Lithium tetrakisborate is the lithium salt of the weakly coordinating anion (B(C₆F₅)₄)^-. Because of its weakly coordinating abilities, lithium tetrakis(pentafluorophenyl)borate makes it commercially valuable in the salt form for the starting material in polymerization reactions and electrochemistry. It is a water-soluble compound. The salt has only been obtained as the etherate, [Li(OEt₂)₃][B(C₆F₅)₄].

Structure and properties

The anion is tetrahedral with B-C bond lengths of approximately 1.65 Angstroms. The salt has only been obtained as the etherate, and the crystallography confirms that four ether molecules are bound to the lithium cation, with Li-O bond lengths of approximately 1.95 Angstroms. This structure gives the Lithium ion a coordination number of 4 with a tetrahedral geometry.

Preparation

The salt was first produced in studies on tris(pentafluorophenyl)boron

Tris(pentafluorophenyl)boron

Trisboron is the chemical compound 3B. The molecule consists of three pentafluorophenyl groups attached in a "paddle-wheel" manner to a central boron atom; the BC3 core is planar. It has been described as the “ideal Lewis acid” because of its versatility and the relative inertness of the B-C bonds...

, a well known Lewis acid

Lewis acid

]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...

ic compound. Combining equimolar ether solutions of pentafluorophenyllithium and tris(pentafluorophenyl)boron gives the lithium salt of tetrakis(pentafluorophenyl)borate, which precipitates as a white solid as the etherate:
₃B + Li(C₆F₅) → [Li(OEt₂)₃][B(C₆F₅)₄]

Since its discovery, many revised syntheses have been described, including an ether-free method.

Reactions

Lithium tetrakis(pentafluorophenyl)borate is primarily used in its salt form as a precursor to cationic transition metal complexes. They are capable of forming highly reactive metal catalysts. Lithium tetrakis(pentafluorophenyl)borate is used for its bulky size in primarily polymerization reactions and electrochemistry. (B(C₆F₅)₄)^- is a weakly coordinating anion, which is compatible with strong electrophiles. This property is essential for the initiation of Ziegler-Natta olefin polymerizations. For example, polymerization by zirconocene catalysts can be initiated with the (B(C₆F₅)₄)^- salt of the carbocation [Ph₃C]⁺ :

In such cases, the required lithium salt of (B(C₆F₅)₄)^- is prepared from Li(B(C₆F₅)₄) and [Ph₃C][B(C₆F₅)₄]. Another use is in the creation of (η5-pentamethylcyclopentadienyl)tin tetrakis(pentafluorophenyl)borate. This cation is then applied as an efficient Ziegler-Natta α-olefin cocatalyst for the polymerization of alkenes.