Experimental and theoretical insights into two tri- and tetranuclear DyIII SMMs reveal the unusual superiority of triangular dodecahedral DyIII over square antiprismatic DyIII as an SIM centre

Abstract

The work in this report deals with the solvent-dependent syntheses, single crystal X-ray structures, dc/ac magnetic investigation and ab initio CASSCF calculations into the magnetic properties of one DyIII3 compound of composition (Et3NH)2[DyIII3L4(NO3)2]NO3·0.6Et2O (1) and one DyIII4 compound of composition [DyIII4L4(μ3-OH)2(NO3)2(dmf)2] (2) derived from a two-pocket ligand H2L = 2-(2-hydroxy-3-ethoxybenzylideneamino)phenol. In the DyIII (terminal)---DyIII(central)---DyIII(terminal) core in 1, a tris(µ-phenoxido) bridging moiety connects the two metal ions in each terminal---central pair. Different bridging moieties in 2 are bis(µ3-hydroxido), bis(µ-phenoxido)-µ3-hydroxido and µ-phenoxido-µ3-hydroxido. Compound 2 has a butterfly-type structure having two DyIII (Dyb) in body positions and two DyIII (Dyw) in wing positions. SHAPE analyses reveal that the coordination geometries are distorted square antiprism for the central DyIII in 1 and the Dyw centres in 2 while they are distorted triangular dodecahedron (TDD) for the terminal DyIII centres in 1 and Dyb centres in 2. DC magnetic measurements reveal strong anisotropy and an indication of weak antiferromagnetic interactions in both 1 and 2. The AC susceptibility data reveal that 1 is an SMM under 1000 Oe DC field while 2 is an SMM under zero DC field and there are two relaxation channels in both which is due to the presence of two types of DyIII. The simulated Ueff values are 32.3 cm-1 for 1 and 65.1 cm-1 for 2. Ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations reveal that the SAPR Dyw centres are more axial (giving Ucal = 162 cm-1) than the TDD Dyb centres (giving Ucal = 69 cm-1) in 2 while, in 1, the SAPR central DyIII is much less axial (giving Ucal = 19 cm-1) than the TDD terminal DyIII centres (giving Ucal = 168.3/163.6 cm-1). The calculations also reveal that the magnetization relaxation occurs via the first excited KD. All of the POLY_ANISO estimated exchange coupling constants are weakly antiferromagnetic. Crystal field parameters have been calculated to corroborate the relative axiality of the metal centres. The assignment of slow and fast relaxation processes to the DyIII centres is made on the basis of theoretical calculations. An unusual outcome regarding the role of coordination environment on the SMM properties has been discussed.