Targeted Synthesis of Heterobimetallic Compounds Containing a Discrete Vanadium(V)−μ-Oxygen–Iron(III) Core

Heterobimetallic compounds [L¹OV^VO→Fe­(metsalophen)­(H₂O)] (<b>1</b>) and [L²OV^VO→Fe­(metsalophen)­(H₂O)]­CH₃CN (<b>2</b>), where H₂L¹ and H₂L² are tridentate dithiocarbazate-based Schiff base ligands, containing a discrete V^V–μ-O–Fe^III angular core have been synthesized for the first time through a targeted synthesis route: confirmation in favor of such a heterobimetallic core structure has come from single-crystal X-ray diffraction analysis and electrospray ionization mass spectrometry

Targeted Synthesis of Heterobimetallic Compounds Containing a Discrete Vanadium(V)−μ-Oxygen–Iron(III) Core

Heterobimetallic compounds [L¹OV^VO→Fe­(metsalophen)­(H₂O)] (<b>1</b>) and [L²OV^VO→Fe­(metsalophen)­(H₂O)]­CH₃CN (<b>2</b>), where H₂L¹ and H₂L² are tridentate dithiocarbazate-based Schiff base ligands, containing a discrete V^V–μ-O–Fe^III angular core have been synthesized for the first time through a targeted synthesis route: confirmation in favor of such a heterobimetallic core structure has come from single-crystal X-ray diffraction analysis and electrospray ionization mass spectrometry

Homo- and Heterometal Complexes of Oxido–Metal Ions with a Triangular [V(V)O–MO–V(V)O] [M = V(IV) and Re(V)] Core: Reporting Mixed-Oxidation Oxido–Vanadium(V/IV/V) Compounds with Valence Trapped Structures

A new family of trinuclear homo- and heterometal complexes with a triangular [V­(V)­O–MO–V­(V)­O] (M = V­(IV), <b>1</b> and <b>2</b>; Re­(V), <b>3</b>] all-oxido–metal core have been synthesized following a single-pot protocol using compartmental Schiff-base ligands, <i>N</i>,<i>N</i>′-bis­(3-hydroxysalicylidene)-diiminoalkanes/arene (H₄L¹–H₄L³). The upper compartment of these ligands with N₂O₂ donor combination (Salen-type) contains either a V­(IV) or a Re­(V) center, while the lower compartment with O₄ donor set accommodates two V­(V) centers, stabilized by a terminal and a couple of bridging methoxido ligands. The compounds have been characterized by single-crystal X-ray diffraction analyses, which reveal octahedral geometry for all three metal centers in <b>1</b>–<b>3</b>. Compound <b>1</b> crystallizes in a monoclinic space group <i>P</i>2₁/<i>c</i>, while both <b>2</b> and <b>3</b> have more symmetric structures with orthorhombic space group <i>Pnma</i> that renders the vanadium­(V) centers in these compounds exactly identical. In DMF solution, compound <b>1</b> displays an 8-line EPR at room temperature with ⟨<i>g</i>⟩ and ⟨<i>A</i>⟩ values of 1.972 and 86.61 × 10^–4 cm^–1, respectively. High-resolution X-ray photoelectron spectrum (XPS) of this compound shows a couple of bands at 515.14 and 522.14 eV due to vanadium 2p_3/2 and 2p_1/2 electrons in the oxidation states +5 and +4, respectively. All of these, together with bond valence sum (BVS) calculation, confirm the trapped-valence nature of mixed-oxidation in compounds <b>1</b> and <b>2</b>. Electrochemically, compound <b>1</b> undergoes two one-electron oxidations at <i>E</i> _1/2 = 0.52 and 0.83 V vs Ag/AgCl reference. While the former is due to a metal-based V­(IV/V) oxidation, the latter one at higher potential is most likely due to a ligand-based process involving one of the catecholate centers. A larger cavity size in the upper compartment of the ligand H₄L³ is spacious enough to accommodate Re­(V) with larger size to generate a rare type of all-oxido heterotrimetallic compound (<b>3</b>) as established by X-ray crystallography

Heterobimetallic μ‑Oxido Complexes Containing Discrete V^V–O–M^III (M = Mn, Fe) Cores: Targeted Synthesis, Structural Characterization, and Redox Studies

Heterobimetallic compounds [L′OV^V(μ-O)­M^IIIL]_<i>n</i> (<i>n</i> = 1, M = Mn, <b>1</b>–<b>5</b>; <i>n</i> = 2, M = Fe, <b>6</b> and <b>7</b>) containing a discrete unsupported V^V–O–M^III bridge have been synthesized through a targeted synthesis route. In the V–O–Mn-type complexes, the vanadium­(V) centers have a square-pyramidal geometry, completed by a dithiocarbazate-based tridentate Schiff-base ligand (H₂L′), while the manganese­(III) centers have either a square-pyramidal (<b>1</b> and <b>3</b>) or an octahedral (<b>2</b> and <b>5</b>) geometry, made up of a Salen-type tetradentate ligand (H₂L) as established by X-ray diffraction analysis. The V–O–Mn bridge angle in these compounds varies systematically from 155.3° to 128.1° in going from <b>1</b> to <b>5</b> while the corresponding dihedral angle between the basal planes around the metal centers changes from 86.82° to 20.92°, respectively. The V–O–Fe-type complexes (<b>6</b> and <b>7</b>) are tetranuclear, in which the two dinuclear V­(μ-O)­Fe units are connected together by apical iron­(III)–aryl oxide interactions, forming a dimeric structure with a pair of Fe–O–Fe bridges. The X-ray data also confirm the VO → M canonical form to contribute predominantly on the overall V–O–M bridge structure. The molecules in solution also retain their heterobinuclear composition, as established by electrospray ionization mass spectrometry and ⁵¹V NMR spectroscopy. Electrochemically, these complexes are quite interesting; the manganese­(III) complexes (<b>1</b>–<b>5</b>) display three successive reductions (processes I–III), each with a monoelectron stoichiometry. Process I is due to a Mn^III/Mn^II reduction (<i>E</i>_1/2 ranges between −0.32 and −0.05 V), process II is a ligand-based reduction, and process III (<i>E</i>_1/2 = ∼1.80 V) owes its origin to a V^VO/V^IVO reduction; all potentials are versus Ag/AgCl. The iron­(III) compounds (<b>6</b> and <b>7</b>), on the other hand, show at least four irreversible processes, appearing at <i>E</i>_pc = −0.20, −1.0, −1.58, and −1.68 V in compound <b>6</b> (processes IV–VII), together with a reversible process (process VIII) at <i>E</i>_1/2 = −1.80 V (Δ<i>E</i>_p = 80 mV). While the first two of these are due to Fe^III/Fe^II reductions at the two iron­(III) centers of these tetranuclear cores, the reversible reduction at a more negative potential (ca. −1.80 V) is due to a V^VO/V^IVO-based electron transfer

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)

Homo- and Heterometal Complexes of Oxido–Metal Ions with a Triangular [V(V)O–MO–V(V)O] [M = V(IV) and Re(V)] Core: Reporting Mixed-Oxidation Oxido–Vanadium(V/IV/V) Compounds with Valence Trapped Structures

A new family of trinuclear homo- and heterometal complexes with a triangular [V­(V)­O–MO–V­(V)­O] (M = V­(IV), <b>1</b> and <b>2</b>; Re­(V), <b>3</b>] all-oxido–metal core have been synthesized following a single-pot protocol using compartmental Schiff-base ligands, <i>N</i>,<i>N</i>′-bis­(3-hydroxysalicylidene)-diiminoalkanes/arene (H₄L¹–H₄L³). The upper compartment of these ligands with N₂O₂ donor combination (Salen-type) contains either a V­(IV) or a Re­(V) center, while the lower compartment with O₄ donor set accommodates two V­(V) centers, stabilized by a terminal and a couple of bridging methoxido ligands. The compounds have been characterized by single-crystal X-ray diffraction analyses, which reveal octahedral geometry for all three metal centers in <b>1</b>–<b>3</b>. Compound <b>1</b> crystallizes in a monoclinic space group <i>P</i>2₁/<i>c</i>, while both <b>2</b> and <b>3</b> have more symmetric structures with orthorhombic space group <i>Pnma</i> that renders the vanadium­(V) centers in these compounds exactly identical. In DMF solution, compound <b>1</b> displays an 8-line EPR at room temperature with ⟨<i>g</i>⟩ and ⟨<i>A</i>⟩ values of 1.972 and 86.61 × 10^–4 cm^–1, respectively. High-resolution X-ray photoelectron spectrum (XPS) of this compound shows a couple of bands at 515.14 and 522.14 eV due to vanadium 2p_3/2 and 2p_1/2 electrons in the oxidation states +5 and +4, respectively. All of these, together with bond valence sum (BVS) calculation, confirm the trapped-valence nature of mixed-oxidation in compounds <b>1</b> and <b>2</b>. Electrochemically, compound <b>1</b> undergoes two one-electron oxidations at <i>E</i> _1/2 = 0.52 and 0.83 V vs Ag/AgCl reference. While the former is due to a metal-based V­(IV/V) oxidation, the latter one at higher potential is most likely due to a ligand-based process involving one of the catecholate centers. A larger cavity size in the upper compartment of the ligand H₄L³ is spacious enough to accommodate Re­(V) with larger size to generate a rare type of all-oxido heterotrimetallic compound (<b>3</b>) as established by X-ray crystallography

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

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

Tetranuclear Hetero-Metal [Co^II₂Ln^III₂] (Ln = Gd, Tb, Dy, Ho, La) Complexes Involving Carboxylato Bridges in a Rare μ₄–η²:η² Mode: Synthesis, Crystal Structures, and Magnetic Properties

A new family of 3d–4f heterometal 2 × 2 complexes [Co^II₂(L)₂(PhCOO)₂Ln^III₂(hfac)₄] (<b>1</b>–<b>5</b>) (Ln = Gd (compound <b>1</b>), Tb (compound <b>2</b>), Dy (compound <b>3</b>), Ho (compound <b>4</b>), and La (compound <b>5</b>)) have been synthesized in moderate yields (48–63%) following a single-pot protocol using stoichiometric amounts (1:1 mol ratio) of [Co^II(H₂L)­(PhCOO)₂] (H₂L = <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-bis­(2-hydroxy-3,5-dimethylbenzyl)­ethylenediamine) as a metalloligand and [Ln^III(hfac)₃(H₂O)₂] (Hhfac = hexafluoroacetylacetone) as a lanthanide precursor compound. Also reported with this series is the Zn–Dy analog [Zn^II₂(L)₂(PhCOO)₂Dy^III₂(hfac)₄] <b>6</b> to help us in understanding the magnetic properties of these compounds. The compounds <b>1</b>–<b>6</b> are isostructural. Both hexafluoroacetylacetonate and benzoate play crucial roles in these structures as coligands in generating a tetranuclear core of high thermodynamic stability through a self-assembly process. The metal centers are arranged alternately at the four corners of this rhombic core, and the carboxylato oxygen atoms of each benzoate moiety bind all of the four metal centers of this core in a rare μ₄–η²:η² bridging mode as confirmed by X-ray crystallography. The magnetic susceptibility and magnetization data confirm a paramagnetic behavior, and no remnant magnetization exists in any of these compounds at vanishing magnetic field. The metal centers are coupled in an antiferromagnetic manner in these compounds. The [Co^II₂Dy^III₂] compound exhibits a slow magnetic relaxation below 6 K, as proven by the AC susceptibility measurements; the activation energy reads <i>U</i>/<i>k</i>_B = 8.8 K (τ₀ = 2.0 × 10^–7 s) at <b>B</b>_DC = 0, and <i>U</i>/<i>k</i>_B = 7.8 K (τ₀ = 3.9 × 10^–7 s) at <b>B</b>_DC = 0.1 T. The [Zn^II₂Dy^III₂] compound also behaves as a single-molecule magnet with <i>U</i>/<i>k</i>_B = 47.9 K and τ₀ = 2.75 × 10^–7 s