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 ∼2.0 and 2.2 Å, 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 (μ ≈ 1.8 μ₆) exhibit axial EPR spectra in CH₂Cl₂ glass at 77 K giving g_II and g_⊥ values of ∼2.18 (A_II = 128 G) and ∼2.07. The data suggest a {d_x2x2-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.