Theoretical investigations of the reactivities of saturated five-membered ring N-heterocyclic carbenes with heavier group 14 elements

The potential energy surfaces for the chemical reactions of group 14 carbenes have been studied using density functional theory (B3LYP/LANL2DZ). Five saturated five-membered-ring N-heterocyclic carbene Dipp[upper bond 1 start]N(CH(2))(2)N(Dipp)E[upper bond 1 end]: (five-ring-E:) species, where E = C, Si, Ge, Sn and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reactions; addition of water, methane insertion, alkene cycloaddition and dimerization, have been used to study the chemical reactivities of these group 14 carbenes. The present 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 the carbene towards chemical reactions. This may be the reason that there have been many instances reported of the synthesis and characterization of stable group 14 five-membered-ring N-heterocyclic carbene species with various alkyl protecting substituents at room temperature. Furthermore, the singlet-triplet energy splitting of the five-ring-E:, 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.     

Preparation,properties and reactivities of novel platinum compounds containing a thiomethoxymethyl group

Oxidative addition of ClCH₂SCH₃ to PtL₄ afforded trans-PtL₂(CH₂SCH₃)Cl (Ia, L = Ph₃P;Ib, L = MePh₂P). Treatment of I with NH₄PF₆ or Et₃OBF₄ in CH₂C1₂ gave ionic species, [PtL₂(CH₂SCH₃)]X (II, L = Ph₃P, MePh₂P, X = BF₄, PF₆), while similar treatment with MeSO₃F in benzene yielded a new type of stable dimethylsulfonium methylide—platinum complex, trans-[PtL₂(CH₂SMe₂)Cl] SO₃F (IIIa, L = Ph₃P; IIIb, L = MePh₂P). Action of H₂O₂ on Ia gave [Pt(Ph₃P)(μ-CH₂SCH₃)C1]₂ (IV) and its triphenylarsine analog, [Pt(Ph₃As)(μ-CH₂SCH₃)C1]₂ (V) was prepared in one step by oxidative addition of ClCH₂SCH₃ to Pt(AsPh₃)₄. The structural difference between [Pt(Ph₃P)(μ-CH₂SCH₃)C1]₂ and Pd(Ph₃P)- (CH₂SCH₃)C1 is discussed in terms of the difference in the ionization potential from d¹⁰ to d⁸ electronic state of metals.     

Visualizing lone pairs in compounds containing heavier congeners of the carbon and nitrogen group elements

In this mini-review, I discuss some recent work on the stereochemistry and bonding of lone pairs of electrons in divalent compounds of the heavier carbon group elements (Sn^II , Pb^II) and in trivalent compounds of the heavier nitrogen group elements (Bi^III). Recently developed methods that permit the real-space visualization of bonding patterns on the basis of density functional calculations of electronic structure, reveal details of the nature of s electron lone pairs in compounds of the heavier main group elements — their stereochemistry and their inertness (or lack thereof). An examination of tetragonalP4/nmm SnO, α-PbO and BiOF, and cubic PbS provides a segue into perovskite phases of technological significance, including ferroelectric PbTiO₃ and antiferroelectric/piezoelectric PbZrO₃, in both of which the lone pairs on Pb atoms play a pivotal r?le.     

Accurate ab initio pair potentials between helium and the heavier group 2 elements

Interactions between the heavier Group 2 metals (Ca, Sr, and Ba) and helium were studied using the well-tempered model core potential method. Accurate pair potentials, calculated at the coupled-cluster level of theory with very large basis sets, were used in bound state calculations. Three bound rovibrational states were found for each complex. The pair-potential parameters were used to predict how each of the metal atoms would be solvated by a helium nanodroplet. The Ca atom is not fully solvated by the droplet and the interaction between the helium and the metal decreases from Ca to Ba. This agrees with the experimental observation that the spectra of these atoms in a nanodroplet are intermediate between the spectra of the free atoms and the spectra in liquid helium.     

Addition of stable nitroxide radical to stable divalent compounds of heavier group 14 elements

The reactions of stable cyclic dialkylgermylene 2 and dialkylstannylene 3 with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical (2 equiv) gave the corresponding 1:2 adducts 4 and 5, respectively, which were characterized by NMR, MS, and X-ray analyses. The kinetics of the stepwise addition of two TEMPO molecules to germylene 2 revealed that the initial addition of TEMPO to 2 was 1010 times slower than the second TEMPO addition to the resulting germyl radical. The origin of the rate difference was discussed on the basis of the qualitative perturbation theory. In contrast to the reactions of 2 and 3, the reaction of dialkylsilylene 1 with TEMPO gave an interesting 1,3-dioxadisiletane derivative.     

Heats of hydrogenation of compounds featuring main group elements and with the potential for multiply bonding

Reaction enthalpies are calculated for the hydrogenation reactions of main group hydrides with the potential for multiple bonding, and thus the unsaturated character of these species is determined. In addition to the global minimum structures, which leave in some cases no hope for even a single E-E bond (E=Group 13, 14, or 15 element), calculations are also performed for geometries with maximum potential for multiple bonding. The trends down the groups and the periods are established. Interpretations have to take several factors into account. These factors sometimes work hand in hand but also against each other. We also include in our survey the species [HGaGaH]2- as a free anion and Na2[HGaGaH] as well as their hydrogenation products [H2GaGaH2]2- and Na2[H2GaGaH2]2-. The results show that the presence of the Na+ ions has a significant impact on their chemistry, and thus suggests that they are involved to a large extent in the bonding. Our results indicate that the compounds should be described as cluster compounds.     

Bis(diphenylphosphido) derivatives of the heavier group 2 elements

A number of heavier alkaline-earth (Ae) diphenylphosphides have been prepared by protonolysis of the corresponding bis(trimethylsilyl)amides and completely characterized in the solid state and in solution. Through careful control of the reaction and workup conditions, preparations performed with tetrahydrofuran (THF) as the only donor solvent crystallize as octahedral tetrakis-solvated THF adducts [Ae(PPh2)2(THF)4] when Ae=Ca and Sr. Both structures are similar and contain the two diphenylphosphide substituents in a trans disposition. In contrast, the analogous reaction performed upon the barium amide yielded a one-dimensional polymeric complex containing both five- and six-coordinated Ba centers in which the polymer is propagated via bridging diphenylphosphide units. Further strictly mononuclear derivatives of the Ca and Ba diphenylphosphides may be straightforwardly prepared by treatment of the THF adducts with 18-crown-6. Although the THF-solvated species are evidently labile to loss of solute, solution (diffusion-ordered spectroscopy NMR, THF-d8) studies indicate that, under catalytically relevant conditions, a similar mononuclear constitution is most likely retained.     

Recent topics of the metallacycles composed of the heavier group 14 elements

This Letter reviews recent advance of metallacycles with chelating Si-, Ge-, and Sn-ligands. Dehydrogenative bond-forming reactions of organosilanes, -germanes, and -stannanes promoted by Pd and Pt complexes afford four- and five-membered metallacycles composed of heavier group 14 elements. It has a couple of advantages such as easier preparation of the starting compounds and reaction procedure than the common metathesis reactions of dianions with transition metal dihalide complexes. These metallacycles are regarded as possible intermediates in catalytic dehydrocoupling polymerizations or as convenient precursors to form discrete oligomers.     

Synthesis and properties of stable 2-metallanaphthalenes of heavier group 14 elements

The first stable neutral stannaaromatic compound, 2-stannanaphthalene , was synthesized by taking advantage of an extremely bulky and efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt). The molecular structure and aromaticity of were discussed on the basis of X-ray crystallographic analysis, NMR, UV-vis, and Raman spectroscopy, cyclic voltammetry, and theoretical calculations. 2-Germanaphthalene , which has a framework similar to that of , was synthesized for comparison, and systematic elucidation was made for the properties of 2-metallanaphthalene systems containing a heavier group 14 element (Si, Ge, or Sn).     

Cyclobutadiene dianions consisting of heavier group 14 elements: synthesis and characterization

The first "heavy" cyclobutadiene dianions (CBD2-), tetrakis(di-tert-butylmethylsilyl)-1,2-disila-3,4-digermacyclobutadiene dianion 22- and tetrakis(di-tert-butylmethylsilyl)tetrasilacyclobutadiene dianion 42-, have been synthesized by the reductive dehalogenation of the corresponding precursors with potassium graphite in THF. Structural characterization of [K+(thf)2]2.22- and [K+(thf)2]2.42- has been achieved by X-ray crystallography, showing that the central four-membered ring is significantly folded and has two eta2-1,3-coordinated potassium cations accommodated above and below the ring. The "heavy" CBD2-'s 22- and 42- were found to be nonaromatic compounds, a conclusion that was supported by nucleus-independent chemical shift (NICS) calculations showing a lack of diatropic ring current effects.     

Reactions of heavier main‐group compounds with hydrogen,ammonia, ethylene and related small molecules

The first reaction between hydrogen and a main‐group compound under ambient conditions was reported in 2005. This unexpected result has been followed by numerous others which show that such reactivity is widespread in unsaturated and multiple bonded main‐group species. These may react spontaneously not only with hydrogen, but also with ethylene, ammonia and related molecules. This account focuses on results from the author's laboratory but also on parallel work by other groups. The link between HOMO‐LUMO separations, symmetry considerations and reactivity of the main‐group species is emphasized as is their similarity in reactivity to transition‐metal organometallic compounds. DOI 10.1002/tcr.201100016     

The heteronuclear bonding between heavier Group 14 elements and transition metals: a novel trioxystannate-iron complex with an unusual stannate fragment

A new trioxystannate-transition metal complex, {[Li][(Me(2)NCH(2)CH(2)O)(3)Sn-Fe(CO)(4)]}(2) (3), exhibiting an unusual type of the heteronuclear bonding between heavier Group 14 elements and transition metals, has been synthesised and completely characterised by NMR, Raman and IR spectroscopy in solution as well as by Raman spectroscopy and X-ray diffraction analysis in the solid state.     

Novel class of heterometallic cubane and boride clusters containing heavier group 16 elements

Thermolysis of an in situ generated intermediate, produced from the reaction of [Cp*MoCl(4)] (Cp* = η(5)-C(5)Me(5)) and [LiBH(4).THF], with excess Te powder yielded isomeric [(Cp*Mo)(2)B(4)TeH(5)Cl] (2 and 3), [(Cp*Mo)(2)B(4)(μ(3)-OEt)TeH(3)Cl] (4), and [(Cp*Mo)(4)B(4)H(4)(μ(4)-BH)(3)] (5). Cluster 4 is a notable example of a dimolybdaoxatelluraborane cluster where both oxygen and tellurium are contiguously bound to molybdenum and boron. Cluster 5 represents an unprecedented metal-rich metallaborane cluster with a cubane core. The dimolybdaheteroborane 2 was found to be very reactive toward metal carbonyl compounds, and as a result, mild pyrolysis of 2 with [Fe(2)(CO)(9)] yielded distorted cubane cluster [(Cp*Mo)(2)(BH)(4)(μ(3)-Te){Fe(CO)(3)}] (6) and with [Co(2)(CO)(8)] produced the bicapped pentagonal bipyramid [(Cp*MoCo)(2)B(3)H(2)(μ(3)-Te)(μ-CO){Co(3)(CO)(6)}] (7) and pentacapped trigonal prism [(Cp*MoCo)(2)B(3)H(2)(μ(3)-Te)(μ-CO)(4){Co(6)(CO)(8)}] (8). The geometry of 8 is an example of a heterometallic boride cluster in which five Co and one Mo atom define a trigonal prismatic framework. The resultant trigonal prism core is in turn capped by two boron, one Te, and one Co atom. In the pentacapped trigonal prism unit of 8, one of the boron atoms is completely encapsulated and bonded to one molybdenum, one boron, and five cobalt atoms. All the new compounds have been characterized in solution by IR, (1)H, (11)B, and (13)C NMR spectroscopy, and the structural types were unambiguously established by crystallographic analysis of 2 and 4-8.     

X-ray diffraction study of the bonding between sulphur and the elements of group IVB. The crystal and molecular structures of methylthiotriphenyl-methane, -silicon, -germanium, -tin and -lead

The X-ray analyses of the compounds methylthiotriphenyl-methane, -silicon, -germanium, -tin, and -lead were undertaken to see if the electronic interactions which affect the X-S bond could influence the molecular geometries. The five structures were solved or by direct methods or by Patterson and Fourier techniques and refined anisotropically to R = 0.053, 0.036, 0.031, 0.041, and 0.070 respectively. The central atom exhibits a distorted tetrahedral coordination; only the propeller shape orientation of the phenyl rings seems to be determined by differences in the electronic nature of the central atoms as well as by steric effects.     

The cation cluster of heavier group 14 elements: a free germyl cation with trishomoaromaticity

The heavier group 14 element cation cluster compound, [(tBu3Si)6Ge10I]+.TTFPB- (3+. TTFPB-; TTFPB- = tetrakis(2,3,5,6-tetrafluorophenyl)borate), was unexpectedly obtained by thermolysis of in situ prepared iodocyclotrigermene, (tBu3Si)3Ge3I, in toluene in the presence of KI and K+.TTFPB-. The molecular structure of 3+.TTFPB- has been unambiguously determined by X-ray crystallography, showing that 3+ is a free germyl cation. The three "naked" cationic germanium atoms in the framework constitute a three-center two-electron (3c-2e) bond with a trishomocyclotrigermenylium character, which is well-supported by a theoretical calculation on the model compound Ge10H7+.     

Localized molecular orbital study on chemical bonding and reactivities of certain Mo-Cu-S cluster compounds

The localized molecular orbitals (LMOs) and Mulliken bond orders for the cluster anions of [Et₄N]₂[Mo₂S₄(edt)₂] (1), [(CH₃CH₂)₄N][Mo₂CuS₄(PPh₃)(edt)₂] (2) and the cluster molecule [Mo₂Cu₂S₄(PPh₃)₂(edt)₂] (3) have been calculated by means of the Edmiston-Ruedenberg energy localization technique under the CNDO/2 approximation. The results of the LMOs calculations show that there are two pairs of (S₁ Mo) bonds each pair of the Mo atoms and the terminal S₁ atoms in the anion of (1). In the addition reactions of the cluster compound (1) with one or two Cu atoms these (S₁ Mo) bonds are sequentially opened and form correspondently the cluster compounds (2) and (3) with the three-centered or four-centered bonds among the copper, sulfur, and molybdenum atoms, accounting for the stronger bonding of the Cu atom to the Mo-S cluster compounds such as (1). This also gives the explanation of chemical bonding for the [2+1] and [2+2x1] synthetic reaction schemes of the compounds (2) and (3). The structural features for the cluster compounds studies are briefly discussed as well.