Small molecule analogues for the specific metal-binding site of lactoferrin. Part 2. Phenolato-complexes of copper(II) and the nature of the charge-transfer transition in the visible region

Abstract

The preparation of the following compounds is described: cis-[CuL₂L¹][L = pentafluorophenolate (pfp), L¹= ethylenediamine (en), NNN′N′-tetramethylethylenediamine (tmen), 2,9-dimethyl-1,10-phenanthroline (dmphen), or 2,2′-bipyridyl (bipy); L = pentachlorophenolate (pcp), L¹= en or dmphen; L = 2,4,6-tri-iodophenolate (tip), L¹= en, tmen, or dmphen; L = 2,4,6-tribromophenolate (tbp), L¹= en, tmen, or dmphen; L = 2,6-di-iodo-4-nitrophenolate (dinp), L¹= en, dmphen, or bipy], trans-[CuL₂L²₂][L = pfp, L²= pyridine (py), imidazole (imH), or 4-dimethylaminopyridine (dmap); L = pcp, pentabromophenolate (pbp), or dinp, L²= py; L = 2,4,6-trichlorophenolate (tcp), L²= dmap; L = tbp; L²= py or imH; L = tip, L²= py or dmap], and [CuL₂L³][L = pfp, L³= collidine (2,4,6-trimethylpyridine)(coll); L = pcp, L³= coll or NN-dimethylformamide (dmf); L = dinp, L³= coll or (dmf)₂]. Spectroscopic studies show most cis-[CuL₂L¹] and trans-[CuL₂L²₂] have tetragonal structures with axial bonding (sometimes weak) by the appropriate ortho-halogeno-groups or, if condensed, by bridging phenolato-groups. For cis-[CuL₂(dmphen)] an out-of-plane arrangement towards distorted tetrahedral exists in solution. The complexes [Cu(pcp)₂(coll)] and [Cu(pcp)₂(dmf)] have low magnetic moments at room temperature, hence binuclear or polymeric structures with bridging phenolato-groups are suggested. All the compounds show a strong charge-transfer (c.t.) absorption in the range 400–550 nm, with an intensity about 1 200 dm³ mol^–1 cm^–1, and weaker d–d bands at ca. 750 nm. By studying the variation in the c.t. absorption energy for this extensive series of compounds, where (a) the phenolate is kept constant and the coligands are varied and (b) the phenolate is varied and the coligands are kept constant, it is concluded that the assignment of the c.t. transition is consistent with a partial transfer of an electron from the P_π orbital on the phenolate oxygen to a d_σ· copper orbital. The c.t. band energies of the complexes are dependent on the nature of the solvent (solvents with low E_T values lowering the c.t. energy), the basicity and number of coligands and phenolato-groups, and the chelate effect (which raises the energy). From these considerations, and e.s.r. data, it is concluded that the copper sites in copper(II) lactoferrin have a CuN₂O₂O′₂ donor set containing two cis tyrosines, one histidine, and one water in the equatorial plane and possibly one histidine and an oxygen-donor ligand (non-ionized phenol) along the axial direction.