Using polyethyleneimine (PEI) as a scaffold to construct mimicking systems of [FeFe]‐hydrogenase: preparation, characterization of PEI‐based materials, and their catalysis on proton reduction

Abstract

Ten branched polymeric materials (PEI‐P‐Fe2s) derived from polyethyleneimine (PEI) functionalized with [Fe2(CO)5]‐units to mimic [FeFe]‐hydrogenase were prepared. Before the functionalization, PEI was first premodified using diphenylphosphinamine (NPPh2) group. In the premodification, three approaches were employed: (i) using PEI with an average molecular weight of 1800 and 600, respectively; (ii) grafting NPPh2 group by either direct reaction of chlorodiphenylphosphine with PEI or Br(CH2)11OPPh2; and (iii) further premodification with BrCH2COOH after immobilization of the NPPh2 group. Reaction of the premodified PEI with diiron hexacarbonyl complexes, [Fe2(μ‐S)2(CO)6] (1), or [Fe2(μ‐S2C2H4)(CO)6] (3) produced 10 functionalized materials, PEI‐P‐Fe2s. These materials were characterized using a variety of spectroscopic techniques, FTIR, NMR, TGA, and cyclic voltammetry. Spectral comparison with two control complexes, [Fe2(μ‐S)2(CO)5PPh3] (2) and [Fe2(μ‐S2C2H4)(CO)5PPh3] (4), suggested that the immobilized diiron units of PEI‐P‐Fe2s were dominantly pentacarbonyl analogous to complexes 2 and 4, although tetracarbonyl units may also exist because the amine groups of PEI could also be involved in substituting CO, as was the NPPh2 group. The catalysis of these materials on proton reduction was examined in 0.1 mol l−1 [NBut4]BF4/DMF containing acetic acid by using cyclic voltammetry. Our results indicated that both the presence of carboxylic acid and dangling the diiron units at the end of a long aliphatic chain improved catalytic efficiency by one‐fold. The improvement was attributed to the increase in flexibility of the catalytic center and enhancement of proton transfer during the catalysis. Copyright © 2013 John Wiley & Sons, Ltd.