Experimental and Computational Studies on the Reactivity and

Article pubs.acs.org/Organometallics

Experimental and Computational Studies on the Reactivity and Binding Mode of Thiophene with N‑Heterocyclic Carbene Iridium Complexes Laura Rubio-Pérez,† E. A. Jaseer,‡ Nestor García,‡ Victor Polo,§ Manuel Iglesias,*,† and Luis A. Oro*,†,‡ †

Departamento de Quı ́mica Inorgánica-ISQCH, Universidad de Zaragoza-CSIC, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain Center of Research Excellence in Petroleum Refining & Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia § Departamento Quı ́mica Fı ́sica-Instituto de Biocomputación y Fı ́sica de Sistemas Complejos (BIFI), Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain ‡

S Supporting Information *

ABSTRACT: The reactivity of thiophene (T), 2-methylthiophene (2-MeT), and benzothiophene (BT) with [Ir(cod)(IPr)(L)]BF4 complexes (L = acetone (1), pyridine (2) or dimethylphenylphosphine (3); IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene) in the presence of molecular hydrogen has been investigated. Under these conditions the 1,5-cyclooctadiene ligand is hydrogenated to cyclooctane, which renders an unsaturated species (Ir-IPr-L) able to coordinate the thiophene moiety. The coordination mode of T, 2-MeT, and BT depends on the nature of the substrate and the ligand trans to the IPr (L). The reaction of 1 with T and 2-MeT leads to dissociation of the acetone ligand to afford the complexes [Ir(H)2(IPr)(η5-T)]BF4 (4) and [Ir(H)2(IPr)(η22-MeT)(κS-2-MeT)]BF4 (5), respectively, but no stable complex is observed on reaction with BT. Analogously to 1, complex 2 does not give a stable complex on reaction with BT, while reaction with 2-MeT yields complex 5 again. Conversely, reaction with T affords a mixture of complexes, [Ir(H)2(IPr)(η2-T)(Py)]BF4 (6) and [Ir(H)2(IPr)(κS-T)2(Py)]BF4 (6′), both featuring a coordinated pyridine ligand. The reaction of 3 with T, 2-MeT, and BT yields in all cases κS complexes, namely [Ir(H)2(IPr)(κST)2(PPhMe2)]BF4 (7), [Ir(H)2(IPr)(κS-2-MeT)2(PPhMe2)]BF4 (8) in equilibrium with [Ir(H)2(IPr)(κS-2-MeT)(PPhMe2)]BF4 (8′), and [Ir(H)2(IPr)(κS-BT)2(PPhMe2)]BF4 (9). Finally, DFT calculations were employed to rationalize the coordination modes of T, 2-MeT, and BT, as well as the tendency of these complexes to undergo hydrogenation instead of hydrogenolysis of the thiophene moiety under catalytic conditions.

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INTRODUCTION The design of organometallic complexes that could mimic traditional heterogeneous catalysts in the hydrodesulfurization reaction was subjected to intensive investigation at the end of the 20th century. Although it was unsuccessful at reaching its ultimate goal, this work provided invaluable information regarding the intimate mechanisms involved in HDS processes.1,2 These studies have focused primarily on the reactivity of thiophene (T), benzothiophene (BT), and dibenzothiophene (DBT) with transition-metal complexes, as they are the most unreactive organosulfur compounds found in fossil fuels. The steps of the HDS process that have been more thoroughly studied are the coordination to the metal center and the C−S bond splitting. The most commonly proposed coordination modes for the thiophene molecule to transition-metal complexes are κS and η5. However, coordination by two and four carbons of the thiophene ring (η2 and η4 coordination modes, respectively) have also been postulated. On the other hand, coordination by the benzene ring in BT has been proposed to occur in a η6 or η4 © XXXX American Chemical Society

fashion in addition to the possibilities mentioned above for T (Chart 1). DBT may present all the previously described coordination modes except for the coordination via the π system of the thiophene ring.3 Authenticated examples of coordination complexes featuring κS-bonded thiophene moieties have been scarcely reported as a result of their low stability, which has limited the study of their Chart 1. Proposed Coordination Modes for T and BT

Received: December 5, 2015

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DOI: 10.1021/acs.organomet.5b00995 Organometallics XXXX, XXX, XXX−XXX

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Organometallics reactivity and spectroscopic properties.4 Regarding the adsorption of thiophene on heterogeneous catalysts, it is worth mentioning that the κS, η2, and η5 interactions were identified by infrared spectroscopy on MoS2 and Co-Mo/ Al2O3.5 Studies on single-crystal surfaces, e. g. Ni(100), Pt(111), Mo(100), and Cu(100), have shown κS and η5 interactions that depend primarily on the metal and temperature.6 The coordination modes of T or BT to the metal center have been proposed to be governed by the nature of the organometallic complex, electron density and steric hindrance being of paramount importance. More encumbered complexes favor κS coordination modes; however, an increase of electron density at the metal center has been reported to favor η2 coordination.2a,7 The coordination mode is of paramount importance because, according to literature data on mononuclear complexes that promote C−S bond splitting,8 κS coordination appears to be key to yield the corresponding C−S activated metallacycle. This reactivity pattern has been rationalized in terms of an increased back-donation from the metal center into a LUMO orbital of the thiophene molecule with C−S antibonding character, which leads to C−S bond splitting and migration of the α-carbon into the metal to afford the insertion product.8a,9 This process is, consequently, facilitated by electron-rich metal centers.8b Noteworthy, the η2 coordination mode may also bring about the C−S bond cleavage thanks to a κS−η2 equilibrium.8g,7 The great success of N-heterocyclic carbene (NHC) complexes in homogeneous catalysis10 prompted us to study their reactivity with the thiophene moiety in detail. In this work, we have studied the coordination chemistry of thiophene, benzothiophene, and 2-methylthiophene with various Ir-NHC complexes. Variable-temperature NMR experiments were performed in order to investigate the labile nature of the Ir− substrate interaction in these complexes. Moreover, these systems have been studied by DFT calculations in order to achieve a better understanding of the reactivity patterns reported here.

Scheme 1. Synthesis of Complexes 3 and 4

sterically hindered phosphines, namely triphenylphosphine or diphenylmethylphosphine, resulted in the formation of a mixture of starting material and unstable phosphine-containing species. Complex 3 shows as the most representative resonances in the 1H NMR spectra two septets that correspond to the two different types of CHMe2 protons at the wingtip groups of the NHC ligand, which appear at δ 3.08 and 2.42 ppm, both of with a coupling constant of 6.8 Hz. This agrees with the fact that four doublets are observed for the methyl groups at the N substituents at δ 1.41, 1.15, 1.12, and 0.98 ppm (JH−H = 6.8 Hz for all). The existence of two different isopropyl groups can be explained in terms of the restricted rotation of the NHC ligand around the C−Ir bond. Assuming that the NCN plane of the NHC ligand is perpendicular to the coordination plane,14 the isopropyl groups at the same diisopropylphenyl substituent must be inequivalent, because one of them would be closer to the olefin and the other to the phosphine. On the other hand, the symmetry plane that matches the coordination plane bisects the NHC, rendering both N substituents equivalent. This behavior contrasts with the fluxionality observed for 2, which at room temperature, probably due to the lower steric hindrance of the pyridine ligand, shows only one broad peak for the CHMe2 protons. In the 13C{1H} NMR spectra, the carbene carbon appears as a doublet due to coupling with the phosphorus atom, at δ 181.9 ppm (2JH−P = 8.7 Hz), while the olefinic carbon atoms of the cod ligand emerge as a doublet at δ 84.5 ppm (2JH−P = 11.7 Hz) and as a singlet at δ 74.8 ppm. The 31P NMR spectra for complex 3 present a peak at δ −16.2 ppm due to the phosphine ligand. The 19F NMR shows a peak at δ ca. −152 ppm for each of the complexes 1−9 that has been assigned to the BF4− counterion, thus supporting their cationic nature. Reactivity of [Ir(cod)(IPr)L]BF4 Complexes with T, 2MeT, and BT. Complexes 1−3 were treated with an excess of thiophene (T), 2-methylthiophene (2-MeT), or benzothiophene (BT) in THF under a dihydrogen atmosphere (1 bar). The hydrogenation of the cod ligand, confirmed by the appearance of 1 equiv of cyclooctane in the 1H NMR spectra, generates in situ an unsaturated Ir(NHC) complex able to react with the corresponding substrate present in solution. The solvent was then evaporated and the residue washed with dry pentane to afford off-white solids in all cases. The use of more coordinating solvents such as diethyl ether resulted in the dissociation of the thiophene moiety and decomposition of the complex. The T, 2-MeT, and BT complexes were studied by NMR using CD2Cl2 as a deuterated solvent. In addition, variable-temperature NMR studies were carried out for some of the complexes in order to investigate their fluxional behavior. Attempts to obtain crystals suitable for X-ray diffraction always resulted in the decomposition of the complex. The lability of the Ir−thiophene bond resulted in extremely unstable complexes. In fact, total decomposition of all isolated complexes was observed within days even when they were

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RESULTS AND DISCUSSION Synthesis of [Ir(cod)(IPr)L]BF4 Complexes. Initial synthetic work aimed at the preparation of various [Ir(cod)(IPr)L] type complexes (cod = 1,5-biscyclooctadiene, IPr = 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Ligands with different donating abilities (L = acetone, pyridine, dimethylphenylphosphine) were employed in order to assess how the electron density at the metal center would influence the coordination chemistry of thiophene and its derivatives. The complex [Ir(cod)(IPr)(Me2CO)]BF4 (1) was synthesized according to a known literature procedure.11 Complexes [Ir(cod)(IPr)(Py)]BF412 (2; Py = pyridine) and [Ir(cod)(IPr)(PMe2Ph)]BF4 (3) were prepared from 1 by substitution of the acetone ligand with pyridine and dimethylphenylphosphine, respectively, in THF at room temperature. Both complexes were isolated as air-stable orange-red powders in good to excellent yields (Scheme 1). The characterization of 2 has been already reported by Hope et al.12a and that of its PF6 derivative by Nolan et al.12b Complexes analogous to 3, of formula [Ir(cod)(NHC)(Phosphine)]+, have been described with the IMes ligand (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2ylidene);12a,13 however, to the best of our knowledge this is the first complex of this class featuring an IPr ligand. In this regard, attempts to prepare complexes analogous to 3 using more B


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Complex [Ir(H)2(IPr)(η2-2-MeT)(κS-2-MeT)]BF4 (5). Analogously to what has been described for the preparation of 4, the reaction of 1 with 2-MeT leads to the displacement of the acetone ligand and formation of a dihydride complex; conversely, in this case, two 2-MeT ligands coordinate to the metal center (5) (Scheme 2). The shifts observed for the thiophene ligands in the 1H NMR spectra at 253 K suggest the presence of two different coordination modes. One of the thiophene rings would be coordinated in a κS fashion, showing peaks for the aromatic protons at δ 7.10, 6.91, and 6.79 ppm, and the other would coordinate in a η2 fashion by one of the two double bonds according to the high-field shifts of the resonances assigned to the aromatic protons: namely, δ 6.06, 5.71, and 4.66 ppm. A η4 coordination mode has been discarded on the basis of NOESY experiments (vide infra) and the difference of ca. 3 ppm in the chemical shifts between both hydride peaks, since almost identical chemical shifts for both hydride resonances would be expected in the case of a η4 coordination mode. The two doublet resonances observed in the high-field region, at δ −15.4 and −18.3 ppm (2JH−H = 5.0 Hz), were assigned to the two inequivalent hydride ligands. The 13C NMR spectra support this postulation with peaks at δ 139.8, 127.1, 125.2, and 123.1 ppm for the κS coordination and δ 108.4, 98.4, 94.1, and 85.6 ppm for the η2 mode of thiophene. The 2DNOESY NMR provides valuable information that supports the proposed structure (Supporting Information). The aromatic protons and the methyl group of the η2-2-MeT show NOE with the methyl and CH protons of the isopropyl moieties, which implies that it must be cis to the IPr ligand. Moreover, the hydride ligand, cis to the η2-2-MeT, presents NOE with only two of the aromatic CHs of η2-2-MeT, thus supporting the η2 coordination mode, since no NOE is observed between the hydride ligand and the third aromatic CH or the methyl group. On the other hand, the κS-2-MeT, trans to the IPr ligand, only shows NOE between its methyl group and the hydride ligand cis to the vacant position, which suggests that the methyl group is oriented toward the vacancy, as this would be the least sterically hindered conformation. The fact that NOE is observed between the hydrides and both 2-MeT ligands supports the fact that both ligands are coordinated to the same Ir center, since both hydrides also belong to the same complex according to the 1H COSY NMR spectra (Supporting Information). The most significant change observed in the variabletemperature 1H NMR is the shift to higher fields of the peak that corresponds to the proton in a position β to the sulfur atom of the η2-coordinated 2-MeT from δ 4.89 ppm at 293 K to δ 4.20 ppm at 183 K, which may be interpreted as a consequence of the stronger interaction between the thiophene ligand and the metal center at lower temperatures (Figure 1). The different reactivity between T and 2-MeT could be ascribed to the greater steric hindrance of the latter, which results in an increased tendency for κS coordination.2a,7 The reaction of 1 with BT does not afford a stable complex even at low temperatures. Complexes [Ir(H)2(IPr)(η2-T)(Py)]BF4 (6) and [Ir(H)2(IPr)(κST)2(Py)]BF4 (6′). The reaction of 2 with thiophene results in the formation of 6 and 6′, the former as the major product (Schemes 3 and 4). The 1H NMR spectra confirm that, in both complexes, the pyridine ligand remains in the coordination sphere of the iridium center. The 13C NMR spectra support the presence of two different species, since one resonance for each

kept inside a glovebox. It is worth noting that all of the thiophene complexes decompose instantaneously in air, thus precluding further analysis by mass spectrometry or microanalysis. In order to facilitate the understanding of the NMR data discussed in the following sections, and for comparison purposes, the chemical shifts in CD2Cl2 of the thiophene moiety in the 1H NMR and 13C NMR spectra of free thiophene, 2-methylthiophene, and benzothiophene are available in the Supporting Information. Complex [Ir(H)2(IPr)(η5-T)]BF4 (4). The reaction of 1 with thiophene affords a dihydride NHC complex with one thiophene ligand coordinated by the π system that should occupy the three vacant positions left, since no coordinated acetone is present in the NMR spectra, which suggests a η5 coordination mode (Scheme 2). Scheme 2. Reactivity of 1 with T, 2-MeT, and BT

The 1H NMR spectra show a mixture of two complexes: 4 and its decomposition product. The latter has been identified as the dimeric complex [{Ir(μ-κCNHC,η6Dipp-IDipp)(H)}2][BF4]2, previously reported by us.11 Attempts to purify the mixture resulted in total decomposition of the complex, probably by loss of the thiophene ligand. Complex 4 is only briefly stable at room temperature; therefore, the NMR characterization was performed at low temperature (253 K). The NMR spectra at temperatures lower than 253 K show no significant changes. Diagnostic peaks for the coordination by the π system are those of the CH protons of thiophene at δ 6.22 and 5.81 ppm in the 1 H NMR spectra and the related carbon atoms at δ 92.7 and 91.6 ppm in the 13C NMR spectra. It is worth noting that the 1 H NMR and 13C NMR spectra of free thiophene in CD2Cl2 show two multiplets centered at δ 7.43 and 7.19 ppm and two singlets at δ 126.9 and 125.1 ppm, respectively. Moreover, the integration of the 1H NMR resonances confirms the presence of one IPr ligand, two hydrides, and only one thiophene ligand. DFT calculations were performed in order to shed light on the type of interaction between the thiophene and the iridium center. The thiophene ring presents the planar structure expected for a η5 coordination mode with three of the carbon atoms slightly closer to the metal center (2.28, 2.30, and 2.54 Å) than the sulfur atom and the fourth carbon atom (2.67 and 2.74 Å, respectively) (Supporting Information). C


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field, δ 92.8 and 91.8 ppm. This seems to suggest that the coordination takes place by the π system, considering that all nuclei in the thiophene moiety are shifted to high field in the 1 H and 13C NMR spectra. Nevertheless, a η4 coordination fashion by both double bonds is highly unlikely, due to the strong trans effect of the hydride ligand.15a,b,e Therefore, the fact that all of the protons and carbon nuclei are similarly shifted in the NMR spectra may be explained by the fluxional nature of the metal−ligand bond. According to the 1H and 13C NMR spectra of 6′, on the other hand, two thiophene molecules are coordinated by the sulfur atom (κS-T), as the resonances are significantly shifted to lower fields: namely, δ 7.29 and 7.11 ppm in 1H NMR and δ 124.7 and 124.0 ppm in 13 C NMR. The pyridine ligands in complexes 6 and 6′ are presumably in positions trans to the IPr ligand because selective 1 H−1H NOESY NMR experiments show a NOE between the CH3 protons of the isopropyl groups and the aromatic protons of the thiophene (see the Supporting Information), which is in agreement with previous examples.15 DFT calculations are in agreement with the NMR data concerning the position of the pyridine ligand. However, the most stable coordination for the thiophene ligand in complex 6 would be the η2 mode by only one of the double bonds, thus leaving a vacant coordination site trans to one of the hydride ligands. The high trans effect of the hydride ligand has been invoked in order to rationalize similar situations in the literature.15a,b,e It is worth noting that our theoretical calculations show that the κS, η1, and η2 coordination modes are very similar in energy, the relative energy between them being less than 2.4 kcal/mol (see the Supporting Information). Therefore, in the case of 6, at higher temperatures we probably observe an average resulting from two different fluxional processes: (i) hydride exchange, which renders the two hydride ligands equivalent, and (ii) the pseudorotation of the thiophene ring, which switches from η2 by one or the other double bond to κS and even to various η1 coordination modes. This would affect the chemical shifts of the four carbon atoms and protons of the thiophene ligand in the 13C and 1H NMR, respectively, resulting in the previously described shielding of these resonances. Variable-temperature 1H NMR seems to support the postulated fluxional processes for 6, as the singlet at δ −24.71 ppm at 263 K, corresponding to the two hydride ligands, broadens at lower temperatures to afford two doublets (JHH = 5.9 Hz) at 183 K. Furthermore, the two peaks assigned to the thiophene protons in complex 6 at 263 K become four broad singlets at lower temperatures (see the Supporting Information). The reactivity of 2 is in contrast with that of 1 in the sense that, in the case of complex 4, the acetone ligand dissociates to give the η5-coordinated thiophene ligand, while in the case of 6 the pyridine ligand remains coordinated, probably due to the stronger bond that the metal forms with pyridine in comparison to acetone. The reactivity of 2 with 2-MeT and BT is analogous to that described for 1 (vide supra). At variance with the reactivity described above, when an excess of T, 2-MeT, or BT is added to a solution of the phosphine derivative 3 in THF, two thiophene moieties coordinate in a κS fashion in positions trans to the two hydrido ligands in the equatorial plane to give complexes 7−9 (Scheme 5).

Figure 1. Variable-temperature 1H NMR of 5.

Scheme 3. Equilibrium between 6 and 6′

Scheme 4. Reactivity of 2 with T, 2-MeT, and BT

of the carbene carbons comes about at δ 153.1 and 153.5 ppm. In the 1H NMR spectra of complex 6 the four aromatic protons of the thiophene ligand are significantly shifted to higher fields, at δ 6.05 and 5.60 ppm at 263 K, and the 13C NMR spectra shows the resonances assigned to the carbon atoms also at high D


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shifts displayed by the carbon atoms in the 13C NMR spectra at 223 K, which appear as two singlets at δ 131.6 and 130.3 ppm. The 31P{1H} NMR spectra show a singlet at δ −27.3 ppm. The 13 C NMR spectra present two singlets at δ 131.6 and 130.3 ppm that are assigned to the two chemically inequivalent carbon atoms of the thiophene ring, while the carbene carbon appears as a doublet due to scalar coupling with the 31P of the phosphine ligand at δ 163.1 ppm (2JC−P = 108.9 Hz). Analysis of the optimized geometrical parameters of complex 7 at the DFT level shows that the Ir−S distances are 2.53 and 2.55 Å, corresponding to a weak metal−sulfur interaction. It is worth noting that, in order to compare the stability of the κS and the η2 coordination modes of thiophene to the iridium center in this scaffold, the energy of [Ir(H)2(IPr)(κS-T)(η2T)(PPhMe2)]BF4 was also calculated, revealing an energy increase of 8 kcal/mol (Supporting Information). Complexes [Ir(H)2(IPr)(κS-2-MeT)2(PPhMe2)]BF4 (8) and [Ir(H)2(IPr)(κS-2-MeT)(PPhMe2)]BF4 (8′). The 1H NMR spectrum of complex 8 shows noticeable differences in comparison to that of 7 at 298 K; while the hydride resonance was observed as a very broad peak at δ −19.0 ppm for 7, a sharp doublet at δ −21.38 ppm emerges for complex 8. Variable-temperature 1H NMR studies on complex 8 (Figure 3) show that the doublet at δ −21.38 ppm at 293 K becomes an

Scheme 5. Reactivity of 3 with T, 2-MeT, and BT

Complex [Ir(H)2(IPr)(κS-T)2(PPhMe2)]BF4 (7). The 1H NMR spectra of complex 7 at 293 K show a broad peak for the hydride ligands at δ −19.0 ppm, while the resonances of the thiophene protons disappear into the baseline. At 223 K two multiplets assigned to the thiophene protons, which integrate to four protons each, emerge at δ 7.03 and 6.27 ppm. When the temperature is decreased below 223 K, no remarkable changes are observed (Figure 2). The peak assigned to the hydride ligands at δ −18.4 ppm resolves at 223 K into a doublet due to coupling with the phosphorus nucleus (2JH−P = 17.2 Hz). The κS coordination of the thiophene ligands is further supported by the low-field

Figure 3. Variable-temperature 1H NMR of 8 and 8′.

apparent doublet of triplets at δ −21.17 ppm at 193 K. This species, which presents a 2:1 ratio of hydride to 2-MeT, formed by loss of one 2-methylthiophene ligand has been named 8′. At 193 K a new doublet (2JH−P = 16.3 Hz) appears at −17.90 ppm that corresponds to 8 on the basis of the relative integration of the hydrides and 2-MeT resonances in 1H NMR. This peak broadens at higher temperatures, eventually disappearing into the baseline. The multiplicity of the hydride resonance at higher field, which corresponds to the hydride ligands in 8′ (Scheme 6), can

Figure 2. Variable-temperature 1H NMR of 7. E


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permits a more intense π back-donation to the coordinated ligand, thus strengthening the Ir−S bond. Hydrogenation vs Hydrogenolysis of the C−S Bond. The reactivity of complex 3 with benzothiophene was tested under 10 bar of hydrogen pressure at 100 °C, since previous reports postulate that the κS coordination of the thiophene moiety promotes hydrogenolysis against hydrogenation pathways.8a,9 When the reaction was performed with 1 mmol of benzothiophene and 0.01 mmol of 3, the formation of 3.3% of hydrogenation product, namely 1,2-dihydrobenzothiophene, was observed after 48 h (3.3 TON); however, no hydrogenolysis of the C−S bond occurs under these reaction conditions. The hydrogenolysis reaction has been proposed to take place by oxidative addition of the C−S bond to yield A, followed by reductive elimination of the alkenyl and the hydride ligand to eventually render intermediate B (Figure 4).16

Scheme 6. Equilibrium between 8 and 8′

be rationalized as two collapsed doublets of doublets attributable to two inequivalent hydride ligands coupled to one another and to the phosphine ligand. A plausible explanation for the chemical inequivalence of the hydrides is that the dissociation of one of the 2-MeT ligands gives a pentacoordinated species with a vacant coordination site trans to one of the hydrides (8′). At high temperature the two hydride ligands would become equivalent by an exchange process that swaps the positions of the 2-MeT ligand and vacant site trans to the two hydrides. An alternative postulation for the structure of this species would be the isomerization of 8 to give a complex with a phosphine cis to the NHC and a 2-MeT ligand coordinated trans to the NHC; however, the large coupling constant expected for a hydride ligand situated in a position trans to a phosphane safely discards this possibility. The equilibrium 8 ↔ 8′ is supported by the appearance of an exchange peak between the two hydride resonances in the 1 H−1H 2D-NOESY NMR and by the fact that the addition of excess 2-MeT drives the equilibrium to the formation of 8. The existence of the postulated equilibrium is supported by the hydride:2-MeT ratio observed in the 1H NMR spectra for 8 and 8′ and the appearance of peaks that correspond to free 2MeT. At 223 K the 13C NMR spectra show one resonance for each of the carbene carbons of the two species, which appear as two doublets due to the coupling with the trans-situated phosphines at δ 162.2 and 167.7 ppm (2JC−P = 113.9 and 2JC−P = 113.9 Hz, respectively). The 31P NMR spectra show two singlets at δ −28.7 and −24.2 ppm for 8 and 8′, respectively. The 13C NMR spectra show peaks for the coordinated thiophene ligands in 8 at δ 145.0, 130.8, 129.2, and 128.6 ppm and those in 8′ at δ 140.0, 125.3, 124.2, and 123.3 ppm. The 1H NMR spectra show three peaks at δ 6.95, 6.87, and 6.23 ppm for 8 and at δ 7.11, 6.91, and 6.79 ppm for 8′. In both cases, 1H and 13C NMR, the resonances in 8 and 8′ are only slightly shifted in comparison to those observed for free 2-MeT. Complex [Ir(H)2(IPr)(κS-BT)2(PPhMe2)]BF4 (9). At variance with complexes 1 and 2, in this case, the BT molecule coordinates to the metal center, forming a stable complex (9). The variable-temperature 1H NMR spectra show patterns very similar to those of 7, with the protons of the thiophene moiety appearing at δ 7.32 and 6.23 ppm and the hydrides at −17.63 ppm as a doublet (2JH−P = 17.6 Hz) at 223 K. The carbon atoms of the thiophene double bond show peaks at δ 129.7 and 129.1 ppm in the 13C NMR spectra, shifted slightly downfield in comparison to free benzothiophene (δ 126.4 and 123.6 ppm). Analogously to 7, the 31P NMR shows a singlet at δ −31.0 ppm. The greater stability of the BT complex formed by reaction with 3 in comparison to those obtained by reaction with 1 or 2 may be rationalized in terms of the higher electron density at the iridium center provided by the phosphine ligand, which

Figure 4. Relative energies calculated for intermediates A−C.

On the other hand, the attack of a hydride ligand at the αcarbon of a coordinated thiophene molecule and subsequent formation of an allyl or alkyl ligand has been proposed in the literature as the initial step for the hydrogenation reaction, which would yield an intermediate analogous to that depicted in Figure 4 (C).17 It is also plausible that the same attack would lead to C−S bond cleavage and the direct formation of B. Theoretical calculations at the DFT level show that the relative energies of intermediates A (48.3 kcal/mol) and B (39.1 kcal/ mol) are markedly higher than that of C (15.3 kcal/mol) and, as a matter of fact, the formation of A or B is not viable under the reaction conditions. It is worth noting that, although the energy barrier for the formation of C is affordable, the hydrogenated product, 1,2-dihydrobenzothiophene in the case under study, is a strongly coordinating ligand that blocks the access of new benzothiophene molecules to the metal center. Thus, the deactivation of the catalyst by the thioether may account for the low TON values obtained for this reaction.

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CONCLUSIONS The study of the reactivity of complexes 1−3 toward thiophene, 2-methylthiophene, and benzothiophene has revealed that the coordination mode of the thiophene moiety and, consequently, the number of coordinated thiophenes F


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Organometallics

Hp‑py), 7.51 (d, 3JH−H = 7.7, 2H, Hm‑IPr), 7.47 (d, 3JH−H = 5.5, 2H, Ho‑py), 7.23 (d, 3JH−H = 7.7, 2H, Hm‑IPr), 7.18 (dd, 3JH−H = 7.7, 5.5, 2H, Hm‑py), 7.17 (s, 2H, CHN), 3.58 and 3.04 (both br, 4H, CHCOD), 3.19 and 2.46 (both sept, 3JH−H = 6.7, 4H, CHMeIPr), 2.0−1.5 (all m, 8H, CH2COD), 1.50, 1.26, 0.91, and 0.81 (all d, 3JH−H = 6.7, 24H, CHMeIPr). 13C{1H}-APT NMR plus HSQC and HMBC (75 MHz, CDCl3, 233 K): δ 175.2 (s, Ir−CIPr), 150.1 (s, Co‑py), 145.4 and 145.3 (both Cq‑IPr), 137.7 (s, Cp‑py), 135.7 (s, CqN), 130.7 (s, Cp‑IPr), 126.4 (s, Cm‑py), 124.8 and 124.4 (both s, Cm‑IPr), 83.2 and 65.5 (both s,  CHCOD), 32.0 and 29.1 (both s, CH2COD), 29.0 and 28.6 (both s. CHMeIPr), 25.9, 25.7, 22.7, and 21.5 (all s, CHMeIPr). 19F NMR (300 NMR, CDCl3, 233 K): δ −152.8 (s, BF4). [Ir(cod)(IPr)(PMe2Ph)]BF4 (3). A solution of [Ir(COD){OC(CH3)2}(IPr)]BF4 (1; 200 mg, 0.240 mmol) in THF (5 mL) was treated with dimethylphenylphosphine (34 μL, 0.240 mmol) and stirred for 2 h at room temperature. The resulting red solution was concentrated to ca. 0.5 mL, and the addition of diethyl ether produced a red solid, which was separated by decantation, washed with diethyl ether, and dried in vacuo. Yield: 88% (193 mg, 0.210 mmol). 1H NMR (400 MHz, CD2Cl2, 298 K): δ 7.48 (t, 3JH−H = 7.8, 2H, Hp‑IPr), 7.39 (dd, 3JH−H = 7.8, 7.6, 2H, Hm‑Ph‑P), 7.33 and 7.10 (d, 3JH−H = 7.8, 4H, Hm‑IPr), 7.32 (t, 3JH−H = 7.8, 1H, Hp‑Ph‑P), 7.30 (s, 2H, CHN), 7.09 (m, 2H, Ho‑Ph‑P), 4.49 and 3.46 (both br, 4H, CHCOD), 2.98 and 2.32 (both sept, 3JH−H = 6.8, 4H, CHMeIPr), 1.7−1.6 (m, 8H, CH2COD), 1.47 (d, 2JH−P = 8.1, 6H, Me-P), 1.30, 1.04, 1.01, and 0.87 (all d, 3JH−H = 6.8, 24H, CHMeIPr). 13C{1H}-APT NMR plus HSQC and HMBC (100 MHz, CD2Cl2, 298 K): δ 181.9 (d, 2JC−P = 8.6, Ir− CIPr), 146.3 and 145.6 (both s, Cq‑IPr), 135.9 (s, CqN), 134.8 (d, 1JC−P = 52.3, CqPh‑P), 133.6 (d, 3JC−P = 12.8, Cm‑Ph‑P), 131.5 (d, 4JC−P = 2.5, Cp‑Ph‑P), 131.3 (s, Cp‑IPr), 129.3 (d, 2JC−P = 10.8, Co‑Ph‑P), 127.2 (s,  CHN), 125.2 and 124.7 (both s, Cm‑IPr), 84.8 (d, 2JC−P = 11.7, 2H,  CHCOD), 75.2 (s, 2H, CHCOD), 30.8, 30.7, 30.6, and 30.5 (all s, CH2COD), 29.9 and 29.7 (both s, CHMeIPr), 27.2, 26.8, 22.1, and 21.7 (all s, CHMeIPr), 15.1 (d, 1JC−P = 32.8, Me-P). 19F NMR (400 NMR, CD2Cl2, 298 K): δ −153.2 (s, BF4). 31P NMR (121 NMR, CD2Cl2, 298 K): δ −16.3 (s, PPhMe2). [Ir(H)2(IPr)(η5-T)]BF4 (4). A solution of [Ir(COD){OC(CH3)2}(IPr)]BF4 (1; 80 mg, 0.150 mmol) in acetone/thiophene (20/1, 5 mL) was stirred under a dihydrogen atmosphere (1 bar) for 30 min. The pale yellow solution obtained was filtered through Celite and dried in vacuo. The resulting residue was washed with pentane to give a beige oil. 1H NMR (300 MHz, CD2Cl2, 253 K): δ 7.81 (t, 3JH−H = 7.7, 2H, Hp‑IPr), 7.60 (d, 3JH−H = 7.7, 4H, Hm‑IPr), 7.46 (s, 2H,  CHN), 6.22 and 5.81 (both m, 4H, HTh), 2.66 (sept, 3JH−H = 6.7, 4H, CHMeIPr), 1.52 and 1.36 (both d, JH−H = 6.7, CHMeIPr), − 16.91 (s, 2H, Ir−H). 13C{1H}-APT NMR plus HSQC and HMBC (75 MHz, CDCl3, 253 K): δ 145.7 (s, Cq‑IPr), 135.3 (s, CqN), 136.5 (s, Ir−CIPr), 131.2 (s, Cp‑IPr), 124.8 (s, Cm‑IPr), 124.7 (s, CHN), 92.7 and 91.6 (both s, CTh), 29.1 (s, CHMeIPr), 24.9 and 22.5 (both s, CHMeIPr). 19F NMR (300 NMR, CDCl3, 253 K): δ −152.2 (s, BF4). [Ir(H)2(IPr)(η2-2-MeT)(κS-2-MeT)]BF4 (5). A solution of [Ir(COD){OC(CH3)2}(IPr)]BF4 (1; 50 mg, 0.060 mmol) in acetone/ 2-methylthiophene (20/1, 5 mL) was stirred under a dihydrogen atmosphere (1 bar) for 1 h. The pale yellow solution obtained was filtered through Celite and dried in vacuo. The resulting residue was washed with pentane to give a beige oil. 1H NMR (300 MHz, CD2Cl2, 253 K): δ 7.64 (t, 3JH−H = 7.6, 2H, Hp‑IPr), 7.43 (d, 3JH−H = 7.6, 4H, Hm‑IPr), 7.28 (s, 2H, CHN), 7.10 (d, 3JH−H = 4.9, 1H, Th2‑a), 6.91 (dd, 3JH−H = 4.9, 3.2, 1H, Th3‑a), 6.79 (d, 3JH−H = 3.2, 1H, Th4‑a), 6.06 (dd, 3JH−H = 3.4, 2.8, 1H, Th3‑b), 5.71 (d, 3JH−H = 2.8, Th4‑b), 4.66 (d, 3 JH−H = 3.4, 1H, Th2‑b), 2.50 (s, 3H, Meb), 2.47 (sept, 3JH−H = 6.9, 4H, CHMeIPr), 2.41 (s, 3H, Mea), 1.37, 1.30, 1.20, and 1.17 (all d, 3JH−H = 6.9, 24H, CHMeIPr), − 15.4 and −18.3 (both d, 3JH−H = 5.0, 2H, Ir− H). 13C{1H}-APT NMR plus HSQC and HMBC (75 MHz, C6D6, 253 K): δ 153.5 (s, Ir−CIPr), 146.0 and 145.7 (both s, Cq‑IPr), 139.8 (s, Th5‑a), 136.7 (s, CqN), 131.2 (s, Cp‑IPr), 127.1 (s, Th3‑a), 125.2 (s, Th4‑a), 124.9 and 124.8 (both s, Cm‑IPr), 123.8 (s, CHN), 123.1 (s, Th2‑a), 108.4 (s, Th5‑b), 98.4 (s, Th3‑b), 94.1 (s, Th2‑b), 85.6 (s, Th4‑b), 29.2 (s, Mea), 29.1 (s, CHMeIPr), 16.5 (s, Meb), 25.1, 24.9, 22.6, and

depend to a great extent on the nature of the ligand system and on subtle changes on the electronic and steric properties of the thiophene derivative. Weak donor ligands such as acetone and pyridine permit various types of coordination by the π system; however, when a strongly donating phosphine ligand is employed, only the κS coordination mode is observed. This is in contrast with previous reports, which suggest that an increased electron density at the metal center favor the η2(C,C) vs the κS coordination mode. We have proved by means of the NMR characterization of complexes 4−9 that the coordination modes of T, 2-MeT, and BT can be easily identified on the basis of the chemical shifts in 1 H and 13C NMR of the thiophene-based ligands when assisted by NOESY experiments. As a general rule it can be concluded that, in comparison to the free ligand, coordination by the π system results in shifts to lower frequencies, while the κS coordination mode results in subtle changes to higher frequencies. Variable-temperature NMR experiments and DFT calculations provide additional information that permits a better understanding of the dynamic behavior of these species. In this regard, the weak metal−substrate bond originates coordination−dissociation equilibria and a fluxional behavior due to low energy differences between various coordination modes. Finally, the use of complex 3 as catalyst for the reduction of benzothiophene resulted in the formation of small amounts of 1,2-dihydrobenzothiophene, resulting from the hydrogenation of the double bond at the thiophene moiety. It is worth noting that no product from the hydrogenolysis of the C−S bond is observed. This behavior can be explained on the basis of the high energy obtained for the hydrogenolysis intermediates by DFT calculations. Therefore, although κS coordination may be important to promote the C−S bond splitting, there are other factors that greatly influence the selectivity of the reaction. For example, the presence of hydride ligands may lead to the nucleophilic attack of a hydride at the α-carbon of a coordinated thiophene moiety as the lowest energy pathway, which would lead to the hydrogenation product.

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EXPERIMENTAL SECTION

General Considerations. All experiments were carried out under an inert atmosphere by using standard Schlenk techniques. The solvents were dried by known procedures and distilled under argon prior to use or obtained oxygen- and water-free from a Solvent Purification System (Innovative Technologies). The starting complexes [Ir(COD)(μ-Cl)]2, [Ir(μ-OMe)(COD)]2, and [Ir(IPr)(OCMe2)]BF4 were prepared according to the literature procedure.18−20 All other chemicals were used as purchased from SigmaAldrich, Merck and J. T. Baker. H2 gas (>99.5%) was obtained from Infra. 1H, 13C{1H}, and 19F spectra were recorded on either a Bruker ARX 300 MHz or a Bruker Avance 400 MHz instrument. Chemical shifts (expressed in parts per million) are referenced to residual solvent peaks (1H, 13C{1H}) and to an external reference of CFCl3 for 19F and of H3PO4 for 31P{1H}. Coupling constants, J, are given in Hz. Spectral assignments were achieved by combination of 1H−1H COSY, 13C APT, and 1H−13C HSQC/HMBC experiments. C, H, and N analyses were carried out in a PerkinElmer 2400 CHNS/O analyzer. [Ir(cod)(IPr)(Py)]BF4 (2). A solution of [Ir(COD){OC(CH3)2}(IPr)]BF4 (1; 200 mg, 0.240 mmol) in THF (5 mL) was treated with pyridine (19 μL, 0.240 mmol) and stirred for 2 h at room temperature. The resulting orange solution was concentrated to ca. 0.5 mL, and the addition of diethyl ether produced an orange solid, which was separated by decantation, washed with diethyl ether, and dried in vacuo. Yield: 88% (181 mg, 0.21 mmol). 1H NMR (300 MHz, CDCl3, 233 K): δ 7.63 (t, 3JH−H = 7.7, 2H, Hp‑IPr), 7.62 (t, 3JH−H = 7.7, 1H, G


Article

Organometallics

6H, Me-P), − 17.90 (d, 1JH−P = 16.3, 2H, Ir−H). 13C{1H}-APT NMR plus HSQC and HMBC (75 MHz, CD2Cl2, 223 K): δ 162.2 (d, 1JC−P = 113.9, Ir−CIPr), 146.1 (s, Cq‑IPr), 145.0 (s, C5‑Th), 137.6 (d, 1JC−P = 55.9, Cq‑Ph‑P), 136.9 (s, CqN), 130.8 (s, C3‑Th), 130.6 (s, Cp‑IPr), 129.2 (s, C4‑Th), 129.7 (d, 2JC−P = 10.5, Co‑Ph‑P), 128.6 (s, C2‑Th), 128.2 (d, 3 JC−P = 10.2, Cm‑Ph‑P), 125.2 (s, Cm‑IPr), 125.1 (br, Cp‑Ph‑P), 124.6 (s,  CHN), 29.0 (s, CHMeIPr), 26.3 and 22.0 (both s, CHMeIPr), 16.7 (d, 1 JC−P = 33.5, Me-P), 13.9 (s, MeTh). 19F NMR (300 NMR, CD2Cl2, 223 K): δ −152.2 (s, BF4). 31P NMR (121 NMR, CD2Cl2, 223 K): δ −28.7 (s, PPhMe2).21 Characterization of 8′. 1H NMR (300 MHz, CD2Cl2, 223 K): δ 7.61 (t, 3JH−H = 7.7, 2H, Hp‑IPr), 7.4−6.7 (m, 5H, HPh‑P), 7.38 (d, 3JH−H = 7.7, 4H, Hm‑IPr), 7.31 (m, 2H, CHN), 7.11 (d, 3JH−H = 5.2, 1H, H4‑Th), 6.91 (dd, 3JH−H = 5.2, 3.8, 1H, H3‑Th), 6.79 (d, 3JH−H = 3.8, 1H, H2‑Th), 2.79 (sept, 3JH−H = 6.3, 4H, CHMeIPr), 2.48 (s, 3H, Me), 1.31 and 1.10 (both d, JH−P = 9.3, 6H, Me-P), 1.17 and 1.05 (both d, 3JH−H = 6.3, 24H, CHMeIPr), − 21.1 (d, 2JH−P = 19.8, 2H, Ir−H). 13C{1H}APT NMR plus HSQC and HMBC (75 MHz, CD2Cl2, 223 K): δ 167.7 (d, 1JC−P = 113.9, Ir−CIPr), 145.8 (s, Cq‑IPr), 140.0 (s, C5‑Th), 137.2 (s, CqN), 134.9 (d, 1JC−P = 45.9, Cq‑Ph‑P), 130.7 (s, Cp‑IPr), 129.9 (d, 2JC−P = 10.5, Co‑Ph‑P), 128.4 (d, 3JC−P = 10.2, Cm‑Ph‑P), 127.0 (s,  CHN), 125.3 (s, C2‑Th), 125.0 (br, Cp‑Ph‑P), 124.5 (s, Cm‑IPr), 124.2 (s, C3‑Th), 123.3 (s, C4‑Th), 29.8 (s, CHMeIPr), 26.3 and 21.7 (both s, CHMeIPr), 25.8 (d, 1JC−P = 35.1, Me-P), 15.2 (s, MeTh). 19F NMR (300 NMR, CD2Cl2, 223 K): δ −152.2 (s, BF4). 31P NMR (121 NMR, CD2Cl2, 223 K): δ −24.2 (t, JP−H = 10.5, PPhMe2).21 [Ir(H)2(IPr)(κS-BT)2(PPhMe2)]BF4 (9). A solution of [Ir(COD)(IPr)(PPhMe2)]BF4 (3; 80 mg, 0.087 mmol) in THF (5 mL) was treated with thianaphthene (70 μL, 10 equiv) and stirred under a dihydrogen atmosphere (1 bar) for 30 min at room temperature. The resulting light yellow solution was concentrated and washed with dry diethyl ether to produce a very sensitive beige solid, which was dried in vacuo: Yield: 60% (51 mg, 0.054 mmol). 1H NMR (300 MHz, CD2Cl2, 223 K): δ 7.79 and 7.00 (both d, 3JH−H = 8.3, 4H, H8,5‑Th), 7.67 (t, 3JH−H = 7.7, 2H, Hp‑IPr), 7.47 and 7.31 (both dd, 3JH−H = 8.3, 7.5, 4H, H7,6‑Th), 7.39 (s, 2H, CHN), 7.38 (d, 3JH−H = 7.7, 4H, Hm‑IPr), 7.32 (d, 3JH−H = 5.8, 2H, H3‑Th), 7.28 (t, 3JH−H = 7.9, 1H, Hp‑Ph‑P), 7.13 (dd, 3JH−H = 8.0, 7.9, 2H, Hm‑Ph‑P), 6.45 (dd, 3JH−H = 8.0, 3 JH−P = 10.7, 2H, Ho‑Ph‑P), 6.23 (d, 3JH−H = 5.8, 2H, H2‑Th), 2.89 (sept, 3 JH−H = 6.7, 4H, CHMeIPr), 1.22 and 1.06 (both d, 3JH−H = 6.7, 24H, CHMeIPr), 0.16 (d, 3JH−H = 9.2, 6H, Me-P), − 17.63 (d, 2JH−P = 17.6, 2H, Ir−H). 13C{1H}-APT NMR plus HSQC and HMBC (75 MHz, CD2Cl2, 223 K): δ 161.3 (d, 1JC−P = 110.6, Ir−CIPr), 146.2 (s, Cq‑IPr), 141.9 and 139.7 (both s, C9,4‑Th), 137.6 (d, 1JC−P = 55.3, Cq‑Ph‑P), 137.1 (s, CqN), 130.8 (s, Cp‑IPr), 130.3 (d, 2JC−P = 10.9, Co‑Ph‑P), 129.7 (s, C2‑Th), 129.1 (s, C3‑Th), 128.0 (d, 3JC−P = 10.2, Cm‑Ph‑P), 127.5 and 126.4 (s, Cp‑Ph‑P), 126.7 and 124.2 (both s, C7,6‑Th), 125.5 and 123.1 (both s, C8,5‑Th), 124.6 (s, Cm‑IPr), 124.3 (s, CHN), 29.8 (s, CHMeIPr), 26.4 and 21.8 (both s, CHMeIPr), 16.5 (d, 1JC−P = 32.9, MeP). 19F NMR (300 NMR, CD2Cl2, 223 K): δ −151.7 (s, BF4). 31P NMR (121 NMR, CD2Cl2, 223 K): δ −31.0 (s, PPhMe2).21 General Procedure for the Catalytic Hydrogenation of Benzothiophene Using Complex 3 as Catalyst. A 25 mL batch reactor with Teflon lining inside was charged with benzothiophene (134 mg, 1 mmol) and 3 (9.1 mg, 0.01 mmol). Then 2 mL of dry toluene added. Then the reactor was closed, vacuum was applied, and the reactor was purged with hydrogen and heated to the corresponding temperature (50 or 100 °C). Then the pressure of H2 gas was adjusted to 3, 10, or 20 bar. The reaction mixture was stirred using a mechanical stirrer. After 24 or 48 h liquid samples were taken after releasing the H2 pressure, without opening the reactor, using a long needle through the sample withdrawal valve. The samples were diluted using 1 mL of tetrahydrofuran and analyzed by GCMS. Computational Details. All DFT theoretical calculations have been carried out using the Gaussian program package.22 The B3LYP method23 has been employed, including the D3 dispersion correction scheme developed by Grimme24 for both energies and gradient calculations and the “ultrafine” grid. The def2-SVP basis set25 has been selected for all atoms. The nature of the stationary points has been

22.4 (all s, CHMeIPr). 19F NMR (300 NMR, CDCl3, 253 K): δ −152.1 (s, BF4).21 [Ir(H)2(IPr)(η2-T)(Py)]BF4 (6) and [Ir(H)2(IPr)(κS-T)2(Py)]BF4 (6′). A solution of [Ir(COD)(IPr)(Py)]BF4 (2; 60 mg, 0.07 mmol) in CD2Cl2 (5 mL) was treated with thiophene (56 μL, 10 equiv) and stirred under a dihydrogen atmosphere (1 bar) for 50 min at room temperature. The resulting light yellow solution was concentrated and washed with pentane to produce a very sensitive beige solid, which was dried in vacuo to afford 37 mg of the mixture of complexes 6 and 6′. Characterization of 6. 1H NMR (300 MHz, CD2Cl2, 263 K): 7.65 (overlapped, Ho‑py), 7.65 (t, 3JH−H = 7.5, 2H, Hp‑IPr), 7.58 (t, 3JH−H = 7.2, 2H, Hp‑py), 7.46 (d, 3JH−H = 7.5, 2H, Hm‑IPr), 7.29 and 7.11 (both d, 3JH−H = 4.0, 8H, Th), 7.12 (m 2H, CHN), 6.90 (dd, 3JH−H = 7.2, 6.4, 2H, Hm‑py), 2.78 (sept, 3JH−H = 6.9, 4H, CHMeIPr), 1.23 and 1.13 (both d, 3JH−H = 6.9, 24H, CHMeIPr), − 24.71 (s, 2H, Ir−H). 13 C{1H}-APT NMR plus HSQC and HMBC (75 MHz, CD2Cl2, 263 K): 154.4 (s, Co‑py), 153.1 (s, Ir−CIPr), 145.8 (s, Cq‑IPr), 136.8 (s, Cp‑py), 136.6 (s, CqN), 131.1 (s, Cp‑IPr), 124.7 and 124.0 (both s, Th), 124.5 (s, Cm‑py), 124.1 (s, Cm‑IPr), 123.8 (s, CHN), 29.1 (s, CHMeIPr), 25.7 and 21.7 (both s, CHMeIPr). 19F NMR (300 NMR, CD2Cl2, 263 K): δ −152.1 (s, BF4). Characterization of 6′. 1H NMR (300 MHz, CD2Cl2, 263 K): δ 8.07 (d, 3JH−H = 5.1, 2H, Ho‑py), 7.77 (t, 3JH−H = 6.7, 1H, Hp‑py), 7.46 (t, 3JH−H = 7.6, 2H, Hp‑IPr), 7.30 (d, 3JH−H = 7.6, 4H, Hm‑IPr), 7.30 (s, 2H, CHN), 7.17 (dd, 3JH−H = 6.7, 5.1, 2H, Hm‑py), 6.05 and 5.60 (both m, 4H, Th), 2.49 (sept, 3JH−H = 7.0, 4H, CHMeIPr), 1.34 and 1.19 (both d, 3JH−H = 7.0, 24H, CHMeIPr), − 17.06 (s, 2H, Ir−H). 13 C{1H}-APT NMR plus HSQC and HMBC (75 MHz, CD2Cl2, 263 K): 154.0 (s, Co‑py), 153.5 (s, Ir−CIPr), 146.5 (s, Cq‑IPr), 137.2 (s, Cp‑py), 137.0 (s, CqN), 130.2 (s, Cp‑IPr), 126.4 (s, Cm‑py), 124.9 (s, Cm‑IPr), 124.0 (s, CHN), 92.8 and 91.8 (both s, CTh), 29.1 (s, CHMeIPr), 25.0 and 22.5 (both s, CHMeIPr). 19F NMR (300 NMR, CD2Cl2, 263 K): δ −152.1 (s, BF4). [Ir(H)2(IPr)(κS-T)2(PPhMe2)]BF4 (7). A solution of [Ir(COD)(IPr)(PPhMe2)]BF4 (3; 80 mg, 0.087 mmol) in THF (5 mL) was treated with thiophene (70 μL, 10 equiv) and stirred under a dihydrogen atmosphere (1 bar) for 30 min at room temperature. The resulting light yellow solution was concentrated and washed with pentane to produce a very sensitive beige solid, which was dried in vacuo. Yield: 60% (51 mg, 0.054 mmol). 1H NMR (300 MHz, CD2Cl2, 223 K): δ 7.59 (t, 3JH−H = 8.0, 2H, Hp‑IPr), 7.36 (s, 2H,  CHN), 7.35 (dd, 3JH−H = 8.3, 8.0, 2H, Hm‑Ph‑P), 7.32 (d, 3JH−H = 8.0, 4H, Hm‑IPr), 7.31 (t, 3JH−H = 8.0, 1H, Hp‑Ph‑P), 7.06 (dd, 3JH−H = 8.3, 3 JH−H = 8.0, 2H, Ho‑Ph‑P), 7.03 (d, 3JH−H = 5.3, 4H, Th2), 6.28 (d, 3JH−H = 5.3, 4H, Th3), 2.75 (sept, 3JH−H = 6.7, 4H, CHMeIPr), 1.18 and 1.15 (both d, 3JH−H = 6.7, 24H, CHMeIPr), 0.91 (d, 2JH−P = 9.4, 6H, Me-P), − 18.4 (d, 1JH−P = 17.2, 2H, Ir−H). 13C{1H}-APT NMR plus HSQC and HMBC (75 MHz, CD2Cl2, 223 K): δ 163.1 (d, 1JC−P = 108.9, Ir− CIPr), 146.6 (s, Cq‑IPr), 136.6 (d, 1JC−P = 52.6, Cq‑Ph‑P), 136.5 (s, CqN), 131.6 (s, CTh‑3), 130.9 (s, Cp‑IPr), 130.5 (d, 2JC−P = 10.5, Co‑Ph‑P), 130.3 (s, CTh‑2), 128.6 (d, 3JC−P = 10.2, Cm‑Ph‑P), 125.6 (d, 4JC−P = 3.2, Cp‑Ph‑P), 125.0 (s, CHN), 124.4 (s, Cm‑IPr), 29.6 (s, CHMeIPr), 26.1 and 21.5 (both s, CHMeIPr), 17.8 (d, 1JC−P = 34.7, Me-P). 19F NMR (300 NMR, CD2Cl2, 223 K): δ −152.3 (s, BF4). 31P NMR (121 NMR, CD2Cl2, 223 K): δ −27.3 (s, PPhMe2). [Ir(H)2(IPr)(κS-2-MeT)2(PPhMe2)]BF4 (8) and [Ir(H)2(IPr)(κS-2MeT)(PPhMe2)]BF4 (8′). A solution of [Ir(COD)(IPr)(PPhMe2)]BF4 (3; 80 mg, 0.087 mmol) in THF (5 mL) was treated with 2methylthiophene (117 mg, 10 equiv) and stirred under a dihydrogen atmosphere (1 bar) for 50 min at room temperature. The resulting light yellow solution was concentrated and washed with pentane to produce a very sensitive beige solid, which was dried in vacuo to afford 43 mg of a mixture of complexes 8 and 8′. Characterization of 8. 1H NMR (300 MHz, CD2Cl2, 223 K): δ 7.68 (t, 3JH−H = 7.7, 2H, Hp‑IPr), 7.35 (d, 3JH−H = 7.7, 4H, Hm‑IPr), 7.27 (m, 2H, CHN), 7.22 (m, 1H, Hp‑Ph‑P), 7.20 (m, 2H, Hm‑Ph‑P), 6.95 (dd, 3JH−H = 5.2, 3.8, 2H, H3‑Th), 6.87 (d, 3JH−H = 3.8, 2H, H2‑Th), 6.81 (dd, 3JH−H = 7.8, 3JH−P = 9.0, 2H, Ho‑Ph‑P), 6.23 (d, 3JH−H = 5.2, 2H, H4‑Th), 2.74 (sept, 3JH−H = 6.3, 4H, CHMeIPr), 1.94 (s, 6H, Me), 1.17 and 1.05 (both d, 3JH−H = 6.3, 24H, CHMeIPr), 0.71 (d, 2JH−P = 9.3, H


Article

Organometallics

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confirmed by analytical frequency analysis. Molecular structures were represented using CYLView software.26

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00995. Theoretical calculations and NMR spectra of compounds 2−9 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for M.I.: [email protected]. *E-mail for L.A.O.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support by the Ministry of Higher Education, Saudi Arabia, in establishment of the Centre of Research Excellence in Petroleum Refining & Petrochemicals at KFUPM (KACST-funded project ART-32-68). The support of the KFUPM under the KACST funded project (ART-32-68) and the KFUPM−University of Zaragoza research agreement are also highly appreciated. This work was further supported by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO CSD20090050, CTQ2011-27593, and CTQ2012-35665 projects) and the Diputación General de Aragón (DGA/FSE-E07). V.P. thankfully acknowledges the resources from the supercomputer “Memento”, technical expertise and assistance provided by BIFI-ZCAM (Universidad de Zaragoza).

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REFERENCES

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