Acidities of closo-1-COOH-1,7-C₂B₁₀H₁₁ and Amino Acids Based on Icosahedral Carbaboranes

Abstract

Carborane clusters are not found in Nature and are exclusively man-made. In this work we study, both experimentally and computationally, the gas-phase acidity (measured GA = 1325 kJ•mol −1 , computed GA = 1321 kJ•mol −1 ) and liquid-phase acidity (measured pK a = 2.00, computed pK a = 1.88) of the carborane acid closo-1-COOH-1,7-C 2 B 10 H 11 . The experimental gasphase acidity was determined with electrospray tandem mass spectrometry (ESI/MS), by using the extended Cooks kinetic method (EKM). Given the similar spatial requirements of the title icosahedral cage and benzene and the known importance of aminoacids as a whole, such a study is extended, within an acid−base context, to corresponding ortho, meta, and para amino acids derived from icosahedral carborane cages, 1-COOH-n-NH 2 -1, n-R with {R = C 2 B 10 H 10 , n = 2, 7, 12}, and from benzene {R = C 6 H 4 , n = 2, 3, 4}. A remarkable difference is found between the proportion of neutral versus zwitterion structures in water for glycine and the carborane derived amino acids.

Predicted Gas‐Phase and Liquid‐Phase Acidities of Carborane Carboxylic and Dicarboxylic Acids

“…At this point, we should emphasize that in a recent work, we measured (and computed) a gas‐phase acidity (GA) of 1325 (1321) kJ⋅mol −1 for 2 a (Figure ), the carboxylic acid derived from icosahedral meta ‐carborane 1‐(COOH)‐1,7‐C 2 B 10 H 11 . However, as we will discuss below, these experiments could not be carried out for the carborane dicarboxylic acids, due to the much larger acidity of the latter.…”