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

This work reports the results of a thermally driven cluster expansion of arachno-1-{η⁵-C<inf>5</inf>Me<inf>5</inf>IrH<inf>2</inf>}B <inf>3</inf>H<inf>7</inf>, 1, with BH<inf>3</inf>•THF. In addition to the previously reported product, arachno-1-{η⁵-C<inf>5</inf>Me <inf>5</inf>IrH}B<inf>4</inf>H<inf>9</inf>, 2, formed at lower temperatures, reaction at 100 °C permits the isolation of four new iridaboranes. Two products, nido-1-(η⁵-C<inf>5</inf>Me<inf>5</inf>Ir)B <inf>5</inf>H<inf>9</inf>, 3, and nido-3-(η⁵-C<inf>5</inf>Me <inf>5</inf>Ir)B<inf>9</inf>H<inf>13</inf>, 4, contain a single Ir atom and five and nine framework boron atoms, respectively. One, nido-3,4-(η⁵- C<inf>5</inf>Me<inf>5</inf>Ir)<inf>2</inf>B<inf>8</inf>H<inf>12</inf>, 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-C<inf>5</inf>Me<inf>5</inf>Ir)<inf>2</inf>B <inf>8</inf>H<inf>13</inf>(μ-BH<inf>2</inf>), 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. © The Royal Society of Chemistry.