Yuichi Shimazaki

Studies on galactose oxidase active site model complexes: effects of ring substituents on Cu(II)-phenoxyl radical formation

Copper(II) complexes of new N 3 O- and N 2 O 2 -donor tripodal ligands bearing one or two o -substituted phenol moieties have been synthesized as models for the galactose oxidase active site. The complexes of 2-[ N -(1-methyl-2′-imidazolylmethyl)- N -(6″-methyl-2″-pyridylmethyl)-aminomethyl)]-4-methyl-6-methylthiophenol (MeSL), [Cu(MeSL)Cl], and N -(6-methyl-2-pyridylmethyl)- N , N -bis(2′-hydroxy-3′,5′-di- tert -butylbenzyl)amine (t-buL2mepy), [Cu(t-buL2mepy)(H 2 O)], have been revealed by X-ray structural analysis to have a square-pyramidal structure with one and two phenolate oxygens in the basal plane, respectively. [Cu(MeSL)Cl] was converted into a Cu(II)- o -methylthiophenoxyl radical species by electrochemical or Ce(IV) oxidation. An o -methoxyphenoxyl radical in a similar complex was considerably more stable than the 2,4-di( tert -butyl)phenoxyl radical. While t-buL2mepy reacted with Cu(ClO 4 ) 2 to give [Cu(t-buL2mepy)(H 2 O)] without disproportionation, an N2O2-donor ligand containing an o -methoxyphenol, a 2,4-di( tert- butyl)phenol, and an N -methylimidazole moiety gave a phenoxyl radical complex exhibiting the characteristic absorption peak at 478 nm as a reddish powder by the reaction with Cu(ClO 4 ) 2 as a result of spontaneous disproportionation. It exhibited a quasi-reversible redox wave at E 1/2 =0.34 V (vs. Ag/AgCl) in CH 3 CN, which is lower than the potentials of the copper complexes of various N 3 O-donor ligands, and oxidized ethanol to acetaldehyde with a low turnover number.

π–π Stacking assisted binding of aromatic amino acids by copper(II)–aromatic diimine complexes. Effects of ring substituents on ternary complex stability

Ternary Cu(ii) complexes containing an aromatic diimine (DA = di(2-pyridylmethyl)amine (dpa), 4,4-disubstituted 2,2-bipyridine (Y(2)bpy; Y = H (bpy), Me, Cl, N(Et)(2), CONH(2) or COOEt) or 2,2-bipyrimidine) and an aromatic amino acid (AA = l-phenylalanine (Phe), p-substituted phenylalanine (XPhe; X = NH(2), NO(2), F, Cl or Br), l-tyrosine (Tyr), l-tryptophan (Trp) or l-alanine (Ala)) were characterized by X-ray diffraction, spectroscopic and potentiometric measurements. The structures of [Cu(dpa)(Trp)]ClO(4).2H(2)O and [Cu((CONH(2))(2)bpy)(Phe)]ClO(4).H(2)O in the solid state were revealed to have intramolecular pi-pi interactions between the Cu(ii)-coordinated aromatic ring moiety, Cu(DA) (Mpi), and the side chain aromatic ring of the AA (Lpi). The intensities of Mpi-Lpi interactions were evaluated by the stability constants of the ternary Cu(ii) complexes determined at 25 degrees C and I = 0.1 M (KNO(3)), which revealed that the stability enhancement of the Cu(DA)(AA) systems due to the interactions is in the order (CONH(2))(2)bpy < bpy < Me(2)bpy < (Et(2)N)(2)bpy with respect to DA. The results indicate that the electron density of coordinated aromatic diimines influences the intensities of the stacking interactions in the Cu(DA)(AA) systems. The Mpi-Lpi interactions are also influenced by the substituents, X, of Lpi and are in linear relationship with their Hammett sigma(p) values with the exception of X = Cl and Br.

Conformational preference of the side chain aromatic ring in Cu(II) and Pd(II) complexes of 2N1O-donor ligands

Abstract Synthetic, structural, and spectroscopic studies were performed on the copper(II) and palladium(II) complexes of various 2N1O-donor tripod-like ligands containing a pyridine or a dimethylamine nitrogen, a tertiary amine nitrogen, and a phenolate oxygen as donor atoms and a pendent indole ring. [Pd(impp)Cl]·3H2O·CH3CN (impp=N-2-pyridylmethyl-N-2-hydroxybenzyl-3-aminomethylindole) and two other Pd(II) complexes and a Cu(II) complex were isolated as crystals and structurally characterized by the X-ray diffraction method. The indole moiety of the Pd(II) complexes was found to be located close to the pyridine ring or the dimethylamino group and away from the phenolate moiety. 1H NMR spectra of the Pd(II) complexes in CD3CN exhibited large upfield shifts of the pyridine and dimethylamino proton signals due to the pendent indole ring, supporting that the complexes in solution have a stacked structure similar to that found in the solid state. These findings indicated that the aromatic ring stacking interactions or CH–π interactions exist between the indole and the pyridine rings or the methyl groups, respectively. [Cu2(impp)2](ClO4)2·H2O was revealed to have a dimeric structure with the indole moiety close to both the Cu(II) center and the pyridine ring, which suggests the possibility that the Cu(II) ion may undergo metal–aromatic ring interactions.

CH⋯Metal(II) axial interaction in planar complexes (metal = Cu, Pd) and implications for possible environmental effects of alkyl groups at biological copper sites

Intramolecular M(II)...H-C interactions (M(II)=Cu(II), Pd(II)) involving a side chain alkyl group of planar d8 and d9 metal complexes of the N-alkyl (R) derivatives of N,N-bis(2-pyridylmethyl)amine with an N3Cl donor set were established by structural and spectroscopic methods. The methyl group from the branched alkyl group (R=2,2-dimethylpropyl and 2-methylbutyl) axially interacts with the metal ion with the M...C and M...H distances of 3.056(3)-3.352(9) and 2.317(1)-2.606(1) A, respectively, and the M-H-C angles of 122.4-162.3 degrees . The Cu(II) complexes showing the interaction have a higher redox potential as compared with those without it, and the (1)H NMR signals of the interacting methyl group in Pd(II) complexes shifted downfield relative to the ligand signals. Dependence of the downshift values on the dielectric constants of the solvents used indicated that the M(II)...H-C interaction is mainly electrostatic in nature and may be regarded as a weak hydrogen bond. Implications for possible environmental effects of the leucine alkyl group at the type 1 Cu site of fungal laccase are also discussed.

Effects of a Side Chain Aromatic Ring on the Reactivity of Copper(I) Complexes with Dioxygen

Stabilization of the bis(μ-oxo)dicopper(III) intermediate is achieved by using a bidentate N-donor ligand with pendent indole rings: N,N-bis[(3-indolyl)methyl]-N-[(2-pyridyl)methyl]amine (BIP). The intermediate decomposes to give products arising from N-dealkylation and a product with a spiro ring composed of two indole rings. This latter product probably results from a radical-coupling reaction.

2-(Imidazol-1-yl)-4-methyl­phenol

The title compound, C10H10N2O, was synthesized to investigate the structure of the Tyr–His residue in the active site of cytochrome c oxidase (CcO). The dihedral angle between the cresol and imidazole moieties is 42.21u2005(4)°, which is similar to that of the tyrosine-linked histidine in bovine heart CcO. In the title compound, the R and S enantiomers of axial chirality are connected by an O—H⋯N hydrogen bond.