ZnII Complexes Research Articles

Bis(4-meth-oxy-chalcone 4-ethyl-thio-semi-carbazon-ato-κ2N1,S)zinc(II): crystal structure and Hirshfeld surface analysis.

The title ZnII complex, [Zn(C19H20N3OS)2] {systematic name: bis-[(N-ethyl-N'-{(Z)-[(2E)-3-(4-meth-oxy-phen-yl)-1-phenyl-prop-2-en-1-yl-idene]amino}-carb-am-im-id-o-yl)sulfanido]zinc(II)}, features a tetra-hedrally coordinated ZnII ion within an N2S2 donor set provided by two N,S-chelating thio-semicarbazone anions. The resulting five-membered Zn,C,N2,S chelate rings adopt different conformations, i.e. almost planar and an envelope with the Zn atom being the flap atom. The configuration about the imine bond within the chelate ring is Z but those about the exocyclic imine and ethyl-ene bonds are E. In the crystal, supra-molecular [100] chains mediated by thio-amide-N-H⋯S(thione) hydrogen bonds and eight-membered thio-amide {⋯HNCS}2 synthons are observed. A range of inter-actions, including C-H⋯O, C-H⋯π, C-H⋯π(chelate ring) and π(meth-oxy-benzene)-π(chelate ring) consolidate the packing. The Hirshfeld surface analysis performed on the title complex also indicates the influence of the inter-actions involving the chelate rings upon the packing along with the more conventional contacts.

The 3rd degree of biomimetism: associating the cavity effect, ZnII coordination and internal base assistance for guest binding and activation.

The synthesis and characterization of a resorcinarene-based tetra(imidazole) ligand is reported. The properties of the corresponding ZnII complex are studied in depth, notably by NMR spectroscopy. In MeCN, acid-base titration reveals that one out of the four imidazole arms is hemi-labile and can be selectively protonated, thereby opening a coordination site in the exo position. Quite remarkably, the 4th imidazole arm promotes binding of an acidic molecule (a carboxylic acid, a β-diketone or acetamide), by acting as an internal base, which allows guest binding as an anion to the metal center in the endo position. Most importantly, the presence of this labile imidazole arm makes the ZnII complex active for the catalyzed hydration of acetonitrile. It is proposed that it acts as a general base for activating a water molecule in the vicinity of the metal center during its nucleophilic attack to the endo-bound MeCN substrate. This system presents a unique degree of biomimetism when considering zinc enzymes: a pocket for guest binding, a similar first coordination sphere, a coordination site available for water activation in the cis position relative to the substrate and finally an internal imidazole residue that plays the role of a general base.

Two zinc(II) coordination complexes based on an asymmetric multidentate ligand: syntheses, structures, selective fluorescence sensing of iron(III) ions and thermal analyses.

The rational selection of ligands is vitally important in the construction of coordination complexes. Two novel ZnII complexes, namely bis(acetato-κO)bis[1-(1H-benzotriazol-1-ylmethyl)-2-propyl-1H-imidazole-κN3]zinc(II) monohydrate, [Zn(C13H15N5)2(C2H3O2)2]·H2O, (1), and bis(azido-κN1)bis[1-(1H-benzotriazol-1-ylmethyl)-2-propyl-1H-imidazole-κN3]zinc(II), [Zn(C13H15N5)2(N3)2], (2), constructed from the asymmetric multidentate imidazole ligand, have been synthesized under mild conditions and characterized by elemental analyses, IR spectroscopy and single-crystal X-ray diffraction analysis. Both complexes exhibit a three-dimensional supramolecular network directed by different intermolecular interactions between discrete mononuclear units. The complexes were also investigated by fluorescence and thermal analyses. The experimental results show that (1) is a promising fluorescence sensor for detecting Fe3+ ions and (2) is effective as an accelerator of the thermal decomposition of ammonium perchlorate.

Synthesis of ZnII and AlIII Complexes of Diaminocyclohexane‐Derived Ligands and Their Exploitation for the Ring Opening Polymerisation of rac‐Lactide

Two series of ligands based on diaminocyclohexane (DACH) have been prepared and successfully complexed to AlIII and ZnII metal centres. Solution and solid‐state studies reveal a tendency to form mono‐ or bis‐ligated forms depending on the steric bulk of the ligand. These complexes have been tested for their ability to ring open rac‐lactide. AlIII based complexes generally gave atactic PLA after a polymerisation time of 1–2 days. The ZnII complexes, were found to be more active and furnished PLA with a slight heterotactic bias.

Dimeric and Trimeric Supramolecular Systems Formed by Donor‒Acceptor Interactions of ZnII, MnIII, and SnIV Porphyrin Complexes

The paper presents the methods of synthesis of dimeric and trimeric compounds formed by donor–acceptor interactions of ZnII, MnIII, and SnIV porphyrin complexes. The structure of the porphyrin ensembles was determined by 1H NMR spectroscopy. The extra coordination properties of the synthesized dimeric and trimeric supramolecular systems in relation to nitrogen-containing compounds were studied and the stability constants of the resulting porphyrin extra complexes were determined.

Heterobimetallic 3d-4f (Zn/La) {6,6′-Dimethoxy-2,2′-[naphthalene-2,3-diylbis(nitrilomethylidyne)]diphenolato}-pyridylzinc(II)-tris(nitrato-O,O′)lanthanum(III) Monohydrate

Herein, we report the synthesis and structural characterization of a complex with extended aromaticity based on a naphthalene-bridged Schiff base complex with two metal binding pockets. In a first step, the Zn(II) complex (ZnL), {6,6′-dimethoxy-2,2′-[naphthalene-2,3-diylbis(nitrilomethylidyne)]diphenolato}a-pyridylzinc(II) was isolated followed by refluxing with lanthanum(III) nitrate to produce the heterobimetallic complex {ZnL[La(NO3)3·H2O]}. Crystals suitable for X-ray diffraction were grown by vapor diffusion of THF into a pyridine solution of the complex. The monoclinic unit cell (P21/n) is commensurately modulated along the c-axis direction and contains four crystallographically independent molecules with pseudo-translational symmetry being broken by slightly differing orientations of included pyridine and THF solvate molecules. The zinc atom adopts a distorted square pyramidal geometry and sits slightly above the ligand plane, whereas the lanthanum atom adopts an all-face capped trigonal prismatic geometry and sits below the ligand plane. Structural characterization of this diamagnetic complex lays the groundwork for applying these synthetic techniques to the near-infrared emitting lanthanides for practical application.

Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements.

Photoredox chemistry with metal complexes as sensitizers and catalysts frequently relies on precious elements such as ruthenium or iridium. Over the past 5 years, important progress towards the use of complexes made from earth-abundant elements in photoredox catalysis has been made. This review summarizes the advances made with photoactive CrIII , FeII , CuI , ZnII , ZrIV , Mo0 , and UVI complexes in the context of synthetic organic photoredox chemistry using visible light as an energy input. Mechanistic considerations are combined with discussions of reaction types and scopes. Perspectives for the future of the field are discussed against the background of recent significant developments of new photoactive metal complexes made from earth-abundant elements.

Tuning of the properties of a picolinic acid-based bispidine ligand for stable copper(II) complexation

The synthesis of the hexadentate bispidine-picolinate ligand L with two tertiary amine, two para-methoxy-substituted pyridine donors (p-OMe) and a picolinate group is reported, together with its NiII, CoII, CuII and ZnII coordination chemistry. The X-ray crystal structures of the CuII and ZnII complexes confirm that the geometries are very similar to those of the unsubstituted ligand. The increased nucleophilicity of the p-OMe substituted pyridine groups leads to a significant decrease in the metal-pyridine bond length but, due to the rigidity of the adamantane-derived bispidine backbone, the other metal-donor bonds are slightly elongated, and this leads to a larger degree of axial distortion, which augments the preference of the ligand for the Jahn-Teller-active CuII center. The structural effects are supported by the dd-electronic and EPR spectra, which indicate that, for CoII, NiII and CuII, the overall ligand field strengths of the unsubstituted ligand L′ and the p-OMe-substituted derivative L are very similar, and for CuII the in-plane ligand field is significantly stronger for the p-OMe-substituted ligand L. The MII/I redox potentials with the optimized ligand L are generally more negative for the CoII, NiII and CuII complexes, indicating that the oxidized form of the complexes takes more profit of the increase in complex stability. For CuII this is analyzed on the basis of the potentiometrically determined stability constants and their correlation with the redox potentials of the substituted and unsubstituted bispidines together with a large series of published data with other ligand systems. It emerges that L is a very selective CuII ligand with a great potential for radiopharmaceutical applications.

"Two-Story" Calix[6]arene-Based Zinc and Copper Complexes: Structure, Properties, and O2 Binding.

A new "two-story" calix[6]arene-based ligand was synthesized, and its coordination chemistry was explored. It presents a tren cap connected to the calixarene small rim through three amido spacers. X-ray diffraction studies of its metal complexes revealed a six-coordinate ZnII complex with all of the carbonyl groups of the amido arms bound and a five-coordinate CuII complex with only one amido arm bound. These dicationic complexes were poorly responsive toward exogenous neutral donors, but the amido arms were readily displaced by small anions or deprotonated with a base to give the corresponding monocationic complexes. Cyclic voltammetry in various solvents showed a reversible wave for the CuII/CuI couple at very negative potentials, denoting an electron-rich environment. The reversibility of the system was attributed to the amido arms, which can coordinate the metal center in both its +II and +I redox states. The reversibility was lost upon anion binding to Cu. Upon exposure of the CuI complex to O2 at low temperature, a green species was obtained with a UV-vis signature typical of an end-on superoxide CuII complex. Such a species was proposed to be responsible for oxygen insertion reactions onto the ligand according to the unusual and selective four-electron oxidative pathway previously described with a "one-story" calix[6]tren ligand.

Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation with a Ni(PS)2 Complex

Tetra‐coordinate NiII and ZnII complexes employing the PS chelates 2‐(diphenylphosphanyl)benzenethiol (HL1) and 2‐(diisopropylphosphanyl)benzenethiol (HL2) have been synthesized and characterized by spectroscopic, structural, and electrochemical methods. All complexes were screened for electrocatalytic activity for hydrogen evolution with acetic acid and hydrochloric acid and hydrogen oxidation in the presence of triethylamine. The nickel complex Ni(L1)2 (1) was found to reduce protons from external acids to generate hydrogen with a turnover frequency of 140 s–1 at an overpotential of 1.1 V and cleave dihydrogen using redox active base triethylamine (Et3N) with a turnover frequency of 23 s–1 at an overpotential of 0.33 V. Ni(L2)2 (3) was also found to be an active catalyst for dihydrogen oxidation, but inefficient for proton reduction due to inaccessible Ni (II/I) reduction in the potential window. Zn(L1)2 (2) and Zn(L2)2 (4) complexes were found to be inadequate for their electrocatalytic behaviour towards both the reactions.

Metal(II) Ion Complexes with 5‐(Pyrazin‐2‐yl)‐2,4‐dihydro‐3H‐1,2,4‐triazole‐3‐thione; Synthesis, Structural Characterization, Acid‐base, and Complexing Properties in Solution

The study reports the synthesis of complexes Co(HL)Cl2 (1), Ni(HL)Cl2 (2), Cu(HL)Cl2 (3), and Zn(HL)3Cl2 (4) with the title ligand, 5‐(pyrazin‐2‐yl)‐1,2,4‐triazole‐5‐thione (HL), and their characterization by elemental analyses, ESI‐MS (m/z), FT‐IR and UV/Vis spectroscopy, as well as EPR in the case of the CuII complex. The comparative analysis of IR spectra of the metal ion complexes with HL and HL alone indicated that the metal ions in 1, 2, and 3 are chelated by two nitrogen atoms, N(4) of pyrazine and N(5) of triazole in the thiol tautomeric form, whereas the ZnII ion in 4 is coordinated by the non‐protonated N(2) nitrogen atom of triazole in the thione form. pH potentiometry and UV/Vis spectroscopy were used to examine CoII, NiII, and ZnII complexes in 10/90 (v/v) DMSO/water solution, whereas the CuII complex was examined in 40/60 (v/v) DMSO/water solution. Monodeprotonation of the thione triazole in solution enables the formation of the L:M = 1:1 species with CoII, NiII and ZnII, the 2:1 species with CoII and ZnII, and the 3:1 species with ZnII. A distorted tetrahedral arrangement of the CuII complex was suggested on the basis of EPR and Vis/NIR spectra.

Facile total synthesis of lysicamine and the anticancer activities of the RuII, RhIII, MnII and ZnII complexes of lysicamine.

Lysicamine is a natural oxoaporphine alkaloid, which isolated from traditional Chinese medicine (TCM) herbs and has been shown to possess cytotoxicity to hepatocarcinoma cell lines. Reports on its antitumor activity are scarce because lysicamine occurs in plants at a low content. In this work, we demonstrate a facile concise total synthesis of lysicamine from simple raw materials under mild reaction conditions, and the preparation of the Ru(II), Rh(III), Mn(II) and Zn(II) complexes 1–4 of lysicamine (LY). All the compounds were fully characterized by elemental analysis, IR, ESI-MS, 1H and 13C NMR, as well as single-crystal X-ray diffraction analysis. Compared with the free ligand LY, complexes 2 and 3 exhibited superior in vitro cytotoxicity against HepG2 and NCI-H460. Mechanistic studies indicated that 2 and 3 blocked the cell cycle in the S phase by decreasing of cyclins A2/B1/D1/E1, CDK 2/6, and PCNA levels and increasing levels of p21, p27, p53 and CDC25A proteins. In addition, 2 and 3 induced cell apoptosis via both the caspase-dependent mitochondrial pathway and the death receptor pathway. in vivo study showed that 2 inhibited HepG2 tumor growth at 1/3 maximum tolerated dose (MTD) and had a better safety profile than cisplatin.

(Aminomethylene)phosphonate Analogues as ZnII Chelators: Synthesis and Characterization

A series of (aminomethylene)phosphonate (AMP) analogues, 8–14, bearing one or two heterocyclic moieties (imidazolyl, pyridyl, and thiazolyl) on the aminomethylene group, were synthesized as potential ZnII chelators. The complexes of analogues 8–14 with ZnII ions were characterized by their stoichiometry, geometry, coordination sites, acid/base equilibria, and stability constants. Analogues 8–14 form stable water‐soluble 2:1 L/ZnII complexes, as established by ZnII titration, monitored by UV/Vis spectrophotometry and by 1H and 31P NMR spectroscopy. Acidity and stability constants were established for each derivative by potentiometric pH titrations. ML2 type ZnII complexes of AMP, bearing either an imidazolyl or pyridyl moiety, 8, 10, and 12, exhibit high log β values (17.68, 16.92, and 16.65, respectively), while for the AMP‐thiazolyl (14) complex with ZnII, log β is 12.53. Generally, ligands 9, 11, and 13, bearing two heterocyclic moieties, present higher log β values (22.25, 21.00, and 18.28, respectively) vs. analogues bearing one heterocyclic moiety. Additionally, based on 1H, 13C, and 31P NMR spectroscopic data, we propose a structure of the AMP‐(Im)2‐ZnII complex in solution, where the ZnII coordination sites involve the phosphonate moiety and both imidazolyl rings of the two binding molecules, forming an octahedral geometry around the ZnII ion. In summary, we propose a new family of water‐soluble high‐affinity ZnII chelators, in particular AMP‐(Im)2, which forms the most stable complex (log β 22).

Bis(N'-{(E)-[(2E)-1,3-di-phenyl-prop-2-en-1-yl-idene]amino}-N-ethyl-carbamimido-thio-ato-κ2N',S)zinc(II): crystal structure and Hirshfeld surface analysis.

The title ZnII complex, [Zn(C18H18N3S)2], (I), features two independent but chemically equivalent mol-ecules in the asymmetric unit. In each, the thio-semicarbazonate monoanion coordinates the ZnII atom via the thiol-ate-S and imine-N atoms, with the resulting N2S2 donor set defining a distorted tetra-hedral geometry. The five-membered ZnSCN2 chelate rings adopt distinct conformations in each independent mol-ecule, i.e. one ring is almost planar while the other is twisted about the Zn-S bond. In the crystal, the two mol-ecules comprising the asymmetric unit are linked by amine-N-H⋯N(imine) and amine-N-H⋯S(thiol-ate) hydrogen bonds via an eight-membered heterosynthon, {⋯HNCN⋯HNCS}. The dimeric aggregates are further consolidated by benzene-C-H⋯S(thiol-ate) inter-actions and are linked into a zigzag supra-molecular chain along the c axis via amine-N-H⋯S(thiol-ate) hydrogen bonds. The chains are connected into a three-dimensional architecture via phenyl-C-H⋯π(phen-yl) and π-π inter-actions, the latter occurring between chelate and phenyl rings [inter-centroid separation = 3.6873 (11) Å]. The analysis of the Hirshfeld surfaces calculated for (I) emphasizes the different inter-actions formed by the independent mol-ecules in the crystal and the impact of the π-π inter-actions between chelate and phenyl rings.

Meso‐Cumulenic 2H‐Corroles from meso‐Ethynyl‐3H‐corroles

AbstractAlkynyl‐substituted 3H‐corrole 9 a was converted to [3]cumulenic 2H‐corrole 10 a by treatment with trimethylsilyl chloride (TMSCl), and 1,3‐butadiyne‐bridged 3H‐corrole dimer 11 b was transformed into [5]cumulene‐bridged 2H‐corrole dimer 12 b by oxidation with PbO2. Both 10 a and 12 b were metalated to form ZnII complexes 10 a‐Zn and 12 b‐Zn. The structures of 10 a‐Zn and 12 b‐Zn show planar conformations with bond‐length alternations that are analogous to those of tetraaryl [n]cumulenes. The cumulenic corrole dimers 12 b and 12 b‐Zn display large NIR absorption bands in the range of 700–1400 nm (maximum ϵ≈1.0×105 m−1 cm−1) owing to the effective π‐conjugation between the two corrole units through the [5]cumulene bridge.

Meso-Cumulenic 2H-Corroles from meso-Ethynyl-3H-corroles.

Alkynyl-substituted 3H-corrole 9 a was converted to [3]cumulenic 2H-corrole 10 a by treatment with trimethylsilyl chloride (TMSCl), and 1,3-butadiyne-bridged 3H-corrole dimer 11 b was transformed into [5]cumulene-bridged 2H-corrole dimer 12 b by oxidation with PbO2 . Both 10 a and 12 b were metalated to form ZnII complexes 10 a-Zn and 12 b-Zn. The structures of 10 a-Zn and 12 b-Zn show planar conformations with bond-length alternations that are analogous to those of tetraaryl [n]cumulenes. The cumulenic corrole dimers 12 b and 12 b-Zn display large NIR absorption bands in the range of 700-1400 nm (maximum ϵ≈1.0×105 m-1 cm-1 ) owing to the effective π-conjugation between the two corrole units through the [5]cumulene bridge.

Synthesis and Crystal Structures of an Unexpected Tetranuclear Zinc(II) Complex and a Benzoquinone Compound Derived from ZnII- and CdII-Promoted Reactivity of Schiff Base Ligands

An unexpected polyhydroxyl-bridged tetranuclear ZnII complex and a benzoquinone compound derived from metal-ion promoted reactivity of Schiff base ligands were synthesized and characterized. The reaction of zinc(II) acetate dihydrate with oxime-type Schiff base ligand HL1 [HL1 = 1-(3-((3,5-dibromosalicylaldehyde)amino)phenyl)ethan-1-one O-benzyl oxime] in methanol, acetone, and acetonitrile resulted in the chemoselective cleavage of the C=N bond of the Schiff base HL1, and then the further addition of acetone to two salicylaldehyde molecules derived from cleavage of the C=N bond in situ α,α double aldol reaction promoted by ZnII ions. The newly formed ligands H4L2 coordinate to four ZnII ions forming a defect-dicubane core structure [ZnII4(H2L2)2(μ3-OCH3)2(μ-OCH3)2(CH3OH)2] (1) bridged exclusively by oxygen-based ligands. The similar ligand HL3 [HL3 = 1-(3-((3,5-dichlorosalicylaldehyde)amino)phenyl)ethan-1-one O-benzyl oxime)] was employed to react with CdII acetate dihydrate under the same reaction conditions. No aldol addition occurred but a unexpected benzoquinone compound 2,5-bis(((3-(1-((benzyloxy)imino)ethyl)phenyl)imino)methyl)-1,4-benzoquinone (2) formed. The results provided interesting insights into one-pot routes involving in situ reactions act as a strategy for obtaining a variety of polymeric/polynuclear complexes which are inconvenient to obtain from directly presynthesizing the ligands.

Steric Enforcement about One Thiolate Donor Leads to New Oxidation Chemistry in a NiSOD Model Complex

Ni-containing superoxide dismutase (NiSOD) represents an unusual member of the SOD family due to the presence of oxygen-sensitive Ni-SCys bonds at its active site. Reported in this account is the synthesis and properties of the NiII complex of the N3S2 ligand [N3S2Me2]3- ([N3S2Me2]3- = deprotonated form of 2-((2-mercapto-2-methylpropyl)(pyridin-2-ylmethyl)amino)-N-(2-mercaptoethyl)acetamide), namely Na[Ni(N3S2Me2)] (2), as a NiSOD model that features sterically robust gem-(CH3)2 groups on the thiolate α-C positioned trans to the carboxamide. The crystal structure of 2, coupled with spectroscopic measurements from 1H NMR, X-ray absorption, IR, UV-vis, and mass spectrometry (MS), reveal a planar NiII (S = 0) ion coordinated by only the N2S2 basal donors of the N3S2 ligand. While the structure and spectroscopic properties of 2 resemble those of NiSODred and other models, the asymmetric S ligands open up new reaction paths upon chemical oxidation. One unusual oxidation product is the planar NiII-N3S complex [Ni(Lox)] (5; Lox = 2-(5,5-dimethyl-2-(pyridin-2-yl)thiazolidin-3-yl)-N-(2-mercaptoethyl)acetamide), where two-electron oxidation takes place at the substituted thiolate and py-CH2 carbon to generate a thiazolidine heterocycle. Electrochemical measurements of 2 reveal irreversible events wholly consistent with thiolate redox, which were identified by comparison to the ZnII complex Na[Zn(N3S2Me2)] (3). Although no reaction is observed between 2 and azide, reaction of 2 with superoxide produces multiple products on the basis of UV-vis and MS data, one of which is 5. Density functional theory (DFT) computations suggest that the HOMO in 2 is π* with primary contributions from Ni-dπ/S-pπ orbitals. These contributions can be modulated and biased toward Ni when electron-withdrawing groups are placed on the thiolate α-C. Analysis of the oxidized five-coordinate species 2ox* by DFT reveal a singly occupied spin-up (α) MO that is largely thiolate based, which supports the proposed NiIII-thiolate/NiII-thiyl radical intermediates that ultimately yield 5 and other products.

Octakis(2-pyridyl)porphyrazine and Its Neutral Metal Derivatives: UV-Visible Spectral, Electrochemical, and Photoactivity Studies.

Tetrapyrrolic porphyrazine macrocycles with externally appended 2-pyridyl rings were synthesized and characterized as to their spectroscopic, electrochemical, and photophysical properties. The investigated compounds are represented as [Py8PzH2], the unmetalated octakis(2-pyridyl)porphyrazine, and the metal complexes [Py8PzM], where M is MgII(H2O), CuII, ZnII, or CoII. The spectroscopic properties and the electrochemical behavior of these compounds were examined in solution of polar (pyridine, dimethyl sulfoxide, and dimethylformamide) and nondonor solvents (CHCl3, CH2Cl2), and the data were compared with those obtained from earlier studies on the related tetrapyrazinoporphyrazines, [Py8TPyzPzM], and the tetraquinoxalinoporphyrazines, [Py8QxPzM], also bearing externally 2-pyridyl rings, and characterized by a more extended central π-conjugated macrocyclic framework. The newly synthesized porphyrazines possess good solubility and are present in their monomeric form in all the examined solvents as shown by UV-visible spectra. The unmetalated species and metal derivatives undergo multiple one-electron reductions within the potential range of the nonaqueous examined solvents. The derivatives with nonredox active metal centers could accept four electrons on the conjugated macrocycle, while the CoII complex was characterized by a single one-electron oxidation and five reductions in DMSO. The photosensitizer activity for the generation of singlet oxygen was also examined for the MgII(H2O) and ZnII complexes in DMF, with measured ΦΔ values being, respectively, 0.42 and 0.64, this latter value indicating the ZnII species as being a promising material for use as anticancer agent in photodynamic therapy.

Effects of anion coordination on the fluorescence of a photo-induced electron transfer (PET) sensor complexed with metal ions

Whether π contacts between the fluorophore of the photoinduced electron transfer (PET) sensor adpa ((N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine) and complexed metal ions or coordinated halide ions control the fluorescence intensity of adpa is further investigated via fluorescence, absorbance and density functional theory (DFT) studies. Three metal-adpa complexes were studied and the crystal structures of [Cd(adpa)I2] (1), [Hg(adpa)I2] (2), [Zn(adpa)Cl2](H2O)2 (3) and [Cd(adpa)S2O3]2 (4) are reported. Unlike Cl−, Br−, SCN− and S2O32−, which cause an increase in fluorescence intensity upon binding to CdII(adpa) due to the disruption of metal-fluorophore π contacts, coordination of I− to the CdII(adpa) complex strongly decreases fluorescence intensity. The same trend was found with the addition of I− to the HgII(adpa) and ZnII(adpa) complexes. However, the causes of fluorescence quenching associated with three metal complexes are not necessarily the same. The structure of 3 shows that the anthracenyl fluorophore does not form a π contact with the Zn, relating to the strong CHEF (chelation enhanced fluorescence) effect ZnII shows with adpa. The absorbance study suggests that the fluorescence quenching of the ZnII(adpa) complex with I− addition is due to collisional quenching. Fluorescence quenching in the Hg(adpa)I2 complex is attributed to the long Hg–N bond of 2.788Å to the central N donor of the adpa in 2, which causes quenching by a PET effect from the weakly coordinated N donor. The quenching mechanism associated with the CdII(adpa)I2 complex seems to be more subtle. Structure 1 shows the nearest Cd⋯C distance to the anthracenyl fluorophore of adpa of 3.554Å, where such long distances are usually associated with increases in fluorescence intensity, rather than quenching. Density functional theory (DFT) studies showed that there is structural change in the excited state of the CdII(adpa)I2 complex, which triggers a reordering of frontier molecular orbitals. This leads to the involvement of charge transfer from the fluorophore to the iodine in the S1→S0 transition, which is forbidden.