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Synthetic and Mechanistic Interrogation of Pd/Isocyanide-Catalyzed Cross-Coupling: π‑Acidic Ligands Enable Self-Aggregating Monoligated Pd(0) Intermediates Brandon R. Barnett,‡ Liezel A. Labios,‡ Julia M. Stauber,‡ Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa* Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla, California 92093-0358, United States S Supporting Information *

ABSTRACT: Despite the large number of judiciously designed ligands that have been exploited in palladium-catalyzed cross-coupling protocols, the incorporation of ligands bearing appreciable π-acidic properties has remained significantly underexplored. Herein, we demonstrate that well-defined and low-coordinate Pd0 complexes supported by m-terphenyl isocyanides function as competent catalysts for the Suzuki−Miyaura cross-coupling of aryl bromides and arylboronic acids. Two-coordinate Pd(CNArDipp2)2 was active for the coupling of unhindered aryl bromides at room temperature in 2-propanol, while increasing the temperature to 60 °C allowed for the use of mono- or di-ortho-substituted aryl bromides. Oxidative addition of the aryl bromide was shown to proceed via a dissociative mechanism, implicating monoligated Pd(CNArDipp2) as the catalytically active intermediate. Attempts to access this fleeting species via activation of the PdII monoisocyanide PdCl(η3-C3H5)(CNArDipp2) with alkoxide base yielded the dinuclear PdI species (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2. Although dinuclear PdI complexes are often produced as off-cycle species when using complexes of the type PdCl(η3-allyl)L as precatalysts, this represents the first time that the comproportionation product (μ-allyl)(μ-Cl)[PdL]2 has been observed to undergo nucleophilic substitution with alkoxide, despite the fact that activating conditions for these precatalysts typically employ alkoxide bases. Remarkably, this alkoxide complex can undergo β-hydride elimination with expulsion of acetone and propene to produce two equivalents of catalytically active Pd(CNArDipp2), which can self-aggregate to yield the isolable tripalladium cluster Pd3(η2-Dippμ-CNArDipp2)3. This cluster is catalytically competent for the Suzuki−Miyaura reaction and functions as a formal source of monoligated Pd(CNArDipp2) in solution.

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INTRODUCTION

In examining the nature of cross-coupling protocols that engender the formation of monoligated Pd0 species, several overarching design principles are evident. First, the supporting donor ligand must bind tightly to the Pd center so as to discourage precipitation of Pd metal and promote oxidative addition. Furthermore, a ligand that presents a very large steric profile is essential in order to foster coordinative unsaturation. More elegant designs, such as biphenyl phosphines which can form Pd−arene π-interactions15,24−27,33,34 and N-heterocyclic carbenes (NHCs) which present a flexible steric profile,12,13 have also proven adept at promoting the formation of monoligated Pd species and catalyzing challenging coupling reactions. However, it is also noted that use of donor ligands that meet these criteria can sometimes have deleterious effects on the transmetalation and/or reductive elimination steps of the catalytic cycle.25,35 It is therefore evident that a very

Palladium-catalyzed cross-coupling reactions have gained a preeminent place in the toolkit of synthetic chemists.1,2 The versatility, generality and functional group tolerance exhibited by such protocols as the Suzuki−Miyaura reaction1−5 and the Buchwald−Hartwig amination6−9 have allowed for the construction of a wide array of scaffolds obtained via the formation of a new carbon−carbon or carbon−nitrogen bond. Nearly all palladium-catalyzed cross-coupling schemes share a mechanistic feature whereby oxidative addition of a C−X bond occurs at a Pd0 center. In some cases, studies have shown that oxidative addition occurs at a 14-electron Pd0L2 species.4,10 However, when sterically encumbering supporting ligands are employed, the active species is often a monocoordinate 12electron Pd 0 −L complex. 11 Such systems can display remarkable activity, as ligand sets and reaction conditions conducive to the formation and stabilization of Pd0L have proven capable of effecting challenging couplings and operating at extremely low catalyst loadings.12−32 © XXXX American Chemical Society

Received: January 13, 2017

A

DOI: 10.1021/acs.organomet.7b00035 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Suzuki−Miyaura Substrate Scope for Room-Temperature Coupling Reactions

a

GC yield (isolated yield).

propensity of isocyanide ligands to undergo migratory insertion processes to yield iminoacyl ligands might be obviated in this system.70 More recently, isolation of the trinuclear complex Pd3(μ−η2:η1-PhNO)3(CNArDipp2)3 (10),71 which was proposed to result from trapping of Pd(CNArDipp2) by nitrosobenzene followed by trimerization, alluded to the ability of the encumbering m-terphenyl isocyanide to support monoligated Pd0 in solution. Accordingly, herein we detail the activity of m-terphenyl isocyanide-supported complexes of palladium in catalyzing the Suzuki−Miyaura cross-coupling reaction. Bis-isocyanide 1 is compatible with a range of electronically diverse and hindered aryl bromides, although di-ortho-substituted arylboronic acids are not tolerated. Oxidative addition of Caryl-Br is shown through kinetic analyses to occur via a dissociative mechanism, indicating that monocoordinate Pd(CNArDipp2) is the active intermediate. The search for highly active catalysts bearing a Pd/isocyanide ratio of 1:1 resulted in the discovery of the dinuclear PdI complex (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2 (4), which is produced upon activation of PdCl(η3-C3H5) (CNArDipp2) (3) with NaOt-Bu in 2-propanol. The observation of an alkoxide-bridged PdI complex is unique in the activation of systems of the type PdCl(η3-allyl)L and allows for the production of 2 equiv of catalytically active Pd(CNArDipp2) from 4 through a β-hydride elimination pathway. Importantly, intermolecular self-trapping of Pd(CNArDipp2) provides access to the isolable tripalladium cluster triangulo-Pd3(η2-Dipp-μCNArDipp2)3 (8), which is also catalytically competent for Suzuki−Miyaura cross coupling. Delineation of Suzuki−Miyaura Cross-Coupling Catalyzed by Pd(CNArDipp2)2 (1). Previously, we reported that Pd(CNArDipp2)2 (1) was a competent catalyst for Suzuki− Miyaura cross-coupling of aryl bromides with phenylboronic acid in THF at room temperature.53 The unoptimized conditions used in this report employed K3PO4 as the base and a relatively high catalyst loading (5 mol %). Optimization of reaction conditions revealed that catalyst loadings of 1 mol %

sensitive interplay between steric and electronic factors must be considered in the design of highly active catalytic species. Despite the numerous potential ancillary ligands that could be used in Pd-catalyzed cross-coupling, triorganophosphines and NHCs have dominated the development of reliable catalytic systems.36−41 These ligands form robust bonds to Pd on account of their strong σ-donor capabilities, and contain steric profiles that can be systematically modulated.42 In contrast, the use of π-acidic ligands in Pd-catalyzed crosscoupling has been significantly underexplored.35,43−47 While the use of strong σ-donor ligands promotes oxidative addition, π-acidic ligands should increase the rate of reductive elimination,48,49 and may also increase catalyst lifetimes by stabilizing the Pd0 oxidation state.43,44 Such features may hold promise in cross-coupling-based polymerizations, where catalyst degradation processes often compete with polymerchain propagation.50−52 Futhermore, although the withdrawal of electron density from Pd0 by π-acidic ligands may slow the oxidative addition step, such a diminution of rate may be beneficial in certain systems which suffer from off-cycle side reactions from PdII intermediates (i.e., homocoupling and hydrodehalogenation).35 In an effort to expand the scope of ligand electronic properties in palladium cross-coupling catalysis, we reported the synthesis of the two-coordinate complex Pd(CNArDipp2)2 (1, ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3), which represented the first monomeric binary isocyanide complex of Pd0.53 The monomeric nature of 1 is enforced by the m-terphenyl isocyanide ancillary ligands,54−65 whereas less encumbering isocyanides have been previously shown to give rise to multinuclear Pd0 aggregates.66−69 Preliminary, unoptimized screenings revealed that 1 could effect the coupling of mono- or di-ortho-substituted aryl bromides with phenylboronic acid at room temperature, affording the corresponding biaryls in high yield (95%).53 Importantly, the isolable PdII intermediate transPdBr(Mes)(CNArDipp2)2 (Mes = 2,4,6-Me3C6H2) was shown to be unchanged upon heating to 80 °C, suggesting that the B


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Organometallics Table 2. Suzuki−Miyaura Substrate Scope for Coupling Reactions Conducted at 60 °C

a

GC yield (isolated yield).

could efficiently effect cross-coupling with high selectivity and yields using NaOt-Bu as the base in 2-propanol solution. At room temperature, unactivated aryl bromides could be coupled with various arylboronic acids bearing methyl- or phenylsubstitution (Table 1, entries 1−10). Use of 1-bromonaphthalene also yielded the corresponding biaryls in good yields (Table 1, entries 11−14). Although these conditions were not suitable for the use of ortho-substituted aryl bromides, increasing the temperature to 60 °C allowed for the coupling of aryl bromide substrates bearing mono- or di-orthosubstitution (Table 2, entries 1−8). In addition, 2,4difluorophenylboronic acid proved a viable coupling partner under these conditions (Table 2, entries 9 and 10), which is notable due to the difficulties associated with using electrondeficient arylboronic acids in Suzuki−Miyaura coupling.72,73 Despite the readiness of 1 to couple unactivated and hindered aryl bromides, aryl chlorides were generally unreactive toward 1 under these conditions. Undoubtedly, this can be traced to the attenuated σ-donor abilities of isocyanides relative to phosphines and NHCs, as very electron-rich Pd0 centers are needed for the oxidative addition of aryl chlorides to proceed under mild conditions. Indeed, the isocyanide-containing Suzuki−Miyaura coupling precatalysts PdCl2(CNR)2 reported by Villemin necessitated harsh conditions (refluxing 1,4dioxane), high catalyst loadings (5 mol %) and long reaction times (18 h) to accommodate activated aryl chlorides as coupling partners.47a Although Luzyanin and Hashmi have reported isocyanide-containing PdII Suzuki−Miyaura precatalysts which can couple aryl chlorides at room temperature, these complexes also employ strongly donating carbene ancillary ligands.47b−d Density functional theory calculations on the monocoordinate Pd(CNArDipp2) fragment reveal the prevalence of π-backbonding interactions between the Pd dxz/ dyz and isocyanide π* orbitals (Figure 1), illustrating how the πacidic isocyanide ligands serve to attenuate the electron-rich

Figure 1. DFT-calculated 4d orbital manifold of the Pd(CNArDipp2) fragment.

nature of the corresponding Pd0L adduct. However, these interactions also serve to strengthen the Pd−CCNR interaction, which should aid in preventing the precipitation of Pd black. It should also be noted that 1 is unable to catalyze the formation of tri- and tetra-ortho-substituted biaryls (Table 2, entries 11, 12, and 14). Although the oxidative addition of C


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Organometallics hindered aryl bromides to 1 readily takes place at 60 °C, attempts to use di-ortho-substituted arylboronic acids as transmetalation reagents were unsuccessful regardless of aryl bromide substitution pattern (Table 2, entries 12 and 13). This observation suggested steric inhibition of the transmetalation step due to the encumbrance of the m-terphenyl isocyanide ligands. Indeed, very bulky ligands can have deleterious effects with regard to coupling of hindered substrates, presumably due to slowed rates of transmetalation.25 Although cross-couplings mediated by preformed Pd0L2 species are often initiated by ligand dissociation, it is known that a very sensitive balance of steric and electronic factors can determine whether the oxidative addition step proceeds via an associative or dissociative mechanism.10,74 In order to interrogate whether cross-coupling in this system proceeds through monoligated Pd(CNArDipp2) as the catalytically active intermediate, a kinetic analysis of the oxidative addition of MesBr by Pd(CNArDipp2)2 (1) was undertaken. Under pseudo-first-order conditions (10 equiv of MesBr) at 25 °C, the consumption of 1 and formation of trans-PdBr(Mes)(CNArDipp2)2 proceeds to completion within 1 h with a rate constant of kobs = 1.47(2) × 10−3 s−1, as assessed by 1H NMR spectroscopy (Figure 2 and

decrease the rate constant of the reaction by more than half (kobs = 5.59(4) × 10−4 s−1). Saturation is reached with 0.5 equiv of CNArDipp2, such that introduction of additional isocyanide has no further effect on the observed rate constant (Table 3). As 1 has been shown to resist ligation of a third isocyanide ligand,53 these observations strongly suggest that oxidative addition proceeds dissociatively, signaling the intermediacy of monoligated Pd(CNArDipp2). As a point of comparison to di-ortho-substituted MesBr, oxidative addition of the unhindered aryl bromide m-XylBr (mXyl =3,5-Me2C6H3) by Pd(CNArDipp2)2 (1) was also examined. Using stoichiometric m-XylBr, this reaction proceeds very cleanly to yield trans-PdBr(m-Xyl) (CNArDipp2)2 (2), which was structurally characterized (Figure S34). Following the reaction of 1 with 10 equiv of m-XylBr by 1H NMR spectroscopy at 25 °C again revealed a first-order dependence on 1 with kobs = 2.46(3) × 10−3 s−1 (Figure 3). While this value of kobs is

Figure 3. Plot of ln[Pd(CNArDipp2)2] vs time, showing the comparative observed rates of oxidative addition of MesBr and mXylBr under pseudo-first-order conditions (10 equiv of aryl bromide).

approximately a factor of 2 larger relative to that for the oxidative addition of MesBr, the fact that both rate constants are of the same order of magnitude strongly suggests that steric inhibition in the transmetalation step is responsible for the inability of this system to produce tri- and tetra-substituted biaryls. Examination of the nature of transmetalation in this system revealed that no biaryl product is obtained when the NaOt-Bu base is omitted from the reaction mixtures. Transmetalation in Suzuki−Miyaura coupling is often believed to proceed from a boronate species formed by complexation of base to the boronic acid.75 In this regard, aryltrifluoroborates have been used as transmetallating agents that do not require addition of an exogenous base.76−78 Notably, however, addition of either K[PhBF3] or [n-Bu4N][PhBF3] (10 equiv) to 2propanol solutions of PdBr(Ar)(CNArDipp2)2 (Ar = Mes, mXyl) does not result in transmetalation or biaryl production, even upon prolonged heating at 60 °C. Addition of water as a co-solvent, which has been shown to improve yields of crosscoupling reactions using aryltrifluoroborates,79 did not alter the reaction outcome, while use of both K[PhBF3] and NaOt-Bu prompted only homocoupling to produce biphenyl. These observations provide circumstantial evidence that transmetalation may instead proceed through a transient Pd alkoxide complex formed from nucleophilic substitution of the alkoxide base at palladium following oxidative addition. Alternatively, as

Figure 2. Plots of ln[Pd(CNArDipp2)2] vs time, showing the rate inhibition imposed by added equivalents of CNArDipp2 in the oxidative addition of MesBr.

Table 3. Observed Rate Constants in the Oxidative Addition of MesBr by 1 under Pseudo-First-Order Conditions (10 equiv of MesBr) as a Function of Added Equivalents of CNArDipp2 equiv CNArDipp2 0 0.10 0.25 0.50 1.0

kobs (s−1) 1.47(2) 5.59(4) 7.24(5) 2.49(4) 2.52(2)

× × × × ×

10−3 10−4 10−5 10−5 10−5

approx. rxn time (h) 1 3 10 >12 >12

Table 3). Plotting ln[1] versus time reveals a linear relationship well past three half-lives, in accord with pseudo-first-order conditions. Strikingly, the introduction of even small amounts of exogenous CNArDipp2 results in a profound diminution of reaction rate. As shown in Figure 2 and Table 3, the addition of 0.1 equiv of CNArDipp2 (with respect to 1) is sufficient to D


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Organometallics Scheme 1. Syntheses of PdCl(η3-C3H5)(CNArDipp2) (3) and Dinuclear Complexes 4−7

Pd hydroxo complexes have been shown to undergo rapid transmetalation with arylboronic acids,80 the intermediacy of Pd(OH)(Ar)(CNArDipp2)2 formed due to adventitious water cannot be discounted. Synthesis and Activation of Complexes Bearing a Single CNArDipp2 Ligand per Palladium Center. Guided by reports of highly competent cross-coupling catalyst precursors bearing a Pd/L (L = PR3, NHC) ratio of 1:1,20−23,28 we targeted a monoisocyanide PdII complex which could undergo reduction to a catalytically active Pd0L species under Suzuki− Miyaura coupling conditions. In particular, we took note of work by Nolan, who showed that complexes of the type PdCl(η3-allyl) (NHC) were highly active cross-coupling catalysts.20,21,81−83 Notably, activation of these mono-NHC PdII complexes could be achieved using NaOt-Bu in 2propanol, 20,21,82 which is the same base and solvent combination utilized in the optimized coupling conditions employed for Pd(CNArDipp2)2 (1). Toward this end, the monoisocyanide complex PdCl(η3-C3H5)(CNArDipp2) (3) was synthesized from [Pd(μ-Cl)(η3-C3H5)]2 and CNArDipp2 (2 equiv) and could be isolated as colorless crystals upon crystallization from a THF/n-pentane solution at −35 °C (Scheme 1). The FTIR spectrum of 3 displays a υ(CN) stretch at 2172 cm−1, which is higher in energy than that of free CNArDipp2 (2118 cm−1)57 and indicates that the isocyanide ligand in 3 functions primarily as a σ-donor. Accordingly, the Pd−CCNR distance in the solid-state structure of 3 (1.988(2) Å, Figure 4) is longer than that observed for 1 (mean = 1.930(3) Å),53 consistent with minimal Pd-to-isocyanide π-backdonation. Screening of the catalytic competency of PdCl(η3-C3H5) (CNArDipp2) (3) under Suzuki−Miyaura coupling conditions was subsequently undertaken using the substrates m-XylBr and PhB(OH)2. After 8 h at room temperature, only 5(1)% of the desired biaryl coupling product had been produced as assessed by GC-MS (Table 4; 5 runs), indicating inefficient activation of precatalyst 3. It is noted that mono-NHC complexes bearing unsubstituted allyl ligands can be sluggish toward activation at room temperature, often requiring high temperatures to yield the desired Pd0L active catalyst.20,83 However, aliquots removed from attempted coupling reactions employing 3 after 1 h do not reveal 1H NMR resonances corresponding to the intact

Figure 4. Molecular structure of PdCl(η3-C3H5)(CNArDipp2) (3).

Table 4. Yields of Biaryl Product in Suzuki−Miyaura Test Reactions Using the Indicated Catalysts and Loadingsa

Pd catalyst

GC yield

Pd(CNArDipp2)2 (1, 1 mol %) Pd(CNArDipp2)2 (1, 0.1 mol %) Pd(CNArDipp2)2 (1, 0.01 mol %) PdCl(η3-C3H5)(CNArDipp2) (3, 1 mol %) (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2 (4, 0.5 mol %) Pd3(η2-Dipp-μ-CNArDiPP2)3 (8, 0.33 mol %)

98(2)% 13(1)% trace 5(1)% 25(5)% 83(8)%

a

All reported yields represent the mean value of at least three independent runs.

precatalyst and instead indicated the formation of a new species that is slow to effect cross-coupling. Stoichiometric reactivity studies were undertaken in order to assess the activation pathway of PdCl(η3-C3H5)(CNArDipp2) (3) under cross-coupling conditions. Addition of NaOt-Bu (1.0 equiv) to a suspension of 3 in 2-propanol proceeds initially to E


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Organometallics

Figure 5. Molecular structures of dinuclear complexes 5, 6, and 7.

mononuclear complex 3 as a starting material, thus lending credence to its proposed constitution. Furthermore, the bridging tert-butoxide complex (μ-C 3 H 5 )(μ-Ot-Bu)[Pd(CNArDipp2)]2 (7) could be accessed upon addition of NaOtBu to a solution of 6 in Et2O (Scheme 1 and Figure 5). As anticipated, the 1H NMR spectrum of 7 displays a singlet at 1.16 ppm integrating to 9 protons and is devoid of the peaks assigned to the OiPr ligand in 4. While 4 and 7 otherwise give rise to similar 1H NMR and IR spectroscopic features, 7 exhibits superior kinetic stability and does not undergo noticeable decomposition over the course of days in Et2O solution at 20 °C. With the formulation of 4 substantiated, its mechanism of formation from PdCl(η3-C3H5)(CNArDipp2) (3) and NaOiPr was examined under conditions where all organic byproducts would be soluble. Accordingly, analysis of the reaction between 3 and 1.0 equiv of NaOiPr in C6D6 (20 °C) after 1 h by 1H NMR spectroscopy and GC-MS revealed formation of acetone, propene, and the chloride-bridged dimer (μ-C3H5)(μ-Cl)[Pd(CNArDipp2)]2 (6), with the latter subsequently undergoing conversion to 4 at longer reaction times (Scheme 2). The

the new complex 4, which contains a magnetically desymmetrized −ArDipp2 environment in its 1H NMR spectrum (Scheme 1). This transformation proceeds to completion over approximately 2 h at room temperature, although the thermal instability of 4 (vida inf ra) rendered isolated yields of pure material consistently low. Yellow single crystals of 4 could be grown from THF at −35 °C. Although unambiguous crystallographic characterization was precluded by severe positional disorder in the solid state, the data were consistent with a dinuclear Pd complex with each Pd center bearing one CNArDipp2 ligand. The 1H NMR spectrum of 4 possessed resonances consistent with the presence of an allyl group and was devoid of an intense singlet expected for incorporation of a tert-butoxide moiety. Interestingly, however, the appearance of a septet at 4.27 ppm integrating to one hydrogen suggested the presence of an isopropoxide group. These observations are consistent with the assignment of 4 as the dinuclear PdI species (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2. This finding is in accord with the known propensity of precatalysts of the type PdCl(η3C3H5)L (L = phosphine, NHC) to form dinuclear PdI complexes upon activation with base,84−91 although 4 is the first, to our knowledge, which incorporates an alkoxide bridging ligand. The isopropoxide ostensibly originates from solvent leveling of the Brønsted base NaOt-Bu, suggesting that OiPr− is the operative base under cross-coupling conditions. Further evidence for the identity of 4 was garnered by synthetic elaboration. Addition of lithium diisopropylamide (LDA) to a thawing Et2O solution of 4 proceeds via substitution of the isopropoxide ligand to give the bridging amide complex (μ-C3 H5)(μ-NiPr2)[Pd(CNArDipp2)]2 (5, Scheme 1). Structural characterization revealed retention of the dinuclear [Pd(CNArDipp2)]2 core, with allyl and diisopropylamido ligands that symmetrically bridge the two Pd centers (Figure 5). In order to exclude the presence of a bridging chloride ligand in 4, the dinuclear complex (μ-C3H5)(μCl)[Pd(CNArDipp2)]2 (6) was independently synthesized through comproportionation of Pd(CNArDipp2)2 (1) with [Pd(μ-Cl)(η3-C3H5)]2 (0.5 equiv, Scheme 1 and Figure 5). Complex 6 gives rise to 1H NMR and IR spectroscopic features which are distinct from those of 4, while the only 1H NMR peaks not arising from the −ArDipp2 groups are due to the allyl ligand. Despite positional disorder of the bridging ligands of 6 imposed by crystallographic symmetry, the data could be successfully modeled with 50:50 two-site disorder of both the allyl and chloride ligands. Importantly, reaction of 6 with NaOiPr in n-pentane/diethyl ether (ca. 5:1) results in elimination of NaCl and formation of (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2 (4, Scheme 1). The spectroscopic features of 4 generated by this route are identical to those observed using

Scheme 2. Proposed Mechanism for the Formation of 4 from 3 and in-Situ-Produced NaOiPr

generation of these products is consistent with a pathway proceeding through β-hydride elimination92,93 of acetone from unobserved Pd(OiPr)(η3-C3H5)(CNArDipp2), which is reasonably generated by alkoxide-for-halide substitution at 3. Reductive elimination of propene from the resulting allylhydride intermediate produces monoligated Pd(CNArDipp2), which can comproportionate with a second equivalent of 3 to F


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Organometallics

is thermally unstable and decomposes in solution over the course of ca. 12 h at room temperature. This behavior differs markedly from the other dinuclear complexes (μ-C3H5)(μN i Pr 2 )[Pd(CNAr D i p p 2 )] 2 (5), (μ-C 3 H 5 )(μ-Cl)[Pd(CNArDipp2)]2 (6), and (μ-C3H5)(μ-Ot-Bu)[Pd(CNArDipp2)]2 (7), which do not undergo noticeable decomposition over this time period. Analysis of the decomposition mixture of 4 by 1H NMR spectroscopy and GC-MS indicated the clean formation of acetone, propene, and a new isocyanide containing species. This complex could be isolated as red crystals upon cooling of a THF solution to −35 °C and was determined to be the trinuclear, tris-isocyanide complex triangulo-Pd3(η2-Dipp-μCNArDipp2)3 (8) by X-ray crystallography (Figure 7). Most importantly, 8 reasonably results from trimerization of the fleeting monoligated species Pd(CNArDipp2), which accounts for the low cross-coupling activity observed when using dimer 4 as a precursor (Scheme 3). In support of this notion, 8 can be

afford 6 (Scheme 2). Nucleophilic substitution of the bridging chloride in 6 with isopropoxide then yields 4 at later stages of reaction. This final step is notable in that the dinuclear Pd(I) complexes (μ-C3H5)(μ-Cl)[PdL]2 (L = phosphine, NHC) produced upon activation of PdCl(η3-C3H5)L are most often generated in the presence of a strong alkoxide base,78−85 yet have not been reported to undergo substitution of the chloride ligand. Allyl isopropyl ether was not detected in the reaction between 3 and NaOiPr, illustrating that neither attack of alkoxide at the allyl ligand nor C−O bond reductive elimination from the putative Pd(OiPr)(η3-C3H5)(CNArDipp2) intermediate are competing pathways in formation of monoligated Pd(CNArDipp2) in this sequence. The inability to directly detect Pd(O i Pr)(η 3 -C 3 H 5 )(CNArDipp2) during the formation of chloride-bridged dimer 6 implies that the rates of β-hydride elimination, C−H reductive elimination, and comproportionation shown in Scheme 2 are faster than that of nucleophilic substitution of 3 by [OiPr]−. We believe that the relatively slow rate of this first step is one of two significant contributing factors to the poor catalytic performance observed when 3 is used as a precursor. In this respect, it is important to note that Hazari and Colacot have demonstrated that monomeric Pd(allyl)(Cl)L complexes (L = NHC, PR3) are activated with strong bases to form monoligated Pd0L intermeditates. However, these intermediate species also rapidly comproportionate with Pd(allyl)(Cl)L catalyst precursors to form dimeric (μ-C3H5)(μ-Cl)[Pd(L)]2 complexes in a deleterious off-cycle reaction that competes with substrate oxidative addition.86−89 Nevertheless, these dinuclear PdI complexes undergo disproportionation at room temperature to regenerate monoligated Pd0L (0.5 equiv of per dinuclear PdI complex), which can subsequently enter the productive catalytic cycle. In contrast, attempted coupling of m-XylBr and phenylboronic acid using pure 4 as a catalyst precursor gave only poor yields (25(5)%; 7 runs) of biaryl product at room temperature (Table 4). This finding indicates that a significant barrier to disproportionation exists for 4, a notion which we ascribe to the differing electronic profiles of NHCs/phosphines versus those of isocyanides. As NHCs and phosphines display σ-donor capabilities superior to those of isocyanides, the Pd centers in 4 are likely to be more Lewis acidic compared to those in (μC3H5)(μ-Cl)[Pd(L)]2 complexes (L = NHC, PR3), effectively discouraging the Pd/bridging-ligand bond breaking which accompanies disproportionation. In addition, the π-acidic character of the isocyanides should serve to stabilize the formally PdI oxidation state, thereby further disfavoring disproportionation relative to PdI NHC/PR3 systems.88 Similar effects were noted by Hazari in (μ-2-R-allyl)(μ-Cl)[Pd(NHC)]2 complexes, where installation of electron withdrawing groups on the allyl ligands resulted in stabilization of the dinuclear PdI complexes due to increased Pd-to-allyl πbackdonation and biased the corresponding divalent PdCl(η3-2R-allyl) (NHC) precursor systems toward deactivation through comproportionation.89 Accordingly, we believe this increased stability of isocyanide-containing PdI also contributes to the low catalytic performance originating from precursor 3. Isolation and Mechanism of Formation of a Trimeric Pd3 Cluster from Self-Aggregation of a Pd0L Intermediate. While pure (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2 (4) does not serve as an efficient catalyst precursor at reasonable reaction times, it does in fact provide a species capable of marginal cross-coupling catalysis (Table 4). As noted above, 4

Scheme 3. Generalized Disproportionation Process for a Complex of the Type (μ-C3H5)(μ-X)[Pd(CNArDipp2)]2 and Proposed Mechanism for the Formation of 8 from 4a

a

Mechanism for formation of 8 from 4 illustrates how the isopropoxide β-hydride elimination pathway allows for the production of 2 equiv of monoligated Pd(CNArDipp2) despite the energetic favorability of comproportionation.

accessed directly by addition of Li[HBEt3] to PdCl(η3C3H5)(CNArDipp2) (3) at low temperature, which results in elimination of propene and LiCl and provides further evidence for self-aggregation of Pd(CNArDipp2) to furnish trimeric 8. Notably, this intermolecular self-aggregation using π-acidic ligands is a complementary trapping strategy for “Pd0L” intermediates, which are most often intercepted in solution by addition of exogenous L-type donor ligands.11,71,81,94,95 As often implicated for Buchwald’s biaryl phosphines, the isolation of 8 demonstrates that the m-terphenyl group can play a complementary role in forming secondary interactions with the unsaturated Pd0 centers, thus protecting against catalyst decomposition and precipitation of Pd black.15,24−27,33,34 Although each of the flanking Dipp rings is magnetically equivalent by 1H NMR spectroscopy at 20 °C, examination of the solid-state structure reveals that they assist in the stabilization of 8 by engaging in η2-C,C interactions with the G


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Organometallics Pd0 centers. The arene rings in 8 appear to act primarily as σdonors, as no significant degree of dearomatization resulting from Pd-to-arene π-back-donation is seen. This is consistent with the lability of the Pd−arene interaction that is reflected in 1 H and 13C{1H} NMR spectra, which for 8 are symmetric and do not contain upfield-shifted arene peaks. While this structural feature is undoubtedly critical in facilitating the aggregation of Pd(CNArDipp2), it should be emphasized that the strong πacidity properties of the ancillary ligand are likely also vital in the stabilization of these highly reactive Pd0 centers, rendering this system an intriguing counterpoint to monoligated Pd0 intermediates supported by biaryl phosphines. With respect to the formation of trimer 8 from pure 4, kinetic analysis by 1H NMR spectroscopy at 45 °C in toluened8 shows that this is a first-order process, as plots of ln[4] versus time are linear past three half-lives, and kinetic runs using 7 or 14 mM solutions of 4 yield rate constants that are identical within error (Figure 6). Importantly, the formation of

Figure 7. Molecular structure of Pd3(η2-Dipp-μ-CNArDipp2)3 (8). Note that all Pd centers and isocyanide ligands are equivalent by crystallographic symmetry. Selected bond distances (Å): Pd−Pd = 2.6353(5); Pd−C1 = 2.102(6); Pd1−C1′ = 2.103(4); Pd1−C15 = 2.479(4); Pd1−C16 = 2.312(4); C15−C16 = 1.411(7); C16−C17 = 1.413(7); C17−C18 = 1.372(7); C18−C19 = 1.402(7); C19−C14 = 1.393(7); C14−C15 = 1.423(6).

complexes (L = phosphine, NHC), which produces 1 equiv of Pd 0 L as well as PdCl(η 3 -C 3 H 5)L, which must be subsequently activated. In the case of 4, formation of 2 equiv of Pd(CNArDipp2) is made possible due to the aptitude of one of the bridging ligands to undergo β-hydride elimination, thereby effecting a net two-electron reduction of the [PdI]2 core. This further punctuates the apparent inertness of the bridging chloride unit in phosphine and NHC (μ-C3H5)(μCl)[PdL]2 complexes toward nucleophilic substitution with alkoxide base.84−89 However, the development of strategies or conditions that foster alkoxide substitution in these species may also enable secondary elimination pathways to become available, thereby allowing dinuclear PdI complexes to rapidly re-enter the cross-coupling catalytic cycle as Pd0L without concomitant production of PdII species. Despite the labile Pd-arene interactions, trinuclear 8 retains its formulation in solution. Electrospray ionization-mass spectrometry (ESI-MS) analysis of acetone solutions shows a molecular ion peak (M+) of the expected mass and isotopic distribution for intact 8 (Figures S1−2). Furthermore, 8 can undergo addition reactions with neutral ligands while retaining a trinuclear formulation (Scheme 4). Addition of CO (3.0 equ i v) r es u l t s in for m at ion of triangulo -Pd 3 (μ CO)3(CNArDipp2)3 (9), which can be isolated as yellow crystals from a diethyl ether solution at −35 °C. Crystallographic characterization (Figure S40) reveals that 9 is isostructural to its triplatinum congener triangulo-Pt3(μ-CO)3(CNArDipp2)3,98 and it correspondingly displays only one υ(CN) (2107 cm−1) and υ(CO) (1871 cm−1) stretch in its IR spectrum consistent with D3h symmetry. In addition, 8 reacts with 3 equiv of nitrosobenzene to yield the known complex Pd3(μ−η2:η1PhNO)3(CNArDipp2)3 (10), which was initially isolated upon reaction of nitrosobenzene with Pd(η 2 -N,O-PhNO)(CNArDipp2)2.71 Importantly, this process substantiates the ability of 8 to mimic the reactivity of “Pd(CNArDipp2)”, as 10

Figure 6. Plot of ln[4] vs time for 7 and 14 mM solutions showing that decomposition is first-order in (μ-C 3 H 5 )(μ-O i Pr)[Pd(CNArDipp2)]2 (4).

both acetone and propene is observed in these solutions by 1H NMR spectroscopy and GC-MS, while no allyl isopropyl ether is detected. We believe that these observations are most reasonably accounted for by β-hydride elimination from transient Pd(OiPr)(η3-C3H5)(CNArDipp2), which can be generated by disproportionation of (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2 (4). As discussed above, the proclivity of dinuclear complexes 4−7 to undergo disproportionation is believed to be significantly attenuated due to the π-acidic properties of the isocyanide ligands. However, fast β-hydride elimination from Pd(OiPr)(η3-C3H5)(CNArDipp2) can ostensibly compete with comproportionation, providing a pathway by which monoligated Pd(CNArDipp2) can be rapidly produced at the expense of the PdII comproportionation partner. With regard to this proposed mechanism, it is also important to note that direct β-hydride elimination from (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2 (4),96 followed by fast C−H reductive elimination from a dinuclear PdI bridging-allyl/bridging-hydride complex97 is also consistent with our kinetic data and the observed organic byproducts and therefore cannot be ruled out. Nevertheless, the unimolecular decomposition of 4 is especially remarkable in that it yields 2 equiv of catalytically active Pd(CNArDipp2). This is in contrast to the typical disproportionation process seen for (μ-C3H5)(μ-Cl)[PdL]2 H


Article

Organometallics

phosphine and NHC ligands commonly employed in Pdcatalyzed cross-coupling and their significantly greater π-acidity properties. As kinetic studies illustrated that oxidative addition of aryl bromides occurs at the monoisocyanide Pd(CNArDipp2), synthetic studies targeting competent catalysts bearing a Pd/ isocyanide ratio of 1:1 were undertaken. It was shown that activation of the monoisocyanide PdCl(η3-C3H5)(CNArDipp2) (3) with alkoxide base first produces the dinuclear Pd(I) complex (μ-C3H5)(μ-OiPr)[Pd(CNArDipp2)]2 (4), which subsequently undergoes slow unimolecular conversion to give monocoordinate Pd(CNArDipp2). The formation of 4 occurs through nucleophilic substitution of (μ-C3H5)(μ-Cl)[Pd(CNArDipp2)]2 (6) with isopropoxide, a process which apparently does not occur for the dinuclear Pd(I) species (μC3H5)(μ-Cl)[PdL]2 (L = phosphine, NHC). β-Hydride elimination from 4 then allows for the production of 2 equiv of catalytically active Pd(CNArDipp2), as opposed to the established disproportionation mechanism for (μ-C3H5)(μCl)[PdL]2, which yields Pd0L and PdIICl(η3-C3H5)L species (L = phosphine, NHC). Most notably, intermolecular trapping of Pd(CNArDipp2) produces the isolable tripalladium cluster triangulo-Pd3(η2-Dipp-μ-CNArDipp2)3 (8). However, despite its trinuclear formulation, 8 functions as a competent crosscoupling precatalyst and acts as a source of the highly reactive monocoordinate Pd(CNArDipp2) fragment in solution. The two primary factors which render 8 isolable are the coordinating abilities of the flanking rings of the m-terphenyl groups and the π-accepting capabilities of the isocyano units, neither of which have received significant attention in cross-coupling catalysis. We envision that these findings will spur interest in the incorporation of ligands bearing these structural and electronic features into other Pd-catalyzed cross-coupling protocols. In addition, these results suggest that the use of strongly π-acidic ancillary ligands can play a complementary role to existing cross-coupling schemes, especially when long catalyst lifetimes or the retention of low-molecular-weight aggregated Pd0 species in solution are desired.

Scheme 4. Reactivity of 8 with CO and Nitrosobenzene

was originally proposed to be generated upon trapping of a monoligated Pd(CNArDipp2) complex with nitrosobenzene followed by trimerization.71 Despite the propensity of 8 to yield new cluster compounds in stoichiometric reactivity studies, it can still function as a competent cross-coupling catalyst. Using the optimized roomtemperature catalytic conditions employed for Pd(CNArDipp2)2 (1) and a loading of 0.33 mol % (1 mol % Pd), trimer 8 can couple m-XylBr and phenylboronic acid to generate the biaryl product in 83(8)% GC yield (Table 4; 4 runs). This yield is marginally lower than that obtained in the coupling of these substrates using 1 mol % 1, which we tentatively ascribe to a rate-limiting cluster fragmentation step. However, the catalytic activity of 8 is far superior to that of PdCl(η3-C3H5)(CNArDipp2) (3), suggesting that 8 is capable of yielding equivalents of “Pd(CNArDipp2)” which can rapidly enter the cross-coupling catalytic cycle. This behavior is reminiscent of that exhibited by the tripalladium isocyanide clusters Pd3(μCNR)3(CNR)3, which in stoichiometric reactivity studies often function as formal equivalents of Pd(CNR)2.66−69 It should be noted that although 8 is relatively stable over the time frame of the cross-coupling reactions it does undergo slow decomposition in solution to give Pd(CNArDipp2)2 (1) and Pd black over the course of approximately 1 week. However, control experiments indicate that low catalyst loadings of 1 (0.1 and 0.01 mol %) give only minimal yields of biaryl product (13(1)% and trace quantities, respectively) in the coupling of m-XylBr and PhB(OH)2 (Table 4), illustrating that in situ produced 1 does not significantly contribute to the catalytic turnover observed in runs using trimeric 8 as the precatalyst.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00035. Synthetic and catalytic procedures, spectroscopic, characterization and kinetic data, results of computational studies (PDF) Cartesian coordinates (XYZ) Crystallographic structure determinations of 2·0.5Et2O, 3·0.5Et2O, 5, 6, 7·C6H5F, 8·13THF, and 9·0.5Et2O (CIF)

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CONCLUSIONS In this report, we have demonstrated that well-defined and isolable Pd complexes supported solely by m-terphenyl isocyanides can catalyze the Suzuki−Miyaura cross-coupling reaction. The bis-isocyanide Pd(CNArDipp2)2 (1) was shown to couple a variety of unactivated aryl bromides, including those bearing mono- or di-ortho substitution, with arylboronic acids under mild conditions. Aryl chlorides were shown to be generally unreactive toward 1, presumably due to both the decreased σ-donor abilities of isocyanides compared to the

AUTHOR INFORMATION

Corresponding Author

*jsfi[email protected]. ORCID

Joshua S. Figueroa: 0000-0003-2099-5984 Author Contributions ‡

B.R.B., L.A.L., and J.M.S. contributed equally to the creation of this work. Notes

The authors declare no competing financial interest. I


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ACKNOWLEDGMENTS We are grateful to the U.S. National Science Foundation for support (CHE-1464978 and Graduate Research Fellowships to B.R.B. and L.A.L). J.S.F is a Camille Dreyfus Teacher−Scholar (2012−2017).

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