[4‐(Ph₃B)‐2,6‐Mes₂Py]⁻: A Sterically Demanding Anionic Pyridine

Abstract

The introduction of a triphenylborate group at the 4-position of 2,6-dimesitylpyridine afforded a sterically demanding anionic pyridine. The charge introduced through the borate group drastically increases its basicity and measurement of its pK value (18.46) revealed a significantly higher value than that of 4-dimethylaminopyridine (17.95). THF ring-opening was observed upon treating its lithium salt with TMSCl, which demonstrates its high nucleophilicity. The mesityl groups at the 2,6-positions are oriented orthogonal to the pyridine ring and do not block the nitrogen atom of the pyridine. The reaction of the protonated pyridine with Li[BH ] yielded the corresponding Lewis acid/base adduct, which shows that the title compound can be used as a monodentate ligand in coordination chemistry. The crystal structures of all the compounds presented in this work are reported.

Strengths of Acids in Acetonitrile

“…The origin of the compounds published by us earlier, as well as the pK a measurement methods, can be found from the following references: [5,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The most comprehensive method description is given in ref [5].…”

Pyridinyl Amide Ion Pairs as Lewis Base Organocatalysts

“…5 Very recently, Boẗtcher et al described the synthesis and characterization of 4-borate pyridine anion 5 intended for use as a strong ligand. 6 The catalytic activity of anionic sulfonamide pyridines 6 and 7 has only been described for the oligomerization of isocyanates in the patent literature. 7 Unfortunately, no comparison has been made to commonly used (neutral) organocatalysts, and characterization was omitted.…”

Electronic Modification of a Sterically Demanding Anionic Pyridine Ligand

“…The electron lone pair of the nitrogen atom of [ 1 ] − was identified in the HOMO‐3 with a calculated energy of −2.99 eV, making [ 1 F ] − a significantly weaker electron donor. This decrease in energy of the electron lone pair of the N py ‐atom is directly reflected in the lower p K a value of 16.44 measured for 1 F ‐H in acetonitrile compared with that for 1 ‐H (p K a =18.46) [8] . However, the p K a value of 1 F ‐H is still four p K a units higher than that for unsubstituted pyridine (p K a =12.53) [18] .…”

“…We already reported the crystal structure of 1 ‐Li(THF) 2 as a contact ion pair with a N py −Li distance of 199.4(3) pm. Reaction of this compound with BH 3 ⋅ SMe 2 gave 1 ‐BH 3 Li(THF) 3 by insertion of the borane into the N py −Li bond (Figure 4, top) [8] . In the solid state, all three hydrogen atoms of the anion [ 1 ‐BH 3 ] − showed short contacts to the lithium atom in the cation [Li(THF) 3 ] + .…”

“…Recently, our group reported the synthesis of the sterically demanding monoanionic pyridine ligand [4‐(Ph 3 B)‐2,6‐Mes 2 py] − ([ 1 ] − ) [8] . All three ligands, the terphenyl, the bulky amide and [ 1 ] − can sterically shield a large volume, however, the bonding motifs of these ligands are quite different (Figure 1).…”