Volume 79, Issue 12 - Special issue dedicated to Professor Hubert Schmidbaur on the occasion of his 90th birthday

Hubert Schmidbaur has significantly influenced the field of gold chemistry. His work on preparing various aurocyclic digold compounds and studying their structures and reactivities has laid the foundation for unique applications in photophysics and homogeneous catalysis. The naming and characterization, both experimental and theoretical, of the aurophilicity phenomenon have led to numerous interdisciplinary applications. The emergence and development of dynamic gold chemistry in the excited state, exemplify this impact. Preparative methodologies, characterization techniques, and qualitative bonding theories have been tested through the rational preparation of ligated, element-centred gold clusters. The potential of this fascinating class of compounds remains largely untapped.

The structures of the cluster carbonyls and the dynamic behaviour they show is explained in terms of the Ligand Polyhedral Model.

A series of 64 ternary equiatomic tetrelides TT ′ Tt ( T = electron-poor transition metal; T ′ = electron-rich transition metal; Tt = Si, Ge) was synthesized by arc-melting and subsequent annealing. The samples were characterized by powder X-ray diffraction and 20 structures were refined from single-crystal X-ray diffractometer data. These tetrelides (mostly silicides) crystallize with TiNiSi-related structures, orthorhombic space groups Pnma . They are ternary ordered, orthorhombically distorted superstructure variants of the aristotype AlB 2 . The precise single-crystal data show distinct differences in the bond lengths and angles within the three-dimensional [ T ′Si] networks as a consequence of different electron densities driven by the electronegativity differences. Systematic solid-state NMR-spectroscopic investigations allow observing the effect of the two transition metal atoms upon the 29 Si isotropic resonance shifts. For the silicides featuring T ′ elements from groups IX and X this value can be predicted from element-specific increments.

The M 2 Al 16 Pt 9 series with M = Ca, La–Nd and Sm–Gd was synthesized from the elements in analogy to the literature known compounds Ce 2 Al 16 Pt 9 and Sr 2 Al 16 Pt 9 . All compounds crystallize in the orthorhombic crystal system with space group Immm and have been characterized by powder X-ray diffraction. For La 2 Al 16 Pt 9 and Eu 2 Al 16 Pt 9 also single-crystal investigations have been conducted. In all cases, Al 3 Pt 2 as well as the members of the RE Al 5 Pt 3 series (YNi 5 Si 3 type, space group Pnma ) have been observed as side phases. Annealing led in all cases to an increase of the side phases, suggesting that the title compounds decompose peritectically. In the crystal structures, polyanionic [Al 16 Pt 9 ] δ – networks can be found with large fourfold capped hexagonal prismatic cavities ( M @Al 6 Pt 6 Al 4 ), which host the respective M atoms. As seen from the unit cell volumes, the La–Nd, Sm and Gd compounds exhibit a formally trivalent oxidation state while the Ca and Eu atoms in Ca 2 Al 16 Pt 9 and Eu 2 Al 16 Pt 9 are divalent.

We synthesized bis(2-chloroethyl)sulfane, S(C 2 H 4 Cl) 2 , and showed its purity by various analytic methods. Its previously unknown crystal structure shows a peculiar intermolecular S⋯S interaction leading to (ClH 4 C 2 ) 2 S⋯S(C 2 H 4 Cl) 2 dimers. The S⋯S interaction could also be reproduced and investigated by quantum chemical calculations. As the C–C bonds have been observed unexpectedly short in the range from 1.505(2) to 1.509(2) Å, the chemical bonds within S(C 2 H 4 Cl) 2 have also been investigated quantum chemically. Intrinsic bonding orbitals (IBOs) showed that while the C–Cl bond is slightly polarized, the C–C and C–S bonds appear purely covalent, as expected. The electron density between the respective two C atoms is decreased by the attached Cl atom so that the two C atoms move closer to each other. High-resolution low-temperature Raman spectra of solid S(C 2 H 4 Cl) 2 are presented, and the bands are assigned with the aid of quantum chemical solid-state calculations.

The crystal structures of two chiral phosphanes, (( R,R )-chiraphos = (2 R ,3 R )-(+)-2,3-bis(diphenylphosphino)butane and R -Tol-BINAP = ( R )-(+)-2,2′-bis(di- p -tolylphosphino)-1,1′-binaphthyl) are unambiguously determined by X-ray crystallography and reported for the first time. The aim is to understand the structural characteristics and their chiral arrangement of the molecules. The ( R,R )-chiraphos structure was solved in the orthorhombic space group P 2 1 2 1 2 1 while R -Tol-BINAP was solved in the triclinic space group P 1. This set of compounds demonstrates attractive intramolecular phenyl ring interactions involving two terminal phenyl rings in ( R,R )-chiraphos, while a terminal tolyl ring and the central binaphthyl ring occurs in R -Tol-BINAP. The crystal structure analyses also revealed that all aromatic units have close C–H⋯aryl centroid weak contacts forming continuous networks of columns and layers in ( R , R )-chiraphos and R -Tol-BINAP respectively.

Orthorhombic Al 3 BO 6 was synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 5 GPa and 1,200 °C, respectively. Its structure has been determined via single-crystal diffraction and is isotypic to M 3 BO 6 ( M  = Cr, Fe) and homeotypic to the natural mineral norbergite (Mg 3 SiO 4 (OH) 2 ). Thereby, we could complement the first report of Al 3 BO 6 by Capponi et al. in 1972 (Capponi, J.-J.; Chenavas, J.; Joubert, J.-C. Eur. J. Mineral. 1972 , 95 , 412–417.) by providing a precise structure description. Al 3 BO 6 crystallizes in the space group Pnma (no. 62) with 4 formula units per unit cell and the lattice parameters a  = 9.5060(5), b  = 8.1058(4) and c  = 4.2899(2) Å. Infrared spectroscopy (powder sample) and high-temperature X-ray powder diffraction further characterize the compound.

Yellow, transparent single crystals of LiEu[BO 3 ] were obtained by serendipity in a solid-state reaction with the educts Eu 2 O 3 , MgBr 2 and Li[BH 4 ] at 750 °C within an arc-welded niobium ampoule enclosed in an evacuated fused silica secondary jacket. LiEu[BO 3 ] crystallizes isotypically to LiBa[BO 3 ] and LiSr[BO 3 ] in the monoclinic space group P 2 1 / n exhibiting the unit cell parameters a = 638.79(4) pm, b = 677.12(4) pm and c = 690.43(4) pm with a monoclinic angle of β = 110.648(3)° for Z = 4. Li + is coordinated by five oxygen atoms from three triangular planar [BO 3 ] 3– anions, while Eu 2+ has seven oxygen neighbors belonging to six ortho- oxoborate groups. A brief synopsis of the crystal structures of other Li M +II [BO 3 ] compounds is given and the Raman spectrum of LiEu[BO 3 ] compares well with the infrared spectrum obtained from the non-isotypic LiCa[BO 3 ].

Micro-spectral analysis and fluorescence decay experiments of beech and oak chips as well as their main constituents lignin and cellulose are reported. While the lignin spectrum shows two fluorescence bands of almost identical intensity around 500 nm and 560 nm, the cellulose spectrum is dominated by a band around 500 with a less pronounced shoulder at 560 nm, whose intensity increases with the amount of residual lignin. Fluorescence decay kinetics are bi-exponential with lifetimes around 0.5 ns and 2.5 ns and an increasing contribution of the short-lived component with an increasing amount of lignin. Both data sets indicate that a quantification of residual lignin in cellulose samples obtained after decomposition of beech wood appears possible in a concentration range of 2–10 %. This may provide a non-destructive and label-free test of the quality of decomposition.

Recently, providing admixtures to improve the performance of the novel low-carbon-blended cement is needed to meet the requirement for environmentally friendly cements. In this study, calcium-silicate-hydrate (C–S–H) nanocomposites which are known as hydration accelerator for Portland cement were employed in such low-carbon cements. The investigation focused on the effect of polycarboxylate ether (PCE) possessing different side-chain lengths on the morphology of C–S–H nanocomposites synthesized from sodium silicate and calcium nitrate via chemical co-precipitation and on their performance (especially early strength enhancement) when applied in a calcined clay-blended cement (OPC substitution rate of 46 wt%). As observed by TEM, addition of PCE effectively controls the size of the C–S–H nanoparticles and delays the transition from C–S–H globules to foils. PCEs with short side-chain (23 EO units) exhibited the highest inhibition effect on the growth of the C–S–H nuclei, which was confirmed by particle size measurements. Additionally, incorporating such C–S–H–PCE nanocomposites into the calcined clay-blended cement (dosage 2.0 % by weight of cement) significantly improved fluidity of the cement pastes. Regarding compressive strength, pristine C–S–H produced only a minor improvement, whereas C–S–H–PCE nanocomposites yielded substantial enhancement, especially after 16 h and 1 day of curing. Remarkably, C–S–H–23PCE pronounced the strongest seeding and strength enhancing effect, owed to its particularly small particle size ( d  = 21.8 ± 0.3 nm). It also significantly accelerated the hydration of the silicate phases (C 3 S and C 2 S) in this blended cement, as was evidenced by isothermal heat flow calorimetry and XRD measurements.

The tin-containing Lewis acid group (C 2 F 5 ) 3 Sn was combined with a phosphonium ylide as base function in one molecule: o -[(C 2 F 5 ) 3 Sn]-C 6 H 4 (Ph) 2 PCMe 2 . It was prepared by reacting the ortho -lithiated triphenylphosphoniumdimethylmethylide ( o -Li-C 6 H 4 )(Ph) 2 PCMe 2 with the stannyl chloride (C 2 F 5 ) 3 SnCl to afford o -[(C 2 F 5 ) 3 Sn]-C 6 H 4 (Ph) 2 PCMe 2 in 86 % yield. It forms a ring structure with a bond between tin and the ylidic carbon atom both in solution and in the solid state and can thus be considered as a hidden frustrated Lewis pair. The title compound is unreactive towards H 2 , CO 2 , CS 2 , PhNCO and decomposes in the presence of SO 2 or NH 3 . However, it is active in the catalytic reduction of CO 2 with pinacolborane, H–BPin. The title compound reacts slowly with H–BPin in an unselective reaction from which a reduction product with cleaved C 2 F 5 group, the phosphonium stannate (II) o -[(C 2 F 5 ) 2 Sn]-C 6 H 4 (Ph) 2 PCHMe 2 , could be obtained in small amounts.

The formation of cyanopyridine (CN-py) adducts of silicon tetrahalides was investigated for 3- and 4-cyanopyridine in combination with SiF 4 and SiCl 4 . Whereas bubbling of SiF 4 through toluene solutions of 3-CN-py and 4-CN-py afforded white precipitates, which should possess the respective composition SiF 4 (CN-py) 2 , addition of SiCl 4 did not cause any precipitation. Upon storage of the toluene solution of SiCl 4 and 4-CN-py at 6 °C for several weeks, some crystals of the composition SiCl 4 (4-CN-py) 2 ⋅ 2 (4-CN-py) ⋅ (toluene) were obtained. The use of SiCl 4 as the solvent (i.e. SiCl 4 in large excess) and dissolving 4-CN-py therein gave access to a crystalline adduct of the composition SiCl 4 (4-CN-py) 2 ⋅ 2 (4-CN-py). 3-CN-py instead recrystallized from SiCl 4 without forming an adduct with the silane. Computational analyses (B2T-PLYP level) of the single-point energy differences between starting materials Si X 4 ( X  = F, Cl) and ‘pyridine’ (‘pyridine’ = pyridine, 3-CN-py, 4-CN-py) and their adducts Si X 4 (‘pyridine’) 2 revealed the tendencies toward adduct formation to decrease in the order SiF 4  > SiCl 4 as well as pyridine >> 4-CN-py > 3-CN-py. For SiCl 4 with 4- and 3-CN-py, the energy of adduct formation (−7.0 and −5.7 kcal mol −1 , respectively) is easily compensated by entropy effects at room temperature. Whereas the former explains as to why cyanopyridines and SiCl 4 may co-exist without noticeable adduct formation, the crystal structures of the adducts SiCl 4 (4-CN-py) 2 ⋅ 2 (4-CN-py) ⋅ (toluene) and SiCl 4 (4-CN-py) 2 ⋅ 2 (4-CN-py) reveal the additional stabilization of these solids by co-crystallization with 4-cyanopyridine, which eventually enabled the isolation of the SiCl 4 (4-CN-py) 2 moiety in a solid. Partial decomposition (hydrolysis) during attempts of recrystallization of SiF 4 (4-CN-py) 2 and SiF 4 (3-CN-py) 2 afforded crystals of the ionic compounds [4-CN-PyH] + [SiF 5 (4-CN-py)] – and [3-CN-PyH] + 2 [SiF 6 ] 2– , respectively.

The reaction of the ferrocene derivative Fe( ƞ 5 -C 5 H 4 CO 2 H)( ƞ 5 -C 5 H 4 CH=CH 2 ) (=fcCO 2 H) ( 1 ) with the zinc reagents [Zn(OAc) 2 ]· x H 2 O (OAc = acetate; 2a , x  = 0; 2b , x  = 2) or ZnEt 2 ( 3 ) in non-aqueous solvents toluene or tetrahydrofuran resulted in the formation of the cluster [Zn 4 O(fcCO 2 ) 6 ] ( 4 ). The structure of 4 in the solid state has been determined by single-crystal X-ray diffraction analysis. Cluster 4 crystallizes in the triclinic space group P 1 ‾ $\overline{1}$ . In 4 , the cluster core is set-up by four zinc(II) ions which are forming the vertices of a tetrahedron with a μ 4 -oxygen atom in its center. Six μ -fcCO 2 units bridge the edges of the tetrahedron. IR spectroscopy confirms with ∆ ν CO2  = 91 cm −1 (∆ ν CO2  =  ν CO2,asym  −  ν CO2,sym ) the μ -bridging character of the fcCO 2 entities. Cyclic voltammetry studies showed a reversible redox event at E° ′ = 245 mV versus FcH/FcH + (FcH = Fe( ƞ 5 -C 5 H 5 ) 2 ) for 4 with the six fcCO 2 redox events superimposed. High-resolution ESI-TOF measurements verified the identity of 4 .

The transformation of gold(I) complexes of the type [AuCl(NHC)] (NHC = N -picoline- N ′-R-functionalized benzimidazolin-2-ylidene ligand; R = Et, 1 ; R = n- Pr, 2 ; R = n- Bu, 3 ) into polynuclear complexes was studied. Chloride abstraction from [AuCl(NHC)] by treatment with silver tetrafluoroborate yielded the digold complexes [Au 2 ( 1 ) 2 ](BF 4 ) 2 –[Au 2 ( 3 ) 2 ](BF 4 ) 2 , featuring two gold atoms coordinated by an NHC carbon atom from one NHC ligand and a picoline nitrogen atom from the second NHC ligand each. Treatment of the gold complexes [AuCl(NHC)] (NHC = 1 − 3 ) with dicarbene silver complexes of type [Ag(NHC) 2 ][AgBr 2 ] (NHC = 1 − 3 ) in the presence of AgBF 4 proceeded under transmetalation of the NHC ligand to gold and formation of discrete heterodinuclear di-NHC complexes [AgAu( 1 ) 2 ](BF 4 ) 2 −[AgAu( 3 ) 2 ](BF 4 ) 2 , where the two NHC donors bind to the gold atoms, while the two pyridine donors bind to the silver atom. Reaction of the gold(I) complex [AuCl( 4 )] ( 4 = N , N ′-dipicoline-functionalized benzimidazolin-2-ylidene) with silver(I) complex [Ag( 4 ) 2 ][AgBr 2 ] in the presence of AgBF 4 yielded the trinuclear heterobimetallic complex [Ag 2 Au( 4 ) 2 ](BF 4 ) 3 featuring the gold atom coordinated by two carbene donors and the two silver atoms coordinated by the two pyridine donors from one ligand 4 . The molecular structures were determined for the homonuclear digold as well as for the heteronuclear Ag I /Au I and Ag I 2 /Au I complexes.

An efficient synthesis of the novel {6-[1,1-di(pyridin-2-yl)ethyl]pyridine-2-yl}2-methyl-1,3-propanediamine ( 2 ) is reported, as well as a reliable large-scale synthesis (of the order of 100 g) of previously known 2,2’-[1-(6-chloropyridin-2-yl)ethane-1,1-diyl]dipyridine ( 4 ); the latter is the starting material for the preparation of the former, as well as a multitude of other polypodal polyamine/polyimine ligands. Both materials, as well as the intermediates in their multi-step syntheses, have been fully characterised. Ligand 2 , in conjunction with ligands 2,2’-(pyridine-2,6-diyl)bis(2-methylpropane-1,3-diamine) ( 1 ) and 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyridine ( 3 ), establishes a series of tetrapodal pentadentate N 5 ligands L of like scaffold, and thus coordination geometry, but graded primary amine/imine donor atom ratios. The iron(II) complexes [Fe( L )NCCH 3 ](OTf) 2 ( L  =  1 : A ; L  =  2 : B ; L  =  3 : C ; OTf = triflate) have been prepared and fully characterised, including X-ray single-crystal structure analyses. The metal-centred one-electron oxidation/reduction potential (Fe II /Fe III ) depends sensitively on the electron donor capacity of the capping ligand used, as demonstrated by cyclic voltammetry. Whereas the acetonitrile ligand in A and C is readily exchanged for a methanol ligand in methanol solution, the resulting complexes showing variable-temperature spin crossover (SCO) in solution, B has been found to be inert to this type of ligand exchange.

This study investigates the thermal expansion properties of argentophilic and aurophilic contacts in coordination complexes of the form [( i Pr NHC) 2 Ag]ClO 4 and [( i Pr NHC) 2 Au]ClO 4 . Using X-ray crystallography distinct structural changes and phase transitions at varying temperatures are revealed. Both complexes exhibit a similar strong temperature dependence of the metallophilic distances. Analysis shows that the shallower, more anharmonic aurophilic interaction is counterbalanced by the smaller vibrational amplitude of the heavier gold atoms, resulting in comparable thermal elongation despite differing atomic dynamics.

The identification of those intermetallic phases which constitute the complex multi-phase system of dental amalgams is rather contradictory in literature. This is intriguing, as the production processes of dental amalgams are standardized since a long time. The reason for the divergence on reported amalgam phases may be due to changes within the microstructure of dental amalgams occurring over time. With a combination of X-ray powder diffraction, Rietveld refinements and X-ray spectroscopy, we were able to show the differences in chemical phases and morphologies between a freshly prepared and a 27-year old dental amalgam sample. Atomic diffusion processes within the amalgam lead to transformation of initially formed intermetallic phases and to dissolution of phases present in the prealloyed metals which react with Hg to the dental amalgam. The main amalgam phases, Ag 1+ x Hg 1− x and Ag 2 Hg 3 , remain stable but change in their relative concentrations: the first increases over time, while the latter decreases. The content of Cu 6 Sn 5 as the third important constituent phase decreases over the years and concentrates in hollow spherical particles. The fourth phase is an fcc Ag–Cu mixed crystal which is only present in the freshly prepared amalgam and dissolves over the years. The silver amalgams form a matrix in which spherical Ag–Cu particles are embedded. Cu atoms migrate from within these particles towards their surface, react to η ′ bronze and are substituted by Hg atoms, whereas the Ag atoms show virtually no mobility.