Reversible Heterolytic Cleavage of the HâH Bond by Molybdenum

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Reversible Heterolytic Cleavage of the H−H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride Shaoguang Zhang, Aaron M. Appel, and R. Morris Bullock* Pacific Northwest National Laboratory, P.O. Box 999, K2-12, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Controlling the heterolytic cleavage of the H−H bond of dihydrogen is critically important in catalytic hydrogenations and in the catalytic oxidation of H2. We show how the rate of reversible heterolytic cleavage of H2 can be controlled, spanning 4 orders of magnitude at 25 °C, from 2.1 × 103 s−1 to ≥107 s−1. Bifunctional Mo complexes, [CpMo(CO)(κ3-P2N2)]+ (P2N2 = 1,5-diaza-3,7-diphosphacyclooctane diphosphine ligand with alkyl/aryl groups on N and P), have been developed for heterolytic cleavage of H2 into a proton and a hydride, akin to frustrated Lewis pairs. The H−H bond cleavage is enabled by the basic amine in the second coordination sphere. The products of heterolytic cleavage of H2, Mo hydride complexes bearing protonated amines, [CpMo(H)(CO)(P2N2H)]+, were characterized by spectroscopic studies and by X-ray crystallography. Variable-temperature 1H, 15N, and 2-D 1H−1H ROESY NMR spectra indicated rapid exchange of the proton and hydride. The t exchange rates are in the order [CpMo(H)(CO)(PPh2NPh2H)]+ > [CpMo(H)(CO)(P Bu2NPh2H)]+ > [CpMo(H)(CO)(PPh2NBn2H)]+ t

t

t

> [CpMo(H)(CO)(P Bu2NBn2H)]+ > [CpMo(H)(CO)(P Bu2N Bu2H)]+. The pKa values determined in acetonitrile range from 9.3 to 17.7 and show a linear correlation with the logarithm of the exchange rates. This correlation likely results from the exchange process involving key intermediates that differ by an intramolecular proton transfer. Specifically, the proton-hydride exchange appears to occur by formation of a molybdenum dihydride or dihydrogen complex, resulting from proton transfer from the pendant amine to the metal hydride. The exchange dynamics are controlled by the relative acidity of the [CpMo(H)(CO)(P2N2H)]+ and [CpMo(H2)(CO)(P2N2)]+ isomers, providing a design principle for controlling heterolytic cleavage of H2.

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INTRODUCTION

prevent the Lewis acid and base centers from forming an adduct. The “unquenched” reactivity of FLPs creates a polarized environment that facilitates heterolytic cleavage of the H−H bond. Some “not-so-frustrated” Lewis acid/base adducts reversibly expose the Lewis acidic and basic centers in an equilibrium, leading to reactivity similar to that of traditional FLPs.21−26 Wass and co-workers called attention27,28 to the relationship between main group FLPs and reactions in which a transition metal functions as a Lewis acid or base.27−36 Transition-metal containing FLPs can be considered in the broader context of metal−ligand bifunctional catalysis37 and metal−ligand cooperation38−41 in catalysis. A recent perspective article reviews the heterolytic cleavage of H2 in the framework of metal-containing FLPs.42 Heterolytic cleavage of the H−H bond in many bifunctional complexes is enabled by an amine as a proton acceptor in the second coordination sphere, and the metal as the hydride acceptor, mimicking the function of hydrogenase.8,9 An important application of the heterolytic cleavage of H2 is ionic hydrogenations,43 a powerful class of reactions that requires generation of a proton and hydride from the H−H bond.7,44−47

The H−H bond is the simplest chemical bond, yet it offers diverse reactivity. 1−4 Cleavage of the H−H bond is a fundamentally important reaction that can occur by homolytic or heterolytic pathways. Heterolytic cleavage of the H−H bond into a proton and hydride (Scheme 1) is a critical process in the Scheme 1. Heterolytic H−H Bond Cleavage

catalytic hydrogenation of ketones,5−7 the oxidation of hydrogen by hydrogenases in nature8,9 and by synthetic molecular catalysts.10−14 Interest in using H2 as a clean fuel produced through sustainable solar and wind energy has propelled research on finding facile ways to cleave the H−H bond. Understanding the fundamental thermodynamics and kinetics of heterolytic H−H bond cleavage and controlling the transfer of the proton and hydride are critically important for the design of new catalysts. Frustrated Lewis pairs (FLPs) based on main group elements have shown remarkable reactivity in heterolytic cleavage of the H−H bond, leading to the development of new classes of hydrogenation catalysts.15−20 Steric hindrance or ring strain © 2017 American Chemical Society

Received: March 27, 2017 Published: May 3, 2017 7376

DOI: 10.1021/jacs.7b03053 J. Am. Chem. Soc. 2017, 139, 7376−7387

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Journal of the American Chemical Society

The amine-bound proton and the metal hydride undergo rapid exchange,63,64,66,69,70 demonstrating reversible cleavage and heterocoupling of the H−H bond. The pKa values quantitatively reveal that tuning the acidity of the complexes controls the rates of reversible heterolytic cleavage by about 4 orders of magnitude. The more acidic [CpMo(H)(CO)(PR2NR′2H)]+ complexes show faster exchange rates of the proton and hydride, demonstrating the ability to control the reaction by changing the substituents on the phosphine and amine.

The proton and hydride are subsequently delivered to the unsaturated substrate, without requiring coordination of the substrate to the metal.37 Tuning the ability of the proton3 and hydride4 to be transferred to a substrate has a strong impact on the kinetics and thermodynamics and consequently on the catalytic activity for ionic hydrogenations. Thus, the ability to understand and control the factors influencing rates of heterolytic cleavage of H2 is important. Heterolytic activation of the H−H bond by Mo complexes is rare;48,49 Berke and co-workers reported heterolytic cleavage of H2 across a MoN bond in [(iPr2PCH2CH2)2N)]Mo(NO)(CO).48,49 The product, [(iPr2PCH2CH2)2NH)]Mo(H)(NO)(CO), was identified by NMR spectroscopy, but it is unstable in the absence of H2. Molybdenum complexes reported by Long, Chang, and co-workers are electrocatalysts for the evolution of H2 in aqueous solution.50−52 Molybdenum or tungsten cationic complexes bearing phosphine or N-heterocyclic carbene ligands add H2 to generate dihydride complexes.53−56 In ionic hydrogenations with these complexes, a Mo−H bond is the proton donor, then the second Mo−H bond functions as a hydride donor.57 To design Mo bifunctional complexes for heterolytic cleavage of H2 with separately tunable proton and hydride donors, we envisioned that incorporating an amine group in the second coordination sphere of the diphosphine ligand could give heterolytic cleavage of the H−H bond.58−67 However, cationic Mo diphosphine complexes bearing an amine group, [CpMo(CO)(κ3-PNP)]+ (PNP = (R2PCH2)2NMe), are unreactive toward H2 addition (Scheme 2).68 These results suggest that the amine is strongly bound to the electrophilic Mo center because of the wide P−Mo−P angle of the flexible PNP ligand.

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RESULTS Synthesis and Characterization of CpMo(H)(CO)(P2N2) Complexes. CpMo(H)(CO)(PPh2NR′2) (PhBnMoH, R′ = Bn; PhPh MoH, R′ = Ph) with Ph groups on the phosphines were synthesized from CpMo(H)(CO)3 and the diphosphine ligand PPh2NR′2 in toluene at 80 °C (Scheme 3), similar to the route Scheme 3. Synthesis of CpMo(H)(CO)(P2N2)

Scheme 2. Heterolytic Cleavage of H2 by a FLP and Mo Complexes

used in the synthesis of CpMo(H)(CO)(PRNMePR) complexes (R = Et, Ph).68 The Mo hydride complex PhBnMoH was isolated in 73% yield, and its structure was confirmed by single-crystal X-ray diffraction (Figure 1, Table S14). A doublet of doublets at −6.83 ppm (2JHP = 57 Hz for cis; 2JHP = 18 Hz for trans) was observed in the 1H NMR spectrum as the hydride resonance of PhBn MoH in CD2Cl2, as expected for hydride ligands coupled to both cis and trans phosphines.71 t t For the analogous CpMo(H)(CO)(P Bu2NR′2) ( BuBnMoH, R′ t

t

t

= Bn; BuPhMoH, R′ = Ph; Bu BuMoH, R′ = tBu) complexes bearing tBu groups on the phosphines, attempted synthesis by t the reaction of CpMo(H)(CO)3 and P Bu2NR′2 in either hexane t

or benzene at 80 °C resulted in a mixture of BuR′MoH and other products that have not been identified. An alternative synthetic t route to BuR′MoH was devised. The Mo chloride complexes,

Inspired by the structure−function relationships established in main group and transition-metal-based FLPs, we anticipated that structural modification of the ligand by introducing ring strain to destabilize the Mo−N coordination might reversibly expose a vacant coordination site for reaction with H2, allowing the Lewis acidic Mo center and the basic amine to provide cooperative heterolytic H−H bond cleavage. In this paper, we report the facile heterolytic cleavage of H2 by cationic Mo complexes, affording Mo hydride complexes bearing a protonated amine, [CpMo(H)(CO)(PR2NR′2H)]+ (Scheme 2).

t

t

t

CpMo(Cl)(CO)(P Bu2NR′2) ( BuBnMoCl, R′= Bn; BuPhMoCl, R′ = Ph;

t

ButBu

MoCl, R′ = tBu), were readily prepared by the reaction t

of CpMo(CO)3Cl and P Bu2NR′2 at 110 °C in toluene, followed by treatment with Me3NO in CH2Cl2, leading to oxidative elimination of one CO ligand. The corresponding Mo hydrides t t BuR′ MoH were prepared from the reaction of BuR′MoCl with 7377


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Journal of the American Chemical Society t

Scheme 4. Formation of [ BuR′Mo]+ and Heterolytic H2 t Cleavage Products [ BuR′Mo H(NH)]+

Figure 1. ORTEP drawing of CpMo(H)(CO)(PPh2NBn2) (PhBnMoH, t

t

upper left), CpMo(Cl)(CO)(P Bu2NPh2) ( BuPhMoCl, upper right), t

t

CpMo(H)(CO)(P Bu2NBn2) ( BuBnMoH, lower left), and CpMo(H)t

t

(CO)(P Bu2NPh2) ( BuPhMoH, lower right) with 30% thermal ellipsoids. Phenyl groups on phosphines, methyls of tert-butyl groups, and hydrogen atoms are omitted for clarity, except for the ipso-C of the phenyl group and Mo−H.

LiBHEt3 in THF (Scheme 3). The X-ray crystallographic t t t structures of PhBnMoH, BuBnMoCl, BuPhMoCl, BuBnMoH, and t BuPh MoH all show four-legged piano-stool geometries and cis geometries of the two phosphines (Figure 1; see SI for the t structure of BuPhMoCl). The P−Mo−P bond angles of these Mo hydrides vary over the narrow range from 74.103(15)° to 75.219(17)°. Synthesis and Characterization of [CpMo(CO)(κ3P2N2)]+, Cationic Mo Diphosphine Complexes Bearing a Bound Amine. The cationic “tuck-in” Mo complexes [CpMot t t (CO)(κ3-P Bu2NR′2)]+[BArF4]− ([ BuBnMo]+, R′ = Bn; [ BuPhMo]+, t

t

Figure 2. ORTEP drawing of [CpMo(CO)(κ3-P Bu2NBn2)]+[BArF4]− t

t

([ BuBnMo]+[BArF4]−, left) and [CpMo(CO)(κ3-P Bu2NPh2)]+[BArF4]− t

([ BuPhMo]+[BArF4]−, right) with 30% thermal ellipsoids. BArF4 anions, methyls of tert-butyl groups, and hydrogen atoms are omitted for clarity.

Scheme 5. Formation of Heterolytic H2 Cleavage Products [PhR′MoH(NH)]+

t

R′ = Ph; [ Bu BuMo]+, R′ = tBu) were prepared by chloride t

abstraction from BuR′MoCl using NaBArF4 (ArF = 3,5-bis(trifluoromethyl)phenyl) in fluorobenzene and were fully characterized (Scheme 4). In the 31P{1H} NMR spectra, the chemical shifts of the 31P NMR resonances range from −2.0 to 6.9 ppm, comparable to other κ3-PNP or κ3-P2N2 metal complexes featuring four-membered phosphacycles.68,70,72 Even though the struct tures of [ BuR′Mo]+ and [CpMo(CO)(κ3-PRNMePR)]+ (Figure 2, Table S15) all show the κ3 coordination mode, with the amine t bound, the P−Mo−P bond angles of [ BuR′Mo]+ (85.636(17)o to 87.75(2)o) are much smaller than the wide P−Mo−P angles found in [CpMo(CO)(κ3-PEtNMePEt)]+ (123.220(16)o) and [CpMo(CO)(κ3-PPhNMePPh)]+ (122.20(2)o).68 The Mo−N bond lengths t t in [ BuBnMo]+ (2.370(3) Å), [ BuPhMo]+ (2.405(2) Å), and t

abstraction from PhBnMoH or PhPhMoH with [Ph3C]+[BArF4]− in fluorobenzene (Scheme 5). The X-ray crystallographic structure of [PhBnMo]+[BArF4]− unambiguously confirmed the connectivity and the κ3-coordination mode, despite some disorder of heavy atoms (see Supporting Information for details). We reported the slow reaction (3 h) of [CpMo(CO)(κ3Et Me Et + P N P )] in neat CD3CN and determined the pseudo-first

t

[ Bu BuMo]+ (2.4583(16) Å) are significantly longer than the values of 2.3083(13) Å in the [CpMo(CO)(κ3-PEtNMePEt)]+, t suggesting the Mo−N bond strength of [ BuR′Mo]+ is weaker. The complex [CpMo(CO)(κ3-PPh2N R′2)]+ ([PhBnMo]+, R′ = Bn; [PhPhMo]+, R′ = Ph) was generated in situ by hydride 7378


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Journal of the American Chemical Society order kinetics of its solvolysis in acetonitrile (ΔH⧧ = 21.6 ± 2.8 kcal/mol, ΔS⧧ = −0.3 ± 9.8 cal/(mol·K)).68 In contrast, t dissociation of the amine in [ BuBnMo]+ and addition of an acetonitrile ligand occurred rapidly at 0 °C, as indicated by a color change from violet to yellow, and was complete within seconds. The high reactivity of electrophilic Mo complexes toward acetonitrile, as found for [CpMo(CO)2(PiPr3)]+[B(C6F5)4]−,73 suggested the dissociation energy of the Mo−N bond in the κ3-Mo-P2N2 complexes [RR′Mo]+ is much lower than that in [CpMo(CO)(κ3-PRNMePR)]+.73,68 Heterolytic Cleavage of H2 to Give Mo Hydride Complexes Bearing a Protonated Amine. Bubbling of H2 (1 atm) into a violet solution of [PhBnMo]+ in fluorobenzene immediately afforded a light orange solution of [PhBnMoH(NH)]+, a Mo hydride complex bearing a protonated amine (Scheme 5). Analogously, reactions of H2 with [PhPhMo]+ generated in situ cleanly formed [PhPhMoH(NH)]+ within seconds. [PhR′MoH(NH)]+ was also prepared by protonation of PhR′MoH with [H(OEt2)2]+[B(C6F5)4]−. [PhPhMoH(NH)]+ slowly releases H2 in THF-d8 or CD3CN solution in hours, presumably through a Mo dihydride or dihydrogen intermediate (Scheme 6). Scheme 6. Proposed Mechanism of Proton-Hydride Exchange through a Mo Dihydrogen or Dihydride Intermediate

Figure 3. ORTEP drawing of [CpMo(H)(CO)(PPh2NBn2H)]+[BArF4]− ([PhBnMoH(NH)]+, top), [CpMo(CO)(H)(PPh2NPh2H)]+[BArF4]− ([PhPhMoH(NH)]+, middle), and [CpMo(H)(CO)-

In CD3CN, [PhR′Mo]+ complexes rapidly bind CD3CN and form [CpMo(CO)(PPh2NR2)(CD3CN)]+, [PhR′Mo(CD3CN)]+, as observed by 31P NMR spectra. Addition of H2 to [PhR′Mo(CD3CN)]+ was not observed in CD3CN solution, indicating the displacement of the acetonitrile ligand by H2 is unfavorable thermodynamically and/or kinetically. Thus, we were not able to measure the hydricity of PhR′MoH or the kinetic barrier for H2 addition to [PhR′Mo(CD3CN)]+ in CD3CN. In contrast, [PhBnMoH(NH)]+ is stable in THF without elimination of H2. In MeCN, it loses H2 slowly, reaching completion after 24 h at room temperature, [PhR′MoH(NH)]+ in MeCN. Addition of H2 to [PhR′Mo(CD3CN)]+ was not observed in CD3CN solution, indicating the displacement of the acetonitrile ligand by H2 is unfavorable thermodynamically, slow kinetically, or both. In contrast, [PhBnMoH(NH)]+ is stable in THF or MeCN solvent without elimination of H2. In MeCN, it loses H2 very slowly, reaching completion after 24 h at room temperature. Deprotonation of [PhR′MoH(NH)]+ by Et3N (pKa = 18.874 for H-NEt3+ in MeCN) generates the corresponding neutral Mo hydride complexes PhR′MoH. Orange crystals of both [PhBnMoH(NH)]+[BArF4]− and PhPh [ MoH(NH)]+[BArF4]− were grown by slow diffusion of pentane into a fluorobenzene solution under H2 (1 atm). The X-ray crystallographic analysis reveals with structures containing both Mo−H and N−H bonds, resulting from heterolytic cleavage of H2. Surprisingly, [PhBnMoH(NH)]+ and [PhPhMoH(NH)]+ adopt different conformational geometries (Figure 3). [PhBnMoH(NH)]+ crystallized as the anti, endo-isomer. The proton and hydride are arranged on different sides of the P−Mo−P plane (H···H distance ∼4.15 Å) instead of being

t

t

(P Bu2NBn2H)]+[BArF4]− ([ BuBnMoH(NH)]+, bottom) with 30% thermal ellipsoids. BArF4 anions, phenyl groups on phosphines, methyls of tert-butyl groups, hydrogen atoms, and cocrystallized fluorobenzene molecules are omitted, except the ipso-C of the phenyl group, Mo−H and N−H.

positioned close to each other on the same side. In contrast, the crystal structure of [PhPhMoH(NH)]+ indicated the syn, endoisomer, with the proton and hydride positioned toward each other (Figure 3, middle). The Mo−H bond length determined by X-ray diffraction is 1.628(9) Å, though the precise location of hydride is subject to the uncertainties associated with X-ray crystallography.75 Setting the N−H distance to 1.000 Å gives an estimated H···H distance of 2.456 Å. This distance is much longer than the short H···H distance of 1.489(10) Å found for dihydrogen bonding in the single crystal neutron diffraction structure of t t [CpC5F4NFe(H)(P Bu2N Bu2H)]+[BArF4]− ([1]+[BArF4]−), in which the N−H bond is positioned in close proximity to the iron hydride in the three-legged piano stool coordination geometry.66 [PhR′MoH(NH)]+ has one additional CO ligand, adopting a four-legged piano stool coordination geometry. The rigid P2N2 ligand could only form the cis configuration instead of trans configuration. As a result, the hydride from heterolytic cleavage of the H−H bond is located trans to one phosphine, instead of cis to two phosphines in [1]+ and [(PPh2NBn2H)Mn(H)(CO) (bppm)]+ ([2]+).69 This coordination geometry in [PhR′MoH(NH)]+ increased the distance between the proton and hydride. Heterolytic cleavage of the H−H bond was also achieved by t addition of H2 to [ BuR′Mo]+, bearing bulky tBu groups on the 7379


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phosphine, affording [ BuR′MoH(NH)]+ (Scheme 5). All three t complexes [ BuR′MoH(NH)]+ are stable in MeCN solution, in the solid state under an N2 atmosphere or under vacuum. t t Crystals of [ BuBnMoH(NH)]+ and [ BuPhMoH(NH)]+ were grown by slow diffusion of pentane into a fluorobenzene solution, and their structures were characterized by X-ray crystallography as syn, endo-isomers (Figures 3 and S3). We propose that [RR′MoH(NH)]+ is formed through intramolecular deprotonation of an unobserved Mo dihydride or dihydrogen intermediate (Scheme 6).76 Attempts to observe [RR′Mo(H)2]+ at −80 °C were unsuccessful, as rapid heterolytic cleavage of H2 generates [RR′MoH(NH)]+. Alternatively, direct H−H bond cleavage by the Lewis acidic Mo and the basic amine, mimicking FLP reactivity, cannot be ruled out, but is considered less likely. We propose that the bound amine in [RR′Mo]+ functions as a hemilabile ligand that stabilizes the electrophilic Mo center and promotes the intramolecular deprotonation, as reported in related iron and manganese complexes.66,69,70 Since H−H bond cleavage was achieved in κ3-Mo-P2N2 complexes [RR′Mo]+, but not in the κ3-Mo-PNP complexes [CpMo(CO)(κ3-PRNMePR)]+ bearing a similar second coordination sphere and Lewis acid−base bifunctionality, we examined the crystal structures to understand the structure−reactivity relationships. To create a vacant coordination site for H2 binding, the amine coordinated to the Mo center must dissociate. In addition, the pendant amine must be correctly positioned and sufficiently basic to deprotonate the intermediate [RR′Mo(H)2]+. Thus, the H2 addition reactivity is strongly affected by the Mo−N binding strength, which is tunable by modulating the ring strain and rigidity of the ligand skeleton and the electronic effects of the ancillary ligand environment around both Mo and the amine. The lack of reactivity of [CpMo(CO)(κ3-PRNMePR)]+ with H2 is attributed to the Mo−N coordination being too strong to dissociate and generate a vacant site for H2 coordination. In the same manner, some Lewis pairs are unable to activate H2 because the acid−base coordination quenches the acidity and basicity, making them inadequately “frustrated”.29,30,77 Consistent with this interpretation, the Mo−N bond length in [CpMo(CO)(κ3PEtNMePEt)]+ is shorter than those in any of the κ3-Mo-P2N2 t complexes [ BuR′Mo]+. The flexible PNP ligand accommodates the strong Mo−N coordination by forming a large P−Mo−P bond angle (123.220(16)° in [CpMo(CO)(κ3-PEtNMePEt)]+).68 Further evidence for the greater structural flexibility of the PEtNMePEt ligands compared to cyclic P2N2 ligand comes from the crystallographic characterization of both cis and trans isomers of CpMo(H)(CO)(κ3-PPhNMePPh).68 In contrast, the t t κ3-Mo-P2N2 complexes, [ BuBnMo]+ and [ BuPhMo]+, have longer Mo−N bond lengths (2.370(3)−2.405(2) Å) and much smaller P−Mo−P bond angles (85.636(17)−87.75(2)o) because of the ring strain in the P2N2 ligand, which destabilizes the metalamine binding and promotes amine dissociation. In addition, steric effects caused by wider P−Mo−P bond angle and kinetic protection by the substituents in the κ3-Mo-PNP complexes might also contribute to the lack of reactivity with H2. The complexes with the P2N2 ligands cannot adopt the preferred geometry that is attained in the κ3-Mo-PNP complexes. Similarly, ring strain of the four-membered ring in some FLPs with a twoatom linker between the Lewis acid and base also promotes the Lewis pair dissociation, leading to cleavage of H2.21,23−26,77 The rates of reaction with H2 depend on the basicity of the t t t amine. While the reaction of H2 with [ BuBnMo]+ or [ Bu BuMo]+

requires about 15 min to reach completion, H2 addition to t

[ BuPhMo]+ is significantly faster, reaching completion in seconds. This difference occurs because the less nucleophilic amine results in a weaker Mo−N coordination. The ability of the pendant amine to switch from the primary coordination sphere to the secondary coordination sphere plays a critical role in heterolytic H−H bond cleavage, which is readily tuned by modification of the electronic characteristics of the substituents on the phosphine and the amine. Determination of the Rates of Reversible Heterolytic Cleavage. Variable-temperature 1H, 31P, 15N, and 2-D NMR spectra of [RR′MoH(NH)]+ reveal rapid exchange of the proton and hydride at 20 °C, indicating that the H−H bond is rapidly, reversibly formed and heterolytically cleaved. The 1H NMR spectrum of [PhBnMoH(NH)]+ in THF-d8 at 20 °C shows a broad singlet integrating to two protons at −0.15 ppm (Figure 4). No separate proton or hydride resonance

Figure 4. Variable-temperature 1H NMR spectra of [PhBnMoH(NH)]+ in THF-d8.

was found in the typical chemical shift regions of Mo(η2-H2)+ (−2 to −5 ppm), Mo(H)2+ (−3 to −6 ppm), a structure containing a Mo−H bond (−5 to −9 ppm) or a N−H bond of the protonated P2N2 ligand (14 to 7 ppm). We assign the resonance at −0.15 ppm in 1H NMR spectrum to the rapidly exchanging N−H and Mo−H resonances. Since only one of the two amines is protonated, the assignment of the structure with dynamic proton-hydride exchanging character was further confirmed by 15N NMR and 1H−15N HSQC spectra. In the 15N NMR spectrum of 15N-labeled [PhBnMoH(NH)]+, the resonance of the protonated amine exhibits a triplet of triplets at −305 ppm (1JHN = 35 Hz, 2JNP = 5.7 Hz), indicating coupling between this 15N nucleus and two equivalent protons. The magnitude of 1JHN is close to the average value of a nonexchanging N−H bond in a protonated amine (∼70 Hz) and negligible 15N coupling (∼0 Hz) to the hydride, showing the rapid proton−hydride exchanging dynamics. The nitrogen of the other pendant amine showed a triplet at −340 ppm with 2JNP = 11.5 Hz and no direct N−H coupling. No “pinched” exo-isomer featuring a bridging proton between two nitrogens of the pendant amine (N−H···N) was observed.78,79 The solution structure of [PhBnMoH(NH)]+ at 20 °C shows dynamic intramolecular proton-hydride exchange of the Mo−H and N−H resonances, in contrast with its anti, endo structure in 7380


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Journal of the American Chemical Society solid state that shows no interaction of the amine-bound proton and Mo hydride. When a THF-d8 solution of [PhBnMoH(NH)]+ was cooled to −20 °C, the broad singlet of the exchanging proton and hydride coalesced into the baseline of the 1H NMR spectrum, indicating a decrease in the rate of dynamic exchange. Decoalescence was reached at −40 °C, and two new broad singlets were observed, assigned to resonances of an amine-bound proton and a Mo hydride. At −80 °C, these two resonances appear as a broad singlet at 6.54 ppm and a doublet of doublets at −6.70 ppm (2JHP = 54 Hz for cis; 2JHP = 21 Hz for trans). In the 15N NMR spectrum, the protonated nitrogen appeared as a broad doublet at −309 ppm with 1JHN = 71 Hz in 15N-labeled [PhBnMoH(NH)]+. A 1H−15N HSQC spectrum of 15N-labeled [PhBnMoH(NH)]+ in THF-d8 at −60 °C exhibits a cross peak between the nitrogen resonance at −309 ppm and the proton resonance at 6.49 ppm, but not with the hydride resonance (Figure S61). 1H−1H NOESY and ROESY NMR spectra both exhibit cross peaks between proton and hydride resonances at −80 °C, suggesting the amine-bound proton and metal hydride still exchange at −80 °C, though with a relatively slow rate (Figures S57 and S58). The observed T1 relaxation time for the individual Mo−H and N−H resonances of [PhBnMoH(NH)]+ are nearly the same (400 ms for N−H and 360 ms for Mo−H at −30 °C), both decreasing from 1.5 s (at −80 °C) to a minimum of 360 ms at −30 °C in THF-d8 (Figure S55). For the averaging Mo−H···H−N resonance at −0.15 ppm in fluorobenzene, the minimum T1 value is 378 ms at −10 °C, which is much shorter than the typical T1 value of Mo hydride complexes (>1 s). Thus, the observed short T1 relaxation time indicates the presence of a Mo−H···H−N interaction in [PhBnMoH(NH)]+ and reveals that the rate of proton-hydride exchange is a function of temperature. Morris and co-workers also observed the resonance of an interacting protonhydride had a shorter T1 relaxation time than that of the metal hydride because of “T1 averaging”.80 The spectroscopic data suggest rapid exchange of the aminebound proton and the Mo hydride in [PhBnMoH(NH)]+ above 20 °C. The dynamic NMR spectra allowed measurement of the kinetics of exchange in different solvents. The pseudo-firstorder rates of exchange of the proton and hydride at 25 °C were determined by simulation of the variable-temperature NMR spectra as k = 3.9 × 105 s−1 (in THF) and 1.7 × 106 s−1 (in CH2Cl2) (Figure 5). Activation parameters were determined from an Eyring analysis, in all cases over at least a 100° range, and are given in Table 1. The rate of exchanging proton and hydride is five times faster in CH2Cl2 than in THF, which could result from the changes in the relative acidities and/or variations in hydrogen bonding in the different solvents. The complex [PhPhMoH(NH)]+, with phenyl groups on the amines, shows strikingly different 1H NMR spectroscopic characteristics. At 20 °C, a 1:2:1 triplet integrating to two protons was observed at −0.51 ppm in CD2Cl2 (−0.81 ppm in fluorobenzene) with 2JHP = 21 Hz, which is assigned as the rapidly exchanging proton and hydride (Figure 6, left). In the 15 N-labeled complex, this resonance appears as a quartet in the 1H NMR spectrum and as a doublet in the 1H{31P} NMR spectrum. In the 15N NMR spectrum, the 15N nucleus coupled to the Mo−H/N−H resonance exhibits a triplet of triplets at −302 ppm (1JHN = 23 Hz, 2JNP = 7.5 Hz) at 20 °C. We propose that the averaging Mo−H/N−H resonance couples with 15N and that the 1JHN is coincidentally the same as the 2JHP value of 21 Hz. Rapid proton-hydride exchange of the Mo−H and N−H

Figure 5. Eyring plot for rates of exchange of the proton and hydride in t

[PhBnMoH(NH)]+ (blue, −70 to 40 °C), [ BuBnMoH(NH)]+ (red, −60 t

t

to 60 °C), and [ Bu BuMoH(NH)]+ (green, −60 to 100 °C) in THF-d8.

resonances was also identified by the cross peak between the nitrogen resonance and the hydride resonance at −0.81 ppm in the 1H−15N HSQC spectrum at 20 °C (Figure S62). Cooling a CD2Cl2 solution of [PhPhMoH(NH)]+ to −90 °C led to broadening of the exchanging proton and hydride resonances, without reaching decoalescence in the 1H NMR spectrum. The triplet observed at 20 °C collapses into a broad singlet at 0.38 ppm at −90 °C, with no separate proton and hydride resonances observed in the typical regions, suggesting that exchange of proton and hydride remains extremely fast, even at −90 °C. The Mo−H/N−H resonance shows temperaturedependent behavior, shifting downfield from −0.51 ppm at 20 °C to 0.38 ppm at −90 °C. Addition of HD gas to a solution of [PhPhMo]+[BArF4]− in fluorobenzene, or adding [D(OEt2)2]+[B(C6F5)4]− to PhPhMoH in CH2Cl2 at 20 °C, gave an equilibrium [PhPhMoH(ND)]+ ⇄ [PhPhMoD(NH)]+. Variable-temperature 1H and 2H NMR spectroscopic studies reveal an equilibrium isotope effect (EIE).10,69,81 At 20 °C, a triplet at −3.73 ppm (1H, 2JPH = 29 Hz) in the 1H NMR spectrum appears near the typical chemical shift of Mo hydride resonances; a resonance was observed at 3.09 ppm in the 2H NMR spectrum. The average of the 1H and 2H chemical shifts (−0.31 ppm) is close to the value of the averaging Mo−H/ N−H resonance of [PhPhMoH(NH)]+ (−0.51 ppm).81 Upon cooling a solution of the HD adduct, the chemical shift exhibiting predominantly metal hydride character gradually shifts upfield from −3.73 ppm (20 °C) to −5.72 ppm (−90 °C) in the 1H NMR spectrum, while the deuterium resonance shifts in the opposite direction, downfield from 3.09 ppm (40 °C) to 5.97 ppm (−70 °C) in the 2H NMR spectrum (Figure S67). Thus, the disparate 1H and 2H chemical shifts of [PhPhMoH(ND)]+ ⇄ [PhPhMoD(NH)]+ result from an EIE that favors the Mo−H/N−D isotopomer energetically over the Mo−D/N−H isotopomer, as a result of the larger zero-point energy differences of the N−H/N−D bonds compared to the Mo−D/Mo−H bonds. At lower temperature, the relative population of [PhPhMoH(ND)]+ increases and that of [PhPh MoD(NH)]+ decreases, indicating that [PhPhMoH(ND)]+ is increasingly thermodynamically favored at lower temperatures. An EIE of 0.22−0.29 at −20 °C is estimated.69 Determination of the rate of reversible heterolytic cleavage of the H−H bond in [PhPhMoH(NH)]+ requires knowledge of the peak separation of the two resonances that are being averaged. Since the “frozen-out” Mo−H and N−H resonances in 7381


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Journal of the American Chemical Society Table 1. Rates and Activation Parametersa of Reversible Heterolytic H−H Bond Cleavage Dynamics solvent

ΔH⧧ (kcal mol−1)

[PhBnMoH(NH)]+

THF-d8 CD2Cl2

13.2 ± 0.7 9.1 ± 0.7

[PhPhMoH(NH)]+

CD2Cl2

t

[ BuBnMoH(NH)]+ t

[ BuPhMoH(NH)]+ t

t

[ Bu BuMoH(NH)]+ a

THF-d8 CD2Cl2

12.0 ± 0.6 10.9 ± 0.7

ΔS⧧ (cal K−1 mol−1) 11.1 ± 2.8 0.5 ± 3.1

3.8 ± 2.3 1.5 ± 2.8

CD2Cl2 THF-d8 CD2Cl2

9.1 ± 1.0 12.7 ± 0.7

−12.9 ± 3.8 0.4 ± 2.9

ΔG⧧ (298 K, kcal mol−1)

k (298 K, s−1)

pKa in CH3CN

9.9 ± 1.1 8.9 ± 1.2

3.9 × 105 1.7 × 106

13.8 ± 0.2

7.1b

4.0 × 107b

9.3 ± 0.1

10.9 ± 0.9 10.4 ± 1.1

6.9 × 104 1.4 × 105

15.3 ± 0.1

8.4b

4.1 × 106b

10.7 ± 0.2

12.9 ± 1.5 12.6 ± 1.1

2.1 × 103 3.8 × 103

17.7 ± 0.1

Reported uncertainties are twice the standard deviation. bEstimated value; see text.

Figure 7. Variable-temperature 1H NMR spectra of [CpMo(H)(CO)-

Figure 6. Variable-temperature 1H NMR spectra of [CpMo(H)(CO)(PPh2NPh2H)]+ ([PhPhMoH(NH)]+, left) and [CpMo(H)(CO)t

t

t

(P Bu2NBn2H)]+ ([ BuBnMoH(NH)]+) in THF-d8.

t

(P Bu2NPh2H)]+ ([ BuPhMoH(NH)]+, right) in CD2Cl2.

The rate of exchange between the proton and hydride at 25 °C was determined from simulation of the dynamic 1H NMR spectra as k = 6.9 × 104 s−1 (THF) and 1.4 × 105 s−1 (CH2Cl2). Changing the substituent on the amine from benzyl to phenyl t in [ BuR′MoH(NH)]+ leads to more rapid exchange dynamics, as found in [PhR′MoH(NH)]+ analogues. The Mo−H/N−H t resonance of [ BuPhMoH(NH)]+ exhibits a 1:2:1 triplet at −1.49 ppm with 2JHP = 24 Hz in CD2Cl2 at 20 °C (Figure 6, right). The Mo−H/N−H resonance is broadened upon cooling the solution and merged into the baseline at −80 °C without showing separate proton and hydride resonances, indicating decoalescence was reached. Variable-temperature 1H and t 2 H NMR spectra of the HD adduct [ BuPhMoH(ND)]+ ⇄

[PhPhMoH(NH)]+ could not be observed at the lowest temperature that the NMR spectrometer could reach, we estimated the rate of the exchange dynamics on the basis of the average chemical shift of Mo−H/N−H resonance (0.38 ppm at −90 °C) and the lowest measured 1H chemical shift of [PhPhMoH(ND)]+ (−5.72 ppm at −90 °C). A minimum peak separation of the Mo−H and N−H resonances of 12.2 ppm is estimated for the “frozen out” structure of [PhPhMoH(NH)]+, which corresponds to a lower limit of the reversible heterolytic H−H bond cleavage rate as 1.4 × 104 s−1 at our lowest temperature, −90 °C (ΔG⧧ < 7.1 kcal mol−1).82 If ΔG⧧ at 25 °C is assumed to have the same value, the proton-hydride exchange dynamic is estimated at a rate of greater than 4.0 × 107 s−1. t Complexes [ BuR′MoH(NH)]+ with bulky, electron-donating t Bu groups on the phosphines also show rapidly exchanging proton and hydride. The Mo−H/N−H resonance of t [ BuBnMoH(NH)]+ shows a very broad singlet at −0.79 ppm at 70 °C in THF-d8 in the 1H NMR spectrum (Figure 7). The spectrum reached decoalescence at 25 °C; the broad singlet was no longer observed. At −80 °C, two widely separated resonances and were observed: a broad singlet at 6.23 ppm for N−H and a doublet of doublets at −7.27 ppm for Mo−H. These resonances both exhibit cross peaks at −70 °C in 1H−1H NOESY and ROESY NMR spectra, suggesting slower exchange dynamics.

t

[ BuPhMoD(NH)]+ show dynamic exchange of H and D between the amine and Mo and an EIE (see Supporting Information for details). Using the average chemical shift of Mo−H/N−H resonance (−0.49 ppm at −60 °C) and the lowest measured 1H t chemical shift of [ BuPhMoH(ND)]+ (−5.03 ppm at −60 °C), a minimum peak separation of the Mo−H and N−H resonances t was estimated as 9.08 ppm for [ BuPhMoH(NH)]+. Thus, the lower limit of exchange rate of proton and hydride in CH2Cl2 is estimated as 1.0 × 104 s−1 at −60 °C (ΔG⧧ < 8.4 kcal mol−1) and 4.1 × 106 s−1 at 25 °C, if ΔG⧧ is the same at both temperatures. Thus, the exchange rate of proton and hydride in 7382


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[ BuPhMoH(NH)]+ is greater than that of [PhBnMoH(NH)]+ but less than that of [PhPhMoH(NH)]+. We expect the rate of the analogous Mo complex bearing a more basic amine would be slower. Indeed, variable-temperature t t 1 H NMR spectra of [ Bu BuMoH(NH)]+, with electron-donating t Bu groups on the amines, shows that it has a slower proton-

Scheme 7. Determination of the pKa of Heterolytic H2 Cleavage Products

t

hydride exchange rate than [ BuBnMoH(NH)]+. The individual resonances for amine-bound proton at 6.05 ppm and Mo hydride at −7.24 ppm were observed in the range between −70 and 10 °C in the 1H NMR spectrum in THF-d8. The decoalescence temperature of 20 °C is higher than that of t

[ BuBnMoH(NH)]+ (0 °C), and the averaging Mo−H/N−H t

ButBu

multiple measurements. The pKa values (9.3−17.7) of these Mo t t complexes are lower than those of [CpC5F4NFe(H)(P Bu2N Bu2H)]+

MoH(NH)] is not observed even at 100 °C resonance of [ (Figure S65). The rate of exchanging proton and hydride at 25 °C was measured as k = 2.1 × 103 s−1 (in THF) and 3.8 × 103 s−1 (in CD2Cl2), about 30 times slower than that of +

t

and [CpC5F4NFe(H2)(P Bu2NBn2)]+ (estimated as 20.7 and 18.3, respectively), which presumably results from the different electronic structure of the metals (d4 for MoII and d6 for FeII) as well as the additional π-acid CO ligand in the Mo complexes.67 Morris has estimated the acidity of several cationic Mo and W hydrides,3,83 and Angelici and co-workers measured the enthalpy of protonation of several classes of metal hydrides.84 Norton and co-workers reported a pKa of 5.6 in MeCN for [CpW(H)2(CO)2(PMe3)]+;85 few pKa values of related cationic Mo dihydride complexes in MeCN have been determined. The pKa values of [RR′MoH(NH)]+ quantitatively show how changing the electronic characteristics of the amine and phosphines tunes the overall acidity of the heterolytic H2 cleavage products. Changing the group on the amine from phenyl to benzyl to t t Bu significantly decreases the acidity of [ BuR′MoH(NH)]+, with

t

[ BuBnMoH(NH)]+. The rates of proton-hydride exchange measured by dynamic 1H NMR spectroscopy give the following order: t

[PhPhMoH(NH)]+ > [ BuPhMoH(NH)]+ > [PhBnMoH(NH)]+ > t

t

t

[ BuBnMoH(NH)]+ > [ Bu BuMoH(NH)]+. The rate of exchanging proton and hydride in [PhPhMoH(NH)]+ is the fastest among this series of Mo-based heterolytic H2 cleavage products, because the relative acidities are very similar for the heterolytic cleavage complex and [PhPhMoH(NH)]+ and the dihydrogen complex [PhPhMo(H2)]+. Complexes bearing more basic amines have lower acidity of the protonated amine, and the more electronrich tBu on the phosphine decreases the acidity of dihydride/ dihydrogen ligand in the intermediate [RR′Mo(H)2]+. In both cases, the higher pKa values of the protonated amine in [RR′MoH(NH)]+ and dihydride/dihydrogen ligand in [RR′Mo(H)2]+ result in a slower rate of exchange between the proton and hydride. Measurement of the pKa Values of [RR′MoH(NH)]+ Complexes in CD3CN. The pKa values of [RR′MoH(NH)]+ were determined in CD3CN by 1H and 31P NMR spectroscopy. The pKa of [PhBnMoH(NH)]+ in CD3CN was determined relative to 2-methylpyridine (pKa = 13.52 for the conjugate acid, 2-methylpyridinium) and 1,3,5-trimethylpyridine (pKa = 14.98 for the conjugate acid).74 In the 1H NMR spectra, the aromatic resonances for each substituted pyridinium and pyridine coalesced into average resonances, and the Cp resonances for [PhBnMoH(NH)]+ and PhBnMoH also coalesced into one resonance, indicating fast exchange. Thus, the ratio of substituted pyridinium to pyridine for the pKa reference and the analogous ratio for [PhBnMoH(NH)]+ and PhBnMoH were both determined from the weighted averages of the chemical shifts. Using these ratios, the equilibrium constant Keq was determined, and a pKa of 13.8 ± 0.2 was obtained from the equation in Scheme 7. The pKa value was also verified by addition of 2,6-lutidinium (pKa = 14.13) into a CD3CN solution of PhBnMoH to attain equilibrium from the reverse direction. t The pKa values of [PhPhMoH(NH)]+, [ BuPhMoH(NH)]+, t

t

t

t

[ Bu BuMoH(NH)]+ as the least acidic complex. Changing the group on the phosphine from phenyl ([PhBnMoH(NH)]+) to a t

t

Bu group ([ BuBnMoH(NH)]+) results in an increase of 1.5 pKa units, because more electron-donating phosphines decrease the acidity of intermediate Mo dihydride complexes. The order of acidity of the heterolytic H2 cleavage products is t PhPh [ MoH(NH)]+ > [ BuPhMoH(NH)]+ > [PhBnMoH(NH)]+ > t

t

t

[ BuBnMoH(NH)]+ > [ Bu BuMoH(NH)]+. This order is exactly the same as the order of the rates of proton/hydride exchange; the acidity shows a linear correlation with the logarithm of the proton-hydride exchange rates (Figure 8). The most acidic complex [PhPhMoH(NH)]+ (pKa = 9.3 ± 0.1) gives the fastest rate of exchanging proton and hydride (>107 s−1 at 25 °C). The t t least acidic complex [ Bu BuMoH(NH)]+ (pKa = 17.7 ± 0.1) is less acidic by approximately 8 pKa units, and its proton-hydride exchange rate is nearly 104 times slower (3.8 × 103 s−1 at 25 °C). Thus, the pKa shows an excellent correlation between the acidity and proton-hydride exchange dynamics.

■

DISCUSSION The cleavage of H2 and the catalytic hydrogenation of unsaturated compounds have been extensively reported in main group systems since the pioneering work by Stephan, Erker, and their co-workers.15−20 Heterolytic cleavage of H2 by transition-metal based frustrated Lewis pairs have been reported. Wass and co-workers reported H2 splitting by the cationic zirconium complex [Cp*2ZrOC6H4PtBu2]+ under mild conditions.29,30 Erker and co-workers developed a cationic zirconocene-amine complex, [Cp*2Zr(OCH2CH2NiPr2)]+, for heterolytic cleavage of H2 and catalytic hydrogenation of alkenes.34 The H2 cleavage in both of these frustrated Lewis

t

[ BuBnMoH(NH)]+, and [ Bu BuMoH(NH)]+ in CD3CN were determined as 9.3 ± 0.1, 10.7 ± 0.2, 15.3 ± 0.1, and 17.7 ± 0.1 by equilibration of 4-bromoaniline (pKa = 9.43 for the conjugate acid),74 aniline (pKa = 10.62 for anilinium),74 1,3,5-trimethylpyridine, and benzylamine (pKa = 16.91 for the conjugate acid),74 respectively. All of these pKa data are averaged values from 7383


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Journal of the American Chemical Society i

Pr, Mes) complexes depends on the size of the substituents on zirconium and the phosphine.29,30 Though [Cp*2ZrOC6H4PtBu2]+ shows no Zr−P interaction and readily adds H2, [Cp2ZrOC6H4PtBu2]+, bearing the less sterically demanding Cp ligand, has a Zr−P bond and does not react with H2, similar to the lack of H2 addition reactivity of [CpMo(CO)(κ3-PNP)]+ because of the strong Mo−N coordination. In our results, the additional ring strain in [CpMo(CO)(κ3P2N2)]+ ([RR′Mo]+) compared to [CpMo(CO)(κ3-PRNMePR)]+ promotes the dissociation of Mo−N bond and enables H2 addition. van Leeuwen and co-workers found that the steric effect of a pyridine ring in [(diphosphine)Pd(2-(diphenylphosphino)-pyridine)]2+ complexes has a significant impact on the activity for heterolytic cleavage of H2.100 The Pd complex bearing a methyl on the 6-position of pyridine ring shows much higher reactivity toward H2 than that of the complex bearing an unsubstituted pyridyl group, probably because the 6-methyl promotes the dissociation of the pyridyl group. All of these results demonstrate that H2 cleavage reactivity in both transitionmetal based FLPs and metal−ligand bifunctional complexes can be tuned by modulating the Lewis acid−base binding. Short proton-hydride distances in some of these metal complexes have been revealed by NMR spectroscopy58,101−106 or by neutron diffraction in one example.66 In only a few cases were the proton-hydride exchange rates experimentally measured. Szymczak and co-workers reported that the doubly deprotonated ruthenium complex heterolytically cleaves H2 via cooperation of metal and pendant oxyl groups, affording 3 as a ruthenium hydride complexes bearing two phenolic hydroxy groups (Figure 9).107 Complex 3 also shows dynamic exchange of two protons and one hydride, and its exchange rate was measured by spin-saturation transfer, corresponding to an

Figure 8. Correlation between the pKa in CD3CN and proton/hydride exchange rates (k) in CD2Cl2 at 25 °C. Triangles show the two complexes whose exchange rates were estimated from the dynamic NMR studies for the complexes formed from HD at low temperature in CD2Cl2, and squares indicate the three complexes whose exchange rates were determined by simulation of variable-temperature 1H NMR spectra in THF-d8.

pair systems could involve direct σ-bond cleavage between acidic and basic centers, similar to that in main group Lewis pairs.86,87 We propose that heterolytic cleavage of H2 by Mo complexes [RR′Mo]+ occurs through a metal−ligand cooperation mechanism38−41 involving reaction of H2 with the metal, forming the dihydride M(H)2+ or dihydrogen M(η2-H2)+ complexes, followed by ligand-mediated intramolecular proton transfer to generate a protonated amine and a metal hydride. Heterolytic cleavage of H2 by many other metal complexes also proceeds by similar mechanisms, such as iron complexes developed by Casey and Guan,7,44 and the iridium complexes with hydroxyl-substituted bipyridine ligands reported by Fujita and co-workers.88,89 A metal can also act as the base to promote inter- or intramolecular proton transfer in H2 cleavage by some bimetallic complexes.90,91 In the broader context of addition of H2 across M−N bonds, Fryzuk and co-workers discovered early examples of the heterolytic cleavage of H2 across iridium amide bonds.92 More recently, Grützmacher and co-workers found rhodium amide bonds that heterolytically cleave H2.93 Cooperative ligand reactivity was observed by Schneider and co-workers in the heterolytic cleavage of H2 in a ruthenium nitride complex, leading to hydrogenolysis of the nitride and producing ammonia.94 In ruthenium95,96 and iridium97 complexes with pyridine-based pincer ligands, Milstein and co-workers used heterolytic cleavage of H2 involving aromatization−dearomatization of the ligand as a key step in catalytic reactions exhibiting a wide scope of reactivity, with many examples documenting the formation of C−H bonds on the ligand. Heterolytic cleavage of H2 is proposed in Fe complexes,98 including Fe complexes that catalyze the dehydrogenation of formic acid.99 In transition-metal based FLPs and metal−ligand bifunctional complexes, the reactivity with H2 can be altered by Lewis acid− base binding or by changing the electronic or steric characteristics of the coordination environment. The ability of the coordinated Lewis acid−base to dissociate and release the “unquenched” reactive vacant coordination site is crucial for H2 binding and activation. For example, Wass and co-workers reported that the kinetics of H2 addition to [CpCp*ZrOC6H4PR2]+ (R = tBu,

Figure 9. Metal complexes formed by heterolytic H2 cleavage, and kinetics of exchange of the proton and hydride. 7384


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Journal of the American Chemical Society extrapolated rate of 519 s−1 at 25 °C. This rate is about four times t

lower than that of [

ButBu

exchange rates of those complexes by simulation of the variabletemperature 1H NMR spectra. The trend of the decoalescence temperature for the averaging proton and hydride from the lowest to the highest of the five Mo complexes in CD2Cl2 again indicates the exchange rates from the highest to the lowest as t [PhPhMoH(NH)]+ > [ BuPhMoH(NH)]+ > [PhBnMoH(NH)]+ >

3 −1

MoH(NH)] in THF-d8 (2.1 × 10 s ).

t

107

+

t

Both complex 3 and [ Bu BuMoH(NH)]+ have negative entropies of activation, −20 ± 1 cal K−1 mol−1 (in CD2Cl2) for 3 and −12.9 ± t t 3.8 cal K−1 mol−1 (in THF-d8) for [ Bu BuMoH(NH)]+. A large ⧧ negative ΔS value would lead to a significant increase in ΔG⧧ at higher temperatures. Andersen and Bergman reported Cp*2Ti(H)(SH) (4), which was generated from heterolytic cleavage of H2 by Cp*2TiS.108 The exchange dynamics of Ti−H and S−H resonances in Cp*2Ti(H)(SH) was measured by 2D EXSY 1H NMR spectroscopy, which gives an exchange rate of 1.2 s−1 at −30 °C. DuBois and co-workers observed rapid proton-hydride exchange in [(PEtNMeHPEt)Fe(H) (dmpm) (CH3CN)]+ (5) and [(PEtNMeHPEt)Ni(H)(PEtNMeHPEt)]+ (6) complexes, both bearing a disphosphine ligand with a pendant amine ligand.63,64 The exchange rates for both complexes were estimated as 104 s−1 at 20 °C, corresponding to an activation barrier of 12 kcal/mol. We developed manganese69,70 and iron65−67 complexes bearing pendant amines as electrocatalysts for oxidation of H2. The Mn complexes have weak Mn−N bonds to the pendant amine that are readily displaced upon reaction with H2 (1 atm), similar to the Mo−N bonds reported here. The iron complex [1]+ and the manganese complex [2]+ both show extremely rapid proton-hydride exchange dynamics at temperatures as low as −80 °C, with no decoalesence observed (Figure 9).66,69 The exchange rates at low temperature for both complexes were estimated as >104 s−1 based on low-temperature NMR data for the complexes formed from HD. Rates over 107 s−1 at 25 °C were estimated, based on assumption that the ΔG⧧ is the same at 25 °C and at −80 °C. The upper limits of the ΔG⧧ for both complexes are around 7 kcal/mol. The averaging proton-hydride resonance in the manganese complex [(PPhNMeHPPh)Mn(H)(CO)(bppm)]+ (7), bearing a PNP ligand, reaches decoalescence at −42 °C.70 A much slower exchange rate of 9.7 × 103 s−1 at 20 °C and a higher ΔG⧧ of 11.8(8) kcal/mol at 25 °C, showing the positioning of amine and the ring flip conformational change, have a large impact on the dynamics of proton-hydride exchange. However, in most of the results mentioned above, only one or two complexes are reported; tuning the kinetics of protonhydride exchange dynamics has not been extensively studied. Our results include the systematic measurement of thermodynamic and kinetic parameters of a series of Mo complexes as heterolytic H2 cleavage products and demonstrate how to tune the kinetics of proton-hydride exchange dynamics. The exchange rates of proton and hydride for [PhPhMoH(NH)]+ t and [ BuPhMoH(NH)]+ at −90 °C are extremely fast. We estimated the exchange rates from the low-temperature NMR data on complexes formed from HD. The lower limit of the rates at 25 °C is scaled from the rates at low temperature based on the assumption that the ΔG⧧ is the same at both low temperature and 25 °C, which could lead to errors in the extrapolated rates. Since the proton-hydride exchange dynamics in [PhPhMoH(NH)]+ and t [ BuPhMoH(NH)]+ do not reach decoalesence even at −90 °C, the estimated values are lower limits of the exchange rates. Since decoalesence of proton and hydride resonances for t

t

t

t

t

[ BuBnMoH(NH)]+ > [ Bu BuMoH(NH)]+, which corresponds to the order of experimentally determined pKa values from the lowest to the highest.

■

CONCLUSIONS Heterolytic cleavage of H2 into a proton and a hydride was achieved in a series of Mo bifunctional complexes, [CpMo(CO)(κ3-PR2NR′2)]+ ([RR′Mo]+). The exchange dynamics of proton and hydride in the complex showing fastest rate ([PhPhMoH(NH)]+, ≥107 s−1 at 25 °C) is nearly 104 times faster t t than that of the complex with slowest rate ([ Bu BuMoH(NH)]+, 2.1 × 103 s−1). We determined that the pKa values of five complexes [RR′MoH(NH)]+ in acetonitrile range from 9.3 to 17.7, leading to a quantitative understanding of how the protonhydride exchange dynamics are controlled by the pKa of the complexes. A more basic amine or a more electron-donating phosphine leads to complexes with a lower acidity and a slower proton-hydride exchange rate. Our investigation into the relation between acidity and kinetic characteristics of bifunctional complexes provides design principles for the rational design FLPs for heterolytic cleavage of H2.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03053. Experimental procedures, synthesis, variable-temperature 1-D and 2-D NMR spectroscopic data for all new compounds, measurement of T1 relaxation time, and pKa determinations by 1H NMR spectroscopy (PDF) X-ray crystallographic data (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Shaoguang Zhang: 0000-0002-0931-321X Aaron M. Appel: 0000-0002-5604-1253 R. Morris Bullock: 0000-0001-6306-4851 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences for support. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy. We thank Dr. Allan Cardenas and Dr. Molly O’Hagan for assistance with the simulation of NMR spectra and Dr. Christopher Zall and Dr. Monte Helm for advice on the refinement of X-ray structures.

t

[PhBnMoH(NH)]+, [ BuBnMoH(NH)]+ and [ Bu BuMoH(NH)]+ was observed at low temperature, and the averaging proton-

■

t

hydride resonances for [PhBnMoH(NH)]+ and [ BuBnMoH(NH)]+ were observed at high temperature, we accurately determined the

REFERENCES

(1) Kubas, G. J. Chem. Rev. 2007, 107, 4152−4205.

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