a-[NH3(CH2)5NH3]SnI4: a new layered perovskite structure

Much attention has been devoted to the synthesis and characterization of organic–inorganic multilayered perovskites owing to the tunability of their structural features.1–5 The physical and structural properties of these compounds can be fine tuned by substituting or tailoring the organic layers or by modulating the thickness of the perovskite inorganic slabs. Applications of layered perovskite materials involve the development of functional electronic, magnetic and optoelectronic materials.2,6,7 Several examples of conducting layered perovskites generally exhibit a trend from semiconducting to metallic behavior with increasing ‘thickness’ of perovskite layers (n).8 This was demonstrated with the synthesis of a family of organic-based layered halide perovskites, (C4H9NH3)2(CH3NH3)n2 1SnnI3n + 1. Layered perovskite structures can be derived by terminating the three dimensional cubic perovskite structure along different crystal faces.9 Among the variety of metal oxides with layered perovskite structures, the common terminating planes are < 001 > , < 110 > and < 111 > . In layered organic–inorganic perovskites the ability to control the termination of perovskite sheets through the proper choice of organic cations was demonstrated by the synthesis of [NH2C(I)NNH2]2(CH3NH3)m2 1SnmI3m + 2. In contrast to the (C4H9NH3)2(CH3NH3)n2 1SnnI3n + 1 family, which consists of < 001 > -terminated perovskite layers, this structural class of Sn(ii) iodides contains perovskite layers terminated along the < 110 > planes. Furthermore, the family of < 110 > -terminated Sn(ii) iodide layered perovskites were found to be more conducting than their < 001 > -terminated analogs. This strongly suggests that termination of the perovskite layers provides an added flexibility in tuning the electronic properties of these layered materials. Another structural element that modulates the properties of these mica-like materials is the dimensionality of the organic layers. Reducing the organic bilayers in the above mentioned perovskites into single layers may be achieved by using a,wsubstituted diamines, thereby increasing the interlayer coupling within the material.1c,3b Eliminating hydrophobic interactions in the organic bilayers leads to improved crystallization and mechanical strength while maintaining the two-dimensionality of the electronic structure. Herein, we report the synthesis and crystal structure of a-[NH3(CH2)5NH3]SnI4 which consists of single layers of 1,5-pentanediammonium ions and an unprecedented inorganic layered perovskite. a-[NH3(CH2)5NH3]SnI4 1 Was prepared by reacting SnI2 (2 mmol), CH3NH3I (1 mmol) and [NH3(CH2)5NH3]I2 (1 mmol) in concentrated HI (3 ml) solution. The HI solution of the organic and tin iodides was prepared at room temperature and heated to 120 °C. The resulting red solution was slowly cooled to 20 °C resulting in the formation of dark-red prismatic-column crystals. The dark-red air stable crystals were preserved in the mother liquor and were found to be of compound 1. As a general precaution, all reactions were carried out under a nitrogen atmosphere and all solvents were degassed before use. The addition of CH3NH3I in the synthesis of 1 is crucial. Analogous reactions without CH3NH3I result in the formation of an orange– red polymorph, b-[NH3(CH2)5NH3]SnI4, which features the commonly observed < 001 > -terminated [SnI4] single perovskite sheets, as in K2NiF4. The ‘templating’ role of CH3NH3 is presumably related to the stabilization and formation of the perovskite structure, as in CH3NH3SnI3, and allows for its termination into layers along different crystallographic planes. Chemical analysis on several crystals of 1 confirmed the stoichiometry obtained from the structural refinement. Incorporation of CH3NH3I results in the formation of higher order layered perovskites [NH3(CH2)5NH3](CH3NH3)xSn1 + xI3x + 1 (x = 1–3).10 IR spectra recorded from thin disks of a-[NH3(CH2)5NH3]SnI4, in the range 500–4000 cm21, feature characteristic IR bands (C–H at 3000–3180 and 1455 cm21, and N–H at 3430–3500 cm21) associated with 1,5-pentanediammonium ions. Measurement of the UV absorption spectra of a thin pressed disk of 1, in the range 200–800 nm at 30 °C, shows a sharp absorption peak centered at 571 nm, in contrast to that observed at 560 nm in < 001 > -terminated b[NH3(CH2)5NH3]SnI4. Thermal measurements indicate that compound 1 and b-[NH3(CH2)5NH3]SnI4, melt incongruently at 167 and 190 °C, respectively. Compound 1 crystallizes in the orthorhombic space group Pbcn.11 The asymmetric unit consists of three SnI4 units (part of a layer of corner-sharing octahedra), and three [NH3(CH2)5NH3] ions. The orthorhombic crystal structure contains organic diammonium cations sandwiched between parallel corrugated sheets, [SnI2I4/2], of corner-shared tin(ii) iodide octahedra. The tin layers can be built-up from SnI6 octahedra linked via corners into nominal zigzag chains, in the manner –cis–trans–trans–cis–trans–trans–, along the crystallographic a-axis. Adjacent parallel chains are further linked through their trans-vertices along the b-axis forming single corrugated [SnI4] sheets along the ab-plane. The corrugated sheets are stacked along the crystallographic c-axis resulting in interlayer sites defined by eight terminal iodine atoms. The 1,5-pentanediammonium ions are located within the nearly orthorhombic interlayer sites forming single organic layers in contrast to the bilayers found in the monoamine compounds, (RNH3)2SnI4. The [SnI4] layer in compound 1 may be described as a combination of the < 001 > and < 110 > terminations and hence considered as the first member of the family of < 330 > -terminated layered perovskites.9a To our knowledge, the remarkable mixed cis–trans < 330 > -terminated perovskite layers is unprecedented in metal oxide and halide perovskite structures. The unprecedented inorganic sheets feature two types of SnI6 octahedra, namely, transand cis-shared, as shown in Fig. 1. The relatively long Sn–I bonds, 3.464(1) Å, in the cis-iodine bridges reflect the significant steric effects present in the cis conformation of the SnI6 octahedra. The bond angles in both types of SnI6 octahedra are slightly distorted from the ideal geometry as expected in low-order layered perovskite structures.4,5,12 These bond angle distortions and the Sn–I bond distance inequalities are reflected in the ‘twists’ and ‘buckles’ of the inter-octahedral linkages and may be attributed to hydrogen-bonding and the stereochemical activity of the Sn(ii) 5s2 lone pairs.12