A conjugated Schiff-base macrocycle weakens the transverse crystal field of air-stable dysprosium single-molecule magnets

Abstract

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 Et₃N produced two belt macrocycle complexes, [Dy(L1^N6)(Ph₃SiO)₂][ClO₄] (1) and [Dy(L1^N6)(Ph₃SiO)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)(Ph₃SiO)₂][(3-Br-C₆H₄)B(C₆H₅)₃] (3R), [Dy(SSSS-L2^N6)(Ph₃SiO)₂][(3-Br-C₆H₄)B(C₆H₅)₃] (3S), [Dy(RRRR-L2^N6)(Ph₃SiO)₂][(4-Br-C₆H₄)B(C₆H₅)₃] (4R) and [Dy(SSSS-L2^N6)(Ph₃SiO)₂][(4-Br-C₆H₄)B(C₆H₅)₃] (4S) (L2^N6 = (2E,5E,8E,11E)-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.