A series of iron(III) complexes of tetradentate tripodal ligands [L1−H2(L7); L1 = tris(pyrid-2-ylmethyl)amine, L2 = bis(pyrid-2-ylmethyl)(pyrid-2-ylethyl)amine, H(L3) = 2-bis(pyrid-2-ylmethyl)aminomethyl-4-nitrophenol, H2(L4) = N,N-bis(2-hydroxy-5-nitrobenzyl)aminomethylpyridine, L5 = tris(benzimidazol-2-ylmethyl)amine, H(L6) = 2-bis(benzimidazol-2-yl)methylaminomethyl-4-nitrophenol, and H2(L7) = N,N-bis(2-hydroxy-5-nitrobenzyl)aminomethylbenzimidazole] have been synthesized. These complexes have been characterized using UV−vis spectral and electrochemical techniques. The single-crystal X-ray structure of [Fe(L3)Cl2] has been determined by standard procedures and refined by least squares to a conventional R factor of 0.054. The violet crystals belong to the triclinic space group P1̄ with Z = 2 and unit cell dimensions a = 7.018(4) Å, b = 9.087 Å, and c = 13.962(5) Å. The iron(III) complex has a pseudo-octahedral N3OCl2 donor set with two chloride ions occupying cis positions; the Fe−O(phenolate) bond is trans to one of the Fe−Npy bonds. The interaction of all the Fe(III) complexes with a variety of monodentate and bidentate heterocyclic bases as well as phenols has been investigated using electronic spectra. The interactions with catecholate anions reveal changes in the phenolate-to-iron(III) charge transfer band, similar to catechol dioxygenase−substrate complexes. The redox behavior of the complexes and their 1:1 adducts with 3,5-di-tert-butylcatechol (H2DBC) has been investigated. All the complexes catalyze the oxidative cleavage of H2DBC by molecular oxygen to yield cis,cis-muconic anhydride. The catalytic activity could be illustrated on the basis of the Lewis acidity of the complex−DBC adducts, derived from the CAT2-/DBC2- → Fe(III) CT band energy and the E 1/2 of the DBSQ/DBC couple, and steric hindrance to substrate binding and by assuming that the product release is the rate-determining phase of the reaction.

A series of high-spin octahedral 1 : 2 iron(I1r) complexes of Schiff bases derived from salicylaldehyde and aromatic amines and the 1 : 1 and 1 : 2 complexes of bis(pyridin-2-yl)-aza and bis(benzimidazol-2-yl)-aza and -thioether ligands have been isolated. The crystal structure of trichloro[bis(pyridin-2-ylmethyl)amine]iron(r11) has been determined. It contains two crystallographically independent molecules in the asymmetric unit cell. In each molecule iron(rI1) possesses a rhombically distorted octahedral co-ordination, constituted by all the three nitrogens of the facial ligand and three chloride ions. The effects of stereochemical and/or donor atom variations on the UV/VIS and EPR spectra and Fe"'-Fe" redox potentials and hence on the Lewis acidity of the iron(m) centre are discussed. The phenolate-to-iron(m) charge-transfer band energy of [FeL' ,]Cl [HL' = N-(pyridin-2-ylmethyl)salicylideneamine] is higher and its Fe"'-Fe" redox potential more negative than those of the corresponding 1 : 1 complex.

The crystal structures of [Cu(bpnp)(SCN)].NH(4)SCN (1), [Cu(bpnp)(CH(3)COO)].CH(3)OH.C(8)H(10) (2), and [Cu(bpnp)ClO(4)] (3) [Hbpnp = 2-(bis(pyrid-2-ylmethyl)aminomethyl)-4-nitrophenol] reveal a distorted square pyramidal geometry around Cu(II) with an unusual axial coordination of phenolate. The mononuclear complex [Cu(bpnp)(SCN)].NH(4)SCN crystallizes in the triclinic space group P&onemacr; with a = 10.796(2) Å, b = 10.804(2) Å, c = 12.559(2) Å, alpha = 71.38(1) degrees, beta = 72.68(1) degrees, gamma = 61.69(1) degrees, and Z = 2. The mononuclear acetate [Cu(bpnp)(CH(3)COO)].CH(3)OH.C(8)H(10) crystallizes in the triclinic space group P&onemacr; with a = 10.480(6) Å, b = 12.116(4) Å, c = 12.547(3) Å, alpha = 98.77(3) degrees, beta = 113.37(3) degrees, gamma = 100.78(3) degrees, and Z = 2. The binuclear perchlorate complex crystallizes in the monoclinic space group C2/c with a = 13.417(3) Å, b = 20.095(2) Å, c= 16.401(2) Å, alpha = 102.21(2) degrees, and Z = 8. The coordination plane in all these complexes is comprised of the tertiary amine and two pyridine nitrogens. The fourth equatorial position is occupied by SCN(-)/CH(3)COO(-) in the mononuclear complexes but by the coordinated phenolate ion from the adjacent molecule in the perchlorate complex, resulting in its dimerization. The unusual occupation of phenolate ion in the axial site is possibly due to the steric constraint at copper imposed by the 5,5,6-chelate ring sequence. The thiocyanate/acetate coordination geometry is reminiscent of the active site of the radical copper enzyme galactose oxidase (GOase) with an axial phenolate and equatorial SCN(-)/CH(3)COO(-) ligands. Further, the present complexes exhibit several spectral features also similar to this enzyme. The addition of chloride or thiocyanate or acetate ions dissociates the dimeric structure of the perchlorate complex to produce the corresponding monomeric derivatives. The study of the interaction of the acetate complex with N(3)(-) and CN(-) ions provide insight into the anion binding properties of the enzyme. The sensitivity of the acetate complex to protons suggests the facile dissociation of the axial phenolate which then acts as a base to bind to protons. The implication of this reaction to the GOase mechanism is discussed.

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