Dioxygen reactivity of iron(ii)-gentisate/1,4-dihydroxy-2-naphthoate complexes of N4 ligands: Oxidative coupling of 1,4-dihydroxy-2-naphthoate

The influence of supporting ligands and co-ligands on the dioxygen reactivity of a series of iron(ii) complexes, [(6-Me₃-TPA)Fe^II(GN-H)]⁺ (1), [(6-Me₃-TPA)Fe^II(DHN-H)]⁺ (1a), [(BPMEN)Fe^II(GN-H)]⁺ (2), [(BPMEN)Fe^II(DHN-H)]⁺ (2a), [(TBimA)Fe^II(GN-H)]⁺ (3), and [(TBimA)Fe^II(DHN-H)]⁺ (3a) (GN-H₂ = 2,5-dihydroxybenzoic acid and DHN-H₂ = 1,4-dihydroxy-2-naphthoic acid) of N4 ligands, is presented. The iron(ii)-gentisate complexes react with dioxygen to afford the corresponding iron(iii) species. On the contrary, DHN-H undergoes oxidative C-C coupling to form [2,2′-binaphthalene]-1,1′,4,4′-tetrone 3-hydroxy-3′-carboxylic acid (BNTHC) on 1a, and [2,2′-binaphthalene]-1,1′,4,4′-tetrone 3,3′-dicarboxylic acid (BNTD) on 2a and 3a. In each case, the reaction proceeds through an iron(iii)-DHN species. The X-ray single crystal structures of [(6-Me₃-TPA)Fe^II(BNTD)] (1^Ox) and [(BPMEN)Fe^II(BNTD)] (2^Ox) confirm the coupling of two DHN-H molecules. The formation of iron(iii) product without any coupling of co-ligand from the complexes, [(BPMEN)Fe^II(HNA)]⁺ (2b) and [(BPMEN)Fe^II(5-OMeSA)]⁺ (2c) (HNA = 1-hydroxy-2-naphthoate, 5-OMeSA = 5-methoxysalicylate) confirms the importance of para-hydroxy group for the coupling reaction. The unusual coupling of DHN-H by the iron(ii) complexes of the neutral N4 ligands is distinctly different from the oxygenolytic aromatic C-C cleavage of DHN by the iron(ii) complex of a facial N3 ligand.