Selectivity of the Highly Preorganized Tetradentate Ligand 2,9-Di(pyrid-2-yl)-1,10-phenanthroline for Metal Ions in Aqueous Solution, Including Lanthanide(III) Ions and the Uranyl(VI) Cation

Preorganization Effects on Eu(III) Ion Coordination by Dipyridyl-Phenanthroline Ligands: A Combined Experimental and Theoretical Analysis

Although 1,10-phenanthroline has been proven to hold a strong complexing capacity for f-block elements and their derivatives have been applied in many fields, research on more highly or completely rigid phenanthroline ligands is still rare due to the challenging syntheses. Here, we reported three tetradentate ligands 2,9-di(pyridin-2-yl)-1,10-phenanthroline (L1), 12-(pyridin-2-yl)-5,6-dihydroquinolino[8,7b][1,10]phenanthroline (L2), and 5,6,11,12-tetrahydrobenzo[2,1-b:3,4-b']bis([1,10]phenanthroline) (L3) with increasing preorganization on the side chain; among which, L3 is fully preorganized. Their complexation reactions with Eu(III) were systematically investigated by electrospray ionization mass spectrometry (ESI-MS), UV-vis titrations, and single-crystal structures. It is found that all three ligands form only 1:1 M/L complexes with Eu(III). The single-crystal structures revealed that the three ligands hold similar coordination modes, while their stability constants determined by UV-vis titrations were L3 (4.80 ± 0.01) > L2 (4.38 ± 0.01) > L1 (3.88 ± 0.01). This trend is supported not only by the thermodynamic stability of rigid ligands compared to free ligands but also by the conclusion that rigid ligands exhibit faster reaction rates (lower energy barrier) than free ligands kinetically. This work is helpful in providing theoretical guidance for the subsequent development of highly preorganized chelating ligands with strong coordination ability and high selectivity for f-block elements.

Tuning Selectivity and Stability in Heteroleptic Lanthanide Adducts by Ligand Design

Terdentate ligands L10-L14 and their heteroleptic [LkLn(hfac)3] complexes (Ln = La, Eu, Gd, Er, or Y; H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) exhibit multifactorial correlations between the ligand's structural frameworks, including their level of preorganization and steric congestion and their affinities and selectivities for catching the trivalent lanthanide containers [Ln(hfac)3]. The polyaromatic ligand scaffolds could be stepwise modulated via lanthanide-template synthetic strategies, using intermolecular rhodium-catalyzed insertion reactions. The increasing level of preorganization along the L10 → L11 → L12 series leads to a duality in which larger thermodynamic formation constants with lanthanides in CD2Cl2 are accompanied by an unexpected decrease in the Ln-N affinities in the solid state, which could be assigned to a limited match between the lanthanide size and the enlarged preorganized cavities. On the contrary, a reduced stability is induced by the connection of additional methyl groups at position 1 of the benzimidazole moieties in L13 and L14, which is accompanied by an optimization of metal-nitrogen bond lengths. This study contributes to the rational design of highly stable neutral heteroleptic lanthanide β-diketonate adducts that resist dissociation in solution, a prerequisite for photophysical applications using these highly luminescent systems at the molecular level.

Evidence for Participation of 4f and 5d Orbitals in Lanthanide Metal-Ligand Bonding and That Y(III) Has Less of This Complex-Stabilizing Ability. A Thermodynamic, Spectroscopic, and DFT Study of Their Complexation by the Nitrogen Donor Ligand TPEN

The formation constants (log K₁) of lanthanide(III) (Ln) ions [except for Pm(III)] and the Y(III) cation have been measured with the ligand TPEN (N,N,N',N'-tetra-2-picolylethylenediamine). These log K₁ values show a typical variation with ionic radius, with a local maximum at Sm(III) and a local minimum at Gd(III), with an overall increase in log K₁ from La(III) to Lu(III) as the ionic radius decreases. The log K₁ for the Y(III)/TPEN complex is much lower than expected from its ionic radius, while the literature log K₁ for Am(III) is much higher. The latter effect is thought to be due to greater covalence in the M-L (metal-ligand) bond than for Ln(III) ions: the low log K₁ for Y(III) is interpreted as being due to lower covalence. The f → f transitions in the Nd(III) and Pr(III) complexes were examined for effects that might indicate the participation of f orbitals in M-L bonding. The intensity of the f → f transitions in the Nd(III)/TPEN complex was greatly increased compared to that of the Nd³⁺ aqua ion, which appeared to be due to additional sharp peaks, possibly parity forbidden transitions where parity rules were broken by covalence in the M-L bond. The Pr(III)/TPEN complex showed that all of the f → f transitions shifted to longer wavelengths by some 5 nm, with modest increases in intensity. The effects seen in the f → f transitions of Nd(III) and Pr(III) with TPEN with its six nitrogen donors were present to a much smaller extent in the EDTA and other complexes with fewer nitrogen donors. The changes in the f → f transitions of the TPEN complexes of Er(III) and Ho(III) were small, suggesting a smaller contribution of f orbitals to M-L bonding in these heavier Ln(III) ions. The intense Laporte allowed f → d transitions in Ce(III) complexes show large shifts to longer wavelengths as complexes of, for example, EDTA with increasing numbers of nitrogen donors, suggesting the participation of both f and d orbitals, or either, in M-L bonding. The nature of M-L bonding in M(III)/TPEN complexes was further investigated via density functional theory calculations.

Fluorescence and Metal-Binding Properties of the Highly Preorganized Tetradentate Ligand 2,2'-Bi-1,10-phenanthroline and Its Remarkable Affinity for Cadmium(II)

The metal-ion-complexing properties of the tetradentate ligand 2,2'-bi-1,10-phenanthroline (BIPHEN) in 50% CH₃OH/H₂O are reported for a variety of metal ions. BIPHEN (with two reinforcing benzo groups in the backbone) was compared to other tetrapyridyls, 2,9-di(pyrid-2-yl)-1,10-phenanthroline (DPP; with one benzo group) and 2,2':6',2″:6″,2‴- quaterpyridine (QPY; with no benzo groups), with levels of preorganization BIPHEN > DPP > QPY. Formation constants were determined by following the variation of the intense π → π* transitions in the absorbance spectra of BIPHEN in the presence of metal ion as a function of the pH. The log K₁ values show that the increased level of preorganization produced by the two benzo groups, reinforcing the backbone of the BIPHEN ligand, leads to increased complex stability with large metal ions (an ionic radius greater than 0.9 Å) compared to the less preorganized tetrapyridines DPP and QPY. In particular, the large Cd^II ion [log K₁(BIPHEN) = 12.7] shows unusual selectivity over the small Zn^II ion [log K₁(BIPHEN) = 7.78]. The order of levels of preorganization BIPHEN > DPP > QPY leads to enhanced selectivity for Sm^III over Gd^III with increased preorganization, which is of interest in relation to separating Am^III from Gd^III in the treatment of radioactive waste. Am^III is very close in ionic radius to Sm^III, so that the size-based selectivity produced by the enhanced preorganization of BIPHEN should translate into enhanced Am^III/Gd^III selectivity. The chelation-enhanced fluorescence (CHEF) effect in BIPHEN complexes is discussed. The CHEF effect in the Zn^II complex is somewhat smaller than that for Cd^II, which is discussed in terms of decreased overlap in the Zn-N bonds formed by the too small Zn^II, leading to a partial photoinduced-electron-transfer quenching of fluorescence. The structure of the complex [Cd(BIPHEN)₂](ClO₄)₂ is reported and shows that the Cd-N bonds are largely normal for the unusual 8-coordination observed, except that steric clashes between the terminal pyridyl groups of each of the BIPHEN ligands, and the rest of the orthogonal BIPHEN ligand, lead to some stretching of the outer Cd-N bonds.

Synthesis and Basicity Studies of Quinolino[7,8-h]quinoline Derivatives

Quinolino[7,8-h]quinoline is a superbasic compound, with a pK_aH in acetonitrile greater than that of 1,8-bis(dimethylaminonaphthalene) (DMAN), although its synthesis and the synthesis of its derivatives can be problematic. The use of halogen derivatives 4,9-dichloroquinolino[7,8-h]quinoline (16) and 4,9-dibromoquinolino[7,8-h]quinoline (17) as precursors has granted the formation of a range of substituted quinolinoquinolines. The basicity and other properties of quinolinoquinolines can be modified by the inclusion of suitable functionalities. The experimentally obtained pK_aH values of quinolino[7,8-h]quinoline derivatives show that N⁴,N⁴,N⁹,N⁹-tetraethylquinolino[7,8-h]quinoline-4,9-diamine (26) is more superbasic than quinolino[7,8-h]quinoline. Computationally derived pK_aH values of quinolinoquinolines functionalized with dimethylamino (NMe₂), 1,1,3,3-tetramethylguanidino (N═C(NMe₂)₂) or N,N,N',N',N″,N″-hexamethylphosphorimidic triamido (N═P(NMe₂)₃) groups are significantly greater than those of quinolino[7,8-h]quinoline. Overall, electron-donating functionalities are observed to increase the basicity of the quinolinoquinoline moiety, while the substitution of electron-withdrawing groups lowers the basicity.

On the preferences of five-membered chelate rings in coordination chemistry: insights from the Cambridge Structural Database and theoretical calculations

The purpose of this review is to give an overview of three important N-bidentate ligands: 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and ethylenediamine (en). We have not attempted to be comprehensive because of the huge amount of activity being done in coordination chemistry using these ligands. Instead we present a full structural and geometrical study by using the Cambridge Structural Database (CSD) combined with theoretical calculations that allow us to parameterize their coordinating properties and ability to coordinate to transition and non-transition metals. More importantly, we illustrate that upon coordination and formation of the five-membered chelate ring, these ligands are able to adapt themselves to the requirements of the different metals by changing the MN distances and NMN angles. Therefore, a redefinition of the preferences of these ligands to metals with large ionic radii is needed. Finally, we will present some facts about the participation of these ligands in inorganic-organic hybrids (IOHs) based on Keggin polyoxometalates (POMs).

Surface-grafted lanthanoid complexes of the tungstosilicate polyanion [SiW12O40]4−: a synthetic, structural and computational investigation

As an extension of our continued interest in the preparation of inorganic–organic hybrids, we report the successful hydrothermal synthesis of sodium tris[triaqua(μ-1,10-phenanthroline-2,9-dicarboxylato)dysprosium(III)] silicododecatungstate dodecahydrate, {[DyNa(C14H6N2O4)3(H2O)9(SiW12O40)]·12H2O}nor Na[Dy(PDA)(H2O)3]3[SiW12O40]·12H2O (1), and sodium aqua tris[tetraaqua(μ-4-hydroxypyridine-2,6-dicarboxylato)praseodymium(III)] silicododecatungstate dodecahydrate, {[NaPr(C7H3NO5)3(H2O)13(SiW12O40)]·12H2O}nor Na(H2O)[Pr(pydc-OH)(H2O)4]3[SiW12O40]·12H2O (2) (in which H2PDA is 1,10-phenanthroline-2,9-dicarboxylic acid and H2pydc-OH is 4-hydroxypyridine-2,6-dicarboxylic acid or chelidamic acid). Both compounds have been characterized using elemental analysis, IR spectroscopy and X-ray diffraction methods. Structural characterization by single-crystal X-ray diffraction reveals that these compounds consist of [SiW12O40]4−Keggin-type polyoxometalates (POMs), where a single {W3O13} triad is decorated with a trinuclear Ln complex. Moreover, the decorated polyanions are involved in a series of intermolecular interactions, such as hydrogen bonds and anion–π interactions, resulting in three-dimensional supramolecular architectures. Density functional theory (DFT) studies were conducted to support these intermolecular interactions in both1and2, and have been rationalized using molecular electrostatic potential (MEP) surface calculations.

Spin state variability in Fe2+ complexes of substituted (2-(pyridin-2-yl)-1,10-phenanthroline) ligands as versatile terpyridine analogues

Fe²⁺ spin crossover complexes [Fe(L)₂]²⁺ (L = substituted (pyridin-2-yl)-1,10-phenanthroline) were prepared and SCO properties were investigated in solution and in the solid state by an experiment and in silico.

A novel highly selective ligand for separation of actinides and lanthanides in the nuclear fuel cycle. Experimental verification of the theoretical prediction

Predicted by DFT simulation dilactams (B) are selective and efficient extractants for the separation of Eu³⁺ and Am³⁺.

Zinc complexes formed by 2,2′-bipyridine and 1,10-phenanthroline moieties combined with 2-azanorbornane: modular chiral catalysts for aldol reactions

Zinc complexes of new hybrid chiral imine ligands based on a bicyclic skeleton appear to be effective modular catalysts in asymmetric aldol reactions.

Controlling the Fluorescence Response of PET Sensors via the Metal-Ion π-Contacting Ability of the Fluorophore: Coumarin, a Weaker π Contacter

The π-contact hypothesis, that quenching of the fluorescence of complexes of photoinduced electron transfer sensors with heavy diamagnetic metal ions may be caused by π contacts between the metal ion and the fluorophore of the sensor, is examined with a study of the fluorescent properties of the sensor 4-[[bis(2-pyridinylmethyl)amino]methyl]-6,7-dimethoxy-1-benzopyran-2-one (cdpa) and the structures of its complexes with some metal ions. The coumarin-type fluorophore of cdpa is a weaker π-contact former than the anthracenyl fluorophore of the analogue adpa (Inorg. Chem. 2014, 53, 9014): only Ag(I), the strongest π contact former, quenches the fluorescence of cdpa, apart from paramagnetic Cu(II) and Ni(II), which quench fluorescence by a redox mechanism not requiring π contacts. The structures of [Ag(cdpa)NO3] (1), [Pb(cdpa)(NO3)2] (2), [Zn(cdpa)(NO3)2] (3), [Cd(cdpa)Cl2]2 (4), [Cd(cdpa)2H2O](NO3)2 (5), and [Hg(cdpa)2H2O](NO3)2 (6) are reported. Structure 1 shows that Ag(I) is the only metal ion studied that forms π contacts with the fluorophore of cdpa in the solid state: Ag···C η(2) π contacts of 3.083 and 3.095 Å, in line with quenching of the fluorescence of the Ag(I)(cdpa) complex. In contrast, Pb(II), Zn(II), and Cd(II) show chelation-enhanced fluorescence in their cdpa complexes, and the structures of 2-4 show that the fluorophore of cdpa in each case forms no π contacts. By contrast, the adpa complexes of Pb(II) and Cd(II) show π contacts with its more strongly π-contacting fluorophore (Inorg. Chem. 2014, 53, 9014). The structures of 5 and 6 show bis-complexes of cdpa: the coordination geometries of Cd(II) and Hg(II) are discussed in relation to the number of covalently bound donor atoms present. The preferred hapticity of π-contacted metal ions is evaluated from the literature structures, suggesting that d(10) metal ions such as Ag(I) and Hg(II), and tetragonally distorted Cu(II) and Pd(II), prefer η(1) and η(2) π contacts, while more ionically bound metal ions such as K(I), Ba(II), and La(III), as well as d(10)s(2) metal ions such as Tl(I), Pb(II), and Bi(III), prefer η(6) contacts.

Complexation of trivalent americium and lanthanides with terdentate 'N' donor ligands: the role of rigidity in the ligand structure

A systematic study on the Ln(3+) complexation behaviour with two terdentate 'N' donor ligands of varying structural rigidity, viz. 5,6-dimethyl-(1,2,4)-triazinylbipyridine (Me2TBipy) and 5,6-dimethyl-(1,2,4)-triazinylphenanthroline (Me2TPhen), is performed in the present work by UV-Vis spectrophotometry, time resolved fluorescence spectroscopy (TRFS) and electrospray ionization mass spectrometric (ESI-MS) studies. These studies indicate the formation of a 1 : 1 complex of La(3+), 1 : 2 complexes of Eu(3+) and Er(3+) with both the ligands. Density functional theoretical (DFT) study is carried out to determine the solution phase structure of the Eu(3+) complex considering the species (from UV-Vis spectrophotometry) and C2v site symmetry around the Eu(3+) ion (from TRFS study). Me2TPhen is found to be a stronger complexing ligand as compared to Me2TBipy irrespective of the Ln(3+) ions. The solid state crystal structure of the La(3+) complex of Me2TPhen is determined using the single crystal X-ray diffraction (SCXRD) technique. The complexation of the trivalent Am(3+) ion is also studied with both these ligands using UV-Vis spectrophotometric titrations which show the formation of 1 : 2 complexes with higher complexation constant values as compared to all the Ln(3+) ions studied, indicating the selectivity of these ligands for the trivalent actinides over the lanthanides.

Quantum chemistry study of uranium(VI), neptunium(V), and plutonium(IV,VI) complexes with preorganized tetradentate phenanthrolineamide ligands

The preorganized tetradentate 2,9-diamido-1,10-phenanthroline ligand with hard-soft donors combined in the same molecule has been found to possess high selectivity toward actinides in an acidic aqueous solution. In this work, density functional theory (DFT) coupled with the quasi-relativistic small-core pseudopotential method was used to investigate the structures, bonding nature, and thermodynamic behavior of uranium(VI), neptunium(V), and plutonium(IV,VI) with phenanthrolineamides. Theoretical optimization shows that Et-Tol-DAPhen and Et-Et-DAPhen ligands are both coordinated with actinides in a tetradentate chelating mode through two N donors of the phenanthroline moiety and two O donors of the amide moieties. It is found that [AnO2L(NO3)](n+) (An = U(VI), Np(V), Pu(VI); n = 0, 1) and PuL(NO3)4 are the main 1:1 complexes. With respect to 1:2 complexes, the reaction [Pu(H2O)9](4+)(aq) + 2L(org) + 2NO3(-)(aq) → [PuL2(NO3)2](2+)(org) + 9H2O(aq) might be another probable extraction mechanism for Pu(IV). From the viewpoint of energy, the phenanthrolineamides extract actinides in the order of Pu(IV) > U(VI) > Pu(VI) > Np(V), which agrees well with the experimental results. Additionally, all of the thermodynamic reactions are more energetically favorable for the Et-Tol-DAPhen ligand than the Et-Et-DAPhen ligand, indicating that substitution of one ethyl group with one tolyl group can enhance the complexation abilities toward actinide cations (anomalous aryl strengthening).

High selectivity towards small copper ions by a preorganized phenanthroline-derived tetradentate ligand and new insight into the complexation mechanism

A preorganized tetradentate phenanthroline-derived amide ligand was found to show high selectivity towards small copper ions.