Investigations of acidity and nucleophilicity of diphenyidithiophosphinate ligands using theory and gas-phase dissociation reactions

Diphenyldithiophosphinate (DTP) ligands modified with electron-withdrawing trifluoromethyl (TFM) substitutents are of high interest because they have demonstrated potential for exceptional separation of AM(3+) from lanthanide(3+) cations. Specifically, the bis(ortho-TFM) (L-1(-)) and (ortho-TFM)(meta-TFM) (L-2(-)) derivatives have shown excellent separation selectivity, while the bis(meta-TFM) (L-3(-)) and unmodified DTP (L-u(-)) did not. Factors responsible for selective coordination have been investigated using density functional theory (DFT) calculations in concert with competitive dissociation reactions in the gas phase. To evaluate the role of (DTP + H) acidity, density functional calculations were used to predict pK(a) values of the free acids (HLn), which followed the trend of HL3 < HL2 < HL1 < HLu. The order of pK(a) for the TFM-modified (DTP+H) acids was opposite of what would be expected based on the e(-)-withdrawing effects of the TFM group, suggesting that secondary factors influence the pK(a) and nucleophilicity. The relative nucleophilicities of the DTP anions were evaluated by forming metal-mixed ligand complexes in a trapped ion mass spectrometer and then fragmenting them using competitive collision induced dissociation. On the basis of these experiments, the unmodified L-u(-) anion was the strongest nucleophile. Comparing the TFM derivatives, the bis(ortho-TFM) derivative L-1(-) was found to be the strongest nucleophile, while the bis(meta-TFM) L-3(-) was the weakest, a trend consistent with the pK(a) calculations. DFT modeling of the Na+ complexes suggested that the elevated cation affinity of the L-1(-) and L-2(-) anions was due to donation of electron density from fluorine atoms to the metal center, which was occurring in rotational conformers where the TFM moiety was proximate to the Na+-dithiophosphinate group. Competitive dissociation experiments were performed with the dithiophosphinate anions complexed with europium nitrate species; ionic dissociation of these complexes always generated the TFM-modified dithiophosphinate anions as the product ion, showing again that the unmodified L-u(-) was the strongest nucleophile. The Eu(III) nitrate complexes also underwent redox elimination of radical ligands; the tendency of the ligands to undergo oxidation and be eliminated as neutral radicals followed the same trend as the nucleophilicities for Na+, viz. L-3(-) < L-2(-) < L-1(-) < L-u(-).