Stabilizing Metal–Organic Polyhedra (MOP): Issues and Strategies

Abstract

Metal–organic polyhedra (MOPs) are discrete, metal–organic molecular entities composed of edge‐sharing molecular polygons or connected molecular vertices. Unlike the infinite metal–organic coordination networks popularized by metal–organic frameworks (MOFs), spherical MOPs, also known as nanocages, nanospheres, nanocapsules, or nanoballs, are obtained through the self‐organization of metal–carboxylate or metal–pyridine/pyrimidine links to afford cage‐like nanoarchitectures. MOPs offer much promise as porous materials owing to their well‐defined structures and solution processability. However, these advantages become moot if their poor aqueous stability and/or guest‐removal‐induced aggregation handicaps remain unaddressed. The concise premise of this contribution limits our discussion to the design principles in action behind recent developments in stable carboxylate MOPs. To highlight the structure–property relationships between the structural and compositional features of these metal carboxylate polyhedra, related scientific challenges and state‐of‐the‐art research directions for further exploration are presented in brief.

Metal‐Organic Frameworks and Metal‐Organic Cages – A Perspective

“…[92] The stabilisation of carboxylate-based MOCs with a view to realising materialsbased applications has recently been reviewed. [93] In solution phase, acetonitrile and water are the two most popular solvents -solubility and stability in the latter vital for realizing many applications. MOCs can be divided into two categories: (i) those able to form in water, [38] and (ii) those that are kinetically but not thermodynamically stable in water (i. e. can persist in water but unable to form in water).…”

Synthesis of Polycarboxylate Rhodium(II) Metal–Organic Polyhedra (MOPs) and their use as Building Blocks for Highly Connected Metal–Organic Frameworks (MOFs)

“…carboxylic acid groups), and then use them as SBBs for the synthesis of MOFs. This is mainly due to the difficulty of controlling the formation of the MOP, rather than extended coordination networks, when the precursor MOP linkers are functionalized with additional coordinating groups; the challenge of preventing any cross‐linking reactions between MOPs that are functionalized with available coordinating sites; [18, 19] and the lack of stability and solubility of the isolated MOPs under the conditions commonly used for MOF synthesis [20] …”

“…This is mainly due to the difficulty of controlling the formation of the MOP, rather than extended coordination networks, when the precursor MOP linkers are functionalized with additional coordinating groups; the challenge of preventing any cross-linking reactions between MOPs that are functionalized with available coordinating sites; [18,19] and the lack of stability and solubility of the isolated MOPs under the conditions commonly used for MOF synthesis. [20] Herein we report the design, synthesis and isolation of two types of polycarboxylate MOPs, and their subsequent use as SBBs to build highly-connected bimetallic MOFs. We chose cuboctahedral Rh II -based MOP as our platform for polycarboxylate MOPs owing to its high stability, symmetry, and multiple sites for functionalization with carboxylic acid groups.…”

Nanotrap Grafted Anion Exchangeable Hybrid Materials for Efficient Removal of Toxic Oxoanions from Water

“…However, during the application process, the bare MOP molecules generally undergo random aggregation or reorganization. 22 This inevitable rearrangement restricts the diffusion of metal-based oxoanions through the aggregated MOP molecules; consequently, the sorption performance efficiency of the pristine MOP molecules is much less. However, the aggregation process of MOP molecules is avoided after covalently grafting nanosized discrete MOP molecules into the crystalline porous COF matrix as the COF matrix tightly anchors the MOP molecules and separates them from each other.…”

“… 18 − 21 However, MOPs are often unable to reach their full potential toward aforementioned applications due to unavoidable aggregation-induced active site blockage upon guest removal in the solid-state. 22 Only a few efforts have been made to overcome such a cardinal issue by encasing MOPs inside porous materials (silica and metal–organic frameworks, MOFs), 23 , 24 but hybridization of a MOP with porous materials has yet remained an unmet challenge.…”