Synthesis and Structure of Isomeric Palladium(II)−Pyrazole Chelate Complexes with and without an N−H Group as Hydrogen Bond Donor

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Abstract
Four new ligands containing a pyrazole ring and either a phosphine or thioether were prepared and converted to their cis-dichloropalladium(II) complexes. Two of the ligands are especially notable for the attachment of a side chain at pyrazole carbon, rather than at nitrogen. The new metal complexes include dichloro[3-(diphenylphosphinomethyl)pyrazole]palladium(II) (1-PdCl2) and dichloro[3-(methylthiomethyl)pyrazole]palladium(II) (2-PdCl2), which both feature an N−H group as a potential proton or hydrogen bond donor. For comparison, isomeric complexes lacking an NH group were prepared: dichloro[1-(diphenylphosphinomethyl)pyrazole]palladium(II) (3-PdCl2) and dichloro[1-(methylthiomethyl)pyrazole]palladium(II) (4-PdCl2). As determined by X-ray crystallography, all four complexes were found to have slightly distorted square planar geometry. Complexes 1-PdCl2 and 2-PdCl2, which contain an NH group, exhibit both intermolecular and intramolecular hydrogen bonding, whereas isomers 3-PdCl2 and 4-PdCl2 do not. Single-crystal X-ray structure determinations on the following compounds are reported: 1-PdCl2, space group P1̄, a = 8.4488(9) Å, b = 8.9175(13) Å, c = 12.731(2) Å, Z = 2, V = 871.8(2) Å3; 2-PdCl2, space group Pbca, a = 10.8827(10) Å, b = 11.7721(7) Å, c = 14.874(2) Å, Z = 8, V = 1905.6 Å3; 3-PdCl2, space group P21/c, a = 20.520(2) Å, b = 12.549(2) Å, c = 13.9784(13) Å, Z = 8, V = 3401.1(6) Å3; 4-PdCl2, space group Pbca, a = 10.6545(10) Å, b = 12.0205(11) Å, c = 14.6474(14) Å, Z = 8, V = 1875.9(3) Å3.