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Norbert W Mitzel

Publications and source records attributed to Norbert W Mitzel.

At least 19 recordsLinked to original sources
PubMed2007

Magnetic face-to-face interaction and electrocommunication in chromium sandwich compounds.

The reaction of [{(C5Me5)CrCl2}2] with [2.2](1,4)cyclophane gave [(C5Me5)Cr{[2.2](1,4)cyclophane}] (1) and [(C5Me5)Cr{[2.2](1,4)cyclophane}Cr(C5Me5)] (2), depending on the reaction conditions. X-ray structure analysis showed 2 to be a ministack which in turn is stacked in the lattice. The chromium atoms are 6.035 A apart, and the distortion of the benzene rings to boat-shaped moieties is less pronounced than in parent [2.2](1,4)cyclophane. The NMR and EPR spectra were consistent with a S=1/2 ground state for 1 and with two interacting S=1/2 centers in 2. Spin density was found in the ligand pi systems, where its sign was negative when the pi system was adjacent to chromium, while on the nonbonded benzene moiety of 1 it was positive. Cyclic voltammograms showed reductions to 1- and 2(2-), as well as oxidations to 1+, 2+, and 2(2+) which were quasireversible, whereas oxidations to 1(2+) and 2(3+) were irreversible. Interaction between the metal ions was revealed by a 260 mV separation of the redox waves belonging to 2+, and 2(2+). Both cations were isolated as [B(C6H5)4]- salts, which in solution decomposed to [2.2](1,4)cyclophane and [(C5Me5)Cr{(eta6-C6H5)B(C6H5)3}] (3). The 1H and 13C NMR spectra of 3 were in accordance with an S=1 ground state. Solid-state magnetic measurements of the dimetallic compounds showed antiferromagnetic interaction with J=-122 cm-1 for 2, J=-31 cm-1 for 2+ (ground state S=1/2), and J=-23.5 cm-1 for 2(2+) (with H=-JS1S2). The decrease of J in the series 2, 2+, and 2(2+) was traced to the number of unpaired electrons and, for the mixed-valent cation 2+, to additional double exchange.

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PubMed2006

Expectation and surprise in group 13 organometallics: molecular vs. polymeric aggregation of Me2Ga and Me2In norcamphor oximates.

Whereas (meso)-[(norcamphor oximato)GaMe2]2 exhibits the dimeric M2O2N2 motif favoured by organo group 13 metal oximates, the Me2In derivative [{(meso)-[(norcamphor oximato)InMe2]2}2]infinity aggregates distinctly different with the In atoms adopting coordination number (CN)=5; compound is the first oximato-based chain-like coordination polymer in group 13 organometallics; it comprises alternating M2O2N2 and M2O2 ring dimers interconnected via two types of a fifth dative bond (O-->In and N-->In) to yield the new {M4O4N4}infinity motif; the calculated energetics of the heteroatom rings involved in and confirm the diverging aggregational behaviour of Ga and In.

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PubMed2006

First mixed hydrazide/hydroxylamide metal aggregates.

The first organometallic clusters of mixed hydrazide/hydroxylamide clusters of zinc, [Zn(MeZn)(4)(HNNMe(2))(2)(ONEt(2))(4)] and {Zn(EtZn)(4)[HNN(CH(2))(5)](2)(ONEt(2))(4)} were synthesized in one-pot synthesis protocols from dialkylzinc solutions, substituted hydrazines and N,N-diethylhydroxylamine; competing for the Zn atoms, the different binding properties of hydrazide and hydroxylamide ligands in these heteroleptic clusters are discussed.

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PubMed2005

Zinc hydrazides and alkoxyhydrazides: organometallic compounds with novel Zn4N8, Zn4N6O and Zn4N4O2 cage structures.

Tetrameric [{RZn(NHNMe2)}4] (R = Me, Et), the first organometallic zinc hydrazides to be described, have been prepared by alkane elimination from dialkylzinc solutions and N,N-dimethylhydrazine. They were characterised by 1H and 13C NMR and IR spectroscopy, mass spectrometry, elemental analysis and X-ray crystallography. The compounds form asymmetric aggregates containing the novel Zn4N8 core; tetrahedra of Zn atoms bear the alkyl groups at Zn, with the triangular faces bridged by NHNMe2 substituents. The NH groups are connected to two Zn atoms, and the NMe2 groups to one. Hydrolysis of the compounds with water gives [(RZn)4(OH)(NHNMe2)3] as products, which also were characterised as described above. Higher yields of these hydroxo clusters were achieved in one-pot syntheses by reaction of dialkylzinc solutions with mixtures of N,N-dimethylhydrazine and water. They contain Zn4N6O cages, in which one hydroxide in the tetrameric hydrazides described above replaces one NHNMe2 group. Similar products can be prepared with alkoxy instead of hydroxy groups, in analogous one-pot syntheses with alcohols. Alcoholysis of [EtZn(NHNMe2)]4 with methanol or ethanol gave zinc trishydrazide monoalkoxides, [(EtZn)4(OR)(NHNMe2)3] (R = Me, Et), which have constitutions analogous to the monohydroxides. The organozinc bishydrazide bisalkoxides [(MeZn)4(NHNMe2)2(OEt)2] and [(EtZn)4(NHNMe2)2(OEt)2] were obtained in one-pot reactions from dialkylzinc solutions with mixtures of the hydrazine and alcohol, and their crystal structures, confirmed by spectroscopic methods in solution, show an unsymmetrical aggregation with the novel Zn4N4O2 cage structure.

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PubMed2005

Organoaluminium complexes with sterically demanding oximato ligands: does a bulky and rigid ligand backbone change the aggregation motif?

The four organoaluminium ketoximates [(2,4-dimethylpentane-3-one oximato)AlMe(2)](2) (1), (meso)-[(norcamphor oximato)AlMe(2)](2) (2), {[(R,R)-camphor oximato]AlMe(2)}(3) (3) and {[(R,S)-fenchone oximato]AlMe(2)}(2) (4) have been prepared by the reactions of the corresponding oximes with trimethylaluminium. All compounds have been fully characterized by means of IR, mass and multi-nuclear NMR spectroscopy ((1)H, (13)C, (27)Al) and by elemental analyses. The crystal structures of three of these compounds (2, 3 and 4) were determined, revealing the aggregation motif of organometallic group 13 oximates to vary from the hitherto predominant six-membered dimeric M(2)N(2)O(2) array to others (six-membered M(3)O(3) core ring in the case of the camphor derivative; five-membered M(2)NO(2) core ring in the case of the fenchone derivative) so far only found e.g. in hydroxylamino complexes of group 13 metals. Furthermore, complexes 3 and 4 exhibit an unusual behaviour in solution, as indicated by upfield-shifted additional signals in their (27)Al NMR spectra, pointing at aluminium atoms in penta- or hexacoordination, respectively.

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PubMed2005

Three-membered ring or open chain molecule - (F3C)F2SiONMe2 a model for the alpha-effect in silicon chemistry.

(F3C)F2SiONMe2 was prepared from LiONMe2 and F3CSiF3. It was characterized by gas IR and multinuclear solution NMR spectroscopy and by mass spectrometry. Its structure was elucidated by single crystal X-ray crystallography and by gas electron diffraction. (It exists as a conformer mixture.) Important findings were extremely acute SiON angles [solid 74.1(1) degrees , gas anti 84.4(32) degrees and gauche 87.8(20) degrees] and short Si...N distances [solid 1.904(2) A]. The bending potential of the SiON unit was calculated at the MP2/6-311++G(3df,2dp) level of theory and appears very flat and highly asymmetric. The calculated atomic charges (NPA) are counterintuitive to the expected behavior for a classical Si-N dative bond, as upon formation of the Si...N bond electron density is transferred mainly from oxygen to nitrogen, while the silicon charge is almost unaffected. Despite the molecular topology of a three-membered ring, the topology of the electron density shows neither a bond critical point between Si and N atoms nor a ring critical point, but the electron density and Laplacian values are related to other hypercoordinate Si compounds. The electronic properties of (F3C)F2SiONMe2 were compared to those of the adduct (F3C)F2(MeO)Si-NMe3, whose properties and structure were also calculated. The charge distribution and Laplacian values along the Si-N vectors in both molecules are similar but not equivalent. (F3C)F2SiONMe2 contains thus a nonclassical Si...N bond, and its properties can be regarded as a new model for the explanation of the old postulate of an alpha-effect in silicon chemistry, explaining the behavior of compounds with geminal Si and N atoms.

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PubMed2005

(Dimethylaminomethyl)trifluorosilane, Me2NCH2SiF3--a model for the alpha-effect in aminomethylsilanes.

F3SiCH2NMe2 was prepared as a model for the investigation of the nature of the alpha-effect in alpha-aminosilanes, by fluorination of Cl3SiCH2NMe2 with SbF3. Under less mild conditions Si--C bond cleavage was also observed, leading to the double adduct F4Si(Me2NCH2SiF3)2, which was characterised by a crystal structure analysis showing that the central SiF4 unit is connected to Me2NCH2SiF3 via SiN dative bonds and FSi contacts. F3SiCH2NMe2 was characterised by multinuclear NMR spectroscopy (1H, 13C, 15N, 19F and 29Si), gas-phase IR spectroscopy and mass spectrometry. It is a dimer in the crystal (X-ray diffraction, crystal grown in situ), held together by two Si--N dative bonds. In solution and in the gas phase the compound is monomeric. The structure of the free molecule, determined by gas-phase electron diffraction, showed that, in contrast to former postulates, there are no attractive SiN interactions. Ab initio calculations have been carried out to explain the nature of the bonding. F3SiCH2NMe2 has an extremely flat bending potential for the Si-C-N angle; the high degree of charge transfer from the Si to the N atoms which occurs upon closing the Si-C-N angle is in the opposite direction to that expected for a dative bond. The topology of the electron density of F3SiCH2NMe2 was analysed. Solvent simulation calculations have shown virtually no structural dependence on the medium surrounding the molecule. The earlier postulate of Si-->N dative bonds in SiCN systems is discussed critically in light of the new results.

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PubMed2005

The experimental gas-phase structures of 1,3,5-trisilylbenzene and hexasilylbenzene and the theoretical structures of all benzenes with three or more silyl substituents.

The structures of 1,3,5-trisilylbenzene and hexasilylbenzene in the gas phase have been determined by electron diffraction, and that of 1,3,5-trisilylbenzene by X-ray crystallography. The structures of three trisilylbenzene isomers, three tetrasilylbenzenes, pentasilylbenzene and hexasilylbenzene have been computed, ab initio and using Density Functional Theory, at levels up to MP2/6-31G*. The primary effect of silyl substituents is to narrow the ring angle at the substituted carbon atoms. Steric interactions between silyl groups on neighbouring carbon atoms lead first to displacement of these groups away from one another, and then to displacement out of the ring plane, with alternate groups moving to opposite sides of the ring. In the extreme example, hexasilylbenzene, the SiCCSi dihedral angle is 17.8(8) degrees .

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PubMed2005

Steric and electronic effects on the conformations of n-butane derivatives with trichlorosilyl, silyl and trichloromethyl groups.

The molecular structure and conformation of 1,1,1,4,4,4-hexachloro-1,4-disilabutane in the gas-phase have been determined by electron diffraction and computational methods. The lowest-energy conformation has the trichlorosilyl groups anti to one another. The gauche conformation also has a shallow potential minimum, but lies about 19 kJ mol-1 above the anti form. Calculations on related butane derivatives, in which terminal methyl groups have been replaced by CCl3, SiH3 and SiCl3 groups, reveal that the conformational preferences are primarily caused by steric interactions between the terminal groups, and that it is the presence of chlorine atoms that destabilises gauche conformations. The electronegativity of the chlorine atoms has only small effects, mainly limited to the SiCl bond lengths.

Butanes↗
PubMed2004

Organoaluminium and -gallium compounds with N,N-diisopropylaminomethyl groups.

The N,N-diisopropylaminomethyl aluminium compound [tBu2AlCH2NiPr2 x LiCl]2(1) and the gallium compounds Li[tBu2Ga(CH2NiPr2)2](2) and [tBu2GaCH2N(H)iPr2]Cl x tBu3Ga (3) were prepared by transmetallation of N,N-diisopropylaminomethyllithium LiCH2NiPr2 with di-tert-butylaluminium or -gallium chloride, and characterised by elemental analyses, multinuclear NMR spectroscopy (1H, 13C, 27Al, 7Li) and IR spectroscopy. The crystal structures have been determined by single crystal X-ray diffraction. Compound aggregates as a centrosymmetric dimer, with two Al-C-N units connected by a frame of two LiCl molecules [Al-Cl 2.367(1), Cl-Li 2.339(4) and 2.374(4), Li-N 1.977(4)A]. Compound 2 is a lithium organogallate with two weak LiN bonds [1.965(7) and 1.937(7)A]. Compound 3 contains two different moieties: tBu3Ga and a [tBu2GaCH2N(H)iPr2]+ cation, which are bridged by a Cl- anion [Ga-Cl 2.445(1) and 2.579(1), HCl 2.362(3)A].

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PubMed2004

Experimental investigations and ab initio studies of selenium(II) dialkanethiolates, Se(SR)2.

Selenium(II) dimethanethiolate, Se(SMe)(2), was synthesized by reaction of SeO(2) with HSMe. Basic spectroscopic data for Se(SMe)(2) and selenium(II) bis(2-methyl-2-propanethiolate), Se(S(t)Bu)(2), were recorded and interpreted with the support of ab initio calculations. Both compounds are thermodynamically unstable relatively to selenium and the corresponding disulfide. The UV/vis spectra of both compounds are qualitatively similar, the two bands being attributed to n(Se)-sigma*(Se-S) transitions. The bands at 369 and 397 cm(-1) in the IR spectra of Se(SMe)(2) and Se(S(t)Bu)(2), respectively, are assigned to nu(as)(SeS(2)). The (77)Se NMR shifts of Se(SMe)(2)(784 ppm) and Se(S(t)Bu)(2)(556 ppm) differ substantially from each other and show positive temperature gradients. Calculations at the GIAO-HF/962+(d) level reproduced the difference of the (77)Se NMR chemical shifts between Se(SMe)(2) and Se(S(t)Bu)(2). At the same level, the effect of conformational changes on (77)Se shifts were studied for Se(SMe)(2). In the solid state Se(SMe)(2) forms long intermolecular SeS contacts while Se(S(t)Bu)(2) does not. Both compounds exhibit anti-conformations of the methyl and tert-butyl groups with respect to the SeS(2) plane. MP2/LANL2DZ(d) geometry optimizations, single point energy and frequency calculations performed for Se(SMe)(2) show, that syn- (C(s)) and anti-conformers (C(2)) represent minima on the potential energy surface, the latter being by 8 kJ mol(-1) lower in energy than the former. Both conformers are stabilized by intramolecular pi-type n(S(1))-sigma*(Se-S(2)) orbital interactions. The energy of the transition state for the mutual conversion of the two conformers was calculated to be 31 kJ mol(-1) above that of the syn conformer, allowing a rapid interconversion of the two conformers at room temperature. Intermolecular interactions between Se(SMe)(2) molecules were also studied by means of calculations at the MP2/LANL2DZ(d) level. For Se(S(t)Bu)(2) MP2/LANL2DZ(d) geometry optimizations and single point energy calculations revealed a C(2)-symmetric anti- and a C(1) symmetric syn-conformer, the latter being 21 kJ mol(-1) higher in energy than the former. Se(SMe)(2) and Se(S(t)Bu)(2) exchange thiolate groups with other selenium(II) dithiolates, tellurium(II) dithiolates and with thiols, if catalytic amounts of p-CH(3)C(6)H(4)SO(3)H are added.

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PubMed2004

Dichlorosilane-dimethyl ether aggregation: a new motif in halosilane adduct formation.

The crystal structures of (H(3)C)(2)O, H(2)SiCl(2) and an adduct of these were determined by low-temperature X-ray crystallography on crystals grown in situ at low temperatures on a diffractometer. The adduct of (H(3)C)(2)O and H(2)SiCl(2) has the composition [(H(3)C)(2)O.H(2)SiCl(2)](2) and contains a four-membered Si(2)O(2) ring, with the Cl atoms pointing to the outside and the Si-H functions pointing to the inner side of the ring. The Si(2)O(2) ring has two longer and two shorter SiO bonds and thus deviates from a square. Quantum chemical calculations give a geometry for [(H(3)C)(2)O.H(2)SiCl(2)](2) which has D(2h) symmetry and allow to obtain an estimate for the adduct formation energies, which are -13.4 kJ mol(-1) for the formation of the mono adduct [(H(3)C)(2)O + H(2)SiCl(2)-->(H(3)C)(2)O.H(2)SiCl(2)], -14.4 kJ mol(-1) for the dimerization of two mono adducts [(H(3)C)(2)O.H(2)SiCl(2)-->[(H(3)C)(2)O.H(2)SiCl(2)](2)] and -41.2 kJ mol(-1) for the reaction 2 (H(3)C)(2)O + 2 H(2)SiCl(2)-->[(H(3)C)(2)O.H(2)SiCl(2)](2). The results are used to rationalize the strongly reduced reactivity of H(2)SiCl(2) towards nucleophilic substitution reactions in (H(3)C)(2)O at low temperatures.

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PubMed2004

Strong intramolecular secondary si.N bonds in trifluorosilylhydrazines.

The simple silylhydrazines F(3)SiN(Me)NMe(2) (1), F(2)Si(N(Me)NMe(2))(2) (2), and F(3)SiN(SiMe(3))NMe(2) (3) have been prepared by reaction of SiF(4) with LiN(Me)NMe(2) and LiN(SiMe(3))NMe(2), while F(3)SiN(SnMe(3))NMe(2) (4) was prepared from SiF(4) and (Me(3)Sn)(2)NNMe(2) (5). The compounds were characterized by gas-phase IR and multinuclear NMR spectroscopy ((1)H, (13)C, (14/15)N, (19)F, (29)Si, (119)Sn), as well as by mass spectrometry. The crystal structures of compounds 1-5 were determined by X-ray crystallography. The structures of free molecules 1 and 3 were determined by gas-phase electron diffraction. The structures of 1, 2, and 4 were also determined by ab initio calculations at the MP2/6-311+G** level of theory. These structural studies constitute the first experimental proof for the presence of strong Si.N beta-donor-acceptor bonds between the SiF(3) and geminal NMe(2) groups in silylhydrazines. The strength of these non-classical Si.N interactions is strongly dependent on the nature of the substituent at the alpha-nitrogen atom of the SiNN unit, and has the order 3>4>1. The valence angles at these extremely deformed alpha-nitrogen atoms, and the Si.N distances are (crystal/gas): 1 104.2(1)/106.5(4) degrees, 2.438(1)/2.510(6) A; 3 83.6(1)/84.9(4) degrees, 2.102(1)/2.135(9) A; 4 89.6(1) degrees, 2.204(2) A.

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PubMed2004

Different modes of aggregation in organoaluminium and -gallium hydroxylamides.

The organoaluminium and -gallium hydroxylamides (Me2GaONMe2)2, (tBu2AlONMe2)2, (tBu2GaONMe2)2 and (Me2AlONiPr2)2 have been prepared by the reaction of the hydroxylamines Me2NOH and iPr2NOH with the trialkylmetal compounds trimethylgallium, tri-tbutylaluminium and tri-tbutylgallium, respectively. All compounds have been characterised by NMR spectroscopy (1H, 13C, 15N, 17O and 27Al), by mass spectrometry and elemental analyses. The crystal structures of the four compounds have been determined, showing that they all form dimers but with different modes of aggregation: (Me2GaONMe2)2 has a Ga2O2N2 six-membered ring, (tBu2AlONMe2)2 and (Me2AlONiPr2)2 have Al2O2 four-membered rings, (tBu2GaONMe2)2 forms a Ga2O2N five-membered ring.

Aluminum↗
PubMed2003

Highly asymmetric coordination in alkenes: gas-phase structures of trans-1,2-dichloro-1,2-disilylethene and 1-bromo-1-silylethene.

The molecular structures of trans-1,2-dichloro-1,2-disilylethene and 1-bromo-1-silylethene have been determined by gas-phase electron diffraction (GED) and ab initio molecular orbital calculations (MP2/6-311G). Both compounds were found to have highly asymmetric coordination around the carbon atoms with [ab initio (r(e))/GED (r(a))] C=C-Cl [117.0/117.0(2) degrees] and C=C-Si [126.2/128.1(1) degrees] in the C(2)(h) structure of trans-1,2-dichloro-1,2-disilylethene and C=C-Br [119.2/120.7(4) degrees] and C=C-Si [125.0/125.0(4) degrees] in the C(s) structure of 1-bromo-1-silylethene. Other important structural parameters for trans-1,2-dichloro-1,2-disilylethene are C=C [135.2/134.5(3) pm], C-Si [189.4/187.9(2) pm], and C-Cl [175.1/174.9(1) pm], and C=C [134.2/133.4(2) pm], C-Si [187.8/187.2(3) pm], and C-Br [191.3/191.0(3) pm] for 1-bromo-1-silylethene. Further ab initio calculations were carried out on CH(2)CRX and trans-(CRX)(2) (R = SiH(3), CH(3), or H; X = H, F, Cl, or Br) to gauge the effects of electron-withdrawing and electron-donating groups on the structures. They reveal some even more distorted structures. The asymmetric appearance of these molecules can largely be accounted for by valence shell electron pair repulsion theory.

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PubMed2002

5-Organyl-5-phosphaspiro[4.4]nonanes: a contribution to the structural chemistry of spirocyclic tetraalkylphosphonium salts and pentaalkylphosphoranes.

Spirocyclic phosphonium salts of the type [(CH(2))(4)P(CH(2))(4)](+) X(-) with X = I(3) (1a), I (1b), picrate (1c), benzoate (1d), and Cl (1e) were prepared from 1,4-diiodobutane and elemental phosphorus followed by metathesis reactions. The crystal structures of 1b and 1c and of 1d(H(2)O) have been determined by X-ray diffraction methods. In the cations of these salts the phosphorus atoms are shared by two five-membered rings in envelop conformations. In the picrate 1c the cations show an unsymmetrical ring folding pattern (point group C(1)), while the geometry of the cations of the iodide 1b and the benzoate hydrate [1d(H(2)O)] approaches the symmetry of point group C(2). These structures can be taken as models for the as yet unknown molecular geometries of the corresponding hydrocarbon (CH(2))(4)C(CH(2))(4) and silane (CH(2))(4)Si(CH(2))(4). Treatment of 1e with organolithium reagents RLi affords spirocyclic pentaorganophosphoranes RP[(CH(2))(4)](2) with R = Me, Et, n-Bu, Vi, and Ph (2a-e) in good (R = Me, Et, n-Bu) to low yields (R = Vi, Ph). The products are isolated as colorless liquids, of which only 2a, 2b, and 2d can be distilled without decomposition. Single crystals of 2a were obtained by low-temperature in situ crystal growth. The molecule has a trigonal bipyramidal configuration with the methyl group in an equatorial position and the two five-membered rings spanning axial/equatorial positions of the polyhedron. Deviations from the standard trigonal bipyramidal geometry are small. The compounds 2a-e are fluctional in solution as demonstrated by NMR spectroscopy.

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