Synthesis, Characterization, And Computational Study of Complexes Containing Pt···H Hydrogen Bonding Interactions

Complexes [Pt­(C6F5)­(bzq)­L] (bzq = 7,8-benzoquinolinate; L = 8-hydroxyquinoline, hqH (1); 2-methyl-8-hydroxyquinoline, hqH′ (2)) have been prepared by replacing the labile acetone ligand in the starting material [Pt­(C6F5)­(bzq)­(Me2CO)]. The 1H NMR spectra of 1 and 2 show that the signals attributable to the hydroxyl proton of the hqH or hqH′ ligands are displaced downfield 2.64 ppm for 1 and 2.74 ppm for 2 with respect to the respective free ligands. Moreover, in both complexes the signals present platinum satellites with J(Pt,H) coupling constant of 67.0 Hz for 1 and 80.6 Hz for 2. All these features are indicative of the existence of Pt···H–O hydrogen bonds in solution for these complexes. The structures of complexes 1 and 2 have been established by an X-ray diffraction study and allow us to confirm the existence of these interactions in the solid state too. Thus, in both cases the hydroxyl hydrogen atom is pointing toward the metal center, and the measured geometric parameters involving this hydrogen are Pt–H = 2.09(4) Å, O–H = 0.94(4) Å, Pt–H–O 162(4)°, for 1, and Pt–H = 2.10(4) Å, O–H = 0.91(4) Å, Pt–H–O 162(4)°, for 2, all of which are fully compatible with a hydrogen bond system. Complexes 1 and 2 and the analogues [Pt­(C6F5)3(hqH)]− (A) and [Pt­(C6F5)3(hqH′)]− (B), prepared some time ago in our laboratory and also showing Pt···H–O hydrogen bonds, have been the object of theoretical calculations to obtain better insight into the Pt···H interactions. Their density functional theory (DFT) calculated structures show excellent agreement with the X-ray determined ones (1, 2, and B). Topological analyses of the electron density function (ρ­(r)) have been performed on the four complexes according to Bader’s Atoms In Molecules theory. These analyses reveal a bond path that relates the platinum atom and the hydroxyl hydrogen atom, as well as the corresponding bond critical points. The values of the Laplacian ∇2ρ­(r) and local energy density H(r) indicate that these are closed shell, electrostatic interactions, but with partial covalence. The deprotonation of the OH fragment in 1 and 2 with BuLi leads to the formation of the unexpected trinuclear complexes (NBu4)­[Li­{Pt­(C6F5)­(bzq)­(L)}2] (L = hq (3), hq′ (4)). The X-ray structures of these have shown a change in the coordination of the deprotonated hq and hq′, which are now bonded to the Pt atoms through their O atoms, and which are bridging the Pt and Li metal atoms.通过置换起始原料[Pt(C₆F₅)(bzq)(丙酮)]中易解离的丙酮配体，成功合成了配合物[Pt(C₆F₅)(bzq)(L)]（其中bzq为7,8-苯并喹啉酸根（7,8-benzoquinolinate）；L分别为8-羟基喹啉（8-hydroxyquinoline, hqH），对应配合物1；以及2-甲基-8-羟基喹啉（2-methyl-8-hydroxyquinoline, hqH'），对应配合物2）。对配合物1和2的氢核磁共振（¹H NMR）谱表征结果显示，相较于各自的游离配体，配合物1与2中hqH、hqH'配体的羟基质子信号分别向低场位移了2.64 ppm与2.74 ppm。此外，两种配合物的信号均呈现铂卫星峰，其中配合物1的铂-氢（J(Pt,H)）耦合常数为67.0 Hz，配合物2则为80.6 Hz。上述所有特征均表明，此类配合物在溶液中存在Pt···H-O氢键相互作用。通过X射线衍射（X-ray diffraction）分析确定了配合物1与2的晶体结构，证实此类相互作用在固态下同样存在。具体而言，两种配合物中的羟基质子均指向金属中心，测得的相关几何参数如下：配合物1的Pt-H键长为2.09(4) Å，O-H键长为0.94(4) Å，Pt-H-O键角为162(4)°；配合物2的Pt-H键长为2.10(4) Å，O-H键长为0.91(4) Å，Pt-H-O键角为162(4)°，所有参数均与氢键体系的特征完全吻合。本课题组此前已合成的类似物[Pt(C₆F₅)₃(hqH)]⁻（A）与[Pt(C₆F₅)₃(hqH')]⁻（B）同样存在Pt···H-O氢键相互作用，本次研究将配合物1、2与A、B一同纳入理论计算范畴，以深入探究Pt···H相互作用的本质。密度泛函理论（DFT）优化得到的配合物结构与X射线衍射测定的结果（1、2与B）具有极佳的一致性。基于巴德分子中原子理论（Bader’s Atoms In Molecules theory），对四种配合物的电子密度函数ρ(r)进行了拓扑分析。分析结果显示，铂原子与羟基质子之间存在键径，同时存在对应的键临界点。电子密度拉普拉斯值∇²ρ(r)与局域能量密度H(r)的计算结果表明，此类相互作用属于闭壳层静电相互作用，但同时带有部分共价性。采用正丁基锂（BuLi）对配合物1与2中的OH片段进行去质子化，意外得到了三核配合物(NBu₄)[Li{Pt(C₆F₅)(bzq)(L)}₂]（L分别为hq，对应配合物3；hq'，对应配合物4）。X射线衍射结构分析显示，去质子化后的hq与hq'配体的配位模式发生改变：它们通过氧原子与铂原子配位，并作为桥联配体连接铂与锂两个金属中心。