Imino iron complexes

Bis(imino)pyridine iron complex 5 as a highly efficient catalyst for a hydrogenation reaction was synthesized by Chirik and coworkers in 2004 [27]. Complex 5 looks like a Fe(0) complex, but detailed investigations into the electronic structure of 5 by metrical data, Mossbauer parameters, infrared and NMR spectroscopy, and DFT calculations established the Fe(ll) complex described as 5 in Fig. 2 to be the higher populated species [28]. 

Scheme 7 Intramolecular arene coordination in bis(imino)pyridine iron complex 10...

Bis(imino)pyridine iron complex 5 acts as a catalyst not only for hydrogenation (see 2.1) but also for hydrosilylation of multiple bonds [27]. The results are summarized in Table 10. The reaction rate for hydrosilylations is slower than that for the corresponding hydrogenation however, the trend of reaction rates is similar in each reaction. In case of tra s-2-hexene, the terminal addition product hexyl (phenyl)silane was obtained predominantly. This result clearly shows that an isomerization reaction takes place and the subsequent hydrosilylation reaction dehvers the corresponding product. Reaction of 1-hexene with H2SiPh2 also produced the hydrosilylated product in this system (eq. 1 in Scheme 18). However, the reaction rate for H2SiPh2 was slower than that for H3SiPh. In addition, reaction of diphenylacetylene as an atkyne with phenylsilane afforded the monoaddition product due to steric repulsion (eq. 2 in Scheme 18). 

The comparison of a bis(imino)pyridine iron complex and a pyridine bis (oxazoline) iron complex in hydrosilylation reactions is shown in Scheme 24 [73]. Both iron complexes showed efficient activity at 23°C and low to modest enantioselectivites. However, the steric hindered acetophenone derivatives such as 2, 4, 6 -trimethylacetophenone and 4 -ferf-butyl-2, 6 -dimethylacetophenone reacted sluggishly. The yields and enantioselectivities increased slightly when a combination of iron catalyst and B(CeF5)3 as an additive was used. 

Scheme 24 The comparison of a bis(imino)pyridine iron complex and a pyridine bis(oxazoline) iron complex for hydrosilylation of ketones...

In 2009, Chirik reported a hydrogen-mediated reductive enyne cyclization catalyzed by the bis(imino)pyridine iron complex 5 (Scheme 37) [119]. In the... 

As an alternative method for the C-C bond formation, oligomerization and polymerization reactions of olefins catalyzed by a bis(imino)pyridine iron complex are also well known (Scheme 40) [121-124]. 

A head-to-head dimerization of a-olefin catalyzed by a bis(imino)pyridine iron complex has been reported by Small and Marcucci [126]. This reaction delivers linear internal olefins (up to 80% linearity) from a-oleftns. The linearity of products, however, depends on the catalyst structure and the reaction conditions. 

Fink and Babik reported that propylene polymerization was achieved by a bis (imino)pyridine iron complex with Ph3C[B(C6p5)]4] and ttialkylaluminium as additives [127]. Both 3-methyl-"butyl and "butyl endgroups were observed by NMR spectrum when ttiisobutylaluminium as an activator was used, whereas the only "propyl endgroup was formed in case of triethylaluminium activation. In addition, this polymerization proceeds two times faster with than without a hydrogen atmosphere, but the value decreases and the M IM value rises up. 

The first report of catalyzed chain growth on aluminum was presented by Samsel et al., using metallocenes of actinides [13] or hafnium [14, 15], Gibson et al. described a detailed study of chain growth using the bis(imino)pyridyl iron complex 1 in combination with several metal alkyls [16], Other systems reported to... 

The reaction mechanism of the very efficient bis(imino)pyridil complexes of iron and cobalt [2,6-(CMe=N(2,6-i-Pr2C6H3))2C5H3N]MCH3+ (M=Fe, Co) has been analyzed computationally [42-44]. The overall mechanism is similar to that depicted above for other systems, but there are some significant... 

Table 1 Optimized polymerization results using iron complexes ligated by unsymmetiical bis(imino)pyiidines bearing benzhydryl derivatives (under a pressure of 10 atm)...

Fig. 1 Structure of bis (iminoaryl)pyridyl iron complexes BI P FeCl2, where R refers to the ortho substituents on the imino aryl part...

Kissin YV, Qian CT, Xie GY, Chen YE (2006) Multi-center nature of ethylene polymerization catalysts based on 2,6-bis(imino)-pyridyl complexes of iron and cobalt. J Polym Sci A Polym Chem 44(21) 6159-6170... 

Ohnishi R, Konakazawa T, Amano J, Fujimura T (2006) Temperatrrre effect on ethylene polymerization with a catalyst prepared by mixitrg Mg(C2H5)(n-C4H9), Al(2H5)i 5CI1.5 and iron(II)bis(imino)pyridyl complex. Polym Bull 56(l) l-8. doi 10.1007/s00289-005-0460-8... 

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