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

Journal Article · Tue Apr 01 04:00:00 EDT 2008 · Inorganic Chemistry

DOI:https://doi.org/10.1021/ic7020897· OSTI ID:932249

Leavitt, Christopher M; Gresham, Garold L; Benson, Michael T; Gaumet, Jean-Jaques; Peterman, Dean; Klaehn, John; Moser, Megan; Aubriet, Frederic; Van Stipdonk, Michael J; Groenewold, Gary S

Diphenyldithiophosphinate (DTP) ligands modified with electron-withdrawing trifluoromethyl (TFM) substitutents are of high interest because they have demonstrated potential for exceptional separation of Am3+ from lanthanide3+ cations. Specifically, the bis(ortho-TFM) (L1-) and (ortho-TFM)(meta-TFM) (L2-) derivatives have shown excellent separation selectivity, while the bis(meta-TFM) (L3)- and unmodified DTP (Lu-) 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 pKa values, which followed the trend of L3 < L2 < L1 < Lu. The order of 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 are influencing the pKa 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. Relative to Na+, the unmodified Lu- anion was the strongest nucleophile. Comparing the TFM derivatives, the bis(ortho-TFM) derivative L1- was found to be the strongest nucleophile, while the bis(meta-TFM) L3- was the weakest, a trend consistent with the pKa calculations. DFT modeling of the Na+ complexes suggested that the elevated cation affinity of the L1- and L2- 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 produced the TFM-modified dithiophosphinate anions, showing again that the unmodified Lu- 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. Lu- > L1- > L2- > L3-.

• Research Organization: Idaho National Laboratory (INL)
• Sponsoring Organization: DOE - SC
• DOE Contract Number: AC07-99ID13727
• OSTI ID: 932249
• Report Number(s): INL/JOU-07-13397
• Journal Information: Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 8 Vol. 47; ISSN 0020-1669; ISSN INOCAJ
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
AFFINITY
ANIONS
ATOMS
CATIONS
DISSOCIATION
ELECTRON DENSITY
EUROPIUM NITRATES
FLUORINE
FUNCTIONALS
MASS SPECTROMETERS
NITRATES
OXIDATION
PH VALUE
RADICALS
SIMULATION
dissociation
dithiophosphinate
electrospray ionization
ligand addition
pKa
separations