The Role of a Dipeptide Outer‐Coordination Sphere on H₂‐Production Catalysts: Influence on Catalytic Rates and Electron Transfer

Abstract

The outer-coordination sphere of enzymes acts to fine-tune the active site reactivity and control catalytic rates, suggesting that incorporation of analogous structural elements into molecular catalysts may be necessary to achieve rates comparable to those observed in enzyme systems at low overpotentials. In this work, we evaluate the effect of an amino acid and dipeptide outer-coordination sphere on [Ni(P(Ph)(2)N(Ph-R)(2))(2)](2+) hydrogen production catalysts. A series of 12 new complexes containing non-natural amino acids or dipeptides was prepared to test the effects of positioning, size, polarity and aromaticity on catalytic activity. The non-natural amino acid was either 3-(meta- or para-aminophenyl)propionic acid terminated as an acid, an ester or an amide. Dipeptides consisted of one of the non-natural amino acids coupled to one of four amino acid esters: alanine, serine, phenylalanine or tyrosine. All of the catalysts are active for hydrogen production, with rates averaging ∼1000 s(-1), 40 % faster than the unmodified catalyst. Structure and polarity of the aliphatic or aromatic side chains of the C-terminal peptide do not strongly influence rates. However, the presence of an amide bond increases rates, suggesting a role for the amide in assisting catalysis. Overpotentials were lower with substituents at the N-phenyl meta position. This is consistent with slower electron transfer in the less compact, para-substituted complexes, as shown in digital simulations of catalyst cyclic voltammograms and computational modeling of the complexes. Combining the current results with insights from previous results, we propose a mechanism for the role of the amino acid and dipeptide based outer-coordination sphere in molecular hydrogen production catalysts.

Electrocatalytic Oxidation of Formate with Nickel Diphosphine Dipeptide Complexes: Effect of Ligands Modified with Amino Acids

“…[18] Differences in redox potentials for all of the complexes are small: E 1/2 values are within 20 mV for both the Ni II/I potentials (-0.83 to -0.85 V) and the Ni I/0 potentials (-1.03 V to -1.05 V), which indicates that the substituents have minimal impact on the metal center. …”

“…[18,19] The dipeptide-functionalized ligands, P 2 with use of standard TBTU/HOBT/ DIPEA peptide coupling methods. [21,22] Nickel complexes were prepared by reacting two equivalents of a dipeptidefunctionalized ligand with one equivalent of [Ni-(MeCN) 6 ] 2+ in acetonitrile, which resulted in a red solution from which a moderately air-sensitive metal complex was obtained in high yield (70-90 %) and high purity.…”

“…[14][15][16][17] We have recently reported that the incorporation of a dipeptide into the outer coordination sphere of these Nibased electrocatalysts results in rates up to five times faster for the production of hydrogen than that of the parent Ni complex. [18][19][20] Amide or acidic/basic amino acid functional groups contribute to the increased rate of hydrogen production. This observation can be best explained by considering that these functional groups help to concentrate water and/or acid near the active site, and they may also aid in proton delivery to the pendant amine; [18] these are features reminiscent of enzymatic function.…”

“…[18][19][20] Amide or acidic/basic amino acid functional groups contribute to the increased rate of hydrogen production. This observation can be best explained by considering that these functional groups help to concentrate water and/or acid near the active site, and they may also aid in proton delivery to the pendant amine; [18] these are features reminiscent of enzymatic function.…”

“…We previously demonstrated that this series of catalysts had a significant impact on hydrogen production. [18][19][20] The catalyst activities ranged over almost an order of magnitude, the fastest catalysts operating five times faster than the unmodified catalysts. In that case, the increase in activity was interpreted to be due to the ability to concentrate water and protons near the active site.…”