Sodium tetracarbonylferrate, better known as Collman’s reagent, is an electron-rich iron complex that has found use in organic synthesis. Namely, in the conversion of alkyl halides/sulfonates to carbonyl-containing compounds such as aldehydes, ketones, amides, and esters, and the selective reduction of conjugated alkenes.[1] The compound is isoelectronic to nickel tetracarbonyl, but less hazardous (because it is a solid as opposed to a volatile liquid, and the LD50 of iron being orders of magnitude larger than nickel).
Although various preparation methods have been reported, Collman’s reagent is typically synthesized from iron pentacarbonyl, molten sodium, and an “electron carrier” such as naphthalene, anthracene, or benzophenone, in a high-boiling ethereal solvent such as dioxane. The protocol from reference 2a (featured below) uses naphthalene as electron carrier, whereas reference 2b uses benzophenone. Both are equally viable. Last time I used it, I purchased it from Millipore-Sigma. However, it seems to be currently unavailable. It is just one of those that may need to be synthesized in-house.
Procedure:[2a] In the glovebox, a 500mL two-neck round-bottom flask with stir bar was charged with freshly-cut sodium (3.45g, 75.0mmol). The flask was sealed, taken out of the glovebox, and attached onto a swivel-frit apparatus attached to a dual-manifold vacuum line under an argon flow. 100mL of anhydrous THF were added, the reaction flask cooled in an ice bath, and a solution of naphthalene (9.90g, 77.0mmol) in 150mL of anhydrous THF was subsequently added. The dark green mixture was stirred, under cooling, for 2h, and then brought down to -78degC (dry-ice/acetone). Freshly distilled iron pentacarbonyl (7.02g, 36.0mmol) in 50mL of THF were added dropwise, under vigorous stirring, and the mixture was allowed to react at -78degC for 1h. The reaction is then allowed to warm up to room temperature, and 50mL of pentane were added to help precipitate the product. Subsequent filtration, washing with pentane (2x30mL), and drying in vacuo, gave the product as a white powder (reported yield: 7.39g, 96%). The contents of the frit were then transferred to the glovebox for storage.
Notes:
- Iron pentacarbonyl can be cheaply purchased from chemical vendors (Strem, Fisher Scientific, Millipore-Sigma). It should be distilled prior to use to maximize reaction yield. All solvents should be thoroughly dried and degassed before use.
- Na2[Fe(CO)4] is a white, highly air/moisture sensitive solid. It is soluble in polar organic solvents such as THF, DMF, NMP, and dioxane.[3] It should be kept in the glovebox freezer to shield it from light and minimize long-term decomposition.
- If iron pentacarbonyl is added to sodium naphthalenide in a 1:1 ratio, the orange Na2[Fe2(CO)8] complex is obtained instead. It forms via comproportionation of Na2[Fe(CO)4] with Fe(CO)5.
- In James Collman’s account,[1] the utility of the reagent was serendipitously discovered when one of his postdocs sought to effect a Wurtz-like homocoupling of methyl iodide by using Na2[Fe(CO)4] as the reducing agent. Subsequent workup gave a crude mixture with the fruity smell of acetaldehyde, which led them to pursue the reaction in more detail.
- In the presence of alkyl halides, the [Fe(CO)4] dianion forms a trigonal bipyramidal complex with bound alkyl and extrusion of sodium halide salt (proceeds in SN2 fashion, favoring primary and secondary alkyl halides; tertiary alkyl halides undergo elimination).[4] This organoiron species undergoes migratory insertion of CO onto the alkyl upon addition of an extra ligand. The resulting species then releases the acyl fragment as an aldehyde, ketone, carboxylic acid, ester, or amide depending of the added reagent. It is for this versatile reactivity that James Collman coined Na2[Fe(CO)] “a transition-metal analog of a Grignard reagent”.[1]
- The potassium analog, K2[Fe(CO)4] can also be prepared. Unlike Collman’s reagent, It is not pyrophoric and easier to handle, but suffers from poor solubility in traditional organic solvents.[5]
[1] Collman, J. P. Disodium Tetracarbonylferrate - a Transition-Metal Analog of a Grignard Reagent. Acc. Chem. Res. 1975, 8, 10, 342-347.
[2] a) Strong, H.; Krusic, P. J.; Filippo, J. S. Sodium Carbonyl Ferrates. Inorg. Synth. 1990, 28, 203-206. b) Finke, R. G.; Sorrell, T. N. Nucleophilic Acylation with Disodium Tetracarbonylferrate. Org. Synth. 1980 , 59, 102.
[3] Pike, R. D. Disodium Tetracarbonylferrate(-II). EROS 2001. DOI: 10.1002/047084289X.rd465.
[4] Collman, J. P.; Finke, R. G.; Cawse, J. N.; Brauman, J. I. Oxidative-addition reactions of the disodium tetracarbonylferrate supernucleophile. J. Am. Chem. Soc. 1977, 99, 8, 2515-2526.
[5] Baby, A.; Brunet, J.-J.; Kindela, F. B.; Neibecker, D. Convenient Preparation and Use of Potassium Tetracarbonylferrate, K2[Fe(CO)4], an Easy-to-Handle Analogue of the Collman Reagent Synth. Commun. 1994, 24, 20, 2827-2834.