Structure and dynamics of dinuclear zirconium(IV) complexes

Synthesis of Hafnium(IV) Polyaminoacetates

The interaction of hafnium(IV) salts (oxide-dichloride, chloride, and bromide) with nitrilotriacetic acid (NTA), diethylenetriamminepentaacetic acid (DTPA), 1,2-diaminocyclohexanetetraacetic acid (CDTA), 1,3-dipropylmino-2-hydroxy N,N,N',N'-tetraacetic acid (dpta), and N-(2-hydroxyethyl)ethylenediamine triacetic acid (HEDTA) has been studied. The corresponding complexes Na₂[Hf(NTA)₂]·3H₂O (1), Na[HfDTPA]·3H₂O (2), [HfCDTA(H₂O)₂] (3), and Na[Hf₂(dpta)₂]·7.5H₂O·0.5C₂H₅OH (4) have been isolated and characterized and their structures have been determined by single crystal X-ray diffraction. Biological studies of [HfCDTA(H₂O)₂] have shown that in 5% glucose solution this complex has low toxicity and good contrasting ability.

Molecular and supramolecular ionic aggregates HxOyzin organic and organometallic crystalline hydrates

Ionic aggregates of the form HxOyz(z≠ 0) have been characterized during an analysis of 245 crystal structures extracted from the Cambridge Structural Database [Allen (2002).Acta Cryst.B58, 380–388]. A systematic nomenclature is proposed for these species. Three modes of hydrogen bonding are described, characterized in part by the distance between contiguous O atoms: normal (NHB; O...O = 2.6–3.0 Å), charge assisted (CAHB; O...O = 2.5 Å) and molecular (MHB; O...O = 2.4 Å). The three modes are consistent with previous reports, our experimental results, and quantum-chemical-optimized geometries and energetics. No evidence is presented concerning the possible existence or stability of these aggregates in solution. Rather, emphasis is placed on the necessity in crystal structure analysis to develop thoroughly existing hydrogen-bonded networks, ignorance of which can lead to erroneous crystal structure models and other physico-chemical data associated with composition and charge balance.

New structures of hydronium cation clusters

Four new hydronium ion structures are investigated by means of quantum mechanical calculations at the DFT/B3LYP6-311+G(2d,2p) level of theory. There exist experimental crystallographic hydronium cations (H11O5 +) of two different geometrical structures, one BEXFEQ (acyclic) and one IYEPEH (cyclic). Molecular calculations reveal their relative stability. Another hydronium cation NEBDII (H15O7 +) when optimized reveals a totally new and unexpected structure. All three optimized structures are shown to be quite stable as judged by their binding energies, and therefore may possibly be found in solution. A main result of this article is the discovery of three new optimized structures of hydronium ions, all of which are preferentially ring structures. The optimized structure of H15O7 + is a cube lacking a vertex. Putting a water molecule at the "empty" vertex leads by energy optimization to a structure of H17O8 + which has the approximate symmetry of a cube. This cubic structure, as judged by its fragments, is one of the most interesting of the hydronium ions studied in this paper. The addition of H3O+ to a group of seven neutral molecules in the hypothetical reaction H3O+ + 7 H2O → H17O8 + induces two water molecules to each capture a proton at the expense of two other water molecules (converting them into hydroxyl anions) leading to a cluster with the formula[ H 3 O + 0.7 ] 3 [ H 2 O ] 3 + 0.1 [ OH - 0.6 ] 2 , where the superscripts are the integrated QTAIM atomic charges (in atomic units) on the respective species (inside the bracket) or on groups of a given species (outside the bracket). The cubic arrangement of 3H3O+.3H2O.2OH- is accompanied with a significant redistribution of charge: Each hydronium cation carries ca. +0.7 au, the hydroxyl anions only around -0.6 au each, while the water molecules remain quasi-neutral with a slight positive charge.

Equilibria and Structure of the Lanthanide(III)-2-hydroxy-1,3-diaminopropane-N,N,N‘,N‘-tetraacetate Complexes: Formation of Alkoxo-Bridged Dimers in Solid State and Solution

The complex formed between 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid (H4L-OH) and Nd3+ at pH 7.5 was found to be a dinuclear dimer in the solid state by X-ray crystallography. In the complex K4[Nd2(L-O)2(H2O)2].14H2O each ligand is coordinated to both Nd3+ atoms with an iminodiacetate group (the Nd3+-Nd3+ distance is 3.9283(8) A). The alcoholic OH groups are deprotonated, and the alkoxo oxygens are coordinated to both Nd3+ in a bridging position. The Nd3+ ions are nine-coordinated with one water molecule per Nd(III) ion in the inner sphere. The complex K4[Nd2(L-O)2(H2O)2].14H2O has an inversion center, and the space group is P1. Two of the K+ counterions are six-coordinated, while the other two K+ ions are eight-coordinated; polar polymeric water-K+ layers are formed between the apolar ligand layers via the bridging water molecules. The dinuclear dimer complexes are also present in aqueous solution. The proton relaxivities of the Gd3+ complex decrease with the increase of pH, and at pH > 6, the low relaxivity values indicate the probable absence of H2O in the inner sphere and the predominance of the eight-coordinated dimer species [Gd2(L-O)2].4- The results of ESI-TOF MS studies of the complexes of La3+, Nd3+, and Lu3+ proved the formation of dinuclear dimers in dilute (0.25 mM) solutions. pH-potentiometric titrations indicate the formation of complexes with 1:1 (Ln(L-OH)-, Ln(HL-OH), and Ln2(L-O)24-) and 2:1 (Ln2(L-O)+) metal-to-ligand ratios. The stability constants of the Ln(L-OH)- species increase from La3+ (log K = 10.19) to Lu3+ (log K = 14.08). The alcoholic OH group of the Ln(L-OH)- species dissociates at unusually low pH values. The pH range of dissociation shifts to lower and lower pH's with the increasing atomic number of the lanthanides. This pH range is about 4-7 for the La3+ complex and 1-4 for the Lu3+ complex. The results of 1D and 2D 1H and 13C NMR studies of the La3+ complex, the number and multiplicity of signals, and the values of coupling constants are in agreement with the dinuclear dimer structure of the complex in solution.