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 Mn^II 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 Mn^II₃[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).