Synthesis, structures, and coordinating properties of phosphole-containing hybrid calixpyrroles

Symmetric and asymmetric hybrid calixpyrroles containing a sigma(4)-phosphole or sigma(4)-2,3-dihydrophosphole unit (symmetric and asymmetric sigma(4)-P,N-2,X-hybrids: X = S, O) were prepared by using acid-promoted condensation reactions of the corresponding sigma(4)-phosphatripyrranes with 2,5-bis(1-hydroxy-1-methylethyl)heteroles. The X-ray crystallographic analyses of the symmetric and asymmetric sigma(4)-P,N-2,X-hybrids show that the cavity sizes of the sigma-P,N-2,S-hybrids are larger than those of the sigma(4)-P,N-2,O-hybrids, mainly reflecting the difference in edge-to-edge distances of the thiophene and furan rings. The symmetric sigma(4)-P,N-2,X-hybrids and the asymmetric sigma(4)-P,N-2,S-hybrid were successfully converted to the corresponding sigma(3) forms by reductive desulfurization at the phosphorus center. Each of the symmetric sigma(3)-P,N-2,X-hybrids was obtained as a mixture of two conformers, where the lone pair of the phosphorus atom is located inside (in) and outside (out) the cavity. The interconversion between the in and out type conformers of the asymmetric sigma(3)-P,N-2,S-hybrid was sufficiently slow to isolate each of them. The complexation reactions of the symmetric sigma(3)-P,N-2,S-hybrid with Au(I), Pt(II), and Pd(II) ions afforded both of the in and out type complexes, where the in type complexes were the thermodynamically favored products. In the complexation reactions of the asymmetric sigma(3)-P,N-2, S-hybrids, the stereochemistry at the phosphorus center was retained to give the in or out type complex exclusively. In the in-in type trans-M(II)-bis(phosphine) complexes (M = Pt, Pd) derived from the symmetric and asymmetric sigma(3)-P,N-2,S-hybrids, the M-Cl fragment is bound above the cavities of the two macrocycles. The crystal structures and the H-1 NMR spectra of these M(II) complexes reveal that the P,N-2,S-hybrid calixpyrroles bind the M-Cl fragments through P-M coordination and cooperative NH-Cl hydrogen-bonding interactions.