Comparative study of the copper(II) cryptates of C-BISTREN and O-BISTREN. Protonation constants, formation constants, and secondary anion bridging by fluoride and hydroxide

Abstract

Potentiometric equilibrium measurements of hydrogen ion concentrations have been employed for the determination of protonation constants and Cu(I1) stability constants of the macrobicyclic ligand C-BISTREN (1,4,10,13,16,22,27,33-octaazabicyclo-[11.11.1 llpentatriacontane) at 25.0 OC in solutions maintained at 0.10 M ionic strength by the addition of sodium perchlorate with and without a 0.010 M concentration of sodium fluoride. The results are compared with the corresponding equilibrium constants previously reported for 0-BISTREN (7,19,30-trioxa-1,4,10,13,16,22,27,33-octaazabicyclo[ 11.1 1.1 llpentatriacontane). The measurements made in the absence of fluoride provide six protonation constants of the ligands, the stability constants of mononuclear and binuclear Cu(I1) complexes, the protonation constants of the mononuclear copper(I1) cryptates, and the hydroxide-bridging constants of the binuclear copper(I1) cryptates. Potentiometric measurements of the ligands in the NaCIO4-NaF medium provide equilibrium constants for bridging of the bifluoride anion, HF2-, by both hexaprotonated ligands and pentaprotonated 0-BISTREN, and fluoride ion bridging in the tetra-, penta-and hexaprotonated 0-BISTREN. Potentiometric measurements of 1:l and 1:2 molar ratios of the ligands to Cu(I1) in the NaC104-NaF medium provide equilibrium constants for fluoride ion bridging between the two copper(I1) centers in the binuclear cryptate, as well as equilibrium constants for fluoride bridging between copper(I1) and protonated nitrogens in the mononuclear copper(I1) cryptate. New data for fluoride anion bridging of the protonated species of 0-BISTREN and its mononuclear and binuclear copper complexes are reported for 0.10 M ionic strength. The comparison between 0-BISTREN and C-BISTREN reveals the high specificity of the binuclear Cu(I1)-0-BIS-TREN cryptate for hydroxide ion bridging. IntroductionThe protonation constants and metal ion stability constants of

Trapped Bifluoride

“…workers reported physical data that indicated such an encapsulation in an azacryptand similar to bis-tren; [3] however, crystallographic corroboration has, to our knowledge, remained elusive. We have isolated and structurally characterized a number of fluoride complexes with both monocyclic and bicyclic amine-based [4] and amide-based [5] anion receptors.…”

Trapped Bifluoride

“…Since Jean‐Marie Lehn first reported the structure of an encapsulated triatomic azide in a bicyclic azacryptand known as bis‐tren, researchers have sought to do the same for another triatomic anion, bifluoride 1–3 Lehn, Martell, and co‐workers reported physical data that indicated such an encapsulation in an azacryptand similar to bis‐tren;3 however, crystallographic corroboration has, to our knowledge, remained elusive. We have isolated and structurally characterized a number of fluoride complexes with both monocyclic and bicyclic amine‐based4 and amide‐based5 anion receptors.…”

Aspects of Anion Coordination from Historical Perspectives

“…further developments) [127,128]. The constants for the equilibrium between the hexaprotonated receptor and HF 2 − were 6.4 and 5.2 logarithmic units for 11 and 18, respectively.…”

“…in the initial work of Martell and Lehn on O-BISTREN (11) and , in which, based on disquisitions about the stability constants, a hydroxide and a fluoride anion were postulated to be included within the metal centers [127,128]. Fabbrizzi and coworkers, among others, have contributed remarkably to this chemistry with several studies and crystal structures.…”

“…The 21-membered macrocycle [21]aneN 7 (67) was shown to be the best catalyst at the examined pHs, with the [24]aneN 8 (68) macrocycle providing also very significant rate enhancements. The relevance of macrocyclic size and charge density on rate of catalysis was later checked by introducing structural modifications such as N-methylation of some of the nitrogen atoms in the macrocycles [18]aneN 6 (36) and [21]aneN 7 (67) [221,222] to yield macrocycles (128)(129)(130). …”