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, {[Mn ^II (dapsc)][Mn ^III (CN) ₆ ][K(H ₂ O) _2.75 (MeOH) _0.5 ]} _n ·0.5n(H ₂ O) (I) and {[Mn ^II (dapsc)][Mn ^III (CN) ₆ ][K(H ₂ O) ₂ (MeOH) ₂ ]} _n (II), based on alternating high-spin Mn ^II (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 U _eff ≈ 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 Mn ^III -CN-Mn ^II spin coupling resulting from a nonmagnetic singlet ground state of orbitally degenerate complexes [Mn ^III (CN) ₆ ] ^3- . The prospects of [Mn ^III (CN) ₆ ] ^3- complex as magnetically anisotropic molecular building block for engineering molecular magnets are critically analyzed.