The use of iron-based catalysts for the water oxidation reaction is highly attractive due to the high abundance of iron. While many molecular catalysts have been made, most show limited activity and short lifetimes. An exception with higher activity was presented by Thummel and co-workers in 2015. Herein we present a study on the feasibility of the coupling of two O centered radicals originating from the two subunits of the dinuclear catalyst. The reaction pathway includes the oxidation to the active species Fe^IV–O–Fe^IV but avoids further high potential oxidations which previous mechanistic proposals have relied on.

Ru(bda)(py)2 (bda = 2,2′-bipyridine-6,6′-dicarboxylate, py = pyridine) has been a significant milestone in the development of water oxidation catalysts. Inspired by Ru(bda)(py)2 and aiming to reduce the use of noble metals, iron (Fe) was introduced to replace the Ru catalytic center in Ru(bda)(py)2. In this study, density functional theory (DFT) calculations were performed on Fe- and Ru(bda)(py)2 catalysts, and a more stable 6-coordinate Fe(bda)(py)2 with one carboxylate group of bda disconnecting with Fe was found. For the first time, theoretical comparisons have been conducted on these three catalysts to compare their catalytic performances, such as reduction potentials and energy profiles of the radical coupling process. Explanations for the high potential of [FeIII(bda)(py)2-H2O]+ and reactivity of [FeV(bda)(py)2-O]+ have been provided. This study can provide insights on Fe(bda)(py)2 from a computational perspective if it is utilized as a water oxidation catalyst. 

To combat the increasing energy demand and climate change, artificial photosynthesis is a promising approach to producing renewables by storing energy into chemicals and fuels. Water oxidation, responsible for offering electrons and protons to the reduction reactions, suffers from slow kinetics. Consequently, developing highly efficient water oxidation catalysts (WOCs) reacting via desirable mechanisms plays an essential role in reaching the goal of artificial photosynthesis. Molecular WOCs in homogeneous catalysis could serve as models to understand the structure-property relationship and the reaction mechanisms owing to their well-defined structures and easily characterized properties. In Chapter 1, a variety of Ru- and Fe-based molecular WOCs are introduced. The two catalytic mechanisms for the O-O bond formation, water nucleophilic attack (WNA) and interaction of two metal-oxos (I2M), are described in Chapter 1.

To understand the catalytic mechanisms for designing better WOCs, various theoretical techniques have been applied. Density functional theory (DFT) method was used to study the molecular properties of catalysts and the reaction energetics. Molecular dynamics (MD) and potential of mean force (PMF) were employed to investigate the behavior of catalysts and to calculate the free energy change along a specific coordinate in the explicit solvent. The theory for computing redox potentials, acid dissociation constants, reaction rate constants, and the descriptions of different solvation models are presented in Chapter 2.

The family of Ru(bda)(py)₂ (bda^2- denotes 2,2'-bipyridine-6,6'-dicarboxylate, py denotes pyridine) complexes exhibits extremely high catalytic activity via the I2M mechanism under acidic conditions. Extensive studies have previously been conducted on the primary and secondary coordination environments. In Chapter 3, the first section focuses on three different modifications on the para-sites of the two pyridines. It is found that the complex with the longer hydrophobic group substituting the para-sites of two pyridines demonstrates the highest activity, which is attributed to the stronger binding energy between two Ru^V monomers. We conclude that the hydrophobic effect is dominating in enhancing the catalytic performance via the I2M mechanism. The second section of Chapter 3 studies the isolated crystal structure of the pseudo seven-coordinate Ru^III-aqua intermediate obtained by connecting two meta-sites of pyridines with an ethylene glycol ether linker. DFT was used to study the formed H-bond network between the distal ligand and Ru(bda)(py)₂. The influence of micro-solvation on the incoming aqua ligand was analyzed in the form of bonds and interactions. Incorporating a distal ligand could be an effective strategy to investigate the outer coordination environment effects. In the third section of Chapter 3, the linkers connecting two meta-sites of pyridines were changed and three more Ru(bda)(py)₂-based catalysts with hydrophobic (aliphatic) and hydrophilic (ethylene glycol ether) linkers of different lengths were synthesized to study the outer coordination environment effects. The hydrophobic ligands lower the potentials slightly by stabilizing the key intermediates. The complex with the longer hydrophobic distal ligand demonstrates the highest TOF with the first-order kinetics. The I2M mechanism is suppressed owing to the limited flexibility of distal ligands validated by the nuclear magnetic resonance spectroscopy and the DFT-calculated energy differences between the conformations of distal ligands in the front and in the back of the bda ligand. The strategy of introducing hydrophobic outer coordination environment could be beneficial to design catalysts involving PCET reactions.

Inspired by the Ru(bda)(py)₂ catalyst and aiming to reduce the usage of Ru, a computational comparative study on Ru(bda)(py)₂ and Fe(bda)(py)₂ is presented in Chapter 4. Fe(bda)(py)₂ was built by directly replacing Ru catalytic center of Ru(bda)(py)₂ with Fe and maintaining the rest of the ligand system. The Fe-based complexes at different valence states prefer higher spin states while the Ru-based complexes are more stable at the lowest spin states. Unlike the Ru, the Fe center disfavors the 7-coordinate structure. Concerning the catalytic performance, the Fe(bda)(py)₂ requires much higher potentials to reach the reactive Fe^V species than the Ru(bda)(py)₂. The 6-coordinate [Fe^V(bda)(py)₂=O]⁺ also has a higher energy barrier of the O-O bond formation via the I2M mechanism than the [Ru^V(bda)(py)₂=O]⁺. We propose that directly substituting the Ru catalytic center with Fe fails to generate a viable catalyst and a significant ligand system modification is required.

An alternative feasible catalytic mechanism is suggested for a highly active dinuclear Fe-based WOC in Chapter 5. The mechanism is proposed where two oxidation reactions are first required to reach the reactive state with calculated potentials matching the onset potential in the experiment. The reactive species is then decomposed by the nucleophilic chloride counter ion attack to produce two Fe-based monomers, which finally form the O-O bond by the radical coupling pathway. The calculated energy barriers and first-order kinetics match well with the experimental observations.

För att bekämpa den ökande energiefterfrågan och klimatförändringarna är artificiell fotosyntes ett lovande tillvägagångssätt för att producera förnybara energikällor genom att lagra energi i kemikalier och bränslen. För att tillhandahålla elektroner och protoner till reduktionsreaktionerna oxideras vatten, en reaktion som lider av långsam kinetik. Följaktligen spelar utveckling av högeffektiva vattenoxidationskatalysatorer (WOC) som reagerar via önskvärda mekanismer en viktig roll för att nå målet med artificiell fotosyntes. Molekylära WOCs i homogen katalys kan fungera som modeller för att förstå struktur-egenskapsförhållandet och reaktionsmekanismerna på grund av deras väldefinierade strukturer och lätt karakteriserade egenskaper. I kapitel 1 introduceras Ru- och Fe-baserade molekylära WOCs. De två katalytiska mekanismerna för O-O-bindningsbildning, nukleofil attack av vatten (WNA) och interaktion mellan två metalloxointermediärer (I2M), beskrivs i kapitel 1.

För att förstå de katalytiska mekanismerna för att designa bättre WOCs, har olika teoretiska tekniker använts. Täthetsfunktionalteori (DFT) användes för att studera de molekylära egenskaperna hos katalysatorer och reaktionsenergierna. Molekyldynamik (MD) och potential of mean force (PMF) användes för att undersöka beteendet hos katalysatorer och för att beräkna den fria energiförändringen längs en specifik koordinat i explicita lösningsmedel. Teorin för beräkning av redoxpotentialer, syradissociationskonstanter, reaktionshastighetskonstanter och beskrivningarna av olika solvatiseringsmodeller presenteras i kapitel 2.

Familjen Ru(bda)(py)₂ (bda^2- betecknar 2,2'-bipyridin-6,6'-dikarboxylat, py betecknar pyridin) komplex uppvisar extremt hög katalytisk aktivitet via I2M-mekanismen under sura förhållanden. Omfattande studier har tidigare gjorts kring de primära och sekundära koordinationssfärerna. I kapitel 3 fokuserar det första avsnittet på tre olika modifieringar av parapositionerna hos de två pyridinerna. Det har visat sig att komplexet med längre hydrofoba grupper i parapositionerna visar den högsta aktiviteten, vilket tillskrivs den starkare bindningsenergin mellan två Ru^V-monomerer. Vi drar slutsatsen att den hydrofoba effekten dominerar för att förbättra den katalytiska prestandan via I2M-mekanismen. Den andra delen av kapitel 3 studerar den isolerade kristallstrukturen av en pseudo-sjukoordinerad Ru^III-aqua-intermediär erhållen genom att koppla två meta-positioner på pyridinerna med en etylenglykoleterlänk. DFT användes för att studera det bildade H-bindningsnätverket mellan den distala liganden och Ru(bda)(py)₂. Inverkan av mikrosolvatisering på den inkommande vattenliganden analyserades i form av bindningar och interaktioner. Att inkorporera en ligand med kedjor på avstånd från metallcentret kan vara en effektiv strategi för att undersöka effekterna på den yttre koordinationsmiljön. I det tredje avsnittet av kapitel 3 ändrades länkarna som förbinder två metapositioner av pyridinerna och ytterligare tre Ru(bda)(py)₂-baserade katalysatorer med hydrofoba (alifatiska) och hydrofila (etylenglykoleter) länkar av olika längd syntetiserades för att studera effekterna av den yttre koordinationsmiljön. De hydrofoba liganderna sänker reduktionspotentialerna något genom att stabilisera intermediärerna. Komplexet med den längre hydrofoba distala liganden visar den högsta reaktionsfrekvensen (TOF) och reagerarar med första ordningens kinetik. I2M-mekanismen undertrycks på grund av den begränsade flexibiliteten hos de distala liganderna, vilket validerats med kärnmagnetisk resonansspektroskopi och de DFT-beräknade energiskillnaderna mellan konformationerna av de distala liganderna i fram- och baksidan av bda-liganden. Strategin att introducera hydrofob yttre koordinationsmiljö kan vara fördelaktig för att designa katalysatorer som involverar PCET-reaktioner.

Inspirerad av Ru(bda)(py)₂-katalysatorn och med syftet att minska användningen av Ru, en jämförande beräkningsstudie av Ru(bda)(py)₂ och Fe(bda)(py)₂ presenteras i kapitel 4. Fe(bda)(py)₂ byggdes genom att direkt ersätta Ru centret i Ru(bda)(py)₂ med Fe och bibehålla resten av ligandsystemet. De Fe-baserade komplexen vid olika valenstillstånd föredrar högre spinntillstånd medan de Ru-baserade komplexen är mer stabila vid de lägsta spinntillstånden. Till skillnad från Ru, är den sju-koordinerade strukturen mindre stabil. När det gäller den katalytiska prestandan kräver Fe(bda)(py)₂ mycket högre potentialer för att nå den reaktiva Fe^V-intemediären än Ru(bda)(py)₂. Den sex-koordinerade [Fe^Vbda)(py)₂=O]⁺ har också en högre energibarriär för O-O-bindningsbildningen via I2M-mekanismen än [Ru^Vbda)(py)₂=O]⁺. Vi föreslår att direkt ersättning av det katalytiska centret Ru med Fe misslyckas med att generera en kompetetiv katalysator och en betydande modifiering av ligandsystemet krävs.

En alternativ katalytisk mekanism föreslås för en högaktiv dinukleär Fe-baserad WOC i kapitel 5. Mekanismen som föreslås kräver först två oxidationsreaktioner för att nå det reaktiva tillståndet, med beräknade potentialer som matchar onset-potentialen i experimentet. Den reaktiva specien sönderdelas sedan av en nukleofil kloridjon, vilket genererar två Fe-baserade monomerer, som slutligen bildar O-O-bindningen genom radikalkopplingsvägen. De beräknade energibarriärerna och första ordningens kinetik matchar väl med de experimentella observationerna.

Isolation of RuIII-bda (17-electron specie) complex with an aqua ligand (2-electron donor) is challenging due to violation of the 18-electron rule. Although considerable efforts have been dedicated to mechanistic studies of water oxidation by the Ru-bda family, the structure and initial formation of the RuIII-bda aqua complex are still controversial. Herein, we challenge this often overlooked step by designing a pocket-shape Ru-based complex 1. The computational studies showed that 1 possesses the crucial hydrophobicity at the RuV(O) state as well as similar probability of access of terminal O to solvent water molecules when compared with classic Ru-bda catalysts. Through characterization of single-crystal structures at the RuII and RuIII states, a pseudo seven-coordinate “ready-to-go” aqua ligand with RuIII...O distance of 3.62 Å was observed. This aqua ligand was also found to be part of a formed hydrogen-bonding network, providing a good indication of how the RuIII-OH2 complex is formed.

The outer coordination sphere of metalloenzyme often plays an important role in its high catalytic activity, however, this principle is rarely considered in the design of man-made molecular catalysts. Herein, four Ru-bda based molecular water oxidation catalysts with well-defined outer spheres are designed and synthesized. Experimental and theoretical studies showed that the hydrophobic environment around the Ru center could lead to thermodynamic stabilization of the high-valent intermediates and kinetic acceleration of the proton transfer process during catalytic water oxidation. By this outer sphere stabilization, a 6-fold rate increase for water oxidation catalysis has been achieved. 

The catalytic activity of the bimolecular reaction was affected by many parameters. Although many efforts have been dedicated to investigate the influence of secondary interactions in pre-organizing catalysts, the hydrophobic effect on Ru-bda-type water oxidation catalysts remains unclear as a result of the lack of an ideal catalytic model. In this work, four catalysts 1–4 with variable hydrophobicity have been synthesized, and cerium(IV)-driven water oxidation results showed that the hydrophobic complexes 3 and 4 outperformed the hydrophilic complex 2. Steric mapping, nuclear magnetic resonance, and differential pulse voltammogram studies indicated that the increase in activity has no correlation with electronic and steric effects but has correlation with hydrophobicity. Molecular dynamics have shown that the modifications of the hydrophobicity on the axial pyridine ligands of the Ru-bda type of catalysts can improve the water oxidation catalytic activity by stabilizing the pre-reactive catalyst dimer.

An improved understanding of the interactions of transition-metal (TM) nanoparticles with Lewis acids/bases will facilitate the design of more efficient catalysts. Therefore, Pt-14, Pt-13, Pt-12, and Ni-12 nanoparticles have been studied at the TPSSh/Def2-TZVP level of density functional theory (DFT). Surface electrostatic potential [V-S(r)] maps are used to analyze the Lewis acidic and basic properties of all nanoparticles and indicate that the interactions of Pt and Ni nanoparticles are governed by sigma(d)-holes and sigma(s) -holes, respectively. Lewis acids (Na+, HF) and a Lewis base (H2O) have been tested as ligands to probe the local interaction proficiencies. The comparison between binding energies and V-S(r) shows that the lowest minimum (V-S,V-min) and highest maximum (V-S,V-max) of V-S(r) on each particle can predict the most favorable binding site for the Lewis acids and base, respectively. V(S,min )can also rank the different binding strengths of Na+ and HF with the nanoparticles. For H2O, the binding strength versus V-S,V-max correlation is better for Ni-12 than for the Pt nanoparticles. This observation is discussed in relation to charge transfer/polarization and structural deformation upon interaction. In light of our findings, we compare the catalytic potential of Ni to the less abundant but more commonly used Pt.

Solid-state luminescent materials that possess reversible fluorescence changes toward external multistimuli are of immense interest because of their potential applications in data storage and sensors. While the recent developments in this field are mainly focused on the pi-conjugated organic molecules. Herein two polymorphic luminescent ionic liquid (IL)-based stimuli-responsive materials were designed by the supramolecular assemblies of an organic-decorated chlorobismuthate anion and a rotationally flexible imidazolium cation, namely, alpha (1)/beta (2)-[Bmmim][BiCl4(2,2'-bpy)] (Bmmim = 1-butyl-2,3-dimethylimidazolium; 2,2'-bpy = 2,2'-bipyridine). Because of the different conformations of the n-butyl chains on the imidazolium cations, tuning of the supramolecular packing structures as well as luminescent colors for 1 and 2 was realized. Single-crystal X-ray diffraction and Hirshfeld surface analyses disclose that the poly-morphism-dependent emission may be attributed to the different weak interactions, especially to the pi-pi interactions between adjacent [BiCl4(2,2'-bpy)](-) anions in two compounds. Additionally, compound 2 could be transformed into 1 spontaneously at ambient conditions, which could be triggered by the moisture in the air. Both of the title compounds could detect NH(3 )vapor selectively through the luminescence "turn-off" method rapidly and reversibly because of the destruction of intermolecular interactions, indicating their stimuli-responsive property toward NH3.

Herein, we report a new type of efficient trifunctional electrocatalyst by in situ coupling amorphous cobalt nitride (CoNx) nanoparticles within three-dimensional (3D) nitrogen-doped graphene aerogel (NGA). The CoNx/NGA nanohybrid with hierarchical porous strucuture guarantees the superior activities toward ORR, OER and HER, due to abundant dual active CoNx. and NxC sites. Impressively, it also exhibits a long lifetime and exceptionally high electrochemical performances as a cathode and an anode in a two-electrode overall water splitting electrolyzer, and also as an air-cathode in a rechargeable Zn-air battery. In addition, the CoNx/NGA-based water splitting electrolyzer and two Zn-air batteries can be integrated together to effectively self-drive electrochemical water splitting device with high gas evolution rates of 186 and 372 mu mol h(-1) for O-2 and H-2, respectively. This work paves a way for designing advanced non-noble multifunctional catalysts, aiming for the real application of energy storage and conversion devices.

Ru(bda)(py)₂ (bda=2,2′-bipyridine-6,6′-dicarboxylate, py=pyridine) has been a significant milestone in the development of water oxidation catalysts. Inspired by Ru(bda)(py)₂ and aiming to reduce the use of noble metals, iron (Fe) was introduced to replace the Ru catalytic center in Ru(bda)(py)₂. In this study, density functional theory (DFT) calculations were performed on Fe- and Ru(bda)(py)₂ catalysts, and a more stable 6-coordinate Fe(bda)(py)₂ with one carboxylate group of bda disconnecting with Fe was found. For the first time, theoretical comparisons have been conducted on these three catalysts to compare their catalytic performances, such as reduction potentials and energy profiles of the radical coupling process. Explanations for the high potential of [Fe^III(bda)(py)₂-H₂O]⁺ and reactivity of [Fe^V(bda)(py)₂-O]⁺ have been provided. This study can provide insights of Fe(bda)(py)₂ from a computational perspective if it is utilized as water oxidation catalyst.