EVIDENCE FOR DIRECT TRANS INSERTION IN A HYDRIDO-OLEFIN RHODIUM COMPLEX - FREE NITROGEN AS A TRAP IN A MIGRATORY INSERTION PROCESS

Reduction of the hydride chloride complex [Rh(H)Cl{CH3C(CH2CH2- P(tBu) (2))(2)}] (4) with NaH under a nitrogen atmosphere results in formatio n of two products: the dinitrogen complex [Rh(N-2)-{CH3C(CH2CH2P(tBu)( 2))(2)}] (2) and the unusual low-valent hydrido-olefin complex, [RhH{C H2=C(CH2CH2P(tBu)(2))(2)}] (3). In the presence of N-2, complexes 2 an d 3 are in equilibrium in solution; 2 is about 2.9 kcal mol(-1) more s table than 3 SN,. Both complexes co-crystallize in the solid state; th ey 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); alpha = 90, beta = 106.50 (3), gamma = 90 degrees). The mechanism of the reversibl e interconversion of 2 and 3 has been studied in detail. Complex 3 und ergoes rapid olefin insertion/beta-hydrogen elimination processes. The insertion rates were measured at different temperatures by saturation transfer NMR experiments, providing evidence for a highly organized l ate transition state (Delta S-not equal approximate to - 40 e.u.), whi ch can be caused by a concerted ''trans mi gration''. This theoretical ly unfavorable process is assisted by a distortion from the ideal squa re-planar configuration,including a decrease of the P-Rh-P angle and s ome bias of the double bond toward the hydride as indicated by the X-r ay crystal structure of 3. Under a nitrogen atmosphere, the intermedia te formed upon olefin insertion is slowly trapped by free dinitrogen t o form complex 2. The dinitrogen dissociation from 2 was found to be t he rate-determining step for the overall interconversion of 2 and 3 (D elta G(298)(not equal) = 24.1 kcal mol(-1)).