Mixed-Spin [2 × 2] Fe4 Grid Complex Optimized for Quantum Cellular Automata

The new pyrazolate-bridged proligand 4-methyl-3,5-bis­{6-(2,2′-bipyridyl)}­pyrazole (MeLH) has been synthesized. Similar to its congener that lacks the backbone methyl substituent (HLH) it forms a robust FeII4 grid complex, [MeL4FeII4]­(BF4)4. The molecular structure of [MeL4FeII4](BF4)4·2MeCN has been elucidated by X-ray diffraction, revealing two high-spin (HS) and two low-spin (LS) ferrous ions at opposite corners of the rhombic metal ion arrangement. SQUID and 57Fe Mössbauer data for solid material showed that this [HS–LS–HS–LS] configuration persists over a wide temperature range, between 7 and 250 K, while spin-crossover sets in only above 250 K. According to Mössbauer spectroscopy a [1HS–3LS] configuration is present in solution at 80 K. Thus, the methyl substituent in [MeL]− leads to a stronger ligand field compared to parent [HL]− and hence to a higher LS fraction both in the solid state and in solution. Cyclic voltammetry of [MeL4FeII4]­(BF4)4 reveals four sequential oxidations coming in two pairs with pronounced stability of the di-mixed-valence species [MeL4FeII2FeIII2]6+ (KC = 3.35 × 108). The particular [HS–LS–HS–LS] configuration as well as the di-mixed-valence configuration, both with identical spin or redox states at diagonally opposed vertices of the grid, make this system attractive as a molecular component for quantum cellular automata.