Cinderella Elements: Strategies to Increase the Stability of Group 1 Complexes by Tailoring Crown Macrocycles

[Abstract] The synthesis and structural characterization of six sodium complexes with bibracchial lariatethers containing aniline or benzimidazole side arms, and derived from 1,7 diaza-12-crown-4, 1,10-diaza-15-crown-5 or 4,13-diaza-18-crown-6, are reported. The X-ray structures of four of these compounds have been obtained. Additionally, the X-ray structures of a sodium macrobicyclic complex derived from 1,10-diaza-15-crown-5, and a potassium complex with a bibracchial lariat ether containing aniline side arms are also reported. Bonding distances as well as the stability constants in acetonitrile solution confirm that the coordination of the pendant arms provides an important contribution to the overall stability of the complexes, particularly when benzimidazole pendants are present rending more stable complexes, even more than cryptand complexes of the same size. Compared with the parent crown ethers, the stability increases when the side arms contain benzimidazole moieties but remains about the same order when aniline side arms are present.Xunta de Galicia; CN2012/01

A merged experimental and theoretical conformational study on alkaline-earth complexes with lariat ethers derived from 4,13-diaza-18-crown-6

[Abstract] Herein, we report the synthesis and structural characterization of alkaline-earth complexes with the bibracchial lariat ethers N,N′-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2) and N,N′-bis(benzimidazol-2ylmethyl)-4,13-diaza-18-crown-6 (L4). The X-ray crystal structures of the Ca(II) and Sr(II) complexes of L2 show the pendant arms of the ligand disposed on opposite sides of the macrocyclic mean plane, which results in an anti conformation in the solid state. A similar anti conformation is also observed for the Mg(II) complex of L4, whereas the Ca(II), Sr(II) and Ba(II) complexes of L4 adopt a syn conformation in the solid state, with the two pendant arms pointing at the same side of the crown moiety. However, a different behavior is observed in solution. Indeed, 1H and 13C NMR spectroscopy, in combination with density functional theory (DFT) calculations performed at the B3LYP level, suggests that the [M(L2)]2+ and [M(L4)]2+ (M = Ca, Sr or Ba) complexes exist in solution as a mixture of syn and anti isomers involved in a dynamic equilibrium. Our results also show that the relative abundance of the syn conformation increases as the ionic radius of the metal ion increases and, furthermore, for a given metal ion the proportion of syn isomer is always higher for L4 complexes than for L2 ones.Xunta de Galicia; PGIDIT06TAM10301PRXunta de Galicia; INCITE09E1R103013E

Solid state and solution structures of alkaline-earth complexes with lariat ethers containing aniline and benzimidazole pendants

[Abstract] Herein we report the synthesis and structural characterization of Mg(II), Ca(II), Sr(II) and Ba(II) complexes with bibracchial lariat ethers derived from 1,7-diaza-15-crown-5 and 1,7-diaza-12-crown-4 containing aniline or benzimidazole pendant arms. The solid state structures of most of them have been determined by using single crystal X-ray crystallography. A coordination number of seven was observed for the Mg(II) complexes in the solid state, while the Ca(II), Sr(II) and Ba(II) complexes are 8-, 9- and 11-coordinate, respectively. The Ca(II), Sr(II) and Ba(II) complexes show a syn conformation, with the two pendant arms of the ligand disposed on the same side of the macrocyclic mean plane. However, the Mg(II) complex with the largest ligand derived from 1,7-diaza-15-crown-5 containing benzimidazole pendants presents an anti conformation in the solid state. 1H and 13C NMR spectroscopy reveal that this conformation is maintained in acetonitrile solution.Xunta de Galicia; IN845B-2010/06

Zn(II), Cd(II) and Pb(II) complexation with pyridinecarboxylate containing ligands

[Abstract] Herein, we report the coordination properties towards Zn(II), Cd(II) and Pb(II) of two hexadentate ligands containing pyridinecarboxylate groups with ethane-1,2-diamine (bcpe) or cyclohexane-1,2-diamine (bcpc) backbones. The X-ray crystal structures of [Zn(bcpe)], [Cd(bcpe)] and [Cd(bcpc)] show hexadentate binding of the ligand to the metal ions, with the coordination polyhedron being best described as a severely distorted octahedron. The X-ray crystal structure of the Pb(II) analogue shows the presence of tetrameric structural units [Pb4(bcpe)4] in which the four Pb(II) ions are bridged by carboxylate oxygen atoms. While in the Zn(II) and Cd(II) complexes the bcpe ligand adopts a twist–wrap (tw) conformation in which the ligand wraps around the metal ion by twisting the pyridyl units relative to each other, for the Pb(II) complex a twist–fold (tf) conformation, where a slight twisting of the pyridyl units is accompanied by an overall folding of the two pyridine units relative to each other is observed. Theoretical calculations performed at the DFT (B3LYP) level on the [Pb(bcpe)] and [Pb(bcpc)] systems indicate that the tf conformation is more stable than the tw form both in the solid state and in aqueous solution. The analysis of the natural bond orbitals (NBOs) indicate that the Pb(II) lone-pair is polarized by a substantial 6p contribution, which results in a hemi-directed coordination geometry around the metal ion. Potentiometric studies have been carried out to determine the protonation constants of the ligands and the stability constants of the complexes with Zn(II), Cd(II), Pb(II) and Ca(II). The replacement of the ethylene backbone of bcpe by a cyclohexylene ring causes a very important increase in the stability constant of the Pb(II) complex (ca. 2.3 logK units), while this effect is less important for Cd(II) (ca. 1.4 logK units). However, the introduction of the cyclohexylene ring does not substantially affect the stability of the Zn(II) and Ca(II) complexes. The ligand bcpc shows Pb/Ca and Cd/Ca selectivities [108.9 and 109.8, respectively] superior to those of extracting agents, such as EDTA, already used in Pb(II) and Cd(II) removal from contaminated water and soils.Galicia. Consellería de Innovación, Industria e Comercio; PGIDIT06TAM10301P

Understanding stability trends along the lanthanide series

[Abstract] The stability trends across the lanthanide series of complexes with the polyaminocarboxylate ligands TETA4− (H4TETA=2,2′,2′′,2′′′‐(1,4,8,11 tetraazacyclotetradecane‐1,4,8,11‐tetrayl)tetraacetic acid), BCAED4− (H4BCAED=2,2′,2′′,2′′′ {[(1,4‐diazepane‐1,4‐diyl)bis(ethane‐2,1‐diyl)]bis(azanetriyl)}tetraacetic acid), and BP18C62− (H2BP18C6=6,6′‐[(1,4,10,13‐tetraoxa‐7,16‐diazacyclooctadecane‐7,16 diyl)bis(methylene)]dipicolinic acid) were investigated using DFT calculations. Geometry optimizations performed at the TPSSh/6‐31G(d,p) level, and using a 46+4fn ECP for lanthanides, provide bond lengths of the metal coordination environments in good agreement with the experimental values observed in the X‐ray structures. The contractions of the Ln3+ coordination spheres follow quadratic trends, as observed previously for different isostructural series of complexes. We show here that the parameters obtained from the quantitative analysis of these data can be used to rationalize the observed stability trends across the 4f period. The stability trends along the lanthanide series were also evaluated by calculating the free energy for the reaction [La(L)]n+/−(sol)+Ln3+(sol)→[Ln(L)]n+/ (sol)+La3+(sol). A parameterization of the Ln3+ radii was performed by minimizing the differences between experimental and calculated standard hydration free energies. The calculated stability trends are in good agreement with the experimental stability constants, which increase markedly across the series for BCAED4−complexes, increase smoothly for the TETA4− analogues, and decrease in the case of BP18C62− complexes. The resulting stability trend is the result of a subtle balance between the increased binding energies of the ligand across the lanthanide series, which contribute to an increasing complex stability, and the increase in the absolute values of hydration energies along the 4f period.Xunta de Galicia; CN2012/01

Binuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes with Schiff-bases derived from crown ether platforms: rare examples of ether oxygen atoms bridging metal centers

[Abstract] Bibracchial lariat ethers L3 and L4, derived from the condensation of N,N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 or N,N′-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 with salicylaldehyde, form binuclear complexes with Co(II), Ni(II), Cu(II) and Zn(II). Our studies show that the different denticity and crown moiety size of the two related receptors give rise to important differences on the structures of the corresponding complexes. Single crystal X-ray diffraction analysis shows that the [Ni2(L3)(H2O)2]2+ and [Cu2(L3)(NO3)]+ complexes constitute a rare example in which an oxygen atom of the crown moiety is bridging the two six coordinate metal ions. In contrast, none of the oxygen atoms of the crown moiety is acting as a bridging donor atom in the [Co2(L4)(CH3CN)2]2+, [Cu2(L4)]2+ and [Zn2(L4)]2+ complexes. This is attributed to the larger size the crown moiety and the higher denticity of L4 compared to L3. In [Co2(L4)(CH3CN)2]2+ the metal ions show a distorted octahedral coordination, while in the Cu(II) and Zn(II) analogues the metal ions are five-coordinated in a distorted trigonal bipyramidal environment. In [Cu2(L3)(NO3)]+ the coordinated nitrate anion acts as a bidentate bridging ligand, which results in the formation of a 1D coordination polymer.Xunta de Galicia; INCITE09E1R103013E

The effect of ring size variation on the structure and stability of lanthanide(III) complexes with crown ethers containing picolinate pendants

[Abstract] The coordination properties of the macrocyclic receptor N,N′-bis[(6-carboxy-2-pyridyl)methylene]-1,10-diaza-15-crown-5 (H2bp15c5) towards the lanthanide ions are reported. Thermodynamic stability constants were determined by pH-potentiometric titration at 25 °C in 0.1 M KCl. A smooth decrease in complexstability is observed upon decreasing the ionic radius of the LnIII ion from La [log KLaL = 12.52(2)] to Lu [log KLuL = 10.03(6)]. Luminescence lifetime measurements recorded on solutions of the EuIII and TbIII complexes confirm the absence of inner-sphere watermolecules in these complexes. 1H and 13C NMR spectra of the complexes formed with the diamagnetic LaIII metal ion were obtained in D2O solution and assigned with the aid of HSQC and HMBC 2D heteronuclear experiments, as well as standard 2D homonuclear COSY and NOESY spectra. The 1H NMR spectra of the paramagnetic CeIII, EuIII and YbIIIcomplex suggest nonadentate binding of the ligand to the metal ion. The syn conformation of the ligand in [Ln(bp15c5)]+ complexes implies the occurrence of two helicities, one associated with the layout of the picolinate pendant arms (absolute configuration Δ or Λ), and the other to the five fivemembered chelate rings formed by the binding of the crown moiety (absolute configuration δ or λ). A detailed conformational analysis performed with the aid of DFT calculations (B3LYP model) indicates that the complexes adopt a Λ(λδ)(δδλ) [or Δ(δλ)(λλδ)] conformation in aqueous solution. Our calculations show that the interaction between the LnIII ion and several donor atoms of the crown moiety is weakened as the ionic radius of the metal ion decreases, in line with the decrease of complex stability observed on proceeding to the right across the lanthanide series.Ministerio de Educación y Ciencia; CTQ2006-07875Ministerio de Educación y Ciencia; CTQ2009-10721Galicia. Consellería de Economía e Industria; INCITE09E1R103013E

Density functional dependence of molecular geometries in lanthanide(III) complexes relevant to bioanalytical and biomedical applications

[Abstract] A set of 15 lanthanide-containing model systems was used to evaluate the performance of 15 commonly available density functionals (SVWN, SPL, BLYP, G96LYP, mPWLYP, B3LYP, BH&HLYP, B3PW91, BB95, mPWB95, TPSS, TPSSh, M06, CAM-B3LYP and wB97XD) in geometry determination, benchmarked against MP2 calculations. The best agreement between DFT optimized geometries and those obtained from MP2 calculations is provided by meta-GGA and hybrid meta-GGA functionals. The use of hybrid-GGA functionals such as BH&HLYP and B3PW91 also provide reasonably good results, while B3LYP provides an important overestimation of the metal–ligand bonds. The performance of different basis sets to describe the ligand(s) atoms, as well as the use of large-core (LC) RECPs and small-core (SC) RECPs, has been also assessed. Our calculations show that SCRECP calculations provide somewhat shorter GdIII–donor distances than the LCRECP approach, the average contraction of bond distances for the systems investigated amounting to 0.033 Å. However, geometry optimizations with the SCRECP (in combination with the mPWB95 functional and the 6-31G(d) basis set for the ligand atoms) take about 15 times longer than the LC counterparts, and about four times longer than MP2/LCRECP/6-31G(d) calculations. The 6-31G(d), 6-311G(d), 6-311G(d,p) or cc-pVDZ basis sets, in combination with LCRECPs, appear to offer an adequate balance between accuracy and computational cost for the description of molecular geometries of LnIII complexes. Electronic energies calculated with the the cc-pVxZ family (x = D-6) indicate a relative fast convergence to the complete basis set (CBS) limit with basis set size. The inclusion of bulk solvent effects (IEFPCM) was shown to provoke an important impact on the calculated geometries, particularly on the metal–nitrogen distances. Calculations performed on lanthanide complexes relevant for practical applications confirmed the important effect of the solvent on the calculated geometries.Ministerio de Educación y Ciencia; CTQ2009-10721Xunta de Galicia; IN845B-2010/06

Conformational study of lanthanide(III) complexes of N-(2-salicylaldiminatobenzyl)-1-aza-18-crown-6 by using X-ray and ab initio methods

[Abstract] A structural study of lanthanide complexes with the deprotonated form of the monobracchial lariat ether N-2-salicylaldiminatobenzyl-aza-18-crown-6 (L4) (Ln = La(III)–Tb(III)) is presented. Attempts to isolate complexes of the heaviest members of the lanthanide series were unsuccessful. The X-ray crystal structures of [Pr(L4)(H2O)](ClO4)2 · H2O · C3H8O and [Sm(L4)(H2O)](ClO4)2 · C3H8O show the metal ion being bound to the eight donor atoms of the ligand backbone. Coordination number nine is completed by the oxygen atom of an inner-sphere water molecule. Two different conformations of the crown moiety (labelled as A and B) are observed in the solid state structure of the Pr(III) complex, while for the Sm(III) complex only conformation A is observed. The complexes were also characterized by means of theoretical calculations performed in vacuo at the HF level, by using the 3-21G∗ basis set for the ligand atoms and a 46 + 4fn effective core potential for lanthanides. The optimized geometries of the Pr(III) and Sm(III) complexes show an excellent agreement with the experimental structures obtained from X-ray diffraction studies. The calculated relative energies of the A and B conformations for the different [Ln(L4)(H2O)]2+ complexes (Ln = La, Pr, Sm, Ho or Lu) indicate a progressive stabilization of the A conformation with respect to the B one upon decreasing the ionic radius of the Ln(III) ion. For the [Ln(L4)(H2O)]2+ systems, most of the calculated bond distances between the metal ion and the coordinated donor atoms decrease along the lanthanide series, as usually observed for Ln(III) complexes. However, our ab initio calculations provide geometries in which the Ln–O(5) bond distance [O(5) is an oxygen atom of the crown moiety] increases across the lanthanide series from Sm(III) to Lu(III).Ministerio de Educación y Ciencia; CTQ2006-0787

Applications of density functional theory (DFT) to investigate the structural, spectroscopic and magnetic properties of lanthanide(III) complexes

[Abstract] Density functional theory (DFT) has become a general tool to investigate the structure and properties of complicated inorganic molecules, such as lanthanide(III) coordination compounds, due to the high accuracy that can be achieved at relatively low computational cost. Herein, we present an overview of different successful applications of DFT to investigate the structure, dynamics, vibrational spectra, NMR chemical shifts, hyperfine interactions, excited states, and magnetic properties of lanthanide(III) complexes. We devote particular attention to our own work on the conformational analysis of LnIII-polyaminocarboxylate complexes. Besides, a short discussion on the different approaches used to investigate lanthanide(III) complexes, i. e. all-electron relativistic calculations and the use of relativistic effective core potentials (RECPs), is also presented. The issue of whether the 4f electrons of the lanthanides are involved in chemical bonding or not is also shortly discussed.Ministerio de Educación y Ciencia; CTQ2009-10721Xunta de Galicia; IN845B-2010/06