Coupling of Aromatic Aldehydes with CO2Me-Substituted TpMe2Ir(III) Metallacyclopentadienes

1 Coupling of Aromatic Aldehydes with CO 2Me- Substituted TpMe2Ir(III) Metallacyclopentadienes Arián E. Roa, †,‡ Verónica Salazar, ‡,* Joaquín López-Serrano, † Enrique Oñate, ∫ José G. Alvarado- Rodríguez, ‡ Margarita Paneque, †,* and Manuel L. Poveda † † Instituto de Investigaciones Químicas (IIQ) and Departamento de Química Inorgánica, CSIC and Universidad de Sevilla, Av. Américo Vespucio 49, 41092 Sevilla, Spain ‡ Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Hidalgo, Carretera Pachuca a Tulancingo Km 4.5, 42184 Mineral de la Reforma, Hidalgo, México ∫ Departamento de Química Inorgánica and Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza and CSIC, 50009 Zaragoza, Spain RECEIVED DATE ABSTRACT: The fully CO 2 Me-substituted aquo-iridacyclopentadiene 1 reacts with a variety of aromatic aldehydes, at 90-120 ºC, with formation of bicyclic Fischer-type carbenes, generated by the transfer of the aldehydic H atom to a α-carbon of the metallacycle and concomitant bonding of the O- atom to the adjacent β-carbon. These carbenes have a thermodynamically favored anti configuration of these C―H and C―O bonds but it is proposed that an unobserved syn carbene is the kinetic primary product, which then easily epimerizes by adventitious water. Milder reaction conditions, 25-60 ºC, allow for the isolation of intermediate O-coordinated aldehyde adducts. While these reactions have been observed for a wide variety of aromatic aldehydes, 2-pyridinecarboxaldehyde behaves differently as the 2 reaction leads to a very stable N-adduct, in spite of two isomeric O-bonded adducts being formed as kinetic products,. INTRODUCTION Aldehydes are very important substrates in synthetic organic chemistry, as they can experience different types of transformations. The simple, normally metal-mediated, aldehyde decarbonylation 1,2 (R—C(O)H → R—H + CO) is receiving increasing interest as more additional examples of the coupling of this process with different reactions are being developed. 3 Many more examples besides this report the use of aldehydes in other processes such as their dimerization to esters (Tishchenko reaction), 4 the preparation of biofuels from carbohydrate feedstock, 5 asymmetric synthesis, 6 the application as CO releasing molecules (CORM’s), 7 etc. Rh(I) and Ir(I) complexes are well known for their ability to effect, either stoicheiometric or catalytically, the decarbonylation of aldehydes. 8 If Ir(III)-R species are considered it has been found that, provided that there is a vacant coordination position easily available, their reaction with aldehydes afford carbonyl-Ir(III) complexes. This is best exemplified by the work of Bergman et al. in a cationic (C 5 Me 5 )Ir system 9 which is summarized in eq 1. (1) In the Tp Me2 Ir(III) system, we have reported on a similar behavior on the reaction of the diene derivative Tp Me2 Ir(CH 2 =C(Me)C(Me)=CH 2 ) with aliphatic and aromatic aldehydes, eventually yielding the Ir(III) carbonyl species A shown in Scheme 1, in a reaction that takes place first by formation of the 16 e - unsaturated Ir(III) species shown in the Scheme and then through a series of kinetic bicyclic Fischer carbene "intermediates" to be commented in more detail below. 10 3 Scheme 1. Reactivity of Tp Me2 Ir(CH 2 =C(Me)C(Me)=CH 2 ) with aldehydes (refer. 10). Throughout the paper, [Ir] represents Tp Me2 Ir (Tp Me2 = hydrotris(3,5-dimethylpyrazolyl)borate). However, a different outcome was observed on the reaction of the complex Tp Me2 Ir(C 2 H 4 ) 2 with the same substrates, in which, through the participation of the 16 e - unsaturated Ir(III) species shown in Scheme 2, an interesting coupling took place. In this case, the aldehyde experienced a nucleophilic attack of an alkenyl ligand, with the eventual formation of an elaborated chelating ligand. 11 Scheme 2. Reactivity of Tp Me2 Ir(C 2 H 4 ) 2 with aldehydes (refer. 11). Very recently, we have described the thermal reactions of the fully CO 2 Me substituted Ir(III) metallacyclopentadiene 1 with aliphatic aldehydes RC(O)H (R = Me, t Bu), which result on the decarbonylation of the organic substrate as depicted in Scheme 3. These findings were rationalized on the basis of a reversible abstraction-donation of the aldehydic hydrogen of the corresponding O-adduct by a α-carbon of the iridacycle (Scheme 3). 12 4 [Ir] E E H2O 1 E E C O H R [Ir] E E CO E E H R E = CO2Me [Ir] E E EE [Ir] E E O E E H OR H R R = Me, tBu - 1 · C O Scheme 3. Reaction of 1 with aliphatic aldehydes (refer. 12). In this contribution, we report on the very different reactivity that complex 1 shows against aromatic aldehydes, which lead to an interesting set of Ir(III) Fischer carbenes that resemble those obtained as intermediates in the reaction of Scheme 1 (see below). RESULTS AND DISCUSSION Formation of bicyclic Fischer carbenes The reaction of the iridacycle 1 13 with benzaldehyde (1.5 equiv., C 6 H 6 , 120 ºC) leads to the bicyclic compound 2a in almost quantitative spectroscopic (NMR) yield (eq 2). (2) As can be observed, 2a is the result of an interesting coupling of the iridacyclopentadiene, after extrusion of water, with the aldehyde. The latter functionality has been transformed into a Fischer carbene ligand with its oxygen being bonded to one of the β-carbons of the original metallacycle and with the aldehydic hydrogen being stereoselectively transferred to the adjacent α carbon, anti with respect to the formed O—C bond. Species 2a has been completely characterized by spectroscopy 5 methods including single crystal X-ray diffraction studies. Inspection of both the 1 H and 13 C NMR spectra immediately reveals the lack of the 2:1 symmetry exhibited by the starting material 1 and thus independent resonances are observed for the nuclei of each of the pyrazolyl arms of the Tp Me2 ligand. In the 1 H NMR spectrum, the most characteristic resonance is a singlet at 5.24 ppm that corresponds to the Ir-CH(E)- proton that derives from the aldehydic H atom. In the 13 C{ 1 H} NMR the supporting sp 3 carbon of this proton resonates at 33.6 ppm ( 1 J CH = 138 Hz) while the carbene carbon gives rise to a signal at 252.3 ppm. Figure 1 shows an ORTEP representation of the molecular structure of 2a, together with some selected bond distances and angles. The carbene ligand has a bond distance Ir—C(16) of 1.929(5) Å while the other two Ir—C bonds have lengths of 2.107(5) Å (Ir—CH(E)-C(E)-) and 2.031(5) Å (Ir— C(E)=C(E)-) respectively, which fall within the expected range for these types of single bonds. 14 FIGURE 1. X-ray structure of compound 2a (50% displacement ellipsoids, H atoms except the H atom on C(23) omitted for clarity). Selected bond lengths (Å) and angles (deg): Ir(1)—C(16) 1.929(5), Ir(1)— C(26) 2.031(5), Ir(1)—C(23) 2.107(5), Ir(1)—N(1) 2.160(4), Ir(1)—N(3) 2.160(4), Ir(1)—N(5) 2.167(4), C(25)—C(26) 1.342(7); C(16)—Ir(1)-C(26) 86.3(2), C(16)—Ir(1)—C(23) 79.0(2), C(26)— Ir(1)—C(23) 77.7(2). 6 Similar reaction outcomes as the one depicted in eq 2 take place with a variety of aromatic aldehydes. Thus, anisaldehyde, p-dimethylaminobenzaldehyde, p-nitrobenzaldehyde, and two thienylaldehydes provide the related compounds 2b-f in excellent spectroscopic yields (eq 3). (3) Probably due to steric reasons, the rotation around the C–Ar bond of the aromatic residue on the carbene ligand is slow, at 25 ºC, on the NMR time scale. For the case of the thienyl-derived compounds only one of the two possible rotamers is present in CDCl 3 solution, and this is the one with the sulfur pointing away from the Tp Me2 ligand. Interestingly in all the carbenes studied, of the four non-equivalent CO 2 Me groups, the methyl resonances appearing at lower field in both the 1 H and 13 C{ 1 H} NMR spectra are broadened. This broadening also affects to the corresponding CO 2 Me nuclei and for that reason they remain unobserved in the 13 C{ 1 H} NMR spectra but can be detected by a weak cross peak in the 1 H- 13 C HMBC spectrum. This behavior is best explained by invoking that the particular CO 2 Me group populates two, or more, conformers that, at ambient temperature, are not in the rapid exchange regime on the NMR time scale, but no efforts have been devoted to either clarify this issue or to positively identify the responsible CO 2 Me group. Electron withdrawing substituents in the aldehyde favors the coupling reaction of eq 3 and thus we have found that 1 and p-NO 2 -C 6 H 4 C(=O)H react in C 6 D 6 solution at 90 ºC, under pseudo-first order conditions (25 mM, 5 equiv. aldehyde), to give 2d with t 1/2 of ca. 30 min, while p-OMe-C 6 H 4 C(=O)H does so, at 120 ºC, with t 1/2 of ca. 1 h. As expected, and following this trend, the more powerful electron-donating NMe 2 substituent retards the coupling even more (120 ºC, t 1/2 of ca. 18 h). Heating 7 compounds 2 at a temperature higher than needed for their formation (150 ºC) only results in a slow decomposition to a complex mixture of unidentified products, at least for the compounds tested, 2a and 2d. In comparing the reactivity, against aromatic aldehydes, of complex 1 and also of the Tp Me2 - iridacyclopentene commented on in the Introduction, two questions arise. First, in route to the carbonyl derivatives Tp Me2 Ir(Ar)(CH 2 C(Me)=CMe 2 )(CO) (A), bicyclic compounds B related to 2 were isolated as "intermediate species" (Scheme 4) where the aldehydic hydrogen was added in a syn disposition with respect to the formed O―C bond and this may be called a "normal" transfer of this hydrogen; so, why this stereochemistry is not observed in species 2? [Ir] Ar C O H[Ir] C O Ar H (a) [Ir] E E Ar C O H E E [Ir] E E C E E H O Ar (b) [Ir] R CO A B [Ir] E E C E H E O Ar [H2O] 2C Scheme 4. Comparison of the bicyclic carbene structures obtained in reference 10 (a) and in this work (b). DFT calculations are clearly in accord with the fact that the anti stereoisomer 2 is more stable than the unobserved syn addition product C (by 6.9 kcal·mol -1 ) and we propose that this species is the primary reaction product, although one that experiences an easy C→2 epimerization by adventitious water acting as a basic transfer catalyst. Evidence for this mechanism was found by heating complex 2a in C 6 D 6 containing CD 3 OD, where complete deuteration of the CH(CO 2 Me) functionality was observed after 1 h at 120 ºC. 8 The second question concerns the lack of a clean decarbonylation of carbenes 2, under forced conditions. In comparison, this process was observed for derivatives B, 10 and more remarkably it was also the case when the same precursor 1 reacted, at 120 ºC, with aliphatic aldehydes (acetaldehyde and t Bu-aldehyde) where the Ir-carbonyl 1·CO and the corresponding alkane were formed 12 (see Schemes 1 and 3 in the Introduction). We have tried to understand this dichotomy through the use of DFT calculations to be described below. Related metallacycles also experience this type of coupling. Thus, the benzo-annelated iridacyclopentadiene 1(C 6 H 4 ) 15 provides the corresponding carbene upon reacting with p-anisaldehyde (eq 4), while the three-CO 2 Me-substituted derivative 1(H) 16 does the same upon reacting with p-NMe 2 - C 6 H 4 C(=O)H (eq 5). Once again, in both complexes the aldehydic hydrogen appears in an anti relationship with respect to the formed O―C moiety. (4) (5) As can be observed, formation of complex 2(H)c implies the selective binding of the aldehydic oxygen atom to the β-carbon that bears the hydrogen atom. This regioselectivity is accompanied by an increase of the reactivity of complex 1(H) with respect to 1 as 2(H)c is formed ca. one order of magnitude faster than the corresponding 2c. This may be the result of the H transfer being subjected to 9 less steric pressure, but also because the attacked Ir-C(E)=C(H)- moiety is less electronically poor. This latter factor complements the reactivity results obtained upon changing the nature of the aldehyde (see above) Compounds 2(C 6 H 4 )b and 2(H)c have been completely characterized from spectroscopic data and, in the case of 2(C 6 H 4 )b, by a single-crystal X-ray structure determination (Figure 3). In this complex the Ir(1)—C(1), Ir(1)—C(16) and Ir(1)—C(10) bond lengths (1.918(6), 2.021(6) and 2.137(6) Å respectively) are in the range expected for Ir-carbene, Ir-alkenyl and Ir-alkyl functionalities. 17 The C— Ir—C bite angles are 87.3(2)° (C(1)—Ir(1)—C(16)), 78.7(2)° (C(1)—Ir(1)—C(10)) and 77.5(2)° (C(10)—Ir(1)—C(16)) for the different metallacycles. Figure 3. X-ray structure of compound 2(C 6 H 4 )b (50% displacement ellipsoids, H atoms except the H atom on C(10) omitted for clarity). Selected bond lengths (Å) and angles (deg): Ir(1)—C(1) 1.918(6), Ir(1)—C(16) 2.021(6), Ir(1)—C(10) 2.137(6), C(16)—C(15) 1.392(9), C(15)—C(9) 1.501(9); C(1)— Ir(1)—C(16) 87.3(2), C(1)—Ir(1)—C(10) 78.7(2), C(10)—Ir(1)—C(16) 77.5(2). 16 Scheme 6. Structures of the carbenes expected from complex 4 on the basis of the results obtained in this paper (left) and in reference 21 (right). The formation of O-aldehyde adducts described in eq 6 is general for a wider variety of iridacycles. Thus, we have studied their formation from the aquoiridacycloheptatriene 5 22 and eq 8 shows the result of the reaction of this complex with p-dimethylaminobenzaldehyde. The new derivative 6c is easily characterized by NMR; the six CO 2 Me groups of the metallacycle give rise to only 3 signals (6 H each). This shows that the molecule keeps the symmetry plane of the starting material after the exchange water-aldehyde has taken place. The chemical shift of the aldehyde H atom (8.16 ppm) indicates a different chemical environment for this nucleus when compared with compound 3c (9.50 ppm), and it is probable that the presence of the 7-membered metallacycle, which adopts a boat conformation, 23 forces the coordinated aldehyde to adopt the rotameric conformation shown in eq 8, in which the pyrazolyl rings shield this proton. (8) This rotamer is also the preferred one in the solid state and Figure 6 shows an ORTEP view of the molecules of this compound, along with some selected bond distances and angles. As can be observed, the metallacycle features a boat structure with the central C=C bond pointing towards the metal centre as previously observed in related species 22,23 The aldehyde ligand is coordinated to the iridium atom by the 17 oxygen atom and the Ir―O(1) bond distance is a normal one of 2.090(2) Å. Also, the two Ir―C(sp 2 ) bond lengths exhibit values in the expected range (2.044(4) and 2.049(4) Å). Figure 6. X-ray structure of compound 6c (50% displacement ellipsoids, H atoms except the H atom on C(16) omitted for clarity). Selected bond lengths (Å) and angles (deg): Ir(1)―N(1) 2.036(3), Ir(1)―N(5) 2.128(3), Ir(1)―N(3) 2.133(3), Ir(1)―C(30) 2.044(4), Ir(1)―C(25) 2.049(4), Ir(1)―O(1) 2.090(2); C(30)―Ir(1)―C(25) 90.69(16), C(30)―Ir(1)―O(1) 91.29(13), C(25)―Ir(1)―O(1) 94.91(13). Another O-adduct 6g, is formed upon the reaction of complex 5 and 2-pyrrolcarboxaldehyde (eq 9). NMR data once more support the solution structure depicted in eq 9 and it has also been confirmed in the solid state by an X-ray structure determination. Figure 7 shows an ORTEP representation of the molecule, along with some selected bond distances and angles. Because of the similarity of complexes 6c and 6g, no further discussion is required, although the presence of a hydrogen bond between the pyrrol NH group and one of the CO 2 Me oxygen atoms, O(8), can be of some interest. 18 (9) Figure 7. X-ray structure of compound 6g (50% displacement ellipsoids, H atoms except the H atoms on C(1) and N(1) omitted for clarity). Selected bond lengths (Å) and angles (deg): Ir(1)―N(2) 2.120(2), Ir(1)―N(4) 2.136(2), Ir(1)―N(6) 2.036(2), Ir(1)―C(6) 2.029(3), Ir(1)―C(21) 2.041(3), Ir(1)―O(1) 2.0824(19); C(6)―C(9) 1.366(4), C(9)―C(12) 1.486(4), C(12)―C(15) 1.340(4), C(15)―C(18) 1.482(4), C(18)―C(21) 1.349(4); C(6)―Ir(1)―C(21) 90.21(10), O(1)―Ir(1)―C(6) 91.65(9), O(1)―Ir(1)―C(21) 93.58(9). Finally and not unexpectedly in view of the structures of compounds 6c and 6g, these compounds do not transfer the aldehydic H atom to the β-carbon of the metallacycle even when heated at 120 ºC for prolonged periods of time. Note that in the case of adducts 3, the aldehydic H atom always points 19 towards the metallacycle, and this probably facilitates the transfer to one of the double bonds; in the case of compounds 6c and 6g, these H atoms are located far from the metallacycle and the required rotameric conformation is probably too crowded to be accessible. DFT calculated mechanism for the formation of bicyclic Fisher carbenes. In a recent paper on the reactivity of complex 1 towards aliphatic aldehydes 12 we proposed a DFT- based mechanism for the observed decarbonylations, in which aldehyde C―H activation with H migration to a α-carbon of the iridacycle, followed by H back-migration onto the alkyl fragment of the resulting acyl ligand were the key steps in the reaction (Scheme 3). In this paper we report a related study to justify why in this case aliphatic aldehydes yield the bicyclic carbene complexes 2 instead of the expected decarbonylation product. This study uses benzaldehyde and a metallic model systems, in which the CO 2 Me fragments of the iridacycle unit of 1 were replaced by hydrogen atoms. QM:MM methods (ONIOM) were used to model the Tp Me2 ligand with the methyl fragments calculated at the molecular mechanics (Uff) level and the rest of the molecule at the QM level. Figure 8 shows the potential energy profiles for the formation of bicyclic carbenes and the unobserved decarbonylation reaction. Both reactions start by water substitution in 1 to yield the benzaldehyde O-adduct 3 C , which has been chosen as the potential energy origin. The calculations indicate that this step is endoergic by less than 1.5 kcal·mol -1 in the gas phase, which is consistent with the isolation of the O-adducts 3. The next step, C―H activation of benzaldehyde by the metallic fragment, requires formation of an unstable σ-H complex (F C ), and the overall barrier for C―H activation from 3 C amounts 32.0 kcal·mol -1 . When the less sterically demanding Tp ligand was used in the calculations instead of Tp Me2 , the only significant difference in the energy profile was the energy barrier for this step, which now is lower at 27.4 kcal·mol - 1 . This step gives an acyl intermediate (G C ) which can either undergo decarbonylation or C―O coupling to form the bicyclic carbene. In the first case, H back migration from the iridacycle onto the C 6 H 5 moiety of the acyl ligand has a high energy barrier (in excess of 40 kcal·mol -1 ). The second pathway requires rotation of the acyl ligand about the Ir―C bond to place the O atom of the aldehyde pointing towards 20 the metallacycle ring. This rotation has a moderate barrier of 10.8 kcal·mol -1 from G C to give the corresponding rotamer G’ C , which is more stable than the former by 3.6 kcal·mol -1 . The last step in this pathway, C―O coupling to give the bicyclic carbene C C is almost barrier-less and the overall reaction is exothermic by 10.8 kcal·mol -1 . As already discussed the resulting species C C is the product of the syn H addition, while the anti stereoisomers (2) are isolated. Full QM calculations on the real system show that formation of 2a vs. the corresponding syn stereoisomer is thermodynamically favorable by ca. 7 kcal·mol -1 . The results of this study suggest that formation of the bicyclic carbenes is kinetically preferred to decarbonylation. Despite the former is reversible, the latter may be prevented in the case of aromatic aldehydes by the high energy barrier required for H back migration from the metallacycle to the aryl moiety of the acyl intermediates (G C ). It should be said that in our previous work 12 the calculated barrier for decarbonylation of acetaldehyde was almost identical to the reported here for benzaldehyde, while in the case of aliphatic aldehydes the expected decarbonylation reactions were observed. Theoretical results at this level of theory (and with the use of model molecules) can be expected to qualitatively agree with experimental results, and although in this case we cannot justify the absence of decarbonylation in reactions with aromatic aldehydes, a few kcal·mol -1 difference in the overall experimental energy barriers would be enough to explain the different reactivity. 21 Figure 8. Calculated potential energy profiles for the formation of the bicyclic Fischer carbene C (red solid line) and the decarbonylation of benzaldehyde (blue dotted line). The inset shows the calculated transition state for benzaldehyde C―H activation (C and H atoms of the Tp Me2 ligand have been omitted for clarity). CONCLUDING REMARKS Three different iridacyclopentadienes, 1, 1(C 6 H 4 ) and 1(H), react with aromatic aldehydes forming initially, at 25-60 ºC, the O-adducts 3, which under more forcing conditions (90-120 ºC) transform into the bicyclic Fischer carbenes 2. These findings strongly contrast with the reactions of 1 with aliphatic aldehydes which, at 120 ºC and through O-adduct intermediates, lead to the Ir-carbonyl adduct 1·CO and the corresponding alkane. In the latter case no bicyclic Fischer carbenes have been observed while no similar decarbonylation process takes place for the aromatic cases. Surprisingly, 2- 22 pyridinecarboxaldehyde behaves quite differently, forming a very stable N-adduct which eventually yields the carbonyl derivative of Scheme 3 under forcing conditions (150 ºC). EXPERIMENTAL SECTION General considerations. All the manipulations were carried out under an inert atmosphere, following Schlenk techniques. The solvents employed were dried before use. The elemental analyses of the new compounds were carried out at the Microanalytical Service of the Instituto de Investigaciones Químicas (Sevilla) (Perkin Elmer Serie II CHNS/O 2400). IR spectra were recorded at Perkin-Elmer system 2000 FT-IR (KBr). NMR instruments were Bruker modes DPX-300, DRX-400 and DRX-500. 1 H and 13 C resonances were referenced with respect to SiMe 4 using the residual protio solvent peaks as internal standard ( 1 H NMR) and the characteristic resonances of the solvent 13 C nuclei ( 13 C NMR). Most of 1 H and 13 C assignments were based in mono and bidimensional experiments ( 13 C{ 1 H}-gated, COSY, NOESY, 1 H - 13 C HMQC and HMBC). For a series of very similar compounds, only some of them were analyzed or investigated by NMR in full. Compounds 1, 1(C 6 H 4 ), 1(H), and 5 were prepared by the procedures described in the literature. Synthesis of the benzaldehyde derivative 2a. To a stirred suspension of compound 1 (0.100 g, 0.126 mmol) in C 6 H 6 (3 mL), freshly destilled benzaldehyde (38.4 µl, 0.37 mmol) was added and the mixture was stirred for 12 h at 120 °C. After this period of time, the solvent was evaporated under vacuo, and the residue was subjected to column chromatography (silica gel) with a mixture of diethyl ether:hexane (10:90) as eluent. Yield: 0.08 g, ca. 70% yield. From a diethyl ether solution of this solid, a yellow crystalline sample of analytical purity was obtained by cooling at -20 ºC. 23 1 H NMR (CDCl 3 25 °C): δ (ppm) 8.30 (d, 1 H, 3 J H-H = 8.0 Hz, H 15 ), 7.52 (t, 1 H, 3 J H-H = 7.3 Hz, H 13 ), 7.26 (t, 1 H, 3 J H-H = 7.7 Hz, H 14 ), 6.93 (d, 1 H, 3 J H-H = 7.9 Hz, H 11 ), 6.84 (t, 1 H, 3 J H-H = 7.7 Hz, H 12 ), 5.84, 5.64, 5.59 (s, 1 H each, CH pz ), 5.24 (s, 1 H, H 7 ), 3.95, 3.66, 3.24, 3.22 (s, the first broad, 3 H each, CO 2 Me), 2.48, 2.44, 2.43, 2.41, 1.35, 0.95 (s, 3 H each, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 252.3 (C 9 ), 180.9, 172.8, 166.5, 162.4 (CO 2 Me), 154.6, 152.1, 150.5, 144.3, 143.4, 143.3 (C qpz ), 144.0 (C 10 ), 136.1 (C 11 ), 134.0 (C 13 ), 128.7 (C 12 ), 128.1 (C 14 ), 125.2 (C 15 ), 108.4, 106.9, 106.3 (CH pz ), 107.6 (C 5 ), 52.4, 51.5, 50.6, 50.3 (the first broad, CO 2 Me), 33.6 (C 7 , 1 J C-H = 138 Hz), 15.67, 15.2, 14.7, 12.9, 12.7, 12.6 (Me pz ). C 3 and C 4 are too weak to be located. Anal. Calcd. for C 34 H 40 BN 6 O 9 Ir: C, 46.42, H, 4.58, N, 9.55. Found: C, 46.42, H, 4.88, N, 9.43. IR (KBr) ν (cm -1 ): (Me) 2951, 2924, (BH) 2531, (CO 2 Me) 1711. Synthesis of carbene 2b. Following the above procedure but employing anisaldehyde, compound 2b was obtained as a yellow solid in ca. 70% yield. 1 H NMR (CDCl 3 25 °C): δ (ppm) 8.30 (dd, 1 H, 3 J H-H = 8.9, J H-H = 2.3 Hz, CH 15 ), 6.84 (dd, 3 J H-H = 8.9, J H-H = 2.3 Hz, CH 11 ), 6.73 (dd, 1 H, 3 J H-H = 8.9, J H-H = 2.7 Hz, CH 14 ), 6.33 (dd, 1 H, 3 J H-H = 8.9, J H-H = 2.7 Hz, CH 12 ), 5.83, 5.62, 5.60 (s, 1 H each, CH pz ), 5.17 (s, 1 H, H 7 ), 3.95, 3.66, 3.25, 3.23 (s, the first broad, 3 H each, CO 2 Me), 3.75 (s, 3 H, OMe), 2.48, 2.44, 2.42, 2.41, 1.35, 1.04 (s, 3 H each, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 248.2 (C 9 ), 181.2, 173.2, 170.5, 162.0 (CO 2 Me), 164.5 (C 13 ), 154.5, 152.5, 150.5, 144.1, 143.3, 143.1 (C qpz ), 138.8 (C 11 ), 137.7 (C 10 ), 127.9 (C 15 ), 113.4 (C 12 ), 113.0 (C 14 ), 108.3, 106.8, 106.2 (CH pz ), 107.2 (C 5 ), 55.3 (OMe), 52.4, 51.7, 50.6, 50.4 (the first broad, CO 2 Me), 33.2 (C 7, 1 J C-H = 137 Hz), 15.8, 15.4, 14.5, 13.1, 12.8, 12.8 (Me pz ). C 3 and C 4 are too weak to 24 be located. Anal. Calcd. for C 35 H 42 BN 6 O 10 Ir: C, 46.20, H, 4.65, N, 9.24. Found: C, 46.22, H, 4.43, N, 9.11. IR (KBr) ν (cm -1 ): (Me) 2953, 2850, (BH) 2530, (CO 2 Me) 1712. Synthesis of carbene 2c. Following the above procedure but employing pdimethylaminobenzaldehyde, compound 2c was obtained as a yellow solid in ca. 55% yield. noe E = CO2Me [Ir] E E C E E H O 34 5 7 9 10 11 13 12 14 15 Me 2 N Mepz Mepz 1 H NMR (CDCl 3 25 °C): δ (ppm) 8.24 (dd, 1 H, 3 J H-H = 9.1, J H-H = 2.2 Hz, CH 15 ), 6.65 (dd, 1 H, 3 J H-H = 9.1, J H-H = 2.2 Hz, CH 11 ), 6.41 (dd, 1 H, 3 J H-H = 9.1, J H-H = 2.2 Hz, CH 14 ), 6.01 (dd, 1 H, 3 J H-H = 9.1, J H- H = 2.2 Hz, CH 12 ), 5.81, 5.60, 5.59 (s, 1 H each CH pz ), 5.08 (s, 1 H, H 7 ), 3.76, 3.64, 3.25, 3.22 (s, the first broad, 3 H each, CO 2 Me), 2.93 (s, 6 H, NMe 2 ), 2.46, 2.42, 2.39, 1.38, 1.16 (s, 1:1:2:1:1, 18 H, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 242.7 (C 9 ), 182.0, 174.0, 162.6 (CO 2 Me), 154.7, 152.6, 151.3, 144.2, 143.4, 143.2 (C qpz ), 154.5 (C 13 ), 139.5 (C 11 ), 133.2 (C 10 ), 128.6 (C 15 ), 110.5 (C 12 ), 110.0 (C 14 ), 108.6, 107.1, 106.5 (CH pz ), 108.2 (C 5 ), 52.7, 51.9, 50.9, 50.7 (the first broad, CO 2 Me), 40.3 (NMe 2 ), 33.2 (C 7 , 1 J C-H = 137 Hz), 16.2, 15.9, 14.7, 13.5, 13.2, 13.2 (Me pz ). C 3 and C 4 and one of the CO 2 Me are too weak to be located. IR (KBr) ν (cm -1 ): (Me) 2923, 2853, (BH) 2527, (CO 2 Me) 1720. Synthesis of carbene 2d. This derivative can be prepared following the same procedure than for the previous carbenes, but employing p-nitrobenzaldehyde. noe E = CO2Me [Ir] E E C E E H O 34 5 7 9 10 11 13 12 14 15 O 2 N Mepz Mepz 25 1 H NMR (CDCl 3 25 °C): δ (ppm) 8.46 (d, 1 H, 3 J H-H = 8.0 Hz, H 15 ), 8.11 (d, 1 H, 3 J H-H = 8.0 Hz, H 14 ), 7.71 (d, 1 H, 3 J H-H = 8.0 Hz, H 12 ), 7.12 (d, 1 H, 3 J H-H = 8.0 Hz, H 11 ), 5.87, 5.69, 5.62 (s, 1 H each, CH pz ), 5.38 (s, 1 H, H 7 ), 3.97, 3.68, 3.26, 3.24 (s, the first broad, 3 H each, CO 2 Me), 2.49, 2.46, 2.45, 2.44, 1.34, 0.91 (s, 3 H each, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 251.3 (C 9 ), 180.7, 172.5, 170.6, 161.7 (CO 2 Me), 154.8, 151.7, 149.9, 144.8, 144.0, 143.8 (C qpz ), 147.7 (C 10 ), 136.2 (C 11 ), 126.1 (C 15 ), 123.4 (C 14 ), 123.0 (C 12 ), 108.7, 107.2, 106.7 (CH pz ), 106.8 (C 5 ), 52.6, 51.8, 50.8, 50.6 (the first broad, CO 2 Me), 34.0 (C 7 ), 15.8, 15.4, 14.8, 13.1, 12.9, 12.7 (Me pz ). C 3 and C 4 are too weak to be located. Synthesis of carbene 2e. This derivative can be prepared following the same procedure than for the previous carbenes, but employing 5-methyl-2-thiophenecarboxaldehyde. 2e was isolated as an orange solid in ca. 75% yield. In this case, the eluent was a mixture of diethyl ether:hexane (50:50). 1 H NMR (CDCl 3 25 °C): δ (ppm) 6.47 (d, 1 H, 3 J H-H = 3.9 Hz, CH 11 ), 6.42 (d, 1 H, 3 J H-H = 3.9 Hz, CH 12 ), 5.84, 5.64, 5.62 (s, 1 H each, CH pz ), 5.17 (s, 1 H, H 7 ), 3.94, 3.66, 3.28, 3.25 (s, the first broad, 3 H each, CO 2 Me), 2.47, 2.44, 2.41, 2.40, 1.39, 1.24 (s, 3 H each, Me pz ), 2.32 (s, 3 H, Me 14 ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 235.7 (C 9 ), 181.3, 173.5, 169.7, 161.9 (CO 2 Me), 154.5, 151.9, 150.5, 143.9, 143.2, 143.2 (C qpz ), 152.8 (C 13 ), 145.7 (C 10 ), 143.3 (C 11 ), 143.2 (C 5 ), 128.1 (C 12 ), 108.3, 106.9, 106.4 (CH pz ), 107.2 (C 5 ), 52.4, 51.5, 50.6, 50.4 (the first broad, CO 2 Me), 33.3 (C 7 ), 16.2 (Me 14 ), 15.7, 15.8, 14.2, 13.0, 12.8, 12.6 (Me pz ). C 3 and C 4 are too weak to be located. Anal. Calcd. for C 33 H 40 BIrN 6 O 9 S: C, 44.05, H, 4.48, N, 9.34, S, 3.56. Found: C, 44.17, H, 4.68, N, 9.22, S, 3.52. IR (KBr) ν (cm -1 ): (Me) 2947, (BH) 2532, (CO 2 Me) 1726, 1711. 32 3 J H-H = 8.6 Hz, H 3 , H 5 ), 6.64 (t, 1 H, 3 J H-H = 7.2 Hz, H 8 ), 5.81, 5.75, 5.45 (s, 1 H each, CH pz ), 3.35, 3.83 (s, 3 H each, CO 2 Me), 3.82 (s, 3 H, OMe), 2.43, 2.42, 2.40, 1.77, 1.75, 1.37 (s, 3 H each, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 201.0 (C(O)H), 174.6, 167.9 (CO 2 Me), 166.3 (C 4 ), 155.8 (C 11 ), 155.0, 150.7, 150.0, 143.4, 143.4, 143.2 (C qpz ), 154.0, 150.7 (C 13 , C 14 ), 147.4 (C 12 ), 137.6 (C 7 ), 134.0 (C 2 ,C 6 ), 128.0 (C 1 ), 123.8 (C 10 ), 123.4 (C 8 ), 122.7 (C 9 ), 114.9 (C 3 , C 5 ), 108.0, 107.2, 106.4 (CH pz ), 55.9 (OMe), 51.6, 50.7 (CO 2 Me), 14.2, 13.4, 13.0, 12.6, 12.5 (1:1:1:2:1, Me pz ). Anal. Calcd. for C 35 H 40 BN 6 O 6 Ir: C, 49.82, H, 4.78, N, 9.96. Found: C, 49.82, H, 4.71, N, 10.25. IR (KBr) ν (cm -1 ): (Me) 2919, 2850, (BH) 2522, (CO 2 Me) 1711, 1687. Synthesis of adduct 3(C 6 H 4 )c. Following the procedure used for compound 3(C 6 H 4 )b, but using pdimethylaminobenzaldehyde, compound 3(C 6 H 4 )c was obtained as a red solid in ca. 60% yield. 1 H NMR (CDCl 3 25 °C): δ (ppm) 9.48 (s, 1 H, C(O)H), 7.50 (d, 2 H, 3 J H-H = 8.7 Hz, H 2 , H 6 ), 7.44 (d, 1 H, 3 J H-H = 7.5 Hz, H 10 ), 7.09 (d, 1 H, 3 J H-H = 7.5 Hz, H 7 ), 6.88 (t, 1 H, 3 J H-H = 7.5 Hz, H 9 ), 6.63 (t, 1 H, 3 J H-H = 7.5 Hz, H 8 ), 6.48 (d, 2 H, 3 J H-H = 8.7 Hz, H 3 , H 5 ), 5.80, 5.74, 5.43 (s, 1 H each, CH pz ), 3.83, 3.37 (s, 3 H each, CO 2 Me), 3.27 (s, 6 H, NMe 2 ), 2.43, 2.41, 2.39, 1.81, 1.79, 1.36 (s, 3 H each, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 197.2 (C(O)H), 174.7, 167.6 (CO 2 Me), 156.0 (C 11 ), 155.2 (C 4 ), 154.7, 150.1, 150.0, 143.0, 142.9, 142.8 (C qpz ), 154.7, 149.8 (C 13 , C 14 ), 147.3 (C 12 ), 137.1 (C 7 ), 130.8, 128.7 (C 2 , C 6 ), 123.3 (C 10 ), 123.0 (C 8 ), 122.7 (C 1 ), 122.1 (C 9 ), 110.8 (C 3 , C 5 ), 107.7, 106.2, 106.2 (CH pz ), 51.3, 50.4 (CO 2 Me), 40.0 (NMe 2 ), 14.1, 14.0, 13.2, 12.9, 12.4, 12.3 (Me pz ). At 25 ºC, the rotation around the Ar–C(=O)H bond is on the slow exchange regime on the 13 C NMR time scale. Anal. 33 Calcd. for C 36 H 43 BIrN 7 O 5 : C, 50.47, H, 5.06, N, 11.44. Found: C, 50.51, H, 5.29, N, 11.52. IR (KBr) ν (cm -1 ): (Me) 2973, 2928, (BH) 252, (CO 2 Me) 1716, 1685. Synthesis of adduct 3(H)c. A sample of compound 3(H) (0.100 g, 0.134 mmol) was dissolved in CH 2 Cl 2 (3 mL) and p-dimethylaminobenzaldehyde was added (0.061 g, 0.40 mmol). The mixture was stirred for 2 h at 60 ºC, the solvent eliminated under vacuo and the residue extracted with Et 2 O. After cooling at -20 ºC, red microcrystals of the title compound were isolated in ca. 75% yield. 1 H NMR (CDCl 3 25 °C): δ (ppm) 9.33 (s, 1 H, C(O)H), 8.04 (s, 1 H, H 7 ), 7.57 (d, 2 H, 3 J H-H = 8.7 Hz, H 2 , H 6 ), 6.53 (d, 2 H, 3 J H-H = 8.7 Hz, H 3 , H 5 ), 5.71, 5.49 (s, 2:1, 3 H, CH pz ), 3.76, 3.42, 3.35 (s, 3 H each, CO 2 Me), 3.06 (s, 6 H, NMe 2 ), 2.39, 1.97, 1.83, 1.64 (s, 3:1:1:1, 18 H, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 197.1 (C(O)H), 176.1, 172.4, 164.4 (CO 2 Me), 174.1, 145.8, 143.5 (CCO 2 Me), 155.4 (C 4 ), 155.4, 150.8, 149.9, 143.2, 142.9, 142.6 (C qpz ), 152.6 (C 7 , 1 J C-H = 160 Hz), 122.4 (C 1 ), 110.9 (v br, C 3 , C 5 ), 107.6, 106.6, 106.0 (CH pz ), 51.1, 50.5, 50.5 (CO 2 Me), 40.0 (NMe 2 ), 13.5, 13.3, 12.9, 12.8, 12.4, 12.4 (Me pz ). C 2 and C 6 were not located because of their broadness. At 25 ºC, the rotation around the Ar–C(=O)H bond is on the intermediate exchange regime on the 13 C NMR time scale. Anal. Calcd. for C 34 H 43 BIrN 7 O 7 : C, 47.22, H, 5.01, N, 11.34. Found: C, 47.26, H, 5.24, N, 11.15. IR (KBr) ν (cm -1 ): (Me) 2921, (BH) 2528, (CO 2 Me) 1697. Synthesis of adduct 4. Compound 1 (0.100 g, 0.126 mmol) was dissolved in CH 2 Cl 2 (3 mL) and 2- pyridinecarboxaldehyde was added (37 µl, 0.38 mmol). After stirring for 2 h at 60 ºC, the solvent was evaporated under vacuo and the residue washed with cold diethyl ether. Crystallization of the resulting 34 dark yellow solid from a mixture of CH 2 Cl 2 and Et 2 O, at -20 ºC, yielded analytically pure 4 in ca. 90% yield. noe E = CO2Me Mepz [Ir] E O E E E H N1 2 3 4 5 1 H NMR (CDCl 3 25 °C): δ (ppm) 9.17 (d, 1 H, 3 J H-H = 5.9 Hz, H 1 ), 7.89 (t, 1 H, 3 J H-H = 7.5 Hz, H 3 ), 7.74 (d, 1 H, 3 J H-H = 7.7 Hz, H 4 ), 7.56 (t, 1 H, 3 J H-H = 7.2 Hz, H 2 ), 6.90 (s, 1 H, C(O)H), 5.82, 5.64 (s, 2:1, 3 H, CH pz ), 3.76, 3.34 (s, 6 H each, CO 2 Me), 2.51, 2.45, 2.16, 1.19 (s, 2:1:1:2, 18 H, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 187.4 (C(O)H), 173.3, 167.2 (1:1, CO 2 Me), 156.5 (C 5 ), 155.5, 151.8 (1:1, CCO 2 Me), 156.1, 151.4, 146.4, 143.9 (1:2:2:1, C qpz ), 154.7 (C 1 ), 138.2 (C 3 ), 128.3 (C 2 ), 127.5 (C 4 ), 109.3, 108.2 (2:1, CH pz ), 51.9, 50.9 (1:1, CO 2 Me), 13.7, 13.4, 13, 12.8 (1:1:2:2, Me pz ). Anal. Calcd. for C 33 H 39 BIrN 7 O 9 : C, 45.00, H, 4.46, N, 11.13. Found: C, 44.99, H, 4.15, N, 11.35. IR (KBr) ν (cm -1 ): (Me) 2943, (BH) 2528, (CO 2 Me) 1720, 1696. Synthesis of adduct 6c. Compound 5 (0.100 g, 0.1 mmol) was dissolved in CH 2 Cl 2 (3 mL), pdimethylaminobenzaldehyde was added (0.045 g, 0.3 mmol) and the mixture was stirred at 60 ºC for 18 h. After this period of time, the solvent was eliminated under vacuo, and the residue extracted with Et 2 O. After cooling at -20 ºC, orange crystals of compound 6c were isolated in ca. 85% yield. 1 H NMR (CDCl 3 25 °C): δ (ppm) 8.16 (s, 1 H, C(O)H), 8.08, 7.12 (d, 1 H each, 3 J H-H = 8.1 Hz, H 2 , H 6 ), 6.73, 6.42 (d, 1 H each, 3 J H-H = 7.9 Hz, H 3 , H 5 ), 5.71, 5.65 (s, 2:1, 3 H, CH pz ), 3.65, 3.07 (s, 2:1, 18 H, CO 2 Me), 3.11 (s, 6 H, NMe 2 ), 2.39, 2.37, 2.18, 1.84 (s, 2:1:1:2, 18 H, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 35 25 o C): δ (ppm) 199.3 (C(O)H), 176.2, 169.3, 167.1 (1:1:1, CO 2 Me), 156.0 (C 4 ), 155.8, 150.9, 143.9, 143.4 (1:2:1:2, C qpz ), 152.1, 136.6, 134.4 (1:1:1, CCO 2 Me), 138.4, 132.3 (C 2 , C 6 ), 123.2 (C 1 ), 112.3, 109.8 (C 3 , C 5 ), 107.0, 106.8 (2:1, CH pz ), 51.9, 51.7, 50.7 (1:1:1, CO 2 Me), 40.3 (NMe 2 ), 17.5, 13.6, 13.5, 12.7 (1:2:1:2, Me pz ). Anal. Calcd. for C 42 H 51 BIrN 7 O 13 : C, 47.37, H, 4.83, N, 9.21. Found: C, 47.36, H, 4.70, N, 9.05. IR (KBr) ν (cm -1 ): (CH 3 ) 2945, (BH) 2528, (CO 2 Me) 1720. Synthesis of adduct 6g. Following the procedure described for compound 6c but using pyrrole-2- carboxaldehyde, a crude residue of a dark yellow material was obtained. The title compound was obtained in ca. 80% yield after washing this solid with cold (-15 ºC) pentane. noe E = CO2Me Mepz [Ir] E EE E E E 1 2 3 4 O H NH 1 H NMR (CDCl 3 25 °C): δ (ppm) 11.84 (s, 1 H, NH), 8.08 (s, 1 H, C(O)H), 7.39 (s, 1 H, H 4 ), 6.84 (s, 1 H, H 2 ), 6.27 (s, 1 H, H 3 ), 5.71, 5.64 (s, 2:1, 3 H, CH pz ), 3.64, 3.62, 3.04 (s, 6 H each, CO 2 Me), 2.37, 2.15, 1.84 (s, 3:1:2, 18 H, Me pz ). 13 C{ 1 H} NMR (CDCl 3 , 25 o C): δ (ppm) 185.5 (C(O)H), 175.7, 170.9, 166.8 (1:1:1, CO 2 Me), 155.6, 150.6, 143.7, 143.4 (1:2:1:2, C qpz ), 152.7, 135.8, 134.3 (1:1:1, CCO 2 Me), 132.8 (C 4 ), 131.9 (C 1 ), 126.4 (C 2 ), 112.8 (C 3 ), 106.7, 106.6 (2:1, CH pz ), 51.2, 51.6, 50.5 (1:1:1, CO 2 Me), 17.3, 13.3, 13.2, 12.5 (1:2:1:2, Me pz ). Anal. Calcd. for C 38 H 45 BIrN 7 O 13 : C, 45.15, H, 4.49, N, 9.70, Found: C, 45.15, H, 4.50, N, 9.47. IR (KBr) ν (cm -1 ): (Me) 2984, 2947, (BH) 2530, (CO 2 Me) 1722. Structural Analysis of Complexes 2a, 3b, and 6c·2CH 2 Cl 2 . X-ray data were collected for all complexes on a Bruker SMART CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube source (Mo radiation, λ = 0.71073 Å) operating at 50 kV and 40 mA. Data were corrected for absorption by using a multi-scan method applied with the SADABS program. 24 The structures of all compounds 36 were solved by heavy atom methods. Refinement, by full-matrix least squares on F 2 with SHELXL97, 25 was similar for all complexes, including isotropic and subsequently anisotropic displacement parameters. The hydrogen atoms were calculated and refined using a restricted riding model. For 6c·2(CH 2 Cl 2 ) we observed a disordered solvent molecule with occupancies 62:38. Crystal data for 2a: C 34 H 40 BIrN 6 O 9 , M W 879.73, orange, irregular block (0.25 × 0.25 × 0.17), triclinic, space group P−1, a: 10.2762(8) Å, b: 11.2916(9) Å, c: 16.7440(13) Å, α: 83.727(2) ° , β : 115.4760(10) ° , γ : 115.4760(10) ° , V = 1826.5(2) Å 3 , Z = 2, D calc = 1.600 g cm −3 , F(000) = 880, T = 295(2) K, µ = 3.716 mm −1 . 22520 measured reflections (2θ: 1.28–26.13 ° , ω scans 0.3 ° ), 7191 unique (R int = 0.0455). Min/max transmission factors: 0.5707/0.4569. Final agreement factors were R 1 = 0.0362 (6125 observed reflections, I > 2σ(I)) and wR 2 = 0.0851. Data/restraints/parameters: 7191/0/470; GoF = 1.113. Largest diff. peak and hole: 1.549 and −0.675 eÅ −3 . Crystal data for 3b: C 35 H 42 BIrN 6 O 10 , M W 909.76, orange, irregular block (0.40 × 0.23 × 0.08), monoclinic, space group P2(1)/c, a: 10.0506(12) Å, b: 18.310(2) Å, c: 22.032(3) Å, β : 100.479(3) ° , V = 3986.9(9) Å 3 , Z = 4, D calc = 1.516 g cm −3 , F(000) = 1824, T = 295(2) K, µ = 3.409 mm −1 . 26177 measured reflections (2θ: 1.46–26.11 ° , ω scans 0.3 ° ), 7863 unique (R int = 0.0820). Min/max transmission factors: 0.7721/0.3425. Final agreement factors were R 1 = 0.0513 (5047 observed reflections, I > 2σ(I)) and wR 2 = 0.1142. Data/restraints/parameters: 7863/0/489; GoF = 1.045. Largest diff. peak and hole: 1.634 and −1.016 eÅ −3 . Crystal data for 6c·2CH 2 Cl 2 : C 44 H 55 BCl 4 IrN 7 O 13 , M W 1234.76, orange, irregular block (0.61 × 0.54 × 0.47), monoclinic, space group P2(1)/n, a: 13.507(3) Å, b: 20.843(4) Å, c: 19.049(4) Å, β : 103.087(5) ° , V = 5223.2(17) Å 3 , Z = 4, D calc = 1.570 g cm −3 , F(000) = 2488, T = 295(2) K, µ = 2.827 mm −1 . 32001 measured reflections (2θ: 1.47–25.15 ° , ω scans 0.3 ° ), 9319 unique (R int = 0.0424). Min/max transmission factors: 0.3500/0.2774. Final agreement factors were R 1 = 0.0288 (6916 observed reflections, I > 2σ(I)) and wR 2 = 0.0654. Data/restraints/parameters: 9319/41/673; GoF = 0.953. Largest diff. peak and hole: 0.987 and −0.520 eÅ −3 . 37 Structural Analysis of Complexes 2(C 6 H 4 )b, 4, and 6g. X-ray data were collected for all complexes on a Bruker Smart APEX I or APEX II CCD diffractometers equipped with a normal focus, 2.4 kW sealed tube source (Mo radiation, λ = 0.71073 Å) operating at 50 kV and 40(6g)/30(2(C 6 H 4 )b, 4) mA. Data were collected over the complete sphere. Each frame exposure time were 10s or 20s (6g) covering 0.3 o in ω. Data were corrected for absorption by using a multiscan method applied with the SADABS program. 24 The structures of all compounds were solved by direct methods. Refinement, by full-matrix least squares on F 2 with SHELXL97, 25 was similar for all complexes, including isotropic and subsequently anisotropic displacement parameters. The hydrogen atoms were observed in the least Fourier Maps or calculated, and refined freely or using a restricted riding model. For 2(C 6 H 4 )b 0.5 molecules of diethyl ether by Ir were found in the least cycles of Fourier in two sites in the asymmetric unit, and were refined with restrained geometry and isotropic displacement parameters. Crystal data for 2(C 6 H 4 )b: C 35 H 40 BIrN 6 O 6 , O 0.5 C 2 H 5 , M W 880.80, orange, irregular block (0.15 x 0.14 x 0.11), monoclinic, space group P2 1 /n, a: 19.2080(9) Å, b: 20.8774(10) Å, c: 22.3156(11) Å, β : 115.4760(10) o , V = 8078.7(7) Å 3 , Z = 8, D calc : 1.448 g cm -3 , F(000): 3544, T = 100(2) K, µ 3.356 mm - 1 ). 87963 measured reflections (2θ: 3-58 o , ω scans 0.3 o ), 21028 unique (R int = 0.0456); min./max. transm. Factors 0.740 /0.862. Final agreement factors were R 1 = 0.0508 (16588 observed reflections, I > 2σ(I)) and wR 2 = 0.1390; data/restraints/parameters 21028/7/ 947; GoF = 1.036. Largest peak and hole 5.522 and -2.987 e/ Å 3 . Crystal data for 4: C 33 H 39 BIrN 7 O 9 , M W 880.72, yellow, irregular block (0.18 x 0.16 x 0.14), monoclinic, space group P2 1 /c, a: 10.4342(7) Å, b: 18.6223(12) Å, c: 18.3236(12) Å, β : 105.6298(8) o , V = 3428.8(4) Å 3 , Z = 4, D calc : 1.706 g cm -3 , F(000): 1760, T = 100(2) K, µ 3.959 mm -1 ). 42204 measured reflections (2θ: 3-58 o , ω scans 0.3 o ), 8407 unique (R int = 0.0412); min./max. transm. Factors 0.742/0.862. Final agreement factors were R 1 = 0.0260 (7076 observed reflections, I > 2σ(I)) and wR 2 = 0.0625; data/restraints/parameters 8407/0/476; GoF = 1.032. Largest peak and hole 1.352 and -0.772 e/ Å 3 38 Crystal data for 6g: C 38 H 45 BIrN 7 O 13 , M W 1010.82, yellow, plate (0.14 x 0.10 x 0.03), monoclinic, space group C2/c, a: 40.649(5) Å, b: 11.7275(14) Å, c: 17.870(2) Å, β : 105.232(2) o , V = 8219.3(17) Å 3 , Z = 8, D calc : 1.634 g cm -3 , F(000): 4064, T = 100(2) K, µ 3.322 mm -1 ). 35762 measured reflections (2θ: 3-58 o , ω scans 0.3 o ), 11447 unique (R int = 0.0329); min./max. transm. Factors 0.711/0.862. Final agreement factors were R 1 = 0.0277 (9212 observed reflections, I > 2σ(I)) and wR 2 = 0.0663; data/restraints/parameters 11447/0/572; GoF = 1.008. Largest peak and hole 1.763 and -0.587 e/ Å 3 . Computational details. All calculations were performed with the GAUSSIAN 09 series of programs 26 using the B3Lyp functional. 27,28 An effective core potential 29 and its associated double-ζ LANL2DZ 30 basis set were used for iridium. C, H, B, N and O atoms where represented by means of the 6-31G(d,p) basis set. 31-33 The steric effects of the methyl fragments of the Tp Me2 ligand were accounted for by means of ONIOM(B3Lyp:UFF) calculations. 34-36 The structures of the reactants, intermediates, transition states, and products were fully optimized in gas phase without any symmetry restriction. Frequency calculations were performed on all optimized structures at the same level of theory to characterize the stationary points and the transitions states, as well as for the calculation of gas-phase enthalpies (H), entropies (S), and Gibbs energies (G) at 298.15 K. The nature of the intermediates connected was determined by perturbing the transition states along the TS coordinate and optimizing to a minimum. ASSOCIATED CONTENT Supporting Information. Cif file for compounds 2a, 2(C 6 H 4 )b, 3b, 4, 6c·2CH 2 Cl 2 and 6g, and tables of the optimized geometries for the calculated species. ACKNOWLEDGMENT 39 Financial support (FEDER and ESF contribution) from the Spanish Ministry of Science (Projects CTQ2010-17476, CTQ2011-23459, and Consolider-Ingenio 2010 CSD2007-00006), the Junta de Andalucía (Grant FQM-119 and Project P09-FQM-4832), the Diputación General de Aragón (E35) and the CONACYT-México (grants 025424 and 84453, and scholarship to A. E. R.) is gratefully acknowledged. J. L.-S. is thankful to the Spanish Ministry of Science and Innovation (MICINN) for a Ramón y Cajal Contract. The use of computational facilities of the Consejo Superior de Investigaciones Científicas (CSIC, Cluster Trueno) and the Supercomputing Centre of Galicia (CESGA) is also acknowledged. Corresponding author *E-mail: [email protected] (M.P.); [email protected] (V.S) REFERENCES (1) (a) Tsuji, J.; Ohno, K. Tetrahedron Lett. 1965, 3969-3971. (b) Ohno, K.; Tsuji, J. J. Am. Chem. Soc. 1968, 90, 99-107. (c) Doughty, D. H.; Pignolet, L. H. J. Am. Chem. Soc. 1978, 100, 7083-7085. 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