Date of Graduation
Eberly College of Arts and Sciences
Brian V. Popp
Jessica M. Hoover
Jeffrey L. Petersen
A significant amount of research has explored the impact of phosphinoborane ligand environments on the structure of the coordination sphere of inorganic and organometallic complexes. Ambiphilic phosphinoborane ligands possess a phosphine and a borane that are attached through a chosen linker. This traditionally involves hydrocarbon linkers and leads to either a conformationally free or a constrained ligand system. In particular, this dissertation will focus on the application of phosphinoborane ligands with flexible alkyl linkers. The goal of this study is to utilize the activation and coordination of Lewis acids in the second coordination sphere to influence reactivity at the transition metal center. In the field of transition metal catalysis, there is a predominant amount of literature in substrate activation that relies on the cooperativity of Lewis bases aiding metal catalysis. The application of frustrated Lewis pairs bearing Lewis acidic moieties to organometallic catalysis may provide new opportunities for small molecule activation and catalyst development. This dissertation explores rhodium and iridium phosphinoborane complexes with the objective of understanding: (1) the role of Lewis acidic pendant boranes in intramolecular interactions on Group IX transition metal complexes and (2) the role of these same Lewis acidic groups on the activation of reagents in catalysis. This work is anticipated to contribute to a better understanding of the function of Lewis acidic moieties in cooperative catalysis with transition metals, but also the stabilizing interactions that lead to the isolation of specific complexes. The limited work on β-phosphinoethylborane ligands with late transition metals has focused on the pendant Lewis acid’s role in stabilizing reactive intermediates and performing difficult bond activations. The overall goal of this dissertation is to understand the role of pendant Lewis acids in a model catalytic system of hydrofunctionalization. This will also help gain insight into bond activations and how these functionalities modify the second coordination sphere of late transition metals. Herein, this dissertation involves the synthesis of late transition metals bearing Lewis acidic β-phosphinoethylborane ligand scaffolds. The interaction of the second coordination sphere Lewis acid and the dynamics involved in this interaction are studied by utilizing a variety of spectroscopic techniques and standard synthetic methods. By relying on 11B NMR spectroscopy, we are able to probe the coordination environment around the Lewis acid. The investigations into the coordination environment led to questions of how changes in the coordination environment of rhodium and iridium complexes with tethered Lewis acids impact catalysis. During the investigation of the hydroboration of styrenyl substrates, the newly synthesized rhodium phosphinoborane catalysts provided high selectivity for the branched products. During this study, vinyldiphenyl phosphine was investigated owing to the limited applications of this substrate in hydroborations with pinacolborane. As a result, we sought to investigate whether pendant Lewis acids were able to temper the Lewis basicity of phosphine species. While the phosphinoborane ligand was not necessary for this catalysis, we were able to achieve regioselective control over product formation with simple rhodium catalysts. The hydroboration products of this reaction were then carried forward for further functionalization. In the previous chapters, the Lewis acidic borane moiety showed a propensity to interact with ligands in the primary coordination sphere of the transition metal center. Therefore, we sought to explore the effect of pendant Lewis acids on stabilizing the oxidative addition products of hydroacylation. These rhodium(III) intermediates are known to undergo decarbonylation, which ultimately leads to catalyst decomposition. We were interested in whether the Lewis acidic ligand frameworks could stabilize these intermediates to allow for the development of hydroacylation catalysts. These fundamental studies will lead to the development of new catalyst systems bearing cooperative ligand motifs.
Nichols, Brian R., "Late-Transition Metal Complexes with Ambiphilic Phosphinoborane Ligands: Synthesis, Characterization and Catalysis." (2018). Graduate Theses, Dissertations, and Problem Reports. 8188.