Modeling Triple Conformational Disorder in a New Crystal Polymorph of cis-Aquabis(l-isoleucinato)copper(II)

The X-ray crystal and molecular structure of a new polymorph
of cis-aquabis­(l-isoleucinato)­copper­(II),
obtained
by recrystallization from an acetic acid–water mixture and
determined at 120 and 295 K, revealed triple dynamic disorder over
one isoleucinato ligand at both temperatures. The complicated multipart
disorder was resolved with the help of extensive computational crystal
structure simulations, which were undertaken for the first time to
interpret disorder in a crystal of bioinorganic compound. The new
polymorph (space group C2) is conformationally polymorphic
with the already known P212121 crystal form. To discover the conformers that can participate
in self-associations in solution, and to rationalize an interplay
of intramolecular and intermolecular interactions in the crystallization
of different conformers, conformational analyses of cis and trans isomers were performed using the same
force field in vacuo, in aqueous solution, and for selected conformers
in P212121 and C2 crystals. Three conformers identified in the disorder
were estimated to form the most favorable intermolecular interactions
in the solid state, and one of them (the most populated conformer
in the disorder) also in aqueous solution. The crystal structure reproduction
of all possible arrangements of the three conformers in C2 unit cell helped to find the most plausible crystal packing motif.

对从乙酸-水混合物中通过重结晶得到的新型顺式水合(l-异亮氨酸酸根)铜(II)多晶体的X射线晶体学和分子结构进行了研究，该多晶体的结构在120和295 K下确定，揭示了在两种温度下均存在跨越一个异亮氨酸酸根配体的三重动态无序。借助广泛的计算晶体结构模拟，成功解析了这种复杂的多元无序，这是首次利用计算方法解释生物无机化合物晶体中的无序现象。该新型多晶型体（空间群C2）与已知的P212121晶体形态在构象上具有多晶性。为了发现能参与溶液中自缔合的构象，并合理化不同构象在结晶过程中分子内和分子间相互作用的相互作用，利用相同的真空、水溶液以及P212121和C2晶体中选定的构象的力场，对顺式和反式异构体的构象进行了分析。在无序中识别出的三种构象被估计为在固态中形成最有利于分子间相互作用的排列，其中之一（无序中最多的构象）在水溶液中也同样如此。对所有可能的三种构象在C2晶胞中的排列的晶体结构再现有助于找到最可能的晶体堆积模式。