INVESTIGATION OF THE FORMATION OF LIGATED DINITROGEN FROM RHENIUM-BOUND ARYLDIAZENIDE

Chemical or electrochemical reduction of the cationic rhenium aryldiazenido complexes [Cp'Re(L(1))(L(2))(p-N(2)C(6)H(4)OMe)][BF4] (1b-7b) generates the corresponding neutral dinitrogen complexes 1a-7a of the type Cp'Re(L(1))(L(2))(N-2) ((a) Cp' Cp; L(1) = L(2) = CO (1a); (b) Cp' = Cp*, L(1) = CO, L(2) = PMe(3) (3a) or P(OMe)(3) (4a); and (c) Cp' Cp*, L(1) = L(2) = CO (2a), PMe(3) (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 (PMe(3))(2) (5b,-1.89 V) > dmpe (6b,-1.74 V) > [P(OMe)(3)](2) (7b,-1.41 V) > (CO)(PMe(3))(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 Cp(2)Co, 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-NHNC(6)H(4)OMe), which is thermally unstable End yields the dinitrogen complex 2a and anisole.