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

Heterobimetallic compounds [L′OVV(μ-O)­MIIIL]n (n = 1, M = Mn, 1–5; n = 2, M = Fe, 6 and 7) containing a discrete unsupported VV–O–MIII 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 (1 and 3) or an octahedral (2 and 5) 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 1 to 5 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 (6 and 7) 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 (1–5) display three successive reductions (processes I–III), each with a monoelectron stoichiometry. Process I is due to a MnIII/MnII reduction (E1/2 ranges between −0.32 and −0.05 V), process II is a ligand-based reduction, and process III (E1/2 = ∼1.80 V) owes its origin to a VVO/VIVO reduction; all potentials are versus Ag/AgCl. The iron­(III) compounds (6 and 7), on the other hand, show at least four irreversible processes, appearing at Epc = −0.20, −1.0, −1.58, and −1.68 V in compound 6 (processes IV–VII), together with a reversible process (process VIII) at E1/2 = −1.80 V (ΔEp = 80 mV). While the first two of these are due to FeIII/FeII 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 VVO/VIVO-based electron transfer.