Armin Berndt

Stable germa- and stannaethenes

Stable germaand stannaethenes have been synthesized by addition of germylenes and stannylenes to a kryptodiborylcarbene. Extreme nmr-deshielding of tin is decisive proof for tricoordinated tin atoms in the stannaethenes. Considerable ylid character of germaand stannaethenes is deduced from nmr-shielding of boron atoms adjacent to the tricoordinated carbon atom. X-ray analysis of a germaand a stannaethene revealed strong distortion from planarity at the C=Ge and C5n bond: twisting of 36* and 61* and pyramidalization at carbon of 5* and 16 as measured by the fold angle. The distances from germanium and tin to the tncoordinated carbon (182.7(4) and 202.5(4) pm) are distinctly shorter (8.4 and 6.4%) than to tetracoordinated carbon atoms.

Radical anions from organoboranes

Abstract Radical anions of boranes, which are isoelectronic with tertiary alkyl, allyl and benzyl radicals, have been characterized by ESR spectroscopy. The remarkable sensitivity of boron coupling constants to substituent effects is taken to indicate that boron is more easily pyramidalized than carbon.

The radical anion from tetra-t-butyltetraborane(4), a new route to t-Bu4B4

Abstract The radical anion from tetra-t-butyltetraborane (4) has been generated from di-t-butyldichlorodiborane (4) (II) and Na/K alloy and identified by ESR spectroscopy. Formation of t-Bu 4 B 4 by reductive dimerization of II is also established by 11 B NMR spectroscopy.

A STANNAETHENE WITH A PLANAR ENVIRONMENT OF THE TRICOORDINATED TIN AND CARBON ATOMS

Addition of the diarylstannylene R″2Sn (R″ = 2−tBu−4,5,6-Me3C6H) to the cryptodiborylcarbene (Me3Si)2C(BtBu)2C furnishes the stannaethene (Me3Si)2C(BtBu)2C=SnR″2 (10). The X-ray structure analysis of 10 reveals a strictly planar environment of the tricoordinated tin and carbon atoms and a slight twisting of the Sn=C double bond.

Two-electron aromatics containing three and four adjacent boron atoms

A two-electron aromatic bis (tris-trimethylsilylmethylene)-substituted tetraborane(4) was found to be a useful precursor for the synthesis of two-electron aromatic tetraboranes(6), triboracyclopropanates,as well as tetraboranes(6) distorted toward triboracyclopropanates with boryl bridges. Bishomo two-electron aromatics with a borata bridge and a protonated borata bridge, respectively, are also presented.

Molecular structures of new compounds with Ge=C and Sn=C double bonds [1]

Addition reactions of the diarylgermylene R2Ge: (R=2-tBu-4,5,6-Me3C6H) and the heteroleptic stannylene RR′Sn: (R′ = Si(SiMe3)3) to the cryptodiborylcarbene (Me3Si)2C(tBuB)2C: furnish the germaethene (Me3Si)2C(tBuB)2C=GeR2 (8) and the stannaethene (Me3Si)2C(tBuB)2C=SnRR′ (12), respectively. The X-ray structure analysis of 8 reveals a twisting of the Ge=C double bond with torsion angles of 33 and 34°. In addition to a slight twisting of the double bond (average torsion angle = 11.9°), compound 12 shows a trans-bending of the substituents with bent angles of 13.2° at the tin and 9.7° at the carbon atom of the Sn=C double bond.

A Five-Membered Ring with Three Negative Charges and Solvent-Free Lithium Counterions

Remarkably short distances to the ring plane are shown by the eta(5)-bound lithium ions in the first compound with a triply negatively charged five-membered ring, 1, which was obtained by reduction of 2 with lithium. R=CH(SiMe(3))(2), Dur=2,3,5,6-tetramethylphenyl.

Two-electron homoaromatics with heteroatom bridges

Abstract Zwitterionic mono- and bis-homoaromatics 2a–f and 5a comprised of positively charged NMe2, P(C6H5)2 or As(C6H5)2 bridges and anionic three-center two-electron (3c2e) delocalized boron heterocyclic units, were prepared and characterized by NMR as well as by X-ray structure analyses. The boron chemical shifts of the trishomoaromatic dianion 12a with an oxygen bridge compare well with those computed ab initio for model 12b. Analysis of the electronic structure of the bishomoaromatic 5u and its anionic analog 11u gives insight into the origin of the trend of increasing effectiveness of BC2, B2C and B3 3c2e bonds: higher electronegativity of carbon vs boron prevents symmetric delocalization in rings with B2C and especially BC2 centers.

Zur konformation sterisch gehinderter allyl-radikale; hyperkonjugation in verdrillten π-radikalen—VII

Abstract For the sterically hindered allyl radicals 2 and 6a , which are produced as model compounds for 1,1,3,3-tetramethylallyl radical, twist angles of about 10° are determined on the basis of their ESR coupling constants. Even the more hindered radicals 8a,b are twisted to only about 16°. These results are in contrast to the interpretation of experimental results regarding 1,1,3,3-tetramethylallyl radical and the corresponding cation.

Very strong anionic homoaromaticity in (deloc-1,3,4)-1-sila-3,4-diboracyclopentane-1-ides, the importance of the energy of the reference system for homoaromatic stabilization energies

( deloc -1,3,4)-1-Sila-3,4-diboracyclopentane-1-ide ( 3a ) was prepared as solvent separated ion pair [Li(Et 2 O)(thf) 3 ][ 3a ] and as contact ion pair [Li(Et 2 O)][ 3a ]. Both were fully characterized by NMR spectroscopy as well as by X-ray structure analyses. Their five-membered rings are strongly distorted as seen from short transannular Si⋯B distances of 206 and 208 pm as compared with 276 pm in the undistorted 1-sila-3,4-diboracyclopentane ( 4c ). This distortion is also found in the unsubstituted prototype 3u by geometry optimizations at the MP2/6-311+G** level of theory. In addition, computations show that a three-center-two-electron (3c2e) bond between the silicon and the two boron atoms is present in the distorted 3u . The planar classical reference molecule 3u * with a 2c2e π bond between the boron atoms is not accessible by computations. Therefore, the energy difference between 3u and 3u * was estimated by isodesmic equations to be about 80 kcal mol −1 (at MP4/6-311+G**), considerably larger than any homoaromatic stabilization energy (HSE) ever discussed. The origin of this huge HSE of 3u is strong electrostatic destabilization of the reference system 3u * due to intramolecular charge separation. The series of bishomoaromatic systems is thus extended by a borderline case of very large HSE. The very small HSE of 1u at the other end of the series is also due to the energy of the reference: 1u * is strongly stabilized by hyperconjugation.