A Carbon-13 and Deuterium NMR Investigation of Solid Platinum−Ethylene Complexes: Zeise's Salt and Pt(η2-C2H4)(PPh3)2

Abstract

The carbon chemical shift (CS) tensors for two platinum−ethylene complexes, ethylene-13C2-bis(triphenylphosphine)platinum(0) and potassium trichloro(ethylene-13C2)platinate(II), have been characterized by the dipolar-chemical shift method and with 2D spin−echo NMR experiments. The carbon CS tensors of the ethylene ligand are significantly modified upon coordination with platinum, particularly for the Pt(0) complex, to which ethylene is strongly coordinated. The most shielded principal component, δ33, perpendicular to the molecular plane in ethylene, is relatively unaffected by coordination; the changes to the CS tensors arise mainly from the increased shielding in the directions corresponding to δ11 and δ22. Hence, the span of the chemical shift tensor decreases from 210 ppm for ethylene (Zilm, K. W.; Conlin, R. T.; Grant, D. M.; Michl, J. J. Am. Chem. Soc. 1980, 102, 6672) to 150 ppm for the Pt(II) complex, and to 48 and 55 ppm, respectively, for the two nonequivalent carbon nuclei of the Pt(0) complex. The orientations of the carbon CS tensor components relative to the 13C,13C dipolar vector are also determined from this analysis. Orientations of the carbon CS tensors in the molecular framework are proposed on the basis of a combination of the experimental results and ab initio calculations using the GIAO method. Deuterium NMR studies of the ethylene-2H4 derivatives of the title compounds are characterized by long 2H T1s and by quadrupolar coupling constants which are comparable in magnitude to that observed for rigid olefins, demonstrating that the ethylene ligand is not subject to significant motion. This conclusion is supported by ab initio calculations which indicate barriers to internal rotation for the ethylene ligand in excess of 80 kJ mol-1 in both complexes.