Experimental and First-Principles NMR Analysis of Pt(II) Complexes With O,O'-Dialkyldithiophosphate Ligands.

Polycrystalline bis(dialkyldithiophosphato)Pt(II) complexes of the form Pt{S2P(OR)2}2 (R = ethyl, iso-propyl, iso-butyl, sec-butyl or cyclo-hexyl group) were studied using solid-state 31P and 195Pt NMR spectroscopy to determine the influence of the alkyl substituents to the structure of the central chromophore. Measured anisotropic chemical shift (CS) parameters for 31P and 195Pt tensors afford more detailed chemical and structural information as compared to isotropic NMR parameters, such as chemical shifts and J-couplings, alone. Advanced theoretical modeling at hybrid DFT level including both crystal lattice and relativistic spin-orbit effects qualitatively reproduced the experimentally observed CS tensors, thus supporting the experimental analysis, as well as providing extensive orientational information for the tensors in the molecular frame. The results for 195Pt CS tensors demonstrated that differences in the alkyl substitutes of the dialkyldithiophosphate ligands in the complexes studied have an insignificant effect on the distorted-square form structure of the central PtS4 core. However, the principal values of 31P chemical shift tensors in the dialkyldithiophosphate ligands do differ significantly, which may be used to distinguish between the different complexes. Relativistic effects (both scalar and spin-orbit) were shown to be important for the NMR parameters of both 31P and 195Pt nuclei. The effects due to the periodic crystal lattice were found to be non-negligible, especially for the CS tensor of the heavy-metal 195Pt isotope. A particular correction model for incorporating lattice effects was adopted to avoid a severe deterioration of the anisotropic parameters of 195Pt due to the high requirements posed on the pseudopotential quality in such calculations. The pseudopotentials available in standard software may be inadequate for periodic calculations of anisotropic NMR parameters of heavy nuclei.