Conformation-Associated C<middle dot><middle dot><middle dot>dz2-PtII Tetrel Bonding: The Case of Cyclometallated Platinum(II) Complex with 4-Cyanopyridyl Urea Ligand

The nucleophilic addition of 3-(4-cyanopyridin-2-yl)-1,1-dimethylurea (1) to cis-[Pt(CNXyl)(2)Cl-2] (2) gave a new cyclometallated compound 3. It was characterized by NMR spectroscopy (H-1, C-13, Pt-195) and high-resolution mass spectrometry, as well as crystallized to obtain two crystalline forms (3 and 3<middle dot>2MeCN), whose structures were determined by X-ray diffraction. In the crystalline structure of 3, two conformers (3A and 3B) were identified, while the structure 3<middle dot>2MeCN had only one conformer 3A. The conformers differed by orientation of the N,N-dimethylcarbamoyl moiety relative to the metallacycle plane. In both crystals 3 and 3<middle dot>2MeCN, the molecules of the Pt(II) complex are associated into supramolecular dimers, either {3A}(2) or {3B}(2), via stacking interactions between the planes of two metal centers, which are additionally supported by hydrogen bonding. The theoretical consideration, utilizing a number of computational approaches, demonstrates that the C<middle dot><middle dot><middle dot>d(z)(2)(Pt) interaction makes a significant contribution in the total stacking forces in the geometrically optimized dimer [3A](2) and reveals the d(z)(2)(Pt)->pi*(PyCN) charge transfer (CT). The presence of such CT process allowed for marking the C<middle dot><middle dot><middle dot>Pt contact as a new example of a rare studied phenomenon, namely, tetrel bonding, in which the metal site acts as a Lewis base (an acceptor of noncovalent interaction).