Reactivity of Oxygen-Bridged Geminal Al/P and Si/P Frustrated Lewis Pairs towards Heterocumulenes.

The two oxygen-bridged geminal frustrated Lewis pairs (FLP) tBu2 P-O-AlBis2 (Bis=CH(SiMe3 )2 ; 1) and tBu2 P-O-Si(C2 F5 )3 (2) were reacted with the heterocumulenes PhNCO, PhOCN, PhNCS, CS2 and PhNSO as well as SO2 . With isocyanate and cyanate, both 1 and 2, form addition products under formation of five-membered rings. With CS2 , isothiocyanate and sulfinylaniline, only 1 forms stable adducts, whereas 2 shows reactivity towards sulfinylaniline, but the product decomposed after a few minutes. The reaction of 1 with SO2 led to partial cleavage of the P-O-Al and Al-C units, as confirmed by X-ray diffraction studies of a complex aggregate. The reaction of 2 with SO2 affords the 1,2-addition product. All adducts were characterized by means of multinuclear NMR spectroscopy, X-ray crystallography and CHN analyses.

Gas-phase equilibrium molecular structures and ab initio thermochemistry of anthracene and rubrene.

Semi-experimental gas-phase structures of anthracene and rubrene (5,6,11,12-tetraphenyltetracene) were determined by means of gas electron diffraction (GED). The use of the flexible restraints in the refinement of the GED data successfully resolves non-equivalent C-C bond lengths. The tetracene core of an isolated rubrene molecule was found to exhibit a twist distortion of about 18°; this is less than DFT calculations predict (30-40°). The modified Feller-Peterson-Dixon method in conjunction with high-level DLPNO-CCSD(T) calculations was employed to resolve the discrepancy between the available experimental gas-phase enthalpies of formation for rubrene. The theoretical value of meets its recent experimental counterpart (765.6 ± 8.4 kJ mol-1) and is in strong disagreement with the previous estimation (882 kJ mol-1).

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.

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.

para-Chlorotetrafluorophenyl-boranes - syntheses and structures of a series of mono- and bidentate Lewis acids.

Mono- and bidentate tri-coordinated boron compounds bearing the yet unexplored para-chlorotetrafluorophenyl substituent have been synthesized and structurally investigated. The acidity of tris(para-chlorotetraphenyl)borane (7) was shown to exceed that of the widely used tris(pentafluorophenyl)borane (BCF) according to the Gutmann-Beckett method. Acid-base adducts of 7 with diethyl ether and triethylphosphine oxide were crystallized. Dihydrogen splitting at room temperature was observed with a Frustrated Lewis Pair consisting of 7 and 2,2,6,6-tetramethylpiperidine (TMP). The hydrogen adduct was isolated and characterized by NMR, SC-XRD and elemental analysis. Its solid-state structure contains a H⋯H distance on the verge of dihydrogen bonding (2.28(6) Å). Bis(para-chlorotetrafluorophenyl)hydridoborane was prepared in situ and shown to be applicable in hydroboration reactions similarly to Piers' borane.

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.