Affinity of the Highly Preorganized Ligand PDA (1,10-Phenanthroline-2,9-dicarboxylic acid) for Large Metal Ions of Higher Charge. A Crystallographic and Thermodynamic Study of PDA Complexes of Thorium(IV) and the Uranyl(VI) ion

The hydrothermal synthesis and structures of [UO2(PDA)] (1) and [Th(PDA)2(H2O)2]·H2O (2) (PDA = 1,10-phenanthroline-2,9-dicarboxylic acid) are reported. 1 is orthorhombic, Pnma, a = 11.1318(7) Å, b = 6.6926(4) Å, c = 17.3114(12) Å, V = 1289.71(14), Z = 4, R = 0.0313; 2 is triclinic, P1̄, a = 7.6190(15) Å, b = 10.423(2) Å, c = 17.367(4) Å, α = 94.93(3)°, β = 97.57(3)°, γ = 109.26(3)°, V = 1278.3(4) Å3, Z = 2, R = 0.0654. The local geometry around the U in 1 is a pentagonal bipyramid with the two uranyl oxygens occupying the apical positions. The donor atoms in the plane comprise the four donor atoms from the PDA ligand (average U−N = 2.558 and U−O = 2.351 Å) with the fifth site occupied by a bridging carboxylate oxygen from a neighboring UO2/PDA individual. The PDA ligand in 1 is exactly planar, with the U lying in the plane of the ligand. The latter planarity, as well as the near-ideal U−O and U−N bond lengths, and O−U−N and N−U−N bond angles within the chelate rings of 1 suggest that PDA binds to the uranyl cation in a low-strain manner. In 2, there are two PDA ligands bound to the Th (average Th−N = 2.694 and Th−O = 2.430 Å) as well as two water molecules (Th−O = 2.473 and 2.532 Å) to give the Th a coordination number of 10. The PDA ligands in 2 are bowed, with the Th lying out of the plane of the ligand. Molecular mechanics calculations suggest that the distortion of the PDA ligands in 2 arises because of steric crowding. UV spectroscopic studies of solutions containing 1:1 ratios of PDA and Th4+ in 0.1 M NaClO4 at 25 °C indicate that log K1 for the Th4+/PDA complex is 25.7(9). The latter result confirms the previous prediction that complexes of PDA with metal ions of higher charge and an ionic radius of about 1.0 Å such as Th(IV) would have remarkably high log K1 values with PDA. The origins of this very high stability are discussed in terms of a synergy between the pyridyl and the carboxylate donor groups of PDA. Metal ions of high charge normally bond poorly with pyridyl donors in aqueous solution because such metal ions require donor groups that are able to disperse charge to the solvent via hydrogen-bonding, which pyridyl groups are unable to do. In PDA, the carboxylates fulfill this need and so enable the high donor strength of the pyridyl groups of PDA to become apparent in the high log K1 for Th(IV) with PDA.

本文报道了配合物[UO₂(PDA)]（1）与[Th(PDA)₂(H₂O)₂]·H₂O（2）的水热合成方法及晶体结构，其中PDA为1,10-邻菲啰啉-2,9-二羧酸（1,10-phenanthroline-2,9-dicarboxylic acid）。配合物1属于正交晶系，空间群为Pnma，晶胞参数a=11.1318(7) Å，b=6.6926(4) Å，c=17.3114(12) Å，晶胞体积V=1289.71(14) ų，Z=4，残差因子R=0.0313；配合物2属于三斜晶系，空间群为P-1，晶胞参数a=7.6190(15) Å，b=10.423(2) Å，c=17.367(4) Å，α=94.93(3)°，β=97.57(3)°，γ=109.26(3)°，晶胞体积V=1278.3(4) ų，Z=2，残差因子R=0.0654。配合物1中铀酰中心U的局部配位几何为五角双锥，两个铀酰氧原子占据顶点位置。平面内的配位原子包含来自PDA配体的四个配位原子（U−N平均键长为2.558 Å，U−O平均键长为2.351 Å），第五个配位位点由相邻的UO₂/PDA单元提供的桥联羧酸根氧原子占据。配合物1中的PDA配体完全共平面，且U原子恰好位于该配体平面内。这种平面构型，加之近乎理想的U−O、U−N键长，以及螯合环内的O−U−N与N−U−N键角，均表明PDA配体以低应变方式与铀酰阳离子结合。在配合物2中，两个PDA配体与Th原子配位（Th−N平均键长为2.694 Å，Th−O平均键长为2.430 Å），另有两个水分子（Th−O键长分别为2.473 Å和2.532 Å）参与配位，使Th原子的配位数达到10。配合物2中的PDA配体发生弯曲，Th原子偏离配体平面。分子力学计算表明，配合物2中PDA配体的畸变源于空间位阻效应。在25 ℃、0.1 M高氯酸钠（NaClO₄）溶液中，对PDA与Th⁴+以1:1摩尔比混合的体系开展紫外光谱研究，结果显示Th⁴+/PDA配合物的一级稳定常数对数log K₁为25.7(9)。该结果证实了此前的预测：电荷较高、离子半径约为1.0 Å的金属离子（如Th(IV)）与PDA形成的配合物，其一级稳定常数对数log K₁会处于极高水平。本文从PDA配体中吡啶基与羧酸根供体基团间的协同效应角度，探讨了该配合物极高稳定性的根源。通常而言，高电荷金属离子在水溶液中与吡啶基供体的配位能力较弱，因为这类金属离子需要能够通过氢键向溶剂分散电荷的配位基团，而吡啶基无法满足这一需求。在PDA配体中，羧酸根恰好弥补了这一缺陷，使得PDA吡啶基的强供体能力得以充分体现，最终使Th(IV)与PDA配合物拥有极高的log K₁值。