Oxygen Reduction Mechanism of Monometallic Rhodium Hydride Complexes

Abstract

The reduction of O2 to H2O mediated by a series of electronically varied rhodium hydride complexes of the form cis,trans-Rh(III)Cl2H(CNAd)(P(4-X-C6H4)3)2 (2) (CNAd = 1-adamantylisocyanide; X = F (2a), Cl (2b), Me (2c), OMe (2d)) was examined through synthetic and kinetic studies. Rhodium(III) hydride 2 reacts with O2 to afford H2O with concomitant generation of trans-Rh(III)Cl3(CNAd)(P(4-X-C6H4)3)2 (3). Kinetic studies of the reaction of the hydride complex 2 with O2 in the presence of HCl revealed a two-term rate law consistent with an HX reductive elimination (HXRE) mechanism, where O2 binds to a rhodium(I) metal center and generates an η(2)-peroxo complex intermediate, trans-Rh(III)Cl(CNAd)(η(2)-O2)(P(4-X-C6H4)3)2 (4), and a hydrogen-atom abstraction (HAA) mechanism, which entails the direct reaction of O2 with the hydride. Experimental data reveal that the rate of reduction of O2 to H2O is enhanced by electron-withdrawing phosphine ligands. Complex 4 was independently prepared by the addition of O2 to trans-Rh(I)Cl(CNAd)(P(4-X-C6H4)3)2 (1). The reactivity of 4 toward HCl reveals that such peroxo complexes are plausible intermediates in the reduction of O2 to H2O. These results show that the given series of electronically varied rhodium(III) hydride complexes facilitate the reduction of O2 to H2O according to a two-term rate law comprising HXRE and HAA pathways and that the relative rates of these two pathways, which can occur simultaneously and competitively, can be systematically modulated by variation of the electronic properties of the ancillary ligand set.

The mechanism and ligand effects of single atom rhodium supported on ZSM-5 for the selective oxidation of methane to methanol

“…Whilst it is mainly studied in palladium hydrides, 39,40 some research has been directed towards rhodium hydrides. 43 Our calculated energy barriers for this step can readily be overcome under the typical reaction conditions of 110 1C to 170 1C; a barrier of 0.65 eV is observed. For Rh(CO), a higher, yet still reasonable, barrier of 1.22 eV is calculated.…”

“…This result is akin to the findings for the respective oxygen covered metal surface systems, where it was found that methane activates preferentially through a surface stabilised mechanism on Rh(111) and an oxygen promoted mechanism on Cu(111). 42,43 Despite this, the barrier observed for a metal oxo species remains moderately reasonable, therefore we must consider the facility of the formation of a rhodium metal oxo species. For the Cu-zeolite system, oxidation of the copper centre is understood as being the rate determining step, 44 and therefore, we computed the thermodynamic favourability of forming a RhQO species with respect to CuQO formation.…”

Experimental and Computational Investigation of the Aerobic Oxidation of a Late Transition Metal-Hydride

“…Here the operation of HAA and HXRE as competitive pathways to generate metal-hydoperoxo species was proposed. 44,49 The intermediacy of the dinuclear rhodium-hydroperoxo species was also supported by isolation of the related iridium complex, Ir 2 II,II (tfepma) 2 (CN t Bu) 2 Cl 3 (OOH), that was formed from Ir 2 II,II (tfepma) 2 (CN t Bu) 2 Cl 3 (H) and oxygen. 43 In Path D, oxygen undergoes oxidation addition at the metal and then a recombination/reductive elimination step to produce the metal-hydroperoxo product.…”

“…In addition, Nocera and co-workers reported that the reduction of O 2 by Rh 2 II,II (tfepma) 2 (CN t Bu) 2 Cl 3 (H) (tfepma = MeN­[P­(OCH 2 CF 3 ) 2 ] 2 ) or cis , trans -Rh III Cl 2 H­(CNAd)­(P­(4-X-C 6 H 4 ) 3 ) 2 (CNAd = 1-adamantylisocyanide, X = F, Cl, Me, or OMe) proceeds with a two-term rate law. Here the operation of HAA and HXRE as competitive pathways to generate metal-hydoperoxo species was proposed. , The intermediacy of the dinuclear rhodium-hydroperoxo species was also supported by isolation of the related iridium complex, Ir 2 II,II (tfepma) 2 (CN t Bu) 2 Cl 3 (OOH), that was formed from Ir 2 II,II (tfepma) 2 (CN t Bu) 2 Cl 3 (H) and oxygen …”

“…For example, in the past 20 years, there have been significant advances in Pd catalyzed aerobic oxidations, − and Pd/O 2 protocols have been developed for a variety of different oxidative transformations, including alcohol oxidation, − oxidative cross-coupling, − and oxidative dehydrogenation. − This progress has been enabled by the extensive mechanistic research into the reaction of palladium complexes with oxygen. For example, there are now several well-defined mechanisms for the insertion of O 2 into Pd–H bonds. − Some information has also been provided concerning the mechanisms of oxygenation of other late metal-hydrides, for example, Ir, − Rh, − and Pt. , …”