The chelate−to−bridging shift of phosphane dipalladacycles: convenient synthesis of double A‐frame tetranuclear complexes
Abstract
Palladacycles of the type [Pd2(Ph2PCH2PPh2-P,P)2(C,N:C,N)] (C,N:C,N = bis(N-2,3,4-trimethoxybenzylidene)-4,4′-sulfonyldianiline or -4,4′-oxydianiline) can undergo a spontaneous slow chelate-to-bridging diphosphane coordination shift in solution. Following this strategy a tailor-made synthetic procedure was devised that culminates in isolation of double A-frame tetranuclear palladium complexes.
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JournalName COMMUNICATION Thisjournalis©TheRoyalSocietyofChemistry20xxJ.Name.,2013,00,1‐3|1 Pleasedonotadjustmargins Pleasedonotadjustmargins a. DepartamentodeQuímicaInorgánica,UniversidadedeSantiagodeCompostela, AvenidadasCienciass/n,15782SantiagodeCompostela,Spain. b. DepartamentodeQuímicaFundamentalandCentrodeInvestigaciónsCientíficas Avanzadas(CICA),FacultadedeCiencias,UniversidadedaCoruña,E‐15071A Coruña,Spain. †Footnotesrelatingtothetitleand/orauthorsshouldappearhere. Electronic Supplementary Information (ESI) available: [Experimental procedure, characterization data, including X‐ray. CCDC 1564895 ( 2a ), 1564896 ( 2b ) and 1819793( 3b ).].SeeDOI:10.1039/x0xx00000x Received00thJanuary20xx, Accepted00thJanuary20xx DOI:10.1039/x0xx00000x www.rsc.org/ Thechelate−to−bridgingshiofphosphanedipalladacycles: ConvenientsynthesisofdoubleA‐frametetranuclearcomplexes. PabloFrieiro−Gomis, a FátimaLucio‐Martínez, a PaulaMunín‐Cruz a ,JuanM.Ortigueira, a M.Teresa Pereira, a, *PaulaPolo‐Ces a ,DignaVázquez‐García b andJoséM.Vila a, * Palladacycles of the type [Pd 2 (Ph 2 PCH 2 PPh 2 − P , P ) 2 ( C , N : C , N )] ( C , N : C , N =bis( N −2,3,4−trimethoxybenzylidene)−4,4´−sulfonyldianiline or −4,4´−oxydianiline) can undergo a spontaneous slow chelate−to−bridging diphosphane coordinationshiftinsolution.Followingthisstrategyatailormade synthetic procedure was devised that culminates in isolation of double A−frame tetranuclear palladium complexes. The chemistry of cyclometallated compounds 1 hasattracted muchresearchinterestinpastyears.Theyareknownfortheir ample applications in numerous fields: organic synthesis, 2 photochemistry, 3 catalysis, 4 andaspotentialbiologicallyactive materials. 5 We now report a chelate‐to‐bridging shift which occursspontaneouslyinsolutionevenatlowtemperaturein palladacycles bearing Ph 2 PCH 2 PPh 2 ‐P,P, (dppm), with formation of double A‐frame complexes; systems that are relatedtothefieldofsupramolecularcoordinationcomplexes (SCCs). 6 Forthecomplexesreportedhereinthechoiceof tetranuclearspeciesasopposedtotheisomeric1D‐polymeric forms results from the experimental data. In the absence of adequatemassspectra,wherecrystalstructuredetermination waspossibletheSCCmetallacyclestructurearised. 7 Wehave previously described formation of an A−frame complex stemmingfroma chelateddppmpalladacycle. 8 Wereasoned that therecould be a conversionbetween both species with the latter being the kinetically stable isomer. This strategy should open up a convenient access to supramolecular palladacycles via attachment of more than one A‐frame arrangementinthemultinuclearcomplex.Inaninitialattempt atthistaskthedinuclearpalladacyleswithphenylenediimine‐ C,N:C,Nfailedtoundergothisconversion; 8 mostprobablydue to the restricted flexibility of the ligand imposed by the rigidnessofthecentralphenyleneringitself,hindering openingofthemetallacycleringneededtoprovidetheextra vacantcoordinationsiteonthemetalforthesecondincoming phosphaneligand.Wethenhypothesizedthatinordertofix the A‒frame disposition a more favorable environment was needed.Inthisregardweenvisionedmoreflexiblebidentate nitrogen‒donor ligands able to supply the needed coordination adaptability of the metal centers and sufficient roomtoaccommodatethephosphanedonors,forwhich purpose bis(N−2,3,4−trimethoxybenzylidene)−4,4´−sulfonyldianiline, a , or −4,4´−oxydianiline, b , were chosen. Accordingly,whatfollowsisoursyntheticstrategyformaking double A‐frame tetranuclear palladium structures based on non‐rigid bidenate imines appropriate for dimetallation bearing‒SO 2 ‒and‒O‒spacers(Scheme1). Scheme1. Reactionsequenceleadingtothesynthesisofthe doubleA−framecomplexes.Thechloridoandbromidoligands comefrom IIa , IIb (Cl)and IIIb (Br)(seeESI). During the NMR determination of 1a in CDCl 3 weobserved thatsignalsotherthanthoseexpectedseemedtoslowlydawn inthe 1 HNMRspectrumof 1a ca.4.0ppmsuggestinganew specieswasdeveloping.Then,theCDCl 3 NMRtubesolutionof 1a waslefttostand,whereuponsingle‐crystalsappeared;an Page 1 of 5 ChemComm
COMMUNICATIONJournalName 2|J.Name.,2012,00,1‐3 Thisjournalis©TheRoyalSocietyofChemistry20xx Pleasedonotadjustmargins Pleasedonotadjustmargins X‐raycrystallographicanalysisshowedthemtobecompound 2a : a novel tetranuclear palladium(II) double A−frame framework bearing two palladium pairs, each bridged by a {( −PCP) 2 / −Cl}group,insupportofourhypothesisthataless rigid imine architecture should allow spanning of the diphosphanesacrossthemetalcenters. Asforthechoiceof 2a versus 1a itshouldbenotedthattwo opposingfactorsconcur.Ontheonehand,favoring 1a isthe thermodynamicstabilityassociatedwith twochelate ringsat palladium: the five− and four−membered metallacycle and phosphanerings,respectively,whilstnonein 2a ;ontheother one, the strain imposed by the four‐membered chelate ring should favor ring opening, enabling intermediates with uncoordinated phosphorus donors, which coupling together should yield the bridging system. Also, the presence of the stronger Pd−PascomparedtothePd −Nbondintermsof Pearson´sconcept, 9 supports 2a . Inviewoftheseresultsandtofurtherclarifythissituationwe proceeded to approach an analogous study and the related ligand b wasconsidered.Ourensuinghypothesiswasthatthe resulting solution of 1b should give rise to 2b over time, provided that some Cl ‐ ion from the precursor remains in solution. Notwithstanding, 1b was kept at very low temperature,c.a.‐40°C,toavoidthe 1b → 2b shiftwhen runningthefirst 1 Hand 31 PNMR.Onapproachingr.t.,c.a.‐10 °C,conversionalreadybegins, 1b slowlytransforminginto 2b withtimeatroomtemperature.Figure1depictschangesover afivedayperiod;inspectionoftheHC=NandMeOresonances putsforwardthesteadyevolution.Lefttostandcompound 2b precipitates as neat single crystals, thus completing the transformation.Attemptstoprepare 2a or 2b directlyfrom the parenthalide‐bridgedcomplexes(seeESI)withdppmwasnot possible; the reaction always proceeds through the chelated species,whichthenundergoesconversionto 2a or 2b. Figure1. Time‐dependent 1 HNMRplotforthe 1b → 2b shiftat r.t.Compound 1b ,*.Compound 2b ,*. Then,the synthesis of 2a wascarriedout inacetone‐d6 ina similar fashion to 2b in order to elucidate whether or not a chlorinatedsolventprovides thechloridoligands.The result wasthatonceagain 1a furnishes 2a andasimilartime‐ a) b) Figure2 .a)Thermalellipsoidplotofthecationfor 2a shownat 30% probability level, inclusive of CDCl 3 solvent molecules. b)Labellingschemefortheasymmetricunit.Hydrogenatoms and phosphane phenyls (in b) have been omitted for clarity. Selectedbonddistances(Å)andangles(deg)for 2a :Pd(1)‐C(1) 1.996(9), Pd(1)‐Cl(1) 2.452(2), Pd(1)‐P(1) 2.334(3), Pd(1)‐P(3) 2.342(3),Pd(2)‐C(1)2.416(2),Pd(2)‐C(17)2.002(10),Pd(2)‐P(2) 2.299(3), Pd(2)‐P(4) 2.326(3), C(1)‐Pd(1)‐P(1) 88.9(3), C(1)‐ Pd(1)‐P(3) 87.8(3), P(1)‐Pd(1)‐Cl(1) 92.6(1), P(3)‐Pd(1)‐Cl(1) 91.54(1). a) b) Figure3 .a)Thermalellipsoidplotofthecationfor 2b shown at 30% probability level. Solvent molecules removed for clarity.b)Labellingschemefortheasymmetricunit.Hydrogen atoms and phosphane phenyls (in b) have been omitted for clarity. Selected bond distances (Å) and angles (deg) for 2b : Pd1−C1 1.997(4), Pd1−P2 2.3228(8), Pd1−P4 2.3331(9), Pd1−Cl1 2.4328(8), Pd2−C82 1.994(4), Pd2−P1 2.3569(9), Pd2−P3 2.3168(9), Pd2−Cl1 2.4607(9), C(1)‐Pd(1)‐P(2) 84.98(10),C(1)‐Pd(1)‐P(4)85.96(10),P(2)‐Pd(1)‐Cl(1)93.40(3), P(4)‐Pd(1)‐Cl(1)94.58(3). Page 2 of 5ChemComm
JournalName COMMUNICATION Thisjournalis©TheRoyalSocietyofChemistry20xxJ.Name.,2013,00,1‐3|3 Pleasedonotadjustmargins Pleasedonotadjustmargins dependent 1 HNMRplotasdepictedinFigure1wasobtained forthe 1a → 2a shift(Figure1ESI).Althoughatthispointitthen seemed clear that the chlorido ligand stemmed from the parent halide‐bridged compounds and not from the solvent, wesoughttopreparetheanalogousA‐framebromidocomplex to ascertain this, as well as to show that the shift may be extended to other bridging halides. Thus, in a similar experiment 3b wasobtained(ESI),asatetranuclearcompound withtwo{Pd ‐Br( ‐PPh 2 CH 2 PPh 2 ) 2 Pd}groups.Figure4shows theORTEPviewforthemoleculeof 3b . The crystal structures of complexes 2a , 2b and 3b provide unequivocal evidence of the 1a → 2a , 1b(IIb) → 2b and 1b(IIIb) → 3b conversionprocesses,andthereproducibilityof thepreparationroutine,ofwhichtothebestofourknowledge therearenoreportedexamples.Figures2,3and4showthe ORTEP view of the molecules of 2a , 2b and 3b , respectively (see ESI). Each asymmetric unit (half‒cation of the complex) may be described as formed by two approximately squareplanarpalladiumcentersbondedthroughtwodppmandone chloridoorbromidobridgingligands.Theorganicligandsare then linked by four Pd−C bonds to thetwo{Pd ‐X( ‐ PPh 2 CH 2 PPh 2 ) 2 Pd}(X=Cl,Br)frameworkstoenvisageacavity which,whereappropriate,hoststhedisorderedchloroform‐d1 ( 2a )andacetone‐d6( 2b )molecules. The bond distances are within the expected values with allowanceforlengtheningofthePd−ClandPd‐Brbondsdueto thehightransinfluenceofthemetallatedphenylcarbonatom. a) b) Figure4. a)Thermalellipsoidplotofthecationfor 3b shownat 30%probabilitylevel.Solventmoleculesremovedforclarity.b) Labelling scheme for the asymmetric unit. Hydrogen atoms and phosphane phenyls (in b) have been omitted for clarity. Selectedbonddistances(Å)andangles(deg)for 3b :Pd1−C20 2.006(4), Pd1−P2 2.3237(10), Pd1−P5 2.3338(11), Pd1−Br3 2.5459(5), Pd2−Br3 2.5722(6), C(20)‐Pd(1)‐P(2) 85.06(11), C(20)‐Pd(1)‐P(5) 85.55(11), P(2)‐Pd(1)‐Br(3) 93.90(3), P(5)‐ Pd(1)‐Br(3)94.24(3). ThePd−Ndistancesca.2.79(1)Å( 2a ),2.74Å( 2b )and2.72Å ( 3b )suggestweakPd∙∙∙Ninteractionsaspreviouslyobserved in the range 2.576(4)‒2.805(5) Å 10 pointing towards a pseudo‒penta‐coordination at palladium. The eightmembered Pd( ‐PCH 2 P) 2 Pdringsadoptapseudo−boat conformation, with the two methylene groups and the bridging chlorido ( 2a , 2b ) or bromido ( 3b ) lying on opposite faces of the Pd 2 P 4 plane. The nearly co−planar metallated phenylringsareessentiallyperpendiculartothePd 2 P 4 plane, presumably in order to minimize repulsions with the dppm phenyl rings; stacking interactions between the phosphane phenyl rings cooperate to stabilize the structure. Analogousinteractionsbetweendppmandthearomaticrings oftheditopiclinkersarenotlikely,duelackofparallelism between the phenyl rings and to the distances between centroids(ESI).TheP‐C‐Panglesca.116.6°( 2a ),115‐117°( 2b ) and( 3b )116‐118°arelargerthanthevalueof106.4°reported for the uncoordinated dppm ligand, 11 probably due to steric requirementstoformthemaincoreoftheA‐framestructure. ThePd‒Pddistancesof3.254Å( 2a ),3.173Å( 2b )and3.222Å ( 3b )precludeanymetal‒metalinteractions. ADFTanalysiswasperformedforthecompounds 1b and 2b usingthepackageofprogramsg09. 12 Inordertocomparethe relativestabilitybetweenthechelate( 1b ,whichpresenttwo conformers)andtheA‐frame( 2b )formsdescribedhere,some distances and angles are also compared to explain the difference in energy (see ESI). The reported geometry and frequency calculations were carried out at the B3LYP/LANL2DZ‐ECP 13 /6‐31g(d) level of theory and no imaginary frequencies were found, confirming the stationary points. The results indicate that the equilibrium between the two formsisdisplacedtowards(ΔG=−171Kcal/mol)theA‐frame conformation that is in agreement with the crystal structure obtainedandtheNMRstudiesperformedonthereaction solution. The A‐frame compound is more stable because the palladiumenvironmentislessconstrainedduetoopeningof theC−C=NandP−C−Pangles,(Δca.4 o and16 o ,respectively). Moreover,thechelateeffectisneutralizedbytheformationof a supramolecular cage that is more stable from an entropic point of view. Figure 5 depicts a graphical representation of therelativeenergiesofthecompoundsunderstudy. Figure 5. Representation of the differing energies for the chelatedandtheA‐framecompounds. Insummary,wehavedesignedauniquesyntheticroutefor achieving double A−frame assemblies from mulnuclear Page 3 of 5 ChemComm
COMMUNICATIONJournalName 4|J.Name.,2012,00,1‐3 Thisjournalis©TheRoyalSocietyofChemistry20xx Pleasedonotadjustmargins Pleasedonotadjustmargins halide−bridgedpalladacyclesviadinuclearchelatedphosphane intermediates. Thisresult representsafurtherenrichment in thechemistryofpalladacyclesSCCsaswellasinthe coordination chemistry of dppm; the crystal structures representing unambiguous evidence of the chelating to bridgingdppmshift.Althoughourexampleshavethusfarbeen with dppm it seems likely that other systems will show the phenomenonandthatcareshouldbetakeninthechoiceof palladacycle and phosphane. For instance, preliminary tests showthatPPh 2 PN(Me)PPh 2 stronglyfavorschelationinterms of the gem‐dimethyl effect, 14 stabilizing the di−chelated compound,thusstoppingtheshift. Thisworkwasmadepossiblethankstothefinancialsupport received from the Xunta de Galica (Galicia, Spain) under the Grupos de Referencia Competitiva Programme (Projects GRC2014/042andGRC2015/009).F.L.−M.thankstheSpanish MinistryofEducation(grantFPU13/05014).Weareindebted toMr.HarryAdamsforhelpinthediscussionofthecrystal structures. Conflictsofinterest Therearenoconflictstodeclare. Notesandreferences 1 (a)M.Albrecht,Chem.Rev.,2010,110,576;(b) I. Omae, Cyclometalation Reactions. Five‐Membered Ring Products as UniversalReagents,Springer,Japan,2014. 2 (a) J. Tsuji, Palladium Reagents and Catalysts, Wiley, Chichester,1995;(b)I.Omae,Applications of Organometallic Compounds,Wiley,Chichester,1998. 3 (a)J.Kalinowski,V. Fattori, M.Cocchiand J.A.G. Williams, Coord.Chem.Rev.,2011,255,2401;(b) Á. Díez, E. Lalinde andM.T.Moreno,Coord.Chem.Rev.,2011,255,2426. 4 (a)J.Dupont,M.PfefferandJ.Spencer,Eur.J.Inorg.Chem., 2001,1917;(b) J. Dupont, C. S. Consorti and J. Spencer, Chem.Rev.,2005,105,2527;(c) S.Doherty,J.G.Knight,N.A. B.Ward,D.M.Bittner,C.Wills,W.McFarlane,W.Cleggand R.W.Harrington,Organometallics,2013,32,1773;(d) S. Chakraborty,P.Bhattacharya,H.DaiandH.Guan,Acc.Chem. Res.,2015,48,1995;(e) Z. Liu and P. J. Sadler, Acc. Chem. Res.,2014,47,1174. 5 (a)A.Habtemariam,B.Watchman,B.S.Potter,R.Palmer,S. Parsons, A. Parkin and P. J. Sadler, J.Chem.Soc.,Dalton Trans.,2001,1306;(b) A.G.QuirogaandC.Navarro Ranninger,Coord.Chem.Rev.,2004,248,119;(c) J. Ruiz, J. Lorenzo, L. Sanglas, N. Cutillas, C. Vicente, M. D. Villa, F. X. Avilés,G.López,V.Moreno,J.PérezandD.Bautista,Inorg. Chem.,2006,45,6347;(d) P.Chellan,S.Nasser,L.Vivas,K. Chibale and G. S. Smith, J. Organomet. Chem., 2010, 695, 2225. 6 (a)D.Preston,A.Fox‐Charles,W.K.C.LoandJ.D.Crowley, Chem. Commun.,2015, 51,9042; (b)T.R.Cook,P.J.Stang, Chem.Rev.2015,115,7001;(c) L.Xu,Y.‐X.Wang,L.‐J.Chen, H.‐B. Yang, Chem. Soc. Rev. 2015, 44, 2148; (d) S. Wang, T. Sawada,K.Ohara,K.YamaguchiandM.Fujita,Angew.Chem. Int.Ed.,2016,55,2063;(e)S.BandiandD.K.Chand, Chemistry – A European Journal, 2016, 22, 10330; (f) M. L. Saha,X.Yan,P.J.Stang,Acc.Chem.Res.2016,49,2527;(g)C. Lescop,Acc.Chem.Res.2017,50,885. 7 (a)J.M.Vila,M.Gayoso,M.TeresaPereira,M.C.Rodríguez,J. M.Ortigueira,M.Thornton‐Pett,J.Organomet.Chem.,1992, 426,267;(b)L.Adrio,J.M.Antelo,J.M.Ortigueira,J.J. Fernández,A.Fernández,M.TeresaPereiraandJ.M.Vila,J. Organomet. Chem., 2009, 694, 1273. (c) Inthecourseof submittingthismanuscripwehavebeenabletodetermine themolecularstructureforIbandthechoicehereisalso theSCCmetallacycle. 8 D. Vázquez‐García, A. Fernández, M. López‐Torres, A. Rodríguez,N.Gómez‐Blanco,C.Viader,J.M.VilaandJ.J. Fernández,J.Organomet.Chem.,2011,696,764.Aprevious example has been reported albeit lacking crystallographic evidence,seeVicente,J.etal,J.Organomet.Chem.2002,663, 164. 9 R.G.Pearson,J.Am.Chem.Soc.,1963,85,3533. 10 (a)J.Granell,D.Sainz,J.Sales,X.SolansandM.Font‐Altaba,J. Chem.Soc.,DaltonTrans.,1986,1785;(b) J. Granell, J. Sales,J.Vilarrasa,J.P.Declercq,G.Germain,C.Miravitllesand X.Solans,J.Chem.Soc.,DaltonTrans.,1983,2441;(c)T.Zenei and Y. Shinsaku, Bull.Chem.Soc.Jpn., 1986, 59, 649; (d) J. Vicente, A. Arcas, D. Bautista and P. G. Jones, Organometallics,1997,16,2127;(e)L. A. Villanueva, K. AbboudandJ.M.Boncella,Organometallics,1991,10,2969; (f)D.Onggo,D.Craig,A.RaeandH.Goodwin,Aust.J.Chem., 1991,44,219;(g)O.Schlager,K.WieghardtandB.Nuber, Inorg.Chem.,1995,34,6449;(h)J. Albert, M. Gomez, J. Granell,J.SalesandX.Solans,Organometallics,1990,9,1405. 11 (a)C.D.Nicola,Effendy,F.Fazaroh,C.Pettinari,B.W.Skelton, N.SomersandA.H.White,Inorg.Chim.Acta,2005,358,720; (b) J. Bruckmann, C. Krüger and F. Lutz, Z. Naturforsch, B: Chem.Sci.,1995,50b,351. 12 M.J.Frischetal.,Gaussian09,Revision D.01Gaussian,Inc., WallingfordCT,2013. 13 P. J. Hay and W. R. Wadt, The Journal of Chemical Physics, 1985,82,270. 14 B.L.Shaw,J.Am.Chem.Soc.,1975,97,3856. Page 4 of 5ChemComm