Changes in ligand coordination mode induce bimetallic C–C coupling pathways

Abstract

Carbon–carbon coupling is one of the most powerful tools in the organic synthesis arsenal. Known methodologies primarily exploit monometallic Pd⁰/Pd^II catalytic mechanisms to give new C–C bonds. Bimetallic C–C coupling mechanisms that involve a Pd^I/Pd^II redox cycle, remain underexplored. Thus, a detailed mechnaistic understanding is imperative for the development of new bimetallic catalysts. Previously, a Pd^II–Me dimer (1) supported by L1, which has phosphine and 1-azaallyl donor groups, underwent reductive elimination to give ethane, a Pd^I dimer, a Pd^II monometallic complex, and Pd black. Herein, a comprehensive experimental and computational study of the reactivity of 1 is presented, which reveals that the versatile coordination chemistry of L1 promotes bimetallic C–C bond formation. The phosphine 1-azaallyl ligand adopts various bridging modes to maintain the bimetallic structure throughout the C–C bond forming mechanism, which involves intramolecular methyl transfer and 1,1-reductive elimination from one of the palladium atoms. The minor byproduct, methane, likely forms through a monometallic intermediate that is sensitive to solvent C–H activation. Overall, the capacity of L1 to adopt different coordination modes promotes the bimetallic C–C coupling channel through pathways that are unattainable with statically-coordinated ligands.

1. Introduction

Palladium-catalyzed C–C bond-forming reactions have revolutionized synthetic chemistry and have been broadly adopted in pharmaceutical manufacturing.^1–7 The vast majority of Pd catalyzed reactions follow the same general mechanism that involves redox cycling of a monometallic catalyst between Pd⁰ and Pd^II (Scheme 1, left).⁸ In contrast, there is a relative dearth of examples that involve redox cycling of a bimetallic catalyst between Pd^I and Pd^II (i.e. Scheme 1, right). While Pd^I complexes are well established as off-cycle precursors to Pd⁰ or as decomposition products,^9,10 the first example of catalysis involving on-cycle bimetallic Pd^I and Pd^II complexes was shown for halide exchange of aryl iodide to aryl bromide.^11,12 The dinuclear catalyst [PdI(Pt-Bu₃)]₂ has since been used extensively,¹³ including in C–E (E = S, Se) bond formation that also follows a bimetallic mechanism.^14–16 Computational studies with other palladium complexes also support bimetallic mechanisms for pyrrole formation,¹⁷ and alkene isomerization¹⁸ reactions. In the context of C–C coupling catalysis, only a few examples with a postulated bimetallic mechanism are known, and little mechanistic detail has been revealed.^19–22 Recently, bimetallic Pd^I and Pd^II complexes supported by bridging 2-phosphinoimidazole ligands were identified as active catalysts for α-arylation to give naphthalene derivatives.²² Computations supported a bimetallic mechanism with redox cycling between Pd^II and Pd^III or Pd^IV and Ar–I oxidative addition occurred at a single Pd atom within the bimetallic framework. Of note, similar higher valent Pd^II/Pd^III or Pd^II/Pd^IV bimetallic catalytic routes are established for C–X²³ or C–C^24,25 bond formations. Further elaboration of mechanistic understanding of bimetallic Pd^I/Pd^II C–C bond formation (e.g. Scheme 1, right), including the influence of ligand structure on reactivity, would greatly facilitate the development of the catalytic methodology.

Scheme 1.

General mechanisms for monometallic (left) and bimetallic (right) palladium catalyzed C–C bond formation.

Reactivity of model compounds for individual reaction steps is a powerful means to gain mechanistic insight. Specifically, Pd^II–R complexes with simple alkyl ligands have provided fundamental insight into the diverse mechanisms for C–C bond formation. While a number of bimetallic palladium products have been characterized, most mechanisms do not involve bimetallic intermediates for the C–C reductive elimination step. For example, bimetallic Pd^I products are formed after light mediated homolysis of a Pd^II–R bond of a monometallic precursor (Scheme 2a).²⁶ Similar coupling of the metal-based reductive elimination products can occur from Pd^II dialkyl complexes following well-established monometallic 1,1-reductive elimination (Scheme 2b).²⁷ The resultant Pd⁰ dimers are favoured if an appropriate bridging ligand, such as bis (dicyclohexylphosphine)methane, is employed.²⁸ Bimetallic intermediates have also been implicated in methyl transfer steps in oxidation-induced C–C bond formation.^29–31 Oxidation of mono or dimethyl Pd^II by an exogenous oxidant or redox non-innocent ligand, induces intermolecular methyl transfer between two monometallic compounds (e.g. Scheme 2c for redox non-innocent ligand).^29–31 In these cases, methyl transfer involves a linear arrangement of Pd–CH₃–Pd, which leads to formation of a high-valent Pd^IV monometallic intermediate. Classic 1,1-reductive elimination then releases the organic product. In the case of the redox-active ligand, a Pd^I dimer is formed after ligand oxidation and complex dimerization.

Scheme 2.

Formation of bimetallic Pd products following C–C bond formation from monometallic precursors.

Only a few examples are known in which both alkyl transfer and C–C reductive elimination occur within a bimetallic palladium structure. The A-frame Pd^II–R dimers, A, (R = Me, Bn, Ph) undergo C–C reductive elimination to give a Pd^I dimer, B (Scheme 3b).^32,33 Notably, the narrow bite angle bis(diphenylphosphino)methane (dppm) ligand effectively stabilizes both the Pd^II and Pd^I bimetallic structures. Experimental mechanistic studies strongly indicated that reductive elimination proceeded via a bimetallic complex. Molecular orbital symmetry precludes a 1,2-elimination mechanism that involves a 4-centred Pd₂C₂ transition state.³⁴ Rather, a mechanism was postulated that involves transfer of R via a bridging interaction to give a dimer with a PdR₂ fragment, which would undergo a classic 1,1-reductive elimination.^32,33 However, the exact nature of such an intermediate, details about the methyl transfer step, the involvement of metal–metal synergy³⁵ or optimal ligand characteristics were not investigated. The methyl substituted A-frame dimer (A where R = Me, X = Cl) gives low yields of ethane, and methane was observed as a major byproduct.³³ Thus, the dppm ligand is not optimal to reliably promote C–C bond formation.

Scheme 3. Bimetallic alkyl transfer and reductive elimination.

Recently, we prepared a Pd^II-methyl dimer 1 that bears the phosphine 1-azaallyl ligand, L1 (Scheme 4).³⁶ The bridging amide nitrogen atoms enforce a bent shape to 1, which is reminiscent of the A-frame dimers (A). On heating, 1 releases ethane without any external oxidant and the ligand L1 has no apparent capacity for redox non-innocence. These observations suggest that the C–C bond forming mechanism is possibly related to the reactivity observed with the bimetallic Pd complexes, A. Reductive elimination from 1 is remarkably well behaved, giving high selectivity for C–C bond formation to give ethane (ca. 80%) over methane (cf. for A ca. 50% ethane; R = Me, X = Cl). This suggests that 1 has advantageous structural characteristics, possibly due to the diverse coordination chemistry of L1. In response to the steric and electronic demands of the metal, the 1-azaallyl fragment in L1 switches from μ-N in 1 to μ-[η²-CC;κ¹-N] and κ¹-N modes in products 2 and 3, respectively, which indicates that L1 is structurally responsive.³⁷ We previously hypothesized that the capacity of L1 to adopt these different coordination modes may induce C–C bond formation. This premise has been exploited extensively for decades with hemilabile ligands that promote substitution chemistry and other organometallic reactions via changes in ligand denticity.^37–41 Indeed, one of the viable catalytic pathways for bimetallic α-arylation, noted above, involves a switch between κ²-P,N and κ¹-P 2-phosphinoimidazole coordination.²²

Scheme 4.

C–C reductive elimination from a Pd^II dimer (1), showing changes in ligand coordination mode. Yields of 2 and 3 are 35 and 50%, respectively based on starting ligand.

The present combined experimental and computational study establishes that the structurally-responsive behaviour of the phosphine 1-azaallyl ligand (L1) is critical to promote bimetallic methyl transfer and C–C reductive elimination. Elucidation of optimal pathways was evaluated computationally using the Growing String Method (GSM)^42–44 developed by one of us (further details in ESI^†). The computational analysis shows that the most viable mechanistic pathways are not accessible if static ligand coordination is enforced. In addition, the origin for methane as the minor byproduct is identified. The presented insights reveal critical considerations to favour productive C–C coupling pathways, which could be exploited in a bimetallic catalytic cycle involving Pd^II and Pd^I intermediates (e.g. 1 and 2, respectively). Namely, a key ligand feature is the ability to readily adopt different coordination modes to maintain a bimetallic motif, which is needed to promote intramolecular methyl transfer and reductive elimination with incipient metal–metal bonding.

2. Results

2.1. Non-structurally responsive phosphine–amido complex

During reductive elimination of ethane from 1, the 1-azaallyl fragment of L1 changes from μ-N coordination mode in 1 to μ-[κ¹-N, η²-CC] and κ¹-N for products 2 and 3, respectively, suggesting that changes in ligand coordination mode may induce C–C bond formation. We sought to probe this hypothesis by investigating the reactivity of a complex bearing L2, the reduced analogue of L1. During C–C bond formation, the ligand structure of L2 would preclude formation of 2, in which the alkene fragment of L1 is needed to bridge the metal centres. We prepared the phosphine amido ligand K[L2] through the reduction of H(L1) with LiAlH₄ and deprotonation with KH. The coordination of L2 to [PdClMe(COD)] was attempted at room temperature and at −30 °C. While complete consumption of K[L2] was observed by ³¹P{¹H} and ¹H NMR spectroscopy, the reaction consistently generated a myriad of new products. Complex 4 was instead prepared in 45% yield following a reaction between K[L2] and [PdClMe(COD)] in the presence of 1.3 equivalents of pyridine (Scheme 5). The ¹H NMR spectrum of 4 has no signals for coordinated pyridine that would be expected for a monometallic pyridine adduct. N–H signals are absent from both the ¹H NMR and the IR spectra, as expected for retention of the amido functionality. A doublet is observed in the ¹H NMR spectrum for the Pd-bound methyl that has a small coupling constant (³J_H–P = 1.8 Hz), which is consistent with a cis orientation to the phosphine donor. The structure of 4 as a dimer was confirmed by single-crystal X-ray diffraction (Scheme 5). The palladium centres were disordered over two sites with the occupancy of the major site refining to a value of 86.4(7)%. The following discussion will pertain to the major component. The two symmetry independent Pd^II centres adopt a distorted⁴⁵ square planar geometry (Pd(1) τ⁴ = 0.12; Pd(2) τ⁴ = 0.13), with the anionic nitrogen atoms bridging the two metal centres. Similar crystallographic inequivalence of the two Pd centres was also observed in the solid-state structure of 1.³⁶ The Pd(1)–N(1)–Pd(2) and Pd(1)–N (2)–Pd(2) angles are 79.30(6) and 80.51(5)°, respectively, which are ca. 2° larger than the analogous angles in 1. The angle between the square planes of 118.76° is ca. 10° wider than that for 1. Despite these minor structural deviations, complex 4 with an iso-butyl amido moiety is a close analogue to 1 with a 1-azaallyl group.

Scheme 5.

Synthesis of phosphine-amido complex, 4, (top) and thermal displacement plot of 4 (bottom). Displacement ellipsoid (50% probability) plot of 4. All hydrogen atoms, and a THF molecule that cocrystallized in the unit cell have been omitted for clarity.

Heating a solution of 4 in benzene at 70 °C for 16 h afforded several new palladium products as judged by ³¹P{¹H} NMR spectroscopy. The variety of products observed from 4 is in contrast to reaction with 1, which gives only 2, 3 and Pd black as the metal-based products. Analysis of the headspace by GC-FID following heating of 4 revealed that ethane and methane were produced in 20 and 80% yields, respectively (1 : 4 ratio), (Scheme 6). This is the inverse selectivity from 1 that gives 76 and 24% yields of ethane and methane, respectively. The change in selectivity could reflect the inability of L2 to adopt a π-type coordination to palladium that is essential for C–C reductive elimination. Alternatively, the alkyl substituent of the amido fragment may be more susceptible to C–H activation chemistry that leads to competitive methane formation. Regardless, the product distribution of 4 vs. 1 shows that the 1-azaallyl fragment in the latter favours C–C reductive elimination.

Scheme 6.

Thermolysis of phosphine-amido complex, 4.

2.2. Assessment of a monomer–dimer equilibrium with 1

We demonstrated previously that 1 exists as a dimer both in the solid- and solution-states.³⁶ However, reaction chemistry of 1 may involve an equilibrium with a ‘three-coordinate’ monomer (Scheme 7). Computed relative energies verifies that the monomers m1 are uphill from 1 by 13.5 kcal mol⁻¹. This corresponds to a K_d of 4.3 × 10⁻³ M at 25 °C, which is consistent with the experimental observation that the dimer 1 is strongly favoured in solution. The three-coordinate metal centre in m1 contains an agostic interaction to a methyl group of the 1-azaallyl group of L1, with a short Pd–H_(CH3) distance (2.215 Å) and a small Pd–H–C bond angle (87.1°). This relatively weak ligation partly compensates for the missing Pd–N bond of 1 that must break to form the monomers.

Scheme 7.

Computed dissociation of dimer 1 to monomers m1. Free energies are given in kcal mol⁻¹.

To determine if 1 stays intact or separates into monomers prior to reductive elimination, a crossover experiment was targeted using a 1 : 1 mixture of 1 and 1-d₆. First, the labeled compound 1-d₆ was prepared in 76% yield by a reaction between K [L1] and [PdCl(CD₃)(COD)]. A 1 : 1 mixture of 1 and 1-d₆ was heated at 70 °C for 2.5 h (Scheme 8), following which the solution was analyzed by ¹H NMR spectroscopy and the headspace was analyzed by GC-MS (Fig. S12 and S13,^† respectively). The ¹H NMR spectrum revealed the formation of C₂H₆ and CH₃CD₃ in a 1 : 2 molar ratio. Formation of C₂D₆ was confirmed by GC-MS, which showed a approximate 1 : 2 : 1 molar ratio of C₂H₆, H₃C–CD₃ and C₂D₆. This mixture of ethane isotopologues indicates that the dimer, 1/1-d₆, does cleave prior to reductive elimination. However, these experiments do not distinguish between a monomer/dimer pre-equilibrium and mechanisms that involve monomer or dimer structures en route to palladium products 2 and 3.

Scheme 8.

Thermolysis of 1 and 1-d₆ for H/D crossover test.

2.3. Formation of the methane as a byproduct

The thermolysis of 1 gives ethane as the major product, but methane is observed in ca. 20% yield. We sought to understand how this byproduct forms in order to guide future ligand re-design strategies. The viability of monomer m1 was established above (section 2.2) through experiments and computations. The monomer bears a C–H agostic bond to a methyl group of L1, and such interactions typically precede C–H bond cleavage by oxidative addition.⁴⁶ The route to form methane from m1 is proposed to involve oxidative addition of the agostic C–H bond to give a square planar Pd^IV-hydride intermediate m1a through a proton shift transition state (TS-m1) with free energy of 28.8 kcal mol⁻¹ (Scheme 9). Reductive elimination from the Pd-hydride m1a releases a methane molecule with free energy of 31.3 kcal mol⁻¹ for the transition state (TS-m1a). An alternate concerted metallation–deprotonation route to C–H activation is possible, but the oxidative addition/reductive elimination sequence described here is more energetically viable by 3 kcal mol⁻¹ (Scheme S4^†). These calculations suggests that C–H activation in monomer m1 could be the source of the observed methane byproduct.

Scheme 9.

Formation of methane from monomer m1. Free energies are relative to dimer 1 and are given in kcal mol⁻¹.

The high energy of the Pd^IV-hydride intermediate m1a, relative to other compounds in the sequence to give methane, suggests attempts toward independent synthesis would be unsuccessful. Additionally, we noted previously that no intermediates were observed during the reductive elimination of 1.³⁶ Owing to the generally higher M–L bond strength for Pt as compared to Pd, the former can often act as more tractable versions of the less stable Pd analogues and can offer a means to isolate or observe relevant intermediates. Thus, we treated [PtClMe(COD)] with K[L1] to afford 5, the Pt^II analogue of 1 (Scheme 10). The ³¹P{¹H} NMR spectrum of 5 revealed a signal for coordinated L1 at δ_P = 20.2 with satellites (¹J_Pt–P = 4399 Hz) concordant with one-bond coupling to platinum. The carbon centre of the 1-azaallyl group that is beta to the amide nitrogen (C2) was found at 120.3 ppm. This strongly indicates that the alkene portion of L1 is not involved in binding to the metal centre, which would give a significantly more upfield C2 shift of ca. 40–75 ppm (Table S1^†).^36,47 Rather, the chemical shift is only ca. 3 ppm downfield from the analogous signal in 1 that has a μ-N binding mode of the 1-azaallyl fragment. A cis arrangement of the phosphine and methyl is supported by a small H–P coupling constant (³J_H–P = 1.9 Hz) observed from the methyl signal in the ¹H NMR spectrum. Collectively, this data supports that 5 is a direct analogue of the Pd^II dimer 1.

Scheme 10.

Preparation of Pt^II dimer, 5.

Heating a benzene solution of 5 at 70 °C for 2.5 h (Scheme 11) gave quantitative conversion to 6, which is observed in the ³¹P{¹H} NMR spectrum as a singlet at δ_P = 28.0 with satellites to platinum (¹J_Pt–P = 5208 Hz). Complex 6 was isolated in a 54% yield. The beta carbon of the 1-azaallyl fragment (C2) is shifted ca. 2 ppm upfield to δ_C = 119.9, which indicates that the 1-azaallyl group coordinates to platinum only through the nitrogen atom. The ¹H NMR spectrum of 6 no longer exhibits a signal corresponding to a platinum-bound methyl. These spectroscopic features are consistent with the assignment of 6 as the Pt analogue of the Pd^II complex 3. The κ²-PN coordination mode of L1 and cis arrangement of the phosphines was confirmed by single-crystal X-ray diffraction of 6 (Scheme 11). Similar to 3, 6 adopts a distorted square planar geometry (τ₄ = 0.15) due to steric repulsion from the 1-azaallyl groups and the phenyl substituents of the phosphines. The only alkane product observed by GC-FID or ¹H NMR spectroscopy was CH₄. These observations suggest that the platinum dimer 6 undergoes C–H activation rather than Csp³–Csp³ reductive elimination to give ethane.

Scheme 11.

Thermolysis of Pt^II-Me dimer 5 to afford Pt^II complex 6 (top) and thermal displacement plot of 6 (bottom). Displacement ellipsoid (50% probability) plot of 6. All hydrogen atoms have been omitted for clarity.

During the conversion of 5 to 6, no intermediates or additional species were observed by ³¹P{¹H} NMR spectroscopy, which prevented direct confirmation of the Pt^IV analogue of m1a. However, additional insight could be gained by comparison of reactions conducted in C₆H₆ and C₆D₆. Heating 5 in C₆D₆ again gave quantitative conversion to 6. No evidence for CH₃D by ¹H or ²H NMR spectroscopy was observed, which suggests that the Pt-Me fragment does not react with solvent directly to give methane. Close inspection of the ²H NMR spectrum of 6 synthesized in C₆D₆ reveals a signal for a deuterated methyl of L1 at ca. 2 ppm, which is not observed for samples of 6 prepared in C₆H₆ (Fig. 1a). Deuteration at this site can be rationalized by solvent activation with the Pt analogue of the proposed intermediate m1c, which forms after C–H activation of an L1 methyl group and methane release (see Scheme 9). Related solvent activation was noted previously for Pt(II)-alkyl compounds,⁴⁸ and the palladium A-frame complexes, D.³³ The product of solvent activation would regenerate the L1 methyl group to give [Pt(Ph)(L1)], which could dimerize with another equivalent of [Pt(Ph)(L1)]. From this compound biphenyl could form as a C–C coupled product, along with 6 (or 6-d₁) as the Pt-based product. Indeed, three signals are observed at 7.45, 7.21, and 7.13 ppm in the ²H NMR spectrum from the thermolysis of 5 in C₆D₆ that correspond to biphenyl-d₁₀ (Fig. 1b). Furthermore, qualitative formation of biphenyl was confirmed by GC-FID, where a signal for biphenyl is consistently observed following thermolysis reactions performed in both C₆H₆ and C₆D₆ (Fig. S18^†). These observations indirectly support the proposal that methane forms from 1 by a sequence involving C–H activation of a methyl substituent of the 1-azaallyl group of L1 (i.e., Scheme 9).

Fig. 1.

(a) ²H NMR spectra following the thermolysis of 5 in C₆D₆ (top), and C₆H₆ (bottom), spectra zoomed in to show δ_D = 6.00–0.00 (92.1 MHz, C₆H₆). (b) ²H NMR spectrum following the thermolysis of 5 in C₆D₆, spectrum zoomed in to show δ_D = 8.50–5.50 (92.1 MHz, C₆H₆).

2.4. Possible pathways for C–C reductive elimination

Reductive elimination from 1 gives two different palladium products, 2 and 3, which may mean that two distinct reaction pathways lead to C–C bond formation. To establish the mechanistic details, we considered three general reductive elimination pathways A–C (Scheme 12). First, despite our preliminary evidence that suggested a radical mechanism is not operative,³⁶ we considered that homolytic Pd–Me bond activation could lead to ethane and the Pd^I dimer 2 (Scheme 12, Path A). Second, cleavage of dimer 1 and methyl transfer could give a monometallic Pd^II dimethyl complex (Path B). Facile methyl transfer between bisphosphine Pd^II complexes has been established⁴⁹ and the anionic nature of [Pd(Me)₂(L1)]⁻ could favour reductive elimination. Third, reductive elimination could proceed via bimetallic complexes that are isomers of 1, such as a methyl-bridged dimer. The monomer–dimer equilibrium established above, could be involved in Paths B or C.

Scheme 12.

Proposed general pathways for reductive elimination from 1.

2.5. Reaction order and Eyring analysis of reductive elimination

To determine the order for reductive elimination, two reactions at different concentrations of 1 in benzene (22 and 62 mM) were heated at 70 °C and monitored over time by ³¹P {¹H} NMR spectroscopy. In each case, a plot of Ln[1] vs. time gave an excellent linear fit (R² > 0.993, 0.973) consistent with a reaction that is first order in 1 (Fig. 2a), which is expected for all of the proposed pathways. Rate constants for the consumption of 1 were determined in the temperature range of 40–70 °C. The activation parameters calculated from an Eyring analysis were ΔH^‡ = 16.79 ± 0.03 kcal mol⁻¹ and ΔS^‡ = −21 ± 1 kcal mol⁻¹ K⁻¹ (Fig. 2b). The moderately large and negative ΔS^‡ is consistent with a highly-ordered transition state with decreased translational and vibrational degrees of freedom as compared to the preceding intermediate, suggesting the formation of new bonds in the rate-determining step. Furthermore, the sign of ΔS^‡ suggests that cleavage of 1 into two monomeric [Pd(L1)Me] fragments is not the rate-determining step, which is consistent with other experimental and computational results. The magnitude of ΔG^‡ (23.1 ± 0.04 kcal mol⁻¹, 25 °C) is consistent with the prior observation³⁶ that the reaction can proceed at room temperature, albeit over the course of several days.

Fig. 2.

(a) Plot of Ln[1] vs. time for two reactions in which the initial concentrations of 1 were 22 mM (red) and 62 mM (blue). (b) Eyring plot of the rate constants acquired from heating a solution of 1 (16 mM) in benzene from 40 to 70 °C.

2.6. Relationship between reductive elimination products 2 and 3

To determine if there is a connection between the pathways affording complexes 2 and 3, separate C₆D₆ solutions were prepared containing 2 or 3 and O=PPh₃ as an internal standard (Scheme 13). These solutions were then subjected to the conditions that give complete thermolysis of 1 and analyzed by ³¹P {¹H} and ¹H NMR spectroscopy after 2.5 h. No change was observed upon heating 3. However, heating 2 resulted in 54% conversion to 3 after 2.5 h. By comparison, 1 completely converts to 2 and 3 within this same timeframe, during which 3 emerges first and the ratio of 2 and 3 is relatively static after ca. 40 min of reaction time. These observations suggest that a small percentage of 3 may be obtained from the thermolysis of 2, but it is unlikely that this pathway exclusively accounts for 3.

Scheme 13.

Thermolysis of 2 and 3.

2.7. Assessment of a radical pathway to reductive elimination

Previously, we found no difference in yield of the palladium products 2 and 3 when reactions were conducted in the light or the dark.³⁶ This indicates that light mediated homolysis of the Pd–CH₃ bond is not a dominant pathway in ethane formation. However, a methyl-transfer step must occur prior to reductive elimination and such reactions can involve uncaged methyl radicals.³¹ To test for radical formation, 1 was heated in the presence of known^29,30,50 H-atom donors 1,4-cyclohexadiene (CHD) or 9,10 dihydroanthracene (DHA), (Scheme 14). In the presence of DHA no change in product selectivity was observed, which indicates that H-atom abstraction did not occur and that an uncaged methyl radical (i.e. Path A, Scheme 12) is not involved in methyl transfer. In contrast, the more potent H-atom donor CHD (cf. BDFE_(C–H): CHD = 67.8; DHA = 76.0 kcal mol⁻¹)⁵⁰ does react with 1 to give methane, likely via interception of an intermediate with methyl-radical character. Despite a large excess of CHD (35 equiv.) methane was not the exclusive organic product, rather ethane and methane were formed in 40 and 60% yields, respectively. This suggests that the two palladium products are formed through distinct mechanisms, where only one has an intermediate that can be intercepted with an H-atom donor to promote homolytic Pd–Me cleavage.

Scheme 14.

Reaction of 1 with H-atom donors.

The calculated free energy for direct homolytic cleavage of the Pd–Me bond in dimer 1 is 35.3 kcal mol⁻¹ (Scheme 15), which further demonstrates that this pathway is prohibitive. As will be discussed in section 2.9, a structural isomer of 1 that appears en route to product 2, species d1c3, can form a methyl radical. This pathway therefore provides an intermediate that explains the observed H-atom abstraction from CHD.

Scheme 15.

Calculated energetics for methyl radical formation. Free energies are relative to dimer 1 and given in kcal mol⁻¹.

2.8. Assessment of a monometallic reductive elimination pathway

To investigate potential Path B (see Scheme 12), involving the monometallic intermediate [Pd(Me)₂(L1)]⁻, independent synthesis of a dimethyl monomer was conducted. Reaction of K [L1] and [Pd(CH₃ )₂(COD)] at −30 °C gave K[7] in near quantitative yields (Scheme 16). The ¹H NMR spectrum of 7 reveals a pair of doublets at δ_H = 0.0 and -0.05 ppm, with H–P coupling constants of ³J_HP = 7.2 and 7.8 Hz respectively. The similar ¹H chemical shifts and coupling values are consistent with other PdMe₂ complexes bearing asymmetric bidentate ligands.^51–53 The ¹H NMR signals correlate by ¹H–¹³C HSQC to doublets at δ_C = −11.6 and 7.9, respectively, with coupling constants of ²J_CP = 6.04 and 117.8 Hz. These coupling constants are consistent with, respectively, cis- and trans-disposed methyls relative to the phosphine donor.^53,54

Scheme 16.

Synthesis of dimethyl monomer K[7].

Two separate solutions of K[7] in acetone-d₆ were prepared, then one was heated at 50 °C for 2.5 h and the other was left at 26 °C for 4 h (Scheme 17a). In both cases, the ³¹P{¹H} NMR spectrum revealed complete consumption of K[7] and a major product (ca. 70%) was observed at δ_P = 40.1, along with several minor products. None of the observed products matched to the spectroscopic signals for 2 or 3. The ¹H NMR spectrum of the decomposition product shows a doublet at δ_H = −1.02, with a small coupling constant (J_HP = 4.0 Hz), consistent with retention of one Pd–CH₃ moiety. Inspection of the headspace by GC-FID revealed a 72% yield of methane, with the balance being ethane. The computed free energies for the formation of methane and ethane from [7]⁻ are −14.9 kcal mol⁻¹ and −1.4 kcal mol⁻¹, respectively (Scheme 17b). The computed preference for methane upon thermolysis of [7]⁻, as well as the experimental observation for the preservation of one of the Pd–CH₃ moieties, suggests that reductive elimination from a [Pd(CH₃)₂(L1)]⁻ intermediate does not occur during thermolysis of 1. Incidentally, simulations also show a relatively high free energy for the formation of methyl radical (37.3 kcal mol⁻¹) from [7]⁻.

Scheme 17.

(a) Thermolysis of K[7]; and (b) computed energies for C–C and C–H bond formation from [7]⁻. Selected atom distances are given in Å.

On the basis of experimental and computational studies we have discounted Path B for reductive elimination, in which ethane is formed from the dimethyl Pd^II monomer [Pd (CH₃)₂(L1)]⁻.

2.9. Assessment of bimetallic pathways to reductive elimination

Based on the studies above, a bimetallic mechanism (i.e. Path C in Scheme 12) for the formation of ethane and both palladium products 2 and 3 is most likely. Therefore, our computational efforts focused on C–C coupling pathways involving bimetallic intermediates that differ based on the coordination geometry of the L1 ligand and the products formed, in which Path C1 (Scheme 18) and C2 (Scheme 19) both form Pd^II(L)₂ (3) and Pd⁰ as products and Path C3 (Scheme 20) forms Pd^I dimer 2. A handful of higher energy (and therefore less feasible) paths were identified and presented in the ESI.^†

Scheme 18.

Path C1: Ethane formation from a pseudo methyl-bridged dimer d1a. Selected atom distances in monomer m1 are given in Å. Computed oxidation states of Pd atoms from LOBA analysis are given in purple. ΔG in kcal mol⁻¹.

Scheme 19.

Path C2: Ethane formation from a bimetallic dimethyl intermediate 8a5. Computed oxidation states of Pd atoms from LOBA analysis are given in purple. ΔG in kcal mol⁻¹.

Scheme 20.

Attempted formation of 8. Conditions: (i) 15 equiv. [Pd (CH₃)₂(cod)], C₆D₆, rt; (ii) 2.0 equiv. [Pd(CH₃)₂(cod)], C₆D₆, rt; (iii) 2.5 equiv. [Pd(CH₃)₂(cod)], C₇D₉, 0°C.

2.10. Path C1: Ethane formation from an asymmetric methyl-bridged dimer

The asymmetric methyl-bridged dimer d1a (12.4 kcal mol⁻¹, Scheme 18) can form via the dimerization of two m1 monomers. d1a contains Pd–C_(CH3) atom distances of 2.08 and 3.11 Å (Table S2^†), which suggests the asymmetric dimer d1a is not a classical methyl-bridged dimer with strong methyl–metal interaction.^55–59 From d1a, two different paths for the generation of ethane are possible. Both mechanisms involve reductive elimination after two methyl groups fully associate to a single Pd, and result in the formation of 3ra, a geometric isomer of the experimentally observed Pd^II(L)₂ product 3. Isomer 3ra is 5.3 kcal mol⁻¹ higher in energy than 3 (Scheme S7^†), and the two species are expected to be in equilibrium. Here the focus is on a pathway from d1a that involves an η³ 1-azaallyl intermediate, since it has a lower rate-limiting barrier of 30.7 kcal mol⁻¹ (Scheme 18) compared a second pathway at 39.5 kcal mol⁻¹ (Scheme S5^†).

The favourable pathway (1 → d1a → d1a1 → d1a5 → d1a6 → 3ra, Scheme 18) involves: (1) methyl transfer to form the Pd^II–Pd^IIMe₂ complex d1a1, (2) isomerization of Pd^II–Pd^IIMe₂ complex d1a1 to form d1a5 (3) C–C coupling to release an ethane molecule and d1a7, and finally (4) release of Pd⁽⁰⁾ and a isomer of Pd^II(L)₂ product (3ra). In the first step, electronic structure analysis suggests direct anionic methyl transfer in methyl-bridged dimer d1a (TS-d1a) leads to a Pd^II–Pd^IIMe₂ dimer (d1a1) with a free energy barrier of 14.7 kcal mol⁻¹. Alternatively, complex d1a4 with a coordinated η³ 1-azaallyl group can appear as an intermediate for methyl transfer. The anionic methyl transfer via TS-d1a4 has a much higher barrier than the path through TS-d1a (32.8 kcal mol⁻¹), but leads to the same intermediate, d1a5. The subsequent C–C reductive elimination from η³ 1-azaallyl complex d1a5 has a barrier of 30.7 kcal mol⁻¹, which is significantly lower than the barrier of C–C reductive elimination from η¹ 1-azaallyl complex d1a1 (39.5 kcal mol⁻¹, Scheme S5^†). After ethane elimination, dissociation of one Pd atom leads to species 3ra.

The free energy of activation for the rate-determining step (methyl transfer, TS-d1a4) in this anionic methyl transfer pathway is 32.8 kcal mol⁻¹, which is a little higher than the C–C coupling step (30.7 kcal mol⁻¹ for TS-d1a5). Additional less favourable pathways with alternate conformations of the 1-azaallyl group of L1 via d1b (a rotamer of d1a) are presented in Scheme S6.^† The free energies of activation for the methyl transfer (TS-d1b3, Scheme S6^†) and the C–C coupling (TS-d1b4, Scheme S6^†) in the analogous pathways are 32.5 and 35.8 kcal mol⁻¹, respectively.

2.11. Path C2: Formation of ethane and product 3 from bimetallic dimethyl intermediate 8a5

A bimetallic intermediate 8a5 with a Pd(Me)₂ moiety can also be formed without ever passing through the bridged-methyl intermediate (8a1 → 8a2 → 8a4 → 8a5 → 8a6 → 3r, Scheme 19). To arrive at this species, phosphine ligand migration (1 → 8a1) results in a transient Pd^II-diphosphine (8a1) with η²-C,C 1-azaallyl coordination to one palladium centre. Intramolecular CH₃ transfer then generates Pd⁰ (diphosphine)Pd^IV(Me)₂ (8a2) with a barrier of 31.4 kcal mol⁻¹ (TS-8a1), which is close to that of the rate-limiting step involved in pathway C1 (TS-d1a4 = 32.8 kcal mol⁻¹, Scheme 18). These similar barriers could be explained by the similar coordination of the phosphine 1-azaallyl group. Electronic structure analysis of 8a2 suggests it is Pd⁰ (diphosphine)Pd^IV(Me)₂, and this species is reached via cationic CH₃ transfer (TS-8a1) instead of anionic CH₃ transfer (i.e. in C1 TS-d1a and TS-d1a4, see Scheme 18).⁶⁰

After forming intermediate 8a2, two processes may occur (Scheme 19): (1) direct C–C reductive elimination to release an ethane molecule (TS-8a2), or (2) a change in 1-azaallyl binding from η²-C,C to κ¹-N to form intermediate 8a4. The direct C–C reductive elimination from Pd⁰ (diphosphine)Pd^IV(Me)₂ intermediate 8a2 has a barrier of 37.3 kcal mol⁻¹ (TS-8a2). Instead of this difficult direct C–C coupling, rearrangement of the ligand in 8a2 relaxes to the symmetric, square-planar Pd^II–Pd^II dimethyl intermediate 8a4, which is over 20 kcal mol⁻¹ downhill from 8a2. 8a4 then rearranges to an asymmetric intermediate 8a5 (3.7 kcal mol⁻¹) with a distorted square-planar structure. This species is consistent with the observations of Csp³–Csp³ reductive elimination from a series of square-planar cis-[PdMe₂(PMe₃)L] complex (L = CH₂CH₂, PMe₃), where the orientation of Csp³–Csp³ bond in the related transition state is perpendicular to the plane of Pd(PMe₃)L.^61,62 A prolonged Pd–Pd atom distance and smaller C_(CH3)–Pd–C_(CH3) bond angle in tetra-coordinated intermediate 8a5 compared to those in intermediate 8a4 are observed (2.939 Å vs. 2.816 Å and 85.9° vs. 88.4°). The barrier for the C–C reductive elimination to form an ethane molecule and Pd⁰–Pd^II complex 8a6 is 29.4 kcal mol⁻¹ (TS-8a5). Once ethane is released, the Pd⁰–Pd^II complex 8a6 leads to Pd⁰ and the geometric isomer of Pd^II(L)₂ product 3, species 3r. After the formation of 3r, ligand rotation leads to generation of the final product Pd^II(L)₂ complex 3. Other analogous pathways for the generation of ethane, Pd⁰, and Pd^II(L)₂ via 8b1 (rotamer of 8a1) are presented in Scheme S9.^† Overall, there are two slow steps in this process: cationic methyl transfer (TS-8a1) and reductive elimination (TS-8a5), each with barriers of around 30 kcal mol⁻¹.

Compared to the formation of ethane and Pd^II(L)₂ (3) from a methyl-bridged dimer d1a (Scheme 18), the path with Pd^II(L)₂Pd^II(Me)₂ dimethyl intermediate 8a5 (8a1 → 8a2 → 8a4 → 8a5 → 3r → 3) in Scheme 19 has a comparable free energy barrier for the rate-determining step (31.4 kcal mol⁻¹ for TS-8a1 vs. 30.7 kcal mol⁻¹ for TS-d1a5). Alternate bimetallic structures that involved bridging via the phosphine rather than the amido of L1 were not energetically viable (Scheme S8^†).

The proposed dimethyl Pd^II intermediate 8a4 in Path C2 is relatively low in energy, which suggests it could be prepared independently. To this end, a solution of 3 was treated with [Pd(Me)₂(COD)] at 0 °C (Scheme 20). After 4 h, ³¹P{¹H} NMR analysis revealed the formation of the Pd^II dimer 1 as the major product in 64% yield. Higher conversion was hindered at longer times or higher temperature by the competitive thermal decomposition of [Pd(Me)₂(COD)]. However, addition of excess palladium dimethyl reagent resulted in improved yields, giving 1 quantitatively at room temperature. In all cases, no evidence for the target bimetallic complex 8 was observed by ³¹P{¹H} or ¹H NMR spectroscopy. Given the calculated energetics, we do not expect that 8 rearranges to 1. Rather, an alternate pathway from the reagents 3 and [Pd (Me)₂(cod)] to 1 is likely operative. Unfortunately, this precludes direct experimental evaluation of 8 as a relevant intermediate in C–C coupling.

2.12. Path C3: Formation of ethane and Pd^I dimer 2 from a ligand-bridged bimetallic intermediate

Having shown possible routes for formation of products complex 3 (Schemes 18 and 19), the formation of dimer 2 and ethane is now considered. This pathway requires intramolecular CH₃ transfer without fragmenting the dimer into monomers, and the coordination of phosphine 1-azaallyl group has a key role in this path (Scheme 21). This process begins with the rotation of phosphine 1-azaallyl anion ligand in Pd^IIMe(L) dimer 1 to form its rotamer, intermediate d1c1, with a barrier of 14.1 kcal mol⁻¹ (TS-1). Next, a change in the bridging coordination mode of the 1-azaallyl fragment from μ-N in d1c1 to μ-[η²-CC:κ¹-N] in d1c2 involves a barrier of 11.4 kcal mol⁻¹ for TS-d1c1. After reorientation of a methyl group in d1c2 (E_(CH3)-isomer), which is necessary prior to methyl transfer, Pd^IIMe–Pd^IIMe isomer d1c3 (Z_(CH3)-isomer) results. A smaller Pd–Pd–C_{(transferred CH3)} bond angle in Z_(CH3)-isomer d1c3 compared to that in d1c2 is observed (80.4° vs. 139.1°), which is in favour of the critical methyl transfer that will lead to the generation of ethane. The homolytic breaking of Pd–Me bond in Pd^II–Pd^II Z_(CH3)-isomer d1c3 is uphill by 3.9 kcal mol⁻¹ (Scheme S10^†), which suggests the possible formation a free methyl radical. This finding suggests that d1c3 may be the intermediate that reacts with 1,4-cyclohexadiene to give methane (cf. Scheme 14). No other more feasible homolytic breaking of Pd–Me bond in intermediates d1a (24.3 kcal mol⁻¹), d1a4 (15.5 kcal mol⁻¹), d1a5 (20.3 kcal mol⁻¹), and 5a1 (35.4 kcal mol⁻¹) are observed.

Scheme 21.

Path C3: Ethane formation from ligand-bridged dimer. Computed oxidation states of Pd atoms from LOBA analysis⁶³ (except those of dimer 2) are given in purple. ΔG°/ΔG^‡ are in kcal mol⁻¹.

Anionic methyl transfer (TS-d1c3) from the Pd^IIMe–Pd^IIMe intermediate d1c3 provides the Pd^II–Pd^II(Me)₂ dimethyl intermediate d1c4. The following C–C reductive elimination from d1c4 generates an ethane molecule and Pd⁰–Pd^II intermediate d1c5 with an overall barrier of 31.1 kcal mol⁻¹ (TS-d1c4) compared to Pd^IIMe(L) dimer 1. This step is rate determining and leads to Pd^I(L) dimer complex 2 (31.3 kcal mol⁻¹ for TS-d1c4), after rearrangement of the 1-azaallyl fragment (TS-d1c6).⁶³

Other analogous pathways for the generation of ethane and Pd^I(L) dimer complex 2 from Pd^IIMe(L) dimer 1 are presented in the ESI (Schemes S13, and S14^†).

3. Discussion

3.1. Monometallic route to methane byproduct

The crossover experiment with 1 and 1-d₆ revealed that a monomer/dimer equilibrium is operative. This is supported by computations that show that the dimer (1) is more stable than two equiv. of the monomer (m1) by 13.5 kcal mol⁻¹. The latter is stabilized by agostic interactions to one of the methyl substituents of the 1-azaallyl group of L1 (Scheme 7).

All three of the viable C–C coupling routes follow a bimetallic mechanism and only one route, C1, relies on monomer m1 formation via the pre-equilibrium. However, m1 is vulnerable to several pathways that can erode selectivity for C–C bond formation to instead favour methane (Scheme 22). One possible pathway involves a dimethyl monomer that was shown through both experiment and computation to undergo C–C and C–H coupling, with preference for methane formation (Scheme 17). A second likely pathway from m1 involves C–H oxidative addition of the ligand to give a high-valent metal-hydride. This was indicated by computations and reactivity with 5, a Pt analogue of 1. Reductive elimination from the presumed Pt-H (from 5) exclusively gives methane and subsequent steps involve solvent activation and formation of M^II(L)₂ (3, M = Pd; 6, M = Pt) (Scheme 9). From palladium monomer m1, the route to give methane is competitive with C–C coupling (C1). These observations indicated that the proximal methyl substituent of the 1-azaallyl moiety is vulnerable to unwanted C–H activation, and that the monomer m1 leads to deleterious reaction pathways.

Scheme 22.

Competing pathways from monomer m1 that can lead to methane formation through methyl transfer (left) or C–H activation (right), as well as the pathway affording ethane and 3 (centre).

3.2. Bimetallic methyl transfer

Experiments confirm that methyl transfer does not involve an uncaged methyl radical. Rather, the computations reveal that intramolecular methyl transfer occurs within a bimetallic structure held together by the phosphine 1-azaallyl ligand (L1), which involves a transition state with a triangular arrangement of methyl and two Pd centres. This is distinct from bimolecular methyl transfer that involves a linear arrangement of Pd–Me–Pd, which leads to intermediates that undergo reductive elimination from a high-valent monometallic complex (i.e., Scheme 2c). Rather, intramolecular methyl transfer is unique to 1 and likely also the A-frame dimers, A (i.e., Scheme 3).

Two qualitatively distinct pathways of methyl transfer are possible that involve cationic or anionic mechanisms, which were assigned based on localized orbital bonding analysis (LOBA; Scheme 23). Additional discussion on the limitations of the LOBA analysis and a complementary assessment are included in the ESI.^† Cationic methyl transfer in the bimetallic Pd^II–Me intermediate (5a1, Path C2, Scheme 19) leads to the mixed valent Pd⁰–Pd^IV(Me)₂ complex 5a2. Anionic methyl transfer on the other hand occurs without a change in oxidation states to reach a bimetallic dimethyl Pd^II(Me)₂ fragment. The two anionic possibilities are realized through TS-d1a4 (Path C1, Scheme 18) and TS-d1c3 (Path C3, Scheme 21), and they ultimately lead to the formation of products 2 and 3. In contrast, the cationic methyl transfer leads only to species 3.

Scheme 23.

(a) The pathways of methyl transfer, and (b) geometric parameters of the Pd–CH₃–Pd fragments in the transition states.

The geometries for cationic and anionic methyl transfer pathways are delineated in Scheme 23. In the cationic path TS-8a1, the two Pd–C_(CH3) bond distances of the transferring methyl are highly asymmetric, being 2.210 and 2.943 Å, respectively. This significantly differs from those in the anionic methyl transfers TS-d1a4 and TS-d1c3, where the two Pd–C_(CH3) bond distances are within approximately 0.1 Å of each other. These differing geometric characteristics, together with the computed oxidation states, make it clear that two very different methyl transfer mechanisms are operative.

3.3. Bimetallic 1,1-reductive elimination

The experimental and computational evidence indicates that C–C reductive elimination from 1 occurs via a bimetallic intermediate. In contrast to the diverging methyl transfer pathways, the pathways for reductive elimination in the formation of ethane are more uniform (Scheme 24). In all cases, the C–C bond forming step involves a concerted 1,1-reductive elimination from a single Pd centre. C–C coupling is characterized by the reduced oxidation state of Pd from Pd(II) to Pd(0), as seen in TS-d1a5 (Scheme 18), TS-d1c4 (Scheme 21), and TS-8a5 (Scheme 19). Similar geometric parameters of the pseudo-symmetric CH₃–Pd–CH₃ fragments are present in these transition states. The slightly longer C_(CH3)–C_(CH3) bond distance (2.048 Å) in TS-d1c4, compared to those in TS-d1a5 (1.969 Å) and TS-8a5 (1.966 Å), is consistent with the relatively high barrier for the C–C coupling (31.3 vs. 30.7 vs. 29.4 kcal mol⁻¹, Scheme 25). The barriers for C–C bond formation are consistent with the elevated temperatures required for the reaction. Reductive elimination from a single metal centre also occurs in bimetallic mechanisms that involve Pd^II/Pd^III or Pd^II/Pd^IV redox cycles^23–25 and this is the proposed pathway^32,33 for C–C bond formation from the A-frame dimers A (Scheme 3).

Scheme 24.

(a) The pathways of ethane formation, and (b) geometric parameters of the CH₃–Pd–CH₃ fragments in the transition states.

Scheme 25.

Three proposed operative bimetallic Csp³–Csp³ coupling pathways (C1–C3) from 1, showing the reductive elimination step to give ethane, and complex 2, or 3 and Pd(0). Colors depict the different coordination modes for the 1-azaallyl group of L1: μ-N (blue), κ¹-N (pink), η³-NCC (red) and μ-(κ¹-N, η²-CC) (green). ΔG values shown are in kcal mol⁻¹ and the values are relative to 1 at 0 kcal mol⁻¹.

3.4. Role of the structurally-responsive 1-azaallyl group

Our initial study³⁶ confirmed the versatile coordination chemistry of phosphine 1-azaallyl ligand L1, based on the structures of 1 and reductive elimination products 2 and 3. The present study shows that the changes in coordination mode induce C–C bond formation. In all three of the viable reaction paths, C1–C3, the 1-azaallyl coordination mode of L1 alters from μ-N to include κ¹-N, η²-CC, η³-NCC and μ-[η²-CC:κ¹-N] (Schemes 18, 19 and 21). The importance of the 1-azaallyl fragment to enable the C–C coupling mechanisms is supported by experimental studies with phosphine amido complex 8 that gives poor selectivity for C–C bond formation. The specific interactions of the 1-azaallyl group are distinct for each of Paths C1–C3 and the impact is most clearly considered for the reductive elimination step (summarized in Scheme 25). In C1 the PdMe₂ fragment of d1a5 is ligated with an η³-NCC 1-azaallyl and the barrier (TS-d1a5, 30.7 kcal mol⁻¹, Scheme 18) is lower than the analogous route with κ¹-N binding (TS-d1a1, 39.5 kcal mol⁻¹, Scheme S5^†) by 8.8 kcal mol⁻¹. During reductive elimination in Path C2 (Scheme 19), one of the 1-azaallyl groups holds the bimetallic intermediate together via a μ-N coordination mode. The other ligand de-coordinates from the PdMe₂ fragment to permit a Pd–Pd interaction giving a barrier (TS-8a5, 29.4 kcal mol⁻¹) that is 7.9 kcal mol⁻¹ lower than an analogous structure with a retained η²-CC interaction to the PdMe₂ moiety (TS-8a2, 37.3 kcal mol⁻¹). Throughout pathway C3, one L1 holds the dimer together through a μ-[η²-CC:κ¹-N] bridging coordination mode (Scheme 21). The alkene of this L1 coordinates to the Pd without the Me groups, and the nature of the bonding interaction changes dramatically through reductive elimination. The second order perturbation energies (E²) from the natural bond orbital (NBO) analyses reveal that the reductive elimination precursor in d1c4 has a relatively weak Pd–alkene interaction dominated by π_c–c → d*_Pd (5.6 kcal mol⁻¹, Scheme S12^†). The analogous interaction is much stronger in intermediate d1c5 that forms following ethane release, as evidenced by the E² values for alkene donation to Pd (π_c–c → d*_Pd, 34.5 kcal mol⁻¹) and backdonation (d_Pd → π*_c–c, 27.6 kcal mol⁻¹, Scheme S12^†). These observations, along with the short Pd–Pd distance, indicate that alkene binding significantly affects reductive elimination by modulating metal–metal synergy. The bimetallic mechanisms proposed herein—intimately dependent on changes in ancillary ligand coordination mode—therefore complement the known monometallic mechanisms for C–C coupling at Pd (see Introduction) and provide an interesting reaction motif for further development.

4. Conclusions

The C–C reductive elimination mechanism of a Pd^II–Me dimer (1) bearing a phosphine 1-azaallyl ligand (L1) was investigated using a combined experimental and computational approach. An equilibrium is operative between dimer 1 and its corresponding monomers, and the latter were deemed responsible for competing methane formation. Thus, suppression of methane to give exclusive C–C bond formation should be possible with a derivative of ligand L1 that induces a greater preference for the dimeric form. A variety of evidence discounted radical or monometallic pathways for C–C bond formation. Instead, three viable bimetallic pathways were proposed and analyzed. In all cases, methyl-transfer affords a PdMe₂ fragment that undergoes 1,1-reductive elimination. A critical aspect of all three viable reductive elimination pathways is the capacity of the phosphine 1-azaallyl ligand (L1) to undergo changes in coordination mode (i.e. five different modes identified). This structurally-responsive feature offers flexibility and versatility in the type of intermediates that can form, specifically stabilization of different bimetallic structures over undesired monometallic compounds. The changes in ligand coordination mode lowers barriers for reductive elimination, which would not be accessible with a statically-coordinated ancillary ligand. Deployment of a structurally-responsive ligand to facilitate bimetallic C–C bond formation is highly unusual and could be exploited in the emerging field of catalysis involving dinuclear Pd^I and Pd^II complexes as on-cycle intermediates.¹³