Sample Originator: S. Yesilot, G. Y. Ciftci, R. A. Shaw, A. Kilic, Michael B. Hursthouse, R. J. Eaton, David B. Davies, Simon J. Coles and S. Besli.



Identification Number:10.5258/ecrystals/60
Date Created:05 December 2001
Deposited On:21 Jan 2008 15:28
Deposited By:A.N. Admin

Depositor Comments

A problem has arisen in using chiral shift reagents (CSR) and chiral solvating agents (CSA) to determine meso and racemic forms of diastereoisomers in which the stereogenic centers of the molecules are separated by achiral spacers. It is found that NMR signals of both meso and racemic forms of diastereoisomers may exhibit doubling on addition of CSR/CSA, which means that unequivocal assignments cannot be made without characterizing the effects for separate meso and racemic forms; this is particularly important for additions of CSR/CSA at relatively low concentrations, which always result in the splitting of some NMR signals of diastereoisomers. The phenomenon is demonstrated in the 31P NMR spectra of meso and racemic forms of three spermine-bridged gem-disubstituted cyclotriphosphazatrienes, 1a-c, and compared with analogous achiral molecules, the per-substituted spermine-bridged cyclotriphosphazatrienes 2a-d. As expected, only one set of 31P NMR signals was observed for the achiral compounds 2a-d, even on addition of CSA. Two sets of 31P NMR ABX multiplets corresponding to meso and racemic diastereoisomers were observed for compounds 1a-c; on addition of CSA, the signals of at least one of the multiplets for each compound separated into more than the expected groups of three lines with an intensity distribution of 2:1:1. To understand this phenomenon, the meso and racemic forms of 1a and 1b and the meso form of 1c have been separated and characterized by X-ray crystallography. On addition of CSA to the racemic forms of 1a and 1b, the 31P NMR spectrum shows the expected doubling of signals, but, unexpectedly, the same is observed for each of the meso forms of 1a-c. Analogous results using both CSA and CSR have been obtained for the meso and racemic forms of the diastereoisomeric piperazine-bridged macrocyclic-phosphazene compound, 3, whereas no effect was observed for the two meso forms of the doubly bridged macrocyclic-phosphazene compound 4. The phenomenon of doubling of the 31P NMR signals of the meso form of singly bridged cyclotriphosphazatrienes, 1a-c and 3, is explained by consideration of the equilibrium in solution of independent complexation of a chiral ligand with molecules that have two chiral cyclophosphazene moieties separated by an achiral spacer group. The results show that the stereogenicity of such diastereoisomeric molecules in solution cannot be characterized unequivocally by NMR measurements on addition of either CSR or CSA.

Data collection parameters

Chemical formulaC16 H34 Cl4 N10 O4 P6
Crystal morphology
Crystal systemMonoclinic
Space group symbolP2(1)/n
Cell length a12.5600(8)
Cell length b9.1084(8)
Cell length c14.2275(11)
Cell angle alpha90.00
Cell angle beta102.860(5)
Cell angle gamma90.00
Data collection temperature120(2)

Refinement results

Solution figure of merit
R Factor (Obs)0.0612
R Factor (All)0.1633
Weighted R Factor (Obs)0.1217
Weighted R Factor (All)0.1539

Citation: Yesilot, S. and Ciftci, G. Y. and Shaw, R. A. and Kilic, A. and Hursthouse, Michael B. and Eaton, R. J. and Davies, David B. and Coles, Simon J. and Besli, S. (2001) University of Southampton, Crystal Structure Report Archive. (doi:10.5258/ecrystals/60)
Export as: oreChem EndNote BibTeX ASCII Citation

Available Files

Final Result










Other Files


Repository Staff Only: item control page