Syntheses and Characterization of Upper Rim 1,2- and 1,3-Diphosphinated Calix[4]arenes and Their Corresponding 1,5-Cyclooctadienylrhodium(I) Complexes: Comparison of the Catalytic Hydroformylation Properties of Terminal Alkenes

Abstract

The 5,11- (1,2-isomer) and 5,17-bis(dialkylphosphino)-25,26,27,28-tetra-n-propoxycalix[4]arene (1,3-isomer) ligands (alkyl = Me, i-Pr) have been prepared and coordinated to Rh(COD)+ fragments (COD = 1,5-cyclooctadiene). The ligands 5,17-bis(diphenylphosphino)-11,23-dibromo-25,26,27,28-tetra-n-propoxycalix[4]arene and 5,11-bis(diphenylphosphino)-25,26,27,28-tetra-n-propoxycalix[4]arene have been coordinated to M(COD)+ (M = Rh, Ir) and RhCl(CO) fragments, as well. On the basis of mass spectrometry and 31P NMR spin−lattice relaxation time measurements (T1), all of the complexes are found to be dimers. Molecular modeling provides evidence that ring stress favors the dimer over the monomer, and the modeled structures for both 1,2- and 1,3-isomers have been corroborated by the comparison of the photophysics of the Ir(COD)+ species at 77 K. The decrease in emission lifetimes of the P2Ir(CC)2+ lumophore in the presence of 1-hexene is more pronounced for the 1,3-isomer, indicating reduced steric hindrance about the metallic center. The catalytic hydroformylation of five terminal alkenes using all six Rh(COD)+ catalyst precursors has been investigated under various conditions in order to extract the turnover frequencies (tof as defined as the number of moles of products divided by the total number of moles of rhodium used per hour) and the ratios of the product distribution (n/i, normal vs internal). The comparison of the data suggests that the basicity and the cone angle of the phosphines influence the n/i ratios. The tof's are generally larger for the 1,2-series for 1-hexene, but depend on the nature of the phosphine for styrene, vinyl acetate, vinyl benzoate, and vinyl p-tert-butylbenzoate. All in all, these complexes are good catalysts with respect to literature data, notably those containing the (i-Pr2P)− groups. During the course of this study, one hydride carbonyl complex has been synthesized and characterized from 1H and 31P NMR, IR, and T1 measurements.