Sodium Congener of the Classical Lithium Methylchromate Dimer: Synthetic, X-ray Crystallographic, and Magnetic Studies of Me₈Cr₂[Na(OEt₂)]₄

One of the milestone structures in the development of transition-metal complexes with metal−metal bonds of multiple bond order was the lithium methylchromate dimer Me8Cr2[Li(donor)]4 (donor = THF or Et2O). Using a simple salt metathesis reaction mixing this compound with sodium tert-butoxide, the sodium congener Me8Cr2[Na(OEt2)]4 has been synthesized as a green crystalline compound and isolated in 51% yield. Its solid-state structure was determined by single-crystal X-ray diffraction. Exhibiting exact crystallographic C4h symmetry, this heavier alkali-metal chromate structure is also dimeric, formally comprising a (Me8Cr4)4− tetranionic core with four peripheral Na+ cations carrying supporting ether ligands. Its salient feature is the long Cr···Cr distance of 3.263(2) Å, which is remarkably elongated compared to that in the lithium THF-solvated congener [1.968(2) Å]. With respect to the methyl C atoms, the Cr coordination is distorted-square-planar. Each Na+ interacts with four methyl C atoms, and there are also some short Na···H(C) contacts. Unlike for lithium chromate, no NMR spectroscopic data could be obtained for sodium chromate. The paramagnetic character of sodium chromate was confirmed by variable-temperature magnetization measurements, which indicated antiferromagnetic behavior.在具有多键级金属-金属键的过渡金属配合物的发展历程中，极具里程碑意义的结构之一为甲基铬酸锂二聚体Me8Cr2[Li(donor)]4（其中donor代表四氢呋喃（THF）或乙醚（Et2O））。通过将该化合物与叔丁醇钠进行简单的盐复分解反应，成功合成了其钠同系物Me8Cr2[Na(OEt2)]4，该产物为绿色结晶态化合物，分离产率达51%。该化合物的固态结构通过单晶X射线衍射得以解析。该重碱金属铬酸盐结构具有严格的晶体学C4h对称性，同样以二聚体形式存在，其形式上由一个(Me8Cr4)4−的四阴离子核与四个外围的Na+阳离子构成，阳离子结合了辅助配位的醚配体。其最显著的特征为Cr···Cr间距长达3.263(2) Å，与该化合物的THF溶剂化锂同系物（Cr···Cr间距为1.968(2) Å）相比，该距离显著伸长。针对甲基碳原子的配位环境而言，铬的配位模式为变形平面正方形。每个Na+与四个甲基碳原子存在相互作用，同时还存在若干短程Na···H(C)接触。与铬酸锂不同，该铬酸钠样品未能获得核磁共振波谱数据。通过变温磁化率测量证实了该铬酸钠的顺磁性特征，测量结果表明其具备反铁磁行为。