Zwitterionic Ammoniumalkenyl Ligands in Metal Cluster Complexes. Synthesis, Structures, and Transformations of Zwitterionic Trimethylammoniumalkenyl Ligands in Hexaruthenium Carbido Carbonyl Complexes.

Ru6(μ6-C)(CO)17, 1, has been shown to react with C2H2 when activated by Me3NO to yield the complexes Ru6C(CO)15(μ-η2-C2H2NMe3), 2, and Ru6C(CO)14(μ3-η4-C4H4NMe3), 3, containing a bridging 2-trimethylammoniumethenyl (C2H2NMe3) ligand and a triply bridging 4-trimethylammoniumbutadienyl (C4H4NMe3) ligand, respectively. Complexes 2 and 3 are formally zwitterionic by virtue of the positive charge on the nitrogen atom and a negative charge that must be assigned formally to the Ru6 cluster. Compound 2 added CO at room temperature to yield the compound Ru6C(CO)16(η1-E-C2H2NMe3), 4, that contains a terminally coordinated zwitterionic 2-trimethylammoniumethenyl ligand. Compounds 2 and 3 eliminated the NMe3 grouping, reversibly, when heated to yield the ethyne cluster complex Ru6C(CO)15(μ3-C2H2), 5, and the butadiendiyl cluster complex Ru6C(CO)14(μ3-η4-C4H4), 6, respectively. Compound 3 was obtained from 2 by addition of C2H2 in the presence of Me3NO. Reaction of 1 with methyl propiolate, HC2CO2Me, yielded the CO2CH3 substituted zwitterionic complex Ru6C(CO)16[η1-E-(C(CO2Me)═C(H)NMe3], 7, which contains the terminally coordinated zwitterionic ligand, C(CO2Me)═C(H)NMe3. Compound 7 eliminated NMe3 and CO when heated to 83 °C to yield the methoxycarbonyl alkyne complex Ru6C(CO)15[μ3-HC2(CO2Me)], 8. All new products, 2-8, were characterized structurally by single-crystal X-ray diffraction analyses.