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Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a -Agostic Ethyl: Structure, Dynamics and Mechanism Máté J. Bezdek, and Paul J. Chirik J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08460 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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Journal of the American Chemical Society

Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a 𝛽-Agostic Ethyl: Structure, Dynamics and Mechanism Máté J. Bezdek and Paul J. Chirik* Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States ABSTRACT: The interconversion of molybdenum ethylene and ethyl complexes by proton-coupled electron transfer (PCET) is described, an unusual transformation in organometallic chemistry. The cationic molybdenum ethylene complex [(PhTpy)(PPh2Me)2Mo(C2H4)][BArF24] ([1-C2H4]+; PhTpy = 4′-Ph-2,2′,6′,2′′-terpyridine, ArF24 = [C6H3-3,5-(CF3)2]4) was synthesized, structurally characterized, and its electronic structure established by a combination of spectroscopic and computational methods. The overall electronic structure is best described as a molybdenum(III) complex with a metallacyclopropane and a redox neutral terpyridine ligand. Addition of the non-classical ammine complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+) to [1-C2H4]+ resulted in a net C–H bondforming PCET reaction to yield the molybdenum ethyl [(PhTpy)(PPh2Me)2Mo(CH2CH3)][BArF24] ([1-CH2CH3]+) and amido [(PhTpy)(PPh2Me)2Mo(NH2)][BArF24] ([1-NH2]+) compounds. The reaction was reversed by addition of 2,4,6-tri-tert-butylphenoxyl radical to [1-CH2CH3]+. The solid-state structure of [1-CH2CH3]+ established a 𝛽-agostic ethyl ligand that is maintained in solution as judged by variable temperature 1H and 13C NMR experiments. A combination of variable-temperature NMR experiments and isotopic labeling studies were used to probe the dynamics of [1-CH2CH3]+ and established restricted 𝛽-agostic -CH3 rotation at low temperature (∆G‡ = 9.8 kcal mol– 1 at -86 °C) as well as ethyl isomerization by 𝛽-hydride elimination-olefin rotation-reinsertion (∆H‡ = 19.3 ± 0.6 kcal mol–1; ∆S‡ = 3.4 ± cal K– 1 mol–1). The 𝛽-(C–H) bond-dissociation free energy (BDFE) in [1-CH2CH3]+ was determined experimentally as 57 kcal mol–1 (THF) supported by a DFT-computed value of 52 kcal/mol (gas phase). Comparison of pKa and electrochemical data for the complexes [1-C2H4]+ and [1-NH3]+ in combination with a deuterium kinetic isotope effect (kH/kD) of 3.5(2) at 23 °C support a PCET process involving initial electron transfer followed by protonation leading to the formation of [1-CH2CH3]+ and [1-NH2]+ or a concerted pathway. The data presented herein provides a structural, thermochemical and mechanistic foundation for understanding the PCET reactivity of organometallic complexes with alkene and alkyl ligands.

INTRODUCTION Redox reactions coupled to the transfer of protons are ubiquitous in biology and have been widely applied in synthesis and energy science. These proton-coupled electron transfer (PCET) processes are commonly mediated by transition metals and include the industrial oxidation of hydrocarbons as well as the 4H+/4e– oxidation of water (H2O) in solar fuel conversion schemes. Following the recognition that PCET reactivity between an H-atom donor (X–H) and acceptor (Y) is dictated by differences in X–H and Y–H bond strengths, the determination of bond dissociation free energies (BDFEs) have proven key for understanding PCET reactivity in transition metal complexes.2d A host of BDFEs are now known for metal complexes across the transition series with PCET typically involving X–H bonds (X = N, O) of aquo, hydroxo or amine ligands bound to the metal center, remote positions in chelating ligands, or transition metal hydrides (Figure 1a). Organometallic complexes with alkene and alkyl ligands are used as precatalysts or implicated as intermediates in important catalytic reactions including alkene polymerization, metathesis and the functionalization of olefins. Despite the ubiquity of PCET, application of this process to the 1H+/1e– interconversion of alkyl and alkene ligands has not been generally recognized or applied as a fundamental transformation in organometallic chemistry. Part of the limitation arises from the lack of information regarding C–H BDFEs in coordinated olefins and transition met1

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al alkyls, with previous research focusing on the thermochemistry of hydride (H+/2e–) transfer from ligands in organometallic complexes. In rare instances, ligand C–H bond strengths have been estimated for alkoxides, benzylic C–H bonds in coordinated arenes , a protonated metallocene as well as in a pincer chelate (Figure 1b). For metal alkene and alkyl complexes, Fryzuk and coworkers proposed intramolecular transfer of hydrogen atoms between ethyl and ethylene ligands in a tantalum complex to account for the experimentally observed scrambling of protons between these sites. Overall, well-defined and fully characterized examples of intermolecular 1H+/1e– PCET reactions with transition metal alkyls to yield olefin complexes have not been described nor have essential thermochemical parameters such as the associated BDFEs been measured. 9

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Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[LnM]–Hx

(a)

- H+, e–

[LnM]–Hx-1

+ H+, e–

Well-Established: PCET Involving X–H Bonds (X = M, N, O) [LnM]–Hx: H N

O Ru

N

NH

2+

H

H N

N N

HN

N

N

HN

N

N

T. J. Meyer

CO

OC

N H

NH J. M. Mayer

Cr

OC

N

N

H

J. R. Norton

SiMe3

n H C

Cl O CH2

Me3Si

L nM

J. W. Bruno

CH2

Fe

H

LnM = (CO)3Cr, (CO)3Mn+, CpFe+

H

H

n = 2+, 1+, 0, 1–

F. G. Bordwell

D. Astruc

(c) This Work: Interconversion of Olefin/Alkyl Complexes by PCET + + + L L L L

L

Mo L

+ H+, e– - H+, e–

L Structure

L

L

L

Mo L

L

Dynamics

H

RESULTS AND DISCUSSION Synthesis, Characterization and PCET Reactivity of the Molybdenum Ethylene Complex [1-C2H4]+. Our studies commenced with the synthesis of the cationic molybdenum ethylene complex [1-C2H4]+. Stirring a benzene solution of (PhTpy)(PPh2Me)2MoCl ([1-Cl]) in the presence of one equiv each of Na[BArF24] and ethylene at room temperature for 18 hours afforded [1-C2H4]+ as a green-brown solid in 88% yield (Figure 2a). The solid-state structure of [1-C2H4]+ was determined by single-crystal X-ray diffraction and established an idealized octahedral geometry with an 𝜂2-ethylene ligand coordinated trans to the central pyridine ring of the terpyridine chelate with apical PPh2Me ligands completing the coordination sphere of molybdenum (Figure 2b). The formally Mo(I) complex has an S = ½ ground state with a measured solid-state magnetic moment of 1.7 µB at 23 °C (magnetic susceptibility balance). Accordingly, [1-C2H4]+ exhibits a rhombic EPR signal in toluene glass at 7 K that was simulated using the g-values gx = 2.122, gy = 2.031, and gz = 1.998 (Figure 2c). The deviation of the observed g-values from ge (ge = 2.002) indicates a principally molybdenum-centered singly-occupied molecular orbital (SOMO), supported by the DFT-computed Mulliken spin density plot of the complex (Figure 2d). The observed g-anisotropy in the EPR spectrum of [1C2H4]+ is characteristic of a low-spin Mo(III) complex and indicates the molybdenum-ethylene bonding interaction is likely best described as a metallacyclopropane. This electronic structure assignment is supported by the elongated ethylene C–C bond distance observed in the solid-state structure of [1-C2H4]+ (1.425(6) Å). 18

(b) Rare: Thermochemical Studies on Ligand C–H Bond Strengths

Nb

structure and dynamics of [1-CH2CH3]+ are described, and a thermochemical study of the 𝛽-(C–H) BDFE is presented. Finally, the mechanism of C–H bond forming PCET reaction between [1-C2H4]+ and [1-NH3]+ is examined.

2+

N

Fe

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Mo

L

CH2

H L Thermochemistry & Mechanism of PCET L

Figure 1. (a) Examples of transition metal PCET reactions involving X–H bonds (X = M, N, O). (b) Selected thermochemical studies on ligand C–H bond strengths. (c) The strategy reported in this work.

Our laboratory has recently reported the non-classical ammine complex, [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+; Ph Tpy = 4′-Ph-2,2′,6′,2′′-terpyridine, ArF24 = [C6H3-3,5-(CF3)2]4) where the ammine N–H bond (BDFEN–H = 46 kcal mol–1) is weaker than the thermodynamic threshold for spontaneous hydrogen evolution (∆G°(H•) = 48.6 kcal mol–1).2d Indeed, warming the compound to 60 ºC resulted in loss of H2 gas and formation of the corresponding molybdenum amido product. Subsequent studies have demonstrated that the ammine complex also serves as an effective H-atom donor and can be used to promote formation of a relatively weak N-H bond in a related molybdenum amido complex [(PhTpy)(PPh2Me)2Mo(NHtBuAr)][BArF24] (tBuAr = 4-tertbutyl-C6H4; BDFEN–H = 46–52 kcal mol–1). The hydrogenation of styrene to ethylbenzene has also been observed using [1NH3]+ as the hydrogen atom source,16 suggesting PCET reactivity was plausible with this compound. The combination of facile 1H+/1e– chemistry from the formally Mo(I) ammine complex [1-NH3]+ together with the observation of olefin reduction prompted exploration of the interconversion of olefin and alkyl complexes separated by a single hydrogen atom (Figure 1c). Observation of such a process requires a kinetically and thermodynamically accessible one-electron redox couple but ultimately uncovers an under recognized fundamental transformation in organometallic chemistry that may be operative or leveraged in catalysis. Here we describe realization of this goal with demonstration that the cationic molybdenum ethylene complex [(PhTpy)(PPh2Me)2Mo(C2H4)][BArF24] ([1-C2H4]+) undergoes PCET and overall accepts an H-atom to yield the 𝛽agostic ethyl [(PhTpy)(PPh2Me)2Mo(CH2CH3)][BArF24] ([1CH2CH3]+) with [1-NH3]+ serving as the H-atom donor. The ACS Paragon Plus Environment

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Journal of the American Chemical Society (a)

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PPh2Me

Ph N N

N Mo Cl

PPh2Me [1-Cl]

1 equiv. C2H4 Na[BArF24]

N N

PhH rt, 18 h - NaCl

+

PPh2Me

Ph

[BArF24]–

N Mo

Ph2MeP [1-C2H4]+

(b)

[1-NH3]+ quantitatively furnished the diamagnetic products of PCET, [1-CH2CH3]+ and [1-NH2]+ over the course of 5 hours at room temperature (Scheme 1; see below for characterization of [1-CH2CH3]+). The reaction was not inhibited by the addition of excess PPh2Me (10 equiv), an observation supporting a PCET pathway involving terpyridine bis(phosphine) molybdenum complexes. The observed reactivity represents an unusual net Hatom addition to the molybdenum olefin complex where the alkyl product was cleanly generated. The C–H bond forming PCET reaction was readily reversed by addition of 1 equiv of tBu3ArO• to [1-CH2CH3]+ in benzene-d6, quantitatively yielding [1-C2H4]+ and tBu3ArOH within minutes at room temperature. While this result defines the range for the strength of the C–H bond formed in [1-CH2CH3]+ as 46-77 kcal mol–1, closer and more detailed examination of the ethyl C–H BDFE formed in [1-CH2CH3]+ as well as the mechanism of PCET with [1-NH3]+ are described in subsequent sections. 22

Scheme 1. Interconversion of [1-C2H4]+ and [1-CH2CH3]+ by PCET PPh2Me

Ph N

(c)

(d)

N

+

PPh2Me

PPh2Me N N

NH2 [1-NH2]+

Figure 2. (a) Synthesis of [1-C2H4]+ by chloride abstraction. (b) Solid-state structure of [1-C2H4]+ with 30% probability ellipsoids. The hydrogen atoms and the [BArF24]– counterion have been omitted for clarity. (c) X-band EPR spectrum of [1-C2H4]+ recorded in toluene glass at 7 K. Collection and simulation parameters: microwave frequency = 9.378 GHz, power = 2.0 mW, modulation amplitude = 4.0 G; gx = 2.122, gy = 2.031, and gz = 1.998. (d) DFT computed spin-density plot for [1-C2H4]+ obtained from Mulliken population analysis in the gas-phase at the B3LYP level of theory.

+ Ph [BArF24]–

PPh2Me N

+

Ph2MeP

+ tBu3ArO C 6D 6 rt, 15 min - tBu3ArOH

THF-d8 rt, 5 h

N Mo

N

[1-C2H4]+ Ph2MeP

PCET

Ph

+ [BArF24]–

Mo

N

NH3

[1-NH3]+

PPh2Me

N Mo

N

+ Ph [BArF24]–

N

+ [BArF24]–

N Mo

CH2 H Ph2MeP [1-CH2CH3]+

Independent Synthesis, Structural Characterization and Dynamics of the 𝛽-Agostic Molybdenum Ethyl Complex [1-CH2CH3]+. Attempts to separate [1-CH2CH3]+ from [1NH2]+ following the PCET reaction were unsuccessful and motivated discovery of an alternate synthetic route. The independent synthesis of [1-CH2CH3]+ was accomplished by a stepwise oxidation-alkylation sequence (Figure 3a). Addition of [Cp2Fe][BArF24] to a toluene solution of [1-Cl] followed by stirring for 18 hours at room temperature furnished With [1-C2H4]+ in hand, the PCET reactivity as an H-atom ac[(PhTpy)(PPh2Me)2Mo(Cl)][BArF24] ([1-Cl]+) as a yellow+ ceptor was explored. Treatment of [1-C2H4] with organic Hgreen solid in quantitative yield. Subsequent alkylation was acatom donors such as cyclohexadiene (CHD) and 2,4,6-tri-tertcomplished by mildly heating an Et2O/benzene solution (1:1, butylphenol (tBu3ArOH) produced no reaction even at elevated v/v) of [1-Cl]+ at 45 °C with Et2Zn for 24 hours, yielding [1temperature (60 °C) and extended reaction times (7 days), sugCH2CH3]+ as a brown solid in 86% yield (Figure 3a). Both the 31 gesting C–H bond formation by PCET is thermodynamically P{1H} and 1H NMR spectra of [1-CH2CH3]+ in benzene-d6 at unfavorable using these reagents (BDFEC–H(CHD) = 68 kcal 23 °C are consistent with C2v molecular symmetry in solution. A mol–1, BDFEO–H(tBu3ArOH) = 77 kcal mol–1). 2d, Accordingly, single 31P{1H} NMR signal was observed at 13.09 ppm and diag+ the cationic molybdenum ammine complex [1-NH3] was selectnostic 1H resonances were observed at 3.33-3.17 and -0.51 ppm ed as the H-atom donor due to an exceedingly weak homolytic Nfor the –CH2– and –CH3 protons of the ethyl ligand, respectively. H bond strength, (BDFEN–H = 46 kcal mol–1)16 and precedent for The solid-state structure of [1-CH2CH3]+ was determined by forming relatively weak N-H bonds when conventional H-atom single-crystal X-ray diffraction and revealed a 𝛽-agostic interacdonors were unsuccessful.17 Because both [1-NH3]+ and [1tion between the molybdenum center and a 𝛽-ethyl hydrogen C2H4]+ have identical terpyridine/ bis(phosphine) coordination (Figure 4). The agostic hydrogen atom (H49A) was located on environments, the reaction chemistry is simplified as nonthe difference map and was refined to a close Mo1–H49A disproductive ancillary ligand exchange is thermoneutral. Accordingtance of 1.99(6) Å. A Mo1–H49A–C49 angle (106.4 deg) was ly, treatment of a THF-d8 solution of [1-C2H4]+ with 1 equiv of observed as well as a shortened ethyl C–C contact (1.452(4) Å) 3 ACS Paragon Plus Environment 21


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consistent with increased sp2 character at the ethyl carbons. While ethyl complexes of molybdenum have been structurally characterized, to our knowledge 𝛽-agostic ethyl ligands have not been observed with this metal. 23

(a) N N [1-Cl]

N Mo Cl PPh2Me

PPh2Me

Ph

PPh2Me

Ph

N

[Cp2Fe][BArF24]

N Mo

N

PhMe rt, 18 h - Cp2Fe

Cl

[1-Cl]+

PPh2Me

(b) Et2Zn

PhH/Et2O (1:1) 45 °C, 24 h - 0.5 ZnCl2 PPh2Me

Ph N N

+

[BArF24]–

+

[BArF24]–

N Mo


Figure 3. (a) Independent synthesis of [1-CH2CH3]+ by oneelectron oxidation-alkylation. (b) Solid-state structure of [1CH2CH3]+ with ellipsoids at 30% probability. Hydrogen atoms (except those connected to C48 and C49), and the [BArF24]– counterion have been omitted for clarity.

The 𝛽-agostic interaction observed in the solid-state structure of [1-CH2CH3]+ was confirmed in solution by low-temperature 1 H and 13C NMR experiments. The 1H NMR spectrum of [1CH2CH3]+ in THF-d8 at –107 °C exhibits the number of resonances consistent with overall Cs molecular symmetry, in agreement with the geometry observed in the solid state. Notably, the 1 H NMR spectrum collected at –107 °C revealed inequivalent non-agostic and agostic methyl protons with resonances at 2.21 and -3.90 ppm, respectively. The 1JC-H coupling constant of the agostic H was determined from low-temperature 13C NMR experiments. The isotopomers [1-13CH2CH3]+ and [1-CH213CH3]+ were prepared by the alkylation of [1-Cl]+ with a mixture of 13Clabelled diethylzinc isotopomers (13CH3CH2)2Zn, 13 13 13 ( CH3CH2)(CH3 CH2)Zn and (CH3 CH2)2Zn. At 23 °C, the 13 C NMR spectrum of the isotopomeric mixture containing [113 CH2CH3]+ and [1-CH213CH3]+ exhibits two prominent signals: a triplet and a quartet assigned to the 𝛼- and 𝛽-carbons of the ethyl ligand at 54.01 and 6.67 ppm with 1JCH of 147.3 Hz and 122.3 Hz, respectively. Collecting the 13C NMR spectrum at -107 °C revealed that the 𝛽-carbon signal approaches higher-order coupling with a significantly reduced 1JCH of 97.2 Hz while the C– H coupling constant for the nonagostic 𝛼-carbon is unchanged (Figure S4). These 1JCH values confirm reduced C–H orbital overlap at the 𝛽-carbon as a result of a 3-centered, 2-electron agostic interaction with molybdenum. 24

25

Figure 4. Alternate view of the solid-state structure of [1CH2CH3]+ along the P1–Mo1–P2 axis with ellipsoids at 30% probability. Hydrogen atoms (except those connected to C48 and C49), the terpyridine 4’-Ph, PPh2Me ligands and the [BArF24]– counterion have been omitted for clarity. Selected bond distances (Å) and angles (deg): Mo1—C48 2.194(2), C48—C49 1.452(4), Mo1—H49A 1.99(6), Mo1—N1 2.2022(18), Mo1—N2 2.0811(18), Mo1—N3 2.1461(19), N1—C5 1.366(3), C5—C6 1.449(3), C6—N2 1.375(3), N2—C10 1.381(3), C10—C11 1.442(3), C11—N3 1.369(3), Mo1—H49A—C49 106.4.

Variable temperature solution NMR studies demonstrated that dynamic processes are operative in [1-CH2CH3]+. The two distinct resonances observed in the low-temperature 1H NMR spectrum of [1-CH2CH3]+ for the agostic and non-agostic 𝛽-methyl protons coalesce at -86 °C, indicative of a 𝛽-methyl rotation dynamic (Scheme 2a). The barrier for this process was estimated by the fast-exchange approximation and was determined to be 9.8 kcal mol–1 at -86 °C. This value is comparable to 𝛽-agostic rotational barriers reported in the range 8.0-8.8 kcal mol–1 for cationic 𝛼-diimine complexes of Ni and Pd relevant to olefin polymerization. Because 𝛽-agostic interactions are commonly invoked as precursors to 𝛽-hydride elimination processes,8 ethyl isomerization dynamics were also investigated for [1-CH2CH3]+ (Scheme 2b). If such a pathway is operative, coalescence of the 𝛼- and 𝛽ethyl 13C NMR signals would be expected at elevated temperatures. Accordingly, rates of ethyl isomerization were measured by 13 C NMR line broadening techniques in toluene-d8 solution containing the isotopomers [1-13CH2CH3]+ and [1-CH213CH3]+ between 60 and 102 °C. Eyring analysis in this temperature range produced activation parameters of ∆H‡ = 19.3 ± 0.6 kcal mol–1 and ∆S‡ = 3.4 ± 1.7 cal K–1 mol–1 for the isomerization (Figure 5). Comparing these values to the activation parameters reported for analogous 𝛽-hydride elimination processes in the cationic 𝛽agostic ethyl complexes of Pd (∆H‡ = 6.1 ± 0.2 kcal mol–1; ∆S‡ = 5.2 ± 0.9 cal K–1 mol–1)27b and Ni (∆H‡ = 13.2 kcal mol–1; ∆S‡ = 4.2 cal K–1 mol–1)27c reveals a comparably modest ∆S‡ but higher enthalpy of activation in [1-CH2CH3]+. 26

27

28

Scheme 2. Dynamic Processes in [1-CH2CH3]+


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Journal of the American Chemical Society (a)

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PPh2Me

Ph N

+

N

N

Mo

N

Ph2MeP

C

Ha

Hc

𝝙G‡

= 9.8 kcal

mol–1,

-86 °C

[BArF24]–

N Mo

N

Hb

+

PPh2Me

Ph

[BArF24]–

Hb

Ph2MeP

C

Hc Ha

Accordingly, IPR was observed for [1-CD2CH3]+, [1CHDCH2D]+ and [1-CH2CHD2]+ that exhibit 𝛽-CH3 resonances at -0.53, -0.70 and -0.90 ppm, respectively (Figure 6). This observation further supports the presence of an agostic interaction while also providing evidence for rapid deuterium scrambling (Scheme 3a).

(b) PPh2Me

Ph N

+

[BArF24]–

N

Ph2MeP

N

𝛽-H elimination

Mo

N

N

CH2 Ph2MeP 𝝙H‡

mol–1

= 19.3 ± 0.6 kcal 𝝙S‡ = 3.4 ± 1.7 cal mol–1 K–1

PPh2Me

Ph N N

N Mo

Ph2MeP

+

[BArF24]–

olefin rotation

+

PPh2Me

Ph N

olefin insertion

* H

*

H

*

[BArF24]–

Mo

N H

+

PPh2Me

Ph

N

[BArF24]–

N Mo

CH2

* H

Ph2MeP

Figure 6. The 𝛽-CH3 region of the 1H NMR spectrum (500 MHz, benzene-d6, 23 °C) of the isotopomeric mixture containing [1CD2CH3]+, [1-CHDCH2D]+ and [1-CH2CHD2]+.

Figure 5. Eyring plot for 𝛽-hydride elimination in [1-CH2CH3]+.

Deuterium labeling experiments were conducted to chemically confirm 𝛽-hydride elimination processes in [1-CH2CH3]+. Monitoring a benzene-d6 containing [1-Cl]+ and 1,1-d2-EtLi revealed scrambling of the deuterium label within minutes at room temperature with statistical formation of the corresponding d2-ethyl isotopomers [1-CD2CH3]+, [1-CHDCH2D]+ and [1CH2CHD2]+ that were identified by isotopic perturbation of resonance (IPR). This technique relies on the zero-point energy differences between terminal/agostic C–H bonds compared to terminal/agostic C–D bonds resulting in an accumulation of the lighter isotope in the agostic position and a noticeable 𝛽-CH3 chemical shift separation for the partially deuterated isotopomers. 29

To provide evidence against the possible deuterium scrambling during transmetalation of 1,1-d2-EtLi to molybdenum, an additional isotopic labeling experiment was conducted that instead involved a PCET pathway (Scheme 3b). Monitoring a benzened6 solution containing [1-C2D4]+ and [1-NH3]+ yielded a statistical isotopomeric mixture containing [1-CHDCD3]+ and [1CD2CHD2]+ within 5 minutes at room temperature. Immediate H incorporation at both the 𝛼- and 𝛽-carbons was thus established, consistent with rapid deuterium scrambling pathways. In the absence of rapid scrambling pathways, exclusive proton incorporation at 𝛽-carbon is expected. Importantly, upon performing each labeling experiment in the presence of excess PPh2Me (10 equiv), product formation was not suppressed and the isotopomeric ratio of products was not perturbed. Taken together, these results are consistent 𝛽-hydride elimination/olefin rotation/reinsertion processes in [1-CH2CH3]+ that proceed through a 7-coordinate, terpyridine bis(phosphine) molybdenum olefin hydride intermediate (Scheme 2b). To probe intermolecular C– H/D exchange that may complicate the interpretation of the labeling experiments, three separate experiments were conducted wherein [1-CD2CD3]+ was treated with [1-C2H4]+ (1 equiv) or [1-CH2CH3]+ (1 equiv) or free ethylene (5 equiv). In all cases, partially deuterated isotopomers were not observed after 48 hours of stirring at 23 °C, ruling out intermolecular C–H/D exchange on the timescale of intramolecular 𝛽-hydride elimination (Figures S9-S11).


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Scheme 3. Deuterium Labeling Experiments to Probe 𝛽-Hydride Elimination in [1-CH2CH3]+ statistical product mixture of isotopomers

(a) +

PPh2Me

Ph N

PPh2Me

Ph

[BArF24]–

N

N

EtLi-(1,1)-d2

Mo

N

+

PPh2Me

Ph

N

PhMe -95 °C→rt, 5 min - LiCl

Cl PPh2Me

N Mo

Ph2MeP

N

fast N

CD2 H

+

N

N Mo

CH2 Ph2MeP

+

CD2 H

N

CH2

PPh2Me

Ph N

+

CHD CHD

H

+

[BArF24]–

N Mo

N

Ph2MeP

[1-CHDCH2D]+

(b)

CH2 CD2

H

[1-CH2CHD2]+

statistical product mixture of isotopomers

+

PPh2Me

Ph N

PPh2Me

Ph

[BArF24]–

N

N

+

Mo

N

NH3 PPh2Me

+

PPh2Me

Ph

[BArF24]–

N

N Mo

N

D 2C Ph2MeP

[1-NH3]+

CD2

N

C 6D 6 rt, 5 min - [Mo] NH2

N Mo

Ph2MeP

+

N

fast N

CD2 H

CD2

+

[BArF24]–

PPh2Me

Ph

N Mo

Ph2MeP

[1-C2D4]+

CD2 H

N

+

N

PPh2Me

30

31

32

35

36

N N

N Mo


+

+

[BArF24]–

N Mo

CD2 Ph2MeP

CHD D

CD2

[1-CHDCD3]+

[1-CD2CHD2]+

Determination of the 𝛽-(C–H) Bond Dissociation Free Energy in [1-CH2CH3]+. Having established the structure and dynamics of [1-CH2CH3]+, thermochemical studies were conducted to probe the 𝛽-(C–H) BDFE. Because [1-C2H4]+ and [1CH2CH3]+ differ only by a hydrogen atom, a thermochemical square scheme can be constructed that defines the BDFE of the 𝛽-(C–H) (Scheme 4). It is important to note that the reorganization energy component of this bond strength definition is expected to be significant, as a new Mo–C interaction stabilizes the alkene product. This interaction will drive C–H bond cleavage processes in [1-CH2CH3]+ and must be overcome in order to achieve C–H bond formation in [1-C2H4]+. Therefore its inclusion in the C–H BDFE definition is essential for understanding the PCET reactivity of [1-CH2CH3]+. The Bordwell equation (Eq. 1) can then be used to quantify the C–H BDFE in terms of a 1-electron redox couple (E°; estimated from E1/2 of a reversible electrochemical wave), the 𝛽-(C–H) pKa and the solvent-specific H+/H• standard reduction potential (CG). The electrochemical behavior of [1-C2H4]+ was therefore of interest and the isolation of the neutral ethylene complex [1-C2H4] was targeted to evaluate the E° and pKa terms in Scheme 4 and to experimentally determine the C–H BDFE in [1-CH2CH3]+ using the Bordwell equation. While thermochemical data for agostic alkyl C–H bonds are principally absent from literature, reports by Milstein, Kirchner and van der Vlugt are notable in demonstrating the deprotonation of agostic aryl C–H bonds in pincer complexes of Rh, Mn, Fe, Ni and Co using NEt3 as a base, thereby defining upper bounds for the C–H pKas in these complexes. More recently, Hulley and coworkers have estimated that 𝜂2-arene coordination results in acidic arene C–H bonds with pKas of 3–6 (MeCN) upon coordination to Pd(II). These results, along with those established for related metal-dihydrogen complexes, suggest that a similar C–H acidification and consequent homolytic bond weakening may arise in 𝛽-agostic alkyl complexes, further motivating a quantitative thermochemical study on the C–H BDFE in [1-CH2CH3]+. 34

PPh2Me

Ph

Ph

33

[BArF24]–

N Mo

Ph2MeP

[1-CD2CH3]+

[1-Cl]+

+

PPh2Me

Ph

[BArF24]–

PPh2Me

Ph - H+

N

pKa + H+

N Mo

N

Ph2MeP [1-C2H4] + e–

BDFEC–H

E°

- e–

PPh2Me

Ph N N

+

N Mo

Ph2MeP

[1-C2H4]+

The cyclic voltammogram (CV) of [1-C2H4]+ was collected in THF solution and exhibits reversible anodic and cathodic waves with half-wave potentials (E1/2) of -0.75 V and -1.35 V (vs Cp2Fe/Cp2Fe+), respectively (Figure 7). While the wave at -0.75 V can be assigned to a one-electron oxidation to [1-C2H4]2+, the second wave at -1.35 V is relevant to Scheme 4 and corresponds to a reduction event furnishing the neutral ethylene complex [1C2H4]. With this reduction potential in hand, the isolation of [1C2H4] was targeted to determine the pKa of its conjugate acid [1CH2CH3]+ and thereby the 𝛽-(C–H) BDFE. Accordingly, oneelectron reduction of [1-C2H4]+ with one equiv of Cp2Co in thawing toluene solution yielded [1-C2H4] in 53% yield after recrystallization (Scheme 5a). The solid-state structure of [1C2H4] was determined by single-crystal X-ray diffraction and the coordination geometry is analogous to [1-C2H4]+. In contrast to the paramagnetic cation however, [1-C2H4] is diamagnetic and exhibits the number of resonances expected for a C2v symmetric complex with a diagnostic triplet at 2.97 ppm (t, 3JPH = 10.4 Hz) assignable to the ethylene hydrogens.

37

BDFE = 1.37pKa + 23.06E° + CG

(1)

Scheme 4. Thermochemical Expression for the 𝛽-(C–H) BDFE in [1-CH2CH3]+


6

Page 7 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society Information). Importantly, while both [1-CH2CH3]+ and [1CH3]+ exhibit agostic interactions, the degree of bond weakening in the latter complex is mitigated by the formation of a potentially unstable, formally Mo(III) methylidene product. Therefore, observation of an agostic interaction does not necessarily imply a significantly weakened bond, wherein the stability of the complex upon H-atom loss must also be considered when estimating the degree of bond weakening. Scheme 5. Synthesis of [1-C2H4] and pKa Equilibration Studies (a)

PPh2Me

Ph

Figure 7. Cyclic voltammogram of [1-C2H4]+ using a glassycarbon working electrode, a platinum wire counter electrode, a silver wire reference electrode, 0.2 M [nBu4N][PF6], and a scan rate of 100 mV/s in THF at 23 °C versus Cp2Fe/Cp2Fe+. The reduction potential (R1) and the oxidation potential (O1) are reported as half-wave potentials (E1/2). For a scan starting at i=0, see the Supporting Information.

N

38

39

PPh2Me

Ph N

Cp2Co N

PhMe -95 °C→rt, 15 min - [Cp2Co][BArF24]

Ph2MeP

(b)

PPh2Me

Ph N

[1-C2H4]

+

[BArF24]–

N

H

CH2

[1-CH2CH3]+

PPh2Me

Ph N N

N

Mo

Ph2MeP

N Mo

Ph2MeP

[1-C2H4]+

N

The C–H pKa of [1-CH2CH3]+ was determined using 1H NMR spectroscopy by addition of 1,8-diazabicyclo(5.4.0)undec7-ene (DBU) as the pKa reference and measurement of the equilibrium concentration ratio with the conjugate base [1-C2H4] (Scheme 5b). An average of three equilibration experiments yielded a pKa of 16.3 in THF for [1-CH2CH3]+. Using the relevant redox couple (E° = -1.35 V, THF) this pKa value enabled calculation of the 𝛽-(C–H) BDFE in [1-CH2CH3]+ as 57 kcal mol–1 in THF, in good agreement with a DFT-computed value of 52 kcal mol–1. These results define both the 𝛽-agostic and terminal 𝛽-(C–H) BDFEs according to the definition in Scheme 4 because a new Mo–C interaction stabilizes the product of Hatom loss from all 𝛽-(C–H) positions and leads to the formation [1-C2H4]+. Therefore, while the experimentally determined 𝛽(C–H) BDFE of 57 kcal mol–1 (THF) likely contains different free energies of reorganization for the agostic and terminal 𝛽-(C– H) bonds, this value is expected to govern the overall thermodynamics of 1H+/1e–PCET reactivity for the [1-CH2CH3]+/[1C2H4]+ pair at ambient conditions independent of which 𝛽-(C– H) bond is involved (see Figure S17). It is important to note that the 𝛽-(C–H) BDFE was measured at ambient conditions where a rapid 𝛽-(CH3) rotation dynamic interconverts agostic and terminal C–H bonds and may attenuate the influence of the 𝛽-agostic interaction on the 𝛽-(C–H) BDFE. The magnitude of this effect may be estimated from the barrier for 𝛽-(CH3) rotation (∆G‡), measured as 9.8 kcal mol–1 at -86°C (see above). Importantly, the 𝛽-(C–H) pKa in [1-CH2CH3]+ is several tens of units lower than that of free alkanes, (16.3 vs. >48)2d contributing to the exceedingly low 𝛽-(C–H) BDFE in [1-CH2CH3]+ that can be compared to a value of 92.9 kcal mol–1 in free ethane.2d As implied by Scheme 4, the loss of an H-atom from [1CH2CH3]+ is driven by the stability of the olefin complex [1C2H4]+, thereby resulting in a net weak C–H bond in the parent metal alkyl. It is therefore instructive to compare the C–H BDFE in [1-CH2CH3]+ to that of the analogous 𝛼-agostic methyl complex [1-CH3]+ to assess whether an agostic interaction gives rise a bond weakening of similarly large magnitude in the absence of an incipient metal-olefin complex. While the preparation of this compound has not yet been realized, the C–H BDFE in [1-CH3]+ was computed to be 69 kcal mol–1 (gas phase, see Supporting

N Mo

N

+ [BArF24]–

N N H THF-d8, rt

+

N

N Mo

[BArF24]–

Ph2MeP [1-C2H4]

Mechanism of PCET Reaction between [1-C2H4]+ and [1-NH3]+. Having determined fundamental thermochemical parameters E° and pKa for [1-CH2CH3]+, the pathway of the PCET reaction forming the 𝛽-(C–H) bond was examined. Shown in Figure 8, three mechanistic possibilities for the PCET reaction between [1-C2H4]+ and [1-NH3]+ were considered: 1) concerted hydrogen atom transfer (HAT; diagonal); 2) electron transfer followed by proton transfer (ET/PT; right-down); or 3) proton transfer followed by electron transfer (PT/ET; downright). Examining whether there is an appropriate match between the relevant one-electron redox couples (E°) or pKas of the reactants at individual ET or PT step of the cycle is an effective approach for probing the thermodynamic accessibility of each PCET pathway. Accordingly, the ET/PT mechanism was first considered. Previously we reported the [1-NH3]+/[1-NH3]2+ couple as -1.09 V (THF, 23 °C, vs Cp2Fe/Cp2Fe+)16 which is in thermodynamic proximity to the [1-C2H4]/[1-C2H4]+ couple of 1.35 V (see above). Therefore, ET from [1-NH3]+ to [1-C2H4]+ is expected to be slightly endoergic with ∆G°ET = +6.0 kcal mol–1. Subsequent PT from [1-NH3]2+ (pKa = 3.6)16 to [1-C2H4] (pKa = 16.3) is favorable, with ∆G°PT = -17 kcal mol–1. Therefore, an ET/PT pathway leading to the formation of [1-CH2CH3]+ and [1-NH2]+ is thermodynamically accessible. To provide support for the PT component of this pathway, the complex [1-NH3]2+ was prepared by our previously reported procedures16 and was treated with one equiv of [1-C2H4]. The reaction cleanly and quantitatively furnished the diamagnetic products [1-CH2CH3]+ and [1-NH2]+ over the course of 1 hour in fluorobenzene solution supporting the thermodynamic and kinetic feasibility of a PT step following initial ET.


7


+

PPh2Me

Ph N

E° = -1.09 Va [1-NH3]+ [1-NH3]2+

N

N

PPh2Me

Ph N

ET

Mo

CONCLUDING REMARKS

N Mo

N

E° = -1.35 V Ph2MeP

[1-C2H4

]+

[1-C2H4] Ph2MeP

[1-NH3]+ pKa 20a

PT

pKa 6d

[1-NH2] PPh2Me

Ph N N

[1-NH3]+

PCET

57 kcal mol–1

BDFEN–Ha 46 kcal mol–1 2+ [1-NH2]+

N

[1-CH2CH3]2+

[1-NH2]

[1-NH2]+

Epc° = -2.40 Vb,c

pKa 3.6a

PT

[1-NH2]+ PPh2Me

Ph N N

ET H 3C

pKa 16.3

E° = -0.76 V

Mo

Ph2MeP

[1-NH3]2+

BDFEC–H

Page 8 of 12

+

N Mo

Ph2MeP

H

CH2

[1-CH2CH3]+

Figure 8. Square scheme showing PCET mechanisms for the conversion of [1-C2H4]+ to [1-CH2CH3]+ (solid arrows), overlaid with thermodynamic parameters contributing to the C–H BDFE (THF) in [1-CH2CH3]+ (gray, dashed arrows). All experimental pKa determinations were conducted in THF solution. E° values reported as half-wave potentials (0.2 M [nBu4N][PF6], 100 mV/s scan rate in THF at 23 °C versus Cp2Fe/Cp2Fe+) unless otherwise noted. a Thermochemical data from Ref. 16. b Peak cathodic potential of a quasi-reversible wave. c Thermochemical data from Ref. 17. d Indirectly calculated from Bordwell equation using experimentally determined BDFEC–H and E° values.

To probe the thermodynamics of the PT/ET pathway in Figure 8, the CV of [1-CH2CH3]+ was collected and exhibits a reversible anodic wave with E1/2 = -0.76 V (THF, 23 °C, vs Cp2Fe/Cp2Fe+) that corresponds to the [1-CH2CH3]+/[1CH2CH3]2+ redox couple (Figure S14). While the isolation of [1CH2CH3]2+ was attempted by treatment of [1-CH2CH3]+ with [Cp2Fe][BArF24], this complex proved too unstable in THF solution for further study. As a result, the pKa of [1-CH2CH3]2+ was indirectly evaluated with the Bordwell equation using the experimentally determined C–H BDFE for [1-CH2CH3]+ (57 kcal mol– 1 ) and the E° value for the [1-CH2CH3]+/[1-CH2CH3]2+ redox couple (-0.76 V). A pKa of 6 (THF) was thus estimated for [1CH2CH3]2+. Therefore, there is a pKa mismatch between [1NH3]+ (pKa = 20)16 and [1-C2H4]+ that renders initial PT between these complexes thermodynamically unfavorable with ∆G°PT ≈ +19 kcal mol–1. Based on these results, a stepwise ET/PT or a concerted hydrogen atom transfer (HAT) mechanism are likely favored in the C–H bond forming PCET reaction between [1-C2H4]+ and [1-NH3]+ over the PT/ET pathway. In an attempt to distinguish the stepwise ET/PT and concerted HAT mechanisms for the PCET reaction between [1-C2H4]+ and [1-NH3]+, a deuterium kinetic isotope effect (KIE; kH/kD) was measured. Upon treatment of [1-C2H4]+ with a 1:1 mixture of [1-NH3]+ and [1-ND3]+, the relative ratio of the product isotopologs [1-CH2CH3]+ and [1-CH2CH2D]+ was determined by 1H NMR spectroscopy (see Supporting Information). Using this approach, a normal, primary KIE of 3.5(2) (23 °C) was measured. While this result does not allow us to rigorously rule out HAT, such pathways are typically favored in the absence of a thermodynamically accessible stepwise mechanism and often exhibit large H/D isotope effects as a consequence.2d Given the reasonable thermodynamic coupling between the 1e– redox potentials of [1-C2H4]+ and [1-NH3]+, we currently favor a mechanistic picture involving stepwise ET/PT for the conversion of the molybdenum olefin complex to the corresponding alkyl.

In summary, a cationic molybdenum ethylene complex [1C2H4]+ supported by terpyridine and bis(phosphine) ligands has been synthesized and its PCET reactivity as an H-atom acceptor has been established. Addition of the non-classical ammine complex [1-NH3]+ to [1-C2H4]+ resulted in a C–H bond forming PCET reaction and furnished the ethyl complex [1-CH2CH3]+ together with the corresponding amido [1-NH2]+. Structural and spectroscopic studies revealed a 𝛽-agostic interaction in [1CH2CH3]+, which undergoes ethyl isomerization via rapid 𝛽hydride elimination, olefin rotation, reinsertion processes. Electrochemical and pKa measurements were conducted as part of a thermochemical square scheme and established an unusually weak 𝛽-(C–H) BDFE of 57 kcal mol–1 in [1-CH2CH3]+. Comparison of the relevant 1e– redox couples and pKa values in [1-C2H4]+ and [1-NH3]+ in combination with a measured deuterium kinetic isotope effect rule out a proton-transfer/electron transfer mechanism in the PCET reaction, and instead support a pathway involving electron transfer/proton transfer or a concerted process. The data presented in this study provide rare insight into the structure, dynamics and PCET reactivity of organometallic complexes with alkene and alkyl ligands and offers a new pathway for their interconversion.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Crystallographic information for [1-Cl]+, [1-C2H4]+, [1-CH2CH3]+ and [1-C2H4] (CIF). Experimental details; characterization data including NMR spectra of complexes; electrochemical data; pKa determinations; computational methods and results (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected] ORCID Máté J. Bezdek: 0000-0001-7860-2894 Paul J. Chirik: 0000-0001-8473-2898

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support was provided by the U.S. Department of Energy, Office of Science, Basic Energy Science (DE-SC0006498). M.J.B. thanks the Natural Sciences and Engineering Research Council of Canada for a predoctoral fellowship (PGS-D). We thank Kenith Conover (Princeton) for assistance with the acquisition of variable temperature NMR data. Professors Robert R. Knowles and Brad P. Carrow (Princeton) are acknowledged for insightful discussions.


8

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9 (a) Tilset, M. The Thermodynamics of Organometallic Systems Involving Electron-transfer Paths. In: Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH, 2001, 677–713 and references therein. (b) Ellis, W. W.; Miedaner, A.; Curtis, C. J.; Gibson, D. 9 ACS Paragon Plus Environment


H.; DuBois, D. L. Hydride Donor Abilities and Bond Dissociation Free Energies of Transition Metal Formyl Complexes. J. Am. Chem. Soc. 2002, 124, 1926−1932. (c) Pitman, C. L.; Finster, O. N. L.; Miller, A. J. M. Cyclopentadiene-mediated hydride transfer from rhodium complexes. Chem. Commun. 2016, 52, 9105−9108. 10 Kerr, M. E.; Zhang, X.-M.; Bruno, J. W. Effects of the Niobium(V) Center on the Energetics of Ligand-Centered Proton and Hydrogen Atom Transfer Reactions in Acyl and Alkoxide Complexes. Organometallics 1997, 16, 3249. 11 (a) Zhang, S. Z.; Bordwell, F. G. Effects of π-Coordinated Transition-Metal Groups on Fluorene Acidities and Homolytic Bond Dissociation Enthalpies. Organometallics 1994, 13, 2920–2921. (b) Trujillo, H. A.; Casado, C. M.; Ruiz, J.; Astruc, D. Thermodynamics of C−H Activation in Multiple Oxidation States: Comparison of Benzylic C−H Acidities and C−H Bond Dissociation Energies in the Isostructural 16−20-Electron Complexes [Fex(η5-C5R5)(η6arene)]n, x = 0−IV, R = H or Me, n = −1 to +3. J. Am. Chem. Soc. 1999, 121, 5674–5686. (c) Trujillo, H. A.; Casado, C. M.; Astruc, D. Thermodynamics of Benzylic C-H Activation in 18- and 19-Electron Iron Sandwich Complexes: Determination of pKa values and Bond Dissociation Energies. J. Chem. Soc., Chem. Commun., 1995, 7–8. (d) 12 Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET. ACS Cent. Sci. 2017, 3, 217−223. 13 Semproni, S. P.; Milsmann, C.; Chirik, P. J. “FourCoordinate Cobalt Pincer Complexes: Electronic Structure Studies and Ligand Modification by Homolytic and Heterolytic Pathways.” J. Am. Chem. Soc. 2014, 136, 9211–9224. 14 Fryzuk, M. D.; Johnson, S. A.; Rettig, S. R. Reaction of [P2N2]Ta=CH2(Me) with Ethylene: Synthesis of [P2N2]Ta(C2H4)Et, a Neutral Species with a β-Agostic Ethyl Group in Equilibrium with an α-Agostic Ethyl Group ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh. J. Am. Chem. Soc. 2001, 123, 1602−1612. 15 For examples of related studies invoking intramolecular H-abstraction in transition alkyl/alkylidene/alkylidyne complexes, see: (a) Kulinkovich, O. G.; Sviridov, S. V.; Vasilevski, D. A. Titanium(IV) Isopropoxide-Catalyzed Formation of 1-Substituted Cyclopropanols in the Reaction of Ethylmagnesium Bromide with Methyl Alkanecarboxylates. Synthesis 1991, 234. (b) Fellmann, J. D.; Schrock, R. R.; Traficante, D. D. 𝛼-Hydride vs. 𝛽-hydride elimination. An example of an equilibrium between two tautomers. Organometallics 1982, 1, 481–484. (c) Cavaliere, V. N.; Crestani, M. G.; Pinter, B.; Pink, M.; Chen, C.-H.; Baik, M.-H.; Mindiola, D. J. Room Temperature Dehydrogenation of Ethane to Ethylene. J. Am. Chem. Soc. 2011, 133, 10700−10703. 16 Bezdek, M. J.; Guo, S.; Chirik, P. J. Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex. Science 2016, 354, 730–733. 17 Bezdek, M. J.; Chirik, P. J. Interconversion of Molybdenum Imido and Amido Complexes by Proton–Coupled Electron Transfer. Angew. Chem. Int. Ed. 2018, 57, 2224–2228.

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18 Bezdek, M. J.; Guo, S.; Chirik, P. J. Terpyridine Molybdenum Dinitrogen Chemistry: Synthesis of Dinitrogen Complexes That Vary by Five Oxidation States. Inorg. Chem. 2016, 55, 3117– 3127. 19 Rossman, G. R.; Tsay, F. D.; Gray, H. B. Spectroscopic and magnetic properties of heptacyanomolybdate(III). Evidence for pentagonal-bipyramidal and monocapped trigonal-prismatic structures. Inorg. Chem. 1973, 12, 824–829. 20 (a) Dewar, M. J. S. A review of π Complex Theory. Bull. Soc. Chim. Fr. 1951, 18, C71 –C79. (b) Chatt, J.; Duncanson, L. A. Olefin co-ordination compounds. Part III. Infra-red spectra and structure: attempted preparation of acetylene complexes. J. Chem. Soc. 1953, 2939–2947. 21 Mader, E. A.; Manner, V. W.; Markle, T. F.; Wu, A.; Franz, J. A.; Mayer, J. M. Trends in Ground-State Entropies for Transition Metal Based Hydrogen Atom Transfer Reactions. J. Am. Chem. Soc. 2009, 131, 4335–4345. 22 The lower bound (46 kcal mol–1) is defined by the N–H BDFE in [1-NH3]+, while the upper bound (77 kcal mol–1) is defined by the O–H BDFE in tBu3ArOH. 23 A CCDC online database search (July 2018) for structures containing the “Mo-CH2CH3” substructure yielded the following relevant results: (a) Byrnes, M. J.; Dai, X.; Schrock, R. R.; Hock, A. S.; Müller, P. Some Organometallic Chemistry of Molybdenum Complexes that Contain the [HIPTN3N]3- Triamidoamine Ligand, {[3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2]3N}3-. Organometallics 2005, 24, 4437−4450. (b) Prout, K.; Cameron, T. S.; Forder, R. A.; Critchley, S. R.; Denton, B.; V. Rees, G. The crystal and molecular structures of bent bis-π-cyclopentadienyl–metal complexes: (a) bisπ-cyclopentadienyldibromorhenium(V) tetrafluoroborate, (b) bisπ-cyclopentadienyldichloromolybdenum(IV), (c) bis-πcyclopentadienylhydroxomethylaminomolybdenum(IV) hexafluorophosphate, (d) bis-πcyclopentadienylethylchloromolybdenum(IV), (e) bis-πcyclopentadienyldichloroniobium(IV), (f) bis-πcyclopentadienyldichloromolybdenum(V) tetrafluoroborate, (g) μ-oxo-bis[bis-π-cyclopentadienylchloroniobium(IV)] tetrafluoroborate, (h) bis-π-cyclopentadienyldichlorozirconium. Acta Cryst. 1974, B30, 2290–2304. (c) Chernega, A.; Cook, J.; Green, M. L. H.; Labella, L.; Simpson, S. J.; Souter, J.; Stephens, A. H. H. New ansa-2,2-bis(-cyclopentadienyl)propane molybdenum and tungsten compounds and intramolecular hydrogen–deuterium exchange in methyl-hydride and ethyl-hydride derivatives. J. Chem. Soc., Dalton Trans. 1997, 3225–3243. (d) Saito, T.; Nishida, M.; Yamagata, T.; Yamagata, Y.; Yamaguchi, Y. Synthesis of Hexanuclear Molybdenum Cluster Alkyl Complexes Coordinated with Trialkylphosphines: Crystal Structures of trans-[(Mo6Cl8)Cl4{P(n-C4H9)3}2] and alltrans-[(Mo6Cl8)Cl2(C2H5)2{P(n-C4H9)3}2]•2C6H5CH3. Inorg. Chem. 1986, 25, 1111−1117. (e) Kuiper, D. S.; Douthwaite, R. E.; Mayol, A.-R.; Wolczanski, P. T.; Lobkovsky, E. B.; Cundari, T. R.; Lam, O. P.; Meyer, K. Molybdenum and Tungsten Structural Differences are Dependent on ndz2/(n+1)s Mixing: Comparisons of (silox)3MX/R (M = Mo, W; silox = tBu3SiO). Inorg. Chem. 2008, 47, 7139−7153. (f) Chisholm, M. H.; Haitko, D. A.; Folting, K.; Huffman, J. C. Preparation and Characterization of 1,2-Dialkyl Compounds of Dimolybdenum and Ditungsten of Formula M2R2(NMe2)4 (M≡M) J. Am. Chem. Soc. 1981, 103, 4046-4053. (g) Khalimon, A. Y.; Simionescu, R.; Kuzmina, L. G.; Howard, J. A. K.;


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Nikonov, G. I. Agostic NSi–H···Mo Complexes: From Curiosity to Catalysis. Angew. Chem., Int. Ed. 2008, 47, 7701−7704. (h) Legzdins, P.; Wassink, B.; Einstein, F. W. B.; Jones, R. H. Cyclopentadienylethyldinitrosylmolybdate Anion, a Novel, Prototypal 19-Electron Nitrosyl Complex. Organometallics 1988, 7, 477–481. (i) Schrauzer, G. N.; Schlemper, E. O.; Liu, N. H.; Wang, Q.; Rubin, K.; Zhang, X.; Long, X.; Chin, C. S. Studies of Molybdenum Compounds. 5. Diethyl(2,2'-bipyridyl)dioxomolybdenum(VI) and Other Higher Dialkyl Derivatives of Dioxomolybdenum(VI). Organometallics 1986, 5, 2452–2456. (j) Bennett, M. J.; Mason, R. Structure and Reactivity of Tricarbonyl-π-cyclopentadienylethylmolybdenum. Proc. Chem. Soc. 1963, 273–274. 24 The 13C-labeled organozinc reagent was prepared by treatment of 1-13C-EtLi with ZnCl2 in analogy to the reported preparation of Et2Zn-d10: Diccianni, J. B.; Heitmann, T.; Diao, T. NickelCatalyzed Reductive Cycloisomerization of Enynes with CO2. J. Org. Chem. 2017, 82, 6895−6903. We have found that during the transmetalation from Li to Zn, the 13C-label scrambles between 1and 2- positions, such that the zinc reagent is likely better described as an isotopomeric mixture containing (13CH3CH2)2Zn, (13CH3CH2)(CH313CH2)Zn and (CH313CH2)2Zn. We note that the exact ratio of this isotopomeric mixture is without consequence for our purposes, as rapid intramolecular isomerization pathways in the molybdenum product equilibrate the 13C-label between 𝛼- and 𝛽ethyl positions to a 1:1 mixture (described in later sections). 25 Brookhart, M.; Green, M. L.; Parkin, G. Agostic interactions in transition metal compounds. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6908−6914. 26 Anet, F. A. L.; Bourn, A. J. R. Nuclear Magnetic Resonance Line-Shape and Double-Resonance Studies of Ring Inversion in Cyclohexane-d11. J. Am. Chem. Soc. 1967, 89, 760–768.  27 (a) Leatherman, M. D.; Svejda, S. A.; Johnson, L. K.; Brookhart, M. Mechanistic Studies of Nickel(II) Alkyl Agostic Cations and Alkyl Ethylene Complexes: Investigations of Chain Propagation and Isomerization in (α-diimine)Ni(II)-Catalyzed Ethylene Polymerization. J. Am. Chem. Soc. 2003, 125, 3068−3081. (b) Shultz, L. H.; Brookhart, M. Measurement of the Barrier to β- Hydride Elimination in a β-Agostic Palladium−Ethyl Complex: A Model for the Energetics of Chain-Walking in (α-Diimine)PdR+ Olefin Polymerization Catalysts. Organometallics 2001, 20, 3975−3982. (c) Xu, H.; White, P. B.; Hu, C.; Diao, T. Structure and Isotope Effects of the bH Agostic (a-Diimine)Nickel Cation as a Polymerization Intermediate. Angew. Chem., Int. Ed. 2017, 56, 1535−1538. (d) Xu, H.; Hu, C.; Wang, T.; Diao, T. Structural Characterization of β-Agostic Bonds in Pd-Catalyzed Polymerization. Organometallics 2017, 36, 4099–4102. 28 Line broadening technique based on: Brookhart, M.; Hauptman, E.; Lincoln, D. 𝛽-Migratory insertion reactions of (𝜂5C5R'5)Rh(L)(C2H4)R+BF4- (R' = hydrogen, methyl; R = hydrogen, ethyl; L = P(OMe)3, PMe3). Comparison of the energetics of hydride versus alkyl migration. J. Am. Chem. Soc. 1992, 114, 10394−10401. 29 (a) Calvert, R. B.; Shapley, J. R. HOs3(CO)10CH3: NMR evidence for a C··H··Os Interaction. J. Am. Chem. Soc. 1978, 100, 7726–7727. (b) Green, M. L. H.; Hughes, A. K.; Popham, N. A.; Stephens, A. H. H.; Wong, L.-L. Nuclear magnetic resonance studies on partially deuteriated transition metal–methyl derivatives. J. Chem. Soc., Dalton Trans. 1992, 3077–3082.

30 (a) Bordwell, F. G.; Bausch, M. J. Acidity-OxidationPotential (AOP) Values as Estimates of Relative Bond Dissociation Energies and Radical Stabilities in Dimethyl Sulfoxide Solution. J. Am. Chem. Soc. 1986, 108, 1979−1985. 31 Vigalok, A.; Uzan, O.; Shimon, L. J. W.; Ben-David, Y.; Martin, J. L.; Milstein, D. Formation of η2 C−H Agostic Rhodium Arene Complexes and Their Relevance to Electrophilic Bond Activation. J. Am. Chem. Soc. 1998, 120, 12539−12544. 32 Himmelbauer, D.; Stöger, B.; Veiros, L. F.; Kirchner, K. Reversible Ligand Protonation of a Mn(I) PCP Pincer Complex To Afford a Complex with an η2-Caryl–H Agostic Bond. Organometallics 2018, DOI: 10.1021/acs.organomet.8b00193. 33 Himmelbauer, D.; Mastalir, M.; Stöger, B.; Veiros, L. F.; Pignitter, M.; Somoza, V.; Kirchner, K. Iron PCP Pincer Complexes in Three Oxidation States: Reversible Ligand Protonation To Afford an Fe(0) Complex with an Agostic C–H Arene Bond. Inorg. Chem. 2018, 57, 7925–7931. 34 Jongbloed, L. S.; Garcia-Lopez, D.; van Heck, R.; Siegler, M. A.; Carbo, J. J.; van der Vlugt, J. I. Arene C(sp2)−H Metalation at NiII Modeled with a Reactive PONCPh Ligand. Inorg. Chem. 2016, 55, 8041−8047. 35 Murugesan, S.; Stöger, B.; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Kirchner, K. A Cobalt(I) Pincer Complex with an η(2) C(aryl)-H Agostic Bond: Facile C-H Bond Cleavage through Deprotonation, Radical Abstraction, and Oxidative Addition. Angew. Chem., Int. Ed. 2016, 55, 3045−3048. 36 Christman, W. E.; Morrow, T. J.; Arulsamy, N.; Hulley, E. B. Absolute Estimates of PdII(η2-Arene) C–H Acidity. Organometallics 2018, 37, 2706–2715. 37 (a) Morris, R. H. Brønsted−Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes. Chem. Rev. 2016, 116, 8588−8654. (b) Kubas, G. J. Activation of Dihydrogen and Coordination of Molecular H2 on Transition Metals. J. Organomet. Chem. 2014, 751, 33−49. 38 Assuming CG = 66 kcal mol–1 in THF. Concerning this constant, see: Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R. H.; Schweitzer, C. T. Effect of the Ligand and Metal on the pKa Values of the Dihydrogen Ligand in the Series of Complexes [M(H2)H(L)2]+, M = Fe, Ru, Os, Containing Isosteric Ditertiaryphosphine Ligands, L. J. Am. Chem. Soc. 1994, 116, 3375–3388. 39 We thank a reviewer for highlighting this point and encouraging us to address it.

TOC: PPh2Me

Ph N N

N Mo

+

PPh2Me

Ph -

H+,

e–

PCET

N N

Structure

Mo

Ph2MeP


Dynamics

PPh2Me

Ph N

N

+ H+, e– Ph2MeP

+

N H

Mo H

CH2

Thermochemistry

+

N

Ph2MeP

Mechanism

11



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Agostic Ethyl - ACS Publications - American Chemical Society

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