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.

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.

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.

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.

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.

A Mechanistic Study of Halogen Addition and Photoelimination from π-Conjugated Tellurophenes.

The ability to drive reactivity using visible light is of importance for many disciplines of chemistry and has significant implications for sustainable chemistry. Identifying photochemically active compounds and understanding photochemical mechanisms is important for the development of useful materials for synthesis and catalysis. Here we report a series of photoactive diphenyltellurophene compounds bearing electron-withdrawing and electron-donating substituents synthesized by alkyne coupling/ring closing or palladium-catalyzed ipso-arylation chemistry. The redox chemistry of these compounds was studied with respect to oxidative addition and photoelimination of bromine, which is of importance for energy storage reactions involving X2. The oxidative addition reaction mechanism was studied using density functional theory, the results of which support a three-step mechanism involving the formation of an initial η(1) association complex, a monobrominated intermediate, and finally the dibrominated product. All of the tellurophene derivatives undergo photoreduction using 430, 447, or 617 nm light depending on the absorption properties of the compound. Compounds bearing electron-withdrawing substituents have the highest photochemical quantum efficiencies in the presence of an alkene trap, with efficiencies of up to 42.4% for a pentafluorophenyl-functionalized tellurophene. The photoelimination reaction was studied in detail through bromine trapping experiments and laser flash photolysis, and a mechanism is proposed. The photoreaction, which occurs by release of bromine radicals, is competitive with intersystem crossing to the triplet state of the brominated species, as evidenced by the formation of singlet oxygen. These findings should be useful for the design of new photochemically active compounds supported by main-group elements.

A Molecular Rotor Possessing an H-M-H "Spoke" on a P-M-P "Axle": A Platinum(II) trans-Dihydride Spins Rapidly Even at 75 K.

A new class of low-barrier molecular rotors, metal trans-dihydrides, is suggested here. To test whether rapid rotation can be achieved, the known complex trans-H2Pt(P(t)Bu3)2 was experimentally studied by (2)H and (195)Pt solid-state NMR spectroscopy (powder pattern changes with temperature) and computationally modeled as a (t)Bu3P-Pt-P(t)Bu3 stator with a spinning H-Pt-H rotator. Whereas the related chloro-hydride complex, trans-H(Cl)Pt(P(t)Bu3)2, does not show rotational behavior at room temperature, the dihydride trans-H2Pt(P(t)Bu3)2 rotates fast on the NMR time scale, even at low temperatures down to at least 75 K. The highest barrier to rotation is estimated to be ∼3 kcal mol(-1), for the roughly 3 Šlong rotator in trans-H2Pt(P(t)Bu3)2.

Amino Acid Chirality and Ferrocene Conformation Guided Self-Assembly and Gelation of Ferrocene-Peptide Conjugates.

The self-assembly and gelation behavior of a series of mono- and disubstituted ferrocene (Fc)-peptide conjugates as a function of ferrocene conformation and amino acid chirality are described. The results reveal that ferrocene-peptide conjugates self-assemble into organogels by controlling the conformation of the central ferrocene core, through inter- versus intramolecular hydrogen bonding in the attached peptide chain(s). The chirality controlled assembling studies showed that two monosubstituted Fc conjugates FcCO-LFLFLA-OMe and FcCO-LFLFDA-OMe form gels with nanofibrillar network structures, whereas the other two diastereomers FcCO-DFLFLA-OMe and FcCO-LFDFLA-OMe exclusively produced straight nanorods and non-interconnected small fibers, respectively. This suggests the potential tuning of gelation behavior and nanoscale morphology by altering the chirality of constituted amino acids. The current study confirms the profound effect of diastereomerism and no influence of enantiomers on gelation. Correspondingly, the diastereomeric and enantiomeric Fc[CO-FFA-OMe]2 were constructed for the study of chirality-organized structures.

Iron(II) complexes containing unsymmetrical P-N-P' pincer ligands for the catalytic asymmetric hydrogenation of ketones and imines.

After their treatment with LiAlH4 and then alcohol, new iron dicarbonyl complexes mer-trans-[Fe(Br)(CO)2(P-CH═N-P')][BF4] (where P-CH═N-P' = R2PCH2CH═NCH2CH2PPh2 and R = Cy or iPr or P-CH═N-P' = (S,S)- Cy2PCH2CH═NCH(Me)CH(Ph)PPh2) are catalysts for the hydrogenation of ketones in THF solvent with added KOtBu at 50 °C and 5 atm H2. Complexes with R = Ph are not active. With the enantiopure complex, alcohols are produced with an enantiomeric excess of up to 85% (S) at TOF up to 2000 h(-1), TON of up to 5000, for a range of ketones. An activated imine is hydrogenated to the amine in 90% ee at a TOF 20 h(-1)and TON 99. This is a significant advance in asymmetric pressure hydrogenation using iron. The complexes are prepared in two steps: (1) a one-pot reaction of phosphonium dimers ([cyclo-(PR2CH2CH(OH)(-))2][Br]2), KOtBu, FeBr2, and Ph2PCH2CH2NH2 (or (S,S)-Ph2PCH(Ph)CH(Me)NH2 for the enantiopure complex) in THF under a CO atmosphere to produce the complexes cis- and trans-[Fe(Br)2(CO)(P-CH═N-P')]; (2) the reaction of these with AgBF4 under CO(g) to afford the dicarbonyl complexes in high yield (50-90%). NMR and DFT studies of the process of precatalyst activation show that the dicarbonyl complexes are converted first to hydride-aluminum hydride complexes where the imine of the P-CH═N-P' ligand is reduced to an amide [P-CH2N-P'](-) with aluminum hydrides still bound to the nitrogen. These hydride species react with alcohol to give monohydride amine iron compounds FeH(OR')(CO)(P-CH2NH-P'), R' = Me, CMe2Et as well as the iron(0) complex Fe(CO)2(P-CH2NH-P') under certain conditions.

[(IPent)PdCl2(morpholine)]: a readily activated precatalyst for room-temperature, additive-free carbon-sulfur coupling.

A series of new, easily activated NHC-Pd(II) precatalysts featuring a trans-oriented morpholine ligand were prepared and evaluated for activity in carbon-sulfur cross-coupling chemistry. [(IPent)PdCl2(morpholine)] (IPent=1,3-bis(2,6-di(3-pentyl)phenyl)imidazol-2-ylidene) was identified as the most active precatalyst and was shown to effectively couple a wide variety of deactivated aryl halides with both aryl and alkyl thiols at or near ambient temperature, without the need for additives, external activators, or pre-activation steps. Mechanistic studies revealed that, in contrast to other common NHC-Pd(II) precatalysts, these complexes are rapidly reduced to the active NHC-Pd(0) species at ambient temperature in the presence of KOtBu, thus avoiding the formation of deleterious off-cycle Pd(II)-thiolate resting states.