Efficient Catalytic Conversion of Dinitrogen to N(SiMe₃)₃ Using a Homogeneous Mononuclear Cobalt Complex

Abstract

Incorporation of the tridentate phosphine-enamidoiminophosphorane onto cobalt(II) produces tetrahedral Co(NpNP iPr )Cl, 1, which upon reduction under dinitrogen generates the T-shaped, paramagnetic Co(I) complex Co(NpNP iPr ), 2. This paramagnetic Tshaped derivative is in equilibrium with the paramagnetic dinitrogen derivative Co(NpNP iPr )-(N 2 ), 3, which can be detected by IR and low-temperature UV−vis spectroscopy. Both 1 and 2 act as homogeneous catalysts for the conversion of molecular nitrogen into tris(trimethylsilyl)amine (N(SiMe 3 ) 3 ) (∼200 equiv, quantified as NH 4 Cl after hydrolysis) in the presence of excess KC 8 and Me 3 SiCl at low temperatures.

Effects of N2Binding Mode on Iron-Based Functionalization of Dinitrogen to Form an Iron(III) Hydrazido Complex

“…Meanwhile, many terminal N 2 complexes of iron that exhibit significantly less ground-state activation of the N-N bond ( v NN > 1900 cm −1 ) have been shown to react with silyl electrophiles and acids both stoichiometrically and catalytically to give silylated and protonated N 2 reduction products. 10,14–16 The observed lack of reactivity at N 2 of bridging 1 could be due to Hypothesis 1) the size of the ligands sterically encumbers approach of electrophiles to the N 2 unit causing N-X bond formation to be kinetically challenging, or Hypothesis 2) the symmetric binding mode of the N 2 leads to a less potent nucleophile since the negative charges on the N-atoms are directly bound to positively charged iron centers. Hypothesis 1 is supported by comparing the spacefilling models of 1 and 3 where the N 2 unit in the bridging complex 1 is almost completely hidden by the steric bulk of the supporting ligand and potassium cations, while in 3 the N 2 units are clearly visible and extend beyond the steric bulk of the supporting ligand (Figure 11).…”

“…In addition to the examples of catalytic N x H y formation, a growing number of catalytic N 2 silylation reactions have been reported. 14 One major advantage of studying reactions between transition metal N 2 complexes and silyl halides is that the silylated N 2 products can be isolated more easily, giving insight into partially reduced dinitrogen species that are analogous to the intermediates in catalytic protonation. 15 Being able to draw parallels between the silylation and protonation chemistries is particularly relevant for first row transition metals where the protonated intermediates are often thermally sensitive.…”

“…16c Lu and coworkers isolated analogous hydrazido(2-) complexes using cobalt and iron metallalumatranes, and calculations suggested that formation of a hydrazido(2-) complex is rate determining; this is consistent with the inability to observe this species during catalysis. 14–16e…”

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

“…The use of cobalt to mediate N 2 silylation was further studied by Fryzuk and Masuda using PNN ‐type frameworks. Complexes [Co(NpNP)] ( 302 ) [241] and [Co(QuiNacNacP)] ( 303 ) [242] (Scheme 60) reduced N 2 in the presence of potassium graphite and trimethylsilyl chloride to afford N(SiMe 3 ) 3 (TONs of 100 and 19 respectively). Additionally, the cobalt analogue 304 of the above mentioned iron PSiP complex 285 (Scheme 56 b) also promoted the catalytic conversion to N(SiMe 3 ) 3 with up to 20.5 TONs [229] …”

Catalytic Silylation of Dinitrogen by a Family of Triiron Complexes

“…This temperature effect on dinitrogen silylation is similar to that reported by Masuda and coworkers for a Co complex. 16 To our knowledge, these triiron compounds are the most effective reported catalytic systems for this reaction, independent of metal ion type or complex nu-clearity (Table S3). 9,16,28,37…”