Catalytic Formic Acid Dehydrogenation and CO₂ Hydrogenation Using Iron PN^RP Pincer Complexes with Isonitrile Ligands

Abstract

It has previously been demonstrated that complexes of the form (iPrPNP)Fe(H)(C≡NR) (iPrPNP = N(CH2CH2P(iPr)2)2–, R = 2,6-dimethylphenyl or 4-methoxyphenyl), which contain a pincer ligand capable of metal–ligand cooperation (MLC), are active for CO2 hydrogenation. Herein, the synthesis and catalytic activity of a second-generation of precatalysts containing a tertiary amine ligand, which cannot participate in MLC, are presented. Specifically, the complexes (iPrPNMeP)Fe(H)(HBH3)(C≡NR) (iPrPNMeP = MeN(CH2CH2P(iPr)2)2, R = 2,6-dimethylphenyl (2a), tert-butyl (2b), or adamantyl (2c)) have been prepared and crystallographically characterized. These complexes are precatalysts for both formic acid dehydrogenation and CO2 hydrogenation to formate, and give improved activity compared to first-generation systems with isonitrile ligands. The second-generation systems 2a–c, however, give inferior activity compared to the related carbonyl complexes (iPrPNP)Fe(H)(CO) and (iPrPNMeP)Fe(H)(HBH3)(CO), which have been previously reported. This study demonstrates that a ligand which can participate in MLC is not universally advantageous for promoting the hydrogenation and dehydrogenation reactions studied in this work and provides guidance for the rational design of improved catalysts for reactions relevant to energy storage.