Correlating axial and equatorial ligand field effects to the single-molecule magnet performances of a family of dysprosium bis-methanediide complexes

Treatment of the new methanediide–methanide complex [Dy(SCS)(SCSH)(THF)] (1Dy, SCS = {C(PPh(2)S)(2)}(2−)) with alkali metal alkyls and auxillary ethers produces the bis-methanediide complexes [Dy(SCS)(2)][Dy(SCS)(2)(K(DME)(2))(2)] (2Dy), [Dy(SCS)(2)][Na(DME)(3)] (3Dy) and [Dy(SCS)(2)][K(2,2,2-cryptand)] (4Dy). For further comparisons, the bis-methanediide complex [Dy(NCN)(2)][K(DB18C6)(THF)(toluene)] (5Dy, NCN = {C(PPh(2)NSiMe(3))(2)}(2−), DB18C6 = dibenzo-18-crown-6 ether) was prepared. Magnetic susceptibility experiments reveal slow relaxation of the magnetisation for 2Dy–5Dy, with open magnetic hysteresis up to 14, 12, 15, and 12 K, respectively (∼14 Oe s(−1)). Fitting the alternating current magnetic susceptibility data for 2Dy–5Dy gives energy barriers to magnetic relaxation (U(eff)) of 1069(129)/1160(21), 1015(32), 1109(70), and 757(39) K, respectively, thus 2Dy–4Dy join a privileged group of SMMs with U(eff) values of ∼1000 K and greater with magnetic hysteresis at temperatures >10 K. These structurally similar Dy-components permit systematic correlation of the effects of axial and equatorial ligand fields on single-molecule magnet performance. For 2Dy–4Dy, the Dy-components can be grouped into 2Dy–cation/4Dy and 2Dy–anion/3Dy, where the former have almost linear C[double bond, length as m-dash]Dy[double bond, length as m-dash]C units with short average Dy[double bond, length as m-dash]C distances, and the latter have more bent C[double bond, length as m-dash]Dy[double bond, length as m-dash]C units with longer average Dy[double bond, length as m-dash]C bonds. Both U(eff) and hysteresis temperature are superior for the former pair compared to the latter pair as predicted, supporting the hypothesis that a more linear axial ligand field with shorter M–L distances produces enhanced SMM properties. Comparison with 5Dy demonstrates unusually clear-cut examples of: (i) weakening the equatorial ligand field results in enhancement of the SMM performance of a monometallic system; (ii) a positive correlation between U(eff) barrier and axial linearity in structurally comparable systems.