Metal complexes with ligand-tethered organo-hydride donor groups - new prospects for efficient multi-electron reduction

Abstract

This research, inspired by metal-containing proteins which bind, and thus activate substrates to hydride transfer from nature s organo-hydride donor (OHD), NADPH, set out to explore the potential of metal complexes with a ligand-tethered OHD or OHD conjugate cation group as new, efficient and green catalysts for the reduction of unsaturated substrates. A myriad of such complexes substituted by nicotinamide, Hantszch ester and benzimidazole groups were synthesised and thoroughly investigated using an array of techniques, in particular (spectro)electrochemistry and X-ray crystallography.

Nicotinamide cations tethered to N-heterocyclic carbene (NHC) ligands of rhodium(I) were found to oxidatively add to the proximal, low-valent rhodium center to afford both monomeric rhodium(III) and dimeric rhodium(II) complexes of a novel bidentate bis(NHC) ligand in which both a pyridylidene and imidazolylidine NHC donor group bind the same metal ion. This was the first report of such metal complexes. The C-metalated nicotinamide cation was found to be inert to hydride addition; C-metalation must be avoided if useful hydride acceptor properties are to be maintained in such a group.

Rhodium(III) half-sandwich complexes bound by ligands with a tethered Hantzsch ester cation group were evaluated as catalysts for the reduction of imine substrates. The complex with a Hantzsch ester cation proximal to the vacant metal coordination site displayed remarkable, enhanced catalytic properties. This was attributed to hydride transfer from metal to Hantszch ester, followed by hydride back transfer to a metal-bound substrate. This catalyst introduces a highly novel, potentially important methodology for the reduction of unsaturated substrates.

During attempted preparation of other Hantzsch ester substituted ligand precursors, a novel synthesis of 2,5-di(2-pyridyl)-1H-pyrroles was uncovered. This new methodology opens the door to the essentially untapped coordination chemistry of these ligand precursors. This new synthesis could be applied to the preparation of novel macrocycles which showed interesting host-guest, photo-, electro- and coordination-chemistry.

Rhodium(III) half-sandwich complexes bound by ligands with a tethered benzimidazolium cation group were found to show significantly enhanced activity as catalysts for the reduction of imine substrates. Ruthenium(II) complexes with a ligand-tethered benzimidazoline OHD group were prepared and show promise as four-electron reductants of unsaturated substrates, including carbon dioxide.