Single-chain magnet behavior in an alternated one-dimensional assembly of a Mn^III 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)](ClO ₄)₂ (1) (TCNQ = tetracyano-p-quinodimethane; 5-TMAM-saltmen = N,N′-(1,1,2,2-tetramethylethylene) bis(5-trimethylammoniomethylsalicylideneiminato)). Compound 1 has a zigzag chain structure packed with adjacent chains with an interchain Mn⋯Mn distance of over 8 Å. As compound 1 contains no crystallization solvent, the void spaces between chains are occupied only by ClO₄^- counter ions. Compound 1 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 1 with an antiferromagnetic exchange coupling, J/k_B≈-96K, between the Mn^III ion and TCNQ radical and with an activated correlation length (Δ_ξ = 26.5K) at low temperatures (50-15 K). The slow relaxation of the magnetization was shown in compound 1 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≈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 1 possesses a single relaxation time (r). As the consequence of the finite size of the chain, the temperature dependence of r presents two activated regimes above and below 4.5 K with τ₀₁ =2.1 × 10-₁₀ s, Δ_τ1 = 94.1 K and τ₀₂ = 6.8× 10^-8s and Δ _τ2 = 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.