NH<sub>3</sub>‑Promoted Ligand Lability in
Eleven-Vertex Rhodathiaboranes
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Abstract
The reaction of the 11-vertex rhodathiaborane,
[8,8-(PPh<sub>3</sub>)<sub>2</sub>-<i>nido</i>-8,7-RhSB<sub>9</sub>H<sub>10</sub>] (<b>1</b>), with NH<sub>3</sub> affords
inmediately the adduct, [8,8,8-(NH<sub>3</sub>)(PPh<sub>3</sub>)<sub>2</sub>-<i>nido</i>-8,7-RhSB<sub>9</sub>H<sub>10</sub>]
(<b>4</b>). The NH<sub>3</sub>–Rh interaction induces
the labilization of the PPh<sub>3</sub> ligands leading to the dissociation
product, [8,8-(NH<sub>3</sub>)(PPh<sub>3</sub>)-<i>nido</i>-8,7-RhSB<sub>9</sub>H<sub>10</sub>] (<b>5</b>), which can
then react with another molecule of NH<sub>3</sub> to give [8,8,8-(NH<sub>3</sub>)<sub>2</sub>(PPh<sub>3</sub>)-<i>nido</i>-8,7-RhSB<sub>9</sub>H<sub>10</sub>] (<b>6</b>). These clusters have been
characterized in situ by multielement NMR spectroscopy at different
temeperatures. The variable temperature behavior of the system demonstrates
that the intermediates <b>4</b>–<b>6</b> are in
equilibrium, involving ligand exchange processes. On the basis of
low intensity signals present in the <sup>1</sup>H NMR spectra of
the reaction mixture, some species are tentatively proposed to be
the <i>bis</i>- and <i>tris</i>-NH<sub>3</sub> ligated clusters, [8,8-(NH<sub>3</sub>)<sub>2</sub>-<i>nido</i>-8,7-RhSB<sub>9</sub>H<sub>10</sub>] (<b>7</b>) and [8,8,8-(NH<sub>3</sub>)<sub>3</sub>-<i>nido</i>-8,7-RhSB<sub>9</sub>H<sub>10</sub>] (<b>8</b>). After evaporation of the solvent and
the excess of NH<sub>3</sub>, the system containing species <b>4</b>–<b>8</b> regenerates the starting reactant, <b>1</b>, thus closing a stoichiometric cycle of ammonia addition
and loss. After 40 h at room temperature, the reaction of <b>1</b> with NH<sub>3</sub> gives the hydridorhodathiaborane, [8,8,8-(H)(PPh<sub>3</sub>)<sub>2</sub>-<i>nido</i>-8,7-RhSB<sub>9</sub>H<sub>9</sub>] (<b>2</b>), as a single product. The reported rhodathiaboranes
show reversible H<sub>3</sub>N-promoted ligand lability, which implies
weak Rh–N interactions, leading to a rare case of metal complexes
that circumvent “classical” Werner chemistry
