O-oximatosilanes: weak beta-donor interactions as secondary bonds

Losehand U, Mitzel NW. O-oximatosilanes: weak beta-donor interactions as secondary bonds. JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS. 1998;(15):2537-2540.The oximatosilanes H3SiON=CMe2, ClH2SiON=CMe2 and H2Si(ON=CMe2)(2) were prepared from the reaction of lithiated acetone oxime with bromosilane and dichlorosilane respectively. They have been characterised by NMR (H-1, C-13, N-15, O-17 and Si-29) and by gas-phase IR spectroscopy. The molecular structures of H3SiON=CMe2 and H2Si(ON=CMe2)(2) have been determined by low-temperature X-ray crystallography. Small Si-O-N angles have been detected in these structures, which are indicative of the formation of weak secondary interactions of the beta-donor-acceptor type between silicon and nitrogen atoms. Important geometrical parameters: H3SiON=CMe2, Si-O 1.673(1), N-O 1.433(1) Angstrom, Si-O-N 106.0(1)degrees, N=C 1.273(2) Angstrom; H2Si(ON=CMe2)(2), Si-O 1.657(1), 1.650(1), N-O 1.466(2), 1.447(2), N=C 1.271(2), 1.266(2) Angstrom, Si-O-N 102.5(1), 107.5(1)degrees. The geometries of H2SiON=CMe2, FH2SiON=CMe2 and ClH2SiON=CMe2 were calculated by optimisations at the MP2/6-311G** level of theory. The Si-O-N angles in FH2SiON=CMe2 and ClH2SiON=CMe2 are predicted to be smaller than that in H3SiON=CMe2. A natural bond orbital analysis describes the beta-donor interaction as weak negative hyperconjugation lp(O)-->sigma*(SiO) and lp(O)-->sigma*(SiH). In general the beta-donor bonds in oximatosilanes are found to be weaker than in the related hydroxyaminosilanes

beta-donor interactions as secondary bonds in dichlorobis(dimethylaminoxy)silane and -germane

Mitzel NW, Losehand U. beta-donor interactions as secondary bonds in dichlorobis(dimethylaminoxy)silane and -germane. EUROPEAN JOURNAL OF INORGANIC CHEMISTRY. 1998;(12):2023-2026.The compounds Cl2Si(ONMe2)(2) and Cl2Ge(ONMe2)(2) were prepared by the reaction of the appropriate tetrachlorides with N,N-dimethylhydroxylamine in the presence of 2,6-lutidine as an auxiliary base. The compounds were obtained as crystalline solids and characterised by elemental analysis, IR and NMR spectroscopy (nuclei H-1, C-13, N-15, O-17, and Si-29). Their molecular structures were determined by single-crystal X-ray diffraction. The compounds are isostructural and both show the presence of weak secondary bonds between the nitrogen and silicon centres (beta-donor bonds), as is indicated by the small angles SiON [1.02.8(1) and 103.7(1)degrees] and GeON [102.0(1)degrees], and the distances Si...N (2.438 and 2.450 Angstrom) and Ge...N (2.519 and 2.520 Angstrom). The conformation is anti-gauche in both compounds (one O-Si-O-N torsion angle being about 180 degrees, the other 60 degrees), but the strength of the beta-donor interaction is not significantly affected by the different antisubstituents at the nitrogen centres, although such behaviour was expected from earlier studies carried out on ClH2SiONMe2. The beta-donor interactions are also weaker than those found in ClH2SiONMe2

Hydroxylaminosilanes: Compounds with beta-donor-acceptor bonds

Losehand U, Mitzel NW. Hydroxylaminosilanes: Compounds with beta-donor-acceptor bonds. INORGANIC CHEMISTRY. 1998;37(13):3175-3182.Two series of compounds, HxSi(ONMe2)(4-x) (3a, 2a, 1a) and HxSi(ONEt2)(4-x) (3b, 2b, 1b) (with x = 1, 2, 3), have been prepared by either the condensation of N,N-dialkylhydroxylamines with halogenosilanes in the presence of; 2,6-lutidine as an auxiliary base or by the more selective reaction of the O-lithiohydroxylamines with halogenosilanes at low temperatures. The compounds are very sensitive to hydrolysis, but are not pyrophoric, and are Stable at ambient temperature to a potentially very exothermic rearrangement into aminosilanoles. The compounds have been characterized by gas-phase IR and solution NMR spectroscopy (H-1,C-13, N-15, O-17, Si-29) and by mass spectrometry. The IR frequencies of the simplest compound H3SiONMe2 (1a) have been assigned by comparison with ab initio frequencies. The NMR data are discussed in the light of P-donor interactions. The Si-29 NMR shifts of the series HxSi(ONMe2)(4-x) are compared with those of the isoelectronic isopropoxysilanes, HxSi(OCHMe2)(4-x), which have been prepared for this purpose. Single crystals of H3SiONMe2 (1a), H3SiONEt2 (1b), H2Si(ONMe2)(2) (2a), and HSi(ONMe2)3 (3a) have been grown by in situ methods, and their structures have been determined by X-ray diffraction. All compounds have small Si-O-N angles, with the minimum of 95.2 degrees occurring with H2Si(ONMe2)(2) (2a). The crystallographic data are compared with the results of ab initio calculations (MP2/6-311G**) and the crystal structure of the isoelectronic H3SiOCHMe2,which has now been deterinined, and the earlier reported Si(OCHMe2)(4). H3SiOCHMe2 crsstalizes in a transition state geometry according to MP2/6-311G** calculations and has a Si-O-C angle of 118.4(1)degrees. The data show clearly the large differences between Si-O-N and Si-O-C angles, which are attributed to the occurrence of P-donor interactions in SiO-N units. The strength of this interaction has been estimatedto be 15 kJ mol(-1) from ab initio calculations

beta-donor bonds in compounds containing SiON fragments

Mitzel NW, Losehand U. beta-donor bonds in compounds containing SiON fragments. ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 1997;36(24):2807-2809

Siloles and germoles modified by partial hypercoordination through aminoxy substituents

Losehand U, Mitzel NW. Siloles and germoles modified by partial hypercoordination through aminoxy substituents. JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS. 2000;7(7):1049-1052.1-Dimethylaminoxy-2,3,4,5-tetraphenyl-1-silacyclopentadiene (1), 1,2-bis(dimethylaminoxy)-2,3,4,5-tetraphenyl-1-silacyclopentadiene (2) and 1,2-bis(dimethylaminoxy)-2,3,4,5-tetraphenyl-1-germacyclopentadiene (3) have been prepared by the reactions of the corresponding chloro-2,3,4,5-tetraphenyl-1-(sila/germa)cyclopentadienes with LiONMe2. They are yellow crystalline materials, which have been identified by multinuclear NMR spectroscopy (H-1, C-13, N-15, O-17, Si-29), mass spectrometry and elemental analysis. The UV-VIS spectra of 1, 2 and 3 show the absorption bands of the silole/germole to be only slightly affected by the Si ... N interaction, which indicates that orbital interactions of the type lp(N)-->sigma*(SiC) do not play a significant role and thus the Si ... N attraction is better interpreted as a dipole interaction. The crystal structures of 2 and 3 reveal planar C4Si and C4Ge rings, with a propeller-like arrangement of the phenyl groups. The SiON and GeON groups contain short Si ... N distances [2.473(3) and 2.503(3) Angstrom in 2, and 2.535(7) to 2.608(7) Angstrom in 3]. 2 adopts a gauche-gauche conformation for the NOSiON backbone, while in 3, two independent molecules are found in the asymmetric unit, one with an anti-gauche conformation and one with a gauche-gauche conformation

Dimethyl-, disilyl- and digermylsulfide: Different intermolecular contacts in the solid state

Mitzel NW, Losehand U. Dimethyl-, disilyl- and digermylsulfide: Different intermolecular contacts in the solid state. ZEITSCHRIFT FUR NATURFORSCHUNG SECTION B-A JOURNAL OF CHEMICAL SCIENCES. 2004;59(6):635-638.The compounds (H3C)(2)S, (H3Si)(2)S and (H3Ge)(2)S have been crystallised in situ on a diffractometer and their crystal structures determined by low-temperature X-ray diffraction. The molecules are present as monomers in the crystals. The aggregation of the molecules through secondary intermolecular contacts in the crystal is different: (H3C)(2)S is weakly associated into dimers by S...S contacts, whereas (H3Si)(2)S and (H3Ge)(2)S form Si...S and Ge...S contacts in an ice-analogous aggregation motif. Important geometry parameters are (H3C)(2)S: C-S 1.794(av) Angstrom, C-S-C 99.2(1)degrees; (H3Si)(2)S: Si-S 2.143(1) Angstrom, Si-S-Si 98.4degrees; (H3Ge)(2)S Ge-S 2.223(2) and 2.230(2) Angstrom, Ge-S-Ge 98.2(1)degrees

beta-donor interactions of exceptional strength in N,N-dimethylhydroxylaminochlorosilane, ClH2SiONMe2

Mitzel NW, Losehand U. beta-donor interactions of exceptional strength in N,N-dimethylhydroxylaminochlorosilane, ClH2SiONMe2. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. 1998;120(29):7320-7327.The compounds ClH2SiONMe2 and ClH2SiONEt2 have been prepared by the reaction of the corresponding O-lithiated hydroxylamines and dichlorosilane. Their identity has been proved by gas-phase IR and solution NMR (H-1, C-13, N-15, O-17, and Si-29) spectroscopy. In contrast to CM2SiONMe2, ClH2SiONEt2 is unstable at ambient temperature and decomposes to give H2SiCl2 and H2Si(ONEt2)(2). ClH2SiONEt2 shows dynamic behavior in the solution as shown by low-temperature NMR. In the crystal ClH2SiONMe2 (low-temperature crystallography) is present as anti-conformer (torsional angle Cl-Si-O-N 180 degrees) and shows an exceptionally small Si-O-N angle of 79.7(1)degrees and a Si ... N distance of 2.028(1) Angstrom, corresponding to a relatively strong Si-N-beta-donor interaction. Ab initio calculations predict a wider Si-O-N angle of 91.6 degrees (MP2/6-311G**). A gas-phase structure determination by electron diffraction shows the presence of two conformers (anti and gauche), which are equal in potential energy. The Si-O-N angle in the anti-conformer is 87.1(9)degrees and that in the gauche-conformer 104.7(11)degrees [Cl-Si-O-N torsion angle: 72.9(28)degrees]. Calculations (MP2/6-31G*) have shown the angle Si-O-N and the energy of the system to be largely dependent on the torsion angle Cl-Si-O-N, with a variation of the angle Si-O-N over a range of 30 degrees during a full rotation of Cl-Si-O-N. Analysis of the calculated charges on the Si and N atoms show the beta-donor interaction not to be simply caused by electrostatic interaction. Natural bond orbital analysis describes a Ip-N --> sigma(Si-X)* type negative hyperconjugation as an important contribution to beta-donor bonding. The silicon substituent in anti position to nitrogen exerts the major effect on the strength of the beta-donor bond. Simultaneous analysis of negative hyperconjugation Ip-O --> sigma(Si-X)* shows the Si-O-N compounds to have similar contributions to this interaction as H3SiOCH3 and questions negative hyperconjugation to be quoted in isolation for the rationalization of the wide bond angles in silyl ethers

Synthesis and molecular structures in the gas phase of N,N-dimethylaminoxy-trimethylsilane and -trimethylgermane

Mitzel NW, Losehand U, Richardson AD. Synthesis and molecular structures in the gas phase of N,N-dimethylaminoxy-trimethylsilane and -trimethylgermane. INORGANIC CHEMISTRY. 1999;38(23):5323-5328.The compounds Me3SiONMe2 and Me3GeONMe2 have been prepared by reacting Me3SiCl and Me3GeBr with LiONMe2. Their identity was proven by gas-phase IR and solution NMR spectroscopy of the nuclei H-1, C-13, N-15, O-17, Si-29, and Ge-73 and by mass spectrometry and elemental analyses. Their molecular structures in the gas phase have been determined by analysis of electron diffraction data augmented by restraints derived from ab initio calculations. The molecules adopt C-s symmetry. Important geometry parameter values for Me3SiONMe2 are: Si-O 1.688(2), Si-Cin-plane 1.870(4), Si-Cout-of-plane 1.872(2), O-N 1.481(6) Angstrom, Si-O-N 107.9(6), O-Si-Cin-plane 102.1(6), O-Si-Cout-of-plane 110.9(7)degrees, for Me3GeONMe2: Ge-O 1.812(2), Ge-Cin-plane 1.949(3), Ge-Cout-of-plane 1.950(2), O-N 1.475(6) Angstrom, Ge-O-N 108.9(7), O-Ge-Cin-plane 101.7(6), O-Ge-Cout-of-plane 110.3(14)degrees. The structure data are interpreted in terms of weak attractive interactions between the nitrogen donor and the silicon/germanium acceptor atoms. The results are discussed in comparison with other structure data from the literature: the strength of a donor-acceptor interaction in Me3SiONMe2 is weaker than in H3SiONMe2 or ClH2SiONMe2, but stronger than in Me3SiON(CF3)(2)

Hexakis(dimethylaminoxy)disiloxane: Insertion of an oxygen atom into the Si-Si bond of a disilane by dimethylaminoxy lithium

Mitzel NW, Losehand U. Hexakis(dimethylaminoxy)disiloxane: Insertion of an oxygen atom into the Si-Si bond of a disilane by dimethylaminoxy lithium. ZEITSCHRIFT FUR NATURFORSCHUNG SECTION B-A JOURNAL OF CHEMICAL SCIENCES. 2001;56(7):630-633.The compound (Me2NO)(3)SiOSi(ONMe2)(3) was prepared by the reaction of LiONMe2 with Si2Cl6, a reaction in which oxygen is inserted into the Si-Si bond. The crystal structure of (Me2NO)(3)SiOSi(ONMe2)(3) was determined and shows the compound to have a linear siloxane core and SiON units with small valence angles at oxygen, caused by weak Si .(.N interactions leading to partially hypercoordinate Si centres

A crystalline diethyl ether adduct of tetrafluorogermane

Mitzel NW, Losehand U, Vojinovic K. A crystalline diethyl ether adduct of tetrafluorogermane. INORGANIC CHEMISTRY. 2001;40(20):5302-5303.The first crystal structure of a bis-ether adduct of a germane was determined from GeF4(OEt2)(2) prepared by the reaction of antimony(III) fluoride and germanium tetrachloride in diethyl ether, and crystallized from the gas phase