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

Abstract

New synthetic routes have been developed to synthesize osmaborane clusters featuring B 4 and B 5 rings in the coordination spheres of osmium. Thermolysis of [Os(PPh 3 ) 3 Cl 2 ], 1 in the presence of excess of [BH 3 •THF] led to the formation of [Os(PPh 3 ) 2 H 2 (η 4 -B 4 H 8 )], 2 along with [HOs(PPh 3 ) 2 B 5 H 10 ], 3.Cluster 2 features a planar tetraborane ring coordinated to an osmium center in an η 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*OsBr 2 ] 2 (Cp* = η 5 -pentamethylcyclopentadienyl), 5 with [LiBH 4 •THF] followed by thermolysis in the presence of [BH 3 •THF] that generated [Cp*Os(η 5 -B 5 H 10 )], 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 B 4 and B 5 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.