Norbert W. Mitzel

On the Molecular and Electronic Structures of AsP3 and P4

The molecular and electronic structures of AsP(3) and P(4) have been investigated. Gas-phase electron diffraction studies of AsP(3) have provided r(g) bond lengths of 2.3041(12) and 2.1949(28) A for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P(4) has been redetermined and provides an updated value of 2.1994(3) A for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP(3) and P(4) and shows that electronically AsP(3) and P(4) are quite similar. Solid-state (75)As and (31)P NMR spectroscopy demonstrate the plastic nature of AsP(3) and P(4) as solids, and an extreme upfield (75)As chemical shift has been confirmed for the As atom in AsP(3). Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP(3) and P(4) can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules.

Cluster self-assembly of di[gold(I)]halonium cations.

Treatment of gold(I) halide complexes of the type L-Au-X [where L = PPh3, PEt3 with X = Cl, Br, I, or L = 2,6-(MeO)2C6H3PPh2 with X = Cl] with AgSbF6 in the molar ratio 2:1 in dichloromethane/tetrahydrofuran at −78°C affords high yields of di[gold(I)]halonium salts of the formula {X[Au(PR3)]2}+ SbF\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{6}^{-}}}\end{equation*}\end{document} (2-8). A determination of the crystal structures of the four triarylphosphine complexes (2-4, 8) revealed the presence of novel tetranuclear dications with a highly symmetrical structure (point group S4) that arises from self-assembly of the dinuclear monocations through a set of four equivalent aurophilic Au–Au interactions. A comparison with two reference structures of corresponding chloronium perchlorate and bromonium tetrafluoroborate salts with monomeric, dinuclear cations shows that the geometry of the latter is greatly altered on dimerization to optimize the interactions between the closed-shell metal centers (Au: 5d10). Weak metallophilic bonding clearly becomes significant only in crystal lattices where anions with a larger ionic radius (SbF\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{6}^{-}}}\end{equation*}\end{document} vs. BF\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{4}^{-}}}\end{equation*}\end{document}, ClO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{4}^{-}}}\end{equation*}\end{document}) reduce the otherwise dominant role of strong interionic Coulomb forces. The results indicate that aurophilic bonding is indeed an ubiquitous, quite dependable mode of intermetallic interactions provided that the right environment is chosen to allow the weak forces to become operative.

A Thallium Coated Dianion: Trigonal Bipyramidal [F2Al(OR)3]2— Coordinated to Three Tl+ Cations in the Ion Pair [Tl3F2Al(OR)3]+[Al(OR)4]— [R = CH(CF3)2]

The reproducible synthesis of the unusual ionic aluminum compd. [Tl3F2Al(OR)3]+[Al(OR)4]- (1) is reported. In the reaction of Li[Al(OR)4] [R = C(H)(CF3)2] with TlF the initially desired Tl[Al(OR)4] only formed with an exact 1:1 stoichiometry, while an excess of TlF led to [Tl3F2Al(OR)3]+[Al(OR)4]- (1). Addnl. the x-ray single crystal structure of the byproduct [(ROH)TlAl(OR)3(m-F)]2 (2) was detd. Compds. 1 and 2 were characterized by x-ray single crystal structure detns. and 1 also by NMR spectroscopy and an elemental anal. In 1 the [Tl3F2Al(OR)3]+ cation forms a trigonal bipyramid with a pentacoordinate aluminum atom. Three Tl+ cations cover the [F2Al(OR)3]2- dianion core and the charge of the resulting [Tl3F2Al(OR)3]+ cation is compensated by a weakly coordinating [Al(OR)4]- anion. Compd. 2 contains a centrosym. [Al(OR)3(m-F)]22- dianion core with pentacoordinate aluminum atoms building a distorted edge sharing double trigonal bipyramid. The [Al(OR)3(m-F)]22- dianion coordinates two [Tl(ROH)]+ cations giving the non charged mol. [(R-OH)TlAl(OR)3(m-F)]2 (2). Based on BP86/SVP (DFT-) and lattice enthalpy calcns. a pathway of the reaction is proposed to rationalize the formation of the [M3F2Al(OR)3]+ cation upon reaction of Li[Al(OR)4] with MF for M = Tl but not for M = Cs (cf Cs+ and Tl+ have very similar ionic radii). Using a suitable Born-Haber cycle and in agreement with the expt., the enthalpies of the reaction of two M[Al(OR)4] with two MF giving [M3F2Al(OR)3]+[Al(OR)4]- and MOR are favorable for M = Tl by 127 kJ/mol but endothermic for the formation of the hypothetical [Cs3F2Al(OR)3]+[Al(OR)4]- by 95 kJ/mol. It is suggested that in the reaction leading to 1 initially Tl[Al(OR)4] is formed, followed by an abstraction of TlOR and Al(OR)3. The latter very strong Lewis acid reacts subsequently with an excess of TlF yielding 1. [on SciFinder (R)]

Dialkylaluminium-, -Gallium-, and -Indium-Based Poly-Lewis Acids with a 1,8-Diethynylanthracene Backbone

Potential host systems based on a rigid 1,8-diethynylanthracendiyl backbone were synthesised by treatment of 1,8-diethynylanthracene with the Group 13 trialkyls AlMe(3), GaMe(3), InMe(3), AlEt(3) and GaEt(3). The resulting products were characterised by IR and multinuclear NMR spectroscopy, elemental analyses and determination of their crystal structures by X-ray diffraction. The compounds are dimeric in the solid state and comprise two M(2)C(2) heterocycles. Depending on the steric demand of the alkyl substituents at the metal atom, different types of binding modes were observed, which can be classified to lie between the ideals of side-on coordination with almost linear primary M-C≡C units and the 3c-2e coordination with symmetrically bridging alkynyl units in M-C-M bonds. As a solution in THF the dimers are broken into monomers and some are found to undergo ligand scrambling reactions.

N,N‐Dimethylaminopropylsilane: A Case Study on the Nature of Weak Intramolecular Si⋅⋅⋅N Interactions

N,N-Dimethylaminopropylsilane H(3)Si(CH(2))(3)NMe(2) was synthesised by the reaction of (MeO)(3)Si(CH(2))(3)NMe(2) with lithium aluminium hydride. Its solid-state structure was determined by X-ray diffraction, which revealed a five-membered ring with an SiN distance of 2.712(2) A. Investigation of the structure by gas-phase electron diffraction (GED), ab initio and density functional calculations and IR spectroscopy revealed that the situation in the gas phase is more complicated, with at least four conformers present in appreciable quantities. Infrared spectra indicated a possible SiN interaction in the Si-H stretching region (2000-2200 cm(-1)), as the approach of the nitrogen atom in the five-membered ring weakens the bond to the hydrogen atom in the trans position. Simulated gas-phase IR spectra generated from ab initio calculations (MP2/TZVPP) exhibited good agreement with the experimental spectrum. A method is proposed by which the fraction of the conformer with a five-membered ring can be determined by a least-squares fit of the calculated to experimental absorption intensities. The abundance of this conformer was determined as 23.7(6) %, in good agreement with the GED value of 24(6) %. The equilibrium SiN distance predicted by theory for the gas-phase structure was highly variable, ranging from 2.73 (MP2) to 3.15 A (HF). The value obtained by GED is 2.91(4) A, which could be confirmed by a scan of the potential-energy surface at the DF-LCCSD[T] level of theory. The nature of the weak dative bond in H(3)Si(CH(2))(3)NMe(2) can be described in terms of attractive inter-electronic correlation forces (dispersion) and is also interpreted in terms of the topology of the electron density.

SARACEN – molecular structures from theory and experiment: the best of both worlds

Structures of molecules in the gas phase, determined experimentally, provide definitive information about their identity, reactivity and other properties, free from intermolecular interactions. Available methods have not been applicable to large and asymmetric molecules. Now the SARACEN (Structure Analysis Restrained by Ab initio Calculations for Electron diffractioN) method, using data from computational methods to complement experimental data, has opened the door to full structure determination for all sufficiently volatile molecules.

Solid‐State Structure of a Li/F Carbenoid: Pentafluoroethyllithium

Lithium carbenoids are versatile compounds for synthesis owing to their intriguing ambiphilic behavior. Although this class of compounds has been known for several years, few solid-state structures exist because of their high reactivity and often low thermal stability. Using cryo X-ray techniques, we were now able to elucidate the first solid-state structure of a Li/F alkyl carbenoid, pentafluoroethyllithium (LiC2F5), finally yielding a prototype for investigating structure-reactivity relationships for this class of molecules. The compound forms a diethyl ether-solvated dimer bridged by a rare C-F-Li link. Complementary NMR spectroscopy studies in solution show dynamic processes and indicate rapid exchange of starting material and product. Theoretical investigations help to understand the formation of the observed unusual structural motif.

CH Activation versus Yttrium–Methyl Cation Formation from [Y(AlMe4)3] Induced by Cyclic Polynitrogen Bases: Solvent and Substituent‐Size Effects

The reaction of 1,3,5-triisopropyl-1,3,5-triazacyclohexane (TiPTAC) with [Y(AlMe(4))(3)] resulted in the formation of [(TiPTAC)Y(Me(3)AlCH(2)AlMe(3))(μ-MeAlMe(3))] by C-H activation and methane extrusion. In contrast, the presence of bulkier cyclohexyl groups on the nitrogen atoms in 1,3,5-tricyclohexyl-1,3,5-triazacyclohexane (TCyTAC) led to the formation of the cationic dimethyl complex [(TCyTAC)(2)YMe(2)][AlMe(4)]. The investigations reveal a dependency of the reaction mechanism on the steric bulk of the N-alkyl entity and the solvent employed. In toluene C-H activation was observed in reactions of [Y(AlMe(4))(3)] with 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) and TiPTAC. In THF molecular dimethyl cations, such as [(TCyTAC)(2)YMe(2)][AlMe(4)], [(TMTAC)(2)YMe(2)][AlMe(4)] and [(TiPTAC)(2)YMe(2)][AlMe(4)], could be synthesised by addition of the triazacyclohexane at a later stage. The THF-solvated complex [YMe(2)(thf)(5)][AlMe(4)] could be isolated and represents an intermediate in these reactions. It shows that cationic methyl complexes of the rare-earth metals can be formed by donor-induced cleavage of the rare-earth-metal tetramethylaluminates. The compounds were characterised by single-crystal X-ray diffraction or multinuclear and variable-temperature NMR spectroscopy, as well as elemental analyses. Variable-temperature NMR spectroscopy illustrates the methyl group exchange processes between the cations and anions in solution.

Mechanism of Host–Guest Complex Formation and Identification of Intermediates through NMR Titration and Diffusion NMR Spectroscopy

The formation of host-guest (H-G) complexes between 1,8-bis[(diethylgallanyl)ethynyl]anthracene (H) and the N-heterocycles pyridine and pyrimidine (G) was studied in solution using a combination of NMR titration and diffusion NMR experiments. For the latter, diffusion coefficients of potential host-guest structures in solution were compared with those of tailor-made reference compounds of similar shape (synthesized and characterized by NMR, HRMS, and in part XRD). Highly dynamic behavior was observed in both cases, but with different host-guest species and equilibria. With increasing concentrations of the pyridine guest, the equilibrium H2⇄H2κ(1)-G1⇄HG2 is observed (in the second step a host dimer coordinates one guest molecule); for pyrimidine the equilibrium H2→H1κ(2)-G1⇄HG2 is observed (the formation of a 1:1 aggregate is the second step).

Lewis Base Induced Reductions in Organolanthanide Chemistry

Recent years have seen a remarkable progress in the molecular chemistry of divalent lanthanides, which was previously restricted to just the three ions Eu, Yb, and Sm. Today even compounds of Tm, Dy, 5] Nd, and La [8] are known. Samarium(II) (usually as SmI2) is in widespread use as a powerful reducing agent in synthetic chemistry. It has a relatively strong negative redox potential (Sm/Sm E1/2 = 1.55 V) compared to europium and ytterbium and is not accesible under mild conditions. It should be noted that the potentials for the reduction M/M are generally more negative for organometallic systems than for reductions in aqueous solution and that both solvent and ligand contributions are important in lanthanide redox chemistry. A striking variant of redox reactivity of samarium compounds was found by Evans and Davis when they investigated pentamethylcyclopentadienyl lanthanide compounds. The tris(pentamethylcyclopentadienyl) complex [(C5Me5)3Sm] was previously thought to be non-existent for steric reasons, but it turned out to behave as a strong oneelectron reducing agent analogous to the divalent compound [(C5Me5)2Sm]. The steric demand of the (C5Me5) ligand facilitates reduction to [(C5Me5)2Sm], whereby the third (C5Me5) unit is oxidized and then dimerizes to give (C5Me5)2. The term “sterically induced reduction” (SIR) [14]