Soluble iron(II) dihalide complexes with labile ligands are useful sources of iron for a large number of applications: Catalytic reactions such as hydrogenation and hydroformylation, mechanistic studies, studies of transition-metal electronic structure, and even material applications in the formation of metal-organic frameworks and nanoparticles. Reports describing the preparation of tetrapyridineiron(II) chloride date as far back as 1905 (and likely even earlier). Although it can easily be made by mixing FeCl2 with pyridine, exclusion of air and moisture are necessary to maximize product purity. Iron oxidation and/or precipitation of the product hydrate are the two biggest issues to watch out for.
Procedure: In the glovebox, a 250mL two-neck round bottom flask with a magnetic stir bar was charged with anhydrous FeCl2 (40g, Xmmol), subsequently attached to a dual-manifold vacuum line, and placed under argon. 150mL of anhydrous methanol were added via cannula and the iron complex was stirred until dissolved. The solution was then transferred via cannula (if FeCl2 did not fully dissolve, care is taken to avoid transferring solid materials) to a 500mL two-neck round-bottom flask containing 150mL of anhydrous pyridine, and attached to a swivel-frit apparatus. Formation of a yellow precipitate immediately ensued, and the mixture was allowed to stand under argon overnight. The yellow, crystalline solid was filtered and dried under vacuum to yield FeCl2(py)4, which was transferred to the glovebox for storage.
Notes:
- Pyridine and methanol can be dried with 3-Angstrom molecular sieves for 48h. Anhydrous FeCl2 can be purchased from chemical vendors (Millipore-Sigma, Strem) but can get expensive if bought in large quantities. Aqueous FeCl2 can be made from metallic iron powder and aqueous HCl. Although FeCl2(py)4 can be synthesized by adding neutral, saturated, aqueous FeCl2 into pyridine, one must take rigorous measures to remove moisture from the crystallized product, which can be tricky.
- The procedure given above is based on three somewhat vague literature protocols.[1-3] A fourth source provided useful information on the synthesis and storage of FeCl2(py)4.[4] Apparently there are more experimental details on the synthesis of the compound in a doctoral dissertation called “A study of the iron (II) halide and pseudohalide complexes of pyridine” by Billy Frank Little, but all attempts to gain access were unsuccessful.
[1] Baudisch, O.; Hartung, W. H. Tetrapyridino-Ferrous Chloride (Yellow Salt). Inorg. Synth. 1939, 1, 184-185.
[2] Golding, R. M.; Mok, K. F.; Duncan, J. F. Magnetic Susceptibility and Mössbauer Results of Some High-Spin Iron(II) Compounds. Inorg. Chem. 1966, 5, 5, 774-778.
[3] Long, G. J.; Clarke, P. J. Inorg. Chem. 1978, 17, 6, 1394-1401.
[4] a) Little, B. F.; Long, G. J. Inorg. Chem. 1978, 17, 12, 3401-3413. b) Little, B. F. A study of the iron (II) halide and pseudohalide complexes of pyridine, Ph.D. Dissertation, University of Missouri-Rolla, Rolla, MO, 1978.