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)

Abstract

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.

EPR Insensitivity of the Metal‐Nitrosyl Spin‐Bearing Moiety in Complexes [LnRuII‐NO·]k

“…[34,35] The presence of states lying close to the radical ground state is also responsible for the rapid relaxation as evident from the EPR silence at room temperature. The relatively large A 2 hyperfine value of 3.8 mT for 14 [36,37] we found no evidence for conversion into a five-coordinate complex in the paramagnetic ruthenium species studied here. The new calculated metal hyperfine coupling constants A n ( 101 Ru) ( Table 2) confirm the failure to observe such splittings experimentally.…”

“…The isotope 99 Ru (12.7% natural abundance, I ϭ 5/2) has a 10% smaller hyperfine coupling [45] and would be still harder to detect. The maximum calculated isotropic hyperfine splitting of 0.94 mT according to Table 2 is only slightly larger than the A iso ( 101 Ru) parameters measured for radical complexes containing cyanoruthenium [37,38] (Յ0.52 mT) or other ruthenium() complex fragments (Յ0.86 mT). [39] Species with metal-centered spin containing ruthenium() [40] are expected to have isotropic metal coupling constants of several milliteslas.…”

A computational study of a cyano-bridged dinuclear ruthenium complex containing the non-innocent methylpyrazinium ligand

“…The bonding between ruthenium(II) and the methylpyrazinium ligand has long been a subject of interest (14,(23)(24)(25)(26)(27)(28)(29). Two relevant crystal structures have been reported.…”

Crystal structures and photoluminescence of the Zn(II) and Cd(II) complexes of N-methylpyrazinium obtained by a less hazardous approach

“…Other than nitrosylium cation NO + which can form a NO + /NO Å redox pair [1], the typical monopositive ligand is N-methylpyrazinium (mpz + ) cation, which was first synthesized by Bahner and Norton [7]. mpz + is a good p-acceptor ligand being easily reduced to the N-methylpyrazinium radical mpz Å (Scheme 1), and the resulting mpz + /mpz Å redox pair can function as an organic analogue of the NO + /NO Å redox system [8]. The mpz + ligand has been used to form some metal complexes, e.g.…”

“…with Fe(II) [9], Ru(II) [10], Os(II) [11], and Cu(I) [12]. Some research groups have investigated this kind of complexes in detail for their interesting electrochemical [8,13], kinetic [14] and nonlinear optical [9d] properties. Although several complexes of mpz + have been prepared [9][10][11][12], to our best knowledge, there is no reported example of group IIB metal complexes.…”