Invited for the cover of this issue is the group of Moritz Schmidt at the Helmholtz‐Zentrum Dresden–Rossendorf. The image depicts the relative strength of bonds from an actinide to a pyrrole‐based ...ligand in comparison with the salen ligand. Read the full text of the article at 10.1002/chem.202102849.
In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as ...the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst–substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.
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•Pnictogen bonding (PnB) as a synthetic tool in coordination chemistry.•Both tri- or pentavalent pnictogen behaves as a PnB donor in coordination compounds.•Mono-, bi- or trifurcated ...PnB donor ability of a pnictogen centre is dependent on the R and Nu at R–Pn⋯Nu.
The pnictogen bond (PnB) is a noncovalent attraction between electrophilic pnictogen atoms, originated from the anisotropic distribution of electron density on Pn in a molecular entity, and a nucleophilic (Nu) region in the same (intramolecular) or another (intermolecular) molecular entity: R–Pn⋯Nu (Pn = N, P, As, Sb or Bi; R = electron withdrawing atom or group; Nu = Ha, Ch, Pn, π-system, anion, etc.). Like the halogen and chalcogen bonds, PnB is a directional noncovalent interaction with a preference for a linear geometry, R–Pn⋯Nu angle approaching 180°. In contrast to the halogen and chalcogen atoms, the pnictogen atoms are able to introduce three electrophilic centres on the Pn atom (on account of the existence of three species bonded to the pnictogen atom), which gives an additional advantage in the use of these weak forces in coordination chemistry. In this review we highlight several recent relevant examples, based on X-ray crystal structure analyses, in which PnB is used as a synthon in the construction and design of coordination and organometallic compounds.
•Recent advancements in using Schiff bases (SBs) as corrosion inhibitors are surveyed.•Coordination bonding between SBs and metallic surfaces has been described.•Effect of polar substituents on ...coordination bonding has also been described with aid of DFT study.•SBs become effective by adsorbing on the metallic surface using through > C = N- moiety.•SBs behave as mixed-type corrosion inhibitors with some cathodic-predominance in few cases.
Schiff bases (SBs) possess the basic feature of compounds that are primarily qualified to test as corrosion inhibitors for different metal/electrolyte systems. SBs adsorb and form corrosion mitigating surface film through their electron rich centers including > C = N– (imine) moiety. The > C = N– (imine) moiety offers strong bonding with the metallic ions because of its π-acceptor properties. It is important to mention that SBs containing polar substituents at suitable positions can form chelates with the central metal ions therefore such SBs are expected to act as superior corrosion inhibitors as compared to the non-substituted SBs. Generally, electron donating substituents such as –OH (hydroxyl), –CH3 (methyl), –NH2 (amino) and –OCH3 (methoxy) are expected to increase the corrosion inhibition potentials of SBs. Halogens are also known to increase electron density at donor sites through their + R-effect. The present review article describes the collection on SBs as corrosion inhibitors for different metal/electrolyte systems. Electronic and molecular structural effects on the corrosion inhibition potential of SBs are discussed in relation to Hammett substituent constants. Interactions of SBs with metallic surfaces at molecular level are discussed in this report. A balanced level of hydrophilicity and hydrophobicity is indispensable for efficient corrosion inhibition activity. Extremely high hydrophobicity has a reducing effect corrosion inhibition performance by decreasing the solubility of the compounds in polar electrolytes.
Cycloisomerizations of enynes are probably the most representative carbon–carbon bond forming reactions catalyzed by electrophilic metal complexes. These transformations are synthetically useful ...because chemists can use them to build complex architectures under mild conditions from readily assembled starting materials. However, these transformations can have complex mechanisms. In general, gold(I) activates alkynes in the presence of any other unsaturated functional group by forming an (η2-alkyne)–gold complex. This species reacts readily with nucleophiles, including electron-rich alkenes. In this case, the reaction forms cyclopropyl gold(I) carbene-like intermediates. These can come from different pathways depending on the substitution pattern of the alkyne and the alkene. In the absence of external nucleophiles, 1,n-enynes can form products of skeletal rearrangement in fully intramolecular reactions, which are mechanistically very different from metathesis reactions initiated by the 2 + 2 cycloaddition of a Grubbs-type carbene or other related metal carbenes. In this Account, we discuss how cycloisomerization and addition reactions of substituted enynes, as well as intermolecular reactions between alkynes and alkenes, are best interpreted as proceeding through discrete cationic intermediates in which gold(I) plays a significant role in the stabilization of the positive charge. The most important intermediates are highly delocalized cationic species that some chemists describe as cyclopropyl gold(I) carbenes or gold(I)-stabilized cyclopropylmethyl/cyclobutyl/homoallyl carbocations. However, we prefer the cyclopropyl gold(I) carbene formulation for its simplicity and mnemonic value, highlighting the tendency of these intermediates to undergo cyclopropanation reactions with alkenes. We can add a variety of hetero- and carbonucleophiles to the enynes in the presence of gold(I) in intra- or intermolecular reactions, leading to the corresponding adducts with high stereoselectivity through stereospecific anti-additions. We have also developed stereospecific syn-additions, which probably occur through similar intermediates. The attack of carbonyl groups at the cyclopropyl carbons of the intermediate cyclopropyl gold(I) carbenes initiates a particularly interesting group of reactions. These trigger a cascade transformation that can lead to the formation of two C–C and one C–O bonds. In the fully intramolecular process, this stereospecific transformation has been applied for the synthesis of natural sesquiterpenoids such as (+)-orientalol F and (−)-englerin A. Intra- and intermolecular trapping of cyclopropyl gold(I) carbenes with alkenes leads to the formation of cyclopropanes with significant increase in the molecular complexity, particularly in cases in which this process combines with the migration of propargylic alkoxy and related OR groups. We have recently shown this in the stereoselective total synthesis of the antiviral sesquiterpene (+)-schisanwilsonene by a cyclization/1,5-acetoxy migration/intermolecular cyclopropanation. In this synthesis, the cyclization/1,5-acetoxy migration is faster than the alternative 1,2-acyloxy migration that would result in racemization.
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•Chalcogen bonding (ChB) as a supramolecular tool in coordination chemistry.•Both di- or tetravalent chalcogen acts as a ChB donor in metal complexes.•The strength and directionality ...of ChB are dependent on the nature of R, Ch and Nu at the R–Ch⋯Nu synthon.
The chalcogen bond (ChB) is defined as a noncovalent interaction between the electron density deficient region (so-called as σ or π hole) of a covalently bonded chalcogen atom and a nucleophilic (Nu) site in the same (intramolecular) or another (intermolecular) molecular entity: R–Ch⋯Nu Ch = O, S, Se or Te; R = C, Pn (pnictogen), Ch, metal, etc.; Nu = lone pair possessing Ha, Ch, Pn or metal atom, π-system, anion, radical, etc.. Like in halogen (Ha) and pnictogen (Pn) bonds, the bond parameters (strength, high directionality and tunability) make ChB a relevant supramolecular tool in the design of the secondary coordination sphere of metal complexes, which concerns an important synthetic strategy in the improvement of functional properties of materials. In this review we discuss/illustrate several relevant examples, taken from the Cambridge Structural Database, in which ChB plays a crucial role in the decoration of the secondary coordination sphere of coordination compounds, controlling molecular conformation, packing and aggregation of tectons, as well as formation of supramolecular 0D aggregates, 1D chains, 2D layers, 3D frameworks, etc.
This review classifies and summarizes the past 10–15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse ...in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.
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