Transformation of SBUs and Synergy of MOF Host-Guest in Single Crystalline State: Ingenious Strategies for Modulating Third-Order NLO Signals.

Central metal exchange can innovatively open the cavity of metal-organic frameworks (MOFs) by alternating the framework topology. Here, the single-crystal-to-single-crystal (SC-SC) transformation is reported from a Co-based MOF {[Co1.25 (HL)0.5 (Pz-NH2 )0.25 (µ3 -O)0.25 (µ2 -OH)0.25 (H2 O)]·0.125 Co·0.125 L·10.25H2 O}n (Co-MOF, L = 5,5'-(1H-2,3,5-triazole-1,4-diyl)diisophthalic acid) into two novel MOF materials, {[Cu1.75 L0.75 (Pz-NH2 )0.125 (µ3 -O)0.125 (µ2 -OH)0.25 (H2 O)0.375 ]•3CH3 CN}n (Cu-MOF) and {[Zn1.75 L0.625 (Pz-NH2 )0.25 (µ3 -O)0.25 (µ2 -O)0.25 (H2 O)1.25 ]•4CH3 CN}n (Zn-MOF), through exchanging the Co2+ in the MOF into Cu2+ or Zn2+ , respectively. The free Co2+ and L4- in the Co-MOF channels fuse with the skeleton during the Co→Cu and Co→Zn exchange processes, leading to the expansion of the channel space and the transformation of the secondary building units (SBUs) to form an adjustable skeleton. The nonlinear optical response results show that the MOFs generated by the exchange of the central metal exhibit different saturable absorption and the self-focusing effect. In addition, loading polypyrrole (PPy) into the MOFs can not only improve the stability of the MOFs but also further optimize the nonlinear optical behavior. This work suggests that SC-SC central metal exchange and the introduction of polymer molecules can tune the nonlinear optical response, which provides a new perspective for the future study of nonlinear optical materials.

A Series of Mesoporous Rare-Earth Metal-Organic Frameworks Constructed from Organic Secondary Building Units.

Further development of metal-organic frameworks (MOFs) requires an establishment of hierarchical interaction within the framework. Herein, we report a series of mesoporous rare-earth (RE) MOFs that are constructed from an unusual 12-connected π-stacked pyrene secondary building unit (SBU) and a typical 12-connected RE6 cluster (RE=Eu, Y, Yb, Tb, Ce). The judicious design of a butterfly-shape pyrene ligand with a tert-butyl substituent enables the formation of the disordered 12-connected organic SBUs on its strong intermolecular π-π interactions. The assembly of 12-connected inorganic cuboctahedron SBUs and 12-connected organic distorted hexagonal prism SBUs generates an unprecedented network that can be further simplified into a 4,4-connected pts net linked from planar square and tetrahedra. This work provides fresh insights into the design and synthesis of frameworks constructed from coordinatively, covalently, and noncovalently linked building units.

Sequential Solid-State Transformations Involving Consecutive Rearrangements of Secondary Building Units in a Metal-Organic Framework.

Solid-state transformations in metal-organic framework (MOF) systems are important phenomena and have led to the creation of new MOF structures. Solid-state transformations from interpenetrated to non-interpenetrated networks involving rearrangement of secondary building units (SBUs) in a single-crystal-to-single-crystal (SCSC) fashion have not been explored to date. Herein, we report the sequential, thermally stimulated solid-state transformations in a barium-organic framework ( UPC-600 ). The two-fold interpenetrated framework of  UPC-600  is converted at 373 K to UPC-601 , a non-interpenetrated framework. This proceeds in a SCSC fashion and involves the rearrangement of two proximate rod-shaped SBUs in different nets to generate a new rod-shaped SBU. At 473 K, a continuous solid-state transformation involving a second rearrangement occurred,  UPC-601  converted to UPC-602  by the rearrangement of the 1D rod-shaped SBU to a 2D layer SBU. This is the first example of such a thermally-driven stepwise transformation involving simultaneous cleavage and regeneration of multiple bonds. This result will enable detailed studies of solid-state transformations, and encourages a deep understanding of the role of solid-state transformations in the synthesis of MOF materials.

Sequential Solid-State Transformations Involving Consecutive Rearrangements of Secondary Building Units in a Metal-Organic Framework (MOF).

Solid-state transformations in metal-organic frameworks (MOFs) are important and have led to the creation of new MOF structures. Solid-state transformations from interpenetrated to non-interpenetrated networks involving rearrangement of secondary building units (SBUs) in a single-crystal-to-single-crystal (SCSC) fashion have not been explored to date. Herein, we report the sequential, thermally stimulated solid-state transformations in a barium-organic framework (UPC-600). The two-fold interpenetrated framework of UPC-600 is converted at 373 K into UPC-601, a non-interpenetrated framework. This proceeds in a SCSC fashion and involves the rearrangement of two proximate rod-shaped SBUs in different nets to generate a new rod-shaped SBU. At 473 K, a continuous solid-state transformation involving a second rearrangement occurred, UPC-601 converted into UPC-602 by the rearrangement of the 1D rod-shaped SBU to a 2D layer SBU. This is the first example of such a thermally driven stepwise transformation involving simultaneous cleavage and regeneration of multiple bonds.

The Chemistry of Nucleation: In Situ Pair Distribution Function Analysis of Secondary Building Units During UiO-66 MOF Formation.

The concept of secondary building units (SBUs) is central to all science on metal-organic frameworks (MOFs), and they are widely used to design new MOF materials. However, the presence of SBUs during MOF formation remains controversial, and the formation mechanism of MOFs remains unclear, due to limited information about the evolution of prenucleation cluster structures. Here in situ pair distribution function (PDF) analysis was used to probe UiO-66 formation under solvothermal conditions. The expected SBU-a hexanuclear zirconium cluster-is present in the metal salt precursor solution. Addition of organic ligands results in a disordered structure with correlations up to 23 Å, resembling crystalline UiO-66. Heating leads to fast cluster aggregation, and further growth and ordering results in the crystalline product. Thus, SBUs are present already at room temperature and act as building blocks for MOF formation. The proposed formation steps provide insight for further development of MOF synthesis.

"Heterogeneity within order" in metal-organic frameworks.

Metal-organic frameworks (MOFs) are constructed by linking inorganic units with organic linkers to make extended networks. Though more than 20 000 MOF structures have been reported most of these are ordered and largely composed of a limited number of different kinds building units, and very few have multiple different building units (heterogeneous). Although heterogeneity and multiplicity is a fundamental characteristic of biological systems, very few synthetic materials incorporate heterogeneity without losing crystalline order. Thus, the question arises: how do we introduce heterogeneity into MOFs without losing their ordered structure? This Review outlines strategies for varying the building units within both the backbone of the MOF and its pores to produce the heterogeneity that is sought after. The impact this heterogeneity imparts on the properties of a MOF is highlighted. We also provide an update on the MOF industry as part of this themed issue for the 150th anniversary of BASF.

Ba3[Sn(OH)6][SeO4]2·3H2O, a hydrated 1:2 double salt of barium hexa-hydroxidostannate(IV) and barium selenate(VI).

Single crystals of tribarium hexa-hydroxidostannate(IV) bis-[selenate(VI)] trihydrate, Ba3H12O17Se2Sn or Ba3[Sn(OH)6][SeO4]2·3H2O, prepared from solid BaSnO3 and aqueous Na2[SeO4] solutions have hexa-gonal (P63) symmetry. The structure consists of four different primary building units: a hexa-hydroxidostannate(IV) ion, two different selenate(VI) ions, all three of point group symmetry C 3, and a mono-capped {BaO9}-square anti-prism of point group symmetry C 1. The secondary building units result from three of the barium coordination polyhedra linked together via common edges. While one of the two tetra-hedral voids formed from these trimeric units is filled by one bidentate, chelating µ2-selenate ion, the other one remains unoccupied as the corresponding second selenate ion only acts as a monodentate, µ1-ligand. SBUs are completed by hexa-hydroxidostannate(IV) ions sharing adjacent edges on the uncapped faces of the three, mono-capped square anti-prisms. These SBUs are arranged into layers via common edges on the uncapped, square faces of the {BaO9} coordination polyhedra in a way that the hexa-hydroxidostannate(IV) ions act as linkage between two neighboring layers.

Why Design Matters: From Decorated Metal Oxide Clusters to Functional Metal-Organic Frameworks.

The opportunity to generate functional solids with defined properties by deliberate design has not been materialized in traditional solid-state chemistry over many decades. The emergence of metal-organic frameworks (MOFs), permanently porous, crystalline solids with defined metrics, has allowed for studying design, synthesis, and properties, which then translated into new applications. Aggregates of metal ions stitched together by multidentate functional groups form such metal oxide clusters and represent the nodes of MOFs. These clusters, termed secondary building units (SBUs), are decorated with organic moieties that provide directionality and can be linked through geometric principles into extended nets using organic molecules (spacers). This concept of reticular chemistry has afforded permanently porous MOFs, and has resulted in over 20,000 structures over the past 20 years. However, there are still only a limited number of symmetric, discrete SBUs commonly used to design and synthesize MOFs. We herein introduce the most important SBUs that have emerged over time together with prototypal MOF structures and their fundamental applications. Both the discovery and the scientific impact will be highlighted alongside advantages and/or drawbacks. In addition, an outlook will be given on how the combination of multiple SBUs can lead to heterogeneous but ordered materials with higher complexity and functionality.

Crystal structure of poly[[(µ3-hydroxido-κ3 O:O:O)(µ3-selenato-κ3 O 1:O 2:O 3)tris-[µ3-2-(1,2,4-triazol-4-yl)acetato-κ3 N 1:N 2:O]tricopper(II)] dihydrate].

The title coordination polymer, {[Cu3(C4H4N3O9)3(SeO4)(OH)]·2H2O} n or ([Cu3(µ3-OH)(trgly)3(SeO4)]·2H2O), crystallizes in the monoclinic space group P21/c. The three independent Cu2+ cations adopt distorted square-pyramidal geometries with {O2N2+O} polyhedra. The three copper centres are bridged by a µ3-OH anion, leading to a triangular [Cu3(µ3-OH)] core. 2-(1,2,4-Triazol-4-yl)acetic acid (trgly-H) acts in a deprotonated form as a µ3-κ3 N 1:N 2:O ligand. The three triazolyl groups bridge three copper centres of the hydroxo-cluster in an N 1:N 2 mode, thus supporting the triangular geometry. The [Cu3(µ3-OH)(tr)3] clusters serve as secondary building units (SBUs). Each SBU can be regarded as a six-connected node, which is linked to six neighbouring triangles through carboxyl-ate groups, generating a two-dimensional uninodal (3,6) coordination network. The selenate anion is bound in a µ3-κ3 O 1:O 2:O 3 fashion to the trinuclear copper platform. The [Cu3(OH)(trgly)3(SeO4)] coordination layers and guest water mol-ecules are linked together by numerous O-H⋯O and C-H⋯O hydrogen bonds, leading to a three-dimensional structure.

Shape-Controlled Synthesis of Metal-Organic Frameworks with Adjustable Fenton-Like Catalytic Activity.

Controllable synthesis of metal-organic frameworks with well-defined morphology, composition, and size is of great importance toward understanding their structure-property relationship in various applications. Herein, we demonstrate a general strategy to modulate the relative growth rate of the secondary building units (SBUs) along different crystal facets for the synthesis of Fe-Co, Mn0.5Fe0.5-Co, and Mn-Co Prussian blue analogues (PBAs) with tunable morphologies. The same growth rate of SBUs along the {100}, {110}, and {111} surfaces at 0 °C results in the formation of spherical PBA particles, while the lowest growth rate of SBUs along the {100} surface resulting from the highest surface energy with increasing reaction temperature induces the formation of PBA cubes. Fenton reaction was used as the model reaction to probe the structure-catalytic activity relation for the as-synthesized catalysts. The cubic Fe-Co PBA was found to exhibit the best catalytic performance with reaction rate constant 6 times higher than that of the spherical counterpart. Via density functional theory calculations, the abundant enclosed {100} facets in cubic Fe-Co PBA were identified to have the highest surface energy and favor high Fenton reaction activity.