Synthesis and properties of cyclic sandwich compounds.

Cyclic nanometre-scale sandwich complexes assembled from individual building blocks were synthesized. Sandwich complexes, in which a metal ion is π-coordinated by two planar aromatic organic rings belong to the foundations of organometallic chemistry. They have been successfully used in a wide variety of applications ranging from catalysis, synthesis and electrochemistry to nanotechnology, materials science and medicine1,2. Extending the sandwich structural motif leads to linear multidecker compounds, in which aromatic organic rings and metal atoms are arranged in an alternating fashion. However, the extension to a cyclic multidecker scaffold is unprecedented. Here we show the design, synthesis and characterization of an isomorphous series of circular sandwich compounds, for which the term 'cyclocenes' is suggested. These cyclocenes consist of 18 repeating units, forming almost ideally circular, closed rings in the solid state, that can be described by the general formula [cyclo-MII(µ-η8:η8-CotTIPS)]18 (M = Sr, Sm, Eu; CotTIPS = 1,4-(iPr3Si)2C8H62-). Quantum chemical calculations lead to the conclusion that a unique interplay between the ionic metal-to-ligand bonds, the bulkiness of the ligand system and the energy gain on ring closure, which is crucially influenced by dispersion interactions, facilitate the formation of these cyclic systems. Up to now, only linear one-dimensional multidecker sandwich compounds have been investigated for possible applications such as nanowires3-10. This textbook example of cyclic sandwich compounds is expected to open the door for further innovations towards new functional organometallic materials.

Thiocarbonyl pseudohalides - the curious case of thiocarbonyl dithiocyanate.

Thiocarbonyl dithiocyanate (1) and chlorothiocarbonyl thiocyanate (2) were synthesized from thiophosgene and ammonium or silver thiocyanate, respectivley. Their crystal structures show syn-anti (1) and syn (2) conformations, which were confirmed in the bulk phases by powder X-ray diffraction, vibrational spectroscopy and DFT calculations. Further calculations explain the isolation of the kintetic reaction products by a lower transition stateas opposed to the thermodynamic reaction products. Reaction of 1 with ethanol gave a dithiobiuret derivative (3). In a proof-ofprinciple study we show that it in turn can be used for the complexation of Ni2+ (4).

Formation and Reactivity of heavy Group 14-15 bonds.

A series of low-valent Group 14-15 compounds were obtained starting from [(Dipp2 NacNac)MCl] (M=Ge-Pb) (I-III) (Dipp2 NacNac=HC{C(Me)N(Dipp)}2 ) and M'E(SiMe3 )2 (M'=Li, E=As; M'=K, E=Sb, Bi) (IV-VI). In the course of this investigations we were able to fully characterize all permutations except Pb-Bi for compounds of the composition [(Dipp2 NacNac)ME(SiMe3 )2 ] (1E : M=Ge, 2E : M=Sn, 3E : M=Pb). Thus, we report the first low valent tetrelene with Sn-Bi bond. All isolated compounds, were examined by NMR spectroscopy, IR spectroscopy and except compound 1As by X-ray structure analysis. Moreover, were examined UV-Vis spectroscopy and investigated the reactivity of these compounds towards different substrates in more detail. Starting with the compound [(Dipp2 NacNac)SnAs(SiMe3 )2 ] (2As ), the reaction with red selenium yields [(Dipp2 NacNac)Sn-Se-As(SiMe3 )2 ] (4) which exhibits a Sn-Se-As chain.

Paramagnetic Nuclear Magnetic Resonance Shifts for Triplet Systems and Beyond with Modern Relativistic Density Functional Methods.

An efficient framework for the calculation of paramagnetic NMR (pNMR) shifts within exact two-component (X2C) theory and (current-dependent) density functional theory (DFT) up to the class of local hybrid functionals (LHFs) is presented. Generally, pNMR shifts for systems with more than one unpaired electron depend on the orbital shielding contribution and a temperature-dependent term. The latter includes zero-field splitting (ZFS), hyperfine coupling (HFC), and the g-tensor. For consistency, we calculate these three tensors at the same level of theory, i.e., using scalar-relativistic X2C augmented with spin-orbit perturbation theory. Results for pNMR chemical shifts of transition-metal complexes reveal that this X2C-DFT framework can yield good results for both the shifts and the individual tensor contributions of metallocenes and related systems, especially if the HFC constant is large. For small HFC constants, the relative error is often large, and sometimes the sign may be off. 4d and 5d complexes with more complicated structures demonstrate the limitations of a fully DFT-based approach. Additionally, a Co-based complex with a very large ZFS and pronounced multireference character is not well described. Here, a hybrid DFT-multireference framework is necessary for accurate results. Our results show that X2C is sufficient to describe relativistic effects and computationally cheaper than a fully relativistic approach. Thus, it allows use of large basis sets for converged HFCs. Overall, current-dependent meta-generalized gradient approximations and LHFs show some potential; however, the currently available functionals leave a lot to be desired, and the predictive power is limited.

Ion-Selective Assembly of Supertetrahedral Selenido Germanate Clusters for Alkali Metal Ion Capture and Separation.

Supertetrahedral chalcogenido (semi)metalate cluster-based frameworks possess high selectivity for alkali metal cations, matching the specific charge density of their inner surfaces, which enables their use as ion-exchange materials. Aggregates of the supertetrahedral chalcogenido metalate cluster offer even new perspectives for metal ion capture and separation. Herein, we report on ionothermal preparation of two corresponding model compounds, (C2C1Im)7[Cs@GeII4(GeIV4Se10)4] (1) and (C2C1Im)10[Na5(CN)6@Cu6(Ge4Se10)4(Cu)] (2). Their formation is reliant on one specific cation type each, Cs+ for 1 and Na+ for 2, thus providing promising separation potential during crystallization. Compound 1 is based on the largest discrete binary selenido germanate cluster reported to date and the first mixed-valent chalcogenido germanate(II/IV) supertetrahedron. Moreover, it adds to the few examples of chalcogenides capable of capturing Cs+ ions. Its high selectivity for Cs+ compared to that of Li+, Na+, K+, and Rb+ was confirmed by single-crystal X-ray diffraction, energy-dispersive X-ray spectroscopy, and electrospray ionization mass spectrometry. Quantum chemical studies indicate that smaller ions, K+ and Rb+, could also be embedded in an isolated cluster assembly, but as the cluster aggregate slightly distorts for crystallization, the selectivity for Cs+ becomes exclusive in the salt. The anionic substructure of compound 2 is based on a two-dimensional network of supramolecular assemblies and exhibits an exclusive preference for Na+. This work thus provides the first comprehensive insight into the selective incorporation of specific alkali metal ions into supramolecular aggregates of supertetrahedral chalcogenide clusters, as a promising basis for new ion trapping techniques─especially for heavy alkali metal ions that pose environmental challenges.

Establishing Family Relations in Group 15 Halogenido Metalates with the Largest Molecular Antimony Iodide Anion.

Studying structurally related families of compounds is a valuable tool in understanding and predicting material properties and has been extensively used for metal halide perovskites. Due to the variable anion structures in group 15 halogenido metalates, similar family relations are still largely missing. Herein, we present compounds featuring the [Sb2n I6n+4 ]4- family of anions, including the first n=5 member in [Hpyz]4 [Sb10 I34 ] (Hpyz=pyrazinium), which contains the largest halogenido pentelate anion reported to date. The optical properties of compounds featuring n=1-5 anions show a clear trend as well as an outlier, a low band gap of 1.72 eV for [Hpyz]4 [Sb10 I34 ], that can be well understood using quantum chemical investigations. Also using SbI3 and [H2 NMe2 ]3 [SbI6 ], a compound featuring a single octahedral [SbI6 ]3- unit, as limiting cases, we show that structure-property relationships can be established in group 15 halogenido metalates in a similar way as in metal halide perovskites, thus providing a framework for understanding new and known compounds in this emerging class of materials.

Fully Tin-Coated Coinage Metal Ions: A Pincer-Type Bis-stannylene Ligand for Exclusive Tetrahedral Complexation.

The synthesis of a novel bis-stannylene pincer ligand and its complexation with coinage metals (CuI , AgI and AuI ) are described. All coinage metal centres are in tetrahedral coordination environments in the solid state and are exclusively coordinated by four neutral SnII donors. 119 Sn NMR provided information about the behaviour in solution. All of the isolated compounds have photoluminescent properties, and these were investigated at low and elevated temperatures. Compared to the free bis-stannylene ligand, coordination to coinage metals led to an increase in the luminescence intensity. The new compounds were investigated in detail through all-electron relativistic density functional theory (DFT) calculations.

Fully Tin-Coated Coinage Metal Ions: A Pincer-Type Bis-stannylene Ligand for Exclusive Tetrahedral Complexation.

Invited for the cover of this issue are the groups of P. W. Roesky (Karlsruhe) and F. Weigend (Marburg). The image depicts coinage metal cores with tetrahedrally coordinating tin atoms. Read the full text of the article at 10.1002/chem.202203583.

Double Addition vs. Ring Closure: Systematic Reactivity Study of CO(NCO)2 and CO(NCS)2 towards Hydrogen Halides.

The reactivity of carbonyl diisocyanate, CO(NCO)2 , and carbonyl diisothiocyanate, CO(NCS)2 with nucleophiles shows different patterns: Whereas carbonyl diisocyanate adds two equivalents of nucleophile forming carbonyl bis(carbamoylhalides), carbonyl diisothiocyanate only adds one equivalent and undergoes intramolecular ring closure, resulting in the formation of substituted thiadiazines. In this study we have reacted both carbonyl diisocyanate and carbonyl diisothiocyanate with the full series of hydrogen halides HF to HI, isolating carbonyl bis(carbamoylfluoride) (1), -chloride (2), -bromide (3), and -iodide (4) as well as (6-chloro-2,3-dihydro-2-thioxo-4H-1,3,5-thiadiazin-4-one (5), and 6-bromo-2,3-dihydro-2-thioxo-4H-1,3,5-thiadiazin-4-one (6). The compounds were analysed by single-crystal X-ray diffraction, NMR spectroscopy, IR and Raman spectroscopy, and elemental analysis. Quantum mechanical calculations show thermodynamic reasons for the differences in reactivity.

[M@Sn14-xSbx]q- (M = La, Ce, or U; x = 6-8; q = 3, 4): Interaction of 4f or 5f Metal Ions with 5p Metal Atoms in Intermetalloid Clusters.

The impact of 4f metal ions Ln3+ (Ln = La or Ce) versus 5f metal ions Un+ (n = 3 or 4) on the compositions and distribution of 5p metal atoms in the cluster shells of endohedral species [M@Sn14-xSbx]q- (M = La, Ce, or U; x = 6-8; q = 3, 4) was studied by means of combined experimental and quantum chemical investigations. While all known f-block metal ion-centered endohedral clusters possessed combinations of larger main group metal atoms so far (Sn/Bi or Pb/Bi), resulting in mixtures of 13- and 14-atom cages, the 14-atom cages reported herein comprise exclusively Sn and Sb atoms and therefore are challenged in accommodating the large 4f and 5f ions. We show that the clusters form in reactions of (Sn2Sb2)2- anions with [Ln(C5Me4H)3] or [U(C5Me4H)3Cl], and that salts of [La@Sn6Sb8]3-, [La@Sn7Sb7]4-, [U@Sn8Sb6]4-, and [U@Sn7Sb7]3- can be isolated from them. The assignment of Sn versus Sb in the encapsulating cage follows a simple rule. Different central atoms cause only slight differences in this regard and with respect to distortions of the cluster shells. The reactions also yielded the salt of the new binary anion (Sn4Sb4)2- that was recently predicted by quantum chemical studies.

Zero-field splitting parameters within exact two-component theory and modern density functional theory using seminumerical integration.

An efficient implementation of zero-field splitting parameters based on the work of Schmitt et al. [J. Chem. Phys. 134, 194113 (2011)] is presented. Seminumerical integration techniques are used for the two-electron spin-dipole contribution and the response equations of the spin-orbit perturbation. The original formulation is further generalized. First, it is extended to meta-generalized gradient approximations and local hybrid functionals. For these functional classes, the response of the paramagnetic current density is considered in the coupled-perturbed Kohn-Sham equations for the spin-orbit perturbation term. Second, the spin-orbit perturbation is formulated within relativistic exact two-component theory and the screened nuclear spin-orbit (SNSO) approximation. The accuracy of the implementation is demonstrated for transition-metal and diatomic main-group compounds. The efficiency is assessed for Mn and Mo complexes. Here, it is found that coarse integration grids for the seminumerical schemes lead to drastic speedups while introducing clearly negligible errors. In addition, the SNSO approximation substantially reduces the computational demands and leads to very similar results as the spin-orbit mean field Ansatz.

Electronic structure and bonding in endohedral Zintl clusters.

Endohedral Zintl clusters-multi-metallic anionic molecules in which a d-block or f-block metal atom is enclosed by p-block (semi)metal atoms-are very topical in contemporary inorganic chemistry. Not only do they provide insight into the embryonic states of intermetallic compounds and show promise in catalytic applications, they also shed light on the nature of chemical bonding between metal atoms. Over the past two decades, a plethora of endohedral Zintl clusters have been synthesized, revealing a fascinating diversity of molecular architectures. Many different perspectives on the bonding in them have emerged in the literature, sometimes complementary and sometimes conflicting, and there has been no concerted effort to classify the entire family based on a small number of unifying principles. A closer look, however, reveals distinct patterns in structure and bonding that reflect the extent to which valence electrons are shared between the endohedral atom and the cluster shell. We show that there is a much more uniform relationship between the total valence electron count and the structure and bonding patterns of these clusters than previously anticipated. All of the p-block (semi)metal shells can be placed on a ladder of total valence electron count that ranges between 4n+2 (closo deltahedra), 5n (closed, three-bonded polyhedra) and 6n (crown-like structures). Although some structural isomerism can occur for a given electron count, the presence of a central metal cation imposes a preference for rather regular and approximately spherical structures which maximise electrostatic interactions between the metal and the shell. In cases where the endohedral metal has relatively accessible valence electrons (from the d or f shells), it can also contribute its valence electrons to the total electron count of the cluster shell, raising the effective electron count and often altering the structural preferences. The electronic situation in any given cluster is considered from different perspectives, some more physical and some more chemical, in a way that highlights the important point that, in the end, they explain the same situation. This article provides a unifying perspective of bonding that captures the structural diversity across this diverse family of multimetallic clusters.

Paramagnetic NMR Shielding Tensors Based on Scalar Exact Two-Component and Spin-Orbit Perturbation Theory.

The temperature-dependent Fermi-contact and pseudocontact terms are important contributions to the paramagnetic NMR shielding tensor. Herein, we augment the scalar-relativistic (local) exact two-component (X2C) framework with spin-orbit perturbation theory including the screened nuclear spin-orbit correction for the EPR hyperfine coupling and g tensor to compute these temperature-dependent terms. The accuracy of this perturbative ansatz is assessed with the self-consistent spin-orbit two-component and four-component treatments serving as reference. This shows that the Fermi-contact and pseudocontact interaction is sufficiently described for paramagnetic NMR shifts; however, larger deviations are found for the EPR spectra and the principle components of the EPR properties of heavy elements. The impact of the perturbative treatment is further compared to that of the density functional approximation and the basis set. Large-scale calculations are routinely possible with the multipole-accelerated resolution of the identity approximation and the seminumerical exchange approximation, as shown for [CeTi6O3(OiPr)9(salicylate)6].

Metal-Free N-H Bond Activation by Phospha-Wittig Reagents.

N-containing molecules are mostly derived from ammonia (NH3 ). Ammonia activation has been demonstrated for single transition metal centers as well as for low-valent main group species. Phosphinidenes, mono-valent phosphorus species, can be stabilized by phosphines, giving so-called phosphanylidenephosphoranes of the type RP(PR'3 ). We demonstrate the facile, metal-free NH3 activation using ArP(PMe3 ), affording for the first time isolable secondary aminophosphines ArP(H)NH2 . DFT studies reveal that two molecules of NH3 act in concert to facilitate an NH3 for PMe3 exchange. Furthermore, H2 NR and HNR2 activation is demonstrated.

A Planar Five-Membered Aromatic Ring Stabilized by Only Two π-Electrons.

Many chemicals known today are partially or fully aromatic, since a ring framework experiences additional stabilization through the delocalization of π-electrons. While aromatic rings with equal numbers of π-electrons and ring atoms such as benzene are particularly stable, those with the minimally required two π-electrons are very rare and yet remain limited to three- and four-membered rings if not stabilized in the coordination sphere of heavy metals. Here we report the facile synthesis of a dipotassium cyclopentagallene, a unique example of a five-membered aromatic ring stabilized by only two π-electrons. Single-crystal X-ray diffraction revealed a planar Ga5 ring with almost equal gallium-gallium bond lengths, which together with computational and spectroscopic data confirm its aromatic character. Our results prove that aromatic stabilization goes far beyond what has previously been assumed as minimum π-electron count in a five-atom ring fragment.

Reactive Solubilization of Heterometallic Clusters by Treatment of (TrBi3 )2- Anions (Tr=Ga, In, Tl) with [Mn{N(SiMe3 )2 }2 ].

Lowering the charge of Zintl anions by (element-)organic substituents allows their use as sources of (semi)metal nanostructures in common organic solvents, as realized for group 15 anions or Ge9 4- and Sn9 4- . We developed a new strategy for other anions, using low-coordinate 3d metal complexes as electrophiles. [K(crypt-222)]+ salts of (TrBi3 )2- anions dissolved in situ in Et2 O and/or THF when reacted with [Mn(hmds)2 ]. Work-up afforded soluble [K(crypt-222)]+ salts of [{(hmds)2 Mn}2 (TlBi3 )]2- (in 1), [{(hmds)2 Mn}2 (Bi2 )]2- (in 2), and [{(hmds)Mn}4 (Bi2 )2 ]2- (in 3) (crypt-222=4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane; Tr=Ga, In, Tl; hmds=N(SiMe3 )2 ), representing rare cases of Zintl clusters with open-shell metal atoms. 1 comprises the first coordination compound of the (TlBi3 )2- anion, 2 features a diamond-shaped {Pn2 M2 } unit, and 3 is a mixed-valent MnI /MnII compound. The uncommon electronic structures in 1-3 and magnetic coupling were studied by comprehensive DFT calculations.

Stimuli Responsive Silylene: Electromerism Induced Reversible Switching Between Mono- and Bis-Silylene.

Electromerism is a very well-known phenomenon in transition metal chemistry. In main group chemistry, this concept has only started getting attention recently. We report stimuli responsive low-valent silicon compounds exhibiting electromerism. A mixed-valent silaiminyl-silylene 1, [LSi-Si(NDipp)L] (L=PhC(Nt Bu)2 ), was synthesized in a single step from amidinate-chlorosilylene. Compound 1 has two interconnected Si atoms in formally +I and +III oxidation states. Upon treatment with Lewis acidic CuI X (X=mesityl, Cl, Br, I), electron redistribution occurs resulting in the formation of [{LSi(NDipp)Si(L)}-CuX], in which both silicon atoms are in the +II formal oxidation state. Removal of the copper center from [{LSi(NDipp)Si(L)}-CuX] by using a Lewis basic carbene led to reformation of the precursor [LSi-Si(NDipp)L]. Thus, the process is fully reversible. This showcases the first example of Lewis acid/base-induced reversible electromerism in silicon chemistry.

Photochemistry with Chlorine Trifluoride: Syntheses and Characterization of Difluorooxychloronium(V) Hexafluorido(non)metallates(V), [ClOF2 ][MF6 ] (M=V, Nb, Ta, Ru, Os, Ir, P, Sb).

A photochemical route to salts consisting of difluorooxychloronium(V) cations, [ClOF2 ]+ , and hexafluorido(non)metallate(V) anions, [MF6 ]- (M=V, Nb, Ta, Ru, Os, Ir, P, Sb) is presented. As starting materials, either metals, oxygen and ClF3 or oxides and ClF3 are used. The prepared compounds were characterized by single-crystal X-ray diffraction and Raman spectroscopy. The crystal structures of [ClOF2 ][MF6 ] (M=V, Ru, Os, Ir, P, Sb) are layer structures that are isotypic with the previously reported compound [ClOF2 ][AsF6 ], whereas for M=Nb and Ta, similar crystal structures with a different stacking variant of the layers are observed. Additionally, partial or full O/F disorder within the [ClOF2 ]+ cations of the Nb and Ta compounds occurs. In all compounds reported here, a trigonal pyramidal [ClOF2 ]+ cation with three additional Cl⋅⋅⋅F contacts to neighboring [MF6 ]- anions is observed, resulting in a pseudo-octahedral coordination sphere around the Cl atom. The Cl-F and Cl-O bond lengths of the [ClOF2 ]+ cations seem to correlate with the effective ionic radii of the MV ions. Quantum-chemical, solid-state calculations well reproduce the experimental Raman spectra and show, as do quantum-chemical gas phase calculations, that the secondary Cl⋅⋅⋅F interactions are ionic in nature. However, both solid-state and gas-phase quantum-chemical calculations fail to reproduce the increases in the Cl-O bond lengths with increasing effective ionic radius of M in [MF6 ]- and the Cl-O Raman shifts also do not generally follow this trend.

Using a Porphyrin Diacid Cation to Stabilize a Square-Pyramidal BiX5 (X = Br, Cl/Br) Unit.

Main-group halogenido metalates are a diverse class of compounds with an intricate structural chemistry and a wide range of applications. Here, we present an unprecedented anion motif in the structural chemistry of halogenido bismuthates, a square-pyramidal BiX5 unit. We show how the porphyrin diacid used as our compounds' counterion is templating the formation of this new motif, suggesting that other strong anion receptors may be able to stabilize unique metalate anions in future work.

Near-Infrared Luminescence in Trinuclear Mixed-Metal Chalcogenolate Complexes of the Types [M2Ti(EPh)6(PPh3)2] (M = Cu, Ag; E = S, Se) and [Na(thf)3]2[Ti(SPh)6].

The optical properties of four new trinuclear chalcogenolato bridged metal complexes [Ag2Ti(SPh)6(PPh3)2], [Na(thf)3]2[Ti(SPh)6], [Cu2Ti(SePh)6(PPh3)2], and [Ag2Ti(SePh)6(PPh3)2] have been investigated by absorption and photoluminescence spectroscopy as well as time-dependent density functional theory (TDDFT) calculations and compared to the results published recently for [Cu2Ti(SPh)6(PPh3)2]. All of these compounds are distinguished by efficient near-infrared luminescence at ∼880-1200 nm in the solid state at low temperatures, which remains quite intense for the copper-titanium complexes at ambient temperature with PL quantum yields of 9.5 and 4.8% at λPL = 1090 and 1240 nm for [Cu2Ti(EPh)6(PPh3)2], E = S, Se, respectively. According to the calculations, a peculiar feature of the lowest-energy electronic transitions in these complexes is their high localization on the metal and chalcogen atoms, with negligible contributions of the "external" ligand groups. Correspondingly, the type of atoms in the M2TiE6 (M = Cu, Ag, Na) core structure determines optical properties such as the absorption and emission wavelengths and PL lifetime.