N₂ Activation in Ni^I–NN–Ni^I Units: The Influence of Alkali Metal Cations and CO Reactivity

Abstract

After single electron reduction of the dinitrogen complex [L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (I) with KC 8 , reaction of the resulting compound K[L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (II) with sodium sand yields KNa[L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (1), which contains two different alkali metal ions. Treatment of I with two equivalents of sodium sand leads to the symmetric complex Na 2 [L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (2). Complexes 1 and 2 were investigated by single crystal X-ray diffraction analysis as well as by Raman spectroscopy, and the results were compared with the data of K 2 [L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (III), which 1169 contains two K + ions. Thus, it became obvious that the nature of the alkali metal ion M in compounds M 2 [L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] has hardly any influence on the degree of NN bond activation. Furthermore, it was shown that treatment of the dinickel(I) complex III with CO leads to the dinickel(0) compound K 2 [L tBu Ni(CO)] 2 (4) and N 2 . Reaction of the unreduced dinickel(I) complex I with CO leads to a more simple replacement of the N 2 ligand and formation of [L tBu Ni(CO)] (3). Scheme 1.Stepwise reduction of I. [1]

Cooperativity Between Low-Valent Iron and Potassium Promoters in Dinitrogen Fixation

“…7 In addition to this mechanistic insight, it reveals that three reduced iron centers acting in a cooperative fashion make N 2 cleavage thermodynamically feasible. As summarized in Figure 9, the K + promoter stabilizes the nitride ligand of the “fixed” tri-iron-bis(nitride) isomers by about 10 kcal/mol relative to the N 2 isomers, and N-N cleavage is only favorable when an added electron and potassium are present.…”

Late Metal Scaffolds that Activate Both, Dinitrogen and Reduced Dinitrogen Species NxHy

“…Investigations of analogous complexes, where one or both K + ions were replaced by Na + ions, showed that the nature of the interacting alkali metal ion does not significantly influence the activation of the NN bond. [168] Interestingly, the same β-diketiminato nickel scaffold also accommodates a variety of other nitrogenous ligands relevant to NH 3 generation, including side-on bonded N 2 H 2 , N 2 H 3 -, N 2 H 4 , and NH 3 ligands (Scheme 13). [171,172] The reaction of a nickel(II) bromide precursor with an excess of hydrazine resulted in the formation of the mononuclear hydrazido(1-) complex [L tBu Ni(η 2 -N 2 H 3 )] (53) and the concomitant precipitation of hydrazinium bromide (N 2 H 5 Br).…”

“…The next successful study on Ni-N 2 complexes was not accomplished until 2005, [166] but nickel got further into the focus of nitrogen fixation research in recent years. [167][168][169][170] A dinitrogen complex with two three-coordinate Ni I units featuring the bulky β-diketiminato ligand and an end-on bridging N 2 molecule [L tBu Ni(μ-η 1 ;η 1 -N 2 )NiL tBu ], (49) was synthesized and reported by our group in 2009. [167] Complex 49 is related to the iron complex (12) mentioned above and the subsequent publication of the corresponding Co complex (46) thus provided for the first time opportunity to compare a series of first-row late transition metal (Fe, Co, Ni) dinitrogen complexes with the same co-ligand, namely β-diketiminate.…”

Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction

“…Interestingly, the authors found that reduction of tBu LNiBr with 1.5 equiv of KC 8 results in the formation of K[ tBu LCo] 2 (μ-N 2 ), which contains a side-on interaction between the bridging N 2 and a single K + on one side [113]. Horn et al found that reacting this partially-reduced complex with sodium metal gives the mixed-alkali product NaK[ tBu LNi] 2 (μ-N 2 ) ( 40 ) [114]. Furthermore, reduction of the previously reported [ tBu LNi] 2 (μ-N 2 ) with excess sodium metal afforded the Na 2 [ tBu LNi] 2 (μ-N 2 ) species ( 41 ), completing a series of reduced Ni-N 2 species that differ only by the intercalated alkali cations.…”