Computational Study of Iridium and

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Combined Experimental/Computational Study of Iridium and Palladium Hydride PP(O)P Pincer Complexes Carmen Martin,†,∥ Sonia Mallet-Ladeira,‡ Karinne Miqueu,*,§ Ghenwa Bouhadir,*,† and Didier Bourissou*,† †

Laboratoire Hétérochimie Fondamentale et Appliquée, Université Paul Sabatier/CNRS UMR 5069, 118 route de Narbonne, 31062 Toulouse, France ‡ Institut de Chimie de Toulouse (FR 2599), Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 9, France § Institut Pluridisciplinaire de Recherche sur l′Environnement et les Matériaux UMR-CNRS 5254, Université de Pau et des Pays de l′Adour, Hélioparc, 2 avenue du Président Angot, 64053 Pau Cedex 09, France S Supporting Information *

ABSTRACT: The diphosphine−phosphine oxide {[o-iPr2P(C6H4)]2P(O)H} (1) has been prepared, and its coordination to Ir and Pd has been explored. Using [IrCl(cyclooctene)2]2, the pincer hydride complex {(o-iPr2PC6H4)2P(O)]IrHCl} (2) is readily obtained by phosphine-assisted P(O)−H bond activation. Coordination of CO to Ir affords the corresponding octahedral complex {(o-iPr2PC6H4)2P(O)]IrHCl(CO)} (3) as a single stereoisomer. The electronic properties of the PP(O)P ligand have been compared with those of related PEP frameworks on the basis of νCO stretching frequencies. Treatment of 1 with [Pd(PtBu3)2] gives the palladium hydride complex {(o-iPr2PC6H4)2P(O)]PdH} (4). The mechanism of P(O)−H bond activation at Pd has been investigated computationally. Complex 4 reacts with methyl acrylate at room temperature, giving {(oiPr2PC6H4)2P(O)]PdCH(Me)CO2Me} (7) as the result of regioselective insertion into the Pd−H bond.

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INTRODUCTION

RESULTS AND DISCUSSION The new trifunctional proligand 1 featuring a central SPO moiety and two lateral phosphine groups was readily prepared on a multigram scale (70% isolated yield) by controlled hydrolysis of the known chlorophosphine (o-iPr2PC6H4)2PCl.4 The 31P{1H} NMR spectrum of 1 displays a doublet at δ −7.1 ppm and a triplet at δ 9.5 ppm (integration 2:1) with a 3JPP coupling constant of 80 Hz. In the 1H NMR spectrum, the proton of the SPO moiety appears as a doublet of triplets at δ 9.8 ppm, with 1JPH and 4JPH coupling constants of 522 and 4 Hz, respectively. Given the interest of pincer iridium hydride complexes,1,5 we were curious to study the reaction of 1 with Ir(I) precursors. Coordination of the two phosphine groups was envisioned to promote the activation of the central P(O)−H bond6 and to afford thereby iridium hydride PP(O)P pincer complexes. Hence, derivative 1 was reacted with [IrCl(COE)2]2 (COE =

Over the past decade, pincer hydride complexes have attracted huge interest and a wide variety of PEP pincer ligands (E = N,1a C,1b Si,1c,d B,1e,f ...) have been reported to form well-defined stable complexes with highly reactive M−H bonds. With the aim to develop new pincer frameworks, we recently started to explore tridentate ligands featuring a central phosphine oxide moiety. 2 , 3 The diphosphine−phosphine oxide (oiPr2PC6H4)2P(O)Ph was shown to display versatile coordination properties. In particular, phosphine chelation was found to promote unusual P(O)−Ph bond activation at Pd and Ni, affording original diphosphine−phosphide oxide PP(O)P complexes.3a,c In this work, we sought to extend further this chemistry and introduced a secondary phosphine oxide (SPO) in the central position of the ligand. As reported hereafter, activation of the P(O)−H bond proceeds readily at Ir and Pd, affording stable pincer hydride complexes whose structure, mechanism of formation, and reactivity have been investigated by a joint experimental/computational approach. © XXXX American Chemical Society

Received: November 18, 2013

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dx.doi.org/10.1021/om401117d | Organometallics XXXX, XXX, XXX−XXX

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cyclooctene) in dichloromethane (Scheme 1). According to 31P NMR monitoring, the reaction is complete within 4 days at

were grown from a dichloromethane/pentane solution at room temperature, and the structure of 3 was analyzed by X-ray diffraction (Figure 2). The iridium center adopts a slightly

Scheme 1. Synthesis of the Iridium Hydride PP(O)P Pincer Complexes 2 and 3

room temperature and gives a single compound 2 characterized by two singlets at δ 43.8 and 138.8 ppm (integration 2:1). The signal at δ 43.8 ppm is associated with the two phosphine groups symmetrically coordinated to iridium. The other signal is diagnostic of a κP-coordinated phosphide oxide moiety:3a,c a huge low field shift (Δδ ∼ 130 ppm) from 1 to 2 and the disappearance of the 1JPH coupling constant. Oxidative addition of the P(O)−H bond to iridium is clearly apparent in 1H NMR. A signal appears in the hydridic region at δ −21.2 ppm (pseudoquadruplet due to similar coupling with the three phosphorus centers, 2JPH = 15 Hz). By comparison with literature data,7,8 we surmised that the geometry of complex 2 significantly deviates from square pyramidal, as typically observed for [(PCsp2P)IrHCl] complexes, but rather resembles that of the diphosphine−silyl complex {(o−Cy2PC6H4)2Si(Me)]IrHCl} (A).6a In the absence of structural characterization, DFT calculations were carried out and the geometry of the actual complex 2 was optimized at the B3PW91/SDD+f (Ir), 6-31G** (other atoms) level of theory (Figure 1). The

Figure 2. Molecular view of complex 3. For clarity, hydrogen atoms are omitted (except that at Ir) and isopropyl/phenyl groups at phosphorus are simplified. Selected bond lengths (Å) and bond angles (deg): P1−Ir 2.259(1), Ir−H1 1.54(3), Ir−Cl 2.495(1), Ir−C5 1.943(3), P1−O1 1.499(2), C5−O2 1.127(3), P2−Ir−P3 154.10(2), P1−Ir−Cl 177.31(2), H1−Ir−C5 178(1), P1−Ir−P2 87.07(2).

distorted octahedral geometry, with meridional coordination of the DPPO ligand. The chloride occupies the position trans to the phosphide oxide moiety, the Ir−Cl distance (2.495(1) Å) is similar to those observed in related (PEP)IrHClCO complexes,11 and the P(O)−Ir distance (2.259(1) Å) falls in the same range as that observed in the related Pd complex [(DPPO)PdPh] (2.277(1) Å).3a,12 The position of the hydride at iridium was unambiguously located in the difference Fourier map. It sits trans to CO (H−Ir−C5 bond angle 178(1)°), and the Ir−H distance equals 1.54(3) Å. The carbonyl group and PO moiety of the DPPO ligand are oriented cis to each other.13 Notably, the coordination of CO and the change in geometry around Ir from 2 to 3 induce a noticeable high-field shift of the phosphide oxide 31P NMR signal (from δ 138.8 ppm in 2 to δ 67.1 ppm in 3). Concomitantly, the hydridic signal shifts to low field, consistent with the presence of a π-accepting CO ligand trans to it, and appears in 3 as a doublet of triplets (2JPH = 16 and 13 Hz) at δ −10.20 ppm.14 The IR spectrum of complex 3 displays characteristic νIrH and νCO bands at 2152 and 2010 cm−1, respectively. The latter stretching frequency falls in the upper range of those reported for related pincer complexes [(PEP)Ir(CO)HCl] (νCO 1976−2030 cm−1 for E = MeSi,1c,d 1976−1990 cm−1 for E = Csp2,1b 1985 cm−1 for E = B1e,f), suggesting that the phosphide oxide is less electron donating than the silyl, aryl, and boryl moieties. However, comparison of the known [(PEP)Ir(CO)HCl] complexes is complicated by the fact that they adopt different stereo arrangements and that not only the central moiety but also the organic backbone of the involved ligands differ one from the other. To facilitate comparison and draw reliable conclusions, we thus turned to the corresponding Ir(I) complexes [(PEP)Ir(CO)] and carried out a computational study on eight complexes differing only in the nature of the central coordination site: E = B, CH, N, P, Si(Me), P(O), P(NPh), P(S). All complexes adopt square-planar geometry with CO trans to E, so that the νCO stretching frequency should ideally convey the electronic properties of the central E moiety. The computed νCO values (Table 1) span over 25 cm−1, with a progressive increase in the series B < Si(Me) < CH < P < N < P(O) < P(S) < P( NPh). This study confirms that the phosphide oxide moiety is less electron donating than the amido, phosphido, alkyl, silyl, and boryl groups. The difference in νCO between P(O) and N is

Figure 1. Molecular views of iridium complex 2 as optimized computationally at the B3PW91/SDD+f (Ir), 6-31G** (other atoms) level of theory: 2* global minima (left) and 2′* isomer located +10.7 kcal/mol higher in energy (right). For clarity, hydrogen atoms (except that at Ir) are omitted. Selected bond lengths (Å) and bond angles (deg): 2*, P1−Ir 2.234, Ir−H 1.559, Ir−Cl 2.407, P2−Ir−P3 167.4, P1−Ir−Cl 139.7, P1−Ir−H 77.9, Cl−Ir−H 142.4; 2′*, P1−Ir 2.273, Ir−H 1.527, Ir−Cl 2.429, P2−Ir−P3 167.8, P1−Ir−Cl 170.1, P1−Ir− H 80.1, Cl−Ir−H 109.7.

most stable structure located on the potential energy surface, 2*, indeed adopts a geometry similar to that determined crystallographically for complex A, with an acute P1−Ir−H bond angle (77.9°) and a wide P1−Ir−Cl bond angle (139.7°).9 In order to assess the electronic properties of the diphosphine−phosphide oxide ligand, a carbonyl complex was prepared. A dichloromethane solution of 2 was bubbled with CO at room temperature, and complex 3 was obtained in nearly quantitative yield as a single stereoisomer.10 Crystals B

dx.doi.org/10.1021/om401117d | Organometallics XXXX, XXX, XXX−XXX

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Table 1. νCO (in cm−1) Predicted by DFT Calculations (B3PW91/SDD+f (Ir), 6-31G** (Other Atoms) Level of Theory) for [(PEP)Ir(CO)] Complexes

E

νCO

P(S) P(NPh) P(O) N P CH Si(Me) B

2044.3 2043.8 2039.3 2030.3 2026.5 2023.2 2022.5 2020.6

Figure 3. Molecular views of complexes 4 (left) and 7 (right). For clarity hydrogen atoms (except that at Pd for 4) and solvate molecules are omitted and isopropyl/phenyl groups at phosphorus are simplified. Selected bond lengths (Å) and bond angles (deg): 4, P1−Pd 2.298(1), P1−O1 1.504(2), Pd−H1 1.65(2), P2−Pd−P3 164.81(2), P1−Pd− H1 176.2(9); 7, P1−Pd 2.276(1), Pd−C5 2.254(3), P1−O1 1.500(2), P2−Pd−P3 159.98(2), P1−Pd−C5 169.40(7).

H bond or alternatively in two steps, involving P-coordinated phosphinous acids as intermediates and subsequent deprotonation. To shed light on the mechanism of P(O)−H bond activation of 1, the formation of complex 4 was investigated by DFT calculations (Figure 4). First, the geometry of the

about the same as that between N and Si(Me). Replacing the O atom at phosphorus for a sulfur15 or imido group induces a further shift of the νCO value by 4−5 cm−1. To generalize the preparation of PP(O)P hydride complexes from 1, we then moved to palladium. Pincer Pd hydride complexes have been known from the early work of Moulton and Shaw,16 and over the past decade, they have attracted increasing attention due to their high reactivity.17 Reaction of the proligand 1 with [Pd(PtBu3)2] for 8 h in toluene at room temperature afforded the desired complex 4 (Scheme 2). No Scheme 2. Synthesis of the Palladium Hydride Complex 4 and Insertion of Methyl Acrylate Leading to 7

Figure 4. Schematic representation of the energy profile for the P(O)−H bond activation at Pd, as calculated at the B3PW91/SDD+f (Pd), 6-31G** (other atoms) level of theory. ΔG values are given in kcal/mol at 25 °C, using the structure of 5* as a reference.

intermediate was detected spectroscopically before oxidative addition took place. After the standard workup, complex 4 was isolated as a white powder in 75% yield. Its 31P{1H} NMR spectrum displays a doublet at δ 87.4 ppm and a triplet at δ 123.0 ppm (2JPP = 15 Hz, integration 2:1). The Pd−H signal appears as a doublet of triplets (2JPH = 225 and 16 Hz) at δ − 2.80 ppm. Crystals of 4 suitable for X-ray diffraction analysis were grown from a toluene solution at room temperature (Figure 3, left). The hydride was unequivocally located in the difference Fourier map. It sits trans to the phosphide oxide moiety (Pd−H 1.65(2) Å,18 Pd−P1 2.298(1) Å), and the geometry around palladium deviates marginally from square planar. Compound 4 represents a rare example of a structurally characterized palladium hydride pincer complex and is the first involving a phosphide oxide ligand. 18,19 Its formation substantiates the generality of our phosphine-chelated P(O)− H bond activation approach. Activation of SPO at late transition metals, palladium in particular, has attracted considerable attention recently.20−22 It is still difficult to provide a detailed mechanistic picture of SPO activation, but recent studies suggest that the transformation may proceed either directly by oxidative addition of the P(O)−

palladium hydride was optimized and the resulting structure (4*) nicely reproduced that determined crystallographically. Another structure (5*) in which the two phosphine groups are coordinated to Pd but the SPO moiety remains intact was located (Table S4, Supporting Information).9 The formation of 4* from 5* is thermodynamically favored (ΔG = −22.3 kcal/ mol). A direct pathway for oxidative addition of the P(O)−H bond to Pd was identified, involving TS5*→4* as the transition state.23 The corresponding activation barrier ΔG⧧ was estimated as 20.2 kcal/mol, a value consistent with a reaction occurring at room temperature. Another route involving a tautomeric form of 1 was then considered,24 given the propensity of secondary phosphine oxides and phosphinous acids to interconvert.25 Coordination of the central >P−OH moiety is energetically favored, and the corresponding Tshaped complex 6* is 9.5 kcal/mol more stable than 5*. The palladium hydride complex 4* may be obtained by oxidative addition of the O−H bond of 6*, but the corresponding barrier is prohibitively high (ΔG⧧ = 56 kcal/mol).26 Accordingly, the formation of complex 4 is likely to result from direct oxidative C

dx.doi.org/10.1021/om401117d | Organometallics XXXX, XXX, XXX−XXX

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access to Ir and Pd hydride complexes. According to IR data, the electronic properties of the tridentate PP(O)P ligands differ slightly from those of related N-, P-, C-, Si- and B-based systems. Reaction of the PP(O)P hydride complex 4 with methyl acrylate provides an interesting example of alkene insertion at Pd.

addition of the P(O)−H moiety of 1 to Pd and is not very likely to involve the corresponding phosphinous acid form >P− O−H.27 Over the past few years, important catalytic transformations have been developed involving pincer Pd hydride complexes as intermediates.17a,e−h Most of these reactions proceed by insertion of π substrates (allenes, dienes, α,β-unsaturated ketones and esters) into Pd−H bonds. To the best of our knowledge, such an elementary step has not been authenticated experimentally so far for pincer complexes.28 However, most relevant is the recent work by Iwasawa and co-workers on PSiP complexes. Starting from a σ(Si−H) complex as a masked form of the pincer Pd hydride, these authors gained indirect evidence for allene insertion, the resulting allyl moiety being transferred from Pd to Si.17e To try to get more insight into such insertion processes at pincer Pd complexes, we explored the reactivity of 4 toward alkenes. No reaction was observed with excess ethylene (4 atm) or styrene (40 equiv) at room temperature.29 However, complex 4 was found to readily react with methyl acrylate. Using 1 equiv of the electron-poor alkene, the reaction is complete within minutes at room temperature and affords complex 7 as the sole product (nearly quantitative yield) (Scheme 2). The 31P NMR spectrum of 7 exhibits a doublet at δ 66.7 ppm and a triplet at δ 123.2 ppm (ratio 2:1, 2JPP = 7 Hz). The regioselectivity of the insertion is clearly apparent from 1H NMR spectroscopy. In addition to the methoxy signal at δ 3.43 ppm (s), two new signals are found at δ 3.35 ppm (m) and 1.47 ppm (pseudo-t, 3JHH = 4JPH = 7 Hz), corresponding to a methine and methyl group, respectively. Thus, complex 7 corresponds to the branched product and the Pd fragment is introduced at the internal position. This regioselectivity is consistent with the polarity of the π(CC) bond of methyl acrylate (Michael-type addition of the hydride) and parallels that observed by Liang et al. with a [(PNP)NiH] pincer complex.28a Crystals of 7 were grown from a toluene/pentane mixture at −25 °C, and an X-ray diffraction study was performed (Figure 3, right). The geometry around Pd only slightly deviates from square planar (P1−Pd−C = 169.40(7)°, P2−Pd−P3 = 159.98(2)°), and the P1−Pd bond length (2.276(1) Å) remains about the same as in the hydride complex 4. The Pd−C distance (2.254(3) Å) falls at the upper limit of those found in the Cambridge Database for pincer Pd alkyl complexes (2.039−2.129 Å), probably due to steric constraints associated with the branched alkyl at Pd and the bulky PiPr2 groups in proximity. The formation of 7 upon reaction of 4 with methyl acrylate provides direct evidence for the insertion of an alkene into a pincer Pd hydride complex. To get more insight about the selectiviy of the insertion of olefins into a Pd−H bond, DFT calculations were performed. The reaction of methyl acrylate with 4 was found to be more favorable thermodynamically (ΔG = −2.3 kcal/mol) than that of ethylene and styrene (ΔG = −0.7 and +11.7 kcal/mol, respectively). The mechanism of formation of 7 was also investigated. The reaction is likely to proceed by direct addition of the Pd−H bond across the CC double bond of methyl acrylate (concerted asynchronous process, activation barrier of 27.5 kcal/mol). Alternative pathways involving temporary dissociation of one flanking phosphine seem unlikely (such dissociation requires about 37 kcal/mol).9

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EXPERIMENTAL SECTION

General Comments. All reactions and manipulations were carried out under an atmosphere of dry argon using standard Schlenk techniques. All solvents were sparged with argon and dried using an MBRAUN Solvent Purification System (SPS). 1H, 13C, and 31P NMR spectra were recorded on a Bruker Avance 500 or 300 spectrometer. Chemical shifts are expressed with a positive sign, in parts per million, calibrated to residual 1H (7.24 ppm) and 13C (77.16 ppm) solvent signals and 85% H3PO4 (0 ppm), respectively. Unless otherwise stated, NMR was recorded at 298 K. The N values corresponding to 1 /2[J(AX) − J(A′X)] are provided when second-order AA′X or AA′MX systems were observed in 1H or 13C NMR spectra.30 Mass spectra were recorded on a Waters LCT mass spectrometer. Bis((odiisopropylphosphino)phenyl)chlorophosphine, (o-iPr2PC6H4)2PCl, was prepared as previously described.4 For atom numbering used in the NMR assignment, see the Supporting Information. Synthesis and Characterization of Compounds. Proligand 1. In a Schlenk flask containing a yellow solution of (o-iPr2PC6H4)2PCl (2.0 g, 6.6 mmol, 1 equiv) in THF (50 mL) was added 30 equiv of H2O (3.5 mL, 0.2 mol). The resulting mixture was stirred for 1 h at room temperature to give a colorless solution. The solvent was evaporated under vacuum, and proligand 1 was extracted with toluene. The solid residue was discarded by filtration, and toluene was evaporated. The resulting white powder was washed with pentane (3 × 5 mL) and dried under vacuum to give 1 (2.0 g, 4.6 mmol, 70% yield). 1 H NMR (500.33 MHz, C6D6): δ 9.78 (dt, 1JPH = 522 Hz, 4JPH = 4 Hz, 1H, P(O)H), 7.97 (m, 2H, H3), 7.25−7.18 (m, 2H, H6), 7.04 (m, 2H, H4), 6.98 (m, 2H, H5), 1.85 (m, 2H, H7), 1.76 (m, 2H, H7), 1.02 (dd, 3 JPH = 15 Hz, 3JHH = 7 Hz, 6H, H8), 0.92 (dd, 3JPH = 15 Hz, 3JHH = 7 Hz, 6H, H8), 0.76 (dd, 3JPH = 11 Hz, 3JHH = 7 Hz, 6H, H8), 0.66 (dd, 3 JPH = 14 Hz, 3JHH = 7 Hz, 6H, H8). 13C{1H} NMR (125.81 MHz, C6D6): δ 141.7 (AA′XM, N = 14.5 Hz, 1JPC = 94 Hz, C1), 140.4 (m, C2), 134.1 (m, C3), 132.4 (dd, 2JPC = 9 Hz, 3JCP = 2 Hz, C6), 130.4 (d, 3 JCP = 2 Hz, C4), 128.5 (d, 3JPC = 11 Hz, C5), 25.7 (d, 1JPC = 14 Hz, C7), 23.9 (d, 1JPC = 11 Hz, C7), 20.6 (d, 2JPC = 15 Hz, C8), 20.4 (d, 2 JPC = 18 Hz, C8), 19.9 (d, 2JPC = 20 Hz, C8), 19.2 (d, 2JPC = 8 Hz, C8). 31 P NMR (202.54 MHz, C6D6): δ −7.1 (d br, 3JPP = 80 Hz, 2P, iPr2P), 9.5 (dt, 1JPH = 522 Hz, 3JPP = 80 Hz, 1P, P(O)H). HRMS (CI, CH4): exact mass (monoisotopic) calcd for [C24H37OP3]H+, 435.2136; found, 435.2123. Anal. Calcd for C24H37OP3: C, 66.35; H, 8.58. Found: C, 66.43; H, 8.87. Mp: 135.0−136.4 °C. Complex 2. In a Schlenk flask containing a yellow solution of [IrCl(COE)2]2 (100 mg, 0.11 mmol, 1 equiv) in dichloromethane (3 mL) was added a solution of proligand 1 (97 mg, 0.22 mmol, 2 equiv) in dichloromethane (5 mL). The resulting mixture was stirred for 4 days at room temperature. The solution was filtered to remove a black precipitate. The solution was concentrated under vacuum, and a yellow solid was obtained when pentane was added at room temperature. The solid was separated by filtration and washed with pentane (3 × 3 mL). A 123 mg amount of complex 2 was obtained (0.18 mmol, 75% yield). 1H NMR (500.33 MHz, CDCl3): δ 8.30 (dd, 3 JHH = 3JPH = 8 Hz, 2H, H6), 7.72 (m, 2H, H3), 7.55 (pseudo-t, 3JHH = 7 Hz, 2H, H5), 7.48 (pseudo-t, 3JHH = 7 Hz, 2H, H4), 3.26 (m, 2H, H7), 3.04 (m, 2H, H7), 1.55 (m, 6H, H8), 1.30 (m, 6H, H8), 1.28 (m, 6H, H8), 1.24 (m, 6H, H8), −21.20 (pseudo-q, 2JPH = 15 Hz, 1H, IrH). 13C{1H} NMR (125.81 MHz, CDCl3): δ 150.5 (AA′XM, N = 16 Hz, 1JPC = 78 Hz, C1), 142.3 (AA′XM, N = 23 Hz, 2JPC = 30 Hz, C2), 130.3 (m, C3 or C4), 130.2 (m, C3 or C4), 129.9 (d, 3JPC = 7 Hz, C5), 129.3 (AA′XM, N = 7 Hz, 2JPC = 4 Hz, C6), 28.9 (AA′X, N = 13 Hz, C7), 25.4 (AA′X, N = 17 Hz, C7), 20.1 (s, C8), 19.2 (s, C8), 18.6 (s, C8), 18.15 (AA′X, N = 2 Hz, C8). 31P{1H} NMR (202.54 MHz,

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CONCLUSION New PP(O)P pincer complexes were prepared from 1. Phosphine-assisted P(O)−H activation gives straightforward D

dx.doi.org/10.1021/om401117d | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

CDCl3): δ 43.8 (s, 2P, iPr2P), 138.8 (s, 1P, PO). IR: νIr−H 2261 cm−1. HRMS (CI, CH 4 ): exact mass (monoisotopic) calcd for [C24H37ClOP3Ir]H+, 661.1437; found, 661.1430. Anal. Calcd for C24H37ClOP3Ir: C, 43.53; H, 5.63. Found: C, 43.85; H, 5.51. Mp: 236.6−238.9 °C. Complex 3. CO was bubbled through a solution of Ir complex 2 (80 mg, 0.12 mmol) in dichloromethane (5 mL) for 30 min. The solvent was evaporated under vacuum, and a yellow solid was obtained. The sample was recrystallized at room temperature in a dichloromethane/pentane mixture (5/1) to give colorless crystals (78 mg, 0.11 mmol, 95% yield). 1H NMR (500.33 MHz, CDCl3): δ 8.32 (pseudo-t, 3JHH = 3JPH = 8 Hz, 2H, H6), 7.76 (m, 2H, H3), 7.57 (pseudo-t, 2H, 3JHH = 8 Hz, H5), 7.47 (pseudo-t, 2H, 3JHH = 8 Hz, H4), 3.40 (m, 2H, H7), 2.80 (m, 2H, H7), 1.49 (AA′XM, N = 7 Hz, 3 JHH = 7 Hz, 6H, H8), 1.45 (AA′XM, N = 7 Hz, 3JHH = 7 Hz, 6H, H8), 1.10 (AA′XM, N = 8 Hz, 3JHH = 7 Hz, 6H, H8), 1.08 (AA′XM, N = 7 Hz, 3JHH = 7 Hz, 6H, H8), −10.20 (dt, 2JPH = 16 Hz, 2JPH = 13 Hz, 1H, Ir-H). 13C{1H} NMR (125.81 MHz, CDCl3): δ 176.0 (m, CO), 154.8 (m, C1), 137.0 (AA′XM, N = 26 Hz, 2JPC = 26 Hz, C2), 131.6 (m, C5), 130.2 (s br, ω1/2 = 17 Hz, C4), 129.9 (m, C6), 129.5 (m, C3), 27.5 (AA′X, N = 15 Hz, C7), 23.3 (AA′X, N = 17 Hz, C7), 18.3 (AA′X, N = 2 Hz, C8), 18.2 (s br, C8), 18.1 (s br, C8), 18.0 (s br, C8). 31P{1H} NMR (202.54 MHz, CDCl3): δ 50.7 (s, 2P, iPr2P), 67.1 (s, 1P, PO). IR (dichloromethane): νIr−H 2152 cm−1 and νIr‑CO 2010 cm−1. HRMS (CI, CH4): exact mass (monoisotopic) calcd for [C24H37ClOP3Ir]H+, 661.1430; found, 661.1421. Anal. Calcd for C25H37ClO2P3Ir: C, 43.51; H, 5.40. Found: C, 43.01; H, 5.09. Mp: 273.5−275.9 °C. Complex 4. In a Schlenk flask containing a yellow solution of Pd[P(tBu)3]2 (350 mg, 0.69 mmol, 1 equiv) in toluene (5 mL) was added a solution of proligand 1 (300 mg, 0.69 mmol, 1 equiv) in toluene (5 mL). The resulting mixture was stirred for 8 h at room temperature to give a red solution. The solvent was concentrated under vacuum, and then pentane was added (8 mL) to give a precipitate. The solid was filtered and washed with pentane (3 × 5 mL). A white solid was obtained (280 mg, 0.52 mmol, 75% yield). The sample was recrystallized at room temperature in toluene to give colorless crystals. 1H NMR (500.33 MHz, C6D6): δ 8.47 (pseudo-t, 3 JHH = 3JPH = 7 Hz, 2H, H6), 7.22−7.10 (m, 4H, H3 and H4), 7.07− 7.00 (m, 2H, H5), 2.13 (m, 4H, H7), 1.28 (m, 6H, H8), 1.14 (m, 6H, H8), 0.84 (m, 6H, H8), 0.56 (m, 6H, H8), −2.80 (dt, 2JPH = 225 Hz, 2 JPH = 16 Hz, 1H, Pd-H). 13C{1H} NMR (125.81 MHz, C6D6): δ 157.6 (AA′XM, N = 21 Hz, 1JPC = 31 Hz, C2), 136.0 (AA′XM, N = 18 Hz, 1JPC = 47 Hz, C1), 131.6 (m, C6), 130.9 (d, 3JCP = 5 Hz, C5), 130.6 (d, 2JCP = 18 Hz, C3), 129.4 (m, C4), 27.8 (AA′X, N = 12 Hz, C7), 22.3 (AA′X, N = 15 Hz, C7), 20.6 (AA′X, N = 3 Hz, C8), 18.8 (m, C8), 18.7 (m, C8), 18.4 (s, C8). 31P{1H} NMR (202.54 MHz, C6D6): δ 87.4 (d, 3 JPP = 15 Hz, 2P, iPr2P), 123.0 (t, 3JPP = 15 Hz, 1P, PO). 31P NMR (202.54 MHz, C6D6): δ 87.4 (br, 2P, iPr2P), 123.0 (d br, 2JPH = 225 Hz, 1P, PO. IR (toluene): νPd−H 1742 cm−1. HRMS (CI, CH4): exact mass (monoisotopic) calcd for C24H36OP3104Pd, 537.1019; found, 537.1010. Anal. Calcd for C24H37OP3Pd: C, 53.29; H, 6.89. Found: C, 53.30; H, 7.00. Mp: 200.7−202.1 °C. Complex 7. In a Schlenk flask containing a solution of Pd complex 4 (100 mg, 0.18 mmol, 1 equiv) in toluene (5 mL) was added 1 equiv of methyl acrylate (16 μL, 0.18 mmol). The resulting mixture was stirred for 10 min at room temperature to give a yellow solution. The solvent was concentrated under vacuum, and then pentane was added (5 mL) to give a precipitate. The solid was separated by filtration and washed with pentane (3 × 5 mL). A white solid was obtained (106 mg, 0.17 mmol, 95% yield). Crystals of 7 suitable for X-ray diffraction analysis were obtained in toluene/pentane at −25 °C. 1H NMR (500.33 MHz, THF-d8): δ 8.42 (pseudo-t, 3JHH = 7 Hz, 3JPH = 7 Hz, 1H, H6), 8.40 (pseudo-t, 3JHH = 7 Hz, 3JPH = 7 Hz, 1H, H6), 7.97 (m, 1H, H3), 7.95 (m, 1H, H3), 7.60 (pseudo-t, 1H, 3JHH = 7 Hz, H5), 7.56 (pseudo-t, 1H, 3JHH = 7 Hz, H5), 7.56−7.50 (m, 2H, H4), 3.43 (s, 3H, CH3OCO), 3.35 (m, 1H, PdCHCH3), 3.26 (m, 1H, H7), 2.95 (m, 1H, H7), 2.75 (m, 1H, H7), 2.67 (m, 1H, H7), 1.47 (pseudo-t, 3JHH = 7 Hz, 4 JPH = 7 Hz, 3H, PdCHCH3), 1.45 (m, 3H, H8), 1.40 (m, 3H, H8), 1.34 (m, N = 7 Hz, 3JHH = 7 Hz, 3H, H8), 1.19 (m, 3H, H8), 1.17 (m,

6H, H8), 1.13 (m, 3H, H8), 0.99 (m, 3H, H8). 13C{1H} NMR (125.81 MHz, THF-d8): δ 183.1 (m, CO), 155.1 (m, C1), 136.5 (dd, 1JPC = 44 Hz, 2JPC = 36 Hz, C2), 135.6 (dd, 1JPC = 44 Hz, 2JPC = 35 Hz, C2), 131.9 (d, 2JCP = 18 Hz, C3), 130.9 (d, 3JCP = 4 Hz, C5), 130.6−130.2 (m, C6), 130.1 (m, C4), 130.0 (s, C4), 48.7 (s, OCH3), 27.8 (dd, 1JCP = 12 Hz, 4JCP = 11 Hz, C7), 25.9 (dd, 1JCP = 12 Hz, 4JCP = 9 Hz, C7), 25.2 (dd, 1JCP = 12 Hz, 4JCP = 10 Hz, C7), 23.7 (dd, 1JCP = 11 Hz, 4JCP = 9 Hz, C7), 20.2 (s br, ω1/2 = 18 Hz, PdCHCH3), 19.1 (AA′X, N = 1 Hz, C8), 19.0 (AA′X, N = 2 Hz, C8), 18.7 (AA′X, N = 2 Hz, C8), 18.3 (AA′X, N = 1 Hz, C8), 17.9 (s, C8), 17.8 (s, C8), 17.7 (s, C8), 16.3 (s br, ω1/2 = 13 Hz, C8), 15.2 (dt, 2JCP = 76 Hz, 2JCP = 7 Hz, CHCO2). 31 P NMR (202.54 MHz, THF-d8): δ 66.2 (d, 3JPP = 7 Hz, 1P, iPr2P), 66.7 (d, 3JPP = 7 Hz, 1P, iPr2P), 123.2 (pseudo-t, 3JPP = 7 Hz, 1P, PO). HRMS (ES+): exact mass (monoisotopic) calcd for [C28H43O3P3106Pd]+, 626.1460; found, 626.1573. Computational Methods. Calculations were carried out with the Gaussian 09 program31 at the DFT level of theory using the hybrid functional B3PW91.32 B3PW91 is Becke’s three-parameter functional, with the nonlocal correlation provided by the Perdew 91 expression. Pd and Ir were treated with the Stuttgart−Dresden set-RECP (relativistic effective core potential) in combination with its adapted basis set. This latter has been augmented by a set of f polarization function.33 All other atoms (C, H, O, P, Cl) have been described with a 6-31G(d,p) double-ζ basis set.34 Geometry optimizations were carried out without any symmetry restrictions; the nature of the extrema (minima or transition state) was verified with analytical frequency calculations. All total energies and Gibbs free energies have been zero-point energy (ZPE) and temperature corrected using unscaled density functional frequencies. The connection between the transition states and the corresponding minima was confirmed by IRC calculations.35 The electronic structure of complex 5* was studied using natural bond orbital analysis (NBO-5 program).36 X-ray Collection and Refinement. Crystallographic data were collected at low temperature (193 or 253 K) on a Bruker-AXS APEXII QUAZAR diffractometer equipped with an air-cooled microfocus source (3 and 7) or on a Bruker-AXS SMART APEXII (4), using Mo Kα radiation (λ = 0.71073 Å). Semiempirical absorption corrections were employed.37 The structures were solved by direct methods (SHELXS-97),38 and all non-hydrogen atoms were refined anisotropically using the least-squares method on F2. The molecular views have been generated with ORTEP3.39 The crystallographic data are given in Table S1 (Supporting Information).

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ASSOCIATED CONTENT

* Supporting Information S

Figures, tables, and CIF files giving multinuclear NMR spectra of 1−4 and 7, computational results and Cartesian coordinates for the optimized structures, and X-ray crystallographic data for CCDC 971577 (3), 971578 (4) ,and 971579 (7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*D.B.: fax, 33 (0)5 61 55 82 04; tel, 33 (0)5 61 55 68 03; email, [email protected]. Present Address ∥

Institut Català d’Investigació Quı ́mica, Avgda. Paı̈sos Catalans 16, 43007 Tarragona, Spain.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Centre National de la Recherche Scientifique, the Université de Toulouse, and the Agence Nationale de la Recherche (ANR-10-BLAN-070901) is gratefully acknowledged. The MCIA (Mesocentre de Calcul Intensif E

dx.doi.org/10.1021/om401117d | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

M.; Dolgushin, F. M.; Ezernitskaya, M. G.; Peregudov, A. S.; Petrovskii, P. V.; Koridze, A. A. Organometallics 2006, 25, 5466. (12) The covalent radii of the two metals are very similar: 1.41 Å for Ir and 1.39 Å for Pd. See: Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.; Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans. 2008, 2832. (13) For the sake of comparison, the geometry of complex 3 was optimized by DFT (see the Supporting Information). Only slight deviations were observed from the crystallographic structure. (14) Such a high-field displacement of the 1H NMR hydridic signal has been noticed upon comparing the two stereoisomers of {[(Ph2PCH2CH2CH2)2Si(Me)]IrHCl(CO)}; see ref 1d. (15) The same trend was observed between Cp(CO)2Fe[P(O)Ph2] and Cp(CO)2Fe[P(S)Ph2]: (a) Treichel, P. M.; Rosenhein, L. D. Inorg. Chem. 1981, 20, 1539. (b) Lorenz, I.-P.; Mürschel, P.; Pohl, W.; Polborn, K. Chem. Ber. 1995, 128, 413. (16) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020. (17) For Heck coupling with a PNP Pd hydride complex, see: (a) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004, 23, 326. For recent studies dealing with stoichiometric O2 and CO2 insertion reactions, see: (b) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 2508. (c) Johansson, R.; Wendt, O. F. Organometallics 2007, 26, 2426. (d) Suh, H. W.; Schmeier, T. J.; Hazari, N.; Kemp, R. A.; Takase, M. K. Organometallics 2012, 31, 8225. For recent catalytic applications involving pincer Pd hydride complexes as putative intermediates, see: (e) Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2008, 130, 15254. (f) Takaya, J.; Iwasawa, N. Organometallics 2009, 28, 6636. (g) Takaya, J.; Sasano, K.; Iwasawa, N. Org. Lett. 2011, 13, 1698. (h) Ding, B.; Zhang, Z.; Liu, Y.; Sugiya, M.; Imamoto, T.; Zhang, W. Org. Lett. 2013, 15, 3690. (18) A Cambridge Database search revealed seven other examples in which the Pd−H distance falls between 1.421 and 1.771 Å. See refs 17c,d and: (a) Fafard, C. M.; Ozerov, O. V. Inorg. Chim. Acta 2007, 360, 286. (b) Boro, B. J.; Duesler, N. D.; Goldberg, K. I.; Kemp, R. A. Inorg. Chem. Commun. 2008, 11, 1426. (c) Steinke, T.; Shaw, B. K.; Jong, H.; Patrick, B. O.; Fryzuk, M. D. Organometallics 2009, 28, 2830. (d) Gregor, L. C.; Chen, C.-H.; Fafard, C. M.; Fan, L.; Guo, C.; Foxman, B. M.; Gusev, D. G.; Ozerov, O. V. Dalton Trans. 2010, 39, 3195. (e) Lansing, R. B.; Goldberg, K. I.; Kemp, R. A. Dalton Trans. 2011, 40, 8950. (19) A Pd hydride PO(P)P pincer was prepared previously from a phosphine−phosphine oxide−phosphonium derivative (chelate-assisted P+−H activation); see ref 3c. (20) For recent reviews, see: (a) Dubrovina, N. V.; Börner, J. Angew. Chem., Int. Ed. 2004, 43, 5883. (b) Ackermann, L. Synthesis 2006, 1557. (c) Ackermann, L.; Born, R.; Spatz, J. H.; Althammer, A.; Gschrei, C. J. Pure Appl. Chem. 2006, 78, 209. (d) Nemoto, T.; Hamada, Y. Tetrahedron 2011, 67, 667. (21) For selected contributions involving group 10 metals, see: (a) Han, L. B.; Tanaka, M. J. Am. Chem. Soc. 1996, 118, 1571. (b) Han, L. B.; Choi, N.; Tanaka, M. Organometallics 1996, 15, 3259. (c) Li, G. Y. Angew. Chem., Int. Ed. 2001, 40, 1513. (d) Li, G. Y. J. Org. Chem. 2002, 67, 3643. (e) Achard, T.; Giordano, L.; Tenaglia, A.; Gimbert, Y.; Buono, G. Organometallics 2010, 29, 3936. (f) Ackermann, L.; Kapdi, A. R.; Schulzke, C. Org. Lett. 2010, 12, 2298. (22) Hydride complexes have rarely been characterized upon reaction of SPO with metal precursors; see ref 21b. Most common is the association of phosphide oxide and phosphinous acid moieties via hydrogen bonding, giving bidentate anionic ligands. (23) A similar pathway had been predicted for related P(O)−Ph bond activation at Pd, the only difference being the participation of weak π interactions between the Ph ring and the Pd center in the latter case; see ref 3a. (24) Tautomerization of the proligand 1 (>P(O)H → >P−O−H) is slightly disfavored thermodynamically (ΔG = 2.9 kcal/mol).

Aquitain) is also gratefully acknowledged for calculation facilities.

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REFERENCES

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dx.doi.org/10.1021/om401117d | Organometallics XXXX, XXX, XXX−XXX