Synthesis and Characterization of NH2C(I)NH22ASnI5 with A = Iodoformamidinium or Formamidinium:  The Chemistry of Cyanamide and Tin(II) Iodide in Concentrated Aqueous Hydriodic Acid Solutions

Crystals of the organic−inorganic compounds NH2C(I)NH22ASnI5 (A = NH2C(I)NH2+ and NH2CHNH2+) have been grown in an inert atmosphere from slowly cooled aqueous hydriodic acid solutions, each with the same starting concentration of dissolved cyanamide and tin(II) iodide. Each compound can be achieved in high purity and yield simply by varying the thermal history of the solution during cooling. If one starts from lower temperatures, cyanamide rapidly undergoes an addition reaction to generate the iodoformamidinium cation, yielding crystals of NH2C(I)NH23SnI5. At higher temperatures, iodoformamidinium is reduced to formamidinium and is ultimately hydrolyzed if heated further or left for long periods of time. When the solution is subjected to an intermediate thermal treatment of 24 h at 80 °C before slow cooling, both cations can be stabilized in the solid-state compound NH2C(I)NH22(NH2CHNH2)SnI5. The NH2C(I)NH23SnI5 structure has recently been reported in the monoclinic space group P21/c and consists of highly distorted SnI6 octahedra which share opposite corners to form extended one-dimensional chains separated by iodoformamidinium cations. The new mixed-cation compound adopts a very similar triclinic (P1) structure, with the lattice parameters a = 6.3635(3) Å, b = 8.8737(5) Å, c = 10.8782(6) Å, α = 111.616(5)°, β = 92.938(4)°, γ = 95.358(4)°, and Z = 1 and with much less distorted SnI6 octahedra and shorter average Sn−I bond lengths. The different local coordination translates into a substantially darker coloration for crystals with A = formamidinium relative to those having A = iodoformamidinium (dark red versus orange). In addition to examining the stability of cyanamide and derivative species in aqueous hydriodic acid solutions, thermal analysis indicates that in an inert atmosphere the iodoformamidinium cation decomposes exothermally in the solid state at approximately 178(2) °C. Ultraviolet illumination studies on the title compounds also demonstrate the photosensitivity of the iodoformamidinium cation within the tin(II) iodide framework.