Abstract
Tetranuclear complexes [M4(LR)4] with M = Ni(II) or Zn(II), with a [2 × 2] grid-type structure, were assembled in good yields and purity from the easily accessible but unprecedented pyrazine-bridged bis(thiosemicarbazone) protoligands (ligand precursors) H2LR (1,4-pyrazine-2,5-bis(R-carbaldehyde-thiosemicarbazone); R = Me, Et, iPr, or Ph). The complexes were characterised in solution by NMR, MS, IR, and UV-Vis absorption spectroscopy and (spectro)electrochemical methods. HR-MS spectra unequivocally reveal that the tetranuclear species are very stable in solution and any measurements represent these species. Only at higher temperatures (fragmentation in solution: MS and in the solid: TG-DTA) or upon the addition of protons (acidic UV-Vis titrations) can the tetrameric entities be decomposed. Single crystal XRD measurement remained preliminary. Rapid loss of co-crystallised solvent molecules within the [2 × 2] grid-type structures resulted in crystals of very poor quality, but the results were qualitatively in line with spectroscopy, electrochemistry, and quantum chemical (DFT) calculations. IR and NMR spectroscopy point clearly to a thiolate coordination of dianionic (deprotonated) ligands. The electrochemistry reveals four electronically coupled and reversible one-electron reductions centred largely at the pyrazine bridges. EPR and UV-Vis spectroelectrochemical measurements in combination with DFT calculation support the assignment.
Highlights
Thiosemicarbazons and their metal complexes are studied intensely since the 1950s, mainly for their interesting biological properties
For the Ni(II) complexes additional can be observed as very long-wavelength shoulders around 700 nm, they might be assigned to nickel(d) absorptions can be observed as very long‐wavelength shoulders around 700 nm, they might be to pyrazine(π*) metal-to-ligand charge transfer (MLCT) transitions
HR-MS spectra unequivocally reveal that the tetranuclear species are very stable and any measurements in solution represent these species
Summary
Thiosemicarbazons and their metal complexes are studied intensely since the 1950s, mainly for their interesting biological properties. The 1,4-pyrazine-bridged bis(thiosemicarbazone) protoligands (ligand precursor) H LR with R = Me (LMe), Et (LEt), iPr (LiPr), or Ph (LPh) used in this study (A); the dianionic ligands (LR)−. 1,4‐Pyrazine has previously shown to be a very suitable ligand to bridge two metal centres and provide effectivehas electronic coupling between atomsligand. Thewhile use of examples with [52,53,54], tetrazine [55,56],moiety and 1,4‐pyrazine [45,46,57,58,59,60] These s-triazine [43]imidazole and pyridazine as a central has been frequently reported, while examples tetranuclear complexes have found from their magneticareand redox with imidazole [55,56],interest and 1,4-pyrazine scarce.
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