Departamento de Química Inorgánica, Facultad de Química, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain.
Inorg Chem. 2012 Feb 6;51(3):1278-93. doi: 10.1021/ic2017038. Epub 2012 Jan 23.
The ring-chain tautomerism of a 2-aryl-1,2,3,4-tetrahydroquinazoline has been exploited to induce reversible changes in the aminal-imine equilibrium, as desired, by coordination of a suitable metal ion. This process was studied by NMR and UV-vis spectroscopies, X-ray crystallography, and molecular modeling approach. The results obtained show that the imine H(2)L(i) undergoes a selective ring-closing reaction upon complexation with Ni(2+). As a result, complexes of the type Ni(HL(a))(2) are obtained, whose chirality arises from the chiral ligand H(2)L(a) and the helicity of the structure. Hence, helical enantiomers form the following racemates: [Δ-C(R,R)N(S,S),Λ-C(S,S)N(R,R)]-Ni(HL(a))(2)·2HOAc and [Δ,Λ-C(S,R)N(R,S)]-Ni(HL(a))(2)·4MeOH. In contrast to the situation observed for Ni(2+), the cyclic tautomer of the ligand, H(2)L(a), undergoes a selective ring-opening reaction upon complex formation with Pd(2+), ultimately yielding Pd(HL(i))(2)·MeOH, in which the open-chain imine ligand is bidentate through the N,O donor set of the quinoline residue. Density functional theory calculations were conducted to provide insight into the different behavior of both coordinated metals (Ni(2+) and Pd(2+)) and to propose a mechanism for the metal-assisted opening/closing reaction of the tetrahydroquinazoline ring.
2-芳基-1,2,3,4-四氢喹唑啉的环链互变异构已被利用，通过合适的金属离子配位，在酰胺-亚胺平衡中诱导所需的可逆变化。通过 NMR 和 UV-vis 光谱、X 射线晶体学和分子建模方法研究了这一过程。结果表明，亚胺 H(2)L(i)在与 Ni(2+)络合时发生选择性闭环反应。因此，得到了 Ni(HL(a))(2)型配合物，其手性源于手性配体 H(2)L(a)和结构的螺旋性。因此，螺旋对映体形成以下外消旋体：[Δ-C(R,R)N(S,S),Λ-C(S,S)N(R,R)]-Ni(HL(a))(2)·2HOAc 和 [Δ,Λ-C(S,R)N(R,S)]-Ni(HL(a))(2)·4MeOH。与观察到的 Ni(2+)情况相反，配体 H(2)L(a)的环状互变异构体在与 Pd(2+)形成配合物时发生选择性开环反应，最终生成 Pd(HL(i))(2)·MeOH，其中开链亚胺配体通过喹啉残基的 N,O 供体组双齿配位。进行密度泛函理论计算以深入了解两种配位金属（Ni(2+)和 Pd(2+)）的不同行为，并提出四氢喹唑啉环的金属辅助开/闭环反应机制。
