Danny Müller

The combination of spin crossover (SCO) with guest incorporation properties has attracted the interest of researchers in the last couple of decades and has led to the design of numerous SCO porous coordination polymers (SCO-PCPs). The most famous class of SCO-PCPs is the Hofmann-type network, which is a very promising material for (chemo)sensing applications. Different strategies have been carried out to expand the classic structure {Fe(pz)[M-II(CN)(4)]} (M = Ni, Pd, Pt) to get larger cavities, but the resulting compounds often showed a poor magnetic behavior. In this work, we present wide-mesh-size spin-switching Hofmann-type networks based on tetrakis-cyanoacetylides synthesized with a newly developed method, resulting in compounds with the general formula {Fe(pz)[M(C3N)(4)]} (M=Ni, Pd, Pt). The compounds were characterized in their structural, magnetic, and spectroscopic properties. They present 5-fold larger cavities and a drastic increase in porosity. The desired hysteretic and guest-dependent spin-crossover behavior is retained, and in situ chemo-switching of the spin state and the memory effect are also observed.

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.

The thermochemical energy-storage material couple CuSO₄/[Cu(NH₃)₄]SO₄ combines full reversibility, application in a medium temperature interval (3 uptake. In order to limit this volume work, as well as enhance the thermal conductivity of the solid material, several composites of anhydrous CuSO₄ with inorganic inert support materials were prepared and characterized with regard to their energy storage density, reversibility of the storage reaction, thermal conductivity, and particle morphology. The best thermochemical energy storage properties were obtained for a 10:1 CuSO₄-sepiolite composite, combining an attractive energy storage density with slightly improved thermal conductivity and decreased bulk volume work compared to the pure salt.

Carboxylic acid functionalized tetrazoles form Fe( ii ) spin crossover materials governed by H-bonding networks.

Thermochemical energy storage is considered as an auspicious method for the recycling of medium-temperature waste heat. The reaction couple Mg(OH)₂–MgO is intensely investigated for this purpose, suffering so far from limited cycle stability. To overcome this issue, Mg(OH)₂, MgCO₃, and MgC₂O₄·2H₂O were compared as precursor materials for MgO production. Depending on the precursor, the particle morphology of the resulting MgO changes, resulting in different hydration behavior and cycle stability. Agglomeration of the material during cyclization was identified as main reason for the decreased reactivity. Immersion of the spent material in liquid H₂O decomposes the agglomerates restoring the initial reactivity of the material, thus serving as a regeneration step.

Invited for the cover of this issue are the groups of Peter Weinberger from TU Wien, Vienna, Austria, and Georg Steinhauser, Leibniz Universität Hannover, Hanover, Germany. The cover image shows the structure of the featured lanthanide azobis[tetrazolide]‐carbonate reaching from Y to Yb to illustrate the “overcoming” of the gadolinium break.

A series of rare earth element (REE) mixed‐anion 5,5′‐azobis(1H‐tetrazol‐1‐ide)‐carbonate ([REE₂(ZT)₂CO₃(H₂O)₁₀]·2H₂O; 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”.

In [Fe(ppditz)₃]X₂, X = BF₄⁻, ClO₄⁻, PF₆⁻ spin crossover complexes the observed cooperativity originates only from the rigidity and internal strain of the ligand.

Unsaturated aryl-and heteroaryl Nl-substituted tetrazoles have been exceedingly difficult to access but are nonetheless highly attractive for the preparation of novel multifunctional spin crossover materials. The development of a Heck cross-coupling protocol, featuring good substrate tolerance for aromatic and heteroaromatic substrates overcoming this synthetic challenge is presented. The combination of aqueous THF, triethylamine, and PdCI2/P(o-tolyl)(3) gave fast reaction rates and good yields. Competition experiments with styrene evidenced a slight preference for reaction with 1-allyl-1H-tetrazole.