d-block coordination compounds
The two achiral ligands tris(1-methyl-1H-imidazol-2-yl)methanol ((mim)3COH) and bis(1-methyl-1H-imidazol-2-yl)(3-methylpyridin-2-yl)methanol ((mim)2(mpy)COH) form on reaction with Fe(BF4)2∙6H2O, the octahedral low-spin complexes [Fe((mim)3COH)2](BF4)2∙MeCN (1) and [Fe((mim)2(mpy)COH)2](BF4)2∙0.5MeCN (2). Both octahedral complexes immediately rearrange to the chiral [Fe4O4]-cubane clusters [Fe4(mim)3CO)4](BF4)4 (3) and [Fe4(mim)3CO)4](BF4)4∙CHCl3 (4), whereas the highly symmetrical 3 crystallizes as racemate, 4 resolves based on the asymmetry introduced by the 2-methylpyridine moiety and crystallizes as an enantiomerically pure sample. Both clusters feature redox active [Fe4O4]-cubane cores with up to four individual accessible states, which directs towards a potential application as electron-shuttle.
"Solvothermal One-Pot Synthesis of a New Family of Chiral [Fe4O4]-Cubane Clusters with Redox Active Cores”, M. Seifried, F.M. Kapsamer, M. Reissner, J.M. Welch, G. Giester, D. Müller, P. Weinberger, Magnetochemistry 2022, 8(9), 95
To increase the supramolecular cooperativity in Fe(II) spin crossover materials based on N1-substituted tetrazoles, a series of ω-(1H-tetrazol-1-yl) carboxylic acids with chain-lengths of C2–C4 were synthesized. Structural characterization confirmed the formation of a strong hydrogen-bond network, responsible for enhanced cooperativity in the materials and thus largely complete spin-state transitions for the ligands with chain lengths of C2 and C4. To complement the structural and magnetic investigation, electronic spectroscopy was used to investigate the spin-state transition. An initial attempt to utilize the bifunctional coordination ability of the ω-(1H-tetrazol-1-yl) carboxylic acids for preparation of mixed-metallic 3d–4f coordination polymers resulted in a novel one-dimensional gadolinium-oxo chain system with the ω-(1Htetrazol-1-yl) carboxylic acid acting as μ2-η2:η1 chelating–bridging ligand.
"Bifunctional Fe(II) Spin Crossover-complexes based on ω-(1H-tetrazol-1-yl) carboxylic acids”, W. Zeni, M. Seifried, C. Knoll, J.M. Welch, G. Giester, B. Stöger, W. Artner, M. Reissner, D. Müller, P. Weinberger, Dalton Trans., 49 (2020) 17183 – 17193
Criteria for a technologically relevant spin crossover (SCO) material include temperature and abruptness. A series of Fe(II) – 1,3-bis((1H tetrazol-1-yl)methyl)bicyclo[1.1.1]pentane SCO complexes with various anions (BF4−, ClO4−, and PF6−) designed using a structure–property based concept is reported. All complexes feature abrupt SCO-behavior with T1/2 between 170 K and 187 K. These materials demonstrate that without stabilizing the effects of incorporated solvents or a hydrogen bond-network, the observed cooperativity during high-spin–low-spin transition is anion independent and originates only from the rigidity and internal strain of the propellane-moiety in the ligand. Spectroscopy and structural investigations of these materials are supported by quantum chemical calculations.
„Cooperativity in spin crossover materials as ligand’s responsibility – investigations of the Fe(II) – 1,3-bis((1H-tetrazol-1-yl)methyl)bicycle [1.1.1]pentane system“, C. Knoll, D. Müller, M. Seifried, G. Giester, J. Welch, W. Artner, K. Hradil, M. Reissner, P. Weinberger, Dalton Trans., 2018, 47, 5553–5557
1-(3-Halopropyl)-1H-tetrazoles and their corresponding FeII spin-crossover complexes have been investigated in a combined experimental and theoretical study. Halogen substitution was found to positively influence the spin transition, shifting the transition temperature about 70 K towards room temperature. Halogens located at the w position were found to be too far away from the coordinating tetrazole moiety to have an electronic impact on the spin transition. The subtle variation of the steric demand of the ligand in a highly comparable series was found to have a comparatively large impact on the spin-transition behavior, which highlights the sensitivity of the effect to subtle structural changes.
„Halogenated Alkyltetrazoles for the Rational Design of FeII Spin-Crossover Materials: Fine-Tuning of the Ligand Size“, D. Müller, C. Knoll, M. Seifried, J. Welch, G. Giester, M. Reissner, P. Weinberger, Chem. Eur. J. 2018, 24, 5271–5280
f-block coordination compounds
Two novel nitrogen-rich lanthanide compounds of 5,5´-(azobis)tetrazolide (ZT) were synthesized and structurally characterized. The dinuclear, isostructural compounds [Ce2(ZT)2CO3(H2O)12] · 4 H2O (1) and [Pr2(ZT)2CO3(H2O)12] · 4 H2O (2) were synthesized via two independent routes. Compound 1 was obtained after partial Lewis acidic decomposition of ZT by CeIV in aqueous solution of (NH4)2Ce(NO3)6 and Na2ZT. Compound 2 was obtained by crystallization from aqueous solutions of Pr(NO3)3, Na2ZT, and Na2CO3. By X-ray diffraction analysis at 200 K, it was found that the trivalent lanthanide cations are bridged by a bidentate carbonato ligand and each cation is further coordinated by six H2O ligands and one ZT ligand thus being ninefold coordinated.
“Controlling complexation behaviour of early lanthanides via the subtle interplay of their Lewis acidity with the chemical stability of 5,5´-(azobis)tetrazolide”, P. Weinberger, G. Giester, G. Steinhauser, Z. Anorg. Allg. Chem., 646 (2020) 1882–1885.
A series of rare earth element (REE) mixed-anion 5,5′-azobis(1H-tetrazol-1-ide)-carbonate ([REE2(ZT)2CO3(H2O)10] · 2 H2O; REE = lanthanides plus yttrium) coordination compounds were synthesized, characterized, and analyzed. Syntheses by simple metathesis reactions under a CO2 atmosphere were carried out at ambient (La–Gd and Ho) and elevated pressures (55 bar; Tb, Dy, Er, Tm, Yb, and Y). The resulting crystalline materials were characterized principally by single-crystal X-ray diffraction and vibrational spectroscopy (infrared and Raman). All materials are structurally isotypic, crystallizing in the space group C2/c and show nearly identical spectroscopic properties for all the elements investigated. Cell parameters, bond lengths, and bond angles differ marginally, revealing only a slight variation coinciding with the lanthanide (Ln) contraction, that is, the change in the ionic radii of the trivalent rare earth elements.
The herein reported series of rare earth element azobis[tetrazolide]-carbonates represents a remarkable exception as they are a series of isotypic REE coordination compounds with tetrazolide-derived ligands unaffected by the “gadolinium break”.
"Azobis[tetrazolide]‐Carbonates of the Lanthanides - Breaking the Gadolinium Break", Müller, D. , Knoll, C. , Herrmann, A. , Savasci, G. , Welch, J. M., Artner, W. , Ofner, J. , Lendl, B. , Giester, G. , Weinberger, P. and Steinhauser, G., Eur. J. Inorg. Chem., 2018, 1969-1975
The crystallization of terbium 5,5’-azobis[1H-tetrazol-1-ide] (ZT) in the presence of trace amounts (ca. 50 Bq, ca. 1.6 pmol) of americium results in 1) the accumulation of the americium tracer in the crystalline solid and 2) a material that adopts a different crystal structure to that formed in the absence of americium. Americium-doped [Tb(Am)(H2O)7ZT]2ZT · 10 H2O is isostructural to light lanthanide (Ce–Gd) 5,5’-azobis[1H-tetrazol-1-ide] compounds, rather than to the heavy lanthanide (Tb–Lu) 5,5’-azobis[1H-tetrazol-1-ide] (e.g., [Tb-(H2O)8]2ZT3 · 6 H2O) derivatives. Traces of Am seem to force the Tb compound into a structure normally preferred by the lighter lanthanides, despite a 108-fold Tb excess. The americium-doped material was studied by single-crystal X-ray diffraction, vibrational spectroscopy, radiochemical neutron activation analysis, and scanning electron microcopy. In addition, the inclusion properties of terbium 5,5’-azobis[1Htetrazol-1-ide] towards americium were quantified, and a model for the crystallization process is proposed.
"Picomolar Traces of Americium(III) Introduce Drastic Changes in the Structural Chemistry of Terbium(III): A Break in the “Gadolinium Break”", J. Welch, D. Müller, C. Knoll, M. Wilkovitsch, G. Giester. J. Ofner, B. Lendl, P. Weinberger, G. Steinhauser, Angew. Chem. Int. Ed.,2017, 56, 13264
The pressure effect on the carbonation behavior of CaO as model compound is studied under mild hydrothermal conditions, as relevant to sustainable geological CO2 sequestration and for potential utilization in thermochemical energy storage. Reaction yields are determined experimentally by means of in-situ powder X-ray diffraction using CaO powder samples in a controlled reaction with CO2 under gas pressures between 1.0 and 5.0 MPa and at temperatures between 298 and 373 K. The results show a two-step conversion of CaO to CaCO3, involving Ca(OH)2 as a reactive intermediate, with differing influences of the microstructures on the individual reaction sub-steps. A kinetic evaluation of the experimental data delivers a high rate-enhancing effect of temperature on the hydration reaction, whereas the CaCO3 formation is strongly dependent on the available CO2 gas pressure. With this systematic investigation the optimal pressure and temperature conditions for this reaction system can be determined delivering a contribution to a sustainable climate and energy management. The pressure effect on the carbonation behavior of CaO as model compound is studied under mild hydrothermal conditions, as relevant to sustainable geological CO2 sequestration and for potential utilization in thermochemical energy storage. Reaction yields are determined experimentally by means of in-situ powder X-ray diffraction using CaO powder samples in a controlled reaction with CO2 under gas pressures between 1.0 and 5.0 MPa and at temperatures between 298 and 373 K. The results show a two-step conversion of CaO to CaCO3, involving Ca(OH)2 as a reactive intermediate, with differing influences of the microstructures on the individual reaction sub-steps. A kinetic evaluation of the experimental data delivers a high rate-enhancing effect of temperature on the hydration reaction, whereas the CaCO3 formation is strongly dependent on the available CO2 gas pressure. With this systematic investigation the optimal pressure and temperature conditions for this reaction system can be determined delivering a contribution to a sustainable climate and energy management.
“Pressure Dependence of the Low Temperature Carbonation Kinetics of Calcium Oxide for Potential Thermochemical Energy Storage Purposes and Sustainable CO2 Fixation”, G. Gravogl, F. Birkelbach, D. Müller, C.L. Lengauer, P. Weinberger, R. Miletich, Adv. Sustainable Syst., (2021) 2100022 (1-11).
Materials with high volumetric energy storage capacities are targeted for high-performance thermochemica energy storage systems. The reaction of transition metal salts with ammonia, forming reversibly the corresponding ammonia-coordination compounds, is still an under-investigated area for energy storage purposes, although, from a theoretical perspective this should be a good fit for application in medium-temperature storage solutions between 25 ◦C and 350 ◦C.
In the present study, the potential of reversible ammoniation of a series of transition metal chlorides and sulphates with gaseous ammonia for suitability as thermochemical energy storage system was investigated. Among the investigated metal chlorides and sulphates, candidates combining high energy storage densities and cycle stabilities were found. For metal chlorides, during the charging / discharging cycles in the presence of ammonia slow degradation and evaporation of the materials was observed. This issue was circumvented by reducing the operating temperature and cycling between different degrees of ammoniation, e.g. in the case of NiCl2 by cycling between [Ni(NH3)2]Cl2 and [Ni(NH3)6]Cl2. In contrast, sulphates are perfectly stable under all investigated conditions.
The combination of CuSO4 and NH3 provided the most promising result directing towards applicability, as the high energy storage density of 6.38 GJ m-3 is combined with full reversibility of the storage reaction and no material degradation over cycling. The results of this comparative systematic material evaluation encourage for a future consideration of the so far underrepresented transition metal ammoniates as versatile thermochemical energy storage materials.
“Medium-temperature thermochemical energy storage with transition metal ammoniates – a systematic comparison”, D. Müller, C. Knoll, G. Gravogl, C. Jordan, E. Eitenberger, G. Friedbacher, W. Artner, J.M. Welch, A. Werner, M. Harasek, R. Miletich, P. Weinberger, Applied Energy, 285 (2021) 116470, 1-11.
Systematic variation of the dehydration temperature and time enables the preparation of highly reactive magnesium oxide for thermochemical energy storage purposes. The reactivity of the MgO, resulting from varying dehydration conditions has been studied by a comparative approach, including reactive surface area, particle morphology and reactivity towards rehydration. For the rehydration an in-situ powder X-Ray diffraction setup is used, allowing for continuous monitoring of Mg(OH)2 formation. The outcome of this investigation was subsequently applied to MgO from natural magnesites to assess the impact of impurities in the material on rehydration reactivity.
“Tuning the performance of MgO for thermochemical energy storage by dehydration – From fundamentals to phase impurities”, D. Müller, C. Knoll, G. Gravogl, W. Artner, J. Welch, E. Eitenberger, G. Friedbacher, M. Schreiner, M. Harasek, K. Hradil, A. Werner, R. Miletich, P. Weinberger, Applied Energy 253, 2019, 113562
Metal carbonates are attractive materials for combining carbon capture and thermochemical energy storage. Carbonate materials feature high decomposition and formation temperatures and may be considered in applications in combination with concentrating solar power. In the present study in-situ P-XRD carbonation (1–8 bar CO2) and reactor-based experiments (1–55 bar CO2) are combined focusing on the effect of elevated CO2 pressures on carbonation of metal oxides. Carbonation of MnO and PbO at CO2 pressures between 8 and 50 bar in the presence of moisture resulted in reaction with CO2, forming the corresponding carbonates at notably lower temperatures than under dry CO2 atmosphere of 1 bar. This enables the application of metal oxide/metal carbonate reaction couples for energy storage at temperatures between 25 and 500 °C. Based on the reversible carbonation/decarbonation of PbO under varying CO2 pressures, an isothermal storage cycle between PbO/PbCO3 · 2 PbO, triggered by changing the CO2 pressure between 2 and 8 bar, was developed.
„Pressure effects on the carbonation of MeO (Me=Co, Mn, Pb, Zn) for thermochemical energy storage“, G. Gravogl, C. Knoll, W. Artner, J. Welch, E. Eitenberger, G. Friedbacher, M. Harasek, K. Hradil, A. Werner, P. Weinberger, D. Müller, R. Miletich, Applied Energy 252, 2019, 113451