Cooperative Bimetallic Effects on New Iridium(III) Pyrazolate Complexes:  Hydrogen−Hydrogen, Carbon−Hydrogen, and Carbon−Chlorine Bond Activations

The reaction of <i>fac</i>-[IrH<sub>2</sub>(NCCH<sub>3</sub>)<sub>3</sub>(P<sup>i</sup>Pr<sub>3</sub>)]BF<sub>4</sub> (<b>1</b>) with potassium pyrazolate gave the binuclear 34-electron complex [Ir<sub>2</sub>(μ-H)(μ-Pz)<sub>2</sub>H<sub>3</sub>(NCCH<sub>3</sub>)(P<sup>i</sup>Pr<sub>3</sub>)<sub>2</sub>] (<b>2</b>). The structure of <b>2</b> was determined by X-ray diffraction. An electrostatic potential calculation located three terminal hydride ligands and one hydride bridging both iridium centers. The feasibility of this arrangement was studied by EHMO calculations. The spectroscopic data for <b>2</b> show that the complex is rigid in solution on the NMR time scale. In solution, the acetonitrile ligand of <b>2</b> dissociates. The activation parameters for this dissociation process in toluene-<i>d</i><sub>8</sub> are Δ<i>H</i><sup>⧧</sup> = 20.9 ± 0.6 kcal mol<sup>-1</sup> and Δ<i>S</i><sup>⧧</sup> = 2.5 ± 1.3 e.u. Reaction of <b>2</b> with various Lewis bases (L) gives the substitution products [Ir<sub>2</sub>(μ-H)(μ-Pz)<sub>2</sub>H<sub>3</sub>(L)(P<sup>i</sup>Pr<sub>3</sub>)<sub>2</sub>] (L = C<sub>2</sub>H<sub>4</sub> (<b>3</b>), CO (<b>4</b>), HPz (<b>5</b>)). The reaction of complex <b>5</b> with C<sub>2</sub>H<sub>4</sub> yields the ethyl derivative [Ir<sub>2</sub>(μ-H)(μ-Pz)<sub>2</sub>(C<sub>2</sub>H<sub>5</sub>)H<sub>2</sub>(HPz)(P<sup>i</sup>Pr<sub>3</sub>)<sub>2</sub>] (<b>6</b>); this reaction is reversible. Complexes <b>2</b> and <b>3</b> react with CHCl<sub>3</sub> to give CH<sub>2</sub>Cl<sub>2</sub> and the compounds [Ir<sub>2</sub>(μ-H)(μ-Pz)<sub>2</sub>H<sub>2</sub>(Cl)(L)(P<sup>i</sup>Pr<sub>3</sub>)<sub>2</sub>] (L = NCCH<sub>3</sub> (<b>7</b>), C<sub>2</sub>H<sub>4</sub> (<b>8</b>)). In the <sup>1</sup>H NMR spectra of <b>2</b><b>−</b><b>6</b>, the signal of the bridging hydride ligand shows two very different <i>J</i><sub>HP</sub> couplings; in contrast, for the chloride complexes <b>7</b> and <b>8</b>, two equal <i>J</i><sub>HP</sub> couplings are observed. NOE and <i>T</i><sub>1</sub> measurements lead to the conclusion that in complexes <b>2</b><b>−</b><b>6</b> the hydride bridges the iridium centers in a nonsymmetric fashion, whereas for <b>7</b> and <b>8</b> the bridge is symmetrical. This structural feature largely influences the reactivity. Compounds <b>2</b> and <b>3</b> undergo H/D exchange under a D<sub>2</sub> atmosphere. Analysis of the isotopomeric mixtures of <b>2</b> reveals downfield isotopic shifts in the <sup>31</sup>P{<sup>1</sup>H} NMR spectrum. Downfield as well as high-field shifts are found for the hydride signals in the <sup>1</sup>H NMR spectrum of partially deuterated <b>2</b>. Further reaction of <b>3</b> with H<sub>2</sub> gave ethane and the dihydrogen complex [Ir<sub>2</sub>(μ-H)(μ-Pz)<sub>2</sub>H<sub>3</sub>(η<sup>2</sup>-H<sub>2</sub>)(P<sup>i</sup>Pr<sub>3</sub>)<sub>2</sub>] (<b>9</b>). Under a deficiency of H<sub>2</sub>, in toluene-<i>d</i><sub>8</sub> solution, <b>9</b> undergoes H/D scrambling with the participation of the solvent. It has also been found that under H<sub>2</sub> complex <b>3</b> catalyzes the hydrogenation of cyclohexene.