Unique Properties and Emerging Applications of Carbolong Metallaaromatics.

ConspectusAromatic compounds are important in synthetic chemistry, biomedicines, and materials science. As a special type of aromatic complex, transition-metal-based metallaaromatics contain at least one transition metal in an aromatic framework. The chemistry of metallaaromatics has seen much progress in computational studies and synthetic methods, but their properties and applications are still emerging. In recent years, we have disclosed a series of metal-centered conjugated polycyclic metallacycles in which a carbon chain is chelated to a metal center through at least three metal-carbon bonds. These are termed carbolong complexes and exhibit good stability to water, oxygen, light, and heat on account of their polydentate chelation and aromaticity, making them easy to handle. Carbolong complexes are not only special π-conjugated aromatics but also organometallics; therefore, they have the properties of both species. In this Account, we showcase the recent advances in their applications based on their different properties.First, carbolong complexes are a special kind of π-conjugated aromatic, with the ability to transmit electrons, allowing them to function as single-molecule conductors and candidates for electron transporting layer materials (ETLs) in solar cells. A series of carbolong complexes have been proved to be useful as achievable ETLs which enhance device performance in both organic solar cells and perovskite solar cells.Second, due to the involvement of d orbitals in the conjugation, carbolong complexes normally exhibit strong and broad absorption, even in some cases extending to the near-infrared region (NIR). The absorbed optical energy can be converted into light, heat, and ultrasound; consequently, carbolong compounds can be used as core moieties in smart materials. For example, 7C carbolong complexes were found to exhibit aggregation-enhanced near-infrared emission (AIEE). Some 12C carbolong complexes have been designed into the core moieties of NIR-responsive polymers, such as cylindrical NIR-responsive materials, self-healing materials, and shape memory materials. In contrast to the stereotypically toxic osmium compounds such as the highly toxic OsO4, some osmium carbolong complexes exhibit low cell cytotoxicity and good biocompatibility; consequently, they also have potential applications in the biomedical area. For example, benefiting from broad absorption in the NIR, 9C and 12C carbolong complexes have been used in photoacoustic imaging and photothermal therapy, respectively. In addition, photodynamic therapeutic applications which take advantage of a carbolong peroxo complex are discussed.Third, as special transition-metal complexes chelated by carbon-based ligands, a carbolong peroxo complex has displayed catalytic activity in the dehydrogenation of alcohols and a bimetallic carbolong complex has been used to catalyze difunctionalization reactions of unactivated alkenes.Overall, aromatic carbolong complexes have been applied to photovoltaics, smart materials, phototherapy, and catalytic reactions. Moving forward, we hope that this Account will shed light on future studies and theoretical research and encourage more discoveries of the properties of other metallaaromatics.

Craig-Hückel Hybrid Aromatic Metalla-dehydro[11]annulenes Constructed by a Formal [10+1] Cycloaddition Reaction.

Aromatic metalla-annulenes are important aromatic compounds, research into which has been mainly concentrated on metal-benzenes and their lower homologues. Reports on their superior homologs are rare, and this has greatly limited the systematic study of their properties. In this work, a series of osma-dehydro[11]annulenes with good air and thermal stability were prepared in high yields through a simple [10+1] strategy, by incorporating a metal fragment into conjugated ten-carbon chains in a one-pot reaction. They are the first monometallic aromatic metalla-[n]annulenes with the ring size larger than 6, and their Craig-Hückel hybrid aromaticity is supported by various physical and computational parameters. Besides, these complexes show versatile reactivities, not only giving further evidence for their aromaticity, but also demonstrating their physical and chemical properties can easily be regulated. This work enriches the metalla-aromatic chemistry, and provides a new avenue for the synthesis of large metalla-annulenes with different ring sizes.

Isolation, Reactivity, and Tunable Properties of a Strained Antiaromatic Osmacycle.

Strain and antiaromaticity in compounds are recognized as two substantial destabilizing features, and consequently, realization of dual destabilizing features in a single molecule is challenging and far more difficult in a single ring. Moreover, transformation of an antiaromatic framework to different antiaromatic or aromatic species is a significant subject in antiaromatic chemistry and has attracted increasing interest. In this work, we isolated a highly strained antiaromatic metallacycle in which a cyclic metal vinylidene unit is embedded. Computational studies revealed its ring strain energies and antiaromatic character and showed that the metal incorporation and the phosphonium substituents play a crucial role in its stabilization. The mechanism of its formation has been illustrated by density functional theory (DFT) calculations and the isolation of a key intermediate. We further discovered diverse reactivities and structural reshuffling of this unusual strained antiaromatic complex according to its two destabilizing characters. We obtained two isomers of metallaindenes fused with oxiranes from the direct oxidation of the metal vinylidene or by nucleophilic addition to an isolated metallacyclocumulene formed by the reaction of metal vinylidene with hydroxide ion, achieving a reconfiguration of the antiaromatic framework. Transformations of the antiaromatic metallacycle by electrophiles to various aromatic metallaindynes have been achieved, and that a condensed Fukui function was employed to confirm the regioselectivity of the electrophilic additions, and the acid/base-induced aromaticity switch along with tunable photophysical properties were investigated. These interesting transformations not only enrich the chemistry of metal vinylidenes and antiaromatics and could also perform potentially as switchable optical materials.

Synthesis and Optoelectronic Applications of dπ -pπ Conjugated Polymers with a Di-metallaaromatic Acceptor.

The development of conjugated polymers especially n-type polymer semiconductors is powered by the design and synthesis of electron-deficient building blocks. Herein, a strong acceptor building block with di-metallaaromatic structure was designed and synthesized by connecting two electron-deficient metallaaromatic units through a π-conjugated bridge. Then, a double-monomer polymerization methodology was developed for inserting it into conjugated polymer scaffolds to yield metallopolymers. The isolated well-defined model oligomers indicated polymer structures. Kinetic studies based on nuclear magnetic resonance and ultraviolet-visible spectroscopies shed light on the polymerization process. Interestingly, the resulted metallopolymers with dπ -pπ conjugations are very promising electron transport layer materials which can boost photovoltaic performance of an organic solar cell, with power conversion efficiency up to 18.28 % based on the PM6 : EH-HD-4F non-fullerene system. This work not only provides a facile route to construct metallaaromatic conjugated polymers with various functional groups, but also discovers their potential applications for the first time.

Metallaaromatic Chemistry: History and Development.

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Halide Ion-Mediated Synthesis of L10-FePt Nanoparticles with Tunable Magnetic Properties.

L10-FePt nanoparticles (NPs) have great potential in areas of advanced magnetic and catalytic applications. Here, we present a facile control route for synthesis of hard magnetic L10-FePt NPs in which halide ions (Cl-, Br-, or I-) were added to the synthetic process to promote the phase transformation. It is confirmed that the strong ionic binding force between halide ions and Fe3+ or Pt2+ ions could facilitate the formation of L10-FePt phase due to favoring growth of FePt NPs in a more thermodynamically stable way, which enables the formation of an ordered structure. L10-FePt NPs with the highest coercivity of 8.64 kOe and saturation magnetization of 64.21 emu/g at room temperature can be directly obtained by controlling the amount of the halide ions. In comparison with conventional solution phase reduction methods, the halide ion-assisted method shows enhanced capability to tune the growth of hard magnetic bimetallic NPs, particularly Pt-based bimetallic NPs.

Visible-Light Induced Direct Synthesis of Polysubstituted Furans from Cyclopropyl Ketones.

In this article, a photoredox protocol for the synthesis of furans via oxidative coupling of olefin generated in situ from cyclopropyl ketones with ketonic oxygen atom is presented. Moreover, bromination of furans in the presence of overstoichiometric oxidant has been achieved with high regioselectivity.

Synthesis and reactivity of mononuclear iron models of [Fe]-hydrogenase that contain an acylmethylpyridinol ligand.

[Fe]-hydrogenase has a single iron-containing active site that features an acylmethylpyridinol ligand. This unique ligand environment had yet to be reproduced in synthetic models; however the synthesis and reactivity of a new class of small molecule mimics of [Fe]-hydrogenase in which a mono-iron center is ligated by an acylmethylpyridinol ligand has now been achieved. Key to the preparation of these model compounds is the successful C-O cleavage of an alkyl ether moiety to form the desired pyridinol ligand. Reaction of solvated complex [(2-CH2CO-6-HOC5H3N)Fe(CO)2(CH3CN)2](+)(BF4)(-) with thiols or thiophenols in the presence of NEt3 yielded 5-coordinate iron thiolate complexes. Further derivation produced complexes [(2-CH2CO-6-HOC5H3N)Fe(CO)2(SCH2CH2OH)] and [(2-CH2CO-6-HOC5H3N)Fe(CO)2(CH3COO)], which can be regarded as models of FeGP cofactors of [Fe]-hydrogenase extracted by 2-mercaptoethanol and acetic acid, respectively. When the derivative complexes were treated with HBF4 ⋅Et2O, the solvated complex was regenerated by protonation of the thiolate ligands. The reactivity of several models with CO, isocyanide, cyanide, and H2 was also investigated.

Syntheses and reactivities of strained fused-ring metallaaromatics containing planar eleven-carbon chains.

Carbolong complexes are one of the primary types of metallaaromatics, and they include metallapentalynes and metallapentalenes. A series of 7C-10C and 12C-carbolong complexes with planar ligand skeletons respectively containing 7-10 and 12 carbon atoms in their backbones, have been previously reported. Herein, two classes of strained substances, metallabenzyne-fused metallapentalenes and metallabenzene-fused metallapentalynes, were prepared, both representing 11C-carbolong complexes with a planar carbon-chain ligand. Furthermore, the former type is also the carbolong derivatives containing a metallabenzyne skeleton, another primary metallaaromatic framework. Metallabenzyne-fused metallapentalenes show versatile reactivities, and the most interesting one is the metal carbyne bond shift from a 6-membered to a more strained 5-membered ring, affording the above-mentioned metallabenzene-fused metallapentalyne. This work makes carbolong chemistry more complete, and provides a method to achieve metallabenzynes, which is anticipated to concurrently advance the development of these two types of metallaaromatics.

Isomerization reactions of metal vinylidene units.

Isomerization reactions of unsaturated molecules offer an efficient strategy in atom-economical synthesis. Although isomerization reactions of unsaturated organic and organometallic compounds, such as alkenes, alkynes, and metal carbynes, have been achieved, those of metal vinylidene units that contain cumulated double bonds have never been reported. Herein, we inaugurally discovered isomerization reactions of metal vinylidene units via protonation and deprotonation reactions of metal carbenes. Experimental and theoretical investigations indicate that the electrical characteristics of substituents on the rings play a crucial role in controlling the formation of metal vinylidene units. The isomerization reactions of metal vinylidene units were driven by thermodynamic forces. Moreover, one of the angles at metal vinylidenes was found as 126.9°, representing the smallest angle in metal vinylidenes and the first cyclic 4d transition metal (Ru) vinylidene complex was successfully isolated. These investigations unveil novel structures and reactivity for metal vinylidenes, offering a fresh perspective on the isomerization reactions of unsaturated molecules containing cumulative unsaturated bonds.

Reversible dimerization of mononuclear models of [Fe]-hydrogenase.

Enzyme models on the catwalk! A series of new model complexes of the active site of [Fe]-hydrogenase have been synthesized and characterized. These complexes are monomeric in solution, but dimeric in the solid state.

Hetero-carbolong chemistry: experimental and theoretical studies of diaza-metallapentalenes.

Four diaza-osmapentalenes were prepared by two-step reactions, through the treatment of an alkyne-coordinated osmium complex with azo compounds, followed by the addition of AgSbF6/CO. Their aromaticity was confirmed by crystal parameters, NMR spectra and theoretical calculations. These complexes are the first diaza-metallapentalenes representing a new class of metallaaromatics.

Experimental and theoretical evidences for the formation of transition metal complexes with five coplanar metal-carbon σ bonds.

The σ bond is an important concept in chemistry, and the metal-carbon (M-C) σ bond in particular is a central feature in organometallic chemistry. Synthesis of stable complexes with five coplanar M-C σ bonds is challenging. Here, we describe the synthesis of two different types of stable complexes with five coplanar M-C σ bonds, and examine the stability of such complexes which use rigid conjugated carbon chains to chelate with the metal center. Density functional theory (DFT) calculations show that the M-C σ bonds in these complexes have primarily a covalent character. Besides the σ nature, there are also a π conjugation component among the metal center and carbons, which causes delocalization. This work expanded the coplanar M-C σ bonds to five.

Reshaping aromatic frameworks: expansion of aromatic system drives metallabenzenoids to metallapentalenes.

Reshaping an aromatic framework to generate other skeletons is a challenging issue due to the stabilization energy of aromaticity. Such reconfigurations of aromatics commonly generate non-aromatic products and hardly ever reshape to a different aromatic framework. Herein, we present the transformation of metallaindenols to metallapentalenes and metallaindenes in divergent pathways, converting one aromatic framework to another with an extension of the conjugation framework. The mechanistic study of this transformation shows that phosphorus ligands play different roles in the divergent processes. Further theoretical studies indicate that the expansion of the aromatic system is the driving force promoting this skeletal rearrangement. Our findings offer a new concept and strategy to reshape and construct aromatic compounds.

A pyridinol acyl cofactor in the active site of [Fe]-hydrogenase evidenced by the reactivity of model complexes.

Understanding the complex: The decomposition reaction of a water-soluble complex (see scheme; 1) in H(2)O confirms the existence of a unique bidentate pyridinol cofactor in [Fe]-hydrogenase. This unique moiety is confirmed for the first time by the decomposition of a well-defined model complex containing a pyridinyl methyl acyl ligand.

Synthesis and characterization of a series of model complexes of the active site of [Fe]-hydrogenase (Hmd).

A series of Fe complexes were synthesized and characterized as small molecule mimics for the active site of [Fe]-hydrogenase (Hmd). The collection includes both structurally new compounds and analogues of previously reported models. These complexes contain the essential ligands of the enzyme, namely, acyl, CO, pyridone, and sulfur ligands. They serve as IR and Mössbauer spectroscopic models for the Fe center in [Fe]-hydrogenase. The field-dependent Mössbauer study of representative model complexes shows that the sign and absolute value of the quadrupole splitting are sensitive to the change in the ligand environment of the Fe center.

Carbolong chemistry: nucleophilic aromatic substitution of a triflate functionalized iridapentalene.

The reactivity of the triflate functionalized iridapentalene 1, [Ir{[double bond, length as m-dash]CHC(CH2C(CO2Me)2CH2)[double bond, length as m-dash]CC[double bond, length as m-dash]CHC(OTf)[double bond, length as m-dash]CH}(CO)(PPh3)2]OTf, with C-, N-, O- and S-centered neutral nucleophiles was studied, leading to the isolation of a wide array of irida-carbolong derivatives. As an extension, a polycyclic complex with a rare six-fused-ring structure was constructed. This strategy provides a new route for the construction of functionalized metallaaromatic complexes, and the resulting iridacycles exhibit broad spectral absorption ranges, making them potential photoelectric materials.

Synthesis and reactivity of iron acyl complexes modeling the active site of [Fe]-hydrogenase.

A [Fe(II)Fe(II)] dithiolate complex containing acyl and carbonyl ligands was synthesized. The diiron complex reacted with phosphine, cyanide, isocyanide, and CO to give monomeric Fe(II) complexes reproducing the first coordination sphere of the active site of [Fe]-hydrogenase (H(2)-forming methylene-tetrahydromethanopterin dehydrogenase, Hmd) in various states. All the acyl and carbonyl carbons in the diiron complex underwent facile isotopic exchange with (13)CO via monomeric Fe tricarbonyl intermediates.

An iron carbonyl pyridonate complex related to the active site of the [Fe]-hydrogenase (Hmd).

A mononuclear iron bis(carbonyl) pyridonate complex (1), which exhibits several common structural features with the active site of the iron-sulfur cluster-free [Fe]-hydrogenase, was synthesized and characterized. Spectroscopic data of 1 suggests a 2+ oxidation state for the Fe ion in the [Fe]-hydrogenase. Complex 1 serves as a precursor to other hydrogenase models.

Bidentate Ru(ii)-NC complexes as catalysts for the dehydrogenative reaction from primary alcohols to carboxylic acids.

Four Ru(ii)-NC complexes were synthesized by one-step processes from the corresponding NC ligands with RuHCl(CO)(PPh3)3. These complexes were tested as catalysts for alcohol dehydrogenative reactions, and complex {(C5H4N)-(C6H4)}RuCl(CO)(PPh3)2 (1) showed the highest activity. With KOH as the nucleophile and 0.5 mol% catalyst loading, a series of carboxylic acids were synthesized in toluene without any oxidant. Catalyst 1 could be transformed to complex {(C5H4N)-(C6H4)}RuH(CO)(PPh3)2 (6) when treated with KOH and benzyl alcohol. Complex 6 further reacted with PhCHO and H2O to generate product {(C5H4N)-(C6H4)}Ru(OCOPh)(CO)(PPh3)2 (7). Complexes 6 and 7 exhibited similar efficiency to complex 1, suggesting that they can be regarded as the catalytic intermediates of 1.