Capturing Re(i) in an neutral N,N,N pincer Scaffold and resulting enhanced absorption of visible light

Structural and Photophysical Trends in Rhenium(I) Carbonyl Complexes with 2,2':6',2″-Terpyridines

This is the first comprehensive review of rhenium(I) carbonyl complexes with 2,2':6',2″-terpyridine-based ligands (R-terpy)-encompassing their synthesis, molecular features, photophysical behavior, and potential applications. Particular attention has been devoted to demonstrating how the coordination mode of 2,2':6',2″-terpyridine (terpy-κ2N and terpy-κ3N), structural modifications of terpy framework (R), and the nature of ancillary ligands (X-mono-negative anion, L-neutral ligand) may tune the photophysical behavior of Re(I) complexes [Re(X/L)(CO)3(R-terpy-κ2N)]0/+ and [Re(X/L)(CO)2(R-terpy-κ3N)]0/+. Our discussion also includes homo- and heteronuclear multicomponent systems with {Re(CO)3(R-terpy-κ2N)} and {Re(CO)2(R-terpy-κ3N)} motifs. The presented structure-property relationships are of high importance for controlling the photoinduced processes in these systems and making further progress in the development of more efficient Re-based luminophores, photosensitizers, and photocatalysts for modern technologies.

Induced fac-mer rearrangements in {M(CO)3}+ complexes (M = Re, 99(m)Tc) by a PNP ligand

The fac-mer rearrangements in [MX3(CO)3]2- (M = Re, 99Tc) induced by a pincer-type ligand (PNP) and a "halide scavenger" are reported. The reactions of fac-[99mTc(CO)3(OH2)3]+ or [99mTcO4]- in saline both yield mer-[99mTc(PNP)(CO)3]+, the first example of a mer-{99mTc(CO)3}+ type complex. In contrast, reactions with terpyridine (terpy) only gave the facial κ2-terpy complexes with Re and 99Tc.

Electrocatalytic reduction of CO2 to CO by a series of organometallic Re(I)-tpy complexes

A series of organometallic Re(I)(L)(CO)3Br complexes with 4'-substituted terpyridine ligands (L) has been synthesised as electrocatalysts for CO2 reduction. The complexes' spectroscopic characterisation and computationally optimised geometry demonstrate a facial geometry around Re(I) with three cis COs and the terpyridine ligand coordinating in a bidentate mode. The effect of substitution on the 4'-position of terpyridine (Re1-5) on CO2 electroreduction was investigated and compared with a known Lehn-type catalyst, Re(I)(bpy)(CO)3Br (Re7). All complexes catalyse CO evolution in homogeneous organic media at moderate overpotentials (0.75-0.95 V) with faradaic yields of 62-98%. The electrochemical catalytic activity was further evaluated in the presence of three Brønsted acids to demonstrate the influence of the pKa of the proton sources. The TDDFT and ultrafast transient absorption spectroscopy (TAS) studies showed combined charge transfer bands of ILCT and MLCT. Amongst the series, the Re-complex containing a ferrocenyl-substituted terpyridine ligand (Re5) shows an additional intra-ligand charge transfer band and was probed using UV-Vis spectroelectrochemistry.

Anticancer and Antibiotic Rhenium Tri- and Dicarbonyl Complexes: Current Research and Future Perspectives

Organometallic compounds are increasingly recognized as promising anticancer and antibiotic drug candidates. Among the transition metal ions investigated for these purposes, rhenium occupies a special role. Its tri- and dicarbonyl complexes, in particular, attract continuous attention due to their relative ease of preparation, stability and unique photophysical and luminescent properties that allow the combination of diagnostic and therapeutic purposes, thereby permitting, e.g., molecules to be tracked within cells. In this review, we discuss the anticancer and antibiotic properties of rhenium tri- and dicarbonyl complexes described in the last seven years, mainly in terms of their structural variations and in vitro efficacy. Given the abundant literature available, the focus is initially directed on tricarbonyl complexes of rhenium. Dicarbonyl species of the metal ion, which are slowly gaining momentum, are discussed in the second part in terms of future perspective for the possible developments in the field.

Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes

Six rhenium(I) κ³N-dicarbonyl complexes with 4'-(4-substituted phenyl)terpyridine ligands were evaluated in their ground and excited states. These complexes, bearing substituents of different electron-donating strengths-from CN to NMe₂-were studied by a combination of transient IR (TRIR), electrochemistry, and IR spectroelectrochemistry, as well as time-dependent density functional theory (TD-DFT). They exhibit panchromatic absorption and can act as stronger photoreductants than their tricarbonyl counterparts. The ground- and excited-state potentials, absorption maxima, and lifetimes (250-750 ps) of these complexes correlate well with the Hammett σ_p substituent constants, showing the systematic effect of remote substitution in the ligand framework. TRIR spectroscopy allowed us to assign the lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous report on analogous κ²N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine, local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with long-lived triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state switching strategy via remote substitution.

In-Depth Study of the Electronic Properties of NIR-Emissive κ3N Terpyridine Rhenium(I) Dicarbonyl Complexes

The structure-properties relationship in a series of carbonyl rhenium(I) complexes based on substituted terpyridine ligands of general formula [Re(κ^xN-Rtpy)(CO)_yL]ⁿ⁺ is explored by both experimental and theoretical methods. In these compounds, the terpyridine ligands adopt both bidentate (κ²N) and terdentate (κ³N) coordination modes associated with three or two carbonyls, respectively. Conversion from the κ²N to the κ³N coordination mode leads to large changes in the absorption spectra and oxidation potentials due to destabilization of the HOMO level of each complex. The absorption profiles of the κ³N complexes cover the whole visible spectra with lower maxima around 700 nm, tailing out to 800 nm, while no emission is observed with Br^- as the axial ligand L. When the axial ligand is modified from the native halide to pyridine or triphenylphosphine, the lowest absorption band is blue-shifted by 60 and 90 nm, respectively. These cationic complexes are near-infrared emitters with emission maxima between 840 and 950 nm for the pyridine compounds and 780-800 nm for the triphenylphosphine compounds.

Towards better understanding of photophysical properties of rhenium(I) tricarbonyl complexes with terpy-like ligands

Series of Re(I) carbonyls complexes were designed and synthesized to explore the impact of the triimine skeleton and number of methoxy groups attached to aryl substituents on their optoelectronic and thermal properties. The chemical structures of the prepared complexes were confirmed by ¹H and ¹³C NMR spectroscopy, HR-MS, elemental anlsysis, and X-ray measurements. DSC measuremtns showed that they melted in the range of 198-325 °C. Some of them form stable molecular glasses with high glass transition temperatures (158-173 °C). Experimentally obtained optical properties were supported by DFT calculations. The UV-Vis spectra display a series of overlapping absorption bands in the range 200-350 nm, and much weaker broad band in the visible spectral region, due to intraligand and charge transfer transitions, respectively. All synthesized complexes were emissive in solution and in solid state as powder. Moreover, when applied in diodes, some of them exhibited ability for emission of light under external voltage with maximum of electroluminescence band located at 591-630 nm.

Pyridylbenzimidazole based Re(i)(CO)3 complexes: antimicrobial activity, spectroscopic and density functional theory calculations

fac-[ReBr(CO)3(L1,2)] L1 = 1-ethyl-2-(pyridin-2-yl)benzimidazole (1) and L2 = 1-[(pyridin-2-yl) benzimidazole]-propyl-sulfonic acid (2), fac-[Re2Br2(CO)6L3] (3) L3 = 1,1'-(hexane-1,6-diyl)bis[2-(pyridin-2-yl)1H-benzimidazole] and fac-[ReBr(CO)3(L4,5-κ2N1N2)] (L4 = 2,6-bis(benzimidazol-2'-yl)pyridine (4) and L5 = 2,6-bis(1-ethyl-benzimidazol-2'-yl)pyridine (5) were synthesized and fully characterized using different spectrocopic and analytical tools. The spectrocopic data showed coordination of L1-3 to fac-ReBr(CO)3 via the benzimidazole and pyridine N-atoms. For 4 and 5, the absence of a two-fold axis of symmetry for L4,5 in the 1H NMR spectra reflect the κ2N1,N2 mode of coordination. The electronic properties of 1-5 were investigated by time-dependent density functional theory calculations in the singlet and triplet states. The ligands and their Re(i) complexes were assessed for their potential antimicrobial activity. Compound 5 was screened against non-malignant cell line (noncancerous human embryonic kidney cell line (HEK293)) as well as evaluated for its blood compatibility.