Homoleptic Mononuclear Tris‐Chelate Complexes of Fe^II, Co^II, Ni^II, and Zn^II Based on a Redox‐Active Imidazolyl‐2‐thione Ligand: Structural and Electrochemical Correlation

Abstract

Five mononuclear tris‐chelate complexes of FeII, CoII, NiII and ZnII, containing the bidentate ligand 3‐methyl‐1‐(2‐pyridyl)imidazolyl‐2‐thione (L), have been synthesised and characterised, namely [MII(L)3](BF4)2, where M = Fe (1), Co (2), Ni (3) and Zn (4), and [FeII(L)3](PF6)2 {1‐(PF6)2}. The complexes have been characterised by standard methods and single‐crystal X‐ray diffraction, showing that all the complexes are isostructural and isomorphs, crystallising in the P1 space group, except for 1‐(PF6)2, which crystallises in the P21/c, as expected due to the different size and symmetry of the anion. The metal centre is found in a distorted meridional‐N3S3 octahedral geometry. In all the complexes, two ligand strands within the same complex cation unit interact via an almost perfect face‐to‐face π–π stacking. The double intramolecular interaction occurs between a pyridyl ring of one ligand strand and an imidazolyl ring of a second ligand strand and vice versa. The FeII and CoII complexes, 1 and 2, are stabilised in the high spin state, based on the bond lengths and angles obtained by SC‐XRD and by VT‐magnetic studies in the solid state and NMR Evans method in CD3CN solution. The cyclic voltammetry behaviour for compounds 1–4 showed an EC mechanism for the oxidation process, in which two ligand strands are electrochemically oxidised, instead of the metal centre, followed by a chemical process (de‐coordination and dimerisation). Whereas, the reduction process involved metal electrodeposition for the FeII, NiII and ZnII complexes. In the case of the CoII complex, the voltammetric response suggested a multistep decomposition process during the electrochemical reduction. Finally, a linear correlation between the oxidation potential of the ligand and the average M–N bond length has been found, permitting the modification of the oxidation potential by selection of the metal centre.

Spin Crossover vs. High‐Spin Iron(II) Complexes in N4S2Coordination Sphere Containing Picolyl‐Thioether Ligands and NCE (E=S, Se and BH3) Co‐Ligands

“…At room temperature the complexes stabilized in the HS‐state show a μ eff value ranging from 4.9 to 5.5 μ B , although for the majority of complexes the value is higher than the expected μ S.O. =4.9 μ B , it is still in good agreement with the high spin state and suggests a considerable orbital contribution to g , [22,27,41] in addition the values are in agreement with the literature [20,42–44] . The room temperature μ eff value for complex 1 a containing the PySBn ligand (Figure 7) indicates that it is stabilized in the HS‐state, the μ eff value is maintained relatively constant down to 25 K, below this temperature zero field splitting (ZFS) is evident.…”

“…All the S‐based ligands contain the same thioether functional group, in combination with amine, pyridine, triazole or pyrazole nitrogen donors, and some of them contain NCE (E=S, Se or BH 3 ) co‐ligands. An advantage of synthesizing S‐containing iron(II) complexes is the protection toward oxidation of the metal centre, as it has been shown that some S‐based ligands are redox non‐innocent, [24–27] preventing the metal centre to oxidize to iron(III), thus ensuring the preservation of the SCO‐properties.…”

High spin iron(ii) complexes based on imidazolyl- and 1,2,3-triazolyl-thione ligands and NCE (E = S, Se or BH3) co-ligands: effect of the S-functional group on the structural and magnetic properties

“…At room temperature the m eff value ranges from 5.00 to 5.41 m B , although for the majority of complexes the value is higher than the expected m S.O. = 4.9 m B , it is still in good agreement with the high spin state and suggests orbital contribution to g, 22,42,43 in addition the values are in agreement with the literature. 6,9,44,45 The family of complexes containing the ImPyS ligand (1a-c, Fig.…”

“…To the best of our knowledge, there is no examples of first-row transition metal complexes containing more than one bidentate imidazolylthione ligand, except for a recent example of our group in which tris-chelate metal complexes of Fe II , Co II , Ni II and Zn II based on ImPyS ligand have been synthesised and characterised showing that this type of ligands are redox active and that the complex redox potential correlates to the M-N bond length, which in turn relates to the ionic radius of the metal ion, thus permitting to predict the complex redox potential. 22 On the other hand the magnetic properties of first row transition metal complexes based on imidazolylthione ligands have been scarcely investigated, 22,[25][26][27][28][29] whereas for 1,2,3-triazolylthione-based complexes there are no examples available, to the best of our knowledge.…”

Albumin Conjugates of Thiosemicarbazone and Imidazole‐2‐thione Prochelators: Iron Coordination and Antiproliferative Activity

“…The crystal structure of the iron complex of imidazole-2thione chelator IT1 (Figures 3, S4, Table S2) also presents FeÀ S and FeÀ N distances consistent with a ferric center as well as an overall cationic charge balanced by a tetrafluoroborate ion. Whereas several imidazole-2-thione ligands were found to stabilize ferrous ions, [31,32] the incorporation of this moiety in a tridentate mercaptobenzaldehyde hydrazone leads to a stable ferric complex. The two ligands coordinate in meridional fashion but again lack the planarity of a fully delocalized π system.…”