From oxido-bridged dimers to hydrogen-bonded assemblies: steric control of self-assembly in cis-dioxovanadium(V)...

cis-Dioxidovanadium(V) (cis-VO₂⁺) moieties are among the most fundamental structural units in vanadium(V) coordination chemistry, yet the factors governing their nuclearity, self-assembly, and crystallization remain poorly understood. Here, we investigate these factors in a family of cis-VO₂⁺ Schiff-base complexes derived from HSHED, the monoanionic coordinating form of N-salicylidene-N'-(2-hydroxyethyl)ethylenediamine, which has recently been identified as a precursor to complexes with potential activity against glioblastoma. The coordination environments were characterized in the solid state by single-crystal X-ray diffraction, in solution by multinuclear NMR and UV-visible spectroscopy, and by density functional theory calculations. Previously reported members of this family crystallize exclusively as oxido-bridged dinuclear complexes containing the characteristic covalent O-V-O-V diamond core. To investigate the factors controlling this aggregation, sterically modified ligands were designed to inhibit oxido-bridge formation. New crystal structures presented here include the oxido-bridged dimer [VO₂(3-OEt-5-Ad-HSHED)]₂ together with two members of this family with sterically hindered Schiff base scaffolds forming systems that undergo different self-assembly processes forming a tetramer, [VO₂(3-t-Bu-HSHED)]₄ and dimer, [VO₂(3,5-di-t-Bu-HSHED)]₂. The [VO₂(3-t-Bu-HSHED)]₄ crystallizes as a H-bonded tetranuclear assembly comprising four crystallographically independent mononuclear complexes. The assembly incorporates four distinct ligand-metal stereoisomers arising from both ligand centered stereochemistry and metal coordination chemistry differences while preserving the characteristic cis-VO₂⁺ coordination environment at each vanadium center. Rather than exhibiting strong stereochemical discrimination during crystallization, the assembly accommodates multiple stereochemical variants within a common H-bonding network. Complementary solution studies using ⁵¹V and ¹H NMR and UV-vis spectroscopy demonstrate that all the complexes form mononuclear species in solution. Collectively, these results show that self-assembly in this family is governed by a hierarchy of structural factors in which the preferred local coordination geometry of the cis-VO₂⁺ unit is strongly conserved and steric effects redirect aggregation from covalent O-V-O-V diamond core formation to H-bonded supramolecular assembly. The crystal structure demonstrates that multiple stereoisomers are incorporated within the same H-bonded assembly, suggesting that the coordination preferences of the cis-VO₂⁺ unit together with the H-bonding network outweigh stereochemical discrimination during crystallization.