End-to-End Rotation of Rhenium-Bound Dinitrogen

The singly 15N-labeled dinitrogen complexes Cp‘Re(CO)(L)(15N14N) (Cp‘ = η5-C5H5, L = CO (1-15Nα) and Cp‘ = η5-C5Me5, L = CO (2-15Nα), PMe3 (3-15Nα), or P(OMe)3 (4-15Nα)) were synthesized from the corresponding aryldiazenido complex [Cp‘Re(CO)(L)(15N14NC6H4OMe)][BF4] by treatment with Ph3C, Cp2Co, Na/Hg, or NaBH4. When these dinitrogen complexes are freshly synthesized, the 15N label is exclusively retained in the metal-bound (Nα) position irrespective of the reagent used. At 291 K 1-15Nα isomerizes in acetone to give a 1:1 mixture of 1-15Nα and 1-15Nβ , as indicated by the intensities of the 15Nα and 15Nβ resonances in the 15N NMR spectra, with rate constant k obs = (48 ± 6) × 10-6 s-1 and ΔG ⧧ = 95.3 ± 0.5 kJ mol-1. For 2-15Nα isomerization occurs similarly, and the temperature dependence over the range 274−291 K yielded ΔG ⧧ 291 = 92.6 ± 0.4 kJ mol-1, ΔH ⧧ = 105.3 ± 6.0 kJ mol-1, ΔS ⧧ = 43.5 ± 21.2 J mol-1 K-1, E a = 106.8 ± 5.4 kJ mol-1, and A = (2.1 ± 1.3) × 10-15 s-1. As a consequence of the linkage isomerization, samples of 1-15N and 2-15N obtained from any of the above syntheses following normal isolation and purification procedures are inevitably 1:1 mixtures of the 15Nα and 15Nβ isotopomers. The dinitrogen complexes 3-15Nα and 4-15Nα do not isomerize at ambient temperature, and isolated samples are the pure 15Nα isotopomer. However, at higher temperatures both 3-15Nα (at 333 K) and 4-15Nα (at 320 K) isomerize to 1:1 15Nα and 15Nβ mixtures. Neither 1-15N nor 2-15N exchanged with bulk 14N2 under pressure at room temperature, nor did 3-15N or 4-15N at 333 or 320 K, respectively, over several half-lives for the isomerization. A mixture of unlabeled 1 and 2-15N showed no evidence of formation of the cross products (unlabeled 2 and 1-15N) at room temperature, and a mixture of unlabeled 4 and 3-15N 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 (η2) dinitrogen complex.