Magnetic behavior in metal tetracyanoethylene molecule-based materials

Several compounds in the metal-tetracyanoethylene (TCNE) family of molecule-based magnets have been studied. The first two are compounds that exhibit similar magnetic behavior as well as share an almost identical synthetic route, namely, MnII(TCNE)3/2(I3)1/2 and MnII(TCNE)I(OH2). They both have a critical temperature of 171 K and exhibit bulk ferrimagnetism. MnII(TCNE)3/2(I3)1/2 is the first known structure of a molecule based magnet to possess μ4-[TCNE]•- in all three dimensions. MnII(TCNE)3/2(I3)1/2 was also shown to exhibit a pressure-induced reversible enhancement of the critical temperature up to a maximum of 273 K. Suprisingly 2-D MnII(TCNE)I(OH2) has the same critical temperature as 3-D MnII(TCNE)3/2(I3)1/2 which may be attributed to significant dipolar interactions between the layers due to the relatively short interlayer separation. The next two compounds are the isostructural MII(TCNE)[C4(CN)8]1/2 (M = Mn, Fe). It was shown via detailed magnetic susceptibility measurements that they both possess an antiferromagnetic ground state with Tcs of 67 and 84 K, respectively. They also both exhibit a reversible ferri- or ferromagnetic pressureinduced magnetic transition. Finally, through a Mean Field (MF) theory analysis, it was shown that expressions that relate the exchange coupling to the critical temperature could be generated for several structure-types of the metal-TCNE family of materials. Using these expressions allowed for the estimation of the exchange coupling for several metal-TCNE compounds.