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Nature of Hydrogen Interactions with Ni(II) Complexes Containing Cyclic

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Nature of Hydrogen Interactions with Ni(II) Complexes Containing Cyclic Nature of hydrogen interactions with Ni(II) SPECIAL FEATURE complexes containing cyclic phosphine ligands with pendant nitrogen 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 proton 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 enzymes. In the electrochem- Fig. 1. Structural features of the active site 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 base 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 deprotonation 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 protons 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 molecule. This can minimize reorgani- differences associated with significantly different distributions in zation energies associated with the approach of an external base electron density and protons within these molecules 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 electrons 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 substituents on nitrogen and phosphorus, these complexes are very active catalysts for either development of hydrogen-based fuel 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 enzyme 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 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0608928104 PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6951–6956 Downloaded by guest on September 23, 2021 Fig. 2. Structure 3. 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 carbon 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.
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