Recent Advances in the Reactions of Cyclic Carbynes

Review Recent Advances in the Reactions of Cyclic Carbynes

Qian Su, Jipeng Ding, Zhihui Du, Yunrong Lai, Hongzuo Li, Ming-An Ouyang, Liyan Song * and Ran Lin *

Key Laboratory of Biopesticide and Chemical Biology (Ministry of Education), College of Plant Protection, Fujian Agriculture and Forestry University, Fuzhou 350002, China; [email protected] (Q.S.); [email protected] (J.D.); [email protected] (Z.D.); [email protected] (Y.L.); [email protected] (H.L.); [email protected] (M.-A.O.) * Correspondence: [email protected] (L.S.); [email protected] (R.L.)

Academic Editors: Evamarie Hey-Hawkins, Santiago Gómez-Ruiz and Goran Kaluderovi´c¯ Received: 6 October 2020; Accepted: 28 October 2020; Published: 30 October 2020

Abstract: The acyclic organic alkynes and carbyne bonds exhibit linear shapes. Metallabenzynes and metallapentalynes are six- or five-membered metallacycles containing carbynes, whose carbine-carbon bond angles are less than 180◦. Such distortion results in considerable ring strain, resulting in the unprecedented reactivity compared with acyclic carbynes. Meanwhile, the aromaticity of these metallacycles would stabilize the ring system. The fascinating combination of ring strain and aromaticity would lead to interesting reactivities. This mini review summarized recent findings on the reactivity of the metal–carbon triple bonds and the aromatic ring system. In the case of metallabenzynes, aromaticity would prevail over ring strain. The reactions are similar to those of organic aromatics, especially in electrophilic reactions. Meanwhile, fragmentation of metallacarbynes might be observed via migratory insertion if the aromaticity of metallacarbynes is strongly affected. In the case of metallapentalynes, the extremely small bond angle would result in high reactivity of the carbyne moiety, which would undergo typical reactions for organic alkynes, including interaction with coinage metal complexes, electrophilic reactions, nucleophilic reactions and cycloaddition reactions, whereas the strong aromaticity ensured the integrity of the bicyclic framework of metallapentalynes throughout all reported reaction conditions.

Keywords: cyclic carbynes; reactivities; metallabenzynes; metallapentalynes

1. Introduction The alkyne (carbon-carbon triple bond), which is a fundamental functional group in organic chemistry, is involved in a large number of reactions in organic chemistry [1–5]. Carbyne complexes, i.e., transition metal complexes with metal-carbon triple bonds, can be described as analogs of alkynes, whereas a transition metal replaces one of the sp carbons. They have attracted considerable attention because of their remarkable features and their significance as catalysts or reagents for various types of organic transformations [5–11]. Part of the reactivities of carbynes paralleled that of their organic parents, such as nucleophilic reactions, electrophilic reactions, photochemistry, oxidation and reduction, reactions with chalcogenides, reactions with unsaturated organic substrates (cycloadditions) and substitution on the α carbon (Figure1). However, the incorporation of metal into the triple would lead to fascinating properties, which were quite different from organic alkynes. For example, late-transition-metal carbyne species were applied as catalysts in alkyne metathesis or alkyne polymerization reactions. Carbynes might be coupled or reactive to other metal complexes to afford bi- or multi-metallic complexes. Ligand substitution is one of the fundamental reactions in organometallic chemistry, which is certainly involved in the reactions of carbynes. In addition, carbyne ligands might be cleaved from metal centers to give various organic products (Figure1).

Molecules 2020, 25, 5050; doi:10.3390/molecules25215050 www.mdpi.com/journal/molecules Molecules 2020, 25, x 2 of 24 Molecules 2020,, 25,, xx 2 of 24 addition, carbyne ligands might be cleaved from metal centers to give various organic products addition, carbyne ligands might be cleaved from metal centers to give various organic products addition,Molecules(Figure 20201). carbyne , 25, 5050 ligands might be cleaved from metal centers to give various organic products2 of 24 (Figure 1).

Figure 1. Reactions of acyclic carbynes. Figure 1. Reactions of acyclic carbynes. The alkyne moiety is normally linear due to the sp hybridization of acetylenic carbon. Ring The alkynealkyne moietymoiety is is normally normally linear linear due due to theto the sp hybridizationsp hybridization of acetylenic of acetylenic carbon. carbon. Ring strainRing strainThe would alkyne be moietyraised fromis normally the introduction linear due of to al thekyne sp moiety hybridization into an oforga acetylenicnic cycle. carbon. As a result, Ring wouldstrain would be raised be fromraised the from introduction the introduction of alkyne of moietyalkyne moiety into an into organic an orga cycle.nic As cycle. a result, As a limitedresult, strainlimited would insights be couldraised be from gained the introductioninto the reactivity of alkyne of organic moiety cyclic into analkynes organic due cycle. to the As instability a result, insightslimited insights could be could gained be into gained the reactivity into the ofreactivity organic cyclicof organic alkynes cyclic due alkynes to the instability due to the caused instability by the limitedcaused byinsights the high could ring be tension gained [1 2,13].into the The reactivity incorporation of organic of a metal cyclic center alkynes into due the acetylenicto the instability carbon highcaused ring by tension the high [12 ring,13]. tension The incorporation [12,13]. The ofincorporation a metal center of intoa metal the acetyleniccenter into carbon the acetylenic would stabilize carbon causedwould stabilizeby the high the ring strained tension cyclic [12,13]. carbon The cyclesincorporation containing of a alkynes.metal center The intocyclic the carbynes acetylenic obtained carbon thewould strained stabilize cyclic the carbon strained cycles cyclic containing carbon alkynes.cycles containing The cyclic alkynes. carbynes The obtained cyclic untilcarbynes now obtained could be woulduntil now stabilize could the be strained classified cyclic as six-memberedcarbon cycles containingmetallabenzynes alkynes. [14–21] The cyclic A and carbynes five-membered obtained classifieduntil now as could six-membered be classified metallabenzynes as six-membered [14–21 ] metallabenzynesA and five-membered [14–21] metallapentalynes A and five-membered [22–24] B untilmetallapentalynes now could be [22–24] classified B (Figure as six-membered 2). In addition, metallabenzynes the first metallapyridyne [14–21] A [25] and was five-membered documented (Figuremetallapentalynes2). In addition, [22–24] the first B (Figure metallapyridyne 2). In addition, [ 25] was the documented first metallapyridyne by Xia and [25] coworkers, was documented providing metallapentalynesby Xia and coworkers, [22–24] providing B (Figure one 2). precious In addition, example the offirst cyclic metallapyridyne carbynes. [25] was documented byone Xia precious and coworkers, example ofproviding cyclic carbynes. one precious example of cyclic carbynes.

Figure 2. Strained metallacycles with carbyne bond. Figure 2. Strained metallacycles with carbyne bond. The difference diﬀerence between between acyclic acyclic carbynes carbynes and and cyclic cyclic carbynes carbynes might might be due be due to the to ring the ringstrain. strain. The The difference between acyclic carbynes and cyclic carbynes might be due to the ring strain. The Thechemical chemical properties properties of cyclic of cyclic carbynes carbynes are summar are summarizedized in Figure in Figure 3. Among3. Among the thelimited limited examples, examples, the chemical properties of cyclic carbynes are summarized in Figure 3. Among the limited examples, the thereaction reaction types types of cyclic of cyclic carbynes carbynes are aresimilar similar to thos to thosee of acyclic of acyclic carbynes carbynes except except that that in most in most cases cases the reaction types of cyclic carbynes are similar to those of acyclic carbynes except that in most cases the the“migratory “migratory insertion” insertion” [26] [26 would] would not not stop stop at atthe the carbene carbene complexes complexes and and lead lead to to the the transformation transformation “migratory insertion” [26] would not stop at the carbene complexes and lead to the transformation of aromatic metallacycles into cyclopentadienescyclopentadienes (Cp) complexescomplexes [[27].27]. of aromatic metallacycles into cyclopentadienes (Cp) complexes [27].

Figure 3. Reactions of cyclic carbynes. Figure 3. Reactions of cyclic carbynes. Carbynes are usually recognized as important intermediates in many transition-metal-catalyzed organic reactions [28,29]. The thorough investigation of the reactivities and properties might provide a Molecules 2020, 25, x 3 of 24

Carbynes are usually recognized as important intermediates in many transition-metal-catalyzed organic reactions [28,29]. The thorough investigation of the reactivities Molecules 2020, 25, 5050 3 of 24 and properties might provide a new perspective for the application of late-transition-metal carbyne species as prosperous catalysts in various organic reactions. The chemistry of metallabenzynes has newbeen perspectivefully illustrated for the and application nicely reviewed of late-transition-metal [14–21]. However, carbyne there is species no review as prosperous paper covering catalysts the inentirety various of cyclic organic carbynes reactions. (metallabenzynes The chemistry and of metallapentalynes). metallabenzynes has This been account fully deals illustrated with them and nicelytogether reviewed for the first [14– 21time]. However, and the scope there has is no been review limited paper to coveringthe reactivities the entirety of metallabenzynes of cyclic carbynes and (metallabenzynesmetallapentalynes, and seeking metallapentalynes). to provide a Thisgeneral account picture deals of with reactivity them together of the formetal–carbon the ﬁrst time triple and thebonds scope or the has entire been limitedaromatic to system. the reactivities of metallabenzynes and metallapentalynes, seeking to provide a general picture of reactivity of the metal–carbon triple bonds or the entire aromatic system. 2. Reactivity of Six-Membered Metallabenzynes 2. Reactivity of Six-Membered Metallabenzynes Metallabenzynes have been recognized as transition metal analogs of benzyne in which a C atomMetallabenzynes is replaced by have an been isolobal recognized transition as transition metal metal fragment. analogs ofThe benzyne chemical in which structure a C atom of is replacedmetallabenzynes by an isolobal exhibits transition aromaticity, metal which fragment. would The lead chemical to the structure similar reactions of metallabenzynes occurring exhibitsin their aromaticity,organic counterparts. which would On leadthe toother the similarhand, the reactions carbyne occurring moiety in(M their≡C, metal-carbon organic counterparts. triple bond) On the is otherreactive hand, to several the carbyne reagents moiety and (M mightC, metal-carbon involve regu triplelar reactions bond) is reactivethat normal to several acyclic reagents carbynes and would might ≡ involveexperience. regular The reactionsfirst metallabenzyne that normal 1 acyclic was secured carbynes by wouldJia [30] experience.and co-workers The ﬁrstin 2001. metallabenzyne Then, many 1interestingwas secured metallabenzynes by Jia [30] and were co-workers produced in and 2001. well Then,-characterized. many interesting (Figure metallabenzynes4) [31–40] Most of were the producedmetallabenzynes and well-characterized. are osmabenzynes (Figure (1–6)4 )[[30–38].31–40 ]Rhenabenzynes Most of the metallabenzynes (7), which are are present osmabenzynes in very (limited1–6)[30 –amounts,38]. Rhenabenzynes were reported (7), which by Jia’s are presentgroup in[39,40]. very limited Moreover, amounts, only were osmabenzynes reported by Jia’sand grouprhenabenzynes [39,40]. Moreover, have been only discovered osmabenzynes to date. and rhenabenzynes have been discovered to date.

Figure 4. Metallabenzynes reported to date.

The cationiccationic osmabenzyne osmabenzyne8 was 8 was formed formed by treatment by treatment of metallabenzyne of metallabenzyne1 [30] with 1 two[30] equivalentwith two HBFequivalent4 in wet HBF dichloromethane.4 in wet dichloromethane. The anionic chlorideThe anionic ligand chloride was replaced ligand bywas the replaced neutral waterby the with neutral the aidwater of Bronstedwith the aid acid of [41 Bronsted]. Similarly, acid the[41]. dicationic Similarly, osmabenzyne the dicationic9 wasosmabenzyne prepared by9 was the replacementprepared by ofthe chloride replacement ligands of chloride in 1 with ligands 2,20-bipyridine in 1 with 2,2 (bipy)′-bipyridine in the presence (bipy) in ofthe thallium presence triﬂate of thallium (TlOTf) triflate [42]. The(TlOTf) exchange [42]. The of phosphineexchange of ligands phosphine was muchligands easier, was andmuch no easier, reagents and and no catalystsreagents wereand catalysts needed. Excesswere needed. PCy3 in Excess reﬂuxing PCy benzene3 in refluxing would benzene simply kickwould out simply the triphenylphospine kick out the triphenylphospine and result in neutral and osmabenzyneresult in neutral10 osmabenzyne[43]. The transformations 10 [43]. The aretransformations depicted in Scheme are depicted1. in Scheme 1. Molecules 2020, 25, x 4 of 24 Molecules 2020, 25, 5050 4 of 24 Molecules 2020, 25, x 4 of 24

Scheme 1. Ligand exchange reactions of osmabenzyne 1. Scheme 1. LigandLigand exchange reactions of osmabenzyne 1. 2.1. Electrophilic Substitution Reaction 2.1. Electrophilic Substitution Reaction 2.1. ElectrophilicTrimethylsilyl Substitution group (TMS) Reaction was usually recognized as the “equivalent” of cationic hydrogen and labileTrimethylsilyl to electrophiles group (TMS) in organic was usually reactions. recognized In the presence as the “equivalent” of excess tetrafluoroboric of cationic hydrogen acid and Trimethylsilyl group (TMS) was usually recognized as the “equivalent” of cationic hydrogen water,labile tometallabenzyne electrophiles in organic1 [30] reactions.was converted In the to presence cationic of excessmetallabenzyne tetrafluoroboric 11 via acid electrophilic and water, and labile to electrophiles in organic reactions. In the presence of excess tetrafluoroboric acid and desilylationmetallabenzyne and 1subsequent [30] was converted ligand exchange. to cationic Sodium metallabenzyne chloride would11 via electrophilicprovide anionic desilylation chloride andion water, metallabenzyne 1 [30] was converted to cationic metallabenzyne 11 via electrophilic tosubsequent avoid ligand ligand exchange exchange. to Sodium facilitate chloride the formation would provide of neutral anionic desilylated chloride ionosmabenzyne to avoid ligand 12. desilylation and subsequent ligand exchange. Sodium chloride would provide anionic chloride ion Treatmentexchange to of facilitate isolated the 11 formation with NaCl of neutral also gave desilylated 12. Bromination osmabenzyne of 12metallabenzyne. Treatment of isolated 1 with 11excesswith to avoid ligand exchange to facilitate the formation of neutral desilylated osmabenzyne 12. elementalNaCl also bromine gave 12. Brominationreadily provides of metallabenzyne brominated osmabenzyne1 with excess 13 elemental, in whichbromine both the readilyMe3Si and provides Cl in Treatment of isolated 11 with NaCl also gave 12. Bromination of metallabenzyne 1 with excess 1brominated have been osmabenzynereplaced by Br13 [41]., in Nitrosation which both of the metallabenzyne Me3Si and Cl 1 in was1 have carried been out replaced in the presence by Br [41 of]. elemental bromine readily provides brominated osmabenzyne 13, in which both the Me3Si and Cl in excessNitrosation nitrosonium of metallabenzyne salt (NOBF1 4was) and carried sodium out chloride in the presence at low oftemperature excess nitrosonium to furnish salt osmabenzyne (NOBF ) and 1 have been replaced by Br [41]. Nitrosation of metallabenzyne 1 was carried out in the presence4 of 14sodium in which chloride the SiMe at low3 substituent temperature at to C4 furnish was replaced osmabenzyne by NO14 [44].in which Nitration the SiMe of metallabenzyne3 substituent atC4 1 took was excess nitrosonium salt (NOBF4) and sodium chloride at low temperature to furnish osmabenzyne placereplaced at C4 by to NO afford [44]. Nitrationosmabenzyne of metallabenzyne 15 under similar1 took reaction place atconditions C4 to afford except osmabenzyne that nitronium15 under salt 14 in which the SiMe3 substituent at C4 was replaced by NO [44]. Nitration of metallabenzyne 1 took (NOsimilar2BF reaction4) was applied. conditions Surprisingly, except that the nitronium biclyclic saltspecies (NO 18BF was) was isolated applied. together Surprisingly, with osmabenzyne the biclyclic place at C4 to afford osmabenzyne 15 under similar reaction2 4 conditions except that nitronium salt 15species. A plausible18 was isolated mechanism together was with rationalized. osmabenzyne Initially,15. A plausibleelectrophilic mechanism substitution was rationalized. of NO2+ produced Initially, (NO2BF4) was applied. Surprisingly,+ the biclyclic species 18 was isolated together with osmabenzyne osmabenzyneelectrophilic substitution 15, whichof might NO2 undergoproduced migratory osmabenzyne insertion15, which reaction might and undergo coordination migratory of insertionanionic 15. A plausible mechanism was rationalized. Initially, electrophilic substitution of NO2+ produced oxygenreaction atom and coordinationto give intermediate of anionic 16 oxygen. Subsequent atom to single give intermediateelectron transfer16. Subsequent generated the single radical electron 17, osmabenzyne 15, which might undergo migratory insertion reaction and coordination of anionic whichtransfer would generated abstract the radical a hydrogen17, which atom would from abstract solvent a hydrogen to give atomthe osmium from solvent complex to give 18 the [44]. osmium The oxygen atom to give intermediate 16. Subsequent single electron transfer generated the radical 17, reactionscomplex 18 of [osmabenzyne44]. The reactions 1 is summarized of osmabenzyne in Scheme1 is summarized 2. in Scheme2. which would abstract a hydrogen atom from solvent to give the osmium complex 18 [44]. The reactions of osmabenzyne 1 is summarized in Scheme 2.

Scheme 2. ElectrophilicElectrophilic reactions reactions of osmabenzyne 1.. Scheme 2. Electrophilic reactions of osmabenzyne 1. Molecules 2020, 25, x 5 of 24 Molecules 2020,, 25,, 5050x 5 of 24

It is well known that C-SiMe3 bonds on an aromatic ring are more reactive than C-H bonds It is well known that C-SiMe3 bonds on an aromatic ring are more reactive than C-H bonds towards electrophiles. In order to further investigate the reactivity of metallabenzynes without towardsIt is electrophiles. well known that In order C-SiMe to3 furtherbonds oninvest an aromaticigate the ringreactivity are more of metallabenzynes reactive than C-H without bonds SiMe3 group, osmabenzyne 3f was subjected to electrophilic conditions (Scheme 3). Bromination and towardsSiMe3 group, electrophiles. osmabenzyne In order 3f was to further subjected investigate to electrophilic the reactivity conditions of metallabenzynes (Scheme 3). Bromination without SiMe and3 group,nitration osmabenzyne were carried3f outwas under subjected similar to electrophilic conditions conditionsand provided (Scheme the identical3). Bromination brominated and nitration product were13 andand carried mononitratedmononitrated out under osmabenzyneosmabenzyne similar conditions 19, respectivelyrespectively and provided [44]. the Treatment identical brominated of osmabenzyne product 3f13 withwithand CF3SO2D in the presence of sodium chloride produced 3,5-dideuterated osmabenzyne 20 [41], which mononitratedCF3SO2D in the osmabenzyne presence of sodium19, respectively chloride [produced44]. Treatment 3,5-dideuterated of osmabenzyne osmabenzyne3f with CF 203SO [41],2D which in the presenceindicatesindicates of thatthat sodium thethe carbonscarbons chloride ofof produced OsOsC oror 3,5-dideuteratedOsOsCH werewere notnot osmabenzyneattackedattacked byby thethe20 [acidacid41], whichunderunder indicates thethe protonationprotonation that the carbonscondition. of OsC or OsCH were not attacked by the acid under the protonation condition.

Scheme 3.3. Electrophilic reactions of osmabenzyne 3f3f..

Chlorination ofof osmabenzyne osmabenzyne1 and 13f withand excess3f with HCl /excessH2O2 furnished HCl/H2O the2 samefurnished 3,5-dichlorinated the same Chlorination of osmabenzyne 1 and 3f with excess HCl/H2O2 furnished the same product3,5-dichlorinated21, while product C2 monochlorinated 21,, whilewhile C2C2 product monochlorinatedmonochlorinated22 could be productproduct obtained 22when couldcould the bebe ratioobtainedobtained of AlCl whenwhen3/H 2thetheO2 wasratio carefully of AlCl controlled3/H2O2 was [44 carefully]. Noteworthy controlled is that [44]. the carbyne Noteworthy moiety is was that attacked the carbyne by oxygen moiety atoms was in ratio of AlCl3/H2O2 was carefully controlled [44]. Noteworthy is that the carbyne moiety was theattacked presence by oxygen of hydrogen atoms peroxidein the presence to aﬀord of metalla-oxirenehydrogen peroxide fragments to afford in metalla-oxirene the chlorination fragments reactions (Schemeinin thethe chlorinationchlorination4). reactionsreactions (Scheme(Scheme 4).4).

Scheme 4. Chlorination reactions of osmabenzynes 1 and 3f. Scheme 4. Chlorination reactions of osmabenzynes 1 andand 3f.. The electrophilic substitution reactions of metallabenzynes resemble the organic arenes and The electrophilic substitution reactions of metallabenzynesllabenzynes resembleresemble thethe organicorganic arenesarenes andand thethe themetal-carbon metal-carbon triple triple bonds bonds stayed stayed intact intactunder under most mostelectrophilic electrophilic conditions conditions except exceptwhen highly when highlymetal-carbon oxidative triple H bondsO was stayed applied. intact Theseunder phenomenamost electrophilic demonstrate conditions the aromaticexcept when properties highly oxidative H2O2 was2 2 applied. These phenomena demonstrate the aromatic properties of ofoxidative metallabenzynes. H2O2 was applied. These phenomena demonstrate the aromatic properties of metallabenzynes. 2.2. Nucleophilic Reaction 2.2. Nucleophilic Reaction The carbyne moiety in metallabenzyne could be attacked by nucleophiles. The ﬁrst nucleophilic The carbyne moiety in metallabenzyne could be attacked by nucleophiles. The first nucleophilic reactionsThe ofcarbyne metallabenzynes moiety in metallabenzyne were explored by could Jia (Scheme be attacked5)[ 42 ].by In nucleophiles. order to increase The the first electrophilicity nucleophilic reactions of metallabenzynes were explored by Jia (Scheme 5) [42]. In order to increase the ofreactions metallacycle, of metallabenzynes the neutral osmabenzyne were explored1 was by transformed Jia (Scheme to dicationic5) [42]. In osmabenzyne order to increase9 by ligand the electrophilicity of metallacycle, the neutral osmabenzyne 1 was transformed to dicationic exchange,electrophilicity which wasof metallacycl quite electrophilice, the andneutral readily osmabenzyne reacted with nucleophiles1 was transformed (Scheme5). Water,to dicationic a weak osmabenzyne 9 by ligand exchange, which was quite electrophilic and readily reacted with nucleophile,osmabenzyne participated 9 by ligand in theexchange, nucleophilic which attack was of quite the carbyne electrophilic atom to and generate readily metallaphenol reacted with23, nucleophiles (Scheme 5). Water, a weak nucleophile, participated in the nucleophilic attack of the whichnucleophiles precipitated (Scheme from 5). theWater, solvent a weak system nucleophile, (H2O/THF participated= 3:4). Desilylation in the nucleophilic was also observedattack of the on carbyne atom to generate metallaphenol 23, which precipitated from the solvent system (H2O/THF = 2 thecarbyne SiMe atom3 adjacent to generate to carbyne metallaphenol carbon, which 23, which might precipitated be attributed from to the the higher solvent nucleophilicity system (H O/THF of the = 3:4). Desilylation was also observed on the SiMe3 adjacent to carbyne carbon, which might be 3:4). Desilylation was also observed on the SiMe3 adjacent to carbyne carbon, which might be Molecules 2020,, 25,, 5050x 6 of 24

attributed to the higher nucleophilicity of the Si-bound carbon. The metallaphenol 23 could be Si-boundtransformed carbon. to acyclic The metallaphenol complex 24 in23 thecould presence be transformed of catalytic to acyclicHBF4. complexProtonation24 in and the enol-ketone presence of catalytictransformation HBF4. might Protonation give intermediate and enol-ketone 25, which transformation was deprotonated might give to afford intermediate the ketone25, whichform 26 was. A deprotonatedsix-electron retro-electrocyclization to aﬀord the ketone form reaction26. A six-electronof 26 would retro-electrocyclization furnish acyclic complex reaction 24. of Moreover,26 would furnishosmabenzyne acyclic could complex be transf24. Moreover,ormed to acyclic osmabenzyne complex could 24 directly be transformed if better solubility to acyclic (H complex2O/THF24 = directly1:15) and if betterlonger solubilityreaction (Htime2O /THF(two =days)1:15) were and longerprovided. reaction The timecombination (two days) of weremethanol provided. and Thepotassium combination carbonate of methanol not only and provided potassium methoxid carbonatee to not conduct only provided the nucleophile methoxide addition to conduct to the nucleophilecarbyne carbon, addition but also to the removed carbyne the carbon, trimethylsilyl but also removedgroup adjacent the trimethylsilyl to the carbyne group carbon, adjacent resulting to the carbynein the similar carbon, product resulting 27 in. theThe similar hydride product provided27. Theby hydrideNaBH4 providedattacked the by NaBHcarbyne4 attacked carbon theof carbyneosmabenzyne carbon 9 of to osmabenzyne give the metallabenzene9 to give the metallabenzene intermediate 28 intermediate, which would28, which undergo would migratory undergo migratoryinsertion reaction insertion and reaction subsequent and subsequent rearrang rearrangementement to afford to acyclopentadienylﬀord cyclopentadienyl complex complex 29. The29. Theregioselectivity regioselectivity of nucleophilic of nucleophilic attack attack is C1 is (carbyne C1 (carbyne carbon) carbon) in Scheme in Scheme 5. 5.

Scheme 5. Nucleophilic reactions of osmabenzyne 9.

Xia andand coworkers coworkers reported reported nucleophilic nucleophilic addition addition to metallabenzynes to metallabenzynes with diﬀerent with regioselectivity different (C3)regioselectivity [36]. They (C3) prepared [36]. They osmabenzyne prepared 4osmabenzyneand subjected 4 itand to subjected nucleophilic it to addition nucleophilic with addition stronger nucleophiles,with stronger suchnucleophiles, as ethyllithium such as (EtLi) ethyllithium or sodium (EtLi) methanethiolate or sodium methanethiolate (NaSMe). Not (NaSMe). surprisingly, Not thesurprisingly, carbyne carbon the carbyne (C3) wascarbon attacked (C3) wa tos generate attacked isoosmabenzene to generate isoosmabenzene30 or 31. If primary30 or 31. amineIf primary was applied,amine was it wouldapplied, kick it would out the kick chloride out the ligand chloride to giveligand the to dicationic give the dicationic osmabenzyne osmabenzyne32, which 32 is, morewhich electrophilic is more electrophilic than 4. Hence, than 4 the. Hence, nucleophilic the nucleophilic attack of primary attack of amine primary on C3 amine and subsequenton C3 and rearrangementsubsequent rearrangement of isoosmabenzene of isoosmabenzene33 furnished the 33 ring-opened furnished the product ring-opened34 or 35 (Schemeproduct6 ).34 or 35 (Scheme 6). Molecules 25 Molecules 2020,, 25,, 5050x 7 of 24 Molecules 2020, 25, x 7 of 24

Scheme 6. Nucleophilic reactions of osmabenzyne 4. Scheme 6. Nucleophilic reactions of osmabenzyne 4.. 2.3. Migratory Insertion Reactions 2.3. Migratory Insertion Reactions Metallabenzenes and metallabenzynes usually exhibit rearrangement to the correspondingcorresponding Metallabenzenes and metallabenzynes usually exhibit rearrangement to the corresponding cyclopentadienyl complexes, which is the majormajor decompositiondecomposition pathway.pathway. The migratorymigratory insertioninsertion cyclopentadienyl complexes, which is the major decomposition pathway. The migratory insertion step involves reductive reductive elimination, elimination, in in which which the the two two carbon carbon atoms atoms adjacent adjacent to metal to metal undergo undergo the step involves reductive elimination, in which the two carbon atoms adjacent to metal undergo the thecoupling coupling reaction reaction to form to form cyclopentadienyl cyclopentadienyl moie moiety.ty. In In2007, 2007, Jia Jia and and Lin Lin [37] [37] discovered thatthat coupling reaction to form cyclopentadienyl moiety. In 2007, Jia and Lin [37] discovered that reduction ofof complexcomplex36 36and and subsequent subsequent cyclometallation cyclometallation gave gave hydrido hydrido osmabenzyne osmabenzyne intermediate intermediate37, reduction of complex 36 and subsequent cyclometallation gave hydrido osmabenzyne intermediate which37, which underwent underwent hydride hydride shift to attackshift to the attack carbyne the carbon carbyne to aﬀ ordcarbon metallabenzene to afford metallabenzene intermediate 38. 37, which underwent hydride shift to attack the carbyne carbon to afford metallabenzene Intermediateintermediate 3838would. Intermediate undergo migratory38 would insertionundergo reactionmigratory and insertion subsequent reaction rearrangement and subsequent to aﬀord intermediate 38. Intermediate 38 would undergo migratory insertion reaction and subsequent cyclopentadienylrearrangement to (Cp)afford complex cyclopentadienyl39 (Scheme (Cp)7). complex 39 (Scheme 7). rearrangement to afford cyclopentadienyl (Cp) complex 39 (Scheme 7).

Scheme 7. Zinc reduction of vinylcarbyne complex 36.. Scheme 7. Zinc reduction of vinylcarbyne complex 36. Metallabenzynes could also undergo migratory in insertionsertion reactions to give carbene complexes. Metallabenzynes could also undergo migratory insertion reactions to give carbene complexes. The substituentsubstituent e ﬀeffectect was was illustrated illustrated by Jiaby andJia and Lin. AsLin. depicted As depicted in Scheme in Scheme8, osmabenzynes 8, osmabenzynes bearing The substituent effect was illustrated by Jia and Lin. As depicted in Scheme 8, osmabenzynes bulkybearing substituents bulky substituents (t-butyl (t or-butyl 1-adamantyl) or 1-adamantyl)para to para the to metal the metal center center would would slowly slowly convert convert to the to bearing bulky substituents (t-butyl or 1-adamantyl) para to the metal center would slowly convert to correspondingthe corresponding osmium osmium carbene carbene complexes complexes40a and 40a40b andat 40b room at room temperature temperature in solution in solution [38]. The [38]. steric The the corresponding osmium carbene complexes 40a and 40b at room temperature in solution [38]. The hindrancesteric hindrance of the of bulky the substituentsbulky substituents might preventmight prevent the osmium the osmium carbene carbene complexes complexes from further from steric hindrance of the bulky substituents might prevent the osmium carbene complexes from transformationfurther transformation [45,46]. [45,46]. further transformation [45,46]. Molecules 2020, 25, 5050 8 of 24 Molecules 2020,, 25,, xx 8 of 24

Scheme 8. SubstituentSubstituent effect eﬀect on migratory insertion reaction.reaction.

The ligand eeffectﬀect playedplayed anan importantimportant role inin thethe rearrangementrearrangement of osmabenzyneosmabenzyne 1 toto thethe corresponding cyclopentadienylcyclopentadienyl complex complex41 ,41 which,, whichwhich was was demonstratedwas demonstrateddemonstrated by Jia [byby43 ]JiaJia as shown[43][43] asas in shownshown Scheme inin9. LigandScheme exchange 9. Ligand reaction exchange of stable reaction osmabenzyne of stable 1osmabenzynewith Mo(CO) 61 would withwith Mo(CO)Mo(CO) replace one66 wouldwould phosphine replacereplace ligand oneone withphosphine carbon ligand monoxide with and carbon deliver monoxide reactive and osmabenzyne deliver reactive intermediate osmabenzyne42, which intermediate underwent migratory42,, whichwhich insertionunderwent involving migratory the insertion two metal-bonded involving carbons the two to metal-bonded give the carbene carbons intermediate to give43 the. Migratory carbene 5 insertionintermediateintermediate of the 43 carbene.. MigratoryMigratory into theinsertioninsertion Os-Cl bondofof thethe in carbenecarbene43 with into ainto subsequent thethe Os-ClOs-Cl rearrangement bondbond inin 43 withwith to η aacoordination subsequentsubsequent wouldrearrangement furnish Cp to η complex55 coordinationcoordination41. wouldwould furnishfurnish CpCp complexcomplex 41..

Scheme 9. Ligand effect eﬀect on migratory insertion reaction. 3. Reactivity of Five-Membered Metallapentalynes 3. Reactivity of Five-Membered Metallapentalynes Recently, Xia and coworkers documented a series of ﬁve-membered cyclic metal carbyne complexes, Recently, Xia and coworkers documented a series of five-membered cyclic metal carbyne i.e., osmapentalynes [47–51] and ruthenapentalynes [52]. The carbine-carbon bond angles in the complexes, i.e., osmapentalynes [47–51] and ruthenapentalynes [52]. The carbine-carbon bond metallapentalynes are around 130◦, which are much smaller than those of the acyclic metal carbynes angles in the metallapentalynes are around 130°, which are much smaller than those of the acyclic (180◦). The large ring strain associated with extreme distortion of the metal carbyne unit lead to the metal carbynes (180°). The large ring strain associated with extreme distortion of the metal carbyne high reactivity of the metal–carbon triple bond, which exhibits a unique performance. unit lead to the high reactivity of the metal–carbonl–carbon tripletriple bond,bond, whichwhich exhibitsexhibits aa uniqueunique 3.1.performance. Formation of Osmapentalyne-Coinage Metal Complexes

3.1. FormationThe metal-carbon of Osmapentalyne-Coinage triple bond possesses Metal an Complexes “alkyne-like” character and can react with an external metal precursor to generate bimetallic adducts, in which osmium-carbon triple bond coordinated with The metal-carbon triple bond possesses an “alkyne-like” character and can react with an the coinageThe metal-carbon metal center. triple Xia reported bond possesses that osmapentalyne an “alkyne-like”44 [48] wascharacter treated and with can cuprous react chloridewith an external metal precursor to generate bimetallic adducts, in which osmium-carbon triple bond toexternal afford metal hetero precursor bimetallic to adduct generate45, inbimetallic which the adducts, chloride in bonded which toosmium-carbon osmium also coordinatedtriple bond coordinated with the coinage metal center. Xia reported that osmapentalyne 44 [48] was treated with withcoordinated copper with atoms. the coinage As different metal silvercenter. or Xia gold reported precursors that osmapentalyne were subjected 44 to [48] osmapentalyne was treated with44, cuprous chloride to afford hetero bimetallic adduct 45, in which the chloride bonded to osmium also thecuprous corresponding chloride to heteroafford bimetallichetero bimetallic adduct adduct46 or 47 45was, in which formed the in chloride a similar bonded way. Theto osmium interaction also coordinated with copper atoms. As different silver or gold precursors were subjected to betweencoordinated cyclic with osmacarbyne copper atoms. with coinage As different metalis si weak,lver whichor gold is supportedprecursors by were the factsubjected that in theto osmapentalyne 44,, thethe correspondingcorresponding heterohetero bimetallicbimetallic adductadduct 46 oror 47 waswas formedformed inin aa similarsimilar way.way. presence of ligands (PPh3 for 45 or (n-Bu)4NCl for 46–47) the bimetallic complex readily dissociates The interaction between cyclic osmacarbyne with coinage metal is weak, which is supported by the andThe interaction osmapentalyne between44 would cyclic osmacarbyne be regenerated with (Scheme coinage 10 metal)[53]. is Ruthenapentalynes weak, which is supported48a–b readily by the fact that in the presence of ligands (PPh3 for 45 or (n-Bu)4NCl for 46–47) the bimetallic complex reactedfact that with in the CuCl presence to afford of similar ligands bimetallic (PPh3 for complexes 45 or (n-Bu)49a and4NCl49b for, except46–47) thatthe thebimetallic chloride complex bonded readily dissociates and osmapentalyne 44 would be regenerated (Scheme 10) [53]. toreadily ruthenium dissociates did not coordinateand osmapentalyne with copper [5244] would be regenerated (Scheme 10) [53]. Ruthenapentalynes 48a–b readily reacted with CuCl to afford similar bimetallic complexes 49a andand 49b,, exceptexcept thatthat thethe chloridechloride bondedbonded toto rutheniumruthenium diddid notnot coordinatecoordinate withwith coppercopper [52][52] Molecules 2020, 25, 5050 9 of 24 Molecules 2020, 25, x 9 of 24

Scheme 10. Reactions of metallapentalynes with coinage metal complexes. 3.2. Electrophilic Reaction 3.2. Electrophilic Reaction Alkynes, which exhibit electrophilic properties, would generate alkenes upon treatment of Alkynes, which exhibit electrophilic properties, would generate alkenes upon treatment of electrophiles. Carbynes and metallapentalynes are both sensitive to acids. The acyclic carbynes electrophiles. Carbynes and metallapentalynes are both sensitive to acids. The acyclic carbynes might generally form carbenes, while the shift of metal–carbon triple bond would be observed in might generally form carbenes, while the shift of metal–carbon triple bond would be observed in metallapentalynes. In most cases, the fused rings of metallapentalynes would be reserved, which might metallapentalynes. In most cases, the fused rings of metallapentalynes would be reserved, which be attributed to the higher stability derived from the aromaticity. might be attributed to the higher stability derived from the aromaticity. Upon treatment of osmapentalynes 50a–c [47] with HBF4 H2O at room temperature, the metal– Upon treatment of osmapentalynes 50a–c [47] with HBF· 4·H2O at room temperature, the carbon triple bond could shift from one ring to another to generate osmapentalynes 51a–c [54]. metal–carbon triple bond could shift from one ring to another to generate osmapentalynes 51a–c The plausible mechanism was that the osmapentalyne was protonated to give the osmapentalene [54]. The plausible mechanism was that the osmapentalyne was protonated to give the intermediate 52 bearing a 16-electron osmium center, which was not stable enough according to the osmapentalene intermediate 52 bearing a 16-electron osmium center, which was not stable enough 18-electron transition metal rule. Subsequent elimination of Ha would furnish osmapentalynes 51a–c according to the 18-electron transition metal rule. Subsequent elimination of Ha would furnish with an 18-electron osmium center. Fortunately, the 16-electron osmapentalenes, complex 52a and 52d, osmapentalynes 51a–c with an 18-electron osmium center. Fortunately, the 16-electron were captured and characterized by the reaction of osmapentalyne 50 or 51 with HBF Et O or AlCl osmapentalenes, complex 52a and 52d, were captured and characterized by the4· 2reaction of3 (Scheme 11). The metal carbyne bond shift reaction of the ruthenapentalyne 48b is similar to that of osmapentalyne 50 or 51 with HBF4·Et2O or AlCl3 (Scheme 11). The metal carbyne bond shift reaction osmapentalynes, as is the mechanism [52]. of the ruthenapentalyne 48b is similar to that of osmapentalynes, as is the mechanism [52]. The halogenation of metallapentalynes [55] would further demonstrate the electrophilicity of metallapentalynes. Treatment of osmapentalyne 51a [47] with ICl or elemental Br2 led to the formation of the corresponding halogenated osmapentalene 55 or 56, respectively, which could be viewed as the ﬁrst examples of metallaiodirenium and metallabromirenium ions (Scheme 12). The well-characterized bromocarbene complex 56 or iodocarbene complex 55 are similar to the generally proposed intermediates in the halogenation of alkynes, which would further demonstrate the “alkyne-like” properties of metal-carbon triple bond in metallapentalynes. The nucleophilic substitution reaction of 55 with Bu4NBr would generate brominated complex 56. However, the application of Bu4NCl to the halogenated osmapentalenes 55–56 would result in the elimination of the halogen cation and the regeneration of osmapentalyne 51a. Moreover, 51a also reacted with elemental selenium at 60 ◦C, leading to the formation of Se-containing osmapentalene 57 [56], which was regarded as the ﬁrst example of σ-aromaticity dominating in an unsaturated Se-containing ring. Molecules 2020, 25, x 10 of 24

Molecules 2020, 25, 5050 10 of 24 Molecules 2020, 25, x 10 of 24

Scheme 11. Metal-carbon triple bond shift of metallapentalynes.

The halogenation of metallapentalynes [55] would further demonstrate the electrophilicity of metallapentalynes. Treatment of osmapentalyne 51a [47] with ICl or elemental Br2 led to the formation of the corresponding halogenated osmapentalene 55 or 56, respectively, which could be viewed as the first examples of metallaiodirenium and metallabromirenium ions (Scheme 12). The well-characterized bromocarbene complex 56 or iodocarbene complex 55 are similar to the generally proposed intermediates in the halogenation of alkynes, which would further demonstrate the “alkyne-like” properties of metal-carbon triple bond in metallapentalynes. The nucleophilic substitution reaction of 55 with Bu4NBr would generate brominated complex 56. However, the application of Bu4NCl to the halogenated osmapentalenes 55–56 would result in the elimination of the halogen cation and the regeneration of osmapentalyne 51a. Moreover, 51a also reacted with elemental selenium at 60 °C, leading to the formation of Se-containing osmapentalene 57 [56], which was regarded as the first example of σ-aromaticity dominating in an unsaturated Se-containing ring. Scheme 11. Metal-carbon triple bond shift of metallapentalynes.

2ICl - - + 2 The halogenation of metallapentalynes[Os] BF4 [55] would furtherI demonstrate the electrophilicity of ICl [Os] metallapentalynes. Treatment of osmapentalyne+ 51a [47] with ICl or elemental Br2 led to the PPh + MeO2C 3 PPh formation of the corresponding halogenated( nosmapentalene-Bu)4NCl MeO2C 55 or 56, respectively,3 which could be viewed as the first examples of51a metallaiodirenium and metallabromirenium55 ions (Scheme 12). The well-characterized bromocarbene complex 56 or iodocarbeneBr2 complex 55 are similar to the generally Se (n-Bu) NBr proposed intermediates in the halogenation of alkynes, which 4would further demonstrate the (n-Bu)4NCl “alkyne-like” properties of metal-carbonSe triple- bond in metallapentalynes. The nucleophilic [Os] BF4 Br + - substitution reaction of 55 with Bu4NBr would generate brominated2 2Brcomplex3 56. However, the + [Os] + application of Bu4NCl toMeO the2C halogenated PPhosmapentalenes3 55–56 would result in the elimination of MeO2C PPh3 the halogen cation and the regeneration57 of osmapentalyne 51a56 . Moreover, 51a also reacted with elemental selenium at 60 °C, leading to the[Os] formation = OsCl(PPh of 3Se-containing)2 osmapentalene 57 [56], which [Os]2 = OsBr(PPh ) was regarded as the first example of σ-aromaticity dominating3 2 in an unsaturated Se-containing ring. Scheme 12. Halogenation of metallapentalynes. 2ICl - - + 2 [Os] BF4 I ICl [Os] The reactions of metal carbynes and+ carbon–carbon triple bonds tended to provide cycloaddition PPh + intermediates or products.MeO2C In contrast, Xia3 and coworkers documented aPPh fantastic reaction between (n-Bu)4NCl MeO2C 3 osmapentalynes and terminal51a alkynes in 2020, giving rise to acyclic55 addition products 59 and 61 (Scheme 13)[57]. High efficiency, regio- and stereoselectivity were observed, furnishing exclusively Br2 trans and anti-Markovnikov productsSe in excellent yields. These(n-Bu) reactions4NBr possess good functional tolerance for both alkynes and osmapentalynes(n-Bu)4NCl and are easy to handle (air atmosphere and ambient Se - conditions). The plausible mechanism[Os] is theBF electrophilic4 Br addition+ of cyclic- carbyne by the terminal [Os]2 2Br3 alkynes under the synergistic effect of protons+ on the basis of experimental observations as well as PPh + MeO2C 3 PPh theoretical calculations. The application of the reactionsMeO2C provides an easy3 access to functionalized 57 56 dπ-pπ conjugated systems, which exhibit great significance in functional materials. [Os] = OsCl(PPh3)2 2 [Os] = OsBr(PPh3)2 Scheme 12. Halogenation of metallapentalynes. Molecules 2020, 25, x 11 of 24

The reactions of metal carbynes and carbon–carbon triple bonds tended to provide cycloaddition intermediates or products. In contrast, Xia and coworkers documented a fantastic reaction between osmapentalynes and terminal alkynes in 2020, giving rise to acyclic addition products 59 and 61 (Scheme 13) [57]. High efficiency, regio- and stereoselectivity were observed, furnishing exclusively trans and anti-Markovnikov products in excellent yields. These reactions possess good functional tolerance for both alkynes and osmapentalynes and are easy to handle (air atmosphere and ambient conditions). The plausible mechanism is the electrophilic addition of cyclic carbyne by the terminal alkynes under the synergistic effect of protons on the basis of experimental observations as well as theoretical calculations. The application of the reactions provides an easy Moleculesaccess to2020 functionalized, 25, 5050 dπ-pπ conjugated systems, which exhibit great significance in functional11 of 24 materials.

Scheme 13. Reactions of alkynes with osmapentalynes. 3.3. Nucleophilic Reaction 3.3. Nucleophilic Reaction The nonlinear distortion of the carbyne carbon angle of the osmapentalyne also facilitates the The nonlinear distortion of the carbyne carbon angle of the osmapentalyne also facilitates the nucleophilic reaction. In contrast to the 16-electron osmapentalene 52 secured by protonation [54], nucleophilic reaction. In contrast to the 16-electron osmapentalene 52 secured by protonation [54], 18-electron osmapentalenes 62 and 63 were achieved by the nucleophilic attack of the methanethioxide 18-electron osmapentalenes 62 and 63 were achieved by the nucleophilic attack of the (MeS ) and the methanoxide (MeO ) anions towards the carbyne carbon of osmapentalyne 50a methanethioxide− (MeS−) and the methanoxide− (MeO−) anions towards the carbyne carbon of under the atmosphere of carbon monoxide. [54] Xia and coworkers reported the transformation of osmapentalyne 50a under the atmosphere of carbon monoxide. [54] Xia and coworkers reported the osmapentalyne to osmafulvenallene 64 (Scheme 14), which constituted the pioneering example of transformation of osmapentalyne to osmafulvenallene 64 (Scheme 14), which constituted the Moleculeswell-characterized 2020, 25, x metallafulvenallenes [58]. 12 of 24 pioneering example of well-characterized metallafulvenallenes [58].

Scheme 14. NucleophilicNucleophilic reactions reactions of osmapentalyne to form 62–64.

The rationale of the mechanism is depicted in Scheme 1515.. The combination of cesium carbonate and methanol would generate methanoxide, which which conducted the the first first nucleophilic addition to a carbyne carbon, affording affording the metallapentalenemetallapentalene intermediate 63.. Subsequent Subsequent coordination coordination with with a 1 terminal alkyne alkyne led led to to the the ππ-alkyne-alkyne complex complex 6565, which, which would would form form the the vinylidene vinylidene η1-complexη -complex 66 via66 isomerization.via isomerization. TheThe α-carbonα-carbon within within the the five-membered five-membered metallacycle metallacycle was was attacked attacked by by the the second methanol molecule. Sequential ring opening, protonation and coordination of the ester group furnished the final fused metallafulvenallene complex 64.

Scheme 15. Proposed mechanism for the formation of 64.

Very recently, oxygenation of osmapentalynes was reported by Xia’s group (Scheme 16). The first example was the oxygenation of the osmium carbolong complex ring [59]. Applying pyridine N-oxide as the oxidant, oxygen atoms were transferred to the metal–carbon triple bond in a nucleophilic reaction. Meanwhile, the labile aldehyde was oxidized to carboxylic acid, affording unprecedented OCCCO-type pentadentate chelates 69. The second example was the direct oxygenation of the metal center in the carbolong complex 70 [60]. The mixture of osmapentalyne 70 and excess sodium methoxide under oxygen atmosphere resulted in almost quantitative formation of osmapentalene 71, in which the dioxygen was bound to the metal center. The plausible mechanism was depicted as the nucleophilic attack at carbyne carbon atom by sodium methoxide, which led to the reduction of metal center and dissociation of chloride, which facilitated the coordination and the following reduction of dioxygen. Molecules 2020, 25, x 12 of 24

Scheme 14. Nucleophilic reactions of osmapentalyne to form 62–64.

The rationale of the mechanism is depicted in Scheme 15. The combination of cesium carbonate and methanol would generate methanoxide, which conducted the first nucleophilic addition to a carbyneMolecules 2020carbon,, 25, 5050 affording the metallapentalene intermediate 63. Subsequent coordination with12 of 24a terminal alkyne led to the π-alkyne complex 65, which would form the vinylidene η1-complex 66 via isomerization. The α-carbon within the five-membered metallacycle was attacked by the second methanol molecule.molecule. Sequential Sequential ring ring opening, opening, protonation protonation and coordinationand coordination of the esterof the group ester furnished group furnishedthe ﬁnal fused the final metallafulvenallene fused metallafulvenallene complex 64 complex. 64.

Scheme 15. ProposedProposed mechanism mechanism for the formation of 64..

Very recently,recently, oxygenation oxygenation of of osmapentalynes osmapentalynes was was reported reported by by Xia’s Xia’s group group (Scheme (Scheme 16). 16). The The ﬁrst firstexample example was thewas oxygenation the oxygenation of the of osmium the osmium carbolong carbolong complex complex ring [59 ring]. Applying [59]. Applying pyridine pyridineN-oxide Nas-oxide the oxidant, as the oxygen oxidant, atoms oxygen were transferredatoms were to thetransferred metal–carbon to the triple me bondtal–carbon in a nucleophilic triple bond reaction. in a nucleophilicMeanwhile, thereaction. labile aldehyde Meanwhile, was oxidizedthe labile to aldehyde carboxylic was acid, oxidized aﬀording to unprecedented carboxylic acid, OCCCO-type affording unprecedentedpentadentate chelates OCCCO-type69. The secondpentadentate example chelates was the direct69. The oxygenation second example of the metal was center the direct in the oxygenationcarbolong complex of the metal70 [60 center]. The in mixture the carbolong of osmapentalyne complex 7070 [60].and The excess mixture sodium of osmapentalyne methoxide under 70 andoxygen excess atmosphere sodium methoxide resulted in under almost oxygen quantitative atmosphere formation resulted of in osmapentalene almost quantitative71, in whichformation the ofdioxygen osmapentalene was bound 71 to, in the which metal center.the dioxygen The plausible was bound mechanism to the was metal depicted center. as the The nucleophilic plausible mechanismattack at carbyne was depicted carbon atom as the by nucleophilic sodium methoxide, attack at which carbyne led carbon to the reductionatom by sodium of metal methoxide, center and whichMoleculesdissociation led 2020 ,to 25 of , the chloride,x reduction which of facilitated metal center the coordination and dissociation and the of following chloride, reduction which facilitated of dioxygen.13 ofthe 24 coordination and the following reduction of dioxygen.

Scheme 16. Oxygenation of osmapentalynes.

The directdirect attackattack of of a freea free isocyanide isocyanide on theon th carbynee carbyne carbon carbon atom atom of the of osmapentalyne the osmapentalyne50a would 50a 2 wouldlead to thelead formation to the formation of η -iminoketenyl of η2-iminoketenyl metallapentalene metallapentalene intermediates intermediates as well as the as unprecedented well as the unprecedentedmetallaindene derivatives metallaindene (Scheme derivatives 17). The steric(Sch hindranceeme 17). ofThe the bulkysteric Nhindrance-substituents of tertthe-butyl bulky or 1-adamantylN-substituents inhibited tert-butyl the or bending 1-adamantyl at the inhibited isocyanide the nitrogen, bending which at the prevented isocyanide the nitrogen, formation which of a 2 preventedη -iminoketenyl the formation structure of and a ledη2-iminoketenyl to the osmapentalene structure72a and–b .led In comparisonto the osmapentalene to bulky isocyanides, 72a–b. In comparisonother less bulky to bulky isocyanides isocyanides, could aﬀotherord theless desired bulky metallacyclopropenimineisocyanides could afford complex the desired73a–e, whichmetallacyclopropenimine were often regarded complex as the intermediates 73a–e, which innucleophile-induced were often regarded carbyne-isocyanide as the intermediates coupling in nucleophile-induced carbyne-isocyanide coupling processes. The intermediates with η2-iminoketenyl ligands had not been isolated and characterized until the η2-iminoketenyl metallapentalenes were documented by Xia and coworkers [61].

Scheme 17. Nucleophilic reactions of 50a with isocyanides.

It is noteworthy that the addition of excess isocyanides at elevated temperature led to metallaindene derivatives, which represented the first metal-bridged polycyclic metallaaromatics [61]. Treatment of metallapentalyne 50a with the first isocyanide provided η2-iminoketenyl metallapentalene 73. The second isocyanide would coordinate with the osmium center to afford intermediate 74, which underwent sequential isocyanide insertion and coordination of a third isocyanide to generate intermediate 75. The exocyclic imine group could kick out one triphenylphosphine and coordinate with the osmium center, followed by subsequent aromatization to furnish metallaindene 77 (Scheme 18). The same research group extended the metallapentalene Molecules 2020, 25, x 13 of 24

Scheme 16. Oxygenation of osmapentalynes.

The direct attack of a free isocyanide on the carbyne carbon atom of the osmapentalyne 50a would lead to the formation of η2-iminoketenyl metallapentalene intermediates as well as the unprecedented metallaindene derivatives (Scheme 17). The steric hindrance of the bulky N-substituents tert-butyl or 1-adamantyl inhibited the bending at the isocyanide nitrogen, which prevented the formation of a η2-iminoketenyl structure and led to the osmapentalene 72a–b. In comparison to bulky isocyanides, other less bulky isocyanides could afford the desired Molecules 2020, 25, 5050 13 of 24 metallacyclopropenimine complex 73a–e, which were often regarded as the intermediates in nucleophile-induced carbyne-isocyanide coupling processes. The intermediates with processes.η2-iminoketenyl The intermediates ligands had with notη 2been-iminoketenyl isolated ligandsand characterized had not been until isolated the and η2-iminoketenyl characterized untilmetallapentalenes the η2-iminoketenyl were documented metallapentalenes by Xia wereand coworkers documented [61]. by Xia and coworkers [61].

Scheme 17. Nucleophilic reactions of 50a with isocyanides.

It isis noteworthy noteworthy that that the additionthe addition of excess of isocyanidesexcess isocyanides at elevated at temperature elevated temperature led to metallaindene led to derivatives,metallaindene which derivatives, represented which the represented first metal-bridged the firstpolycyclic metal-bridged metallaaromatics polycyclic metallaaromatics [61]. Treatment 2 of[61]. metallapentalyne Treatment of metallapentalyne50a with the first isocyanide50a with the provided first isocyanideη -iminoketenyl provided metallapentalene η2-iminoketenyl73. Themetallapentalene second isocyanide 73. The would second coordinate isocyanide withwould the coordinate osmium with center the to osmium afford intermediatecenter to afford74, whichintermediate underwent 74, which sequential underwent isocyanide sequential insertion isocyanide and coordination insertion of aand third coordination isocyanide toof generate a third intermediateMoleculesisocyanide 2020 , 25to75, x. generate The exocyclic intermediate imine group 75 could. The kick exocyclic out one triphenylphosphineimine group could and coordinatekick out14 with ofone 24 thetriphenylphosphine osmium center, followedand coordinate by subsequent with the aromatizationosmium center, to followed furnish metallaindene by subsequent77 aromatization(Scheme 18). Theandto furnish samemetallaindene researchmetallaindene system group 77 extendedto (Scheme ruthenium the 18). metallapentaleneexcept The same that theresearch η2 and-iminoketenyl group metallaindene extended species systemthe were metallapentalene tonot ruthenium observed except[62]. that the η2-iminoketenyl species were not observed [62].

Scheme 18. Nucleophilic reactions of 50a to form metal-bridged metallaindene.

The metal-carbonmetal-carbon triple triple bond bond moiety moiety in carbyne in carbyne shifted shifted complex complex51a is also 51a sensitive is also to nucleophilicsensitive to attacknucleophilic (Scheme attack 19). Thus,(Scheme products 19). Thus,78 and products79 were 78 formed and 79 by were treating formed osmapentalyne by treating51a osmapentalynewith sodium hydrosulﬁde51a with sodium and benzylamine,hydrosulfide respectivelyand benzylamine, [47]. Noteworthy respectively is that[47]. in Noteworthy the presence is of that aniline in andthe presence of aniline and cesium carbonate, osmapentalyne 51a was transformed to the novel metal-bridged polycyclic aromatic system 80, in which the metal center is shared by three aromatic five-membered rings [63]. The reaction involved the nucleophilic addition of aniline to the carbyne moiety and subsequent C-H bond activation (oxidative addition), providing complex 80a. Treatment of osmapentalyne 51a and phenol produced the similar product 80b via a paralleled mechanism. A number of metal-bridged polycyclic aromatics, including different substitutions on the aromatic ring or larger fused-ring systems, were achieved with the same synthetic strategy [63–65].

BF - Cl PhXH, 4 - Cs CO [Os]4 BF4 2 3 + X H 4 MeO2C PPh3 [Os] + 51a MeO2C PPh3 80a:X=NH BnNH2, 80b:X=O NaSH Cs2CO3

S - 4 BF4 - [Os] N 4 BF + Bn [Os] 4 + MeO2C PPh3 MeO2C PPh3 4 78 [Os] = Os(PPh3)2 79 Scheme 19. Nucleophilic reactions of osmapentalyne 51a to form 78–80.

In the presence of arene nucleophile X, the lactone-fused osmapentalyne 81 [66] was converted to metal bridgehead polycyclic π conjugate systems 82. Further transformations with t-BuOK or cyclohexyl isocyanide would lead to the neutral complex 83 or coordinated complex 84, respectively (Scheme 20). The delocalization of the above mentioned three metalla-aromatics could be used to switch the charge transport pathway of single-molecule junctions and thus tune the charge transport abilities significantly [67], which shed light on the potential applications of metal-bridged polycyclic aromatics in materials science. Molecules 2020, 25, x 14 of 24 and metallaindene system to ruthenium except that the η2-iminoketenyl species were not observed [62].

Scheme 18. Nucleophilic reactions of 50a to form metal-bridged metallaindene.

The metal-carbon triple bond moiety in carbyne shifted complex 51a is also sensitive to Molecules 2020, 25, 5050 14 of 24 nucleophilic attack (Scheme 19). Thus, products 78 and 79 were formed by treating osmapentalyne 51a with sodium hydrosulfide and benzylamine, respectively [47]. Noteworthy is that in the presencecesium carbonate, of aniline osmapentalyne and cesium51a carbonate,was transformed osmapentalyne to the novel 51a metal-bridgedwas transformed polycyclic to the aromatic novel metal-bridgedsystem 80, in which polycyclic the metal aromatic center system is shared 80 by, in three which aromatic the metal ﬁve-membered center is shared rings by [63 three]. The aromatic reaction five-memberedinvolved the nucleophilic rings [63]. addition The reaction of aniline involved to the carbynethe nucleophilic moiety and addition subsequent of aniline C-H bondto the activation carbyne (oxidativemoiety and addition), subsequent providing C-H bond complex activation80a. (oxidative Treatment addition), of osmapentalyne providing51a complexand phenol 80a. Treatment produced ofthe osmapentalyne similar product 51a80b andvia phenol a paralleled produced mechanism. the similar A number product of 80b metal-bridged via a paralleled polycyclic mechanism. aromatics, A numberincluding of di metal-bridgedﬀerent substitutions polycyclic on the aromatics, aromatic including ring or larger different fused-ring substitutions systems, on were the achievedaromatic withring orthe larger same fused-ring synthetic strategy systems, [63 were–65]. achieved with the same synthetic strategy [63–65].

BF - Cl PhXH, 4 - Cs CO [Os]4 BF4 2 3 + X H 4 MeO2C PPh3 [Os] + 51a MeO2C PPh3 80a:X=NH BnNH2, 80b:X=O NaSH Cs2CO3

S - 4 BF4 - [Os] N 4 BF + Bn [Os] 4 + MeO2C PPh3 MeO2C PPh3 4 78 [Os] = Os(PPh3)2 79 Scheme 19. NucleophilicNucleophilic reactions of osmapentalyne 51a to form 78–80. In the presence of arene nucleophile X, the lactone-fused osmapentalyne 81 [66] was converted In the presence of arene nucleophile X, the lactone-fused osmapentalyne 81 [66] was converted to metal bridgehead polycyclic π conjugate systems 82. Further transformations with t-BuOK or to metal bridgehead polycyclic π conjugate systems 82. Further transformations with t-BuOK or cyclohexyl isocyanide would lead to the neutral complex 83 or coordinated complex 84, respectively cyclohexyl isocyanide would lead to the neutral complex 83 or coordinated complex 84, respectively (Scheme 20). The delocalization of the above mentioned three metalla-aromatics could be used to (Scheme 20). The delocalization of the above mentioned three metalla-aromatics could be used to switch the charge transport pathway of single-molecule junctions and thus tune the charge transport switch the charge transport pathway of single-molecule junctions and thus tune the charge transport abilities signiﬁcantly [67], which shed light on the potential applications of metal-bridged polycyclic abilities significantly [67], which shed light on the potential applications of metal-bridged polycyclic aromaticsMolecules 2020 in, 25 materials, x science. 15 of 24 aromatics in materials science.

Scheme 20. Nucleophilic reactions of osmapentalyne 81 and further transformation.

The ruthenium-carbon triple bond moiety moiety in in ruthenapentalynes ruthenapentalynes showed showed nucleophilic nucleophilic reactivity. reactivity. The reactionreaction of of ruthenapentalyne ruthenapentalyne48a with48a sodiumwith sodium thiophenoxide thiophenoxide under carbon under monoxide carbon atmospheremonoxide resultedatmosphere in the resulted generation in the of generation the nucleophilic of the addition nucleophilic product addition ruthenapentalene product ruthenapentalene85. On the basis 85 of. theOn richthe basis and unique of the reactivityrich and unique of ruthenapentalynes, reactivity of ruthenapentalynes, Xia and coworkers Xia designed and coworkers and carried designed out fantastic and cascadecarried out cyclization fantastic reactions cascade with cyclization bidentate reactions nucleophiles with (sodiumbidentate cyanate nucleophiles or sodium (sodium dicyanamide) cyanate or to asodiumﬀord annulation dicyanamide) complexes to afford86 annulationor 87, respectively complexes (Scheme 86 or 2187)[, respectively52]. (Scheme 21) [52].

Scheme 21. Nucleophilic reactions of ruthenapentalyne 43.

A plausible cascade cyclization mechanism is depicted in Scheme 22 [52]. The first cyanate ion initiated the nucleophilic attack at the carbyne carbon of 48a, leading to the formation of ruthenapentalene intermediate 88, which could be further attacked by the second cyanate ion to give intermediate 89. The Cl ligand dissociates from intermediate 89 to form the intermediate 90, which contains a vacant site at the ruthenium center. The subsequent coordination of the OCN group with the metal center could result in the final polycyclic complex 86. The mechanism to access the annulation product 87 might parallel that of the above cascade cyclization reaction, additionally, the nucleophilic attack was initiated by the N(CN)2 ion. Molecules 2020, 25, x 15 of 24

Scheme 20. Nucleophilic reactions of osmapentalyne 81 and further transformation.

The ruthenium-carbon triple bond moiety in ruthenapentalynes showed nucleophilic reactivity. The reaction of ruthenapentalyne 48a with sodium thiophenoxide under carbon monoxide atmosphere resulted in the generation of the nucleophilic addition product ruthenapentalene 85. On the basis of the rich and unique reactivity of ruthenapentalynes, Xia and coworkers designed and Moleculescarried 2020out, 25fantastic, 5050 cascade cyclization reactions with bidentate nucleophiles (sodium cyanate15 of or 24 sodium dicyanamide) to afford annulation complexes 86 or 87, respectively (Scheme 21) [52].

Scheme 21. Nucleophilic reactions of ruthenapentalyne 4343..

AA plausible cascade cyclization cyclization mechanism mechanism is isdepicted depicted in inScheme Scheme 22 22[52].[ 52 The]. Thefirst ﬁrstcyanate cyanate ion ioninitiated initiated the thenucleophilic nucleophilic attack attack at atthe the carbyne carbyne carbon carbon of of48a48a, ,leading leading to to the the formation formation of of ruthenapentaleneruthenapentalene intermediate intermediate 8888, ,which which could could be be further further attacked attacked by the by second the second cyanate cyanate ion to ion give to giveintermediate intermediate 89. The89 .Cl The ligand Cl ligand dissociates dissociates from intermediate from intermediate 89 to form89 to the form intermediate the intermediate 90, which90 , whichcontains contains a vacant a vacant site at sitethe atruthenium the ruthenium center. center. The subsequent The subsequent coordination coordination of the ofOCN the group OCN group with withthe metal the metal center center could could result result in the in the final ﬁnal polycyclic polycyclic complex complex 8686. The. The mechanism mechanism to to access access the the annulationannulation product 8787 mightmight parallel parallel that that of ofthe the above above cascade cascade cyclization cyclization reaction, reaction, additionally, additionally, the thenucleophilicMolecules nucleophilic 2020, 25attack, x attack was was initiated initiated by the by theN(CN) N(CN)2 ion.2 ion. 16 of 24

SchemeScheme 22.22. Proposed mechanism for the formation of 86.. 3.4. Cycloaddition Reactions 3.4. Cycloaddition Reactions The metallapentalynes exhibit rich metal carbyne reactivities due to the extreme strain in the fused The metallapentalynes exhibit rich metal carbyne reactivities due to the extreme strain in the five-membered ring containing a metal carbyne bond. The driving force for the cycloaddition reaction fused five-membered ring containing a metal carbyne bond. The driving force for the cycloaddition of metallapentalynes with the alkynes could be attributed to the release of the large ring strain in the reaction of metallapentalynes with the alkynes could be attributed to the release of the large ring five-membered ring of metallapentalynes. Xia and coworkers documented a number of cycloaddition strain in the five-membered ring of metallapentalynes. Xia and coworkers documented a number of reactions involving the metal-carbon triple bond in metallapentalynes [52,66,68–73]. cycloaddition reactions involving the metal-carbon triple bond in metallapentalynes [52,66,68–73]. Treatment of the cationic osmapentalyne 51a with propionic acid or ethoxyacetylene provided Treatment of the cationic osmapentalyne 51a with propionic acid or ethoxyacetylene provided the planar products 91a and 91b, respectively, through unprecedented [2+2] cycloaddition reactions the planar products 91a and 91b, respectively, through unprecedented [2+2] cycloaddition reactions of alkynes with a late-transition-metal carbyne complex [68]. It is noteworthy that two classical of alkynes with a late-transition-metal carbyne complex [68]. It is noteworthy that two classical anti-aromatic frameworks, cyclobutadiene and pentalene, were stabilized by incorporating one anti-aromatic frameworks, cyclobutadiene and pentalene, were stabilized by incorporating one transition metal fragment into the anti-aromatic systems, thus providing novel aromatic metallacycles. transition metal fragment into the anti-aromatic systems, thus providing novel aromatic Complexes 91a and 91b constituted the first example of [2+2] cycloaddition products formed between metallacycles. Complexes 91a and 91b constituted the first example of [2+2] cycloaddition products aformed late-transition-metal between a late-transition-metal carbyne with alkynes. carbyne Further with alkynes. inserti on Further was not inserti observed on was in not the observed presence in of excessthe presence alkynes, of thus excess preventing alkynes, thethus formation preventing of largerthe formation metallacycles. of larger The metallacycles. reaction could The be extendedreaction tocould early-transition-metal be extended to early-transition-metal carbyne complexes. ca Treatmentrbyne complexes. of ruthenapentalyne Treatment 93of ruthenapentalynewith ethoxyacetylene 93 furnishedwith ethoxyacetylene complex 94 ,furnished which constitute complex the 94, firstwhich [2+ constitute2] cycloaddition the first reaction[2+2] cycloaddition of ruthenium reaction carbyne of complexruthenium with carbyne an alkyne complex [52]. with The an first alkyne [2+2] [52]. cycloaddition The first reaction[2+2] cycloaddition of metallapentalyne reaction 51aof withmetallapentalyne nitrosoarenes 51a (Ar–N with= nitrosoarenesO) was documented (Ar–N=O) by was Xia docu et al.mented [69], abyffording Xia et well-characterizedal. [69], affording well-characterized metallapentalenoxazetes 92a–c (Scheme 23). Faster reaction rate was observed provided that the electron-donating substituents were introduced to the para-position of the nitrosophenyl ring (92b–c), which could be attributed to the enhanced nucleophilicity of the nitrogen atom. Molecules 2020, 25, 5050 16 of 24 metallapentalenoxazetes 92a–c (Scheme 23). Faster reaction rate was observed provided that the electron-donating substituents were introduced to the para-position of the nitrosophenyl ring (92b–c), whichMoleculesMolecules could 2020 2020, ,25 be25, ,x attributedx to the enhanced nucleophilicity of the nitrogen atom. 1717 of of 24 24

SchemeScheme 23. 23.23. The The [2[2+2][2+2]+2] cycloaddition cycloadditioncycloaddition reactionsreactions ofof metallapentalynesmetallapentalynes withwith alkynesalkynes andand nitrosoarenes.nitrosoarenes.

TheThe [2 [2+2][2+2]+2] cycloadditions cycloadditionscycloadditions of osmapentalynesofof osmapentalynesosmapentalynes with withwith alkynes alkynesalkynes were furtherwerewere further investigatedfurther investigatedinvestigated with diff erentwithwith reactiondifferentdifferent conditions. reactionreaction conditions.conditions. In the presence InIn thethe presencepresence of water ofof and waterwater oxygen, andand oxygen,oxygen, osmapentalyne osmapentalyneosmapentalyne and arylacetylene andand arylacetylenearylacetylene were 18 18 transformedwerewere transformedtransformed to the firsttoto thetheα-metallapentalenofuran firstfirst αα-metallapentalenofuran-metallapentalenofuran95. Furthermore, 9595.. Furthermore,Furthermore,O labeling 18OO labelinglabeling experiments experimentsexperiments suggest thatsuggestsuggest the oxygen thatthat thethe atom oxygenoxygen in the furanatomatom ringinin the comesthe furanfuran from ringring water comescomes [66]. Withfromfrom the waterwater synthetic [66].[66]. methodology WithWith thethe syntheticsynthetic in hand, modificationsmethodologymethodology wereinin hand,hand, carried momo outdificationsdifications to install olefinicwerewere groupscarriedcarried inoutout the metallapentalenofurans,toto installinstall olefinicolefinic groupsgroups which ininwere thethe thenmetallapentalenofurans,metallapentalenofurans, subjected to polymerization whichwhich werewere to provide thenthen subjectesubjecte metallapolymersdd toto polymerizationpolymerization exhibiting to excellentto provideprovide stimuli-responsive metallapolymersmetallapolymers propertiesexhibitingexhibiting [ excellent74excellent,75]. The stimuli-responsivestimuli-responsive lactone-fused metallapentalynes propertipropertieses [74,75].[74,75].81 and TheThe96 lactonelactonewere formed-fused-fused if metallapentalynesmetallapentalynes tetrafluoroboric acid 8181 wasandand also9696 werewere present. formedformed Interestingly, ifif tetrafluoroborictetrafluoroboric the metal–carbon acidacid waswas alsoalso triple present.present. bond in Interestingly,Interestingly, osmapentalynes thethe metal–carbonmetal–carbon51a was shifted tripletriple to anotherbondbond inin five-membered osmapentalynesosmapentalynes ring 51a51a in complexwaswas shiftedshifted81 and toto another96anotheraccompanied five-memberedfive-membered by the formation ringring inin complexcomplex of new lactone 8181 andand ring 9696 (Schemeaccompaniedaccompanied 24). byby thethe formationformation ofof newnew lactonelactone ringring (Scheme(Scheme 24).24).

Scheme 24. Synthesis of complexes 81, 95–96. SchemeScheme 24.24. Synthesis Synthesis ofof complexescomplexes 81 81,, 9595––9696. .

InIn 2018,2018, XiaXia andand coworkerscoworkers advancedadvanced thethe abovabovee [2+2][2+2] cycloadditioncycloaddition toto establishestablish thethe firstfirst exampleexample ofof aa [2+2+2][2+2+2] cycloadditiocycloadditionn reactionreaction ofof anan alkynealkyne withwith aa late-transition-metallate-transition-metal carbynecarbyne 4 6 complexcomplex [48].[48]. UponUpon treatmenttreatment ofof NHNH4PFPF6,, neutralneutral osmapentalyneosmapentalyne 4444 underwentunderwent [2+2][2+2] cycloadditioncycloaddition withwith thethe firstfirst alkyne.alkyne. Meanwhile, Meanwhile, thethe chloridechloride ligand ligand of of thethe osmiumosmium centercenter waswas kicked kicked outout toto afford afford a a Molecules 2020, 25, 5050 17 of 24

In 2018, Xia and coworkers advanced the above [2+2] cycloaddition to establish the ﬁrst example of a [2+2+2] cycloaddition reaction of an alkyne with a late-transition-metal carbyne complex [48].

UponMolecules treatment 2020, 25, x of NH4PF6, neutral osmapentalyne 44 underwent [2+2] cycloaddition with the18 firstof 24 alkyne. Meanwhile, the chloride ligand of the osmium center was kicked out to afford a cationic intermediatecationic intermediate97, which 97 exhibited, which exhibited potent reactivity potent reactivity to be inserted to be byinserted the second by the alkyne, second aff alkyne,ording non-planaraffording non-planar eleven-carbon eleven-carbon framework framework98. Subsequent 98. Subsequent migratory migratory insertion furnishedinsertion furnished the thermally the stablethermallyη5-Cp stable complex η5-Cp99 complex(Scheme 99 25 (Scheme). 25).

SchemeScheme 25.25. TheThe [2[2+2+2]+2+2] cycloaddition cycloaddition of of metallapentalyne metallapentalyne 4444 withwith alkynes. alkynes.

In 2019,2019, thethe samesame researchresearch groupgroup reportedreported thethe formalformal [2[2+2+2]+2+2] cycloaddition reaction of a metal-carbyne triple bond with nitriles, demonstratingdemonstrating the “alkyne-like” properties of metal–carbon triple bonds.bonds. Treatment Treatment of of osmapentalyne osmapentalyne51a 51awith with 2,2-diphenylacetonitrile 2,2-diphenylacetonitrile in the in presence the presence of sodium of hydroxidesodium hydroxide resulted resulted in the formation in the formation of tricyclic of metallapyrazinetricyclic metallapyrazine100 (Scheme 100 (Scheme26)[70]. 26) [70].

SchemeScheme 26.26. TheThe [2[2+2+2]+2+2] cycloaddition cycloaddition of of metallapentalyne metallapentalyne 51a51a withwith nitriles. nitriles.

The reactionreaction mechanismmechanism waswas rationalizedrationalized asas followsfollows (Scheme(Scheme 2727).). The deprotonation of 2,2-diphenylacetonitrile with with sodium sodium hydroxide hydroxide provided provided the ketenimine the ketenimine intermediate intermediate as a nucleophile, as a whichnucleophile, could attackwhich thecould carbyne attack carbon the carbyne to give carbon intermediate to give101 intermediate. The chloride 101 ligand. The chloride was kicked ligand out andwas replacedkicked out with and second replaced molecule with second of 2,2-diphenylacetonitrile molecule of 2,2-diphenylacetonitrile to afford intermediate to afford102, whichintermediate would π facilitate102, which the would 6 electrocyclization facilitate the 6 reactionπ electrocyclization to furnish metallapyrazine reaction to furnish complex metallapyrazine100. complex 100. Very recently, Xia’s group enriched the “alkyne like” reactivity of metal–carbon triple bond. Osmapentalyne 103 was prepared from the deprotonation of metallapentalene, furnishing the first cyclic metal carbyne in tricyclic metalla-aromaticsPh with a bridgehead metal, which was subjected CN Ph Ph C to a range of diversely functionalized azides.OH ExcellentPh2C yields, regioselectivity and compatibility OH C Cl Ph Cl Ph Cl Ph were observed to provide an unprecedented- Ph series of polycyclicN 4 aromatics (104aCN–e), in which a 4 BF4 C N [Os]4 Ph [Os] 4 Ph C N [Os] Ph Ph + bridgehead metal center was shared+ by four five-membered aromatic+ rings, thus representing a PPh MeO C PPh MeO2C PPh3 MeO2C PPh3 typical “metalla-click”2 reaction (Scheme3 28)[71]. The2 construction of polycyclicCl aromatics that 51a 101 share a bridgehead atom with more than three rings has never101 been accomplished before this work, Ph Ph Ph Ph which therefore broke the record. PhPh Ph C Ph Ph C Ph2C N C N Ph N - N 4 Electrocyclization N 4 BF - [Os]4 N [Os]4 BF4 + + MeO C PPh3 MeO C PPh3 MeO2C PPh3 MeO2C PPh3 102 [Os]4 = Os(PPh ) 100 [Os] = Os(PPh3)2 Scheme 27. Mechanism of [2+2+2] cycloaddition of metallapentalyne 51a with nitriles. Molecules 2020, 25, x 18 of 24 cationic intermediate 97, which exhibited potent reactivity to be inserted by the second alkyne, affording non-planar eleven-carbon framework 98. Subsequent migratory insertion furnished the thermally stable η5-Cp complex 99 (Scheme 25).

Scheme 25. The [2+2+2] cycloaddition of metallapentalyne 44 with alkynes.

In 2019, the same research group reported the formal [2+2+2] cycloaddition reaction of a metal-carbyne triple bond with nitriles, demonstrating the “alkyne-like” properties of metal–carbon triple bonds. Treatment of osmapentalyne 51a with 2,2-diphenylacetonitrile in the presence of sodium hydroxide resulted in the formation of tricyclic metallapyrazine 100 (Scheme 26) [70].

Scheme 26. The [2+2+2] cycloaddition of metallapentalyne 51a with nitriles.

The reaction mechanism was rationalized as follows (Scheme 27). The deprotonation of 2,2-diphenylacetonitrile with sodium hydroxide provided the ketenimine intermediate as a nucleophile, which could attack the carbyne carbon to give intermediate 101. The chloride ligand was kicked out and replaced with second molecule of 2,2-diphenylacetonitrile to afford intermediate

102Molecules, which2020 ,would25, 5050 facilitate the 6π electrocyclization reaction to furnish metallapyrazine complex18 of 24 100.

Ph CN Ph Ph C OH 2 Cl C Cl Ph Molecules 2020, 25, x - Ph N CN 19 of 24 Molecules 2020, 25, x 4 BF4 C N [Os]4 Ph 19 of 24 [Os] Ph + + PPh Very recently,MeO 2CXia’s group enrichedPPh3 the “alkyneMeO like”2C reactivity of metal–carbon3 triple bond. Very recently, Xia’s group enriched the “alkyne like” reactivity of metal–carbonCl triple bond. Osmapentalyne 103 was prepared51a from the deprotonation of101 metallapentalene, furnishing the first Osmapentalyne 103 was prepared from the deprotonation of metallapentalene, furnishing the first cyclic metal carbyne in tricyclicPh Ph metalla-aromatics with a bridgehead metal, which was subjected to a cyclic metal carbyne in tricyclic metalla-aromatics with a bridgeheadPh metal,Ph which was subjected to a range of diversely functionalizedC azides. Excellent yields, regioselectivityPh and compatibility were range of diverselyPh 2functionalizedC azides. Excellent yields, regioselectivity and compatibility were observed to provideC an unprecedenteN d series of polycyclicPh aromaticsN (104a–e), in which a observed to provide Nan unprecedented Electrocyclizationseries of polycyclic aromatics (104a–-e), in which a bridgehead metal center was[Os] 4shared by four five-membered aromaticN [Os] rings,4 thusBF4 representing a bridgehead metal center was shared+ by four five-membered aromatic rings,+ thus representing a typical “metalla-click”MeO C reaction (SchemePPh3 28) [71]. The constructionMeO C of polycyclicPPh aromatics3 that share typical “metalla-click”2 reaction (Scheme 28) [71]. The construction2 of polycyclic aromatics that share 102 a bridgehead atom with more than three rings has4 never been accomplished before100 this work, which a bridgehead atom with more than three rings[Os] has =never Os(PPh been3)2 accomplished before this work, which therefore broke the record. therefore broke the record. Scheme 27.27. Mechanism of [2[2+2+2]+2+2] cycloaddition cycloaddition of of metallapentalyne metallapentalyne 51a51a withwith nitriles. nitriles.

Scheme 28. Metalla-click reactions. Scheme 28. Metalla-click reactions. During the preparation of this manuscript, Xia and coworkers secured the Simmons-Smith During thethe preparation preparation of thisof th manuscript,is manuscript, Xiaand Xia coworkers and coworkers secured secured the Simmons-Smith the Simmons-Smith reactions reactions of substrates with metal–carbon triple bonds for the first time [72] (Scheme 29). reactionsof substrates of withsubstrates metal–carbon with metal–carbon triple bonds fortrip thele ﬁrstbonds time for [72 the] (Scheme first time29). Osmapentalyne[72] (Scheme 29).51a Osmapentalyne 51a was transformed into cyclopropenation product 105a in the presence of Osmapentalynewas transformed 51a into was cyclopropenation transformed productinto cyclopropenation105a in the presence product of classical 105a in Fukuyama the presence reagent. of classical Fukuyama reagent. The substrate scope was examined with electron-withdrawing classicalThe substrate Fukuyama scope wasreagent. examined The substrate with electron-withdrawing scope was examined substituents with electron-withdrawing to give the expected substituents to give the expected cyclopropene products 105b–d in good yields. When the substituentscyclopropene to products give the105b expe–d ctedin good cyclopropene yields. When products the substituents 105b–d in were good switched yields. to When carbinols, the substituents were switched to carbinols, the Fukuyama reagent failed, and the more reactive Shi’s substituentsthe Fukuyama were reagent switched failed, to andcarbinols, the more the reactiveFukuyama Shi’s reagent reagent failed, was and used the to more furnish reactive the desired Shi’s reagent was used to furnish the desired product 107. These results demonstrate that the reactions reagentproduct was107. used These to results furnish demonstrate the desired that product the reactions 107. These were results sensitive demonstrate to electronic that eﬀect, the which reactions was were sensitive to electronic effect, which was substantiated by theoretical calculations. Based on the weresubstantiated sensitive by to theoreticalelectronic effect, calculations. which Basedwas substantiated on the results by achieved, theoretical the calculations. Simmons–Smith Based reactions on the results achieved, the Simmons–Smith reactions were also extended to ruthenapentalynes [72]. resultswere also achieved, extended the to Simmons–Smith ruthenapentalynes reactions [72]. were also extended to ruthenapentalynes [72].

Scheme 29. Simmons-Smith reactions. Scheme 29. Simmons-Smith reactions. Unprecedented [3+1] cycloadditions of metalla-azirines with terminal alkynes were Unprecedented [3+1] cycloadditions of metalla-azirines with terminal alkynes were documented by Xia’s [73] group (Scheme 30). Osmapentalyne 50a was transformed into the documented by Xia’s [73] group (Scheme 30). Osmapentalyne 50a was transformed into the metalla-azirine 108 in the presence of sodium azide. The reaction was the first synthesis of a metalla-azirine 108 in the presence of sodium azide. The reaction was the first synthesis of a metalla-azirine by [2+1] cycloaddition reaction of a metal carbyne with an azide and paralleled the metalla-azirine by [2+1] cycloaddition reaction of a metal carbyne with an azide and paralleled the first [3+2] cycloadditions of late-transition-metal carbyne with organic azides [71]. In the presence of first [3+2] cycloadditions of late-transition-metal carbyne with organic azides [71]. In the presence of AgBF4, a series of substituted terminal alkynes as well as metalla-azirine 108 were transformed to AgBF4, a series of substituted terminal alkynes as well as metalla-azirine 108 were transformed to Molecules 2020, 25, 5050 19 of 24

Unprecedented [3+1] cycloadditions of metalla-azirines with terminal alkynes were documented by Xia’s [73] group (Scheme 30). Osmapentalyne 50a was transformed into the metalla-azirine 108 in the presence of sodium azide. The reaction was the first synthesis of a metalla-azirine by [2+1] cycloaddition reaction of a metal carbyne with an azide and paralleled the first [3+2] cycloadditions Moleculesof late-transition-metal 2020, 25, x carbyne with organic azides [71]. In the presence of AgBF4, a series20 of of24 substituted terminal alkynes as well as metalla-azirine 108 were transformed to various tetracyclic variouscomplexes tetracyclic109a–d. Moreover,complexes the 109a five–d coordinating. Moreover, atomsthe five lie coordinating in the equatorial atoms plane lie in in the the CCCCX-type equatorial (Xplane= N, in O, the S) CCCCX-type carbolong complexes (X = N, O, obtained S) carbolong and the complexes reaction provides obtained a and valuable the reaction supplement provides to the a valuableconstruction supplement of planar to pentadentate the construction chelates. of planar pentadentate chelates.

O - O BF4 N RO [Os]4 + MeO2C PPh3 109a

- S BF4 S N 4 Cl Cl Cl- [Os] - N 4 Cl + [Os] [Os]4 NaN3 MeO2C PPh3 + AgBF4 + 109b MeO2C PPh3 CH Cl MeO C PPh3 2 2 2 rt rt 50a - 108 N N BF4 N [Os]4 + MeO2C PPh3 109c

- BF4 NH2 HN N [Os]4 + 4 MeO2C PPh3 [Os] =Os(PPh3)2 109d Scheme 30. Formal [3[3+1]+1] cycloadditions. cycloadditions.

4. Conclusions 4. Conclusions Fantastic molecular architectures characterized with aromaticity have drawn considerable attention Fantastic molecular architectures characterized with aromaticity have drawn considerable from the synthetic community, thus producing a huge number of aromatic structures with elegance attention from the synthetic community, thus producing a huge number of aromatic structures with as well as variety. As commonly sensed, the organic alkyne exhibits a linear shape and the bond elegance as well as variety. As commonly sensed, the organic alkyne exhibits a linear shape and the angles around the acetylenic carbons are 180 . Many efforts have been devoted to the chemistry bond angles around the acetylenic carbons ◦are 180°. Many efforts have been devoted to the of cyclic complexes containing metal-carbon triple bonds, in which the carbine-carbon bond angles chemistry of cyclic complexes containing metal-carbon triple bonds, in which the carbine-carbon were less than 180 since the first example of metallabenzyne in 2001. Cyclic complexes containing bond angles were less◦ than 180° since the first example of metallabenzyne in 2001. Cyclic complexes carbynes could be classified into six-membered metallacarbynes and five-membered metallapentalynes. containing carbynes could be classified into six-membered metallacarbynes and five-membered The carbine-carbon bond angles of metallabenzynes are around 148 . Such a record was held for 12 metallapentalynes. The carbine-carbon bond angles of metallabenzynes◦ are around 148°. Such a years until the discovery of metallapentalynes, whose carbine-carbon bond angles were bent to around record was held for 12 years until the discovery of metallapentalynes, whose carbine-carbon bond 130 , far away from the ideal value of 180 . Such extreme distortion results in considerable large ring angles◦ were bent to around 130°, far away◦ from the ideal value of 180°. Such extreme distortion strain, resulting in the unprecedented high reactivity. results in considerable large ring strain, resulting in the unprecedented high reactivity. The reaction types of metallabenzynes could be classified into four types, such as ligand The reaction types of metallabenzynes could be classified into four types, such as ligand exchange reactions, electrophilic reactions, nucleophilic reactions and migratory insertion reactions. exchange reactions, electrophilic reactions, nucleophilic reactions and migratory insertion reactions. Ligand exchange reactions are the fundamental reactions in organometallic chemistry and were Ligand exchange reactions are the fundamental reactions in organometallic chemistry and were unsurprisingly involved in the reactions of metallabenzynes and metallapentalynes. Due to the unsurprisingly involved in the reactions of metallabenzynes and metallapentalynes. Due to the ambiphilic properties of carbynes, both electrophilic and nucleophilic reactions were observed in ambiphilic properties of carbynes, both electrophilic and nucleophilic reactions were observed in the the reactions of metallabenzynes and metallapentalynes, which resemble the classical metal carbyne reactions of metallabenzynes and metallapentalynes, which resemble the classical metal carbyne complex. Metallabenzynes exhibited excellent aromatic properties and the whole cyclic system acted as complex. Metallabenzynes exhibited excellent aromatic properties and the whole cyclic system acted an entire entity under common electrophilic conditions, thus resembling the electrophilic substitution as an entire entity under common electrophilic conditions, thus resembling the electrophilic substitution reactions displayed upon organic aromatics. The carbyne moiety stayed intact, whereas the aromatic rings were attacked by electrophiles. One exception that was observed was that the carbyne moiety was epoxidized upon treatment of hydrogen peroxide. In most cases, the nucleophiles would attack the carbyne carbon. However, “softer” nucleophiles might attack the para position of the six-membered ring system, thus demonstrating the aromaticity of metallabenzynes. Meanwhile, the aromaticity of osmabenzynes was strongly affected by the corresponding substituents of metallacycles and ligands of metal center, leading to the decomposition of metallacarbyne ring systems to form cyclopentadiene complexes via migratory insertion reactions. Molecules 2020, 25, 5050 20 of 24 reactions displayed upon organic aromatics. The carbyne moiety stayed intact, whereas the aromatic rings were attacked by electrophiles. One exception that was observed was that the carbyne moiety was epoxidized upon treatment of hydrogen peroxide. In most cases, the nucleophiles would attack the carbyne carbon. However, “softer” nucleophiles might attack the para position of the six-membered ring system, thus demonstrating the aromaticity of metallabenzynes. Meanwhile, the aromaticity of osmabenzynes was strongly affected by the corresponding substituents of metallacycles and ligands of metal center, leading to the decomposition of metallacarbyne ring systems to form cyclopentadiene complexes via migratory insertion reactions. In sharp contrast, the ring systems of metallapentalynes were never broken throughout all the reported reactions, which might be attributed to the higher stability of metallapentalynes derived from their strong aromaticity despite the extremely strained bond angles of metallapentalynes. The metal–carbon triple bond in metallapentalynes possessed “alkyne-like” character, which was demonstrated by the formation of osmapentalyne-coinage metal complexes and cycloaddition reactions. Recent advances in the reactivity of a series of cyclic complexes containing carbynes described herein deepened our understanding of the nature of metallabenzynes and metallapentalynes and resulted in the production of a number of novel metallacycles. However, the research on the reactivity of metallabenzynes and metallapentalynes is still limited compared with that of organic alkynes and metal-carbon triple bonds. Additionally, the reactivity studies of these unique species need further exploration, and the isolation of other metalla-aromatics is also anticipated via the corresponding research. Considering that metallacycles are regarded as intermediates in many transition-metal-catalyzed reactions, this study provides a new perspective for the application of late-transition-metal carbyne species as catalysts. The unique structure leads to novel properties. Novel metallacycles derived from the reactivity studies of metallapentalynes exhibited great application prospects in various areas, such as near-infrared dyes [47], photothermal therapy [74–77], phototherapy [78], and self-healing materials [79], enabling their potential applications in materials science and biomedicine. Future developments of metallabenzynes and metallapentalynes and exploration of their potential applications in catalysis [60], materials science and biomedicine would not only extend our perception of aromaticity in metallacycles, but also further enrich fundamental organometallic chemistry.

Author Contributions: Investigation, Q.S.; writing—original draft preparation, J.D. and Z.D.; writing—review and editing, Y.L., H.L. and M.-A.O.; supervision, L.S. and R.L. The manuscript was written through contributions of all authors. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Natural Science Foundation of Fujian Province of China, grant number 2020J01526; the Foundation of Department of Education of Fujian Province, grant number JT180114; the Fujian Agriculture and Forestry University Foundation for Distinguished Young Scholars, grant number XJQ201623; the Foundation of Key Laboratory of Biopesticide and Chemical Biology, grant number Keylab2019-03. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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