Studies of chelation. Part 9. Cobalt complexes of 1-[(substituted phenyl)azo]-2-naphthol and 1-[(substituted phenylimino)methyl]-2-naphthol ligands. Tautomerism and reactivity

Abstract

Spectroscopic characterisation (¹H and ¹³C n.m.r., i.r., Raman, and u.v.–visible) of various 1-[(substituted phenyl)-azo]-2-naphthols, R-Haz (R = 4′-Me, 4′-OMe, 4′-Cl, H, 2′-OMe, 2′-Me, 2′-Cl, and 2′-Br), and 1-[(substituted phenylimino) methyl]-2-naphthols, R-Hsb (R = 4′-OMe, 4′-Me, 4′-Cl, H, 4′-Br, 4′-CF₃, 4′-CN, 4′-NO₂, 2′-OMe, 2′-Me, 2′-CHMe₂, 2′-Cl, 2′-CN, and 2′-NO₂), in the solid state and in solution in both polar and non-polar solvents has shown that the hydrogen-bonded keto-tautomer is dominant in polar solvents. Reaction of cobalt(II) salts with R-Haz and with R-Hsb in ethanol produces [CoL₂], cis-[Co(HL)₂X₂], and fac-[CoL₃](L = R-az or R-sb). The tautomeric form of the ligand in these complexes is established by vibrational spectroscopy and shown to be predominantly keto (hydrazone, ketoenamine) in [Co(R-az)₂], in [Co(R-Haz)₂X₂], and [Co(R-Hsb)₂X₂]( X = Cl or Br) but predominantly enolimine in [Co(R-sb)₂]. The interconversion of [CoL₂] and [Co(HL)₂X₂] is established generally: the intermediate, five-co-ordinate [Co(R-Hsb)(R-sb)X] is identified in certain cases (R = 4-OMe, 4′-Me, or H). In the chelation process, proton loss follows ring closure and is succeeded by tautomerisation where this occurs. The facility with which the chelating ligand deviates from coplanarity of the benzene and naphthalene rings is a critical feature of the chelation process. The formation of [Co(R-az)₃] from [Co(R-az)₂] and from [Co(R-Haz)₂X₂] in the presence of air occurs easily and is acid catalysed. [Co(R-sb)₃] is formed only under vigorously oxidising conditions. The isoelectronic ligands R-Haz and R-Hsb should not be regarded as identical in their reactivity towards cobalt(II).