Triple-Stranded Helicates of Zinc(II) and Cadmium(II) Involving a New Redox-Active Multiring Nitrogenous Heterocyclic Ligand: Synthesis, Structure, and Electrochemical and Photophysical Properties

The protonated form [H₂(L)]­(CF₃SO₃)₂ (<b>1</b>) of a new redox-active bis-bidentate nitrogenous heterocyclic ligand, viz., 3,3′-dipyridin-2-yl­[1,1′]­bi­[imidazo­[1,5-<i>a</i>]­pyridinyl] (L), and its zinc­(II) and cadmium­(II) complexes (<b>2</b> and <b>3</b>) have been synthesized and characterized by single-crystal X-ray diffraction analysis. In the solid state, both <b>2</b> and <b>3</b> have triple-stranded helical structures involving ligands that experience twisting and bending to the extent needed by the stereoelectronic demand of the central metal ion. The metal centers in the zinc­(II) complex [Zn₂(L)₃]­(ClO₄)₄ (<b>2</b>) are equivalent, each having a distorted octahedral geometry, flattened along the <i>C</i>₃ axis with a Zn1···Zn1# separation of 4.8655(13) Å. The cadmium complex [Cd₂(L)₃(H₂O)]­(ClO₄)₄ (<b>3</b>), on the other hand, has a rare type of helical structure, showing coordination asymmetry around the metal centers with a drastically reduced Cd1···Cd2 separation of 4.070 Å. The coordination environment around Cd1 is a distorted pentagonal bipyramid involving a N₆O donor set with the oxygen atom coming from a coordinated water, leaving the remaining metal center Cd2 with a distorted octahedral geometry. The structures of <b>2</b> and <b>3</b> also involve anion−π- and CH−π-type noncovalent interactions that play dominant roles in shaping the extended structures of these molecules in the solid state. In solution, these compounds exhibit strong fluxional behavior, making the individual ligand strands indistinguishable from one another, as revealed from their ¹H NMR spectra, which also provide indications about these molecules retaining their helical structures in solution. Electrochemically, these compounds are quite interesting, undergoing ligand-based oxidations in two successive one-electron steps at <i>E</i>_1/2 of ca. 0.65 and 0.90 V versus a Ag/AgCl (3 M NaCl) reference. These molecules are all efficient emitters in the red and blue regions because of ligand-based π*−π fluorescent emissions, tuned appropriately by the attached Lewis acid centers

Triple-Stranded Helicates of Zinc(II) and Cadmium(II) Involving a New Redox-Active Multiring Nitrogenous Heterocyclic Ligand: Synthesis, Structure, and Electrochemical and Photophysical Properties

The protonated form [H₂(L)]­(CF₃SO₃)₂ (<b>1</b>) of a new redox-active bis-bidentate nitrogenous heterocyclic ligand, viz., 3,3′-dipyridin-2-yl­[1,1′]­bi­[imidazo­[1,5-<i>a</i>]­pyridinyl] (L), and its zinc­(II) and cadmium­(II) complexes (<b>2</b> and <b>3</b>) have been synthesized and characterized by single-crystal X-ray diffraction analysis. In the solid state, both <b>2</b> and <b>3</b> have triple-stranded helical structures involving ligands that experience twisting and bending to the extent needed by the stereoelectronic demand of the central metal ion. The metal centers in the zinc­(II) complex [Zn₂(L)₃]­(ClO₄)₄ (<b>2</b>) are equivalent, each having a distorted octahedral geometry, flattened along the <i>C</i>₃ axis with a Zn1···Zn1# separation of 4.8655(13) Å. The cadmium complex [Cd₂(L)₃(H₂O)]­(ClO₄)₄ (<b>3</b>), on the other hand, has a rare type of helical structure, showing coordination asymmetry around the metal centers with a drastically reduced Cd1···Cd2 separation of 4.070 Å. The coordination environment around Cd1 is a distorted pentagonal bipyramid involving a N₆O donor set with the oxygen atom coming from a coordinated water, leaving the remaining metal center Cd2 with a distorted octahedral geometry. The structures of <b>2</b> and <b>3</b> also involve anion−π- and CH−π-type noncovalent interactions that play dominant roles in shaping the extended structures of these molecules in the solid state. In solution, these compounds exhibit strong fluxional behavior, making the individual ligand strands indistinguishable from one another, as revealed from their ¹H NMR spectra, which also provide indications about these molecules retaining their helical structures in solution. Electrochemically, these compounds are quite interesting, undergoing ligand-based oxidations in two successive one-electron steps at <i>E</i>_1/2 of ca. 0.65 and 0.90 V versus a Ag/AgCl (3 M NaCl) reference. These molecules are all efficient emitters in the red and blue regions because of ligand-based π*−π fluorescent emissions, tuned appropriately by the attached Lewis acid centers

Ligand-Induced Tuning of the Oxidase Activity of μ‑Hydroxidodimanganese(III) Complexes Using 3,5-Di-<i>tert</i>-butylcatechol as the Substrate: Isolation and Characterization of Products Involving an Oxidized Dioxolene Moiety

Oxidase activities of a μ-hydroxidodimanganese­(III) system involving a series of tetradentate capping ligands H₂L^R₁,R₂ with a pair of phenolate arms have been investigated in the presence of 3,5-di-<i>tert</i>-butylcatechol (H₂DBC) as a coligand cum-reductant. The reaction follows two distinctly different paths, decided by the substituent combinations (R₁ and R₂) present in the capping ligand. With the ligands H₂L^{<i>t</i>‑Bu,<i>t</i>‑Bu} and H₂L^{<i>t</i>‑Bu,OMe}, the products obtained are semiquinonato compounds [Mn^III(L^{<i>t</i>‑Bu,<i>t</i>‑Bu})­(DBSQ)]·2CH₃OH (<b>1</b>) and [Mn^III(L^{<i>t</i>‑Bu,OMe})­(DBSQ)]·CH₃OH (<b>2</b>), respectively. In the process, molecular oxygen is reduced by two electrons to generate H₂O₂ in the solution, as confirmed by iodometric detection. With the rest of the ligands, viz., H₂L^Me,Me, H₂L^{<i>t</i>‑Bu,Me}, H₂L^{Me,<i>t</i>‑Bu}, and H₂L^Cl,Cl, the products initially obtained are believed to be highly reactive quinonato compounds [Mn^III(L^R₁,R₂)­(DBQ)]⁺, which undergo a domino reaction with the solvent methanol to generate products of composition [Mn^III(L^R₁,R₂)­(BMOD)] (<b>3</b>–<b>6</b>) involving a nonplanar dioxolene moiety, viz., 3,5-di-<i>tert</i>-butyl-3-methoxy-6-oxocyclohexa-1,4-dienolate (BMOD^–). This novel dioxolene derivative is formed by a Michael-type nucleophilic 1,4-addition reaction of the methoxy group to the coordinated quinone in [Mn^III(L^R₁,R₂)­(DBQ)]⁺. During this reaction, molecular oxygen is reduced by four electrons to generate water. The products have been characterized by single-crystal X-ray diffraction analysis as well as by spectroscopic methods and magnetic measurements. Density functional theory calculations have been made to address the observed influence of the secondary coordination sphere in tuning the two-electron versus four-electron reduction of dioxygen. The semiquinone form of the dioxolene moiety is stabilized in compounds <b>1</b> and <b>2</b> because of extended electron delocalization via participation of the appropriate metal orbital(s)

Nonoxido Vanadium(IV) Compounds Involving Dithiocarbazate-Based Tridentate ONS Ligands: Synthesis, Electronic and Molecular Structure, Spectroscopic and Redox Properties

A new series of nonoxido vanadium­(IV) compounds [VL₂] (L = L¹–L³) (<b>1</b>–<b>3</b>) have been synthesized using dithiocarbazate-based tridentate Schiff-base ligands H₂L¹–H₂L³, containing an appended phenol ring with a <i>tert</i>-butyl substitution at the 2-position. The compounds are characterized by X-ray diffraction analysis (<b>1</b>, <b>3</b>), IR, UV-vis, EPR spectroscopy, and electrochemical methods. These are nonoxido V^IV complexes that reveal a rare distorted trigonal prismatic arrangement around the “bare” vanadium centers. Concerning the ligand isomerism, the structure of <b>1</b> and <b>3</b> can be described as intermediate between <i>mer</i> and <i>sym-fac</i> isomers. DFT methods were used to predict the geometry, <b>g</b> and ⁵¹V <b>A</b> tensors, electronic structure, and electronic absorption spectrum of compounds <b>1</b>–<b>3</b>. Hyperfine coupling constants measured in the EPR spectra can be reproduced satisfactorily at the level of theory PBE0/VTZ, whereas the wavelength and intensity of the absorptions in the UV-vis spectra at the level CAM-B3LYP/gen, where “gen” is a general basis set obtained using 6-31+g­(d) for S and 6-31g for all the other elements. The results suggest that the electronic structure of <b>1</b>–<b>3</b> can be described in terms of a mixing among V-<i>d</i>_<i>xy</i>, V-<i>d</i>_<i>xz</i>, and V-<i>d</i>_<i>yz</i> orbitals in the singly occupied molecular orbital (SOMO), which causes a significant lowering of the absolute value of the ⁵¹V hyperfine coupling constant along the <i>x</i>-axis. The cyclic voltammograms of these compounds in dichloroethane solution display three one-electron processes, two in the cathodic and one in the anodic potential range. Process A (<i>E</i>_1/2 = +1.06 V) is due to the quasi-reversible V­(IV/V) oxidation while process B at <i>E</i>_1/2 = −0.085 V is due to the quasi-reversible V­(IV/III) reduction, and the third one (process C) at a more negative potential <i>E</i>_1/2 = −1.66 V is due to a ligand centered reduction, all potentials being measured vs Ag/AgCl reference

Pentanuclear 3d–4f Heterometal Complexes of M^II₃Ln^III₂ (M = Ni, Cu, Zn and Ln = Nd, Gd, Tb) Combinations: Syntheses, Structures, Magnetism, and Photoluminescence Properties

A new family of pentanuclear 3d–4f heterometal complexes of general composition [Ln^III₂(M^IIL)₃(μ₃-O)₃H]­(ClO₄)·<i>x</i>H₂O (<b>1</b>–<b>5</b>) [Ln = Nd, M = Zn, <b>1</b>; Nd, Ni, <b>2</b>; Nd, Cu, <b>3</b>; Gd, Cu, <b>4</b>; Tb, Cu, <b>5</b>] have been synthesized in moderate yields (50–60%) following a self-assembly reaction involving the hexadentate phenol-based ligand, viz., <i>N</i>,<i>N</i>-bis­(2-hydroxy-3-methoxy-5-methylbenzyl)-<i>N</i>^′,<i>N</i>^′-diethylethylenediamine (H₂L). Single-crystal X-ray diffraction analyses have been used to characterize these complexes. The compounds are all isostructural, having a 3-fold axis of symmetry that passes through the 4f metal centers. The [M^IIL] units in these complexes are acting as bis-bidentate metalloligands and, together with μ₃-oxido bridging ligands, complete the slightly distorted monocapped square antiprismatic nine-coordination environment around the 4f metal centers. The cationic complexes also contain a H⁺ ion that occupies the central position at the 3-fold axis. Magnetic properties of the copper­(II) complexes (<b>3</b>–<b>5</b>) show a changeover from antiferromagnetic in <b>3</b> to ferromagnetic 3d–4f interactions in <b>4</b> and <b>5</b>. For the isotropic Cu^II–Gd^III compound <b>4</b>, the simulation of magnetic data provides very weak Cu–Gd (<i>J</i>₁ = 0.57 cm^–1) and Gd–Gd exchange constants (<i>J</i>₂ = 0.14 cm^–1). Compound <b>4</b> is the only member of this triad, showing a tail of an out-of-phase signal in the ac susceptibility measurement. A large-spin ground state (<i>S</i> = 17/2) and a negative value of <i>D</i> (−0.12 cm^–1) result in a very small barrier (8 cm^–1) for this compound. Among the three Nd^III₂M^II₃ (M = Zn^II, Ni^II, and Cu^II) complexes, only the Zn^II analogue (<b>1</b>) displays an NIR luminescence due to the ⁴F_3/2 → ⁴I_11/2 transition in Nd^III when excited at 290 nm. The rest of the compounds do not show such Nd^III/Tb^III-based emission. The paramagnetic Cu^II and Ni^II ions quench the fluorescence in <b>2</b>–<b>5</b> and thereby lower the population of the triplet state

Dinuclear Iron(III) and Cobalt(III) Complexes Featuring a Biradical Bridge: Their Molecular Structures and Magnetic, Spectroscopic, and Redox Properties

Bis-bidentate ligand H₄L^<i>B</i> featuring two <i>o</i>-amidophenol noninnocent units was used to synthesize novel binuclear complexes [(L^<i>R</i>)­M^III(•L^<i>B</i>•)­M^III(L^<i>R</i>)]­(ClO₄)₂, M = Fe (<b>1</b>) and Co (<b>2</b>, <b>3</b>), with HL^<i>R</i> (R = CH₃, Cl) being the facially coordinating tetradentate coligands. Upon the synthesis, the fully reduced amidophenolate form of the ligand (L^<i>B</i>)^4– becomes oxidized, resulting in the formation of a rare example of a biradical (•L^<i>B</i>•)^2– bridge connecting two metal ions, as supported by X-ray crystallography. The electronic structures of the complexes have been probed by Mössbauer spectroscopy, magnetic susceptibility measurements, and electron paramagnetic resonance (EPR) spectroscopy. Species <b>1</b> contains two high-spin Fe­(III) ions (<i>S</i> = 5/2) each coupled strongly antiferromagnetically (|<i>J</i>| > 150 cm^–1; <b>Ĥ</b> = −<i>2J</i><b>Ŝ</b>₁<b>Ŝ</b>₂) with a semiquinone π-radical (<i>S</i> = 1/2) form of the bridging (•L^<i>B</i>•)^2– ligand. The effective <i>S</i> = 2 spins of each [Fe­(III)+R^●] monomeric unit are then weakly ferromagnetically coupled with <i>J</i> = +0.22 cm^–1. Species <b>2</b> and <b>3</b> reveal very similar electronic structures: the low-spin Co­(III) ion is diamagnetic, which leaves the two-spin carriers at the bridging (•L^<i>B</i>•)^2– biradical to display an isotropic EPR signal at <i>g</i> = 1.995 for <b>2</b> (1.993 for <b>3</b>) in solution at room temperature and in the frozen state with no hyperfine structure. The weak half-field signal at <i>g</i> = 3.988 for <b>2</b> (3.978 for <b>3</b>) was also observed at 17 K for the spin-forbidden |Δ<i>M</i>_S| = 2 transition due to ferromagnetically coupled <i>S</i> = 1/2 spins (<i>J</i> = +47 cm^–1) of the bridging biradical. The compounds show rich electrochemistry, displaying two (<b>1</b>) or four (<b>2</b>, <b>3</b>) one-electron reversible processes. Normal and differential pulse voltammetry as well as constant potential coulometry, combined with EPR experiments, confirmed that the observed electron transfers are all localized at the bridging noninnocent (•L^<i>B</i>•)^2– ligand