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

Abstract

The redox-active fac-[Mo^V(mp)₃]⁻ (mp: o-mercaptophenolato) bearing asymmetric O- and S-cation binding sites can bind with several kinds of metal ions such as Na⁺, Mn^II, Fe^II, Co^II, Ni^II, and Cu^I. The fac-[Mo^V(mp)₃]⁻ metalloligand coordinates to Na⁺ to form the contact ion pair {Na⁺(THF)₃[fac-Mo^V(mp)₃]} (1), while a separated ion pair, n-Bu₄N[fac-Mo^V(mp)₃] (2), is obtained by exchanging Na⁺ with n-Bu₄N⁺. In the presence of asymmetric binding-sites, the metalloligand reacts with Mn^IICl₂·4H₂O, Fe^IICl₂·4H₂O, Co^IICl₂·6H₂O, and Ni^IICl₂·6H₂O to afford UV-vis-NIR spectra, indicating binding of these guest metal cations. Especially, for the cases of the Mn^II and Co^II products, trinuclear complexes, {M(H₂O)(MeOH)[fac-Mo^V(mp)₃]₂}·1.5CH₂Cl₂ (3·1.5CH₂Cl₂ (M = Mn^II), 4·1.5CH₂Cl₂ (M = Co^II)), are successfully isolated and structurally characterized where the M are selectively bound to the hard O-binding sites of the fac-[Mo^V(mp)₃]⁻. On the other hand, a coordination polymer, {Cu^I(CH₃CN)[mer-Mo^V(mp)₃]}_n (5), is obtained by the reaction of fac-[Mo^V(mp)₃]⁻ with [Cu^I(CH₃CN)₄]ClO₄. In sharp contrast to the cases of 1, 3·1.5CH₂Cl₂, and 4·1.5CH₂Cl₂, the Cu^I in 5 are selectively bound to the soft S-binding sites, where each Cu^I is shared by two [Mo^V(mp)₃]⁻ with bidentate or monodentate coordination modes. The second notable feature of 5 is found in the geometric change of the [Mo^V(mp)₃]⁻, where the original fac-form of 1 is isomerized to the mer-[Mo^V(mp)₃]⁻ in 5, which was structurally and spectroscopically characterized for the first time. Such isomerization demonstrates the structural flexibility of the [Mo^V(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.