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.
Grotjahn
Douglas B
DB
Department of Chemistry, 5500 Campanile Drive, San Diego State University, San Diego, CA 92182-1030, USA. grotjahn@chemistry.sdsu.edu
Van
Sang
S
Combs
David
D
Lev
Daniel A
DA
Schneider
Christian
C
Incarvito
Christopher D
CD
Lam
Kim-Chung
KC
Rossi
Gene
G
Rheingold
Arnold L
AL
Rideout
Marc
M
Meyer
Christoph
C
Hernandez
Genaro
G
Mejorado
Lupe
L