Ipso Bond Research Articles

Polymerisation of polar olefins promoted by organometallic cobalt complexes: free radical or coordinative process?

Organometallic cobalt(III) complexes, namely [Co(R) 2(NN) 2][X] (NN=2,2′-bipyridine, 1,10-phenanthroline and 4,7-dimethyl-1,10-phenanthroline, R=methyl, benzyl; X −=BF 4 − , PF 6 − , SbF 6 − ) have been obtained in good yield as stable orange crystalline compounds, by reduction of CoCl 2 with NaBH 4 in methanol in the presence of NN and the appropriate organic halide. The crystal structure of the complexes [Co(CH 3) 2(bipy) 2][PF 6] and [Co(CH 2Ph) 2(bipy) 2][PF 6] shows an octahedral coordination geometry for cobalt with a cis disposition of the two chelating ligands and the organic groups σ-bonded to the cobalt atom. In particular, the benzyl derivative shows stacking interactions between the phenyl ring of the benzyl groups and the planar bidentate ligands. NMR investigation confirms the cis coordination geometry in solution. Moreover, in the benzyl derivatives a restricted rotation of the phenyl group around the CC ipso bond is observed. These complexes efficiently promote the polymerisation of polar olefins, such as acrylonitrile, in the dark and with no addition of radical traps. Molecular weight and polydispersities determination suggest a catalytic controlled radical mechanism.

Carbon–silicon hyperconjugation: X-ray structural study of N-methyl-4-trimethylsilylmethylpyridinium triflate

Hyperconjugation between the CH 2Si bond and the charged pyridinium π system in the N-methyl 4-trimethylsilylmethyl pyridinium ion ( 8), manifests in the crystal structure as lengthening of the CH 2Si bond, shortening of the CH 2C ipso bond and smaller but systematic effects on the CC bond distances within the pyridinium ring. The structural effects are supported in solution by systematic effects on the 29Si– 13C and 13C– 13C one-bond coupling constants.

1,4-Diaza-1,3-diene (DAD) complexes of early transition elements. Syntheses, structures and molecular dynamics of mono- and bis(η 5-cyclopentadienyl)titanium-, zirconium- and hafnium(DAD) complexes. Crystal- and molecular structures of CpTi(DAD)CH 2Ph, [CpTi(DAD)] 2O, CpZr[(DAD)(N ∩O)] and Cp 2Hf(DAD)

Treatment of CpTiCl 3 and CpZrCl 3(THF) 2 with one equivalent magnesium in the presence of 1,4-diaza-1,3-dienes (R 1NCR 2CR 2NR 1 (R 1,R 2-DAD; R 1=C 6H 4-2-Me, C 6H 4-4-Me, C 6H 4-4-OMe, R 2=H, Me, Ph) yields the monomeric titanium complexes CpTi(R 1,R 2-DAD)Cl ( 2, R 1=C 6H 4-4-OMe, R 2=H; 3, R 1=C 6H 4-2-Me, R 2=Me; 4, R 1=C 6H 4-4-OMe, R 2=Me; 5, R 1=C 6H 4-4-Me, R 2=Ph), and the chloro bridged dimeric zirconium complexes [CpZr(R 1,R 2-DAD)Cl] 2 ( 6, R 1=C 6H 4-4-OMe, R 2=H; 7, R 1=C 6H 4-4-OMe, R 2=Me; 8, R 1=C 6H 4-4-Me, R 2=Ph). Both the half-sandwich complexes of DAD ligands bearing alkyl ( 3, 4 and 7) and aryl ( 5, 8) substituents at the inner carbon atoms and the complexes without substituents at this DAD positions ( 2, 6) prefer the σ 2,π-coordination geometry with a supine conformation of the heterodiene. Alkylation of the new half-sandwich DAD complexes with one equivalent of PhCH 2MgCl or one equivalent of MeMgI affords the benzyl and methyl derivatives CpM(R 1,R 2-DAD)CH 2Ph ( 9, M=Ti, R 1=C 6H 4-4-OMe, R 2=Me; 10, M=Zr, R 1=C 6H 4-4-OMe, R 2=Me) and CpZr(R 1,R 2-DAD)Me ( 11, R 1=C 6H 4-4-Me, R 2=Ph). An X-ray study of the benzyl derivative 9 reveals that the alkylation does not change appreciably the DAD bonding parameters in comparison with the starting chloride complex 4. The monomeric half-sandwich zirconium complex CpZr(R 1,R 2-DAD)(N ∩O) ( 12, R 1=C 6H 4-4-OMe, R 2=H) which has been prepared by reaction of 6 with the chelating acetylacetoneiminate compound Na[(C 6H 4-4-Me)NC(Me)-CHC(Me)O] (Na[N ∩O]) as well as the oxygen bridged complex [CpTi(R 1,R 2-DAD)] 2O ( 13, R 1=C 6H 4-2-Me, R 2=Me) which has been formed by hydrolysis of 3 also keep the supine conformation of the heterodiene ligand with respect to the Cp group. Temperature dependent NMR spectra of a series of different titanocene DAD complexes Cp 2Ti(R 1,R 2-DAD) (R 1=Ph, C 6H 4-2-Me, C 6H 4-4-Me, C 6H 4-4-OMe, 1-C 10H 7, R 2=Me; 14– 18) have been used to estimate the energy barrier of the thermal induced inversion of the folded diazametallacyclopentene rings and to identify rotameric isomers derived from restricted rotation of the C 6H 4-2-Me and the 1-C 10H 7 group about the NC ipso bond of the DAD ligand. Accordingly, complexes 16 and 18 and also the half-sandwich complexes 3 and 13 adopt mixtures of meso and rac rotamers. Finally, the crystal structure of Cp 2Hf(R 1,R 2-DAD) ( 21, R 1=R 2=Ph) is reported.

Ortho-Lithiated benzyl diorganophosphines [ o-(R 2PCH 2)C 6H 4Li(Et 2O)], R=Ph, Me. Synthesis, structural characterization, and reactions

( o-Lithiobenzyl)dimethylphosphine, [ o-(Me 2PCH 2)C 6H 4Li(Et 2O)] 2 ( 7), is shown by crystal structure analysis to be dimeric, the two lithium atoms bridging the phenyl ortho-carbon atoms in a four-membered ring structure. A distorted tetrahedral coordination sphere at lithium is completed by coordination of one phosphino group and a molecule of diethylether (triclinic, space group P1̄, a=10.986(3), b=11.160(3), c=12.368(4) Å, α=86.04(2), β=89.51(2), γ=83.04(2)°, Z=2). [ o-(Me 2PCH 2)C 6H 4Li(Et 2O)] 2 ( 7) cleanly reacts in donor solvents like diethylether to the thermodynamically more stable (α-lithiobenzyl)dimethylphosphine [Me 2PCHLi(Et 2O)C 6H 5] 2 ( 9) with the same overall composition. Crystal structure analysis of the latter shows each lithium atom to be now bonded in an η 2 fashion to the C benzylC ipso bond of one anion, the diethylether molecule, and to the phosphino group of the second anion thereby resulting in a dimeric structure with a central six-membered ring (triclinic, space group P1̄, a=8.482(3), b=9.388(3), c=9.482(3) Å, α=95.42(2), β=91.36(2), γ=104.20(2)°, Z=1). ( o-Lithiobenzyl)diphenylphosphine undergoes the same reaction to the corresponding α-lithiobenzylphosphine upon addition of strong donors like N, N, N′, N′-tetramethylethylenediamine (tmeda). Upon addition of Cp 2TiCl 2, ( o-lithiobenzyl)diphenylphosphine reacts to form 1,2-bis[ o-(diphenylphosphino)phenyl]ethane, [ o-(Ph 2P)C 6H 4CH 2] 2 ( 10), the formation of which is thought to proceed by oxidation of the anion [ o-(Ph 2PCH 2)C 6H 4] − by Cp 2TiCl 2 to the radical which undergoes a 1,3 shift of the diphenylphosphino group from the sp 3 benzyl carbon atom to the sp 2 ortho-carbon atom of the phenyl ring to form the more stable benzyl radical. Dimerization of the latter ultimately leads to [ o-(Ph 2P)C 6H 4CH 2] 2 ( 10) whose molecular structure was determined in the solid state (monoclinic, space group P2 1/ n, a=12.217(2), b=8.382(1), c=15.004(2) Å, β=90.81(1)°, Z=2). The same reaction occurs upon oxidation of ( o-lithiobenzyl)diphenylphosphine with ferricinium tetrafluoroborate, [Cp 2Fe] +BF 4 −, thus corroborating the proposed reaction sequence with the transition metal species acting solely as oxidizing agents.

Molecular and electronic structure of some silacyclopropabenzenes: the reversed Mills–Nixon effect

Molecular and electronic structure of silacyclopropabenzenes are examined by the MP3(fc)/6-31G* theoretical model. The salient feature of these compounds is alternation of bond distances of the aromatic fragment in the reversed Mills–Nixon sense i.e. the annelated CC bonds are shortened, whilst the ortho bonds are stretched. This is in harmony with the π-electron partial bond localization as evidenced by the π-bond order analysis, which in turn indicates that the π-effect dominates over the σ-electron rehybridization effect. Concentration of the π-density in the ipso bond(s) seems to be triggered by hyperconjugation with SiH 2 group(s). The latter mode of interaction is responsible for a decrease in the strain energy of silacyclopropabenzenes relative to the corresponding cyclopropabenzenes. It is shown that the total strain energy is a simple additive function of the number and types of the fused small rings involving SiH 2 group(s).

Stereoselective pinacol coupling of planar chiral (benzaldehyde)Cr(CO) 3, (benzaldimine)Cr(CO) 3, ferrocenecarboxaldehyde and (dienal)Fe(CO) 3 complexes with samarium diiodide

An intermolecular pinacol coupling of the planar chiral tricarbonylchromium complexes of o-substituted benzaldehydes or benzaldimines with samarium(II) diiodide in THF produces exclusively threo 1,2-diols or 1,2-diamines in an optically pure form, while the corresponding racemic o-substituted benzaldehyde or benzaldimine chromium complexes give a mixture of threo and erythro pinacol coupling products in a various ratio depending upon the nature of o-substituent. Similarly, planar chiral 2-substituted ferrocenecarboxaldehydes and (dienal)Fe(CO) 3 produce the corresponding 1,2-diols with high stereoselectivity. The generated transition metal-complexed ketyl radical intermediates are configurationally stable with restriction to a rotation about C αC ipso bond.

Ortho-substituted aryl diamido complexes of zirconium: observation of rotameric isomers

Zirconium complexes bearing a series of ortho-substituted aryl diamido ligands [2,6-(RNCH 2) 2NC 5H 3] 2− (R = 2-PhC 6H 4 (BPhP); 2- i PrC 6H 4 (BMPP); 2- i BuC 6H 4 (BMBP), 2- i Pr-6-MeC 6H 3 (MPPP)) have been synthesized. The mixed amide complexes [2,6-(RNCH 2) 2NH 5H 3]Zr(NMe 2) 2 are prepared in high yield from 2,6-(RHNCH 2) 2NC 5H 3 and Zr(NMe 2) 4. The mixed amides react with excess ClSiMe 3 to afford the dichlorides [2,6-(RNCH 2) 2NC 5H 3]ZrCl 2 in nearly quantitative yield. Dimethyl complexes are prepared from [2,6-(RNCH 2) 2NC 5H 3]ZrCl 2 and 2 equiv. of MeMgCl. NMR spectroscopy has been used to identify rotameric isomers derived from restricted rotation about the NC ipso bond of the ligand. The aryl groups in (BPhP)ZrX 2 complexes freely rotate at all temperatures (−80 to +80°C) while (BMPP)ZrX 2 (X  NMe 2, Cl, Me) derivatives adopt meso and rac rotamers at low temperatures. In contrast, (BMBP)ZrX 2 and (MPPP)ZrX 2 (X  NMe 2, Cl, Me) compounds are locked at all temperatures. (BMBP)ZrCl 2 is isolated as a single isomer, likely the meso rotamer, while (MPPP)ZrCl 2 is a near statistical mixture of meso and rac isomers.

Molecular and electronic structure of 1,2-disilacyclobutabenzenes. Ab initio molecular orbital and density functional study

The geometric and electronic structure of 1,2-disilacyclobutabenzene and its polyannelated analogues is examined by the MP2(fc)/6-31G * and B3LYP/6-31G * methods. It is found that the bond distances alternate in the benzene fragment, the ipso bond being the longest. This finding is rationalised by the rehybridization at the carbon junction atoms. The average C C distances are larger than in the free benzene, reflecting an interesting blow-up phenomenon. The latter is interpreted by the π-electron charge transfer interaction between the highest occupied MOs of the benzene ring and the φ * (SiH 2) antibonding fragment orbitals of the SiH 2 groups. The destabilisation energy of these systems was examined by the homodesmic reactions. It is shown that the total strain energy is additive to a good approximation, depending on the number of annelated four-membered rings.

Energetics and Site Specificity of the Homolytic C−H Bond Cleavage in Benzenoid Hydrocarbons: An ab Initio Electronic Structure Study

Electronic structure calculations carried out at the BLYP/6-311G** level of theory accurately predict the dissociation energy of the C−H bond in benzene. The analogous energies of the homolytic C−H bond cleavage in the other nine polycyclic aromatic hydrocarbons (PAHs) are found to be governed almost entirely by steric factors, the hydrogens from congested regions of the PAHs being removed preferentially. The removal of hydrogens is accompanied by highly regular changes in the molecular geometries, namely a widening of the ipso bond angle by ca. 6.0° and a concomitant shortening of the adjacent C−C bonds by ca. 0.02 Å. These observations suggest an almost complete localization of the unpaired σ electrons on single carbon atoms and the separation of the local σ and π effects in the aryl radicals under study. This localization is confirmed by the computed charges and spin populations of atoms in the phenyl, 1-naphthalenyl, and 2-naphthalenyl radicals. In contrast with their UHF counterparts, the UBLYP electronic wave functions are only mildly spin contaminated.

2-η3-Allyl-1,1,1-tricarbonyl-μ-(η1:η6-diphenylmethyl)chromiumpalladium(Pd–Cr) at 130 K

The introduction of the phenyl substituent at the benzyl C atom of the arene, π-coordinated by the Cr atom, does not cause any significant changes in the geometry of the binuclear complex which contains a direct Pd-Cr bond [average length 2.768(1) A for the two independent molecules]. The Pd-C benzyl , Pd-C ipso and C benzyl -C ipso bond lengths [averages 2. 138(3), 2.547(2) and 1.452(3) A, respectively] and other geometric features indicate that the title compound may be treated as a superposition of two canonical structures, one of which implies π coordination of the Pd atom by the exocylic C benzyl -C ipso bond whereas the second involves only Pd-C benzyl σ bonding

Molecular geometry and ring deformation of trimethylsilylbenzenes: Gas-phase electron-diffraction study of 1,3-bis(trimethylsilyl)benzene and 1,3,5-tris(trimethylsilyl)benzene

The molecular geometries of 1,3-C 6H 4(SiMe 3) 2 ( 2) and 1,3,5,-C 6H 3(SiMe 3) 3 ( 3) have been determined by gas-phase electron diffraction. The mean CC bond lengths are slightly larger than those in benzene, and the endocyclic ipso bond angles C6C1C2 are considerably less than 120°, in accord with the electron-releasing inductive effect of the SiMe 3 substituent and the predictions of the VSEPR model. The SiC (methyl) bonds are longer than the SiC(aryl) bonds by about 0.02 Å (see below). Only limited information concerning conformational preference could be obtained from the electron-diffraction data but this did not hinder the determination of the other structural characteristics. The following main bond length ( r g) and angle parameters (with estimated total errors) have been obtained at nozzle temperatures of 86 and 105°C, respectively. For 2: (CC) mean 1.405±0.003, (SiC) mean 1.87 9± 0.004, Δ(SiC)  SiC(methyl)  SiC(aryl) = 0.018 ± 0.008, SiC(methyl) 1.884 ± 0.004, SiC(aryl) 1.866 ± 0.007 Å C6C1C2 116.2±0.6°, C4C5C6 119.5° (assumed), CSiC 109.5° (assumed); for 3: CC 1.410±0.003, (SiC) mean 1.881 ± 0.004, Δ(SiC) 0.016±0.006, SiC(methyl) 1.885±0.004, SiC(aryl) 1.869±0.006 Å, C6C1C2 117.0±0.6°, CSiC 109.5° (assumed). On the basis of available structural information on three trimethylsilyl-substituted benzene derivatives, and assuming additivity, the angular ring deformation impact of the trimethylsilyl substituent has been estimated. The predicted endocyclic angles for trimethylsilylbenzene are (starting from the ipso angle) 116.8, 122.2, 119.9, and 119.0°.