Reactions of Tetracyclone Molybdenum Complexes with Electrophilic Alkynes: Cyclopentadienone-Alkyne Coupling and Alkyne Coordination

The reactions of the complexes [Mo(CO)2(η4-C4Ph4CO)2] and [Mo(CO)3(NCMe)(η4-C4Ph4CO)] with the alkynes dimethyl acetylenedicarboxylate (DMAD; RC≡CR where R = CO2Me) and methyl propiolate (RC≡CH) have been studied. In the case of DMAD, the initial product is the green carbonyl complex [Mo(CO)(RC≡CR)(η5,σ-C4Ph4COCR═CR)] (3), in which two alkyne molecules have been incorporated: one is linked to the carbonyl group of the tetracyclone ligand, whereas the other is π-bound to the metal as a four-electron donor. Oxidation of this compound affords yellow [Mo(O)(RC≡CR)(η5,σ-C4Ph4COCR═CR)] (8). When the π-acceptor carbonyl ligand is replaced by the π-donor oxo group, the alkyne ligand changes orientation: it lies parallel to the Mo–CO bond in 3 but perpendicular to the Mo═O group in 8. Analogous complexes (9, 10) were isolated in the case of methyl propiolate; each exists as a mixture of two isomers depending on the orientation of the unsymmetrical alkyne ligand.