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

Four lithium magnesiate complexes (<b>2</b>–<b>5</b>) containing the dianionic (<i>rac</i>)-BIPHEN ligand have been prepared and characterized using X-ray crystallography and NMR spectroscopy. (THF)<sub>3</sub>·Li<sub>2</sub>Mg­{(<i>rac</i>)-BIPHEN}<sup><i>n</i></sup>Bu<sub>2</sub>, <b>2</b>, (THF)<sub>3</sub>·Li<sub>2</sub>Mg­{(<i>rac</i>)-BIPHEN}­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>, <b>3</b>, and (THF)<sub>2</sub>·Li<sub>2</sub>Mg­{(<i>rac</i>)-BIPHEN}<sup><i>neo</i></sup>Pe<sub>2</sub>, <b>4</b>, have been prepared by complexation of the appropriate dialkylmagnesium compound with <i>in situ</i> prepared Li­(<i>rac</i>)<i>-</i>BIPHEN in a mixture of hydrocarbon/THF. For all structures, the Mg centers are four-coordinate (and retain the alkyl groups); however, in <b>2</b> and <b>3</b> the two Li centers have different coordination spheres (one binding to one THF molecule, the other to two). The solid-state structures of <b>2</b> and <b>3</b> are essentially isostructural with that of <b>4</b> except that both Li atoms in this molecule have equivalent coordination spheres. The solution behaviors of these three molecules have been studied by <sup>1</sup>H, <sup>13</sup>C, and DOSY NMR spectroscopy. During the synthesis of <b>2</b>, it was discovered that a (<i>rac</i>)-BIPHEN-rich (or <i>n</i>-butyl-free) lithium magnesiate, (THF)<sub>4</sub>Li<sub>2</sub>Mg­{(<i>rac</i>)-BIPHEN}­fo<sub>2</sub>, <b>2b</b>, could be isolated. The lithium precursor to <b>2</b>–<b>5</b>, (THF)<sub>4</sub>·Li<sub>4</sub>{(<i>rac</i>)-BIPHEN)}<sub>2</sub>, <b>1</b>, has also been isolated. Within the molecular structure of this tetranuclear complex, there are three different Li coordination environments. Finally, <b>2</b> 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 <b>5</b>, (THF)<sub>2</sub>·Li<sub>2</sub>Mg­{(<i>rac</i>)-BIPHEN}­(2-pyridyl)<sub>2</sub>, was isolated in high yield.