Mixed-Spin [2 x 2] Fe-4 Grid Complex Optimized for Quantum Cellular Automata

Abstract

The new pyrazolate-bridged proligand 4-methyl-3,5-bis{6-(2,2'-bipyridyl)}pyrazole ((LH)-L-Me) has been synthesized. Similar to its congener that lacks the backbone methyl substituent ((LH)-L-H) it forms a robust Fe-4(II) grid complex, [(L4Fe4II)-L-Me](BF4)(4). The molecular structure of [(L4Fe4II)-L-Me](BF4)(4)center dot 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 Fe-57 Mossbauer 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 Mossbauer spectroscopy a [1HS-3LS] configuration is present in solution at 80 K. Thus, the methyl substituent in [L-Me](-) leads to a stronger ligand field compared to parent [L-H]- and hence to a higher LS fraction both in the solid state and in solution. Cyclic voltammetry of [(L4Fe4II)-L-Me](BF4)(4) reveals f(o)ur sequential oxidations coming in two pairs with pronounced stability of the di-mixed-valence species [(L4Fe2Fe2III)-L-Me-Fe-II](6+) (K-C = 3.35 x 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.