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The strategic design and synthesis of two isomeric Cu II complexes, [CuL A ] and [CuL B ], of asymmetrically dicondensed N 2 O 3 -donor Schiff-base ligands (where H 2 L A and H 2 L B are N -salicylidene- N '-3-methoxysalicylidenepropane-1,2-diamine and N -3-methoxysalicylidene- N '-salicylidenepropane-1,2-diamine, respectively) have been accomplished via a convenient Cu II template method. These two complexes have been used as metalloligands for the synthesis of three pairs of Cu-Ln isomeric complexes [CuL(μ-NO 3 )Ln(NO 3 ) 2 (H 2 O)]·CH 3 CN (for complexes 1A - 3A , L = L A , and for complexes 1B - 3B , L = L B and Ln = Gd, Tb, and Dy, respectively), all of which have been characterized structurally. In all six isomorphous and isostructural complexes, the decacoordinated Ln III centers and pentacoordinated Cu II centers possess sphenocorona and square-pyramidal geometries, respectively. The isomeric pair of Cu-Gd compounds shows field-induced slow relaxation of magnetization, although they present the typical isotropic behavior of Gd III complexes, indicating that slow relaxation is not due to the usual energy barrier originating from the magnetic anisotropy. The isostructural derivatives with the ion-anisotropic lanthanides Tb III and Dy III do not show slow magnetic relaxation with or without a direct-current bias field, demonstrating that the magnetic response of the isotropic system Cu II -Gd III occurs through different mechanisms than the rest of the Ln cations.

In the present work, four new heterometallic coordination complexes, {[(CuL) 2 Mn(nic)(H 2 O) 2 ](ClO 4 )(0.5H 2 O)} n ( 1 ), {[(CuL) 2 Cd(nic)(H 2 O) 2 ](ClO 4 )(H 2 O)} n ( 2 ), [(CuL) 2 Mn(nic) 2 ]·2CH 3 OH ( 3 ), and [(CuL) 2 Cd(nic) 2 ]·2CH 3 OH ( 4 ) (where H 2 L = N , N '-bis(α-methylsalicylidene)-1,3-propanediamine and nic = nicotinate ion), have been synthesized and characterized by single-crystal X-ray crystallography. In complexes 1 and 2 , the nicotinate ion acts as a bifunctional linker (N,O donor) and joins the linear trinuclear nodes to form 1D polymeric chains. However, in complexes 3 and 4 , the nicotinate ion uses only the oxygen atoms of the carboxylic acid (O donor) to bind to the metal centers, forming discrete linear trinuclear units, while the pyridyl nitrogen (N donor atom) remains free. The dc magnetic susceptibility measurements show that the Cu II and Mn II ions are antiferromagnetically coupled in both 1 and 3 , with exchange coupling constants ( J Mn-Cu ) of -20.57 ± 0.08 and -9.38 ± 0.08 cm -1 , respectively. Among the four complexes, 1 and 3 show catechol oxidase and phenoxazinone synthase like catalytic activities. The turnover numbers ( k cat ) of complexes 1 and 3 for catecholase activity are 1121 and 720 h -1 , respectively, at an optimum pH of 8.0 and for phenoxazinone synthase activity are 429 and 398 h -1 , respectively, at an optimum pH of 9.7. The higher k cat values of 1 for both reactions are attributable to a water molecule coordinated to the central Mn II atom that facilitates the substrate-catalyst binding. An ESI-mass spectral analysis indicates that trinuclear heterometallic species, e.g., [(CuL) 2 Mn(nic)(H 2 O)] + for 1 and [(CuL) 2 Mn(nic)] + for 3 , are the active species that bind to the substrate, and on that basis, probable mechanisms through the formation of radical intermediates have been proposed.