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

A wide range of osmium-polyhydride complexes stabilized by the POP-pincer ligand xant­(PiPr2)2 (9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) have been synthesized through cis-OsCl2{κ-S-(DMSO)4} (1, DMSO = dimethyl sulfoxide). Treatment of toluene solutions of this adduct with the diphosphine, under reflux, leads to OsCl2{xant­(PiPr2)2}­(κ-S-DMSO) (2). The reaction of 2 with H2 in the presence of Et3N affords OsH3Cl­{xant­(PiPr2)2} (3), which can be also prepared by addition of xant­(PiPr2)2 to toluene solutions of the unsaturated d4-trihydride OsH3Cl­(PiPr3)2 (5). Complex 3 reductively eliminates H2 in toluene at 90 °C. In the presence of dimethyl sulfoxide, the resulting monohydride is trapped by the S-donor molecule to give OsHCl­{xant­(PiPr2)2}­(κ-S-DMSO) (6). The reaction of 2 with H2 is sensible to the Brønsted base. Thus, in contrast to Et3N, NaH removes both chloride ligands and the hexahydride OsH6{xant­(PiPr2)2} (7), containing a κ2-P-binding diphosphine, is formed under 3 atm of hydrogen at 50 °C. Complex 7 releases a H2 molecule to yield the tetrahydride OsH4{xant­(PiPr2)2} (8), which can be also prepared by reaction of OsH6(PiPr3)2 (9) with xant­(PiPr2)2. Complex 8 reduces H+ to give, in addition to H2, the oxidized OsH4-species [OsH4(OTf)­{xant­(PiPr2)2}]+ (10, OTf = trifluoromethanesulfonate). The redox process occurs in two stages via the OsH5-cation [OsH5{xant­(PiPr2)2}]+ (11). The metal oxidation state four can be recovered. The addition of acetonitrile to 10 leads to [OsH2(η2-H2)­(CH3CN)­{xant­(PiPr2)2}]2+ (12). The deprotonation of 12 yields the osmium­(IV) trihydride [OsH3(CH3CN)­{xant­(PiPr2)2}]+ (13), which is also formed by addition of HOTf to the acetonitrile solutions of 8. The latter is further an efficient catalyst precursor for the head-to-head (Z)-dimerization of phenylacetylene and tert-butylacetylene. During the activation process of the tetrahydride, the bis­(alkynyl)­vinylidene derivatives Os­(CCR)2(=CCHR)­{xant­(PiPr2)2} (R = Ph (14), tBu (15)) are formed.