A tricyclic phosphine has been generated that has a rigid molecular backbone with the P atoms exclusively bound to C(sp ) atoms as well as a very large Tolman angle and buried volume. It is an interesting new ligand in coordination chemistry (Au, Pd complexes) and shows unusual insertion reactions into its endocyclic P-C bonds facilitated by its inherent molecular strain.

Reaction of the Al/P-based frustrated Lewis pair (FLP) Mes 2 P-C(CH-C 6 H 5 )-Al(CMe 3 ) 2 1 with heavier chalcogens (E = S, Se and Te) yielded by oxidation of the P atoms the respective phosphorus(V) compounds Mes 2 P(E)-C(CH-C 6 H 5 )-Al(CMe 3 ) 2 (2a, E =S, 2b, E = Se, 2c, E = Te) in good yield. The chalcogen atoms were coordinated to the Lewis-acidic Al atoms, which, in the case of the dark red Te compound 2c, resulted in a stabilization of the P−Te bond. Unique fourmembered, slightly puckered P-C-Al-E heterocycles were formed with P−E bond lengths in the normal range of terminal chalcogen atoms and comparatively long Al−E bonds, which are consistent with relatively weak Al−E interactions. Both the Se and more pronounced the Te compound formed dimers in the solid state as a result of closed-shell chalcogen−chalcogen interactions. While the Se···Se distance was only slightly shorter than the sum of the van der Waals radii (3.8 Å), the Te···Te separation (3.33 Å) was relatively short and in the characteristic range of significant intermolecular Te···Te interactions, which may result from a double donor−acceptor interaction. Quantum chemical calculations suggested a Te−Te bond energy of about −16.5 kcal/mol. The corresponding oxygen derivative could not be isolated in a pure form, but it may be formed by thermal decomposition of the new FLP adduct 1·ONMe 3 .

The Al/P-based frustrated Lewis pair (FLP) Mes P-C(AltBu) =C(H)Ph (1; Mes=mesityl) reacted as an efficient two-electron reductant with benzil to afford a cis-enediolate that was coordinated to the FLP through P-O and Al-O bonds and the formation of a seven-membered heterocycle (2). The phosphorus atom is oxidised from +III to +V. Similar heterocycles (3 a to 3 f) were formed if 1 was treated with various enones (acrolein, acrylate, acrylamide). The resulting enolates are bound to the FLP through P-C and Al-O bonds. Cyclopropenone gave an adduct (4) with the C=O bond coordinated by P and Al. Ynones gave a fascinating variety of different structures. 1,3-Diphenylprop-2-yn-1-one afforded a remarkable allene-type moiety with two cumulated C=C bonds (5); 3-hexyn-2-one yielded a ligand with two conjugated C=C bonds by C-H bond activation at the carbonyl methyl group (7); and 4-(trimethylsilyl)-3-butyn-2-one reacted by C-H bond cleavage, formation of an enolate group with a terminal C=C bond, and shift of the proton to the P atom (8). The C≡C bond was not affected. Allene compound 5 rearranged at elevated temperature and in daylight through the formation of a tricyclic compound by C-H bond activation and C-C bond formation. DFT calculations on this unusual rearrangement suggest insertion of the central allene C atom into the C-H bond of a methyl group and the intermediate formation of a C ring.

Treatment of the Al/P-based frustrated Lewis pair (FLP) Mes2P–C(AltBu2)=C(H)-Ph (1) with benzophenone afforded the simple 1:1 adduct (4) with a O=CPh2 molecule coordinated to the aluminum atom by an Al←O donor-acceptor bond. Steric repulsion may prevent an interaction between the electrophilic carbonyl carbon atom and the Lewis-basic phosphorus atom. 4 is unstable in solution at room temperature, the coordination to aluminium increases the polarisation of the carbonyl group and favours its reduction. As suggested by quantum chemical calculations, a C–H bond of a tBu group approaches the electrophilic center and facilitates β-hydride elimination with the release of isobutene and the formation of an Al–OCHPh2 ligation. An intact O=CPh2 molecule completes the coordination sphere of the metal atom (5). The second tBu group at aluminium reacts similarly by the selective formation of an Al(OCHPh2)2 moiety (6). The thermodynamics of adduct formation and the mechanism of the hydride shift have been evaluated by quantum chemical DFT calculations.

The geometrically constrained tricyclic phosphine 3b has two annulated five-membered PC4 heterocycles with the P atom in a bridgehead position. Treatment with (THF)­M­(CO)5 (M = Cr, Mo, and W) afforded M­(CO)5 complexes 4–6, of which the Cr and Mo compounds showed in heat the unusual insertion of a carbonyl C atom into a P–C bond of one of the strained PC4 rings. Ring expansion yielded a six-membered PC5 heterocycle with an exocyclic CO group and afforded new phosphine ligand 8 that contains annulated five- (1×) and six-membered (2×) rings. Its Mo complex (7) was obtained by treatment of starting phosphine 3b with excess Mo­(CO)6 in hot toluene. Phosphine 8 was removed from the metal atom by addition of the diphosphine Ph2PC2H4PPh2. Treatment of 8 with AlBr3 resulted in enolization and an increased nucleophilicity of the β-C atom, which causes dimerization by P–C bond formation to yield a barrelane type molecule (9).