SYNTHESIS AND CHARACTERIZATION OF [NH2C(I)=NH2](2)ASNI(5) 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)=NH2](2)ASnI(5) (A = NH2C(I)=NH2+ and NH2CH=NH2+) have been grown in an inert atmosphere from slowly cooled aqueous hydriodic acid solutions, each with the sa me starting concentration of dissolved cyanamide and tin(II) iodide. E ach compound can be achieved in high purity and yield simply by varyin g the thermal history of the solution during cooling. If one starts fr om lower temperatures, cyanamide rapidly undergoes an addition reactio n to generate the iodoformamidinium cation, yielding crystals of [NH2C (I)=NH2](3)SnI5. At higher temperatures, iodoformamidinium is reduced to formamidinium and is ultimately hydrolyzed if heated further or lef t for long periods of time. When the solution is subjected to an inter mediate thermal treatment of 24 h at 80 degrees C before slow cooling, both cations can be stabilized in the solid-state compound [NH2C(I)=N H2](2)(NH2CH=NH2)SnI5. The [NH2C(I)=NH2](3)SnI5 structure has recently been reported in the monoclinic space group P2(1)/c and consists of h ighly distorted SnI6 octahedra which share opposite corners to form ex tended one-dimensional chains separated by iodoformamidinium cations. The new mixed cation compound adopts a very similar triclinic (P1) str ucture, with the lattice parameters a = 6.3635(3) Angstrom, b = 8.8737 (5) Angstrom, c = 10.8782(6) Angstrom, alpha = 111.616(5)Angstrom, bet a = 92.938(4)degrees, gamma =95.358(4)degrees, 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 darke r coloration for crystals with A = formamidinium relative to those hav ing A iodoformamidinium (dark red versus orange). In addition to exami ning the stability of cyanamide and derivative species in aqueous hydr iodic acid solutions, thermal analysis indicates that in an inert atmo sphere the iodoformamidinium cation decomposes exothermally in the sol id state at approximately 178(2) degrees C. Ultraviolet illumination s tudies on the title compounds also demonstrate the photosensitivity of the iodoformamidinium cation within the tin(II) iodide framework.