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)₃X (where M = Mn, X = Br or M = Re, X = Cl and R = -CF₃, -CN, -Ph, -PhOH, -NMe₂) 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)₃Cl (dpbpy = 4,6-diphenyl-2,2'-bipyridine) and Re(hpbpy)(CO)₃Cl (hpbpy = 4-(2-hydroxy-phenyl)-6-phenyl-2,2'-bipyridine). Electrochemical behaviors of the complexes in acetonitrile under Ar and their catalytic performances for CO₂ 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 CO₂ transformation have been analyzed. The complexes carrying only electron withdrawing groups like -CF₃, -CN totally lose their catalytic activities toward CO₂ reduction, whereas the symmetric -NMe₂ substituted and push-pull systems (containing both -NMe₂ and -CF₃) still display electrocatalytic current enhancement under CO₂ 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'-NMe₂)(CO)₃Cl 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.

Keywords: CO2 electroreduction; DFT calculations; Mn complexes; Re complexes; bipy ligands; electron-donating; electron-withdrawing; homogeneous catalysis.

Scheme 1

Chemical sketches of the investigated complexes, where M = Mn, X = Br, or M = Re, X = Cl.

Figure 1

(A) molecular structure of Re(dpbpy)(CO)₃Cl (2e, structure A), (B) molecular structure of Re(hpbpy)(CO)₃Cl (2f), (C) chain formed by O–H···Cl intermolecular contacts in the crystal structure of 2f, and (D) comparison of DFT optimized (light blue) and X-ray (brown) structures of 2f.

Figure 2

CVs of 1 mM solutions of 1a−1g Mn complexes in MeCN/0.1 M TBAPF₆ at GCE, scan rate 200 mVs⁻¹ under Ar. CV of the reference fac-Mn(bpy)(CO)₃Br is in black.

Figure 3

CVs of 1 mM solutions of 2a−2g Re complexes in MeCN/0.1 M TBAPF₆ at GCE, scan rate 200 mVs⁻¹ under Ar. CV of the reference fac-Re(bpy)(CO)₃Cl is in black.

Figure 4

Plot of the calculated vs. experimental standard reduction potentials for 2a−2g.

Figure 5

CVs of 0.5 mM solutions of 1d−1g Mn complexes in MeCN/0.1 M TBAPF₆ at GCE, scan rate 200 mVs⁻¹ under Ar, under CO₂ and with H₂O (5%v). CV in black is the electrolyte saturated with CO₂.

Figure 6

CVs of 0.5 mM solutions of 2d−2g Re complexes in MeCN/0.1 M TBAPF₆ at GCE, scan rate 200 mVs⁻¹ under Ar, under CO₂ and with 5%v MeOH. CV in black is the electrolyte saturated with CO₂.