Microwave-Assisted Synthesis of Tricarbonyl Rhenacarboranes: Steric and Electronic Effects on the 1,2 → 1,7 Carborane Cage Isomerization

A series of tricarbonyl rhenacarboranes {[M][Re(CO)3(RR‘C2B9H9)]} (M = Na, K) were synthesized in water using
microwave radiation with reaction times of less than 15 min. The novel complexes were isolated in good yields
(57−94%) as either 3,1,2-(R = H:  R‘ = CH2Pyr 6; R‘ = CH2Cy, 20) or 2,1,8-(R = H:  R‘ = H, 4; R‘ = CH2PyrMe
12; R‘ = CH2PyrH, 13; R‘ = Pyr, 15; R‘ = Ph, 17; R = R‘ = Bn, 19) metallacarboranes and characterized by
multinuclear (1H, 11B, 13C) and NOE NMR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray
crystallography in the case of compounds 12 and 13. Carborane cage isomerization from the original 1,2 configuration
to the 1,7 orientation occurred in cases where significant steric crowding was present at the metal center. Incorporation
of a methylene spacer between the carborane cage and the six-membered ring as in 7 and 20 decreased steric
strain such that the 3,1,2 configuration was maintained. Conversion of the 3,1,2 complex 6 to the 2,1,8 isomers 12
and 13 takes place at room temperature upon methylation or protonation of the pyridyl ring, indicating that electronic
effects also play a significant role in the isomerization process.