Evidence for direct trans insertion in a hydrido-olefin rhodium complex - Free nitrogen as a trap in a migratory insertion process

Reduction of the hydrido chloride complex [Rh(H)C1{CH₃C(CH₂CH₂P(1Bu)₂)₂}] (4) with NaH under a nitrogen atmosphere results in formation of two products: the dinitrogen complex [Rh(N₂)-{CH₃C(CH₂CH₂P(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 2.9 kcalmol 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 from 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).