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.