Expansion of iridaborane clusters by addition of monoborane. Novel metallaboranes and mechanistic detail

Abstract

This work reports the results of a thermally driven cluster expansion of arachno-1-{η⁵-C₅Me₅IrH₂}B₃H₇, 1, with BH₃·THF. In addition to the previously reported product, arachno-1-{η⁵-C₅Me₅IrH}B₄H₉, 2, formed at lower temperatures, reaction at 100 °C permits the isolation of four new iridaboranes. Two products, nido-1-(η⁵-C₅Me₅Ir)B₅H₉, 3, and nido-3-(η⁵-C₅Me₅Ir)B₉H₁₃, 4, contain a single Ir atom and five and nine framework boron atoms, respectively. One, nido-3,4-(η⁵-C₅Me₅Ir)₂B₈H₁₂, 5, contains two Ir atoms and eight framework boron atoms. Their structures are predicted by the electron counting rules to be a nido-iridahexaborane, 3, nido-iridadecaborane, 4, and nido-diiridadecaborane, 5. The accuracy of these predictions in each case is established experimentally by spectroscopic characterization in solution and structure determinations in the solid state. A less stable metallaborane has been identified and the available spectroscopic and crystallographic information are consistent with the formulation nido-3,4-(η⁵-C₅Me₅Ir)₂B₈H₁₃(µ-BH₂), 6, i.e., a species containing an exopolyhedral bridging BH group. These new observations, along with earlier ones on ruthenaborane cluster systems, are used to fully define a general mechanism for a cluster expansion reaction, i.e., addition of borane to form an exopolyhedral adduct followed by cage insertion.