Metallosupramolecular Helicates and Tetrahedra: transition metal-directed assembly of polypyridyl ligands

Glasson, Christopher R.K. (2009) Metallosupramolecular Helicates and Tetrahedra: transition metal-directed assembly of polypyridyl ligands. PhD thesis, James Cook University.

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This thesis reports the synthesis of a range of polypyridyl ligands and their subsequent incorporation into transition metal-directed assembly experiments. These latter experiments were designed to assess the viability of subsequent metal-template reductive amination procedures for the preparation of pseudocryptands, mono- and dinuclear cryptates and larger polycyclic compounds.

The synthesis of polypyridyl derivatives for use in the current project employed a range of modern coupling procedures, including Stille and Suzuki cross-couplings. The former was used to synthesise a range of bipyridines, notably the regioselective cross-coupling between 2,5-dibromopyridine and 2-trimethylstannyl-5-methylpyridine to afford 5-bromo-5'-methylbipyridine (I) in high yield. As well, the reaction of 2- trimethylstannyl-5-methylpyridine and 6,6'-dichloro-3,3 -bipyridine in a bis-Stille cross-coupling allowed the synthesis of 5,5'-dimethyl-2,2';5',5";2",2""-quaterpyridine (II), often in yields in excess of 90 %. Alternatively, quaterpyridine II could be synthesised by two other methods: a Ni(0)-homocoupling reaction or a modified Suzuki coupling, both using bromobipyridine I as the starting material.

The interaction of quaterpyridine II with a range of transition metals, including Fe(II), Co(II), Ni(II) and Ru(II) was investigated. The resulting metal-complexes were characterised using a combination of NMR techniques, ESI-HRMS, X-ray crystallography and elemental analysis. The more labile first row transition metals yielded M₄L₆ host-guest complexes of type [M₄(II)₆()anion]⁷⁺ (where M = Fe(II), Co(II) and Ni(II) and anion = [FeCl₄]⁻, BF₄⁻ and PF₆⁻). There is also evidence that the [Fe₄(II)₆]⁸⁺ host encapsulates [FeCl₄]₂⁻, a rare example of the inclusion of a doubly charged species. Interestingly, a series of ¹⁹F NMR experiments revealed that the [Fe₄(II)₆]⁸⁺ host selectively binds PF₆⁻ over BF₄⁻; an observation that most probably reflects a size based recognition process. Furthermore, a successful synthetic procedure for isolation of the empty cage (free of an encapsulated anion) was developed, indicating that anion templation is not essential for the formation of [Fe₄(II)₆]⁸⁺.

The interaction of quaterpyridine II with RuCl₃ in ethylene glycol using microwave heating was found to yield a rare dinuclear helicate, [Ru₂(II)₃]⁴⁺, in 36% yield. The racemate of this product was resolved by cation exchange chromatography on C-25 Sephadex with 0.1 M (-)-O,O' -dibenzoyl-L-tartaric acid as eluent. Circular dichroism measurements were made to assess the success of the separation of the two enantiomers and the crystallisation of enantiopure material has allowed the assignment of the M-[Ru₂(II)₃]⁴⁺ and P-[Ru₂(II)₃]⁴⁺ forms using X-ray crystallography. In turn, an equilibrium dialysis experiment with calf thymus DNA indicated that M-[Ru₂(II)₃]⁴⁺ binds preferentially over the P-[Ru₂(II)₃]⁴⁺. Furthermore, the use of a Sepharose-immobilized AT dodecanucleotide column resulted in the successful separation of the M-and P-enantiomers; M-[Ru₂(II)₃]⁴⁺ was strongly retained whilst P-[Ru₂(II)₃]⁴⁺ essentially eluted with the solvent front. Less efficient (but still satisfactory) separations were observed with other DNA motifs; for example, on employing a GC 12-mer and bulge and hairpin sequences. In each case M-[Ru₂(II)₃]⁴⁺ bound to the column more strongly than the P-enantiomer.

To investigate the effect that rigidly bridged quaterpyridines might have on analogous octahedral metal-directed assembly outcomes, quaterpyridines III and IV were synthesised. These ligands were prepared in high yield by bis-Suzuki coupling reactions between bromobipyridine I and appropriate bis-pinacol-diboronic esters using microwave heating.

The interaction of quaterpyridines III and IV with octahedral metal ions resulted in mixtures of [M₂L₃]⁴⁺ and [M₄L₆]⁸⁺ complexes (M = Fe(II) or Ni(II) and L = III or IV). [Fe₂L₃]⁴⁺ and [Fe₄L₆]⁸⁺ were adequately inert to allow their chromatographic separatation and subsequent characterization. A level of control over the relative ratio of these products was demonstrated using a combination of reaction times and the degree of dilution employed for their synthesis; short reaction times and high dilution favoured the formation of [M₂L₃]⁴⁺ (e.g. [Ni₂(III)₃]⁴⁺), while long reaction times and normal dilution favoured the formation of [M₄L₆]⁸⁺ (e.g. [Fe₄(III)₆()PF₆]⁷⁺). Interestingly, M₂L₃ and M₄L₆ complexes incorporating quaterpyridines III and IV are fluorescent. With respect to the latter, on interaction with BPh₄- the larger tetrahedron, [Fe₄(IV)₆]⁸⁺, yields a change in fluorescence (a fluorescent signal). These observations suggest that complexes incorporating ligands III and IV might find application as fluorescent sensors.

The isolation of a number of interesting M₂L₃ and M₄L₆ complexes led to the possibility that analogous metal-directed assembly procedures employing appropriately substituted quaterpyridines, related to III and IV, might allow for the metal-template synthesis of dinuclear cryptates and larger tetranuclear polycyclic species. With this in mind a number of bipyridyl and quaterpyridyl derivatives were synthesized with salicyloxy functionality to allow for subsequent reductive amination procedures. In this regard, dialdehydes V – VII were synthesised and reacted with Fe(II) in a 2:3 ratio. The resulting products were characterised by NMR and ESI-HRMS, revealing a series of M₂L₃ and M₄L₆ precursor complexes, including [Fe₄(V)₆](BF₄)₈, [Fe₂(VI)₃](PF₆)₈, [Fe₄(VI)₆](PF₆)₈, [Fe₄(VII)₆](PF₆)₈ and [Fe₄(VII)₆](PF₆)₈. As was the case for the interaction of quaterpyridines III and IV with Fe(II), the interaction of VI and VII with Fe(II) yielded mixtures of M₂L₃ and M₄L₆ complexes; the product ratio of which could also be controlled.

Preliminary experiments revealed that reductive amination of [Fe₂(VI)₃](PF₆)₈ using NH₄OAc and NaCNBH₃ in acetonitrile yields the dinuclear cryptate [Fe₂(L¹)](PF₆)₈ (L¹ = the corresponding cryptand). Reductive amination of the precursor complexes [Fe₄(V)₆](BF₄)₈ and [Fe₄(VII)₆](PF₆)₈ under these same conditions revealed the production of the unique tetranuclear polycyclic species [Fe₄(L²)](BF₄)₈ and [Fe₄(L³)](PF₆)₈ (L² and L³ = the corresponding metal-free polycyclic ligands). The successful syntheses of the latter species required a total of twelve successive in situ imine condensation/reduction reactions from a total of fourteen components.

Item ID: 31890
Item Type: Thesis (PhD)
Keywords: coupling procedures; polypyridyl ligands synthesis; reductive amination procedures; metal-directed assembly procedures
Date Deposited: 30 Apr 2014 04:22
FoR Codes: 03 CHEMICAL SCIENCES > 0302 Inorganic Chemistry > 030207 Transition Metal Chemistry @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970103 Expanding Knowledge in the Chemical Sciences @ 100%
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