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

Shanta Dhar, Pattubala A.N. Reddy, Munirathinam Nethaji, Subramony Mahadevan, Manas K. Saha, Akhil R. Chakravarty

Research output: Contribution to journalArticle

88 Scopus citations

Abstract

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 μ6) exhibit axial EPR spectra in CH2Cl2 glass at 77 K giving gII and g values of ∼2.18 (AII = 128 G) and ∼2.07. The data suggest a {dx2x2-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.

Original languageEnglish (US)
Pages (from-to)3469-3476
Number of pages8
JournalInorganic Chemistry
Volume41
Issue number13
DOIs
StatePublished - Jul 1 2002
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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