Synthesis and Structural Characterization of Dinitrogen Chromium Complexes with Triamidoamine Ligands Possessing Bulky Substituents, and Nitrogen Fixation by These Complexes

Chromium complexes with triamidoamine derivatives bearing bulky substituents at the terminal positions of the ligands, tris(2-(3-pentylamino)ethyl)amine (H₃L^Pen) and tris(3-dicyclohexylmethylaminoethyl)amine (H₃L^Cy), have been prepared: [{Cr(L^Pen)}₂(m-N₂)] (1), [{CrK(L^Pen)(m-N₂)(Et₂O)}₂] (2), [CrCl(L^Pen)] (3), [Cr(L^Cy)] (4), [CrK(L^Cy)(m-N₂)(18–crown–6)(THF)] (5(THF)), and [CrCl(L^Cy)] (6)). The preparation of these complexes has been confirmed by X-ray diffraction analysis. Complexes 1, 2, and 5(THF) have coordinated dinitrogen molecules, with N–N bond lengths of 1.185(3), 1.174(9), and 1.162(3) Å, respectively. These lengths are significantly elongated compared to that of free dinitrogen molecule (1.10 Å), indicating that the N₂ ligands are activated. The n(¹⁴N–¹⁴N) values of 1, 2, and 5(THF) are 1715 cm^-1 (n(¹⁵N–¹⁵N): 1651 cm^-1) for 1 (Raman, in solution), 1787, 1743 cm^-1 (n(¹⁵N–¹⁵N) 1728, 1687 cm^-1) for 2 (IR, in solid), and 1824 cm^-1 (n(¹⁵N–¹⁵N) 1757 cm^-1) for 5(THF) (IR, in solid), respectively. These values are markedly smaller than free nitrogen (2331 cm^-1), confirming that the dinitrogen is interacting with the metal ions and activated. The structures of 2 and 5(THF) in solution have also been studied by ¹H NMR and solution IR spectroscopies. ¹H NMR spectra of these complexes have revealed that the peaks of 2 and 5(THF) have been observed in the diamagnetic region, whereas those for the other complexes (1, 3, 4, and 6) have exhibited paramagnetic shifts. The reactions of these complexes with K[C₁₀H₈] and HOTf under N₂ in THF have yielded hydrazine and a small amount of ammonia, however, they have not been catalytic. The ¹H NMR and IR spectra of the products obtained by reacting 1 or 3 with reductant K in THF under N₂ atmosphere have indicated that 2 has been formed based on spectral agreement. Similarly, upon examining for 4 or 6, it has been confirmed that a species similar to 5(THF) has been generated.