Synthesis, Crystal Structures and Magnetic Properties of MIICuII Chains (M=Mn and Co) with Sterically Hindered Alkyl-Substituted Phenyloxamate Bridging Ligands

A series of neutral oxamato-bridged heterobimetallic chains of general formula [MCu(L-x)(2)(S)(2)]center dot p S center dot qH(2)O [p=0-1, q=0-2.5; L-1=N-2,6-dimethylphenyloxamate, S=DMF with M=Mn (1a) and Co (1b); L-2=N-2,6-diethylphenyloxamate, S=DMF with M=Mn (2a) and Co (2b) or S=DMSO with M=Mn (2c) and Co (2d); L-3=N-2,6-diisopropylphenyloxamate, S=DMF with M=Mn (3a) and Co (3b) or S=DMSO with M=Mn (3c) and Co (3d)] were prepared by treating the corresponding anionic oxamatocopper(II) complexes [Cu(L-x)(2)](2-) (x=1-3) with M2+ cations (M=Mn and Co) in DMF or DMSO as the solvent. The single-crystal X-ray structures of 2a and 3a reveal the occurrence of well-isolated, zigzag, oxamato-bridged manganese(II)-copper(II) chains. The intrachain Cu center dot center dot center dot Mn distances across the oxamato bridge are 5.3761(7) and 5.4002(17) angstrom for 2a and 3a, respectively, whereas the shortest interchain Mn center dot center dot center dot Mn distances are 9.4475(16) and 8.1649(14) angstrom for 2a and 3a, respectively. All of these (MCuII)-Cu-II chains (M=Mn and Co) exhibit ID ferrimagnetic behaviour with moderately strong intrachain antiferromagnetic coupling between the square-planar Cu-II and octahedral high-spin M-II ions across the oxamato bridge [-J=31.4-35.2 and 33.4-44.8 cm(-1), respectively; H=Sigma(i)-JS(M,i)(S-Cu,S-i+S-Cu,S-i-1)]. Only the (CoCuII)-Cu-II chains show slow magnetic relaxation effects characteristic of single-chain magnets (SCMs). Analysis of the magnetic relaxation dynamics of 3d shows a thermally activated mechanism (Arrhenius law dependence) with values of the pre-exponential factor (tau(0)=2.6 x 10(-9) s) and activation energy (E-a = 7.7 cm(-1)) that are typical of SCMs. In contrast, two relaxation regimes are observed for 2d in different temperature regions (tau(0) = 3.2 x 10(-10) s and E-a = 24.7 cm(-1) for T < 4.5 K and tau(0)=3.2 x 10(-14) s and E-a = 37.5 cm(-1) for T > 4.5 K).