Platinum(II) and Palladium(II) Complexes of Bis(diphenylphosphino)calix[4]arene Tetrabenzyl Ether: Fluxional Behavior Caused by Two Motions

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 PdCl 2 (cod) under dilute conditions results in formation of the trans-coordinated dinuclear complex [PdCl 2 · 3 ] 2 ( 4 ). The X-ray structure of 4 reveals that the two phosphinocalix[4]arene molecules are linked by the two PdCl 2 fragments. On the other hand, reaction of 3 with PtCl 2 (cod) affords the cis-chelating mononuclear complex PtCl 2 · 3 ( 5 ). The X-ray structure, low-temperature NMR measurements, and geometry optimization of 5 reveal that the structure in solution at −80 °C 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( η 3 -C 3 H 5 )(cod)]BF 4 to yield the corresponding cationic complex [Pd( η 3 -C 3 H 5 )· 3 ]BF 4 ( 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 °C, their NMR spectra measured at 20 °C indicate a C 2 v 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).