Osmaborane Clusters with B4 and B5 Rings Stabilized in the Coordination Sphere of {OsLn} (Ln = η5-C5Me5 or H2(PPh3)2)

New synthetic routes have been developed to synthesize osmaborane clusters featuring B4 and B5 rings in the coordination spheres of osmium. Thermolysis of [Os(PPh3)3Cl2], 1 in the presence of excess of [BH3<middle dot>THF] led to the formation of [Os(PPh3)2H2(eta 4-B4H8)], 2 along with [HOs(PPh3)2B5H10], 3. Cluster 2 features a planar tetraborane ring coordinated to an osmium center in an eta 4 fashion. Cluster 3 can be considered an osmium analogue of hexaborane(10), in which the osmium center is situated at the base of the pentagonal pyramid geometry. In a different synthetic protocol, we have carried out the metathesis reaction of [Cp*OsBr2]2 (Cp* = eta 5-pentamethylcyclopentadienyl), 5 with [LiBH4<middle dot>THF] followed by thermolysis in the presence of [BH3<middle dot>THF] that generated [Cp*Os(eta 5-B5H10)], 6. Cluster 6 has a planar pentaborane ring that is stabilized in the coordination sphere of osmium, making it a boron analogue of osmocene. Clusters 2 and 6 are the first examples of structurally characterized planar B4 and B5 rings, respectively, that are stabilized in the coordination sphere of osmium. All the synthesized molecules were characterized using multinuclear NMR and IR spectroscopy, mass spectrometry, and single crystal X-ray diffraction analyses. Theoretical calculations were carried out to visualize the electronic structures and bonding scenarios in 2 and 6.