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 ((Me)LH) has been synthesized. Similar to its congener that lacks the backbone methyl substituent ((H)LH) it forms a robust Fe(II)4 grid complex, (Me)L4Fe(II)44. The molecular structure of (Me)L4Fe(II)44·2MeCN 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 (57)Fe 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 (Me)L leads to a stronger ligand field compared to parent (H)L and hence to a higher LS fraction both in the solid state and in solution. Cyclic voltammetry of (Me)L4Fe(II)44 reveals four sequential oxidations coming in two pairs with pronounced stability of the di-mixed-valence species (Me)L4Fe(II)2Fe(III)2 (K(C) = 3.35 × 10(8)). 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.