Diiron complex

Evans found that molecular hydrogen was efficiently generated by the reaction of a simple diiron complex [CpFe(CO)2]2 (Fp2) with acetic acid (pA a = 22.3) in acetonitrile [202]. Electrochemical simulations revealed that Ep2, [CpEe(CO)2] (Fp ), and [CpFe(CO)2H] (FpH) were key intermediates in this catalytic mechanism (Scheme 61). Reduction of Fp2 produces both an Fp anion and an Fp radical, which is further reduced to give an Fp anion. The oxidation of the Fp anion by proton affords FpH. This protonation was found to be the rate-limiting step. The dimerization of the FpH generates Fp2 and H2. Alternatively, the FpH is reduced to afford the FpH anion, which is subsequently protonated... 

Very recently, Hammarstrom, Ott, and coworkers found that the catalytic activity was significantly increased (up to 200 TON based on the iron complex) when a diiron complex having a 3,6-dichlorobenzene-1,2-dithiolate ligand (Fig. 10) instead of a benzylazadithiolate ligand was used as a WRC (see ref. [64] in [232]). 

Fig. 10 Diiron complex having a 3,6-dichlorobenzene- 1,2-dithiolate ligand as a catalyst for the hydrogen generation...

Two different types of zinc-porphyrins coordinated diiron complex act as catalysts for the photochemical reduction hydrogen evolution from water. In this system... 

The first reported diiron complex possessing nuclease activity actually did not operate by a true hydrolytic mechanism, although the products... 

The second oxidation, which involves the Fe(III)/Fe(IV) change, is accompanied by a relatively slow degradation of the electrogenerated Fe(IV) complex. The electrochemical access to the high-valent Fe(IV) oxidation state is rather uncommon, even if a few bis(/x-oxo)diiron complexes have displayed the (III,III)/(III/IV) redox changes.116,117... 

The double iron-mediated arylamine cyclization provides a highly convergent route to indolo[2,3-fc]carbazole (Scheme 16). Double electrophilic substitution of m-phenylenediamine 34 by reaction with the complex salt 6a affords the diiron complex 35, which on oxidative cyclization using iodine in pyridine leads to indolo[2,3-b]carbazole 36 [98].Thus,ithasbeen demonstrated that the bidirectional annulation of two indole rings can be applied to the synthesis of indolocarbazoles. 

The first isolable, albeit binuclear, butatrienylidene complexes, the cationic diiron complexes 8, were likewise prepared by addition of an electrophile E+ to neutral butadiynyl complexes. Instead of mononuclear butadiynyl complexes, binuclear C4-bridged butadiyndiyl complexes 7 were used as the starting complexes by Lapinte et al. (Scheme 3.4) [18]. Complexes 8 were characterized by multinuclear NMR, IR, UV-vis, and Mossbauer spectroscopies, mass spectrometry and cyclic voltammetry. 

The tetrathiomolybdate anion [MoS4]2 acts as a chelating ligand toward iron in both diiron complexes [Fe2(SR)2(NO)4] and tetrairon complexes [Fe4S3(NO)7] and [Fe4S4(NO)4] to yield two paramagnetic iron-nitrosyls which have been assigned (55) the constitutions 15 and 16 ... 

In contrast to the stepwise reduction of [Fe4S4(NO)4], [Fe4Se4(N0)4], and the heterometallic cubanes 19 and 21, cyclic voltammetry of the diiron complex [Fe2(TePh)2(NO)4] showed (52) a single reversible two-electron wave corresponding to reduction to [Fe2(TePh)2(NO)4]2, salts of which were subsequently isolated by chemical reduction. This dianion is a 36-electron species, and hence is isoelectronic with the neutral cobalt complexes [Co2(SR)2(NO)4] (26) it thus lacks an Fe-Fe bond. 

Cyclopropylidene complexes are rare. In fact, there is only one report of isolated (/(-cyclo-propylidene)diiron complexes which are stable in the solid state at room temperature vide infray. More recently, a labile cyclopropylidene tungsten complex has been isolated which decomposes above -40 °C42. In all other reports21,110 cyclopropylidene complexes are proposed as transient intermediates. 

Unusual bridging (//-cyclopropyIidene)diiron complexes having a tetrahedral carbene carbon have been studied as model intermediates in carbon-carbon bond formation in the Fischer-Tropsch synthesis248. The cyclopropylidene complexes cis- and trans-[Cp(Co)Fe]2(/(-Co)(//-C,H4) were readily prepared by cyclopropanation in ether, of the corresponding cis- and mww-vinylidene complexes [CpCoFe](//-CO)(//-CH2) with diazomethane in the presence of CuCl (equation 181). Both isomers are air stable in the solid state. Solutions of the complexes are air stable for several hours, provided they are kept in the dark. The pure //-cyclopropylidene isomers slowly interconvert in solution, like their parent /z-vinylidene and other alkylidene complexes. The final equilibrium ratio cis .trans = 4.8 1 is reached after two weeks. 

Two literature examples serve to illustrate these procedures. The first involves rates of competitive ligand exchange reactions and relates to a single reaction condition, wherein irgy/ifwd rati°s are measured at a series of different scan rates. The second involves ligand substitution of an organometallic diiron complex, and relates to a set of different reaction conditions (nucleophile concentrations), wherein the irev/ifwd rat ° ls measured for each concentration at a single scan rate. 

Figure 23.15 shows the CV scans obtained after successive additions of CH3CN to a 0.98 mM solution of CpfFe2(CO)2(p-CO)2 in CH2C12. The one-electron oxidation at approximately +0.4 V is that of the reactant diiron complex. Equations 23.23 (E) and 23.22 (C) constitute the EC mechanism. The reduction wave at -0.85 V is that of the ultimate reaction product, Cp Fe(CO)2(NCCH3) +, and although it is a useful indicator of the reaction product, it is not used in the calculations to obtain k. ... 

The first example of iron-catalyzed asymmetric oxidation of sulfides was described by Fontecave and coworkers in 1997 [163]. An oxo-bridged diiron complex, which contained (—)-4,5-pinenebipyridine as chiral ligand, was reported to catalyze sulfide oxidations with H202 in acetonitrile, having the potential to transfer an oxygen atom directly to the substrates. However, the enantioselectivity of this process remained rather low (<40% ee, Scheme 3.53). 

Scheme 3.53 Oxo-bridged diiron complex for sulfide oxidations.

A number of molecular mechanics studies of metal-cyclopentadienyl complexes have been reported recently. The systems studied include linear metallocenes (in particular ferrocene), ferrocene derivatives (such as complexes with substituted cy-clopentadienyl ligands, bis(fulvalene)diiron complexes, ferrocenophanes and mixed-ligand complexes with carbonyls and phosphines), and nonlinear cyclopentadienyl complexes 8,153,221 231]. 

Fig. 2. Medium pressure NMR sample tubes containing solutions of the diiron complexes, pressurized with 10 bar D2 and were exposed to sunlight on the windowsill.

---