Family of Cofacial Bimetallic Complexes of a Hexaanionic Carboxamide Cryptand

A series of coordination compounds has been prepared comprising manganese, iron, nickel, and zinc bound by a hexaanionic cryptand where carboxamides are anionic N-donors. The metal complexes have been investigated by X-ray crystallography, and possess metal centers in trigonal monopyramidal geometries with intermetallic distances spanning dMn,avg = 6.080 Å to dNi,avg = 6.495 Å. All complexes featuring trigonal monopyramidal metal(II) ions crystallize in Cc, and feature extended three-dimensional networks composed of cryptate anions linked by bridging potassium countercations. We also report the first solid state structure of the free cryptand ligand, which features no guest in its cavity and which possesses an extended hydrogen-bonding network. SQuID magnetometry data of the metal complexes reveal weak antiferromagnetic coupling of the metal centers. Only the diiron(II) complex exhibits reversible electrochemistry, and correspondingly, its chemical oxidation yields a powder formulated as the diiron(III) congener. The insertion of cyanide into the intermetallic cleft of the diiron(II) complex has been achieved, and comparisons of its solid state structure to the recently reported dicobalt(II) analogue are made. The antiferromagnetic coupling between the diiron(II) and the dicobalt(II) centers when bridged by cyanide does not increase significantly relative to the unbridged congeners. A one-site model satisfactorily fits Mössbauer spectra of unbridged diiron(II) and diiron(III) complexes whereas a two site fit was needed to model the iron(II) centers that are bridged by cyanide.

本研究制备了一系列配位化合物，其以六阴离子穴状配体（hexaanionic cryptand）作为配位骨架，结合了锰、铁、镍与锌四种金属离子，该配体以酰胺作为阴离子型氮给体。本研究通过X射线晶体学（X-ray crystallography）对该系列金属配合物进行结构表征，结果显示其金属中心均呈现三角单锥几何构型，金属间平均距离范围为dMn,avg=6.080 Å至dNi,avg=6.495 Å。所有含三角单锥型二价金属离子的配合物均以空间群Cc结晶，并形成由穴状阴离子通过桥联钾抗衡阳离子连接而成的延伸三维网络结构。本研究还首次报道了游离穴状配体的固态结构：其空腔内未包合任何客体分子，且存在延伸的氢键网络。通过超导量子干涉磁强计（SQuID magnetometry）对金属配合物的磁学性质进行测试，结果显示金属中心间存在弱反铁磁耦合作用。仅二铁(II)配合物表现出可逆电化学行为，相应地，对其进行化学氧化可得到组成为二铁(III)同系物的粉末产物。本研究实现了将氰根插入二铁(II)配合物金属间隙的合成策略，并将该配合物的固态结构与近期报道的二钴(II)类似物进行了对比分析。当以氰根桥联时，二铁(II)与二钴(II)中心之间的反铁磁耦合强度相较于未桥联的同系物并无显著提升。未桥联的二铁(II)及二铁(III)配合物的穆斯堡尔谱（Mössbauer spectra）可通过单位点模型进行良好拟合，而氰根桥联的铁(II)中心则需采用双位点拟合模型进行解析。