Four mononuclear dysprosium complexes, namely, [Dy(thd)₃(MeOH)₂] (1), [Dy(thd)₃(Phen)] (2), [Dy(thd)₃(Dpq)] (3) and [Dy(thd)₃(Dppz)] (4) (thd = 2,2,6,6-tetramethyl-3,5-heptanedione; Phen = 1,10-phenanthroline; Dpq = dipyrazine[2,3-f:2′,3′-h]quinoxaline; and Dppz = dipyrido[3,2-a:2′,3′-c]phenazine), have been designed and isolated by reactions of Hthd with DyCl₃·6H₂O and corresponding auxiliary capping ligands such as Phen, Dpq and Dppz. The crystal structure analysis exhibits that Dy(III) ions in complexes 1–4 adopt distorted square antiprism geometry with D_4d symmetry. Magnetic analysis reveals that all complexes 1–4 exhibit slow magnetic relaxation with the barrier heights (U_eff/k_B) of 45.1 K, 98.4 K, 128.9 K and 89.4 K, respectively. The butterfly magnetic hysteresis loops were observed up to 5 K for complex 2. The co-relationship between the barrier heights and the structure in terms of the Dy–O_β-diketone bond lengths, the quantify distortion degree (σ_α² or σ_Φ²) and the deviation angles (θ) of magnetic easy axes for the SAP coordination environment were investigated and discussed.