Low Oxidation State Heavy P-Block Metal Compounds Supported by Di(amido) Chelating Ligands
Authors:   , Schwamm Ryan 
Published: November 2021

<p>The work presented in this thesis describes the synthesis and stabilisation of heavy p-block elements (defined herein as being those with 5s/p and 6s/p valence electrons) in low oxidation states using sterically demanding ligands based on a di(amido)siloxane framework ([(O{SiMe2N(R)}2]2-, abbrev. [(NONR)]2-).  Chapter 1 gives a general introduction to the heavy p-block elements and discusses a number of concepts that define the molecular chemistry of these elements. A brief introduction into low oxidation state main group chemistry is provided and the importance of sterically demanding ligands in this field of research is introduced. The di(amido)siloxane ligand framework utilised in this work is introduced, with common coordination modes and characteristic properties discussed.  Chapter 2 discusses the chemistry of low oxidation state bismuth complexes and follows a recent report by our group on the first structurally authenticated bismuth(II) radical •Bi(NONAr). The synthesis of a series of bismuth(III) monochloride species Bi(NONR)Cl (R = tBu, Ph, 2,6-Me2C6H3 (Ar’), 2,6-iPr2C6H3 (Ar) and 2,6-(CHPh2)2-4-tBu-C6H2 (Ar‡)) is discussed, and the steric properties of the ligand systems evaluated. In the case of the R = tBu and Ar‡ derivatives, reduction of the bismuth(III) monochloride gave the dibismuthane [Bi(NONtBu)]2 and bismuth(II) radical •Bi(NONAr‡), respectively. Further reduction of the bismuth centres resulted in the formation of rare and unprecedented multimetallic bismuth compounds containing [Bin]n+ cores. These include the Bi4 cluster compound Bi4(NONAr)2, in which the bismuth atoms exist in an unprecedented mixed valent arrangement and may be assigned oxidation states of 0, +1 or +2, and the tribismuthane cluster [Bi3(NONtBu)2]-, which features the first structurally characterised Bi3 chain. The utility of the di(amido) ligand plays a key role in the formation of many of these compounds, with Bi-N bond cleavage suggested to be a key step in many of the reaction pathways.  Chapter 3 discusses the reactivity of the bismuth(II) complexes [Bi(NONtBu)]2, •Bi(NONAr) and •Bi(NONAr‡) which feature either a Bi-Bi bond or a bismuth-centred radical. Initial experiments parallel reported reactivity with halogen radical sources (N-bromosuccinimide or iodine), chalcogens (S, Se, Te) and the stable nitroxyl radical (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), resulting in oxidative addition to generate bismuth(III) complexes. In the latter case, the isolated reaction products, Bi(NONR)(OTEMP), were used to access the catalytic coupling of TEMPO and phenylsilane. Subsequent investigations into the reactivity of the bismuth(II) species revealed the selective activation of white phosphorus (P4) and terminal aromatic alkynes by •Bi(NONAr), generating the bismuth(III) complexes [Bi(NONAr)]2(P4) and [Bi(NONR)]2(HC=C(C6H4-4-X)), respectively. In both cases, a temperature dependent equilibrium is observed. In contrast, the dibismuthane [Bi(NONtBu)]2 and more encumbered bismuth radical •Bi(NONAr‡) do not react with these substrates, demonstrating the importance of the nature of the bismuth centre (i.e. dibismuthane vs. bismuth radical) and ligand bulk on the reactivity of these systems.  Chapter 4 describes the synthesis and characterisation of a series of low oxidation state antimony compounds. A series of distibanes supported by the (NONR)-framework were prepared from the reaction of antimony(III) chloride species Sb(NONR)Cl with magnesium(I) reducing agents [(BDIAr§)Mg]2 (Ar§ = 2,4,6-Me3C6H3 or Ar). When R = tBu, Ph or 2,6-Me2C6H3 (Ar’), a distibane [Sb(NONR)]2 is obtained, featuring a Sb-Sb single bond. While the tBu and Ph derivatives contained typical Sb-Sb single bonds, the bonding in the Ar’ derivative is elongated, significantly longer than in all other reported distibanes. The weakness of this bond is highlighted in a reaction with P4, which shows activation of the P4 tetrahedron and P-P bond cleavage. In contrast, reduction of the bulkier Ar derivative (Ar = 2,6-iPr2C6H3) with the magnesium(I) reagents results in formation of the distibene [Sb(NONR)Mg(BDIAr§)]2, featuring a Sb=Sb bond.  Chapter 5 describes the synthesis and characterisation of low oxidation state indium compounds supported by the (NONAr)-ligand. A number of indium(III) chloride species supported by either the (NONAr)-ligand or the retro-Brook rearranged (NNOAr)-ligand (NNOAr = [RN{Me2SiO}{Me2SiN(R)}) were synthesised. In all cases, an equivalent of lithium chloride was retained in the molecular structure, allowing isolation of the indate complexes In(NONAr)(μ-Cl)2Li(Et2O)2, [Li(THF)4][In(NONAr)Cl2] and In(NNOAr.Li(THF)3)Cl2. Attempts to reduce these complexes using a hydride source were unsuccessful, instead yielding the corresponding indium(III) hydride species [Li(THF)4][In(NONAr)H2] and In(NNOAr.Li(THF)3)H2, respectively. Reduction of the (NONAr)-supported indium(III) chloride complexes using alkali reducing agents allowed access to the diindane [In(NONAr)]2, featuring an In-In single bond, and the first example of an anionic N-heterocyclic indene. The latter species is isovalent with N-heterocyclic carbenes and is a potential pre-cursor for indium-metal bonding formation. In addition, this compound is of interest as a source of nucleophilic indium.  Finally, Chapter 6 provides a summary of the results presented in this thesis and a brief overview of the future direction of this field of research.</p>