Homomeric chains of intermolecular bonds scaffold octahedral germanium perovskites.

Perovskites with low ionic radii metal centres (for example, Ge perovskites) experience both geometrical constraints and a gain in electronic energy through distortion; for these reasons, synthetic attempts do not lead to octahedral [GeI6] perovskites, but rather, these crystallize into polar non-perovskite structures1-6. Here, inspired by the principles of supramolecular synthons7,8, we report the assembly of an organic scaffold within perovskite structures with the goal of influencing the geometric arrangement and electronic configuration of the crystal, resulting in the suppression of the lone pair expression of Ge and templating the symmetric octahedra. We find that, to produce extended homomeric non-covalent bonding, the organic motif needs to possess self-complementary properties implemented using distinct donor and acceptor sites. Compared with the non-perovskite structure, the resulting [GeI6]4- octahedra exhibit a direct bandgap with significant redshift (more than 0.5 eV, measured experimentally), 10 times lower octahedral distortion (inferred from measured single-crystal X-ray diffraction data) and 10 times higher electron and hole mobility (estimated by density functional theory). We show that the principle of this design is not limited to two-dimensional Ge perovskites; we implement it in the case of copper perovskite (also a low-radius metal centre), and we extend it to quasi-two-dimensional systems. We report photodiodes with Ge perovskites that outperform their non-octahedral and lead analogues. The construction of secondary sublattices that interlock with an inorganic framework within a crystal offers a new synthetic tool for templating hybrid lattices with controlled distortion and orbital arrangement, overcoming limitations in conventional perovskites.

Organic Polar Crystals, Second Harmonic Generation, and Piezoelectric Effects from Heteroadamantanes in the Space Group R3m.

Polar crystalline materials, a subset of the non-centrosymmetric materials, are highly sought after. Their symmetry properties make them pyroelectric and also piezoelectric and capable of second-harmonic generation (SHG). For SHG and piezoelectric applications, metal oxides are commonly used. The advantages of oxides are durability and hardness - downsides are the need for high-temperature synthesis/processing and often the need to include toxic metals. Organic polar crystals, on the other hand, can avoid toxic metals and can be amenable to solution-state processing. While the vast majority of polar organic molecules crystallize in non-polar space groups, we found that both 7-chloro-1,3,5-triazaadamantane, for short Cl-TAA, and also the related Br-TAA (but not I-TAA) form polar crystals in the space group R3m, easily obtained from dichloromethane solution. Measurements confirm piezoelectric and SHG properties for Cl-TAA and Br-TAA. When the two species are crystallized together, solid solutions form, suggesting that properties of future materials can be tuned continuously.

A Boron Scan of Ethyl Acetoacetate Leads to Versatile Building Blocks.

Discovered in the 19th century, ethyl acetoacetate has been central to the development of organic chemistry, including its pedagogy and applications. In this study, we present borylated derivatives of this venerable molecule. A boron handle has been installed at either α ${{\rm \alpha }}$ - or ß ${\beta }$ -position of acetoacetate by homologation of acyl-MIDA (N-methyliminodiacetic acid) boronates with diazoacetates. Either alkyl or boryl groups were found to migrate with regiochemistry being a function of the steric bulk of the diazo species. Boryl ß ${{\rm \beta }}$ -ketoesters can be further modified into borylated pyrazolones and oximes, thereby expanding the synthetic toolkit and offering opportunities for additional modifications.

Synthesis, Characterization, and Catalytic Exploration of Mononuclear Mo(VI) Dioxido Complexes of (Z)-1-R-2-(4',4'-Dimethyl-2'-oxazolin-2'-yl)-eth-1-en-1-ates.

The coordination chemistry of the title ligands with Mo metal centers was investigated. Thus, the synthesis and characterization (NMR, X-ray diffraction) of four mononuclear formally Mo(6+) complexes of (Z)-1-R-2-(4',4'-dimethyl-2'-oxazolin-2'-yl)-eth-1-en-1-ates (L: R = -Ph, -Ph-p-NO2, -Ph-p-OMe and -t-Bu), derived from the part enols (LH), is described. The resulting air-stable MoO2L2 complexes (1-4) exist, as shown by single-crystal X-ray diffraction experiments, in the cis-dioxido-trans(N)-κ2-N,O-L conformation in the solid state for all four examples. This situation was further probed using semi-empirical PM6(tm) calculations. Complexes 1-4 represent the first Mo complexes of this ligand class and, indeed, of Group 6 metals in general. Structural and spectroscopic comparisons were made between these and related Mo(6+) compounds. Complex 1 (R = -Ph) was studied for its ability to selectively catalyze the production of poly-norbornene from the monomer in the presence of MAO. This, unfortunately, only resulted in the synthesis of insoluble, presumably highly cross-linked, polymeric and/or oligomeric materials. However, complexes 1-4 were demonstrated to be highly effective for catalyzing benzoin to benzil conversion using DMSO as the O-transfer agent. This catalysis work is likewise put into perspective with respect to analogous Mo(6+) complexes.

Rigid Conjugated Diamine Templates for Stable Dion-Jacobson-Type Two-Dimensional Perovskites.

Hybrid organic-inorganic perovskites (HOIPs) have garnered widespread interest, yet stability remains a critical issue that limits their further application. Compared to their three-dimensional (3D) counterparts, two-dimensional (2D)-HOIPs exhibit improved stability. 2D-HOIPs are also appealing because their structural and optical properties can be tuned according to the choice of organic ligand, with monovalent or divalent ligands forming Ruddlesden-Popper (RP) or Dion-Jacobson (DJ)-type 2D perovskites, respectively. Unlike RP-type 2D perovskites, DJ-type 2D perovskites do not contain a van der Waals gap between the 2D layers, leading to improved stability. However, bifunctional organic ligands currently used to develop DJ-type 2D perovskites are limited to commercially available aliphatic and single-ring aromatic ammonium cations. Large conjugated organic ligands are in demand for their semiconducting properties and their potential to improve materials stability further. In this manuscript, we report the design and synthesis of a new set of larger conjugated diamine ligands and their incorporation into DJ-type 2D perovskites. Compared with analogous RP-type 2D perovskites, DJ 2D perovskites reported here show blue-shifted, narrower emissions and significantly improved stability. By changing the structure of rings (benzene vs thiophene) and substituents, we develop structure-property relationships, finding that fluorine substitution enhances crystallinity. Single-crystal structure analysis and density functional theory calculations indicate that these changes are due to strong electrostatic interactions between the organic templates and inorganic layers as well as the rigid backbone and strong π-π interaction between the organic ligands themselves. These results illustrate that targeted engineering of the diamine ligands can enhance the stability of DJ-type 2D perovskites.

Boramidine: A Versatile Structural Motif for the Design of Fluorescent Heterocycles.

Sodium cyanoborohydride-derived N-alkylnitriliumboranes were found to be versatile precursors for the synthesis of novel boron-containing heterocycles. The reaction between N-alkylnitriliumboranes and 2-aminopyridines, imidazoles, oxazoles, or isoxazoles leads to the incorporation of the [B-C] motif into a five-membered boramidine, which exists as a mixture of Z and E isomers. The resulting heterocycles are blue fluorescent in both the solid state and in solution with ca. 2700-8400 cm-1 Stokes shifts and quantum yields in the 65-74% range in water and in the 42-84% range in organic solvents. The combination of photophysical properties, structural tunability, stability, and solubility in various media is expected to find application in a range of disciplines.

Why Diorganyl Zinc Lewis Acidity Dramatically Increases with Narrowing C-Zn-C Bond Angle.

The Lewis acidity of a metal center is influenced not only by the electronic properties of the bonded ligands but also by the bond angles, which we suggest to be important for zinc diorganyls. Molecular orbital correlation predicts that a narrower C-Zn-C bond angle of the R2Zn fragment lowers its lowest unoccupied molecular orbital (LUMO) and increases its Lewis acidity, such that it binds added ligands more strongly. Computations on Me2Zn(bipy) (bipy = 2,2'-bipyridine) yield that, for every 10° of C-Zn-C narrowing close to tetrahedral geometry, the Zn-N distance shortens by 0.027 Å (0.048 Å per 10° for the range 180-90°) and that the LUMO of the Me2Zn fragment drops by 0.24 eV. A total of 10 dialkyl zinc complexes of bipy or 4,4'-di-tert-butyl-2,2'-bipyridine are crystallographically characterized here. Structure correlations (published and new data) confirm the link between the C-Zn-C angle and Zn-N distance. Principal component analysis provides a detailed picture of the correlated distortions. Relevance for zinc fingers/zinc enzymes is discussed.

A One-Step Preparation of Tetradentate Ligands with Nitrogen and Phosphorus Donors by Reductive Amination and Representative Iron Complexes.

The synthesis and use of the first examples of unsymmetrical, mixed phosphine donor tripodal NPP2' ligands N(CH2CH2PR2)2(CH2CH2PPh2) are presented. The ligands are synthesized via a convenient, one pot reductive amination using 2-(diphenylphosphino)ethylamine and various substituted phosphonium dimers in order to introduce mixed phosphine donors substituted with P/P', those being Ph/Cy (2), Ph/iPr (3), Ph/iBu (4), Ph/o-Tol (5), and Ph/p-Tol (6). Additionally, we have developed the first known synthesis of a symmetrical tripodal NP3 ligand N(CH2CH2PiBu2)3 using bench safe ammonium acetate as the lone nitrogen source (7). This new protocol eliminates the use of extremely dangerous nitrogen mustard reagents typically required to synthesize NP3 ligands. Some of these tetradentate ligands and also P2NN' ligands N(CH2-o-C5H4N)(CH2CH2PR2)2 (P2NN'-Cy, R = Cy; P2NN'-Ph, R = Ph) prepared by reductive amination using 2-picolylamine are used in the synthesis and reactions of iron complexes. FeCl2(P2NN'-Cy) (8) undergoes single halide abstraction with NaBPh4 to give the trigonal bipyramidal complex [FeCl(P2NN'-Cy)][BPh4] (9). Upon exposure to CO(g), complex 9 readily coordinates CO giving [FeCl(P2NN'-Cy)(CO)][BPh4] (10), and further treatment with an excess of NaBH4 results in formation of the hydride complex [Fe(H)(P2NN'-Cy)(CO)][BPh4] (11). Our previously reported complex FeCl2(P2NN'-Ph) undergoes double halide abstraction with NaBPh4 in the presence of the coordinating solvent to give [Fe(NCMe)2(P2NN'-Ph)][BPh4]2 (12). Ligand 3 can be coordinated to FeCl2, and upon sequential halide abstraction, treatment with NaBH4, and exposure to an atmosphere of dinitrogen, the dinitrogen hydride complex [Fe(H)(NPP2'-iPr)(N2)][BPh4] (13) is isolated. Our symmetrical NP3 ligand 7 can also be coordinated to FeCl2 and, upon exposure to an atmosphere of CO(g), selectively forms [FeCl(NP3)(CO)][BPh4] (14) after salt metathesis with NaBPh4. Complex 14 can be treated with an excess of NaBH4 to give the hydride complex [Fe(H)(NP3)(CO)][BPh4] (15), which can further be deprotonated/reduced to the Fe(0) complex Fe(NP3)(CO) (16) upon treatment with an excess of KH.

Hydrogen Bond Assisted l to d Conversion of α-Amino Acids.

l to d conversion of unactivated α-amino acids was achieved by solubility-induced diastereomer transformation (SIDT). Ternary complexes of an α-amino acid with 3,5-dichlorosalicylaldehyde and a chiral guanidine (derived from corresponding chiral vicinal diamine) were obtained in good yield as diastereomerically pure imino acid salt complexes and were hydrolysed to obtain enantiopure α-amino acids. A combination of DFT computation, NMR spectroscopy, and crystal structure provide detailed insight into how two types of strong hydrogen bonds assist in rapid epimerization of the complexes that is essential for SIDT.

Synthesis and Biochemical Evaluation of Nicotinamide Derivatives as NADH Analogue Coenzymes in Ene Reductase.

Nicotinamide and pyridine-containing conjugates have attracted a lot of attention in research as they have found use in a wide range of applications including as redox flow batteries and calcium channel blockers, in biocatalysis, and in metabolism. The interesting redox character of the compounds' pyridine/dihydropyridine system allows them to possess very similar characteristics to the natural chiral redox agents NAD+ /NADH, even mimicking their functions. There has been considerable interest in designing and synthesizing NAD+ /NADH mimetics with similar redox properties. In this research, three nicotinamide conjugates were designed, synthesized, and characterized. Molecular structures obtained through X-ray crystallography were obtained for two of the conjugates, thereby providing more detail on the bonding and structure of the compounds. The compounds were then further evaluated for biochemical properties, and it was found that one of the conjugates possessed similar functions and characteristics to the natural NADH. This compound was evaluated in the active enzyme, enoate reductase; like NADH, it was shown to help reduce the C=C double bond of three substrates and even outperformed the natural coenzyme. Kinetic data are reported.

Enantioselective Hydrogenation of Activated Aryl Imines Catalyzed by an Iron(II) P-NH-P' Complex.

Chiral amines are key building blocks in synthetic chemistry with numerous applications in the agricultural and pharmaceutical industries. Asymmetric imine hydrogenation, particularly with iridium catalysts, is well developed. However, imine reduction still remains challenging in the context of replacing such a precious metal with a cheap, nontoxic, and environmentally friendly substitute such as iron. Here, we report that an unsymmetrical iron P-NH-P' catalyst that was previously shown to be effective for the asymmetric hydrogenation of aryl ketones is also a very effective catalyst for the asymmetric hydrogenation of prochiral aryl imines activated with N-diphenylphosphinoyl or N-tosyl groups. The P-NH-P' abbreviation stands for (S,S)-PPh2CHPhCHPhNHCH2CH2PiPr2. Density functional theory results suggest that, surprisingly, the NH group on the catalyst activates and orients the imine to hydride attack by hydrogen bonding to the PO or SO group on the imine nitrogen, as opposed to the imine nitrogen itself. This may explain why N-Ph and N-Bu imines are not hydrogenated.

Synthesis and Characterization of Readily Modified Poly(aryl)(alkoxy)stannanes by use of Hypercoordinated Sn Monomers.

The synthesis and solid-state molecular structures of two dichlorido(aryl)(alkyl) tin compounds, 5 and 8, both key intermediates to tunable polystannane architectures, are reported. The materials were further investigated by single-crystal XRD and a DFT analysis of their preferential "open and closed" geometries. Conversion of said compounds to their dihydride analogues was undertaken, followed by their application as monomers for polystannane polymer synthesis. The properties of two asymmetrical polystannanes prepared by transition-metal-catalyzed dehydropolymerization of dihydrido(aryl)alkylstannanes (6 and 9) were investigated. The first product was the structurally simple, modest molecular weight, semi-crystalline light- and moisture-stable polystannane 10 with NMR (119 Sn) evidence of prominent Sn←O hypercoordination along the polymer backbone. The second was the lower molecular weight, tosylated four-coordinate polystannane 11 with no evidence of hypercoordination. Differential scanning calorimetry (DSC) of polymer 10 revealed a reversible semi-crystalline nature, whereas an amorphous character was detected for polymer 11. Polystannane 10 was also found to be exceptionally stable to both moisture and light (>6 months) and a promising candidate for the design of readily modified (i.e., tunable) polystannane materials.

Reversible Solution π-Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical.

Reversible solution π-dimerization is observed in the stable neutral phenoxyl radical 2,6-bis-(8-quinolylamino)-4-(tert-butyl)phenoxyl baqp and is spectroscopically characterized. This behavior, not previously observed for π-extended phenoxyl radicals, is relevant to the formation of long multicenter bonding in the π-dimer at low temperature akin to previously reported phenalenyl radicals. Our experimental data are supported in a quantitative manner by results from density functional theory (DFT) and ab initio molecular orbital theory calculations. Our theoretical results indicate that the solution dimer features strong bonding interactions between the two phenoxyl rings but that the stability of the dimer is also related to dispersion interactions between the flanking nearly parallel quinolyl rings.

AgI -Induced Switching of DNA Binding Modes via Formation of a Supramolecular Metallacycle.

The histidine derivative L1 of the DNA intercalator naphthalenediimide (NDI) forms a triangular AgI complex (C2). The interactions of L1 and of C2 with DNA were studied by circular dichroism (CD) and UV/Vis spectroscopy and by viscosity studies. Different binding modes were observed for L1 and for C2, as the AgI complex C2 is too large in size to act as an intercalator. If AgI is added to the NDI molecule that is already intercalated into a duplex, higher order complexes are formed within the DNA duplex and cause disruptions in the helical duplex structure, which leads to a significant decrease in the characteristic CD features of B-DNA. Thus, via addition of a metal we show how a classic and well-known organic intercalator unit can be turned into a partial metallo insertor. We also show how electrochemical impedance spectroscopy (EIS) can be used to probe DNA binding modes on DNA films that are immobilized on gold surfaces.

An Approach to the 9-Oxo-10-oxabicyclo[5.3.0]dec-2-ene Core of the Guaianolide and Pseudoguaianolide Sesquiterpenes via a Domino Electrocyclic Ring-Opening/Carboxylic Acid Trapping of a gem-Dibromocyclopropane.

A domino silver(I)-promoted electrocyclic 2π-disrotatory electrocyclic ring-opening/intramolecular nucleophilic trapping of [ n.1.0]-dibromocyclopropanes by tethered carboxylic acids results in cyclization to butyrolactones fused to six- and seven-membered carbocycles. In the case of bicyclic [4.3.0] lactones the cis-fused stereoisomer was formed, whereas for the bicyclic [5.3.0] lactones the trans-fused stereoisomer was formed. Optimal conditions for the reaction used silver(I) trifluoroacetate (2.0 equiv) in trifluoroethanol or with added pyridine (2.0 equiv) and NaPF6 (5.0 equiv). The dibromocyclopropane precursors were made through cyclopropanation with in situ-generated dibromocarbene. The trans-fused lactones are potentially useful building blocks for pseudoguaianolide, guaianolide, and xanthanolide total synthesis. A computational study on the conformational preferences of these systems indicates that the trans-fused bicyclic [5.3.0] butyrolactones are lower in energy than the corresponding cis-fused lactones at the B3LYP/cc-pVTZ level of theory.

Polynuclear Cu4L4 Copper(II) Aminyl Radical Coordination Complexes.

We describe the structural features and magnetic properties of two polynuclear copper(II) complexes containing a redox-active ligand. These neutral complexes each bear the formula RL4Cu4 (R = tBu, Me) with the ligand in a dianion-aminyl radical oxidation state. X-ray data and density functional theory calculations support an aminyl-type radical character in these complexes, making these the first polynuclear metal aminyl radical complexes.

Versatile Synthesis of Siloxy Silicon Tetrabenzotriazacorroles and Insight into the Mode of Macrocycle Formation.

Tetrabenzotriazacorroles (Tbcs) are a family of molecules related to phthalocyanines but have the unique ability to intensely absorb both blue and red light. Here, we report the synthesis of four novel silicon tetrabenzotriazacorrole derivatives (SiTbcs) with varying sized axial ligands. SiTbcs are formed starting from bis(hydroxy) silicon phthalocyanine ((OH)2-SiPc) via a simple in situ axial functionalization and reductive chemical process using magnesium metal and the respective chlorosilane in pyridine. Systematic probing of the reaction conditions revealed that the reaction is acid-promoted and that the formation of the Tbc macrocycle occurs at temperatures as low as 40 °C. Results imply this chemistry can be extended to SiTbcs with any axial ligands using pyridine hydrochloride as an acid source. Single crystals of all compounds were grown and showed significant π-π interactions between the macrocycles in the solid state. Optical, electrochemical, and thermal characterization of these materials is also described. The SiTbcs exhibit interesting highly oxidative electrochemistry as well as high thermal stability and tunable phase transition behavior.

Dechlorinated Analogues of Dechlorane Plus.

Degradation products of the chlorinated additive flame retardant Dechlorane Plus (DP) have been discovered globally. However, the identity of many of these species remains unknown due to a lack of available analytical standards, hindering the ability to quantitatively measure the amounts of these compounds in the environment. In the present study, synthetic routes to possible dechlorinated DP derivatives were investigated in an effort to identify the environmentally significant degradation products. The methano-bridge chlorines of anti- and syn-DP were selectively replaced by hydrogen atoms to give six new hydrodechlorinated DP analogues. The identity and absolute configuration of all of these compounds were confirmed by GC-MS, NMR spectroscopy, and X-ray diffraction studies. These compounds were observed in sediment samples from streams and rivers in relatively rural areas of Ontario and are thus environmentally relevant.

Proof of Concept Studies Directed Towards Designed Molecular Wires: Property-Driven Synthesis of Air and Moisture-Stable Polystannanes.

Polystannanes with azobenzene moieties designed to protect the Sn-Sn backbone from light- and moisture-induced degradation are described. The azo-stannyl precursor 3 (70 %) is converted in good yields (88-91 %) to the mono- (4), and dichlorostannanes (5), by sequential chlorination, followed by further reduction of 5 to the dihydride (6) using NaBH4 (78 %). All stannanes were characterised by NMR (1 H, 13 C, 119 Sn) spectroscopy and HRMS; in addition, 3, 4 and 5 were structurally elucidated using X-ray diffraction analysis. Metal-free dehydrocoupling of 6 at RT leads exclusively to homopolymer (7-i) displaying an initial solution 119 Sn NMR signal (δ=-196 ppm) that migrates to -235 ppm after 10 days (7-f). In contrast, metal-catalyzed dehydrocoupling of 6 in toluene at RT leads directly 7-f. Random co-polymers formed from 6 and (nBu)2 SnH2 at 4:1 (8 a) and 1:1 (8 b) ratios were compared to the alternating polystannane (9) prepared by the reaction of 6 with (nBu)2 Sn(NEt2 )2 . DFT calculations of 3-6 indicate that hypercoordination at Sn is influenced by substituents and by solvation. Homopolymer 7 was found to have unprecedented moisture and light stability in the solid state for >6 months.

Unsymmetrical Iron P-NH-P' Catalysts for the Asymmetric Pressure Hydrogenation of Aryl Ketones.

The reductive amination of α-dialkylphosphine acetaldehydes with enantiopure ß-aminophosphines is a new, versatile route to unsymmetrical tridentate (pincer) ligands P-NH-P'. Four new ligands PR2 CH2 CH2 NHCHR'CHR''PPh2 (R=iPr, Cy, R'=Ph, CH(CH3 )2 , R''=Ph, H) prepared in this way are used to make the iron(II) complexes mer-FeCl2 (CO)(P-NH-P') and mer-FeCl(H)(CO)(P-NH-P'). The hydride complex with the rigid ligand with R'=R''=Ph is an efficient and highly enantioselective homogeneous asymmetric pressure hydrogenation (APH) catalyst. Prochiral aryl ketones are reduced under mild conditions (THF, 0.1 mol % catalyst, 1 mol % KOtBu, 5-10 bar, 50 °C) to the (S)-alcohols, usually in enantiomeric excess (ee) greater than 90 %. DFT calculations provided transition-state structures for the enantiodetermining hydride-transfer step.