Synthesis and Small Molecule Reactivity of <i>trans</i>-Dihydride Isomers of Ru(NHC)₂(PPh₃)₂H₂ (NHC = N‑Heterocyclic Carbene)

Addition of IMe₄ (1,3,4,5-tetra­methyl­imidazol-2-ylidene) to Ru­(PPh₃)₃­HCl (in the presence of H₂) or Ru­(PPh₃)₄­H₂ gave the all-trans isomer of Ru­(IMe₄)₂­(PPh₃)₂­H₂ (<b>1a</b>), whereas 1,3-diethyl-4,5-dimethylimidazol-2-ylidene (IEt₂­Me₂) reacted with Ru­(PPh₃)₄­H₂ to form <i>cis</i>,<i>cis</i>,<i>trans</i>-Ru­(IEt₂­Me₂)₂­(PPh₃)₂­H₂ (<b>2b</b>). H/D exchange of <b>1a</b> with C₆D₆ (elevated temperature) or D₂ (room temperature) gave Ru­(IMe₄)₂­(PPh₃)₂­HD (<b>1a-HD</b>) and Ru­(IMe₄)₂­(PPh₃)₂­D₂ (<b>1a-D</b>_<b>2</b>). CO reacted with <b>1a</b> to give a mixture of Ru­(IMe₄)₂­(PPh₃)­(CO)­H₂ (<b>3</b>) and Ru­(IMe₄)₂­(CO)₃ (<b>4</b>); <b>2b</b> reacted in a similar manner, although more slowly, allowing isolation of the monocarbonyl species Ru­(IEt₂­Me₂)₂­(PPh₃)­(CO)­H₂ (<b>5</b>). Insertion of CO₂ into one of the Ru–H bonds of <b>1a</b> and <b>2b</b> generated mixtures of major and minor isomers of the κ²-formate complexes Ru­(IMe₄)₂­(PPh₃)­(OCHO)H (<b>7</b>/<b>8</b>) and Ru­(IEt₂Me₂)₂(PPh₃)­(OCHO)H (<b>9</b>/<b>10</b>). The hydridic nature of <b>1a</b> and <b>2b</b> was apparent by their reactivity toward MeI, which gave [Ru­(IMe₄)₂­(PPh₃)₂­H]I (<b>11</b>), Ru­(IEt₂­Me₂)₂­(PPh₃)­HI (<b>12</b>), [Ru­(IEt₂­Me₂)₂­(PPh₃)₂­H]I (<b>13</b>), and Ru­(IEt₂­Me₂)­(PPh₃)₂­HI (<b>14</b>). Complexes <b>1a</b>, <b>2b</b>, <b>5</b>, <b>9</b>, <b>11</b>, <b>13</b>, and <b>14</b> were structurally characterized