Carbeneiridium Complexes

Review Iridaaromatics via Methoxy(alkenyl)carbeneiridium Complexes

Maria Talavera 1,2 and Sandra Bolaño 2,*

1 Department of Chemistry, Humboldt–Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany; [email protected] 2 Departamento de Química Inorgánica, Universidade de Vigo, Campus Universitario, 36310 Vigo, Spain * Correspondence: [email protected]

Abstract: This review describes the development of a versatile methodology to synthesize polycyclic metallaaromatic hydrocarbons based on iridium, as well as the studies that helped us to determine and understand what is required in order to broaden the scope and the selectivity of the methodology and stabilize the complexes obtained. This methodology aims to open the door to new materials based on graphene fragments.

Keywords: metallaaromatics; iridium; PAHs

1. Introduction The interest of graphene lies in its structure and unusual physical properties, which turn polycyclic aromatic hydrocarbons (PAHs) and nanographenes, small graphene fragments, into the subject of several research in the last decades [1]. Studying PAHs allows for correlating their geometrical and electronic structure with their properties, which can Citation: Talavera, M.; Bolaño, S. then be applied to larger structures. It has been found that the introduction of defects or Iridaaromatics via Methoxy irregularities in the structure of PAHs modifies their properties and consequently broadens (alkenyl)carbeneiridium Complexes. their field of application. Among these defects, the most common are deviations from Molecules 2021, 26, 4655. https:// planarity due to edges, the introduction of non-six-member rings, doping with heteroatoms doi.org/10.3390/molecules26154655 or the inclusion of substituents for their functionalization [2–5]. The introduction of these irregularities in PAHs can lead to new materials improving Academic Editors: results and even opening up new applications with respect to their non-defective analogues. Maurizio Peruzzini and With this in mind, over the last decade our research group has focused its interest on the Luca Gonsalvi development of new PAHs by incorporating a transition metal in order to create polycyclic metallaaromatic hydrocarbons (PMAHs) by replacing a CH unit with a transition metal. Received: 10 July 2021 These new systems are considered to be doped PAHs and could potentially introduce Accepted: 28 July 2021 Published: 31 July 2021 a slight deviation of the plane. This has led us to develop a synthetic and versatile methodology that allows us to obtain PMAHs with different structures and sizes, as this is the key to obtaining new and improved properties for their application in the field of Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in materials. published maps and institutional affil- In this review, we focus on the studies which led us to the successful development iations. and understanding of a methodology for synthesizing iridium based PMAHs.

2. Methoxy(alkenyl)carbeneiridium Complexes The synthesis of methoxycarbenes is a well-known process which is achieved by the nucleophilic addition of methanol to a vinylidene or allenylidene complex [6,7]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. One useful methodology to obtain these products is the activation of terminal alkynes This article is an open access article and propargylic alcohols with a suitable precursor. In this regard, complexes with la- distributed under the terms and bile ligands, such as ethylene, cyclooctadiene or acetonitrile, are convenient starting conditions of the Creative Commons materials [8]. Therefore, the acetonitrile complexes [IrCp*Cl(NCMe)(PPh2Me)]PF6 (1) Attribution (CC BY) license (https:// and [IrCp*(NCMe)2(PPh2Me)](PF6)2 (2) obtained from [IrCp*Cl2(PPh2Me)] (Scheme1) creativecommons.org/licenses/by/ promised to be suitable precursors. It was observed that the chosen salt for the abstrac- 4.0/). tion of the chlorido ligand played a key role in the product selectivity. Therefore, AgPF6

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Molecules 2021, 26, 4655 2 of 14 [IrCp*(NCMe)2(PPh2Me)](PF6)2 (2) obtained from [IrCp*Cl2(PPh2Me)] (Scheme 1) promised to be suitable precursors. It was observed that the chosen salt for the abstraction of the chlorido ligand played a key role in the product selectivity. Therefore, AgPF6 promotes promotes a double methathesis (independent of the stoichiometry) while TlPF6 selectively a double methathesis (independent of the stoichiometry) while TlPF6 selectively leads to leads to the monoacetonitrile derivative [9,10]. the monoacetonitrile derivative [9,10].

SchemeScheme 1. 1.Synthesis Synthesis of of the the acetonitrile acetonitrile complexes complexes1 1and and2 2and and their their reactivity reactivity towards towards propargylic propargylic alcohols. alcohols.

TheThe monoacetonitrile monoacetonitrile complex complex1 reacted1 reacted with with 1,1-diphenyl-2-propyn-1-ol 1,1-diphenyl-2-propyn-1-ol in methanol, in metha- yieldingnol, theyielding expected methoxy(alkenyl)carbeneiridiumthe expected methoxy(alkenyl)carbeneiridium complex [IrCp*Cl{=C(OMe)CH= complex [IrCp*Cl{=C(OMe)CH=CPh2}(PPh2Me)]PF6 (3). When the reaction was performed in di- CPh2}(PPh2Me)]PF6 (3). When the reaction was performed in dichloromethane, the forma- tionchloromethane, of an unstable the allenylidene formation of complex an unstable4 was allenylidene observed, whichcomplex by 4 addition was observed, of methanol which yielded,by addition as proposed, of methanol complex yielded,3 (Scheme as proposed,1)[9]. complex 3 (Scheme 1) [9]. Interestingly,Interestingly,the the bisacetonitrile bisacetonitrile complexcomplex2 2was was capable capable of of catalytically catalytically activating activating propargylicpropargylic alcohols alcohols in in order order to to produce produce the the corresponding corresponding aldehydes aldehydes through through a a Meyer– Meyer– SchusterSchuster rearrangement,rearrangement, insteadinstead ofof thethe expectedexpected organometallicorganometallic derivativederivative (Scheme(Scheme1 ).1). TheThe mechanism mechanism proposed proposed for for the the reaction reaction involved involved the the initial initial formation formation of of a a vinylidene vinylidene complexcomplex which which underwent underwent an an intramolecular intramolecular nucleophilic nucleophilic addition addition of of the the hydroxo hydroxo group group toto thethe αα-carbon-carbon ofof thethe vinylidenevinylidene [10]. [10]. This This mechanism mechanism excluded excluded the the formation formation of ofan an al- allenylidenelenylidene complex complex and, and, therefore, therefore, hampered thethe nucleophilicnucleophilic attackattack of of methanol methanol to to form form methoxy(alkenyl)carbenes.methoxy(alkenyl)carbenes. TheThe reactivity reactivity of of complex complex3 3towards towards strong strong bases bases and and amines amines was was studied. studied. Thus, Thus, an an excessexcess of of KO KOtButBu deprotonated deprotonated the the alkenyl alkenyl moiety moiety leading leading to to the the neutral neutral methoxyallenyl methoxyallenyl complexcomplex [IrCp*Cl{C(OMe)=C=CPh [IrCp*Cl{C(OMe)=C=CPh22}(PPh}(PPh22Me)] ((55)) (Scheme(Scheme2 2).). This This reaction reaction is is reversible, reversible, andand the the addition addition of of HBF HBF4 4regenerated regenerated cation cation complex complex3 3[ 9[9].]. On On the the other other hand, hand, reactivity reactivity withwith amines amines did did not not provide provide the the expected expected results. results. It It is is known known that that primary primary or or secondary secondary aminesamines can can react react with with alkoxycarbenes alkoxycarbenes to to provide provide aminocarbenes aminocarbenes through through an aminolysisan aminolysis re- actionreaction [11 –[11–13].13]. However, However, this this reaction reaction only on tookly took place place when when an aqueous an aqueous ammonia ammonia solution solu- wastion used was andused the and aminocarbene the aminocarbene complex complex [IrCp*Cl{=C(NH [IrCp*Cl{=C(NH2)CH=CPh2)CH=CPh2}(PPh2}(PPh2Me)]PF2Me)]PF6 (6) 6 could(6) could be isolated be isolated (Scheme (Scheme2). Instead, 2). Instead, reactivity reac withtivity amines with amines underwent underwent a heterolytic a heterolytic cleav- 3 agecleavage of the carbon(spof the )-oxygencarbon(sp bond3)-oxygen to generate bond the acylto complexgenerate [IrCp*Cl{C(O)CH=CPh the acyl complex2} (PPh[IrCp*Cl{C(O)CH=CPh2Me)] (7) and the release2}(PPh of2Me)] a mixture (7) and of the methylammonium release of a mixt saltsure of (Scheme methylammonium2)[ 9]. DFT calculationssalts (Scheme were 2) performed[9]. DFT calculations in order to were explain performed the different in order reactivity, to explain and the the presence different ofreactivity, an equilibrium and the between presence electronic, of an equilibrium steric and between cooperative electronic, effects steric was and revealed cooperative [14]. Calculationseffects was revealed showed [14]. that theCalculations aminolysis showed reaction that is the not aminolysis the dominant reaction reaction is not with the these dom- particularinant reaction alkoxycarbene with these complexes. particular alkoxycarbene complexes.

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SchemeScheme 2.2. ReactivityReactivity ofof thethe methoxy(alkenyl)carbeneiridiummethoxy(alkenyl)carbeneiridium complexcomplex3 3..

3.3. IridanaphthaleneIridanaphthalene ComplexComplex andand Itsits EvolutionEvolution BesidesBesides the the unexpected unexpected chemistry chemistry displayed displayed by complexby complex3 towards 3 towards amines, amines, the complexes the com- [IrCp*Cl{=C(OMe)CH=CPhplexes [IrCp*Cl{=C(OMe)CH=CPh2}(L)]PF62}(L)]PF(L = PPh6 (L2Me = (PPh3), PMe2Me 3((38),)) PMe [15] have3 (8))[15] been have proved been to beproved active to towards be active orthomethalation towards orthomethalati of a phenyl substituenton of a phenyl of the substituent methoxy(alkenyl)carbene of the meth- ligand.oxy(alkenyl)carbene Thus, abstraction ligand. of the Thus, chlorido abstractio ligand,n inof complexesthe chlorido3 and ligand,8, by in addition complexes of a silver3 and salt,8, by promoted addition theof a intramolecular silver salt, promoted C–H bond the activation intramolecular of a phenyl C–H substituentbond activation of the of carbene a phe- ligand,nyl substituent leading to of the the formation carbene ligand, of a new leading Ir–C bond to the resulting formation in twoof a newnew iridanaphthaleneIr–C bond result- complexes,ing in two 9newand iridanaphthalene10, respectively, bearing complexes, a phenyl 9 and substituent 10, respectively, in the iridanaphthalene bearing a phenyl moiety sub- (Schemestituent 3in) [ the16]. iridanaphthalene moiety (Scheme 3) [16].

Scheme 3. Synthesis and evolution of the iridanaphthalene complexes 9 and 10. [Ir] = [IrCp*L]PF6. SchemeScheme 3.3. SynthesisSynthesis andand evolutionevolution ofof thethe iridanaphthaleneiridanaphthalene complexescomplexes9 9and and10 10.. [Ir][Ir] == [IrCp*L]PF[IrCp*L]PF6..

ComplexesComplexes9 9 andand 1010 representedrepresented thethe thirdthird exampleexample ofof metallanaphthalenemetallanaphthalenecomplexes complexes described,described, andand the the second second with with iridium iridium as the as transition the transition metal [ 17metal,18]. Previously,[17,18]. Previously, Paneque etPaneque al. had reportedet al. had the reported synthesis the of synthesis the ﬁrst iridanaphthalene of the first iridanaphthalene complex through complex an oxidative through ringan oxidative contraction ring reaction contraction of a benzannelatedreaction of a benzannelated iridacycloheptatriene iridacycloheptatriene complex [18]. complex The X- ray[18]. structure The X-ray of structure complexes of9 complexes·BPh4, 10 and 9·BPh104·,BPh 10 and4 present 10·BPh C–C4 present distances C–C between distances 1.35 be- andtween 1.47 1.35 Å, and comparing 1.47 Å, wellcomparing with those well of with naphthalene, those of naphthalene, suggesting that suggesting the delocalized that the πdelocalized-bonding extends π-bonding over extends both rings. over Inboth these rings. structures, In these thestructures, iridium the atom iridium is displaced atom is fromdisplaced the nearly from co-planarthe nearly carbon co-planar atoms carbon by 0.066(5) atoms and by 0.176(9)0.066(5) Å and for 0.176(9)9·BPh4 andÅ for10 ·9·BPhBPh4,4 respectivelyand 10·BPh4, [ 16respectively]. The values [16]. of The theiridanaphthalene values of the iridanaphthalene complexes with complexes BPh4 as counteranion with BPh4 as are signiﬁcantly smaller than those reported in the literature on other iridanaphthalene complexes, which are very similar to the value observed for 10 (0.689(5) Å) [18,19].

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Molecules 2021, 26, 4655 4 of 14 counteranion are significantly smaller than those reported in the literature on other iridanaphthalene complexes, which are very similar to the value observed for 10 (0.689(5) Å) [18,19]. InIn thethe studystudy ofof metallaaromaticmetallaaromatic complexes,complexes, X-rayX-ray diffractiondiffraction andand NMRNMR spectroscopyspectroscopy areare thethe mostmost widelywidely usedused techniques,techniques, asas theythey provideprovide informationinformation aboutabout thethe structurestructure andand naturenature ofof thesethese complexescomplexes [[20].20]. InIn complexescomplexes 99 andand 1010,, twotwo featuresfeatures areare determineddetermined inin orderorder toto confirmconfirm thethe formationformation ofof thethe cycle:cycle: (i)(i) thethe absenceabsence ofof thethe protonproton resonanceresonance ofof thethe 11 activatedactivated C–HC–H bondbond inin thethe 1HH NMR spectra,spectra, and;and; (ii)(ii) thethe deshieldingdeshielding togethertogether withwith thethe C,PC,P 2 2 1313 11 couplingcoupling inin thethe carboncarbon resonance resonance ( δ(δ155 155 ppm, ppm, J2C-PJC-P == 1010 Hz)Hz)at at the the 13C{C{1H}H} NMRNMR spectraspectra whenwhen bondedbonded toto thethe iridiumiridium atomatom [[16].16]. AsAs has happened in in many many metallaaromatic metallaaromatic systems, systems, 9 and9 and 1010 resultedresulted in the in the presen- pre- sentationtation of a of low a low thermal thermal stability. stability. While While thethe literature literature described described the therearrangement rearrangement of the of theiridaaromatic iridaaromatic complexes complexes into into the thethermodynamically thermodynamically stable stable cyclopentadienyl cyclopentadienyl derivatives derivatives[21,22], [21 ,complexes22], complexes 9 and9 and 10 10evolvedevolved into into 3-phenylindanone 3-phenylindanone when when heat heat was appliedapplied (Scheme(Scheme3 3))[ [16].16]. DifferentDifferent experimentsexperiments conductedconducted inin orderorder toto elucidateelucidate thethe mechanismmechanism ofof thisthis transformationtransformation havehave allowedallowed forfor thethe observationobservation ofof thethe expectedexpected indenylindenyl complexescomplexes 3 [IrCp*{η 3-(C H )(OMe)(Ph)}(L)]PF (L = PPh Me (11), PMe (12)) as reaction intermedi- [IrCp*{η3-(C9H55)(OMe)(Ph)}(L)]PF6 6(L = PPh2Me2 (11), PMe3 (123 )) as reaction intermediates. ates.Therefore, Therefore, the iridanaphthalene the iridanaphthalene complexes complexes 9 and9 and 10 may10 may undergo undergo carbene carbene migratory migratory in- Therefore, the iridanaphthalene1 complexes 9 and 10 may undergo carbene3 migratory in- insertion, yielding1η indenyl complexes which would evolve to the3 η intermediates 11 sertion, yielding η1 indenyl complexes which would evolve to the η3 intermediates 11 and and 12 through a hapticity switch. Finally, the heterolytic cleavage of the O-CH3 bond 12 through a hapticity switch. Finally, the heterolytic cleavage of the O-CH3 bond would would occur and hydrogenation of the indenone would take place in order to yield the occur and hydrogenation of the indenone would take place in order to yield the final 3- final 3-phenylindanone (Scheme4). In order to find the hydrogen source, a reaction was phenylindanone (Scheme 4). In order to find the hydrogen source, a reaction was also also performed in deuterated solvents as well as in strictly water-free conditions. These performed in deuterated solvents as well as in strictly water-free conditions. These studies studies suggested the involvement of the methyl group in the hydrogenation reaction [16]. suggested the involvement of the methyl group in the hydrogenation reaction [16].

SchemeScheme 4.4. MechanismMechanism proposalproposal forfor thethe formationformation ofof 3-phenylindanone3-phenylindanone from complexescomplexes 9 and 1010.. 6 [Ir] = [IrCp*L]PF6. [Ir] = [IrCp*L]PF6.

4.4. EffectEffect ofof thethe γγ-Carbon-carbon Substituents Substituents TheThe understandingunderstanding and and control control of the of synthetic the synthetic methodology, methodology, with methoxy(alkenyl) with meth- carbeneiridiumoxy(alkenyl)carbeneiridium complex as the complex starting as material,the starting will material, allow for will the allow accessing for the of accessing different andof different stable PMAHs. and stable Thus, PMAHs. it was Thus, expected it was expected that the ﬁnal that the product final product would depend would depend on the substituentson the substituents at a γ spatial at a γ spatial arrangement arrangement of the precursorof the precursor methoxy(alkenyl)carbeneiridium methoxy(alkenyl)carbenei- and,ridium therefore, and, therefore, the key the would key would be in thebe in choice the choice of the of propargylic the propargylic alcohol. alcohol. In order In order to gainto gain a better a better insight insight into into the the effect effect of of the theγ γcarbon carbon substituent, substituent, methoxy(alkenyl)carbene methoxy(alkenyl)carbene complexescomplexes withwith twotwo differentdifferent substituentssubstituents havehave beenbeen described,described, whichwhich maymay resultresult inin aa mixturemixture ofof thethe EE andand ZZ isomersisomers (Scheme(Scheme5 ).5).

Scheme 5. Possible E and Z isomers in methoxy(alkenyl)carbeneiridium complexes. [Ir] = SchemeScheme 5.5. Possible E andand ZZ isomersisomers inin methoxy(alkenyl)carbeneiridium methoxy(alkenyl)carbeneiridium complexes. complexes. [Ir][Ir] == 3 + [IrCp*Cl(PMe)3]+. [IrCp*Cl(PMe)3] .

When the substituents on the γ carbon are a methyl and phenyl group (13), only the E isomer was formed, as the methyl group is located close to the iridium atom. With this spatial arrangement, AgPF6 treatment leads to C–H bond activation of the methyl

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Molecules 2021, 26, 4655 5 of 14 When the substituents on the γ carbon are a methyl and phenyl group (13), only the E isomer was formed, as the methyl group is located close to the iridium atom. With this spatial arrangement, AgPF6 treatment leads to C–H bond activation of the methyl group, group,obtaining obtaining the iridacyclopenta-1,3-diene the iridacyclopenta-1,3-diene complex complex 14 (Scheme14 (Scheme 6). Complex6). Complex 14 can react14 withcan reacta base with in order a base to inyield order an iridacyclopenta-2,4-diene to yield an iridacyclopenta-2,4-diene complex 15 [23]. complex The same15 [23 behavior]. The samewas observed behavior at was complex observed 16,at where complex the phenyl16, where group the phenylwas substituted group was with substituted a spirobifluo- with arene spirobiﬂuorene moiety (SBF) moiety and complexes (SBF) and 17 complexesand 18 were17 obtainedand 18 were (Scheme obtained 6) [24]. (Scheme Likewise,6)[ com-24]. Likewise,plex 19, bearing complex two19 ,methyl bearing groups, two methyl also perfor groups,med also the performed C–H bond the activation, C–H bond yielding activation, 20. t yieldingAfter, the20 treatment. After, the of treatment 20 with K oftBuO,20 with the KiridacyclopentadieneBuO, the iridacyclopentadiene complex 21 complexwas formed21 was(Scheme formed 6). (SchemeHowever,6). two However, hydrogen two atoms hydrogen on γ atoms carbon on didγ carbonnot afford did a not stable afford iridacyclo- a stable iridacyclobutadienebutadiene complex [23]. complex [23].

Scheme 6. Formation of iridacyclopentadiene complexes. [Ir] = [IrCp*(PMe3)]PF6. Scheme 6. Formation of iridacyclopentadiene complexes. [Ir] = [IrCp*(PMe3)]PF6.

ToTo better better understand understand the the importance importance of theof the isomerism isomerism at the at methoxy(alkenyl)carbenethe methoxy(alkenyl)car- forbene the for synthesis the synthesis of iridaaromatic of iridaaromatic complexes, complexes, DFT calculations DFT calculations were performed were performed (B3LYP level).(B3LYP Thus, level). in Thus, complexes in complexes13 and 1613, and the calculations16, the calculations predicted predicted a single a product:single product: the E isomer.the E isomer. This wasThis duewas todue a higherto a higher stability stability of the of theE isomers E isomers over over the theZ Zisomers isomers in in these these complexescomplexes withwith aa difference difference of of 3.94 3.94 kcal/mol kcal/mol andand 4.854.85 kcal/mol,kcal/mol, respectively.respectively. However,However, if if thethe methyl methyl group group in 16in was16 was substituted substituted with awith phenyl a phenyl group, thegroup,∆G wouldthe ΔG be would 0.70 kcal/mol be 0.70 whichkcal/mol would which allow would for allow the formation for the formation of small of amounts small amounts of the Z ofisomer the Z isomer and therefore and there- a mixturefore a mixture of iridaaromatic of iridaaromatic complexes complexes [24]. These [24]. results These showed results that showed the spatial that the arrangement spatial ar- israngement crucial for is the crucial methodology for the methodology and that the preferenceand that the of preference one isomer of over one another isomer is over clearly an- influencedother is clearly by the influenced lower steric by hindrance.the lower steric hindrance. ItIt waswas notnot possible,possible, inin thethe aboveabove examples,examples, toto establishestablish thethe effecteffect thatthat anan aliphaticaliphatic substituentsubstituent ofof the theγ carbonγ carbon may havemay onhave the formationon the formation of the corresponding of the corresponding metallaaromaticmetallaaromatic compound compound if, instead ofif, obtaininginstead of theobtainingE isomer, the oneE isomer, had the oneZ isomer.had the Thus,Z isomer. the methoxy(alkenyl)carbeneThus, the methoxy(alkenyl)carbene22 bearing a22 2-naphtyl bearing a and 2-naphtyl a tert-butyl and ligand a tert-butyl was synthesized. ligand was Duesynthesized. to the steric Due hindrance to the steric of the hindrancetert-butyl of group, the tert only-butyl the Egroup,isomer only was the observed, E isomer which was wouldobserved, facilitate which the would C–H facilitate bond activation the C–H of bond the aromaticactivation substituent. of the aromatic However, substituent. treat- mentHowever, with treatment silver salt with did notsilver provide salt did the not expected provide iridaaromaticthe expected iridaaromatic complex, and complex, instead 2 theandη insteadcomplex the η232 complexwas identified 23 was (Schemeidentified7)[ (Scheme25]. Although 7) [25]. Although complex complex23 might 23 be might the intermediatebe the intermediate for the for formation the formation of the of iridaaromatic the iridaaromatic complex, complex, further further evolution evolution to the to correspondingthe corresponding iridaphenantrene iridaphenantrene was wa nots not observed. observed. Notably, Notably, when when the thetert tert-butyl-butyl sub-sub- stituentstituent inin 22 was replaced replaced by by a a phenyl phenyl group, group, the the C– C–HH bond bond activation activation took took place place at the at themethoxy(alkenyl)carbene methoxy(alkenyl)carbene 24 and24 and the thecorresponding corresponding iridaaromatic iridaaromatic compound compound 25 was25 was ob- obtainedtained (Scheme (Scheme 7).7). Fukui indices determined that C1 was the most reactive position in 22, leading to the unique formation of 23, despite the free rotation of the naphthalenyl substituent. In contrast, complex 24 presented the higher nucleophilic Fukui index for C4, which was too far away from the iridium nucleus (Ir-C4 distance = 5.08 Å) leading to the activation of C3 which resulted in an iridaanthracene compound, 25 (see also Section 5.3). In addition, DFT calculations (B3LYP level) determined that the C1–H bond activation of 22 to give an iridaphenantrene complex would not be favored by 2.80 kcal/mol, possibly due to the steric hindrance of the final product. However, the effect of the tert-butyl moiety was further determined by comparison with complex 24 [25]. In order to analyze electronic delocalization, different methodologies exist [26–28], including the anisotropy of the induced

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PF6 [Ir] OMe C 1 C C H 4 t R = Bu 3

Molecules 2021, 26, 4655 6 of 14 23 [Ir] OMe C Cl 1 C C H current4 density (ACID). ACID calculations showed that the electron density in 24 was 3 delocalized alongR the wholeR = system, Ph contrary to whatOMe was observed in 22. Similarly, the [Ir] R = tBu (22) LUMO orbital for 22 was less widespread throughout4 3 the molecule than in 24 (Figure1) and the HOMO-LUMOR = Ph (24) gap was bigger, supporting a low reactivity of the tert-butyl derivative. These ﬁndings endorsed the high inﬂuence of the electron density delocalization on the 1 Molecules 2021, 26, x FOR PEER REVIEWformation of this family of iridaaromatic complexes, which would be, therefore, promoted6 of 15 by second substituents such as aromatic rings. 25

2 Scheme 7. Activation of the methoxy(alkenyl)carbene complexesPF 226 and 24 yielding the η complex 23 and the iridaaromatic complex 25, respectively. [Ir][Ir] = [IrCp*(PMeOMe 3)]PF6. C 1 Fukui indices determined that C1 was the mostC reactive position in 22, leading to the C H 4 unique formation of 23, despitet the free rotation of the naphthalenyl substituent. In con- R = Bu 3 trast, complex 24 presented the higher nucleophilic Fukui index for C4, which was too far

away from the iridium nucleus (Ir-C4 distance23 = 5.08 Å) leading to the activation of C3 [Ir] OMe which resulted inC an iridaanthracene compound, 25 (see also Section 5.3). In addition, DFT Cl calculations1 (B3LYP level) determined that the C1–H bond activation of 22 to give an iridaphenantrene complexC would not be favored by 2.80 kcal/mol, possibly due to the steric C H 4 hindrance3 of the final product. However, the effect of the tert-butyl moiety was further R R = Ph OMe determined by comparison with complex 24 [25].[Ir] In order to analyze electronic delocali- R = tBu (22) 4 3 zation, differentR = Ph (24) methodologies exist [26–28], including the anisotropy of the induced current density (ACID). ACID calculations showed that the electron density in 24 was delo-

calized along the whole system, contrary to what1 was observed in 22. Similarly, the LUMO orbital for 22 was less widespread throughout the molecule than in 24 (Figure 1) and the HOMO-LUMO gap was bigger, supporting a low reactivity of the tert-butyl derivative. 25 These findings endorsed the high influence of the electron density delocalization on the 2 SchemeformationScheme 7. 7.Activation Activationof this family of of the the of methoxy(alkenyl)carbene methoxy(alkenyl)carbene iridaaromatic complexes, complexes complexes which22 would 22and and24 be,24yielding yielding therefore, the theη promoted2ηcomplex complex 23 and the iridaaromatic complex 25, respectively. [Ir] = [IrCp*(PMe3)]PF6. 23byand second the iridaaromatic substituents complex such as25 aromatic, respectively. rings. [Ir] = [IrCp*(PMe3)]PF6. Fukui indices determined that C1 was the most reactive position in 22, leading to the unique formation of 23, despite the free rotation of the naphthalenyl substituent. In contrast, complex 24 presented the higher nucleophilic Fukui index for C4, which was too far away from the iridium nucleus (Ir-C4 distance = 5.08 Å) leading to the activation of C3 which resulted in an iridaanthracene compound, 25 (see also Section 5.3). In addition, DFT calculations (B3LYP level) determined that the C1–H bond activation of 22 to give an iridaphenantrene complex would not be favored by 2.80 kcal/mol, possibly due to the steric hindrance of the final product. However, the effect of the tert-butyl moiety was further determined by comparison with complex 24 [25]. In order to analyze electronic delocalization, different methodologies exist [26–28], including the anisotropy of the induced current density (ACID). ACID calculations showed that the electron density in 24 was delo- FigureFigure 1.1. ACIDACID representationsrepresentations (left)(left) andand LUMOLUMO orbitalsorbitals (right)(right) ofof complexescomplexes22 22and and24 24. . calized along the whole system, contrary to what was observed in 22. Similarly, the LUMO orbital for 22 was less widespread throughout the molecule than in 24 (Figure 1) and the 5.5. StabilityStability ofof thethe MetallaaromaticMetallaaromatic SystemsSystems HOMO-LUMO gap was bigger, supporting a low reactivity of the tert-butyl derivative. BesidesBesides establishingestablishing thethe requirementrequirement ofof twotwo aromaticaromaticsubstituents substituentsat atthe theγ γcarbon carbonof of theThese methoxy(alkenyl)carbene findings endorsed the ligand,high influence it is also of important the electron to analyze density the delocalization effect of the metal,on the the methoxy(alkenyl)carbene ligand, it is also important to analyze the effect of the metal, information order to studyof this differentfamily of topologies iridaaromatic and complexes, improve the which stability would of these be, therefore, metallaaromatics promoted byby incorporating second substituents different such substituents as aromatic in rings. the aromatic cycle.

Figure 1. ACID representations (left) and LUMO orbitals (right) of complexes 22 and 24.

5. Stability of the Metallaaromatic Systems Besides establishing the requirement of two aromatic substituents at the γ carbon of the methoxy(alkenyl)carbene ligand, it is also important to analyze the effect of the metal,

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in order to study different topologies and improve the stability of these metallaaromatics by incorporating different substituents in the aromatic cycle.

5.1. Effect of the Aromatic Ring Substituents The low thermal stability of the iridanaphthalene complexes 9 and 10 (see Section 3) reduces their possible applications in materials sciences. Therefore, it is important to assess the effect of different substituents in the phenyl rings of the iridaaromatic moiety in Molecules 2021, 26, 4655 order to enhance their stability. While Haley and co-workers reported the higher tendency7 of 14 to the rearrangement of iridabenzene complexes when they bear bulky ligands [21,22], our group focused on the electronic effects on the stability of the iridanaphthalene system [29]. Thus, the propargylic alcohols HC≡C(OH)(Ph)(p-R-C6H4) with R = NO2, OMe or CH3 Thus, the propargylic alcohols HC≡C(OH)(Ph)(p-R-C6H4) with R = NO2, OMe or CH3 where chosen in order to assess the electronic effect of such substituents on the formation where chosen in order to assess the electronic effect of such substituents on the formation of the substituted 3-phenylindanone derivatives. The similar steric hindrance between of the substituted 3-phenylindanone derivatives. The similar steric hindrance between the the non-substituted and the nitro- or methyl-substituted phenyl rings did not allow for non-substituted and the nitro- or methyl-substituted phenyl rings did not allow for ob- obtaining selectively a unique methoxy(alkenyl)carbeneiridium isomer. Consequently, the taining selectively a unique methoxy(alkenyl)carbeneiridium isomer. Consequently, the C–H bond activation reaction of the mixture of the E- and Z-isomers 26·NO2/CH3 gave C–H bond activation reaction of the mixture of the E- and Z-isomers 26·NO2/CH3 gave rise rise to their respective iridanaphthalenes 27 and 28 (Scheme8). However, when one of to their respective iridanaphthalenes 27 and 28 (Scheme 8). However, when one of the the aromatic rings bears a methoxy group, the ratio between the E- and Z- isomers is not aromatic rings bears a methoxy group, the ratio between the E- and Z- isomers is not pro- proportional, resulted in obtaining the E-isomer E-26·OMe in higher amounts, which may portional, resulted in obtaining the E-isomer E-26·OMe in higher amounts, which may be bedue due to toelectronic electronic effects effects rather rather than than the the steric steric hindrance hindrance due due to totheir their location location in ina remote a remote positionposition (Scheme(Scheme8 8).).

Scheme 8. Formation of complexes 27 and 28 from Z/E isomers of 26. [Ir] = [IrCp*(PMe3)]PF6. Scheme 8. Formation of complexes 27 and 28 from Z/E isomers of 26. [Ir] = [IrCp*(PMe3)]PF6.

AsAs happened for for 1010 thesethese mixtures mixtures of iridanaphthalene of iridanaphthalene complexes complexes are not are stable not in stable a insolution a solution evolving evolving to the indanone to the indanone derivative derivativess[29]. The study [29]. of The their study evolution of their established evolution establisheda direct correlation a direct between correlation the electronic between prop theerties electronic of the propertiesgroups in the of iridanaphthalene the groups in the iridanaphthalenemoiety and the final moiety indanone, and thewhere ﬁnal 27·OMe indanone, possessing where a 27π-donor·OMe grouppossessing (-OMe) a πat-donor po- groupsition (-OMe)6 of the metallanaphthalene at position 6 of the backbone metallanaphthalene proved to be backbone the most provedstable iridanaphtha- to be the most stablelene. On iridanaphthalene. the other hand, the On nitro theother group hand, is an electron-withdrawing the nitro group is an electron-withdrawinggroup through reso- groupnance throughand induction resonance which and facilitates induction the whichisomerization facilitates of complex the isomerization 27·NO2 into of 28·NO complex2 when located in position 6 of the naphthalene ring. This process was proposed to take 27·NO2 into 28·NO2 when located in position 6 of the naphthalene ring. This process wasplace proposed through tothe take initial place formation through of the a carbanion initial formation from the of complex a carbanion 27·NO from2, followed the complex by a proton abstraction right after a rotation over the σ bond between Cγ and the substituted 27·NO2, followed by a proton abstraction right after a rotation over the σ bond between phenyl ring. Then, the rotations and electronic rearrangements would yield complex Molecules 2021, 26, x FOR PEER REVIEWCγ and the substituted phenyl ring. Then, the rotations and electronic rearrangements8 of 15 28·NO2 (Scheme 9)[29]. would yield complex 28·NO2 (Scheme9)[29].

Scheme 9. Isomerization mechanism of 27·NO2 into 28·NO2. [Ir] = [IrCp*(PMe3)]PF6. Scheme 9. Isomerization mechanism of 27·NO2 into 28·NO2. [Ir] = [IrCp*(PMe3)]PF6.

5.2. Effect of the Transition Metal In spite of the broad range of metals used to synthesized metallabenzenes [30], their higher analogues have been mainly described with iridium[18,19,31–33] together with an example of osmium[17] and ruthenium [34]. However, rhodium, the iridium congener, has been never found to form a metallabenzene complex. This fact suggests that not only the electronic properties of the metal play a role in the synthesis of metallaaromatic complexes but also the steric ones. In order to study the effect of the transition metal in the methodology developed for the synthesis of iridanaphthalene complexes, the methoxy(alkenyl)carbene rhodium complex 29, analogous to 3, was synthesized from the corresponding monoacetonitrile complex. However, treatment of complex 29 with AgPF6 did not afford the expected rhodanaphthalene, producing instead the η3 complex 30 together with a 15% of the η5 complex 31, due to the hapticity change of the indenyl ligand.[35] The formation of 30 resembles the evolution observed in the iridanaphthalene complexes 9 and 10, which would suggest the formation of a very unstable rhodanaphthalene complex as intermediate (Scheme 10).

Scheme 10. Implementation of the methodology using rhodium as transition metal. [Rh] = [RhCp*(PMe3)]PF6.

Although the rhodanaphthalene complexes were not possible to stabilize, the synthesis of other family of metallaaromatic complexes, the rhodafurans, was achieved.

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Scheme 9. Isomerization mechanism of 27·NO2 into 28·NO2. [Ir] = [IrCp*(PMe3)]PF6.

5.2.5.2. EffectEffect ofof thethe TransitionTransition MetalMetal InIn spitespite ofof thethe broadbroad rangerange ofof metalsmetals usedusedto tosynthesized synthesizedmetallabenzenes metallabenzenes [[30],30],their their higherhigher analoguesanalogues havehave beenbeen mainly mainly described described with with iridium iridium[18,19,31–33] [18,19,31–33] togethertogether withwith anan exampleexample of of osmium osmium[17] [17] and and ruthenium ruthenium [34 [34].]. However, However, rhodium, rhodium, the the iridium iridium congener, congener, has beenhas been never never found found to form to form a metallabenzene a metallabenzene complex. complex. This This fact suggestsfact suggests that notthat onlynot only the electronicthe electronic properties properties of the of metal the metal play aplay role a in role the in synthesis the synthesis of metallaaromatic of metallaaromatic complexes com- butplexes also but the also steric the ones. steric ones. InIn orderorder to study the the effect effect of of the the transition transition metal metal in in the the methodology methodology developed developed for forthe the synthesis synthesis of iridanaphthalene of iridanaphthalene complexe complexes,s, the methoxy(alkenyl)carbene the methoxy(alkenyl)carbene rhodium rhodium com- complexplex 29, analogous29, analogous to 3, to was3, wassynthesized synthesized from from the corresponding the corresponding monoacetonitrile monoacetonitrile com- complex.plex. However, However, treatment treatment of ofcomplex complex 2929 withwith AgPF AgPF6 6diddid not not afford thethe expectedexpected 3 5 rhodanaphthalene,rhodanaphthalene, producingproducing insteadinstead thethe ηη3 complexcomplex 3030 togethertogether withwith aa 15%15% ofof the theη η5 complexcomplex 3131,, duedue toto the the hapticity hapticity change change of of the the indenyl indenyl ligand ligand.[35] [35]. TheThe formationformation ofof 3030 resemblesresembles thethe evolutionevolution observedobserved inin thethe iridanaphthaleneiridanaphthalene complexescomplexes 99 andand 1010,, whichwhich wouldwould suggest suggest the the formation formation of of a a very very unstable unstable rhodanaphthalene rhodanaphthalene complex complex as intermediateas intermedi- (Schemeate (Scheme 10). 10).

SchemeScheme 10.10. ImplementationImplementation ofof thethe methodologymethodology usingusing rhodiumrhodium asas transition transition metal. metal. [Rh][Rh] == 3 6 [RhCp*(PMe[RhCp*(PMe3)]PF)]PF6.

AlthoughAlthough the the rhodanaphthalene rhodanaphthalene complexes complexes were were not not possible possible to stabilize, to stabilize, the synthesis the syn- ofthesis other of family other of family metallaaromatic of metallaaromatic complexes, complexes, the rhodafurans, the rhodafurans, was achieved. was Therefore, achieved. if the reaction of 29 with the silver salt is performed in the presence of a terminal alkyne, the insertion into the Rh=C bond takes place, forming the rhoda-1,3,5-hexatriene complexes 32– 33 [35]. The Z-selectivity of complexes 32–33 would allow the intramolecular coordination

of the oxygen to the rhodium nucleus, with the concomitant loss of CH3PF6 in order to obtain new rhodafuran complexes 34–35 (Scheme 11). These complexes represent one of the few examples of rhodafuran derivatives after Yamamoto and co-workers reported the reaction of RhCp*Cl2(PPh3) towards alkynes in the presence of KPF6 and water and Tani et al. described the reactivity of alkynes with rhodium carbonyl complexes [36,37]. Molecules 2021, 26, x FOR PEER REVIEW 9 of 15

Therefore, if the reaction of 29 with the silver salt is performed in the presence of a terminal alkyne, the insertion into the Rh=C bond takes place, forming the rhoda-1,3,5-hexatriene complexes 32–33 [35]. The Z-selectivity of complexes 32–33 would allow the intramolecular coordination of the oxygen to the rhodium nucleus, with the concomitant loss of CH3PF6 in order to obtain new rhodafuran complexes 34–35 (Scheme 11). These complexes represent one of the few examples of rhodafuran derivatives after Yamamoto and Molecules 2021, 26, 4655 co-workers reported the reaction of RhCp*Cl2(PPh3) towards alkynes in the presence9 of 14of KPF6 and water and Tani et al. described the reactivity of alkynes with rhodium carbonyl complexes [36,37].

SchemeScheme 11.11. Effect ofof thethe involvementinvolvement ofof alkynesalkynes ininthe the studied studied methodology. methodology. [Rh][Rh] == [RhCp*(PMe3)]PF6. [RhCp*(PMe3)]PF6.

TheseThese results results support support the the need need for for the the use use of of iridium iridium as transitionas transition metal metal for thefor the synthe- syn- sisthesis of these of these metallanaphthalene metallanaphthalene complexes. complexes. However, However, the adjustmentthe adjustment of the of methodologythe methodol- byogy including by including terminal terminal alkynes alkynes allows forallows a new for and a new versatile and wayversatile of achieving way of theachieving synthesis the ofsynthesis rhodafurans of rhodafurans with a broad with range a broad of substituents. range of substituents.

5.3.5.3. EffectEffect ofof thethe TopologyTopology InIn spitespite ofof thethe successfulsuccessful development of of the the methodology methodology in in the the synthesis synthesis of of irida- iri- danaphthalenes,naphthalenes, the the synthesis synthesis of la ofrger larger structures structures is necessary is necessary in order in order to improve to improve the prop- the propertieserties of PAHs. of PAHs. However, However, the the development development of ofhigher higher analogues analogues of of metallanaphthalene metallanaphthalene is isa abarely-studied barely-studied field ﬁeld and and only only a afew few examples examples are are known known [32–34,38]. [32–34,38]. InIn order order to to obtain obtain higher higher analogues, analogues, such such as iridaanthraceneas iridaanthracene or iridaphenantrene or iridaphenantrene deriva- de- tives,rivatives, propargylic propargylic alcohols alcohols bearing bearing at least at one least naphthyl one naphthyl group at groupγ position at γ position were synthesized. were syn- Asthesized. was stated As was in Section stated4 ,in the SectionE/Z isomerism4, the E/Z atisomerism the methoxy(alkenyl)carbeneiridium at the methoxy(alkenyl)carbenei- com- plexesridium is complexes crucial, however, is crucial, the sterichowever, hindrance the steric difference hindrance between difference a phenyl between and a naphthyl a phenyl substituentsand a naphthyl is not substituents high enough is to not hamper high the enou formationgh to hamper of both the isomers formation and, consequently, of both isomers the corresponding iridaaromatics (Scheme 12) [39]. and, consequently, the corresponding iridaaromatics (Scheme 12) [39]. However, the naphthyl group can be bonded to the γ carbon through two different positions, leading to a variety of products. On the one hand, when coordinated at 2-position, an iridaanthracene complex 25 and the iridanaphthalene complex 36 were obtained (Scheme 12, bottom) [25]. Complex 25 represents the second example of a metallaanthracene described in literature. Frogley and Wright reported in 2016 the ﬁrst metallaanthracene complex, also bearing iridium as metal center [32]. In contrast to the α-iridaanthracene complex 25, Wright’s complex is a tethered-β-iridaanthracene. Although it was not possible to obtain X-ray data of 25, its geometry optimization by DFT calculations showed that both iridaanthracene complexes show similar structural data which are also comparable to the purely organic anthracene. Regarding 25, DFT calculations determined that two rotational isomers for the corresponding methoxy(alkenyl)carbene complex 24 were possible, due to the free rotation of the naphthyl substituent (Figure2). This effect would lead to not only 25, but also an iridaphenantrene derivative 25b, which was never experimentally observed. Although DFT calculations showed that the most stable rotational isomer was the precursor of the iridaphenantrene complex, the energy difference of the isomers was too small (∆G = 0.88 kcal/mol) compared to the higher stability of 25, which is 6.41 kcal/mol more stable than 25b, probably due to the larger steric hindrance of 25b (Figure2)[25]. Molecules 2021, 26, 4655 10 of 14 Molecules 2021, 26, x FOR PEER REVIEW 10 of 15

SchemeScheme 12. 12.Synthesis Synthesis ofof higherhigher analoguesanalogues [Ir] [Ir] = = [IrCp*(PMe[IrCp*(PMe33)]PF)]PF66.

OnHowever, the other the hand, naphthyl the naphthyl group can group be bonded can be bonded to the γ at carbon 1-position through getting two rid different of the rotationalpositions, isomerism.leading to a variety of products. On the one hand, when coordinated at 2-position,Thus, an iridaanthracene despite the steric complex hindrance, 25 and activation the iridanaphthalene at the C–H bond complex of the 36 1-naphthyl were obtained substituent(Scheme led 12, to bottom) the first [25]. iridaphenantrene Complex 25 represents complex 37 the(Scheme second 12 example, middle) of [39 a]. metallaanthra- Interestingly, thecene activation described at thein literature. phenyl group Frogley gave and rise Wright to two diastereoisomersreported in 2016 forthe thefirst iridabinaphthyl metallaanthra- complexcene complex,38, observable also bearing by NMR iridium spectroscopy, as metal center due to [32]. the hinderIn contrast rotation to the about α-iridaanthra- the biaryl bondcene complex as well as 25 the, Wright’s iridium complex chiral center. is a tethered- This atropisomerismβ-iridaanthracene. was Although also found it was inthe not iridaphenanthrenepossible to obtain X-ray complex data39 offormed 25, its bygeometry the activation optimization of a symmetric by DFT calculations propargylic alcoholshowed bearingthat both two iridaanthracene 1-naphtyl substituents complexes (Scheme show similar12, top) structural [39]. A theoretical data which study are determinedalso compa- thatrable the to rotational the purely barriers organic around anthracene. the biaryl Regarding bond in 3825,and DFT39 calculations, as well as their determined aromaticity, that 0 aretwo comparable rotational isomers to the 1,1 for-binaphthalene the corresponding and themethoxy(alkenyl)carbene calculated energy differences complex between 24 were Spossible,,M and S ,dueP isomers to the canfree be rotation related of to the the naph experimentallythyl substituent observed (Figure product 2). This ratios effect [39]. would Prior tolead the to formation not only 25 of, 37butand also39 an, low iridaphenantrene temperature studies derivative allowed 25b, forwhich the was characterization never experi- 2 ofmentally the corresponding observed. Althoughη intermediates, DFT calculations which inshowed contrast that to the complex most stable23 (see rotational Section 4iso-), weremer was capable the ofprecursor performing of the the iridaphenantrene C–H bond activation complex, in order the energy to obtain difference the iridaaromatic of the iso- complexes.mers was too small (ΔG= 0.88 kcal/mol) compared to the higher stability of 25, which is 6.41 Inkcal/mol addition, more Liu stable et al. applied than 25b our, probably methodology due to in the order larger to obtain steric iridafluoranthene hindrance of 25b derivatives (Scheme 13)[33]. Thus, methoxy(alkenyl)carbene complexes, such as 40 and 42, (Figure 2) [25]. are capable of performing C–H bond activation in order to give the iridaaromatic complexes 41 and 43, respectively. Interestingly, when an asymmetric propargylic alcohol is used, only one methoxy(alkenyl)carbene iridium complex 42 is obtained, and therefore, only one iridafluoranthene complex 43. The authors proposed the unlikely activation of the remote fused ring to provide a seven-member metallacycle, which might be less favored than the obtained six-member ring [33]. This selectivity is consistent with our theoretical and experimental work using aromatic moieties at γ carbon with the ortho positions substituted (see Section 5.4)[40], as well as the effect of their steric hindrance, which would hinder the formation of the other Z-isomer.

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Molecules 2021, 26, x FOR PEER REVIEW 12 of 15

only one iridafluoranthene complex 43. The authors proposed the unlikely activation of the remote fused ring to provide a seven-member metallacycle, which might be less favored than the obtained six-member ring [33]. This selectivity is consistent with our theoretical and experimental work using aromatic moieties at γ carbon with the ortho positions FigureFiguresubstituted 2. 2.Energy Energy (see profile profile Section for for the 5.4)[40], the formation formation as well of of iridaanthracene iridaanthraceneas the effect complexof complextheir 25steric vs.25 vs. iridaphenanthrenehindrance, iridaphenanthrene which complex would com- 25bplexhinder[Ir] 25b = the [IrCp*(PMe[Ir] formation= [IrCp*(PMe3)]PF of6. 3 )]PFthe 6other. Z-isomer.

On the other hand, the naphthyl group can be bonded at 1-position getting rid of the rotational isomerism. Thus, despite the steric hindrance, activation at the C–H bond of the 1-naphthyl substituent led to the first iridaphenantrene complex 37 (Scheme 12, middle) [39]. Interest- ingly, the activation at the phenyl group gave rise to two diastereoisomers for the iridabinaphthyl complex 38, observable by NMR spectroscopy, due to the hinder rotation about the biaryl bond as well as the iridium chiral center. This atropisomerism was also found in the iridaphenanthrene complex 39 formed by the activation of a symmetric propargylic alcohol bearing two 1-naphtyl substituents (Scheme 12, top) [39]. A theoretical study determined that the rotational barriers around the biaryl bond in 38 and 39, as well as their aromaticity, are comparable to the 1,1′-binaphthalene and the calculated energy differences between S,M and S,P isomers can be related to the experimentally observed product ratios [39]. Prior to the formation of 37 and 39, low temperature studies allowed for the characterization of the corresponding η2 intermediates, which in contrast to complex 23 (see Section 4), were capable of performing the C–H bond activation in order to obtain the iridaaromatic complexes. In addition, Liu et al. applied our methodology in order to obtain iridafluoranthene derivatives (Scheme 13) [33]. Thus, methoxy(alkenyl)carbene complexes, such as 40 and SchemeScheme 13. 13.Synthesis Synthesis of of some some iridaﬂuoranthene iridafluoranthene derivatives. derivatives. [Ir] [Ir] = = [IrCp*(PMe [IrCp*(PMe)]PF3)]PF6.. 42, are capable of performing C–H bond activation in order to give the 3iridaaromatic6 com- plexesThe 41 iridafluorantheneand 43, respectively. complexes Interestingly, showed when a red-shift an asymmetric in the UV-Vis propargylic spectra alcohol for the is used,higher only conjugated one methoxy(alkenyl)carbene derivatives such as 43 ,iridium while substitution complex 42 at is 41obtained, did not andhave therefore, effect. In addition, CV measurements showed that only the first reduction was reversible. The substitution with electrondonating or electronwithdrawing groups at 41 caused a cathodic or anodic shift in the reduction potential, respectively, while 43 derivatives showed anode shift for the reduction and cathode shift for the oxidation potentials [33].

5.4. Special Systems The chemistry of metallaaromatic complexes is not only limited to metallabenzenes, metallanaphthalenes or higher analogues [30,41–45]. As has been previously described by other groups, different topologies can produce properties which could be used in specific applications [30,43,46]. Therefore, the introduction of systems such as the spirobifluorene moiety (SBF) could be useful to generate new properties with optoelectronic applications. As described in Section 4, the higher steric hindrance of the SBF substituent with respect to the methyl group, was the key for the formation of the methoxy(alkenyl)carbene isomer 16 and, therefore, a five-membered metallacycle 17 upon C–H bond activation [24]. If the methyl group is substituted by a mesityl group, the E methoxy(alkenyl)carbene isomer with the SBF moiety near the iridium atom, 44, is 1.85 kcal/mol more stable than the Z isomer,[24] which leads to a selective synthesis of 44. Subsequent activation of the C–H bond led to the first spirobifluorene metallaaromatic complex 45 (Scheme 14) [40].

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The iridaﬂuoranthene complexes showed a red-shift in the UV-Vis spectra for the higher conjugated derivatives such as 43, while substitution at 41 did not have effect. In addition, CV measurements showed that only the ﬁrst reduction was reversible. The substitution with electrondonating or electronwithdrawing groups at 41 caused a cathodic or anodic shift in the reduction potential, respectively, while 43 derivatives showed anode shift for the reduction and cathode shift for the oxidation potentials [33].

5.4. Special Systems The chemistry of metallaaromatic complexes is not only limited to metallabenzenes, metallanaphthalenes or higher analogues [30,41–45]. As has been previously described by other groups, different topologies can produce properties which could be used in specific applications [30,43,46]. Therefore, the introduction of systems such as the spirobifluorene moiety (SBF) could be useful to generate new properties with optoelectronic applications. As described in Section4, the higher steric hindrance of the SBF substituent with respect to the methyl group, was the key for the formation of the methoxy(alkenyl)carbene isomer 16 and, therefore, a five-membered metallacycle 17 upon C–H bond activation [24]. If the methyl group is substituted by a mesityl group, the E methoxy(alkenyl)carbene Molecules 2021, 26, x FOR PEER REVIEWisomer with the SBF moiety near the iridium atom, 44, is 1.85 kcal/mol more stable13 of than 15 the Z isomer [24], which leads to a selective synthesis of 44. Subsequent activation of the C–H bond led to the first spirobifluorene metallaaromatic complex 45 (Scheme 14)[40].

Scheme 14. Spirobifluorene containing complexes. [Ir] = [IrCp*(PMe3)]PF6. Scheme 14. Spirobiﬂuorene containing complexes. [Ir] = [IrCp*(PMe3)]PF6.

ComplexComplex45 45showed showedinteresting interesting properties properties as as a a potential potential material material in in optoelectronics. optoelectronics. It presentedIt presented a high a high thermal thermal stability stability above above 473 473 K and K and higher higher aromatic aromatic character, character, as wellas well as a hinderas a hinder rotational rotational barrier barrier 20 times 20 times higher higher than biarylthan biaryl systems systems38 and 3839 and. In 39 addition,. In addition, a 1.1eV a red-shift1.1eV red-shift in the UV/Visin the UV/Vis spectrum spectrum compared compared to the spirobifluorene to the spirobifluorene was also was observed also ob- [40]. served[40]. 6. Conclusions 6. ConclusionsIn recent years our versatile synthetic methodology has allowed us to obtain different topologiesIn recent of metallaaromatic years our versatile complexes, synthetic suchmethodology as iridanaphthalenes, has allowed us iridaphenanthrenes, to obtain different iridaanthracene,topologies of metallaaromatic rhodafurans orcomplexes, the spirobifluorene such as iridanaphthalenes, iridanaphthalene. iridaphenanthrenes, iridaanthracene,The key aspect rhodafurans of our methodology or the spirobifluorene is the precursor, iridanaphthalene. a methoxy(alkenyl)carbene complex, whichThe key by aspect C–H of bond our activationmethodology of one is the of precursor, the ortho carbons a methoxy(alkenyl)carbene of an aromatic substituent com- onplex,γ carbon which leadsby C–H to bond the formation activation of of a one metallaaromatic of the ortho carbons compound. of an Inaromatic orderto substituent guarantee theon successγ carbon of leads this methodology,to the formation we of have a metallaaromatic studied the influence compound. of different In order substituents to guarantee at thetheγ successcarbon, of the this metallic methodology, center, the we substituents have studied on the the influence aromatic of cycle different and the substituents possibility ofat usingthe γ itcarbon, to obtain the different metallic topologies.center, the Therefore,substituents it wason the concluded aromatic that: cycle (1) and it is the necessary possi- forbility one of of using the substituentsit to obtain different on the γ topologies.carbon to possessTherefore, a large it was steric concluded hindrance that: in 1) order it is tonecessary obtain a for single one Zof ortheE substituentsisomer; (2) theon the existence γ carbon of to a secondpossess substituent,a large steric also hindrance aromatic, in ensuresorder to the obtain formation a single of theZ or metallaaromatic E isomer; 2) the complex existence due of toa second its capability substituent, to delocalize also aro- the electronicmatic, ensures density the among formation the wholeof the metallaaromatic complex; (3) in this complex case, atdue least to its one capability of them needto delo- not becalize substituted the electronic in the densityortho positions; among the (4) thewhole increase comp oflex; aromatic 3) in this rings case, increases at least one stability; of them (5) theneed most not favoredbe substitutedortho position in the ortho for C–H positions; bond activation4) the increase is the of one aromatic that produces rings increases the least stability; 5) the most favored ortho position for C–H bond activation is the one that produces the least stressed cycle; and 6) the capacity of the methodology to synthesize different topologies is very broad. All together gives the opportunity to synthesize analogues to polycyclic aromatic hydrocarbons, PMAHs, which are small fragments of graphene doped with a transition metal and thus to study their new properties and consequently their applications in the field of materials.

Author Contributions: Writing—original draft preparation, M.T., S.B.; writing—review and editing, M.T., S.B.; visualization, M.T., S.B.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Xunta de Galicia (ED431F 2016/005) and IBEROS (0245_IBEROS_1_E). Acknowledgments: The authors acknowledge collaborations and fruitful discussions in this research area with Jorge Bravo and Soledad García-Fontán (UVigo) and the FA3 group of the Univer- sidade de Vigo and thank them for all their support. Important contributions to these experimental and theoretical studies have been made by many coworkers and students, mainly Ángeles Peña- Gallego, José L. Alonso Gómez, Raquel Pereira-Cameselle, Jesús Castro, José M. Hermida-Ramón, Vanesa Arias-Coronado as well as Krystal M. Cid-Seara. The authors are thankful to the Supercom- puting Center of Galicia (CESGA) for the generous allocation of computer time and Universidade de Vigo-CACTI services for recording the NMR and UV/Vis spectra and collecting X-ray data.

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stressed cycle; and (6) the capacity of the methodology to synthesize different topologies is very broad. All together gives the opportunity to synthesize analogues to polycyclic aromatic hydrocarbons, PMAHs, which are small fragments of graphene doped with a transition metal and thus to study their new properties and consequently their applications in the ﬁeld of materials.

Author Contributions: Writing—original draft preparation, M.T., S.B.; writing—review and editing, M.T., S.B.; visualization, M.T., S.B.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Xunta de Galicia (ED431F 2016/005) and IBEROS (0245_IBERO S_1_E). Acknowledgments: The authors acknowledge collaborations and fruitful discussions in this research area with Jorge Bravo and Soledad García-Fontán (UVigo) and the FA3 group of the Universidade de Vigo and thank them for all their support. Important contributions to these experimental and theoretical studies have been made by many coworkers and students, mainly Ángeles Peña-Gallego, José L. Alonso Gómez, Raquel Pereira-Cameselle, Jesús Castro, José M. Hermida-Ramón, Vanesa Arias-Coronado as well as Krystal M. Cid-Seara. The authors are thankful to the Supercomputing Center of Galicia (CESGA) for the generous allocation of computer time and Universidade de Vigo-CACTI services for recording the NMR and UV/Vis spectra and collecting X-ray data. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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