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

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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, complexes2and3are in equilibrium in solution;2is about 2.9kcalmol−1more stable than3+ 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 of2and3has been studied in detail. Complex3undergoes 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 “transmigration”. 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 of3. Under a nitrogen atmosphere, the intermediate formed upon olefin insertion is slowly trapped by free dinitrogen to form complex2. The dinitrogen dissociation from2was found to be the rate‐determining step for the overall interconversion of2and3(δG≠298= 24.1 kcalmol−1).

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