The majority of our energy needs depends on non-sustainable fuels: coal, oil and natural gas. Carbon dioxide that comes from fossil fuels accounts for 65% of greenhouse gas emission that plays a major role in climate change. Therefore, the development of an alternative carbon cycle that includes production of sustainable fuels should be the priority for the future of our planet. The electrochemical CO2 reduction is a promising pathway from both economic and environmental points of view. The main challenge to implement this technology at an industrial scale is to find the catalyst that will be robust, selective with high conversion efficiency. This thesis described multiple approaches to solving this problem. One of the most effective electrocatalytic systems for the electrochemical reduction of CO2 to CO was described in Chapter 2 of this thesis, where Re(tBu-bpy)(CO)3Cl molecular catalyst was incorporated into the structure of MWCNTs and obtain all advantages of both homogeneous and heterogeneous catalysis. This system is considered to be one of the most selective and robust among other developed CO2 electroreduction systems. This heterogeneous system was developed under the support of NASA/JPL as an alternative for the current technology that is being tested on planet Mars, where the CO2 gas represents 90-95% of the Martian atmosphere. The Mars Oxygen In-Situ Resource Utilization Experiment is better known as MOXIE reduces CO2 to CO and produces Oxygen gas that is vital for supporting human life during the interstellar missions. This device utilizes yttria-stabilized zirconia and operates at 700C. Thus, the development of an alternative electrochemical device that operates at room temperature is of a great interest and a serious challenge. Chapter 2 of this thesis describes the main breakthrough in the development of a key part of this device that represents the cathode at which selective CO2 reduction to CO occurs. The development of the anode that will electrochemically produce O2 and building a complete device are among future challenges.

Chapter 1. This section presents an overview of literature pertaining to low-valent/low-oxidation state tin hydrides, transition metal coordinated tin hydrides, and group 10 trinuclear metal cluster synthesis, characterization, and reactivity. Chapter 2. Treatment of the trichlorotin-capped trinuclear nickel cluster, [Ni3(dppm)3(μ3-Cl)(μ3-SnCl3)], 1, with 4 eq. NaHB(Et)3 yields a μ3-SnH capped trinuclear nickel cluster, [Ni3(dppm)3(μ3-H)(μ3-SnH)], 2 [dppm = bis(diphenylphosphino)methane]. Single-crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and computational studies together support that cluster 2 is a divalent tin hydride. Complex 2 displays a wide range of reactivity including oxidative addition of bromoethane across the Sn center. Addition of 1 eq. iodoethane to complex 2 releases H2 (g) and generates an ethyltin-capped nickel cluster with a μ3-iodide, [Ni3(dppm)3(μ3-I)(μ3-Sn(CH2CH3))], 4. Notably, insertion of alkynes into the Sn–H bond of 2 can be achieved via addition of 1 eq. 1-hexyne to generate the 1-hexen-2-yl-tin-capped nickel cluster, [Ni3(dppm)3(μ3H)(μ3-Sn(C6H11))], 5. Addition of H2 (g) to 5 regenerates the starting material, 2, and hexane. The formally 44-electron cluster 2 also displays significant redox chemistry with two reversible one-electron oxidations (E = −1.3 V, −0.8 V vs. Fc0/+) and one-electron reduction process (E = −2.7 V vs. Fc0/+) observed by cyclic voltammetry. Chapter 3. H2 and D2 exchange by a μ3-SnH, μ3-H capped trinuclear nickel cluster, [Ni3(dppm)3(μ3-H)(μ3-SnH)], 1, (dppm = bis(diphenylphosphino)methane) was investigated through a variety of nuclear magnetic resonance (NMR) spectroscopy studies. Mechanistic studies reveal reversible exchange upon addition of D2/H2 between 1 and the deuteride species, [Ni3(dppm)3(μ3-D)(μ3-SnD)], 1D. Furthermore, these studies suggest a conformational change at the Ni-H functionality upon addition of H2. Variable temperature (VT) NMR studies of 1 demonstrate a temperature dependence of the Sn-H and the Ni-H resonances, with the Ni-H experiencing the largest perturbation in chemical shift. Further mechanistic insights were obtained with VT NMR studies performed under an atmosphere H2 and HD. Overall, these studies provide evidence of H2/D2 exchange by complex 1 and suggest a novel mechanism for hydrogen activation and exchange processes. Chapter 4. Spectroscopic and structural comparisons are made between a series of (OR)3Sn capped trinuclear nickel clusters. The synthesis of the (OEt)3Sn and (OPh)3Sn capped nickel clusters, [Ni3(dppm)3(μ3-Cl)(μ3-Sn(OEt)3)] (1), [Ni3(dppm)3(μ3-Cl)(μ3-Sn(OPh)3)] (2), was realized by the treatment of [Ni3(dppm)3(μ3-Cl)(μ3-SnCl3] with 6 eq. of KOEt or NaOPh, respectively. Treatment of complex 1 with 10 eq. glycerol results in the synthesis of the (C3H5O3)Sn capped trinuclear nickel cluster, [Ni3(dppm)3(μ3-Cl)(μ3-Sn(C3H5O3))] (3). The crystallographic studies of these clusters allow a detailed structural comparison. In addition, cyclic voltammetric data were obtained for complexes 1, 2, and 3 and their electrochemical properties are compared. Complex 1 and 3 exhibit reversible oxidation and reduction events. Complex 2 displays a reversible oxidation and a pseudo-reversible reduction. Chapter 5. The focus of this chapter is to briefly describe project ideas based on main group coordinated polynuclear clusters that future researchers may find worthwhile.

As global anthropogenic carbon dioxide (CO2) emissions continue to rise, there is a need not only to reduce production of CO2, but also an opportunity to use it as a substrate for value-added products. One viable solution is to reduce CO2 in the two proton, two electron coupled process to produce carbon monoxide (CO), which can in turn be utilized to recreate hydrocarbon fuels. One of the most active and selective molecular electrocatalysts for the reduction of CO2 to CO is Re(2,2′-bipyridine)(CO)3Cl (Re-bpy) and derivatives thereof. The best method to study electrocatalysts is cyclic voltammetry (CV), which affords both kinetic and thermodynamic information about catalysis. CV is the main technique used to characterize substituent, labile ligand, and Brønsted acid effects on Re-bpy based catalysts, which show increased activity with electron donating 4,4′-substituents and moderate Brønsted acids such as 2,2,2-trifluoroethanol and phenol. The Re-bpy catalyst motif is also extended to Group 6 Mo and W metals, which are not as active as their Group 7 counterparts due to high overpotentials and product poisoning of the catalyst.

To build a fundamental understanding of how molecular catalysts interact with surfaces, Re-bpy derivatives were bound to Au substrates and studied by sum frequency generation spectroscopy (SFG). While cyano substituents deactivated the molecular catalyst, they adsorbed onto Au surfaces, allowing for determination of molecular orientation on the surface as well as characterization of surface-molecule vibratinal communication. Thiol groups were subsequently employed on the bpy ligand for both Re and Mn catalysts to create a covalent attachment to Au surfaces. These groups did not deactivate the molecular catalysts and reproducibly create monolayers on Au surfaces. Further studies are needed in order to fully understand the implications of surface bound Re-bpy based catalysis as well as apply the design principles learned from Re-bpy systems to future molecular electrocatalysts.

Efficient electrocatalytic reduction of small molecules such as CO2, N2, and even protons relies upon the precise delivery of electrons and protons to an active site. The coordination of these reactions can involve sophisticated ligands controlling the coordination spheres around a reactive center. The incorporation of a pendant base for proton shuttling is one way to achieve such control. In order to explore the reactivities of a class of complexes, Ni(P2N2), versatile syntheses are required.

Seven 1,5-diaza-3,7-diphosphacyclooctane (P2N2) ligands with alkyl- and carbonyl- substituted phosphines have been synthesized via two versatile methods that allows for improved control of the phosphine substituent. The methyl-, ethyl-, benzyl-, 3-propanoic acid, 3-propanamide, and ethyl 3-propanoate-phosphino- substituted phosphine P2N2 ligands (PMe2NPh2, PEt2NPh2, PBn2NPh2, PPA2NPh2, PAmide2NPh2, PEtE2NPh2, PEtE2NPhOMe2) were synthesized and characterized by 31P{1H} NMR, 1H NMR, and elemental analysis, and their corresponding [Ni(PR2NPh2)2](BF4)2 complexes were synthesized and characterized by 31P{1H} NMR, 1H NMR, and electrochemistry, and their corresponding Ni(PR2NR’2)X2 (X= Cl-, BF4-, CF3SO3-, PF6-) complexes have been synthesized and characterized by 31P{1H} NMR, 1H NMR, X-ray crystallography, and elemental analysis

The nickel complexes of P2N2 ligands synthesized and characterized by these new methods were investigated for hydrogen evolution and formate oxidation electrocatalysis. The newly synthesized complexes with alkyl- and carbonyl- phosphine substituents rapidly evolve hydrogen and oxidize formate more slowly than previously synthesized cyclohexyl- and phenyl- substituted phosphine ligands, due to a more negative reduction potential leading to a decrease in the driving force of formate oxidation and increase for hydrogen evolution.

Nickel is an attractive metal for use in a wide variety of catalytic transformations that normally require precious metals. Of particular interest is the reduction of CO2 to formate, which is a potential hydrogen storage medium, fuel, and chemical building block. The nickel bis-diphosphine class of complexes has recently been studied for this application. However, it has been found that existing nickel bis-diphosphine complexes cannot produce hydrides that are capable of reducing CO2 to form formate.

In this dissertation, I first provide a detailed analysis of the thermodynamic relationships which are responsible for the inadequate reduction power of hydrides formed by known nickel bis-diphosphines. It is established that by designing complexes with a more negative reduction potential than that of a typical nickel bis-diphosphine complex, more powerfully reducing hydrides may be formed. N-heterocyclic carbene (NHC) ligands are chosen as a substitute for phosphine ligands in this system, as NHC ligands are generally more electron donating than typical phosphine ligands, and are thus expected to result in complexes with more negative reduction potentials.

By direct analogy to known nickel bis-diphosphine complexes, homoleptic nickel bis(NHC) complexes are synthesized. These are found to form sterically hindered, highly reducing Ni(0) compounds upon reduction. These Ni(0) compounds are the first group 10 M(0) (M = nickel, palladium, platinum) tetracarbene complexes reported. They display an unusual sawhorse geometry in the Ni(0) state.

In order to address the electrochemical irreversibility displayed the nickel tetracarbene complexes, a less sterically encumbering bis(NHC) ligand linked by a propyl group is chosen. This ligand is found to provide access to the [Ni(NHC2)]2+ fragment, which in turn provides access to a new Ni(II)(NHC2)¬L2 class of complexes. Four Ni(II)(NHC2)¬(bipyridine) complexes are synthesized and characterized. It is found that a rigid, twice-linked bis(NHC) ligand is capable of imparting electrochemical reversibility to these systems.

This new bis(NHC) ligand is used to synthesize the first Ni(II)(NHC2)(diphosphine) complex. This compound is isolated in both the Ni(II) and Ni(0) oxidation states. Strategies to further develop this system in order to be suitable for CO2 reduction are discussed. Future experiments involving the [Ni(NHC2)]2+ fragment are also suggested.

This dissertation details efforts in designing and tuning catalysts that have the ability to engage CO2 through a metal center and functional groups in the second coordination sphere. We attempted to improve a dinuclear copper complex that has the ability to engage CO2 through two metal centers and was previously reported as a CO2 reduction catalysts. Electron donating substitutions were made on the ligands in attempts to improve rates for the electrocatalytic reduction of CO2. These modifications led to changes in the reduction potentials and the structures of the complexes, but did not produce significant improvements in turnover frequencies or overpotentials for CO2 reduction.

We attempted to improve rhodium disphosphine catalysts for the conversion of CO2 to HCOO– by introducing ligands with proton relays. Several [Rh(P2N2)2]+ complexes were synthesized. They were structurally characterized as square planar with slight tetrahedral distortions and exhibited a reversible 2e– Rh(I/–I) redox couple in voltammetric studies. We synthesized the HRh(P2N2)2 complexes and structurally characterized them as having distorted trigonal bipyramidal geometry. The hydricities of several of the HRh(P2N2)2 complexes were measured using equilibration experiments monitored by 31P NMR. The HRh(P2N2)2 complexes are among the most hydridic complexes the 16 e– M(diphosphine)2 class.

We compared the activity of the [Rh(P2N2)2]+ complexes for catalytic CO2 hydrogenation to formate to a [Rh(diphosphine)2]+ complex of a similar hydricity that lacked pendant amines. We found that, despite the strong reducing power of the HRh(P2N2)2 complexes, the non-pendant-amine-containing Rh complex was the best catalyst for CO2 hydrogenation. We also tested these complexes for their electrochemical CO2 reduction activity. While these complexes are energetic enough to react with CO2 when reduced, they are unstable under the high potentials necessary for their reduction.

We tested Ru pincer complexes that are known to hydrogenate esters via a mechanism that involves co-operative metal-ligand interactions for their ability to reduce CO2 and methylformate electrochemically. We also synthesized and tested an Fe analogue of these complexes for electrochemical reactivity. We found that these complexes are promising candidates for further study as CO2 reduction catalysts.

Chemical exchange dynamics are often studied using a Bloch equation analysis of coalescing line shapes in nuclear magnetic resonance spectroscopy (NMR). A similar treatment has been applied to systems exchanging on the infrared (IR) vibrational timescale, where line shape analysis of coalesced FTIR and Raman spectra allow the study of reactions occurring in picoseconds (ps). In practice however, there are comparatively few examples of line shape analysis by vibrational spectra as inhomogeneous broadening, solvent environment fluctuations or other dynamic processes in addition to chemical exchange can contribute to the overall vibrational line shape. This work examines two model systems displaying exchange dynamics on the vibrational timescale and the use of vibrational spectroscopy to describe their ultrafast dynamics. The first system details electron transfer dynamics in dimers of oxo-centered triruthenium clusters that are linked by non-covalent interactions, where we have shown how electron transfer across hydrogen bonds occurs on (or approaches) the vibrational time scale. While these studies have important implications in understanding long range ET in biological systems, the importance of non-covalent interactions also extends throughout the chemical sciences and as they readily affect the stability of artificial supramolecular structures, and selectivity of catalysts. In this dissertation the fundamental relationship between non-covalent molecular interactions and ET is examined to gain new understanding of ET processes occurring across hydrogen bonds. We were also able to show that electron delocalization across hydrogen bonds imparts substantial stability (~5 kcal mol–1) to the hydrogen bonding interaction.

The second system, a penta-coordinate ruthenium dithitene complex seeks to examine vibrational lineshape coalescence and the application of an optical Bloch analysis in systems exhibiting ultrafast dynamics. The ruthenium complex undergoes structural rearrangement between three distinct structural isomers that differed only in the orientation of a carbonyl ligand about the metal center. In methylene chloride, the three isomers 1) square pyramidal equatorial, 2) trigonal bipyramidal, and 3) square pyramidal axial were found to exchange through the meta-stable trigonal bipyriamidal intermediate within picoseconds. This study was a direct validation of the mechanism and timescale of Berry pseudorotation – the pairwise exchange of ligands in a pentacoordinate complex – a process that was described nearly fifty years ago, and to our knowledge, was the first observation of an ultrafast dynamic equilibrium involving two distinct structural isomers and the intermediate connecting them.

Carbon dioxide reduction has been an increasingly popular research in the renewable energy development as it can be used to store the solar energy in the form of chemical energy in liquid fuels, like gasoline and diesel. There are two main catalytic approaches to overcome the thermodynamically unfavored conversion of carbon dioxide (CO2) to carbon-based species, such as carbon monoxide and format: photochemical reduction using direct sunlight, and electrochemical reduction using electricity generated by solar panels.

In a typical photochemical system using rhenium or manganese bipyridine catalysts, previous work has been done on ligand modification to improve the quantum yield of carbon monoxide (CO) and other carbon species production. The work presented in this dissertation focuses on the structural modification of these catalysts to eliminate dimerization of manganese bipyridine catalyst upon first reduction and facilitate electron transfer from singly reduced photosensitizer to catalyst through non-covalent supramolecular assembly. In the former method, the bromide ligand of the manganese bipyridine catalyst (Mnbpy(CO)3Br) was replaced with a cyanide ligand (Mnbpy(CO)3CN) to reach an alternative reaction mechanism, in which disproportionation of two singly reduced manganese bipyridine catalyst occurs to give the active species without dimerization. In the latter method, electron transfer between the singly reduced photosensitizer and the catalyst is facilitated by the closer proximity of the two through non-covalent hydrogen bonding. Both method, unexpectedly, discovered the role of solvents in photocatalysis on product selectivity.

One of the biggest obstacles of electrochemical reduction of carbon dioxide in large-scale application is the immobilization of catalysts onto electrode surface. Most of the attachment methods in the literature face the issues of catalysts detachment and deactivation, and poor electrical contact between the catalyst and the electrode. A novel solvent-free synthetic method was invented to embed a top carbon dioxide electrocatalyst iron porphyrin into covalent organic frameworks. The COF-modified electrode demonstrated good activity for production of CO under electrocatalytic conditions in acetonitrile (MeCN) compared to the control electrode with only adsorbed iron porphyrins.

To create fully sustainable energy systems requires multiple stages of engagement across different fields. The work presented in this dissertation represents a small survey of two major branches of sustainable chemistry – the improved efficiency for converting from renewably sourced fuels to power and the utilization of renewable sources of electricity to produce valuable chemicals from renewable carbon feedstocks. Electrochemistry and photoelectrochemistry are powerful tools for interconversion between chemical and electrical energy and for using light to drive chemical change.

Light can be used to overcome both the kinetic and thermodynamic barriers found in electrochemistry. In a proposed fuel cell device, the feasibility of using multi-carbon fuels like n-hexane at ambient temperatures and pressures may be made possible by using light activated materials to overcome the kinetic barriers to fuel oxidation. The energy to produce the light may be sourced from the fuel cell itself and fed back into the oxidation process. In an electrolysis reaction, light may be used to drive the uphill reaction by creating sufficient photovoltage and photocurrent. In a paired electrolysis, CO2 and a lignin derived substrate syringaldehyde are simultaneously upgraded using a customized photovoltaic device to drive both catalytic processes. In both of these implementations, electricity plays the crucial role of connecting light energy to chemical change and provides two examples where it can be used in sustainable chemical methods.

Electrocatalytic reduction of carbon dioxide (CO2) is a profoundly challenging problem that is of interest, not only as a means of counteracting unsustainable emissions of CO2, but also as a method for the development of renewable fuels. Rhenium and manganese bipyridine tricarbonyl complexes are among the most active and robust catalysts for proton-coupled CO2 reduction to carbon monoxide (CO). X- ray Absorption Spectroscopy studies are reported to reveal the electronic ground state of the Re catalysts, which help explain origins for high selectivity for CO2 reduction over proton reduction. Stopped-flow mixing in tandem with rapid-scan IR spectroscopy is utilized to probe the direct reaction of the Re catalysts with CO2, observing, for the first time, the binding of CO2 to these catalysts.

Manganese bipyridine catalysts are desirable, in comparison with their Re analogs, due to the earth-abundance of Mn and the ability for these catalysts to operate at lower overpotentials. One distinct difference between these Mn catalysts and their Re counterparts is a high tendency for dimerization after one-electron reduction, which contributes to the potential necessary to access their active state and to limiting their catalytic activity. Synthetic modification of the bipyridine ligand (by adding bulky mesityl groups) is used to completely eliminate dimerization for these Mn complexes, allowing the active catalyst to be generated at a 300 mV more positive potential than in typically Mn bipyridine complexes. CO2 reactivities in the presence of weak Brønsted acids, strong Brønsted acids, and Lewis acids have been explored in order to encourage this bulky Mn catalyst to reduce CO2 at low overpotentials. Mechanistic tools, including IR-spectroelectrochemistry, are described to gain insight into these unique catalytic processes.

In order to further enhance stability and facilitate product separation, the use of metal-organic frameworks (MOFs) is explored as a means of anchoring molecular catalysts on a heterogeneous platform. A Mn bipyridine catalyst attached to a highly robust Zr(IV)-based MOF is used to enhance photochemical CO2 reduction. By utilizing an iron porphyrin catalyst, anchored into the linkers of a MOF thin film, we demonstrate, in a proof of principle, electrochemical CO2 reduction by this heterogenized molecular catalyst.