Niobium(III) chloride explained

Niobium(III) chloride also known as niobium trichloride is a compound of niobium and chlorine. The binary phase NbCl₃ is not well characterized but many adducts are known.

Synthesis

thumb|left|184px|Structure of NbCl₃(dimethoxyethane)(3-hexyne).^[1] Nb₃Cl₈ is produced by reduction of niobium(V) chloride with hydrogen, or just by heating.

Salt-free reduction of dimethoxyethane solution of NbCl₅ with 1,4-disilyl-cyclohexadiene in the presence of 3-hexyne produces the coordination complex NbCl₃(dimethoxyethane)(3-hexyne):

NbCl₅ + C₆H₆(SiMe₃)₂ + C₂Et₂ + dme → NbCl₃(dme)(C₂Et₂) + C₆H₆ + 2 Me₃SiCl

An impure dimethoxyethane (dme) adduct of niobium trichloride was produced by reduction of a dme solution of niobium pentachloride with tributyltin hydride:^[2]

NbCl₅ + 2 Bu₃SnH + MeOCH₂CH₂OMe → NbCl₃(MeOCH₂CH₂OMe) + 2 Bu₃SnCl

Structure

Nb₃Cl₈ has a hexagonal close packed array of chloride ions. Triangles of niobium occur in octahedral spaces in the chloride array. The compositions with higher chloride have some niobium atoms missing from the structure, creating vacancies and giving rise to nonstoichiometric compounds. NbCl₄ has this pattern of vacancies stretched until the niobium atoms are in pairs rather than triangles. So NbCl₃ can be considered as a solid solution of Nb₃Cl₈ and Nb₂Cl₈.^[3]

The colour of niobium trichloride varies depending on the niobium:chloride ratio. NbCl_2.67 is green, while NbCl_3.13 is brown.^[4]

Reactions

When heated to over 600 °C niobium trichloride disproportionates to niobium metal and niobium pentachloride.

Adducts

NbCl₃(dimethoxyethane) has received significant attention as a reagent for reductive coupling of carbonyls and imines.^[5] It is sold as a 1,2-dimethoxyethane complex. Nb(III) adducts are also known for 1,4-dioxane and diethyl ether.

Niobium(III) chloride forms a series of compounds with the formula Nb₂Cl₆L_x with Nb=Nb double bond. With tertiary phosphines and arsines, the complexes are edge-share bioctahedra, e.g., Nb₂Cl₆(PPhMe₂)₄.^[6] Thioethers form adducts with one bridging thioether (R₂S). These face-sharing bioctahedra have the formula Nb₂X₆(R₂S)₃ (X = Cl, Br).

Notes and References

1. 10.1021/acs.accounts.8b00638. Salt-Free Reduction of Transition Metal Complexes by Bis(trimethylsilyl)cyclohexadiene, -dihydropyrazine, and -4,4′-bipyridinylidene Derivatives . 2019 . Tsurugi . Hayato . Mashima . Kazushi . Accounts of Chemical Research . 52 . 3 . 769–779 . 30794373 . 73505603 .
2. Book: 10.1002/9780470132609.ch28. Niobium(III) and (IV) Halide Complexes. Inorganic Syntheses. 119–123. Pedersen. Steven F.. Hartung. Jack B.. Roskamp. Eric J.. Dragovich. Peter S.. 1992. 28. 9780470132609.
3. Book: Cotton . F. Albert . Wilkinson . Geoffrey . Advanced Inorganic Chemistry: A Comprehensive Text . 1966 . John Wiley . 927.
4. Book: Gutmann . Viktor . Halogen Chemistry . 1967 . Elsevier . 978-0-323-14847-4 . 157 . en.
5. Encyclopedia: 10.1002/047084289X.rt202s. Trichloro(1,2-dimethoxyethane)niobium. Encyclopedia of Reagents for Organic Synthesis. 2001. Roskamp. Carrie A.. Roskamp. Eric J.. 0471936235.
6. Sharma . Sangeeta . Vermani . O. P. . Narula . A. K. . Synthesis and Structural Studies of Complexes of Niobium(III) Chloride with Ditertiary Phosphines . Indian Journal of Chemistry, Section A . January 1996 . 35A . 1 . 0975-0975.