Nature of hydrogen interactions with Ni(II)complexes containing cyclic phosphineligands with pendant nitrogen basesAaron D. Wilson*, R. K. Shoemaker*, A. Miedaner†, J. T. Muckerman‡, Daniel L. DuBois§, and M. Rakowski DuBois*¶

*Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309; §Division of Chemical Sciences, Pacific Northwest NationalLaboratory, Richland, WA 99352; †National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401; and ‡Brookhaven NationalLaboratory, P.O. Box 5000, Upton, NY 11973

Edited by John E. Bercaw, California Institute of Technology, Pasadena, CA, and approved January 8, 2007 (received for review October 9, 2006)

Studies of the role of proton relays in molecular catalysts for theelectrocatalytic production and oxidation of H2 have been carriedout. The electrochemical production of hydrogen from protonatedDMF solutions catalyzed by [Ni(P2

PhN2Ph)2(CH3CN)](BF4)2, 3a (where

P2PhN2

Ph is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane),permits a limiting value of the H2 production rate to be determined.The turnover frequency of 350 s�1 establishes that the rate of H2

production for the mononuclear nickel catalyst 3a is comparable tothose observed for Ni-Fe hydrogenase enzymes. In the electrochem-ical oxidation of hydrogen catalyzed by [Ni(P2

CyN2Bz)2](BF4)2, 3b

(where Cy is cyclohexyl and Bz is benzyl), the initial step is thereversible addition of hydrogen to 3b (Keq � 190 atm�1 at 25°C). Thehydrogen addition product exists as three nearly isoenergetic isomers4A–4C, which have been identified by a combination of one- andtwo-dimensional 1H, 31P, and 15N NMR spectroscopies as Ni(0) com-plexes with a protonated amine in each cyclic ligand. The nature ofthe isomers, together with calculations, suggests a mode of hydrogenactivation that involves a symmetrical interaction of a nickel dihy-drogen ligand with two amine bases in the diphosphine ligands.Single deprotonation of 4 by an external base results in a rearrange-ment to [HNi(P2

CyN2Bz)2](BF4), 5, and this reaction is reversed by the

addition of a proton to the nickel hydride complex. The small energydifferences associated with significantly different distributions inelectron density and protons within these molecules may contributeto their high catalytic activity.

catalysis � hydrogen oxidation � hydrogen production

The catalytic interconversion of H2 with two protons and twoelectrons plays an important role in the metabolism of

various bacteria and algae, and it is important to the futuredevelopment of hydrogen-based fuel cells and solar hydrogenproduction technologies. Recent structural studies of Fe-onlyand Ni-Fe hydrogenase enzymes have demonstrated that com-plexes of these relatively inexpensive and common metals candisplay high activities for this reaction (1–8). This has led to thehope that replacement of platinum in fuel cells for the hydrogenoxidation reaction could be achieved with simple syntheticcatalysts based on iron or nickel, and many synthetic dinucleariron complexes, developed as structural models for the enzymeactive site, have been studied to explore their fundamentalproperties and catalytic potential (9–20). Structural features ofthe Fe-only hydrogenase catalytic site are depicted in structure1 of Fig. 1, and a dihydrogen molecule is shown in the putativebinding site on the distal iron atom. It has also been proposedthat the central atom of the three atom backbone of thedithiolate bridge is a N atom, and that this amine plays a centralrole in the heterolytic cleavage reaction (1). In this processdihydrogen is split to form a hydride ligand coordinated to ironand a proton coordinated to the amine as shown in structure 2.

The structural features of the hydrogenase active site and itsproposed mechanism of operation suggest that the followingconsiderations should be important in designing molecular

catalysts for this reaction. (i) The heterolytic cleavage of H2should be at or near equilibrium to avoid high-energy interme-diates. This implies the hydride (H�) acceptor ability of themetal and the proton (H�) acceptor ability of the base must beenergetically matched to provide enough energy to drive theheterolytic cleavage of H2, but this reaction should not bestrongly exergonic. (ii) A pendant base should be incorporatedinto the second coordination sphere of the catalyst to serve as aproton relay to shuttle protons from the central metal to theexterior of the catalyst molecule. This can minimize reorgani-zation energies associated with the approach of an external basefor proton transfer. (iii) The nitrogen atom of the proton relayshould be precisely positioned to assist the heterolytic cleavageof H2 as shown in Fig. 1.

In previous studies we have attempted to sequentially incor-porate each of these features into synthetic molecular catalystsfor H2 oxidation and production (21, 22). This resulted in thesynthesis of nickel complexes with the general structure shownfor 3 (Fig. 2). Depending on the substituents on nitrogen andphosphorus, these complexes are very active catalysts for eitherH2 production {3a, [Ni(P2

PhN2Ph)2(CH3CN)](BF4)2, R � R� �

phenyl} or H2 oxidation {3b, [Ni(P2CyN2

Bz)2](BF4)2, R � cyclo-hexyl, R� � benzyl} (22). These nickel-based synthetic catalystsallow a more detailed understanding of the relationship betweenstructure and activity to be developed for this simple butimportant redox reaction.

In this article, we examine additional aspects of both thecatalytic hydrogen evolution reaction and the hydrogen oxida-tion cycle. In our previous report, our study of the rate of

Author contributions: D.L.D. and M.R.D. designed research; A.D.W. and J.T.M. performedresearch; R.K.S. and A.M. contributed new reagents/analytic tools; A.D.W., R.K.S., J.T.M.,D.L.D., and M.R.D. analyzed data; and A.D.W., D.L.D., and M.R.D. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

¶To whom correspondence should be addressed at: Department of Chemistry and Bio-chemistry, University of Colorado, 215 UCB, Boulder, CO 80309. E-mail: mary.rakowski-dubois@colorado.edu.

This article contains supporting information online at www.pnas.org/cgi/content/full/0608928104/DC1.

© 2007 by The National Academy of Sciences of the USA

Fig. 1. Structural features of the active site of Fe-only hydrogenases and theproposed activation of hydrogen.

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hydrogen production was limited by the instability of the catalystat high concentrations of trif lic acid. We now report kineticstudies of the catalytic reaction carried out in the presence of amilder acid in which the catalyst displays long-term stability.These studies permit a limiting value of the H2 production rateto be determined and establish that the rate of H2 production forthe mononuclear nickel catalyst 3a is comparable to thoseobserved for Ni-Fe hydrogenase enzymes. To obtain furtherinsights into possible structures of the doubly protonated anddoubly reduced intermediate formed during the catalytic pro-duction of H2 by 3a, we examine the initial step in the catalyticoxidation of hydrogen and report our characterization of theisomeric products formed upon the addition of hydrogen to 3b.The structures illustrate a novel pathway for hydrogen oxidation/production facilitated by multiple pendant bases positioned neara redox-active metal center. Our results suggest that both thepositioning and the number of pendant bases may be importantfor achieving high catalytic rates for H2 oxidation and productionfor these nickel complexes.

ResultsRate of Electrocatalytic Hydrogen Formation. In previous studies wehave shown that 3a catalyzes H2 production from trif lic acid,CF3SO3H, in acetonitrile solutions (22). However, at trif lic acidconcentrations above �0.1 M, the complex is unstable. Disso-ciation of the protonated diphosphine ligands is a likely mode ofdecomposition under these strongly acidic conditions. Thisbehavior prevented an accurate assessment of the intrinsicturnover frequency for 3a, but a value of 130 sec�1 could beobserved for a 0.1 M acid concentration. To achieve the optimumrate and performance of an electrocatalyst for proton reduction,the acid strength should be selected to match the pKa of thecatalyst. Although the pKa of the phenylamine base in 3a has notbeen measured directly, the pKa value can be estimated to be �7in acetonitrile on the basis of thermodynamic studies of relatedcomplexes (K. Fraze, A.D.W., M.R.D., and D.L.D., unpublisheddata). Protonated dimethylformamide in acetonitrile (pKa �6.1) (23) is a significantly weaker acid than trif lic acid in the samesolvent (pKa � 2.6) (23), and provides a good match in pKavalues with catalyst 3a. Complex 3a was found to be quite stableat high concentrations of H�-DMF/DMF in acetonitrile, wherethe pH approaches the pKa value. Less than 10% decompositionof 3a has been observed by 1H and 31P NMR spectroscopy after10 days in a mixture of 0.62 M H�-DMF and 0.62 M DMF inacetonitrile-d3.

The catalytic activity of 3a using a buffer solution of proton-ated dimethylformamide/dimethylformamide (H�-DMF/DMF,1:1) in acetonitrile was studied by cyclic voltammetry. Fig. 3shows a series of cyclic voltammograms recorded at increasingH�-DMF concentrations. The peak current (ip) associated withthe Ni(II/I) couple in the absence of acid (not resolved in initialscan at scale shown here) is much smaller than the catalyticcurrent (ic) measured in the presence of acid. A plot of the ic/ip

ratio vs. acid concentration is shown in the inset of Fig. 3. It canbe seen that as the acid concentration increases, the catalyticcurrent initially increases and then becomes independent of acidconcentration. The linear region observed at low acid concen-trations indicates a second order process in acid, and the acidindependent region indicates saturation with acid and a rate-limiting step such as H2 elimination or an intramolecular protontransfer. As described previously (22), the ic/ip ratio can be usedto calculate a turnover frequency (24–27). For the acid inde-pendent region of the plot, a limiting value of 350 s�1 has beendetermined for this system at 22°C.

Characterization of H2�[Ni(PCy2NBz

2)2](BF4)2. The preceding studiesare consistent with H2 elimination from the catalyst as therate-determining step in the production of H2, but intermediatesbefore hydrogen elimination were not directly observable byspectroscopic methods. To obtain more information on thenature of the intermediate steps, we turned to the study ofintermediates observed for the reverse reaction, H2 oxidation.We have recently reported that the Ni(II) complex[Ni(PCy

2NBz2)2](BF4)2, 3b, serves as a catalyst for the electro-

chemical oxidation of hydrogen in the presence of a base (22).The proposed catalytic cycle for this complex together withintermediate species to be discussed in this article are summa-rized in Scheme 1.

When one atmosphere of hydrogen is added to an acetone oracetonitrile solution of 3b in the absence of an external base,spectroscopic data support the formation of a hydrogen additionproduct, 4. The hydrogen addition is reversible and an equilib-rium constant of 190 � 20 atm�1 at 21°C in acetonitrile has beenmeasured (22).

The 31P NMR spectrum of the hydrogen addition product, 4,indicates that a mixture of isomers is present, but the positionsof the added hydrogens were not initially determined. Althoughthe product isomers are not stable enough for isolation, it hasbeen possible in further studies of this system to obtain detailedinformation about the nature of the isomers by heteronuclearand two-dimensional NMR techniques.

The 31P NMR spectrum of the hydrogen addition product inacetone-d6, shown in Fig. 4a, shows both broad and sharpresonances. When this NMR solution is cooled to �70°C, thepeaks become sharper, Fig. 4b, and resonances for severalisomers are observed. The spectrum includes two AB patternscentered at 19.5 and �7.5 ppm and two singlets at 16.6 and �10.2ppm. In the 1H NMR spectrum of this mixture recorded at

Fig. 2. Structure 3.

Fig. 3. Cyclic voltammograms of a 3.2 � 10�4 M solution of[Ni(PPh

2NPh2)2(CH3CN)](BF4)2, 3a, with increasing concentrations of H�-

DMF(OTF)/DMF (1:1) in acetonitrile. Conditions were as follows: scan rate of 50mV/s, acetonitrile solvent, 0.3 M NEt4BF4 as supporting electrolyte, glassycarbon working electrode. Inset shows values of ic/ip vs. the concentration ofthe buffer H�-DMF(OTF)/DMF in acetonitrile.

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�70°C, no nickel hydride resonances are observed at character-istic upfield shifts. Although the resonances for the P2N2 ligandsobscure many regions of the proton spectrum, new broad singletsare observed at 6.8 ppm and near 15 ppm.

Synthesis and Characterization of a Single Isomer, 4A. Furtherinterpretation of these data is facilitated by the observation thatwhen an acetone-d6 solution of 3b is first cooled to �70°C andthen reacted with hydrogen, as described in the Experimental,only one isomer of the hydrogen addition product is formed. Thisisomer 4A displays the AB pattern in the 31P NMR spectrum at22.4 and 17.2 ppm with JP-P � 34 Hz. Simulation of the spectrumhas established that the pattern is actually consistent with theexpected AA�BB� spin system and a listing of coupling constantsis given in supporting information (SI) Table 1. Once again nonickel hydride resonances are detected in the upfield region ofthe 1H NMR spectrum of 4A, but the broad singlet at 6.8 ppmis observed in this spectrum.

Additional experiments in which HD and D2 are added toprecooled solutions of 3a at �70°C provide more information onthis system. The addition of deuterium resulted in the formationof the single isomer 4A(D2). The 31P NMR shows a similar ABpattern as 4A(H2), but the two doublets are shifted to slightlylower chemical shifts of 21.8 and 16.9 ppm (see Fig. 5a for H2 and5b for D2). When HD is used instead of D2, the 31P spectrum ofthe product shows two AB patterns for phosphorus chemical

shifts influenced by both hydrogen and deuterium (Fig. 5c). Theobservation of two AB patterns indicates that rapid HD ex-change is not occurring in this system at �70°C, as this shouldlead to one averaged AB spectrum. Consequently, the absenceof a nickel hydride signal in the 1H NMR does not appear to bea result of Ni-H/N-H exchange. The resonance at 6.8 ppm in the1H NMR spectrum of 4A(H2) decreases to about half its intensityin the spectrum of 4A(HD) and disappears in the spectrum of4A(D2) (SI Fig. 8 a–c). The chemical shift of this resonance isconsistent with an NH functional group.

On the basis of these data, 4A is proposed to be a tetrahedralNi(0) complex with a protonated amine in each cyclic ligand, asshown in Scheme 1. The proposed structure is symmetric alonga C2 axis bisecting the PB-Ni-PB� and PA-Ni-PA� angles and isconsistent with both the 31P and 1H NMR data. To provideadditional support for this structure, the 15N labeled nickelcomplex was synthesized by using a labeled form of the ligandPCy

2NBz2 prepared from 15N-benzylamine. The 15N NMR spec-

trum of the hydrogen addition product formed at �70°C showstwo resonances of about equal intensity at 74.7 and 43.5 ppm.The chemical shifts are consistent with assignment of theresonances to quaternary ammonium and tertiary amine nitro-gens, respectively (28). A two-dimensional 1H/15N NMR [gra-dient heteronuclear single quantum coherence (gHSQC)] ex-periment (SI Fig. 9) established that the resonance at 74.7 ppmis coupled to a proton resonance near 6.8 ppm, and in the 1HNMR spectrum, this resonance is observed as a doublet with1JNH � 71.6 Hz, SI Fig. 8d. No proton coupling was observed inthe gradient heteronuclear single quantum coherence spectrumfor the 15N resonance at 43.5 ppm. The combined spectroscopicdata are all consistent with the proposed structure of 4A. It ispossible that in this structure the Ni(0) center is further stabi-lized by N-H–Ni(0) interactions. We have no direct spectro-scopic evidence for this, but the relative 31P NMR chemical shiftsare suggestive of such interactions, as discussed in the nextsection.

Characterization of Additional Isomers. As the solution of 4A isslowly warmed above �70°C, resonances for additional isomersare observed in the 31P NMR spectrum. The isomers are allsimilar in energy and are also assigned as diprotonated Ni(0)complexes, related to 4A by proton transfers, conformationalchanges of the ligand chelate rings, and/or inversions at theamine nitrogens. The heteronuclear NMR data, discussed below,provide support for the structures shown below for 4B and 4C inFig. 6.

As the solution is warmed, in addition to the resonances of 4A,the AB pattern near �7 ppm and the singlet at 17.4 ppm are also

Scheme 1

Fig. 4. 400-MHz 31P NMR spectra of 4 formed at room temperature inacetone-d6 (a) and the same sample solution cooled to �80°C (b).

Fig. 5. 31P NMR spectra of 4A(H2) (a), 4A(D2) (b), and 4A(HD) (c) recorded at400 MHz in acetone-d6 at �70°C.

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observed in the 31P NMR spectrum. In acetone-d6 the integra-tions for these two resonances are very close to 1:1, suggestingthat they correspond to a single compound, 4B. This conclusionis also supported by the fact that the relative integrations forthese two resonances remained relatively constant when hydro-gen addition was carried out in acetonitrile, dichloromethane,and dimethylformamide at room temperature, whereas ratios ofisomers 4A and 4C varied significantly. The large difference inthe 31P NMR chemical shifts of the two ligands in isomer 4Bprovides support for a N-H–Ni interaction involving the ligandchelate in the boat conformation. Such an interaction results inthe formation of two five-membered rings, and such ring systemsare known to result in significant downfield shifts of the 31PNMR resonances compared with those in six-membered ringsystems (29).

The 15N NMR spectrum for 4B in acetone-d6 shows tworesonances that are very close in chemical shifts to those of 4A,as summarized in SI Table 2. In addition, a third 15N resonanceis observed at 47.0 ppm. In the 1H NMR spectrum at �70°C,singlets at 7.0 and 14.6 ppm are assigned to the N-H protons inthe two inequivalent ligands. For the 15N-labeled complex, thesplitting of the proton resonance near 7 ppm is obscured byphenyl resonances, but the downfield resonance is split into atriplet at 15.4 ppm with a coupling constant of 31 Hz. Thecoupling between the 1H and 15N resonances is confirmed by thegradient heteronuclear single quantum coherence spectrum.Similar downfield 1H chemical shifts and JN-H values have beenobserved previously for the proton stabilized by two amines inprotonated Proton Sponge [1,8-bis(dimethylamino)naphtha-lene] and related derivatives (30).

The last isomer to form as the solution is warmed to roomtemperature is 4C, which shows a singlet in the 31P NMRspectrum at �9.2 ppm. The 1H and 15N NMR data, summarizedin SI Table 2, are consistent with a tetrahedral structure with D2dsymmetry in which each proton is stabilized by two amines of thechelate rings in chair conformations as shown in Fig. 6.

The broad resonances observed at 19.5 and �7.5 ppm in theroom temperature 31P NMR spectrum shown in Fig. 4a indicatethat exchange processes are occurring for isomers 4A and 4B. Inaddition, the coupling of a proton to two 15N nuclei in isomers4B and 4C may indicate that each NH proton is exchangingrapidly between a single pair of 15N atoms, or it could indicatestructures in which the proton symmetrically bridges the two 15Natoms. Although further information regarding the nature andmechanism of these exchange processes is certainly of interest,it is beyond the scope of the present work.

Deprotonation of 4. The isomers of 4 are rapidly converted to theNi(II) hydride [HNi(PCy

2NBz2)2](BF4), 5, upon deprotonation

with triethylamine in acetonitrile, as shown by the bottomreaction in Scheme 1. The deprotonation reaction is reversible,and the hydride complex 5 is converted to the doubly protonatedNi(0) derivatives, 4A–4C, upon reaction with tetrafluoroboricacid or anisidinium tetrafluoroborate. This equilibrium repre-sents an unusual pH-dependent intramolecular redox event inwhich protonation leads to a formal two-electron reduction of

the metal while deprotonation results in a two-electron metaloxidation. We propose that protonation of one ligand leads to asignificant decrease in electron density at nickel. As a result thenickel hydride becomes more acidic and the hydrogen is trans-ferred as a proton to an adjacent base, forming the isomers of theNi(0) product. Consistent with this interpretation is the obser-vation that the cyclic voltammogram for 4 in acetonitrile showsa significant anodic shift in potentials relative to 3b. The cyclicvoltammogram for the isomers of 4 shows two irreversible waveswith peak potentials at 0.0 and �0.4 V vs. ferrocene, whereas thereduction potentials for the Ni(II/I) and (I/0) couples for 3b areobserved at �0.80 and �1.28 V (22). Although the waves for 4are irreversible, they indicate that large changes in electrondensity occur at nickel as a result of protonating the nitrogenatoms of the diphosphine ligands.

DiscussionComplex 3a is an exceptionally effective catalyst for the elec-trochemical production of hydrogen, displaying high rates andlong lifetimes. The turnover frequency of 350 s�1, determined at22°C, is comparable to the catalytic rates of 500–700 s�1 reportedfor H2 production at 30°C for Ni-Fe hydrogenases (31, 32). At thehigher acid concentrations shown in Fig. 3, H2 elimination or anintramolecular proton transfer appears to be the rate-limitingstep in the catalytic cycle. Similarly, hydrogen addition appearsto be the rate-determining step in the catalytic oxidation of H2

by 3b. In the latter case, oxidation of H2 occurs readily within thecoordination sphere of catalyst 3b to give an isomeric series ofNi(0) products containing a protonated nitrogen atom in each ofthe cyclic ligands. The nature of the first observable intermedi-ate, 4A, presents a surprising contrast to that observed previouslyfor [Ni(PNP)2]2�, 6, where PNP is Et2PCH2N(Me)CH2PEt2

(21). In the latter case, an intramolecular heterolytic cleavage ofH2 occurs when 6 reacts with H2 to form [HNi(PNHP)(PNP)]2�,7, as shown in Eq. 1.

[Ni(PNP)2](BF4)2

6� H23 �HNi(PNHP)(PNP)](BF42

7

[1]

The differences in structures 4 and 7 demonstrate that seeminglysubtle differences in the diphosphine ligand structure thatinvolve positioning of the nitrogen bases can result in signifi-cantly different electron distributions during hydrogen addition.

In our spectroscopic studies that led to the identification of 4and 7, no evidence for the initial interaction with hydrogen toform a nickel-dihydrogen intermediate has been obtained foreither system. However, DFT calculations on model complexesof both [Ni(PNP)2]2� and [Ni(P2

RN2R�)2]2� derivatives (where

phosphine and nitrogen substituents are replaced with hydro-gens) have indicated that an initial intermediate is a Ni(II)dihydrogen complex. In the case of the PNP complex, thedihydrogen ligand is somewhat unsymmetrically coordinatedwith one hydrogen showing a closer approach to the pendantamine of the ligand (22). In the case of the P2N2 complex, acompletely symmetrical dihydrogen complex, as shown inScheme 1, is observed computationally as the initial intermedi-ate. This dihydrogen intermediate lies 2.1 kcal/mol above theenergy of the reactants, whereas the corresponding dihydride lies15.0 kcal/mol higher in energy. The Ni(0) complex with twoprotonated nitrogen atoms analogous to 4 has a calculatedenergy of �2.5 kcal/mol with respect to the reactants in com-parison with an experimental free energy of �3.1 kcal/molobserved for H2 addition to 3b.

Fig. 6. Proposed structures for isomers 4B and 4C.

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The heterolytic cleavage of H2 is generally thought to occur by anasymmetric polarization of a dihydrogen molecule to form an H�

and an H� species. The heterolytic cleavage of H2 by 6 to form 7shown in Eq. 1 is an example of such a reaction. A transition statefor this reaction has been calculated [structure 8 (Fig. 7)] in whichthe H-H bond of the coordinated dihydrogen ligand is weakened asthe new bonds with the proton and hydride acceptor sites areformed (22). The observation that 4A is the first detectable inter-mediate in the addition of H2 to 3b is interesting because it suggeststhat a distinctly different mechanism for H2 activation may occur forthis complex. A novel feature of 3b is that two amine nitrogens areheld in positions that could allow for simultaneous interaction ofboth bases with the incoming H2 molecule, as shown by theproposed dihydrogen intermediate in Scheme 1. If such a symmetrictransition state occurs, then the heterolytic cleavage achieved by 3bwould not involve a H2 molecule that is strongly polarized as H�H�.Instead the polarization would occur by the removal of twoelectrons by nickel from a symmetric transition state in which bothof the hydrogen atoms gradually develop a more positive charge.Similarly, the production of H2 by complex 3a may involve thereverse process. The proposed symmetric mechanism of H2 oxida-tion/production implies that cooperative interactions of the dihy-drogen ligand with both the metal center and multiple proton relaysincorporated in the second coordination sphere contribute to thehigh activity observed for these nickel-based molecular catalysts.

Summary. Complex 3a is an extremely effective catalyst for theelectrochemical production of hydrogen with a turnover fre-quency (350 s�1) that is comparable to that of the nickel-ironhydrogenase enzymes. Kinetic studies indicate that the rate-determining step involves a doubly protonated and doublyreduced intermediate. To provide insight into possible structuresof this intermediate, H2 addition to 3b has been studied.Complex 3b oxidizes H2 to form a mixture of Ni(0) products inwhich an amine in each of the cyclic ligands is protonated. Threenearly isoenergetic Ni(0) isomers, 4A–4C, are suggested on thebasis of heteronuclear and two-dimensional NMR techniques.The structure of the first intermediate 4A suggests that both ofthe pendant nitrogen atoms in 3b may simultaneously interact ina symmetric manner with H2. This may be important forstabilizing the dihydrogen intermediate and for the subsequentcleaving of H2 with a concerted reduction of Ni(II) to Ni(0). Thecontrast between these products and the closely related PNPstructure 7, which contains both a nickel hydride and a proton-ated nitrogen atom, demonstrates that subtle differences in thediphosphine ligand structure can result in very different protonand electron distributions. The cooperative interactions that leadto a delicate balance in the distribution of protons and electronsbetween nickel and the pendant bases of the second coordina-tion sphere are believed to be important factors in the highcatalytic activity of these systems.

Materials and MethodsComplexes 3a and 3b were synthesized according to a publishedprocedure (22). 15N (95%) labeled benzylamine was purchasedfrom Aldrich. NMR spectra were recorded on a Varian Inova400-MHz spectrometer, operating at 400.159 MHz for 1H ob-servation. 1H NMR chemical shifts are reported relative totetramethylsilane using residual solvent protons as a secondaryreference. 31P chemical shifts are reported relative to externalphosphoric acid, and 15N chemical shifts are reported relative toexternal ammonia, referenced indirectly to the frequency of thedeuterated solvent. Two-dimensional 1H-15N field gradient het-eronuclear single quantum coherence experiments were ac-quired in pure-phase mode with broadband 15N decouplingduring detection by using pulsed-field gradients for coherenceselection, optimized for an average 1H-15N heteronuclear cou-pling of 80 Hz. Reported sample temperatures for low-temperature experiments were calibrated by using a standard100% methanol sample and calculated by using utilities providedin the VNMR 6.1C software (Varian). One-dimensional 15NNMR spectra were acquired with continuous broadband irradi-ation of the protons to provide sensitivity enhancement fromboth the NOE interaction and from decoupling JNH.

Cyclic voltammetry experiments were carried out on a CypressSystems computer-aided electrolysis system under an N2 or H2

atmosphere on acetonitrile solutions containing 0.3 M Bu4NBF4.The working electrode was a glassy carbon disk, and the counterelectrode was a glassy carbon rod. A silver wire was used as apseudoreference electrode. Ferrocene was used as an internalstandard, and all potentials are referenced to the ferrocene/ferrocenium couple. To determine the rate of proton reduction by3a, cyclic voltammograms were recorded at 50 mV/s as aliquots ofa 1:1 buffer solution of [H�-DMF]OTF/DMF were added (up to�0.3 M) to 3a (3.2 � 10�4 M) in acetonitrile. The values of ic/ip vs.[H�-DMF] were plotted as shown in Fig. 3.

Synthesis of PCy215NBz

2. A procedure slightly modified from theligand synthesis reported previously was used. A 250-mlSchlenk f lask was charged with cylohexylphosphine (2.32 g,0.02 mol), fresh paraformaldehyde (1.21 g, 0.04 mol), anddegassed ethanol (100 ml). The resulting suspension wasimmersed in a hot oil bath (80°C) and stirred for 15 minresulting in a clear solution. Under magnetic stirring a solutionof 15N-benzylamine (2.2 ml, 0.02 mol) in ethanol (30 ml) wasadded dropwise to the hot solution over a period of 60 min.The reaction mixture turned slightly cloudy when 25 ml of thebenzylamine solution had been added. Completion of theaddition and further heating resulted in a clear solution thatwas heated overnight at 75°C to form a white precipitate. Thesolution was cooled to room temperature, solvent volume wasreduced on a vacuum line to �50 ml, and product was filteredvia a cannula stick and dried on a vacuum line. The yield was2.75 g, 55%. Reducing the volume of the filtrate solution gaveanother fraction of product. 31P NMR(CDCl3): 41 ppm (s).

Low-Temperature NMR Studies. A solution of [Ni(PCy2NBz

2)2](BF4)2 (15 mg, 0.012 mmol) in actone-d6 was cooled to �78°C,and H2 (3 ml, 0.11 mmol) was added through a gas-tight syringe.The solution was shaken and allowed to warm to approximately�10°C until it reacted. After the solution changed from purpleto colorless, it was cooled again to �78°C. Within minutes, thesolution was monitored by 31P NMR at �70°C.

DFT Calculations. The all-electron DFT calculations were carriedout with the Gaussian 03 program by using the 6–31G(d,p) basisand the hybrid B3LYP method.

Fig. 7. Structure 8.

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A.D.W. thanks Prof. G. Girolami for helpful insights and discussions.This work was supported by National Science Foundation Grant CHE-0240106. D.L.D. acknowledges the support of the Chemical Sciences

Program of the Office of Basic Energy Sciences of the U.S. Departmentof Energy. The Pacific Northwest National Laboratory is operated byBattelle for the U.S. Department of Energy.

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