Electronic Effects of Substituents on fac-M(bpy-R)(CO)3 (M = Mn, Re) Complexes for Homogeneous CO2 Electroreduction

Abstract

Synthesis and characterization of 14 new 2,2′-bipyridine metal complexes fac -M(bpy-R)(CO) 3 X (where M = Mn, X = Br or M = Re, X = Cl and R = -CF 3 , -CN, -Ph, -PhOH, -NMe 2 ) are reported. The complexes have been characterized by NMR, IR spectroscopy and elemental analysis. Single crystal X-Ray diffraction structures have been solved for Re(dpbpy)(CO) 3 Cl (dpbpy = 4,6-diphenyl-2,2′-bipyridine) and Re(hpbpy)(CO) 3 Cl (hpbpy = 4-(2-hydroxy-phenyl)-6-phenyl-2,2′-bipyridine). Electrochemical behaviors of the complexes in acetonitrile under Ar and their catalytic performances for CO 2 reduction with added water and MeOH have been investigated by cyclic voltammetry and controlled potential electrolysis. The role of the substituents on the electrochemical properties and the related over potentials required for CO 2 transformation have been analyzed. The complexes carrying only electron withdrawing groups like -CF 3 , -CN totally lose their catalytic activities toward CO 2 reduction, whereas the symmetric -NMe 2 substituted and push-pull systems (containing both -NMe 2 and -CF 3 ) still display electrocatalytic current enhancement under CO 2 atmosphere. The complexes carrying a phenyl or a phenol group in position 4 show catalytic behaviors similar to those of simple M-bpy systems. The only detected reduction product by GC analysis is CO: for example, fac -Re (bpy-4,4′-NMe 2 )(CO) 3 Cl gives CO with high faradic efficiency and a TON of 18 and 31, in absence of external proton source and with 5% MeOH, respectively. DFT calculations were carried out to highlight the electronic properties of the complexes; results are in agreement with experimental electrochemical data.

Photochemical CO2 Reduction Using Rhenium(I) Tricarbonyl Complexes with Bipyridyl‐Type Ligands with and without Second Coordination Sphere Effects

“…All rhenium compounds were prepared as previously reported. [49][50][51][52][53] Bipyridine ligands were dissolved with [ReCl(CO) 5 ] in anhydrous toluene. The reaction mixtures were then refluxed for 4 hours at 110°C.…”

“…We previously investigated the electrochemical CO 2 reduction performance of six Re complexes (1)(2)(3)(4)(5)(6) shown in Scheme 1. [49][50][51][52][53] Complexes 4 and 6 have an OH group in the second coordination sphere of the Re center. Detailed investigations revealed that the OERS of both 4 and 6 dissociates a coordinated Cl À anion with net loss of 1 = 2 of H 2 to form a neutral six-coordinate species with an internal ReÀ O (phenoxy) bond.…”

Molecular Catalysts with Intramolecular Re–O Bond for Electrochemical Reduction of Carbon Dioxide

“…Multisweep voltammograms ( Figure S12 ) show decreasing current for reduction I and a complete loss of reduction II without accumulation of any distinct redox processes. For comparison, the multisweep CV of [Re(4,6-diphenyl-2,2′-bpy)(CO) 3 Cl] 11 is also shown. This complex differs only by the absence of the −OH group and represents the electrochemical behavior expected of [Re(bpy)(CO) 3 Cl]-type complexes.…”

“… 7 After the pioneer work of Lehn in the 1980s that first reported the capability of fac -Re(bpy)(CO) 3 Cl (bpy = 2,2′-bipyridine) in homogeneous solution to selectively reduce CO 2 to CO, 8 many new transition metal complexes were discovered. 5 , 9 , 10 Among them, those derived from the original Rebpy and the corresponding Mn-bpy type received considerable interest 11 − 34 due to performances and stability, especially when chemically bound to the electrode surface. 12 , 25 , 35 − 39 The transition from the homogeneous to the heterogeneous approach, which in some cases displays superior stability and durability, 40 can find considerable support whenever the intimate molecular mechanism responsible for the catalysis is initially deeply investigated in the homogeneous phase.…”

“…The concept of a local proton source applied to the electrochemical reduction of CO 2 41 has been recently extended not only to the Mn-bpy system 14 , 16 , 23 but also to the Rebpy complexes. 1 , 11 , 42 − 44 Scheme 1 shows a set of Mn and Re catalysts bearing pendent phenolic groups near the metal center acting as intramolecular proton sources. We initially reported fac -Mn(pdbpy)(CO) 3 Br 14 , 16 ( Mn-2OH ), and recently we studied the effect of the two OH functionalities on fac -Re(pdbpy)(CO) 3 Cl 13 ( Re-2OH ; pdbpy = 4-phenyl-6-(phenyl-2,6-diol)-2,2′-bipyridine).…”