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 (iPrLH2) with excess ZnR2 (R = Et, Me) in toluene solution forms the dizinc derivatives
iPrLZn2Et2 (1) and iPrLZn2Me2 (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
ZnPh2 with N,N‘-bis(2-(dimethylamino)ethyl)dibenzofuran-4,6-diamine (MeLH2) forms MeLZn2Ph2 (3), which was isolated in 73% yield. The analogous reaction using iPrLH2 instead of
MeLH2 forms iPrLZn2Ph2 (4), which was isolated in 56% yield. The solid-state structure of 3
(two independent molecules) reveals a parallelogram-shaped [Zn2(μ-Ph)2]2+ core, with the
overall molecule having approximate (noncrystallographic) C2 symmetry and a short
intermetal separation (2.7279(6) and 2.7809(6) Å). In contrast, the 1H NMR spectrum of 3
(CD2Cl2) indicates overall C2v symmetry; this is likely due to cleavage of the phenyl bridges
in solution. The solid-state structure of 4, which features the bulkier iPr-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 MeLZn2(Ph)(OCHPh2) (5) in 87% isolated yield.
The solid-state structure of 5 reveals a symmetric and puckered [Zn2(μ-Ph)(μ-OCHPh2)]2+
core with a short Zn−Zn separation of 2.6977(13) Å. In CD2Cl2 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 1H 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 MeLZn2(OCHPh2)2 (6) in 67% isolated yield.
The related species iPrLZn2(OCHPh2)2 (7) was formed in good yield by the reaction of 2 equiv
of benzaldehyde with 4 at 70 °C for 24 h.