Synthesis, Characterization, and Ligand Exchange Reactivity of a Series of First Row Divalent Metal 3-Hydroxyflavonolate Complexes

A series of divalent metal flavonolate complexes of the general formula [(6-Ph2TPA)M(3-Hfl)]X (1−5-X; X = OTf− or ClO4−; 6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; M = Mn(II), Co(II), Ni(II), Cu(II), Zn(II); 3-Hfl = 3-hydroxyflavonolate) were prepared and characterized by X-ray crystallography, elemental analysis, FTIR, UV−vis, 1H NMR or EPR, and cyclic voltammetry. All of the complexes have a bidentate coordinated flavonolate ligand. The difference in M−O distances (ΔM−O) involving this ligand varies through the series, with the asymmetry of flavonolate coordination increasing in the order Mn(II) ∼ Ni(II) < Cu(II) < Zn(II) < Co(II). The hypsochromic shift of the absorption band I (π→π*) of the coordinated flavonolate ligand in 1−5-OTf (relative to that in free anion) increases in the order Ni(II) < Mn(II) < Cu(II) < Zn(II), Co(II). Previously reported 3-Hfl complexes of divalent metals fit well with this ordering. 1H NMR studies indicate that the 3-Hfl complexes of Co(II), Ni(II), and Zn(II) exhibit a pseudo-octahedral geometry in solution. EPR studies suggest that the Mn(II) complex 1-OTf may form binuclear structures in solution. The mononuclear Cu(II) complex 4-OTf has a distorted square pyramidal geometry. The oxidation potential of the flavonolate ligand depends on the metal ion present and/or the solution structure of the complex, with the Mn(II) complex 1-OTf exhibiting the lowest potential, followed by the pseudo-octahedral Ni(II) and Zn(II) 3-Hfl complexes, and the distorted square pyramidal Cu(II) complex 4-OTf. The Mn(II) complex [(6-Ph2TPA)Mn(3-Hfl)]OTf (1-OTf) is unique in the series in undergoing ligand exchange reactions in the presence of M(ClO4)2·6H2O (M = Co, Ni, Zn) in CD3CN to produce [(6-Ph2TPA)M(CD3CN)n](X)2, [Mn(3-Hfl)2·0.5H2O], and MnX2 (X = OTf− or ClO4−). Under similar conditions, the 3-Hfl complexes of Co(II), Ni(II), and Cu(II) undergo flavonolate ligand exchange to produce [(6-Ph2TPA)M(CD3CN)n](X)2 (M = Co, Ni, Cu; n = 1 or 2) and [Zn(3-Hfl)2·2H2O]. An Fe(II) complex of 3-Hfl, [(6-Ph2TPA)Fe(3-Hfl)]ClO4 (8), was isolated and characterized by elemental analysis, FTIR, UV−vis, 1H NMR, cyclic voltammetry, and a magnetic moment measurement. This complex reacts with O2 to produce the diiron(III) μ-oxo compound [(6-Ph2TPAFe(3Hfl))2(μ-O)](ClO4)2 (6).

一系列通式为[(6-Ph2TPA)M(3-Hfl)]X（1−5-X；X = OTf⁻ 或 ClO4⁻；6-Ph2TPA = N,N-双((6-苯基-2-吡啶基)甲基)-N-((2-吡啶基)甲基)胺；M = Mn(II)、Co(II)、Ni(II)、Cu(II)、Zn(II)；3-Hfl = 3-羟基黄酮醇盐）的二价金属黄酮醇盐配合物，通过X射线晶体衍射（X-ray crystallography）、元素分析、傅里叶变换红外光谱（Fourier Transform Infrared Spectroscopy, FTIR）、紫外-可见吸收光谱（Ultraviolet-Visible Spectroscopy, UV−vis）、氢核磁共振（¹H Nuclear Magnetic Resonance, ¹H NMR）或电子顺磁共振波谱（Electron Paramagnetic Resonance, EPR）以及循环伏安法（Cyclic Voltammetry）合成并表征。所有目标配合物中的黄酮醇盐配体均以双齿模式配位。该配体相关的M−O键长差值（ΔM−O）在系列配合物中呈现系统性变化，黄酮醇盐配位的不对称性按Mn(II) ≈ Ni(II) < Cu(II) < Zn(II) < Co(II)的顺序逐步递增。在1−5-OTf系列中，配位黄酮醇盐配体的吸收带I（π→π*跃迁）相较于游离阴离子的蓝移程度，按Ni(II) < Mn(II) < Cu(II) < Zn(II)、Co(II)的顺序依次增大。此前已报道的二价金属3-Hfl配合物与该规律高度契合。
氢核磁共振研究表明，Co(II)、Ni(II)和Zn(II)的3-Hfl配合物在溶液中呈假八面体几何构型。电子顺磁共振波谱研究显示，Mn(II)配合物1-OTf在溶液中可能形成双核结构。单核Cu(II)配合物4-OTf具有畸变的四方锥几何构型。黄酮醇盐配体的氧化电位取决于所配位的金属离子以及配合物的溶液结构，其中Mn(II)配合物1-OTf的氧化电位最低，其次为假八面体构型的Ni(II)与Zn(II) 3-Hfl配合物，最后是畸变四方锥构型的Cu(II)配合物4-OTf。
该系列中独特的Mn(II)配合物[(6-Ph2TPA)Mn(3-Hfl)]OTf（1-OTf），在氘代乙腈（CD3CN）溶剂中与M(ClO4)2·6H2O（M = Co、Ni、Zn）发生配体交换反应，生成[(6-Ph2TPA)M(CD3CN)n](X)2、[Mn(3-Hfl)2·0.5H2O]以及MnX2（X = OTf⁻ 或 ClO4⁻）。在相似反应条件下，Co(II)、Ni(II)和Cu(II)的3-Hfl配合物则发生黄酮醇盐配体交换，生成[(6-Ph2TPA)M(CD3CN)n](X)2（M = Co、Ni、Cu；n = 1或2）与[Zn(3-Hfl)2·2H2O]。
此外，3-Hfl的二价铁(Fe(II))配合物[(6-Ph2TPA)Fe(3-Hfl)]ClO4（8）被成功分离，并通过元素分析、傅里叶变换红外光谱、紫外-可见吸收光谱、氢核磁共振、循环伏安法以及磁矩测量完成表征。该配合物可与氧气(O2)反应生成双核铁(III) μ-氧桥化合物[(6-Ph2TPAFe(3Hfl))2(μ-O)](ClO4)2（6）。