POP-Pincer Osmium-Polyhydrides: Head-to-Head (<i>Z</i>)‑Dimerization of Terminal Alkynes

A wide range of osmium-polyhydride complexes stabilized by the POP-pincer ligand xant­(P^<i>i</i>Pr₂)₂ (9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) have been synthesized through <i>cis</i>-OsCl₂{κ-<i>S</i>-(DMSO)₄} (<b>1</b>, DMSO = dimethyl sulfoxide). Treatment of toluene solutions of this adduct with the diphosphine, under reflux, leads to OsCl₂{xant­(P^<i>i</i>Pr₂)₂}­(κ-<i>S</i>-DMSO) (<b>2</b>). The reaction of <b>2</b> with H₂ in the presence of Et₃N affords OsH₃Cl­{xant­(P^<i>i</i>Pr₂)₂} (<b>3</b>), which can be also prepared by addition of xant­(P^<i>i</i>Pr₂)₂ to toluene solutions of the unsaturated <i>d</i>⁴-trihydride OsH₃Cl­(P^<i>i</i>Pr₃)₂ (<b>5</b>). Complex <b>3</b> reductively eliminates H₂ in toluene at 90 °C. In the presence of dimethyl sulfoxide, the resulting monohydride is trapped by the <i>S</i>-donor molecule to give OsHCl­{xant­(P^<i>i</i>Pr₂)₂}­(κ-<i>S</i>-DMSO) (<b>6</b>). The reaction of <b>2</b> with H₂ is sensible to the Brønsted base. Thus, in contrast to Et₃N, NaH removes both chloride ligands and the hexahydride OsH₆{xant­(P^<i>i</i>Pr₂)₂} (<b>7</b>), containing a κ²-<i>P</i>-binding diphosphine, is formed under 3 atm of hydrogen at 50 °C. Complex <b>7</b> releases a H₂ molecule to yield the tetrahydride OsH₄{xant­(P^<i>i</i>Pr₂)₂} (<b>8</b>), which can be also prepared by reaction of OsH₆(P^<i>i</i>Pr₃)₂ (<b>9</b>) with xant­(P^<i>i</i>Pr₂)₂. Complex <b>8</b> reduces H⁺ to give, in addition to H₂, the oxidized OsH₄-species [OsH₄(OTf)­{xant­(P^<i>i</i>Pr₂)₂}]⁺ (<b>10</b>, OTf = trifluoromethanesulfonate). The redox process occurs in two stages via the OsH₅-cation [OsH₅{xant­(P^<i>i</i>Pr₂)₂}]⁺ (<b>11</b>). The metal oxidation state four can be recovered. The addition of acetonitrile to <b>10</b> leads to [OsH₂(η²-H₂)­(CH₃CN)­{xant­(P^<i>i</i>Pr₂)₂}]²⁺ (<b>12</b>). The deprotonation of <b>12</b> yields the osmium­(IV) trihydride [OsH₃(CH₃CN)­{xant­(P^<i>i</i>Pr₂)₂}]⁺ (<b>13</b>), which is also formed by addition of HOTf to the acetonitrile solutions of <b>8</b>. The latter is further an efficient catalyst precursor for the head-to-head (<i>Z</i>)-dimerization of phenylacetylene and <i>tert</i>-butylacetylene. During the activation process of the tetrahydride, the bis­(alkynyl)­vinylidene derivatives Os­(CCR)₂(=CCHR)­{xant­(P^<i>i</i>Pr₂)₂} (R = Ph (<b>14</b>), ^<i>t</i>Bu (<b>15</b>)) are formed