Formazanate Complexes of Hypervalent Group 14 Elements as Precursors to Electronically Stabilized Radicals

The stability of molecular radicals containing main‐group elements usually hinges on the presence of bulky substituents that shield the reactive radical center. We describe a family of Group 14 formazanate complexes whose chemical reduction allows access to radicals that are stabilized instead by geometric and electron‐delocalization effects, specifically by the square‐pyramidal coordination geometry adopted by the Group 14 atom (Si, Ge, Sn) within the framework of the heteroatom‐rich formazanate ligands. The reduction potentials of the Si, Ge, and Sn complexes as determined by cyclic voltammetry become more negative in that order. Examination of the solid‐state structures of these complexes suggested that their electron‐accepting ability decreases with increasing size of the Group 14 atom because a larger central atom increases the nonplanarity of the ligand‐based conjugated π‐electron system of the complex. The experimental findings were supported by density‐functional calculations on the parent complexes and the corresponding radical anions.     

Chemistry of Complexes of Group 14 Elements Based on Redox-Active Ligands of the <Emphasis Type="Italic">o</Emphasis>-Iminoquinone Type

Specific features of the synthesis and structures of the complexes of Group 14 elements (Si, Ge, Sn, Pb) with o-iminoquinone ligands are discussed. The chemical reactions of the above indicated compounds accompanied by the transformation of the redox-active ligand in the coordination sphere of the complex- forming agent are considered.     

杂硼原子簇XB6+ (X=C,Si, Ge,Sn, Pb)的稳定性和芳香性

利用从头算MP2方法和密度泛函理论B3LYP和B3PW91方法, 研究了杂硼原子簇XB₆⁺ (X=C, Si, Ge, Sn, Pb)的结构、稳定性及化学键合情况. 对C, Si, Ge, B使用6-311+G(d)基组, 对Sn和Pb使用LANL2DZ赝势基组. 研究结果表明, 具有C_s对称性的假平面XB₆⁺ (X=C, Si, Ge, Sn, Pb)结构是势能面上的全域极小点, 其稳定性要高于C_6v对称性的锥形结构和C₂对称性的假锥形结构. 在B3LYP水平上, 对这些异构体的势能面的极小点进行了自然键轨道(NBO)的分析; 对最稳定构型的最高占据分子轨道(HOMO)和最低空轨道(LUMO)能级差、分子轨道(MO)和核独立化学位移(NICS)进行了计算和讨论. 分析了杂原子和硼原子间、相邻硼原子间的键合情况, 讨论了最稳定构型的芳香性质.     

Density functional theory study on the possibility of Si‐, Ge‐, and Sn‐doped carbon nanotubes as efficient support materials for platinum

PBEPBE‐D3 calculations were performed to investigate how platinum (Pt) interacts with the internal and external surfaces of single‐walled pristine, Si‐, Ge‐, and Sn‐doped (6,6) carbon nanotubes (CNTs). Our calculations showed that atomic Pt demonstrates stronger binding strength on the external surfaces than the internal surface adsorption for the same type of nanotube. In cases of external surface adsorptions, Si‐, Ge‐, and Sn‐doped CNTs show comparable binding energies for Pt, at least 1.40 eV larger than pristine CNT. This enhancement can be rationalized by the strong covalent interactions between Pt and X? C (X = Si, Ge, and Sn) pairs based on structural and projected density of states analysis. In terms of internal surface adsorptions, Ge and Sn doping could significantly enhance the binding of Pt. Pt atom shows much more delocalized and bonding states inside Ge‐ and Sn‐doped CNTs, indicating multiple‐site interaction pattern when atomic Pt is confined inside the nanotubes. However, the internal surface of Si‐doped CNT presents limited enhancement in Pt adsorption with respect to that of pristine CNT because of their similar binding geometries. © 2016 Wiley Periodicals, Inc.     

Probing the electronic structure,chemical bonding,and excitation spectra of [CuE]+/0/− (E = 14 group element) diatomics employing DFT and ab initio methods

The electronic structure, chemical bonding, and excitation spectra of neutral, cationic, and anionic diatomic molecules of Cu and 14 group elements formulated as [CuE]^+/0/? (E = C, Si, Ge, Sn, Pb) were investigated by density functional theory (DFT) and time‐dependent (TD)‐DFT methods. The electronic and bonding properties of the diatomics analyzed by natural bond orbital (NBO) analysis approch revealed a clear picture of the chemical bonding in these species. The spatial organization of the bonding between Cu and E atoms in the [CuE]^+/0/? (E = Si, Ge, Sn, Pb) molecules can easily be recognized by the cut‐plane electron localization function representations. Particular emphasis was given on the absorption spectra of the [CuE]^+/0/? which were simulated using the results of TD‐DFT calculations employing the hybrid Coulomb‐attenuating CAM‐B3LYP functional. The absorption bands have thoroughly been analyzed and assignments of the contributing principal electronic transitions associated to individual excitations have been made. © 2012 Wiley Periodicals, Inc.     

Metavalent Bonding in 2D Chalcogenides: Structural Origin and Chemical Mechanisms

An unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which involves violation of the 8-N rule. Precise mechanisms of MVB and the relevance of lone pair of Group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB. Here, we present first-principles theoretical analysis of the nature of bonding in five distinct 2D lattices of Group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out-of-plane expression of the lone pair versus in-plane bonding can be systematically explored. While their honeycomb lattices respecting the 8-N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in-plane, with cationic lone pair activating the out-of-plane structural puckering that controls their relative stability. Anomalies in Born-effective charges, dielectric constants, Grüneisen parameters occur only in their in-plane behaviour, confirming MVB is confined strictly to 2D and originates from p-p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures.     

Central‐Atom Size Effects on the Methyl Torsions of Group XIV Tetratolyls

The Group XIV tetratolyl series X(C₆H₄‐CH₃)₄ (X=C, Si, Ge, Sn, Pb) were studied by using inelastic neutron scattering to measure the low‐energy phonon spectra to directly access the methyl‐group torsional modes. The effect of increased molecular radius as a function of the size of the central atom was shown to have direct influence on the methyl dynamics, reinforced with the findings of molecular dynamics and contact surface calculations, based upon the solid‐state structures. The torsional modes in the lightest analogue were found to be predominantly intramolecular: the Si and Ge analogues have a high degree of intermolecular methyl–methyl group interactions, whilst the heaviest analogues (Sn and Pb) showed pronounced intermolecular methyl interactions with the whole phonon bath of the lattice modes.     

On the formation of smaller p‐block endohedral fullerenes: Bonding analysis in the E@C20 (E = Si,Ge, Sn,Pb) series from relativistic DFT calculations

Experimentally characterized endohedral metallofullerenes are of current interest in expanding the range of viable fullerenic structures and their applications. Smaller metallofullerenes, such as M@C₂₈, show that several d‐ and f‐block elements can be efficiently confined in relatively small carbon cages. This article explores the potential capabilities of the smallest fullerene cage, that is, C₂₀, to encapsulate p‐block elements from group 14, that is, E = Si, Ge, Sn, and Pb. Our interest relates to the bonding features and optical properties related to E@C₂₀. The results indicate both s‐ and p‐type concentric bonds, in contrast to the well explored endohedral structures encapsulating f‐block elements. Our results suggest the E@C₂₀ series to be a new family of viable endohedral fullerenes. In addition spectroscopic properties related to electron affinity, optical, and vibrational were modeled to gain further information useful for characterization. Characteristic optical patterns were studied predicting a distinctive first peak located between 400 and 250 nm, which is red‐shifted going to the heavier encapsulated Group 14 atoms. Electron affinity properties expose different patterns useful to differentiate the hollow C₂₀ fullerene to the proposed p‐block endohedral counterparts. © 2017 Wiley Periodicals, Inc.     

[MAl5]+(M=Si,Ge,Sn,Pb)体系结构和稳定性的理论研究

利用B3LYP和CCSD(T)(单点)方法, 研究了含Si, Ge, Sn, Pb的六原子体系[MAl₅]⁺中各个异构体的结构及稳定性. 研究结果表明, 尽管与[CAl₅]⁺一样也具有18个价电子, 但[MAl₅]⁺(M=Si, Ge, Sn, Pb)体系并不存在具有平面五配位结构的异构体, 其能量的全局极小点为具有C_s对称性的蝶形异构体Int1, 这是由于中心原子M(Si, Ge, Sn, Pb)较大的体积显著破坏了[MAl₅]⁺中平面五配位结构的稳定性所致.     

Shifting Paradigms: Electrostatic Interactions and Covalent Bonding

The role of electrostatic interactions in covalent bonding of heavier main group elements has been evaluated for the exemplary set of molecules X₂H₂ (X=C, Si, Ge, Sn, Pb). Density functional calculations at PBE/QZ4P combined with energy decomposition procedures and kinetic energy density analyses have been carried out for a variety of different structures, and two factors are responsible for the fact that the heavier homologues of acetylene exhibit doubly hydrogen‐bridged local minimum geometries. For one, the extended electronic core with at least one set of p orbitals of the Group 14 elements beyond the first long period is responsible for favorable electrostatic E–H interactions. This electrostatic interaction is the strongest for the isomer with two bridging hydrogen atoms. Secondly, the H substituent does not posses an electronic core or any bonding‐inactive electrons, which would give rise to a significant amount of Pauli repulsion, disfavoring the doubly bridged isomer. When one of two criteria is not met the unusual doubly bridged structure no longer constitutes the energetically preferred geometry. The bonding model is validated in calculations of different structures of Si₂(CH₃)₂.     

Low-Valent Group 14 Phosphinidenide Complexes [({SIDipp}P)2M] Exhibit P–M pπ–pπ Interaction (M=Ge,Sn, Pb)

Herein, the synthesis of new low-valent Group 14 phosphinidenide complexes [({SIDipp}P)₂M ] exhibiting P–M pπ–pπ interactions (SIDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene, M=Ge, Sn, Pb), is presented. These compounds were investigated by means of structural, spectroscopic, and quantum-chemical methods. Furthermore, the monosubstituted compounds [(SIDippP)MX]₂ (M=Sn, X=Cl; M=Pb, X=Br) are presented, which show dimeric structures instead of multiple bonding interaction.     

Der Übergang vom „geordneten”︁ Anti-PbCl2-Gitter zum Anti-PbFCl-Gitter: Ternäre Phasen ABX der Erdalkalimetalle mit Elementen der 4. Hauptgruppe (A = Ca,Sr, Ba; B = Mg; X = Si,Ge, Sn,Pb)

The transition of the “ordered” anti-PbCl₂ lattice in the anti-PbFCl lattice: The ternary phases ABX of the alkaline earths with main group IV elements (A = Ca, Sr, Ba; B = Mg; X = Si, Ge, Sn, Pb) The compounds CaMgX, SrMgX and BaMgX (X = Si, Ge, Sn, Pb) were synthesized and their structures determined. CaMgX and SrMgX crystallize in the “ordered” Anti-PbCl₂-type and are therefore related to the binary compounds Ca₂X(X = Si, Ge, Sn, Pb), which form the Anti-PbCl₂-type too. The phases BaMgX build up the Anti-PbFCl-structure. The relations of these two different structures are discussed in respect to the radii of the components.     

An N‐Heterocyclic Boryloxy Ligand Isoelectronic with N‐Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners

Introduced here is a new type of strongly donating N‐heterocyclic boryloxy (NHBO) ligand, [(HCDippN)₂BO]^? (Dipp=2,6‐diisopropylphenyl), which is isoelectronic with the well‐known N‐heterocyclic iminato (NHI) donor class. This 1,3,2‐diazaborole functionalized oxy ligand has been used to stabilize the first acyclic two‐coordinate dioxysilylene and its Ge, Sn, and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X‐ray crystallography and density‐functional theory, enabling analysis of periodic trends: the potential for the [(HCDippN)₂BO]^? ligand to subtly vary its electronic‐donor capabilities is revealed by snapshots showing the gradual evolution of arene π coordination on going from Si to Pb.     

A computational study of the reactivities of four-membered heavy carbene systems

The potential energy surfaces for the chemical reactions of four-membered N-heterocyclic group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ). Five four-membered group 14 heavy carbene species, (i-Pr)(2) NP(NR)(2) E:, in which E = C, Si, Ge, Sn, and Pb, were chosen as the model reactants in this work. Also, four kinds of chemical reactions, C-H bond insertion, water addition, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these group 14 four-membered N-heterocyclic carbene species. Basically, our present theoretical work predicts that the larger the ∠NEN bond angle of the four-membered group 14 heavy carbene species, the smaller the singlet-triplet splitting, the lower the activation barrier, and, in turn, the more rapid, its chemical reactions to various chemical species. Moreover, our theoretical investigations suggest that the relative carbenic reactivity decreases in the order: C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is its four-membered carbene toward chemical reactions. As a result, our results predict that the four-membered group 14 heavy carbene species (E = Si, Ge, Sn, and Pb) should be more kinetically stable than the observed carbene species and, thus, can be also readily synthesized and isolated at room temperature. Furthermore, the singlet-triplet energy splitting of the four-membered group 14 carbene species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.     

Mechanism of abstraction reactions of dimetallenes (R2X=XR2; X = C, Si, Ge, Sn, Pb) with halocarbons: a theoretical study

Potential energy surfaces for the abstraction reactions of dimetallenes with halocarbons have been studied using density functional theory (B3LYP). Five dimetallene species, (SiH(3))(2)X=X(SiH(3))(2), where X = C, Si, Ge, Sn, and Pb, have been chosen in this work as model reactants. The present theoretical investigations suggest that the relative dimetallenic reactivity increases in the order C=C < Si=Si < Ge=Ge < Sn=Sn < Pb=Pb. That is to say, for halocarbon abstractions there is a very clear trend toward lower activation barriers and more exothermic reactions on going from C to Pb. Moreover, for a given dimetallene, the overall barrier heights are determined to be in the order CF(4) > CCl(4) > CBr(4) > CI(4). That is, the heavier the halogen atom (Y), the more facile its abstraction from CY(4). Halogen abstraction is always predicted to be much faster than the abstraction of a CY(3) group irrespective of the dimetallene or halocarbon involved. Our model conclusions are consistent with some available experimental findings. Furthermore, both a configuration mixing model based on the work of Pross and Shaik and bonding dissociation energies can be used to rationalize the computational results. These results allow a number of predictions to be made.     

Norbornyl cations of group 14 elements

Norbornyl cations of the group 14 elements Si --> Pb have been synthesized from substituted 3-cyclopentenemethyl precursors by intramolecular addition of transient cations to the C=C double bond of the 3-cyclopentenemethyl substituent (pi-route to norbornyl cations). The norbornyl cations 4a (E = Si, R = Me), 4e (E = Si, R = Et), 4f (E = Si, R = Bu), 4g (E = Ge, R = Bu), 4h (E = Sn, R = Bu), and 4i (E = Pb, R = Et) have been identified by their characteristic NMR chemical shifts (4a,e,f, delta((29)Si) = 80-87, delta((13)C)(CH=) = 149.6-150.6; 4g, delta((13)C)(CH=) = 144.8; 4h, delta((119)Sn) = 334, delta((13)C)(CH=) = 141.5; 4i, delta((207)Pb) = 1049, delta((13)C)(CH=) = 138). The significant deshielding of the vinylic carbon atoms (Deltadelta((13)C)) relative to those of the precursor (Deltadelta((13)C) = 19.3-20.3 (4a,e,f), Deltadelta((13)C) = 14.6 (4g), Deltadelta((13)C) = 11.1 (4h), Deltadelta((13)C) approximately 8 (4i)) and the small J coupling constants between the element and the remote vinyl carbons in the case of 4h and 4i (J(CSn) = 26 Hz, J(CPb) = 16 Hz) give experimental evidence for the intramolecular interaction and the charge transfer between the positively charged element and the remote C=C double bond. The experimental results are supported by quantum mechanical calculations of structures, energies, and magnetic properties for the norbornyl cations 4a,b (E = Ge, R = Me), 4c (E = Sn, R = Me), 4d (E = Pb, R = Me), and 4e,f at the GIAO/B3LYP/6-311G(3d,p)//MP2/6-311G(d,p) (Si, Ge, C, H), SDD (Sn, Pb) level of theory. The calculated (29)Si NMR chemical shifts for the silanorbornyl cations 4a,e,f (delta((29)Si) = 77-93) agree well with experiment, and the calculated structures of the cations 4a-f reveal their bridged norbornyl cation nature and suggest also for the experimentally observed species 4a,e-i a formally 3 + 1 coordination for the element atom with the extra coordination provided by the C=C double bond. This places five carbon atoms in the close vicinity of the positively charged element atom. The group 14 element norbornyl cations 4a,e-i exhibit only negligible interactions with the aromatic solvent, and they are, depending on the nature of the element group, stable at room temperature in aromatic solvents for periods ranging from a few hours to days. In acetonitrile solution, the intramolecular interaction in the norbornyl cations 4a,e-h breaks down and nitrilium ions with the element in a tetrahedral environment are formed. In contrast, reaction of acetonitrile with the plumbyl cation 4i forms an acetonitrile complex, 10i, in which the norbornyl cation structure is preserved. The X-ray structure of 10i reveals a trigonal bipyramidal environment for the lead atom with the C=C double bond of the cyclopentenemethyl ligand and the nitrogen atom of the acetonitrile molecule in apical positions. Density functional calculations at the B3LYP/6-311G(2d,p)//(B3LYP/6-31G(d) (C, H), SDD (Si, Ge, Sn, Pb)) + DeltaZPVE level indicate that the thermodynamic stability of the group 14 norbornyl cations increases from Si to Pb. This results in a relative stabilization for the plumbanorbornyl cation 4d compared to tert-butyl cation of 52.7 kcal mol(-)(1). In contrast, the intramolecular stabilization energy E(A) of the norbornyl cations 4a-d decreases, suggesting reduced interaction between the C=C double bond and the electron-deficient element center in the plumbacation compared to the silacations. This points to a reduced electrophilicity of the plumbacation compared to its predecessors.     

Theoretical investigations of the reactivities of cationic six-membered carbene analogues of group 14 elements

The potential energy surfaces for the chemical reactions of cationic six-membered group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ) and CCSD (CCSD/LANL2DZ//B3LYP/LANL2DZ) methods. Five six-membered group 14 cationic heavy carbene species, [HC(CMeNPh)2E:](+), where E = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reaction, C-H bond insertion, multiple bond cycloaddition, dimerization, and O-H bond insertion, have been used to study the chemical reactivities of these group 14 cationic carbene species. Basically, our present theoretical work predicts that the larger the angle NEN bond angle and the smaller the singlet-triplet splitting of the carbene, the lower its activation barriers will be and, in turn, the more rapid are its chemical reactions with other species. Moreover, the theoretical investigations suggest that the relative carbenic reactivity decreases in the order C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is its cationic carbene toward chemical reaction. As a result, we predict that the cationic six-membered group 14 carbene species (E = C, Si, Ge, Sn, and Pb) should be stable, readily synthesized, and isolated at room temperature. Our computational results are in good agreement with the available experimental observations. Furthermore, the singlet-triplet energy splitting of the carbene, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict its reactivities. The results obtained allow a number of predictions to be made.     

Stable Di‐ and Tri‐coordinated Carbon(II) Supported by an Electron‐Rich β‐Diketiminate Ligand

Complexes of the ubiquitous β‐diketiminates (NacNac) ligands have been reported with most elements of the periodic table, including Group 14 Si, Ge, Sn, and Pb. The striking absence of carbon representatives has been attributed to the extreme electrophilicity of the putative C‐NacNac adducts. An electron enriched 2,4‐(dimethylamino)diketiminato backbone is described, which allowed for the synthesis and isolation of such stable pyrimidin‐1,3‐diium and pyrimidinium‐2‐ylidene salts. Structural and preliminary reactivity studies are reported, including an air‐stable gold complex. An unforeseen original class of stable N‐heterocyclic carbenes and, more generally, the potential of electron‐rich NacNac patterns for taming highly electrophilic centers are showcased.     

Structural and electronic properties of Al12X+ (X=C, Si, Ge, Sn, and Pb) clusters

Using the first-principles method with the generalized gradient approximation, the authors have studied the structural and electronic properties of Al(12)X(+) (X=C, Si, Ge, Sn, and Pb) clusters in detail. The ground state of Al(12)C(+) is a low symmetry C(s) structure instead of an icosahedron. However, the Si, Ge, Sn, and Pb atom doped cationic clusters favor icosahedral structures. The ground states for Al(12)Si(+) and Al(12)Ge(+) are icosahedra, while the C(5nu) structures optimized from an icosahedron with a vertex capped by a tetravalent atom have the highest binding energy for Al(12)Sn(+) and Al(12)Pb(+) clusters. The I(h) structure and the C(5nu) structure are almost degenerate for Al(12)Ge(+), whose binding energy difference is only 0.03 eV. The electronic properties are altered much by removing an electron from the neutral cluster. The binding strength of a valence electron is enhanced, while the binding energy of the cluster is reduced much. Due to the open electronic shell, the band gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital are approximately 0.3 eV for the studied cationic clusters.     

Topographical analyses of homonuclear multiple bonds between main group elements

Recent experiments have resulted in the completion of the series of Group 14 and Group 15 element double-bond systems, R(n)E=ER(n) (E = C - Pb, n = 2; E = N - Bi, n = 1). Furthermore, new families of multiple-bonded species have been discovered, such as the radical anion [RSnSnR](-) , the close ion pairs [RE(mu2Na)2ER] (E = Ge,Sn), and a digallyne [RGa(mu2Na)2GaR] for which a Ga=Ga triple bond was formulated. Some of these compounds show classical multiple bond features (i.e. the dipnictogens RE=ER, E=N-Bi) in the sense that planar structures with short E-E distances are observed. However, many (i.e. R2E=ER2, E = Si - Pb) do not behave as expected for compounds with multiple bonds. They have trans bent structures, show enormous variation in their E-E distances, and some dissociate easily under E-E bond cleavage in solution. These properties raised doubts as to whether these compounds can be formulated as multiple-bonded systems. Using the electron localization function (ELF) it is possible to clearly show the topographical similarities between classical and nonclassical multiple bonds; ELF divides these systems into unslipped (classical) and slipped (nonclassical) systems. ELF can also be employed to confirm the nonexistence of multiple bonds. Therefore, topographical analyses using ELF are useful to categorize a bonding system. In particular, the bonds in the heavier Group 14 double systems and the Ga-Ga bond in digallyne are clearly shown by this method as slipped double and triple bonds, respectively.