Structure and excited-state dynamics of dimeric copper(i) photosensitizers investigated by time-resolved X-ray and optical transient absorption spectroscopy.

Time-resolved X-ray (tr-XAS) and optical transient absorption (OTA) spectroscopy in the picosecond time scale coupled with Density Functional theory (DFT) and X-ray absorption near-edge structure (XANES) calculations are applied to study three homoleptic Cu(i) dimeric chromophores with ethyl and longer propyl spacers, denoted as [Cu2(mphenet)2]Cl2 (C1), [Cu2(mphenet)2](ClO4)2 (C2) and [Cu2(mphenpr)2](ClO4)2 (C3) (where mphenet = 1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane and mphenpr = 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane). Tr-XAS analysis after light illumination at ∼ 100 ps illustrate the formation of a flattened triplet excited state in all 3 complexes. Optical transient absorption (OTA) analysis for C1 monitored in water and C2 and C3 measured in acetonitrile reveals distinct excited-state lifetimes of 169 ps, 670 ps and 1600 ps respectively. These differences are associated to changes in the solvent (comparing C1 and C2) and the flexibility of the ligand to adapt after Cu flattening upon excitation (C2 and C3). Our results are important for the improved structural dynamics of these types of Cu-based dimeric compounds, and can guide the integration of these chromophores into more complex solar energy conversion schemes.

Reactivity of CuIN4 Flattened Complexes: Interplay between Coordination Geometry and Ligand Flexibility.

The relation between redox activity and coordination geometry in CuIN4 complexes indicates that more flattened structures tend to be more reactive. Such a preorganization of the ligand confers to the complex geometries closer to a transition state, which has been termed the "entatic" state in metalloproteins, more recently extending this concept for copper complexes. However, many aspects of the redox chemistry of CuI complexes cannot be explained only by flattening. For instance, the role of ligand flexibility in this context is an open debate nowadays. To analyze this point, we studied oxidation properties of a series of five monometallic CuI Schiff-base complexes, [CuI(Ln)]+, which span a range of geometries from a distorted square planar (n = 3) to a distorted tetrahedron (n = 6, 7). This stepped control of the structure around the CuI atom allows us to explore the effect of the flattening distortion on both the electronic and redox properties through the series. Experimental studies were complemented by a theoretical analysis based on density functional theory calculations. As expected, oxidation was favored in the flattened structures, spanning a broad potential window of 370 mV for the complete series. This orderly behavior was tested in the reductive dehalogenation reaction of tetrachloroethane (TCE). Kinetic studies show that CuI oxidation by TCE is faster as the flattening distortion is higher and the oxidation potentials of the metal are lower. However, the most reactive complex was not the more planar, contradicting the trend expected from oxidation potentials. The origin of this irregularity is related to ligand flexibility and its connection with the atom/electron transfer reaction path, highlighting the need to consider effects beyond flattening distortion to better understand the reactivity of this important class of complexes.

Influence of the substituent on the phosphine ligand in novel rhenium(i) aldehydes. Synthesis, computational studies and first insights into the antiproliferative activity.

Cyrhetrenyl aldehyde derivatives [(η5-C5H4CHO)Re(CO)2PR3] with R = methyl (Me, 2a), phenyl (Ph, 2b), and cyclohexyl (Cy, 2c) were synthesized by a photochemical reaction from the starting material [(η5-C5H4CHO)Re(CO)3] (1) and the corresponding phosphines. The complexes were fully characterized by FT-IR, 1H, 13C and 31P NMR spectroscopy, elemental analysis and mass spectrometry. The molecular structures of 2a-c have also been determined. Electronic structure calculations by TD-DFT and electrochemical studies are in sound agreement with the effect of the substitution of one carbonyl group by a phosphine ligand. Additionally, the antiproliferative activity of complexes 1 and 2a-c was studied on the human cancer cell lines HT-29 and PT-45 using an MTT assay. Out of all new compounds, only the triphenylphosphine derivative 2b exhibited pronounced cytotoxic effects on both cell lines, being ca. 1.5 times more potent than cisplatin.

Steric and Electronic Factors Affecting the Conformation of Bimetallic CuI Complexes: Effect of the Aliphatic Spacer of Tetracoordinating Schiff-Base Ligands.

A family of six homoleptic [CuI (Ln )]2 (ClO4 )2 and six heteroleptic [CuI (Ln )(PPh3 )2 ]2 (ClO4 )2 bimetallic complexes, in which Ln are bis-Schiff base ligands with alkyl spacers of variable length (n=2-7 -CH2 -), were prepared to evaluate the role of the spacer on the formation of helicates or mesocates. In the homoleptic series, spectroscopic and theoretical studies indicate that preferences for a conformation are based on energetic parameters, mainly, the establishment of noncovalent interactions. The odd-even nature of the spacers preconditions the superposition of the aromatic rings to allow the juxtaposition necessary for noncovalent interactions, whereas the increase of the length reduces the strength of such interactions. Consequently, complexes with even-spacers of short length were identified as helicates in solution, [CuI (Ln )]22+ (n=2, 4). Complexes [CuI (Ln )]22+ (n=3-7) dissociate in solution to produce the monometallic complexes in equilibrium, [CuI (Ln )]+ . The stability of the bimetallic species is discussed in terms of their conformations. The set of heteroleptic complexes was prepared to evaluate the reach of the "odd-even rule" in the solid, which is based on the "zig-zag" arrangements of the spacers. Based on crystallographic information, "S-" and "C"-type conformations of Ln are related to even and odd spacers, respectively. This trend is considered in addition to other factors to explain preferences for either a mesocate or helicate conformation in the homoleptic series.