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

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