No abstract

As shown by the X-ray data, in the crystal structure complex 1 has a distorted trigonal bypyramidal geometry. However, variable temperature NMR studies revealed a fluxional behavior in solution (Scheme S2). Scheme S2 When the temperature is increased from-40 °C to 25 °C, the signals for the olefins reach a coalescence point and appear as one broad signal at 1.84 ppm. Importantly, when the same sample is then cooled down beyond the coalescence point (back to −40 °C), the characteristic signals for 1 observed at low temperature appear again. These results are in agreement with those reported by Jonas et al. (Angew. Chem. Int. Ed. 1980, 19, 520) which suggest a fluxional behavior of the ligands. A putative mechanism is depicted in Scheme S3. Scheme S3 S7 Oxidation state of the Nickel center: The structure of complex 1 contains 3 largely elongated olefins bound to the Ni center with distances of 1.42, 1.46 and 1.47 Å. This experimental observation is in agreement to the Dewar-Chatt-Duncanson model, in which the olefins can be described by the continuum from metallacyclopropane to olefin. Additionally, the Ni center is sharing electrons with the Li atom, thus making two metallic Ni-Li bonds, which indicates that electrondensity at the Ni is reserved to form two Li-Ni bonds. In the literature, these descriptions of Ni-olefin complexes usually refer to the dichotomy of Ni(0)-olefin/Ni(II) metallacyclopropane. 2 However, our case represents a twofold down in oxidation state of this dichotomy and hence, a rare and unique form of a Ni(0) metallacyclopropane. S8 4 Stoichiometric Experiments Scheme S4 Reaction A: A Schlenk tube was charged with complex 1 (18.0 mg, 0.029 mmol) via argon trousers and dissolved in precooled THF (1 mL) at 0 °C. The solution was cooled to −78 °C and (E)-(2bromovinyl)benzene (5.4 mg, 3.4 µL, 0.029 mmol, 1 eq.)was added via a Hamilton syringe. After stirring for 30 minutes, hexylMgBr (55 µL, 1.05 M, 0.058 mmol, 2 eq.) was added and the reaction was stirred overnight at −20 °C. The reaction was quenched with water and decane (10 µL, 7.3 mg, 0.051 mmol) was added as an internal standard. GC FID analysis showed 32 % product formation, along with styrene (15%) and substrate homocoupling (22%). Reaction B: A Schlenk tube was charged with complex 1 (22.6 mg, 0.036 mmol) via argon trousers and dissolved in freshly dried (Na/K alloy) and precooled d 8-THF (1 mL) at 0 °C. The solution was cooled to −78 °C and (E)-(2-bromovinyl)benzene (4.1 µL, 6.6 mg, 0.036 mmol, 1 eq.) was added via a Hamilton syringe. The reaction was stirred for 30 minutes, quenched by addition of water and rinsed with EtOAc.

Sixteen-electron Ni(0) complexes bearing trans-stilbene derivative ligands have been shown to display a high degree of stability toward oxidation in the solid state. A structural analysis of a unique family of tris Ni(0) stilbene complexes revealed a remarkable effect of the steric hindrance of the substituents at the para position of the stilbene unit to temperature, oxidation, and degradation in solution. From these analyses, Ni( 4-tBu stb) 3 arose as a long-term air-, bench-. and temperature-stable Ni(0) complex. Importantly, Ni( 4-tBu stb) 3 presents faster kinetic profiles and a broader scope as a Ni(0) source, thus outperforming the previously described Ni( 4-CF3 stb) 3 in a variety of relevant Ni-catalyzed transformations.

Investigations into the mechanism of the low-temperature C­(sp2)–C­(sp3) Kumada cross-coupling catalyzed by highly reduced nickel-olefin-lithium complexes revealed that 16-electron tris­(olefin)­nickel(0) complexes are competent catalysts for this transformation. A survey of various nickel(0)-olefin complexes identified Ni­(nor)3 as an active catalyst, with performance comparable to that of the previously described Ni-olefin-lithium precatalyst. We demonstrate that Ni­(nor)3, however, is unable to undergo oxidative addition to the corresponding C­(sp2)–Br bond at low temperatures (<−40 °C), thus indicating that the canonical cross-coupling cycle is not operative under this condition. Instead, such binary nickel(0)-olefin complexes capitalize on the long-known Lewis acidity of the Ni(0) center when it is coordinated to olefins to accommodate the incoming charge from the Grignard reagent, forming a nickel(0)-alkylmagnesium complex. We demonstrate that this unique heterobimetallic complex is now primed for reactivity, thus cleaving the C­(sp2)–Br bond and ultimately delivering the C­(sp2)–C­(sp3) bond in high yields.