Iron(i) complexes of 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline (H2dophen) as electrocatalysts for carbon dioxide reduction. X-Ray crystal structures of [Fe(dophen)Cl]2·2HCON(CH3)2 and [Fe(dophen)(N-MeIm)2]ClO4 (N-MeIm = 1-methylimidazole)

Abstract

The crystal structure of two iron complexes of 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline (H₂dophen) [Fe(dophen)Cl]₂·2HCON(CH₃)₂ [1·2HCON(CH₃)₂] and [Fe(dophen)(N-MeIm)₂]ClO₄ [2] (N-MeIm = 1-methylimidazole) have been determined: 1·2HCON(CH₃)₂, monoclinic, space group P2₁/n, a = 11.141(4), b = 15.519(4), c = 13.387(3) Å, β = 93.76(2)°, Z = 4; 2, triclinic, space group P, a = 10.293(1), b = 12.395(3), c = 12.400(6) Å, α = 105.04(4), β = 90.54(4), γ = 99.93(1)°, Z = 2. The cyclic voltammograms of 1 and 2 in dimethylformamide show that the iron complexes undergo three successive reversible reductions with E_1/2 = −0.80 V, −2.02 V, −2.45 V for 1 and −0.75 V, −2.03 V and −2.45 V for 2vs. the ferrocenium/ferrocene couple (Cp₂Fe^+/0) respectively. The first two couples are assigned as the Fe(III)/Fe(II) and Fe(II)/Fe(I) couples, whereas the couple at −2.45 V is assigned to the reduction of the dophen ligand. The Fe(I) species are active towards CO₂ reduction. Electrolysis of CO₂ in the presence of 1 or 2 at −2.0 V vs. Cp₂Fe^+/0 gave a mixture of carbon monoxide, formate and oxalate, with formate being the major product. The rate of CO₂ reduction was enhanced by the addition of 1,1,1-trifluoroethanol or methanol as the proton source to the electrolyte. Iron carbonyl and iron formato species were detected as intermediates by in-situ FTIR spectroelectrochemistry.