Widely differing stabilities of molecule-bridged cyanodiiron(III, II) species in non-aqueous solvents

The redox systems [(NC)(5)Fe(mu-L)Fe(CN)(5)](6- /5- /4-) with L = pyrazine (1(n)), 4,4'-bipyridine (2(n)) or 1,2-bis(4-pyridyl)ethene (3(n)), and [(NC)(4)Fe(mu-bmtz)Fe(CN)(4)](5 - /4- /3 -) (4(n)) with bmtz = 3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine were studied in acetonitrile and other aprotic solvents. The comproportionation constants K-c for the mixed-valent diiron(III, II) intermediates vary from less than 10 for 3(5-) via 10(6.5) for 1(5-) to 10(14.1) for 4(3-). Intervalence charge transfer band maxima lie between 1270 nm (2(5-)) and 2475 nm 1(5-). In comparison with previous results from aqueous solutions, the system 1(n) in particular shows increased metal-metal interaction. W-Vis and IR spectroelectrochemistry suggest a localized/delocalized borderline sitation for 1(5-) in acetonitrile, with only slightly weaker metal-metal coupling than in the related Creutz-Taube ion. The results illustrate how the established strong solvent sensitivity of cyanoiron complexes can be exploited in mixed-valence chemistry. (C) 1999 Elsevier Science S.A. All rights reserved.