Schlenk's early "free" carbanions

Area of Science:

• Organometallic Chemistry
• Solid-State Chemistry
• Computational Chemistry

Background:

• Wilhelm Schlenk's early 20th-century work on highly reactive carbanions.
• The challenge of isolating and characterizing unstable ionic compounds.

Purpose of the Study:

• To re-examine the crystal structure of Schlenk's triphenylmethyl and benzyl tetramethylammonium salts.
• To investigate charge delocalization and intermolecular interactions within these crystalline carbanions.
• To computationally determine the Brønsted acidity of tetramethylammonium cation.

Main Methods:

• X-ray crystallography to analyze the structure of [Ph(3)C(-)][Me(4)N(+)] and [PhCH(2) (-)][Me(4)N(+)].
• Analysis of phenyl group orientations and intermolecular interactions (C-H...C, C-H...pi).
• G2(MP2) computational method to calculate the absolute Brønsted acidity of tetramethylammonium cation (Me(4)N(+)).

Main Results:

• Crystal structure reveals planar trigonal geometry for the triphenylmethyl anion (Ph(3)C(-)) with varied phenyl group orientations.
• Observed charge delocalization correlates with phenyl group orientations and specific C-H...pi interactions.
• Calculated Brønsted acidity of Me(4)N(+) is 287.7 kcal mol(-1), yielding an estimated pK(a) of 29.6.

Conclusions:

• The structural data supports significant charge delocalization in the isolated carbanions.
• The calculated high pK(a) value of Me(4)N(+) explains the remarkable stability and isolation of these early "free" carbanion salts.
• Revisiting Schlenk's work highlights the importance of structural and computational analysis in understanding chemical stability.