Rational Enantioselective Design of Chiral Heterobimetallic Single‐Chain Magnets: Synthesis, Crystal Structures and Magnetic Properties of Oxamato‐Bridged M II Cu II Chains (M=Mn, Co)

A new series of neutral oxamato‐bridged M II Cu II chiral chains of general formula [MCuL x (S) m (H 2 O) n ] ⋅ a S ⋅ b H 2 O [L 1 =( M )‐1,1′‐binaphthalene‐2,2′‐bis(oxamate) with M=Mn ( 1 a ) and Co ( 1 b ); L 2 =( P )‐1,1′‐binaphthalene‐2,2′‐bis(oxamate) with M=Mn ( 2 a ) and Co ( 2 b )] and the analogous racemic chains of formula [MCuL 3 (S) m (H 2 O) n ] ⋅ a S ⋅ b H 2 O [L 3 =1,1′‐binaphthalene‐2,2′‐bis(oxamate) with M=Mn ( 3 a ) and Co ( 3 b )] have been prepared by reaction of the corresponding dianionic oxamatocopper(II) complex [Cu(L x )] 2− with Mn 2+ or Co 2+ cations in either dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). Solid circular dichroism (CD) spectra of the bimetallic chain compounds were recorded to establish their chiral and enantiomeric nature. They exhibit maximum positive and negative Cotton effects, each pair of enantiomeric chains being non‐superimposable mirror images. The crystal structures of the Mn II Cu II ( 1 a – 3 a ) and the Co II Cu II ( 1 b and 2 b ) chain compounds were solved by single‐crystal X‐ray diffraction methods. Our attempts to obtain X‐ray quality crystals of 3 b were unsuccessful. The values of the shortest interchain Mn⋅⋅⋅Mn and Co⋅⋅⋅Co distances are indicative of a good isolation of neighbouring chains in the crystal lattice, which is caused by the bulky aromatic ligand. Although all the Mn II Cu II and Co II Cu II chains exhibit ferrimagnetic behaviour (− J MnCu =18.9–26.6 cm −1 and − J CoCu =19.5–32.5 cm −1 ), only the enantiopure Co II Cu II chains ( 1 b and 2 b ) show slow magnetic relaxation at low temperatures ( T B =0.6–1.8 K), which is a characteristic of single‐chain magnets (SCMs) and is related to the magnetic anisotropy of the high‐spin Co II ion. Analysis of the SCM behaviour of 1 b and 2 b , based on Glauber’s theory for an Ising one‐dimensional system, shows a thermally activated mechanism for the magnetic relaxation (Arrhenius law dependence). The energy barriers ( E a ) to reverse the magnetisation direction are 8.2 ( 1 b ) and 8.1 cm −1 ( 2 b ), whereas the pre‐exponential factor ( τ 0 ) is 1.9×10 −8 ( 1 b ) and 6.0×10 −9  s ( 2 b ). Interestingly, the racemic Co II Cu II chain analogue, 3 b , showed no evidence of SCM behaviour.