Plutonium(IV) sequestration: Structural and thermodynamic evaluation of the extraordinarily stable cerium(IV) hydroxypyridinonate complexes

Ligands containing the 1-methyl-3-hydroxy-2(1H)-pyridinone group (Me-3,2-HO PO) are powerful plutonium-(IV) sequestering agents. The Ce(IV) complexes o f bidentate and tetradentate HOPO ligands have been quantitatively studied as models for this sequestration. The complexes Ce(L-1)(4), Ce(L-2)(4), Ce( L-3)(2), and Ce(L-4)(2) (L-1 = Me-3,2-HOPO; L-2 = PR-Me-3,2-HOPO; L-3 = 5LI -Me-3,2-HOPO; L-4 = 5LIO-Me-3,2-HOPO) were prepared in THF solution from Ce (acac)(4) and the corresponding ligand. The complex Ce(L-4)(2) was also pre pared in aqueous solution by air oxidation of the Ce(III) complex [Ce(L-4)( 2)](-). Single-crystal X-ray diffraction analyses are reported for Ce(L-1)( 4). 2CHCl(3) [P (1) over bar (no. 2), Z = 2, a = 9.2604(2) Angstrom, b = 12 .1992(2) Angstrom, c 15.9400(2) Angstrom, alpha = 73.732(1)degrees, beta = 85.041(1)degrees, gamma = 74.454(1)degrees],Ce(L-3)(2). 2CH(3)OH [P2(1)/c ( no. 14), Z = 4, a = 11.7002(2) Angstrom, b = 23.0033(4) Angstrom. c = 15.71 55(2) Angstrom, beta = 96.149(1)degrees], Ce(L-4)(2). 2CH(3)OH [P (1) over bar (no. 2), Z = 2, a = 11.4347(2) Angstrom, b = 13.8008(2) Angstrom, c = 1 5.2844(3) Angstrom, alpha = 101.554(1)degrees, beta = 105.691(1)degrees, ga mma = 106.746(1)degrees], and Ce(L-4)(2). 4H(2)O [P2(1)/c (no. 14), Z = 4, a = 11.8782(1) Angstrom, b = 22.6860(3) Angstrom, c = 15.2638(1) Angstrom, beta = 96.956(1)degrees]. A new criterion, the shape measure S. has been in troduced to describe and compare the geometry of such complexes. It is defi ned as S = min[((1/m)Sigma(i=1)(m)(delta(i) - theta(i))(2))(1/2)], where m is the number of edges, delta(i) is the observed dihedral angle along the i th edge of delta (angle between normals of adjacent faces), theta(i) is the same angle of the corresponding ideal polytopal shape theta, and min is th e minimum of all possible values. For these complexes the shape measure sho ws that the coordination geometry is strongly influenced by small changes i n the ligand backbone or solvent. Solution thermodynamic studies determined overall formation constants (log beta) for Ce(L-2)(4), Ce(L-3)(2), and Ce( L-4)(2) of 40.9, 41.9, and 41.6, respectively. A thermodynamic cycle has be en used to calculate these values from the corresponding formation constant s of Ce(III) complexes and standard electrode potentials. From the formatio n constants and from the protonation constants of the ligands, extraordinar ily high phl values for Ce(IV) are generated by these tetradentate Ligands (37.5 for Ce(L-3)(2) and 37.0 for Ce(L-4)(2)). The corresponding constants for Pu(TV) are expected to be substantially the same.