Room-Temperature Carbon–Sulfur Bond Activation by a Reactive

Apr 21, 2015 - The reactivity of [Pd(dippe)(μ-H)]2 (1) and [(μ-dippe)Pd]2 (2) (dippe = 1,2-bis(diisopropylphosphino)ethane) toward C–S bonds in th...
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Room-Temperature Carbon−Sulfur Bond Activation by a Reactive (dippe)Pd Fragment Lloyd Munjanja, William W. Brennessel, and William D. Jones* Department of Chemistry, University of Rochester, P.O. Box 270216, 491, Rochester, New York 14627-0216, United States S Supporting Information *

ABSTRACT: The reactivity of [Pd(dippe)(μ-H)]2 (1) and [(μ-dippe)Pd]2 (2) (dippe = 1,2-bis(diisopropylphosphino)ethane) toward C−S bonds in thiophene derivatives and thioethers was investigated, which led to C−S bond activation products. The thiapalladacycles derived from thiophenic substrates were fully characterized by 1H, 31P, and 13C NMR spectroscopy, elemental analysis, and X-ray diffraction. The stability of the C−S insertion products was probed by performing competition experiments which follow the thermodynamic stability order (dippe)Pd(κ2C,Sbenzothiophene) (6) > (dippe)Pd(κ2C,S-dibenzothiophene) (8) > (dippe)Pd(κ2C,S-thiophene) (3). The reactivity of the thiapalladacycles with small molecules such as H2, CO, and alkynes was investigated.

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INTRODUCTION

article extends the study of reactivity of 1 toward various thiophenic substrates.

Interest in C−S bond activation has risen due to the significance of hydrodesulfurization (HDS) in the petroleum refining industry. HDS is the catalytic process for the removal of sulfur from organosulfur compounds (organosulfur compounds account for ∼5 wt % in petroleum) under 150−600 psi of hydrogen at 300−450 °C.1 Current technologies of Ni/Mo (or Mo/Co) heterogeneous catalysts reduce the sulfur content of oil from 1−5% to ∼300 ppm in fuels.1 With new mandates by the Environmental Protection Agency to reduce sulfur content from transportation fuels to 10 ppm or lower,2 there is a greater demand for the development of new catalysts that can meet this new challenge. Consequently, homogeneous transition-metal catalysts serve as models for C−S cleavage by providing an understanding of how the thiophene substrates react with the metal center to yield thiametallacycles. We have previously reported the C−S activation of thiophenes by a hydride-bridged nickel3 dimer and a platinum−hydride dimer4 to form ring-opened thiophene adducts. Work in our laboratory with [Ni(dippe)(μ-H)]2 (dippe = 1,2-bis(diisopropylphosphino)ethane) found that it is possible to activate the carbon−sulfur bond in thiophene, benzothiophene, dibenzothiophene, 4-methylbenzothiophene, and other cyclic sulfur compounds to yield the corresponding thianickelacycles. An analogous platinum complex, [(dippe)PtH]2, was found to activate thiophene over the course of 10 days at 70 °C. As part of our ongoing studies related to C−S bond activation with group 10 metal complexes, we became interested in studying the analogous dihydride palladium complex 1 to compare its reactivity with nickel and platinum. Garcia et al. have previously reported reactions of a variety of substituted thiophenes with Pd(PEt3)3.5 [Pd(dippe)(μ-H)]2 (1) was first generated in situ by our group and has been shown to react with thiophene to give 3 (eq 1), although both reactant and product were found to be very unstable.4 This © XXXX American Chemical Society

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RESULTS AND DISCUSSION Preparation of the Palladium(I) Hydride Dimer 1 and Dinuclear Pd(0) Species 2. We generated the bridging hydrido complex [Pd(dippe)(μ-H)]2 (1) in situ from a palladium-chloro precursor6 and sodium triethylborohydride. Here we show that [Pd(dippe)(μ-H)]2 (1) and the dinuclear Pd(0) species [(μ-dippe)Pd]2 (2)7 remain in an equilibrium with a relative ratio of 9:1, respectively, when a suspension of [(dippe)PdCl2] is treated with 2 equiv of NaHBEt3 in toluene (Scheme 1). The 31P{1H} NMR spectrum shows two singlet resonances, one at δ 61.3 which has been assigned to 1 and the other resonance at δ 33.0 corresponding to [(μ-dippe)Pd]2 (2). The 1H NMR spectrum (THF-d8) of the in situ solution shows a quintet centered at δ −2.00 (JP−H = 40 Hz) assigned to 1, indicative of hydride coupling to four equivalent phosphorus atoms. Efforts to isolate 1 in pure form have so far been unsuccessful, since this complex decomposes readily under vacuum. When a frozen toluene solution (−196 °C) containing a mixture of 1 and 2 was subjected to freeze−pump−thaw degas, 2 was formed exclusively by the loss of H2 from 1. Subsequently, addition of 1 atm of H2 to 2 establishes the initial equilibrium between 1 and 2. We propose that the Received: March 12, 2015

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Scheme 3. Trapping the 14-Electron (dippe)Pd0 Species A

Scheme 1. Formation of [Pd(dippe)(μ-H)]2 (1) and [(μdippe)Pd]2 (2) in Situ

corresponding C−S bond insertion adducts exclusively with the complete disappearance of 1 and 2, as detailed in the following section. Reactivity of a Mixture Containing 1 and 2 with Thiophenic Substrates. The reactivity of a mixture containing 1 and 2 toward C−S activation was examined. Insertion of palladium into the carbon−sulfur bonds was achieved for most, but not all, of the substrates tested (Figure 1).

equilibrium between 1 and 2 is established via a reactive 14electron intermediate species, (dippe)Pd(0) (A) (Scheme 1). Neither A nor dippe ligand was observed in the 31P{1H} NMR spectrum even at low temperatures. 31P{1H} NMR spectra taken over a course of several days indicated that the amounts of 1 and 2 decreased as the concentration of two new complexes increased. These complexes were identified as the thermodynamically stable species 4 and 5, as judged by a singlet at δ 45.5 and two multiplets at δ 49.5 and 54.8, respectively, in the 31P{1H} NMR spectra (Scheme 2).8 The solution shows no precipitate but is opaque brown. Scheme 2. Formation of [(dippe)2Pd] (4) and [Pd2(dippe)2(μ-dippe)] (5) from a Mixture of 1 and 2

Figure 1. Summary of complexes synthesized using a mixture containing [Pd(dippe)(μ-H)]2 (1) and [(μ-dippe)Pd]2 (2). Reported yields are isolated yields unless otherwise noted.

In support of the formation of the 14-electron intermediate species A, Fink and co-workers observed the formation of the mixed dimer (μ-dippe)(μ-dcpe)Pd after the dissolution of equimolar amounts of [(μ-dippe)Pd]2 and [(μ-dcpe)Pd]2 (dcpe = 1,2-bis(dicyclohexylphosphino)ethane).7 Consequently, treatment of a THF solution mixture of 1 and 2 with a slight excess of dippe ligand converts both complexes to 4, indicating that 1 and 2 in the above experiment interconvert via A, which was trapped by the dippe ligand to give 4 exclusively as shown in Scheme 3. Palladium colloid is presumably also formed in the reaction in Scheme 2, as the opaque brown solution9 becomes transparent brown upon stirring over elemental mercury. Since both 1 and 2 plus H2 disproportionate in solution to afford a coordinatively unsaturated nucleophile, A, addition of an organosulfur compound to the solution instantly yields the

Jones and co-workers showed previously that reduction of [(dippe)PdCl2] in neat thiophene gave the C−S insertion adduct 3.4 Similarly, the reaction of a solution containing a mixture of 1 and 2 and an excess of benzothiophene at room temperature afforded a white solid identified analytically and spectroscopically as (dippe)Pd(κ2C,S-benzothiophene) (6). A 31 1 P{ H} NMR spectrum of complex 6 shows a pair of doublets at δ 73.5 and 66.7 with J = 26.2 Hz, consistent with a mononuclear, asymmetric ring-opened structure. X-ray-quality crystals were grown by layering a solution of 6 in CH2Cl2 (−30 °C) with hexanes (Figure 2). The geometry of 6 is slightly distorted square planar. The dihedral angle between planes B

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Figure 3. ORTEP drawing of (dippe)Pd(κ2C,S-3-methylbenzothiophene) (7). Selected bond lengths (Å): Pd(1)−C(1), 2.059(3); Pd(1)−P(1), 2.2805(11); Pd(1)−S(1), 2.2816(11); Pd(1)−P(2), 2.3262(11); S(1)−C(4), 1.764(4); C(1)−C(2), 1.307(5); C(2)− C(3), 1.455(6); C(3)−C(4), 1.398(6). Selected bond angles (deg): C(1)−Pd(1)−P(1), 90.67(10); C(1)−Pd(1)−S(1), 90.80(10); P(1)− Pd(1)−S(1), 176.90(4); C(1)−Pd(1)−P(2), 175.98(10); P(1)− Pd(1)−P(2), 85.59(4); S(1)−Pd(1)−P(2), 93.01(4).

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Figure 2. ORTEP drawing of (dippe)Pd(κ C,S-benzothiophene) (6). Selected bond lengths (Å); Pd(1)−C(1), 2.032(3); Pd(1)−P(1), 2.2554(9); Pd(1)−S(1), 2.2793(8); Pd(1)−P(2), 2.3144(9); S(1)− C(4), 1.734(3); C(1)−C(2), 1.321(4); C(2)−C(3), 1.454(4); C(3)− C(4), 1.411(4). Selected bond angles (deg): C(1)−Pd(1)−P(1), 91.65(9); C(1)−Pd(1)−S(1), 93.58(9); P(1)−Pd(1)−S(1), 170.97(4); C(1)−Pd(1)−P(2), 173.47(10); P(1)−Pd(1)−P(2), 86.44(3); S(1)−Pd(1)−P(2), 89.14(3).

and NMR spectroscopic results were consistent with a monomeric ring-opened structure. X-ray-quality crystals were grown from a solution of 8 in THF at room temperature (Figure 4). The metallacyclic complex 8 is highly distorted

P1−Pd−P2 and S1−Pd−C1 is 9.8(1)° and corresponds to a slight fold as opposed to a twist. The Pd−P2 bond (2.3144(9) Å) is slightly longer than the Pd−P1 bond (2.2554(9) Å), which is characteristic of similar asymmetric (dippe)Pd complexes.10,18 It is postulated that the Pd−P2 bond is longer due to the trans effect of C(sp2) being much greater than that of S. The bite angle for the chelating dippe ligand is 86.44(3)°, comparable to that of complex 3 of 86.19(4)°,4 as well as to those of other (dippe)Pd complexes.8,18,20 The metallacycle complex 6 is square planar with an RMS deviation of 0.011 Å. Similar to the carbon−sulfur activation chemistry of [(dippe)NiH]2,3 palladium inserts exclusively into the vinyl C−S bond of benzothiophene in lieu of the aryl C−S bond. Attempts to react a mixture of 1 and 2 with 2methylbenzothiophene were unsuccessful, attributed to the vinyl C−S bond being obstructed by the methyl group at the 2position. Theoretical studies carried out in our group on the binding of platinum metal to thiophenes showed that π coordination to the double bond adjacent to the C−S bond is an essential step before insertion into the C−S bond.11 In contrast, when 3-methylbenzothiophene was subjected to the same reaction conditions, C−S bond activation took place, yielding a monomeric complex, 7, as shown by the X-ray structure (Figure 3). A plausible explanation for this observation is that for 3-methylbenzothiophene the methyl substituent is positioned away from the palladium center, thereby allowing π coordination to the double bond adjacent to the C−S bond before C−S cleavage. However, no π complex could be detected prior to product formation. Comparably to 6, complex 7 is nearly square planar. The RMS deviation of the metallacycle moiety is 0.015, indicating its planarity. The dihedral angle between the planes P1−Pd−P2 and S1−Pd−C1 is 3.1(1)°. The trans effect (C(sp2) ≫ S) is also noticeable in complex 7 by the inequivalent Pd−P bonds with Pd−P2 at 2.3262(11) Å being longer than Pd−P1 at 2.2805(11) Å. The reaction of a mixture of 1 and 2 with dibenzothiophene at room temperature led to the solid white product (dippe)Pd(κ2C,S-dibenzothiophene) (8) in 45 min. X-ray diffraction

Figure 4. ORTEP drawing of (dippe)Pd(κ2C,S-dibenzothiophene) (8). Selected bond lengths (Å): Pd(1)−C(1), 2.069(7); Pd(1)−P(1), 2.271(2); Pd(1)−P(2), 2.319(2); Pd(1)−S(1), 2.353(2); S(1)−C(4), 1.766(8); C(1)−C(2), 1.376(10); C(2)−C(3), 1.498(10); C(3)− C(4), 1.400(10); Selected bond angles (deg): C(1)−Pd(1)−P(1), 93.1(2); C(1)−Pd(1)−P(2), 178.3(2); P(1)−Pd(1)−P(2), 86.30(7); C(1)−Pd(1)−S(1), 85.7(2); P(1)−Pd(1)−S(1), 169.65(7); P(2)− Pd(1)−S(1), 94.56(7).

from planarity due to the rigidity of the biphenyl moiety, for which the angle between the two biphenyl ring planes is 40.7(2)°. The trans effect (C(sp2) ≫ S) is again apparent by the inequivalent Pd−P bonds: Pd−P1 at 2.271(2) Å and Pd− P2 at 2.319(2) Å. Solution and solid forms of complex 8 gradually decompose within a couple of weeks to form 4 and 5 (+DBT) at room temperature. In contrast to the chemistry of [(dippe)NiH]2, C

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Organometallics C−S activation of 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene with a mixture of of 1 and 2 were unsuccessful. The fact that dibenzothiophene reacts indicates that intermediate A can find access to the C−S bond that is ultimately cleaved. It is somewhat surprising, therefore, that the unhindered C−S bond of 4-methyldibenzothiophene does not react. A mixture of 1 and 2 instantly reacts with thianthrene and thioxanthene at room temperature to give the C−S insertion products (dippe)Pd(κ2C,S-thianthrene) (9) and (dippe)Pd(κ2C,S-thioxanthene) (10), respectively. Colorless crystals of both 9 and 10 were grown by slow evaporation of a solution in THF at −30 °C (Figures 5 and 6). Solutions of solid products 9

Figure 6. ORTEP drawing of (dippe)Pd(κ2C,S-thioxanthene) (10). Selected bond lengths (Å): Pd(1)−C(1), 2.0525(13); Pd(1)−P(1), 2.2696(4); Pd(1)−P(2), 2.3332(4); Pd(1)−S(1), 2.3439(4); S(1)− C(5), 1.7613(13); C(1)−C(2), 1.400(2); C(2)−C(3), 1.515(2); C(3)−C(4), 1.506(2); C(4)−C(5), 1.4067(19). Selected bond angles (deg): C(1)−Pd(1)−P(1), 89.19(4); C(1)−Pd(1)−P(2), 172.38(4); P(1)−Pd(1)−P(2), 87.242(13); C(1)−Pd(1)−S(1), 97.14(4); P(1)− Pd(1)−S(1), 165.921(13); P(2)−Pd(1)−S(1), 87.840(13); C(5)− S(1)−Pd(1), 117.48(5).

Figure 5. ORTEP drawing of (dippe)Pd(κ2C,S-thianthrene) (9). Selected bond lengths (Å); Pd(1)−C(1), 2.053(2); Pd(1)−P(1), 2.2471(6); Pd(1)−P(2), 2.3114(6); Pd(1)−S(1), 2.3530(6); S(1)− C(4), 1.753(2); S(2)−C(3), 1.774(2); S(2)−C(2), 1.786(2); C(1)− C(2), 1.384(3); C(3)−C(4), 1.412(3). Selected bond angles (deg): C(1)−Pd(1)−P(1), 91.82(6); C(1)−Pd(1)−P(2), 177.43(6); P(1)− Pd(1)−P(2), 86.86(2); C(1)−Pd(1)−S(1), 90.52(6); P(1)−Pd(1)− S(1), 174.85(2); P(2)−Pd(1)−S(1), 90.63(2); C(4)−S(1)−Pd(1), 115.19(8).

Hz) started forming at the expense of 3. Within a few minutes 3 disappeared and 6 formed. Free thiophene was formed from the decomposition of 3, as indicated by the 1H NMR spectrum. This indicates that the benzothiophene adduct 6 is thermodynamically more stable than the thiophene adduct 3 and that 3 is very labile. Alternatively, equimolar amounts of 3 and 6 were dissolved in THF and the reaction was monitored by NMR spectroscopy. Complex 3 was observed to completely decompose over 12 h, leaving 6 in solution in addition to free thiophene. For [(dippe)Ni] adducts, the thiophene and the benzothiophene C−S insertion products were in equilibrium with Keq ≈ 110 (eq 3).3 On the other hand, (PEt3)2Pt(κ2C,S-benzothiophene) was found to be 10 times more stable than the corresponding (PEt3)2Pt(κ2C,S-thiophene).12

and 10 are stable indefinitely in moisture and air. Both structures show highly distorted metallacycles due to the ringbridging sulfur and methylene groups favoring twists of the biphenyl ring planes of 80.69(6) and 83.23(5)° in 9 and 10, respectively. The deviation from planarity at the metal center of the planes P1−Pd−P2 and S1−Pd−C1 is 5.0(1)° for 9 and 14.19(4)° for 10. Similarly to the other palladium C−S insertion products (6−8), the Pd−P2 bond is longer than the Pd−P1 bond in both complexes 9 and 10 due to the C(sp2) ≫ S trans effect. The reactivity of the Pd(dippe) fragment A with thiophenic substrates occurs rapidly at room temperature. This can be compared with the reactions of Pd(PEt3)3 with thiophenes, which requires heating to 70 °C for 10 min to 3 h.5 Relative Thermodynamic Stabilities of the Thiapalladacycles Derived from Thiophene, Benzothiophene, and Dibenzothiophene. The reaction of (dippe)Pd(κ2C,Sthiophene) (3) with 1 equiv of benzothiophene in THF was probed (eq 2). Initially 31P{1H} NMR spectroscopy showed a pair of doublets: δ 74.59 and 67.10 (J = 25.5 Hz), attributed to 3. However, with the addition of benzothiophene, 6 with a pair of doublets at δ 73.52 (d, J = 26.2 Hz) and δ 66.86 (d, J = 26.2

In a separate experiment, the reaction of (dippe)Pd(κ2C,Sdibenzothiophene) (8) with 1 equiv of benzothiophene in THF affords 6 and free dibenzothiophene in 12 h (eq 4). Initially, 31 1 P{ H} NMR spectroscopy showed two pairs of doublets at δ 71.86 (d, J = 19.4 Hz) and δ 68.60 (d, J = 19.4 Hz), attributed to 8. In 12 h the initial set of doublets was replaced by another pair of doublet resonances at δ 73.52 (d, J = 26.2 Hz) and δ 66.86 (d, J = 26.2 Hz) for 6. Free dibenzothiophene was D

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benzothiophene from 6 was observed, with the formation of a thermally stable palladium complex, 11 (eq 6). The reaction

observed in the 1H NMR spectrum. This result establishes the stability of 6 in comparison to 8. In addition, this shows that reaction of a mixture of 1 and 2 with dibenzothiophene to form 8 is reversible but that 8 loses dibenzothiophene more slowly than 3 loses thiophene. In another related experiment equimolar amounts of 3 and 8 were mixed together in THF and the reaction was monitored by NMR spectroscopy over the course of 1 h. Complete decomposition of 3 was observed with formation of free thiophene. Complex 8 remains in solution. Alternatively, adding 1 equiv of dibenzothiophene to a solution of 3 results in complete formation of 8 along with free thiophene after 1 h. This observation establishes that complex 8 is more thermodynamically stable than complex 3 in solution (eq 5).

was monitored by 31P{1H} NMR spectroscopy over 8 h. Disappearance of the pair of doublet resonances at δ 73.52 (d, J = 26.2 Hz) and δ 66.86 (d, J = 26.2 Hz) for 6 was seen with simultaneous growth of a singlet at δ 66.5 for complex 1116 (eq 6). Free benzothiophene was observed by 1H NMR spectroscopy. Similar palladium18 and nickel19 complexes with πcoordinating alkynes have been reported in the literature. Attempts to carbonylate complex 6 at 1 atm of CO to functionalize benzothiophene to 2H-thiochromen-2-one were not successful. Instead, the reaction yielded a thermodynamically stable dicarbonyl palladium complex, (dippe)Pd(CO)2 (12),20 (eq 7) and free benzothiophene. A plausible

In light of these experimental results, the thermodynamic stability of the thiapalladacycles was established as 6 ≫ 8 > 3. A similar trend was seen by Garcia and co-workers in their C−S activation studies of thiophene, benzothiophene, and dibenzothiophene using tris(triethylphosphine)platinum(0).12 Studies with [Ni(dippe)(μ-H)]2 in our group also show a related trend.3 Reactivity of 6 with Small Molecules. Since complex 6 was the most reluctant to reductively eliminate benzothiophene, its reactivity with small molecules was studied. In our search to model HDS processes we have discovered that thiametallacycles formed from the insertion of metals such as rhodium or iridium1,13 and platinum12 into benzothiophenes usually react with H2 to give 2-ethylthiophenol,14 2,3dihydrobenzothiophene,15 or ethylbenzene.16 We decided to carry out similar experiments with H2 to probe HDS with complex 6. The disappearance of complex 6 was observed in the 31P{1H} NMR spectrum when 6 was treated with H2 at 1− 5 atm, with simultaneous generation of the decomposition products 4 and 5. No other intermediates were observed by spectroscopy. 1H NMR spectroscopy showed no signals due to benzothiophene hydrogenation or desulfurization products. Spectroscopic data showed the formation of free benzothiophene. Garcia and co-workers12 have concluded that HDS reactions in platinum systems are promoted by the use of hydridic agents. With this knowledge we explored the use of various hydride sources to achieve HDS with the palladium system. Et3SiH, NaBH4, and LiAlH4 were reacted with 6, but unfortunately none of these produced any HDS products, as evidenced by NMR spectroscopy and GC-MS. Only free benzothiophene was observed in these studies. Palladacycles have since been known to insert alkynes17 into the Pd−C(sp2) and Pd−C(sp3) bonds. We investigated thiapallacycle alkyne insertion by the reaction of 6 with 1 equiv of diphenylacetylene. Complete reductive elimination of

explanation for this observation would be that the nucleophilic intermediate fragment A would be more likely to bind strongly to carbon monoxide via back-donation, hence favoring 12 instead of a carbonylated benzothiapalladacycle. Reaction of a Mixture of 1 and 2 with Ph2S. Although C−S bonds of thioethers are relatively weak, their reactions with transition metals have been less commonly studied. Recently, we reported C−S insertions into aryl, vinyl, and allyl sp2 and sp3 carbon centers of thioethers using [(dippe)Pt(NBE)] (NBE = norbornene) as a source of platinum(0).21 Reactions with [(dippe)Pt(NBE)] were done under relatively harsh conditions with temperatures >100 °C and long reaction times in contrast to the observations in this study with a mixture of 1 and 2. Addition of excess diphenyl sulfide to a mixture of 1 and 2 formed a light yellow solid, the C−S insertion adduct 13, which shows a characteristic pair of doublet resonances in the 31P{1H} NMR spectrum at δ 61.4 and 59.4 with J = 19.6 Hz. Complex 13 is stable indefinitely under an inert atmosphere at room temperature and was unchanged after heating at 80 °C in C6D6 for more than 8 h. Acyclic palladium complexes derived from the C−S activation of thioethers were more responsive to CO insertions than the cyclic palladium complexes derived from activation of thiophenes. A plausible explanation for this observation is that there may be a thermodynamically unfavorable insertion of CO in the cyclic complexes due to either the introduction of ring strain or a decrease in aromaticity, both of which would be absent upon CO insertion in the acyclic complexes. Hence, treatment of (dippe)Pd(SPh)(Ph) (13) with 1 atm of CO for 12 h affords new palladium complexes in solution. Complex 13 in either benzene or THF converted to four major phosphoruscontaining products that were observed in the 31P{1H} NMR spectrum. The products were identified by NMR spectroscopy and X-ray diffraction as (dippe)Pd(SPh)2 (14), (dippe)Pd(SPh)(COPh) (15), 12, and 4 (eq 8). E

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evaporation from a THF solution showed a cocrystallized mixture of 15 and 14 (∼7:1, respectively) (Figure 7). The two monodentate ligand sites were each modeled as a disorder of ligands SPh and C(O)Ph (0.79:0.21 and 0.33:0.67, for the two sites, respectively).

In addition to the Pd-containing products, formation of the organic thioester product S-phenyl benzothioate (6%) as well as the reductive elimination product of 13 diphenyl sulfide (3%) was observed in the 1H NMR spectrum. This reaction shows the success of carbonylation of thioethers to thioesters using palladium, although in a very inefficient fashion. Similar carbonylation reactions of thioethers have been shown by Hillhouse and co-workers using [(2,2-bipyridine)Ni(1,5-cyclooctadiene)].22 The mixture of the complexes in eq 8 could not be separated by any method attempted, and the 1H NMR spectra were too complicated to assign the resonances to individual species. The 31 1 P{ H} NMR (C6D6) spectrum of the products in eq 8 revealed the presence of previously independently synthesized 4 and 12; thus, we decided to independently synthesize complexes 14 and 15 for comparison. Addition of an excess of diphenyl disulfide to a mixture of 1 and 2 afforded a yellow solid with a singlet resonance at δ 85.0 in the 31P{1H} NMR (C6D6) spectrum (eq 9), matching the data for 14 in eq 8. Figure 7. ORTEP drawings of (dippe)Pd(SPh)2 (14) (top) and (dippe)Pd(SPh)(COPh) (15) (bottom) Selected bond lengths (Å): Pd(1)−C(7), 2.046(3); Pd(1)−C(7′), 2.051(7); Pd(1)−P(1), 2.2883(4); Pd(1)−P(2), 2.3362(4); Pd(1)−S(1′), 2.3876(14); Pd(1)−S(1), 2.4038(6).

Complex 15 was independently synthesized from the C−S bond activation of S-phenyl benzothioate by a mixture of 1 and 2 to form a yellow solid containing 15 and 14 coproduced in a 4:1 ratio, respectively (eq 10). Solid-state IR spectroscopy

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CONCLUSIONS In summary, we have synthesized and characterized C−S activation adducts of benzothiophene, dibenzothiophene, 3methylbenzothiophene, thianthrene, thioxanthene, diphenyl sulfide, and S-phenyl benzothioate using the reactive proposed intermediate (dippe)Pd0 (A). The thermodynamic stability of the thiapalladacycles has been studied via competition experiments to show that (dippe)Pd(κ2C,S-benzothiophene) (6) is much more stable than (dippe)Pd(κ2C,S-dibenzothiophene) (8) and (dippe)Pd(κ2C,S-thiophene) (3). Activation of the C− S bond of the thioether diphenyl sulfide to yield (dippe)Pd(SPh)(Ph) (13) was explored. Carbonylation of complex 13 produced a mixture of complexes: (dippe)Pd(SPh)2 (14), (dippe)Pd(SPh)(COPh) (15), (dippe)Pd(CO)2 (12), and (dippe)2Pd (4) but most importantly the carbonylated organic product S-phenyl benzothioate in low yield.

showed that the carbonyl stretch in complex 15 appears at 1604 cm−1, which differs from the free S-phenyl benzothioate carbonyl stretch at 1663 cm−1. Attempts to separate 14 and 15 were unsuccessful. X-rayquality crystals grown from the yellow solid by slow F

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tube, and the reaction was monitored by NMR spectroscopy over the course of 12 h. Initially, 31P NMR spectroscopy showed two sets of pairs of doublets at δ 73.52 (d, J = 26.2 Hz) and 66.86 (d, J = 26.2 Hz) attributed to 6 and at δ 71.86 (d, J = 19.4 Hz) and 68.60 (d, J = 19.4 Hz) for 8, respectively. In the course of 12 h, 8 gradually disappeared, leaving behind 6 and free dibenzothiophene in solution. Complex 4 was also observed in the 31P{1H} NMR spectrum. Establishing the Relationship between (dippe)Pd(κ2C,Sthiophene) (3) and (dippe)Pd(κ2C,S-dibenzothiophene) (8). Equimolar amounts of 3 (15 mg, 0.033 mmol) and 8 (17 mg, 0.033 mmol) were dissolved in 0.6 mL of THF-d8 in a J. Young NMR tube, and then the reaction was monitored by 31P NMR spectroscopy over the course of 12 h. Complex 8 remained in solution while 3 decomposed to form free thiophene and complex 4. General Procedure for the Synthesis of the C−S Insertion Products. Under a nitrogen atmosphere, NaHBEt3 (91 μL of 1 M solution in toluene, 0.091 mmol) was added slowly to a stirred suspension of [(dippe)PdCl2] (20 mg, 0.046 mmol) in 0.5 mL of toluene and 4 equiv of substrate. The resulting dark red solution was stirred for 30−45 min and then filtered through Celite on a fine glass frit. The solvent was evaporated under vacuum overnight to yield C−S insertion products. The products were washed with 6 mL of either hexane or pentane. (dippe)Pd(κ2C,S-benzothiophene) (6). Complex 6 was synthesized (7.5 mg, 32%) using 4 equiv of benzothiophene (24 mg, 0.18 mmol). 1 H NMR (400 MHz, THF-d8): δ 7.66 (d, J = 7.8 Hz, 1H), 7.57−7.26 (m, 2H), 7.11 (d, J = 7.6 Hz, 1H), 6.82 (dt, J = 38.1, 7.1 Hz, 2H), 2.60−2.24 (m, 4H), 2.11−1.79 (m, 4H), 1.40−1.13 (m, 24H). 13 C{1H} NMR (126 MHz, THF-d8): δ 138.91 (s), 138.04 (s), 135.87 (d, J = 9.1 Hz), 131.38 (s), 130.50 (s), 129.78 (d, J = 10.3 Hz), 124.11 (s), 119.62 (s), 22.71−22.29 (m), 20.35−19.45 (m), 18.86 (d, J = 5.0 Hz), 18.60 (d, J = 5.1 Hz), 17.66 (s), 17.39 (s), 13.41 (s). 31P{1H} NMR (162 MHz, THF-d8): δ 73.52 (d, J = 26.2 Hz), 66.86 (d, J = 26.2 Hz) Anal. Calcd (found) for C22H38PdP2S·0.3CH2Cl2: C, 50.68 (50.43); H, 7.36 (7.34). (dippe)Pd(κ2C,S-3-methylbenzothiophene) (7). Complex 7 (20% NMR yield) could not be isolated in pure form by any method attempted. 13C and 1H NMR spectra could not be obtained. However, 31 1 P{ H} NMR spectroscopy showed the presence of 7 (major product) and complexes 2, 4, and 5 as minor products. Surprisingly, the oily reddish brown product formed crystals, which were confirmed by X-ray diffraction as 7 (see the Supporting Information for X-ray diffraction details). 31P{1H} NMR (162 MHz, THF-d8, 25 °C): δ 70.20 (d, J = 26.3 Hz), 65.31 (d, J = 26.3 Hz). (dippe)Pd(κ2C,S-dibenzothiophene) (8). Complex 8 was synthesized (7.6 mg, 30%) using 4 equiv of dibenzothiophene (33 mg, 0.18 mmol). 1H NMR (500 MHz, CD2Cl2): δ 8.20 (br, s, 2H), 7.88 (br, s, 2H), 7.49 (br, s, J = 2.6 Hz, 4H), 2.50−2.27 (m, 4H), 2.01−1.87 (m, 2H), 1.72−1.59 (m, 2H), 1.40−1.11 (m, 24H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 139.95 (s), 136.07 (s), 127.34 (s), 124.98 (s), 123.33 (s), 122.15 (s), 26.69 (d, J = 26.7 Hz), 25.27 (d, J = 17.7 Hz), 24.81−24.34 (m), 20.40−19.84 (m), 19.09−18.46 (m). 31P{1H} NMR (162 MHz, THF-d8, 25 °C): δ 71.86 (d, J = 19.4 Hz). 68.60 (d, J = 19.4 Hz) Anal. Calcd (found) for C26H40PdP2S: C, 56.47 (56.24); H, 7.29 (7.21). (dippe)Pd(thianthrene) (9). Complex 9 was synthesized (15.7 mg, 59%) using 4 equiv of thianthrene (39 mg, 0.18 mmol). 1H NMR (500 MHz, THF-d8): δ 7.42 (d, J = 7.5 Hz, 1H), 7.33 (d, J = 7.7 Hz, 1H), 7.16−7.04 (m, 2H), 6.86 (t, J = 6.6 Hz, 1H), 6.70 (dt, J = 31.1, 7.1 Hz, 2H), 6.57 (t, J = 6.9 Hz, 1H), 2.62−2.32 (m, 3H), 2.21−1.76 (m, 5H), 1.45 (dd, J = 17.3, 7.1 Hz, 3H), 1.39−1.28 (m, 9H), 1.28−1.18 (m, 6H), 1.15 (dd, J = 10.7, 7.0 Hz, 3H), 0.39 (dd, J = 14.5, 6.9 Hz, 3H). 13 C{1H} NMR (126 MHz, THF-d8): δ 165.67 (d, J = 4.4 Hz), 164.61 (d, J = 4.4 Hz), 153.33 (s), 144.75 (s), 137.67 (s), 134.79 (s), 134.60 (d, J = 8.8 Hz), 134.21 (s), 133.36 (d, J = 7.0 Hz), 128.03 (d, J = 7.9 Hz), 127.75 (s), 120.65 (s), 27.54 (d, J = 24.3 Hz), 26.36 (d, J = 19.1 Hz), 24.10−23.55 (m), 22.45 (d, J = 22.4 Hz), 21.39 (d, J = 6.2 Hz), 20.59 (s), 20.46 (d, J = 4.8 Hz), 20.32 (d, J = 5.3 Hz), 20.06 (d, J = 6.0 Hz), 19.60 (d, J = 4.0 Hz), 18.95 (s), 18.40 (s), 17.16 (s), 16.02 (d, J = 6.6 Hz). 31P{1H} NMR (162 MHz, THF-d8, 25 °C): δ 74.37 (d, J =

EXPERIMENTAL SECTION

General Procedures. All operations and routine manipulations were performed under a nitrogen atmosphere, either on a high-vacuum line using modified Schlenk techniques or in a Vacuum Atmospheres Corp. Dri-Lab. Benzene-d6 and THF-d8 were purchased from Cambridge Isotope Laboratories, Inc. Prior to use, benzene-d6 and THF-d8 were distilled under vacuum from a dark purple solution of benzophenone ketyl and stored in an ampule with a Teflon valve. CD2Cl2 was dried with CaH2 and stored over molecular sieves. THF and toluene were distilled from dark purple solutions of benzophenone ketyl and stored over molecular sieves in an ampule with a Teflon valve. Thiophene (99+%) was purchased from SigmaAldrich Co. and dried under CaCl2. (CH3CN)2PdCl2, benzothiophene, dibenzothiophene, 3-methylbenzothiophene, 2-methyldibenzothiophene, thioxanthene, thianthrene, diphenyl disulfide, and diphenyl sulfide were purchased from Sigma-Aldrich Co. and used without further purification. S-Phenyl benzothioate23 and dippe7 ligand were synthesized via the reported literature procedures. All 1H and 13C NMR spectra were collected on either a Bruker Avance 400 or Avance 500 MHz spectrometer. All chemical shifts were reported in ppm (δ) relative to tetramethylsilane and referenced to the chemical shifts of residual solvent resonances (C6HD5, δ 7.16 or 128.0). While 1H chemical shifts are given to three decimal places (±0.4 Hz), these values can vary slightly with concentration and temperature. 13C shifts are given to two decimal places (±1 Hz). 31 1 P{ H} NMR spectra were referenced relative to external H3PO4. Elemental analysis was performed by the University of Rochester using a PerkinElmer 2400 Series II elemental analyzer in CHN mode. GCMS spectra were recorded on a Shimadzu QP2010 GCMS instrument. Infrared spectra were recorded in the solid state on a Thermo Scientific Nicolet 4700 FT-IR spectrometer equipped with a smart orbit diamond attenuated total reflectance (ATR) accessory. Establishing the Equilibrium between Pd(dippe)(μ-H)]2 (1) and [(μ-dippe)Pd]2 (2). NaHBEt3 (45.4 μL of 1 M solution in toluene) was added slowly to a stirred suspension of [(dippe)PdCl2] (10 mg, 22.7 μmol) to give a dark red solution. Two singlet resonances were observed at δ 61.3 (1) and 33.1 (2) in a ratio of 9:1 in the 31 1 P{ H} NMR spectrum. (The same ratio was seen in C6H6 or THF solvent.) After freeze/pump/thaw degassing of the solution, 1 gradually disappeared as 2 grew in. New chemical shifts were also observed as minor decomposition products, identified as [Pd(dippe)2] (4) and [Pd2(dippe)2(μ-dippe)] (5), which show a singlet resonance at δ 45.5 and two multiplets at δ 49.5 and 54.8, respectively. Addition of H2 (1 atm) to a solution of 2 resulted in 1 gradually growing back, establishing the equilibrium ratio between 1 and 2 of 9:1, respectively. Establishing the Relationship between (dippe)Pd(κ2C,Sthiophene) (3) and (dippe)Pd(κ2C,S-benzothiophene) (6). Equimolar amounts of 3 (15 mg, 0.033 mmol) and 6 (17 mg, 0.033 mmol) were dissolved in 0.6 mL of THF-d8, and the reaction was monitored via 31P{1H} NMR spectroscopy. Initially two sets of pairs of doublets were observed in the 31P{1H} NMR spectrum: one at δ 74.59 and δ 67.10 (J = 25.5 Hz) attributed to 3 and another set at δ 73.52 (d, J = 26.2 Hz) and δ 66.86 (d, J = 26.2 Hz) for 6. The dark red solution mixture was monitored for 12 h, over which time 3 decomposed, leaving behind 6 as the only C−S activated product and free thiophene. It was also observed that the concentration of 4 increased (singlet at δ 46.03 in the 31P{1H} NMR spectrum) throughout the experiment. Alternatively, a solution of 3 (15 mg, 0.033 mmol) was dissolved in 0.6 mL THF-d8 and 1 equiv of benzothiophene (4.4 mg, 0.033 mmol) was introduced. Within a few minutes the complete disappearance of resonances for 3 and the growth of a pair of doublets at δ 73.52 (d, J = 26.2 Hz) and δ 66.86 (d, J = 26.2 Hz) for 6 was observed via 31P{1H} NMR spectroscopy. Complex 4 was also observed in the 31P{1H} NMR spectrum. Free thiophene was formed from the decomposition of 3. Establishing the Relationship between (dippe)Pd(κ2C,Sdibenzothiophene) (8) and (dippe)Pd(κ2C,S-benzothiophene) (6). Equimolar amounts of (dippe)Pd(κ2C,S-dibenzothiophene) (15 mg, 0.027 mmol) and (dippe)Pd(κ2C,S-benzothiophene) (14 mg, 0.027 mmol) were dissolved in 0.6 mL of THF-d8 in a J. Young NMR G

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Organometallics

the tips of thin glass optical fibers and mounted on a Bruker SMART CCD platform diffractometer for data collection.24 For each crystal a preliminary set of cell constants and an orientation matrix were calculated from reflections harvested from three orthogonal wedges of reciprocal space. Full data collections were carried out using Mo Kα radiation (0.71073 Å, graphite monochromator) with frame times ranging from 25 to 60 s and at a detector distance of approximately 4 cm. Randomly oriented regions of reciprocal space were surveyed: four to six major sections of frames were collected with 0.50° steps in ω at four to six different φ settings and a detector position of −38° in 2θ. The intensity data were corrected for absorption.25 Final cell constants were calculated from the xyz centroids of approximately 4000 strong reflections from the actual data collections after integration.26 Structures were solved using SIR201127 and refined using SHELXL2014.28 Space groups were determined on the basis of systematic absences, intensity statistics, or both. Direct-methods solutions were calculated, which provided most non-hydrogen atoms from the E map. Full-matrix least-squares/difference Fourier cycles were performed which located the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. Full-matrix least-squares refinements on F2 were run to convergence. In structures with multiple molecules in the asymmetric unit, only one is shown in the figures and referred to in the discussions because either the second independent molecule was disordered or the two independent molecules were metrically identical. Full details for each structure can be found in the Supporting Information.

25.9 Hz), 67.60 (d, J = 25.9 Hz). Anal. Calcd (found) for C26H40PdP2S2·0.25C6D6: C, 54.63 (55.24); H, 6.92 (6.38). (dippe)Pd(thioxanthene) (10). Complex 10 was synthesized (11.9 mg, 46%) with 4 equiv of thioxanthene (36 mg, 0.18 mmol). 1H NMR (400 MHz, THF-d8): δ 7.22 (d, J = 7.6 Hz, 1H), 7.12 (t, J = 7.0 Hz, 1H), 6.99 (d, J = 7.3 Hz, 1H), 6.92−6.87 (m, 1H), 6.74−6.58 (m, 4H), 4.53 (d, J = 11.2 Hz, 1H), 3.65 (d, J = 11.1 Hz, 1H), 2.69−2.07 (m, 4H), 2.00−1.77 (m, 4H), 1.52−1.21 (m, 18H), 1.14 (dd, J = 10.5, 7.0 Hz, 3H), 0.30 (dd, J = 14.4, 7.0 Hz, 3H). 13C{1H} NMR (126 MHz, THF-d8): δ 154.24 (d, J = 4.3 Hz), 148.29 (d, J = 4.7 Hz), 144.77 (d, J = 8.3 Hz), 142.28 (s), 137.82 (d, J = 8.1 Hz), 134.37 (d, J = 8.3 Hz), 128.15 (s), 126.80 (d, J = 7.6 Hz), 125.98 (s), 124.64 (d, J = 6.7 Hz), 123.94 (s), 121.66 (s), 49.99 (s), 27.66 (d, J = 25.0 Hz), 24.09 (t, J = 22.0 Hz), 21.60 (dd, J = 38.2, 12.9 Hz), 20.62−20.03 (m), 19.53 (s), 18.50 (s), 16.87 (s), 16.22 (d, J = 6.8 Hz). 31P{1H} NMR (162 MHz, THF-d8): δ 70.53 (d, J = 22.8 Hz), 64.16 (d, J = 22.9 Hz) Anal. Calcd (found) for C27H42PdP2S: C, 57.19 (57.35); H, 7.47 (7.54). (dippe)Pd(Ph)(SPh) (13). Complex 13 was synthesized (10.5 mg, 41%) with 4 equiv of diphenyl sulfide (30.1 μL, 0.18 mmol). 1H NMR (400 MHz, C6D6): δ 7.60 (d, J = 7.2 Hz, 2H), 7.51 (t, J = 7.2 Hz, 2H), 7.04−6.83 (m, 6H), 2.10 (td, J = 14.6, 7.2 Hz, 2H), 1.75 (td, J = 14.5, 7.2 Hz, 2H), 1.37 (dd, J = 16.0, 7.2 Hz, 8H), 0.91 (dd, J = 13.0, 7.0 Hz, 8H), 0.73 (dd, J = 15.6, 7.1 Hz, 12H). 13C{1H} NMR (126 MHz, C6D6): δ 162.55 (dd, J = 122.6, 7.9 Hz), 146.93−146.76 (m), 138.72 (s), 136.12 (s), 127.29 (s), 127.20 (s), 122.64 (s), 25.20 (d, J = 3.7 Hz), 25.05 (d, J = 10.1 Hz), 23.75 (t, J = 21.8 Hz), 20.30 (t, J = 17.6 Hz), 20.12 (d, J = 5.0 Hz), 19.22 (d, J = 3.2 Hz), 18.77 (s), 17.91 (s) 31 1 P{ H} NMR (162 MHz, C6D6): δ 61.38 (d, J = 19.9 Hz), 59.35 (d, J = 19.6 Hz). Anal. Calcd (found) for C26H42PdP2S: C, 56.26 (56.45); H, 7.63 (7.65). (dippe)Pd(SPh)2 (14). Complex 14 was synthesized (15.2 mg, 56%) with 4 equiv of diphenyl disulfide (39 mg, 0.18 mmol). 1H NMR (400 MHz, CD2Cl2): δ 7.30 (d, J = 7.3 Hz, 4H), 6.86 (m, 6H), 2.57 (s, 4H), 1.89 (m, 4H), 1.28 (m, 24H). 13C{1H} NMR (126 MHz, CD2Cl2) δ146.64 (s), 134.24 (s), 127.17 (s), 122.08 (s), 26.77 (d, J = 24.7 Hz), 22.85−22.09 (m), 20.42 (s), 18.88 (s). 31P{1H} NMR (162 MHz, CD2Cl2): δ 82.08 (s). Anal. Calcd (found) for C26H42PdP2S2: C, 53.19 (52.08); H, 7.21 (6.58). (dippe)Pd(SPh)(COPh)(15). A 4 equiv amount of S-phenyl benzothioate (39 mg, 0.18 mmol) was used. The aromatic region of the 1H NMR and 31P{1H} NMR spectra of the yellow solid confirmed coproduction of 15 (81%) and 14 (19%). The 1H NMR spectrum’s aliphatic region was difficult to characterize due to the presence of dippe ligand resonances from both complexes 14 and 15 (see the Supporting Information for spectra). 1H NMR (aromatic region) of 15 (400 MHz, THF-d8): δ 7.84 (d, J = 6.3 Hz, 2H), 7.14 (d, J = 7.2 Hz, 3H), 7.04 (d, J = 7.7 Hz, 2H), 6.54 (d, J = 7.3 Hz, 3H). 13C NMR (126 MHz, CD2Cl2): δ 146.40 (dd, J = 7.7, 2.8 Hz), 145.99−145.52 (m), 135.01 (s), 130.82 (s), 129.81 (s), 127.88 (s), 127.11 (s), 122.52 (s), 25.10 (d, J = 14.1 Hz), 23.33 (t, J = 22.8 Hz), 19.87 (d, J = 5.7 Hz), 19.74−19.49 (m), 18.79 (s). 31P{1H} NMR (162 MHz, THF-d8): δ 65.09 (d, J = 31.1 Hz), 58.36 (d, J = 31.0 Hz). The missing CO resonance of 15 in the 13C{1H} NMR spectrum was confirmed by IR (ATR, cm−1): νCO(stretch) 1604, differing from the carbonyl resonance of free S-phenyl benzothioate at 1663 cm−1. Carbonylation of Complex 13. A J. Young NMR tube was loaded with 11 (29.6 mg, 0.053 mmol) dissolved in 0.5 mL of C6D6. As a reference 1,4-dioxane (4.5 μL, 0.053 mmol) was dissolved in the solution subsequently. The tube was freeze/pump/thaw degassed before exposing the solution to 1 atm of CO. The reaction solution was then heated to 80 °C for 12 h. The reaction was judged complete after 12 h by the complete disappearance of a pair of doublets in the 31 1 P{ H} NMR spectrum at δ 61.38 (d, J = 19.9 Hz) and δ 59.35 (d, J = 19.6 Hz) for complex 13. Simultaneous formation of the new products 4 (3%), 12 (30%), 14 (12%), 15 (46%), S-phenyl benzothioate (6%), and diphenyl sulfide (3%) was observed by 31P{1H} and 1H NMR spectroscopy (yields by NMR). General Description of X-ray Structural Determinations: Single-Crystal X-ray Crystallography. Crystals were placed onto

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ASSOCIATED CONTENT

* Supporting Information S

Tables, figures, and CIF files giving crystallographic information, including data collection parameters, bond lengths, fractional atomic coordinates, and anisotropic thermal parameters, for complexes 6−10, 14, 15, and (dippe)Pd(Cl)2 (CCDC deposition nos. 1050732−1050738 and 1052414) and NMR spectra for new products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for W.D.J.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Science Foundation (NSF Grant CHE-1360985) for their support of this work. This paper is dedicated to the memory of Gregory L. Hillhouse, a giant in group 10 organometallic chemistry.

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REFERENCES

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DOI: 10.1021/acs.organomet.5b00194 Organometallics XXXX, XXX, XXX−XXX