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Introduction
Until 2001, the ligand Fp, was produced by reducing [CpFe(CO)2]2 with strong agents such as Na/Hg and Na/K or electrochemical reduction at negative potentials. Fp is a useful ligand because it can be easily acylated/alkylated by a suitable electrophile. Photochemical reduction of [CpFe(CO)2]2 had been studied, but none by an electron donor. Using photochemistry to synthesize metal carbonyl complexes is a valued technique in organometallic chemistry.

Photochemical Reaction
It was discovered that [CpFe(CO)2]2 could be photochemically reduced by 1-benzyl-1,4-dihydronicotinamide dimer, (BNA)2, a two electron donor. This reaction happens when [CpFe(CO)2]2 is in its primary form, the cis-two carbonyl bridged isomer. In this synthesis, (BNA)2 and [CpFe(CO)2]2 are irradiated with UV-visible light at a wavelength of 350 nm to produce Fp, [CpFe(CO)2]-, and BNA+ (figure 1). This wavelength is the λmax of (BNA)2 and is crutial for the formation of [CpFe(CO)2]- without the loss of a carbonyl. Absorption was used to monitor the reaction with a disappearance of the band due to [CpFe(CO)2]2 and appearance of a new band at 450 nm corresponding to [CpFe(CO)2]-. Researchers also tried reducing [Cp*Fe(CO)2]2 ,where Cp* is (η5-C5Me5), but hardly any reduction occurred.



Reaction Mechanism
Using voltammetry, fluorescence, laser flash photolysis, absorption, and electron spin resonance the reaction mechanism was speculated. First 3(BNA)2* is produced by intersystem crossing. Once 3(BNA)2* is produced there is an electron transfer and fast carbon-carbon bond cleavage to [CpFe(CO)2]2 to form BNA·, BNA+, and [CpFe(CO)2]2·. Lastly, an iron-iron bond cleavage gives the final products [CpFe(CO)2]-, and BNA+ (figure 2).



Intermediate Complexes
More extensive studies on the electrochemical oxidation of [CpFe(CO)2]- were reported. It had been established that the voltammertic pattern on [CpFe(CO)2]- contained two major waves. One wave corresponds to the one-electron oxidation of [CpFe(CO)2]- and the other wave corresponds to the reduction of [CpFe(CO)2]2. Researchers were able to conclude from cyclic voltammetry that radical [CpFe(CO)2]· is formed via a one electron oxidation of the anion. The anion then dimerizes to [CpFe(CO)2]2, the unbridged version, which is then reduced via a two electron reduction to once again generate [CpFe(CO)2]- (figure 3). Overall, the researchers were able to establish an intermediate radical, [CpFe(CO)2]·, via a one electron oxidation in the cycle to photochemically generate Fp.



Conclusion
Thanks to this discovery there is now yet another way to generate the Fp ligand from [CpFe(CO)2]2 which is useful in a handful of organometallic reactions.