N−H···Cl₂−M Synthon as a Structure-Directing Tool:  Crystal Structures of Some Perchlorometallates

A series of perchlorometallate salts, namely, [4,4‘-H₂diazastilbene][PdCl₄] 1; [H₂-N-(4-pyridyl)isonicotinamide][MCl₄], M = Pt(II) 2, M = Pd(II) 2a; [H₂-N,N‘-bis(4-pyridyl)urea][MCl₄], M = Pt(II) 3, M = Pd(II) 3a; [H₂-N-(3-pyridyl)isonicotinamide][MCl₄], M = Pt(II) 4, M = Pd(II) 4a; [H₂-N-(4-pyridyl)nicotinamide][PtCl₄] 5; [H₂-N,N‘-bis(3-pyridyl)urea][PtCl₄] 6, have been synthesized and analyzed by single-crystal X-ray diffraction to study the frequency of occurrence, robustness, and reliability (as structure directing tools) of the bifurcated hydrogen bonding of the type N−H···Cl₂−M (synthon A). The results indicate that synthon A is indeed quite robust and reliable as structure-directing tool when the interacting cationic and anionic species are rigid. Structural parameters for synthon A indicate that the N−H···Cl₂M bifurcated hydrogen-bonding moiety is generally asymmetric and a “face approach” is more preferred in salts (4−6) derived from dications having angular cationic topology, whereas a nearly “edge approach” is preferred in salts (1−3a) derived from dications having linear cationic topology. Hydrogen-bonding interactions of the type N/C−H···Pt are present in all the Pt salts except in 4, whereas no such interactions are observed in the cases of Pd salts. The metal center of the anionic moiety seems to have a profound effect on the molecular geometry of the cationic species as well as on the overall supramolecular architecture of the salts.