Structure and photochemistry of di-tert-butyldiphosphatetrahedrane.

Di-tert-butyldiphosphatetrahedrane (tBuCP)2 (1) is a mixed carbon- and phosphorus-based tetrahedral molecule, isolobal to white phosphorus (P4). However, despite the fundamental significance and well-explored reactivity of the latter molecule, the precise structure of the free (tBuCP)2 molecule (1) and a detailed analysis of its electronic properties have remained elusive. Here, single-crystal X-ray structure determination of 1 at low temperature confirms the tetrahedral structure. Furthermore, quantum chemical calculations confirm that 1 is isolobal to P4 and shows a strong largely isotropic diamagnetic response in the magnetic field and thus pronounced spherical aromaticity. A spectroscopic and computational study on the photochemical reactivity reveals that diphosphatetrahedrane 1 readily dimerises to the ladderane-type phosphaalkyne tetramer (tBuCP)4 (2) under irradiation with UV light. With sufficient thermal activation energy, the dimerisation proceeds also in the dark. In both cases, an isomerisation to a 1,2-diphosphacyclobutadiene 1' is the first step. This intermediate subsequently undergoes a [2 + 2] cycloaddition with a second 1,2-diphosphacyclobutadiene molecule to form 2. The 1,2-diphosphacyclobutadiene intermediate 1' can be trapped chemically by N-methylmaleimide as an alternative [2 + 2] cycloaddition partner.

ortho-Phenylene-bridged phosphorus/silicon Lewis pairs.

A series of five ortho-phenylene-bridged phosphorus-silicon Lewis pairs was synthesized, with phosphorus bearing isopropyl groups while the substituents at the silicon atom vary (-CH3, -Cl, -F). Possible interactions between Lewis acid and base were investigated both experimentally (NMR, XRD) and theoretically to determine the influence of the different substituents. Calculated ortho-interaction energies (OIEs) show a stabilizing interactions between the acidic and basic units which were also found for the meta- and para-interaction energies (MIEs and PIEs, respectively), indicating stabilization resulting not from direct acid-base interaction but from electronic interactions through the ring. Further spectroscopic (NMR, XRD) and theoretical (NBO, QTAIM, SAPT) investigations confirmed the absence of direct interactions between silicon and phoshorus.

Frustrated Lewis pair chemistry of hydride sponges.

An improved synthesis for 1,8-bis(dimethylboranyl)naphthalene (1, hydride sponge) was developed avoiding the use of tin(IV) reagents. The related 1,2-bis(dimethylboranyl)benzene (2) was prepared. 1 combined with 1,8-bis(dimethylamino)naphthalene (3, proton sponge) is a Frustrated Lewis pair (FLP) that forms adducts [1-EHn-1][3-H] with the protic compounds EHn = H2O, NH3, H2S, PH3, H2Se, HCN. Their structures show the chelation of the deprotonated substrates, except for CN- (binds to one B atom of 1). The mechanisms of formation of [1-EHn-1][3-H] were explored by NMR spectroscopy. Similar reactions took places for the FLP system 2 + 3, but the adducts [2-EHn-1][3-H] are less stable; only the PH3 adduct was isolated and AsH3 forms a chelated adduct [2-NC(CD3)AsH2][3-H]. FLP 1 + 3 does not react with molecular hydrogen, but the formal adduct [1-H][3-H] was isolated via salt metathesis from K[1-H] and [3-H]Cl; it is stable towards water, atmospheric oxygen and up to 100 °C; its thermal decomposition proceeds without formation of H2. The stabilities of both, the mixture 1 + 3 + H2 and the formal adduct [1-H][3-H], allow concluding that hydrogen activation of FLP 1 + 3 is kinetically prevented.

Synthesis, structural and photophysical properties of dimethylphosphino(perfluoro-)phenylene-based gold(I) dimers.

Starting with 1,2-dibromobenzene and 1,2,3,4-tetrafluorobenzene, dimethyl(2-(trimethylstannyl)phenyl)phosphane, Me2P(o-C6H4)SnMe3, and dimethyl-[2,3,4,5-tetrafluoro-6-(trimethylstannyl)phenyl]phosphane, Me2P(o-C6F4)SnMe3, were synthesized and used in tin-gold exchange reactions to prepare two gold(I) dimers, bis[(2-dimethylphosphino)phenyl]di-gold(I), [Au2(µ-2-C6H4PMe2)2], and bis[(2-dimethylphosphino)-3,4,5,6-tetrafluorophenyl]di-gold(I), [Au2(µ-2-C6F4PMe2)2], respectively. Both tin precursor molecules, as well as the gold(I) complexes, were characterized by multinuclear NMR spectroscopy, CHN analysis and X-ray diffraction experiments. Both gold(I) dimers were further investigated by the means of computational as well as photophysical methods.

Monitoring dynamic pre-crystallization aggregation processes in solution by VT-DOSY-NMR spectroscopy.

Equimolar mixtures of pyridine (Py) with para-halotetrafluoropyridine (BrTFP and ITFP) were investigated by VT-diffusion NMR experiments. The formation of a halogen-bond-stabilized ITFP·Py complex was detected upon cooling a solution in methylcyclohexane-d14 to 260 K; this allows monitoring a halogen-bond-driven aggregation process preceding crystallization in solution.

The Structure of Bis(catecholato)silanes: Phase Adaptation by Dynamic Covalent Chemistry of the Si-O Bond.

Catechols occupy a unique role in the structural, bio-, and geochemistry of silicon. Although a wealth of knowledge exists on their hypercoordinate complexes, the structure of tetracoordinate bis(catecholato)silane, Si(catH)2 1, has been enigmatic since its first report in 1951. Indeed, the claim of a planar-tetracoordinated silicon in 1 triggered a prominent debate, which is unsettled to this day. Herewith, we present a comprehensive structural study on 1 and derivatives in the gas phase by electron diffraction, in a neon matrix by IR spectroscopy, in solution by diffusion NMR spectroscopy, and in the solid-state by X-ray diffraction and MAS NMR spectroscopy, complemented by high-level quantum-chemical computations. The compound exhibits unprecedented phase adaptation. In the gas phase, the monomeric bis(catecholato)silane is tetrahedral, but in the condensed phase, it is metastable toward oligomerization up to a degree controllable by the type of catechol, temperature, and concentration. For the first time, spectroscopic evidence is obtained for a rapid Si-O σ-bond metathesis reaction. Hence, this study sorts out a long-lasting debate and confirms dynamic covalent features for our Earth's crust's most abundant chemical bond.

A Zwitterionic Phosphonium Stannate(II) via Hydrogen Splitting by a Sn/P Frustrated Lewis-Pair and Reductive Elimination.

The reactivity of the frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (1) was investigated with respect to the activation of elemental hydrogen. The reaction of 1 at elevated hydrogen pressure afforded the intramolecular phosphonium stannate(II) (F5 C2 )2 SnCH2 PH(tBu)2 (3). It was characterized by means of multinuclear NMR spectroscopy and single crystal X-ray diffraction. NMR experiments with the two isotopologues H2 and D2 showed it to be formed via an H2 adduct (F5 C2 )3 HSnCH2 PH(tBu)2 (2) and the subsequent formal reductive elimination of pentafluoroethane; this is supported by DFT calculations. Parahydrogen-induced polarization experiments revealed the formation of a second product of the reaction of 1 with H2 , [HP(tBu)2 Me][Sn(C2 F5 )3 ] (4), in 1 H NMR spectra, whereas 2 was not detected due to its transient nature.

Bromination Mechanism of closo-1,2-C2 B10 H12 and the Structure of the Resulting 9-Br-closo-1,2-C2 B10 H11 Determined by Gas Electron Diffraction.

9-Br-closo-1,2-C2 B10 H11 has been prepared and its gas-phase structure has been examined by means of gas electron diffraction. The structure of the carbaborane core is similar to the structure of the parent compound, which is of C2v symmetry. A DFT-based search for the corresponding reaction pathway of the bromination of closo-1,2-C2 B10 H12 revealed that the catalytic amount of aluminum reduces the barrier of the initial attack of the bromination agent toward the negatively charged part of the icosahedral carbaborane, i. e., the first transition state, from about 40 to about 27 kcalmol-1 . The Br-Br bond is weakened by an intermediate binding to the large π-hole on the aluminum atom of AlBr3 , which is the driving force for the AlBr3 -catalyzed bromination.

Spectroscopic Properties, Conformation and Structure of Difluorothiophosphoryl Isocyanate in the Gaseous and Solid Phase.

Difluorothiophosphoryl isocyanate, F2P(S)NCO was characterized with UV/vis, NMR, IR (gas and Ar-matrix), and Raman (liquid) spectroscopy. Its molecular structure was also established by means of gas electron diffraction (GED) and single crystal X-ray diffraction (XRD) in the gas phase and solid state, respectively. The analysis of the spectroscopic data and molecular structures is complemented by extensive quantum-chemical calculations. Theoretically, the Cs symmetric syn-conformer is predicted to be the most stable conformation. Rotation about the P-N bond requires about 9 kJ mol-1 and the predicted existence of an anti-conformer is dependent on the quantum-chemical method used. This syn-orientation of the isocyanate group is the only one found in the gas phase and contained likewise in the crystal. The overall molecular structure is very similar in gas and solid, despite in the solid state the molecules arrange through intramolecular O⋅⋅⋅F contacts into layers, which are further interconnected by S⋅⋅⋅N, S⋅⋅⋅C and C⋅⋅⋅F contacts. Additionally, the photodecomposition of F2P(S)NCO to form CO, F2P(S)N, and F2PNCO is observed in the solid Ar-matrix.

Structures and Properties of trans-1,3,3,3-Tetrafluoro- propene (HFO-1234ze) and 2,3,3,3-Tetrafluoropropene (HFO-1234yf) Refrigerants.

The refrigerant trans-1,3,3,3-tetrafluoropropene (HFO-1234ze) is used as a replacement for former cooling agents that have been phased-out due to their global warming potential or ozone depleting potential. Although it is used on a large scale, only a few vibrational data and no structural data of HFO-1234ze are known. We report structure determinations based on low-temperature single-crystal X-ray diffraction data as well as gas-phase diffraction data of HFO-1234ze and HFO-1234yf (2,3,3,3-tetrafluoropropene). Furthermore, vibrational spectra of HFO-1234ze in all phases are described. The results are discussed together with quantum-chemical calculations on the PBE0/cc-pVTZ level of theory. Combustion experiments of HFO-1234ze show carbonyl difluoride, carbon dioxide and hydrogen fluoride to be the main combustion products.

London dispersion-driven hetero-aryl-aryl interactions in 1,2-diaryldisilanes.

Non-covalent aryl-aryl interactions for the molecular structures of three 1,2-diaryltetramethyldisilanes X5C6-(SiMe2)2-C6Y5 (X ≠ Y; X, Y = H, F, Cl) were studied by single crystal X-ray and gas electron diffraction as well as by SAPT calculations. Aryl-aryl interactions cause all to adopt exclusively rather untypical eclipsed syn-conformers in the gas phase, and C6F5-(SiMe2)2-C6Cl5 also in the solid state.

Aryl-Aryl Interactions in (Aryl-Perhalogenated) 1,2-Diaryldisilanes.

Three 1,2-diaryltetramethyldisilanes X5 C6 -(SiMe2 )2 -C6 X5 with two C6 H5 , C6 F5 , or C6 Cl5 groups were studied concerning the importance of London dispersion driven interactions between their aryl groups. They were prepared from 1,2-dichlorotetramethyldisilane by salt elimination. Their structures were determined in the solid state by X-ray diffraction and for free molecules by gas electron-diffraction. The solid-state structures of the fluorinated and chlorinated derivatives are dominated by aryl-aryl interactions. Unexpectedly, Cl5 C6 -(SiMe2 )2 -C6 Cl5 exists exclusively as an eclipsed syn-conformer in the gas phase with strongly distorted Si-C6 Cl5 units due to strong intramolecular interactions. In contrast, F5 C6 -(SiMe2 )2 -C6 F5 reveals weaker interactions. The contributions to the total interaction energy were analyzed by SAPT calculations.

Solid-State and Gas-Phase Structures and Energetic Properties of the Dangerous Methyl and Fluoromethyl Nitrates.

An improved synthesis of the simplest nitric acid ester, methyl nitrate, and a new synthesis of fluoromethyl nitrate use the metathesis of the corresponding iodomethanes with silver nitrate. Both compounds were identified by spectroscopy and the structures determined for in situ grown crystals by X-ray diffraction as well as in the gas phase by electron diffraction. Fluorination leads to structures with shorter C-O and N-O bonds, has an energetically destabilizing effect and increases friction sensitivity, but decreases detonation performance.

Halogen Bonds of Halotetrafluoropyridines in Crystals and Co-crystals with Benzene and Pyridine.

The structures of the three para-substituted halotetrafluoropyridines with chlorine, bromine, and iodine have been determined in the solid state (X-ray diffraction). The structures of these compounds and that of pentafluoropyridine were also determined in the gas phase (electron diffraction). Structures in the solid state of the bromine and iodine derivatives exhibit halogen bonding as a structure-determining motif. On the way to an investigation of halogen bond formation of halotetrafluoropyridines in the solid state with the stronger Lewis base pyridine, co-crystals of benzene adducts were investigated to gain an understanding of the influence of aryl-aryl interactions. These co-crystals showed halogen bonding only for the two heavier halotetrafluoropyridines. In the pyridine co-crystals halogen bonding was observed for all three para-halotetrafluoropyridines. The formation of homodimers and heterodimers with pyridine is also supported by quantum-chemical calculations of electron density topologies and natural bond orbitals.

Atomically Precise Expansion of Unsaturated Silicon Clusters.

Small- to medium-sized clusters occur in various areas of chemistry, for example, as active species of heterogeneous catalysis or as transient intermediates during chemical vapor deposition. The manipulation of stable representatives is mostly limited to the stabilizing ligand periphery, virtually excluding the systematic variation of the property-determining cluster scaffold. We now report the deliberate expansion of a stable unsaturated silicon cluster from six to seven and finally eight vertices. The consecutive application of lithium/naphthalene as the reducing agent and decamethylsilicocene as the electrophilic source of silicon results in the expansion of the core by precisely one atom with the potential of infinite repetition.

Preparation and Properties of Chlorosulfuryl Chloroformate, ClC(O)OSO2Cl.

The novel chlorosulfuryl chloroformate, ClC(O)OSO2Cl, was prepared by the reaction of CCl4 and SO3. Alternatively, the compound was obtained from the direct insertion reaction of SO3 to Cl2C═O. The latter reaction constitutes also a confirmation of the proposed mechanism for the former one. Density functional theory and MP2 theoretical approximations predict the existence of two conformers, syn-gauche and syn-anti, depending on the value adopted by the dihedral angles ϕ(Cl-C-O-S) and ϕ(C-O-S-Cl). The structure of the syn-gauche conformer was determined by gas-phase electron diffraction (GED). The existence of the syn-anti conformer can be neither confirmed nor excluded by the GED experiment. Vibrational spectra (vapor-phase and argon-matrix Fourier transform infrared and liquid Fourier transform Raman) were interpreted by an equilibrium mixture between both conformers.

The perfluorinated alcohols c-C6F11OH, c-C6F10-1,1-(OH)2 and c-C6F10-1-(CF3)OH.

The thermally unstable α-fluoroalcohol undecafluorocyclohexanol (c-C6F11OH) was prepared by addition of hydrogen fluoride to the corresponding ketone. c-C6F10(CF3)OH was obtained by protonation of its alkoxide [NMe4]+[C7F13O]-. Decafluorocyclohexane-1,1-diol (c-C6F10(OH)2) was prepared by acidic workup of the corresponding alkoxide [NMe4]+[C6F11O]- with sulfuric acid, which yielded (c-C6F10(OH)2) and fluorosulfonic acid. The structures of c-C6F10(CF3)OH·2H2O and of (c-C6F10(OH)2) were elucidated by single-crystal X-ray and gas-phase electron-diffraction studies.

Disulfuryl Dichloride ClSO2 OSO2 Cl: A Conformation and Polymorphism Chameleon.

Disulfuryl dichloride ClSO2 OSO2 Cl was characterized by vibrational spectroscopy in the gaseous and liquid phase as well as in the Ar-matrix. By varying the temperature, certain bands could be assigned to several conformers. Gas-phase electron diffraction revealed a dominance of the anti-conformer at ambient temperature. The same conformation was found in the solid state. Via the in situ technique for crystallization, not less than four different modifications were identified. Among these different modifications, the structural parameters of the molecules remain relatively constant, but the aggregation pattern changes. Although the molecules aggregate by chlorine⋅⋅⋅oxygen contacts in each modification, the geometrical parameters of these interaction show significant differences and were evaluated and are in part inconsistent with the halogen bonding concept.

Tris(perfluorotolyl)borane-A Boron Lewis Superacid.

Tris[tetrafluoro-4-(trifluoromethyl)phenyl]borane (BTolF) was prepared by treating boron tribromide with tetrameric F3 CC6 F4 -CuI . The F3 CC6 F4 -CuI was generated from F3 CC6 F4 MgBr and copper(I) bromide. Lewis acidities of BTolF evaluated by the Gutmann-Beckett method and calculated fluoride-ion affinities are 9 and 10 %, respectively, higher than that of tris(pentafluorophenyl)borane (BCF) and even higher than that of SbF5 . The molecular structures of BTolF and BCF were determined by gas-phase electron diffraction, that of BTolF also by single-crystal X-ray diffraction.

Tetranitromethane: A Nightmare of Molecular Flexibility in the Gaseous and Solid States.

After numerous attempts over the last seven decades to obtain a structure for the simple, highly symmetric molecule tetranitromethane (C(NO2 )4 , TNM) that is consistent with results from diffraction experiments and spectroscopic analysis, the structure has now been determined in the gas phase and the solid state. For the gas phase, a new approach based on a four-dimensional dynamic model for describing the correlated torsional dynamics of the four C-NO2 units was necessary to describe the experimental gas-phase electron diffraction intensities. A model describing a highly disordered high-temperature crystalline phase was also established, and the structure of an ordered low-temperature phase was determined by X-ray diffraction. TNM is a prime example of molecular flexibility, bringing structural methods to the limits of their applicability.