Department of Chemistry and Center for Theoretical and Computational Chemistry, University of Tromsø, 9037 Tromsø, Norway.
J Phys Chem B. 2010 Nov 25;114(46):15380-8. doi: 10.1021/jp107692m. Epub 2010 Oct 27.
Presented herein is a first density functional theory (DFT) (ZORA, STO-TZP) survey of ruthenium(IV) porphyrins with monoanionic nitrogen ligands, modeled after experimentally observed ruthenium porphyrin bis(amido), bis(methyleneamido), and bis(pyrazolato) complexes. Three exchange correlation functionals--PW91, OLYP, and B3LYP, which often behave somewhat differently--provide good, consistent descriptions of the lowest singlet and triplet states. For ruthenium porphyrin bis(amido) and bis(methyleneamido) complexes, the calculations reproduce the experimentally observed S = 0 ground states, with the triplet states only a few tenths of an electron-volt higher in energy. The singlet-triplet energy gaps decrease somewhat along the series PW91 > OLYP > B3LYP. Molecular orbital (MO) analyses also provide a qualitative explanation for the singlet ground states of these complexes, which may be contrasted with the triplet states of heme protein compound II intermediates and their synthetic iron(IV) models. Amido and methyleneamido ligands have a single π-lone pair, unlike hydroxide, alkoxide, and thiolate ligands, which have two. The former therefore engage in a single π-bonding interaction with one of the Ru d(π) orbitals, resulting in an S = 0 d(4) electronic configuration. In contrast, the O or S ligands present in compound II engage in π-bonding with both d(π) orbitals, resulting in an S = 1 ground state. For the ruthenium(IV) bis(methyleneamido) complexes, our MO analysis indicates a somewhat different bonding description, relative to that proposed by the experimental researchers, who invoked Ru(d(π)) → N(methyleneamido)(π*) backbonding to explain Ru-N(methyleneamido) multiple bond character. Instead, we found that the metal-methyleneamido π-bonding almost exclusively involves N-to-Ru π-donation and thus is qualitatively very similar to metal-amido π-bonding. Ruthenium(IV) bis(pyrazolato) complexes provide rare examples of ruthenium(IV) centers with all-nitrogen ligation that are paramagnetic. OLYP successfully captures this "inverse" spin state energetics; PW91 and B3LYP do so less well.
本文呈现了基于实验观察的钌卟啉双（酰胺）、双（亚甲基酰胺）和双（吡唑啉）配合物，对具有单阴离子氮配体的钌（IV）卟啉进行了第一性原理密度泛函理论（DFT）（ZORA，STO-TZP）研究。三种交换相关泛函——PW91、OLYP 和 B3LYP，它们的表现通常略有不同——对最低单重态和三重态提供了良好、一致的描述。对于钌卟啉双（酰胺）和双（亚甲基酰胺）配合物，计算结果再现了实验观察到的 S = 0 基态，而三重态仅高出几个十分之一个电子伏特。随着系列的进行，PW91 > OLYP > B3LYP，单重态-三重态能隙略有减小。分子轨道（MO）分析也为这些配合物的单重基态提供了定性解释，这可以与血红素蛋白化合物 II 中间体及其合成铁（IV）模型的三重态进行对比。酰胺和亚甲基酰胺配体只有一个 π-孤对电子，与氢氧化物、烷氧基和硫醇盐配体不同，后者有两个。因此，前者与一个 Ru d(π) 轨道发生单一的 π 键合相互作用，导致 S = 0 d(4)电子构型。相比之下，化合物 II 中存在的 O 或 S 配体与两个 d(π) 轨道发生 π 键合，导致 S = 1 基态。对于钌（IV）双（亚甲基酰胺）配合物，我们的 MO 分析表明，与实验研究人员提出的观点相比，存在略微不同的键合描述，实验研究人员援引 Ru(d(π)) → N(methyleneamido)(π*) backbonding 来解释 Ru-N(methyleneamido)多重键特性。相反，我们发现金属-亚甲基酰胺 π 键合几乎完全涉及 N 到 Ru 的 π 供体，因此在性质上与金属酰胺 π 键合非常相似。钌（IV）双（吡唑啉）配合物提供了具有全氮配体的钌（IV）中心的罕见实例，它们是顺磁性的。OLYP 成功捕获了这种“反转”自旋态能学；PW91 和 B3LYP 则不然。
