Nature of interactions with Ni(II) SPECIAL FEATURE complexes containing cyclic ligands with pendant bases

Aaron 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 National Laboratory, Richland, WA 99352; †National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401; and ‡Brookhaven National Laboratory, 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 relays in molecular catalysts for the electrocatalytic production and oxidation of H2 have been carried out. The electrochemical production of hydrogen from protonated Ph Ph DMF solutions catalyzed by [Ni(P2 N2 )2(CH3CN)](BF4)2, 3a (where Ph Ph P2 N2 is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H2 production rate to be determined. ؊1 The turnover frequency of 350 s establishes that the rate of H2 production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase . In the electrochem- Fig. 1. Structural features of the of Fe-only hydrogenases and the Cy Bz proposed activation of hydrogen. ical oxidation of hydrogen catalyzed by [Ni(P2 N2 )2](BF4)2,3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the ؊1 atm at 25°C). The 190 ؍ reversible addition of hydrogen to 3b (Keq catalysts for this reaction. (i) The heterolytic cleavage of H2 hydrogen addition product exists as three nearly isoenergetic isomers CHEMISTRY should be at or near equilibrium to avoid high-energy interme- 4A–4C, which have been identified by a combination of one- and diates. This implies the hydride (HϪ) acceptor ability of the two-dimensional 1H, 31P, and 15N NMR spectroscopies as Ni(0) com- metal and the proton (Hϩ) acceptor ability of the must be plexes with a protonated amine in each cyclic ligand. The nature of energetically matched to provide enough energy to drive the the isomers, together with calculations, suggests a mode of hydrogen heterolytic cleavage of H , but this reaction should not be activation that involves a symmetrical interaction of a nickel dihy- 2 strongly exergonic. (ii) A pendant base should be incorporated drogen ligand with two amine bases in the diphosphine ligands. Single of 4 by an external base results in a rearrange- into the second coordination sphere of the catalyst to serve as a Cy Bz proton relay to shuttle from the central metal to the ment to [HNi(P2 N2 )2](BF4), 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy exterior of the catalyst . This can minimize reorgani- differences associated with significantly different distributions in zation energies associated with the approach of an external base density and protons within these may contribute for proton transfer. (iii) The nitrogen atom of the proton relay to their high catalytic activity. should be precisely positioned to assist the heterolytic cleavage of H2 as shown in Fig. 1. catalysis ͉ hydrogen oxidation ͉ hydrogen production In previous studies we have attempted to sequentially incor- porate each of these features into synthetic molecular catalysts for H oxidation and production (21, 22). This resulted in the he catalytic interconversion of H with two protons and two 2 2 synthesis of nickel complexes with the general structure shown plays an important role in the metabolism of T 3 various bacteria and algae, and it is important to the future for (Fig. 2). Depending on the on nitrogen and , these complexes are very active catalysts for either development of hydrogen-based cells and solar hydrogen Ph Ph H2 production {3a, [Ni(P2 N2 )2(CH3CN)](BF4)2,Rϭ RЈϭ production technologies. Recent structural studies of Fe-only Cy Bz phenyl} or H2 oxidation {3b, [Ni(P2 N2 )2](BF4)2,Rϭ cyclo- and Ni-Fe hydrogenase enzymes have demonstrated that com- Јϭ plexes of these relatively inexpensive and common metals can hexyl, R benzyl} (22). These nickel-based synthetic catalysts display high activities for this reaction (1–8). This has led to the allow a more detailed understanding of the relationship between hope that replacement of platinum in fuel cells for the hydrogen structure and activity to be developed for this simple but oxidation reaction could be achieved with simple synthetic important redox reaction. catalysts based on iron or nickel, and many synthetic dinuclear In this article, we examine additional aspects of both the iron complexes, developed as structural models for the catalytic hydrogen evolution reaction and the hydrogen oxida- active site, have been studied to explore their fundamental tion cycle. In our previous report, our study of the rate of properties and catalytic potential (9–20). Structural features of the Fe-only hydrogenase catalytic site are depicted in structure Author contributions: D.L.D. and M.R.D. designed research; A.D.W. and J.T.M. performed 1 of Fig. 1, and a dihydrogen molecule is shown in the putative research; R.K.S. and A.M. contributed new reagents/analytic tools; A.D.W., R.K.S., J.T.M., binding site on the distal iron atom. It has also been proposed D.L.D., and M.R.D. analyzed data; and A.D.W., D.L.D., and M.R.D. wrote the paper. that the central atom of the three atom backbone of the The authors declare no conflict of interest. dithiolate bridge is a N atom, and that this amine plays a central This article is a PNAS Direct Submission. role in the heterolytic cleavage reaction (1). In this process ¶To whom correspondence should be addressed at: Department of Chemistry and Bio- dihydrogen is split to form a hydride ligand coordinated to iron chemistry, University of Colorado, 215 UCB, Boulder, CO 80309. E-mail: mary.rakowski- and a proton coordinated to the amine as shown in structure 2. [email protected]. The structural features of the hydrogenase active site and its This article contains supporting information online at www.pnas.org/cgi/content/full/ proposed mechanism of operation suggest that the following 0608928104/DC1. considerations should be important in designing molecular © 2007 by The National Academy of Sciences of the USA

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hydrogen production was limited by the instability of the catalyst at high concentrations of triflic acid. We now report kinetic Fig. 3. Cyclic voltammograms of a 3.2 ϫ 10Ϫ4 M solution of Ph Ph ϩ studies of the catalytic reaction carried out in the presence of a [Ni(P 2N 2)2(CH3CN)](BF4)2, 3a, with increasing concentrations of H - milder acid in which the catalyst displays long-term stability. DMF(OTF)/DMF (1:1) in acetonitrile. Conditions were as follows: scan rate of 50 mV/s, acetonitrile solvent, 0.3 M NEt4BF4 as supporting electrolyte, glassy These studies permit a limiting value of the H2 production rate to be determined and establish that the rate of H production for working electrode. Inset shows values of ic/ip vs. the concentration of 2 the buffer Hϩ-DMF(OTF)/DMF in acetonitrile. the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. To obtain further insights into possible structures of the doubly protonated and ratio vs. acid concentration is shown in the inset of Fig. 3. It can doubly reduced intermediate formed during the catalytic pro- be seen that as the acid concentration increases, the catalytic duction of H2 by 3a, we examine the initial step in the catalytic current initially increases and then becomes independent of acid oxidation of hydrogen and report our characterization of the concentration. The linear region observed at low acid concen- isomeric products formed upon the addition of hydrogen to 3b. trations indicates a second order process in acid, and the acid The structures illustrate a novel pathway for hydrogen oxidation/ independent region indicates saturation with acid and a rate- production facilitated by multiple pendant bases positioned near limiting step such as H2 elimination or an intramolecular proton a redox-active metal center. Our results suggest that both the transfer. As described previously (22), the ic/ip ratio can be used positioning and the number of pendant bases may be important to calculate a turnover frequency (24–27). For the acid inde- Ϫ for achieving high catalytic rates for H2 oxidation and production pendent region of the plot, a limiting value of 350 s 1 has been for these nickel complexes. determined for this system at 22°C.

Results Cy Bz Characterization of H2⅐[Ni(P 2N 2)2](BF4)2. The preceding studies Rate of Electrocatalytic Hydrogen Formation. In previous studies we are consistent with H2 elimination from the catalyst as the have shown that 3a catalyzes H2 production from triflic acid, rate-determining step in the production of H2, but intermediates CF3SO3H, in acetonitrile solutions (22). However, at triflic acid before hydrogen elimination were not directly observable by concentrations above Ϸ0.1 M, the complex is unstable. Disso- spectroscopic methods. To obtain more information on the ciation of the protonated diphosphine ligands is a likely mode of nature of the intermediate steps, we turned to the study of decomposition under these strongly acidic conditions. This intermediates observed for the reverse reaction, H2 oxidation. behavior prevented an accurate assessment of the intrinsic We have recently reported that the Ni(II) complex Ϫ1 Cy Bz turnover frequency for 3a, but a value of 130 sec could be [Ni(P 2N 2)2](BF4)2, 3b, serves as a catalyst for the electro- observed for a 0.1 M acid concentration. To achieve the optimum chemical oxidation of hydrogen in the presence of a base (22). rate and performance of an electrocatalyst for proton reduction, The proposed catalytic cycle for this complex together with the should be selected to match the pKa of the intermediate species to be discussed in this article are summa- catalyst. Although the pKa of the phenylamine base in 3a has not rized in Scheme 1. been measured directly, the pKa value can be estimated to be Ϸ7 When one atmosphere of hydrogen is added to an acetone or in acetonitrile on the basis of thermodynamic studies of related acetonitrile solution of 3b in the absence of an external base, complexes (K. Fraze, A.D.W., M.R.D., and D.L.D., unpublished spectroscopic data support the formation of a hydrogen addition data). Protonated dimethylformamide in acetonitrile (pKa ϭ product, 4. The hydrogen addition is reversible and an equilib- 6.1) (23) is a significantly weaker acid than triflic acid in the same rium constant of 190 Ϯ 20 atmϪ1 at 21°C in acetonitrile has been solvent (pKa ϭ 2.6) (23), and provides a good match in pKa measured (22). values with catalyst 3a. Complex 3a was found to be quite stable The 31P NMR spectrum of the hydrogen addition product, 4, at high concentrations of Hϩ-DMF/DMF in acetonitrile, where indicates that a mixture of isomers is present, but the positions the pH approaches the pKa value. Less than 10% decomposition of the added were not initially determined. Although of 3a has been observed by 1H and 31P NMR spectroscopy after the product isomers are not stable enough for isolation, it has 10 days in a mixture of 0.62 M Hϩ-DMF and 0.62 M DMF in been possible in further studies of this system to obtain detailed acetonitrile-d3. information about the nature of the isomers by heteronuclear The catalytic activity of 3a using a buffer solution of proton- and two-dimensional NMR techniques. ated dimethylformamide/dimethylformamide (Hϩ-DMF/DMF, The 31P NMR spectrum of the hydrogen addition product in 1:1) in acetonitrile was studied by cyclic voltammetry. Fig. 3 acetone-d6, shown in Fig. 4a, shows both broad and sharp shows a series of cyclic voltammograms recorded at increasing resonances. When this NMR solution is cooled to Ϫ70°C, the ϩ H -DMF concentrations. The peak current (ip) associated with peaks become sharper, Fig. 4b, and resonances for several the Ni(II/I) couple in the absence of acid (not resolved in initial isomers are observed. The spectrum includes two AB patterns scan at scale shown here) is much smaller than the catalytic centered at 19.5 and Ϫ7.5 ppm and two singlets at 16.6 and Ϫ10.2 1 current (ic) measured in the presence of acid. A plot of the ic/ip ppm. In the H NMR spectrum of this mixture recorded at

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31 Fig. 5. P NMR spectra of 4A(H2) (a), 4A(D2) (b), and 4A(HD) (c) recorded at 400 MHz in acetone-d6 at Ϫ70°C.

Scheme 1 shifts influenced by both hydrogen and (Fig. 5c). The observation of two AB patterns indicates that rapid HD ex- change is not occurring in this system at Ϫ70°C, as this should Ϫ70°C, no nickel hydride resonances are observed at character- to one averaged AB spectrum. Consequently, the absence istic upfield shifts. Although the resonances for the P2N2 ligands of a nickel hydride signal in the 1H NMR does not appear to be obscure many regions of the proton spectrum, new broad singlets a result of Ni-H/N-H exchange. The resonance at 6.8 ppm in the 1 are observed at 6.8 ppm and near 15 ppm. H NMR spectrum of 4A(H2) decreases to about half its intensity

in the spectrum of 4A(HD) and disappears in the spectrum of CHEMISTRY Synthesis and Characterization of a Single Isomer, 4A. Further 4A(D2) (SI Fig. 8 a–c). The chemical shift of this resonance is interpretation of these data is facilitated by the observation that consistent with an NH functional . Ϫ when an acetone-d6 solution of 3b is first cooled to 70°C and On the basis of these data, 4A is proposed to be a tetrahedral then reacted with hydrogen, as described in the Experimental, Ni(0) complex with a protonated amine in each cyclic ligand, as only one isomer of the hydrogen addition product is formed. This shown in Scheme 1. The proposed structure is symmetric along isomer 4A displays the AB pattern in the 31P NMR spectrum at a C2 axis bisecting the PB-Ni-PBЈ and PA-Ni-PAЈ angles and is ϭ 31 1 22.4 and 17.2 ppm with JP-P 34 Hz. Simulation of the spectrum consistent with both the P and H NMR data. To provide has established that the pattern is actually consistent with the additional support for this structure, the 15N labeled nickel expected AAЈBBЈ spin system and a listing of coupling constants complex was synthesized by using a labeled form of the ligand Cy Bz 15 15 is given in supporting information (SI) Table 1. Once again no P 2N 2 prepared from N-benzylamine. The N NMR spec- nickel hydride resonances are detected in the upfield region of trum of the hydrogen addition product formed at Ϫ70°C shows the 1H NMR spectrum of 4A, but the broad singlet at 6.8 ppm two resonances of about equal intensity at 74.7 and 43.5 ppm. is observed in this spectrum. The chemical shifts are consistent with assignment of the Additional experiments in which HD and D2 are added to resonances to quaternary ammonium and tertiary amine nitro- precooled solutions of 3a at Ϫ70°C provide more information on gens, respectively (28). A two-dimensional 1H/15N NMR [gra- this system. The addition of deuterium resulted in the formation dient heteronuclear single quantum coherence (gHSQC)] ex- 31 of the single isomer 4A(D2). The P NMR shows a similar AB periment (SI Fig. 9) established that the resonance at 74.7 ppm 1 pattern as 4A(H2), but the two doublets are shifted to slightly is coupled to a proton resonance near 6.8 ppm, and in the H lower chemical shifts of 21.8 and 16.9 ppm (see Fig. 5a for H2 and NMR spectrum, this resonance is observed as a doublet with 31 1 5b for D2). When HD is used instead of D2, the P spectrum of JNH ϭ 71.6 Hz, SI Fig. 8d. No proton coupling was observed in the product shows two AB patterns for phosphorus chemical the gradient heteronuclear single quantum coherence spectrum for the 15N resonance at 43.5 ppm. The combined spectroscopic data are all consistent with the proposed structure of 4A.Itis possible 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 shifts are suggestive of such interactions, as discussed in the next section.

Characterization of Additional Isomers. As the solution of 4A is slowly warmed above Ϫ70°C, resonances for additional isomers are observed in the 31P NMR spectrum. The isomers are all similar in energy and are also assigned as diprotonated Ni(0) complexes, related to 4A by proton transfers, conformational changes of the ligand chelate rings, and/or inversions at the amine . The heteronuclear NMR data, discussed below, provide support for the structures shown below for 4B and 4C in Fig. 6. Fig. 4. 400-MHz 31P NMR spectra of 4 formed at room temperature in As the solution is warmed, in addition to the resonances of 4A, acetone-d6 (a) and the same sample solution cooled to Ϫ80°C (b). the AB pattern near Ϫ7 ppm and the singlet at 17.4 ppm are also

Wilson et al. PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6953 Downloaded by guest on September 23, 2021 the metal while deprotonation results in a two-electron metal oxidation. We propose that protonation of one ligand to a significant decrease in electron density at nickel. As a result the nickel hydride becomes more acidic and the hydrogen is trans- ferred as a proton to an adjacent base, forming the isomers of the Ni(0) product. Consistent with this interpretation is the obser- vation that the cyclic voltammogram for 4 in acetonitrile shows a significant anodic shift in potentials relative to 3b. The cyclic Fig. 6. Proposed structures for isomers 4B and 4C. voltammogram for the isomers of 4 shows two irreversible waves with peak potentials at 0.0 and Ϫ0.4 V vs. ferrocene, whereas the reduction potentials for the Ni(II/I) and (I/0) couples for 3b are observed in the 31P NMR spectrum. In acetone-d the integra- 6 observed at Ϫ0.80 and Ϫ1.28 V (22). Although the waves for 4 tions for these two resonances are very close to 1:1, suggesting are irreversible, they indicate that large changes in electron that they correspond to a single compound, 4B. This conclusion is also supported by the fact that the relative integrations for density occur at nickel as a result of protonating the nitrogen these two resonances remained relatively constant when hydro- atoms of the diphosphine ligands. gen addition was carried out in acetonitrile, dichloromethane, and dimethylformamide at room temperature, whereas ratios of Discussion isomers 4A and 4C varied significantly. The large difference in Complex 3a is an exceptionally effective catalyst for the elec- the 31P NMR chemical shifts of the two ligands in isomer 4B trochemical production of hydrogen, displaying high rates and provides support for a N-H–Ni interaction involving the ligand long lifetimes. The turnover frequency of 350 sϪ1, determined at chelate in the boat conformation. Such an interaction results in 22°C, is comparable to the catalytic rates of 500–700 sϪ1 reported the formation of two five-membered rings, and such ring systems for H2 production at 30°C for Ni-Fe hydrogenases (31, 32). At the 31 are known to result in significant downfield shifts of the P higher acid concentrations shown in Fig. 3, H2 elimination or an NMR resonances compared with those in six-membered ring intramolecular proton transfer appears to be the rate-limiting systems (29). step in the catalytic cycle. Similarly, hydrogen addition appears 15 The N NMR spectrum for 4B in acetone-d6 shows two to be the rate-determining step in the catalytic oxidation of H2 4A resonances that are very close in chemical shifts to those of , by 3b. In the latter case, oxidation of H2 occurs readily within the 15 as summarized in SI Table 2. In addition, a third N resonance coordination sphere of catalyst 3b to give an isomeric series of 1 Ϫ is observed at 47.0 ppm. In the H NMR spectrum at 70°C, Ni(0) products containing a protonated nitrogen atom in each of singlets at 7.0 and 14.6 ppm are assigned to the N-H protons in the cyclic ligands. The nature of the first observable intermedi- 15 the two inequivalent ligands. For the N-labeled complex, the ate, 4A, presents a surprising contrast to that observed previously splitting of the proton resonance near 7 ppm is obscured by 2ϩ for [Ni(PNP)2] , 6, where PNP is Et2PCH2N(Me)CH2PEt2 phenyl resonances, but the downfield resonance is split into a (21). In the latter case, an intramolecular heterolytic cleavage of triplet at 15.4 ppm with a coupling constant of 31 Hz. The ϩ H occurs when 6 reacts with H to form [HNi(PNHP)(PNP)]2 , coupling between the 1H and 15N resonances is confirmed by the 2 2 7, as shown in Eq. 1. gradient heteronuclear single quantum coherence spectrum. Similar downfield 1H chemical shifts and J values have been N-H [Ni(PNP) ](BF ) ϩ H 3 ͓HNi(PNHP)(PNP)](BF ͒ observed previously for the proton stabilized by two amines in 2 4 2 2 4 2 protonated Proton Sponge [1,8-bis(dimethylamino)naphtha- 6 7 lene] and related derivatives (30). [1] The last isomer to form as the solution is warmed to room 31 temperature is 4C, which shows a singlet in the P NMR The differences in structures 4 and 7 demonstrate that seemingly Ϫ 1 15 spectrum at 9.2 ppm. The H and N NMR data, summarized subtle differences in the diphosphine ligand structure that in SI Table 2, are consistent with a tetrahedral structure with D2d involve positioning of the nitrogen bases can result in signifi- symmetry in which each proton is stabilized by two amines of the cantly different electron distributions during hydrogen addition. chelate rings in chair conformations as shown in Fig. 6. Ϫ In our spectroscopic studies that led to the identification of 4 The broad resonances observed at 19.5 and 7.5 ppm in the and 7, no evidence for the initial interaction with hydrogen to room temperature 31P NMR spectrum shown in Fig. 4a indicate form a nickel-dihydrogen intermediate has been obtained for that exchange processes are occurring for isomers 4A and 4B.In either system. However, DFT calculations on model complexes addition, the coupling of a proton to two 15N nuclei in isomers of both [Ni(PNP) ]2ϩ and [Ni(P RN RЈ) ]2ϩ derivatives (where 4B and 4C may indicate that each NH proton is exchanging 2 2 2 2 phosphine and nitrogen substituents are replaced with hydro- rapidly between a single pair of 15N atoms, or it could indicate structures in which the proton symmetrically bridges the two 15N gens) have indicated that an initial intermediate is a Ni(II) atoms. Although further information regarding the nature and dihydrogen complex. In the case of the PNP complex, the mechanism of these exchange processes is certainly of interest, dihydrogen ligand is somewhat unsymmetrically coordinated it is beyond the scope of the present work. with one hydrogen showing a closer approach to the pendant amine of the ligand (22). In the case of the P2N2 complex, a Deprotonation of 4. The isomers of 4 are rapidly converted to the completely symmetrical dihydrogen complex, as shown in Cy Bz Ni(II) hydride [HNi(P 2N 2)2](BF4), 5, upon deprotonation Scheme 1, is observed computationally as the initial intermedi- with triethylamine in acetonitrile, as shown by the bottom ate. This dihydrogen intermediate lies 2.1 kcal/mol above the reaction in Scheme 1. The deprotonation reaction is reversible, energy of the reactants, whereas the corresponding dihydride lies and the hydride complex 5 is converted to the doubly protonated 15.0 kcal/mol higher in energy. The Ni(0) complex with two Ni(0) derivatives, 4A–4C, upon reaction with tetrafluoroboric protonated nitrogen atoms analogous to 4 has a calculated acid or anisidinium tetrafluoroborate. This equilibrium repre- energy of Ϫ2.5 kcal/mol with respect to the reactants in com- sents an unusual pH-dependent intramolecular redox event in parison with an experimental free energy of Ϫ3.1 kcal/mol which protonation leads to a formal two-electron reduction of observed for H2 addition to 3b.

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Complexes 3a and 3b were synthesized according to a published SPECIAL FEATURE procedure (22). 15N (95%) labeled benzylamine was purchased from Aldrich. NMR spectra were recorded on a Varian Inova 400-MHz spectrometer, operating at 400.159 MHz for 1H ob- servation. 1H NMR chemical shifts are reported relative to tetramethylsilane using residual solvent protons as a secondary reference. 31P chemical shifts are reported relative to external phosphoric acid, and 15N chemical shifts are reported relative to external , referenced indirectly to the frequency of the deuterated solvent. Two-dimensional 1H-15N field gradient het- Fig. 7. Structure 8. eronuclear single quantum coherence experiments were ac- quired in pure-phase mode with broadband 15N decoupling during detection by using pulsed-field gradients for coherence The heterolytic cleavage of H2 is generally thought to occur by an selection, optimized for an average 1H-15N heteronuclear cou- asymmetric polarization of a dihydrogen molecule to form an Hϩ Ϫ pling of 80 Hz. Reported sample temperatures for low- and an H species. The heterolytic cleavage of H2 by 6 to form 7 temperature experiments were calibrated by using a standard shown in Eq. 1 is an example of such a reaction. A transition state 100% methanol sample and calculated by using utilities provided for this reaction has been calculated [structure 8 (Fig. 7)] in which in the VNMR 6.1C software (Varian). One-dimensional 15N the H-H bond of the coordinated dihydrogen ligand is weakened as NMR spectra were acquired with continuous broadband irradi- the new bonds with the proton and hydride acceptor sites are ation of the protons to provide sensitivity enhancement from formed (22). The observation that 4A is the first detectable inter- both the NOE interaction and from decoupling JNH. mediate in the addition of H2 to 3b is interesting because it suggests Cyclic voltammetry experiments were carried out on a Cypress that a distinctly different mechanism for H activation may occur for 2 Systems computer-aided electrolysis system under an N2 or H2 this complex. A novel feature of 3b is that two amine nitrogens are atmosphere on acetonitrile solutions containing 0.3 M Bu4NBF4.

held in positions that could allow for simultaneous interaction of The working electrode was a glassy carbon disk, and the counter CHEMISTRY both bases with the incoming H2 molecule, as shown by the electrode was a glassy carbon rod. A silver wire was used as a proposed dihydrogen intermediate in Scheme 1. If such a symmetric pseudoreference electrode. Ferrocene was used as an internal transition state occurs, then the heterolytic cleavage achieved by 3b ϩ Ϫ standard, and all potentials are referenced to the ferrocene/ would not involve a H2 molecule that is strongly polarized as H H . ferrocenium couple. To determine the rate of proton reduction by Instead the polarization would occur by the removal of two 3a, cyclic voltammograms were recorded at 50 mV/s as aliquots of electrons by nickel from a symmetric transition state in which both a 1:1 buffer solution of [Hϩ-DMF]OTF/DMF were added (up to Ϫ4 of the hydrogen atoms gradually develop a more positive charge. Ϸ0.3 M) to 3a (3.2 ϫ 10 M) in acetonitrile. The values of ic/ip vs. ϩ Similarly, the production of H2 by complex 3a may involve the [H -DMF] were plotted as shown in Fig. 3. reverse process. The proposed symmetric mechanism of H2 oxida- Cy 15 Bz tion/production implies that cooperative interactions of the dihy- Synthesis of P 2 N 2. A procedure slightly modified from the drogen ligand with both the metal center and multiple proton relays ligand synthesis reported previously was used. A 250-ml incorporated in the second coordination sphere contribute to the Schlenk flask was charged with cylohexylphosphine (2.32 g, high activity observed for these nickel-based molecular catalysts. 0.02 mol), fresh paraformaldehyde (1.21 g, 0.04 mol), and degassed ethanol (100 ml). The resulting suspension was Summary. Complex 3a is an extremely effective catalyst for the immersed in a hot oil bath (80°C) and stirred for 15 min electrochemical production of hydrogen with a turnover fre- resulting in a clear solution. Under magnetic stirring a solution Ϫ quency (350 s 1) that is comparable to that of the nickel-iron of 15N-benzylamine (2.2 ml, 0.02 mol) in ethanol (30 ml) was hydrogenase enzymes. Kinetic studies indicate that the rate- added dropwise to the hot solution over a period of 60 min. determining step involves a doubly protonated and doubly The reaction mixture turned slightly cloudy when 25 ml of the reduced intermediate. To provide insight into possible structures benzylamine solution had been added. Completion of the of this intermediate, H2 addition to 3b has been studied. addition and further heating resulted in a clear solution that Complex 3b oxidizes H2 to form a mixture of Ni(0) products in was heated overnight at 75°C to form a white precipitate. The which an amine in each of the cyclic ligands is protonated. Three solution was cooled to room temperature, solvent volume was nearly isoenergetic Ni(0) isomers, 4A–4C, are suggested on the reduced on a vacuum line to Ϸ50 ml, and product was filtered basis of heteronuclear and two-dimensional NMR techniques. via a cannula stick and dried on a vacuum line. The yield was The structure of the first intermediate 4A suggests that both of 2.75 g, 55%. Reducing the volume of the filtrate solution gave 31 the pendant nitrogen atoms in 3b may simultaneously interact in another fraction of product. P NMR(CDCl3): 41 ppm (s). a symmetric manner with H2. This may be important for Cy Bz stabilizing the dihydrogen intermediate and for the subsequent Low-Temperature NMR Studies. A solution of [Ni(P 2N 2)2] cleaving of H2 with a concerted reduction of Ni(II) to Ni(0). The (BF4)2 (15 mg, 0.012 mmol) in actone-d6 was cooled to Ϫ78°C, contrast between these products and the closely related PNP and H2 (3 ml, 0.11 mmol) was added through a gas-tight syringe. structure 7, which contains both a nickel hydride and a proton- The solution was shaken and allowed to warm to approximately ated nitrogen atom, demonstrates that subtle differences in the Ϫ10°C until it reacted. After the solution changed from purple diphosphine ligand structure can result in very different proton to colorless, it was cooled again to Ϫ78°C. Within minutes, the and electron distributions. The cooperative interactions that lead solution was monitored by 31P NMR at Ϫ70°C. to a delicate balance in the distribution of protons and electrons between nickel and the pendant bases of the second coordina- DFT Calculations. The all-electron DFT calculations were carried tion sphere are believed to be important factors in the high out with the Gaussian 03 program by using the 6–31G(d,p) basis catalytic activity of these systems. and the hybrid B3LYP method.

Wilson et al. PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6955 Downloaded by guest on September 23, 2021 A.D.W. thanks Prof. G. Girolami for helpful insights and discussions. Program of the Office of Basic Energy Sciences of the U.S. Department This work was supported by National Science Foundation Grant CHE- of Energy. The Pacific Northwest National Laboratory is operated by 0240106. D.L.D. acknowledges the support of the Chemical Sciences Battelle for the U.S. Department of Energy.

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