Heterobimetallic
μ‑Oxido Complexes Containing
Discrete V<sup>V</sup>–O–M<sup>III</sup> (M = Mn, Fe)
Cores: Targeted Synthesis, Structural Characterization, and Redox
Studies
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Abstract
Heterobimetallic compounds [L′OV<sup>V</sup>(μ-O)M<sup>III</sup>L]<sub><i>n</i></sub> (<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<sup>V</sup>–O–M<sup>III</sup> 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 (H<sub>2</sub>L′), 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 (H<sub>2</sub>L) 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 VO → 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 <sup>51</sup>V 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<sup>III</sup>/Mn<sup>II</sup> reduction (<i>E</i><sub>1/2</sub> ranges between −0.32 and −0.05 V), process
II is a ligand-based reduction, and process III (<i>E</i><sub>1/2</sub> = ∼1.80 V) owes its origin to a V<sup>V</sup>O/V<sup>IV</sup>O 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><sub>pc</sub> = −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><sub>1/2</sub> = −1.80 V (Δ<i>E</i><sub>p</sub> = 80 mV). While the first two of these are due to Fe<sup>III</sup>/Fe<sup>II</sup> 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<sup>V</sup>O/V<sup>IV</sup>O-based
electron transfer