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Pyrazolate‐Based Dinucleating Ligands in L 2 M 2 Scaffolds: Effects of Bulky Substituents and Coligands on Structures and M···H−C Interactions
Author(s) -
Röder Jens C.,
Meyer Franc,
Kaifer Elisabeth,
Pritzkow Hans
Publication year - 2004
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.200300778
Subject(s) - chemistry , isopropyl , metal , ligand (biochemistry) , aryl , crystallography , nickel , ion , stereochemistry , metal ions in aqueous solution , electron paramagnetic resonance , palladium , medicinal chemistry , nuclear magnetic resonance , organic chemistry , alkyl , catalysis , biochemistry , receptor , physics
A series of nickel( II ) and palladium( II ) complexes [L 2 M 2 ] 2+ have been prepared and structurally characterized, where L is a pyrazolate ligand with bulky 2,6‐dimethyl‐ or 2,6‐di(isopropyl)anilinomethyl side arms. Coordinating counter anions such as chloride can bind to axial sites of the dinickel species in a solvent‐dependent process, giving rise to five‐coordinate high‐spin metal ions. In the case of weakly coordinating anions, the metal ions are found in roughly square‐planar environments, and the structures are governed by the tendency of the bulky aryl groups to avoid each other, which forces the methyl or isopropyl substituents in the aryl 2‐ and 6‐positions to approach the metal ions from the axial directions. This leads to drastic low‐field shifts of the respective 1 H NMR signals, e.g. δ = 7.86 ppm for the isopropyl −CH which comes in close proximity to the low‐spin nickel( II ) center. The relevance of such low‐field NMR resonances of protons close to the axial sites of d 8 metal ions for possible three‐center four‐electron M ··· H−C hydrogen bonds involving the filled d z 2orbital of the metal ion is discussed. In the present case, attractive M ··· H interactions are assumed to be of no major significance. This was corroborated by the structure of a further [L 2 Ni 2 ] 2+ type complex where the anilinomethyl side arms bear only a single 2‐isopropyl group, which was found rotated away from the metal. Additional spectroscopic and electrochemical properties of the various complexes are reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)