From N-heterocyclic imine to N-heterocyclic boryloxy derived main group species

In this thesis titled “From N-Heterocyclic Imine to N-Heterocyclic Boryloxy Derived Main Group Species”, we present in a series of four chapters, an overarching theme regarding the transformation of the well-known N-heterocyclic imine (NHI) ligand to its isoelectronic N-heterocyclic boryloxy (NHBO) counterpart, and their employment to tame unprecedented main group species.
Chapter One
Chapter one represents an introduction to ligands based around the N-heterocyclic carbenes (NHCs) framework. Carbenes were once considered as laboratory curiosities. Since the introduction of the ‘bottleable’ NHCs, they have been elevated to compounds of immense practical importance. While the NHCs have been widely regarded as privileged ligands, their strong nucleophilicity and huge steric profile can be transferred via the carbene centre to an appended p block element. This allows the design of new NHC derived ligands such as the N-heterocyclic imines (NHIs) and N-heterocyclic olefins (NHOs). The use of NHIs and NHOs to stabilize unprecedented main group species are described. Furthermore, such a concept can be extended to the N-heterocyclic boryl (NHB) anions, which are isoelectronic group 13 carbene analogues. NHB derived systems, such as the N-heterocyclic borylamino (NHBA) ligands can be designed, and their applications towards group 14 systems are documented.
Chapter Two
Carbonyl cations are among the most commonly invoked reactive intermediates in organic synthesis. While Olah pioneered superacids to provide a “stable ion” environment for their study in situ, isolated examples are rare. Here, we disclose successive protonation of an NHI derived carbonyl compound (IDippN)2CO, to the monocation [(IDippN)(IDippNH)CO]+, and the doubly protonated dication [(IDippNH)2CO]2+. [(IDippN)(IDippNH)CO]+ represents a rare example of an N-protonated carbonyl cation and [(IDippNH)2CO]2+ the first example of a superelectrophilic carbonyl dication. All three compounds have been characterized by X-ray crystallography and IR spectroscopy, revealing stepwise strengthening of the C=O bond on protonation. The unique stability of these systems is attributed to the enhanced basicity and steric profile provided by the NHI substituents. In addition, we report the related singly NHI-stabilized cation [IDippNCO]+. Crystallographic and DFT analyses provide insight into the interaction between the carbonyl fragment and the NHI, which reveals that the [CNCO]+ unit (isoelectronic to CCCO) can be described as an acylium ion masked as a cumulene. We further describe complete oxide (O2−) ion abstraction of (IDippN)2CO to generate the first linear carbodication, [(IDippN)C(NIDipp)]2+, containing a cumulative [CNCNC]2+ unit, and thus could be described as a carbodication masked as a cumulene, based on crystallographic and DFT analyses.
Chapter Three
We introduce a new type of strongly donating N-heterocyclic boryloxy (NHBO) ligand, [(HCDippN)2BO]−, which is isoelectronic with the well-known NHI ligand. This 1,3,2-diazaborole functionalized oxy ligand can be employed to stabilize the first acyclic two-coordinate dioxysilylene and its Ge, Sn and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X-ray crystallography and DFT, enabling analysis of periodic trends: the potential for the [(HCDippN)2BO]− ligand to subtly vary its electronic donor capabilities is illustrated via snapshots revealing gradual evolution of arene p coordination on going from Si to Pb.
Chapter Four
We disclose the synthesis and structural characterization of the first acid-free anionic oxoborane, [K(2.2.2-crypt)][(HCDippN)2BO], which is isoelectronic with classical carbonyl compounds. The oxoborane can readily be accessed from its borinic acid by a simple deprotonation/sequestration sequence. Crystallographic and DFT analyses support the presence of a polarized terminal B=O double bond. Subsequent p bond metathesis converts the B=O bond to a heavier B=S containing system, affording the first anionic thioxoborane [K(2.2.2-crypt)][(HCDippN)2BS], isoelectronic with thiocarbonyls. Facile B=O bond cleavage can also be achieved to access B−H and B−Cl bonds, and via a remarkable oxide (O2−) ion abstraction to generate a borenium cation [(HCDippN)2B(NC5H5)][OTf]. By extension, the oxoborane can act as an oxide transfer agent to organic substrates, a synthetic role traditionally associated with transition metal compounds. Hence we show that B−O linkages, which are often considered to be thermodynamic sinks, can be activated under mild conditions towards bond cleavage and transfer, by exploiting the higher reactivity inherent in the B=O double bond.

Advisor: Aldridge, S