N−H···Cl<sub>2</sub>−M Synthon as a Structure-Directing Tool:  Crystal Structures of Some Perchlorometallates

A series of perchlorometallate salts, namely, [4,4‘-H<sub>2</sub>diazastilbene][PdCl<sub>4</sub>] <b>1</b>; [H<sub>2</sub>-<i>N</i>-(4-pyridyl)isonicotinamide][MCl<sub>4</sub>], M = Pt(II) <b>2</b>, M = Pd(II) <b>2a</b>; [H<sub>2</sub>-<i>N</i>,<i>N</i>‘-bis(4-pyridyl)urea][MCl<sub>4</sub>], M = Pt(II) <b>3</b>, M = Pd(II) <b>3a</b>; [H<sub>2</sub>-<i>N</i>-(3-pyridyl)isonicotinamide][MCl<sub>4</sub>], M = Pt(II) <b>4</b>, M = Pd(II) <b>4a</b>; [H<sub>2</sub>-<i>N</i>-(4-pyridyl)nicotinamide][PtCl<sub>4</sub>] <b>5</b>; [H<sub>2</sub>-<i>N</i>,<i>N</i>‘-bis(3-pyridyl)urea][PtCl<sub>4</sub>] <b>6</b>, 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···Cl<sub>2</sub>−M (synthon <b>A</b>). The results indicate that synthon <b>A</b> is indeed quite robust and reliable as structure-directing tool when the interacting cationic and anionic species are rigid. Structural parameters for synthon <b>A</b> indicate that the N−H···Cl<sub>2</sub>M bifurcated hydrogen-bonding moiety is generally asymmetric and a “face approach” is more preferred in salts (<b>4</b>−<b>6</b>) derived from dications having angular cationic topology, whereas a nearly “edge approach” is preferred in salts (<b>1</b>−<b>3a</b>) 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 <b>4</b>, 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.