31P{1H} NMR spectroscopic and X-ray crystallographic studies of conformational dynamics in cis-M{Ph2P(CH2CH2O)5CH2CH2PPh2-P,P′} (M=Mo(CO)4, PtCl2) metallacrown ethers

Broad 31P{1H} NMR resonances are observed for cis-M{Ph2P(CH2CH2O)5CH2CH2PPh2-P,P′} (M=Mo(CO)4, 3, and PtCl2, 6) metallacrown ethers at 295 K. Variable temperature 31P{1H} NMR studies of these metallacrown ethers have demonstrated that this is due to exchange between symmetrical and unsymmetrical conformers. Simulation of the 31P{1H} NMR spectra of 6 using the gNMR™ program yields an Ea of 34 kJ mol−1, ΔH≠ of 32 kJ mol−1 and ΔS≠ of −146 J mol-K−1 for the exchange process. The relative amounts of the symmetrical and unsymmetrical conformers in solutions of 3 (90:10) and 6 (42:58) are quite different. The X-ray crystal structures of 3 (symmetrical) and 6 (unsymmetrical) are consistent with the relative amounts of conformers in solution. This suggests that the steric bulk of the metal center is determining the relatively stabilities of the symmetrical and unsymmetrical conformers. The square planar platinum center can accommodate the more crowded unsymmetrical structure while axial ligands in the octahedral molybdenum center favor the more open, symmetrical structure.