Synthesis, Crystal Structures and Magnetic Properties of M II Cu II 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 ] ⋅ p  S ⋅ q  H 2 O [ p =0–1, q =0–2.5; L 1 = N ‐2,6‐dimethylphenyloxamate, S=DMF with M=Mn ( 1 a ) and Co ( 1 b ); L 2 = N ‐2,6‐diethylphenyloxamate, S=DMF with M=Mn ( 2 a ) and Co ( 2 b ) or S=DMSO with M=Mn ( 2 c ) and Co ( 2 d ); L 3 = N ‐2,6‐diisopropylphenyloxamate, S=DMF with M=Mn ( 3 a ) and Co ( 3 b ) or S=DMSO with M=Mn ( 3 c ) and Co ( 3 d )] were prepared by treating the corresponding anionic oxamatocopper(II) complexes [Cu(L x ) 2 ] 2− ( x =1–3) with M 2+ cations (M=Mn and Co) in DMF or DMSO as the solvent. The single‐crystal X‐ray structures of 2 a and 3 a reveal the occurrence of well‐isolated, zigzag, oxamato‐bridged manganese(II)–copper(II) chains. The intrachain Cu ⋅⋅⋅ Mn distances across the oxamato bridge are 5.3761(7) and 5.4002(17) Å for 2 a and 3 a , respectively, whereas the shortest interchain Mn ⋅⋅⋅ Mn distances are 9.4475(16) and 8.1649(14) Å for 2 a and 3 a , respectively. All of these M II Cu II chains (M=Mn and Co) exhibit 1D 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 =∑ i − J S M, i ( S Cu, i + S Cu, i −1 )]. Only the Co II Cu II chains show slow magnetic relaxation effects characteristic of single‐chain magnets (SCMs). Analysis of the magnetic relaxation dynamics of 3 d shows a thermally activated mechanism (Arrhenius law dependence) with values of the pre‐exponential factor ( τ 0 =2.6×10 −9  s) and activation energy ( E a =7.7 cm −1 ) that are typical of SCMs. In contrast, two relaxation regimes are observed for 2 d in different temperature regions ( τ 0 =3.2×10 −10  s and E a =24.7 cm −1 for T <4.5 K and τ 0 =3.2×10 −14  s and E a =37.5 cm −1 for T >4.5 K).