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

Abstract

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 N2 2– 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.

Snapshots of Early-Stage Quantitative N2 Electrophilic Functionalization

“…To verify whether 1 , 1-crypt , and 4-H/H are intermediates en route to NH 3 and N 2 H 4 , we tested their reactions with reductants and acids. Indeed, NH 3 and N 2 H 4 were detected as the final protonation products, , albeit with low yields (Table S1). Interestingly, we found that in the presence of excess Me 3 SiCl and KC 8 , 1 and 3-SiMe 3 can catalyze silylation of the dinitrogen ligand to furnish N(SiMe 3 ) 3 (for details, see the Supporting Information).…”

Catalytic dinitrogen reduction to hydrazine and ammonia using Cr(N2)2(diphosphine)2 complexes

“…While these examples highlight new discoveries using [M(N 2 ) 2 (P–P) 2 ] (M = Mo, W) complexes, catalytic N 2 reduction with analogous Cr compounds are limited. Recent reports highlighted the utility of molecular Cr complexes using a variety of ligand architectures for N 2 activation, 8 a ,13 functionalization, 14 or catalytic N 2 silylation. 15 However, molecular Cr complexes that catalyze the direct reduction of N 2 to NH 3 are rare.…”