Dioxygen adducts of rhodium N-heterocyclic carbene complexes

Abstract

Rhodium complexes functionalized by N-heterocyclic carbene ligands react with dioxygen to form adducts. Depending on the specifics of the ancillary ligands, oxygen binds to Rh either as a peroxide to form a fully oxidized Rh(III) complex, or as singlet dioxygen in a Rh(I) square planar complex. We have shown through analysis of a series of compounds, some previously published and some novel, that the presence of additional ligands that would support the formation of an octahedral geometry, as typically found with Rh(III) complexes, is critical for formation of the peroxide. In addition, we have demonstrated through DFT studies, that the potential energy surface with regard to the O-O bond length is relatively shallow, which provides a rationale for the distribution of bond lengths observed for apparently similar complexes analyzed by crystallography.

Pyridine versus acetonitrile coordination in rhodium–N-heterocyclic carbene square-planar complexes

“…The coordination of dioxygen to transition metal complexes very often results in the formation of monodentated dioxygen adducts { 1 O 2 } or peroxo {O 2 2− } complexes. 23 In this particular case, complex 18 is better described as a tetracoordinated Rh I { 1 O 2 } 24 species whereas 19 points to a Rh III {O 2 2− } 25 hexacoordinated compound. Suitable single crystals for X-ray determination were obtained by slow diffusion of n-hexane into saturated toluene solutions of 18 and 19.…”

Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

“…The specific binding mode depends on the structure of the surrounding ligands, the availability of coordination sites at the metal, and the pattern and energies of the frontier metal d-orbitals. − In monometallic complexes, the accessible oxidation states of the metal also play a role in determining the dioxygen-binding mode. ,, The traditional names of η 1 -superoxo and η 2 -peroxo structures imply 1 e – and 2 e – oxidations of the metal center, respectively. There was, however, significant debate many years ago about the appropriate assignment of the metal and dioxygen oxidation states in these adducts. ,,,,, Recently, Kennepohl and co-workers have revisited this question, using X-ray and Raman spectroscopies with DFT calculations to show that, at least in some cases, there is little charge transfer from M to O 2 in peroxo adducts. , The dioxygen-binding mode can also influence trends in dioxygen reactivity (c.f. refs , , and ), making it an important parameter to consider when designing ORR catalysts.…”

“…9,170,175,176,180,181 Recently, Kennepohl and co-workers have revisited this question, using X-ray and Raman spectroscopies with DFT calculations to show that, at least in some cases, there is little charge transfer from M to O 2 in peroxo adducts. 182,183 The dioxygen-binding mode can also influence trends in dioxygen reactivity (c.f. refs 168, 178, and 179), making it an important parameter to consider when designing ORR catalysts.…”

Oxygen transfer reactivity mediated by nickel perfluoroalkyl complexes using molecular oxygen as a terminal oxidant

“…Oxidation of organic substrates using molecular oxygen as a terminal oxidant is an important process in many biological processes 1 and in industrial aerobic oxidation catalysis where oxygen or air is considered as an inexpensive, “green” oxidant that produces no hazardous waste products. 2 Precious metal catalysts have been extensively used in enabling aerobic oxidation of hydrocarbon substrates or stoichiometric O 2 activation, typically based on Pt, 3 Pd, 4 Ru, 5 Rh, 6 Ir, 7 etc. At the same time, metalloenzymes capable of aerobic oxygenation typically contain more abundant first row transition metals, which led to the development of bioinspired model complexes capable of O 2 activation, most commonly based on Cu, 8 Fe, 9 Mn, 10 and Co. 11 Although Ni is known to be present at the active site of several O 2 -reactive enzymes, 12 examples of synthetic “nickel oxygenase” reactivity in which Ni complexes assist in the transfer of O-atoms from O 2 to a substrate remain scarce.…”