Treatment of the Ru(II) precursor cis,cis,trans-[RuCl2(dmso-S)2(PTA)2] (1, PTA=1,3,5-triaza-7-phosphaadamantane) with 2,2’-bipyridine (bpy) in refluxing ethanol selectively affords cis,cis-[Ru(bpy)Cl2(PTA)2] (2), whereas with pyridine (py), under the same conditions, it gives trans,cis,cis-[RuCl2(PTA)2(py)2] (6). The slightly less stable stereoisomer of 2, cis,trans-[Ru(bpy)Cl2(PTA)2] (3), is obtained selectively through a different
synthetic route. Isomers 2 and 3 are thermally stable, but cleanly equilibrate upon irradiation of an aqueous solution of either one with blue light. Intrigued by the stereoisomeric outcome in the preparations of this homogeneous set of complexes, we also investigated 2, 3, and 6 (and the monopyridine complex trans,mer-[RuCl2(py)(PTA)3] (7)) through a topological analysis of the electron density map using the quantum theory of atoms in molecules (QTAIM). The wealth of acquired experimental and calculated data allow us to discuss the stereochemical preferences of the [RuCl2(PTA)2(2 L)] complexes (2 L=bpy or 2py) in terms of electronic and steric contributions. The results of this speculative study on model
complexes are transferable to similar systems. As an example, our findings from the reactivity of 1 towards pyridine allowed us to prepare the 2+2 pyridylporphyrin metallacycle trans,cis,cis-[RuCl2(PTA)2(4’-cisDPyP)]2 (10, 4’-cisDPyP=5,10-(4’-pyridyl)-15,20-(phenyl)-porphyrin), whose X-ray molecular structure is also reported.
