Tetracyanometalates of Ni, Pd, and Pt with Cyclic Diquaternary Cations of 2,2‘-Bipyridine and 1,10-Phenanthroline. A Vibrational, Crystallographic, and Theoretical Study of Intermolecular Weak Interactions

The compounds (bpy-dq)[Ni(CN)₄]·2H₂O (1), (bpy-dq)[Pd(CN)₄]·2H₂O (2), and ((bpy-dq)[Pt(CN)₄]·2H₂O (3) (bpy-dq = (C₁₂H₁₂N₂)²⁺, 6,7-dihydrodipyrido[1,2-a:2‘,1‘-c]pyrazinediium) and (phen-dq)[Ni(CN)₄] (4), (phen-dq)[Pd(CN)₄]·H₂O (5), and (phen-dq)[Pt(CN)₄]·H₂O (6) (phen-dq = (C₁₄H₁₂N₂)²⁺, 5,6-dihydropyrazino[1,2,3,4-lmn]-1,10-phenanthrolinediium) have been synthesized and characterized by X-ray diffraction. The three bipyridinium diquaternary salt derivatives are isostructural. The crystal structures of these dihydrated compounds consist of columns formed by alternating anion complexes and diquaternary cations, π−π interacting through cyanide ligands and the aromatic rings, and stabilized by an extended hydrogen-bond network. On the other hand, the packing in the phenanthrolinium diquaternary salt derivatives is strongly dependent on the hydration degree. Thus, the anhydrous [Ni(CN)₄]^2- compound presents a laminar arrangement and the hydrated salts show a columnar packing, similar but not the same as compounds 1−3. The anhydrous form of compound 5 is isostructural with compound 4. Vibrational (IR, Raman) and thermogravimetric studies of these compounds have been carried out. Finally, DFT calculations have been performed on the isolated tetracyanometalate anions and diquaternary cations to assign the fundamental modes in the vibrational spectra. The intermolecular weak interactions were studied through the analysis of the charge density by using the theory of atoms in molecules (AIM).