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.
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/vis/near-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 CH(3)CN), whereas the localized Fe(III)/Fe(II) formulation is appropriate for the bpym-bridged system (K(c) = 10(5.4) in CH(3)CN and 10(6.4) in CH(2)Cl(2)). 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.
Glöckle
Markus
M
J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, 18223 Prague, Czech Republic.
Kaim
Wolfgang
W
Katz
Néstor E.
NE
García Posse
Mónica
M
Cutin
Edgardo H.
EH
Fiedler
Jan
J