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Conformationally rigidified tetraCMPO derivatives have been prepared from calix[4]arene bis(crown ether) 4a in which adjacent oxygens are bridged at the narrow rim by two diethylene glycol links. Acylation of the tetraamine 4c with the CMPO-active ester 5b gave the tetraphosphine oxide 6a, while the tetraphosphinate 6b and the tetraphosphonate 6c were obtained by Arbuzov reaction of tetrabromoacetamido derivative 7 with PhP(OEt)2 or P(OEt)3. The extraction ability of these CMPO derivatives was checked for selected lanthanides and actinides and compared with the analogous compounds 1b, 10b and 10d derived from calix[4]arene tetrapentyl ether. All rigidified bis(crown ether) ligands are more effective extractants than their pentyl ether counterparts and require only 1/10 of the concentration (cL= 10 4M) to obtain the same distribution coefficients, while with CMPO itself a 2,000-fold concentration is necessary. This could be a consequence of a better preorganisation of the ligating functions owing to the rigidity which on the other hand did not change the observed selectivity for americium (DAm/DEu=9-19) and for light lanthanides over heavy ones. NMR relaxivity titration curves show that the complex of Gd3+ with ligand 6a is highly oligomerised in anhydrous acetonitrile over a large range of ligand:metal concentration ratios. Nuclear magnetic relaxation dispersion (NMRD) profiles also showed that large oligomers were formed, and their mean tumbling times were deduced from the Solomon-Bloembergen-Morgan equations. The NMR spectra of dia- and paramagnetic lanthanide complexes with 6a agreed with the presence of two conformers with an elongated calix[4]arene skeleton in which the distances between opposite methylene groups are different. Contrary to what was observed with ligand 2a, the addition of nitrate ions does not labilize the metal complexes, presumably because of the rigidification effect of the ether bridges. Single-crystal X-ray structures were obtained for the active ester 5b and for diphenylphosphorylacetic acid 5a.