Investigations of Diketiminate-Supported Iron and Cobalt Chalcogenide Complexes: Small Molecule Activation and Electronic Structure

This thesis describes the preparation, detailed characterization, and reactivity of iron and cobalt complexes designed to understand molecular redox transformations. Despite their essential roles in the redox chemistry of small molecules (for example, O2, CO2, N2), the mechanisms by which metal chalcogenide species orchestrate important reactions remain poorly understood. Chapter 1 examines the relative kinetics of C-H activation by heterobimetallic and homobimetallic oxo complexes. Inspired by natural systems like methane monooxygenase that use two metal centers to cooperatively bind and activate O2, we describe a structural, spectroscopic, and computational analysis to characterize bimetallic Fe/Co and Co/Co complexes that differ in a single metal center. We interrogate mechanisms of C-H activation to quantify the influence of the metal on C-H activation, and our results demonstrate that exchange of a single cobalt atom with iron to generate a Fe/Co heterobimetallic complex results in much faster C–H bond activation. These findings corroborate previous proposals of heterobimetallic oxo species in redox reactions, and include the first structurally characterized heterobimetallic oxo complex of any two transition metals. Chapter 2 interrogates CO2 reduction by a diketiminate supported cobalt(I) complex. We report a reaction that converts CO2 to fully characterized CO and carbonate products, and conduct kinetics experiments to determine the rate law of this reaction. Computations conducted by collaborators supplement the experiment work, and suggest the intermediacy of a dicobalt oxo. Through independent synthesis of this proposed intermediate, we demonstrate the kinetic competency of the dicobalt oxo in the reduction of CO2 by the cobalt(I) complex. This chapter reveals a well-characterized mechanistic picture for cobalt mediated CO2 reduction. Chapter 3 describes synthetic [4Fe-3S] iron-sulfur clusters with unusual structural, electronic, and reactivity properties. This chapter evaluates one hypothesis for enabling reactivity in iron-sulfur clusters: that three-coordinate Fe sulfide, possibly formed by rupture of weak Fe-S bonds at tetrahedral sites, serves as a binding site in clusters like nitrogenase. We describe the synthesis of new [4Fe-3S] clusters that feature the first example of an iron site supported only by three sulfide ligands. Detailed spectroscopic characterization and computational analysis is discussed, which reveals an unusual electronic structure in [4Fe-3S] clusters. Biomimetic reactivity of [4Fe-3S] demonstrates that three-coordinate iron in an all-sulfide coordination sphere is a viable precursor to substrate binding by iron-sulfur clusters. Chapter 4 reports the synthesis of a [2Fe-1S] cluster with a reactivity pattern that leads to various other iron-sulfur clusters. We describe the stabilization of the otherwise highly reactive [2Fe-1S] complex using phosphine ligands to protect low-coordinate iron-sulfide, which enables its thorough characterization. Through phosphine removal, a highly reactive species is formed that can be elaborated into higher nuclearity iron-sulfur clusters, including the [4Fe-3S] cluster described in Chapter 3 in addition to a partially characterized [10Fe-8S] cluster with a previously unobserved geometry. Chapter 5 offers a perspective on the possible implications of the [4Fe-3S] cluster family for the mechanism of nitrogenase and for the electronic structure of iron-sulfur clusters more generally. We describe connections between one [4Fe-3S] cluster and a recently reported crystal structure of a nitrogenase cofactor trapped during catalytic turnover. The electronic structure of [4Fe-3S] is placed in the context of known mixed-valent iron-sulfur clusters. Chapter 5 closes with a discussion of the opportunities afforded by [4Fe-3S] and other synthetic clusters for understanding electronic structure and reactivity in biological iron-sulfur cofactors

One Word: Polymers. The Development of Ruthenium-Based Catalysts for Ring Opening Metathesis Polymerization

The formation of carbon-carbon bonds is a challenging and important transformation in synthetic chemistry. In 2005 the Nobel Prize in Chemistry was awarded to Robert Grubbs for his work in the development of olefin metathesis, a reaction that forms two new carbon double bonds. One application of this reaction, ring opening metathesis polymerization (ROMP), has become especially entrenched in industry for its efficient use of inexpensive starting materials in an atom economical way. Our talk will focus on catalyst development for and applications of the ROMP reaction, demonstrating the utility of fundamental organometallic principles in rational catalyst design

Formation of Chlorosilyl Pincer-Type Rhodium Complexes by Multiple Si–H Activations of Bis(phosphine)/Dihydrosilyl Ligands

The synthesis and metalation of two bis­(phosphine)/dihydrosilyl ligands at rhodium­(I) sources is reported. Irrespective of the substitution at silicon (diaryl versus diamino), multiple Si–H activations and chloride migration afford tridentate bis­(phosphine)/chlorosilyl complexes of Rh­(I). For the diarylsilyl ligand, reaction with a cationic rhodium­(I) triflate precursor gives the analogous triflatosilyl complex. The [P₂Si]­H₂ proligands and their Rh complexes provide distinct opportunities for exploring metal/silicon cooperative reactivity

Formation of Chlorosilyl Pincer-Type Rhodium Complexes by Multiple Si–H Activations of Bis(phosphine)/Dihydrosilyl Ligands

The synthesis and metalation of two bis­(phosphine)/dihydrosilyl ligands at rhodium­(I) sources is reported. Irrespective of the substitution at silicon (diaryl versus diamino), multiple Si–H activations and chloride migration afford tridentate bis­(phosphine)/chlorosilyl complexes of Rh­(I). For the diarylsilyl ligand, reaction with a cationic rhodium­(I) triflate precursor gives the analogous triflatosilyl complex. The [P₂Si]­H₂ proligands and their Rh complexes provide distinct opportunities for exploring metal/silicon cooperative reactivity

Iron and Cobalt Diazoalkane Complexes Supported by β‑Diketiminate Ligands: A Synthetic, Spectroscopic, and Computational Investigation

Diazoalkanes are interesting redox-active ligands and also precursors to carbene fragments. We describe a systematic study of the binding and electronic structure of diphenyldiazomethane complexes of β-diketiminate supported iron and cobalt, which span a range of formal d-electron counts of 7–9. In end-on diazoalkane complexes of formally monovalent three-coordinate transition metals, the electronic structures are best described as having the metal in the +2 oxidation state with an antiferromagnetically coupled radical anion diazoalkane as shown by crystallography, spectroscopy, and computations. A formally zerovalent cobalt complex has different structures depending on whether potassium binds; potassium binding gives transfer of two electrons into the η²-diazoalkane, but the removal of the potassium with crown ether leads to a form with only one electron transferred into an η¹-diazoalkane. These results demonstrate the influence of potassium binding and metal oxidation state on the charge localization in the diazoalkane complexes. Interestingly, none of these reduced complexes yield carbene fragments, but the new cobalt­(II) complex L^tBuCoPF₆ (L^tBu = bulky β-diketiminate) does catalyze the formation of an azine from its cognate diazoalkane, suggesting N₂ loss and transient carbene formation

Reduction of CO2 by a masked two-coordinate cobalt(i) complex and characterization of a proposed oxodicobalt(ii) intermediate

Fixation and chemical reduction of CO2 are important for utilization of this abundant resource, and understanding the detailed mechanism of C–O cleavage is needed for rational development of CO2 reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(I) complex, LtBuCo (where LtBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO2, which yields two products of C–O cleavage, the cobalt(I) monocarbonyl complex LtBuCo(CO) and the dicobalt(II) carbonate complex (LtBuCo)2(μ-CO3). Kinetic studies and computations show that the κN,η6-arene isomer of LtBuCo rearranges to the κ2N,N′ binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to LtBuCo. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO2 proceed through bimetallic mechanisms, and DFT and highly correlated domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations reveal the cooperative effects of the two metal centers during facile C–O bond rupture. A plausible intermediate in the reaction of CO2 with LtBuCo is the oxodicobalt(II) complex LtBuCoOCoLtBu, which has been independently synthesized through the reaction of LtBuCo with N2O. The rapid reaction of LtBuCoOCoLtBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by LtBuCo. LtBuCoOCoLtBu is a novel example of a thoroughly characterized molecular cobalt–oxo complex where the cobalt ions are clearly in the +2 oxidation state. Its nucleophilic reactivity is a consequence of high charge localization on the μ-oxo ligand between two antiferromagnetically coupled high-spin cobalt(II) centers, as characterized by DFT and multireference complete active space self-consistent field (CASSCF) calculations

Spin Isomers and Ligand Isomerization in a Three-Coordinate Cobalt(I) Carbonyl Complex

Hemilabile ligands, which have one donor that can reversibly bind to a metal, are widely used in transition-metal catalysts to create open coordination sites. This change in coordination at the metal can also cause spin-state changes. Here, we explore a cobalt­(I) system that is poised on the brink of hemilability and of a spin-state change and can rapidly interconvert between different spin states with different structures (“spin isomers”). The new cobalt­(I) monocarbonyl complex L^tBuCo­(CO) (<b>2</b>) is a singlet (^<b>1</b><b>2</b>) in the solid state, with an unprecedented diketiminate binding mode where one of the CC double bonds of an aromatic ring completes a pseudo-square-planar coordination. Dissolving the compound gives a substantial population of the triplet (^<b>3</b><b>2</b>), which has exceptionally large uniaxial zero-field splitting due to strong spin–orbit coupling with a low-lying excited state. The interconversion of the two spin isomers is rapid, even at low temperature, and temperature-dependent NMR and electronic absorption spectroscopy studies show the energy differences quantitatively. Spectroscopically validated computations corroborate the presence of a low minimum-energy crossing point (MECP) between the two potential energy surfaces and elucidate the detailed pathway through which the β-diketiminate ligand “slips” between bidentate and arene-bound forms: rather than dissociation, the cobalt slides along the aromatic system in a pathway that balances strain energy and cobalt–ligand bonding. These results show that multiple spin states are easily accessible in this hemilabile system and map the thermodynamics and mechanism of the transition