Incorporation\nof Pendant Bases into Rh(diphosphine)₂ Complexes: Synthesis,\nThermodynamic Studies, And Catalytic\nCO₂ Hydrogenation Activity of [Rh(P₂N₂)₂]⁺ Complexes

Abstract

A series of five [Rh­(P₂N₂)₂]⁺ complexes (P₂N₂ = 1,5-diaza-3,7-diphosphacyclooctane)\nhave been synthesized and characterized: [Rh­(P^Ph₂N^Ph₂)₂]⁺ (<b>1</b>), [Rh­(P^Ph₂N^Bn₂)₂]⁺ (<b>2</b>), [Rh­(P^Ph₂N^PhOMe₂)₂]⁺ (<b>3</b>), [Rh­(P^Cy₂N^Ph₂)₂]⁺ (<b>4</b>), and [Rh­(P^Cy₂N^PhOMe₂)₂]⁺ (<b>5</b>). Complexes <b>1</b>–<b>5</b> have been\nstructurally characterized as square planar rhodium bis-diphosphine\ncomplexes with slight tetrahedral distortions. The corresponding hydride\ncomplexes <b>6</b>–<b>10</b> have also been synthesized\nand characterized, and X-ray diffraction studies of HRh­(P^Ph₂N^Bn₂)₂ (<b>7</b>), HRh­(P^Ph₂N^PhOMe₂)₂ (<b>8</b>) and HRh­(P^Cy₂N^Ph₂)₂ (<b>9</b>) show that the\nhydrides have distorted trigonal bipyramidal geometries. Equilibration\nof complexes <b>2</b>–<b>5</b> with H₂ in the presence of 2,8,9-triisopropyl-2,5,8,9-tetraaza-1-phosphabicyclo­[3,3,3]­undecane\n(Verkade’s base) enabled the determination of the hydricities\nand estimated p<i>K</i>_a’s of the Rh­(I)\nhydride complexes using the appropriate thermodynamic cycles. Complexes <b>1</b>–<b>5</b> were active for CO₂ hydrogenation\nunder mild conditions, and their relative rates were compared to that\nof [Rh­(depe)₂]⁺, a nonpendant-amine-containing\ncomplex with a similar hydricity to the [Rh­(P₂N₂)₂]⁺ complexes. It was determined that the\nadded steric bulk of the amine groups on the P₂N₂ ligands hinders catalysis and that [Rh­(depe)₂]⁺ was the most active catalyst for hydrogenation of CO₂ to formate.