Effect of steric encumbrance of tris(3-phenylpyrazolyl)borate on the structure and properties of ternary copper(II) complexes having N,N-Donor heterocyclic bases

Complexes of formulation [Cu(Tp(Ph))(L)](ClO4) (1-4), where Tp(Ph) is anionic tris(3-phenylpyrazolyl)borate and L is N,N-donor heterocyclic base, viz. 2,2'-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyridoquinoxaline (dpq, 3), and dipyridophenazine (dppz, 4), are prepared from a reaction of copper(II) acetate-hydrate with KTp(Ph) and L in CH2Cl2 and isolated as perchlorate salts. The complexes are characterized by analytical, structural, and spectral methods. The crystal structures of complexes 1-4 show the presence of discrete cationic complexes having the metal, Tp(Ph), and L in a 1:1:1 ratio and a noncoordinating perchlorate anion. The complexes have a square-pyramidal 4 + 1 coordination geometry in which two nitrogens of L and two nitrogens of the Tp(Ph) ligand occupy the basal plane and one nitrogen of Tp(Ph) binds at the axial site. Complexes 3 and 4 display distortion from the square-pyramidal geometry. The Cu-N distances for the equatorial and axial positions are similar to2.0 and 2.2 Angstrom, respectively. The phenyl groups of Tp(Ph) form a bowl-shaped structure that encloses the {CuL} moiety. The steric encumbrance is greater for the bpy and phen ligands compared to that for dpq and dppz. The one-electron paramagnetic complexes (mu approximate to 1.8 mu(B)) exhibit axial EPR spectra in CH2Cl2 glass at 77 K giving g(parallel to) and g(perpendicular to) values of similar to2.18 (A(parallel to) = 128 G) and similar to2.07. The data suggest a {d(x2-y2)}(1) ground state. The complexes are redox-active and display a quasireversible cyclic voltammetric response for the Cu(II)/Cu(I) couple near 0.0 V versus SCE with an i(pc)/i(pa) ratio of unity in CH2Cl2 or DMF-0.1 M TBAP. The E-1/2 values of the couple vary in the order 4 > 3 > 2 > 1. A profound effect of steric encumbrance caused by the Tp(Ph) ligand is observed in the reactivity of 1-4 with the calf thymus (CT) and supercoiled (SC) DNA. Complexes 2-4 show similar binding to CT DNA. The propensity for the SC DNA cleavage varies as 4 > 3 > 2. The bpy complex does not show any significant binding or cleavage of DNA. Mechanistic investigations using distamycin reveal minor groove binding for 2 and 3 and a major groove binding for 4. The scission reactions that are found to be inhibited by hydroxyl radical scavenger DMSO are likely to proceed through sugar hydrogen abstraction pathways.