Experimental section. All manipulations of air-sensitive materials were performed with rigorous exclusion of oxygen and moisture in flamed Schlenk-type glassware on a dual-manifold Schlenk line, interfaced to a high-vacuum line (10 -5 torr), or in a nitrogen-filled Vac-Atmosphere glove-box with a high-capacity recirculator (<1 ppm O 2 and H 2 O). All solvents were freeze-pump-thaw degassed on the high-vacuum line, dried over Na/K alloy, and vacuum-transferred to a dry storage flask containing

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... tivated molecular sieves. B(C 6 F 5 ) 3 was purified by sublimation (40-60 °C, 10 -5 torr). NMR samples were prepared in oven-dried J-Young NMR tubes. All NMR experiments were performed on a Bruker Avance DRX 400 instrument. Referencing by residual solvent signals is relative to TMS. Mixing time values for the Overhauser experiments were in the range 400-800 ms. The [ONNO] ligand was synthesized according to the literature. 1 Synthesis of 1: 564 mg (1.0 mmol) of the [ONNO] ligand were weighted in a Schlenk flask and dissolved in 20 mL of dry toluene. To this solution, 1.3 mL of a buthyl-lithium solution (1.6M in hexane, 2.1 mmol) were added at 0°C. The mixture was allowed to reach room temperature and stirred for 1 hour. Solid zirconium tetrachloride (233 mg, 1.1 mmol) was added and the resulting white suspension was stirred for 2 hours at 75 °C. A white precipitate formed. 0.85 mL of a methylmagnesium bromide solution (3M in diethyl ether, 2.5 mmol) were added. The obtained dark mixture was stirred overnight. The solvent was evaporated in vacuum, the resulting solid was dissolved in fresh toluene and the solid was filtered off through a little bed of celite. The filtrate, a colourless solution, was evaporated in vacuum to afford the desired compound, as a pale yellow powder (yield: 70%). 1 H NMR (C 6 D 6 , 296 K, J in Hertz, δ in ppm, the indexes "a" and "b" refer to the "in-plane" and "out-of-plane" protons, respectively): δ 7.60 (d, 3 J 3p'-