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 (H3LPen) and tris(2-dicyclohexylmethylaminoethyl)amine (H3LCy), are prepared: [{Cr(LPen)}2(μ-N2)] (1), [{CrK(LPen)(μ-N2)(Et2O)}2] (2), [CrCl(LPen)] (3), [Cr(LCy)] (4), [CrK(LCy)(μ-N2)(18-crown-6)(THF)] (5(THF)), and [CrCl(LCy)] (6). The preparation of these complexes is 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 a free dinitrogen molecule (1.10 Å), indicating that the N2 ligands are activated. The ν(14N–14N) values of 1, 2, and 5(THF) are 1715 cm−1 for 1 (Raman, in solution), 1787, 1743 cm−1 for 2 (IR, in solid), and 1824 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 is activated. The structures of 2 and 5(THF) in solution are also studied by 1H NMR and solution IR spectroscopies. 1H NMR spectra of these complexes reveal that the peaks of 2 and 5(THF) are observed in the diamagnetic region, whereas those for the other complexes (1, 3, 4, and 6) exhibit paramagnetic shifts. The reactions of these complexes with K[C10H8] and HOTf under N2 in THF yield hydrazine and a small amount of ammonia; however, they are not catalytic. The 1H NMR and IR spectra of the products obtained by reacting 1 or 3 with reductant K in THF under N2 atmosphere indicate that 2 is formed based on spectral agreement. Similarly, upon examining for 4 or 6, it is confirmed that a species similar to 5(THF) is generated.