Reductive nitrosylation of tetraoxometalates. Part 15. Generation of Mn(NO)3+, Mn(NO)2+, and Mn(NO)22+moieties directly from MnO4–. Synthesis, characterization, and electrochemistry of cyanonitrosyl derivatives of manganese

Abstract

In an aqueous–alkaline medium MnO4– undergoes an apparently single-step nitrosylation reaction, in the presence of an excess of NH2OH·HCl and CN–, to afford complexes containing Mn(NO)2+, Mn(NO)22+(hitherto unknown), or Mn(NO)3+ moieties. This is evident from the products K3[Mn(NO)(CN)5]·2H2O (1), [PPh4]2[Mn(NO)2(CN)4]·H2O, (2), [NMe4]2[Mn(NO)(CN)4]·2H2O (3), and isomeric [Mn(NO)(CN)3(L–L)][(4a), (1Oa), for L–L = 2,2′-bipyridine; (4b), (10b), for L–L = 1,10-phenanthroline]. The last compounds contain formally MnII in a spin-quartet ground state, as confirmed by their variable-temperature magnetic susceptibility and e.s.r. data. The reductive nitrosylation also generates Mn2(NO)24+ present in MnII3[Mn2(NO)2(CN)10](5) or [PPh4]Mn2[Mn2(NO)2(CN)9](9) and Mn2(NO)25+ in [PPh4]3[Mn2(NO)2(CN)8](6a). Complex (6a) is an one-electron paramagnet, shows an highly featured e.s.r. spectrum, and exhibits a rather irreversible cyclic voltammetric oxidative response at an apparent E0298 of 0.98 V vs. saturated calomel electrode (s.c.e.) corresponding to the process Mn2(NO)25+⇌ Mn2(NO)26+. The reaction when conducted in a near neutral medium, produces besides (10a), (1Ob), and (5), another product of possible composition R3[Mn2(NO)2(CN)8]·3H2O [R = PPh4(6b), NMe4(7) or Cs (8)] obviously containing a Mn2(NO)25+ core. The product (6b) is an isomer of (6a) and shows an e.s.r. spectrum having even more features than that of (6a). Compound (6b), however, shows a reversible cyclic reponse [E0298= 0.85 V vs. s.c.e. (0.47 V vs. ferrocene–ferrocenium] assigned to the electrode process shown for (6a).