Binuclear rhenium(I) complexes with 1,2-bis(4,4'-methyl-[2,2']bipyridyl)-ethane and 1,2-bis(4,4'-methyl-[2,2']bipyridyl)-dodecane as bridging ligands and their mononuclear analogues have been synthesized and characterized by their spectroscopic and electrochemical properties. First reduction potentials and luminescence properties as well as the reductive quenching of the emissive state with TEOA were not affected by the alkyl linker. By means of a detailed comparison of the photocatalytic CO(2) reductions of the monometallic and the bimetallic complexes a great beneficial effect on the activity depending on the proximity of the centres was found. In high dilution the overall kinetics in the CO(2) photoreduction of mononuclear complexes are clearly monometallic. If the proximity of the centres is adjusted according to the lifetime of the OER (one electron reduced species) the photocatalytic activity is greatly improved showing a clear bimetallic mechanism. In the binuclear rhenium complexes, both the facile generation of a free coordination site and binuclear interactions for effective two electron transfer can be realized.

Photocatalytic reduction of CO2 with rhenium(I) bipyridine complexes has been studied for several decades. Nonetheless, important parameters affecting the catalytic performance remain elusive to date. By using the standard catalyst [Re(dmb)(CO)3Cl] (dmb=4,4′‐dimethyl‐2,2′‐bipyridine), the effect of catalyst concentration and irradiation intensity is studied in detail and important correlations are revealed. The decomposition of the catalyst is investigated, and two main deactivation pathways are proposed, both of which involve the one‐electron‐reduced species and are likely to be valid for other homogeneous photocatalysts as well. The rate of deactivation is linked to the relative concentration of 1) the catalyst in its electronic ground state, 2) the catalyst in its excited state, 3) the one‐electron‐reduced species, and 4) quencher radicals. Adequate tuning of catalyst concentration and irradiation intensity leads to the highest quantum yield (Φ=0.53) reported to date for a single‐molecule system.

Mononuclear iridium(III) complexes [Ir(mppy)(tpy)X] (mppy = 4-methyl-2-phenylpyridine, X = Cl, I) and binuclear analogues with various bis(2-phenylpyridin-4-yl) bridging ligands were synthesized and characterized by their spectroscopic and electrochemical properties. Kinetic measurements concerning the photocatalytic two electron reduction of CO2 to CO were investigated in order to determine the influence of intermolecular interactions between two active centers. A detailed comparison between the monometallic and the bimetallic complexes indicates an enhanced lifetime (TON) of the covalently linked complexes, causing an increased overall conversion of CO2. Additionally the deactivation pathways of the catalysts are examined.

Abstract:This review gives an overview on the principles of light-promoted homogeneous redox catalysis in terms of applications in CO 2 conversion. Various chromophores and the advantages of different structures and metal centers as well as optimization strategies are discussed. All aspects of the reduction catalyst site are restricted to CO 2 conversion. An important focus of this review is the question of a replacement of the sacrificial donor which is found in most of the current publications. Furthermore, electronic parameters of supramolecular systems are reviewed with reference to the requisite of chromophores, oxidation and reduction sites.

A trinuclear complex consisting of one [Ru(dmb)3]2+ (dmb=4,4′‐dimethyl‐2,2′‐bipyridine) (Ru) and two [Re(dmb)(CO)3Cl] (Re) building blocks, [Re(CO)3Cl(dmb−dmb)Ru(dmb)(dmb−dmb)Re(CO)3Cl](PF6)2 (Re−Ru−Re), is presented. Photophysical properties of Re−Ru−Re and the individual components with different or no covalent linkages are thoroughly investigated and compared. To elucidate the role of the single covalent bonds, photocatalytic reduction of CO2 is performed with the trinuclear complex and a series of model systems featuring systematic absence of linkages between the metal centers. Photoluminescence spectra and quantum yields reveal efficient energy transfer from the excited state of Re to Ru if these fragments are covalently linked. Moreover, intramolecular electron transfer from the one‐electron reduced species of Ru to Re occurs if there is covalent bonding, leading to a higher photostability and thus the highest turnover number in photocatalytic CO2 reduction of 199 for the trinuclear complex Re−Ru−Re within the systems under investigation. Optimized experimental conditions reveal the highest turnover number (315) reported to date for ReI/RuII‐based homogeneous catalysts in photocatalytic CO2 reduction.

A trimetallic Ir(iii) based complex () was synthesized and fully characterized by spectroscopic and electrochemical methods. A detailed comparison to its mono- () and bimetallic () analogue regarding the photocatalytic reduction of CO2 is outlined. In particular, the effect of intramolecular quenching, provided by ethyl tethers, was investigated. Moreover, the relationship between the photophysical properties, the lifetime of the excited state, the quenching efficiency and the catalytic performance is presented. Notably, the covalent linkage of the Ir(iii) moieties within the three-armed ligand structure (complex ) leads to a twofold increase of the turn over number (TON) compared to its monometallic analogue . Taking in account the quantum efficiency of 10% and the TONCO = 60 (per Ir(iii) center), complex is a highly active Ir(iii) based photocatalyst.

The recently discovered ring-opening polymerization (ROP) of β-butyrolactone (β-BL) using Cr III (salphen) as catalyst converts racemic β-BL to isotactic enriched poly(hydroxybutyrate) (PHB). These achiral complexes arrange themselves in a dimeric sandwich-like structure entrapping the growing polymer chain and the monomer. The polymerization mechanism discussed based on previous DFT calculations is supported by kinetic studies in here. Furthermore, the influence of different substitution patterns in salphen complexes on polymerization of β-BL was investigated. In addition, effects of different polymerization starters and additives on the broad molecular weight distribution of PHB have been investigated, which show the limitations of this catalysis.

Two novel rhenium(I) phosphinine complexes [Re(CO) (2) were synthesized and the molecular structure of both was determined by single crystal X-ray diffraction. In compound 1 the two coordinated phosphinine ligands are arranged in a cis position, whereas compound 2 with three phosphinine ligands crystallizes in a meridional structural motif.Density functional theory investigations were executed to examine the relative stabilities of both complexes 1 and 2.