Investigations on the Coupling of Ethylene and Alkynes in Tp Me2 Ir Compounds: Water as an Effective Trapping Agent

1 FULL PAPER DOI: 10.1002/chem.200((……)) Investigations on the Coupling of Ethylene and Alkynes in TpMe2Ir Compounds: Water as an Effective Trapping Agent Eleuterio Álvarez,[a] Margarita Paneque,*[a] Cristina M. Posadas,[a] Manuel L. Poveda,*[a] Nuria Rendón,[a] and Kurt Mereiter[b] Abstract: The reaction of the bis(ethylene) complex TpMe2Ir(C2H4)2 (1) (TpMe2 = hydrotris(3,5- dimethylpyrazolyl)borate) with 2 equiv. of dimethyl acetylenedicarboxylate, MeO2CCCCO2Me (DMAD), in CH2Cl2 at 25 ºC gives the hydridealkenyl species Tp Me2 IrH(C(R)=C(R)C(R)=C(R)CH=CH 2 ) (2, R = CO2Me) in high yield. A careful study of this system has established the active role of a number of intermediates in route to 2. The first of these is the Ir(I) complex TpMe2Ir(C2H4)(DMAD) (4) formed by substitution of one of the ethylene ligands in 1 by a molecule of DMAD. 4 reacts further with another equivalent of the alkyne with formation of the unsaturated metallacyclopentadiene “ Tp Me2 Ir(C(R)=C(R)C(R)=C(R)) ” which can be trapped by added water to give adduct 7, or react with the C2H4 present in solution generating complex 2. This last step has been shown to proceed by insertion of ethylene into one of the Ir—C bonds of the metallacyclopentadiene and subsequent -H elimination. Complex 1 reacts sequentially with 1 equiv. of DMAD and 1 equiv. of methyl propiolate, HCCCO2Me (MP), in the presence of water, with regioselective formation of the unsymmetrical iridacyclopentadiene Tp Me2 Ir(C(R)=C(R)C(H)=C(R))(H 2 O) (9). Complex 9 reacts with ethylene giving a hydride-alkenyl complex 10, related to 2, in which the C2H4 has inserted regiospecifically into the Ir— C(R) bond that bears the CH functionality. Heating CH2Cl2 solutions of either 2 or 10 allows the formation of the allyl species 3 or 11, respectively, by simple stereoselective migration of the hydride ligand into the C alkenyl carbon and concomitant bond reorganization of the resulting organic chain. All the compounds described herein have been characterized by microanalysis, IR, and NMR spectroscopy and for the case of 3, 7, 7·CO, 8·NCMe, 9, 9·NCMe and 10, also by single crystal X-ray diffraction studies. Keywords: C-C coupling · iridacyclopentadienes · C2H4 insertion · C-H activation · water complexes Introduction Metallacyclopentadienes[1] are very interesting organometallic species which are involved, as intermediates, in a number of important catalytic or stoichiometric processes mediated by transition metal complexes, as is the case of the cyclotrimerization of alkynes (to give benzene derivatives),[2] alkynes-nitriles (for the synthesis of pyridines),[3] alkynes-olefins (formation of cyclohexadienes),[3,4] etc.[1h,5] In this contribution, we report on the coupling of two molecules of DMAD (dimethyl acetylenedicarboxylate, MeO2CC≡CCO2Me) in the TpMe2Ir system (TpMe2 = hydrotris(3,5-dimethylpyrazolyl)borate),[6] to give an iridacyclopentadiene[7] which completes the 18-electron metal count by coordination of a molecule of water. Substitution and insertion reactions of this complex have been investigated, as well as the synthesis and reactivity of a related unsymmetrical iridacyclopentadiene that results from the regioselective coupling of DMAD and MP (MP = methyl propiolate, HC≡CCO2Me). Part of this work has been published in preliminary form.[8] Results and Discussion The addition of 2 equiv. of DMAD to a solution of the bis(ethylene) complex TpMe2Ir(C2H4)2 (1)[9] in CH2Cl2, at room temperature, produces immediate consumption of 1 and formation of a new compound, 2, as deduced from the NMR spectra of the reaction mixture [Eq. (1)]. The reaction is stereospecific, and the spectroscopic yield of this species is higher than 85%. The chelating [a] Dr. E. Álvarez, Dr. M. Paneque, Dr. C. M. Posadas, Prof. Dr. M. L. Poveda, Dr. N. Rendón, Instituto de Investigaciones Químicas and Departamento de Química Inorgánica Consejo Superior de Investigaciones Científicas (CSIC) and Universidad de Sevilla Av. Américo Vespucio 49, Isla de la Cartuja, 41092 Sevilla (Spain) Fax: (+34)954460565 E-mail: [email protected] [b] Dr. K. Mereiter Department of Chemistry, Vienna University of Technology, Getreidemarkt 9/164, A-1060 Vienna (Austria) Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. 2 hydrocarbyl-alkene ligand in compound 2 formally results from the C—C coupling of two molecules of DMAD and a vinyl fragment, which derives itself from one of the ethylene ligands in 1. B NNN NNN Ir H [Ir] = Tp Me2 Ir = CH 2 Cl 2, 25 °C [Ir] R RR R 2 2DMAD [Ir] 1 DMAD = RR (1) H -C 2 H 4 R=CO 2 Me Although compound 2 is the major product in this reaction, attempts to obtain an analytical and spectroscopically pure sample of it failed, due to its slow transformation into a new allyl derivative 3 (see below). Either by crystallization or by column chromatography, species 2 is always isolated as a mixture with 3. Nevertheless, it survives in solution long enough as to be properly characterized by spectroscopy, either in the mixture with 3 or in the crude product from the reaction. The presence of the hydride ligand is shown by a high-field resonance at -16.91 ppm in the 1H NMR spectrum, while the π-coordinated vinyl moiety exhibits the pattern of resonances expected for a ligand of this type:[10] three multiplets centered at 6.39, 3.63 and 3.07 ppm, with trans and cis 3JHH coupling constants of 11.5 and 9.8 Hz, respectively (see Experimental Section for assignments). 13C NMR data and a full set of two-dimension experiments are also in agreement with the structure proposed and, in particular, the NOESY spectrum confirms the coordination of the olefin to the metal through the face indicated. Complex 2 cleanly transforms into the alkyl-allyl 3 when solutions of 2 are warmed at 60 °C [Eq. (2)], and 3 is also obtained, in almost quantitative yield, when the reaction depicted in Eq. (1) is performed in CH2Cl2 at 60 °C. The formation of 3 is the neat result of the stereospecific transfer of the hydride ligand to the α-carbon atom of the alkenyl end of the chelating ligand in 2. Different pathways could in principle be envisaged for this transformation; nevertheless, we propose that it takes place by direct migration of the hydride to the alkenyl carbon, with concerted or stepwise bond rearrangement along the chain. Compound 3 exhibits 1H NMR resonances due to the -coordinated allyl moiety[9] at 7.07 (CH), 4.09 and 2.94 (CH2) ppm, the corresponding C atoms producing signals in the 13C NMR spectrum at 91.2 (1JCH = 170 Hz) and 25.2 ppm (1JCH = 155 and 166 Hz) while the remaining allylic carbon appears at 53.0 ppm. The Ir-bonded alkyl carbon, Ir— C(H)(CO2Me)R resonates at 12.0 ppm (1JCH = 135 Hz). This compound has been additionally characterized by an X-ray diffraction structure analysis (see below). 3 CH 2 Cl 2, 60 °C [Ir] R RR R 2 H [Ir] R RR R H (2) R=CO 2 Me The reaction shown in Eq. (1) has been monitored by NMR, at low temperature. When 2 equiv. of DMAD are added to a solution of 1 in CD2Cl2 at -40 ºC, the 1 H NMR spectrum recorded immediately after mixing shows quantitative formation of a new compound, which has been characterized by NMR, at 0 ºC, as the Ir(I) adduct TpMe2Ir(C2H4)(MeO2CCCCO2Me) (4) [Eq. (3)]. Its 1H and 13C{1H} NMR spectra show local Cs symmetry for the TpMe2 ligand and all the protons and both carbon nuclei of the ethylene ligand are equivalent, generating a singlet in each spectrum (3.14 ppm for 1H and 51.1 ppm for 13C{1H}). The 11B{1H} NMR spectrum shows a singlet at 32.5 ppm, and this value[11] compares well with the corresponding chemical shift recorded for complex 1 (32.9 ppm), which has been shown previously to be a 18-e- species with the TpMe2 ligand κ3 coordinated both in the solid state and in solution.[9] Compound 4 is generated by substitution of one of the ethylene ligands in 1 by a molecule of DMAD, a process that also occurs when this derivative reacts with one equivalent of soft Lewis bases (CO and PR3; R3 = Me3, Me2Ph, Et3).[12] These reactions have been proposed to occur by an associative mechanism, by means of a change on the coordination mode of the TpMe2 ligand, from κ3 to κ2.[12,13] CD 2 Cl 2, -40 °C 2DMAD [Ir] 1 [Ir] 4 R R (3) R=CO 2 Me However, and unlike other derivatives of composition TpMe2Ir(C2H4)(L) (L = CO, PR3)[12] and related Tp species,[13,14] which adopt a rigid trigonal bipyramidal structure with the, slowly rotating in the NMR time scale, ethylene ligand occupying the equatorial position, and hence giving rise to an AA’BB’ spin system in the 1H NMR spectra, compound 4 is fluxional, and its ethylene ligand seems to be fast rotating, even at -40 ºC. This difference can be attributed to the fact that the ethylene ligand is occupying an axial position in the trigonal bipyramid (Fig. 1), where the π back donation would be weaker than when in the equatorial place.[15] The adoption of this structure is also supported by a comparison of the 13C NMR chemical shift of the ethylene ligand in the series of compounds TpMe2Ir(C2H4)(L) (L = PMe3, CO, C2H4 and DMAD) (see Table 1). For the case of L = C2H4, all the ethylene carbon atoms resonate at 26.2 ppm. As already reported, this compound is fluxional, and this signal represents the average of the chemical shifts corresponding to the axial and equatorial positions, and thus, the 13C{1H} NMR spectrum recorded for 1 in the solid state exhibits two signals for the ethylene ligands, at 48 and 5 ppm (axial and equatorial, respectively).[9] Probably, the preference of the DMAD for the equatorial position in 4 is because it is a better -acceptor than the ethylene ligand. Although it can not be deduced from the data available, it is proposed that the DMAD ligand does not rotate around the Ir—DMAD axis, due to its strong π-acceptor character. Finally on this respect, the related Tp derivative TpIr(C2H4)(DMAD)[7c] has also been reported to be fluxional, and the authors suggest a stereochemistry analogous to the one proposed herein for 4. 3 Ir N N N NN N HB CO 2 Me CO 2 Me Figure 1. Proposed structure of complex 4. Table 1. Solution Chemical Shifts for the 13C Nuclei of the Ethylene Ligands in Compounds of Composition TpMe2Ir(C2H4)(L). L PMe3 CO C2H4 DMAD δ C2H4 (ppm) -8.1 0.6 26.2[a] 51.1 [a] 48 and 5 ppm in the solid state. Compound 4 is stable up to 10 °C; above this temperature, and under the conditions of Eq. (3), it evolves to a mixture of species. Monitoring the reaction by 1H NMR at 20 ºC reveals the formation of 2 together with other unidentified species, although they subsequently disappear, and finally 2 is present in ca. 90% spectroscopic yield. As we were unable to directly characterize any of the intermediate(s) species en route to 2, and in order to obtain further information about this system, we decided to seek alternative ways to prepare likely intermediates in the formation of 2. In a recent preliminary communication,[8a] we have reported that the iridacycloheptatriene 6, to be described in full detail elsewhere, is obtained by the reaction of the Ir-dimethylbutadiene derivative 5 with 3 equiv. of DMAD in CH2Cl2 [Eq. (4)]. Interestingly, the presence of a large excess of water (≥ 10 equiv.) in the reaction mixture prevents the incorporation of the third equivalent of alkyne, yielding the water stabilized[16] iridacyclopentadiene derivative 7 [Eq. (5)], that obviously has been formed by the coupling of two DMAD molecules in the metal coordination sphere.[7] As expected, the water ligand present in 7 is labile, as demonstrated by the substitution reactions carried out with NCMe and CO, that yield the corresponding derivatives 7·NCMe and 7·CO. With excess of DMAD, an insertion reaction into one of the Ir—C bonds takes place and compound 6 is formed (Scheme 1). All these metallacyclopentadienes have been fully characterized by spectroscopy (see Experimental Section) and in addition, the solidstate structures of 7 and 7·CO have been determined by X-ray crystallography (see below). [Ir] [Ir] R RR R RR H 2 O CH 2 Cl 2, 60 °C 3DMAD(H 2 O) 6 5 (4) R=CO 2 Me [Ir] [Ir] H 2 O CH 2 Cl 2, 60 °C 3DMAD,10H 2 O 7 5 R R R R (5) R=CO 2 Me L, 60-90 ºC [Ir] L 7·L(L = NCMe, CO) R R R R R=CO 2 Me [Ir] H 2 O 7 R R R R [Ir] R RR R RR H 2 O 6 DMAD C 6 H 12 100 ºC, 18 h Scheme 1. Synthesis of complex 7 and its reactivity with Lewis bases and DMAD. More interesting, in the present context, is the reaction of 7 with ethylene which, at 60 ºC, results in the clean formation of the already described compound 3 [Eq. (6)], supporting the intermediacy of an iridacyclopentadiene species in the formation of compound 2 (or 3) starting from 1 (or 4). Furthermore, if ethylene is bubbled through a solution of 7 in CDCl3, and the mixture kept at room temperature, NMR monitoring shows the slow formation of a new species 8 (Scheme 2). Compound 8 is stable at room temperature and forms metastable solutions in chloroform from where it eventually precipitates and, after dissolution in acetone-d6, could be appropriately characterized by NMR. The more characteristic signals for this compound are those corresponding to the two adjacent CH2 groups: multiplets at 3.37, 2.90 (IrCH2CH2) and 2.71, 2.17 (IrCH2CH2) ppm in the 1H NMR spectrum and resonances at 35.5 (IrCH2CH2) and -2.6 (IrCH2CH2) ppm in the 13C{1H} NMR spectrum. Probably, compound 8 is formed via displacement of water by a molecule of ethylene to give A (Scheme 2), followed by the insertion of this ligand into one of the equivalent Ir—C bonds. It is worth of mention the facility with which the ethylene inserts into the Ir—C bond of the presumed intermediate A. In the case of the related, isolable, compound with the unsubstituted Tp ligand,[7c] no insertion was observed up to 110 ºC. This is in agreement with the known decreased reactivity of the TpIr compounds, as compared with the related TpMe2Ir ones, in activation processes. In the TpMe2Ir system we have observed cases in which C2H4 easily inserts into an Ir—C bond, for example in its reaction with TpMe2Ir(C6H5)2(N2),[17] but also found complexes that are unreactive in this respect, as the iridacyclopentene T p Me2 I r ( CH 2 C ( Me ) =C ( Me ) CH 2 ) ( C 2 H 4 ) which, upon heating, dissociates the coordinated ethylene;[18] or the very reactive TpMe2Ir(H)(CH=CH2)(C2H4), which has been shown to yield a C4 chain by the coupling of the two C2 ligands, although by following a mechanistic pathway different from the insertion of ethylene into the Ir—C bond.[9] The water ligand in 8 is also labile and the corresponding adducts 8·NCMe and 8·CO are easily obtained [Eq. (7)]. These complexes have been fully characterized by spectroscopy and, in addition, by an X-ray study carried out with 8·NCMe (see below). Finally and as expected, heating solutions of compound 8 at 40 °C promotes its transformation into the allyl derivative 3 [Eq. (8)]. 4 [Ir] H 2 O 7 R R R R CH 2 Cl 2, 60 °C C 2 H 4 3 [Ir] R RR R H (6) R = CO 2 Me [Ir] H 2 O 7 R R R R20 °C C 2 H 4 [Ir] H 2 O R R 8 R R [Ir] R R R R A H 2 O CDCl 3 R = CO 2 Me Scheme 2. Formation of the alkyl-allyl derivative 3 and trapping of an intermediate as the water adduct 8. 8 L,20-60 ºC [Ir] H 2 OR R R R 8·L (L =NCMe, CO) [Ir] LR R R R R=CO 2 Me (7) 83 40 °C d6-acetone [Ir] H2OR R R R [Ir] R RR R H (8) R = CO2Me Compounds 7 or 8 were not detected during the course of the transformation depicted in Eq. (1), and this negative evidence is probably due to the absence of enough H2O in the reaction mixture. In fact, if the reaction is carried out in the presence of an excess of H2O, compound 7 is formed in almost quantitative spectroscopic yield [Eq. (9)] and, despite of the presence of the evolved ethylene in the reaction flask, compound 2 is not observed. The larger amount of water vs. ethylene competes effectively for the coordination to iridium, preventing the incorporation of the olefin. CH 2 Cl 2, 25 °C 2 DMAD, 10 H 2 O [Ir] 1 [Ir] H 2 O 7 R R R R (9) R = CO 2 Me Considering all the observations commented so far, the mechanism depicted in Scheme 3 can be proposed for the formation of compounds 2 and 3, in which compounds 7 and 8 have also been included. From the experimental data we cannot say which of the intermediates A or B (B is depicted as an unsaturated 16-e- Ir(III) species rather than as having a 18-e- bis(carbene) Ir(I) structure[2c,7b]) is formed directly from 4 (in both cases we presume that the second molecule of DMAD enters the coordination sphere in an associative process, as is the norm in substitution reactions of Tp’Ir(I) species). If B is formed first, then there is a competence for coordination to this intermediate, of the evolved ethylene (to give A) and the water present in the reaction mixture (to give 7). On the contrary, if A is the favored kinetic species, the coordinated ethylene would be dissociatively exchanged by H2O under the reaction conditions, before C2H4 insertion into one of the Ir—C bonds of the metallacycle takes place. The facility with which this insertion takes place, and the observation that, under certain conditions, the iridacycloheptatriene 6 is observed in this reaction (see below), and other data obtained in our laboratory in systems related to this one, to be discussed elsewhere, suggest the initial formation of intermediate B. Whichever is the first species formed, it is clear that it is the result of the oxidative coupling of two molecules of DMAD bonded simultaneously to iridium, and this process is likely governed by the high tendency of the Tp’Ir(I) derivatives to oxidize to Ir(III), and more importantly by the mutually cis disposition adopted by these ligands in the purported intermediate that also contains a fac-type TpMe2-Ir linkage. This is in agreement with the experimental and theoretical studies carried out with the system [Ir(PR3)3(alkyne)2]+, which indicate that the phosphines have to occupy a fac disposition for the coupling of the two alkynes to take place.[7b] Recently, it has been reported that a bis(ethylene)iridium(I) derivative stabilized by a mer tripodal nitrogen donor ligand, reacts with DMAD giving rise to a stable bis(alkyne)iridium(I) complex, even in the presence of NCMe.[19] [Ir] 1 DMAD [Ir] R R[Ir] R R R R [Ir] H 2 O R R R R DMAD H 2 O 1 < 10 °C 47 > 10 °C 40 °C C 2 H 4 -H elim. H-migr. C 2 H 4 ins. -C 2 H 4 [Ir] R R R R -H 2 O 3 [Ir] R RR R H [Ir] R R R R B A (?) -C 2 H 4 DMAD > 10 °C (?) [Ir] R RR R 2 H C [Ir] H 2 O R R 8 R R H 2 O -H 2 O R = CO 2 Me Scheme 3. Mechanism proposed for the formation of complex 3 from the bis(ethylene) derivative 1 and DMAD with inclusion of all the intermediate species isolated. 5 In order to complete this study, some deuteration experiments have been carried out. First, we have prepared, at low temperature, complex 4 by addition to 1 of only one equivalent of DMAD, to add afterwards an excess (≥ 6 equiv.) of DMAD-d6. After warming at 60 °C, compound 3-d6 is formed and the corresponding 1H NMR spectrum exhibits resonances for the four CO2Me groups of intensity half of the observed in compound 3 [Eq. (10)]. This indicates that 4 does not interchange with free DMAD, and also reflects the symmetry of the intermediates A and B. Secondly, we have found that two equivalent CO2Me groups present in 7 can be selectively replaced by CO2CD3 by a transesterification process in CD3OD, catalyzed by acid. Although not confirmed, it is highly probable, on steric grounds, that the deuterated positions are those shown in Scheme 4. When 7-d6 is subjected to reaction with C2H4, no scrambling of the labels is observed, as two CO2Me resonances are absent in the 1H NMR spectra of compound 3-d6, probably those shown in Scheme 4. This experiment rules out any interconversion between the iridacyclopentadiene and a bis(alkyne)Ir(I) species [Eq. (11)] and this is in accord with the findings observed in a related system.[7a] 3-d 6 CD 2 Cl 2, 60 °C >6 DMAD-d 6 [Ir] 4 R R [Ir] R-d 1.5 R-d 1.5 R-d 1.5 R-d 1.5 H (10) R = CO 2 Me [Ir] H2O 7 R R R R [Ir] H2O 7-d6 R R-d3 R-d3 R CD3OD H+ 3-d6 [Ir] R R-d3 R-d3 R H C2H4 60 ºC R = CO2Me Scheme 4. Partial deuteration of complex 7 by a transesterification reaction in CD3OD. [Ir] H 2 O 7 R R R R (11) [Ir] R R R R D R = CO 2 Me Mention has to be done to the different reactivity observed when the reaction of compound 1 is carried out, in solution at 60 ºC, with 3 equiv. of DMAD, and where, under the appropriate conditions, the iridacycloheptatriene 6 is generated. This result is almost independent on the solvent employed (C6H12, CH2Cl2 or CHCl3), but strongly depends on the conditions of the reaction. Thus, if it is carried out in deuterated chloroform or dichloromethane (in cyclohexane the starting material is not soluble enough), in a NMR tube, compound 3 is formed in almost quantitative spectroscopic yield (>90%). By contrast, if the reaction is performed, starting with the same amount of compound and solvent in a much bigger sealed flask (hence with much bigger atmosphere volume), both 3 and 6 are formed, in ca. 1:1 ratio. This seems to indicate that the iridacyclopentadiene intermediate B of Scheme 3 is less reactive towards DMAD than towards ethylene (as mentioned before, the H2O adduct 7 reacts with C2H4 at 25 ºC, while its reaction with DMAD requires much higher temperatures), and this may be due to the lesser tendency of the Ir(III) intermediate (already electron deficient) to bind the alkyne, less electron-donating than ethylene. At this point it seems appropriate to make another comparison with the related system of the unsubstituted Tp ligand, TpIr(C2H4)2- DMAD (Scheme 5).[7c] In this case, the first step is also the formation of the Ir(I) adduct Tp(C2H4)(DMAD), a compound that is stable at room temperature, and that when heated in acetonitrile or other solvents like THF forms an iridacyclopent-2-ene, by oxidative coupling of ethylene and DMAD. This coupling is not observed in the case of the corresponding TpMe2 derivative 4, which, as already mentioned, decomposes at >10 ºC in the presence of 1 equiv. of C2H4 and 1 equiv. of DMAD to give 2 (in the absence of DMAD 4 evolves by a vinylic C—H activation process to be reported elsewhere). If DMAD is present, the Tp-iridacyclopentene structure can be transformed, under appropriate conditions, into an iridacyclopentadiene analogous to our proposed intermediate A. As already mentioned, this compound is very stable and highly reluctant to experience ethylene insertion into the Ir—C bonds of the metallacycle, no transformation taking place even at 100 ºC, this fact highlighting once more the higher reactivity of the TpMe2 derivatives in comparison with the Tp ones. Ir R R R R Ir O R R TpIr R R THF 60 ºC Tp Tp DMAD C 6 H 6 , 60 ºC toluene-d 8 100 ºC, 13 h no reaction R = CO 2 Me Scheme 5. Formation of an iridacyclopent-2-ene and an iridacyclopentadiene in the system TpIr-C2H4-DMAD. We have also studied the reaction of compound TpMe2Ir(C2H4)(DMAD) (4) with methyl propiolate (MP), a terminal alkyne. In principle, a reactivity similar to that observed for DMAD could be advanced, although the asymmetry of MP may give rise to two different coupled regioisomers. Also, the presence of the terminal C—H bond in MP could promote C—H activation reactions.[20,21] The sequential addition to 1, at low temperature, of 1 equiv. of DMAD and 1 equiv. of MP in the presence of added water (10 equiv.), produces selectively, upon warming to room temperature, the iridacyclopentadiene 9 [Eq. (12)] in which the CH of the MP occupies a  position in the ring, as deduced by its 1H NMR chemical shift of 7.75 ppm (in this kind of Ir-complexes an alkenyl CH in  position would give a signal further downfield, even at 10 ppm).[20] As expected, the water ligand in 9 is labile, and the compounds 9·NCMe and 9·CO are readily synthesized by its 6 reaction with an excess of L [Eq. (13)]. The new iridacyclopentadienes have been fully characterized by microanalysis and spectroscopy (IR, 1H and 13C NMR studies) and in addition, the solid-state structure of 9 and 9·NCMe have been determined by X-ray crystallography (see below). [Ir] H2O 9 R H R R 1 DMAD [Ir] 1 [Ir] 4 R R THF -20 °C 1 MP, >10 H2O THF25 °C (12) -20 ºC R = CO2Me [Ir] H 2 O 9 R H R R [Ir] L 9·L (L=NCMe, CO) R H R R L, 25 ºC R=CO 2 Me (13) Complex 9 reacts with ethylene, at room temperature, with the regio- and stereoselective formation of the hydrido-olefin derivative 10 (Scheme 6). In this case, the formation of this kind of species seems to be very favorable, since the purported intermediate iridacyclohexadiene related to 8 has not been observed, even in the presence of excess of added water. Sequential addition of DMAD and MP to compound 1, in the absence of water, at low temperature, followed by stirring at 25 ºC for 15 min, also yields complex 10, in almost quantitative yield. Unlike 2, 10 is very stable, can be properly purified, and it has been completely characterized, including an X-ray structure determination (see below), which firmly establishes that the insertion of the ethylene has taken place regioselectively into the Ir—C bond of 9 adjacent to the C—H functionality. In accord with the high stability of this derivative, isomerization to the corresponding allyl derivative 11 requires quite forcing conditions [Eq (15)]. The NMR data obtained for this compound are in agreement with the structure depicted, but the compound experiences some kind of fluxional process, probably conformational in origin, which is responsible for the broadening of some of the resonances, in both 1H and 13C{1H} NMR spectra (see Experimental Section). 11 [Ir] R RH R H [Ir] H 2 O 9 R H R R [Ir] R RH R 10 H C 2 H 4 CH 2 Cl 2 25 °C CH 2 Cl 2 80 °C 24 h R = CO 2 Me Scheme 6. Consecutive formation of complexes 10 and 11 from the iridacyclopentadiene 9 and C2H4. X-Ray diffraction studies Table 2 presents crystal data and data collection details for all compounds analyzed in this section. Complex 3: Figure 2 shows an ORTEP view of a molecule of 3 while Table 3 gives selected bond lengths and angles. The sp3 carbon atom bonded to iridium forms an iridium-carbon bond with a length of 2.12 Å, a value typical of an Ir—C single bond.[22] The η3- allyl—Ir interaction is characterized by Ir—C bond distances of Ir— C(1), 2.16; Ir—C(2), 2.10 and Ir—C(3), 2.18 Å and the C(1)— C(2)—C(3) angle of 121.4º is clearly in accord with almost pure sp2 hybridization at the C(2) carbon. INSERT TABLE 2 HERE Figure 2. X-ray structure of complex 3 (thermal ellipsoids drawn at the 50% probability level). Table 3. Selected bond lengths [Å] and angles [º] for complex 3 Ir—N(6) 2.214(2) Ir—C(2) 2.098(2) Ir—C(3) 2.181(2) C(5)—C(6) 1.507(3) Ir—C(1) 2.164(2) C(3)—C(4) 1.482(3) Ir—N(2) 2.139(2) C(2)—C(3) 1.442(3) Ir—C(6) 2.118(2) C(1)—C(2) 1.411(3) Ir—N(4) 2.106(2) C(4)—C(5) 1.366(3) N(2)—Ir—N(6) 87.61(7) C(1)—C(2)—C(3) 121.4(2) N(4)—Ir—N(2) 90.68(7) C(2)—C(3)—C(4) 116.4(2) N(4)—Ir—N(6) 80.62(7) C(5)—C(4)—C(3) 118.5(2) C(6)—Ir—C(1) 87.48(9) C(4)—C(5)—C(6) 117.4(2) C(6)—Ir—C(3) 79.86(8) C(5)—C(6)—Ir 110.06(15) C(1)—Ir—C(3) 69.86(9) C(2)—C(3)—Ir 67.21(12) 7 Complexes 7 and 7·CO: In Figures 3 and 4, the molecular structures of complexes 7 and 7·CO are represented, while Table 4 collects selected bond lengths and angles. The first compound crystallized with ca. 1.25 molecules of additional water which form hydrogen bonds with the Ir—OH2 moiety. The iridacyclopentadiene units in 7 and 7·CO are almost planar, characterized by C—Ir—C bite angles of 79.3 and 78.6º, respectively, and all the Ir—C bond distances fall between 2.00 and 2.06 Å as expected for sp2 carbons.[22] Interestingly, for complex 7 the Ir—N(pyrazolyl) bond trans with respect to the hard water ligand is shorter (2.03 Å) than the other two (2.16 Å av.) but this effect is rather diminished in 7·CO. Figure 3. X-Ray structure of complex 7 (thermal ellipsoids drawn at the 50% probability level). Figure 4. X-Ray structure of complex 7·CO (thermal ellipsoids drawn at the 20% probability level. Table 4. Selected Bond Lengths (Å) and Angles (deg) for Complexes 7 and 7·CO. 7 7·CO Ir—N(12) 2.171(5) 2.151(3) Ir—N(22) 2.157(5) 2.128(3) Ir—N(32) 2.035(4) 2.111(3) Ir—C(42) 2.001(6) 2.055(4) Ir—C(52) 2.037(6) 2.067(4) Ir—L 2.091(3) 1.852(4) C(42)—C(43) 1.368(8) 1.358(6) C(43)—C(53) 1.463(9) 1.463(5) C(52)—C(53) 1.375(8) 1.355(6) C(42)—Ir—C(52) 79.3(2) 78.64(16) C(42)—Ir—L 90.19(19) 85.45(17) C(52)—Ir—L 87.72(18) 84.99(17) Complex 8·NCMe: Figure 5 shows an ORTEP view of this compound and Table 5 collects selected bond lengths and angles. The Ir—C(alkenyl) distance (2.02 Å) compares well with those commented for the iridacyclopentadienes 7 and 7·CO while the corresponding Ir—CH2 is longer at 2.09 Å. In turn, this later bond is slightly shorter than the Ir—C(alkyl) present in complex 3 which supports a CO2Me substituent. Of the three Ir—N(pyrazolyl) bonds, the one trans with respect to the NCMe is shorter (2.05 Å) than the other two (2.16 Å av.), and in that way the acetonitrile behaves like the H2O ligand, another hard donor, in 7. 8 Figure 5. X-Ray structure of complex 8·NCMe (thermal ellipsoids drawn at the 20% probability level. Table 5. Selected bond lengths [Å] and angles [º] for complex 8·NCMe Ir—N(12) 2.164(4) C(41)—C(42) 1.545(6) Ir—N(22) 2.157(4) C(42)—C(52) 1.496(6) Ir—N(32) 2.049(3) C(53)—C(62) 1.477(6) Ir—C(41) 2.091(4) C(52)—C(53) 1.368(6) Ir—C(63) 2.025(4) C(62)—C(63) 1.354(6) Ir—N(71) 1.976(3) N(71)—C(72) 1.133(5) N(22)—Ir—N(12) 84.47(13) C(62)—C(63)—Ir 127.6(3) N(32)—Ir—N(12) 88.30(13) C(42)—C(41)—Ir 117.3(3) N(32)—Ir—N(22) 89.85(16) C(52)—C(42)—C(41) 110.6(4) C(63)—Ir—C(41) 94.02(17) C(53)—C(52)—C(42) 121.8(4) N(71)—Ir—C(41) 92.68(18) C(52)—C(53)—C(62) 122.4(4) N(71)—Ir—C(63) 90.91(16) C(63)—C(62)—C(53) 124.4(4) Complex 9: The structure of this complex is shown in Figure 6, while selected bond lengths and angles are collected in Table 6. Once again the iridacyclopentadiene ring is almost planar and the Ir—N(pyrazolyl) bond trans to the H2O is shorter (2.04 Å) than the other two (2.15 Å av.). As commented in section Results, the Ir— C(alkenyl) bond that supports the CH functionality is more reactive against C2H4 insertion than the other similar one present in this molecule. From the corresponding bond distances, 2.04 vs. 2.02 Å, it may be concluded that the first bond is slightly weaker than the second and this may be the reason for the different reactivity. However, we have found that methyl propiolate inserts into the ring of 9 with the opposite regioselectivity.[23] Figure 6. X-Ray structure of complex 9 (thermal ellipsoids drawn at the 40% probability level). Table 6. Selected Bond Lengths (Å) and Angles (deg) for complex 9. Ir—N(1) 2.142(3) Ir—O(1) 2.103(2) Ir—N(3) 2.041(3) C(16)—C(17) 1.363(5) Ir—N(5) 2.155(3) C(17)—C(18) 1.464(4) Ir—C(16) 2.021(3) C(18)—C(19) 1.357(4) Ir—C(19) 2.039(3) C(16)-Ir(1)-C(19) 79.45(12) C(16)-Ir(1)-N(1) 177.72(11) C(16)-Ir(1)-N(3) 93.49(11) C(19)-Ir(1)-N(1) 99.04(11) C(19)-Ir(1)-N(3) 96.87(11) O(1)-Ir(1)-N(1) 89.46(10) C(16)-Ir(1)-O(1) 88.74(11) C(16)-Ir(1)-N(5) 97.52(12) C(19)-Ir(1)-O(1) 86.28(11) C(19)-Ir(1)-N(5) 172.68(11) N(3)-Ir(1)-O(1) 176.43(10) O(1)-Ir(1)-N(5) 86.99(10) 9 Complex 9·NCMe: An ORTEP view of this molecule is shown in Figure 7 and selected bond length and angles are collected in Table 7. These are very similar to those found in 9 and will not be commented with the exception of the two Ir—C(alkenyl) bond distances. Interestingly, and in comparison with 9, for the carbon that supports the CH group the Ir—C distance is much longer (2.03 Å) than the other one (1.97 Å). Figure 7. X-Ray structure of complex 9·NCMe (thermal ellipsoids drawn at the 30% probability level). Table 7. Selected bond lengths [Å] and angles [º] for complex 9·NCMe Ir—N(22) 2.158(5) Ir—N(61) 1.991(5) Ir—N(12) 2.157(5) C(41)—C(53) 1.410(9) Ir—N(32) 2.044(4) C(52)—C(53) 1.364(8) Ir—C(42) 2.033(5) C(41)—C(42) 1.351(8) Ir—C(52) 1.974(7) N(61)—C(62) 1.144(7) N(12)-Ir-N(22) 82.98(17) C(52)-Ir-C(42) 77.1(2) N(32)-Ir-N(12) 89.91(17) C(53)-C(52)-Ir 118.6(5) N(32)-Ir-N(22) 88.38(18) C(41)-C(42)-Ir 114.6(4) N(61)-Ir-C(42) 87.8(2) C(42)-C(41)-C(53) 115.8(6) C(52)-Ir-N(61) 87.3(2) C(52)-C(53)-C(41) 111.5(6) Complex 10: The molecular structure of this compound is shown in Figure 8, while selected bond length and angles are collected in Table 8. The Ir—C(41) bond distance at 2.02 Å is in the expected range for an Ir—C(alkenyl) species while, for the olefin bonded to Ir, the C(45)—C(46) separation of 1.49 Å is much closer to the corresponding for a single C—C bond (1.54 Å) than for a double one (1.34 Å), and this probably reflects a quite strong Irolefin bond. Figure 8. X-Ray structure of complex 10 (thermal ellipsoids drawn at the 40% probability level). Table 8. Selected Bond Lengths (Å) and Angles (deg) for complex 10 Ir—N(12) 2.193(2) Ir—H(1I) 1.600 Ir—C(45) 2.180(3) C(44)—C(45) 1.495(4) Ir—N(22) 2.162(2) C(42)—C(43) 1.460(4) Ir—C(46) 2.138(3) C(45)—C(46) 1.391(5) Ir—N(32) 2.090(2) C(41)—C(42) 1.363(4) Ir—C(41) 2.015(3) C(43)—C(44) 1.343(4) N(22)-Ir-N(12) 87.79(9) C(46)-Ir(1)-C(45) 37.56(13) N(32)-Ir-N(12) 88.19(9) C(42)-C(41)-Ir 129.1(2) N(32)-Ir-N(22) 82.82(9) C(44)-(45)-Ir 116.8(2) C(41)-Ir-H(1I) 94.7(14) C(41)-C(42)-C(43) 123.4(3) C(41)-Ir-C(46) 92.93(13) C(44)-C(43)-C(42) 127.1(3) C(41)-Ir-C(45) 90.64(12) C(43)-C(44)-C(45) 126.0(3) C(45)-Ir-H(1H) 69.5(10) C(46)-C(45)-C(44) 123.0(3)