Inclusion Potential, Polymorphism, and Molecular Isomerism of Metal Dibenzoylmethanates Coordinated with 2-Methylpyridine†

In this study, new complexes of metal(II) (Ni, Zn, Cd) dibenzoylmethanates (DBMs)
coordinated with 2-methylpyridine (2-MePy) were prepared and studied for host properties
as a function of their structure and molecular isomerism. The novel [Ni(2-MePy)2(DBM)2]
complex host exists as the trans isomer in five structurally distinct phases comprising two
guest-free polymorphs as well as inclusion compounds with benzene, carbon tetrachloride,
chlorobenzene, and toluene. All architectures arise from van der Waals packing, the host
materials having guest species located in channels or cages. The stability of inclusions
correlates with the shape of the cavity space. For zinc dibenzoylmethanate only one 2-MePy
unit coordinates to give the [Zn(2-MePy)(DBM)2] complex that does not exhibit inclusion
properties. With cadmium dibenzoylmethanate, two polymorphic forms of the composition
[Cd(2-MePy)2(DBM)2] were isolated. A metastable form contains complex molecules in the
trans isomeric state, while a stable form contains cis isomers. As earlier, the ability to form
clathrates was attributed only to trans-configured molecules of this type; the presence of a
more stable cis form, with a trans-to-cis phase conversion enthalpy gain of as much as 17.2(6)
kJ/mol, explains the failure to isolate any inclusion compounds of the Cd complex. The
precedent of isolating both cis and trans isomers for the same complex of metal dibenzoylmethanate host is discussed in the context of designing novel materials where the potential
for inclusion may be switched through molecular-level control of the isomeric state.

在本研究中，通过2-甲基吡啶（2-MePy）配位的金属（II）二苯甲酰甲烷（DBMs）新复合物（Ni、Zn、Cd）被制备并研究，以探究其结构异构和分子异构对宿主性质的影响。新型[Ni(2-MePy)2(DBM)2]复合物宿主以顺式异构体存在于五个结构上各异的相中，包括两种无客体分子的多形体以及与苯、四氯化碳、氯苯和甲苯的包合化合物。所有结构均源自范德华堆积，宿主材料中的客体物种位于通道或笼中。包合物的稳定性与空腔空间的形状相关。对于锌二苯甲酰甲烷，只有一个2-MePy单元与配位，形成不具包合性质的[Zn(2-MePy)(DBM)2]复合物。至于镉二苯甲酰甲烷，分离出了两种组成[Cd(2-MePy)2(DBM)2]的多晶形态。一种亚稳态包含顺式异构体的复杂分子，而一种稳定形态包含反式异构体。与之前类似，形成笼状化合物的能力仅归因于此类反式构型的分子；更稳定的顺式形式的出现，其反式至顺式的相变焓变为高达17.2(6) kJ/mol，解释了为何无法分离出镉复合物的任何包合化合物。在讨论通过分子级对异构态的控制来设计能够切换包合潜力的新型材料时，提出了对同一金属二苯甲酰甲烷宿主复合物的顺式和反式异构体同时分离的先例。