Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.     

Effect of butterfly-shaped sulfur-bridged ligand and counter anions on the catalytic activity and diastereoselectivity of organobismuth complexes

In the direct Mannich reaction and synthesis of α,β-unsaturated ketones, the use of organobismuth complexes as catalysts leads to high diastereoselectivity and products of single trans conformation. In this paper, we illustrate the relationship between structure and catalytic activity as well as diastereoselectivity of organobismuth complexes having a 5,6,7,12-tetrahydrodibenz [c,f][1,5]thiobismocine framework as well as bearing a butterfly-shaped sulfur-bridged ligand and tunable anions. With the exposed bismuth center acting as a Lewis acid site and the uncoordinated lone pair electrons of sulfur as a Lewis base site, the cationic organobismuth complexes work as bifunctional Lewis acid/base catalysts. Due to the steric influence of the butterfly-shaped structure and synergistic effect of Lewis acid and Lewis base centers, the complexes can direct substrate attack in organic synthesis. By adjusting the electron-withdrawing ability of the counter anions, the S-Bi bond strength can be regulated, leading to a significant change in Lewis acidity and Lewis basicity as well as catalytic activity. Through synergistic modulation of the above effects, one can control the diastereoselectivity of the organobismuth complexes for the generation of a single diastereoisomer.     

The Behavior of Acid Halides Toward Lewis Acids and Lewis Bases

Reactions of some typical acid halides of carbonic and trithiocarbonic acids and of orthophosphoric and sulfuric acids with Lewis acids and Lewis bases are compared. Acylium, perfluoroacylium, thioacylium, and even sulfonylium ions are obtainable with Lewis acids. It is possible by conductivity measurements and by electronic and above all IR spectroscopic investigations to determine whether the 1:1 adducts of acid halides and Lewis compounds are acylium or sulfonylium salts or donor-acceptor complexes. In the reaction with Lewis bases, the halogen atom in the acid halide is replaced by the electron donor, generally with formation of nonpolar molecular compounds or complexes.     

Late‐Transition‐Metal Complexes as Tunable Lewis Bases

Syntheses of the first heteroleptic N‐heterocyclic carbene (NHC)–phosphane platinum(0) complexes and formation of the corresponding Lewis acid–base adducts with aluminum chloride is reported. The influence of N‐heterocyclic carbenes on tuning the Lewis basic properties of the metal complexes was judged from spectroscopic, structural, and computational data. Conclusive experimental evidence for the enhanced Lewis basicity of NHC‐containing complexes was provided by a transfer reaction.     

Discotic salicylaldimato metal complexes exhibiting columnar mesophases and their mixtures with rod-like salicylaldimato complexes

The synthesis and mesomorphism of a series of discotic metal complexes of salicylaldimine ligands is reported. None of the ligands was mesomorphic, but the longer chain length complexes showed a Col_h phase as evidenced by optical microscopy. Phase diagrams were constructed using rod- and disk-like salicylaldimato complexes, where one was designed to have Lewis acid characteristics and the other Lewis base properties. It was hoped that an associative Lewis acid/base interaction would prevent phase separation of the two components, which may point a way to the realization of a mixture with a biaxial nematic phase. The form of the phase diagrams is discussed.     

Quantitative evaluation of Lewis acidity of metal ions with different ligands and counterions in relation to the promoting effects of Lewis acids on electron transfer reduction of oxygen

The g(zz) values of ESR spectra of superoxide (O(2)(.-) complexes of metal ion salts acting as Lewis acids with different ligands and counterions were determined in acetonitrile at 143 K. The binding energies (DeltaE) of (O(2)(.-)/Lewis acid complexes have been evaluated from deviation of the g(zz) values from the free spin value. The DeltaE value is quite sensitive to the difference in the counterions and ligands of metal ion salts acting as Lewis acids. On the other hand, the fluorescence maxima of the singlet excited states of 10-methylacridone/Lewis acid complexes are red-shifted as compared with that of 10-methylacridone, and the relative emission energies (Deltahnu(f)) vary significantly depending on the Lewis acidity of metal ion salts with different counterions and ligands. The promoting effects of Lewis acids were also examined on electron transfer from cobalt(II) tetraphenylporphyrin to oxygen in acetonitrile at 298 K, which does not occur in the absence of Lewis acids under otherwise the same experimental conditions. Both DeltaE and Deltahnu(f) values are well correlated with the promoting effects of Lewis acids on the electron transfer reduction of oxygen. Such correlations indicate that DeltaE and Deltahnu(f) values can be used as quantitative measures of Lewis acidity of metal ion salts with different ligands and counterions. The Lewis acidity thus determined can also be applied to predict the promoting effects of Lewis acids on organic synthesis.     

Investigation of Lewis Acid versus Lewis Base Catalysis in Asymmetric Cyanohydrin Synthesis

The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}₂] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO₃ or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}₂O] and triphenylphosphine oxide, a non‐linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron‐deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case.     

Donor-acceptor complexes of borazines

Donor-acceptor complexes of borazine (BZ) and its substituted derivatives with Lewis acids (A = MCl(3), MBr(3); M = B, Al, Ga) and Lewis bases (D = NH(3), Py) have been theoretically studied at the B3LYP/TZVP level of theory. The calculations showed that complexes with Lewis bases only are unstable with respect to dissociation into their components, while complexes with Lewis acids only (such as aluminum and gallium trihalides) are stable. It was shown that formation of ternary D→BZ→A complexes may be achieved by subsequent introduction of the Lewis acid (acceptor A) and the Lewis base (donor D) to borazine. The nature of substituents in the borazine ring, their number, and position were shown to have only minor influence on the stability of ternary D→BZ→A complexes due to the compensation effect. Much weaker acceptor properties of borazine are explained in terms of large endothermic pyramidalization energy of the boron center in the borazine ring. In contrast to borazine, binary complexes of the isoelectronic benzene were predicted to be weakly bound even in the case of very strong Lewis acids; ternary DA complexes of benzene were predicted to be unbound. The donor-acceptor complex formation was predicted to significantly reduce both the endothermicity (by 70-95 kJ mol(-1)) and the activation energy (by 40-70 kJ mol(-1)) for the borazine hydrogenation. Thus, activation of the borazine ring by Lewis acids may be a facile way for the hydrogenation of borazines and polyborazines.     

Chelation-controlled radical chain reactions studied by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) is a novel tool for the investigation of chemical reactions in solution and for the direct detection and identification of reactive intermediates. The tributyltin hydride mediated addition of tert-butyl iodide to dimethyl 2-cyclohexyl-4-methyleneglutarate (2) in the presence of Lewis acids was investigated by ESI-MS using a microreactor coupled on-line to an ESI mass spectrometer. For the first time we have been able to show that transient radicals in radical chain reactions can be detected unambiguously under steady-state conditions in the reaction solution and can be characterized by ESI-MS/MS and accurate mass determination. The detection of different heterodimer radical complexes by ESI-MS/MS has provided new insights into the mechanism of Lewis acid controlled radical chain reactions. Dimeric chelate complexes of glutarates, such as 2 and 3, and Lewis acids, like Sc(OTf)3, MgBr2OEt2 and LiClO4, were observed as well as higher aggregates with additional equivalents of Lewis acid. Evidence for a dynamic equilibrium of the complexes in solution was found by NMR spectroscopy. The ESI-MS investigation of the chelation of glutarate 2 with various Lewis acids has led to the conclusion that the tendency for Lewis acids to form dimeric chelate complexes and higher aggregates has an important effect on the stereoselective outcome of the radical reactions.     

Halogen bond and its counterparts: Bent's rule explains the formation of nonbonding interactions

Ab Initio calculations were carried out on complexes of F(3)CCl. The Cl center may act as the Lewis acid as well as the Lewis base. Hence various interactions are analyzed for several complexes of F(3)CCl: halogen bond, dihalogen bond, halogen-hydride bond, hydrogen bond, and others. It was found that Lewis acid-Lewis base interactions lead to the electron charge redistribution being in agreement with Bent's rule. This rule explains the formation of nonbonding interactions. For example, if the C-Cl bond acts as the Lewis acid thus the increase of the s-character in C-orbital of C-Cl is observed as a result of complexation. However, if the C-Cl bond acts as the Lewis base thus the mentioned above s-character decreases. Numerous analogies between the halogen bond and the hydrogen bond are analyzed and various relationships between energetic, geometrical and the natural bond orbitals method (NBO) parameters are shown.     

Lewis Acid-Assisted Molybdenum(VI) Complexes with S,N-bidentate Ligands to Reduce Nitrate

Two readily-accessible molybdenum complexes with low sterically hindered S, N-bidentate ligands were designed for the reduction of nitrate to nitrite. The Lewis acid Sc(III) acted as a significant co-catalyst to enhance the catalytic efficiency by activating the N−O bond of nitrate. This study indicates that molybdenum complexes in cooperation with a Lewis acid offer functional Mo-catalyst systems, which present functional artificial models of the natural molybdenum enzymes.     

全氟烷基磺酸盐和全氟烷基磺酰亚胺盐的多相化催化研究

传统的Lewis酸催化剂在环境的压力下受到挑战,全氟烷基磺酸盐和全氟烷基磺酰亚胺盐作为均相、高效的Lewis酸催化剂在有机合成中受到人们的关注.为了简化分离操作,人们对全氟烷基磺酸盐和全氟烷基磺酰亚胺盐的多相化进行了研究,并已取得巨大进展.本文综述了全氟烷基磺酸盐和全氟烷基磺酰亚胺盐分别负载在有机载体、无机载体以及离子液体上的多相化催化最新研究进展,简要概括了其制备方法和催化活性,并对其催化应用前景进行了展望.     

Multi-step detection of cyanide ion by a bis(dimesitylboryl)dibenzoazaborine

A bis(dimesitylboryl)dibenzoazaborine formed Lewis acid-base type complexes with up to two cyanide ions in stepwise fashion. The complex formation constants for cyanide ion were larger than those for fluoride ion, despite the higher affinity of fluoride ion to boron atoms than that of cyanide ion.     

Paramagnetic complexes of 9, 10-anthraquinone and 9-fluorenone on the metal oxide surface

Paramagnetic complexes of 9, 10-anthraquinone and 9-fluorenone adsorbed on the surface of calcium, magnesium, zinc, zirconium, and aluminum oxides and modified Al₂O₃ as well as on mixed oxides were studied by ESR and electron-nuclear double resonance. Radical anions that do not interact with Lewis acid sites are generated on the surfaces of oxides with electrondonating properties (CaO, MgO). Paramagnetic complexes of the anthraquinone or fluorenone radical anion with Lewis acid sites (coordinatively unsaturated metal cations) are formed in other cases. Several types of similar complexes can be formed. Mechanisms of interaction of the probe molecules with the metal oxide surface were proposed.     

Quantifying cooperative intermolecular interactions for improved carbon dioxide capture materials

We have optimized the geometry and calculated interaction energies for over 100 different complexes of CO(2) with various combinations of electron accepting (Lewis acid) and electron donating (Lewis base) molecules. We have used the recently developed explicitly correlated coupled cluster singles doubles and perturbative triples [CCSD(T)-F12] methods and the associated VXZ-F12 (where X = D,T,Q) basis sets. We observe only modest changes in the geometric parameters of CO(2) upon complexation, which suggests that the geometry of CO(2) adsorbed in a nanoporous material should be similar to that of CO(2) in gas phase. When CO(2) forms a complex with two Lewis acids via the two electron rich terminal oxygen atoms, the interaction energy is less than twice what would be expected for the same complex involving a single Lewis acid. We consider a series of complexes that exhibit simultaneous CO(2)-Lewis acid and CO(2)-Lewis base intermolecular interactions, with total interaction energies spanning 14.1-105.9 kJ mol(-1). For these cooperative complexes, we find that the total interaction energy is greater than the sum of the interaction energies of the constituent complexes. Furthermore, the intermolecular distances of the cooperative complexes are contracted as compared to the constituent complexes. We suggest that metal-organic-framework or similar nanoporous materials could be designed with adsorption sites specifically tailored for CO(2) to allow cooperative intermolecular interactions, facilitating enhanced CO(2) adsorption.     

Lewis Acid Coupled Electron Transfer of Metal–Oxygen Intermediates

Redox‐inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal–oxygen intermediates, such as metal–oxo and metal–peroxo complexes. The mechanisms of the oxidative C?H bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)–oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)₃ have been unified as rate‐determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)₃) by iron(III)–oxo complexes. All logarithms of the observed second‐order rate constants of Lewis acid‐promoted oxidative C?H bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)–oxo complexes exhibit remarkably unified correlations with the driving forces of proton‐coupled electron transfer (PCET) and metal ion‐coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)₃ to the metal–oxo moiety has been confirmed for Mn^IV–oxo complexes. The enhancement of the electron‐transfer reactivity of metal–oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox‐inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)–peroxo complexes, resulting in acceleration of the electron‐transfer reduction but deceleration of the electron‐transfer oxidation. Such a control on the reactivity of metal–oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca²⁺ in the oxidation of water to dioxygen by the oxygen‐evolving complex in photosystem II.     

On the Conformation and Structure of Organometal Complexes in the Solid State: Two Studies Relevant to Chemical Synthesis

Conformational preferences of two classes of organometal complexes have been surveyed by inspecting the Cambridge Structural Database (CSD). Lewis acid carbonyl complexes demonstrate a variety of coordination geometries, depending on the electronic and steric requirements of the carbonyl ligands and the nature of the Lewis acid. Similarly, the solid-state conformation of various π-bonding ligands in metal acyl, metal nitrosyl, metal acetylene, and metal imido complexes is revealed. These insights have stimulated the development of a conformational model that is based on considerations of π-bond hybridization and frontier molecular orbital theory. The analyses are relevant to the mechanism and transition structures of many synthetically important transformations. A deeper understanding of the conformational properties of organometal complexes, based on accurate structural information, will likely expedite the design and improvement of metal-mediated processes.     

Design of Stereoelectronically Promoted Super Lewis Acids and Unprecedented Chemistry of Their Complexes

A new family of stereoelectronically promoted aluminum and scandium super Lewis acids is introduced on the basis of state‐of‐the‐art computations. Structures of these molecules are designed to minimize resonance electron donation to central metal atoms in the Lewis acids. Acidity of these species is evaluated on the basis of their fluoride‐ion affinities relative to the antimony pentafluoride reference system. It is demonstrated that introduced changes in the stereochemistry of the designed ligands increase acidity considerably relative to Al and Sc complexes with analogous monodentate ligands. The high stability of fluoride complexes of these species makes them ideal candidates to be used as weakly coordinating anions in combination with highly reactive cations instead of conventional Lewis acid–fluoride complexes. Further, the interaction of all designed molecules with methane is investigated. All studied acids form stable pentavalent‐carbon complexes with methane. In addition, interactions of the strongest acid of this family with very weak bases, namely, H₂, N₂, carbon oxides, and noble gases were investigated; it is demonstrated that this compound can form considerably stable complexes with the aforementioned molecules. To the best of our knowledge, carbonyl and nitrogen complexes of this species are the first hypothetical four‐coordinated carbonyl and nitrogen complexes of aluminum. The nature of bonding in these systems is studied in detail by various bonding analysis approaches.     

Lewis Acids as Activators in CBS‐Catalysed Diels–Alder Reactions: Distortion Induced Lewis Acidity Enhancement of SnCl4

The effect of several Lewis acids on the CBS catalyst (named after Corey, Bakshi and Shibata) was investigated in this study. While ²H NMR spectroscopic measurements served as gauge for the activation capability of the Lewis acids, in situ FT‐IR spectroscopy was employed to assess the catalytic activity of the Lewis acid oxazaborolidine complexes. A correlation was found between the Δδ(²H) values and rate constants k_DA, which indicates a direct translation of Lewis acidity into reactivity of the Lewis acid–CBS complexes. Unexpectedly, a significant deviation was found for SnCl₄ as Lewis acid. The SnCl₄–CBS adduct was much more reactive than the Δδ(²H) values predicted and gave similar reaction rates to those observed for the prominent AlBr₃–CBS adduct. To rationalize these results, quantum mechanical calculations were performed. The frontier molecular orbital approach was applied and a good correlation between the LUMO energies of the Lewis acid–CBS–naphthoquinone adducts and k_DA could be found. For the SnCl₄–CBS–naphthoquinone adduct an unusual distortion was observed leading to an enhanced Lewis acidity. Energy decomposition analysis with natural orbitals for chemical valence (EDA‐NOCV) calculations revealed the relevant interactions and activation mode of SnCl₄ as Lewis acid in Diels–Alder reactions.     

Slow Reactant–Water Exchange and High Catalytic Performance of Water‐Tolerant Lewis Acids

³¹P nuclear magnetic resonance (NMR) spectroscopic measurement with trimethylphosphine oxide (TMPO) was applied to evaluate the Lewis acid catalysis of various metal triflates in water. The original ³¹P NMR chemical shift and line width of TMPO is changed by the direct interaction of TMPO molecules with the Lewis acid sites of metal triflates. [Sc(OTf)₃] and [In(OTf)₃] had larger changes in ³¹P chemical shift and line width by formation of the Lewis acid–TMPO complex than other metal triflates. It originates from the strong interaction between the Lewis acid and TMPO, which results in higher stability of [Sc(OTf)₃TMPO] and [In(OTf)₃TMPO] complexes than other metal triflate–TMPO complexes. The catalytic activities of [Sc(OTf)₃] and [In(OTf)₃] for Lewis acid‐catalyzed reactions with carbonyl compounds in water were far superior to the other metal triflates, which indicates that the high stability of metal triflate–carbonyl compound complexes cause high catalytic performance for these reactions. Density functional theory (DFT) calculation suggests that low LUMO levels of [Sc(OTf)₃] and [In(OTf)₃] would be responsible for the formation of stable coordination intermediate with nucleophilic reactant in water.