The dominance of a self-condensed conjugated macrocycle over a [2 + 2] conventional macrocycle in weakening the transverse crystal field and boosting axiality provides a new route to construct high-performance air-stable lanthanide SMMs.
Self-condensation of bifunctional 8-aminoquinoline-2-carbaldehyde templated by dysprosium( iii) ions followed by the replacement of axial ligands with triphenylsilanol in the presence of Et 3N produced two belt macrocycle complexes, [Dy(L1 N6)(Ph 3SiO) 2][ClO 4] (1) and [Dy(L1 N6)(Ph 3SiO) 2][OTf] (2) (L1 N6 = 2,5,8-triaza-1(8,2),4,7(2,8)-triquinolinacyclononaphane-2,5,8-triene). For comparison, other kinds of belt macrocycle complexes [Dy( RRRR-L2 N6)(Ph 3SiO) 2][(3-Br-C 6H 4)B(C 6H 5) 3] (3R), [Dy( SSSS-L2 N6)(Ph 3SiO) 2][(3-Br-C 6H 4)B(C 6H 5) 3] (3S), [Dy( RRRR-L2 N6)(Ph 3SiO) 2][(4-Br-C 6H 4)B(C 6H 5) 3] (4R) and [Dy( SSSS-L2 N6)(Ph 3SiO) 2][(4-Br-C 6H 4)B(C 6H 5) 3] (4S) (L2 N6 = (2 E,5 E,8 E,11 E)-3,5,9,11-tetraaza-1,7(2,6)-dipyridina-4,10(1,2)-dicyclohexanacyclododecaphane-2,5,8,11-tetraene) were also prepared using the same procedure except that the equatorial macrocycle was built from a [2 + 2]-condensation of cyclohexane-1,2-diamine and pyridine-2,6-dicarbaldehyde. These structurally similar complexes have almost identical axial Dy–O distances but distinguishable equatorial Dy–N distances, i.e. longer Dy–N distances were observed in 1 and 2, suggesting a weaker equatorial crystal field. Magnetism measurements reveal a typical zero-field single-molecule magnet (SMM) behavior with the effective energy barriers ( U eff) up to 1732(43) K, 1680(9) K, 1363(22) K/1415(24) K and 1369(34) K/1434(29) K for 1, 2, 3R/3S and 4R/4S, respectively. The experimental results are supported by ab initio calculations, which exhibit relaxation of the magnetization via higher excited states in 1 and 2 as the consequence of a weaker equatorial crystal field and a less distorted coordination geometry, which efficiently hampers the transverse components of magnetization. This work unequivocally evidenced the dominance of L1 N6 over L2 N6 in weakening the transverse crystal field, providing a new route to construct high-performance air-stable lanthanide SMMs.