Mapping the Assembly of Metal–Organic Cages into Complex Coordination Networks

Abstract

Structural transformations of supramolecular assemblies play an important role in the synthesis of complex metal-organic materials. Nonetheless, often little is known of the assembly pathways that lead to the final product. This work describes the conversion of cubic metal-organic polyhedra to connected-cage networks of varying topologies. The neutral cubic cage assembly of formula {Pd [PO(NiPr) ]} (PZDC) has been synthesized from {Pd [(NiPr) PO](OAc) (OH)} ⋅2 (CH ) SO and 2,5-pyrazenedicarboxilic acid (PZDC-2H). This 42-component self-assembly is the largest known among the neutral cages with Pd ions. The cage contains twenty-four vacant carboxylate O-sites at the PZDC ligands that are available for further coordination. Post-assembly reactions of the cubic cage with Fe and Zn ions produced cage-connected networks of dia and qtz topologies, respectively. During these reactions, the discrete cubic cage transforms into a network of tetrahedral cages that are bridged by the 3D metal ions. The robustness of the [Pd {[PO(NiPr) }] molecular building units made it possible to map the post-assembly reactions in detail, which revealed a variety of intermediate 1D and 2D cage networks. Such step-by-step mapping of the transformation of discrete cages to cage-connected frameworks is unprecedented in the chemistry of coordination driven assemblies.

Polyanionic Imido‐P(V) Ligands: From Transition Metal Complexes to Coordination Driven Self‐Assemblies

“…A comparison of the structure of the cubic cage 58 and the tetrahedral cages found in the 3D-assemblies (and in the intermediates) show that the primary coordination of the Pd 3 X motifs and the PZDC ligands is same for both of them. However, the radius of canopy for the [(Pd 3 X)(PZDC) 3 ] 3À segment, measured at the ends of the three PZDC ligands is smaller for tetrahedral cages (5.5 Å) than in the cubic cage [55] Figure 19. Molecular structures of (a) discrete assembly of 63, (b) 1-D helical assembly of 64 and (c) crystal structure of hetero-cubane 65.…”

“…In this effort, a 42-component cubic cage assembly, 58 was synthesized based on these imido-Pd 3 PBUs and 2,5-pyrazine dicarboxylate linkers (Figure 16). [55] The cubic assembly of formula [(Pd 17b, f). In both frameworks the basic structural unit is the heterocubane of the type [M 4 (Pd 3 X) 4 (PZDC) 6 ] in which the M(II) ions bridge between the tetrahedral faces by coordinating with the free carboxylate O-atoms in their triangular faces.…”

“…However, heating the defect cube assembly of 61 at 70 °C for 24 h yields the 3D-assembly of 59. [55]…”

Coordination driven architectures based on metalloligands offering appended carboxylic acid groups

“…Boomishankar and co-workers 78 have demonstrated an interesting transformation of a cubic cage [(Pd 3 (X) 8 (Y) 12 ] (X = PO(NiPr) 3 and Y = 2,5-pyrazenedicarboxylate) 37 into a 3D framework (37a). Treatment of [Pd 3 X] 3+ with 2,5-pyrazenedicarboxylic acid (Y) yielded a cubic cage metalloligand 37.…”

“…The examples presented herein not only illustrated the importance of appended carboxylic acid groups in dictating the coordination properties of a metalloligand but also in controlling the dimensionality of ions; adapted from ref. 78 .…”