The mpz(+)/mpz(*) Pair as Organic Analogue of the NO(+)/NO(*) Ligand Redox System (mpz = N-Methylpyrazinium). A Combined Electrochemical and Spectroscopic Study (UV/Vis/NIR, IR, EPR) of Complexes [(mpz)M(CN)(5)](2)(-)(/3)(-) (M = Fe, Ru, Os).
The oxidation and reduction behavior of complexes [(mpz)M(CN)(5)](2)(-) between the group 8 pentacyanometalates(II) and the N-methylpyrazinium ion mpz(+) was studied using spectroelectrochemical techniques (UV/vis/NIR, IR, EPR) in nonaqueous and, in part, aqueous media. As a further source of information on the electronic structure, we studied the variable solvatochromism of MLCT absorption bands in the precursor ions [(mpz)M(CN)(5)](2)(-). The mpz-centered one-electron reduction showed a tendency for concomitant loss of cyanide (EC process) to yield ions [(mpz)M(CN)(4)](2)(-), the lability increasing in the order M = Os << Ru < Fe. In contrast to the EPR spectra obtained from reduction with S(2)O(4)(2)(-) in aqueous media, which showed no evidence for close mpz(*)/metal association, the intra muros electrolysis experiments in DMSO or acetonitrile yielded the following results: The EPR spectrum that was previously assigned to [(mpz)Fe(CN)(5)](3)(-) is now ascribed to [(mpz)Fe(CN)(4)](2)(-). Whereas the dissociatively inert osmium radical complex [(mpz)Os(CN)(5)](3)(-) is distinguished by a broad EPR signal with a rather large value g(iso) = 2.0157, the EPR spectrum of [(mpz)Ru(CN)(4)](3)(-) reveals well-resolved separate coupling with the (99)Ru and (101)Ru isotopes in natural abundance. Spectroelectrochemistry and reactivity patterns of the complex ions [(mpz)M(CN)(5)](2)(-) are reminiscent of results for corresponding nitrosyl complexes [(NO)M(CN)(5)](2)(-), which suggests that the mpz(+)/mpz(*) ligand redox pair can function as an organic analogue of the NO(+)/NO(*) redox system.
Waldhör
Eberhard
E
J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic.
Kaim
Wolfgang
W
Olabe
José A.
JA
Slep
Leonardo D.
LD
Fiedler
Jan
J