Slow Magnetic Relaxation, Antiferromagnetic Ordering, and Metamagnetism in MnII(H2dapsc)-FeIII(CN)6 Chain Complex with Highly Anisotropic Fe-CN-Mn Spin Coupling

Reactions of [Mn(H(2)dapsc)Cl-2]H2O (dapsc=2,6-diacetylpyridine bis(semicarbazone)) with K-3[Fe(CN)(6)] and (PPh4)(3)[Fe(CN)(6)] lead to the formation of the chain polymeric complex {[Mn(H(2)dapsc)][Fe(CN)(6)][K(H2O)(3.5)]}(n)1.5n H2O (1) and the discrete pentanuclear complex {[Mn(H(2)dapsc)](3)[Fe(CN)(6)](2)(H2O)(2)}4 CH(3)OH3.4 H2O (2), respectively. In the crystal structure of 1 the high-spin [Mn-II(H(2)dapsc)](2+) cations and low-spin hexacyanoferrate(III) anions are assembled into alternating heterometallic cyano-bridged chains. The K+ ions are located between the chains and are coordinated by oxygen atoms of the H(2)dapsc ligand and water molecules. The magnetic structure of 1 is built from ferrimagnetic chains, which are antiferromagnetically coupled. The complex exhibits metamagnetism and frequency-dependent ac magnetic susceptibility, indicating single-chain magnetic behavior with a Mydosh-parameter phi=0.12 and an effective energy barrier (U-eff/k(B)) of 36.0 K with tau(0)=2.34x10(-11) s for the spin relaxation. Detailed theoretical analysis showed highly anisotropic intra-chain spin coupling between [Fe-III(CN)(6)](3-) and [Mn-II(H(2)dapsc)](2+) units resulting from orbital degeneracy and unquenched orbital momentum of [Fe-III(CN)(6)](3-) complexes. The origin of the metamagnetic transition is discussed in terms of strong magnetic anisotropy and weak AF interchain spin coupling.