Site-selective nitrenoid insertions utilizing postfunctionalized bifunctional rhodium(ii) catalysts.

We report a new strategy for the preparation of dirhodium(ii) complexes with the general formula Rh2(A)4 that allows the isolation of a dirhodium tetracarboxylate complex with a free amino group available for postfunctionalization. The postfunctionalization of this complex enables the incorporation of a variety of functional groups, including double and triple bonds as well as nucleophilic moieties, thus paving the way to new classes of polymeric as well as bifunctional catalysts, and polymetallic complexes. Furthermore, we demonstrate that a urea containing dirhodium(ii) complex enables site-selective nitrenoid insertions by remote hydrogen bonding control.

Control of Excited-State Conformations in B,N-Acenes.

We present a new concept to control the conformations of molecules in the excited state through harvesting negative hyperconjugation. The strategy was realized with the 2,3,1,4-benzodiazadiborinane scaffold, which was prepared by a new synthetic procedure. Photochemical studies identified dual light emission, which was assigned to well-defined conformers. The emission at longer wavelength can be switched off by restricting the rotational degrees of freedom in the solid state as well as by controlling the energy levels of the excited states through adjusting the solvent polarity.

An air-stable bisboron complex: a practical bidentate Lewis acid catalyst.

We report an air-stable bisboron complex as an efficient catalyst for the inverse electron-demand Diels-Alder (IEDDA) reaction of 1,2-diazine as well as 1,2,4,5-tetrazine. Its stability towards air and moisture was demonstrated by NMR studies enabling its application in organic transformations without glovebox. A one-pot procedure for its synthesis was developed starting from 1,2-bis(trimethylsilyl)benzene greatly enhancing its practicality. Comparative reactions were carried out to evaluate its catalytic activity in IEDDA reactions of diazine including phthalazine as well as 1,2,4,5-tetrazine.

Directed C-H Bond Oxidation of Bridged Cycloalkanes Catalyzed by Palladium(II) Acetate.

We have developed a synthesis of 1,2-substituted adamantane carboxylic acids and further bridged cycloalkanes (cage compounds) by palladium acetate-catalyzed C-H bond oxidation. Acetoxylation of cycloalkane framework was performed using picolylamide as a directing group. Modification of the substrate, ligand design and variation of reaction conditions enabled us to study the mechanism of acetoxylation of aliphatic compounds. Post-functionalization reactions and cleavage of the directing group were developed. For the first time the synthesis and characterization of a ß-C3 -tri-substituted adamantane derivatives was achieved.

Stable Organic Neutral Diradical via Reversible Coordination.

We report the formation of a stable neutral diboron diradical simply by coordination of an aromatic dinitrogen compound to an ortho-phenyldiborane. This process is reversible upon addition of pyridine. The diradical species is stable above 200 °C. Computations are consistent with an open-shell triplet diradical with a very small open-shell singlet-triplet energy gap that is indicative of the electronic disjointness of the two radical sites. This opens a new way of generating stable radicals with fascinating electronic properties useful for a large variety of applications.

Intramolecular London Dispersion Interaction Effects on Gas-Phase and Solid-State Structures of Diamondoid Dimers.

The covalent diamantyl (C28H38) and oxadiamantyl (C26H34O2) dimers are stabilized by London dispersion attractions between the dimer moieties. Their solid-state and gas-phase structures were studied using a multitechnique approach, including single-crystal X-ray diffraction (XRD), gas-phase electron diffraction (GED), a combined GED/microwave (MW) spectroscopy study, and quantum chemical calculations. The inclusion of medium-range electron correlation as well as the London dispersion energy in density functional theory is essential to reproduce the experimental geometries. The conformational dynamics computed for C26H34O2 agree well with solution NMR data and help in the assignment of the gas-phase MW data to individual diastereomers. Both in the solid state and the gas phase the central C-C bond is of similar length for the diamantyl [XRD, 1.642(2) Å; GED, 1.630(5) Å] and the oxadiamantyl dimers [XRD, 1.643(1) Å; GED, 1.632(9) Å; GED+MW, 1.632(5) Å], despite the presence of two oxygen atoms. Out of a larger series of quantum chemical computations, the best match with the experimental reference data is achieved with the PBEh-3c, PBE0-D3, PBE0, B3PW91-D3, and M06-2X approaches. This is the first gas-phase confirmation that the markedly elongated C-C bond is an intrinsic feature of the molecule and that crystal packing effects have only a minor influence.

London Dispersion Enables the Shortest Intermolecular Hydrocarbon H···H Contact.

Neutron diffraction of tri(3,5-tert-butylphenyl)methane at 20 K reveals an intermolecular C-H···H-C distance of only 1.566(5) Å, which is the shortest reported to date. The compound crystallizes as a C3-symmetric dimer in an unusual head-to-head fashion. Quantum chemical computations of the solid state at the HSE-3c level of theory reproduce the structure and the close contact well (1.555 Å at 0 K) and emphasize the significance of packing effects; the gas-phase dimer structure at the same level shows a 1.634 Å C-H···H-C distance. Intermolecular London dispersion interactions between contacting tert-butyl substituents surrounding the central contact deliver the decisive energetic contributions to enable this remarkable bonding situation.

Triflic Acid Promoted Decarboxylation of Adamantane-oxazolidine-2-one: Access to Chiral Amines and Heterocycles.

We have developed a one-step procedure to a variety of chiral lipophilic and conformationally rigid amines and heterocycles by decarboxylation of adamantane-oxazolidine-2-one. Triflic acid or aluminum triflate promote the addition of diverse nucleophiles to the oxazolidine-2-one moiety accompanied by the release of carbon dioxide. The resulting amine or heterocycle is then protonated/metalated by the catalyst (promotor). Additionally, the starting racemic material, adamantane-oxazolidine-2-one, was resolved into single enantiomers using a chiral auxiliary to access enantio-enriched products and to study the racemization pathway of chiral 1,2-disubstituted adamantane derivatives.

Reactions with oleum under harsh conditions: characterization of the unique [M(S2O7)3]2- ions (M=Si, Ge, Sn) in A2[M(S2O7)3] (A=NH4, Ag).

The reactions of group 14 tetrachlorides MCl(4) (M=Si, Ge, Sn) with oleum (65% SO(3)) at elevated temperatures lead to the unique complex ions [M(S(2)O(7))(3)](2-), which show the central M atoms in coordination with three chelating S(2)O(7)(2-) groups. The mean distances M-O within the anions increase from 175.6(2)-177.5(2) pm (M=Si) to 186.4(4)-187.7(4) pm (M=Ge) to 201.9(2)-203.5(2) pm (M=Sn). These distances are reproduced well by DFT calculations. The same calculations show an increasing positive charge for the central M atom in the row Si, Ge, Sn, which can be interpreted as the decreasing covalency of the M-O bonds. For the silicon compound (NH(4))(2)[Si(S(2)O(7))(3)], (29)Si solid-state NMR measurements have been performed, with the results showing a signal at -215.5 ppm for (NH(4))(2)[Si(S(2)O(7))(3)], which is in very good agreement with theoretical estimations. In addition, the vibrational modes within the [MO(6)] skeleton have been monitored by Raman spectroscopy for selected examples, and are well reproduced by theory. The charge balance for the [M(S(2)O(7))(3)](2-) ions is achieved by monovalent A(+) counter ions (A=NH(4), Ag), which are implemented in the syntheses in the form of their sulfates. The sizes of the A(+) ions, that is, their coordination requirements, cause the crystallographic differences in the crystal structures, although the complex [M(S(2)O(7))(3)](2-) ions remain essentially unaffected with the different A(+) ions. Furthermore, the nature of the A(+) ions influences the thermal behavior of the compounds, which has been monitored for selected examples by thermogravimetric differential thermal analysis (DTA/TG) and XRD measurements.

The unprecedented tetrakis-(disulfato)-silicate anion [Si(S2O7)4]4-, its germanium congener [Ge(S2O7)4]4-, and the tris-(disulfato)-metallates [M(S2O7)3]2- (M = Si, Ge, Ti), stabilized by divalent counter cations B2+ (B = Sr, Ba, Pb).

The reaction of oleum (65% SO(3)) with the tetrachlorides of silicon, germanium, and titanium, respectively, led to the complex disulfates Sr(2)[M(S(2)O(7))(4)] (M = Si, Ge), Ba[M(S(2)O(7))(3)] (M = Si, Ge, Ti) and Pb[M(S(2)O(7))(3)] (M = Ge, Ti) if strontium, barium, and lead were used as divalent counter cations. The strontium compounds exhibit the unique tetrakis-(disulfato)-metallate anions [M(S(2)O(7))(4)](4-) with the silicon and germanium atoms in octahedral coordination of two chelating and two monodentate disulfate groups. All of the other compounds display tris-(disulfato)-metallate anions [M(S(2)O(7))(3)](2-) with three chelating disulfate groups surrounding the M atoms. Thermoanalytical investigations on the germanium compounds Sr(2)[Ge(S(2)O(7))(4)] and Ba[Ge(S(2)O(7))(3)] revealed their decomposition in multi-step processes leading to a mixture of BSO(4) and BGe(4)O(9) (B = Sr, Ba), while the thermal degradation of Pb[Ti(S(2)O(7))(3)] yields PbTiO(3). For selected examples, IR data are additionally presented.

The elusive tetrasulfate anion [S4O13](2-).

Sulfates united: the unique tetrasulfate S(4)O(13)(2-) anion was observed in the structure of (NO(2))(2)[S(4)O(13)] that forms in the reaction of N(2)O(5) with SO(3). Theoretical investigations show that the anion is a stable member of the polysulfate series [S(n)O(3n+1)](2-), which was investigated up to n=11.

The unique bis-(disulfato)-aurate anion [Au(S2O7)2](-): synthesis and characterization of Li[Au(S2O7)2] and Na[Au(S2O7)2)].

The reaction of Au(OH)(3) and oleum (65% SO(3)) in the presence of M(2)SO(4) (M = Li, Na) afforded yellow single crystals of Li[Au(S(2)O(7))(2)] (triclinic, P ̅1, Z = 1, a = 532.20(3), b = 649.69(4), c = 836.72(5) pm, α = 107.982(2)°, ß = 90.171(2)°, γ = 102.583(2)°, V = 267.80(3) Å(3)) and Na[Au(S(2)O(7))(2)] (monoclinic, P2(1)/n, Z = 2, a = 533.31(3), b = 1193.38(7), c = 907.67(5) pm, ß = 98.548(3)°, V = 571.26(6) Å(3)). Both compounds exhibit the unprecedented [Au(S(2)O(7))(2)](-) anion in which a square planar coordination of the central gold atom is achieved by the chelating attachment of two disulfate groups. The disulfates were characterized by means of IR spectroscopy and DTA/TG measurements. For both compounds, the decomposition occurs via several steps and is finished at about 450 °C at the stage of elemental gold and the sulfates M(2)SO(4) (M = Li, Na), as revealed by X-ray powder diffraction of the residues.