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·H2O}n (2), [Mn(bpy)2][Cr(bpy)(CN)4]2·5H2O (3), {[Mn(dca)(bpy)(H2O)][Cr(bpy)(CN)4]·H2O}n (4) (dca = N(CN)2-), {Mn(N3)(CH3OH)[Cr(bpy)(CN)4]·2H2O}n (5), and {Mn(bpy)(N3)(H2O)[Cr(bpy)(CN)4]·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 (1D) 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 CrIII and MnII ions in all compounds. All of the chain compounds exhibit metamagnetic behaviors with different critical temperatures (Tc) and critical fields (Hc; 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 = −JSCr·SMn). It is observed that the antiferromagnetic interaction through the Mn−N−C−Cr bridge increases as the Mn−N−C angle (θ) decreases to the range of 155−180°, obeying an empirical relationship: J = −40 + 0.2θ. This result suggests that the best overlap between t2g (high-spin MnII) and t2g (low-spin CrIII) occurs at an angle of ∼155°.