Synthesis and Reactivity of a Nucleophilic Palladium(II) Carbene

Organometallics , 2014, 33 (21), pp 6059–6064 ... and DFT calculations indicate that the interaction between palladium and carbon is best described ...
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Synthesis and Reactivity of a Nucleophilic Palladium(II) Carbene Cezar C. Comanescu and Vlad M. Iluc* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Two formal palladium carbene complexes, [PC(sp2)P]Pd(PR3) (3: R = Me; 4: R = Ph) were isolated and characterized from [PC(sp3)H2P] ([PC(sp3)H2P] = bis[2-(diisopropylphosphino)phenyl]methane, iPr2P-C6H4-CH2-C6H4PiPr2). Structural studies and DFT calculations indicate that the interaction between palladium and carbon is best described as a single bond, associated with nucleophilic character at that carbon atom. The characteristics of 3 were probed by reactions with electrophiles (MeI), acids (MeOH and HCl), and paratoluidine.

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Scheme 1. Synthesis of [PC(sp2)P]Pd(PR3)

INTRODUCTION Metal carbene complexes have long interested chemists due to their fascinating reactivity and applicability in organic and organometallic reactions.1−3 In general, carbenes are classified as nucleophilic or electrophilic, but, ultimately, the reactivity of a carbene metal complex is dictated by an interplay between the ability of the carbene substituents and of the metal to release electrons into the empty p orbital of the carbene carbon.4 Although palladium is an important metal in catalysis,5−16 its carbene complexes are heavily represented by N-heterocyclic carbenes,17−19 which have electrophilic character.20−30 Unlike those, nucleophilic carbenes are rare and, with one exception,21 are stabilized by heteroatoms.26,31,32 We now report a study of a palladium(II) carbene complex that explores its nucleophilic and basic character.

the central cyclometalated carbon atom at a downfield-shifted triplet resonance (51.84 ppm, 2JCP = 2.9 Hz); the phosphorus nuclei resonate at 50.20 ppm compared to −5.87 ppm in the free ligand. X-ray crystallography indicated that the solid-state structure of 2 (Figure 1) is similar to its solution structure. The palladium(II) center adopts a slightly distorted square-planar geometry with P−Pd−P and C−Pd−Cl angles of 159.397(19)° and 172.80(6)°, respectively. Further reaction of 2 with a strong base, K[N(SiMe3)2], in the presence of a trialkyl or triaryl phosphine (PR3 = PMe3, PPh3) led through dehydrohalogenation to the formation of the palladium carbene complexes [PC(sp2)P]Pd(PR3) (3: R = Me; 4: R = Ph). Carbenes 3 and 4 were characterized by 31P{1H} NMR (THF-d8, 298 K) spectroscopy, which indicates the presence of an AX2 spin system, diagnostic of the carbene formulation: δ(A) is −33.88 ppm (t, 2JPP = 43.2 Hz, −PMe3) for 3 and 15.60 ppm (t, 2JPP = 41.4 Hz, −PPh3) for 4; δ(X2) is 62.99 ppm (d, 2JPP = 43.2 Hz, −PiPr2) for 3 and 58.85 ppm (d, 2 JPP = 41.4 Hz, −PiPr2) for 4. While no 13C NMR peaks were found for the carbene carbons, the absence of the benzylic proton from 2 was confirmed by 1H NMR spectroscopy and is in agreement with 31 P{1 H} NMR spectroscopic data. Single-crystal X-ray diffraction (Figures 2 and 3) was used to investigate the

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RESULTS AND DISCUSSION Recently, Piers et al. reported the isolation of a nickel carbene from [PC(sp3 )HP]Ni(II)Br ([PC(sp3)HP] = bis[2-(diisopropylphosphino)phenyl]methyl, iPr2P-C6H4-CH-C6H4PiPr2)33−36 by dehydrohalogenation with potassium bis(trimethylsilyl)amide (K[N(SiMe3)2]) in the presence of PPh3. The central carbene atom is nucleophilic and strongly σ donating; that design allowed 1,2-additions across the Ni−C bond via heterolytic splitting of E−H substrates (E−H = H2, NH3, H2O, R−OH, PhCCH) to yield the corresponding [PC(sp3)HP]Ni−E complexes.34 As part of our metal−ligand cooperation program,37,38 we became interested in the reactivity of the palladium analogue of [PC(sp2)P]Ni(PPh3). Heating a mixture of iPr2P-C6H4-CH2C6H4-PiPr2 (1) with (COD)PdCl2 led to the isolation of the precursor [PC(sp3)HP]PdCl (2) in 71% yield (Scheme 1). Alternatively, 2 can be isolated in similar yields by thermally induced dehydrohalogenation of [PC(sp3)H2P]PdCl2.39 Complex 2 displays spectral patterns typical of a symmetrical species. The backbone proton shows a singlet resonance in the 1H NMR spectrum at 6.23 ppm and exhibits no long-range phosphorus coupling.26 The 13C{1H} NMR spectrum shows © XXXX American Chemical Society

Received: June 27, 2014

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Figure 1. Molecular structure of [PC(sp3)HP]PdCl (2) with thermal ellipsoids at 50% probability. Most hydrogen atoms were omitted for clarity. Selected distances (Å) and angles (deg): Pd−Cl = 2.3937(5), Pd−P(1) = 2.3170(5), Pd−P(2) = 2.2592(5), Pd−C = 2.0738(19), P(1)−Pd−Cl = 101.350(18), Cl−Pd−P(2) = 93.765(18), Cl−Pd−C = 172.80(6), P(2)−Pd−P(1) = 159.397(19), P(1)−Pd−C = 82.08(6), C−Pd−P(2) = 84.55(6), C(12)−C−Pd = 115.84(13), C(22)−C−Pd = 109.46(12), C(12)−C−C(22) = 116.24(17).

Figure 3. Molecular structure of [PC(sp2)P]Pd(PPh3) (4) with thermal ellipsoids at 50% probability. Hydrogen atoms were omitted for clarity. Selected distances (Å) and angles (deg): Pd−C = 2.0755(11), Pd−P(1) = 2.3040(3), Pd−P(2) = 2.3480(3), Pd−P(3) = 2.3993(3), P(1)−Pd−C = 81.73(3), P(2)−Pd−C = 81.97(3), P(3)−Pd−C = 177.96(3), P(1)−Pd−P(3) = 96.748(11), P(2)−Pd− P(3) = 99.581(10), P(1)−Pd−P(2) = 163.655(11), C(11)−C−Pd = 118.68(8), C(21)−C−Pd = 119.40(8), C(11)−C−C(21) = 121.85(10).

bond between the respective atoms. These findings agree with a pronounced single-bond character of the Pd−C bond in both 3 and 4. Density functional theory calculations using Gaussian03 (B3LYP functional, LANL2DZ basis set) were performed on a carbene model complex of 3 and 3′, in which the isopropyl phosphine groups were replaced by methyls. Analysis of the frontier molecular orbitals (Figure 4) indicates that the HOMO

Figure 2. Molecular structure of [PC(sp2)P]Pd(PMe3) (3) with thermal ellipsoids at 50% probability. Hydrogen atoms were omitted for clarity. Selected distances (Å) and angles (deg): Pd−C = 2.086(4), Pd−P(1) = 2.3018(11), Pd−P(2) = 2.3081(11), Pd−P(3) = 2.3525(11), P(1)−Pd−C = 81.92(11), P(2)−Pd−C = 82.53(11), P(3)−Pd−C = 176.21(11), P(1)−Pd−P(3) = 95.93(4), P(2)−Pd− P(3) = 99.74(4), P(1)−Pd−P(2) = 164.24(4), C(12)−C−Pd = 118.4(3), C(22)−C−Pd = 118.6(3), C(12)−C−C(22) = 123.0(3).

Figure 4. Molecular orbitals of 3′: HOMO (left) and HOMO−10 (right).

solid-state molecular structures of both complexes. Compounds 3 and 4 contain a three-coordinate carbon that is bound to palladium and the two inherited framework phenyl rings in the o,o′-positions. The Pd−C distance (2.086(4) Å in 3, 2.0755(11) Å in 4) is rather long compared to values corresponding to PdC bonds, which typically range from 2.005(5)40 to 2.020(5) Å;41 however, the present values are smaller than those corresponding to a single Pd−C bond (ca. 2.15 Å).26,42,43 Interestingly, the Pd−C distance barely changes from 2.0738(19) Å in the parent [PC(sp3)HP]PdCl (2) to 2.086(4) Å in 3 and 2.0755(11) Å in 4. The carbene carbon is sp2 hybridized, with angles of 123.0(3)°, 118.6(3)°, and 118.4(3)° in 3 and 118.68(8)°, 121.85(10)°, and 119.40(8)° in 4; the C carbene −C aryl distances (1.427(5), 1.436(6) 1.4348(15), and 1.4389(15) Å) are consistent with a single

contains an antibonding π-type interaction between a carbene carbon p orbital and the appropriate symmetry d orbital of palladium. The bonding component of this π bond was found in HOMO−10. In addition, HOMO−18 shows the expected σ interaction. The frontier molecular orbitals described here are reminiscent of those reported for [SP(Ph)2CP(Ph)2S]Pd(PPh3).32 The net single-bond character for the palladium− carbon interaction is also supported by its bond order of 0.89 calculated using natural bond order (NBO) analysis.44 Furthermore, NBO analysis indicates that the partial charges are qC = −0.38 and qPd = +0.23. Reactivity studies were undertaken to probe the computational results. Accordingly, the reaction of 3 with an electrophile, MeI (Scheme 2), leads to a square-planar Pd(II) pincer complex, [PC(sp3)MeP]PdI (5), in which the methyl B

dx.doi.org/10.1021/om500682s | Organometallics XXXX, XXX, XXX−XXX

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Scheme 2. Reactivity of 3 Indicating Its Nucleophilic and Basic Character

(Figure 6) is similar to its solution structure. The Pd(II) center exhibits a square-planar geometry, while the Pd−OMe distance

group adds to the carbene carbon and the iodide adds to palladium. Both NMR spectroscopy and X-ray crystallography confirm the structure of 5 (Figure 5).

Figure 6. Molecular structure of [PC(sp3)HP]Pd(OMe) (6) with thermal ellipsoids at 50% probability. Most hydrogen atoms were omitted for clarity. Selected distances (Å) and angles (deg): Pd−C = 2.074(5), Pd−P(1) = 2.2596(17), Pd−P(2) = 2.2944(16), Pd−O(3) = 2.055(4), P(1)−Pd−C = 84.81(17), P(2)−Pd−C = 82.57(16), O(3)−Pd−C = 177.9(2), P(1)−Pd−O(3) = 95.20(17), P(2)−Pd− O(3) = 98.10(15), P(1)−Pd−P(2) = 157.64(6), C(11)−C−Pd = 115.2(4), C(21)−C−Pd = 109.4(4), C(11)−C−C(21) = 117.3(5).

is 2.055(4) Å. There are only two other examples of methoxide complexes supported by PCP pincer ligands in which the central carbon is sp 2 hybridized, 2,6-(bis((di-tertbutylphosphino)methyl)phenyl)palladium(II) methoxide (Pd−O distance of 2.084 Å) 4 5 and (2,6-bis((diisopropylphosphino)methyl)phenyl)palladium methoxide (Pd−O distance of 2.087 Å),46 and no examples of analogous complexes in which the central carbon is sp3 hybridized. In the [PC(sp3)HP]Pd(OMe) complex reported herein, the Pd−O distance is 2.055(4) Å, slightly shorter than those two examples, consistent with the electron-donating ability of the central C(sp3)H moiety, which strengthens the Pd−O bond.45,46 Compound 3 functions as a base also in reactions with amines. Its reaction with p-toluidine (p-H2N-C6H4-CH3) leads through N−H bond cleavage to backbone carbon protonation and addition of the p-toluidyl moiety to the palladium center, forming the anilide [PC(sp3)HP]Pd(NHC6H4CH3) (7) in near-quantitative yield. Compound 7 was characterized by 1H NMR spectroscopy, which shows an amide proton resonating at 1.62 ppm, the methyl toluidine as a singlet at 2.39 ppm, and the benzylic proton at 5.86 ppm, which is shifted upfield from the parent pincer chloride 2. A single resonance was recorded in the corresponding 31P{1H} NMR spectrum at δ 46.35 ppm. X-ray crystallography (Figure 7) indicates a square-planar palladium(II) center and a Pd−N distance of 2.0860(14) Å, which is similar to those found in analogous Pd(II) amides (2.085 to 2.097 Å).47−49 In conclusion, we described the isolation and characterization of two formal palladium carbene complexes, 3 and 4. Structural studies and DFT calculations indicate that the interaction between palladium and carbon is best described as a single bond, associated with nucleophilic character at that carbon atom. Consequently, the reactions of 3 with an electrophile, MeI, and acids, MeOH and HCl, agree with this assignment.

Figure 5. Molecular structure of [PC(sp3)MeP]PdI (5) with thermal ellipsoids at 50% probability. Most hydrogen atoms were omitted for clarity. Selected distances (Å) and angles (deg): Pd−C(1) = 2.0978(17), Pd−P(1) = 2.3127(5), Pd−P(2) = 2.2700(5), Pd−I = 2.6857(3), P(1)−Pd−C(1) = 83.72(5), P(2)−Pd−C(1) = 85.40(5), I−Pd−C(1) = 173.91(5), P(1)−Pd−I = 94.951(13), P(2)−Pd−I = 96.925(14), P(1)−Pd−P(2) = 164.924(17), C(11)−C(1)−Pd = 107.45(11), C(21)−C(1)−Pd = 115.06(11), C(11)−C(1)−C(21) = 114.58(14), C(11)−C(1)−C(2) = 108.91(13), C(21)−C(1)−C(2) = 105.82(14), C(2)−C(1)−Pd = 104.41(11).

The nucleophilic character of 3 is related to its basic character, and two acids, HCl and MeOH, were used to probe it. Protonation of 3 with HCl leads to regeneration of 2 by heterolytic cleavage of the H−Cl bond and addition across the Pd−C bond. Complex 3 reacts with methanol at room temperature (Scheme 2) to give [PC(sp3)HP]Pd(OMe) (6). This reaction is also similar to the E−H additions described by the Piers group with the analogous nickel complex.34 X-ray crystallography indicated that the solid-state structure of 6 C

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Schlenk tube resulted in the formation of complex 2 in a similar yield. H NMR (300 MHz, C6D6): δ 7.32 (d, 3JHH = 7.8 Hz, 2H, ArH), 7.17−7.06 (m, 4H, ArH), 6.97 (t, 3JHH = 7.3 Hz, 2H, ArH), 6.23 (s, 1H, benzylic backbone), 2.62−2.50 (m, 2H, −CH(CH3)2), 2.43−2.33 (m, 2H, −CH(CH3)2), 1.55−1.40 (m, 12H, −CH(CH3)2), 1.15 (dd, 3 JHP = 13.2, 3JHH = 5.9 Hz, 3H, −CH(CH3)2), 1.08 (dd, 3JHP = 12.9, 3 JHH = 5.6 Hz, 3H, −CH(CH3)2). 31P{1H} NMR (121 MHz, C6D6): δ 50.20. 13C{1H} NMR (75 MHz, C6D6): δ 158.97 (t, JCP = 15.0 Hz, ArC), 134.80 (t, JCP = 16.6 Hz, ArC), 132.48 (s, ArC), 130.57 (s, ArC), 127.39 (t, JCP = 9.0 Hz, ArC), 125.73 (t, JCP = 3.2 Hz, ArC), 51.84 (t, 2 JCP = 2.9 Hz, benzylic backbone), 25.86 (t, JCP = 10.0 Hz, −CH(CH3)2), 25.35 (t, JCP = 11.9 Hz, −CH(CH3)2), 19.34 (t, JCP = 2.8 Hz, −CH(CH3)2), 19.09 (t, JCP = 2.3 Hz, −CH(CH3)2), 18.52− 18.02 (m, −CH(CH3)2). 13C NMR (75 MHz, C6D6): δ 51.84 (d, JCH = 129.5 Hz, benzylic backbone). Anal. Calcd for C25H37ClP2Pd: C, 55.46; H, 6.89. Found: C, 55.42; H, 6.59. Synthesis of [PC(sp2)P]Pd(PMe3) (3). In a 20 mL scintillation vial, 216.6 mg of 2 (0.4 mmol) and 61 mg of PMe3 (0.8 mmol) were mixed in THF and stirred for 5 min prior to the addition of 0.61 mL of K[N(SiMe3)2] (0.66 M in toluene, 0.4 mmol). The solution turned brown, and the mixture was left stirring for an additional 3 h. The volatiles were removed under reduced pressure, and the solid residue was triturated with n-pentane. The resulting solid was dissolved in a minimum amount of THF, and the concentrated solution was passed through a plug of Celite. This concentrated solution was left to crystallize at −35 °C to yield analytically pure 3 (209 mg, 87%). 1H NMR (400 MHz, THF-d8): δ 7.28 (d, 3JHH = 8.6 Hz, 2H, ArH), 6.63− 6.58 (m, 2H, ArH), 6.35 (ddd, J = 8.3, 2.6, 1.3 Hz, 2H, ArH), 5.69 (t, J = 6.9 Hz, 2H, ArH), 2.50−2.38 (m, 4H, −CH(CH3)2), 1.50 (d, 2JHP = 6.4 Hz, 9H, −P(CH3)3), 1.30 (dd, 3JHP = 13.3, 3JHH = 6.9 Hz, 12H, −CH(CH3)2), 1.24 (dd, 3JHP = 16.4, 3JHH = 7.2 Hz, 12H, −CH(CH3)2). 31P{1H} NMR (162 MHz, THF-d8): δ 62.99 (d, 2JPP = 43.2 Hz, −PiPr2), −33.88 (t, 2JPP = 43.2 Hz, −PMe3). 13C{1H} NMR (101 MHz, THF-d8): δ 164.00 (td, JCP = 19.6, 2.8 Hz, ArC), 132.13 (s, ArC), 130.16 (d, JCP = 1.1, ArC), 116.92 (td, JCP = 22.2, 10.8 Hz, ArC), 115.95 (td, JCP = 12.1, 7.1 Hz, ArC), 110.39 (t, JCP = 3.7 Hz, ArC), 27.79 (t, JCP = 11.0 Hz, −CH(CH3)2), 21.20 (t, JCP = 3.3 Hz, −CH(CH3)2), 20.27 (dt, JCP = 18.8, 1.6 Hz, −P(CH3)3), 19.26 (s, −CH(CH3)2). Anal. Calcd for C28H45P3Pd: C, 57.88; H, 7.81. Found: C, 57.42; H, 7.75. Synthesis of [PC(sp2)P]Pd(PPh3) (4). In a 20 mL scintillation vial, 216.6 mg of 2 (0.4 mmol) and 104.9 mg of PPh3 (0.4 mmol) were mixed in THF and stirred for 5 min prior to the addition of 0.61 mL of K[N(SiMe3)2] (0.66 M in toluene, 0.4 mmol). The solution turned brown, and the mixture was left stirring for an additional 3 h. The volatiles were then removed under reduced pressure, and the solid obtained was triturated with n-pentane. The solid residue was dissolved in a minimum amount of toluene, and this solution was passed through a plug of Celite. Analytically pure 4 was isolated by crystallization at −35 °C (224 mg, 73%). 1H NMR (500 MHz, THFd8): δ 7.85−7.76 (m, 6H, Ar −P(C6H5)3), 7.51−7.42 (m, 9H, −P(C6H5)3), 7.19 (d, 3JHH = 7.0 Hz, 2H, ArH), 6.50−6.45 (m, 2H, ArH), 6.37 (dd, 3JHP = 8.2, 3JHH = 7.0 Hz, 2H, ArH), 5.69 (t, J = 6.9 Hz, 2H, ArH), 1.64 (tddd, J = 9.2, 6.8, 4.8, 2.3 Hz, 4H, −CH(CH3)2), 0.98 (dd, 3JHP = 13.1, 3JHH = 6.8 Hz, 12H, −CH(CH3)2), 0.93 (dd, 3 JHP = 16.4, 3JHH = 7.3 Hz, 12H, −CH(CH3)2). 31P{1H} NMR (162 MHz, THF-d8): δ 58.85 (d, 2JPP = 41.4 Hz, −PiPr2), 15.61 (t, 2JPP = 41.4 Hz, −PPh3). 13C{1H} NMR (101 MHz, THF-d8): δ 162.83 (t, J = 17.9 Hz, ArC), 135.56 (d, JCP = 27.5 Hz, ArC), 134.98 (d, JCP = 12.9 Hz, ArC), 133.90 (d, JCP = 18.4 Hz, ArC), 132.88 (s, ArC), 131.31 (d, JCP = 1.6 Hz, ArC), 131.09 (s, ArC), 129.33 (d, JCP = 8.9 Hz, ArC), 129.03 (d, JCP = 7.5 Hz, ArC), 119.42−118.71 (m, ArC), 116.24 (td, JCP = 12.0, 6.3 Hz, ArC), 111.00 (s, ArC), 26.03 (t, JCP = 10.5 Hz, −CH(CH3)2), 21.01 (t, JCP = 3.0 Hz, −CH(CH3)2), 18.71 (s, −CH(CH3)2). Anal. Calcd for C43H51P3Pd: C, 67.32; H, 6.70. Found: C, 67.41; H, 6.78. Reaction of [PC(sp2)P]Pd(PMe3) (3) with HCl. In a 20 mL scintillation vial, 29.1 mg of carbene 3, [PC(sp2)P]PdPMe3 (0.05 mmol), was dissolved in THF. Subsequently, 1 mL of 0.1 M HCl (in 1

Figure 7. Molecular structure of [PC(sp3)HP]Pd(NHC6H4CH3) (7) with thermal ellipsoids at 50% probability. Most hydrogen atoms were omitted for clarity. Selected distances (Å) and angles (deg): Pd−C = 2.0878(15), Pd−P(1) = 2.2618(5), Pd−P(2) = 2.3223(4), Pd−N = 2.0860(14), P(1)−Pd−C = 84.48(4), P(2)−Pd−C = 81.99(4), N− Pd−C = 177.89(6), P(1)−Pd−N = 94.92(4), P(2)−Pd−N = 98.11(4), P(1)−Pd−P(2) = 161.054(15), C(11)−C−Pd = 116.43(10), C(21)−C−Pd = 109.56(10), C(11)−C−C(21) = 116.35(13), Pd−N−C(51) = 127.47(12).

Current efforts focus on applying these findings to the activation of inert substrates.

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EXPERIMENTAL SECTION

General Remarks. Experiments were performed under an N2 atmosphere using glovebox techniques. Solvents were dried by passing through a column of activated alumina followed by storage under dinitrogen. All commercial chemicals were used as received, except where specified otherwise. Pd(COD)Cl2 was purchased from SigmaAldrich. Deuterated solvents were obtained from Cambridge Isotope Laboratories. THF-d8 was dried over sodium, while C6D6 was dried by refluxing over dry CaH2 and filtered prior to use. NMR spectra were recorded on a Varian 300 spectrometer at ambient probe temperature or Bruker 400 and 500. Chemical shifts are reported in ppm relative to residual internal protonated solvent for 1H and 13C{1H} NMR spectra; J values are given in Hz. All assignments are based on one-dimensional 1 H and 13C experiments unless otherwise noted. Compound 1 was synthesized according to a literature procedure.33 CHN analyses were performed on a CE-440 elemental analyzer or by Midwest Microlab, LLC. Gaussian 03 (revision D.02) was used for all reported calculations.50 The B3LYP (DFT) method was used to carry out the geometry optimizations on model compounds specified in the text using the LANL2DZ basis set. The validity of the true minima was checked by the absence of negative frequencies in the energy Hessian. Synthesis of [PC(sp3)HP]PdCl (2). In a 20 mL scintillation vial, 240 mg of 1 (0.599 mmol) was dissolved in a minimum amount of THF and left stirring for 5 min at ambient temperature. Independently, 164.58 mg of Pd(COD)Cl2 (0.57 mmol) was dissolved in THF and stirred for 5 min prior to addition to the solution of 1. The reaction turned yellow within 15 min and was then transferred to a Schlenk tube and heated at 70 °C for 48 h outside of the box. The color progressively became lighter with time, and by the end of the thermal treatment, it was almost cream. After bringing the Schlenk tube back into the box, the volatiles were removed under reduced pressure and the residue was triturated several times with hexanes and Et2O. The solid residue was kept under vacuum for 3 h to yield 221.9 mg of crude product (71.9% yield). Complex 2 was crystallized by layering a saturated toluene solution with a small amount of n-pentane at −35 °C, in 57% overall yield (175.9 mg). Alternatively, heating a solution of [PC(sp3)H2P]PdCl239 in THF for 48 h at 70 °C in a D

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Organometallics

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161.21 (s, ArC), 159.86 (t, JCP = 14.9 Hz, ArC), 134.63 (t, JCP = 16.6 Hz, ArC), 132.16 (s, ArC), 130.18 (s, ArC), 129.84 (s, ArC), 127.82− 127.54 (m, ArC), 124.86 (t, JCP = 3.1 Hz, ArC), 116.11 (s, ArC), 115.83 (s, ArC), 49.98 (s, -C(H)Pd), 26.34 (t, J = 9.8 Hz, −CH(CH3)2), 24.67 (t, JCP = 11.5 Hz, −CH(CH3)2), 20.96 (s, pC6H4-CH3), 19.01 (t, JCP = 2.9 Hz, −CH(CH3)2), 18.47 (t, JCP = 2.9 Hz, −CH(CH3)2), 18.29 (s, −CH(CH3)2), 17.97 (s, −CH(CH3)2). Anal. Calcd for C32H45NP2Pd: C, 62.79; H, 7.41; N, 2.29. Found: C, 62.53; H, 7.38; N 2.20. Crystal Data for [PC(sp3)HP]PdCl (2). Single crystals were obtained by layering a concentrated toluene solution of 2 with npentane at −35 °C in a glovebox. The resulting crystals were pale yellow, almost colorless blocks. For 2: C25H37ClP2Pd; Mr = 541.34; monoclinic; space group P2(1)/n; a = 11.2355(9) Å; b = 13.5693(10) Å; c = 17.3504(13) Å; α = 90°; β = 107.5300(10)°; γ = 90°; V = 2522.4(3) Å3; Z = 4; T = 120(2) K; λ = 0.710 73 Å; μ = 0.978 mm−1; dcalc = 1.426 g·cm−3; 34 058 reflections collected; 4439 unique (Rint = 0.0265); giving R1 = 0.0203, wR2 = 0.0473 for 4003 data with [I > 2σ(I)] and R1 = 0.0244, wR2 = 0.0485 for all 4439 data. Residual electron density (e−·Å−3) max/min: 1.249/−0.916. Crystal Data for [PC(sp2)P]Pd(PMe3) (3). Single crystals were obtained from a concentrated THF solution at −35 °C in a glovebox. The resulting crystals were very dark (almost black) thin needles. For 3: C28H45P3Pd; Mr = 580.95; monoclinic; space group P2(1)/c; a = 17.038(3) Å; b = 9.1693(14) Å; c = 19.255(3) Å; α = 90°; β = 111.415(2)°; γ = 90°; V = 2800.4(8) Å3; Z = 4; T = 120(2) K; λ = 0.710 73 Å; μ = 0.848 mm−1; dcalc = 1.378 g·cm−3; 26 200 reflections collected; 4925 unique (Rint = 0.0580); giving R1 = 0.0399, wR2 = 0.0856 for 4090 data with [I > 2σ(I)] and R1 = 0.0536, wR2 = 0.0889 for all 4925 data. Residual electron density (e−·Å−3) max/min: 2.626/ −2.666. Crystal Data for [PC(sp2)P]Pd(PPh3) (4·C7H8). Single crystals were obtained from a concentrated toluene solution at −35 °C in a glovebox. The resulting crystals were dark brown-red blocks. For 4· C7H8: C50H59P3Pd; Mr = 859.28; monoclinic; space group P2(1)/n; a = 10.6451(6) Å; b = 19.8859(11) Å; c = 20.3451(12) Å; α = 90°; β = 102.1540(8)°; γ = 90°; V = 4210.3(4) Å3; Z = 4; T = 120(2) K; λ = 0.710 73 Å; μ = 0.589 mm−1; dcalc = 1.356 g·cm−3; 73 275 reflections collected; 11 416 unique (Rint = 0.0221); giving R1 = 0.0209, wR2 = 0.0539 for 10 585 data with [I > 2σ(I)] and R1 = 0.0235, wR2 = 0.0555 for all 11 416 data. Residual electron density (e−·Å−3) max/min: 0.469/−0.492. Crystal Data for [PC(sp3)MeP]PdI (5). Single crystals were obtained from a concentrated Et2O solution layered with n-hexane at −35 °C in a glovebox. The resulting crystals were bright yellow, cubic blocks. For 5: C26H39IP2Pd; Mr = 646.81; orthorhombic; space group Pca2(1); a = 14.7756(18) Å; b = 12.1643(15) Å; c = 14.8269(18) Å; α = 90°; β = 90°; γ = 90°; V = 2664.9(6) Å3; Z = 4; T = 120(2) K; λ = 0.710 73 Å; μ = 1.986 mm−1; dcalc = 1.612 g·cm−3; 62 173 reflections collected; 7529 unique (Rint = 0.0308); giving R1 = 0.0160, wR2 = 0.0364 for 7239 data with [I > 2σ(I)] and R1 = 0.0179, wR2 = 0.0371 for all 7529 data. Residual electron density (e−·Å−3) max/min: 0.846/ −0.630. Crystal Data for [PC(sp3)HP]Pd(OMe) (6). Single crystals were obtained from a concentrated diethyl ether solution at −35 °C in a glovebox. The resulting crystals were yellow-green blocks. For 6: C26H40OP2Pd; Mr = 536.92; monoclinic; space group P2(1); a = 9.3988(10) Å; b = 15.9775(16) Å; c = 9.8203(10) Å; α = 90°; β = 118.2420(15)°; γ = 90°; V = 1299.2(2) Å3; Z = 2; T = 120(2) K; λ = 0.710 73 Å; μ = 0.852 mm−1; dcalc = 1.373 g·cm−3; 19 305 reflections collected; 5435 unique (Rint = 0.0551); giving R1 = 0.0516, wR2 = 0.1239 for 5067 data with [I > 2σ(I)] and R1 = 0.0563, wR2 = 0.1267 for all 5435 data. Residual electron density (e−·Å−3) max/min: 3.424/ −3.295. Crystal Data for [PC(sp3)HP]Pd(NHC6H4CH3) (7). Single crystals were obtained from a saturated Et2O solution at −35 °C in a glovebox. The resulting crystals were yellow-green blocks. For 7: C32H45NP2Pd; Mr = 612.03; monoclinic; space group P2(1)/c; a = 10.6205(12) Å; b = 13.0337(14) Å; c = 21.568(2) Å; α = 90°; β = 96.867(2)°; γ = 90°; V = 2964.2(6) Å3; Z = 4; T = 120(2) K; λ = 0.710 73 Å; μ = 0.755

diethyl ether) was added via syringe, when an instant color change from dark brown to very light yellow occurred, along with some precipitate being formed. After filtering off the resulting phosphonium salt and solvent removal under reduced pressure, the [PC(sp3)HP]PdCl complex 2 was obtained in near-quantitative yield (25.4 mg, 94% yield). 1H and 31P NMR data showed that the product was analytically pure. Synthesis of [PC(sp3)MeP]PdI (5). A 23.2 mg amount of carbene 3 (0.04 mmol) was mixed in a 20 mL scintillation vial with 5.7 mg of CH3I (0.04 mmol) in THF. An instant color change to yellow occurred upon addition. After removal of the volatiles under reduced pressure and recrystallization from a concentrated n-hexane solution, 25.2 mg of 5 was obtained in near-quantitative yield (97%). 1H and 31P NMR spectroscopies show the product to be analytically pure. 1H NMR (400 MHz, C6D6): δ 7.50 (d, 3JHH = 8.0 Hz, 2H, ArH), 7.20 (dt, J = 7.6, 3.8 Hz, 2H, ArH), 7.10 (ddd, J = 8.1, 2.2, 1.1 Hz, 2H, ArH), 6.95 (t, J = 7.3 Hz, 2H, Ar), 2.74−2.63 (m, 2H, −CH(CH3)2), 2.62− 2.53 (m, 2H, −CH(CH3)2), 2.04 (t, 4JHP = 4.1 Hz, 3H, −C(CH3)), 1.40 (ddd, J = 16.9, 15.8, 7.6 Hz, 12H, −CH(CH3)2), 1.16 (dd, J = 14.4, 7.2 Hz, 6H, −CH(CH3)2), 0.94 (dd, J = 14.9, 7.3 Hz, 6H, −CH(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 47.73 (s). 13C{1H} NMR (101 MHz, C6D6): δ 163.80 (t, JCP = 14.4 Hz, ArC), 135.90 (t, JCP = 15.4 Hz, Ar), 133.03 (s, Ar), 130.07 (s, Ar), 127.41 (t, JCP = 9.4 Hz, ArH), 126.01 (t, JCP = 3.0 Hz, ArH), 74.08 (t, 2JCP =7.1 Hz, −Ar2CCH3), 40.63 (s, −C(CH3)), 28.18 (t, JCP = 12.4 Hz, −CH(CH3)2), 26.96 (t, JCP = 10.4 Hz, −CH(CH3)2), 20.29 (t, JCP = 2.4 Hz, −CH(CH3)2), 19.73 (t, JCP = 1.8 Hz, −CH(CH3)2), 19.41 (s, −CH(CH 3 ) 2 ), 18.84 (s, −CH(CH 3 ) 2 ). Anal. Calcd for C26H39IP2Pd: C, 48.28; H, 6.08. Found: C, 48.61; H, 6.05. Synthesis of [PC(sp3)HP]Pd(OMe) (6). To a solution of 23.2 mg of 3 (0.04 mmol) in 5 mL of THF was added dropwise a 0.2 mL CH3OH solution in THF (0.2 M, 0.04 mmol). A rapid color change from brown to yellow was observed, and after an additional 20 min of stirring at ambient temperature the volatiles were removed under reduced pressure. The solid residue was triturated with n-pentane and kept under vacuum for 1 h. Analytically pure 6 was isolated by recrystallization from Et2O at −35 °C to yield 16.1 mg (75%). 1H NMR (400 MHz, C6D6): δ 7.29 (d, 3JHH = 7.8 Hz, 2H, ArH), 7.11 (dt, J = 7.3, 3.6 Hz, 2H, ArH), 7.06 (t, J = 7.5 Hz, 2H, ArH), 6.92 (t, J = 7.3 Hz, 2H, ArH), 5.69 (s, 1H, backbone H), 4.45 (s, 3H, −OCH3), 2.45− 2.36 (m, 2H, −CH(CH3)2), 2.35−2.27 (m, 2H, −CH(CH3)2), 1.47 (dd, 3JHP = 13.9, 3JHH = 6.5 Hz, 6H, −CH(CH3)2), 1.42 (dd, 3JHP = 14.1, 3JHH = 6.5 Hz, 6H, −CH(CH3)2), 1.13 (dd, 3JHP = 14.9, 3JHH = 7.5 Hz, 6H, −CH(CH3)2), 1.09 (q, J = 7.1 Hz, 6H, −CH(CH3)2). 31 1 P{ H} NMR (162 MHz, C6D6): δ 45.10 (s). 13C{1H} NMR (101 MHz, C6D6): δ 160.34 (t, JCP = 15.1 Hz, ArC), 134.69 (t, JCP = 16.7 Hz, ArC), 132.49 (s, ArC), 130.29 (s, ArC), 127.95 (t, JCP = 8.3 Hz, ArC), 125.21 (t, JCP = 3.1 Hz, ArC), 61.25 (s, backbone C), 46.47 (s, −OCH3), 26.09 (t, JCP = 9.4 Hz, −CH(CH3)2), 25.29 (t, JCP = 11.2 Hz, −CH(CH3)2), 19.52 (t, JCP = 3.5 Hz, −CH(CH3)2), 18.79 (t, JCP = 1.6 Hz, −CH(CH3)2), 18.71 (t, JCP = 3.0 Hz, −CH(CH3)2), 18.62 (s, −CH(CH3)2). Anal. Calcd for C26H40OP2Pd: C, 58.16; H, 7.51. Found: C, 58.41; H, 7.40. Synthesis of [PC(sp3)HP]Pd(NHC6H4CH3) (7). A solution of 10.7 mg of p-toluidine (C7H9N, 107.15 g mol−1, 0.1 mmol) in 5 mL of THF was added dropwise to a solution containing 58.1 mg (C28H45P3Pd, 0.1 mmol) of 3 in 5 mL of THF. The color changed rapidly from brown to light yellow. After 5 more minutes, the volatiles were removed under reduced pressure, the residue was triturated with n-pentane, and the volatiles were removed again. The light orange powder obtained in 74% yield, consisting of 44.2 mg of [PC(sp3)HP]Pd(NHC6H4CH3) (C32H44P2Pd, 597.07 g mol−1), was analytically pure. 1H NMR (400 MHz, C6D6): δ 7.32 (dt, J = 8.5, 1.0 Hz, 2H, ArH), 7.13−7.03 (m, 6H, ArH), 6.90 (dd, J = 7.7, 7.0 Hz, 2H, ArH), 6.86 (d, 3JHH = 8.3 Hz, 2H, ArH), 5.86 (s, 1H, −C(H)Pd), 2.39 (s, 3H, −p-C6H4-CH3), 2.25 (m, 2H, −CH(CH3)2), 2.18−2.08 (m, 2H, −CH(CH3)2), 1.62 (s, 1H, −Pd(NH)−), 1.32 (dd, 3JHP = 15.4, 3JHH = 8.2 Hz, 6H, −CH(CH3)2), 1.12 (dd, 3JHP = 15.4, 3JHH = 8.2 Hz, 6H, −CH(CH3)2), 1.00−0.93 (m, 12H, −CH(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 47.13 (s). 13C{1H} NMR (101 MHz, C6D6): δ E

dx.doi.org/10.1021/om500682s | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

mm−1; dcalc = 1.371 g·cm−3; 32 599 reflections collected; 6044 unique (Rint = 0.0227); giving R1 = 0.0198, wR2 = 0.0483 for 5465 data with [I > 2σ(I)] and R1 = 0.0242, wR2 = 0.0502 for all 6044 data. Residual electron density (e−·Å−3) max/min: 0.383/−0.238.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra for compounds 2−7, crystallographic tables and details (CIF) for compounds 2−7, complete ref 50, a text file of all computed molecules, Cartesian coordinates in a format for convenient visualization. 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 We thank Dr. Allen Oliver for crystallographic assistance. This work was partially supported by the University of Notre Dame. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (ACS PRF no. 53536-DNI3).

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REFERENCES

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dx.doi.org/10.1021/om500682s | Organometallics XXXX, XXX, XXX−XXX