Quantum-chemical calculation of NMR chemical shifts of organic molecules: III. Intramolecular coordination effects on the 31P NMR chemical shifts of phosphorylated N-vinylazoles

Abstract

Theoretical and experimental studies on magnetic shielding of the phosphorus nucleus in trichloro-[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ 6 -pyrazolo-[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide showed that intramolecular coordination of the phosphorus atom in the chlorophosphonium group to the nitrogen atom in the pyrazole ring leads to upfield shift of the phosphorus signal (to δ P 170 ppm) and that the contribution of the spin-orbital contribution to the 31 P chemical shift reaches 15%. Relativistic effects and effects of the medium are determining in the theoretical calculation of 31 P NMR chemical shifts. * For communication II, see [1].The range of 31 P chemical shifts is known to exceed 500 ppm, which ensures high informative value of 31 P NMR spectroscopy in structural studies on organophosphorus compounds [2, 3]. One of the most interesting aspects of 31 P NMR spectroscopy is related to effects arising from intra-and intermolecular coordination of phosphorus to heteroatoms possessing an unshared electron pair (UEP). The effect of coordination may attain a considerable value comparable with the overall range of variation of phosphorus chemical shifts. For example, phosphorus pentachloride, which is a known electrophilic phosphorylating agent, both in crystal and in polar solvents undergoes dissociation into ions, whereas in nonpolar solvents it exists as five-coordinate phosphorus species, and the δ P value changes from -300 to 80 ppm, depending on the degree of coordination [4] (Scheme 1).In addition to available experimental data, up-todate quantum-chemical methods make it possible to get a deeper insight into the relation between δ P and the structure of organophosphorus derivatives with different coordination numbers of the phosphorus atom. The present communication continues the series of studies involving ab initio calculations on shielding constants [1,5]; in this work we examined effects of intramolecular coordination of the phosphorus atom in trichloro[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) (I) and 1,1,1,1-tetrachloro-1H-1λ 6 -pyrazolo[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide (II). Phosphorylation of 1-vinyl-1H-pyrazole with PCl 5 gave Z isomer of salt I, which was stable at room temperature; it underwent Z-E isomerization at elevated temperature (Scheme 2). This process is facilitated by the presence of hydrogen chloride in the reaction mixture (via reversible hydrochlorination of the double bond. Phosphorylation of 1-vinyl-1H-pyrazole with an equimolar amount of PCl 5 favors formation of structure Z-II with six-coordinate phosphorus atom, which may be regarded as dipolar ion with intramolecular coordination.

Quantum-chemical calculations of chemical shifts in NMR spectra of organic molecules: V. Stereochemical structure of unsaturated phosphonic acids dichlorides from 31P NMR spectral data

“…We demonstrated in the previous reports [1,2] that combining the experimental measurements and theoretical calculations of the chemical shifts in the 31 Р NMR spectra made possible a deeper understanding of the nature of coordination effects arising at the formation of intra-and intermolecular complexes of the phosphorylated nitrogen heterocyclic compounds. In this research we applied this approach to the study of the spatial arrangement of unsaturated phosphonic acids dichlorides containing in their structure the POCl 2 group linked to double bonds C=C and C=N, namely, of 2-(pyrazolyl) ethenylphosphonic acid dichloride (I) obtained by the oxidation of 2-(pyrazolyl)ethenyltrichlorophosphonium hexachlorophosphorate [3], N-dichlorophosphonylimin obenzoic acid chloride (II) prepared by procedure [5], N'-dichlorophosphoryl-N,N-dimethylchloroformamidine (III) obtained by the phosphorylation of N,N-dimethylurea with phosphorus pentachloride [4], , and also of 1-trichlorophosphazo-1-chloro-2-dichlorophosphoryl-2-azaethene (IV), the product of the reaction of urea with phosphorus pentachloride [6].…”

“…As showed previous studies [1,2,11,12] the accounting for the spin-orbital coupling is necessary for the correct calculation of the shielding constants of the 31 Р nucleus in the phosphorus halides, for it fundamentally affected the accuracy of the calculation of 31 Р NMR chemical shifts.…”

“…In the study of the structure of compounds I-IV two ____________________ * For communication IV, see [1]. stereochemical problems should be solved: establishing the confi guration at the double bonds С=С and С=N and the investigation of the internal rotation around the P-N bond, and in compounds I, III, IV, around the N-C sp2 bond.…”

“…In the table the theoretical values of 31 Р NMR chemical shifts δ calc (ppm) are presented referred to external 85% aqueous H 3 PO 4 recalculated by the equation taken from [13] that we have successfully used in our previous publications [1,2,12,14]:…”

Quantum-chemical calculations of NMR chemical shifts of organic molecules: IV. Effect of intermolecular coordination on 31P NMR shielding constants and chemical shifts of molecular complexes of phosphorus pentachloride with azoles

“…We previously studied by theoretical methods the effect of intramolecular coordination on 31 P NMR shielding constants (chemical shifts) of [2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ 5 -pyrazolo[1,2-a][1,2,3]-diazaphosphol-8-ium-1-ide [1]. While developing our studies on theoretical calculations of 31 P chemical shifts [1][2][3], in the present work we examined molecular complexes of phosphorus pentachloride with some azoles, N-vinylimidazole (I) and 1-allyl-3,5-dimethylpyrazole (II).…”