Carbon Monoxide Induced Double Cyclometalation at the Iridium

Fabian Reiß , Kai Altenburger , Lisanne Becker , Kathleen Schubert , Haijun Jiao , Anke Spannenberg , Dirk Hollmann , Perdita Arndt , Uwe Rosenthal...
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Carbon Monoxide Induced Double Cyclometalation at the Iridium Center S. M. Wahidur Rahaman, Shrabani Dinda, Tapas Ghatak, and Jitendra K. Bera* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India S Supporting Information *

ABSTRACT: Bubbling of CO into a dichloromethane solution of [Ir(COD)(CH3CN)2][BF4] followed by the addition of 2-phenyl-1,8-naphthyridine (LH) at room temperature results in the bis-cyclometalated IrIII complex [Ir(C∧N)2(CO)(LH)][BF4] (C∧N = L). The observed cyclometalation contradicts the classical role of CO, which is to hinder oxidative addition by lowering electron density on the metal. DFT calculations reveal that the first cyclometalation involves oxidative addition of the ligand. Subsequently, preferential electrophilic activation of the second ligand followed by elimination of dihydrogen affords the bis-cyclometalated IrIII complex.

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INTRODUCTION The growing interest in cyclometalation reactions originates from their ability to provide easy access to organometallic compounds with a metal−carbon σ bond.1 The reaction typically involves initial coordination of the ligand to the metal via a donor group followed by intramolecular activation of the C−H bond. Depending on the electronic nature of the metal and the set of ligands bound to it, several possible pathways have been suggested for the C−H bond cleavage.2 Oxidative addition (OA) is favored for electron-rich, low-valent transition metals, leading to the corresponding alkyl or aryl hydride complex with concomitant formal two-electron oxidation of the metal. Transition metals that lack sufficient electron density for OA employ alternate mechanisms which include σ-bond metathesis, 1,2-addition, radical activation, and electrophilic activation.3 For an electron-deficient metal bearing an alkyl or hydride ligand, the σ-bond metathesis mechanism has been invoked. Electrophilic activation is often facilitated by the use of a Brønsted base such as acetate, carbonate, or hydride, which is either added externally or bound to the metal center to begin with. Bis-cyclometalated IrIII complexes constitute an important class of compounds, owing to their rich photophysical properties.4 The most prominent and widely studied complexes are the ones which contain the “Ir(ppy)2” (ppyH = 2phenylpyridine) unit. These are conveniently synthesized from the chloro-bridged IrIII dimer [{(ppy)2Ir}2(μ-Cl)2]. The double cyclometalation in this dimer is achieved by refluxing IrCl3·nH2O and ppyH in an alcohol−water mixture at elevated temperature.5 Acetate-assisted cyclometalation reactions under mild conditions are widely known for a variety of IrIII complexes.6 In contrast, cyclometalation at IrI center is surprisingly rare. The “{Ir(COD)}+” unit exhibits reluctance toward OA of the C−H bond. Only in select cases have cyclometalated IrIII hydride complexes been isolated.7 © 2012 American Chemical Society

The OA of the C−H bond to a transition metal involves electron donation from the C−H σ orbital to a vacant metal d orbital and back-donation from a filled metal d orbital to the C−H σ* orbital. A π-acceptor CO withdraws electron density from the metal center and thus normally hinders OA and facilitates reductive elimination (RE).8 In contrast with this classical behavior, Milstein, Martin, and co-workers reported CO-promoted OA of the arene C−H bond to a cationic RhI complex containing a phosphine-based pincer ligand.9 We have observed double cyclometalation on a simple IrI system at room temperature which is facilitated by CO. The reaction protocol that allows CO-induced double cyclometalation is described, and a mechanistic scheme is outlined with the aid of DFT calculations.

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RESULTS AND DISCUSSION Synthesis. Reaction of NP-R (2-R-1,8-naphthyridine) with [Ir(COD)(CH3CN)2][BF4] allowed the isolation of [Ir(COD)(NP-R)2][BF4] (R = Ph, ferrocenyl (Fc) and 2(OH)Ph (1)) (Scheme 1). Full characterizations of the compounds [Ir(COD)(NP-Ph)2][BF4] and [Ir(COD)(NPFc)2][BF4] have been reported earlier.10 The molecular structure of 1 is very similar to that of those two complexes. The cationic unit consists of a central Ir having one η2,η2-bound COD and two NP ligands coordinated through the N8 nitrogens in a square-planar arrangement (Figure S1, Supporting Information). A continuous flow of CO into [Ir(COD)(NP-R)2][BF4] afforded different compounds, depending on the nature of the substituent R and the choice of solvent. For methyl and phenyl substituents, [Ir2(CO)4(μ-NPR)2][BF4]2 (R = Me, Ph) were isolated in dichloromethane.10 These dinuclear compounds have a characteristic dark blue Received: June 7, 2012 Published: July 24, 2012 5533

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Scheme 1. Reactions of [Ir(COD)(CH3CN)2](BF4) with NP-R and CO

color and very low solubility in common organic solvents. On the other hand, bulky ferrocenyl and napthyl substituents led to direct replacement of COD by two CO ligands, resulting in [Ir(CO)2(NP-R)2][BF4] (R = Fc, naphthyl (2)), respectively. The molecular structure of 2 reveals a square-planar Ir complex containing two carbonyls and two NP ligands bound via N8 nitrogens (Figure S2, Supporting Information). It should be noted here that the ferrocenyl analogue [Ir(CO)2(NP-Fc)2][BF4] was isolated by employing chlorobenzene as a solvent. The identical reaction in dichloromethane provided the metal− metal singly bonded diiridium(II) compound [Ir2(CO)4(Cl)4(NP-Fc)2], in which the chlorides are shown to originate from the solvent (Scheme 1).10 Reversal of the reaction sequence, that is, bubbling CO through a dichloromethane solution of [Ir(COD)(CH3CN)2][BF4] followed by the addition of NP-Ph, provided the unusual double cyclometalated complex [Ir(NP-C6H4)2(NP-Ph)(CO)](BF4) (3). The molecular structure of 3 (Figure 1) reveals a cationic complex consisting of a central Ir in an octahedral geometry occupied by two cyclometalating (C∧N) ligands, one CO, and one NP-Ph. The cyclometalation occurs through the N1 nitrogen of the naphthyridine and the ortho phenyl carbon. A NP-Ph ligand coordinates to the metal via the NP N8 nitrogen to complete the octahedral geometry around the metal. The Ir−C (Ir1−C15 = 2.048(5) Å and Ir1−C21 = 2.026(5) Å) and Ir−N (Ir1−N1 = 2.210(4) Å and Ir1−N3 = 2.150(4) Å) distances are consistent with bond parameters of other bis-cyclometalated Ir(III) compounds.4,11 One significant difference, however, is worth mentioning. The cis-C,C/cis-N,N disposition of the cyclometalating ligands is in sharp contrast to the normal cis-C,C/trans-N,N configurations observed for the majority of these compounds.4a,b The NMR data of 3 conform to the X-ray structure. Two cyclometalated carbons show signals at δ 174 and 171 ppm in 13 C NMR spectra. The carbonyl carbon appears at δ 207 ppm. The IR stretching frequency at 2086 cm−1 confirms the presence of a single CO. The ESI-MS spectrum of complex 3 exhibits signals at m/z (z = 1) 649, 631, and 603 attributed to [M − -(NP-Ph) + (H2O)]+ (17%), [M − (NP-Ph)]+ (100%), and [M − (NP-Ph) − CO]+ (60%) fragments.

Figure 1. ORTEP diagram (30% probability thermal ellipsoids) of the cationic unit of 3 with important atoms labeled. Hydrogen atoms are omitted for the sake of clarity. Selected bond distances (Å) and angles (deg): Ir1−C1 = 1.846(5), Ir1−C21 = 2.026(5), Ir1−C15 = 2.048(5), Ir1−N3 = 2.150(4), Ir1−N5 = 2.180(4), Ir1−N1 = 2.210(4); C1− Ir1−C21 = 95.0(2), C1−Ir1−C15 = 89.0(2), C21−Ir1−C15 = 92.9(2), C1−Ir1−N3 = 173.96(19), C21−Ir1−N3 = 79.12(18), C15−Ir1−N3 = 89.99(17), C1−Ir1−N5 = 94.99(18), C21−Ir1−N5 = 89.90(18), C15−Ir1−N5 = 174.91(17), N3−Ir1−N5 = 86.36(15), C1−Ir1−N1 = 86.46(18), C21−Ir1−N1 = 171.04(18), C15−Ir1−N1 = 78.28(19), N3−Ir1−N1 = 99.16(15), N5−Ir1−N1 = 98.79(15).

The emission spectrum of 3 shows blue emission at 405 and 427 nm upon irradiation at 300 nm in acetonitrile (Figure S5a, Supporting Information). The excited-state lifetime of 1.2 μs is consistent with emission from a triplet excited state. A clear vibronic structure indicates that the emissive state is most likely to be 3LC (ligand-centered) involving the cyclometalating ligands (C∧N). This is characteristic of bis-cyclometalated IrIII compounds containing photophysically innocent ancillary ligands.4 Reaction with the pyridine analogue 2-phenylpyridine (Hppy) under identical conditions also led to cyclometalation. All attempts to obtain crystals of the product suitable for X-ray diffraction were unsuccessful. However, the 1H and 13C NMR spectra revealed the double-cyclometalated complex [Ir5534

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Scheme 2. Mechanistic Pathways for Double Cyclometalation Supported by DFT Calculationsa

a

Ea = activation energy (kcal/mol).

Figure 2. Optimized structures of intermediates and transition states.

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(Ir−H1 = 1.546 Å). The transition state TSA→B connecting A and B has one imaginary frequency of −809.4 cm −1 corresponding to the movement of hydrogen from carbon to the metal center with short Ir···C/H (2.083/1.607 Å) and elongated C−H separations (1.649 Å). The coordination of an acetonitrile to B completes the stable octahedral intermediate C. Two distinct possibilities for the second C−H bond activation have been considered, which include a second OA of the C−H resulting in an IrV dihydride intermediate (defined as path A) and an electrophilic activation of the C−H to give an IrIII dihydrogen intermediate (defined as path B). Several groups have shown that an associative process could proceed in one of these two ways.13 The preference for a particular pathway depends on the metal oxidation state and the ligand properties. The agostic interaction results in enhanced C−H acidity due to forward electron donation from the C−H σ orbital to the empty metal orbital. Electrophilic activation may operate when this electron donation is significant, which is followed by proton abstraction by a base to complete the cyclometalation. Alternatively, OA takes place when the backdonation from the metal to the C−H σ* orbital is sufficiently high. Both pathways are discussed independently in the following section, and their energy profiles are compared in Figure 3.

(ppy)2(CH3CN)(CO)](BF4) (4), which was further characterized by IR, mass, and elemental analysis. A pair of signals at δ 169 and 165 ppm in 13C NMR spectra indicates the double cyclometalation. The carbonyl carbon appears at δ 197 ppm. The IR stretching frequency at 2037 cm−1 confirmed the presence of CO. The ESI-MS spectrum of complex 4 reveals the molecular ion signal at m/z 570 (10%). The other prominent signals are attributed to [M − CH3CN]+ (100%) and [M − CH3CN − CO]+ (64%) at m/z (z = 1) 529 and 501, respectively. The elemental analysis data correspond to an acetonitrile coordinated to the metal. The emission spectrum of 4 shows blue emission at 455 and 482 nm upon irradiation at 300 nm in acetonitrile (Figure S5b, Supporting Information) with a corresponding excited-state lifetime of 1.2 μs. The emission pattern is similar to that of 3 and likely originates from a 3LC emissive state. This is further confirmation of double cyclometalation at the Ir center. Clearly, no C−H activation takes place when COD is bound to the metal. X-ray structures reveal ubiquitous N8 coordination of NP ligands in the solid state that put the aryl C−H far away from the metal. Even for a possible N1 coordination, the bulky COD is not likely to allow a close approach of the ortho C−H to the metal. In contrast, linearly bound CO ligands may allow an agostic interaction between the aryl C−H and the Ir while the latter is engaged to N1 of the ligand. The result is possibly C−H OA to the metal, resulting in a cyclometalated IrIII hydride complex. The second cyclometalation can occur either via electrophilic activation or by another OA. Finally the loss of dihydrogen would lead to the bis-cyclometalated compound 3. Mechanism of Double Cyclometalation. We realize that our proposal of first C−H OA is counterintuitive, since it contradicts the classical role of the π-acceptor CO, which withdraws electron density from the metal and thus inhibits OA. Further, the mechanism for the second cyclometalation is not immediately obvious. To gain insight into the double cyclometalation mechanism, a computational study was undertaken using the DFT/B3LYP level of theory. 2-Phenylpyridine (Hppy) was considered as the model ligand, which reduces the computational cost and represents a generalized situation. The use of Hppy is justified, since it afforded a similar biscyclometalated compound. A mechanistic scheme supported by DFT calculations is shown in Scheme 2. The optimized structures of all the species are given in Figure 2. Replacement of COD in [Ir(COD)(CH3CN)2][BF4] by two CO followed by ligand addition led to a mononuclear dicarbonyl complex [Ir(CO)2(Hppy)] (A), which is considered as a potential starting intermediate. The C−H can approach the metal in two ways, via front-phase or the rear-phase Ir···C−H interactions (Figure S6, Supporting Information). However, they constitute a pair of enantiomers, and their bond parameters and relative energies are identical. Hence, we restricted our study to only one case (rear-phase). Examination of the metrical parameters of A reveals strong agostic interaction of the aryl C−H bond to the metal (Ir···C/H = 2.387/2.482 Å) (Figure 2). A long C−H distance (1.094 Å) and an acute Ir−H−C angle (72°) favor OA with an activation barrier of 15.3 kcal/mol (Scheme 2). The intermediacy of similar M···C−H agostic complexes (or C−H σ complexes) has been proposed and identified along the reaction coordinates for C−H activation.6,12 The resultant OA product B is a cyclometalated IrIII hydride species having a square-pyramidal geometry in which the hydride (H1) occupies an axial position

Figure 3. Comparative energy profiles for different pathways and the corresponding transition states. Energies are in kcal/mol (not to scale).

The double-cyclometalated product necessitates coordination of the second Hppy. For this purpose, C must lose acetonitrile and carbonyl(s) from the metal coordination sphere. We computed the five-coordinated hydride intermediate D, which undergoes ready OA to form the octahedral IrV intermediate E. It should be noted here that both carbonyls and acetonitrile are released from C to accommodate an incoming ligand. The six-coordinated species D′ with an additional CO attached at an equatorial site was also computed. However, all our attempts to find a low-energy C−H OA step from D′ failed, probably because of the difficulty associated with a sevencoordinated species in the TS. The geometry of D consists of a cyclometalated ppy, coordination of a Hppy via pyridine N accompanied by an agostic C−H interaction to the metal (Ir···C4/H4 = 2.328/2.482 Å), and finally an axial hydride H1 5536

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orbitals of CO promote M → C−H σ*, although to a lesser extent than for σ CO orbitals (Figure S10, Supporting Information). These interactions are attributed to COpromoted cyclometalation at RhI pincer complexes. The effect is more pronounced, as shown in this work, for the bis-CO system than for the mono-CO congener. In the present work, the cationic {Ir(CO)2}+ allows a close approach of the aryl C−H (Hppy) to the metal, which is unlikely to occur when COD is bound. We tentatively propose that CO, particularly that trans to the aryl C−H bond, acts as a weak dπ(M) → pπ acceptor but a good donor (σ and π) to the metal, in line with the Milstein and Martin scheme explained above. As a consequence, the OA ensues, leading to first cyclometalation. To examine the relative efficacy of CO for OA as compared to acetonitrile, we carried out computational work on three additional hypothetical systems (Scheme 3). The OA for bis-

trans to the Hppy nitrogen. The OA of the C−H to D leads to the IrV dihydride intermediate E via the transition state TSD→E, corresponding to an activation barrier of 24.1 kcal/mol. The TSD→E connecting D and E has one imaginary frequency of −610.0 cm−1 and involves the movement of the aryl hydrogen (H4) away from the carbon (C4) and toward the metal (Figure 3). Among the two hydrides in E, one (H4) is trans to the ipsoC at an equatorial position of the octahedron, while the second (H1) is trans to the pyridine N of the other ppy at an axial position. Subsequently, reductive elimination of H2 followed by coordination of CO and acetonitrile gives the doublecyclometalated product F. The electrophilic mechanism does not involve an expansion of coordination number around the metal. Therefore, we computed the pseudo-octahedral species G, in which a Hppy is coordinated axially via pyridine N, featuring an agostic Ir···C− H (2.335/1.893 Å) interaction. The loss of one CO and acetonitrile from C is necessary to accommodate another Hppy. The carbonyls on a high-valent IrIII are incompatible, and an acetonitrile trans to a hydride is anticipated to be labile. With the geometry of the final product F in mind, a CO is placed at an axial site in G, necessitating a switch of place for the hydride. The geometry of G was found to be most suited for electrophilic activation of the second Hppy, affording the octahedral IrIII dihydrogen complex H. The metal-bound hydride plays an important role in facilitating the electrophilic activation process. The conversion G → H is an exothermic process (−13.6 kcal/mol), and the activation energy barrier is only 3.1 kcal/mol. The corresponding TS connecting G and H (TSG→H) has one imaginary frequency of −1015.3 cm−1 related to movements of the aryl hydrogen (H4) and the metal-bound hydride (H1) toward each other (Figure 3). In the transition state, considerable shortening of the Ir−C (2.335 to 2.137 Å), Ir−H4 (1.893 to 1.659 Å), and H1−H4 separations (1.965 to 1.398 Å) are noted with respect to G. The dihydrogen ligand in H occupies an equatorial site trans to the ipso-C of the first ppy ligand. The distance between Ir and the η2-H2 centroid is 1.923 Å, which is similar to the literature values.14 Conversion of H to the final product F is a downhill process which involves the release of H2 followed by acetonitrile coordination. Between these two plausible pathways, the oxidative route demands a much higher activation energy in comparison to the electrophilic pathway (24.1 vs 3.1 kcal/mol). In addition, the formation of the IrV dihydride intermediate E is highly endothermic, whereas the formation of the IrIII dihydrogen species H via electrophilic C−H activation is an exothermic process (Figure 3). Hence, an electrophilic activation mechanism is most likely to be operative for the second cyclometalation. Milstein, Martin, and co-workers have recently analyzed the effect of CO on the OA of the aryl C−H bond in cationic RhI complexes.9b It is argued that the metal d orbital energies are lowered in the cationic complex relative to CO molecular orbitals, resulting in weak metal→CO back-donation and enhanced CO → metal electron donation. Detailed electronic structure analyses revealed that the competing σ(CO) → dz2(M) and σ(C−H) → dz2(M) push−push unfavorable interactions is relieved by bending both CO ligands away from their idealized positions. This causes significant alterations in orbital interactions between the metal, C−H, and CO: (1) the C−H → M σ donation is facilitated by redistributing metal electron density (dz2) to π* CO; (2) dxz(M) → σ*(C−H) is assisted by dominant σ donation from both CO's; (3) filled π

Scheme 3. Activation Energies for Oxidative Addition Reactions of Hppy with Different IrI Systemsa

a

Ea = activation energy (kcal/mol).

CH3CN is almost barrierless (0.42 kcal/mol). Replacement of an CH3CN (cis) by CO has no significant effect on the activation energy. However, a CO trans to the aryl C−H bond makes the transition state more energy demanding (11.61 kcal/ mol), which is only 3.7 kcal/mol lower than the bis-CO system. Evidently, the acetonitrile-solvated IrI system would have been the best candidate for cyclometalation; however, a method to access such species does not exist presently.

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CONCLUSION In conclusion, we have reported CO-induced double cyclometalation at the Ir center. The synthetic protocol involves sequential introduction of CO followed by the cyclometalating ligand to a dichloromethane solution of [Ir(COD)(CH3CN)2][BF4]. DFT calculations reveal that the first cyclometalation involves C−H OA of the ligand followed by a preferential electrophilic activation of the second ligand. Efforts to access a wide variety of bis-cyclometalated IrIII complexes from a “{IrI(COD)}” precursor via a CO-induced route are being actively pursued in our laboratory.

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EXPERIMENTAL SECTION

Materials and Methods. All manipulations were carried out under an inert atmosphere with the use of standard Schlenk-line techniques. Solvents were dried by conventional methods, distilled under nitrogen, and deoxygenated prior to use. IrCl3·nH2O was 5537

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purchased from Arora Matthey, India, and 2-phenylpyridine (Hppy) was purchased from Sigma-Aldrich. 2-Phenyl-1,8-naphthyridine (NPPh),15 2-naphthyl-1,8-naphthyridine (NP-npl),15 and [Ir(COD)(μCl)]216 were synthesized according to the literature procedures. Elemental analyses were carried out by using a Thermo quest CE instrument, Model EA/110 CHNS-O elemental analyzer. Infrared spectra were recorded in the range 4000−400 cm−1 on a Vertex 70 Bruker spectrophotometer on KBr pellets. 1H NMR spectra were obtained on a JEOL-JNM LAMBDA 500 model. 1H NMR chemical shifts were referenced to the residual hydrogen signal of the deuterated solvents. ESI-MS analyses were performed on a Waters Micromass Quattro Micro triple-quadrupole mass spectrometer. ESI-MS of all complexes were recorded in acetonitrile. Emission spectra were recorded using a Fluorolog FL3-21(Horiba Jobin Yvon) spectrofluorometer equipped with a xenon flash lamp and also using a PTI QuantaMaster Model QM-4 scanning spectrofluorometer equipped with a 75 W xenon lamp, emission and excitation monochromators, excitation correction unit, and a PMT detector for both visible and near-IR regions. Computational Details. Full geometry optimization and frequency calculations were performed at the gradient-corrected DFT level using the three-parameter fit of the exchange-correlation potential suggested by Becke, in conjunction with the correlation functional suggested by Lee, Yang, and Parr (B3LYP)17 as implemented in Gaussian 03.18 The structures reported are either minima (NIMAG = 0) or transition states (NIMAG = 1) on the potential energy surface. The double-ζ basis set of Hay and Wadt (LanL2DZ) with effective core potential (ECP)19 was used for the Ir. Remaining atoms including H, C, N, and O were described using the 6-31G(d,p) basis sets.20 Solvent effects were accounted using acetronitrile (ϵ = 36.64) by means of geometry optimization on all stationary structures obtained from gas-phase calculations with a conductor like polarizable continuum model (PCM).21 Synthesis of 1. TlBF4 (51 mg, 0.18 mmol) was added to an acetonitrile solution (10 mL) of [IrCl(COD)]2 (54 mg, 0.08 mmol), and the yellow mixture was stirred for 30 min. The TlCl precipitate was removed by Schlenk filtration; NP-Ph (73 mg, 0.33 mmol) was added to the filtrate and stirred for 8 h at room temperature. The resulting solution was concentrated under vacuum, and 15 mL of diethyl ether was added with stirring to induce precipitation. The solid residue obtained was washed with diethyl ether and dried under vacuum. Crystals suitable for X-ray study were obtained by layering hexane onto a dichloromethane solution of the compound. Yield: 106 mg (90%, based on iridium). 1H NMR (CD3CN, δ, ppm): 9.08 (s, 2H, NP), 8.63 (d, 4H, NP), 8.42 (d, 2H, NP), 8.29 (d, 2H, NP), 8.12 (d, 2H, NP), 7.71 (m, 4H, Ph), 7.12 (m, 2H, Ph), 4.07 (br, 2H, COD), 2.55 (s, 2H, COD), 1.88 (br, 2H, COD), 1.71 (br, 2H, COD), 1.51 (br, 4H, COD). 13C{1H} NMR (125 MHz, CD3CN): δ 171.01, 165.12, 163.51, 161.32, 159.28, 156.23, 155.43, 153.24, 151.75, 150.52, 147.42, 145.23, 142.72, 141.45, 139.23, 138.28, 137.21, 135.52, 134.72, 131.23, 129.42, 128.23, 127.39, 125.35, 123.43, 121.24, 120.21, 119.11, 80.43 (CH, COD), 79.35 (CH, COD), 78.34 (CH, COD), 77.12 (CH, COD), 45.42 (CH2, COD), 43.52 (CH2, COD), 41.23 (CH2, COD), 39.32 (CH2, COD). IR (KBr, cm−1): ν(C−H, COD): 2925; ν(BF4−): 1055. Anal. Calcd for C36H32BF4IrN4O2: C, 51.99; H, 3.88; N, 6.74. Found: C, 51.95; H, 3.80; N, 6.71. Synthesis of 2. TlBF4 (64 mg, 0.22 mmol) was added to an acetonitrile solution (10 mL) of [IrCl(COD)]2 (67 mg, 0.10 mmol), and the yellow mixture was stirred for 30 min. The TlCl precipitate was removed by Schlenk filtration; NP-npl (105 mg, 0.41 mmol) was added to the filtrate and stirred for 8 h at room temperature. The resulting solution was concentrated under vacuum, and 15 mL of diethyl ether was added with stirring to induce precipitation. The solid residue obtained was washed with diethyl ether, dried under vacuum, and dissolved in dichloromethane. Subsequently carbon monoxide was bubbled for 10 min through the solution, and the resultant red solution was stirred for 2 h at room temperature. The solution was then concentrated under vacuum, and hexane (15 mL) was added with stirring to induce precipitation. The solid residue obtained was washed with diethyl ether and dried under vacuum. Crystals were grown by

layering hexane onto the dichloromethane solution of the compound. Yield: 128 mg (85%, based on iridium). 1H NMR (CD3CN, δ, ppm): 9.04 (m, 1H, NP), 8.25 (d, 1H, npl), 8.06 (dd, 1H, NP), 8.03 (s, 1H, npl), 7.90 (d, 1H, NP), 7.72 (d, 2H, NP), 7.48-7.43 (q, 2H, npl), 7.39−7.35 (q, 1H, npl), 7.31−7.27 (q, 2H, npl). 13C{1H} NMR (125 MHz, CD3CN): δ 205.15, 154.80, 153.25, 144.13, 143.22, 141.67, 137.73, 136.54, 135.53, 129.61, 127.32, 127.19, 126.23, 121.92, 121.27, 119.27, 117.93, 117.11, 115.25. IR (KBr, cm−1): ν(CO) 2065. ν(BF4−) 1056. Anal. Calcd for C38H24BF4IrN4O2: C, 53.84; H, 2.85; N, 6.61. Found: C, 53.75; H, 2.81; N, 6.57. Synthesis of 3. TlBF4 (82 mg, 0.28 mmol) was added to an acetonitrile solution (10 mL) of [IrCl(COD)]2 (90 mg, 0.13 mmol), and the mixture was stirred for 30 min. The TlCl precipitate was removed by Schlenk filtration, affording a yellow solution. Acetonitrile was removed under vacuum to obtain an oily residue. The highly moisture- and air-sensitive compound [Ir(COD)(MeCN)2](BF4) was redissolved in dichloromethane. Subsequently, carbon monoxide was bubbled for 10 min, turning the solution green. NP-Ph (85 mg, 0.42 mmol) was immediately added to it, and the solution was continued to stir for 36 h at room temperature. The yellow solution was concentrated, and hexane was added to induce precipitation. The solid residue obtained was washed with hexane and dried under vacuum. Crystals suitable for an X-ray study were obtained by layering hexane onto a dichloromethane solution of the compound. Yield: 126 mg (85%, based on iridium). IR (KBr, cm−1): ν(CO) 2086, ν(BF4−) 1060. 1H NMR (CD3CN, δ): 9.38 (br, 1H), 9.09 (br, 2H), 8.79−8.73 (m, 1H), 8.60−8.44 (m, 7H), 8.33 (d, 4H, J = 10 Hz), 7.62−7.79 (m, 8H), 7.63(d, 5H, J = 10 Hz). 13C{1H} NMR (125 MHz, CD3CN): δ 207.09, 173.71, 161.22, 159.56, 148.06, 147.90, 147.27, 143.90, 143.63, 142.85, 142.69, 142.46, 142.26, 141.75, 141.09, 140.67, 140.28, 140.15, 139.98, 139.77, 138.35, 137.65, 134.46, 132.33, 131.82, 129.65, 129.48, 129.32, 129.13, 128.80, 128.52, 128.24, 126.72, 125.38, 124.99, 124.40, 124.24, 123.83, 123.44, 123.24, 122.64, 122.49, 120.84. ESI-MS: m/z (z = 1) 631 [Ir(C6H4-NP)2(CO)]+, 603 [Ir(C6H4−NP)2]+, 649 [Ir(C6H4-NP)2(CO)(H2O)]+. Anal. Calcd for C43H28BF4IrN6O: C, 55.91; H, 3.06; N, 9.10. Found: C, 55.84; H, 2.95; N, 9.01. Synthesis of 4. Compound 4 was synthesized following a procedure similar to the synthesis of 3 using [IrCl(COD)]2 (100 mg, 0.15 mmol), TlBF4 (90 mg, 0.31 mmol), and Hppy (23 mg, 0.15 mmol). Yield: 126 mg (87%, based on iridium). IR (KBr, cm−1): ν(CO) 2037, ν(BF4−) 1067. 1H NMR (CD3CN, δ): 9.05 (d, 1H), 8.63 (d, 1H), 8.25 (t, 1H), 8.11 (d, 1H), 7.96 (br, 1H), 7.71(m, 1H), 7.65 (m, 5H), 7.52−7.25 (m, 5H). 13C NMR (125 MHz, CD3CN): δ 196.72, 169.28, 164.46, 141.46, 139.46, 136.75, 136.56, 132.55, 131.04, 130.58, 129.29, 128.32, 125.64, 125.52, 125.28, 125.13, 124.68, 123.84, 123.59, 122.94, 122.86, 120.16, 119.80. ESI-MS: m/z (z = 1): 570 [Ir(ppy)2(CO)(CH3CN)]+, 529 [Ir(ppy)2(CO)]+, 501 [Ir(ppy)2]+. Anal. Calcd for C25H19BF4IrN3O: C, 45.74; H, 2.92; N, 6.40. Found: C, 45.65; H, 2.85; N, 6.35.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallography details for the compounds and atomic coordinates of the intermediates and the transition states. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Department of Science and Technology (DST) of India and Council of Scientific and Industrial Research (CSIR) of India. J.K.B. thanks 5538

dx.doi.org/10.1021/om300506v | Organometallics 2012, 31, 5533−5540

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the DST for the Swarnajayanti fellowship. S.M.W.R. and S.D. thank the DST of India and T.G. thanks UGC of India for fellowships.

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DEDICATION Dedicated to Professor Luis A. Oro. REFERENCES

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