Influence of Mixed Substituents on the Macrocyclic Ring Distortions of Free Base Porphyrins and Their Metal Complexes

Crystal structures of a series of free base porphyrins, 2,3,12,13-tetra(cyano/chloro/bromo)-5,7,8,10,15,17,18,20-octaphenylporphyrin solvates [H2(TPP(Ph)4(CN)4)·3(C2H4Cl2), H2(TPP(Ph)4Cl4)·2(CH3OH), and H2(TPP(Ph)4Br4)·2THF·1.5(CH3OH)], were determined to examine the influence of mixed antipodal β-pyrrole substitution on the stereochemistry of the porphyrin macrocycle. Nonplanarity of the porphyrin skeleton increases with an increase in size of the X group at the β-pyrrole positions, and the root-mean-square deviation of the core atoms follows the order CN (0.508 Å) < Cl (0.687 Å) < Br (0.792 Å). The normal-coordinate decomposition analysis of the free-base structures shows dramatic substituent- (X-)dependent out-of-plane distortions featuring saddling combined with a ruffled conformation in H2(TPP(Ph)4(CN)4), while it is predominantly saddled geometry in H2(TPP(Ph)4X4) (X = Cl, Br) structures. For H2(TPP(Ph)4X4) (X = Cl, Br) structures, the core elongation is along the antipodal pyrroles bearing halogen groups, and in the case of the H2(TPP(Ph)4(CN)4) structure, it is along the other antipodal pyrroles with phenyl groups. However, the average core, N···N separation along the transannular pyrrole direction follows the trend H2(TPP(Ph)4(CN)4) (4.134(4) Å) < H2(TPP(Ph)4Cl4) (4.184(5) Å) < H2(TPP(Ph)4Br4) (4.205(5) Å). The bond lengths of the 24-atom core are comparable, but its bond angles showed significant differences along the antipodal direction bearing β-pyrrole with X groups when compared to the other transannular pyrrole direction. The four-nitrogen porphyrin core (N4H2) exhibited weak intramolecular hydrogen bonding and also intermolecular interactions. Interestingly, H2(TPP(Ph)4Cl4)·2(CH3OH) shows an extended chain structure involving hydrogen-bonding interactions between the CH3OH···OHCH3 (O···O) and CH3OH···core (N4H2) interactions. The nonplanar geometry of these free base porphyrin rings suggests a more predominant influence of steric crowding of the peripheral substituents rather than intermolecular interactions. The four-coordinated Ni(TPP(Ph)4(CN)4)·C6H14·0.5(C2H4Cl2) complex shows an enhanced ruffling of the macrocycle along with the saddled conformation relative to more saddle-shaped H2(TPP(Ph)4(CN)4)·3(C2H4Cl2) structure. The crystal structure of the Zn(TPP(Ph)4Cl4)(Py)·(C2H4Cl2) complex features distorted square-pyramidal geometry with the reduction in the nonplanarity of the core in contrast to its free base porphyrin structure. Normal-coordinate-decomposition analysis for the out-of-plane displacement of the core atoms in the Ni(TPP(Ph)4(CN)4) structure showed enhanced ruffling combined with saddling of the macrocycle while Zn(TPP(Ph)4Cl4)(Py) exhibited mainly saddling when compared to their corresponding free base porphyrin structures. The nonplanar distortion in the Ni(TPP(Ph)4(CN)4)·(C6H14)·0.5(C2H4Cl2) complex indicates that the contracted porphyrin core and the weak intermolecular interactions are responsible for the nonplanar geometry of the macrocyclic ring.

一系列自由碱基卟啉（free base porphyrin）的晶体结构，即2,3,12,13-四(氰基/氯/溴)-5,7,8,10,15,17,18,20-八苯基卟啉溶剂化物[H₂(TPP(Ph)₄(CN)₄)·3(C₂H₄Cl₂)、H₂(TPP(Ph)₄Cl₄)·2(CH₃OH)以及H₂(TPP(Ph)₄Br₄)·2THF·1.5(CH₃OH)]，已被解析测定，旨在考察对顶β-吡咯混合取代对卟啉大环骨架立体化学的影响。卟啉骨架的非平面性随β-吡咯位取代基X的尺寸增大而增强，核心原子的均方根偏差（root-mean-square deviation）遵循CN(0.508 Å) < Cl(0.687 Å) < Br(0.792 Å)的顺序。对自由碱基卟啉结构的简正坐标分解（normal-coordinate decomposition）分析显示，取代基（X）对平面外畸变（out-of-plane distortions）存在显著调控作用：H₂(TPP(Ph)₄(CN)₄)的畸变兼具鞍型扭曲（saddling）与褶皱构象（ruffled conformation），而H₂(TPP(Ph)₄X₄)（X=Cl、Br）的构象则以鞍型扭曲为主。对于H₂(TPP(Ph)₄X₄)（X=Cl、Br）结构，卟啉核心的伸长沿带有卤素取代基的对顶吡咯方向；而H₂(TPP(Ph)₄(CN)₄)的核心伸长则沿另一组带有苯基取代基的对顶吡咯方向。不过，跨环（transannular）吡咯方向的平均核心N···N间距遵循H₂(TPP(Ph)₄(CN)₄)(4.134(4) Å) < H₂(TPP(Ph)₄Cl₄)(4.184(5) Å) < H₂(TPP(Ph)₄Br₄)(4.205(5) Å)的趋势。24原子卟啉核心的键长较为相近，但相较于另一组跨环吡咯方向，带有β-吡咯X取代基的对顶方向的键角存在显著差异。四氮卟啉核心（N₄H₂）表现出弱分子内氢键（intramolecular hydrogen bonding）与分子间相互作用。值得注意的是，H₂(TPP(Ph)₄Cl₄)·2(CH₃OH)呈现出延伸链结构，其氢键相互作用包括CH₃OH···OHCH₃（O···O）以及CH₃OH···核心（N₄H₂）的相互作用。这些自由碱基卟啉环的非平面几何构型表明，外围取代基的空间拥挤效应相较于分子间相互作用占据主导地位。四配位的Ni(TPP(Ph)₄(CN)₄)·C₆H₁₄·0.5(C₂H₄Cl₂)配合物相较于H₂(TPP(Ph)₄(CN)₄)·3(C₂H₄Cl₂)更偏向鞍型的结构，其大环的褶皱程度更高，同时兼具鞍型构象。Zn(TPP(Ph)₄Cl₄)(Py)·(C₂H₄Cl₂)配合物的晶体结构呈现畸变四方锥几何构型（square-pyramidal geometry），与对应的自由碱基卟啉结构相比，其核心的非平面性有所降低。对Ni(TPP(Ph)₄(CN)₄)结构核心原子平面外位移的简正坐标分解分析显示，与对应自由碱基卟啉结构相比，该配合物的大环兼具更高程度的褶皱与鞍型扭曲；而Zn(TPP(Ph)₄Cl₄)(Py)则主要呈现鞍型扭曲。Ni(TPP(Ph)₄(CN)₄)·C₆H₁₄·0.5(C₂H₄Cl₂)配合物中的非平面畸变表明，收缩的卟啉核心与弱分子间相互作用是导致该大环环非平面几何构型的原因。