Paramagnetic Resonance of Cobalt(II) Trispyrazolylmethanes and Counterion Association

Paramagnetic resonance studies (EPR, ESEEM, ENDOR, and NMR) of a series of cobalt(II) bis-trispyrazolylmethane tetrafluoroborates are presented. The complexes studied include the parent, unsubstituted ligand (Tpm), two pyrazole-substituted derivatives (4Me and 3,5-diMe), and tris(1-pyrazolyl)ethane (Tpe), which includes a methyl group on the apical carbon atom. NMR and ENDOR establish the magnitude of ¹H hyperfine couplings, while ESEEM provides information on the coordinated ¹⁴N. The data show that the pyrazole 3-position is more electron rich in the Tpm analogues, that the geometry about the apical atom influences the magnetic resonance, and that apical atom geometry appears more fixed in Tpm than in Tp. NMR and ENDOR establish that the BF₄^- counterion remains associated in fluid solution. In the case of the Tpm^3,5Me complex, it appears to associate in solution, in the same position it occupies in the X-ray structure.