The Synthesis, Characterization, and Fluxional Behavior of a Hydridorhodatetraborane

The octahydridotriborate anion plays a crucial role in the field of polyhedral boron chemistry, facilitating the synthesis of higher boranes and the preparation of diverse transition metal complexes. Among the stable forms of this anion, CsB₃H₈ (or (n-C₄H₉)₄N)[B₃H₈] have been identified. These salts serve as valuable precursors for the synthesis of metallaboranes, wherein the triborate anion acts as a ligand coordinating to the metal center. In this study, we have successfully synthesized a novel rhodatetraborane dihydride, [Rh(η²-B₃H₈)(H)₂(PPh₃)₂] (1), which represents a Rh(III) complex featuring a bidentate chelate ligand fasormed by B₃H₈^-. Extensive characterization of this rhodatetraborane complex has been performed using NMR spectroscopy in solution and X-ray diffraction analysis in the solid state. Notably, the complex exhibits intriguing fluxional behavior, which has been investigated using NMR techniques. Moreover, we have explored the reactivity of complex 1 towards pyridine (py) and dimethylphenylphosphine (PMe₂Ph). Our findings highlight the labile nature of this four-vertex rhodatetraborane as it undergoes disassembly upon attack from the corresponding Lewis base, resulting in the formation of borane adducts, LBH₃, where L = py, PMe₂Ph. Furthermore, in these reactions, we report the characterization of new cationic hydride complexes, such as [Rh(H)₂(PPh₃)₂ (py)]⁺ (2) and [Rh(H)₂(PMe₂Ph)₄]⁺. Notably, the latter complex has been characterized as the octahydridotriborate salt [Rh(H)₂(PMe₂Ph)₄][B₃H₈] (3), which extends the scope of rhodatetraborane derivatives.