Platinum(II) and palladium(II) complexes of bis(diphenylphosphino)calix[4]arene tetrabenzyl ether: Fluxional behavior caused by two motions

摘要

The synthesis and complexation behavior of a novel bis-phosphine ligand based on a calix[4]arene scaffold, 5,17-bis(diphenylphosphino)-25,26,27,28-tetrabenzyloxycalix[4]arene (3), are reported. Reaction of 3 with PdCl2(cod) under dilute conditions results in formation of the trans-coordinated dinuclear complex [PdCl2.3](2) (4). The X-ray structure of 4 reveals that the two phosphinocalix[4]arene molecules are linked by the two PdCl2 fragments. On the other hand, reaction of 3 with PtCl2(cod) affords the cis-chelating mononuclear complex PtCl2.3 (5). The X-ray structure, low-temperature NMR measurements, and geometry optimization of 5 reveal that the structure in solution at -80 degreesC and in solid-state has a C-1 symmetry where the P-Pt-P plane is inclined and the calix[4]arene moiety is twisted. It was also found that 3 reacts with [Pd(eta(3)-C3H5)(cod)]BF4 to yield the corresponding cationic complex [Pd(eta(3)-C3H5).3]BF4 (6) quantitatively. Low-temperature NMR measurements suggest that 6 exists as a mixture of two stereoisomers (6a and 6b) possessing a C-1 symmetry. Although both 5 and 6 exhibit C-1 structure at -80 degreesC, their NMR spectra measured at 20 degreesC indicate a C-2v symmetry for 5 and a C-s symmetry for 6. These fluxional behaviors of 5 and 6 in solution are caused by two separable motions: rollover motion of the coordination plane (R motion) and twist motion of the calix[4]arene scaffold (T motion). [References: 71]