Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Hydroxyl-group functionalized phenanthroline diimides as efficient masking agents for Am(III)/Eu(III) separation under harsh conditions

The separation of Lns(III) from radioactive Ans(III) in high-level liquid waste remains a formidable hydrometallurgical challenge. Water-soluble ligands are believed to be new frontiers in the search of efficient Lns/Ans separation ligands to close the nuclear fuel cycles and dealing with current existing nuclear waste. Currently, the development of hydrophilic ligands far lags behind their lipophilic counterparts due to their complicated synthetic procedures, inferior extraction performances, and acid tolerances. In this paper, we have showed a series of hydroxyl-group functionalized phenanthroline diimides were efficient masking agents for Am(III)/Eu(III) separation under high acidity (˃ 1 M HNO3). Record high SFEu(III)/Am(III) of 162 and 264 were observed for Phen-2DIC2OH and Phen-2DIC4OH in 1.25 M HNO3 which represents the best Eu(III)/Am(III) separation performance at this acidity. UV-vis absorption, NMR and TRLFS titrations were conducted to elucidate the predominant of 1:1 ligand/metal species under extraction conditions. X-ray data of both the ligand and Eu(III) complex together with DFT calculations revealed the superior extraction performances and selectivities. The current reported hydrophilic ligands were easy to prepare and readily to scale-up, acid tolerant and highly efficient, together with their CHON-compatible nature make them promising candidates in the development of advanced separation processes.

Synthesis of temporin L hydroxamate-based peptides and evaluation of their coordination properties with iron(III )

Ferric iron is an essential nutrient for bacterial growth. Pathogenic bacteria synthesize iron-chelating entities known as siderophores to sequestrate ferric iron from host organisms in order to colonize and replicate. The development of antimicrobial peptides (AMPs) conjugated to iron chelators represents a promising strategy for reducing the iron availability, inducing bacterial death, and enhancing simultaneously the efficacy of AMPs. Here we designed, synthesized, and characterized three hydroxamate-based peptides Pep-cyc1, Pep-cyc2, and Pep-cyc3, derived from a cyclic temporin L peptide (Pep-cyc) developed previously by some of us. The Fe³⁺ complex formation of each ligand was characterized by UV-visible spectroscopy, mass spectrometry, and IR and NMR spectroscopies. In addition, the effect of Fe³⁺ on the stabilization of the α-helix conformation of hydroxamate-based peptides and the cotton effect were examined by CD spectroscopy. Moreover, the antimicrobial results obtained in vitro on some Gram-negative strains (K. pneumoniae and E. coli) showed the ability of each peptide to chelate efficaciously Fe³⁺ obtaining a reduction of MIC values in comparison to their parent peptide Pep-cyc. Our results demonstrated that siderophore conjugation could increase the efficacy and selectivity of AMPs used for the treatment of infectious diseases caused by Gram-negative pathogens.

N-oxide ligands for selective separations of lanthanides: insights from computation

Preorganized ligands such as bis-lactam-1,10-phenanthroline (BLPhen) show unique selectivity trends across the lanthanide series, indicating the synergistic effects of both N and O donors in complexing with lanthanides. We hypothesize that by replacing amide functional groups with an N-oxide functionality would open the door to new ligand architectures with improved selectivities. To test this idea, we computationally examined mixed N,O-donor ligands containing pyridinic N and N-oxide groups and evaluated their relative aqueous La(iii)/Ln(iii) selectivity by computing free energy changes for the exchange reaction between the designed ligands and a reference ligand. Three novel ligands show promise as excellent extractant agents in selectively separating trivalent lanthanides. The extent of conjugation (and hyperconjugation), the complex geometry, and the electron accumulations on the two O-donors of the N-oxide groups are found to be important factors in dictating the selectivity trends.

Quantifying the Influence of Covalent Metal-Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

Qualitative differences in the reactivity of trivalent lanthanide and actinide complexes have long been attributed to differences in covalent metal-ligand bonding, but there are few examples where thermodynamic aspects of this relationship have been quantified, especially with U³⁺ and in the absence of competing variables. Here we report a series of dimeric phosphinodiboranate complexes with trivalent f-metals that show how shorter-than-expected U-B distances indicative of increased covalency give rise to measurable differences in solution deoligomerization reactivity when compared to isostructural complexes with similarly sized lanthanides. These results, which are in excellent agreement with supporting DFT and QTAIM calculations, afford rare experimental evidence concerning the measured effect of variations in metal-ligand covalency on the reactivity of trivalent uranium and lanthanide complexes.

Complexation Behaviors of a Tridentate Phenanthroline Carboxamide Ligand with Trivalent f-Block Elements in Different Anion Systems: A Thermodynamic and Crystallographic Perspective

The counteranion has a strong influence on the complexation behavior of tridentate phenanthroline carboxamide ligands with actinides and lanthanides, but the thermodynamic and underlying interaction mechanism at the molecular level is still not clear. In this work, a tridentate ligand, N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (Et-Tol-PTA), was synthesized, and the effects of different anions (Cl^-, NO₃^-, and ClO₄^-) on the complexation behavior of Et-Tol-PTA with typical lanthanides were thoroughly studied by using ¹H nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-vis) spectrophotometry, and single-crystal X-ray diffraction. The NMR spectroscopic titration of Lu(III) showed that there were three species (1:1, 2:1, and 3:1 ligand-metal complexes) formed in Cl^- solution systems while two species (2:1 and 1:1) were formed in NO₃^- and ClO₄^- solution systems. When Et-Tol-PTA was titrated with La(III), two species (2:1 and 1:1) were formed in NO₃^- systems and only one species (1:1) was formed in Cl^- and ClO₄^- systems. In addition, the stability constant was determined via UV-vis spectroscopic titration, which showed that the complexation strength between Et-Tol-PTA and Eu(III) decreased in the following order: ClO₄^- > NO₃^- > Cl^-. This indicated that Et-Tol-PTA had the strongest complexation ability with Eu(III) in the ClO₄^- system. The structures of Et-Tol-PTA complexed with EuCl₃, Eu(NO₃)₃, and Eu(ClO₄)₃ were further elucidated by single-crystal X-ray diffraction and agreed well with the results of UV-vis titration experiments. The results of this work revealed that the mechanisms of complexation of lanthanides with the asymmetric ligand Et-Tol-PTA were strongly affected by different anionic environments in solution and in the solid state. These findings may lead to the improvement of the separation of trivalent actinides and lanthanides in nuclear waste.

Comparative Investigation into the Complexation and Extraction Properties of Tridentate and Tetradentate Phosphine Oxide-Functionalized 1,10-Phenanthroline Ligands toward Lanthanides and Actinides

Two new phosphine oxide-functionalized 1,10-phenanthroline ligands, tetradentate 2,9-bis(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-BPPhen, L¹ ) and tridentate 2-(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-MPPhen, L² ), were synthesized and studied comparatively for their coordination with trivalent actinides and lanthanides. The complexation mechanisms of these two ligands toward trivalent f-block elements were thoroughly elucidated by NMR spectroscopy, UV/vis spectrophotometry, fluorescence spectrometry, single-crystal X-ray diffraction, solvent extraction, and theoretical calculation methods. NMR titration results demonstrated that 1 : 1 and 1 : 2 (metal to ligand) lanthanides complexes formed for L¹ , whereas 1 : 1, 1 : 2 and 1 : 3 lanthanide complexes formed for L² in methanol. The formation of these species was validated by fluorescence spectrometry, and the corresponding stability constants for the complexes of Nd^III with L¹ and L² were determined by using UV/vis spectrophotometry. Structures of the 10-coordinated 1 : 1-type complexes of EuL¹ (NO₃ )₃ and [EuL² (NO₃ )₃ (H₂ O)] Et₂ O in the solid state were characterized by X-ray crystallography. In solvent-extraction experiments, L¹ exhibited extremely strong extraction ability for both Am^III and Eu^III , whereas L² showed nearly no extraction toward Am^III or Eu^III due to its high hydrophilicity. Finally, the structures and bonding natures of the complex species formed between Am^III /Eu^III and L¹ /L² were analyzed in DFT calculations.

Experimental evaluation of the stabilization of the COT orbitals by 4f orbitals in COT2Ce using a Hubbard model

Significant orbital mixing is rare in lanthanide complexes because of the limited radial extent of the 4f orbitals, which results in a generally small stabilization due to 4f orbital interactions. Nevertheless, even a small amount of additional stabilization could enhance lanthanide separations. One lanthanide complex in which orbital mixing has been extensively studied both experimentally and computationally is cerocene, COT₂Ce, where COT is cyclooctatetraene. This compound has a singlet ground state with a low-lying, triplet excited state. Previous fluorescence studies on trimethylsilyl-substituted cerocenes indicate the triplet state is 0.4 eV higher in energy than the singlet state. In addition, computational studies predict that the triplet is 0.3 to 1 eV higher in energy than the singlet. The synthesis of highly pure COT₂Ce by Walter and Andersen allowed its physical properties to be accurately measured. Using these measurements, we evaluate the stabilization of the 4f orbitals using two, independent approaches. A Hubbard model is used to evaluate the stabilization of the ground state due to orbital mixing. This stabilization, which is also the singlet-triplet gap, is -0.29 eV using this model. This gap was also from the temperature independent paramagnetism of COT₂Ce, which yielded a value of -0.32 eV.

Complexes of Th(IV) with neutral O-N-N-O hybrid ligands: a thermodynamic and crystallographic study

The thermodynamics of Th(iv) complexes with N,N,N',N'-tetramethyl-2,2'-bipyridine-6,6'-dicarboxamide (TMBiPDA) and N,N,N',N'-tetramethyl-1,10-phenanthroline-2,9-dicarboxamide (TMPhenDA) in CH3OH/10%(v)H2O (CH3OH : H2O = 9 : 1 by volume) were determined by spectrophotometry and calorimetry. The ligand TMBiPDA/TMPhenDA coordinates with the central Th atom by the tetradentate (O-N-N-O) mode, which is validated by 1H NMR in solution and crystallography in the solid. The single crystal X-ray diffraction data show that ten-coordinated thorium coordinates with two ligand molecules and two solvent molecules (water or methanol). Both ThL and ThL2 complexes (L = TMPhenDA or TMBiPDA) were detected in solution. In thermodynamics, the formation of all complexes is driven by both enthalpy and entropy. In a comparison, enthalpy is more favorable to the formation of TMBiPDA complexes, while entropy is more favorable to the formation of TMPhenDA complexes; the entropy advantages of the TMPhenDA complexes override the enthalpy advantages of the corresponding TMBiPDA complexes, giving the TMPhenDA complexes higher stability constants than the TMBiPDA complexes. In crystallography, ligand distortions occur in ThL2 complexes, and TMBiDA distorts more than TMPhenDA does; the Th-O and Th-N bonds involving TMBiPDA are slightly shorter than those involving TMPhenDA.

A New Tetradentate Mixed Aza-Thioether Macrocycle and Its Complexation Behavior towards Fe(II), Ni(II) and Cu(II) Ions

A new tetradentate mixed aza-thioether macrocyclic ligand 2,6-dithia[7](2,9)-1,10-phenanthrolinophane ([13]ane(phenN₂)S₂) was successfully synthesized. Reacting metal precursors [Fe(CH₃CN)₂(OTf)₂], Ni(ClO₄)₂·6H₂O, and Cu(ClO₄)₂·6H₂O with one equivalent of [13]ane(phenN₂)S₂ afforded [Fe([13]ane(phenN₂)S₂)(OTf)₂] (1), [Ni([13]ane(phenN₂)S₂)](ClO₄)₂ (2(ClO₄)₂), and [Cu([13]ane(phenN₂)S₂)(OH₂)](ClO₄)₂ (3(ClO₄)₂), respectively. The structures of [13]ane(phenN₂)S₂ and all of its metal complexes were investigated by X-ray crystallography. The [13]ane(phenN₂)S₂ was found to behave as a tetradentate ligand via its donor atoms N and S.

Gas-Phase Complexes of Americium and Lanthanides with a Bis-triazinyl Pyridine: Reactivity and Bonding of Archetypes for F-Element Separations

Bis-triazinyl pyridines (BTPs) exhibit solution selectivity for trivalent americium over lanthanides (Ln), the origins of which remain uncertain. Here, electrospray ionization was used to generate gas-phase complexes [ML₃]³⁺, where M = La, Lu, or Am and L is EtBTP 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)-pyridine. Collision-induced dissociation (CID) of [ML₃]³⁺ in the presence of H₂O yielded a protonated ligand [L(H)]⁺ and hydroxide [ML₂(OH)]²⁺ or hydrate [ML(L-H)(H₂O)]²⁺, where (L-H)^- is a deprotonated ligand. Although solution affinities indicate stronger binding of BTPs toward Am³⁺ versus Ln³⁺, the observed CID process is contrastingly more facile for M = Am versus Ln. To understand the disparity, density functional theory was employed to compute potential energy surfaces for two possible CID processes, for M = La and Am. In accordance with the CID results, both the rate determining transition state barrier and the net energy are lower for [AmL₃]³⁺ versus [LaL₃]³⁺ and for both product isomers, [ML₂(OH)]²⁺ and [ML(L-H)(H₂O)]²⁺. More facile removal of a ligand from [AmL₃]³⁺ by CID does not necessarily contradict stronger Am³⁺-L binding, as inferred from solution behavior. In particular, the formation of new bonds in the products can distort kinetics and thermodynamics expected for simple bond cleavage reactions. In addition to correctly predicting the seemingly anomalous CID behavior, the computational results indicate greater participation of Am 5f versus La 4f orbitals in metal-ligand bonding.

Unraveling the complexation mechanism of actinide(iii) and lanthanide(iii) with a new tetradentate phenanthroline-derived phosphonate ligand

The separation and complexation mechanisms of actinide(iii) and lanthanide(iii) with a new phenanthroline-derived phosphonate ligand were studied experimentally and theoretically.

Theoretical insights into selective separation of trivalent actinide and lanthanide by ester and amide ligands based on phenanthroline skeleton

Phenanthroline based ligands have shown potential performance for partitioning trivalent actinides from lanthanides. In this work, we have explored four ester and amide ligands based on the phenanthroline skeleton and elucidated the separation mechanism between Am(iii) and Eu(iii) ions. The molecular geometries and extraction reactions of the metal-ligand complexes were modeled by using scalar-relativistic density functional theory. The results show that the amide based ligands have stronger coordination ability with the metal ions than the corresponding ester based ligands. According to the thermodynamic results, ligands N,N'-diethyl-N,N'-ditolyl-2,9-diamide-1,10-phenanthroline (L2) and N,N'-(1,10-phenanthroline-2,9-diyl)bis(N-ethyl-P-methyl-N-(p-tolyl)phosphinic amide) (L4) appear to have the strongest complexing ability, which is supported by the result of electrostatic potential (ESP) and the M-OL bond orders. Moreover, ligand L2 has excellent selectivity for Am(iii)/Eu(iii) among the four ligands. Additionally, the bonding properties between the metal ions and the ligands reveal that the Am(iii)/Eu(iii) selectivity stems from the Am-N bonds with more covalent character, which is supported by the analysis of the hardness of the ligands and the bond orders. This work provides useful information for understanding the Am(iii)/Eu(iii) selectivity of phenanthroline derived ligands bearing ester and amide groups.

Binding affinity of pyridines with AmIII/CmIII elucidated by density functional theory calculations

In recent decades, N-heterocyclic ligands have been extensively used in the separation of lanthanides/actinides, whereas the selective extraction of amercium or curium has been very challenging. Using density functional theory calculations, this study is devoted to the investigation of the binding affinity of a series of modelling pyridine ligands with Am^III and Cm^III. The structure-property correlations between the amercium and curium systems and the binding affinity were obtained, and promising strategies for efficient separation of Am^III/Cm^III were proposed.

Thermodynamics of plutonium(iii) and curium(iii) complexation with a N-donor ligand

The complexation of Pu(iii) and Cm(iii) with a soft N-donor ligand was investigated using the van't Hoff method, microcalorimetry and DFT calculations. The studies revealed that the strength of the actinide-ligand bond as given by the enthalpic contribution drastically decreases on going from Pu(iii) to Cm(iii), while the complex stability remains nearly constant.

Quantification of f-element covalency through analysis of the electron density: insights from simulation

The electronic structure of f-element compounds is complex due to a combination of relativistic effects, strong electron correlation and weak crystal field environments. However, a quantitative understanding of bonding in these compounds is becoming increasingly technologically relevant. Recently, bonding interpretations based on analyses of the physically observable electronic density have gained popularity and, in this Feature Article, the utility of such density-based approaches is demonstrated. Application of Bader's Quantum Theory of Atoms in Molecules (QTAIM) is shown to elucidate many properties including bonding trends, orbital overlap and energy degeneracy-driven covalency, oxidation state identification and bond stability, demonstrating the increasingly important role that simulation and analysis play in the area of f-element bond characterisation.

Complexation of a macrocyclic ligand, 2,6-di (N-methyl)formamide-calix[4]pyridine, with Eu(III) and extraction of Eu(III) and Am(III)

Complexation of a new macrocyclic compound, 2,6-dimethylformamide-calix[4]pyridine (L1 ), with Eu(III) was studied by spectrophotometry. Stability constants of the Eu(III)/L1 complex in different solvents were determined. The results reveal that L1 forms moderately strong complexes with Eu(III) and other lanthanides in aprotic solvents and shows little binding ability with transition metals. Moreover, the binding strength of L1 weakens significantly in protic solvents. Using 2-bromodecanoic acid as the synergistic reagent, L1 extracts Am(III) and Eu(III) successfully with a separation factor of SFAm/Eu=1.3, and the distribution ratios of Am(III) and Eu(III) increases as the aqueous acidity is decreased. DFT computational studies were conducted to corroborate the solvent extraction data, and compare the coordination properties of Am(III)/Eu(III) complexes with L1 and a related, 2,6-diformamide-calix[4]pyridine (L2 ). The computational results suggest that L2 could form stable complexes [ML]3+ and ML(NO3)3 [where M represent Am(III) or Eu(III)] in aqueous phase, in sharp contrast to the case of L1 where such complexes in aqueous phase are not stable.

New insights into the selectivity of four 1,10-phenanthroline-derived ligands toward the separation of trivalent actinides and lanthanides: a DFT based comparison study

To understand the correlation between the chemical structures of N-ligands and Ln/An separation capability, we selected four phenanthroline-derived ligands as examples to probe why some ligands work whereas others fail.

A phenanthroline-derived ligand and its complexation with Pd(ii): from ligand design, synthesis and Pd(ii) complexes structures to its application

This paper reports the first example of a phenanthroline-derived bis-opened-triazine ligand showing high selectivity and fast extraction rate toward Pd(ii).

Exploring electronic effects on the partitioning of actinides(iii) from lanthanides(iii) using functionalised bis-triazinyl phenanthroline ligands

Revealing how soft N-type donor ligands achieve minor actinide selectivity is fundamental in the design of new and improved extractants for advanced future fuel cycles.