Fe -N2 complexes

Holland and Peters earlier reported examples of three- and four-coordinate N2 complexes with iron in the formal +1 oxidation state, but in neither case were terminally bonded N2 species identified or isolated dinuclear end-on Fe-NN-Fe species were inevitably obtained (99,100). Therefore, Peters and coworkers recently began to turn their attention to new derivatives of the classic Sacconi-type tripodal ligands to achieve access to a terminally bound Fe-N2 complex. Indeed, by employing the mono-anionic ligand L8 — [f2-Ph2PC6H4)3Si] (Fig. 10) they were able to generate the desired complex [Fe(L8)(N2)] (10) (101). 

In view of the considerable discussion of nitrogen complexes in earher papers, I should mention in passing that infrared spectra have also been most useful in our studies of heme-nitrogen complexes in continuation of a recent observation that azido protohemin may be converted to an Fe-N2 complex in pyridine solution 14),... 

In order to mimic the sulfur-rich coordination sphere in the active site of nitrogenase, Holland and coworkers prepared a molecular model system, in which upon reduction an Fe-S bond is broken and N2 coordinates (Scheme 58). " Complex 229 represents the first synthetic Fe—N2 complex, in which iron is bonded both to sulfur as well as carbon atoms. The terminally coordinated N2 ligand is significantly activated with Vnn = 1880 cm which is the lowest value observed so far for Fe complexes with terminal Fe-N2 ligands.The thiolate ligands act as powerful electron donors enabling substantial backbonding from the Fe d-orbitals into the Jt -N2 orbitals. 

Creutz SE, PetersJC. Catalytic reduction ofN2 to NH3 by an Fe-N2 complex featuring a C-atom anchor. J Am Chem Soc. 2014 136 1105-1115. 

This section on N2 complexes concludes with a brief mention of a Mossbauer study of the Fe/N2 system (8). The N2 stoichiometries were... 

Other iron-imido complexes have also been reported. Holland and coworkers reported the synthesis of the imidoiron(III) complex [L FeNAd] [40, 41]. This imidoiron(III) complex has not been isolated and was found to convert to a purple high-spin iron(III) complex. It has an S = 3/2 ground state from EPR measurement. Based on the results of QM/MM computations, [L EeNAd] is a three-coordinated complex with an Fe-N distance of 1.68 A and has a nearly linear Fe=N-C unit with Fe-N-C angle of 174.1°. Chirik and coworkers made use of liable ligands to prepare iron-imido complexes by treatment of C PDI)-Fe(N2)2 ( PDI = (2,6- Pr2CgH3N = CMe)2C5H3N) with a series of aryl azides [47]. 

Fe-S complexes have important functions in today s living systems, in enzymes such as the ferredoxins and oxidoreductases, as well as in electron transport proteins. It is striking that these redox reactions mainly involve elements and compounds such as CO, H2 and N2, which were probably also components of the primeval Earth s atmosphere. Thus, the assumption of an active involvement of Fe-S clusters in a (hypothetical) Fe-S world in processes which finally led to biogenesis appears completely reasonable We now have a background to the theory of the chemoau-totrophic origin of life . 

Metal-promoted 1,2-migration of silyl groups in silylalkynes results in the formation of silylvinylidenes which are subsequently readily desilylated. " Alkynyl-substituted silylvinylidenes have been obtained from silylated diynes and the Fe(N2)(CO)2 P(OMe)3 2/ Fe(CO)2[P(OMe)3]2 2(/t-N2) reagent and similar species are implicated in the reactions of several Group 8 metal complexes with mono- and bis-trialkylsilyl diynes. " ... 

This type of double-seal gave very good protection against O2. Fe(III) protoporphyrin IX was prepared after the method of Falk (10). The Fe(III) complex was converted to Fe(II) using the method oF Brault and Rougee (11) in the N2-H2 atmosphere glove bag. 

Fig. 16. Structures involved in the decomposition of the adduct intermediate resulting from the reaction of [Fe(CN)r,NO]2 with NH3. (a) Only the formation of the linear, q1-N2 complex is considered, (b) The formation of T 2-N2 as an intermediate step is also included. The structures correspond to single points on the potential hypersurface, calculated at the B3LYP/6-31G level. Relative energies are not drawn to scale. Arrows indicate changes in the molecule that lead to the next step.

Figure 6.4 Example of a two-coordInate iron complex, [Fe N(SiMePh2)2 2]- ° Iron and nitrogen atoms are shown as black spheres, silicon atoms are grey and carbon atoms are white. Selected bond lengths and an angle Fe-NI 1.90, Fe-N2 1.91 N-Fe-N 172. F ...

The first investigations on iron-catalyzed Michael reactions utilized Fe(acac)3 as catalyst. However, this metal complex is itself catalytically almost inactive. Yields of only up to 63% could be achieved, if BF3OEt2 is used as a co-catalyst [55], Polystyrene-bound Fe(acac)3 catalysts were also reported to give yields up to 63% [56], FeCl3 was used as a co-catalyst for clay-supported Ni(II). Yields achieved with this heterogeneous system ranged from 40 to 98% [57]. The double Michael addition of acrylonitrile to ethyl cyanoacetate is smoothly catalyzed by a complex generated from [Fe(N2) (depe)2] [depe = l,2-bis(diethylphosphano)ethane]. At 23 °C and after 36h, an 88% yield is obtained with 1 mol% of this Fe(0) catalyst [58]. 

E Coordinating N2 to Mononuclear Fe(II) Complexes The Role of Spin Barriers... 

A Hypothetical Cycle for N2 Reduction with [Fe(S )] Complexes and Reversible Redox Reactions of Diazene Complexes / 662... 

Scheme 37 shows a hypothetical cycle for the N2 reduction in the coordination sphere of [Fe(S )] complexes (7e-7g) (47), emphasizing the dominance of iron and sulfur in all nitrogenase cofactors. 

A different rendering of the relevant core atoms of the dication [Fe, N2H2, S(thiolate) donors] shows that the dicationic diazene complex B and the N2 complex C are potential redox isomers or valence tautomers (Scheme 38). 

It can be conceived that intramolecular electron transfer from N2H2 to the Fe(III) centers, cleavage of NH bonds, and formation of SH bonds converts 44 into 45, which is a doubly protonated N2 complex. Deprotonation of 45 would yield the neutral N2 complex 46. The reversal of this sequence would convert 46 into 43 and realize the first 2 H+/2 e reduction step of N2 fixation. Twofold protonation of 46 gives 45 in which all atoms necessary to form the neutral diazene complex 43 have already taken their positions. The above mentioned anodic redox potential shift upon protonation (cf. Section IV.F) could enable us to reduce species C at relatively mild redox potentials, in contrast with the neutral species 46, which might be irreducible when it is an 18 VE complex. 

Closely related are the -acyl complexes of iron (115). These have been prepared by three methods. Loss of N2 from (112) in the presence of an alkyl halide gives oxidative addition see Oxidative Addition) to yield ) -acyl complexes for some phosphine ligands. Sodium amalgam reduction of Fe halide complexes (116), and exposme of the resulting (117) to an alkyl halide, gives similar complexes (equation 24). Finally, placement of iron alkyl (aryl) complexes (118) under a CO atmosphere results in insertion to afford complexes (119) (equation 25). ... 

Liquid and supercritical noble gases are ideal media for studying H2 species since H2 is completely miscible under these conditions. The group VI metal carbonyl hydrides, M(CO)5(H2), and cis-Cr(CO)4(H2)2 were formed photochemically in liquid Xe/If2 mixtures. H2 and N2 complexes of a number of half-sandwich compounds were formed in supercritical Xe. These included (C Rn)M(CO)2Y, where R = H, Me, M = Mn, Re (n = 5), Cr (n = 6), and (C4H4)Fe(CO)2Y, where Y = H2 or N2. In all cases except Re, nonclassical H2 complexes were formed. ... 

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