ABSTRACT
A series of Ni-based electrocatalysts, [Ni(7P(Ph)2N(C6H4X))2](BF4)2, featuring seven-membered cyclic diphosphine ligands incorporating a single amine base, 1-para-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane (7P(Ph)2N(C6H4X), where X = OMe, Me, Br, Cl, or CF3), have been synthesized and characterized. X-ray diffraction studies have established that the [Ni(7P(Ph)2N(C6H4X))2](2+) complexes have a square planar geometry, with bonds to four phosphorus atoms of the two bidentate diphosphine ligands. Each of the complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni(II/I) couple, with turnover frequencies ranging from 2400 to 27,000 s(-1) with [(DMF)H](+) in acetonitrile. Addition of water (up to 1.0 M) accelerates the catalysis, giving turnover frequencies ranging from 4100 to 96,000 s(-1). Computational studies carried out on the [Ni(7P(Ph)2N(C6H4X))2](2+) family indicate the catalytic rates reach a maximum when the electron-donating character of X results in the pKa of the Ni(I) protonated pendant amine matching that of the acid used for proton delivery. Additionally, the fast catalytic rates for hydrogen production by the [Ni(7P(Ph)2N(C6H4X))2](2+) family relative to the analogous [Ni(P(Ph)2N(C6H4X)2)2](2+) family are attributed to preferred formation of endo protonated isomers with respect to the metal center in the former, which is essential to attain suitable proximity to the reduced metal center to generate H2. The results of this work highlight the importance of precise pKa matching with the acid for proton delivery to obtain optimal rates of catalysis.
ABSTRACT
A series of dipeptide substituted nickel complexes with the general formula, [Ni(P(Ph)(2)N(NNA-amino acid/ester)(2))(2)](BF(4))(2), have been synthesized and characterized (P(2)N(2) = 1,5-diaza-3,7-diphosphacyclooctane, and the dipeptide consists of the non-natural amino acid, 3-(4-aminophenyl)propionic acid (NNA), coupled to amino acid/esters = glutamic acid, alanine, lysine, and aspartic acid). Each of these complexes is an active electrocatalyst for H(2) production. The effects of the outer-coordination sphere on the catalytic activity for the production of H(2) were investigated; specifically, the impact of sterics, the ability of the side chain or backbone to protonate and the pK(a) values of the amino acid side chains were studied by varying the amino acids in the dipeptide. The catalytic rates of the different dipeptide substituted nickel complexes varied by over an order of magnitude. The amino acid derivatives display the fastest rates, while esterification of the terminal carboxylic acids and side chains resulted in a decrease in the catalytic rate by 50-70%, implicating a significant role of protonated sites in the outer-coordination sphere on catalytic activity. For both the amino acid and ester derivatives, the complexes with the largest substituents display the fastest rates, indicating that catalytic activity is not hindered by steric bulk. These studies demonstrate the significant contribution that the outer-coordination sphere can have in tuning the catalytic activity of small molecule hydrogenase mimics.
