Syntheses, Characterizations, Crystal Structures, and Protonation Reactions of Dinitrogen Chromium Complexes Supported with Triamidoamine Ligands.

A novel dinitrogen-dichromium complex, [{Cr( L Bn )} 2 (μ-N 2 )] ( 1 ), has been prepared from reaction of CrCl 3 with a lithiated triamidoamine ligand ( Li 3 L Bn ) under dinitrogen. The X-ray crystal structure analysis of 1 revealed that it is composed of two independent dimeric Cr complexes bridged by N 2 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 N 2 . Complex 1 was identified to be a 5-coordinated high spin Cr(IV) complex by Cr K-edge XANES measurement. The 1 H 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 N 2 2- ligand are strongly antiferromagnetically coupled. Reaction of complex 1 with 2.3 equiv of Na or K gave chromium complexes with N 2 between the Cr ion and the respective alkali metal ion, [{CrNa( L Bn )(N 2 )(Et 2 O)} 2 ] ( 2 ) and [{CrK( L Bn )(N 2 )} 4 (Et 2 O) 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( L Bn )(N 2 )(15-crown-5)] ( 4 ) and [CrK( L Bn )(N 2 )(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 NH 3 and/or N 2 H 4 . 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.