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Thermochemistry of a Cobalt Complex with Ionisable Pyrazole Protons
Author(s) -
Wilken Mona,
Würtele Christian,
Kügler Merle,
Chrobak Frank,
Siewert Inke
Publication year - 2018
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201800347
Subject(s) - chemistry , cobalt , pyrazole , protonation , aqueous solution , ligand (biochemistry) , coordination sphere , inorganic chemistry , metal , crystallography , hydrogen bond , molecule , medicinal chemistry , stereochemistry , ion , organic chemistry , biochemistry , receptor
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 .