Influence of the Leaving Group on C–H Activation Pathways in

Jun 26, 2018 - A series of palladium pincer complexes supported by (iPr2P-C6H4)2CH2 (PC(sp3)H2P = bis(2-(diisopropylphosphanyl)phenyl)methane) was ...
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Influence of the Leaving Group on C−H Activation Pathways in Palladium Pincer Complexes Melissa R. Hoffbauer, Cezar C. Comanescu, Brittany J. Dymm, and Vlad M. Iluc* Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States

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ABSTRACT: A series of palladium pincer complexes supported by (iPr2P-C6H4)2CH2 (PC(sp3)H2P = bis(2(diisopropylphosphanyl)phenyl)methane) was isolated and characterized. Different modes of C−H activation were observed for [{PC(sp3)H2P}PdX2] (X = Cl, OTf, Me, OAc). The thermal treatment of [{PC(sp3)H2P}PdX2] (X = Cl, OTf) induced a backbone C−H bond activation to generate the respective [{PC(sp3)HP}PdX] complex, while under similar reaction conditions [{PC(sp3)H2P}PdMe2] underwent a C−C reductive elimination that led to the formation of a Pd(0) dimer, [{PC(sp3)H2P}Pd]2. For the analogous diacetate complex [{PC(sp3)H2P}Pd(OAc)2], the C−H activation occurred at the phosphine isopropyl methine group to generate a palladacycle.

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INTRODUCTION Pincer ligands of the general formula YCY (Y = S, N, P, etc.) are typically monoanionic, six-electron-donor ligands (Figure 1), which have been extensively used in catalysis and materials

localized at the backbone carbon were also reported by our group,31 and their reactivity was studied.32−38 In some cases, the predicted C−H bond activation does not occur during the generation of pincer metal complexes; rather, an intramolecular activation of the isopropyl methyl group of the phosphine donor may occur.39 Furthermore, isopropyl methine activation is not common. C−H activations at this position are reported to undergo a [1,5]-H or [1,4]-H shift40−42 or deprotonation;43 examples of agostic interactions involving a C−H bond are also known.44,45 However, activation of a methine C−H bond of isopropyl groups is rare and is not known for pincer complexes. Although examples of four-membered rings are relatively common,46,47 three-membered metallacycles are scarce. Two such examples were reported:48,49 only one contained crystallographic data.48 Protasiewicz and co-workers showed that the reaction of trans-[(R3P)2Pd(OAc)2] (R = isopropyl) with [Me2(H)NPh][B(C6F5)4] in CH2Cl2 led to a cationic palladium acetate complex, [(R3P)2Pd(κ2-O,O′-OAc)][B(C6F5)4], featuring a chelating acetate, which underwent cyclopalladation upon treatment with excess Na2CO3 to yield a three-membered palladacycle, which was isolated as a pyridine adduct and fully characterized (Figure 2).48 Equilibrium mixtures of the acetate complexes and the C−H activation products were obtained upon dissolving [(R3P)2Pd(κ2-O,O′-OAc)][B(C6F5)4] in CH3CN, pyridine, or 4-tertbutylpyridine (Figure 2).49 The widespread use of the palladium acetate starting material in catalytic C−H bond

Figure 1. Examples of palladium pincer metallacycles (X = halide, OTf, OAc; R = alkyl).

science.1−13 While many studies focus on metal complexes of the 2,6-disubstituted arene skeleton PC(sp2)P,14−17 aliphatic backbones of the type PC(sp3)HP have also been used (Figure 1),18−23 leading to unique reactivity and enhanced catalytic activity in C−C bond formation reactions.24 Synthetic routes to the corresponding metal complexes include C−H activation, oxidative addition, trans-cyclopalladation, and transmetalation.25 In general, the activation of the backbone C−H bond occurs to produce the monoanionic ligand,26,27 but under certain conditions multiple C−H activations are also possible: a report by Piers et al. describes the synthesis of a nickel complex supported by (iPr2P-C6H4)2CH (PC(sp3)HP = bis(2(diisopropylphosphanyl)phenyl)methyl)28,29 that underwent dehydrohalogenation by K[N(TMS)2] in the presence of PPh3 to yield a carbene complex, (PCcarbeneP)Ni(PPh3).30 Analogous Pd(II) carbenes exhibiting nucleophilic character © XXXX American Chemical Society

Received: April 17, 2018

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To probe OTf as a leaving group, [{PC(sp3)H2P}Pd(OTf)2] (3) was synthesized through the treatment of 1 with 2 equiv of AgOTf (Scheme 1). Crystallization by slow vapor diffusion of n-pentane into a CH2Cl2 solution afforded single crystals of 3 (Figure 3). The diphosphine ligand maintains a cis Figure 2. Synthetic routes to three-membered metallacycles.48

functionalization has facilitated discussion surrounding the role of the acetate molecule throughout these processes.50−63 Our group observed the C−H activation of cis-[{PC(sp3)H2P}PdCl2] upon a thermal treatment to extrude HCl and generate the desired pincer complex.31 Though the transformation was smooth, we became interested in the leaving group’s effect on C−H bond activation. Herein, we report the synthesis of a series of [{PC(sp3)H2P}PdX2] complexes (X = OTf, OAc, Me). The thermal treatment of [{PC(sp3)H2P}PdX2] (X = OTf, Me) leads to [{PCsp3HP}Pd(OTf)] through a backbone C−H activation (X = OTf) or to a previously reported C−C reductively eliminated species (X = Me), [{PC(sp3)H2P}Pd]2.64 Interestingly, when X = OAc, [{PC(sp3)H2P}Pd(OAc)2], we observe a surprising and rare case of C−H activation of the isopropyl methine group, CH(CH3)2, on the phosphine arm, yielding [{PC(sp3)H2P}actPd(OAc)], which features a three-membered palladacycle. A series of kinetic experiments were conducted to elucidate the rate and process by which these activations proceed.

Figure 3. Thermal ellipsoid (50% probability level) representation of [{PC(sp3)HP}Pd(OTf)2] (3). Most hydrogen atoms, the solvent molecule, and the isopropyl groups are omitted for clarity. The isopropyl substituents are depicted in wireframe. Selected distances (Å) and angles (deg): Pd−P(1) = 2.2605(12), Pd−P(2) = 2.2760(12), Pd−O(51) = 2.142(3), Pd−O(61) = 2.151(3), O(51)−Pd−P(1) = 89.99(10), O(51)−Pd−O(61) = 86.05(14), O(61)−Pd−P(2) = 86.29(10), P(1)−Pd−P(2) = 97.73(4).

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coordination, inherited from the dichloride complex 1, with a square-planar geometry at the metal center and the sum of angles around palladium of 360.06(3)°. The additional steric bulk of the triflate ligands reduces the P−Pd−P angle from 100.02(5)° in 1 to 97.73(4)°. Other metrical parameters are as expected. For instance, the average Pd−P distance of 2.2680(12) Å is typical for palladium diphosphine complexes, and the average Pd−O distance of 2.147(3) Å lies within the expected range for Pd−O(triflate) bonds in related structures.65,66 The solid-state molecular structure is consistent with the solution structure determined by NMR spectroscopy. The backbone protons exhibit an anagostic interaction,67−69 as was reported for 1,64 with the endo proton resonating at 6.94 ppm in the 1H NMR spectrum as a doublet of triplets (2JH−H = 14.9 Hz, 4JH−P = 1.9 Hz) and the exo proton at 4.22 ppm as a doublet ( 2 JH−H = 14.9 Hz), showing no phosphorus coupling.70−72 The corresponding 31P NMR spectrum exhibits a single broad resonance at 54.53 ppm, indicative of equivalent phosphorus environments and a dynamic behavior in solution. The 19F NMR spectrum confirms the equivalence of the six fluorine atoms of the triflate groups and shows a sharp singlet at −80.57 ppm. The 13C{1H} NMR spectrum displays a triplet resonance at 43.11 ppm (3JC−P = 9.6 Hz), corresponding to the backbone carbon that experiences a long-range coupling to the equivalent 31P nuclei. Mild heating of 3 in CH2Cl2 induces a backbone C−H bond activation to generate the corresponding pincer, [{PC(sp3)HP}Pd(OTf)] (4). Interestingly, kinetic data obtained for the conversion of 3 to 4 are more complicated, perhaps due to a dissociative mechanism through which a triflate anion dissociates to generate a Pd(II) cationic solvated species. In a subsequent step, the triflate anion deprotonates the ligand backbone, thereby promoting cyclopalladation. The half-life at 55 °C in CDCl3 decreased significantly (5 min), which is

RESULTS AND DISCUSSION We reported the synthesis and characterization of cis[{PC(sp3)H2P}PdCl2] (1)64 and observed the C−H activation of the backbone methylene group upon thermal treatment, leading cleanly to [{PC(sp3)HP}PdCl] (2) and HCl evolution (Scheme 1). To measure the rate, the conversion of 1 to 2 in CDCl3 at 55 °C with a trimethoxybenzene standard was tracked. Kinetic data revealed that the reaction followed firstorder trends, indicating a concerted process (k = 0.011 min−1). Scheme 1. Synthesis of [{PC(sp3)HP}PdCl] (2), [{PC(sp3)H2P}Pd(OTf)2] (3), and [{PC(sp3)HP}Pd(OTf)] (4)

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ligand on the Cbackbone−Pd distance is small: 2.0569(15) Å in 4 vs 2.0738 (19) Å in 2. Monomeric and dimeric rhodium triflate complexes supported by this PC(sp3)P ligand or the tert-butyl analogue were previously reported.74 These complexes were synthesized either by ligand C−H activation from the [(COE)2Rh(OTf)]2 starting material or by ligand exchange using TMSOTf. The cis-dimethyl complex [{PC(sp3)H2P}PdMe2] (5) was obtained from the reaction of [{PC(sp3)H2P}PdCl2] (1) with 2 equiv of MeLi in Et2O at −78 °C (Scheme 2). The 1H NMR spectrum of 5 revealed that the backbone methylene protons experience different chemical environments, suggestive of an anagostic interaction between one of the backbone protons and the metal center; the exo proton displays a doublet at 3.50 ppm (2JHH = 14.4 Hz), while the endo proton resonates as a doublet of triplets at 7.15 ppm (2JHH = 14.0 Hz, 4JHP = 3.4 Hz). Two methine peaks are identified at 2.73 ppm as a multiplet, while the other two appear as a broad multiplet in the 1.55− 1.36 ppm region. The 31P NMR spectrum revealed a single resonance at 23.30 ppm, confirming the chemical equivalency of the two phosphorus nuclei. Additionally, the 13C{1H} NMR spectrum displays a triplet for the backbone carbon at 43.23 (3JCP = 12.3 Hz). The solid-state molecular structure of 5 (Figure 5) shows that the metal center displays a square-planar geometry, with the sum of angles around palladium of 360.86(6)°. The two Pd−C distances are similar, averaging 2.0916 Å. The Pd−P distances, averaging 2.3792 Å, are almost equal and are remarkably longer than those in 1, 2, or similar compounds previously reported by our group.64 The P−Pd−P angle of 102.829(13)° is also larger than the corresponding value in 1 (100.02(5)°). Due to its thermal instability, 5 undergoes a C−C reductive elimination at ambient temperature to form a previously characterized Pd(0) dimer, [{PC(sp3)H2P}Pd]2 (6), and ethane.32,64 The C−H activated complex could not be formed via manipulation of thermal parameters. However, through photolysis of 5 (UV radiation, 245 nm) a mixture of 6 and the desired monomethyl complex [{PC(sp3)HP}PdMe] (7) was observed. The latter was formed by C−H bond activation with subsequent loss of CH4.

relatively unsurprising due to the enhanced leaving group capabilities exhibited by the triflate moiety. Compound 4 can also be obtained by a salt metathesis reaction between [{PC(sp3)HP}PdCl] (2) with 1 equiv of AgOTf (Scheme 1), a process similar to the synthesis of the nickel analogue [{PC(sp3)HP}Ni(OTf)] from the corresponding nickel bromide [{PC(sp3)HP}NiBr].73 The 31P NMR spectrum indicates that the two 31P nuclei are equivalent and are represented by a sharp singlet at 51.81 ppm. The backbone proton attached to the corresponding carbon resonates as a singlet at 6.29 ppm in the 1H NMR spectrum, similar to the case for the chloride derivative 2. Due to the rigidity of the pincer framework and the relative orthogonal orientation of the backbone C−H vector to the P−C−P plane, no H−P coupling is observed throughout the series of [{PC(sp3)HP}PdX] complexes reported herein. The 13C{1H} NMR spectrum displays a triplet resonance for the cyclometalated carbon at 47.82 ppm (2JC−P = 3.5 Hz). The solid-state molecular structure (Figure 4) confirms the NMR assignments. The influence of the OTf

Figure 4. Thermal ellipsoid (50% probability level) representation of [{PC(sp3)HP}PdOTf] (4). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pd−C = 2.0569(15), Pd−O(31) = 2.1986(10), Pd−P(1) = 2.3346(4), Pd−P(2) = 2.2962(4), C−C(21) = 1.524(2), C−C(11) = 1.527(2), P(2)−Pd− P(1) = 158.612(15), C−Pd−O(31) = 175.49(6), C−Pd−P(2) = 84.42(4) C−Pd−P(1) = 82.26(4), O(31)−Pd−P(2) = 97.99(3), O(31)−Pd−P(1) = 96.49(3).

Scheme 2. Synthesis and Decomposition Studies of 5

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Figure 6. Thermal ellipsoid (50% probability level) representation of [{PC(sp3)HP}PdMe] (7). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pd−C(2) = 2.119(2), Pd−C(1) = 2.139(2), Pd−P(1) = 2.2899(6), Pd−P(2) = 2.2578(6), C(2)−Pd−C(1) = 176.13(10), P(2)−Pd−P(1) = 162.44(2), C(2)−Pd−P(2) = 94.27(7), C(2)−Pd−P(1) = 99.41(7), C(1)−Pd−P(2) = 84.79(7), C(1)−Pd−P(1) 82.25(7).

Figure 5. Thermal ellipsoid (50% probability level) representation of [{PC(sp3)H2P}PdMe2] (5). Most hydrogen atoms and the solvent molecule are omitted for clarity. The isopropyl substituents are depicted in wireframe. Selected distances (Å) and angles (deg): Pd− C(1) = 2.0883(14), Pd−C(2) = 2.0949(15), Pd−P(2) = 2.3732(4), Pd−P(1) = 2.3853(4), C(1)−Pd−C(2) = 82.17(7), C(1)−Pd−P(2) = 88.97(4), C(2)−Pd−P(1) = 86.89(5), P(2)−Pd−P(1) = 102.829(13).

Scheme 3. Synthesis of 9 and 10

To support its formation through the photolysis of 6, [{PC(sp3)HP}PdMe] (7) was also synthesized via an alternate route: i.e., salt metathesis from [{PC(sp3)HP}PdCl] (2) and MeLi (eq 1). Its 1H NMR spectrum features a triplet

resonance at 0.46 ppm (3JHP = 5.2 Hz) corresponding to the methyl group, while the backbone proton resonates at 5.51 ppm, significantly upfield in comparison to the corresponding value of the parent chloride complex 2 (6.23 ppm). The 13C NMR spectrum indicates that the methyl carbon appears as a triplet at −17.11 ppm (2JCP = 10.6 Hz), while the backbone carbon displays a singlet at 58.31 ppm. The solid-state molecular structure (Figure 6) reveals a square-planar geometry at the palladium center (sum of angles 360.72(28)°), with typical Pd−P and Pd−C distances. Though the synthesis of [{PC(sp3)H2P}Pd(OAc)2] (8) is analogous to that of [{PC(sp3)H2P}Pd(OTf)2] (3) and [{PC(sp3)H2P}PdMe2] (5),52 8 undergoes a more complicated C−H activation process. At 80 °C, 8 converted to a mixture of complexes, including trace amounts of the backbone C−H activation product [{PC(sp3)HP}Pd(OAc)] (9). Heating 8 at 60 °C in CDCl3 for 1 h (Scheme 3) leads to a quantitative conversion to a new asymmetric product, represented by cis coupling doublets at 29.36 and 11.32 ppm (JPP = 29.5 Hz). The P−P coupling constant in the 31P NMR spectrum is typical for a cis orientation of the diphosphine ligand and is not consistent with the formation of a pincer complex, which would exhibit only one signal; this hints at the C−H activation of the isopropyl phosphine arms. This hypothesis is corroborated by the presence of three multiplets

attributed to the C−H methine protons in the corresponding H NMR spectrum (2.47−2.40, 2.28, and 1.97−1.88 ppm) and the presence of both backbone protons resonating at 3.29 ppm (d, 2JHH = 14.3 Hz, exo backbone hydrogen) and 6.95 ppm (endo proton, ddd, 4JHP = 1.9 Hz; 4JHP = 5.3 Hz; 2JHH = 14.0 Hz, exhibiting coupling to the two inequivalent phosphorus nuclei and to the exo proton).75 The loss of AcOH is confirmed by the acidic proton observed in the crude reaction mixture resonating at 12.08 ppm in the 1H NMR spectrum; one of the methyls in the cyclometalated isopropyl can be identified as an isolated quartet upfield at −0.13 ppm (dd, JHP = 20.6 Hz, JHP = 7.1 Hz). The solid-state molecular structure confirmed the solution structure [iPr2PC(sp3)H2PiPrCMe2]Pd(OAc)], [{PC(sp3)H2P}actPd(OAc)] (10, Figure 7). The geometry around Pd(II) is distorted from square planar, with a sum of angles of 359.74(13)°, and individual angles that strongly deviate from 90°. While the O(1)−Pd−P(2) angle is 97.32(5)° and O(1)− Pd−C(31) measures 104.50(9)°, the three-membered palladacycle features an exceptionally acute C(31)−Pd−P(1) angle of 47.35(7)°, and the diphosphine backbone bends to accommodate a wide P(1)−Pd−P(2) angle of 110.96(3)°; this value is larger than the P−Pd−P angle of 101.06(3)° for 8 but lower than the largest P−Pd−P angle of 113.33(5)°, which was 1

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through a six-membered transition state allows an almost barrierless proton transfer.55 It is important to note that, although the deprotonation of aryl groups by acetate in palladium complexes has been reported widely,54 the only other example besides that reported herein of a phosphine isopropyl methine deprotonation is that described by Protasiewicz and co-workers.48 Synthesis of [{PC(sp3)HP}Pd(OAc)] (9) was achieved by a salt metathesis reaction between 2 and AgOAc (Scheme 3). Compound 9 was fully characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. The 1H NMR spectrum shows the backbone proton resonating at 6.08 ppm, as a sharp singlet, and the three equivalent protons from the acetate methyl group at 2.33 ppm. The iPr methyl protons appear as quartets in the expected alkyl region, 1.02−1.35 ppm, while the methine protons display multiplets at 2.44 (2H) and 2.29 ppm (2H). The phosphorus nuclei are equivalent, as confirmed by a sharp singlet at 49.68 ppm in the 31P NMR spectrum, with a slight shift upfield from the corresponding value (50.20 ppm) in the parent complex 2. In the 13C{1H} NMR spectrum, the cyclometalated carbon appears as a triplet at 46.48 ppm (3JC−P = 1.8 Hz), while the sp2 carbon of the acetate, CH3COO, resonates at 175.44 ppm as a singlet. The solid-state molecular structure (Figure 9) indicates that the

Figure 7. Thermal ellipsoid (50% probability level) representation of [{PC(sp3)H2P}actPd(OAc)] (10). Most hydrogen atoms and the solvent molecule are omitted for clarity. The isopropyl substituents are depicted in wireframe. Selected distances (Å) and angles (deg): Pd−O(1) = 2.1516(19), Pd−P(1) = 2.1645(7), Pd−P(2) = 2.3451(7), P(1)−C(31) = 1.749(3), Pd−C(31) = 2.191(3), O(1)− Pd−P(1) = 151.46(5), O(1)−Pd−C(31) = 104.50(9), P(1)−Pd− C(31) = 47.35(7), P(1)−Pd−P(2) = 110.96(3), C(31)−Pd−P(2) = 158.17(7), O(1)−Pd−P(2) = 97.32(5).

found in a trigonal-planar Pd(0) species, [{PC(sp3)H2P}Pd(dba)].64 As mentioned before, three-membered palladacycles are rare, and only one crystallographically confirmed example of C−H bond activation in a iPr3P group exists to date.48 However, a few three-membered metallacycles containing Pd, P, and C have been reported, with P−Pd−C angles in the range of 41.19−47.99°.76−81 Furthermore, the metrical parameters of [{PC(sp3)H2P}actPd(OAc)] are similar to those reported by Protasiewicz and co-workers for [(iPr3P)(iPr2CMe2)Pd(py)].48 We propose that the formation of 10 occurs because of the proximity of the acetate to the phosphine isopropyl methine proton. The noncoordinated oxygen atom on the acetate can reach the methine hydrogen, favoring deprotonation. The reaction takes place in a concerted fashion, as confirmed by NMR spectroscopy, which supported first-order kinetics (0.012 min−1). The acetate likely acts as a base to deprotonate the isopropyl methine to induce the formation of the threemembered palladacycle [{PC(sp3)H2P}actPd(OAc)] (Figure 8). This proposed mechanism takes into account, besides the

Figure 9. Thermal ellipsoid (50% probability level) representation of [{PC(sp3)HP}Pd(OAc)] (9). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): O(31)−Pd = 2.1305(15), C(31)−O(31) = 1.297(3), C(31)−O(32) = 1.233(3), Pd−C = 2.066(2), Pd−P(1) = 2.2784(7), Pd−P(2) = 2.3169(6), C− Pd−O(31) = 175.17(8), P(1)−Pd−P(2) = 156.99(2), C−Pd−P(1) = 84.15(6), O(31)−Pd−P(1) = 95.90(5), C−Pd−P(2) = 81.73(6), O(31)−Pd−P(2) = 99.68(5).

Pd(II) center exhibits a square-planar geometry, with the sum of angles around the metal center of 361.46(22)°. The average Pd−P distance of 2.2976(13) Å is common for palladium diphosphines. The Pd−O(31) distance of 2.1305(15) Å is in line with previously reported Pd−O distances in Pd−OAc metal complexes.17,46,64 The P−Pd−P angle of 156.99(2)° is slightly smaller than in the corresponding chloride and triflate complexes. An analogous Ni complex, [{PC(sp3)HP}Ni(OAc)], reported by Piers was synthesized from acetic acid and the nickel hydroxide [{PC(sp3)HP}Ni(OH)].82 A mixture of the backbone C−H activation product (9) and complex 10 were observed in trace amounts through photolysis of 8 for 6 h using a mercury lamp. These results indicate that neither

Figure 8. Proposed C−H activation mechanism in 8.

results described by Protasiewicz and co-workers,48 the findings of Davies, Macgregor, and co-workers, who reported DFT calculations of the cyclopalladation of dimethylbenzylamine (DMBA-H) by Pd(OAc)2. Their results showed that the reaction proceeds by an initial agostic C−H bond interaction between palladium and the ortho proton, which enables a subsequent facile intramolecular deprotonation by the coordinated acetate. This ambiphilic metal ligand activation E

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Organometallics

ArC), 125.45 (d, J = 47.2 Hz, ArC), 120.07 (q, J = 319.3 Hz, −OSO2CF3), 43.11 (t, J = 9.6 Hz, backbone CH2), 30.11 (d, J = 25.2 Hz, CH(CH3)2), 28.40 (d, J = 28.1 Hz, CH(CH3)2), 22.34 (d, J = 2.2 Hz, CH(CH3)2), 22.28 (s, CH(CH3)2), 22.01 (s, CH(CH3)2), 20.87 (d, J = 3.5 Hz, CH(CH3)2). Anal. Calcd for C28.5H41Cl3F6O6P2PdS2 (3·1.5CH2Cl2, 932.46 g/mol): C, 36.71; H, 4.43. Found: C, 36.98; H, 4.61. Synthesis of [{PC(sp3)HP}Pd(OTf)] (4). Method A. A solution of [{PC(sp3)H2P}Pd(OTf)2] (3; 161.0 mg, 0.2 mmol) in 10 mL of CH2Cl2 was heated at 40 °C for 24 h in a Schlenk flask. The 1H and 31 1 P{ H} NMR spectra showed quantitative formation of the product 4. The solution was filtered through a plug of Celite, and the volatiles were removed under reduced pressure. The resulting yellow powder was analytically pure by 1H and 31P{1H} NMR spectroscopy. Yield: 131 mg, 0.2 mmol, quantitative. Method B. To a solution of [{PC(sp3)HP}PdCl] (2; 54.1 mg, 0.1 mmol) in 5 mL of Et2O was added a suspension of AgOTf (25.7 mg, 0.1 mmol) in 5 mL of Et2O. The reaction mixture was stirred at room temperature for 2 h. The solution was filtered, the residue was washed with Et2O, and the volatiles were removed under reduced pressure. The product, 4, was obtained in near-quantitative yield (62.2 mg, 0.095 mmol). Data for 4 are as follows. 1H NMR (400 MHz, C6D6): δ 7.11−7.07 (m, 2H, ArH), 7.06 (d, J = 7.7 Hz, 2H, ArH), 7.01 (td, J = 7.4, 1.1 Hz, 2H, ArH), 6.96 (t, J = 7.2 Hz, 2H, ArH), 6.29 (s, 1H, backbone CH), 2.70−2.57 (m, 2H, CH(CH3)2), 2.43−2.30 (m, 2H, CH(CH3)2), 1.37−1.25 (m, 12H, CH(CH3)2), 1.01 (dd, J = 13.4, 6.3 Hz, 6H, CH(CH3)2), 0.95 (dd, J = 14.5, 7.4 Hz, 6H, CH(CH3)2). 19F NMR (376 MHz, C6D6): δ −79.97 (s). 31P{1H} NMR (162 MHz, C6D6): δ 51.81 (s). 13C{1H} NMR (101 MHz, C6D6): δ 157.10 (t, J = 14.5 Hz, ArC), 132.38 (s, ArC), 131.53 (t, J = 18.0 Hz, ArC), 130.72 (s, ArC), 127.08 (t, J = 9.0 Hz, ArC), 126.40 (t, J = 3.3 Hz, ArC), 121.24 (dd, J = 638.8, 318.6 Hz, − OSO2CF3), 47.82 (t, J = 3.5 Hz, backbone CH), 25.40 (t, J = 9.7 Hz, CH(CH3)2), 25.24 (t, J = 11.5 Hz, CH(CH3)2), 19.47 (t, J = 3.3 Hz, CH(CH3)2), 18.72 (s, CH(CH3)2), 18.27 (t, J = 2.6 Hz, CH(CH3)2), 17.58 (s, CH(CH3)2). 1 H NMR (300 MHz, CD2Cl2): δ 7.53 (dtd, J = 5.3, 3.8, 1.4 Hz, 2H, ArH), 7.38 (tdd, J = 7.4, 2.4, 1.2 Hz, 2H, ArH), 7.31 (ddd, J = 7.3, 1.4, 0.7 Hz, 2H, ArH), 7.27−7.22 (m, 2H, ArH), 6.33 (s, 1H, backbone CH), 2.83−2.62 (m, 4H, CH(CH3)2), 1.42−1.30 (m, 18H, CH(CH3)2), 1.15 (td, J = 8.2, 7.0 Hz, 6H, CH(CH3)2). 19F NMR (376 MHz, CH2Cl2): δ −81.35. 31P{1H} NMR (162 MHz, CD2Cl2): δ 54.24. 13C{1H} NMR (75 MHz, CD2Cl2): δ 157.64−156.87 (m, ArC), 132.62 (s, ArC), 131.41 (s, ArC), 131.04 (s, ArC), 127.42 (t, J = 9.0 Hz, ArC), 126.70 (t, J = 3.4 Hz, ArC), 126.24−113.48 (m, -OSO2CF3), 48.20 (t, J = 3.6 Hz, backbone CH), 26.26−24.98 (m, CH(CH3)2), 19.68 (t, J = 3.3 Hz, CH(CH3)2), 19.05 (s, CH(CH3)2), 18.36 (t, J = 2.6 Hz, CH(CH3)2), 18.03 (s, CH(CH3)2). 13C NMR (75 MHz, CD2Cl2): δ 48.21 (d, JCH = 133.2 Hz, backbone CH). Anal. Calcd for C26H37F3O3P2PdS (655.00 g/mol): C, 47.68; H, 5.69. Found: C, 47.73; H, 5.84. Synthesis of [{PC(sp3)H2P}PdMe2] (5). A cold solution of MeLi (2 mL, 0.1 M in Et2O, 0.2 mmol, −78 °C) was added to a cold solution of [{PC(sp3)H2P}PdCl2] (1; 57.8 mg, 0.1 mmol, −78 °C) in 5 mL of Et2O. The mixture was stirred at −78 °C for 2 h and then warmed to ambient temperature. The volatiles were removed under reduced pressure, and the crude residue was extracted with n-pentane. This pentane solution was filtered over Celite, and the volatiles were removed under reduced pressure to yield 5 as a white powder. Analytically pure 5 was isolated from a concentrated Et2O solution at −35 °C. Yield: 48.9 mg, 0.091 mmol, 91%. Data for 5 are as follows. 1 H NMR (400 MHz, C6D6): δ 7.27−7.20 (m, 2H, ArH), 7.15 (dt, JHH = 14.0 Hz, JHP = 3.4 Hz, 1H, backbone endo CH(H)), 7.10−7.06 (m, 2H, ArH), 7.01−6.92 (m, 4H, ArH), 3.50 (d, JHH = 14.4 Hz, 1H, backbone exo CH(H)), 2.73 (dtd, J = 14.2, 7.1, 4.3 Hz, 2H, CH(CH3)2), 1.55−1.36 (m, 2H, CH(CH3)2), 1.19 (dd, J = 12.2, 7.1 Hz, 6H, CH(CH3)2), 1.11 (dd, J = 12.5, 7.1 Hz, 6H, CH(CH3)2), 1.02 (dd, J = 15.1, 7.1 Hz, 6H, CH(CH3)2), 0.89−0.82 (m, 6H, Pd(CH3)2), 0.73 (dd, J = 9.8, 7.0 Hz, 6H, CH(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 23.30 (s). 13C{1H} NMR (101 MHz, C6D6): δ 146.25−145.46 (t, J = 8.0 Hz, ArC), 134.26 (t, J = 9.8 Hz, ArC),

photolysis nor thermal treatment was a viable synthetic method to produce 9.

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CONCLUSIONS A series of palladium pincer complexes supported by PC(sp3)H2P was isolated and characterized. When [{PC(sp3)H2P}PdX2] (X = Cl, OTf) is heated, the activation of the backbone C−H bond occurs to yield [{PC(sp3)HP}PdX]. Kinetic data indicate that the rate of C−H activation increases with the presence of a stronger leaving group. The thermally induced C−C reductive elimination of cis-[{PC(sp3)H2P}PdMe2] generated a Pd(0) dimer, [{PC(sp3)H2P}Pd]2. The formation of the desired backbone C−H activated product can be observed as a mixture of products through photolysis. Finally, on exposure to elevated temperatures, the analogous diacetate species [{PC(sp3)H2P}Pd(OAc)2] yields a rare three-membered palladacycle, which is formed through a concerted C−H activation of a phosphine isopropyl methine group.

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EXPERIMENTAL SECTION

General Considerations and Physical Methods. Experiments were performed under a N2 atmosphere using glovebox techniques. All commercial chemicals were used as received, except where specified otherwise. Pd(COD)Cl2 and Pd(OAc)2 were purchased from Sigma-Aldrich. Characterization data of [{PC(sp3)H2P}PdCl2] (1),64 [{PC(sp3)HP}PdCl] (2),31 [{PC(sp3)H2P}Pd]2 (6),64 [{PC(sp3)H2P}Pd(OAc)2] (8),64 and [{PC(sp3)HP}PdH] (14)32 were compared with data previously reported. Deuterated solvents were obtained from Cambridge Isotope Laboratories. CD2Cl2 and CDCl3 were dried over molecular sieves, while C6D6 was dried by refluxing over dry CaH2 and filtered prior to use. The solvents THF, Et2O, toluene, n-pentane, and hexanes were dried using a solvent column purification system.83 NMR spectra were recorded on a Varian 300 spectrometer at ambient probe temperature or on Bruker 400 and 500 instruments. Chemical shifts are reported in ppm relative to residual internal protio solvent for 1H and 13C{1H} NMR spectra and with respect to a TMS standard when CDCl3 is used. The 31P{1H} chemical shifts are reported in ppm relative to H3PO4. Coupling constants are given in Hz. All assignments are based on onedimensional 1H, 13C{1H}, and 31P{1H} experiments unless otherwise noted. Single-crystal X-ray diffraction was conducted on Bruker Kappa and Duo instruments with Mo Kα X-ray sources. CHN analyses were performed on a CE-440 Elemental Analyzer or by Midwest Microlab, LLC. Synthesis of [{PC(sp3)H2P}Pd(OTf)2] (3). To a solution of [{PC(sp3)H2P}PdCl2] (1; 57.8 mg, 0.1 mmol) in 5 mL of Et2O was added a suspension of 51.4 mg of AgOTf (0.2 mmol) in 5 mL of Et2O. The reaction mixture was stirred at room temperature for 2 h. The volatiles were removed under reduced pressure, and the residue was extracted with CH2Cl2 several times and filtered over Celite. The volatiles were removed under reduced pressure to give crude 3 as a yellow powder. Yield: 76.4 mg, 0.095 mmol, 95%. The 1H NMR and 31 1 P{ H} NMR spectra showed this product to be pure. Recrystallization was achieved by slow vapor diffusion of n-pentane into a concentrated CH2Cl2 solution of 3, which yielded yellow crystals of suitable quality for X-ray diffraction. Data for 3 are as follows. 1H NMR (400 MHz, CD2Cl2): δ 7.63−7.54 (m, 6H, ArH), 7.43−7.37 (m, 2H, ArH), 6.94 (dt, J = 14.9, 1.9 Hz, 1H, endo CH(H)), 4.22 (d, J = 14.9 Hz, 1H, exo CH(H)), 3.17 (dq, J = 14.1, 7.0 Hz, 2H, CH(CH3)2), 1.91 (d, J = 6.8 Hz, 2H, CH(CH3)2), 1.73 (dd, J = 17.2, 7.0 Hz, 6H, CH(CH3)2), 1.65 (dd, J = 19.1, 7.3 Hz, 6H, CH(CH3)2), 1.46 (dd, J = 17.9, 6.9 Hz, 6H, CH(CH3)2), 1.01 (dd, J = 15.9, 6.5 Hz, 6H, CH(CH3)2). 31P NMR (162 MHz, CD2Cl2): δ 54.53 (s). 19F NMR (376 MHz, CD2Cl2): δ −80.57 (s). 13C NMR (101 MHz, CD2Cl2): δ 144.76 (d, J = 10.2 Hz, ArC), 133.65 (d, J = 2.4 Hz, ArC), 133.50 (d, J = 8.8 Hz, ArC), 133.05 (s, ArC), 127.74 (d, J = 8.2 Hz, F

DOI: 10.1021/acs.organomet.8b00237 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

= 10.8, 6.3, 2.4 Hz, 2H, ArH), 6.95 (ddd JHP = 1.9 Hz; JHP = 5.3 Hz; JHH = 14.0 Hz, 1H, backbone endo CH(H)), 6.87 (td, J = 7.5, 1.2 Hz, 2H, ArH), 6.81 (t, J = 7.2 Hz, 1H, ArH), 3.29 (d, J = 14.3 Hz, 1H, backbone exo CH(H)), 2.49 (s, 3H, OCOCH3), 2.47−2.40 (m, 1H, CH(CH3)2), 2.28 (ddt, J = 13.6, 11.0, 6.8 Hz, 1H, CH(CH3)2), 1.97− 1.88 (m, 1H, CH(CH3)2), 1.73 (dd, J = 12.9, 12.0 Hz, 3H, CH(CH3)2), 1.53 (dd, J = 16.0, 6.9 Hz, 3H, −CH3), 1.47 (dd, J = 13.8, 12.3 Hz, 3H, −CH3), 1.34 (dd, J = 14.7, 5.6 Hz, 3H, −CH3), 1.30 (dd, J = 15.2, 5.1 Hz, 3H, −CH3), 0.87 (dd, J = 10.6, 7.0 Hz, 3H, −CH3), 0.70 (dd, J = 15.1, 7.1 Hz, 3H, −CH3), −0.13 (dd, J = 20.6, 7.1 Hz, 3H, −CH3). 31P{1H} NMR (202 MHz, C6D6): δ 29.36 (d, JPP = 29.5 Hz), 11.32 (d, JPP = 29.4 Hz). 13C{1H} NMR (126 MHz, C6D6): δ 175.10 (s, -OCOCH3), 148.76 (d, J = 13.4 Hz, ArC), 147.80 (d, J = 18.2 Hz, ArC), 136.28 (s, ArC), 133.64 (d, J = 19.4 Hz, ArC), 133.13 (d, J = 9.5 Hz, ArC), 132.13 (d, J = 6.2 Hz, ArC), 131.36 (s, ArC), 131.05 (d, J = 2.4 Hz, ArC), 128.99 (d, J = 1.1 Hz, ArC), 126.16 (d, J = 7.9 Hz, ArC), 124.65 (d, J = 3.7 Hz, ArC), 121.80 (d, J = 49.1 Hz, ArC), 43.59 (dd, J = 61.4, 36.8 Hz, backbone CH2), 39.41 (dd, J = 15.0, 12.1 Hz, Pd−C(CH3)2), 25.79 (d, J = 9.3 Hz, CH(CH3)2), 25.36 (dd, J = 33.0, 4.7 Hz, CH(CH3)2), 24.87 (s, −OCOCH3), 24.54 (dd, J = 5.8, 4.4 Hz, CH(CH3)2), 22.90 (t, J = 6.5 Hz, −CH3), 22.76 (d, J = 15.7 Hz, −CH3), 20.95 (d, J = 10.3 Hz, −CH3), 20.11 (t, J = 4.5 Hz, −CH3), 19.87 (d, J = 7.5 Hz, −CH3), 18.01 (d, J = 11.4 Hz, −CH3), 16.97 (d, J = 2.3 Hz, −CH3), 16.82 (d, J = 5.6 Hz, −CH3). Anal. Calcd for C27H40O2P2Pd (564.98 g/mol): C, 57.40; H, 7.14. Found: C, 56.81; H, 6.88. Thermolysis of [{PC(sp3)H2P}PdMe2] (5). A solution containing 16.1 mg of [{PC(sp3)H2P}PdMe2] (5, 0.3 mmol) in 0.5 mL of C6D6 was left at 20 °C in a J. Young NMR tube. 1H and 31P{1H} spectra showed a complete conversion to [{PC(sp3)H2P}Pd]2 (6) within 24 h. The removal of volatiles under reduced pressure and trituration with n-pentane afforded 14.2 mg of 6 in near-quantitative yield (94%). Kinetic Measurement for the Conversion of 1 to 2. A stock solution of the standard, trimethoxybenzene, was prepared in 5 mL of deuterated chloroform (0.028 M) and stored at −35 °C. [{PC(sp3)H2P}PdCl2] (1; 10 mg, 0.017 mol) was dissolved in 0.6 mL of the trimethoxybenzene/CDCl3 stock solution and placed in a J. Young NMR tube. An initial 1H NMR spectrum was taken, and the concentration of compound 1 was recorded (0.023 M). The temperature of the probe was increased to 55 °C, and the remainder of the data points were collected. The decay of the isopropyl protons (CH(CH3)2) was integrated against the methyl hydrogens of the standard. Spectra were taken until no starting material remained. Microsoft Excel was used to plot the acquired data. Kinetic Measurement for the Conversion of 3 to 4. A stock solution of the standard, trimethoxybenzene, was prepared in 5 mL of deuterated chloroform (0.028 M) and stored at −35 °C. [{PC(sp3)H2P}Pd(OTf)2] (3; 10 mg, 0.012 mol) was dissolved in 0.6 mL of the trimethoxybenzene/CDCl3 stock solution and placed in a J. Young NMR tube. An initial 1H NMR spectrum was taken, and the concentration of compound 3 was recorded (0.018 M). The temperature of the probe was increased to 55 °C, and the remainder of the data points were collected. The decay of the isopropyl protons (CH(CH3)2) was integrated against the methyl hydrogens of the standard. Spectra were taken until no starting material remained. Microsoft Excel was used to plot the acquired data. Kinetic Measurement for the Conversion of 8 to 10. A stock solution of the standard, trimethoxybenzene, was prepared in 5 mL of deuterated CDCl3 (0.028 M) and stored at −35 °C. [{PC(sp3)H2P}Pd(OAc)2] (8; 10 mg, 0.016 mol) was dissolved in 0.6 mL of the trimethoxybenzene/CDCl3 stock solution and placed in a J. Young NMR tube. An initial 1H NMR spectrum was taken, and the concentration of compound 8 was recorded (0.026 M). The temperature of the probe was increased to 55 °C, and the remainder of the data points were collected. The decay of the isopropyl protons (CH(CH3)2) was integrated against the methyl hydrogens of the standard. Spectra were taken until no starting material remained. Microsoft Excel was used to plot the acquired data.

132.34 (t, J = 3.4 Hz, ArC), 131.75 (s, ArC), 128.90 (s, ArC), 125.50 (t, J = 1.6 Hz, ArC), 43.23 (t, J = 12.3 Hz, backbone CH2), 24.52 (t, J = 5.0 Hz, CH(CH3)2), 23.35 (dd, J = 10.3, 8.1 Hz, CH(CH3)2), 21.20 (t, J = 3.6 Hz, CH(CH3)2), 20.69 (s, CH(CH3)2), 20.40 (s, CH(CH3)2), 20.12 (s, CH(CH3)2), 4.44 (dd, J = 103.5, 14.6 Hz, Pd(CH3)2). Anal. Calcd for C27H44P2Pd (537.02 g/mol): C, 60.39; H, 8.26. Found: C, 59.18; H, 7.44. Compound 5 undergoes C−C reductive elimination in solution at room temperature to give [{PC(sp3)H2P}Pd]2 (6). Data for 6 are as follows. Anal. Calcd for C50H76P4Pd2 (1013.89 g/mol): C, 59.23; H, 7.56. Synthesis of [{PC(sp3)HP}PdMe] (7). A cold solution of MeLi (1 mL, 0.1 M in Et2O, 0.1 mmol, −78 °C) was added to a cold solution of [{PC(sp3)HP}PdCl] (1, 54.1 mg, 0.1 mmol, −78 °C) in 5 mL of Et2O. The mixture was stirred at −78 °C for 2 h and then warmed to room temperature. The volatiles were removed under reduced pressure, and the crude residue was extracted with n-pentane. This pentane solution was filtered over Celite, and the volatiles were removed under reduced pressure to yield 7 as a pale yellow powder. Analytically pure 7 was isolated from a concentrated Et2O solution at −35 °C. Yield: 45.8 mg, 0.088 mmol, 88%. Data for 7 are as follows. 1 H NMR (400 MHz, C6D6): δ 7.48 (d, J = 7.8 Hz, 2H, ArH), 7.20− 7.07 (m, 4H, ArH), 6.90 (t, J = 7.4 Hz, 2H, ArH), 5.51 (s, 1H, backbone CH), 2.37−2.20 (m, 4H, CH(CH3)2), 1.23 (q, J = 7.6 Hz, 6H, CH(CH3)2), 1.16 (q, J = 7.5 Hz, 6H, CH(CH3)2), 1.05 (q, J = 7.1 Hz, 6H, CH(CH3)2), 0.98 (q, J = 7.2 Hz, 6H, CH(CH3)2), 0.46 (t, J = 5.2 Hz, 3H, PdCH3). 31P{1H} NMR (162 MHz, C6D6): δ 49.00 (s). 13C{1H} NMR (101 MHz, C6D6): δ 160.74 (t, J = 15.0 Hz, ArC), 137.24 (t, J = 17.0 Hz, ArC), 131.88 (s, ArC), 129.85 (s, ArC), 128.41−128.05 (m, ArC), 123.66 (t, J = 3.2 Hz, ArC), 58.31 (backbone CH), 26.27 (t, J = 9.8 Hz, CH(CH3)2), 25.22 (t, J = 11.6 Hz, CH(CH3)2), 19.65 (t, J = 3.2 Hz, CH(CH3)2), 19.06 (t, J = 2.9 Hz, CH(CH3)2), 18.71 (t, J = 1.8 Hz, CH(CH3)2), 18.66 (t, J = 1.8 Hz, CH(CH3)2), − 17.11 (t, J = 10.6 Hz, PdCH3). Anal. Calcd for C26H40P2Pd (520.97 g/mol): C, 59.94; H, 7.74. Found: C, 60.16; H, 8.09. Synthesis of [{PC(sp3)HP}PdOAc] (9). A suspension of AgOAc (13.4 mg, 0.08 mmol) in 5 mL of Et2O was added to a solution of [{PC(sp3)HP}PdCl] (2; 43.2 mg, 0.08 mmol) in 5 mL of Et2O. The mixture was stirred at room temperature for 1 h. The cloudy solution was filtered through a plug of Celite, and the volatiles were removed under reduced pressure. The residue was extracted with n-pentane, and the combined extracts were filtered through a plug of Celite. The volatiles were removed under reduced pressure. Analytically pure 9 was isolated by layering a concentrated toluene solution with npentane at −35 °C. Yield: 41.8 mg, 0.074 mmol, 78%. Data for 9 are as follows. 1H NMR (500 MHz, C6D6): δ 7.24 (dd, J = 7.8, 0.5 Hz, 2H, ArH), 7.04−6.98 (m, 4H, ArH), 6.87 (t, J = 7.4 Hz, 2H, ArH), 6.08 (s, 1H, backbone −CH), 2.44 (dqd, J = 13.8, 6.9, 2.9 Hz, 2H, CH(CH3)2), 2.33 (s, 3H, −OCOCH3), 2.29 (dddd, J = 16.4, 9.3, 4.6, 2.2 Hz, 2H, CH(CH3)2), 1.35 (dd, J = 15.5, 8.3 Hz, 6H, CH(CH3)2), 1.26 (dd, J = 15.5, 8.3 Hz, 6H, CH(CH3)2), 1.02 (dq, J = 9.5, 7.4 Hz, 12H, CH(CH3)2). 31P{1H} NMR (202 MHz, C6D6): δ 49.68 (s). 13 C{1H} NMR (101 MHz, C6D6): δ 175.44 (s, −OCOCH3), 159.16 (t, J = 15.1 Hz, ArC), 133.66 (t, J = 17.0 Hz, ArC), 132.20 (s, ArC), 130.17 (s, ArC), 127.33 (t, J = 8.8 Hz, ArC), 125.26 (t, J = 3.1 Hz, ArC), 46.48 (t, J = 1.8 Hz, backbone CH), 25.66 (t, J = 9.8 Hz, CH(CH3)2), 24.96 (t, J = 11.2 Hz, CH(CH3)2), 24.90 (s, −OCOCH3), 18.99 (t, J = 3.2 Hz, CH(CH3)2), 18.58 (t, J = 3.0 Hz, CH(CH3)2), 18.19 (s, CH(CH3)2), 18.15 (s, CH(CH3)2). Anal. Calcd for C27H40O2P2Pd (564.98 g/mol): C, 57.40; H, 7.14. Found: C, 57.37; H, 7.22. Synthesis of [{PC(sp3)H2P}actPd(OAc)] (10). A solution of [{PC(sp3)H2P}Pd(OAc)2] (8; 100 mg, 0.159 mmol) in CHCl3 was heated to 50 °C for 3 days in a Schlenk flask. The volatiles were removed under reduced pressure, and the residue was dissolved in CH2Cl2. Crystalline [{PC(sp3)H2P}actPd(OAc)] (10) was obtained from a concentrated CH2Cl2 solution layered with n-pentane at −35 °C. Yield: 57.8 mg, 0.102 mmol, 64%. A small amount of a contaminant cocrystallized along with 10. Data for 10 are as follows. 1 H NMR (500 MHz, C6D6): δ 7.12−7.04 (m, 3H, ArH), 7.00 (ddd, J G

DOI: 10.1021/acs.organomet.8b00237 Organometallics XXXX, XXX, XXX−XXX

Article

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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.8b00237. NMR spectra for compounds 3−5, 7, 9, and 10, kinetic measurements, and X-ray crystallographic data (PDF) Accession Codes

CCDC 1433160−1433165 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for V.M.I.: [email protected]. ORCID

Vlad M. Iluc: 0000-0001-6880-2470 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.M.I. acknowledges support from the National Science Foundation (NSF) CAREER Program (CHE-1552397). We are grateful to Dr. Allen Oliver for crystallographic assistance.

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REFERENCES

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DOI: 10.1021/acs.organomet.8b00237 Organometallics XXXX, XXX, XXX−XXX