Ligand-Induced Reductive Elimination of Ethane from Azopyridine

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Cite This: J. Am. Chem. Soc. 2018, 140, 11408−11415

Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes Andrey E. Rudenko,‡ Naomi E. Clayman,‡ Katherine L. Walker,‡ Jana K. Maclaren,† Paul M. Zimmerman,∥ and Robert M. Waymouth*,‡ ‡

Department of Chemistry, Stanford University, Stanford, California 94305-5080, United States Stanford Nano Shared Facilities, Stanford University, 476 Lomita Mall, Stanford, California 94305, United States ∥ Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, United States J. Am. Chem. Soc. 2018.140:11408-11415. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/12/18. For personal use only.

†

S Supporting Information *

ABSTRACT: Reductive elimination (RE) is a critical step in many catalytic processes. The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl) from Pd(II) species is considerably faster than RE of saturated alkyl groups. Pd(II) dimethyl complexes ligated by chelating diimine ligands are stable toward RE unless subjected to a thermal or redox stimulus. Herein, we report the spontaneous RE of ethane from (azpy)PdMe2 complexes and the unique role of the redox-active azopyridine (azpy) ligands in facilitating this reaction. The (azpy)PdMe2 complexes are air- and moisture-stable in the solid form, but they readily produce ethane upon dissolution in polar solvents at temperatures from 10 °C to room temperature without the need for an external oxidant or elevated temperatures. Experimental and computational studies indicate that a bimolecular methyl transfer precedes the reductive elimination step, where both steps are facilitated by the redox-active azopyridine ligand.

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INTRODUCTION The reductive elimination of C−C bonds is a fundamental reaction in organometallic chemistry and a key step in crosscoupling reactions.1−5 While the reductive elimination of sp2 Pd−C bonds is relatively facile,6 the reductive elimination of sp3−sp3 bonds is typically more challenging. Early work by Yamamoto7 and Stille8,9 demonstrated reductive elimination of ethane at elevated temperatures (60 °C) from dimethyl Pd centers ligated by monodentate phosphine ligands. For multidentate chelating phosphorus9 or nitrogen ligands (bipy, phen) the reductive elimination of sp3−sp3 bonds typically requires promotion of Pd(II) dialkyls to higher oxidation states to facilitate reductive elimination. Mechanistic10−17 and computation studies11,18,19 suggest that initial oxidation of Pd(II)MeX (X = halide or Me) to Pd(III) is followed by bimolecular methyl transfer to generate Pd(IV)Me2 species, which are the key intermediates leading to ethane. For other transition metals (Zr,20,21 Co,22 Fe23−25), reductive elimination can be promoted by the presence of redox-active ligands.26,27 We28,29 and others30−33 have demonstrated that azopyridine34,35 ligands exhibit redox-active behavior when coordinated to transition metals. The low-lying NN π* orbital of azopyridine allows ligand reduction by one electron to generate a radical anion or two electrons to form a hydrazido dianion. The redox-chemistry of azopyridine can be taken advantage of to stabilize high-valent metal complexes. Herein, we demonstrate the facile reductive elimination of ethane from LPdMe2 complexes bearing redox-active azopyr© 2018 American Chemical Society

idine ligands (eq 1). Mechanistic experimental and computational studies imply that this reductive elimination proceeds by a rapid methyl group transfer facilitated by the redox-active azopyridine ligand.

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RESULTS AND DISCUSSION Initial attempts to synthesize the (azpy)PdMe2 complex (azpy = (E)-5-methyl-6-(phenyldiazenyl)pyridine) complex by alkylation of (azpy)PdMeCl with either MeMgBr or AlMe3 in THF at −78 °C36 were unsuccessful. When these reaction mixtures were warmed to 0 °C, elimination of ethane was observed, accompanied by the formation of paramagnetic Pd species, the free azopyridine ligand, and Pd black. An alternate synthesis of (azpy)PdMe2 was devised that took advantage of the low solubility of these complexes in nonpolar solvents. Treatment of (cod)PdMe2 (cod = 1,5-cyclooctadiene) and the azopyridine ligand14 in hexanes or pentane at −20 °C for 1−10 days Received: June 23, 2018 Published: August 30, 2018 11408

DOI: 10.1021/jacs.8b06398 J. Am. Chem. Soc. 2018, 140, 11408−11415

Article

Journal of the American Chemical Society

azopyridine ligand by 0.557 Å. For 4, the presence of the ortho-diethyl substituents results in a perpendicular orientation of the N-aryl substituent relative to the N−C−N−N azopyridine plane (Figure 2), positioning the ethyl groups above and below the Pd square plane. Compound 4 exhibits a N3−N1−C6−C5 dihedral angle of 77.6° whereas the dihedral angles for arylazopyridine complexes lacking ortho substituents are less than 39° (31° for compound 1, 39° for compound 2 and 38° for compound 6). As described below, this structural feature has significant consequences on the course of the reductive elimination. Complex 5 was not characterized crystallographically, but analysis of the solution structure by 2-D 1H NMR (COSY, NOESY) suggest that Pd coordinates to the two bipyridyl nitrogens, whereas for 6 Pd is coordinated to one pyridyl nitrogen and one of the azo nitrogen ligands, analogous to that of 1. Dissolution of (azpy)Pd(CH3)2, 1, in THF-d8 leads to the reductive elimination of ethane at room temperature. Analysis of the rate of decay of 1 in THF-d8 indicates that the reductive elimination is second order in Pd (Figures S6B, S7C and S8C, Supporting Information). Analysis of the reaction profile by 1H NMR reveals that ≤0.5 equiv of ethane is produced per molecule of 1 (Figure 3). Ethane quantification was

without stirring produced black crystals of (azpy)PdMe2, 1, in 61% isolated yield (eq 2). 1 is soluble in polar solvents (dichloromethane (DCM), CH3CN, chloroform, tetrahydrofuran (THF), acetone, etc.) but is almost insoluble in hexane or pentane.

To assess the role of the azopyridine ligand on the reductive elimination, a series of modified azopyridine ligands and the corresponding LPd(CH3)2 complexes were prepared and investigated (Figure 1). Complexes 2−6 were prepared analogously to that of 1 from the ligand and (cod)PdMe2 and isolated in yields of 32−78%.

Figure 1. LPd(CH3)2 complexes with modified azopyridine ligands.

Complexes 1, 2, 4 and 6 were characterized crystallographically, and an ORTEP representation of 4 is given in Figure 2. The structural parameters of 1, 2, 4 and 6 are comparable to those of the related (2-phenylazopyridine)PdCl233 and (azpy)Pd(CH3)Cl (Figure S62, Supporting Information) except that the Pd center of 1 sits slightly out of the plane defined by the N−C−N−N plane of the

Figure 3. Reaction profile of 1 in THF-d8 at 25 °C. Starting concentration of 1 is 0.1 M.

accomplished with a calibration curve of ethane speciation in solution and in the headspace of an NMR tube (page S33, Supporting Information). Approximately 30% of the azopyridine ligand was liberated along with paramagnetic Pdcontaining products and Pd black. Analysis of the reaction mixture (after cessation of ethane release) by EPR spectroscopy revealed a signal corresponding to a single unpaired electron with g = 2.0080 (Figure S57, Supporting Information), which is similar to that observed for the radical anion [(azpy)PdCl2]•− that was characterized33 as a ligand-centered radical bound to Pd. A crossover reaction conducted in the dark with a 1:1 mixture of (azpy)Pd(CH3)2, 1, and (azpy)Pd(CD3)2, 1-d6, yields a statistical mixture of CH3CH3, CH3CD3 and CD3CD3 (Figures S9 and S10, Supporting Information). Analysis of this reaction by 1H NMR revealed that the scrambling of methyl groups to generate (azpy)Pd(CH3)(CD3) occurs faster than reductive elimination. These results indicate that methyl group exchange between the Pd centers is facile under these conditions.10,37−39

Figure 2. ORTEP representation (50% probability ellipsoids) of 4. Selected bond distances (Å): Pd1−C17 2.025(2), Pd1−C18 2.055(2), Pd1−N1 2.0807(17), Pd1−N2 2.2110(16), N1−N3 1.261(2). 11409


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cessation of ethane evolution, the slow release of ligand is observed over the course of 22 h. In the case of 5, in DCM-d2, the generation of ethane is not accompanied by methane or semideuteromethane evolution. As observed with 1, when a 1:1 mixture of 5 and (AzoBipy)Pd(CD3)2 was dissolved in DCM-d2, reductive elimination of ethane yielded a statistical mixture of CH3CH3, CH3CD3 and CD3CD3. In contrast, reductive elimination from 6 proceeds similarly to that of 1 and is accompanied by formation of free ligand and Pd black. It is likely that the ortho-isopropyl-substituted pyridine is unable to coordinate effectively to Pd and stabilize the paramagnetic Pd products. The lack of methane as a byproduct during RE of 1 or 5 indicates that reductive elimination is unlikely to proceed by a radical mechanism involving uncaged methyl radicals generated by Pd-CH3 homolysis.10,43 Furthermore, the addition of ∼23 equiv of dihydroanthracene (DHA) to a 0.3 mM DCM-d2 solution of 5 had no effect on course of the reductive elimination and did not result in the production of methane (Supporting Information, Figure S21). As the Pd-containing products of reductive elimination from 5 are paramagnetic, the reaction mixture was analyzed by EPR spectroscopy of the reaction mixture after reductive elimination. The EPR spectrum measured at 77K in DCM reveals a single peak with g = 2.0055 (Supporting Information, Figure S59), consistent with that expected for an phenylazopyridine radical anion bound to Pd.33 As these paramagnetic Pd species appeared to be more stable than those derived from 1, the Pd containing products generated from the reductive elimination of ethane from 5 were analyzed by X-ray photoelectron spectroscopy (XPS) and compared to that of 5 and Pd(PPh3)4, a representative Pd(0) compound. The XPS of the isolated powder sample of 5 exhibited two peaks at 342.8 and 337.4 eV (Supporting Information, Figure S61), consistent with binding energies characteristic of Pd(II)44,45 (Pd(II) is reported to exhibit transitions at 343.2 and 338.0 eV,45 while Pd(PPh3)4 exhibits binding energies of 341.1 and 335.8 eV, Figure S61, Supporting Information). The reductive elimination of ethane from a 3 mM DCM solution of 5 was monitored by NMR and after ∼13 h, after ethane evolution had ceased, a black crystalline precipitate formed. Analysis of this paramagnetic sample by XPS revealed two peaks at 343.3 and 338.1 eV, binding energies that are most consistent with Pd(II). On the basis of these data, we tentatively assign this species as 5a, an azobipyridine palladium(II)−CH3 coordinated to the phenylazobipyridyl radical anion ligand (Figure 5).

The reductive elimination of ethane occurs readily for complexes 1, 2, 3, 5 and 6 upon dissolution in polar solvents (DCM, THF) at room temperature. The reductive elimination from LPdMe2 is strongly influenced by both steric and electronic effects; compounds ligated with more electronwithdrawing phenylazopyridine ligands undergo more rapid reductive elimination. Under comparable experimental conditions ([Pd]0 = 0.05M, THF-d8, 25 °C), the reductive elimination of ethane from complex 3, bearing the p-NO2 substituent, is approximately 7 times faster than that for 1 (t1/2 = 1349 vs 9353 s, respectively). Steric effects also play a role: the more sterically demanding o-diethyl complex (Et2-azpy)PdMe2, 4, is stable when dissolved in solution and does not undergo reductive elimination of ethane over the course of 10 h at room temperature. A 0.05 M solution of 4 in either DCM-d2 or THF-d8 was stable over the course of 10 h with no noticeable release of ethane over that time. At higher temperatures (70 °C), 4 decomposes in solution with the generation of methane and other gaseous products consistent with Pd-Me bond homolysis, as previously reported for (bipy)PdMe2.40 The reticence of 4 to undergo reductive elimination is proposed to a be a consequence of steric demands of the ortho-diethyl groups which shield the apical positions of the Pd centers. Related arguments have been invoked to rationalize the unique behavior of the Brookhart Pd polymerization catalysts,41 where sterically demanding aryl diimine ligands inhibit bimolecular chaintransfer reactions. The observation that the o-diethyl complex, 4, does not undergo reductive elimination is consistent with the hypothesis that a bimolecular step is required for reductive elimination. Complexes bearing the 2-phenylazobipyridyl ligands (5 and 6) were prepared to assess whether, upon reductive elimination, the presence of a pendant ligand might stabilize the Pd products of reductive elimination.42 The reductive elimination of ethane from complex 5 occurred more rapidly compared to 1; analysis of the reaction profile by 1H NMR (Figure 4) revealed that the reductive elimination of 0.5 equiv of ethane (relative to 5) occurs over the course of ∼3 h without release of the AzoBipy ligand and Pd black, consistent with the hypothesis that the presence of an additional N-donor stabilizes the reduced Pd species. Nevertheless, following the

Figure 5. Proposed structure of 5a, the product of reductive elimination.

To provide insight on the unusual features of this reductive elimination, quantum mechanical chemical simulations employing the reaction discovery tools of the Zimmerman group were used to identify an energetically feasible series of elementary steps that were then fully optimized using density functional theory;46−48 single-point computations provide energetics at the DLPNO-CCSD(T) level of theory49 (see computational details, Supporting Information). An initial set

Figure 4. Reaction profile of 5 in DCM-d2 at 10 °C. Starting concentration of 5 is 0.0012 M. This experiment was duplicated with same results. 11410


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Figure 6. Reaction profile for the reductive elimination of ethane from 1, leading to dimeric product, S = CH3CN. Top right: Modeled transition state for the methyl transfer step of RE (TS-MT).

(CH3)(S) fragment (2.440 Å) than the (azpy)Pd(CH3)2(S) fragment (2.474 Å) (Figure 6). Bimolecular methyl transfer reactions have been invoked for oxidatively induced reductive elimination from related LPdMe2 and LPtMe2 complexes. While these reactions are typically proposed to involve the transfer of methyl cations between an electrophilic LPd(III)Me2 center and LPd(II)Me2,10−12,17,19,38,39,50,54 recent experimental13−16 and computational studies18 have suggested that methyl radical exchange, as proposed here, is also a viable pathway for the interchange of Pd methyl groups. The DFT calculations (Figure 6) indicate that intermolecular methyl transfer generates two neutral species, the square-planar (azpy)Pd(CH3)(S) 1a and the octahedral (azpy)Pd(CH3)3(S) 1b. The HOMOs of both 1a and 1b are primarily ligand-based with spin-densities of 0.9 (Figure 7), indicative of considerable radical anion character to the phenylazopyridine ligands. The long N−N azo bond lengths calculated for both 1a and 1b (1.355 and 1.357 Å, respectively, Supporting Information, page S63) are also consistent with that expected35 for the ligand radical anion. Thus, the octahedral and square planar products can be formulated as (azpy-·)Pd(IV)(CH3)3 and (azpy-·)Pd(II)(CH3)(S) species. This suggests that the intermolecular methyl transfer results in the disproportionation of (azpy)Pd(II)(CH3)2 to Pd(II) and Pd(IV), which are ligated by azopyridine radical anions. The simulations also reveal the critical role of the azpy ligand in facilitating the methyl transfer and reductive elimination steps. For instance, methyl transfer between two (bipy)Pd(CH3)2 molecules is calculated to have a barrier of over 40 kcal/mol, more than 10 kcal/mol higher than that with azpy. In contrast, the azpy ligand allows the bimetallic pair to reach increased oxidation states, facilitating the square-pyramidal Pd geometries in the transition structure of TS-MT. Natural bond order analysis confirms this hypothesis by showing that the azpy ligands of the (azpy)Pd(CH3)(S) and the (azpy)Pd(CH3)2 fragments both build up negative charge at the

of simulations were carried out to assess the barriers for the direct reductive elimination from the monomeric (azpy)Pd(CH3)2. These calculations predict an activation barrier of 29.4 kcal/mol (25 °C), which is inconsistent with the rapid reductive elimination observed at these temperatures. A more energetically accessible reaction pathway for reductive elimination was identified (Figure 6) that involves the intermolecular transfer of the methyl group (TS-MT) to generate the neutral species, (azpy)Pd(CH3)(S) 1a and (azpy)Pd(CH3)3(S) 1b. Intermolecular methyl group exchange has been proposed for the oxidatively induced reduction elimination of ethane from other classes of LPdMe2 and LPtMe2 complexes.10,11,13,14,50 This first step, which involves a formal disproportionation of PdII to PdI and PdIII is assisted by solvent coordination to each Pd, to give (azpy)Pd(CH3)S 1a and (azpy)Pd(CH3)3S 1b, with S = CH3CN. Reductive elimination subsequently occurs with simultaneous dissociation of coordinated solvent from the trimethyl Pd complex (TS-RE), producing an (azpy)Pd(Me)ethane σ-complex. The solvent-assisted intermolecular methyl transfer step is calculated to have a barrier of 19.7 kcal/mol, and the reductive elimination of ethane from (azpy)Pd(Me)3S is calculated to have a similar barrier of 19.0 kcal/mol. Following this step, the calculations indicate that the reduced LPdMe complexes dimerize to generate 1d, a Pd−Pd dimer, an unusual species that is calculated to contain an unsupported Pd−Pd bond.51 Related Pd−Pd species have been previously observed as intermediates in catalysis and in reductive elimination.52,53 More detailed analysis of the bonding parameters and frontier molecular orbitals of the transition states and products reveal the important role of the redox-active phenylazopyridine ligand34,35 in facilitating both the intermolecular methyl transfer as well as the reductive elimination of ethane. The calculations indicate that the methyl group transfers as a methyl radical via an unsymmetrical transition state where the bridging methyl group is slightly closer to the (azpy)Pd11411


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SOMO occupations. This greater interaction is due to a new Pd−Pd bond in 1d, which deserves additional attention due to its unusual electronic structure. Figure 8 shows the interactions between the high-lying valence orbitals in the Pd dimer 1d. Electronic coupling of the

Figure 8. Orbital interactions for dimeric complex, 1d. Figure 7. Calculated singly occupied molecular orbital (SOMO) of 1a (top) and 1b (bottom).

azpy π systems is facilitated by through-bond coupling of the metal centers, rather than simply the geometric proximity of the two ligands in 1d (nearest N−N distance is 5.08 Å). This electronic coupling is mediated by bonding interactions between dxy and dz2 orbitals on the pair of Pd centers. Each of these d orbitals is in turn coupled to the π* orbitals of their respective azpy ligands. Because of the significant coupling and near-degeneracy of these four orbitals in 1d, a spectrum of bonding and antibonding orbitals appears when the four are mixed. The lowest energy of these (bottom right of Figure 8) is an all-bonding π*−σ−π* orbital, which RAS-SF predicts to contain 1.59 e−. The three remaining orbitals are slightly higher in energy, and contain 0.34, 0.15 and 0.03 e−, respectively. Overall, this state is multiconfigurational, with a dominant (∼80% of the wave function) electronic configuration consisting of a bonding d−d σ orbital interacting with the two azpy π* orbitals. As with TS-MT and 1a−1b, the electronic state of the 1d dimer exhibits an unusual delocalized electronic structure due to the ability of the azpy ligand to strongly interact with high-lying metal d orbitals. As the experimental and computational studies had implicated the formation of the neutral LPd(CH3)(CH3CN) complex 1a as a product of methyl transfer and reductive elimination (Figure 6), we investigated the cyclic voltammetry of the monomethyl cation (Et2-azpy)PdMe(CH3CN)(OTf) in an effort to generate an analog of this species independently and assess its stability (Figure 9). The cation (Et2-azpy)-

transition state by 0.42 and 0.15 e−, respectively, on the NNCN atoms of the azopyridine ligand. The calculations indicate that the bipy ligand accommodates less charge, explaining the great difference in reactivity among the two bidentate ligands. The participation of the azpy ligand in the reductive elimination of ethane from 1b to generate 1c is also indicated by the DFT calculations, which reveal considerable radical anion character for the azpy ligands for both 1b and 1c (Figure S68, Supporting Information). The restricted-active-space spin-flip (RAS(h,p)-nSF) level of theory55,56 provided additional insight into the electronic structures of TS-MT, 1a−1b and 1d. This technique provides a multiconfigurational picture of strongly correlated states at relatively low cost, and therefore is ideally suited for examining ligand-centered radical character. Table 1 summarizes the Table 1. Spin Gaps and Orbital Occupancies for Key Intermediates of Figure 5 Occupancies (Singlet) TS-MT 1a−1b 1d

T−S Gap

SOMO #1

SOMO #2

0.003 eV 0.000 eV 0.012 eV

1.06 1.001

0.94 0.999 see Figure 7

results of these computations, and the Supporting Information contains images of the associated SOMOs (natural orbitals of the RAS-SF computation). The lowest energy electronic states are biradicals, and therefore RAS-SF provides the same qualitative electronic structure as the above DFT simulations, with ligand-centered radicals in TS-MT and 1a−1b (RAS-SF assigns the (azpy)PdMe2 species as a closed-shell, singlet ground state). Additionally, RAS-SF gives the triplet−singlet gap and the SOMO occupancies. The T−S gap is small: 3 meV at TS-MT and zero at the intermediate, indicating weakly interacting SOMOs at the TS, and noninteracting SOMOs at the methyl-transferred intermediate (1a−1b). In 1d, the two SOMOs interact to a greater degree, resulting in a slightly larger T−S gap (12 meV) and corresponding noninteger

Figure 9. (a) Cyclic voltammetry (CV) of [(Et2-azpy)PdMe(CH3CN)][OTf] referenced vs Fc/Fc+. (b) Corresponding Randles−Sevcik plot of the first reduction wave. 11412


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Figure 10. Palladium-containing ions observed by ESI-MS with L = azpy, AzoBipy.

Analysis of the reaction mixture by ESI-MS after the cessation of ethane elimination revealed additional ions centered at m/z 662.0452, m/z 692.0918 and m/z 718.1059. Each of these additional ions correspond to dimeric (azpy)Pd species with varying levels of methylation. The ions at m/z 662.0452 corresponds to the formula [(azpy)(CH3)Pd− Pd(NCCH3)(azpy)]+, that at m/z 692.0918 to [(azpy) (CH3)2Pd−Pd(CH3)(NCCH3)(azpy)]+ and that at m/z 718.1061 to [(azpy)(CH3)(NCCH3)Pd−Pd(CH3)(NCCH3)(azpy)]+. Notably, the ion at m/z 718.1061 is in excellent agreement to the product 1d identified through computational studies. Collision induced dissociation (CID) coupled tandem mass spectrometry of the ion at m/z 663.0464,61 corresponding to one of the Pd dimers, revealed fragmented ions at m/z 622.0199 and m/z 605.9886 corresponding to loss of CH3CN and CH4. The observation that the daughter ions retained two Pd atoms indicates that the energy of the CID-induced collisions was insufficient to disassociate the dimer, consistent with a Pd−Pd bond for the parent ion. When a sample of 5 is analyzed by ESI-MS, analogous ions are observed (Figure 10). Many of the observed ions do not have MeCN coordinated, presumably because the AzoBipy can take three coordination sites on the palladium center. The ions centered at m/z 762.0663 also agree with the Pd−Pd compound predicted by computational studies to be the result of RE.

PdMe(CH3CN)(OTf) exhibits two well-separated reversible one-electron reductions at −0.788 and −1.23 V (vs Fc+/Fc0, in CH3CN, Figure 9). The reversibility of the first reduction of the cation indicates that the neutral monomethyl species (analogous to 1a) is stable, at least on the CV time-scale. This reduction is slightly negative of that previously reported (−0.59 V vs Fc+/Fc0, in CH3CN) for (azpy)PdCl2.33 On the basis of the DFT calculations of 1a and the analysis of the related reduction of (azpy)PdCl2,33 this reduction is proposed to occur predominantly on the azopyridine ligand.28 Ligand based reductions have been previously observed in related Pdazopyridine complexes.33,57 To provide further evidence of the products of reductive elimination from these complexes, we utilized high-resolution electrospray mass spectrometry (ESI-MS)15,58−60 in positive ion mode to identify the Pd-containing products generated following reductive elimination of ethane. ESI-MS spectra were collected on samples of 1 immediately upon dissolution in DCM, as well as the reaction mixture after ethane evolution had ceased (Figure 10). Samples were diluted into CH3CN prior to analysis. Upon dissolution of 1 in DCM and dilution with CH3CN, two major envelopes of ions are observed centered at m/z 359.0467 and m/z 515.1150 (corresponding to the Pd-106 isotope), which are formulated as [(azpy)Pd(CH3)(NCCH3)]+ and [(azpy)2Pd(CH3)−(NCCH3)]+, respectively. As no ions corresponding to the cation of 1b ([(azpy)Pd(CH3)3(NCCH3)]+) were observed, we attribute these ions to the ionization of 1a by loss of a methyl anion (Me−). 11413


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CONCLUSION In summary, the redox-active azpy ligand has a significant influence on the reactivity of Pd dialkyl species and facilitates the reductive elimination of ethane from (azpy)PdMe2 complexes under mild conditions. In polar solvents, (azpy)PdMe2 complexes spontaneously produce ∼0.5 equiv of ethane and form paramagnetic species. Mechanistic and computational studies are consistent with a bimolecular mechanism involving methyl radical transfer and reductive elimination from the neutral (azpy)PdMe3(S) complex, in which both steps are facilitated by the redox-active azopyridine ligand. The paramagnetic products of RE are unstable, but can be partially stabilized with appropriate tridentate ligands. Further studies are underway to assess the potential role of redox-active ligands in catalytic reactions involving reductive elimination.

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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/jacs.8b06398. Synthetic procedures, ligand exchange experiment description, NMR, UV−vis, XPS spectra, CV traces, HRMS, elemental analysis, computational details (PDF) X-ray crystallographic data for C21H24N4Pd (CIF) X-ray crystallographic data for C14H17N3Pd (CIF) X-ray crystallographic data for C15H16F3N3Pd (CIF) X-ray crystallographic data for C18H25N3Pd (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Naomi E. Clayman: 0000-0002-4894-2174 Katherine L. Walker: 0000-0002-7924-8185 Paul M. Zimmerman: 0000-0002-7444-1314 Robert M. Waymouth: 0000-0001-9862-9509 Present Address

A.E.R.: 455 Forest St., Marlborough, MA 01752 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the Office of Naval Research (N00014-14-1-0551). A.E.R. acknowledges Franklin Research Grant from American Philosophical society. We thank Prof. E. I. Solomon for use of the UV−vis spectrophotometer. Part of this work was performed at Stanford Nano Shared Facilities (SNSF) and Vincent Coates Foundation Mass Spectrometry Laboratory, Stanford University Mass Spectrometry.

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Ligand-Induced Reductive Elimination of Ethane from Azopyridine

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