Combined Experimental and Theoretical Investigation of Ligand and Anion Controlled Complex Formation with Unprecedented Structural Features and Photoluminescence Properties of Zinc(II) Complexes

By using two potential tridentate ligands, HL1 [4-chloro-2-[(2-morpholin-4-yl-ethylimino)-methyl]-phenol] and HL2 [4-chloro-2-[(3-morpholin-4-yl-propylimino)-methyl]-phenol], which differ by one methylene group in the alkyl chain, four new ZnII complexes, namely, [Zn­(L2H)2]­(ClO4)2 (1), [Zn­(L1)­(H2O)2]­[Zn­(L1)­(SCN)2] (2), [Zn­(L1)­(dca)]n (3), and [Zn2(L1)2(N3)2(H2O)2] (4) [where dca = dicyanamide anion] were synthesized and structurally characterized. The results indicate that the slight structural difference between the ligands, HL1 and HL2, because of the one methylene group connecting the nitrogen atoms provokes a chemical behavior completely different from what was expected. Any attempt to isolate the Zn­(L2) complexes with thiocyanato, dicyanamido, and azide was unsuccessful, and perchlorate complex 1 was always obtained. In contrast, with HL1 we obtained structural diversity on varying the anions, but we failed to isolate the analogous perchlorate complex of HL1. Single-crystal X-ray analyses revealed that the morpholine nitrogen of ligand L2 is protonated and thus does not take part in coordination with ZnII in complex 1. On the other hand, the morpholine nitrogen of L1 is coordinated to ZnII in 2–4. Of these, 2 and 4 are rare examples of a cocrystallized cationic/anionic complex and of a dinuclear complex bridged by a single azide, respectively. Some of these unexpected findings and some interesting noncovalent interactions leading to the formation of dimeric entities in solid-state compound 4 were rationalized by a DFT approach. Photoluminescence properties of the complexes as well as the ligands were investigated in solution at ambient temperature and at 77 K. The very fast photoinduced electron transfer (PET) from the nitrogen lone pair to the conjugated phenolic moiety is responsible for very low quantum yield (Φ) exhibited by the ligands, whereas complexation prevents PET, thus enhancing the Φ in the complexes. The origin of the electronic and photoluminescence properties of the ligands and complexes was assessed in light of theoretical calculations.

本研究以两个烷基链相差一个亚甲基（methylene group）的三齿配体（tridentate ligand）HL1[4-氯-2-[(2-吗啉-4-基-乙基亚氨基)甲基]-苯酚]和HL2[4-氯-2-[(3-吗啉-4-基-丙基亚氨基)甲基]-苯酚]为配体，成功合成并结构表征了四种新型二价锌（Zn(II)）配合物，分别为[Zn(L2H)₂](ClO₄)₂(1)、[Zn(L1)(H₂O)₂][Zn(L1)(SCN)₂](2)、[Zn(L1)(dca)]ₙ(3)以及[Zn₂(L1)₂(N₃)₂(H₂O)₂](4)【其中dca为二氰胺根阴离子】。研究结果表明，由于HL1与HL2之间仅存在一个连接氮原子的亚甲基的微小结构差异，二者表现出与预期完全不同的化学行为。所有尝试分离得到含硫氰酸根（thiocyanato）、二氰胺根（dicyanamide）或叠氮根（azide）的Zn(L2)配合物的工作均告失败，最终始终只能得到高氯酸盐（perchlorate）配合物1。与此形成鲜明对比的是，使用HL1作为配体时，通过改变阴离子种类可获得结构多样的配合物，但未能分离得到HL1对应的高氯酸盐类似配合物。单晶X射线衍射分析（single-crystal X-ray analyses）结果显示，配合物1中配体L2的吗啉环氮原子发生了质子化（protonated），因此未参与与Zn(II)的配位（coordination）作用；而在配合物2~4中，配体L1的吗啉环氮原子则与Zn(II)形成配位键。其中，配合物2和4分别为罕见的共结晶阴阳离子配合物（cocrystallized cationic/anionic complex）以及由单个叠氮根桥联的双核配合物（dinuclear complex）。本研究借助密度泛函理论（Density Functional Theory, DFT）方法，对部分意外的实验结果以及促使固态配合物4形成二聚体实体的有趣非共价相互作用进行了合理化阐释。我们在室温与77 K的溶液体系中，测试了各配体与配合物的光致发光性质。配体之所以表现出极低的量子产率(Φ)，是因为其氮原子孤对电子向共轭酚羟基单元发生了超快的光诱导电子转移（Photoinduced Electron Transfer, PET）；而配合物的形成阻断了该光诱导电子转移过程，因此提升了体系的量子产率。本研究结合理论计算，对配体与配合物的电子结构及光致发光性质的起源进行了分析与评估。