Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes.
Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.
Department of Chemistry, 5500 Campanile Drive, San Diego State University, San Diego, CA 92182-1030, USA. grotjahn@chemistry.sdsu.edu
Douglas B
DB
Grotjahn
Sang
S
Van
David
D
Combs
Daniel A
DA
Lev
Christian
C
Schneider
Christopher D
CD
Incarvito
Kim-Chung
KC
Lam
Gene
G
Rossi
Arnold L
AL
Rheingold
Marc
M
Rideout
Christoph
C
Meyer
Genaro
G
Hernandez
Lupe
L
Mejorado