A Magneto-Structural Study of a Hexanuclear (VIV═O)6-Complex and a Tetranuclear Mixed-Valent [VIII2VIV2] Species

Syntheses, crystal structures, and magnetic properties are reported for a hexametallic and a mixed-valent tetrametallic vanadium cluster, namely [Na(L(1))(6)(V(IV)=O)(6)]ClO(4) (1) and [(L(2))(2)V(III)(2)V(IV)(2)(mu-Cl)(6)Cl(4)] (2), where H(2)L(1) represents N-methyldiethanolamine and H(2)L(2) N'N'-di(3,5-di-tert-butyl-salicylidene)-1,3-diaminobenzene. The structure of the cation in 1 is comprised of a hexagon of six (V(IV)=O) units, in which a sodium ion is encapsulated in the center of the hexagon. Thus, the hexametalate ring contains six d(1) ions, each of which is in an octahedral environment of O(5)N atoms. Magnetic studies reveal the cluster to exhibit ferromagnetic exchange interactions (J = +16.7 +/- 0.3 cm(-1)), which is rationalized by the orthogonal neighboring xy planes of d(1) V(IV)=O ions. The structure of complex 2 can be considered as two mixed-valent face-sharing bioctahedral units of [V(III)(mu-Cl)(3)V(IV)] bridged by two m-phenylene linkers. The V(1)(III)...V(2)(IV) distances of av. 3.10 A preclude metal-metal bonding. Variable-temperature magnetic susceptibility data are fitted to obtain the parameters J(1) = -47.6 cm(-1), J(2) = +1.5 cm(-1), g(V(III)) = 1.95, g(V(IV)) = 1.85 (H = -2J x S(i).S(j)). The ferromagnetic coupling J(2) operates at a distance of approximately 7.09 A between two vanadium centers through the spin polarization mechanism due probably to the topology (1,3-substitution) of the m-phenylene bridges. The X-band electron paramagnetic resonance (EPR) spectrum analyzed with S(t) = 1 shows that the mixed-valent V(III)V(IV) species has indeed been formed and the tetranuclear core remains intact in solution. The hyperfine coupling constants indicate the localized mixed-valence nature of 2.