Incorporation of Pendant Bases into Rh(diphosphine)₂ Complexes: Synthesis, Thermodynamic Studies, And Catalytic CO₂ Hydrogenation Activity of [Rh(P₂N₂)₂]⁺ Complexes

A series of five [Rh­(P₂N₂)₂]⁺ complexes (P₂N₂ = 1,5-diaza-3,7-diphosphacyclooctane) have 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 structurally characterized as square planar rhodium bis-diphosphine complexes with slight tetrahedral distortions. The corresponding hydride complexes <b>6</b>–<b>10</b> have also been synthesized and 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 hydrides have distorted trigonal bipyramidal geometries. Equilibration of 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 (Verkade’s base) enabled the determination of the hydricities and estimated p<i>K</i>_a’s of the Rh­(I) hydride complexes using the appropriate thermodynamic cycles. Complexes <b>1</b>–<b>5</b> were active for CO₂ hydrogenation under mild conditions, and their relative rates were compared to that of [Rh­(depe)₂]⁺, a nonpendant-amine-containing complex with a similar hydricity to the [Rh­(P₂N₂)₂]⁺ complexes. It was determined that the added 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