The synthesis and study of bridging heterocyclic ligands

This thesis describes the synthesis and study of thirty four multidentate heterocyclic bridging ligands, twenty four of which are new compounds. A majority of these ligands incorporate a di-2-pyridyl coordination motif and are capable of chelation to a metal centre with the formation of a six-membered chelate ring, in contrast to the five-membered chelate rings formed by the majority of bridging ligands previously described in the literature. Four ligands were synthesised that represent the first examples of bridging heterocyclic ligands incorporating a [3]radialene core. The ligands described in this thesis are divided into three sections: those prepared from dipyridylmethane precursors, ligands with a di-2-pyridylamine chelating motif, and tripodal ligands, which are capable of facial coordination to an octahedral metal atom. The coordination and metallosupramolecular chemistry of these ligands were investigated with several different metal atoms, predominantly silver(I), copper(II) and palladium(II). With the dipyridylmethyl ligands and the di-2-pyridylarnine-based ligands, both discrete and polymeric structures were obtained, including dinuclear complexes, [2+2] dimeric complexes, molecular polyhedra and one-dimensional coordination polymers. Some novel structures, including a hexanuclear silver cage with an encapsulated fluoride anion and a triply cyclopalladated compound, are described. The tripodal ligands showed a preference for forming discrete complexes with M2L and M2L2 compositions, which included a new type of helicate. Bis(2,2'-bipyridyl)ruthenium complexes of the di-2-pyridylmethane-based and di-2-pyridylamine-based ligands are described and structurally characterised. Despite forming dinuclear complexes in many cases with palladium and copper, all the multidentate di-2-pyridylmethane-based ligands were surprisingly resistant to chelating their intended number of ruthenium atoms. Visible absorption spectroscopy and cyclic voItammetry was used to probe the nature of the metal-ligand and metal-metal interactions in these complexes. In combination with NMR spectroscopy, mass spectrometry and elemental analysis, X-Ray crystallography was used to characterise many of the complexes and metallosupramolecular species produced. The crystal structures of five ligands and fifty one complexes are described

Synthesis and coordination chemistry of 2-(di-2-pyridylamino)pyrimidine; structural aspects of spin crossover in an Fe(II) complex

This paper was accepted on February 26 20122-(Di-2-pyridylamino)pyrimidine (L), a potentially ditopic tetradentate ligand, was synthesized from commercially available di-2-pyridylamine and 2-chloropyrimidine. Despite being capable of bridging two metal atoms with bidentate chelation of both metal centres, L prefers to chelate or bridge through the more basic pyridyl donors of the di-2-pyridylamine moiety. Mononuclear trans-[Fe(NCS)2(L)2] and [Cu(L)2(H2O)](BF4)2•H2O complexes, and a discrete [Ag2(L)4](PF6)2 metallomacrocycle were isolated and structurally characterized by X-ray crystallography. A mononuclear palladium complex [PdCl2(L)]•(solvate), where solvate = ½H2O or CH2Cl2, was also readily obtained in 71% yield. One example of the ligand acting as a bis(bidentate) bridging ligand was observed in a dinuclear [(PdCl2)2(L)]•¾H2O complex that was obtained only in very low yield (ca. 3%) from the reaction that produced [PdCl2(L)]•½H2O. trans-[Fe(NCS)2(L)2] undergoes a temperature dependent HS-LS (HS = high spin; LS = low spin) crossover at ca. 205 K that was 2 observed by X-ray crystallography and magnetic measurements and attempts were made to understand the structural basis of this process. Despite efforts to isolate examples of L bridging two iron(II) centres, only the mononuclear trans-[Fe(NCS)2(L)2] species could be obtained.Rachel S. Crees, Boujemma Moubaraki, Keith S. Murray, and Christopher J. Sumb

Synthesis of Triple-Stranded Diruthenium(II) Compounds

A series of ligands containing a 1,4-disubstituted 1,2,3-triazole unit have been used for the formation of triple-stranded dinuclear Ru(II) complexes. In contrast to the previously reported complexes of labile metals, the use of inert Ru(II) enabled stereoisomeric mixtures of triple-stranded diruthenium(II) complexes to be accessed. The chromatographic resolution of the enantiomers of a reported helicate containing a more rigid 1,4-xylyl spacer was carried out on cellulose. The ligand spacer was modified and as the flexibility increased the production of isomeric mixtures was detected; the mesocate and helicate forms were separated when an n-propyl spacer was used. This pair of diastereomers was found to exhibit photoconversion, a unique observation for Ru(II) compounds of this type. Partial separation via chromatographic resolution was achieved for compounds containing an n-butyl spacer, and the presence of a mesocate/helicate pair confirmed

Studying manganese carbonyl photochemistry in a permanently porous metal–organic framework

First published 15 Aug 2023. OnlinePublMn(diimine)(CO)₃X (X = halide) complexes are critical components of chromophores, photo- and electrocatalysts, and photoactive CO-releasing molecules (photoCORMs). While these entities have been incorporated into metal–organic frameworks (MOFs), a detailed understanding of the photochemical and chemical processes that occur in a permanently porous support is lacking. Here we site-isolate and study the photochemistry of a Mn(diimine)(CO)₃Br moiety anchored within a permanently porous MOF support, allowing for not only the photo-liberation of CO from the metal but also its escape from the MOF crystals. In addition, the high crystallinity and structural flexibility of the MOF allows crystallographic snapshots of the photolysis products to be obtained. We report these photo-crystallographic studies in the presence of coordinating solvents, THF and acetonitrile, showing the changing coordination environment of the Mn species as CO loss proceeds. Using time resolved experiments, we report complementary spectroscopic studies of the photolysis chemistry and characterize the final photolysis product as a possible Mn(II) entity. These studies inform the chemistry that occurs in MOF-based photoCORMs and where these moieties are employed as catalysts.Rosemary J. Young, Michael T. Huxley, Lingjun Wu, Jack Hart, James O'Shea, Christian J. Doonan, Neil R. Champness and Christopher J. Sumb

Solar Energy Storage by Molecular Norbornadiene–Quadricyclane Photoswitches:Polymer Film Devices

Devices that can capture and convert sunlight into stored chemical energy are attractive candidates for future energy technologies. A general challenge is to combine efficient solar energy capture with high energy densities and energy storage time into a processable composite for device application. Here, norbornadiene (NBD)–quadricyclane (QC) molecular photoswitches are embedded into polymer matrices, with possible applications in energy storing coatings. The NBD–QC photoswitches that are capable of absorbing sunlight with estimated solar energy storage efficiencies of up to 3.8% combined with attractive energy storage densities of up to 0.48 MJ kg −1 . The combination of donor and acceptor units leads to an improved solar spectrum match with an onset of absorption of up to 529 nm and a lifetime (t 1/2 ) of up to 10 months. The NBD–QC systems with properties matched to a daily energy storage cycle are further investigated in the solid state by embedding the molecules into a series of polymer matrices revealing that polystyrene is the preferred choice of matrix. These polymer devices, which can absorb sunlight and over a daily cycle release the energy as heat, are investigated for their cyclability, showing multicycle reusability with limited degradation that might allow them to be applied as window laminates

Protecting-group-free site-selective reactions in a metal–organic framework reaction vessel

Site-selective organic transformations are commonly required in the synthesis of complex molecules. By employing a bespoke metal-organic framework (MOF, 1·[Mn(CO)3N3]), in which coordinated azide anions are precisely positioned within 1D channels, we present a strategy for the site-selective transformation of dialkynes into alkyne-functionalized triazoles. As an illustration of this approach, 1,7-octadiyne-3,6-dione stoichiometrically furnishes the mono-“click” product N-methyl-4-hex-5’-ynl-1’,4’dione-1,2,3-triazole with only trace bis-triazole side-product. Stepwise insights into conversions of the MOF reaction vessel were obtained by X-ray crystallography, demonstrating that the reactive sites are “isolated” from one another. Single-crystal to singlecrystal transformations of the Mn(I)-metalated material 1·[Mn(CO)3(H2O)]Br to the corresponding azide species 1·[Mn(CO)3N3] with sodium azide, followed by a series of [3+2] azide-alkyne cycloaddition reactions, are reported. The final liberation of the “click” products from the porous material is achieved by N-alkylation with MeBr, regenerating starting MOF 1·[Mn(CO)3(H2O)]Br, and the organic products characterized by NMR spectroscopy and mass spectrometry. Once the dialkyne length exceeds the azide separation, site selectivity is lost, confirming the critical importance of isolated azide moieties for this strategy. We postulate that carefully designed MOFs can act as physical protecting groups to facilitate other site-selective and chemoselective transformations

Chelation-driven fluorescence deactivation in three alkali earth metal MOFs containing 2,2’-dihydroxybiphenyl-4,4’-dicarboxylate

First published online 04 Sep 2013Three new metal-organic frameworks (MOFs) have been synthesised from alkali earth metal ions of increasing ionic radii (Mg, Ca and Sr) and 2,2’-dihydroxybiphenyl-4,4’-dicarboxylic acid (H4diol). The distinct coordination environments, framework topologies and the non-coordinated diol moieties accessed are a result of using differently sized metal ions for MOF synthesis which affects the ability of the diol moieties to chelate the metal. Detailed structural analysis of [Sr3(H2diol)3(DMF)5], [Ca3.5(Hdiol)(H2diol)2(DMF)5] and [Mg(H2diol)(DMF)2] show distinctive variations in variable temperature expansion/contraction properties and porosity. In addition, [Sr3(H2diol)3(DMF)5] and [Ca3.5(Hdiol)(H2diol)2(DMF)5] display a broad fluorescence emission (λmax = ~435 nm) under ultraviolet light due to the presence of non-coordinated biphenyl-diol moieties within the structures, while chelation of Mg by the diol pocket in [Mg(H2diol)(DMF)2] leads to quenching of the ligand fluorescence.Damien Rankine, Tony D. Keene, Christopher J. Sumby and Christian J. Doona

Does functionalisation enhance CO2 uptake in interpenetrated MOFs? An examination of the IRMOF-9 series

The effect of pore functionalisation (-I, -OH, -OCH3) on a series of topologically equivalent, interpenetrated metal-organic frameworks (MOFs) was assessed by both simulation and experiment. Counter-intuitively, a decreased affinity for CO2 was observed in the functionalised materials, compared to the non-functionalised material. This result highlights the importance of considering the combined effects of network topology and chemical functionality in the design of MOFs for enhanced CO2 adsorptionRavichandar Babarao, Campbell J. Coghlan, Damien Rankine, Witold M. Bloch, Gemma K. Gransbury, Hiroshi Sato, Susumu Kitagawa, Christopher J. Sumby, Matthew R. Hill and Christian J. Doona

Bridging ligands comprising two or more di-2-pyridylmethyl or amine arms: alternatives to 2,2'-bipyridyl-containing bridging ligands

Bridging ligands incorporating 2,2'-bipyridine as a chelating component have been utilised for several decades and are widely employed in coordination chemistry, supramolecular chemistry and materials synthesis. Such ligands form stable 5-membered chelate rings upon coordination to a metal. Two related chelating units, di-2-pyridylamine and di-2-pyridylmethane, which form 6-membered chelate rings when coordinated to a metal, have been studied far less as components of bridging ligands but have recently garnered significant levels of attention. Of around 140 reports on the incorporation of these moieties into bridging ligands some 75% have been published in the last 15 years. This review covers the synthesis of bridging ligands containing di-2-pyridylamine and di-2-pyridylmethane chelating moieties, and a survey of their coordination and supramolecular chemistry. Applications of the resulting systems as structural and functional models of enzyme active sites, and spin-crossover materials, and for investigations into anion-π interactions are covered. © 2011 Elsevier B.V.Christopher J. Sumbyhttp://www.elsevier.com/wps/find/journaldescription.cws_home/500845/description#descriptio

Solar energy storage at an atomically defined organic-oxide hybrid interface

Molecular photoswitches provide an extremely simple solution for solar energy conversion and storage. To convert stored energy to electricity, however, the photoswitch has to be coupled to a semiconducting electrode. In this work, we report on the assembly of an operational solar-energy-storing organic-oxide hybrid interface, which consists of a tailor-made molecular photoswitch and an atomically-defined semiconducting oxide film. The synthesized norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) was anchored to a well-ordered Co3O4(111) surface by physical vapor deposition in ultrahigh vacuum. Using a photochemical infrared reflection absorption spectroscopy experiment, we demonstrate that the anchored CNBD monolayer remains operational, i.e., can be photo-converted to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC). We show that the activation barrier for energy release remains unaffected by the anchoring reaction and the anchored photoswitch can be charged and discharged with high reversibility. Our atomically-defined solar-energy-storing model interface enables detailed studies of energy conversion processes at organic/oxide hybrid interfaces