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.

采用X射线晶体学和核磁共振波谱技术，已成功制备并表征了含有双阴离子（对位）-BIPHEN配体的锂镁酸盐复合物（2-5）。(THF)3·Li2Mg{(rac)-BIPHEN}nBu2，2，(THF)3·Li2Mg{(rac)-BIPHEN}(CH2SiMe3)2，3，以及(THF)2·Li2Mg{(rac)-BIPHEN}neoPe2，4，通过将相应的二烷基镁化合物与现场制备的Li(rac)-BIPHEN在烃/THF混合物中配位而合成。对于所有结构，镁中心均为四配位（并保留了烷基团）；然而，在2和3中，两个锂中心具有不同的配位环境（一个与一个THF分子配位，另一个与两个配位）。2和3的固态结构基本上与4的结构相同，除了该分子中的两个锂原子具有等效的配位环境。这些三种分子的溶液行为已通过1H、13C和DOSY核磁共振波谱技术进行研究。在合成2的过程中，发现了一种富含(rac)-BIPHEN（或无丁基）的锂镁酸盐(THF)4Li2Mg{(rac)-BIPHEN}fo2，2b，可以分离出来。2-5的锂前体(THF)4·Li4{(rac)-BIPHEN)}2，1，也已分离。在此四核复合物的分子结构中，存在三种不同的锂配位环境。最后，2已显示出作为与2-溴吡啶进行卤素-金属交换反应试剂的潜力。通过X射线晶体学和核磁共振波谱技术对反应中的结构化学进行了探究。有机金属中间体吡啶化镁5，(THF)2·Li2Mg{(rac)-BIPHEN}(2-吡啶基)2，以高产量分离出来。