Coordination Properties of Tris(2-carboxyethyl)phosphine, a Newly Introduced Thiol Reductant, and Its Oxide

Acid−base properties and metal-binding abilities of tris(2-carboxyethyl)phosphine (TCEP), a newly introduced thiol
group protectant, were studied in solution, using potentiometry, 1H and 31P NMR, and UV−vis spectroscopy, and
also in the solid state by X-ray diffraction. Stability constants of complexes of the P-oxide of TCEP (TCEPO) were
established by potentiometry. The list of metal ions studied included Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II). Cu(II)
catalyzed oxidation of TCEP to TCEPO. For all other systems ML complexes were found as major species at
neutral pH with TCEP and TCEPO. Monoprotonated MHL species were also detected in weakly acidic conditions
for all TCEP complexes and for the Pb(II) complex of TCEPO, while hydrolytic MH-1L complexes were found for
TCEP at the weakly alkaline pH range. The NiL4 complex was found to form at excess of TCEP. Overall, the
complexes were found to be rather weak, with log βML values around 3−5 for TCEP and 1.5−2.5 for TCEPO. The
phosphorus pKa value for TCEP, 7.68, suggests that it can be a good buffer for studies at physiological pH.