The synthesis of closo- and nido-(aminoalkyl)dicarbaboranes: a re-examination of contradictory literature reports, crystal structure of [7-{H3N(CH2)3}-7,8-C2B9H11]·NH2NH2

Abstract

The reaction of Li[1-^tBuMe₂Si-1,2-C₂B₁₀H₁₀] with N-(bromoethyl)phthalimide or N-3-(bromopropyl)phthalimide generates unusual carboranyl heterocycles, resulting from nucleophilic substitution followed by insertion of a phthalimide carbonyl into the C–Si bond. The structure of one was determined by single crystal X-ray diffraction. Reaction of the heterocycles with ⁿBu₄NF affords closo-1-{C₆H₄(CO)₂N(CH₂)_n}-1,2-C₂B₁₀H₁₁ (1bn = 2 and 1cn = 3) together with the anions nido-[7-{C₆H₄(CO)₂N(CH₂)_n}-7,8-C₂B₉H₁₁]⁻ as minor side-products on prolonged reaction. The prolonged reaction of 1b and 1c with hydrazine results in deboronation to give hydrazine solvates of zwitterionic nido-[7-{H₃N(CH₂)_n}-7,8-C₂B₉H₁₁]. A single crystal X-ray diffraction study for one reveals an elegant dimeric architecture supported by hydrazine-bridged hydrogen bonds. The reaction of the heterocycles with ethanolic KOH results in cluster deboronation and partial deprotection of the amine group to give nido-[7-(2-O₂CC₆H₄CONH)(CH₂)_n-7,8-C₂B₉H₁₁]²⁻ as potassium salts which can be metathesised to less hygroscopic Me₃NH⁺ salts. The molecular structure of one of the latter displays hydrogen bonding generating a dimeric unit. Complete deprotection of the amine function in these salts by water–HCl gives zwitterionic aminoalkylcarboranes nido-7-{H₃N(CH₂)_n}-7,8-C₂B₉H₁₁.