ReviewSynthesis, characterization and dynamic behavior of some iridiumcarbonyl cluster complexes derived from Ir4(CO)12 with N-, P- andC-donor ligands: A surveyAugusto Tassana,, Mirto Mozzona, Giacomo Facchinb, Alessandro Dolmellac, Serena DettidaDipartimento di Ingegneria Industriale, via Marzolo 9, 35131 Padova, ItalybIstituto per lEnergetica e le Interfasi IENI-CNR, via Marzolo 9, 35131 Padova, ItalycDipartimento di Scienze del Farmaco, via Marzolo 5, 35131 Padova, ItalydInstitute of Ecosystem Study ISE-CNR, via Moruzzi 1, 56122 Pisa, Italyarti cle i nfoArticle history:Received 4 August 2014Received in revised form 9 September 2014Accepted 14 September 2014Available online 28 September 2014Keywords:Iridium clustersCarbonylIntramolecular dynamicabstractThe synthesis of iridium dodecacarbonyl cluster derivatives Ir4(CO)12 with donor ligand such as amine,phosphites, hydrido and cyclic mono and dioxycarbene, NMR and X-ray characterization and uxionalbehavior study in solution at variable temperature is briey reviewed. 2014 Elsevier B.V. All rights reserved.Augusto Tassan initiated its research activity in the Chemistry Department of the Venice University. He then moved to the University of Padova,Industrial Chemistry Institute, under the supervision of Prof. R. Ros and R.A. Michelin, and collaborating with the Prof. R. Roulet of EPFL in Lausanne.His research focuses on the synthesis of new organometallic platinum and iridium clusters, with particular interest on NMRcharacterizationMirto Mozzon took a degree of Industrial Chemistry at the University of Padova with full marks. He became then CNR researcher in the group headedby Prof. U. Belluco, and nally researcher at the University of Padova, under the supervision of Prof. R. A. Michelin. He is now Associate Professor. Hemaintained a research cooperation whit Prof. A.J.L. Pombeiro at Instituto Superior Tecnico in Lisbon. He coauthored about 80 papers published onpeerreviewed international journals in Inorganic and Organometallic Chemistry. He also lled 4 International Patents and written 2 students book.http://dx.doi.org/10.1016/j.ica.2014.09.0140020-1693/ 2014 Elsevier B.V. All rights reserved.Corresponding author. Tel.: +39 498275518.E-mail address: augusto.tassan@unipd.it (A. Tassan).Inorganica Chimica Acta 424 (2015) 91102ContentslistsavailableatScienceDirectInorganica Chimica Actaj our nal homepage: www. el sevi er . com/ l ocat e/ i caContents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 922.13CO-enrichment of tetrairidium carbonyl clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933. Diamine derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 934. Monoamine derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945. Hydrido derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946. Cyclic mono and dioxycarbene derivatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 967. Phosphites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 988. Intramolecular dynamics of [Ir4(CO)12] derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1021. IntroductionThechemistryofiridiumcarbonyls, notably, thechemistryofclusters derived fromIr4(CO)12, were developed along variousresearchlines. Amongtheinvestigatedtopics, wecanmentionthe stereochemistryof the ligands [1], the uxional processesoccurringinsolution[2], thestudiesonthekineticsofcarbonylsubstitution reactions [3], the modellisation of metal surfaces forabsorptionreactions of unsaturated substrates [4], the use ofsuchmaterials as catalysts or precursors inthehydrogenationprocessesof hydroformylationof unsaturatedorganicmolecules[5].Along with these perspectives, Garlaschelli and co-workers [6]prepared the starting complex Ir4(CO)12from IrCl3nH2O inethyleneglycol monomethylethermediumunderaCOgasowwith more than 80% yield.TheIRspectrumof theobtainedmixtureshowsthetypicalbands of terminal carbonyls in the range 21142000 cm1.A few years later Pruchnik et al. [7] reported an even more ef-cient method (ca. 95% yield) to obtain Ir4(CO)12 by reacting IrCl33H2O with formic acid in autoclave at 100 C for 12 h.Tri- and tetra-substituted derivatives of Ir4(CO)12can beobtainedingoodyieldbymeans of thedirect reactionof thetetrairidium complex with different ligands (L). Further studies ofsubstitutionreactions haveidentied as aprocess made ofthreeconsecutive steps:Ir4CO12 L !COIr4CO11L L !COIr4CO10L L !COIr4CO9LGiacomo Facchin studied chemistry at the Universit degli Studi di Padova (Italy) and completed his PhD in 1979. After a post doc term with Prof. R.J.Angelici at the Iowa State University he joined the Italian National Research Council (CNR) where is currently Senior Researcher at the Istituto perlEnergetica e le Interfasi (IENI). His scientic activity mainly focuses on organometallic and coordination chemistry, nanostructurated materials andmaterials containing metallic nanoparticles.Alessandro Dolmella entered the Department of Pharmaceutical and Pharmacological Sciences in 1990 to work in the research group headed byProf. M. Nicolini, studying radiopharmacy and computational chemistry. He is presently interested in bioinorganic and coordination chemistry, with aspecial emphasis on transition metals complexes.Serena Detti graduated in chemistry at the University of Pisa in 1996, under the supervision of Prof. F. Calderazzo and G. Pampaloni. She received herPhD inChemistry at theSwiss Federal Instituteof Technologyof Lausannein 2002, working inthe groupof Prof. R. Rouletin theeld of metalscarbonyl clusters. Sheworkedat theItalianforensicscienceserviceandlater shedevotedtoresearchonnanotechnology, studyingpotentialinteractions of metal nanostructures and the environment.92 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102Intherst step, thesubstitutionreactionfollowsasecond-order kinetics law. It is characterized by an associative mechanism,and three of the (previously) all terminal CO undergo a structuralrearrangement that transformsthemintobridgingCOunitsonthe basal iridium atoms. The following two steps have an associa-tive-to-dissociative substitutionmechanism. Factors as basicityand the bulkness of the incoming ligand do not vary the processkinetics [3].Mono- or di-substituted derivatives of Ir4(CO)12 cannot insteadbeisolatedbydirect synthesis fromthetetrairidiumcomplex,becauseof itsinsolubilityincommonorganicsolventsandalsobecause the reaction requires high temperatures (80120 C).Accordingly, alternative routes had to be sought. An opportunity,although with low yields, is given by the reduction of Ir(CO)2(H2-NC6H4Me-p)Cl by zinc metal in the presence of carbon monoxideand the ligands [8]; another one is offered by the reaction of Ir4-H(CO)11 with the appropriate ligands [9].Still another possibility was explored by Chini et al., who pre-paredtheanioniccomplexesofthetype[Ir4(CO)11X](X = BrorI) [10]. Since the latter are soluble and more reactive than iridiumdodecacarbonyl, they can undergo replacement reactions of a COwith several ligands, including PF3 and SO2 [11], olen [12], phos-phines [13], arsines [14], yielding various complexes.In the following sections, we provide a brief account on the syn-thesis, characterization and dynamic behavior of new tetrairidiumclusters with H-, N-, P-, and C-donor ligands derived fromIr4(CO)12.2.13CO-enrichment of tetrairidium carbonyl clustersThe molecular structure and the stereochemistry of metal car-bonyl derivatives might be investigated by means of 13C NMR anal-ysis. However, a problem arises because the exchange reactions donotalwaysoccurwithfree13CO;whileforsomemetalcarbonylclusters, suchasCo4(CO)12andRh4(CO)12, thisexchangeiseasy,thereactioninvolvingOs3(CO)12andIr4(CO)12occur onlywithmore difculty. The main obstacle is the low solubility of the clus-ters, which forces the exchange reactions to take place in heteroge-neous phase and makes themextremely slow, even at highpressures and high temperatures.Tassanandco-workershavereported[15]twosimpleproce-durestomakethe13COexchangeprocesseasier. Therst oneinvolvestheuseofanionicclusters;thesecondonerequirestheuseofthewell-known decarbonylatingagenttrimethylamine-N-oxide, Me3NO [1618], in the presence of free13CO.The rst method (a two-step process) is illustrated in Scheme 1.At thebeginning,as reported by Chini etal. [10],thereaction ofIr4(CO)12 with NEt4I in THF at 70 C leads to the formation of theanioniciridiumtetracarbonyl [Ir4(CO)11I]. Thesecondstep, thedisplacement of iodide by 13CO, occurs in THF at room temperatureand affords the enriched Ir4(CO)12 cluster.Both reactions occur with more than 90% yield. The degree oftherstenrichmentA1isgivenbythemolarfractionofcoordi-nated13CO and can be calculated from Eq. (1) below:A1 b 11A012 0:09266 1whereA0andbare, respectively, themolarfractionsfornaturalabundance13CO(0.01108) andfor used13CO(0.99). Asecondenrichment step can then be carried out, again, in THF at room tem-perature as outlined in Eq. (2):NEt4Ir4CO11 13CO ! Ir4CO12 # NEt4I 2i.e., by repeating the reaction path described in Scheme 1, this timeusing the enriched Ir4(CO)12as starting material. The molar frac-tion of13CO can be calculated from Eq. (3) below:A2 b 11A112 0:16743 3Further A3, A4, . . . Ax values can then be calculated from the fol-lowing Eq. (4):Ax b 11Ak112k 1; 2; 3; . . . 4Eq. (4) has basically the form:Ak f Ak1 5where f is a linear function that converges to the point (b = 0.99), avalue which veries the equation t = f(t).As mentioned above, the alternative direct method uses Me3NOas decarbonylating reagent, according to the following scheme:TherstreactionoftheScheme2iscarriedoutat 30 CinTHF, with a Me3NO/cluster stoichiometric ratio and a slight excessof13CO, affordingtheanionic[Ir4(CO)11I]complex. Thesubse-quentreactionisperformedatroomtemperatureandyieldsthetetrairidium carbonyl complex in more than 90% yield. In general,this kind of reaction allows the preparation of a large number of13CO-enrichediridiumcarbonylclustercomplexeswithdifferentligands, including monodentate phosphines, diphosphines orarsines. The uxional behavior of all these species can be readilyanalyzed by13C NMR (eq. (6)).Ir4CO12nLn 13CO Me3NO ! Ir4CO12nLn CO2NMe36Theexchangerate12CO13COwas determinedbyfollowing,through IR spectrometry, the enrichment reaction of the complexIr4(CO)11(PPh3)inCH2Cl2. Asexpected, aslongas13COincreasesthere is a lowering of the CO stretching frequencies. Changing from99%12CO to 98%13CO, the IR spectrum presents the same overallshapeandthesamenumberof bands, however, withashiftof47.048.5 cm1for terminal carbonyls, and of 40 cm1foredge-bridgingCO. Thesefrequencyshiftsareinagreementwiththe values shown by the12CO and13CO ligands, whose vibrationsarerelatedonlytotheirreducedmassesandnotinuencedbythe coupled cluster moiety.m12CO m13CO m13CO 1 l13CO l12CO ! " #1=273. Diamine derivativesTherst diaminederivatives of tetrairidiumdodecacarbonylhavebeensynthesizedbyTassanandco-workers[19]. Initially(Scheme3), [Ir4(CO)11X](X = BrorI)reactswithalargeexcessof aromatic ligand in presence of Ag+(one equivalent) in dichloro-Scheme 1. Reaction path for the synthesis of13CO-enriched tetrairidium dodeca-carbonyl. Mixture of12CO and13CO.Scheme 2. Alternative synthesis of13CO-enriched tetrairidium dodecacarbonyl.A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102 93methane at lowtemperature (30 C); the diamines are thenobtained after a disproportion reaction at room temperature.The conguration of the complexes (Fig. 1) has been assignedon the basis of IR and13C{1H} NMR spectroscopies at lowtemperature (180 K). The complexes showsix resonances: forexample in the case of N-N = 1,10-phenantroline the13C{1H}NMR carbonyls a at 226.6 (relative intensity 2), b at 200.1 (r.i. 1),d (r.i. 2), c (r.i. 2), e (r.i. 2) and g (r.i. 1). The IR spectroscopic datashowthepresenceofterminal carbonyls(2070s, 2043vs, 1995s)and bridged carbonyls (1830m and 1797m).The structure proposed in Fig. 1 has been conrmed by singlecrystal X-ray diffraction analysis (Fig. 2). The complex presents atetrahedral cluster of iridiumatoms and a distributionof COligands similar to that found in many other derivatives of Ir4(CO)12[20]. TheIrCbonddistancesbecomeshorterinpresenceofthediamino groups. The diamino ligands chelate thecluster througha basal iridium via the lone-pairs of nitrogen atoms lying in axialandradial positions. TheIr2Ir3andIr2Ir4distancesinvolvingtheiridiumatom(Ir2)whichischelatedthediamineligandarecomparable to the unbridged IrIr bond distances [20].4. Monoamine derivativesThe work on diamino ligands was extended by preparing com-plexes with monoamine [21], notably, with pyridine (7), 4-methyl-pyridine(8), 4-ter-buthylpyridine(9), 3,5-dimethylpyridine(10)and3,4-dimethylpyridine(11). Inthiscase, [Ir4(CO)11Br]reactsreadily with an excess of aromatic monoamine and one equivalentof AgBF4inCH2Cl2at 25 C. Theproducts areobtainedwith6071% yield, after recrystallization from a CH2Cl2/MeOH mixture.NEt4Ir4CO11Br AgBF4 L ! Ir4CO11L AgBrwhere L monoamine ligands 8TheIRspectraofthesecompounds, inCH2Cl2solution, showeither the presence of the characteristic terminal CO bands(21001950 cm1) and also two adsorptions in the region of bridg-ing CO. However, by replacing the CH2Cl2 solvent with cyclohex-ane, thebandsof bridgingCOdisappear. Thiscanbeexplained(Scheme4)byassumingthepresenceofatleasttwospeciesinsolution: an isomer with all terminals ligands (A), an isomer withbridging COand theamine in axial position (B) and anotheronewith the amine group in equatorial position (C).Likewise, the13C NMR analysis of compound (8) enriched with13CO ca. 20% reveals two sets of signals in 36/73 ratio. The rst setcan be attributed to the 8B isomer, with the monoamine ligand inaxialposition, thesecondandmoreabundantonetoisomer8A.Thesameanalysisforcompound9showsthepresenceofthreesets of signals originated by the three isomers (A, B, C) in 42/55/3 ratio, respectively.The X-ray crystal structure of Ir4(CO)11(4-methylpyridine)(Fig. 3) shows that the molecule contains a nearly tetrahedral Ir4coreandallterminalligands, asresultingalsofromtheanalysisof the 13C NMR spectra. The Ir1Ir4 distance, in a pseudo-trans posi-tionwithrespecttotheamineligand, islower(2.659(6) )thanthe average value found for the remaining metalmetal distances(2.687(17) ); this may be due to the weaker sigma-trans inuencewith respect to the carbonyl ligand.5. Hydrido derivativesKnown hydrido derivative of tetrairidiumdodecacarbonylare [H2Ir4(CO)10]2[10], [HIr4(CO)11]2[22], and the neutralorthometalled compound [HIr4(CO)7(Ph2PCH@CHPPh2)(PhC6H4PCH@CHPPh2)] [18] reported by Albano et al. that shows a bridginghydridebetweentwoiridiumatomswithIrAHbondlengthsof1.71and1.76 . Withrespecttosimilarderivatives, itisworthnoting that the deprotonated formof the dppmdiphosphineligand, bis(diphenylphosphino)methanide[(Ph2P)2CH], hasbeenused for its ability to behave as a two-, four- or six electrons donor[23]. Infact, simpledeprotonationof dppmligandwithabaseaffords the preparation of new hydride iridium cluster derivatives[24]. The reaction underlined below (Eq. (9)) is carried out with anexcess of KOH dry powder in dichloromethane at 20 C and givesthe product with 76% yield:Ir4CO10l-dppm 2KOH PPNCl! PPNIr4CO9l3-Ph2P2CH KCl KHCO39Since in the IR spectrum there are no bands due to bridging car-bonyls, this complex shows in solution and solid state a symmetrywith only terminals CO ligands. At 173 K the13C NMR shows thatthe apicals CO are already slowly exchanging and at 203 K the onlyuxionalmechanismobservedarisesfromtherotationofapicalcarbonyls.Thecrystal structureof [PPN][Ir4(CO)10(l3-(Ph2P)2CH)]showsCs symmetry, with all terminals CO and with the ligand [(Ph2P)2-[NEt4][Ir4(CO)11X]+ (N-N)-AgX-30C[Ir4(CO)11{1-(N-N)}]-1/2Ir4(CO)10{2-(N-N)}+ 1/2Ir4(CO)12 +1/2 (N-N)R.T.Scheme 3. Synthesis of diamine derivatives of tetrairidium dodecacarbonyl. (N-N)= 1,10-phenantroline (1); 4,7-dimetylphenantroline (2); 5,6-dimetylphenantroline(3); 3,4,7,8-tetrametylphenantroline (4); 2,20-dipiridine (5); 4,40-dimetyl-2,20-dipir-idine (6).aa bcdcee gdLLFig. 1. Proposed structure for the diamine derivatives. (L-L = N-N).Fig. 2. X-ray structure of Ir4(CO)11(1,10-phentroline) 1.94 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102CH]face bridging withthe plane P(2)ACAP(3) eclipsedwithrespecttotheplaneIr(3)AIr(2)AIr(4). TheIrAIrdistance(meanvalue = 2.700(1) ) is shorter than the one found for Ir4(CO)7(l-CO)3-(l-(Ph2P)2CHMe) (2.729(1) ). This nding has been sup-ported by other structural data, conrming that the CO-unbridgedmetal bondsareshorter thanthosebridged[25]. Likewise, thehydrido-cluster [PPN][HIr4(CO)9(l-dppm)] is obtained in 85% yieldaccordingtothefollowingreaction, (Eq. (10)) byusingalargeexcessof1,8-diazobicylo[5,4,0]undec-7-ene(DBU)asbaseunderCO atmosphere in wetCH2Cl2at 20 Cand [Ir4(CO)10(l-dppm)]as starting material [24]:Ir4CO10l-dppm DBU H2O PPNCl! PPNHIr4CO9l-dppm CO2 DBUHCl 10Also for this hydrido compound the geometry in solution and inthe solid state have been found to be the same, with three bridgingCO, the remaining CO terminal and the hydride ligand coordinatedin axial position. Interestingly, in the1H NMR spectrum the meth-ylene CH2 protons signals of dppm originate an ABX2 spin systemdue to the inequivalence of the two protons. While the proton HBshows a chemical shift of 2.69 ppm, the HAis observed at6.03 ppm, indicating a strongly deshielded nucleus. This cluster isuxional insolution. TherstCOscrambling process takesplaceat200 Kandinvolvesthebasalcarbonylsa, b, dandf;asecondprocesstakesplaceat220 Kandinvolvestherotationof apicalcarbonyls e and g.The crystal structure of the hydrido-cluster [PPN][HIr4(CO)9(l-dppm)] showsCssymmetry, withthreebridgingCOonthebasal plane and the bidentate diphosphine ligand taking two axialpositions. Themeanvalueof theIrAIr distance(2.769(3) ) islonger than that reported for the [PPN][Ir4(CO)10((Ph2P)2CH)],while the IrAH distance (2.08(6) ) is longer than those found formonometalliccomplexes[24]. Theexactlocationofthehydrideligand in the complex could not be successfully dened byconventional X-raydiffractionanalysis. Consequently, aneutrondiffraction experiment was performed at the Institute LaueLangevininGrenoble[26]. Fig. 4illustratestheoutcomeof thisexperiment. TheIrAHdistancefound is1.618(14) anditistherst experimental determination of an iridium cluster. Comparisonwith the value above reported of 2.08(6) proves the latter to beincorrect and, conrms the predictive power of ab initiocalculations [26], and, at the same time, highlights once more thelimitsof conventional X-raydiffractionanalysisindeningtheposition of light atoms in the proximity of heavy ones.Asdescribedabove, Ir4(CO)10(l-dppm)quicklyreactswithanexcess of KOHto give [Ir4(CO)10(l-(PPh2P)2CH)], which in turn con-verts into the decarbonylated anion [PPN][Ir4(CO)9(l3-(PPh2P)2CH)],whereasthehydrido-derivativeIr4H(CO)9(l-dppm)isobtainedifthe same reaction is carried out with an excess of DBU. Detti andco-workers [27] further studied this reaction in dichloromethanewithanexcess of DBUandPPNCl, usingdifferent phosphinesand arsines, such as: bis(diphenylphosphino)methane, 1,1-bis(diphenylphosphino)ethane, 1,2-bis(diphenylphosphino)ethane, 1,3-bis(diphenylphosphino)propane and bis(diphenylarsino)meth-ane. All the corresponding hydrido-complexes (12, 13, 14, 15, 16,respectively) were obtained with more than 75% yield. TheproposedmechanismrequiresthenucleophilicattackofOHonthe metal carbonyl, as in Scheme 5:Onthecontrary, thereactionofIr4(CO)10(l-dppmMe)carriedout in presence of an excess of DBU, but without PPNCl, the hydr-ido-compound(13a)(75%yield, seeScheme6)with[DBUH]+ascounterionwas obtainedtogether witha secondaryderivative(13b, 1%) with [DBUMe]+. Using Ir4(CO)10(L) (L = dppm, dppe, dppp,dpam) as starting materials and the same reaction conditions usedforIr4(CO)10(l-dppmMe) doesnot leadtotheformationof theanalogous hydrides-anions. Theexplanationappears tobethatthe nucleophilic attack by such a strong base as DBU on a diphos-phinic chain produces a lack of a site and this is related with theweakacidityofthemethyl group[28]. Thecompounds(1216)showthecharacteristicIRbandsinthebridgingCOregion. The31P {1H} NMR spectra have only one signal for the diphosphine, likethe starting complexes. The hydrido ligand is located in axial posi-tion and presents a single1H NMR signal at low eld (15 ppm).Finally, the low-temperature13C{1H} NMR spectra obtained fromenriched compounds show the typical pattern of carbonyls.Themolecularstructuresofcompounds(13)withcounterion[DBUH]+and [DBUMe]+respectively, and (14, 15) with [PPN]+areillustratedinFig. 5. All structures showthe diphosphinic andhydrideligandsinaxial positionwithrespecttotheIr1AIr2AIr3plane that also accommodates three bridging CO units.La abc cd de egfa abc cd de eghL6 65L1 1223 34 4A B CScheme 4. Possible arrangements for amine ligands.Fig. 3. ORTEP view of the complex Ir4(CO)11(4-methylpyridine) 8.A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102 95The investigation of structural data highlights that the hydrideligandhasastrongertrans-inuencecomparedtotheCOgroup.As for thebond distances,the Ir3AIr4 distanceis longer than theremainingIrAIrbondsandalsolongerthanthosefoundinthestartingcompounds. Incontrast, theIrAPdistancesareshorterthan those observed in Ir4(CO)10(l-dppmMe) and Ir4(CO)10(l-dppp). As expected, thedeterminationof hydridebondlengthproveddifcult[26]. TheIrAHbondlengthis1.32(5) anditisshorter thanthat foundfor [HIr4(CO)9(l-dppm)](1.618(14) )by neutron diffraction.6. Cyclic mono and dioxycarbene derivativesTassan and co-workers [29] have reported the synthesis and theinvestigationoftheuxional dynamicsandtheX-raymolecularstructuresofaseriesof newdioxycarbenecompoundsobtainedfromIr4(CO)11(L)(L = PtBu3(17), PPh3(18,19)andIr4(CO)10(LAL)(LAL = Ph2PCH2PPh2 (20), norbornadiene (21) and 1,5-cyclooctadi-ene(22,23)). ThestartingphosphinederivativeIr4(CO)11(PtBu3)was obtained by reacting Ir4(CO)11(norborn-2ene) [15] with a stoi-chiometric amount of tri(ter-butyl)phosphine (PtBu3) in dichloro-methane. The31P{1H}NMRspectrumshowsasingleresonancefor phosphine at 65.9 ppm. The values of the coordination chemicalshift (Dd = dcoord. dfreephosphine) [13] of 2.6 ppm suggests that thephosphine lies in axial position. This idea is supported by the pres-ence in the13C{1H} NMR spectrum of two bands in the radial COdomain, one of which, f, Scheme 7, shows a coupling of 8.1 Hz withthe phosphorus atom, and, inthe apical ligands eld, 27.1 Hzpseudo-trans-coupling of CO g with the same atom.ThereactionofIr4(CO)11(PtBu3)withoxirane2-bromoethanolandsodiumbromideas catalyst leads totheformationof themonocyclic dioxocarbene derivative Ir4CO10PtBu3COCH2CH2O. The IR spectrumof this complex shows the presenceof three bands (at 1862, 1819 and 1795 cm1) due to bridging COthat are typical of complexes having a ground state C3v symmetry.The31P{1H} NMR at 230 K exhibits two resonances,d = 62.29 and64.56 ppm, duetotwodifferentisomers, AandB(ratio = 28:72;17; see Scheme 7); the latter may be separated by TLC.Fromvalues of the calculatedcoordinationchemical shifts,Dd = 1.3 and 1.0 ppm, it is possible to infer that in isomer A thephosphineandcarbeneligandsarebothinaxial position, whileinisomer Btheter-butyl is inaxial andthecarbeneinradialposition. Likewise, the13C{1H} NMR spectrum in CD2Cl2 at 230 Kpresent two sets of signals. Those relating tomajor isomer B areFig. 4. Left: ORTEP drawing of the [HIr4(CO)9(l-dppm)] (12) obtained from X-rays diffraction. Right: Structure obtained from neutron diffraction.IrCOH2O -H+Ir COOHIr H +CO2Scheme 5. The formation of hydride-derivatives by nucleophilic attack of OH onmetal carbonyl.CHCH3PPIrIr+ dbuPPIrIrC CH3 CH(CH3)PPIrIrPPIrIrC H CH2PPIrIr-dbuH+-dbuCH3++H++H+75 % 1 %Scheme 6. The formation of hydride-derivatives with two possible types of direct attack of dbu.96 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102identied by the carbene chemical shift at 212.04 ppm (COO groupinradial position); the198.91 ppmvalueof theminority(28%)isomerindicatesthatinthiscasethecarbeneoccupiesanaxialposition. The molecular structure of 17Ashows that the fouriridiumatoms denearegular tetrahedronandthephosphineand carbene ligands are axially bonded to two vicinal Ir atoms ofthe basal plane. The values of the dihedral angles between the tet-rahedronbaseontheplaneIr1AIr2ACO12, Ir1AIr2ACO13andIr2AIr3ACO23[7.7(5), 0.7(7)and2.4(6), respectively]suggestanasymmetrical bridgingof the COunits. The reactionof Ir4(CO)11(PPh3) [13] with a large excess of oxirane, NaBr and 2-bro-moethanol givescompounds18and19with37and40%yield,respectively. Thethreebands at 9.68,10.10and20.36 ppm(42:39:19 ratios) in the31P{1H} spectrum of18 at 183 Kidentifythreepossibleisomers, 18A18C. Theresonancesat 9.68and10.10 ppmwereassignedtoPPh3inaxial position(18Aand18B) becausetheylooklikethestartingcomplex(dax = 11.08forIr4(CO)11(PPh3));thetwoisomersdifferforcarbeneposition,as17Aand17Babove. Theresonanceat20.36 ppmiscoherentwitha radial coordinationof thePPh3moietyandbelongs toisomer18C(seeScheme7). Whenthe31P{1H}NMRspectrumiscollectedat 310 K, theabovethreeresonances coalesceintoabroadsignal, anindicationthat theisomersundergostructuralrearrangement according to merry-go-round and change ofbasalface ofCO. Afurtherconrmationoftheexistenceofthethree isomers 18A18C is given by the 13C{1H} spectrum. The lattershows three separate sets of resonances with 42:39:19 ratios, eachone with eleven resonances in the areas typical of bridging and ter-minal CO.For the dioxycarbene (19) the31P{1H} NMR spectrum at 183 Khasthreesignalsat 7.08, 9.81and19.71 ppmwith55:34:11ratios, thus indicating the existence of three isomeric forms19A19B19C (see Scheme 8). The Dd (0.2 and 2.9 ppm) suggestan axial coordination of PPh3 (19A, 19B) and the value of 26.6 ppmFig. 5. (A,B) Molecular structures of [HIr4(CO)9(dppmMe)] 13a with [DBUH+] and [DBUMe+] 13b as counterion, respectively; (C,D) molecular structures of [HIr4(CO)9(dppe)]14 and [HIr4(CO)9(dppp)] 15, respectively, both with [PPN]+as counterion.Lbab'cd' de egf *Cc'bab'cd' de' egL *CLbab'c*C de' egf c'17A18A18C 17B18BScheme 7. Possible arrangements of ligands. L = PtBu3 (17A17B), PPh3 (18A18B),*C = COCH2CH2O.A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102 97aradial-coordinatedofPPh3(19C). Thelow-temperature13C{1H}spectrumis similar tothat obtainedpreviously(for compound18), indicatingthepresenceof threeedge-bridging, tworadialandthreeapical COunits. Ontheother hand, thetwosignalsrelatedtoCOOAshowthatinisomer19Atheyholdoneradialandoneaxial positionontwoseparateIratoms;inisomer19Bthey also are placed in radial and axial positions, but on the samebasal iridium atom; nally, in the minority isomer 19C the phos-phine is radial, and both COOA groups take axial positions.The compound 20, where the phosphine is dppm, can be pre-pared in a similar manner to that reported for PtBu3. The31P spec-troscopic data show two signals at 51.14 and 57.25 ppm with23/77 ratio, compared with d = 52.2 ppm for the starting cluster.These data are consistent with the diphosphine being coordinatedin axialaxial positions [13]; hence, the two isomers differ only bythe position of the COOA group. Scheme 9 and Fig. 6 show the X-raycrystal structure, conrmingtheresultsofthespectroscopicanalysis. The cluster has Cs symmetry, with the four iridium atomsdening a regular tetrahedron, three bridging CO on the basal faceIr1AIr2AIr3 and the carbene and diphosphine ligands in axial posi-tions. The mean IrAP distance of 2.300(3) is in agreement withknown data [13,18,30].Also the carbene derivatives obtained with olenic ligands (nor-bornadiene, 21, and1,5-cyclooctadiene22, 23) canbereacting[Ir4(CO)11Br]withsuitableolenandthecomplexeshavebeencharacterized bymeansofIRand13CNMRspectroscopy. Cluster21presentstwoisomerswith89:11ratios, wherethecarbeneligand binds to an axial and to a radial position, respectively. Com-pounds22and23areobtainedwith50%and23%yield, respec-tively. The IR spectra of both clusters show bands of bridging andterminal CO. The13CNMRspectrumof compound22at 200 Kshows two sets of resonances (10 signals, relative intensities18:82). Compound 23 shows three sets of signals. The rst is givenbytwocarbenesholdingaradial andanaxial positionontwoseparate Ir atoms; the second refers to a couple of carbenes againplacedinradial andaxial positions, but onthesameIr atom;nally, thethirdoneindicatestwoaxially-bondedcarbenesontwo separate basal iridium atoms.7. PhosphitesThereactionofanionicclusters[Ir4(CO)11Br]withphosphiteligandssuchasphenyl-dimethoxyphosphine, diphenyl-methoxy-phosphine anddiphenyl-phenoxyphosphine have beeninvesti-gatedbyDetti et al. [30,31]. Thebromideis displacedbyoneequivalent of phosphite at room temperature, giving the monosub-stituted products [Ir4(CO)11{L}] [L = PPh(OMe)2 24; PPh2(OMe) 25andPPh2(OPh)26]. Anexcessofligandaffordsthedisubstitutedcompounds[Ir4(CO)10{L2}][L = PPh(OMe)227;PPh2(OMe)28andPPh2(OPh) 29]. The monosubstitutedcomplexes 2426canbeobtained with 3560% yield. The IR spectra collected in dichloro-methane solution show two m(CO) stretching bands below1900 cm1, indicatingthepresenceof bridgingCOligands. The31P{1H}spectraobtainedat195 KinCD2Cl2solutionshowonlyone resonance, suggesting the presence of a single isomer. Besides,the13CO-enriched(ca. 30%)13CNMRspectraof all compoundspoint tothepresenceof twoaxial, threebridging, threeradialandthreeapical carbonyl groups, indicatingthat thephosphitecoordinates through an axial position.The crystal structure of 26 and the selected labeling scheme areshown in Fig. 7. The molecule contains a nearly tetrahedral Ir4 core,with three CO units bridging to the basal face and with the phos-phiteligandsinaxial position. Thepresenceofagoodrdonorsuch as the diphenyl-phenoxyphosphine makes the Ir4AIr2, Ir4AIr3distances (mean 2.755 ) longer than the Ir1AIr2, Ir1AIr3andc a'ab*C he' egfc a'ab*Ch*Ce' egfcha'abde' eg*C19B 19A 19CC*PPh3PPh3PPh3*CScheme 8. Arrangements of ligands in the dicarbene derivatives.Ph2PPPh2bbbh fg egf *Cc Ph2PPPh2bbbfg egf*C20A 20BScheme 9. Structure of Ir4(CO)9(dppm)(COCH2CH2O)Fig. 6. ORTEP plot (30% of probability) of compound 20.Fig. 7. ORTEP view of the molecular structure of [Ir4(CO)11{PPh2(OPh)}] 26. Thermalellipsoids at 50% probability.98 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102Ir2AIr3ones(mean2.707 ). TheIrACdistancesarecomparablewiththoseof theotheriridiumclusters[32]. TheIrAPdistanceof 2.301(2) is greater thanthat foundinIr4(CO)11{P(OMe)3}(2.258 ), but shorterthanthoseobservedforphosphines(thatis, 2.311 for PMe3 [32], 2.335 for PPh3 [33]). As expected, thedifferences of the bond distances between phosphites andphosphines derive from the high p-withdrawing character of thephosphites with respect to the phosphines.Thethermodynamicparameters for theisomerizationA MBhave been determined integrating the31P{1H} NMR signalsrecorded at variable temperature (185300 K) in toluene-d8 solu-tion, because in this solvent the two isomers are present in similarproportions. Inthe185210 Ktemperaturerangetheexchangebetween the two populations is slow and it is possible to calculatethe rate constant Keq = [A]/[B] at different temperatures. The linearregression of logKeq versus 1/T allows to determine the differenceenergy between thetwo isomers. Thecalculated thermodynamicparameters are: DHeq = 2.132 0.155 kJ mol1, DSeq = 0.014 0.005 kJ K1mol1, DGeq = 1.970 0.155 kJ mol1.The variable-temperature (190300 K)13C{1H} NMR spectra inCD2Cl2 solution of compound A, the only isomer formed in this sol-vent (Scheme 10), was carriedout to investigate its uxionalbehavior. Byanalyzingthe spectra obtainedbetween190and230 K, it was possible to identify only merry-go-round processesof basal CO groups (bridging and radial, Scheme 10B). In the 230300 K range, it was not possible to dene the other two processes:face exchange and rotation of apical carbonyls, because the peakshinting atthetwo processes wereoverlapped (see Scheme10A),besides, above 300 K the compound decomposed. A simulation ofthe NMR spectra by means of the Exchange program [34] allowedto calculate the activation energy of the process at severaltemperatures.By using the Eyring linear regression equation we found for thisprocess: DG= 44.3 0.8 kJ mol1at 298 K; DH= 37.3 + 0.8 -kJ mol1;DS= 23.6 3.5 J K1mol1. InTable1, thevaluesofthe calculated activation energies for the merry-go-roundprocess are comparedwiththose experimentally obtainedforsimilarcompounds. Thesedataindicatetheeffect of theligandbulk, that is, the effect of increasing the angle between the basalplane and the iridium-carbonyl bond.The infrared spectra of compounds (2729) collected indichloromethane solution show two m(CO) stretching bands below1900 cm1, indicating the presence of bridging carbonyl ligands inall complexes. The31P{1H} spectra have been carried out in CD2Cl2solution at 195 K and reveal two resonances due of the radial andaxial phosphorous. In addition, the analysis of13CO-enriched (ca.30%)13CNMR spectra ofall compounds point to thepresence oftwoaxial, threebridging, tworadial andthreeapical carbonylgroups, indicatingthat twophosphiteunitscoordinatethroughan axial and a radial position.Thecrystal structuresof27and29andtheselectedlabelingschemes are shown in Fig. 8. The two molecules contain a nearlytetrahedral Ir4 core with three CO units bridging to the basal faceand withthe phosphiteligands in axial andradial positions. TheaverageIrAIrdistancefor27is2.724 , avalueconsistentwiththosefoundforrelatedcompoundssuchasIr4(CO)10{P(OMe)3}2,2.728 , andIr4(CO)10(PPh3)2, 2.739 [33], andalsointheIrAIrdistance range of dodecacarbonyl derivatives, but longer than thatof Ir4(CO)12 (2.693 ). The Ir2AIr3 bond (2.702(10) ) (see Fig. 8) isconsiderably shorter than Ir2AIr4 (2.7399(7) ) and Ir3AIr4(2.7419(6) ). Moreover, as observed for Ir4(CO)10(PPh3)2, the dis-tancesbetweentheiridiumatomsofthebasalplanandtheoneinapical position(Ir1) are all different: 2.7367(6) (Ir2AIr1),2.7159(6) (Ir4AIr1) and 2.7089(6) (Ir3AIr1). The IrAP distancesfor P4 (radial) and P2 (axial), are 2.262(2) and 2.251(2) , respec-tively. They are shorter than those found in bis-diphenylphosphinoderivatives [33], because the two AOCH3 groups make the ligand agood p-accepter.Themetalmetal bonddistancesinthebasal plane(Ir2AIr3;where the bound phosphorus atoms are located) for 29 are longerthan the other (2.770 versus 2.755 Ir3AIr4, and 2.762 Ir2AIr4),andtheIr1AIr4areshorterthantheother(2.735versus2.7678Ir1AIr3 and 2.7624 Ir1AIr2).8. Intramolecular dynamics of [Ir4(CO)12] derivativesMost studies on the uxional behavior of the tetrahedral clusterof iridium covers the migration ofcarbon monoxide. Thismigra-tion has been described, in particular, with the models developedbyCotton[35,36]andbyJohnsonandBeneld[37,38]. Therstisnamedmerry-go-round anddescribestheexchangeofsitesaroundthemetal backbone; thesecondis calledLPM, LigandPolyhedralModel, anddescribestheexchangeoftheCOsiteasthe result of a rotation (or libration) of the metallic skeleton withinthe envelope of the ligands whose donor atoms form the vertices ofapolyhedronwhichcandeform(icosahedral Manticubeoctahe-dral Micosahedral, for example).Therstexperimental evidence, IRandNMR, of themerry-go-round process has been obtained by the Roulets group duringthestudiesof [Ir4(CO)9(l3-1,3,5-trithiane], wheretheunbridgedisomer (A), which is in general the transition state of themerry-go-round, was foundbothinsolidstateandinsolution(see Fig. 9) [39].Overtheyears, alotofmonosubstitutedtetrairidiumderiva-tiveswithCssymmetryofgeneral formula[Ir4(CO)11L](L = PEt3,PAr3, PMePh2, PHPh2, PH2Ph2, PPh3, P(OMe)3, P(OPh)3, etc.)havebeen investigated [40,2]. Roulet and co-workers further deepenedthe studies about the intramolecular dynamics of iridium carbonylclusters by analyzing the solution and the solid state behavior ofbidentatedonorligands, suchas:1,1-bis-(methylthio)ethane30,ethylidenebis(diphenylphosphine) 31 and propane-1,3-diyl-bis(diphenylphosphine)] 32 [41]. The [Ir4(CO)10(l2-(MeS)2CHMe)](30) hasagroundstategeometrywithonlyterminal COunits;on the contrary, compounds 31 and 32 show three edge-bridgingCO groups, both in solution and solid state.The crystallographic analysis for compound 30 shows a tetrahe-dral metal core of Cs symmetry and only terminal CO ligands andthe S-atoms in axialaxial positions; it is one of the few Ir clustersab be egPPh2(OPh)d dfc ce egPPh2(OPh)d dfc cd dA B Scheme 10. Two-isomer equilibrium for compound 26.Table 1Activation parameters at 298 K; h = Tolmans cone angle [4].L h(deg) DG(kJ mol1)PPh3145 45.6 0.4PPh2(OPh) 139 44.3 0.8PPh2(OMe) 132 42.8 0.8P(OMe)3107 37.5 0.4A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102 99without any bridging CO. The CAO bond lengths are in the typicalrange for terminal CO groups. Interestingly, complex (30) can existintwoconformations(AandB). Uponcoordinationtotheaxialpositions, the ligand forms a ve-member ring, where C1 may stayapartfromtheIr1AIr2AIr3planeorliebeneath. Ofcourse, eachoneof theseconformersmayalsohavetwoisomers(aandb),dependingonwhethertheMegroupboundtoC1isalsoplacedaway or under the Ir1AIr2AIr3 plane (Scheme 11).As mentioned above, complexes 31 and 32 both have a struc-turewiththreebridgingCOonthebasal triangleIr1AIr2AIr3,andadiphosphineligandboundtoaxial positions(seeFig. 10).Compound31has anAa conformation, withthe CAMe bondroughly parallel to the Ir1AIr2AIr3 triangular face.Complexes 31 and 32 both have Cs symmetry, but while for 31thephenylmoietiesP(1)and(P2)arenotrelatedbysymmetry;complex32showsamirrorplanepassingthroughIr3AIr4andIr1AIr2 bond. The reason of the difference between the two com-plexes both, must be ascribed to intramolecular steric effects andadifferent hydrogenbondingnetwork, whichhasalreadybeendescribed [42].The1H NMR spectrum of compound 30 shows one quartet andone doublet relative to HACAMe that indicate the conformation ofthe coordinated ligand; moreover, the presence of a singlet for thetwo SAMe shows the mirror symmetry of the complex. The13CO-enriched (30%)13C{1H} NMR spectrum in CD2Cl2 at 177 K ofcompound30presentssixresonancesforterminal COunitsat:167.7 (a), 164.9 (b), 164.1 (c), 158.8 (d), 157.2 (g) and154.3(e) ppm, withrelativeintensities2:2:1:2:2:1. The2D-EXSYFig. 8. ORTEP view of the molecular structure of 27 and 29. Thermal ellipsoids at 50% probability.a aab be eebg ggc cc cc cS SSS SSA BFig. 9. Ir4(CO)9(l3-1,3,5-trithiane in solution, unbridged (A) and bridged (B) isomer.Ir4Ir3Ir1XIr2XC1HCH3HCH3abConformers AIr4Ir3Ir1XIr2XC1H3CHCH3HabConformers BScheme 11. The two conformation A and B with the two possible isomers a and b (X = SCH3, PCH3).100 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91102spectrumin CD2Cl2at 215 K shows one intense cross-peak,between 164.9 and 158.8 ppm, indicating the dynamic connectiv-ityb Ma Md(seeFig. 11), andoneless intenseat 164.1and158.8 ppm indicating c Md exchange; a third g Me exchange pro-cess takes place at 270 K. The exchange b Ma Md corresponds tothe merry-go-round of the six CO groups about the Ir1AIr2AIr3triangular face; the second and third exchanges involve only twosites, that is, those arising from the rotation of three CO residingonthemirror planeIr3andIr4. Thefreeactivationenthalpiescalculated by Eyring linear regression equation at 298 K are:DG1= 42.6 0.4, DG2= 47.0 0.4 and DG3= 58.0 0.8 kJ mol1.The cluster 31 has the same geometry both in solution and inthesolidstate. The13CO-enriched(30%)13C{1H}NMRspectrumof compound 31 shows seven resonances at: 223.5 (a), 203.8 (b),179.5(f), 171.3(d), 163.8(c), 162.2(e) and157.7(g) ppm, withrelative intensities 1:2:2:1:1:1:2. The 2D-EXSY spectrum of 31 inTHFat 215 Kis similar tothat of 32[18]. Thelowest energyprocess, again, the merry-go-round one, involves with rateconstant k1 the a, b,f and d CO units. At 247 K, the signal for COc starts to broaden (rate constant k2), while at 287 K emerges theexchange between CO g and e. The free activation enthalpies, calcu-latedbyEyringlinearregressionequationare:DG1= 38.7 0.4,DG2= 50.4 0.4 and DG3= 60.1 0.6 kJ mol1.The unobserved intermediate of merry-go-round incomplexes 31 and 32 has a geometry with all CO in terminal posi-tions. It may be that the transition state for the merry-go-roundprocess has a semi-bridged geometry, as already reported [43]. Theenergybarrierof merry-go-round processforcompound31isFig. 10. 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