Determination of Rhodium–Alkoxide Bond Strengths in Tp′ Rh

Article pubs.acs.org/IC

Determination of Rhodium−Alkoxide Bond Strengths in Tp′Rh(PMe3)(OR)H Jing Yuwen, 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 active fragment [Tp′Rh(PMe3)], generated from a thermal precursor Tp′Rh(PMe3)(CH3)H, underwent oxidative addition of water and alcohols to give O−H adducts of the type Tp′Rh(PMe3)(OR)H (R = H, Me, Et, nPr, nBu, CH2Ph, iPr, c-pentyl, CH2CF3, CH2CH2OH) at ambient temperature. These activation products eliminate water or alcohols in benzene, which allows determination of the relative metal−oxygen bond energies by using previously established kinetics techniques. Analysis of the relationship between the relative M−O bond strengths and O−H bond strengths showed a linear correlation with RM−O/O−H of 0.97 (3) for aliphatic alcohols. The two extraordinary substrates (R = CH2CF3, CH2CH2OH) both have stronger M−O bonds than would be predicted from this trend, suggesting the stabilization of the M−O bond when an electron-withdrawing substituent is present as previously seen in M−C bond strengths. In addition, the O−H activation products of aliphatic alcohols are thermally unstable at 80 °C, as rearrangement to form Tp′Rh(PMe3)H2 from β-elimination is observed after 1 or 2 d. Benzyl alcohol and 2,2,2-trifluoroethanol activation products were stable. For benzyl alcohol, although the O−H activation product was kinetically favored, the C−H activation products of the benzene ring were thermodynamically preferred.

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INTRODUCTION Oxidative addition of alcohols to transition-metal complexes leading to the formation of hydrido(alkoxy) complexes has received much attention over the past few decades. These species are common intermediates in many catalytic cycles such as carbalkoxylation of olefins and alkyl halides, alcohol dehydrogenations, and carbonyl compound hydrogenations.1−3 Water splitting is also of special interest due to its potential relevance to solar energy conversion and green chemistry.4,5 Therefore, direct observation and isolation of metal alkoxides and hydroxyalkyl complexes is important to compare their reactivities toward further dehydrogenation. Although polar O−H bonds in alcohols and water are easily broken heterolytically in comparison with the unreactive nonpolar C−H bond, only a few cases of hydrido(alkoxy)metal6−15 or hydrido(hydroxy)9,15−22 complexes have been prepared via oxidative addition of O−H bond to the metal. The challenges in isolating alcohol addition products are due to unfavorable formation of a labile M−OR bond and/or the following decomposition from β-hydride elimination or insertion reactions.23 In the early 1980s, Otsuka and co-workers investigated the reversible addition of water to PtL3 (L = PEt3, P(iPr)3)24 and HRh(PiPr3)3.25 Miles and Lappert reported oxidative addition of MeOH and EtOH to the alkylgermanium(II) compound [Ge{CH(SiMe3)2}2] at room temperature (RT). Although hydrido-alkoxygermanium(IV) compounds are generally unstable, a crystal structure of Ge{CH(SiMe3)2}2(OEt)H was obtained at −78 °C.14 Later, in 1989, Bergman reported O−H activation © 2016 American Chemical Society

intermediates Cp*Ir(PMe3)(OR)H (R = Me or Et) generated by irradiation of Cp*Ir(PMe3)H226 in liquid xenon containing methanol or ethanol at −60 to −75 °C. However, irradiation in the presence of isopropanol or tert-butyl alcohol gave C−H insertion products Cp*Ir(PMe3)(CH2CH(Me)OH)H and Cp*Ir(PMe3)(CH2C(Me)2OH)H, respectively, independent of temperature.27 In 1993, Merola observed that reaction of [Ir(COD)(PMe3)3]Cl and tert-butyl alcohol resulted in the formation of a hydroxyiridium hydride complex and isobutylene. Oxidative addition of phenols and carboxylic acids to [Ir(COD)(PMe3)3]Cl gave O−H addition products. However, oxidative addition of primary alcohols to [Ir(COD)(PMe3)3]Cl gave a complicated variety of products.28 In 2002, Milstein and co-workers reported the oxidative addition of O−H bonds of water, primary, and secondary alcohols to (C8H14)IrCl(PMe3)3 (C8H14 = cyclooctene), which produced mer-cis-HIr(OR)Cl(PMe3)3 (R = H, Me, Et, 1-pentyl, 2-propyl) as the only kinetic products.7 A detailed mechanistic study was performed, revealing that π-donation by the chloro ligand stabilized the transition state and governed the stereochemical course of the reaction, with hydrogen bonding also assisting in the O−H cleavage. In 2009, Zessin and co-workers first reported oxidative addition of ethanol and water at a gallium(I) center at −78 °C, and a crystal structure was provided for the ethanol addition product Ga({N(dipp)CMe}2CH)(OEt)H.13 Received: August 17, 2016 Published: September 7, 2016 9482

DOI: 10.1021/acs.inorgchem.6b01992 Inorg. Chem. 2016, 55, 9482−9491

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Inorganic Chemistry

reversible O−H bond cleavage was kinetically favored. Addition of the aryl C−H bond was suggested as the thermodynamic products for phenol and benzyl alcohol, while β-hydride elimination to form Tp′Rh(PMe3)H2 is the thermodynamic product for other aliphatic alcohols.

Later, in 2015, the Jancik group synthesized Ga({N(dipp)CMe}2CH)(OCH2CCH)H in tetrahydrofuran (THF) at −108 °C; however, it could not be crystallized in pure form, but a crystal structure of its hydrolysis product Ga({N(dipp)CMe}2CH)(OCH2CCH) (OH) was provided.29 In 1998, the first water and methanol addition products at tin were reported by Pörschke. The crystal structure of {CH(SiMe3)2}Sn(OH)H was provided, and a detailed mechanistic study indicated that in solution, with an excess of water and methanol present, rapid nucleophilic exchange reactions proceed at the SnIV center by an SN2-type mechanism.16 Recently, Aldridge and co-workers investigated oxidative addition of water to [Sn{B(NDippCH)2}2] (Dipp = 2,6-C6H3iPr2). Although the resulting SnIV products are prone to further reaction leading to B−O reductive elimination, a crystal structure of Sn{B(NDippCH)2}2(OH)H was obtained. The molecular structure is similar to that previously reported for {CH(SiMe3)2}Sn(OH)H.22 While most of these O−H bond additions were related to a single metal center, oxidative addition of water to the dinuclear Ir(I) complexes [Ir2(μ-Cl)2(diphosphine)2] to give [Ir2(μ-OH)2(diphosphine)2H2] has been demonstated.30,31 Wayland reported a rare example of a bi-metalloradical complex, •Rh(m-xylyl)Rh•, reactivity with methanol, where selectivities of O−H versus C−H bond activation at rhodium(II) were compared both qualitatively and quantitatively.32 Fully selective H−OCH3 bond activation was observed in neat methanol to afford the methoxide product Rh−OCH3, which isomerized to hydroxymethyl species Rh−CH2OH in an equilibrium favoring the latter by ∼8.5 kcal mol−1. Oxidative additions of alcohols to early late hetero-bimetallic complexes [(Cp2Zr) (Cp*Ir(μ-NtBu))]33 and [{Cp*Ta(CH2SiMe3)2}(Cp*IrH2)]34 have also been reported. The addition of p-cresol and tBuOH to [(Cp2Zr) (Cp*Ir(μ-NtBu)] resulted in the immediate formation of O−H addition product [{Cp2Zr(OAr)}(Cp*Ir(μ-NtBu)(μ-H)] and [{Cp2Zr(OtBu)}-{Cp*Ir(μ-NtBu)}(μ-H)], respectively, at the zirconium center. Alcoholysis of [{Cp*Ta(CH2SiMe3)2}(Cp*IrH2)] with MeOH and i PrOH would disrupt the Ta−Ir bimetallic structure, whereas reaction with tBuOH and phenols give [{Cp*Ta(OtBu) (CH2SiMe3)}(Cp*IrH2)] and [{Cp*Ta(OR)2}-(Cp*IrH2)] (R = C6H4-4-tBu, C6H4-4-Br). The bond dissociation energies of the O−H bonds in water and alcohols are relatively high compared to those of C−H bonds in organic compounds.35 However, this does not necessarily mean that water and alcohol oxidative additions are more challenging thermodynamically than oxidative addition of the weaker C−H bonds. It has been observed that the cleavage of stronger C−H bonds is thermodynamically preferred because of the formation of stronger C−M bonds.36−38 Therefore, the question for water and alcohols is whether the strengths of the M−O and the M−H bonds that are formed are enough to compensate for the O−H bonds that are broken. The data for relative M−O bond strengths are in short supply, and this work helps to fill this vacancy. We recently reported that the [Tp′Rh(L)] (L = CNneopentyl, PMe3, P(OMe)3) fragment served as an active species to selectively activate C−H bonds in functionalized hydrocarbons.36−38 Although thermodynamic driving forces to form metal−carbon bonds are influenced by the proximity of electron-withdrawing substituents to the carbon attached to the metal center, these functional groups remain untouched during the reaction. Here, we report on reactivity of the [Tp′Rh(PMe3)] fragment toward water and alcohols, where

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RESULTS AND DISCUSSION A thermally labile complex Tp′Rh(PMe3)(CH3)H (1) was prepared in situ to generate the active fragment [Tp′Rh(PMe3)] as previously reported.37 Thermolysis of 1 with hydrocarbons has proven to be a general method leading to selective C−H activation. Here we examine its reactivity with organic substrates containing a hydroxyl functional group. Activation of Water at [Tp′Rh(PMe3)]. Addition of water to a solution of 1 led to complete conversion to Tp′Rh(PMe3) (OH)H (2a; Figure 1). The hydride signal of 2a displays

Figure 1. Thermal ellipsoid drawing of Tp′Rh(PMe3) (OH)Br (2a-Br). Ellipsoids are shown at the 50% probability level, and hydrogen atoms (except OH) were omitted for clarity.

a doublet of doublets at δ −16.493 (1JRh−H = 20.5 Hz, 2JP−H = 30.0 Hz) in the 1H NMR spectrum (cf. 1JRh−H = 23.6 Hz, 2 JP−H = 33.6 Hz for 1). The resonance for the Rh−OH appears as a broad singlet at δ −3.38. The phosphorus resonance of 2a features a doublet at δ 7.991 with 1JRh−P = 136.4 Hz (cf. 1JRh−P = 147.7 Hz for 1). Treatment of 2a with bromoform yielded Tp′Rh(PMe3)(OH)Br (2a-Br) in 83% yield. The identity of 2a-Br was confirmed by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction (Figure 1). As with many 9483

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analysis, and single-crystal X-ray diffraction (Figure 2). The structure of 2b-Br again shows the typical octahedral geometry with a bond distance of 1.965(3) Å for Rh−OCH3 resembling that seen for the Rh−OH bond in 2a-Br. Heating of a methanol solution of 2b at 80 °C for 2 d led to ∼40% conversion of 2b to 3. A very small amount of Tp′Rh(CO)H2 (doublet at δ −12.925, 1JRh−H = 18 Hz)39 was observed in the 1H NMR spectrum. A small doublet of doublets seen at δ −15.158 (1JRh−H = 18.5 Hz, 2JP−H = 26 Hz) in the 1H NMR spectrum may arise from the product of formaldehyde C−H addition to the rhodium complex. The formation of 3 presumably occurred by way of β-hydride elimination of 2b to liberate CH2O, followed by decarbonylation of CH2O to give Tp′Rh(CO)H2.40−42 The formation of formaldehyde was not detected by gas chromatography−mass spectrometry (GCMS), which may be attributed to the high reactivity of the newly formed formaldehyde with the rhodium complex. Reaction of Tp′Rh(PMe3)(CH3)H with CD3OD gave Tp′Rh(PMe3)(OCD3)D (5a) as the major product (74%; eq 3).

Tp′Rh(L)(X)(Y) analogues, the crystal structure of 2a-Br displays an octahedral geometry around the metal center. The bond distance for Rh−OH is 2.007(4) Å, slightly shorter than Rh−CH3 bond (2.062(6) Å) in Tp′Rh(PMe3)(CH3)Cl. Activation of Linear Aliphatic Alcohols. Reaction of 1 with methanol overnight at RT gave O−H oxidative addition product Tp′Rh(PMe3)(OMe)H (2b) in 95% yield (Figure 2).

The existence of a small amount of Tp′Rh(PMe3)(OCD3)H (5b; 24%) suggests that the competing pathway of anionic ligand exchange (MeO− for Me−) is less preferred over reductive elimination of methane, although the former was measured to be 6 times faster than the latter in the reaction of H[(OH)Ir(PMe3)3Cl] with CD3OD.10 A tiny amount of 3 (2%) is also observed. Cleavage of the O−H bond was also observed in reactions of 1 with other primary alcohols. Addition of ethanol to 1 led to the formation of Tp′Rh(PMe3)(OEt)H (2c) in 98% NMR yield, and 1.5% of the dihydride 3 was observed. The 1H NMR signals of 2c are very similar to those of 2b, and the ethoxy group features a triplet at δ 1.483 for the methyl hydrogens and two quintets at δ 3.701 and δ 3.838 for the methylene hydrogens. Treatment of 2c with N-chlorosuccinimide yielded a mixture of products, where the major species is attributed to the chloro derivative Tp′Rh(PMe3)(OEt)Cl (2c-Cl). A pure form of complex 2c-Cl was not attained due to decomposition during chromatography, but the assignment of 2c-Cl is consistent with spectral data from an independent preparation from reaction of Tp′Rh(PMe3)Cl2 with sodium ethoxide. Heating of an ethanol solution of 2c at 80 °C for 2 d led to ∼25% conversion of 2c to Tp′Rh(PMe3)H2 (3). In addition, a small amount of Tp′Rh(CO)H2, a small doublet of doublets at δ −15.156 (1JRh−H = 18 Hz, 2JP−H = 25 Hz) and a triplet at δ −15.797 (J = 25 Hz) were observed. These products may arise from the addition of acetaldehyde (formed via β-elimination of ethoxide) to the rhodium complex. The formation of acetaldehyde was observed by GCMS. Clean conversion to O−H addition product Tp′Rh(PMe3)(OnPr)H (2d) (99%) was observed in the reaction of 1-propanol with 1 at RT after standing overnight. The 1 H NMR spectrum displays a typical hydride resonance at

Figure 2. Thermal ellipsoid drawing Tp′Rh(PMe3) (OMe)Br (2b-Br). Ellipsoids are shown at the 50% probability level, and hydrogen atoms were omitted for clarity.

The reaction was almost clean, with small amounts (1.5%) of a known dihydride species Tp′Rh(PMe3)H2 (3) observed at δ −17.086 (dd, 1JRh−H = 19.2 Hz, 2JP−H = 35.8 Hz) and 3.5% of a hydride species observed at δ −18.283 (dd, 1JRh−H = 26 Hz, 2 JP−H = 32 Hz). This latter species was tentatively assigned as the C−H activation product of Tp′Rh(PMe3) (CH2OH)H (4) based on the typical coupling constants for rhodium alkyl hydride resonances (for the analogous complex Tp′Rh(PMe3) (CH2OCH3)H, the hydride appeared at δ −17.87 with 1JRh−H = 24.8 Hz, 2JP−H = 31.4 Hz37). The 1H NMR signal for Rh−OCH3 appears as a singlet at δ 3.410. The hydride of 2b has a characteristic doublet of doublets at δ −16.356, shifted downfield compared with 1 (cf. δ −18.14).37 The rhodium− hydride coupling constant of 19.3 Hz and phosphorus−hydride coupling constant of 28.8 Hz are both smaller than alkyl analogues. A smaller Rh−P coupling constant of 138 Hz was also found for 2b in 31P{1H} NMR (1JRh−P ≈ 150 Hz for alkyl analogues). Addition of bromoform to the reaction solution led to formation of the air-stable halogenated species of Tp′Rh(PMe3) (OMe)Br (2b-Br) in 87% yield. The structure of 2b-Br was fully characterized by NMR spectroscopy, elemental 9484

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Inorganic Chemistry δ −16.361 (dd, 1JRh−H = 19.8 Hz, 2JP−H = 28.8 Hz) as well as the n-propoxy signals at δ 1.085, 1.880, 3.526, and 3.652. Heating of 2d at 80 °C in 1-propanol led to formation of 3 (15%) as well as Tp′Rh(CO)H2, a doublet of doublets at δ −15.140 (1JRh−H = 18.5 Hz, 2JP−H = 25 Hz), and a triplet at δ −15.906 (J = 25 Hz) after 2 d. Addition of 1-butanol at RT overnight yielded Tp′Rh(PMe3)(OnBu)H (2e; 98.5%) as well as trace amounts of

doublet of doublets at δ −17.566 (dd, 1JRh−H = 22 Hz, 2JP−H = 29 Hz), and another one shows a triplet δ −19.289 (J = 26 Hz) in the 1H NMR spectrum.

3 (1.5%; eq 4). The structure of 2e is suggested from 1H and P{1H} NMR spectra by analogy to the resonances of 2b−d. Heating of the products at 80 °C in n-BuOH resulted in 12.3% conversion of 2e to 3 after 2 d. Tp′Rh(CO)H2, a very small doublet of doublets at δ −15.171, and a triplet at δ −15.924 (J = 25 Hz) were also observed. The 1H NMR spectrum of reaction of Tp′Rh(PMe3)(CH3)H with butyraldehyde overnight at RT showed a triplet at δ −15.928 (J = 25 Hz), providing support for the previous hypothesis that aldehydes produced by β-hydride elimination from Tp′Rh(PMe3)(OR)H had reacted with [Tp′Rh(PMe3)] to give new hydridecontaining products. From previous studies, activation of the aryl C−H bond is kinetically and thermodynamically favored compared with activation of alkyl and alkenyl C−H bonds. Addition of benzyl alcohol to 1 at RT for 3 h generated Tp′Rh(PMe3)(OCH2Ph) H (2f; 90%) as the main product. Longer reaction times yielded a mixture of aryl C−H activation products. The 1H NMR spectrum displays a typical hydride resonance at δ −16.102 (dd, 1JRh−H = 19 Hz, 2JP−H = 28 Hz) as well as an AB quartet phenylmethoxy signal at δ 4.598. The resonances for the aryl hydrogens were located from δ 7.067 to δ 7.571, overlapping with the residual free benzyl alcohol. Heating of 2f at 80 °C in a mixed solution of benzyl alcohol and cyclohexane for 1 d led to the formation of a small amount of Tp′Rh(CO)H2 as well as minor products displaying a series of overlapping doublet of doublets around δ −16.9, comparable to the hydride resonance for Tp′Rh(PMe3)(C6H5)H (the hydride appeared at δ −16.9 with 1JRh−H = 25 Hz, 2JP−H = 31 Hz), suggesting the formation of o-, m-, and p-aryl C−H activation products. Activation of Secondary Alcohols. In addition to the work done with the linear alcohols, activations of branched and cyclic alcohols were also investigated. Reaction of 1 in neat 2-propanol overnight at RT yielded formation of O−H addition product Tp′Rh(PMe3) (OCH(CH3)2)H (2g), featuring a typical hydride resonance at δ −16.215 (dd, 1JRh−H = 20 Hz, 2 JP−H = 27.5 Hz) in the 1H NMR spectrum. The attached isopropoxy ligand was suggested by two doublets at δ 1.514 (JH−H = 5.5 Hz) and δ 1.581 (JH−H = 5.6 Hz) for the six hydrogens of the methyl groups and a septet at δ 3.807 for the methine hydrogen atom. After the mixture was heated at 80 °C for 11 h in isopropanol, 3 was formed as well as at least two unidentified rhodium−hydride complexes. One shows a 31

Reaction of c-pentanol gave Tp′Rh(PMe3)(cyclopentoxy)H (2h) as the major product after 2 h at RT as well as C−H activation products Tp′Rh(PMe3)(2-hydroxycyclopentyl)H (6a) and Tp′Rh(PMe3)(3-hydroxycyclopentyl)H (6b), similar to the reaction of THF with Tp′Rh(PMe3)(CH3)H (eq 7).

The structure of 2h was inferred from the characteristic hydride resonance showing a doublet of doublets at δ −16.383 with similar coupling constants of 1JRh−H = 20 Hz and 2JP−H = 27 Hz to other Tp′Rh(PMe3)(OR)H complexes. Also, typical phosphorus signals were observed (δ 7.41, d, 1JRh−P = 141.9 Hz). Heating the products at 80 °C in a cyclopentanol/hexane mixture resulted in formation of 3 as well as two same rhodium−hydride complexes observed after heating of Tp′Rh(PMe3)(OiPr)H in isopropanol. Activation of Phenol. Compared with benzyl alcohol, cleavage of the aryl C−H bonds of phenol is more kinetically and thermodynamic favored. Reaction of 1 with excess phenol in THF resulted in the O−H cleavage product Tp′Rh(PMe3) (OPh)H (2i) as well as a series of C−H cleavage products. When all of 1 reacted at RT for 24 h, 85% of 2i was obtained as well as 15% of Tp′Rh(PMe3)(hydroxyphenyl)H mixture (eq 8). Longer reaction time yielded more Tp′Rh(PMe3)(hydroxyphenyl)H products. Compared to aliphatic alcohol products 2b−h, the hydride resonance for 2i was shifted downfield to δ −15.272 with smaller coupling constants of 1JRh−H = 17.5 Hz and 2JP−H = 26.5 Hz. The resonances for the aryl hydrogens 9485

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Table 1. Rates of Reductive Elimination of Water and Alcohols from Tp′Rh(PMe3) (OR)H in C6D6 at 30 °C ROH H2O methanol ethanol 1-propanol 1-butanol 2-propanol cyclopentanol benzyl alcohol CF3CH2OH ethylene glycol CD3OD

were located from δ 6.7 to 7.5. Several additional hydride species located ca. δ −17.05 were observed as presumed C−H activation products of isomeric Tp′Rh(PMe3)(C6H4OH)H complexes. The major formation of 2i again proved the kinetic favorability of breaking O−H bonds rather than aryl C−H bonds at RT. Activation of Fluoroalcohols and Ethylene Glycol. The tolerance of functionalities in alcohol substrates was also investigated in this work. Reaction of 1 in 2,2,2-trifluoroethanol showed the formation of Tp′Rh(PMe3)(OCH2CF3)H (2j) in 99% yield and 1% of the dihydride 3 after reaction overnight at RT (eq 9). The 1H NMR resonances of 2j display a typical

a b

kre(ROH), s−1 a 5.07(18) 6.21(28) 5.38(12) 6.17(5) 6.05(12) 4.69(12) 8.18(14) 2.75(4) 1.75(2) 6.46(18) 4.35(9)

× × × × × × × × × × ×

−5

10 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−7 10−6 10−5

t1/2, h

ΔGre‡, kcal mol−1 b

3.80 3.10 3.58 3.12 3.18 4.11 2.35 7.00 1100.2 29.8 4.43

23.71(2) 23.59(3) 23.68(1) 23.59(1) 23.61(1) 23.76(2) 23.43(1) 24.08(1) 27.13(1) 24.95(2) 23.81(1)

Errors in elimination rates are reported as standard deviations. Errors in ΔGre‡ calculated using σG = (RT/kre)σk.

activation energies for the reductive elimination. Generally, reductive elimination of water and alcohols in 2a−k have higher barriers than methane elimination in Tp′Rh(PMe3) (CH3)H by 0.8−4.5 kcal·mol−1. Phenol did not undergo reductive elimination in C6D6, even when heated to 80 °C. Comparison of the rates for methanol elimination from Tp′Rh(PMe3)(OCH3) H versus Tp′Rh(PMe3)(OCD3)D led to the kinetic isotope effect (KIE = kH/kD) of 1.43(9). Competitive Selectivity Experiments. The relative selectivity of the fragment [Tp′Rh(PMe3)] for O−H activation of the various substrates was determined by photolysis of Tp′Rh(PMe3)H2 (3) in a mixture of two substrates. The ratio of the two substrates was measured by 1H NMR analysis before irradiation. The competition experiments were performed with only 10 min of photolysis at 10 °C, so the product ratio represented the kinetic products of the reaction. The product distribution was determined on the basis of the relative areas of the corresponding hydride resonances by 1H NMR spectroscopy in deuterated solvent (THF-d8 or C6D12). The relative competitive rates k1/k2 reported in Table 2 were calculated on

doublet of doublets at δ −16.167 (1JRh−H = 18.0 Hz, 2JP−H = 27.5 Hz). A quartet at δ 3.682 was seen for the methylene group with a fluorine−hydrogen coupling constant of 10.0 Hz. The incorporation of fluorine was confirmed from the 19F NMR spectrum, where a triplet at δ −13.70 with the same JH−F of 9.9 Hz. Similar to 2a−i, the phosphorus resonance of 2j appears as a doublet at δ 3.688 with JRh−P = 133.4 Hz. Reaction of 1 with ethylene glycol gave O−H addition product of Tp′Rh(PMe3)(OCH2CH2OH)H (2k) (98%) as well as a small amount of 3 (eq 10). The 1H NMR spectrum

Table 2. Kinetic Selectivity Data Determined from Competition Experiments

of 2k displays a doublet of doublets at δ −16.254 for the metal hydride. Two sets of multiplets at δ 3.569 and 3.869 were observed for the two CH2 groups. The 31P{1H} NMR spectrum of 2k shows a doublet at δ 4.492 with a typical JRh−P of 135.8 Hz as seen in 2a−j. Reductive Elimination of Tp′Rh(PMe3) (OR)H. The rates for reductive elimination of complexes 2a−h, 2j, and 2k were determined by monitoring the first-order disappearance of the hydride resonance in C6D6 at 30.0 °C by 1H NMR spectroscopy (see Supporting Information, Tables S2−S23). Table 1 summarizes the rate constants kre(ROH) and the

substrates

k1/k2a

ΔΔGoa‡ b (kcal mol−1)

benzene/water benzene/methanol benzene/ethanol benzene/1-propanol benzene/1-butanol benzene/2-propanol benzene/cyclopentanol benzene/benzyl alcohol benzene/CF3CH2OH benzene/ethylene glycol

0.46 0.87 0.44 0.25 0.18 2.10 0.48 0.61 2.14 0.86

−0.43 −0.08 −0.46 −0.77 −0.95 0.42 −0.41 −0.28 0.43 −0.09

Experiments were conducted with 10 min of irradiation at 10 °C. Errors in rate ratio estimated at 5% for proton NMR integration, giving σG = (RT/ratio)σratio = 0.05RT ≈ 0.03 kcal mol−1 (see Figures S51−S60). a b

a per-molecule basis using eq 11, where I1/I2 is the integrated area of the hydride resonances, and n2/n1 is the mole ratio of the two substrates (subscript 1 refers to benzene, and subscript 2 represents the other competing substrate). The differences in free energies of activation ΔΔGoa‡ can be calculated using eq 12. 9486

DOI: 10.1021/acs.inorgchem.6b01992 Inorg. Chem. 2016, 55, 9482−9491

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Inorganic Chemistry The results of competition between 2a−k and benzene are included in Table 2.

k1/k 2 = (I1/I2)(n2 /n1)

(11)

ΔΔG‡oa = RT ln(k1/k 2)

(12)

ΔH ° = [DPh − H + DRh − OR ] − [DRO − H + DRh − Ph ]

Drel = (DRh − OR − DRh − Ph ) = ΔH ° + DRO − H − DPh − H = ΔG° + RT ln(6/#H) + DRO − H − DPh − H

Figure 4. Plot of relative M−O bond strengths vs O−H bond strengths for Tp′Rh(PMe3)(OR)H. The line is fit to water and alcohols excluding CF3CH2OH and ethylene glycol. Experimental O−H bond strengths were used for all substrates except benzyl alcohol and ethylene glycol. O−H bond strength in benzyl alcohol and ethylene glycol were estimated from the calculated (M06-2X) bond energy, since the experimental values are unavailable.

Figure 3. Free-energy diagram for activation of O−H bonds with fragment [Tp′Rh(PMe3)].

oxygen−hydrogen bond energies, the relative metal−oxygen bond energies in the activation products Tp′Rh(PMe3) (OR)H can be determined using eqs 14 and 15 (see Table 3).

positive correlation indicates that the Rh−O bond strengths could be inferred from the corresponding O−H bond strengths in ROH. As in C−H activation, activation of a strong O−H bond will also give a strong M−O bond in the product. A slope of 0.97 (3) was observed for RM−O/O−H, indicating that the Rh−O bond strength varies in direct proportion to the O−H bond strength, which is different from RM−C/C−H, where values larger than unity were observed (∼1.5).36−38 Activation of 2,2,2-trifluoroethanol and ethylene glycol gave stronger Rh−OCH2CF3 bond and Rh−OCH2CH2OH bonds than would be expected based on the O−H bond strengths, as these points lie above the parent line of plain alcohols. One of the earliest and most famous correlations of X−H and M−X bond strengths was reported by Bryndza and Bercaw in 1987. In this study, they compared a series of derivatives of Cp*Ru(PMe3)2X and (dppe)PtMeX complexes by equilibrating the compounds with various alcohols and amines. They found a good linear correlation with a slope RM−X/X−H = 1.0.44 Marks examined a series of metal−alkoxide bond strengths in Cp*2ZrX2 complexes and found a linear correlation. The slope can be estimated from the figure as RZr−O/O−H = 1.3.45 Mayer also examined alkoxide and amide exchange in Re(O) (MeCCMe)2X complexes. While there was some scatter in the plotted data, the observed slope RRe−X/X−H was close to 1.46 More recently, Campbell examined bond dissociation energies of alkoxides attached to a Pt(111) surface. The alkoxide− surface bond energies showed a very good correlation with RPt−O/O−H = 1.47 Consequently, it appears that for a variety of

Table 3. Kinetics and Thermodynamics Data for Tp′Rh(PMe3) (OR)H D(RO−H)a

ΔΔGoa‡ vs PhH

ΔG0 vs PhH

Drel(M−OR)

Ph OH OMe OEt OnPr OnBu OiPr O-cyclopentyl OCH2Ph OCH2CH2OH OCH2CF3

112.9 118.81 105.20 105.40 103.50 103.30 105.70 104.80 100.70b 102.90b 107.00

0.00 −0.43 −0.08 −0.46 −0.77 −0.95 0.42 −0.41 −0.28 0.43 −0.09

0.00 −5.83 −6.31 −5.84 −5.61 −5.42 −6.64 −6.14 −5.62 −5.45 −2.76

0.0 0.7 −12.9 −12.3 −14.0 −14.0 −12.8 −13.2 −16.8 −14.8 −7.6

a

All values in kilocalories per·mole. O−H bond strengths are from experimental data.43 bThe bond strengths for benzyl alcohol and ethylene glycol are not experimentally known; therefore, the value was estimated from the calculated (M06-2X) bond energy, since experimental value is unavailable.

Note here the statistic contribution was included in the calculation to account for the number difference of hydrogen atoms available to activate (six for benzene, one for ROH, and two for water and ethylene glycol). ΔG° = ΔG‡Rh − Ph + ΔΔG‡oa − ΔG‡Rh − OR

(15)

Plotting of these relative Rh−O bond strengths versus the O−H bond strengths gave a good linear relationship for all the alcohols without additional heterosubstituents (Figure 4). The

Analysis of Rhodium−Oxygen Bond Strengths. Combining these kinetic barriers to reductive elimination (ΔG‡re) with kinetic selectivities (ΔΔG‡oa) allows the calculation of the driving force of O−H activation relative to benzene activation (Figure 3 and eq 13). With the experimentally known

R

(14)

(13) 9487

DOI: 10.1021/acs.inorgchem.6b01992 Inorg. Chem. 2016, 55, 9482−9491

Article

Inorganic Chemistry

The reaction was complete after standing overnight at RT. The solvent was removed in vacuo, and the resulting white residue was dissolved in C6D6. Activation of Water. Approximately 10 mg (0.019 mmol) of 1 was synthesized in situ from Tp′Rh(PMe3)(Me)Cl and Cp2ZrH2. The zirconium-free form of 1 was purified by flash chromatography with trace amounts of Tp′Rh(PMe3)(furanyl)H and Tp′Rh(PMe3)(Cl)H (