Cobalt complexes pincer

Interestingly, the activity of the corresponding cobalt catalyst possessing a pincer-type ligand is higher than that of the iron complex. In addition, the cobalt complex also acts as a catalyst in asymmetric mtermolecular cyclopropanations. 

Note All cobalt(I), cobalt(II) and cobalt(III) pincer carbene complexes are neutral and soluble in common organic solvents (THF, toluene, but not hexanes). 

Electronic Structures of Reduced Manganese, Iron, and Cobalt Complexes Bearing Redox-Active Bis(imino)pyridine Pincer Ligands... 

C—H borylation of indoles has been reported by a number of groups. Chirik and coworkers utilized pincer-ligated cobalt complexes with N-methyhndole and recorded the C-2 borylated indole (133) as the major product (2014JAC4133). The catalysts employed demonstrated high catalysis turnover and low catalytic loading and also demonstrated efficacy with other electron-rich heteroarenes (furan, thiophene, benzofuran) as well as electron-deficient pyridines. More recendy, platinum-NHC complexes have been used in the selective C—H borylation of indoles (2015JAC12211). The authors reported higher isolated yields with this... 

In 2015, Kempe and co-workers reported the first cobalt-catalyzed A-alkylation of (hetero)arylamines with alcohols under mild conditions (80 °C) with a relatively low catalyst loading (2 mol%) (Elq. 32) [133]. In 2016, Zhang and co-workers developed another pincer cobalt complex 19, which is an active catalyst for both imination and A-alkylation reactions of aromatic and aliphatic amines with alcohols under base-free conditions [134]. By using complex 19, monoalkylated amine products rather than the imines can be selectively achieved by simply adding 4 A molecular sieves to the reaction mixture (Eq. 33). 

Recently, Obligacion, Semproni and Chirik demonstrated the first example of a C—H borylation using pincer-ligated cobalt complexes." Treatment of benzofuran... 

These studies raised several fundamental questions about the effects of pincer substituents on the overall electronic structure of the resulting cobalt halide or alkyl compound. First, why are the electronic stractures of N-aryl and N-alkyl substituted bis(imino)pyridine cobalt chloride complexes different. Second, why for the N-alkyl derivatives is the chloride compound spin crossover and the methyl diamagnetic. What role does the distortion of the hgand play on the overall electronic structure ... 

Recently Chirik et al. reported the synthesis of cobalt pincer complexes (such as 85) that were shown to be effective catalysts for the borylation of substituted pyri-dines 76 with B2pin2 (Scheme 10.29). Use of this catalyst allowed access to novel boronate esters 87A and 87B. 2-Substituted pyridines underwent smooth monobory-lation to afford the C4 product as the major positional isomer (88-91). 

The chemistry of pincer complexes of the group 9 has been dominated by the study of the derivatives with rhodium and iridium. To date, the reactivity of cobalt pincer compounds with carbon monoxide has not been reported, mainly due to the fact that the number of pincer complexes derived from cobalt is reduced [17]. Thus, the interest in the Rh and Ir PCP pincer complexes is mainly owing to the fact that they have been proved efficient in the catalytic dehydrogenation of aliphatic C—H bonds this is particularly true in the case of the iridium PCP pincer complexes [3j. 

This complex has been obtained by adding an excess of MeLi onto the corresponding dibromide cobalt derivative with the same tridentate ligand, or more cleanly onto the monobromide cobalt A -heterocyclic pincer dicarbene derivative. The complex displayed a square-planar structure. This seems to be the first example of a Co(i)-alkyl complex stabilized by a CNC pincer ligand and lacking any tr-acccptor coligands. 

Substitution of noble metals in catalysts for cheap and abundant materials is of obvious importance, and many works concern the preparation and study of the latter systems. Thus, cobalt(II) alkyl complexes of aliphatic PNP pincer ligands (Fig. 12) are active precatalysts for the hydrogenation of ketones and the acceptorless dehydrogenation of alcohols under mild conditions in this case the alcohol dehydrogenation likely proceeds through a cobalt(I)/(III) redox cycle. ... 

Figure 12 Cobalt(ll) complex with a PNP pincer ligand as a catalyst precursor for acceptorless dehydrogenation of alcohols." " ...

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