Evidence for DirecttransInsertion in a Hydrido‐Olefin Rhodium Complex—Free Nitrogen as a Trap in a Migratory Insertion Process

Abstract

Reduction of the hydrido chloride complex [Rh(H)Cl{CH₃C(CH₂CH₂-P(tBu)₂)₂}] (4) with NaH under a nitrogen atmosphere results in formation of two products: the dinitrogen complex [Rh(N₂)-{CH₃C(CH₂P(tBu)₂)₂}] (2) and the unusual low-valent hydrido-olefin complex, [RhH{CH₂C(CH₂CH₂P(tBu)₂)₂}] (3). In the presence of N₂, complexes 2 and 3 are in equilibrium in solution; 2 is about 2.9kcalmol⁻¹ more stable than 3 + N₂. Both complexes co-crystallize in the solid state; they occupy the same crystallographic site in the crystal lattice (P2-(1)/c; Z = 4; a = 12.173(2), b = 14.121 (3), c = 15.367 (3); α = 90, β = 106.50(3), γ = 90°). The mechanism of the reversible interconversion of 2 and 3 has been studied in detail. Complex 3 undergoes rapid olefin insertion/β-hydrogen elimination processes. The insertion rates were measured at different temperatures by saturation transfer NMR experiments, providing evidence for a highly organized late transition state (δS≠≈︂ – 40 e.u.), which can be caused by a concerted “trans migration”. This theoretically unfavorable process is assisted by a distortion from the ideal square-planar configuration, including a decrease of the P-Rh-P angle and some bias of the double bond toward the hydride as indicated by the X-ray crystal structure of 3. Under a nitrogen atmosphere, the intermediate formed upon olefin insertion is slowly trapped by free dinitrogen to form complex 2. The dinitrogen dissociation from 2 was found to be the rate-determining step for the overall interconversion of 2 and 3 (δG≠₂₉₈ = 24.1 kcalmol⁻¹).