Experimental X-ray Charge-Density Studies─A Suitable Probe for Superconductivity? A Case Study on MgB2.

Case studies of 1T-TiSe2 and YBa2Cu3O7-δ have demonstrated that X-ray diffraction (XRD) studies can be used to trace even subtle structural phase transitions which are inherently connected with the onset of superconductivity in these benchmark systems. However, the utility of XRD in the investigation of superconductors like MgB2 lacking an additional symmetry-breaking structural phase transition is not immediately evident. Nevertheless, high-resolution powder XRD experiments on MgB2 in combination with maximum entropy method analyses hinted at differences between the electron density distributions at room temperature and 15 K, that is, below the Tc of approx. 39 K. The high-resolution single-crystal XRD experiments in combination with multipolar refinements presented here can reproduce these results but show that the observed temperature-dependent density changes are almost entirely due to a decrease of atomic displacement parameters as a natural consequence of a reduced thermal vibration amplitude with decreasing temperature. Our investigations also shed new light on the presence or absence of magnesium vacancies in MgB2 samples─a defect type claimed to control the superconducting properties of the compound. We propose that previous reports on the tendency of MgB2 to form non-stoichiometric Mg1-xB2 phases (1 - x ∼ 0.95) during high-temperature (HT) synthesis might result from the interpretation of XRD data of insufficient resolution and/or usage of inflexible refinement models. Indeed, advanced refinements based on an Extended Hansen-Coppens multipolar model and high-resolution X-ray data, which consider explicitly the contraction of core and valence shells of the magnesium cations, do not provide any significant evidence for the formation of non-stoichiometric Mg1-xB2 phases during HT synthesis.

HTD2: a single-crystal X-ray diffractometer for combined high-pressure/low-temperature experiments at laboratory scale.

High-pressure (HP) X-ray diffraction experiments at low temperature (LT) require dedicated instruments as well as non-standard sample environments and measuring strategies. This is especially true when helium cryogenic temperatures below 80 K are targeted. Furthermore, only experiments on single-crystalline samples provide the prerequisites to study subtle structural changes in the p-T phase diagram under extreme LT and HP conditions in greater detail. Due to special hardware requirements, such measurements are usually in the realm of synchrotron beamlines. This contribution describes the design of an LT/HP diffractometer (HTD2) to perform single-crystal X-ray diffraction experiments using a laboratory source in the temperature range 400 > T > 2 K while applying pressures of up to 20 GPa.

Chromous siloxides of variable nuclearity and magnetism.

Treatment of Cr[N(SiMe3)2]2(thf)2 with HOSiR3 (R = Et, iPr) in THF afforded the bridged CrII siloxide complexes Cr3(OSiEt3)2(µ-OSiEt3)4(thf)2 and Cr2(OSiiPr3)2(µ-OSiiPr3)2(thf)2 in high yield. Exposure of these compounds to vacuum in aliphatic solvents led to the loss of coordinated THF and to the formation of the homoleptic chromous siloxides Cr4(µ-OSiEt3)8 and Cr3(OSiiPr3)2(µ-OSiiPr3)4, respectively, in moderate to high yield. Use of TMEDA as a potentially bidentate donor molecule gave the monomeric cis-coordinated siloxide Cr(OSiiPr3)2(tmeda) (tmeda = N,N,N',N'-tetramethylethane-1,2-diamine). Oxidation of Cr2(OSiiPr3)2(µ-OSiiPr3)2(thf)2 with CHI3 and C2Cl6 produced the trigonal bipyramidal chromic compound CrIII(OSiiPr)3(thf)2 and asymmetrically coordinated Cr2Cl3(OSiiPr3)3(thf)3, respectively. Magnetic measurements (Evans and SQUID) hinted at (a) antiferromagnetic interactions between the CrII centres, (b) revealed higher effective magnetic moments (µeff) for cis-coordinated monomeric heteroleptic complexes compared to trans-coordinated ones, and (c) pointed out the highest (µeff) for the tetranuclear complex Cr4(µ-OSiEt3)8 (6.26µB, SQUID, 300 K; Cr⋯Cradjacent avg. 2.535 A).

Beyond Takai's Olefination Reagent: Persistent Dehalogenation Emerges in a Chromium(III)-µ3 -Methylidyne Complex.

Reaction of CHI3 with six equivalents of CrCl2 in THF at low temperatures affords [Cr3 Cl3 (µ2 -Cl)3 (µ3 -CH)(thf)6 ] as the first isolable high-yield CrIII µ3 -methylidyne complex. Substitution of the terminal chlorido ligands via salt metathesis with alkali-metal cyclopentadienides generates isostructural half-sandwich chromium(III)-µ3 -methylidynes [CpR 3 Cr3 (µ2 -Cl)3 (µ3 -CH)] (CpR =C5 H5 , C5 Me5 , C5 H4 SiMe3 ). Side and decomposition products of the Cl/CpR exchange reactions were identified and structurally characterized for [Cr4 (µ2 -Cl)4 (µ2 -I)2 (µ4 -O)(thf)4 ] and [(η5 -C5 H4 SiMe3 )CrCl(µ2 -Cl)2 Li(thf)2 ]. The Cl/CpR exchange drastically changed the ambient-temperature effective magnetic moment µeff from 9.30/9.11 µB (solution/solid) to 3.63/4.32 µB (CpR =C5 Me5 ). Reactions of [Cr3 Cl3 (µ2 -Cl)3 (µ3 -CH)(thf)6 ] with aldehydes and ketones produce intricate mixtures of species through oxy/methylidyne exchange, which were partially identified as radical recombination products through GC/MS analysis and 1 H NMR spectroscopy.

Dispersion Forces Drive the Formation of Uranium-Alkane Adducts.

Single-crystal cryogenic X-ray diffraction at 6 K, electron paramagnetic resonance spectroscopy, and correlated electronic structure calculations are combined to shed light on the nature of the metal-tris(aryloxide) and η2-H, C metal-alkane interactions in the [((t·BuArO)3tacn)UIII(Mecy-C6)]·(Mecy-C6) adduct. An analysis of the ligand field experienced by the uranium center using ab initio ligand field theory in combination with the angular overlap model yields rather unusual U-OArO and U-Ntacn bonding parameters for the metal-tris(aryloxide) interaction. These parameters are incompatible with the concept of σ and π metal-ligand overlap. For that reason, it is deduced that metal-ligand bonding in the [((t·BuArO)3tacn)UIII] moiety is predominantly ionic. The bonding interaction within the [((t·BuArO)3tacn)UIII] moiety is shown to be dispersive in nature and essentially supported by the upper-rim tBu groups of the (t·BuArO)3tacn3- ligand. Our findings indicate that the axial alkane molecule is held in place by the guest-host effect rather than direct metal-alkane ionic or covalent interactions.