Geometry constrained main-group compounds for the activation of small molecules

• Main Author: Lo, Siu Kwan
• Other Authors: Goicoechea, Jose
• Published: University of Oxford 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.757879

A series of base-supported group 13 complexes bearing N,N-bis(3,5-di-tert-butyl-2-phenolate)amide pincer ligand, (ONO)^3-, are reported (E(ONO)·L (E = B, Al, Ga; L = Lewis base). B(ONO) type compounds were formed in-situ and are capable of THF activation. Alternatively, a NMe₃ stabilised boron compound was shown to activate the O-H bond of water and ethanol via frustrated Lewis pair (FLP) type reactivity. The aluminium analogue of E(ONO)·L displays a bimetallic structure in its molecular structure and was shown to be capable of O-H bond cleavage of ethanol across the Al-N bond. Finally, a neutral gallium hydride species, Ga(H){ON(H)O}, was syntheisised which is capable of activating aldehydes to form gallium alkoxide compounds. These alkoxide compounds could be employed as an active catalyst for the hydroboration with benzaldehyde. Group 14 geometrically constrained anionic compounds, [E(ONO)]^- (E = Ge, Sn), can be prepared via a two-step process. In the case of [Ge(ONO)]^-, it undergoes facile addition of water and ethanol to form trigonal bipyramidal germanium(IV) complexes. In the case of [Sn(ONO)]^-, it undergoes a transmetallation reaction with gallium chloride allowing to access a bimetallic {Ga(ONO)·THF}₂ complex. In addition, four novel neutral constrained geometry group 14 species bearing a xanthene based ligand framework, E(NON), are also reported. The LUMOs of such species are chemically accessible using KO^tBu. The arsenic analogue, As(ONO), was prepared and shown that it exhibits no reactivity towards ammonia in contrast to its phosphorus analogue which gives rise to a phosphorus(V) complex, highlighting the increased stability of the +3 oxidation state on descending group 15, and the difficulty associated with accessing arsenic(V) complexes. The HOMO and LUMO of As(ONO) were accessible by KNu (Nu = O^tBu and NPh₂) and ROTf (R = H, Me), respectively. In addition, we report the synthesis of a phosphorus(III) complex bearing the 2,2'-[iminobis(methylene)]-bis[4-methoxy-phenol] ligand, (P(ONO*)), which undergoes hydrolysis reactions. The lone pair on phosphorus was accessible using borane to afford Lewis acid-base adducts.