Mono and dinuclear rhodium, iridium and ruthenium complexes containing chelating 2,2′-bipyrimidine ligands: Synthesis, molecular structure, electrochemistry and catalytic properties

The mononuclear cations [(η⁵-C₅Me₅)RhCl(bpym)]⁺ (<b>1</b>), [(η⁵-C₅Me₅)IrCl(bpym)]⁺ (<b>2</b>), [(η⁶-p-Pr<i>i</i>C₆H₄Me)RuCl(bpym)]⁺ (<b>3</b>) and [(η⁶-C₆Me₆)RuCl(bpym)]⁺ (<b>4</b>) as well as the dinuclear dications [{(η⁵-C₅Me₅)RhCl}₂ (bpym)]²⁺ (<b>5</b>), [{(η⁵-C₅Me₅)IrCl}₂ (bpym)]²⁺ (<b>6</b>), [{(η⁶-p-Pr^<i>i</i>C₆H₄Me)RuCl}₂ (bpym)]²⁺ (<b>7</b>) and [{(η⁶-C₆Me₆)RuCl}₂ (bpym)]²⁺ (<b>8</b>) have been synthesised from 2,2′-bipyrimidine (bpym) and the corresponding chloro complexes [(η⁵-C₅Me₅)RhCl₂]₂, [(η⁵-C₅Me₅)IrCl₂]₂, [(η⁶-Pr^<i>i</i>C₆H₄Me)RuCl₂]₂ and [(η⁶-C₆Me₆)RuCl₂]₂, respectively. The X-ray crystal structure analyses of [<b>3</b>][PF₆], [<b>5</b>][PF₆]₂, [<b>6</b>][CF₃SO₃]₂ and [<b>7</b>][PF₆]₂ reveal a typical piano-stool geometry around the metal centres; in the dinuclear complexes the chloro ligands attached to the two metal centres are found to be, with respect to each other, cis oriented for <b>5</b> and <b>6</b> but <i>trans</i> for <b>7</b>. The electrochemical behaviour of <b>1–8</b> has been studied by voltammetric methods. In addition, the catalytic potential <b>of 1–8</b> for transfer hydrogenation reactions in aqueous solution has been evaluated: All complexes catalyse the reaction of acetophenone with formic acid to give phenylethanol and carbon dioxide. For both the mononuclear and dinuclear series the best results were obtained (50 °C, pH 4) with rhodium complexes, giving turnover frequencies of 10.5 h⁻¹ for <b>1</b> and 19 h⁻¹ for <b>5</b>.