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

Two new series of neutral oxamato-bridged heterobimetallic chains of general formula [MCu(L-x)(2)]. mDMSO (m=0-4) (L-1=N-2-methyl-phenyloxamate, M=Mn (1a) and Co (1b); L-2 = 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 Mn2+ or Co2+ cations in DMSO. The crystal structures of [CoCu(L-2)(2)(H2O)(2)] (2b') and [CoCu(L-3)(2)(H2O)(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 (CuCo)-Co-... distance through the oxamate bridge is 5.296(1) angstrom in 2b' and 5.301(2) angstrom in 3B', while the shortest interchain (CoCo)-Co-... distance is 5.995(5) angstrom in 2b' and 8.702(3) angstrom 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 (MnCuII)-Cu-II and (CoCuII)-Cu-II chains exhibit ferrimagnetic behaviour with moderately strong intrachain antiferromagnetic coupling (-J(Mn.Cu) =24.7-27.9 cm(-1) and -J(CoCu) = 35.0-45.8 cm(-1); H= Sigma - J(M.Cu)S(M,i)S(Cu.i)), only the (CoCuII)-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 Coll ion. The blocking temperatures Tu 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 Ea to reverse the magnetisation direction are 38.0 (3b) and 16.3cm(-1) (W), while the preexponential factors tau(0) are 2.3x10(-11) (3b) and 4.0x10(-9) s (3b').