Coordination site-dependent cation binding and multi-responsible redox properties of Janus-head metalloligand, [MoV(1,2-mercaptophenolato)3]

The redox-active fac-[MoV(mp)3]− (mp: o-mercaptophenolato) bearing asymmetric O- and S-cation binding sites can bind with several kinds of metal ions such as Na+, MnII, FeII, CoII, NiII, and CuI. The fac-[MoV(mp)3]− metalloligand coordinates to Na+ to form the contact ion pair {Na+(THF)3[fac-MoV(mp)3]} (1), while a separated ion pair, n-Bu4N[fac-MoV(mp)3] (2), is obtained by exchanging Na+ with n-Bu4N+. In the presence of asymmetric binding-sites, the metalloligand reacts with MnIICl2·4H2O, FeIICl2·4H2O, CoIICl2·6H2O, and NiIICl2·6H2O to afford UV-vis-NIR spectra, indicating binding of these guest metal cations. Especially, for the cases of the MnII and CoII products, trinuclear complexes, {M(H2O)(MeOH)[fac-MoV(mp)3]2}·1.5CH2Cl2 (3·1.5CH2Cl2 (M = MnII), 4·1.5CH2Cl2 (M = CoII)), are successfully isolated and structurally characterized where the M are selectively bound to the hard O-binding sites of the fac-[MoV(mp)3]−. On the other hand, a coordination polymer, {CuI(CH3CN)[mer-MoV(mp)3]}n (5), is obtained by the reaction of fac-[MoV(mp)3]− with [CuI(CH3CN)4]ClO4. In sharp contrast to the cases of 1, 3·1.5CH2Cl2, and 4·1.5CH2Cl2, the CuI in 5 are selectively bound to the soft S-binding sites, where each CuI is shared by two [MoV(mp)3]− with bidentate or monodentate coordination modes. The second notable feature of 5 is found in the geometric change of the [MoV(mp)3]−, where the original fac-form of 1 is isomerized to the mer-[MoV(mp)3]− in 5, which was structurally and spectroscopically characterized for the first time. Such isomerization demonstrates the structural flexibility of the [MoV(mp)3]−. Spectroscopic studies strongly indicate that the association/dissociation between the guest metal ions and metalloligand can be modulated by solvent polarity. Furthermore, it was also found that such association/dissociation features are significantly influenced by coexisting anions such as ClO4− or B(C6F5)4−. This suggests that coordination bonds between the guest metal ions and metalloligand are not too static, but are sufficiently moderate to be responsive to external environments. Moreover, electrochemical data of 1 and 3·1.5CH2Cl2 demonstrated that guest metal ion binding led to enhance electron-accepting properties of the metalloligand. Our results illustrate the use of a redox-active chalcogenolato complex with a simple mononuclear structure as a multifunctional metalloligand that is responsive to chemical and electrochemical stimuli.