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(TpPh)(L)](ClO4) (1−4), where TpPh 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 KTpPh 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, TpPh, 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 TpPh ligand occupy the
basal plane and one nitrogen of TpPh 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 TpPh 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 μB) exhibit axial EPR spectra in CH2Cl2 glass at 77 K giving g∥ and g⊥ values of ∼2.18 (A∥ = 128 G)
and ∼2.07. The data suggest a {dx2-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 ipc/ipa ratio of unity in
CH2Cl2 or DMF−0.1 M TBAP. The E1/2 values of the couple vary in the order 4 > 3 > 2 > 1. A profound effect of
steric encumbrance caused by the TpPh 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.