Date of Award
Open Access Dissertation
Chemistry and Biochemistry
College of Arts and Sciences
In organometallic chemistry, boron-centered ligand systems represent a powerful synthetic strategy for the promotion of a variety of catalytic transformations. These ligands offer a versatile array of bonding motifs, which can function as borane, boratrane, boryl, borate, or borylenes type ligands, and are likely the strongest σ-donors in the anionic series of ligands based on boron, carbon, nitrogen, and oxygen. Carboranes, which are icosahedral boron clusters of the type C2B10H12, offer unique potential as metal ligands, such as their bonding to exohedral metal centers being dominated by two-center-two-electron σ-bond interactions, their three-dimensional cluster structure providing considerable steric shielding to a metal center, and the ability to have up to five vicinal vertex-metal interactions. Such interactions can potentially stabilize low-coordinate metal center configurations, isolate intermediates of catalytic transformations, and serve as a scaffold for multimetallic architectures which can lead to metal-metal and ligand-metal cooperative reactivity.
In this dissertation, our experimental efforts in utilizing POBOP-H (POBOP = 1,7-OP(i-Pr)2-m-carboranyl as a carborane backbone pincer ligand is detailed. In the preparation of its rhodium complexes, for example, (POBOP)Rh(I)(Ph) is formed. This is the first example of a B-carboranyl metal aryl complex, which is a likely intermediate in B-C coupling interactions. In the presence of acetonitrile, this complex cleanly converts to (POB(BPh)OP)Rh(H)(I)(CH3CN), which proceeds through an unprecedented cascade reductive elimination/oxidative addition process where the B-carboranyl and phenyl group reductively eliminate to form a new B-Ph group. Subsequently, the vicinal B-H bond of the boron cluster is activated to form a new B-carboranyl bond and a new rhodium hydride. This is also the first example of the stereoselective mono-B-phenylation of carborane.
Metalation of the POBOP pincer ligand framework with ruthenium led to the synthesis of complexes featuring exohedral metal-boron bonds, bridging vicinal B-H···Ru three-center-two-electron interactions, and metal-hydride functionalities. (POBOP)Ru(H)(PPh3) was found to exhibit an unprecedented rapid intramolecular boryl to borane exchange between the metal bound hydride and the vicinal B-H bond of the carborane cluster. This proceeds through the shuttling of the metal center between two adjacent boron atoms at temperatures above -50 °C. (POBOP)Ru(H)(PPh3) can also be considered as an isolated intermediate of metal promoted B-H bond activations which helps demonstrate the observation of various regioisomerizations that occur in metal promoted carborane functionalizations.
The chelating (POBOP)H pincer framework has also been shown to enforce close contact of a metal center to multiple cage vertices. This was realized by the reaction of the POBOP ligand with a ruthenium carbonyl precursor. This is the first example of a transition metal (BB)-carboryne complex which contains two borons connected to a single exohedral metal. The reactivity of the highly strained B-Ru bonds showed that they themselves acted as nucleophilic reaction centers with a variety of electrophiles (e.g. HCl, I2, HCCCO2Et, EtCCEt, and BH3SMe2) displaying ligand- and metal-centered reactivity.
Eleazer, B. J.(2018). Synthesis And Reactivity Of Carborane Backbone Pincer Complexes. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/4912