Studies of Two- and Three-Coordinate Boron Cations

The reactivity of dicoordinate boron (borinium) cations is largely unexplored. Following the introductory chapter, four research chapters centre on homoleptic bis(amido)borinium and borenium complexes. In Chapter 2, [(iPr2N)2B][B(C6F5)4] was prepared and proved much more electrophilic than the well-studied B(C6F5)3. This cation rapidly hydroborated alkyne, nitrile, ketone, or diazomethane substrate despite there being no evidence of any boron-hydride species. Computational studies suggest a Meerwein–Ponndorf–Verley reduction-like mechanism via hydride migration from a diisopropylamide ligand. Chapter 3 reports the synthesis of fluoroboranes FB(N(SiMe3)R) (R = SiMe3, tBu, C6F5, o-tol, Mes, Dipp), which showed diverging reactivity upon treatment with [Et3Si][B(C6F5)4]. When R = SiMe3 or tBu, fluoride abstraction led to borinium salt formation. Phosphines abstracted [SiMe3]+ and H+ from [((Me3Si)2N)2B]+, yielding [Me3SiPR’3][B(C6F5)4] and [HPR’3][B(C6F5)4] (R’ = Me, tBu); the adduct of [((Me3Si)2N)2BPMe3]+ was also observed. With the bis(anilido)fluoroboranes, [Et3Si]+ instead abstracted Me-, leading to complex mixtures. When R = Dipp, the unusual salt [(μ-F)(SiMe2N(Dipp))2BMe][B(C6F5)4] was isolated. Chapter 4 describes the chelate complexes FB(RNSiMe2CH2)2 (R = Dipp, (1-Ad)). 19F{nB} NMR spectroscopy revealed that 19F chemical shifts changed for n = 10 or 11. [B(DippNSiMe2CH2)2]+ was characterized in situ and is a proposed intermediate in the reduction of FB(DippNSiMe2CH2)2 to HB(DippNSiMe2CH2)2 with HSiEt3. The chelate-borinium cation [B((1 Ad)NSiMe2CH2)2][B(C6F5)4] was isolated and crystallographically characterized. The final research chapter models geometrically constrained “borinium” complexes binding to anionic ligands containing NN and NP fragments. XB(DippNCH)2 and XB(DippNCH2)2 (X = NH2, NHNH2) were prepared. Following protonation, the amides were readily substituted by the conjugate base of a strong acid, but hydrazides were not. The congeners Cl2PNHB(DippNCH)2 and XClPN(SiMe3)B(DippNCH)2 (X = Cl, O3SCF3) were prepared. XSiMe3 elimination yielded (ClPNB(DippNCH)2)2. NH2NHB(DippNCH2)2, Cl2PN(SiMe3)B(DippNCH)2 and (ClPNB(DippNCH)2)2 were characterized crystallographically. The enclosed thesis represents the most comprehensive work to date on the reactions of bis(amido)borinium cations.