Investigation of a Homoleptic Nickel(II) Complex: Synthesis, Crystal Structure, Computational Insights, and Antimicrobial Efficacy

A homoleptic nickel­(II) complex, [Ni­(L)2], where HL = 2-methoxy-6-[(methylimino)­methyl]­phenol, synthesized via the condensation of bis­(3-methoxysalicylaldehyde)­nickel­(II) with methylamine in methanol, was structurally characterized by spectroscopic techniques and single crystal X-ray diffraction. The peaks at 232 and 275 nm in the UV–vis spectrum correspond to the π → π* transition, while 374 nm is assigned to the n → π* transition. The characteristic FTIR bands associated with Schiff base formation were noted at 1609 cm–1 (−CN). The 1H NMR spectrum displayed prominent signals at 7.26 ppm (azomethine H), 6.06 ppm (aromatic H), and 3.78 ppm (methyl protons). TGA/DTA studies revealed significant thermal stability of the nickel­(II) complex up to 260 °C, with a final residue of NiO/C at 519 °C. The mass spectra showed a primary molecular ion peak at m/z 387.08, along with the fragmentation pattern. Powder X-ray diffraction confirmed the purity of the nickel­(II) complex powder. The single-crystal X-ray diffraction revealed that the complex crystallizing in a monoclinic space group P21/n, has a pseudo centrosymmetric square planar geometry featuring Ni–O and Ni–N bond lengths with mean values of 1.8264(18) Å and 1.91(2) Å, respectively. Density functional theory (DFT) calculations yielded a HOMO–LUMO gap of 3.53 eV, a chemical potential of −3.31 eV, and an electrophilicity index of 3.11 eV, indicating high stability and reactivity. Hirshfeld surface analysis identified H···H (53.6%) and C···H (25.6%) as the dominant contributors, while NCI plots revealed strong van der Waals and hydrogen bonding interactions. Biological studies, conducted through antimicrobial assays, demonstrate commendable biological activity at significantly low MIC/MBC/MFC values of 30 μg/mL. Molecular docking studies revealed the highest binding affinities with E. coli DNA gyrase B (−8.03 kcal/mol, Ki = 29 μM) among the bacterial targets and C. tropicalis phospholipid synthase (−7.20 kcal/mol, Ki = 97 μM) among the fungal targets. These findings suggest that the synthesized nickel­(II) complex is a promising agent for antibacterial and antifungal applications.以甲醇为溶剂，通过双(3-甲氧基水杨醛)合镍(II)与甲胺的缩合反应合成了同配体镍(II)配合物（homoleptic nickel(II) complex）[Ni(L)₂]，其中HL=2-甲氧基-6-[(甲亚氨基)甲基]苯酚。采用光谱技术及单晶X射线衍射（single crystal X-ray diffraction）对该配合物的结构进行了表征。紫外-可见（UV–vis）光谱中232 nm与275 nm处的吸收峰对应π→π*跃迁，374 nm处的吸收峰归属于n→π*跃迁。傅里叶变换红外（FTIR）光谱中，希夫碱（Schiff base）生成的特征谱带出现在1609 cm⁻¹处，对应-C=N键。氢核磁共振（¹H NMR）谱显示，7.26 ppm处存在显著的甲亚胺氢信号，6.06 ppm处为芳香氢信号，3.78 ppm处为甲基质子信号。热重-差热（TGA/DTA）分析结果表明，该镍(II)配合物在260 ℃下具备良好的热稳定性，最终在519 ℃时生成NiO/C残余物。质谱分析显示，其分子离子主峰的质荷比m/z为387.08，并给出了对应的碎片离子碎裂模式。粉末X射线衍射（powder X-ray diffraction）证实了该镍(II)配合物粉末的纯度。单晶X射线衍射结果显示，该配合物结晶于单斜空间群P2₁/n，具有准中心对称的平面正方形几何构型，Ni-O键与Ni-N键的平均键长分别为1.8264(18) Å和1.91(2) Å。密度泛函理论（DFT）计算结果表明，该配合物的最高占据分子轨道-最低未占据分子轨道（HOMO–LUMO）能隙为3.53 eV，化学势为-3.31 eV，亲电性指数为3.11 eV，说明其具有较高的稳定性与反应活性。希尔施费尔德表面分析（Hirshfeld surface analysis）显示，H···H（53.6%）与C···H（25.6%）相互作用为主要的分子间相互作用；非共价相互作用图（NCI plots）则揭示了较强的范德华相互作用与氢键相互作用。通过抗菌实验（antimicrobial assays）开展的生物学研究表明，该配合物在低至30 μg/mL的最小抑菌浓度/最小杀菌浓度/最小真菌浓度（MIC/MBC/MFC）下即表现出优异的生物活性。分子对接（molecular docking）研究显示，在细菌靶标中，该配合物与大肠杆菌DNA回旋酶B（E. coli DNA gyrase B）的结合亲和力最高（结合能为-8.03 kcal/mol，抑制常数Ki=29 μM）；在真菌靶标中，与热带假丝酵母菌磷脂合酶（C. tropicalis phospholipid synthase）的结合亲和力最高（结合能为-7.20 kcal/mol，抑制常数Ki=97 μM）。上述研究结果表明，本研究合成的镍(II)配合物是一种极具应用前景的抗菌与抗真菌试剂。