Synthesis, Structure, and Magnetic Properties of 1D {[Mn III (CN) 6 ][Mn II (dapsc)]} n Coordination Polymers: Origin of Unconventional Single-Chain Magnet Behavior.

Two one-dimensional cyano-bridged coordination polymers, namely, {[MnII (dapsc)][MnIII (CN)6 ][K(H2 O)2.75 (MeOH)0.5 ]}n ·0.5n(H2 O) (I) and {[MnII (dapsc)][MnIII (CN)6 ][K(H2 O)2 (MeOH)2 ]}n (II), based on alternating high-spin MnII (dapsc) (dapsc = 2,6-diacetylpyridine bis(semicarbazone)) complexes and low-spin orbitally degenerate hexacyanomanganate(III) complexes were synthesized and characterized structurally and magnetically. Static and dynamic magnetic measurements reveal a single-chain magnet (SCM) behavior of I with an energy barrier of Ueff ≈ 40 K. Magnetic properties of I are analyzed in detail in terms of a microscopic theory. It is shown that compound I refers to a peculiar case of SCM that does not fall into the usual Ising and Heisenberg limits due to unconventional character of the MnIII -CN-MnII spin coupling resulting from a nonmagnetic singlet ground state of orbitally degenerate complexes [MnIII (CN)6 ]3- . The prospects of [MnIII (CN)6 ]3- complex as magnetically anisotropic molecular building block for engineering molecular magnets are critically analyzed.