Stereoselective Access to Diverse Alkaloid-Like Scaffolds via an Oxidation/Double-Mannich Reaction Sequence.

Sequential oxidative cleavage and double-Mannich reactions enable the stereoselective conversion of simple norbornenes into complex alkaloid-like structures. The products undergo a wide range of derivatization reactions, including regioselective enol triflate formation/cross-coupling sequences and highly efficient conversion to an unusual tricyclic 8,5,5-fused lactam. Overall, the process represents a formal one-atom aza-ring expansion with concomitant bridging annulation, making it of interest for the broader derivatization of alkene feedstocks.

Synthesis, structure and stereodynamics of atropisomeric N-chloroamides.

Atropisomeric N-chloroamides were efficiently accessed by electrophilic halogenation of ortho-substituted secondary anilides. The stereodynamics of atropisomerism in these novel scaffolds was interrogated by detailed experimental and computational studies, revealing that racemization is correlated with amide isomerization. The stereoelectronic nature of the amide was shown to significantly influence racemization rates, with potentially important implications for other C-N atropisomeric scaffolds.

Lithium, sodium and potassium enolate aggregates and monomers: syntheses and structures.

In this Article, we report the syntheses and comparative structural studies of lithium, sodium, and potassium anthracen-9-yl enolates, as their aggregates (Li, Na: hexamer; K: tetramer) and ligand-stabilized monomers (for Li and Na). The monomers add new members to the rare collection of group-1 metal monomeric enolates. Moreover, the series covers different group-1 metal cations (Li+, Na+ and K+) and aggregate sizes, allowing comparative structural studies to elucidate how the metal identity and aggregate size influence the enolate structure.

A State of Independents: Rationalizing the High Z' Crystal Structures of Shikimate Esters.

Z' is a parameter used to denote the number of symmetry-independent molecules in the asymmetric unit of a crystal structure. High Z' (>1) crystal structures are relatively uncommon and are thought to arise through competition between intermolecular interactions of similar strength. As such high Z' crystal structures are challenging to predict and new examples are valuable in improving understanding in the field. Herein, we report the X-ray crystal structures of a series of shikimate esters, the asymmetric units of which exhibit high Z' values. Of special interest is the crystal structure of methyl shikimate, the asymmetric unit of which comprises 12 independent molecules; Z' = 12. This uncommonly large Z' value arises through a combination of factors, including the intrinsic homochirality of the molecule, the conformational inflexibility of the cyclohexene ring, the presence of multiple hydrogen bonding motifs, and both the cis- and trans-conformers of the ester moiety. Comparison of the X-ray crystal structures of shikimic acid, methyl shikimate, ethyl shikimate, and iso-propyl shikimate suggests that instances of high Z' in this series correlate with specific hydrogen bonding motifs influenced by the steric bulk of the ester. The results of this study provide important insights into factors that influence the formation of organic crystal structures where the value of Z' is greater than 1.

Improving the Conductivity of Amide-Based Small Molecules through Enhanced Molecular Packing and Their Application as Hole Transport Mediators in Perovskite Solar Cells.

Organic-inorganic hybrid halide perovskite solar cells (PSCs) have attracted substantial attention from the photovoltaic research community, with the power conversion efficiency (PCE) already exceeding 26%. Current state-of-the-art devices rely on Spiro-OMeTAD as the hole-transporting material (HTM); however, Spiro-OMeTAD is costly due to its complicated synthesis and expensive product purification, while its low conductivity ultimately limits the achievable device efficiency. In this work, we build upon our recently introduced family of low-cost amide-based small molecules and introduce a molecule (termed TPABT) that results in high conductivity values (∼10-5 S cm-1 upon addition of standard ionic additives), outperforming our previous amide-based material (EDOT-Amide-TPA, ∼10-6 S cm-1) while only costing an estimated $5/g. We ascribe the increased optoelectronic properties to favorable molecular packing, as shown by single-crystal X-ray diffraction, which results in close spacing between the triphenylamine blocks. This, in turn, results in a short hole-hopping distance between molecules and therefore good mobility and conductivity. In addition, TPABT exhibits a higher bandgap and is as a result more transparent in the visible range of the solar spectrum, leading to lower parasitic absorption losses than Spiro-OMeTAD, and has increased moisture stability. We applied the molecule in perovskite solar cells and obtained good efficiency values in the ∼15% range. Our approach shows that engineering better molecular packing may be the key to developing high-efficiency, low-cost HTMs for perovskite solar cells.

Vesicles, fibres, films and crystals: A low-molecular-weight-gelator [Au(6-thioguanosine)2]Cl which exhibits a co-operative anion-induced transition from vesicles to a fibrous metallo-hydrogel.

We describe a simple coordination compound of Au(I) and 6-thioguanosine, [Au(6-tGH)2]Cl, that has a rich self-assembly chemistry. In aqueous solution, the discrete complex assembles into a supramolecular fibre and forms a luminescent hydrogel at concentrations above about 1 mM. Below this concentration, the macromolecular structure is a vesicle. Through appropriate control of the solvent polarity, the gel can be turned into a lamellar film or crystallised. The molecular structure of [Au(6-tGH)2]Cl was determined using single crystal X-ray diffraction, which showed bis-6-thioguanosine linearly coordinated through the thione moiety to a central Au(I) ion. In the vesicles, the photoluminescence spectrum shows a broad, weak band at 550 nm owing to aurophilic interactions. Co-operative self-assembly from vesicle to fibre is made possible through halogen hydrogen bonding interactions and the aurophilic interactions are lost, resulting in a strong photoluminescence band at 490 nm with vibronic structure typical of an intraligand transition. The vesicle-fibre transition is also revealed by a large increase of ellipticity in the circular dichroism spectrum with a prominent peak near 390 nm owing to the helical structure of the fibres. Atomic force microscopy shows that at the same time as fibres form, the sample gels. Imaging near the vesicle-fibre transition shows that the fibres form between vesicles and a mechanism for the transition based on vesicle collisions is proposed.

Reduction of K+ or Li+ in the Heterobimetallic Electride K+[LiN(SiMe3)2]e.

Given their very negative redox potential (e.g., Li+ → Li(0), -3.04 V; K+ → K(0), -2.93 V), chemical reduction of Group-1 metal cations is one of the biggest challenges in inorganic chemistry: they are widely accepted as irreducible in the synthetic chemistry regime. Their reduction usually requires harsh electrochemical conditions. Herein we suggest a new strategy: via a heterobimetallic electride intermediate and using the nonbinding "free" electron as reductant. Based on our previously reported K+[LiN(SiMe3)2]e- heterobimetallic electride, we demonstrate the reducibility of both K+ and Li+ cations. Moreover, we find that external Lewis base ligands, namely tris[2-(dimethylamino)ethyl]amine (Me6Tren) or 2,2,2-cryptand, can exert a level of reducing selectivity by preferably binding to Li+ (Me6Tren) or K+ (2,2,2-cryptand), hence pushing the electron to the other cation.

Conformational study into N-alkyl-N'-aryl ureas to inform drug discovery.

Ureas are an important functional group in small molecule drugs as well as having wider applications in organic chemistry. Understanding of their conformation is of critical importance for rational design of urea-containing bioactive compounds. Whilst the conformational preferences of biaryl ureas have been extensively studied, very little attention has been paid to alkylated analogues. We carried out a systematic study of N-aryl (phenyl and pyridyl)-N'-cyclopentyl ureas with differing N-methylation patterns using Well Tempered Metadynamics at a semi-empirical level in implicit water (GBSA) using Well-Tempered Metadynamics to generate their conformational free-energy landscapes. Geometries and energetics of the most relevant configurations are further refined using DFT level of theory. Validation for the computation was obtained by synthesis of all 8 analogues followed by conformational studies by X-ray crystallography and NMR. These findings reveal that the methylation pattern significantly affects the conformational preference of the system. Most notably, N-phenyl-N'-cyclopentyl urea is shown to adopt both the trans-trans, and cis-trans conformations with equal energy and that the cis-trans conformation can be significantly stabilised by the presence of an internal hydrogen bond to the N'-hydrogen. This study will be of utility for the design of N-alkyl-N'-aryl ureas as drug candidates.

Stereoselective two-carbon ring expansion of allylic amines via electronic control of palladium-promoted equilibria.

General methodologies enabling the two-carbon homologation of pyrrolidine and piperidine systems have yet to be developed. Herein we report that palladium-catalysed allylic amine rearrangements enable efficient two-carbon ring expansion of 2-alkenyl pyrrolidine and piperidines to their azepane and azocane counterparts. Conditions are mild, tolerant of a range of functional groups and the process can occur with high enantioretention. The products formed undergo a range of orthogonal transformations, making them ideal scaffolds for the creation of compound libraries.

A thirty-year old mystery solved: identification of a new heptatungstate from non-aqueous solutions.

A new isopolyoxotungstate has been characterised, thirty years since the first spectroscopic evidence of its existence. The heptatungstate [W7O24H]5-, containing a {W5} lacunary Lindqvist unit fused to a ditungstate fragment, has significant stability and is only the third isopolytungstate structure to be obtained from non-aqueous systems.

A Window into the Workings of anti-B18H22 Luminescence-Blue-Fluorescent Isomeric Pair 3,3'-Cl2-B18H20 and 3,4'-Cl2-B18H20 (and Others).

The action of AlCl3 on room-temperature tetrachloromethane solutions of anti-B18H22 (1) results in a mixture of fluorescent isomers, 3,3'-Cl2-B18H20 (2) and 3,4'-Cl2-B18H20 (3), together isolated in a 76% yield. Compounds 2 and 3 are capable of the stable emission of blue light under UV-excitation. In addition, small amounts of other dichlorinated isomers, 4,4'-Cl2-B18H20 (4), 3,1'-Cl2-B18H20 (5), and 7,3'-Cl2-B18H20 (6) were isolated, along with blue-fluorescent monochlorinated derivatives, 3-Cl-B18H21 (7) and 4-Cl-B18H21 (8), and trichlorinated species 3,4,3'-Cl3-B18H19 (9) and 3,4,4'-Cl3-B18H19 (10). The molecular structures of these new chlorinated derivatives of octadecaborane are delineated, and the photophysics of some of these species are discussed in the context of the influence that chlorination bears on the luminescence of anti-B18H22. In particular, this study produces important information on the effect that the cluster position of these substitutions has on luminescence quantum yields and excited-state lifetimes.

Monomeric lithium and sodium silylbenzyl complexes: syntheses, structures, and CO bond olefination.

Herein we report the syntheses, structures and reactivity studies of two new monomeric alkali metal silylbenzyl complexes stabilised by a tetradentate amine ligand, tris[2-(dimethylamino)ethyl]amine (Me6Tren). The two complexes, namely [MR'(Me6Tren)] (R': CH(Ph)(SiMe3)) (2-Li: M = Li; 2-Na: M = Na), exhibit significant different coordination modes according to their metal identity (Li: σ-coordination; Na: π-coordination). Reactivity studies of 2-Li and 2-Na reveal that they are efficient in promoting a widely-used class of organic functional group interconversion: CO bond olefination of ketones, aldehydes and amides, to produce tri-substituted internal alkenes.

Pentafluorosulfanyl-functionalised BODIPY push-pull dyes for p-type dye-sensitized solar cells.

We report a push-pull BODIPY-based dye functionalised with an electronegative SF5 group at the meso position for applications in photocathodes in tandem dye-sensitized solar cells (DSSCs). The push-pull character enhances charge-transfer from the mesoporous NiO cathode surface towards the redox mediator. A Knoevenagel condensation reaction was used to introduce the carboxylic acid to anchor the dye to the oxide surface, via a styryl linker which increases the conjugation in the molecule and shifts the absorption to the red. The room-temperature synthesis and high yields, make the dye promising for manufacture on a large scale. The dye was applied in p-DSSCs giving a power conversion efficiency (0.066%), a short circuit photocurrent (J SC) of 3.84 mA cm-2, open circuit voltage (V OC) of 58 mV and fill factor of 30%.

A BF2 Chelate Exhibiting Excimer-like Fluorescence with an Unusually Large Stokes Shift in the Crystalline Phase.

The target mono-BF2 complex is weakly emissive in fluid solution because radiationless decay of the excited-singlet state is promoted through an intramolecular N⋅⋅⋅H-N hydrogen bond. The lack of mirror symmetry for this compound is attributed to vibronic effects, as reported previously for the bis-BF2 complex (BOPHY). Red-shifted fluorescence is observed from single crystals, the emission quantum yield approaching 30 % with a fluorescence lifetime of 2 ns. The large Stokes shift of 5,700 cm-1 helps minimize self-absorption. Crystallography indicates that the internal fold and twist angles are increased substantially in the crystal, but the hydrogen bond is weakened relative to solution. The crystal structure is compiled from pairs of head-to-tail molecules having a shift of ca. 4.1 Šand closest approach of ca. 3.5 Å. These molecular pairs are arranged in columns, which, in turn, assemble into sheets. The proximity favors excitonic coupling between individual molecules, with the coupling strength obtained by analysis of the absorption spectrum reaching ca. 1,000 cm-1 . Both the ideal dipole approximation and the extended dipole methodology seriously overestimate the coupling strength, but the atomic transition charge density procedure leads to good agreement with experiment. Emission is attributed to the closely coupled molecular pair functioning in an excimer-like manner with the exciton trapped in a local minimum. Increasing temperature causes a slight blue shift and loss of fluorescence.

Stereochemical Control of Secondary Benzamide-based BODIPY Emitters.

Aromatic amides can be used to construct light-harvesting materials with valuable optical properties. The amide bond is formed using well-known coupling agents in near quantitative yield, as illustrated here through the synthesis of two boron dipyrromethene derivatives bearing an amide linkage. The primary concern with acyl amides is rotation around the C-N bond, leading to cis and trans isomers. Using NMR spectroscopy, quantum chemical calculations and critical comparison to simpler benzamides, the stereochemistry of the target compounds has been addressed. The N-cyclohexyl derivative gave diffraction quality crystals that established a trans geometry for the amide bond. Quantum chemical calculations support the trans geometry as being the lowest-energy structure in solution but indicate that inversion of the aryl ring is an important structural feature. Indeed, rotation around the C(sp2 )-C(aryl) bond has a strong influence on the solution-phase NMR spectra. The amide connection has minimal effect on the photophysical properties.

Li vs Na: Divergent Reaction Patterns between Organolithium and Organosodium Complexes and Ligand-Catalyzed Ketone/Aldehyde Methylenation.

Organosodium chemistry is underdeveloped compared with organolithium chemistry, and all the reported organosodium complexes exhibit similar, if not identical, reactivity patterns to their lithium counterparts. Herein, we report a rare organosodium monomeric complex, namely, [Na(CH2SiMe3)(Me6Tren)] (1-Na) (Me6Tren: tris[2-(dimethylamino)ethyl]amine) stabilized by a tetra-dentate neutral amine ligand Me6Tren. Employing organo-carbonyl substrates (ketones, aldehydes, amides, ester), we demonstrated that 1-Na features distinct reactivity patterns compared with its lithium counterpart, [Li(CH2SiMe3)(Me6Tren)] (1-Li). Based on this knowledge, we further developed a ligand-catalysis strategy to conduct ketone/aldehyde methylenations, using [NaCH2SiMe3]∞ as the CH2 feedstock, replacing the widely used but hazardous/expensive C═O methylenation methods, such as Wittig, Tebbe, Julia/Julia-Kocienski, Peterson, and so on.

Conductivity in Thin Films of Transition Metal Coordination Complexes.

Two coordination complexes have been made by combining the dithiolene complexes [M(mnt)2]2- (mnt = maleonitriledithiolate; M = Ni2+ or Cu2+) as anion, with the copper(II) coordination complex [Cu(Stetra)] (Stetra = 6,6'-bis(4,5-dihydrothiazol-2-yl)-2,2'-bipyri-dine) as cation. The variation of the metal centers leads to a dramatic change in the conductivity of the materials, with the M = Cu2+ variant (Cu-Cu) displaying semiconductor behavior with a conductivity of approximately 2.5 × 10-8 S cm-1, while the M = Ni2+ variant (Ni-Cu) displayed no observable conductivity. Computational studies found Cu-Cu enables a minimization of reorganization energy losses and, as a result, a lower barrier to the charge transfer process, resulting in the reported higher conductivity.

Phosphido-borane-supported stannates.

The reactions between SnCl2 and three equivalents of the alkali metal phosphido-borane complexes [R2P(BH3)]M yield the corresponding tris(phosphido-borane)stannate complexes [LnM{R2P(BH3)}3Sn] [R2 = iPr2, LnM = (THF)3Li (2Li), (Et2O)Na (2Na), (Et2O)K (2K); R2 = Ph2, LnM = (THF)Li (3Li), (THF)(Et2O)Na (3Na), (THF)(Et2O)K (3K); R2 = iPrPh, LnM = (THF)4Li (4Li)]. In each case X-ray crystallography reveals an anion consisting of a trigonal pyramidal tin centre coordinated by the P atoms of the phosphido-borane ligands. These tris(phosphido-borane)stannate anions coordinate to the alkali metal cations via their BH3 hydrogen atoms in a variety of modes to give monomers, dimers, and polymers, depending on the alkali metal and the substituents at the phosphorus centres. In contrast, reactions between SnCl2 and three equivalents of [tBu2P(BH3)]M (M = Li, Na) gave the known hydride [M{tBu2P(BH3)}2SnH], according to multinuclear NMR spectroscopy.

Elucidating Solution-State Coordination Modes of Multidentate Neutral Amine Ligands with Group-1 Metal Cations: Variable-Temperature NMR Studies.

Multidentate neutral amine ligands play vital roles in coordination chemistry and catalysis. In particular, these ligands are used to tune the reactivity of Group-1 metal reagents, such as organolithium reagents. Most, if not all, of these Group-1 metal reagent-mediated reactions occur in solution. However, the solution-state coordination behaviors of these ligands with Group-1 metal cations are poorly understood, compared to the plethora of solid-state structural studies based on single-crystal X-ray diffraction (SCXRD) studies. In this work, we comprehensively mapped out the coordination modes with Group-1 metal cations for three multidentate neutral amine ligands: tridentate 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN), tetradentate tris[2-(dimethylamino)ethyl]amine (Me6Tren), and hexadentate N,N',N″-tris-(2-N-diethylaminoethyl)-1,4,7-triaza-cyclononane (DETAN). The macrocycles in the Me3TACN and DETAN are identified as the rigid structural directing motif, with the sidearms of DETAN providing flexible "on-demand" coordination sites. In comparison, the Me6Tren ligand features more robust coordination, with the sidearms less likely to undergo the decoordinating-coordinating equilibrium. This work will provide a guidance for coordination chemists in applying these three ligands, in particular, the new DETAN ligand to design metal complexes which suit their purposes.

Spontaneous Decomposition of an Extraordinarily Twisted and Trans-Bent Fully-Phosphanyl-Substituted Digermene to an Unusual GeI Cluster.

Ditetrelenes R2 E=ER2 (E=Si, Ge, Sn, Pb) substituted by multiple N/P/O/S-donor groups are extremely rare due to their propensity to disaggregate into their tetrylene monomers R2 E. We report the synthesis of the first fully phosphanyl-substituted digermene {(Mes)2 P}2 Ge=Ge{P(Mes)2 }2 (3, Mes=2,4,6-Me3 C6 H2 ), which adopts a highly unusual structure in the solid state, that is both strongly trans-bent and highly twisted. Variable-temperature 31 P{1 H} NMR spectroscopy suggests that 3 persists in solution, but is subject to a dynamic equilibrium between two conformations, which have different geometries about the Ge=Ge bond (twisted/non-twisted) due to a difference in the nature of their π-stacking interactions. Compound 3 undergoes unprecedented, spontaneous decomposition in solution to give a unique GeI cluster {(Mes)2 P}4 Ge4 ⋅5 CyMe (7).