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-tetrapropoxycalix[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-1 ₄Pr 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).