Calix[4]pyrrolato Stannate(II): A Tetraamido Tin(II) Dianion and Strong Metal-Centered σ-Donor

Anionic, metal-centered nucleophiles are emerging compounds with unique reactivities. Here, we describe the isolation and full characterization of the first tetraamido tin(II) dianion, its behavior as ligand towards transition metals, and its reactivity as a tin-centered nucleophile. Experimental values such as the Tolman electronic parameter (TEP) and computations attest tin-located σ-donor ability exceeding that of carbenes or electron-rich phosphines. Against transition metals, the stannate(II) can act as η1 - or η5 -type ligand. With aldehydes, it reacts by hydride substitution to give valuable acyl stannates. The reductive dehalogenation of iodobenzene indicates facile redox pathways mediated by halogen bond interaction. Calix[4]pyrrolato stannate(II) represents the first example of this macrocyclic ligand in low-valent p-block element chemistry.

Metathesis of P=C Bonds Catalyzed by N-Heterocyclic Carbenes

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p-block elements. A high-yielding P=C bond metathesis of phosphaalkenes (ArP=CPh₂ , Ar=Mes, o-Tol, Ph) has been discovered that is catalyzed by N-heterocyclic carbenes (NHC=Me₂ IMe, Me₂ Iⁱ Pr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)_n ; n=3, 4, 5, 6] and Ph₂ C=CPh₂ . Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o-Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo-(ArP)₂ CPh₂ and cyclo-(ArP)₄ CPh₂ have also been observed. This work represents a rare application of non-metal-based catalysts for transformations involving main-group elements.

Bridged Aromatic Oxo- and Thioethers with Intense Emission in Solution and the Solid State

In this contribution, we report on a class of emitters based on bridged oxo- and/or thioethers revealing striking photoluminescence properties in fluid solution and in the solid state. In total, nine compounds were investigated concerning their photophysical properties, which were interpreted by quantum chemical calculations. To our delight, we discovered compounds possessing nearly identical photoluminescence quantum yields (ΦF ) in solution and in the solid state, which has been rarely reported so far. Besides these efforts, we shed light on the influence of polymorphism and solvent polarity on the emission properties. In addition, an in-depth X-ray diffractometric analysis was conducted to correlate molecular packing in the crystal with differences in the photophysical properties.

B-B vs. B-H Bond Activation in a (μ-Hydrido)diborane(4) Anion upon Cycloaddition with CO2 , Isocyanates, or Carbodiimides

The intriguing (μ‐hydrido)diboranes(4) with their prominent pristine representative [B₂H₅]⁻ have mainly been studied theoretically. We now describe the behavior of the planarized tetraaryl (μ‐hydrido)diborane(4) anion [1H]⁻ in cycloaddition reactions with the homologous series of heterocumulenes CO₂, iPrNCO, and iPrNCNiPr. We show that a C=O bond of CO₂ selectively activates the B−B bond of [1H]⁻, while the μ‐H ligand is left untouched ([2H]⁻). The carbodiimide iPrNCNiPr, in contrast, neglects the B−B bond and rather adds the B‐bonded H⁻ ion to its central C atom to generate a formamidinate bridge across the B₂ pair ([3]⁻). As a hybrid, the isocyanate iPrNCO combines the reactivity patterns of both its congeners and gives two products: one of them ([4H]⁻) is related to [2H]⁻, the other ([5]⁻) is an analog of [3]⁻. We finally propose a mechanistic scenario that rationalizes the individual reaction outcomes and combines them to a coherent picture of B–B vs. B–H bond activation.

Sulfur Tetrahydride and Allied Superhydride Clusters: When Resonance Takes Precedence

Sulfur offers a variety of bonding surprises compared to the parent oxygen atom of the chalcogen family. In the present work, we employ standard quantum chemistry methods to characterize formation of previously unrecognized sulfur tetrahydride (C_4v -symmetric SH₄ ) from hydrogen sulfide (H₂ S) and molecular hydrogen (H₂ ) on the ground state potential energy surface. The unusual intramolecular interactions of SH₄ defy Lewis-like bonding conceptions, exhibiting the dominance of resonance-type donor-acceptor delocalizations well beyond those of SF₄ (C_2v sawhorse geometry) and other known tetrahalides. The distressed character of SH₄ bonding also leads to exotic intermolecular structural motifs in clusters of pure (SH₄ )_n and mixed (SH₄ ⋅⋅⋅H₂ S)_n composition. We evaluate structural, spectroscopic, and electronic properties for various 2D/3D coordination patterns and discuss how (SH₄ ⋅⋅⋅H₂ S)_n -type building blocks may relate to recent experimental studies of superconductivity in high-pressure materials of "SH₃ " stoichiometry.

Easy Access to Enantiomerically Pure Heterocyclic Silicon-Chiral Phosphonium Cations and the Matched/Mismatched Case of Dihydrogen Release

Phosphonium ions are widely used in preparative organic synthesis and catalysis. The provision of new types of cations that contain both functional and chiral information is a major synthetic challenge and can open up new horizons in asymmetric cation-directed and Lewis acid catalysis. We discovered an efficient methodology towards new Si-chiral four-membered CPSSi* heterocyclic cations. Three synthetic approaches are presented. The stereochemical sequence of anchimerically assisted cation formation with B(C6 F5 )3 and subsequent hydride addition was fully elucidated and proceeds with excellent preservation of the chiral information at the stereogenic silicon atom. Also the mechanism of dihydrogen release from a protonated hydrosilane was studied in detail by the help of Si-centered chirality as stereochemical probe. Chemoselectivity switch (dihydrogen release vs. protodesilylation) can easily be achieved through slight modifications of the solvent. A matched/mismatched case was identified and the intermolecularity of this reaction supported by spectroscopic, kinetic, deuterium-labeling experiments, and quantum chemical calculations.

Two coordination compounds of SnCl2 with 4-methyl-pyridine N-oxide

In the solid-state structures of catena-poly[[di-chlorido-tin(II)]-μ2-(4-methyl-pyridine N-oxide)-κ2 O:O], [SnCl2(C6H7NO)] n , 1, and di-chlorido-bis-(4-methyl-pyridine N-oxide-κO)tin(II), [SnCl2(C6H7NO)2], 2, the bivalent tin atoms reveal a seesaw coordination with both chlorine atoms in equatorial and the Lewis base mol-ecules in axial positions. While the Sn-Cl distances are almost identical, the Sn-O distances vary significantly as a result of the different bonding modes (μ2 for 1, μ1 for 2) of the 4-methyl-pyridin-N-oxide mol-ecules, giving rise to a one-dimensional coordination polymer for the 1:1 adduct, 1, and a mol-ecular structure for the 1:2 adduct, 2. The different coordination modes also influence the bonding parameters within the almost planar ligand mol-ecules, mostly expressed in N-O-bond lengthening and endocyclic bond-angle widening at the nitro-gen atoms. Additional supra-molecular features are found in the crystal structure of 2 as two adjacent mol-ecules form dimers via additional, weak O⋯Sn inter-actions.

Understanding the Uniqueness of 2p Elements in Periodic Tables

The Periodic Table, and the unique chemical behavior of the first element in a column (group), were discovered simultaneously one and a half centuries ago. Half a century ago, this unique chemistry of the light homologs was correlated to the then available atomic orbital (AO) radii. The radially nodeless 1s, 2p, 3d, 4f valence AOs are particularly compact. The similarity of r(2s)≈r(2p) leads to pronounced sp-hybrid bonding of the light p-block elements, whereas the heavier p elements with n≥3 exhibit r(ns) ≪ r(np) of approximately -20 to -30 %. Herein, a comprehensive physical explanation is presented in terms of kinetic radial and angular, as well as potential nuclear-attraction and electron-screening effects. For hydrogen-like atoms and all inner shells of the heavy atoms, r(2s) ≫ r(2p) by +20 to +30 %, whereas r(3s)≳r(3p)≳r(3d), since in Coulomb potentials radial motion is more radial orbital expanding than angular motion. However, the screening of nuclear attraction by inner core shells is more efficient for s than for p valence shells. The uniqueness of the 2p AO is explained by this differential shielding. Thereby, the present work paves the way for future physical explanations of the 3d, 4f, and 5g cases.

Heavy Metals Make a Chain: A Catenated Bismuth Compound

Catenation is common for the light main-group elements whereas it is rare for the heavy elements. Herein, we report the first example of a neutral molecule containing a Bi₄ chain. It is prepared in a one-step reaction between bismuth trichloride and bis(diisopropylphosphino)amine in methanol suspension. The same reaction carried out in dichloromethane gives quite different products. All products have been characterized spectroscopically and using single-crystal X-ray analysis.

Octacarbonyl Ion Complexes of Actinides [An(CO)8 ]+/- (An=Th, U) and the Role of f Orbitals in Metal-Ligand Bonding

The octacarbonyl cation and anion complexes of actinide metals [An(CO)₈ ]^+/- (An=Th, U) are prepared in the gas phase and are studied by mass-selected infrared photodissociation spectroscopy. Both the octacarbonyl cations and anions have been characterized to be saturated coordinated complexes. Quantum chemical calculations by using density functional theory show that the [Th(CO)₈ ]⁺ and [Th(CO)₈ ]^- complexes have a distorted octahedral (D_4h ) equilibrium geometry and a doublet electronic ground state. Both the [U(CO)₈ ]⁺ cation and the [U(CO)₈ ]^- anion exhibit cubic structures (O_h ) with a ⁶ A_1g ground state for the cation and a ⁴ A_1g ground state for the anion. The neutral species [Th(CO)₈ ] (O_h ; ¹ A_1g ) and [U(CO)₈ ] (D_4h ; ⁵ B_1u ) have also been calculated. Analysis of their electronic structures with the help on an energy decomposition method reveals that, along with the dominating 6d valence orbitals, there are significant 5f orbital participation in both the [An]←CO σ donation and [An]→CO π back donation interactions in the cations and anions, for which the electronic reference state of An has both occupied and vacant 5f AOs. The trend of the valence orbital contribution to the metal-CO bonds has the order of 6d≫5f>7s≈7p, with the 5f orbitals of uranium being more important than the 5f orbitals of thorium.

Reply to Correspondence on "Core Electron Topologies in Chemical Compounds: Case Study of Carbon versus Silicon"

In their Correspondence, von Szentpály, Schwarz, Stoll, and Werner claim that the main conclusions of our Communication previously published in this journal are based on computational artifacts and oversimplified models. We clarify the justification of our simple one-electron model to describe one-electron physics, and refute their criticism based on what they call "computational artifacts." We remind that our main conclusion on the crucial role of qualitative changes in core electron wavefunctions is evidenced not only by wavefunction topologies the complainants cling to, but also by several other physical observables, which remain unrefuted. Hence, the conclusions of our original Communication stand.

Correspondence on "Core Electron Topologies in Chemical Compounds: Case Study of Carbon versus Silicon"

The conclusions of a recent Communication of Yoshida, Raebiger, Shudo, and Ohno published in this journal, that varying core orbital topologies with minuscule negative tails upon bond formation determine the different chemistries of carbon and silicon and affect ionization energies, excitation energies and bond properties of molecules, are now shown to be based on computational artifacts and oversimplified models. The all-electron wave function uniquely determines the observables, while its representation by one-electron orbital products depends on the details of the chosen approximation and therefore need to be considered with great care.

Generation and EPR Spectroscopy of the First Silenyl Radicals, R2 C=Si. -R: Experiment and Theory

The first two persistent silenyl radicals (R₂ C=Si^. -R), with a half-life (t_1/2 ) of about 30 min, were generated and characterized by electron paramagnetic resonance (EPR) spectroscopy. The large hyperfine coupling constants (hfccs) (a(²⁹ Si_α )=137.5-148.0 G) indicate that the unpaired electron has substantial s character. DFT calculations, which are in good agreement with the experimentally observed hfccs, predict a strongly bent structure (∡C=Si-R=134.7-140.7°). In contrast, the analogous vinyl radical, R₂ C=C^. -R (t_1/2 ≈3 h), exhibits a small hfcc (a(¹³ C_α )=26.6 G) and has a nearly linear geometry (∡C=C-R=168.7°).

Revisiting a Historical Concept by Using Quantum Crystallography: Are Phosphate, Sulfate and Perchlorate Anions Hypervalent?

There are many examples of atoms in molecules that violate Lewis' octet rule, because they have more than four electron pairs assigned to their valence. These atoms are referred to as hypervalent. However, hypervalency may be regarded as an artifact arising from Lewis' description of molecules, which is based on the assumption that electrons are localized in two-center two-electron bonds and lone pairs. In the present paper, the isoelectronic phosphate (PO₄ ^3- ), sulfate (SO₄ ^2- ) and perchlorate (ClO₄ ^- ) anions were examined with respect to the concept of hypervalency. Lewis formulas containing a hypervalent central atom exist for all three anions. Based on X-ray wavefunction refinements of high-resolution X-ray diffraction data of representative crystal structures (MgNH₄ PO₄ ⋅6 H₂ O, Li₂ SO₄ ⋅H₂ O, and KClO₄ ), complementary bonding analyses were performed. In this way, experimental information from the new field of quantum crystallography validate long-known facts, or refute long-standing misunderstandings. It is shown that the P-O and S-O bonds are highly polarized covalent bonds and, thus, the increase in the valence population following three-center four-electron bonding is not sufficient to yield hypervalent phosphorus or sulfur atoms, respectively. However, for the highly covalent Cl-O bond, most bonding indicators imply a hypervalent chlorine atom.

Core Electron Topologies in Chemical Compounds: Case Study of Carbon versus Silicon

The similarities and differences between carbon and silicon have attracted the curiosity of chemists for centuries. Similarities and analogies can be found in their saturated compounds, but carbon exhibits a cornucopia of unsaturated compounds that silicon (and most other elements) cannot replicate. While this qualitative difference is empirically well known, quantum chemistry has previously only described quantitative differences related to orbital overlap, steric effects, or orbital energies. We study C₂ and Si₂ and their hydrides X₂ H_2n (X=C, Si; n=1, 2, 3) by first-principles quantum chemical calculation, and find a qualitative difference in the topologies of the core electrons: carbon has the propensity to alter its core electron topology when forming unsaturated compounds, and silicon has not. We draw a connection between the core electron topologies and ionization energies, and identify other elements we expect to have similarly flexible core topologies as carbon.

Triple Bonds Between Iron and Heavier Group 15 Elements in AFe(CO)3- (A=As, Sb, Bi) Complexes

Heteronuclear transition-metal-main-group-element carbonyl complexes of AsFe(CO)₃^- , SbFe(CO)₃^- , and BiFe(CO)₃^- were produced by a laser vaporization supersonic ion source in the gas phase, and were studied by mass-selected IR photodissociation spectroscopy and advanced quantum chemistry methods. These complexes have C_3v structures with all of the carbonyl ligands bonded on the iron center, and feature covalent triple bonds between bare Group 15 elements and Fe(CO)₃^- . Chemical bonding analyses on the whole series of AFe(CO)₃^- (A=N, P, As, Sb, Bi, Mc) complexes indicate that the valence orbitals involved in the triple bonds are hybridized 3d and 4p atomic orbitals of iron, leading to an unusual (dp-p) type of transition-metal-main-group-element multiple bonding. The σ-type three-orbital interaction between Fe 3d/4p and Group 15 np valence orbitals plays an important role in the bonding and stability of the heavier AFe(CO)₃^- (A=As, Sb, Bi) complexes.

Diversity of Chemical Bonding and Oxidation States in MS4 Molecules of Group 8 Elements

The geometric and electronic ground-state structures of 30 isomers of six MS₄ molecules (M=Group 8 metals Fe, Ru, Os, Hs, Sm, and Pu) have been studied by using quantum-chemical density functional theory and correlated wavefunction approaches. The MS₄ species were compared to analogous MO₄ species recently investigated (W. Huang, W.-H. Xu, W. H. E. Schwarz, J. Li, Inorg. Chem. 2016, 55, 4616). A metal oxidation state (MOS) with a high value of eight appeared in the low-spin singlet T_d geometric species (Os,Hs)S₄ and (Ru,Os,Hs)O₄ , whereas a low MOS of two appeared in the high-spin septet D_2d species Fe(S₂ )₂ and (slightly excited) metastable Fe(O₂ )₂ . The ground states of all other molecules had intermediate MOS values, with S^2- , S₂^2- , S₂^1- (and O^2- , O^1- , O₂^2- , O₂^1- ) ligands bonded by ionic, covalent, and correlative contributions. The known tendencies toward lower MOS on going from oxides to sulfides, from Hs to Os to Ru, and from Pu to Sm, and the specific behavior of Fe, were found to arise from the different atomic orbital energies and radii of the (n-1)p core and (n-1)d and (n-2)f valence shells of the metal atoms in row n of the periodic table. The comparative results of the electronic and geometric structures of the MO₄ and MS₄ species provides insight into the periodicity of oxidation states and bonding.

Carbones as Ligands in Novel Main-Group Compounds E[C(NHC)2 ]2 (E=Be, B+ , C2+ , N3+ , Mg, Al+ , Si2+ , P3+ ): A Theoretical Study

Quantum chemical calculations of the main-group compounds E[C(NHC^Me )₂ ]₂ (E=Be, B⁺ , C²⁺ , N³⁺ , Mg, Al⁺ , Si²⁺ , P³⁺ ) have been carried out using density functional theory at the BP86/def2-TZVPP and BP86-D3(BJ)/def2-TZVPP levels of theory. The geometry optimization at BP86/def2-TZVPP gives equilibrium structures with two-coordinated species E and bending angles C-E-C between 152.5° (E=Be) and 110.5° (E=Al). Inclusion of dispersion forces at BP86-D3(BJ)/def2-TZVPP yields a three-coordinated beryllium compound Be[C(NHC^Me )₂ ]₂ as the only energy minimum form. Three-coordinated isomers are found besides the two-coordinated energy minima for the boron and carbon cations B[C(NHC^Me )₂ ]₂⁺ and C[C(NHC^Me )₂ ]₂²⁺ . The three-coordinated form of the boron compound is energetically lower lying than the two-coordinated form, while the opposite trend is calculated for the carbon species. The theoretically predicted bond dissociation energies suggest that all compounds are viable species for experimental studies. The X-ray structure of the benzoannelated homologue of P[C(NHC^Me )₂ ]₂³⁺ that was recently reported by Dordevic et al. agrees quite well with the calculated geometry of the molecule. A detailed bonding analysis using charge and energy decomposition methods shows that the two-coordinated neutral compounds Be[C(NHC^Me )₂ ]₂ and Mg[C(NHC^Me )₂ ]₂ possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHC^Me )₂ serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHC^Me )₂ C←E→C(NHC^Me )₂ (E=Be, Mg). Dative bonds in which the carbones C(NHC^Me )₂ are donor ligands are suggested for the cations (NHC^Me )₂ C→E←C(NHC^Me )₂ (E=B⁺ , Al⁺ ). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHC^Me )₂ C]⁺ -E-[C(NHC^Me )₂ ]⁺ (E=C, Si, N⁺ , P⁺ ).

Comparison of Hydrogen and Gold Bonding in [XHX](-) , [XAuX](-) , and Isoelectronic [NgHNg](+) , [NgAuNg](+) (X=Halogen, Ng=Noble Gas)

Quantum chemical calculations at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels have been carried out for the title compounds. The electronic structures were analyzed with a variety of charge and energy partitioning methods. All molecules possess linear equilibrium structures with D∞h symmetry. The total bond dissociation energies (BDEs) of the strongly bonded halogen anions [XHX](-) and [XAuX](-) decrease from [FHF](-) to [IHI](-) and from [FAuF](-) to [IAuI](-) . The BDEs of the noble gas compounds [NgHNg](+) and [NgAuNg](+) become larger for the heavier atoms. The central hydrogen and gold atoms carry partial positive charges in the cations and even in the anions, except for [IAuI](-) , in which case the gold atom has a small negative charge of -0.03 e. The molecular electrostatic potentials reveal that the regions of the most positive or negative charges may not agree with the partial charges of the atoms, because the spatial distribution of the electronic charge needs to be considered. The bonding analysis with the QTAIM method suggests a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg](+) and to the halogen atoms in [XHX](-) . The covalent character of the bonding in the gold systems [NgAuNg](+) and [XAuX](-) is smaller than in the hydrogen compound. The energy decomposition analysis suggests that the lighter hydrogen systems possess dative bonds X(-) →H(+) ←X(-) or Ng→H(+) ←Ng while the heavier homologues exhibit electron sharing through two-electron, three-center bonds. Dative bonds X(-) →Au(+) ←X(-) and Ng→Au(+) ←Ng are also diagnosed for the lighter gold systems, but the heavier compounds possess electron-shared bonds.

On the energetics of P–P bond dissociation of sterically strained tetraamino-diphosphanes

Thermochemical data for the homolytic P–P bond fission in tetraaminodiphosphanes (R₂N)₂P–P(NR₂)₂ were determined experimentally and computationally. The results confirm that radical formation is favoured by entropic and structural relaxation effects, and disfavoured by dispersion forces. Unlike aminophosphenium cations, the radicals display no strong preference for a planar (R₂N)₂P unit.

Unexpected 1,2-migration in metallasilabenzenes: theoretical evidence for reluctance of silicon to participate in π bonding

Density functional theory (DFT) calculations were carried out to investigate the 1,2-migration in metallasilabenzenes. The results suggested that the chloride migration of metallabenzenes is unfavorable due to the loss of aromaticity in the nonaromatic analogues. In sharp contrast, such a migration in metallasilabenzenes is favorable due to the reluctance of silicon to participate in π bonding. The migration of hydride and methyl group from the metal center to the silicon atom in metallasilabenzenes is computed to be also feasible. In addition, the π donor ligand and the third row transition metal can stabilize metallasilabenzenes. Thus, such a migration becomes less favorable thermodynamically and kinetically. These findings could be very helpful for synthetic chemists to realize the first metallasilabenzene.