Using a Radical Straitjacket to Enforce an Ultrashort Dy–Dy Distance and Thus Enhance Dipolar Interactions in a Dy$^{III}$$_2$ Dimer

We present three air-stable radical-containing compounds, [M$^{III}$$_2$(phsq)$_4$(NO$_3$)$_2$(MeOH)$_2$]·2MeOH (M = Dy (1), Y (2), Gd (3); Hphsq = 9,10-phenanthrenesemiquinone), in which strong intramolecular inter-radical interactions force two lanthanide ions into close proximity and isolate them from the lattice. Magnetic measurements on (1) showed it to be an SMM with maxima in the ac susceptibility up to 18 K (1000 Hz). Modeling the magnetic data using three different analytical approaches reveals the absence of zero-field quantum tunneling of magnetization (ZFQTM) as a relaxation pathway. This is a result of significant internal magnetic fields identified by ab initio calculations that appear to quench the ZFQTM. This can be verified from measurements performed on an Y-doped sample (1a), which show that strong ZFQTM is present in the DyY analogue.