Disodium tetracarbonylferrate

Synthesis

The reagent was reported by Cooke in 1970. The current synthesis entails the reduction of a solution of iron pentacarbonyl in tetrahydrofuran by sodium naphthenide. The efficiency of the synthesis depends on the quality of the iron pentacarbonyl.

Fe(CO)₅ + 2 Na → Na₂[Fe(CO)₄] + CO

When a deficiency of sodium is used, the reduction affords octacarbonyl diferrate:

2 Fe(CO)₅ + 2 Na → Na₂[Fe₂(CO)₈] + 2 CO

Other synthesis forms to make Collman's Reagent from Fe(CO)₅ are listed below.

Fe(CO)₅ + Na-Hg +THF → Na₂[Fe(CO)₄]Fe(CO)₅ + Na + Dioxane + PhCOPh → Na₂[Fe(CO)₄]Fe(CO)₅ + Na/PhCOPh + THF → Na₂[Fe(CO)₄]

Another way to synthesize Collman's Reagent is to use FeCl₃.

FeCl₃ + Na(C₁₀H₈) + 4CO + THF → +Na → Na₂[Fe(CO)₄]

These synthesis pathways are extremely useful in preparing Collman's Reagent if the typical reagents to make it are not available.

Reactions

The reagent was originally described for the conversion of primary alkyl bromides, RBr, to the corresponding aldehydes in a two-step, "one-pot" reaction:

Na₂[Fe(CO)₄] + RBr → Na[RFe(CO)₄] + NaBr

This solution is then treated sequentially with PPh₃ and then acetic acid to give the aldehyde, RCHO.

Disodium tetracarbonylferrate can be used to convert acid chlorides to aldehydes. As for Cooke’s early discovery, an iron acyl complex undergoes protonolysis to give the aldehyde.

Na₂[Fe(CO)₄] + RCOCl → Na[RC(O)Fe(CO)₄] + NaClNa[RC(O)Fe(CO)₄] + HCl → RCHO + "Fe(CO)₄" + NaCl

Disodium tetracarbonylferrate reacts with alkyl halides (RX) to produce alkyl complexes:

Na₂[Fe(CO)₄] + RX → Na[RFe(CO)₄] + NaX

Such iron alkyls can be converted to the corresponding carboxylic acid and acid halides:

Na[RFe(CO)₄] + O₂, H⁺ →→ RCO₂H + Fe...Na[RFe(CO)₄] + 2 X₂ → RC(O)X + FeX₂ + 3 CO + NaX

One attraction of these methods is the low cost of the iron carbonyl as well as the fact that the procedures are relatively “green” because the side product is iron-based.