First examples of heteroleptic dipyrrin/η⁵-pentamethylcyclopentadienyl rhodium/iridium(III) complexes and their catalytic activity

Abstract

Heteroleptic pentamethylcyclopentadienyl rhodium/iridium(III) complexes imparting dipyrrins as co-ligands with the general formulations [(η⁵-C₅Me₅)MCl(L)] [(M = Rh(III) or Ir(III); L = 5-(4-cyanophenyl)dipyrromethene, cydpm; 5-(4-nitrophenyl)dipyrromethene, ndpm; and 5-(4-benzyloxyphenyl)dipyrromethene, bdpm] have been synthesized. Reactivity of the complexes [(η⁵-C₅Me₅)MCl(L)] (M = Rh(III), Ir(III); L = ndpm and cydpm) with various species, viz., sodium azide (NaN₃), ammonium thiocyanate (NH₄SCN), triphenylphosphine (PPh₃), 4,4′-bipyridine (bpy), and bis(diphenylphosphino)hexane (dpph), has been examined. Resulting complexes have been characterized by elemental analyses and spectral and electrochemical studies. Molecular structures of the representative complexes [(η⁵-C₅Me₅)RhCl(cydpm)], [(η⁵-C₅Me₅)RhCl(ndpm)], [(η⁵-C₅Me₅)Rh(PPh₃)(cydpm)]SO₃CF₃, and [(η⁵-C₅Me₅)Ir(PPh₃)(ndpm)]SO₃CF₃ have been determined crystallographically. The complexes [(η⁵-C₅Me₅)MCl(L)] [(M = Rh(III) or Ir(III) and L = cydpm, ndpm, or bdpm) effectively catalyze reduction of terephthalaldehyde to 4-hydroxymethybenzaldehyde in the presence of HCOOH/CH₃COONa in water under aerobic conditions. Among these complexes the rhodium complex [(η⁵-C₅Me₅)RhCl(ndpm)] is most effective in this regard.