Rational synthesis and magnetic properties of a family of low-dimensional heterometallic Cr-Mn complexes based on the versatile building block [Cr(2,2′-bipyridine)(CN)4]-

Six heterometallic compounds based on the building block [Cr(bpy)(CN)(4)](-) (bpy = 2,2'-bipyridine) with secondary and/or tertiary coligands as modulators, {Mn(H2O)(2)[Cr(bpy)(CN)(4)](2)}(n) (1), {Mn(bpy)(H2O)[Cr(bpy)(CN)(4)](2)center dot H2O}(n) (2), [Mn(bpy)(2)][Cr(bpy)(CN)(4)](2)center dot 5H(2)O (3), {[Mn(dca)(bpy)(H2O)][Cr(bpy)(CN)(4)]center dot H2O}(n) (4) (dca = N(CN)(2)(-)), {Mn(N-3)-(CH3OH)[Cr(bpy)(CN)(4)](2)center dot 2H(2)O}(n) (5), and {Mn(bpy)(N-3)(H2O)[Cr(bpy)(CN)(4)]center dot H2O}(2) (6), have been prepared and characterized structurally and magnetically. X-ray crystallography reveals that the compounds 1, 2, 4, and 5 consist of one-dimensional (1 D) chains with different structures: a 4,2-ribbon-like chain, a branched zigzag chain, a 2,2-CC zigzag chain, and a 3,3-ladder-like chain, (1b) respectively. It also reveals that compound 3 has a trinuclear [MnCr2] struture, and compound 6 has a tetranuclear [Mn2Cr2] square structure. Magnetic studies show antiferromagnetic interaction between Cr-III and Mn-II ions in all compounds. All of the chain compounds exhibit metamagnetic behaviors with different critical temperatures (T-C) and critical fields (H-C; at 1.8 K): 3.2 K and 3.0 kOe for 1; 2.3 K and 4.0 kOe for 2; 2.1 K and 1.0 kOe for 4; and 4.7 K and 5.0 kOe for 5, respectively. The noncentrosymmetric compound 2 is also a weak ferromagnet at low temperature because of spin canting. The magnetic analyses reveal Cr-Mn intermetallic magnetic exchange constants, J, of -4.7 to -9.4 cm(-1) (H = -JS(Cr)center dot S-Mn). It is observed that the antiferromagnetic interaction through the Mn-N-C-Cr bridge increases as the Mn-N-C angle (theta) decreases to the range of 155-180 degrees, obeying an empirical relationship: J = -40 + 0.2 theta. This result suggests that the best overlap between t(2g) (high-spin Mn-II) and t(2g) (low-spin Cr-III) occurs at an angle of similar to 155 degrees.