Structural Studies of (rac)‑BIPHEN Organomagnesiates and Intermediates in the Halogen–Metal Exchange of 2‑Bromopyridine

Four lithium magnesiate complexes (2–5) containing the dianionic (rac)-BIPHEN ligand have been prepared and characterized using X-ray crystallography and NMR spectroscopy. (THF)₃·Li₂Mg­{(rac)-BIPHEN}ⁿBu₂, 2, (THF)₃·Li₂Mg­{(rac)-BIPHEN}­(CH₂SiMe₃)₂, 3, and (THF)₂·Li₂Mg­{(rac)-BIPHEN}^neoPe₂, 4, have been prepared by complexation of the appropriate dialkylmagnesium compound with in situ prepared Li­(rac)-BIPHEN in a mixture of hydrocarbon/THF. For all structures, the Mg centers are four-coordinate (and retain the alkyl groups); however, in 2 and 3 the two Li centers have different coordination spheres (one binding to one THF molecule, the other to two). The solid-state structures of 2 and 3 are essentially isostructural with that of 4 except that both Li atoms in this molecule have equivalent coordination spheres. The solution behaviors of these three molecules have been studied by ¹H, ¹³C, and DOSY NMR spectroscopy. During the synthesis of 2, it was discovered that a (rac)-BIPHEN-rich (or n-butyl-free) lithium magnesiate, (THF)₄Li₂Mg­{(rac)-BIPHEN}­fo₂, 2b, could be isolated. The lithium precursor to 2–5, (THF)₄·Li₄{(rac)-BIPHEN)}₂, 1, has also been isolated. Within the molecular structure of this tetranuclear complex, there are three different Li coordination environments. Finally, 2 has already shown promise as a reagent in a halogen–metal exchange reaction with 2-bromopyridine. The structural chemistry at play in this reaction was probed by X-ray crystallography and NMR spectroscopy. The organometallic intermediate pyridyl-magnesiated 5, (THF)₂·Li₂Mg­{(rac)-BIPHEN}­(2-pyridyl)₂, was isolated in high yield.