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), 1 and 2; Re­(V), 3] all-oxido–metal
core have been synthesized following a single-pot protocol using compartmental
Schiff-base ligands, N,N′-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 1–3. Compound 1 crystallizes in a monoclinic space group P21/c, while both 2 and 3 have more symmetric structures with orthorhombic space group Pnma that renders the vanadium­(V) centers in these compounds
exactly identical. In DMF solution, compound 1 displays
an 8-line EPR at room temperature with ⟨g⟩
and ⟨A⟩ 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 2p3/2 and
2p1/2 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 1 and 2. Electrochemically, compound 1 undergoes
two one-electron oxidations at E
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 (3) as established by X-ray crystallography.