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.

Carbon dioxide reduction by an Al-O-P frustrated Lewis pair.

The reaction of tBu2P(O)H with Bis2AlH (Bis = CH(SiMe3)2) afforded the adduct tBu2P(H)-O-Al(H)Bis2 (3). It slowly releases H2 to form the first oxygen-bridged geminal Al/P frustrated Lewis pair tBu2P-O-AlBis2. It is capable of reversibly binding molecular hydrogen to afford 3, shown by NMR and H/D scrambling experiments, and forms a 1,2-adduct with CO2. Importantly, the H2 adduct 3 reduces CO2 in a stoichiometric reaction leading to the formic acid adduct tBu2P(H)-O-Al(CO2H)Bis2. The formation of the different species was explored by density functional theory calculations which provide support for the experimental results. All products were characterized by NMR spectroscopy as well as X-ray diffraction experiments and elemental analyses.

Synthesis of bi- and tetradentate poly-Lewis acids by hydroalumination of poly-alkynyl-anthracene derivatives.

Photo-dimers of 1,5-diethynylanthracene and 1,5-bis[(trimethylsilyl)ethynyl]anthracene, obtained by UV radiation of the monomers, were treated with the bulky aluminium hydride [(SiMe3)2HC]2AlH in hydroalumination reactions. Hydroalumination of the tetraethynyl-substituted anthracene photo-dimers (syn and anti) led to fourfold aluminium-functionalized Janus-like products with two aluminium functions on each side oriented towards the same direction. The reactions of the monomeric anthracene derivatives with [(SiMe3)2HC]2AlH afforded semi-flexible bidentate Lewis acids, which are interesting building blocks for molecular chains bearing multiple Lewis-acidic functions. The aluminium-functionalized anti-photo-dimer was treated with various donor molecules to gain insight into its host-guest chemistry. All poly-Lewis acids and their adducts were characterized by X-ray diffraction experiments, multinuclear NMR spectroscopy and by elemental analyses.

Bidentate Lewis Acids Derived from o-Diethynylbenzene with Group 13 and 14 Functions.

Starting from 1,2-diethynylbenzene, a series of bidentate Lewis acids was prepared by means of hydrometalations, in particular hydrosilylation, hydroboration, hydroalumination and terminal metalation based on group 13 and 14 elements. In the case of terminal alkyne metalation, the Lewis-acidic gallium function was introduced using triethylgallium under alkane elimination. A total of six different Lewis acids based on a semiflexible organic scaffold were prepared, bearing -SiClMe2 , -SiCl2 Me, -SiCl3 , -B(C6 F5 )2 , -AlBis2 (Bis=bis(trimethylsilyl)methyl) and -GaEt2 as the corresponding functional units. In all cases, the Lewis acid functionalisation was carried out twice and the products were obtained in good to excellent yields. In the case of the twofold gallium Lewis acid, a different structural motif in the form of a polymer-like chain was observed in the solid state. All new bidentate Lewis acids were characterised by multinuclear NMR spectroscopy, CHN analysis and X-ray diffraction experiments.

A P-H functionalized Al/P-based frustrated Lewis pair - hydrophosphination of nitriles, ring opening with cyclopropenones and evidence of P[double bond, length as m-dash]C double bond formation.

Hydroalumination of R-P(H)-C[triple bond, length as m-dash]C-tBu with bulky H-Al[CH(SiMe3)2]2 afforded the new P-H functionalized Al/P-based frustrated Lewis pair R-P(H)-C[[double bond, length as m-dash]C(H)-tBu]-AlR2 [R = CH(SiMe3)2; FLP 7]. A weak adduct of 7 with benzonitrile (8) was detected by NMR spectroscopy, but could not be isolated. tert-Butyl isocyanide afforded a similar, but isolable adduct (9), in which the isocyanide C atom was coordinated to aluminium. The unique reactivity of 7 became evident from its reactions with the heteroatom substituted nitriles PhO-C[triple bond, length as m-dash]N, PhCH2S-C[triple bond, length as m-dash]N and H8C4N-C[triple bond, length as m-dash]N. Hydrophosphination of the C[triple bond, length as m-dash]N triple bonds afforded imines at room temperature which were coordinated to the FLP by Al-N and P-C bonds to yield AlCPCN heterocycles (10 to 12). These processes depend on substrate activation by the FLP. Diphenylcyclopropenone and its sulphur derivative reacted with 7 by addition of the P-H bond to a C-C bond of the strained C3 ring and ring opening to afford the fragment (Z)-Ph-C(H)[double bond, length as m-dash]C(Ph)-C-X-Al (X = O, S). The C-O or C-S groups were coordinated to the FLP to yield AlCPCX heterocycles (13 and 14). The thiocarbonyl derived compound 14 contains an internally stabilized phosphenium cation with a localized P[double bond, length as m-dash]C bond, a trigonal planar coordinated P atom and a short P[double bond, length as m-dash]C distance (168.9 pm). Insight into formation mechanisms, the structural and energetic properties of FLP 7 and compounds 13 and 14 was gained by quantum chemical DFT calculations.