Harriott Nowell

Photoreactivity examined through incorporation in metal−organic frameworks

Metal-organic frameworks, typically built by bridging metal centres with organic linkers, have recently shown great promise for a wide variety of applications, including gas separation and drug delivery. Here, we have used them as a scaffold to probe the photophysical and photochemical properties of metal-diimine complexes. We have immobilized a M(diimine)(CO)(3)X moiety (where M is Re or Mn, and X can be Cl or Br) by using it as the linker of a metal-organic framework, with Mn(II) cations acting as nodes. Time-resolved infrared measurements showed that the initial excited state formed on ultraviolet irradiation of the rhenium-based metal-organic framework was characteristic of an intra-ligand state, rather than the metal-ligand charge transfer state typically observed in solution, and revealed that the metal-diimine complexes rearranged from the fac- to mer-isomer in the crystalline solid state. This approach also enabled characterization of the photoactivity of Mn(diimine)(CO)(3)Br by single-crystal X-ray diffraction.

Highly porous and robust scandium-based metal–organic frameworks for hydrogen storage

The metal-organic frameworks NOTT-400 and NOTT-401, based on a binuclear [Sc(2)(μ(2)-OH)(O(2)CR)(4)] building block, have been synthesised and characterised; the desolvated framework NOTT-401a shows a BET surface area of 1514 m(2) g(-1) with a total H(2) uptake of 4.44 wt% at 77 K and 20 bar.

Near-critical water, a cleaner solvent for the synthesis of a metal-organic framework

The microporous metal–organic framework {[Zn2(L)]·(H2O)3}∞ (H4L = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene) has been synthesised using near-critical water (300 °C) as a cleaner alternative to toxic organic solvents. A single crystal X-ray structure determination confirms that the complex incorporates tetrahedral Zn(II) centres bridged through the carboxylate anions to form a binuclear building block, which extends into a one dimensional chain along the c axis. Four L4− ligands bind to each Zn(II) centre and cross-link the one dimensional chains along both a and b axes to afford a three dimensional network structure incorporating pores of ca. 4.3 A in diameter. The complex shows high thermal stability up to 425 °C by gravimetric thermal analysis, and on desolvation, displays a high adsorption enthalpy of 11.0 kJ mol−1 for H2 uptake at zero coverage, consistent with the narrow pore diameter for the framework.

Chemical control of spin propagation between heterometallic rings

We present a synthetic, structural, theoretical, and spectroscopic study of a family of heterometallic ring dimers which have the formula [{Cr(7)NiF(3)(Etglu)(O(2)CtBu)(15)}(2)(NLN)], in which Etglu is the pentadeprotonated form of the sugar N-ethyl-D-glucamine, and NLN is an aromatic bridging diimine ligand. By varying NLN we are able to adjust the strength of the interaction between rings with the aim of understanding how to tune our system to achieve weak magnetic communication between the spins, a prerequisite for quantum entanglement. Micro-SQUID and EPR data reveal that the magnetic coupling between rings is partly related to the through-bond distance between the spin centers, but also depends on spin-polarization mechanisms and torsion angles between aromatic rings. Density functional theory (DFT) calculations allow us to make predictions of how such chemically variable parameters could be used to tune very precisely the interaction in such systems. For possible applications in quantum information processing and molecular spintronics, such precise control is essential.

A Perylene Diimide Rotaxane: Synthesis, Structure and Electrochemically Driven De‐Threading

The first example of a [2]-rotaxane in which a perylene diimide acts as a recognition site has been synthesised and characterised. The interlocked nature of the compound has been verified by both NMR studies and an X-ray structure determination. Electrochemical investigations confirm that the nature of the redox processes associated with the perylene diimide are modified by the complexation process and that it is possible to mono-reduce the [2]-rotaxane to give a radical anion based rotaxane. Further reduction of the compound leads to de-threading of the macrocycle from the reduced PTCDI recognition site. Our synthetic strategies confirm the potential of PTCDI-based rotaxanes as viable targets for the preparation of complex interlocked species.

Tailoring porosity and rotational dynamics in a series of octacarboxylate metal-organic frameworks

Significance A family of stable porous materials incorporating organic linkers and Cu(II) cations is reported. Their pores can be altered systematically by elongation of the ligands allowing a strategy of selective pore extension along one dimension. These materials show remarkable gas adsorption properties with high working capacities for CH4 (0.24 g g−1, 163 cm3 cm−3 at 298 K, 5–65 bar) for the most porous system. The mechanism of rotation of the organic groups in the solid state has been analyzed by NMR spectroscopy and rotational rates and transition temperatures analyzed. Significantly, we show that framework dynamics can be controlled by ligand design only, and this paves the way to understanding the role of molecular rotors within these materials. Modulation and precise control of porosity of metal-organic frameworks (MOFs) is of critical importance to their materials function. Here we report modulation of porosity for a series of isoreticular octacarboxylate MOFs, denoted MFM-180 to MFM-185, via a strategy of selective elongation of metal-organic cages. Owing to the high ligand connectivity, these MOFs do not show interpenetration, and are robust structures that have permanent porosity. Interestingly, activated MFM-185a shows a high Brunauer–Emmett–Teller (BET) surface area of 4,734 m2 g−1 for an octacarboxylate MOF. These MOFs show remarkable CH4 and CO2 adsorption properties, notably with simultaneously high gravimetric and volumetric deliverable CH4 capacities of 0.24 g g−1 and 163 vol/vol (298 K, 5–65 bar) recorded for MFM-185a due to selective elongation of tubular cages. The dynamics of molecular rotors in deuterated MFM-180a-d16 and MFM-181a-d16 were investigated by variable-temperature 2H solid-state NMR spectroscopy to reveal the reorientation mechanisms within these materials. Analysis of the flipping modes of the mobile phenyl groups, their rotational rates, and transition temperatures paves the way to controlling and understanding the role of molecular rotors through design of organic linkers within porous MOF materials.

Manganese(II) and copper(II) nitrate bis-imidazole coordination polymers: dimensionality and product morphology

The reaction of a bis-imidazole ligand, N,N′-bis(3-(1-imidazolyl)propyl)-1,4,5,8-naphthalene-tetracarboxylicdiimide (BIPNDI), with either Mn(NO3)2 or Cu(NO3)2 affords coordination polymers of formula {[Mn(BIPNDI)3](NO3)2}∞ or [M(BIPNDI)2(NO3)2]∞ (M = Mn or Cu). Single crystal X-ray diffraction studies confirm the structures of supramolecular isomers of {[Mn(BIPNDI)3](NO3)2}∞ (1,2), both of which contain two-dimensional coordination polymer sheets, and a one-dimensional coordination polymer [Mn(BIPNDI)2(NO3)2]∞ (3). Reaction of either Mn(NO3)2 or Cu(NO3)2 with BIPNDI exploiting rapid precipitation affords sub-micron coordination particles of either rod-shaped (4, Mn) or spherical (5, Cu) morphology depending on the metal salt used. SEM and DLS studies were used to probe the morphology and solution behaviour of the coordination particles. Spectroscopic investigations indicate that the rod-shaped, Mn-based particles comprise two-dimensional coordination polymers in contrast to the spherical, Cu-based particles which are composed of one-dimensional coordination polymers.

DetOx: a program for determining anomalous scattering factors of mixed-oxidation-state species

Overlapping absorption edges will occur when an element is present in multiple oxidation states within a material. DetOx is a program for partitioning overlapping X-ray absorption spectra into contributions from individual atomic species and computing the dependence of the anomalous scattering factors on X-ray energy. It is demonstrated how these results can be used in combination with X-ray diffraction data to determine the oxidation state of ions at specific sites in a mixed-valance material, GaCl(2).