Schlenk's early "free" carbanions.
Nearly a century ago, Schlenk published the syntheses and isolation of two most remarkable and unstable complexes: cryst. [Ph3C-][Me4N+] and [PhCH2-][Me4N+]. The crystal structure of the first complex contains a free Ph3C- ion, which displays the expected planar trigonal geometry at its central C atom. The Ph groups are not oriented in the typical propeller arrangement, but instead display various orientations with respect to the mol. plane. These orientations can be directly related to the extent of charge delocalization and correlate well with other structural characteristics related to charge delocalization. The crystal structure also shows a network of C-Hd+.tplbond.Cd- and C-H.tplbond.p interactions. Only C-H.tplbond.p interactions to the most neg. charged Ph rings are obsd. The abs. Broensted acidity of Me4N+ is calcd. by the G2(MP2) method (287.7 kcal mol-1) and is compared to the calcd. acidity of Me4P+ (268.4 kcal mol-1). On this basis, the pKa value for Me4N+ is estd. at 29.6. This makes the existence, and esp. Schlenk's early isolation, of the free carbanions [Ph3C-][Me4N+] and [PhCH2-][Me4N+] quite noteworthy.
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