Spin States of the Cobalt(II) Complex with Bis(pyrazol-3-yl)pyridine and pH-Sensitive Functional Groups and Its Deprotonated Derivatives

The reaction of 2,6-bis(pyrazol-3-yl)pyridine H₂L bearing pH-sensitive imino groups with cobalt perchlorate hexahydrate followed by the addition of 1,8-diazabicyclo[5.4.0]undec-7-ene affords the cobalt(II) complex [Co(H₂L)₂](ClO₄)₂ (I) and its doubly deprotonated analog [Co(HL)₂]⁰ (II). The compounds are characterized by NMR spectroscopy, mass spectrometry, and XRD. The influence of the deprotonation of ligand H₂L with a chosen base on the spin state of the cobalt(II) ion in a solution is studied by in situ NMR spectroscopy. Complex I is shown to retain its high-spin state in the whole temperature range accessible in deuterated methanol (200–325 K) both before and after the deprotonation of all pH-sensitive imino groups. However, the doubly deprotonated form of complex I (complex II·2MeOH) exists in the crystal (CIF file CCDC no. 2351546) in the low-spin state, which is the first example of the pH-induced stabilization of this state for the cobalt(II) complexes with 2,6-bis(pyrazol-3-yl)pyridines.