Syntheses and spectral characterizations of tricarbonylchromium complexes of calix[4]arenes

The 1:1 arene–tricarbonylchromium complexes were synthesized for the first time from four conformational isomers of 25,26,27,28-tetra propoxycaIix [4]arene (14Pr) and Cr(CO)6. The 1:2, 1:3 and 1:4 complexes of cone-14Pr were also synthesized by increasing the ratio of Cr(CO)6 against cone-14Pr. The 1:2 complex was a mixture of two compounds, which were identified as distal and proximal isomers of cone-14Pr·2Cr(CO)3. The relative stability of the 1:1 complex was estimated via its oxidation by CuCl2. The pseudo-first-order rate constants appeared to be in the order of 1,3-alternate-14Pr > 1,2-alternate-14Pr > cone-14Pr > 2,6-dimethylanisole. The results indicate that the 14Pr·Cr(CO)3 complexes are less stable than the corresponding monomeric complex. The 1:1 complexes with partial-cone-14Pr and 1,2-alternate-14Pr were demonstrated to be a pair of racemates by 1H NMR spectroscopy using a chiral shift reagent (Pirkle's reagent).