Experimental and Theoretical Studies of Acid−Base Equilibria of Substituted 4-NitropyridineN-Oxides

Abstract

By using the potentiometric titration method, acidity constants in the polar aprotic solvent acetonitrile (in the form of pK a AN values) of cations obtained by protonation of 13 substituted 4-nitropyridine N-oxides and cationic homoconjugation constants (K BHB + ) of the cationic acids conjugated with the N-oxides studied have been determined. A correlation has been established between the tendency toward cationic homoconjugation (expressed as log K BHB + ) and the basicity of the N-oxides in acetonitrile (pK a AN ). Further, by using ab initio methods at the RHF and MP2 levels utilizing the Gaussian 6-31G* basis set, energies and Gibbs free energies have been determined of protonation and formation of homocomplexed cations stabilized by O‚‚‚H‚‚‚O bridges in the gas phase. The calculated protonation energies, ∆E prot , and Gibbs free enthalpies, ∆G prot , in vacuo have been found to correlate well with the acid dissociation constants (expressed as pK a AN values) of protonated N-oxides, whereas the calculated energies, ∆E BHB + , and Gibbs free energies, ∆G BHB + , of homoconjugation do not correlate with the cationic homoconjugation constant values determined in acetonitrile.

Electronic spectra and crystal structure of dinuclear Cu(II) ‘paddle wheel’ acetate complex containing axially coordinated 3-methyl-4-nitropyridine-N-oxide

“…From the large class of the methyl derivatives of 4-nitropyridine-N-oxide only one compound, that with one methyl group positioned at C(3) interacts with copper(II) acetate Scheme 2 The ligand field splitting patterns for d 9 configuration in D 4h , D 4h distorted and C 4v symmetries (graphically presented according to Boudreaux data [42]) Table 5 The reflectance spectra in the range 20,000-45,000 cm -1 for [Cu(OAc) 2 (3-mnpn)] 2 with the isolation of a solid, crystalline product. The reaction results in the formation of the dinuclear [Cu(OAc) 2 (L)] 2 complex with atypically short Cu-Cu bond, which was confirmed by preliminary X-ray diffraction studies.…”

“…Substituted pyridine N-oxides (mnpn) are an interesting group of compounds applied as catalysts [1][2][3][4][5], reactive intermediates and drugs in pharmaceutical chemistry [6][7][8][9]. Recently, it was found that they can also be a potentially useful in nonlinear optical devices [10,11].…”

Potentiometric studies of acid–base interactions in substituted 4-nitropyridine N-oxide systems

“…The synthesis of the methyl derivatives of 4-nitropyridine N-oxide was described previously [14][15][16][17]. Both the perchlorates of the 4-nitropyridine N-oxides and auxiliary materials for potentiometric measurements were prepared and purified as described in papers [14][15][16][17] and references therein.…”

“…Both the perchlorates of the 4-nitropyridine N-oxides and auxiliary materials for potentiometric measurements were prepared and purified as described in papers [14][15][16][17] and references therein. Also the procedures used for purification of acetone and acetonitrile as well as the equipment used for potentiometric titrations were described in these papers [14][15][16][17].…”

“…Moreover, the measurements in systems with two of the compounds (with R = 2MeNH and 2EtNH) were conducted in a solvent of moderate basicity, acetonitrile (AN) with DN = 14.9 in which the pK AN a and K AN BHB þ values for the 11 remaining N-oxides studied here were determined previously [14]. This allowed us to perform a correlational analysis of the pK a values of the protonated N-oxides in acetone and acetonitrile, beside that in acetone and water (using pK a in water [14] obtained from a linear relationship between pK a values of the protonated pyridine N-oxides in acetonitrile and water).…”