Stepwise construction of [Os6(CO)18(µ6-P)]–, a hexanuclear osmium cluster monoanion with trigonal-prismatic co-ordination for phosphorus: X-ray crystal structures of [Os6(µ-H)2(CO)20(MeCN)(µ3-PH)] and [PPh3Me][Os6(CO)18(µ6-P)]

Abstract

Treatment of the labile cluster [Os₃(CO)₁₁(MeCN)] with PH₃ affords the substituted product [Os₃(CO)₁₁(PH₃)](1) in high yield. Subsequent reaction of (1) with Na₂CO₃ in MeOH, followed by acidification, gives the hydrido phosphido cluster [Os₃(µ-H)(CO)₁₀(µ-PH₂)](2). When (2) is heated to 45–60 °C in the presence of [Os₃(CO)₁₁(MeCN)] a hexanuclear complex with the formulation [Os₆(µ-H)₂(CO)₂₁(µ₃-PH)](3) is obtained. If this reaction is repeated using [Os₃(CO)₁₀(MeCN)₂] instead of [Os₃(CO)₁₁(MeCN)], an acetonitrile-containing product, [Os₆(µ-H)₂(CO)₂₀(MeCN)(µ₃-PH)](4), is obtained. An X-ray analysis of (4) shows that two Os₃ triangular units are linked by a µ₃-phosphinidene ligand, which symmetrically bridges an Os–Os edge of one triangle and is terminally co-ordinated to one Os atom of the second triangle. When (3) is treated with a weak base, such as [N(PPh₃)₂]Cl or [PPh₃Me] Br, deprotonation to the corresponding cluster monoanion [Os₆(µ-H)(CO)₂₁(µ₃-PH)]^–(5) occurs. Treatment of (5) with a weak acid regenerates (3) in quantitative yield. Thermolysis of (3) leads to a closing up of the metal framework, affording the cluster [Os₆(µ-H)(CO)₁₈(µ₆-P)], which readily deprotonates to give the anion [Os₆(CO)₁₈(µ₆-P)]^–(7) in the presence of [N(PPh₃)₂] Cl or [PPh₃Me]Br. The same anion (7) may also be obtained by direct thermolysis of (5). An X-ray analysis of the [PPh₃Me]⁺ salt of (7) confirms that the phosphorus occupies an interstitial site in a trigonal-prismatic hexaosmium framework, and co-ordinates to all six metal atoms with an average Os–P distance of 2.31 (1)Å. Proton and ³¹P n.m.r. data on all the new clusters are presented, and the position of the phosphorus resonance in the ³¹P n.m.r. spectrum is related to the changes in the environment of the phosphorus atom.