Ajaykamal Tamilarasan, Palaniandavar Mallayan
Department of Chemistry, Bharathidasan University Tiruchirapalli 620 024 Tamil Nadu India
RSC Adv. 2023 Aug 18;13(35):24674-24690. doi: 10.1039/d3ra04834a. eCollection 2023 Aug 11.
Three new nickel(ii)-flavonolate complexes of the type Ni(L)(fla) 1-3, where L is the tripodal 4N ligand tris(pyrid-2-ylmethyl)amine (tpa, L1) or (pyrid-2-ylmethyl)bis(6-methylpyrid-2-ylmethyl)amine (6-Me-tpa, L2) or tris(-Et-benzimidazol-2-ylmethyl)amine (Et-ntb, L3), have been isolated as functional models for Ni(ii)-containing quercetin 2,4-dioxygenase. Single crystal X-ray structures of 1 and 3 reveal that Ni(ii) is involved in π-back bonding with flavonolate (fla), as evident from enhancement in C[double bond, length as m-dash]O bond length upon coordination [H(fla), 1.232(3); 1, 1.245(7); 3, 1.262(8) Å]. More asymmetric chelation of fla in 3 than in 1 [Δ = (Ni-O - Ni-O): 1, 0.126; 3, 0.182 Å] corresponds to lower π-delocalization in 3 with electron-releasing -Et substituent. The optimized structures of 1-3 and their geometrical isomers have been computed by DFT methods. The HOMO and LUMO, both localized on Ni(ii)-bound fla, are highly conjugated bonding π- and antibonding π*-orbitals respectively. They are located higher in energy than the Ni(ii)-based MOs (HOMO-1, d; HOMO-2/6, d), revealing that the Ni(ii)-bound fla rather than Ni(ii) would undergo oxidation upon exposure to dioxygen. The results of computational studies, in combination with spectral and electrochemical studies, support the involvement of redox-inactive Ni(ii) in π-back bonding with fla, tuning the π-delocalization in fla and hence its activation. Upon exposure to dioxygen, all the flavonolate adducts in DMF solution decompose to produce CO and depside, which then is hydrolyzed to give the corresponding acids at 70 °C. The highest rate of dioxygenase reactivity of 3 (: 3 (29.10 ± 0.16) > 1 (16.67 ± 0.70) > 2 (1.81 ± 0.04 × 10 M s)), determined by monitoring the disappearance of the LMCT band in the range 440-450 nm, is ascribed to the electron-releasing -Et substituent on bzim ring, which decreases the π-delocalization in fla and enhances its activation.
已分离出三种新型的Ni(L)(fla) 1 - 3型镍（II）-黄酮醇配合物，其中L为三脚架型四氮配体三（吡啶 - 2 - 基甲基）胺（tpa，L1）或（吡啶 - 2 - 基甲基）双（6 - 甲基吡啶 - 2 - 基甲基）胺（6 - Me - tpa，L2）或三（ - Et - 苯并咪唑 - 2 - 基甲基）胺（Et - ntb，L3），作为含镍（II）的槲皮素2,4 - 双加氧酶的功能模型。1和3的单晶X射线结构表明，镍（II）与黄酮醇（fla）参与了π - 反馈键合，这从配位时C = O键长的增加可以明显看出[H(fla)，1.232(3)；1，1.245(7)；3，1.262(8) Å]。与1相比，3中fla的螯合更不对称[Δ = (Ni - O - Ni - O)：1，0.126；3，0.182 Å]，这对应于3中具有给电子 - Et取代基时较低的π - 离域。通过密度泛函理论（DFT）方法计算了1 - 3及其几何异构体的优化结构。最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）都位于与镍（II）结合的fla上，分别是高度共轭键合的π轨道和反键π*轨道。它们的能量比基于镍（II）的分子轨道（HOMO - 1，d；HOMO - 2/6，d）更高，这表明与镍（II）结合的fla而非镍（II）在暴露于氧气时会发生氧化。计算研究结果与光谱和电化学研究相结合，支持氧化还原惰性的镍（II）参与与fla的π - 反馈键合，调节fla中的π - 离域从而激活它。在暴露于氧气时，DMF溶液中的所有黄酮醇加合物都会分解生成CO和缩酚酸，然后缩酚酸在70°C水解生成相应的酸。通过监测440 - 450 nm范围内配体到金属电荷转移（LMCT）带的消失来确定，3的双加氧酶反应活性最高（3（29.10 ± 0.16）> 1（16.67 ± 0.70）> 2（1.81 ± 0.04 × 10 M s）），这归因于苯并咪唑环上的给电子 - Et取代基，它减少了fla中的π - 离域并增强了其活性。
