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Sodium and caesium cation-exchanged mesoporous MCM-41 are shown t o be mild, selective, water-stable and recyclable catalysts for the base-catalysed Knoevenagel condensation and acid-catalysed acetalization and aldol condensation, whereas caesium acetate-impregnated MCM-41 shows strong base activity in the Michael addition.

Starting from CpVCl2(PMe3)2 (1) the paramagnetic mono- and bis(phenylethynyl) complexes CpVCl2 - n (C⋮CPh) n (PMe3)2 [n = 1 (2), 2 (3)] have been synthesized. Both compounds were characterized by X-ray diffraction and were found to be thermally more stable than other CpVIII hydrocarbyl complexes. The CpVIII bis(phenyl) complex CpVPh2(PMe3)2 (5) decomposes at ambient temperature through β-hydrogen abstraction to give the first isolated vanadium benzyne complex, CpV(η2-C6H4)(PMe3)2 (6). The molecular structure of 6 indicates that this compound can best be described as a high-spin d2 vanadium(III) benzometallacyclopropene. The cyclopropene character is expressed in the reactivity of 6, showing insertion of unsaturated substrates. Insertion of diphenylacetylene gives CpV(η2-PhCCPhC6H4)(PMe3)2 (7), with a planar vanadaindene structure, whereas terminal alkenes CH2CRR‘ (R = H, Me; R‘ = H, Me) insert regioselectively to give β-substituted metallaindanes CpV(η2-CH2CRR‘C6H4)PMe3 (R and R‘ = H, 8a; R = H, R‘ = Me, 8b; R and R‘ = Me, 8c), which show a low thermal stability in case one of the β-substituents is a hydrogen. With t-BuCN double insertion followed by rearrangement of the intermediate diazametallacycle gives the isoindolenine-substituted vanadium(III) imido complex CpV[NC(t-Bu)N=C(t-Bu)C6H4](PMe3)2 (9). The benzyne complex 6 reacts with dihydrogen to form the known triple-decker CpV(C6H6)VCp through partial hydrogenation of the benzyne ligand.

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