Heterometallic Ru2Co2 [2 × 2] Grid with Localized Single Molecule Magnet Behavior.

Metal complexes with a [n × n] gridlike structure are discussed as attractive building blocks for various materials chemistry applications in molecular nanotechnology and electronics, which often rely on the grids' magnetic and redox properties. Most of the known metallogrids are homometallic, though heterometallic systems that comprise two or more different metals promise higher level functionalities. However, heterometallic [n × n] grids are relatively rare, mostly because of the more challenging synthetic strategies. To that end a new heterometallic [2 × 2] grid complex [L4Ru2Co2](BF4)4 (2) based on a known pyrazolate-bridged bis(tridentate) compartmental N-donor ligand [L]- is presented in this work, along with its doubly oxidized congener [L4Ru2Co2](BF4)6 (3). In order to prevent scrambling of the different metal ions, a stepwise synthetic approach was implemented in which an inert RuII "corner complex" [(HL)2Ru](BF4)2 (1) was isolated first, followed by addition of the more labile CoII. This exclusively yields the desired [L4Ru2Co2]4+ with anti-topology, viz., with the RuII and CoII ions situated at opposite corners of the [2 × 2] grid, as confirmed by single crystal X-ray diffraction. 2 can be sequentially oxidized four times, first at the Co vertices and then at the Ru vertices. 1H NMR spectroscopy as well as ESI mass spectrometry evidenced integrity of the [L4Ru2Co2]4+/6+ grids in solution. Structural and magnetic analyses revealed that paramagnetic 2 features LS-RuII and HS-CoII ions (LS = low-spin, HS = high-spin) whereas LS-RuII and LS-CoIII ions are present in diamagnetic 3. The LS-RuII ions in 2 serve to magnetically isolate the HS-CoII whose coordination geometry is strongly distorted from octahedral. A large and negative axial zero-field splitting value (D = -64 cm-1) for the local S = 3/2 ions is shown to lead to single molecule magnetic (SMM) properties characterized by a barrier to spin inversion of Ueff = 8.8 cm-1 and a single relaxation process with τo = 3.1 × 10-5 s. Transition metal [2 × 2] grid complexes showing SMM behavior are extremely rare, and this is the first heterometallic 3d/4d grid system featuring such a magnetic signature.