Reductive Electrochemistry of Rhodium Porphyrins. Disproportionation of Intermediary Oxidation States

Abstract

The reduction of rhodium(III) porphyrins in polar aprotic solvents is a two-electron irreversible reaction yielding directly the Rh(I) complex. The cause of this irreversibility is not the metal−metal dimerization of the initially formed Rh(II) complex as believed earlier but rather deligation which generates a secondary Rh(II) species easier to reduce than the starting Rh(III) porphyrin. This is confirmed by the fact that sterically encumbered porphyrins, such as those bearing cross-trans basket-handle superstructures which forbid the approach of two molecules at bonding distance, exhibit the same behavior as simple rhodium porphyrins. The occurrence of such an ECE−disproportionation process, seldom observed in the redox chemistry of metallo-porphyrins or similar complexes, is probably related to the tendency of the rhodium atom to shift out of the porphyrin plane, particularly at the Rh(I) oxidation state. It is remarkable that strong and soft ligands, e.g., tertiary phosphines, annihilate the disproportionation of the rhodium(II) complex.

Solution Catalytic Cycle of Incompatible Steps for Ambient Air Oxidation of Methane to Methanol

“…19 In an argon (Ar) environment and noncoordinating solvent, 1,2-difluorobenzene (1,2-DFB), 23 with 0.1 M tetrabutylammonium perchlorate (TBAClO 4 ), a cyclic voltammogram (CV) of 1d on a platinum (Pt) working electrode displays quasi-reversible behavior at an electrode potential E appl = −1.26 V vs standard calomel electrode (SCE) (Figure 2B), consistent with a previous report that the Rh II species can be regenerated by electrochemistry. 24,25 In the presence of O 2 , a catalytic cathodic wave was observed on a glassy carbon electrode preceding the Rh III /Rh II redox couple ( E appl < – 0.7 V vs SCE) (green trace in Figure 2B), while the CV trace in the absence of 1d yielded no such activity (Figure S2). Previous literature report the generation of superoxide and peroxide as the immediate products both in solution or electrochemically when O 2 and Rh II porphyrin are in a stoichiometric ratio.…”

Determinant Role of Electrogenerated Reactive Nucleophilic Species on Selectivity during Reduction of CO2 Catalyzed by Metalloporphyrins

“…A strong affinity for axial ligation for the Rh has been observed experimentally and has been held responsible for its peculiar electrochemical behavior. 53 , 54 An indication for the presence of axial ligands in the experiment is the fact that the onset potential for the CO2RR at pH = 3 and higher ( E < −1.2 V vs NHE), is substantially more negative than the computed reduction potential for the bare catalyst ( E ≈ −0.2 V vs NHE). The presence of axial ligands is known to substantially shift the reduction potential toward more negative potentials (the reduction potential computed for the pentacoordinated hydroxo complex [Rh(III)P(OH)] is E ≈ −0.8 V vs NHE).…”

Electrochemical synthesis of bimetallic Au@Pt nanoparticles supported on gold film electrode by means of self-assembled monolayer

“…However, concern regarding to high cost of direct use of these metals or thin films of the metals has also aroused interest and led to intensive investigations on molecular-based catalysts capable of catalyzing proton reduction at low overpotentials [4,5].…”