A salt of a metal−pyrylium σ-complex, pentacarbonyl-3-(2-methoxy-5,6-diphenylpyrylium)manganese(I) tetrafluoroborate (1BF 4 ), has been prepared by the action of Mn(CO)5BF4 on methyl propiolate and diphenylacetylene. The pyrylium complex shows unusual resistance to electrophilic cleavage of the Mn−C bond by H+ and a low aptitude for migratory insertion reactions. The complex was studied by spectroscopic, voltammetric, and controlled potential electrolysis methods. 1BF 4 displays two ligand-based one-electron reductions (E 1° = −1.33 V, E pc,2 = −2.41 V vs Fc), the first of which is Nernstian. Its reactivity is enhanced by one-electron reduction, and it undergoes subsequent dimerization in THF (k 273K = 285 M-1 s-1) and CH2Cl2 (k 273K = 260 M-1 s-1) and a CO dissociation equilibrium (K ≈ 1.4 × 105, k = 0.7 s-1). In the presence of triphenylphosphine, a slow electrocatalytic CO-substitution reaction occurs for which a mechanism and rate constants have been determined by digital simulation. When 1BF4 is reduced in the presence of HSnPh3 and a proton donor, another electrocatalytic process results, presumably forming H2 at the electrode surface.

The syntheses and characterization of three cationic iron-pyrylium complexes (2 +, [2-OMe-3-Fp-5,6-Ph2(C5HO)]+; 3 +, [2-OMe-3-Fp-6-Ph(C5H2O)]+; 4 +, [2-OMe-3-Fp‘-6-Ph (C5H2O)]+) that result from the sequential addition of methyl propiolate and PhC⋮CR (R = Ph, H) to FpBF4 (Fp = η5-C5H5(CO)2Fe) or Fp‘PF6 (Fp‘ = η5-(C5H4Me)(CO)2Fe) are presented. The initially formed cationic Fp-methyl propiolate complex is shown to convert into a complicated mixture that includes the corresponding vinylidene and alkynyl complexes. The iron-pyrylium complexes have been characterized by 1H NMR and IR spectroscopy, elemental analysis, FAB-MS, electrochemical, and fiber-optic-IR and ESR spectroelectrochemical techniques. These metal-pyrylium complexes undergo electron transfer at the pyrylium ring rather than at the metal center. The subsequent chemistry of the neutral species 2 • , 3 • , and 4 • is strongly influenced by the nature of the substituents on the ring. The sterically protected complex 2 + undergoes chemically and electrochemically reversible reductions. The complexes 3 + and 4 + undergo dimerization reactions after reduction presumably through the 4-position of the pyrylium ring.