End-to-end rotation of rhenium-bound dinitrogen

The singly N-15-labeled dinitrogen complexes Cp'Re(CO)(L)((NN)-N-15-N-14) (Cp' = eta(5)-C5H5, L = CO (1,N-15(alpha)) and Cp' = eta(5)-C(5)Me(5), L = CO (2-N-15(alpha)), PMe(3) (3-N-15(alpha)), or P(OMe)(3) (4-N-15(alpha))) were synthesized from the corresponding aryldiazenido complex [Cp'Re(CO)(L)((15)N(14)NC(5)H(4)OMe)]-_ [BF4] by treatment with Ph(3)C, Cp(2)Co, Na/Hg, or NaBH4. When these dinitrogen complexes are freshly synthesized, the N-15 label is exclusively retained in the metal-bound (N-alpha) position irrespective of the reagent used. At 291 K 1-N-15(alpha) isomerizes in acetone to give a 1:1 mixture of 1-N-15(alpha) and 1-N-15(beta), as indicated by the intensities of the N-15(alpha) and N-15(beta) resonances in the N-15 NMR spectra, with rate constant K-obs = (48 +/- 6) x 10(-6) s(-1) and Delta G(double dagger) = 95.3 +/- 0.5 kJ mol(-1). For 2-N-15(alpha) isomerization occurs similarly, and the temperature dependence over the range 274-291 K yielded Delta G(291)(double dagger) = 92.6 +/- 0.4 kJ mol(-1), Delta H-double dagger = 105.3 +/- 6.0 kJ mol(-1), Delta S-double dagger = 43.5 +/- 21.2 J mol(-1) K-1, E(a) = 106.8 +/- 5.4 kJ mol(-1), and A = (2.1 +/- 1.3) x 10(-15) s(-1). As a consequence of the linkage isomerization, samples of 1-N-15 and 2-N-15 obtained from any of the above syntheses following normal isolation and purification proceduresare inevitably 1:1 mixtures of the N-15(alpha) and N-15(beta) isotopomers. The dinitrogen complexes 3-N-15(alpha) and 4-N-15(alpha) do not isomerize at ambient temperature, and isolated samples are the pure N-15(alpha) isotopomer. However, at higher temperatures both 3-N-15(alpha) (at 333 K) and 4-N-15(alpha) (at 320 K) isomerize to 1:1 N-15(alpha) and N-15(beta) mixtures. Neither 1-N-15 nor 2-N-15 exchanged with bulk N-14(2) under pressure at room temperature, nor did 3-N-15 or 4-N-15 at 333 or 320 K, respectively, over several half-lives for the isomerization. A mixture of unlabeled 1 and 2-N-15 showed no evidence of formation of the cross products (unlabeled 2 and 1-N-15) at room temperature, and a mixture of unlabeled 4 and 3-N-15 showed no cross-product formation at 333 K. It is concluded that linkage isomerization of the dinitrogen ligand is nondissociative and intramolecular. The proposed mechanism is end-to-end rotation through the intermediacy of a side-on (eta(2)) dinitrogen complex.