POP–Rhodium-Promoted C–H and B–H Bond Activation and C–B Bond Formation

The square-planar monohydride complex RhH­{xant­(PⁱPr₂)₂} (<b>1</b>; xant­(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) activates C–H bonds of arenes. Heating of benzene solutions of <b>1</b> at 80 °C affords RhPh­{xant­(PⁱPr₂)₂} (<b>2</b>). Under the same conditions, toluene gives Rh­(<i>m</i>-tolyl)­{xant­(PⁱPr₂)₂} (<b>3a</b>) and Rh­(<i>p</i>-tolyl)­{xant­(PⁱPr₂)₂} (<b>3b</b>) in a 78:22 molar ratio, whereas <i>m</i>-xylene leads to Rh­(C₆H₃-3,5-Me₂)­{xant­(PⁱPr₂)₂} (<b>4</b>). At room temperature, fluorobenzene and 1,3-difluorobenzene generate Rh­(C₆H₄-2-F)­{xant­(PⁱPr₂)₂} (<b>5</b>) and Rh­(C₆H₃-2,6-F₂)­{xant­(PⁱPr₂)₂} (<b>6</b>), respectively. Complex <b>1</b> also promotes the B–H bond activation of pinacolborane (HBpin) and catecholborane (HBcat). The reactions initially give the <i>trans</i>-dihydride derivatives RhH₂(BR₂)­{xant­(PⁱPr₂)₂} (BR₂ = Bpin (<b>7</b>), Bcat (<b>8</b>)), which lose H₂ to afford the square-planar Rh­(BR₂)­{xant­(PⁱPr₂)₂} (BR₂ = Bpin (<b>9</b>), Bcat (<b>10</b>)). Complex <b>2</b> reacts with HBpin to regenerate <b>1</b> and to give Ph-Bpin. Similarly, complex <b>9</b> regenerates <b>1</b> and gives Ph-Bpin by reaction with benzene. In agreement with these transformations, complex <b>1</b> catalyzes the direct C–H borylation of arenes. The selectivity of the process appears to be governed by the kinetic of the C–H bond activations of the arenes. Benzylic borylation is not observed for methylbenzenes. The first X-ray structure of a square-planar rhodium–boryl complex is also reported