borate Complex - American Chemical Society

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Oxidative Addition of Chlorohydrocarbons to a Rhodium Tris(pyrazolyl)borate Complex Yunzhe Jiao, William W. Brennessel, and William D. Jones* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *

ABSTRACT: The reactive fragment [Tp′Rh(PMe3)], generated from the thermal precursor Tp′Rh(PMe3)(Me)H, is found to cleave the C−Cl bonds of chlorohydrocarbons under mild conditions. Reaction with chloromethane gives clean formation of an initial C−H activation product, which rearranges to form the C−Cl activation product at 30 °C. Reaction with dichloromethane or benzyl chloride gives a mixture of C−Cl activation products as well as products from chlorination. Reaction with chlorocyclohexane gives a mixture of intermediates from C−H activation, which react further upon heating to give a C−Cl cleavage product as well as the β-chloride elimination product Tp′Rh(PMe3)(Cl)H plus cyclohexene. Complete conversion from a C−H activation product to a C−Cl activation product was observed in the reaction with 1,2-dichloroethylene, where β-elimination is circumvented. Activation of 1-chlorobutane, 1,2dichloroethane, or 1,4-dichlorobutane gives a mixture of C−Cl activation products as well as Tp′Rh(PMe3)(Cl)H plus olefin. Similar to the case for activation of methylene chloride, C−Cl activation and hydride/chloride exchange was observed in the reaction with benzyl chloride, where C−H activation was not seen. The reaction with chlorobenzene gives isomeric species resulting from C−H activation, which react further to give the corresponding chloride derivatives upon heating. Reaction with pentachlorobenzene gives a cyclometalated product from C−H bond cleavage in the phosphine ligand. These reactions are compared and contrasted with related photoreactions with the [Tp′Rh(CNneopentyl)] analogue, where C−H activation is solely observed in most cases. Mechanistic studies suggest the spectator ligand dependent reactivity relies greatly on the dissociation energy of the Tp′Rh−L bond.

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well as in the related DFT calculations by Hall.24 The selectivity can be different, as seen in the reactions of the rhodium analogue (PNP)RhI with haloarenes, giving exclusive C−Hal bond addition.25 The absence of C−H activation is consistent with dominating thermodynamic factors relative to small kinetic selectivities, as indicated by DFT calculations.26 In comparison with activations of aryl halides, oxidative additions of alkyl halides are less common. A typical activation of a C−Hal bond by Ir(I) is the oxidative addition reaction of an alkyl halide to Vaska’s complex.27 Cundari calculated CH3−H vs CH3−X preferences where X = N, O, F, P, S, and Cl using an Ir(PH3)2(H) model, finding a significant thermodynamic preference for C−X activation but a kinetic preference for C−H activation.28 Our group investigated the activation of chloroalkanes using a rhodium tris(pyrazolyl)borate species.29,30 [Tp′Rh(CNneopentyl)] has been used for selective C−H activations of alkanes, alkenes, arenes, and alkynes.31−35 Even in reactions with functionalized hydrocarbons, C−H activation was still preferred to C−X activation (X = Cl, CN), although the latter would usually lead to a much more robust product. Similar to the case for the isonitrile derivative, highly regioselective cleavage of C−H bonds by [Tp′Rh(PMe3)] was also observed in the activation of substrates with functional groups.36,37

INTRODUCTION Selective C−H activation of functionalized substrates has attracted much attention recently due to the possibility for incorporation of versatile functional groups such as halo,1−3 cyano,4,5 and hydroxo6−8 in organic synthesis. In these cases, the functionalities remain unreactive under suitable conditions, allowing the targeted C−H bonds to be cleaved to form more complicated organic molecules. However, competitive C−X (X = CN, Cl, Br, I, OTf) bond activation can take place, especially when it involves breaking a weak C−X bond or forming a strong M−X bond.9−18 Therefore, investigation of the factors that control the preference of bond cleavage is crucial to the design of optimal routes in transition-metal-catalyzed processes. Halohydrocarbons are among the most commonly accessible functionalized feedstocks, which have been involved in many oxidative addition reactions of C−H and/or C−Hal bonds with unsaturated metal centers. There are a number of experimental and computational studies focusing on the C−Hal bond cleavage by zerovalent d10 metals.9,10,19−21 Recently, the group 9 metals have been extensively reported for activation of C−H bonds in the presence of C−Hal bonds. For example, Milstein has described an exclusive activation of the ortho C−H bonds in chloro- and bromobenzene via the cationic pincer complex [(PNP*)Ir]+.22 The kinetically favored C−H activation and thermodynamic preference for C−Hal activation of PhCl was observed by Ozerov23 using an analogous (PNP)IrI system as © 2015 American Chemical Society

Received: February 16, 2015 Published: April 15, 2015 1552

DOI: 10.1021/acs.organomet.5b00131 Organometallics 2015, 34, 1552−1566

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Organometallics For example, in the activation of fluoroalkanes and fluoroarenes, the C−F bond remained unreactive to [Tp′Rh(PMe3)],36,38 although it was cleaved in perfluoropyridine.39 In both examples, the halogen substituent gave additional stabilization to the resulting product Tp′Rh(L)(RX)(H) (L = CN-neopentyl, PMe3) on the basis of the relative distance between the substituent and the metal center, consistent with an inductive effect.29,34,36 To compare the influence of the spectator ligand in these reactions, we report here the activation of a wide range of chlorohydrocarbons using the analogous fragment [Tp′Rh(PMe3)]. Some interesting results on selectivity control of C−H vs C−Cl cleavage have been uncovered.

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RESULTS AND DISCUSSION Reactions with Chloroalkanes. Thermolysis of Tp′Rh(PMe3)(Me)H (1) or irradiation of Tp′Rh(PMe3)H2 (2) at ambient temperature generates the active fragment [Tp′Rh(PMe3)], which has been found to react with various types of C−H bonds of alkanes, alkenes, arenes, and alkynes.38 It has been reported that thermal exchange of 1 with CH3F resulted in the formation of the C−H activation product Tp′Rh(PMe3)(CH2F)H, which underwent reductive elimination of CH3F in benzene at 67 °C.38 Similarly, reaction of 1 with CH3Cl after several days at room temperature cleanly forms the C−H activation product Tp′Rh(PMe3)(CH2Cl)H (3) (eq 1), which contains a characteristic hydride resonance at

Figure 1. 31P{1H} NMR spectra for the activation of methylene chloride by [Tp′Rh(PMe3)]: (a) thermolysis of 1 in CH2Cl2 after 18 h at room temperature followed by replacement of CH2Cl2 by C6D6; (b) photolysis of 2 in CH2Cl2 after 20 min at −20 °C followed by replacement of CH2Cl2 by C6D6.

δ −17.479 with typical coupling constants (1JRh−H = 24 Hz, 2 JP−H = 29 Hz) (cf. for (Tp′Rh(PMe3)(CH3)H: 1JRh−H = 24 Hz, 2JP−H = 34 Hz). The two methylene hydrogen atoms are diastereotopic with an H−H coupling constant of ∼7 Hz. Addition of carbon tetrachloride to 3 gave the air-stable derivative Tp′Rh(PMe3)(CH2Cl)Cl (3-Cl), the structure of which was determined by X-ray analysis. 3 was expected to reductively eliminate chloromethane in benzene, like other Tp′Rh(PMe3)(R)H complexes. However, the known species Tp′Rh(PMe3)(CH3)Cl (1-Cl)38 was the only product formed after 1 month at room temperature (τ1/2 = 10.0 days at 30 °C; ΔG⧧ = 26.20(1) kcal mol−1). As the reaction took place in neat benzene-d6 and Tp′Rh(PMe3)(C6D5)D was not observed, isomerization of 3 to 1-Cl must therefore proceed via an intramolecular pathway. A competition experiment showed a kinetic preference for benzene over chloromethane by 0.3 kcal mol−1 in the C−H activation barriers (eq 2). In comparison to methane with a kmethane/kbenzene ratio of 1/2.55,38 activation of chloromethane is slightly faster than activation of methane (1.5×). With [Tp′Rh(CNneopentyl)], chloromethane reacted 2.5 times faster than methane.29 Reaction of 1 with methylene chloride resulted in a nonhydridic major product after standing overnight at 22 °C (eq 3). The structure was confirmed as Tp′Rh(PMe3)(CH2Cl) Cl (3-Cl) from C−Cl bond activation (72%) by NMR spectroscopy and X-ray crystallography (Figures 1a and 2). C−Cl activation of CH2Cl2 was also observed during irradiation of Tp′Rh(CNR)(PhNCNR) (R = neopentyl) in methylene

Figure 2. ORTEP drawing of the molecular structure of 3-Cl. All hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.

chloride.29 Other products that formed were assigned as the two known species Tp′Rh(PMe3)MeCl (1-Cl, 17%) and Tp′Rh(PMe3)Cl2 (2-Cl2, 11%). 3-Cl can also be generated by photolysis of 2 in methylene chloride at low temperature (eq 4, Figure 1b). Oxidative addition of the C−Cl bond of dichloromethane40−45 and chloroform46 to rhodium(I) has been observed previously. From a previous report,29 photolysis of Tp′Rh(CNR)(η2PhNCNR) (R = neopentyl) in 1-chlorobutane gave Tp′Rh(PMe3)(CH2(CH2)3Cl))H with no activation of the C−Cl bond at −20 °C. However, reaction of 1 with 1-chlorobutane at room temperature afforded a mixture of 2-Cl and the stable product 4-Cl in a 2.4/1 ratio after 3 days (eq 5). 1553


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Tp′Rh(PMe3)(1-chlorocyclohexyl)H resulting from α-C−H activation, one with the chlorine “up” (between two pyrazole rings), and one with the chlorine “down” (between the phosphine and hydride ligands). Hindered rotation around the rhodium−cyclopentyl bond was seen previously in Tp′Rh(PMe3)(c-pentyl)H.38 Their stabilities relative to the labile complexes 5a and 1 is also consistent with this assignment, as seen in Tp′Rh(CN-neopentyl)(CHClR)H derivatives with α-chloro substituents.29 5b,c can convert to 5-Cl and 2-Cl in a 1/3 ratio after 1 h at 70 °C (Figures 3c and 4c). These observations, especially the major quantity of 2-Cl, indicate that C−H activation is kinetically favored over C−Cl activation. However, the C−H activation products are too reactive to be observed, as the subsequent positional isomerization and rapid β-Cl elimination (cyclohexene was observed by GC-MS) dominates over isomerization to the C−Cl oxidative addition product. Like 4-Cl, 5-Cl can also be independently synthesized from the reaction of 2-Cl2 and (c-hexyl)MgCl. The structure of 5-Cl was characterized by NMR spectroscopy and X-ray analysis (Figure 5). Reactions with Dichloroalkanes. Previously, C−Cl bonds were observed to remain inert during photolysis of Tp′Rh(CNneopentyl)(η2-PhNCNR) in 1,4-dichlorobutane. Different results were found here in the activation of nongeminal dichlorohydrocarbons at [Tp′Rh(PMe3)]. Reaction of 1 with 1,2-dichloroethane after 3 days at room temperature gave the new non-hydridic product 6′-Cl as well as the two known species 2-Cl and 2-Cl2 in a 1/2.8/2.6 ratio (eq 7).

The NMR spectra of 4-Cl were consistent with those of Tp′Rh(PMe3)(n-butyl)Cl, which was independently synthesized from the reaction of 2-Cl2 and n-BuMgCl Grignard reagent. NMR resonances of 4-Cl resemble those of the known alkyl chloride species Tp′Rh(PMe3)(n-pentyl)Cl.38 The formation of 2-Cl as the major product suggested the existence of the C−H addition adduct Tp′Rh(PMe 3 )(CH(CH 2 Cl)CH2CH3)H (4), which underwent fast β-chloride elimination to give 2-Cl. Additional evidence to support this assumption is the observation of 1-butene in the reaction by GC-MS and 1H NMR spectroscopy. Activation of secondary C−H bonds was seen in the reaction of 1 with chlorocyclohexane at room temperature, which led to the formation of a mixture of three hydridic species (5a−c), the C−Cl activation product Tp′Rh(PMe3)(c-hexyl)Cl (5-Cl), and 2-Cl in a ratio of 2/1/6 (eq 6, Figures 3a and 4a). On

Chloroethylene and ethylene were seen by GC-MS and 1H NMR spectroscopy. The pure form of 6′-Cl was isolated and identified as Tp′Rh(PMe3)(η1-CHCH2)Cl by NMR spectroscopy and X-ray analysis (Figure 6). Note that the formation of Tp′Rh(PMe3)(CH2CH2Cl)Cl (6-Cl) from C−Cl cleavage of 1,2-dichloroethane was not seen in this reaction. The 1H NMR spectrum of 6′-Cl displays the geminal methylene resonances as two doublets at δ 5.24 and 5.66. The methine hydrogen has a characteristic splitting pattern of an overlapping dddd at δ 7.95 due to coupling with rhodium, phosphorus, and the two neighboring methylene hydrogen atoms. These typical resonances corresponded to a singlet at δ 122.43 for the methylene carbon and a doublet of doublets at δ 147.99 for the methine carbon in the 13C{1H} NMR spectrum. The 31P{1H} NMR spectrum of 6′-Cl shows a doublet at δ 1.415 with a rhodium−phosphorus coupling constant of 126 Hz. The crystal structure of 6′-Cl reveals an octahedral coordination sphere around the rhodium center with a κ3-Tp′ ligand. The C(16)−C(17) distance is 1.270(3) Å, indicating the presence of a double bond. Correspondingly, the Rh− C(16) distance (2.019(2) Å) is typical for a Rh−C(sp2) bond (cf. Rh−C(sp2) distance in Tp′Rh(PMe3)(CHCHtBu)Br: 2.008(2) Å38). The proposed mechanism for formation of these products is shown in eq 8. The reaction of 1 with dichloroethane initially proceeds via C−H activation and C−Cl activation to give two transient species 6 and 6-Cl. 6-Cl is

Figure 3. 1H NMR spectra for the reaction of 1 with chlorocyclohexane: (a) after 1 day at room temperature; (b) on heating at 30 °C after 16 h; (c) on heating at 70 °C after 1 h. The asterisks denote the two isomeric residues Tp′Rh(PMe3)(furanyl)H from the preparation of 1.

monitoring of the reaction at different temperatures, 5a and the starting material signals disappeared after 16 h at 30 °C, while hydride resonances for 5b,c remained at δ −17.301 (JRh−H = 24 Hz, JP−H = 31 Hz) and δ −17.879 (JRh−H = 26 Hz, JP−H = 29 Hz) (in chlorocyclohexane), corresponding to the two sets of doublet signals at δ 4.29 (J = 151 Hz), δ 3.28 (J = 153 Hz) in the 31P{1H} NMR spectrum (Figures 3a,b and 4a,b). 5a is assigned as arising from C−H activation at the 3- or 4-position of the chlorocyclohexane, as it goes away quickly. 5b,c can be assigned as the two α-chloro rotamers of 1554


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Figure 4. 31P{1H} NMR spectra for the reaction of 1 with chlorocyclohexane: (a) after 1 day at room temperature; (b) on heating at 30 °C after 16 h; (c) on heating at 70 °C after 1 h. The asterisks denote the two isomeric residues Tp′Rh(PMe3)(furanyl)H from the preparation of 1.

Figure 6. ORTEP drawing of the molecular structure of 6′-Cl. All hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.


fragment to give the C−Cl cleavage product 6′-Cl. The reaction was more complicated than that seen with the isocyanide analogue [Tp′Rh(CN-neopentyl)], which gives exclusive formation of Tp′Rh(CN-neopentyl)HCl in the photolysis with 1,2-dichloroethane.29 Activation of 1,4-dichlorobutane gave a mixture of the products 7-Cl and 2-Cl in a 1.2/1 ratio (eq 9). The structure of 7-Cl was confirmed as Tp′Rh(PMe3)((CH2)4Cl)Cl from C−Cl

thermally labile, and β-chloride elimination occurs rapidly to give the stable complex 2-Cl2 and ethylene. Conversion of 6 could follow an intramolecular C−Cl activation to give 6-Cl and/or undergo a similar β-Cl elimination to form 2-Cl. The released chloroethylene is rapidly seized by the reactive Rh

oxidative addition by solid-state X-ray analysis (Figure 7).29 As expected from β-Cl elimination, 4-chloro-1-butene was observed by GC-MS and by 1H NMR spectroscopy. Although no direct C−H activation product was observed, internal C−H oxidative addition was assumed to be an initial or a competitive 1555


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Figure 8. ORTEP drawing of the molecular structure of 8-Cl (cis). All hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.


(Figure 8, modeled as an isomeric disorder of cis to trans in a 87/13 ratio). Reactions with Chloroarenes. Similar to the case for activation of methylene chloride, reaction of 1 with benzyl chloride led to formation of 1-Cl, 2-Cl2, and the new nonhydridic species 9-Cl in nearly equal amounts (eq 11). The new

step in comparison to C−Cl activation, followed by rapid β-chloride elimination to give the stable species 7-Cl and 2-Cl. Additionally, reaction of 1 in cis-1,2-dichloroethylene overnight gave an initial product distribution of three non-hydridic species (8-η2, 8-Cl-cis, and 8-Cl-trans) and the hydridic species 8 in a 1.0/2.9/1.1/2.8 ratio (eq 10). The observation of a minor

product contains a 31P{1H} resonance at δ −0.85 with a Rh−P coupling constant of 131 Hz, typical for species of the type Tp′Rh(PMe3)(R)Cl. 9-Cl was isolated and the structure confirmed by X-ray analysis as the C−Cl oxidative addition product Tp′Rh(PMe3)(benzyl)Cl (Figure 9). The resonances for the two diastereotopic benzylic hydrogens appear as a

quantity of 2-Cl (18%) is from the side reaction in preparing 1, as the possibility of β-chloride elimination can be excluded in this case.38 8 may be assigned as the C−H adduct Tp′Rh(PMe3)(C(Cl)CHCl)H with a hydride resonance as a triplet at δ −17.55 (1JRh−H = 2JP−H = 25 Hz) as well as a 31 1 P{ H} resonance at δ 3.21 (1JRh−P = 138 Hz). Additional evidence for this assignment comes from the observation that 8 can convert to 8-Cl-cis and 8-Cl-trans with a half-life of ∼2 h at 70 °C (ΔG⧧ = 26.5 kcal mol−1), comparable to the reductive elimination rate of Tp′Rh(PMe3)(CHCH2tBu)H38 (ΔG⧧ = 28.0 kcal mol−1). In comparison to the complexes Tp′Rh(PMe3)(X)(Y), the 31 1 P{ H} resonance of 8-η2 shows a characteristic downfield shift (for RhI) at δ 12.40, as well as a large coupling constant of 153 Hz, leading to its assignment as the π-bound rhodium complex Tp′Rh(PMe3)(η2-CHClCHCl) (cf. 31P{1H} for Tp′Rh(PMe3)(η2-H3CCCCH3): δ 10.66, J = 160 Hz37). As expected, complete conversion of 8 and 8-η2 to form 8-Cl-cis and 8-Cl-trans was observed after 2 days at 70 °C in neat cis-1,2-dichloroethylene. Heating of the initial product mixture in neat C6D6 also led to growth of 8-Cl-cis and 8-Cl-trans without formation of Tp′Rh(PMe3)(C6D5)D, indicating an intramolecular pathway for rearrangement of 8 and 8-η2 to 8-Cl-cis and 8-Cl-trans. 8-Cl-cis and 8-Cl-trans were cocrystallized, and their structures were confirmed as the C−Cl cleavage products Tp′Rh(PMe3)(HCCHCl)Cl by X-ray analysis

Figure 9. ORTEP drawing of the molecular structure of 9-Cl. All hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level. 1556


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Organometallics doublet of doublets at δ 4.528 (2JRh−H = 3 Hz, 2JH−H = 11 Hz) and a doublet of triplets at δ 5.117 (2JRh−H = 3JP−H = 3 Hz, 2 JH−H = 11 Hz) in the 1H NMR spectrum. The methylene carbon displays a doublet of doublets at δ 16.37 (1JRh−C = 8 Hz, 2 JP−C = 21 Hz) in the 13C{1H} NMR spectrum. The resonances at δ 7.010 and 7.552 are assigned to the five hydrogen atoms on the phenyl ring. In addition to the aliphatic chlorohydrocarbons, aryl chlorides were also investigated for reactivities with 1. Reaction with chlorobenzene at room temperature for 1 day gave five isomeric sets of hydride-containing species 10a−e, with hydride resonances at δ −15.988 to −16.884 and corresponding phosphorus signals at δ 2.80−1.83 (Figure 10). 2-Cl is also

Tp′Rh(PMe3)(2-C6H4F)H, which appears at δ −16.32.36 Heating the mixture of C−H activation products led to interconversion between these hydrides, leaving 10a,b as the most stable isomers Tp′Rh(PMe3)(2-C6H4Cl)H after 6 h at 140 °C.34,36 Continued heating did not afford clean formation of the single product Tp′Rh(PMe3)(Ph)Cl as expected. Instead, with the consumption of the hydride species, the series of non-hydridic complexes 10a-Cl−10c-Cl grew up at 140 °C associated with three new sets of doublets at δ −2.17 to −2.51 (J = ∼125 Hz) in 31P{1H} NMR (Figure 11). 10a-Cl− 10c-Cl can presumably be attributed to the chloro derivatives Tp′Rh(PMe3)(C6H4Cl)Cl. Only the two stable rotamers Tp′Rh(PMe3 )(2-C6 H4Cl)Cl (10a-Cl and 10b-Cl) were observed after heating for 2 weeks. Pentachlorobenzene was selected for examination, as it contains only one possible C−H activation position. 1 was completely consumed in pentachlorobenzene after 4 days, resulting in a mixture of the one non-hydride species 11-Cl (76%) as well as a small amount of 2-Cl residue (24%) (eq 12).

The absence of hydride resonances from C−H activation was probably due to the large steric hindrance from two α-chlorine atoms. The sole product 11-Cl was characterized as the cyclometalated structure Tp′Rh(η2-CH2PMe2)Cl. The 31P{1H} resonance of 11-Cl displays a doublet with a large upfield chemical shift at δ −32.87 and an unusually small Rh−P coupling constant of 96 Hz. The two phosphine methyl groups appeared as two doublets at δ 1.789 and 1.864 with 2JP−H = 13 Hz in the 1H NMR spectrum, which corresponded to two doublets at δ 8.16 (1JP−C = 41 Hz) and 10.32 (1JP−C = 22 Hz) in the 13C{1H} NMR spectrum. The methylene group appeared as two broad doublets at δ 2.101 and 3.093 with 2JP−H = 8 Hz in the 1H NMR spectrum and a characteristic doublet of doublets

Figure 10. 1H NMR spectra for reaction of 1 with chlorobenzene at varying temperatures.

seen as the residue from the preparation of 1. 10a−e can be assigned as the C−H oxidative addition products Tp′Rh(PMe3)(C6H4Cl)H by comparison to the hydride resonance of

Figure 11. 31P{1H} NMR spectra for reaction of 1 with chlorobenzene at varying temperatures. 1557


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Organometallics at δ −8.96 (1JRh−C = 16 Hz, 1JP−C = 21 Hz) in the 13C{1H} NMR spectrum. The crystal structure of 11-Cl clearly displays the η2 coordination of the P−CH2 bond to the rhodium center, rendering a largely distorted octahedral geometry (Figure 12).

hydrocarbons by either exchange with 1 or photolysis of 2. Even in activation of relatively inert pentane or cyclopentane substrates, most of 1 remained unreactive prior to decomposition. To compare the stability of 11 with other hydridic species of Tp′Rh(PMe3)(R)H, the first-order rate of reductive elimination from 11 was measured in C6D6 at 30 °C by monitoring the disappearance of the hydride resonance in the 1 H NMR spectrum (eq 14). The half-life of 7.5 h suggests that

11 is actually more robust than 1 (τ1/2 = 0.6 h). In light of the stability of 11, it should have been observed in reactions of 1 with inert alkanes, unless its formation is kinetically disfavored due to geometrical limitations or exposure to the alkane solvent (i.e., an alkane σ complex forms). The hypothesis of formation of 11 prior to 11-Cl was tested by a control experiment in which the reaction of 11 with pentachlorobenzene was monitored under the same conditions as reaction of 1 with C6Cl5H. As expected, complete conversion of 11 to 11-Cl was observed after 1 week. This reaction rate of 11 with pentachlorobenzene is comparable to the rate of reaction in eq 12, suggesting the intermediacy of 11 in the reaction of 1 with pentachlorobenzene. DFT Study of C−Cl Activation in the Rearrangement of Tp′Rh(PMe3)(CH2Cl)H. Density functional theory (DFT) calculations were used to determine the reaction pathways and transition states for both C−H and C−Cl bond activations using the model reaction of [Tp′Rh(PMe3)] with chloromethane. The direct C−H reductive elimination/C−Cl oxidative addition pathway via two three-centered transition states and two high-energy η1 intermediates is described in Figure 13. The calculated energies are consistent with the experimental observations, in which C−H activation is kinetically preferred while C−Cl activation is thermodynamically favored. However, the calculated activation barrier for C−Cl activation to form 1-Cl from 3 is uphill by 36.4 kcal mol−1, which is much higher than the experimental value of 26.2 kcal mol−1 from the kinetic measurement. Moreover, calculations of a similar pathway on the reaction of [Tp′Rh(CNMe)] with chloromethane show little ligand dependence for C−H/C−Cl selectivities (see the Supporting Information for details), which is in contrast to the fact that C−Cl activation was not observed in the rearrangement of Tp′Rh(CNMe)(CH2Cl)H at room temperature or at 80 °C.29 Goldman and Choi also found an overestimation of the activation barrier in their calculations of C−F addition of CH3F to a [(PCP)Ir] fragment.50 The activation barrier was lowered by introducing an indirect pathway where C−H bond cleavage occurred initially, followed by α-fluorine migration to form the methylidene fluoride hydride intermediate (PCP)Ir(CH2)(F)H before giving the C−F activation product (PCP)Ir(CH3)F. On the basis of the analogy between chloromethane and fluoromethane activation, a similar carbene intermediate is proposed here with related migration transition states in the pathway from S2 to S1 (Figure 14). The combination of the methylidene complex Tp′Rh(CH2)(Cl)H (S5) with free trimethylphosphine was found to have a lower energy than (κ3-Tp′)(PMe3)Rh(CH2)(Cl)H or (κ2-Tp′)(PMe3)Rh(CH2)(Cl)H. The RhCH2 distance of 1.83 Å is

Figure 12. ORTEP drawing of the molecular structure of 11-Cl. Hydrogen atoms, except on C16, are omitted for clarity. Ellipsoids are shown at the 50% probability level.

The Rh(1)−Cl(1) distance is 2.34 Å, slightly shorter than that of 1-Cl (2.36 Å). The restricted three-membered ring contains Rh(1)−P(1) and Rh(1)−C(16) distances of 2.21 and 2.14 Å, respectively, both shorter than those of 1-Cl by 0.05 and 0.08 Å, respectively. The similar rhodacycle Tp′Rh(η2-C6H42-PMe2)H was previously reported via intramolecular C−H activation from photolysis of Tp′Rh(PPhMe2)H2 or heating of Tp′Rh(PPhMe2)(C6D5)D.36 A search of the Cambridge Structural Database shows 30 transition-metal complexes containing similar M−C−P rings, and only 3 of them contain Rh−CH2−P ring structures.47−49 The formation of 11-Cl is interpreted in terms of the sterically crowded pentachlorobenzene being resistant to reaction with the unsaturated fragment [Tp′Rh(PMe3)] generated from 1. Therefore, the neighboring phosphine C−H bond was recruited for activation, affording the hydride intermediate Tp′Rh(η2-CH2PMe2)H (11). The hydride then underwent chlorination by reaction with excess pentachlorobenzene to give 11-Cl. To test this proposal, 11 was independently prepared by reducing 11-Cl with Cp2ZrH2 in THF (eq 13). The structure of 11 is representative, with an

upfield hydride resonance at δ −21.135 as a doublet of doublets with a small Rh−H coupling constant of 16 Hz and a slightly large P−H coupling constant of 35.7 Hz (cf. hydride resonance for 1: δ −18.14 with 1JRh−H = 24 Hz and 2JP−H = 34 Hz). Similar to the case for 11-Cl, the 31P{1H} NMR of 11 possesses a doublet at δ −35.24 with a small JRh−P value of 113 Hz. 11 was expected to be very labile, as it had never been observed as an intermediate in the C−H activation of other 1558


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Figure 13. Free energy landscape for C−H and C−Cl bond activation of chloromethane with [Tp′Rh(PMe3)] (free energies in kcal mol−1 at 298 K). Gas-phase values are shown in red (italics), and PCM-corrected values are shown in black. All energies are relative to the C−Cl activation product (S1). The energies of [Tp′Rh(PMe3)] + chloromethane are 25.6 kcal mol−1 (gas phase) and 26.1 kcal mol−1 (PCM).

Figure 14. Free energy landscape for the rearrangement reaction of Tp′Rh(PMe3)(CH2Cl)H to Tp′Rh(PMe3)(CH3)Cl (free energies in kcal mol−1 at 298 K). Gas-phase values are shown in red (italics), and PCM-corrected values are in black. All energies are relative to the C−Cl activation product (S1), and intermediates S3 and S4 both include an external PMe3.

significantly shorter than the Rh−C single bond. The two other intermediates Tp′Rh(CH3)Cl (S3) and Tp′Rh(CH2Cl)H (S4) were optimized by removing phosphine from the corresponding ground states of Tp′Rh(PMe3)(CH3)Cl (S1) and Tp′Rh(PMe3)(CH2Cl)H (S2). By using a potential energy surface scan beginning with S3 and increasing one of the C−H bond distances of Rh−CH3, we found a transition state (TS1) for C−H cleavage and connected S3 and S5 in an α-hydrogen migration. The Rh−H distance of 1.63 Å in the transition state is only slightly longer than the Rh−H bond distance of 1.55 Å in S5, indicative of a strong Rh−H interaction in TS1. Similarly, a transition state (TS2) for α-chlorine migration between rhodium and the methylidene carbon was located using a potential energy scan that increased the C−Cl distance beginning with S4 and proceeding to S5. All optimized stationary points along the reaction pathway are shown in Figure 15, and selected structural parameters are shown in Table 1. A complete list of the free energies for each stationary point is shown in Table 2.

In the gas phase, the C−Cl activation product (S1) lies 21.6 kcal mol−1 lower in energy than the C−H activation product (S2) (Figure 14). Application of a solvation model (PCM, benzene) slightly increases the selectivity to 22.6 kcal mol−1 in energy. The two transition states TS1 and TS2 are close in energy, indicating that thermodynamic factors play a larger role in favoring C−Cl formation. In comparison to the experimental C−Cl activation barrier (ΔG = 26.2 kcal mol−1), DFT calculations seem to underestimate the barrier as 16.1 kcal mol−1 in the gas phase (PCM, 14.7 kcal mol−1). Along the reaction coordinate, ligand dissociation is most responsible to account for the energy gap for C−Cl activation. Chloride dissociation was not examined, as these reactions were conducted in benzene solvent, which would strongly disfavor ion formation. To analyze the ligand influence in C−Cl activation, DFT calculations have also been conducted in the rearrangement reaction from Tp′Rh(CNMe)(CH3)Cl (S6) to Tp′Rh(CNMe)(CH2Cl)H (S7). As shown in Figure 14, the spectator 1559


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Figure 15. Optimized structures of stationary points on the potential energy surface for the rearrangement reaction of Tp′Rh(L)(CH2Cl)H to Tp′Rh(L)(CH3)Cl (L = PMe3, CNMe3).

Table 1. Optimized Structural Parameters (Å, deg) for the rearrangement reaction of Tp′Rh(L)(CH2Cl)H to Tp′Rh(L)(CH3)Cl (L = PMe3, CNMe3).a

a

complex

Rh−C

Rh−Cl

C−Cl

C−Hb

Rh−Hb

Cl−Rh−C

H−Rh−Cb

Rh−Nc

S1 S2 S3 S4 S5 S6 S7 TS1 TS2

2.07 2.04 2.05 2.00 1.83 2.08 2.05 1.85 1.86

2.42 3.40 2.36 2.57 2.39 2.41 3.38 2.39 2.45

3.12 1.90 3.09 1.93 2.97 3.18 1.89 2.95 2.61

1.09 2.54 1.10 2.44 2.36 1.09 2.55 1.75 2.38

2.61 1.53 2.56 1.54 1.55 2.63 1.54 1.63 1.53

87.75 29.36 88.90 48.04 88.19 89.91 29.37 87.19 73.05

23.57 89.18 24.42 86.47 87.71 23.35 89.16 59.87 88.43

2.36 2.35 2.26 2.29 2.20 2.26 2.26 2.20 2.29

Optimized structural parameters (Å) for free chloromethane: C−H, 1.09; C−Cl, 1.83. bShortest Rh−H distance. cLongest Rh−N bond length.

Table 2. Free Energies (kcal mol−1) for Stationary Points in the Rearrangement Reaction of Tp′Rh(L)(CH2Cl)H to Tp′Rh(L)(CH3)Cl (L = PMe3, CNMe3) (B3LYP) ΔG

methylene species has a much higher activation barrier of 34.6 kcal mol−1 in the gas phase (PCM, 33.5 kcal mol−1) relative to that from the phosphine analogue S2, consistent with the experimentally observed ligand influence. The dissociative mechanism did not greatly lower the barrier for C−Cl activation, as a similar activation barrier (37.4 kcal mol−1 in the gas phase, 37.4 kcal mol−1 in benzene) was calculated for the direct oxidative addition pathway (see the Supporting Information). The selectivity difference in C−Cl activation is attributed to the relative ease of ligand dissociation from Tp′Rh(L)(CH2Cl)H (L = PMe3 vs CNMe). To test the hypothesis of ligand dissociation in Figure 14, we monitored the kinetics of rearrangement of Tp′Rh(PMe3)(CH2Cl)H (3) in C6D6 at 30 °C with the addition of free trimethylphosphine. The first trial of adding trimethylphosphine (10 equiv) to 3 led to the formation (73%) of the known compound (κ2-Tp′)Rh(PMe3)3 as the major product after 4 days. Therefore, only 0.1 equiv of trimethylphosphine was added in a separate experiment to give a clean conversion

ΔG

complex

gas phase

PCM

complex

gas phase

PCM

S1 S3 + PMe3 TS1+ PMe3 S5 + PMe3 TS2 + PMe3 S4 + PMe3 S2

0.00 10.17 37.73 33.09 37.60 31.57 21.60

0.00 8.97 37.23 32.68 37.56 33.02 22.57

S6 S3 + CNMe TS1 + CNMe S5 + CNMe TS2 + CNMe S4 + CNMe S7

0.00 29.33 56.89 52.26 56.76 50.74 22.34

0.00 27.63 55.89 51.34 56.22 51.68 23.52

ligand L only participates in the first step of ligand dissociation from Tp′Rh(L)(CH3)Cl. Therefore, the same set of intermediates (S3−S5) and transition states (TS1 and TS2) was used together with the free methyl isocyanide to connect S6 and S7 (Figures 15 and 16). C−Cl cleavage from S7 via the 1560


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Figure 16. Free energy landscape for the rearrangement reaction of Tp′Rh(CNMe)(CH2Cl)H to Tp′Rh(CNMe)(CH3)Cl (free energies in kcal mol−1 at 298 K). Gas-phase values are shown in red (italics), and PCM-corrected values are shown in black. All energies are relative to the C−Cl activation product (S6). filter and a 270−370 nm low-pass filter. All 1H, 13C{1H}, and 31P{1H} NMR spectra were collected on either a Bruker Avance 400 or an Avance 500 MHz spectrometer. All 1H chemical shifts are reported in ppm (δ) relative to the chemical shift of residual solvent (benzene-d6 (δ 7.160), CDCl3 (δ 7.260), or cyclohexane-d6 (δ 1.400)). 13C{1H} NMR spectra were referenced to benzene-d6 (δ 128.00) or CDCl3 (δ 77.16). 31P{1H} NMR spectra were referenced to external H3PO4 (δ 0.00). 1H chemical shifts are given to three decimal places (±0.4 Hz), which is the typical resolution employed while recording these spectra. 13C{1H} and 31P{1H} shifts are given to two decimal places (±1 Hz), which is the typical resolution employed while recording these spectra. These values can vary slightly with concentration and temperature. All kinetic plots and least-squares error analysis were done using Microsoft Excel. Elemental analysis was performed by the University of Rochester using a PerkinElmer 2400 Series II elemental analyzer in CHN mode. GC-MS spectra were recorded on a Shimadzu QP2010 instrument. A Bruker-AXS SMART platform diffractometer equipped with an APEX II CCD detector was used for X-ray crystal structure determination. Room temperature is defined as 22 °C. Computational Details. The crystal structures of 1-Cl and 3-Cl were used as the starting points for the calculations. The only simplication used was the replacement of neopentyl isocyanide with methyl isocyanide for S6 and S7 in Figures 15 and 16 and Tables 1 and 2. This substitution has been used in previous studies and does not have a large impact on the electronics of the rhodium fragment.52 The gas-phase structures were fully optimized in redundant internal coordinates,53 with density functional theory (DFT) and a wave function incorporating Becke’s three-parameter hybrid functional (B3),54 along with the Lee−Yang−Parr correlation functional (LYP).55 All calculations were performed using the Gaussian09 package.56 The Rh, Cl, and P atoms were represented with the effective core pseudopotentials of the Stuttgart group and the associated basis sets improved with a set of f polarization functions for Rh (α = 1.350) and a set of d polarization functions for Cl (α = 0.640) and P (α = 0.387).57,58 The remaining light atoms (C, H, B, and N) were represented by a 6-31G** basis set.59 The geometry optimizations were performed without any symmetry constraints, and the local minima and the transition states were checked by frequency calculations. For each transition-state structure, the intrinsic reaction coordinate (IRC) routes were calculated in both directions toward the corresponding minima. Because of the polarity of the structures, the solvent effects on their relative stabilities were evaluated by calculating the free energies of the solvation in terms of the polarizable continuum model (PCM). The solution-phase structures were also fully optimized to include the solvent effect in geometries. The self-consistent reaction field (SCRF) calculations using the PCM-UA0 solvation model60 were

of 3 to 1-Cl. In comparison to the reaction in the absence of added phosphine (k = [8.00(10)] × 10−7 s−1), phosphine addition slows the first-order decay of 3 by a factor of ∼3 (k = [2.93(14)] × 10−7 s−1). The inhibition by free phosphine suggests that the spectator ligand must dissociate before or during the rate-determining step for C−Cl cleavage, although the inhibition is not as strong as one would expect on the basis of the calculations.

■

CONCLUSIONS In summary, we report that the complex Tp′Rh(PMe3)(CH3)H (1) serves as a thermal precursor to activate C−H and C−Cl bond of chlorohydrocarbons at ambient temperature. Reactions of 1 with chloro-substituted hydrocarbons give C−H oxidative addition complexes as the kinetic products, which can rearrange to form products from C−Cl cleavage and/or β-chloride elimination. The mechanism of C−Cl activation in chloromethane was investigated by kinetic studies and DFT calculations. A plausible C−Cl activation pathway is associated with the generation of a methylidene intermediate after losing the spectator ligand. The ligand dissociation energy is likely to be the key factor in determining the activation barrier for C−Cl bond cleavage.

■

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. Chloromethane was purchased from Aldrich and used straight from lecture bottles. Dichloromethane was dried over activated alumina and was distilled prior to use. 1-Chlorobutane was purchased from Fluka and was dried and degassed prior to use. Chlorocyclohexane, cis-1,2-dichloroethylene, 1,4-dichlorobutane, chlorobenzene, pentachlorobenzene, and carbon tetrachloride were purchased from Aldrich Chemical Co. and dried over 4A molecular sieves prior to use. 1,2-Dichloroethane was purchased from Fisher Scientific and was dried and degassed prior to use. Benzene-d6 and cyclohexane-d12 were purchased from Cambridge Isotopes and were distilled under vacuum from CaH2 and stored in ampules with Teflon valves. Preparations of Tp′Rh(PMe3)(CH3)H (1)38 and Tp′Rh(PMe3)H2 (2)51 have been reported. All photolysis experiments were performed using a 200 W Hg(Xe) arc lamp purchased from Oriel, which was fitted with a water-filled IR 1561


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Activation of 1-Chlorobutane. A resealable NMR tube was charged with a solution of 1 (∼10 mg, 0.019 mmol) in 0.5 mL of 1-chlorobutane. The resulting colorless solution was kept at room temperature and monitored by NMR spectroscopy. After 3 days, the reaction was complete and resulted in a mixture of Tp′Rh(PMe3)(Cl) H (70%) and 4-Cl (30%) by 1H and 31P{1H} NMR spectroscopy. 1-Butene (56, 55, 41, 39 m/z) was observed as the byproduct of the reaction by GC-MS. It was also observed in the 1H NMR spectrum of the reaction mixture (∼11% by integration). Preparation of Tp′Rh(PMe3)(n-butyl)Cl (4-Cl). n-Butylmagnesium chloride (40 μL of a 2 M solution in THF, 0.080 mmol) was added dropwise to 40 mg (0.073 mmol) of 2-Cl2 in 15 mL of THF. Upon addition of the Grignard reagent, the solution changed from orange to yellow. The reaction mixture was stirred for 20 min. The reaction mixture was quenched with a saturated solution of NH4Cl(aq) until all had reacted to give a clear solution. The volatiles were removed under vacuum, the resulting residue was dissolved in a minimum amount of CH2Cl2, and the solution was filtered through Celite and layered with hexane for recrystallization. Light orange-yellow crystals were collected (39.3 mg, 95%) and dissolved in C6D6. Data for 4-Cl are as follows. 1H NMR (400 MHz, C6D6): δ 0.875 (m, 4 H, 2 × CH2), 1.103 (d, 2JP−H = 10.1 Hz, 9 H, PMe3), 1.525 (t, 3 H, CH3), 2.075 (s, 3 H, pzCH3), 2.094 (s, 3 H, pzCH3), 2.174 (s, 3 H, pzCH3), 2.275 (s, 3 H, pzCH3), 2.677 (s, 3 H, pzCH3), 2.742 (m, 1 H, RhCH2), 2.907 (s, 3 H, pzCH3), 4.015 (m, 1 H, RhCH2), 5.552 (s, 1H, pzH), 5.639 (s, 1 H, pzH), 5.693 (s, 1 H, pzH). 13C{1H} NMR (500 MHz, CDCl3): δ 12.97 (s, 1 C, pzCH3), 13.08 (s, 1 C, pzCH3), 13.83 (s, 1 C, pzCH3), 14.17 (s, 1 C, pzCH3), 14.26 (s, 1 C, pzCH3), 16.17 (s, 1 C, pzCH3), 16.17 (s, 1 C, n-butyl), 16.73 (d, 1JP−C = 32.9 Hz, 3 C, P(CH3)3), 18.12 (dd, 1JRh−C = 7.4 Hz, 2JP−C = 20.5 Hz, 1 C, Rh−C), 24.87 (s, 1 C, n-butyl), 34.63 (s, 1 C, n-butyl), 107.71 (s, 1 C, pzCH), 107.80 (d, 4 JP−C = 4.0 Hz, 1 C, pzCH), 108.83 (s, 1 C, pzCH), 142.72 (d, 4JP−C = 2.5 Hz, 1 C, pzCq), 143.61 (s, 1 C, pzCq), 144.81 (s, 1 C, pzCq), 151.38 (s, 1 C, pzCq), 151.82 (s, 1 C, pzCq), 151.89 (d, 3JP−C = 4.4 Hz, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ −0.036 (d, 1 JRh−P = 130.6 Hz). Anal. Calcd (found) for C22H40BClN6PRh· hexane0.14: C, 47.23 (47.23); H, 7.28 (7.27); N, 14.47 (14.42) (n-hexane seen in NMR spectrum; Figure S7, Supporting Information). Activation of Chlorocyclohexane. A resealable NMR tube was charged with a solution of 1 (∼10 mg, 0.019 mmol) in 0.5 mL of chlorocyclohexane. The resulting colorless solution was kept at room temperature and monitored by NMR spectroscopy. After 1 day, the reaction was almost complete and resulted in a mixture of Tp′Rh(PMe3)(Cl)H, 5-Cl, and other hydridic species (see text) in a ratio of 5.8/1/1.9. The reaction mixture was then heated to 70 °C for 2 h, resulting in the formation of Tp′Rh(PMe3)(Cl)H (77%) and 5-Cl (23%) by 1H and 31P{1H} NMR spectroscopy. Cyclohexene (82, 67, 54, 41, 39 m/z) was observed as the byproduct of the reaction by GC-MS. Preparation of Tp′Rh(PMe3)(c-hexyl)Cl (5-Cl). A procedure similar to that for 4-Cl was employed, except that cyclohexylmagnesium chloride was used as the Grignard reagent (53% yield). Data for 5-Cl are as follows. 1H NMR (400 MHz, C6D6): δ 1.145 (d, 2JP−H = 10.0 Hz, 9 H, PMe3), 1.227 (m, 2 H, c-hexyl), 1.519 (m, 4 H, c-hexyl), 1.753 (br, 1 H, c-hexyl), 2.005 (m, 2 H, c-hexyl), 2.148 (s, 3 H, pzCH3), 2.163 (s, 3 H, pzCH3), 2.187 (s, 3 H, pzCH3), 2.257 (s, 3 H, pzCH3), 2.637 (s, 3 H, pzCH3), 2.781 (m, 1 H, c-hexyl), 3.056 (s, 3 H, pzCH3), 3.877 (m, 1 H, RhCH), 5.579 (s, 1H, pzH), 5.606 (s, 1 H, pzH), 5.670 (s, 1 H, pzH). 13C{1H} NMR (500 MHz, C6D6): δ 12.88 (s, 1 C, pzCH3), 13.07 (s, 1 C, pzCH3), 13.80 (s, 1 C, pzCH3), 15.95 (s, 1 C, pzCH3), 16.04 (s, 1 C, pzCH3), 16.06 (d, 1JP−C = 32.3 Hz, 3 C, P(CH3)3), 16.70 (s, 1 C, pzCH3), 28.18 (s, 1 C, c-hexyl), 30.49 (s, 1 C, c-hexyl), 31.19 (s, 1 C, c-hexyl), 31.96 (dd, 1JRh−C = 6.7 Hz, 2JP−C = 21.3 Hz, 1 C, Rh−C), 38.00 (s, 1 C, c-hexyl), 40.03 (s, 1 C, c-hexyl), 107.97 (s, 1 C, pzCH), 108.46 (d, 4JP−C = 3.5 Hz, 1 C, pzCH), 110.15 (s, 1 C, pzCH), 142.71 (d, 4JP−C = 2.5 Hz, 1 C, pzCq), 143.20 (s, 1 C, pzCq), 144.64 (s, 1 C, pzCq), 151.22 (s, 1 C, pzCq), 151.82 (s, 1 C, pzCq), 152.74 (d, 3JP−C = 4.1 Hz, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ −0.011 (d, 1JRh−P = 135.5 Hz). Anal. Calcd (found) for C24H42BClN6PRh·0.2(hexane): C, 49.46 (49.28); H, 7.38 (7.25);

carried out for the gas-phase optimized structures as well as the PCM optimized structures. The dielectric constant in the PCM calculations was set to ε = 2.2706 to simulate benzene as the solvent medium used in the experimental study. Gibbs free energies have been calculated at 298.15 K and 1 atm. Activation of Chloromethane. An ∼40 mg portion of 1 (0.076 mmol) was dissolved in 1 mL of C6D12 and transferred to a medium-wall Teflon-valved NMR tube, followed by pressurization with chloromethane at 50 psi. The tube was kept at room temperature, and the starting material was almost depleted after 1 day, as monitored by NMR spectroscopy. Some white precipitates formed during the reaction. The solution phase was separated from the solid phase by filtration. The NMR spectra of the two portions (liquid and redissolved solid) display identical resonances indicative of Tp′Rh(PMe3)(CH2Cl)H (3) (NMR yield: 95%) as the major product. However, the precipitates contains a small fraction of the known species Tp′Rh(PMe3)(Cl)H. Treatment of the solution of 3 with CCl4 led to the formation of Tp′Rh(PMe3)(CH2Cl)Cl (3-Cl) (46%), Tp′Rh(PMe3)Cl2 (2-Cl2) (29%), and Tp′Rh(PMe3)(CH3)Cl (1-Cl) (25%) after 1 week at room temperature. Data for 3 are as follows. 1H NMR (500 MHz, C6D6): δ −17.479 (1JRh−H = 23.9 Hz, 2JP−H = 28.6 Hz, 1 H, RhH), 1.332 (d, 2JP−H = 9.5 Hz, 9 H, PMe3), 2.097 (s, 3 H, pzCH3), 2.115 (s, 3 H, pzCH3), 2.184 (s, 3 H, pzCH3), 2.217 (s, 3 H, pzCH3), 2.258 (s, 3 H, pzCH3), 2.789 (s, 3 H, pzCH3), 4.794 (dt, 2 JRh−H = 2.3 Hz, 3JP−H = 3JH−H = 6.5 Hz, 1 H, RhCH2), 4.887 (dd, 2 JRh−H = 3.6 Hz, 3JH−H = 7.0 Hz, 1 H, RhCH2), 5.582 (s, 1 H, pzH), 5.603 (s, 1 H, pzH), 5.708 (s, 1H, pzH). 13C{1H} NMR (500 MHz, C6D6): δ 12.70 (s, 1 C, pzCH3), 13.01 (s, 1 C, pzCH3), 13.04 (s, 1 C, pzCH3), 15.51 (s, 1 C, pzCH3), 15.66 (s, 1 C, pzCH3), 16.69 (s, 1 C, pzCH3), 19.07 (d, 1JP−C = 31.4 Hz, 3 C, P(CH3)3), 38.58 (dd, 1JRh−C = 13.4 Hz, 2JP−C = 32.0 Hz, 1 C, Rh−C), 106.43 (s, 1 C, pzCH), 106.44 (d, 4JP−C = 2.7 Hz, 1 C, pzCH), 107.35 (s, 1 C, pzCH), 143.14 (d, 4 JP−C = 2.7 Hz, 1 C, pzCq), 143.84 (s, 1 C, pzCq), 144.31 (s, 1 C, pzCq), 149.29 (s, 1 C, pzCq), 150.04 (s, 1 C, pzCq), 150.56 (s, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ 4.83 (d, 1JRh−P = 147.2 Hz). Activation of Methylene Chloride. Method A. An ∼10 mg portion (0.019 mmol) of 1 was synthesized in situ, dissolved in 0.5 mL of methylene chloride, and transferred to a resealable 5 mm NMR tube. The reaction was complete after 18 h at room temperature, at which point the solvent was removed under vacuum and the residue dissolved in C6D6. The crude products contained as the major species Tp′Rh(PMe3)(CH2Cl)Cl (3-Cl) (72%) as well as small fractions of 1-Cl (17%) and 2-Cl2 (11%). 3-Cl (7 mg, 66%) was isolated as a light yellow solid and purified by chromatography on silica gel using 5/1 hexane/THF as the eluent. 2-Cl was recrystallized from methylene chloride/hexanes at room temperature to give light yellow crystals. Method B. A resealable NMR tube was charged with 5 mg (0.010 mmol) of 2 and 0.5 mL of CH2Cl2. The colorless solution was cooled to −20 °C in an isopropyl alcohol/N2 bath and photolyzed for 20 min. The solvent was removed from the resulting pale yellow solution under vacuum to give a mixture of Tp′Rh(PMe3)(CH2Cl)Cl (3-Cl) (67%), Tp′Rh(PMe3)(Cl)H (2-Cl) (25%), and 2 (8%) by 1H and 31P{1H} NMR spectroscopy. Data for 3-Cl are as follows. 1H NMR (500 MHz, C6D6): δ 0.975 (d, 2JP−H = 10.4 Hz, 9 H, PMe3), 2.054 (s, 3 H, pzCH3), 2.151 (s, 3 H, pzCH3), 2.242 (s, 3 H, pzCH3), 2.252 (s, 3 H, pzCH3), 2.609 (s, 3 H, pzCH3), 3.076 (s, 3 H, pzCH3), 5.229 (ddd, 2JRh−H = 2.2 Hz, 3JH−H = 6.7 Hz, 3JP−H = 8.8 Hz, 1 H, RhCH2), 5.531 (s, 1H, pzH), 5.632 (s, 1 H, pzH), 5.646 (s, 1 H, pzH), 6.173 (dd, 2JRh−H = 3.2 Hz, 3JH−H = 6.5 Hz, 1 H, RhCH2). 13C{1H} NMR (500 MHz, C6D6): δ 12.78 (s, 1 C, pzCH3), 12.91 (s, 1 C, pzCH3), 13.34 (s, 1 C, pzCH3), 14.45 (s, 1 C, pzCH3), 15.46 (d, 1 JP−C = 33.4 Hz, 3 C, P(CH3)3), 16.24 (s, 1 C, pzCH3), 16.35 (s, 1 C, pzCH3), 37.88 (dd, 1JRh−C = 9.9 Hz, 2JP−C = 25.3 Hz, 1 C, Rh−C), 108.15 (d, 4JP−C = 4.5 Hz, 1 C, pzCH), 108.74 (s, 1 C, pzCH), 109.09 (s, 1 C, pzCH), 142.46 (d, 4JP−C = 2.5 Hz, 1 C, pzCq), 144.47 (s, 1 C, pzCq), 144.63 (s, 1 C, pzCq), 152.01 (s, 1 C, pzCq), 152.81 (d, 3 JP−C = 4.5 Hz, 1 C, pzCq), 153.57 (s, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ 1.08 (d, 1JRh−P = 123.3 Hz). Anal. Calcd (found) for C19H33BCl2N6PRh: C, 40.67 (40.89); H, 5.93 (5.89); N, 14.98 (14.91). 1562


Article

Organometallics

Tp′Rh(PMe3)(trans-HCCHCl)Cl (8-Cl-trans), Tp′Rh(PMe 3)(C(Cl)CHCl)H (8), and Tp′Rh(PMe 3)(η 2-HC(Cl)CHCl) (8-η2) as well as the residue of 2-Cl with a ratio of 2.9/1.1/2.8/ 1.0/1.7. The reaction mixture was then heated to 70 °C for 2 days, resulting in the formation of 8-Cl-cis and 8-Cl-trans (2.0/0.6/1 relative to 2-Cl) by 1H and 31P{1H} NMR spectroscopy. 1 (∼50 mg, 0.095 mmol) was used for a larger scale reaction to isolate 8-Cl-cis and 8-Cl-trans (total yield 25.3 mg, 46%) from 2-Cl by chromatography on silica gel using 5/1 hexane/acetone as the eluent. Light yellow crystals were formed by slow evaporation of the THF solution layered with hexane. The chloroethenyl ligand was modeled as disordered with two orientations (87/13, cis/trans) in the X-ray structure. Data for 8 are as follows. 1H NMR (500 MHz, 1,2-dichloroethylene): δ −17.55, (dd, 1JRh−H = 2JP−H = 24.6 Hz). 31P{1H} NMR (400 MHz): δ 3.205 (d, 1 JRh−P = 138.3 Hz). Data for 8-η2 are as follows. 31P{1H} NMR (400 MHz): δ 12.395 (d, 1JRh−P = 152.9 Hz). Data for 8-Cl-cis are as follows. 1H NMR (400 MHz, C6D6): δ 1.042 (d, 2JP−H = 10.5 Hz, 9 H, PMe3), 2.096 (s, 3 H, pzCH3), 2.154 (s, 3 H, pzCH3), 2.210 (s, 3 H, pzCH3), 2.258 (s, 3 H, pzCH3), 2.688 (s, 3 H, pzCH3), 2.840 (s, 3 H, pzCH3), 5.586 (s, 1H, pzH), 5.644 (s, 1 H, pzH), 5.667 (s, 1 H, pzH), 6.387 (d, 3JH−H = 6.0 Hz, 1 H, RhCHCHCl), 7.723 (septet 1 H, J = 3.1 Hz, RhCH). 13C{1H} NMR (500 MHz, C6D6): δ 12.81 (s, 1 C, pzCH3), 12.92 (s, 1 C, pzCH3), 13.39 (s, 1 C, pzCH3), 15.46 (s, 1 C, pzCH3), 15.78 (d, 1JP−C = 33.7 Hz, 3 C, P(CH3)3), 16.32 (s, 1 C, pzCH3), 17.12 (s, 1 C, pzCH3), 107.44 (d, 4JP−C = 4.6 Hz, 1 C, pzCH), 107.89 (s, 1 C, pzCH), 108.73 (s, 1 C, pzCH), 124.05 (s, 1 C, CHCl), 136.08 (dd, 1JRh−C = 13.3 Hz, 2JP−C = 28.6 Hz, 1 C, Rh−C), 142.07 (d, 4JP−C = 2.5 Hz, 1 C, pzCq), 144.24 (s, 1 C, pzCq), 144.41 (s, 1 C, pzCq), 152.81 (s, 1 C, pzCq), 153.15 (d, 3JP−C = 4.3 Hz, 1 C, pzCq), 153.64 (s, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ 0.61 (d, 1JRh−P = 118.0 Hz). Data for 8-Cl-trans are as follows. 1H NMR (400 MHz, C6D6): δ 0.981 (d, 2JP−H = 10.6 Hz, 9 H, PMe3), 1.991 (s, 3 H, pzCH3), 2.073 (s, 3 H, pzCH3), 2.141 (s, 3 H, pzCH3), 2.241 (s, 3 H, pzCH3), 2.627 (s, 3 H, pzCH3), 2.840 (s, 3 H, pzCH3), 5.450 (s, 1H, pzH), 5.482 (s, 1 H, pzH), 5.509 (s, 1 H, pzH), 7.781 (dt, 1 H, 3JH−H = 13.1 Hz, 3JRh−H = 4JP−H = 3.5 Hz, RhCH), 8.147 (d, 3 JH−H = 7.2 Hz, 1 H, RhCHCHCl). 13C{1H} NMR (500 MHz, C6D6): δ 12.57 (s, 1 C, pzCH3), 12.87 (s, 1 C, pzCH3), 13.27 (s, 1 C, pzCH3), 14.90 (s, 1 C, pzCH3), 15.95 (d, 1JP−C = 34.3 Hz, 3 C, P(CH3)3), 16.12 (s, 1 C, pzCH3), 17.06 (s, 1 C, pzCH3), 108.20 (d, 4 JP−C = 4.5 Hz, 1 C, pzCH), 109.11 (s, 1 C, pzCH), 111.116 (s, 1 C, pzCH), 129.93 (s, 1 C, CHCl), 136.46 (dd, 1JRh−C = 12.4 Hz, 2JP−C = 27.0 Hz, 1 C, Rh−C), 142.53 (d, 4JP−C = 2.2 Hz, 1 C, pzCq), 144.24 (s, 1 C, pzCq), 144.98 (s, 1 C, pzCq), 151.75 (s, 1 C, pzCq), 152.93 (d, 3JP−C = 4.0 Hz, 1 C, pzCq), 153.49 (s, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ 1.31 (d, 1JRh−P = 118.1 Hz). Anal. Calcd (found) for C20H33BCl2N6PRh· 0.4(hexane): C, 44.20 (44.28); H, 6.30 (6.40); N, 13.44 (13.94). Activation of Benzyl Chloride. A resealable NMR tube was charged with a solution of 1 (∼10 mg, 0.019 mmol) in 0.5 mL of benzyl chloride. The resulting colorless solution was kept at room temperature and monitored by NMR spectroscopy. After the reaction mixture stood overnight, the reaction was complete and resulted in a mixture of Tp′Rh(PMe3)(benzyl)Cl (9-Cl) (35%), Tp′Rh(PMe3)(CH3)Cl (34%), and Tp′Rh(PMe3)(Cl)H (31%). 1 (∼90 mg, 0.171 mmol) was used for a larger scale reaction to prepare a pure form of 9-Cl (48.1 mg, 47%) by chromatography on silica gel using 5/1 hexane/THF as the eluent. 9-Cl was recrystallized from methylene chloride/hexanes at room temperature to give orange-yellow crystals. Data for 9-Cl are as follows. 1H NMR (400 MHz, C6D6): δ 0.966 (d, 2 JP−H = 10.3 Hz, 9 H, PMe3), 1.936 (s, 3 H, pzCH3), 2.148 (s, 3 H, pzCH3), 2.214 (s, 3 H, pzCH3), 2.270 (s, 3 H, pzCH3), 2.673 (s, 3 H, pzCH3), 2.752 (s, 3 H, pzCH3), 4.528 (dd, 1 H, 2JRh−H = 2.6 Hz, 2 JH−H = 11.2 Hz, RhCH2), 5.117 (dt, 1 H, 2JRh−H = 3JP−H = 3.5 Hz, 2 JH−H = 11.2 Hz, RhCH2), 5.443 (s, 1 H, pzH), 5.539 (s, 1 H, pzH), 5.670 (s, 1H, pzH), 7.010 (m, 3 H, Ph), 7.552 (d, 2 H, J = 6.2 Hz, Ph). 13 C{1H} NMR (500 MHz, C6D6): δ 12.85 (s, 1 C, pzCH3), 12.98 (s, 1 C, pzCH3), 13.61 (s, 1 C, pzCH3), 14.86 (s, 1 C, pzCH3), 16.12 (s, 1 C, pzCH3), 16.16 (d, 1JP−C = 32.9 Hz, 3 C, P(CH3)3), 16.37 (dd, 1 C, 1 JRh−C = 7.9 Hz, 2JP−C = 21.0 Hz, RhCH2), 17.03 (s, 1 C, pzCH3), 108.46

N, 13.73 (13.56) (n-hexane seen in NMR spectrum; Figure S10, Supporting Information). Activation of 1,2-Dichloroethane. A resealable NMR tube was charged with a solution of 1 (∼10 mg, 0.019 mmol) in 0.5 mL of 1,2-dichloroethane. The resulting colorless solution was kept at room temperature and monitored by NMR spectroscopy. After 3 days, the reaction was complete and resulted in a mixture of 2-Cl (44%), 2-Cl2 (41%), and Tp′Rh(PMe3)(η1-CHCH2)Cl (6′-Cl) (15%) by 1H and 31 1 P{ H} NMR spectroscopy. Chloroethylene (62, 27, 26 m/z) and ethylene (28, 27, 26 m/z) were seen by GC-MS and by 1H NMR spectroscopy (∼15% C2H3Cl and ∼10% C2H4 by integration). 1 (∼200 mg, 0.380 mmol) was used for a larger scale reaction to isolate 6′-Cl (36.6 mg, 18%) by chromatography on silica gel using 5/1 hexane/THF as the eluent. 6′-Cl was recrystallized from methylene chloride/hexanes at room temperature to give light yellow crystals. Data for 6′-Cl are as follows. 1H NMR (500 MHz, C6D6): δ 1.09 (d, 2 JP−H = 10.4 Hz, 9 H, PMe3), 2.08 (s, 3 H, pzCH3), 2.11 (s, 3 H, pzCH3), 2.18 (s, 3 H, pzCH3), 2.29 (s, 3H, pzCH3), 2.70 (s, 3 H, pzCH3), 2.88 (s, 3 H, pzCH3), 5.24 (d, 3JH−H = 16.4 Hz, 1H, RhCHCH2), 5.52 (s, 1 H, pzH), 5.66 (d, 3JH−H = 8.3 Hz, 1H, RhCHCH2), 5.57 (s, 1 H, pzH), 5.70 (s, 1 H, pzH), 7.95 (nonet (overlapped dddd), 1 H, RhCH). 13C{1H} NMR (500 MHz, C6D6): δ 12.76 (s, 1 C, pzCH3), 12.93 (s, 1 C, pzCH3), 13.47 (s, 1 C, pzCH3), 15.75 (s, 1 C, pzCH3), 15.98 (d, 1JP−C = 33.8 Hz, 3 C, P(CH3)3), 16.17(s, 1 C, pzCH3), 17.69 (s, 1 C, pzCH3), 108.43 (s, 1 C, pzCH), 108.01 Hz (d, 4JP−C = 4.9 Hz, 1 C, pzCH), 108.85 (s, 1 C, pzCH), 122.43 (s, 1 C, CH2), 142.41 (d, 4JP−C = 2.8 Hz, 1 C, pzCq), 143.96 (s, 1 C, pzCq), 144.94 (s, 1 C, pzCq), 147.99 (dd, 1JRh−C = 12.0 Hz, 2 JP−C = 25.1 Hz, 1 C, Rh−C), 151.82 (s, 1 C, pzCq), 152.88 (d, 3 JP−C = 3.8 Hz, 1 C, pzCq), 153.12 (s, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ 1.415 (d, 1JRh−P = 125.9 Hz). Anal. Calcd (found) for C20H34BClN6PRh: C, 44.59 (44.55); H, 6.36 (6.36); N, 15.60 (15.56). Ethylene (28, 27, 26 m/z) and chloroethylene (62, 27 m/z) were detected as the byproducts of the reaction by GC-MS. Activation of 1,4-Dichlorobutane. A resealable NMR tube was charged with a solution of 1 (∼10 mg, 0.019 mmol) in 0.5 mL of 1,4-dichlorobutane. The resulting colorless solution was kept at room temperature and monitored by NMR spectroscopy. After 2 days, the reaction was complete and resulted in a mixture of 2-Cl (73%) and Tp′Rh(PMe3)((CH2)4Cl)Cl (7-Cl) (27%) by 1H and 31P{1H} NMR spectroscopy. A larger scale reaction of 1 (∼100 mg, 0.190 mmol) led to the formation of a different ratio of 2-Cl (45%), and 7-Cl (55%). 7-Cl (30.4 mg, 27%) was isolated by chromatography on silica gel using 10/1 hexane/THF as the eluent. 7-Cl was recrystallized from THF/hexanes at room temperature to give light yellow crystals. Data for 7-Cl are as follows. 1H NMR (500 MHz, C6D6): δ 1.084 (d, 2 JP−H = 10.2 Hz, 9 H, PMe3), 1.155 (m, 1 H, CH2), 1.419 (m, 1 H, CH2), 1.812 (m, 2 H, CH2), 2.061 (s, 3 H, pzCH3), 2.100 (s, 3 H, pzCH3), 2.160 (s, 3 H, pzCH3), 2.260 (s, 3 H, pzCH3), 2.576 (m, 1 H, RhCH2), 2.657 (s, 3 H, pzCH3), 2.830 (s, 3 H, pzCH3), 3.132 (m, 2 H, CH2), 3.810 (m, 1 H, RhCH2), 5.557 (s, 1 H, pzH), 5.651 (s, 1 H, pzH), 5.682 (s, 1H, pzH). 13C{1H} NMR (500 MHz, C6D6): δ 12.86 (s, 1 C, pzCH3), 12.91 (s, 1 C, pzCH3), 13.49 (s, 1 C, pzCH3), 14.59 (s, 1 C, pzCH3), 15.94 (d, 1JP−C = 33.1 Hz, 3 C, P(CH3)3), 15.96 (s, 1 C, pzCH3), 16.19 (s, 1 C, pzCH3), 17.20 (dd, 1JRh−C = 7.9 Hz, 2JP−C = 21.1 Hz, 1 C, Rh−C), 30.05 (s, 1 C, CH2), 35.47 (s, 1 C, CH2), 44.85 (s, 1 C, CH2), 108.04 (d, 4JP−C = 4.1 Hz, 1 C, pzCH), 108.34 (s, 1 C, pzCH), 108.84 (s, 1 C, pzCH), 142.77 (d, 4JP−C = 2.6 Hz, 1 C, pzCq), 144.08 (s, 1 C, pzCq), 144.75 (s, 1 C, pzCq), 151.55 (s, 1 C, pzCq), 152.40 (d, 3JP−C = 4.5 Hz, 1 C, pzCq), 153.11 (s, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ 0.15 (d, 1JRh−P = 129.7 Hz). Anal. Calcd (found) for C22H39BCl2N6PRh: C, 43.81 (44.34); H, 6.52 (6.58); N, 13.93 (13.76). 4-Chlorobutene (90, 55, 41 m/z) was seen as the byproduct of the reaction by GC-MS. Activation of cis-1,2-Dichloroethylene. A resealable NMR tube was charged with a solution of 1 (∼10 mg, 0.019 mmol) in 0.5 mL of cis-1,2-dichloroethylene. The resulting colorless solution was kept at room temperature and monitored by NMR spectroscopy. After the mixture stood overnight, the reaction was complete and resulted in a mixture of Tp′Rh(PMe 3 )(cis-HCCHCl)Cl (8-Cl-cis), 1563


Article

Organometallics (s, 1 C, pzCH), 108.64 (d, 4JP−C = 4.1 Hz, 1 C, pzCH), 108.76 (s, 1 C, pzCH), 124.26 (s, 2C, Ph), 130.04 (s, 3 C, Ph), 142.80 (d, 4JP−C = 2.4 Hz, 1 C, pzCq), 143.49 (s, 1 C, pzCq), 145.34 (s, 1 C, pzCq), 151.46 (d, 2 JRh−C = 1.4 Hz, 1 C, Rh-CH2CPh), 152.42 (s, 1 C, pzCq), 152.69 (s, 1 C, pzCq), 153.10 (d, 3JP−C = 4.4 Hz, 1 C, pzCq). 31P{1H} NMR (400 MHz, C6D6): δ −0.85 (d, 1JRh−P = 130.6 Hz). Anal. Calcd (found) for C25H38BClN6PRh: C, 49.82 (49.48); H, 6.35 (6.42); N, 13.94 (13.33). Activation of Chlorobenzene. A resealable NMR tube was charged with a solution of 1 (∼10 mg, 0.019 mmol) in 0.5 mL of chlorobenzene. The resulting colorless solution was kept at room temperature and at 70 and 140 °C and monitored by 1H and 31 1 P{ H} NMR spectroscopy. The spectra obtained showed a mixture of activation products, as described in the text. Data for 10a,b are as follows. 1H NMR (400 MHz, C6D6): δ −16.840 (dd, 1JRh−H = 25.3 Hz, 2 JP−H = 29.7 Hz, RhH); δ −16.853 (dd, 1JRh−H = 24.8 Hz, 2JP−H = 29.8 Hz, RhH). 31P{1H} NMR (400 MHz, C6D6): δ 2.80 (d, 1JRh−P = 142.8 Hz); δ 2.57 (d, 1JRh−P = 142.8 Hz). Data for 10c are as follows. 1H NMR (400 MHz, C6D6): δ −16.884 (dd, 1JRh−H = 25.3 Hz, 2JP−H = 30.2 Hz, RhH). 31P{1H} NMR (400 MHz, C6D6): δ 2.65 (d, 1JRh−P = 143.2 Hz). Data for 10d are as follows. 1H NMR (400 MHz, C6D6): δ −15.988 (dd, 1JRh−H = 22.7 Hz, 2JP−H = 24.7 Hz, RhH). 31P{1H} NMR (400 MHz, C6D6): δ 1.83 (d, 1JRh−P = 140.7 Hz). Data for 10e are as follows. 1H NMR (400 MHz, C6D6): δ −16.173 (dd, 1JRh−H = 22.6 Hz, 2JP−H = 25.5 Hz, RhH). 31P{1H} NMR (400 MHz, C6D6): δ 0.98 (d, 1JRh−P = 136.4 Hz). Data for 10a-Cl, 10b-Cl, and 10c-Cl are as follows. 31P{1H} NMR (400 MHz, C6D6): δ −2.17 (d, 1JRh−P = 124.3 Hz); δ −2.37 (d, 1JRh−P = 124.6 Hz); δ −2.51 (d, 1JRh−P = 127.1 Hz). Activation of Pentachlorobenzene. A resealable NMR tube was charged with a solution of 1 (∼10 mg, 0.019 mmol) and pentachlorobenzene (15 mg, 0.10 mmol) in 0.5 mL THF. The resulting colorless solution was kept at room temperature and monitored by NMR spectroscopy. After 4 days, the reaction was complete and resulted in a mixture of Tp′RhCl(η2-CH2PMe2) (11-Cl) (76%) and 2-Cl (24%). The pure form of 11-Cl (5.8 mg, 60%) was isolated by chromatography on silica gel using 5/1 hexane/THF as the eluent. 1 (∼208 mg, 0.395 mmol) was used for a larger scale reaction to prepare pure 11-Cl (134.4 mg, 67%). 11-Cl was recrystallized from methylene chloride/hexanes at room temperature to give red-orange crystals. Data for 11-Cl are as follows. 1H NMR (400 MHz, CDCl3): δ 1.789 (d, 2JP−H = 13.3 Hz, 3 H, P(CH3)2), 1.864 (d, 2JP−H = 13.5 Hz, 3 H, P(CH3)2), 2.042 (s, 3 H, pzCH3), 2.101 (bd, 2JP−H = 8.5 Hz, 1 H, CH2PMe2), 2.319 (s, 3 H, pzCH3), 2.407 (s, 3 H, pzCH3), 2.418 (s, 3 H, pzCH3), 2.448 (s, 3 H, pzCH3), 2.471 (s, 3 H, pzCH3), 3.093 (br d, 2JP−H = 8.4 Hz, 1 H, CH2PMe2), 5.557 (s, 1H, pzH), 5.821 (s, 1 H, pzH), 5.908 (s, 1 H, pzH). 13C{1H} NMR (500 MHz, CDCl3): δ −8.96 (dd, 1JRh−C = 15.8 Hz, 1JP−C = 21.2 Hz, 1 C, Rh−C), 8.16 (d, 1 JP−C = 40.7 Hz, 1 C, P(CH3)2), 10.32 (d, 1JP−C = 22.0 Hz, 1 C, P(CH3)2), 12.32 (s, 1 C, pzCH3), 12.60 (s, 1 C, pzCH3), 13.95 (s, 1 C, pzCH3), 14.72 (s, 1 C, pzCH3), 15.54 (d, 3JRh−C = 1.5 Hz, 1 C, pzCH3), 16.13 (s, 1 C, pzCH3), 106.33 (d, 4JP−C = 2.0 Hz, 1 C, pzCH), 106.59 (s, 1 C, pzCH), 109.65 (s, 1 C, pzCH), 143.32 (s, 1 C, pzCq), 143.56 (d, 4JP−C = 1.9 Hz, 1 C, pzCq), 145.91 (s, 1 C, pzCq), 150.97 (s, 1 C, pzCq), 152.02 (br, 1 C, pzCq), 152.64 (s, 1 C, pzCq). 31 1 P{ H} NMR (400 MHz, CDCl3): δ −32.87 (d, 1JRh−P = 96.2 Hz). Anal. Calcd (found) for C18H30BClN6PRh: C, 42.34 (42.92); H, 5.92 (6.08); N, 16.46 (16.05). Preparation of Tp′Rh(η2-CH2PMe2)H (11). A resealable NMR tube was charged with a slurry of 11-Cl (10 mg, 0.020 mmol) and Cp2ZrH2 (21.9 mg, 0.098 mol) in 0.6 mL of THF. The resulting colorless solution was kept at room temperature and monitored by NMR spectroscopy. The reaction was complete after 1 h, and the zirconium complexes were removed by chromatography using 5/1 hexane/THF as the eluent. The resulting products contain a mixture of Tp′RhH(η2-CH2PMe2) (11) (67%), Tp′Rh(PMe3)(Cl)H (26%), and 2 (7%). Data for 11 are as follows. 1H NMR (400 MHz, C6D6): δ −21.135 (dd, 1JRh−H = 16.3 Hz, 2JP−H = 35.7 Hz, RhH), 0.898 (m, 1 H, CH2PMe2), 1.256 (d, 2JP−H = 11.9 Hz, 3 H, P(CH3)2), 1.418 (d, 2JP−H = 11.3 Hz, 3 H, P(CH3)2), 1.499 (m, 1 H, CH2PMe2), 1.973 (s, 3 H, pzCH3), 2.218 (s, 3 H, pzCH3), 2.287 (s, 3 H, pzCH3),

2.311 (s, 3 H, pzCH3), 2.329 (s, 3 H, pzCH3), 2.424 (s, 3 H, pzCH3), 5.607 (s, 1H, pzH), 5.700 (s, 1 H, pzH), 5.770 (s, 1 H, pzH). 31P{1H} NMR (400 MHz, C6D6): δ −35.24 (d, 1JRh−P = 112.8 Hz). Reaction of 11 with Pentachlorobenzene. 11 was prepared as described above from ∼5 mg (0.0095 mmol) of 11-Cl. A 25 mg portion of pentachlorobenzene was added to the solution of 11 in 0.5 mL of THF. The reaction mixture was placed in a resealable NMR tube at room temperature and was monitored by 1H and 31P{1H} NMR spectroscopy. The spectra showed no reactivity in the first day. The decrease of 11 and the growth of 11-Cl were observed in the following days, and the reaction was clean and complete after 1 week. Competition Experiment of Chloromethane/Benzene. A solution of 2 (8 mg, 0.017 mol) dissolved in 0.3 mL of cyclohexaned12 was placed in a resealable NMR tube. A 10 μL portion of benzene was added and freeze−pump−thaw degassed 3× prior to pressurization of the tube with chloromethane at 60 psi. A 1H NMR spectrum of the sample was taken to record the mole ratio of the two substrates. The sample was irradiated (270−370 nm) for 10 min at 10 °C, and another 1H NMR spectrum was immediately collected. The ratio of C−H activation products was measured by integrating the hydride resonances in 1H NMR spectrum. kbenzene/kchloromethane = (Ibenzene/ Ichloromethane)(nchloromethane/nbenzene) = 0.353 × 4.82 = 1.70. ΔΔGoa⧧ = RT ln(kbenzene/kchloromethane) = 0.30 kcal mol−1. Kinetics of Elimination Reactions of Chloromethane from Complex 3 to give 1-Cl. Complex 3 (∼10 mg) was dissolved in 0.6 mL of C6D6, followed by 0.5 μL of hexamethyldisiloxane added as an internal standard. This solution was placed in a resealable NMR tube and heated in a 30.0 °C heating block. 1H and 31P{1H} NMR spectra were recorded at regular intervals. Kinetic analysis was performed by integration of the decreasing hydride resonance relative to the signal for hexamethyldisiloxane. A first-order decay of the concentration of 3 was plotted against time to give the rate of reductive elimination of chloromethane, and Excel was used to determine the rate constant. The reaction was followed for ∼3 half-lives. For kinetics with added trimethylphosphine, a separate NMR tube was prepared with the same solution as above and 0.1 equiv of PMe3 before heating. Kinetics of Reductive Elimination Reactions from Complex 11. Complex 11 was prepared from 10 mg of 11-Cl as described above. The reaction products were dried and dissolved in 0.6 mL of C6D6, followed by 0.5 μL of hexamethyldisiloxane added as an internal standard. This solution was transferred into a resealable NMR tube, which was placed in a preheated NMR spectrometer at 30.0 °C. The decrease of the hydride resonance of 11 relative to internal standard was recorded by 1H spectroscopy at regular intervals. A plot of the concentration of 11 vs time showed a first-order decay to give the rate of reductive elimination, and Excel was used to determine the rate constant. The reaction was followed for >2 half-lives. X-ray Crystal Structure Determinations. Data were collected on a Bruker SMART APEX II CCD Platform diffractometer at 100.0(1) K. The data collection was carried out using Mo Kα radiation. Data were collected in accord with the parameters in the Supporting Information. The structures were solved using SIR 97 and refined using SHELXL-97, SHELXL-2012, or SHELXL-2013.61−64 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.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF and AVI files giving NMR data, kinetic data, details of X-ray crystallographic determinations of 3-Cl, 5-Cl, 6′-Cl, 7-Cl−9-Cl, and 11-Cl (CCDC deposition nos. 1020315−1020321), coordinates and energies for calculated complexes, the complete ref 56, and movie files of six transition states. This material is available free of charge via the Internet at http://pubs.acs.org. 1564


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Organometallics

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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 authors thank the U.S. Department of Energy (grant no. FG02-86ER-13569) for its support of this work. We acknowledge the Center for Integrated Research Computing at the University of Rochester for providing the necessary computing systems and personnel to enable the research presented in this paper, and the CENTC Elemental Analysis Facility at the University of Rochester, funded by NSF CHE1205189.

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