Tuning the oxidation state and magnetic and coordination behaviour of iron and cobalt complexes by O/S variation in mono-thio and dithio-oxamide chelating ligands

New iron and cobalt complexes coordinated with the ligands Me2pipto (N,N′-dimethyl-piperazine-3-oxo-2-thione) and Me2pipdt (N,N′-dimethyl-piperazine-2,3-dithione, S,S′), differing in one sulphur substituting the oxygen atom, have been prepared and characterized. The reaction with Me2pipto and iron salts affords the heteroleptic [FeIII(Me2pipto)2Cl2]+ or the homoleptic [FeII(Me2pipto)3]2+ cationic complexes (isolated as tetrafluoroborate salts 1 and 2) depending on the employed iron source: FeCl3 or Fe2(SO4)3 respectively. The corresponding reaction with CoCl2 as the metal source allowed us to obtain [CoII(Me2pipto)3](BF4)2 (3). By reacting FeCl3, Me2pipto and KSCN in the molar ratio 1 : 1 : 4, [FeIII(Me2pipto)(NCS)4]− is obtained and isolated as a Ph4P+ salt (4). Upon using the Me2pipdt ligand, the reaction with FeCl3 affords [FeII(Me2pipdt)3]2+ which is isolated as a tetrafluoroborate salt (5). The same cation is found in the polyiodide salt [FeII(Me2pipdt)3](I3)1.8(I)0.2 (6) obtained by reacting iron-metal powders with Me2pipdt and I2 mixtures. Through a similar reaction by using cobalt-metal powders, [CoIII(Me2pipdt)3]2(I3)2(I)4·2I2 (7) is obtained. Structural results show that in all these compounds the metal ions are in a pseudo-octahedral coordination geometry and that bond distances are consistent with the presence of iron(III) in 1 and 4 and iron(II) in 2 and cobalt(III) in 7. Magnetic susceptibility measurements show that the metals are in a high spin state in all Me2pipto complexes and a low spin state in Me2pipdt. The observed differences are relatable to the different σ-donor and π-acceptor capabilities of the ligands tuned by the S and O donor atoms. Results from DFT calculations using B3LYP and OLYP as functionals are in agreement with the observed magnetic behaviour of the complexes.