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RESEARCH PRODUCT

Ligand Design for Heterobimetallic Single-Chain Magnets: Synthesis, Crystal Structures, and Magnetic Properties of MIICuII (M=Mn, Co) Chains with Sterically Hindered Methyl-Substituted Phenyloxamate Bridging Ligands

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Two new series of neutral ox-amato-bridged heterobimetallic chains of general formula [MCu(L x ) 2 ]- m DMSO (m=0-4) (L 1 =N-2-methyl-phenyloxamate, M=Mn (1a) and Co (1 b); L2 = N-2,6-dimethylphenyloxamate, M=Mn (2a) and Co (2b); L 3 = N-2,4,6-trimethylphenyloxamate, M= Mn (3a) and Co (3b)) have been prepared by reaction between the corresponding anionic oxamatocopper(II) complexes [Cu(L x ) 2 ] 2- with Mn 2+ or Co 2+ cations in DMSO. The crystal structures of [CoCu(L 2 ) 2 (H 2 O) 2 ] (2b') and [CoCu(L 3 ) 2 (H 2 O) 2 ]·4H 2 O (3b') have been solved by single-crystal X-ray diffraction methods. Compounds 2b' and 3b' adopt zigzag and linear chain structures, respectively. The intrachain Cu···Co distance through the oxamate bridge is 5.296(1) A in 2b' and 5.301(2) A in 3b', while the shortest interchain Co···Co distance is 5.995(5) A in 2b' and 8.702(3) A in 3b', that is, the chains are well isolated in the crystal lattice due to the presence of the bulky methyl-substituted phenyl groups. Although both Mn II Cu II and Co II Cu II chains exhibit ferrimagnetic behaviour with moderately strong intrachain antiferromagnetic coupling (-J Mn,Cu =24.7-27.9 cm -1 and -J Co,Cu = 35.0-45.8cm -1 ; H= E -J M,Cu S M,i S Cu,i ), only the Co II Cu II chains show slow magnetic relaxation at low temperatures (T B <3.5 K), which is characteristic of single-chain magnets (SCMs) because of the high magnetic anisotropy of the Co II ion. The blocking temperatures T B along this series of chains vary according to the steric hindrance of the aromatic substituent of the oxamate ligand in the series L 1 <L 2 <L 3 . Analysis of the SCM behaviour for 3b and 3b' on the basis of Glauber's theory for a one-dimensional Ising system showed a thermally activated mechanism for the magnetic relaxation (Arrhenius law dependence). The activation energies E a to reverse the magnetisation direction are 38.0 (3b) and 16.3 cm -1 (3b'), while the preexponential factors To are 2.3 x 10 -11 (3b) and 4.0 x 10 -9 s (3b').