Triptycene Derivatives: From Their Synthesis to Their Unique Properties

Since the first preparation of triptycene, great progress has been made with respect to its synthesis and the understanding of its properties. Interest in triptycene-based systems is intense; in recent years, advances in the synthetic methodology and properties of new triptycenes have been reported by researchers from various fields of science. Here, an account of these new developments is given and placed in reference to earlier pivotal works that underpin the field. First, we discuss new approaches to the synthesis of new triptycenes. Progress in the regioselective synthesis of sterically demanding systems is discussed. The application of triptycenes in catalysis is also presented. Next, progress in the understanding of the relations between triptycene structures and their properties is discussed. The unique properties of triptycenes in the liquid and solid states are elaborated. Unique interactions, which involve triptycene molecular scaffolds, are presented. Molecular interactions within a triptycene unit, as well as between triptycenes or triptycenes and other molecules, are also evaluated. In particular, the summary of the synthesis and useful features will be helpful to researchers who are using triptycenes as building blocks in the chemical and materials sciences.

Dicoordinate Au(I)–Ethylene Complexes as Hydroamination Catalysts

A series of gold(I)–ethylene  complexes containing a family of bulky phosphine ligands has been prepared. The use of these sterically congested ligands is crucial to stabilize the gold(I)–ethylene bond and prevent decomposition, boosting up their catalytic performance in the highly underexplored hydroamination of ethylene. The precatalysts bearing the most sterically demanding phosphines showed excellent results reaching full conversion to the hydroaminated products under low ethylene pressure (1 bar). Kinetic analysis together with density functional theory (DFT) calculations revealed that the assistance of a second molecule of the nucleophile as a proton shuttle is preferred even when using an extremely congested cavity-shaped Au(I) complex.

Low Oxidation State Heavy P-Block Metal Compounds Supported by Di(amido) Chelating Ligands

<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>

Low Oxidation State Heavy P-Block Metal Compounds Supported by Di(amido) Chelating Ligands

<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>

Diversity of N-triphenylacetyl-L-tyrosine solvates with halogenated solvents

The structure of N-triphenylacetyl-L-tyrosine (C29H25NO4, L-TrCOTyr) is characterized by the presence of both donors and acceptors of classical hydrogen bonds. At the same time, the molecule contains a sterically demanding and hydrophobic trityl group capable of participating in π-electron interactions. Due to its large volume, the trityl group may favour the formation of structural voids in the crystals, which can be filled with guest molecules. In this article, we present the crystal structures of a series of N-triphenylacetyl-L-tyrosine solvates with chloroform, namely, L-TrCOTyr·CHCl3 (I) and L-TrCOTyr·1.5CHCl3 (III), and dichloromethane, namely, L-TrCOTyr·CH2Cl2 (II) and L-TrCOTyr·0.1CH2Cl2 (IV). To complement the topic, we also decided to use the racemic amide N-triphenylacetyl-DL-tyrosine (rac-TrCOTyr) and recrystallized it from a mixture of chloroform and dichloromethane. As a result, rac-TrCOTyr·1.5CHCl3 (V) was obtained. In the crystal structures, the amide molecules interact with each other via O—H...O hydrogen bonds. Noticeably, the amide N—H group does not participate in the formation of intermolecular hydrogen bonds. Channels are formed between the TrCOTyr molecules and these are filled with solvent molecules. Additionally, in the crystals of III and V, there are structural voids that are occupied by chloroform molecules. Structure analysis has shown that solvates I and II are isostructural. Upon loss of solvent, the solvates transform into the solvent-free form of TrCOTyr, as confirmed by thermogravimetric analysis, differential scanning calorimetry and powder X-ray diffraction.

Facile Synthesis and Redox Behavior of an Overcrowded Spirogermabifluorene

A spirogermabifluorene that bears sterically demanding 3,3′,5,5′-tetra(t-butyl)-2,2′-biphenylene groups (1) was obtained from the reaction of in-situ-generated 2,2′-dilithiobiphenylene with GeCl2·(dioxane). The solid-state structure and the redox behavior of 1 were examined by single-crystal X-ray diffraction analysis and electrochemical measurements, respectively. The sterically hindered biphenyl ligands endow 1 with high redox stability and increased electron affinity. The experimental observations were corroborated by theoretical DFT calculations.

Efficient Bulky Organo-Zinc Scorpionates for the Stereoselective Production of Poly(rac-lactide)s

The direct reaction of the highly sterically demanding acetamidinate-based NNN′-scorpionate protioligand Hphbptamd [Hphbptamd = N,N′-di-p-tolylbis(3,5-di-tertbutylpyrazole-1-yl)acetamidine] with one equiv. of ZnMe2 proceeds in high yield to the mononuclear alkyl zinc complex [ZnMe(κ3-phbptamd)] (1). Alternatively, the treatment of the corresponding lithium precursor [Li(phbptamd)(THF)] with ZnCl2 yielded the halide complex [ZnCl(κ3-phbptamd)] (2). The X-ray crystal structure of 1 confirmed unambiguously a mononuclear entity in these complexes, with the zinc centre arranged with a pseudotetrahedral environment and the scorpionate ligand in a κ3-coordination mode. Interestingly, the inexpensive, low-toxic and easily prepared complexes 1 and 2 resulted in highly efficient catalysts for the ring-opening polymerisation of lactides, a sustainable bio-resourced process industrially demanded. Thus, complex 1 behaved as a single-component robust initiator for the living and immortal ROP of rac-lactide under very mild conditions after a few hours, reaching a TOF value up to 5520 h−1 under bulk conditions. Preliminary kinetic studies revealed apparent zero-order dependence on monomer concentration in the absence of a cocatalyst. The PLA materials produced exhibited narrow dispersity values, good agreement between the experimental Mn values and monomer/benzyl alcohol ratios, as well as enhanced levels of heteroselectivity, reaching Ps values up to 0.74.