Phenylation of benzaldehyde by dizinc organometallics supported by binucleating bis(amidoamine) ligands

Several new dizinc organometallics that are supported by dianionic bis(amidoamine) ligands are reported. The reaction of N,N'-bis(2-(diisopropylamino)ethyl)dibenzofuran-4,6-diamine ( i P r LH 2 ) with excess ZnR 2 (R = Et, Me) in toluene solution forms the dizinc derivatives i P r LZn 2 Et 2 (1) and i P r LZn 2 Me 2 (2), which were isolated in 69% and 72% yields, respectively. Both 1 and 2 feature three-coordinate zinc centers in the solid state. Reaction of 2 equiv of ZnPh 2 with N,N'-bis(2-(dimethylamino)ethyl)dibenzofuran-4,6-diamine ( M e LH 2 ) forms M e LZn 2 -Ph 2 (3), which was isolated in 73% yield. The analogous reaction using i P r LH 2 instead of M e LH 2 forms i P r LZn 2 Ph 2 (4), which was isolated in 56% yield. The solid-state structure of 3 (two independent molecules) reveals a parallelogram-shaped [Zn 2 (μ-Ph) 2 ] 2 + core, with the overall molecule having approximate (noncrystallographic) C 2 symmetry and a short intermetal separation (2.7279(6) and 2.7809(6) A). In contrast, the 1 H NMR spectrum of 3 (CD 2 Cl 2 ) indicates overall C 2 v symmetry; this is likely due to cleavage of the phenyl bridges in solution. The solid-state structure of 4, which features the bulkier i Pr-substituted ligand, reveals the absence of Ph bridges. Instead, each molecule of 4 has a pair of essentially noninteracting three-coordinate Zn centers. Benzene solutions of 3 react with 1 equiv of benzaldehyde to form the addition product M e LZn 2 (Ph)(OCHPh 2 ) (5) in 87% isolated yield. The solid-state structure of 5 reveals a symmetric and puckered [Zn 2 (μ-Ph)(μ-OCHPh 2 )] 2 + core with a short Zn-Zn separation of 2.6977(13) A. In CD 2 Cl 2 solution, the Ph bridge of 5 undergoes reversible cleavage. This process followed by rotation about the Zn-Ph bond leads to exchange of the inequivalent ortho (and meta) protons of the μ-Ph ligand. Variable-temperature 1 H NMR spectroscopic data indicates that this exchange occurs with ΔG = 11.5(1) kcal mol - 1 (-50 °C). At 70 °C, benzene solutions of 5 react with 1 equiv of benzaldehyde to form the double-addition product M e LZn 2 (OCHPh 2 ) 2 (6) in 67% isolated yield. The related species i P r LZn 2 (OCHPh 2 ) 2 (7) was formed in good yield by the reaction of 2 equiv of benzaldehyde with 4 at 70 °C for 24 h.