Strongly coloured thiocyanate frameworks with perovskite-analogue structures

Inorganic Metal Thiocyanates

Metal thiocyanates were some of the first pseudohalide compounds to be discovered and adopt a diverse range of structures. This review describes the structures, properties, and syntheses of the known binary and ternary metal thiocyanates. It provides a categorization of their diverse structures and connects them to the structures of atomic inorganic materials. In addition to this description of characterized binary and ternary thiocyanates, this review summarizes the state of knowledge for all other binary metal thiocyanates. It concludes by highlighting opportunities for future materials development.

Selective Crystallization of Linkage Isomers, [RhIII(NCS)(SCN)5]3- and [RhIII(SCN)6]3-, to Investigate Structural Trans Influence and Thermal Stability

Linkage isomers of homoleptic complexes, [RhIII(SCN)6]3- and [RhIII(NCS)(SCN)5]3-, formed in aqueous solution were successfully separated by employing methyltriphenylphosphonium (MePPh3+) and 1-ethylquinolinium (EtQu+) ions as countercations, respectively. The single-crystal X-ray analysis of (MePPh3)3[RhIII(SCN)6] (1) indicated that all of the SCN- ligands coordinate to the RhIII ion by S atoms with an octahedral symmetry, where the average bond length of Rh-S is 2.374(7) Å. On the other hand, the RhIII ion of (EtQu)3[RhIII(NCS)(SCN)5]·H2O (2) is coordinated by five S atoms and one N atom of the SCN- ligands with a C4v symmetry. Structural trans influence was observed in the shorter bond length of Rh-S at the trans position of Rh-N. The Rh-S bond length is 2.3398(13) Å significantly shorter than those of 1 by ca. 0.04 Å, although DFT calculations based on the crystal structures indicated that the effective bond order of Rh-N is higher than those of Rh-S. Thermal stability examination by thermogravimetric and differential thermal analyses (TG/DTA) and IR spectroscopy indicated that the linkage isomerization of [RhIII(SCN)6]3- to [RhIII(NCS)(SCN)5]3- proceeded after melting around 174 °C. These results clearly indicate that [RhIII(NCS)(SCN)5]3- is thermodynamically more stable than [RhIII(SCN)6]3- in solid states, although further linkage isomerization hardly occurs.

Predicting Distortion Magnitudes in Prussian Blue Analogues

Based on simple electrostatic and harmonic potential considerations, we derive a straightforward expression linking the composition of a Prussian blue analogue (PBA) to its propensity to undergo collective structural distortions. We demonstrate the existence of a threshold value, below which PBAs are undistorted and above which PBAs distort by a degree that is controlled by a geometric tolerance factor. Our analysis rationalizes the presence, absence, and magnitude of distortions in a wide range of PBAs and distinguishes their structural chemistry from that of other hybrid perovskites.

Nonlinear Optical and Magnetic Properties of FeII-SCN-HgII Isomers: Centrosymmetric Layers and Chiral Networks

Research on isomers is highly desirable due to their prospective role in better understanding of physicochemical properties of similar systems and further development of multifunctional molecular materials. Iron(II) and tetra(thiocyanato)mercury(II) ions self-assembled in the presence of 2-acetylpyridine (2-acpy) excess to form two {[Fe(2-acpy)][Hg(μ-SCN)₄]}_n isomers: two-dimensional (2D) centrosymmetric layers with folded ring structural motifs (1) and three-dimensional (3D) chiral networks with right- or left-handed {···Fe-NCS-Hg-SCN···}_∞ helixes (2). New methods of designing and synthesizing functional thiocyanate-bridged materials have been proposed. In addition, the similarity between 1 and 2 allowed for the description of subtle changes in IR and UV-visible spectra. Moreover, 2 shows spontaneous resolution, and it crystallizes in the noncentrosymmetric space group P2₁, leading to the occurrence of nonlinear optical activity in circular dichroism studies and second harmonic generation (SHG). At room temperature, the SH susceptibility for powder sample 2 reached 6.0 × 10^-11 esu. Ab initio calculations indicated the electric polarization vector and the crystallographic twofold screw axis pass through the aromatic ring. Magnetic studies for 1 and 2 revealed high-spin iron(II) with zero-field splitting at low temperatures. Analysis of magnetic data gave |D| = 37.45 cm^-1, |E/D| = 5.59 cm^-1, and ⟨g⟩ = 2.15 for 1, |D| = 36.78 cm^-1, |E/D| = 4.92 cm^-1, and ⟨g⟩ = 2.18 for 2, and information about the orientation of magnetic anisotropy vectors for both compounds.

Magnetic order in a metal thiocyanate perovskite-analogue

Metal thiocyanate perovskite-analogues are a growing class of materials, but although they contain paramagnetic cations there have been no reports of their magnetic properties. Due to the large separations between...

The Structural Complexity of Perovskites

A recent research direction related to ABX3 perovskites is the use of molecules on the A and/or X-site, a development that has proved fruitful for photovoltaics, (improper) ferroelectrics and barocalorics....

Photoluminescent coordination polymer bulk glasses and laser-induced crystallization

Over centimeter-sized luminescent coordination polymer glasses were fabricated. They showed high transparency (over 80%) and strong green emission at room temperature. The glass-to-crystal transformation by laser irradiation was demonstrated.

Mixed-Metal Cu-Zn Thiocyanate Coordination Polymers with Melting Behavior, Glass Transition, and Tunable Electronic Properties

The solid-state mechanochemical reactions under ambient conditions of CuSCN and Zn(SCN)₂ resulted in two novel materials: partially Zn-substituted α-CuSCN and a new phase Cu_xZn_y(SCN)_x+2y. The reactions take place at the labile S-terminal, and both products show melting and glass transition behaviors. The optical band gap and solid-state ionization potential can be adjusted systematically by adjusting the Cu/Zn ratio. Density functional theory calculations also reveal that the Zn-substituted CuSCN structure features a complementary electronic structure of Cu 3d states at the valence band maximum and Zn 4s states at the conduction band minimum. This work shows a new route to develop semiconductors based on coordination polymers, which are becoming technologically relevant for electronic and optoelectronic applications.

Tilt and shift polymorphism in molecular perovskites

Molecular perovskites, i.e. ABX₃ coordination polymers with a perovskite structure, are a chemically diverse material platform for studying fundamental and applied materials properties such as barocalorics and improper ferroelectrics. Compared to inorganic perovskites, the use of molecular ions on the A- and X-site of molecular perovskites leads to new geometric and structural degrees of freedom. In this work we introduce the concept of tilt and shift polymorphism, categorising irreversible perovskite-to-perovskite phase transitions in molecular perovskites. As a model example we study the new molecular perovskite series [(nPr)₃(CH₃)N]M(C₂N₃)₃ with M = Mn²⁺, Co²⁺, Ni²⁺, and nPr = n-propyl, where different polymorphs crystallise in the perovskite structure but with different tilt systems depending on the synthetic conditions. Tilt and shift polymorphism is a direct ramification of the use of molecular building units in molecular perovskites and as such is unknown for inorganic perovskites. Given the role of polymorphism in materials science, medicine and mineralogy, and more generally the relation between physicochemical properties and structure, the concept introduced herein represents an important step in classifying the crystal chemistry of molecular perovskites and in maturing the field.

A spin-crossover phenomenon in a 2D heterometallic coordination polymer with [Pd(SCN)4]2- building blocks

Two new two-dimensional (2D) coordination polymers, [Fe^II(L)₂{Pd^II(SCN)₄}] (L₁ = 3-(9-anthracenyl)-pyridine (1) and L₂ = 4-(9-anthracenyl)-pyridine (2)), were constructed by employing square-planar [Pd(SCN)₄]^2- building blocks. Compound 1 exhibits a complete spin-crossover (SCO) behaviour under normal atmospheric pressure, and represents the first SCO example in a 2D system containing [Pd(SCN)₄]^2- units. In contrast, compound 2 only shows paramagnetic behaviour at measured temperatures. It is clear that the fine-tuning of the monodentate ligand can modulate the ligand field and packing fashions, which sheds light on developing new SCO materials.

Controlling multiple orderings in metal thiocyanate molecular perovskites A x {Ni[Bi(SCN)6]}

We report four new A-site vacancy ordered thiocyanate double double perovskites, , A = K+, NH4 +, CH3(NH3)+ (MeNH3 +) and C(NH2)3 + (Gua+), including the first examples of thiocyanate perovskites containing organic A-site cations. We show, using a combination of X-ray and neutron diffraction, that the structure of these frameworks depends on the A-site cation, and that these frameworks possess complex vacancy-ordering patterns and cooperative octahedral tilts distinctly different from atomic perovskites. Density functional theory calculations uncover the energetic origin of these complex orders and allow us to propose a simple rule to predict favoured A-site cation orderings for a given tilt sequence. We use these insights, in combination with symmetry mode analyses, to show that these complex orders suggest a new route to non-centrosymmetric perovskites, and mean this family of materials could contain excellent candidates for piezo- and ferroelectric applications.

Tilting and twisting in a novel perovzalate, K3NaMn(C2O4)3

A unique variant on the perovskite structure, K₃NaMn(C₂O₄)₃, has been identified with unconventional octahedral tilting, interpenetration of two topologically identical perovskite-like frameworks and an unusual, twisted oxalate ligand.

Structural insights into a new family of three-dimensional thiocyanate-bridged molecular double perovskites

Four new three-dimensional thiocyanate-bridged molecular double perovskites with bent Cd–S–C angles in a narrow distribution range reveal highly distorted frameworks with a relatively strong structural rigidity.

Strengthening the Magnetic Interactions in Pseudobinary First-Row Transition Metal Thiocyanates, M(NCS)2

Understanding the effect of chemical composition on the strength of magnetic interactions is key to the design of magnets with high operating temperatures. The magnetic divalent first-row transition metal (TM) thiocyanates are a class of chemically simple layered molecular frameworks. Here, we report two new members of the family, manganese(II) thiocyanate, Mn(NCS)₂, and iron(II) thiocyanate, Fe(NCS)₂. Using magnetic susceptibility measurements on these materials and on cobalt(II) thiocyanate and nickel(II) thiocyanate, Co(NCS)₂ and Ni(NCS)₂, respectively, we identify significantly stronger net antiferromagnetic interactions between the earlier TM ions-a decrease in the Weiss constant, θ, from 29 K for Ni(NCS)₂ to -115 K for Mn(NCS)₂-a consequence of more diffuse 3d orbitals, increased orbital overlap, and increasing numbers of unpaired t_2g electrons. We elucidate the magnetic structures of these materials: Mn(NCS)₂, Fe(NCS)₂, and Co(NCS)₂ order into the same antiferromagnetic commensurate ground state, while Ni(NCS)₂ adopts a ground state structure consisting of ferromagnetically ordered layers stacked antiferromagnetically. We show that significantly stronger exchange interactions can be realized in these thiocyanate frameworks by using earlier TMs.

The structures of ordered defects in thiocyanate analogues of Prussian Blue

We report the structures of six new divalent transition metal hexathiocyanatobismuthate frameworks with the generic formula , M = Mn, Co, Ni and Zn. These frameworks are defective analogues of the perovskite-derived trivalent transition metal hexathiocyanatobismuthates MIII[Bi(SCN)6]. The defects in these new thiocyanate frameworks order and produce complex superstructures due to the low symmetry of the parent structure, in contrast to the related and more well-studied cyanide Prussian Blue analogues. Despite the close similarities in the chemistries of these four transition metal cations, we find that each framework contains a different mechanism for accommodating the lowered transition metal charge, making use of some combination of Bi(SCN)6 3- vacancies, MBi antisite defects, water substitution for thiocyanate, adventitious extra-framework cations and reduced metal coordination number. These materials provide an unusually clear view of defects in molecular framework materials and their variety suggests that similar richness may be waiting to be uncovered in other hybrid perovskite frameworks.

Perovskite-related ReO3-type structures

Materials with the perovskite ABX3 structure play a major role across materials chemistry and physics as a consequence of their ubiquity and wide range of useful properties. ReO3-type structures can be described as ABX3 perovskites in which the A-cation site is unoccupied, giving rise to the general composition BX3, where B is typically a cation and X is a bridging anion. The chemical diversity of such structures is extensive, ranging from simple oxides and fluorides, such as WO3 and AlF3, to complex structures in which the bridging anion is polyatomic, such as in the Prussian blue-related cyanides Fe(CN)3 and CoPt(CN)6. The same ReO3-type structure is found in metal-organic frameworks, for example, ln (im)3(im = imidazolate) and the well-known MOF-5 structure, where the B-site cation is polyatomic. The extended 3D connectivity and openness of this structure type leads to compounds with interesting and often unusual properties. Notable among these properties are negative thermal expansion (for example, ScF3), photocatalysis (for example, CoSn(OH)6), thermoelectricity (for example, CoAs3) and superconductivity in a phase that is controversially described as SH3 with a doubly interpenetrating ReO3 structure. We present an account of this exciting family of materials and discuss future opportunities in the area.

Rare-earth-containing perovskite nanomaterials: design, synthesis, properties and applications

As star material, perovskites have been widely used in the fields of optics, photovoltaics, electronics, magnetics, catalysis, sensing, etc. However, some inherent shortcomings, such as low efficiency (power conversion efficiency, external quantum efficiency, etc.) and poor stability (against water, oxygen, ultraviolet light, etc.), limit their practical applications. Downsizing the materials into nanostructures and incorporating rare earth (RE) ions are effective means to improve their properties and broaden their applications. This review will systematically summarize the key points in the design, synthesis, property improvements and application expansion of RE-containing (including both RE-based and RE-doped) halide and oxide perovskite nanomaterials (PNMs). The critical factors of incorporating RE elements into different perovskite structures and the rational design of functional materials will be discussed in detail. The advantages and disadvantages of different synthesis methods for PNMs will be reviewed. This paper will also summarize some practical experiences in selecting suitable RE elements and designing multi-functional materials according to the mechanisms and principles of REs promoting the properties of perovskites. At the end of this review, we will provide an outlook on the opportunities and challenges of RE-containing PNMs in various fields.

Tilts and shifts in molecular perovskites

A survey of the rigid unit modes in molecular perovskites is presented, showing how the prevalence of conventional tilts, unconventional tilts and columnar shifts vary across the different classes of molecular perovskites.

Creation and stabilisation of tuneable open metal sites in thiocyanato-bridged heterometallic coordination polymers to be used as heterogeneous catalysts

A series of thiocyanato-bridged heterometallic coordination polymers with a 3D reticular network have been synthesised by the reaction of [Pt^IV(SCN)₆]^2- with M^II ions to form {M^II[Pt^IV(SCN)₆]}_n and {[M^II(CH₃OH)₂][Pt^IV(SCN)₆]}_n (M^II = Mn^II, Fe^II, Co^II, Ni^II or Cu^II) in water and methanol, respectively. Single-crystal X-ray analyses revealed the absence of open metal sites in {M^II[Pt^IV(SCN)₆]}_ns and the formation of potential open metal sites at the M^II ions of {[M^II(CH₃OH)₂][Pt^IV(SCN)₆]}_ns by the coordination of methanol. One of the two coordinating methanol molecules in {[Co^II(CH₃OH)₂][Pt^IV(SCN)₆]}_n was replaced with pyridine to stabilise the open metal sites, because the methanol molecules are too labile to maintain open metal sites in water. The heterogeneous catalysis of coordination polymers with and without open metal sites was examined for organophosphate hydrolysis and photocatalytic water oxidation to clarify the requisites for heterogeneous catalysts.

Structure and thermal expansion of the distorted Prussian blue analogue RbCuCo(CN)6

The structure and thermal expansion of the Prussian blue analogue RbCuCo(CN)6 has been determined via neutron and X-ray powder diffraction. The system crystallises in Cccm and harbours three coexisting distortions relative to the parent Fm3[combining macron]m structure, which leads to anisotropic thermal expansion with a near-zero component in one direction. The difficulties associated with determining octahedral tilt systems in Prussian blue analogues and related double molecular perovskites are discussed.