PtII-Mediated Imine–Nitrile Coupling Leading to Symmetrical (1,3,5,7,9-Pentaazanona-1,3,6,8-tetraenato)Pt(II) Complexes Containing the Incorporated 1,3-Diiminoisoindoline Moiety

Treatment of trans-[PtCl2(NCR)2] (1; R = Et (1a), Ph (1b)) with 1,3-diiminoisoindoline (2) gives access to the corresponding symmetrical (1,3,5,7,9-pentaazanona-1,3,6,8-tetraenato)­Pt­(II) complexes [PtCl­{NHC­(R)­NC­(C6H4)­NCNC­(R)NH}] (3). The reactions of 1 with one equivalent of 1,1,3,3-tetramethylguanidine (4), 1,3-diphenylguanidine (6), or acetone oxime (8) leads to the formation of mixed asymmetrical Pt­(II) complexes trans-[PtCl2{NHC­(R)­NC­(NMe2)2}­(NCR)] (5), [PtCl­{NHC­(R)­NC­(NHPh)NPh}­(NCR)] (7), or trans-[PtCl2{NHC­(Ph)­ONCMe2}­(NCPh)] (9), respectively, as a result of nucleophilic addition to one of the nitrile ligands in 1. Treatment of 5, 7, and 9 with one equivalent of 2 leads to complexes 3. The complexes were characterized by IR, 1H, 13C­{1H}, and 195Pt NMR (for 3) spectroscopies, ESI+-MS, elemental analyses, and X-ray diffraction (for 3). Complex 3a has an asymmetric unit with five independent Pt molecules of the same chemical composition and two molecules of water, resulting in a total of 40 molecules of the complex and sixteen guest water molecules per unit cell. Theoretical calculations revealed that the most plausible mechanism of formation of complexes 3 includes stepwise nucleophilic addition of 2 to one of the nitrile ligands in 1, a first cyclization upon formation of the Pt–N bond and elimination of HCl, and a second nucleophilic addition/cyclization.