High-pressure studies of macrocycle coordination complexes

Chapter 1: An introduction is given to high-pressure crystallography with the experimental design and equipment required outlined. The basic theory that underpins X-ray diffraction and structure solution is covered with emphasis given to points that raise considerations for high-pressure crystallographic studies; key software and their uses are briefly introduced. A literature survey of molecular coordination complexes under pressure is given that provides a detailed view of the typical phenomena observed and interrogated in such work. Chapter 2: Recrystallisation of [PdCl2([9]aneS2O)] ([9]aneS2O = 1-oxa-4,7-dithiacyclononane), 1, and [PtCl2([9]aneS2O)], 2, by diffusion of Et2O vapour into solutions of these complexes in CH3NO2 has yielded three phases of 1 and two phases of 2. The phase of 1, herein designated α-1, was obtained under ambient conditions. A second phase, designated β-1, was initially also obtained by this method; following the advent of a third phase, γ-1, all subsequent efforts over a period of a year to crystallise β-1 yielded either γ-1, which was typically obtained by carrying out the recrystallisation at elevated temperature, or α-1, commonly found throughout the study. This persistent absence of a phase which could initially be crystallised with ease led to the conclusion that β-1 was an example of a ‘disappearing polymorph’. The first phase obtained of 2, designated α-2, was obtained by recrystallisation under ambient conditions and is isomorphous and isostructural with α-1. The second phase, β-2, was obtained by slight elevation of the recrystallisation temperature and was found to be isomorphous and isostructural with β-1. Subsequently, β-2 was used to seed the growth of the disappearing polymorph β-1. No third phase of 2 ("γ-2") has been isolated. Density functional theory calculations were employed to aid in rationalising this behaviour. Chapter 3: The three reported phases of the mononuclear macrocyclic Pd(II) complex [PdCl2([9]aneS2O)] (1) were each studied up to pressures exceeding 90 kbar using high pressure single crystal X-ray diffraction. The α and γ phases both exhibited smooth compression of the unit cell parameters with third-order Birch-Murnaghan bulk moduli of 14.4(8) and 7.6(6) GPa, respectively. Between 68.1 and 68.7 kbar β-1 was found to undergo a reversible transformation to a phase denoted β’ and characterised by a tripling of the unit cell volume. Across the phase transition, rearrangement of the conformation of the bound macrocycle at two of the resulting three unique sites gave rise to an extensively disordered structure. This phenomenon was largely owed to a close and approximately linear C−H···H−C approach between macrocycles. Density functional theory calculations were employed to further understand the high-pressure behaviour of the phases. Cooling from 290 to 90 K in complementary variable temperature crystallographic studies revealed similar effects as ca. 5 kbar pressure. Chapter 4: The two reported phases of the mononuclear macrocyclic Pt(II) complex [PtCl2([9]aneS2O)] (2) were each studied up to pressures exceeding 90 kbar using high pressure single crystal X-ray diffraction. The α phase exhibited smooth compression of the unit cell parameters with third-order Birch-Murnaghan bulk modulus of 11.8(5) GPa. Between 65.2 and 69.9 kbar β-2 was found to undergo an incomplete rearrangement of the macrocycle that was not characterised by a phase transition as seen for the corresponding Pd(II) phase. The β phase was also indicated to be more resistant to compression than the α phase with a third-order Birch-Murnaghan bulk modulus of 13.5(5) GPa. The conformational rearrangement was again rationalised by a close and approximately linear C−H···H−C approach between macrocycles. Density functional theory calculations were employed to further understand the high-pressure behaviour of these two phases and why β-1 and β-2 might differ. Cooling from 290 to 90 K in complementary variable temperature crystallographic studies again revealed similar effects as ca. 5 kbar pressure. Chapter 5: The previously unreported solvate [Pd([9]aneS3)2](PF6)2·2CH3NO2 is studied using high-pressure crystallography, high-pressure solid-state UV/vis spectroscopy and density function theory calculations to interrogate the piezochromism previously observed by this group. Up to 49.3 kbar, gradual sky blue to dull green piezochromism was observed with considerable compression of the elongated axial Pd···S interactions. A reversible P21/c → P-1 phase transition with doubling of the unit cell volume was observed between 49.3 and 51.0 kbar. This was accompanied by a dull green to orange stepwise piezochromism and characterised by an organised reorientation of the coordination axes in 50 % of the cations. The phase transition had a range of effects on the axial interactions which remained compressible in the high-pressure phase. No further piezochromism was clearly observed. Density functional theory calculations showed a fair match with experimentally obtained spectra and strongly indicated that the piezochromism is primarily owed to compression of the axial interaction. These calculations also indicated that outer-sphere effects further modulate the piezochromism, but gave no evidence for a cause of the phase transition. The phase transition was thus rationalised as a response to the large value of the PV term of the Gibb’s free energy associated with the transition. The ambient pressure structures of two other previously unreported solvates are also reported. Chapter 6: The templated polyiodide framework [Ag([18]aneS6)]I7 were studied to ca. 45 kbar. Each of the two crystals employed in this study underwent two phase transitions: at ca. 11 kbar an R-3m → R3m transition was observed in both crystals. This appeared to be ferroelectric in nature and was associated with a change in bonding of the polyiodide network from 3∞[I−·(I2)6/2] to 3∞[I7−]. Analysis of the calculated Mayer bond orders for the catenating I−···I2 interactions supported this description of the bonding. Compression of the first phase appears essentially the same for both crystals; compression through the second phase differed between crystals and the second phase transition, at ca. 40 kbar in both cases, resulted in differing monoclinic phases. The second transition was associated with the ordering of the conformation of the macrocycle: one phase appeared ordered from the perspective of the refinement and the macrocycle adopted the previously unseen [84114] conformation by Dale analysis. The other phase appears disordered from the perspective of the refinement but would appear to comprise alternating enantiomers of the [333333] conformer. The differing conformation of the macrocycle in the third phase was taken as indicative of differing major components in the disordered lower phases. This point of difference in turn rationalised the different response to pressure in the second phase: compression of the second phase is limited by interactions between cations while the first phase is dependent on the catenating I−···I2 interactions which are identical between crystals. Chapter 7: A summary of the key findings of this body of work is given along with suggested avenues for future studies. This thesis closes with the wider reaching considerations that are highlighted by this body of work

Polymorphism and structural variety in Sn(II) carboxylate coordination polymers revealed from structure solution of microcrystals

The crystal structures of four coordination polymers constructed from Sn(II) and polydentate carboxylate ligands are reported. All are prepared under hydrothermal conditions in KOH or LiOH solutions (either water or methanol–water) at 130−180 °C and crystallize as small crystals, microns or less in size. Single‐crystal structure solution and refinement are performed using synchrotron X‐ray diffraction for two materials and using 3D electron diffraction (3DED) for the others. Sn2(1,3,5‐BTC)(OH), where 1,3,5‐BTC is benzene‐1,3,5‐tricarboxylate, is a new polymorph of this composition and has a three‐dimensionally connected structure with potential for porosity. Sn(H‐1,3,5‐BTC) retains a partially protonated ligand and has a 1D chain structure bound by hydrogen bonding via ─COOH groups. Sn(H‐1,2,4‐BTC) contains an isomeric ligand, benzene‐1,2,4‐tricarboxylate, and contains inorganic chains in a layered structure held by hydrogen bonding. Sn2(DOBDC), where DOBDC is 2,5‐dioxido‐benzene‐1,4‐dicarboxylate, is a new polymorph for this composition and has a three‐dimensionally connected structure where both carboxylate and oxido groups bind to the tin centers to create a dense network with dimers of tin. In all materials, the Sn centers are found in highly asymmetric coordination, as expected for Sn(II). For all materials phase purity of the bulk is confirmed using powder X‐ray diffraction, thermogravimetric analysis, and infrared spectroscopy

Halogen-substituted ureas for anion binding: solid state and solution studies

Herein, we report the synthesis and the anion binding properties of a family of N,N′-diphenylureas L1-L15, bearing on the aromatic ring(s) halogens (chlorine and iodine) and/or nitro or trifluoromethyl electron-withdrawing groups. The analysis of the crystal structures obtained from single crystal X-ray diffraction experiments shows that self-assembled chains or tapes connected via N–H···O hydrogen bonds are the most commonly adopted arrangements for this type of molecules in the crystal lattice. In the presence of anion guests or solvent molecules with competing hydrogen bond donors and acceptors, other supramolecular arrangements can be observed. Solution studies conducted in DMSOd 6/0.5% H2O by means of 1H-NMR titrations show the formation of 1:1 adducts with all receptors. The different observed affinities of the receptors for the anion guests were rationalised in terms of steric hindrance of the substituents on the phenyl rings and their electron-withdrawing properties

Asymmetric phase diagram and dimensional crossover in a system of spin-spin- 1/2 dimers under applied hydrostatic pressure

We present the magnetic and structural properties of [Cu(pyrazine)0.5 (glycine)]ClO4 under applied pressure. As previously reported, at ambient pressure this material consists of quasi-two-dimensional layers of weakly coupled antiferromagnetic dimers which undergo Bose-Einstein condensation of triplet excitations between two magnetic field-induced quantum critical points (QCPs). The molecular building blocks from which the compound is constructed give rise to exchange strengths that are considerably lower than those found in other S=1/2 dimer materials, which allows us to determine the pressure evolution of the entire field-temperature magnetic phase diagram using radio-frequency magnetometry. We find that a distinct phase emerges above the upper field-induced transition at elevated pressures and also show that an additional QCP is induced at zero-field at a critical pressure of pc=15.7(5),kbar. Pressure-dependent single-crystal X-ray diffraction and density functional theory calculations indicate that this QCP arises primarily from a dimensional crossover driven by an increase in the interdimer interactions between the planes. While the effect of quantum fluctuations on the lower field-induced transition is enhanced with applied pressure, quantum Monte Carlo calculations suggest that this alone cannot explain an unconventional asymmetry that develops in the phase diagram

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(N2S2)Fe(CO)2(CN)2], [Ni(S4)Fe(CO)2(CN)2] and [Ni(N2S3)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(N2S3)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 to −1.67 V vs Fc+/Fc and [Ni(N2S3)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(N2S3)Fe(CO)2(CN)2]+. The SOMO in [Ni(N2S3)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(N2S3)Fe(CO)2(CN)2] and its [Ni(N2S3)] precursor, together with calculations on the oxidized [Ni(N2S3)Fe(CO)2(CN)2]+ and [Ni(N2S3)]+ 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.

Halogen-substituted ureas for anion binding: solid state and solution studies

Herein, we report the synthesis and the anion binding properties of a family of N,N′-diphenylureas L1-L15, bearing on the aromatic ring(s) halogens (chlorine and iodine) and/or nitro or trifluoromethyl electron-withdrawing groups. The analysis of the crystal structures obtained from single crystal X-ray diffraction experiments shows that self-assembled chains or tapes connected via N–H···O hydrogen bonds are the most commonly adopted arrangements for this type of molecules in the crystal lattice. In the presence of anion guests or solvent molecules with competing hydrogen bond donors and acceptors, other supramolecular arrangements can be observed. Solution studies conducted in DMSOd 6/0.5% H2O by means of 1H-NMR titrations show the formation of 1:1 adducts with all receptors. The different observed affinities of the receptors for the anion guests were rationalised in terms of steric hindrance of the substituents on the phenyl rings and their electron-withdrawing properties

Heterobimetallic [NiFe] complexes containing mixedCO/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(N2S2)Fe(CO)2(CN)2], [Ni(S4)Fe(CO)2(CN)2] and [Ni(N2S3)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(N2S3)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 to −1.67 V vs Fc+/Fc and [Ni(N2S3)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(N2S3)Fe(CO)2(CN)2]+. The SOMO in [Ni(N2S3)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(N2S3)Fe(CO)2(CN)2] and its [Ni(N2S3)] precursor, together with calculations on the oxidized [Ni(N2S3)Fe(CO)2(CN)2]+ and [Ni(N2S3)]+ 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.

Structural origins of dielectric anomalies in the filled tetragonal tungsten bronze, Sr2NaNb5O15

The tetragonal tungsten bronze, Sr2NaNb5O15, shows promise for application in high-temperature high-efficiency capacitors vital for the sustainable energy revolution. Previously, the structural complexity of this and related materials has obscured the mechanisms underpinning two large anomalies in relative permittivity (εr) which give rise to their exceptionally broad dielectric response. We comprehensively investigate the structural evolution from −173 to 627 °C, combining electron, X-ray and neutron diffraction, electron microscopy, and first principles electronic structure calculations to unambiguously identify the structural origins of both anomalies. The peak in εr at 305 °C is associated with a polar-nonpolar phase transition, wherein cations displace along the c-axis. Guided by DFT, we identify a further transition upon cooling, associated with the second peak at −14 °C, linked to the softening of an in-plane polar distortion with a correlation length limited by ferroelastic nano-domains arising from rigid-unit-like tilting of NbO6 octahedra at high temperature, imparting relaxor-like behaviour. Thus, the two dielectric anomalies in Sr2NaNb5O15 are associated with two distinct crystallographic phase transitions and their interplay with a microstructure that arises from a third, non-polar structural distortion. Chemical control of these will enable development of tuneable materials with dielectric properties suitable for high-temperature energy storage applications

Symmetry-informed design of magnetoelectric coupling in the manganite perovskite CeBaMn2O6

Magnetoelectric multiferroics hold great promise for the development of new sustainable memory devices. However, practical applications of many existing multiferroic materials are infeasible due to the weak nature of the coupling between the magnetic and electrical orderings, meaning new magnetoelectric multiferroics featuring intrinsic coupling between their component orderings are sought instead. Here, we apply a symmetry-informed design approach to identify and realize the new manganite perovskite CeBaMn2O6 in which magnetoelectric coupling can be achieved via an intermediary non-polar structural distortion. Through first-principles calculations, we demonstrate that our chosen prototype system contains the required ingredients to achieve the desired magnetoelectric coupling. Using high-pressure/high-temperature synthesis conditions, we have been able to synthesize the CeBaMn2O6 perovskite system for the first time. Our subsequent neutron and electron diffraction measurements reveal that the desired symmetry-breaking ingredients exist in this system on a nanoscopic length scale, enabling magnetoelectric nanoregions to emerge within the material. Through this work, we showcase the potential of the new CeBaMn2O6 perovskite material as a promising system in which to realize strong magnetoelectric coupling, highlighting the potential of our symmetry-informed design approach in the pursuit of new magnetoelectric multiferroics for next-generation memory devices

Structural chemistry of metal coordination complexes at high pressure

The application of pressures of up to about 10 GPa may induce significant geometric, configurational, conformational and packing changes in molecular solids. This review highlights and describes recent advances in high pressure studies of coordination complexes, many of which have been conducted at synchrotrons or other central facilities. The main focus is on the wide range of geometric changes which occur with pressure. In some cases these changes have associated physical effects, and the review describes materials exhibiting negative linear compressibility, spin cross-over phenomena, magnetism and molecular conduction, as well as detailing the exciting possibilities for future developments in this area of research