Investigation of the formation of ligated dinitrogen from rhenium-bound aryldiazenide

Article abstract of DOI:10.1021/om00010a032

Chemical or electrochemical reduction of the cationic rhenium aryldiazenido complexes [Cp′Re(L₁)(L₂)(p-N₂C₆H ₄OMe)][BF₄] (1b-7b) generates the corresponding neutral dinitrogen complexes 1a-7a of the type Cp′Re(L₁)(L₂)(N₂) ((a) Cp′ = Cp; L₁ = L₂ = CO (1a); (b) Cp′ = Cp*, L₁ = CO, L₂ = PMe₃ (3a) or P(OMe)₃ (4a); and (c) Cp′ = Cp*, L₁ = L₂ = CO (2a), PMe₃ (5a), dmpe (6a), or P(OMe)₃ (7a)). Evidence is presented in support of a proposed mechanism which involves one-electron reduction of the aryldiazenido complex cation in 1b-7b to the neutral 19-electron intermediate, followed by C-N bond homolysis to give 1a-7a plus the p-methoxyphenyl radical that proceeds to form anisole by hydrogen atom abstraction from the solvent. The cathodic reduction peak potentials observed for 1b-7b by cyclic voltammetry are increasingly negative in the sequence of coligands (PMes)₂ (5b, -1.89 V) > dmpe (6b, -1.74 V) > [P(OMe)₃]₂ (7b, -1.41 V) > (CO)(PMe₃) (3b, -1.24 V) > (CO)[P(OMe)₃] (4b, -0.98 V) > (CO)₂ (2b, -0.62 V). The value for the dicarbonyl Cp* complex 2b is more negative than for the corresponding Cp complex 1b (-0.46 V). These potentials are a good indication of the success of chemical reduction: all 1b-7b are reduced to 1a-7a by Na/Hg in THF, whereas Cp₂Co, which has a smaller reduction potential than Na/Hg, only reduces 1b, 2b, and 4b. The reaction of 2b with NaBH₄ is shown by full spectroscopic characterization to produce the aryldiazene complex Cp*Re(CO)₂(p-NHNC₆H₄OMe), which is thermally unstable and yields the dinitrogen complex 2a and anisole.