Bis imino pyridine Iron Complexes

Based on the potential isolobal relationship between [Fe(CO)3] and [C PDI)Fe], we sought to prepare a formally iron(O) bis(itriino)pyridine complex with a thermally labile ligand. Dinitrogen seemed an ideal target ligand that could readily be 

Stoichiometric experiments were conducted with the functionalized alkenes to further explore iron-substrate interactions and gauge relative coordination affinities. Several bis(imino)pyridine iron amine and ketone compounds were isolated and studied using a combination of X-ray diffraction, NMR and Mossbauer spectroscopy and established electronic structures similar to that for ( PDI)Fe(N2)2-In the absence of H2, diallyl ether and allyl ethyl ether underwent facile C-0 bond cleavage and yielded a near equimolar mixture of the corresponding iron allyl and alkoxide complexes [89]. Our group has recently published a comprehensive study on these types of reachons and discovered rare examples of C-0 bond cleavage in saturated esters [89]. 

Given the wealth of intereshng reachvity observed with ( TDl)Fe(N2)2, attention has been devoted to improving the performance of bis(imino)pyridine iron 

The competition for coordination of the different arenes was examined as a function of solvent. For relatively non-coordinating solvents such as pentane, hexane, diethyl ether and mesitylene, exclusive formation of the Tj -aryl compound was observed. In more coordinating media, such as benzene, toluene, THF or cyclohexene, increased amounts of the Ti -phenyl complex were obtained. These 

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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. 

Further investigations revealed that this hydrogenation is accelerated in pentane solution. These results are shown in brackets in Table 3 [31]. Under optimized reaction conditions high catalyst TOF up to 5,300 were achieved when 10 was used. In the absence of both hydrogen and nitrogen, 10 was converted into the q -arene complexes such as the bis(imino)pyridine iron q -phenyl complex, 10-Phenyl, and the corresponding q -2,6-diisopropylphenyl complex, 10-Aryl, in the 85 15 ratio in... 

In contrast, the related diamines 45 and 46 [cf. (3,5-/-Pr2C6H2-4-NH2)2CH2 above] give, on treatment with 2,6-diacetylpyridine, oligomeric polyimines that can be readily complexed with iron dichloride to afford 47 and 48, respectively (Fig. 14). Notably, on activation with MAO, 47 and 48 are active catalysts for ethylene polymerisation and indeed perform more efficiently at elevated temperatures than those of the original bis(imino)pyridine iron precatalyst 5 [166],... 

Attempts were also made to observe or isolate catalyticaUy relevant intermediates. Addition of one equivalent of PhCsCPh to ( PDI)Fe(N2)2 yielded a red paramagnetic solid identified as the iron Tj -alkyne complex, which was also characterized by X-ray diffraction. Treatment of the isolated product with 1 atm of H2 initially yielded stilbene, followed by 1,2-diphenylethane, establishing its catalytic competency (Scheme 4.8). Our laboratory has recently conducted experiments to elucidate the stability of related bis(imino)pyridine olefin complexes [87]. Treatment of C PDl)Fe(N2)2 vvith 1-hexene under a dinitrogen atmosphere resulted in slow conversion to n-hexane and the paramagnetic, NMR-silent intramolecular... 

Scheme 4.10 Synthesis of bis(imino)pyridine iron bis(chelate) complexes.

The catalytically active species formed by the treatment of 2,6-bis(imino) pyridine iron(II) chloride complexes with MAO is generally proposed to be a highly reactive monomethylated iron(ll) cation [LFe-Me] (L = 2,6-bis(imino)pyridine ligand) bearing a weakly coordinating counteranion [Me-MAO]. Both monochloride and monoalkyl cationic species are expected to be present in the solution, their relative concentration depending on the MAO/Fe ratio [24]. 

It is useful to compare the molecular and electronic stmctures of the bis(imino)pyridine iron dinitrogen complexes with those of the ruthenium congeners reported by Berry and coworkers [32]. Stirring K -( PDI)Ru(q -arene) complexes in non-arene solvents under a dinitrogen atmosphere furnished the bimetallic ruthenium dinitrogen complex [( PDI)Ru]2(P2> l dimeric... 

Figure 7.6 Electronic structure of bis(imino)pyridine iron dinitrogen and diethyl ether complexes over four oxidation states.

An analogous series of bis(imino)pyridine iron dicarbonyl complexes was also synthesized (Figure 7.7) [38]. Unlike ( PDI)FeN2, ( PDI)Fe(CO)2 exhibits clean and reversible electrochemistry, allowing quantitative measurement of the potentials for one-electron oxidation and reduction. The combined metrical, spectroscopic, and computational data established that the cationic iron dicarbonyl complex [( PDI)Fe(CO)2][BAr 4j is best described as a low-spin iron(I) compound with a neutral bis(imino)pyridine chelate. Both EPR and DFT calculations support a principally iron-based SOMO with significant d 2 character. In the limit where the neutral iron dicarbonyl ( PDI)Fe(CO)2 is viewed in its low-spin Fe(II) canonical form, one-electron oxidation to [( PDI)Fe(CO)2] is pincer-based and results in a net reduction of the metal to Fe(I). 

Kurokawa, H., Matsuda, M., Fujii, K., Ishihama, Y, Sakuragi, T, Ohshima, M., and Miura, H. 2007. Bis(imino)pyridine iron and cobalt complexes immobilized into interlayer space of fluorotetrasilicic mica Highly active heterogeneous catalysts for polymerization of ethylene. Chemistry Letters 36 1004—1005. 

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