylide formations. Additionally, it has been suggested that certain intermediates in NiFe hydrogenases contain a nickel-iron bond. In light of the successful applications of dirhodium complexes in organic chemistry, as well as the role metal-metal bonds play in biology, the design of synthetic bimetallic complexes with mid-to-late first-row transition metals is of great interest. Yet, few examples of mid-to-late first-row transition metal complexes exhibiting metal-metal bonding have been reported, and even more strikingly, very few mid-to-late heterobimetallic complexes have been prepared. In the second chapter of this thesis, the synthesis and characterization of an isostructural series of dicobalt, cobalt-iron, cobalt-manganese, diiron, and iron-manganese complexes supported by a new binucleating ligand is disclosed. The diiron compound has a much shorter crystallographic metal-metal distance than the other four complexes. Experimental and theoretical work suggests that the short iron-iron distance is due to the full delocalization of the d orbitals, which leads to an S = 3 ground state. This is in contrast to the other four bimetallics, in which the magnetic interactions are modeled as high-spin metal centers that antiferromagnetically couple. In the third chapter, the synthesis and characterization of a dicobalt organometallic complex and a series of organometallic aluminum-cobalt complexes is described. Isostructural dicobalt benzyl and aluminum-cobalt benzyl compounds are compared using experiment and theory. A series of C-C bond forming experiments from the reaction of R-X compounds with the metal-cobalt benzyl complexes suggests that both the dicobalt compound and the aluminum-cobalt compound are capable of one-electron chemistry, whereas only the aluminum-cobalt complex undergoes two-electron reactions. These results are explained by the electronic structure of the two compounds: the aluminum-cobalt complex has the aluminum(III)cobalt(I) oxidation state, whereas calculations suggest that the dicobalt complex is cobalt(II)cobalt(II). In the fourth chapter, the synthesis and characterization of a series of hexairon and tetrairon clusters related by one-, two-, or three-electron redox steps is reported. In the fifth chapter, the role of some of these clusters in the dioxygen reactivity of a diiron(II) complex is revealed.