RESUMO
A novel dinitrogen-dichromium complex, [{Cr(LBn)}2(µ-N2)] (1), has been prepared from reaction of CrCl3 with a lithiated triamidoamine ligand (Li3LBn) under dinitrogen. The X-ray crystal structure analysis of 1 revealed that it is composed of two independent dimeric Cr complexes bridged by N2 in the unit cell. The bridged N-N bond lengths (1.188(4) and 1.185(7) Å) were longer than the free dinitrogen molecule. The elongations of N-N bonds in 1 were also supported by the fact that the ν(N-N) stretching vibration at 1772 cm-1 observed in toluene is smaller than the free N2. Complex 1 was identified to be a 5-coordinated high spin Cr(IV) complex by Cr K-edge XANES measurement. The 1H NMR spectrum and temperature dependent magnetic susceptibility of 1 indicated that complex 1 is in the S = 1 ground state, in which two Cr(IV) ions and unpaired electron spins of the bridging N22- ligand are strongly antiferromagnetically coupled. Reaction of complex 1 with 2.3 equiv of Na or K gave chromium complexes with N2 between the Cr ion and the respective alkali metal ion, [{CrNa(LBn)(N2)(Et2O)}2] (2) and [{CrK(LBn)(N2)}4(Et2O)2] (3), respectively. Furthermore, the complexes 2 and 3 reacted with 15-crown-5 and 18-crown-6 to form the respective crown-ether adducts, [CrNa(LBn)(N2)(15-crown-5)] (4) and [CrK(LBn)(N2)(18-crown-6)] (5). The XANES measurements of complexes 2, 3, 4, and 5 revealed that they are high spin Cr(IV) complexes like complex 1. All complexes reacted with a reducing agent and a proton source to form NH3 and/or N2H4. The yields of these products in the presence of K+ were higher than those in the presence of Na+. The electronic structures and binding properties of 1, 2, 3, 4, and 5 were evaluated and discussed based on their DFT calculations.
RESUMO
Dinitrogen-divanadium complexes with triamidoamine ligands, 1-3, were synthesized and characterized by resonance Raman, UV-vis, and NMR spectroscopy and elemental and X-ray structure analyses. X-ray structure analyses reveal that all three of the complexes have a dimeric structure with a µ-N2 ligand (N-N bond length 1.200-1.221 Å). Resonance Raman and NMR spectra of 1-3 in solution show that these complexes maintain a dimeric structure in benzene and toluene solutions. 15N NMR spectra of 1 and 3 have peaks assignable to µ-N2 ligands at 33.4 and 27.6 ppm, respectively, but 2 does not have a similar peak under the same conditions. In 51V NMR spectra, the peaks of vanadium ions were observed at -173.3, -143.8, and -240.2 ppm, respectively, which are in a higher magnetic field region in comparison to those of dinitrogen-divanadium complexes reported previously. The structure and electronic properties of 1 are supported by DFT calculations. Additionally, all complexes react with excess amounts of M[C10H8] (M = Na, K) and the proton sources HOTf, HCl, and [LutH]OTf (Lut = 2,6-dimethylpyridine) to produce ammonia without hydrazine. The ammonia produced was evaluated as an ammonium salt by 1H and 15N NMR spectroscopy. The yield of NH3 produced in the reaction of 1 with Na[C10H8] and HOTf under N2 was 151% (per V atom).