Beyond Traditional Superatom Ligands and Cores Evan Ambrose Doud Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2020 © 2019 Evan Ambrose Doud All Rights Reserved Abstract Beyond Traditional Superatom Ligands and Cores Evan Ambrose Doud This dissertation summarizes my research in the Roy group on the development, synthesis, and study of new N-heterocyclic carbene (NHC) based ligands and nickel phsophinidene core compositions of molecular clusters, also known as superatoms. Chapter 1 introduces superatoms as atomically precise and discreet building blocks for use in the design and synthesis of novel materials. A brief history as well as selected synthetic strategies of superatoms will be introduced. The relevant materials properties of superatoms as well as their dependence on core composition and ligand structure will be discussed. Next, the use of superatoms with specialized or functionalizable ligands to synthesize new materials will be demonstrated. This chapter details the importance of the superatom ligands and core composition is the foundation that the subsequent chapters builds upon in developing these two areas. Chapter 2 introduces a functionalized NHC as a potential superatom ligand. While not necessary for all superatom ligands, ligands that enable electronic access to the superatom core are attractive. In this chapter, the conductance of potential NHC based ligands are probed through the scanning tunneling microscope-based break-junction (STM-BJ) method. A novel method of forming single molecule junctions in situ was used and these ligands are found to display a length dependent conductance with strong coupling to the Au electrode, confirming their potential use as ligands for superatoms. Chapter 3 describes the nature of the NHC–M bond in self-assembled monolayers (SAMs) on a Au(111) surface using high-resolution spectroscopy and theoretical calculations. This study was performed as a result of challenges and questions encountered during the work of Chapter 2. The results obtained from this study explore an important structure-function relationship of NHC ligands and have broader impact in materials chemistry beyond superatoms. Chapter 4 explores the synthesis of superatoms with NHC ligands beyond simple imidazolium-based NHCs. This chapter describes the two primary synthetic techniques used and the synthesis of NHC-ligated superatoms. This work is also ongoing and characterization is limited to crude single crystal X-ray diffraction structures and select NMRs. Finally, Chapter 5 details the use of uncommon organocyclophosphine reagents to synthesis novel nickel-phosphinidene molecular clusters, a potential new superatom. In this chapter the synthesis of a family of nickel-phosphinidene molecular clusters is described and studied. A potential application of these molecular clusters is explored through the thermolytic conversion to the industrially relevant Ni2P. Table of Contents List of Figures and Tables.............................................................................................................. iv Acknowledgments...................................................................................................................... xviii Chapter 1: Superatoms in Materials Science .................................................................................. 1 1.1 Preface................................................................................................................................... 1 1.2 Introduction ........................................................................................................................... 1 1.3 Overview of Clusters as Superatoms .................................................................................... 4 1.4 Superatom Assembly and Superatomic Crystal Properties ................................................ 10 1.5 Multicomponent Covalent Superatomic Materials ............................................................. 23 1.6 Conclusion and Outlook ..................................................................................................... 28 1.7 References ........................................................................................................................... 29 Chapter 2: In Situ Formation of N-Heterocyclic Carbene-Bound Single Molecule Junctions ..... 39 2.1 Preface................................................................................................................................. 39 2.2 Introduction ......................................................................................................................... 39 2.3 In Situ Formation of NHC-Bound Single Molecule Junctions ........................................... 41 2.4 Conductance Dependence on MI Identity ........................................................................... 44 2.5 Length Dependence of Conductance .................................................................................. 46 2.6 Computational Studies ........................................................................................................ 48 2.7 Conclusion and Outlook ..................................................................................................... 51 2.8 Synthetic Details ................................................................................................................. 51 2.9 Single Crystal X-Ray Diffraction ....................................................................................... 70 2.10 Additional STM-BJ Data .................................................................................................. 77 i 2.11 Additional Computational Data ........................................................................................ 78 2.12 References ......................................................................................................................... 85 Chapter 3: Determination of the Structure and Geometry of N-Heterocyclic Carbenes on Au(111) Using High-Resolution Spectroscopy ........................................................................................... 91 3.1 Preface................................................................................................................................. 91 3.2 Introduction ......................................................................................................................... 91 3.3 Compounds Studied and Monolayer Formation ................................................................. 94 3.4 NEXAFS Studies ................................................................................................................ 95 3.5 Computational Investigation of Binding Geometry ............................................................ 97 3.6 XPS Investigation of the NHC Bound Surface ................................................................... 99 3.7 Conclusion and Outlook ................................................................................................... 105 3.8 Synthetic Details ............................................................................................................... 106 3.9 NEXAFS Sample Preparation........................................................................................... 107 3.10 Details of XPS and NEXAFS Measurements ................................................................. 108 3.11 Details of DFT Calculations ........................................................................................... 109 3.12 Additional Experimental and Theoretical Data .............................................................. 110 3.13 References ....................................................................................................................... 114 Chapter 4: NHCs as Superatomic Ligands ................................................................................. 119 4.1 Preface............................................................................................................................... 119 4.2 Introduction ....................................................................................................................... 119 4.3 Selection of NHC Ligands and NHC Library ................................................................... 120 4.4 Synthetic Strategy ............................................................................................................. 122 4.5 Co6S8(PPh3)6 Ligand Exchange Experiments ................................................................... 123 ii 4.6 Ligand Exchange Using Site-differentiated Co6Se8(PEt3)6-x(CO)x (x = 1 or 2) Superatoms ................................................................................................................................................. 124 4.7 Ligand Exchange Using the Oxidized [Co6Se8(PEt3)6-x(CO)x][PF6] (x = 1 or 2) ............. 127 i 4.8 Attempts at Functionalization of Co6Se8(PEt3)5(6Br-D PrBI) .......................................... 130 4.9 Effects of Bulky N-substituents ........................................................................................ 131 4.10 Conclusions and Outlook ................................................................................................ 133 4.11 Synthetic Details ............................................................................................................. 134 4.12 NMR Data ......................................................................................................................