Syntheses, Structures, and Coordination Chemistry of Phosphole-Containing Hybrid Calixphyrins:  Promising Macrocyclic P,N₂,X-Mixed Donor Ligands for Designing Reactive Transition-Metal Complexes

The syntheses, structures, and coordination chemistry of phosphole-containing hybrid calixphyrins (P,N2,X-hybrid calixphyrins) and the catalytic activities of their transition-metal complexes are reported. The 5,10-porphodimethene type 14π-P,(NH)2,X- and 16π-P,N2,X-hybrid calixphyrins (X = O, S, NH) are prepared via acid-promoted dehydrative condensation between a σ4-phosphatripyrrane and the corresponding 2,5-bis[hydroxy(phenyl)methyl]heteroles followed by DDQ oxidation. Both spectroscopic and crystallographic data of the hybrid calixphyrins have revealed that the conformation and size of the macrocyclic platforms as well as the oxidation state of the π-conjugated pyrrole−heterole−pyrrole (N−X−N) units vary considerably depending on the combination of heteroles. The σ3-P,(NH)2,S- and σ3-P,N2,S-hybrids react with Pd(OAc)2 and Pd(dba)2, respectively, to afford the same Pd(II)−P,N2,S-hybrid complex, in which the calixphyrin platform is regarded as a dianionic ligand. In the complexation with [RhCl(CO)2]2 in dichloromethane, the σ3-P,N2,S-hybrid behaves as a neutral ligand to afford an ionic Rh(I)−P,N2,S-hybrid complex, whereas the σ3-P,N2,NH-hybrid behaves as an anionic ligand to produce Rh(III)−P,N3-hybrid complexes. In the latter reaction, it is likely that a neutral Rh(I)−P,N3-hybrid complex, generated as a highly nucleophilic intermediate, undergoes C−Cl bond activation of the solvent. The complexation of AuCl(SMe2) with the σ3-P,N2,X-hybrids (X = S, NH) leads to the formation of the corresponding Au(I)−monophosphine complexes. The spectral data and crystal structures of these metal complexes exhibit the hemilabile nature of the phosphole-containing hybrid calixphyrin platforms derived from the flexible phosphole unit and the redox active N−X−N units. The hybrid calixphyrin−palladium and −rhodium complexes catalyze the Heck reaction and hydrosilylations, respectively, implying that the metal center in the core is capable of activating the substrates under appropriate reaction conditions. The present results demonstrate the potential utility of the phosphole-containing hybrid calixphyrins as a new class of macrocyclic P,N2,X-mixed donor ligands for designing highly reactive transition-metal complexes.