Mononuclear to Polynuclear Transition Induced by Ligand Coordination: Synthesis, X-ray Structure, and Properties of Mono-, Di-, and Polynuclear Copper(II) Complexes of Pyridyl-Containing Azo Ligands

Reactions of two hydrated cupric salts (CuCl2·2H2O and Cu(ClO4)2·6H2O) with three azopyridyl ligands, viz.
2-[(arylamino)phenylazo]pyridine [aryl = phenyl (HL1a), p-tolyl (HL1b), and 2-thiomethyl phenyl (HL1c)], 2-[2-(pyridylamino)phenylazo]pyridine (HL2), and 2-[3-(pyridylamino)phenylazo]pyridine (HL3), afford the mononuclear
[CuClL1] (1), dinuclear [Cu2X2L22]n+ (X = Cl, H2O, ClO4; n = 0, 1; 2, 3), and polynuclear [CuClL3]n (4) complexes,
respectively, in high yields. Representative X-ray structures of these complexes 1−4 are reported. X-ray structure
analysis of 4 reveals an infinite 1D zigzag chain that adopts a saw-tooth-like structure. Variable-temperature
cryomagnetic measurements (2−300 K) on the complexes 2−4 have revealed weak magnetic interactions between
the copper centers with J values −1.04, 9.88, and −1.31 cm-1, respectively. Positive ion ESI mass spectra of the
soluble complexes 1−3 are studied which provide the evidence for the integrity of the complexes also in solution.
Visible range spectra of the complexes 1−3 in solution consist of intense and broad transitions in the range 700−600 nm. The solid-state spectrum of the insoluble copper complex 4, on the other hand, shows a structured band
near 700 nm. The intensities of the transitions of the dinuclear complexes are much higher than those of the
corresponding mononuclear copper complexes. Redox properties of the present copper complexes are reported.
Notably, the dinuclear complex, 3, displays two successive redox processes:  CuIICuII ⇌ CuIICuI ⇌ CuICuI. It
catalyzes aerial oxidation of L-ascorbic acid. The catalytic cycle is most effective up to H2A/3 (H2A = L-ascorbic
acid) molar ratio of 20:1.

将两种水合铜(II)盐（CuCl₂·2H₂O和Cu(ClO₄)₂·6H₂O）分别与三种偶氮吡啶类配体反应，即2-[(芳氨基)苯基偶氮]吡啶[芳基为苯基(HL1a)、对甲苯基(HL1b)和2-甲硫基苯基(HL1c)]、2-[2-(吡啶基氨基)苯基偶氮]吡啶(HL2)以及2-[3-(吡啶基氨基)苯基偶氮]吡啶(HL3)，可高收率得到单核配合物[CuClL1](1)、双核配合物[Cu₂X₂L2₂]ⁿ⁺（X=Cl、H₂O、ClO₄；n=0、1；2、3）以及多核配合物[CuClL3]ₙ(4)。本文报道了配合物1~4的代表性X射线晶体结构。对配合物4的X射线结构分析显示，其为无限一维锯齿形链状结构，呈类锯齿状构型。对配合物2~4开展2~300 K范围内的变温低温磁学测试，结果表明铜中心之间存在弱磁相互作用，对应的磁耦合常数J分别为-1.04、9.88和-1.31 cm⁻¹。对可溶性配合物1~3进行正离子模式电喷雾电离(ESI)质谱表征，证实了其在溶液中仍保持结构完整性。配合物1~3的溶液可见光谱在600~700 nm区间内展现出强而宽的吸收带。而不溶性铜配合物4的固态光谱则在700 nm附近出现结构化吸收带。双核配合物的吸收峰强度远高于对应单核铜配合物。本文还报道了上述铜配合物的氧化还原性质。值得注意的是，双核配合物3展现出两步连续的氧化还原过程：CuⅡCuⅡ ⇌ CuⅡCuⅠ ⇌ CuⅠCuⅠ。该配合物可催化抗坏血酸的需氧氧化反应，当H₂A与3的摩尔比（H₂A为抗坏血酸）不超过20:1时，催化循环效率达到最佳。