Thorium(IV) Complexes of Bidentate Hydroxypyridinonates¹

Abstract

The coordination chemistry of actinide(IV) ions with hydroxypyridinone ligands has been initially explored by examining the complexation of Th(IV) ion with bidentate PR-1,2-HOPO (HL(1)()), PR-Me-3,2-HOPO (HL(2)()), and PR-3,4-HOPO-N (HL(3)()) ligands. The complexes Th(L(1)())(4), Th(L(2)())(4), and Th(L(3)())(4) were prepared in methanol solution from Th(acac)(4) and the corresponding ligand. Single-crystal X-ray diffraction analyses are reported for the free ligand PR-Me-3,2-HOPO (HL(2)()) [Ponemacr;, Z = 8, a = 8.1492(7) A, b = 11.1260(9) A, c = 23.402(2) A, alpha = 87.569(1) degrees, beta = 86.592(1) degrees, gamma = 87.480(1) degrees ], and the complex Th(L(2)())(4).H(2)O [Pna2(1) (No. 33), Z = 4, a = 17.1250(5) A, b = 12.3036(7) A, c = 23.880 (1) A]. A comparison of the structure of the metal complex Th-PR-Me-3,2-HOPO with that of free ligand PR-Me-3,2-HOPO reveals that the ligand geometry is the same in the free ligand and in the metal complex. Amide hydrogen bonds enhance the rigidity and stability of the complex and demonstrate that the Me-3,2-HOPO ligands are predisposed for metal chelation. Solution thermodynamic studies determined overall formation constants (log beta(140)) for Th(L(1)())(4), Th(L(2)())(4), and Th(L(3)())(4) of 36.0(3), 38.3(3), and 41.8(5), respectively. Species distribution calculations show that the 4:1 metal complex Th(L)(4) is the dominant species in the acidic range (pH < 6) for PR-1,2-HOPO, in weakly acidic to physiological pH range for PR-Me-3,2-HOPO and in the high-pH range (>8) for PR-3,4-HOPO-N. This finding parallels the relative acidity of these structurally related ligands. In the crystal of [Th(L(2)())(4)].H(2)O, the chiral complex forms an unusual linear coordination polymer composed of linked, alternating enantiomers.

Structurally Characterized Quadruple‐Stranded Bisbidentate Helicates

“…As expected from repulsion energy calculations and previous observations, [14,15] the bidentate units are symmetrically bonded in a capped square-antiprism geometry (Figure 1 b) 2 ]. Note that in the 10-coordinate Th IV complex of bis(4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3-H-pyrazolonato-O,O')-bis(nitrato-O,O')-bis(triphenylphosphine oxide-O) complex, [19] the bond lengths between the Th IV ion and the two oxygen donors of a bidentate 4-acyl-pyrazolonate ligand are quite similar at 2.435 and 2.447 , respectively.…”

“…In addition, repulsion energy calculations predicted that the capped square antiprism isomer with a C 4 symmetry axis is the discrete potential energy minimum for [M IV (bidentate) 4 (monodentate)] complexes. [14,15] In such a geometry, the monodentate ligand occupies the cap position and the four equivalent bidentate ligands are wrapped around the nine-coordinate M IV atoms, thus forming capped square antiprisms with the bidentate ligands spanning the edges linking the two square faces of the square antiprism (Figure 1 b).…”

“…The tricapped trigonal prism (D 3h ) and capped square antiprism (C 4v ) are the most stable geometries, and the energy difference between the two is very small as predicted by repulsion energy calculations. [14,15] We thus employed the shape measure S to assess this subtle difference [18] and found that in both helicates the nine-coordinate geometries can be assigned as being closest to the monocapped square antiprism (C 4v ) based on the shape measure calculation.…”

Pyrone derivatives and metals: From natural products to metal-based drugs

“…On the other hand pyrone-based complexes can act as a metal ion source, due to their weaker bidentate binding, and have potential for the delivery and release of metals, fine-tunable by the substitution pattern of the pyrone backbone. The order of stability for complexes of different hydroxypyridonates is the following: 3-hydroxy-4-pyridonates > 3-hydroxy-2-pyridonates > 1-hydroxy-2-pyridonates [36,56,57].…”

Nanomaterials for Environmental Remediation

“…The actinide-binding affinities of similar carboxylate-based ligand systems drop off at lower pHs [78,79]. The HOPO class of ligands is the result of 20þ years of ligand design and development by Ken Raymond's group at Berkeley, and these ligands are superb actinide binders [82][83][84]. Incorporation of the HOPO ligands into the SAMMS superstructure has resulted in a superior class of actinide sorbent material [80,81].…”