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)center dot center dot center dot H-C interactions (M(II)=Cu(II), Pd(II)) involving a side chain alkyl group of planar d(8) and d(9) 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 center dot center dot center dot C and M center dot center dot center dot H distances of 3.056(3)-3.352(9) and 2.317(1)-2.606(1) angstrom, 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 H-1 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)center dot center dot center dot 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 I Cu site of fungal laccase are also discussed. (C) 2007 Elsevier Inc. All rights reserved.