Apical Carbon Atom Research Articles

A topological study of the haxacoordinated carbon in the pentagonal-pyramidal benzene and hexamethylbenzene dications

The nature of the hexacoordinated carbon in the pentagonal-pyramidal benzene and hexamethylbenzene dications was investigated using the Atoms in Molecules (AIM’s analysis) and the electron localization function (ELF) analysis. The following conclusions were reached from the AIM analysis: (i) six bond critical points linked to the apical carbon atom were found; (ii) at the equilibrium geometry, no cage critical point was observed; and (iii) Bader’s charges are consistent with a cyclopentadiene cation linked to a [C-R]+ fragment. The topological analysis of the ELF shows a near ring basin between the apical carbon and the pentadiene ring.

Application of Hetero-Triphos Ligands in the Selective Ruthenium-Catalyzed Transformation of Carbon Dioxide to the Formaldehyde Oxidation State

Due to the increasing demand for formaldehyde as a building block in the chemical industry as well as its emerging potential as feedstock for biofuels in the form of dimethoxymethane and the oxymethylene ethers produced therefrom, the catalytic transformation of carbon dioxide to the formaldehyde oxidation state has become a focus of interest. In this work, we present novel ruthenium complexes with hetero-triphos ligands, which show high activity in the selective transformation of carbon dioxide to dimethoxymethane. We substituted the apical carbon atom in the backbone of the triphos ligand platform with silicon or phosphorus and optimized the reaction conditions to achieve turnover numbers as high as 685 for dimethoxymethane. The catalytic systems could also be tuned to preferably yield methyl formate with turnover numbers of up to 1370, which in turn can be converted into dimethoxymethane under moderate conditions.

Paramagnetic Resonance of Cobalt(II) Trispyrazolylmethanes and Counterion Association.

Paramagnetic resonance studies (EPR, ESEEM, ENDOR, and NMR) of a series of cobalt(II) bis-trispyrazolylmethane tetrafluoroborates are presented. The complexes studied include the parent, unsubstituted ligand (Tpm), two pyrazole-substituted derivatives (4Me and 3,5-diMe), and tris(1-pyrazolyl)ethane (Tpe), which includes a methyl group on the apical carbon atom. NMR and ENDOR establish the magnitude of 1H hyperfine couplings, while ESEEM provides information on the coordinated 14N. The data show that the pyrazole 3-position is more electron rich in the Tpm analogues, that the geometry about the apical atom influences the magnetic resonance, and that apical atom geometry appears more fixed in Tpm than in Tp. NMR and ENDOR establish that the BF4- counterion remains associated in fluid solution. In the case of the Tpm3,5Me complex, it appears to associate in solution, in the same position it occupies in the X-ray structure.

Theoretical insights into the structure of the η5-(Cp∗)C+ cation

Using both ab initio and density functional theory (DFT) calculations in combination with basis sets of triple- and quadruple-ξ quality we investigate the molecular and electronic structure of the half-sandwich η5-(Cp∗)C+ cation. The results indicate that the lowest-energy rotamer is a C5-symmetric ground state singlet while the triplet state is Cs-symmetric and ∼68kcal/mol higher in energy. A topological analysis of the theoretical electron density based on the quantum theory of atoms in molecules (QTAIM) indicates that the η5-type bonding between the apical carbon atom and Cp∗ comprises five bond paths each bearing a bond critical point of type (3,−1).

Syntheses and characterization of iron(II) and iron(III) complexes of a tripodal ligand derived from tris(2-aminoethyl)methane

The trihydrochloride salt of tris(2-aminoethyl)methane (tram·3HCl) was deprotonated in methanolic potassium hydroxide and reacted with three molar equivalents of imidazole-2-carboxaldehyde to give a new Schiff base ligand, HC(CH 2CH 2N CH-2ImH) 3. The ligand, H 3(1), was reacted in situ with iron(II)chloride tetrahydrate. Addition of excess sodium perchlorate resulted in the isolation of the dark red [FeH 3(1)](ClO 4) 2·KClO 4 . The neutral emerald green iron(III) tripodal complex, Fe(1), was prepared by the aerial oxidation of the iron (II) complex on addition of three equivalents of potassium hydroxide. The complexes are characterized by EA, IR, ESI-MS, Mössbauer, magnetic susceptibility and single crystal XRD. The spectroscopic and structural data support a low spin assignment for both the iron(II) and iron(III) complexes at 295 K. The overall conformation of the tram backbone in these complexes has the apical carbon atom, C ap, pointed away from the iron atom with an average non-bonded distance of 3.83 Å. However, C ap is distorted from tetrahedral geometry toward trigonal monopyramidal. This is indicated by a narrowing of the H–C ap–C angles, an expansion of the C–C ap–C angles and a compression along the C–H axis so that C ap approaches the plane defined by its three carbon substituents. Two unusual supramolecular features are exhibited in [FeH 3(1)](ClO 4) 2·KClO 4 . These are a polymeric [K(ClO 4) 3 2−] n anion and a bidentate hydrogen bonding donor, N imine C H–C imidazole–N imidazole H, on each arm of the tripodal ligand. Density Functional Theory (DFT) calculations using the B3LYP functional were performed on the low spin and high spin states of both complexes. B3LYP correctly predicts that the low spin state is favored in both systems and closely matches the important metrical parameters that are indicative of spin state. B3LYP shows that the C ap-out conformation of the tram backbone would be nearly identical in the low and high spin forms.

Syntheses, Reactivity and DFT Studies of Group 2 and Group 12 Metal Complexes of Tris(pyrazolyl)methanides Featuring “Free” Pyramidal Carbanions

Reactions of HC(Me2pz)3 with Grignard reagents, dialkyl magnesium compounds and dimethylzinc are reported, together with a DFT study on some of the aspects of this chemistry. Reactions of HC(Me2pz)3 with MeMgX (X=Cl or Br) gave the half-sandwich zwitterionic compounds [Mg((Me)Tpmd)X] (X=Cl (2) or Br (3); (Me)Tpmd(-)=[C(Me2pz)3](-)). Addition of HCl to 2 gave the structurally characterised half-sandwich compound [Mg{HC(Me2pz)3}Cl2(thf)] (4). The zwitterionic sandwich compound [Mg(MeTpmd)2] (5) formed in low yields in the reaction of MeMgX with HC(Me2pz)3 but was readily prepared from HC(Me2pz)3 and either MgnBu2 or MgPh2. The structurally characterised compound 5 contains two "naked" sp3-hybridised carbanions fully separated from the dicationic metal centre. Only by using MgPh2 as starting material could the half-sandwich compound [Mg(MeTpmd)Ph(thf)] (6) be isolated. The zwitterionic sandwich compound 5 reacted with HOTf (OTf(-)=[O3SCF3](-)) to form the dication [Mg{HC(Me2pz)3}2]2+ (7(2+)), which was structurally characterised. Pulsed field gradient spin-echo (PGSE) diffusion NMR spectroscopy revealed both compounds to be intact in solution. In contrast to the magnesium counterparts, HC(Me2pz)3 reacted only slowly with ZnMe2 (and not at all with ZnPh2) to form the half-sandwich zwitterion [Zn(MeTpmd)Me] (8), which contains a cationic methylzinc moiety separated from a single sp3-hybridised carbanion. Density functional calculations on the zwitterions [M(MeTpmd)Me] and [M(MeTpmd)2] (M=Mg, Zn) revealed that the HOMO in each case is a (Me)Tpmd-based carbanion lone pair. The kappa 1C isomers of [M(MeTpmd)Me] were calculated to be considerably less stable than their kappa 3N-bound counterparts, with the largest gain in energy for Mg due to the greater ease of electron transfer from metal to the (Me)Tpmd apical carbon atom on formation of the zwitterion. Moreover, the computed M-C bond dissociation enthalpies of the kappa 1C isomers of [M(MeTpmd)Me] are considerably higher than expected by simple extrapolation from the corresponding computed H-C bond dissociation enthalpy.

Synthesis and crystal structure of diorganotin(IV) complexes with pyruvic acid benzoylhydrazone and pyruvic acid salicylhydrazone

Six diorganotin esters of Schiff-base ligands formulated as [R2SnLY]2, where L1 is C6H5CON2C(CH3)CO2 with Y = CH3CH2OH, R = mClC6H4CH2 (1), oFC6H4CH2 (2), pFC6H4CH2 (3) and L2 is 2-HOC6H4CON2C(CH3)CO2 with Y = CH3OH, R = oFC6H4CH2 (4), pFC6H4CH2 (5), mClC6H4CH2 (6) have been prepared and characterized by elemental analysis, IR, 1H and 119Sn NMR spectra. The crystal structures of complexes 1 and 4 have been determined by X-ray single crystal diffraction. The structure analyses reveal that the Sn atom in both 1 and 4 is seven-coordinate in distorted pentagonal bipyramid geometries with a planar SnO4N unit and two apical aryl carbon atoms, thus forming a dimeric molecule, which sits on a crystallographic center of symmetry. Intramolecular or intradimeric hydrogen bonds contribute to the stability and compactness of the crystal structures.

A novel extended covalent tripod for assembling triple-helical nine-co-ordinated lanthanide(iii) podates: the rational design of a single conformerElectronic supplementary information (ESI) available: tables of molecular peaks obtained by ESI-MS (Table S1), elemental analyses (Table S2), structural data for the lanthanide co-ordination sphere in [Eu(L13)](ClO4)3 (19, Table S3) and paramagnetic 1H NMR shifts (Table S4). Figures showing views of [Eu(L13)]3+ along

The connection of a methyl group to the apical carbon atom of a semi-rigid extended tripod provides the novel podand 1,1,1-tris-{2-[2-(6-diethylcarbamoyl-pyridin-2-yl)-1-ethyl-1H-benzoimidazol-5-ylmethoxy]-ethyl}-ethane (L13). Reaction with lanthanide(III) in acetonitrile produces stable and dynamically inert C3-symmetrical podates [Ln(L13)]3+ (Ln = La–Lu) in which LnIII is nine-co-ordinate in a pseudo-tricapped trigonal prismatic site. Electron-induced nuclear relaxation in the paramagnetic complex [Nd(L13)]3+ shows that the capping carbon atom adopts the exo conformation with the methyl group pointing outside the cavity of the podand. The crystal structure of [Eu(L13)](ClO4)3 confirms the exclusive formation of the exo conformer, which contrasts with the mixture of two endo conformers reported for the non-methylated analogue. Structural analysis combined with photophysical data in [Ln(L13)](ClO4)3 (Ln = La, Eu, Gd, Tb) point to negligible mechanical coupling mediated by the flexible four-atoms aliphatic chains between the apical carbon atom and the unsymmetrical tridentate binding units. However, the exo conformation of the tripod in [Eu(L13)]3+ removes some interactions between the high-frequency CH oscillators and the metal-centred excited electronic levels, thus leading to larger lifetime and improved quantum yield in solution.

Hemispiroalkaplanes: hydrocarbon cage systems with a pyramidal-tetracoordinate carbon atom and remarkable basicity

A new class of saturated hydrocarbons, in which a spiropentane-type unit is bound by a cyclic hydrocarbon, has been investigated by using ab initio molecular orbital calculations at the B3-LYP and MP2 levels. These molecules have been given the trivial name hemispiroalkaplanes. Hemialkaplanes, which are analogous molecules built-up from a neopentane-type unit and a cyclic hydrocarbon, have also been examined. The hemispiroalkaplanes are predicted to contain a pyramidal-tetracoordinate carbon atom that possesses a lone pair of electrons. Protonation at this apical carbon atom is found to be highly favourable, resulting in a remarkably high basicity for a saturated hydrocarbon. The proton affinities of the hemispiroalkaplanes are calculated to be more than 1170 kJmol(-1), even greater than that of the diamine "proton sponges". Structural parameters, heats of formation and strain energies for the novel hydrocarbons are detailed.

Absolute proton affinities of biphenyl and its derivatives

The spatial structure of biphenyl 1 is studied by the semiempirical AM1 and ab initio HF/6-31G* and MP2(fc)/6-31G* theoretical models. The resulting bond distances are in good agreement with the X-ray structure. The calculated dihedral angle is in accordance with the value observed by the electron diffraction technique (ϕ = 45°). Its large value is a compromise between the steric hindrance effect and the π-electron conjugation. The estimated barrier heights for the internal rotation are very low however. Theoretical values are in accordance with the available experimental evidence. The calculated proton affinity (PA) obtained by the scaled (AM1)sc model and by using the MP2 level of theory compares very well with the experimental value. It is some 13 kcal mol–1 higher than the reference PA value of benzene, because of the strong resonance interaction between the two phenyl rings. The increase in the π-electron conjugation energy triggered by protonation overcomes the steric repulsion between H atoms thus decreasing the twist angle by some 20°. The apical carbon atom, placed para to the coannular CC bond, is most susceptible to the proton attack. On the other hand, the PA values for ipso and meta carbons are substantially lower since an amplification of the inter-ring conjugation interaction is then precluded. The PA increments for the CH3 group and F atom monosubstituted biphenyls are determined by using the (AM1)sc approach. They are employed in estimating proton affinities of a number of polysubstituted biphenyls applying a very simple additivity formula based on the independent substituent approximation (ISA). It is shown that the performance of the additivity rule is very good. Variation in the PA of substituted biphenyls is rationalized in terms of the conjugation effect and repulsion between hydrogen atoms or substituents attached to the face-to-face ortho positions of the neighbouring rings (steric effect).Finally, the proton affinity of fluorene possessing a “frozen” planar biphenyl moiety is calculated and compared with that of the paradigmatic Mills–Nixon (MN) system—indan. It is found that PA values of the former compound are determined by the MN and resonance effects, the latter being predominant. The most basic site in fluorene is the C(4) atom where both effects act in a synergistic way.

A novel iron sequestering agent: synthesis and iron-chelating properties of 1,1,1,-tris(3-hydroxy-2-oxo-1,2-dihydro-1-pyridylpropoxymethyl) ethane

A new hexadentate chelating agent with three 3-hydroxy-2-(1H)-pyridinone moieties combined through flexible aliphatic chains emanating from an apical carbon atom was found to have very high affinity for Fe(III). The synthesis of the ligand is described: the acid dissociation constants and the stability constants of its iron (III) complex were determined potentiometrically and through spectrophotometric competition with EDTA. The flexibility of the ligand and of its Fe(III) complex were confirmed by molecular mechanics.

New square-pyramidal organoantimony(V) compounds; crystal structures of (biphenyl-2,2′-diyl)phenylantimony(V) dibromide, dichloride and diisothiocyanate, Sb(2,2′-C12H8)PhX2 (X = Br, Cl or NCS), and of octahedral SbPh(o-O2C6Cl4)Cl2 ·OEt2

Oxidative addition between (biphenyl-2,2′-diyl)phenylantimony(III), Sb(2,2′-C 12 H 8 )Ph 1 and Br 2 or SO 2 Cl 2 gave Sb(2,2′-C 12 H 8 )PhBr 2 2 and Sb(2,2′-C 12 H 8 )PhCl 2 3, respectively, while the corresponding fluoride Sb(2,2′-C 12 H 8 )PhF 2 4 and the thiocyanate Sb(2,2′-C 12 H 8 )Ph(NCS) 2 5 were obtained by metathesis reactions between 3 and KF and KSCN, respectively. Compounds 2 and 3 are isostructural but, in contrast to the closely related SbPh 3 X 2 species, individual molecules have square-pyramidal geometry. Again in contrast to SbPh 3 X 2 compounds, secondary antimony–halogen interactions trans to the apical carbon atom lead to solid-state dimers, implying Lewis acidity at antimony. Antimony in the thiocyanate 5 showed similar square-pyramidal geometry with N-bonded thiocyanate groups but bridging by one thiocyanate again gives dimers in the solid. Oxidative addition between SbPhCl 2 and tetrachloro-ortho-benzoquinone in ether solution gave the tetrachlorocatechol analogue of 3 as a six-co-ordinate ether solvate, SbPh(o-O 2 C 6 Cl 4 )·OEt 2 6. If the weak bond to ether is ignored, antimony again has square-pyramidal geometry but formation of the adduct again points to antimony Lewis acidity. A non-solvated substituted catecholate, SbPh(o-O 2 C 6 H 2 Bu t 2 -3,5)Cl 2 8, was also synthesized but crystals suitable for X-ray diffraction could not be obtained.

Optimised preparation of 1,5‐dialkylsemibullvalenes

AbstractAskani's synthesis of the 1,5‐dialkylsemibullvalenes 1a and c was optimised on a gram scale. The as yet unknown trimethylenesemibullvalene 1b was obtained according to the same protocol which also greatly improved a recent synthesis of barbaralane (2). Allylic brominations of the dienes 5 + 6 with N‐bromosuccinimide yielded complex mixtures of 7–10. Depending on the substitution pattern at the apical carbon atoms C‐1 and C‐5 of the bicyclo[3.3.0]octadiene system, the intermediate allylic radicals are attacked preferentially either from the exo (R = Me) or from the endo face [R, R = (CH2)3, (CH2)4]. Reductive cyclisations of the dibromides 9 and exo,exo‐15 were carried out with a sonicated suspension of an activated zinc‐copper couple in tetraglyme. Sweeping of the volatile products from the reaction mixtures with nitrogen at elevated temperatures allowed the convenient isolation of the semibullvalenes 1 and barbaralane (2) in reasonable purities (92–97%) and yields (57–82%).

Reactions of the alkylidyne cluster [Os 3(μ-H) 2(CO) 9( μ3-CNC 5H 4CHCH 2)]. Crystal structures of [Os 3(μ-H) 2(CO) 9( μ3-CNC 5H 4CHCH 2)], [Os 3(μ-H) 2(CO) 9( μ3-CNC 5H 4- μ- η2-CHCH)Os 3(μ-H)(CO) 10] and [Os 3(μ-H) 2(CO) 9( μ- 3-CNC 5H 4-C(O)CH 3)

The neutral triosmium alkylidyne cluster [Os 3(μ-H) 2(CO) 9( μ 3-CNC 5H 4CHCH 2)] ( 1) has been made in good yield by the reaction of [Os 3(μ-H) 3(CO) 9( μ 3-CCl)] with one equivalent of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in the presence of an excess of 4-vinylpyridine. The reaction of 1 with [Os 3(CO) 10(NCMe) 2] in refluxing n-hexane affords the linked cluster [Os 3(μ-H) 2(CO) 9( μ 3-CNC 5H 4- μ- η 2-CHCH Os 3( μ-H)(CO) 10] ( 2) in in moderate yield, while the Wacker-type reaction of 1 with a trace amount of water in the presence of Wilkinson's catalyst RhCl(PPh 3) 3 in refluxing dichloromethane yields the 4-acetylpyridine derivative of the corresponding alkylidyne cluster [Os 3(μ-H) 2(CO) 9( μ 3-CNC 5H 4C(O)CH 3)] ( 3). The structures of compounds 1–3 have been determined by single crystal X-ray diffraction studies. The structure of 1 involves a triosmium alkylidyne metal core with the 4-vinylpyridine moiety bonded to the μ 3-bridging alkylidyne carbon atom. In the case of 2 the triosmium alkylidyne metal core is linked to another triosmium cluster unit via a 4-vinylpyridine moiety, while 3 contains an alkylidyne metal core with the 4-acetylpyridine ligand attached to the apical carbon atom. The M.L.C.T. transitions of all three complexes show strong solvent dependency, displaying unusually large negative solvatochromism in a wide range of organic solvents. This suggests that a less polar excited state is produced by photoexcitation.

Triosmium alkylidyne clusters containing monophosphine and chiral diphosphine ligands. Crystal and molecular structures of [Os3(µ-H)2(CO)9(µ3-CPBunPh2)] and (R)-[Os3(µ-H)2(CO)8{µ3-CPPh2CH(Me)CH2PPh2}

The cluster [Os3(µ-H)3(CO)9(µ3-CCl)] reacted with excess of butyldiphenylphosphine, in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu), to give the complex [Os3(µ-H)2(CO)9(µ3-CPBunPh2)]1. The optically active diphosphine (R)-Ph2PCH(Me)CH2PPh2 reacted similarly to give two isomers, (R)-[Os3(µ-H)2(CO)9{µ3-CPPh2CH(Me)CH2PPh2}]2a and (R)-[Os3(µ-H)2(CO)9{µ3-CPPh2CH2CH(Me)PPh2}]2b, which have been spectroscopically characterised. Complex 2a readily undergoes decarbonylation, followed by the formation of Os–P bond, to afford an octacarbonyl chiral cluster complex (R)-[Os3(µ-H2)(CO)8{µ3-CPPh2CH(Me)CH2PPh2}]3. The molecular structures of 1 and 3 have been established by X-ray crystallography. Complex 1 contains a triosmium alkylidyne metal core with the BunPh2P moiety bonded to the apical carbon atom through the phosphorus atom. On the other hand, the structure of 3, which also contains a phosphine-substituted µ3-C ligand bonded to three osmium atoms at the base, is characterised by the formation of a six-membered osmacycle with the centre of chirality being retained in the phosphine ligand of the complex.

Hydroformylation of 1-octene in the presence of cobalt alkylidynecarbonyl clusters

The hydroformylation of 1-octene in the presence of Co3(CO)9(μ-CR) (R=H, Me, Ph, CO2Me, CO2Et, CO2Pri, CO2But, Cl, Br, OMe) alkylidynecarbonyl clusters, as well as triphenylphosphine derivatives of these complexes and heteronuclear Co2Ni compounds have been studied. The nature of the catalytically active species in hydroformylation, as well as the processes of their formation and transformation during the reaction, have been established by means of IR spectroscopy. The effects of the reaction conditions, the nature of the substituent at the apical carbon atom, the electron donating phosphine substituent, and the substituent in the metal cluster framework have been discussed.

Fe(CO) 4XXX radical anion: theoretical study of the electronic structure and magnetic properties

The g tensor of the short-lived radical Fe(CO) 4 XXX was measured in frozen 2MeTHF glasses ( g ⊥ = 2.0707, g ∥ = 2.0039, g av = 2.0484, g iso = 2.0486). A spin unrestricted SCF-X α-SW study of the radical in a C 3v structure supports the experimental findings, showing a singly occupied molecular orbital (SOMO) of symmetry a 1 largely delocalized on the equatorial CO groups, confirms a net positive Δ g ⊥. for the tensor and shows a spin density located mainly on the apical carbon atom.

First example of polytopal dominance of square pyramidal geometry in organotin pentacoordination: crystal structure of tribenzyl(2-thiolatopyridine-N-oxide)tin(IV)

The coordination geometry about the central atom in monomeric tribenzyl(2-thiolatopyridine-N-oxide)tin(IV) is a unique square pyramid, with one benzyl carbon atom occupying he apical position and the basal plane containing the other two benzyl carbon atoms along with the oxygen and sulfur atom of the chelating ligand; the tin atom is displaced out of the basal plane in the direction of the apical carbon atom by 0.64(1) Å.

Reaction of hydroxydiamantanes with chloroethylenes in sulfuric acid

Reaction of hydroxydiamantanes with 1,1-dichloroethylene in concentrated sulfuric acid leads to complex reaction mixtures, containing predominantly diamantylacetic and diamantanebisacetic acids. The same reaction with trichloroethylene affords diamantylchloroacetic and diamantanebischloroacetic acids. In all these products the substituents are bonded to the apical and secondary carbon atoms of the diamantane skeleton. For steric reason, acids with carboxyl in the medial position are not formed.

Heteronucleare clustersysteme : XIX. Structuruntersuchungen an tetraedrischen carbonylcobalt-clustern; ein beitrag zur frage der bildung von carbonylbrücken

The tetrahedral carbonylcobalt clusters (CO) 9Co 3CC 6H 2(CH 3) 3(II), (CO) 8P(CH) 3) 3Co 3Co(CO) 3 (III) and (CO) 7[P(CH 3) 2C 6H 5] 2Co 3Fe(H)(CO) 2P(CH 3) 2C 6H 5 (IV) were synthesized, and investigated by single-crystal X-ray diffraction methods together with the known compound (CO) 9Co 3CCl (I). The results of the structure determinations show that the mesityl-substituted methylidyne-cluster II can be arranged structurally between the non-bridged cluster I and the two CO-bridged clusters III and IV. The o-CH 3 substituents of the mesityl group act as pseudo ligands of the apical carbon atom and push the neighbouring CO ligands into the Co 3-plane: one condition for the formation of bridges as they exist in the compounds III and IV.