The FeIII/FeII vs Fe2 2.5 Formulation in Mixed-Valent Species [(NC)4Fe(BL)Fe(CN)4]3-, BL = 2,2‘-Bipyrimidine and 3,6-Bis(2-pyridyl)-1,2,4,5-tetrazine. Distance and Size Do Not Always Matter

• Published in: Inorganic chemistry 1999-07, Vol.38 (14), p.3270-3274
• Main Authors: Glöckle, Markus, Kaim, Wolfgang, Katz, Néstor E, García Posse, Mónica, Cutin, Edgardo H, Fiedler, Jan
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic990249l

Two molecule-bridged bis(tetracyanoiron) complexes [(NC)4Fe(BL)Fe(CN)4] n -, BL = 2,2‘-bipyrimidine (bpym) and 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz), were studied by EPR, UV/vis/near-IR and IR spectroelectrochemistry (n = 2−5) in aprotic media. Following these results, the Fe2 2.5 formulation is the correct valence description for the metal centers in the bptz-bridged trianion (K c = 1014.6 in CH3CN), whereas the localized FeIII/FeII formulation is appropriate for the bpym-bridged system (K c = 105.4 in CH3CN and 106.4 in CH2Cl2). In contrast to common perception, the larger tetrazine-bridged system with the longer metal−metal distance exhibits much stronger metal−metal coupling than the bpym-bridged analogue, a consequence of the very low lying π* LUMO of tetrazines with high MO coefficients at the coordinating nitrogen centers.