Design, synthesis, and characterization of transition metal compounds using binucleating and bifunctional ligands: strategies for the multi-electron reduction of small molecules

Abstract

The work described in this thesis focuses on two strategies to promote multi-electron redox within simple inorganic systems. The first involves electronic coupling between two mid-to-late first-row transition metals in the form of strong metal-metal bonds. The second involves tethering a redox-active cofactor to a mononuclear transition metal site via a bifunctional ligand platform. In Chapter 1, the background and theory relevant to achieving multi-electron reactivity using bimetallic complexes are discussed, with a particular focus on the electronic coupling between first-row transition metals. In Chapter 2, the electronic structure reinvestigation of two "trigonal lantern" bimetallic complexes is discussed. These complexes, Fe2(DPhF)3 and Co2(DPhF)3, (DPhF = N,N'-diphenylformamidinate) contain remarkably short metal-metal distances of 2.23 Å and 2.38 Å, respectively, while also possessing large magnetic moments indicative of strong, ferromagnetic coupling between the two metal centers. The electronic structures of these molecules have been studied by a variety of physical and theoretical methods. The molecules have energetically well-isolated high-spin electronic configurations of S = 7/2 and S = 5/2, respectively, and fully delocalized M(1.5)M(1.5) oxidation states. The strong metal-metal bonding, electronic delocalization, and high-spin states are shown to be interrelated, resulting from the distribution of electrons in a molecular orbital manifold that has very small orbital energy differences, engendered by the weak-field ligands and trigonal coordination geometry. In Chapter 3, the synthesis and characterization of new bimetallic complexes a new, chelating tris(amidinato)amine are discussed. This ligand provides a similar ligand environment to the original "trigonal lanterns" but also contains a single axial donor that differentiates the two metal sites. The homobimetallic dicobalt complex of this ligand, Co2LPh, has been synthesized and possesses an even shorter Co-Co distance, at 2.29 Å, than Co2(DPhF)3. In addition, a heterobimetallic iron-cobalt complex, FeCoLPh, has also been prepared that contains the shortest known Fe-Co distance, at 2.18 Å. The positions of the iron and cobalt atoms in this complex were assigned by anomalous dispersion methods; these reveal that the compound is essentially a single species, rather than a mixture of heterobimetallic isomers, and that the cobalt selectively occupies the tetracoordinate "bottom" site, bound to the axial nitrogen donor while the iron occupies the tricoordinate "top" site. Both Co2LPh and FeCoLPh possess high-spin electronic configurations and strong metal-metal bonds. While the asymmetric ligand environment creates a distinct polarization of the molecular orbitals and oxidation states, the metal-metal bonding is largely unaffected and is qualitatively similar to that observed in Fe2(DPhF)3 and Co2(DPhF)3. In Chapter 4, the design and coordination chemistry of a bifunctional ligand system containing a reversible organic hydride donor group is presented. This ligand tethers a redox-active phenanthridinium group to a phosphine donor in order to facilitate bifunctional reactivity, in which the hydride donor and an appended metal center react cooperatively to activate and reduce substrates. Palladium dichloride complexes containing two such ligands have been prepared: these can be cleanly interconverted between hydride-"loaded" and -"unloaded" forms by reaction with hydride acceptors and donors. In addition, lower-coordinate palladium complexes have been studied that can react with dihydrogen (H2) and show intriguing exchange of the hydrides between positions at the metal and ligand.