Iron NHC complexes

The ability of iron-NHC complexes to serve as free radical and ROP catalysts has also been demonstrated. In 2000, Grubbs described the first set of iron-NHC... 

Recent developments on iron catalysis have shown the enormous potential of this metal in the reduction of carbonyl groups with hydrogen or using transfer hydrogen and silanes as reducing agents [69]. However, the use of well-defined iron-NHC complexes as catalysts for these reduction processes is rather undeveloped [72]. 

Hilt and coworkers developed radical addition/cyclization sequences of styrene oxides 42 and styrenes, dienes, 1,3-enynes, and acrylates 43 catalyzed by three catalyst systems (A) 20 mol% of the Fe(dppe)Cl2 complex [103], (B) FeCl2 in the presence of 10 mol% PPh3/10 mol% NHC 44 [103], or (C) 20 mol% of (salen)iron(II) complex 45 (Fig. 9) [104, 105]. In all three methods NEt3 and zinc were used as... 

It is interesting to note, that apart from the impressive series of transition metal complexes with monodentate NHCs, tetracarbene palladium(II) and platinum(II) as well as hexacarbene iron(III) complexes with bi- and tridentate chelating carbene ligands were prepared. 

It is somewhat ironic that the trend in metal NHC complex chemistry reverts back to the old traditional protocols of Ofele and Wanzlick, the reaction of an imidazohum salt with a transition metal complex in the presence of a weak base. Ofele and Wanzlick had the base incorporated in the transition metal complex, Lin did the same, but it can be provided externally as KjCOj, NaOAc, NEtj, pyridine or almost any other known base. The driving force... 

Figure 10.8 Synthesis of iron(II) complexes bearing tetramethylcyclopentadienyl-functionalized NHC ligands.

Figure 10.9 (a-d) Iron(II) complexes with Cp-tethered NHC hgands. 

HALF-SANDWICH IRON AND NICKEL NHC COMPLEXES AS CATALYSTS FOR REDUCTIONS... 

Blom B, Tan G, Enthaler S, Inoue S, EppingJD, Driess M. Bis-N-heterocychc carbene (NHC) stabilized ti -arene iron(0) complexes synthesis, structure, reactivity, and catalytic activity.. Tm Chem Soc. 2013 135 18108-18120. 

Although not as frequent as in iridium or ruthenium complexes, C-H activation of Af-aryl substituents in palladium-NHC complexes is also known to occur and have an influence on catalytic reactions. Danopoulos and coAvorkers showed that NHC complexes in which the Af-aryl moiety contained only one substituent underwent metalation at the aromatic C-H bond. Depending on the reaction conditions, one or both of the NHC ligands would be metalated. " Even iron complexes, which are not generally susceptible to ortAo-metalation, were shown to undergo C-H activation of the NHC ligand. ... 

Cycloaddition of aUcynes catalysed by transition metals is one of the most efficient and valuable ways to prepare benzene and pyridine systems [12], Among the possible catalytic systems able to catalyse this reaction, cobalt and iron complexes containing NHCs as ligands have shown high catalytic activity in the intramolecular cyclotrimerisation of triynes 36 (Scheme 5.10) [13]. The reaction was catalysed with low loading of a combination of zinc powder and CoC or FeClj with two or three equivalents of IPr carbene, respectively. 

Mandemack KW, Bazylinski DA, Shanks III WC, Bullen TD (1999) Oxygen and iron isotope studies of magnetite produced by magnetotactic bacteria. Science 285 1892-189 Mann S, Sparks NHC, Couling SB, Larcombe MC, Franke RB (1989) Crystallochemical characterization of magnetite spinels prepared from aqueous solution. J Chem Soc Far Trans 85 3033-3044 Matthews A, Zhu XK, O Nions K (2001) Kinetic iron stable isotope fractionation between iron (-II) and (-III) complexes in solution. Earth Planet Sci Lett 192 81-92... 

Bielawski et al. described the synthesis and properties of NHC 15 introducing a novel carbene architecture incorporating a l,l -disubstituted ferrocene moiety as backbone [71 ]. In its metal complexes [71,72] this carbene ligand supported a long-range electronic communication between the iron center and the carbene coordinated transition metal. 

A new approach for the generation of NHCs has been reported using a cyclopentadi-ene(arene)iron complex (17).22 The method converts imidazolium to the free carbene in the presence of oxygen (peroxyradical anion is the base deprotonating the imidazolium salt). A colour change and precipitation of the oxidized iron complex are evidence for the reaction outcome. 

Cyclotrimerization of triynes has been reported using cobalt or iron complexes of NHCs without CO and Cp ligands.31 Benzannulated compounds have been isolated with up to quantitative yields. 

The unusual rotated structure of the enzyme active site is achieved in a mixed-valent Fe(II)Fe(I) complex, which uses the unique orientation of a bulky NHC ligand to protect the open site on the rotated iron. 

Within the same triad, iron has the smaller ionic radius compared with ruthenium [20], although electronically and structurally the two elements should form the same complexes. They essentially do, but when a second pincer carbene ligand is coordinated, it coordinates with one NHC moiety in abnormal coordination mode [476] as opposed to ruthenium, where both pincer carbene ligands are coordinated normally [468], In the absence of coordinating anions (BPh instead of bromide) octahedral cationic complexes are formed instead of square pyramidal neutral ones (see Figure 3.158). The additional n-donor ligand compensates the positive charge electronically. 

Figure 4.17 Synthesis of a homoleptic iron(ll) carbene complex with a phenoxy functionalised NHC ligand.

Piano-stool iron complexes bearing NHC have been obtained by substitution of a CO ligand from [(Cp)Fe(CO)2X] (56), as illustrated in Scheme 9. When X was a methyl group, the reaction, performed at room temperature, afforded... 

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