Communication pubs.acs.org/Organometallics
Comparison of [Ni(PPh2NPh2)2(CH3CN)]2+ and [Pd(PPh2NPh2)2]2+ as Electrocatalysts for H2 Production Eric S. Wiedner* and Monte L. Helm Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: The complexes [Ni(PPh2NPh2)2(CH3CN)]2+ and [Pd(PPh2NPh2)2]2+, where PPh2NPh2 is 1,5-diphenyl-3,7-diphenyl1,5-diaza-3,7-diphosphacyclooctane, are compared as electrocatalysts for H2 production under identical experimental conditions. With [(DMF)H]+ as the acid in acetonitrile solution, [Pd(PPh2NPh2)2]2+ afforded a turnover frequency (TOF) of 230 s−1 for formation of H2 under dry conditions and a TOF of 640 s−1 when H2O was added. These rates are similar to the TOFs of 590 s−1 (dry) and 720 s−1 (wet) that were previously measured for [Ni(PPh2NPh2)2(CH3CN)]2+ using [(DMF)H]+. The [Ni(PPh2NPh2)2(CH3CN)]2+ and [Pd(PPh2NPh2)2]2+ complexes both exhibited large current enhancements when treated with trifluoroacetic acid (TFA). At a TFA concentration of 1.8 M, TOF values of 5670 and 2060 s−1 were measured for [Ni(PPh2NPh2)2(CH3CN)]2+ and [Pd(PPh2NPh2)2]2+, respectively. The fast rates observed using TFA are, in part, attributed to homoconjugation of TFA in acetonitrile solutions, which decreases the effective pKaMeCN of the acid. In support of this hypothesis, dramatically lower rates of H2 production were observed using p-anisidinium, which has a pKaMeCN value comparable to that of TFA but does not homoconjugate significantly in acetonitrile solutions.
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enewable energy technologies are expected to become increasingly important as the global energy demand rises.1,2 One factor that limits widespread adoption of wind and solar power is the intermittent nature of these energy sources. A possible solution is to convert the electricity generated from these power sources into a chemical fuel such as H2. While platinum metal is an excellent electrocatalyst for the production of H2, its high cost has thus far limited its economic viability. Consequently, many groups have investigated the ability of molecular complexes based on inexpensive metals to function as electrocatalysts for the production of H2.3−7 Our laboratory has reported the use of [Ni-
Figure 1. (a) General structure of [Ni(PR2NR′2)2(CH3CN)] 2+
(PR2NR′2)2(CH3CN)]2+ complexes as electrocatalysts for the
complexes. (b) General structure of [Pd(PR2NR′2)2]2+ complexes. (c) Exo-exo to endo-endo isomerization; organic groups on P and N not shown.
PR2NR′2 is 8,9
a 1,5-diaza-3,7-diphosphaproduction of H2, where cyclooctane ligand (Figure 1a). Mechanistic studies have indicated that the pendant amines of [Ni(PR2NR′2)2(CH3CN)]2+ facilitate intra- and intermolecular proton transfer to and from the metal center and the surrounding solvent medium. Spectroscopic and computational studies have provided evidence that the turnover frequency (TOF) for H2 formation is determined by a rate-limiting isomerization from catalytically nonproductive exo isomers to productive endo isomers (Figure 1c).10−12 As a result of this mechanistic feature, the TOF is highly dependent on the nature of the acid substrate employed for catalysis.13 For example, a strong acid will protonate the pendant amines rapidly, but its weak conjugate base will not deprotonate the isomers © 2014 American Chemical Society
effectively, thus slowing down the isomerization process.14 Increased rates of H2 formation are observed for the [Ni(P R 2 N R ′ 2 ) 2 (CH 3 CN)] 2+ catalysts upon addition of water,13,15,16 which suggests that water speeds the exo to endo isomerization process. Special Issue: Organometallic Electrochemistry Received: November 1, 2013 Published: January 24, 2014 4617
dx.doi.org/10.1021/om4010669 | Organometallics 2014, 33, 4617−4620
Organometallics
Communication
Kubiak et al. recently reported a series of [Pd(PR2NR′2)2]2+ complexes (Figure 1b) as potential electrocatalysts for reduction of CO2 in the presence of 1,1,1-trifluoroacetic acid (TFA).17 These complexes were found to be inactive for CO2 reduction, but they were active for H2 production. The rates of electrocatalytic H2 formation for the palladium complexes were reported to be significantly slower than those for analogous nickel complexes. For example, a TOF of
