Characterisation of redox states of metal-organic frameworks by growth on modified thin-film electrodes.

The application of metal-organic framework (MOF) materials in electrochemical and electrochromic devices remains rare. One of the main reasons for this is the inability to readily access their detailed electrochemistry. The inherent insolubility of these materials does not allow interrogation by traditional solution-based electrochemical or spectroscopic methods. In this study, we report a straightforward alternative approach to the spectroelectrochemical study of MOFs. We have used two systems as exemplars in this study, MFM-186 and MFM-180. The method involves chemical modification of a working electrode to attach MOF materials without using corrosive reagents such as inorganic acids or bases which otherwise could limit their application in device development. MFM-186 demonstrates the formation of a stable radical species [MFM-186]˙+ on electrochemical oxidation, and this has been characterised by electrochemical, spectroelectrochemical and EPR spectroscopic techniques coupled to DFT analysis.

Tailoring the Shape Memory Properties of Segmented Poly(ester urethanes) via Blending.

Thermoplastic segmented polyurethanes (PUs) can exhibit shape memory behavior, if they feature multiple kinds of physical cross-links that can be dissociated at different temperatures. This is the case if the hydrogen-bonded hard phase is joined with soft segments that can partially crystallize, so that the melting transition acts as the memory switch. For applications in the biomedical field, it is important that the fixation and recovery temperatures can be minutely controlled. We show here that this tailoring can be easily achieved by formulating a commercial PU featuring poly(1,4-butylene adipate) (PBA) as a crystallizable segment (PBA-PU) with either PBA or poly(ε-caprolactone) (PCL) of moderate molecular weight. We show that the nature of the end groups and the processing conditions dictate if there is any reaction between the components or if the product is merely a blend. Interestingly, in either case, the addition of PBA or PCL causes nucleation and thereby a noteworthy increase of the crystallization temperature of the switching element from below to above ambient temperature, so that excellent shape fixity (∼98%) can be achieved at 37 °C. The melting temperature is maintained above 50 °C and significant increases in strength and modulus are achieved. The new materials platform is well suited for applications in which a shape is to be fixed at physiological temperature.

Heterobimetallic [NiFe] Complexes Containing Mixed CO/CN- Ligands: Analogs of the Active Site of the [NiFe] Hydrogenases.

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging, and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN- ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni( N2 S2)Fe(CO)2(CN)2], [Ni( S4)Fe(CO)2(CN)2], and [Ni( N2 S3)Fe(CO)2(CN)2] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)2(CN)2} unit. X-ray crystallographic studies on [Ni( N2 S3)Fe(CO)2(CN)2], supported by DFT calculations, are consistent with a solid-state structure containing distinct molecules in the singlet ( S = 0) and triplet ( S = 1) states. Each cluster exhibits irreversible reduction processes between -1.45 and -1.67 V vs Fc+/Fc and [Ni( N2 S3)Fe(CO)2(CN)2] possesses a reversible oxidation process at 0.17 V vs Fc+/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a NiIIIFeII formulation for [Ni( N2 S3)Fe(CO)2(CN)2]+. The singly occupied molecular orbital (SOMO) in [Ni( N2 S3)Fe(CO)2(CN)2]+ is based on Ni 3dz2 and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a NiIIIFeII formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni( N2 S3)Fe(CO)2(CN)2] and its [Ni( N2 S3)] precursor, together with calculations on the oxidized [Ni( N2 S3)Fe(CO)2(CN)2]+ and [Ni( N2 S3)]+ forms suggests that the binding of the {Fe(CO)(CN)2} unit to the {Ni(CysS)4} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.

Dynamic nuclear polarisation by thermal mixing: quantum theory and macroscopic simulations.

A theory of dynamic nuclear polarisation (DNP) by thermal mixing is suggested based on purely quantum considerations. A minimal 6-level microscopic model is developed to test the theory and link it to the well-known thermodynamic model. Optimal conditions for the nuclear polarization enhancement and effects of inhomogeneous broadening of the electron resonance are discussed. Macroscopic simulations of nuclear polarization spectra displaying good agreement with experiments, involving BDPA and trityl free radicals, are presented.

A Ni(i)Fe(ii) analogue of the Ni-L state of the active site of the [NiFe] hydrogenases.

[Ni(L(1))Fe((t)BuNC)4](PF6)2 is a robust Ni(II)Fe(II) complex that undergoes a reversible one-electron reduction. Spectroscopic and theoretical studies show that [Ni(L(1))Fe((t)BuNC)4](+) is an unprecedented Ni(I)Fe(II) species that reproduces the electronic configuration of the Ni-L state of the [NiFe] hydrogenases.

The vibrational bound states of isomerising disilyne.

Full-dimensional variational calculations are reported for the isomerising disilyne molecule, Si2H2. Large-scale calculations using coordinates based on orthogonal satellite vectors permitted the computation of excited vibrational state energies and wavefunctions for all four isomeric forms: dibridged Si(H2)Si, monobridged Si(H)SiH, disilavinylidene H2SiSi, and trans-bent HSiSiH. Energies and wavefunctions have been determined for the lowest 2400 totally symmetric vibrational states; this set includes highly excited states above all three chemically relevant isomerisation barriers--up to about 8300 cm(-1) above the (dibridged) ground state. States strongly localised in the dibridged, monobridged, and disilavinylidene regions of the potential energy surface have been found as well as many partially or fully delocalised states. For the trans-bent form, only partially localised states have been identified. Comparisons are made with similar literature calculations on the isovalent acetylene-vinylidene system HCCH/H2CC.

Catalytic hydrogen production by a Ni-Ru mimic of NiFe hydrogenases involves a proton-coupled electron transfer step.

A combined electrochemical and theoretical study suggests that hydrogen evolution from weak acids catalyzed by a structural mimic of the active site of NiFe hydrogenases [Ni(xbsms)Ru(C6Me6)Cl](+) proceeds through proton-coupled electron transfer steps.

The potential energy surface of isomerising disilyne.

A (semi-)global, analytical potential energy surface is reported for the ground electronic state of the isomerising disilyne molecule, Si2H2. The surface reproduces well ab initio energies calculated at the CCSD(T) level with a cc-pV(Q+d)Z basis set for over 50 000 symmetrically unique molecular geometries. Of these ab initio points, 33 000 were used in a least-squares fit to determine the parameters of the analytical surface and the remainder to provide an independent test/validation set. The fitted surface includes: the four known isomeric forms of disilyne, dibridged, monobridged, disilavinylidene and trans-bent; the three most important transition states and four other critical points. The surface reproduces accurately existing experimental spectroscopic data for the dibridged and monobridged isomers and predictions are made for the disilavinylidene and trans-bent forms. The surface has the correct symmetry properties with respect to permutation of like atoms and is suitable for detailed dynamics studies of the isomerising Si2H2 system. Also reported is a systematic investigation of the critical points using the CCSD(T) and MRCI methods and basis sets up to 6-zeta quality: the effects of core-correlation, augmentation with diffuse functions and tight-d functions have been studied. The basis sets include the correlation consistent core-valence, cc-pCV(n+d)Z, basis sets recently developed by Yockel and Wilson [Theor. Chem. Acc., 2008, 120, 119]. Very good agreement is obtained between the theoretical and experimental equilibrium geometries, rotational constants and three available vibration frequencies for the dibridged isomer and for the rotational constants of the monobridged isomer. Multireference character, as measured by the T1 diagnostic, is found to vary significantly across the 12 critical points investigated.