Crystal and Molecular Structure of Manganese(II) Lapacholate, a Novel Polymeric Species Undergoing Temperature-Reversible Metal to Ligand Electron Transfer

Lapachol (2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphtoquinone) (HLap, C15H14O3) reacts with Mn2+ producing a novel polymeric complex with formula: [Mn(Lap)2]n. Two ligands chelate the metal through their ortho oxygen (O1, O2) moiety while two para oxygens, from other Lap ligands, complete the octahedral coordination sphere. Thus far, all reported Lap metal complexes are mononuclear, lack the metal-trans-quinonic (para) oxygen binding and have Lap as a bidentate ligand. Synthesis, X-ray diffraction, IR, and UV−visible spectroscopic properties, thermogravimetric analysis, and differential thermal analysis of this complex are reported along with a density functional theory study describing electron transfer from the Mn to the Lap ligand at low temperature. X-ray structure determinations at 125, 197, and 300 K describe the progressive trend of a Mn contribution to the Mn−O1 bond length as a function of T. The Mn−O1 bond distance increases with temperature and may be therefore associated with a semiquinonate action at low T by the carbonyl O1 donor (and corresponding to MnIII). It transforms to a more classical coordinative bond at room T and stabilizes a MnII species; this is a reversible phenomenon involving MnII−MnIII oxidation states.

拉帕醇（Lapachol，2-羟基-3-(3-甲基-2-丁烯基)-1,4-萘醌，HLap，C₁₅H₁₄O₃）与二价锰离子（Mn²+）反应，生成分子式为[Mn(Lap)₂]ₙ的新型聚合物配合物。两个Lap配体通过其邻位氧（O1、O2）基团螯合金属中心，另外两个来自其他Lap配体的对位氧则完成该配合物的八面体配位环境。迄今为止，所有已报道的Lap基金属配合物均为单核结构，不存在金属-反式醌（对位）氧结合模式，且Lap仅作为双齿配体参与配位。本工作报道了该配合物的合成、X射线衍射（X-ray diffraction）、红外光谱（IR）、紫外-可见（UV−visible）光谱性质、热重分析及差热分析，并开展了密度泛函理论（Density Functional Theory, DFT）研究，阐明了低温下Mn向Lap配体的电子转移过程。通过在125、197和300 K下测定X射线晶体结构，揭示了Mn对Mn−O1键长的影响随温度变化的渐进趋势：Mn−O1键长随温度升高而增加，该现象可归因于低温下羰基O1供体的半醌式配位作用（对应Mn³+物种）；在室温下，该键则转变为更典型的配位键，稳定Mn²+物种。这是一个涉及Mn²+与Mn³+氧化态互变的可逆过程。