29Si NMR Chemical Shift Tensor and Electronic Structure of 7-Silanorbornadienes

Abstract

The structure and bonding of 7-silanorbornadienes was investigated using X-ray Diffraction (XRD), solid-state NMR spectroscopy and density functional calculations. The solid state structures of four benzo-7-silanorbornadienes (4a, c, d, e) and of one dibenzo-7-silabenzonorbornadiene (5a) are reported and compared with the results of previous structural investigations. The most prominent features of the molecular structures of all 7-silanorbornadienes are very long Si-C(bridgehead) bonds (d(SiC)=190.6-196.8 pm) and very acute CSiC bond angles α (α(CSiC)=78.7-83.9°). All newly investigated 7-silanorborndienes show for tetracoordinated silicon nuclei extremely deshielded 29 Si NMR resonances (δ 29 Si=65.6-31.6). Solid State NMR investigations for 7-silanorbornadienes anti-4a, b reveal highly anisotropic chemical shift tensors of axial or nearly axial symmetry (4a: δ 11 =161, δ 22 =δ 33 =−11; 4b: δ 11 =113, δ 22 =14, δ 33 =−15). The dominating, strongly deshielding δ 11 component is oriented almost perpendicular to the plane spanned by the two bridgehead carbon atoms and the bridging silicon atom.The DFT calculations suggest that the origin of the strong deshielding is a small energy difference between the frontier orbitals, which are strongly localized at the silicon atom. In addition the computations reveal that both the long SiC bonds and the strongly deshielded 29 Si NMR chemical shift are direct consequences of the bonding situation in 7-silanorbornadienes which are characterized by through space interaction of the C=C double bonds and by hyperconjugation between the SiC σ-bonds and the unoccupied orbitals of the C=C double bonds.

Dibenzosilanorbornadienyl Cations and Their Fragmentation into Silyliumylidenes

“…13 Therefore, this route is particularly well suited for the synthesis of dibenzo-7-silanorbornadienes with bulky terphenyl substituents. 14 Dibenzosilanorbornadienes 12 were obtained in satisfying isolated yields (Scheme 2) and were fully characterized by NMR spectroscopy and X-ray diffraction analysis of suitable crystals (see Figure 1, Table 1, and the Supporting Information). The relatively low field shifted resonances in the 29 Si NMR spectra (δ( 29 Si) = 31.6 (12a), 31.5 (12b)) are characteristic for 7-silanorbornadienes.…”

“…Both features, the deshielded 29 Si nuclei and the long endocyclic SiC bonds, are typical of the 7-silanorbornadiene subunit, and they are indications of the occurrence of σ−π* conjugation in these compounds. 14,15 Addition of benzene to an equimolar mixture of trityl tetrakispentafluorophenyl borate ([Ph 3 C][B(C 6 F 5 ) 4 ]) and dibenzo-7-silanorbornadiene 12a results in a biphasic reaction mixture, characteristic of benzene solutions of the [B(C 6 F 5 ) 4 ] − salts. Monitoring of the reaction progress by 1 H NMR spectroscopy of the lighter nonpolar phase indicates a slow hydride transfer and selective formation of the terphenylsubstituted 7-silanorbornadienyl borate 11[B(C 6 F 5 ) 4 ], which was obtained as the sole product after 14 h at room temperature (Scheme 3, Figure 2a).…”

“…The regioselectivity of the C 10 -deprotonation increases with the steric bulk of the applied base and of the substituent R at the silicon atom . Therefore, this route is particularly well suited for the synthesis of dibenzo-7-silanorbornadienes with bulky terphenyl substituents . Dibenzosilanorbornadienes 12 were obtained in satisfying isolated yields (Scheme ) and were fully characterized by NMR spectroscopy and X-ray diffraction analysis of suitable crystals (see Figure , Table , and the Supporting Information).…”

“…Dibenzosilanorbornadienes 12 were obtained in satisfying isolated yields (Scheme ) and were fully characterized by NMR spectroscopy and X-ray diffraction analysis of suitable crystals (see Figure , Table , and the Supporting Information). The relatively low field shifted resonances in the 29 Si NMR spectra (δ( 29 Si) = 31.6 ( 12a ), 31.5 ( 12b )) are characteristic for 7-silanorbornadienes. − From a structural point of view, the long endocyclic Si–C 1/4 bonds ( d (SiC 1/4 ) = 192.7 pm ( 12a ), 192.4 pm ( 12b )) are noticeable. Both features, the deshielded 29 Si nuclei and the long endocyclic SiC bonds, are typical of the 7-silanorbornadiene subunit, and they are indications of the occurrence of σ–π* conjugation in these compounds. , …”

“… a In benzene- d 6 at room temperature. b Data from ref . c Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl. d Ter* = 2,6-bis(2,4,6-tri- iso -propylphenyl)phenyl. e In toluene- d 8 at room temperature. f In chlorobenzene- d 5 at room temperature. g 29 Si NMR chemical shifts calculated at GIAO/B3LYP/IGLOIII//PCM/M06-2X/6-311+G(d,p). h Structural parameter obtained at M06-2X/6-311+G(d,p) …”