Terrance J. Hadlington

Low coordinate germanium(II) and tin(II) hydride complexes: efficient catalysts for the hydroboration of carbonyl compounds.

This study details the first use of well-defined low-valent p-block metal hydrides as catalysts in organic synthesis. That is, the bulky, two-coordinate germanium(II) and tin(II) hydride complexes, L(†)(H)M: (M = Ge or Sn, L(†) = -N(Ar(†))(SiPr(i)3), Ar(†) = C6H2{C(H)Ph2}2Pr(i)-2,6,4), are shown to act as efficient catalysts for the hydroboration (with HBpin, pin = pinacolato) of a variety of unactivated, and sometimes very bulky, carbonyl compounds. Catalyst loadings as low as 0.05 mol % are required to achieve quantitative conversions, with turnover frequencies in excess of 13u2009300 h(-1) in some cases. This activity rivals that of currently available catalysts used for such reactions.

A singly bonded amido-distannyne: H2 activation and isocyanide coordination

The first amido-distannyne, L(†)SnSnL(†) (L(†) = -N(Ar(†))(SiPr(i)3), Ar(†) = C6H2{C(H)Ph2}2Pr(i)-2,6,4), has been prepared and shown to possess a very long Sn-Sn single bond; the compound activates dihydrogen under ambient conditions to give L(†)Sn(μ-H)2SnL(†), and reacts with Bu(t)NC to give a stable adduct complex, L(†)(Bu(t)NC)SnSn(CNBu(t))L(†).

Extremely Bulky Amido and Amidinato Complexes of Boron and Aluminium Halides: Synthesis and Reduction Studies

An extremely bulky secondary amine, HN(Ar†)(SiPr3i) HL† (Ar†u2009=u2009C6H2{C(H)Ph2}2Pri-2,6,4) has been synthesised and deprotonated with KH in toluene, to afford the potassium amide [KL†(η6-toluene)], which was structurally authenticated. Reaction of this with BBr3 and AlBr3, reproducibly gave the crystallographically characterised amido bromo-borane, [L†B(H)Br], and aluminacycle, [AlBr2{κ2-C,N-N(H)(SiPr3i){C6H2[CPh2][C(H)Ph2]Pri-2,6,4}}], respectively, via ligand C–H activation processes. The known secondary amines, HN(Dip)(Mes) (HLMes) and HN(Dip)(Trip) (HLTrip) (Dipu2009=2,6-diisopropylphenyl, Mesu2009=u2009mesityl, Tripu2009=u20092,4,6-triisopropylphenyl), have been structurally characterised, and deprotonated to give the in situ generated lithium amides, [Li(LMes)] and [Li(LTrip)]. Reaction of these with BBr3 and AlBr3 has given the amido group 13 element halide complexes, [LMesBBr2] and [LAlBr2(THF)] (Lu2009=u2009LMes or LTrip), the crystal structures of all of which have been determined. Synthetic routes to two new bulky amidine pro-ligands, ArNu2009=u2009C(But)-N(H)Ar, Aru2009=u2009C6H2{C(H)Ph2}2Me-2,6,4 (Piso*H) or C6H2Pr2i(CPh3)-2,6,4 (Piso″H), have been developed, and the compounds crystallographically characterised. Deprotonation of Piso″H gave the potassium amidinate, [K(Piso″)], which was reacted with BBr3 to give [(Piso″)BBr2]. Reaction of Piso″H with AlMe3 afforded [(Piso″)AlMe2], which, when treated with I2 yielded [(Piso″)AlI2], the crystal structure of which was determined. Reductions of all of the prepared amido and amidinato group 13 element(iii) halide complexes were attempted using a variety of reducing reagents, with a view to prepare boron(i) or aluminium(i) complexes. While these were not successful, this study does offer synthetic inorganic chemists a variety of new very bulky anionic N-donor ligands, and boron/aluminium halide complexes thereof, for use in their own research.

Synthesis of a Metallo-Iminosilane via a Silanone-Metal π-Complex

Facile oxygenation of the acyclic amido-chlorosilylene bis(N-heterocyclic carbene) Ni0 complex [{N(Dipp)(SiMe3 )ClSi:→Ni(NHC)2 ] (1; Dipp=2,6-i Pr2 C6 H4 ; N-heterocyclic carbene=C[(i Pr)NC(Me)]2 ) with N2 O furnishes the first Si-metalated iminosilane, [DippN=Si(OSiMe3 )Ni(Cl)(NHC)2 ] (3), in a rearrangement cascade. Markedly, the formation of 3 proceeds via the silanone (Si=O)-Ni π-complex 2 as the initial product, which was predicted by DFT calculations and observed spectroscopically. The Si=O and Si=N moieties in 2 and 3, respectively, show remarkable hydroboration reactivity towards H-B bonds of boranes, in the former case corroborating the proposed formation of a (Si=O)-Ni π-complex at low temperature.

Synthesis and crystal structures of two bulky bis(amido)germylenes

Abstract A new bulky secondary amine, HN(Dip){C(H)Ph2} (Dip=C6H3Pri2-2,6), has been prepared. This and a related amine, HN(Dip)(Mes) (Mes=mesityl), have been utilized in the formation of bulky lithium amides, which when added to a source of GeCl2 have yielded two new, bulky bis(amido)germylenes, [Ge{N(Dip)(Mes)}2] and [Ge{N(Dip)[C(H)Ph2]}2], both of which have been crystallographically characterized and shown to be two-coordinate monomers.

Stoichiometric Reactivity and Catalytic Applications of Heavier Tetrylene Derivatives

Whilst the spotlight has been on the synthesis and reactivity of the heavier alkyne analogues (LEEL, E = Si–Pb) in regards to low-oxidation state group 14 chemistry, the reactivity of heavier carbene analogues (L2E:) has also seen considerable interest. Specifically, small-molecule activation in the context of catalysis has seen much attention. Advances in the area of small-molecule activation by group 14 element(II) complexes will be discussed herein, followed by our efforts in this regard toward their applications in efficient, well defined catalytic regimes.

The Use of a Bulky Boryl-Substituted Amide Ligand in Low-Oxidation State Group 14 Element Chemistry

This chapter describes the synthesis and application of a novel boryl silyl amide ligand in low-oxidation state, low-coordinate group 14 chemistry. This is, in part, related to chmistry discussed in previous chapters: the synthesis of group 14 halide complexes using monodentate ligands (Chap. 2), the synthesis and reactivity of heavier alkyne analogues (Chap. 3), and the synthesis and reactivity of heavier carbene analogues (Chaps. 4 and 5). The application of this novel ligand system, however, has allowed for the isolation of the first acyclic bis(amido) silylene, tin(II) catalysed ketone hydrosilylation, and related significant discoveries, truly highlighting the importance of ligand design in such fundamental chemistry research.

Reactivity of Low-Coordinate Group 14 Element(II) Hydride Complexes

The addition of element-hydrogen bonds across unsaturations (i.e. hydroelementation) is of paramount importance in organic synthesis. The application of group 14 element-hydrogen (E–H) bonds in this regard, however, has largely relied upon transition-metal (TM) catalysts or radical mechanisms. Recent developments in the synthesis of low-valent group 14 element hydride species has allowed for such reactivity in the absence of a catalyst or initiator. This chemistry will be dicussed in this chapter, leading to our research which has taken this a step further, involving facile E-H bond addtion to unactivated unsaturates such as alkenes, in some cases reversibly. Such reactivity is implicit in countless known catalytic cycles.

Synthesis and Reactivity of Heavier Alkyne Analogues Stabilised by Extremely Bulky Amide Ligands

Over recent decades, numerous landmark discoveries in the area of main-group chemistry have been realised. Of most relevance to this thesis are the advances that have been made in regards to complexes incorporating the heavier group 14 elements in unusual co-ordination environments and oxidation states. This work has led us to a far greater understanding of discrepancies between the chemistries of these heavier elements and carbon. This chapter discusses these recent advances, and our more recent efforts towards this end, specifically regarding the heavier alkyne analogues, LEEL (L = a bulky ligand; E = Si-Pb).

The Development of Extremely Bulky Amide Ligands and Their Application to the Synthesis of Group 14 Element(II) Halide Complexes

As described in Chap. 1, the isolation of some of the first examples of low-oxidation state main-group (MG) complexes has involved the use of sterically demanding and poly-dentate ligand systems, which kinetically stabilise reactive element centres. Two main classes of mono-anionic ligand have been successful applied in such MG chemistry: chelating ligands and mono-dentate ligands. Here, some of the more important examples of such ligands will be discussed, as will their use in the synthesis of group 14 element halide complexes, alongside our efforts in this regard.