Structure, Bonding, and Electrochemical Studies of Osmaboranes Comprising Planar B4 Ring

Abstract

An osmaborane featuring a planar [B4H8]2– ring has been synthesized and structurally characterized. Prolonged thermolysis of the intermediate generated from the reaction of [Os(PPh3)3Cl2] (1) with dppf [dppf = 1,1′-Bis(diphenylphosphino) ferrocene] in the presence of excess [BH3·THF] afforded [Os(dppf)H2(η4–B4H8)] (2) along with [H(dppf)OsB5H10] (3) and [{HOs(dppf)}2(μ-H)B4H7] (4). Complex 2 contains a planar [B4H8]2– ring and is an osmium analog of pentaborane(9). Complex 3 is the osmium analog of hexaborane(10). On the other hand, complex 4 adopts a nido monocapped square pyramidal geometry which is structurally analogous to [{HOs(PPh3)2}2(μ-H)B4H7] (5), obtained from the prolonged thermolysis of [Os(PPh3)3Cl2] (1) with excess of [BH3·THF] alongside the earlier reported [Os(PPh3)2H2(η4–B4H8)] (I) and [H(PPh3)2OsB5H10] (II). Surprisingly, no analogous products were obtained using dppm [dppm = 1,2-bis(diphenylphosphino)methane] and dppe [dppe = 1,2-bis(diphenylphosphino)ethane]. All synthesized molecules were characterized through multinuclear NMR and IR spectroscopy and mass spectrometry. Single-crystal X-ray diffraction analyses of clusters 2 and 5 established their solid-state structures, while theoretical investigations elucidated the bonding in 2. In addition, comparative electrochemical studies of 2 and I were carried out to examine electronic communication and to assess the influence of the redox-active dppf ligand on the electronic properties of the complex.