Coupled High Spin Co II Ions Linked by Symmetrical Double Hydrogen Bonds: Role of a Slowly Relaxing Cu II Impurity in Interrupting the Co II –Co II Exchange Interaction

Co II and Cu II ions are two paramagnetic transition metal ions showing different relaxation rates ( ν ), with ν Co II >> ν Cu II . To measure the isotropic exchange constant ( J ) between high spin Co II ions and between Cu II ions and high spin Co II ions, we performed magnetic and EPR measurements complemented with computational calculations on pure trans ‐diaqua‐bis(picolinato‐ N , O )‐cobalt(II) dihydrate ( 1 ), on the Cu II ion doped compound of 1 (2) , and on the Cu II ion doped compound in a Zn II matrix isomorphous to 1 . The temperature dependence of the Cu II EPR resonance lines of 2 induced by the fast relaxing Co II ion host as well as the evaluation of the temperature‐independent Co II –Co II and Cu II –Co II isotropic exchange interaction are analyzed. We determined an antiferromagnetic interaction [ J Co–Co = –1.07(1) cm –1 ] associated with a double symmetrical hydrogen bond bridge and another one [| J | Co–Cu = 0.0015(2) cm –1 ] associated with a double but asymmetrical hydrogen bond bridge, which are in line with the values obtained from computational calculations. This work shows that EPR spectroscopy can advantageously be used to evaluate weak exchange interactions between distinct metal ions with different relaxation rates by using the fact that those metal–metal interactions that broaden the EPR resonance line, which are described by matrices with a trace of zero, are averaged out at high temperatures.