Ruthenacarboranes from the reaction of nido-1,2-(Cp*RuH)2B3H7 with HC [triple bond] CCO2Me, Cp* = eta5-C5Me5. Hydrometalation, alkyne incorporation, and functional group modification via cooperative metal-boron interactions within a metallaborane cluster framework

Reaction of nido-1,2-(Cp*RuH)2B3H7, 1, and methyl acetylene monocarboxylate under kinetic control generates nido-1,2-(Cp*Ru)2(mu-C[[CO2Me]Me])B3H7 (a pair of geometric isomers, 3 and 5) and nido-1,2-(Cp*Ru)2(1,3-mu-C[[CH2CO2Me]H])B3H7, 4, which display the first examples of exo-cluster mu-alkylidene Ru-B bridges generated by hydrometalation of an alkyne on the cluster framework. Both 3 and 5, but not 4, rearrange into arachno-2,8-mu(C)-5-eta1(O)-Me[CO2Me]C-1,2-(Cp*Ru)2B3H7, 2, in which an unprecedented intramolecular coordination of the carbonyl oxygen atom of the alkyne substituent to a boron framework site opens the ruthenaborane skeleton. Compound 2, in turn, is an intermediate in the formation of the ruthenacarborane nido-1,2-(Cp*Ru)2-3-OH-4-OMe-5-Me-4,5-C2B2H5, 12, in which the carbonyl-oxygen double bond has been cleaved as its oxygen atom inserts into a B-H bond and the carbonyl carbon inserts into the metallaborane framework. In a parallel reaction pathway, nido-1,2-(Cp*Ru)2-5-CO2Me-4,5-C2B2H7, 6, nido-1,2-(Cp*Ru)2-4-B(OH)2-5-CO2Me-4,5-C2B2H6, 16, and nido-1,2-(Cp*Ru)2(mu-H)(mu-BH2)-3-(CH2)2CO2Me-CO2Me-4,5-C2B2H4 (a pair of geometric isomers, 7 and 14, which contain an unusual Ru-B borane bridge) are formed. On heating, 7 rearranges to yield nido-1,2-(Cp*Ru)2-3-(CH2)2CO2Me-4-BH2-5-CO2Me-4,5-C2B2H5, 13, whereas 14 converts to nido-1,2-(Cp*Ru)2-3-(CH2)2CO2Me-4-CO2Me-4,5-C2B2H6, 8. Under thermodynamic control, nido-1,2-(Cp*Ru)2-4,5-B[(CH2)2CO2Me]CO(MeO)[C(CH2)CO2Me]-4,5-C2B2H6, 11, is the major product accompanied by lesser amounts of 6 and 1,2-(Cp*Ru)2-4-OMe-5-Me-4,5-C2B2H6, 10. Compound 11 features a five-membered heterocycle containing a boron atom. The structure of 7, which is an intermediate in the formation of 11, provides the basis for an explanation of this complex condensation of three alkynes. A previously unrecognized role for an exo-cluster bridging borene generated from the metallaborane skeleton by addition of the alkyne is also a feature of this chemistry. Reinsertion or loss of this boron fragment accounts for much of the chemistry observed. NMR experiments reveal labile intermediates, and one has been sufficiently characterized to provide mechanistic insight on the early stages of the alkyne-metallaborane addition reaction. All isolated compounds have been spectroscopically characterized, and most have been structurally characterized in the solid state.