Reversible Heterolytic Cleavage of the H−H Bond by MolybdenumComplexes: Controlling the Dynamics of Exchange Between Protonand HydrideShaoguang 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 theH−H bond of dihydrogen is critically important in catalytichydrogenations and in the catalytic oxidation of H2. We showhow the rate of reversible heterolytic cleavage of H2 can becontrolled, spanning 4 orders of magnitude at 25 °C, from2.1 × 103 s−1 to ≥107 s−1. Bifunctional Mo complexes,[CpMo(CO)(κ3-P2N2)]

+ (P2N2 = 1,5-diaza-3,7-diphosphacy-clooctane 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 cleavageis enabled by the basic amine in the second coordination sphere. The products of heterolytic cleavage of H2, Mo hydride complexesbearing 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

exchange rates are in the order [CpMo(H)(CO)(PPh2NPh2H)]

+ > [CpMo(H)(CO)(PtBu

2NPh2H)]

+ > [CpMo(H)(CO)(PPh2NBn

2H)]+

> [CpMo(H)(CO)(PtBu

2NBn

2H)]+ > [CpMo(H)(CO)(P

tBu2N

tBu2H)]

+. The pKa values determined in acetonitrile range from 9.3 to17.7 and show a linear correlation with the logarithm of the exchange rates. This correlation likely results from the exchange processinvolving key intermediates that differ by an intramolecular proton transfer. Specifically, the proton-hydride exchange appears to occurby formation of a molybdenum dihydride or dihydrogen complex, resulting from proton transfer from the pendant amine to the metalhydride. 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.

■ INTRODUCTION

The H−H bond is the simplest chemical bond, yet it offersdiverse reactivity.1−4 Cleavage of the H−H bond is afundamentally important reaction that can occur by homolyticor heterolytic pathways. Heterolytic cleavage of the H−H bondinto a proton and hydride (Scheme 1) is a critical process in the

catalytic hydrogenation of ketones,5−7 the oxidation of hydrogenby hydrogenases in nature8,9 and by synthetic molecularcatalysts.10−14 Interest in using H2 as a clean fuel producedthrough sustainable solar and wind energy has propelled researchon finding facile ways to cleave the H−H bond. Understandingthe fundamental thermodynamics and kinetics of heterolyticH−H bond cleavage and controlling the transfer of the protonand 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 ofthe H−H bond, leading to the development of new classesof hydrogenation catalysts.15−20 Steric hindrance or ring strain

prevent the Lewis acid and base centers from forming an adduct.The “unquenched” reactivity of FLPs creates a polarized environ-ment that facilitates heterolytic cleavage of the H−Hbond. Some“not-so-frustrated” Lewis acid/base adducts reversibly exposethe Lewis acidic and basic centers in an equilibrium, leading toreactivity similar to that of traditional FLPs.21−26

Wass and co-workers called attention27,28 to the relationshipbetween main group FLPs and reactions in which a transitionmetal functions as a Lewis acid or base.27−36 Transition-metalcontaining FLPs can be considered in the broader context ofmetal−ligand bifunctional catalysis37 and metal−ligand cooper-ation38−41 in catalysis. A recent perspective article reviews theheterolytic cleavage of H2 in the framework of metal-containingFLPs.42

Heterolytic cleavage of the H−H bond in many bifunctionalcomplexes is enabled by an amine as a proton acceptor in thesecond coordination sphere, and the metal as the hydrideacceptor, mimicking the function of hydrogenase.8,9 Animportant application of the heterolytic cleavage of H2 is ionichydrogenations,43 a powerful class of reactions that requiresgeneration of a proton and hydride from the H−H bond.7,44−47

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

Scheme 1. Heterolytic H−H Bond Cleavage

Article

pubs.acs.org/JACS

© 2017 American Chemical Society 7376 DOI: 10.1021/jacs.7b03053J. Am. Chem. Soc. 2017, 139, 7376−7387

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The proton and hydride are subsequently delivered to theunsaturated substrate, without requiring coordination of thesubstrate to the metal.37 Tuning the ability of the proton3 andhydride4 to be transferred to a substrate has a strong impacton the kinetics and thermodynamics and consequently on thecatalytic activity for ionic hydrogenations. Thus, the ability tounderstand and control the factors influencing rates of heterolyticcleavage 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 ofH2 across a MoN 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 inthe absence of H2. Molybdenum complexes reported by Long,Chang, and co-workers are electrocatalysts for the evolution ofH2 in aqueous solution.50−52 Molybdenum or tungsten cationiccomplexes bearing phosphine or N-heterocyclic carbene ligandsadd H2 to generate dihydride complexes.53−56 In ionic hydro-genations with these complexes, a Mo−H bond is the protondonor, then the second Mo−H bond functions as a hydridedonor.57 To design Mo bifunctional complexes for heterolyticcleavage of H2 with separately tunable proton and hydridedonors, we envisioned that incorporating an amine group inthe second coordination sphere of the diphosphine ligand couldgive 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 un-reactive toward H2 addition (Scheme 2).68 These results suggestthat the amine is strongly bound to the electrophilic Mo centerbecause of the wide P−Mo−P angle of the flexible PNP ligand.

Inspired by the structure−function relationships established inmain group and transition-metal-based FLPs, we anticipated thatstructural modification of the ligand by introducing ring strainto destabilize the Mo−N coordination might reversibly expose avacant coordination site for reaction with H2, allowing the Lewisacidic Mo center and the basic amine to provide cooperativeheterolytic H−H bond cleavage.In this paper, we report the facile heterolytic cleavage of H2 by

cationic Mo complexes, affording Mo hydride complexes bearinga protonated amine, [CpMo(H)(CO)(PR2N

R′2H)]

+ (Scheme 2).

The amine-bound proton and the metal hydride undergo rapidexchange,63,64,66,69,70 demonstrating reversible cleavage andheterocoupling of the H−H bond. The pKa values quantitativelyreveal that tuning the acidity of the complexes controls the rates ofreversible heterolytic cleavage by about 4 orders of magnitude.Themore acidic [CpMo(H)(CO)(PR2N

R′2H)]

+ complexes showfaster exchange rates of the proton and hydride, demonstratingthe ability to control the reaction by changing the substituents onthe phosphine and amine.

■ RESULTSSynthesis and Characterization of CpMo(H)(CO)(P2N2)

Complexes. CpMo(H)(CO)(PPh2N

R′2) (PhBnMoH, R′ = Bn;

PhPhMoH, R′ = Ph) with Ph groups on the phosphines weresynthesized from CpMo(H)(CO)3 and the diphosphine ligandPPh

2NR′2 in toluene at 80 °C (Scheme 3), similar to the route

used in the synthesis of CpMo(H)(CO)(PRNMePR) complexes(R = Et, Ph).68 The Mo hydride complex PhBnMoH was isolatedin 73% yield, and its structure was confirmed by single-crystalX-ray diffraction (Figure 1, Table S14). A doublet of doublets at−6.83 ppm (2JHP = 57 Hz for cis; 2JHP = 18 Hz for trans) wasobserved in the 1H NMR spectrum as the hydride resonance ofPhBnMoH in CD2Cl2, as expected for hydride ligands coupled toboth cis and trans phosphines.71

For the analogous CpMo(H)(CO)(PtBu

2NR′2) (

tBuBnMoH, R′= Bn;

tBuPhMoH, R′ = Ph;tButBuMoH, R′ = tBu) complexes

bearing tBu groups on the phosphines, attempted synthesis by

the reaction of CpMo(H)(CO)3 and PtBu

2NR′2 in either hexane

or benzene at 80 °C resulted in a mixture oftBuR′MoH and other

products that have not been identified. An alternative synthetic

route totBuR′MoH was devised. The Mo chloride complexes,

CpMo(Cl)(CO)(PtBu

2NR′2) (

tBuBnMoCl, R′= Bn;tBuPhMoCl, R′ =

Ph;tButBuMoCl, R′ = tBu), were readily prepared by the reaction

of CpMo(CO)3Cl and PtBu

2NR′2 at 110 °C in toluene, followed

by treatment with Me3NO in CH2Cl2, leading to oxidativeelimination of one CO ligand. The corresponding Mo hydridestBuR′MoH were prepared from the reaction of

tBuR′MoCl with

Scheme 2. Heterolytic Cleavage of H2 by a FLP and MoComplexes

Scheme 3. Synthesis of CpMo(H)(CO)(P2N2)

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LiBHEt3 in THF (Scheme 3). The X-ray crystallographicstructures of PhBnMoH,

tBuBnMoCl,tBuPhMoCl,

tBuBnMoH, andtBuPhMoH all show four-legged piano-stool geometries andcis geometries of the two phosphines (Figure 1; see SI for the

structure oftBuPhMoCl). The P−Mo−P bond angles of these

Mo hydrides vary over the narrow range from 74.103(15)° to75.219(17)°.Synthesis and Characterization of [CpMo(CO)(κ3-

P2N2)]+, Cationic Mo Diphosphine Complexes Bearing a

Bound Amine. The cationic “tuck-in” Mo complexes [CpMo-

(CO)(κ3-PtBu

2NR′2)]

+[BArF4]− ([

tBuBnMo]+, R′ = Bn; [tBuPhMo]+,

R′ = Ph; [tButBuMo]+, R′ = tBu) were prepared by chloride

abstraction fromtBuR′MoCl using NaBArF4 (ArF = 3,5-bis-

(trifluoromethyl)phenyl) in fluorobenzene and were fully charac-terized (Scheme 4). In the 31P{1H} NMR spectra, the chemicalshifts of the 31P NMR resonances range from −2.0 to 6.9 ppm,comparable to other κ3-PNP or κ3-P2N2 metal complexes featuringfour-membered phosphacycles.68,70,72 Even though the struc-

tures of [tBuR′Mo]+ and [CpMo(CO)(κ3-PRNMePR)]+ (Figure 2,

Table S15) all show the κ3 coordination mode, with the amine

bound, the P−Mo−P bond angles of [tBuR′Mo]+ (85.636(17)o to

87.75(2)o) are much smaller than the wide P−Mo−P angles foundin [CpMo(CO)(κ3-PEtNMePEt)]+ (123.220(16)o) and [CpMo-(CO)(κ3-PPhNMePPh)]+ (122.20(2)o).68 The Mo−N bond lengths

in [tBuBnMo]+ (2.370(3) Å), [

tBuPhMo]+ (2.405(2) Å), and

[tButBuMo]+ (2.4583(16) Å) are significantly longer than thevalues of 2.3083(13) Å in the [CpMo(CO)(κ3-PEtNMePEt)]+,

suggesting the Mo−N bond strength of [tBuR′Mo]+ is weaker.

The complex [CpMo(CO)(κ3-PPh2N

R′2)]

+ ([PhBnMo]+,R′ = Bn; [PhPhMo]+, R′ = Ph) was generated in situ by hydride

abstraction from PhBnMoH or PhPhMoH with [Ph3C]+[BArF4]

−

in fluorobenzene (Scheme 5). The X-ray crystallographicstructure of [PhBnMo]+[BArF4]

− unambiguously confirmed theconnectivity and the κ3-coordination mode, despite somedisorder of heavy atoms (see Supporting Information for details).We reported the slow reaction (3 h) of [CpMo(CO)(κ3-

PEtNMePEt)]+ in neat CD3CN and determined the pseudo-first

Figure 1. ORTEP drawing of CpMo(H)(CO)(PPh2N

Bn2) (

PhBnMoH,

upper left), CpMo(Cl)(CO)(PtBu

2NPh

2) (tBuPhMoCl, upper right),

CpMo(H)(CO)(PtBu

2NBn

2) (tBuBnMoH, lower left), and CpMo(H)-

(CO)(PtBu

2NPh

2) (tBuPhMoH, lower right) with 30% thermal ellipsoids.

Phenyl groups on phosphines, methyls of tert-butyl groups, andhydrogen atoms are omitted for clarity, except for the ipso-C of thephenyl group and Mo−H.

Scheme 4. Formation of [tBuR′Mo]+ and Heterolytic H2

Cleavage Products [tBuR′Mo H(NH)]+

Figure 2. ORTEP drawing of [CpMo(CO)(κ3-PtBu

2NBn

2)]+[BArF4]

−

([tBuBnMo]+[BArF4]

−, left) and [CpMo(CO)(κ3-PtBu

2NPh

2)]+[BArF4]

−

([tBuPhMo]+[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)]+

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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,

dissociation of the amine in [tBuBnMo]+ and addition of an

acetonitrile ligand occurred rapidly at 0 °C, as indicated by acolor change from violet to yellow, and was complete withinseconds. The high reactivity of electrophilic Mo complexestoward acetonitrile, as found for [CpMo(CO)2(P

iPr3)]+-

[B(C6F5)4]−,73 suggested the dissociation energy of the Mo−N

bond in the κ3-Mo-P2N2 complexes [RR′Mo]+ is much lower thanthat in [CpMo(CO)(κ3-PRNMePR)]+.73,68

Heterolytic Cleavage of H2 to Give Mo HydrideComplexes Bearing a Protonated Amine. Bubbling of H2(1 atm) into a violet solution of [PhBnMo]+ in fluorobenzeneimmediately afforded a light orange solution of [PhBnMoH(NH)]+,a Mo hydride complex bearing a protonated amine (Scheme 5).Analogously, reactions of H2 with [PhPhMo]+ generatedin situ cleanly formed [PhPhMoH(NH)]+ within seconds.[PhR′MoH(NH)]+ was also prepared by protonation of PhR′MoHwith [H(OEt2)2]

+[B(C6F5)4]−. [PhPhMoH(NH)]+ slowly releases

H2 in THF-d8 or CD3CN solution in hours, presumably througha Mo dihydride or dihydrogen intermediate (Scheme 6).

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 thedisplacement of the acetonitrile ligand by H2 is unfavorablethermodynamically and/or kinetically. Thus, we were not ableto measure the hydricity of PhR′MoH or the kinetic barrier forH2 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 atroom temperature, [PhR′MoH(NH)]+ in MeCN. Addition of H2to [PhR′Mo(CD3CN)]

+ was not observed in CD3CN solution,indicating the displacement of the acetonitrile ligand by H2 isunfavorable thermodynamically, slow kinetically, or both. Incontrast, [PhBnMoH(NH)]+ is stable in THF or MeCN solventwithout elimination of H2. In MeCN, it loses H2 very slowly,reaching completion after 24 h at room temperature. Deproto-nation of [PhR′MoH(NH)]+ by Et3N (pKa = 18.8

74 for H-NEt3+ in

MeCN) generates the corresponding neutral Mo hydridecomplexes PhR′MoH.Orange crystals of both [PhBnMoH(NH)]+[BArF4]

− and[PhPhMoH(NH)]+[BArF4]

− were grown by slow diffusion ofpentane into a fluorobenzene solution under H2 (1 atm). TheX-ray crystallographic analysis reveals with structures containingbothMo−H andN−H bonds, resulting from heterolytic cleavageof H2. Surprisingly, [

PhBnMoH(NH)]+ and [PhPhMoH(NH)]+

adopt different conformational geometries (Figure 3).[PhBnMoH(NH)]+ crystallized as the anti, endo-isomer. Theproton and hydride are arranged on different sides of theP−Mo−P plane (H···H distance ∼4.15 Å) instead of being

positioned close to each other on the same side. In contrast, thecrystal structure of [PhPhMoH(NH)]+ indicated the syn, endo-isomer, with the proton and hydride positioned toward each other(Figure 3, middle). TheMo−Hbond length determined by X-raydiffraction is 1.628(9) Å, though the precise location of hydride issubject to the uncertainties associated with X-ray crystallog-raphy.75 Setting the N−H distance to 1.000 Å gives an estimatedH···H distance of 2.456 Å. This distance is much longer than theshort H···H distance of 1.489(10) Å found for dihydrogenbonding in the single crystal neutron diffraction structure of

[CpC5F4NFe(H)(PtBu

2NtBu

2H)]+[BArF4]

− ([1]+[BArF4]−), in which

the N−H bond is positioned in close proximity to the iron hydridein the three-legged piano stool coordination geometry.66

[PhR′MoH(NH)]+ has one additional CO ligand, adopting afour-legged piano stool coordination geometry. The rigid P2N2ligand could only form the cis configuration instead of transconfiguration. As a result, the hydride from heterolytic cleavage ofthe H−H bond is located trans to one phosphine, instead of cis totwo phosphines in [1]+ and [(PPh2N

Bn2H)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

addition of H2 to [tBuR′Mo]+, bearing bulky tBu groups on the

Scheme 6. ProposedMechanism of Proton-Hydride Exchangethrough a Mo Dihydrogen or Dihydride Intermediate

Figure 3.ORTEP drawing of [CpMo(H)(CO)(PPh2NBn

2H)]+[BArF4]

−

([PhBnMoH(NH)]+, top), [CpMo(CO)(H)(PPh2N

Ph2H)]

+[BArF4]−

( [ P h P hMoH (NH ) ] + , m i d d l e ) , a n d [CpMo(H ) (CO) -

(PtBu

2NBn

2H)]+[BArF4]

− ([tBuBnMoH(NH)]+, bottom) with 30%

thermal ellipsoids. BArF4 anions, phenyl groups on phosphines, methylsof tert-butyl groups, hydrogen atoms, and cocrystallized fluorobenzenemolecules are omitted, except the ipso-C of the phenyl group, Mo−Hand N−H.

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phosphine, affording [tBuR′MoH(NH)]+ (Scheme 5). All three

complexes [tBuR′MoH(NH)]+ are stable in MeCN solution, in

the solid state under an N2 atmosphere or under vacuum.

Crystals of [tBuBnMoH(NH)]+ and [

tBuPhMoH(NH)]+ weregrown by slow diffusion of pentane into a fluorobenzenesolution, and their structures were characterized by X-raycrystallography as syn, endo-isomers (Figures 3 and S3).We propose that [RR′MoH(NH)]+ is formed through

intramolecular deprotonation of an unobserved Mo dihydrideor dihydrogen intermediate (Scheme 6).76 Attempts to observe[RR′Mo(H)2]

+ at −80 °C were unsuccessful, as rapid heterolyticcleavage of H2 generates [

RR′MoH(NH)]+. Alternatively, directH−H bond cleavage by the Lewis acidic Mo and the basic amine,mimicking FLP reactivity, cannot be ruled out, but is consideredless likely. We propose that the bound amine in [RR′Mo]+

functions as a hemilabile ligand that stabilizes the electrophilicMo center and promotes the intramolecular deprotonation, asreported in related iron and manganese complexes.66,69,70

Since H−H bond cleavage was achieved in κ3-Mo-P2N2complexes [RR′Mo]+, but not in the κ3-Mo-PNP complexes[CpMo(CO)(κ3-PRNMePR)]+ bearing a similar second coordi-nation sphere and Lewis acid−base bifunctionality, we examinedthe crystal structures to understand the structure−reactivityrelationships. To create a vacant coordination site for H2 binding,the amine coordinated to the Mo center must dissociate. Inaddition, the pendant amine must be correctly positioned andsufficiently basic to deprotonate the intermediate [RR′Mo(H)2]

+.Thus, the H2 addition reactivity is strongly affected by theMo−Nbinding strength, which is tunable by modulating the ring strainand rigidity of the ligand skeleton and the electronic effects of theancillary 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 todissociate and generate a vacant site for H2 coordination. In thesamemanner, some Lewis pairs are unable to activate H2 becausethe acid−base coordination quenches the acidity and basicity,making them inadequately “frustrated”.29,30,77 Consistent withthis interpretation, the Mo−N bond length in [CpMo(CO)(κ3-PEtNMePEt)]+ is shorter than those in any of the κ3-Mo-P2N2

complexes [tBuR′Mo]+. The flexible PNP ligand accommodates

the strong Mo−N coordination by forming a large P−Mo−Pbond angle (123.220(16)° in [CpMo(CO)(κ3-PEtNMePEt)]+).68

Further evidence for the greater structural flexibility of thePEtNMePEt ligands compared to cyclic P2N2 ligand comesfrom the crystallographic characterization of both cis and transisomers of CpMo(H)(CO)(κ3-PPhNMePPh).68 In contrast, the

κ3-Mo-P2N2 complexes, [tBuBnMo]+ and [

tBuPhMo]+, have longerMo−N bond lengths (2.370(3)−2.405(2) Å) and much smallerP−Mo−P bond angles (85.636(17)−87.75(2)o) because ofthe ring strain in the P2N2 ligand, which destabilizes the metal-amine binding and promotes amine dissociation. In addition,steric effects caused by wider P−Mo−P bond angle and kineticprotection by the substituents in the κ3-Mo-PNP complexesmight also contribute to the lack of reactivity with H2. Thecomplexes with the P2N2 ligands cannot adopt the preferredgeometry that is attained in the κ3-Mo-PNP complexes. Similarly,ring strain of the four-membered ring in some FLPs with a two-atom linker between the Lewis acid and base also promotes theLewis pair dissociation, leading to cleavage of H2.

21,23−26,77

The rates of reaction with H2 depend on the basicity of the

amine. While the reaction of H2 with [tBuBnMo]+ or [

tButBuMo]+

requires about 15 min to reach completion, H2 addition to

[tBuPhMo]+ is significantly faster, reaching completion in seconds.This difference occurs because the less nucleophilic amine resultsin a weaker Mo−N coordination. The ability of the pendantamine to switch from the primary coordination sphere to thesecondary coordination sphere plays a critical role in heterolyticH−H bond cleavage, which is readily tuned by modificationof the electronic characteristics of the substituents on thephosphine and the amine.

Determination of the Rates of Reversible HeterolyticCleavage. Variable-temperature 1H, 31P, 15N, and 2-D NMRspectra of [RR′MoH(NH)]+ reveal rapid exchange of the protonand 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

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 bondof the protonated P2N2 ligand (14 to 7 ppm). We assign theresonance at −0.15 ppm in 1H NMR spectrum to the rapidlyexchanging N−H and Mo−H resonances.Since only one of the two amines is protonated, the assignment

of the structure with dynamic proton-hydride exchangingcharacter was further confirmed by 15N NMR and 1H−15NHSQC spectra. In the 15N NMR spectrum of 15N-labeled[PhBnMoH(NH)]+, the resonance of the protonated amineexhibits a triplet of triplets at −305 ppm (1JHN = 35 Hz, 2JNP =5.7 Hz), indicating coupling between this 15N nucleus and twoequivalent protons. The magnitude of 1JHN is close to the averagevalue 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 twonitrogens of the pendant amine (N−H···N) was observed.78,79The solution structure of [PhBnMoH(NH)]+ at 20 °C showsdynamic intramolecular proton-hydride exchange of the Mo−Hand N−H resonances, in contrast with its anti, endo structure in

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

in THF-d8.

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solid state that shows no interaction of the amine-bound protonand Mo hydride.When a THF-d8 solution of [PhBnMoH(NH)]+ was cooled to

−20 °C, the broad singlet of the exchanging proton and hydridecoalesced into the baseline of the 1H NMR spectrum, indicatinga decrease in the rate of dynamic exchange. Decoalescence wasreached at −40 °C, and two new broad singlets were observed,assigned to resonances of an amine-bound proton and a Mohydride. At −80 °C, these two resonances appear as a broadsinglet 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 NMRspectrum, the protonated nitrogen appeared as a broaddoublet 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 peakbetween the nitrogen resonance at −309 ppm and the protonresonance at 6.49 ppm, but not with the hydride resonance(Figure S61). 1H−1H NOESY and ROESY NMR spectra bothexhibit cross peaks between proton and hydride resonances at−80 °C, suggesting the amine-bound proton and metal hydridestill 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), bothdecreasing from 1.5 s (at −80 °C) to a minimum of 360 ms at−30 °C in THF-d8 (Figure S55). For the averagingMo−H···H−Nresonance at −0.15 ppm in fluorobenzene, the minimum T1 valueis 378 ms at −10 °C, which is much shorter than the typical T1value of Mo hydride complexes (>1 s). Thus, the observed shortT1 relaxation time indicates the presence of a Mo−H···H−Ninteraction in [PhBnMoH(NH)]+ and reveals that the rate ofproton-hydride exchange is a function of temperature. Morris andco-workers also observed the resonance of an interacting proton-hydride had a shorter T1 relaxation time than that of the metalhydride because of “T1 averaging”.

80

The spectroscopic data suggest rapid exchange of the amine-bound proton and the Mo hydride in [PhBnMoH(NH)]+ above20 °C. The dynamic NMR spectra allowed measurement of thekinetics of exchange in different solvents. The pseudo-first-order rates of exchange of the proton and hydride at 25 °C weredetermined by simulation of the variable-temperature NMRspectra as k = 3.9 × 105 s−1 (in THF) and 1.7 × 106 s−1

(in CH2Cl2) (Figure 5). Activation parameters were determinedfrom an Eyring analysis, in all cases over at least a 100° range, andare given in Table 1. The rate of exchanging proton and hydrideis five times faster in CH2Cl2 than in THF, which could resultfrom the changes in the relative acidities and/or variations inhydrogen bonding in the different solvents.The complex [PhPhMoH(NH)]+, with phenyl groups on the

amines, shows strikingly different 1H NMR spectroscopiccharacteristics. At 20 °C, a 1:2:1 triplet integrating to twoprotons was observed at −0.51 ppm in CD2Cl2 (−0.81 ppm influorobenzene) with 2JHP = 21 Hz, which is assigned as therapidly exchanging proton and hydride (Figure 6, left). In the15N-labeled complex, this resonance appears as a quartet inthe 1H NMR spectrum and as a doublet in the 1H{31P} NMRspectrum. In the 15N NMR spectrum, the 15N nucleus coupledto 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 proposethat the averagingMo−H/N−H resonance couples with 15N andthat the 1JHN is coincidentally the same as the 2JHP value of 21 Hz.Rapid proton-hydride exchange of the Mo−H and N−H

resonances was also identified by the cross peak between thenitrogen resonance and the hydride resonance at −0.81 ppm inthe 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 hydrideresonances, without reaching decoalescence in the 1H NMRspectrum. The triplet observed at 20 °C collapses into a broadsinglet at 0.38 ppm at −90 °C, with no separate proton andhydride resonances observed in the typical regions, suggestingthat exchange of proton and hydride remains extremely fast, evenat −90 °C. The Mo−H/N−H resonance shows temperature-dependent behavior, shifting downfield from−0.51 ppm at 20 °Cto 0.38 ppm at −90 °C.Addition of HD gas to a solution of [PhPhMo]+[BArF4]

− influorobenzene, 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 NMRspectroscopic 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 1HNMR spectrum appears near the typical chemical shift ofMo hydride resonances; a resonance was observed at 3.09 ppm inthe 2H NMR spectrum. The average of the 1H and 2H chemicalshifts (−0.31 ppm) is close to the value of the averaging Mo−H/N−H resonance of [PhPhMoH(NH)]+ (−0.51 ppm).81 Uponcooling a solution of the HD adduct, the chemical shift exhibitingpredominantly metal hydride character gradually shifts upfieldfrom−3.73 ppm (20 °C) to−5.72 ppm (−90 °C) in the 1HNMRspectrum, while the deuterium resonance shifts in the oppositedirection, downfield from 3.09 ppm (40 °C) to 5.97 ppm(−70 °C) in the 2H NMR spectrum (Figure S67). Thus, thedisparate 1H and 2H chemical shifts of [PhPhMoH(ND)]+ ⇄[PhPhMoD(NH)]+ result from an EIE that favors theMo−H/N−Disotopomer energetically over the Mo−D/N−H isotopomer, as aresult of the larger zero-point energy differences of the N−H/N−Dbonds compared to the Mo−D/Mo−H bonds. At lower tem-perature, the relative population of [PhPhMoH(ND)]+ increasesand that of [PhPhMoD(NH)]+ decreases, indicating that[PhPhMoH(ND)]+ is increasingly thermodynamically favored atlower temperatures. An EIE of 0.22−0.29 at−20 °C is estimated.69

Determination of the rate of reversible heterolytic cleavage ofthe H−H bond in [PhPhMoH(NH)]+ requires knowledge of thepeak separation of the two resonances that are being averaged.Since the “frozen-out” Mo−H and N−H resonances in

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

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

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

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[PhPhMoH(NH)]+ could not be observed at the lowesttemperature that the NMR spectrometer could reach, weestimated the rate of the exchange dynamics on the basis ofthe average chemical shift of Mo−H/N−H resonance (0.38 ppmat −90 °C) and the lowest measured 1H chemical shift of[PhPhMoH(ND)]+ (−5.72 ppm at −90 °C). A minimum peakseparation of the Mo−H and N−H resonances of 12.2 ppm isestimated for the “frozen out” structure of [PhPhMoH(NH)]+,which corresponds to a lower limit of the reversible heterolyticH−H bond cleavage rate as 1.4 × 104 s−1 at our lowesttemperature, −90 °C (ΔG⧧ < 7.1 kcal mol−1).82 If ΔG⧧ at 25 °Cis assumed to have the same value, the proton-hydride exchangedynamic is estimated at a rate of greater than 4.0 × 107 s−1.Complexes [

tBuR′MoH(NH)]+ with bulky, electron-donatingtBu groups on the phosphines also show rapidly exchang-ing proton and hydride. The Mo−H/N−H resonance of

[tBuBnMoH(NH)]+ shows a very broad singlet at −0.79 ppm at70 °C in THF-d8 in the 1H NMR spectrum (Figure 7). Thespectrum reached decoalescence at 25 °C; the broad singlet wasno longer observed. At−80 °C, two widely separated resonancesand were observed: a broad singlet at 6.23 ppm for N−H and adoublet of doublets at −7.27 ppm for Mo−H. These resonancesboth exhibit cross peaks at −70 °C in 1H−1H NOESY andROESY NMR spectra, suggesting slower exchange dynamics.

The rate of exchange between the proton and hydride at 25 °Cwas determined from simulation of the dynamic 1H NMRspectra 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

in [tBuR′MoH(NH)]+ leads to more rapid exchange dynamics,

as found in [PhR′MoH(NH)]+ analogues. The Mo−H/N−Hresonance of [

tBuPhMoH(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 coolingthe solution and merged into the baseline at −80 °C withoutshowing separate proton and hydride resonances, indicatingdecoalescence was reached. Variable-temperature 1H and2H NMR spectra of the HD adduct [

tBuPhMoH(ND)]+ ⇄

[tBuPhMoD(NH)]+ show dynamic exchange of H and D betweenthe amine and Mo and an EIE (see Supporting Information fordetails). Using the average chemical shift of Mo−H/N−Hresonance (−0.49 ppm at −60 °C) and the lowest measured 1H

chemical shift of [tBuPhMoH(ND)]+ (−5.03 ppm at −60 °C), a

minimum peak separation of the Mo−H and N−H resonances

was estimated as 9.08 ppm for [tBuPhMoH(NH)]+. Thus, the

lower limit of exchange rate of proton and hydride in CH2Cl2 isestimated 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 bothtemperatures. Thus, the exchange rate of proton and hydride in

Table 1. Rates and Activation Parametersa of Reversible Heterolytic H−H Bond Cleavage Dynamics

solvent ΔH⧧ (kcal mol−1) ΔS⧧ (cal K−1 mol−1) ΔG⧧ (298 K, kcal mol−1) k (298 K, s−1) pKa in CH3CN

[PhBnMoH(NH)]+THF-d8 13.2 ± 0.7 11.1 ± 2.8 9.9 ± 1.1 3.9 × 105 13.8 ± 0.2CD2Cl2 9.1 ± 0.7 0.5 ± 3.1 8.9 ± 1.2 1.7 × 106

[PhPhMoH(NH)]+ CD2Cl2 7.1b 4.0 × 107b 9.3 ± 0.1

[tBuBnMoH(NH)]+

THF-d8 12.0 ± 0.6 3.8 ± 2.3 10.9 ± 0.9 6.9 × 104 15.3 ± 0.1CD2Cl2 10.9 ± 0.7 1.5 ± 2.8 10.4 ± 1.1 1.4 × 105

[tBuPhMoH(NH)]+ CD2Cl2 8.4b 4.1 × 106b 10.7 ± 0.2

[tButBuMoH(NH)]+

THF-d8 9.1 ± 1.0 −12.9 ± 3.8 12.9 ± 1.5 2.1 × 103 17.7 ± 0.1CD2Cl2 12.7 ± 0.7 0.4 ± 2.9 12.6 ± 1.1 3.8 × 103

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

Figure 6. Variable-temperature 1H NMR spectra of [CpMo(H)(CO)-(PPh

2NPh

2H)]+ ([PhPhMoH(NH)]+, left) and [CpMo(H)(CO)-

(PtBu

2NPh

2H)]+ ([

tBuPhMoH(NH)]+, right) in CD2Cl2.

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

(PtBu

2NBn

2H)]+ ([

tBuBnMoH(NH)]+) in THF-d8.

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[tBuPhMoH(NH)]+ is greater than that of [PhBnMoH(NH)]+ butless than that of [PhPhMoH(NH)]+.We expect the rate of the analogous Mo complex bearing a

more basic amine would be slower. Indeed, variable-temperature1H NMR spectra of [

tButBuMoH(NH)]+, with electron-donatingtBu groups on the amines, shows that it has a slower proton-

hydride exchange rate than [tBuBnMoH(NH)]+. The individual

resonances for amine-bound proton at 6.05 ppm and Mohydride 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

[tBuBnMoH(NH)]+ (0 °C), and the averaging Mo−H/N−Hresonance of [

tButBuMoH(NH)]+ is not observed even at 100 °C(Figure S65). The rate of exchanging proton and hydrideat 25 °C was measured as k = 2.1 × 103 s−1 (in THF) and3.8 × 103 s−1 (in CD2Cl2), about 30 times slower than that of

[tBuBnMoH(NH)]+.The rates of proton-hydride exchange measured by

dynamic 1H NMR spectroscopy give the following order:

[PhPhMoH(NH)]+ > [tBuPhMoH(NH)]+ > [PhBnMoH(NH)]+ >

[tBuBnMoH(NH)]+ > [

tButBuMoH(NH)]+. The rate of exchangingproton and hydride in [PhPhMoH(NH)]+ is the fastest among thisseries of Mo-based heterolytic H2 cleavage products, becausethe relative acidities are very similar for the heterolytic cleavagecomplex and [PhPhMoH(NH)]+ and the dihydrogen complex[PhPhMo(H2)]

+. Complexes bearing more basic amines havelower acidity of the protonated amine, and the more electron-rich tBu on the phosphine decreases the acidity of dihydride/dihydrogen ligand in the intermediate [RR′Mo(H)2]

+. In bothcases, the higher pKa values of the protonated aminein [RR′MoH(NH)]+ and dihydride/dihydrogen ligand in[RR′Mo(H)2]

+ result in a slower rate of exchange between theproton 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 determinedrelative to 2-methylpyridine (pKa = 13.52 for the conjugate acid,2-methylpyridinium) and 1,3,5-trimethylpyridine (pKa = 14.98for the conjugate acid).74 In the 1H NMR spectra, the aromaticresonances for each substituted pyridinium and pyridinecoalesced into average resonances, and the Cp resonances for[PhBnMoH(NH)]+ and PhBnMoH also coalesced into oneresonance, indicating fast exchange. Thus, the ratio of substitutedpyridinium to pyridine for the pKa reference and the analogousratio for [PhBnMoH(NH)]+ and PhBnMoH were both determinedfrom the weighted averages of the chemical shifts. Using theseratios, the equilibrium constant Keq was determined, and a pKa of13.8 ± 0.2 was obtained from the equation in Scheme 7. ThepKa value was also verified by addition of 2,6-lutidinium (pKa =14.13) into a CD3CN solution of PhBnMoH to attain equilibriumfrom the reverse direction.

The pKa values of [PhPhMoH(NH)]+, [tBuPhMoH(NH)]+,

[tBuBnMoH(NH)]+, and [

tButBuMoH(NH)]+ in CD3CN weredetermined as 9.3± 0.1, 10.7± 0.2, 15.3± 0.1, and 17.7± 0.1 byequilibration of 4-bromoaniline (pKa = 9.43 for the conjugateacid),74 aniline (pKa = 10.62 for anilinium),74 1,3,5-trimethylpyr-idine, and benzylamine (pKa = 16.91 for the conjugate acid),74

respectively. All of these pKa data are averaged values from

multiple measurements. The pKa values (9.3−17.7) of these Mo

complexes are lower than those of [CpC5F4NFe(H)(PtBu

2NtBu

2H)]+

and [CpC5F4NFe(H2)(PtBu

2NBn

2)]+ (estimated as 20.7 and 18.3,

respectively), which presumably results from the differentelectronic structure of the metals (d4 for MoII and d6 for FeII) aswell as the additional π-acid CO ligand in the Mo complexes.67

Morris has estimated the acidity of several cationic Mo and Whydrides,3,83 and Angelici and co-workers measured the enthalpyof protonation of several classes of metal hydrides.84 Norton andco-workers reported a pKa of 5.6 in MeCN for [CpW-(H)2(CO)2(PMe3)]

+;85 few pKa values of related cationic Modihydride complexes in MeCN have been determined. The pKavalues of [RR′MoH(NH)]+ quantitatively show how changingthe electronic characteristics of the amine and phosphines tunesthe overall acidity of the heterolytic H2 cleavage products.Changing the group on the amine from phenyl to benzyl totBu significantly decreases the acidity of [

tBuR′MoH(NH)]+, with

[tButBuMoH(NH)]+ as the least acidic complex. Changing thegroup on the phosphine from phenyl ([PhBnMoH(NH)]+) to atBu group ([

tBuBnMoH(NH)]+) results in an increase of 1.5 pKa

units, because more electron-donating phosphines decrease theacidity of intermediate Mo dihydride complexes.The order of acidity of the heterolytic H2 cleavage products is

[PhPhMoH(NH)]+ > [tBuPhMoH(NH)]+ > [PhBnMoH(NH)]+ >

[tBuBnMoH(NH)]+ > [

tButBuMoH(NH)]+. This order is exactlythe same as the order of the rates of proton/hydride exchange;the acidity shows a linear correlation with the logarithm ofthe proton-hydride exchange rates (Figure 8). The most acidiccomplex [PhPhMoH(NH)]+ (pKa = 9.3 ± 0.1) gives the fastestrate of exchanging proton and hydride (>107 s−1 at 25 °C). The

least acidic complex [tButBuMoH(NH)]+ (pKa = 17.7± 0.1) is less

acidic by approximately 8 pKa units, and its proton-hydrideexchange rate is nearly 104 times slower (3.8 × 103 s−1 at 25 °C).Thus, the pKa shows an excellent correlation between the acidityand proton-hydride exchange dynamics.

■ DISCUSSIONThe cleavage of H2 and the catalytic hydrogenation ofunsaturated compounds have been extensively reported inmain group systems since the pioneering work by Stephan, Erker,and their co-workers.15−20 Heterolytic cleavage of H2 bytransition-metal based frustrated Lewis pairs have been reported.Wass and co-workers reported H2 splitting by the cationiczirconium complex [Cp*2ZrOC6H4P

tBu2]+ under mild con-

ditions.29,30 Erker and co-workers developed a cationiczirconocene-amine complex, [Cp*2Zr(OCH2CH2N

iPr2)]+, for

heterolytic cleavage of H2 and catalytic hydrogenation ofalkenes.34 The H2 cleavage in both of these frustrated Lewis

Scheme 7. Determination of the pKa of Heterolytic H2Cleavage Products

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pair systems could involve direct σ-bond cleavage between acidicand basic centers, similar to that in main group Lewis pairs.86,87

We propose that heterolytic cleavage of H2 by Mo com-plexes [RR′Mo]+ occurs through a metal−ligand cooperationmechanism38−41 involving reaction of H2 with the metal, formingthe dihydride M(H)2

+ or dihydrogen M(η2-H2)+ complexes,

followed by ligand-mediated intramolecular proton transfer togenerate a protonated amine and a metal hydride. Heterolyticcleavage of H2 by many other metal complexes also proceedsby similar mechanisms, such as iron complexes developedby Casey and Guan,7,44 and the iridium complexes withhydroxyl-substituted bipyridine ligands reported by Fujita andco-workers.88,89 A metal can also act as the base to promoteinter- or intramolecular proton transfer in H2 cleavage by somebimetallic complexes.90,91

In the broader context of addition of H2 across M−N bonds,Fryzuk and co-workers discovered early examples of theheterolytic cleavage of H2 across iridium amide bonds.92

More recently, Grutzmacher and co-workers found rhodiumamide bonds that heterolytically cleave H2.

93 Cooperative ligandreactivity was observed by Schneider and co-workers in theheterolytic cleavage of H2 in a ruthenium nitride complex,leading to hydrogenolysis of the nitride and producingammonia.94 In ruthenium95,96 and iridium97 complexes withpyridine-based pincer ligands, Milstein and co-workers usedheterolytic cleavage of H2 involving aromatization−dearomatiza-tion of the ligand as a key step in catalytic reactions exhibiting awide scope of reactivity, with many examples documenting theformation of C−H bonds on the ligand. Heterolytic cleavage ofH2 is proposed in Fe complexes,98 including Fe complexes thatcatalyze the dehydrogenation of formic acid.99

In transition-metal based FLPs and metal−ligand bifunctionalcomplexes, the reactivity with H2 can be altered by Lewis acid−base binding or by changing the electronic or steric characteristicsof the coordination environment. The ability of the coordinatedLewis acid−base to dissociate and release the “unquenched”reactive vacant coordination site is crucial for H2 binding andactivation. For example, Wass and co-workers reported that thekinetics of H2 addition to [CpCp*ZrOC6H4PR2]

+ (R = tBu,

iPr, Mes) complexes depends on the size of the sub-stituents on zirconium and the phosphine.29,30 Though[Cp*2ZrOC6H4P

tBu2]+ shows no Zr−P interaction and readily

adds H2, [Cp2ZrOC6H4PtBu2]

+, bearing the less stericallydemanding Cp ligand, has a Zr−P bond and does notreact with H2, similar to the lack of H2 addition reactivityof [CpMo(CO)(κ3-PNP)]+ because of the strong Mo−Ncoordination.In our results, the additional ring strain in [CpMo(CO)(κ3-

P2N2)]+ ([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 stericeffect of a pyridine ring in [(diphosphine)Pd(2-(diphenylphos-phino)-pyridine)]2+ complexes has a significant impact on theactivity for heterolytic cleavage of H2.

100 The Pd complex bearinga methyl on the 6-position of pyridine ring shows much higherreactivity toward H2 than that of the complex bearing anunsubstituted pyridyl group, probably because the 6-methylpromotes the dissociation of the pyridyl group. All of theseresults demonstrate that H2 cleavage reactivity in both transition-metal based FLPs and metal−ligand bifunctional complexes canbe 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 caseswere the proton-hydride exchange rates experimentally meas-ured. Szymczak and co-workers reported that the doublydeprotonated ruthenium complex heterolytically cleaves H2 viacooperation of metal and pendant oxyl groups, affording 3 as aruthenium hydride complexes bearing two phenolic hydroxygroups (Figure 9).107 Complex 3 also shows dynamic exchangeof two protons and one hydride, and its exchange rate wasmeasured by spin-saturation transfer, corresponding to an

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

Figure 8. Correlation between the pKa in CD3CN and proton/hydrideexchange rates (k) in CD2Cl2 at 25 °C. Triangles show the twocomplexes whose exchange rates were estimated from the dynamicNMR studies for the complexes formed from HD at low temperature inCD2Cl2, and squares indicate the three complexes whose exchange rateswere determined by simulation of variable-temperature 1H NMRspectra in THF-d8.

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extrapolated rate of 519 s−1 at 25 °C. This rate is about four times

lower than that of [tButBuMoH(NH)]+ in THF-d8 (2.1 × 103 s−1).

Both complex 3107 and [tButBuMoH(NH)]+ have negative entropies

of activation,−20± 1 cal K−1 mol−1 (in CD2Cl2) for 3 and−12.9±3.8 cal K−1 mol−1 (in THF-d8) for [

tButBuMoH(NH)]+. A largenegative ΔS⧧ value would lead to a significant increase in ΔG⧧ athigher temperatures.Andersen and Bergman reported Cp*2Ti(H)(SH) (4), which

was generated from heterolytic cleavage of H2 by Cp*2TiS.108

The exchange dynamics of Ti−H and S−H resonances inCp*2Ti(H)(SH) was measured by 2D EXSY 1H NMRspectroscopy, which gives an exchange rate of 1.2 s−1 at−30 °C. DuBois and co-workers observed rapid proton-hydrideexchange in [(PEtNMeHPEt)Fe(H) (dmpm) (CH3CN)]

+ (5) and[(PEtNMeHPEt)Ni(H)(PEtNMeHPEt)]+ (6) complexes, bothbearing 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 pendantamine that are readily displaced upon reaction with H2 (1 atm),similar to the Mo−N bonds reported here. The iron complex[1]+ and themanganese complex [2]+ both show extremely rapidproton-hydride exchange dynamics at temperatures as low as−80 °C, with no decoalesence observed (Figure 9).66,69 Theexchange rates at low temperature for both complexes wereestimated as >104 s−1 based on low-temperature NMR data forthe complexes formed fromHD. Rates over 107 s−1 at 25 °Cwereestimated, based on assumption that the ΔG⧧ is the same at25 °C and at −80 °C. The upper limits of the ΔG⧧ for bothcomplexes are around 7 kcal/mol. The averaging proton-hydrideresonance in the manganese complex [(PPhNMeHPPh)Mn(H)-(CO)(bppm)]+ (7), bearing a PNP ligand, reaches decoales-cence at −42 °C.70 A much slower exchange rate of 9.7 × 103 s−1

at 20 °C and a higherΔG⧧ of 11.8(8) kcal/mol at 25 °C, showingthe 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 ortwo complexes are reported; tuning the kinetics of proton-hydride exchange dynamics has not been extensively studied.Our results include the systematic measurement of thermody-namic and kinetic parameters of a series of Mo complexes asheterolytic H2 cleavage products and demonstrate how to tunethe kinetics of proton-hydride exchange dynamics.The exchange rates of proton and hydride for [PhPhMoH(NH)]+

and [tBuPhMoH(NH)]+ at −90 °C are extremely fast. We

estimated the exchange rates from the low-temperature NMRdata on complexes formed from HD. The lower limit of the ratesat 25 °C is scaled from the rates at low temperature based on theassumption that theΔG⧧ is the same at both low temperature and25 °C, which could lead to errors in the extrapolated rates. Sincethe proton-hydride exchange dynamics in [PhPhMoH(NH)]+ and

[tBuPhMoH(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

[PhBnMoH(NH)]+, [tBuBnMoH(NH)]+ and [

tButBuMoH(NH)]+

was observed at low temperature, and the averaging proton-

hydride resonances for [PhBnMoH(NH)]+ and [tBuBnMoH(NH)]+

were observed at high temperature, we accurately determined the

exchange rates of those complexes by simulation of the variable-temperature 1H NMR spectra. The trend of the decoalescencetemperature for the averaging proton and hydride from thelowest to the highest of the five Mo complexes in CD2Cl2 againindicates the exchange rates from the highest to the lowest as

[PhPhMoH(NH)]+ > [tBuPhMoH(NH)]+ > [PhBnMoH(NH)]+ >

[tBuBnMoH(NH)]+ > [

tButBuMoH(NH)]+, which corresponds tothe order of experimentally determined pKa values from thelowest to the highest.

■ CONCLUSIONSHeterolytic cleavage of H2 into a proton and a hydride wasachieved in a series of Mo bifunctional complexes, [CpMo-(CO)(κ3-PR

2NR′2)]

+ ([RR′Mo]+). The exchange dynamics ofproton and hydride in the complex showing fastest rate([PhPhMoH(NH)]+, ≥107 s−1 at 25 °C) is nearly 104 times fasterthan that of the complex with slowest rate ([

tButBuMoH(NH)]+,2.1 × 103 s−1). We determined that the pKa values of fivecomplexes [RR′MoH(NH)]+ in acetonitrile range from 9.3 to17.7, leading to a quantitative understanding of how the proton-hydride exchange dynamics are controlled by the pKa of thecomplexes. A more basic amine or a more electron-donatingphosphine leads to complexes with a lower acidity and a slowerproton-hydride exchange rate. Our investigation into the relationbetween acidity and kinetic characteristics of bifunctionalcomplexes provides design principles for the rational designFLPs for heterolytic cleavage of H2.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/jacs.7b03053.

Experimental procedures, synthesis, variable-temperature1-D and 2-D NMR spectroscopic data for all newcompounds, measurement of T1 relaxation time, and pKadeterminations by 1H NMR spectroscopy (PDF)X-ray crystallographic data (CIF)

■ AUTHOR INFORMATIONCorresponding Author*morris.bullock@pnnl.govORCIDShaoguang Zhang: 0000-0002-0931-321XAaron M. Appel: 0000-0002-5604-1253R. Morris Bullock: 0000-0001-6306-4851NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe 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 NorthwestNational 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 NMRspectra and Dr. Christopher Zall and Dr. Monte Helm for adviceon the refinement of X-ray structures.

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

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