Thermochemistry of a Cobalt Complex with Ionisable Pyrazole Protons

Herein, we present the thermodynamic analysis of a cobalt complex with a new pentadentate N‐donor ligand bearing four ionisable pyrazole protons in aqueous solution. A detailed analysis of the Co II complex [Co( L )(X)] +/2+ in the solid state revealed that the 6 th ligand X at the metal centre depends on the cobalt source employed. Small anions such as Cl – and NO 3 – coordinate to the metal ion, while larger anions that are weaker hydrogen‐bond acceptors are found in the second coordination sphere of the complex and instead a solvent molecule coordinates. However, in aqueous KCl solution, the sixth ligand is always chloride forming [Co( L )Cl]Cl, 1 Cl . pH dependent species distribution studies revealed a p K a of 7.3(3) for the first ionisable pyrazole proton in the cobalt(II) complex and 6.0(3) in the cobalt(III) complex (methanol/H 2 O mixture). That is the oxidation state has a fairly minor influence on the p K a of the pyrazole proton. The Co III /Co II redox pair of the complex with the fully protonated ligand exhibits a potential of 0.78 V vs. NHE. The BDFE of the hypothetical H‐atom abstraction step of [Co II ( L )Cl] + forming [Co III ( L H –1 )Cl] + was determined to equal 336 kJ mol –1 .