Single-chain magnet behavior in an alternated one-dimensional assembly of a MnIII Schiff-base complex and a TCNQ radical

An alternated 1:1 chain compound of a Mn-III salen derivative and the TCNQ monoradical was synthesized: [Mn(5-TMAMsaltmen)(TCNQ)](ClO4)(2) (1) (TCNQ=tetra-cyano-p-quinodimethane; 5-TMAM-saltmen = N,N-(1,1,2,2-tetramethyleth-ylene) bis(5-trimethylammoniomethyl-salicylideneiminato)). Compound I has a zigzag chain structure packed with adjacent chains with an interchain Mn center dot center dot center dot Mn distance of over 8 angstrom. As compound I contains no crystallization solvent, the void spaces between chains are occupied only by ClO4- counter ions. Compound I has a structure reminiscent of what has been observed in the family of Mn-III(porphyrin)-TCNE or -TCNQ compounds reported previously by Miller and co-workers and we demonstrate herein its unique single-chain magnet behavior among this family of compounds. The direct current (dc) magnetic measurements established the one-dimensional nature of compound I with an antiferromagnetic exchange coupling, J/k(B) approximate to -96 K, between the Mn-III ion and TCNQ radical and with an activated correlation length (Delta(xi) = 26.5 K) at low temperatures (50-15 K). The slow relaxation of the magnetization was shown in compound I by the field hysteresis of the magnetization observed below 3.5 K (with a coercive field up to 14 kOe at 1.8 K). Single-crystal magnetization measurements demonstrated the uniaxial symmetry of this compound and allowed an estimation of the anisotropy field, H-a approximate to 97 kOe. The absence of magnetic ordered phase or spin-glass behavior was established by heat-capacity calorimetry measurements that exhibit no abnormality of C-p between 0.5 K and 10 K. The study of the magnetization relaxation by combined ac (alternating current) and dc techniques showed that compound I possesses a single relaxation time (tau). As the consequence of the finite size of the chain, the temperature dependence of tau presents two activated regimes above and below 4.5 K with tau(01) = 2.1 x 10(-10) s, Delta(tau 1) = 94.1 K and tau(02) = 6.8 x 10(-8) s and Delta(tau 2) = 67.7 K, respectively. The detailed analysis of these dynamics properties together with the correlation length, allows an unambiguous demonstration of the single-chain magnet behavior in 1.