Silylanions: Inversion Barriers and NMR Chemical Shifts

Flock, Michaela; Marschner, Christoph

doi:10.1002/1521-3765(20020301)8:5<1024::aid-chem1024>3.0.co;2-u

Cited by 30 publications

(16 citation statements)

References 37 publications

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“…The molecular structure and numbering of 6 with 30% probability thermal ellipsoids; all hydrogens have been removed for clarity; selected bond lengths [Å ] and bond angles [8] with estimated standard deviations: K(1)Ã/Si(1) 3.4404 (13); K(2)Ã/Si (4) 3.4717 (14); Si(4)Ã/Si (8) 2.3364(11); Si(4)Ã/Si (5) 2.3407 (11); Si(4)Ã/Si (3) 2.3496 (18); Si(6)Ã/Si(1) 2.3438 (18); Si(6)Ã/Si (5) 2.3484 (11); Si(1)Ã/Si (7) 2.3402(11); Si(1)Ã/Si (2) 2.3535 (11); Si(2)Ã/Si (3) 2.3554 (11); Si(8)Ã/Si(4)Ã/Si (5) 102.36(4); Si(8)Ã/Si(4)Ã/Si (3) 102.06(4); Si(5)Ã/Si(4)Ã/Si (3) 101.59(4); Si(8)Ã/Si(4)Ã/K (2) 114.20(4); Si(5)Ã/Si(4)Ã/K(2) 100.93(4); Si(3)Ã/Si(4)Ã/ K(2) 131.46(4); Si(1)Ã/Si(6)Ã/Si (5) 111.07(4); Si(7)Ã/Si(1)Ã/Si (6) 102.38(4); Si(7)Ã/Si(1)Ã/Si (2) 102.02(4); Si(6)Ã/Si(1)Ã/Si (2) 100.73(4); Si(6)Ã/Si(1)Ã/K(1) 115.41(4); Si(2)Ã/Si(1)Ã/K(1) 117.13(4); Si(1)Ã/Si(2)Ã/ Si (3) 112.67(4); Si(4)Ã/Si(3)Ã/Si (2) 109.46(4); Si(4)Ã/Si(5)Ã/Si (6) 110.92(4). substituted silylanions [9] facilitates the formation of 7 which can be considered to be an oligosilyldianion isomeric to 6 with inverted configuration at position 1.…”

Section: Resultsmentioning

confidence: 94%

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Route Si6 revisited

Fischer

Konopa

Ully

et al. 2003

Journal of Organometallic Chemistry

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129

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Section: Resultsmentioning

confidence: 94%

“…The alkylation reaction gives the trans -and cis isomers (9) in a ratio of 3:1. It can be assumed that after the first methylation step the second one is not as fast as the protonation reaction.…”

Section: Resultsmentioning

confidence: 99%

Route Si6 revisited

Fischer

Konopa

Ully

et al. 2003

Journal of Organometallic Chemistry

Self Cite

129

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“…Previous studies on organosilicon compounds show that DFT calculations at the B3LYP level result in structures and relative energies very similar to those obtained at the MP2 level of theory demanding less time and computer resources 22. 23 We used the hybrid functional B3LYP together with 6‐31+G(d) basis sets also for characterizing the stationary points calculating the vibrational frequencies. Minima only have real frequencies while transition structures possess exactly one imaginary frequency.…”

Section: Methodsmentioning

confidence: 96%

Chair, Boat and Twist Conformation of Dodecamethylcyclohexasilane and Undecamethylcyclohexasilane: A Combined DFT and Raman Spectroscopic Study

et al. 2006

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The conformations of dodecamethylcyclohexasilane Si6Me12 and undecamethylcyclohexasilane Si6Me11H have been investigated by ab initio calculations employing the B3LYP density functional with a 6-31+G(d) basis set. Local minima as well as transition structures were calculated with imposed symmetry constraints. For Si6Me12, three unique minima, which correspond to the chair, twist and boat conformations were located with relative zero-point-vibration-corrected energies of 0.0, 7.8 and 11.4 kJ mol(-1). A half-chair conformation with four coplanar silicon atoms connects the chair and twisted minima via an energy barrier of 16.0 and 8.2 kJ mol(-1), respectively. A second transition structure with a barrier of 3.9/0.3 kJ mol(-1) connects the twist with the boat structure. Solution Raman spectra of Si6(CH3)12 and Si6(CD3)12 fully corroborate these results. Below -40 degrees C, the symmetric SiSi ring breathing vibration is a single line, which develops a shoulder (originating from the twist conformer) at longer wavelengths whose intensity increases with increasing temperature. From a Van't Hoff plot, the chair/twist enthalpy difference is 6.6+/-1.5 kJ mol(-1) for Si6(CH3)12 and 6.0+/-1.5 kJ mol(-1) for Si6(CD3)12, which is in reasonable agreement with the ab initio results. Due to the low barrier, the boat conformation cannot be observed, because either the lowest torsional vibration level lies above it or a rapid interconversion between the twist and boat conformations occurs, resulting in averaged Raman spectra. For Si6Me11H, six local minima were located. The chair with the hydrogen atom in the axial position (axial chair) is the global minimum, followed by the equatorial chair (+1.9 kJ mol(-1)) and the three twist conformers (+5.3, +8.0 and +8.1 kJ mol(-1)). The highest local minimum (+11.9 kJ mol(-1)) is a C(s) symmetric boat with the hydrogen atom in the equatorial position. Two possible pathways for the chair-to-chair interconversion with barriers of 13.9 and 14.5 kJ mol(-1) have been investigated. The solution Raman spectra in the SiSi ring breathing region clearly show that below -50 degrees C only the axial and equatorial chairs are present, with an experimental deltaH-value of 0.46 kJ mol(-1). With increasing temperature a shoulder develops which is attributed to the combined twist conformers. The experimental deltaH-value is 6.9 kJ mol(-1), in good agreement with the ab initio results. Due to the low interconversion barriers, the various twist conformers cannot be detected separately.

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“…Several mono-and dianionic oligosilylpotassium salts were prepared by this method: 97 (Me 3 Si) 3 SiK, 97a (Me 3 Si) 3 Si-Si(SiMe 3 ) 2 K(C 7 H 8 ), 97a Given the general tendency of the heavy group 14 anions to adopt pyramidal geometry, it should be recognized that the diagnostic planarity of 8a,b − • Li + has both steric and electronic origins. 100 The other reason contributing to the overall flattening of the structures is the intramolecular Li· · ·(C-H) agostic interaction, as shown by the Li· · ·CH 3 (t-Bu) bonding contacts of 2.518(5)-2.595(6)Å (for 8a − • Li + ) (Figure 3.5). Electronically, flattening around the anionic centers stems from the decrease in the inversion barrier because of the electropositive σ -donating silyl substituents [16.3 kcal/mol for (H 3 Si) 3 Si − ].…”

Section: Scheme 320mentioning

confidence: 99%

Heavy Analogs of Carbanions: Si‐, Ge‐, Sn‐ and Pb‐Centered Anions

2010

Organometallic Compounds of Low‐Coordinate Si, Ge, Sn and Pb

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Standard organic chemistry textbooks typically define a carbanion as a highly reactive species with a trivalent carbon that has a lone electron pair and bears a negative charge. In contrast to electron-deficient carbenium ions (see Chapter 1), carbanions, featuring eight valence electrons around the central carbon, are electron rich being strong nucleophiles and Lewis bases, typically more powerful than amines. The best-known examples of carbanionic species are definitely the Grignard reagents RMgX and organolithium reagents RLi, readily accessible and extremely useful organometallic derivatives extensively applied in synthetic organic chemistry. Known for more than a hundred years, metal salts of carbanions have been thoroughly studied from the viewpoint of their fundamental properties, such as basicity, stability, ion-pairing behavior and aggregation states.Carbanions contain an sp 3 -hybridized anionic carbon adopting a trigonal-pyramidal geometry with one of its tetrahedral valencies occupied by a lone electron pair. Such geometry is highly reminiscent of that of amines, for example NH 3 , which is isoelectronic with the simplest methyl anion CH 3 − . This was indeed confirmed by PES measurements suggesting a barrier to inversion and pyramidal structure for CH 3 − , 1 that agreed well with the computational data, revealing an H-C-H bond angle for the methyl anion of 109.4 • . 2 The preference for the pyramidal geometry of carbanions can be readily understood from simple orbital considerations. Thus, in a planar carbanion the lone pair would occupy a pure p-orbital, whereas in a pyramidalized carbanion the lone pair orbital has a substantial s-character (close to sp 3 ) gaining an additional stabilization through the

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Silylanions: Inversion Barriers and NMR Chemical Shifts

Cited by 30 publications

References 37 publications

Route Si6 revisited

Route Si6 revisited

Chair, Boat and Twist Conformation of Dodecamethylcyclohexasilane and Undecamethylcyclohexasilane: A Combined DFT and Raman Spectroscopic Study

Heavy Analogs of Carbanions: Si‐, Ge‐, Sn‐ and Pb‐Centered Anions

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