Modulating Nucleation and Crystal Growth of Tin Perovskite Films for Efficient Solar Cells.

The performance of tin halide perovskite solar cells (PSCs) has been severely limited by the rapid crystallization of tin perovskites, which usually leads to an undesirable film quality. In this work, we tackle this issue by regulating the nucleation and crystal growth of tin perovskite films using a small Lewis base additive, urea. The urea-SnI2 interaction facilitates the formation of larger and more uniform clusters, thus accelerating the nucleation process. Additionally, the crystal growth process is extended, resulting in a high-quality tin perovskite film with compact morphology, increased crystallinity, and reduced defects. Consequently, the efficiency of tin PSCs is significantly increased from 10.42% to 14.22%. This work highlights the importance of manipulating the nucleation and crystal growth of tin perovskites to realize efficient tin PSCs.

Synergy of machine learning and density functional theory calculations for predicting experimental Lewis base affinity and Lewis polybase binding atoms.

Investigation of Lewis acid-base interactions has been conducted by ab initio calculations and machine learning (ML) models. This study aims to resolve two critical tasks that have not been quantitatively investigated. First, ML models developed from density functional theory (DFT) calculations predict experimental BF3 affinity with Pearson correlation coefficients around 0.9 and mean absolute errors around 10 kJ mol-1. The ML models are trained by DFT-calculated BF3 affinity of more than 3000 adducts, with input features readily obtained by rdkit. Second, the ML models have the capability of predicting the relative strength of Lewis base binding atoms in Lewis polybases, which is either an extremely challenging task to conduct experimentally or a computationally expensive task for ab initio methods. The study demonstrates and solidifies the potential of combining DFT calculations and ML models to predict experimental properties, especially those that are scarce and impractical to empirically acquire.

Chiral 4-MeO-Pyridine (MOPY) Catalyst for Enantioselective Cyclopropanation: Attenuation of Lewis Basicity Leads to Improved Catalytic Efficiency.

The design of pyridine-derived organocatalysts aims at the increase of their Lewis basicity, however such an approach is not always efficient. For example, strongly Lewis basic DMAP is completely inefficient as catalyst in the cyclopropanation reaction. Herein we disclose an alternative approach that relies on attenuation of DMAP Lewis basicity. Specifically, the replacement of 4-dimethylamino substituent in DMAP for 4-MeO group delivered a highly efficient catalyst for cyclopropanation of electron-deficient olefins with α-bromoketones. Kinetic studies provide compelling evidence that the superior catalytic efficiency of 4-MeO pyridine (MOPY) is to be attributed to the favorable balance between Lewis basicity and leaving group ability. The use of chiral, enantiomerically pure MOPY catalyst has helped to achieve high enantioselectivities (up to 91 : 9 er) in the previously unreported pyridine-catalyzed cyclopropanation reaction.

Base-Stabilized Gallium Sulfides and Selenides Supported by a Bis(oxazolinyl)(phenyl)methanide Ligand.

Gallylene supported by a bis(oxazolinyl)(phenyl)methanide (Boxm) ligand was synthesized and structurally characterized. The reaction of this gallylene with triphenylphosphine sulfide/selenide yielded dimeric gallium sulfide and selenide. These compounds could be converted to monomeric terminal sulfide and selenide by coordination of an external Lewis base such as an N-heterocyclic carbene (NHC or IMe4) and 4-dimethylaminopyridiene (DMAP). These doubly-base-stabilized gallium sulfide/selenide reacted with phenyl isocyanate to give the corresponding cycloadducts by releasing the Lewis base, indicating the formation of a single-base-stabilized gallium sulfide/selenide intermediate.

Synthesis of ß-Amino Acid Derivatives via Enantioselective Lewis Base Catalyzed N-Allylation of Halogenated Amides with Morita-Baylis-Hillman Carbonates.

Trifluoro- and trichloroacetamides serving as pronucleophiles undergo enantioselective Lewis base catalyzed N-allylation with Morita-Baylis-Hillman carbonates to produce enantioenriched ß-amino acid derivatives. The reactions proceed as a kinetic resolution to give the allylation products and the remaining carbonates in good yields and high enantioselectivity. The obtained products are amenable to diastereoselective derivatization to produce a library of spiro-isoxazoline lactams.

An N-Heterocyclic Quinodimethane: A Strong Organic Lewis Base Exhibiting Diradical Reactivity.

We report the preparation of a new organic σ-donor with a C6H4-linker between an N-heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N-heterocyclic quinodimethane (NHQ). The aromatization of the C6H4-linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N-heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head-to-head dimerization by C-C coupling of the methylidene groups. DFT computations indicate that this reaction proceeds via an open-shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N-heterocyclic bis-quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent (E1/2=-1.71 V vs. Fc/Fc+) and exhibits a remarkable small singlet-triplet gap of ΔES→T=4.4 kcal mol-1.

Deoxygenative Geminal Silylboration of Amides Using Silylboronates: Synthesis and Use of α-Boryl-α-Silylalkylamines.

α-Silylalkylamines and α-borylalkylamines are versatile synthetic intermediates and attractive scaffolds found in pharmaceutical drugs and agrochemicals. Despite great progress on synthetic methods for preparation of α-silylalkylamines or α-borylalkylamines, there are no general strategies for preparation of α-boryl-α-silylalkylamines and the reactivity has not been explored. Here we report deoxygenative geminal silylboration of amides using silylboronates in the presence of alkoxide base catalyst, producing α-boryl-α-silylalkylamines. The silicon and boron groups in α-boryl-α-silylalkylamines are found to be utilized to chemoselective transformations, such as protonation and alkylation. This protocol serves various α-silylalkylamines and α-borylalkylamines from readily available amides.

Design, Preparation, and Implementation of Axially Chiral Benzotetramisoles as Lewis Base Catalysts for Asymmetric Cycloadditions.

Developing novel catalysts with potent activity is of great importance in organocatalysis. In this study, we designed and prepared a new class of benzotetramisole Lewis base catalysts (AxBTM) that have both central and axial chirality. This unique feature of these catalysts results in a three-dimensional microenvironment with multi-layers of chirality. The performance of the developed catalysts was tested in a series of cycloaddition reactions. These included the AxBTM-catalyzed (2+2) cycloaddition between α-fluoro-α-aryl anhydride with imines or oxindoles, and the sequential gold/AxBTM-catalyzed (4+2) cycloaddition of enynamides with pentafluorophenyl esters. The interplay between axial and central chirality had a collaborative effect in regulating the stereochemistry in these cycloadditions, leading to high levels of stereoselectivity that would otherwise be challenging to achieve using conventional BTM catalysts. However, the (2+2) and (4+2) cycloadditions have different predilections for axial and central chirality combinations.

Enantioselective Synthesis of Planar-Chiral Sulfur-Containing Cyclophanes by Chiral Sulfide Catalyzed Electrophilic Sulfenylation of Arenes.

An efficient catalytic asymmetric electrophilic sulfenylation reaction for the synthesis of planar-chiral sulfur-containing cyclophanes has been developed for the first time. This was achieved by using a new Lewis base catalyst and a new ortho-trifluoromethyl-substituted sulfenylating reagent. Using the substrates with low rotational energy barrier, the transformation proceeded through a dynamic kinetic resolution, and the high rotational energy barrier of the substrates allowed the reaction to undergo a kinetic resolution process. Meanwhile, this transformation was compatible with a desymmetrization process when the symmetric substrates were used. Various planar-chiral sulfur-containing cyclophanes were readily obtained in moderate to excellent yields with moderate to excellent enantioselectivities (up to 97 % yield and 95 % ee). This approach was used to synthesize pharmaceutically relevant planar-chiral sulfur-containing molecules. Density functional theory calculations showed that π-π interactions between the sulfenyl group and the aromatic ring in the substrate play a crucial role in enantioinduction in this sulfenylation reaction.

Enantioselective Construction of Eight-Membered N-Heterocycles from Simple 1,3-Dienes via Pd(0) Lewis Base Catalysis.

We report herein an unprecedented enantioselective (4+4) cycloaddition of simple 1,3-dienes with azadienes for the construction of fused eight-membered N-heterocycles. In this transformation, the π-Lewis basic Pd(0) catalyst achieves activation of 1,3-dienes to induce nucleophilic addition to azadienes followed by ring cyclization via a selective terminal allylic substitution. Furthermore, highly efficient and diastereoselective derivatizations of the eight-membered rings provide a facile access to diverse enantiopure fused tetra- to hexacyclic compounds with potential application in medicinal chemistry.

Using the phospha-Michael reaction for making phosphonium phenolate zwitterions.

The reactions of 2,4-di-tert-butyl-6-(diphenylphosphino)phenol and various Michael acceptors (acrylonitrile, acrylamide, methyl vinyl ketone, several acrylates, methyl vinyl sulfone) yield the respective phosphonium phenolate zwitterions at room temperature. Nine different zwitterions were synthesized and fully characterized. Zwitterions with the poor Michael acceptors methyl methacrylate and methyl crotonate formed, but could not be isolated in pure form. The solid-state structures of two phosphonium phenolate molecules were determined by single-crystal X-ray crystallography. The bonding situation in the solid state together with NMR data suggests an important contribution of an ylidic resonance structure in these molecules. The phosphonium phenolates are characterized by UV-vis absorptions peaking around 360 nm and exhibit a negative solvatochromism. An analysis of the kinetics of the zwitterion formation was performed for three Michael acceptors (acrylonitrile, methyl acrylate, and acrylamide) in two different solvents (chloroform and methanol). The results revealed the proton transfer step necessary to stabilize the initially formed carbanion as the rate-determining step. A preorganization of the carbonyl bearing Michael acceptors allowed for reasonable fast direct proton transfer from the phenol in aprotic solvents. In contrast, acrylonitrile, not capable of forming a similar preorganization, is hardly reactive in chloroform solution, while in methanol the corresponding phosphonium phenolate is formed.

N-Heterocyclic Olefin Type Ionic Liquid with Innate Soft Lewis-Base Character as an Effective Additive for Hybrid Quasi-2D and 3D Perovskite Solar Cells.

In optimizing perovskites with ionic liquid (IL), the comparative study on Lewis acid-base (LAB) and hydrogen-bonding (HB) interactions between IL and perovskite is lacking. Herein, methyl is substituted for hydrogen on 2-position of imidazolium ring of N-heterocyclic carbene (NHC) type IL IdH to weaken HB interactions, and the resulting N-heterocyclic olefin (NHO) type IL IdMe with softer Lewis base character is studied in both hybrid quasi-2D (Q-2D) and 3D perovskites. It is revealed that IdMe participates in constructing high-quality Q-2D perovskite (n = 4) and provides stronger passivation for 3D perovskite compared with IdH. Power conversion efficiency (PCE) of Q-2D PEA2 MA3 Pb4 I13 perovskite solar cells (PVSCs) is boosted to 17.68% from 14.03%. PCE and device stability of 3D PVSCs enhances simultaneously. Both theoretical simulations and experimental results show that LAB interactions between NHO and Pb2+ take the primary optimization effects on perovskite. The success of engineering LAB interactions also offers inspiration to develop novel ILs for high-performance PVSCs.

Anion-Dependent Reactivity of Mono- and Dinuclear Boron Cations.

The dinuclear bis(N-heterocyclic carbene) borane adduct 2 rapidly reacts with tritylium salts at room temperature but the outcome is strongly impacted by the respective counter-ion. Using tritylium tetrakis(perfluoro-tert-butoxy)aluminate affords - depending on the solvent - either the bis(boronium) ion 4 or the hydride-bridged dication 5. In case of tritylium hexafluorophosphate, however, H/F exchange occurs between boron and phosphorus yielding the dinuclear BF3 adduct 3 along with phosphorus dihydride trifluoride. H/F exchange also takes place when using the mononuclear N-heterocyclic carbene BH3 adduct 6 and hence provides a facile route to PH2 F3 , which is usually synthesized in more complex reaction sequences regularly involving toxic hydrogen fluoride. DFT calculations shed light on the H/F exchange between the borenium ion and the [PF6 ]- counter-ion and the computed mechanism features only small barriers in line with the experimental observations.

Intermolecular Triazene Alkene Cycloaddition via Lewis Base Catalysis: Access to Diverse Trifluoromethylated Pyrazolines.

A novel approach to chemoselective synthesis of biologically important CF3 -subsituted pyrazolines was developed via a Lewis base catalyzed intermolecular triazene cycloaddition reaction of an array of terminal/internal alkenes with CF3 CHN2 . This strategy features a catalytic amount of 1,8-diazabicyclo[5.4.0]undec-7-ene, high yields (up to 95 %), wide substrate scope and excellent functional group tolerance (54 examples). Importantly, we preformed scaffold diversification of a panel of known pharmaceuticals, natural products, and bioactive heterocycles to generate the corresponding pyrazoline derivatives with potential broad bioactivities for further development.

SN 2 versus E2 Competition of Cyclic Ethers.

We have quantum chemically studied the influence of ring strain on the competition between the two mechanistically different SN 2 and E2 pathways using a series of archetypal ethers as substrate in combination with a diverse set of Lewis bases (F- , Cl- , Br- , HO- , H3 CO- , HS- , H3 CS- ), using relativistic density functional theory at ZORA-OLYP/QZ4P. The ring strain in the substrate is systematically increased on going from a model acyclic ether to a 6- to 5- to 4- to 3-membered ether ring. We have found that the activation energy of the SN 2 pathway sharply decreases when the ring strain of the system is increased, thus on going from large to small cyclic ethers, the SN 2 reactivity increases. In contrast, the activation energy of the E2 pathway generally rises along this same series, that is, from large to small cyclic ethers. The opposing reactivity trends induce a mechanistic switch in the preferred reaction pathway for strong Lewis bases from E2, for large cyclic substrates, to SN 2, for small cyclic substrates. Weak Lewis bases are unable to overcome the higher intrinsic distortivity of the E2 pathway and, therefore, always favor the less distortive SN 2 reaction.

Latent Pronucleophiles in Lewis Base Catalysis: Enantioselective Allylation of Silyl Enol Ethers with Allylic Fluorides.

Lewis base catalyzed allylations of C-centered nucleophiles have been largely limited to the niche substrates with acidic C-H substituted for C-F bonds at the stabilized carbanionic carbon. Herein we report that the concept of latent pronucleophiles serves to overcome these limitations and allow for a variety of common stabilized C-nucleophiles, when they are introduced as the corresponding silylated compounds, to undergo enantioselective allylations using allylic fluorides. The reactions of silyl enol ethers afford the allylation products in good yields and with high degree of regio/stereoselectivity as well as diastereoselectivity when cyclic silyl enol ethers are used. Further examples of silylated stabilized carbon nucleophiles that undergo efficient allylation speak in favor of the general applicability of this concept to C-centered nucleophiles.

Cage-Shaped Phosphites Having C3 -Symmetric Chiral Environment: Steric Control of Lewis Basicity and Application as Chiral Ligands in Rhodium-Catalyzed Conjugate Additions.

Designing chiral ligands with an axial symmetry higher than C2 -rotational symmetry is one of the most crucial approaches to improving enantioselectivity in asymmetric synthesis. Herein, C3 -symmetric chiral cage-shaped phosphites are reported. Their Lewis basicity and chiral environment are precisely controlled by the tethered group. The cage-shaped phosphites successfully worked as chiral ligands in Rh-catalyzed asymmetric conjugate additions, realizing acceptable yields with excellent enantioselectivity, and were used to synthesize a pharmacologically important molecule.

Low-Temperature Preparation of High-Quality Perovskite Polycrystalline Films via Crystallization Kinetics Engineering.

Preparation of lead halide perovskite polycrystalline films at a low annealing temperature is highly restricted by their intrinsically large crystallization activation energy, which hinders the conversion of the precursors/intermediates to perovskites and yields as-prepared polycrystals with tiny grain sizes and terrible crystal quality. Herein, we demonstrate through in-situ, real-time spectroscopic studies that both the nucleation and crystal growth kinetics can be improved without the need for a high annealing temperature by treating the film with thiourea, as accounted for by the reduced activation energy. As a consequence, the thiourea-treated perovskite polycrystalline film exhibits larger grain sizes and greater crystallinity than the untreated one. More importantly, owing to the synergistic effect of the promoted crystallization kinetics and the passivation of surface defects, the low-temperature prepared films treated with thiourea even present more prominent photophysical properties than those fabricated by using the conventional high-temperature method. The strategy of crystallization kinetics engineering proposed in this work paves the way for fabricating high-quality perovskite polycrystalline films in a low-temperature manner.

Urea additive improves the performance of low bandgap tin-lead perovskite solar cells.

Recently, narrow bandgap tin-lead mixed perovskite solar cells (PSCs) have become a research hotspot because they can be applied in tandem cells to break the Shockley-Queisser radiative limit of the single junction PSCs. However, the introduction of tin, on the one hand, makes the crystal quality of perovskite thin film worse, leading to the increase of film defects; on the other hand, the easy oxidation of divalent tin also leads to the increase of defect states, which seriously affects the photoelectric conversion efficiency of tin-lead cell devices. Good crystallization and low defect density of perovskite layer are very important to ensure good light absorption and photogenerated carrier generation and transport. Here, we adjust the crystallization of tin-lead perovskite films by a Lewis base-urea (CO(NH2)2), which significantly increases the grain size and improves the film morphology. At the same time, because of the Lewis base property of urea, the uncoordinated Pb2+and Sn2+defects of Lewis acids in the tin-lead films are effectively passivated, and the occurrence of non-radiative recombination in the films is reduced. Under the dual effects of improving crystallization and passivating defects, the photoelectric performance of tin-lead perovskite solar cell devices is significantly improved to 18.1% compared with the original device of 15.4%.

Cooperative Chiral Lewis Base/Palladium-Catalyzed Asymmetric Syntheses of Methylene-Containing δ-Lactams.

We herein report a two-step approach for the enantioselective synthesis of novel chiral δ-lactams. By using a cooperative chiral ITU/achiral Pd-catalyst system, this protocol proceeds via an asymmetric α-allylation of activated aryl esters first, followed by an acid-mediated lactam formation. A variety of differently substituted products could be obtained with usually high levels of enantioselectivities and in reasonable yields (16 examples, up to 98 : 2 er and 73 % yield over two steps). In addition, further utilizations of the products via transformations of the exocyclic double bond were successfully carried out as well.