Date of Award

Spring 2022

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Dmitry V. Peryshkov

Abstract

Exploration of transition metal complexes with metal-boron bonds for the cooperative bond activation of small molecules in catalysis is one of the recent developments in novel synthetic strategies. The use of boron clusters as a modular platform for the formation of metal-boron bond has been successfully utilized by our and other groups. Carboranes are icosahedral boron-carbon molecular clusters that have gained attraction as multifunctional ligands due to their unique geometry, electronic properties, and versatility of functionalization methods. Their 3-D structure exerts a steric bulk around the exohedral substituent, either metal or non-metal center. Furthermore, due to the cluster geometry, five of the cluster atoms are always in proximity with the exohedral metal center vertex resulting in the possibility of 3-center-2-electron B–H…M bridging interactions that support the 2-center-2-electron metal-boryl -bond. The use of rigid tridentate pincer ligands with carborane backbone and heteroatom donor groups allowed the synthesis of transition metal B-carboranyl complexes through the activation of relatively inert cluster B–H bonds. The unique geometry of the cluster with the pincer donor arms has resulted the highly strained, electron rich metal-boron bonds that have the potential to act as nucleophilic reaction centers in the bond activation of small molecules.

Transition metal benzyne complexes have been intensively studied due to their ability of small molecule activation. The benzyne chemistry inspired the synthesis of the transition metal carboryne complexes, which can be considered as 3-D inorganic boron-based analogues of benzynes, in many cases similar reactivity. The highly strained and electron-rich metal-boron bonds in the three-membered (BB)>Ru metallacycle can act as nucleophilic reaction centers in bond activation of small molecules.

In this dissertation, a novel synthetic stratergy was employed in synthesizing (BB)>Ru carboryne complexes via neutral ruthenium carborane dichloride complex as an intermediate, which allowed to install several classes of auxiliary ligands in trans- position to boryls: nitriles and phosphines. The replacement of strongly bound π-accepting carbonyls by these predominantly σ-donating and more labile ligands opened several new reactivity manifolds. New examples of the reactivity of the ruthenium carboryne bis(benzonitrile) complex in C-C bond activation of terminal alkynes, C-H bond activation of the phenyl group, N=N bond activation of azides and C-H and C-C bond activation of aldehydes wasstudied, in addition to the unique reactivity observed in ruthenium carboryne bis(diphenylphosphine) complex for the aldehydes.

In addition, the presence of an exohedral 2-center-2-electron B-Ru bond and a 3- center-2-electron B-H…Ru bond in the same metal complex with the metal hydride resulted in catalytic activity in the transfer hydrogenation reaction. A series of boryl hydride complexes (POBOP)Ru(H)(L) [L=PPh3, PHPh2, PEt3] were synthesized, which were used as a platform for the assembly of heterometallic complexes.

Functionalization of the carbon vertex of the carborane cluster is carried out using alkali metal-containing strongly basic reagents to generate carbon-centered nucleophiles. The careful utilization of metal chelating ligands to capture and remove the metal center from the metalated carbon vertex of the cluster generated strong carbon-centered nucleophiles with uncompensated negative charge on the carbon atom in these separated ion pairs. The alkali metal cation was separated from the deprotonated cluster using metalchelating crown ethers, which allowed us for the first time to isolate, structurally and spectroscopically characterize the highly nucleophilic “naked” 3-D carborane carbanion. Analogously, the doubly deprotonated 1,1′-bis(o-carborane) yielded similar dianionic “naked” carbanionic species serving as the first example of carbdianion.

Rights

© 2022, Hindurangalage Don Asela Chathumal Jayaweera

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