Temperature Effect on Ancillary μ‐Carbonato Ligand Modes in Hydroxy Naphthoquinonato Copper(II) Complex: An EPR Spectroscopic and Magnetic Coupling Evidences

Abstract

Thermal, magnetic and electrochemical properties of the complex 1, [Cu₂ (μ‐CO₃) (ONQ)₂ (4HNQ)₂] are discussed [where ONQ=2‐oxido‐1, 4‐napthoquinone and 4HNQ=4‐hydroxy‐1, 2‐naphthoquinone]. The coordination of the ligand is in tautomeric forms viz. ONQ and 4HNQ in 1. The carbonate anion acts as a bridge between two Cu (II) centers showing bidentate (μ‐η¹‐η¹‐CO₃ ^2‐) coordination mode with distorted square based pyramidal geometry for each Cu(II) ion. Temperature‐dependent zero field splitting (zfs) interactions (D and E) are resolved from X‐ (300K and 77 K) and Q‐band (300K) EPR spectra. At 300K, E>D and at 77K, D>E. It reveals temperature dependent modes of coordination viz. syn‐anti and anti‐anti of ancillary ligand in 1. Variable temperature magnetic susceptibility (SQUID) measurements show a weak antiferromagnetic coupling between the two Cu (II) centers. The susceptibility data was best fitted with HDVV model and the coupling constant found to be J=−4.8 cm⁻¹ with agreement factor R=2.41∗10⁻⁸. Metal based redox couple assigned at E_½=+0.18 V with ΔE=0.125 V corresponds to two electrons transfer in the Cu (II)⇔Cu (0) redox couple.

Keywords:

• lawsone
• quinones
• antiferromagnetic exchange
• carbonato bridged complexes
• zero field splitting

We thank the late Prof. O. Kahn for valuable discussions and for SQUID data measurements on the complex. Salunke‐Gawali S. is thankful to UGC for awarding the SRF. Rane S.Y. is grateful to the CSIR, New Delhi, India (01(1686)/00/EMR‐II) for the grant. We thank to Prof. S. B. Padhye, Head, Department of Chemistry, University of Pune for providing CV facility.