Structural Determination of an Unusual Cu^I‐Porphyrin‐π‐Bond in a Hetero‐Pacman Cu‐Zn‐Complex

Abstract

The synthesis and characterization of a hetero‐dinuclear compound is presented, in which a copper(I) trishistidine type coordination unit is positioned directly above a zinc porphyrin unit. The close distance between the two coordination fragments is secured by a rigid xanthene backbone, and a unique (intramolecular) copper porphyrin‐π‐bond was determined for the first time in the molecular structure. This structural motif was further analyzed by temperature‐dependent NMR studies: In solution at room temperature the coordinative bond fluctuates, while it can be frozen at low temperatures. Preliminary reactivity studies revealed a reduced reactivity of the copper(I) moiety towards dioxygen. The results adumbrate why nature is avoiding metal porphyrin‐π‐bonds by fixing reactive metal centers in a predetermined distance to each other within multimetallic enzymatic reaction centers.

An Iron Porphyrin Complex with Pendant Pyridine Substituents Facilitates Electrocatalytic CO2Reduction via Second Coordination Sphere Effects

“…Ligand Py 2 XPH 2 was synthesized via demetallation of known compound Py 2 XPZn [7] followed by iron insertion according to established procedures [8] . The demetallation procedure was best achieved by using half‐concentrated sulfuric acid, which led to quantitative conversion.…”

“…has shown that the addition of Lewis acidic metals to the electrochemical reaction mixture of iron porphyrins improves overall catalytic performance facilitating the CO 2 reduction reaction [37] . As well as acting as hydrogen bonding residues to promote catalysis, the bispyridylamine‐based hanging group of the Py 2 XPFe complex can function as an additional coordination sphere for a second metal [7] . Kojima et al.…”

“…Metalloligand Py 2 XPZn was synthesized according to a recently published procedure. [7] Solvents were purified and dried according to standard procedures or directly taken from a MBraun solvent purification system (e. g. THF, acetonitrile or toluene). Unless otherwise stated, all reactions which are sensitive towards air or moisture were carried out under dry argon by using Schlenk techniques.…”

“…Recrystallization from a methanol/CHCl 3 solution gives crystalline product that is suitable to be investigated with x-ray crystallography (see SI). Synthesis of the Py 2 XPFeCl: Following known procedures [7] Py 2 XPH 2 (59 mg, 0.055 mmol) and FeCl 2 (28 mg, 0.22 mmol) were dissolved in 15 mL dry DMF and heated at 110°C overnight. After cooling, 10 mL of a 1 M hydrochloric acid solution was added and stirred for one hour at room temperature.…”

“…[37] As well as acting as hydrogen bonding residues to promote catalysis, the bispyridylaminebased hanging group of the Py 2 XPFe complex can function as an additional coordination sphere for a second metal. [7] Kojima et al have also reported on a Ni II complex bearing an S 2 N 2 -type ligand with peripheral pyridine substituents, which has shown to coordinate Mg 2 + in solution and display improved CO 2 to CO reduction reactivity compared with the parent complex. [38] With this in mind, we sought to investigate the introduction of a second Lewis acidic metal coordinating in-situ in the vacant site of the Py 2 XPFe complex to improve catalytic performance.…”