A mesoionic nitrogen-donor ligand: Structure, iridium coordination, and catalytic effects

Article abstract of DOI:10.1039/c7dt04555g

A mesoinic pyridylideneamide ligand (PYA) was synthetized and fully characterized and coordinated to an iridium(iii) center. This ligand represents the first example of a mesoionic N-donor ligand. Structural and spectroscopic analysis revealed unique properties which were exploited in chemically driven water oxidation catalysis.

Full text of DOI:10.1039/c7dt04555g

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January 2016 Pages 1–398
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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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COMMUNICATIONꢀ
ꢀ
Aꢀmesoionicꢀnitrogen-donorꢀligand:ꢀstructure,ꢀiridiumꢀ
coordination,ꢀandꢀcatalyticꢀeffectsꢀ
a
b
b
a
MiquelꢀNavarro, ꢀMoꢀLi, ꢀStefanꢀBernhard ꢀandꢀMartinꢀAlbrecht *
Receivedꢀ00thꢀJanuaryꢀ20xx,ꢀ
Acceptedꢀ00thꢀJanuaryꢀ20xxꢀ
DOI:ꢀ10.1039/x0xx00000xꢀ
www.rsc.org/ꢀ
ꢀ
Aꢀ mesoinicꢀ pyridylideneamideꢀ ligandꢀ (PYA)ꢀ wasꢀ synthetizedꢀ andꢀ Weꢀ haveꢀ beenꢀ intriguedꢀ byꢀ theꢀ donorꢀ abilityꢀ ofꢀ so-calledꢀ
6
–12
fullyꢀcharacterizedꢀandꢀcoordinatedꢀtoꢀanꢀiridium(III)ꢀcenter.ꢀThisꢀ pyridylideneꢀamideꢀ(PYA)ꢀligandsꢀ(Fig.ꢀ1b), ꢀwhichꢀdisplayꢀaꢀ
ligandꢀrepresentsꢀtheꢀfirstꢀexampleꢀofꢀaꢀmesoionicꢀN-donorꢀligand.ꢀ neutralꢀasꢀwellꢀasꢀaꢀzwitterionicꢀlimitingꢀresonanceꢀform.ꢀWeꢀ
Structuralꢀ andꢀ spectroscopicꢀ analysisꢀ revealedꢀ uniqueꢀ propertiesꢀ haveꢀdemonstratedꢀthatꢀdependingꢀonꢀvariousꢀfactorsꢀ(solvent,ꢀ
whichꢀ wereꢀ exploitedꢀ inꢀ chemicallyꢀ drivenꢀ waterꢀ oxidationꢀ chargeꢀatꢀmetal),ꢀthisꢀPYAꢀligandꢀadoptsꢀitsꢀstructureꢀandꢀhenceꢀ
1
3,14
catalysis.ꢀ
providesꢀelectronicꢀflexibility.
ꢀWeꢀhaveꢀnowꢀexploitedꢀtheꢀ
syntheticꢀ versatilityꢀ ofꢀ theꢀ PYAꢀ ligandsꢀ toꢀ modifyꢀ theꢀ donorꢀ
propertiesꢀofꢀtheꢀPYAꢀunitꢀbyꢀchangingꢀtheꢀpositionꢀofꢀtheꢀamideꢀ
unitꢀ (Fig.ꢀ 1c)ꢀ toꢀ theꢀ metaꢀ position.ꢀ Thisꢀ PYAꢀ isomerꢀ lacksꢀ anꢀ
unchargedꢀ resonanceꢀ formꢀ andꢀ thereforeꢀ representsꢀ aꢀ
mesoionicꢀN-donorꢀligandꢀsystem.ꢀ
Metal–ligandꢀ bondꢀ interactionsꢀ serveꢀ bothꢀ toꢀ modulateꢀ theꢀ
electronicꢀpropertiesꢀofꢀtheꢀmetalꢀcenterꢀandꢀtoꢀinfluenceꢀtheꢀ
stericꢀ environmentꢀ ofꢀ theꢀ metalꢀ coordinationꢀ sphere,ꢀ thusꢀ
providingꢀexcellentꢀcontrolꢀoverꢀtheꢀstructureꢀandꢀreactivityꢀofꢀ
theꢀ coordinatedꢀ metalꢀ center.ꢀ Therefore,ꢀ theꢀ designꢀ andꢀ
developmentꢀofꢀnewꢀligandꢀmotifsꢀisꢀcrucialꢀtoꢀadvanceꢀtheꢀareaꢀ
ꢀ
1
ofꢀ organometallicꢀ chemistryꢀ andꢀ homogeneousꢀ catalysis. ꢀ Aꢀ
particularlyꢀ intriguingꢀ classꢀ ofꢀ rareꢀ ligandsꢀ areꢀ so-calledꢀ
mesoionicꢀ compounds,ꢀ i.e.ꢀ speciesꢀ whichꢀ cannotꢀ beꢀ
representedꢀbyꢀaꢀneutralꢀLewisꢀstructure,ꢀandꢀwhichꢀfeatureꢀaꢀ
formallyꢀ negativeꢀ andꢀ positiveꢀ chargeꢀ thatꢀ areꢀ mutuallyꢀ
2
conjugated. ꢀMesoionicꢀcarbenesꢀ(Figureꢀ1a)ꢀconstituteꢀaꢀclassꢀ
ofꢀsuchꢀmesoionicꢀligands,ꢀinꢀwhichꢀtheꢀligandꢀisꢀboundꢀtoꢀtheꢀ
metalꢀ centerꢀ throughꢀ aꢀ carbenicꢀ carbon,ꢀ whichꢀ isꢀ formallyꢀ aꢀ
carbanionꢀ thatꢀ isꢀ chargeꢀ balancedꢀ byꢀ aꢀ conjugatedꢀ iminiumꢀ
3
–5
cation. ꢀ Theseꢀ mesoionicꢀ carbenesꢀ areꢀ aꢀ subclassꢀ ofꢀ N-
heterocylicꢀ carbenesꢀ andꢀ haveꢀ beenꢀ extensivelyꢀ studiedꢀ asꢀ
stronglyꢀ donatingꢀ neutralꢀ C-donorꢀ ligandsꢀ duringꢀ theꢀ lastꢀ
decade.ꢀEvenꢀthoughꢀnitrogenꢀdonorꢀligandsꢀhaveꢀbeenꢀwidelyꢀ
studiedꢀ andꢀ usedꢀ inꢀ organometallicꢀ chemistry,ꢀ mesoionicꢀ N-
donorꢀligandsꢀhaveꢀnotꢀbeenꢀknownꢀsoꢀfar.ꢀ
ꢀ
ꢀ
Herein,ꢀ weꢀ describeꢀ theꢀ firstꢀ mesoionicꢀ N-donorꢀ ligand,ꢀ itsꢀ
ꢀ
coordinationꢀabilityꢀandꢀitsꢀimpactꢀforꢀcatalyticꢀtransformations.ꢀ Figureꢀ1ꢀRepresentationꢀofꢀC4-imidazolylideneꢀ(I),ꢀC4-triazolylideneꢀ(II)ꢀandꢀ
pyridylideneꢀ(III)ꢀasꢀexamplesꢀofꢀmesoionicꢀligandsꢀ(a),ꢀlimitingꢀresonanceꢀ
structuresꢀofꢀaꢀpara-PYAꢀunitꢀ(b)ꢀandꢀaꢀmesoionicꢀmeta-PYAꢀunitꢀ(c).
ꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
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Schemeꢀ1ꢀSynthesisꢀofꢀtheꢀmesoionicꢀpyridylideneꢀamideꢀ3ꢀandꢀitsꢀiridiumꢀcomplexꢀ4ꢀ
Theꢀsynthesisꢀofꢀtheꢀmeta-pyridiniumꢀamideꢀsaltꢀasꢀprecursorꢀofꢀ wasꢀunequivocallyꢀconfirmedꢀbyꢀsingleꢀX-rayꢀdiffractionꢀanalysisꢀ
theꢀmesoionicꢀPYAꢀligandꢀwasꢀaccomplishedꢀinꢀtwoꢀsimpleꢀsteps.ꢀ (Figureꢀ2).ꢀ
1
5,16ꢀ
Inꢀanalogyꢀtoꢀtheꢀsynthesisꢀofꢀtheꢀknownꢀpara-PYAꢀsystem.
Accordingly,ꢀ reactionꢀ ofꢀ 3-aminopyridineꢀ andꢀ theꢀ substitutedꢀ almostꢀco-planarꢀarrangementꢀofꢀtheꢀpyridylideneꢀunitꢀandꢀtheꢀ
benzoylꢀ chlorideꢀ yieldedꢀ theꢀ correspondingꢀ amideꢀ ꢀ inꢀ goodꢀ amideꢀgroup,ꢀasꢀindicatedꢀbyꢀtheꢀsmallꢀCCO–N–Cβ’–Cα’ꢀangleꢀofꢀ
ꢀ
Theꢀ molecularꢀ structureꢀ ofꢀ theꢀ mesoionicꢀ PYAꢀ revealsꢀ anꢀ
1
1
7ꢀ
yieldꢀ(85%,ꢀSchemeꢀ1). ꢀSubsequentꢀmethylationꢀwithꢀMeIꢀwasꢀ 13.7°.ꢀTheꢀbondꢀlengthsꢀwithinꢀtheꢀpyridylꢀheterocycleꢀdoꢀnotꢀ
selectiveꢀ andꢀ tookꢀ placeꢀ exclusivelyꢀ atꢀ theꢀ pyridineꢀ nitrogen,ꢀ showꢀaꢀdistinctꢀalteration,ꢀindicatingꢀaꢀconjugatedꢀsystemꢀwithꢀ
1
evenꢀ whenꢀ anꢀ excessꢀ ofꢀ alkylatingꢀ agentꢀ wasꢀ used. ꢀ Theꢀ substantialꢀaromaticꢀcharacter.ꢀAꢀsimilarꢀbondingꢀsituationꢀwasꢀ
8
+
pyridiniumꢀ saltꢀ
resolutionꢀ MSꢀatꢀ 303.1339ꢀ (theoreticalꢀ valueꢀ 303.1329ꢀ amu),ꢀ C–Cꢀ bondsꢀ withinꢀ theꢀ heterocycleꢀ (averageꢀ C–Cꢀ bondꢀ isꢀ
andꢀtheꢀappearanceꢀofꢀaꢀnewꢀsingletꢀforꢀtheꢀN–CH ꢀgroupꢀ( ꢀ=ꢀ 1.383(13)ꢀÅꢀinꢀ2ꢀandꢀ1.383(15)ꢀÅꢀinꢀ ,ꢀTableꢀ1).ꢀInꢀcontrast,ꢀtheꢀ
.41)ꢀalongꢀwithꢀaꢀdiagnosticꢀdownfieldꢀshiftꢀofꢀtheꢀpyridiniumꢀ analogousꢀpara-PYAꢀshowsꢀmarkedꢀbondꢀlengthꢀalterationꢀwithꢀ
protonsꢀbyꢀaboutꢀ0.5ꢀppmꢀwhenꢀcomparedꢀtoꢀtheꢀanalogousꢀ shortꢀ C –C ꢀ bondsꢀ (averageꢀ 1.36ꢀ Å)ꢀ andꢀ longꢀ C –C ꢀ bondsꢀ
resonancesꢀofꢀtheꢀneutralꢀpyridineꢀprecursorꢀ ,ꢀcorroboratedꢀ (averageꢀ 1.43ꢀ Å). ꢀ Whileꢀ theꢀ freeꢀ para-PYAꢀ thereforeꢀ
theꢀformationꢀofꢀtheꢀpyridiniumꢀsaltꢀ .ꢀAnꢀamideꢀN–Hꢀbendingꢀ demonstratesꢀsubstantialꢀdieneꢀcharacter,ꢀtheꢀmesoionicꢀmeta-
modeꢀ wasꢀ observedꢀ atꢀ 1666ꢀ cm ꢀ inꢀ theꢀ IRꢀ spectrum.ꢀ Theꢀ PYAꢀ3ꢀisꢀpredominantlyꢀaromatic.ꢀInꢀagreementꢀwithꢀthisꢀnotion,ꢀ
molecularꢀstructureꢀofꢀpyridiniumꢀsaltꢀ ꢀwasꢀfurtherꢀconfirmedꢀ theꢀNPYA–CpyrꢀbondꢀinꢀtheꢀmesoionicꢀPYAꢀ ꢀisꢀsubstantiallyꢀlongerꢀ
2ꢀ showedꢀ theꢀ pertinentꢀ [M–I] ꢀ signalꢀ inꢀ high- notedꢀforꢀtheꢀprecursorꢀpyridiniumꢀsaltꢀ2,ꢀwithꢀessentiallyꢀequalꢀ
3
d
H
3
4
α
β
β
γ
1
5
1
2
–
1
2
3
byꢀ X-Rayꢀ diffractionꢀ analysisꢀ (Figureꢀ 2).ꢀ Amideꢀ deprotonationꢀ thanꢀ inꢀ theꢀ freeꢀ para-PYAꢀ ligandꢀ (1.381(2)ꢀ Åꢀ vsꢀ 1.353(2)ꢀ Å),ꢀ
wasꢀaccomplishedꢀwithꢀaqueousꢀNaOHꢀbyꢀaꢀsimpleꢀliquid-liquidꢀ whichꢀindicatesꢀaꢀhigherꢀsingle-bondꢀcharacterꢀasꢀexpectedꢀforꢀ
extractionꢀ withꢀ CH
pyridinumꢀsaltꢀ .ꢀTheꢀorganicꢀlayerꢀcontainedꢀtheꢀfreeꢀmesoionicꢀ
ligandꢀ ,ꢀ whichꢀ wasꢀ isolatedꢀ asꢀ anꢀ air-stableꢀ yellowishꢀ demonstratedꢀ withꢀ anꢀ iridium(III)ꢀ precursor.ꢀ Additionꢀ ofꢀ theꢀ
hygroscopicꢀsolidꢀinꢀaꢀgoodꢀyieldꢀ(60%).ꢀFormationꢀofꢀtheꢀfreeꢀ freeꢀmesoionicꢀPYAꢀ ꢀtoꢀ[Ir(Cp)*Cl ꢀinꢀtheꢀpresenceꢀofꢀNaOAcꢀ
PYAꢀwasꢀindicatedꢀbyꢀtheꢀdisappearanceꢀofꢀtheꢀamideꢀproton,ꢀ inducedꢀ alsoꢀ cyclometalation,
asꢀwellꢀasꢀanꢀupfieldꢀshiftꢀofꢀaboutꢀ0.4ꢀppmꢀforꢀtheꢀheterocyclicꢀ complexꢀ ꢀ asꢀ anꢀ air-stableꢀ solidꢀ thatꢀ wasꢀ purifiedꢀ byꢀ columnꢀ
2
Cl
2
ꢀ fromꢀ aꢀ 2ꢀ Mꢀ NaOHꢀ solutionꢀ ofꢀ theꢀ aꢀmesoionicꢀstructure.ꢀꢀ
2
ꢀ
MetalꢀcoordinationꢀofꢀtheꢀmesoionicꢀN-donorꢀligandꢀ3ꢀwasꢀ
3
3
2 2
]
1
9–21
ꢀ andꢀ affordedꢀ theꢀ iridiumꢀ
4
1
protonꢀresonancesꢀinꢀtheꢀ HꢀNMRꢀspectrum.ꢀInꢀaddition,ꢀtheꢀN– chromatographyꢀ andꢀ obtainedꢀ inꢀ 77%ꢀ yield.ꢀ Cyclometallationꢀ
HꢀbendingꢀfeatureꢀwasꢀabsentꢀinꢀtheꢀIRꢀspectrum.ꢀTheꢀmethoxy- wasꢀevidencedꢀbyꢀtheꢀsplitꢀofꢀtheꢀresonancesꢀdueꢀtoꢀtheꢀOMeꢀ
1
groupsꢀintoꢀ3ꢀdistinctꢀsingletsꢀinꢀtheꢀ HꢀNMRꢀspectrum.ꢀSingleꢀ
substitutedꢀ mesoionicꢀ PYAꢀ
3
ꢀ readilyꢀ crystallizedꢀ fromꢀ CH
2
Cl
2
ꢀ
uponꢀ slowꢀ diffusionꢀ ofꢀ pentane,ꢀ andꢀ theꢀ molecularꢀ structureꢀ crystalꢀX-rayꢀdiffractionꢀanalysisꢀyieldedꢀaꢀmolecularꢀstructureꢀofꢀ
complexꢀ
4
ꢀ thatꢀ showsꢀ theꢀ classicalꢀ three-leggedꢀ piano-stoolꢀ
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Figureꢀ1ꢀORTEPꢀrepresentationꢀofꢀpyridiniumꢀsaltꢀ2,ꢀPYAꢀfreeꢀligandꢀ3ꢀandꢀiridiumꢀcomplexꢀ4ꢀ(50%ꢀprobability;ꢀhydrogenꢀatomsꢀandꢀcocrystallizedꢀsolventꢀ
omittedꢀforꢀclarity).
geometryꢀ withꢀ theꢀ iridiumꢀ centerꢀ inꢀ aꢀ pseudo-tetrahedralꢀ
geometryꢀ (Fig.ꢀ 2).ꢀ Theꢀ bondꢀ lengthsꢀ andꢀ anglesꢀ aroundꢀ theꢀ identicalꢀenergiesꢀforꢀtheꢀC=Oꢀstretchꢀbandꢀ(
iridiumꢀcenterꢀareꢀessentiallyꢀidenticalꢀtoꢀtheꢀmetricsꢀofꢀsimilarꢀ suggestingꢀ noꢀ perturbationꢀ ofꢀ theꢀ C=Oꢀ moietyꢀ uponꢀ
ꢀ
Infraredꢀspectroscopicꢀanalysisꢀofꢀcompoundsꢀ
2–4ꢀshowedꢀ
–
1
n
COꢀ=ꢀ1585±1ꢀcm ),ꢀ
1
5,16
PYAꢀiridiumꢀcomplexes.
theꢀ complexꢀ theꢀ NPYA–Cpyrꢀ bondꢀ lengthꢀ isꢀ considerablyꢀ largerꢀ amideꢀ C=Oꢀ bondꢀ wasꢀ notedꢀ forꢀ theꢀ para-PYAꢀ analoguesꢀ
1.394(4)ꢀÅ)ꢀthanꢀinꢀtheꢀisomericꢀparaꢀPYAꢀcomplexꢀ(1.378(3)ꢀÅ),ꢀ althoughꢀ theseꢀ compoundsꢀ displayꢀ twoꢀ bandsꢀ inꢀ thisꢀ region,ꢀ
CE
suggestingꢀ aꢀ moreꢀ pronouncedꢀ singleꢀ bondꢀ characterꢀ ofꢀ thisꢀ whichꢀwereꢀattributedꢀtoꢀcarbonylꢀandꢀimineꢀC=Nꢀvibrationsꢀ(n ꢀ
ꢀAsꢀnotedꢀforꢀtheꢀfreeꢀligand,ꢀalsoꢀinꢀ deprotonationꢀorꢀmetalation.ꢀSimilarly,ꢀlittleꢀperturbationꢀofꢀtheꢀ
(
–
1
–1
bondꢀinꢀtheꢀmesoionicꢀN-donor.ꢀItꢀisꢀworthꢀnotingꢀthatꢀtheꢀbondꢀ =ꢀ1642±3ꢀcm ꢀandꢀ1590±5ꢀcm ;ꢀEꢀ=ꢀO,ꢀN).ꢀꢀ
lengthsꢀinꢀtheꢀpyridylideneꢀringꢀdoꢀnotꢀalterꢀandꢀareꢀidenticalꢀtoꢀ Weꢀhaveꢀexploitedꢀtheꢀpropertiesꢀofꢀthisꢀuniqueꢀmesoionicꢀ
thoseꢀofꢀtheꢀfreeꢀPYAꢀ ,ꢀsuggestingꢀveryꢀlittleꢀperturbationꢀofꢀ N-donorꢀligandꢀinꢀtheꢀiridium-catalyzedꢀwaterꢀoxidation,ꢀoneꢀofꢀ
ꢀ
3
theꢀ pyridylideneꢀ unitꢀ ofꢀ thisꢀ N-donorꢀ ligandꢀ uponꢀ metalꢀ theꢀ currentꢀ bottleꢀ necksꢀ forꢀ theꢀ developmentꢀ ofꢀ efficientꢀ
coordination.ꢀSuchꢀbehaviorꢀsupportsꢀaꢀmesoionicꢀnotion,ꢀwhileꢀ artificialꢀphotosyntheticꢀprocesses.ꢀWaterꢀoxidationꢀwasꢀdrivenꢀ
4 2 3 6
theꢀpyridylideneꢀsubstituentꢀinꢀtheꢀpara-PYAꢀrespondsꢀtoꢀmetalꢀ byꢀ(NH ) [Ce(NO ) ]ꢀ(CAN)ꢀasꢀsacrificialꢀchemicalꢀoxidant,ꢀwhichꢀ
coordinationꢀ byꢀ aꢀ partialꢀ lossꢀ ofꢀ theꢀ dieneꢀ configuration.ꢀ isꢀ inactiveꢀ onꢀ itsꢀ ownꢀ inꢀ theꢀ oxidationꢀ ofꢀ water.ꢀ Catalyticꢀ
Moreover,ꢀtheꢀpyridylꢀringꢀisꢀmarkedlyꢀtwistedꢀoutꢀofꢀtheꢀamideꢀ applicationꢀisꢀpromotedꢀbyꢀtheꢀgoodꢀsolubilityꢀofꢀtheꢀmesoionicꢀ
planeꢀinꢀcomplexꢀ
4
,ꢀasꢀdemonstratedꢀbyꢀtheꢀlargeꢀCCO–NPYA–Cβ’
α’ꢀdihedralꢀangleꢀofꢀ49.374°.ꢀThisꢀlargeꢀangleꢀsuggestsꢀnegligibleꢀ immediateꢀ gasꢀ formationꢀ wasꢀ observedꢀ andꢀ quantifiedꢀ
ꢀ electronꢀ densityꢀ overlapꢀ betweenꢀ theꢀ amideꢀ andꢀ theꢀ dynamicallyꢀwithꢀaꢀhome-builtꢀpressureꢀtransducerꢀsetupꢀandꢀ
–
PYAꢀ iridiumꢀ complexꢀ 4ꢀ inꢀ water.ꢀ Uponꢀ additionꢀ ofꢀ CANꢀ
C
p
2
2
23
pyridylideneꢀresidues. ꢀInꢀcontrast,ꢀthisꢀdihedralꢀangleꢀinꢀtheꢀ calibratedꢀ byꢀ GC. ꢀ Quantitativeꢀ analysisꢀ revealedꢀ turnoverꢀ
–
1
pyridinumꢀ saltꢀ
ligandꢀ ꢀ (13.668°)ꢀ areꢀ muchꢀ smaller.†ꢀ Presumably,ꢀ stericꢀ mostꢀ previouslyꢀ evaluatedꢀ C,N-bidentateꢀ chelatedꢀ orꢀ
repulsionꢀ withꢀ theꢀ Cp*ꢀ ligandꢀ forcesꢀ theꢀ tiltingꢀ ofꢀ theꢀ monodentateꢀiridiumꢀcomplexes, ꢀthoughꢀnotꢀrecord-high.
2ꢀ (22.560°)ꢀ andꢀ especiallyꢀ theꢀ freeꢀ mesoionicꢀ frequenciesꢀupꢀtoꢀ1500ꢀh ꢀ(Tableꢀ2,ꢀentryꢀ1)ꢀwhichꢀisꢀfasterꢀthanꢀ
3
2
4
25,26
ꢀ
pyridylideneꢀring.ꢀ
ꢀ
Theꢀ catalystꢀ demonstratesꢀ excellentꢀ robustnessꢀ andꢀ utilizesꢀ
CANꢀ quantitatively,ꢀ reachingꢀ almostꢀ theꢀ theoreticalꢀ turnoverꢀ
2
limitꢀ wꢀ ithꢀ4600ꢀTONꢀatꢀ20ꢀµMꢀcatalystꢀconcentrationꢀ(entryꢀ2). ꢀ
7
Tableꢀ1ꢀBondꢀlengthꢀdistancesꢀ(Å)ꢀandꢀanglesꢀ(°)ꢀaroundꢀtheꢀPYAꢀmoietyꢀofꢀ
pyridiniumꢀsaltꢀ2,ꢀmesoionicꢀfreeꢀbaseꢀ3,ꢀandꢀmesoionicꢀPYAꢀcomplexꢀ4.ꢀ
a
Tableꢀ2ꢀPreliminaryꢀcatalyticꢀwaterꢀoxidationꢀdataꢀusingꢀcomplexꢀ4. ꢀ
–
1
ꢀ
2ꢀ
3ꢀ
4ꢀ
Entryꢀ
[
4
]/µMꢀ
CAN/complexꢀ
7360ꢀ
18440ꢀ
TONꢀ TOFmax/h ꢀ
Ir–NPYA
ꢀ
-ꢀ
-ꢀ
2.093(3)ꢀ
2.050(4)ꢀ
2.4141(10)ꢀ
1.394(4)ꢀ
1.351(4)ꢀ
1.349(4)ꢀ
1.371(5)ꢀ
1.390(5)ꢀ
1.376(5)ꢀ
1.395(5)ꢀ
49.374ꢀ
1
2
ꢀ
ꢀ
49.9ꢀ
19.9ꢀ
1810ꢀ
4600ꢀ
1500ꢀ
1210ꢀ
Ir–CPh
Ir–Clꢀ
ꢀ
-ꢀ
-ꢀ
a
-ꢀ
-ꢀ
Measurementsꢀwereꢀperformedꢀinꢀaꢀsealedꢀ40ꢀmLꢀEPAꢀvialꢀcontainingꢀ10ꢀmLꢀ
.40ꢀMꢀCANꢀsolutionꢀbufferedꢀinꢀ1MꢀHNO ꢀandꢀcomplexꢀ4.ꢀO ꢀevolutionꢀwasꢀ
dynamicallyꢀ monitoredꢀ withꢀ digitalꢀ manometryꢀ andꢀ endꢀ pointsꢀ wereꢀ
calibratedꢀ byꢀ gasꢀ chromatography;ꢀ TOFmaxꢀ valuesꢀ wereꢀ determinedꢀ byꢀ
calculatingꢀtheꢀrateꢀofꢀchangeꢀofꢀtheꢀgeneratedꢀoxygenꢀoverꢀtimeꢀfromꢀtheꢀ
kineticꢀtrace.ꢀ
0
3
2
N
N
N
PYA–Cβ’
pyr–C
pyr–ꢀCα’
ꢀ
1.405(2)ꢀ
1.352(2)ꢀ
1.344(2)ꢀ
1.366(3)ꢀ
1.384(3)ꢀ
1.385(2)ꢀ
1.397(2)ꢀ
22.560ꢀ
1.381(2)ꢀ
1.339(2)ꢀ
1.354(2)ꢀ
1.368(2)ꢀ
1.396(2)ꢀ
1.373(2)ꢀ
1.396(2)ꢀ
13.668ꢀ
α
ꢀ
ꢀ
C
C
C
C
α β
–ꢀC ꢀ
α’–ꢀCβ’
ꢀ
β
γ
–ꢀC ꢀ
β’–C
γ
ꢀ
ꢀ
Inꢀ summary,ꢀ weꢀ haveꢀ synthetized,ꢀ isolatedꢀ andꢀ fullyꢀ
C(O)–N–Cβ’–Cα’
ꢀ
characterizedꢀ theꢀ firstꢀ mesoionicꢀ nitrogen-donorꢀ ligand.ꢀ
Modulationꢀ ofꢀ theꢀ pyridylideneꢀ ringꢀ inꢀ aꢀ simpleꢀ pyridylideneꢀ
amid ꢀe ꢀligandꢀbyꢀchangingꢀtheꢀamideꢀpositionꢀfromꢀtheꢀknownꢀ
para-ꢀ toꢀ theꢀ meta-positionꢀ affordedꢀ anꢀ air-stableꢀ meta-PYAꢀ
ꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ3ꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ

Pleas De ꢀad l to oꢀ nn oT tr ꢀaa nd sj au cs tti ꢀomn as rginsꢀ
Page 4 of 5
COMMUNICATIONꢀ
JournalꢀNameꢀ
Forꢀ pioneeringꢀ workꢀ onꢀ variousꢀ mesoionicꢀ C-dVoienwoArrꢀtilcilgeaOnndlinse,ꢀ
5
ligand,ꢀ whichꢀ representsꢀ aꢀ newꢀ classꢀ ofꢀ N-donorꢀ system.ꢀ
Structuralꢀ andꢀ spectroscopicꢀ analysisꢀ revealꢀ theꢀ mesoionicꢀ
propertiesꢀofꢀthisꢀligand.ꢀWhileꢀweꢀhaveꢀexploitedꢀtheꢀspecificꢀ
propertiesꢀofꢀthisꢀligandꢀinꢀiridium-catalyzedꢀwaterꢀoxidation,ꢀ
theꢀ mesoionicꢀ propertiesꢀ areꢀ expectedꢀ toꢀ haveꢀ muchꢀ widerꢀ
impactꢀ onꢀ aꢀ varietyꢀ ofꢀ redoxꢀ reactions.ꢀ Inꢀ particularꢀ theꢀ
synthesisꢀofꢀtheꢀpyridiniumꢀsaltꢀprecursorꢀisꢀhighlyꢀconvenientꢀ
andꢀallowsꢀforꢀintroducingꢀaꢀvarietyꢀofꢀfunctionalꢀfeatures.ꢀThisꢀ
diversityꢀcombinedꢀwithꢀtheꢀfacileꢀaccessibilityꢀofꢀtheꢀfreeꢀbaseꢀ
ligandꢀprovidesꢀinterestingꢀopportunitiesꢀforꢀapplicationꢀinꢀfirstꢀ
rowꢀtransitionꢀmetalꢀcatalysisꢀandꢀforꢀassistingꢀsingleꢀelectronꢀ
transferꢀpathways.ꢀ
see:ꢀ(a)ꢀS.ꢀGründemann,ꢀA.ꢀKovacevic,ꢀM.ꢀAlbrecht,ꢀJ.ꢀW.ꢀFaller,ꢀ
DOI: 10.1039/C7DT04555G
R.ꢀ H.ꢀ Crabtree,ꢀ J.ꢀ Am.ꢀ Chem.ꢀ Soc.ꢀ 2002,ꢀ 124,ꢀ 10473;ꢀ (b)ꢀ P.ꢀ
Mathew,ꢀA.ꢀNeels,ꢀM.ꢀAlbrecht,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.ꢀ2008,ꢀ130,ꢀ
1
3534;ꢀ(c)ꢀY.ꢀHan,ꢀH.ꢀV.ꢀHuynh,ꢀChem.ꢀCommun.ꢀ2007,ꢀ1089;ꢀ
(d)ꢀE.ꢀStander-Gobler,ꢀO.ꢀSchuster,ꢀG.ꢀHeydenrych,ꢀS.ꢀCronje,ꢀ
E.ꢀ Tosh,ꢀ M.ꢀ Albrecht,ꢀ G.ꢀ Frenking,ꢀ H.ꢀ G.ꢀ Raubenheimer,ꢀ
Organometallicsꢀ2010,ꢀ29,ꢀ5821.ꢀ
M.ꢀE.ꢀDosterꢀandꢀS.ꢀA.ꢀJohnson,ꢀAngew.ꢀChem.ꢀInt.ꢀEd.ꢀ2009,ꢀ
48,ꢀ2185.ꢀ
M.ꢀE.ꢀDoster,ꢀJ.ꢀA.ꢀHatnean,ꢀT.ꢀJeftic,ꢀS.ꢀModiꢀandꢀS.ꢀA.ꢀJohnson,ꢀ
J.ꢀAmꢀChem.ꢀSoc.ꢀ2010,ꢀ132,ꢀ11923.ꢀ
M.ꢀ E.ꢀ Dosterꢀ andꢀ S.ꢀ A.ꢀ Johnson,ꢀ Organometallicsꢀ 2013,ꢀ 32,ꢀ
6
7
8
9
4
174.ꢀ
Q.ꢀShi,ꢀR.ꢀJ.ꢀThatcher,ꢀJ.ꢀSlattery,ꢀP.ꢀS.ꢀSauari,ꢀA.ꢀC.ꢀWhitwood,ꢀ
P.ꢀC.ꢀMcGowanꢀandꢀR.ꢀE.ꢀDouthwaite,ꢀChem.ꢀEur.ꢀJ.ꢀ2009,ꢀ15,ꢀ
ꢀ Theꢀauthorsꢀgratefullyꢀacknowledgeꢀfinancialꢀsupportꢀfromꢀ
theꢀEuropeanꢀResearchꢀCouncilꢀ(GrantꢀCoGꢀ615653)ꢀandꢀfromꢀ
1
1346.ꢀ
theꢀSwissꢀNationalꢀScienceꢀFoundationꢀ(Grantꢀ200021_162868ꢀ 10 J.ꢀSlattery,ꢀR.ꢀJ.ꢀThatcher,ꢀQ.ꢀShiꢀandꢀR.ꢀE.ꢀDouthwaite,ꢀPureꢀ
Appl.ꢀChem.ꢀ2010,ꢀ82,ꢀ1663.ꢀ
1 R.ꢀJ.ꢀThatcher,ꢀD.ꢀG.ꢀJohnson,ꢀJ.ꢀSlatteryꢀandꢀR.ꢀE.ꢀDouthwaite,ꢀ
Chem.ꢀEur.ꢀJ.ꢀ2012,ꢀ18,ꢀ4329.ꢀ
2 W.ꢀP.ꢀD.ꢀBoyd,ꢀL.ꢀJ.ꢀWrightꢀandꢀM.ꢀN.ꢀZafar,ꢀInorg.ꢀChem.ꢀ2011,ꢀ
50,ꢀ10522.ꢀ
3 V.ꢀLeigh,ꢀD.ꢀJ.ꢀCarleton,ꢀJ.ꢀOlguin,ꢀH.ꢀMüller-Bunz,ꢀL.ꢀJ.ꢀWrightꢀ
andꢀM.ꢀAlbrecht,ꢀInorg.ꢀChem.ꢀ2014,ꢀ53,ꢀ8054.ꢀ
4 K.ꢀF.ꢀDonnelly,ꢀC.ꢀSegarra,ꢀL.-X.ꢀShao,ꢀR.ꢀSuen,ꢀH.ꢀMüller-Bunzꢀ
andꢀM.ꢀAlbrecht,ꢀOrganometallicsꢀ2015,ꢀ34,ꢀ4076.ꢀ
5 M.ꢀ Navarro,ꢀ M.ꢀ Li,ꢀ H.ꢀ Müller-Bunz,ꢀ S.ꢀ Bernhardꢀ andꢀ M.ꢀ
AlbrechtꢀChem.ꢀEur.ꢀJ.ꢀ2016,ꢀ22,ꢀ6740.ꢀ
andꢀR’equipꢀProjectsꢀ206021_128724ꢀandꢀ206021_170755).ꢀWeꢀ
thankꢀtheꢀgroupꢀofꢀChemicalꢀCrystallographyꢀofꢀtheꢀUniversityꢀ
ofꢀBernꢀforꢀX-rayꢀanalysisꢀofꢀallꢀreportedꢀstructures.ꢀ
1
1
1
1
1
Conflictsꢀofꢀinterestꢀ
Thereꢀareꢀnoꢀconflictsꢀtoꢀdeclare.ꢀ
Notesꢀandꢀreferencesꢀ
16 M.ꢀNavarro,ꢀC.ꢀA.ꢀSmithꢀandꢀM.ꢀAlbrecht,ꢀInorg.ꢀChem.ꢀ2017,ꢀ
,ꢀ11688.ꢀ
5
6
†
ꢀ
TheꢀX-rayꢀstructureꢀanalysisꢀareꢀnotꢀunambiguousꢀinꢀjustifyingꢀ
theꢀ termꢀ mesoionicꢀ forꢀ thisꢀ typeꢀ ofꢀ ligands.ꢀ Fromꢀ
considerationꢀ ofꢀ theꢀ bondingꢀ situationꢀ inꢀ theꢀ pyridylꢀ
heterocycleꢀ (noꢀ C-Cꢀ bondꢀ lengthꢀ alteration)ꢀ suggestꢀ asꢀ aꢀ
prevailingꢀ polarꢀ structureꢀ aꢀ cationicꢀ chargeꢀ onꢀ theꢀ pyridineꢀ
nitrogenꢀ (orꢀ moreꢀ exactlyꢀ onꢀ itsꢀ methylꢀ substituent),ꢀ andꢀ aꢀ
negativeꢀchargeꢀonꢀNPYA.ꢀTheꢀrepresentationꢀwithꢀoneꢀpolarꢀ
structureꢀwouldꢀargueꢀinꢀfavorꢀofꢀaꢀzwitterionicꢀterminology,ꢀ
andꢀnotꢀaꢀmesoionicꢀsystem.ꢀInꢀcontrast,ꢀtheꢀflexibilityꢀofꢀtheꢀ
C–NPYAꢀ bondꢀ lengthꢀ (cfꢀ Tableꢀ 1),ꢀ andꢀ theꢀ distinctꢀ dihedralꢀ
anglesꢀofꢀtheꢀamideꢀandꢀtheꢀpyridineꢀheterocycleꢀsuggestꢀnon-
negligibleꢀ chargeꢀ delocalizationꢀ andꢀ henceꢀ supportsꢀ aꢀ
mesoionicꢀ terminology.ꢀ Complementaryꢀ studiesꢀ inꢀ solutionꢀ
andꢀtheꢀsolidꢀstateꢀwillꢀbeꢀneededꢀtoꢀshedꢀfurtherꢀlightꢀonꢀtheꢀ
1
7 T.ꢀvanꢀDijk,ꢀS.ꢀBurck,ꢀM.ꢀK.ꢀRong,ꢀA.ꢀJ.ꢀRosenthal,ꢀM.ꢀNieger,ꢀJ.ꢀ
C.ꢀSlootwegꢀandꢀK.ꢀLammertsma,ꢀAngew.ꢀChem.ꢀInt.ꢀEd.ꢀ2014,ꢀ
53,ꢀ9068.ꢀꢀꢀ
1
8 S.ꢀ Robert-Piessard,ꢀ G.ꢀ Leꢀ Baut,ꢀ J.ꢀ Courant,ꢀ J.ꢀ D.ꢀ Brion,ꢀ L.ꢀ
Sparfel,ꢀ S.ꢀ Bouhayat,ꢀ J.ꢀ Y.ꢀ Petit,ꢀ R.ꢀ Y.ꢀ Sanchez,ꢀ M.ꢀ Juge,ꢀ N.ꢀ
GrimaudꢀandꢀL.ꢀWelin,ꢀEur.ꢀJ.ꢀMed.ꢀChem.ꢀ1990,ꢀ25,ꢀ9.ꢀ
9 D.ꢀL.ꢀDavies,ꢀS.ꢀM.ꢀA.ꢀDonaldꢀandꢀS.ꢀA.ꢀMacgregor,ꢀJ.ꢀAm.ꢀChem.ꢀ
Soc.ꢀ2005,ꢀ127,ꢀ13754.ꢀ
0 D.ꢀL.ꢀDavies,ꢀS.ꢀM.ꢀA.ꢀDonald,ꢀO.ꢀAl-Duaij,ꢀJ.ꢀFawcett,ꢀC.ꢀLittleꢀ
andꢀS.ꢀA.ꢀMacgregor,ꢀOrganometallicsꢀ2006,ꢀ25,ꢀ5976.ꢀ
1 M.ꢀAlbrecht,ꢀChem.ꢀRev.ꢀ2010,ꢀ110,ꢀ576.ꢀ
1
2
2
2
2 Theꢀ CCO–NPYA–Cβ’–Cα’ꢀ dihedralꢀ angleꢀ isꢀ largerꢀ thanꢀ theꢀ
correspondingꢀ dihedralꢀ angleꢀ forꢀ theꢀ iridiumꢀ para-PYAꢀ
analogueꢀcomplexꢀ(36.568°).ꢀ
(
de)localizationꢀofꢀchargesꢀinꢀthisꢀligandꢀsystem.ꢀ
2
3 N.ꢀD.ꢀMcDaniel,ꢀF.ꢀJ.ꢀCoughlin,ꢀL.ꢀL.ꢀTinkerꢀandꢀS.ꢀBernhard,ꢀJ.ꢀ
Am.ꢀChem.ꢀSoc.ꢀ2008,ꢀ130,ꢀ210.ꢀ
ꢀ
1
R.ꢀ J.ꢀ Lundgrenꢀ andꢀ M.ꢀ Stradiotto,ꢀ Ligandꢀ Designꢀ inꢀ Metalꢀ
Chemistry:ꢀ Reactivityꢀ andꢀ Catalysis.ꢀ Johnꢀ Wileyꢀ &ꢀ Sons:ꢀ
Chichester,ꢀUK,ꢀ2016.ꢀ
2
4
I.ꢀCorbucci,ꢀA.ꢀMacchioniꢀandꢀM.ꢀAlbrecht,ꢀIridiumꢀComplexesꢀ
inꢀWaterꢀOxidationꢀCatalysis,ꢀinꢀIridium(III)ꢀinꢀOptoelectronicꢀ
andꢀ Photonicsꢀ Applicationsꢀ (edꢀ E.ꢀ Zysman-Colman),ꢀ Johnꢀ
2
MesoionicꢀcompoundsꢀareꢀdefinedꢀbyꢀIUPACꢀasꢀ“dipolarꢀfive-ꢀ
Wileyꢀ&ꢀSons,ꢀLtd,ꢀChichester,ꢀUK,ꢀ2017.ꢀ
5 Upꢀ toꢀ 6,000ꢀ h ꢀ haveꢀ beenꢀ reported,ꢀ see:ꢀ I.ꢀ Corbucci,ꢀ A.ꢀ
Petronilho,ꢀH.ꢀMüller-Bunz,ꢀL.ꢀRocchigiani,ꢀM.ꢀAlbrechtꢀandꢀA.ꢀ
ꢀ
(
possiblyꢀ six-)ꢀ memberedꢀ heterocyclicꢀ compoundsꢀ inꢀ whichꢀ
–1
2
2
2
bothꢀtheꢀnegativeꢀandꢀtheꢀpositiveꢀchargeꢀareꢀdelocalized,ꢀforꢀ
whichꢀ aꢀ totallyꢀ covalentꢀ structureꢀ cannotꢀ beꢀ written,ꢀ andꢀ
whichꢀcannotꢀbeꢀrepresentedꢀsatisfactorilyꢀbyꢀanyꢀoneꢀpolarꢀ
structure.ꢀTheꢀformalꢀpositiveꢀchargeꢀisꢀassociatedꢀwithꢀtheꢀ
ringꢀatoms,ꢀandꢀtheꢀformalꢀnegativeꢀchargeꢀisꢀassociatedꢀwithꢀ
ringꢀ atomsꢀ orꢀ anꢀ exocyclicꢀ nitrogenꢀ orꢀ chalcogenꢀ atom”.ꢀ
IUPAC.ꢀCompendiumꢀofꢀChemicalꢀTerminology,ꢀ2ndꢀEd.ꢀGoldꢀ
Bookꢀ Compiledꢀ byꢀ A.ꢀ D.ꢀ McNaught,ꢀ A.ꢀ Wilkinson,ꢀ Blackwellꢀ
Scientificꢀ Publications,ꢀ Oxford,ꢀ 1997.ꢀ XMLꢀ on-lineꢀ correctedꢀ
version:ꢀhttp://goldbook.iupac.orgꢀ(2006-)ꢀcreatedꢀbyꢀM.ꢀNic,ꢀ
J.ꢀ Jirat,ꢀ B.ꢀ Kosataꢀ updatesꢀ compiledꢀ byꢀ A.ꢀ Jenkins.ꢀ ISBNꢀ 0-
Macchioni,ꢀACSꢀCatal.ꢀ2015,ꢀ5,ꢀ2714.ꢀ
6 G.ꢀM.ꢀRodriguez,ꢀA.ꢀBucci,ꢀR.ꢀHutchinson,ꢀG.ꢀBellachioma,ꢀC.ꢀ
Zuccaccia,ꢀ S.ꢀ Giovagnoli,ꢀ H.ꢀ Idrissꢀ andꢀ A.ꢀ Macchioni,ꢀ ACSꢀ
EnergyꢀLett.ꢀ2017,ꢀ2,ꢀ105.ꢀ
7 TheꢀslightꢀreductionꢀofꢀTOFꢀatꢀlowerꢀcatalystꢀloadingꢀhasꢀbeenꢀ
discussedꢀpreviously,ꢀseeꢀrefꢀ15.ꢀ
ꢀ
ꢀ
ꢀ
9
678550-9-8.ꢀꢀꢀ
3
4
Forꢀaꢀreview,ꢀsee:ꢀO.ꢀSchuster,ꢀL.ꢀYang,ꢀH.ꢀG.ꢀRaubenheimerꢀ
andꢀM.ꢀAlbrecht,ꢀChem.ꢀRev.ꢀ2009,ꢀ109,ꢀꢀ3445.ꢀꢀ
Forꢀaꢀdiscussionꢀonꢀeffectsꢀofꢀmesoionicꢀsystems,ꢀsee:ꢀ(a)ꢀR.ꢀH.ꢀ
Crabtree,ꢀCoord.ꢀChem.ꢀRev.ꢀ2013,ꢀ257,ꢀ755;ꢀ(b)ꢀꢀꢀM.ꢀAlbrecht,ꢀ
Adv.ꢀOrganomet.ꢀChem.ꢀ2014,ꢀ62,ꢀ111.ꢀ
4
ꢀ|ꢀJ.ꢀName.,ꢀ2012,ꢀ00,ꢀ1-3ꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
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Page 5 of 5
Pleas De ꢀad l to oꢀ nn oT tr ꢀaa nd sj au cs tti ꢀomn as rginsꢀ
JournalꢀNameꢀ
ꢀCOMMUNICATIONꢀ
ForꢀToCꢀuseꢀonlyꢀ
View Article Online
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
DOI: 10.1039/C7DT04555G
ꢀ
ꢀ
ꢀ
Aꢀfirstꢀexampleꢀofꢀaꢀmesoionicꢀligandꢀwithꢀaꢀnitrogenꢀcoordinationꢀ
siteꢀwasꢀpreparedꢀbyꢀaꢀsimpleꢀandꢀrobustꢀsynthesisꢀandꢀisꢀshownꢀ
toꢀ haveꢀ excellentꢀ propertiesꢀ forꢀ promotingꢀ iridium-catalyzedꢀ
waterꢀoxidation.ꢀ
ꢀ
ꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ5ꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ