Role of the transition metal in metallaborane chemistry. reactivity of (Cp*ReH₂)₂B₄H₄with BH₃·thf, CO, and Co₂ (CO)₈

Abstract

The reaction of Cp*ReCl₄, [Cp*ReCl₃]₂, or [Cp*ReCl₂]₂ (Cp* = η⁵-C₅Me₅) with LiBH₄ leads to the formation of 7-skeletal-electron-pair (7-sep) (Cp*ReH₂)₂ (B₂H₃)₂ (1) together with Cp*ReH₆. Compound 1 is metastable and eliminates H₂ at room temperature to generate 6-sep (Cp*ReH₂)₂B₄H₄ (2). The reaction of 2 with BH₃·thf produces 7-sep (Cp*Re)₂B₇H₇, a hypoelectronic cluster characterized previously. Heating of 2 with 1 atm of CO leads to 6-sep (Cp*ReCO)(Cp*ReH₂)B₄H₄ (3). Both 2 and 3 have the same bicapped Re₂B₂ tetrahedral cluster core structure. Monitoring the reaction of 2 with CO at room temperature by NMR reveals the formation of a 7-sep, metastable intermediate, (Cp*ReCO)(Cp*ReH₂)(B₂H₃)₂ (4), which converts to 3 on heating. An X-ray structure determination reveals two isomeric forms (4-cis and 4-trans) in the crystallographic asymmetric unit which differ in geometry relative to the disposition of the metal ancillary ligands with respect to the Re−Re bond. The presence of these isomers in solution is corroborated by the solution NMR data and the infrared spectrum. In both isomers, the metallaborane core consists of fused B₂Re₂ tetrahedra sharing the Re₂ fragment. On the basis of similarities in electron count and spectroscopic data, 1 also possesses the same bitetrahedral structure. The reaction of 2 with Co₂ (CO)₈ results in the formal replacement of the four rhenium hydrides with a 4-electron Co₂ (CO)₅ fragment, thereby closing the open face in 2 to produce the 6-sep hypoelectronic cluster (Cp*Re)₂Co₂(CO)₅B₄H₄ (5). These reaction outcomes are compared and contrasted with those previously observed for 5-sep (Cp*Cr₂)₂B₄H₈.