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

Magnetic exchange and charge transfer mechanism in dimeric (1) [Cu(Lw)(2)H2O](2) and monomeric (2). [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 (1) are fitted to Heisenberg's isotropic spin pair (S'=1.1) model and tetranuclear (S'= 1/2, 1/2, 1/2, 1/2) spins in a two dimensional model. Both models result in intradimer antiferromagnetic coupling (J=-44.4 cm(-1)) between [Cu(NSQ)](2) units and ferromagnetic coupling (zJ(1)=28.5 cm(-1)) between one Cu(II)NSQ coordinated unit (where NSQ=naphtho-semiquinone). 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=3.04 Angstrom. The monomeric complex (2) is devoid of such interactions due to the bulky methyl group at C3 position of quinonoid ring. Cyclic voltammograms of (1) and (2) 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 NSQdouble left right arrowCAT (catechol) charge transfer leads to radical coordination in (1). However quasireversible NQdouble left right arrowNSQ 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).