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

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.

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...

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.

Metal thiocyanate hybrid perovskites can have multiple simultaneous complex orderings and combining these orderings appropriately can produce non-centrosymmetric structures.

Heterometallic coordination polymers as heterogeneous electrocatalysts

Heterometallic coordination polymers have been rapidly developed as heterogeneous electrocatalysts. This review highlights the synthesis strategies of these polymers and the relationships between structures and electrocatalytic performances.

Heterometallic coordination polymers: Treatment activity on diabetic foot by reducing the excess inflammatory response in the plantar tissue

Two new heterometallic coordination polymers formulated as {[LaCo(2.5-pydc)2(H2O)4]Cl·4H2O}n (1, 2,5-H2pydc = pyridine-2,5-dicarboxylic acid) and {[LaCo2Cl3(4,4′-bpdc)2(H2O)3]·H2O}n (2, 4,4′-H2bpdc = 2,2′-bipyridyl-4,4′-dicarboxylic acid) have been solvothermally synthesized using the coordination between rare earth/transition metal ions and pyridinedicarboxylic acids. To develop new candidates for enhancing the therapeutic effect of improved negative pressure suction for diabetic foot, the activities of compounds 1 and 2 were studied. First, the ELISA (enzyme linked immunosorbent assay) detection kit was used and the inflammatory cytokines in the plantar tissue were measured. Besides, the TLR-4-NF-κB signaling pathway was measured with real-time reverse transcription-polymerase chain reaction.

Crystal transformation in Mn(II) metal-organic frameworks based on a one-dimensional chain precursor

The solvothermal reaction of Mn(ii) salts and 5-((4'-(tetrazol-5''-yl)benzyl)oxy)isophthalic acid (H3L) affords an Mn(ii) based coordination polymer Mn(H2L)2(H2O)2 (1), which possesses a one-dimensional (1D) chain structure. Using 1 as the precursor, three Mn(ii) metal-organic frameworks, Mn3L2(2,2'-bpy)2·5H2O (2), Mn3L2(H2O)4 (3), and Mn4L2(HL)(H2O)5·0.5H2O (4), with three-dimensional (3D) networks can be obtained by different strategies of crystal transformation. Upon introduction of 2,2'-bipyridine (2,2'-bpy) as the ligand and 2,2'-biquinoline-4,4'-dicarboxylic acid as the structural-directing agent, 1 undergoes irreversible crystal transformation into 2 and 3, respectively, and 1 can be transformed into 4 by increasing the reaction temperature. Interestingly, the irreversible structural transformation of 3 into 2 can be carried out by adding a 2,2'-bpy ligand. Notably, after the removal of coordinated water molecules, 1 and 3 exhibit good catalytic performance for the cyanosilylation reaction even at 0 °C.

Single Open Sites on FeII Ions Stabilized by Coupled Metal Ions in CN-Deficient Prussian Blue Analogues for High Catalytic Activity in the Hydrolysis of Organophosphates

CN-deficient Prussian blue analogues (PBAs), [M^N(H₂O)_x]_y[Fe^II(CN)₅(NH₃)] (M^N = Cu^II, Co^II, or Ga^III), were synthesized and examined as a new class of heterogeneous catalysts for hydrolytic decomposition of organophosphates often used as pesticides. The active species of the CN-deficient PBAs were mainly C-bound Fe^II ions with only single open sites generated by liberation of the NH₃ ligand during the catalytic reactions. [Cu^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] showed higher catalytic activity than [Co^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and [Ga^III(H₂O)][Fe^II(CN)₅(NH₃)], although N-bound Cu^II species has been reported as less active than Co^II and Ga^III species in conventional PBAs. IR measurements of a series of the CN-deficient PBAs after the catalytic reactions clarified that a part of the NH₃ ligands remained on [Co^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and that hydrogen phosphate formed as a product strongly adsorbed on the Fe^II ions of [Ga^III(H₂O)][Fe^II(CN)₅(NH₃)]. Hydrogen phosphate also adsorbed, but weakly, on the Fe^II ions of [Cu^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)]. These results suggest that heterogeneous catalysis of the Fe^II ions with single open sites were tuned by the M^N ions through metal-metal interaction.

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 M^III[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, M_Bi 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.

H-Bonded and metal(ii)-organic architectures assembled from an unexplored aromatic tricarboxylic acid: structural variety and functional properties

This study reports the application of an aromatic tricarboxylic acid, 2,5-di(4-carboxylphenyl)nicotinic acid (H3dcna) as a versatile and unexplored organic building block for assembling a new series of metal(ii) (M = Co, Ni, Zn, Fe, and Mn) complexes and coordination polymers, namely [M(Hdcna)(phen)2(H2O)]·H2O (M = Co (1), Ni (2)), [Zn(μ-Hdcna)(phen)]n (3), [Co(μ-Hdcna)(bipy)(H2O)2]n·nH2O (4), [Zn2(μ-Hdcna)2(bipy)2(H2O)4]·6H2O (5), [Zn(μ3-Hdcna)(H2biim)]n (6), [Ni2(Hdcna)2(μ-bpb)(bpb)2(H2O)4] (7), [Fe(μ4-Hdcna)(μ-H2O)]n·nH2O (8), and [Mn3(μ5-dcna)2(bipy)2(H2O)2]n·2nH2O (9). Such a diversity of products was hydrothermally prepared from the corresponding metal(ii) salts, H3dcna as a principal multifunctional ligand, and N-donor mediators of crystallization (1,10-phenanthroline, phen; 2,2'-bipyridine, bipy; 2,2'-biimidazole, H2biim; or 1,4-bis(pyrid-4-yl)benzene, bpb). The obtained products 1-9 were fully characterized by standard methods (elemental analysis, FTIR, TGA, PXRD) and the structures were established by single-crystal X-ray diffraction. These vary from the discrete monomers (1, 2) and dimers (5, 7) to the 1D (3, 4, 6) and 2D (8, 9) coordination polymers (CPs). Structural and topological characteristics of hydrogen-bonded or metal-organic architectures in 1-9 were highlighted, revealing that their structural multiplicity depends on the type of metal(ii) source and crystallization mediator. Thermal stability as well as luminescent, magnetic, or catalytic properties were explored for selected compounds. In particular, the zinc(ii) derivatives 3, 5, and 6 were applied as efficient heterogeneous catalysts for the cyanosilylation of aldehydes with trimethylsilyl cyanide at room temperature. The catalytic reactions were optimized by tuning the different reaction parameters (solvent composition, time, catalyst loading) and the substrate scope was also explored. Compound 5 revealed superior catalytic activity leading to up to 75% product yields, while maintaining its original performance upon recycling for at least four reaction cycles. Finally, the obtained herein products represent the unique examples of coordination compounds derived from H3dcna, thus opening up the use of this multifunctional tricarboxylic acid for generating complexes and coordination polymers with interesting structures and functional properties.