The Fe-III/Fe-II vs Fe-2(2.5) formulation in mixed-valent species [(NC)(4)Fe(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

摘要

Two molecule-bridged bis(tetracyanoiron) complexes [(NC)(4)Fe(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/visear-IR and IR spectroelectrochemistry (n = 2-5) in aprotic media. Following these results, the Fe-2(2.5) formulation is the correct valence description for the metal centers in the bptz-bridged trianion (K-c = 10(14.6) in CH3CN), whereas the localized Fe-III/Fe-II formulation is appropriate for the bpym-bridged system (K-c = 10(5.4) in CH3CN and 10(6.4) 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 pi* LUMO of tetrazines with high MO coefficients at the coordinating nitrogen centers. [References: 35]