Tetrakis(1-norbornyl)cobalt(IV) Knowpia

Tetrakis(1-norbornyl)cobalt(IV)
Names
	Other names (T-4)-Tetrakis(bicyclo[2.2.1]hept-1-yl)cobalt
Identifiers
CAS Number	36333-80-9 ;
3D model (JSmol)	Interactive image;
	InChI InChI=1S/4C7H11.Co/c41-2-7-4-3-6(1)5-7;/h46H,1-5H2; Key: FULAMMPKLQBENQ-UHFFFAOYSA-N;
	SMILES C1CC2CCC1(C2)[Co](C(CC1)2CCC1C2)(C(CC1)2CCC1C2)C(CC1)2CCC1C2;
Properties
Chemical formula	C28H44Co
Molar mass	439.593 g·mol−1
Appearance	brown crystals
Melting point	100 °C (decomposes)
Solubility	soluble in THF
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Tetrakis(1-norbornyl)cobalt(IV) is an air-sensitive organometallic compound of cobalt. It was first synthesized by Barton K. Bower and Howard G. Tennent in 1972^[1] and is one of few compounds in which cobalt has a formal oxidation state of +4.

Preparation edit

Tetrakis(1-norbornyl)cobalt(IV) is formed the reaction of CoCl₂•THF with 1-norbornyllithium (norLi) in n-pentane under an inert atmosphere.^[1] The cobalt(II) chloride-THF adduct is prepared from Soxhlet extraction of anhydrous CoCl₂ with THF, and the organolithium reagent is prepared from the reaction between 1-chloro-norbornane and lithium metal.

{\ce {{2CoCl2.THF}+4norLi->[{\text{pentane}}][]{[Co(nor)4]}+{Co}+4{LiCl}+2THF}}

The compound can then be purified by recrystallization.

Properties edit

The complex is a thermally stable homoleptic tetraorganylcobalt(IV) complex with exclusively σ-bonding ligands. It was the first low-spin complex with tetrahedral geometry to be isolated.^[2]^[3]^[4]

Stability edit

The exceptional stability of the complex is in large part due to its inability to undergo either α- or β-hydride elimination. The α-position of the metal (corresponding to the 1-position of the norbornyl ligand) has no more hydrogen atoms, while hydride elimination from the β-position would yield an energetically unfavorable double bond on a bridgehead atom (Bredt's rule). Moreover, the bulky norbornyl ligands sterically shield the central atom, hindering ligand substitutions as well as homolysis.^[1]^[5]

The rare d⁵ low-spin configuration in a tetrahedral ligand field is possible because the ligand is so strongly σ-donating that the gap between the e und t₂ orbitals is raised sufficiently to overcome the spin pairing energy. The resulting configuration is e⁴t₂¹, with magnetic measurements showing paramagnetism consistent with only one unparied electron.^[1]^[3]^[4]

Cobalt(III) and cobalt(V) derivatives edit

The reaction between CoCl₂•THF and 1-norbornyllithium (norLi) also allows the formation of a cobalt(III) complex: if a mixture of diethyl ether and THF is used as the solvent in place of n-pentane, the resulting disproportionation reaction affords the complex tetrakis(1-norbornyl)cobaltate(III), which crystallizes out of solution with solvated lithium counterions, along with elemental cobalt.^[4]^[6]

{\ce {3{CoCl2.THF}+8{norLi}+5{THF}->[{} \atop {\ce {Et2O/THF}}]2{[Li(THF)4][Co(nor)4]}+{Co}+LiCl}}

The compound is air-sensitive, has a green color and is paramagnetic, with two unpaired electrons, again indicating a low-spin tetrahedral configuration (d⁶, e⁴t₂²).^[6]^[4]

The corresponding cobalt(V) complex is prepared by oxidizing tetrakis(1-norbornyl)cobalt(IV) with Ag[BF₄] in THF and crystallizes with tetrafluoroborate as the counterion.^[6]^[4]

Co(nor)₄ + AgBF₄ → [Co(nor)₄]BF₄ + Ag

This complex :[Co(nor)₄]⁺ is the first cobalt(V) complex to be isolated. Again the configuration is low-spin (d⁴, e⁴t₂⁰).^[2]^[4]^[6]

References edit

^ ^a ^b ^c ^d B. K. Bower and H. G. Tennent (1972). "Transition metal bicyclo[2.2.1]hept-1-yls". J. Am. Chem. Soc. 94 (7): 2512–2514. doi:10.1021/ja00762a056.
^ ^a ^b Holleman, Arnold F.; Wiberg, E.; Wiberg, N. (2007). Lehrbuch der anorganischen Chemie (102nd ed.). Berlin. p. 1695. ISBN 978-3-11-017770-1. OCLC 180963521.{{cite book}}: CS1 maint: location missing publisher (link)
^ ^a ^b E. K. Byrne, D. S. Richeson and K. H. Theopold (1986). "Tetrakis(1-norbornyl)cobalt, a low spin tetrahedral complex of a first row transition metal". J. Chem. Soc., Chem. Commun. (19): 1491–1492. doi:10.1039/C39860001491.
^ ^a ^b ^c ^d ^e ^f E. K. Byrne, K. H. Theopold (1989). "Synthesis, characterization, and electron-transfer reactivity of norbornyl complexes of cobalt in unusually high oxidation states". J. Am. Chem. Soc. 111 (11): 3887–3896. doi:10.1021/ja00193a021.
^ Riedel, Erwin; Alsfasser, R.; Janiak, C.; Klapötke, T. M.; Meyer, H.-J. (2007). Moderne Anorganische Chemie. Berlin • New York: Walter de Gruyter. p. 718. doi:10.1515/9783110206852. ISBN 978-3-11-020685-2.
^ ^a ^b ^c ^d E. K. Byrne, K. H. Theopold (1987). "Redox chemistry of tetrakis(1-norbornyl)cobalt. Synthesis and characterization of a cobalt(V) alkyl and self-exchange rate of a Co(III)/Co(IV) couple". J. Am. Chem. Soc. 109 (4): 1282–1283. doi:10.1021/ja00238a066.