Metallosupramolecular assemblies : development of novel cyclometalated Pt(II) and Ir(III)-based capsules
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Inspired by nature’s use of self-assembled systems to carry out virtually all biological processes, chemists have taken to building simplified synthetic systems that mimic the biotic world. Although transition metal-ligand interactions are rarely used for the purpose of biological self-assembly, they have several advantages over other weak noncovalent interactions, such as pronounced directionality and significant strength. These particular attributes have allowed chemists to construct a comprehensive library of self-assembled polygons and polyhedra, using different transition metal-ligand motifs. Many of these supramolecular assemblies possess cavities of defined shape and size, which are able to accommodate guest molecules. It has further been realised that isolation of guest species from the bulk phase can lead to many interesting functions, such as containment, sensing and catalysis. Herein, a new self-assembly strategy has been used to construct novel cyclometalated Pt cages and assembly of the first known [Ir(ppy)2]-based capsule has also been achieved. Chapter 1 includes an introduction to metallosupramolecular assemblies, followed by a comprehensive review of three-dimensional architectures with accessible cavities, their synthetic strategies and applications. Chapter 2 reports on the construction of novel Pt(II)-based trigonal prisms using an unusual, kinetically controlled protocol. By exploiting asymmetric cyclometalated 2-phenylatopyridine based platinum corner units that possess both labile and non-labile cis-coordination sites, trigonal prismatic stereoisomeric architectures have been selectively prepared by altering the sequence of addition of ditopic 4,4′-bipy (4,4′-bipyridine) and tritopic tpt (2,4,6-tris(4-pyridyl)-1,3,5-triazine) molecular structural components using a template free method. Collision-induced-dissociation mass spectrometry experiments were used to differentiate between the structural isomers due to their significantly different fragmentation profiles. Chapter 3 describes the synthesis and characterisation of the first molecular capsule based on an [Ir(ppy)2]+ 90° metallosupramolecular acceptor unit. Initial work focused on pyridine-based donor ligands from which an Ir2L2 metallamacrocycle was assembled. However, when the highly conjugated tpt “panels” were used, due to postulated constraints in the dihedral angle, self-assembly of the Ir6tpt4 octahedral was unsuccessful. The constraints in the dihedral angle were eliminated by swapping pyridine for nitrile-based ligands and following the development of a method to resolve rac-[(Ir(ppy)2Cl)2] into its enantiopure forms, homochiral Ir6tcb4 (tcb = 1,3,5-tricyanobenzene) octahedral capsules where realised. Photophysical studies on the Ircapsules have shown that the ensemble of cooperative, weakly coordinating ligands can lead to luminescence not present in the comparative mononuclear analogues. X-ray crystallographic analysis revealed that the Ir capsules possess cavities large enough to accommodate 4 triflate counterions. Through a series of titration experiments the ability of the capsules to act as anion sensors was also exposed. Further exploration into the host-guest chemistry of the Ir6tcb4 capsule is reported in Chapter 4. Subsequent experiments have shown that self-assembly is highly dependent on the counterions associated with the system. While a number of different anions (OTf-, BF4 -, ClO4 -, PF6 -) facilitate the formation of the same octahedral scaffold, when triflimide was employed as a bulkier counterion, no capsule was observed. On subsequent addition of smaller counterions, such as triflate, the same Ir6tcb4 cage assembles, demonstrating that the anions also act as templates. Kinetic stability experiments, undertook by monitoring the rate of scrambling of the Δ and Λ-[Ir(ppy)2]+ components within the preformed ensembles, show that the Ir capsules are up to 1.4×104 times more stable than their mononuclear analogues. The counter anions were also observed to play a crucial role in the capsule’s stability with measured scrambling half-lives ranging from 4.7 mins with tetrafluoroborate to as long as 4.5 days with triflate. In contrast, the rate of ligand exchange in simple mononuclear complexes, as ascertained using EXSY NMR experiments, was found to be approximately independent of the associated anion.