Carbonyl insertion into metal-boron based clusters: pathway to a rhodathiacarborane

The reaction of the ten-vertex hydridorhodathiaborane [2,2,2-(H)(PPh₃)₂-closo-2,1-RhSB₈H₈] (1) with excess of carbon monoxide at room temperature yields a novel eleven-vertex rhodathiacarborane [1,1-(CO)(PPh₃)-3-(OH)-closo-1,2,3-RhSCB₈H₈] (4). The formation of 4 involves the substitution of a PPh₃ ligand by CO at the rhodium centre and the subsequent insertion of a second CO molecule into the polyhedral cage to form a hydroxy-functionalized carbon vertex (C-OH). In addition to 4, the reaction affords the CO-ligated ten-vertex adducts [6,6-(CO)(PPh₃)-9-(PPh₃)-arachno-6,5-RhSB₈H₉] (5a) and [9,9-(CO)(PPh₃)-8-(PPh₃)-arachno-9,6-RhSB₈H₉] (5b). While a mixture of 5a and 5b can also be synthesized from the PPh₃-adduct [6,6-(PPh₃)₂-9-(PPh₃)-arachno-6,5-RhSB₈H₉] (2), compound 2 does not yield 4 upon treatment with CO. This suggests that the transformation of CO into a C-OH vertex is mediated by a specific CO-Rh/PPh₃ intermediate. Similarly, the reaction of the 2-methyl-pyridine adduct, [6,6-(PPh₃)₂-9-(2-Mepy)-arachno-6,5-RhSB₈H₉] (3) with CO selectively yields the CO-ligated isomer [6,6-(CO)(PPh₃)-9-(2-Mepy)-arachno-6,5-RhSB₈H₉] (6). All the compounds were characterized by multielement NMR spectroscopy and mass spectrometry, with 4, 5a and 6 further analysed by single-crystal X-ray diffraction. Variable temperature NMR and DFT calculations for 4 reveal a fluxional process characterized by the rotation of the {Rh(CO)(PPh₃)} unit relative to the thiacarborane cage. These findings underscore the versatility of rhodathiaborane platforms for the activation and incorporation of small molecules via metal-polyhedron cooperation.