A novel magnetic exchange mechanism in<b> </b>bridged copper (II) coordinations of naturally occurring spin carriers viz. <i>ortho-</i>functionalized<i> p</i>-naphthoquinones

Abstract

Magnetic exchange and charge transfer mechanisms in dimeric (<b>1</b>) [Cu(Lw)₂H₂O] and monomeric (<b>2</b>), [Cu(phth)₂] complexes of lawsone (2-hydroxy-1,4-naphthoquinone) and phthiocol (3-methyl-2-hydroxy 1,4-naphthoquinone) are described. Magnetic susceptibility measurements of (<b>1</b>) are fitted to Heisenberg's isotropic spin pair (S' = 1,1) model and

tetranuclear (S' = ½, ½,½, ½) spins in a two dimensional model. Both models result in intradimer antiferromagnetic coupling (<i>J</i> = -44.4 cm^-1) between [Cu(NSQ)]₂ units and ferromagnetic coupling (<i>zJ</i>₁ = 28.5 cm^-1) between one Cu(II)NSQ coordinated unit (where NSQ = naphtho-semiquinone).<br>

Latter interaction leads to a triplet ground state and triplet-triplet intradimer interaction governs the ‘‘quintet state’’ at room temperature, as detected from X- and Q-band EPR studies. The zero- field split interaction (0.024 cm^-1) exhibits

contributions from D_pd (-0.071 cm^-1) and combined D_d and D_anti (+0.095 cm^-1) interactions. The intradimer exchange is due to a stacked (S' = 1,1) spin pair with r = 3.04Å. The monomeric complex (2) is devoid of such interactions due to the bulky methyl group at C3 position of quinonoid ring.<br>

Cyclic voltammograms of (<b>1</b>) and (<b>2</b>) show an anodic peak at +0.22 V, assigned to Cu(0) → Cu(II) charge transfer. Only one redox couple at E_1/2= -0.687 V due to NSQ CAT (catechol) charge transfer leads to radical coordination in (1). However, quarsireversible NQ NSQ redox couple at E_1/2 = -0.525 V together with an irreversible NSQ → CAT reduction peak in (2) at E_1/2= -0.75 V indicate fully the presence of oxidized (NQ) i.e. naphthoquinone ligation in (2).