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} n Bu2, 2, (THF)3·Li2Mg­{(rac)-BIPHEN}­(CH2SiMe3)2, 3, and (THF)2·Li2Mg­{(rac)-BIPHEN} neo Pe2, 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.