Yuan-Zhu Zhang,† Song Gao,*,† Zhe-Ming Wang,† Gang Su,‡ Hao-Ling Sun,† and Feng Pan†

State Key Laboratory of Rare Earth Materials Chemistry and Applications and PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China, and College of Physical Sciences, Graduate School of the Chinese Academy of Sciences, P.O. Box 3908, Beijing 100039, P. R. China

Received November 11, 2004

Abstract:

    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°.